Batch and Flow Nanomanufacturing of Large Quantities of Colloidal Silver and Gold Nanocrystals Using Deep Eutectic Solvents

Significant process intensification of nanoparticle synthesis is required to meet the demands of industrial applications. Unfortunately, conventional nanoparticle synthetic methods (i.e., those using traditional solvents) are largely unsustainable long term due to inefficient reagent, solvent, personnel, and time utilization. As one possible path forward, we demonstrate the benefits of a dimethylammonium nitrate-polyol deep eutectic solvent (DES) system paired with oleylamine for the rapid and facile production of significant quantities of high-quality silver and gold nanoparticles dispersible in hydrophobic organic solvents such as toluene. We investigate three potential polyols as the hydrogen bond donor of the DES (i.e., triethylene glycol, ethylene glycol, glycerol), provide a temperature semioptimization, and investigate product quality using both batch and continuous flow reaction formats. These DESs possess extraordinary metal dissolution properties and are capable of producing monodispersed coinage metal colloids at remarkably high concentrations (i.e., 1000 and 400 mM silver and gold content, respectively) in essentially a quantitative yield. As an illustration of the consequence of this high metal concentration, a 1 mL reaction employing 1000 mM AgNO3 produces 107 mg of silver nanoparticles within minutes, some 1000- to 4000-fold higher than a conventional aqueous synthesis performed using the same reaction volume. Likewise, translation to continuous flow millifluidic synthesis at a moderate flow rate of 0.24 mL min(-1) at the same metal loading yields 26 mg of silver nanoparticles per minute. This amounts to 37.4 g of isolable nanoparticles daily, a substantial output, which can be amplified further by operating multiple continuous flow reactors in parallel. Finally, we demonstrate the efficient phase transfer of the oleylamine-stabilized gold nanoparticles from nonpolar solvent (toluene) into aqueous solution by applying a ligand-exchange surface reaction using 11-mercaptoundecanoic acid with full preservation of nanoparticle size, morphology, monodispersity (8.8% RSD), and colloidal stability. This research points to an exceptional future for DESs as high-performance fluids in scalable, sustainable, and flow processes toward the intensification of nanomanufacturing efforts.