Microscale droplets covered by amphiphilic gold nanoparticles with various ligand ratios and concentrations

We describe a simple and reliable route for forming microscale droplets covered with amphiphilic gold nanoparticles (AuNPs) functionalized with mixed hydrophilic and hydrophobic ligands on the AuNP surface. When an aqueous solution of amphiphilic AuNPs was mixed with hexane, the hexane (or water) droplets covered with amphiphilic AuNPs in the water (or hexane) phase were created, in which amphiphilic AuNPs in water phase migrated to the hexane-water interface. The extent of AuNP migration to the interface could be tuned by the ratio of the hydrophobic to hydrophilic ligands on the AuNP surface and the overall concentration of AuNPs. Importantly, the extent of the AuNP migration to the interface depending on the AuNP concentration followed the Langmuir isotherm model well. The equilibrium constant and maximum uptake of AuNPs at the interface were estimated using the Langmuir isotherm model. To the best of our knowledge, this is the first report of the equilibrium constant determination of amphiphilic AuNPs at the liquid-liquid interface. Based on the maximum uptake of AuNPs at the hexane-water interface, we demonstrated that the AuNPs could be closely packed at the interface.