Aggregation and Precipitation of Gold Nanoparticle Clusters in Carbon Dioxide-Gas-Expanded Liquid Dimethyl Sulfoxide

Previous post-synthesis nanoparticle processing techniques have demonstrated that carbon dioxide (CO2)-gas-expanded liquids can be used as a tool to controllably induce nanoparticle precipitation and size-selective fractionation. Until now, these previous studies have been limited to using nonpolar solvents (e.g., hexane) in CO2-gas-expanded liquids as a method to control nanoparticle precipitation by exploiting subtle changes in dispersive forces. We demonstrate a facile, post-synthesis nanoparticle processing technique to controllably aggregate gold (Au) nanoparticles into supraparticle clusters using CO2-gas-expanded dimethyl sulfoxide (DMSO), a polar, aprotic solvent, as well as the subsequent ability to precipitate these Au nanoparticle clusters from the DMSO solvent media at moderate applied CO2 pressures. Dispersions of Au nanoparticles have been prepared in three different DMSO solutions (where DMSO can serve as both the solvent and a stabilizing ligand): (i) neat DMSO, (ii) DMSO + oleic acid, and (iii) DMSO + stearic acid. UV-vis spectroscopy was used to monitor the surface plasmon resonance band of the Au nanoparticle dispersions where it was determined that the post-synthesis aggregation of the Au nanoparticles in DMSO is a direct function of the applied CO2 pressure. Furthermore, it was observed that the subsequent precipitation of the Au nanoparticle clusters in DMSO was induced by the applied CO2 pressure whereby the Au nanoparticle clusters would precipitate as a function of time (at a specified pressure). These findings illustrate the markedly different dispersibility of nanoparticles in CO2-gas-expanded nonpolar and polar solvent systems.