When nanoparticle size and molecular geometry matter: Analyzing the degree of surface functionalization of gold nanoparticles with sulfur heterocyclic compounds

Abstract

The degree of surface functionalization of colloidal dispersions of Au nanoparticles (AuNPs) by sulfur heterocyclic compounds is determined using localized surface plasmon resonance spectroscopy (LSPRS), morphological characterization, and electrodynamic modeling. We demonstrate that AuNP surface coverage is the result of the interplay between the molecular geometry of ligands and the Au nanoparticle diameter. For the same ligand, it is found that the surface density of molecules (σ) is almost constant or slightly decreases as the Au nanoparticle diameter increases, while for the same NP size the degree of surface coverage increases as the length of the molecule increases. The chemical interaction between the ligand molecules and the AuNP surface is demonstrated using surface enhanced Raman scattering (SERS) and 1H NMR. For the same average AuNP size, the full width at half maximum (fwhm) of the LSPR is significantly greater for derivatized AuNPs, a feature that shows the important role played by chemical interface damping in NP surface functionalization. The strong chemical stability of the modified AuNPs is further probed against KCN etching, indicating that there is a close correspondence between σ and the slow kinetics of the decay of the LSPR peak. © 2012 American Chemical Society.