Electrochemical and impedance spectroscopy studies of various diazonium salts on a glassy carbon electrode

Article abstract of DOI:10.1007/s00706-010-0335-z

The derivatization of a glassy carbon electrode surface was achieved with and without electrochemical reduction of various diazonium salts in acetonitrile solutions. The surfaces were characterized, before and after their attachment, by cyclic voltammetry and electrochemical impedance spectroscopy to evidence the formation of a coating on the carbon surface. The results were indicative of the presence of substituted phenyl groups on the investigated surface. Also, the effects of diazonium thin films at the surface of a glassy carbon electrode, modification time, and salt concentration on their electrochemical responses in the presence of the Fe(CN)₆^3-/4- probe were investigated. Electrochemical impedance measurements indicated that the kinetics of electron transfer is slowed down when the time and the concentration used to modify the glassy carbon electrode are increased. We therefore modified a glassy carbon surface via its derivatization with and without electrochemical reduction of various diazonium salts in acetonitrile solution. Springer-Verlag 2010.

Full text of DOI:10.1007/s00706-010-0335-z

Monatsh Chem (2010) 141:841–846
DOI 10.1007/s00706-010-0335-z
ORIGINAL PAPER
Electrochemical and impedance spectroscopy studies of various
diazonium salts on a glassy carbon electrode
Maryam Khoshroo Abbas A. Rostami
•
Received: 4 November 2009 / Accepted: 16 May 2010 / Published online: 15 June 2010
Ó Springer-Verlag 2010
Abstract The derivatization of a glassy carbon electrode
surface was achieved with and without electrochemical
reduction of various diazonium salts in acetonitrile solu-
tions. The surfaces were characterized, before and after
their attachment, by cyclic voltammetry and electrochem-
ical impedance spectroscopy to evidence the formation of a
coating on the carbon surface. The results were indicative
of the presence of substituted phenyl groups on the
investigated surface. Also, the effects of diazonium thin
films at the surface of a glassy carbon electrode, modifi-
cation time, and salt concentration on their electrochemical
technological applications in many different areas such as
sensors, molecular electronics, electrocatalysis, and bio-
medical applications [1]. Methods leading to the formation
of organic layers covalently bound to conductive surfaces
are relatively scarce. Most of them use electrochemistry to
induce the attachment [2–11]. In the electrochemical
method, a solution-based radical is generated at the elec-
trode (substrate) surface by reduction or oxidation of the
modifier. The radical appears to couple to the surface via
formation of a covalent bond. The most studied reaction is
reduction of an aryldiazonium cation to yield an aryl rad-
ical which binds to the surface [12]. This modification
process is very attractive because it avoids the use of
oxidative conditions which can lead to the damaging oxi-
dation of the carbon substrate [13] and also because it
allows the presence of selected functional groups on the
aryl groups. A specific substituent on the aryl group can be
used to provide useful properties to the modified electrode
and to elaborate more complex chemical structures that can
have various applications [14, 15]. For example, this
method has been used for covalent binding of enzymes and
proteins and study of the voltammetric differentiation of
dopamine and ascorbic acid [16]. Some results concerning
the attachment of diazonium salts on glassy carbon and
metals without electrochemical induction have been
reported in recent papers [17, 18]. This spontaneous
grafting has been also observed on silicium, gallium arse-
nide (GaAs), palladium [19], carbon black, and carbon
nanotube surfaces [20, 21]. Compared with electrochemi-
cal grafting, this procedure provides a simpler and more
versatile method for the elaboration of organic layers
attached to the substrate. Its easy implementation is very
attractive in the industrial field, as indicated by the large
number of patents applications for the modification of
carbon black and the production of inks.
3
-/4-
responses in the presence of the Fe(CN)6
probe were
investigated. Electrochemical impedance measurements
indicated that the kinetics of electron transfer is slowed
down when the time and the concentration used to modify
the glassy carbon electrode are increased. We therefore
modified a glassy carbon surface via its derivatization with
and without electrochemical reduction of various diazo-
nium salts in acetonitrile solution.
Keywords Cyclic voltammetry ꢀ Charge transfer ꢀ
Diazo compounds ꢀ Glassy carbon ꢀ
Electrochemical impedance spectroscopy
Introduction
During the past two decades, surface modification via the
attachment of organic molecules and nanosized materials
with specific end functionalities has been the subject of
substantial research which is driven by a large number of
M. Khoshroo ꢀ A. A. Rostami (&)
Faculty of Chemistry, University of Mazandaran, Babolsar, Iran
e-mail: rostami@umz.ac.ir
123

