Cobalt salophen-modified carbon-paste electrode incorporating a cationic surfactant for simultaneous voltammetric detection of ascorbic acid and dopamine

A carbon-paste electrode (CPE) is modified by incorporating cobalt-5-nitrosalophen (CoNSal) and tetraoctylammonium bromide (TOAB). The mechanism of electrocatalytic oxidation of dopamine (DA) and ascorbic acid (AA) at the surface of the CoNSal-modified CPE containing various percents of TOAB is thoroughly investigated by cyclic and differential pulse voltammetry. In solutions of pH 5.0, in which all studies are performed, DA exists as the positively charged species whereas AA is mainly as the neutral form. Therefore, the favorable ionic interaction (electrostatic repulsion) between the cationic form of DA and the cationic surfactant (TOA(+)) caused increasing the overvoltage for DA and positive shift in its anodic peak potential. Results of these studies showed that by increasing the weight percent of TOAB in the matrix of the modified CPE, Delta E-p for oxidation of AA and DP is increased, which makes it suitable for simultaneous voltammetric detection of AA and DA. However, hydrophobicity of TOAB at the surface of the modified electrode shows a great influence on the sensitivity of the voltammetric responses, especially for DA. Therefore, an optimum 2 wt.% of TOAB together with 3 wt.% of CoNSal was used for construction of the modified electrode and two well-resolved anodic wave for AA and DA were obtained in both cyclic and differential pulse voltammetric techniques. A linear range of 1 X 10(-6) to 1 X 10(-4) M for AA in the absence and also presence of a constant concentration of 1 X 10(-4) M DA is resulted using the modified electrode. The linear range for DA in a constant concentration of 1 X 10(-4) M AA was in the rang of 1 X 10(-6) to 1 X 10(-4) M. The preparation of the modified electrode is,e very easy and renewed by simple polishing. This electrode has very good reproducibility (R.S.D. <= 3%), high stability in its voltammetric response (more than two month without any considerable change in response) and low detection limit (micromolar) for both AA and DA. The prepared electrode is successfully applied for the voltammetric detection of AA and DA in pharmaceutical preparations. (c) 2006 Elsevier B.V. All rights reserved.