Carbon paste electrode incorporating multi-walled carbon nanotube/cobalt salophen for sensitive voltammetric determination of tryptophan

Abstract

A modified carbon paste electrode is prepared by incorporating multi-walled carbon nanotube (MWCNT) and cobalt salophen (CoSal). A mixture of fine graphite powder with 10 wt% of MWCNT is applied for the preparation of carbon paste (by dispersing in Nujol) and finally modified with CoSal. The electrocatalytic oxidation of tryptophan (Trp) is investigated on the surface of the MWCNT/CoSal-modified electrode using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Using the modified electrode, the kinetics of the electro-oxidation of Trp is considerably enhanced, by lowering the anodic overpotential through a catalytic fashion. The mechanism of electrochemical behavior of Trp at the surface of modified electrode is analyzed by investigating the effect of sweep rate and pH on the responses of CV method. The prepared modified electrode showed voltammetric responses with high sensitivity and selectivity for Trp, which makes it very suitable for sub-micromolar detection of Trp. The electrode is able to completely resolve the voltammetric response of Trp from potentially interfering species, e.g. ascorbic acid (AA) and cysteine, which can be oxidized effectively on the surface of the modified electrodes with transition metal complexes. The voltammetric studies using the modified electrode showed three well-resolved anodic peaks for AA, cysteine and Trp, revealing the applicability of the electrode for simultaneous electrochemical detections of these compounds in mixtures. A linear dynamic range of 5 × 10 −5 to 5.0 × 10 −7 M for Trp is obtained in buffered solutions with pH 4.0. The linear range of the calibration curve remains constant in binary mixture solutions of Trp and cysteine or AA. By use of modified electrode, the voltammetric detection limit for Trp in DPV measurements was 1.0 × 10 −7 M. The reproducibility of the electrode response, based on 6 measurements during 1 month, was 2.2% for the slope of the calibration curve. High sensitivity and selectivity together with sub-micromolar detection limit of the electrode response make it very suitable for the determination of trace amounts of Trp in pharmaceutical and clinical preparations.