Template directed synthesis of highly organized functional biomimetic silica nanostructures

Silica is an important mineral in technological and biological applications. Many
protocols have been developed for the synthesis of complex silica architectures.
Most prominent is the silicification approach, where polymers build up the templates
for the revealed polymer/silica structures.
The current thesis demonstrates that star-shaped polymers and block
copolymers are efficient templates for the fabrication of silica particles with spherical
or raspberry-like morphology. The shape of the resulting particles depends on the
pre-formed or self-assembled polymer structure and on the polymer chemistry.
In this work, we used two different templates: Star-shaped polymers were
synthesized by polymerizing individual arms via ATRP from a silsesquioxane core (2-
2.5 nm) as the connecting point. Those polymers build up star-like structures in
solution. On the other hand, a linear diblock copolymer poly(ethyleneimine)-bpoly(
ethylene glycol) (PEI-b-PEG) self-assembles into simple spherical aggregates.
PEI-b-PEG as well as the star-shaped polymer poly(N,N-dimethylaminoethyl
methacrylate) (PDMAEMA), results in rather small spherical particles (d = 20–30nm)
after silicification. The PDMAEMA/silica nanoparticles appear to have low electron
density interior domains, resulting from the silsesquioxane core in a polymer-rich
region. Furthermore, the silica particles synthesized using the star-shaped polymer
template poly[2-(methacryloyloxy)ethyl] trimethylammonium iodide (PMETAI) result in
raspberry-like structures also with a low electron density core embedded in a silica
layer and an average diameter of 50 nm. The external raspberry-bulbs investigated
by electron microscopy and small-angle X-ray scattering (SAXS) exhibit the length of
10 nm. This leads to the assumption of individual polymer arms encapsulated in
silica, since the number of bulbs is roughly related to the number of polymer chains
connected to the silsesquioxane core.
As the amino groups of the polymers catalyze the hydrolysis and condensation
reaction of the silicon alkoxide precursor TEOS, no additional catalysts are required
e.g. ammonium hydroxide solution. The reaction can take place under ambient
conditions compared to other silica nanoparticles production methods (Stöber
method or microemulsion method), where solvents or surfactants are required. Time resolved ζ-potential and pH measurements, dynamic light scattering, and electron
microscopy reveal that silica shell formation proceeds differently if PDMAEMA or
PMETAI are used as templates.
The ability to trap compounds by electrostatic interactions is an advantage of
the star-shaped polymers. The encapsulation and trapping of the fluorescent dye
sulforhodamine G can be monitored by fluorescence correlation spectroscopy (FCS)
and confocal microscopy. Electron paramagnetic resonance spectroscopy (EPR)
proves the trapping of the paramagnetic copper species Cu(OTf)2.
The process of encapsulating the protein hemoglobin can be monitored by
FCS, after labeling with the fluorescent dye 5(6)-carboxyfluorescein Nhydroxysuccinimide
ester (6-FAM). The UV-Vis measurements of hemoglobin
trapped in the silica shell confirm that the activity of the protein towards CN- and CO
remains intact.
The definite encapsulation of hemoglobin and the protective shielding by the
silica shell against digesting-enzymes can be monitored by UV-Vis spectroscopy.
The enzyme trypsin digests only the accessible proteins, which are free in solution,
on the silica surface, or not completely encapsulated.
To complete the multifunctional template-directed polymer/silica nanoparticles,
surface functionalization of the silica shell can be performed by a post-synthetic step
in a one-pot synthesis. This procedure implicates a facile approach to functionalize
silica with amine groups, without any previous washing steps, which avoid
unnecessary aggregation of particles before the functionalization step.