Accumulation and reactivity of the redox protein cytochrome c in mesoporous films of TiO2 phytate

The formation of well-defined nanofilm deposits of TiO2 nanoparticle phytates based on the "directed assembly" methodology is demonstrated. Alternate exposure of a tin-doped indium oxide electrode surface to aqueous solutions of TiO2 nanoparticles (3-4% in HNO3, ca. 6 nm diameter) and phytic acid (40 mM, at pH 3) causes layer-by-layer growth of a three-dimensional mesoporous structure. Cytochrome c in aqueous phosphate buffer (pH 7) is readily accumulated into the mesoporous TiO2 phytate film predominantly due to electrostatic binding of the positively charged protein to the negatively charged interfacial phytic acid. Voltammetric data for the reversible reduction and reoxidation of cytochrome c suggest strong adsorption and "ideal" thin film behavior over a wide range of conditions. Voltammetric data are analyzed quantitatively based on the model of a finite diffusion zone. For a TiO2 phytate modified electrode immersed in aqueous 0.1 M phosphate buffer (pH 7), strong accumulation (a 3 order of magnitude increase in concentration) of cytochrome c, an apparent standard rate constant for electron transfer, k(apparent)(0) = 3 x 10(-8) m s(-1), and an effective diffusion coefficient for cytochrome c within the mesoporous structure, D-effective = 2 x 10(-14) m(2) s(-1), are obtained. The redox processes within the nanoporous membrane, which are relatively insensitive to impurities and strongly affected by the electrolyte concentration in the aqueous buffer solution, are proposed to be dominated by electron "hopping" between adjacent cytochrome c molecules.