Electrocatalysis of dopamine in the presence of uric acid and folic acid on modified carbon nanotube paste electrode

Article abstract of DOI:10.1016/S1872-2067(12)60734-7

A chemically modified carbon paste electrode (CPE), consisting of 2,2'-[(1E)-(1,2-phenylenebis (azanylylidene)] bis(methanylylidene)]bis(benzene- 1,4-diol) (PBD) and multiwalled carbon nanotubes (CNTs), was used to study the electrocatalytic oxidation of dopamine using cyclic voltammetry, chronoamperometry, and differential pulse voltammetry (DPV). First, the electrochemical behavior of the modified electrode was investigated in buffer solution. Then the diffusion coefficient, electrocatalytic rate constant, and electron-transfer coefficient for dopamine oxidation at the surface of the PBD-modified CNT paste electrode were determined using electrochemical approaches. It was found that under optimum conditions (pH = 7.0), the oxidation of dopamine at the surface of such an electrode occurred at about 200 mV, lower than that of an unmodified CPE. DPV of dopamine at the modified electrode exhibited two linear dynamic ranges, with a detection limit of 1.0 μmol/L. Finally, DPV was used successfully for the simultaneous determination of dopamine, uric acid, and folic acid at the modified electrode, and detection limits of 1.0, 1.2, and 2.7 μmol/L were obtained for dopamine, uric acid, and folic acid, respectively. This method was also used for the determination of dopamine in a pharmaceutical preparation using the standard addition method.

Full text of DOI:10.1016/S1872-2067(12)60734-7

Chinese Journal of Catalysis 35 (2014) 201–209
ava i l a b l e a t w w w. s c i e n c ed i r e c t . c o m
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e /c h n j c
Article
Electrocatalysis of dopamine in the presence of uric acid and folic
acid on modified carbon nanotube paste electrode
Mohammad Mazloum‐Ardakania,*, Mahboobe Abolhasania, Bibi‐Fatemeh Mirjalilia,
Mohammad Ali Sheikh‐Mohsenib, Afsaneh Dehghani‐Firouzabadia, Alireza Khoshrooa
a Department of Chemistry, Faculty of Science, Yazd University, Yazd, Iran
b Shahid Bakeri High Education Center of Miandoab, Urmia University, Urmia, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
A chemically modified carbon paste electrode (CPE), consisting of 2,2'‐[(1E)‐(1,2‐phenylenebis
(azanylylidene)]bis(methanylylidene)]bis(benzene‐1,4‐diol) (PBD) and multiwalled carbon nano‐
tubes (CNTs), was used to study the electrocatalytic oxidation of dopamine using cyclic voltamme‐
try, chronoamperometry, and differential pulse voltammetry (DPV). First, the electrochemical be‐
havior of the modified electrode was investigated in buffer solution. Then the diffusion coefficient,
electrocatalytic rate constant, and electron‐transfer coefficient for dopamine oxidation at the sur‐
face of the PBD‐modified CNT paste electrode were determined using electrochemical approaches.
It was found that under optimum conditions (pH = 7.0), the oxidation of dopamine at the surface of
such an electrode occurred at about 200 mV, lower than that of an unmodified CPE. DPV of dopa‐
mine at the modified electrode exhibited two linear dynamic ranges, with a detection limit of 1.0
µmol/L. Finally, DPV was used successfully for the simultaneous determination of dopamine, uric
acid, and folic acid at the modified electrode, and detection limits of 1.0, 1.2, and 2.7 µmol/L were
obtained for dopamine, uric acid, and folic acid, respectively. This method was also used for the
determination of dopamine in a pharmaceutical preparation using the standard addition method.
© 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Received 25 August 2013
Accepted 22 October 2013
Published 20 February 2014
Keywords:
Electrocatalysis
Dopamine
Uric acid
Folic acid
Carbon nanotube
Voltammetry
Published by Elsevier B.V. All rights reserved.
1. Introduction
very sensitive but require compressing systems, tempera‐
ture‐controlling systems, separation systems, and other spec‐
Dopamine (DA) is an important neurotransmitter. It belongs
to the catecholamine group and plays a significant role in the
central nervous, renal, hormonal, and cardiovascular systems
[1,2]. Dysfunction of the dopaminergic system in the central
nervous system has been related to neurological disorders such
as schizophrenia and Parkinson’s disease [3]. Various com‐
monly usable analytical methods for DA and its analogs have
therefore been developed in the past. Some examples of these
methods are rapid liquid chromatography/tandem mass spec‐
trometry [4], chromatography methods [5–7], and capillary
electrophoresis mass spectrometry [8]. These methods are
trophotometric or electric detection systems. Electrochemical
detection of DA has attracted much interest because of the im‐
portance of DA in the central nervous system. A significant
problem in DA determination is fouling effects caused by ac‐
cumulation of reaction products, which form electropolymer‐
ized films on the electrode surface [9]. One promising approach
to overcoming problems arising from fouling of the biological
substrate is the use of electrocatalysis at chemically modified
electrodes (CMEs) [10–12].
