Electroless deposition of bis(4'-(4-pyridyl)-2,2':6',2"-terpyridine) iron(II) thiocyanate complex onto carbon nanotubes modified glassy carbon electrode: Application to simultaneous determination of ascorbic acid, dopamine and uric acid

Article abstract of DOI:10.1590/S0103-50532011000300009

A glassy carbon (GC) electrode modified by multi-walled carbon nanotubes (MWNTs) and bis(pyterpy)iron(II) thiocyanate complex (pyterpy = 4'-(4-pyridyl)-2,2':6',2"-terpyridine) was investigated by voltammetric methods in acetate buffer solution (pH 5). Performances of the iron(II)-complex/MWNTs modified electrode were evaluated with differential pulse voltammetry and scanning electron microscopy (SEM). The modified glassy carbon electrode shows an excellent electrochemical response for ascorbic acid (AA), dopamine (DA) and uric acid (UA). The differential pulse voltammetry oxidation peaks for AA and DA, DA and UA, AA and UA are separated by 210, 136 and 346 m V, respectively. This separation permits the simultaneous determination of AA, DA and UA. The anodic peak currents of AA, DA and UA increase linearly with concentration in the range of 1.10×10^-5-1.50×10 ^-3 mol L^-1, 9.0×10^-7-1.20×10 ^-3 mol L^-1 and 2.00×10^-6-1. 50×10^-3 mol L^-1, respectively, with a correlation coefficient (r) always higher than 0.998. In addition, the modified electrode also shows good sensitivity and stability. Satisfactory results were achieved for the determination of AA in vitamin C tablets, DA in dopamine injection solution and UA in human blood serum samples.

Full text of DOI:10.1590/S0103-50532011000300009

J. Braz. Chem. Soc., Vol. 22, No. 3, 468-477, 2011.
Printed in Brazil - ©2011 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Article
A
Electroless Deposition of bis(4’-(4-Pyridyl)-2,2’:6’,2’’-terpyridine)iron(II) Thiocyanate
Complex onto Carbon Nanotubes Modified Glassy Carbon Electrode: Application to
Simultaneous Determination of Ascorbic Acid, Dopamine and Uric Acid
M. A. Kamyabi,* O. Narimani and H. H. Monfared
Department of Chemistry, Zanjan University, P. O. Box 45195-313 Zanjan, Islamic Republic of Iran
Um eletrodo de carbono vítreo modificado por nanotubos de carbono de paredes múltiplas
(NTCPM) e pelo complexo tiocianato de bis(piterpi) ferro(II) (piterpi = 4’-(4-piridil)-2,2’:6’,2”-
terpiridina) foi investigado através de métodos voltamétricos em solução de tampão acetato
(pH 5). A performance do eletrodo modificado pelo complexo de ferro(II)/NTCPM foi avaliada
por voltametria de pulso diferencial e microscopia eletrônica de varredura (MEV). O eletrodo de
carbono vítreo modificado apresentou uma excelente resposta eletroquímica para ácido ascórbico
(AA), dopamina (DA) e ácido úrico (AU). Os picos de oxidação em voltametria de pulso diferencial
para AA e DA, DA e AU, AA e AU são separados por 210, 136 e 346 mV, respectivamente. Esta
separação permite a determinação simultânea deAA, DA eAU. As correntes de pico anódico para
AA, DA e AU aumentam linearmente com a concentração na faixa 1,10×10-5-1,50×10-3 mol L-1,
9,0×10-7-1,20×10-3 mol L-1 e 2,00×10-6-1,50×10-3 mol L-1, respectivamente, com um coeficiente de
correlação (r) sempre maior do que 0.998. Além disto, o eletrodo modificado também apresentou
boa sensibilidade e estabilidade. Resultados satisfatórios foram alcançados para a determinação
de AA em tabletes de vitamina C, DA em solução de dopamina de injeção, e AU em amostras de
sangue humano.
