Supercritical CO2 based silica coating of gold nanoparticles using water-in-oil microemulsions

A method is developed for the formation of a silica coating on nanoparticles using supercritical CO2, in which supercritical CO2 acts both as the antisolvent and as a reactant. A water-in-oil microemulsion of an aqueous sodium silicate solution in an organic solvent is injected into supercritical CO2 by means of a micronozzle, forming small droplets. Supercritical CO2 rapidly extracts the solvent from the droplet and reacts with the exposed surfactant-supported aqueous sodium silicate, forming silica. When gold nanoparticles are suspended in the sodium silicate solution, a silica coating on those nanoparticles is obtained. The particle size is controlled by controlling the coagulation process, i.e., how fast the sticky sodium silicate is reacted into hard silica. The reaction rate is simply controlled by adjusting the CO2 concentration. Particles in the size range 30-300 nm were obtained by changing the CO2 pressure from subcritical to supercritical with a coating thickness of as low as 20 nm. The particle size increases linearly with the CO2 molar volume. The water-to-surfactant mole ratio in the microemulsion also influences the silica coating.