{Electrocatalytic determination of levodopa in presence of cabergoline using carbon paste electrode modified with graphene quantum dots/2-chlorobenzoyl ferrocene/ionic liquid:}
http://dx.doi.org/10.5599/jese.1133 81
J. Electrochem. Sci. Eng. 12(1) (2021) 81-90; http://dx.doi.org/10.5599/jese.1133
Open Access : : ISSN 1847-9286
www.jESE-online.org
Original scientific paper
Electrocatalytic determination of levodopa in presence of
cabergoline using carbon paste electrode modified with
graphene quantum dots/2-chlorobenzoyl ferrocene/ionic liquid
Peyman Mohammadzadeh Jahani
School of Medicine, Bam University of Medical Sciences, Bam, Iran
Corresponding author: peymanjahani1234@gmail.com
Received: October 8, 2021; Accepted: November 3, 2021; Published: November 17, 2021
Abstract
The electrochemical sensor was fabricated for the simultaneous determination of
levodopa and cabergoline using carbon paste electrode (CPE) modified with graphene
quantum dots (GQD), 2-chlorobenzoyl ferrocene (2CBF) and ionic liquid (IL). Then, the
electrochemical behavior of levodopa alone and simultaneously with cabergoline at the
surface of GQDs/2CBF/IL/CPE was investigated in phosphate buffer solution (PBS). Under
optimal PBS, pH=7 condition, oxidation peak current has been found proportional to
levodopa concentration in the range between 0.07 μM and 500.0 μM, with the limit of
detection (LOD) of 0.02 μM (S/N=3). Outputs showed that at GQDs/2CBF/IL/CPE surface,
the levodopa and cabergoline oxidation peaks are separated by the potential difference
of 200 mV. In addition, it was found that this modified electrode possesses acceptable
sensitivity, selectivity, stability and repeatability. All these properties were sufficient to
allow simultaneous detection of levodopa and cabergoline in real samples at the surface
of GQDs/2CBF/IL/CPE. This was supported by the successful application of this electro-
chemical sensor electrode for the determination of levodopa and cabergoline in urine,
serum, and cabergoline tablets.
Keywords
Electrochemical sensor; chemically modified electrode, levodopa, cabergoline.
Introduction
There is a continuously growing interest in electrochemical sensors and bio-sensors for drugs in
environmental, food, and agricultural analyses. This is probably a result of both, electrochemical
behavior of biomolecules and advancement in electrochemical testing [1-4]. Hence, merging the
rapid, selective, sensitive, precise, affordable, as well as miniaturized electrochemistry-based
sensing handhelds with biochemistry, proteomics, nanotechnology, molecular biology, and drug
analyses resulted in developing electrochemical sensors [5-9].
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http://dx.doi.org/10.5599/jese.1133
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mailto:peymanjahani1234@gmail.com
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It is a well-known fact that the carbon paste electrode (CPE) has a widespread utilization for
electrochemical determinations of diverse biological and pharmaceutical species, which resulted
from low residual currents and noise, simplified construction, wider cathodic and anodic potential
range, fast surface renewal, and low cost. In addition, the CPE surface can be easily chemically
modified via the addition of diverse materials to enhance the selectivity, quickness, and sensitivity
of determinations [10-13].
Generally, the chemical modification of an inert substrate electrode with mediators offered
significant advantages in the development of electrochemical sensors [14]. Redox-active sites
shuttle electrons between the analyte solution and the substrate electrode, which is frequently in
line with a considerable diminishing of the activation overpotential for the corresponding
electrochemical reaction. Other advantages of chemically modified electrodes over the unmodified
substrate electrodes are their lower susceptibility to the surface fouling and formation of oxides at
their surfaces.
When choosing modifying materials, it is important to know that functional mediators should
exhibit lower relative molar mass and possess reversible, rapid and regenerable reaction at low
potentials. Furthermore, they should be pH-independent, highly stable in oxidized and reduced forms,
unreactive to oxygen, and nontoxic. Beyond the most successful mediators, it was already shown that
mediators based on ferrocene and its derivatives met most of thementioned criteria [14-16].
