American Journal of Anal yt ical Chemistry, 2010, 1, 102-112
doi:10.4236/ajac.2010.13014 Published Online November 2010 (
Copyright © 2010 SciRes. AJAC
Electrochemical Synthesis of Three-Dimensional
Polyaniline Network on 3-Aminobenzenesulfonic Acid
Functionalized Glassy Carbon Electrode and Its
Lei Zhang*, Qiuhua Lang, Zhige Shi
Department of Chemistry, College of Life and Environment Sciences, Shanghai Normal University,
Shanghai, China
Received July 24, 2010; revised August 2, 2010; accepted August 23, 2010
The electrochemical synthesis of three-dimensional (3D) polyaniline (PAN) network structure on
3-aminobenzenesulfonic acid (ABSA) functionalized glassy carbon electrode (GCE) and its electro-catalytic
oxidation towards ascorbic acid (AA) had been studied. ABSA was first covalently grafted on GCE surface
via the direct electrochemical oxidation of ABSA on GCE, which was followed by the electrochemical po-
lymerization of aniline on the ABSA functionalized GCE. Then PAN-ABSA composite film modified GCE
(PAN-ABSA/GCE) was obtained. Scanning electron microscope (SEM), X-ray photoelectron spectroscopy
(XPS), electrochemical impedance spectroscopy (EIS) and electrochemical techniques had been employed to
characterize the obtained electrodes. Due to the effective doping of ABSA in PAN, the redox electro-activity
of PAN had been extended to neutral and even the basic media, thus, the PAN-ABSA composite film modi-
fied GCE could be used for electro-catalytic oxidation of AA in 0.1 M phosphate buffer solution (PBS, pH
6.8). At PAN-ABSA/GCE the oxidation over-potential of AA shifted from 0.39 V at GCE to 0.17 V with a
greatly enhanced current response. The electro-catalytic oxidation peak current of AA increased linearly with
the increasing AA concentration over the range of 5.00 × 10-4-1.65 × 10-2 M with a correlation coefficient of
0.9973. The detection limit (S/N = 3) for AA was 1.16 × 10-6 M. Chronoamperometry had also been em-
ployed to investigate the electro-catalytic oxidation of AA at PAN-ABSA/GCE. The modified electrode had
been used for detecting AA in real samples with satisfactory results.
Keywords: Polyaniline, 3-Aminobenzenesulfonic Acid, Ascorbic Acid, Aniline, Electrochemical
1. Introduction
As a conducting polymer, PAN has received much inter-
est due to its facile synthesis, good environmental stabil-
ity, ease of conductivity control by changing the oxida-
tion and protonation states and inexpensive monomers
[1-4]. These properties make it attractive for certain ap-
plications, such as energy conversion, electrochromic
devices, sensors, corrosion prevention [5-7] and electro-
catalysis [8-11]. However, when medium pH value is
more than 4, PAN exhibits low conductivity and poor
redox activity. It is clear that the influence of medium pH
on the conductivity and redox activity of PAN is very
important. The potential range of the electro-activity for
PAN decreases with increasing pH value [12], and its
redox peaks disappear in the cyclic voltammetry when
medium pH 5. Thus, in general, the redox potential of
species to be oxidized and reduced by PAN is limited
within the potential range in which PAN itself is elec-
trochemical active; this restricts greatly its application in
other fields, such as bioelectrochemistry, which normally
requires a neutral pH environment. To extend its applica-
tion in solutions with higher pH values, three kinds of
methods have been explored and used widely: the first
way of improving the pH dependence of PAN reactivity
was performed by using the sulfonation of PAN with
fuming sulfuric acid treatment to prepare the self-doped
PAN, whose conductivity is independent of pH in the
aqueous acid solutions of pH 7.5 [13,14]; the second
way of extending the pH dependence of PAN elec-
tro-reactivity is to perform the polymerization of aniline
in the presence of other organic acids, such as camphor-
sulphonic acid (CSA) [15], -naphthalenesulfonic acid
(NSA) [16], 5-sul-phosalicylic acid (SPA) [17], dode-
cylbenzenesulfonic acid (DBSA) and p-toluenesulfonic
acid (p-TSA) [18]; and the third method is via the syn-
thesis of self-doped PAN by electrochemical or chemical
copolymerization of aniline and its derivatives (ortho-,
meta-substituted aniline) bearing ionogenic functional-
ities, such as sulphonic [19-21], carboxylic acids [19,22]
and hydroxyl groups [23].
