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Functional mesoporous silica materials obtained via supramolecular soft templating

Boek - Dissertatie

Silica materials exist in all shapes and sizes, from crystalline to amorphous networks, from dense to porous materials with pores ranging from sub-nanometer to several nanometers. This work will focus on the ordered mesoporous silica materials. These materials are thus, mesoporous, i.e. having pores ranging 2 – 50 nm; contain mostly silica; and the order refers to regularity in pore arrangement, not the structure of the silica framework, which normally is amorphous. These ordered mesoporous silicates are characterized by their well-defined pore structure, high surface area and total porous volume. Due to these properties, these materials receive a lot of attention as promising materials in various applications such as (catalyst) carrier materials, electronics or as a release reservoir for drug delivery in pharmaceutical applications. Evidently, all these different applications require precise pore size control. By careful controlling synthesis parameters such as temperature, pH, chemical composition, etc., material properties such as pore size can be tuned to the desired dimensions. The synthesis of these materials however, require harsh synthesis conditions, together with costly processing such as the requirement for elevated temperatures over prolonged periods of time. Recently a new ordered mesoporous material was developed at the Centre for Surface Chemistry and Catalysis, and aptly coined COK-12. This new material is formed using cheap starting materials, under environmentally friendly conditions such as an aqueous environment and quasi neutral pH conditions. Moreover, synthesis occurs at room temperature or slightly higher temperatures and time is greatly reduced as the formation is almost instantaneous. These factors are interesting, considering relevant industrial up-scaling. More specifically, COK-12 is synthesized by mixing a sodium silicate solution to a citrate buffered solution of triblock copolymer, namely Pluronic  P123. Pluronic P123 is composed of a central hydrophobic polymer block of polypropylene oxide, flanked by two polyethylene oxide blocks. In aqueous solutions, P123 forms spherical micelles, having a hydrophobic core stabilized by a hydrophilic palisade layer. When adding sodium silicate to the buffered solution, Pluronic P123 acts as a template around which the silica will concentrate, forming an intricate silicate framework. In this work the COK-12 synthesis is explored further, taking advantage of the mild synthesis conditions. Firstly, the Pluronic P123 template was replaced by a different organic surfactant from the same triblock copolymer family. This polymer has similar properties as the P123 but differs in the length of the hydrophilic stabilizing side chains. Where Pluronic P123 has a chemical formula of (ethylene oxide)20-(propylene oxide)70-(ethylene oxide)20, F127 [(EO)102-(PO)70-(EO)102] has much longer ethylene oxide chains. This will affect the outcome of the final material. By replacing P123 in the buffered solution with F127 and tuning of the synthesis, a new ordered mesoporous material, named COK-19 is formed. This material is composed of spherical mesopores of 7 – 11 nm, separated by an amorphous silica wall of ca. 8 – 9 nm. The spherical mesopores are organized in a cubic close packing giving the material an Fm-3m symmetry. The formation mechanism of this new material was investigated using small-angle X-ray scattering, showing the cooperative self-assembly of Pluronic micelles and silicate precursors. During formation, silicate species concentrate around the individual Pluronic micelles, wrapping them in a silica layer of approximately 4 nm. When a critical silicate concentration around the micelles is reached, silica wrapped micelles start aggregating. These Pluronic core – silica shell nanoparticles stack in a spherical close packed fashion. Post-synthesis heating of the synthesis mixture allows control over pore size, making this a preferred choice regarding pore tuning. In a second part, the potential of this synthesis was explored regarding its use in pharmaceutical applications. More and more newly discovered active pharmaceutical compounds are hydrophobic, reducing their aqueous solubility and consequently their bioavailability in the human body. The uniform pore size and large porous volume makes these silica materials an ideal storage and release material. However, loading pharmaceuticals into a carrier material requires cumbersome loading procedures and the use of organic solvents. Here, the functionality of the triblock copolymer surfactant is used to the fullest by combining its ability to form mesoporous materials with its micellar surfactant properties to solubilize the hydrophobic compounds. Flurbiprofen is used as a model pharmaceutical compound and is solubilized in the buffered surfactant solution. In a second step, sodium silicate is added wrapping the flurbiprofen swollen micelles in a silica layer. The formed silicate material is less ordered than the previously synthesized COK-12 or COK-19 materials due to the interactions between flurbiprofen and the Pluronic  surfactant. Flurbiprofen molecules exhibit a high mobility inside the carrier material. In other words, flurbiprofen remains solubilized inside the hydrophobic core of the surfactant template, which in turn is wrapped in a silicate layer. Simulating release of flurbiprofen in biological relevant conditions in gastric and intestinal media is controlled by the pH of the media and the pKa and solubility of flurbiprofen. Under acidic conditions the carboxylic group of flurbiprofen is protonated, enforcing the hydrophobic-hydrophobic interactions between flurbiprofen and the micelle core. In contrast, under neutral conditions, above the pKa of flurbiprofen, the carboxylic group is deprotonated, making flurbiprofen hydrophilic, expelling flurbiprofen from the hydrophobic environment and consequently from the silicate structure. The use of amphiphilic molecules as templates around which silica can be wrapped was expanded further, by using proteins as template instead of synthetic organic polymers. The mild conditions of near neutral pH and ambient temperature allow the use of proteins as templates, without risking denaturing the protein or loosing protein functionality. Moreover, wrapping proteins in a silica layer improves protein stability by restricting the mobility of the protein, preventing unfolding or denaturation. Several other techniques exist to stabilize protein in a silica layer, or silica matrix, often requiring chemical modification of the protein surface, harsh conditions, or the use of organic solvents. b-casein was used as a model protein. The amphiphilic b-casein self-assembles into micelles in aqueous solutions. Using a modified synthesis in which sodium silicate is added to a citrate buffered b-casein solution, discrete nanocapsules of b-casein micelles wrapped in a silica layer are obtained.
Jaar van publicatie:2015
Toegankelijkheid:Closed