Tuning and Quantifying the Dispersibility of Gold Nanocrystals in Liquid and Supercritical Solvents

Abstract

The application of nanomaterials relies on the ability to synthesize, purify, transport, and deposit them in a controllable fashion. The capacity to adjust the density, and thus the solvent strength, of a supercritical or near-critical fluid can be used to tune reaction and separation processes as well as to assemble nanomaterials in a controllable fashion. Herein we demonstrate and quantify density-tunable and reversible size-dependent dispersibility of octanethiol-stabilized gold nanocrystals with a size of 3.7 ± 2.2 nm in near-critical and supercritical solvents as a way to show the significant potential of these fluids for nanomaterials processing. This study introduced discrete variations on the pressure of nanocrystal dispersions in compressed ethane and propane at temperatures of 25, 45, and 65 °C until they reached a saturation region, at which point actual measurements of nanocrystal dispersibility were obtained using UV−vis absorption spectroscopy. Transmission electron microscopy (TEM) was employed to correlate the dispersibility results with the actual size of the nanoparticle fractions at different densities. The results showed that stable dispersions of nanocrystals could be obtained at pressures as low as 50 atm in both solvents. Compressed ethane in its liquid or supercritical state was found to provide better dynamic tunability, whereas propane provided higher dispersibility of these nanocrystals under the studied pressure−temperature conditions. Two theoretical models, the total interaction theory and Chrastil equation, are briefly presented as a means of interpreting the experimental observations. It was determined that dispersibility depends strongly on the nanocrystal size, solvent density, and carbon chain length of the solvent. These results clearly show that selected supercritical fluids can be remarkably effective for the manipulation of nanoparticles.