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 ×10–6 cm2 s–1) of L-cysteine was also estimated using chronoamperometry. The electron-transfer coefficient, α(=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 ×10–5 –4.9 ×10–3 M and 2.0 ×10–5 –2.8 ×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 (2σ) were determined to be 9.9 ×10–6 M and 5 ×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, Lserine, L-threonine, L-cystine, cysteamine and gluthathione, on the current response of the sensor were examined. Theobtained 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.
