Biomimetic Preparation and Morphology Control of Mesoporous Silica - Biomimetics: Advancing Nanobiomaterials and Tissue Engineering - page 308

13.4.1.1 MCM-41

A typical synthesis of MCM-41 requires a minimum of four reagents: a
solvent (water and/or ethanol), a silica precursor (tetraethyl orthosilicate
(TEOS), tetramethyl orthosilicate (TMOS), tetrabutyl orthosilicate (TBOS)),
and an ionic (anionic or cationic) or non-charged surfactant, and a cat-
alyst. Depending on the protocol, the reaction could occur in an acidic
or basic medium, with different silica/surfactant ratios. The mixture is
stirred, aged at room temperature or around 100°C, and placed in a static
autoclave for several hours. The surfactant template is removed by calci-
nation. Novel routes have been proposed based either on a nonsurfactant
templated method [45, 46] or on a polyelectrolyte/hexadecyltrimethyl
-ammonium bromide method [47].
An alternate route [48, 49] involves a microwave treatment of a precur-
sor gel fi rst heated by microwave radiation around 100-150°C followed by
conventional heating. The advantages of the microwave-assisted prepara-
tion over the conventional hydrothermal method is the signifi cant decrease
in reaction time, and a more homogeneous heating which leads to better
control of the texture and morphology. However, it has been superseded
by faster methods which produce within a few hours the same, if not bet-
ter, quality materials.

13.4.1.2 SBA-15

Zhao et.al reported the syntheses of well-ordered hexagonal mesoporous
silica structures (SBA-15) with tunable large uniform pore sizes (up to ~ 30
nm) which are obtained by use of amphiphilic block copolymers [50, 51]
as organic structure-directing agents. In particular, poly(alkylene oxide)
triblock copolymers such as poly(ethylene oxide)-poly(ethylene oxide)-
poly(ethylene oxide) (PEO-PPO-PEO) are good candidates, because of
their mesostructural ordering properties, amphiphilic character, low-cost
commercial availability, and biodegradability. Using aqueous acidic con-
ditions (pH ~ 1) and dilute triblock copolymer concentrations, SBA-15 has
been synthesized with a highly ordered two-dimensional (2D) hexagonal
(p6 mm) mesostructure and thick, uniform silica walls (3 ~ 6nm). The
thick silica walls, in particular, are different from thinner-walled MCM-41
structures made with conventional cationic surfactants, and lead to greater
hydrothermal stability on the part of SBA-15. The pore size and the thick-
ness of the silica wall can be adjusted by varying the heating temperature
(35 ~ 140°C) and time (11 ~ 72 hours) of SBA-15 in the reaction solution.
SBA-15 can be synthesized over a range of reaction mixture compo-
sitions and conditions. Use of concentrations of block copolymer higher
than 6 wt% yields only silica gel or produces no precipitation of silica,
whereas concentrations of copolymer below 0.5 wt% result in only amor-
phous silica. Preparation of SBA-15 has been achieved with reaction tem-
peratures between 35 ° C and 80 ° C. At room temperature, only amorphous