11

Square-wave voltammetry: the most advanced

electroanalytical technique

Valentin MirValentin MirččeskieskiInstitute of Chemistry Institute of Chemistry

Faculty of Natural Sciences and MathematicsFaculty of Natural Sciences and Mathematics

““Ss Cyril and Methodius” University, SkopjeSs Cyril and Methodius” University, Skopje

Republic of MacedoniaRepublic of Macedonia

2

t / s

E /

V

Esw

E

Square-wave voltammetry (SWV) is a pulsed voltammetric technique. The potential modulation consists of a train of equal potential pulses superimposed on a staircase potential ramp.

t / s

E /

V

Square-Wave Voltammetry: Potential Modulation

Red Ox + e

•

Ox + e Red

•

f = 1/vE f

A single potential cycle consisting of a two equal potential pulses superimposed on a single potential tread in two opposite (anodic and cathodic) directions. The current is measured at the end of each pulse in order to discriminate against the capacitate current and to extract only the faradic response of the electrode reaction. Properties of the potential modulation are: Esw – SW amplitude (pulse height); E –potential step; – duration of a single potential cycle; f - frequency of the pulses.

3

0.15 0.1 0.05 0 0.05 0.1 0.150.4

0.2

0

0.2

0.4

0.6

net p

f p

bp

Ep

0 200 400 600 800 1000 1200 1400 16004

3

2

1

0

1

2

3

4

54.31

3.725

j

1.5 103

1 j

Variation of the current with the time in the course of the experiment

4

t0

I

If / Ic >> 1(sampling point)

Faradaic current I f

(due to electrode reaction)

Capacitive current, Ic (due to charging - formation of the double layer)

Faradaic vs. capacitive current in the course of a single potential pulse

5

SW voltammogram

0.2 0.1 0 0.1 0.20.4

0.2

0

0.2

0.4

0.6

0.723

0.212

net p

f p

bp

0.190.19 Ep

net

f

b

net = f - b

Net component, calculated (not measured!) as a difference between the forward and backward components

Forward component measured at the end of each pulse with odd serial number (i.e., 1st, 3rd, etc.;

Backward component measured at the end of each pulse with even serial number (i.e., 1st, 3rd, etc.;

6

Time window of the voltammetric experiment

SWV

Scan rate: v = f E

Example: E = 0.1 mV, f = 200 Hz

v = 0.020 V/s

= 1/f = 5 msExample:

E = 0.1 mV, f = 500 Hz

v = 0.050 V/s

t = 2 ms

CV

For 300 mV potential pathFor 300 mV potential path

vv = 60 V/s = 60 V/s

vv = 150 V/s = 150 V/s

7

A technique for mechanistic, kinetic and analytical application

-0.4

-0.2

0

0.2

0.4

0.6

0.8

-0.4 -0.2 0 0.2 0.4

-0.4

-0.2

0

0.2

0.4

0.6

0.8

-0.4 -0.2 0 0.2 0.4

-0.4

-0.2

0

0.2

0.4

0.6

0.8

-0.4 -0.2 0 0.2 0.4

-0.6

-0.3

0

0.3

0.6

0.9

-0.35 0.15

-0.6

-0.3

0

0.3

0.6

0.9

-0.35 0.15

-0.6

-0.3

0

0.3

0.6

0.9

-0.35 0.15

An electrode reaction of a dissolved redox couple

Surface confined electrode reaction

irrevrersible

irrevrersible

quasirev. reversible

quasirev. reversible

8

-0.2

0

0.2

0.4

0.6

0.8

-0.4 -0.2 0 0.2 0.4-0.2

0

0.2

0.4

0.6

0.8

-0.4 -0.2 0 0.2 0.4-0.2

0

0.2

0.4

0.6

0.8

-0.4 -0.2 0 0.2 0.4

-0.4

-0.2

0

0.2

0.4

0.6

0.8

-0.5 0 0.5

-0.2

0

0.2

0.4

0.6

0.8

-0.5 0 0.5

-0.2

0

0.2

0.4

0.6

0.8

-0.5 0 0.5

EC mechanism

ECE mechanism

9

Electrode mechanisms

I. Electrode reaction of an immobilized redox coupe (surface electrode reaction);

II. Electrode mechanism involving formation of an insoluble compound with the electrode material;

10

Ox(ads)

Red(ads)

Oxbulk

Redbulk

ne- Diffusion mass transport is neglected

Ox(ads) + ne- Red(ads)

Ele

ctr

od

e

Reaction scheme for the electrode reaction of an immobilized redox coupe (surface confined electrode reaction)

11

Toward electrode kinetic measurements: Modeling and application

Application:

Protein-film voltammetry;

Electrochemicaly active drugs;

Simple adsorbates (many organic compounds);

Azodies; Metal complexes; Organometalic

compounds; Surface modified

electrodes; Voltammetry of solid

micro- particles etc.

