Electrocatalytic oxidation and determination of homocysteine at nanotubes-modified carbon paste electrode using dopamine as a mediator

Article abstract of DOI:10.2298/JSC121019022M

A carbon paste electrode modified with multiwall carbon nanotubes (MWCNTPE) was prepared to study the electrocatalytic activity of dopamine (DP) in the presence of homocysteine (HCy) and it was used for the determination of HCy. The diffusion coefficient of HCy (D = 6.79×10^-6cm ²s^-1),and the kinetic parameters of its oxidation, such as electron transfer coefficient (a = 0.46), and rate constant (k_h= 7.44×10²dm³mol^-1s^-1)were also determined using electrochemical approaches. Under the optimum pH of 5.0, the peak current of oxidation of HCy at MWCNTPE in the presence of DP occurred at a potential of about 530 mV and the results showed that the oxidation peak current of HCy at the modified carbon nanotubes electrode was higher than on the unmodified electrode. The peak current of differential pulse voltammograms of HCy solutions increased linearly in the range 3.0-600 μM HCy with a detection limit of 2.08 μM HCy. This method was also examined for determination of HCy in physiological serum and urine samples.

Full text of DOI:10.2298/JSC121019022M

J. Serb. Chem. Soc. 78 (10) 1595–1607 (2013)
JSCS–4521
UDC 577.112.386+544.6.076.32–
039.6:544.4:543.552
Original scientific paper
Electrocatalytic oxidation and determination of homocysteine at
nanotubes-modified carbon paste electrode using dopamine as a
mediator
1
2
*
SAFOORA MOHAMMADZADEH and MASOUD FOULADGAR
1 Department of Chemistry, Payame Noor University, P. O. Box 19395-3697 Tehran, Iran and
2 Department of Biochemistry, Falavarjan Branch, Islamic Azad University, Falavarjan, Iran
(Received 29 October 2012, revised 11 January 2013)
Abstract: A carbon paste electrode modified with multiwall carbon nanotubes
(MWCNTPE) was prepared to study the electrocatalytic activity of dopamine
(DP) in the presence of homocysteine (HCy) and it was used for the deter-
mination of HCy. The diffusion coefficient of HCy (D = 6.79×10-6 cm2 s-1),
and the kinetic parameters of its oxidation, such as electron transfer coefficient
(α = 0.46), and rate constant (kh = 7.44×102 dm3 mol-1 s-1) were also
determined using electrochemical approaches. Under the optimum pH of 5.0,
the peak current of oxidation of HCy at MWCNTPE in the presence of DP
occurred at a potential of about 530 mV and the results showed that the
oxidation peak current of HCy at the modified carbon nanotubes electrode was
higher than on the unmodified electrode. The peak current of differential pulse
voltammograms of HCy solutions increased linearly in the range 3.0–600 μM
HCy with a detection limit of 2.08 µM HCy. This method was also examined
for determination of HCy in physiological serum and urine samples.
Keywords: homocysteine; dopamine; electrocatalytic effect; carbon nanotubes,
carbon paste electrode; voltammetry.
INTRODUCTION
Homocysteine (HCy), as an important sulfur-containing amino acid, plays a
1
considerable role in many biochemical processes. HCy was discovered by Butz
2
and Du Vigneaud in 1932 by heating methionine in sulfuric acid. The produced
compound was found to have chemical properties similar to those of cysteine.
The concentration of HCy can be an independent risk factor for disease in
3,4
human society. Mild hyperhomocysteinemia has no signs but it provides the
conditions for developed premature coronary disease in the third or fourth decade
4
of life. Thus, early determination of the HCy plasma concentration is necessary
and essential. Based on clinical studies, the normal total plasma HCy concen-
*Corresponding author. E-mail: Fouladgar@iaufala.ac.ir
doi: 10.2298/JSC121019022M
1595

