Hollow particle synthesis is an active research field due to its many industrial, biological and chemical applications. Typical synthesis methods include emulsion-based techniques and template-assisted approaches using colloidal particles or vesicles as the core template. In this paper we discuss a new fabrication scheme for a facile hollow particle synthesis that also allows for the capture and retention of introduced material or molecules inside the hollow colloids.

In this study we report results on the formation of hollow silica particles that have entrapped fluorescent quantum dots. We have used both and oil-in-water as well as a water-in-oil suspension approach. In the latter system the oil phase consists of a poly-dimethyl-siloxane (PDMS) that also contains a metal-organic silica precursor, tetraethyl-ortho-silicate (TEOS) the corresponding water phase consists of a surfactant, cetyltrimethylammoniun bromide (CTAB); while in the former a dimethyldiethoxysilane (DMDES) polymerized PDMS oligomer is ionized at the interface. The charge at the interface in both cases provides the reaction and growth region for the silica shell. The precursor must hydrolyze before it can form silica compounds and that can only happen readily at the o/w or w/o interface. The charged molecules provide an additional driving force for localizing the reaction at the interface. Control of pH is essential to minimize the gelation time of the hydrolyzed TEOS and thus creating a stable, hardened shell. The rate at which the shell grows is dependent on the pH of the system. This also affects the thickness and morphology of the shell.

In synthesizing these silica shells we have also encapsulated in the core fluorescing semiconductor nanoparticles, CdSe/ZnS quantum dots (QDs). In either dispersion system, the distributed phase provides the vestibule for the encapsulated nanoparticles. As the silica shell is formed at the oil-water interface the QDs are trapped in the core. For the oil-in-water dispersion we use a hydrophobically modified QDs. The hydrophobic nature of the QD surface stabilizes their dispersion in an oil phase. The hydrophobic termination of the QDs is provided by a TriOctyl-PhosphineOxide (TOPO) and octadecylamine adsorbed cap. This cap is chemisorbed against the ZnS shell of the QDs and extends its hydrophobic tails into the oil phase, resulting in the stabilization of dispersion. For the water-in-oil suspension, the QD surface must be rendered hydrophilic in order to stabilize the water-based suspension. To do so we use polyvinylpyrolidine (PVP) adsorbed on the hydrophobic QDs. The resulting aqueous/QD dispersion is stable.