UC San Diego

As nanoparticle (NP) surfaces are highly reactive, NPs can interact with environmental and biological systems and undergo complicated surface transformation processes, including ligand adsorption, ligand displacement reactions, surface oxidation. These transformations can significantly alter the physiochemical properties of NPs. Although surface transformations have been proposed in previous studies, information on the details of these mechanisms and the impact on NP behavior remains unclear.

In this dissertation, in-situ ATR–FTIR was employed to better understand surface transformation on oxide NPs (i.e. TiO2). Displacement reactions of ascorbic acid, citric acid, and bovine serum albumin by humic acid show three different behaviors ranging from complete displacement, partial displacement, and no displacement, respectively. The detailed chemistry analyzed using two-dimensional correlated spectroscopy indicate the formation of different types of adsorption modes including outer sphere and inner sphere complexation during the reaction. These reactions are important to understand if NPs are to be used for in-situ environmental remediation. For example, we through combined in-situ ATR–FTIR and molecular dynamic simulations studied how fulvic acid (FA) alters the performance of polyvinylpyrrolidone coated magnetite NPs in crude oil clean-up. Additionally, the effects of surface coatings on TiO2 NP reactivity associated with the formation of reactive oxygen species (ROS) was investigated. The results indicate that the generated ROS on TiO2 NPs under light can be completely quenched by BSA but partially by FA.

Another focal point of this dissertation is on surface oxidation of CuS NPs. The details of oxidation were investigated via in-situ ATR–FTIR in oxygen under various relative humidity (RH) and the formation of sulfate on the surface at higher relative humidity was observed. These surface species were confirmed by microscopic methods including high resolution transmission electron microscope and atomic force microscope infrared-spectroscopy. X-ray photoelectron spectroscopy data suggests that CuS NPs show little oxidation under dry conditions (RH < 2%), while surface oxidation occurs stepwise with increasing RH resulting in distinct products.

Overall, these studies presented in this dissertation provide valuable insights into the details of mechanisms associated with surface transformation and impacts on the reactivity and properties of NPs.