Biomedical Engineering Reference
In-Depth Information
nanoparticles via coprecipitation methods [5, 6, 24], although in order to achieve
a better dispersion the magnetite particles are often modifi ed after their precipita-
tion [25] .
7.2.2.2
Inorganic Core with Inorganic Shell
The coating of iron oxide magnetic nanoparticles with polymers and surfactants
does not always provide a suitable protection, mainly because the organic-coated
iron oxides are not stable in air, and are easily leached by acidic solution, resulting
in their demagnetization. Another drawback of polymer-coated magnetic nanopar-
ticles is the relatively low intrinsic stability of the coating at higher temperatures,
a problem which is enhanced by the possible catalytic action of the metallic cores
[1]. Therefore, the development of other methods for protecting magnetic nanopar-
ticles against deterioration is warranted. Precious metals can be coated onto mag-
netic nanoparticles through reactions in microemulsion, redox transmetalation,
iterative hydroxylamine seeding, or other methods, so as to protect the cores
against oxidation. Although the use of several inorganic coating materials has been
investigated, the use of silica and gold protective coatings is most effective, and
this will be discussed in the following sections.
7.2.2.2.1
Coating with Gold
The preparation of iron oxide nanoparticles coated
with gold is of special interest, not only because gold stabilizes the iron core, but
also because of the potential applications of these nanoparticles in sensors, drug
delivery, and biodetection technologies. Gold is an excellent candidate because of
its easily reductive preparation, high chemical stability, biocompatibility, and its
affi nity for binding to amine and thiol terminal groups in organic molecules.
Ideally, gold nanoshells should be thin enough so as to barely alter the magnetic
properties of the magnetite core. Detailed below is a variety of synthetic methods
for the direct attachment of gold shell with the iron core. These different examples
were chosen because of the very interesting structures and morphology of the
gold-iron nanoparticle produced. Currently, however, the method of accomplish-
ing a controlled coating of individual magnetite nanoparticles, and the tunability
of their shell thickness, remain largely unresolved.
Yu
et al.
[26] prepared dumbbell - like Fe
3
O
4
@Au nanoparticles, by decomposing
iron pentacarbonyl, Fe(CO)
5
, over the surface of gold nanoparticles in the presence
of oleic acid and oleylamine (Figure 7.1). The mixture was heated under refl ux
(
300 °C), followed by room-temperature oxidation under air. The size of the Au
particles, which was tuned at 2 - 8 nm for Au and 4 - 20 nm for Fe
3
O
4
, was controlled
by adjusting the temperature at which the HAuCl
4
was injected, or the HAuCl
4
/
∼
Fe(CO)
5
1. Decomposition
2. Oxidation
Fe
3
O
4
Au
Au
Figure 7.1
The formation of dumbbell - like Fe
3
O
4
@Au nanoparticles [26] .
