| Summary: | The widespread use of metal nanoparticles in different fields has raised many doubts regarding their possible toxicity to living organisms and the accumulation and discharge of metals in fish species. Among these nanoparticles, titanium dioxide (TiO<sub>2</sub>) and cerium oxide (CeO<sub>2</sub>) nanoparticles have mainly been employed in photocatalysis and water depuration. The aim of this research was to evaluate the potential toxic effects, after a co-exposure of TiO<sub>2</sub>-3%CeO<sub>2</sub> nanoparticles, on zebrafish development, using an acute toxicity test. Increasing concentrations of TiO<sub>2</sub>-3%CeO<sub>2</sub> nanoparticles were used (0.1-1-10-20 mg/L). The heartbeat rate was assessed using Danioscope<sup>TM</sup> software (version 1.2) (Noldus, Leesburg, VA, USA), and the responses to two biomarkers of exposure (Heat shock proteins-70 and Metallothioneins) were evaluated through immunofluorescence. Our results showed that the co-exposure to TiO<sub>2</sub>-3%CeO<sub>2</sub> nanoparticles did not affect the embryos’ development compared to the control group; a significant difference (<i>p</i> < 0.05) at 48 hpf heartbeat for the 1, 10, and 20 mg/L groups was found compared to the unexposed group. A statistically significant response (<i>p</i> < 0.05) to Heat shock proteins-70 (Hsp70) was shown for the 0.1 and 1 mg/L groups, while no positivity was observed in all the exposed groups for Metallothioneins (MTs). These results suggest that TiO<sub>2</sub>-3%CeO<sub>2</sub> nanocomposites do not induce developmental toxicity; instead, when considered separately, TiO<sub>2</sub> and CeO<sub>2</sub> NPs are harmful to zebrafish embryos, as previously shown.
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