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MOHAMMADZADEH and FOULADAR
tration range has been confirmed as 5–15 µM in the fasting state and elevated
level of HCy has been classified in three range, moderate (16–30 µM) medium
5,6
(30–100 µM) and severe (>100 µM). The total plasma HCy includes, 75 %
homocysteine bound to various proteins via disulfide bonds and the remaining 25
% free HCy includes oxidized homocysteine dimers (homocystine) or as homo-
cysteine–cysteine hetero dimers. Only 1–2 % of HCy is in the reduced state and
is distributed among different tissues, cells, and intracellular compartments. In
addition, small metabolic changes of HCy affect many physiological concentra-
7,8
tions.
According to many researches, elevated plasma HCy concentrations are
known to be a factor implicated in the appearance and progression of various
4,9,10
diseases, such as coronary artery and cerebro vascular disease.
To date, due to the importance of the determination of HCy, different tech-
niques have been employed its measurement. The direct methods included the
11,12
enzyme immunoassay (EIA),
the fluorescence polarization immunoassay
13–15
16,17
(FPIA),
chromatography (GC),
and the enzyme-linked immunosorbent assay (ELISA).
Gas
18–21
22,23
liquid chromatography (LC),
high-performance
24–31
28,32,33
liquid chromatography (HPLC),
capillary electrophoresis (CPE)
and
1,5,34–41
electrochemical methods
fall in the category of indirect methods. The
advantages of electrochemical methods, such as sensitivity, selectivity, facile,
high performance, low cost, high accuracy and precision, aroused the attention of
1,5,35,41–51
many researchers.
Other methods such as immunoassay kits and chro-
matographic methods require sample preparation that consumes a long time, are
1,5,35
expensive and necessitate the use environmentally hazardous solvents.
In the present work, we applied a carbon nanotubes-modified graphite paste
electrode for determination of HCy using dopamine (DP) as an electrocatalytic
mediator. The nanotubes interconnect the graphite powder particles together to
form a continuous conductive network. The small diameters of the nanotubes
enabled the nanotubes to be homogeneously distributed in the thin electrode
material and to introduce a larger surface area to react with the electrolyte. The
improved electrical conductivity of the electrode was related to the high electrical
conductivity of the tubes, and the function of the electrical bridge between the
graphite particles. In the presence of carbon nanotubes, the anodic signal
increased, which could be due to the increase in the active surface of electrode
52–61
and increase the conductivity of the electrode.
Effective parameters on ano-
dic signal were studied and a method was proposed for determination of HCy in
biological serum and urine samples.
EXPERIMENTAL
Reagents and apparatus
All employed chemical were of analytical grade and purchased from Merck, Aldrich and
Fluka. Homocysteine (≥95 %) and DP (99 %) were purchased from Aldrich.

DETERMINATION OF HOMOCYSTEINE AT CARBON NANOTUBES MODIFIED PASTE ELECTRODE
1597
Spectrally pure graphite powder (particle size <50 µm) from Merck and multiwall carbon
nanotubes (> 90 %, MWCNTs, d×l, 100–70 nm×5–9 μm) from Fluka were used as the sub-
strate for the preparation of the carbon paste electrode as a working electrode (WE). High
viscosity paraffin (ρ = 0.88 kg L-1) from Merck was used as the pasting liquid for the
preparation of the paste electrodes.
Universal buffer solutions (0.04 M acetic acid, boric acid, phosphoric acid and 0.2 M
sodium hydroxide) contain the 0.1 M potassium chloride with different pH values were pre-
pared to examine and provide the optimum pH value.
All aqueous solutions were prepared from deionized water by passing through a Milli-Q
water purification system (resistance >18 MW cm-1).
The homocysteine and DP solutions, 1.0×10-2 M, were prepared daily by dissolving
0.0068 g homocysteine, respectively, 0.0095 g dopamine in the buffer solution and then
diluting to 5 ml in two volumetric flasks. The solution was kept in a refrigerator at 4 °C in the
dark. More dilute solutions were prepared by serial dilution of these solutions with water.
All electrochemical experiments including cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) were performed using a computerized potentiostat/galvanostat
(Autolab PGSTAT101, Eco Chem. Utrecht, The Netherlands). A Pentium IV computer con-
trolled all settings and data processing of the system. All the electrochemical studies were
performed at 25±1 °C. The system was connected to a three-electrode cell assembly consisting
of a 50 mL glass cell containing an Ag/AgCl electrode as the reference electrode (RE), a
platinum wire counter electrode and a MWCNTPE working electrode. All of the measured
potentials are reported vs. the Ag/AgCl reference electrode. The pH of the solutions was
controlled with a Metrohm pH meter (model 827). The electrode prepared with carbon nano-
tubes was characterized by scanning electron microscopy (SEM) (Seron Tech. AIS 2100).
Preparation of the modified electrode (MWCNTPE)
A mixture of 0.10 g carbon nanotubes (CNT) and 0.9 g graphite powder was hand mixed
using a pestle and mortar. To achieve a uniform mixture, diethyl ether was added to the
mixture, the mixture was mixed again, and the diethyl ether allowed adequately to evaporate.
Paraffin oil, about of 0.4 g, was added to the mixture and mixed well for 20 min until a uni-
formly wetted paste was obtained. The prepared paste was inserted into a glass tube (internal
radius 2.2 mm) with a copper wire in it for electrical connection. A fresh surface was obtained
after pushing out an excess the paste and polishing it on a weighing paper.
For comparison, a uniform carbon paste electrode (CPE) was obtained in the same way
without addition of CNT to the mixture.
Electrochemical procedure
The MWCNTPE as working electrode was located in an electrochemical cell containing
10 ml of buffer solution (universal pH 5 and 0.10 M KCl) including 400 µM DP to record the
voltammetric measurements. The voltamogramms were recorded with a scan rate 20 mV s-1 to
give the blank signal from initial to final potentials of 0.1 mV to 0.7 mV vs. the Ag/AgCl
reference electrode. Subsequently, different amounts of HCy solution were added to the cell
and the same procedure was repeated to give the analytical signal. Differential pulse voltam-
metry (DPV) curves were recorded with a pulse height and pulse width of 50 mV and 0.005 s,
respectively, using the same procedure to obtain blank and analytical signals. The difference
between the blank and the analytical signal was obtained as a net peak current that was pro-
portional to the concentration of HCy. Calibration curves were constructed by plotting the net
peak current vs. concentration of HCy.