In recent years, researchers were also focused on designing and synthesizing nanomaterials
for various applications due to their unique physical and chemical properties [17-21]. In this context,
graphene quantum dots (GQDs) have been introduced as small units of graphene with sizes smaller
than 30 nm. GQDs are zero-dimensional substances that combine both carbon dots (CDs) and
graphene features [22]. Therefore, researchers considered GQDs in diverse areas because of the
confinement of quantum and edge impacts that cause specific electronic, optoelectronic, photo-
electric, larger surfaces, and better conductive characteristics. With these unique features, GQDs
have been introduced as a useful material for electrochemical sensors. GQDs contributed to larger
surface areas in contact with the analyte. As the electroactive surface area is highly significant in
electrochemistry, researchers predicted that modifying electrodes by GQDs would increase
electrochemical reaction rates [23,24].
Room-temperature ionic liquids (ILs) exhibit encouraging features for electrochemical
utilizations, like high ionic conductivity and non-volatility [25,26]. Due to high ionic conductivity,
broader electrochemical window, and faster ion mobility, ILs are frequently employed as
electrolytes, binders, and solvents in modified electrode electrochemical preparations. According to
the outputs, ILs can enhance response sensitivity and simplify direct electron transfer of diverse
electroactive compositions [27,28].
In this work, CPE has been modified with all three mentioned materials (ferrocene, GQDs and IL)
to form an efficient sensor electrode for detecting and determining levodopa alone or in the
presence of cabergoline.
Levodopa is one of the catecholamines with an alkylamine chain bond to a benzene ring with 2-
hydroxyl groups. Levodopa that is chosen to treat Parkinson’s disease is metabolized by one
enzymatic reaction (dopa-decarboxylase) to dopamine and compensates for diminished dopamine
in the brain [29]. Actually, Parkinson’s disease has been considered one of the progressive
neurological disorders, which happens in the case of brain failure for the production of sufficient
dopamine, which results in tremor, muscles rigidity or stiffness, slow movement (bradykinesia) as
well as imbalance. However, it is not possible to directly administer levodopa because it cannot
P. Mohammadzadeh Jahani J. Electrochem. Sci. Eng. 12(1) (2021) 81-90
http://dx.doi.org/10.5599/jese.1133 83
permeate the blood-brain barrier. Hence, levodopa that could be taken orally will be utilized for
providing a resource of dopamine and for treating Parkinson’s disease for relieving the symptoms
in a majority of the patients at the early phases of the disease. Consequently, researchers designed
various analytical procedures to determine levodopa [30-32].
Cabergoline is one of the ergot alkaloid derivatives for treating Parkinson’s disease as the
dopamine agonist. This drug is applied as the only agent for treating early Parkinson’s disease and
the adjunct to levodopa in the late phase of the disease. It was confirmed that using this drug
delayed the initiation of the levodopa-induced motoric consequences and decreased total levodopa
dose crucial for sufficient control over the symptoms of the disease. Finally, its reverse impacts could
be compared with the effects of other dopamine agonists like bromocriptine [33,34]. Therefore, the
combined treatment of cabergoline and levodopa is usually utilized to treat the disease. In general,
the oral administration of cabergoline in a dosage >1 mg/day would be commonly prescribed to
treat Parkinson’s disease. Moreover, researchers used radio-immunoassay and indicated that the
level of cabergoline in plasma in the healthy participants receiving a single oral dosage equal to 0.6
mg had been ranged between 80 and 800 pg/mL [34].
The present research detected levodopa in the aqueous buffer solution by the developed
GQDs/2CBF/IL/CPE sensor. Also, analytical functions of the modified electrode for quantifying
levodopa in the presence of cabergoline was assessed. Ultimately, the developed electrochemical
sensor has been utilized to determine cabergoline and levodopa in some real samples.
