Tungsten oxide-based bulk and nanocrystalline materials are widely used as photocatalytic and photo- and electrochromic materials, as well as materials for biomedical applications. In our work, we focused our attention on the effect of sodium cations on the structure and photochromic properties of the WO₃@PVP aqueous sols. To establish the effect, the sols were synthesized by either simple pH adjusting of sodium or ammonium tungstates’ solutions, or using an ion exchange technique to remove the cations from the materials to the greatest possible extent. We showed that the presence of sodium cations in WO₃@PVP favors the formation of reduced tungsten species (W⁺⁵) upon UV irradiation of the materials, strongly affecting their photochromic and photocatalytic properties. The pronounced photoreductive properties of WO₃@PVP sols in photocatalytic reactions were demonstrated. Due to photoreductive properties, photochromic sols of tungsten oxide can act as effective photoprotectors in photooxidation processes. We believe that our work provides a considerable contribution to the elucidation of photochromic and redox phenomena in WO₃-based materials.

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.

Their unique physicochemical properties and multi-enzymatic activity make CeO₂ nanoparticles (CeO₂ NPs) the most promising active component of the next generation of theranostic drugs. When doped with gadolinium ions, CeO₂ NPs constitute a new type of contrast agent for magnetic resonance imaging, possessing improved biocatalytic properties and a high level of biocompatibility. The present study is focused on an in-depth analysis of the enzyme-like properties of gadolinium-doped CeO₂ NPs (CeO₂:Gd NPs) and their antioxidant activity against superoxide anion radicals, hydrogen peroxide, and alkylperoxyl radicals. Using an anion-exchange method, CeO₂:Gd NPs (~5 nm) with various Gd-doping levels (10 mol.% or 20 mol.%) were synthesized. The radical-scavenging properties and biomimetic activities (namely SOD- and peroxidase-like activities) of CeO₂:Gd NPs were assessed using a chemiluminescent method with selective chemical probes: luminol, lucigenin, and L-012 (a highly sensitive luminol analogue). In particular, gadolinium doping has been shown to enhance the radical-scavenging properties of CeO₂ NPs. Unexpectedly, both bare CeO₂ NPs and CeO₂:Gd NPs did not exhibit SOD-like activity, acting as pro-oxidants and contributing to the generation of reactive oxygen species. Gadolinium doping caused an increase in the pro-oxidant properties of nanoscale CeO₂. At the same time, CeO₂:Gd NPs did not significantly inhibit the intrinsic activity of the natural enzyme superoxide dismutase, and CeO₂:Gd NPs conjugated with SOD demonstrated SOD-like activity. In contrast to SOD-like properties, peroxidase-like activity was observed for both bare CeO₂ NPs and CeO₂:Gd NPs. This type of enzyme-like activity was found to be pH-dependent. In a neutral medium (pH = 7.4), nanoscale CeO₂ acted as a prooxidant enzyme (peroxidase), while in an alkaline medium (pH = 8.6), it lost its catalytic properties; thus, it cannot be regarded as a nanozyme. Both gadolinium doping and conjugation with a natural enzyme were shown to modulate the interaction of CeO₂ NPs with the key components of redox homeostasis.

This paper reports on a comprehensive study of the UV-shielding properties (namely, the sun protection factor and the factor of protection against UV-A radiation) and cytotoxicity (including photocytotoxicity) of amorphous and crystalline cerium(IV) phosphates. It has been shown that cerium(IV) phosphate NH₄Ce₂(PO₄)₃ is characterised by UV-shielding properties that are comparable to those of nanocrystalline TiO₂ and CeO₂. Moreover, cerium(IV) phosphates did not show toxicity towards cell cultures of NCTC L929 line mouse fibroblasts and human mesenchymal stem cells, in a wide range of concentrations, and even enhanced the proliferative activity of the latter. In a model study of the photoprotective properties of cerium(IV) phosphates on human mesenchymal stem cells, the pronounced protective effect of NH₄Ce₂(PO₄)₃ was observed, which was comparable to the shielding action of nanocrystalline CeO₂. The results have shown that tetravalent cerium phosphates can be considered as promising UV-filters for sunscreen applications.