American Journal of Anal yt ical Chemistry, 2011, 2, 289-293
doi:10.4236/ajac.2011.22036 Published Online May 2011 (
Copyright © 2011 SciRes. AJAC
Simultaneous Determination of Dopamine and L-Ascorbic
Acid by Modified Carbon Paste Electrode with Ni (II)
Cyclam Complex
Leila Farzin1, Mohammad Reza Milani Hosseini2
1Environmental Laboratory, Nuclear Science Research School, Nuclear Science & Technology Research Institute,
Atomic Energy Organization of Iran (AEOI), Tehran, Iran
2Analytical Department, Chemistry faculty, Iran University of Science and Technology,
Tehran, Iran
Received November 8, 2010; revised January 10, 2011; accepted May 16, 2011
The electroanalysis of dopamine (DA) and ascorbic acid (AA) by square wave voltammetry has been per-
formed at a modified carbon paste electrode with macrocyclic ligand 1,4,8,11-tetraazacyclotetradecane (cy-
clam) and monolayer of Ni (II) cyclam. In pH 7.2 buffer solutions, the electrostatic reaction of AA with
di-positive monolayer shifts the oxidation potential to less positive potential, while the electrostatic repulsion
of DA with the monolayer shifts the oxidation potential of DA to more positive potential. The separation
between the oxidation peaks of AA and DA at the present di-positive monolayer modified electrode (252 mV)
was larger than that (187 mV) at the cyclam modified electrode. In addition, the catalytic oxidation of AA by
oxidized DA has been advantageously eliminated at the modified carbon paste electrode with cyclam and Ni
(II) cyclam complex. Thus, the determination of DA in the presence of an excess of AA is possible with the
present modified electrodes.
Keywords: Macrocyclic Compound, Carbon Paste Electrode, Dopamine, Ascorbic Acid
1. Introduction
Dopamine (DA) and ascorbic acid (AA) are compounds
of great biomedical and neurochemical interest playing a
potential role in human metabolism. DA is one of the
most significant catecholamine, functioning as a neuro-
transmitter in the central nervous system and a medica-
ment to drug addiction and Parkinson’s disease [1,2]. It
affects brain processes that control movement, emotional
response, and ability to experience pleasure and pain.
AA is a water-soluble vitamin that is widely required in
metabolism. It has been used in the prevention and
treatment of common cold, mental illness, cancer and
Aids [3]. In mammalian brain DA and AA coexists in the
extracellular fluids.
There are various determination methods includes ul-
traviolet spectroscopy (UV) [4], high performance liquid
chromatography (HPLC) [5,6], capillary electrophoresis
(CE) [7] and electrochemical approaches [8-10]. Because
both DA and AA are oxidisable compounds, their detec-
tion can be made by electrochemical methods based on
anodic oxidation. When a potential is applied at the elec-
trode, ascorbic acid is also oxidized to dehydroascorbic
acid, which undergoes further chemical reaction to form
the gem-diol (Figure 1). As is known the oxidation po-
tential is pH dependent [11] (The first pKa is at 4.17 and
the second is at 11.57).
Dopamine is oxidized to form dopaminequinone with
the liberation of two electrons (Figure 2). It is generally
believed that direct redox reactions of these species at
bare electrodes are irreversible and therefore require high
overpotentials. Moreover the direct redox reactions of
these species at the bare electrodes take place at very
similar potentials and often suffer from a pronounced
Figure 1. Electrooxidation of ascorbic acid.
Figure 2. Electrooxidation of dopamine.
fouling effect, which results in rather poor selectively
and reproducibility. Thus the simultaneous determination
of DA and AA is of critical importance in the field of
biochemistry and medical treatment [12].
The use of chemically modified electrodes greatly in-
creases the selectivity and sensitivity toward these ana-
lytes. The development of voltammetric sensors for the
detection of neurotransmitters in the extracellular fluid of
the central nervous system has received much interest in
the past few decades. So, many different strategies have
been employed for the modification of the electrode sur-
face [13]. A simple method for preparing electrochemi-
cal modified electrodes (CME) is based on doping car-
bon paste with the biocatalyst [14-16]. The most impor-
tant advantage of the “mixed catalyst-carbon paste elec-
trode” is the substantial reduction of response time ow-
ing to the absence of a layer that hinders mass transport.
The catalyst is an integral part of the sensing element,
and hence the electrode responds rapidly to changes in
the level of the substrate.
The goal of this study was to develop a production
method for modified carbon paste electrodes with mac-
rocyclic compounds for selective measurement of neuro-
transmitters in physiological pH (7.2). This modified
electrode with Ni (II) 1, 4, 8, 11-tetraazacyclotetradecane
has been used for Simultaneous Determination of DA
and AA. The electrochemical behavior of a broad family
of macrocyclic complexes of nickel has been studied by
Busch et al. [17].
2. Materials and Methods
2.1. Chemical and Reagents
All chemicals used were of analytical-reagent grade.
