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.

The ability to process and purify engineered nanomaterials using near critical or supercritical fluids (NcFs or ScFs) has enormous potential for the application at various stages of the development of green nanomaterials. The dispersibility of octanethiol-stabilized gold nanocrystals of different core sizes is explored, which were chosen to serve as model nanomaterials of general interest in compressed ethane and propane over a wide range of fluid conditions. Both solvents have enormous potential for the environmentally benign processing and transport of engineered nanomaterials due to their nominal toxicity and high degree of tunability and processability that can essentially eliminate solvent waste. The dispersibility is determined by measuring the absorption spectra of dispersions of various sizes of nanocrystals in NcFs. To better understand the obtained results three models, the total interaction theory, the sedimentation coefficient equation, and the Chrastil method, are discussed. Nanoparticle dispersibility versus density plots are strongly dependent on nanoparticle size and solvent conditions, with the dispersion of larger nanocrystals more dependent on changes of pressure or density at a given temperature. For the range of nanoparticle sizes studied, compressed ethane at 25 degrees C leads to a greater tunability of nanoparticle dispersion when compared with compressed propane at 65 degrees C. For equivalent pressures, compressed propane is found to provide better solubility than ethane due to its higher density. The results quantitatively demonstrate that NcFs can offer pressure-tunable, size-selective control of nanoparticle solvation and transport at easily obtainable temperature and pressure conditions. These capabilities provide clear advantages over conventional solvents and direct application to various nanomaterials processes, such as synthesis, separation, transport, and purification of nanocrystals.

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.