Highly sensitive voltammetric sensor based on NiO nanoparticle room temperature ionic liquid modified carbon paste electrode for levodopa analysis

Article abstract of DOI:10.1016/j.molliq.2015.04.023

This paper describes the development of 1-methyl-3-butylimidazolium chloride ionic liquid-NiO nanoparticle modified carbon paste electrode (MBICl/NiO/NPs/CPE) for the voltammetric determination of levodopa (l-DOPA) in real samples. We describe the synthes

Full text of DOI:10.1016/j.molliq.2015.04.023

Journal of Molecular Liquids 208 (2015) 78–83
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Highly sensitive voltammetric sensor based on NiO nanoparticle room
temperature ionic liquid modified carbon paste electrode for
levodopa analysis
Masoud Fouladgar ⁎, Hassan Karimi-Maleh , Vinod Kumar Gupta c,d,e,⁎⁎
a,
b
a
Department of Biochemistry, Falavarjan Branch, Islamic Azad University, Isfahan, Iran
Department of Chemistry, Graduate University of Advanced Technology, Kerman, Iran
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India
Center for Environment and Water, The Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
b
c
d
e
Department of Applied Chemistry, University of Johansburg, Johansburg, South Africa
a r t i c l e i n f o
a b s t r a c t
Available online xxxx
This paper describes the development of 1-methyl-3-butylimidazolium chloride ionic liquid-NiO nanoparticle
modified carbon paste electrode (MBICl/NiO/NPs/CPE) for the voltammetric determination of levodopa
(L-DOPA) in real samples. We describe the synthesis and characterization of NiO/NPs with different methods
Keywords:
1-Methyl-3-butylimidazolium chloride
such as transmission electron microscopy (TEM); energy-dispersive X-ray spectroscopy (EDS) and X-ray diffrac-
NiO nanoparticles
Levodopa analysis
Modified electrode
tion (XRD). The electrochemical oxidation of L-DOPA occurred in a pH-dependent 2e− and 2H+ process, and the
electrode reaction followed a diffusion-controlled pathway. The oxidation peak potential of LDOPA on the MBICl/
NiO/NPs/CPE appeared at 450 mV, which was about 90 mV decrease of the overpotential compared to that ob-
tained on the traditional carbon paste electrode (CPE) and the oxidation peak current was increased for about
3
.0 times. The electrochemical parameter such as charge transfer coefficient (α = 0.73) was calculated. The
−
1
−1
linear response range and detection limit were found to be 0.7–900 μmol L and 0.4 μmol L , respectively
using the differential pulse voltammetry method (DPV). The results showed that the proposed sensor is highly
selective, sensitive with a fast response for L-DOPA analysis.
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
Nevertheless, each technique has often suffered from diverse disad-
vantages with regard to cost and selectivity, the use of organic solvents,
complex sample preparation procedures, or long analysis time. Electro-
chemical methods provide useful alternatives since they allow faster,
cheaper, and safer analysis [10,11].
Materials in nanosize such as graphene, type of carbon nanotubes,
nanoparticles and nanocomposite have also been incorporated into
modified electrode as a voltammetric sensor in the recent years
L-DOPA is a medication used to treat Parkinson's disease. This type of
disease is associated with low levels of dopamine in the brain. L-DOPA is
turned into dopamine in the body and therefore increases levels of this
chemical. Also, L-DOPA is used to treat the stiffness, tremors, spasms,
and poor muscle control of Parkinson's disease. Levodopa is also used
to treat these same muscular conditions when they are caused by
drugs such as chlorpromazine, fluphenazine, perphenazine, and others
[12–20]. In between of nano-based materials, metal oxide nanoparticles
and especially NiO/NPs are important multifunctional material with ap-
plications such as sensors. The various applications of NiO are due to the
specific chemical, surface and microstructural properties of this material
[
1]. According to the above points, many attempts have been made to
determine L-DOPA in biological and pharmaceutical conditions. Numer-
ous analytical methods have been developed to determine of L-DOPA in
different sample matrices, such as HPLC [2], spectrofluorimetry [3], flow
injection analysis [4], fluorescence spectrometry [5], photokinetic [6]
and voltammetric [7–9] methods.
