Tailored siloxane nanocomposites: Synthesis and applications as nanocarriers for drugs, metal particles, and catalysts

Inorganic/Organic (I/O) hybrid nanocomposites are a rapidly growing field of science in pursuit of novel materials with improved properties and performances. Owing to the remarkable combination of inorganic and organic segments into one material, I/O hybrid nanocomposites promise to have the potential for numerous useful applications, which are not offered by inorganic or organic materials alone. In our approach to synthesize such hybrid materials, we have used cyclic and cubic siloxanes as hydrophobic, inorganic cores and frameworks for the generation of I/O hybrid materials by introducing hydrophilic, organic arms such as polyethylene glycols. The reaction was accomplished via Rh-catalyzed dehydrogenative alcoholysis coupling reaction between a cyclic siloxane (D4H) or cubic siloxane (Q8M 8H) and polyethylene glycol (PEG). The resultant silicones were analyzed by various spectroscopy techniques such as IR and NMR ( 1H, 13C, 29Si), which showed successful substitutions in quantitative yields. Our hypothesis was that the amphiphilic nature of the resultant alkoxy siloxanes would generate well-defined siloxane networks and micelles when dissolved in selective solvents above a certain concentration. We envisioned that the micelles can be employed as nanoreactors not only for generation of metal nanoparticles, but also as stabilizers for generated colloids. Secondly, hydrophilic polyethylene glycol addition on the siloxane core may allow solubility of hybrid composites and self-assembly into micelles in aqueous media. This property of hybrid composites makes them potential candidates as drug delivery carriers. The micellization of synthesized I/O hybrid nanocomposites was monitored with transmission electron microscope (TEM), which revealed well-defined siloxane network micellar structures. Furthermore, we believe that hybrid nanocomposites can be expanded to further generations by introducing additional siloxane units through glycol bridges giving rise to dendrimers.;To establish their role as nanoreactors, I/O hybrid micellar networks of first and second generation were for preparation of nanosized metallic particles, via in-situ reduction of metal salts at mild laboratory conditions. Silver (Ag), palladium (Pd), and platinum (Pt) nanoparticles of size 1-5 nm were prepared in this way. Characterization of nanoclusters was carried out using a number of analysis techniques, such as TEM and UV-vis, which revealed non-aggregated metal nanoparticles with narrow size distributions confirming the role of hybrid composites as excellent reducing agents and effective stabilizers as well. Catalytic application of Pt-nanoclusters was demonstrated by using them as effective catalyst for hydrosilylation of Si-H bonds of heptamethyltrisiloxane (HMTS) with double bonds of various alkenes. In most of the cases, quantitative and selective attachment of alkenes was achieved under mild reaction conditions without any noticeable side reactions. The scope of I/O hybrid nanocomposites as drug carriers was probed by encapsulation of a fluorescent dye in micelles in water solutions. Microscopic studies of encapsulated samples confirmed the potential and efficiency of hybrid nanocomposites as excellent drug carriers in aqueous media. Furthermore, grafting of octakis (dimethylsiloxy)-T 8-silsesquioxane (Q8M8H) with various alcohols was carried out in order to establish a potential route to POSS (polyhedral oligomeric silsesquioxanes) materials. Spectroscopy techniques such as IR, and NMR (1H, 13C, 29Si) spectroscopy were used to analyze the covalent attachments and quantitative grafting was achieved. TGA analysis showed that the resultant POSS materials had higher thermal stability.