American Journal of Anal yt ical Chemistry, 2011, 2, 582-588
doi:10.4236/ajac.2011.25066 Published Online September 2011 (http://www.SciRP.org/journal/ajac)
Selective Detection of Dopamine in the Presence of
Ascorbic Acid at Poly (m-Aminobenzene Sulfonic Acid)
Gamze Erdoğdu1,*, Mehmet Mutluhan Mutlu2
1Department of Chemistry, Faculty of Arts and Sciences, İnönü University, Malatya, Turkey
2Department of Chemistry, Faculty of Arts and Sciences, Harran University, Şanlıurfa, Turkey
Received August 4, 2010; revised October 26, 2010; accepted May 1, 2011
Abstract
Poly (m-aminobenzene sulfonic acid, m-ABSA) films were electrochemically prepared by cyclic voltam-
metry (CV) in 0.1 mol L–1 KCl solution. The dopamine (DA) selectivity of polymeric electrodes prepared at
the different thicknesses was examined in the presence of ascorbic acid (AA). The results showed that the
modified electrode showed an excellent electrocatalytical effect towards oxidation of dopamine (DA) and
ascorbic acid (AA). Electrostatic interaction between the negatively charged poly(m-ABSA) film and either
cationic DA species or anionic AA species favorably contributed to the redox response of DA and AA.
Moreover, the regular and repetitive responses for dopamine were obtained even in the presence of the some
interfering substances such as ascorbic acid, NaCl, NaClO4, Na2SO4 ,NaNO3 and KCl.
Keywords: Poly (m-Aminobenzene Sulfonic Acid), Dopamine, Ascorbic Acid
1. Introduction
Among the catecholamines, DA has attracted most inter-
est, because the change in DA levels has proved to be a
very effective route toward understanding brain func-
tions, such as learning and memory formation, and
physiological and pathological process of Parkinson’s
disease [1,2]. All attempts to measure neurotransmitters,
particularly DA, in the brain with voltammetric proce-
dures require the use of several strategies to improve the
qualitative and quantitative aspects of these measure-
ments. The main problem associated with in vivo meas-
uring of levels of this monoamine is the very low DA
concentration (10–8 - 10–6 M) and the large excesses of
interfering species such as ascorbic acid (AA) (about 0.1
M). As well known, a major problem encountered with
the detection of DA is the interference from ascorbic acid
(AA), which largely coexists with DA in brain issue and
has an overlapping oxidation potential on the solid elec-
trodes, so it is very difficult to determine DA directly [3].
In order to resolve this problem, many different strate-
gies have been used to modify the electrode, which in-
clude self-assembled monolayer [4], electrochemical pre-
treatment [5], polymer film [6-17] and covalent modified
[18]. It is well known that DA exists as a cation at
physiological pH 7.0, while AA exists as an anion [19].
It is a possible way to overcome this problem by coating
the electrodes with a thin film of Nafion [20-22]. The
SO3
– of Nafion film can repel AA anion to eliminate
interference of AA. The kind of electrode usually suffers
from a slow response due to low diffusion coefficients
[23,24] of analytes in the films. The other method is to
cover the electrode surface with double-layer film
[25-27]. This kind of modified electrode is coated first
with an electroactive material having catalytic effect on
the oxidation of DA and then with a Nafion layer. An-
other approach is to cover the electrode surface with
electropolymerized films.
In this paper, we apply m-ABSA as a modifier to fab-
ricate polymer modified electrodes by electropolymeri-
zation method. In m-ABSA, there are electron-rich N
atom and high electron density of sulfonic group. Hence,
the poly (m-ABSA) film is negatively charged. The mo-
dified electrodes show an electrocatalytic activity for the
oxidation of DA and AA. The obvious separation of po-
tential to DA and AA can be obtained at modified elec-
trode. It means that AA has no interference for detection
DA. On the contrary, poly(m-ABSA) film modified
glassy carbon electrode was utilized for electrocatalytic
effect on the electrooxidation of DA in the presence of
AA at physiological pH. The modified electrode showed
good stability and reproducibility.

