Mesoporous silicates applications

Optical sensors based on UV-visible and fluorescence spectrophotometry and a visual color change in a material are other directions for mesoporous silicates application (Melde et al. 2008). Several examples of such sensors are presented in Table 8.2. Usually optical detection of gases in mesoporous silica-based sensors takes place through the use of an incorporated dye. In particular, oxygen sensing... 

Nucleic acids, DNA and RNA, are attractive biopolymers that can be used for biomedical applications [175,176], nanostructure fabrication [177,178], computing [179,180], and materials for electron-conduction [181,182]. Immobilization of DNA and RNA in well-defined nanostructures would be one of the most unique subjects in current nanotechnology. Unfortunately, a silica surface cannot usually adsorb duplex DNA in aqueous solution due to the electrostatic repulsion between the silica surface and polyanionic DNA. However, Fujiwara et al. recently found that duplex DNA in protonated phosphoric acid form can adsorb on mesoporous silicates, even in low-salt aqueous solution [183]. The DNA adsorption behavior depended much on the pore size of the mesoporous silica. Plausible models of DNA accommodation in mesopore silica channels are depicted in Figure 4.20. Inclusion of duplex DNA in mesoporous silicates with larger pores, around 3.8 nm diameter, would be accompanied by the formation of four water monolayers on the silica surface of the mesoporous inner channel (Figure 4.20A), where sufficient quantities of Si—OH groups remained after solvent extraction of the template (not by calcination). 

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

Much effort has thus been expended in two other directions. Many mesoporous silicates have been prepared containing silicon and a second element, e. g. titanium, vanadium, zirconium, and a range of other first row transition metals, and their uses in catalysis have been examined [14-16]. The scope of this section is, however, to cover organic modification of the silica framework, both by postsynthesis grafting and in-situ functionalization, and to discuss applications of the products as basic catalysts. 

From their discovery in 1992, the field of mesoporous materials has grown in an exponential way with increasing successful synthetic methods for siHca, metal oxides, phosphates, and so on. The relatively low hydrothermal stabiHty of mesoporous phases is perhaps the most critical consideration for their appH-cation, in particular for heterogeneous catalysis. Likewise, the incorporation of chemical functionalities into mesoporous materials has enlarged their appHca-tions. Future research efforts will likely address the development of novel synthetic methods to better control such functionaHzation while avoiding the loss of textural and structural properties of these materials. The morphological control of mesoporous siliceous solids and the incorporation of chemical functionaHties are also of great interest to yield materials with novel properties and prospective applications. 

The MCM-type materials belong to a new family of ordered, mesoporous silicate/aluminosilicate prepared by hydrothermal formation of silica gels in the presence of surfactant templates (Beck et al., 1992). They were discovered only recently, by Beck et al. in 1992, and hold promise for a number of interesting applications. Hence they are included in this chapter. 

The above methods are applicable to many kinds of mesoporous silicas, and we used siliceous FSM-16 and FIMM-1 in our work. 

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