Adsorption Patterns of Gold Nanoparticles on Methyl-Terminated Self-Assembled Monolayers

Abstract

Understanding the adsorption of gold nanoparticles (AuNPs) on self-assembled monolayers (SAMs) is important because an assembly of the AuNPs-SAM-gold substrate provides easily controllable metal metal junctions, in which fascinating phenomena such as electron tunneling and surface-enhanced Raman scattering can occur. In this work, we report strikingly different adsorption patterns of AuNPs on methyl-terminated SAMs. In contrast to the general belief that the terminal functional groups determine the surface properties of SAMs, the AuNPs adsorb on the surfaces of SAMs of 4-methyl-benzenethiols (MET) in a uniform, dispersed fashion, whereas aggregated adsorption is observed on the surfaces of SAMs of alkanethiols. We explore the effects of the terminal methyl group orientation, structural properties of SAMs, such as orderliness and packing density, and surface energies on the adsorption pattern differences. Dispersed or aggregated adsorption is determined by whether the citrate anions on the AuNPs are retained or removed by the SAM surfaces during the adsorption and thus, is critically dependent on the extent of the interactions between the AuNPs and SAMs. Direct interactions between the AuNPs and hydrophobic surfaces of alkanethiol SAMs strip the AuNPs of their citrate layers, leading to aggregated adsorption. For the less hydrophobic MBT SAMs, water mediates and softens the adsorption of the AuNPs. As a result, the citrate anions are retained on the AuNP surfaces, leading to dispersed adsorption of AuNPs. Forced interactions between the AuNPs and the MBT SAM surfaces by vigorous stirring yield aggregated adsorption, supporting our model.