Hybrid Mesoporous Materials: mimicking natural cavities

PAULA C. ANGELOMÉ; GALO J. A. A. SOLER-ILLIA

San Carlos de Barilcohe

Encuentro; PASI 2005 on Bionspired Materials and Molecular Machines; 2005

Hybrid organic-inorganic materials are one of the most promising fields in Materials Chemistry. In particular, the incorporation of organic or biological functions to porous inorganic solid frameworks leads to the formation of an inorganic-organic composite that could mimic a natural cavity. The case of nanocavities is especially interesting because they allow the possibility of having in a small and limited space one or more functions, reproducing natural conditions found in enzymatic sites or vesicles. One of the most interesting potential uses is the determination of environmental conditions of a specific biological behaviour. In fact, it is possible to include a function which can specifically interact with biological systems (such as a protein, an antigen, DNA fragments, a cell, etc) and a second function in order to impart specific medium conditions (hydrofilicity, hydrofobicity, acidity…) to the cavity. In order to obtain nanocavities, organized mesoporous materials are ideal, because they present ordered arrays of monodispersed pores. Silica and transition metal mesoporous oxides have been synthesised by following different chemical routes and can be obtained as xerogels, powders, films or fibers with high surface areas (200-1000 m2/g), a variety of mesostructures (2D- or 3D- hexagonal, cubic, local…), pore diameters (3-12 nm) and amorphous or nanocrystalline walls. Following this approach, mostly silica-based hybrid materials bearing one organic function have been synthesized by co-condensation or post grafting using silane derivatives. While this approach permits to irreversibly anchor organic functions to the framework, for some applications such as controlled delivery or transferring of chemical information, it would be desirable to control the degree of grafting from firmly anchored functions to loosely bound ones. Furthermore, there are only few examples of incorporation of two or more organic functions in mesoporous silica powders. In this work, we report a strategy leading to functionalised transition metal oxides, which are interesting for their electronic, optical and chemical stability properties, as well as for their ability to be complexed by a great variety of anchoring groups. Post-grafting processes permit to attach several organic functions to titania or zirconia mesoporous films with a wide choice of grafting strengths, from stable (phosphate, phosphonate) to labile (hydroxyphenols). Moreover, bifunctional hybrid mesoporous films and powders with mixed oxide frameworks have been produced by using a two-step strategy. In the first step, a mixed oxide is obtained by co-condensation of R-Si(OEt)3 and a TM centre (Ti or Zr) in the presence of a template (which creates the mesopores). Organic functions included (amino, thiol or phenyl) are conserved after template removal and free M(IV) sites are left behind in the pore surface. A second function is then added by post-functionalisation with molecules containing the desired R’ and a suitable grafting group able to perform selective complexation of the TM centres. This procedure leads to highly ordered hybrid mesoporous films and powders presenting two different functions at the pore surface, and represents the first step in order to obtain an array of ordered cavities that could mimic natural environments.