8
42
M. Khoshroo, A. A. Rostami
The aim of the present study was attachment of diazo-
250
nium salts with different substituents on the phenyl group
to a glassy carbon surface with and without electrochemi-
cal reduction in nonaqueous solution and investigation of
2
1
00
50
GC-Cl
3
-/4-
Fe(CN)
behavior at the modified electrode surface.
6
For this purpose, the following diazonium salts were
chosen: 4-(trifluoromethyl)benzene diazonium tetrafluorobo-
100
GC-Bare
GC-CF 3
50
rate (CF -PD), 4-chlorobenzene diazonium tetrafluorobo-
3
rate (Cl-PD), 4-nitrobenzene diazonium tetrafluoroborate
0
(
NO -PD), and 4-diazobenzenesulfonic acid (SO H-PD).
2 3
GC-NO2
The modified electrodes are referred to as GC-X where GC
is the glassy carbon substrate and X is the substituent on
the phenyl group. The cyclic voltammetry (CV) and elec-
trochemical impedance spectroscopy (EIS) techniques
were used to detect the presence of the grafted molecular
groups on the glassy carbon electrode.
-
50
-
-
100
150
GC-SO H
3
900
400
-100
-600
E / mV
Fig. 1 Responses of ferri/ferrocyanide on the modified glassy carbon
electrodes at E for 270 s
p
Results and discussion
2
50
200
50
100
Electrochemical response of the redox system
on grafted electrodes with electrochemical induction
1mM-270 s
1
The behavior of ferri/ferrocyanide redox system on the GC
electrode was investigated for different layers grafted by
electrochemical reduction of diazonium salts. In addition to
the nature of the substituent on the diazonium moiety, the
diazonium reduction time and the concentration were var-
ied. The reduction time was set at 10, 30, 90, and 270 s, the
concentration both 1 and 5 mM, and the electrochemical
modification potential was the cathodic peak potential, Ep,
of each diazonium salt. The ferri/ferrocyanide couple in
1mM-10 s
GC-Bare
5
0
0
5mM-270 s
-
50
-100
150
5mM-10 s
-
0
.1 M KCl aqueous solution presents a reversible system
900
400
-100
-600
E / mV
with DE = 0.081 V on bare GC electrode. Figure 1 illus-
p
trates the responses of the ferri/ferrocyanide redox system
2
Fig. 2 Responses of ferri/ferrocyanide on GC-NO at different times
and concentrations
on the modified glassy carbon electrodes at E for 270 s.
p
The various diazonium layers on the glassy carbon
electrode exhibit different blocking behavior for oxidation
3
Assessment on the blocking efficiency of the grafted
layers on a glassy carbon electrode at various times was
-/4-
and reduction reactions of the Fe(CN)
redox system.
6
3
-/4-
For the same grafting conditions, the blocking effect is more
also evaluated by EIS in a 1 mM Fe(CN)
6
solution.
important for the NO -modified electrode. The absence of
2
Figure 3 presents the complex impedance plots for a bare
3
-/4-
the electrochemical response of the Fe(CN)
couple was
electrode and (4-nitrophenyl)-modified glassy carbon
-/4-
6
3
6
first attributed to the hydrophobicity of the film and later to
the change of GC electronic properties during reductive
grafting leading to a decrease of the electron transfer rate
electrodes in the presence of 1 mM Fe(CN)
at various
reduction times. The impedance plot for the bare electrode
is characterized by a semicircle at high frequency and a
low frequency Warburg line at an angle of 45° (Fig. 3a).
The impedance plots for glassy carbon electrodes modi-
fied by electrochemical reduction of a (4-nitrophenyl)
[
18]. Results from Fig. 2 and Table 1 show that the blocking
effects increase (e.g., relative electroactivity, I , decreases
rel
and surface coverage, h, increases) with increasing con-
centration and reduction time. This trend is observed for all
grafted layers and demonstrates that it is possible to obtain
relative control of the layer thickness by controlling the
diazonium reduction time [22].
diazonium salt at E at various times differ significantly
p
from that of the bare electrode. The diameter of the semi-
circle increases with increasing electrolysis time (see
Fig. 3b). The Warburg line is not observed for the modified
1
23

