Biomedical Engineering Reference
In-Depth Information
(A)
(B)
Organic
solvent
Water
Water
Organic solvent
: Hydrophilic part,
: Hydrophobic part
FIGURE 4.3
Synthesis of template-assisted silica nanoparticles by formation of micelles and inverse micelles. (A) A micelle
in aqueous solution and (B) inverse micelle in organic solvents such as CTAB, AOT, phospholipids, and block
copolymers (PS-b-PVP, where PS is polystyrene and PVP is polyvinylpyridine) are normally used for their
formation. By adjusting the proportions of water, organic solvent, and surfactant, the size and shape of the
micelles can be tightly and reproducibly controlled.
A second important consideration regarding the method of synthesis is the potential need to
remove residual surfactant before application. Fumed silica is essentially surfactant free following
synthesis; however, both the sol
gel and template methods will require purification, the removal
of excess surfactants or reactants. For example, the template-assisted method results in surfactants
such as CTAB (cetyl trimethylammonium bromide) and AOT (aero OT, dioctyl sulfosuccinate
sodium salt). CTAB will have to be completely removed before biological application as free
CTAB results in acute toxicity at the cellular level [18] . Purification can be as simple as centrifuga-
tion, acid extraction, or osmosis. Repeated purification steps such as washing and redispersion after
centrifugation may be important to achieve a neutral pH and/or completely remove the surfactants/
contaminants necessary to avoid unintended secondary effects on biological systems.
4.3.3 Composites and functionalization
In addition to pure silica-based materials, silica is also used in combination with other materials to
increase biocompatibility. A common scenario is to coat a metal core (e.g., gold, silver, iron oxide,
or cobalt ferrite (see Table 4.1 )), with a silica shell. The shell may function to decrease acute toxic-
ity of certain metals such as cadmium and lead (e.g., quantum dots) or allow for easy surface
modification (such as antibody conjugation). In addition, the selective removal of the core can be
used to synthesize hollow typed nanoparticles [9] suitable for delivery of compounds such as thera-
peutics. Despite masking the potentially toxic core with an inert silica shell, the potential long-term
negative effects of a metal core may preclude their general use in humans. However, these types of
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