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.
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