Ag Nanostructures: from Fabrication to Interaction · cycling in 8M KOH. (Appendix C). 130 Table 5.1. Analysis of the solution by GFAA after carrying out CV experiments Ag surfaces

Ag Nanostructures: from Fabrication to Interaction

by

Farkhondeh Fathi

A thesis submitted in conformity with the requirements for the degree of the requirements for the degree of Doctor of Philosophy

Chemistry Department University of Toronto

© Copyright by Farkhondeh Fathi (2013)

ii

Ag Nanostructures: from Fabrication to Interaction

Farkhondeh Fathi

Doctorate of philosophy

Chemistry Department

University of Toronto

2013

Abstract

Electrochemical cycling of Ag surfaces under basic conditions has led to the discovery of a new,

simple, rapid and cost effective method for the preparation of nano-structured Ag surfaces with

features ranging from 30–150 nm in diameter. Our results indicated that during cyclic voltammetry,

the surface was oxidized, resulting in the formation of soluble Ag complexes which were re-

deposited as elemental Ag nanostructures (NSs) on the cathodic scan. The electrochemical

properties of the Ag NSs were greatly affected by the presence of organophosphonates and other

additives (vide infra), which also influenced the growth of nanostructures. The interaction of these

Ag NSs with malathion and paraoxon were explored in more detail using Surface Enhanced Raman

Spectroscopy. Results showed that generally smaller nanosized features resulted in high quality

surface Raman enhancement. Next, Ag NSs’ properties in the presence of organophosphonates was

investigated in tap water and apple juice in order to address issues related to matrix effects and

potential interference from constituents in solution. Electrochemical and localized surface plasmon

resonance results demonstrated the ability to detect organophosphonates in real samples, albeit at a

lower limit of detections and without any selectivity to any particular organophosphonate. Next, the

morphology and corrosion behaviour of Ag surfaces was explored in the absence and presence of

surfactants and capping agents. Results demonstrated the protection of Ag surfaces against

iii

corrosion in the presence of Tween-20, while potassium citrate presence enhanceed corrosion of

silver surfaces, resulting in the formation of a pitted surface with smaller Ag NSs. Lastly, the

interactions of 2-cyano-3-(2′-(5′,10′,15′,20-tetraphenyl porphyrinato zinc-(II))yl) acrylic acid-

modified Ag NSs with biomolecules were explored using spectro-electrochemical techniques. The

photocurrent response of porphyrin-modified Ag NSs was quenched by the addition of adenosine-

5’-monophosphate (AMP), guanosine-5’-monophosphate (GMP) and cytidine5’-monophosphate

(CMP), with a quenching efficiency of 80%, 68% and 48% for AMP, CMP and GMP, respectively.

iv

Dedication

To my beloved parents and sisters for their endless love and affections, my best friends Zeinab

Mehdipour and Mahnoosh Zebardast for their support and friendship and to anyone else who has

ever dared to work with extremely toxic chemicals.

v

Acknowledgments

First and foremost, praises and thanks to God, the Almightly, for his showers of blessings through

my life.

I would like to express my deepest appreciation to my advisor, Professor Heinz-Bernhard Kraatz,

for supporting me during my PhD studies. Bernie’s kindness and intelligence continue to inspire

me.

My sincere gratitude also goes to my thesis advisory and exam committees; Professor Aaron

Wheeler, Professor Gilbert Walker, Professor Ulrich Krull and Dr. Sylvie Morin who generously

offered their time to provide me with valuable comments toward my thesis improvement.

I am also grateful to the collaborators for lending me their expertise; Dr. Francois Lagugne-

Labarthet at Western University, Professor Yi-Tao Long and his group members at East China

University of Science and Technology. Special thanks to Yi-Lun Ying and Kong Cong.

I thank all the past and present members of Kraatz’s group. Special thank to Meghan Schlitt,

Chandrika Vijayaratnam, Sowole Modupeola and Dr. Piotr M. Diakowsky.

My profound gratitude goes also to the support staff in both chemistry departments of University of

Western Ontario and University of Toronto. Special thanks to Dr. Todd Simpson and Tim

Goldhawk at University of Western Ontario and Bill Nicholson and Tony Adamo at Department of

Physical and Environmental Sciences at University of Toronto Scarborough.

I deeply thank my parents and sisters for their unconditional love and support and my best friends,

Zeinab and Mahnoosh, for their encouragement, inspiration and wonderful friendship. I also reserve

special thank to Karram, Leishman, Spring-Doelman, Sarah and Elliot Rossiter families for all their

support and kindness during my stay in Canada.

Finally I am extending my appreciation to all the people who have supported me during this journey

directly or indirectly.

vi

Table of Contents

Abstract ................................................................................................................................................ ii

Dedication ........................................................................................................................................... iv

Acknowledgments................................................................................................................................ v

Table of Contents ................................................................................................................................ vi

List Of Publications ............................................................................................................................ xi

List of Tables ..................................................................................................................................... xii

List of Schemes ................................................................................................................................. xiii

List of Figures ................................................................................................................................... xiv

List of Abbreviations ....................................................................................................................... xxii

Chapter 1. Silver Nanostructure Fabrication and Some Applications ................................................. 1

1.1 Introduction ............................................................................................................................... 1

1.1.1 Nanotechnology ................................................................................................................ 1

1.1.2 Nanostructure Materials .................................................................................................... 2

1.1.3 Metal Nanostructures ........................................................................................................ 2

1.2 Synthesis of Silver Nanoparticles and Nanostructures ............................................................. 3

1.2.1 Chemical Reduction Method ........................................................................................... 3

1.2.2 Thermal, Sonochemical or PhotoChemical Decomposition of an Organometallic

Complex ............................................................................................................................................... 5

1.2.3 Biochemical Methods ...................................................................................................... 6

1.2.4 Electrochemistry .............................................................................................................. 7

1.3 Overview of Ag NSs Application ........................................................................................... 13

1.3.1 Electroanalysis Using Ag NSs ........................................................................................ 13

1.3.1.1 Colloidal Ag Application for Electroanalysis ........................................................ 14

1.3.1.2 Application of Electrodeposited Ag NSs for Electroanalysis ................................ 17

vii

1.3.2 Application of Ag NSs for Plasmonic Purposes ............................................................. 20

1.3.2.1 Colloidal Nanostructures ....................................................................................... 22

1.3.2.2 Self-assembled Metal Nanostructures .................................................................... 22

1.3.2.3 Lithography Technique .......................................................................................... 24

1.4 Objective ................................................................................................................................. 26

1.5 References ............................................................................................................................... 27

Chapter 2. Chemical Behavior of Electrochemically-Generated Nanostructured Silver Surfaces .... 31

2.1. Introduction ............................................................................................................................ 32

2.2 Experimental Methods ............................................................................................................ 33

2.2.1 Reagents .......................................................................................................................... 33

2.2.2 Electrochemistry ............................................................................................................. 34

2.2.3. Surface Characterization ................................................................................................ 34

2.3 Results and Discussion ........................................................................................................... 34

2.4 Conclusions ............................................................................................................................. 44

2.5 Acknowledgement .................................................................................................................. 44

2.6 References ............................................................................................................................... 45

Chapter 3. Studies of the Interaction Two Organophosphonates with Nanostructured Silver Surfaces

............................................................................................................................................................ 47

3.1 Introduction ............................................................................................................................. 48


3.2.1 Reagents ......................................................................................................................... 49


3.2.3 Surface Characterization ................................................................................................. 50

3.2.4 Surface Enhanced Raman Spectroscopy Measurements ................................................ 50


viii

3.3.1 Surface Preparation ........................................................................................................ 50

3.3.2 Study of Paraoxon and Malathion................................................................................... 51

3.2.3 The Effect of Ag Nanostructure Size on SERS Enhancement Factor ............................ 59

3.2.4 Limit of Detection in SERS ............................................................................................ 60

3.3 Conclusions ............................................................................................................................. 61

3.4 Acknowledgement .................................................................................................................. 62

3.5 References ............................................................................................................................... 62

Chapter 4. Dual Localized Scanning Plasmon Resonance and Electrochemical Investigations of

Organophosphorus Insecticides Presence .......................................................................................... 64

4.1 Introduction ............................................................................................................................. 65


4.2.1 Reagents .......................................................................................................................... 66

4.2.3 Optical Measurements .................................................................................................... 67

4.2.4 Surface Preparation ......................................................................................................... 67

4.2.5 Immobilization of Insecticides ........................................................................................ 67

4.2.6 Surface Area Measurement ............................................................................................. 68


4.3.1 Electrochemical Studies of OP-modified Ag NSs .......................................................... 68

4.3.2 Localized Surface Plasmon Resonance .......................................................................... 71

4.4 Conclusion .............................................................................................................................. 73

4.5 Acknowledgements ................................................................................................................. 73

4.6 References .............................................................................................................................. 73

Chapter 5. Effects of Surfactants on Electrochemically Prepared Ag Nanostructures ...................... 75

5.1 Introduction ............................................................................................................................ 76


ix

5.2.1 Reagents .......................................................................................................................... 77


5.2.3 Surface Characterization ................................................................................................. 78

5.2.4 Graphite Furnace Atomic Absorption ............................................................................. 78

5.2.5 Transmission Electron Microscopy ................................................................................ 79

5.2.6 Mass Spectroscopy.......................................................................................................... 79


5.3.1 Surface Preparation ......................................................................................................... 79

5.3.2 Electrochemical Impedance Spectroscopy ..................................................................... 80

5.3.3 GFAA Measurement ....................................................................................................... 81

5.3.4 Analysis of the Surface Morphology .............................................................................. 82

5.4 Conclusion .............................................................................................................................. 86

5.5 Acknowledgements ................................................................................................................. 87

5.6 References ............................................................................................................................... 87

Chapter 6. Tailoring Zinc Porphyrin to the Ag Nanostructure Substrate: an Effective Approach for

Photoelectrochemical Studies in the Presence of Mononucleotides .................................................. 92

6.1 Introduction ............................................................................................................................. 93

6.2 Experimental Section .............................................................................................................. 94

6.2.1 Reagents .......................................................................................................................... 94


6.2.3 Electrode Modifications .................................................................................................. 95

6.2.4 Photocurrent Measurements of Functionalized Electrodes ............................................. 95

6.2.5 UV-Vis Spectroscopy ..................................................................................................... 95

6.2.6 X-ray Photoelectron Spectroscopy ................................................................................. 96

6.2.7 Time-of-flight Secondary Ion Mass Spectrometry ......................................................... 96

x

6.2.8 Surface Enhanced Raman Spectroscopy Measurements ................................................ 96


6.3.1 Electrochemical Characterization of Porphyrin Modified Ag NSs ................................ 96

6.3.2 Surface Characterization of Porphyrin Modified Ag NSs .............................................. 98

6.3.3 Photocurrent Response of the Porphyrin Functioned Ag NSs Surface......................... 100

6.3.4 Photocurrent Action of the Functionalized Electrode ................................................... 102

6.3.5 Nucleotide Interaction Effect on the Photocurrent Response of Por-Ag NSs Electrode

.......................................................................................................................................................... 104

6.4 Conclusions ........................................................................................................................... 106

6.5 Acknowledgements ............................................................................................................... 106

6.6 References ............................................................................................................................. 107

Chapter 7. Conclusion and Future Outlook ..................................................................................... 111

7.1 References ............................................................................................................................. 113

Appendix A. Supplementary Information for Chapter 2 ................................................................. 114

Appendix B. Supplementary Information for Chapter 3 ................................................................. 124

Appendix C. Supplementary Information for Chapter 4 ................................................................. 130

Appendix D. Supplementary Information for Chapter 5 ................................................................. 140

Appendix E. Supplementary Information for Chapter 6 .................................................................. 151

Copyright Acknowledgements......................................................................................................... 157

xi

List Of Publications

1 Fathi, F., Vijayaratnam, C., Kraatz, H.B., Dual Localized Scanning Plasmon Resonance and

Electrochemical Investigations of Organophosphorus Insecticides Presence, Submitted.

2 Fathi, F., Kraatz, H.B., Effects of Surfactants on Electrochemically Prepared Ag

Nanostructures, Analyst (accepted).

3 Fathi F., Kong, C., Wang, Y., Xie, Y., Kraatz, H.B., Tailoring Zinc Porphyrin to the Ag

Nanostructure Substrate: an Effective Approach for Photoelectrochemical Studies in the

Presence of Mononucleotides, Analyst, 2013, 138, 3380 – 3387 (Highlithed on Journal Cover).

4 Fathi, F., Lagugné-Labarthet, F., Pedersen, D.B., Kraatz, H.B., Studies of the Interaction of

Two Organophosphonates with Nanostructured Silver Surfaces, Analyst, 2012, 137, 4448-4453.

5 Fathi, F., Schlitt, M., Pedersen, D.B., Kraatz, H.B., Chemical Behavior of Electrochemically-

Generated Nanostructured Silver Surfaces, Langmuir, 2011, 27, 12098–12105.

6 Li, Y.T., Qu, L.L, Li, D.W., Song, Q.X., Fathi, F., long, Y.T., Rapid and Sensitive In-situ

Detection of Polar Antibiotics in Water Using a Disposable Ag-Graphene Sensor based on

Electrophoretic Preconcentration and Surface Enhanced Raman Spectroscopy, Biosens

Bioelectron, 2012, 43, 94-100.

http://www.academia.edu/2301272/Chemical_Behavior_of_Electrochemically-Generated_Nanostructured_Silver_Surfaces

http://www.academia.edu/2301272/Chemical_Behavior_of_Electrochemically-Generated_Nanostructured_Silver_Surfaces

xii

List of Tables

Table 1.1. Summary of the Ag NSs shapes, LSPR absorption peaks and

applications.

23

Table A.2.1. The nano particle sizes (nm) and their standard deviation for

different concentrations of KOH. (Appendix A).

117

Table 3.1. SERS spectrum assignment of the Ag foil surface cycled in 10

mM paraoxon in 8 M KOH.

54

Table 3.2. Interpretation of SERS spectra from adsorbed thiophosphonate

(Scheme 3.5b) on Ag foil surface cycled in 10 mM malathion in

8 M KOH.

58

Table C.4.1. Results for real surface area of nanostructured Ag surfaces after

cycling in 8M KOH. (Appendix C).

130

Table 5.1. Analysis of the solution by GFAA after carrying out CV

experiments Ag surfaces for 16 cycles, in the range of -0.5 to 0.9

V.

85

Table D.5.1. Impedance parameter values. 141

Table 6.1. Atomic percentage of immobilized porphyrin film and C 1s XPS

results.

100

xiii

List of Schemes

Scheme 2.1. Chemical drawing of Tabun (left) and diethylcyanophsphonate

(DECP, right).

33

Scheme 3.1. Chemical structure of paraoxon (left) and malathion (right). 51

Scheme 3.2. Hydrolysis of malathion in alkaline solution to form an

alkylthiophosphate and a thiolate species.

56

Scheme 3.3. Proposed complexation of surface bound Ag (I) generated

electrochemically during cycling ions by a thiophosphate group.

58

Scheme 4.1. Chemical structure of ethion 1, fenthion 2 and malathion 3. 68

Scheme 5.1. Chemical structure of the potassium citrate 1, PVP 2 and Tween-

20 3.

79

Scheme 5.2. Proposed formation of Ag nanostructures (a) particle coalescence

mechanism in the presence of citrate and KOH (b) proposed

Ostwald ripening on the Ag surface after redeposition of Ag NSs.

84

Scheme 6.1. Chemical structure of the Zn-porphyrin 1, adenosine-5’-

monophosphate 2, cytidine 5-monophosphate 3, and guanosine

5’-monophosphate 4.

101

Scheme 6.2. Schematic view of the surface modification. After porphyrin

immobilization on the surface (A), the nucleoside

monophosphate is added (B).

103

Scheme 7.1. Interaction of desire analyte with recognition element modified

Ag NSs surface in the matrix environment gives electrochemical

signal.

113

xiv

List of Figures

Figure 1.1. TEM images and histograms for Ag NPs in different systems. 4

Figure 1.2. Photochemical fabrication of Ag NPs with various morphologies. 6

Figure 1.3. Biologically fabricated spherical Ag NPs in the presence of

Chenopodium album leaf extract as a reducing agent.

7

Figure 1.4. Electrochemcial Synthesis of Ag NPs. 8

Figure 1.5. Electrochemical synthesis of Ag NPs. 9


Figure 1.7. Electrochemical formation of Ag NPs by Ag anode dissolution

and its reduction at cathode electrode in distilled water.

11



Figure 1.10. Fabrication of modified ITO electrode:with Ag NPs. 14

Figure 1.11. Electrochemical detection of Cl-

by Ag NPs modified ITO

electrode.

15

Figure 1.12. Electrochemical response of Ag NPs modified Ag electrode. 16

Figure 1.13. Electrochemical detection of H2O2. 17

Figure 1.14. Detection of Cr(VI) by electrodeposited Ag NSs. 18

Figure 1.15. Schematic representation of electrode preparation for detetion of

DNA Using Ag NSs.

19

Figure 1.16. The DPV response of modified GC electrode with Ag NPs to

DNA hybridation.

20

Figure 1.17. The oscillation of conduction electrons and their relative move

from the positively charged nuclei.

21

xv

Figure 1.18. Assembly technique for Au and Ag NPs monolayers. 22

Figure 1.19. Scheme for two fabrication methods for SERS substrates

production using EBL technique.

25

Figure 2.1. A typical cyclic voltammetry scan of Ag foil in a solution of 8 M

KOH at a scan rate of 0.15 Vs-1

in a potential range of -0.5 and

0.9 V vs. Ag/AgCl.

35

Figure 2.2. Results of a combined SEM and EDX study of the effects of

electrochemical cycling a Ag surface at a scan rate of 0.15 Vs-1

in

the potential range of 0.5 and 0.9 V vs. Ag/AgCl for 15 CV scans

in 8 M KOH.

36

Figure 2.3. XPS of the white nanostructured material showing signals at

binding energies of 368.3 and 374.0 eV characteristic of the Ag

3d3/2 and 3d5/2 signals of Ag (0).

37

Figure 2.4. XRD pattern spectrum of the deposited Ag NPs on the surface of

Ag foil formed by electrochemical cycling.

38

Figure 2.5. SEM study of the effects of electrochemical cycling of the silver

foil in a potential range of -0.5 and 0.9 V vs. Ag/AgCl at a scan

rate of 0.150 mVs-1

at a supporting electrolyte concentration of 8

M KOH.

39

Figure 2.6. SEM study of the effects of supporting electrolyte concentration

on the surface morphology of the deposited Ag NP film.

41

Figure 2.7. Electrochemical behavior of Ag exposure a) KCN (no KCN

added _____

; 1 pM KCN ----; 100 µM KCN …..

) and b) DECP (no

DECP added _____

; 1pM DECP ----; 100µM DECP …..

) in 1M

KOH at a scan rate of 0.100 Vs-1

in a potential range of -0.5 and

0.9 V vs. Ag/AgCl.

42

Figure 2.8. SEM images of the silver surface as a function of KCN and

DECP addition.

43

Figure A.2.1.

Particle size distribution for prepared Ag NPs after exposure to 8

M KOH at 0.15 Vs-1

between -0.5 and 0.9 V vs. Ag/AgCl for 1

(a), 5 (b), 10 (c) and 15 (d) CV cycles respectively (Appendix A).

114

xvi

Figure A.2.2. Particle size distribution for prepared Ag NPs after exposure to 1

M KOH at 0.15 Vs-1

between -0.5 and 0.9 V vs. Ag/AgCl for 1

(a), 5 (b), 10 (c) and 15(d) CV cycles respectively (Appendix A).

115

Figure A.2.3. Particle size distribution for prepared Ag NPs after exposure to

0.1 M KOH at 0.15 Vs-1

between -0.5 and 0.9 V vs. Ag/AgCl for

1 (a), 5 (b), 10 (c) and 15(d) CV cycles respectively (Appendix

A).

116

Figure A.2.4. Particle size distribution for prepared Ag NPs after exposure to

0.01 M KOH at 0.15 Vs-1

between -0.5 and 0.9 V vs. Ag/AgCl

for 15 CV cycles (Appendix A).

116

Figure A.2.5. SEM Images of Ag NP-decorated ITO (Appendix A). 117

Figure A.2.6. EDX analysis of ITO surface after exposure to 0.5 M K [Ag (CN)

2] solution at 0.15 Vs-1

between 0.5 and 0.9 V vs. Ag/AgCl for 15

CV scans, shows that metallic silver deposits on the surface due

to cycling and reducing K [Ag (CN) 2] (Appendix A).

118

Figure A.2.7. Mass spectra for 0.5 M K[Ag(CN)2] in water as a control test

(Appendix A).

119

Figure A.2.8. Mass spectra for the solution of 100 µM KCN in 1 M KOH after

exposure to Ag bulk and cycling at 0.10 Vs-1

between 0.5 and 0.9

V vs. Ag/AgCl for 15 CV scans (Appendix A).

120

Figure A.2.9. SEM images of the silver surface as a function of KCN addition

(a) 10 pM, b) 100 pM, c) 1 nM d) 10 nM e) 100 nM f) 1 µM g)

10 µM ) (Appendix A).

121

Figure A.2.10. SEM images of the silver surface as a function of DECP addition

a) 10 pM, b) 100 pM, c) 1 nM d) 10 nM e) 100 nM f) 1 µM g) 10

µM ) (Appendix A).

122

Figure A.2.11. Chronoamperometry measurements and SEM images for two

steps of Ag oxidation, (a) oxidation of Ag to Ag (I) and (b)

oxidation of Ag (I) to Ag (II) vs. Ag/AgCl in 8 M KOH Solution

(Appendix A).

123

Figure A.2.12. A typical cyclic voltammetry scan of Ag foil in a solution of 1.0

M KOH at a scan rate of 0.100 Vs-1

in a potential range of -0.5

and 0.9 V vs. Ag/AgCl (Appendix A).

123

xvii

Figure 3.1. Cyclic voltammograms of Ag foil in 8 M KOH (_____

) and after

addition of 10 mM paraoxon in 8 M KOH (…….

).

52

Figure 3.2. SEM image Ag foil cycled in 10 mM Paraoxon in 8 M KOH. 52

Figure 3.3. Raman spectrum of pure paraoxon and SERS spectrum of the Ag

foil surface cycled in 10 mM paraoxon in 8 M KOH.

53

Figure 3.4. Effects of malathion interaction with Ag NSs on the cyclic

voltammogram of Ag foil.

55

Figure 3.5. SEM image of the Ag foil cycled in 10 mM malathion in 8 M

KOH.

56

Figure 3.6. Raman spectrum of pure malathion and SERS spectrum of the

Ag foil surface after cycling in 10 mM malathion in 8 M KOH.

57

Figure 3.7. SERS spectra of Ag foil surfaces after cycling in 10 mM

paraoxon in 8 M KOH.

59

Figure 3.8. SERS spectra of the Ag foil surface cycled in a wide range of

Malathion concentration (1 mM - 1 pM) in 8 M KOH.

61

Figure B.3.1. SEM images of the initial Ag foil (Appendix B). 124

Figure B.3.2. Effects of paraoxon addition on the cyclic voltammograms of

gas-phase generated Ag NPs on ITO (Appendix B).

124

Figure B.3.3. SEM images of the Ag NPs decorated ITO cycled in (a) 8 M

KOH, (b) 10 mM Paraoxon in 8 M KOH (Appendix B).

125

Figure B.3.4.

Effects of malathion addition on the cyclic voltammograms of

gas-phase generated Ag NPs on ITO (Appendix B).

125

Figure B.3.5. SEM image of the Ag NPs decorated ITO cycled in 10 mM

malathion in 8 M KOH (Appendix B).

126

Figure B.3.6. EDX analysis from the Ag foil cycled in 10 mM Malathion in 8

M KOH (Appendix B).

126

Figure B.3.7. Electrospray mass spectroscopy in positive mode of AgNO3 with

malathion in water (Appendix B).

127

xviii

Figure B.3.8. Electrospray mass spectroscopy in positive mode of AgNO3 with

malathion in water (Appendix B).

128

Figure B.3.9. SERS spectrum of the Ag foil surface cycled in a wide range of

paraoxon concentration (1 mM- 1 pM) in 8 M KOH (Appendix

B).

128

Figure 4.1. SWVs of modified Ag NSs substrates with OPs in water (A) and

apple juice (B).

69

Figure 4.2. Bode plots of modified Ag NSs with OPs in tap-water (A) and

apple juice (B).

70

Figure 4.3. LSPR of OP modified Ag NSs.. 72

Figure C.4.1. Typical underpotential deposition (UPD) graph (Appendix C). 130

Figure C.4.2. Electrochemical response of modified Ag NSs substrates with

various concentration of ethion (Appendix C).

131


various concentration of fenthion (Appendix C).

132


various concentration of malathion (Appendix C).

133

Figure C.4.5. Data fitting of Bode plots for modified Ag NSs with 100 nM (A)

10 µM (B), 100 µM (C) and 1mM (D) ethion (Appendix C).

134

Figure C.4.6. Data fitting of Bode plots for modified Ag NSs with 10 µM (A),

100 µM (B) and 1mM (C) fenthion (Appendix C).

135

Figure C.4.7. Data fitting of Bode plots for modified Ag NSs with 10 µM (A),

100 µM (B) and 1mM (C) malathion (Appendix C).

136

Figure C.4.8. Data fitting of Bode plots for modified Ag NSs with 100 µM (A)

and 1mM (B) Ops in tap-water (Appendix C).

137

Figure C.4.9. Data fitting of Bode plots for modified Ag NSs with 100 µM (A)

and 1mM (B) Ops in apple juice (Appendix C).

137

Figure C.4.10. LSPR response of modified Ag NSs with different concentrations

of ethion (Appendix C).

138

xix


of fenthion (Appendix C).

138


of malathion(Appendix C).

139

Figure 5.1. Bode plots log Z versus log f after cycling a Ag working

electrode surface 16 times in the 0.1 M KOH (a) and 0.01 M

KOH (b) in the presence and absence capping agents and

surfactant.

80

Figure 5.2. SEM study of the effects of capping agents and surfactants on the

surface morphology of deposited Ag NS film.

83

Figure D.5.1. Fitting of impedance spectra. 140

Figure D.5.2. Calibration curve of AgNO3 solution for GFAA measurements

(Appendix D).

141

Figure D.5.3. The film thickness of formed deposited Ag NSs on the Ag

surface after cycling Ag foil in the potential range of -0.4 to 0 V

vs Ag/AgCl at a scan rate of 150 mVs-1

in the 0.1 M KOH (a)

and in the 0.1M KOH with citrate (b) (Appendix D).

142

Figure.D.5.4. Mass spectra for 0.01M KOH after exposure to Ag bulk and

cycling at 150 mVs-1


CV scans (Appendix D).

143

Figure D.5.5. Mass spectra for 0.01M KOH after exposure to Ag bulk and




144





145

Figure D.5.7. Mass spectra for 0.01M KOH with citrate after exposure to Ag

bulk and cycling at 150 mVs-1

between 0.5 and 0.9 V vs.

Ag/AgCl for 16 CV scans (Appendix D).

146





147

xx





148





149

Figure D.5.8.11. TEM images from the solution of 0.01 M KOH (a), 0.01M KOH

with citrate (b), 0.01M KOH with PVP and 0.01M KOH with

Tween-20 after cycling Ag for 16 times in the range of -0.5 to 0.9

V vs Ag/AgCl (Appendix D).

