Voltammetric methods and electrodes. Electroanalytical methods Interfacial methods Bulk methods...

Voltammetric methods and electrodes

Electroanalytical methods

Interfacial methods

Bulk methods

Static methodsI ~ 0

Dinamic methodsI # 0

Conductometry(G = 1/R)

Conductometric Titrations (volume)

Potentiometry(E)

Potentiometric titrations (volume)

Constant current

Coulometric Titrations(Q = It)

Electrogravimetry(wt)

Controlled potential

Electrogravimetry (wt)

Amperometric titrations (volume)

Voltammetry[ I = f (E) ]

Const. electrode potential

coulometry (Q = ∫0

1 idt

Introduction

Five Important interrelated concepts to understand electrochemistry: (1) the electrode’s potential determines the analyte’s form at the electrode’s surface;

(2) the concentration of analyte at the electrode’s surface may not be the same as its concentration in bulk solution;

(3) in addition to an oxidation–reduction reaction, the analyte may participate in other reactions;

(4) current is a measure of the rate of the analyte’s oxidation or reduction; and

(5) we cannot simultaneously control current and potential.

L. Faulkner, Understanding electrochemistry: some distinctive concepts,” J. Chem. Educ., 60, 262 (1983) ; P. T. Kissinger, A. W. Bott, Electrochemistry for the Non-Electrochemist,” Current Separations, 20, 51 (2002)

1) The Electrode’s Potential Determines the Analyte’s Form

Fig. Redox ladder diagram for Fe3+/Fe2+ and for Sn4+/ Sn2+ redox couples. The areas in blue show the potential range where the oxidized forms are the predominate species; the reduced forms are the predominate species in the areas shown in pink. Note that a more positive potential favors the oxidized forms. At a potential of +0.500 V (green arrow) Fe3+ reduces to Fe2+, but Sn4+ remains unchanged.

2) Interfacial Concentrations May Not Equal Bulk Concentrations

Fig. Concentration of Fe3+ as a function of distance from the electrode’s surface at (a) E = +1.00 V and (b) E = +0.500 V. The electrode is shown in gray and the solution in blue.

Nernst Equation

E = Eo + 0.0592/n + log [ox]/[red]

Fe3+ + e Fe2+

E = Eo + 0.0592/1 + log [Fe3+]/[Fe2+]

3) The Analyte May Participate in Other Reactions

Fe3+ + OH- FeOH2+

4) We Cannot Simultaneously Control Both Current and Potential

5) Controlling and Measuring Current and Potential

Controlled Potential Methods (Voltammetry)

Fig. Flow patterns and regions of interest near the work electrode in hydrodynamic voltammetry

Controlled Potential Methods (Voltammetry)

O + ne R (1)

E = Eo` + 0.0592/n + log CsO / Cs

R (2)

E = potential applied to electrode (mV)

Eo`= formal reduction potential of the couple vs Eref

n = number of electrons in reaction (1)

CsO = surface concentration of species O

CsR = surface concentration of species R

where

Table. Relationship of E to surface concentrations#

E, mV CsO / Cs

R

236 10,000/1

177 1,000/1

118 100/1

59 10/1

0 1/1

-59 1/10

-118 1/100

-177 1/1,000

-236 1/10,000

For a reversible system, n = 1, Eo`= V

The current at an electrode is related to the flux (rate of mass transfer) of material to the electrode

Considering x = and C = CO – CsO

Where = Nernst diffusion layer

(3)

4(4)

(5)

Where ilc is limiting cathodic current and CsO is zero

Cyclic Voltammetry

FeIII(CN)63- + e FeII(CN)6

4- (1)

FeII(CN)64- FeIII(CN)6

3- (2)

E = Eo` + 0.0592/1 log [FeIII(CN)63-] / [FeII(CN)6

4-] (3)

Eo` = E1/2 = (Epa + Epa)/ 2 (4)

Nernst Equation for a reversible system

Ep = Epa – Epc = 0.059/ n (5)

The peak current for a reversible system is described by Randles-

Sevcic Equation for the forward sweep for the first cicle:

ip = 2.69 105 n2/3 A D1/2 1/2 C (6)

Where: ip = peak current (A); n = number of electrons; A = electrode

area (cm2); D = diffusion coefficient (cm2 / s); = scan rate (V /s) and

C = concentration (mol / cm3)

Cronoamperometria

Figura (A) Representação esquemática da aplicação de potencial em voltametria de pulso diferencial. A corrente é amostrada em S1 e S2 e a

diferença entre elas é registrada; (B) Voltamograma de pulso diferencial.

SCHOLZ, F., ed (2005). Electroanalytical methods. New York: Springer.

