Electrochemical determination of methyldopa in pharmaceutical samples using graphene oxide- phosphomolybdic acid/pencil graphite electrode

Methyldopa, (a-methyl-3,4-dihydroxyphenylalanine), is an antihypertensive agent and the best factor for the treatment of high blood pressure in pregnancy time [1]. The determination of methyldopa has been reported by several methods including titrimetry [2], chromatography [3] and spectroscopy [4]. Because of the importance of methyldopa in biological fluid, it is important to develop simple, accurate, low level and rapid method to the determination of this drug in pharmaceutical and biological samples. Electrochemical methods have advantages such as low detection limit, high sensitivity and repeatability [5]. In this study a modified pencil graphite (PGE) electrode was used as an excellent electrochemical sensor for the selective and sensitive low level determination of methyldopa in different samples. The PGE was modified with graphene oxide (GO) and phosphomolybdic acid (PMo12) as a coating layer on its surface by spin coating method. The electron transfer reaction between electrolyte and electrode interfacial surface were studied using electrochemical impedance spectroscopy (EIS) in 50 mM [Fe(CN)6]3-/4- with 0.1 M KCl. The obtained results showed that Rct for the GO/PGE is larger than PMo12-GO/PGE and much smaller than that of Bare/PGE, suggesting GO successfully enhanced electron transfer in PMo12-GO modified electrode. The modifier was analysed by SEM, XRD and EDX which showed C, P, O and Mo peaks. Methyldopa can be determined by differential pulse voltammetric (DPV) technique in the linear range of 4.9×10-10 M to 1.0×10-7 M and the obtained relevant detection limit for the sensor was 1.2×10-10 M. The proposed sensor was used for the determination of methyldopa in pharmaceutical and urine samples.