[21–24].
Room temperature ionic liquids can possess archetypical properties
such as high intrinsic conductivity, high thermal stability, low volatility,
high polarity, high viscosity and wide electrochemical windows [25]. On
the other hand, RTIL is a suitable binder for preparation of carbon paste
electrodes or modification of other solid state electrodes such as glassy
carbon [26–36].
Here, we describe a linear sweep voltammetry, chronoamperometry,
electrochemical impedance spectroscopy and differential pulse
voltammetric studies of L-DOPA at a MBICl/NiO/NPs/CPE. Compared
⁎
⁎
Corresponding author.
Correspondence to: V.K. Gupta, Department of Chemistry, Indian Institute of
⁎
Technology Roorkee, Roorkee 247667, India.
E-mail addresses: fouladgar@iaufala.ac.ir (M. Fouladgar), vinodfcy@gmail.com,
vinodfcy@iitr.ac.in (V.K. Gupta).
http://dx.doi.org/10.1016/j.molliq.2015.04.023
0167-7322/© 2015 Elsevier B.V. All rights reserved.

M. Fouladgar et al. / Journal of Molecular Liquids 208 (2015) 78–83
79
with the unmodified carbon paste electrode using nonconductive paraffin
as the binder, the MBICl/NiO/NPs/CPE exhibited high conductivity by
using conductive MBICl and NiO/NPs for fabricating the electrode. The di-
rect electrochemical behaviors of L-DOPA on this new MBICl/NiO/NPs/CPE
were carefully investigated and further applied to the L-DOPA content
detection in real samples with satisfactory results.
firmly into one glass tube as described above to prepare MBICl/NiO/
NPs/CPE.
2.5. Preparation of real samples
Tap water samples were stored in a refrigerator immediately after
collection. Ten milliliters of the sample was centrifuged for 15 min at
1500 rpm. The supernatant was filtered using a 0.45 μm filter and
then diluted 5-times with the PBS pH = 6.0. The solution was trans-
ferred into the voltammetric cell to be analyzed without any further
pretreatment.
2
. Experimental
2
.1. Chemicals
All chemicals were of A.R. grade and were used as received without
Urine samples were stored in refrigerator immediately after collec-
tion (from the Kerman Health Centre from four health man). Ten milli-
liters of the sample was centrifuged for 20 min at 1500 rpm. The
supernatant was filtered out using a 0.45 μm filter and then diluted 5
times with the PBS (pH 6.0). The solution was transferred into the
voltammetric cell to be analyzed without any further pretreatment. A
food and pharmaceutical serum sample was used for real sample analy-
sis without any further pretreatment. The standard addition method
was used for the determination of L-DOPA in real samples.
any further purification. Doubly distilled water was used throughout.
Levodopa was purchased from Fluka. Sodium hydroxide and phosphoric
acid were purchased from Merck. Also, high viscosity paraffin (d =
−
1
0
.88 kg L ) from Merck was used as the pasting liquid for the prepara-
tion of the carbon paste electrodes.
−
2
−1
A 1.0 × 10
mol L
L-DOPA solution was prepared daily by
dissolving 0.197 g of L-DOPA (from Merck) in water and the solution
was diluted to 100 mL with water in a 100-mL volumetric flask. The so-
lution was kept in a refrigerator at 4 °C in the dark. More dilute solutions
were prepared by serial dilution with buffer solution.
Pharmaceutical serum samples were purchased from local market
and used without any further pretreatment.
Phosphate buffer (sodium dihydrogen phosphate and disodium
−
1
monohydrogen phosphate plus sodium hydroxide, 0.1 mol L
solutions (PBS) with different pH values were used.
