- Metal oxide nanoparticles (MONPs) are manufactured at the greatest rate of any class of nanomaterial due to their wide variety of industrial, commercial, and environmental applications. The sustainable use of MONPs requires a balance of careful consideration of their potential negative environmental impacts with the effective exploitation of their unique properties in situations where the benefit of their use outweighs the risk. Currently, the study of the environmental behavior and toxicity of MONPs, as well as the understanding of their efficacy in environmental remediation applications, would benefit from the understanding of how MONPs behave in complex aquatic media. MONPs are particularly difficult to study in environmentally-relevant matrices like natural water samples or within organisms due to the presence of elevated levels of the naturally occurring metal oxide.
In this dissertation, I directly address this challenge through the development of gold core-labeled TiO2 NPs (Au@TiO2 NPs). These labeled NPs were characterized and compared to their unlabeled counterparts, and it was determined that the size, crystal structure, and stability in media up to 500 mM was not significantly changed by the presence of the gold core label. The Au@TiO2 NPs were quantified at low concentrations in complex natural matrices containing elevated levels of background titanium, using both inductively-coupled plasma optical emission spectroscopy and instrumental neutron activation analysis.
To contextualize these Au@TiO2 NPs within the broader context of the available MONP labeling strategies, I prepared a critical review of the current literature. Four major categories of labeling techniques are described: fluorescent dyes, radioactive isotopes, stable isotopes, and dopant and core/shell labels. The advantages and disadvantages of each technique are presented. Recommendations are made for characterization that should be performed on labeled and unlabeled MONPs to ensure mechanistic conclusions drawn in experiments are not affected by the presence of the label. Guidance is provided on choosing and applying a labeling strategy for a given study.
Finally, I examine the properties and behavior of TiO2 NPs in water treatment applications. The relative influence of elemental surface composition, crystal structure, ROS generation capacity, aggregation state, and water chemistry on the photocatalytic degradation of methylene blue dye is presented. Due to sample contamination, no correlations were found between the molecular-level surface properties determined by x-ray photoelectron spectroscopy and overall treatment efficacy. The most important property determining the extent of decolorization of the dye solutions was the inherent ROS generation capacity of the TiO2 NPs, which in this study was controlled by the crystal structure. Other important properties were the aggregation state of the TiO2 NPs in solution, and the pH stability of the media during the treatment process.
Overall, this dissertation provides both justification and methodologies for the study of TiO2 NPs in complex, environmentally-relevant aquatic matrices.