Ion Pairing as the Main Pathway for Reducing Electrostatic Repulsion among Organothiolate Self-assembled on Gold Nanoparticles in Water

Abstract

Organothiol binding to gold nanoparticles (AuNPs) in water proceeds through a deprotonation pathway in which the sulfur-bound hydrogen (RS-H) atoms are released to solution as protons and the organothiol attach to AuNPs as negatively charged thiolate. The missing puzzle pieces in this mechanism are (i) the significance of electrostatic repulsion among the likely charged thiolates packed on AuNP surfaces, and (ii) the pathways for the ligand binding system to cope with such electrostatic repulsion. Presented herein are a series of experimental and theoretical evidence that ion pairing, the coadsorption of negatively charged thiolate and positively charged cations, is a main mechanism for the system to reduce the electrostatic repulsion among the thiolate self-assembled onto AuNP surfaces. This work represents a significant step forward in the comprehensive understanding of organothiol binding to AuNPs.