Oil-in-water synthesis of hollow-shell mesoporous peapod-like silicates: Electron microscopy insights

Abstract

Hollow mesoporous silica nanoparticles (HMSN) receive a remarkable attention for their many advantages in several applications. Hollow-shell mesoporous peapod-like silicates were synthesized based on the recently introduced drug structure-directing agent (DSDA) concept. The anionic DSDA was formed by the amidation of L-tryptophan with palmitoyl chloride. 3-Aminopropyltrimethoxysilane (APS) was employed as co-structure directing agent (CSDA). The interactions among DSDA, CSDA and silica source promoted the formation of the oil-in-water emulsions. We hypothesized that the emulsions aggregated in micelles that merged into sheet-like structures that curved, bent and closed to form spherical hollow structures. Finally, the hollow structures fused between them to form the final hollow-shell peapod-like silicates with 0.6–2.5 μm in length. Interestingly, the formation of inner lamellar-like mesoporous pillars separated by large mesovoids with sizes around 50 nm was promoted. The role of the APS is determinant in the synthesis of the hollow morphologies, since its absence disrupts the formation of HMSNs. The changes in the organic component packing and the charge density matching between the silica framework, continuously transforming, and the surfactant headgroups are postulated as the driving forces in determining the particles morphology. Moreover, based on an original TEM sample preparation, perpendicular views over the long axis, and mainly over the short axis were acquired, proving the symmetry of revolution of these materials along their axis. Perpendicular slices of the nanoparticles demonstrated the internal hollows and the inner crossing walls with lamellar-like mesoporosity. These materials are expected to have potential applications in fields including, batteries, adsorption, and mainly in biomedicine processes.