Abstract
The fabrication of mesoporous silica microcapsules with a highly controlled particle size ranging in the micrometer size presents a major challenge in many academic and industrial research areas, such as for the developement of smart drug delivery systems with a well controlled loading and release of (bio)active molecules. Many studies based on the solvent evaporation or solvent diffusion methods have been developed during the last two decades in order to control the particle size, which is often found to range at a sub-micrometer scale. Droplet-based microfluidics proved during the last decade a powerful tool to produce highly monodisperse and mesoporous silica solid microspheres with a controllable size in the micrometer range. We show in the present study, in contrast with previous microfluidic-assisted approaches, that a better control of the diffusion of the silica precursor sol in a surrounding perfluorinated oil phase during the silica formation process allows for the formation of highly monodisperse mesoporous silica microcapsules with a diameter ranging in the 10 micrometer range. We show also, using optical, scanning and transmission electron microscopies, small angle x-ray diffraction and BET measurements, that the synthesized mesoporous silica microcapsules exhibit a soft-like thin shell with a thickness of about 1 μm, across which 5.9 nm sized mesopores form a well-ordered hexagonal 2D network. We suggest and validate experimentally a model where the formation of such microcapsules is controlled by the solvent evaporation process at the droplet-air interface.
Highlights
Well-ordered mesoporous silica materials with a controllable nanopore size and structure are of a significant importance in many academic and industrial research areas
It is highly valuable to develop a soft template method which allows for the synthesis of highly monodisperse mesoporous hollow silica microcapsules with a size ranging in the micrometer range and allowing for a large volume of the inner chamber
In order to reduce the rather long time required for silica condensation, Chokkalingam et al.[17] developed another one-step approach, which enabled to achieve highly mesoporous silica microspheres with a large specific surface area, ca. 820 m2/g, by triggering a quick condensation of sol droplets inside the microfluidic channels by means of electrodes embedded inside the microfluidic device
Summary
Well-ordered mesoporous silica materials with a controllable nanopore size and structure are of a significant importance in many academic and industrial research areas. Mesoporous silica microspheres and microcapsules have potential applications in drug-delivery[1,2,3], catalysis[4], biosensing[5] and/or tissue bio-engineering[6], see for a detailed review refs[7,8] The fabrication of such microparticles relies mostly on methods derived from the so-called ESE (Emulsion Solvent Evaporation) approach, developed a decade ago by Andersson et al.[9], which is based on the combination of the well-known Stöber sol-gel technique[10] and the dispersing in a continuous oil phase of a silica precursor solution (sol) droplets. This concentration is for instance 20 times smaller than the one used by Chokkalingam et al.[17] and it allowed for the reducing of the diffusion and the dispersion of both solvent and sol while droplets flow along the microfluidic channel and tubing
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