Functional Mesoporous Silica Nanoparticles with a Core-Shell Structure for Controllable Drug Delivery - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 50

cores providing more space to load drug molecules. On the other hand, the
penetrating mesopores in the shells guarantee the facile transport of drug
molecules into and out of the hollow cores (Zhu, Shi, Chen, et al. 2005; Zhu,
Shi, Li, et al. 2005; Zhu, Shi, Shen, Chen, et al. 2005; Zhu, Shi, Shen, Dong, et
al. 2005; Zhao et al. 2009). Therefore, HMS nanoparticles have attracted great
and increasing attention for drug delivery.
3.2.1 Preparation of Hollow Mesoporous Silica Nanoparticles
To date, great efforts have been devoted to the preparation of HMS nanopar-
ticles with controllable particle and pore sizes (Li et al. 2003; Tan and Rankin
2005; Zhu, Shi, Chen, et al. 2005; Zhu, Shi, Shen, Dong, et al. 2005; Zhao et
al. 2009; Chen, Chen, Guo, et al. 2010; Du et al. 2011; Fang et al. 2011; Zhu
et al. 2011; Lim et al. 2012). Traditional preparation methodologies of HMS
nanoparticles are so-called soft-/hard-templating routes (Li et al. 2003; Tan
and Rankin 2005; Zhu, Shi, Chen, et al. 2005; Zhu, Shi, Shen, Dong, et al.
2005; Zhao et al. 2009; Du et al. 2011; Zhu et al. 2011; Lim et al. 2012) including
the fabrication of uniform soft/hard templates and deposition of the meso-
porous silica shells. The soft templates could be emulsion drops or micellar
aggregates. The hard templates could be Fe 2 O 3 nanoparticles, polystyrene
spheres, or carbon spheres. The hollow cores could be obtained by remov-
ing the templates via calcination or dissolution using suitable reagents or
solvents. In general, the structural parameters of HMS nanoparticles via the
soft-/hard-templating routes can be regulated by the adjustment of synthetic
conditions (e.g., reaction time, temperature, precursor concentration, etc.) to
endow materials with desired structures and properties. Zhu, Shi, Chen, et
al. (2005) reported for the first time a facile route to HMS spheres with pen-
etrating pore channels across the shells by an approach using PVP aggre-
gates and cetyltrimethylammonium bromide (CTAB) as cotemplates. Here,
PVP aggregates and CTAB surfactant serve as the templates for the cores
and mesoporous structure, respectively (see Figure 3.1). Later, they also syn-
thesized HMS nanoparticles with particle diameter of ca. 100 nm using the
colloidal carbon spheres as templates (Zhu, Shi, Shen, et al. 2005). The par-
ticle size and shell thickness of HMS nanoparticles can be turned through
changing the carbon spheres and the addition of a silica source.
Recently, many efforts have also been made to develop new strategies for
the preparation of HMS nanoparticles (Chen, Chen, Guo, et al. 2010; Fang et
al. 2011). Chen, Chen, Guo, et al. (2010) have developed a simple synthetic
strategy, namely, the structure difference-based selective etching process, to
prepare HMS nanoparticles, in which the silica core was selectively etched
away from the silica core/mesoporous silica shell structure while the meso-
porous silica shell was kept almost intact (see Figure 3.2). For this silica core-
shell template, the composition between core and shell is the same, while the
structures are different. This leads to different dissolution behaviors of the
core and shell in Na 2 CO 3 solution (Route A) and in ammonia solution under