pH- and Photo-Switched Release of Guest Molecules from Mesoporous Silica Supports

This paper proposes a new nanoscopic molecular movable gate-like functional hybrid system consisting of nanoscopic MCM-41-based material functionalized onto pore outlets with a saccharide derivative capable of interacting with boronic acid functionalized gold nanoparticles (AuNPs) acting as nanoscopic caps. The gating mechanism involves the reversible reaction between polyalcohols and boronic acids to form boronate esters. Functionalized AuNPs thus act as a suitable nanoscopic cap via the reversible formation of the corresponding boroester bonds with the saccharicle derivative anchored on the external surface of the mesoporous silica-based solid. The developed platform operates in aqueous solution and can be triggered by two simple external stimuli such as pH changes or light. The hydrolysis of the boroester bond takes place at pH 3, which results in rapid delivery of the safranine cargo from the pore voids into the aqueous solution. However, at pH 5 the pores are capped with nanoparticles and the delivery is strongly inhibited. The kinetics of the delivery was studied at pH = 3, assuming a simple diffusion process and that the kinetics of guest release from the pore voids of the hybrid material can be explained by the Higuchi model. It is possible to deliver the cargo in small portions by carrying out on-off aperture cycles via changing the pH from 3 to 5. AuNPs also open the possibility of employing light as a suitable stimulus for release procedures using the AuNPs' capacity for raising their temperature locally by absorption of laser light. The plasmonic heating using a Nd:YAG laser at 1064 nm results in the cleavage of the boronic ester linkage that anchors the nanoparticles to the surface of the mesoporous silica-based material, allowing the release of the entrapped guests. Further studies also demonstrated that it is possible to fine-tune the amount of cargo delivered by simply controlling the laser irradiation opening the possibility to designing laser-induced pulsatile release supports.