The above sulfonated PAN and the copolymers exhibit
good redox activity in neutral and even alkaline aqueous
solutions. However, to study the electrochemical proper-
ties of the conducting polymer synthesized chemically,
they usually were first dissolved in organic solvents and
then the solution was coated on an electrode substrate
followed by evaporation. This procedure for coating film
is not only quite inconvenient and the obtained film is
easy to fall off from the electrode surface, but also the
dopants, such as the organic acids embedded in PAN,
can leak out from the PAN film when continuous poten-
tial scanning is performed, this phenomenon also hap-
pens for the doped PAN film synthesized by electro-
chemical procedure. Furthermore, the copolymerization
of aniline and its derivatives presents lower polymeriza-
tion rates than that for aniline polymerization, depending
on the monomers concentration ratio in the polymeriza-
tion solution; and the substituted anilines make it diffi-
cult for the polymerization of aniline [20,23].
To overcome these problems, in this study, ABSA
molecule monolayer was first covalently grafted on GCE
surface by carbon-nitrogen bond to form the ABSA
functionalized glassy carbon electrode (ABSA/GCE) via
electrochemical oxidation, then the direct electropoly-
merization of aniline was carried out on ABSA/GCE to
form the 3D PAN networks structure modified GCE
(PAN-ABSA/GCE). Due to the covalent binding of
ABSA on GCE and the formation of PAN-ABSA com-
posite film, it is difficult for the PAN film to fall off from
electrode surface and thus improve the stability of the
modified electrode. The study showed that the PAN
doped by the sulfonic acid functionalities in ABSA
molecules exhibited greatly improved electrochemical
activity in neutral and even up to the basic medium, and
thus the PAN-ABSA/GCE can be used for the elec-
tro-catalytic oxidation of AA in pH 6.8 PBS. It is also
intended to evaluate the diffusion coefficient of AA and
rate constant for electro-oxidation reaction of AA at
PAN-ABSA/GCE by using chronoamperometric tech-
2. Experimental
2.1. Chemicals and Solutions
Aniline (Sigma) was purified by distillation with zinc
dust under vacuum and stored at 4 in dark. ABSA
(Sigma) and acetonitrile (ACN, Shanghai Chemical Re-
agent Company) were used as received. AA (Fluka
Chemicals Co.) solution was prepared immediately prior
to use in 0.1 M phosphate buffer solution (PBS, pH 6.8).
Sulfuric acid solutions (pH 1-3), 0.1 M acetate (pH 4-5),
0.1 M PBS (pH 6-8) and 0.1 M borate (pH 9) buffers
were used. Other reagents were all of analytical grade
and used as received. Twice distilled water was used for
the preparation of all the above solutions. The solutions
were thoroughly deoxygenated by bubbling highly puri-
fied nitrogen and a nitrogen atmosphere was maintained
over the solutions. All experiments were carried out at
room temperature ( 21).
2.2. Apparatus
The morphology of the polymer film was characterized
using a JXA 840 field emission scanning electron mi-
croscope (JEOL, Japan). X-ray photo-electron spectros-
copy (XPS) was recorded on an ESCALAB-MK spec-
trometer (VG Co., UK).
Electrochemical measurements were performed on a
CHI 660C electrochemical workstation (CH Instruments,
USA) with a three-electrode electrochemical cell. Work-
ing electrodes were glassy carbon electrode (Ø = 3 mm)
and PAN-ABSA/GCE. The reference electrode was a
saturated calomel electrode (SCE) and all potentials re-
ported herein were referred to the SCE. A platinum sheet
(1 cm × 0.5 cm) served as auxiliary electrode.
