Application of a modified graphene nanosheet paste electrode for voltammetric determination of methyldopa in urine and pharmaceutical formulation
© Beitollahi et al.; licensee Springer. 2014
Received: 7 January 2014
Accepted: 17 March 2014
Published: 28 May 2014
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
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
Calibration plot and limit of detection
Comparison of the efficiency of some modified electrodes used in the electrocatalysis of methyldopa
Dynamic range (M)
Limit of detection (M)
Ferrocene monocarboxylic acid
2.0 × 10−7 to 1.0 × 10−4
8.0 × 10−8
Molaakbari et al. ()
5.0 × 10−8 to 4.0 × 10−5
1.0 × 10−8
Shahrokhian and Rastgar ()
Cellulose acetate/ionic liquids
3.48 × 10−5 to 3.7 × 10−4
5.5 × 10−6
Moccelini et al. ()
Carbon nanotube paste
1.0 × 10−7 to 2.1 × 10−4
4.8 × 10−8
Tajik et al. ([2013a])
8.0 × 10−8 to 5.0 × 10−4
5.0 × 10−8
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
The application of 2,7-BFGPE for determination of methyldopa in methyldopa tablet and urine samples
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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