A general method to coat colloids w

A general method to coat colloids with silica is described. The amphiphilic, nonionic polymer poly(vinylpyrrolidone)(PVP) was adsorbed to various colloidal particles such as small gold colloids, gold-shellsilica-core particles, small and large silver colloids, boehmite rods, gibbsite platelets, and positively ornegatively charged polystyrene. After this functionalization the stabilized particles could be transferredto a solution of ammonia in ethanol and directly coated with smooth and homogeneous silica shells ofvariable thickness by addition of tetraethoxysilane in a seeded growth process. The length of the polymerused strongly influences the stability of the colloids and the homogeneity and smoothness of the initialsilica coating. This method is especially useful for colloidal particles that cannot be covered directly withSiO2 by a Sto¨ber-like growth process. Compared to methods known from the literature for the coating ofsuch particles, this new method is faster and requires neither the use of silane coupling agents nor aprecoating step with sodium silicate, which is poorly reproducible.