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

The adsorption of human serum albumin (HSA) onto nanocrystalline TiO2 elect rodes was studied by electrochemical impedance spectroscopy (EIS) in functi on of pH and electrode potential. The characterization and physico-chemical properties of the TiO2 electrode were investigated by scanning electron mi croscopy (SEM), UV-photoelectron spectroscopy (UPS), cyclic voltammetry and capacitance measurements. The impedance response of the particulate TiO2 e lectrode/protein interface was fitted using an equivalent circuit model to describe the adsorption process. The adsorbed protein layer, which is forme d as soon as the protein is injected into the solution and becomes in conta ct with the electrode, was investigated as a function of electrode potentia l and solution pH. The measurements were performed under pseudo-steady-stat e and steady-state conditions, which gave information about the different s tates of the system. With the pseudo-steady state measurements, it was poss ible to determine two rate constants of the protein adsorption process, whi ch correspond to two different states of the protein. The shortest one was associated with the first contact. between the protein and the substrate an d the second relaxation time, with the protein suffering an structural rear rangement due to the interaction with the TiO2 electrode. It was detected t hat at sufficiently long times (approx. 1 h, where the system was under ste ady state conditions), a quasi-reversible protein adsorption mechanism was established. The measurements performed as a function of frequency under st eady-state conditions, an equivalent circuit with a Warburg element gave th e better fitting to data taken at - 0.585 V closer to the oxide flat band p otential 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 prote in adsorption, which renders to be potential- and pH-dependent. Low capacit y values were obtained in the whole potential range, which were measured in the presence and in the absence of the protein layer, The capacity depende nce on potential and pH were associated with the generation of surface stat es on TiO2. A surface state concentration of 4.1 X 10(18) cm(-2) was obtain ed by relating the parallel capacitance with oxide surface states arising f rom the protein-oxide interaction. (C) 2001 Elsevier Science B.V. All right s reserved.