Biomedical Engineering Reference
In-Depth Information
microfl uidics with magnetic nanosensors. There are myriad challenges to the suc-
cessful development of diagnostic assays with high sensitivity and specifi city, and
robust performance characteristics. The fundamental challenges include an ability
to detect low-expression cancer biomarkers in complex biological samples, and to
accurately and precisely quantitate the levels of these analytes.
Advances are continually being made in nanotechnology that will enable us
to engineer precisely the critical features of magnetic nanoparticles, such as
their composition, size, and surface chemistry. In this chapter, we review some of
the most recent research and development efforts to optimize the biomedical
application of magnetic nanoparticles. The text does not delve into the specifi c
chemical reactions involved in the synthesis protocols, but rather focuses on the
design schemes for the magnetic nanoparticle platforms to improve their
functionality.
In order to facilitate an understanding of the unique opportunities afforded
by magnetic nanoparticles for biomedical application, the chapter begins with
a brief overview of the fundamental concepts of magnetism and the physico-
chemical properties of magnetic nanoparticles, as well as the principles behind
magnetic resonance. Subsequently, the challenges of magnetic nanoparticle
design for systemic administration and the progress that is being made to over-
come those challenges are discussed. Finally, several technological developments
in magnetic nanoparticle-based platforms for
in vitro
molecular diagnostics are
highlighted.
5.2
Physico-Chemical Properties of Magnetic Nanoparticles
When placed in an applied magnetic fi eld with strength
H
, materials exhibit an
induced magnetization
M
characterized by
M
=
is the magnetic
susceptibility. Materials with magnetic moments aligned parallel to
H
and suscep-
tibilities on the order of 10
− 6
to 10
− 1
are described as paramagnetic. At very small
sizes (usually
χ
H
, where
χ
<
50 nm), iron oxide nanoparticles [typically magnetite (Fe
3
O
4
) or
maghemite (
- Fe
2
O
3
) nanocrystals with the oxygen ions forming a cubic lattice
with iron cations located at the interstices] exhibit unique magnetic properties
through quantum tunneling of magnetization, which gives rise to a single mag-
netic domain behavior and one large magnetic moment. As a result of the align-
ment of electron spins in the single domain, the individual magnetic dipole
moments of ferromagnetic nanoparticles are several orders of magnitude larger
than those of paramagnetic materials and, hence, exhibit superparamagnetism.
That is, the coupling interactions between the electrons within these single mag-
netic domains result in much higher magnetic susceptibilities than those of
paramagnetic materials. Thus, a relatively weak magnetic fi eld - for example,
35 Gauss - would be suffi cient to align the dipole moments of superparamagnetic
nanoparticles to form essentially a large dipole magnet while the fi eld is turned
on.
γ
