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The rapid development of nanotechnology has resulted in an increasing number of nanomaterial-based consumer products and industries. Because of their unique physical properties, nanomaterials have dramatically transformed the function and application of commercial products, including wound dressings, cosmetics, detergents, food packaging, drug delivery, biosensors, and antimicrobial coatings [1]. Recently, titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles (NPs) have gained popularity as inorganic physical sunscreens because they can reflect and scatter UVA and UVB radiations while preventing skin irritation and disruption of the endocrine system typically induced by chemical UV filters. Also, these NPs may be transparent and pleasant to touch [1,2]. However, safety concerns regarding their utilization in consumer products have recently emerged. Reports have suggested that sunscreen NPs induce cyto- and genotoxicity through oxidative stress [3]. Zvyagin et al [4] and Tilman et al [5] have shown that TiO2 and ZnO NPs could not penetrate the deep layers of healthy adult skin. In contrast, Wu et al [6] demonstrated that TiO2 NPs could enter the deep layers of porcine epidermis as well as hairless mouse skin. Because the impact of NPs on humans is poorly understood, no clear regulation has been implemented for NPs among international authorities.
The International Cooperation on Cosmetic Regulation define a nanomaterial in cosmetics as an insoluble, intentionally manufactured ingredient with one or more dimensions ranging from 1 nm to 100 nm in the final formulation. In addition, the nanomaterial must be sufficiently stable and persistent in biological media to enable potential interactions with biosystems [7]. In 2012, the International Organization for Standardization underlined that the physicochemical characterization of nanomaterials was critical for the identification of test materials before toxicological assessment (ISO/TR13014). Physicochemical parameters include particle size/particle size distribution, aggregation/agglomeration state, shape, surface area, composition, surface chemistry, surface charge, and solubility/dispersibility [8]. A safety guideline on nanomaterials in cosmetics issued by the United States Food and Drug Administration [9] recommended that the product be evaluated by analyzing these physicochemical properties. NPs may aggregate when added to cosmetics, making their characteristics in the final products essential.
Sunscreen formulations are very complex and opaque, hindering NP detection and characterization. Finding appropriate analytical methods to achieve this characterization without product modification...