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A novel surface plasmon resonance enhanced total internal reflectionellipsometric application: electrochemically grafted isophthalic acid nanofilmon gold surface

Zafer €Ust€unda�g,a Mustafa O�guzhan Ca�glayan,b Remziye G€uzel,c Erhan Pisxkind and Ali Osman Solak*ef

Received 16th June 2010, Accepted 1st February 2011

DOI: 10.1039/c0an00410c

The scope of this study is to modify a Surface Plasmon Resonance (SPR) sensor slide with isophthalic

acid to evaluate the possible application on the detection of copper(II) ions in aqueous media by total

internal reflection ellipsometry. A gold sensor surface was modified by an electrochemical diazonium

reduction modification method. The modified surfaces are characterized with cyclic voltammetry (CV)

and ellipsometry. Isophthalic acid monolayer modified gold slides were used for in situ detection of

aqueous Cu2+ solution with the SPR enhanced total internal reflection ellipsometry (SPRe-TIRE)

technique. Layer formation, pH dependency of adsorption, sensor response of the SPRe-TIRE and

isothermal kinetic parameters were examined. A high dependency on the number of CV cycles in the

monolayer–multiple layer transition was observed. The suggested sensor gave a linear response over

a wide range of Cu2+ concentrations. It was also reported that adsorption on the SPRe-TIRE sensor

gave Langmuir adsorption model behavior.

Introduction

Monitoring of macromolecule adsorption and determination of

micro- and nano-structure and optical properties of thin films are

of main research areas for bioengineering applications, analytical

sensors, etc.1 Ellipsometry has become an attractive technique

for thin film applications because of its high sensitivity and the

possibility of making in situ measurements at solid/liquid inter-

faces. There are numerous examples of ellipsometric applications

in the literature so as to determine the adsorption parameters of

molecules, ions and their interaction kinetics.2 In last decades,

several researchers have proposed some sensor applications of

ellipsometry.3

The ellipsometric technique is based on polarization changes

in reflected light from a surface at a defined angle of incidence

(i.e. near Brewster angle) of polarized light. In ellipsometric

measurement the complex reflectance ratio (r) is measured (eqn

(1))eqn (1))

aDumlupınar University, Faculty of Art and Science, Dept. of Chemistry,K€utahya, TurkeybCumhuriyet University, Faculty of Eng, Dept. of Chemical Eng., Sivas,TurkeycDicle University, Faculty of Art and Science, Dept. of Chemistry,Diyarbakır, TurkeydHacettepe University, Faculty of Eng, Dept. of Chemical Eng., Beytepe,Ankara, TurkeyeKyrgyz-Turk Manas University, Faculty of Eng., Dept. of Chem. Eng.,Bishkek, KyrgyzstanfAnkara University, Faculty of Science, Dept. of Chemistry, Tandogan,Ankara, Turkey. E-mail: aliosman.solak@gmail.com

1464 | Analyst, 2011, 136, 1464–1471

r ¼ Xr

Xi

(1)

where Xr and Xi are the state of polarization of the reflected and

incident light, respectively. For optically isotropic samples, this

ratio can be written as

r ¼ Rp

Rs

¼ tanj expðiDÞ (2)

where Rp and Rs are the complex reflection coefficients (from the

Fresnel equation) for the components of polarized light parallel

and perpendicular to the plane of incidence, respectively; D is the

phase shift which is induced by the reflection and j is the ratio of

magnitudes of the total reflection coefficients. These parameters

(D and j) are ellipsometric angles and are determined by ellip-

sometric measurements.

From the primary data measured by ellipsometer expressed as

either D or j, the thickness of the surface film d, and/or the

dielectric constants of the film are calculated by classical ellip-

sometric method.4

Total internal reflection (TIR) occurs above the critical angle

and evanescent field5 which penetrates into the dielectric

(e.g. organic layer/solution interface) medium. By probing this

evanescent field by ellipsometry, the D parameter can highly be

correlated to surface interactions by total internal reflection

ellipsometry (TIRE). SPRe-TIRE is a special case of TIRE

application in which a thin metal layer (e.g. Au or Ag for SPR

excitation) is deposited on two dielectric interfaces to obtain the

SPR phenomenon over the entire surface.

