Fabrication of spherical colloidal particles of metal oxides doped with semiconductor nanocrystals

Abstract

Inorganic hollow microspheres have been investigated extensively for their potential applications as encapsulants for the controlled release of drugs, dyes and cosmetics, and as carriers of bioactive materials. Composites of metal oxides with fluorescent nanoparticles can be utilized also as bio-imaging or optical coding. Here, we present the fabrication of monodisperse hollow titania microspheres and silica particles in submicrometer scales incorporated with fluorescent nanocrystals. Several approaches have been employed to form metal oxide hollow structures, including layer by layer (LbL) deposition on colloidal templates and emulsion method. The emulsion-assisted method, compared to template method, is efficient to produce hollow spheres but with polydisperse particle size. In this study, titania shells have been formed by interfacial sol-gel reaction of titanium n-butoxide mingled with emulsion droplets of octanoic acid. In particular, we used high-throughput microfluidic devices to produce monodisperse emulsion droplets. In this strategy, nanoparticles such as semiconductor nanocrystals and iron oxide nanoparticles can be incorporated onto titania shells without additional steps or ligand exchange of nanoparticles. In addition, we have studied also the incorporation of nanoparticles into inorganic spheres. Silica particles of a few hundred nanometers doped with semiconductor nanocrystals are fabricated by ligand exchange. Specifically, ethanol soluble nanocrystals modified with hydroxyl and siloxane groups can be embedded onto silica particles as a silane coupling agent.