)
2.6. Recommended procedure
MBICl/NiO/NPs/CPE was polished with a white and clean paper. To
prepare a blank solution, 10.0 mL of the buffer solution (PBS, pH 6.0)
was transferred into an electrochemical cell. The initial and final poten-
tials were adjusted to 0.35 and 0.65 V vs. Ag/AgCl, respectively. DPV was
recorded with a pulse height and a pulse width of 100 mV to give the
blank signal and labeled as Ipb. Then, different amounts of L-DOPA
solution were added to the cell, using a micropipette, and the DPV
was recorded again to get the analytical signal (Ips). Calibration curve
was constructed by plotting the catalytic peak current vs. the L-DOPA
concentration.
2
.2. Apparatus
Voltammetric investigations were performed in an analytical
system, μ-Autolab with (μ3AUT 71226) PGSTAT (Eco Chemie, the
Netherlands). The system was run on a PC using NOVA software. A con-
ventional three-electrode cell assembly consisting of a platinum wire as
an auxiliary electrode and an Ag/AgCl/KClsat electrode as a reference
electrode was used. The working electrode was either an unmodified
carbon paste electrode (CPE), NiO/NPs/CPE, MBICl/CPE or MBICl/NiO/
NPs/CPE. X-ray powder diffraction studies were carried out using a
STOE diffractometer with Cu-Ka radiation (k = 1.54 Å). Samples for
transmission electron microscopy (TEM) analysis were prepared by
evaporating a hexane solution of dispersed particles on amorphous
carbon coated copper grids.
3. Results and discussion
3.1. NiO nanoparticle characterization
Fig. 1A shows X-ray patterns of the pristine NiO nanoparticle. The
diffraction angles at 2θ = 37.26, 43.42, 63.86, 75.47 and 78.87, can be
assigned to (111), (200), (220), (311) and (222) planes of the NiO
nanoparticle (Cubic phase). The crystallite sizes were estimated using
the Scherrer formula:
2
.3. Synthesis procedure of NiO/NPs
To prepare the NiO/NPs, in a typical experiment, a 0.4 M aqueous
3 2 2
solution of Ni (NO ) ·6H O and a 0.6 M aqueous solution of sodium
hydroxide (NaOH) was prepared in distilled water. Then, the beaker
containing NaOH solution was heated at the temperature of about
D ¼ kλ=ðβ cosθÞ
ð1Þ
7
3 2
0 °C. The Ni (NO )2·6H O solutions were added dropwise (slowly for
2
.0 h) to the above heated solution under high-speed stirring. The
where λ is the X-ray wavelength, θ is the Bragg's angle, and β is the full
width of the diffraction line at half of the maximum intensity. For NiO/
NPs at 350 °C the corresponding crystallite size of ~18 nm was measured.
Fig. 1B shows EDAX analysis for NiO nanoparticles in this study.
Result shows that the presence of Ni and O elements confirms the
synthesis of NiO/NPs carefully. The morphology of the as-grown nano-
structures was characterized by TEM. Typical TEM micrograph of the
NiO/NPs is shown in Fig. 1C. Presences of dark point in this figure in
nanoscale size confirm synthesis of this nanoparticle carefully. It is
clear that in this case, NiO nanoparticle was successfully prepared.
beaker was sealed at this condition for 2 h. The precipitated Ni(OH)
was cleaned with deionized water and ethanol then calcined at 350 °C
for 2.0 h for synthesis of NiO/NPs.
2
2
.4. Preparation of the sensor
MBICl/NiO/NPs/CPE was prepared by hand-mixing of 0.90 g of
graphite powder and 0.10 g NiO/NPs plus paraffin and mixed well for
0 min until a uniformly wetted paste was obtained. The paste was
5
2
then packed into a glass tube (geometric surface area 0.091 cm ). Elec-
trical contact was made by pushing a copper wire down the glass tube
into the back of the mixture. When necessary, a new surface was obtain-
ed by pushing an excess of the paste out of the tube and polishing it on a
weighing paper. MBICl/NiO/NPs/CPE was prepared by mixing of 0.15 g
of MBICl, 0.85 g of the liquid paraffin, 0.1 g of NiO/NPs, and 0.90 g of
graphite powder. Then the mixture was mixed well for 50 min until a
uniformly wetted paste was obtained. A portion of the paste was filled
3.2. Electrochemical investigation
According to the previous report [9], we anticipated that the redox
response of L-DOPA would be pH dependent (see Scheme 1). In order
to ascertain this, the voltammetric response of L-DOPA was obtained
in solutions with varying pH from 4.0 to 7.0 (Fig. 3 inset) at a surface
of MBICl/NiO/NPs/CPE. Result shows that the potential (E) of the

80
M. Fouladgar et al. / Journal of Molecular Liquids 208 (2015) 78–83
Fig. 2. Potential–pH curve for electrooxidation of 250 μmol L− of L-DOPA at MBICl/NiO/
NPs/CPE with a scan rate of 100 mV s−1. Inset: influence of pH on linear sweep voltammo-
grams of L-DOPA at a surface of the modified electrode (pH 4–7, respectively).