Figura (1) Forma de aplicação de potencial na voltametria de onda quadrada; (2) Voltamogramas de onda quadrada esquemáticos para um sistema reversível (A) e para um sistema totalmente irreversível (B).

SOUZA, D.; MACHADO, S. A. S.; AVACA, L. A. Química Nova, Vol. 26, 81-89, 2003.LOVRIC, M.; KOMORSKY- LOVRIC, S.; MIRCESKI, V. Square Wave Voltammetry. ed. (2007), Berlin: Springer.

Eletrodo de diamante dopado com boro8000 ppm; 0,72 cm2

Glassy carbon electrode

Eletrodos de carbono vítreo da Tokai Carbon Co

-3.0 3.0

0

1M H2SO4

1M NaOH

1M H2SO4

1M KCl

1M NaOH

Pt

Hg

1M HClO4

0.1M KCl

C

0.5 M H2SO4BDD

- 1.5 +2.5

Approximate potential ranges for platinum, mercury, carbon and boro-doped diamond electrodes

Glassy carbon electrode application

Aplicação de EQM em sistema FIAAplicação de EQM em sistema FIA

Eletrodo base:

Eletrodo de carbono vítreo

Preparação do Eletrodo:

Ciclagem de potencial entre -0,2 e 0,6 V (vs. Ag/Cl) em solução de 1,0 mmol L-1

FeCl3.6H2O e 10 mmol L-1 de K3[Fe(CN)6]

Funcionamento

[Fe(CN)6]4- [Fe(CN)6]3- + e-

[Fe(CN)6]3- + AA [Fe(CN)6]4-

Comportamento voltamétrico do sistema

Aplicação de EQM em sistema FIAAplicação de EQM em sistema FIA

Anodic stripping voltammetric determination of copper(II) using a functionalized carbon nanotubes paste electrode modified with crosslinked chitosan

Janegitz, B.C., Marcolino-Junior, L.H., Campana-Filho, S.P.,Faria, R.C., Fatibello-Filho,O. Sensors and Actuators B, 142, 260 (2009)

49

Comparison: with and without carbon nanotube functionalization

Anodic stripping voltammetry

•80 % CNTs (w/w) + 20 % nujol (w/w)

•-0.2V for 270 s

• 25 mV s-1

Figure XX - Linear voltammograms obtained with electrodes containing functionalized nanotubes not (A) and functionalized (B), in 0.1 mol L-1 NaNO3 solution in the presence of Cu2 + 9.0 x 10-5 mol L-1.

-0.2 0.0 0.2 0.4 0.6 0.8-50

0

50

100

150

200

250

300

I/

A

E / V vs. Ag/AgCl

-0.2 0.0 0.2 0.4 0.6 0.8

0

50

100

150

200

250

300

I/A

E/ V vs. Ag/AgCl

Carbon Nanotubes Functionalized

(A) (B)

50

Anodic stripping voltammetry

•-0.2V por 270 s•25 mV s-1

Figura XX -. Stripping voltammetry for EPN (A), EPNM-QTS (B), EPNM-QTS-GA (C) and EPNM-QTS-ECH (D) in 0.1 mol L-1 NaNO3 solution in the presence of Cu2 + 9.0 x 10-5 mol L-1., = 25mV s-1, a 25ºC.

-0.2 0.0 0.2 0.4 0.6 0.8-10

0

10

20

30

I/A

E/V vs. Ag/AgCl

A

D

EPNM-QTS-ECH

Analytical CurveAnalytical Curve

Figura XX - Voltammograms obtained for the construction EPNM-QTS-ECH with 15% (w/w) QTS-ECH in 0.05 mol L-1 NaNO3 solution.

Figura XX - Analytical curve:

• 7.93 x 10-8 a 1.6 x 10-5 mol L-1

L D =1.06 x10-8 mol L-1 ,

LQ= 7.93 x 10-8 mol L-1

RSD= 3.12%

-0.3 -0.2 -0.1 0.0 0.1 0.2

-4

0

4

8

12

16

I/

A

E/ V vs. Ag/AgCl

1

6

-0.2 0.0 0.2 0.4 0.6 0.8-50

0

50

100

150

200

250

300

I/

A

E / V vs. Ag/AgCl

Concentration de Cu (II) (mol L-1)

Sample Method

comparative*

Method

proposed

Erro relativo %

Urine samples0.50 ± 0.03 0.52 ± 0,09 4,.0

2.4 ± 0.2 2.3 ± 0.1 -4.1

Industrial Waste3.5 ± 0.2 3.6 ± 0.1 1.0

10.7 ± 0.2 11.1 ± 0.1 3.6

Determination of Cu2+

Voltammetric determination of ciprofibrate using a glassy carbon electrode modified with functionalized carbon nanotube within a poly (allylamine hydrochloride) film

ClCl

O

CH3

CH3

COOH

o The ciprofibrate is a fibrate and present Antilipemic effect (lipid

lowering);

o Fibrates are indicated for patients who, after tests confirmed that the

increase in endogenous triglecirideos is due to poor nutrition;

o A possible interest in determining the ciprofibrate addition to quality

control of drugs;

Figure XXX - Ciprofibrate molecular estructure.

