Understanding the Structure of Silver Nanoparticles and Its Influence on Surface Reactivity

Abstract

Silver nanoparticles (Ag NPs) have proven to be useful for biomedical applications, as the ability to fine-tune properties such as size, shape, composition, and surface structure is a distinct advantage for their associated antibacterial properties. Their surface structure is especially important, as it can influence NP stabilization as well as govern any further surface reactivity. Despite the huge amount of research being performed on metal NPs in general, their surface structure is a much less studied parameter, partly because the averaging effect from the non-surface atoms of NPs makes it difficult to isolate. However, with the employment of element-specific techniques such as X-ray absorption spectroscopy (XAS), the surface structure of metal NPs can be more easily investigated. The research presented in this thesis explored the surface structures of Ag-based NPs using XAS, in order to gain fundamental and practical insight to their physicochemical properties and surface reactivity. In particular, the structures of thiolate-protected Ag NPs in both organic and aqueous solutions were investigated and were found to be highly sulfidized at their surfaces. The sulfidized surface structures also caused reduced antibacterial activity in comparison to more metallic surface structures. In addition to monometallic Ag NPs, a series of bimetallic AgAu NPs was also examined. The atomic structure within the cores of each type of AgAu NP was dependent on the molar ratios of the metals, yet each type had a similar surface structure that was predominantly composed of Ag. The AgAu NPs exhibited lower antibacterial activity compared to pure Ag NPs, but also showed significantly reduced cytotoxicity, thereby demonstrating their potential for therapeutic applications. Furthermore, the surface structures of chloride-adsorbed, Ag-coated anisotropic Au NPs were examined to understand ligand and alloy effects on NP stabilization. The surface structures were found to be sensitive to their formation mechanism and facet type, which sheds light on the mechanism of Ag- and chloride-induced NP growth. Overall, this thesis contributes to a better understanding of how ligands and alloying can control the surface properties and reactivity of Ag-based NPs.