Surface binding properties of tetraoctylammonium bromide-capped gold nanoparticles

Abstract

Surface interaction of gold nanoparticles with solvents and functionalized organic molecules is probed using the changes in the surface plasmon absorption band. A red shift in the surface plasmon band is observed with increase in solvent dielectric constant for solvents that do not complex with metal surface. A plot of the square of the observed position of the surface plasmon bands of Au nanoparticles in these solvents as a function of medium dielectric function shows a linear dependence. Interestingly, the surface plasmon band position remains unaffected in polar solvents (with nonbonding electrons) and is attributed to the direct interaction of these solvents with the gold surface. Binding of gold nanoparticles with organic molecules containing functional groups such as -SCN or -NH₂ were found to dampen the surface plasmon bands. Binding of phenyl isothiocynate (PITC) and ¹⁵ N- labeled benzylamine to gold nanoparticles was probed using NMR techniques, and in both cases, two sets of signals were observed, one from the free molecules and the other from the molecules complexed to Au nanoparticles. Surface binding with organic molecule facilitates the formation of ordered array of gold nanoparticles.