Experimental
Instruments and chemicals
An Auto-lab potentiostat/galvanostat (PGSTAT 302N, Eco Chemie, The Netherlands) was utilized
for electrochemical experimentations and monitored with general-purpose electrochemical system
software. A traditional three-electrode cell was used at 25±1 °C. A platinum wire, a conventional
Ag/AgCl/KCl (3.0 M KCl) electrode, and GQDs/2CBF/IL/CPE were employed as the auxiliary, reference,
and the working electrodes, respectively. A Metrohm 710 pH meter was used to measure pH.
Cabergoline, levodopa, and all remaining analytical grade reagents were obtained from Merck
(Darmstadt, Germany). Finally, ortho-phosphoric acid and its salts (KH2PO4, K2HPO4, K3PO4) were
utilized to prepare 0.1 M phosphate buffer solution (PBS) with pH ranging from 2.0 to 9.0.
Electrode preparation
Based on the research design, GQDs/2CBF/IL/CPE was prepared by dissolution of 0.01 g 2CBF in
3 mL diethyl ether. Afterward, this solution was mixed into 0.1 g GQDs and 0.89 g graphite powder
composed of pestle and mortar. Then, 0.6 mL of paraffin and 0.3 mL of IL (n-hexyl-3-
methylimidazolium hexafluoro phosphate) were added to the mentioned mix and shaken for 15
minutes until a uniform paste was achieved. In the next stage, the paste was packed into a glass
tube (ca. 3.4 mm i.d. and 10 cm long), with a copper wire located at the carbon paste to ensure
electrical contact. Afterward, a fresh surface was attained via impelling an excessive paste outside
the tube and polishing with a weighing paper.
It should be noted that in order to compare the materials, bare CPE was analyzed with GQDs/CPE
(without 2CBF and IL), 2CBF/CPE (without GQDs and IL), and GQDs/2CBF/CPE (without IL), which
were all procured similarly.
The surface areas of GQDs/2CBF/IL/CPE and bare CPE were obtained by CVs of 1 mM K3Fe(CN)6 in
0.1 M PBS, recorded at different scan rates. Using the Randles-Ševčik formula [35] for
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GQDs/2CBF/IL/CPE, the electrode surface was calculated as 0.35 cm2, about 3.9 times greater than
bare CPE.
Preparation of real samples
Urine samples were stored in a refrigerator upon the collection. Then, 10 mL of the specimens were
centrifugated for 15 min at 2,000 rpm, and afterward, a 0.45 µm filter was applied to purify the
supernatant. In the next step, various contents of the supernatant solution were transferred into 25
ml volumetric flasks and diluted to the marks with PBS (pH 7.0). The diluted urine specimens were
spiked with various levels of cabergoline and levodopa. Finally, the recommended process was used
to analyze the amounts of cabergoline and levodopa, employing the standard addition technique.
The serum samples were prepared similarly by centrifugation, filtration and dilution with PBS
(pH 7.0). Afterward, the diluted specimen of the serum sample has been injected with various
contents of cabergoline and levodopa. Next, the newly recommended procedure and standard
addition technique were used to analyze cabergoline and levodopa.
Five cabergoline tablets were ground, and then 100 mg of the obtained powder was dissolved in
25 mL water through ultra-sonication. In the next stage, diverse contents of the above solution were
transferred in the cell and diluted by PBS. The standard addition technique was used to determine
the contents of cabergoline and levodopa in the tablets.
Results and discussion
Electrochemical features of levodopa at the surface of GQDs/2CBF/IL/CPE
For studying the electrochemical oxidation behaviour of levodopa, which is pH-dependent
reaction according to the oxidation mechanism presented in Scheme 1, finding an optimal pH value
would be of high importance for achieving acceptable outputs.
Scheme 1. Electrochemical oxidation mechanism of levodopa at the surface of the modified electrode
Therefore, we used the modified electrode to run experiments using 100 m of levodopa in 0.1 M
PBS of different pH values, ranging from 2.0 to 9.0. According to Figure 1, showing oxidation peak
current values of 100 M levodopa at GQDs/2CBF/IL/CPE in dependence on pH of the solution, the
most acceptable output was observed for electro-oxidation of levodopa at pH of 7.0.
