Transcript Kompleksni soedinenija

Square-wave voltammetry:
the most advanced
electroanalytical technique
Valentin Mirčeski
Institute of Chemistry
Faculty of Natural Sciences and Mathematics
“Ss Cyril and Methodius” University, Skopje
Republic of Macedonia
1
Square-Wave Voltammetry: Potential Modulation
Red  Ox + e
Esw
DE
•
t
•
Ox + e  Red
t/s
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.
f = 1/t
v = DE f
t/s
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); DE
–potential step; t – duration of a single potential
cycle; f - frequency of the pulses.
2
Variation of the current with the time in the course of the experiment
4.31
5
4
3
2
1
j
0
1
2
3
 3.725
0
Ep
0.05
0.1
0.15
4
0
1
200
400
600
800
j
1000
1200
1400
1600
3
1.510
3
Faradaic vs. capacitive current in the course of a single potential
pulse
Faradaic current I f
(due to electrode
reaction)
If / Ic >> 1
(sampling point)
I
Capacitive current, Ic
(due to charging formation of the
double layer)
0
t
4
SW voltammogram
0.723
net
net = f - b
0.6
Net component,
calculated (not
measured!) as a
difference between the
forward and backward
components
f
0.4
Forward component
measured at the end of
each pulse with odd
serial number (i.e., 1st,
3rd, etc.;
 net p
f p
0.2
 bp
0
b
Backward component
measured at the end of
each pulse with even
serial number (i.e., 1st,
3rd, etc.;
0.2
 0.212
0.4
0.2
 0.19
0.1
0
Ep
0.1
0.2
0.19
5
Time window of the voltammetric experiment
SWV
CV
Scan rate: v = f DE
Example:
DE = 0.1 mV, f = 200 Hz
For 300 mV potential path
v = 0.020 V/s
t = 1/f
v = 60 V/s
t = 5 ms
Example:
DE = 0.1 mV, f = 500 Hz
v = 0.050 V/s
v = 150 V/s
t = 2 ms
6
A technique for mechanistic, kinetic and analytical application
An electrode reaction of a dissolved redox couple
-0.4
0.8 irrevrersible
0.8
0.6
0.6
0.6
0.4
0.4
0.4
0.2
0.2
0.2
0
0
0
-0.2
0
-0.2
0.2
0.4
-0.4
-0.4
-0.2
0
-0.2
quasirev.
0.2
0.4
0.8
-0.4
-0.2
0
-0.2
reversible
0.2
0.4
-0.4
-0.4
Surface confined electrode reaction
0.9
irrevrersible
0.9
0.6
0.6
0.3
0.3
0
0
-0.35
0.15
-0.35
quasirev.
0.9
reversible
0.6

0.3
0
0.15
-0.35
0.15
-0.3
-0.3
-0.3
-0.6
-0.6
-0.6
7
EC mechanism
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.2
0
0
0
0.8
0.6
0.4
-0.4
-0.2
0
0.2
0.4
-0.4
-0.2
0
0.2
0.4
-0.4
0
0.2
0.4
-0.2
-0.2
-0.2
-0.2
ECE mechanism
0.8
0.6
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.4
0.2
0
-0.5
0
-0.2
-0.4
0.5
0
0
-0.5
0
-0.2
0.5
-0.5
0
0.5
-0.2
8
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;
9
Reaction scheme for the electrode reaction of an immobilized redox
coupe (surface confined electrode reaction)
Ox(ads)

Oxbulk
ne-
Red(ads)
Diffusion mass
transport is neglected

Redbulk
Ox(ads) + ne-  Red(ads)
10
Toward electrode kinetic measurements: Modeling and application
Application:
t =0
t 0
Γ Ox = Γ  , Γ Re d = 0
Γ Ox  Γ Re d = Γ