1598
MOHAMMADZADEH and FOULADAR
Preparation of real sample
Urine sample was stored in a refrigerator (at 4 °C) immediately after collection. Ten
milliliters of the sample was centrifuged for 15 min at 1500 rpm and the supernatant solution
was filtered through a 0.45 µm filter. Then, the solution was diluted ten times with buffer
solution (pH 5.0 and 0.10 M KCl) and was transferred into the electrochemical cell, where it
was analyzed without any further pretreatment. The standard addition method was used for the
determination of HCy in real samples.
Physiological serum solutions (0.9 % NaCl) were analyzed directly after 1:1 diluted with
buffer solution (pH 5 and 0.10 M KCl) without any further preparation.
RESULTS AND DISCUSSION
SEM characterization of electrodes
The SEM image of the CPE and MWCNTPE has been shown in the Fig. 1. As
can be seen, multiwall carbon nanotubes were dispersed to the carbon paste matrix
and were caused increasing the surface area of the active sites of the electrode.
Fig. 1. SEM Image of A) CPE and B) MWCNTPE.
Electrochemical behavior of dopamine (DP) on MWCNTPE
The electrochemical behavior of DP on MWCNTPE in buffer solution (pH
5.0 and 0.10 M KCl) was characterized by cyclic voltammetry. The cyclic
voltammogram (Fig. 2) of DP showed well-defined and reproducible anodic and
cathodic peaks with a peak separation potential of ΔE = E − E = 620 mV.
p
pa
pc
This value was greater than the value of 59/n mV for reversible behavior and was
indicative of quasi-reversible behavior. The influence of potential scan rate (20 to
−1
500 mV s ) on the anodic peak current of DP was studied by cyclic voltam-
metry. The peak currents of DP grew with increasing scan rate and the anodic
1/2
peak current increased linearly with square root of the scan rate (I = 0.986v
–
2
– 0.534, R = 0.995). Such a dependence shows that the oxidation reaction of DP
62
on the MWCNTPE is diffusion-controlled (Fig. 2).
pH Effect
As the oxidation of DP is accomplished with the exchange of two electrons
and two protons, the dependence of the redox reaction of DP on pH could be con-

