Hemoglobin adsorption into TiO2 phytate multi-layer films: particle size and conductivity effects

Hemoglobin (molecular weight 64.5 kDa, isoelectric point 7.4) in 0.1 M phosphate buffer solution at pH 5.5 readily adsorbs onto mesoporous TiO2 phytate films, which have been formed in a layer-by-layer deposition process from TiO2 nanoparticles (ca. 6-10 mn diameter) and phytic acid at tin-doped indium oxide (ITO) electrodes. Quartz crystal microbalance data, voltammetry, and SEM evidence are consistent with hemoglobin adsorption only into the outer TiO2 phytate surface layer. The size of the tetrametric hemoglobin protein (ca. 6 nm diameter) appears to be too big for a homogeneous film to form. The modified ITO electrode immersed in 0.1 M phosphate buffer solution at pH 5.5 allows reversible electron transfer for hemoglobin to be observed with a midpoint potential of 0.01 vs. SCE. Characteristic TiO2 phytate film thickness and pH effects are observed with both thicker films and lower proton activity causing 'decoupling' of the protein redox chemistry due to a reduced electrical conductivity of the TiO2 phytate film connecting hemoglobin with the electrode. This is the first example of a bi-layer nanofilm structure where the underlying TiO2 phytate film controls the electrochemical properties of the hemoglobin modified top-layer. (C) 2004 Elsevier B.V. All rights reserved.