Biomedical Engineering Reference
In-Depth Information
engineering nanomaterials; for biological systems, the toxicity of nanomaterials
represents an ongoing and enormous challenge. Many species present in complex
media may be capable of undergoing ligand exchange reactions on the nanopar-
ticle surfaces, displacing the surface molecules and potentially leading to particle
insolubility and agglomeration. To address the issue of particle stability, one strat-
egy has been to encapsulate nanoparticles in a complete shell of another material
(often denoted as core@shell or core-shell). Nanoparticles have been encased in
a wide range of materials from polymers, noble metals, and glass (these are dis-
cussed below). The composite materials are designed to protect the magnetism of
the core material and, at the same time, to incorporate an outer shell surface that
can serve as a robust handle for further chemical modifi cations. In this section,
each of the major types of shell formation, together with some details of recent
studies in this area with more exotic materials, will be described.
14.3.5.1
Encapsulation: Silica ( SiO
2
)
Probably the most well known and widely applied method of nanoparticle encap-
sulation is the formation of glass or silica shell. Siloxane chemistry (see Section
14.3.4) provides robust surface bonds to many metal oxide nanoparticles; oxide-
based materials represent the bulk of the reports describing the encapsulation of
magnetic particles by silica shells. These core@SiO
2
materials have the added
advantage of well-known silica surface chemistry, and a wide range of chemically
functional chlorosilanes and siloxanes are commercially available. However, the
silica shell thickness can be diffi cult to control and many synthetic techniques do
not selectively coat individual nanoparticles but instead result in aggregates of
many particles conjoined by the silica.
The synthesis of core@SiO
2
materials is straightforward and, although many
variations exist, is generally known as the Stöber process (this is shown schemati-
cally in Figure 14.4 ) [99] . Commercially available tetraethylorthosilicate ( TEOS ) is
combined with a sample of metal oxide nanoparticles that have been dispersed in
water or water/ethanol. Polymerization is initiated by the addition of aqueous
Figure 14.4
General reaction scheme for the St ö ber synthesis,
during which an alcoholic solution containing water, ammonia
and an alkyl silicate are reacted to form silica spheres of
relatively uniform size [99].
