Synergic effect of graphene quantum dots and room temperature ionic liquid for the fabrication of highly sensitive voltammetric sensor for levodopa determination in the presence of serotonin

Article abstract of DOI:10.1016/j.molliq.2017.04.123

Levodopa, one of the prescribed medications in Parkinson disease, is electrochemically determined here. A room temperature ionic liquid/graphene quantum dots (GQDs) modified carbon paste electrode (RTIL-GQDs/CPE) was used for the electrochemical determina

Full text of DOI:10.1016/j.molliq.2017.04.123

Accepted Manuscript
Synergic effect of graphene quantum dots and room temperature
ionic liquid for the fabrication of highly sensitive voltammetric
sensor for levodopa determination in the presence of serotonin
Afsaneh L. Sanati, Farnoush Faridbod, Mohammad Reza Ganjali
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DOI:
Reference:
S0167-7322(17)30571-8
doi: 10.1016/j.molliq.2017.04.123
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23 April 2017
26 April 2017
Please cite this article as: Afsaneh L. Sanati, Farnoush Faridbod, Mohammad Reza Ganjali
, Synergic effect of graphene quantum dots and room temperature ionic liquid for the
fabrication of highly sensitive voltammetric sensor for levodopa determination in the
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ACCEPTED MANUSCRIPT
Synergic effect of graphene quantum dots and room
temperature ionic liquid for the fabrication of highly
sensitive voltammetric sensor for levodopa determination in
the presence of serotonin
Afsaneh L. Sanati,a Farnoush Faridbod*,a, Mohammad Reza Ganjalia,b
aCenter of Excellence in Electrochemistry, School of Chemistry, College of Science, University of Tehran,
Tehran, Iran
bBiosensor Research Center, Endocrinology & Metabolism, Molecular-Cellular Sciences
Institute, Tehran University of Medical Sciences, Tehran, Iran
E-mail: faridbodf@khayam.ut.ac.ir
Tel.:+982161113813
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Abstract
Levodopa, one of the prescribed medication in Parkinson disease, is electrochemically
determined here. A room temperature ionic liquid/graphene quantum dots (GQDs) modified
carbon paste electrode (RTIL-GQDs/CPE) was used for the electrochemical determination of
levodopa. The electro-oxidation mechanism of levodopa at RTIL-GQDs/CPE was also studied.
The obtained data showed that the electro-oxidation of levodopa at the RTIL-GQDs/CPE is
greatly facilitated, which is attributed to high electrical conductivity of GQDs and RTIL. The
electrochemical oxidation of levodopa at the RTIL-GQDs/CPE is a two proton–two electron
process. The RTIL-GQDs/CPE showed a good electron mediating behavior followed by well
separated oxidation signals of levodopa and serotonin. Square wave voltammetric peak current
shows a linear relationship correspond to the concentrations of levodopa in a linear range of 0.05
to 250.0 μM with detection limit of 10.0 nM. The RTIL-GQDs/CPE shows excellent ability to
determination of levodopa in real samples.
Keywords: Levodopa, Serotonin, Graphene quantum dots, Room temperature ionic liquid
1. Introduction
Levodopa is an amino acid which plays an important role in humans, some animals and plants.
The body is able to make it through biosynthesis from the amino acid L-tyrosine. Levodopa is a
precursor of catecholamines neurotransmitters such as dopamine, norepinephrine, and
epinephrine. Furthermore, L-DOPA itself mediates neurotrophic factor release by the brain.
Levodopa is one of the most important catechol drugs suggested for treatment of Parkinson’s
disease alone or in combination with Carbidopa [1]. Levodopa can help the movements of the
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body by improving muscle control [2, 3]. Levodopa has shown some side effects in treatment of
Parkinson’s disease such as hypotension, arrhythmias, nausea, hair loss and disorientation and
confusion. Moreover, levodopa directly affects the serotonin systems. Serotonin neurons are
capable of taking up levodopa and decarboxylating it to dopamine [4]. Therefore, simultaneous
determination of levodopa and serotonin is very important in blood and urine sample in
Parkinson’s disease. Up to now, some analytical methods have been used for the analysis of L-
DOPA, such as high performance chromatography [5], spectrophotometry [6],
chemiluminecence [7] and electrochemical sensors [8, 9]. However, electrochemical methods
can be a superior methods of neurotransmitters determination due to the miniaturization of
technology and ability of online analysis. Due to this fact, many researchers focused on
designing new electrochemical sensors to measure these compounds more selective and sensitive
[10-20].