SPR is used to monitor the changes in the thickness or

refractive index of ultra-thin films of metal surfaces. SPR occurs

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at the interface of the thin film and is highly sensitive to refractive

index (i.e. dielectric constant) changes at this medium.6

When the SPR effect is combined with ellipsometry in TIRE, it

yields a highly sensitive sensor technique (SPRe-TIRE) because

of the sensitivity enhancement due to SPR waves propagating at

the metal surface.7 This five-year-old method has been used for

detecting genomic DNA adsorption,8 low molecular weight

toxins,9 pesticides and herbicides10 interactions between

biomolecules and functionalized polymer surfaces,11 amd anti-

body interactions.12 Also there are few studies, to improve the

ellipsometric sensor response in the TIRE setup.13

Metal and carbon surface nanofilm modification studies such

as self-assembled monolayers (SAMs), diazonium salts reduc-

tion, Langmuir–Blodgett films and layer-by-layer surface modi-

fications have gained more interest in recent years.14–17 Metal

surface modifications by electrochemical techniques are impor-

tant due to imparting novel and specific properties to the inter-

faces. A modified surface provides new applications, for instance

in sensor researches, surface functionalization studies, chemical

and electrochemical analysis,18 corrosion,19 molecular elec-

tronics.20 etc. Although various modification methods are

available, electrochemical diazonium salt modification is very

important in terms of covalent attachment on the substrate

surface. Electrochemical surface modification yields generally

stable attachment and the grafting of aromatic organic molecules

has a well established mechanism.21 Although electrochemical

methods offer film formation control on the substrate surface

through changing the cycle number and potential excursion

during modification,22 the derivatized surfaces will seldom have

sufficiently well defined monolayers suitable for sensors and

molecular electronics uses.23,24

Copper is essential for health25 and is needed for both struc-

tural and catalytic roles in certain enzymes to function in the

human body.26–28 On the other hand, too little or too much

copper in the body may cause health problems such as Menke’s

syndrome,29 celiac disease,30 cystic fibrosis,31 short bowel

syndrome,32 and Wilson’s disease.33 Therefore many different

techniques for the measurement of copper concentration have

been reported in the literature, performed by RP-HPLC with

UV-Vis detection,34 atomic absorption spectrometry after pre-

concentration on N,N-(4-methyl-1,2-phenylene)diquinoline-2-

carboxamidenaphthalene,35 square wave voltammetry36 and

cyclic voltammetry on a modified Au electrode with a SAM of 3-

mercaptopropionic acid.37 These techniques in general require

expensive equipment, pretreatment and preconcentration steps

for the analyte. Thus, a simple, rapid, selective and sensitive

method is a goal for the detection of metal ions. The method of

Total Internal Reflection Ellipsometry (TIRE) conjoins the

advantages of high sensitivity of spectroscopic ellipsometry and

selectivity of surface plasmon resonance (SPR). Interaction of an

analyte/ligand complex can be followed with this technique,

supplying kinetic data in real time.

The purpose of this study is to modify the gold surface by the

electrochemical reduction of 3,5-dicarboxylbenzenediazonium

tetrafluoroborate (diazonium salt of 5-aminoisophthalic acid or

5-diazoisophthalic acid tetrafluoroborate) salt in acetonitrile

containing 0.1 M tetrabutylammoniumtetrafluoroborate

(TBATFB) to use it as a Cu2+ sensor. The selected molecule is

expected to give a 3,5-dicarboxyphenyl film which is pH sensitive

This journal is ª The Royal Society of Chemistry 2011

and can be used as a surface ligand for most transition metals.

