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Iron nanoparticles (NPs) have been proposed as a tool in very different fields such as environmental remediation and biomedical applications, including food fortification against iron deficiency, even if there is still concern about their safety. Here, we propose Xenopus laevis embryos as a suitable model to investigate the toxicity and the bio-interactions at the intestinal barrier of Fe 3 O 4 and zerovalent iron (ZVI) NPs compared to Fe(II) and (III) salts in the 5 to 100 mg Fe/L concentration range using the Frog Embryo Teratogenesis Assay in Xenopus (FETAX). Our results demonstrated that, at concentrations at which iron salts induce adverse effects, both iron NPs do not cause acute toxicity or teratogenicity even if they accumulate massively in the embryo gut. Prussian blue staining, confocal and electron microscopy allowed mapping of iron NPs in enterocytes, along the paracellular spaces and at the level of the basement membrane of a well-preserved intestinal epithelium. Furthermore, the high bioaccumulation factor and the increase in embryo length after exposure to iron NPs suggest greater iron intake, an essential element for organisms. Together, these results improve the knowledge on the safety of orally ingested iron NPs and their interaction with the intestinal barrier, useful for defining the potential risks associated with their use in food/feed fortification.

Silver nanoparticles (AgNPs) are among the most exploited antimicrobial agents and are used in many consumer products. Size and surface reactivity are critical physico-chemical properties responsible for NPs toxicity, and surface coatings, often used to functionalize or stabilize AgNPs, can influence their toxic profile and biocompatibility. In the current study the developmental toxicity of (1) negatively charged citrate-coated AgNPs (Cit-AgNPs), (2) positively charged branched polyethylenimine-coated AgNPs (BPEI-AgNPs), and (3) Ag + (from 0.0625 to 0.75 mg Ag/L) was investigated by the standard Frog Embryo Teratogenesis Assay - Xenopus (FETAX). In order to identify the most sensitive developmental phase, embryos were also exposed during different embryonic stages. Morphological and bio-physical studies were performed to characterize tissue lesions and NP uptake. The results suggest that Ag + was strongly embryo-lethal. Contrary to Cit-AgNPs, the positively charged BPEI-AgNPs exert a concentration-dependent effect on lethality and malformations of embryos. The BPEI-AgNPs showed the highest teratogenic index (TI = 1.6), pointing out the role of functional coating in determining the developmental hazard. The highest susceptibility to BPEI-AgNPs was during early embryogenesis, when embryos are still enclosed in the fertilization envelope, and the post-stomodeum opening stages, when NPs ingestion occurs. In BPEI-AgNPs treated larvae, the histological examination revealed irregular intestinal diverticula coupled with edematous connective tissue. Small NPs aggregates are mapped throughout the intestinal mucosa and secondary target organs by two-photon excitation microscopy. We conclude that a teratogenic risk may be associated with BPEI-AgNPs exposure, but the modality of NP-tissue interactions and the teratogenic mechanism need further investigations to be better defined.