Electrochemical sensors and biosensors for pharmaceutical, food, agricultural and environmental analyses have been growing rapidly due to electrochemical behavior of drugs and biomolecules and partly due to advances in electrochemical measuring systems. In the present work, we describe the preparation of a new electrode composed of graphen (G) modified with 2,7-bis(ferrocenyl ethyl) fluoren-9-one (2,7-BFGPE) and investigate its performance for the electrocatalytic determination of methyldopa in aqueous solutions.
Experimental section was carried out using cyclic voltammetry, square wave voltammetry and chronoamperometry.
Under the optimized conditions (pH 7.0), the square wave voltammetric peak current of methyldopa increased linearly with methyldopa concentration in the ranges of 9.0 × 10−8 to 5.0 × 10−4 M. The detection limit was 5.0 × 10−8 M methyldopa. The diffusion coefficient (D= 9.35 × 10−6 cm2/s) and electron transfer coefficient (α=0.52) for methyldopa oxidation were also determined.
The method shows the development of a sensor for selective and sensitive determination of methyldopa. This sensor was successfully applied to determine the methyldopa in some real samples.
Methyldopa is an antihypertensive agent that is used in the treatment of high blood pressure or hypertension, especially when it is complicated with renal disease. Its antihypertensive properties are primarily due to its action on the central nervous system. Methyldopa inhibits the enzyme DOPA decarboxylase, which converts l-DOPA into dopamine, and is a precursor for norepinephrine and subsequently epinephrine. It is converted to α-methyl norepinephrine in adrenergic nerve terminals, and its antihypertensive action appears to be due to its stimulation of central adrenal receptors, which reduces sympathetic tone and produces a fall in blood pressure. The therapeutic concentration of methyldopa in human plasma is usually in the range of 0.1 to 0.5 mg L−1, and its average terminal elimination half-life is 2 h (Kwan et al. ; Myhre et al. ). Clearly, detection and quantification of methyldopa is an important feature in pharmaceutical and clinical procedures (Rezaei et al. ; Gholivand and Amiri ). Several analytical procedures have been reported for the analysis of methyldopa in bulk form, pharmaceutical form, or biological fluids. These include titrimetry, chromatography, kinetic methods, spectrophotometry, and H NMR. However, 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 selectivity, that make 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 detecting methyldopa and various other analytes without requiring tedious pretreatments (Athanasiou-Malaki and Koupparis ; Tajik et al. [2013a]; Shahrokhian and Rastgar ; Molaakbari et al. ; Moccelini et al. ).
The use of carbon paste as an electrode was initially reported in 1958 by Adams. In afterward researches, a wide variety of modifiers including enzymes, polymers, and nanomaterials have been used with these versatile electrodes. Carbon paste electrodes (CPEs) are widely applicable in both electrochemical studies and electroanalysis, thanks to their advantages such as very low background current (compared to solid graphite or noble metal electrodes), facility to prepare, low cost, large potential window, simple surface renewal process, and easiness of miniaturization. Besides the advantageous properties and characteristics listed previously, the feasibility of incorporating different substances during paste preparation (which results in the so-called modified carbon paste electrode) allows the fabrication of electrodes with desired composition and, hence, with predetermined properties (Tajik et al. [2013b]; Khoobi et al. ; Mokhtari et al. ; Díaz et al. ; Gholivand and Mohammadi-Behzad ; Mazloum-Ardakani et al. ; Thomas et al. [2013a]; Raoof et al. ; Dönmez et al. ; Raoof et al. [2006a]).
Electrochemical methods using chemically modified electrodes (CMEs) have been widely used as sensitive and selective analytical methods for the detection of trace amounts of biologically important compounds. One of the most important properties of CMEs is their ability to catalyze the electrode process via the significant decrease of overpotential with respect to the unmodified electrode. With respect to the relatively selective interaction of the electron mediator with the target analyte in a coordinated fashion, these electrodes are capable of considerably enhancing the selectivity of electroanalytical methods (Beitollahi et al. [2011a]; Luo et al. ; Taleat et al. ; Huo et al. ; Thomas et al. [2013b]; Raoof et al. [2006b]; Oliveira et al. ; Beitollahi et al. [2011b]; Thomas et al. [2013c]; Mohammadi et al. ; Sanghavi et al. ; [Beitollahi et al. 2014]; Li et al. ; Ghoreishi et al. ; Yildiz et al. ).
As a new kind of two-dimensional carbon material, graphene has attracted increasing attention due to its unique properties including high surface area, excellent electrical conductivity, quick electron mobility at room temperature, high mechanical strength, and ease for functionalization (Joon et al. ). Graphene-based electrochemical sensors have been proved to possess excellent electrocatalytic ability and good performances (Ping et al. ; Ma et al. ; Silva et al. ; Zhu et al. ; Sun et al. ; Xi et al. ).
