Preparation of uniform Au@SiO2 particles by direct silica coating on citrate-capped Au nanoparticles

Preparation of Au@SiO2 particles by direct silica coating on three different sized citrate-capped Au nanoparticles (17, 25 and 33 nm) with Stِber method was investigated in this work. It was found that the uniformity of the resulting Au@SiO2 particles was related to both the sizes of the Au nanoparticles and the concentration of citrate during the particle synthesis. When the citrate concentration during the particle synthesis was low, the 25 and 33 nm Au nanoparticles could be well dispersed in the Stِber system, thus resulting the formation of uniform Au@SiO2 particles containing single core. However, small Au nanoparticles (17 nm) were identified to show poor stability in the Stِber system even under low citrate concentration, the silica coating must be performed in a pre-hydrolyzed Stِber system to get the uniform Au@SiO2 particles. This approach was also applicable to citrate capped Ag nanoparticles. After removal of the excess citrate in the Ag prepared by citrate reduction, uniform Ag/SiO2 particles containing single core colloids could also be prepared by the direct silica coating.