Electrochemical low-level detection of l-tryptophan in human urine samples: use of pencil graphite leads as electrodes for a fast and cost-effective voltammetric method

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Electrochemical low-level detection of l-tryptophan (l-Trp), a precursor to the neurotransmitter serotonin, was performed at a practical sensor based on pencil graphite electrode. The electrode chosen is disposable, economical, and eco-friendly as well as sensitive and selective when used for adsorptive stripping differential pulse voltammetry (DPAdSV) in the presence of uric acid and also in urine samples. The electrochemical oxidation of l-Trp was found to be partly adsorption controlled under diffusion conditions. The optimum values for the pH (3.0), accumulation potential (+ 0.3 V), and accumulation time (5.0 s) were investigated and used in the voltammetric analysis. Under the optimum conditions, the anodic DPAdSV peak current of l-Trp linearly increased with the concentration of l-Trp in the presence of uric acid and also in urine. The electrode used exhibited a wide working range from 0.154 to 200.0 µM with a detection limit of 0.046 µM. The method developed was applied to the analysis of l-Trp in the human urine sample with acceptable recoveries.