Biomedical Engineering Reference
In-Depth Information
of precursors, flow injection syntheses, electrospray syntheses and mechanochemical
processes (Laurent et al.
2008
; Lin et al.
2006
; Zheng et al.
2010
).
The simplest and most efficient chemical pathway to obtain magnetic particles
is probably the coprecipitation technique. Iron oxides, either in the form of magnetite
(Fe
3
O
4
) or maghemite (
g-
Fe
2
O
3
), are usually prepared by aging stoichiometric mix-
ture of ferrous and ferric salts in aqueous alkaline medium. The chemical reaction
of Fe
3
O
4
formation is usually written as follows:
Fe
2
+
+
2Fe
3
+
+
8OH
−
→+
Fe O 4H O
34 2
However, magnetite (Fe
3
O
4
) is not very stable and is sensitive to oxidation which
results in the formation of maghemite (
g-
Fe
2
O
3
).
The main advantage of the coprecipitation process is that a large amount of
nanoparticles can be synthesized. However, the control of particle size distribution
is limited. The addition of chelating organic anions (carboxylate or
a-
hydroxy
carboxylate ions, such as citric, gluconic, or oleic acids) or polymer surface com-
plexing agents (dextran, carboxydextran, starch, or polyvinyl alcohol) during the
formation of magnetite can help to control the size of the nanoparticles. According
to the molar ratio between the organic ion and the iron salts, the chelation of these
organic ions on the iron oxide surface can either prevent nucleation and then lead
to larger particles or inhibit the growth of the crystal nuclei, leading to small nano-
particles (Berger et al.
1999
; Laurent et al.
2008
).
Classical coprecipitation method generates particles with a broad size distribu-
tion. Synthesis of iron oxide nanoparticles with more uniform dimensions can be
performed in synthetic and biological nanoreactors, such as water-swollen reversed
micellar structures in non-polar solvents, apoferritin protein cages, dendrimers,
cyclodextrins, and liposomes (Laurent et al.
2008
).
Hydrothermal syntheses of magnetite nanoparticles are performed in aqueous
media in reactors or autoclaves where the pressure can be higher than 2,000 psi
(ca 13.8 MPa) and the temperature can be above 200°C. In this process, the reaction
conditions, such as solvent, temperature, and time, usually have important effects
on the products. The particle size of magnetite powders increased with a prolonged
reaction time and the higher water content resulted in the precipitation of larger
magnetite particles (Laurent et al.
2008
).
The sol-gel process is a suitable wet route to the synthesis of nanostructured
metal oxides. This process is based on the hydroxylation and condensation of
molecular precursors in solution, originating a “sol” of nanometric particles.
Further condensation and inorganic polymerization leads to a three-dimensional
metal oxide network denominated wet gel. Because these reactions are performed
at room temperature, further heat treatments are needed to acquire the final crystalline
state. The main parameters that influence the kinetics, growth reactions, hydrolysis,
condensation reactions, and consequently, the structure and properties of the gel are
solvent, temperature, nature and concentration of the salt precursors employed, pH,
and agitation (Laurent et al.
2008
).
The polyol process is a versatile chemical approach for the synthesis of nano-
and microparticles with well-defined shapes and controlled sizes. Selected polyols
(for example, polyethylene glycol) used as solvents exhibit high dielectric constants,
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