American Journal of Anal yt ical Chemistry, 2011, 2, 262-269
doi:10.4236/ajac.2011.22032 Published Online May 2011 (
Copyright © 2011 SciRes. AJAC
Determination of Dopamine in Presence of Uric Acid at
Poly (Eriochrome Black T) Film Modified Graphite Pencil
Umesh Chandra, Bahaddurghatta Eshwaraswamy Kumara Swamy*, Ongera Gilbert, Sathish Reddy,
Bailire Sheena Sherigara
Department of PG Studies and Research in Industrial Chemistry, Kuvempu University, Karnataka, India
Received December 26, 2010; revised April 1, 2011; accepted April 2, 2011
A simple commercial graphite pencil electrode was used to investigate the electrochemical oxidation of do-
pamine. The electropolymerised film of eriochrome black T was prepared on the surface of graphite pencil
electrode by using cyclic voltammetry technique. The prepared electrode exhibits an excellent electrocata-
lytical activity towards the determination of dopamine. The effects of concentration, pH and scan rate were
investigated. Simultaneous detection of dopamine and uric acid was investigated by using both cyclic volt-
ammetric and differential pulse voltammetry technique. The modified electrode was also used for the detec-
tion of dopamine in injection.
Keywords: Poly (Eriochrome Black T), Graphite Pencil Electrode, Dopamine, Uric Acid, Cyclic
Voltammetry, Differential Pulse Voltammetry
1. Introduction
Dopamine is an important neurotransmitter in the amyg-
dala, a phylogenetically older structure of the brain,
which is thought to play a critical role in limbic, cogni-
tive and neuroendocrine functions [1,2]. Serious diseases
such as Schizophrenia and Parkinsonism may result by
loss of DA-containing neurons [3-8]. Patient with this
disease shows a low level of DA. Therefore, determina-
tion of DA concentration has become important. Many
methods were introduced to determine DA, such as spec-
troscopy, chromatography and electrochemistry [9-13].
Because of its electrochemical activity, DA can also be
determined with electrochemical method [7,14]. Uric
acid (UA) is the primary end product of purine metabo-
lism in the human body [15]. In a healthy human being,
the typical concentration of UA in urine is around 2 mM
and in the blood is in between 120 μM to 450 μM ranges
[16,17]. Extreme abnormalities of UA levels are symp-
tomic of several diseases, such as, cardiovascular disease
[18], hyperuricaemia, uric acid stones [19], gout and Le-
seh-Nyhan syndrome [20]. Thus accurate determination
of UA concentration is of great importance. Recently,
electrochemical sensors have attracted much attention
due to their advantages of simplicity, cheapness, fast
analysis along with high sensitivity and selectivity [21].
The oxidation potential of DA and UA are very close
such that, the bare electrode often suffers from fouling
effects. The chemically modified electrodes have the
ability to detect both DA and UA selectively [22,23].
The electropolymer film coated electrodes are playing
an important role in sensor field. Ongera et al. studied
the simultaneous determination of dopamine in presence
of ascorbic acid at electropolymer modified carbon paste
electrode [3,4,21]. Gabriela Broncova et al. used poly
(neutral red) modified electrode for determination of
citrate in soft drinks [24]. M. Pandurangachar et al. pre-
pared poly (patton’s and reeder’s) film coated carbon
paste electrode for simultaneous detection of dopamine
[7]. Sarah M. Kirwan studied the electrochemical prop-
erties of AA and H2O2 at Poly (o-phenylenediamine) film
modified Platinum-iridium electrodes alloy wires [25].
In the present work, poly (EBT) film coated graphite
pencil electrode is used for the selective detection of DA
in the presence of UA. The electrochemical study of
EBT was done by our research group [26]. Many electro-
chemical experiments have been done by electropoly-
merizing the EBT indicator and discussing their voltam-
metric behavior by modifying at glassy carbon electrode
[27-29]. The structure of EBT was shown in Scheme 1.
The graphite pencil electrode (GPE) has been success-
fully applied to analyses of certain compounds in recent
years [30-34]. The GPE is relatively new type of carbon
electrode, it is less expensive, more convenient, and re-
newable compare to the commonly used CPE or GCE
[35]. The electropolymerization film of eriochrome black
T (EBT) was coated on the surface of GPE by cyclic
voltammetry (CV). The electrode had high concentration
of negative-charged function group 3 and electron
rich oxygen atom on its surface. The poly (EBT) film
coated GPE showed excellent electocatalytical activity
towards the selectivity and sensitivity of DA in the pres-
ence of UA in the range of pH 5.0 in acetate buffer solu-
tion. The peak to peak separation between DA and UA
was 158 mV. This was large enough to identify the DA
and UA individually.