150

Figure 6.1. Electrochemical behavior of Ag NSs before and after

modification with porphyrin.

97

Figure 6.2. Positive secondary ion mass spectrum of the porphyrin film on

Ag NSs surface.

99

Figure 6.3. Representative XPS spectrum of the Ag NSs surface after

immobilization of porphyrin.

99

Figure 6.4. Photocurrent response of bare Ag (a), bare Ag NSs (b), modified

Ag (c) and modified Ag NSs (d) with porphyrin in 0.4 M sodium

acetate buffer (pH = 6.4).

102

Figure 6.5. Absorption of 25 µM porphyrin in THF (solid line) and the

photocurrent response of the Por-Ag NSs electrode (Square

spots) measured in the 0.4 M sodium acetate buffer solution (pH

6.4) containing 10 mM EDTA.

103

Figure 6.6. Photocurrent response of the Por-Ag NSs electrode to different

nucleotides in sodium acetate buffer solution (pH 6.4) containing

EDTA in the absence (a) and present (b) of nucleotides.

104

Figure 6.7. Photocurrent quenching percentage after addition of different

nucleotides.

105

Figure E.6.1. Raman spectrum of pure 2-Cyano-3-(2′-(5′,10′,15′,20-

tetraphenylporphyrinato Zinc-(II))yl)acrylic acid and SERS

spectrum of the Ag foil surface cycled 5 times in 8 M KOH

(Appendix E).

151

xxi

Figure E.6.2. Positive secondary ion mass spectrum of the porphyrin film on

Ag NSs surface (Appendix E).

152

Figure E.6.3. Angle resolved x-ray photoelectron spectroscopy (ARXPS) of

modified Ag NSs with porphyrin (Appendix E).

153

Figure E.6.4. A series of Zn 2p spectra in different angles (Appendix E). 153

Figure E.6.5. A series of N1s spectra in different angles (Appendix E). 154

Figure E.6.6. A series of C1s spectra in different angles (Appendix E). 154

Figure E.6.7. A series of O1s spectra in different angles (Appendix E). 155

Figure E.6.8. A series of Ag 3d 3/2 and Ag 3d 5/2 spectra in different angles

(Appendix E).

155

Figure E.6.9. The effect of various electron donors on the photocurrent

response of the modified Por-Ag NSs (Appendix E).

156

xxii

List of Abbreviations

AJ Apple juice

AMP Adenosine-5’-monophosphate disodium hexahydrate

ARXPS Angle resolved X-ray photoelectron spectroscopy

Cdl Double layer capacitance

Cf Film capacitance

CPE Constant phase element

CV Cyclic voltammetry

CT Charge transfer

CWAs Chemical warfare agents

CMP Cytidine 5-monophosphate sodium salt

DNA Deoxy ribonucleic acid

DSS Dioctyl sodium sulfosuccinate

DMF N,N-dimethylformamide

DPV Differential pulse voltammetry

DECP Diethylcyanophosphonate

EIS Electrochemical impedance spectroscopy

EM Electromagnetic mechanism

EBL Electron beam lithography

EDX Energy-dispersive X-ray

EMS Electrospray mass spectrometry

EI Electroionization

EDTA Ethylenediaminetetraacetic acid

FIB Focused ion beam

xxiii

GC Glassy carbon

GFAA Graphite furnace atomic absorption

GMP Guanosine 5’-monophosphate disodium salt

ITO Indium tin oxide

LSPR Localized surface plasmon resonance

LSV Linear sweep voltammetry

LOD Limit of detection

MWCNTs Multiwalled carbon nanotubes

MV Methyl viologen dichloride

NSs Nanostructures

NPs Nanoparticles

OP Organophosphonate

OHP Outer Helmholtz plane

PVP Polyvinylpyrolidone

PDDA Polydiallyldimethylammonium chloride

PBS Phosphate buffer saline

PAA Poly(trans-3(3-pyridyl)acrylic acid)

PMMA Poly(methyl methacrylate)

Por Porphyrin

Rct Charge transfer resistance

Rs Solution resitance

Rf Film resistance

SERS Surface enhanced Raman spectroscopy

SDS Sodium dodecyl sulfate

SWCNT Single walled carbon nanotubes

xxiv

SEM Scanning electron microscopy

TGA Thioglyconicacid

Tween-20 Polyoxyethylene(20) sorbitan monolaurate

TEM Transmittance electron microscopy

THF Tetrahydrofuran

TOF-SIMS Time-of-flight secondary ion mass spectrometry

UPD Underpotenial deposition

W Warburg impedance

W Water

XRD X-ray diffraction

XPS X-ray photoelectron spectroscopy

1

Chapter 1 Silver Nanostructure Fabrication and Some Applications

1.1 Introduction

Synthesis and application of metal nanoparticles has attracted much interest among various areas

of science and engineering in recent decades. Because of potential application of nanometals in

various areas it is critical to understand their interaction with different additives in order to

development of devices which provide better efficiency compared with their crystalline form.

Therefore the main focus of research described in this thesis was to investigate the interaction of

electrochemically fabricated nanostructured Ag surfaces with various additives such as

organophosphates, surfactants, capping agents and biomolecules. This work provided new

information related to the interaction of these additives with Ag NSs and highlighted their

potential applications as SERS, localized surface plasmon resonance (LSPR) and chemosensor

substrates.

1.1.1 Nanotechnology

―There is plenty of room at the bottom‖, this statement by Nobel physicist, Richard Feyman on

December 29th

1959, during his presentation at American physical society meeting, is widely

accepted as the flint that ignited the present ―nano‖ age.1 The prefix ―nano‖, ―dwarf‖ in Greek,

described as one billionth in the metric system, is numerically expressed as a factor of 10-9

m or

0.000000001 m.2 To put this scale in perspective, the average diameter of a human hair is about

75000 nm.3

Establishing a solid definition for nanotechnology is difficult since it is often tailored to fit the

researchers and their fields. The general definition by the national nanotechnology initiative 3 is

as follows:

2

Nanotechnology involves the development of research and technology in the range of 1 to

100 nm.

Nanotechnology is the fabrication and application of structures that have novel properties

because of their small size.

Nanotechnology is established on the ability to control or manipulate on the atomic scale.

1.1.2 Nanostructure Materials

Nanostructured materials, or synonymously nanophase materials, nanocrystalline materials or

supramolecular solids, are materials with a few nanometer sizes at least in one direction. 4-5

These materials can be divided into three categories.

The first class embraces materials with decreased dimensions and/or dimensionality in the form

of isolated, substrate-supported or embedded nano-size particles, thin wires or thin films.4

The second class are materials where the nanometer-sized microstructure are limited to the

surface region of a bulk material with a thin nanometer size.4

The third category is bulk solids with nanosize microstructures. It means their chemical

composition, atomic arrangement and/or the size of their building blocks changes on a length

scale of a few nanometers throughout the bulk.4

Nanostructured materials show electronic, physical and chemical properties which are

substantially different from their bulk counterparts. These unique properties are due to their large

surface area which alter their physical and thermodynamic properties.6-8

One of the major

categories of nanostructure materials is metal nanostructures which is the focus of our

introduction.

1.1.3 Metal Nanostructures

Coinage metals such as silver (Ag), gold (Au) and copper (Cu) have had an important role

throughout human civilization. In fact, few other materials have been more involved in human

3

life and history.9 Metals remain important today despite the popularity of silicon and polymeric

materials and new uses are continually being discovered in nanoscience.9

Metal nanostructures were first used as decorative materials.10

One example is the famous

Lycurgus cup (4th

century AD), which is currently held at the British museum.11

It contains a very

small quantities (~70 nm) of nano Ag and Au in the approximate ratio of 14:1, which are the

source of the unique feature of the cup where the color changes depending on the light’s angle of

incidence. It looks red when light is radiated from inside while it appears green once viewed in

reflected light.11

Among noble metals, Ag has attracted extensive interest of the scientific community due to its

outstanding electrical and thermal conductivity, unique qualities in terms of plasmonic abilities

and low cost.12

On the downside, Ag nanostructures (NSs) or more accurately Ag+ ions, detach

from the surface which are considered to be toxic.12

Since the properties of metal nanostructures and also Ag mainly rely on their size and shape,

many techniques have been developed to produce a wide variety of nanostructures with tailored

characteristics.12

These techniques include chemical reduction, photochemistry, sonochemistry,

thermochemistry, biochemistry and electrochemistry.13

The major route reported in the literature

is formation of metal nanoparticles by chemical reduction method.14

We discuss here briefly the

Ag NSs synthesis methods and their advantages and disadvantages.

1.2 Synthesis of Silver Nanoparticles and Nanostructures

1.2.1 Chemical Reduction Method

Transition metal salt reduction is the most popular applied technique for fabrication of nano

metal colloidal suspensions due to its relative ease of synthesis. This route was presented by

Faraday in 1857.15

The advantages of the chemical reduction method are mild reaction

conditions, simple separation procedures, low energy consumption, high yields and

approximately short reduction times.16

4

Synthesis of Ag NSs by chemical reduction was proposed by Lee and Meisel in 1982 for the first

time.17

They used a solution of AgNO3 added dropwise to vigorously stirred solution of NaBH4

in ice. A 1% polyvinyl alcohol was added during the reduction and the mixture was boiled to

decompose of excess NaBH4. In another set of experiment AgNO3 was dissolved in H2O and

brought to boiling then a 1% solution of citrate was added and the solution was kept boiling for 1

hr. Their technique is still a popular strategy for fabrication of Ag colloids.

Figure 1.1. TEM images and histograms for Ag NPs in different systems: (A and B) DSS (dioctyl sodium

sulfosuccinate)-water-dodecane; (C and D) SDS (sodium dodecyl sulfate)-water-isoamylalcohol-cyclohexane.

Interestingly particle size and size distribution are affected by solvent, reducing and surfactant agents.1 ―Reproduced

from Material Science and Engineering B, Vol.142, W. Zhang, X. Qiao, Synthesis of silver nanoparticles—Effects of

concerned parameters in water/oil microemulsion, p. 1-15, Copyright (2007), with permission from Elsevier.‖

In all of the chemical reduction techniques for Ag NSs fabrication, firstly nucleation is started

from a supersaturated solution. The formed nuclei may further enlarge through a diffusion

5

mechanism. Secondly particles are formed by primary particle agglomeration. This process may

be facilitated by altering the solutions’ ionic strength, or pH.16

The final morphology of the

fabricated Ag NSs is controlled by experimental conditions, addition of surfactants and reducing

agents.16

Figure 1.1 demonstrates the effect of surfactant application on the Ag NPs size and

dispersity. Interestingly, as seen in Figure 1.1 the application of bis(2-ethylhexyl)sulfosuccinate

has resulted to the smaller diameter and more disperse Ag NPs while lauryl sodium sulfate

resulted to larger Ag NPs with lower dispersity in two different microemultion systems due to

their different structural features and charge properties.

1.2.2 Thermal, Sonochemical or PhotoChemical Decomposition of

an Organometallic Complex

For the methodology presented in this section, Ag NSs with a wide range of morphologies are

produced by decomposition of either Ag or its organometallic salt using light, heat or ultrasonic

radiation effects.18-21

These techniques are appealing as precise control could be gained by metal

ion decomposition on the Ag seed surface and consequently potential control of the final

morphology and size of Ag NSs would be easier.22

These approaches could be used to fabricate

high-quality Ag NSs with different shapes including prisms23

, decahedrons24

, right bipyramids

25

and tetrahedrons26

from a precursor solution of Ag salt in the presence of reducing or stabilizing

agents (see Figure 1.2). In Figure 1.2, Ag nanoprism with the average edge length of 72±8 nm

have been produced by photoexcitation of a colloidal Ag NP passivated with sodium citrate and

bis(p-sulphonato-phenyl)phenylphosphine dihydrate dipotassium using visible light.23

Furthermore decahedral Ag NSs are fabricated as the result of photochemical transformation of

AgNO3 with NaBH4 in the presence of polyvinylpyrolidone (PVP) and citrate as stabilizers and

arginine as a photochemical promoter.24

In addition, fabrication of Ag bypiramides with edge

length 131±12 nm has been reported by irradiating a solution of AgNO3 containing citrate, bis(p-

sulfonatophenyl) phenylphosphine dehydrate dipotassium salt and NaOH using a halogen lamp

and a bandpass filter at 550±20 nm.26

Moreover, Tetrahedron Ag NSs with the average edge

length of 118±18 nm have been produces by illuminating a solution of AgNO3 containing

tartrate, citrate and PVP using a 70-W sodium.26

6

Figure 1.2. Photochemical fabrication of Ag NPs with various morphologies: (A) shows dual laser excitation Ag

seeds produced Ag prism with the edge length of 72±8 nm, scale bar is 200 nm, (B) demonstrates production of

decahedral Ag NPs with size of 57±4 nm by photochemical reduction of AgNO3, scale bar is 100 nm, (C) shows

photochemical production of right bypiramids Ag NPs with size of 131±12 nm, scale bar is 300 nm (inset of C: a

higher magnification view, with sacel bar 100 nm), (D) demonstrates the photochemical formation of tetrahedron Ag

NPs with the average edge length of 118±18 nm, the scale bar is 1 µm. (A) ―Reproduced by permission from

Macmillan Publishers Ltd: [Nature] (Controlling anisotropic nanoparticle growth through plasmon excitation, R.Jin,

Y.C. Cao, E. Hao, G. S. Metraux, G.C. Schatz, C.A. Mirkin, Nature 2003, 425, 487-490), Copyright (2003).‖ (B)

―Reproduced with permission from (B. Pietrobon, V. Kitaev, Photochemical synthesis of monodispese size-

controlled silver decahedral nanoparticles and their remarkable optical properties, Chem. Mater. 20, 5186-5190).

Copyright (2008), American Chemical Society.‖ (C) ―Reproduced with permission from John Wiley and Sons (J.

Zhang, S. Li, J. Wu, G. Schatz, A.C. Mirkin, Plasmon-mediated synthesis of silver triangular bipyramids, Angew.

Chem. Int. Ed. 2009, 48, 7787-7791).‖ (D) ―Reproduced with permission from (J. Zhou, J. An, B. Tang, S. Xu, Y.

Cao, B. Zhao, W. Xu, J. Chang, J.R. Lombardi, Growth of tetrahedral silver nanocrystals in aqueous solution and

their SERS enhancement, Langmuir 2008, 24, 10407-10413). Copyright (2008), American Chemical Society.‖

The main disadvantages of these methods are being expensive, multi-step, energy intensive and

they require the use of hazardous chemicals.27

1.2.3 Biochemical Methods

AgNO3 is the most common salt form of Ag used to produce Ag NSs by this method.27-28

Bacteria, microbes, fungus, proteins and plant parts such as seed, root, leaf are being used as

7

reducing agents for fabrication of Ag NSs in this method.27-31

A variety of Ag NSs morphology

has been reported by use of this method such as hexagonal, twinned cubic and spherical.28-30

Figure 1.3. shows the fabrication of spherical Ag NPs within the range of 10-30 nm by using the

leaf extract of Chenopodium album as a mild reducing agent from a AgNO3 solution.It has been

reported that COOH group in the plant extract acted as functional reducing groups.30

Figure 1.3. Biologically fabricated spherical Ag NPs in the presence of Chenopodium album leaf extract as a

reducing agent: TEM image (A) and size distribution histogram (B). ―Reproduced from Colloids and Surfaces A:

Physiochem. Eng. Aspects, Vol.369, A.Dhar Dwivedi, K. Gopal, Biosynthesis of silver and gold nanoparticles using

Chenopodium album leaf extract, p. 27-33, Copyright (2010), with permission from Elsevier.‖

The main disadvantages of biological techniques for Ag NSs preparation are application of

expensive materials, long preparation time and complicated procedures.27

1.2.4 Electrochemistry

An electrochemical method for metal NSs fabrication was first applied by Reetz and Helbig in

1994.32

They used a two-electrode set up in which the sacrificed electrode was a bulk metal

which was converted into metal NSs by anodic dissolution and cathodic deposition in the mixture

of acetonitrile and tetrahydrofuran media. Tetrahydroammonium salt was employed as the

supporting electrolyte and stabilizer for the metal clusters which prevented metal powder

agglomeration on the cathode.32-33

This method was then tailored for fabrication of 2-7 nm Ag

nanoparticles (NPs) in acetonitrile including tetrabutylammonium salts (see Figure 1.4).

8

Figure 1.4. Electrochemcial Synthesis of Ag NPs: (A):TEM image of Ag NPs produced by linear voltammetry

method at the current density of -1.35 mA.cm-2 with the average diameter of 6±0.7 nm in acetonitril in the presence

of tetrabutylammonium acetate, (B): size distribution of Ag NPs and their average size. The scale bar is 20 nm.

―Reproduced with permission from (L.R. Sanchez, M.C. Blanco, M.A. Lopez-Quintela, Electrochemical synthesis of

silver nanoparticles, Chem. Mater. 20, 5186-5190). Copyright (2008), American Chemical Society.‖

However it was necessary to deoxygenate the solution to avoid the oxidation of formed Ag NSs

and oxygen interference in the electrochemical process as they applied voltage in the range of -

2.5 to -0.5 V. 34

Yin et al reported the preparation of 1-35 nm spherical Ag NPs with the similar

strategy using Pt as a working electrode in KNO3 as the supporting electrolyte in the presence of

polyvinylpyrolidone (PVP). 35

Application of PVP significantly increases Ag particle formation

and decreases Ag deposition rate, therefore Ag NPs are more dispersed. Figure 1.8 demonstrates

CVs of platinum rod electrode in the presence and absence of PVP. The reduction peak at 0.53 V

is related to the Ag ions electrodepostion on Pt surface which are oxidized again at 0.36 V. A

comparison of CVs in the presence and absence of PVP demonstrates a decrease of current in the

presence of PVP compared to its absence. In the PVP-free solution, Ag ions almost completely

deposited on the Pt electrode after reduction, while in the presence of PVP, the reduced Ag ions

are devided into two parts: one part is electrodeposited on the Pt electrode, and the other part is

reduced to Ag NPs stabilized by PVP and remain in the solution, therefore the decrease is seen in

the oxidation peak.

9

Figure 1.5. Electrochemical synthesis of Ag NPs: (A) Cyclic voltammograms for the Pt electrode in 0.1 M KNO3

and 0.005 M AgNO3 with and without PVP. The presence of PVP resulted the decrease of oxidation peak due to

stabilizing Ag ions in the form of Ag NPs in the solution. ( B and C) are TEM image and distribution of forming Ag

NPs in the solution containing PVP. ―Reproduced with permission from (B. Yin, H. Ma, S. Wang, Electrochemical

synthesis of silver nanoparticles under protection of poly(N-vinylpyrrolidone), J. Phys. Chem. B 107, 8898-8904).

Copyright (2003), American Chemical Society.

The formed Ag NPs are quite spherical and their size is in the range of 1-35 nm (see Figure 1.5 ).

Their method requires the continuous stirring of the solution to avoid nanoparticle aggregation, in

addition the use of sodium dodecyl benzensulfonate is required as the co-stabilizer of Ag NPs in

the solution.35

10

A similar method was also employed for Ag NPs synthesis by Starowicz et al.36

A saturated

solution of AgNO3 in ethanol was deaerated by argon and a potential of -0.6 to 0.8 V was applied

to the working electrode in a three electrode system cell. As the result 20 nm Ag NPs were

precipitated in the solution (see Figure 1.6). The cyclic voltammetry (CV) studies were done for

Ag in a solution of NaNO3-C2H5OH. Ag was replaced by Pt in one experiment to find out more

details about the process. Ag oxidation happened above 0.4V in the anodic area and converted it

to Ag+ and again reduced in the cathodic wave. The higher potential for Ag was obtained without

compensation of the system resistivity; while CV shows a sharp increase for Pt above 0.6 V,

which could be attributed to ethanol oxidation.The size of Ag NPs is of the order of

approximately 20 nm by this method.36

Figure 1.6. Electrochemical synthesis of Ag NPs : (A) shows the CVs for Ag and Pt electrode in NaNO3-C2H5OH

solution with the scan rate of 0.01667 V/s. The scans with compensation of system resistivity are labled. (B) TEM

image of obtaining Ag NPs with a size of 20 nm. ―Reproduced from Electrochemistry Communication, Vol. 8, M.

Starowicz, B. Stypula, J. Banas, Electrochemical synthesis of silver nanoparticles, p. 227-230, Copyright (2006),

with permission from Elsevier.‖

Khaydarov et al. synthesized 2-20 nm near spherical Ag NPs in the presence of PVP. In their

method a constant voltage of 20V was applied in the temperature range of 20-95oC between two

Ag electrodes under intensive stirring (Figure 1.7); however the polarity of the current between

two electrodes had to be switched every 30-300 s.37

11

Figure 1.7. Electrochemical formation of Ag NPs by Ag anode dissolution and its reduction at cathode electrode in

distilled water. The release of O2 and H2 is due to oxidation and reduction of water at the anode and cathode

respectively. ―With kind permission from Springer Science+Business Media, J. Nanopart. Res., 11, 2009, 1193-

1200, Electrochemical method for the synthesis of silver nanoparticles, R. A. Khaydarov, R. R. Khaydarov, O.

Gapurova, Y. Estrin, T. Scheper, figure# 1‖

Figure 1.8. Electrochemical synthesis of Ag NPs: TEM image and size distribution of formed Ag NPs (A and B) in

the solution. The scale bar is 100 nm. C and D demonstrate SEM image and size distribution of deposited Ag NPs on

the cathodic electrode. The scale bar is 1 µm and 100 nm for C and D respectively. ―With kind permission from

Springer Science+Business Media, J. Nanopart. Res., 11, 2009, 1193-1200, Electrochemical method for the synthesis

of silver nanoparticles, R. A. Khaydarov, R. R. Khaydarov, O. Gapurova, Y. Estrin, T. Scheper, figure# 5, 8 and 9‖

12

In their technique as shown in Figure 1.7, Ag was dissolved by oxidation and migrated to the

cathode where it was reduced to zero-valent Ag atoms. Some of the Ag ions reduced on the

electrode surface with the average size of 40 nm, while some formed a solution of colloidal Ag

NPs with the average size of 7.3±3.1 nm after treatment with H2O2 (Figure 1.8).

Recently Rabinal et al. reported the preparation of Ag NPs with average size of 5.3 nm in the

presence of thioglyconicacid (TGA) as the capping and stabilizing agent in N,N-

dimethylformamide (DMF) as the solvent.38

they used two Ag electrodes as the anode and

cathode connected to DC power (150V). Ag NPs were prepared by polarizing the two electrodes

in dimethylfuran under a nitrogen atmosphere (Figure 1.9).

Figure 1.9. Electrochemical synthesis of Ag NPs: (A) demonstrates an experimental set up for electrochemical

synthesis of Ag NPs in the presence of 0.5M TGA in DMF solvent. (B) is typical TEM image of formed Ag NPs

with the average size of 5.3 nm in the solution. ―Reproduced from Journal of Alloys and Compounds, Vol. 562,

M.K. Rabinal, M.N. Kalasad, K. Praveen Kumar, V.R. Bharadi, A.M. Bhikshavartimath, Electrochemical synthesis

and optical properties of organically capped silver nanoparticles, p. 43-47, Copyright (2013), with permission from

Elsevier.‖

All of these electrochemical strategies lead to the fabrication of pure Ag NPs compared to other

methods where purification is required. Also these techniques are free of toxic reducing agents

such as NaBH4 which are reactive and corresponded to a high level of biological and

environmental risks.39

However the electrochemical methods still have some limitations such as

blocking the cathode by Ag NPs deposition; therefore there is still an important issue in the

research trends to find simpler, ecofriendly and more cost-effective strategies.37

13

1.3 Overview of Ag NSs Application

There has been a diverse range of applications for Ag NSs in different fields including catalysis,

40,41 water treatment,

42 medicine,

43 bioengineering,

44 textile engineering,

45 electronics,

46 and

optics.47

Two main applications of Ag NSs are employing them as electroanalysis and plasmonic

materials due to its unique properties over other noble metals and enhanced electrical and thermal

conductivity in nanosize compared to its crystalline form.12,48

Here some of its applications are

summarized as it would be the main focus of my thesis.

1.3.1 Electroanalysis Using Ag NSs

Electroanalysis is one of the most cost effective and facile techniques to use for determining

species in the solution compared to other methods such as chromatography, spectroscopy and

luminescence.48-49

Various electrochemical techniques such as cyclic voltammetry (CV),

stripping voltammetry, chronoamperometry, linear sweep voltammetry (LSV), electrochemical

impedance spectroscopy (EIS) and differential pulse voltammetry (DPV) have been applied to

leverage a selective and sensitive electrochemical responses.48,49

In this context application of nanostructured electrodes for electroanalysis have four main

advantages over macroelectrodes: high surface area, easy mass transport, catalysis effects and

control over local microenvironments.50

Specially large surface areas provide more active spots

leading to the higher signal to noise response.49

However, due to high reactivity of NSs, they

would passivate quickly because of aerial oxidation.48,51

Ag as a constructive electrode material is recognized as an important metal in the electroanalysis

of various analytes such as biomolecules, halides and toxic compounds.49

Moreover it is much

cheaper and less rare than gold which makes it more feasible for use as the electrode materials.49

Ag can be used in two forms for electroanalysis: colloidal form and electrodeposited onto a

substrate. In the next section, we have a look at these two forms of Ag NSs for electroanalysis of

different analytes.

14

1.3.1.1 Colloidal Ag Application for Electroanalysis

As it has been mentioned in the former paragraph there is a wide range of techniques for

fabrication of colloidal Ag NSs with diverse morphologies. There are some reports which used

them as electrode materials for electroanalytical purposes.

Recently a multilayer Ag NSs modified on indium tin oxide (ITO) surface has been applied for

the detection of chloride anions.52

In this method, ITO was firstly modified with 3-

aminopropyltriethoxysilane and then polydiallyldimethylammonium chloride (PDDA) was

employed as a bridging ligand for deposition of Ag onto the ITO surface (Figure 1.10).

Figure 1.10. Fabrication of modified ITO electrode:with Ag NPs: (A) demonstrates the fabrication steps of

modifield ITO electrode with Ag NPs. Ag NPs were deposited on 3-aminopropyltriethoxysilane modified ITO

surface by PDDA as bridging agent (B) is typical SEM image after modification of ITO surface with 4 layers of Ag

NPs. The scale bar is 100 nm. ―Reproduced from Journal of Electrochemical Chemistry, Vol.665, L. Chu, X. Zhang,

Electrochemical detection of chloride at the multilayer nano-silver modified indium-tin oxide thin electrodes, p. 26-

32, Copyright (2012), with permission from Elsevier.‖

Then the electrochemical behaviour of the fabricated ITO electrode was studied in the presence

of Cl- ions by the linear sweep voltammetry in the presence of Br

-, I

- and SCN

-. As it is seen in

Figure 1.11 an extra oxidation peak was seen in the presence of Br- and I

- in the voltammogram

but SCN- had no response peak. The response was linear in the range of 1×10

-8 to 1×10

-6 molL

-1

and the limit of detection (LOD) was reported to be 5.2×10-2

molL-1

.

