On the photocatalytic reduction of MU tetrazolium salt on the surface of TiO2 nanoparticles: Formazan production kinetics and mechanism

Hypothesis: The MU [3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium-bromide] cell cytotoxicity indicator is photocatalytically reduced on the surface of TiO2 nanoparticles in phosphate-buffered-saline (PBS) environment. We hypothesize that specific phosphate adsorption may be used to modulate the efficiency of the TiO2 MU reaction through colloidal and semiconductor-liquid interface processes. Experiments: The TiO2 MU reaction kinetics was studied in PBS, with respect to photocatalyst and MIT concentrations and irradiation wavelength. The effects of PBS and electron scavengers (Fe3+ ions) on reaction efficiency and the role of colloidal surface charge in the photocatalytic process were investigated. The structural and spectroscopic characteristics of relevant TiO2-formazan systems were studied by X-ray diffraction, transmission electron microscopy and IR-spectroscopy. Findings: The reaction was pseudo-first order with respect to photocatalyst and showed a negative and fractional partial order with respect to MU. Formazan production rates were directly proportional to radiation wavelength and TiO2 concentration and inversely proportional to the MIT initial concentration. The addition of Fe3+ ions, as well as the absence of PBS, induced strong reaction inhibition. Reaction efficiency and catalyst Zeta potential were enhanced by Na2HPO4 (PBS component) and showed a maximum around the phosphate concentration 0.005 M. Findings: Structural/spectroscopic characterization confirmed the formation of amorphous MU-formazan on the surface of TiO2 and the TiO2-phosphate binding. (C) 2015 Elsevier Inc. All rights reserved.