1
electrode with the oxidation peak current of 9.1 and 3.6 μA, respectively.
In addition, at the surface of bare MBICl/CPE, the oxidation peak that ap-
peared at 520 mV with the peak current was 6.6 μA (Fig. 3, curve b),
which indicated that the presence of MBICl in CPE could enhance the
peak currents and decrease the oxidation potential. A substantial nega-
tive shift of the currents starting from oxidation potential for L-DOPA
and dramatic increase of current of L-DOPA indicated the catalytic abil-
ity of MBICl/NiO/NPs/CPE to L-DOPA oxidation. Comparison of oxidation
peak current and peak potential between MBICl/NiO/NPs/CPE and CPE
shows 90 mV decreasing in overpotential and increasing about 3.0
Fig. 1. A) XRD patterns of as-synthesized NiO nanoparticles. B) EDAX analysis for NiO
nanoparticle. C) TEM image of NiO nanoparticles.
redox couple was pH dependent, with a slope of 0.0675 mV/pH unit at
2
5 °C which was equal to the anticipated Nernstian value for a two elec-
tron, two proton electrochemical reaction (see Fig. 2). It can be seen that
maximum value of the peak current was appeared at pH 6.0, so this
value was selected throughout the experiments.
times in oxidation current at a surface of modified electrode.
−1
The influence of potential scan rate (ν) on I
p
of 300 μmol L
of L-
DOPA at the MBICl/NiO/NPs/CPE was studied by LSV at various sweep
rates (Fig. 4 inset). As shown in Fig. 5, the peak currents of L-DOPA
Current density derived from the linear sweep voltammograms of
−
1
5
00 μmol L of L-DOPA (pH 6.0) at the surface of different electrodes
grow with the increasing of scan rates and there are good linear rela-
is shown in Fig. 3 (inset). The results show that the presence of NiO/
NPs and MBICl together causes the increase of the electrode. The direct
electrochemistry of L-DOPA on the modified electrode was investigated
by the LSV method. Fig. 3 shows linear sweep voltammograms of
−1/2
tionships between the peak currents and ν
showed a positive shift in E
. The recorded LSVs
p
, which is confirming the kinetic limitation
in the electrochemical reaction. The result shows that the electrode
process is controlled under the diffusion step. At higher scan rate, the
dependence of the peak potential (Epa) and ln (ν) showed a linear
relationship with a regression equation of:
−
1
5
00 μmol L of L-DOPA at pH 6.0 at the surface of different electrodes
−
1
with a scan rate of 100 mV s . MBICl/NiO/NPs/CPE exhibited signifi-
cant oxidation peak current around 450 mV with the peak current of
ꢀ
ꢁ
1
0.9 μA (Fig. 3, curve a). The L-DOPA oxidation peak potential at NiO/
2
−1
Ep ¼ 0:023 lnðνÞ þ 0:403 r ¼ 0:9908; E in V; ν in V s
ð2Þ
p
CPE and at CPE observed around 480 and 540 mV vs. the reference
Fig. 3. Linear sweep voltammograms of a) MBICl/NiO/NPs/CPE, b) MBICl/CPE, c) NiO/NPs/
CPE and d) CPE in the presence of 500 μmol L− of L-DOPA at pH 6.0, respectively. Inset:
the current density derived from linear sweep voltammogram responses of
1
−
1
Scheme 1. Electrochemical oxidation mechanism for levodopa at a surface of electrode.
