(158b) Manipulating Size-Selective Dispersability of Gold Nanoparticles in Gas Expanded Liquid Systems Using Ligand/Solvent Steric Effects

Nanotechnology is a field which involves the study of the conversion of two and three dimensional assemblies of molecular scale building blocks, into nanomaterials/nanoparticles. It is an important topic in modern research due to its multidisciplinary applicability and future potential in many underdeveloped applications. Nanoparticles of most materials have unique size-dependent physicochemical properties due to quantum effects and a high surface area to volume ratio. However, the current methods used to produce monodisperse nanoparticles have several operational shortcomings such as the use of expensive equipment, large quantities of solvents and low throughputs. The synthesis procedures used to generate monodisperse nanoparticles usually necessitate then use of expensive reagents, high temperatures and can result into nanoparticles having non-uniform morphologies. A 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 CO₂pressure, increasingly smaller sized nanoparticles can be controllably precipitated from the dispersion.

The aim of this particular study is to investigate how the steric nature of a solvent and ligand affects the ligand-solvent interaction and thus the size dependent dispersability of gold nanoparticles in a GXL system. The nanoparticles precipitate from GXLs due in part to collapsing of the ligands under worsening solvent conditions. It is hypothesized that bulky, branched ligands should collapse to a lesser extent in an alkyl solvent due to less favorable solvent-ligand interactions when compared to straight-chain ligands and result in lower precipitation pressures. It is attempted in this study to increase the efficiency and effectiveness of this GXL process by combining branched ligands and branched solvents. Gold nanoparticles with a broad size distribution were synthesized using the popular Brust-Schiffrin method with various branched alkyl ligands, dispersed in alkyl solvents of varying structure, and then precipitated using the GXL technique. n-Hexane was chosen as the control solvent and the effect of several of its isomers on the precipitation and fractionation process will be discussed. The precipitation characteristics of ligand-stabilized gold nanoparticles were quantified by measuring the intensity of the surface plasmon resonance band for 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 pressures necessary to induce precipitation, the pressure range over which the nanoparticles precipitate, and the fractionation efficiency from each of these solvents provides new physicochemical insights that help further our fundamental understanding of the GXL precipitation phenomenon.