Chemistry Reference
In-Depth Information
4.2.2
Fluorescent Silica Nanoparticles and Other Oxide
Nanoparticles
Silica nanoparticles can be easily produced by applying either the
protocol developed by Stöber or the microemulsion techniques.
Stöber's protocol consists of alkaline hydrolysis-condensation of
tetraethyl orthosilicate (TEOS) added in a large volume of ethanol
[15]. The high dilution and the aqueous NH
OH to TEOS ratio are
key parameters for yielding monodisperse silica spheres in a large
range of size (from 20 to 800 nm). The control of the size, when
microemulsion techniques are applied, is ensured by the volume
of micelles, which behave as nanoreactors [16]. Moreover, these
synthesis protocols can be exploited for the encapsulation of metal
oxide nanoparticles because of the great affinity between the
surface of these particles and the polysiloxane shell precursors.
Besides their simplicity, both methods are very attractive because
multifunctional nanoparticles can be obtained by minor variations
in the composition of the polysiloxane precursors and/or by post-
functionalization. Indeed a large variety of polysiloxane precursors
are available for envisaging an accurate control of the composition
of the silica nanoparticles and for anticipating a functionalization
before and/or after the hydrolysis-condensation. The production
of multifunctional silica nanoparticles can be performed in several
steps for the elaboration of multilayered nanostructures [17].
Each layer and the surface after derivatization contribute to the
multifunctional character of the nanoparticles since they can
each be designed for expressing a peculiar feature. For instance,
aminopropyltriethoxysilane (APTES) was widely used in mixture
with TEOS for introducing in silica nanoparticle, in each layer or
on the surface amino groups, which can act as grafting site for the
immobilization of organic dyes, hydrophilic molecules for improving
the colloidal stability in aqueous biological media, biotargeting
groups for increasing the accumulation of the nanoparticles in the
zone of interest within the organism (Scheme 4.3). Van Blaaderen
4
et
al.
demonstrated that the conjugation of fluorescein isothiocyanate
to APTES before the hydrolysis-condensation allows the formation
of fluorescent silica nanoparticles in which the organic dyes are
distributed
in the whole nanoparticles or in the thickness
of the polysiloxane shell, whereas the post-functionalization (i.e.,
after the hydrolysis-condensation) of the amino groups leads to the
either