15

Figure 1.11. Electrochemical detection of Cl- by Ag NPs modified ITO electrode: The LSV response of various

concentrations of Cl- in the presence of I- (A), Br- (B) and SCN- (C) with concentration of 1.0 ×10-6 M at modified

ITO electrode with Ag NPs. (A) Cl- (1-6): 0.1,1,4,6,8,10 × 10-6 M; (B) Cl- (1-6): 0.5, 1, 4, 6, 8, 10 × 10-6 M; (C) Cl-

(1-6): 1,2, 4, 6, 8, 10 × 10-6 M. Inset shows the relation between the peak heights versus various concentration of Cl-.

(D) Is calibration curve of Cl- at modified electrode under optimized conditions at 1.0 × 10-8, 1.0× 10-7, 2.0 × 10-7, 4.0

× 10-7, 6.0 × 10-7, 8.0 × 10-7 and 1.0 × 10-6 M. ―Reproduced from Journal of Electrochemical Chemistry, Vol.665, L.

Chu, X. Zhang, Electrochemical detection of chloride at the multilayer nano-silver modified indium-tin oxide thin

electrodes, p. 26-32, Copyright (2012), with permission from Elsevier.‖

The biocompatibility of some modified Ag NSs has made them excellent candidates for exploring

the electron transfer processes of biomolecules as well.49

It has been reported that Ag NSs

modified Ag electrode increase the electron transfer process between cytochrome c and Ag

16

electrode compared with the bulk Ag electrode.53

A linear relationship was observed between the

peak current and cytochrome c in the range of 8nM-3µM with 2.3 nM as the LOD using the linear

sweep voltammogram (LSV) technique (see Figure 1.12).

Figure 1.12. Electrochemical response of Ag NPs modified Ag electrode: (A) Linear sweep voltammograms of 0.1

M Cytochrome c on the modified electrode with Ag NPs in acetate buffer solution (pH 5.0) at scan rates of 10, 30, 70,

100, 150, 200, 250 and 300 mVs-1 (from inner curve to our curve respectively). (B) demonstrates the oxidation peak

current vs. scan rate.This linear behavior between oxidation current and scan rate indicates the adsorption-controlled

of the electrochemical oxidation of cytochrome c. ―Reproduced from Electrochimia Acta, Vol. 53, L. Lin, P. Qiu, X.

Cao, L. Jin, Colloidal silver nanoparticles modified electrode and its application to the electroanalysis of Cytochrome

c, p. 5368-5372, Copyright (2008), with permission from Elsevier.‖

As seen in Figure 1-12, there is a linear relation between scan rate and oxidation peak in the range

of 10 and 300 mVs-1

, indicating the adsorption-controlled of cytochrome c oxidation on the

electrode.

Furthermore due to the significant role of H2O2 as an intermediate in bio systems there has been

numerous research on developing sensors based on nanometals by amperometric technique, as this

technique has a good sensitivity and low detection limit. In this context there are several reports

on using deposited colloidal Ag NSs on single walled carbon nanotubes (SWCNT),54

glassy

carbon (GC) electrode,55-58

and Au59

for detection of H2O2 by amperometry.

Figure 1.13 shows the typical amperometric response of deposited Ag NPs on GC for different

concentrations of H2O2 at a working potential of -0.3 V in N2 saturated 0.2 M phosphate buffer

saline (PBS) buffer (pH 6.5).55

17

Figure 1.13. Electrochemical detection of H2O2:Typical steady-state amperometric response of the modified GC

electrode with Ag NPs for detection of H2O2 in the stirred N2-saturated 0.2M PBS buffer at pH 6.5 at applied potential

of -0.3 V. The inset shows calibration curve for different concentration of H2O2―Reproduced from Electrochimia

Acta, Vol. 56, W.B. Lu, F. Liao, Y.L. Chang, X.P. Sun, Hydrothermal synthesis of well-stable silver nanoparticles

and their application for enzymeless hydrogen peroxide detection, p. 2295-2298, Copyright (2011), with permission

from Elsevier.‖

Once a drop of H2O2 was added to the solution, the reduction current increased to reach a stable

value. The sensor could attain to 95% of the steady state current within 2s which is a fast

amperometric response. The inset in Figure 1.13 demonstrates the calibration curve of the sensor.

The linear range is 1×10-4

M to 0.18 M and the limit of detection of H2O2 with this method is

around 3.39×10-5

M. Also this non-enzymic sensor is more stable than enzyme based biosensor

for H2O2.

1.3.1.2 Application of Electrodeposited Ag NSs for Electroanalysis

Ag NSs have been easily deposited using electrochemical techniques usually by potentiometric

methods such as pulsed or constant potential applications.49

There are reports on using these

prepared Ag NSs for sensing purposes which would be summarized below. Recently

electrodeposited dendiritic Ag NSs were used for amperometric detection of nitrate by Hu et al. 60

The modified electrode was able to detect nitrate as low as 2µM with a linear range of 2-1000µM

18

at pH 7.0. The selectivity of this electrode toward nitrate was examined in the presence of various

anions and the results showed the amperometric response change of ±10% compared to the

solution without interference.60

Detection of Cr(VI) has been reported by Renedo et al. using

electrodeposited Ag NSs (see Figure 1.14).61

Figure 1.14. Detection of Cr(VI) by electrodeposited Ag NSs: (A) typical SEM image of electrodeposited Ag NPs on

carbon screen printed electrode from 0.10 mM AgClO4 solution in Britton-Robinson pH 2. (B) Diffrential pulse

voltammetram for Cr(VI) modified screen printed carbon electrode by Ag NPs: (──) blank and (− −) Cr(VI)= 1 µM.

The peak at potential about -0.19 V could be attributed to the reduction of Cr (VI) to Cr (III). Potentials are versus an

Ag/AgCl reference electrode. ―Reproduced from Talanta, Vol. 76, O.D. Renedo, G.R. Espelt, N. Astorgano, M.J.

Arcos-Martinez, Electrochemical determination of chromium(VI) using metallic nanoparticle-modified carbon

screen-printed electrodes,p. 854-858, Copyright (2008), with permission from Elsevier.‖

As shown in Figure 1.14, Cr(VI) was detected by electrodeposited Ag NSs on screen printed

electrodes using DPV and the measurements demonstrated a LOD of 8.5×10-7

M for chromium. In

their technique Ag NSs were deposited from an agitating solution of AgClO4 by employment of -

0.8 V (vs Ag/AgCl) for 400 s.61

The benefits of such methods are facile production of disposable

electrodes and having better selectivity by attaching-enzymes, polymers, complexing agents onto

the electrode.49

In another report electrodeposited Ag NSs on a GC electrode was applied for nitrophenolic

compound electroreduction.62

Ag NSs were electrodeposited on GC by pulsing potential between

-0.6 V to 0.2 V with a duration of t1 = 50 ms and t2 = 50 ms for 200 cycles in an deoxygenated

5mM AgNO3 solution. The results show much faster kinetics over polycrystalline Ag and less

negative reducing potential compared to GC electrode.62

19

Zhang et al. utilized electrodeposited AgNPs on GC modified with multiwalled carbon nanotubes

with carboxyl groups (MWCNTs-COOH) for deoxyribinuleic acid (DNA) sensing (See Figure

1.15).63

Figure 1.15. Schematic representation of electrode preparation for detetion of DNA Using Ag NSs. A composite of

Ag NPs and poly(trans-3(3-pyridyl)acrylic acid) was prepared and deposited on GC electrode by 5’thiol linker. DPV

was used for DNA detection by using adriamysin as an electroactive indicator. ―Reproduced from Talanta, Vol. 387,

Y. Zhang, K. Zhang, H. Ma, Electrochemical DNA biosensor based on silver nanoparticles/poly(3-(3-pyridyl) acrylic

acid)/carbon nanotubes modified electrode,p. 13-19, Copyright (2009), with permission from Elsevier.‖

As it is seen in Figure1.15, firstly poly(trans-3(3-pyridyl)acrylic acid) (PAA) film was deposited

on the modified GC electrode by CV, and then Ag NPs were deposited on the modified electrode.

Next this electrode immersed into the solution of probe DNA. 0.1% SDS solution was used

afterwards to wash off unimmobolized DNA. DPV was applied to monitor DNA hybridization

using adriamycin (chemotherapy drug) as an electroactive agent. This electrode has displayed an

excellent performance for DNA hybridization monitoring in a range of 9.0 pM to 9.0 nM with a

LOD of 3.2 pM with a high sensitivity, stability and reproducibility (see Figure 1.18).63

20

Figure 1.16. The DPV response of modified GC electrode with Ag NPs to DNA hybridation. The DPV was recorded

in the presence of different concentration of complementary oligonucleotides: (a) 0 M; (b) 9.0× 10-12 M; (c) 4.8 × 10-

11 M; (d) 9.0 × 10-11 M; (e) 4.8 × 10-10 M and (f) 9.0 × 10-9 M. The inset depicted the increase of peak current (∆I) vs

logarithm of complimentary oligonucleotides’ concentration. The pulse amplitude is 50 mV, pule period is 0.2 s and

pulse width is 50ms in DPV. ―Reproduced from Talanta, Vol. 387, Y. Zhang, K. Zhang, H. Ma, Electrochemical

DNA biosensor based on silver nanoparticles/poly(3-(3-pyridyl) acrylic acid)/carbon nanotubes modified electrode,p.

13-19, Copyright (2009), with permission from Elsevier.‖

Also, similar to colloidal Ag NSs, these are several reports on application of electrodeposited Ag

NSs on different substrates such as GC64-67

and Au68

for H2O2 detection by using amperometric

technique.These electrodes show similar linear range (10-2

-10-6

) and LOD (10-6

-10-7

) towards

H2O2 64-68

1.3.2 Application of Ag NSs for Plasmonic Purposes

One of the supreme properties of nanoscale nanometals compared to their bulk materials is their

interactions with light.12

Light interaction with the metal surfaces leads to the movement of free

electrons in a pool of fixed positive ions as a background. This electron mobility creates a plasma;

the free electron plasma. Light acts as an external electro-magnetic field and once it is applied to a

metal, the conduction electrons transport collectively this collective movement to the metal

surface is known as the LSPR.69,70

Figure 1.17 shows the plasmon oscillation of the conduction

electrons.71

21

Fabrication ease, cost and plasmonic properties range, all influence the usefulness of a metal for

plasmonic applications. In this context hardly any other material is similar to Ag in plasmonic

properties. 12

Ag supports a wide range of surface plasmon, (300-1200 nm), which includes the

visible and near infrared regions.72

This collective oscillation of the free electrons in Ag NSs has

been leveraged in numerous applications such as Raman scattering and LSPR.

Figure 1.17. The oscillation of conduction electrons and their relative move from the positively charged nuclei.

―Reproduced with permission from (K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, The optical properties of

metal nanoparticles: the influence of size, shape, and dielectric environment, J. Phys. Chem. B 107, 668-677.).

Copyright (2003), American Chemical Society.‖

Moreover the discovery of surface enhanced Raman spectroscopy (SERS) by Fleischman in 1974

caused a worldwide effort to explore its origin, enhance it and employ it in a broad range of

applications from plasmonics to diagnostics.70,73-75

There are two proposed mechanism for

explanation of the SERS phenomenon: electromagnetic mechanism (EM) which described the

surface electron movement in the substrate and chemical mechanism that corborates to the charge

transfer (CT) between the substrate and the analyte molecules. Usually EM mechanism have the

larger enhancement contribution and in contrast with CT has the uniform effects across all

molecule types, therefore mainly EM process is tuned and modified through optimization of the

SERS substrates.74,75

SERS EM enhancement is usually controlled by SPR of the substrate when it

is irradiated with light. In this context nanostructured substrates have more SERS enhancement

over their bulk compartment due to raising the incident laser field leading to more total photon-

molecule interactions. Furthermore they enhance the Raman scattered light intensity resulting in

detection of more scattered photons.75

Therefore, SERS and LSPR have long been used to

research the fundamentals, which focus on investigating the conformation, structure and charge

transfer of molecules such as biomolecules or for applied purposes which deal with detection and

diagnosistics.76,77

In doing so there has been a number of diverse methods for plasmonic substrate

fabrication used for sensing, including colloidal metal nanostructures, self-assembled metal

22

nanostructures on the solid substrates and lithography techniques.12, 75-80

Here we summarized

substrate fabrication techniques using metal NSs.

1.3.2.1 Colloidal Nanostructures

Previously we discussed all methods to fabricate Ag nanostructures which embrace other noble

metals as well. These nanostructures not only could be used for electroanalysis but also are useful

as plasmonic substrates. They were employed as colloidal form for plasmonic purposes as well.75-

79 Nanostructures are produced easily through regular wet chemistry and provide a diverse range

of morphologies. However the employment of either dispersed or aggregated metal NSs as

plasmonic substrates are limited due to their insufficient enhancement factor and

reporoducibility.76

1.3.2.2 Self-assembled Metal Nanostructures

For self-assembly of nanostructures on the solid platform (Si, glass, ITO, Au film, polymers) the

idea is to employ the cross-linking molecules for immobilization. Therefore it can anchored to the

solid platform by one of the functional groups and the others would be free for binding to the

metal nanostructures.75-76,78

With this technique the control of SERS hot spot (areas of highly

localized strong electromagnetic filed leading to enhanced localized activities) might be easier

than when they are randomly dispersed or aggregated metal NSs in the solution.75

This method

was first proposed by Natan and co-workers in 1995.81

(Figure 1.18).

Figure 1.18. Assembly technique for Au and Ag NPs monolayers, X = CN, NH2, 2-pyridyl, P(C6H5)2, or SH; R=CH3

or CH2CH3. ―From [R.G. Freeman, K.C. Grabar, K.J. Allison, R.M. Bright, J.A. Davis, A.P. Guthrie, M.B. Hommer,

M.A. Jackson, P.C. Smith, D.G. Walter, M.J. Natan, Self-assembled metal colloid monolayers: An approach to SERS

substrates, Science 1995, 267, 1629-1632.) Adapted with permission from AAAS‖

23

Table 1.1. Summary of the Ag NSs’ shapes, LSPR absorption peaks and applications

aThe main absorption peak (nm). bAssembly means NSs have been deposited on other solid platforms for the

proposed applications. ―Adapted with permission from (M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q.

Zhang, D. Qin, Y. Xia, Controlling the synthesis and assembly of silver nanostructures for plasmonic applications,

Chem. Rev. 111, 3669-3712.). Copyright (2011), American Chemical Society.‖

Shapes Illustration LSPRa Applications

b

Sphere and quasi-sphere

320-450

SERS, LSPR sensing,

assembly

Cube and truncated cube

400-480 SERS, LSPR sensing,

assembly

Tetrahedron and

truncated tetrahedron 350-450 SERS

Octahedron and truncated

tetrahedron

400-500 Assembly

Bar

350-900 SERS

Spheroid

350-900 SERS

Right bipyramid

500-700 -

Beam

- Electron Transport

Decahedron

350-450 -

Wire and rod

380-460 Wave guiding,

electronics, SERS,

assembly

Polygonal plates and disc

350-1000 SERS, LSPR sensing

Branched structures

400-110 SERS

Hollow structures

380-800 SERS, LSPR sensing

24

The concept behind Natan’s work was expanded afterward and there has been numerous work

based on this fabrication.82-84

This method was used for the preparation of multilayer metal NSs,

sometimes called 3D structures.76,

85,86

In doing so after immobilization of the first layer of Metal

NSs based on Figure 1.18, the substrate would immerse inside the cross-linking solution and again

would repeat the process to deposit lateral layers of metal NSs on the substrates.76

Brolo and

coworkers have used this technique to deposit Ag NSs on the glass substrate.87

They found that

there is an optimum number of NS deposition on the substrate for having the maximum SERS

enhancement, also the SERS signal is 2~3 orders higher than the monolayer of deposited Ag NSs.

Figure 1.18. demonstrates the preparation steps for such substrates.87

The application of metals as

the substrates for NSs deposition in self-assembly techniques have provided more signal

enhancement due to surface Plasmon coupling between metal NSs and metal film.76, 88

In this

context various shapes of Ag NSs have been synthesized by previously mentioned techniques and

their plasmon properties have been examined against different analytes.12

Table 1.1 demonstrates

a sketch of various Ag NSs which has applied as LSPR or SERS substrate in colloidal or self-

assembled forms.12

As shown in Table 1.1 various Ag NSs have demonstrated activity in the

specific plasmonic area which make them suitable for various applications.

1.3.2.3 Lithography Technique

This method is categorized as a ―top down‖ technique compared to the self-assembled method as

a ― bottom up‖ method.76

The most common lithography technique to prepare SERS substrates is

electron beam lithography (EBL) as it is able to produce nanostructure features to be small enough

for being SERS-active.75-76

In the EBL technique an electron beam of 10-50 keV focused on a

solid substrate, which is usually SiOx/Si wafers covered by photoresistor material 75-76

Poly(methyl methacrylate) (PMMA) is the most common resistors for this method.89

Typically

after etching the resistor by EBL there are two procedures for fabrication of SERS-active

substrates as shown in figure 1.3.89

In the right-hand side the metal vapour evaporated on the

patterned resist and the resist peeled off afterwards. With this method the metal between the NSs

was removed by lift-off therefore the remaining metal NSs are similar to metal island films. On

the left-hand side after EBL etching, application of HF baths leading to more enhancement in the

25

depth of the exposed patterns. Then the remained resist removed and vapour deposition of the

metal provides a nano-patterned metallic substrate (see Figure 1.19).89

Figure 1.19. Scheme for two fabrication methods for SERS substrates production using EBL technique. The right

hand process demonstrates the deposition of metal right after the e-beam exposition. Then the remained photoresistor

casts out and the metals remains as nano-islands on the substrate.On left-hand side, the chemical etching is used to

generate more depth features on the substrate, then the resistor removed and the metal deposits on substrate.

―Reprinted from Sensors and Actuators B, Vol. 51, M. Kahl, E. Voges, S. Kostrewa, C. Viets, W. Hill Periodically

structured metallic substrates for SERS, p. 285-291, Copyright (1998), with permission from Elsevier.‖

Based on the various methods proposed for plasmonic substrate fabrication, features for an ideal

SERS substrate are as follows:78

1- The substrate should have a high level of sensitivity and SERS activity. This can be gained

by the precise control of nanostructures’ size (more than 50nm) and interparticle gap (less

than 10 nm). Therefore as the result the LSPR frequency of the substrate can be matched

to the frequency of incident laser leading to maximizing the enhancement.

2- The substrate should be uniform to provide an approximate deviation less than 20%. This

can be attained by having a relatively ordered arrangement of the nanoparticles on the

substrate.

3- The substrate should be stable and reproducible even after a long shelf time and deviation

for the different batches of prepared substrated should be less than 20%.

4- The substrate should be unstained and clean to provide an appropriate media for the study

of weak adsorbates as well or even unknown materials.

26

Unfortunately it is still quite tough to fabricate substrates which meet all of these requirements.

Therefore based on the application purpose one has to compromise with some aspects. For

instance for biological purposes a highly clean and substantially enhanced substrate is usually

required while for quantitative analysis a highly reproducible and uniform substrate is

required.77,78

So there are still lots of research which aims to make an ideal SERS substrate or

fabricate a good one for a specific purpose. In addition there is still a need to develop easier, more

time and cost effective as well as greener techniques to generate such substrates.

1.4 Objective

In the first set of electrochemical experiments a new technique for fabrication of nanostructured

Ag surface was discovered. The objective of the research described in this thesis firstly was to

study the effect of various experimental conditions on the morphology of these electrochemically

prepared Ag NSs. Next the interaction of in-situ electrochemically prepared Ag NSs with

chemical warfare agents (CWAs) mimics specifically cyanide releasing organophosphonates was

studied and in more detail the hypothesis of cyanide mediated dissolution of Ag NSs was

explored. Following this, the potential of these nanostructured Ag substrate were evaluated as the

plasmonic substrates for SERS detection of organophosphonate based pesticides. Then the

nanostructured Ag substrates were utilized in the matrix environment, tap water and juice, to

evaluate their potential for rapid sensing of these organophosphate based pesticides using

localized surface palsmon resonance and electrochemical techniques. Due to the effect of different

additives on the morphology of these Ag NSs, the effect of various capping agents and surfactants

on the morphology and corrosion behavior of these substrates was evaluated which has addressed

the corrosion in nanoscale. Lastly the photoelectrochemical behavior of modified Ag NSs with

porphyrin was explored in the presence of mononucleotides to address the usefulness for

chemosensory substrates.

27

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31

Chapter 2 Chemical Behavior of Electrochemically-Generated

Nanostructured Silver Surfacesa

As mentioned in the objectives, one of the critical issues is to understand the interaction

mechanism of cyanide releasing organophosphonates by electrochemical and morphological

studies of Ag NSs in the presence of them. In order to address this question, the electrochemical

behavior of gas generated Ag NPs on ITO and Ag foil was explored in the absence and present of

cyanide and diethylcyanophosphonate (DECP) as a cyanide releasing organophosphonate. The

first attempts lead to the discovery of a new electrochemical method for fabrication of Ag NSs,

therefore this work focus on the effect of different parameters on their properties and application

of them to answer the raised questions.

a This chapter has been taken from the published article. ―Reprinted with permission from (Chemical

Behavior of Electrochemically-Generated Nanostructured Silver Surfaces‖, F. Fathi, M. Schlitt, D.B.

Pederson, H.B. Kraatz, Langmuir 2011, 27, 12098–12105). Copyright (2011) American Chemical

Society.‖ <DOI: 10.1021/la2015379>. Prof. H.B. Kraatz suggested the idea and I am the major contributer

to this work in terms of the experimental and written work. Schlitt assisted me in doing some of the

experiments and data interpretation. Pederson read and offered helpful advice. The final manuscript was

published after revision by Prof. Kraatz. The manuscript is used here verbatim in the optimized thesis

format.

32

2.1. Introduction

In recent years, the synthesis of metal nanostructures has attracted much interest in different fields

of chemistry due to their physicochemical properties, which differ significantly from macroscopic

metal phases.1,2

Nanostructured Ag, like other nano metals, is very different from its bulk

counterpart in physicochemical properties.3,4

Nano-Ag is also extremely stable and has electrical

conductivity significantly higher than other metals. These properties make Ag attractive as an

electrode material in electrochemical cells.4 In previous studies, Ag nanostructures have been used

for the electrochemical detection of ammonia,5 thiocyanate,

6 hydrogen peroxide,

7 and cyanide.

8,9

Previously, a number of Ag nanostructures with various sizes and shapes have been fabricated in

aqueous and non-aqueous media with different methods including chemical reduction of Ag

precursors, photochemical methods, electron bombardment, laser ablation and electrochemical

methods. Among these methods, electrochemical strategies typically provide a more economical,

easily adaptable, and higher purity of nanostructured Ag surfaces.10,11

An electrochemical method

for silver nanoparticle synthesis was first proposed by Reetz and Helbig in 1994. In their work,

silver metal sheets were dissolved anodically and metal salts were formed as an intermediate.

These were subsequently reduced and deposited as silver nanoparticles on the cathode. 12

Based

on this work, Sanchez et al. reported the dissolution of a metallic Ag anode in a non-protic solvent

in the presence of a stabilizer as a method for producing Ag nanoparticles/structures on Pt

surfaces.13

Similarly, Shang et al. reported the application of a high negative voltage to an aerated

solution of AgNO3 and KNO3 in the presence of a stabilizer. The result was the formation of 20 to

40 nm Ag nanoparticles on indium tin oxide surfaces. Extended application of the potential led to

the formation of aggregated Ag nanostructures.14

Kaydarov et al. reported the formation of Ag

NPs at a constant potential between two Ag electrodes under stirring. In this approach, the current

polarity between the electrodes was changed every 30 to 300 s.15

It is important to notice that no

method is ideal and all of those mentioned have drawbacks, such as the need for organic solvents

or chemical stabilizers, heating of the solution, de-aeration of the solution. In this context, we also

mention the use of roughened metal surfaces for surface-enhanced Raman spectroscopy (SERS)

studies, which makes it critical to develop simple, cheap, and reproducible methods for their

preparation.16-18

33

Here we present a simpler method for the preparation of nanostructured Ag surfaces based on

cyclic voltammetry (CV) using a conventional three-electrode set up with a Ag working electrode.

The nanostructured Ag surfaces prepared were then explored for the electrochemical detection of

cyanide ions and diethylcyanophosphonate (DECP), a tabun mimic (Scheme 1) that can dissociate

to release cyanide. Our own interest in this topic stems from a study involving gas-phase

generated Ag NPs deposited on indium tin oxide which underwent significant morphological

changes in the presence of DECP.19

In that study it was hypothesized that dissolution of Ag NPs is

driven by a reaction with cyanide. To substantiate this proposed mechanism of cyanide detection

it was necessary to carry out a more detailed investigation that goes beyond previous studies of

Ag surfaces in alkaline20-22

and alkaline–cyanide solutions,9, 23-27

and that evaluates changes on the

surface topography, at the nanoscale, caused by cyanide ions reacting with Ag2O to form

[Ag(CN)2]- under cyclic voltammetry conditions.

PN O

CN

O

P

O

O

CN

O

Scheme 2.1. Chemical drawing of Tabun (left) and diethylcyanophosphonate (DECP, right).

Here we present our results of a combined electrochemical and structural study that shows, a) that

electrochemical cycling leads to the formation of a nano-structured surface, b) that the grain size

of the nano-structured surface is affected by cyanide and DECP concentration and c) that the

electrochemical properties of this surface is affected significantly by cyanide concentration and

the concentration of DECP.

2.2 Experimental Methods

2.2.1 Reagents

Diethylcyanophosphonate (DECP, Aldrich), KOH (Caledon), and KCN (Fischer) were used as

received. Deionized water (18.2 MΩ·cm resistivity) from a Millipore Milli-Q system was used

throughout this work. K [Ag (CN) 2] was purchased from Strem Chemicals and used as received.

34

The Ag foil (thickness = 0.28 mm, 99.9% metal basis) and platinum wire were purchased from

Alfa Aesar.


Cyclic voltammetry experiments were performed using a CHI660B electrochemical workstation

(CH instruments Inc.), using a 10 mL homemade Teflon cell having a three-electrode setup. All

experiments were carried out in different aqueous KOH solutions (0.001-8.0 M). A coiled Pt wire

(0.25 mm diameter, Alfa Aesar, Ward Hill, MA, 99.9% metal basis) was used as an auxiliary

electrode and silver foil was used as the working electrode. Ag/AgCl (3 M KCl, CH Instruments,

Inc.) served as the reference electrode, which was connected to the electrochemical cell via a

homemade agar salt bridge (1 M KNO3). All electrochemical measurements were carried out in a

grounded Faraday cage.

2.2.3. Surface Characterization

The silver surface morphology was investigated at the Nanofabrication Facility of the University

of Western Ontario using a scanning electron microscope (SEM) (Leo 1540XB FIB/SEM and Leo

1530 SEM) that was equipped with energy dispersive X-ray spectroscopy for composition

analysis. X-ray diffraction analysis (XRD: Rigaku Rata flex 300 RC X-ray diffractometer using a

Co source) was employed to determine the crystal structure of the deposits on the surface. X-ray

photoelectron spectroscopy (XPS) (Kratos Axis Ultra spectrometer) was performed at the Surface

Science center of University of Western Ontario, with Al Kα (15 mA, 14 kV) as the photo source

for structural analysis. The size of Ag NPs was measured by ImageJ 1.43 software.