Functionalization of MWCNTs in acid solution (H2SO4/HNO3 3:1)

Carbon nanotubes dispertion in PAH solution [dispertion]=1mg mL-1

Film formation on the elcetrode by casting technique (20 μL)

(a)

(b)

(c)

(d)

Figure XX - PAH SEM images (a) and (b); MWCNTs/PAH SEM image (c) and (d)

Linear equation: i= -0.700 + 4.75 x 104 x C

Concentration range: 13.3 to132 mol L-1

Detection Limit: 8.34 mol L-1

Figure XX - Analytical curve obteined for ciprofibrate determination in phosphate buffer solution 0,01 mol L-1 by VPD. = 12 mV s-1, A= 60 mV, t= 100 ms

Analytical curve

Sample Label value (mg) DPV

method

HPLC

method

REc1

A 100 100 ± 3 98±5 2

B 100 99 ± 4 100±7 -1

C 100 99 ± 6 100±4 -1

D 100 100 ± 6 104±2 -4

Table XX - Ciprofibrate determination in pharmaceuticals formulations using GCE-MWCNTs/PAH and standard method

REc1 = 100 x (VPD value – Reference method value) / Reference method value

Inhame (Alocasia macrorhiza)

Abacate (Persea americana)

Batata inglesa (Solanum tuberosum)

Frutas e vegetais empregados como fontes da PFO (POLIFENOL OXIDASE) e PER (PEROXIDADE) em bioreatores e biossensores.

Abobrinha (Cucurbita pepo)

Alcachofra (Cynara scolymus L.)

Banana (Musa paradisiaca)

Batata doce (Ipomoea batatas L.

Lam.)

Berinjela (Solanum melongena)

Cara (Dioscorea bulbifera)

Coco (Cocus nucifera L.)

Jaca (Artocarpus integrifolia L.)

Mandioca (Manihot utilissima)

Nabo (Brassica

campestre ssp.)Pêssego

(Prunus persica)Rabanete

(Raphanus sativus)

Enzimas São proteínas que agem como catalisadores biológicos:

enzimaComposto AComposto A

Composto BComposto B

Centro ativo ou sítio catalítico Não há consumo ou modificação

permanente da enzima

Emil Fisher, década 50

Daniel Kosland, 1970

Modelo chave-fechadura

Modelo Encaixe induzido

E e S se deformam quando em contato (alteração

conformacional), para otimizar o encaixe

Biossensor para glicose - Radiometer®

OH

+O2+ H+3 PFOOH

OH

+ H2O

OHOH

+ 1/2 O2

OO

H2O+

o-quinona

Catecol

Catecol

Fenol

Fig. Esquema de um biossensor

PFO

Low cost PortabilityPracticality

64

Screen-printed electrodes

“screen-printed” or “silk-screen” Technology the possibility of mass production Extremely low cost Simplicity Complete electrochemical system

WorkCounter

Reference

65

Screen-printed electrodes

Figura 5: Struture of screen printed electrodes

Substrates

Plastic materials (Polyester)

Ceramics

Metals

Work electrode

•Addition

•Deposition

Metalic films

Nanoparticles

Carbon nanotubes

Enzymes

Polymers

Complexation agents

66

Screen-printed electrodes

Carbon nanotubes

Boron-doped diamond (BDD)

Carbon glassy (CG)

Metallic films

etc

Copper

Gold

Iridium

Antimony

Bismuth

Etc.

67

New Materials

2002Vytras et al.Pauliukaite et al.

Carbon paste modified with Bi2O3

2003Wang et al.