Uric acid (UA) is a primary end product of purine metabo‐
lism. Abnormal levels of UA are symptoms of several diseases
* Corresponding author. Tel: +98‐351‐8211670; Fax: +98‐351‐8210644; E‐mail: mazloum@yazduni.ac.ir
DOI: 10.1016/S1872‐2067(12)60734‐7 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 2, February 2014

Mohammad Mazloum‐Ardakani et al. / Chinese Journal of Catalysis 35 (2014) 201–209
such as gout, hyperpiesia, and Lesch‐Nyhan syndrome [13].
(PBD) and studied its electrochemical behavior in a PBD‐modi‐
fied CNT paste electrode (PBDCNPE). The experimental results
showed that PBD has appropriate redox behavior for use as a
good modifier in the construction of redox‐active‐modified
electrode. We therefore investigated the suitability of this mod‐
ified electrode as a new electrocatalyst for the electrocatalysis
of DA; satisfactory results were obtained. In addition, we eval‐
uated the analytical performance of the modified electrode for
DA quantification in the presence of UA and FA. Finally, to
demonstrate the catalytic potential of this modified electrode
for electro‐oxidation of DA in real samples, we examined this
method for the voltammetric determination of DA in ampoule
preparations.
Monitoring of the concentration of UA in biological fluids can
therefore be used as an early warning of the presence of these
diseases. Colorimetric, enzymatic, and electrochemical methods
are used to determine the concentration of UA [14,15]. Colori‐
metric methods are unreliable for accurate determination of
UA concentration. Although determination of UA by enzymatic
methods is promising because of their high selectivities, this
methodology is inherently expensive and does not have a good
detection limit. Electrochemical methods for the determination
of UA are more selective, less expensive, and less time‐con‐
suming than other methods [16].
Folic acid (FA) is a water‐soluble vitamin B, which helps to
build healthy cells. A deficiency of FA is a common cause of
anemia and is thought to increase the likelihood of heart attack
and stroke. Many studies suggest that diminished folate status
is associated with enhanced carcinogenesis because FA, along
with vitamin B12, participates in nucleotide synthesis, cell divi‐
sion, and gene expression [17]. Periconceptual supplementa‐
tion of FA has been demonstrated to significantly reduce the
incidence and recurrence of neural tube defects such as spina
bifida in women [18]. A survey of the literature reveals that
there are various methods available for the determination of
FA, including liquid chromatography [19,20], flow‐injection
chemiluminometry [21], isotope dilution‐liquid chromatog‐
raphy/tandem mass spectrometry [22], and spectrophotomet‐
ric methods [23]. FA is an electroactive component, so some
electrochemical methods have been reported for its determina‐
tion [24]. Electrochemical methods are more desirable than
other techniques because they are convenient and low cost.
Carbon‐based electrodes are among the most commonly
used electrodes in voltammetric analysis because of their low
cost, wide potential windows, low electrical resistances, and
versatility of chemical modification [25]. The use of carbon
paste as an electrode has been applied in the preparation of
CMEs for several purposes, such as electrochemical sensors for
the analysis of biologically important compounds [26,27] and
for electrocatalysis [28–30]. The chemical modification of elec‐
trodes using electron‐transfer mediators is an interesting area
of analytical chemistry [31]. One of the most important effects
of any mediator is the reduction in the overpotential required
for electrochemical reactions, which enhances the sensitivity
and selectivity of the method [32,33]. In one type of modified
electrode, known as electrocatalytic modified electrodes, a
redox‐active modifier uses as an electrocatalyst to catalyze
oxidation of a substance.
2. Experimental
2.1. Apparatus and reagents
The electrochemical measurements were performed using a
potentiostat/galvanostat (SAMA 500 electroanalyzer system,
Iran). A three‐electrode cell was used at 25 ± 1 °C. A saturated
calomel electrode (SCE), a platinum wire, and the PBDCNPE
were used as the reference, auxiliary, and working electrodes,
respectively. All potentials in this work are reported versus
SCE. The pH measurements were carried out using a Metrohm
model 691 pH/mV meter. All solutions were prepared using
doubly distilled water. DA, UA, FA, and other reagents were
analytical grade (Merck). Phosphate buffer solutions (0.1
mol/L) were prepared from H3PO4‐NaH2PO4 (0.1 mol/L), and
the pH was adjusted with H3PO4 or NaOH (0.1 mol/L). Graphite
paste was prepared from two main components, i.e., graphite
powder (Merck) and paraffin oil (DC 350, Merck, density = 0.88
g/cm3).