2.3 Results and Discussion

Cyclic voltammetry (CV) experiments were carried out using Ag foil immersed in 8 M KOH

solution and a number of CV scans were recorded in the potential range of -0.5 to 0.9 V vs

Ag/AgCl. Essentially, there are no noticeable changes in the CV curves after extended cycles. A

http://www.uwo.ca/fab/Equipment/Characterization/1540XB/1540XB.html

http://www.uwo.ca/fab/Equipment/Characterization/1530/1530.html


35

typical CV is shown in Figure 2.1 and compares well with the CVs reported before by Burke.20

The electrochemical behaviour of Ag in alkaline solution is complex and the formation of Ag2O

was reported earlier to involve at least three steps.20-22

The first step has been assigned to either

the formation of monolayer Ag2O or AgOH or dissolution of material as [Ag(OH)2]-. The second

step was attributed to electro-dissolution of [Ag(OH)2]-, formation of Ag2O or to the sublayer

trapping of O atoms in the Ag surface. The third step was attributed to nucleation and 3-D growth

of Ag2O on the base layer.20-22

This is followed by formation of AgO on top of the Ag2O layer,

which is thought to be formed by advancing nucleation and 3-D growth stages or by direct

nucleation, 3-D growth and Ostwald ripening.20-22, 28

Due to some loss of material from the

surface, as described above, there is a charge imbalance between the charge of the first anodic and

second anodic peaks. In addition, some of the Ag may be dissolved as AgO- and AgO

+ in the

anodic sweep and the product may be cathodically reduced and redeposited on the surface as Ag

metal at the end of every CV cycle with other silver oxide forms.20-22

We have some indication for

a dissolution process and have observed the formation of a black particulate in solution. We did

not analyze this material further.

Accordingly, signals were assigned to the various oxidation events of Ag to Ag2O and at the

higher potential to AgO as labeled in Figure 2.1. On the cathodic sweep, these oxides are reduced

stepwise back to Ag (0).

Figure 2.1. A typical cyclic voltammetry scan of Ag foil in a solution of 8 M KOH at a scan rate of 0.15 Vs-1 in a

potential range of -0.5 and 0.9 V vs. Ag/AgCl. On the anodic sweep, peaks are observed due to the oxidation of Ag to

the Ag (I) and Ag (II) oxides. On the cathodic sweep, these oxides are reduced back to Ag (0).

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-0.02

-0.01

0.00

0.01

0.02

E/V (vs.Ag/AgCl)

j/ m

A c

m-2

Ag Ag2O

Ag2O AgO

AgO Ag2O

Ag2O Ag

36

After the completion of the CV cycles, the silver foil was no longer silvery and shiny but had a

matt white appearance. This material could be scraped off the Ag foil and appeared as a loose

white powder, opposite to what is expected of oxidation since silver oxides are dark materials in

the bulk.29

Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX),

X-ray photon spectroscopy (XPS), and X-ray powder diffraction (XRD) studies of this material

were implemented to determine the structure and morphology of the white material as well as its

chemical composition. Figure 2.2 shows the SEM image of the Ag foil surface after

electrochemical cycling in the potential range of -0.5 to 0.9 V vs. Ag/AgCl in 8 M KOH.

In comparison to a fresh Ag surface, the image shows a roughened surface with nanoscopic

deposits, which are readily removed from the surface. EDX analysis showed that the deposit

consists of silver only without any significant contributions from other elements other than some

small carbon and oxygen contaminations, presumably due to CO2 adsorption onto the silver

surface or hydrocarbon impurities.30

Figure 2.2. Results of a combined SEM and EDX study of the effects of electrochemical cycling a Ag surface at a

scan rate of 0.15 Vs-1 in the potential range of 0.5 and 0.9 V vs. Ag/AgCl for 15 CV scans in 8 M KOH. Please note

the roughness of the surface. EDX analysis (right) shows a composition of 98.83% Ag and 1.17% C.

37

XPS analysis of the white material showed two peaks at 368.3 and 374.0 eV at core binding

energies typical for the Ag 3d5/2 and 3d 3/2 signals of elemental silver (Figure 2.3). The literature

values for Ag (0) are Ag 3d5/2 and 3d 3/2 is 368.0 ± 0.2 and 374.0 ± 0.2 eV.30-32

In comparison,

binding energies for Ag2O (Ag 3d5/2 = 367.3 ± 0.2 eV)30-32

and for AgO (Ag 3d5/2 = 367.7 ± 0.2 eV

and Ag 3d 3/2 = 373.2 ± 0.2 eV)30,31

are sufficiently different so that the white nanostructured

surface material can be identified as a deposit of elemental silver.

Further investigation of the silver nanomaterial by XRD shows (Figure 2.4) that the silver exhibits

reflections that can be indexed to a face centered cubic structure. The diffraction patterns around

2Ө values of 44.5, 51.9, 76.4 and 93.0 correspond to the planes (111), (200), (220) and (311) of

silver, respectively.33

Figure 2.3. XPS of the white nanostructured material showing signals at binding energies of 368.3 and 374.0 eV

characteristic of the Ag 3d3/2 and 3d5/2 signals of Ag (0).

Furthermore the higher intensity ratio of (111) over the others indicates the enrichment of the Ag

(111) plane in Ag NPs. This can presumably be caused by defects in the form of steps and kinks

in the Ag (111) plane.34

0

1000

2000

3000

4000

5000

360365370375380

Co

unts

Per

Seco

nd

s (c

ps)

Binding Energy (eV)

Ag 3d5/2

Ag 3d3/2

38

Figure 2.4. XRD pattern spectrum of the deposited Ag NPs on the surface of Ag foil formed by electrochemical

cycling. The XRD pattern of the material exhibit 2Ө values of 44.5, 51.9, 76.4 and 93.0 corresponding to the planes

(111), (200), (220) and (311), respectively, of face centered cubic silver (insert; JCPDS card file No. 4-783].

The electrochemical cycling of a silver electrode in the potential range from -0.5 to 0.9 V vs.

Ag/AgCl in 8 M KOH as supporting electrolyte clearly results in the formation of a

nanostructured metallic silver surface that based on Conway et. al. work, the roughness factor for

this roughened nanostructure surface might be 1.5.35

Two obvious factors potentially influencing

the formation of the nanostructured surface and the thickness of the nanostructured layer are the

number of electrochemical cycles and the concentration of the supporting electrolyte KOH.

Figure 2.5 shows the SEM images acquired as the number of electrochemical cycles was varied.

Both the size of the Ag grains on the surface and the thickness of the nanostructured layer

deposited on unperturbed bulk silver are affected by the electrochemical cycling. After only one

scan, a layer of silver is deposited with a grain size of approximately 31 ± 9 nm (Figure 2.5a) and

a uniform thickness of the deposit of about 300 nm. The population of Ag grains or nanoparticles

is not particularly monodisperse but the number of fused particles is small (see also Figure A.2.1.

Appendix A). After five cycles, the particles are clearly fused and the thickness of the deposit

remains uniformly distributed across the surface with a thickness of about 630 nm with a clear

boundary between the nanostructured Ag overlaying the bulk silver phase below.

39

Figure 2.5. SEM study of the effects of electrochemical cycling of the silver foil in a potential range of -0.5 and 0.9 V

vs. Ag/AgCl at a scan rate of 0.150 mVs-1 at a supporting electrolyte concentration of 8 M KOH. After only 1 cycle

(frames a and e), there are well-defined Ag grains and a clear boundary between the nanostructured Ag and the

underlaying bulk Ag. After 5 cycles (frames b and f): the grain size and film thickness increase. Cycles 10 (frames c

and g) and 15 (frames d and h): the grains are now strongly fused and there is no longer any clear demarcation

between the nanostructured surface material and the underlayer.

40

After 10 and 15 cycles, the particles on the surface continue to fuse and there is no clear

demarcation between the nanostrucured surface and the bulk silver. From the cross-section images

showing a clear boundary between the nanostructured surface and the bulk silver, it is clear that

the Ag nanostructures are the product of continuous deposition/oxidation at growing Ag grains

during the anodic and cathodic sweeps.

Figure 2.6 shows a series of SEMs at a KOH concentration of 8.0 M (a), 1.0 M (b), 0.1 M (c),

0.01 M (d) and 0.001 M (e). As seen, lowering the concentration of KOH resulted in a less

compact Ag NP film being formed. Particles are spaced out more and the silver foil underlayer is

clearly visible. At a concentration of 0.001 M KOH, no significant accumulation of Ag NPs was

observed. These results combined with the cycling data confirm the disruption of Ag surfaces that

occurs with electrochemical cycling under basic conditions.20

In terms of mechanism, the cross

sectional data shown in Figure 2.5 demonstrate that the nanoparticles formed are the result of a re-

deposition process, as opposed to a direct roughening of the Ag electrode surface. The observed

deposition of layers of nanoparticles up to 650 nm in thickness is also indicative of a significant

concentration of dissolved Ag. These aqueous ions are likely in the form of hydroxides,

(Ag(OH)n)-n+1

, as no nanoparticle formation occurs in the absence of significant hydroxide

concentrations (e.g. Figure 2.6e). Decreasing the electrolyte concentration results in a higher

ohmic resistance. Thus, under these conditions the surface is not exposed to the same potential

range (see also Figure A.2.2. and Table A.2.1.). Having successfully formed Ag nanostructures,

their response to potassium cyanide and diethylcyanophosphonate (DECP) exposure was of

interest. A previous study on gas-phase Ag NPs deposited onto ITO substrates has demonstrated

that the addition of DECP decreases the amount of Ag NPs present on indium tin oxide surfaces.19

That study speculated that cyanide anions released upon hydrolysis of DECP will interact with Ag

surfaces, or the various silver oxides formed during cycling, causing formation of dissolved

dicyanoagentate [Ag(CN)2]- according to equation 2.1.

AgO + 2CN- + H2O + e

- [Ag(CN)2]

- + 2OH

- (2.1)

Alternatively, Ag can be oxidized in the presence of O2 and cyanide according to

2 Ag + 4 CN- + ½ O2 + H2O → 2 [Ag(CN)2]

- + 2 OH

- (2.2)

41

Figure 2.6. SEM study of the effects of supporting electrolyte concentration on the surface morphology of the

deposited Ag NP film. Shown are the results after 15 CV cycles over a potential range of -0.5 and 0.9 V vs. Ag/AgCl

at a scan rate of 0.150 mVs-1 at a supporting electrolyte concentration of a) 8.0 M, b) 1.0 M, c) 0.1 M, d) 0.01 M, and

e) 0.001 M KOH. The grain size at 1.0 M KOH is clearly smaller compared to the 8.0 M KOH solution..

As can be seen in Figure 2.7, the addition of either KCN or DECP to the supporting electrolyte (1

M KOH) had a significant effect on the electrochemical behavior of the system. In a report by

Taheri et al., a decrease in the cathodic current was chosen as an analytical signal for cyanide

determination. They assumed that the cathodic current decrease results from the formation of a

silver cyanide complex which prevents the oxidation of the Ag surface.9 Previous speciation

analysis of silver cyanide complexes in an alkaline cyanide solution has shown that the

42

predominant ion is [Ag (CN)2]- at a pH > 5 and CN

- concentration of less than 10 mM. [Ag

(CN)3]2-

is the predominant species when the [CN-] > 20 mM and [Ag (CN)4]

3- dominates only for

high cyanide concentrations exceeding 2.5 M. The composition of species such as [Ag (OH)2]- and

[Ag (OH)(CN)]- are small in comparison with the previous silver cyanide complexes

mentioned.36,37

Previous mechanistic studies of Ag dissolution in a cyanide solution have shown

that at a low concentrations of cyanide (less than 0.1 M), Ag dissolution follows first order

kinetics. The formation and decomposition of the surface complex [Ag(CN)2]- is involved in the

charge transfer step which is typically rate-limiting.38,39

Consistent with previous work, the

addition of either KCN or DECP resulted in a decrease in the redox peak intensity, as shown in

Figure 2.7. The similar effect of both chemicals suggests analogous chemistry, indicating that CN-

is the active species and hydrolysis of DECP is occurring.

Figure 2.7. Electrochemical behavior of Ag exposure to a) KCN (no KCN added _____; 1 pM KCN ----; 100 µM KCN …..) and b) DECP (no DECP added _____; 1pM DECP ----; 100µM DECP …..) in 1M KOH at a scan rate of 0.100 Vs-1

in a potential range of -0.5 and 0.9 V vs. Ag/AgCl. Please note that for CN- and DECP addition, the peak intensities

decrease. This is rationalized by the passivation of the silver surface by CN- decreasing the anodic peaks. The

cathodic peaks decrease in intensity due to the dissolution of the material as [Ag(CN)2]-.

In the context of Taheri et al., the observed decrease in CV intensity (Figure 2.7) is expected to

result from the formation of a redox inactive silver cyanide coating that prevents the oxidation of

the Ag electrode and effectively passivates the surface.9CN

- does have a higher affinity for Ag

surfaces than the hydroxide anion.40

Alternatively, formation of significant quantities of aqueous

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-0.02

-0.01

0.00

0.01

0.02

j/ m

A c

m-2

E/V (vs.Ag/AgCl)

Ag Ag2O

Ag2O AgO

AgO Ag2O

Ag2O Ag

(a)

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-0.02

-0.01

0.00

0.01

0.02

j/ m

A c

m-2

E/V (vs.Ag/AgCl)

Ag Ag2O

Ag2O AgO

AgO Ag2O

Ag2O Ag

(b)

43

[Ag(CN)2]- could also explain the loss in redox activity.

33,34 In support of the latter, the presence

of [Ag(CN)2]-

was confirmed by taking mass spectra (MS) of samples of the cyanide/KOH

solution after exposure to Ag surfaces subjected to CV experiments (see Figure A.2.7. and A.2.8,

Appendix A). Interestingly, [Ag (CN)2]- can serve as a source of elemental silver and at

sufficiently high concentrations, elemental silver is deposited (see Figure A.2.6., Appendix A),

under experimental conditions, however, it appears to behave as a relatively redox-inactive sink

for Ag.

Figure 2.8. SEM images of the silver surface as a function of KCN and DECP addition. Images were recorded after a

total of 15 electrochemical cycles in the range of -0.5 and 0.9 V vs. Ag/AgCl at a KOH concentration of 1.0 M and a

scan rate of 0.100 Vs-1. a) shows the SEM image of a Ag surface exposed to KOH only and the formation of a

nanostructured Ag film; b) addition of KCN (1 pM) causes significantly smaller grains; c) increased concentration of

KCN (100 µM). The grains are smaller and less evenly distributed across the surface; d) DECP (1 pM) addition

affects the grains; e) at a DECP concentration of 100 µM the surface is very heterogeneous.

44

Clearly the addition of CN- either by direct addition of KCN or by addition of DECP followed by

hydrolysis, alters the electrochemical properties of the nanostructured Ag foil surface. The effect

of cyanide on surface topography is evident from SEM images. Figure 2.8 shows a series of such

SEM images recorded as a function of added KCN and DECP (see Figure A.2.9. and A.2.10). As

seen, exposure of the Ag surface to 1M KOH results in a significant growth of NPs on the surface

but as KCN and DECP are added to the solution a decrease in the amount of deposited Ag NPs on

the surface is observed. At higher concentrations of CN-, the freshly formed Ag nanostructures

begin to dissolve. This effect increases as the concentration of KCN and DECP increases in the

solution. The observation is consistent with the dissolution of the silver as [Ag(CN)2]-, as

suggested above.

2.4 Conclusions

A nanostructured Ag surface has been prepared by electrochemical cycling of Ag foil, in a

potential range of -0.5-0.9V vs Ag/AgCl, in KOH solution. The surface morphology prepared has

been shown to depend on both the KOH concentration and the number of electrochemical cycles.

Repeated electrochemical cycling was found to increase grain size on the surface as well as blur

the boundary between bulk and nanostructured surface. At low hydroxide concentrations no

nanostructures were observed.

Ag nanostructures fabricated by electrochemical cycling of Ag foil in highly basic conditions

were found to be sensitive to the addition of KCN or DECP. This chemistry is similar to that

previously observed to occur on gas-phase Ag nanoparticles deposited onto ITO electrodes. In

both systems, loss of redox activity can be used for the sensitive detection of cyanide.

2.5 Acknowledgement

We thank the University of Western Ontario for financial support. In addition, we would like to

thank the following individuals for their invaluable help with SEM and EDX (Dr. Todd Simpson

and Dr. Tim Goldhawk Western’s Nanofabrication Center), with XPS (Dr. Mark Biesinger,

45

Science Surface Science Western) center, with XRD (Dr. Kimberly R. Law in Department of

Earth Science, UWO) and with Mass spectroscopy (MS) (Dr.Doug Hairsine).

Supplementary Information: Supplementary material for this chapter can be found in Appendix

A.

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37 G. Senanayake, Hydrometallurgy 2006, 81, 75-85.

38 R.Y. Bek, N.A.R. Kov, J. Electroanal. Chem. 1998, 447, 109-115.

39 G. Baltrunas, E. Pakalniene, Chimija (Vilnius) 2002, 13, 64-69.

40 G. Baltrunas, G.S. Popkirov, R.N. Schindler, J. Electrocanal. Chem. 1997, 435, 95-101.

47

Chapter 3 Studies of the Interaction Two Organophosphonates with

Nanostructured Silver Surfacesb

In chapter 2 the discovery of a new electrochemical method for preparing silver surfaces that are

nanostructured was demonstrated. Studies indicate that surface structures can be generated ranging in

size from 31 ± 9 nm to 152 ± 48 nm. The size is affected by the concentration of the supporting

electrolyte and the number of cyclic voltammetry scans. Early results indicate that these structures are

formed by oxidation of surface of silver to soluble silver salts, followed by reductive deposition of Ag

and growth of nanostructures. Additives greatly affect growth and surface roughness. For example, the

addition of cyanide reduces the redeposition leading to a smoother surface. Furthermore, simple

organophosphonates appear to influence the growth of the nanostructures and give rise to reproducible

changes in the electrochemical properties of the system that suggest the possibility of electrochemical

sensing of organophosphonates. In this chapter the SERS enhancement of these Nanostructured Ag

surfaces after exposure to two insecticides, malathion and paraoxon, would evaluate. The results of this

study will allow us to shed light on the interaction between these two pesticides with nanostructured Ag

surfaces. In addition the potential of these Ag NSs for application as SERS substrates would be

evaluated.

b This chapter has been taken from the published article ―Studies of the interaction two

organophosphonates with nanostructured silver surfaces‖, F. Fathi, F. Lagugne-Labarthet, D.B. Pederson,

H.B. Kraatz, Analyst, 2012, 137, 4448-4453.Reproduced by permission of the Royal Society of Chemistry.

Copyright 2012, < DOI: 10.1039/C2AN35641D>. Prof. H.B. Kraatz suggested the idea and I am the major

contributer to this work in terms of the experimental and written work. Lagugne assited me in doing SERS

experiments and interpretation of data. Pederson offered helpful advice. The final manuscript was

published after revision by Prof. Kraatz and Dr. Lagugne-Labarthet. The manuscript is used here verbatim

in the optimized thesis format.

48

3.1 Introduction

Detection of thin films and monolayers using metallic nanoparticles or nanostructures is the basis

of numerous sensing technologies involving electrochemical and optical measurements as

diagnostic techniques for a number of applications requiring extreme sensitivity.1-3

Significant

efforts to synthesize and optimize silver nanostructures (NSs) have therefore emerged due to their

potential for sensing applications in critical fields such as in biomedical sciences, pollution

monitoring or homeland security testing.1,2,4,5

The size, the shape and the interactions between

metallic NSs are factors that affect their electrochemical and optical properties.1,2

For instance the

plasmon frequency of such metallic particles depends not only on the nature of the metal but also

on the size of the particles and the spacing between adjacent particles thus requiring, for any

optical measurement involving surface plasmon, a matching wavelength of excitation to benefit

from the maximum enhancement conditions of the electromagnetic field in the vicinity of the

particles.6

Another critical challenge is to develop an easy, cheap and reproducible method for preparing Ag

nanostructured substrates with high stability to use as a sensing material for surface enhanced

Raman spectroscopy (SERS)7,8

. More specifically the aggregation of the particles often result in a

weaker sensitivity whereas optimal gap between nanostructures, generally in the range of a few

nanometers, generally results in larger electromagnetic enhancement up to 10 order of

magnitude.9-11

Various approaches have been taken to prepare Ag nanostructured platforms such

as self assembly12,13

, chemical etching14

and electrochemical deposition.10,15,16

All these strategies

often require valuable chemicals, multiple steps and difficult fabrication methods that produce Ag

NSs with limited applications in SERS or electrochemical sensing.9,15,17

In a previous report, we

laid the ground work for the present report and outlined an electrochemical method for producing

highly stabilized Ag nanostructured substrates in an alkaline solution that can be used for the

electrochemical detection of the cyanide anion and of diethylcyanophsphonate.18

This approach has led to the synthesis of Ag NSs in a wide range of sizes that are stable and can

be used for electrochemical and SERS detection. Such nanoparticles were tested in the presence of

paraoxon and malathion, two organophosphonates molecules. Both molecules are insecticides that

can be found in small but detectable quantities in many fruits and vegetables. A fast and reliable

49

detection of these cholinesterase inhibitors is therefore a matter of public safety and interest.

Widely used strategies for organophosphonate detection include liquid or gas chromatography,

high performance liquid chromatography and mass spectrometry.19

Electrochemical and SERS

measurements are also suitable for the fast, cost-effective, in-situ and sensitive analysis of

organophosphonates.20

For SERS studies, the presence of a localized surface plasmon resonance

(LSPR) is known to enhance greatly the detection limit for these surfaces measurements.21

In the

present work, we investigate the interaction of nanostructured Ag surfaces with

organophosphonates by electrochemical methods and by SERS and report on the effects of

nanostructure/particle size on SERS, in order to optimize the fabrication conditions for the SERS

platforms. The concentration of the organophosphonates was varied from 10 mM to 1 pM and a

limit of detection of 10 pM for malathion hydrolysis product was observed.


3.2.1 Reagents

KOH (Caledon), paraoxon and malathion (both Sigma-Aldrich) were used as received. Deionized

water (18.2 MΩ·cm resistivity) from a Millipore Milli-Q system was used throughout this work.

The Ag foil (thickness = 0.28 mm, 99.9% metal basis) and platinum wire were purchased from

Alfa Aesar.


Cyclic voltammetry (CV) experiments were performed using a CHI660B electrochemical

workstation (CH instruments Inc.), using a 10 mL homemade Teflon cell in a three-electrode

setup for holding the silver foil. A coiled Pt wire (0.25 mm diameter, Alfa Aesar, Ward Hill, MA,

99.9% metal basis) was used as an auxiliary electrode and Ag/AgCl (3 M KCl, CH Instruments,

Inc.) served as a reference electrode, which was connected to the electrochemical cell via a

homemade agar salt bridge (1 M KNO3). All electrochemical measurements were carried out in a

grounded Faraday cage.

50

3.2.3 Surface Characterization

The silver surface morphology was investigated using scanning electron microscopy (SEM) (Leo

1540XB FIB/SEM and Leo 1530 SEM) which was equipped with energy dispersive X-ray

spectroscopy for composition analysis.

3.2.4 Surface Enhanced Raman Spectroscopy Measurements

The Raman spectra were recorded using a Labram HR 800 (Horiba) combined with an Olympus

IX71 microscope. The used objective was a 40x with a numerical aperture of 0.75. The use of

higher numerical aperture objective (100x, 0.9 NA) led to quick sample degradation. The

irradiation wavelength was set to 632.8 nm and the typical acquisition time was set to 10-20 s per

spectrum while the laser intensity was typically about 250 W at the sample. A series of 20

spectra was recorded for each sample to ensure the reproducibility of the measurements. For

sample with very low concentration, i.e. less than 10 nM, acquisition time was longer (up to 300

s) and similar laser intensity was used (250 W). All the spectra reported here are normalized

with respect to acquisition time and laser intensity.


3.3.1 Surface Preparation

A series of electrochemical experiments were conducted to produce Ag nanostructures from a

polished Ag surface. The SEM image of initial Ag foil has been shown in supporting information

(Figure B.3.1., Appendix B). A standard cyclovoltammetry experiment was carried out using a

three-electrode setup with the Ag surface as the working electrode. The Ag surface was cycled for

1, 5, 10 and 15 CV cycles with a potential range of -0.5 and 0.9 V vs. Ag/AgCl at a scan rate of

150 mVs-1

at a supporting electrolyte concentration. Cyclic voltammetry experiments were

conducted in the presence of paraoxon and malathion with concentrations varying from 10 mM to

1 pM in 8 M KOH.




51

3.3.2 Study of Paraoxon and Malathion

Using these modified metallic surfaces, we have performed a series of SERS experiments to

detect paraoxon and malathion (Scheme 3.1) and estimate both the ideal electrochemical

preparation methods as well as the detection limit for these two organophosphonates that are

currently used as insecticides and have been shown to be responsible for a range diseases.22

Scheme 3.1. Chemical structure of paraoxon (left) and malathion (right).

Figure 3.1 shows a comparison of cyclic voltammograms of freshly prepared Ag NSs in the

presence and absence of 10 mM paraoxon in 8 M KOH. The presence of paraoxon causes a

significant reduction in all redox-signal current intensities because of its interaction with the Ag

surface. These results can be compared to those previously reported on gas-phase generated Ag

NPs on ITO that also display a reduction of the current intensity in the CV curves (Figure B.3.2.,

Appendix B).23

The examination of the surface by scanning electron microscopy (SEM) shows

morphological changes of the Ag NSs after exposure to paraoxon followed by electrochemical

cycling in the range of -0.5 to 0.9 V vs Ag/AgCl. The aggregation of Ag particles and the increase

of the size of the formed Ag NSs to 13510 nm in the presence of paraoxon in 8 M KOH is

observed and can be compared to 8910 nm for the size of Ag NSs in the presence of 8 M KOH

as reported previously.18

However, as shown in Figure 3.2, the size distribution and density of the

Ag nano particles is quite homogeneous over the surface as opposed to our previous approaches

on gas generated particles over ITO that shows large structural defects (needle like particles) and

inhomogeneous distribution (Figure B.3.3., Appendix B).

The paraoxon exposed Ag surfaces were studied by Raman spectroscopy. Due to the presence of

the roughened metal surface, SERS enhancements are expected.24

As shown in Figure 3.3, the

SERS of the Ag surface exposed to 10 mM solution of paraoxon can be directly compared to the

Raman spectrum of pure paraoxon.25-27

52

Figure 3.1 Cyclic voltammograms of Ag foil in 8 M KOH (_____) and after addition of 10 mM paraoxon in 8 M KOH

(…….). The CVs were recorded at a scan rate of 150 mVs-1 in the potential range of -0.5 and 0.9 V vs. Ag/AgCl after

15 CV scans.

Figure 3.2 SEM image Ag foil cycled in 10 mM Paraoxon in 8 M KOH. Image were recorded after a total of 15 CV

cycles in the range of -0.5 and 0.9 V vs. Ag/AgCl at a scan rate of 150 mVs-1.