500 μmol L of L-DOPA at pH 6.0 at the surface of different electrodes.

M. Fouladgar et al. / Journal of Molecular Liquids 208 (2015) 78–83
81
phase element corresponding to the double-layer capacitance, and the
charge transfer resistance associated with the oxidation of low valence
mediator species. W is a finite-length Warburg short-circuit term coupled
to Rct
.
Chronoamperometric measurements of L-DOPA at MBICl/NiO/NPs/
CPE were carried out by setting the working electrode potential at
6
00 mV vs. Ag/AgCl/KClsat for the various concentrations of L-DOPA in
buffered aqueous solutions (pH 6.0) (Fig. 6A). For an electroactive ma-
terial (L-DOPA in this case) with a diffusion coefficient of D, the current
observed for the electrochemical reaction at the mass transport limited
condition is described by the Cottrell equation. Experimental plots of I
−/12
vs. t
were employed, with the best fits for different concentrations
of L-DOPA (Fig. 6B). The slopes of the resulting straight lines were
then plotted vs. L-DOPA concentration. From the resulting slope
and Cottrell equation the mean value of the D was found to be
−
6
2
1
.76 × 10 cm /s.
Fig. 4. Plot of Ipa versus ν1/2 for the oxidation of L-DOPA at MBICl/NiO/NPs/CPE. Inset shows
linear sweep voltammograms of L-DOPA at MBICl/NiO/NPs/CPE at different scan rates
3.3. Analytical parameters
−
1
(from inner to outer) of 5, 30, 45, 60, 80 and 100 mV s in 0.1 M phosphate buffer, pH 6.0.
DPV has a much higher current sensitivity and better resolution
than linear sweep voltammetry [37–49], the DPV was used for the
voltammetric determination of L-DOPA (Fig. 7 inset). The results
showed that the plot of the peak current vs. L-DOPA concentration
according to the following equation:
h
ꢀ
ꢁ
i
0=
1=2 −1
−1
Epa ¼ E þ m 0:78 þ ln D ks
–0:5 lnm þ ðm=2Þ lnðνÞ
ð3Þ
ð4Þ
was linear for 0.7–900 μmol L of L-DOPA (Fig. 7), with a regression
equation of I
p
(μA) = (0.0045 ± 0.0002) C
L
-DOPA + (0.1003 ± 0.0031)
2
with
(r = 0.9983, n = 17), where C is the concentration of L-DOPA in
−
1
−1
μmol L . The detection limit was obtained as 0.4 μmol L of L-DOPA
according to the definition of YLOD = Y + 3σ.
m ¼ RT=½ð1–αÞn F:
α
B
The value of m = 0.046 is calculated from Eq. (3). Therefore, the
electron transfer coefficient (α) is approximately 0.72 for the quasi-
reversible electrode process.
3
.4. Real sample analysis
In order to evaluate the analytical applicability of the proposed sen-
The electrochemical characterization of CPE, NiO/NPs/CPE, MBICl/CPE
and MBICl/NiO/NPs/CPE was carried out by means of electrochemical im-
sor, also it was applied to the determination of L-DOPA in biological
samples such as water, serum and urine. Standard addition method
was used for measuring L-DOPA concentration in the samples. The re-
sults are given in Table 1; which confirm that the modified electrode
retained its efficiency for the determination of L-DOPA in real samples.
−
1
pedance spectroscopy. The Nyquist plots for L-DOPA (500 μmol L
)
show a significant difference in the response for all the four electrodes
as shown in Fig. 5. A semicircle with larger diameter is observed for CPE
2
6
in the frequency range of 10 –10 Hz. However, the diameter of semicir-
cle diminished with the employment of MBICl/NiO/NPs/CPE. This implies
that charge transfer resistance of the electrode surface decreases and the
charge transfer rate increases on employing MBICl/NiO/NPs/CPE.
