Photoexcitation of aqueous suspensions of titanium dioxide nanoparticles: an electron spin resonance spin trapping study of potentially oxidative reactions.

It is well-established that exposure of aqueous suspensions of titanium dioxide (TiO(2)) nanoparticles to ultraviolet A (UVA) light produces reactive oxygen species which leads to biological damage. However, there is disagreement in the literature as to the exact nature of these species and how they are formed. Using a number of different spin traps (i.e. PBN, POBN, DMPO, DEPMPO), we have shown that the primary damaging species produced on irradiation of an aqueous suspension of TiO(2) is the hydroxyl radical, which is formed at the valence band hole under both aerobic and hypoxic conditions. Hydroxyl radical production is enhanced by the presence of oxygen which probably reacts with the conduction band electrons or resultant Ti(3+), inhibiting hole-electron recombination, although we find no evidence of reaction of oxygen to form free superoxide radical anions or of the formation of any other radical at that site. The present results suggest that the resulting O(2)(•-) species may not be as labile as previously thought and may possibly undergo further reduction to the O(2)(2-) dianion. Hydroxyl radicals formed at the surface of the TiO(2) readily react with substrates containing an abstractable hydrogen to produce secondary radicals that, in biological systems, could lead to cell damage.