2.2. Synthesis of 2,2'‐[1,2‐phenylenediyl‐
bis(nitrilomethylidene)]bis(4‐hydroxyphenol)
1,2‐Phenylenediamine (0.15 g, 1.4 mmol) was added to a
mixture of 2,5‐dihydroxybenzaldehyde (0.35 g, 2.5 mmol) in
methanol; the mixture was stirred for 30 min. The progress of
the reaction was monitored using thin‐layer chromatography.
After the reaction was complete, the red solid product was re‐
moved by filtration and washed with cold methanol. The pure
desired Schiff base was obtained in 96% yield. The Schiff base
product was identified from its physical and spectroscopic data.
Red solid. Yield: 96%. Mp: 270–272 °C. Anal. Calcd: C 68.9, H
4.6, N 8.04; Found: C 68.7, H 4.9, N 7.7. IR (KBr, cm−1): υ
3250–3500 (s, br, 2OH), 1619 (s, C=N), 1572, 1488 (Ar), 1289
Nanostructures with large specific surface areas provide
important and feasible platforms for catalysis [34], separation
[35], sorption [36], sensing [37], and fuel cells [38]. Carbon
nanotubes (CNTs) are one of the most actively studied materi‐
als because of their finite small size, high specific surface area,
high porosity, and unique physical, chemical, and electrical
properties [39,40]. In electrocatalysis, the use of nanomaterials
and CNTs significantly enhances the electron‐transfer kinetics
and mass transport [41–43].
1
(s, C–O). H NMR (400 MHz/DMSO‐d6): δ 12.13 (br, 2OH, in‐
tramolecular hydrogen bonding), 9.10 (br, 2OH), 8.79 (s, 2CH
imine), 7.40 (dd, 2H, Ar, J1 = 8.2 Hz, J2 = 2.3 Hz), 7.37 (dd, 2H, Ar,
J1 = 8.3 Hz, J2 = 2.3 Hz), 7.02 (d, 2H, Ar, J = 2.8 Hz), 6.86 (dd, 2H,
Ar, J1 = 8.1 Hz, J2 = 2.7 Hz), 6.78 (d, 2H, Ar, J = 8.8 Hz). 13C NMR
(100 MHz/DMSO‐d6): δ 164.28, 153.73, 150.04, 142.98, 128.03,
121.77, 120.28, 119.88, 117.38, 117.27. MS: m/z = 348 (M+, 3),
212 (10), 129 (14), 92 (78), 93 (14), 80 (42), 77 (47), 65 (100).
UV/λmax (nm): 360 (s), 260 (w).
In this study, we synthesized 2,2'‐[(1E)‐(1,2‐phenylenebis
(azanylylidene))bis(methanylylidene)]bis(benzene‐1,4‐diol)

Mohammad Mazloum‐Ardakani et al. / Chinese Journal of Catalysis 35 (2014) 201–209
2.3. Preparation of electrodes as electrocatalysts
buffer (pH 7.0, 0.1 mol/L) are shown in Fig. 1(a). A pair of re‐
versible peaks are observed at Epa = 0.200 V and Epc = 0.100 V
versus SCE, and ΔEp = (Epa − Epc) was 0.100 V. The electrode
process was quasi‐reversible, with ΔEp, greater than that ex‐
pected for a reversible system. Figure 1(b) shows that the an‐
odic and cathodic peak currents (Ip) were linearly dependent
on υ at scan rates of 10–800 mV/s. A linear correlation was
obtained between the peak current and the scan rate, indicat‐
ing that the control of the redox process was diffusion inde‐
pendent (Fig. 1(b)).
An approximate estimate of the surface coverage (Γ) of the
modified CPE (mol/cm2) was made by the method used by
Sharp et al. [44]. According to this method, the peak current is
related to the surface concentration of electroactive species by
the following equation:
The PBDCNPEs were prepared by mixing 0.94 g of graphite
powder, 0.03 g of PBD, 0.03 g of CNT, and 0.7 mL of paraffin oil
with a mortar and pestle until a uniformly wetted paste was
obtained. These amounts of materials were obtained by opti‐
mization. The paste was then packed into the end of a glass
tube (ca. 3.5 mm i.d. and length 10 cm). A copper wire inserted
into the carbon paste provided an electrical contact. When
necessary, the surface of the carbon paste was polished with a
smooth paper to obtain a shiny appearance. For comparison, a
PBD‐modified carbon paste electrode (CPE) without CNTs
(PBDCPE), a CNT paste electrode without PBD (CNPE), and an
unmodified CPE, i.e., without PBD and CNT, were also prepared
in the same way.