][

d

dd

d

0

0,0

Re

Re

Re

Re

dOxs

d

Ox

dOx

dOx

ΓeΓeknFA

InFA

I

t

ΓnFA

I

t

Γ

ΓΓΓt

ΓΓΓt

12

Inet

Net dimensionless SW voltammograms simulated for different reversibility of the electrode reaction

irreversible quasireversible region reversible

= ks / f

increases

Dimensionless current = I/(nFA*f )

13

0.2 0.1 0 0.1 0.20.3

0.2

0.1

0

0.1

0.2

0.3

0.4

0.50.48

0.221

Icp 1

Iap 1

Inet p 1

0.20.195 Ep

0

0.4

0.8

-2 -1 0 1 2log)

p

Quasireversible maximum and the SW response at the quasireversible maximum

14

Synchronisation of the rate of the redox transformation withthe SW frequency!

The origin of the quasireversible maximum: Chronoamperometry of the surface eelectrode reaction

0 20 40 60 80 1000

100

200

300

400

500

If j 1

If j 10

If j 15

If j 20

j

ks = f

f = 250 Hz, = 0.5ks = 500 s-1

ks = 375 s-1

ks = 25 s-1

dim

ensi

onle

ss c

urr

ent

t

15

Simple methodology for using the quasireversible maximum for redox kinetic measurements

max = ks / fmax

max calculated by the

model fmax measured in the

experiment

ks = max fmax

16

0.2 0.15 0.1 0.05 0 0.05 0.1 0.15 0.20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.80.769

8.176 109

Inet p 1

Inet p 2

Inet p 3

Inet p 4

Inet p 5

Inet p 6

Inet p 7

Inet p 8

0.20.195 Ep

Splitting of the net SW response for fast and reversible surface electrode reaction

increases

17

0.2 0.1 0 0.1 0.21

0.5

0

0.5

10.769

0.591

Inet p 1

Icp 1

Iap 1

0.20.195 Ep

0.2 0.1 0 0.1 0.20.6

0.4

0.2

0

0.2

0.4

0.60.576

0.54

Inet p 2

Icp 2

Iap 2

0.20.195 Ep

log() = 0

0.2 0.1 0 0.1 0.20.3

0.2

0.1

0

0.1

0.2

0.30.283

0.282

Inet p 5

Icp 5

Iap 5

0.20.195 Ep

log() = 0.1 log() = 0.4

The Origin of the Splitting

18

40

80

120

160

200

240

35 55 75 95 115

Experimental systems that have been analyzed on the base of quasireversible maximum and the splitting:

Cytochrome C; Alyzarine red-S;Probucole;2-propylthiouracil;Fluorouracil;Molybdenum(VI)-phenantroline-fulvic acid;Azobenzene;Methilene blue,….;

The The dependence of the splitting on the SW amplitudedependence of the splitting on the SW amplitude

Esw / mV

Ep /

mV

19

alizarin

vitamin K2

vitamin B12

Examples of surface confined electrode reactions Examples of surface confined electrode reactions

20

Comparison of theoretical (□) and experimental (○) net peak currents for alizarin as a function of pH.

