152
H. Khurshid et al.
Another salient feature that can be elucidated from the simulations is the distinct
behavior of the inner and outer surfaces along the hysteresis loop. By looking only
at the global shape of the hysteresis loops of surface spins, it might be concluded
that surface spins behave similarly in solid and hollow MNPs since the coercive field
and loop shifts for the surface contribution are almost identical for both morphologies. However, the core contributions are different and, therefore, the difference in
magnetic behavior between both morphologies have to ascribed to the additional
inner surface spins in the hollow nanoparticles. In fact, by computing separately the
contribution of the spins at the core and at the inner and outer surfaces to the total
magnetization this can be corroborated. The results presented in Fig. 6.9b show that
inner surface spins are more easily magnetized than spins at the outer surface, and
that their associated hysteresis loops present a reduced vertical shift. The reversal
behavior of the interior spins is clearly influenced by the existence of additional
surface spins at the inner surface of the hollow nanoparticle. A further confirmation
can be obtained by detailed inspection of snapshots taken (see insets in Fig. 6.9b
for two representative examples) at different points of the hysteresis loop. These
provide a clear indication that the change in dynamic properties of spins at the inner
and outer surfaces is a consequence of the different range of effective energy barriers
governing their relaxation.
6.6 Applications
Hollow MNPs, in general, present a lot of promising applications in a wide variety
of areas including biomedicine, data storage, capacitors, etc. [22, 27, 65].
On the field of biomedicine, one of the most promising applications of hollow
MNPs is in targeted drug delivery. This is a method of delivering medication to
a patient based on the use of MNPs that can be externally guided (i.e., through
magnetic fields), so that they release the therapeutic drug only into the affected
area that needs to be treated, maximizing this way the efficiency of the medication
and minimizing collateral damage [66] (see Fig. 6.10). Conventionally, the drug is
attached onto the surface of the MNPs and released by an external stimuli (e.g.,
pH change, heat, etc.) once in the target area. For an efficient MNPs mediated drug
delivery, several factors need to be optimized, including the magnetic response of the
MNPs against an external magnetic field (which is proportional to their saturation
magnetization), the drug loading capacity of the MNPs (which depends on their
surface area), the drug release capacity and rate, etc. To this respect, hollow MNPs
present a series of advantages compared to conventional solid MNPs for drug release
treatment. On the one hand, the surface area available to attach drugs on hollow
MNPs is much higher than in the case of solid MNPs, because of the existence of
both the inner and outer surfaces, increasing their loading capacity. Since the drug
can be encapsulated inside the hollow MNP instead of being attached to the surface,
the drug is effectively “camouflaged” and protected on its way to the target [26].
All these results have led to several groups investigating different types of hollow
H. Khurshid et al.
Another salient feature that can be elucidated from the simulations is the distinct
behavior of the inner and outer surfaces along the hysteresis loop. By looking only
at the global shape of the hysteresis loops of surface spins, it might be concluded
that surface spins behave similarly in solid and hollow MNPs since the coercive field
and loop shifts for the surface contribution are almost identical for both morphologies. However, the core contributions are different and, therefore, the difference in
magnetic behavior between both morphologies have to ascribed to the additional
inner surface spins in the hollow nanoparticles. In fact, by computing separately the
contribution of the spins at the core and at the inner and outer surfaces to the total
magnetization this can be corroborated. The results presented in Fig. 6.9b show that
inner surface spins are more easily magnetized than spins at the outer surface, and
that their associated hysteresis loops present a reduced vertical shift. The reversal
behavior of the interior spins is clearly influenced by the existence of additional
surface spins at the inner surface of the hollow nanoparticle. A further confirmation
can be obtained by detailed inspection of snapshots taken (see insets in Fig. 6.9b
for two representative examples) at different points of the hysteresis loop. These
provide a clear indication that the change in dynamic properties of spins at the inner
and outer surfaces is a consequence of the different range of effective energy barriers
governing their relaxation.
6.6 Applications
Hollow MNPs, in general, present a lot of promising applications in a wide variety
of areas including biomedicine, data storage, capacitors, etc. [22, 27, 65].
On the field of biomedicine, one of the most promising applications of hollow
MNPs is in targeted drug delivery. This is a method of delivering medication to
a patient based on the use of MNPs that can be externally guided (i.e., through
magnetic fields), so that they release the therapeutic drug only into the affected
area that needs to be treated, maximizing this way the efficiency of the medication
and minimizing collateral damage [66] (see Fig. 6.10). Conventionally, the drug is
attached onto the surface of the MNPs and released by an external stimuli (e.g.,
pH change, heat, etc.) once in the target area. For an efficient MNPs mediated drug
delivery, several factors need to be optimized, including the magnetic response of the
MNPs against an external magnetic field (which is proportional to their saturation
magnetization), the drug loading capacity of the MNPs (which depends on their
surface area), the drug release capacity and rate, etc. To this respect, hollow MNPs
present a series of advantages compared to conventional solid MNPs for drug release
treatment. On the one hand, the surface area available to attach drugs on hollow
MNPs is much higher than in the case of solid MNPs, because of the existence of
both the inner and outer surfaces, increasing their loading capacity. Since the drug
can be encapsulated inside the hollow MNP instead of being attached to the surface,
the drug is effectively “camouflaged” and protected on its way to the target [26].
All these results have led to several groups investigating different types of hollow