Graphene quantum dots are derivative of graphene nanosheet with unique properties such as high
surface area and good electrical conductivity, which can be a good candidates for fabrication of
electrochemical sensors [21]. Recently, GQDs, besides their remarkable optical properties, were
used as a modifier for increasing electrical conductivity of voltammetric sensors. Shiddiky et al.
reported a highly sensitive immunosensor for EpCAM biomarkers using GQDs modified
electrode. The proposed sensor showed a 100 fg/mL limit of detection for epithelial cell adhesion
molecule analysis [22]. Roushani and Abdi reported a novel voltammetric sensor based on GQDs
and riboflavin for determination of persulfate with detection limit of 0.2 μM [23].
On the other hand, room temperature ionic liquids are highly conductive binders for modification
of carbon paste electrodes [24-29]. Base on the previous studies, it seems that coupling of RTIL
and GQDs may be useful for fabrication of highly sensitive electrochemical sensor in
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voltammetric analysis. Hence, in this study, we describe fabrication of a novel voltammetric
sensor based on carbon paste electrode modified by synthesized GQDs and 1-butyl-3-
methylimidazolium hexafluorophosphate (as a binder) for determination of levodopa in the
presence of serotonin. The analytical feasibility of RTIL-GQDs/CPE is examined by measuring
of levodopa in urine and serum samples.
2. Materials and methods
2.1. Apparatus
An Autolab Electrochemical system consisting of a counter electrode (Pt wire), a working
electrode (RTIL-GQDs/CPE, 0.284 cm2) and a reference electrode (an Ag/AgCl/KClsat) was used
for electrochemical experiments. A Transmission electron microscopy (Philips, CM30, 300 Kv)
was used for morphological investigation.
2.2. Reagents and materials
Levodopa, serotonin hydrochloride, phosphoric acid, sodium hydroxide, citric acid were
purchased from Sigma-Aldrich Company. Graphite powder and paraffin oil were purchased from
Merck Company. Levodopa stock solution (0.01 M) was prepared by dissolving 0.197 g
levodopa in 100 mL double-distilled water. Serotonin solution (0.01 M) was prepared by
weighing 0.212 g of serotonin hydrochloride to 100.00 mL of double-distilled water.
2.3. Real sample preparation
Pharmaceutical serum was purchased from local market (Iranian Blood Transfusion
Organization) and was used for real sample determination without any pretreatment except
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protein lysis by acid. Urine samples were collected from a safe woman and stored in refrigerator.
Five mL of the urine was centrifuged for 35 min at 400 rpm was filtered out using a 0.45 μm
filter. The sample diluted 50 times and was used for real sample analysis by standard addition
method.
2.4. Sensor fabrication
RTIL-GQDs/CPE was prepared by mixing of 0.18 g of 1-butyl-3-methylimidazolium hexafluoro
phosphate, 0.82 g of paraffin, 0.07 g of GQDs, and 0.93 g of graphite powder. In continuous,
previous mixture was mixed about 1 h and a portion of the paste was filled into end of glass tube
which is in contact with a copper wire as a conductive matter.
2.5. Synthesis of GQDs
Citric acid was used as a carbon source for the synthesis of GQDs through pyrolysis method. For
this goal, suitable amount of citric acid (2 g) was weighed and transferred in to a beaker and was
heated at 200 ˚C for a few minutes until citric acid converts to a liquid phase. The heating
continued for 30 min more until the solution color changed from colorless to orange which
indicated GQD was synthesized.
3. Results and discussion
3.1. TEM Investigation
For investigation of GQDs morphology we used TEM method as a powerful technique in
characterization of nanomaterial. As can be seen in figure 1, GQDs were synthesized in sheet
shape and in the size ~3-5 nm.
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“Here Figure 1”
3.2. Electrochemical behavior of levodopa at a surface of RTIL-GQDs/CPE
Standard solution of levodopa (300 μM) was used for pH optimization in levodopa
determination by RTIL-GQDs/CPE employing cyclic voltammetry (CV) (Fig. 2 inset) [30]. It
was observed that best electrochemical oxidation current obtained at pH=7.0 and also with
increasing pH value, the oxidation peak potential shifted towards less positive values, which
proposing the involvement of protons in the reaction. Figure 2 shows the plot of peak potential
vs. pH for levodopa in the pH range of 5.0–8.0. As can be seen, a slope of −0.050V was obtained
for levodopa which confirmed equal number of electrons and protons in electro-oxidation of
levodopa (Scheme 1).
