Electrochemical behavior of human serum albumin-TiO2 nanocrystalline electrodes studied as a function of pH. Part 2: Presence of [Fe(CN)6]3-/4- redox couple in solution

The interaction of human serum albumin (HSA) with the nanocrystalline Ti/TiO2 electrode surface was previously studied using electrochemical techniques in 0.1 M NaCl at different solution pH. In this report, we analyze the influence of the protein adsorption on the interface kinetics at different concentrations of Fe(CN)63-/4- redox couple and electrode potential in a pH range between 3.0 and 10.0 using Electrochemical Impedance Spectroscopy and Cyclic Voltammetry. The impedance spectrum of the redox couple-Ti/TiO2 nanocrystalline electrode interface in the presence and absence of HSA into the bulk solution was fitted using different equivalent circuits, which describe changes due to the HSA adsorption process. Around the flat band potential of the nanocrystalline Ti/TiO2 electrode (−0.20 V vs. saturated calomel electrode), an increase in capacitance element values when protein was absorbed at the electrode surface was observed. Simultaneously, a decrease in the resistance of charge transfer process of the Fe(CN)63-/4- redox couple, Rct, due to the protein adsorption was also observed. At 1.00 V electrode potential and at solution pH equal to HSA isoelectric point (IEP = 4.7), an increase in the Rct was observed and it was related to a predominant protein blocking effect. Two time constants, τH associated to charge transfer-related processes and τD associated to diffusion processes, were obtained from the fitting of the impedance data under potentiostatic conditions, and a protein diffusion coefficient value of 4.28 × 10−11 m2 s−1 was determined. The variation of the τH as a function of solution pH was interpreted, considering the chemical reactions between the surface oxide acid-base groups and the carboxylic or amine groups of the HSA protein.