Biomedical Engineering Reference
In-Depth Information
have lateral dimensions in the nanometer scale and longitudinal dimensions
which range from hundreds of nanometers to hundreds of microns. Such charac-
teristics give aspect ratios (length to diameter) of up to several thousands. The
shape of the particles strongly affects the magnetic anisotropy, opening new per-
spectives in the control and tuning of magnetic properties [10 - 12] .
In this chapter, we will describe approaches to synthesis and magnetic charac-
terization of 0-D and 1-D nanostructured materials, with some attention to inter-
mediate dimensionality, as magnetic nanocomposite materials and nanoparticles
with core- shell morphology.
Many research investigations have been conducted on nanostructured magnetic
metals such as Fe, Co, and Ni [6, 13-16]. However, nanostructured
metal oxides
are more stable and allow for a relative tunability of the magnetic properties, thus
demonstrating great potential for applications. In particular, iron oxides with
spinel structures (MeFe
2
O
4
, Me = Fe, Co, Mg, Mn, Zn, etc.) have been intensively
studied, both in terms of the fundamental relationships between their magnetic
properties and crystal chemistry [4, 17-21], and for applications in fi elds as diverse
as catalysis, medical diagnostics, drug delivery, and environment protection
[22 - 26] .
In the present chapter, we will discuss this very large and promising class of
materials, with special regard to the most common spinel iron oxides - that is,
magnetite (Fe
3
O
4
) and maghemite (
- Fe
2
O
3
). Although during the past few years
many reviews have been published on this class of materials, their expanding fi eld
of applications, especially in biomedicine and diagnostics [23, 24, 26], and the
explosive progress in the ability to tune and control their magnetic properties,
requires continuous updating. Moreover, it is fundamental to investigate in more
detail the close link existing between the magnetic properties and the preparation
method of the material, a feature which up to now has been undervalued but which
can greatly affect the magnetic behavior of the nanomaterials [27].
There are two different approaches to the synthesis of nanostructured materials:
the “ top - down ” approach, which utilizes physical methods; and the “ bottom - up ”
approach, which employs solution-phase colloidal chemistry. The advantage of the
physical methods is the production of a large quantity of nanomaterials, but the
synthesis of uniform-sized nanoparticles and their size control is very diffi cult to
achieve using the top-down approach. In contrast, colloidal chemical synthetic
methods are more suited to the synthesis of uniform nanoparticles with controlled
particle size, shape, structure, and composition [28]. Hence, in this chapter we will
restrict discussions to solution-phase colloidal chemical methods.
In order to design magnetic materials for specifi c applications and set up
convenient synthesis procedures, a basic knowledge of magnetism in nanostruc-
tures is essential. In Section 12.2, the fundamental concepts of magnetism
are introduced. The direct correlation between crystalline structure, morphology
(size, size distribution, and shape) and magnetic properties relevant to applications
is discussed on the basis of magnetic anisotropy. Attention will also be given
to the most common approaches used to study the magnetic behavior of
nanostructures.
γ