DETERMINATION OF HOMOCYSTEINE AT CARBON NANOTUBES MODIFIED PASTE ELECTRODE
1599
cluded. The voltammetric response of DP showed that with increasing pH of the
solution, the peak potential shifted to less positive potentials. In addition, the
effect of pH solution on the electrocatlytic oxidation of HCy in the presence of
DP at the surface of MWCNTPE was investigated using cyclic voltammetry. The
optimum pH was investigated by varying the pH from 3.0 to 6.0. As can be seen
in Fig. 3, maximum peak current was obtained at pH 5.0. Hence, this pH was
selected as optimum pH for further experiments.
Fig.2. Plot of I vs. ν1/2 for 400 µM DP at the MWCNTPE. Inset: cyclic voltammograms at
various scan rates of a) 50, b) 100, c) 150, d) 200, e) 300 and f) 400 mV s-1
in the universal buffer (pH 5).
Fig. 3. Effect of pH on the
electrocatalytic current.
Optimization of the dopamine (DP) concentration
The effect of DP concentration on the peak currents was studied by varying
its concentration in the range of 100–600 μM at pH 5.0. The electrocatalytic

1600
MOHAMMADZADEH and FOULADAR
response was optimized and the maximum value of the oxidation peak current
was achieved by increasing the DP concentration up to 400 μM. This value was
selected as the optimum concentration for further experiments.
Electrocatalytic oxidation of HCy on MWCNTPE
HCy, as an oxidizable compound, can be detected by electrochemical
8
methods based on its anodic oxidation. The voltammetric response of HCy at
MWCNTPE was investigated in presence and absence of DP in pH 5.0 buffer
−1
solution using cyclic voltammetry at a scan rate of 20 mV s . The recorded cyc-
lic voltammograms of HCy in the absence of DP did not show a significant redox
peak for HCy (Fig. 4, curve d). Moreover, a pair of well-defined redox peaks was
observed as the voltammetric behavior of DP (400 µM) in the absence of HCy
(Fig. 4, curve c). Upon the addition of 500 µM HCy, the anodic peak current
increased sharply and no cathodic current was observed in the reverse scan (Fig.
4, curve a). Therefore, HCy can be detected in the presence of DP potential about
0.53 V vs. Ag/AgCl at the surface of a MWCNTPE. As can be seen, the oxi-
dation of HCy on a CPE in presence of DP (Fig. 4, curve b) has a lower peak cur-
rent compare to that on the MWCNTPE. In the other words, the presence of
multiwall carbon nanotubes increased the conductivity and effective surface area
53,54
of the electrode for the oxidation of HCy.
Based on the the obtained infor-
mation, the electrocatalytic mechanism showen in Scheme 1 is suggested for the
oxidation of HCy. In the first step, DP is oxidized on the surface of the
MWCNTPE. Then, the oxidized form of DP is reduced by HCy, whereby HCy is
8
converted to its oxidized form.
Fig.4. Cyclic voltammograms recorded at a scan rate of 20 mV s-1 in universal buffer solution
pH 5.0. a) 400 µM DP at the MWCNTPE in the presence of 500 µM HCy, b) as a) but at the
CPE, c) 400 µM DP at the MWCNTPE, d) 500 µM HCy at the MWCNTPE
in the absence of DP.

DETERMINATION OF HOMOCYSTEINE AT CARBON NANOTUBES MODIFIED PASTE ELECTRODE
1601
Scheme 1. Electrocatalytic mechanism for the oxidation of HCy
at the surface of the modified electrode.
Cyclic voltammograms of 500 µM HCy in presence of 400 µM DP at pH 5.0
were recorded in the potential range −0.1 to 0.6 V at various sweep rates. With
−1
increasing scan rate from 5 to 40 mV s , the peak currents of HCy increaed (Fig.
5, inset). The results showed, there was a linear relationship between the peak
1/2
current and the square root of the scan rate (ν ) with the regression equation:
1/2
−1 1/2
I (µA) = 1.621v (mV s ) + 0.820
2
(R = 0.997)
Fig. 5. Plot of I vs. ν1/2 for 500 µM HCy in the presence of 400 µM DP at the MWCNTPE.
Inset: cyclic voltammograms recorded at various scan rates: a) 5, b) 10, c) 15, d) 20, e) 30 and
f) 40 mV s-1 in the universal buffer (pH 5.0).