Figure 1. Ip vs. pH curve obtained from DPVs of GQDs/2CBF/IL/CPE in solution containing
100.0 μM of levodopa in 0.1 M PBS of different pH (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 and 9.0)
P. Mohammadzadeh Jahani J. Electrochem. Sci. Eng. 12(1) (2021) 81-90
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Figure 2 represents CV responses for different electrodes in solutions with and without 100.0 µM
of levodopa. CVs of bare CPE in the blank solution and solution containing 100.0 µM of levodopa are
presented by curves a and b, respectively. CVs of GQDs/2CBF/IL/CPE in blank and levodopa solutions
are presented as curves c and f, respectively. The rest two curves are CVs recorded for partially
modified electrodes in levodopa solutions, i.e., GQDs/CPE (curve d), and 2CBF/CPE (curve e).
Figure 2 demonstrates that in solutions containing levodopa, the potential of the anodic peak is
approximately 820 mV for oxidizing levodopa at the bare CPE surface (curve b) and 645 mV at the
surface of GQDs/2CBF/IL/CPE (curve f), what is a difference of 175 mV.
E / mV vs. Ag/AgCl/KCl
Figure 2. CVs (10 mVs-1) of unmodified and modified CPEs in 0.1M PBS (pH 7.0): (a) unmodified CPE without
levodopa, and (b) with 100.0 µM levodopa; (c) GQDs/2CBF/IL/CPE without levodopa and (f) with 100.0 µM
levodopa; (d) GQDs/CPE and (e) 2CBF/CPE with 100.0 µM levodopa
The highest oxidation currents were observed for the surface of 2CBF/CPE (curve e) and
particularly for GQDs/2CBF/IL/CPE (curve f). The significant increase of the anodic peak current for
GQDs/2CBF/IL/CPE compared to either GQDs/CPE or 2CBF/CPE, implies the influence of ionic liquids
(ILs) present on CPE. IL/CPE has some benefits like quick transfer of electrons, suitable antifouling
traits, greater conductivity, and the catalytic nature of ILs. Therefore, IL mass has been inserted into
the carbon and paraffin oil that connect the granules. Thus, IL/CPE conductivity has been
considerably improved, which is consistent with the electrochemistry outputs of the present study.
Finally, GQDs on the IL surface largely enlarged the electrochemical responses likely caused by the
potential features of the GQDs, such as the greater surface area, stronger chemical stability, and
suitable electrical conductivity.
Impacts of the scan rate
The effectiveness of the potential scan rate on the oxidation current of levodopa is presented in
Figure 3, showing linear sweep voltammograms of GQDs/2CBF/IL/CPE in 0.1 M PBS, pH 7.0 with 100.0
μM of levodopa. It is obvious from Figure 3 that higher scan rates resulted in enhanced oxidation peak
current values. Additionally, it has been found that Ip is linearly related to the square root of the
potential scan rate (ν1/2), demonstrating that levodopa oxidation is the diffusion-controlled process
(Figure 3, inset A). In addition, the electrocatalytic mechanism (EC′) has been shown by the plot of the
scan rate normalized current (Ip/ν1/2) against the scan rate (Figure 3, inset B).
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E / mV vs. Ag/AgCl/KCl
Figure 3. Linear sweep voltammograms of
GQDs/2CBF/IL/CPE in 0.1 M PBS, pH 7.0 with 100.0 μM
levodopa, at distinct scan rates (1-9 correspond to 5,
10, 20, 30, 40, 50, 60, 70, and 80 mV s-1). Insets: (A)
variation of anodic peak current with ν1/2; (B) variation
of scan rate normalized currents (Ip/ν1/2) with scan rate
Chronoamperometric measurements
The analysis of chronoamperometry for levodopa specimens at GQDs/2CBF/IL/CPE was
performed at 0.7 V. Figure 4 displays chronoamperometric outputs of diverse concentrations of
levodopa in PBS, pH 7.0. In addition, the Cottrell equation was applied for the chronoamperometric
analysis of electroactive moieties reaction based on the mass transfer restricted conditions [35]:
I = nFAD1/2Cbπ-1/2t-1/2 (1)
In Eq. (1), D is diffusion coefficient (cm2 s-1), Cb stands for the bulk concentration of analyte
(mol cm−3), A is electrode surface area (cm2), n is the number of transferred electrons, and t is time.
