Biomedical Engineering Reference
In-Depth Information
The stabilizing surfactants typically used for each of the described methods are
fatty acids of at least six carbon atoms. Oleic acid is the most widely reported, as
it is believed that the unsaturated bonds in the chain contribute to the particle' s
stabilization. Although other surfactants have often been reported, all of the
nanoparticles generated using a high-temperature decomposition method typically
yield nanoparticles with hydrophobic ligand shells. Regardless of the stabilizing
ligand used, a water-soluble moiety is frequently desired, and modifi cation of the
as-prepared nanoparticles by thermal decomposition can be carried out in a
number of ways (for more detail, see Section 9.2.2).
9.2.1.3
Other Synthetic Methods
As noted in Table 9.1, several other methods are available that generate superpara-
magnetic iron oxide nanoparticles. Although some of their drawbacks make them
less desirable techniques, these routes are often preferred for
in situ
modifi cations
over the multistepped, post- synthetic modifi cation approach.
The
microemulsion technique
utilizes a ternary system of water, oil, and surfactant
that allows for the formation of uniform-sized droplets in which the surfactant-
stabilized nanoparticles are formed. The most commonly used method for syn-
thesizing nanoparticles in this manner is that of
reverse microemulsion
(water in
oil, w/o), where the hydrophilic acid head group is solubilized in the water droplet
with the long-chain fatty acids dispersed in the oil phase. The size of the nanopar-
ticles can be tuned by varying the water/oil/surfactant ratios. Under optimal condi-
tions, the dispersity of these nanoparticles is narrow. Whilst the preparation of
bare particles using this technique is less practical, either polymer or inorganic
shells can be incorporated
in situ
using this technique (see Section 9.2.2). Again,
the most signifi cant drawbacks are the low material yield and the large volume of
solvents required.
The
hydrothermal method
also has some benefi ts, particularly when tailoring the
surface coatings of the nanoparticles. Here, nanoparticles are formed by placing
all of the reactants (e.g., iron salt, 1,2-diol, stabilizing surfactant) into an autoclave
and heating for a defi ned period of time, depending on the desired fi nal nanopar-
ticle size. Although this method is still fairly new, and very few reports of synthetic
modifi cations have been made, it shows much promise for the formation of car-
bon-encased iron oxide (as described in more detail in Section 9.2.2).
9.2.2
Functionalization of Magnetic Nanoparticles
For most sensing applications, the nanoparticle's utility lies in both its magnetic
character and its surface functionality. Although the preparation of iron oxide
cores of various sizes, shapes, and magnetic susceptibility is relatively straightfor-
ward (see Section 9.2.1), further modifi cations are generally necessary to add the
required affi nity ligand to the particle surface for sensing applications. Surface
modifi cation can have a mild to dramatic effect on the core magnetic character;
consequently, the selection of an appropriate surface coating must take these
