Voltammetric and potentiometric study of cysteine at cobalt(II) phthalocyanine modified carbon-paste electrode

Cyclic voltammetry and potentiometry were used to investigate the electrochemical behavior of cysteine at a chemically modified electrode prepared by incorporating cobalt(II) phthalocyanine [Co(II)Pc] into carbon paste matrix. The modified electrode showed high electrocatalytic activity toward cysteine; the overpotential for the oxidation of cysteine was decreased by more than 100 mV, and the corresponding peak current increased significantly. The electrocatalytic process was highly dependent on the pH of the supporting electrolyte. The peak currents decreased when the pH was raised to 6 and totally disappeared at pH≥ 7, resulting from the autocatalytic oxidation of cysteine by Co(II)Pc at the electrode surface. Therefore, at pH values of 6 to 8, the modified electrode was used as a potentiometric sensor for quantitative measurement of cysteine in the presence of oxygen in air saturated solutions. In fact, the Co(II)Pc/Co(I)Pc couple acts as a suitable mediator for indirect oxidation of cysteine by dissolved oxygen at approximately neutral pH values. Under the optimized conditions, the potentiometric response of the modified electrode was linear against the concentration of cysteine in the range of 0.6 μM to 2 mM. The limit of detection was found to be 0.5 μM. The potentiometric response time was ≤15 s. The electrode showed long term stability; the standard deviation of the slope obtained after repeated calibration during a period of two months was 2.8% (n = 7). Application of the electrode in a recovery experiment for the determination of cysteine added to a synthetic serum sample is described.