The isophthalic acid modified gold surface, IPA-Au, was char-

acterized by cyclic voltammetry (CV) using a ferricyanide redox

probe. IPA-Au was used for copper determination in aqueous

media and the kinetics of the complex formed by Cu2+ at the

modified IPA-Au surface as a ligand by Surface Plasmon Reso-

nance enhanced Total Internal Reflection Ellipsometry. Cu2+

adsorption from aqueous solutions of Cu2+ was monitored using

ellipsometry having a TIRE setup (i.e. flow cell and TIRE optical

setup). Sensor linearity and sensitivity were determined as the

main sensor parameters. Cu2+ adsorption on the modified surface

was also monitored in situ and equilibrium adsorption kinetics

were examined by application of Langmuir isotherm models.

Experimental

Reagents and chemicals

In all experiments, ultrapure water with a resistance of 18.3 MU

cm (Human Power 1+ Scholar purification system) was used for

preparation of the aqueous solutions, the cleaning of the glass-

ware and the polishing of the electrodes. Chemicals used in this

study, i.e. potassium ferricyanide (K3Fe(CN)6) (Sigma-Aldrich),

acetonitrile (MeCN) (Sigma-Aldrich), isopropyl alcohol (Sigma-

Aldrich), copper acetate (Merck), NaNO2 (Merck), activated

carbon (Sigma-Aldrich), TBATFB (Fluka), phosphoric acid,

boric acid, sodium hydroxide, sodium acetate and glacial acetic

acid for buffer solution preparations and nitric acid (Merck),

5-aminoisophthalic acid (Aldrich), aluminium oxide polishing

materials (Baikowski, USA) were reagent grade and used as

received without further purification. Solutions were deaerated

by purging with purified argon gas (99.999%) for 7–8 min prior to

the electrochemical experiments. An argon blanket was main-

tained over the solutions to supply an inert atmosphere during

voltammetric measurements. All electrochemical experiments

were performed at room temperature (25 � 1) �C.

Materials and instrumentation

Cyclic voltammetric experiments were carried out using a BAS-

100B electrochemical analyzer (Bioanalytical Systems, West

Lafayette, IN, USA) equipped with a BAS C3 Cell Stand. A

conventional three-electrode cell was used for all electrochemical

measurements. Either an Ag/AgCl/KClsat or an Ag/AgNO3 (0.01

M) reference electrode was used in aqueous and non-aqueous

media, respectively. To prepare the Ag/AgNO3 (0.01 M) refer-

ence electrode, pure solid AgNO3 was dissolved in 0.1 M

TBATFB in MeCN to obtain a 0.01 M AgNO3 inner solution. A

platinum wire counter electrode was used in all voltammetric

measurements. Gold surfaces used for SPRe-TIRE study were

prepared as follows: (1) SF10 glass slides (2.5 cm � 2.5 cm) were

cleaned in a boiling solution of HNO3 (1 : 3 v/v) for 30 min

before coating to remove oily residues and carbon depositions;

(2) to get clean surfaces before PVD coating, surfaces were

treated with oxygen plasma in a plasma chamber for 30 min; and

(3) for SPR phenomena at a 532 nm light source, a 32 nm Au

layer was deposited onto an adhesive Cr (3 nm) layer in the PVD

chamber. The SF10/Cr film/Au film layer combination has

a surface plasmon resonance dip at 63� angle of incidence for the532 nm light source.

Analyst, 2011, 136, 1464–1471 | 1465

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Modification of gold surface

For CV characterization, gold electrodes (BAS, MF2014) were

cleaned prior to modification according to the well-established

procedure reported elsewhere.38 The gold electrode were

prepared to remove the oxide and other functionalities by pol-

ishing first with fine wet emery papers (Buehler with grain size of

4000) and then with 0.3 mm, 0.1 mm and 0.05 mm alumina slurry

made from aluminium suspension solution on a Buehler polish-

ing microcloth. After removal of trace alumina from the surface

by rinsing with water and brief cleaning in an ultrasonic bath

(Bandelin RK 100, Germany) with water then an isopropyl

alcohol + MeCN mixture purified over activated carbon, the

gold electrode was rinsed withMeCN to remove any physisorbed

and unreacted materials from the electrode surface.