A glassy carbon (GC) electrode modified by multi-walled carbon nanotubes (MWNTs) and
bis(pyterpy)iron(II) thiocyanate complex (pyterpy = 4’-(4-pyridyl)-2,2’:6’,2’’-terpyridine) was
investigated by voltammetric methods in acetate buffer solution (pH 5). Performances of the
iron(II)-complex/MWNTs modified electrode were evaluated with differential pulse voltammetry
and scanning electron microscopy (SEM). The modified glassy carbon electrode shows an
excellent electrochemical response for ascorbic acid (AA), dopamine (DA) and uric acid (UA).
The differential pulse voltammetry oxidation peaks for AA and DA, DA and UA, AA and UA
are separated by 210, 136 and 346 mV, respectively. This separation permits the simultaneous
determination of AA, DA and UA. The anodic peak currents of AA, DA and UA increase linearly
with concentration in the range of 1.10×10-5-1.50×10-3 mol L-1, 9.0×10-7-1.20×10-3 mol L-1 and
2.00×10-6-1.50×10-3 mol L-1, respectively, with a correlation coefficient (r) always higher than
0.998. In addition, the modified electrode also shows good sensitivity and stability. Satisfactory
results were achieved for the determination of AA in vitamin C tablets, DA in dopamine injection
solution and UA in human blood serum samples.
Keywords: multi-wall carbon nanotubes, bis(pyterpy)iron(II) thiocyanate complex, ascorbic
acid, dopamine, uric acid
Introduction
in many important life processes. It is one of the most
important vitamins, due to its antioxidant and pH regulator
properties often being added to various food products and
pharmaceuticals.1,2 Ascorbic acid exists in mammalian brain
in the presence of several other neurotransmitter amines
including dopamine.
Dopamine plays an important role as a neurotransmitter
(NTM) in the body. A deficiency in this NTM has been
related to serious neurological disorders such as Parkinson
Recently there has been a considerable effort in
the development of voltammetric methods for the
determination of ascorbic acid (AA), dopamine (DA) and
uric acid (UA) in biological samples. Ascorbic acid is a
water-soluble vitamin, and is a compound that takes part
*e-mail: makamyabi@gmail.com

Vol. 22, No. 3, 2011
Kamyabi et al.
469
disease and Schizophrenia.3-5 As a result, there is a
continuing interest in the development of fast, sensitive and
selective methods for the detection of DA. In particular,
there has been much interest in the electrochemical
methods because DA is electrochemically active and these
methods are inexpensive and easy to employ. However, the
determination of DA is complicated by the coexistence
of many interfering compounds such as AA and UA,
with AA being of particular significance.6 AA is the most
problematic because it is oxidized at similar potentials to
DA and AA is present in concentrations that are several
hundred times higher than DA in the living organism.
Furthermore, the oxidized dopamine product, dopamine-
quinone, can be chemically reduced by AA to DA, which
can be reoxidised at the electrode surface.7
Uric acid is a primary product of purine metabolism.
Its abnormal concentration level in the human body
causes many diseases, such as gout, hyperuricaemia
and Lesch-Nyan disease. Elevated UA concentration in
serum causes kidney damage and cardiovascular disease.
Therefore, the research of UA determination is of great
importance in reality.
electrochemical research.33-38 The direct electron transfer of
enzymes such as cytochrome C,39 glucose oxidase (GOx),40
catalase,41 horseradish peroxidase, myoglobin42 and
hemoglobin43,44 can be observed in the presence of carbon
nanotube. In addition, due to its ability of fast electron
transfer, carbon nanotube also shows electrocatalytic
activities towards many substances such as hydrogen
peroxide (H2O2), NADH, ascorbic acid, dopamine,
catechol, homocysteine and thiocytosine.45-51 The good
catalytic activities of these molecules as well as their good
biocompatibility make possible to apply it in the fabrication
of excellent biosensors.
In the current study, we report the electrochemical
behavior of ascorbic acid, dopamine and uric acid on the
surface of the iron(II) complex-MWNT modified GC
electrode. It has been found that the modified glassy carbon
electrode shows an excellent electrochemical response for
ascorbic acid, dopamine and uric acid. This capability of
the modified glassy carbon electrode makes it possible to
determine AA, DA and UA simultaneously.
