Potential of (NH ₄ ) ₂ Ce(PO ₄ ) ₂ ·H ₂ O Phase for the Next Generation of Sunscreens: UV-Shielding Properties, Cytotoxicity and Biochemical Behavior

Recently, it has been recognized that crystalline ceric phosphates are promising candidates for use in sunscreen cosmetics, as inorganic UV filters. The present paper provides deeper insights into the UV-shielding properties, biocompability, radical-scavenging properties and redox activity of the crystalline double ammonium ceric phosphate (NH₄)₂Ce(PO₄)₂·H₂O. A strong correlation was established between conditions of hydrothermal synthesis and the functional characteristics of this compound. Suspensions containing (NH₄)₂Ce(PO₄)₂·H₂O with an intermediate crystallite size of ∼70 nm had higher values of sun protection factor (SPF) and protection factor against UV-A radiation (UVAPF) than other cerium phosphates, and even than traditional UV filters─titanium and zinc oxides. It was shown that (NH₄)₂Ce(PO₄)₂·H₂O particles at concentrations of 0.125-1.0 mg/mL, upon direct contact with human keratinocytes (HaCaT) or human mesenchymal stem cells, reduced their metabolic activity, while providing a photoprotective effect when exposed to ultraviolet radiation. The UV-shielding property of ammonium ceric phosphate at a concentration of 1.0 mg/mL preserved almost 100% of human mesenchymal stem cells. For the first time, the biochemical properties of (NH₄)₂Ce(PO₄)₂·H₂O and another double phosphate, NH₄Ce₂(PO₄)₃, both containing cerium in a tetravalent state, were compared. Both phosphates demonstrated radical-scavenging properties in relation to alkylperoxyl radicals. (NH₄)₂Ce(PO₄)₂·H₂O was found to have prooxidant activity toward hydrogen peroxide that was comparable with that of ceria nanoparticles, while NH₄Ce₂(PO₄)₃ demonstrated the lowest prooxidant action.