Stober synthesis

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 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. 

Segmented gas-liquid (Taylor) flow was used for particle synthesis within the liquid slugs. Tetraethylorthosilicate in ethanol was hydrolyzed by a solution of ammonia, water and ethanol (Stober synthesis) [329]. The resulting silicic acid monomer Si (OH)4 is then converted by polycondensation to colloidal monodisperse silica nanoparticles. These particles have industrial application, for example, in pigments, catalysts, sensors, health care, antireflective coatings and chromatography. 

However, if we use the expressions for the classical nucleation rate, we also do not find the large increase in radius that is possible. Furthermore, Weres et al. (57) state that addition of NaCl to a homogeneous nucleating solution of silicic acid in water has only a moderate effect if the concentrations are below 1 M. Addition of salt to the reaction mixture of the Stober process has, however, very large effects. Clearly, classical nucleation cannot explain all the features of the Stober synthesis either. 

In the nineties, Ohmori and Matijevic used Stober synthesis [61] for the homogeneous coating of hematite (a-Fe203) particles [69], adapting the method subsequently for the preparation of silica coated iron [70]. This procedure was later improved by Thies-Wessie etal. [71]. 

Fig. 7 Stober synthesis of dye coated silica nanoparticles (DCSNs)...

The photophysical properties of dye molecule in DDSNs can also be tuned by exploiting plasmonic effects, that is by growing the silica nanoparticle around a metal core. Experimentally, such sophisticated structures are achieved by carrying out the Stober synthesis in the presence of preformed metal nanoparticles stabilized... 

Among metal oxide compositions, the chemistry of silica is the most widely studied and weU established, from the classical Stober synthesis of a silica sphere [23] to the recently developed surfactant-mediated synthesis of mesoporous silica [24, 25]. These developments were mainly enabled by the availability of stable silica precursors, their controllable hydrolysis and condensation, and the stable oxidation state of silica. Based on previously established chemistries, sol-gel or microemulsion methods were used to synthesize silica-shell-based core-shell NPs. 

Riccio, D.A., Nugent, J.L., Schoenfisch, M.H. Stober synthesis of nitric oxide-releasing S-Nitrosothiol-modifled silica particles. Chem. Mater. 23(7), 1727-1735 (2011)... 

Currently, microelectronics relies on bulk sihca as an important dielectric material that is often used as an insulating template for further reactivity. On the nanoscale, sUica can be synthesized by polymerizing silicic acid in an aqueous system, or through hydrolysis and condensation of silicon alkoxides in the Stober synthesis [51]. The mechanism of these two methods is unique. The first method is dominated by monomers and tetra-functionalized species, such that the resultant sihcate sols are uniform, which means that they are fully hydrolyzed and grow by monomer addition. In contrast, for the second method, di- and tri-functionalized species are dominant for alkoxides. Regardless of the synthesis used, these particles induce a fractal interior with minimal morphological control due to their common template, ammonium hydroxide [51]. 

Rossi, L.M., Shi, L, Quina, F.H. and Rosenzweig, Z. (2005) Stober synthesis of monodispersed luminescent silica nanoparticles for bioanalytical assays. Langmuir, 21, 4277-80. 

For the Stober synthesis the following components were mixed 0.05 g PS colloids, 0.66 ml ammonia and 0.5 g tetraethoxysilane in 1.5 ml ethanol. Scale bar 0.5 p.m. Reproduced by permission of The Royal Society of Chemistry [31]... 

To investigate the growth process in more detail we imaged aliquots of the reaction mixture after different reaction times, Fig. 4. Already after 20 min, 30 nm sized silica particles appeared on the surface of the PS colloids. After this initial fast growth, the silica grain size steadily increased until it approached 55 5 nm after 3 h. Thereafter, the size remained almost constant. The growth process is limited by lack of reaction material. To prevent detachment of the silica particles, the rough colloids were coated with a silica shell. Therefore, they were immersed in a water solution of poly (allylamine hydrochloride) and a Stober synthesis was... 

Bamakov, Y. A., Yu, M. H., and Rosenzweig, Z. 2005. Manipulation of the magnetic properties of magnetite-silica nanocomposite materials by controlled Stober synthesis. Langmuir 21 7524-7527. 

Several preparation methods of uniform solid nanoparticles have been proposed in the literature. Generally, they can be classified as belonging to one of two groups the first one is a group of methods in which the nanoparticle synthesis requires the presence of a surfactant, while in the second one the preparation procedure takes place in the absence of surfactant. Surfactant-free methods include Stober synthesis [5] and solidification in spontaneously formed emulsions (the Ouzo effect) [6]. When surfactant is used during the synthesis the solidification occurs within the emulsion droplets. 

A variant of the Stober synthesis, in which the only reaction medium used was water, was presented by Miller et al. [12]. The surfactant-free synthetic route for the fabrication of silicone nanospheres via a two-step process was developed (1) acid-catalyzed hydrolysis and condensation of 3-mercap-topropyltrimethoxysilane, followed by (2) base-catalyzed condensation [12]. This procedure resulted in a rapid formation of emulsion droplets with a narrow size distribution, which undergo further condensation with the formation of soUd spheres with an equcdly narrow size distribution. By Vcuying the total monomer concentration the control over the pcuTicle size in the rjuige of 50 nm - 3 pm cm be achieved [13]. 

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