Experimental
AA, DA and UA usually coexist in physiological
samples,8 but there is an overlapping oxidation potential
on the solid electrodes. Therefore it is essential to develop
simple and rapid methods for their determination in routine
analysis.9-14
Reagents
The ligand 4’-(4-pyridyl)-2,2’:6’,2’’-terpyridine
(pyterpy) and [Fe(pyterpy)2](SCN)2 complex (Scheme 1)
were synthesized, purified and characterized as
reported.52 A brief methodology for the synthesis of
this ligand is as follows: i) 1,5-bis(2-pyridyl)-3-(4-
pyridyl)-1,5-pentanedione 2-acetylpyridine (8.4 mL)
and 4-pyridinecarboxaldehyde (3.0 mL) are dissolved
in ethanol (35 mL) in a 100 mL round-bottomed flask
and a solution of sodium hydroxide (2.0 g) in water
(25 mL) added. The mixture is stirred for 1 h at room
temperature and then 30 mL of water added. This should
give an off-white precipitate of 1,5-bis(2-pyridyl)-3-(4-
pyridyl)-1,5-pentanedione. Collect this by filtration, wash
well with water and a little cold ethanol, and dry in a
desiccator. ii) 4’-(4-pyridyl)-2,2’: 6’,2”-terpyridine (pytpy)
Heat a solution of 1,5-bis(2-pyridyl)-3-(4-pyridyl)-1,5-
pentanedione (0.40 g) and ammonium acetate (5.0 g) in
50 mL of ethanol to reflux for 2 h. Allow the solution to
cool and add 50 mL of water to precipitate the product.
Recrystallise the product from a small amount of ethanol.
Multiwall carbon nanotubes, (MWNTs) (1-8 nm inner
diameter, 2-25 nm outer diameter and BET of 285 m2 g-1),
and 1 μm length were obtained from research institute
of petroleum industry of Iran. All chemicals were of
analytical-reagent grade from Merck or Fluka and were
used without further purification. Triply distilled water was
Among many methods for determination of UA, DA and
AA in biological samples, voltammetric method has been
shown to be a powerful tool. It is generally believed that
direct redox reactions of these species on the bare electrodes
are irreversible and high overpotentials are usually required
for their amperometric detections. Moreover, the direct
redox reactions of these species at the bare electrodes
take place at very similar potentials and often suffer from
a pronounced fouling effect, which results in rather poor
selectivity and reproducibility. Correspondingly, various
approaches have been developed to solve the problems.15-22
Carbon nanotubes (CNTs) with single or multiple walls
have attracted considerable attention of the researchers
since they were discovered by Iijima in 1991.23 They are
molecular-scale wires with high electrical conductivity,
high chemical stability, and extremely high mechanical
strength and modulus.24
Utilization of these properties has lead to application
of carbon nanotubes as scanning probes,25 electron field
emission sources,26 actuators,27 nanoelectronic devices,28
batteries,29 nanotubes-reinforced materials,30 potential
hydrogen storage material31 and chemical sensors.32
Due to its good electronic properties and electric
conductivity, carbon nanotube has also been applied in

470
Electroless Deposition of bis(4’-(4-pyridyl)-2,2’:6’,2’’-terpyridine)iron(II) Thiocyanate Complex
J. Braz. Chem. Soc.
2+
N
N
N
N
N
MeOH
2SCN−
N
N
Fe
+ Fe(SCN)2
N
N
2 N
N
N
used to prepare buffer and reagent solutions. Stock solutions
of AA, DA and UA were freshly prepared as required
in 0.10 mol L-1 of appropriate buffer solution. In these
experiments, 0.10 mol L-1 acetate was used for pH 4 and 5,
and 0.10 mol L-1 phosphate for pH 3, 6 and 7. The prepared
solutions were purged with pure nitrogen gas (99.999%) for
5 min before the voltammetric measurements.Voltammetric
experiments were carried out in the buffered solutions
of AA, DA and UA, deoxygenated by purging the pure
nitrogen. During the experiments, nitrogen gas was passed
over the surface of the test solutions in order to avoid
entrance of oxygen into the solution.
