Electrodeposition of Pt-Ru nanoparticles on multi-walled carbon nanotubes: Application in sensitive voltammetric determination of methyldopa

Article abstract of DOI:10.1016/j.electacta.2011.09.023

A modified glassy carbon electrode, prepared by potentiostatic electrodeposition of platinum-ruthenium nanoparticles (Pt-RuNPs) onto a multi-walled carbon nanotube (MWCNT) layer, offers dramatic improvements in the stability and sensitivity of voltammetric responses toward methyldopa (m-dopa) compared to glassy carbon electrodes individually coated with MWCNT or Pt-RuNPs. The surface morphology and nature of the hybrid film (Pt-RuNPs/MWCNT) deposited on glassy carbon electrodes was characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. A remarkable enhancement in the microscopic area of the electrode together with the catalytic role of the composite modifier resulted in a considerable increase in the peak current (110 times) and a negative shift (-200 mV) in the oxidation peak potential of m-dopa. The mechanism of the electrocatalytic process on the surface of the modified electrode was analyzed via cyclic voltammograms at various potential sweep rates and pHs of the buffer solutions. Differential pulse voltammetry was applied and shown to provide a very sensitive analytical method for the determination of sub-micromolar amounts of m-dopa, for which a linear dynamic range of 0.05-40 μM and a detection limit of 10 nM was obtained. The modified electrode was successfully used for accurate determination of trace amounts of m-dopa in pharmaceutical and clinical preparations.

Full text of DOI:10.1016/j.electacta.2011.09.023

Electrochimica Acta 58 (2011) 125–133
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrodeposition of Pt–Ru nanoparticles on multi-walled carbon nanotubes:
Application in sensitive voltammetric determination of methyldopa
Saeed Shahrokhiana,b,∗, Shokoufeh Rastgara
a Department of Chemistry, Sharif University of Technology, Tehran 11155-9516, Iran
b Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 April 2011
Received in revised form 4 September 2011
Accepted 7 September 2011
Available online 6 October 2011
A modified glassy carbon electrode, prepared by potentiostatic electrodeposition of platinum–ruthenium
nanoparticles (Pt–RuNPs) onto a multi-walled carbon nanotube (MWCNT) layer, offers dramatic improve-
ments in the stability and sensitivity of voltammetric responses toward methyldopa (m-dopa) compared
to glassy carbon electrodes individually coated with MWCNT or Pt–RuNPs. The surface morphology and
nature of the hybrid film (Pt–RuNPs/MWCNT) deposited on glassy carbon electrodes was characterized
by scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spec-
troscopy (EIS) techniques. A remarkable enhancement in the microscopic area of the electrode together
with the catalytic role of the composite modifier resulted in a considerable increase in the peak current
(110 times) and a negative shift (−200 mV) in the oxidation peak potential of m-dopa. The mechanism of
the electrocatalytic process on the surface of the modified electrode was analyzed via cyclic voltammo-
grams at various potential sweep rates and pHs of the buffer solutions. Differential pulse voltammetry was
applied and shown to provide a very sensitive analytical method for the determination of sub-micromolar
amounts of m-dopa, for which a linear dynamic range of 0.05–40 ␮M and a detection limit of 10 nM was
obtained. The modified electrode was successfully used for accurate determination of trace amounts of
m-dopa in pharmaceutical and clinical preparations.
Keywords:
Ruthenium nanoparticles
Platinum nanoparticles
Multi-walled carbon nanotube
Methyldopa
Voltammetry
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
metallic NPs on MWCNT surfaces show promise as nanocatalysts
for use in chemically modified electrodes [4–11].
Recent activities in analytical chemistry have focused on the
development of nano-scaled particles for sensor applications. Due
to their small size (normally in the range of 1–100 nm), there
are significant advantages to using nanoparticle-modified elec-
trodes compared to traditional micro-electrodes. These include
enhanced diffusion based on convergent rather than linear diffu-
sion at the smaller nanoparticles (NPs), high active surface areas,
improved selectivity, improved catalytic activity, higher signal-to-
noise ratios and unique electrochemical properties. The use of NPs
also provides control over the local microenvironment [1]. The
substrates for supporting metal nanoparticles have a considerable
influence on their morphology (size and distribution) and response
characteristics. Carbon nanotubes have intrinsic properties that
include high surface areas, unique physicochemical properties,
and high electrical conductivities, and their inherent size and hol-
low geometry make them extremely attractive as supports for
heterogeneous catalysis [2,3]. Furthermore, uniform dispersion of
Several methods for depositing metal NPs on carbon nanotubes
have been proposed [12–16]. Among them, electrochemical tech-
niques are particularly attractive for the electro-deposition of
metallic nanostructures. Various experimental parameters includ-
ing deposition potential, time and solution composition can be
manipulated to control the nucleation and growth rate of metal
NPs. Finally, electrochemical deposition is a clean, rapid and facile
synthesis that facilitates control over the size and distribution of
NPs [17,18].
