Hydrophilic/hydrophobic features of TiO ₂ nanoparticles as a function of crystal phase, surface area and coating, in relation to their potential toxicity in peripheral nervous system

The hydrophilic/hydrophobic properties of a variety of commercial TiO ₂ nanoparticles (NP), to be employed as inorganic filters in sunscreen lotions, were investigated both as such (dry powders) and dispersed in aqueous media. Water uptake and the related interaction energy have been determined by means of adsorption microcalorimetry of H ₂O vapor, whereas dispersion features in aqueous solutions were investigated by dynamic light scattering and electrokinetic measurements (zeta potential). The optimized dispersions in cell culture medium were employed to assess the possible in vitro neuro-toxicological effect on dorsal root ganglion (DRG) cells upon exposure to TiO ₂-NP, as a function of crystal phase, surface area and coating. All investigated materials, with the only exception of the uncoated rutile, were found to induce apoptosis on DRG cells; the inorganic/organic surface coating was found not to protect against the TiO ₂-induced apoptosis. The risk profile for DRG cells, which varies for the uncoated samples in the same sequence as the photo-catalytic activity of the different polymorphs: anatase-rutile>anatase≫rutile, was found not to be correlated with the surface hydrophilicity of the uncoated/coated specimens. Aggregates/agglomerates hydrodynamic diameter was comprised in the ∼200-400nm range, compatible with the internalization within DRG cells.