“Here Figure 2&Scheme 1”
Active surface area of RTIL-GQDs/CPE, RTIL/CPE, GQDs/CPE and CPE were determined by
standard solution of 1.0 mM [Fe(CN)6]-3/-4 in the presence of 0.1 M KCl by Randles–Sevcik
equation and cyclic voltammograms signals. They were 0.22, 0.20, 0.18 and 0.1256 cm2 for
RTIL-GQDs/CPE, RTIL/CPE, GQDs/CPE and CPE, respectively. The obtained data confirmed
highly electrical conductivity of GQDs and RTIL.
The linear sweep voltammograms of 500 μM levodopa at all four different electrodes are given
in Figure 3. As can be seen in figure 3 the moving from unmodified carbon paste electrode
(curve d) to RTIL-GQDs/CPE (curve a), the oxidation peak current of levodopa increases from
8.84 μA to 42.1 μA. On the other hand, oxidation peak potential for levodopa reduces from 460
mV (for carbon paste electrode) to 400 mV (for RTIL-GQDs/CPE). On the other hand, current
density data for all of the four electrodes has been presented in figure 3 inset. These data confirm
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electrical conductivity of two mediators and facile electro-oxidation of levodopa at a surface of
RTIL-GQDs/CPE.
“Here Figure 3”
In the next step, we studied the effect of scan rate on the oxidation peak current of levodopa (Fig.
4 inset). The obtained data confirmed that the anodic peak potential shifted to more positive
value for levodopa oxidation with increasing scan rates and also peak current has a linear relation
with ν1/2 that suggest diffusion control for electro-oxidation of levodopa (Fig. 4).
“Here Figure 4”
Tafel plot (for oxidation of 300 μM levodopa at a surface of RTIL-GQDs/CPE) was used for
calculation of charge transfer coefficient (Fig. 5). As we know, the slope of the Tafel plot is
equal to 2.3RT/ n(1−α)F which comes up to 0.2149 Vdecade−1. We obtained α as 0.86.
“Here Figure 5”
Electro-oxidation of 300 μM and 500 μM levodopa at the RTIL-GQDs/CPE was investigated by
employing chronoamperometry for the determination of the diffusion coefficient (D) (Fig. 6 A).
The plot of current (I) vs. t-1/2 was showed a linear relationship (obtained from
chronoamperometry data) (Fig. 6 B). According to the Cottrell equation and obtained slops from
Fig. 6B, the diffusion coefficient could be estimated 2.36×10−5 cm2/s.
“Here Figure 6”
Validation of the RTIL-GQDs/CPE for determination of standard levodopa and serotonin was
examined via evaluation of limit of detection (LOD), linear dynamic range (LDR) and
selectivity. For levodopa, a plot of peak current vs. potential was linear in the concentration
range of 0.05-250 μM with the LOD (S/N= 3) of 10 nM while the linear dynamic range obtained
for serotonin was 0.5−500 μM with limit of detection of 0.1 μM.
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Application of RTIL-GQDs/CPE for simultaneous determination of levodopa and serotonin was
checked by simultaneous concentration changing of two compounds at an optimum condition.
Two separated square wave voltammograms were obtained at potential ~400 mV and 720 mV
relative to levodopa and serotonin, respectively that is enough for simultaneous determination of
two compounds in mixture samples (Fig. 7A). As can be seen in Figure 7 B&C, the sensitivity
for levodopa or serotonin analysis in the presence of other compound is very similar to
sensitivity of them in dynamic range investigation. This point confirmed simultaneous
determination of levodopa and serotonin can be done at a surface of RTIL-GQDs/CPE without
any interference.
“Here Figure 7”
Stability of the RTIL-GQDs/CPE was tested by keeping the proposed sensor at best pH value for
20 days and then the square wave voltammograms of 10 μM + 20 μM levodopa and serotonin
were recorded and compared with their signals at a first day. 1.2% decrease in levodopa and
serotonin signal after 20 days was observed which confirmed good stability of RTIL-
GQDs/CPE. Also four different RTIL-GQDs/CPEs were fabricated with same procedure for
investigation of reproducibility. RSD for the RTIL-GQDs/CPEs signals in the presence of 10 μM
+ 20 μM levodopa and serotonin for seven analysis was calculated to be 3.5% that confirmed
high reproducibility in electrode fabrication.
