Biomedical Engineering Reference
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sizes and the ratio of seeds/Fe(acac)
3
could further generate IOMNPs with sizes
up to 20 nm. Transmission electron microscopy (TEM) and high-resolution TEM
(HRTEM) indicate that the size-controllable IOMNPs were monodisperse and
single crystalline (Fig. 2.4c). The 16 nm Fe
3
O
4
NPs produced through this route
display the saturation magnetization
M
s
at 82 emu·g
−1
(Fe
3
O
4
), which is very close
to the value of bulk Fe
3
O
4
(90 emu·g
−1
(Fe
3
O
4
)). Fe
3
O
4
NPs > 20 nm were also
reported
via
the decomposition of Fe(acac)
3
in the presence of only oleic acid in
benzyl ether [35]. Their sizes in the range of 20-160 nm were controlled by the
reaction time and concentration of Fe(acac)
3
. But the Fe
3
O
4
NPs > 20 nm show fer-
rimagnetism at room temperature and are difficult to stabilize under the synthetic
conditions.
apart from Fe(acac)
3
, iron oleate is also a common precursor used to synthesize
iron oxide NPs with controllable sizes [36]. It is prepared from iron chloride
(Fecl
3
·6H
2
O) and sodium oleate in a mixture solvent of ethanol, water, and hexane at
70°c. Thermal decomposition of iron oleate in the reflux of different organic sol-
vents produced IOMNPs with different sizes, for example, 5, 9, 12, 16, and 22 nm
IOMNPs were yielded in the solvent of 1-hexadecene (b.p. 274°c), octyl ether
(b.p. 287°c), 1-octadecene (b.p. 317°c), 1-eicosene (b.p. 330°c), and trioctylamine
(b.p. 365°c), respectively. However, these IONPs are not the pure Fe
3
O
4
or γ-Fe
2
O
3
, but
contain their mixture in the form of (γ-Fe
2
O
3
)
1−
x
(Fe
3
O
4
)
x
where
x
varies with the size.
The IOMNP's size effect on MRI contrast was first demonstrated by cheon's
group [32, 37]. They synthesized 4, 6, 9, and 12 nm Fe
3
O
4
NPs
via
the decomposition
of Fe(acac)
3
(Fig. 2.4d). The as-synthesized NPs were made water soluble after
ligand exchange with dMSa. The dMSa-protected Fe
3
O
4
NPs were dispersed in
phosphate-buffered saline (PBS). The magnetizations of the dMSa-capped Fe
3
O
4
NPs at 1.5
T
were measured to be 25 (4 nm) to 43 (6 nm), 80 (9 nm), and 102 emu·g
−1
(Fe) (12 nm). consistent with the magnetization data, their
T
2
-weighted MR images
(same iron concentration) show significant change from white to black with the size
increasing from 4 to 12 nm, suggesting the much enhanced MRI sensitivity in larger
NPs (Fig. 2.4e). Using a 1.5
T
MRI scanner, the relaxivity
r
2
values were calculated
to be 78, 106, 130, and 218 mM
−1
·s
−1
for 4, 6, 9, and 12 nm dMSa-protected Fe
3
O
4
NPs, respectively, as shown in Figure 2.4f.
2.4.2
composition control
The magnetism of Fe
3
O
4
is based on the inverse spinel structure in the form of
(Fe
3+
)
T
(Fe
2+
Fe
3+
)
O
O
4
. Fe
2+
and half Fe
3+
occupy the octahedral (
O
) sites, while the
other half Fe
3+
take the tetrahedral (
T
) sites. as the spins from the two classes of Fe
3+
cancel each other, the overall magnetic moment of each unit of (Fe
3+
)
T
(Fe
2+
Fe
3+
)
O
O
4
is from the
d
6
configuration of Fe
2+
, which is 4 µ
B
in high spin state. considering that
the dopant substitution of Fe
2+
with M
2+
(M = Mn, co, Ni, Zn, etc.) is capable of
changing the overall magnetic moment, synthesis of ferrites provides another method
to tune the performance of IOMNP contrast agent [32, 34].
The controlled synthesis of ferrite MFe
2
O
4
MNPs can be directly modified from
the abovementioned methods used to produce Fe
3
O
4
NPs. High-quality 3-20 nm
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