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T. Vemulkar and R. P. Cowburn
of differences in anisotropy, the presence of interparticle interactions and shape on
hyperthermia efficiency.
Inspired by work into bacterial magnetosomes [135], this study demonstrates
through experiment the effect of shape and nanoparticle concentration on the SLP
and also looks to numerical calculations of the hysteresis loops of the different particle
types to draw connections between the structural and magnetic properties of each
particle type, and the measured hyperthermia results.
Focussing first on establishing well-characterized nanoparticle formulations as the
core of this study, the team fabricated iron oxide nanocubes of 20 and 40 nm in size,
as well as nanospheres of 20 nm in size. The different shaped nanoparticles have the
same crystal structure (as characterized by transmission electron microscopy (TEM))
and the same size, and the two different sizes of nanocubes allow for a comparison
of the effect of size while keeping the shape constant.
The interparticle interaction of the nanocubes is clear in the formation of chainlike aggregates verified by TEM imaging, even in the absence of an applied magnetic
field. The effect of these aggregates may seem evident in the concentration dependence of the SLP for both the 20 and 40 nm nanocubes, as well as in the plateauing
of the field-cooled curves for the SLP measurements which is not expected for a
system of non-interacting particles [134]. Since the SLP decreases with concentration, it seems to suggest from this study that minimizing the interparticle interactions may be of benefit. It was also found via dynamic light scattering measurements
that the nanocubes self-assembled into chain-like structures at remanence while the
nanospheres did not, showing the importance of shape in interparticle interactions.
A curious point of note, however, is that in the simulations conducted it was shown
that the SLP appears to monotonically increase with increasing chain length up to ten
particles, which would be expected to occur in the samples of higher concentration.
Why then does the SAR not similarly increase in the samples where longer chains
of particles are to be expected? It may perhaps be the case that minimal ordering on
the scale of simple short chains is desirable, but at high concentrations interactions
between chains may lead to large clusters that reduce SLP efficiency. A better understanding of the effect of such interparticle interactions is clinically relevant because
of the localization of a high dose of nanoparticles in a tumour mass for therapy.
The 20 nm nanocubes were then compared to 20 nm nanospheres, and it was
found that there was a 20% increase in the SAR compared to the spheres both
experimentally, and this quite closely matched Monte-Carlo simulations of the SAR
of the two systems that considered dipolar interactions. The nanocubes were estimated to have an anisotropy approximately 25% higher than the nanospheres at room
temperature accounting for dipolar interactions. The higher anisotropy was attributed
primarily to the higher surface area of the cubic particles that leads to a higher surface
anisotropy contribution since exchange bias effects were shown to be non-existent
and a homogenous oxide composition was assumed.
Tuning the shape of nanoparticles thus can have a significant effect on their effectiveness in hyperthermia applications. However, this work highlights some interesting
questions about interparticle interactions and their effect on the SAR. Aggregates of
particles in chains, for example, have an anisotropy that can be assigned to the entire
T. Vemulkar and R. P. Cowburn
of differences in anisotropy, the presence of interparticle interactions and shape on
hyperthermia efficiency.
Inspired by work into bacterial magnetosomes [135], this study demonstrates
through experiment the effect of shape and nanoparticle concentration on the SLP
and also looks to numerical calculations of the hysteresis loops of the different particle
types to draw connections between the structural and magnetic properties of each
particle type, and the measured hyperthermia results.
Focussing first on establishing well-characterized nanoparticle formulations as the
core of this study, the team fabricated iron oxide nanocubes of 20 and 40 nm in size,
as well as nanospheres of 20 nm in size. The different shaped nanoparticles have the
same crystal structure (as characterized by transmission electron microscopy (TEM))
and the same size, and the two different sizes of nanocubes allow for a comparison
of the effect of size while keeping the shape constant.
The interparticle interaction of the nanocubes is clear in the formation of chainlike aggregates verified by TEM imaging, even in the absence of an applied magnetic
field. The effect of these aggregates may seem evident in the concentration dependence of the SLP for both the 20 and 40 nm nanocubes, as well as in the plateauing
of the field-cooled curves for the SLP measurements which is not expected for a
system of non-interacting particles [134]. Since the SLP decreases with concentration, it seems to suggest from this study that minimizing the interparticle interactions may be of benefit. It was also found via dynamic light scattering measurements
that the nanocubes self-assembled into chain-like structures at remanence while the
nanospheres did not, showing the importance of shape in interparticle interactions.
A curious point of note, however, is that in the simulations conducted it was shown
that the SLP appears to monotonically increase with increasing chain length up to ten
particles, which would be expected to occur in the samples of higher concentration.
Why then does the SAR not similarly increase in the samples where longer chains
of particles are to be expected? It may perhaps be the case that minimal ordering on
the scale of simple short chains is desirable, but at high concentrations interactions
between chains may lead to large clusters that reduce SLP efficiency. A better understanding of the effect of such interparticle interactions is clinically relevant because
of the localization of a high dose of nanoparticles in a tumour mass for therapy.
The 20 nm nanocubes were then compared to 20 nm nanospheres, and it was
found that there was a 20% increase in the SAR compared to the spheres both
experimentally, and this quite closely matched Monte-Carlo simulations of the SAR
of the two systems that considered dipolar interactions. The nanocubes were estimated to have an anisotropy approximately 25% higher than the nanospheres at room
temperature accounting for dipolar interactions. The higher anisotropy was attributed
primarily to the higher surface area of the cubic particles that leads to a higher surface
anisotropy contribution since exchange bias effects were shown to be non-existent
and a homogenous oxide composition was assumed.
Tuning the shape of nanoparticles thus can have a significant effect on their effectiveness in hyperthermia applications. However, this work highlights some interesting
questions about interparticle interactions and their effect on the SAR. Aggregates of
particles in chains, for example, have an anisotropy that can be assigned to the entire