Next, the interference of some biological or metal ions compounds on oxidation signal of 10.0
μM levodopa was checked (error less than ±5.0% is acceptable). 1000-fold of glucose,
-
methionine, alanine, ascorbic acid (after addition of ascorbic oxidase), Na+, Cl-, NO3 and 300-
fold of uric acid had no effect on the levodopa analysis.
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For investigation the performance of the RTIL-GQDs/CPE in the complex matrix determination
of levodopa in urine and serum samples was done through standard addition method. The
obtained data were shown in Table 1. As can be seen, the RTIL-GQDs/CPE has a good quality
for analysis of two compounds in the real samples.
“Here Table 1”
4. Conclusion
A highly sensitive and good selective electrochemical sensor based on GQDs and RTIL was
fabricated for analysis of levodopa in the presence of serotonin. Combination of GQDs and RTIL
showed unique properties for modification of carbon paste electrode. The RTIL-GQDs/CPE
successfully resolves the overlapped oxidation peaks of levodopa and serotonin by about 320
mV. The RTIL-GQDs/CPE showed good ability for analysis of levodopa and serotonin in real
samples.
Acknowledgments
The authors are grateful to the Research Council of University of Tehran for the financial
support of this work.
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Figures and Scheme Captions
Fig. 1 TEM image of as-synthesized GQDs.
Fig. 2 Plot of Ep vs. pH for RTIL-GQDs/CPE. Inset; cyclic voltammograms of RTIL-GQDs/CPE
at various buffered pHs in the presence of 300 μM levodopa.
Fig. 3 Linear sweep voltammograms of (a) RTIL-GQDs/CPE; (b) RTIL/CPE; (c) GQDs/CPE
and (d) CPE in the buffer solution pH=7.0 in the presence of 500 µM levodopa. Inset: the current
density derived from cyclic voltammograms responses of 500 μM levodopa at pH 7.0 at the
surface of different electrodes.
Fig. 4 Variation of anodic peak current vs. v1/2 for electro-oxidation of 700 µM levodopa. Inset;
cyclic voltammograms of RTIL-GQDs/CPE in 0.1 mol L−1 PBS (pH 7.0) containing 700 µM
levodopa at various scan rates; a-h correspond to 2.0, 5.0, 10.0, 18.0, 25.0, 40.0, 80.0 and 90.0
mV s−1, respectively.
Fig. 5 Tafel plot for RTIL-GQDs/CPE in 0.1 M PBS (pH 7.0) in the presence of 700 µM
levodopa and scan rate 10 mVs−1.
Fig. 6 A) Chronoamperograms obtained at the RTIL-GQDs/CPE in the presence of (a) 300 and
(b) 500 µM levodopa at pH 7.0. B) Plots of I vs. t-1/2 obtained from chronoamperograms.
Fig. 7 A) Square wave voltammograms of RTIL-GQDs/CPE in pH 7.0 containing different
concentrations of levodopa and serotonin (from inner to outer) mixed solutions of: (a):
3.0.0+3.0; (b): 10.0+20.0; (c): 20.0+40.0; (d): 35.0+ 65.0; (e) 50.0+95.0 and (f) 55.0+1100.0 μM
levodopa and serotonin, respectively. (B) plot of the peak currents as a function of levodopa
concentration and (C) plot of the peak currents as a function of serotonin concentration.
Scheme 1. Mechanism for electro-oxidation of levodopa
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Scheme 1
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Table 1 Determination of levodopa and serotonin in real samples (number of replicate measurments=5).
Sample
Added
Added
Found
levodopa
(µM)
Found
serotonin
(µM)
levodopa serotonin
(µM)
---
(µM)
---
Pharmaceutical
serum
<LOD
<LOD
10.00
---
20.00
---
9.87±0.44
<LOD
20.45±0.67
<LOD
Urine
20.00
40.00
20.67±0.87
40.89±1.22
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Graphical Abstract
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Highlights
 Synthesis of graphene quantum dots as high conductive mediator for electrochemical
sensor
 Using hybrid of graphene quantum dots and ionic liquid for the modification of electrode
 Simultaneous determination of levodopa and serotonin in trace level
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