54. Thakur, Ranjit; Gupta, Ram B. Supercritical CO2 Based Silica Coating of Gold Nanoparticles Using Water-in-Oil Microemulsions. Industrial & Engineering Chemistry Research (2005), 44(9), 3086-3090.
A method is developed for the formation of a silica coating on nanoparticles using supercrit. CO2, in which the CO2 acts both as the antisolvent and reactant. A water-in-oil microemulsion of an aq. sodium silicate soln. in an org. solvent is injected into supercrit. CO2 by using a micronozzle to form small droplets. Supercrit. CO2 rapidly exts. the solvent from the droplet and reacts with the exposed surfactant-supported aq. sodium silicate to form silica. When gold nanoparticles are suspended in the sodium silicate soln., a silica coating on those nanoparticles is obtained. The particle size is controlled by controlling the coagulation process, i.e., how rapid the sticky sodium silicate is reacted into hard silica. The reaction rate is simply controlled by adjusting the CO2 concn. Particles in the size range 30-300 nm were obtained by changing the CO2 pressure from subcrit. to supercrit. with a coating thickness of as low as 20 nm. The particle size increases linearly with increasing CO2 molar volume. The water-to-surfactant mole ratio in the microemulsion also influences the silica coating.