This implies that the paraoxon molecules are presumably adsorbed on the surface of Ag NSs

without any hydrolysis nor specific surface orientation (Figure 3.3 and Table 3.1). Such

observation provides a rationale for the decrease in the current as shown (Figure 3.1) due to

increased film resistance. The adsorption of paraoxon during the CV cycles is responsible for the

changes in the morphology of the Ag nanostructures as well as for the formation of needle-like

structures on ITO (Figure B.3.3., Appendix B).

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-0.03

-0.02

-0.01

0.00

0.01

0.02

j/m

A c

m-2

E/V (Vs. Ag/AgCl)

Ag2O AgO

AgO Ag2O

Ag2O Ag

Ag Ag2O

1 µm

53

.

Figure 3.3 Raman spectrum of pure paraoxon and SERS spectrum of the Ag foil surface cycled in 10 mM paraoxon

in 8 M KOH. Spectra were recorded after a total of 15 CV cycles in the range of -0.5 and 0.9 V vs. Ag/AgCl at a scan

rate of 150 mVs-1. No baseline correction was performed on the Raman spectra.

Since the relative intensities of the peaks of the SERS spectrum for paraoxon on the Ag surface

are similar to pure paraoxon, one can assume that paraoxon is adsorbed without any specific

orientation with respect to the silver surface. It is important to point out that the SERS spectra of

the adsorbed film were obtained with short acquisition times and low laser intensity, yet with an

excellent signal-to-noise ratio. This contrasts with SERS measurements on gas generated Ag

particles on ITO that yield weak Raman signal. It also highlights the importance of the

electrochemically prepared Ag surfaces for surface enhanced Raman spectroscopy and

demonstrates the correlation between the Raman enhancement with the size and the distribution of

the Ag particles over the surface.

Analogous measurements were performed in the presence of Malathion. For Ag foil in the

presence of malathion, the intensity of the first oxidation peak (Ag Ag2O) increases

significantly, but the corresponding reduction wave remains virtually unchanged. The intensity of

the second oxidation peak (Ag2O AgO) and its redox couple both decrease.

54

Table 3.1. SERS spectrum assignment of the Ag foil surface cycled in 10 mM paraoxon in 8 M KOH. The spectrum

was recorded after a total of 15 electrochemical cycles in the range of -0.5 and 0.9 V vs. Ag/AgCl at a scan rate of

150 mVs-1.

Raman shift (cm -1

) Mode descriptions

619 out of plane ring movement

636 NO2 scissor, CC bending

732 NO2 scissor, CC bending

812 Ring Breath

857 NO2 scissor (Ar-NO2)

1110 C-H bend (in plane)/ NO2 asymmetric stretch

1164 C-H bend (in plane)/ NO2 asymmetric stretch, in plane para-nitrophenol vibration

bands + rock (C14, C12 –H) + strech(C15,C13-H) + T( O-C14 or12-C15 or13 –H)

1229 P=O stretch

1348 symmetry stretching NO2

1448 ring deformation

1524 unsymmetric stretching NO2

1592 phenyl ring vibration

2935 C15 or 13 –H stretching

2977 C14 or 12 –H stretching

3088 C-H phenyl stretching

In addition, a shift to higher oxidation potentials is noticeable for both oxidation waves (Ag

Ag2O) and (Ag2O AgO) (Figure 3.4). This result contrasts with the electrochemical behaviour

of gas-phase generated Ag NPs on ITO and that of Ag foil in the presence of malathion are quite

different. For the gas-phase generated Ag NPs on ITO, the intensity of all electrochemical signals

decreased significantly (Figure B.3.4., Appendix B) which is similar to our previous work18

. In

addition, shifts in the redox signals were observed with the exception of that of the Ag(0) Ag(I)

oxidation wave (Figure B.3.4., Appendix B).

55

Figure 3.4 Effects of malathion interaction with Ag NSs on the cyclic voltammogram of Ag foil. The overlay of the

cyclic voltammograms for Ag foil in 8.0 M KOH (_____) and after addition of 10 mM malathion in 8 M KOH (…….).

CVs were recorded at a scan rate of 150 mVs-1 in the potential range of -0.5 and 0.9 V vs. Ag/AgCl after 15 CV scans.

The surface morphology of the Ag NP-decorated ITO after exposure to malathion is more

spherical with an average size of 81 nm (Figure B.3.5., Appendix B), but with smaller surface

density as compared to which is a novel and totally different from all previous observations.

18,23We suspect that malathion hydrolyzes in the alkaline solution to produce a sulfur-containing

leaving group, causing passivation of the surface as is indicated in the hydrolysis pathway shown

in Scheme 3.2, resulting in the formation of a thiolate leaving group (a) and dimethyl

thiophosphonate (b). Both may react with silver surfaces giving adsorbates. In addition, initial

interaction of the surface may also involve the thiophosphate of malathion group, followed by a

surface based hydrolysis reaction. Shown in Figure 3.5, the surface morphology of Ag foil after

exposure to malation in alkaline solution indicates that the particle size of Ag NSs on a Ag foil is

unaffected by the presence of malathion (8.0 M KOH: 895 nm; 8.0 M KOH/10 mM malathion:

885 nm), but there is a noticeable black discolouration on the Ag surface.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-0.03

-0.02

-0.01

0.00

0.01

0.02

j/m

A c

m-2

E/V (Vs. Ag/AgCl)

Ag2O AgO

AgO Ag2O

Ag2O Ag

Ag Ag2O

56

Scheme 3.2. Hydrolysis of malathion in alkaline solution to form an alkylthiophosphate and a thiolate species.

The distribution of the silver nanoparticles as well as their density over the silver foil is quite

homogeneous. We carried out additional studies on Ag foil to evaluate the interaction of Ag NPs

with malathion. EDX analysis (Figure B.3.6, supplementary information chapter 3, appendix B)

shows that in addition to Ag, C and O, the presence of sulfur and phosphorus is observed on the

surface.

Figure 3.5 SEM image of the Ag foil cycled in 10 mM malathion in 8 M KOH. Image was recorded after a total of 15

CV cycles in the range of -0.5 and 0.9 V vs. Ag/AgCl at a scan rate of 150 mVs-1.

SERS studies were carried out to evaluate the nature of the adsorbed material on the Ag surface.

As shown in Figure 3.6, the SERS spectra clearly indicates stretching modes for

dimethylthiosphosphonate9, 25-27, 29-31

(Figure 3.6 and Table 3.2). But no C=O or P-S stretching

modes were observed (C=O: 1740 cm-1

and P-S: 490-528 cm-1

).

1 µm

57

Figure 3.6. Raman spectrum of pure malathion and SERS spectrum of the Ag foil surface after cycling in 10 mM

malathion in 8 M KOH. The spectrum was recorded after a total of 15 CV cycles in the range of -0.5 and 0.9 V vs.

Ag/AgCl at a scan rate of 150 mVs-1. No baseline correction was performed on the Raman spectra.

A new peak, at 1585 cm-1

, that was not reported in the pure malathion spectrum, was observed in

the SERS spectra and is presumably be related to the formation of complex between Ag (I) and

dimethyl thiophosphonate as indicated in Scheme 3.3. It is interesting to note that malathion in

aqueous solution of AgNO3 forms a thiophosphonate complex of Ag (I). The electrospray mass

spectrum of such a reaction mixture shows signals due to Ag-thiosphosphonate and Ag-

dimethylthiosphosphonate (Figure B.3.7., Appendix B). This is another evidence for the formation

of the supposed structure when Ag NSs exposed to malathion in alkaline solution. From these

results we suggest that the adsorbed material originates from the hydrolysis of malathion. The

thiosphosphonate formed in the hydrolysis will be able to coordinate to Ag (I) on the surface

forming a complex as indicated in Scheme 3.3.

58

Table 3.2. Interpretation of SERS spectra from adsorbed thiophosphonate (Scheme 3.5b) on Ag foil surface cycled in

10 mM malathion in 8 M KOH.

Raman shift (cm -1

) Mode descriptions

635 P=S stretching

685 P=S stretching

815 In phase P-O-C stretch/ P-O stretch

983 P-O stretch

1043 P-O-C vibration

1074 Out of phase P-O-C stretch

1190 P=O stretching

1342 POS- stretching

1442 P-O-CH3 symmetric deformation

1505 P-O-CH3 asymmetric deformation

1585 POS- Ag+

Such complex should passivate the surface, and will finally result in a reduction of the second

oxidation wave (Ag(I) Ag (II)).

Scheme 3.3. Proposed complexation of surface bound Ag(I) generated electrochemically during cycling ions by a

thiophosphate group. The proposed interactions involve formation of a -O,S complex bridging between adjacent

Ag(I) sites, a O,S-chelate, or O- or S-coordinated monodentate complexes.

59

Our studies clearly indicate dissociation of malathion under the reaction conditions in the presence

of Ag nanostructured surfaces. This will limit the amount of material chemisorbed onto the Ag

surfaces in the case of malathion. In sharp contrast, paraoxon is not dissociatively adsorbed and

can therefore accumulate in much larger amounts on Ag surfaces.

3.2.3 The Effect of Ag Nanostructure Size on SERS Enhancement

Factor

Next, we probed the effects of particle size on the SERS enhancement. We reported previously

that the number of CV scans will greatly affect the size of the Ag NSs.

Figure 3.7. SERS spectra of Ag foil surfaces after cycling in 10 mM paraoxon in 8 M KOH. The spectra was

recorded after 1(a) , 5(d), 10 (c), and 15 (b) electrochemical cycles in the range of -0.5 and 0.9 V vs. Ag/AgCl at a

scan rate of 150 mV. Spectra were acquired with the exact same conditions using a 0.75 NA, 40X objective. The laser

power was of 100 µW, and the acquisition time of 20 s for each spectrum. A Baseline correction was applied to the

series of spectra.

60

Generally, low number of CV scans will result in smaller particles in low density, while

increasing the number of CV scans will increase the size and density of the nanoscopic features on

a Ag surface.18

Thus, a series of Ag surfaces were prepared in which the number of CV scans was

varied between 1 and 15 CV scans in the potential range of -0.5 – 0.9 V at a scan rate of 150 mVs-

1. Shown in Figure 3.7 are the Raman spectra acquired from four different surfaces that were

cycled in 8.0 M KOH in the presence of 10 mM paraoxon. The spectra collected after 1, 5, 10, and

15 CV scans, correspond to particle sizes of 315, 497, 637 and 8910 nm, respectively.

Samples after 10 and 5 scans exhibit the larger Raman enhancement (Figure 3.7 c and d). All the

samples show unambiguously Raman SERS activity with minimum background. The SERS

recorded after 5 cycles displays the best spectral details for mid-sized particle size and consistency

of the thickness of the particle deposit.18

SERS recorded for 5 and 10 CV cycles indicate virtually

identical signal.

3.2.4 Limit of Detection in SERS

In order to examine the sensitivity limit of our method, surfaces with various concentrations of

paraoxon and malathion in the range of 10 mM to 1 pM were prepared. Paraoxon could be

detected down to 10 nM concentration (Figure B.3.9., Appendix B) which can be compared to

alternative analytical methods based on functionalized colloidal Ag Nps in the presence of

acetylcholin enzyme which reported a detection limit down to 18 nM concentration.28

For

malathion, the most intense characteristic peak (P-O Stretching mode at 983 cm-1

) was measured

with concentration as low as 10 pM as shown in Figure 3.8.

This indicates a high sensitivity of our surface preparation method together with standard Raman

conditions. The sensitivity appears to be dependent on the molecule of interest. The reactivity of

malathion with the surface appears to lead to better Raman SERS detection at lower

concentration.

61

Figure 3.8. SERS spectra of the Ag foil surface cycled in a wide range of Malathion concentration (1 mM - 1 pM) in

8 M KOH. Spectra were recorded after a total of 15 electrochemical cycles in the range of -0.5 and 0.9 V vs. Ag/AgCl

at a scan rate of 150 mVs-1. No baseline correction was performed on the Raman spectra.

3.3 Conclusions

Silver nanostructured surfaces are sensitive to the addition of organophosphonates, resulting in

significant changes in the electrochemical behaviour and the surface morphology. SERS has

shown to be critical in evaluating the interactions between malathion and paraoxon, with the Ag

surfaces in more detail and has allowed us to shed light on the adsorbates. While paraoxon

adsorbs on the silver surface without any structural change, it appears that malathion undergoes

hydrolysis and the adsorbed species is in fact a thiophosphonate. Interestingly, the SERS method

using these in-situ electrochemically fabricated Ag NSs substrates is able to detect paraoxon and

hydrolysis product of malathion as low as 10 pM for malathion which highlights the interest of

electrochemically prepared Ag NSs as effective SERS substrates. A lower CV cycling number

provides smaller and homogeneous surface features, resulting in high quality SERS spectra. This

opens a new window to detect low concentration pesticide using simple preparation approach and

non-critical conditions for the Raman measurements.

62

3.4 Acknowledgement

This work was carried out under Public Works Contract W7702-09R218/001/EDM. The authors

wish to gratefully acknowledge the Nanofabrication Facility at Western University for their

invaluable help with SEM and EDX (Dr. T. Simpson and T. Goldhawk). We acknowledge D.

Hairsine for his help in Mass spectroscopy.


B.

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64

Chapter 4 Dual Localized Scanning Plasmon Resonance and

Electrochemical Investigations of Organophosphorus Insecticides Presence

c

In chapters 2 and 3, we investigated and extracted detailed information about the interaction of

organophorsphorus (OP) compounds with electrochemically prepared Ag NSs. In addition,

plasmon resonance properties of Ag NSs made them suitable SERS substrates that provide

superior enhancement of Raman signals and tunability of the size of the nanostructural features. In

this chapter, we expand the application of these Ag NSs and address questions related to rapid

sensing of OPs in real samples, such as tap-water, or apple juice. We explore this topic exploiting

electrochemical and localized surface plasmon techniques focusing on sulfur-contining OPs.

c This chapter has been taken from the submitted article ―Dual Localized Scanning Plasmon Resonance and

Electrochemical Investigations of Organophosphorus Insecticides Presence‖ F. Fathi, Vijayaratnam, C., H.B. Kraatz,

Submitted. Prof. H.B. Kraatz suggested the idea and I am the major contributor to this work in terms of the

experimental and written work. Vijayarantnam assited me to repeat experiments.

65

4.1 Introduction

Organophosphorus (OP) compounds containing a thiophosphoryl (P=S) functional group are

generally used as insecticides for agricultural applications.1,2

They exhibit a high level of toxicity

due to disabling of the central nerveous system by irreversibly binding to acetyl cholinesterase.2

Continual application of OP compounds in farm lands worldwide has led to their presence as the

residual not only in agricultural products, but also has resulted to their seepage into underground

water sources which is causing significant problems.3,4

Currently gas and liquid chromatography5-6

and mass spectroscopy7 are used for OPs residual

detection, also there has been some reports on development of biosensors based on the

cholinestrase activity inhibition,8-11

electrochemical, fluorescence, colorimetric and quartz crystal

microbalance;2,3,12,13

however there is still a continued high demand of developing new materials

which interacts strongly with OPs and can be used for quick monitoring of them in robust or

complicated conditions using more portable, sensitive and low cost techniques from practical

perspective.14,15

In this context, the use of metal nanoparticles (NPs), in particular silver NPs provides a new

analytical angle and has potential for significant advances. Different methodologies have been

applied for the synthesis of Ag NSs and interestingly they have found applications in

electrochemical and localized surface plasmon resonance (LSPR) techniques for sensing of a

broad range of analytes, are readily available at a low cost and pose no issues related to

fabrication.16-23

Previously we reported a simple electrochemical method for fabrication of nanostructured Ag

substrates.24

In addition these Ag nanostructures (NSs) were successfully applied for detection of

two OPs, paraoxon and malathion at ppm levels using surface enhanced Raman spectroscopy.25

This result has prompted us to evaluate the use of LSPR for OP detection on Ag NSs and here we

describe the results of our study into the use of LSPR for monitoring the presence of the three

organophosphate pesticides ethion, fenthion and malathion (Scheme 4.1).

66


4.2.1 Reagents

KOH, KNO3, Na2SO4, H2SO4, and ethanol were purchased from Caledon and used as received.

CdSO4, Ru[(NH3)6]Cl3, ethion, fenthion malathion were purchased from Sigma-Aldrich and used

as received. Apple juice (Simply Food) was used as received. Deionized water (18.2 MΩ·cm

resistivity) from a Millipore Milli-Q system was used throughout this work. The Ag foil

(thickness = 0.28 mm, 99.9% metal basis) and platinum wire (0.25 mm diameter, 99.9% metal

basis) were purchased from Alfa Aesar.


All electrochemical experiments, including cyclic voltammetry (CV), under potential deposition

(UPD), square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS)

were performed using a CHI660B electrochemical workstation (CH Instruments Inc.), using a 10

mL homemade Teflon cell in a three-electrode setup for holding the silver foil. A coiled Pt wire

was used as an auxiliary electrode and Ag/AgCl (3 M KCl, CH Instruments, Inc.) served as a

reference electrode. The reference electrode always was separated from the cell by a miniature

homemade salt bridge (agar and KNO3) to minimize the contamination of the electrochemical cell

with Cl- ions. All measurements were carried out at room temperature (23 ± 2

oC) in an enclosed

and grounded Faraday cage. Electrochemical measurements were performed in 1.0 M KNO3

containing 1.0 mM Ru[(NH3)6]Cl3. The SWV measurements were carried out in the potential

range of -0.4-0 V with a step potential of 0.004V, frequency at 10 Hz, quiet time at 2 s and a pulse

amplitude of 20 mV. All EIS measurements were performed in the frequency range of 1 Hz to

100 kHz with an AC amplitude of 5 mV. The initial potential was adjusted to -0.170 V for all

measurements. The measured EIS by ZSimpWin 3.22 (Princeton Applied Research) and the data

were presented in Bode plots. Underpotential deposition (UPD) of cadmium were carried out in an

aqueous solution of 0.1 M Na2SO4 and 6 mM CdSO4. The pH was adjusted to pH = 5 by H2SO4

addition. UPD measurements were carried out in the range of -1.200 to 0.200 V (vs Ag/AgCl) at

the scan rate of 10 mVs-1

and a quiet time of 30 s.

67

Importantly, all measurements were repeated for a minimum of three times with separate

electrodes to obtain statistically significant results.

4.2.3 Optical Measurements

The optics system set up is composed of a spectrophotometer (USB-4000-UV-vis), a tungsten

halogen light source (LS-1-LL, wavelength range 200-1100 nm), a fiber probe bundle (fiber core

diameter 400 μm, wavelength range 300-1100 nm) and WS-1 diffuse reflectance standard, all

purchased from Ocean Optics (Dunedin, USA). The diffuse reflectance standard was used as a

Lambertian reference surface. The UV-vis probe was placed close to the working electrode

surface so that incident light was reflected upon hitting the surface and then reflected back to the

detector situated in the light probe. The intensity and wavelengths of spectral peaks were recorded

as modifications were made to the surface. The probe height was held constant (~1 mm above the

sample surface) throughout this study. All experiments were performed at room temperature.


A nanostructured Ag surface was prepared by a standard CV experiment using a three-electrode

setup with the silver surface as the working electrode. Ag surface was washed for 30 s in the

boiling ethanol before and after roughening process and then rinse with Milli-Q water. The Ag

was cycled for 5 CV cycles in a potential range of -0.5 and 0.9 V vs. Ag/AgCl at a scan rate of

150 mVs-1

at 8.0 M KOH solution.

4.2.5 Immobilization of Insecticides

Solution of the insecticides ethion, fenthion and malathion (Scheme 4.1) were prepared at various

concentration in dried ethanol and then were immobilized onto the roughened Ag surface during

their incubation for 72 hr at 5oC. The surface was thoroughly washed using absolute ethanol and

Milli-Q water after incubation. The electrode was dried in the air and immediately transferred to

the electrochemical cell for the measurement. Tests were repreated in apple juice and tap water

with organophosphonates added to the test solution.

68

4.2.6 Surface Area Measurement

Since changes in the experimental conditions affect the size of the Ag NSs,24

the active surface

area needed to be determined. This was achieved by underpotential deposition (UPD) of

cadmium. Our results show the highest surface area (5.90±0.10 cm2) was achieved after 5 cycles

and these are used for all further studies (see supplementary information for chapter 6 in appendix

C, Figure C.4.1 and Table C.4.1).


4.3.1 Electrochemical Studies of OP-modified Ag NSs

The electrochemical properties of this system were investigated by SWV and EIS in the potential

range of -0.4 – 0.0 V (vs Ag/AgCl) in 1.0 M KNO3 containing 1.0 mM concentration of the

cationic redox probe Ru[(NH3)6]Cl3 before and after OP (Scheme 4.11) immobilization on the Ag

NSs substrates.

Scheme 4.1. Chemical structure of ethion 1, fenthion 2 and malathion 3.

EIS allows the characterization of the electrical properties of the interface.26,27

This approach has

been employed previously to examine various sensing and biosensing systems.28,29

The sensitivity

of these substrates toward ethion using SWV and EIS is as low as 100 nM, while it is 10 µM for

fenthion and malathion in ethanolic solution (see Figures C.4.2., C.4.3. and C.4.4., Appendix C).

Upon incubation of the Ag NSs substrates in the solution containing OPs (ethanolic solution, tap

water and apple juice) the current signal in either SWV is decreased presumably due to the

formation of an OP thin film on the NS surface, which increases the resistance to charge transfer

(see Figure 4.1 and 4.2).

69

It appears that the signal intensity decreases as the number of S atoms in the backbone of the OP

increases along the order of ethion > malathion ≈ fenthion. Presumabaly this is due to stronger

interactions between the Ag surface and the OP as the number of interacting sites in the molecule

increase.29,30

Previous studies on the detection of OPs by differential pulse voltammetry reported

detection limits of 10-12

M, 10-8

M and 10-9

M for ethion, fenthion and malathion respectively

using modified glassy carbon electrode with phenazine compounds under N2 gas.3 However, the

responses were measured in a buffer under controlled conditions and not in complex matrices.

Figure 4.1. SWVs of modified Ag NSs substrates with OPs in water (A) and apple juice (B). In both graphs lines are

as following, bare Ag NSs (solid lines), modified with tap-water or apple juice (dash lines), modified with 100µM

OPs in tap-water or apple juice (dot lines) and modified with 1mM OPs in tap-water or apple juice (dash dot dot

lines). SWVs were performed in the potential range of -0.4 to 0 V vs Ag/AgCl at a scan rate of 0.1 Vs-1. with a step

potential of 0.004V, frequency at 10 Hz, quiet time at 2 s and a pulse amplitude of 0.02 V.in a solution of 1M KNO3

containing 1 mM [Ru(NH3)6]Cl3 as redox probe. The current signal intensity decreases in both cases with OPs

concentration enhancement. (C) demonstrated the relation between current intensity and OPs concentration in water

(W) and apple juice (AJ). The current intensity decrease with OPs concentration enhancement is slightly more for tap-

water than apple-juice, which could be because of less complex matrix.

70

To further explore the electrochemical characteristics of the films, EIS studies were carried out.

EIS provides information about the physical properties of organic film which can be interepreted

in terms of capacitative and resistive properties with the help of an equivalent circuit.31

Figure 4.2 shows the Bode plots for OP-modified Ag NSs substrates in tap-water and apple juice

and Figure 4.2C shows a modified Randles’ circuit used to fit the experimental EIS data.

Figure 4.2. Bode plots of modified Ag NSs with OPs in tap-water (A) and apple juice (B). In both graphs lines are as

following, bare Ag NSs (solid lines), modified with tap-water or apple juice (dash lines), modified with 100µM OPs

in tap-water or apple juice (dot lines) and modified with 1mM OPs in tap-water or apple juice (dash dot dot lines).

EIS measurements were performed in a solution of 1M KNO3 containing 1 mM [Ru(NH3)6]Cl3 as redox probe. They

were acquired at the formal half potential of the [Ru(NH3)6]Cl3 (-0.17 V vs. Ag/AgCl) at 5 mV amplitude and in the

0.1 Hz to 100 kHz range. (C) demonstrates a modified Randles’s equivalent circuit: Rs, the solution resistance; Cdl,

the double layer capacitance; Rct, the charge transfer resistance and CPE is the constant phase element. (D)

demonstrates the relation between Rct and OPs concentration, clearly Rct increases due to OPs concentration’s

enhancement.

71

In the proposed circuit, Rs accounts for the solution resistance of the electrolyte solution and is a

function of the supporting electrolyte concentration. The Rs is in series with a circuit which used

to describe the OP/solution interface. The Cdl in the circuit demonstrating the double layer

capacitance, Rct represents the OP film/solution interface related to the the Ru(III/II) redox

process. The constant phase element, CPE, represents the frequency dispersion of the

pseudocapacitance due to the surface inhomogenity.31

CPE impedance is given by

ZCPE=1/(Q(j.ω)n , where Q is the frequency-independent constant correlating to the redox

properties of the surface, j=-11/2

, ω represents the anqular frequency, and the exponent n arises

from the slope of Log Z vs logf (-1 ≤ n ≤ 1). The CPE acts as a pure resistance for n = 0; the CPE

behaves as a pure capacitor for n = 1, and CPE behaves as an inductor for n = -1. The CPE is

attributed to a Warburg impedance for n = 0.5, that is incorporated with the mass transport

because of diffusion of ions at the electrode/solution interface. The calculated impedance

spectrum are shown with solid line and are in good agreement with the experiemnatl data points

(see supplementary Information appendix C, Figure C.4.5-C.4.9). The increase of OP

concentration in any of the solutions tested gives rise to an increase of the charge transfer

resistance Rct (Figure 4.2D and Figure C.4.3-4.5). These results are compatible with the SWVs

results as well, suggesting detection of P=S containing OPs to a concentration as low as 100 µM.

Previously work on the detection of OPs by EIS using interdigited gold electrodes modified with

benzodipyrido [3,2-a:2'3'-c] phenazine demonstrated sensitivity as low as 1mM for ethion, 1 µM

for malathion and 1 pM for fenthion.2 The results of the EIS study reported here are comparable

for ethion, but more work is required to optimize the system for fenthion and malathion by

modification of the Ag NSs substrates.

4.3.2 Localized Surface Plasmon Resonance

Next, the LSPR behaviour of OP-modified AgNSs was examined. Earlier results showed that

nanostructured-Ag substrates exhibit an outstanding surface plasmon properties which make them

a suitable candidate for use as SERS substrate.25

The results for the addition of OPs to Ag NSs surface and their corresponding changes in

absorbance intensity compared to control surface are shown in Figure 4.3 (see also Appendix C,

72

Figure C.4.10-C.4.12). Our LSPR results show that the 400 nm peak intensity decreases by

increasing the OP concentration. The slight reduction of peak intensity in pure tap-water and juice

may arise as a result of the absorbance of other compounds in the solution on the surface. All

LSPR results are compatible with electrochemical results. To the best of our knowledge, there has

not been any report to apply LSPR for the detection of these organophosphates.

Figure 4.3. LSPR response of OP modified Ag NSs. (A) Absorbance intensity against wavelength to show decrease

in Fresnel signal as different OP concentration bind to Ag NSs surface in tap-water (A) and apple juice (B) for bare

Ag NSs (solid black line), modified Ag NSs with pure tap-water (A) or apple juice (B) (dash line), modified Ag NSs

with 100 µM OPs in tap-water (A) and apple juice ( dot line) and modified Ag NSs with 1mM OPs in tap-water (A)

and apple juice ( solid gray line). (C) shows the relation between relative absorbacne reduction with ennhancement of

OPs concentration in tap-water and apple juice. The change in reflectance intensity was monitored at 400 nm.