Methyldopa (m-dopa) (l-␣-methyl-3,4-dihydrophenylalanine)
is an antihypertensive agent that is used in the treatment of high
blood pressure (HBP) or hypertension (HTN), especially when it
is complicated with renal disease. Its hypertensive proprieties are
primarily due to its action on the central nervous system. M-
dopa inhibits the enzyme DOPA decarboxylase, which converts
L-DOPA into dopamine and is a precursor for norepinephrine
(noradrenaline) and subsequently epinephrine (adrenaline). It is
converted to ␣-methyl norepinephrine in adrenergic nerve ter-
minals, and its antihypertensive action appears to be due to its
stimulation of central adrenal-receptors, which reduces sympa-
thetic tone and produces a fall in blood pressure. The therapeutic
concentration of m-dopa in human plasma is usually in the range
∗
Corresponding author at: Department of Chemistry, Sharif University of Tech-
nology, Tehran 11155-9516, Iran. Tel.: +98 21 66005718; fax: +98 21 66012983.
E-mail address: shahrokhian@sharif.edu (S. Shahrokhian).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.09.023

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S. Shahrokhian, S. Rastgar / Electrochimica Acta 58 (2011) 125–133
of 0.1–0.5 mg/L, and its average terminal elimination half-life is
2 h [19,20]. Clearly, detection and quantification of m-dopa is
an important feature in pharmaceutical and clinical procedures
[21–23]. Several analytical procedures have been reported for the
analysis of m-dopa in bulk form, pharmaceutical form or biologi-
cal fluids. These include titrimetry, chromatography [24,25], kinetic
methods [23,26], spectrophotometry [22,27] and H NMR [28]. How-
ever, these methods have disadvantages, including high costs, long
analysis times, the requirement of complex and tedious sample
pretreatments and, in some cases, a low sensitivity and selec-
tivity that makes them unsuitable for a routine analysis. On the
other hand, electrochemical methods have attracted great interest
because of their simplicity, rapidness and high sensitivity in detect-
ing m-dopa and various other analytes without requiring tedious
pre-treatments [29–34].
In the present work, we synthesized a composite of MWC-
NTs modified with Pt–Ru nanoparticles by simple electrochemical
deposition onto MWCNT films pre-casted on the surfaces of
glassy carbon electrodes (GCE). Scanning electron microscopy
(SEM), energy dispersive X-ray spectroscopy (EDS), electrochemi-
cal impedance spectroscopy (EIS) and cyclic voltammetry (CV) were
used to characterize the electrode surface. The experimental results
indicate that the Pt–RuNPs–MWCNT hybrid exhibits an increased
electroactive surface area and enhanced electron-transfer at the
electrode surface. The resulting electrochemical sensor is conve-
nient, sensitive and applicable to the determination of m-dopa
concentrations in pharmaceutical and clinical assays.
morphologies of MWCNTs and Pt–RuNPs electrodeposited on
MWCNTs were obtained using a scanning electron microscope
(SEM), and the composition(s) of Pt–RuNPs–MWCNT/GC electrodes
were investigated by energy dispersive X-ray spectroscopy (EDS)
(RONTEC, QUANTAX). Scanning electron microscopy (SEM) images
were obtained with a VEGA\\TESCAN SEM that operated at an
accelerating voltage of 15 kV. The tip of a glassy carbon electrode
was used for SEM measurements. Before doing any SEM charac-
terization, it was necessary to completely dry the surface of the
modified electrode. The tips of the modified electrodes were held
in sample holders/stubs by adhesives, and a thin gold coating
(20–50 nm) was sputtered on the surface of the modified glassy
carbon electrode to make it more conductive before SEM analy-
sis. All the requirements mentioned for SEM were also necessary
for EDS experiments. A digital pH/mV/Ion meter (CyberScan model
2500) was used for the preparation of the buffer solutions. Elec-
trochemical impedance spectroscopy (EIS) measurements were
performed with a Potentiostat/Galvanostat EG&G model 273A
(Princeton Applied Research, USA) equipped with a Frequency
Response Detector model 1025 (Power Suite software), which was
used with a frequency between 100 MHz and 10 kHz and a 5 mV
rms sinusoidal modulation (effective value of a varying AC ampli-
tude) in 0.1 M KCl solution containing 1 mM of both K4Fe(CN)6 and
K3Fe(CN)6 (1:1 mixture) at the E1/2 of the [Fe(CN)6]3−/4− (0.13 V vs.