EIS measurements were performed in 0.1 M KNO3 so-
lution containing 1 mM K3Fe(CN)6 and 1 mM K4Fe(CN)6
using an alternating current voltage of 5 mV. Impedance
measurements were performed at an open potential in the
frequency range from 0.1 to 100,000 Hz.
2.3. Preparation of the Modified Electrode
GCE electrodes were hand-polished for 3 min on a wet
soft polishing cloth with alumina powder (0.5 and 0.03 μm,
successively). The alumina powder was kept wet with
twice distilled water during polishing. The electrodes
were rinsed with water between each step and at the end
Copyright © 2010 SciRes. AJAC
of polishing. After sonicating in absolute ethanol and
water for 2 min successively, the mirror-like GCE was
dried with a fluid of highly purified nitrogen. The dried
GCE was then treated with cyclic scanning in the poten-
tial range of 0.6-1.5 V at 0.06 V/s for 10 scans in ACN
solution containing 0.001 M ABSA and 0.1 M NaClO4.
To remove any physisorbed, unreacted materials from
electrode surface, the electrode was rinsed carefully with
ethanol and water and was sonicated for 2 min in pH 6.8
PBS. The ABSA-modified GCE (ABSA/GCE) was ob-
Then ABSA/GCE was immersed in 1.0 M HClO4 so-
lution containing 0.1 M aniline and treated by poten-
tial-cycling between –0.2 and 0.85 V at 0.02 V/s for 50
cycles. After rinsing with water, the electrode was cycled
again as above for 3 scans to further polymerize the ani-
line monomer absorbed on/inside the film in 1.0 M
HClO4 solution. Then PAN-ABSA composite film modi-
fied GCE (PAN-ABSA/GCE) was obtained and stored in
0.1 M pH 6.8 PBS at 4 for use.
3. Results and Discussion
3.1. Electrode Preparation
3.1.1. Preparation of GCE
Previous workers have reported that the pre-treated
GCEs by using chemical and/or electrochemical proce-
dures exhibit somewhat electro-catalytic effect towards
the redox of some electro-active substances, and indi-
cated that this catalytic property has been resulted from
the generation of oxygen-containing functional groups
[11,12,24,25]. Therefore, prior to modification, GCE was
usually pre-treated via electrochemical and/or chemical
oxidation in strong acid medium, such as H2SO4, HClO4.
This process can produce oxygen-containing functional-
ities, such as, carbonyl, carboxylate, quinoid, and hy-
droxyl radical species, etc [11], on GCE surface and even
inside the carbon substrate. In this study, we aimed to
investigate the electrochemical activity and electrocata-
lysis of PAN doped by sulphonic acids groups in ABSA
molecules, thus it’s necessary to avoid the influence of
these oxygen-containing functional groups on GCE sur-
face. Therefore, the GCE used should not be activated by
chemical or electrochemical methods (potential scan or
polarized with positive potential) in strong acid medium.
And it’s also noted that, to avoid the exposing of GCE in
air and thus the possible oxidation of carbon, after pol-
ishing and washing, the GCE should immediately be
dried with a fluid of highly purified nitrogen, and im-
mersed in ABSA solution for grafting.
3.1.2. Coval e n t Gr af ting of ABSA on GCE
Figure 1(A) shows the cyclic voltammograms (CVs) of
Figure 1. (A) CVs of GCE in ACN containing 0.001 M
ABSA; (B) CVs of ABSA/GCE in ACN. Supporting elec-
trolyte: 0.1 M NaClO4, Scan rate: 0.06 V/s.
GCE in ACN solution containing 0.001 M ABSA and
0.1 M NaClO4 as supporting electrolyte. It can be seen
from Figure 1(A) that there is a broad, irreversible an-
odic peak at 1.28 V in the first cycle, and no cathodic
peak is observed on the reverse scan, this indicates that
the species obtained after the first electron transfer un-
dergoes a chemical reaction. During this process, the
amino group in ABSA was first oxidized electrochemi-
cally to turn into its corresponding cation radical via a
one-electron oxidation; then, these cation radicals form
C-N covalent bonds at carbon electrode surface [26,27].