Figure 4A shows experimental results of I vs. t−1/2, reflecting the best fit for distinct concentrations
of levodopa.
Figure 4. Chronoamperograms of
GQDs/2CBF/IL/CPE in 0.1 M PBS, pH 7.0 for dif-
ferent levodopa concentrations (1-4 correspond
to 0.1, 0.6, 1,5 and 3.0 mM). Inset A: I plot vs. t-1/2
from chronoamperograms 1 to 4. Inset B: slope
plot of straight line vs. concentration of levodopa
Afterward, final slopes relative to the straight lines in Figure 4A were drawn versus levodopa
concentration (Figure 4B). Using the Cottrell equation and resultant slopes, the mean value of D was
calculated as 7.45×10-5cm2/s.
Calibration plot and detection limit
Considering the oxidation peak currents for different concentrations of levodopa with
GQDs/2CBF/IL/CPE, levodopa can be quantitatively analysed in the water solution. The modified
P. Mohammadzadeh Jahani J. Electrochem. Sci. Eng. 12(1) (2021) 81-90
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electrode (GQDs/2CBF/IL/CPE) was used as a working electrode in the concentration range of
levodopa from 0.07 to 500.0 µM M in 0.1 M PBS, pH 7.0, and differential pulse voltammetry (DPV)
measurements were performed due to DPV merits like more reasonable sensitivity and more
acceptable performance in analytical utilizations. The recorded DPVs (step potential=0.001 V,
amplitude=0.02 V) are presented in Figure 5, showing that peak currents are linearly related to
levodopa concentration ranging from 0.07 to 500.0 µM, with the correlation coefficient equal to
0.9999. Also, the detection limit, Cm, of levodopa was obtained using the following equation:
Cm = 3sb / m (2)
In Equation (2), m is the slope of the calibration plot (0.1003 μA μM-1) and sb is the standard
deviation of the blank response obtained from 20 replicate measurements of the blank solution.
The detection limit was calculated as 0.02 μM.
E / mV vs. Ag/AgCl/KCl
Figure 5. DPVs of GQDs/2CBF/IL/CPE in 0.1 M
PBS, pH 7.0 with distinct concentrations of
levodopa (1–9 correspond to 0.07, 5.0, 20.0, 50.0,
100.0, 200.0, 300.0, 400.0, and 500.0 μM). Inset:
peak current plot vs. levodopa concentration
Concurrent detection of levodopa and cabergoline
No study has been reported on the use of CPE modified with GQDs/2CBF/IL for simultaneous
detection of cabergoline and levodopa. In addition, because the electrochemical detection of
levodopa in the presence of cabergoline at unmodified electrodes would have a drawback of
interference with cabergoline due to the comparative oxidation capacity of both samples, we tried
to separate two analyte peaks. This stage has been proceeded by concurrent changes in the analyte
concentration and recording DPVs. It is shown in Figure 6 that specific anodic peaks have been
observed at 630 and 830 mV for oxidizing levodopa and cabergoline, respectively, which confirms
the use of GQDs/2CBF/IL/CPE, by which it becomes possible to detect the analytes with no
interference between them.
Stability of GQDs/2CBF/IL/CPE
According to the research design, the stability of GQDs/2CBF/IL/CPE was tested via holding it in PBS
pH 7.0 for 20 days. Then, the cyclic voltammogram was registered in the presence of 50.0 µM levodopa
after cycling the potential fifteen times at 50 mV s−1. The measured CV was then compared with that
observed before submersion. According to the findings, the levodopa oxidation peak did not change
the peak potential value, while peak current value showed a certain decrease (≤2.5 %) compared to
the initial response, reflecting acceptable stability of GQDs/2CBF/IL/CPE.