Ag/AgCl). Voltammetric experiments were carried out in buffered
solutions of m-dopa that were deoxygenated by purging with pure
nitrogen (99.999% from Roham Gas Company). Nitrogen gas was
also flowed over the surface of the test solutions during the exper-
iments.
2. Experimental
2.3. Preparation of the Pt–RuNPs–MWCNT/GCEs
2.1. Materials
Multi-walled carbon nanotubes were treated with a 1:3 (v/v)
mixture of HNO3 (65%) and H2SO4 (98%) at ca. 100 ◦C for 45 min
to obtain more edge sites and a better dispersion of nanotubes by
the creation of carboxylate groups. Before the modification step,
each GCE was polished with alumina slurry (0.1 ␮m) on a polishing
cloth, rinsed thoroughly with water, sonicated in water for 5 min
and air dried. A 2.0 mg portion of functionalized MWCNTs was dis-
persed in 2.0 mL of doubly distilled water and ultrasonicated for
15 min until a homogenous suspension of MWCNTs was obtained.
For preparation of the MWCNT/GCE electrode, 3 ␮L of the pre-
pared black suspension was applied directly onto a well-polished
GCE using a micropipette and left to dry at room temperature. In
the next step, the MWCNT/GCE electrode was immersed in 0.5 M
H2SO4 aqueous solution containing 1.3 mM ruthenium (III) chloride
and 1.3 mM hexachloroplatinic acid. The electrochemical deposi-
tion of the Pt–RuNPs was conducted for 15 s at −0.3 V (vs. Ag/AgCl).
The resulting electrode, denoted as Pt–RuNPs–MWCNT/GCE, was
removed and rinsed with water. Before voltammetric measure-
ments, the modified electrode was cycled five times between 0
and 1 V (scan rate 100 mV s−1) in a phosphate buffer solution of
pH 3.0 to obtain a reproducible response. This modified electrode
was also cycled 10 times in a buffer solution of pH 3.0 between
measurements to remove any adsorbed m-dopa on the surface of
the electrode and renew the electrode surface. The modification of
the electrode surface was refreshed daily.
Multi-walled carbon nanotubes synthesized by catalytic chemi-
cal vapor deposition (CVD, purity > 95%) and with an outer diameter
(o.d.) of 10–20 nm, an inner diameter (i.d.) of 5–10 nm and a
tube length of 0.5–200 nm were obtained from Nanostructured &
Amorphous Materials (Houston, TX, USA). RuCl3 was purchased
from Across Chemical Company, France. Methyldopa (m-dopa,
Supplement 1) was purchased from Aldrich, USA. Stock solutions of
m-dopa were freshly prepared as required in 0.1 M of the prepared
buffer solutions. All other chemicals were of analytical reagent
grade and were obtained from Merck. All aqueous solutions were
prepared with doubly distilled de-ionized water. A concentration
of 0.1 M acetate was used for preparation of solutions of pHs 4 and
5, and 0.1 M phosphate was used for solutions of pHs 3, 6 and 7.
Tablets of m-dopa (250 mg per tablet) were purchased from local
pharmacies. Fresh frozen human blood serum was obtained from
the Iranian Blood Transfusion Organization. Two percent (v/v) pure
methanol was added to the serum sample. After vortexing each of
the samples for 2 min, the precipitated proteins were separated
by centrifugation for 10 min at 10 000 rpm. Next, the sample was
diluted 10-fold, spiked with different amounts of standard m-dopa
without extraction or further treatments and used for the recov-
ery tests in the spiked samples. Each sample was run in triplicate,
and the relative standard deviation (RSD) for each sample was
calculated.
2.2. Apparatus
3. Results and discussion
Electrodeposition of Pt–RuNPs on MWCNTs and voltammet-
ric experiments were performed using a Metrohm potentio-
stat/galvanostat model 797VA. A conventional three-electrode
3.1. Surface characterization of the Pt–RuNPs–MWCNT/GCE
system was used with
(GCE, unmodified or modified, diameter = 2 mm),
Ag/AgCl reference electrode and a Pt wire counter electrode. The
a
glassy carbon working electrode
The surface morphology of the Pt–RuNPs–MWCNT/GCE was
characterized by SEM. As shown in Fig. 1A, a MWCNT Cover/Layer
without aggregation was observed on the electrode surface,
a
saturated

S. Shahrokhian, S. Rastgar / Electrochimica Acta 58 (2011) 125–133
127
Fig. 1. SEM images with different resolutions of (A) MWCNT and (B) Pt–RuNPs–MWCNT modified glassy carbon electrodes.