(H2SO4).39 Finally, the electrode was heated for 5 min at
50 °C in oven. MWNTs (4 mg) were dispersed in 10 mL
dimethyl sulfoxide (DMSO) by ultrasonic agitation for
about 30 min to give a 0.4 mg mL-1 black suspension. The
MWNTs modified electrode was prepared by casting 30 μL
of the MWNTs on the surface of GC electrode, which was
dried in air for 24 h at room temperature. When the DMSO
was volatilized, a MWNTs film was formed. The iron(II)
complex-MWNTs-modified GC electrode was prepared
by the following procedure. The MWNTs-modified GC
electrode was dipped in 1 mmol L-1 acetonitrile solution
of bis(pyterpy)iron(II) thiocyanate complex for 60-120 s.
The modified electrode, then, was rinsed with distilled
water and was dried for 1 h at room temperature. The
iron(II) complex-MWNTs-modified GC electrode was
then dipped in 0.1 mol L-1 acetate buffer solution (pH 5),
and consecutive cyclic potential scan ( 15-20 cycles) were
performed in the potential range -1.0 to +1.0V with a scan
rate of 100 mV s-1 to obtain a stable cyclic voltammogram.
The pretreatment of activation is believed to result in the
increase in surface roughness by removal of the surface
contaminants and inhibitory layers which hinder electron
transfer.39,53,54 This activation also breaks the end caps of
carbon nanotubes, exposing new carbon plane and leading
to electrocatalytic activity.55 The modified electrode was
stored in pH 5 buffer solution at 4 °C before use.
Apparatus
Voltammetric experiments were performed with
a Metrohm computrace voltammetric analyzer model
757 VA. A conventional three-electrode system including
glassy carbon electrode modified with MWNTs and
bis(pyterpy)iron(II) thiocyanate complex as working
electrode, anAg/AgCl (saturated KCl) and a platinum wire
were used as reference and counter electrode, respectively.
A digital pH/mV meter model 780 Metrohm was applied
for the preparation of the buffer solution. A Carl Zeiss
CEM 902A, Germany, device was used for transmission
electron microscopy (TEM) and scanning electron
microscopy (SEM) was carried out on a Philips device
(XL30). Electrochemical experiments were carried out at
room temperature 25.0 0.1 °C.
Results and Discussion
Characterization of the iron(II)-complex/MWNTs-modified
GC electrode
Preparation of the modified electrode
The GC (glassy carbon) electrode (2 mm diameter) was
carefully polished with aluminum on polishing cloth. The
electrode was placed in ethanol and subjected to vibration
to remove adsorbed particles. Then 15 cycle scans were
carried out in the potential range of -2.0 to +2.0 V versus
reference electrode in a solution of 1.0 mol L-1 sulfuric acid
The iron(II)-complex/multi-walled nanotubes modified
GC electrode was first characterized by TEM and SEM.
Figure 1 (A and B) shows a typical TEM image of the
MWNTs which were dispersed in DMF by sonication. It is
clear that MWNTs are highly entangled with the diameter
of several tens of nanometers. A typical scanning electron

Vol. 22, No. 3, 2011
Kamyabi et al.
471
micrograph image of the iron(II)-complex/MWNTs-
modified GC film electrode surface, is shown in Figure 1c.
It is clearly seen that the glassy carbon disk surface is
completely and homogeneously coated by the iron(II)-
complex/MWNTs composite film.
Electrochemical behavior of AA, DA and UA at the iron(II)-
complex/MWNTsmodified GC electrode
The cyclic voltammograms of ascorbic acid, dopamine
and uric acid at a bare GC and the iron(II)-complex/
MWNTs-modified GC electrodes are shown in Figure 2.
The oxidation peak potentials of AA, DA and UA are
very close to each other at a bare GC electrode (Figure 2a).
The electrochemical reactions of AA, DA and UA are
also quasireversible or irreversible. This finding indicates
the slow electron transfer rates for the oxidation of these
biomolecules at a bare GC electrode. Such sluggish electron
Figure 2. Cyclic voltammograms of ascorbic acid (0.5 mmol L-1),
dopamine (100 μmol L-1) and uric acid (0.5 mmol L-1) at (A) bare GC
electrode (B) the iron(II)-complex/MWNTs modified electrode in
0.1 mol L-1 acetate buffer solution (pH 5.0). Scan rate was 100 mV s-1.
