Silica nanocoils

Figure 3.13 (Top) TEM micrographs of (a-i) silver nanocoils prepared by backfilling, (j-k) helical mesoporous silica grown inside nanochannels of different sizes (indicated below each micrograph). (Bottom) Schematic of differing confined nanostructural evolution with decreasing diameter of the nanochannels [56],...

The newly developed nanosilica discussed here can adopt many forms, including nanocoils, helices, nanoparticles and nanotubes, that differ widely from the other more traditional silica materials that have long been recognized. For example. 

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. 

Figure 2.2 A silica nanocoil in two states-expanded and compressed. The inner or protected surface and the outer or exposed surface are shown.

Figure 2.3 Illustration of an asymmetric silica nanocoil catalyzed with a cubic cobalt oxide nanoparticle. Note the edges of a marquise-like cross-section.

Silica nanocoils/springs Asymmetric Yes (Au) Liquid vapor-soUd growth mechanism. Milder temperatures needed to produce two types of nanowire single and multiple intertwined [20]... 

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. 

In recent years, asymmetric nanosihca have not been widely used, most likely due to a lack of convenient ways in which they can be manipulated. Nonetheless, these materials should have an effective application potential in asymmetric catalysis, although their sizes (e.g. nanocoils) must be tightly controlled. The details of asymmetric colloids of many compositions have been discussed. For example, Correa-Duarte et al. described the creation of asymmetric functional colloids through selective hemisphere modification [54], a method which could easily be expanded for the modification of silica nanoparticles and asymmetric nanosihca. 

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. 

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