Effects of nano-scale TiO2, ZnO and their bulk counterparts on zebrafish: Acute toxicity, oxidative stress and oxidative damage

The acute toxicity and oxidative effects of nano-scale titanium dioxide, zinc oxide and their bulk counterparts in zebrafish were studied. It was found that although the size distribution of nanoparticles (NPs) was similar to that of the bulk particles in suspension, the acute toxicity of the TiO2 NPs (96-h LC50 of 124.5 mg/L) to zebrafish was greater than that of the bulk TiO2, which was essentially non-toxic. The acute toxicities observed for ZnO NPs, a bulk ZnO suspension, and a Zn2+ solution were quite similar to each other (96-h LC50 of 4.92, 3.31 and 8.06 mg/L, respectively). In order to explore the underlying toxicity mechanisms of NPs, ·OH radicals generated by NPs in suspensions and five biomarkers of oxidative effects, i.e. superoxide dismutase, catalase activities, malondialdehyde, reduced glutathione and protein carbonyl were investigated. Results showed that after the illumination for 96 h, the quantities of ·OH in the NP suspensions were much higher than ones in the bulk particles suspensions. The malondialdehyde content of zebrafish gills exposed to either illumination or dark were 217.2% and 174.3% of controls, respectively. This discrepancy indicates the occurrence of lipid peroxidation which is partly due to the generation of ·OH. In contrast, exposure to 5 mg/L ZnO NPs and bulk ZnO suspension induced oxidative stress in the gills without oxidative damage. Oxidative effects were more severe in the livers, where the protein carbonyl content, in the light and dark groups exposed to 50 mg/L TiO2 NPs, was 178.1% and 139.7% of controls, respectively. The malondialdehyde levels in the liver of fish exposed to 5 mg/L ZnO NPs and bulk ZnO were elevated (204.2% and 286.9% of controls, respectively). Additionally, gut tissues exhibited oxidative effects after exposure to NP suspensions. These results highlight the importance of a systematic assessment of metal oxide NP toxicity mechanisms.

Research Highlights
► Toxicity and oxidative effects of nano and bulk ZnO and TiO2 on zebrafish.
► Toxicity mechanism of ZnO and TiO2 NPs differed.
► Activities of diverse NPs in various tissues differed.
► Metal ions released by nano and bulk ZnO contributed partly to toxicity.
► Extracellular *OH generated by nano-TiO2 hurts gill cell membranes directly.