A novel cation-binding TiO2 nanotube substrate for electro-catalysis and bioelectro-catalysis

TiO2 nanotubes (8-20 nm outer diameter and 3-5 nm inner diameter) grown via alkaline hydrothermal synthesis are characterised and compared to 6 nm diameter TiO2 (anatase) nanoparticles. Zeta potential, voltammetric, and titration experiments reveal that, in contrast to anatase nanoparticles (p.z.c. ca. 6), TiO2 nanotubes carry a stronger negative surface charge (p.z.c. ca. 3, acidic protons ca. 2 x 10(-3) mol g(-1), electrostatic cation adsorption sites in neutral solution ca. 7 x 10(-5) mol g(-1)) and, under neutral conditions, offer electrostatic binding sites for cations. When immobilised onto an inert boron-doped diamond substrate, TiO2 nanotubes show electrochemical reactivity due to reversible Ti(IV) reduction, which is very similar to that observed for anatase nanoparticles. Three cationic redox systems, Meldola's blue, Ni2+, and cytochrome c, are immobilised on the TiO2 nanotube surface; the binding ability and the number of binding sites are quantified voltammetrically. Redox proteins, such as cytochrome c, adsorb readily and irreversibly. Well-defined voltammetric signals for the immobilised protein are observed in an aqueous buffer. TiO2 nanotubes are shown to be novel, inert substrates for both inorganic and biological electrocatalysts. (c) 2005 Elsevier B.V. All rights reserved.