Ip = n2F2AΓꢀ/4RT
(1)
3. Results and discussion
where n represents the number of electrons involved in the
reaction, A (cm2) is the surface area of the PBDCNPE, Γ
(mol/cm2) is the surface coverage, and the other symbols have
their usual meanings. The surface concentration of PBD calcu‐
lated from the slope of the anodic peak current versus scan rate
(Fig. 1(b)) is Γ = 8.2 × 10−8 mol/cm2 for n = 2.
3.1. Electrochemical behavior of PBDCNPE
PBD is insoluble in aqueous media; we prepared the
PBDCNPE and studied its electrochemical properties in a buff‐
ered aqueous solution using cyclic voltammetry (CV). The CVs
for the modified electrode at different scan rates in phosphate
Laviron [45] derived general expressions for the linear po‐
tential sweep voltammetric response of surface‐confined elec‐
40
(13)
(a)
(b)
Ipa
30
0
y = 0.0395x + 2.177
R2 = 0.9915
30
20
10
Ipc
-30
y = -0.0346x - 0.9767
R2 = 0.9891
0
225
450
675
900
(mV/s)
0.3
0.2
0.1
0.0
(c)
Epa
0
(1)
Epc
-10
-20
-30
-40
0.5
1.0
1.5
2.0
2.5
3.0
3.5
log(V/s)
0.3
(d)
Epa
y = 0.080x + 0.024
R2 = 0.995
0.2
0.1
0.0
y = -0.086x + 0.293
R2 = 0.99
Epc
0
50
100
150
200
250
300
350
2.0
2.4
2.8
3.2
log(V/s)
E (mV)
Fig. 1. (a) CVs of PBDCNPE in phosphate buffer (pH 7.0, 0.1 mol/L) at scan rates of 10 (1), 20 (2), 50 (3), 70 (4), 80 (5), 100 (6), 120 (7), 200 (8),
300 (9), 400 (10), 600 (11), 800 (12), and 1200 (13) mV/s; (b) Variations in Ip with scan rate; (c) Variations in Ep with logarithm of scan rate; (d)
Magnification of the same plots for high scan rates.

Mohammad Mazloum‐Ardakani et al. / Chinese Journal of Catalysis 35 (2014) 201–209
troactive species:
logks = αlog(1 − α) + (1 − α)logα –
log(RT/nαFυ) – α(1 − α)nαF∆Ep/2.3RT
A plot of Ep as a function of logυ yields a straight line with a
slope equal to 2.3RT/(1 – α)nF for the anodic peak (Fig. 1(d)).
The values of α and ks for PBD oxidation were determined to be
0.31 and 1.17 s−1, respectively, using such a plot and Eq. (2).
12
(5)
(4)
10
8
(2)
6
(2)
(1)
4
(3)
2
3.2. Effect of pH on peak potential
0
The voltammetric behavior of the PBDCNPE was character‐
ized at various pH values using CV. Figure 2 shows the CVs of
the modified electrode in solutions at various pH values rang‐
ing from 3.0 to 10.0. The anodic peak potential was pH de‐
pendent. The inset in Fig. 2 shows E˚ʹ as a function of pH. The
results show that the slope (E˚ʹ/pH) is −52.2 mV/pH over the
pH range from 2.0 to 10.0. This slope was close to the Nernstian
value of −59.2 mV for a two‐electron, two‐proton process [46].
Two protons are therefore transferred in the redox reaction in
the pH range 3.0–10.0.
-2
0.0
0.2
0.4
0.6
E (V)
Fig. 3. CVs of (1) CPE in 0.1 mol/L phosphate buffer solution (pH 7.0)
and (2) in 0.2 mmol/L DA; (3) as (1) for PBDCNPE; (4) and (5) as (2)
at the surfaces of PBDCPE and PBDCNPE, respectively. In all cases, the
scan rate is 25 mV/s.