Electrochemical and impedance spectroscopy studies
843
Table 1 Relative electroactivity Irel (%) and surface coverage h (%) obtained for aryl-modified glassy carbon electrodes at different concen-
trations of diazonium salts
I
rel (%)
h (%)
1
0 s
30 s
90 s
270 s
10 s
30 s
90 s
270 s
GC-CF
GC-Cl (1 mM)
GC-SO H (1 mM)
GC-NO
GC-CF
3
(1 mM)
51.6
50.1
43.5
–
44.1
40.0
31.1
–
43.2
38.1
–
42.8
98.17
98.66
99.40
99.50
99.11
99.74
99.81
99.87
99.28
99.68
99.82
99.84
99.50
99.83
99.84
99.90
99.45
99.74
99.84
99.87
99.72
99.85
99.89
99.94
99.65
99.82
99.86
99.96
99.78
99.88
99.92
99.98
–
–
3
2
(1 mM)
(5 mM)
–
–
3
49.2
–
43.0
–
39.7
–
36.7
–
GC-Cl (5 mM)
GC-SO H (5 mM)
GC-NO (5 mM)
3
–
–
–
–
2
–
–
–
–
group. The impedance data are in good agreement with the
CV results of Fig. 2. To give more detailed information
about the electrode/solution interface, the Randles circuit
was chosen to fit the obtained impedance data. The electrical
equivalent circuits for bare and modified electrodes are
shown in Fig. 4. They comprise the solution resistance, Rs,
the charge transfer resistance, RCT, the double layer capac-
itance, C , capacitance of the coated layer, C , and
-
2500
A
-
-
-
2000
1500
1000
dl
c
resistance of the coated layer, Rc.
The impedance data were analyzed by using the elec-
trical equivalent circuits and the results are shown in
Table 2. The table shows that the charge transfer resistance
increases dramatically following modification of the glassy
carbon electrode from a value of 81 X for the bare elec-
-
500
0
5
0
500
1000
1500
2000
2500
trode to 3.2 9 10 X after electrochemical modification for
Z' / Ω
270 s from a 5 mM (4-nitrophenyl)diazonium at Ep.
For the same 1 mM diazonium concentration, an
-
-
-
-
-
-
-
-
800000
700000
600000
500000
400000
300000
200000
100000
0
B
increase of RCT was noticed when the electrolysis time
increased from 10 to 270 s. A similar trend is observed in
the other groups, although the amplitude of the difference
in RCT values is different. The increase of RCT with the
electrolysis time demonstrates that the film becomes more
compact as the treatment time increases.
270 s
9
0 s
Electrochemical response of redox probe on grafted
electrodes without electrochemical induction
3
0 s
1
0 s
The characterization of the modified surface proceeded
with the study of the barrier properties of chemically
3-/4-
0
100000 200000300000 400000 500000 600000 700000800000
grafted layers by using Fe(CN)6
as redox probe. The
Z' / Ω
modification of the different diazonium layers without
electrochemical induction was accomplished with the
procedure described in the ‘‘Experimental’’ section, by
immersing the sample in a 10 mM deaerated diazonium
solution for 20 h. Samples were then analyzed by CV and
EIS methods. After immersion, the reacted electrodes were
Fig. 3 Complex impedance plots for a bare (a) and a modified glassy
carbon electrode with 1 mM 4-nitrophenyl (GC-NO ) (b) in the
at various reduction times
2
3
6
-/4-
presence of 1 mM Fe(CN)
electrodes. Qualitatively, the increase of the semicircle
indicates that the electrode kinetics becomes slower as the
glassy carbon electrode is modified with a substituted phenyl
3
-/4-
transferred into a Fe(CN)
solution for electrochemical
6
analysis. Figure 5 shows a representative set of cyclic
123