The gold slide (i.e. gold coated SF10) substrates were first

cleaned by repeated rinsing with deionized water and MeCN.

They were then further cleaned with a mixture of NH3 (25%, v/v),

H2O2 (30%, v/v), and ultrapure water having a volume ratio of

1 : 1 : 5 at a temperature of 70 �C for 20 min. The cleaned gold

surfaces were rinsed with MeCN to remove any physisorbed

materials from the electrode surface prior to modification. The

IPA nanofilm on the gold surface was checked for stability in

MeCN using the Fe(CN)63� redox probe and it was found to be

quite stable for several hours.

The precursor for the synthesis of 3,5-dicarbox-

ylbenzenediazonium tetrafluoroborate (IPA-DAS) was 5-ami-

noisophthalic acid, and this precursor was synthesized according

to the procedure as described elsewhere.38 Cleaned gold surfaces

(i.e. electrodes) were then dipped into the 1.0� 10�3 M IPA-DAS

solution and derivatized by cyclic voltammetry scanning from

+0.2 V to �0.8 V at a 200 mV s�1 scan rate for 1–5 scans. The

modified surfaces were taken out of the solution and immediately

rinsed with MeCN to remove the unreacted precursors. The

argon stream dried surfaces were then stored in MeCN until

characterization and application.

Characterization of surfaces by CV

The isophthalic acid modified gold (IPA-Au) surface was char-

acterized with potassium ferricyanide (in BR buffer containing

0.1 KCl, pH¼ 2.20 and 4.27) Fe(CN)63� redox probe at bare and

IPA-Au electrode surfaces (scan rate of 200 mV s�1, vs.Ag/AgCl/

KClsat).

Characterization of surfaces by ellipsometry

IPA functionalized Au surfaces were characterized by thickness

measurements using ellipsometry (ELX-02C/01R model, 532 nm

laser source, Germany). Prior to measurements, the surface was

blown with a stream of nitrogen. Ellipsometric data, Delta (D)

and Psi (j) were collected on 9 different zones having a 50 mm �50 mm area at 3 different regions of the surface. Ellipsometric

data were then modeled to get information on the IPA layer

formation on the gold surface using the instrument’s software. A

five-layer model was applied, air/organic layer/Au film/Cr film/

substrate (SF10), to investigate the effect of the number of

modification cycles on layer formation. Refractive indices of

1.7379 for the SF10 glass substrate, 3.0390 and k ¼ 3.330 for the

Cr layer, 0.4137 and k ¼ 2.4083 for the Au layer, 1.4600 for the

1466 | Analyst, 2011, 136, 1464–1471

IPA organic layer and 1.0000 for air are assigned, assuming that

thickness and refractive indices are reasonably correlated for all

films.

SPRe-TIRE application: Cu2+ adsorption on modified gold

surface

Cu2+ adsorption on gold surface was in situ monitored by an

ellipsometer having a flow cell with a volume of 50 mL. Flowcell

arrangement is in Kretschmann geometry with a 60� equilateraltriangle SF10 prism in optical contact with the SF10 glass slide

modified with IPA. An index matching oil (n ¼ 1.7379) is used

between the prism and the glass slide. In order to get SPR action,

all measurements were done at an angle of incidence of 63�

(i.e. above the critical angle). The ellipsometric parameter of the

phase shift (D) was measured vs. time during the interaction to

get real time kinetic data. During the measurements, the flow rate

of buffer and Cu2+ solutions was maintained at 20 mL min�1

(i.e. retention time, t, is 5 min including dead volume) and all

measurements were done at room temperature.