In the next 4 scans, the oxidation peak shows a negative
shift from cycle to cycle with a quickly decreased current
response. While from the sixth potential scan, the anodic
peak potential keeps unchanged, the current response
decreases somewhat till the tenth scan. The gradual de-
Copyright © 2010 SciRes. AJAC
crease of the oxidation current is ascribed to the passiva-
tion of the carbon electrode; this passivation is related to
the grafting of ABSA onto GCE. This binding process is
almost complete after 10 cycles. As a primary amine-
containing compound, the above electrochemical-che-
mical (EC) reaction for GCE modification can be pro-
posed as Scheme 1.
To investigate the binding of ABSA on the surface of
GCE, XPS analysis was used to characterize the fabri-
cated ABSA/GCE (Figure 2). It can be seen from Fig-
ure 2 that, the maximal peak at 400.2 eV is assigned to
the characteristic peak of N(1s) environment, which is
also consistent with the formation of C-N bond between
the amine cation radical and the aromatic moiety of GCE
surface [26,27]; and the peak at 163.3 eV shows the
presence of S(2p). This verifies the attachment of ABSA
on GCE surface.
Figure 2. XPS spectra for ABSA/GCE showing (A) the N(1s)
peak and (B) the S(2p) peak.
Scheme 1
Cyclic voltammetry can also be used to confirm the
modification of ABSA on GCE: once ABSA has been
bonded on GCE surface, its reduction should be observed.
After being washed and ultrasonicated carefully, the
ABSA/GCE was put into ACN solution containing 0.1 M
NaClO4 as supporting electrolyte for potential scan
(Figure 1(B)). As can be seen, the ABSA/GCE presents
a well-defined reduction wave at –0.99 V, which corre-
sponds to the reduction of the amine groups attached on
GCE surface. However, with continuous potential scan
the reduction peak current diminishes gradually till up to
the forth cycle, which may be due to the re-protonation
of the radical anion. And based on the area of the reduc-
tion peak the number of molecules bonded on GCE can
be deduced. The surface coverage (Γ) of ABSA on GCE
surface can be estimated by integrating the reduction
peak of the CV according to Q = nFAΓ, where Q is the
charge involved in the reaction, n is the number of elec-
tron transferred, F is the Faraday’s constant, and A is the
geometric area of GCE. This procedure generates the
surface coverage of 3.7 × 10-12 mol/cm, indicating the
monolayer coverage of ABSA on GCE [28].
3.2. Morphology of the Polymer Film
The polymerization of aniline was performed by poten-
tial-cycling between –0.2 and 0.85 V at 0.02 V/s for 50
cycles in which the potential was cycled to positive limit
of 0.85 V. This potential is just positive of the onset for
aniline oxidation, and the lower scan rate which leads to
the gentle formation of aniline cation radicals, ensures
the slow deposition/self-assembly of PAN upon repeated
potential cycling. The obtained PAN-ABSA composite
film by the above procedure appears to be dark green and
uniform by visual inspection. SEM was used to investi-
gate the morphology of the composite film on GCE sur-
face (Figure 3). It can be seen from Figure 3 that, a
non-periodic three-dimensional networks structure is
obtained. The diameter of the PAN fibril is in the range
of 10-300 nm, and the average distance between the
contact points is more than 300 nm. The detailed mecha-
nism for the formation of the three dimensional networks
is not clear now. However, it can be assumed that the
polymerization occurs at the pinholes already present in
the ABSA monolayer, or the monomer partitions into the
ABSA monolayer where it undergoes oxidation. That is,
Copyright © 2010 SciRes. AJAC
Copyright © 2010 SciRes. AJAC
3.3. Electrochemical Impedance Spectroscopy
EIS can give information about the impedance changes
of the electrode surface with different modifiers. EIS
include a semicircle part and a linear part, the semicircle
part at high frequencies corresponding to the electron
transfer limited process and the linear part at low fre-
quencies to the diffusion process. Figure 4 shows the
EIS results for GCE, ABSA/GCE, and PAN-ABSA/GCE
at open circuit. To understand clearly the electrical prop-
erties of the electrodes/solution interfaces, the Randle’s
equivalent circuit (inset of Figure 4, top) was chosen to
fit the obtained impedance data [29]. In Randle’s circuit,
it was assumed that the resistance to charge transfer (Rct)
and the diffusion impedance (W) were both in parallel to
the interfacial capacity (Cdl). This parallel combination
of Rct and Cdl gives rise to a semicircle in the complex
plane plot of Z" against Z, the semicircle diameter equals
the charge transfer resistance (Rct). This resistance exhib-
its the electron transfer kinetics of the redox probe at
electrode interface. It can be seen from the inset of Fig-
ure 4 (bottom, curve a) that Fe(CN)6
3-/4- exhibits a very
low charge transfer resistance at GCE. After modifying
Figure 3. SEM image of the 3D PAN networks.