indicating that the MWCNTs were homogeneously dispersed on the
surface of the GCE. The diameters of the MWCNTs were approxi-
mately 20–50 nm. As can be seen in Fig. 1B, the Pt–RuNPs deposited
on the MWCNTs were spherical, with an average diameter of
approximately 55 nm (40–80 nm), and well distributed.
differential pulse voltammetry (CV and DPV). Fig. 3A depicts CVs
for the electrochemical oxidation of 10 ␮M m-dopa at the sur-
face of different electrodes including bare GCE and GCE coated
with MWCNT, Pt–RuNPs and Pt–RuNPs–MWCNT, respectively, in
a 0.1 M phosphate buffer solution of pH 3.0 using a scan rate
of 100 mV s−1. The m-dopa CVs display a broad oxidation peak
at 0.65 V on the bare GCE. However, a reversible behavior was
obtained for m-dopa within the potential window from 0.25 to
0.8 V on the surface of the modified GCEs. Compared to the bare
GCE, these voltammograms exhibit an oxidation peak current that
is enhanced by a factor of 40, 30 and 100 at the MWCNT/GCE,
Pt–RuNPs/GCE and Pt–RuNPs–MWCNT/GCE, respectively. On the
other hand, a 190 mV negative shift in the peak potential, together
with enhanced sharpness of the anodic wave, indicate that the
high surface area and electrocatalytic effect of the composite film
caused a remarkable enhancement of the kinetics of the electrode
process. This modified electrode also exhibited greater electro-
chemical activity toward the oxidation of m-dopa compared to
its MWCNT and Pt–RuNPs counterparts, revealing a synergis-
tic response rather than merely a combination of the individual
nanostructures. The DPV peak currents of m-dopa on the surface
of the MWCNT/GCE, Pt–RuNPs/GCE and Pt–RuNPs–MWCNT/GCE
show remarkable enhancements of approximately 50, 26 and
110 times, respectively, as large as those on the bare GCE
(Fig. 3B).
The EDS results shown in Fig. 2 show that Pt, Ru, C and Au
were the major elements on the electrode surface. The Au was
obtained from the gold coating of the Pt–RuNPs–MWCNT/GC sur-
face during SEM analysis. The EDS results clearly show that Pt and
Ru are co-electrodeposited on the surface of MWCNT films. Fur-
thermore, they indicate that Ru and Pt were not equally deposited
on the MWCNT-GC electrode in terms of mass percent even under
the same precursor salts concentrations. Therefore, loadings of Ru
nanoparticles were less than those of Pt deposits at the deposition
potential. This may be related to the fact that the reduction poten-
tials of Pt precursor salts are higher than the reduction potentials
for Ru precursor salts [7,35]. Thus, one would expect the reduc-
tion of the Pt metal to be more effective during the simultaneous
deposition of Pt and Ru.
3.2. Electrochemical characterization of the
Pt–RuNPs–MWCNT/GCE
The electrochemical response characteristics of the modified
electrode with regard to m-dopa were investigated via cyclic and

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S. Shahrokhian, S. Rastgar / Electrochimica Acta 58 (2011) 125–133
Fig. 2. The EDS pattern of Pt–RuNPs–MWCNT/GCE.
We also recorded DPVs of 4 ␮M m-dopa oxidation on the
semicircle diameter of the Nyquist plot and can be used to describe
the kinetics polarization of the electrode process. Fig. 4 shows
the Nyquist plots of K3Fe (CN)6/K4Fe(CN)6 at the bare GCE, the
MWCNT/GCE and the Pt–RuNPs–CNT/GCE. A semicircle with a large
diameter is observed at the surface of the bare GCE. The diameter
of this semicircle is reduced at the Pt–RuNPs–CNT/GCE compared
to the CNT/GCE and bare GCE, and the linear region is extended to
more frequencies, suggesting that the surface of the Pt–RuNPs–CNT
modified electrode exhibits lower electron-transfer resistance and
greatly increases the electron transfer rate.
surface of Pt–MWCNT/GCEs and Ru–MWCNT/GCEs. As shown in
Fig. 3C, modification of the surface of the MWCNT/GCE with
bimetallic Pt–Ru nanoparticles, in comparison to independent con-
tributions from Ru or Pt nanoparticles, improved the voltammetric
response toward m-dopa, increased the current (∼2 times) and
decreased the anodic potential of the oxidation peak (∼30 mV).
According to previous reports [7,36–39], deposition of bimetal-
lic Pt–Ru nanoparticles at the surface of the electrode has shown
improvement with respect to the separate deposition of Pt or Ru.
