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The growing global production of zinc oxide nanoparticles (ZnONPs) suggests a realistic increase in the environmental exposure to such a nanomaterial, making the knowledge of its biological reactivity and its safe-by-design synthesis mandatory. In this study, the embryotoxicity of ZnONPs (1–100 mg/L) specifically synthesized for industrial purposes with different sizes, shapes (round, rod) and surface coatings (PEG, PVP) was tested using the frog embryo teratogenesis assay-Xenopus (FETAX) to identify potential target tissues and the most sensitive developmental stages. The ZnONPs did not cause embryolethality, but induced a high incidence of malformations, in particular misfolded gut and abdominal edema. Smaller, round NPs were more effective than the bigger, rod ones, and PEGylation determined a reduction in embryotoxicity. Ingestion appeared to be the most relevant exposure route. Only the embryos exposed from the stomodeum opening showed anatomical and histological lesions to the intestine, mainly referable to a swelling of paracellular spaces among enterocytes. In conclusion, ZnONPs differing in shape and surface coating displayed similar toxicity in X. laevis embryos and shared the same target organ. Nevertheless, we cannot exclude that the physico-chemical characteristics may influence the severity of such effects. Further research efforts are mandatory to ensure the synthesis of safer nano-ZnO-containing products.

The impact of nano metal-oxides on human and environmental health is predicted to be increasing. Among metal oxides, nano ZnO (nZnO) is retained one of the most dangerous. Recently nZnO has invaded the market for its UV protective and antibacterial properties, that make it suitable for a wide range of application for functional coating formulations to protect wood, plastics, textiles from UV and microbial degradation. Previous data already showed that nZnO has a powerful embryotoxic potential on X. laevis and that it was able to mainly affect gut development. It was clearly demonstrated that nZnO produced severe lesions at the intestinal mucosa and potentially cross the gut barrier reaching the underlying tissues. In this work we used Xenopus laevis embryos to characterize the embryotoxic and teratogenic potential of nZnO according to the modulation of NP size and surface charge, as well as to the irradiation conditions. To optimize the stability of the NP suspensions and to achieve useful NP-surface functionalization, we worked in strict connection with a private nanotech company with both R&D and commercial activities. The purpose was to provide mechanistic data on nZnO ecotoxicology and to suggest criteria to design safer Zinc oxide NPs. We demonstrated that nZnO-induced embryotoxicity was mediated by NPs’ own reactivity rather than ion dissolution and that it is strongly associated with the modality of the biological interactions at the nano-level, which at last depend upon the physical and chemical NP surface properties. These properties are also at the base of the induced oxidative potential by nZNO, which is also very efficiently modulated by light irradiation. Finally NP dimension, and especially surface charge, played a crucial role in determining the embryotoxic potential and the intestinal translocation and lesions of nZnO. The present results showed how a comprehensive knowledge of the nZnO physical and chemical properties, affecting the interactions at