Functional Mesoporous Silica Nanoparticles with a Core-Shell Structure for Controllable Drug Delivery - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 48

silica is known to cause adverse health effects such as silicosis, whereas
amorphous silica is considered to be biocompatible and nontoxic (Park et al.
2011; Uboldi et al. 2012). Hence, amorphous silica, especially in the form of
nanoparticles, has been employed in a wide range of industrial and biomedi-
cal applications including cosmetics, food additives, and drug delivery sys-
tems (Hirsch et al. 2003; Huang et al. 2005; Dekkers et al. 2010; Shi et al. 2010).
In general, amorphous silica is highly inert and stable in comparison with
organic systems such as liposomes, dendrimers, and polymers, which can
be suitable carriers for a wide range of drug delivery systems in the physi-
ological environment (Barbé et al. 2004). On the other hand, amorphous sil-
ica nanoparticles are degradable and undergo hydrolysis to form silicic acid,
Si(OH) 4 , in aqueous solution. In vivo , silicic acid from the degradation of silica
nanoparticles can diffuse through the bloodstream or lymph, and is excreted
through urine (Finnie et al. 2009). Furthermore, for the application in drug
delivery, silica nanoparticles can be surface functionalized with adequate
organic reagents due to the abundant reactive silanol groups (-OH) on the
surface and, by doing so, an active layer for the absorption of desired drug
molecules could be generally produced.
Currently, the simplest way to synthesize amorphous silica nanoparti-
cles is the Stöber method based on the sol-gel technique (Stöber et al. 1968).
Using this procedure, amorphous silica nanoparticles with a large variety of
sizes can be synthesized by controlling the synthetic conditions. However,
the pore volume and surface area of these nanoparticles are usually lower,
which limits the drug adsorption capacity on silica nanoparticles. Therefore,
amorphous silica nanoparticles with high surface area and pore volume are
pursued for high-efficient drug delivery.
3.1.2 Mesoporous Silica Nanoparticles
Ordered mesoporous silica materials, a new family of molecular sieves called
M41S, were discovered by Mobil researchers in 1992 (Kresge et al. 1992). They
are synthesized in the presence of assembled surfactant micelle templates,
which serve as structure-directing agents for polymerizing silica component
by electrostatic interaction. The most well-known and common mesoporous
silica includes MCM-41, MCM-48, and SBA-15. Mesoporous silica materi-
als are amorphous and have unique properties including high surface area,
large pore volume, uniform and tunable pore size, nontoxic nature, well-
defined surface properties for functionalization, and good biocompatibil-
ity (He and Shi 2011). On the one hand, textural properties of mesoporous
silica (such as high surface area, large pore volume, and mesoporous chan-
nels with tunable pore size) provide the possibility to load a high amount of
drugs within mesoporous silica carriers (Muñoz et al. 2003). On the other
hand, there are abundant silanol groups on the surfaces of mesoporous
channels and the outer surfaces of mesoporous silica nanoparticles, which
facilitate the surface functionalization to allow for a better control over the