8
44
M. Khoshroo, A. A. Rostami
Cdl
C
salts are uncharged, so these layers are more hydrophilic
than the (4-sulfophenyl)-derived layer. The difference
between RCT of the modified electrodes in their neutral state
is consistent with the existence of an influence of the
hydrophobic character of the grafted layers and the intensity
c
R
s
R
R
c
ct
Modified Electrode
of their blocking effect. The NO -modified GC electrode
2
C
exhibits a significant blocking effect (highest RCT value).
The layers obtained from spontaneous grafting will be of
little use if one looks for a protective barrier (against dif-
fusion of species, etc.), but they could find applications in
systems where a very compact coverage of the surface is
not necessary, such as attachment of enzymes to the sur-
face. In this case an easier electron transfer to the surface
would even be an advantage, for example, in the use of
tethered redox probes as catalysts for redox enzymes.
In conclusion, the EIS measurements reveal that the
layers obtained without electrochemical induction have a
dl
Rs
R
ct
Bare Electrode
Fig. 4 Equivalent circuits used for analysis of the electrochemical
impedance data
voltammograms for various grafted layers on a glassy
carbon electrode.
3
-/4-
lower blocking effect toward the Fe(CN)
redox
6
Differences in blocking properties were observed among
the different diazonium salts used. This points to an
influence of the substituent of the phenyl rings either on the
formation of layer or on its resulting properties. The best
results, in terms of blocking properties, were obtained with
reaction. Also, these measurements show that the grafted
layers did not afford a very compact coverage in compar-
ison with electrochemical grafted layers. However these
chemical layers could find applications in electroanalysis.
Additionally, as grafting can be limited in some parts of a
composite surface, this process could be interesting for
further industrial applications.
(
4-nitrophenyl)diazonium salt. On the contrary, a weak
modification of the electrochemical behavior of the probe
is visible when the glassy carbon electrode is exposed to
[
4-(trifluoromethyl)phenyl]diazonium solution. Results
obtained with glassy carbon electrodes chemically grafted
are also compared with the glassy carbon electrode without
electrochemical induction. Chemically grafted layers were
Experimental
Reagents
3
-/4-
efficient barriers to the Fe(CN)6
probe in solution and
they completely hinder the corresponding electron transfer
processes. The Nyquist plots obtained for the modified
electrodes are shown in Fig. 6.
Acetonitrile (ACN), tetrabutylammonium tetrafluoroborate
(TBABF ), potassium ferricyanide, potassium chloride,
4
hydrochloric acid, para-substituted anilines used for dia-
zonium salt synthesis, sodium nitrite, tetrafluoroboric acid,
4-nitrophenyldiazonium tetrafluoroborate, and 4-(trifluo-
romethyl)phenyldiazonium tetrafluoroborate were used as
received from Fluka.
A simplified representation allows the charge transfer
resistance to be determined from the diameter of the
semicircle. The charge transfer resistances of the various
modified electrodes are shown in Table 3. Impedance data
show that the charge transfer resistance increases dramati-
cally with increasing semicircle diameter. Obviously, these
data are consistent with the results obtained from CV
measurements. The layers grafted with (4-nitrophenyl)-,
Synthesis of Cl-PD and SO H-PD
3
The para-substituted aniline (17 mmol) was dissolved in
3
11.5 cm of 50% w/w aqueous tetrafluoroboric acid
(4-chlorophenyl)-, and [4-(trifluoromethyl)phenyl]diazonium
Table 2 RCT (X) obtained values for aryl-modified glassy carbon electrodes at different concentrations of diazonium salts
1
1
mM
0 s
5 mM
10 s
30 s
90 s
270 s
30 s
90 s
270 s
3
3
4
4
4
4
4
4
4
4
4
5
4
4
4
5
4
4
4
5
4
4
4
5
4
4
4
5
5
5
5
5
GC-CF
GC-Cl
3
5.9 9 10
8.1 9 10
1.1 9 10
2.2 9 10
1.4 9 10
3.1 9 10
3.5 9 10
5.2 9 10
1.7 9 10
3.8 9 10
5.6 9 10
2.1 9 10
3.35 9 10
6.1 9 10
8.1 9 10
2.8 9 10
2.1 9 10
4.2 9 10
5.8 9 10
1.0 9 10
3.9 9 10
6.6 9 10
7.0 9 10
2.0 9 10
4.2 9 10
7.0 9 10
8.1 9 10
2.6 9 10
1.3 9 10
2.5 9 10
2.8 9 10
3.2 9 10
GC-SO
3
H
GC-NO
2
1
23

