(356b) Facile Separation of Gold Nanoparticles in Tunable Solvents

Nanoparticles are of recent interest due to their unique properties (e.g., catalytic, optical, electrical, magnetic, etc.) seen at the nanoscale which are not seen at larger, bulk sizes. Gold nanoparticles are of particular interest because they demonstrate high catalytic activities at diameters below ~20 nm which completely vanish by ~40 nm. Nanoparticles can be used to produce various pharmaceutical, agricultural, and potentially commodity chemicals but their high cost of recovery makes them economically undesirable for use. Herein, we propose a facile separation method using moderate pressures of CO₂ to utilize solution properties and perform selective extractions in tunable solvents (SETS). SETS are homogeneous mixtures of an organic solvent and water which have the inherent property to split into a heterogeneous mixture under moderate CO₂ pressure. A 60/40 volume percent mixture of acetonitrile and water to disperse nanoparticles was chosen for these experiments and the split occurs between 9.6-11.3 bar of CO₂.  Poly(vinylpyrrolidone) (PVP) was chosen as a stabilizing ligand because it is soluble in the homogeneous SETS mixture, can preferentially partition the nanoparticles to the water phase over the organic in the heterogeneous SETS mixture, and allows access to the metal surface for reactive compounds. We synthesized gold nanoparticles of different average sizes (1, 7.7, 9.6 nm) and performed four different heat treatments on them (no heat treatment, 40, 50, 60˚C). Gold nanoparticles preferentially extracted into the water phase for all experiments and the 7.7 nm particles heat treated at 50˚C had the greatest recovery (100 ± 7%) based on UV-vis absorbance data collected from each phase. The water phase containing the nanoparticles was decanted from the organic phase and the nanoparticles remained catalytically active. This demonstrates that SETS are a potentially sustainable approach to homogeneous catalyst recovery.