pH-Responsive Silica Coatings: A Generic Approach for Smart Protection of Colloidal Nanoparticles

The silica coating of nanoparticles and of complex bioentities such as viruses has gathered significant scientific interest due to the versat i l i t y of silica-based materials. These nano-structured core-shell architectures have emerged as a tool for drug delivery and biospecimen preservation. Yet, the absence of straightforward methods for removing the silica coating limits the applicabi l i t y of this class of functional materials. For several drug delivery applications as well as for fast dissolution of silica nanomaterials, an acidic environment is frequently used . Acid-sensitive silica nanoparticles are extensively explored, but easily prepared acid-sensitive silica coatings are rare. In this context, we herein report a simple approach for coating polystyrene (PS) nanoparticles as model systems with a pH-responsive silica framework . The core-shell pH-responsive silica coatings were constructed by a seeded sol-gel route using tetraethyl orthosilicate (TEOS) and a pH-sensitive dumine organosilane. Carboxylate-modified PS beads with 100 nm were successfully used for the coating process, and the thickness of the silica shel l was tuned between 7 and 20 nm by adjusting the TEOS concentration. The introduction of the diimine organosilane in the silica framework resulted in PS@SiO2* core-shell nanoparticles with raspberry-like shells instead of a smooth and homogeneous morphology. According to the mechanism, this was due to a combination of the surface charge of silica colloids and the rapid self-condensation of the organosilane moieties. When this acid-sensitive silica framework was exposed to an acidic environment (pH 5.0), the consequent hydrolysis of the imine bonds promoted a fast degradation of the silica coating , exposing the bare PS nanoparticles. The described results provide proof-of-concept evidence that pH-responsive silica coatings are a promising platform for a range of applications in drug delivery in an acidic environment, sensing, and as a protective synthetic coating that could be removed on demand.