In the present work, we describe the preparation of a new electrode composed of graphene (G) modified with 2,7-bis(ferrocenyl ethyl)fluoren-9-one (2,7-BFGPE) and investigate its performance for the electrocatalytic determination of methyldopa in aqueous solutions.
Apparatus and chemicals
The electrochemical measurements were performed with an Autolab potentiostat/galvanostat (PGSTAT-302 N, Eco Chemie, Utrecht, The Netherlands). The experimental conditions were controlled with General Purpose Electrochemical System (GPES) software. A conventional three-electrode cell was used at 25°C ± 1°C. An Ag/AgCl/KCl (3.0 M) electrode, a platinum wire, and 2,7-BFGPE were used as the reference, auxiliary, and working electrodes, respectively. A Metrohm 691 pH/Ion Meter (Utrecht, The Netherlands) was used for pH measurements.
All solutions were freshly prepared with double-distilled water. Methyldopa and all other reagents were of analytical grade from Merck (Darmstadt, Germany). Graphite powder and paraffin oil (DC 350, density = 0.88 g cm−3) as the binding agent (both from Merck) were used for preparing the pastes. The buffer solutions were prepared from orthophosphoric acid and its salts in the pH range of 2.0 to 11.0. 2,7-BF was synthesized in our laboratory as reported previously (Raoof et al. [2006a]).
Synthesis of graphene nanosheets
Graphene nanosheets were synthesized from natural graphite flakes based on the modified Hummers and Offeman's method (Hummers and Offeman ). In a typical synthesis process, 1.0 g of pristine graphite flakes was immersed in 50 mL of formic acid and then sonicated for 2 h at room temperature. The resulting graphite plates were washed with acetone and then dried in an oven at 95°C for 12 h. Then, 100 mL H2SO4 (95%) was added into a 500-mL flask and cooled by immersion in an ice bath followed by stirring. About 1.0 g of treated graphite powder and 0.5 g NaNO3 were added under vigorous stirring to avoid agglomeration. After the graphite powder was well dispersed, 3 g KMnO4 was added gradually under stirring and cooling so that the temperature of the mixture was maintained below 10°C. The mixture was stirred for 2 h and diluted with deionized double-distilled water (in an ice bath). After that, 25 mL 15% H2O2 was slowly added to the mixture until the colour of the mixture changed to brilliant yellow, indicating fully oxidized graphite. The as-obtained graphite oxide slurry was re-dispersed in deionized double-distilled water and then exfoliated to generate graphene oxide nanosheets by sonication for 2 h. Then, the solution was filtered and washed with diluted HCl solution to remove metal ions. Finally, the product was washed with deionized double-distilled water until the solution became acid free and dried under vacuum at 50°C. A typical transmission electron microscopy (TEM) image of the synthesized graphene nanosheets is shown in Figure 1.
Preparation of the electrode
2,7-BFGPEs were prepared by hand-mixing 0.01 g of 2,7-BF with 0.89 g graphite powder and 0.1 g G with a mortar and pestle. Then, approximately 0.7 mL of paraffin was added to the above mixture and mixed for 20 min until a uniformly wetted paste was obtained. The paste was then packed into the end of a glass tube (ca. 3.4 mm i.d. and 10 cm long). A copper wire inserted into the carbon paste provided the electrical contact. When necessary, a new surface was obtained by pushing an excess of the paste out of the tube and polishing with a weighing paper.
For comparison, 2,7-BF-modified CPE (2,7-BFCPE) without G, G paste electrode (GPE) without 2,7-BF, and unmodified CPE in the absence of both 2,7-BF and G were also prepared in the same way.
Results and discussion
Electrochemical behavior of 2,7-BFGPE
2,7-BFGPE was constructed and its electrochemical properties were studied in a 0.1 M phosphate-buffered saline (PBS; pH 7.0) by cyclic voltammetry (CV). The experimental results show well-defined and reproducible anodic and cathodic peaks related to the 2,7-bis(ferrocenyl ethyl)fluoren-9-one/2,7-bis(ferricenium ethyl)fluoren-9-one redox system, with Epa, Epc, and E°´ of 320, 260, and 290 mV vs. Ag/AgCl/KCl (3.0 M) respectively. The observed peak separation potential (ΔEp = Epa − Epc) of 60 mV was greater than the value of 59/n mV expected for a reversible system (Bard and Faulkner ), suggesting that the redox couple of 2,7-BF in 2,7-BFGPE has a quasi-reversible behavior in aqueous medium.
