The effects of conductivity and electrochemical doping on the reduction of methemoglobin immobilized in nanoparticulate TiO 2 films

Abstract

Methemoglobin (bovine) is immobilized from aqueous phosphate buffer (pH 5.5) solution into thin porous TiO 2 (anatase) films at ITO electrode surfaces. Films of TiO 2 are produced in a deposition process employing 40 nm diameter TiO 2 nanoparticles suspended in dry methanol followed by calcination. The pore size in these films is sufficient for methemoglobin (ca. 6 nm diameter) to diffuse into the porous structure (over several hours) and to remain immobilized in electrochemically active form. The electrochemical reduction of methemoglobin immobilized in TiO 2 and immersed in aqueous phosphate buffer at pH 5.5 is observed in two steps with (i) a small quasi-reversible voltammetric response at − 0.16 V vs. SCE (Process 1) and (ii) an irreversible reduction peak at ca. − 0.5 V vs. SCE (Process 2). The irreversible response is recovered only after slow chemical re-oxidation of hemoglobin to methemoglobin. At sufficiently negative applied potential “electrochemical doping” of the TiO 2 host is observed to lead to a considerably enhanced reduction Process 1. TiO 2 can be temporarily switched from a non-conducting (irreversible electron transfer) into a conducting (reversible electron transfer) state.