Synthesis of monodisperse mesoporous silica hollow microcapsules and their release of loaded materials.

Monodisperse mesoporous silica hollow capsules (MSHCs) with sizes ranging from 570 to 75 nm were synthesized using the sol-gel method combined with the template-assisted method. Monodisperse polystyrene (PS) particles were used as templates for the hollow structure of the MSHCs, and cylindrical micelles of cationic surfactant were used to create mesopores across the shell of the MSHC. To obtain MSHCs with a degree of polydispersity in diameter comparable to that of the PS templates and spherical in shape with uniform shell thicknesses, the conditions for the synthesis were systematically examined. It was found that the ranges of the reaction conditions to obtain such MSHCs had to be narrow because (1) the colloidal stability of the particles must be maintained before and after the sol-gel reaction and (2) the rate of the silica formation during the reaction must be regulated to attain sufficient shell thickness to retain the hollow structure and to achieve smooth surfaces. The sustained release of dye molecules loaded in the MSHCs was confirmed, indicating that our MSHC is a candidate for use as a drug carrier in drug delivery systems or as a container for microreactors.