Existence of adsorbed hydrogen on the Pt surface has been impli-
cated in blocking the metal ions’ access to the electrode surface,
thereby inhibiting metal deposition. The disappearance of hydro-
gen adsorption/desorption peaks in the presence of Ru and the
high current in the double-layer region ascribed to the adsorption
of oxygen-containing species on Ru atoms led to more avail-
able sites for metal deposition and, consequently, improvement
in the voltammetric response toward m-dopa. Electrochemical
impedance spectroscopy (EIS) was employed to investigate the
impedance changes and the interfacial characterization of the
electrode surface during the modification process. The electron-
transfer resistance (Rct) at the electrode surface is equal to the
3.3. Optimization of the film thickness
3.3.1. Choice of the amount of MWCNTs
The effect of the amount of MWCNTs on the electrochemical
response toward m-dopa was investigated via cyclic voltamme-
try. The results show a marked increase in the oxidation peak
current of 4 ␮M m-dopa obtained using the MWCNT/GCE when
the amount of MWCNTs in the suspension was increased from 0
to 3 ␮L. In our investigations, the optimal m-dopa sensitivity was
obtained under conditions employing 3 ␮L of the MWCNT suspen-
sion (Fig. 5). Greater suspension volumes resulted in inappropriate
100
A
B
C
100
120
100
80
80
60
40
20
0
90
80
70
60
-20
-40
60
0.30
0.35
0.40
0.45
0.50
0.55
0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
E vs (Ag/AgCl)/V
E vs (Ag/AgCl)/V
E vs (Ag/AgCl)/V
Fig. 3. (A) CVs and (B) DPVs of m-dopa (10 ␮M) on the surface of a bare GCE (inset), a MWCNT/GCE (dashed line), a Pt–RuNPs/GCE (dotted line) and a Pt–RuNPs–MWCNT/GCE
(solid line) and (C) DPVs of MD (4 ␮M) on the surface of a RuNP–MWCNT/GCE (dotted), a PtNs–MWCNT/GCE (dashed line) and a Ru–Pt–MWCNT/GCE (solid line) in 0.1 M
PBS (pH 3.0), scan rate = 100 mV s−1 and pulse amplitude = 50 mV.

S. Shahrokhian, S. Rastgar / Electrochimica Acta 58 (2011) 125–133
129
the particles’ density remains approximately constant [18,40]. Each
NP has an independent diffusion layer and experiences convergent
diffusion rather than linear diffusion, resulting in high signal-to-
noise and high sensitivity. However, when the sizes of the NPs
are increased, their independent diffusion layers interact, reduc-
ing the overall mass transfer rate of the electroactive material to
the NP layer. Therefore, the current is less than that predicted for
isolated NPs. If there is a great enough increase in the NPs size(s),
the overlapping diffusion fields of neighboring particles results in
linear rather than convergent diffusion, leading to a decrease in the
peak current of m-dopa at the surface of the modified electrode
[1,41–43]. For these reasons, the maximum sensitivity for m-dopa
is obtained with a deposition time of 15 s.
3.4. Effect of potential scan rate
The electrochemical mechanism can usually be elucidated using
CVs at different potential sweep rates. Fig. 7A shows CVs of 0.1 mM
m-dopa at a modified electrode in a buffered solution of pH 3.0 at
different scan rates from 10 to 400 mV s−1. A slope of 0.957 for the
linear variation of the logarithm of the peak current with respect
to the logarithm of the sweep rate (Supplement 2A) describes a
predominantly adsorptive mechanism for the reaction of m-dopa
on the surface of the Pt–RuNPs–MWCNT film [44].
Fig. 4. Nyquist diagram (−Zꢀꢀ vs. Zꢀ) for the EIS measurements in 1 mM
K3Fe(CN)6/K4Fe(CN)6 and 0.1 M KCl at E1/2 = 0.13 V for a bare GCE (ꢀ), a MWCNT/GCE
(ꢁ) and a Pt–RuNPs–MWCNT/GCE (᭹).
The investigations herein show that increasing the potential
scan rate results in negative and positive shifts of the oxidation and
reduction peak potentials, respectively, which are linearly depen-
dent on the log ꢀ (Supplement 2B). According to the Laviron theory
[45], a plot of Ep vs. log ꢀ yields two straight lines with slopes
of −2.3RT/˛nF and 2.3RT/(1 − ˛)nF for the reduction and oxida-
tion peaks, respectively. Based on these linear relationships, the
charge transfer coefficient (˛) and the electron transfer number in
the rate-determining step (n) can be calculated as 0.50 and 2 using
the following equation:
mechanical properties and decreased the adherence of the MWCNT
film onto the GCE surface. Such effects led to a remarkable decrease
in the sensitivity and reproducibility of the modified electrode
response.
