Nanoparticle properties depend heavily on their size and size distribution. While solution based synthesis techniques are popular due to their simplicity, they often result in particles with a wide size range. Our group has developed a technique for the size-selective fractionation of ligand-capped nanoparticles that utilizes the tunable thermophysical properties of gas-expanded liquids (J. Phys. Chem. B, 109, 2005). This nanoparticle size separation process is based on the controlled reduction of the solvent strength of an organic phase nanoparticle dispersion through increases in concentration of the antisolvent CO₂ via pressurization. These changes in solvent strength affect the subtle balance between the osmotic forces of repulsion (originating from the solvation of the nanoparticle ligand tails) and the van der Waals forces of attraction between different sized particles necessary for a stable dispersion. As such, increasingly smaller sizes of particles can be controllably precipitated from the dispersion through modest increases in CO₂ pressure, resulting in the separation of particle dispersions typically ranging from 2nm to 12nm into ±1nm fractions. Recently, we have demonstrated effective separations of metal or semiconductor nanoparticles using a novel spiral tube apparatus within a high pressure vessel. This initial apparatus was capable of handling ~200 μL (2 mg) quantities of the nanoparticle dispersions. It is desirable to scale up this process to handle larger quantities for applications such as catalysts, sensors, semiconductors, and optics. The scaled up apparatus consists of three vertically mounted high pressure vessels connected with high pressure needle valves that allow for sequential isolation and separation of the fractionated nanoparticle dispersion. This process, operated at room temperature and pressures between 400 PSI and 700 PSI, can result in a size-selective separation of ~20 mL (200 mg) of a nanoparticle dispersion, a scale up factor of 100. This process can be easily modified by increasing the number of the vessels or by increasing the size of the vessels allowing for more fractions or separation of larger samples, respectively. Comparisons of the results for the size-selection separation of a silver nanoparticle dispersion in both the original process and the scaled up process will be discussed.

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