| Description |
A detailed understanding of the conformation of adsorbed molecules and regional surface functionalization of metal nanoparticles (MNPs) is challenging for nanometersize (10 - 100 m) materials and necessary for fundamental studies and applications. The studies are motivated by open questions related to surface chemistry of noble MNPs. Although citrate-stabilized gold NPs (AuNPs) have been widely used, the citrate layer is not well-understood. Thiols have been suggested to displace citrate anions adsorbed on metal surfaces due to strong gold-sulfur interaction, but quantitative experimental evidence of the extent of ligand-exchange has not been reported. Whereas asymmetrically-functionalized AuNPs are utilized for nanoparticle assembly due to the interparticle coupling of localized surface plasmons, the interface between asymmetric nanoparticles in single assemblies has not been studied. Noble MNPs with sizes smaller than citrate-stabilized AuNPs also need to be surface-modified for stability in water for biological applications. The dissertation presents investigations of the chemical and physical properties of gold and silver NPs (AgNPs) related to ligand adsorption at the metal surface. Firstly, self-assembled layers of citrate adsorbed on AuNP (111), (110), and (100) surfaces were proposed, based on geometric considerations and spectroscopic investigations by infrared (IR) and X-ray photoelectron spectroscopy (XPS). Adsorption characteristics of citrate are the unique structure of adsorbed species, intermolecular interactions through hydrogen bonds and van der Waals attractions, bilayer formation, surface coverage, nanoparticle-stabilization role, and chirality. Secondly, IR and XPS studies showed coadsorption of thiolate on the surface of citrate-stabilized AuNPs. Steric, chelating effects and intermolecular interactions are the origins of the strong adsorption of citrate on AuNP surfaces. Surface coverage was determined from XPS analyses. Thirdly, an exclusive placement of Raman probe molecules (4-nitrobenzenethiol) at junctions (hot-spots) of AuNP dimers was achieved through an asymmetric functionalization approach. The orientation of asymmetric nanoparticles in dimers and the location of Raman probes were investigated using surface-enhanced Raman scattering (SERS). A linear correlation of SERS signal with hot-spot population and a SERS enhancement factor are presented. Lastly, AuNPs and AgNPs of small sizes (< 5 nm) were synthesized in water using poly(allylamine). Ligand-exchange by thiols was demonstrated by IR spectroscopy and transmission electron microscopy analyses. |