3.3.2. Choice of deposition time for the MWCNT/GCE
The effect of the number of deposited Pt–RuNPs on the peak
current was investigated using DPV, the corresponding results of
which are shown in Fig. 6. The oxidation peak current of 4 ␮M
m-dopa obtained with Pt–RuNPs–MWCNT/GCEs increased with
increasing deposition time from 5 to 15 s. However, when the
deposition time was longer than 15 s, the peak current gradually
decreased. As previously reported, increasing the deposition time
increases the average particle size of Pt–Ru nanoparticles, while
ka
kc
˛
log
= log
,
1 − ˛
where ka and kc are the slopes of the straight lines for Ep,a and Ep,c
vs. log ꢀ, respectively.
3.5. Influence of pH
To achieve the optimum pH and to evaluate the ratio of elec-
trons and protons involved in the anodic oxidation of m-dopa on
the surface of the modified electrode, the electrochemical behav-
ior of m-dopa was investigated in buffered solutions of varying
pH (pH = 3–8). The anodic peak potential shifted negatively with
increasing pH, suggesting the participation of H+ in the oxida-
tion process (Fig. 7B). The slope of the linear variation of Ep,a vs.
pH (−67.40 mV/pH) clearly indicates that equal numbers of elec-
trons and protons are involved in the electro-oxidation of m-dopa
(Supplement 3A) [46]. Integrating the results obtained in pH and
scan rate studies, the electrochemical oxidation of m-dopa at the
Pt–RuNPs–CNT/GCE should be a two-electron and two-proton pro-
cess. From the results of cyclic voltammetric investigations, which
are shown in Fig. 7B, one may conclude that the optimum pH for
further measurements is 3.0 (Supplement 3B).
40
20
0
-20
-40
-60
3.6. Effect of the time and potential of adsorption
The modified electrode was inserted in a buffered solution of
m-dopa (pH = 3.0) for a specified time after which multi-cyclic
voltammograms were obtained. In the first scan, anodic and
cathodic peaks corresponding to m-dopa were observed, and these
gradually disappeared with subsequent cycles. On the other hand,
increasing the adsorption time caused the m-dopa peak currents
0.2
0.3
0.4
0.5
0.6
0.7
0.8
E vs (Ag/AgCl)/V
Fig. 5. Effect of the drop size of the MWCNT suspension cast on the GCE surface on
the cyclic voltammograms of 4 ␮M m-dopa (down to up: 2, 3, 4 and 5 ␮L).

130
S. Shahrokhian, S. Rastgar / Electrochimica Acta 58 (2011) 125–133
26
110
100
90
B
A
24
22
20
18
16
14
12
10
8
d
e
c
b
f
a
80
70
6
60
0
10
20
30
0.3
0.4
0.5
0.6
t/s
E vs (Ag/AgCl)/V
Fig. 6. (A) Effect of deposition time on the DP voltammograms of 4 ␮M m-dopa on the surface of Pt–RuNPs–MWCNT/GCEs (a–f: 0, 5, 10, 15, 20 and 30 s), (B) The plot of Ip,a
vs. deposition time.
to increase. These observations suggest the formation of adsorbed
species on the electrode surface, indicating that m-dopa is chemi-
cally adsorbed on the electrode surface (through electrostatic and
␲–␲ interactions) and then diffused through the porous CNT layer.
Such behavior has been previously reported by Compton et al. as
a model of the inter-layer diffusion regime through the porous
layer of the CNT on the electrode surface [47,48]. Therefore, time
and potential are considered two paramount parameters for the
adsorption step that can affect the sensitivity of determinations by
changing the amount of adsorbed m-dopa at the modified electrode
surface. Increasing the adsorption time increased the peak current
(Fig. 8). The results indicate that, before reaching the equilibrium
adsorption, longer accumulation times (up to 200 s) enhance the
amount of adsorbed analyst. At longer times, saturation of the elec-
trode surface is achieved.
the oxidation peak current remained almost constant when the
adsorption potential was applied. Therefore, adsorption of m-dopa
was carried out under open circuit conditions.
3.7. Differential-pulse voltammetric determinations,
reproducibility evaluations
A highly sensitive electrochemical method with a low detec-
tion limit, DPV was used to determine trace amounts of m-dopa
under optimized experimental conditions at the surface of the
Pt–RuNPs–MWCNT/GCE (Fig. 9A). The results show that the oxi-
dation peak currents were proportional to the concentrations
of m-dopa over two intervals in the range of 0.05–4.0 ␮M and
4.0–40 ␮M in 0.1 M phosphate buffer solution at pH 3.0 (Fig. 9B).
