Biomedical Engineering Reference
In-Depth Information
nanoparticles were synthesized by thermal decomposition at high temperature,
using an iron organic precursor in an organic medium and following the proce-
dure described in Section 12.4.1.2. Oleic acid was used as the surfactant, and in
all cases the Fe : oleic acid molecular ratio was maintained at 1 : 3. The report
referred to three samples of different particle size, with XRD patterns showing all
three to consist of highly crystalline magnetite particles. A subsequent TEM
analysis showed that the particles were poly-disperse below 20% and consisted
of a mixture of diamond, cubic, and triangular shapes [102]. Magnetization
versus temperature measurements showed the
M
s
values to be almost size-
independent and close to the saturation magnetization of the bulk material
(
M
s
bulk
1
) [30, 47, 51]. The latter result was surprising as
M
s
bulk
in
Fe
3
O
4
was not achieved up to a particle diameter of 150 nm [103]. This suggested
that the crystal fi eld associated with the new O
2−
surface ligands of oleic acid coor-
dinated to Fe surface cations resembled that of the Fe bulk cations, and this
induced a reduction in the surface magnetic disorder.
H
c
was seen to increase with
the increase in particle size, most likely due to shape anisotropy associated with
the faceted growth of the particles [102]. These results supported the idea that the
oleic acid molecules were bonded covalently to the nanoparticles, and were able
to reduce the surface spin disorder.
=−
78
90
emug
−
12.4.1.4
Anisometric Maghemite (
γ
- Fe
2
O
3
) Particles
As described in Section 12.2.2, the morphology and shape of magnetic nanocrys-
tals represent potential tools for tuning the magnetic properties of the materials.
The formation of monodisperse, spherical magnetite nanoparticles by using
spherical micelles as nanoreactors was detailed in Section 12.4.1.1, when low
surfactant concentrations were used. Conversely, by increasing the surfactant
concentration and/or by adding salts or alcohols, the micelles were seen to grow
to a rod-like shape, as predicted by the micellization theory [104]. This anisometric
shape could be taken as an ideal template for preparing anisometric structures.
Moreover, the surfactant molecules present in the system may selectively interact
with preferred crystallographic planes of the forming inorganic nanostructures,
thus originating an oriented growth of crystalline nuclei with new morphologies
[105, 106] .
In this connection, Wang and coworkers [83] reported a systematic variation of
the shape of
- Fe
2
O
3
nanostructures by using an anionic surfactant, sodium dodecyl
sulfate (SDS), in SDS/
n
-octanol/water microemulsions. By adjusting only two
parameters - reaction time and SDS concentration - the group succeeded in con-
trolling the formation of rods and multipod shapes, and in studying the infl uence
of anisometric shape on the magnetic properties.
The samples were prepared by the addition of FeSO
4
to a microemulsion formed
by mixing (using an ultrasonic oscillator) SDS, deionized water and 1-octanol, and
setting the pH value at 10.8 by the dropwise addition of an ammonia solution. The
mixture was then held at 40 °C for 3, 4, and 5 h under continuous stirring. The
precipitate was then separated and purifi ed.
γ