Figure 1. TEM images of (A, B) MWNT in DMF and (C) SEM image of
the iron(II)-complex/MWNTs composite film over GC electrode.

472
Electroless Deposition of bis(4’-(4-pyridyl)-2,2’:6’,2’’-terpyridine)iron(II) Thiocyanate Complex
J. Braz. Chem. Soc.
transfer kinetics may be related to the electrode fouling
caused by the deposition ofAA, DA, UA and their oxidation
products on the electrode surface.56 For dopamine, the
cathodic and anodic peaks appeared on GCE at 192 mV and
420 mV, respectively. The peak potential separation is about
280 mV. Moreover, on the electrode surface, DA is oxidized
at about the same potential as AA; and the homogeneous
catalytic oxidation of AA with DA in the vicinity of the
electrode surface causes the main interference of AA for
the detection of DA. Therefore, the AA oxidation peak is
not distinguishable in the presence of DA.8 In a comparison
with the GC electrode, the reversibility of DA at the iron(II)-
complex/MWNTs8 modified GC electrode is considerably
improved, so that the anodic and cathodic peak potentials
separation becomes 70 mV, and the ipa/ipc ratio becomes
near unity (Figure 2b). The peak potential due to the
oxidation of AA occurs at 180 mV (vs. Ag/AgCl, KCl,
3 mol L-1) which is about 200 mV more negative than the
GC electrode. In the case of UA, a sharp oxidation peak at
465 mV and a small reduction peak at 418 mV occurred at
the iron(II)-complex/MWNTs-modified GC electrode, i.e.,
the negative shift with much enhanced anodic peak current
in comparison with the GC electrode.
These findings showed high enhancement in the
electron transfer rates of AA, DA and UA on the iron(II)-
complex/MWNTs-modified GC electrode (Figure 2b).
Apparent peak shapes for AA, DA and UA at the iron(II)-
complex/MWNTs-modified GC electrode are improved
against those of the GC electrode. So the well-shaped
peaks of these species could be observed in the presence of
iron(II)- complex/MWNTs-modified GC and provided an
excellent electrochemical reactivity. Moreover, no fouling
of the modified electrode was observed due to the oxidation
of AA, DA or UA.
pH (Figure 3). As Figure 3 shows, in the pH range 3-9 the
anodic peak potential difference (Epa) in the instance of
DA and UA, was almost constant. However, the iron(II)-
complex showed a better electrocatalytic activity for the
oxidation of all three components in solution with a pH
equal to 5 which had the maximum anodic peak potential
differences and the sensitivities were also enhanced by
working at this pH where the currents are maximum.
Figure 3. Anodic peak potential differences (Epa) between AA-DA and
DA-UA of ascorbic acid (0.5 mmol L-1), dopamine (100 μmol L-1) and uric
acid (0.5 mmol L-1) at the iron(II)-complex/MWNTs modified electrode
in 0.1 mol L-1 acetate buffer solution (pH 5.0). Scan rate was 100 mV s-1.
The pH effects on peak potentials (Epa) for the
oxidation of AA, DA and UA on the iron(II)-complex/
MWNTs-modified GC electrode are shown in Figure 4.
All species Epa shifted to lower potential by increasing
Dependence of the voltammetric resolution of AA, DA and
UA at the iron(II)-complex/MWNTs modified GC electrode
on pH
To optimize the electrochemical response of the
modified electrode for the electrocatalytic oxidation ofAA,
DA and UA, the effects of pH on the electrode response
and the oxidation potential were investigated by cyclic
voltammetry in the solution containing 1.0 mmol L-1 of
analyte. With increasing pH (3-9), the peak potentials of
all three species AA, DA and UA were shifted to more
negative values (not shown). This is a consequence of a
deprotonation step involved in the oxidation processes
and is facilitated at higher pH. For AA and DA, the anodic
peak potential difference (Epa) increased up to pH 5 with
increasing in pH and then decreased with more increase in
Figure 4. Dependence of the oxidation peak potentials of 0.5 mmol L-1
ascorbic acid, 100 μmol L-1 dopamine and 0.5 mmol L-1 uric acid on pH
at the iron(II)-complex/MWNTs modified GC electrode in 0.1 mol L-1
acetate buffer solution (pH 5.0). Scan rate was 100 mVs-1.

