Stober particles

Figure 8.2e had the characteristics of particles that are smaller than 70-nm radius and are synthesized from concentrations of TES around 0.17 A/. These smaller Stober particles are only roughly spherical and possess a grainy irregular surface (Figure 8.2d). However, the particles A2 prepared from a low concentration of TES and a high concentration of water (Table 8.1) are small, but almost perfect spheres with very smooth surfaces. Only the polydispersity is very high.

In Figure 8.2g a typical larger Stober particle (A6) is shown. These particles are smooth and almost perfect spheres. Comparing the shape of the Ludox particles... 

Figure 8.2b) and the Compol particles (Figure 8.2f) with the Stober particles of about the same size (Figure 8.2a and 8.2d) shows only small differences. The particles not synthesized from TES are slightly more spherical, and the surfaces are somewhat less rough. 

Interestingly, it was found that gold particles were not produced with monodisperse amorphous Si02 particles prepared by the method of Stober et al. [26]. Flence, silica... 

Thomas R. G. (1972). An interspecies model for retention of inhaled particles, page 405 in Assessment of Airborne Particles, Mercer, T. T., Morrow, P. E. and Stober, W., Eds. (Charles C Thomas, Springfield, Illionis). 

The use of silica particles in bioapplications began with the publication by Stober et al. in 1968 on the preparation of monodisperse nanoparticles and microparticles from a silica alkoxide monomer (e.g., tetraethyl orthosilicate or TEOS). Subsequently, in the 1970s, silane modification techniques provided silica surface treatments that eliminated the nonspecific binding potential of raw silica for biomolecules (Regnier and Noel, 1976). Derivatization of silica with hydrophilic, hydroxylic silane compounds thoroughly passivated the surface and made possible the use of both porous and nonporous silica particles in all areas of bioapplications (Schiel et al., 2006). 

Fluorescent silica nanoparticles, called FloDots, were created by Yao et al. (2006) by two synthetic routes. Hydrophilic particles were produced using a reverse micro-emulsion process, wherein detergent micelles formed in a water-in-oil system form discrete nanodroplets in which the silica particles are formed. The addition of water-soluble fluorescent dyes resulted in the entrapment of dye molecules in the silica nanoparticle. In an alternative method, dye molecules were entrapped in silica using the Stober process, which typically results in hydrophobic particles. Either process resulted in luminescent particles that then can be surface modified with... 

Ow et al. (2005) developed an improved method of incorporating fluorescent molecules into silica particles using a modified Stober synthesis, which resulted in both enhanced fluorescence and photostability of the encapsulated dyes. In this two-stage procedure, reactive organic dyes... 

The method used here was essentially that described by Stober et al. (4) which involves the room-temperature hydrolysis of tetra-ethylsilicate in ethanol containing varying amounts of ammonia and water, the latter two components control the final particle morphology and size. The method has been modified slightly by Bridger (2,5) and by van Helden and Vrij (3). 

The Stober method is also known as a sol-gel method [44, 45], It was named after Stober who first reported the sol-gel synthesis of colloid silica particles in 1968 [45]. In a typical Stober method, silicon alkoxide precursors such as tetramethylorthosili-cate (TMOS) and tetraethylorthosihcate (TEOS), are hydrolyzed in a mixture of water and ethanol. This hydrolysis can be catalyzed by either an acid or a base. In sol-gel processes, an acidic catalyst is preferred to prepare gel structure and a basic catalyst is widely used to synthesize discrete silica nanoparticles. Usually ammonium hydroxide is used as the catalyst in a Stober synthesis. With vigorous stirring, condensation of hydrolyzed monomers is carried out for a certain reaction time period. The resultant silica particles have a nanometer to micrometer size range. 

In 1956 Gerhard Kolbe (1) published the first results that showed that spherical silica particles could be precipitated from tetraethoxysilane in alcohol solutions when ammonia was present as the catalyzing base. Several years later, in 1968, StOber, Fink, and Bohn (2) continued in this research area and published the frequently cited original article for the preparation of monodispersed silica particles form alkoxide solutions. StOber et al. improved the precipitation process and described the formation of exceptionally monodispersed silica particles. The final particle size could be controlled over a wide range from about 50 nm to 1 1/2 p,m. Variations of the particle size could be achieved by different means, e.g., temperature, water and ammonia concentration, type of alcohol (solvent), TEOS (tetraethoxysilane) concentration, or mixing conditions. 

The original process was described by Stober, Fink, and Bohn (2). A suitable alk-oxysilane is reacted in the corresponding alcohol, water, and ammonia mixture. Usually the reaction is performed at room temperature, but higher or lower temperatures can also be applied, if so desired. Stober et al. described the influence of different alkoxides and alcohols as well as the water and ammonia concentration on the resulting particle size. A more specific example is provided next. 

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.)...

An interesting modification of the Stober silica process has been described by Unger et al. (50). By using a mixture of TEOS and an alkyltriethoxysilane they were able to synthesize monodispersed porous silica particles. The porosity is created by the alkyl groups, which act like space holder. After calcination/burnout of the organics, a well-defined porosity is left behind in the silica particles. The materials are used for very fast high-pressure liquid chromatography. 

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. 

LaMer, Monomer Addition Growth Model. Most of the recent publications (13,18,37,43-45) concerning the Stober silica precipitation describe a first-order hydrolysis of TEOS as the rate-limiting process in the silica particle precipitation. The second reaction step, the condensation reaction, was found to be faster by at least a... 

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... 

In 1968, Stober et al. (18) reported that, under basic conditions, the hydrolytic reaction of tetraethoxysilane (TEOS) in alcoholic solutions can be controlled to produce monodisperse spherical particles of amorphous silica. Details of this silicon alkoxide sol-gel process, based on homogeneous alcoholic solutions, are presented in Chapter 2.1. The first attempt to extend the alkoxide sol-gel process to microemul-sion systems was reported by Yanagi et al. in 1986 (19). Since then, additional contributions have appeared (20-53), as summarized in Table 2.2.1. In the microe-mulsion-mediated sol-gel process, the microheterogeneous nature (i.e., the polar-nonpolar character) of the microemulsion fluid phase permits the simultaneous solubilization of the relatively hydrophobic alkoxide precursor and the reactant water molecules. The alkoxide molecules encounter water molecules in the polar domains of the microemulsions, and, as illustrated schematically in Figure 2.2.1, the resulting hydrolysis and condensation reactions can lead to the formation of nanosize silica particles. 

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