Effects of pH and initial concentration of Cu2+ on Cu2+

adsorption

The effect of pH on adsorption was studied at pH 4.0, pH 4.5 and

pH 5.0 buffer in 0.2 M sodium acetate/0.2 M acetic acid buffer.

Binding was monitored in situ during the course of adsorption

and the response is presented as the relative sensor response (i.e.

D, degree). The effect of the initial concentration on adsorption

was monitored in situ at pH 4.5 (optimum pH for adsorption) by

aqueous copper solutions having concentrations of 5 � 10�8 M,

1 � 10�7 M, 1 � 10�6 M, 1 � 10�5 M and 1 � 10�4 M. From the

equilibrium response, the adsorption isotherm is also detected at

room temperature.

Results and discussion

Modification of the gold surface and electrochemical

characterization of the IPA-Au surface by CV

Gold surface modification was performed in acetonitrile (con-

taining 0.1 M TBATFB) by the reduction of IPA-DAS using

cyclic voltammetry. In Fig. 1a, the cyclic voltammograms for the

modification of the Au electrode to generate IPA-Au surfaces

was shown. An irreversible cathodic peak was observed in the

first scan of the five-cycled modification voltammograms. This

peak is attributed to the formation of a radical by the reduction

of IPA-DAS and covalently grafting this radical to the Au

surface, accompanied by N2 gas evolution.39–41 In the second and

subsequent scans, reduction of IPA-DAS was almost blocked as

a result of the formation of the IPA film at the electrode surfaces.

This type of modification voltammogram for the aryl diazonium

salt reduction is very common and is characteristic for the

covalent bonding of organic molecules to the various carbona-

ceous and metallic substrates.20,38,42–44 Although one peak is

expected in the modification voltammograms, sometimes multi-

peaks have also been observed when reducing diazonium salts by

cyclic voltammetry on metal and carbon surfaces. No satisfac-

tory explanation of this phenomenon has been provided yet, and

it is still a debated question. In two recent papers, a full

This journal is ª The Royal Society of Chemistry 2011

Fig. 1 (a) Modification voltammograms of the gold surfaces in 1.0 �10�3 M 3,5-dicarboxylbenzenediazonium tetrafluoroborate (IPA-DAS)

in acetonitrile (containing 0.1 M TBATFB). Scan rate is 200 mV s�1, vs.

Ag/AgNO3 (0.01 M). (b) Electrochemical behavior of potassium ferri-

cyanide (in BR buffer solution, pH ¼ 2.20 and 4.27, containing 0.1 M

KCl) at the bare Au and IPA-Au (scan rate: 200 mV s�1, vs. Ag/AgCl/

KClsat in BR buffer solution in 0.1 M KCl).

Fig. 2 Stability of the IPA film grafted to the Au electrode when it is

sonicated (a) in water, (b) in acetonitrile for 20 min and on bare Au.

Voltammograms were acquired vs. Ag/AgCl/KClsat in 5 � 10�4 M Fe

(CN)63� (in BR buffer containing 0.1 KCl, pH ¼ 4.27) on the modified

and bare Au surfaces. Scan rate was 200 mV s�1.

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discussion has been given in explaining the splitting of the CV

peak due to the crystallographic site on the gold and carbon

surfaces and cleanness of the bare surfaces before modifica-

tion.45,46

Small currents observed in the following scans after the first

probably originated from the modification of pinholes which are

left uncovered during the first scan. To be sure of complete

coverage of the surface, the 5 cycle-scan modification was found

to be sufficient, as shown in Fig. 1a. The peak current diminished

almost completely during the subsequent scans as an indicator of

a passivated surface against IPA-DAS. The passivation of the

electrode surface implies a compact and complete monolayer

formation.47,48

The grafting of the IPA film to the Au surfaces was confirmed

by the comparison of cyclic voltammograms of Fe(CN)63� at the

bare and modified surfaces in two different pH values of 2.20 and

4.27: one is less than pKa1 and the other is greater than the pKa2

of 1,3-benzendicarboxylic acid in solution, which are 3.62 and

4.60, respectively.49 The magnitude of the current describes what

the grafted layer at the surface does to the electron transfer rate

of the Fe(CN)63�. In other words, the intensity of the current for

This journal is ª The Royal Society of Chemistry 2011

a redox probe is an indication of the conductivity of the modified

surface which is inversely related to the blocking properties of the

film present at the surface.50 The electrochemistry of Fe(CN)63�

used as the redox probe for the characterization of the IPA-Au

surface is shown in Fig. 1b.

