Electrolyte interactions with ligand functionalized gold nanoparticles (AuNPs) have broad implication to a wide range of applications in nanoparticle research field. Among a wide range of electrolytes, halides, nitrates, borohydrides, and sulfides are used to study the AuNP interfacial interactions. Although there are many studies on AuNP interactions with anionic species (halides, nitrates, borohydrides, and sulphides), there is limited information on AuNP interactions with metallic cations. Therefore, studying the nanoparticle interfacial interactions with both anionic and metallic cation species is highly important. The research reported here is focused on deepening the understanding of electrolyte interactions with ligand functionalized AuNPs in aqueous solutions. The stability of citrate-residues on AuNPs against ligand displacement has been controversial. In the first study, we demonstrated the direct experimental evidence for the simultaneous adsorption of both citrate-residues and solution impurities onto citrate-reduced AuNPs by using AuNPs synthesized with deuterated citrate in combination with the surface-enhanced Raman spectroscopic (SERS) analysis. The citrate-residues can be readily displaced from AuNPs by a wide range of specific and non-specific ligands including organosulfur and electrolytes. In the second study, we investigated the charge state and the mechanism of silver ion binding onto organothiol functionalized AuNPs. Mechanistic study reveals that silver binding onto AuNPs proceeds predominantly through reactive pathways with proton generations providing the first direct experimental evidence that Ag+ can disrupt the Au-S binding and enhance the mobility of the organothiols on AuNPs. Ligand displacement from AuNPs is important in a wide range of applications. Complete and non-destructive removal of ligands from AuNPs is important and challenging due to the strong Au-S binding and the steric hindrance imposed by ligand overlayer on AuNPs. In the final study, we investigated hydrogen sulphide (HS-), an anionic thiol as an effective ligand to induce complete and non-destructive removal of ligands from aggregated AuNPs. The new insights and methodologies presented in this dissertation are important for studying the electrolyte interfacial interactions with ligand functionalized AuNPs which have a broad impact on nanoparticle surface chemistry