Electrochemical and impedance spectroscopy studies
845
2
1
1
00
50
00
Table 3 DEp (mV), Irel (%), h (%), and RCT (X) values for aryl-
modified glassy carbon electrodes without electrochemical induction
GC-NO2
a
p
DE
(mV)
I
rel (%)
h (%)
RCT (X)
GC-CF3
GC-Bare
GC-Cl
4
4
4
4
GC-CF
GC-Cl
3
672
841
887
44.1
41.2
33.2
–
99.20
99.61
99.65
99.78
c
1.5 9 10
2.8 9 10
3.1 9 10
5.0 9 10
5
0
3
GC-SO H
GC-NO
2
Not observable
0
a
a
p p p
DE is the peak potential separation, i.e., E - E (where a = anode,
GC-SO3H
-
50
c = cathode)
-
-
100
150
potentials were reported versus the reference electrode of
Ag/AgCl (saturated KCl). The glassy carbon electrode
surface was cleaned by polishing with 0.05-lm alumina
slurry successively. After each polishing, the electrode was
carefully washed with distilled water twice. Then, elec-
trochemical modification of glassy carbon electrode was
900
400
-100
-600
E / mV
Fig. 5 Responses of ferri/ferrocyanide on the modified glassy carbon
electrodes after immersing in a 10 mM deaerated diazonium solution
for 20 h
carried out in acetonitrile containing 0.1 M TBABF and 1
4
or 5 mM diazonium salts for various times (10, 30, 90, and
270 s). For all films, the value of modification potential
(Eapp) was determined from the respective diazonium salt
solution. After the solution was cooled to -3 °C, 2.4 g
(
34 mmol) of NaNO , dissolved in cold water, was slowly
2
red
added to the mixture [23]. The solution was filtered and
then cooled below -3 °C to favor crystallization and again
filtered by suction, washed with ice water and cold ether,
and finally dried under vacuum. The synthesized diazo-
nium salts were kept in a desiccator at 3 °C, and the
presence of diazonium functional group was verified by the
observation of the characteristic absorption band at about
voltammetric reduction peak potential (Ep ). The sponta-
neous grafting consists in dipping cleaned electrodes in
10 mM diazonium salts in ACN solution over a period of
20 h. After immersion, substrates were rinsed with distilled
water. The solution containing electroactive probe was
ferricyanide/ferrocyanide (1 mM, 0.1 M KCl, adjusted at
pH 7 with a 1 M KOH solution) for the various films. The
relative electroactivity of the modified electrode, which is
inversely related to its blocking properties, was quantified
by CV in the presence of redox probe species. The
parameter of Irel is defined as:
-
1
2
,290 cm in the IR spectra [24].
Electrode preparation and modification
The working electrode was a glassy carbon (GC) electrode
and a platinum wire was utilized as counter electrode. All
Irelð%Þ ¼ Ipa with a film=Ipa for a bare electrode ꢁ 100
ð1Þ
where Ipa is the intensity of the anodic peak current. Also,
-
-
-
-
-
-
-
-
80000
70000
60000
50000
40000
30000
20000
10000
0
GC-NO2
the increase in the charge transfer resistance can be related
to the electrode coverage and is given by the following
equation [25]:
o
CT
ð1 ꢂ hÞ ¼ R =RCT
ð2Þ
where h is the apparent electrode coverage, assuming that
all currents are passed via bare spots on the electrode, and
GC-SO3H
ꢃ
CT
R
and RCT represent the charge transfer resistance
measured on a bare and on a modified glassy carbon
electrode, respectively.
GC-Cl
GC-CF3
Instrumentation and procedure
-
10000 10000 30000 50000 70000 90000 110000 130000 150000
Z' / Ω
Electrochemical measurements were performed in a one-
compartment cell with a three electrode configuration.
Cyclic voltammetry (CV) was carried out with use of a
potentiostat/galvanostat (EG&G Model 263 A, USA), with
Fig. 6 Complex impedance plots for various modified glassy carbon
after immersing in a
3
6
-/4-
electrodes in the presence of 1 mM Fe(CN)
0 mM deaerated diazonium solution for 20 h
1
123

8
46
M. Khoshroo, A. A. Rostami
a PC and electrochemical setup that was controlled with M
70 software. Electrochemical impedance spectroscopy
9. Brooksby PA, Downard AJ (2005) J Phys Chem B 109:8791
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1
1
2
(
(
EIS) was performed with a frequency response detector
EG&G Model 1025, USA), with a PC and electrochemical
setup that was controlled with M 398 software. An ac
voltage of 5 mV in amplitude using a frequency range from
1
1
1
5
0 MHz to 65 kHz was superimposed on the dc potential
and applied to the studied electrodes. The dc potential was
-/4-
16. Taleat Z, Ardakani MM, Naeimi H, Beitollahi H, Mand N, Zare
HR (2008) Anal Sci 24:1039
3
always set up at the formal potential of Fe(CN)6
.
1
1
1
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Vautrin-Ul C (2005) Chem Mater 17:491
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Experimental data of the electrochemical impedance plot
were analyzed by using Boukamp software. The error for
the all simulated quantities was less than 5%.
1
0. Bahr JL, Tour JM (2001) Chem Mater 13:3523
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