Optical waveguide spectroscopy study of the transport and binding of cytochrome c in mesoporous titanium dioxide electrodes.

Abstract

The increasing interest in using nanoporous and mesoporous films for a range of applications including sensors and solar cells has created a concomitant demand for the development of analytical methodologies to study the films in situ. Especially important is the ability to quantify the filling of such films with guest molecules, be they dyes or proteins. In this paper we have demonstrated a simple methodology for the highly sensitive monitoring of nanoporous titania films as protein is both adsorbed in, and removed from the film matrix: optical waveguide mode spectroscopy. Porous titania films of thickness between 0.5 and 1.5 μm were created on gold slides by the deposition of a colloidal suspension of titanium dioxide nanoparticles with a phytate binder. Absorption of cytochrome c into the highly porous titanium structure was followed by both optical waveguide spectroscopy (OWS) and electrochemistry. OWS showed that saturation occurred when 27% of the pore volume was filled with cytochrome c and initial diffusion coefficients of 10⁻¹⁷ m² s⁻¹ were measured for cytochrome c within the pores. Electrochemical measurements displayed good agreement with OWS measurements and showed that the protein permeated all the way through the porous film to the gold substrate. The results further demonstrate that the OWS method is more than two orders of magnitude more sensitive than classical electrochemical methods conventionally used to study such systems, and is not dependent on having a redox-active adsorbate.