RETRACTED ARTICLE: A Voltammetric Sensor for the Simultaneous Determination of l-Cysteine and Tryptophan Using a p-Aminophenol-Multiwall Carbon Nanotube Paste Electrode

Two amino acids, L-cysteine and tryptophan, could be simultaneously determined in an aqueous solution (pH 6.0) using a carbon nanotube paste electrode (CNPE) modified with p-aminophenol as a mediator. The results indicate that the electrode is efficient in terms of its electrocatalytic activity for the oxidation of L-cysteine, leading to an overpotential reduced by more than 550 mV. Using differential pulse voltammetry, we could measure L-cysteine and tryptophan in one mixture independently from each other by a potential difference of about 600 mV. Electrochemical techniques are used to determine the diffusion coefficients, kinetic parameters such as electron transfer coefficient, and the rates of electro-oxidation of L-cysteine at the surface of the p-aminophenol-modified CNPE. The peak current is found to depend linearly on L-cysteine and tryptophan concentrations within the ranges of 0.5 – 100 μmol L–1 L-cysteine and 10.0 – 300 μmol L–1 tryptophan. The detection limits for L-cysteine and tryptophan are found to be 0.3 and 5.7 μmol L–1, respectively. The proposed method is also used for the determination of L-cysteine and tryptophan in urine, river water, blood plasma, and serum samples using standard addition methods.