dΓ Ox
I
=
dt
nFA
dΓ Re d
I
=
dt
nFA
I
= k s e   [ Γ Ox  e Γ Re d ]
nFA








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.
11
Net dimensionless SW voltammograms simulated for different
reversibility of the electrode reaction
Dimensionless current
 = I/(nFAG*f )
w
increases
w = ks / f
irreversible
quasireversible region
reversible
Inet
12
Quasireversible maximum and the SW response at the
quasireversible maximum
0.48
0.5
0.4
0.8
0.3
0.2
Dp
Icp  1
Iap  1
0.4
0.1
Inetp  1
0
0.1
0
0.2
-2
-1
0
log(w)
1
2
 0.221 0.3
0.2
 0.195
0.1
0
Ep
0.1
0.2
0.2
13
The origin of the quasireversible maximum:
Chronoamperometry of the surface eelectrode reaction
500
f = 250 Hz,  = 0.5
ks = 500 s-1
400
ks = 375 s-1
If j 1
If j 1 0
300
If j 1 5
If j 2 0
200
ks = f
100
ks = 25 s-1
0
0
20
40
60
80
100
j
Synchronisation of the rate of the redox transformation with
the SW frequency!
14
Simple methodology for using the quasireversible maximum for
redox kinetic measurements
wmax = ks / fmax
wmax
calculated by the model
fmax
measured in the experiment
ks = wmax fmax
15
Splitting of the net SW response for fast and reversible surface
electrode reaction
0.769
0.8
w increases
0.7
Inetp  1 0.6
Inetp  2
Inetp  3
0.5
Inetp  4
Inetp  5
0.4
Inetp  6
Inetp  7
0.3
Inetp  8
0.2
0.1
8.17610
9
0
0.2
 0.195
0.15
0.1
0.05
0
Ep
0.05
0.1
0.15
0.2
0.2
16
The Origin of the Splitting
0.769
1
0.576
0.6
0.283
0.3
0.2
0.4
0.5
0.2
Inetp  1
Icp  1
0.1
Inetp  2
Icp  2
0
Iap  1
Inetp  5
Icp  5
0
Iap  2
0
Iap  5
0.2
0.1
0.4
0.2
0.5
 0.591
1
0.2
 0.195
0.1
0
0.1
Ep
log(w) = 0
0.2
0.2
 0.54 0.6
0.2
 0.195
0.1
0
Ep
log(w) = 0.1
0.1
0.2
0.2
 0.282 0.3
0.2
0.1
 0.195
0
0.1
Ep
0.2
0.2
log(w) = 0.4
17
The dependence of the splitting on the SW amplitude
240
Experimental systems that have
been analyzed on the base of
quasireversible maximum and
the splitting:
200
160
Cytochrome C;
Alyzarine red-S;
Probucole;
2-propylthiouracil;
Fluorouracil;
Molybdenum(VI)-
120
80
40
35
55
75
Esw / mV
95
115
phenantroline-fulvic acid;
Azobenzene;
Methilene blue,….;
18
Examples of surface confined electrode reactions
alizarin
vitamin B12
vitamin K2
19
Comparison of theoretical (□) and experimental (○) net peak currents
for alizarin as a function of pH.
20
Mo(VI)-phenantroline-fulvic acid system
i
5 10
-5 10
b
-8
-7
-1.5 10
I / A
I / A
0
-5 10
-8
i
-7
f
-1 10
-8
-2.5 10
-7
-7
-3.5 10
-1.5 10
-7
-0.7
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.6
-0.5
-0.4
-0.3
-0.2
E/V
E/V
ks = 8  2 s-1;  = 0.41  0.05
n=2
21
Splitting of the net SW response of methylene blue under the influence
of the SW amplitude
methylene blue
amplitude
increases
3,7-bis(Dimethylamino)phenothiazin-5-ium chloride
22
Square wave voltammetry of azurin immobilized on 1-decanethiolmodified gold
Azurin – a blue
copper protein
23
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
E vs E
0
0'
0.1
0.2
0.3
/V
famotidine
Electrode mechanism
Fam(ads)  FamH+(ads)
FamH+(ads) + e-  Fam(ads) + H(aq)
24
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
25
Reaction scheme of an electrode reaction involving formation of
chemical bonds with the electrode
ne-
S
S
S
S
S
S
S
S
S
Application:










Sulfur containing amino
acids;
Glutathione and other
cysteine containing peptides
and proteins;
Mercaptans;
Thyroxin;
Thiourea;
Thioethers;
Phorphyrins;
Flavins;
Sulphide;
Iodide etc.
26
Modeling
c(L)
 c(L)
=D
t
x 2
t = 0, x  0 :
c( L) = c  ( L), Γ ( HgL) = 0
2
I
 c( L) 
t  0, x = 0 :
D
=


2 FA
 x  x =0
dΓ ( HgL)
I
=
dt
2 FA

I
   Γ (HgL)


(

= ks e 
 e c(L)x =0 
2 FA
 rs

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)
27
Qvazireversible maximum of the cathodic stripping reaction
Dimensionless current
 = I / (nFAc*(Df )1/2 )
ks = kmaxD1/4 fmax3/4 rs-1/2
rs = 1 cm
precision ± 10 %
28
Cathodic stripping voltammetry of glutathione
100
pH = 5.6
-0.35
80
ks = 5  0.2 cm s-1
-0.25
60
I /m A
pH = 7.0
40
20
pH = 8.5
-0.05
0
0
0.05
500
1000
1500
2000
f / Hz
-0.200 -0.300 -0.400 -0.500 -0.600 -0.700
E/V
29
Cathodic stripping voltammetry of glutathione in the presence of
copper
-0.20
-0.23
I / mA
Without Cu2+
I / mA
-0.10
With Cu2+
-0.13
-0.05
0
-0.03
ks = 5.22 cm s-1
0.10
-0.300
-0.500
E/V
-0.700
ks < 0.11 cm s-1
0.07
-0.300
-0.500
-0.700
E/V
30
Influence of different cations on the SW net peak currents of
glutathione
-28
Cu
-18
8
D I p 10 / A
-23
Mg
-13
Ba
Ca
-8
Zn
-3
-7
-6
-5
-4
-3
-2
-1
0
2+
log(c (M ) / M)
31
The influence of the metal ions on the morphology of the film deposited
on the electrode
ne-
S
S
Mx+
S
Mx+
S
Additional Interactions:
attraction
repulsion
complexation
Mx+
S
S
32
Cathodic stripping mechanism coupled with a chemical reaction
-2.5
-5
(1)
experimental
-4
(2)
-1.5
-3
I /μA
theoretical
-2
-1
-2
-0.5
-1
0
0
0.5
1
(3)
1
2
0
-0.2
-0.4
-0.6
E vs Ag/AgCl / V
-0.8
-1
0.1
0
-0.1
-0.2
-0.3
-0.4
1.5
6-mercaptopurine-9-D-riboside in the
presence of nickel(II) ions
A(aq) = L(aq)
L(aq) + Hg(l) = HgL (s) + 2e-
33
Cyclic Square-Wave Voltammetry: a technique of the future
1
0.637
0.2
0.19
0.5
0.1
k
0
net p  1
0
 0.1
 0.2
 0.2
0
1
110
3
3
210
k
3
 0.5
310
3
310
 0.636
1
 0.2
 0.3
0
Ep
0.2
0.29
34
35
36