Stober silica particles

Fig. 2.1.2 Reaction rate of the Stober silica particle growth reaction at a constant TEOS concentration of 0.2 mol/L each data triplei indicates the reaction temperature of 293, 313. and 333 K, respectively. (From Ref. 37.)...

Numerous techniques have been applied for the characterization of StOber silica particles. The primary characterization is with respect to particle size, and mostly transmission electron microscopy has been used to determine the size distribution as well as shape and any kind of aggregation behavior. Figure 2.1.7 shows a typical example. As is obvious from the micrograph, the StOber silica particles attract a great deal of attention due to their extreme uniformity. The spread (standard distribution) of the particle size distribution (number) can be as small as 1%. For particle sizes below SO nm the particle size distribution becomes wider and the particle shape is not as perfectly spherical as for all larger particles. Recently, high-resolution transmission electron microscopy (TEM) has also revealed the microporous substructure within the particles (see Fig. 2.1.8) (51), which is further discussed in the section about particle formation mechanisms. 

Fig. 2.1.7 Transmission electron micrograph of StOber silica particles.

Fig. 2.1.8 Transmission electron micrograph showing the internal structure of Stober silica particles. (From Ref. 51.)...

Fig. 2.1.10 Adsorption of gases on/in StOber silica particles. (Courtesy of Horst Reichert, University of Mainz, Germany.)...

StOber silica particles also show a low density of the powder as precipitated. All reported literature values are at or below a density of 2.0 g cm-3, and van Helden et al. (14,15) reported values of as low as 1.61 g cm-3. These results are in accordance with the previously discussed microporosity and TEM substructure in the particles. 

Only at calcination temperatures above 800°C does the density increase to the literature value of amorphous silica of 2.2 to 2.25 g cm-1. The exact microstructure within the Stober silica particles depends very much on the specific precipitation conditions, which are discussed in more detail in section 2.1.4. 

VSi MAS-NMR experiments by van Blaaderen et al. (11), Labrosse et al. (51), Humbert (52), and Davis et al. (53) have indicated the same porous microstruclure within the Stober silica particles as observed by TEM and the surface area analysis. The publications reported high values for the Q1 and the Q2 species, which are an indication of a very open internal structure or molecular network. Q" values of approximately 65%, Q1 of 30%, and Q2 of about 5% were reported. 

Several growth and formation mechanisms have been proposed for the formation of monodispersed Stober silica particles. Silica in general is an extremely well-studied system, and there are numerous publications with respect to the hydrolysis and condensation reaction. At present there are two major formation mechanisms that have been used to explain the formation of Stober silica particles. 

Second, nucleation and growth of Stober silica particles is modeled by a controlled aggregation mechanism of subparticles, a few nanometers in size, as for example presented by Bogush and Zukoski (19). Colloidal stability, nuclei size, surface charge, and diffusion and aggregation characteristics are the important parameters in this model. 

The formation of ordered sphere-packing structures was observed in certain rheological experiments as just described. Due to the extremely uniform size of the particles, an ordered dense packing structure will develop during sedimentation of the Stober silica particles (see Fig. 2.1.12) when the dispersion is either sterically or electrostatically stabilized. The gemstone opal is essentially based on this principle (80-88). A transmission election replica picture is shown in Figure 2.1.13. The uniform... 

The second step consisted of the coating of several Stober silica particles with APS and the synthesis of a new kind of particles from a mixture of APS and TES. The colloidal dispersions obtained were also characterized by the techniques just mentioned. 

Particle Morphology (SLS, DLS, and TEM). In this section the results of the Stober silica particles are presented, followed by the Ludox and Compol particles, and finally the particles made with APS. Then, the results relevant to a tentative formation and growth model are discussed. 

H. Boukari, G. G. Long, and M. T. Harris. Polydispersity during the formation and growth of the stober silica particles from small-angle x-ray scattering measurements. J. Colloid Interface Set, 229 129-139, 2000... 

Bogush and co-workers [24,26,27] investigated the Stdber synthesis using electron microscopy, conductivity measurements, and the (small) change in reaction medium volume. They concluded that all the TES hydrolyzes completely in the first few minutes and that the Stober silica particles are formed through a... 

Szekeres et al. (2003) prepared monolayers of Stober silica particles on the surface of water and deposited onto glass substrates by the Langmuir-Blodgett method. Prior to film formatiou the surface of the silica particles was methoxylated by washing with methanol at room temperature. This reaction... 

Xu et al. coated Stober silica particles with polyethylene glycol (PEG) in order to improve the biocompatibUity of those particles and verified this improvement using a protein adsorption test. The article then further describes possible applications of encapsulation reagents for diagnosis, analysis or other measurements inside active biological systems. 

Nozawa, K., Gailhanou, H., Raison, L., Panizza, P., Ushiki, H., Sellier, E., Delville, J.P. and Delville, M.H. (2005) Smart control of monodisperse Stober silica particles effect of reactant addition rate on growth process. Langmuir, 21,1516-23. 

Efficient intracellular delivery of the anticancer drug camptothecin (CPT) by hollow silica/titania nanoparticles has been reported [114]. Monodispersed hollow nanoparticles (about 50 nm) were prepared by titania coating of Stober silica particles followed by silica dissolution and redeposition in an ammonia solution (Figure 11.13). Surface modification of these particles with an antibody herceptm, a... 

Figure 20.3 DNP-enhanced CP (a) and CP MAS-NMR (b) spectra recorded on Stober silica particles functionalized with aminopropyl groups.

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