Biomedical Engineering Reference
In-Depth Information
nanoparticles can be redispersed in water, without the need for further surfactants,
to produce stable magnetic fl uids [61, 132] .
Another approach to silica coating is that of microemulsion [135-137], when
micelles or inverse micelles are used to deposit and control the coating. Water- in -
oil microemulsions require three components, namely water, oil, and amphiphilic
surfactant molecules. During the process, the surfactant lowers the interfacial
tension between the water and oil, which results in the formation of a transparent
solution. In this case, the water nanodroplets present in the bulk oil phase serve
as nanoreactors for the synthesis and coating of nanoparticles. As an example, Tan
et al.
reported the preparation of iron oxide nanoparticles with a uniform silica
coating as thin as 1 nm, by using a base-catalyzed hydrolysis and the polymeriza-
tion reaction of TEOS in a microemulsion [135]. One advantage of the microemul-
sion method is that it also facilitates the incorporation of biological macromolecules,
since the nanocomposites formed are porous [138]. A number of interesting iron
oxide-based magnetic nanocomposites with silica-enriched surface layers have
been prepared using a modifi ed microemulsion technique, which involved the
aerosol pyrolysis of an iron ammonium citrate/TEOS solution. This approach led
to the production of hollow magnetic spheres and nanomagnets, dispersed in
dense submicrospherical silica cages [139, 140].
Finally, one quite successful approach to coating is based on the deposition of
silica from silicic acid solutions. This technique is relatively easy to apply, and also
allows the thickness of the silica coating to be controlled by changing the ratio of
SiO
2
/Fe
3
O
4
, or by repeating the coating procedure when necessary [128, 140]. One
of the earliest reports on this method was made by Philipse
et al.
, who dispersed the
bare magnetic nanoparticles by using tetramethylammonium hydroxide to form a
stable magnetic fl uid that was subsequently treated with sodium silicate [128].
Ferumoxsil [17], a well-known, orally administered clinical contrast agent (see
Section 4.5) is based on silica-coated magnetite particles, which are functionalized
with [3 - (2 aminoethylamino)propyl]trimethoxysilane [141, 142] . Amino - silane -
functionalized silica-coated nanoparticles can be quite easily further functional-
ized. As an example, in one report amino-silane coatings on magnetic nanoparticles
were activated using glutaraldehyde, which served as a linker for the binding of
Hepama-1, a humanized monoclonal antibody directed against liver cancer, This
process resulted in new immunomagnetic nanoparticles for the targeted MRI of
liver cancer [130] .
Thus, whilst silica coating represents a very convenient and widely used approach
for the protection and stabilization of magnetic nanoparticles, it does have
certain drawbacks. For example, silica is not stable under basic conditions and is
usually porous; consequently, oxygen and other species may be able to diffuse
through the materials, with the resultant oxidation and deterioration of the mag-
netic core.
Coating with inert precious metals represents another effective means to protect
the magnetic cores against oxidation and to stabilize the aqueous solutions, and
several such methods have been reported. Reverse micelle (microemulsion)
methods can be used to deposit a gold coating on iron nanoparticles [36, 143-145];
in one example, a series of iron nanoparticles coated with gold of varying shell
