Asymmetric silica nanomaterials

In this chapter we review the current state of research on symmetric and asymmetric silica nanomaterials, or nanosUica, in terms of their synthesis, characterizahon and applications. Both, catalyticaUy and noncatalytically grown nanosilica are discussed, and several types of application for these nanomaterials are described. For asymmetric nanosihca, the focus is on helical nanosilica such as silica nanocoils and other heh-cal nano silica, whereas for symmetric silica nanomaterials the discussion covers more general forms of nanosilica, including nanoparticles, mesoporous nanomaterials and sihca nanotubes. 

Helical nanomaterials Asymmetric No Cosurfactants used to modify synthesis of regular mesoporous silica nanomaterials [26]... 

Another area which was initiated during last year is development of chiral metal oxide based nanomaterials such as chiral Ti02 nanofibres and chiral ZrOj nanotubes. It is anticipated that these chiral metal oxide nanostructures will find very important applications as asymmetric catalysts. In addition the progress in the fabrication of mesoporous silica based chiral nanostructures e.g. helical architectures) should open new opportunities in chiral separation of enantiomeric compounds. 

From the mechanical application point of view, asymmetric nanomaterials such as silica nanocoils should have a much greater compressibihty and expansibUity than symmetric nanowires or nanorods. This unique property may be employed to create unique appUcations. For example, the magnetically controlled opening and closing of pitches in the helical coils may be used as a drug deUvery vehicle, although until now this property has not been utilized, most likely due to the lack of manipulation of these asymmetric nanomaterials. 

The preparation of asymmetric nanosilica differs slightly from that of their symmetric counterparts, the main difference being an involvement of templates or anisotropic catalysts [28]. The addition of these materials initiates an asymmetric growth of nanomaterials, thus producing helical nano materials. In the template nanosiUca, a further treatment of as-made silica-like nanomaterials is required to convert them into real siUca nanomaterials. However, in the catalytic growth of heUcal nanomaterials such as nanocoils, no post-synthesis treatment is required. 

The condensation of sihca can also be controlled with surfactants or, in other words, a template effect, as demonstrated by many groups. This is because the hydrophobic force created by high-density surfactants on the surface of silica nanofibers or nanotubes is sufficient to twist the nanomaterials to become helical or asymmetric. 

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