G. ERDOĞDU ET AL.
583
2. Experimental
2.1. Materials
m-aminobenzen sulfonic acid was purchased from Merck
and was used without any further purification. All the
other chemicals used such as dopamine hydrochloride,
ascorbic acid and KCl were of analytical grade and pur-
chased either from Sigma Chemical Company (St. Louis,
MO, USA) or from Merck (Darmstad, Germany). Mo-
nomer solutions were purged with nitrogen gas for about
10 min before polymerization and the solution was blan-
keted with the same gas during electropolymerization.
Amperometric measurements were performed in a PBS
(phosphate buffer salts, pH = 7.0) solution. Dopamine
and ascorbic acid solution were prepared freshly before
experiment. All aqueous solutions were prepared with
deionized and doubly distilled water.
2.2. Instrumentation
An electrochemical workstation BAS 100 W (Bioanaly-
tical Systems, Inc. West Lafeyette, IN, USA) equipped
with a personel computer was used for electropolymeri-
zation, cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) experiments. All electrochemical
studies were performed using a conventional three-
electrode system consisted of a bare or polymer modified
glassy carbon working electrode (geometric area: 6.85
mm2), a Ag/AgCl reference electrode and a Pt wire coil
auxiliary electrode. All electrolysis and voltammetric
experiments were made at room temperature. In the cy-
clic voltammetry experiments the scan rate was 50 mV/s.
2.3. Preparation of Poly (m-Aminobenzene
Sulfonic Acid) Film
Prior to electrochemical modification, the bare GCE with
a diameter of 0.3 µm was polished with diamond pastes
and alumina slurry down to 0.05 µm on a polishing cloth.
Then it was rinsed with double-distilled water, and
sonicated in 1:1 nitric acid, acetone and double-distilled
water for 10 min, respectively. Then it was electro-
chemically activated by using 20 times cycling potential
sweeps in the range of −0.5 to 2.0 V in 0.1M H2SO4 so-
lution at a scan rate of 100 mV/s. After being cleaned,
the electrode was immersed in 0.1 M KCl solution con-
taining 5.0 mM m-ABSA and was conditioned by cyclic
sweeping from –2.0 to 2.5 V at 50 mVs–1 for 14 scans.
After that, the modified electrode was electroactivated by
cyclic voltammetry from –0.5 to 0.5 V at 100 mVs–1 in
pH 7.0 PBS.
3. Results and Discussion
3.1. Electropolymerization of m-ABSA at the
GCE Surface
Voltammograms of 5.0 mM m-ABSA in 0.1 M KCl so-
lution at a GCE are shown in Figure 1. From the first
Figure 1. Repetitive cyclic voltammograms of 5.0 mM m-ABSA in 0.1 M KCl solution. Initial potential: –1.5 V; terminal po-
tential: +2.5 V; sensitivity: 1.0 × 10-4 A V-1.

G. ERDOĞDU ET AL.
584
cycle, an anodic peak (a) at. 0.510 V increased gradually
until the third cycle, and then a new anodic peak (c) ap-
peared at 1.027 V. A cathodic peak (b) appeared in the
first cycle with a potential at −0.510 V, then a new ca-
thodic peak (c) appeared at 0.199 V. Then larger peaks
were observed on continuous scanning, reflecting the
continuous growth of the film. These facts indicated that
m-ABSA was deposited on the surface of GCE by elec-
tropolymerization. After modification, the poly(m-ABSA)
film electrode was carefully rinsed with doubly distilled
water and then stored in air and prepared to use later.
3.2. Electrochemical Response of DA and AA at
Poly(M-ABSA) Film Modified Electrode
Figure 2 shows cyclic voltammograms of DA and AA in
pH 7.0 phosphate buffer solution at a bare GCE and a
poly (m-ABSA) film modified GCE. The electrochemi-
cal response of DA and AA at the bare GCE produce an
anodic peak at the potentials of 0.192 and 0.184 V, re-
spectively; the peak potentials are very close and nearly
overlap. But at the poly (m-ABSA) film modified elec-
trode, it could be observed that the modification shifts
the oxidation potentials of DA and AA toward signify-
cantly negatively potentials. In pH 7.0 PBS, DA exists as
a cation with a positively charged amino group (pKa 8.9)
[28] while m-ABSA is nonprotonated. Hence, the oxida-
tion of DA might be ascribed to the electrostatic attract-
tion interaction between DA cations and the high elec-
tron density of sulfonic group of m-ABSA, such an in-
teraction would lead to the increase in concentration of
DA around the surface of the modified electrode. Similar
results also reported in literatures [29].