aniline monomers were firstly oxidized to their cation
radicals, and assuming a distribution of isolated sites,
these cation radicals act as “nuclear sites” to direct the
“linear growth/polymerization” of other aniline mono-
mers in the same direction, and thus the bamboo-like
fibrils were formed. With continuous increasing of the
bamboo-like fibrils in length, these PAN fibrils must
grow out from the ABSA monolayer and the adjacent
PAN fibrils may grow to some “maximal length” and
then tend/happen to contact together to form the stable
star-like structure. The star-like wires tend to connect
together by dendrites to form two-dimensional and fur-
ther the three-dimensional networks.
with ABSA, the Rct increases dramatically to about
14300 (Figure 4, curve b), indicating that ABSA
monolayer film hinders the charge transfer. While, when
PAN was polymerized on ABSA/GCE surface, the Rct
decreases to about 1350 (inset of Figure 4, top, curve
c), which indicates that PAN plays a role similar to a
Figure 4. EIS plots of GCE (curve a), ABSA/GCE (curve b), and PAN-ABSA/GCE (curve c) in the presence of 1.0 mM
Fe(CN)63-/4- containing 0.1 M KNO3. Frequency range is from 0.1 Hz to 100 KHz. Inset (top, right) is the Randle’s equivalent
ircuit. c
Copyright © 2010 SciRes. AJAC
conductive wire or electron-conducting tunnel and thus
makes electron transfer easier [30,31]. The differences of
electrodes indicate that PAN-ABSA composite film has
been effectively attached on GCE surface.
3.4. Redox Electroactivity of PAN-ABSA/GCE
To verify the improved electro-activity of the PAN doped
by the sulfonic acid functionalities in ABSA molecules,
the CVs of PAN-ABSA/GCE in solutions with different
pH values have been performed (Figure 5). It can be
seen from Figure 5 that PAN shows three pairs of sepa-
rate redox peaks in strong acid medium (pH 1). Among
these redox waves, the first sets of peaks located at 0.15
V is assigned to the transformations of leucoemeraldine
to emeraldine salt, and the second pair of redox waves
around 0.78 V is due to the transition from emeralding
salt to the pernigraniline state. The third pair of peaks in
middle is attributed to the defects in the linear structure
of the polymer [32], this pair of peaks almost disappear
when the pH values of solutions increase up to 3. When
medium pH 5, the two sets of peaks overlap into one
pair of peaks, which corresponds to the reaction between
leucoemeraldine and pernigraniline. This is due to the pH
dependence of the reaction between emeraldine and per-
nigraniline. The redox electroactivity of ABSA-doped
PAN still remains at pH 9, indicating the remarkable
extension for the electro-activity of PAN. This im-
provement is due to the incorporation of organic acid
dopant ABSA into PAN film deposited onto the elec-
trode surface. The presence of sulfonic acid groups can
change the micro-environment of PAN backbone: it can
be assumed that when there is an increase in medium pH,
the –H+ ion in –SO3H groups can take a role of “buffer”
Figure 5. CVs of PAN-ABSA/GCE in buffer solutions with
different pH values. Scan rate: 0.05 V/s.
and maintain the local pH value to some extent in PAN
film near the electrode surface. Thus, the PAN doped by
ABSA can still exhibit good electrochemical activity in
solution with higher pH value.
