Mechanisms and effects of zinc oxide nanoparticle transformations on toxicity to zebrafish embryos

The effect of transformations on the toxicity of zinc oxide nanoparticles (ZnO NPs) and the underlying mechanisms are unclear, limiting our understanding of the realistic impacts of ZnO NPs in the environment. Three kinds of ZnO NP transformations were induced through interaction with sulfur (sulfidation) and phosphate (phosphation) and association with hydrous ferric oxide (Zn-FeOOH). Pristine and transformed ZnO NPs were characterized using X-ray diffraction and transmission electron microscopy. Changes in toxicity as a result of transformation were evaluated using an embryonic zebrafish assay. Toxicity was reduced in descending order by phosphation, sulfidation, and Zn-FeOOH with increasing molar ratios of S/Zn, PO₄/Zn, and Fe/Zn, respectively. Reduced toxicity correlated well with the zinc concentration and addition of ethylenediaminetetraacetic acid completely eliminated the toxicity, indicating that the remaining zinc ions in bulk solution were responsible for the toxicity of transformed ZnO NPs. The maximum toxicity reduction was observed after phosphation, attributable to the inhibition of zinc release by phosphates rapidly enclosing ZnO NPs. Zn-FeOOH does not effectively reduce the zinc concentration because no significant morphological changes occur during the transformation. We demonstrated that transformations modified the physicochemical properties of ZnO NPs, modulating their ability to reduce zinc ions, and governing toxicity to zebrafish embryos.