Synthesis of Au–silica core–shell particles by sol–gel process

This paper describes an amine free sol–gel method for silica coating of Au nanoparticles and shows an X-ray image of a colloid solution of the silica coated Au (Au–SiO₂) particles. The Au nanoparticles that had a size of 16·9±1·2 nm were prepared through a conventional citrate reduction method. The silica coating was performed with a sol–gel reaction of tetraethylorthosilicate (TEOS) catalysed with NaOH in the presence of Au nanoparticles. The silica shell thickness was varied from 37 to 68 nm for TEOS concentrations of 1×10⁻³–20×10⁻³M at 4·3×10⁻⁵M of Au, 10·7M of H₂O and 1·0×10⁻³M of NaOH. The optical properties of the Au–SiO₂ particle colloid solution were related to the refractive index around the Au particles and the intensity of scattering from silica shells. The as prepared colloid solution could be concentrated up to an Au concentration of 4·3×10⁻²M with salting out. The concentrated colloid solution showed a high contrast X-ray image.