2. Experimental Section
2.1. Reagents
The pencil-lead rods were HB 0.5 mm in diameter and 6
cm length purchased from local bookstore. 25 mM erio-
chrome black T stock solution was prepared in double
distilled water. 10 × 10–4 M DA stock solution was pre-
pared by dissolving in 0.1 M perchloric acid solution. 10
× 10–3 M stock solution of UA was prepared by dissolv-
ing in 0.1 M sodium hydroxide solution. Sodium acetate
and acetic acid were used to prepare the 0.2 M acetate
buffer solution. Chemicals mentioned above were all of
analytical grade. The water used in the preparation of
solutions was double distilled water.
2.2. Apparatus
The electrochemical experiments were carried out using
a model-201 electroanalyser (EA-201 chemilink system).
All the experiments were carried out in a conventional
three electrochemical cell. The electrode system contained
a working electrode was bare GPE and poly (EBT)
Scheme 1. Structure of eriochrome black T.
filmmodified GPE (0.5 mm in diameter), a platinum wire
as counter electrode and saturated calomel electrode as
reference electrode.
2.3. Preparation of Poly (EBT) Modified GPE
The 1 mM EBT was placed in the electrochemical cell
with 0.05 M H2SO4. The GPE was scanned by immers-
ing 3 mm length in that solution (from –400 to 1400 mV)
at 100 mVs–1 for 20 times. After this, the same GPE was
enforced under sweeping from –400 to 1400 mV at 100
mVs–1 for multiple cycles (20 cycles) in the solution
containing 1 mM EBT with 0.01 M NaOH.
3. Results and Discussion
3.1. Electrochemical Polymerization of EBT on
Figure 1 showed the cyclic voltammogram for electro-
polymerisation of EBT on the surface of GPE in the range
from –400 to 1400 mV at the sweep rate of 100 mVs–1 at
20 multiple cycles. Before the electropolymerisation pro-
cess the GPE was pretreated by scanning in the solution
containing 1 × 10–3 M EBT in 0.05 M H2SO4 for 20
times in the same potential range. After this the electrode
was made to undergo multiple cycles in 0.01 M NaOH
containing 1 × 10–3 M EBT. During the process of multi-
ple cycles the voltammogram has gradually descended
with increase of cyclic time. This indicates that the poly
(EBT) film was formed and deposited on the surface of
GPE [27-29].
3.2. Electrocatalytic Response of DA at Poly
(EBT) Modified GPE
DA being an easily oxidizable catecholamine, showed
Figure 1. Cyclic voltammogram of preparation of poly
(EBT) film coated GPE containing 1 mM EBT in 0.01 M
NaOH at 20 cycles with sweep rate of 100 m·Vs1.
Copyright © 2011 SciRes. AJAC
quasi-reversible voltammogram with supporting electro-
lyte 0.2 M acetate buffer of pH 5.0 at 50 mV·s–1 scan rate
for bare GPE in the potential range of –200 to 600 mV.
Figure 2 showed a pair of redox peak for bare GPE
(dashed line), with Epa 346 mV and Epc 198 mV (vs
SCE). The separation of redox peaks (Ep) was found to
be 148 mV and the ratio of redox peak current (Ipa/Ipc)
was 1.625, which were the characteristics of a quasire-
versible electrode process. The formal peak potential
(E0), which is the midpoint of Epa and Epc was obtained
as 272 mV. However, at the poly (EBT) modified GPE a
pair of redox peaks are obtained with strong increased in
both Ipa and Ipc (solid line). The poly (EBT) modified
GPE reduced the over potential which occurred for bare
GPE. The Epa and Epc were located at 275 mV and 229
mV respectively. At poly (EBT) modified GPE the Epa
was shifted negatively upto 71 mV and Epc was posi-
tively upto 31 mV. The Ep was found to be 40 mV,
which was on accordance with a Nernst reversible be-
haviour and identified that number of electrons involved
in the reaction was about equal to two. The ratio (Ipa/Ipc)
was 1.1 and the E0 was 252 mV. The shifting of redox
peak potentials and approaching of Ipa/Ipc towards 1,
were shows the excellent electrocatalytic activity of poly
(EBT) film coated GPE for detection of DA. Thus, the
voltammogram obtained for DA at poly (EBT) modified
GPE was reversible with excellent enhancement of oxi-
dation and reduction peak currents.