Vol. 22, No. 3, 2011
Kamyabi et al.
473
pH. The calculated [∂Epa/∂pH] = -24.7 for AA, suggests
a two-electron, one-proton oxidation process.57,58 However,
the calculated [∂Epa/∂pH] for DA and UA are -57.2,
-59.0 mV pH-1, respectively. These are close to that of
expected for monoelectronic/monoprotonic electrode
reaction (-59.2 mV pH-1 at 25 °C) and shows the electron
transfer was accompanied by an equal number of protons.
The stability of electrocatalytic activity of the modified
GC electrode towards oxidation of AA, DA and UA were
checkedseparatelybyrepetitivescanningintheacetatebuffer
pH 5.0 solutions of each of species, between 0.0 to 0.7 V at
scan rate of 100 mV s-1. The peak current was considered as
a factor in indicating the stability of the modified electrode
at various conditions of operation. The anodic peak current
showed a small decrease (about 2-3%, not shown) after
twenty cyclic voltamograms in the pH 5.0 acetate buffer
solution. In addition, the electrode could also withstand if
stored in solution for a period of time (at least 2 week) and
the electrocatalytic currents remained almost unchanged.
These phenomena show that the modified electrode is stable
in aqueous solution. The high stability of the adsorbed
iron(II)-complex against desorption in aqueous solution, is
related to the chemical and mechanical stability of nanotube
film. This stability probably is due to the strong interaction
of aromatic groups of the iron(II)-complex with π-staking of
carbon nanotubes and the possible interaction between the
iron(II)-complex and activated carbon nanotubes.52
Figure5.DifferentialpulsevoltammogramsofmixtureofAA(0.5mmolL-1),
DA (100 μmol L-1) and UA (0.5 mmol L-1) at (A) a bare GC electrode,
(B) MWNTs modified GC electrode, (C) the iron(II)-complex/MWNTs
modified GC electrode in 0.1 mol L-1 acetate buffer solution (pH 5.0).
Scan rate was 100 mV s-1.
Differential pulse voltammetry
The linear ranges for the determination of AA,
DA and UA using DPV were 1.10×10-5-1.50×10-3,
9.00×10-7-1.20×10-3 and 2.00×10-6-1.50×10-3 mol L-1,
respectively. The limit of detection equals 3 sb m-1,
where sb is the standard deviation of the blank signal
and mol L-1 is the slope of the calibration curve for
the proposed method. The theoretical detection limits,
of the proposed method, for AA, DA and UA were
8.00×10-6, 2.00×10-7, and 1.0×10-6 mol L-1, respectively.
The electrooxidation processes of AA, DA and UA in a
mixture when the concentration of one species changed,
while those of the other two species were kept constant
were also investigated. Figure 6 shows the DPV for the
determination ofAA, DA, and UA in the mixture while the
concentration of one species is being changed, and those
of the other two species were kept constant. The slope of
the linear regression line for the calibration graph of each
species is nearly equal to that without the other species,
indicating that they do not interfere in the determination
of each other. Table 1 shows a comparison between
previously reported modified electrodes for determination
of AA, DA and UA and the iron(II)-complex/MWNTs
Differential pulse voltammetry (DPV) was used for
simultaneous determination of AA, DA and UA at the
iron-complex/MWNTs modified GC electrode because of
its higher current sensitivity and better resolution than the
cyclic voltammetry. For this purpose, DPV was carried out
in the potential range of -50 to 550 mV. Figure 5 shows the
DPV of a mixture ofAA (0.5 mmol L-1), DA (0.1 mmol L-1)
and UA (0.5 mmol L-1) at a bare GC electrode, MWNTs-
modified GC electrode and the iron-complex/MWNTs-
modified GC electrode in 0.1 mol L-1 acetate buffer solution
(pH 5). It is obvious that the oxidation peak potentials of
AA, DA and UA are very close to each other at a bare GC
electrode and MWNTs-modified GC electrode (Figure 5,
curvesA and B). Three well-defined peaks at about 72, 298,
and 460 mV vs. Ag/AgCl (KCl, 3.0 mol L-1) were observed
at the iron(II)-complex/MWNTs modified electrode
(Figure 5, curve C), which correspond to the differential
pulse voltammograms of AA, DA, and UA, respectively.
