Effect of the ligand shell composition on the dispersibility and transport of gold nanocrystals in near-critical solvents.

The development of more efficient and environmentally benign methods for the synthesis and manipulation of nanomaterials has been a major focus of research among the scientific community. Supercritical (ScFs) and near-critical fluids (NcFs) offer numerous advantages over conventional solvents for these purposes. Among them, ScFs and NcFs offer dramatic reductions in the volume of organic waste typically generated during advanced material processes with the feasibility of changing a number of physicochemical properties by discrete variations in solvent pressure or temperature. In this work, we study the dispersibility of gold nanocrystals with a 3.7 nm core size stabilized by different ligand shells in NcF ethane and propane over a wide range of densities by fine-tuning the pressure of these fluids. Dispersibility vs density plots are obtained by following the variation in the surface plasmon resonance (SPR) absorption spectra of the nanoparticles. To understand the results obtained in this study, three models are briefly discussed: the total interaction theory, the sedimentation coefficient equation, and the Chrastil method. The dispersibility and behavior of the nanocrystals with variations in fluid density are strongly dependent on the surface chemistry of the nanocrystal and the solvent employed. A correlation between measured dispersibility values and calculated sedimentation coefficients was observed in both compressed solvents. In addition, we successfully applied the Chrastil equation to predict and describe the dispersibility of gold nanocrystals with different shells as a function of density, determining that the reason for the high stabilities of some of the nanocrystal dispersions is the strong solvent-nanocrystal interactions. While NcF propane showed higher nanocrystal dispersibilities, using NcF ethane led to improved tunability of nanoparticle dispersions formed in the pressure range studied. Therefore, with a judicious selection of the fluid, NcFs seem to offer a remarkable advantage over conventional solvents for manipulation of nanomaterials, which could be applied to transport, purification, and separation of nanocrystals.