In addition, the long-term stability of 2,7-BFGPE was tested over a 3-week period. When CVs were recorded after the modified electrode was stored in atmosphere at room temperature, the peak potential for methyldopa oxidation was unchanged and the current signals showed less than 2.1% decrease relative to the initial response. The antifouling properties of the modified electrode toward methyldopa oxidation and its oxidation products were investigated by recording the cyclic voltammograms of the modified electrode before and after use in the presence of methyldopa. Cyclic voltammograms were recorded in the presence of methyldopa after having cycled the potential 20 times at a scan rate of 10 mV s−1. The peak potentials were unchanged, and the currents decreased by less than 2.3%. Therefore, at the surface of 2,7-BFGPE, not only the sensitivity increased, but the fouling effect of the analyte and its oxidation product also decreased.
Influence of pH
The electrochemical behavior of methyldopa is dependent on the pH value of the aqueous solution, whereas the electrochemical properties of the 2,7-bis(ferrocenyl ethyl) fluoren-9-one/2,7-bis(ferricenium ethyl)fluoren-9-one (Fc/Fc+) redox couple are independent on pH. Therefore, pH optimization of the solution seems to be necessary in order to obtain the electrocatalytic oxidation of methyldopa. Thus, the electrochemical behavior of methyldopa was studied in 0.1 M PBS in different pH values (2.0 < pH < 11.0) at the surface of 2,7-BFGPE by CV. It was found that the electrocatalytic oxidation of methyldopa at the surface of 2,7-BFGPE was more favored under neutral conditions than in acidic or basic medium. This appears as a gradual growth in the anodic peak current and a simultaneous decrease in the cathodic peak current in the CVs of 2,7-BFGPE. Thus, pH 7.0 was chosen as the optimum pH for electrocatalysis of methyldopa oxidation at the surface of 2,7-BFGPE.
Electrocatalytic oxidation of methyldopa at a 2,7-BFGPE
Figure 2 depicts the CV responses for the electrochemical oxidation of 0.35 mM methyldopa at the unmodified CPE (curve b), GPE (curve d), 2,7-BFCPE (curve e), and 2,7-BFGPE (curve f). As it is seen, while the anodic peak potential for methyldopa oxidation at the GPE and unmodified CPE are 480 and 520 mV, respectively, the corresponding potential at 2,7-BFGPE and 2,7-BFCPE is approximately 320 mV. These results indicate that the peak potential for methyldopa oxidation at 2,7-BFGPE and 2,7-BFCPE shift by approximately 160 and 200 mV toward negative values compared to GPE and unmodified CPE, respectively. However, 2,7-BFGPE shows much higher anodic peak current for the oxidation of methyldopa compared to 2,7-BFCPE, indicating that the combination of G and the mediator (2,7-BF) has significantly improved the performance of the electrode toward methyldopa oxidation. In fact, 2,7-BFGPE, in the absence of methyldopa, exhibited a well-behaved redox reaction (Figure 2, curve c) in 0.1 M PBS (pH 7.0). However, there was a drastic increase in the anodic peak current in the presence of 0.35 mM methyldopa (curve f), which can be related to the strong electrocatalytic effect of 2,7-BFGPE toward this compound (Bard and Faulkner ).
The effect of scan rate on the electrocatalytic oxidation of methyldopa at 2,7-BFGPE was investigated by linear sweep voltammetry (Figure 3). As can be observed in Figure 3, the oxidation peak potential shifted to more positive potentials with increasing scan rate, confirming the kinetic limitation in the electrochemical reaction. Also, a plot of peak height (Ip) vs. the square root of scan rate (ν1/2) was found to be linear in the range of 2 to 100 mV s−1, suggesting that, at sufficient overpotential, the process is diffusion- rather than surface-controlled (Figure 3A) (Bard and Faulkner ). A plot of the scan rate-normalized current (Ip/ν1/2) vs. scan rate (Figure 3B) exhibits the characteristic shape typical of an EC process (Bard and Faulkner ).
Figure 4 shows the linear sweep voltammograms (LSVs) of an 2,7-BFGPE obtained in 0.1 M PBS (pH 7.0) containing 20.0 μM methyldopa, with a sweep rate of 2 mV s−1. The points show the rising part of the voltammogram (known as the Tafel region), which is affected by the electron transfer kinetics between methyldopa and 2,7-BFGPE. If deprotonation of methyldopa is a sufficiently fast step, the number of electrons involved in the rate-determining step can be estimated from the slope of the Tafel plot. The inset of Figure 4 shows a Tafel plot that was drawn from points of the Tafel region of the LSV. The Tafel slope of 0.122 V obtained in this case agrees well with the involvement of one electron in the rate-determining step of the electrode process, assuming a charge transfer coefficient of α = 0.52 (Bard and Faulkner ).