Double distilled, deionized water (Milli-Q system, Mil-
lipore, Japan) was used for preparation of all solutions.
1,4,8,11-tetraazacyclotetradecane (cyclam), dopamine
hydrochloride and ascorbic acid were bought from Fluka
and used as such. All the voltammetric studies were car-
ried out in phosphate buffer (potassium phosphate were
also used as a supporting electrolyte). Buffer solutions
were prepared from orthophosphoric acid and its salts in
the pH range of 3-9.
2.2. Apparatus
The voltammetric system used for the studies was Auto-
lab PSTAT 10 potentiostat joined to a Metrohm 663 VA.
Square wave voltammetry was carried out in a three-
electrode cell. Silver/silver chloride (3 moldm–3 KCl), a
platinum wire and a bare or modified electrode were
used as reference, counter and working electrodes, re-
spectively. The pH values were measured with a digital
pH meter MK VI (systronics).
2.3. Preparation of Bare Carbon Paste Electrode
The bare carbon paste electrode was prepared by hand
mixing of graphite powder and silicon oil at a ratio 70:30
(w/w) in an agate mortar until a homogenous paste was
obtained. The paste was then tightly packed into a PVC
tube (3 mm internal diameter) and the electrical contact
was provided by a copper wire connected to the end of
tube. The bare carbon paste electrode was polished suc-
cessively with 0.3 and 0.05 µm Al2O3 slurry on emery
paper. It was then rinsed with doubly distilled water and
sonicated in 1 + 1 HNO3, acetone and doubly distilled
water for 10 min, respectively.
2.4. Preparation 1of Modified Carbon Paste
The macrocyclic nickel complex (nickel (II) 1,4,8,11-
tetraazacyclotetradecane) was synthesized, purified and
characterized by elemental analysis and 13C-NMR and IR
spectral measurements according to reported procedure
The carbon paste electrodes were prepared as before
with 5% of the modifier in graphite-silicon oil matrix and
used in conjunction with an Ag/AgCl reference electrode
and a platinum counter electrode. The thickness of modi-
fied carbon paste was controlled in the range of 4 - 6
2.5. Procedure
The electrochemical experiments were performed in a 25
cm3 electrolytic cell with 10 cm3 solutions. All measure-
ments were conducted at room temperature and under a
nitrogen atmosphere. Nitrogen gas was bubbled through
the solution for 30 min prior to each electrochemical
Solutions of various pHs were tested with phosphate
buffer. The square-wave voltammograms were recorded
for the unmodified electrode and the electrode modified
with cyclam (CME-1) in 0.5 M phosphate buffer solution
(pH = 7.2) containing DA and AA at different scan rates.
Then, another set of experiments was carried out to study
the effect of catalysis by the incorporated metal ion in
the macrocyclic ring on the electrode modified with
nickel (II) macrocyclic complex (CME-2).
Copyright © 2011 SciRes. AJAC
3. Results and Discussion
3.1. Electrochemical Behavior of AA and DA on
The square-wave voltammograms obtained for the oxi-
dation of AA and DA at the CME-1 and the bare elec-
trode are shown in Figure 3. At the bare electrode when
both AA and DA coexist, only one voltammetric peak is
obtained for both analytes. Thus it is impossible to de-
termine the individual concentrations from the broad
voltammetric peak. Moreover the catalytic oxidation of
AA by the oxidized DA [19] enhances the oxidation peak
current of DA at the bare electrode. The precise deter-
mination of DA in the presence of AA is not possible
because of this catalytic oxidation. As the AA concentra-
tion, for example, in the extracellular fluid is very high,
this mediated oxidation would affect the accurate deter-
mination of DA and this unwanted catalytic oxidation
needs to avoid. The present modified electrode clearly
separates the merged voltammetric peaks of AA and DA
and the mediated oxidation of AA by oxidized DA has
been successfully eliminated. At these electrodes the AA
oxidation occurs well before the DA oxidation potential
and hence the mediated oxidation would not be expected.
Since the voltammetric peak of DA is well separated
from the AA peak, the determination of DA in the pres-
ence of AA is possible with the present modified elec-
trode. The separation between the oxidation peaks of AA
and DA at the CME-1 in phosphate buffer solution (pH =
7.2) was 187 mV.
3.2. Electrochemical Behavior of AA and DA on
Figure 4 shows the oxidation of AA and DA at the bare
Figure 3. Square-wave voltammograms obtained for the
oxidation of DA and AA (2 × 105 M) at (a) bare and (b)
CME-1 in 0.5 M phosphate buffer (pH = 7.2).
Figure 4. Square-wave voltammograms obtained for the
oxidation of DA and AA (2 × 105 M) at (a) bare and (b)
CME-2 in 0.5 M phosphate buffer (pH = 7.2).
carbon paste electrode and the modified carbon paste
electrode with nickel (II) macrocyclic complex (CME-2).
As can be readily seen from Figure 4 a negative shift in
the AA oxidation potential can be the electrostatic inter-
action of AA with the positively charged monolayer.
