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

Abstract

The silica coating of nanoparticles and of complex bioentities such as viruses has gathered significant scientific interest due to the versatility of silica-based materials. These nanostructured 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 applicability 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 diimine organosilane. Carboxylate-modified PS beads with 100 nm were successfully used for the coating process, and the thickness of the silica shell 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.