We have recently scaled up a process from the benchtop test tube level to application scale that allows for the size-selective fractionation of ligand-stabilized metal and semiconductor nanoparticles that utilizes the pressure tunable physico-chemical properties of CO₂ gas expanded liquids (GXL's). The nanoparticle size separation technique is based on the controlled reduction of the solvent strength of organic phase nanoparticle dispersions through increases in concentration of the anti-solvent CO₂ via pressurization. These changes in solvent strength affect the subtle balance between the osmotic repulsive forces (due to the solvation of the stabilizing nanoparticle ligand tails) and the van der Waals forces of attraction between different sized particles necessary to maintain a stable dispersion. Through modest changes in CO₂ pressure, increasingly smaller sized nanoparticles can be controllably precipitated from the dispersion, resulting in the separation of particle dispersions typically ranging from 2nm to 12nm into ±1nm fractions.

Previously, we have demonstrated an effective separation of metal (Au and Ag) and semiconductor (CdSe/ZnS) nanoparticles at the bench scale using a novel glass spiral tube apparatus within a high pressure vessel. However, this initial apparatus was only capable of separating mg quantities of nanoparticle dispersions. It was desirable to design a new apparatus capable of processing larger quantities of monodisperse nanoparticles for applications in catalysis, sensors, semiconductors, and optics. This new scaled up apparatus consists of three vertically mounted high pressure vessels connected in series with high pressure needle valves that allow for sequential isolation and separation of the fractionated nanoparticle dispersions. This process, operated at room temperature and CO₂ pressures between 28 bar and 49 bar, can result in a batch or semi-continuous size selective separation of larger volumes of the nanoparticle dispersions, resulting in a scale up factor of more than 200 over the previous process. Performing separations at this scale illustrates that it is possible to separate large quantities of nanoparticle dispersions using this technique. This process can be easily modified by increasing the number of fractionation vessels, by increasing the size of the vessels, by adjusting the ΔP at each stage, or through changes in ligand type or solvent choice. The efficiency of separation of several nanoparticle systems such as metals and semiconductors will be discussed including the effects of different nanoparticle ligands and other design parameters.