Lastly, it is worth nothing that the approval levels of malathion, fenthion and ethion by the US

Food and Drug Administration are 0.2 mM, 0.36 µM and 1.04 nM, respectively.2 Thus, our results

73

are promising for the rapid monitoring of presence of OPs in tap-water or apple juice by

electrochemical and LSPR techniques.

4.4 Conclusion

In summary interaction of OPs with Ag NSs leads to impedance enhancement, current intensity

reduction in SWV and LSPR absorbance decrease. The obtained results are as a proof of sensing

OPs’ category containing P=S. However for having selectivity and lower detection limits,

modificiation of Ag NSs surfaces with a linker that selectively captures just one of the OPs at the

same time is proposed. This work opens new doors for using these Ag NSs surfaces as a useful

platform for dual electrochemical and LSPR sensing of OPs in the complicated matrix.

4.5 Acknowledgements

We would like to thank the University of Toronto Scarborough for financial support of this

research.


C.

4.6 References

1 http//: www.epa.gov/pesticides/food/pest.html

2 B.B. Narakathu, W. Gua, S.O. Obare, M.Z. Atashbar, Sensor Acuate B 2011, 158, 69-74.

3 W. Guo, B.J. Engelman, T.L. Haywood, N.B. Blok, D.S. Beaudois, S.O. Obare, Talanta

2011, 87, 276-283.

4 G. Liu, Y. Lin, Anal. Chem. 2007, 77, 5894-5901.

5 B. Jin, L. Xie, Y. Guo, G. Pang, Food Res. Inter. 2012, 46, 399-409.

6 J. Fenik, M. Jankiewicz, M. Biziuk, Trends in Anal. Chem. 2011, 30, 814-826.

74

7 M.R. Gravett, F.B. Hopkins, M.J. Main, A.J. Self, C.M. Timperley, A.J. Webb, M.J.

Baker, Anal. Methods 2013, 5, 50-53.

8 X. Sun, X. Wang, Biosens. Bioelectron. 2010, 25, 2611-2614.

9 W. Zhao, P.Y. Ge, J.J. Xu, H.Y. Chen, Environ. Sci. Technol. 2009, 43, 6724-6729.

10 S. Samadi, H. Sereshti,Y. Assadi, J. Chromatogr. A. 2012, 1219, 61-65.

11 D. Lu, Y. Yang, X. Luo, C. Sun, Anal. Methods 2013, 5, 1721-1732.

12 C. De, T.A. Samuels, T.L. Haywood, G.A. Anderson, K. Campbell, K. Fletcher, D.H.

Murray, S.O. Obare, Tetrahedron Lett. 2010, 51, 1754-1757.

13 D.D. Erbahar, I. Gurol, G. Gumus, E. Musluogla, Z.Z. Ozturk, V. Ahsen, M. Harbeck,

Sensors Acuate B: Chem. 2012, 173, 562-568.

14 M. Trojanwicz, Electroanalysis 2002, 14, 1311-1328.

15 M. Zourob, K.G. Ong, K.F. Zeng, F. Monffonk, C.A. Grimes, Analyst 2007, 132, 338-343.

16 J. Yguerabide, E.E. Yguerabide, Anal. Biochem. 1998, 262, 137–156.

17 S.K. Srivastava, V. Arora, S. Sapra, B.D. Gupta, Plasmonics 2012, 7, 261-268.

18 A.J. Haes, R.P. VanDuyne, J. Am. Chem. Soc. 2002, 124, 10596-10604.

19 K.S. Lee, M.A. El-Sayed, J. Phys. Chem. B 2006, 110, 19220-19225.

20 A. Kumaravel, M. Chandrasekaran, Sensors Acuat B: Chem. 2011, 158, 319-326.

21 A. Kumaravel, M. Chandrasekaran, Sensors Acuat B: Chem. 2012, 174, 380-388.

22 L. Kashefi-Kheyrabadi, M.A. Mehrgardi, Biosens. Bioelectron. 2012, 37, 94-98.

23 W. Song, H. Li, H. Liu, Z. Wu, a D. Xu, Electrochem. Comm. 2013, 31, 16-19.


25 F. Fathi, F. Lagugné-Labarthet, D.B. Pedersen, H.B. Kraatz, Analyst 2012, 137, 4448-

4453.

26 R.P. Janek, W.R. Fawcett, A. Ulman, J. Phys. Chem. B 1997, 101, 8550-8558.

27 43 E. Boubour, R.B. Lennox, Langmuir 2000, 16, 7464-7470.

28 44 J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Chem. Rev. 2005,

105, 1103-1169.

29 45 E. Katz, I. Willner, Electroanalysis 2003, 15, 913-947.

30 R. A. Bell, J.R. Kramer, Environ. Toxicol. Chem. 1999, 18, 9-22.

31 E. Barsoukov, R. MacDonald, Impedance Spectroscopy: Theory, Experiment, and

Applications, 2nd Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005, Ch. 2, pp. 37-39.

75

Chapter 5 Effects of Surfactants on Electrochemically Prepared Ag

Nanostructuresd

As discussed in earlier chapters, previous studies have described the morphological features of the

electrochemically prepared Ag NSs and the effects on their properties. It was also discussed that

the addition of compounds as diverse as cyanide or organophosphonates influence the

morphology of nanostructured surfaces, sometimes leading to the dissolution of Ag NSs. It was

suspected that corrosion of the surface is responsible for changes in the surface morphology;

however, detailed corrosion studies were lacking. In order to gain more insight into how additives

affect the morphology of electrochemically prepared silver nanostructures, we decided to carry

out a more detailed study evaluation of the corrosion behaviour of these Ag surfaces. In this

chapter, impedance spectroscopy and scanning electron microscopy are used to evaluate the

effects of common caping agents and surfactants on the corrosion behaviour and the resulting

morphology of silver surfaces.

d This chapter has been taken from the accepted article ―Effects of surfactants on electrochemically

prepared Ag nanostructures‖ F. Fathi, H.B. Kraatz, Analyst, accepted, <DOI: 10.1039/C3AN00933E>..

Prof. H.B. Kraatz suggested the idea and I am the major contributor to this work in terms of the

experimental and written work.

76

5.1 Introduction

The shape and morphology of Ag nanostructures (NSs) have a tremendous effect on their physical

and chemical properties.1-9

Thus, a considerable focus was placed on controlling the morphology

of AgNSs. 1-4, 10-16

With the introduction of capping agents, the morphology of nanostructures can

be influenced by manipulating the surface free energy of a particular crystal facet through

chemisorption.17,18

This has been explored for the design and fabrication of a large variety of

different NSs with various shapes that are optimized for specific experiments by different

additives, including capping agents, stabilizers and surfactants.2, 5-9,10-16

In this context, several

studies have focused on the mechanisms of interactions of AgNSs with citrate and

polyvinylpyrolidone (PVP) as the capping agents and stabilizers, to better control their

morphology or to produce nanostructure shapes with a high level of monodispersity.9,19-23

It has

been demonstrated that PVP can bind selectively to Ag(100) to reduce its surface free energy

compared to the Ag(111) facet. This leads to the formation of nanocubes and nanobars, including

structures with prominent {100} faces.11, 24-26

In contrast, citrate appears to bind less strongly to

Ag(100) than to Ag(111), leading to nanostructures formation with their {111} faces exposed on

the surface.11,27

While Tween-20 has been utlilized in AgNSs preparation, no details were reported

on the effects of NS morphology.28,29

Given the use of metal NSs as electrode materials, in

electronics, optoelectronic, as sensor application and energy related devices,30-34

it is important to

develop at least a basic understanding of their chemical reactivity, including corrosion events that

are ultimately responsible for the creation of the NS features. Ultimately, corrosion may play an

important role in device failure.30,35-37

There have been a number of reports on the corrosion of

bulk Ag in the presence of chloride, sulfide, ozone and UV irradiation. The results indicated that

the formation of Ag2S causes tarnishing of the Ag surface and consequently results in corrosion.

Also, the Ag surface corrodes in the presence of chloride due to the formation of soluble silver-

chloride complexes. UV radiation of ozone produces atomic oxygen which quickly forms silver

oxide on the surface and corrodes it.38-45

Due to larger surface area of Ag NSs, their surface may

be more vulnerable to corrosion once exposed to corrosive environments. More recently a few

studies have been performed to better understand the corrosion of Ag NSs in the presence of

chloride or sulfide at high potentials in aqueous solution.35, 46-48

This is one of a major limitations

in silver applications: when it is exposed to the atmosphere, significant corrosion can occur that

77

weakens the integrity of structures, such as vital power contacts. Yacaman et al have studied the

corrosion behaviour of colloidal PVP-stabilized Ag nanoparticles.35

Chang et al reported that

covering Ag NSs with polypyrrole reduces electrochemical corrosion and sulfur tarnishing.49

Studies on Ag nanowires and nanoparticles demonstrate that corrosion at the nanoscale is similar

to that of bulk silver.50

In order to prevent or decrease corrosion, a number of organic and inorganic corrosion inhibitors

were examined.51-54

Essentially, inhibitor adsorption on the metal surface can significantly change

the corrosion properties of the metals. For example, Tween-20 significantly decreases the

corrosion of cold rolled steel in acidic media.52

PVP has chelating properties and can be dissolved

in aqueous and organic media. It has been applied to protect carbon steel in neutral and alkaline

solution against a corrosion. The results demonstrated that PVP formed a protective film on the

carbon steel/solution interface, leading to double layer thickness enhancement and this resulted in

corrosion inhibition.53

In addition, it has been reported that citrate reduces corrosion in steel due to

interactions of the carboxyl groups with the metal surface forming a protective film formation.55

To our knowledge, there are no corrosion studies that explore the effects capping agents or

surfactants. Here, we explore the influence of (i) the capping agents tripotassium citrate and PVP,

and (ii) the surfactant Tween-20 on the morphology and corrosion behaviour of electrochemically

in situ prepared Ag NSs. These surfactants are environmental friendly, cost-effective and readily

available. Previous studies demonstrate their usefulness as corrosion inhibitors for various metals

in different environments.51-54


5.2.1 Reagents

KOH (Caledon), tripotassium citrate (Sigma-Aldrich), polyvinylpyrolidone (PVP) (MP

biomedicals) and polyoxyethylene(20) sorbitan monolaurate (Tween-20) (Biobasic Inc.) were

used as received. Deionized water (18.2 MΩ·cm resistivity) from a Millipore Milli-Q system was

used throughout this work. The Ag foil (thickness = 0.28 mm, 99.9% metal basis) and platinum

wire were purchased from Alfa Aesar.

78


Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments were

performed using a CHI660B electrochemical workstation (CH instruments Inc.), using a 10 ml

homemade Teflon cell in a three-electrode setup for holding the silver foil. A coiled Pt wire (0.25

mm diameter, Alfa Aesar, Ward Hill, MA, 99.9% metal basis) was used as an auxiliary electrode

and Ag/AgCl (3 M KCl, CH Instruments, Inc.) served as a reference electrode, which was

connected to the electrochemical cell via a homemade agar salt bridge (1 M KNO3). All

electrochemical measurements were carried out in a grounded Faraday cage. All CV experiments

were conducted at a scan rate of 150 mVs-1

in the potential range of -0.5 to 0.9 V. All EIS

measurements were performed in the frequency range of 0.01 Hz to 100 kHz with AC amplitude

of 10 mV. The initial potential was adjusted to 0 V vs. Ag/AgCl for all measurements. The

experimental EIS data were analyzed and fitted to an appropriate equivalent circuit using

ZSimWin 2.0 (Echem software).

5.2.3 Surface Characterization

The morphology of the silver surface was investigated using scanning electron microscopy (SEM)

(Leo 1540XB FIB/SEM and Leo 1530 SEM) which was equipped with energy dispersive X-ray

spectroscopy for composition analysis.

5.2.4 Graphite Furnace Atomic Absorption

The atomic absorption measurements were carried out using Thermo Scientfic ICE3500 AA

Spectrometer with a GFS35 Graphite Furnace with a Deuterium Background Correction. The

source was set to 328.1nm using silver hollow cathode lamp. The drying, ash, atomic and cleaning

temperature were 100o, 450

o, 1100

o and 2500

oC respectively.

79

5.2.5 Transmission Electron Microscopy

Transmission electron microscopy (TEM) measurements were done using Hitachi 7500 at the

Centre for the Neurobiology of Stress, University of Toronto.

5.2.6 Mass Spectroscopy

Electrospray Mass Spectrometry (ESI) measurements were done by using Broker maxis Q-ToF

Ultra High Resolution Mass Spectrometer. Capillary voltage was set on 2400 V, neublizer

pressure was set on 0.3 bar and nitrogen as a drying gas was used with a flow of 4 L/min at 180o

C.



AgNS surfaces were prepared from polished Ag working electrodes by cyclic voltammetry (CV)

in an aqueous KOH solution in a potential range of -0.5 to 0.9 V versus Ag/AgCl at a scan rate of

150 mVs-1

.55

The influence of capping agents and the surfactant (see Scheme 5.1) was

investigated keeping identical electrochemical conditions for all CV experiments. Samples were

evaluated after 16 CV cycles.

Scheme 5.1. Chemical structure of the potassium citrate 1, PVP 2 and Tween-20 3.

80

5.3.2 Electrochemical Impedance Spectroscopy

We carried out impedance spectroscopic studies in KOH solution at an applied potential of 0 mV

with a modulation amplitude of 10 mV. Bode plots were used to examine the corrosion behaviour

of AgNSs in the presence and absent of potassium citrate, PVP and Tween-20 (Figure 5.1 a,b).

The fitted data using an appropriate Randle circuit (Figure 5.1c) are presented in Appendix D

Figure D.5.3.1. With the help of a modified Randles’ equivalent circuit, it was possible to model

the impedance behaviour of the surfaces and express them in terms of equivalent circuitry

elements.

Figure 5. 1. Bode plots log Z versus log f after cycling a Ag working electrode surface 16 times in the 0.1 M KOH (a)

and 0.01 M KOH (b) in the presence and absence of capping agents and surfactant. (c) is a modified Randles’

equivalent circuit: Rs, the solution resistance; Cdl, the double layer capacitance; Rct, the charge transfer resistance;

CPE, the constant phase element; Rf, film resistance and W is the Warburg impedance, representing a diffusion

element. EIS were acquired at the potential of 0.0 V vs. Ag/AgCl at a modulation amplitude of 10 mV and in the 0.01

Hz to 100 kHz range. In both graphs, EIS results are shown for several additives: citrate (−∙∙─∙∙−), surfactant-free

(_______), PVP (∙∙∙∙∙∙), Tween-20 (-----). The addition of citrate decreases the impedance and addition of Tween-20

increases impedance in both cases, while PVP does not have any significant effect on the impedance.

In the equivalent circuit, Rct represents the resistance between the outer Helmholtz plane (OHP)

and the electrode surface. The solution resistance, Rs, represents the resistance between the

81

working electrode, the counter, and the reference electrode. The double layer capacitance (Cdl) is

defined as the ability of the Helmholtz plane to accumulate charges. The constant phase element

(CPE) is used to represent the frequency dispension of the pseudo capacitance resulting from the

surface inhomogeneity.56

Here CPE is constant phase element of metal/solution interface. The

impedance of the CPE (ZCPE) is represented by the following relationship ZCPE=1/(Q(j.ω)n),

where Q is the frequency independent constant to referring to the redox properties of the surface,

j=-11/2

, ω is the angular frequency and the exponent n originates from the slope of log Z vs log f (-

1 < n < 1). The CPE acts as a pure resistance for n = 0, while it behaves as a pure capacitance for

n = 1. At n = -1, CPE functions as an inductor. Once n = 0.5, CPE acts as a Warburg impedance,

which is associated with the mass transport due to the diffusion of ions at the electrode/solution

interface.56

Rf represents the film pores resistance and W denotes Warburg impedance, which

represents a diffusive contribution due to the diffusion of ions at the electrode/solution interface.

All fitting parameters are summarized in the ESI (see Table D.5.2., Appendix D). Our EIS studies

demonstrate that at both KOH concentrations, the addition of Tween-20 increases the impedance.

This suggests that the surface is protected from corrosion, while citrate has an opposite effect and

a significant reduction of impedance, which indicates corrosion of the silver surface. Interestingly,

the addition of PVP does not have any significant effect on the corrosion behaviour of Ag in

alkaline solutions. The slight difference between corrosion in 0.01 and 0.1M KOH is the result of

concentration differences (pH effects) and not related to corrosion effects.

5.3.3 GFAA Measurement

The Ag content of the solution was measured by Graphite Furnace Atomic Absorption (GFAA)

after cycling the Ag surface in aqueous KOH solution for 16 CV scans as described in the

experimental section. A range of silver nitrate solutions were used as the standard solutions (see

Appendix D, Figure D.5.2.) and the results are presented in Table 5.1. As seen in this table, in the

presence of surfactants, the concentration of Ag ions increased. This is more remarkable in the

presence of PVP and Tween-20, which could be related to: a) formation of a protective film due to

adsorption onto the Ag NSs surface; and b) solution density enhancement. These factors reduce

formation of soluble Ag complexes. The increase in the impedance of system after the addition of

Tween-20 suggests film formation. However, to gain a better understanding of the effects of

82

surfactants on Ag NSs surface, and to provide a more thorough explanation of the observed

results, it is important to evaluate the surface morphology of Ag surfaces that were exposed to

these additives.

5.3.4 Analysis of the Surface Morphology

In order to investigate the effect of additives on the morphology of the AgNSs after addition of

Tween-20, potassium citrate, and PVP, scanning electron microscopy (SEM) studies were carried

out (Figure 5.2). Based on our previous studies,55

we learned that decreasing the KOH

concentration caused the formation of less compact Ag NSs film due to higher ohmic resistance.

The addition of citrate to the solution resulted in the formation of smaller AgNSs features while at

the same time resulting in more pitting of the surface compared to KOH only. Previous studies

have shown that, in the presence of citrate, a silver citrate complex is formed, which presumably

leads to the formation of larger clusters. 9,22

Both the ionic strength of the solution and the cluster charge are presumably factors that control

the particle size in a wide range of KOH concentrations in the presence of citrate.9,23

The negatively charged citrate adsorbs onto the surface of the AgNSs and produces an electrostatic

repulsion layer around them preventing the formation of larger clusters.9, 20-23

When the cluster

reaches an optimum size, the repulsion of the citrate layer around the cluster prohibits more

aggregation of AgNPs, so they deposit on the Ag surface and grow through the Ostwald ripening

mechanism.57

Thus, the smaller particles are adsorbed onto the bigger particles and reduced

electrochemically on the surface, resulting in formation of the nanostructured Ag surface. This Ag

cluster reduction on the surface is facilitated by the potential reduction of the metal surface in

comparison to the solution57

(Scheme 5.2). The net result is a surface with smaller surface features.

It is interesting to note that focused ion beam (FIB) studies demonstrate that the thickness of the

formed AgNS film does not depend on the presence of the additive (see Figure D.5.3. Appendix D).

The formation of Ag-citrate complexes were demonstrated by EI-mass spectroscopy analysis of the

solution (see Figure D.5.4., Appendix D).

83

Figure 5.2. SEM study of the effects of capping agents and surfactants on the surface morphology of deposited Ag

NS film. Demonstrated are the results after 16 CV cycles over the potential range of -0.5 to 0.9 V vs Ag/AgCl at a

scan rate of 0.150 mVS-1 at supporting electrolyte concentration of 0.1 M KOH (a) , with citrate (b), with PVP (c),

with Tween-20 (d), 0.01M KOH (e), with potassium citrate (f), with PVP (g) and with Tween-20. Potassium citrate

leads to smaller nanofeatures, PVP results to the smaller and less compact NS film and Tween-20 addition forms

spherical NS Ag with higher distance between Ag nanoparticles.

84

Scheme 5.2. Proposed formation of Ag nanostructures (a) particle coalescence mechanism in the presence of citrate

and KOH (b) proposed Ostwald ripening on the Ag surface after redeposition of Ag NSs.

Furthermore because of the formation of Ag-citrate clusters, more Ag ions remain in solution.

This leads to less deposition of Ag nanoparticles on the surface and thus less silver is available

for the electrochemical reduction back to elemental silver. Thus more pitting due to corrosion is

apparent in the SEM images of the surface (Figure 5.2 b, f ). In the presence of citrate, the

impedance decreases, which clearly indicates corrosion of the Ag surface (Figure 5.1 a, b).

Moreover analysis of the solution for Ag content by GFAA after performing CV confirmed the

presence of significant concentrations of Ag ions in the solution upon citrate addition (Table 5.1).

The addition of PVP does not have any significant effect on the corrosion behaviour, but

obviously the morphology of Ag NSs change (Figure 5.1 a, b). A less compact AgNSs film

formed on the Ag surface in the presence of PVP (Figure 5.2 c, g). The PVP can interact with Ag

via O or N donor sites forming Agxy+

-PVP that are trapped into PVP molecules and the Ag

surface electrolyte interface by taking electrons produced by Agmo-PVP and deposited on the Ag

surface.3,21,57-61

PVP is known to increase the viscosity of the electrolyte and decreases the

diffusion rate of Ag ions toward the surface.57

Together, these two factors reduce the rate of the

Ag deposition at the Ag surface within the experimental timeframe.

85

Table 5.1. Analysis of the solution by GFAA after carrying out CV experiments Ag surfaces for 16 cycles, in the

range of -0.5 to 0.9 V. The experiments were carried out under two separate sets of KOH concentrations of 0.1 and

0.01 M KOH and the addition of citrate, PVP and Tween-20.

Solution Concentration (nM)

0.1 M KOH 65.0

0.1 M KOH with K-citrate 158.0

0.1 M KOH with PVP 433.0

0.1 M KOH with Tween-20 352.0

0.01 M KOH 103.0

0.01 M KOH with K-Citrate 185.0

0.01 M KOH with PVP 791.0

0.01 M KOH with Tween-20 301.0

GFAA data, summarized in Table 5.1, shows that in the presence of PVP, the amount of Ag ions

in the solution has increased, presumably due to an increase in solution viscosity and PVP-Ag

interaction. Consequently, by increasing the rate of Ag nucleus formation and reducing the rate of

deposition, clusters have less time during the experiment window to grow through the Ostwald

ripening mechanism after slow deposition on the Ag surface. This results in smaller Ag NPs on

the surface (Figure 5.2 c, g). Also, because of the hydrophobic nature of PVP, a hydrophobic

layer surrounding the Ag ions which keeps them more separate.59

This effect of PVP is stronger

than the repulsion produced by the citrate layer around Ag clusters. Therefore, the use of PVP as

a capping agent appears to form more separate and finer Ag NSs on the surface than citrate.

Therefore, less compact Ag NSs films are formed on the Ag surface in the presence of PVP

compared to citrate or the solution without any additives.

Tween-20 appears to have a strong effect on the morphology of AgNSs resulting in a very fine

spherical deposits of Ag particles on the surface (Figure 5.2 d, h). It has been reported that using

even small amounts of surfactants in the synthesis of colloidal silver nanoparticles reduces their

size and makes them more spherical.19,62

Surfactants can even be used as metal-corrosion

86

inhibitors.52,53,63,64

To our knowledge, the effects of surfactants on AgNSs or their effects as a

corrosion inhibitors for silver metal has not been reported. For this reason, we have investigated

the effect Tween-20 on the Ag NSs morphology. Based on our EIS results as shown in the Bode

plots (Figure 5.1 a, b), the impedance increases in the presence ofTween-20, indicate passivation

and a lowered corrosion. It changes the electrolyte double-layer structure when they are adsorbed

on the metal surface. Thus with the surface of the metal insulated from the solution, the corrosion

becomes significantly more difficult.65,66

Some insight into the role of the adsorption of the

surfactant was proposed by Chen et al.67

It was proposed that the hydrophilic heads of the

surfactants adsorbs onto the NP surfaces while the hydrophobic tails are oriented to the exterior.67

This effect creates a layer that inhibits the transference of ions, thus surface pitting and formation

of AgNSs by silver redeposition are reduced. Thus only a thin layer of redeposited AgNPs can be

seen on the Ag surface and particles are more spherical compared to controls in alkaline solution

(Figure 5.2d, h). The presence of nano species in 0.01M KOH solution in the absence and

presence of surfactants have been approved by doing TEM (see Figure D.5.5., Appendix D).

5.4 Conclusion

In summary, the corrosion of silver surfaces in basic solution is affected by the addition of

capping agents and surfactants. The presence of citrate causes significant corrosion of silver

surfaces as is demonstrated by EIS and SEM studies, which is in contrast to corrosion inhibition

by citrate of steel. While it was reported to be a corrosion inhibitor agent for steel. Tween-20 has

a protective effect on Ag and its behaviour is similar to previous reports on cold rolled steel.

Results of the impedance study clearly shows significantly less corrosion, presumably by forming

a protective outer layer on the metal surface, which protect the surface from corrosion. This also

has significant effects on re-deposition of material on the surface, which is impeded significantly.

Interestingly, PVP alters the morphology of the AgNS surface, but its presence does not affect

the corrosion behaviour. The addition of additives when preparing an AgNS surface by

electrochemical cycling provides a facile method to further control the size of the surface

features.

87


Financial support from NSERC and the University of Toronto are gratefully acknowledged. We

also wish to thank Tony Adamo, Robert (Bob) Temkin (University of Toronto Scarborough), and

Dr. Todd Simpson (Western University) for help with the SEM experiments.

Supplementary Information: Supplementary material for this chapter can be found in appendix

D.

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92

Chapter 6 Tailoring zinc Porphyrin to the Ag Nanostructure Substrate: an

Effective Approach for Photoelectrochemical Studies in the Presence of Mononucleotides

E

Both chapters 2 and 3 focused on exploring the use of electrochemically fabricated nanostructured

Ag surfaces for electrochemical sensing and as substrates for SERS. Importantly, it has been

displayed that this method is simple, rapid, cost effective, and reproducible. Moreover in chapter 3

it was clearly demonstrated that all Ag NSs surfaces display a superb enhancement, but lower

cycling which provide access to smaller and consistent surfaces features will result in high quality

SERS. These results pose a number of questions that is explored in this chapter. In doing so the

interaction of Ag nanostructured surfaces with biomolecules through electrochemistry and surface

chemistry studies is explored to understand the mode of absorbtion and elucidate potential

reactions of the biomolecules with these surfaces. The next issue that will be addressed relates to

the observed electrochemical changes of the nanostructured Ag surfaces in the presence of

potential reactions of the biomolecules with these surfaces. This would shed light on the potential

of these nanostructures surfaces for using as a biomolecules sensing substrates.

E This chapter has been taken from the accepted article ―Tailoring zinc porphyrin to the Ag nanostructure

substrate: an effective approach for photoelectrochemical studies in the presence of mononucleotides‖ F.

Fathi, C. Kong, Y. wang, Y. Xie, Y.T. Long, H.B. Kraatz, Analyst, 2013, 138, 3380-3387.Reproduced by

permission of the Royal Society of Chemistry. Copyright 2013, < DOI: 10.1039/C3AN00156C>. Prof. H.B.

