XANES and SR-XRF study of skin as a barrier to ultra-fine nanocrystals of TiO2

Abstract

Nanocrystalline TiO2 is commonly used in cosmetic industry as a photoprotective agent. With recent advances in nanomaterial processing, the size of TiO2 crystals decreased into the nanometre regime. There is no satisfactory evidence that crystals of such small size are harmless to the human population. An EU project NANODERM has been launched where several techniques have been applied to investigate the possiblity of particle penetration through the protective horny layer into vital skin regions. Skin biopsies of the animal and human skin have been collected after exposition to formulations containing TiO2 nanocrystals. The Ti depth distributions were measured by electron and ion microscopy. The microscopy studies did not detect penetration into vital tissue of healthy skin what does not exclude a possibility that TiO2 could penetrate pathological skin with lowered barrier efficiency. Due to literature the physical effect of the UV irradiation of the TiO2 nanoparticle is the shift from 4th to 3rd oxidation state of the Ti. Titanium at 3rd oxidation state interact with environment producing free radicals and Reactive Oxygen Species. In order to quantify the oxidation state shift, XANES experiments were carried out with commercially available TiO2 nanocrystals (6–100 nm size), both in anatase and rutile phase. The samples were irradiated with X-rays with, and without accompanying UV illumination at the NSLS X27A beam line. The corresponding XANES spectra were registered and the absorption edge was compared in UV–illuminated and not illuminated spectra. A shift of about 1 eV in the absorption edge position of the rutile sample exposed to UVA light (365 nm, 20 mW/cm2) has been measured and attributed to the changed electron configuraion. However, the direction of the shift detected in measured samples was opposite to the expected.