Biomedical Engineering Reference
In-Depth Information
example, of a stable Fe
3
O
4
particle dispersion (ferrofl uid) prepared by a facile
method. As many methods involve the preparation of stable ferrofl uids in organic
solvents, a general procedure that allows the transform of hydrophobic particles
to hydrophilic particles is provided in Section 12.3.2.2.
12.3.2.1
Surfactant - Assisted Dehydration
Water - stable, citrate - capped magnetite (Fe
3
O
4
) nanoparticle dispersions have been
prepared by the reaction at 100 °C of iron sulfate (FeSO
4
) with sodium citrate in
alkaline water [74]. In a typical procedure, 1 mmol C
6
H
5
Na
3
O
7
· 2H
2
O (citric acid,
trisodium salt dehydrate), 4 mmol NaOH and 0.2 mol NaNO
3
were mixed in 19 ml
deionized water. The mixture was heated to 100 °C, and formed a transparent
solution. A 1 ml aliquot of FeSO
4
· 4H
2
O (2 mmol) aqueous solution was added
rapidly to the solution and the suspension maintained with stirring at 100 ° C for
1 h. The precipitate was separated from the mother liquor by using a magnet,
washed several times with water and then redispersed in water. The diameter of
the Fe
3
O
4
particles could be controlled in the range 20-40 nm by varying the
experimental parameters, with 20 nm particles showing the greatest stability in
water. The concentration of Fe
2+
was found to be a key factor for controlling Fe
3
O
4
particle size; a reduction from 0.1 to 0.02
M
allowed an increase in particle size,
from approximately 20 to 40 nm. The reason for this is that the ferrous concentra-
tion strongly affects the nucleation and growth rate of Fe
3
O
4
nanoparticles. A
higher initial Fe
2+
concentration leads to smaller particles due to the formation of
a larger number of seeds, providing a higher particle concentration and thus
smaller particles.
Fe
3
O
4
is formed as a result of the dehydration reaction of ferrous hydroxide and
ferric oxohydroxide (as represented by Equation 12.7), in which the latter com-
pound is produced by the partial oxidation of ferrous hydroxide by O
2
dissolved in
water:
(
)
+
Fe OH
2
FeOOH
→
Fe O
+
2
H O
(12.7)
2
34
2
- Fe
2
O
3
cannot exactly be
excluded, the remarkable point of this method is that, due to the presence of well-
capped citrate molecules on the particle surface, the particles acquire a stability in
water which exceeds one month.
Although a partial superfi cial oxidation of Fe
3
O
4
to
γ
12.3.2.2
Hydrophobic - Hydrophilic Phase Transfer
When used in biomedical applications, hydrophobic metal oxide nanoparticles
must fi rst be transferred to the aqueous phase. This is not a minor problem, as
phase transference often results in multiple nanoparticles being collectively coated
within an envelope of the coating [75]. However, a collective coating can negate
any benefi t of the initial particle uniformity. Many methods have been developed
for this, although most are not suffi ciently general to be used on any system [76];
alternatively, the result may be low concentrations of nanoparticles being phase-
transferred. Krishnan [77] has recently reported a method that allows for high
