The preparation of ferrocene-modified carbon paste electrode by incorporation of a mediator into a graphite powder–paraffin oil matrix is described. The suitability of this chemically modified electrode for electrocatalytic oxidation of L-cysteine was studied in buffered solution by cyclic voltammetry and double potential step chronoamperometry. It has been found that, under optimum conditions (pH ¼ 7:00), the oxidation of L-cysteine at the surface of such an electrode occurs at a potential about 500 mV less positive than on an unmodified carbon paste electrode. The kinetic parameters such as electron transfer coefficient and catalytic reaction rate constant K0 h were determined using various electrochemical approaches. The electrocatalytic oxidation peak current of L-cysteine at the surface of this modified electrode was used for determination of L-cysteine by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Linear analytical curves were obtained in the ranges of 105 M (= mol dm3)–103 M and 8:9 106 M–2 104 M of Lcysteine concentration with CV and DPV methods, respectively. The detection limits were determined as 6:5 106 M and 4:7 106 M by CV and DPV methods. This method was also examined for determination of L-cysteine in samples, such as Soya protein powder, serum and plasma of human blood, using recovery and standard addition methods.
