Long-time and short-time investigation of the electrode interface through electrochemical impedance measurements. Application to adsorption of human serum albumin onto glassy carbon rotating disc electrode

Abstract

Abstract Two methods were used to investigate the adsorption of a human protein onto glassy carbon under well-defined conditions, i.e. in 0.1 M phosphate buffer (pH = 7.4), at T = 25°C, in the presence of equimolar (5 × 10−4 M) ferricyanide + ferrocyanide ions as steady-state indicator, with a three-electrode potentiostatic system at a bias potential of −0.67 V (SCE), under the laminar shear flow obtained by a carbon disc electrode rotated at 700 rev min−1. The protein concentration was 1 g dm−3. With the long-time method, the behaviour of the interface in its entirely was studied through the time-evolution of the impedance diagrams over a wide frequency range (sine-wave perturbation from 40 mHz to 10 kHz). With the short-time method, the time-evolution of the double-layer capacitance was determined at only one frequency, starting from the imaginary part of the impedance, on the basis of the well-known interface equivalent circuit without diffusion. However, investigation of the whole interface shoed that diffusion was actually indicated and the double-layer capacitance exhibited a fractional power-law (constant phase element) and was therefore frequency-dependent. The contribution of the characteristic electrical parameters of the interface in the overall impedance was determined. Use of the two methods justified the choice of the frequency value in the second method and showed the necessity of performing complete investigations of the interface over a wide frequency range before any capacitance measurements.