Abstract
Inorganic UV absorbers such as titanium oxide and zinc oxide are in common use in formulations of sunscreens and cosmetics. These materials strongly absorb UV, and show no UV induced degradation over time. However, the dissipation mechanism of the UV energy is not often considered. It is demonstrated that the primary de-excitation mechanism of inorganic sunscreen components is via the surface of the particle and leads to the creation of free radicals. Substantial free radical generation is observed under simulated solar energy with all sunscreen grades of titanium oxide and zinc oxide. Free radicals are implicated in a number of potential health issues such as skin aging. Doping trap centres into titanium oxide and zinc oxide results in the total elimination of free radical generation under simulated solar energy. The trap centres provide sites within the oxide via which it is energetically favourable for photogenerated charge carriers to de-excite. Thus the charge is confined on sites within the host lattice and cannot migrate to the particle surface and create free radicals. The absorbed UV energy is converted to long wavelength light and heat. This effect is demonstrated by electron spin resonance, a primary measure of free radical generation, and by a plasmid nicking assay. The plasmid nicking assay demonstrates the effect of free radicals on supercoiled DNA.
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