Biomedical Engineering Reference
In-Depth Information
7.3
Magnetic Nanoparticle Tags
Magnetic particles are commonly used in a variety of different applications such as
cell sorting, magnetic resonance imaging, data storage, environmental remediation,
and other applications. Here, we focus on using MNPs as tags for proteomic analysis
in biomedicine. In this section, the MNP design requirements for applications to
magnetically responsive nanosensors are discussed. The ideal design parameters,
however, are often in conflict, thus requiring optimization choices to be made.
For example, the highest signal per particle would originate from relatively large
magnetic particles (on the order of 1m or larger). Larger particles, however, are
not necessarily optimal because they tend to settle as they lack colloidal stability
and they have significantly slower diffusion times. Therefore, in selecting the ideal
MNP tag, competing factors must be considered and trade-offs must be made.
7.3.1
Superparamagnetism
An important design requirement for this technology is that the magnetic tags must
not aggregate, chain, or precipitate during the course of any given experiment. Thus,
the particles used in magnetically responsive and proximity-based detection systems
ideally should be superparamagnetic, where the volume of the ferromagnetic core
is so small that thermal energy alone is large enough to cause the magnetic moment
of the cores to fluctuate rapidly. The average magnetic moment over time of any
given superparamagnetic core is therefore zero, resulting in zero remnant moment.
However, when an external magnetic field is applied, the nanoparticles magnetize
with a much greater magnetic susceptibility than paramagnetic materials.
More specifically, a superparamagnetic material is a magnetic material of such
small size that at temperatures below the blocking temperature, it behaves like a
paramagnetic material. As the size of these superparamagnetic particles increase,
they lose their superparamagnetic nature and become ferromagnetic. This limit is
known as the “superparamagnetic radius.” This superparamagnetic radius can be
calculated by the following equation:
v
0
e
f V
k
B
T
P
D
(7.2)
where P is the probability per unit time that the magnetization will change direction,
v
0
is the attempt frequency (
10
9
s
1
), fV is the free-energy barrier that the
particle must overcome in order for the moment of the particle to switch directions,
k
B
is Boltzmann's constant, and T is the temperature. Therefore, if iron oxide
nanoparticles, for example, are to be utilized in our assay, it is favorable to use
them at sizes smaller than the critical size (so they remain in the superparamagnetic
regime). The challenge in using such small MNPs, however, is that as the size of the
