Acute and developmental toxicity of metal oxide nanoparticles (ZnO, TiO2, Fe2O3, and CuO) in Xenopus laevis

ZnO, TiO2, Fe2O3, and CuO nanomaterials (20-100 nm) were evaluated to determine toxicity of acute exposure using the Frog Embryo Teratogenesis Assay Xenopus (FETAX) protocol. Acute exposure to the tested metal oxide nanomaterials did not increase mortality or decrease hatchability, however these nanomaterials did inhibit growth. Exposure to either TiO2 or Fe2O3 inhibited growth of tadpoles with exposure to 1000 mg/L of metal oxide. Exposure to CuO or ZnO inhibited growth of Xenopus laevis at concentrations of 10 mg/L or greater. Copper oxide and ZnO exposure also produced malformations in tadpoles, and malformation EC values were determined from the malformation data. An EC15 of 39.29 mg/L CuO and an EC50 of 10.26 mg/L ZnO was observed for total malformations.

Due to the toxicity observed with ZnO and CuO nanomaterials exposure, chronic developmental assays were conducted with ZnO and CuO. Xenopus laevis tadpoles were exposed to metal oxide nanomaterials from larval stage through completion of metamorphosis. Nanomaterials were dispersed via sonication methods into reconstituted moderately hard water test solutions. A flow-through system was utilized to decrease the likelihood of the depletion in metal oxide concentration.

Exposure to 2 mg/L ZnO nanoparticles significantly increased mortality incidence to 40% and negatively affected metamorphosis of Xenopus laevis. Tadpoles exposed to 2 mg/L ZnO nanoparticles experienced slower progression of staging resulting in tadpoles with an average stage of 56 at the conclusion of the study which was significantly lower than control tadpole staging. Tadpoles exposed to 0.125 mg/L ZnO nanoparticles experienced faster stage progression along with larger body measurements indicating that low dose exposure to ZnO nanoparticles can be beneficial to growth and metamorphosis of Xenopus laevis.

Chronic exposure of 0.3 mg/L CuO nanoparticles caused significant mortality and affected the rate of metamorphosis by slowing stage progression. Exposure to lower concentrations of CuO nanoparticles induced increased stage progression and body measurements, indicating that low dose exposure can have beneficial effects on metamorphosis. Tadpoles exposed to 0.15 mg/L CuO nanoparticles experienced similar stage progression and growth as control tadpoles for the majority of exposure.