Since AA is negatively charged in neutral aqueous solu-
tion (pH = 7.2), the electrostatic interaction is expected
between AA and Ni 2+ redox centers of the monolayer
and it would favor the oxidation of AA.
In contrast, DA exists in the cationic form physiologi-
cal pH (pKa 8.9). It is repelled by the Ni2+ redox centers.
Hence, it cannot enter the monolayer to the same extent
as AA, and the interference with the determination of
DA is diminished. These results indicated that the prob-
lem of the overlapped voltammetric responses of DA
with AA, due to their coexistence in real biological fluids
can be effectively overcome by use of dipositive mono-
layer of Ni (II) cyclam.
The monolayer modified electrode successfully re-
solves the merged voltammetric peaks of AA and DA
and the peaks are separated enough (252 mV separation)
to determine the concentration of each analytic.
3.3. Effect of Film Thickness on the
Voltammetric Response
The thickness of complex film directly controls the elec-
trode performance. The optimum film thickness reflects
compromise between mechanical stability and residual
Nevertheless, the high residual current remains a se-
vere limitation of these modified electrodes. The residual
current tends to be high when maximum catalyst is used
for modification of carbon paste electrode. The film
thickness was varied by preparing the electrodes with dif-
ferent wt% of catalyst. The electrode prepared with 5 wt%
macrocyclic complex or ligand, shows the best perform-
ance. When the films were too thin, the limited amount of
Copyright © 2011 SciRes. AJAC
catalyst loaded apparently affect the sensitivity. Whereas,
when the films were too thick, residual current increased
remarkably. So, electrodes prepared with the optimum of
catalyst (5 wt%), were used in all experiments.
3.4. Optimization of the Solution pH
Figure 5 shows the ΔEp versus pH plots for CME-2 in
the phosphate buffer with various pHs (in range of 3 - 9).
The electrochemical reaction can be induced and mo-
nitored by voltammetry to quantify the concentration of
ascorbic acid in solution.
The voltammograms obtained with the Ni (II) macro-
cyclic for solutions containing L-ascorbic acid in strongly
acidic media (e.g. pH2) showed that L-ascorbic acid did
not couple catalytically with the Ni (II) macrocyclic.
Therefore, optimization of the solution pH was necessary
in order to obtain a catalytic couple. A variation in the
electrolyte pH will result in variations in the formal po-
tential of L-ascorbic acid. Therefore, the thermodynamic
driving force for the catalysis will vary with the pH,
making the peak currents and the shapes of the voltam-
mograms at different pH values. The anodic peak cur-
rents increased with an increase in the pH up to 6.6, and
then gradually decreased up to pH 9. So, the most opti-
mized pH for catalytic oxidation of AA is 6.6. However,
in this pH, electron transfer kinetic for the oxidation of
DA was found to be rather sluggish owing to the electro-
static repulsion between positively charged DA (pKa 8.9)
and Ni (II). So, the electrostatic repulsion of DA and
with di-positive monolayer shifts the oxidation potential
of DA to more positive potential, while the electrostatic
reaction of AA with the monolayer shifts the oxidation
potential to less positive potential. In this study, all of the
measurements were carried out at the physiological pH
(7.2) that it is much near to optimized pH.
3.5. Effect of Scan Rate on Peak Currents of DA
and AA
The results show an initial linearity which curves off at
Figure 5. Plot of ΔEp vs pH for 2 × 103 M ascorbic acid and
dopamine obtained by CME-2 in the 0.5 M phosphate buf-
higher scan rates. It suggests that the reaction is initially
diffusion controlled, but at faster scan rates the electron
transfer becomes rate determining. It appears from these
data that a scan rate of 100 mVs–1 was used for the pur-
pose of simultaneous determination of DA and AA. In
addition, the Ep values are shifted to more positive val-
3.6. Calibration Curve and Reproducibility
The calibration plots for the oxidation of AA and DA
were linear for a wide range of concentration (1-100 μM
for AA and 1.5 - 100 μM for DA at the CME-2).
To characterize the reproducibility of the CME-2, re-
petitive measurementregeneration cycles were carried
out. The results of 15 successive measurements showed a
relative standard deviation of 4.1% and 3.8% for 50 μM
ascorbic acid and 50 μM dopamine.
4. Conclusions
Redox processes of organic compounds often have slow
charge transfer rates, leading to poorly defined voltam-
metric responses. Modification of the electrode surface
by a redox mediator reduces the over-potential for the
redox processes. The modified carbon paste electrodes
with cyclam and Ni (II) cyclam monolayer have been
successfully applied to the determination of ascorbic acid
and dopamine. The separation between the oxidation
peaks of AA and DA at the CME-2 (252 mV) was larger
than that (187 mV) at the CME-1.
The carbon paste approach permits convenient mixing
of different ligands and as the ligand is homogeneously
mixed in the bulk of the paste, renewal of the surface is
done simply by pressing out the paste from syringe,
which is easier and faster. Moreover, the electrode so
fabricated can be stored for about six months in an air-
tight container.
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