An ionic electrochemical probe, Fe(CN)63�, was used to

interrogate the monolayer–solution interface properties and

charged nature of the film surface in different solution pHs.

Fig. 1b shows the electrochemical behavior of Fe(CN)63� at the

IPA-Au surfaces. Voltammograms were acquired in solutions

having pH values below and above the pKa values of the iso-

phthalic acid film, assuming that the solution and surface-

confined pKa1 values of isophthalic acid were the same. Total loss

of Fe(CN)63� oxidation peaks in a solution of pH that was

greater than expected pKa can be explained by the anionic charge

repulsion between the deprotonated –COO� groups at the

surface and Fe(CN)63� anions. In the case of lower pH solutions

(pH ¼ 2.20 for modified Au) the rate of electron transfer was

higher than that in solutions having pH values greater than the

pKa. This behavior is good evidence of the presence of the pH

sensitive isophthalic acid layer at the Au surface.

To examine the stability of the isophthalic acid film on the gold

surface, the potassium ferricyanide redox probe test was per-

formed by following the ferricyanide voltammogram after the

sonication of the modified surface in an ultrasonic bath. Soni-

cation was performed both in ultrapure water and in acetonitrile

for 20 min. Cyclic voltammograms of the IPA modified gold

surfaces subjected to sonication are given in Fig. 2. As Fig. 2

shows, even after 20 min sonication, the electrochemistry of the

ferricyanide species is dead showing that the IPA film still exists

at the surface. Hence, it was concluded that the IPA film on the

gold surface is quite stable towards water, acetonitrile and severe

sonication during 20 min.

Characterization of surface formation by ellipsometry

Overlayer formation on the modified substrate surface upon

electrochemical modification was also monitored using

Analyst, 2011, 136, 1464–1471 | 1467

Fig. 4 Ellipsometric 2D images of IPA modified surfaces: (a) bare Au

surface, (b) IPA-Au surface after 1 cycle, (c) IPA-Au surface after 3

cycles, (d) IPA-Au surface after the 5 cycle modification.

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ellipsometry by thickness measurements. Fig. 3 shows the surface

thickness vs. the number of cycles during the modification by CV.

During the first two cycles, the relative thickness was approxi-

mately 1.3 nm which corresponds approximately to a monolayer

by comparing the theoretical thickness. The theoretical thickness

of the IPA monolayer is 0.61 nm (calculated by molecular

mechanics 2 – MM2 – modeling). Over the two cycles, a multi-

layer formation was observed. This behavior was also supported

by 2D ellipsometric images (Fig. 4). In Fig. 4(a–d) the surface

morphology was demonstrated as the number of modification

cycles increased. In Fig. 4b, monolayer formation was predicted

due to the homogenous appearance of the surface for 1 cycle, in

accordance with surface thicknesses. In Fig. 4c and Fig. 4d, non-

homogenous surfaces were observed due to layer-by-layer

deposition which was assumed as multilayer formation by

comparing the theoretical surface thickness. It should be noted

that the relative intensity changes in 2D ellipsometric images

show relative thickness differences and anisotropy on the surface

(as given in the arbitrary scale).

In order to minimize clusters on the surface and to increase the

number of free active sites, monolayer formation was preferred

for sensor applications. Accordingly, to get compact monolayer

formation, cyclic voltammetric modification with 2 cycles was

applied during the further experiments.

