Silica precursor

The sol-gel processes are based on the polycondensation reactions that proceed with participation of silicic acid. There are three main opportunities to perform them. They are considered below. 

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The mechanical incorporation of active nanoparticles into the silica pore structure is very promising for the general synthesis of supported catalysts, although particles larger than the support s pore diameter cannot be incorporated into the mesopore structure. To overcome this limitation, pre-defined Pt particles were mixed with silica precursors, and the mesoporous silica structures were grown by a hydrothermal method. This process is referred to as nanoparticle encapsulation (NE) (Scheme 2) [16] because the resulting silica encapsulates metal nanoparticles inside the pore structure. 

The acidic conditions of standard SBA-15 synthesis [35] cause the precipitation of metal nanoparticles without silica encapsulation, or the formation of amorphous silica due to the presence of the polymer used for nanoparticle synthesis. Therefore, the SBA-15 framework was synthesized under neutral condition using sodium fluoride as a hydrolysis catalyst and tetramethylorthosilicate (TMOS) as the silica precursor. Pt particles with different sizes were dispersed in the aqueous template polymer solution sodium fluoride and TMOS were added to the reaction mixture. The slurry aged at 313 K for a day, followed by an additional day at 373 K. Pt(X)/SBA-15-NE (X = 1.7, 2.9, 3.6, and 7.1nm) catalysts were obtained by ex-situ calcination (see Section 3.2). TEM images of the ordered... 

Scheme 2. Encapsulation of size- and shape-controlled Pt nanoparticles under neutral hydrothermal synthesis conditions of SBA-15. Silica templating block copolymers and silica precursors were added to PVP-protected Pt nanoparticle solutions and subjected to the standard SBA-15 silica synthesis conditions. Neutral, rather than acidic pH conditions were employed to prevent particle aggregation and amorphous silica formation [16j. (Reprinted from Ref. [16], 2006, with permission from American Chemical Society.)...

Scheme 5.6 Assembly of hybrid microcapsules. Oppositely charged layers of protamine and silica precursor are alternately deposited on an enzyme-containing CaC03 bead, producing a core-shell microcapsule...

Fig. 3.2 Structural formulas of the silica precursors that are discussed in the chapter. Special features in the structures are shown in bold.

In a procedure developed by Carturan et al. [79,93] the alcohol is removed in a gas flow. This is also used to deliver the silica precursor. An alkoxide is initially evaporated to saturate a heated inert gas which serves as a carrier. When the gas... 

The realization of the reasons for poor biocompatibility of general alkoxides with biopolymers led to the development of approaches to minimize or eliminate the problem of the detrimental effect of alcohols. This can be done in two ways modification of the sol-gel processing or the silica precursor. This is considered in some detail below. 

The alcohol evaporation under vacuum, the treatment of cells and microcapsules by silica precursor supplied by gas carrier and infiltration of silica nanoparticles, whose are discussed above, are the examples of such modified approaches. There are two original methods whose would be well to consider. 

However, this study is of great importance since Gill and Ballesteros demonstrated first by numerous examples [46,82,101] that the exchange of alcohol with polyols improves the compatibility of the sol-gel processing to biopolymers. This showed a method for modification of the silica precursors. 

An example of the appropriate application of organically-modified silica precursors is alkoxides with an alkyl group. When methyltrimethoxy- or methyl-triethoxysilane (Figure 3.2) was added in formulations to increase the hydro-phobicity of ORMOSILs, it resulted in a better enzymatic activity of lipases immobilized in the alkyl-modified silica than in a hydrophilic matrix fabricated by means ofTEOS alone [51,80,129-133]. Similarly, an increased stability of lipase from Candida antarctica B was observed after its immobilization in a silica matrix... 

Organically-modified silica precursors have three alkoxy groups. As a result, alcohol is separated after the hydrolysis. Its appearance causes a detrimental effect on biopolymers typical to that ofalkoxides. The next disadvantage of these precursors is the absence of universality. They are appropriate only in particular cases. 

One-Stage Approach Based on a Silica Precursor with Ethylene Glycol Residues... 

Fig. 3.4 Schematic drawings illustrating the nucleation of silica precursor and possible mechanisms of reactions leading to silica formation. (A) The precursor hydrolysis. THEOS nucleates on a polysaccharide macromolecule through hydrogen bonds that are formed with hydroxy groups in the biomacromolecule. Per-...