3.3. The Effect of Scan Rate
The effect of scan rate on the anodic peak current of DA
was studied at poly (EBT) modified GPE by using CV
technique (Figure 3(a)). The scan rate was increased
from 50 to 400 mV/s. the anodic peak current was in-
creased with increase in scan rate. The graph of Ipa vs
scan rate was plotted (Figure 3(b)). The resulted graph
Figure 2. Cyclic voltammogram of 1 × 104 M DA in 0.2 M
acetate buffer solution of pH 5.0 at bare GPE (dashed line)
and poly(EBT) film coated GPE (solid line).
Figure 3. (a) Variation of scan rate for DA at poly (EBT)
film coated GPE (a-h; 50 mVs1 to 400 mVs1). (b) Graph
of current vs square root of scan rate.
showed excellent linearity with correlation co-efficient
of 0.9997. This result showed that the electrode process
was adsorption controlled.
3.4. Effect of pH
The electrochemical response of DA at poly (EBT) film
coated GPE is generally pH dependent. The voltammo-
grams of DA were recorded at 0.2 M acetate buffer solu-
tions of different pH by cyclic voltammetric method.
Figure 4(a) demonstrates the pH dependence of DA at
poly (EBT) film coated GPE at sweep rate of 50 mV·s–1.
The both anodic and cathodic peak potentials were
shifted to less positive side with increasing in the pH
values. The anodic peak potential of DA shifted from
331 mV to 249 mV with respect the pH from 3.8 to 5.6.
The potential diagram was constructed by plotting the
graph of calculated E0 vs pH of the solution (Figure
4(b)). The graph has good linearity with a slope of 45
mV/pH this behaviour is nearly obeyed the Nernst Equa-
tion for two electron and two proton transfer reaction.
Copyright © 2011 SciRes. AJAC
Figure 4. (a) Cyclic voltammogram of DA at different pH (a
- e; 3.8, 4.2, 4.6, 5.0, 5.6 pH). (b) Graph of E0 vs pH.
3.5. Effect of Concentration of DA
The electrocatalytic oxidation of DA was carried out by
varying its concentration at poly (EBT) modified GPE.
Figure 5(a) showed that, by increasing the concentration
of DA, the electrochemical anodic and cathodic peak
current goes on increasing with shifting Epa towards
positive and Epc towards negative direction. DA from
0.1 × 10–3 to 0.5 × 10–3 concentrations showed the Epa
was increased from 275 mV to 305 mV. The graph of
anodic peak current vs concentration of DA was plotted
(Figure 5(b)). The anodic peak current was proportional
to concentration of DA in the above range.
3.6. Simultaneous Determination of DA and UA
by Cyclic Voltammetry
UA is present along with DA in mammalian brain. The
concentration of UA is much higher than that of DA.
Since, the oxidation potential of UA is nearly same as
Figure 5. (a) Cyclic voltammogram of DA at different con-
centration (a-e; 0.1, 0.2, 0.3, 0.4, 0.5 × 103 M). (b) Graph of
current vs concentration of DA.
that of DA result in an overlapped voltammetric response
at bare CPE. However, the poly (EBT) modified GPE
has ability to separate the oxidation peak potentials of
DA and UA. Figure 6(a) showed the voltammograms
were recorded for individual UA at both bare poly (EBT)
film modified GPE at sweep rate of 50 mV·s–1 in the
potential range from –200 mV to 700 mV. The voltam-
mogram of UA has Epa, which was found to be 452 mV
in a pH 5.0 acetate buffer solution at bare GPE (dashed
line). However the Epa was shifted at 424 mV at poly
(EBT) modified GPE (solid line) with strong enhance-
ment of Ipa. The plot of Epa vs pH was linear and slope
was found to be –0.054 mV. The Ipa of uric acid was
proportional to the v with correlation coefficient (r)
0.9995 (data was not shown).
