Stober method

Generally, two common methods, the Stober method and the reverse microemulsion method are used for synthesis of silica nanoparticles. As derivatives of a sol-gel process, both methods involve hydrolysis of a silicon alkoxide precursor to form a hydroxysilicate followed by polycondensation of the hydroxysilicate to form a silica nanoparticle [44]. 

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. 

The Stober method can be used to form core-shell silica nanoparticles when a presynthesized core is suspended in a water-alcohol mixture. The core can be a silica nanoparticle or other types of nanomaterials [46, 47]. If the core is a silica nanoparticle, before adding silicon alkoxide precursors, the hydroxysilicates hydrolyzed from precursors condense by the hydroxide groups on the surface of the silica cores to form additional layers. If the core is a colloid, surface modification of the core might be necessary. For example, a gold colloid core was modified by poly (vinylpyrrolidone) prior to a silica layer coating [46]. 

The size of silica nanoparticles affects their physical, chemical, electronic, and optical properties. Proper size of silica nanoparticles is crucial for design of silica-based nanomaterials. In Stober methods, the size of silica nanoparticles is adjusted by changing the type of organic solvent, the amount of silicon alkoxide, and the... 

Figure 11.1 TEM images of silica nanoparticles of different sizes (a) 20 nm, (b) 50 nm, (c) 200 nm, (d) 290 nm, prepared with the Stober method [scale bar at the comer of (d) corresponds to 300 nm].

A recent example of how silica nanoparticles prepared by the Stober method can be used as supports was the work of Gao et al., who derivatized a silica nanoparticle first with APTMS and then with acryloyl chloride to form reactive vinyl groups.74 A polymer shell with sites imprinted with the template, TNT, was then formed around the silica nanoparticle using conventional acrylic organic polymerization procedures. The capacity and binding kinetics were shown to be significantly better than traditional imprinted particles.74... 

Polystyrene (PS) spheres were prepared with emulsifier fiee emulsion polymerization. Silica spheres were prepared following the modified StOber method. All chemicals were used without further purification. 

Following the same synthesis principle, Jaroniec et al. reported the preparation of a series of CS by carbonization of phenolic resin spheres via the modified Stober method. As shown in Fig. 2.17, all samples show spherical morphology with the average diameter of 570, 420, 370, and 200 nm for as-synthesized CS-6, CS-6-CD-4, CS-6-CD-8, CS-6-CD-12 carbon spheres, respectively. The particle... 

Liu J, Qiao SZ, Liu H, Chen J, Orpe A, Zhao D, Lu GQ (2011) Extension of the Stober method to the preparation of monodisperse resorcinol-formaldehyde resin polymer and carbon spheres. Angew Chem Int Ed 50 5947-5951... 

Figure 5.17 Schematic synthesis of silica nanoparticles by the Stober method (top) and via reverse-phase microemulsion (bottom). The scale bars represent 1000 nm and 500 nm, respectively. Adapted from ref. 51 with permission from the Royal Society of Chemistry.

Fig. 9 Van Blaaderen modification of the Stober method for the synthesis of dye doped silica nanoparticles...

The Stober method was brilliantly modified by van Blaaderen who had the idea of co-condensing fluorescent molecules with the monomeric tetraethoxysilane (TEOS) precursor during the growing step in the nanoparticles synthesis, yielding systems in which organic dyes are covalently linked to the silica matrix [67, 68]. These architectures are commonly addressed as DDSN and present a high versatility since different species can be inserted inside the nanoparticles and, moreover, the surface is still available for further functionalization (Fig. 9). 

The reverse microemulsion method is based on the controlled hydrolysis of tetra-ethoxysilane (TEOS) molecules and their ammonia catalysed condensation like the Stober method, but the reaction milieu is in this case a stable and macroscopically isotropic dispersion of a surfactant and water in a hydrocarbon. In this system the hydrolysis is confined inside the aqueous nuclei where precursors condense to form the nanoparticles. Optimized synthetic protocols and experimental conditions allow one to obtain nanoparticle samples in the dimensional range of about 15-200 nm [70, 76] (Fig. 11). 

The silica nanoparticles of about 40-nm diameter were produced by the Stober method [9] from ethanol, ammonium hydroxide and tetraethyl orthosilicate. One kind of sample was used as formed, (hydrophiUc sample), the other was surface-treated by trimethyl-silyl-A,A-dimethylcarbamate, in order to render the particles hydrophobic. The samples were used from the alcosols. The solid content of the sols was determined from the amount of solid residuum after evaporating the solvent at 80 °C. The diameter of the particles was determined from the transmission electron microscope (TEM) image of layers transferred on Formvar-coated grids. Details of sample preparation and characterization have been described in previous papers [10, 11]. 

Another method for preparing silica sols is the process described by Stober, and co-workers, known as the SFB or Stober method (see ref. 18 (p. 111)). The starting material here is a tetraalkylorthosilicate in ethanol solution water and ammonia are also present. Tetraethylorthosilicate (TEOS) is generally used. The first step is the hydrolysis of TEOS according to the following ... 

Yttrium silicate cerium phosphors of superior properties were synthesized by Marsh et al. (2002). A precipitate was obtained by adding excess ammonia to a sol obtained from a mixture of tetraethyl orthosilicate (TEOS) in ethanol and yttrium nitrate in concentrated HNO3 solution. The precipitate was stirred in 2-propanol with cerium nitrate as the precursor for the dopant (see above for syntheses with dopant salt addition), dried and fired at 1600°C/2 h. Apparently the particles were not monodisperse (as in the ammonia-mediated synthesis in the so-called StOber method see Gel Microspheres Precipitated through pH Control ). 

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