3.2.1. Effect of Scan Rate and pH on Oxidation of DA
The effect of scan rate on the oxidation peak current of
1.0 mM DA was studied. With the scan rate increasing,
the anodic peak current (ipa) increased. A good linearity
between the square root of scan rate and ipa was obtained
within the range of 10 to 200 mVs–1. The linear regres-
sion equation was ip (10 µA) = −0.9991 + 2.0446 V1/2
(mVs-1) with the correlation coefficient r2 = 0.9945. The
result indicates that the electrode process is controlled by
the diffusion of DA.
The effect of pH on the peak potential and current was
investigated by differential pulse voltammetry in the
presence of 1 mM DA in 0.1 M phosphate buffer solu-
tion. The peak potential (Epa) shifted negatively when the
pH changed from acid over neutral to basic. The plot of
Epa versus pH shows linearity in the pH range of 5.0 to
8.0 with a slope of –23.51 mV pH–1, revealing that the
proportion of the electron and proton involved in the
reactions is 1:1. Because the DA oxidation is a two-
electron process, the number of protons involved is also
predicted to be two. This accords with the mechanism of
DA oxidation as reported previously [30,31] It can also
be seen from Figure 3 that the peak current of DA
reached a maximum at pH 7.0 and then decreased with
the change of pH. Because pH 7.0 was the physiological
condition and the response current of DA was the highest
at this pH, it was chosen as the experiment pH value in
the electrochemical detection of DA.
(a)
(b)
Figure 2. Differential pulse voltammograms of DA and AA
at a bare GCE (a) and a poly(m-ABSA) film modified GCE
(b) in 0.1 M phosphate buffer solution (pH 7.0). (a) a: blank;
b: 1 mM DA; c: 1 mM AA. (b) a: blank; b: 1 mM DA; c:
1mM DA + 1 mM AA.

G. ERDOĞDU ET AL.
585
Figure 3. Differential pulse voltammograms of 1.0 mM DA
at poly (m-ABSA) modified electrode at different pH values
(from a to e: 3.0, 5.0, 7.0, 9.0 and 11.0, respectively). The
inset shows the dependence of the oxidized current versus
pH of solution.
3.2.2. Interference and Reproducibility Study
In the extracellular fluid of the central nervous system,
DA exists in only a nanomolar to micromolar range (0.01
- 1 µM) [32], whereas the concentration of AA is very
high (100 - 500 µM) [33]. As the main interference, AA
hinders the accurate detection of DA because the oxi-
dized DA product, dopamine-o-quinone, can be catalyti-
cally reduced to DA by AA that again becomes available
for oxidation (as can be seen in Figure 2(b), the DA oxi-
dized current increased when AA exists); however, when
the concentration ratio of AA/DA is greater than 1, this
interference is constant [34]. We carefully examined the
oxidation currents of DA at the poly(m-ABSA) modified
GCE in the presence of increasing concentrations of AA
(Figure 4(a)). There is no obvious change in the oxida-
tion currents of DA when the concentration of AA
changed (when the concentration of DA was 50 µM, the
concentration ratio of AA/DA was 10–20). Moreover,
there was hardly any response for AA oxidation at the
poly(m-ABSA) modified electrode. As can be seen in
Figure 4(b), the oxidation currents of DA increased pro-
portionally with DA’s concentration while the peak cur-
rent of AA remained constant, indicating that the poly
(m-ABSA) electrode was sensitive only to DA.
This means that in the real biological matrixes, where
the AA level is usually more than three orders of magni-
tude larger than DA, the poly(m-ABSA) film modified
electrode could be used for the determination of DA in
the real sample. We also examined the influence of other
substances on the signals of the DA and found that no
interference occurred in the presence of the following
substance: 1000-fold NaCl, 1000-fold NaNO3, 1000-fold
NaClO4, 1000-fold Na2SO 4 and 1000-fold KCl.
