Electrocatalytic oxidation and highly selective voltammetric determination of L-cysteine at the surface of a 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone modified carbon paste electrode

A carbon paste electrode (CPE) chemically modified with 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone (4-FEPEMCPE) was employed to study the electrocatalytic oxidation Of L-cysteine using cyclic voltammetry, differential pulse voltammetry and double potential step chronoamperometry as diagnostic techniques. The diffusion coefficient (D 7.863 x 10(-6) cm(2) s(-1)) Of L-Cysteine was also estimated using chronoamperometry. The electron-transfer coefficient, a 0.40), for L-Cysteine at the surface of 4-FEPEMCPE was determined using cyclic voltammetry technique. It was found that under an optimum pH (= 7.00), the oxidation Of L-Cysteine at the surface of such an electrode occurred at a potential of about 350 mV less positive than that of an unmodified CPE. The catalytic oxidation peak currents represented a linear dependence on the L-Cysteine concentration. Linear analytical curves were obtained in the ranges of 9.0 x 10(-5) - 4.9 x 10(-3) M and 2.0 x 10(-5) - 2.8 X 10(-3) M of L-cysteine with correlation coefficients of 0.9981 and 0.9982 in cyclic voltammetry and differential pulse voltammetry, respectively. The detection limits (2 sigma) were determined to be 9.9 X 10(-6) M and 5 x 10(-6) M with cyclic voltammetry and differential pulse voltammetry, respectively. The influences of twenty other amino acids, such as glutamine, L-glutamic acid, L-glysine, L-histidine, L-isoleucine, L-leucine, L-arginine hydrochloride, L-aspargine, L-aspartic acid, S-carboxy methyl-L-Cysteine, L-methionine, L-phenyl alanine, L-proline, L-serine, L-threonine, L-Cystine, cysteamine and gluthathione, on the current response of the sensor were examined. The obtained results did not show any influence on the analytical signal Of L-Cysteine by these amino acids (except for cysteamine). The method was also used for the selective determination Of L-Cysteine in patient-blood plasma and some pharmaceutical preparations by using standard addition method.