Study of human serum albumin-TiO2 nanocrystalline electrodes interaction by impedance electrochemical spectroscopy

The adsorption of human serum albumin (HSA) onto nanocrystalline TiO2 electrodes was studied by electrochemical impedance spectroscopy (EIS) in function of pH and electrode potential. The characterization and physico-chemical properties of the TiO2 electrode were investigated by scanning electron microscopy (SEM), UV-photoelectron spectroscopy (UPS), cyclic voltammetry and capacitance measurements. The impedance response of the particulate TiO2 electrode/protein interface was fitted using an equivalent circuit model to describe the adsorption process. The adsorbed protein layer, which is formed as soon as the protein is injected into the solution and becomes in contact with the electrode, was investigated as a function of electrode potential and solution pH. The measurements were performed under pseudo-steady-state and steady-state conditions, which gave information about the different states of the system. With the pseudo-steady state measurements, it was possible to determine two rate constants of the protein adsorption process, which correspond to two different states of the protein. The shortest one was associated with the first contact. between the protein and the substrate and the second relaxation time, with the protein suffering an structural rearrangement due to the interaction with the TiO2 electrode. It was detected that at sufficiently long times (approx. 1 h, where the system was under steady state conditions), a quasi-reversible protein adsorption mechanism was established. The measurements performed as a function of frequency under steady-state conditions, an equivalent circuit with a Warburg element gave the better fitting to data taken at - 0.585 V closer to the oxide flat band potential and it was associated with protein diffusion. Experimental results obtained at only one frequency as a function of potential could be fitted to a model that takes into account non-specific and probable specific protein adsorption, which renders to be potential- and pH-dependent. Low capacity values were obtained in the whole potential range, which were measured in the presence and in the absence of the protein layer, The capacity dependence on potential and pH were associated with the generation of surface states on TiO2. A surface state concentration of 4.1 X 10(18) cm(-2) was obtained by relating the parallel capacitance with oxide surface states arising from the protein-oxide interaction. (C) 2001 Elsevier Science B.V. All rights reserved.