Peak separations of 226, 162 and 388 mV between DA and
AA, DA and UA, and UA and AA, respectively, allow us
to detect AA, DA and UA simultaneously by using DPV.

474
Electroless Deposition of bis(4’-(4-pyridyl)-2,2’:6’,2’’-terpyridine)iron(II) Thiocyanate Complex
J. Braz. Chem. Soc.
Fiure 6. (A) Differential pulse voltammograms of AA at the iron(II)-complex/MWNTs modified GC electrode in the presence of 100 μmol L-1 DA and
0.5 mmol L-1 UA in 0.1 mol L-1 acetate buffer, pH 5.0, AA concentrations (from a to j): 0.16, 0.33, 0.75, 1.17, 1.58, 2.00, 2.42, 2.83, 3.25 and 3.50 mmol L-1.
(B) Differential pulse voltammograms of DA at the iron(II)-complex/MWNTs modified GC electrode in the presence of 0.5 mmol L-1 AA and 0.5 mmol L-1
UA in 0.1 mol L-1 acetate buffer, pH 5.0. DA concentrations (from a to g): 0.05, 0.16, 0.34, 0.50, 0.67, 0.83, and 1.00 mmol L-1. (C) Differential pulse
voltammograms of UA at the iron(II)-complex/MWNTs modified GC electrode in the presence of 0.5 mmol L-1 AA and 100 μmol L-1 DA in 0.1 mol L-1
acetate buffer, pH 5.0. UA concentrations (from a to i): 0.20, 0.40, 0.60, 0.80, 1.00, 1.20, 1.40, 1.60 and 1.80 mmol L-1.
Table 1. Comparison of analytical parameters of several modified electrodes for AA, DA and UA determination
Electrode
Method
Analyte
Linear range
(μmol L-1)
Limit of
detection
(μmol L-1)
Sensitivity
(μA μmol-1 L)
Reference
Modified carbon paste electrode by tetrabromo-p
benzoquinone
DPV
DPV
CV
AA
DA
UA
10-600
10-100
10-100
0.62
-
-
0.005
0.0074
0.0022
19
34
33
39
Poly (3-(5-chloro-2-hydroxyphenylazo)-4,5
dihydroxynaphthalene-2,7-disulfonic acid) film
AA
DA
UA
5-240
5-280
0.1-180
1.43
2.9
0.16
0.013
0.057
0.353
Oxidation in mild
acidic media
AA
DA
UA
197-988
1.97-9.78
19.7-97.8
-
-
-
-
-
-
Dopamine solutions-phosphate buffer
DPV
AA
DA
UA
25-500
1-20
2.5-20
13
0.11
1.4
0.007
0.006
0.09
Novel choline and acetylcholine modified glassy
carbon
DPV
DPV
DA
AA
0.7-5
7-90
0.3
0.9
-
-
14 I
54
CPE/CNF/Pdnano
AA
DA
UA
50-4000
0.5-160
2-200
15
0.2
0.7
-
-
-
Pt/PF/Pdnano
DPV
DPV
AA in presence
of ACOP
DA in presence
of ACOP
50-1000
7.1
5.92
59
0.5-100
0.5
0.0213
Iron(II)-complex / MWNTs/GC
AA
DA
UA
11-1500
0.9-1200
2-1500
8
0.2
1
0.0118
0.059
0.0027
(This work)

Vol. 22, No. 3, 2011
Kamyabi et al.
475
modified GC electrode. As can be seen the proposed
modified electrode shows somewhat similar (or worse)
performances, in some cases, or superior ones, in most
cases, than the previously reported modified electrodes.
