Solvent and Ligand Effects on the Localized Surface Plasmon Resonance (LSPR) of Gold Colloids

Abstract

Cetylpyridinium chloride(CPC)-stabilized gold organosol in toluene has been prepared by using a two-phase (water−toluene) extraction of AuCl4- followed by its reduction with sodium borohydride in the presence of the surfactant, CPC. The surfactant-stabilized gold nanoparticles were exploited to examine their optical properties when exposed to various solvent systems and ligands by measuring the changes in the localized surface plasmon resonance (LSPR) spectrum. It was seen that the position of the surface plasmon band of metal nanoparticles is greatly influenced by the solvents and the ligands under consideration. The surface plasmon absorption maxima modulates/varies between 520 and 550 nm for gold nanoparticles, depending on the refractive index of the solvent. The significant discovery presented here is that λmax of the LSPR shifts to the blue by 3 nm for the increase of one carbon atom in the alcohol chain. Cationic and anionic surfactants of different chain lengths induce changes in the optical properties of gold nanoparticles, whereas zwitterionic amino acid molecules do not incite remarkable changes in the LSPR spectrum. The λmax of the LSPR gradually shifts to the red with the increase in chain length for both the cationic and anionic surfactants indicating specific binding of the surfactant molecules around the gold particles. Binding of three model compounds (1-dodecylamine, 1-dodecanol, and 1-dodecanethiol) indicates their relative affinity toward the gold surface that corroborate the HSAB (Hard−Soft Acid−Base) principle.