1602
MOHAMMADZADEH and FOULADAR
This equation confirms that the process is diffusion controlled (Fig. 5). In
addition, by increasing in the scan rate and following the shift of the oxidation
peak potential towards more positive potential, the kinetic limitation of the
63
electrochemical reaction was verified.
In order to obtain information on the rate determining step, Tafel plot was
−1
obtained from raising part of the current–voltage curve at scan rate of 20 mV s
(Fig. 6). The slope of the Tafel plot was equal to 2.3RT/n(1−α)F, which was
−1
0.108 V decade . Using this data and assuming n = 1, then α = 0.46.
Fig. 6. Tafel plot for 500 µM HCy in the presence of 400 µM DP (pH 5.0) at the MWCNTPE.
Chronoamperometric study
The electrochemical response of HCy at the MWCNTPE was investigated
using various concentrations of HCy by chronoamperometry. The chronoampero-
grams were obtained in buffered solution by employing and adjusting a double-
step potential from 0.3 (first step) to 0.7 V (second step) vs. Ag/AgCl (Fig. 7).
−1/2
The linearity of the electrocatalytic current vs. t
indicates that the current
must have been under diffusion control. The inset in Fig. 7 shows the linear
–1/2
relationships between I vs. t
for different concentrations of HCy. The Cottrell
Equation is given by:
nFAc D
I =
(1)
πt
where I is the current (A), n the number of electrons involved in the process, F is
2
the Faraday constant, A is the area of the electrode (cm ), c is the initial
–3
2 –1
concentration of HCy (mol cm ), D is the diffusion coefficient (cm s ) and t is
time (s).

DETERMINATION OF HOMOCYSTEINE AT CARBON NANOTUBES MODIFIED PASTE ELECTRODE
1603
Fig.7. Chronoamperograms obtained at MWCNTPE in the absence (a), and in the presence of
b) 100, c) 300, d) 700, e) 900 and f) 1000 µM HCy in buffer solution (pH 5.0). Inset (A):
charge–time curves; inset (B): the Cottrell plot for data from the chronoamperograms; inset
(C): dependence of IC/IL on t1/2 derived from the chronoamperogramic data.
From the Cottrell Equation and the obtained slope values for different
concentrations of HCy, the diffusion coefficient could be calculated and was
−6
2 −1 62
found to be 6.79×10 cm s .
In addition, the chronoamperometry method was applied to obtain the rate
constant of the catalytic oxidation (k ) of HCy. For this purpose, the Galus
h
Method was used:
1/2 1/2
1/2
1/2
I /I = π
C L
γ
= π (k ct)
(2)
h
where I is catalytic current of HCy in the presence of DP at the MWCNTPE, I
C
L
is the limiting current in the absence of HCy, and t is the time elapsed (s). Based
1/2
on the slope of the I /I vs. t plots, k could be obtained for different con-
C L
2
3
centrations of HCy. The average value of k was calculated to be 7.44×10 dm
h
–1 –1
mol s .
Analytical features of the method
Differential pulse voltammetry method (DPV) was used for preparation of
the calibration plot. The DP voltammograms were recorded for the determination
of HCy in the buffer solution (pH 5.0 and 0.10 M KCl). The electrocatalytic
current of MWCNTPE increased with increasing HCy concentration in the pre-

1604
MOHAMMADZADEH and FOULADAR
sence of 400 µM DP. The results showed that there is a linear dynamic range of
3.0–600 µM for HCy.
2
I (μA) =0.008c
+0.873, R =0.997
HCy
The limit of detection was 2.08 µM and the relative standard deviations
−1
(RSD) for six replicated analyses of 200 and 400 µM HCy at 20 mV s were
1.15 and 2.3 %, respectively.
Interference study
To evaluate the ability of proposed method for determination of HCy in real
samples, such as urine, the interference effects due to some potential interfering
species that are commonly found in urine, were studied under the optimum
conditions with 400 µM HCy at pH 5.0. The tolerance limit was taken as the
maximum concentration of foreign substances that caused no more than ±5 %
relative error in the determination of HCy. Some cations may form complexes
with HCy or DP and disrupt or effect on electrocatalytic mechanism, but no inter-
2+
2+
2+
2+
ference was observed for the following cations: Pb , Ni , Ba and Ca (1000
2+
fold) or Fe (100 fold). The results are presented in Table I.
TABLE I. Interference study
Tolerance limit (M/MHCy
)
Species
Pb2+, NO32-, Ni2+ , SO42-, Ba2+, Cl-, Ca2+, glucose, methionine,
1000
phenylalanine, ethanol, fructose, lactose, sucrose, urea
100
50
10
1
Fe2+
Citric acid
Uric acid
Ascorbic acid, L-cysteine
Determination of HCy in real samples
The utility of the proposed sensor for the electrocatalytic determination of
HCy was investigated in physiological serum solution and urine samples. The
centrifuged urine sample and physiological serum were diluted ten and two
times, respectively, with buffer and then various amounts of HCy were added to
the physiological serum and urine samples. The recovery of HCy was obtained
using the standard addition method. The results are given in Table II, from which
it could be seen the proposed method had good recoveries of the spiked HCy.
TABLE II. Determination of HCy in real samples
Found SD (n = 3)
μM
Biological
sample
HCy added
Expected
Recovery
%
No.
μM
μM
1
2
3
Serum
–
–
< Detection limit
50.6 1.1
50.0
100.0
50.0
150.0
101.2
99.3
148.9 1.2