Effect of pH on Cu2+ adsorption at the IPA-Au surface

IPA-Au surfaces were used as an SPRe-TIRE sensor for Cu2+

adsorption after modification. For this purpose, the pH depen-

dency of Cu2+ adsorption on IPA-Au from aqueous media was

investigated. Metal ion adsorption is directly affected by the pH

of the medium. By applying various pH buffers, the effect of pH

on Cu2+ adsorption at the IPA-Au surface was monitored online

by TIR ellipsometry. In Fig. 5, the relative change of delta (D)

values (in degrees) upon Cu2+ adsorption by the TIRE sensor was

represented. Although it was suitable to compare the kinetic

parameters of binding to select the optimum pH, the relative

response difference at equilibrium (over 10 min) was compared

due to similar kinetic characteristics. From the figure, it is seen

Fig. 3 Effect of number of CV cycles on the thickness of the IPA layer

during modification.

1468 | Analyst, 2011, 136, 1464–1471

that the equilibrium relative response at pH 4.5 (approximately

over 4�) was higher than the response observed at both pH 4.0

and pH 5.0 (approximately at 3�). As shown in Fig. 5, the

optimum pH was 4.5 in acetic acid/acetate buffer at room

temperature. Accordingly, the sensor response and detection

limit studies were conducted at pH 4.5 for IPA-Au sensor

surfaces for Cu2+ adsorption.

Effect of initial Cu2+ concentration on adsorption

The sensor ability of the IPA-Au surface in the SPRe-TIRE

method and the effect of the initial Cu2+ concentration on

adsorption were investigated at various concentrations in the

range of 5 � 10�8 to 1 � 10�4 M in acetic acid/acetate buffer

having a pH of 4.5 (i.e. the optimum value). The in situ

adsorption kinetics are shown in Fig. 6. Relative sensor response

increased with increasing concentration of Cu2+, as expected.

More favorable adsorption kinetics were observed at higher Cu2+

concentrations. Also, from the adsorption kinetics, a plateau was

Fig. 5 SPRe-TIRE sensor response for Cu2+ adsorption on the IPA-Au

surface at pH 4.0, pH 4.5 and pH 5.0 at room temperature. The Cu2+

concentration was 1 � 10�5 M.

This journal is ª The Royal Society of Chemistry 2011

Fig. 6 SPRe-TIRE sensor response for Cu2+ adsorption on the IPA-Au

surface at different initial concentrations (pH was 4.5 at room tempera-

ture).

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observed for all concentrations, and from these results a Lang-

muir model was selected for modeling adsorption data which is

intended to simulate favorable and single layer adsorption (i.e.

complex formation) conditions.

For this purpose, the equilibrium sensor response (as the

relative sensor response, D) was calculated from the TIRE data

and these data were implemented in the Langmuir adsorption

model. The Langmuir isotherm can be written as in the linearized

form:

1

Ne

¼ 1

CeN�bþ 1

N� (3)

where Ce is the equilibrium concentration of Cu2+ in solution

(M);Ne is the amount of adsorbed Cu2+ on a unit surface of IPA-

Au; N* is the maximum capacity of the adsorbent of the system

and b is an adsorption equilibrium constant related to the energy

of the sorption.51 It is known that the D parameter is related to

the surface coverage or surface density of the molecules for

a system in which only a single molecule deposition occurs with

Fig. 7 Langmuir isotherm at room temperature for Cu2+ adsorption on

the IPA-Au surface, determined by TIRE.

This journal is ª The Royal Society of Chemistry 2011

known dielectric constants. Therefore, the relative change in D is

proportional to the surface mass density (ng mm�2 etc.).52 It is

possible to assume that Cu2+ complexation on the sensor surface

was single species adsorption with known dielectric constants.

Thus, we correlated the sensor response, as relative change in D

with Ne, to fit the adsorption data to the Langmuir model.

