Silica-Based Nanoparticles: Design and Properties - Advanced Fluorescence Reporters in Chemistry and Biology - page 233

2.1.2 Reverse Microemulsion Method
The reverse microemulsion method is another common method for synthesis of
silica nanoparticles. This synthetic process takes more time than the St¨ber method.
However, the reverse microemulsion method can overcome the limitations of the
St¨ber method [ 25 , 55 - 57 ] by producing silica nanoparticles with a uniform size
distribution. Furthermore, this method can efficiently dope dye molecules into the
silica nanoparticles. Reverse microemulsion is also known as water-in-oil micro-
emulsion. In this system, oil (or a nonpolar organic solvent), water, surfactant and
cosurfactant form an isotropic, transparent and thermodynamically stable single
phase liquid mixture (Fig. 1 ). In a typical reverse microemulsion, micelles contain-
ing water droplets are stabilized by surfactant/cosurfactant molecules, which reduce
the interfacial tension between water and the organic solvent. When doping dye
molecules into the silica nanoparticles, the water soluble dye will be dissolved in the
water droplet and eventually doped into the silica nanoparticles. Since water and the
hydrolysis catalyst (mostly ammonium hydroxide) exist in the micelles, the added
silicon alkoxide cannot be hydrolyzed until it diffuses into the micelles. Therefore,
the polycondensation of the hydroxysilicates takes place within the micelles. The
growth of silica nanoparticles is confined by the size of micelles. The resultant silica
nanoparticles have narrow size distribution. However, the size of the nanoparticles is
restricted by the size of micelles [ 25 , 56 ]. It is difficult to produce large silica
nanoparticles [ 25 ]. In addition, the surfactant used in a reverse microemulsion
method cannot be completely removed from the silica nanoparticles.
2.1.3 Manipulation of the Size of Silica Nanoparticles
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 St¨ber methods, the size of silica nanoparticles is adjusted
by changing the type of organic solvent, the amount of silicon alkoxide, and the
Fig. 1 Schematic diagram of water-in-oil microemulsion for synthesis of silica nanoparticles