Voltammetric studies of a cobalt(II)-4-methylsalophen modified carbon-paste electrode and its application for the simultaneous determination of cysteine and ascorbic acid

A carbon-paste electrode (CPE) chemically modified with the cobalt(II)-4-methylsalophen (CoMSal) as a Schiff base complex was used as a highly sensitive and fairly selective electrochemical sensor for simultaneous determination of minor mounts of ascorbic acid (AA) and cysteine. This modified electrode shows very efficient electrocatalytic activity for anodic oxidation of both AA and cysteine via substantially decreasing of anodic overpotentials for both compounds. The mechanism of electrochemical oxidation of both AA and cysteine using CoMSal-modified electrode was thoroughly investigated by cyclic voltammetry and polarization studies. Results of cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using this modified electrode show two well-resolved anodic waves for the oxidation of AA and cysteine, which makes it possible for simultaneous determination of both compounds. A linear range of 1 x 10(-4) to 5 x 10(-7) M for cysteine in a constant concentration of 1 x 10(-4) M AA in buffered solution (as a background electrolyte) was obtained from DPV measurements using this electrode. The linear range, which is obtained for AA in the presence of 1 x 10(-4) M cysteine, was in the range of 1 x 10(-4) to 1 x 10(-6) M. The modified electrode has good reproducibility (RSD less than or equal to 2.5%), low detection limit (sub-micromolar) and high sensitivity for the detection of both AA and cysteine with a very high stability in its voltammetric response. Differential pulse voltammetry using the modified electrode exhibited a reasonable recovery for a relatively wide concentration range of cysteine spiked to a synthetic human serum sample. (C) 2004 Elsevier Ltd. All rights reserved.