3.5. Electro-Oxidation of AA at PAN-ABSA/GCE
Figure 6(A) shows the CVs of GCE (a), PAN-ABSA/GCE
(b) and ABSA/GCE (c) in 0.1 M PBS pH 6.8 containing
5.0 × 10-3 M AA, respectively. It can be seen from Figure
6(A) that the oxidation peak of AA is broad, irreproduci-
ble at 0.39 V with Ep Ep/2 = 0.16 V at GCE (curve a), and
Figure 6. (A) (a) CVs of GCE, (b) PAN–ABSA/GCE and (c)
ABSA/GCE in 0.1 M PBS pH 6.8 containing 5.00 × 10-3 M
AA; (B) CVs of PAN-ABSA/GCE in 0.1 M PBS pH 6.8 con-
taining different concentrations of AA. AA contents from a
to i are: 5.00 × 10-4, 2.50 × 10-3, 4.50 × 10-3, 6.50 × 10-3, 8.50 ×
10-3, 1.05 × 10-2, 1.25 × 10-2, 1.45 × 10-2, and 1.65 × 10-2 M,
respectively. Scan rate: 0.06 V/s. Inset of (B) shows the cali-
bration plot of ip versus cAA.
the oxidation peak current is about 46 μA. While, the
oxidation current ( 65 μA) increases greatly and the
peak potential shifts negatively to 0.17 V with EpEp/2 =
0.046 V at PAN-ABSA/GCE (curve b). The obviously
increased peak current and the decrease in oxidation
overpotential of 0.22 V for AA indicate the effective
electro-catalytic function of PAN-ABSA/GCE towards
the oxidation of AA. The shift in the anodic overpoten-
tial is due to a kinetics effect, thus a substantial increase
in the rate of electron transfer from AA is observed,
which indicates the improvement in the reversibility of
the electron transfer processes. And also, as a compari-
son, the electrochemical behavior of AA at ABSA/GCE
has been investigated (curve c), it can be seen from curve
c that the oxidation peak potential of AA has a slightly
negative shift (0.35 V) and the oxidation peak current is
nearly unchanged (47 μA) compared with that at GCE.
This indicates that the obviously electrocatalytic oxida-
tion of AA is mainly due to the doped PAN by ABSA
with sulfonic acid functionalities.
Figure 6(B) shows the CVs of PAN-ABSA/GCE in
0.1 M PBS pH 6.8 containing different concentrations of
AA. It can be seen that the oxidation peak current in-
creases with increasing of AA concentration in the solu-
tion. The inset of Figure 6(B) shows that the anode peak
current is linearly dependent on the AA concentration
from 5.00 × 10-4 M to 1.65 × 10-2 M, the equation of lin-
ear regression is Ipa (μA) = 20.2 + 0.62cAA with a correla-
tion coefficient of 0.9973.
To further investigate the transport characteristics of
AA at the modified electrode, the CVs of PAN-ABSA/GCE
in 3.5 × 10-3 M AA at different scan rates have been
shown in Figure 7(A). It can be seen from Figure 7(A)
that the oxidation peak potential of AA shifts to more
positive with increasing scan rate; this indicates that
there is a kinetics limitation in the reaction between the
redox sites of PAN-ABSA composite film and AA.
However, the voltammetric peak currents for the cata-
lytic oxidation of AA at PAN-ABSA/GCE are propor-
tional to the square root of scan rates in the range of
50-900 mV/s (Figure 7(A), inset), indicating that the
electrode reaction is a diffusion-controlled process.
