(649d) Tuning the Precipitation and Fractionation of Nanoparticles in Gas-Expanded Liquids

Size-based fractionation of nanoparticles remains a non-trivial task for the preparation of significant quantities of well-defined nanomaterials for certain applications and fundamental studies. Through the use of the pressure-tunable physico-chemical properties of CO₂-expanded liquids, a rapid, precise, and environmentally sustainable size-selective fractionation of significant quantities of ligand-stabilized nanoparticles is possible through simple variations in applied CO₂ pressure (Saunders, S. R. and C. B. Roberts (2009). Nanotechnology (47): 475605). Unfortunately, an applied CO₂ pressure upwards of 35 bar is needed to precipitate dodecanethiol-stabilized gold nanoparticles from a hexane dispersion. However, this work demonstrates that the precipitation and size-selective fractionation of nanoparticles can be tuned by varying the length of the stabilizing ligand, the surface coverage of the stabilizing ligand, or the composition of the solvent. For example, the addition of a certain amount of a liquid antisolvent (e.g. acetone) to a stable dispersion of nanoparticles in hexane can be performed prior to CO₂ addition such that the solvent strength of the liquid mixture (e.g. hexane + acetone) is reduced to near the threshold of precipitation. As such, these liquid solvent mixtures require only an applied CO₂ pressure of 20 bar to induce nanoparticle precipitation, as opposed to the higher pressures necessary for the same nanoparticle dispersion in the neat hexane solvent. Moreover, changing the stabilizing ligand from dodecanethiol to hexanethiol allowed for precipitation and hence fractionation of nanoparticles to occur at applied CO₂ pressures of less than 5 bar. This paper will demonstrate that this technique allows for greater control of the stability of the nanoparticle dispersions allowing for greater precision when processing.