Silica shells

In addition to tire standard model systems described above, more exotic particles have been prepared witli certain unusual properties, of which we will mention a few. For instance, using seeded growtli teclmiques, particles have been developed witli a silica shell which surrounds a core of a different composition, such as particles witli magnetic [12], fluorescent [13] or gold cores [14]. Anotlier example is tliat of spheres of polytetrafluoroetliylene (PTFE), which are optically anisotropic because tire core is crystalline [15]. 

Vogt, C., Toprak, M.S., Muhammed, M., Laurent, S., Bridot, J.L. and Muller, R.N. (2010) High quality and tuneable silica shell-magnetic core nanopartides. Journal of Nanoparticle Research, 12 (4), 1137-1147. 

Ge, J.P., Zhang, Q., Zhang, T.R. and Yin, Y.D. (2008) Core-satellite nanocomposite catalysts protected by a porous silica shell controllable reactivity, high stability, and magnetic recyclability. Angewandte Chemie International Edition, 47 (46), 8924-8928. 

Ikeda, S., Kobayashi, H., Ikoma, Y., Harada, T., Yamazaki, S., and Matsumura.M. (2009) Structural effects of titanium(IV) oxide encapsulated in a hollow silica shell on photocatalytic activity for gas-phase decomposition of organics. Applied Catalysis A General, 369 (1-2), 113-118. 

As mentioned earlier, biological systems have developed optimized strategies to design materials with elaborate nanostructures [6]. A straightforward approach to obtaining nanoparticles with controlled size and organization should therefore rely on so-called biomimetic syntheses where one aims to reproduce in vitro the natural processes of biomineralization. In this context, a first possibility is to extract and analyze the biological (macro)-molecules that are involved in these processes and to use them as templates for the formation of the same materials. Such an approach has been widely developed for calcium carbonate biomimetic synthesis [13]. In the case of oxide nanomaterials, the most studied system so far is the silica shell formed by diatoms [14]. 

Fig. 5.1 SEM micrograph of the silica shell of a Coscinodiscus sp. diatom cell...

In the preparation of 15 nm core-shell fluorescent silica particles, Ow et al. (2004) reported that the naked core (2.2 nm) alone produced a fluorescence intensity of less than the free dye in solution, presumably due to dye quenching. However, upon addition of the outer silica shell around the core, the brightness of the particles increased to 30 times that of the free dye (using tetramethylrhodamine-5-(and 6)-isothiocyanate (TRITC)). They speculate that shell may protect the core from solvent effects, as evidenced by a lack of spectral shift upon changing the solvent in which the particles are suspended. 

As an example of composite core/shell submicron particles, we made colloidal spheres with a polystyrene core and a silica shell. The polar vapors preferentially affect the silica shell of the composite nanospheres by sorbing into the mesoscale pores of the shell surface. This vapor sorption follows two mechanisms physical adsorption and capillary condensation of condensable vapors17. Similar vapor adsorption mechanisms have been observed in porous silicon20 and colloidal crystal films fabricated from silica submicron particles32, however, with lack of selectivity in vapor response. The nonpolar vapors preferentially affect the properties of the polystyrene core. Sorption of vapors of good solvents for a glassy polymer leads to the increase in polymer free volume and polymer plasticization32. 

Tovmachenko OG, Graf C, van den Heuvel DJ, van Blaaderen A, Gerritsen HC (2006) Fluorescence enhancement by metal-core/silica-shell nanoparticles. Adv Mater 18 91-95... 

Recently, the design of multiple-layer DDSNs becomes popular [61]. These multiple-layer configurations can be categorized as a core-shell structure. Frequently, this core-shell structure involves metal enhancement. The dye-doped silica exists either as a fluorescent core coated with a silica shell or as a fluorescent shell on a metal core. These new designs have provided DDSNs with advanced fluorescence... 

Figure 5 shows two typical core-shell structures (a) contains a metal core and a dye doped silica shell [30, 32, 33, 78-85] and (b) has a dye doped silica core and a metal shell [31, 34]. There is a spacer between the core and the shell to maintain the distance between the fluorophores and the metal to avoid fluorescence quenching [30, 32, 33, 78-80, 83]. Usually, the spacer is a silica layer in this type of nanostructures. Various Ag and Au nanomaterials in different shapes have been used for fluorescence enhancement. Occasionally, Pt and Au-Ag alloys are selected as the metal. A few fluorophores have been studied in these two core-shell structures including Cy3 [30], cascade yellow [78], carboxyfluorescein [78], Ru(bpy)32+ [31, 34], R6G [34], fluorescein isothiocyanate [79], Rhodamine 800 [32, 33], Alexa Fluor 647 [32], NIR 797 [82], dansylamide [84], oxazin 725 [85], and Eu3+ complexes [33, 83]. 

Fig. 5 Cores-shell structure of metal and DDSN complexes, (a) Metal core/ silica shell, (b) silica core/ metal shell...

The liquid-phase reaction kinetics of doped molecules in silica nanomatrixes was conducted using the metalation of meso-tetra (4-Ai,Ai,Ai-trimethylanilinium) porphyrin tetrachloride (TTMAPP) with Cu(II) as a model. To demonstrate the effect of the silica nanomatrix on the diffusion, pure silica shells with varied thickness were coated onto the same silica cores, which doped the same amount of TTMAPP molecules. The Cu(II) from the suspension could penetrate into the silica nanomatrixes and bind to the TTMAPP. The reaction rate of TTMAPP metalation with Cu(II) was significantly slower than that in a bulk solution. The increase in the thickness of the silica resulted in a consistent decrease of reaction rates (Fig. 8). 

Figure 3.17 Formation of a silica shell on the surface of citrate-stabilized Au NPs (left). micrographs of silica-coated Au NPs (right). Reprinted with permission from reference [174]. Copyright 1996 American Chemical Society.

Coombs, J., and Volcani, B. E. Studies on the biochemistry and fine structure of silica-shell formation in diatoms. Chemical changes in the wall of Navicula pelliculosa during its formation. Planta (Berl.) 82, 280-292 (1968). 

To date, the vast majority of work deals with hydrophilic GNRs functionalized by electrostatic attraction of CTAB molecules, anionic polyelectrolytes, or covalent binding of hydrophilic thiols. Hydrophobic GNRs are mainly produced by depositing octadecyl-trimethoxysilane as a coating agent onto the nanorod surface either in the presence [178, 179] or absence of a silica shell [180]. 

Diatomite A sedimentary rock formed by the burial and diagenesis of sediments consisting of abundant microscopic and amorphous silica shells from marine microorganisms. 

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