The electrical equivalent circuits (from the electrodes in the presence of
3
.5. Stability and reproducibility of the modified electrode
The stability and reproducibility of any sensor are two important an-
alytical parameters for application of a suggestion sensor. Our experi-
ments showed that after MBICl/NiO/NPs/CPE was stored for 4 weeks
at 4 °C, only a small decrease of peak current sensitivity with a relative
L-DOPA) compatible with the impedance spectra are shown in Fig. 5
inset. In this circuit, Rs, Q, and Rct represent solution resistance, a constant
Fig. 5. Nyquist plots of MBICl/NiO/NPs/CPE (a), MBICl/CPE (b), NiO/NPs/CPE (c), and
CPE (d) in the presence of 500 μmol L− of L-DOPA. Conditions: pH, 6.0; Edc, +0.5 V vs.
Ag/AgCl; Eac, 5 mV; frequency range, 0.1–100000 Hz. The equivalent circuit compatible
with the Nyquist diagram in the absence and presence of L-DOPA.
1
Fig. 6. A) Chronoamperograms obtained at MBICl/NiO/NPs/CPE in the presence of a) 50;
b) 100; and c) 150 μmol L− of L-DOPA in the buffer solution (pH 6.0). B) Cottrell's plot
for the data from the chronoamperograms.
1

82
M. Fouladgar et al. / Journal of Molecular Liquids 208 (2015) 78–83
Table 2
Interference study for the determination of 10.0 μmol L− of L-DOPA under the optimized
1
conditions.
Selected compounds for interference study
Tolerant limits
L
-DOPA
(WSubstance/W )
a
Glucose, fructose, lactose, sucrose, ascorbic acid
900
−
+
+
−
2+
2+
2−
, Al , NH
4
+, F−, Na+ 600
3+
Br , K , Li , Cl , Ca , Mg , SO
4
−
and ClO
4
,
Tryptophan, valine, methionine, lucine, histidine, glutamic
500
acid, alanine, glycine, phenylalanine
Uric acid, ammonia
Dopamine
50
20
Starch
Saturation
a
After addition of 1.0 mM ascorbic acid oxidase to cell.
overpotential of oxidation of L-DOPA was about 90 mV with 3.0-fold in-
crement in the oxidation peak current when using MBICl/NiO/NPs/CPE
as a sensor. Result confirms that the presence of NiO/NPs with high
surface area and MBICl with high conductivity structure can increase
sensitivity proposed sensor for L-DOPA analysis. The proposed sensor
was successfully applied to the L-DOPA detection in real samples.
Fig. 7. The plots of the electrocatalytic peak current as a function of L-DOPA concentration.
Inset shows the DPVs of MBICl/NiO/NPs/CPE in 0.1 mol L− of phosphate buffer solution
pH 6.0) containing different concentrations of L-DOPA. From inner to outer correspond
to 0.7, 1.0, 5.0, 18.0, 40.0, 62.0, 100.0, 150.0, 200.0, 250.0, 350.0, 450.0, 550.0, 650.0, 700,
1
(
1
8
00 and 900.0 μmol L− of L-DOPA.
standard deviation (RSD) of 1.5% (for 20.0 μmol L− of L-DOPA) was ob-
served. This showed good stability of the modified electrode. Further-
more, the reproducibility of the determination was performed
1
Acknowledgments
The authors gratefully acknowledge the Islamic Azad University,
Falavarjan Branch, Isfahan, Iran.
−
1
with seven successive scans in the solution containing 20.0 μmol L
of L-DOPA. The RSD values were found to be 2.0% for the analyte, indicat-
ing good reproducibility of the modified electrode. The electrode can be
immersed in an aqueous media for 1.5 h with stable response. After that,
the background current began to increase, which may be due to the
partly leakage of ionic liquid from the electrode and the roughness of
the electrode surface increases gradually.
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1
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Table 1
Determination of L-DOPA in real samples (n = 3).
Sample
Added
μmol L−1)
Expected
Founded
Recovery
(%)
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1
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1
2
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3
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