Chronoamperometric measurements of methyldopa at 2,7-BFGPE were carried out by setting the working electrode potential at 0.4 V (at the first potential step) and at 0.1 V (at the second potential step) vs. Ag/AgCl/KCl (3.0 M) for the various concentrations of methyldopa in buffered aqueous solutions (pH 7.0) (Figure 5). For an electroactive material (methyldopa 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 (Bard and Faulkner ). Experimental plots of I vs. t−1/2 were employed, with the best fits for different concentrations of methyldopa (Figure 5A). The slopes of the resulting straight lines were then plotted vs. the methyldopa concentration (Figure 5B). From the resulting slope and Cottrell equation, the mean value of D was found to be 9.35 × 10−6 cm2 s−1.
Calibration plot and limit of detection
The Square Wave Voltammetry (SWV) method was used to determine the concentration of methyldopa (initial potential = 0.06 V, end potential = 0.6 V, step potential = 0.001 V, amplitude = 0.02 V, frequency = 10 Hz) (Figure 6). The plot of peak current vs. methyldopa concentration consisted of two linear segments with slopes of 0.657 and 0.024 μA μM−1 in the concentration ranges of 0.09 to 15.0 μM and 15.0 to 500.0 μM, respectively. The decrease in sensitivity (slope) of the second linear segment is likely due to kinetic limitation (Bard and Faulkner ). The detection limit (3σ) of methyldopa was found to be 50.0 nM. These values are comparable with values reported by other research groups for electrocatalytic oxidation of methyldopa at the surface of chemically modified electrodes by other mediators (Table 1).
The influence of various substances as compounds potentially interfering with the determination of methyldopa was studied under optimum conditions. The potentially interfering substances were chosen from the group of substances commonly found with methyldopa in pharmaceuticals and/or in biological fluids. The tolerance limit was defined as the maximum concentration of the interfering substance that caused an error of less than ±5% in the determination of methyldopa. According to the results, l-lysine, glucose, NADH, acetaminophen, uric acid, l-asparagine, l-serine, l-threonine, l-proline, l-histidine, l-glycine, l-tryptophan, l-phenylalanine, lactose, saccharose, fructose, benzoic acid, methanol, ethanol, urea, caffeine, Mg2+, Al3+, NH4+, Fe2+, Fe3+, F−, SO42−, and S2− did not show interference in the determination of methyldopa. However, levodopa, carbidopa, dopamine, and ascorbic acid with equal molar concentration make interference. Although ascorbic acid showed interference, this interference could be minimized, if necessary, by using ascorbic oxidase enzyme, which exhibits a high selectivity to the oxidation of ascorbic acid.
Real sample analysis
Determination of methyldopa in methyldopa tablet and urine samples
In order to evaluate the analytical applicability of the proposed method, it was also applied for the determination of methyldopa in methyldopa tablet (each tablet containing 250 mg methyldopa from Darou-Pakhsh, Tehran, Iran). The results for determination of methyldopa in the methyldopa tablet are given in Table 2. Satisfactory recovery of the experimental results was found for methyldopa. The reproducibility of the method was demonstrated by the mean relative standard deviation (R.S.D.).
Also, in order to evaluate the analytical applicability of the proposed method, it was applied for the determination of methyldopa in urine samples. Known amounts of methyldopa were added to the urine sample, and its concentrations were estimated with the proposed method. The urine sample was found to be free from methyldopa. Therefore, different amounts of methyldopa were spiked to the sample and analyzed by the proposed method. The results for determination of methyldopa in real samples are given in Table 2. Satisfactory recovery of the experimental results was found for methyldopa. The reproducibility of the method was demonstrated by the mean R.S.D.
2,7-BFGPE was prepared and used for the investigation of the electrochemical behavior of methyldopa. Two pairs of well-defined redox peaks were obtained at 2,7-BFGPE. 2,7-BFGPE showed excellent electrocatalytic activity for the redox of methyldopa. Compared with the bare electrode, the oxidation current of methyldopa increased greatly and the oxidation peak potential shifted negatively by 200 mV. This sensor showed a wide linear range (0.09 to 500.0 μM) with good detection limit (0.05 μM) for methyldopa. This sensor was successfully applied to determine methyldopa in some real samples.
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The authors declare that they have no competing interests.
Hadi Beitollahi, Somayeh Tajik and Hossein Asadi performed the fields of experimental sections and analysis of data. Pourya Biparva synthesized the graphene nanosheets. All authors read and approved the final manuscript.
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Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Beitollahi, H., Tajik, S., Asadi, M.H. et al. Application of a modified graphene nanosheet paste electrode for voltammetric determination of methyldopa in urine and pharmaceutical formulation.
J Anal Sci Technol5, 29 (2014). https://doi.org/10.1186/s40543-014-0029-y