One of the problems of determination of DA by the
bare electrode is the fouling of electrode surface, but in
our experiment we did not notice the inhibition of the
activity of the modifier toward DA detection; the peak
current of DA remained constant after the scan cycles of
(a)
(b)
Figure 4. (a) Differential pulse voltammograms at poly
(m-ABSA) modified GCE in pH 7.0 phosphate buffer solu-
tion containing 50 µM DA in the presence of different con-
centrations of AA: a. 0 µM b. 500 µM c. 600 µM d. 700 µM
e. 800 µM f) 900 µM and g) 1000 µM. (b) Differential pulse
voltammograms at poly (m-ABSA) modified GCE in pH 7.0
phosphate buffer solution containing 1 mM µM AA in the
presence of different concentrations of DA: a. 0 µM b. 5 µM
c. 10 µM d. 15 µM e. 20 µM f. 25 µM and g. 30 µM.

586 G. ERDOĞDU ET AL.
cyclic voltammograms up to 7 times (see Figure 5). This
can be explained as shown by the equations in Scheme 1
[13]. When the DA is oxidized (Equation (1)), its oxi-
dation product, dopaminequinone, can undergo follow-
up ring closure reaction (Eqaution (2)), leading to le-
ucodopaminechrome [35], which in turn is oxidized to
dopaminechrome (Equation (3)). It is required that the
protonated side chain of dopaminequinone move toward
the quinone ring. But at the poly(m-ABSA) modified
electrode, the high density of negatively charged groups
in the film is likely to immobilize the chain or at least su-
ppress its mobility and, thus, prevent the reaction given
by Equation (2) and, consequently, all of the follow-up
reactions.
3.2.3. Determination of DA
The determination of DA concentration at the poly (m-A-
BSA) modified electrode was performed with differential
pulse voltammetry. The anodic peak current was linear to
DA concentration in the ranges of 1.0 × 10–7 to 5.0 × 10–5
M and 5.0 × 10–5 to 1.0 × 10–4 M. The linear regression
(1)
(2)
(3)
Scheme 1. Possible reaction procedure of DA on the elec-
trode.
Figure 5. Repetitive differential pulse voltammograms of 1
mM DA at poly(m-ABSA) modified electrode in 0.1 M
phosphate buffer solution (pH = 7.0).
Table 1. Determination of DA in Injections.
DA specified
(×10–5 mol L–1)Added Found (×10–5 mol L–1)
R.S.D.
(×10–5 mol L–1, n=5)
Recovery
(%)
5.00 1.06.09 3.82 101.5
5.00 2.06.98 3.45 99.71
5.00 3.07.97 3.02 99.63
equations were ip (10 µA) = 0.9879 + 0.4874 C (10 µM)
(r2 = 0.9937) and ip (10 µA) = 4.3189 + 0.3456 C (10
µM) (r2 = 0.9882), respectively. The detection limit was
5.0 × 10–9 M.
The relative standard deviation of 7 successive scans
was 3.5% for 1.0 × 10–5 M DA, indicating that the poly
(m-ABSA) modified electrode had excellent reproduci-
bility. Furthermore, the stability of the modified electrode
was investigated.
3.2.4. Analytical Applications
The injections of DA were analyzed by the standard
addition method. The results are shown in Table 1. The
recovery and relative standard deviation values were
acceptable, showing that the proposed methods could be
efficiently used for the determination of DA in injec-
tions.
4. Conclusions
This study has indicated that the poly(m-ABSA) film
exhibits highly electrocatalytic activity to the oxidation
of DA. The modified electrode provides greater sensitiv-
ity and selectivity in the determination of DA. The inter-
ference of AA could be eliminated due to the very fa-
vorable electrostatic interaction between the negatively
charged poly(m-ABSA) film and cationic species of DA
or anionic species of AA in phosphate buffer solution at
pH 7.0. Moreover, the modified electrode showed good
reproducibility and stability. The proposed methods can
be applied to the detection of DA in the presence of ex-
cess AA in real samples.
5. Acknowledgements
This study was financially supported by the Research
Fund Unit of İnönü University (Grant no APYB: 2008/
48).
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