Biomedical Engineering Reference
In-Depth Information
Fig. 9.2
Mn-doped ferrite for DMR detection
.(
a
) Transmission electron microscope (TEM)
images of MnFe
2
O
4
MNPs prepared in our laboratory. The particles have a narrow size distribution
and consisted of a single crystal (
insets
). (
b
) The particles in (
a
) showed superparamagnetic behav-
ior at room temperature (Adopted from [
28
]. Copyright 2009 National Academy of Sciences, USA)
(Fig.
9.2
). Because of their larger magnetic core, these MnFe
2
O
4
nanoparticles
assumed high relaxivities with r
2
values approaching 420 s
1
:mM
1
[metal] (equal
to 6
s
1
per particle), more than eight times greater than CLIO nanopar-
ticles in metal basis (50 s
1
mM
1
[metal] or 7
10
14
L
10
16
L s
1
per particle) [
28
].
9.3.2
Fe-Core MNPs
Ferromagnetic metals, instead of their oxides, have been suggested as an ideal
constituent of MNPs for their superior magnetization. However, while Fe-core
MNPs can achieve high r
2
relaxivities [
41
], these monometallic MNPs are extremely
reactive and require protective layers to prevent rapid oxidation.
Recently, a unique 16-nm Fe-core/ferrite-shell MNP (Fe@ferrite) has been
developed for DMR applications (Fig.
9.3
)[
42
]. The particle consists of an elemen-
tal iron core (not iron oxide) and a protecting oxide shell. A novel synthetic route
was established, as summarized below, that allows for the preparation of large yet
monodisperse Fe MNPs and the growth of protective ferrite shells around existing
Fe MNPs (Fig.
9.3
a).
•
Larger Fe cores.
Fe MNPs were formed by thermally decomposing metal
complexes ŒFe.CO/
5
in the presence of surfactant (oleylamine) under air-free
condition. By increasing the reaction temperature during Fe MNP synthesis,
the particle size could be proportionally increased [
42
]. The phenomenon can
be attributed to the higher reactivity of Fe ions at elevated temperatures during
particle formation [
43
]. Applying this approach, Fe MNPs with diameters up to
18 nm could be prepared while maintaining the relative size variations <5%.
