Coexistence of silver and titanium dioxide nanoparticles: enhancing or reducing environmental risks?

Due to their bactericidal and photocatalytic characteristics, silver nanoparticles (Ag NPs) and titanium dioxide nanoparticles (TiO 2 NPs) are widely used in the fields of environment and physiology. Once these untreated nanoparticles are released into an aquatic environment and encounter one another, there is more uncertainty about their fate and ecotoxicological risks compared with the single nanoparticles. To expand our knowledge of the health and environmental impacts of nanoparticles, we investigated the possible risk of the co-existence of TiO 2 NPs and Ag NPs in an aquatic environment using ciliated protozoa ( Tetrahymena pyriformis ) as an aquatic animal model. In this study, silver ion (Ag + ) release and physicochemical properties, as well as their effect on oxidative stress biomarkers, were monitored. Continuous illumination (12,000   lx) led to the 20.0% decrease in Ag + release in comparison with dark conditions, while TiO 2 NPs and continuous illumination resulted in decreasing the Ag + concentration to 64.3% in contrast with Ag NPs-only suspensions. Toxicity tests indicated that different illumination modes exerted distinct effects of TiO 2 NPs on the toxicity of Ag NPs: no effects, antagonism and synergism in dark, natural light and continuous light, respectively. In the presence of 1.5   mg/L (18.8   μM) TiO 2 NPs, the toxicity of 1.5   mg/L (13.9   μM) Ag NPs was reduced by 28.7% and increased by 6.93% in natural light and 12,000   lx of continuous light, respectively. After culturing in 12,000   lx continuous light for 24   h, SOD activity of the light control surged to 1.96 times compared to the dark control ( P   <   0.001). TiO 2 NPs induced a reduction of CAT activity by an average of (36.1   ±   1.7) % in the light. In the natural light reductions in the toxicity of Ag, NPs decrease Ag + concentrations via adsorption of Ag + onto TiO 2 NPs surfaces. The enhancement of Ag NPs toxicity can contribute to the formation of activated TiO 2 –Ag NPs complexes in continuous light. The existence of TiO 2 NPs in various illumination modes changed the surface chemistry of Ag NPs and then led to different toxicity effects. TiO 2 NPs reduce the environmental risks of Ag NPs in natural light, but in continuous light, TiO 2 NPs enhance the environmental risks of Ag NPs.