Biomedical Engineering Reference
In-Depth Information
area is the development of synthetic methods by which the surface chemistry of
magnetic particles can be modifi ed, tuned to specifi c applications, and understood
at the molecular level. Some of the chemical methods used to prepare magnetic
nanoparticles are briefl y described in this chapter, with attention being focused
mainly on the approaches developed to render the particles chemically and biologi-
cally functional.
14.2
Magnetic Nanoparticle Synthesis
Common high-temperature synthetic methods utilize the thermal decomposition
[21, 24, 26, 27] or thermal reductive decomposition [22, 23, 25] of metal precursors
in the presence of a stabilizing ligand (or surfactant) to yield metal nanoparticles
with diameters ranging from 3 to 15 nm. Typical metal precursors include carbonyl
[21] and acetylacetonate ( acac ) [25] complexes (e.g., Fe(CO)
5
, Co(acac)
2
and
Fe(acac)
3
). The ligands are generally long-chain carboxylic acids and/or amines
(e.g., oleic acid and oleylamine). These surfactants are required both to mediate
growth during the reaction and to prevent agglomeration of the prepared particles.
Shorter-chain lengths are sometimes employed to facilitate surface modifi cation
post synthesis [32], but generally chains with at least six carbons are necessary to
provide suffi cient stabilization [24, 25, 32]. The synthesis and modifi cation of
magnetic nanoparticles is a vigorously studied fi eld that continues to expand
rapidly, and the key reports outlined in this chapter are representative of a much
larger and growing body of work.
Modifi cation of the reaction parameters tunes the nanoparticle size and
shape - and therefore the magnetic moment - of a nanoparticle sample. The mag-
netic moment is intrinsically related to the composition: the incorporation of
Co
2+
into an Fe oxide matrix (i.e., CoFe
2
O
4
) increases the magnetic anisotropy rela-
tive to an Fe
3
O
4
iron oxide nanoparticle of equivalent size. The insertion of Mn
2+
(i.e., MnFe
2
O
4
), however, will cause a decrease in the anisotropy [33]. The boiling
point (BP) of the solvent (i.e., the temperature of the reaction), the relative molar
quantities of metal and ligand, mixing rate, and time each further affect the par-
ticle size and morphology. For example, when synthesizing CoFe
2
O
4
particles,
changing the solvent from phenyl ether (BP
∼
265 ° C, 30 min) to benzyl ether (BP
∼
298 °C, 2 h) causes a 7 nm increase in nanoparticle diameter [25]. It has been
shown during the synthesis of MnFe
2
O
4
nanoparticles that a surfactant to Fe ratio
of 3 : 1 yields spherical particles, while a 1 : 3 ratio results in the synthesis of cube-
like particles [34]. These examples illustrate that the choice of reaction parameters
exerts a clear control over the particles' size, shape, composition, and magnetic
properties.
Over the past decade, a wide range of magnetic nanoparticle compositions
and structures has been synthesized. The most common of these materials
are the Fe oxides (Fe
2
O
3
and Fe
3
O
4
), known for their potential for biological com-
patibility, and their corresponding ferrites (e.g., MnFe
2
O
4
and CoFe
2
O
4
). Metals
