Protein adsorption on nanoporous TiO2 films: a novel approach to studying photoinduced protein/electrode transfer reactions

We have investigated the use of nanoporous TiO2 films as substrates for pro tein immobilisation. Such films are of interest due to their high surface a rea, optical transparency, electrochemical activity and ease of fabrication . These films moreover allow detailed spectroscopic study of protein/electr ode electron transfer processes. We find that protein immobilisation on suc h films may be readily achieved from aqueous solutions at 4 degreesC with a high binding stability and no detectable protein denaturation. The nanopor ous structure of the film greatly enhances the active surface area availabl e for protein binding (by a factor of up to 850 for an 8 mum thick film). W e demonstrate that the redox state of proteins such as immobilised cytochro me-c (Cyt-c) and haemoglobin (Hb) may be modulated by the application of an electrical bias potential to the TiO2 film, without the addition of electr on transfer mediators. The binding of Cyt-c on the TiO2 films is investigat ed as a function of film thickness, protein concentration, protein surface charge and ionic strength. We demonstrate the potential use of immobilised Hb on such TiO2 films for the detection of dissolved CO in aqueous solution s. We further show that protein/electrode electron transfer may be initiate d by UV bandgap excitation of the TiO2 electrode. Both photooxidation and p hotoreduction of the immobilised proteins can be achieved. By employing pul sed UV laser excitation, the interfacial electron transfer kinetics can be monitored by transient optical spectroscopy, providing a novel probe of pro tein/electrode electron transfer kinetics. We conclude that nanoporous TiO2 films may be useful both for basic studies of protein/electrode interactio ns and for the development of novel bioanalytical devices such as biosensor s.