These two linear ranges have different slopes (sensitivities) due to
the different amounts of m-dopa adsorbed on the electrode sur-
face in these two regions. It can be proposed that the adsorption
The effect of the potential was also investigated from 0.4 to
−0.5 V. The results show that, compared to open circuit conditions,
200
80
A
B
150
100
50
60
3
8
40
20
0
0
-20
-40
-60
-50
-100
-150
0.0
0.2
0.4
0.6
0.8
0.3
0.4
0.5
0.6
0.7
0.8
vs
vs
(Ag/AgCl)/V
E
(Ag/AgCl )/V
E
Fig. 7. (A) Effect of the potential sweep rate on the CV response to 0.1 mM m-dopa (10–400 mV s−1) in 0.1 M PBS. (B) CVs of 10 ␮M m-dopa at a Pt–RuNPs–MWCNT/GCE in
various pHs (from 3 to 8) of buffer solutions, scan rate = 100 mV s−1
.

S. Shahrokhian, S. Rastgar / Electrochimica Acta 58 (2011) 125–133
131
35
120
110
100
90
B
A
30
25
20
15
10
5
80
70
60
0
0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60
0
50
100
t
150
200
250
300
/
s
E vs (Ag/AgCl )/V
accumulation
Fig. 8. (A) Effect of accumulation time (sec) on the DPV of 10 ␮M m-dopa on the surface of the Pt–RuNPs–MWCNT/GCE. The supporting electrolyte was a 0.1 M phosphate
buffer solution of pH 3, and the pulse amplitude was 50 mV. (B) Plot of Ip,a vs. accumulation time.
of m-dopa directly onto the MWCNTs, as monolayer coverage at
low concentrations (0.05–4 ␮M) and as a multilayer at higher con-
centrations (4–40 ␮M), caused the calibration curve to break. At
concentrations higher than 40 ␮M, saturation of the modified elec-
trode surface is obtained, and the m-dopa anodic peak current
considerably decreases [49,50].
The linear regression equations related to two regions are
inserted in the curves in Fig. 9. In these measurements, a detec-
tion limit of 10 nM (S/N = 3) is obtained, indicating that the system
is sensitive toward trace amounts of m-dopa. Table 1 compares
response characteristics of the introduced modified electrode with
those of previously reported in the literature for the determi-
nation of m-dopa, showing that, due to its low detection limit
and very wide linear dynamic range, the present method is pre-
ferred to the others for the successful determination of m-dopa.
To study the reproducibility of the electrode preparation proce-
dure, five electrodes modified by the same fabrication procedure
were prepared and used for the determination of 1 ␮M and 10 ␮M
solutions of m-dopa. The RSD for the electrodes’ peak currents
(average of three determinations on each electrode) was calcu-
lated to be 3.05% and 3.00%. To determine the precision of the
method, electrochemical experiments were repeated 5 times with
the same Pt–RuNPs–MWCNT/GCE in solutions containing l ␮M
and 10 ␮M of m-dopa. The RSDs were calculated to be 2.33%
and 2.86%, respectively. In this work, any adsorbed m-dopa was
counteracted by applying ten CVs in supporting electrolyte (PBS)
before each measurement, thereby obviating fouling problems.
The modified electrode surface renewed in this way exhibited
excellently reproducible responses. Thus, the results herein indi-
cate that the modified electrode has high reproducibility and
excellent repeatability in both the preparation procedure and
the voltammetric determinations. In addition, after storing the
Pt–RuNPs–MWCNT/GCE in air for two weeks, the electrode main-
tained 95.3% of its initial peak-current response for an m-dopa
120
2
A
B
i
(µA)=5.466 C(µM)+0.096, R =0.998
p,a
110
100
90
40
30
20
10
0
80
70
2
i
(µA)= 0.568 C(µM) + 20.250, R =0.996
60
p,a
50
0.2
0.3
0.4
0.5
0.6
0
10
20
30
40
/µmol dm-3
E vs (Ag/AgCl)/V
C
m-dopa
Fig. 9. (A) DPVs for various m-dopa concentrations in the range of 0.05–40 ␮M in 0.1 M PB solution (pH 3), and (B) corresponding linear calibration curve (peak current vs.
m-dopa concentration). Pulse amplitude = 50 mV.

132
S. Shahrokhian, S. Rastgar / Electrochimica Acta 58 (2011) 125–133
Table 1
Comparison of different methods with the present work for the determination of m-dopa.