The Figure 6(b) showed the voltammogram for solu-
tion containing mixture of both 0.1 × 10–3 DA and 0.5 ×
10–3 M UA in pH 5.0 acetate buffer solution at sweep
rate of 50 mV·s–1 from potential range 0 to 700 mV. The
bare GPE (dashed line) showed only one broad anodic
Copyright © 2011 SciRes. AJAC
Figure 6. (a) Cyclic voltammogram of 0.5 × 103 M UA at
bare GPE (dashed line) and at poly (EBT) film coated GPE
(solid line) in 0.2 M acetate buffer solution of pH 5.0. (b)
Simultaneous determination of 1 × 104 M DA and 0.5 ×
103 M UA at bare GPE (dashed line) and at poly (EBT)
film coated GPE (solid line).
peak but not cathodic peak. The poly (EBT) modified
GPE has able to separate the oxidation peaks of DA and
UA by showing two well separated anodic peaks and one
cathodic peak (solid line). The electrocatalytical anodic
peak of DA was obtained at 271 mV and UA was found
to be at 429 mV. The cathodic peak for dopamine was
found to be 235 mV. The separation between DA-UA
was found to be 158 mV.
3.7. Simultaneous Detection of DA and UA by
Differential Pulse Voltammetry
DPV was used for the determination of DA and UA at
poly (EBT) modified GPE because of its higher current
sensitivity and better resolution than CV. The simultane-
ous study was carried out in the potential range from 0 to
600 mV (Figure 7(a)). The DPV showed the simultaneous
Figure 7. (a) Differential pulse voltammogram for simulta-
neous detection of 0.1 mM DA and 0.5 mM UA at poly
(EBT) film coated GPE. (b) DPVs of (a-f; 0.1, 0.15, 0.17, 0.2,
0.25 and 0.3 × 103 M) DA in 0.2 M ABS of pH 5.0 in the
presence of 1 × 103 M UA at poly (EBT) film modified GPE.
(c) DPVs of (a-f; 0.2, 0.4, 0.6, 1.1, 1.6 and 2.0 × 103 M) UA
in 0.2 M ABS of pH 5.0 in the presence of 0.1 × 103 M DA
at poly (EBT) film modified GPE.
Copyright © 2011 SciRes. AJAC
determination of DA and UA with well separated two
anodic peaks corresponding to their oxidation could be
possible at poly (EBT) modified GPE. The 0.1 × 10–3 M
DA showed its Epa at 252 and 0.5 × 10–3 M UA was at
414 mV. The peak separation between DA and UA was
162 mV which was greater when comparing to peak se-
paration occurred by CV.
The simultaneous determination of DA and UA in the
mixture was carried out at poly (EBT) film coated GPE
when concentration of one species changed, whereas
another one remained kept constant. From the Figure
7(b), it can be seen that the peak current of DA was pro-
portional to its concentration, which was increased from
0.1 × 10–3 M to 0.3 × 10–3 M when keeping the concen-
tration of UA 1 × 10–3 M. There was no change in the
peak current and peak potential occurred for UA. Simi-
larly in the Figure 7(c) keeping the concentration of DA
constant, the UA concentration was varied from 0.2 ×
10–3 M to 2.0 × 10–3 M. The oxidation peak current of
UA increases with increase in its concentrations. The
detection limit was calculated and found to be 0.08 μM
for DA concentration [36].
3.8. Analytical Application
The modified electrode was applied to the determination
of dopamine hydrochloride injection. The DA injection
sample purchased from sterile specialities India Private
Ltd with a specified content of DA of 40.0 mg/mL. The
sample was used after suitable dilution. The results were
shown in Table 1. The recovery and R.S.D. were ac-
ceptable, showing that the proposed methods could be
efficiently used for the determination of DA in injections
with recovery in the range 99.25% - 101.25%.
4. Conclusions
In this work, poly (EBT) film coated GPE was used to
investigate the electrochemical response of an interesting
neurotransmitter DA. The poly (EBT) film coated GPE
enhanced both anodic and cathodic peak current strongly.
The poly (EBT) film coated GPE showed excellent se-
lective and electrocatalytic activity towards the oxidation
of DA in the presence of and UA. The poly (EBT) film
coated GPE has very low detection limit 0.08 μM. Hence,
Table 1. Detection of DA in injection samples (n = 5).
Sample Content
1 4.0 3.97 2.4 99.25
2 4.0 3.99 2.0 99.75
3 4.0 4.05 2.2 101.25
poly (EBT) modified GPE is acting as a good sensor for
the detection of DA. We hope our modified electrode can
be used for investigation of other neurotransmitters. The
proposed methods can be applied to the detection of DA
in injection.
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