Biomedical Engineering Reference
In-Depth Information
nanoparticles with a narrow size distribution and good crystallinity of individual
and dispersible magnetic iron oxide nanoparticles; however, because the process
is complicated and requires an inert atmosphere, it often takes from a few hours
to several days to synthesize the nanoparticles. In some of these preparations, it
is also unknown whether the synthesized nanoparticles can be suspended in
aqueous media, which is important in biological applications [1].
7.2.1.3
Microemulsions
Microemulsions are clear, isotropic, thermodynamically stable liquid media.
These systems are composed of at least three components - two immiscible
phases (generally water and oil) and a surfactant. It was proposed that, when water
and oil collided, they would form a microemulsion. Then, when two microemul-
sions carrying appropriate reactants were put together they would form a
surfactant-stabilized microcavity (with diameters typically ranging from 1 to
50 nm), which provide a confi nement effect that limits particle nucleation, growth,
and agglomeration. The reaction would then take place inside the microcavity
containing the reactants, wherein these microdroplets would continuously collide,
coalesce, and break again, with a precipitate fi nally forming in the micelles [16,
17]. By adding a solvent, such as acetone or ethanol, to the microemulsions,
the precipitate can be extracted by either fi ltering or centrifuging the mixture.
The size of the reverse micelle is determined by the molar ratio of water to
surfactant [17] .
An example of the microemulsion method was demonstrated by Pileni [18] and
coworkers. In their method, ferrous dodecyl sulfate, Fe(DS)
2
micellar solution was
used to produce nanosized magnetic nanoparticles with average sizes from 4 to
12 nm, and standard deviation ranging from 0.2 to 0.3. The size of the synthesized
magnetic nanoparticle can be controlled by the micellar concentration and the
temperature [18] .
Although many types of magnetic nanoparticle have been synthesized in a con-
trolled manner using the microemulsion method, the particle size and shapes
usually vary widely. The working window for synthesis in microemulsions is
usually quite narrow, and the yield of nanoparticles is low compared to other
methods, such as thermal decomposition and coprecipitation. Furthermore,
because large amounts of solvent are necessary to synthesize appreciable amounts
of material, microemulsion is not a very effi cient process and is rather diffi cult to
scale- up.
7.2.1.4
Pyrolysis
Pyrolysis is the decomposition of a material by heating it in the absence of oxygen.
Maghemite nanoparticles are usually prepared via a pyrolytic method by reacting
Fe
3+
salt and a reducing agent. In nanoparticle preparation, pyrolysis can be per-
formed in two ways:
•
Spray pyrolysis: This produces ultrafi ne particles that are usually aggregated. In
this process, a solution of precursors serves as the starting materials; these are