DETERMINATION OF HOMOCYSTEINE AT CARBON NANOTUBES MODIFIED PASTE ELECTRODE
1605
TABLE II. Continued
Found SD (n = 3)
μM
Biological
sample
HCy added
Expected
Recovery
No.
μM
μM
%
4
5
6
7
Urine
–
–
< Detection limit
48.7±0.6
–
50.0
50.0
100.0
50.0
100.0
200.0
97.4
102.2
99.3
102.2±0.7
198.6±0.8
CONCLUSIONS
In this paper, DP was proposed as a homogeneous mediator for deter-
mination of HCy by electrochemical oxidation at a multiwall carbon nanotubes
graphite paste electrode (MWCNTPE). The electrochemical behavior of HCy
was studied using cyclic voltammetry, differential pulse voltammetry and chro-
noamperometry in a buffer solution (pH 5.0 and 0.10 M KCl). The peak current
of HCy oxidation was at 0.53 V on the MWCNTPE in the presence of DP, while
in the absence of DP oxidation of HCy did not occur at the surface of the carbon
nanotube graphite paste electrode. The kinetic parameters of the electrocatalytic
process and the diffusion coefficients of HCy in an aqueous solution were deter-
mined. The proposed method has a good linear dynamic range and was applied
for the determination of HCy in biological serum and urine samples.
Acknowledgments. The authors gratefully acknowledge the National Foundation for
Iranian Elites, the research councils of Islamic Azad University, Falavarjan branch, and
Payame Noor University, Iran, for their support of this work.
И З В О Д
ЕЛЕКТОКАТАЛИТИЧКА ОКСИДАЦИЈА И ОДРЕЂИВАЊЕ ХОМОЦИСТЕИНА НА
ЕЛЕКТРОДИ ОД УГЉЕНИЧНЕ ПАСТЕ МОДИФИКОВАНОЈ ДОПАМИНОМ КАО
МЕДИЈАТОРОМ
SAFOORA MOHAMMADZADEH1 и MASOUD FOULADGAR2
1
Department of Chemistry, Payame Noor University, P. O. Box 19395-3697 Tehran, Iran и
2
Department of Biochemistry, Falavarjan Branch, Islamic Azad University, Falavarjan, Iran
Електрода од угљеничне пасте која је модификована вишеслојним угљеничним
наноцевима припремљена је за испитивање електрокаталитичке активности допамина у
присуству хомоцистеина и коришћена за одређивање хомоцистеина у раствору. На
основу електрохемијских мерења одређен је коефицијент дифузије хомоцистеина (D =
= 6,79×10-6 cm2 s-1) и кинетички параметари за његову оксидацију, као што су коефи-
цијент прелаза (α = 0.46) и константа брзине (kh = 7,44×102 dm3 mol-1 s-1). При опти-
малној вредности pH 5,0 максимум струје оксидације хомоцистеина на електроди од
угљеничне пасте модификованој допамином постиже се на потенцијалу од око 530 mV.
Запажено је да је тај максимум виши него на немодификованој електроди. Струјни мак-
симум за оксидацију хомоцистеина одређен диференцијалном пулсном волтаметријом
повећавао се линеарно у опсегу концетрације 3,0–600,0 μM са границом детекције 2,08
μM хомоцистеина. Ова метода је такође тестирана за одређивање хомоцистеина у
физиолошком серуму и у узорцима урина.
(Примљено 29. октобра 2012, ревидирано 11. јануара 2013)

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MOHAMMADZADEH and FOULADAR
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