Experimental data fitted to the Langmuir model were shown in

Fig. 7. In this form, the linear regression coefficient was 0.9923

for modeling. 1/N* is determined from the intercept (in the

equation the ‘a’ value) and 1/b is determined from slope of the

graph (in the equation the ‘b’ value).

Determination of sensor linearity and detection limit

From the equilibrium data obtained at various concentrations,

the sensor response was also examined (Fig. 8). Sensor properties

were evaluated in terms of linearity of response and detection

limit in this study. There was a linear sensor response zone

between 5� 10�8 and 1� 10�5 M with a correlation coefficient of

0.9893. The overall sensor response was logarithmic for the entire

Cu2+ concentration range used in this study.

A semi-logarithmic sensor response was acceptable due to

sensor linearity. According to the curve in Fig. 8, it was clear that

the sensor can give reliable and linear results below 1 � 10�8 M

for Cu2+ adsorption on the IPA-Au surface using the SPRe-

TIRE setup. It can be also suggested that a linear zone below 1�10�6 M was yielded from the TIRE setup due to at least

a 100-fold high sensitivity difference from the SPR setup. The

other linear zone above 1� 10�6 M is yielded from SPR behavior

which has limited sensitivity compared to the TIRE setup. SPR

signals are relatively less sensitive to dielectric constants

compared to total internal reflection ellipsometry in which the

phase shift upon reflection measured instead of the phase

intensity

The theoretical detection limit can be determined from the

intersection of the sensor response fitting function. The calcu-

lated detection limit was (6.7 � 0.2) �10�9 M for this system.

Fig. 8 Relative response for the SPRe-TIRE sensor against the molar

concentration of Cu2+ ions (inset) and its logarithm.

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Finally, as a highly sensitive technique, the SPRe-TIRE

method shows promise in determining ions in solutions at very

low concentrations. In a similar emphasis, Yang and co-workers

studied Cu(II) determination by using Osteryoung square wave

voltammetry.53 In their study, Gly–Gly–His has been covalently

attached on to a gold surface with the help of a 3-mercapto-

propionic acid self-ordered monolayer. Their detection limit on

the thin film was 0.2 ppt for Cu(II) (approximately 3.1 � 10�11

M), which is better than our detection limit. On the other hand,

Freire and Kubota modified a Au electrode with a SAM of

3-mercaptopropionic acid and evaluated a highly sensitive vol-

tammetric sensor for copper ions by CV that showed an attrac-

tive ability to efficiently preconcentrate traces of Cu(II) from

solution, allowing a very simple and reproducible method at

levels down to ppq (parts per quadrillion with a detection limit of

1.8 � 10�14 M (1.1 pg L�1)).37

The emphasis of this study is searching the applicability of

SPRe-TIRE sensors on ion detection in aqueous media. As

known, conventional ellipsometric sensor techniques use

molecular agglomeration on the sensor surface to detect layer

characteristics such as thickness. From the measured data,

explanation of modification on the sensor surface is tried by

mathematical models. Because the electronic changes in layers

result only changes in the D parameter, which is one of the main

parameters measured in ellipsometric measurements, in this

study, the use of this parameter was improved by using the SPRe-

TIRE setup in order to detect metal ions in aqueous media.

Conclusions

An IPA modified gold electrode was constructed by the elec-

trochemical reduction of a diazonium salt in acetonitrile. The

surface of the modified electrode was characterized by CV and

ellipsometry and proved that IPA is successfully grafted at the

Au surface to form an IPA-Au film. This surface was suggested

as an SPRe-TIRE sensor to detect Cu2+ ions at extremely low

concentrations. The electrochemical response of the IPA-Au

electrode was tested for the determination of Cu2+ ions with

a calculated detection limit of (6.7 � 0.2) � 10�9 M and a linear

response for the 10�8 to 10�5 M region.

Acknowledgements

This work was supported by Ankara University Scientific

Research Fund with Project Grant number 09B4240012, and

TUBITAK (Scientific and Technological Research Council of

Turkey) project with a number of 106T622.

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