Kraatz suggested the idea and I am the major contributer to this work in terms of the experimental and

written work. Kong assited me to do experiments. Wang synthesized the porphyrin compound. Xie

provided us the porphyrin compound and offered some helpful advice. The final manuscript was

submitted after revision by Prof. Kraatz and Prof. Long. The manuscript is used here verbatim in the

optimized thesis format.

93

6.1 Introduction

In recent years, there has been a great interest to exploit metallic nanostructures (NSs), typically

silver or gold, to enhance optical absorption. 1,2

This optical activity of metal NSs could be gained

either by direct excitation or indirect excitation through a bonded chromophore molecules to

them.3 Incorporation of metal NSs with photochemically active molecules leads to enhancement

of photocurrent response in a range of incident photon wavelength. This phenomenon is

attributed to the combination of interband transitions in the metal NSs along with the surface

plasmon resonance excitation. 4,5

Charge transfer between photoactive molecules and nearby

metallic NSs has been extensively studied. 6-13

Modulated optical and electrochemical properties

resulting from these interactions are desirable for applications including molecular sensing, 14,15

hydrogen production, 16,17

photoenergy conversion,18-21

and electrocatalysis.22

We were

particularly interested in exploring the properties of electrochemically prepared Ag NSs in the

presence of protective molecules. These structures are readily prepared by carrying out cyclic

voltammetry in aqueous KOH, providing access to a NSs of various grain sizes. 23

Among

photoactive molecules, porphyrins and their derivatives are of particular interest and have been

examined extensively in the past for potential applications including light harvesting and various

electron transfer and catalytic reactions. 6, 22, 24

There are a number of reports of immobilized

porphoryins on metal surfaces 24-27

and metallic nanoparticles. 28-30

The latter has received

significant attention due to the potential application in photodynamic therapy. 31

Surface

modifications by physi- and chemisorption were reported leading to the formation of thin films

that were explored for their potential applications in nanoscience, molecular electronics and

photonics. 32-34

Chemical modifications of metal surfaces often lead to the formation of stable

films. Amongst metal substrates, gold modifications have been studied extensively and gold-thiol

chemistry has been successfully used to form a multitude of thin films. 35-36

While modification

of silver surfaces are well known, they are generally less explored. Silver-carboxylates are often

used for film formation. 37

While there are also reports of porphyrins on silver nanoparticles that

mostly investigate spectroscopy features, 38-40

porphyrin films on nanostructured Ag surfaces are

less known and their properties have not been explored. Here, we explore the properties of

electrochemically fabricated Ag NSs as a substrate for photo-electrochemical studies involving

the cyanoacrylic acid modified Zn-tetraphenyl porphyrin 2-cyano-3-(2′-(5′,10′,15′,20-

94

tetraphenylporphyrinato zinc(II))yl)acrylic acid. This particular porphyrin was chosen due to its

proven potential for light harvesting and having a carboxylate group that can be anchored to the

silver surface. 6

In our studies, we explore the effect of the nanostructure on the photo-electrochemistry of the

films and compare them to films prepared on a bulk Ag surface. As part of this study we explore

the effect of the addition of three nucleoside monophosphates AMP, CMP and GMP on the

photoelectrochemical response of the modified Ag NSs with porphyrin. Our studies show that the

porphyrin films exhibit significant differences as a function of nucleoside monophosphate.

6.2 Experimental Section

6.2.1 Reagents

Potassium hydroxide, sodium acetate and potassium nitrate (Shanghai Lingfeng Chemical

Reagent Co. Ltd), ethylenediaminetetraacetic acid (EDTA) (Sinopharm Chemical Co. Ltd.),

hexaammine ruthenium(III) chloride (Sigma-Aldrich), glucose, galactose, fructose, mannose,

hydroquinone, ascorbic acid, acetic acid, tetrahydrofuran (THF) (Shanghai

Aladdin Chemical Reagent Co. Ltd.), adenosine-5’-monophosphate disodium hexahydrate

(AMP), guanosine 5’-monophosphate disodium salt (GMP) and cytidine 5-monophosphate

sodium salt (CMP) (Sangon Biotech (Shanghai) Co. Ltd.) were used without further purification.

2-Cyano-3-(2′-(5′,10′,15′,20-tetraphenylporphyrinato zinc(II))yl)acrylic acid was synthesized

based on the literature.6 Deionized water (18.2 MΩ·cm resistivity) from a Millipore Milli-Q

system was used throughout this work. The Ag foil (thickness = 0.28 mm, 99.9% metal basis)

and platinum wire were purchased from Alfa Aesar, MA.


Electrochemcial experiments were performed using a CHI660C electrochemical workstation

(Shanghai Chenhua Co. Ltd., China). A coiled Pt wire (0.25 mm diameter, Alfa Aesar, Ward

Hill, MA, 99.9% metal basis) was used as an auxiliary electrode and Ag/AgCl (3 M KCl, CH

Instruments, Inc.) served as a reference electrode. The experimental EIS data were analyzed and

95

fitted to an appropriate equivalent circuit by using EIS ZSimWin 2.0 (Echem software).

6.2.3 Electrode Modifications

Ag NSs were prepared based on our previous reported method by cycling a silver foil in 5M

KOH. 23

the size of these particles are 49 ± 7 nm and base on our previous study have a consistent

thickness over the surface. 23

These properties induced the highest enhancement in surface

enhanced raman spectroscopy (SERS) compared to the other particle sizes,41

so we use them for

the current study as the substrate. In the next steps, the resulting silver nanostructures were

cleaned by dipping them into boiling ethanol for 1 min and rinsing with Milli-Q water. Next, the

Ag NSs surfaces were incubated in 1 mM solution of zinc porphyrin in THF for 12-18 hours.

The resulting surfaces were examined by Raman spectroscopy. SERS results show the successful

modification of the silver surface existence of porphyrin on the Ag NSs surface even after

washing the surface with solvent (see Figure E.6.1., Appendix E).

6.2.4 Photocurrent Measurements of Functionalized Electrodes

All photocurrent experiments were performed on a CHI 660c electrochemical workstation

(Shanghai Chenhua Co. Ltd., China). The functionalized silver electrode in the quartz cell was

irradiated by a Xenon lamp (LE-SP-LS-XE 500, Shenzhen Leo-photoelectric Co., Ltd. China)

operated at 500 W, through a home-made light chopper in which the shutter chopping frequency

was controlled manually. The photocurrent response of the functionalized Ag NSs electrode as a

working electrode was measured using an amperometric I-t curve with an Ag/AgCl reference

electrode and a Pt wire counter electrode, in 0.4 M sodium acatate buffer (pH 6.4) solution. The

frequency of the on and off phases of the light source was controlled with the light chopper. The

photocurrent action spectrum was obtained with the method except that the light source is

equipped with a filter to get a monochrome light.

6.2.5 UV-Vis Spectroscopy

UV-vis absorption spectra were taken with an Ocean Optics USB2000 equipped with miniature

fiber-optic spectrometer in the 190–1700 nm region.

96

6.2.6 X-ray Photoelectron Spectroscopy

Measurements (ThermoFisher, E. Greanstead) was performed at the Surface Interface Ontario

center of University of Toronto, with Al Kα (15 µm, 200 eV) as the photo source for analysis.

The same conditions have been applied for core level measurements except pass energy (PE)

which is 30 ev for core level measurements.

6.2.7 Time-of-flight Secondary Ion Mass Spectrometry

Positive and negative static SIMS measurements were conducted using ToF-SIMS spectrometer

(IONTOF Gmbh, Muenster). The sample was bombarded by a low primary ion dose (Bi 3+

< 1013

cm-2

) and low flux (10 pA-5nA). The positive Tof-SIMS are presented because they contain a

greater amount of information regarding our area of interest.

6.2.8 Surface Enhanced Raman Spectroscopy Measurements

All Raman spectra were recorded using a Labram HR 800 (Horiba) combined with an Olympus

IX71 microscope. The used objective was a 50x. The irradiation wavelength was set to 514 nm

and the typical acquisition time was set to 10s per spectrum while the laser intensity was typically

about 3.5 mW at the sample.


6.3.1 Electrochemical Characterization of Porphyrin Modified Ag

NSs

The redox properties of this system were examined by cyclic voltammetry in the region of -0.4–

0 V (vs Ag/AgCl) in sodium acetate buffer (Figure 6.1 A). There is not any significant change in

the current response before and after immobilization of porphyrin, suggesting neither the

porphyrin on the Ag NSs substrates nor does Ag NSs have any redox activity in this region.

Therefore we use cationic redox probe, [Ru(NH3)6]3+

to investigate the change in the

97

electrochemical properties before and after porphyrin immobilization on the Ag NSs. Next, the

porphyrin-modified surface was studied by cyclic voltammetry (Figure 6.1 B) and

electrochemical impedance spectroscopy (EIS) in the presence of [Ru(NH3)6]3+

as a cationic

redox probe (Figure 6.1 C).

Figure 6.1. Electrochemical behavior of Ag NSs before and after modification with porphyrin. (A) CVs of Ag NSs

(a) and Por-Ag NSs (b) in sodium acetate buffer solution (pH 6.4) in the potential range of -0.4 to 0 V vs Ag/AgCl at

a scan rate of 0.1 Vs-1. There is not any significant change in the redox properties of Ag NSs before and after

immobilization with porphyrin in this region.(B) Typical CVs of bare Ag NSs (a), bare Ag (b) modified Ag (c) and

modified Ag NSs (d) in a solution of 1M KNO3 containing 1 mM [Ru (NH3)6]Cl3 as redox probe at a scan rate of 0.1

Vs-1 in the potential range of -0.4 to 0 V vs Ag/AgCl.Ag NSs exhibit higher sensitivity compared to Ag in the

present and absence of phorphyrin due to their larger surface area. In addition decreasing current in both (c) and (d)

are due to resistance enhancement because of porphyrin film immobilization on the Ag and Ag NSs respectively (C)

Nyquist plots for Ag NSs (a) and Por-Ag NSs (b) in 1M KNO3 containing 1 mM [Ru (NH3)6]Cl3 as redox probe.

Data points represent experimental results while solid lines correspond to spectra calculated for an equivalent circuit

shown as an inset. The electrochemical impedance spectra were acquired at the formal half potential of the [Ru

(NH3)6]Cl3 (-0.17 V vs. Ag/AgCl) at 5 mV amplitude and in the 0.1 Hz to 100 kHz range. (D) is a modified Randles’

equivalent circuit: Rs, the solution resistance; Cdl, the double layer capacitance; Rct, the charge transfer resistance;

CPE, the constant phase element; Cf, the film capacitance and Rf is the film resistance.

98

EIS is a powerful technique which allows characterizing the electrical interfacial properties, in

which a small sinusoidal voltage is applied (5 mV) in order to cause minimal perturbation to the

system.42-44

This approach has been used successfully to study a series of sensing and biosensing

systems..45,46

The impedance results were rationalized with the help of a modified Randles’

circuit. A proposed equivalent circuit is depicted in Figure 6.1 D. Rs represents for the solution

resistance of the electrolyte solution and is determined by supporting electrolyte concentration.

Cdl is the double layer capacitance of the external porphyrin/electrolyte interface. Rct accounts

for film/solution interface and represents to the charge-transfer resistance related to the Ru (III/II)

redox process. CPE is the constant phase element, which is used to represent the frequency

dispersion of the pseudocapacitance resulting from the surface inhomogenity..47

Cf is the

capacitance of the internal porphyrin/solution interface, and Rf is the resistance that expands

between the porphyrin and the Ag NSs substrate. The redox couple is reversibly oxidized at the

Ag NSs surface (Figure 6.1B). The formation of the porphyrin film on the Ag NSs surface

increases the resistance, leading to a slight reduction of the current response in the CV and the

EIS due to changes in the diffusion controlled charge transfer process.

6.3.2 Surface Characterization of Porphyrin Modified Ag NSs

ToF-SIMS and XPS were performed to characterize the adsorption mode and the thickness of the

porphyrin film.The most characteristic fragments observed in ToF-SIMS spectra were shown in

Figure 6.2 (also see Figure E.6.2., Appendix E). The porphyrin molecule exhibits a signal at m/z

= 773.2 (Figure 6.2 B). The multiples observed upon the region of interest in the Tof-SIMS are

related to the central metal atom isotope (64

Zn (48.6%), 66

Zn (27.9%), and 68

Zn (18.8%)) and 13

C

(1.1%). The peak at m/z = 324.7 corresponds to Ag3 cluster (Figure 6.2 A). In addition, two

signals at m/z = 310.8 and 257.8 are attributed to Ag2C4O2NH3 and Ag2CO2 respectively (Figure

6.2 A), which suggests that the porphyrin system is coordinated to the surface through the

carboxylate group. This is in line with previously observed carboylate films are observed

respectively which indicate porphyrin has adsorbed on the surface through carboxyl group.48,49

The thickness of this porphyrin film relies on the photoelectron attenuation from the underlying

substrate by immobilized porphyrin50

and is calculated based on equation 6.1. In this equation I is

an XPS signal from the substrate covered with porphyrin. Io is the intensity of signal from the

99

clean substrate. λ is the escape depth (the attenuation length), Ѳ is the angle between the plane of

the surface and the detector and d, here is the thickness of zinc porphyrin50

. Therefore the

calculated thickness based on the XPS results (see Figure E.6.3., Appendix E) is 0.3-0.45 nm.

I/Io= exp[-d/λcosѲ] (6.1)

Figure 6.2. Positive secondary ion mass spectrum of the porphyrin film on Ag NSs surface. The peaks at m/z =

324.7 u (A) , 310.8 u (A), 257.8 u (A) and 773.2 u (B) are attributed to Ag3 cluster, Ag2C4O2NH3, Ag2CO2 and

porphyrin respectively.

Figure 6.3. Representative XPS spectrum of the Ag NSs surface after immobilization of porphyrin. A shows the

survey spectrum.which signals Zn 2p, N 1s, C 1s and O 1s are indicative of immobilized surface with porphyrin

which is anchored by carboxylate group to the substrate. 51-54 Ag 3d signals are attributed to Ag NSs substrate.23 B

shows the C 1s core level spectrum together with fitted data. The fiiting data shows three peaks at 286.7, 288.3 and

289.0 which are assigned to C-OH, -C=O and O-C=O, respectively.51

100

XPS survey reveals characteristic peaks for C 1s, Ag 3d, N 1s, O 1s and Zn 2p which are

indicative of porphyrin adsorbtion on the surface (Figure 6.3 A and also see Figure E.6.4.-C.6.8.,

Appendix E). The best fitted C1s spectra results shows a peak at 285.0 which can be attributed to

C-C and C-H bonding groups. The peaks at 286.7, 288.3 and 289.0 (Figure 3.3 B) are assigned to

C-OH, -C=O and O-C=O, respectively.51

This is in good agreement with literature reports on the XPS properies of carboxylate group on

Ag surface (Table 6.1), described by Daniels et. al.51

Table 6.1. Atomic percentage of immobilized porphyrin film and C 1s XPS results.

Binding energy (eV) Assignment Atom %

285.0 C-C/C-H C 1s: 73.31

286.7 -C-O- Ag 3d: 16.38

288.3 -C=O N 1s: 1.29

289.0 -O-C=O O 1s : 8.12

Zn 2p: 0.91

Integration of the XPS signals from the survey spectrum allows us to quantify the response from

individual elements and express it in atom%. XPS revealed characteristic Zn 2p at 1021.0 ev

(Figure 6.3A and also see Figure E.6.4., Appendix E) and N 1s at 398.0 ev (Figure 6 B and also

see Figure E.6.5., Appendix E) signals which are indicative of metal-centered porphyrins.52-54

Furthurmore based on bonding energies C 1s (see Figure E.6.6., Appendix E) and O 1s (Figure

E.6.7., Appendix E) signals are indicative of the carboxylate group presence on the Ag NSs

surface 51

Ag 3d signal (Figure E.6.8., Appendix E) is attributed to the metallic Ag. 23

So all in all

appearance of Zn 2p, N 1s, C1s and O 1s clearly proved the attachment of zinc porphyrin

molecules on nanostructured Ag surface.

6.3.3 Photocurrent Response of the Porphyrin Functioned Ag NSs

Surface

To investigate the effect of Ag NSs on the photocurrent response, the photoresponse of it was

compared to Ag foil in the presence and absence of porphyrin (Scheme 6.1). As shown in Figure

6.1, the Ag NSs electrode demonstrates stronger photocurrent enhancement compared to Ag foil.

This behavior could be attributed to excitation of coherent swinging of the free electrons in the

101

metallic nanostructures as a result of light irradiation.55

Also as can be seen in Figure 6.4, the

functionalized Ag NSs electrode with porphyrin (por-Ag NSs), gives the strongest photocurrent

intensity amongst all electrodes. The current intensity of this electrode is 2 times higher than the

functionalized Ag foil and 7 times stronger than the non-functionalized one.

Scheme 6.1. Chemical structure of the Zn-porphyrin 1, adenosine-5’-monophosphate 2, cytidine 5-monophosphate 3,

and guanosine 5’-monophosphate 4.

These phenomena were attributed to surface plasmon polariton resonance excitation in the Ag

nanostructures combined with interparticle coupling effects which increase the light absorption of

porphyrin and further photocurrent response of the functionalized electrode.56,57

Moreover,

compared to a polycrystalline Ag surface, the Ag NSs provide more adsorption sites for the

porphyrin due to the nanostructured nature of the surface, which is thought to enhance the

photocurrent. For having a more stable photocurrent system and boosting of por-Ag NSs

photocurrent response, a wide range of various sacrified electron donors were examined to

investigate their effect on the photocurrent enhancement (Figure E.6.9., Appendix E).

102

Figure 6.4. Photocurrent response of bare Ag (a), bare Ag NSs (b), modified Ag (c) and modified Ag NSs (d) with

porphyrin in 0.4 M sodium acetate buffer (pH = 6.4). Ag NSs exhibit a higher photocurrent response compared to Ag

in the present and absence of phorphyrin. ―On‖ stands for light irradiation, while ―Off‖ represents blockage of light

irradiation.

Based on the results, EDTA was chosen as a sacrifice electron donor, which exhibits highest

photocurrent enhancement after hydroquinone. The mechanism of EDTA a sacrificial electron

donor was reported before.58

It has been stated that EDTA injects electrons into the HOMO

orbital of excited porphyrin molecular levels and oxidized. Then the extra electron moves from

HOMO to LUMO level and transfers to the metallic surface .58

6.3.4 Photocurrent Action of the Functionalized Electrode

Since there are Ag NSs and porphyrin on the functionalized electrode, the photoexcited electron

source as a main unit to generate photocurrent should be elucidated (Scheme 6.2). For this

purpose the modified electrode was irradiated using the light in different wavelength and

compared to the curve which is UV-vis spectra of 25 µM porphyrin solution. Figure 6.5

demonstrates the UV-vis spectra of porphyrin solution along with the photocurrent density of

modified Ag NSs electrode irridiated by light source with changing wavelength under working

conditions.

103

Scheme 6.2. Schematic view of the surface modification. After porphyrin immobilization on the surface (A), the

nucleoside monophosphate is added (B). The phosphate group is likely to interact with the Ag surface. 60-62

Figure 6.5. Absorption of 25 µM porphyrin in THF (solid line) and the photocurrent response of the Por-Ag NSs

electrode (Square spots) measured in the 0.4 M sodium acetate buffer solution (pH 6.4) containing 10 mM EDTA.

The error bars for photocurrent action responses represent the standard deviation of triplicate measurements (n=3).

The strong peak at 434 nm and two small peaks between 500-700 nm can be assigned to the Soret

and the Q band of the porphyrin ring respectively. 58,59

The action spectrum (Figure 6.5) shows

the same spectral features, indicating that the porphyrin is the photoactive species on the

functionalized surface.

(A) (B)

Light

e-

e-

Light

-- - - - -- - - - -- - - - -- - - - -

104

6.3.5 Nucleotide Interaction Effect on the Photocurrent Response

of Por-Ag NSs Electrode

The photocurrent response of modified Por-Ag NSs in the presence of three different nucleotides

including AMP, CMP and GMP was examined (Figure 6.6). The photocurrent changing profile

before and after addition of these nucleotides (1mM) in the buffer solution containing EDTA was

demonstrated in Figure 6.7. We define ΔI as the current change of the electrode before and after

light irridiation. The order of photocurrent quenching is AMP > CMP > GMP (Figure 6.7).

Figure 6.6. Photocurrent response of the Por-Ag NSs electrode to different nucleotides in sodium acetate buffer

solution (pH 6.4) containing EDTA in the absence (a) and present (b) of nucleotides. The order of photocurrent

quenching is AMP>CMP > GMP respectively.

105

At least two factors may explain the photocurrent quenching results: a) the structural differences

between the nucleotides and b) reduction potential of nucleotides. The complexation may prevent

the contact of electron donors and porphyrin to the AgNSs, which block electron transfer from

the porphyrin to the AgNSs surface. However, there should exist the π-π stacking phenomenon

between the purine and pyrimidine groups of nucleotides with the porphyrin. Their difference in

nucleoside structure cause various face to face π-π stacking abilities between porphyrin and

nucleobases, and further result in differences in the complex’s blocking electron transfer process.

24, 63, 64 AMP and GMP have a double ring structure that leads to stronger π-π stacking

interactions with porphyrin, while CMP has only single ring structure resulting to a weaker π-π

overlap with porphyrin.64

Besides, AMP and CMP have intention to be reduced in the

experimental conditions. Previous studies illustrated that adenine and cytosine and their

nucleotides and nucleosides exhibit a single, large and pH dependent reduction polarographic

wave at E1/2 = -0.975 to 0.084 V and at E1/2 = -1.125 to 0.073 V, respectively.65

Figure 6.7. Photocurrent quenching percentage after addition of different nucleotides. AMP has the highest

quenching effect and GMP the lowest. Error bars represent the standard deviation of triplicate measurements.

The adenine reduction wave has a constant height up to pH 4-5 and then starts to decrease with

pH increasing.65

While guanosine and its nucleotides and nucleosides reduction are occurred at

highly negative potentials under similar conditions.65

Therefore AMP not only has stronger π-π

106

stacking interaction with porphyrin, but also has tendency to reduction in the applied potential

which causes electron pulling from porphyrin and more photocurrent quenching. Although CMP

has weaker π-π stacking interaction with porphyrin compared with AMP, but its tendency for

reduction overweights and leads to more photocurrent quenching than GMP and places it as the

second order in photocurrent quenching effect. The last one in this order is GMP which does not

have any tendency for reduction in the experimental condition and only its π-π stacking

interaction causes quenching.

6.4 Conclusions

In summary, modification of a nanostructured Ag surface with a porphyrin exhibits a

significantly enhanced photocurrent compared to a monocrystalline Ag substrate. The

enhancement of the photocurrent is an interesting property that is linked to the nanostructured

nature of the Ag surface. It is interesting to note that the photocurrent is modified by the presence

of the nucleotides, presumably due to the interaction of the nucleotides with the modified surface.

Specific porphyrin/nucleotide interactions may play a role. This work represents a new aspect of

Ag NSs substrates and highlights their usefulness as a transducer for potential chemosensor

systems.


This was funded by NSERC. We would like to thank University of Toronto, the National Science

Fund for Distinguished Young Scholars (21125522), the Major Research Plan of the National

Natural Science Foundation of China (91027035), and the Fundamental Research Funds for the

Central Universities (Grant No. WK1013002) in China for financial support of this work. F.F. is

the recipient of a travel scholarship from the University of Toronto.

Supplementary Information: Supplementary material for this chapter can be found in appendix

E.

107

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Chapter 7 Conclusion and Future Outlook

The research in this thesis was initiated to address questions arising from earlier work in the

Kraatz group exploring the behaviour of gas generated Ag NPs.1 Initial results of the interaction

of diethyl cyanophosphonate, a tabun mimic, on the gas generated Ag NSs in alkaline solution

clearly showed noticeable changes in the morphology and electrochemical properties of the Ag

NPs compared to those of polycrystalline silver surfaces.1 While a mechanism involving formal

oxidation in the presence of CN- of the substrate was postulated, it was not investigated in detail.

Essentially, the work described in this thesis focuses on this issue: exploring the formation of Ag

NSs under a variety of conditions. Experimental conditions, including the supporting electrolyte

concentration and number of cyclic voltammetry scans were studied with a focus on the effects

on the morphology of the Ag NSs.

In Chapter 2, it was demonstrated that the size of the Ag NSs is dependent on the OH-

concentration and the number of CV scans. A lower OH- concentrations and a low number of CV

scans result in the formation of smaller nanostructured features that are deposited on top of bulk

silver. It was hypothesized that this approach would be of use for the fabrication of substrates for

SERS studies. Chapter 3 addressed this issue directly and examined the effects of

organophosphonate deposition using a combination of SERS and electrochemical studies.

Overall, the interaction of organophosphates or their hydrolysis products with Ag NSs substrates

were detected using SERS, and limit of detection results were comparable with previously SERS

results for the same organophoshates using different type of substrates. The results from this

work provided us with a new analytical aspects of these nanostructured Ag surfaces with a high

potential for significant advancement. This prompted us to utilize them to probe the response to

organophosphonates in a series of complex media, including apple juice. As can be expected,

these surfaces do not display any particular selectivity to a particular organophosphonate.

Adsorption of the phosphonate or its degradation product results in a increase of electrochemical

response. In order to achieve any level of selectivity of the electrochemical response, a

recognition element may need to be introduced. However, this was not in the scope of this thesis.

112

Chapter 5 brought into focus the corrosion behaviour of silver in the presence of potassium

citrate, PVP, and Tween-20. Potassium citrate is a known corrosion inhibitor for steel2, PVP is a

known corrosion inhibitor for carbon steel 3, 4

and Tween-20 protects against corrosion of cold

rolled steel and nickel. 5, 6

Our studies discovered that citrate enhanced corrosion of Ag NSs in

contrast to its protective behaviour for steel. However Tween-20 has a protective effect and

reduced the corrosion of Ag NSs which is align with other results, using it as corrosion inhibitor.

5, 6

Next, in chapter 6, Ag NSs were modified with porphyrin as a recognition element that may

confer selectivity to the surfaces which would be useful to develop a selective sensing surface.

The results of this work clearly depicted that porphyrin modified Ag NSs surfaces not only show

better photoelectrochemical response compared to their Ag bulk compartment, but also acts

selectively to a particular analyte. Nevertheless, more work has to be done in order to broaden the

scope of this work.

Future directions should include two main areas of research. Examining the Ag NSs surface

against different analytes is proposed to make sure of its potential for application as SERS

substrates including a wide range of analyte with an acceptable limit of detection. This may

require further modification of the surface morphology features, which could be done by using

different capping agents or surfactants. Some investigation also needs to be done with regard to

the effect of surfactant concentration, temperature, time exposure and pH on the morphological

features of Ag NSs surfaces and their corrosion behaviour. The results of these investigations can

assist us to use a prepared Ag NSs substrate with specific morphological features for the specific

desire category of analytes in order to have the highest SERS response with lowest limit of

detection. Furthermore, the results will assist us to have a better sense of corrosion behaviour of

Ag in nanoscale in different conditions for providing better protection and having a longer

lifetime when it is used in different appliances with minimum environmental impacts. Next, as

the results showed the surface chemistry of Ag NSs is identical to bulk Ag, so we can apply the

same chemistry for modification of Ag NSs with different recognition elements to use it for

selective sensing of different analytes using electrochemical techniques (See Scheme 7.1).

