Template directed synthesis of highly organized functional biomimetic silica nanostructures

Abstract

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 (Stober 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.