Biomedical Engineering Reference
In-Depth Information
−
3
+
+
+⎯→
2
8
OH
2
Fe
Fe
⎯⎯
Fe O
+
4
H O
(9.1)
34
2
Adjustment of the ratios of Fe
3+
to Fe
2+
, the iron salt source (chlorides, sulfates,
nitrates), the solution pH, and the presence of an organic stabilizing ligand
will all have an effect on the fi nal size, shape, dispersion, and stability of the
Fe
3
O
4
nanoparticles formed [14]. As unprotected Fe
3
O
4
nanoparticles are vulner-
able to oxidation, a controlled oxidation can be purposely carried out to form
-
Fe
2
O
3
(maghemite) nanoparticles by dispersion in an acidic medium and heating
in the presence of iron(III) nitrate. The
γ
- Fe
2
O
3
nanoparticles obtained in this
manner are more stable over a broader pH range and are more resistant to
aggregation, although with a lower saturation magnetization than the Fe
3
O
4
nanoparticles.
Preparations of iron oxide nanoparticles using this technique can generate
superparamagnetic nanoparticles which range from 2 to 17 nm in diameter,
simply by adjusting the various synthetic conditions [14]. The typical measured
saturation magnetization of unfunctionalized Fe
3
O
4
nanoparticles depends
strongly on the overall size, and also appears to depend on the preparation tem-
perature [26], although reported values are typically between 30 and 70 emu g
− 1
,
slightly lower than the experimental value of 82 emu g
− 1
obtained for bulk Fe
3
O
4
.
The specifi c surface area of nanoparticles of
γ
7 nm diameter is up to 124 m
2
g
− 1
,
with higher values being obtained from smaller nanoparticles, albeit with the
trade-off of lowered saturation magnetization values. Nanoparticles prepared
using this method can be functionalized with a sensing moiety of interest either
in situ
or immediately following synthesis, and are described in more detail in
Section 9.2.2 .
∼
9.2.1.2
Thermal Decomposition
The synthesis of magnetic nanoparticles using a thermal decomposition method
offers the most versatility, as the technique allows for the greatest degree of control
over the particle size, shape, size distribution, and crystallinity. An enhanced
control over particle saturation magnetization and susceptibility is also possible,
and has great importance for many sensing applications. The preparation of
monodisperse Fe
3
O
4
and
- Fe
2
O
3
by the high-temperature decomposition of an
organic iron complex has been reported [29-31], where an iron precursor [e.g.,
Fe(III) acetylacetonate, Fe(II) oleate, Fe(CO)
5
] is dispersed in a high-boiling solvent
(e.g., benzyl, octyl, or phenyl ether) in the presence of a stabilizing surfactant (e.g.,
oleic or lauric acid) in an inert atmosphere. Sun
et al.
described the decomposition
of Fe(III) acetylacetonate in the presence of oleic acid, oleylamine, and 1,2-hexa-
decandiol in either phenyl or benzyl ether. Under varying conditions of heating
ramp rate and refl ux times, nanoparticles of various sizes with a very narrow size
dispersity were produced. This method of preparing nanoparticles can also be
scaled up, with no effect on the quality of material produced. If larger particle sizes
are desired, the smaller (4-6 nm-diameter) nanoparticles may be used as seeds in
the synthesis of nanoparticles of up to 20 nm diameter.
γ
