Functionalized mesoporous silica structures

Functionalized Mesoporous Silica Structures Towards Novel Drug Delivery Systems... 

Fig. 27.5. Mesoporous silica nanoparticles as novel drug delivery systems, (a) Cocondensation method to form functionalized mesoporous silica structures in a surfactant template synthesis, (b) TEM image of mesoporous silica nanoparticles and sketch of a novel drug delivery particle which contains functionalized pores, closed by a gate, and is decorated with ligands for cell targeting, (c) Cell targeting by ligand-receptor interaction at the cell membrane, endosomal uptake and controlled release after pH change from early to late endosome...

Methylrheniumtrioxide (MTO) can straightforwardly be immobilized using copolymers consisting of polystyrene and polyvinyl pyridine. The authors proposed two possible structures 18 shown in Scheme 4.10. In the presence of H2O2 these catalysts are efficient and selective heterogeneous catalysts for the epoxidation of alkenes. It was shown that their activity is maintained for at least five recycling experiments [23 dj. In analogy to this work, bipyridyl-functionalized mesoporous silica can be employed for the immobilization of MTO [75]. 

An alternative method to synthesize organically functionalized mesoporous silica phases is the co-condensation method (also called one-pot synthesis or direct synthesis). Historically, the classical co-condensation reaction involved tetraalkoxysilane [(RO Si (predominantly TEOS or TMOS)] and a terminal organotrialkoxysilane of the type (R 0)3Si-R in the presence of structure-directing agents (Fig. 3.9, top). Meanwhile, not only bis- and multi-silylated precursors are used, that is, precursors of the type [(R 0)3Si]mR, m > 2 (Fig. 3.9, bottom), together with TMOS/TEOS, but... 

Similar to higher organisms combining muscles with skeletons to attain mobility, the addition of flexible organic functionalities to the rigid mesoporous silica structures enables the formation of gated systems. Organic species can be covalently attached to silica with relative ease. The main sources for the functionalization of silica are... 

Another photochemically based gating system is the one by Brinker and coworkers in which the cis-trans isomerization of azobenzene was applied.69 (Triethoxysilylpropylureido)azobenzene was co-condensed with TEOS by evaporation-induced self-assembly to yield a functionalized mesoporous silica thin film. The propylureidoazobenzene groups inside of the mesopores could then be iso-merized from their stable elongated trans structure to the more compact cis geometry upon UV irradiation. The trans geometry could be recovered by thermal treatment. The authors suggested that the results of the transitions from trans to cis could lead to an increase in pore size of approximately 0.7 nm and an increase of the polarity from 0 to 3 D. This could be applied to regulation of ion transport however, they did not confirm such expectations.69... 

Structural Properties of the Organically Functionalized Mesoporous Silica Materials... 

Han, L., Sakamoto, Y., Terasaki, O., Li, Y., and Che, S. (2007) Synthesis of carboxylic group functionalized mesoporous silicas (CFMSs) with various structures. /. 

Direct synthesis (one-pot synthesis or co-condensation) [43,44] is an alternative method to synthesize organically functionalized mesoporous silica phases. It involves the hydrolysis and condensation, in the presence of an SDA and under basic [95,96] or acidic [97,98] conditions, of tetraalkoxysilanes (typically tetrae-thoxysilane (TEOS)) and a nonbridged organosilane R —Si(OR)3 (route B, Figure 18.2), to yield organically modified PMSs. After the bulk structure is... 

Sol-gel processing forms the basis for various routes employed for the fabrication of a wide diversity of functional materials. To impart a structural organization at various length scales, the syntheses are performed using templates. Most consist of a self-organized ensemble of surfactants and co-polymers [1-10]. They have been successfully applied to control the geometry and dimensions of pores that are periodically arranged as in the initial structures. Mesoporous silica materials of the MCM family, which were first synthesized by a team from the Mobil oil company [11,12], are a well-known example. 

Control over the material s shape at the nanoscale enables further control over reactants access to the dopant, and ultimately affords a potent means of controlling function which is analogous to that parsimoniously employed by Nature to synthesize materials with myriad function with a surprisingly low number of material s building blocks. A nice illustration is offered by the extrusion catalytic polymerization of ethylene within the hexagonal channels of MCM-41 mesoporous silica doped with catalyst titanocene.36 The structure is made of amorphous silica walls spatially arranged into periodic arrays with high surface area (up to 1400 m2g 1) and mesopore volume >0.7 mLg-1. In this case, restricted conformation dictates polymerization the pore diameter... 

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