The critical role of light in moderating microbial stress due to mixtures of engineered nanomaterials

Exposure to light is a key parameter that influences the chemical interactions and microbial stress of engineered nanomaterials (ENMs). Yet, the photochemistry and phototoxic stress responses of many ENMs and ENM mixtures have not been adequately detailed. We exposed E. coli bacteria to binary combinations of n-TiO₂ (a stable metal oxide) with different plasmonic metal ENMs (n-Ag, n-Au, or n-Pt) in a natural aqueous medium under light and dark conditions. Using ATP level and cell membrane integrity probes, we measure the toxic stress due to these ENM mixtures. We previously found that under dark conditions, n-TiO₂ attenuates the toxic stress due to low concentrations of n-Ag (<20 μg L^-1) via adsorption of Ag⁺. However, mixtures containing n-Au or n-Pt produce no difference in toxic stress responses with and without n-TiO₂ in the dark. In contrast, under simulated solar irradiation, we observed that n-Ag, n-Au, and n-Pt each with n-TiO₂ cause synergistic toxic stress. The n-Ag/n-TiO₂ combination causes the greatest phototoxicity, followed by n-Au/n-TiO₂ and then n-Pt/n-TiO₂. Measurements of photocatalytic production of reactive oxygen species reveal that irradiation of n-Ag or n-Au with n-TiO₂ yields synergistic production of superoxide anion and hydrogen peroxide. The photochemical interactions of these ENMs, governed by metal ENM solubility and localized surface plasmon resonance, provide mechanistic insight into the amplified photoactivity of mixtures.