(397k) Augmenting the Size-Selective Fractionation of Metal and Metal Oxide Nanoparticles Using a Modified Gas-Expanded Liquid Process

The term nanotechnology is employed to describe the creation and manipulation of materials with structural features in between those of atoms and bulk materials (two and three dimensional assemblies of molecular scale), with at least one dimension in the nanometer range. It is an important topic in modern research due to its multidisciplinary applicability and future potential in many underdeveloped applications. Nanoparticles have unique size-contingent physicochemical properties which arise due to quantum effects and properties such as chemical reactivity and melting point which are attributed to the nanoparticle size. The synthesis and post-synthesis processing procedures used to generate monodisperse nanoparticles usually necessitate the use of expensive reagents & equipment, large quantities of solvents and high temperatures. A post-synthesis processing technique to size-selectively precipitate and fractionate ligand-stabilized gold and silver nanoparticle dispersions in organic solvents has been developed in our laboratory which utilizes the pressure tunable physicochemical properties of CO₂ Gas-eXpanded Liquids (GXLs). This size-selective fractionation technique is based on the controlled reduction of the solvent strength through increases in the concentration of CO₂ (a known nonsolvent for aliphatic stabilizing ligands) via pressurization. These changes in solvent strength affect the subtle balance between the osmotic repulsive forces and the van der Waals forces of attraction between differently sized nanoparticles necessary to maintain a stable dispersion. Through modest changes in CO₂ pressure, increasingly smaller sized nanoparticles can be controllably precipitated from the dispersion.

Previous studies have shown that using metal (gold) nanoparticles with branched ligands, it is possible to manipulate the process pressure and hence the size-selective fractionation of metal nanoparticles using the GXL system. However, employing the same apparatus that was used in the previous studies, various anomalies were observed when size-selective fractionation of metal oxide (iron oxide) nanoparticles was attempted. These anomalies included non-uniform, location-specific precipitation of nanoparticles and poor fractionation results when using iron-oxide nanoparticles coated with fatty acid ligands. This particular study aims to increase the efficiency and effectiveness of the GXL process through the development of several process modifications. These modifications include improvements in the cascaded vessel apparatus, such as conversion into a packed column and using a metal jacket to cause particles with lower Hamaker constant values to adhere more effectively. The precipitation characteristics of ligand-stabilized gold nanoparticles were quantified by measuring the absorbance of the nanoparticle dispersion using UV-vis spectroscopy at various levels of CO₂ pressurization. The characterization of these nanoparticles was performed using transmission electron microscopy (TEM) to analyze their size distribution and hence judge the efficacy of the size-selective fractionation. The pressure range over which the nanoparticles precipitate and the fractionation efficiency from each of these process modifications provides new insights into the GXL fractionation process that help further our fundamental understanding of the GXL precipitation phenomenon.