Bismuth film electrode (BiFE) electrodeposited in CG

68

Bismuth film

• Good cathodic potential window

• Interference of dissolved oxygen is minimal

• Low toxicity

• Electrochemical behavior is similar to that of mercury

69

Bismuth film

70

MEV-FEG

Figura 10: Micrographs of the BiFE A) 10000x B) 50000x

A) B)

Bismuth film electrode

= =

Copperplate

3-electrodesscheme

Insulatingfilm

Definitionof the

superficial area

Agdeposit

Bideposit

Bi Filmmini-sensor

= =

Copperplate

3-electrodesscheme

Insulatingfilm

Definitionof the

superficial area

Agdeposit

Bideposit

Bi Filmmini-sensor

Bismuth film electrode for anodic stripping SWV lead determination

A B

C

(A): PalmSens and (B): DropSens potentiostats and (C) BiSPE preparation

72

Confecção do minissensor

120 °C durante 200 s

FeCl3 0,50 mol L-1 em meio de HCl 0,10 mol L-1 durante 15-20 minutos.

electrode

Bismuth film electrode

tt-type connector for printers

Bismuth redox process

-0,6 -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2-0,03

-0,02

-0,01

0,00

0,01

0,02

I

II

I /

E/ V vs. Ag/AgCl ( KCl 3,0 mol L-1)

Figura 7: Cyclic voltammogram for 0.02 mol L-1 Bi(NO3)3 in 0.10 mol L-

1 acetate buffer (pH 4,5) solution as electrolyte support; the work electrode is a platinum foil and scan rate of 10 mV s-1.

74

I Bi3+ + 3e- Bi0 -0.30 V II Bi3+ Bi0 + 3e- 0.08 V

Filme de bismuto -0.18 V vs. Ag/AgCl (3.0 mol L-1 KCl) during 200 s 0.02 mol L-1 Bi(NO3)3, 1.0 mol L-1 HCl in 0.15 mol L-1 Sodium citrate.

-0,8 -0,7 -0,6 -0,5 -0,40

4

8

12

16

20

24

28

pa

/ A

E / V vs. Ag/AgCl

0 1 2 3 4 50

4

8

12

16

pa

/ A

[Pb2+] / mol L-1

Anodic stripping voltammograms of 9.9 x 10-8 – 8.3 x 10-6 lead (LD of 5.8 x 10-8 M) in 0.1 M acetate buffer (pH 4.5), using square-wave mode. Deposition at -

1.1 V for 2 min; pulse amplitude of 28 mV; increment of potential of 3 mV and frequency of 15 Hz.

Determination of lead

Bismuth film electrode (BiFE) for paraquat determination

In 0.1 mol L-1 HAC pH 4,5, using differential pulse voltammetry.

Figueiredo-Filho, L. C. et al, Electroanalysis, 22, 1260 (2010)

DPV para determinação de Paraquat

Besides of paraquat can be determined simultaneously Cd2+ e Pb2+.

Determination of PQ in six natural water samples by BIFE and HMDE (reference).

Samples* /

µ mol L-1 HMDE BIFE ER (%)

A1 59.03 ± 0.06 58.73 ± 0.03 -0.51

A2 58.74 ± 0.01 59.23 ± 0.00 0.84

A3 58.35 ± 0.03 57.41 ± 0.02 -1.61

A4 29.36 ± 0.08 29.56 ± 0.03 0.68

A5 29.23 ± 0.05 27.97 ± 0.02 -4.31

A6 27.95 ± 0.05 29.51 ± 0.02 5.58

*The SD (±) was calculated from three replicates.

Filme de bismuto -0,18 V vs. Ag/AgCl (KCl 3,0 mol L-1) durante 200 s Bi(NO3)3 0,02 mol L-1, HCl 1,00 mol L-1 e citrato de sódio 0,15 mol L-1

Cola de prata Bright Silver Epoxy (BSE) + Gray Silver Hardener (GSH). Após a aplicação na superfície de cobre esperou-se 24 horas para a cura da cola

79

Confecção do minissensor

Figura- Etapas da confecção do minissensor

• Atrazina (ATZ) (2-cloro-4-etilenodiamino-6-isopropilamino-s-triazina)

• Pertence a classe das triazinas

• Composto polar, fracamente básico de coloração branca

N

N

N

NHHN CHC2H5

Cl

CH2 3

Figura. Fórmula estrutural da Atrazina80

Atrazina

-1,2 -1,0 -0,8 -0,6 -0,4

-9

-6

-3

0

I /

E / V vs. Ag/AgCl ( 3,0 mol L-1)

Figura 11- Voltamograma obtido para uma solução de Atrazina 4,00 x 10-5 mol L-1, utilizando tampão acetato 0,10 mol L-1, pH *4,5 em 15 % v/v de etanol como eletrólito suporte. * pH condicional

81

Comportamento eletroquímico da Atrazina (ATZ)

N

N

N

NHCHCH3CH2NH

Cl

CH3

CH3

H+N

N+

N

NHCHCH3CH2NH

Cl

CH3

CH3H

e-

N

H

Cl

NHCHCH3CH2NHCH3

CH3

*e- N

N

N

NHCHCH3CH2NH

CH3

CH3

+ Cl-