Studies on the electrochemical behavior of a cystine self-assembled monolayer modified electrode using ferrocyanide as a probe

The preparation and electrochemical characterization of a cystine self-assembled monolayer (cys-SAM) onto a gold electrode is reported. The electron transfer of the Fe(CN)63-/4- couple at the modified electrode surface was studied at different pH values by using cyclic voltammetry and electrochemical impedance spectroscopy. A reversible voltammogram for the redox couple was observed at the cys-SAM surface, indicating that the electron transfer is not restrained. Electrochemical impedance spectroscopy was used to evaluate the electron transfer dependence on the protonation of acidic/basic groups of the adsorbed thiol and an increase of the resistance was observed at higher pH solution values because of the repulsion between negatively charged groups. Calculated values of charge transfer resistance increased with the solution pH from 600 to 1700 Ω cm2 in the pH range comprising 4.9 and 8.0. Quartz Crystal Microbalance experiments were performed for characterizing the cystine coverage and results showed a 58 ng cm−2 mass increase during immersion of a gold electrode in a 1 × 10−3 mol L−1 cystine solution, corresponding to 1.4 × 1014 molecules/cm2 (Γ = 2.3 × 10−10 mol cm−2). The influence of cystine concentration on the rate of adsorption was also investigated by cyclic voltammetry experiments.