Determination of AA in vitamin C injection, DA in
dopamine hydrochloride injection solutions and UA in
human blood serum
In order to demonstrate the capability of this modified
electrode for the catalytic oxidation of AA, DA and UA in
real samples, we examined this ability in the voltammetric
determination of AA, DA and UA in some pharmaceutical
preparations, such as vitamin C injection solution (standard
content 100 mg mL-1 AA, 5 mL per injection), (Daro Pakhsh
Co.), dopamine hydrochloride injection (DHI) solution
(standard content of 40 mg mL DA, 5 mL per injection),
(Rasht Co.) and human blood serum. Human blood serum
samples were centrifuged before the experiment. All
samples were diluted with acetate buffer (pH 5.0) and
then appropriate amounts of these diluted samples were
transferred to the electrochemical cell to determine each
species using DPV. The standard addition technique was
employed for AA, DA and UA determination. The results
of AA, DA and UA determinations in the real samples and
the spiked samples with AA, DA or UA standard solutions
are shown in Tables 3 and 4. The recovery and precision
were acceptable, revealing that the modified electrode could
be efficiently applied for determination ofAA, DA and UA
in pharmaceutical samples.
Interferences
For investigating the interference, several compounds
were selected. If the tolerance limit was taken as the
maximum concentration of the foreign substances, which
causes an approximately 5% relative error, for 0.5 mmol L-1
AA, 0.1 mmol L-1 DA, and 0.5 mmol L-1 UA, no interference
was observed for the following compounds (μ mol L-1): K+,
Ca2+, Mg2+, Zn2+, starch, citric acid, cysteine, glucose. The
results are listed in Table 2.
Table 2. Interferences of some foreign substances for 0.5 mmol L-1 AA,
100 μmol L-1 DA, and 0.5 mmol L-1 UA
Foreign substances
Starch
Tolerance level (μmol L-1)
400
250
100
40
K+, Mg2+, Ca2+, Zn2+
Citric acid
Cysteine
Glucose
50
Table 3. Recovery results obtained for determination of AA and DA and the spiked AA and DA in injection solutions (n = 5)
AA labeled concentration (mmol L-1)
Content
Added concentration (mmol L-1)
AA found (mmol L-1)a
Recovery (%)
0.00
0.47 0.013
0.97 0.014
1.47 0.02
94
0.50
97
0.50
1.00
98
1.50
2.00
1.98 0.02
99
97
2.42 0.022
DA found (mM)a
DA labeled concentration (mmol L-1)
Content
added concentration (mmol L-1)
Recovery (%)
0
(9.80 0.21)×10-3
(2.90 0.32)×10-2
(5.00 0.28)×10-2
(7.10 0.34)×10-2
(9.10 0.34)×10-2
98
97
2.00×10-2
4.00×10-2
6.00×10-2
8.00×10-2
1.00×10-2
100
101
101
aResults are expressed as mean value SD, based on five replicate.
Table 4. Recovery results obtained for determination of UA and the spiked UA in human blood serum (n = 5)
Sample detected UA (mmol L-1)
Content
Added concentration (mmol L-1)
UA Found (mmol L-1)a
Recovery (%)
5.00×10-1
1.00
(9.10 0.21)×10-1
1.43 0.32
101
102
98
4.00×10-1
1.50
1.86 0.28
2.00
2.42 0.34
101
aResults are expressed as mean value SD, based on five replicate.

476
Electroless Deposition of bis(4’-(4-pyridyl)-2,2’:6’,2’’-terpyridine)iron(II) Thiocyanate Complex
J. Braz. Chem. Soc.
Conclusions
15. Kalimuthu, P.; John, S. A.; Talanta 2010, 80, 1686.
16. Noroozifar, M.; Khorasani-Motlagh, M.; Taheri, A.; Talanta
2010, 80, 1657.
We have demonstrated the possibility of using the
iron(II)-complex/MWNTs modified electrode for the
simultaneous determination of UA, DA and AA. The
modified electrode showed excellent sensitivity, selectivity
and anti-fouling properties. The differential pulse
voltammetry oxidation peaks for AA and DA, DA and
UA, AA and UA are separated by 210, 136 and 346 mV,
respectively. Therefore, simultaneous or independent
measurements of the three analytes are possible without
any interference. The proposed methods can be applied to
the determination of DA, AA and UA in real samples with
satisfactory results.
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The authors wish to express their gratitude to the
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