Moreover, the plot of the scan rate-normalized current
(I/υ1/2) versus scan rate (Figure 7(B)) shows the typical
shape of an electrochemical-chemical (EC) catalytic
3.6. Chronoamperometric Measurements
To investigate the electrode process of AA at PAN-
ABSA/GCE, chronoamperometric measurements for AA
with different concentrations at PAN-ABSA/GCE by
setting the working electrode potential at 0.70 V have
Figure 7. ( A) CVs of PAN-ABSA/G CE in 0.1 M PBS pH 6. 8
containing 3.50 × 10-3 M AA at different scan rates. Scan
rates from a to j are 50, 100, 200, 300, 400, 500, 600, 700,
800 and 900 mV/s, respectively. Inset shows the calibration
plot of Ip versus
1/2; (B) Plot of the anodic current function
1/2) versus scan rate (
been shown in Figure 8(A), the inset of Figure 8(A)
shows the current responses of different concentrations
of AA at fixed time of 8, 16 and 24 s, respectively. It can
be seen from plots (a), (b) and (c) that the slopes of the
calibrations decrease with the increasing time elapsed
after potential step application. However, there is an al-
most similar intersection between the currents measured
at different time and AA concentrations. The typical I-t
curve in Figure 8(A) indicates that the currents observed
should be controlled by the diffusion of AA in solution.
Thus, the current corresponding to the electrochemical
reaction obeys Cottrell’s law [33]:
1/ 21/ 21/ 2
nFAD ct
Copyright © 2010 SciRes. AJAC
Figure 8. (A) Chronoamperograms of PAN-ABSA/GCE in
solutions containing different concentrations of AA. AA
contents from a to g are 0, 5.00 × 10-4, 2.50 × 10-3, 4.50 × 10-3,
6.50 × 10-3, 8.50 × 10-3, and 1.05 × 10-2 M, respectively . Inset
shows the dependenc e of the fixed-time current observed at
(a) 8 s, (b) 16 s and (c) 24 s, respectively; (B) Dependence of
Icat/IL on the t1/2 derived from the data of chronoampero-
grams of curve a and c in (A).
where D and c0 are the diffusion coefficient (cm2/s) and
bulk concentration (mol/cm3), respectively. Based on (1),
the plot of I versus t1/2 is a straight line, and the slope of
such lines can be used to estimate the diffusion coeffi-
cient of AA. The mean value of D is found to be 4.7 ×
10-7 cm2/s.
Chronoamperometry can also be employed to evaluate
the catalytic rate constant for the reaction between AA
and the redox sites of surface confined PAN film ac-
cording to the method [34]:
1/2 1/21/21/2
cat Lerf expII
 
where Icat is catalytic current of AA at PAN-ABSA/GCE,
IL is the limiting current in the absence of AA, and γ =
kc0 t (c0 is the bulk concentration of AA) is the argument
of the error function. When γ exceeds 2, the error func-
tion is almost equal to 1 and therefore the above equation
can be reduced to:
1/2 1/21/2
cat L0
 
ct (3)
where t is the time elapsed (s). Based on the slope of the
Icat/IL versus t1/2 plot, k can be obtained for a given AA
concentration. One such plot is shown in Figure 8(B)
constructed from the chronoamperogram of PAN-ABSA/
GCE in the absence and presence of 2.50×10-3 M AA, the
catalytic rate constant (k) can thus be estimated to be
7.42 × 105 cm3/(mol s) based on this plot’s slop. Ac-
cording to this method, the corresponding k values can
also be obtained for AA with different concentrations.
The mean value of k in AA concentration range of 5.00 ×
10-4-1.05 × 10-2 M is 7.51 × 105 cm3/(mol s).
3.7. Analytical Characterization
The present doped-PAN modified GCE is useful in
sensing AA in real samples. A tablet of vitamin c (10 mg
per tablet) was first dissolved in 40 mL 0.1 M PBS pH
6.8, after filtration the AA solution was transferred into
100 mL volumetric flask and diluted to volume with 0.1
M PBS pH 6.8, thus a AA solution with content of 5.68 ×
10-4 M (100 μg/mL) was obtained. Then 4.0 mL of the
sample solution was placed into the electrochemical cell
and the concentration of AA was detected using the cali-
bration method. The results are listed in Table 1. To as-
certain the validity of the proposed procedure based on
the PAN-ABSA/GCE, the samples were spiked with
certain amounts of standard AA solution (0.05 M) and
then the total amount of AA was measured (Table 1).