113

Scheme 7.1. Interaction of the desire analyte with recognition element modified Ag NSs surface in the presence of

various interferences gives electrochemical signal.

The ideal modifier should be cheap, stable, easy to prepare, immobilized, and selective in the

presence of interferences. This approach will allow us to not only to evaluate surface properties

by electrochemical techniques step by step; but also put us one more step forward to development

of a small handy electrochemical sensor for detection of desire analyte using cheaper materials.

The last step would include performing quantitative work for each project including measurement

of limit of detection, standard errors and calibration curves.

7.1 References

1. A.J. Marenco, D.B. Pederson, S. Wang, M.W.P. Petryk, H.B. Kraatz, Analyst 2009, 134,

2021-2027.

2. B. Shaymala, S. Rajendran, Eur. Chem. Bull. 2012, 1, 150-157.

3. L.A. Al Juhaiman, A.A. Mustafa, W.K. Mekhamer, Int. J. Electrochem. Sci. 2012, 7,

8578-8596.

4. S.K. Selvaraji, A.J. Kennedy, A.J. Amalraj, S. Rajendran, N. Palaniswamy, Corrosion

Rev. 2004, 22, 219-232.

5. G. Mu, X. Li, J. Colloid. Inteface Sci. 2005, 289, 184-192.

6. A. Abdallah, A.Y. El-Etre, Port. Electrochim. Acta 2003, 21, 315-326.

114

Appendix A Supplementary Information for Chapter 2

Figure A.2.1. Particle size distribution for prepared Ag NPs after exposure to 8 M KOH at 0.15 Vs-1

between -0.5 and 0.9 V vs. Ag/AgCl for 1 (a), 5 (b),10 (c) and 15 (d) CV cycles respectively. The particle

size increases with increasing the number of CV cycles.

0

20

40

60

80

Ab

un

da

nce

Particle Size (nm)

(a)

0

20

40

60

80

Abun

dance

Particle Size (nm)

(b)

0

20

40

60

80

Abun

dance

Particle Size (nm)

(c)

0

20

40

60

80

Abun

dance

Particle Size (nm)

(d)

115

Figure A.2.2. Particle size distribution for prepared Ag NPs after exposure to 1 M KOH at 0.15 Vs-1

between -0.5 and 0.9 V vs. Ag/AgCl for 1 (a), 5 (b),10 (c) and 15(d) CV cycles respectively. The particle

size increases with increasing the number of CV cycles.

0

20

40

60

80A

bun

dance

Particle Size (nm)

(a)

0

20

40

60

80

Abun

dance

Partcle Size (nm)

(b)

0

10

20

30

40

50

60

Abund

ance

Particle Size (nm)

(c)

0

20

40

60

80

Abun

dance

Particle Size (nm)

(d)

116

Figure A.2.3. Particle size distribution for prepared Ag NPs after exposure to 0.1 M KOH at 0.15 Vs-1

between -0.5 and 0.9 V vs. Ag/AgCl for 1 (a), 5 (b), 10 (c) and 15(d) CV cycles respectively. The

particle size increases with increasing the number of CV cycles.

Figure A.2.4. Particle size distribution for prepared Ag NPs after exposure to 0.01 M KOH at 0.15 Vs-1

between -0.5 and 0.9 V vs. Ag/AgCl for 15 CV cycles. The particle just fused together and formed aggregated

particles on the surface.

0

20

40

60

80A

bu

nd

an

ce

Particle Size (nm)

(a)

0

20

40

60

80

Abun

dance

Particle Size (nm)

(b)

0

20

40

60

80

Abun

dance

Particle Size (nm)

(c)

0

20

40

60

80

Abun

dance

Particle Size (nm)

(d)

0

20

40

60

80

Abund

ance

Particle Size (nm)

117

Table A.2.1. The nano particle sizes (nm) and their standard deviation for different concentrations of KOH.

There is not any consistency between concentration and particle size, just with increasing the CV cycles the

particle size increase due to particle fusing.

Figure A.2.5. SEM Images of Ag NP-decorated ITO. Each surface was exposed to 15 CV cycles at

0.15 Vs-1 between -0.5 and 0.9 V vs. Ag/AgCl. (a) exposed to 8 M KOH, (b) exposed to 10 mM KCN

in 8 M KOH (c) exposed to 10 mM DECP in 8 M KOH. The aggregated Ag NPs will dissolve after

exposure to KCN or DECP.

CV scan

numbers 8 M KOH 1 M KOH 0.1 M KOH 0.01M KOH

1 31 ± 9 50 ± 20 47 ± 15 -

5 49 ± 15 57 ± 18 64 ± 18 -

10 63 ± 20 60 ± 15 64 ± 20 -

15 89 ± 27 69 ± 22 82 ± 24 152 ± 48

118

Figure A.2.6. EDX analysis of ITO surface after exposure to 0.5 M K [Ag (CN) 2] solution at 0.15 Vs-1

between 0.5 and 0.9 V vs. Ag/AgCl for 15 CV scans, shows that metallic silver deposits on the surface

due to cycling and reducing K [Ag (CN) 2].

Area Ag (%) In (%) S (%)

Spectrum 1 97.15 2.28 0.57

119

Figure A.2.7. Mass spectra for 0.5 M K[Ag(CN)2] in water as a control test. [Ag (CN) 2]- ion (158.9 and 160.9 for

two Ag isotopes).

120

Figure A.2.8. Mass spectra for the solution of 100 µM KCN in 1 M KOH after exposure to Ag bulk and cycling at

0.10 Vs-1 between 0.5 and 0.9 V vs. Ag/AgCl for 15 CV scans . The presence of [Ag (CN) 2]- ion (158.9 and 160.9

for two Ag isotopes), improves our proposed hypothesis that Ag dissolves as [Ag (CN) 2]-complex and so the

intensity of cathodic peak decreases after addition of CN- (spectrum Figure A.2.7 is a control test).

121

Figure A.2.9. SEM images of the silver surface as a function of KCN addition ( a) 10 pM, b) 100 pM, c) 1 nM d) 10

nM e) 100 nM f) 1 µM g) 10 µM ) . Images were recorded after a total of 15 electrochemical cycles in the range of -

0.5 and 0.9 V vs. Ag/AgCl at a KOH concentration of 1.0 M and a scan rate of 0.100 Vs-1. A-g show increasing the

concentration of KCN causes significantly smaller grains, dissolving formed Ag Ns and making the surface more

heterogeneous.

122

Figure A.2.10. SEM images of the silver surface as a function of DECP addition, a) 10 pM, b) 100 pM, c) 1 nM d)

10 nM e) 100 nM f) 1 µM g) 10 µM ) . Images were recorded after a total of 15 electrochemical cycles in the range

of -0.5 and 0.9 V vs. Ag/AgCl at a KOH concentration of 1.0 M and a scan rate of 0.100 Vs-1. A-g show increasing

the concentration of DECP causes significantly smaller grains, dissolving formed Ag Ns and making the surface

more heterogeneous.

123

Figure A.2.11. Chronoamperometry measurements and SEM images for two steps of Ag oxidation, (a) oxidation of

Ag to Ag (I) and (b) oxidation of Ag (I) to Ag (II) vs. Ag/AgCl in 8 M KOH Solution.

Figure A.2.12. A typical cyclic voltammetry scan of Ag foil in a solution of 1.0 M KOH at a scan rate of 0.100 Vs-1

in a potential range of -0.5 and 0.9 V vs. Ag/AgCl. On the anodic sweep, peaks are observed due to the oxidation of

Ag to the Ag (I) and Ag (II) oxides. On the cathodic sweep, these oxides are reduced back to Ag (0).

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-0.02

-0.01

0.00

0.01

0.02

E/V (vs.Ag/AgCl)

j/ m

A c

m-2

Ag Ag2O

Ag2O AgO

AgO Ag2O

Ag2O Ag

124

Appendix B Supplementary Information for Chapter 3

Figure B.3.1. SEM images of the initial Ag foil.

Figure B.3.2. Effects of paraoxon addition on the cyclic voltammograms of gas-phase generated Ag NPs on ITO.

The overlay of the cyclic voltammograms for Ag NPs-decorated ITO in 8.0 M KOH (_____) and after addition of 10

mM paraoxon in 8 M KOH (…….).The intensity of all redox signals in the presence of paraoxon decreases

significantly. CVs were recorded at a scan rate of 0.15 Vs-1 in the potential range of -0.5 and 0.9 V vs. Ag/AgCl after

15 CV scans.

1 µm

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-0.004

-0.002

0.000

0.002

0.004

0.006

j/m

A c

m-2

E/V (Vs. Ag/AgCl)

Ag2O AgO

AgO Ag2O

Ag2O Ag

Ag Ag2O

125

Figure B.3.3. SEM images of the Ag NPs decorated ITO cycled in (a) 8 M KOH, (b) 10 mM Paraoxon in 8 M

KOH. Images were recorded after a total of 15 electrochemical cycles in the range of -0.5 and 0.9 V vs. Ag/AgCl at

a scan rate of 150 mV. Aggregation of Ag particles and the formation of additional needle-like structures are

observed.

Figure B.3.4. Effects of malathion addition on the cyclic voltammograms of gas-phase generated Ag NPs on

ITO. The overlay of the cyclic voltammograms for Ag NPs-decorated ITO in 8.0 M KOH (_____) and after

addition of 10 mM malathion in 8 M KOH (…….). CVs were recorded at a scan rate of 0.15 Vs-1 in the

potential range of -0.5 and 0.9 V vs. Ag/AgCl after 15 CV scans. The intensity of all electrochemical signals

decreased significantly.In addition, shifts in the redox signals were observed with the exception of that of the

Ag(0) Ag(I) oxidation wave.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-0.004

-0.002

0.000

0.002

0.004

0.006

j/m

A c

m-2

E/V (Vs. Ag/AgCl)

Ag2O AgO

AgO Ag2O

Ag2O Ag

Ag Ag2O

126

Figure B.3.5. SEM image of the Ag NPs decorated ITO cycled in 10 mM malathion in 8 M KOH. image was

recorded after a total of 15 electrochemical cycles in the range of -0.5 and 0.9 V vs. Ag/AgCl at a scan rate of 150

mV. Interestingly, the surface morphology of the Ag NP-decorated ITO after exposure to malathion is more

spherical with an average size of 81 nm.

Figure B.3.6. EDX analysis from the Ag foil cycled in 10 mM Malathion in 8 M KOH. Images were recorded after

a total of 15 electrochemical cycles in the range of -0.5 and 0.9 V vs. Ag/AgCl at a scan rate of 150 mV. The results

show the composition of Sulphur (1.66 %), phosphorus (0.68%), O (61.57%), C (15.69%), K (19.11% ) and Ag

(1.84%).

1 µm

127

Figure B.3.7. Electrospray mass spectroscopy in positive mode of AgNO3 with malathion in water. The 227 and 229

peaks show the presence of AgSPO2H2. The 354.8, 356.8 and 358.8 peaks show the presence of Ag2OSP(CH3O)2.

The 372.8, 374.8 and 376.8 peaks show the presence of Ag2OSP(CH3O)2 and H2O. The 602.7, 604.7, 606.7 and

608.6 peaks show the presence of Ag3(OSP(CH3O)2)2. The proposed complexation structures of these multinuclear

Ag structures have been shown in scheme 2.3.

128

Figure B.3.8. Electrospray mass spectroscopy in positive mode of AgNO3 with malathion in water. (a) and (b) are

the expansion of spectrum in Figure B.3.7. The 227 and 229 peaks show the presence of AgSPO2H2. The 354.8,

356.8 and 358.8 peaks show the presence of Ag2OSP(CH3O)2. The 372.8, 374.8 and 376.8 peaks show the presence

of Ag2OSP(CH3O)2 and H2O. The 602.7, 604.7, 606.7 and 608.6 peaks show the presence of Ag3(OSP(CH3O)2)2.

The proposed complexation structures of these multinuclear Ag structures have been shown in scheme 2.3.

129

Figure B.3.9. SERS spectrum of the Ag foil surface cycled in a wide range of paraoxon concentration (1 mM pM)

in 8 M KOH. Spectra were recorded after a total of 15 electrochemical cycles in the range of -0.5 and 0.9 V vs.

Ag/AgCl at a scan rate of 150 mV.These spectra series show a limit of detection of 10 nM for paraoxon.

130

Appendix C Supplementary Information for Chapter 4

Figure C.4.1. Typical underpotential deposition (UPD) graph. Cadmium deposited on nanostructured-Ag surface

from a solution of CdSO4 ( 6× 10 -3 M) and Na2SO4 (0.1M) at pH=5. The CVs were recorded at a scan rate of 0.01

V.s-1 in the potential range of -1.2 and 0.2 V vs. Ag/AgCl.

Table C.4.1. Results for real surface area of nanostructured Ag surfaces after cycling in 8M KOH. The surface area

of polycrystalline Ag foil which is exposed to KOH solution is 0.063 cm2 before roughening.

Number of cycles 1 5 10 15

Surface (cm2) 4.93±0.32 5.90±0.10 4.12±0.72 5.09±0.23

131

Figure C.4.2. Electrochemical response of modified Ag NSs substrates with various concentration of ethion. (A)

SWVs , (B) Bode plots of modified Ag NSs substrates with various concentration of ethion; bare Ag NSs (black

solid line), 100 nM ethion (gray solid line), 10µM ethion (dash line), 100µM ethion (dot line) and 1mM ethion (dash

dot dot line) in dry ethanol. SWVs were performed in the potential range of -0.4 to 0 V vs Ag/AgCl at a scan rate of

0.1 Vs-1. with a step potential of 0.004V, frequency at 10 Hz, quiet time at 2 s and a pulse amplitude of 0.02 V in a

solution of 1M KNO3 containing 1 mM [Ru (NH3)6]Cl3 as redox probe The current signal intensity decreases

slightly in with ethion concentration enhancement up to 10µM. 100 nM have a similar response to 10µM. EIS

measurements (Bode plots) were acquired at the formal half potential of the [Ru (NH3)6]Cl3 (-0.17 V vs. Ag/AgCl)

at 5 mV amplitude and in the 0.1 Hz to 100 kHz range. Here the impedance increase with enhancement of ethion

concentration. (C) demonstrates the relation between current and ethion concentration, (D) shows the relation

between Rct and ethion concentration. Briefly in (C) enhancement of ethion concentration decreases the current due

to enhancement of Rct as is seen in (D).

C

132

Figure C.4.3. Electrochemical response of modified Ag NSs substrates with various concentration of fenthion. (A)

SWVs , (B) Bode plots of modified Ag NSs substrates with various concentration of fenthion; bare Ag NSs (black

solid line), 10µM fenthion (dash line), 100µM fenthion (dot line) and 1mM fenthion (dash dot dot line) in dry

ethanol. SWVs were performed in the potential range of -0.4 to 0 V vs Ag/AgCl at a scan rate of 0.1 Vs-1. with a

step potential of 0.004V, frequency at 10 Hz, quiet time at 2 s and a pulse amplitude of 0.02 V in a solution of 1M

KNO3 containing 1 mM [Ru (NH3)6]Cl3 as redox probe The current signal intensity decreases slightly in with

fenthion concentration enhancement. EIS measurements (Bode plots) were acquired at the formal half potential of

the [Ru (NH3)6]Cl3 (-0.17 V vs. Ag/AgCl) at 5 mV amplitude and in the 0.1 Hz to 100 kHz range. Here the

impedance increase with enhancement of fenthion concentration. (C) demonstrates the relation between current and

fenthion concentration, (D) shows the relation between Rct and fenthion concentration. Briefly in (C) enhancement

of fenthion concentration decreases the current due to enhancement of Rct as is seen in (D).

C D

133

Figure C.4.4. Electrochemical response of modified Ag NSs substrates with various concentration of malathion. (A)

SWVs , (B) Bode plots of modified Ag NSs substrates with various concentration of malathion; bare Ag NSs (black

solid line), 10µM malathion (dash line), 100µM malathion (dot line) and 1mM malathion (dash dot dot line) in dry

ethanol. SWVs were performed in the potential range of -0.4 to 0 V vs Ag/AgCl at a scan rate of 0.1 Vs-1. with a

step potential of 0.004V, frequency at 10 Hz, quiet time at 2 s and a pulse amplitude of 0.02 V in a solution of 1M

KNO3 containing 1 mM [Ru (NH3)6]Cl3 as redox probe The current signal intensity decreases slightly in with

malathion concentration enhancement. EIS measurements (Bode plots) were acquired at the formal half potential of

the [Ru (NH3)6]Cl3 (-0.17 V vs. Ag/AgCl) at 5 mV amplitude and in the 0.1 Hz to 100 kHz range. Here the

impedance increase with enhancement of malathion concentration. (C) demonstrates the relation between current

and malathion concentration, (D) shows the relation between Rct and malathion concentration. Briefly in (C)

enhancement of malathion concentration decreases the current due to ehhancement of Rct as is seen in (D).

C D

134

Figure C.4.5. Data fitting of Bode plots for modified Ag NSs with 100 nM (A) 10 µM (B), 100 µM (C) and 1mM

(D) ethion. Data points represent experimental results while solid lines correspond to spectra calculated for an

equivalent circuit shown in Figure 4.2D.

135

Figure C.4.6.Data fitting of Bode plots for modified Ag NSs with 10 µM (A), 100 µM (B) and 1mM (C) fenthion.


shown in Figure 4.2D.

136

Figure C.4.7. Data fitting of Bode plots for modified Ag NSs with 10 µM (A), 100 µM (B) and 1mM (C) malathion.



137

Figure C.4.8. Data fitting of Bode plots for modified Ag NSs with 100 µM (A) and 1mM (B) Ops in tap-water.



Figure C.4.9. Data fitting of Bode plots for modified Ag NSs with 100 µM (A) and 1mM (B) Ops in apple juice.



138

Figure C.4.10. LSPR response of modified Ag NSs with different concentrations of ethion. (A) Absorbance

intensity against wavelength to show decrease in Fresnel signal as different ethion concentration bind to Ag NSs

surface. Graphs are for bare Ag NSs (solid black line), modified Ag NSs with 100 nM ethion (dash dot dot line),

10µM ethion (dash line), 100 µM ethion ( dot line) and 1mM ethion ( solid gray line). (B) demonstrates the relation

between relative absorbance percentage decrease with enhancement of ethion concentration. The change in

reflectance intensity was monitored at 400 nm.

Figure C.4.11. LSPR response of modified Ag NSs with different concentrations of fenthion. (A) Absorbance

intensity against wavelength to show decrease in Fresnel signal as different fenthion concentration bind to Ag NSs

surface. Graphs are for bare Ag NSs (solid black line), modified Ag NSs with 10 µM fenthion (dot line), 100 µM

fenthion ( dash line) and 1mM fenthion ( solid gray line). (B) demonstrates the relation between relative absorbance

percentage decrease with enhancement of fenthion concentration. The change in reflectance intensity was monitored

at 400 nm.

A B

AB

139

Figure C.4.12. LSPR response of modified Ag NSs with different concentrations of malathion.. (A) Absorbance

intensity against wavelength to show decrease in Fresnel signal as different malathion concentration bind to Ag NSs

surface. Graphs are for bare Ag NSs (solid black line), modified Ag NSs with 10 µM malathion (dot line) 100 µM

malathion (dash line) and 1mM malathion ( solid gray line). (B) demonstrates the relation between relative

absorbance percentage decrease with enhancement of malathion concentration. The change in reflectance intensity

was monitored at 400 nm.

A B

140

Appendix D Supplementary Information for Chapter 5

Figure D.5.1. Fitting of Impedance spectra. Impedance spectra after cycling Ag in 0.1 M KOH: (a) with citrate (b),

with PVP (c), with Tween-20 (d) and 0.01 M KOH: (e), with citrate (f), with PVP (g), with Tween-20 (h). Data

points represent experimental results while solid lines correspond to spectra calculated for an equivalent circuit

shown in Figure 1C. EIS were acquired at the potential of 0 V vs. Ag/AgCl at 10 mV amplitude and in the 0.01 Hz

to 100 kHz range.

141

Table D.5.1. Impedance parameter values extracted from the fitting (Figure 1C) to the equivalent circuit for

impedance spectra recorded in pure 0.1M KOH (a) and with citrate (b), PVP (c), Tween-20 (d) and in pure 0.01M

KOH (e) and with citrate (f), PVP (g) and Tween-20 (h).

Figure D.5.2. Calibration curve of AgNO3 solution for GFAA measurements.

Solution Rs/Ω Cdl/µF Rct/Ω Q/mS.s-n

n Rf/Ω W/ mS.s-0.5

a 7.21±1.75 0.02±0.00 85.51±9.80 0.32±0.00 0.88±0.00 15200.00±483.16 8.84±0.25

b 5.46±0.10 0.03±0.00 56.89±0.29 0.27±0.03 0.86±0.00 12776.67±155.03 2.21±0.10

c 9.61±0.28 0.02±0.00 93.23±0.64 0.25±0.04 0.85±0.00 13210.00±1167.8

0

88.60±15.3

1

d 4.76±1.23 0.03±0.00 94.34±1.54 0.08±0.02 0.63±0.03 22600.00±1456.6

4 0.02±0.00

e 65.53±2.08 - 735.50±2.40 0.09±0.01 0.92±0.00 14465.00±403.05 0.56±0.29

f 13.53±1.00 0.01±0.00 146.05±0.07 0.23±0.04 0.88±0.01 9155±811.76 2.92±0.97

g 73.39±1.05 - 810.3±0.88 0.06±0.00 0.91±0.00 12903.33±657.29 10.10±0.75

h 78.03±3.27 - 1038.00±1.4

1 0.08±0.03 0.74±0.06 2172±156.97 0.04±0.01

142

Figure D.5.3. The film thickness of formed deposited Ag NSs on the Ag surface after cycling Ag foil in the

potential range of -0.4 to 0 V vs Ag/AgCl at a scan rate of 150 mVs-1 in the 0.1 M KOH (a) and in the 0.1M KOH

with citrate(b). There is a slightly thicker film of the nanostructured Ag film after addition of citrate due to the

corrosion enhancement of the Ag leading to more dissolution and deposition of Ag at the same time window.

143

Figure.D.5.4. Mass spectra for 0.01M KOH after exposure to Ag bulk and cycling at 150 mVs-1 between 0.5 and

0.9 V vs. Ag/AgCl for 16 CV scans. The presence of [Ag(OH)2H2O]- ion (160.9076 and 162.9058 for two Ag

isotopes) (see expanded spectrum in Figure D.5.5) and [AgO(OH)2H2O]2- ion (176.8823, 178.8806) (see expanded

spectrum in Figure D.5.6), corroborate the dissolution of Ag in the alkaline solution.

144

Figure D.5.5. Mass spectra for 0.01M KOH after exposure to Ag bulk and cycling at 150 mVs-1 between 0.5 and 0.9

V vs. Ag/AgCl for 16 CV scans. The presence of [Ag(OH)2H2O]- ion (160.9076 and 162.9058 for two Ag isotopes),

corroborates the dissolution of Ag in the alkaline solution.

145

Figure D.5.6. Mass spectra for 0.01M KOH after exposure to Ag bulk and cycling at 150 mVs-1 between 0.5 and

0.9 V vs. Ag/AgCl for 16 CV scans. The presence of [AgO(OH)2H2O]2- ion (176.8823, 178.8806), corroborates the

dissolution of Ag in the alkaline solution.

146

Figure D.5.7. Mass spectra for 0.01M KOH with citrate after exposure to Ag bulk and cycling at 150 mVs-1

between 0.5 and 0.9 V vs. Ag/AgCl for 16 CV scans. The presence of [AgC6H5O7(OH)3(H2O)4]5- (419.8582-422-

3596 for isotopes), AgC12H11O135- (471.5053-472.8393 for isotopes) and Ag[(C6H7O7)2(H2O)4]

5- (572.7986-

575.2987 for isotopes) (See expanded spectra in figure D.5.8-10), determined the presence of Ag citrate cluster

formation in the solution after cycling Ag in the alkaline solution containing citrate.

147

Figure D.5.8. Mass spectra for 0.01M KOH after exposure to Ag bulk and cycling at 150 mVs-1 between 0.5 and 0.9

V vs. Ag/AgCl for 16 CV scans. The presence of [AgC6H5O7(OH)3(H2O)4]5- (419.8582-422-3596 for isotopes),

determined the presence of Ag citrate cluster formation in the solution after cycling Ag in the alkaline solution

containing citrate.

148


between 0.5 and 0.9 V vs. Ag/AgCl for 16 CV scans. The presence of AgC12H11O135- (471.5053-472.8393 for

isotopes), determined the presence of Ag citrate cluster formation in the solution after cycling Ag in the alkaline

solution containing citrate.

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between 0.5 and 0.9 V vs. Ag/AgCl for 16 CV scans. The presence of Ag[(C6H7O7)2(H2O)4]5- (572.7986-575.2987

for isotopes), determined the presence of Ag citrate cluster formation in the solution after cycling Ag in the alkaline

solution containing citrate.

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Figure D.5.11. TEM images from the solution of 0.01 M KOH (a), 0.01M KOH with citrate (b), 0.01M KOH with

PVP and 0.01M KOH with Tween-20 after cycling Ag for 16 times in the range of -0.5 to 0.9 V vs Ag/AgCl. The

results show the presence of nano species in the solution which approved the release of Ag nanoparticles to the

solution from the Ag NSs surfaces.

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Appendix E Supplementary Information for Chapter 6

Figure E.6.1. Raman spectrum of pure 2-Cyano-3-(2′-(5′,10′,15′,20-tetraphenylporphyrinato Zinc-

(II))yl)acrylic acid and SERS spectrum of the Ag foil surface cycled 5 times in 8 M KOH. Spectra were recorded

after washing the surface with solvent (tetra hydro furan). No baseline correction was performed on the Raman

spectra. The SERS shows that the porphyrin has been modified on the surface.

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Figure E.6.2. Positive secondary ion mass spectrum of the porphyrin film on Ag NSs surface. A, B and C do not

contain any specific peaks indicating porphyrin immobilization on Ag NSs.

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Figure E.6.3. Angle resolved x-ray photoelectron spectroscopy (ARXPS) of modified Ag NSs with porphyrin. The

figure shows the atomic percentage of substrate and immobilized porphyrin in different angles.

Figure E.6.4. A series of Zn 2p spectra in different angles. It shows binding energy at 1021.0 eV.

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Figure E.6.5. A series of N1s spectra in different angles. It shows a binding energy at 398.0 eV.

Figure E.6.6. A series of C1s spectra in different angles. It show binding energy at 288.3 eV for C1s.

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Figure E.6.7. A series of O1s spectra in different angles. It shows a peak around 532.5 eV for O 1s.

Figure E.6.8. A series of Ag 3d 3/2 and Ag 3d 5/2 spectra in different angles. It shows two peaks around 368 and

374 eV which are indicative of metallic silver.

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Figure E.6.9. The effect of various electron donors on the photocurrent response of the modified Por-Ag NSs. As

can be seen in the graph photocurrent response has the highest amount in the present of hydroquinone, EDTA,

galactose and manose. It has moderate response after addition of fructose and glucose.

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