PBDCPE. The combination of CNTs and PBD therefore definite‐
ly improved the characteristics of DA oxidation. The PBDCNPE,
in phosphate buffer (pH 7.0, 0.1 mol/L) and without DA in so‐
lution, exhibited a well‐behaved redox reaction (curve (3));
upon addition of 0.2 mmol/L DA, there was a significant in‐
crease in the anodic peak current (curve (5)), indicating a
strong electrocatalytic effect [46]. So, it is concluded that elec‐
trocatalytic behavior occurs for DA oxidation at the PBDCNPE
surface via an EC' catalytic mechanism (Scheme 1). In this
mechanism, DA is oxidized in a catalytic chemical reaction (C')
by the oxidized form of PBD (PBDox), which is produced via an
electrochemical reaction (E). Therefore, when the PBD is oxi‐
dized at a potential of 180 mV, DA can also be oxidized at this
potential [33].
3.3. Electrocatalytic oxidation of DA at PBDCNPE
Figure 3 shows the cyclic voltammetric responses from the
electrochemical oxidation of 0.2 mmol/L DA at the PBDCNPE
(curve (5)), PBDCPE (curve (4)), and CPE (curve (2)). As
shown, the anodic peak potentials for DA oxidation at the
PBDCNPE and PBDCPE were about 180 mV, whereas at the
unmodified CPE, the peak potential was about 450 mV. From
these results, it was concluded that the best electrocatalytic
effect for DA oxidation was observed at the PBDCNPE. The peak
potential of DA oxidation at the PBDCNPE shifted by about 270
mV toward negative values compared with that at the unmodi‐
fied CPE.
3.4. Effect of scan rate on oxidation of DA
A comparison of DA oxidations at the PBDCPE and the
PBDCNPE shows a significant enhancement of the anodic peak
current at the PBDCNPE relative to that obtained at the
The effect of the scan rate on the electrocatalytic oxidation
of 0.2 mmol/L DA at the PBDCNPE was investigated using line‐
ar sweep voltammograms (Fig. 4(a)). The oxidation peak po‐
tential shifts with increasing scan rates towards a more posi‐
tive potential, confirming kinetic control of the electrochemical
reaction.
A plot of peak height (Ip) against the square root of the scan
rate (v1/2) in the range 5–100 mV/s was constructed (Fig. 4(b));
it was found to be linear, suggesting that at sufficient overpo‐
tential, the process is diffusion rather than surface controlled. A
plot of the sweep‐rate‐normalized current (Ip/v1/2) versus
sweep rate (Fig. 4(c)) exhibits the characteristic shape of an
EC'cat process. Also, from the points in the rising part of the
0.6
20
y = -0.0522x + 0.5313
0.4
R2 = 0.9999
0.2
15
0.0
-0.2
10
0
5
10
15
pH
5
0
-
-
2e
+
-
2H
PBD
PBDox
E
-5
(1)
0.4
(8)
HO
HO
O
O
-0.2
0.0
0.2
0.6
+ PBDox
NH2
C'
+ PBD
NH2
E (V)
Fig. 2. CVs (at 100 mV/s) of PBDCNPE at buffered pH values of pH = 3
(1), 4 (2), 5 (3), 6 (4), 7 (5), 8 (6), 9 (7), and 10 (8). Inset: Plot of E''
versus pH.
Scheme 1. Electrocatalytic reaction mechanism for DA oxidation at the
PBDCNPE surface.

Mohammad Mazloum‐Ardakani et al. / Chinese Journal of Catalysis 35 (2014) 201–209
voltammogram (known as the Tafel region), which is affected
ments for DA at the PBDCNPE. It shows the current‐time pro‐
files obtained at a working electrode potential of 300 mV for
various concentrations of DA. In the chronoamperometric
studies, we determined the diffusion coefficient of DA at the
PBDCNPE based on the Cottrell equation [46]. Under diffusion
control, a plot of I versus t−1/2 will be linear, and the value of D
can be obtained from the slope. Inset (a) in Fig. 5 shows the
experimental plots with the best fits for the different DA con‐
centrations used. The slopes of the resulting straight lines were
by the electron‐transfer kinetics between DA and the
PBDCNPE, a charge‐transfer coefficient of α = 0.51 was ob‐
tained for DA oxidation.
3.5. Chronoamperometric measurements
Chronoamperometry was used to investigate the processes
at the CMEs. Figure 5 shows chronoamperometric measure‐
30.0
(b)
22.5
(a)
(10)
25
20
15
10
5
15.0
y = 2.5477x + 0.9796
R2 = 0.9993
7.5
0.0
0
2
4
6
8
10
12
1/2 (mV/s)1/2
3.2
3.0
2.8
2.6
(c)
(1)
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0
50
(mV/s)
100
E (V)
Fig. 4. (a) Linear sweep voltammograms of PBDCNPE in 0.1 mol/L phosphate buffer (pH 7.0) containing 0.2 mmol/L DA at scan rates of 5 (1), 15
(2), 20 (3), 25 (4), 30 (5), 40 (6), 50 (7), 60 (8), 80 (9), and 100 (10) mV/s; (b) Variations in electrocatalytic current with the square root of scan
rate; (c) Variations in scan‐rate‐normalized current (Ip/v1/2) with scan rate.