Fig. 3.5 Representation of a scheme of an experiment (upper set of drawings) and the obtained experimental results presented as AFM images (middle part) and cross-sectional profiles (bottom) that provides evidence of silica nucleation and shell formation on biopolymer macromolecules. Scheme of experiment. This includes the following main steps. 1. Protection of the mica surface against silica precipitation. It was covered with a fatty (ara-chidic) acid monolayer transferred from a water substrate with the Langmuir-Blodgett technique. This made the mica surface hydrophobic because of the orientation of the acid molecules with their hydrocarbon chains pointing outwards. 2. Adsorption of carbohydrate macromolecules. Hydrophobically modified cationic hydroxyethylcellulose was adsorbed from an aqueous solution. Hydrocarbon chains of polysaccharide served as anchors to fix the biomacromolecules firmly onto the acid monolayer. 3. Surface treatment by silica precursor. The mica covered with an acid mono-...

Additional evidence for silica nucleation on biopolymer macromolecules was furnished by experiments in which solutions of proteins were studied by dynamic light scattering. As an illustration, Figure 3.6 shows the relative intensity of light scattering versus the diameter of the scattering particles in solution with 1 wt.% of bovine serum albumin. Curve 1 presents the initial state where the protein was not yet treated with silica precursor. The measured... 

Fig. 3.6 The relative intensity of dynamic light scattering vs. the diameter of scattering particles for a solution with 1 wt.% bovine serum albumin (1) and the same solution after addition of 3 wt.% of THEOS (2). Before the measurements, the solutions were left at ambient temperature for a week. The drawings are a schematic representation of a protein macromolecule before and after the treatment by silica precursor.

The introduced THEOS did not bring about precipitation in protein solutions. This behavior differs from that observed with common silica precursors. For example, TEOS added in such small amounts caused precipitation. By using THEOS, we could prepare homogeneous mixtures. When its amount introduced into the albumin solution was less than 5 wt.%, there was no transition to a gel state (Table 3.1). A gradual increase in THEOS concentration resulted in a rise in the solution viscosity. The transition to a gel state took place as soon as a critical concentration was reached. Its value, as demonstrated in Ref. 

The ethylene glycol-containing silica precursor has been combined, as mentioned above, with most commercially important polysaccharides and two proteins listed in Table 3.1. In spite of the wide variety of their nature, structure and properties, the jellification processes on addition of THEOS to solutions of all of these biopolymers (Scheme 3.2) had a common feature, that is the formation of monolithic nanocomposite materials, proceeding without phase separation and precipitation. The syner-esis mentioned in a number of cases in Table 3.1 was not more than 10 vol.%. It is worthwhile to compare it with common sol-gel processes. For example, the volume shrinkage of gels fabricated with the help of TEOS and diglyceryl silane was 70 and 53 %, respectively [138,141]. 

The one-stage process with THEOS proceeds differently (Figure 3.7B). The difference is in the absence of sol nanoparticles in the initial solution. There are entangled macromolecular chains (stage 1, Figure 3.7B). The silica precursor is introduced in a biopolymer solution as a monomer. The experimental results available to date (see Section 3.4.2) demonstrate that instead of sol formation there... 

In fact, such biomimetic molecules demonstrate the ability to tailor the growth of silica nanoparticles in a way that is very similar to diatom-extracted species. However, they demonstrate the same limitations in terms of morphological control of nanoparticle assembly. This is because the diatom shell architecture results not only from interactions of silica precursors with templating molecules but also benefits from a cell-driven molding of the vesicular compartment where silicification occurs [29]. Thus, it is very likely that diatom-like synthetic silica will only be achieved when such confinement/molding effects are taken into account in the design of biomimetic experiments [30]. 

In contrast, such approaches have been much more developed with proteins, termed silicateins, that have been extracted from some silicified sponges [38]. The success of these approaches probably originates from the fact that the reactivity of these proteins towards silica precursors differs significantly from the processes occurring in diatoms. Whereas silaffins and poly-amines activate silica formation via electrostatic interactions due to the presence of positively-charged ammonium... 

Time-resolved in situ Small Angle Neutron Scattering (SANS) investigations have provided direct experimental evidence for the initial steps in the formation of the SBA-15 mesoporous material, prepared using the non-ionic tri-block copolymer Pluronic 123 and TEOS as silica precursor. Upon time, three steps take place during the cooperative self-assembly of the Pluronic micelles and the silica species. First, the hydrolysis of TEOS is completed, without modifications of the Pluronic spherical micelles. Then, when silica species begin to interact with the micelles, a transformation from spherical to cylindrical micelles takes place before the precipitation of the ordered SBA-15 material. Lastly, the precipitation occurs and hybrid cylindrical micelles assemble into the two-dimensional hexagonal structure of SBA-15. 

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