A Comparative Study of Cerium(III) and Cerium(IV) Phosphates for Sunscreens

Crystalline cerium(III) phosphate (CePO₄), cerium(IV) phosphates, and nanocrystalline ceria are considered to be promising components of sunscreen cosmetics. This paper reports on a study in which, for the first time, a quantitative comparative analysis was performed of the UV-shielding properties of CePO₄, Ce(PO₄)(HPO₄)_0.5(H₂O)_0.5, and CePO₄/CeO₂ composites. Both the sun protection factor and protection factor against UV-A radiation of the materials were determined. Ce(PO₄)(HPO₄)_0.5(H₂O)_0.5 was shown to have a sun protection factor of 2.9, which is comparable with that of nanocrystalline ceria and three times higher than the sun protection factor of CePO₄. Composites containing both cerium dioxide and CePO₄ demonstrated higher sun protection factors (up to 1.8) than individual CePO₄. When compared with the TiO₂ Aeroxide P25 reference sample, cerium(III) and cerium(IV) phosphates demonstrated negligible photocatalytic activity. A cytotoxicity analysis performed using two mammalian cell lines, hMSc and NCTC L929, showed that CePO₄, Ce(PO₄)(HPO₄)_0.5(H₂O)_0.5, and nanocrystalline ceria were all non-toxic. The results of this comparative study indicate that cerium(IV) phosphate Ce(PO₄)(HPO₄)_0.5(H₂O)_0.5 is more advantageous for use in sunscreens than either cerium(III) phosphate or CePO₄/CeO₂ composites, due to its improved UV-shielding properties and low photocatalytic activity.