60
40
20
0
40
30
20
10
0
125
100
75
50
25
0
y = 32.324x + 8.1669
y = 29.51x + 5.6445
y = 24.742x + 4.8582
y = 22x + 4.2551
y = 17.857x + 2.81
y = 15.284x + 1.5994
y = 12.488x - 0.3269
(a)
(b)
y = 16.937x + 8.48
R2 = 0.9939
(7)
(1)
0.5
1.0
1.5
t1/2(s1/2
2.0
2.5
0.0
0.5
1.0
[DA] (mmol/L)
1.5
)
15
10
5
y = 5.5782x + 3.3615
(c)
y = 4.687x + 2.9183
y = 3.9021x + 2.5492
y = 3.3527x + 1.9742
y = 2.5728x + 1.9764
y = 1.989x + 1.6808
y = 1.0191x + 1.6166
(8)
0
0.5
1.0
1.5
2.0
2.5
3.0
t1/2(s1/2)
(1)
0
10
20
30
t (s)
Fig. 5. Chronoamperograms obtained at PBDCNPE in phosphate buffer solution (pH 7.0, 0.1 mol/L) for DA concentrations of 0.0 (1), 0.2 (2), 0.4
(3), 0.6 (4), 0.8 (5), 1.0 (6), 1.2 (7), and 1.4 (8) mmol/L DA. Insets: (a) plots of I versus t−1/2 obtained from chronoamperograms (2)–(8), (b) plot of
slopes of straight lines against DA concentration, and (c) dependence of IC/IL on t1/2 derived from chronoamperograms.

Mohammad Mazloum‐Ardakani et al. / Chinese Journal of Catalysis 35 (2014) 201–209
plotted versus the DA concentration (Fig. 5, inset b). From the
slope of this curve, the value of D was found to be 1.52 × 10−6
cm2/s.
Chronoamperometry can also be used to evaluate the cata‐
lytic rate constant, k, for the reaction between DA and the
PBDCNPE according to Galus’s method [47]:
24
18
12
6
30
20
10
0
y = 0.0156x + 15.447
R2 = 0.999
y = 0.0922x + 8.035
R2 = 0.9919
IC/IL = γ1/2[π1/2erf(γ1/2) + exp(−γ)/γ1/2
]
(3)
where IC is the catalytic current of DA at the PBDCNPE, IL is the
limiting current in the absence of DP, and γ = kCbt (Cb is the bulk
concentration of DA) is the argument of the error function. In
the cases where γ exceeds 2, the error function is almost equal
to 1, so the above equation can be reduced to:
0
250 500 750 1000 1250
[DA] (mol/L)
(9)
(1)
IC/IL = π1/2γ1/2 = π1/2(kCbt)1/2
(4)
where t is the time elapsed (s). The above equation can be used
to calculate the rate constant of the catalytic process (k). Based
on the slope of the IC/IL versus t1/2 plot, k can be obtained for a
given DA concentration. Such plots obtained from the chrono‐
amperograms in Fig. 5 are shown in inset (c). From the values
of the slopes, the average value of k was found to be 4.36 × 103
mol−1 L s−1. The value of k explains the sharp feature of the cat‐
alytic peak observed for catalytic oxidation of DA at the surface
of the PBDCNPE. Finally, the heterogeneous rate constant of the
catalytic reaction was calculated to be k = 3.586 × 10−1 cm/s.
0
0.0
0.2
0.4
E (V)
0.6
0.8
Fig. 6. DPVs of PBDCNPE in 0.1 mol/L phosphate buffer solution (pH
7.0) containing DA concentrations of 30 (1), 40 (2), 60 (3), 80 (4), 100
(5), 200 (6), 400 (7), 600 (8), and 800 (9) µmol/L. Inset: plots of elec‐
trocatalytic peak current as a function of DA concentration.
FA simultaneously. The use of the PBDCNPE for the simultane‐
ous determination of DA, UA, and FA was demonstrated by
simultaneously changing the concentrations of DA, UA, and FA.
