Biomedical Engineering Reference
In-Depth Information
7.2
Synthesis
Molecular diagnostics have provided new strategies for tailoring therapies to fi t
the needs of each cancer patient's unique biology (i.e., individualized cancer
therapy). Today, both diagnostic and therapeutic agents are increasingly prepared
with controlled properties for specialized uses, although for these agents to
perform properly it is important that the synthesized particles have an almost
uniform size and shape. The synthesis of core-shell magnetic nanoparticles
includes: (i) synthesizing the magnetic core; (ii) forming the core- shell magnetic
nanoparticles with an organic or inorganic coating; and (iii) functionalizing the
core-shell magnetic nanoparticles for biological applications.
7.2.1
Formation of the Magnetic Core
Several reviews have discussed the synthesis and characterization of the core of
magnetic nanoparticles [1, 2]. In this chapter, the discussion will be limited to
synthesis of the superparamagnetic iron oxide nanoparticles (SPIONs) most com-
monly used for biological applications. Magnetite (Fe
3
O
4
) and maghemite (
- Fe
2
O
3
)
are the two most common materials; magnetite is oxidized very easily to maghemite
so, for the sake of simplicity, we will here discuss maghemite as the iron oxide
nanoparticles, unless otherwise noted. In later sections, we will briefl y outline the
most common synthesis methods, the general synthetic mechanism, the param-
eters that affect the properties of the nanoparticles, and their advantages and
disadvantages over the other methods. Some examples of each method will also
be provided.
γ
7.2.1.1
Coprecipitation from Solution
The easiest and most convenient way to synthesize iron oxides (Fe
3
O
4
or
- Fe
2
O
3
)
in the nanometer range is via coprecipitation. In this process, Fe
2+
or Fe
3+
is added
with a base under inert atmosphere or at elevated temperatures, and the magnetic
particle is then precipitated. Two main methods are available for synthesizing
maghemite via coprecipitation:
•
The fi rst method is based on the partial oxidation of ferrous hydroxide via the
addition of Fe
2+
, a base, and a mild oxidizing agent. For example, Sugimoto
et al.
[3] reacted FeSO
4
with KOH in the presence of nitrate ion (the mild oxidizing
agent). The resultant gelatinous suspension was maintained at 90 °C in the
presence of air for several hours, in order to oxidize the magnetite to maghemite.
In this way, spherical magnetite particles with average diameters ranging from
0.03 to 1.1
γ
m, were obtained [3] .
•
The second method involves ageing stoichiometric mixtures of ferrous and
ferric ions in aqueous solution. For example, Massart
et al.
[4] reacted alkaline
Fe
2+
- Fe
3+
solutions to produce maghemite particles with good yield.
μ
