Multi-scale microporous silica microcapsules from gas-in water-in oil emulsions

Controlling the surface area, pore size and pore volume of microcapsules is crucial for modulating their activity in applications including catalytic reactions, delivery strategies or even cell culture assays, yet remains challenging to achieve using conventional bulk techniques. Here we describe a microfluidics-based approach for the formation of monodisperse silica-coated micron-scale porous capsules of controllable sizes. Our method involves the generation of gas-in water-in oil emulsions, and the subsequent rapid precipitation of silica which forms around the encapsulated gas bubbles resulting in hollow silica capsules with tunable pore sizes. We demonstrate that by varying the gas phase pressure, we can control both the diameter of the bubbles formed and the number of internal bubbles enclosed within the silica microcapsule. Moreover, we further demonstrate, using optical and electron microscopy, that these silica capsules remain stable under particle drying. Such a systematic manner of producing silica-coated microbubbles and porous microparticles thus represents an attractive class of biocompatible material for biomedical and pharmaceutical related applications.