Fate, Transformations, and Toxicity of Nanoparticles in Freshwaters

Nanoparticles are being researched and developed at a rapid pace, and the knowledge in the literature of the impacts that these nanoparticles may have on freshwater ecosystems is limited. The advent of particle stabilisation via the addition of simple polymeric coatings adds further complexity to this issue, as most environmental studies to date have focused on ‘bare’ nanoparticles dispersed in relatively simple media. The gaps between what is known already about ‘bare’ nanoparticles and the ecological risk posed by functionalised nanoparticles in complex riverine waters needs to be addressed. In this thesis, these gaps were investigated by first setting out to synthesise well-characterised model probe nanoparticles for assays into the fate and transformations of silver and gold nanoparticles in relevant media. Coated and uncoated ceria nanoparticles were then utilised to investigate the potential toxicity-modulating effects of particle coatings and natural riverine sediments using natural waters. The preparation of core-shell-shell Ag@Au@Ag nanoparticles was explored using a seed- mediated hydrothermal synthesis method. Galvanic replacement was found to occur, wherein Ag° atoms were oxidised to afford reduction of Au3+, resulting in the formation of unwanted hollow alloy nanostructures. Various reducing agents were explored to preferentially reduce and deposit Au on the Ag seeds to produce core-shell nanoparticles, with limited success. Further, calculations and experimental observations indicated that the nanoscale Kirkendall effect was potentially significant and diffusion between the two miscible metals was rapid, negatively impacting the integrity of the core-shell nanoparticles. The importance of precise nanoparticle characterisation was highlighted. Finally, the absence of a suitable synthesis purification technique prevented the recovery of the small proportion of successfully-converted seed nanoparticles. The focus of the thesis was subsequently shifted to consider the fate and transformations of commercially- available nano-ceria. The relative insolubility of these materials eliminated particle dissolution as a pathway of toxicity and allowed investigation of particle-associated toxicity mechanisms.