In vitro photoprotective efficacy and photostability of synthesized star-shaped ZnO nanoaggregates associated with ethylhexyl methoxycinnamate and butyl methoxydibenzoylmethane

Abstract

The heightened susceptibility to skin cancer correlates with exposure to ultraviolet (UV) radiation, which can induce various cutaneous injuries. Inorganic UV filters, like zinc oxide (ZnO), are extensively utilized in sunscreens owing to their capacity to scatter and reflect UV radiation. The efficacy of inorganic UV filters can be augmented across a wider spectrum through synergistic combinations with other active compounds, such as organic UV filters. In this study, we synthesized and evaluated the in vitro effectiveness of star-shaped ZnO nanoaggregates. The samples were obtained employing a simple and greener precipitation method in an aqueous solution. The synthesized ZnO nanoaggregates were characterized through X-ray diffraction, UV/Vis spectroscopy, and transmission electron microscopy. The characterization suggests the aggregation of nanocrystals, with hexagonal wurtzite geometry, enabling the formation of star-shaped particles. The irregular (non-flat) surface and high surface area combined with light absorption in the UVA region make the material susceptible to the application of sun protection. Sunscreens formulated with the synthesized ZnO_ng in conjunction with ethylhexyl p-methoxycinnamate (EHMC) and/or butyl methoxydibenzoylmethane (BMDBM) were evaluated for in vitro sun protection factor (SPF) and critical wavelength (cλ) using diffuse reflectance spectrophotometry with an integrated sphere. Photostability assessments were also conducted using artificial UV radiation. Sunscreen formulations containing ZnO_ng in combination with EHMC and BMDBM exhibited a substantial increase in in vitro SPF, approximately 990 % (from SPF 26 to 285). Furthermore, the synthesized ZnO_ng demonstrated higher in vitro efficacy compared to commercial ZnO active ingredients. Although all samples experienced reductions in SPF values during the photostability assay, ZnO_ng + EHMC + BMDBM retained a broad-spectrum profile (SPF > 15 and cλ > 370 nm). Based on the distinctive properties and in vitro performance of star-shaped ZnO_ng, we propose that this synthesized inorganic UV filter could serve as an alternative for enhancing the SPF of sunscreen systems while reducing the concentrations of organic UV filters.

Keywords: Inorganic UV filter; Nanoaggregate synthesis; Photostability; Sun protection factor; Sunscreen; Zinc oxide.