Method
Linear range (M)
Detection limit (M)
Electrode
Reference
DP voltammetrya
DP voltammetry
1H NMRc
1.00 × 10−7 to 2.00 × 10−5
5.00 × 10−8 to 4.00 × 10−5
1.39 × 10−4 to 4.20 × 10−4
4.65 × 10−6 to 2.32 × 10−4
1.85 × 10−4 to 16.70 × 10−4
4.80 × 10−8
1.00 × 10−8
7.40 × 10−6
1.80 × 10−6
–
Ppyox/NFR/Aub
Pt–RuNPs–MWCNT/GC
–
–
–
[22]
This work
[28]
[27]
[23]
FISd
Kinetic method
a
Differential pulse voltammetry.
b
Ppyox/NFR/Au: polypyrrole film was prepared by incorporation of nuclear fast red (NFR) as the doping anion during the electropolymerization of pyrrole onto a gold
(Au) electrode.
c
1H nuclear magnetic resonance spectroscopy.
Flow injection spectrophotometry.
d
Table 2
concentration of 1 ␮M, demonstrating the long-term stability of
this thin-film modifier on the surface of the GCE during determi-
nations in aqueous solutions.
Results of the recovery analysis of m-dopa spiked tablet samples.
No.
Spiked (␮M)
Expected (␮M)
Founda (␮M)
Recovery (%)
1
2
3
4
5
6
7
8
9
0
0.06
0.1
0.5
0.8
2
4
10
20
0.1
0.16
0.2
0.6
0.9
2.1
4.1
10.1
20.1
0.0984
0.1480
0.1870
0.6330
0.9260
2.0130
3.80
–
92.50
93.50
105.5
102.88
95.85
92.68
100.19
94.13
3.8. Analytical applications
To evaluate the applicability of the proposed method
in determinations in pharmaceutical preparations, the
Pt–RuNPs–MWCNT/GCE was used to determine the m-dopa
content of “methyldopa” tablets as real pharmaceutical samples
using the DPV method. Three tablets containing labeled amounts
of 250 mg m-dopa were accurately weighed, ground to a fine
powder, diluted in PBS to a nominal concentration of 0.1 ␮M
and then spiked with a standard solution of m-dopa in the range
of 4.0–40 ␮M. The voltammetric responses and corresponding
calibration plots of peak currents vs. concentrations are shown in
Fig. 10A and B. Using the standard addition method, the m-dopa
content was determined to be 249.92 mg with a RSD of 3.2%
(n = 3) per tablet, which is very close to the labeled amount of
250 mg. It is concluded that the tablet matrix does not result in any
interferences in the electrochemical analysis of m-dopa. Moreover,
for accuracy studies, the recoveries of the spiked tablet solutions at
8 concentration levels were evaluated (Table 2). The results are in
the range of 92.50–105.50%, which are acceptable levels according
to the US FDA Guidance for industry (bioanalytical method valida-
tion) [51]. The modified electrode was also used to analyze human
blood serum samples using the standard addition method. Because
there is binding between m-dopa and albumin, it was necessary
10.12
18.92
a
Average of three replicate measurements (rounded).
Table 3
Results of the recovery analysis of m-dopa spiked plasma samples.
No.
Spiked (␮M)
Founda (␮M)
Recovery (%)
1
2
3
4
5
0
–
–
0.2
0.6
1
0.19 0.14
0.62 0.05
1.09 0.09
2.36 0.22
96.66
102.90
109.00
118.00
2
a
Mean standard deviation (n = 3) (rounded).
to disrupt their interaction before the analysis. To do this, 2% (v/v)
pure methanol was added to the serum sample, which was then
thoroughly mixed. The resulting sample was diluted 10-fold and
spiked with different amounts of standard m-dopa in the range
of 0.2–2 ␮M (DPs are shown in Supplement 4). In these tests, no
120
A
B
40
110
100
90
35
30
80
70
25
60
2
(µA)= 0.502 C (µM) + 20.44, R =0.994
i
p,a
50
20
0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60
10
20
30
40
/µmol dm-3
E vs (Ag/AgCl)/V
C
m-dopa
Fig. 10. (A) DPVs for the standard addition of different amounts of m-dopa (4, 5, 6, 8, 10, 20 and 40 ␮M) to a real sample solution of m-dopa diluted with 0.1 M PB solution
(pH 3), and (B) the plot of peak current vs. added concentration of m-dopa. Pulse amplitude = 50 mV.

S. Shahrokhian, S. Rastgar / Electrochimica Acta 58 (2011) 125–133
133
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calculated by obtaining the recovery of known amounts of m-dopa
spiked in the serum solutions at 4 concentrations. The results show
an average recovery of 106.64% for m-dopa added to the serum
samples (Table 3).
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Acknowledgements
The authors gratefully acknowledge the support of this work by
the Research Council and the Center of Excellence for Nanostruc-
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Appendix A. Supplementary data
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