Rattle-Type Silica Colloidal Particles Prepared by a Surface-Protected Etching Process

This paper explores the capability of the "surface-protected etching" process for the creation of rattle-type SiO₂@void@SiO₂ colloidal structures featuring a mesoporous silica shell and a mesoporous movable silica core. The surface-protected etching process involves stabilization of the particle surface using a polymer ligand, and then selective etching of the interior to form hollow structures. In this paper, this strategy has been extended to the formation of rattle-like structures by etching SiO₂@SiO₂ core-shell particles which are synthesized by a two-step sol-gel process. The key is to introduce a protecting polymer of polyvinylpyrrolidone (PVP) to the surface of both core and shell in order to tailor their relative stability against chemical etching. Upon reacting with NaOH, the outer layer silica becomes a hollow shell as only the surface layer is protected by PVP and the interior is removed, while the core remains its original size thanks to the protection of PVP on its surface. This process can be carried out at room temperature without the need of additional templates or complicated heterogeneous coating procedures. The etching process also results in the rattle-type colloids having mesoscale pores with two distinct average sizes. In our demonstration of a model drug delivery process, such mesoporous structures show an interesting two-step elution profile which is believed to be related to the unique porous rattle structures.