Oxidation-induced surface deposition of tannic acid: towards molecular gates on porous nanocarriers for acid-responsive drug delivery

Simple construction of porous nanocarriers with surface coatings for gated release is of great significance in the current drug delivery research. In the present study, tannic acid (TA) derived metal-phenolic network (MPN) and boronate-phenolic network (BPN) coatings on the surfaces of 70 nm-sized mesoporous silica nanoparticles (MSN) were developed by separating the TA deposition and the crosslinking into two steps. Surprisingly, the highly efficient deposition of TA on MSN by surface interactions was observed by UV-vis spectra, thermogravimetric analysis, and nitrogen sorption. The interactions stem from the oxidation of TA by superoxide anions from MSN suspension, as evidenced by fluorescence spectra and HPLC. The pore openings (similar to 3.9 nm) were partially occluded by TA deposition (at a weight ratio of 6 wt% TA), which circumvented the requirement of multi-layer coating for sufficient pore sealing. Moreover, it played significant roles in constructing the MPN or BPN structures after TA deposition, as well as regulating the release of a hydrophilic model drug (DOX) from the mesopores. The MPN-containing MSN particles (MSN@TA-Zn) possessed higher pH sensitivity as compared with its counterpart incorporating BPN (MSN@TA-Zn-BDBA), which gave rise to more pronounced pH-enhanced drug release kinetics. The particles were shown to deliver DOX to cancer cells for inducing efficient cytotoxicity. Therefore, these results would inspire simple controlled release systems through responsive pore gating.