The recovery rates of the spiked samples were estimated
to be between 94.3% and 97.0%.
3.8. Interference Study
The influence of some foreign species on the detection of
5.0 × 10-4 M AA has been investigated. The tolerance
limit was taken as the maximum concentration of the
foreign substances, which caused an approximately ± 5%
relative error in the determination. The tolerated concen-
tration of some foreign substances was 0.1 M for Na+,
Cl-1 and K+; 0.05 M for Mg2+ and Ca2+; 0.005 M for
L-lysine, glucose, glycin, L-asparagine and glutamic acid.
However, the presence of NADH, dopamine and uric
acid will interfere with the sensing of AA. This is because
Copyright © 2010 SciRes. AJAC
Table 1. Determination and recoveries of AA in Vitamin C
1 400 391.2 88.1 471.6 96.6
2 400 389.8 132.2 516.1 97.0
3 400 388.7 176.2 549.8 95.4
4 400 390.6 220.3 585.4 94.3
Average 390.1 95.8
athe mean value of 5 determination results.
that the doped-PAN modified GCE also shows elec-
tro-catalytic activity towards the electro-oxidation of
these species. The details about the electro-oxidation of
NADH, dopamine and uric acid at PAN-ABSA/GCE are
under investigation.
3.9. Electrode Stability, Reproducibility and
Detection Limit
The long-term stability of the modified electrode has been
checked by measuring the current response for detection
of 5.00 × 10-4 M AA from day to day during a storage in
0.1 M PBS pH 6.8 at 4. During the first day the current
response had no apparent change and in the next five days
the signal decreased about 7% of its initial response, and
12% for one month. The current values used for evaluating
the stability of PAN-ABSA/GCE are the averages of 5
successive measurements of 5.00 × 10-4 M AA.
Repetitive measurements have been carried out in so-
lutions containing 5.0 × 10-4 M AA to characterize the
reproducibility of PAN-ABSA/GCE. The results of 11
successive measurements showed a relative standard
deviation of 2.7% for AA, indicating that the modified
electrode was not subject to surface fouling by the oxida-
tion products of AA. The fabrication reproducibility of
five electrodes, prepared independently, shows an ac-
ceptable extent with a relative standard deviation of 3.4%
for the determination of 5.0 × 10-4 M AA.
To evaluate the detection limit of the proposed ana-
lytical procedure, the responses of 11 reagent blank sam-
ples have been measured; the standard derivation (δ) was
calculated to be 2.4 × 10-3. Based on the slope (S) of the
working curve of 6.2 μA/mM, the detection limit (3δ/S)
can be estimated to be 1.16 μM.
4. Conclusions
This study reported the electrochemical synthesis of 3D
PAN networks on ABSA functionalized GCE and its
electro-catalytic oxidation towards AA. XPS, SEM, EIS
and electrochemical techniques have been employed to
characterize the obtained electrodes. The effective dop-
ing of PAN by sulfonic functionalities in ABSA mole-
cules changes the microenvironment of PAN backbone
structure and maintains the local pH at a lower value.
Therefore, the electrochemical activity of PAN was ex-
tended to neutral medium and even to basic media. In pH
6.8 PBS, PAN-ABSA/GCE shows good electro-catalytic
activity towards the oxidation of AA via a surface
layer-mediated charge transfer. Due to the covalent
grafting of ABSA layer on GCE surface, the PAN-ABSA
composite film exhibits good stability and reproducibil-
ity. The proposed procedure can be used for the deter-
mination of AA in real sample with satisfactory results.
5. Acknowledgements
This work was supported by the Innovation Program of
Shanghai Municipal Education Commission (09YZ161)
and the Natural Science Foundation of Shanghai
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