Ultraviolet irradiation of titanium dioxide in aqueous dispersion generates singlet oxygen

We previously reported that irradiation of titanium dioxide (TiO 2 ) in ethanol generates both singlet oxygen ( 1 O 2 ) and superoxide anion (O 2 ·- ) as measured by EPR spectroscopy. The present study describes the production of reactive oxygen species upon irradiation of TiO 2 in aqueous suspension as determined by EPR spectroscopy using 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TMP) and 5,5- dimethyl-pyrroline-N-oxide (DMPO). Photoproduction of 1 O 2 by suspended TiO 2 , detected as 2,2,6,6-tetramethyl-4-piperidone-N-oxyl (4-oxo-TEMPO), was measured in water and deuterium oxide (D 2 O) in the presence or absence of sodium azide (NaN 3 ) and under air or argon atmospheres. Production of a DMPO-OH adduct was examined in 4-oxo-TMP containing medium in the presence or absence of dimethyl sulfoxide (DMSO). The signal for the DMPO spin adduct of superoxide anion was not observed in aqueous conditions. Kinetic analysis revealed that 1 O 2 was produced at the surface of irradiated TiO 2 in aqueous suspension as was observed in ethanol. Kinetic analysis revealed that the formation of DMPO-OH adduct reflects oxidation of DMPO by 1 O 2 rather than the trapping of the hydroxyl radical produced by the reaction of photo-exited TiO 2 and water. The production of large amounts of 1 O 2 by TiO 2 in aqueous suspension as compared to those in ethanol and possible formation of hydroxyl radical in aqueous suspension but not in alcohol, suggest that irradiation of TiO 2 in aqueous environments is biologically more important than that in non-aqueous media.