Non-UV-Induced Radical Reactions at the Surface of TiO2 Nanoparticles That May Trigger Toxic Responses

Titania is generally considered to be an inert and safe material. Several studies, however, have reported that nanosized TiO2 may elicit toxic effects. In some cases the observed adverse effects have been related to free radicals. Although new studies mainly concern irradiated titania, the role and the mechanisms of the generation of free radicals by TiO2 in the absence of UV irradiation are not well known. The purpose of the present study is to investigate the free-radical-generation mechanisms by nano- and micronsized anatase or rutile powders under normal laboratory illumination or in the dark by means of a spin-trapping/ESR spectroscopy technique. This technique is used to identify the nature and the amount of free radicals released in solution, and in the solid-state to characterise the paramagnetic centres at the surface of particles that may participate in the reactions. The following radical-generating mechanisms have been considered: 1) the generation of oxygenated free radicals (HO2 center dot, O-2(center dot-), HO center dot) following the reaction of TiO2 with oxygen, water or H2O2 and 2) the generation of carbon-centred radicals following the cleavage of the C-H bond in a model molecule. Although no free radicals were detected in a simply buffered solution, anatase and rutile generated O-2(center dot-) and HO center dot, respectively, in the presence of H2O2. Both polymorphs were also active in the cleavage of the C-H bond. Although the formation of O-2(center dot-) appears to be related to exposure to sunlight, the generation of HO center dot and carbon-centred free radicals also occurs in the dark. When samples of equal surface area were tested, micron- and nanosized anatase was found to react in the same way indicating that a reduction in diameter does not generate new kinds of reactive sites. The data presented herein may have implications in the assessment of the health risk associated with the exposure to TiO2 nanoparticles and in the ecotoxicological impact following their possible leakage into the environment.