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E / mV vs. Ag/AgCl/KCl
Figure 6. DPVs of GQDs/2CBF/IL/CPE in 0.1 M
PBS, pH 7.0, with various concentrations of
levodopa and cabergoline (1–6 correspond to
0.0 + 8.0, 20.0 + 50.0, 50.0 + 125.0, 100.0 + 250.0,
200.0 + 500.0, and 400.0 + 900.0 µM of levdopa
and cabergoline. Inset A: Ip plot vs. levodopa
concentration. Inset B: Ip plot vs. cabergoline
concentration
Interference study
This study examined the possible effects of various materials as compounds that might have a po-
tential interference with cabergoline and levodopa detection under optimized conditions with 50.0 µM
levodopa at pH 7.0. It should be noted that potentially interfering materials have been selected from a
group of materials usually observed with levodopa pharmaceuticals and/or biological fluids. The limit of
tolerance has been described as the highest concentration of the interfering material, which gives rise
to less than ±5 % error in detecting levodopa. As shown by the outputs, glucose, lactose, fructose,
sucrose, ethanol, citric acid, methanol, Fe3+, Mg2+, Fe2+, Al3+, SO42-, CO32-, NH4+, F-, Cl-, glycine, alanine,
methionine, folic acid, phenylalanine, urea, and the saturated starch solution had no interference with
cabergoline and levodopa detection. On the other side, ascorbic acid, dopamine, norepinephrine, and
epinephrine with the same concentration showed some interference with cabergoline and levodopa
detection. Even though ascorbic acid had interfered, it might be omitted if required via the ascorbic
oxidase enzyme that has an excellent selectivity to oxidizing the ascorbic acid.
Analysis of real samples
For assessing the usability of the developed modified electrode for determination of levodopa
and cabergoline in real samples, GQDs/2CBF/IL/CPE was applied for biological fluids and drugs, i.e.,
urine, serum and cabergoline tablets. Consequently, a standard addition procedure was employed,
and outputs are presented in Table 1. As seen, reasonable recovery of levodopa and cabergoline, as
well as reproducible outcomes were obtained based on the mean relative standard deviation (RSD).
Table 1. Levodopa and cabergoline concentrations determined by GQDs/2CBF/IL/CPE in real samples (n=5).
Concentration, M
Recovery, % RSD, %
Sample Spiked Found
Levodopa Cabergoline Levodopa Cabergoline Levodopa Cabergoline Levodopa Cabergoline
Urine
0 0 - - - - - -
5.0 7.5 4.9 7.7 98.0 102.7 3.5 1.9
10.0 12.5 10.1 12.3 101.0 101.0 2.7 2.4
Serum
0 0 - - - - - -
6.0 10.0 6.1 9.8 101.7 98.0 2.3 3.1
12.0 15.0 11.7 15.5 97.5 103.3 2.9 1.7
Cabergoline
tablet
0 0 - 3.5 - - - 3.2
5.0 2.5 5.1 5.9 102.0 98.3 3.4 1.9
12.0 7.5 11.7 11.3 97.5 102.7 2.4 2.7
P. Mohammadzadeh Jahani J. Electrochem. Sci. Eng. 12(1) (2021) 81-90
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Conclusions
An electrochemical sensor with high sensitivity, selectivity, and stability based on GQDs/2CBF/IL
modified CPE was successfully fabricated. GQDs/2CBF/IL/CPE showed much better performance
when compared with bare CPE, GQDs/CPE and 2CBF/CPE. The developed GQDs/2CBF/IL/CPE
showed excellent catalyzing effect for levodopa oxidation and found suitable for simultaneous
determination of levodopa and cabergoline. The anodic peak currents were linear to levodopa and
cabergoline concentrations in the reasonable concentrations ranging from 0.07 to 500.0 µM,
respectively. In addition, GQDs/2CBF/IL/CPE sensor can be favorably employed for determining
levodopa as well as cabergoline in real samples.
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