Silica microcapsules (hollow spheres) were readily prepared by an interfacial reaction using a water/oil/water (W/O/W) emulsion system. A W/O emulsion consisting of an aqueous solution of sodium silicate (WP-1) and an n-hexane solution of Tween 80 and Span 80 (OP) was added to another aqueous solution of a precipitant (WP-2). The reaction of sodium silicate with a precipitant to form silica on this W/O/W emulsion system (WP-1/OP/WP-2) forms the hollow structure spontaneously. Therefore, no core material often utilized in silica hollow sphere fabrication was required in this process. The formation and the particle size of the silica microcapsule depended on the precipitant employed. When NH 4 HCO 3 was used as precipitant, the particle size of the silica microcapsule was successfully controlled by the volume ratio of WP-1/OP/WP-2, the rotation number of the homogenizer, and the concentration of sodium silicate in WP-1. However, this control of the particle size was not achieved when other precipitants such as NH 4 Cl were used. In the case of NH 4 HCO 3 , silica formation takes place at the outer interface between OP and WP-2 on the W/O/W emulsion system. On the other hand, when NH 4 Cl is utilized, silica is yielded at the inner interface between WP-1 and OP. These differences of reaction mechanisms of sodium silicate among precipitants were important factors in the preparation and properties of microcapsules.

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Artificial synthesis of hollow cell walls of diatoms is an ultimate target of nanomaterial science. The addition of some water-soluble polymers such as sodium polymethacrylate to a solution of water/oil/water emulsion system, which is an essential step of the simple synthetic procedure of silica hollow spheres (microcapsules), led to the formation of silica hollow spheres with nano-macroholes (>100 nm) in their shell walls, the morphologies of which are analogous to those of diatom earth.

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Phase transition from vaterite to calcite is a general behavior of CaCO3 materials. The interfacial reaction method using water-in-oil-in-water (W/O/W) emulsion we reported before is an effective method to produce hollow CaCO3 particles (microcapsules) with vaterite crystalline structure. These vaterite CaCO3 microcapsules underwent the phase transition to calcite in various aqueous solutions. When some proteins were mixed in the solution used for the phase transition, their encapsulations were achieved satisfactorily at room temperature regardless of their molecular weight. Insulin, lysozyme, bovine serum albumin, and chicken IgY were successfully encapsulated into the phase transition CaCO3 particles, while the encapsulation of lysozyme was unsuccessful by the interfacial reaction method. Protein included in the phase transition CaCO3 particle was not discharged by simple water washing but by the dissolution of the CaCO3 matrix in an acid solution, being advantageous to a responsive protein delivery technology. The recycle uses of the insulin solution used for the encapsulation improved the utilization efficiency of insulin. It was ascertained that the phase transition of vaterite CaCO3 microcapsule to calcite is a simple, general, and convenient method to encapsulate proteins into CaCO3 small particles under very mild conditions.