The DPV results, with three well‐distinguished anodic peaks at
potentials of 130, 360, and 700 mV, corresponding to the oxi‐
dation of DA, UA, and FA, respectively, showed that simultane‐
ous determination of DA, UA, and FA at the PBDCNPE was pos‐
sible (Fig. 7). In contrast, the bare electrode could not separate
the voltammetric signals of these substances and an overlap‐
ping voltammogram was obtained for the analytes.
The sensitivity of the modified electrode toward the oxida‐
tion of DA was found to be 0.0906 μA μmol−1 L, whereas the
sensitivity toward DA in the absence of UA and FA was found to
be 0.092 μA μmol−1 L. It is interesting that the sensitivities of
the modified electrode toward DA in the absence and presence
of UA and FA were virtually the same; this indicates that the
oxidation processes of DA, UA, and FA at the PBDCNPE were
independent, therefore simultaneous or independent meas‐
urements of the three analytes are possible without any inter‐
ference. If UA or FA affected the DA signal, the above‐men‐
tioned slopes would be different.
3.6. Differential pulse voltammetry investigations and limit of
detection
Differential pulse voltammetry (DPV) was used to deter‐
mine the concentration of DA. Figure 6 shows the DPVs ob‐
tained for the oxidation of different concentrations of DA at the
PBDCNPE. The dependence of the peak current on the DA con‐
centration is shown in the inset of Fig. 6. This inset clearly
shows that the plot of peak current versus DA concentration
consists of two linear segments with different slopes, corre‐
sponding to two different substrate concentration ranges. The
decrease in sensitivity (slope) in the second linear range is
caused by kinetic limitations. From analysis of these data, we
estimated that the lower limit of detection of DA is 1.0 μmol/L.
This value is comparable to these reported by other research
groups (Table 1).
3.7. Simultaneous determination of DA, UA, and FA
The main objective of this study was to detect DA, UA, and
Table 1
Comparison of some electrochemical procedures used in DA determination.
Scan rate
(mV/s)
Dynamic range
(μmol/L)
Detection limit
(μmol/L)
Electrode
Modifier
pH
Ref.
CPE
CPE
CPE
GCE
CPE
Poly (rhodamine B)
poly (l‐arginine)
Poly (naphthol green B)
MWCNT
7.0
5.6
7.0
8.0
6.5
5
50
100
50
6–1000
50–100
5–270
3–200
17–1900
3.99
0.5
0.25
0.8
[48]
[49]
[50]
[51]
[52]
100
2.0
Horseradish peroxidase immobilized on
PEGylated polyurethane nanoparticles
CPE
CPE
SDS micelles
PBD and MWCNT
7.0
7.0
80
100
8–134
30–800
3.7
1.0
[53]
this work

Mohammad Mazloum‐Ardakani et al. / Chinese Journal of Catalysis 35 (2014) 201–209
30
25
20
15
10
5
30
(a)
(b)
20
(7)
y = 0.0157x + 14.701
R2 = 0.9947
10
y = 0.0906x + 6.9075
R2 = 0.9892
(6)
0
0
0
0
200
400
600
(5)
[DA] (mol/L)
30
20
10
0
(c)
y = 0.03x + 5.896
R2 = 0.9878
(4)
y = 0.0733x - 0.5811
R2 = 0.9926
(3)
(2)
(1)
200
400
[UA] (mol/L)
600
800
30
20
10
0
(d)
y = 0.0147x + 6.88
R2 = 0.9957
y = 0.034x + 2.864
R2 = 0.9999
0
0.0
0.2
0.4
0.6
0.8
1.0
400
800
1200
[FA] (mol/L)
E (V)
Fig. 7. (a) DPVs of PBDCNPE in 0.1 mol/L phosphate buffer solution (pH 7.0) containing different concentrations (μmol/L) of DA + UA + FA mixed
solutions: (1) 50.0 + 70.0 + 100.0, (2) 80.0 + 112.0 + 160.0, (3) 100.0 + 140.0 + 200.0, (4) 200.0 + 280.0 + 400.0, (5) 300.0 + 420.0 + 600.0, (6)
400.0 + 560.0 + 800.0, and (7) 500.0 + 700.0 + 1000.0. (b), (c), and (d) are plots of peak currents as a function of DA, UA, and FA concentration,
respectively.
3.8. Interference study
The antifouling properties of the modified electrode toward
DA oxidation and its oxidation products were investigated by
recording the CVs of the modified electrode before and after
use in the presence of DA. CVs were recorded in the presence of
DA after cycling the potential 10 times at a scan rate of 25
mV/s. The peak potentials were unchanged, and the currents
decreased by less than 2.3%. This showed that at the surface of
the PBDCNPE, the sensitivity increases and the fouling effects
of the analyte and its oxidation product also decrease.