21

-1.5 10-7

-1 10-7

-5 10-8

0

5 10-8

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2

I / A

E / V

ib

if

-3.5 10-7

-2.5 10-7

-1.5 10-7

-5 10-8

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2

I / A

E / V

Mo(VI)-phenantroline-fulvic acid systemMo(VI)-phenantroline-fulvic acid system

ks = 8 2 s-1; = 0.41 0.05 n = 2

22

Splitting of the net SW response of methylene blue under the Splitting of the net SW response of methylene blue under the influence of the SW amplitudeinfluence of the SW amplitude

amplitude increasesmethylene bluemethylene blue

3,7-bis(Dimethylamino)-phenothiazin-5-ium chloride

23

Square wave voltammetry of azurin immobilized on 1-decanethiol-modified gold

Azurin – a blue copper protein

24

Square wave voltammetry of famotidin: catalytic hydrogen evolution reaction from adsorbed state

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3

E vs E0' / V

Electrode mechanism

Fam(ads) FamH+(ads)

FamH+(ads) + e- Fam(ads) + H(aq)

famotidine

25

Square wave voltammetry of 2-guanidinobenzimidazole : another example for the catalytic hydrogen evolution reaction

from adsorbed state

SWV DPV LSV

LOD [mol L-1] 0.035 0.14 0.2

LOQ [mol L-1] 0.1 0.4 0.6

26

Ele

ctro

de

S

Reaction scheme of an electrode reaction involving formation of Reaction scheme of an electrode reaction involving formation of chemical bonds with the electrodechemical bonds with the electrode

ne- Application: Sulfur containing amino

acids; Glutathione and other

cysteine containing peptides and proteins;

Mercaptans; Thyroxin; Thiourea; Thioethers; Phorphyrins; Flavins; Sulphide; Iodide etc.

S

S

S

SSS

S

S

27

Modeling

0s

s

0

2

2

(L)e(HgL)

e2

2d

)(d

2

)(:0,0

0)(),()(:0,0

(L)(L)

x

x

cr

Γk

FA

I

FA

I

t

HgLΓ

FA

I

x

LcDxt

HgLΓLcLcxt

x

cD

t

c

HgL (s) + 2e- Hg(l) + L2-(aq)

HgL2(s) + 2e- Hg(l) + 2L-(aq)

HgL (s) + 2e- L2-(ads) + Hg(l)

L2-

(aq)

 

HgL2(s) + 2e- 2L-(ads) + Hg(l)

2L-

(aq)

28

4

6

8

10

12

14

16

18

20

-2 -1 0 1 2 3

log( )

p

precision ± 10 %

Qvazireversible maximum of the cathodic stripping reaction

ks = kmaxD1/4 fmax

3/4 rs-1/2 rs

= 1 cm

Dimensionless current = I / (nFAc*(Df )1/2 )

29

Cathodic stripping voltammetry of glutathione

-0.700-0.600-0.500-0.400-0.300-0.200

-0.35

-0.25

-0.05

0.05

E / V

I /m

A

0

20

40

60

80

100

0 500 1000 1500 2000

pH = 5.6

pH = 7.0

pH = 8.5

f / Hz

I p f

-1 /

mA

s

ks = 5 0.2 cm s-1

30

-0.700-0.500-0.300

-0.20

-0.10

-0.05

0

0.10

E / V

I /

mA

-0.700-0.500-0.300

-0.23

-0.13

-0.03

0.07

E / V

I /

mA

Cathodic stripping voltammetry of glutathione in the presence of copper

Without Cu2+ With Cu2+

ks = 5.22 cm s-1 ks < 0.11 cm s-1

31

-28

-23

-18

-13

-8

-3

-7 -6 -5 -4 -3 -2 -1 0

log(c (M2+) / M)

I p

10

8 / A

Zn

Ca

Ba

Mg

Cu

Influence of different cations on the SW net peak currents of glutathione

32

Ele

ctr

od

e

S

S

S

S

S

S

The influence of the metal ions on the morphology of the film deposited on the electrode

ne-

Mx+

Mx+

Mx+

Additional Interactions:

attractionrepulsioncomplexation

33

-5

-4

-3

-2

-1

0

1

2

-1-0.8-0.6-0.4-0.20E vs Ag/AgCl / V

I /μ

A

(1)

(2)

(3)

A(aq) = L(aq) L(aq) + Hg(l) = HgL (s) + 2e-

Cathodic stripping mechanism coupled with a chemical reaction

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

-0.4-0.3-0.2-0.100.1

experimental theoretical

6-mercaptopurine-9-D-riboside in the presence of nickel(II) ions

34

Cyclic Square-Wave Voltammetry: a technique of the future

0 1 103 2 10

3 3 103

0.2

0.1

0

0.1

0.20.19

0.2

k

3 1031 k

0.2 0 0.21

0.5

0

0.5

10.637

0.636

net p 1

0.290.3 Ep

35

36