Outsized Effects: Mixtures of Photoactive Nanomaterials Create Emergent Systems and Unexpected Biological Stress under Environmental Conditions

Wilke, Carolyn

doi:https://doi.org/10.21985/n2-h5mx-wc73

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Outsized Effects: Mixtures of Photoactive Nanomaterials Create Emergent Systems and Unexpected Biological Stress under Environmental Conditions

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Due to their widespread use and subsequent release, ENMs may exist as mixtures in the natural environment where their chemical interactions can control the toxic stress to exposed microorganisms. Will complex environmental mixtures containing ENMs produce microbial stress that is synergistic, attenuated, or simply additive relative to single ENM exposure, and how does light influence these outcomes? To investigate this question, E. coli bacteria were exposed to binary combinations of nano-TiO2 (a stable metal oxide with photocatalytic properties) with different plasmonic metal ENMs (n-Ag, n-Au, or n-Pt) or n-Ag2S (the transformation product of n-Ag formed under reducing conditions with sulfide) in a natural aqueous medium under light and dark conditions. ATP levels and cell membrane integrity were used as indicators to observe the toxic effects caused by these ENM mixtures. Under dark conditions n-TiO2 attenuates the stress induced by low concentrations of n-Ag (<20 µg L-1) via adsorption of Ag+. For n-Au and n-Pt, no difference in stress with and without n-TiO2 was observed. In contrast, under simulated solar irradiation, binary mixtures of n-Ag, n-Au, and n-Pt with n-TiO2 provoked synergistic stress effects in E. coli, with the n-Ag/n-TiO2 combination causing the greatest enhancement in toxic effects. The photochemistry of these mixtures under light was probed by measuring the photocatalytic production of reactive oxygen species. Measurements of dissolved Ag and STEM images suggest that under irradiation a new hybrid Ag/TiO2 nanomaterial forms, contributing to its greatly enhanced production of hydrogen peroxide. The photoactivity of the different combinations of ENMs were explained by the differing stabilities and surface plasmon resonance wavelengths of the metal ENMs. n-Ag2S is generally considered stable and benign. Yet it will likely occur and interact with other nanomaterials such as n-TiO2, for instance, in waters receiving wastewater treatment effluents. There, the presence of light and n-TiO2, which produce oxidizing conditions, may lead to the release of Ag+ from n-Ag2S. Under dark conditions, sulfidation increases the threshold concentration for bacterial stress. Yet, under simulated solar irradiation, n-Ag2S and n-TiO2 together induce synergistic decreases in bacterial ATP because of their enhanced production of reactive oxygen species. Together, all of these studies highlight the need to evaluate ecotoxicity of mixtures rather than single ENMs and reveal the critical role of light in the moderating the chemistry and toxic effects of mixtures of photoactive nanomaterials. Overall, the work presented in this thesis reveals that mixtures of nanomaterials create emergent systems with interactions and cumulative stress not easily predicted by those of the original pristine ENMs. In particular, these studies illuminate the transformations of Ag, which is dynamic under environmental conditions and cycles through several forms: metallic, dissolved, complexed, and sulfidized. Ultimately, this work reveals that the environmental transformations of ENMs do not necessarily render them benign and can even increase their microbial stress. Still, the application of nanomaterials has continued to increase without a full understanding of the unintended consequences of their release to the environment. Many other types of nanomaterials that will likely form mixtures in the environment still require investigation and questions remain as to the chronic effects of ENMs in natural systems and how they may restructure microbial communities and contribute to the spread of antimicrobial resistance.

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