The influence of various foreign species on the determina‐
tion of 0.1 mmol/L DA was investigated. The tolerance limit
was taken as the maximum concentration of the foreign sub‐
stances, which caused an approximately ±5% relative error in
the determination. The tolerated concentrations of foreign
substances were 1.0 mol/L for Na+, Cl−, F−, S2−, CO32−, HCO3 ,
−
NO3 , and K+; 1 mmol/L for UA, FA, captopril, and N‐acetyl cys‐
−
teine; and 0.1 mmol/L for ascorbic acid, isoprenaline, epineph‐
rine, and levodopa.
3.10. Real sample analysis
3.9. Repeatability and stability of PBDCNPE
To demonstrate the catalytic oxidation of DA in real sam‐
ples, we used voltammetric determination of DA in a DA am‐
poule purchased from local sources. The determination of DA
in the ampoule samples was carried out using multipoint
standard addition to prevent any matrix effects. The amount of
unknown DA in the ampoule was obtained by extrapolating the
plot. The average amount of DA in the injection was found to be
0.97 mg, with a recovery of 97%, a value in good agreement
with the nominal value on the ampoule label (1.0 mg).
Also, the applicability of the PBDCNPE for simultaneous de‐
termination of DA, UA, and FA in real samples was investigated
using the electrode in mixture solutions. Table 2 shows the
percentage recoveries of DA, UA, and FA in synthetic solutions
with the PBDCNPE using the standard addition method; the
results are good for DA, UA, and FA.
The ability to generate a reproducible electrode surface was
examined using CV data from five separately prepared
PBDCNPEs obtained at the optimum solution pH. The calculat‐
ed relative standard deviations for various parameters
(1%–4%) indicated that the surface reproducibility was satis‐
factory. This degree of reproducibility is virtually the same as
that expected for an ordinary carbon paste surface [54]. In ad‐
dition, the long‐term stability of the PBDCNPE was tested over
a 3‐week period. When CVs were recorded after the modified
electrode was stored in the atmosphere at room temperature,
the peak potential for DA oxidation was unchanged, and the
current signals showed a decrease of less than 2.3% relative to
the initial response.

Mohammad Mazloum‐Ardakani et al. / Chinese Journal of Catalysis 35 (2014) 201–209
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Recoveries of DA, UA, and FA in synthetic solutions with PBDCNPE
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Synthetic solutions
(µmol/L )
Found
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(%)
DA UA
FA
—
—
400
—
DA
202.2
98.7
UA
—
—
—
FA
—
—
401.8
—
DA
UA
—
—
—
FA
—
—
200
100
200
—
—
—
101.1
98.70
99.45
198.9
100.45
—
200 400
201.1 398.6
100.55 99.65
200 400 400
150 300 300
100 200 200
203.1 397.5 399
148.7 302.5 298.5
102.5 201.5 198.5
101.55 99.37 99.75
99.13 100.83 99.50
102.5 100.75 99.25
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A CPE modified with PBD and CNTs was fabricated and used
for electrocatalytic determination of DA. The electro‐oxidation
of DA at the surface of the PBDCNPE occurred at a potential
about 270 mV less positive than that for the bare CPE. The use
of the PBDCNPE for the simultaneous determination of DA, UA,
and FA was demonstrated. The detected potential differences
of 230, 570, and 340 mV between DA−UA, DA−FA, and UA−FA,
respectively, were large enough to determine DA, UA, and FA
individually and simultaneously. Finally, this electrode was
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Acknowledgements
The authors wish to thank the Yazd University Research
Council, the IUT Research Council and Excellence in Sensors for
financial support of this research.
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Graphical Abstract
Chin. J. Catal., 2014, 35: 201–209 doi: 10.1016/S1872‐2067(12)60734‐7
Electrocatalysis of dopamine in the presence of uric acid and folic
acid on modified carbon nanotube paste electrode
O
+ 2H+
O
NH2
Mohammad Mazloum‐Ardakani*, Mahboobe Abolhasani,
Bibi‐Fatemeh Mirjalili, Mohammad Ali Sheikh‐Mohseni,
Afsaneh Dehghani‐Firouzabadi, Alireza Khoshroo
Yazd University, Iran;
HO
HO
Urmia University, Iran
NH2
PBDox
Dopamine is oxidized in a catalytic chemical reaction by the oxidized
form of a modifier (PBDox), produced via an electrochemical reaction at
PBD
a
multiwalled‐carbon‐nanotube‐modified carbon paste electrode
MWCNT in CPE
(MWCNT in CPE).

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