Nowadays silica particles are commercially available
in all sizes ranging from nanometer to micrometer;
however, several researchers still synthetize particles on
their own. Two main methods have been developed for
silica-particle formation: the sol-gel method and the
microemulsion method.23 In 1968, however, Stöber and
Fink24 introduced a simple synthesis of monodisperse
spherical silica particles starting with tetraethyl orthosilicate
(TEOS 98 %), deionized water, ammonia (25 %)
and absolute ethanol (99.9 %) as the alkoxide precursor,
hydrolyzing agent, catalyst and solvent. In the process,
two mother solutions are prepared, one containing
ammonia–water, and the other containing TEOS–ethanol.
The two solutions are mixed in a thermostatically
controlled water bath (50 ± 1) °C. After 60 min, the
resulting spheres are separated from the liquid phase
with centrifugation and then ultrasonically dispersed in
deionized water. Finally, the particles can be dried in an
oven at 50 °C to obtain white powder. Note that using
this method, the final particle size critically depends on
the reagent concentrations, molar ratio and reaction
temperature. As shown in Figure 1 good monodispersity
of silica spheres can be obtained with this method.25