Combined Toxicity of Nano-ZnO and Nano-TiO2: From Single- to Multinanomaterial Systems

Previous studies on the toxicity of engineered nanomaterials (ENMs) have been primarily based on testing individual ENMs, so little is known about the interactions and combined toxicity of multiple ENMs. In this study the toxicity of chemically stable nano-TiO2 and soluble nano-ZnO was investigated individually and in combination, by monitoring bacterial cell membrane integrity and ATP levels in a natural aqueous medium (Lake Michigan water). Both nano-TiO2 and nano-ZnO damage bacterial cell membranes under simulated solar irradiation (SSI), but their phototoxicity is not additive. Nano-ZnO at 1 mg/L, for example, surprisingly eliminates the damaging effect of nano-TiO2 at 10 mg/L. This phenomenon does not correlate with reactive oxygen species production, but is explained by a reduced extent of bacteria/nano-TiO2 contact in the presence of both nano-ZnO and dissolved zinc. The presence of nano-ZnO also exerts a significant decrease in bacterial ATP levels both under SSI and in the dark, a stress effect not captured by measuring bacterial cell membrane integrity. This inhibitory effect of nano-ZnO, however, is reduced somewhat by nano-TiO2 due to the adsorption of Zn2+. Therefore, our results reveal that nanoparticle interactions and surface complexation reactions alter the original toxicity of individual nanoparticles and that comprehensive assessments of potential ENM toxicity in the environment require careful integration of complex physicochemical interactions between ENMs and various biological responses.