To
investigate the role of magnetic anisotropy on magnetic hyperthermia
heating efficiency at low field conditions, Mn, MnZn, and MnCo-ferrite
nanoparticles were synthesized using the hydrothermal method. The
coercive field temperature dependence method was used to determine
the blocking temperature distribution of the particles by considering
the temperature dependence of anisotropy and magnetization and the
random anisotropy axis configuration. The data allowed one to estimate
the room-temperature quasi-static superparamagnetic diameter, which
was found to be lower than the theoretical value. Magnetic hyperthermia
experiments of the magnetic nanocolloids at 522 kHz indicated that
soft nanomagnets heat more efficiently at clinically relevant conditions.
The heating performance was found to decrease at the higher fraction
of blocked nanoparticles. For instance, samples with similar size
distribution and mean diameter of 10 nm, at a field amplitude of only
120 Oe (9.6 kA m–1), showed a decrease of specific
loss power of 56% for the Mn-ferrite and 93% for the MnCo-ferrite
in comparison with the MnZn-ferrite nanoparticle. The fractions of
blocked particles of the MnZn, Mn, and MnCo-ferrite were 5, 10, and
25%, respectively, at room temperature.
The magnetic response of nanostructures plays an important role on biomedical applications being strongly influenced by the magnetic anisotropy. In this work we investigate the role of temperature, particle concentration and nanoparticle arrangement forming aggregates in the effective magnetic anisotropy of Mn-Zn ferrite-based nanoparticles. Electron magnetic resonance and coercivity temperature dependence analyses, were critically compared for the estimation of the anisotropy. We found that the temperature dependence of the anisotropy follows the Callen-Callen model, while the symmetry depends on the particle concentration. At low concentration one observes only an uniaxial term, while increasing a cubic contribution has to be added. The effective anisotropy was found to increase the higher the particle concentration on magnetic colloids, as long as the easy axis was at the same direction of the nanoparticle chain. Increasing even further the concentration up to a highly packed condition (powder sample) one observes a decrease of the anisotropy, that was attributed to the random anisotropy axes configuration.
Understanding high-field amplitude electromagnetic heat loss phenomena is of great importance, in particular in the biomedical field, since the heat-delivery treatment plans might rely on analytical models that are only valid at low field amplitudes. Here, we develop a nonlinear response model valid for single-domain nanoparticles of larger particle sizes and higher field amplitudes in comparison to linear response theory. A nonlinear magnetization expression and a generalized heat loss power equation are obtained and compared with the exact solution of the stochastic Landau-LifshitzGilbert equation assuming the giant-spin hypothesis. The model is valid within the hyperthermia therapeutic window and predicts a shift of optimum particle size and distinct heat loss field amplitude exponents. Experimental hyperthermia data with distinct ferrite-based nanoparticles, as well as third harmonic magnetization data supports the nonlinear model, which also has implications for magnetic particle imaging and magnetic thermometry.
Collective
magnetic relaxation of coupled nanoparticle’s
magnetic moments and its influence in magnetic nanoparticle hyperthermia
(MNH) therapy are studied by combining experimental data, numerical
simulations, and theoretical approaches. Frequency-dependent MNH of
Mn-ferrite nanoparticles with different particle sizes and different
nanoparticle arrangements, controlled by medium pH and surface coating,
revealed that the hyperthermia efficiency could increase or decrease
depending on the nanoparticle’s organization within the aggregate.
Effective relaxation times of ∼10–7 s were
obtained for heat generation that are not explained by Brownian or
single-particle Néel relaxation. In particular, we propose
a theoretical approach that is a generalization of the Allia–Knobel
phenomenological model that allows us to build magnetic regime diagrams
and find the conditions for single-particle relaxation (superparamagnetic,
interacting superparamagnetic, and single-particle blocked regimes)
and collective magnetic relaxation. The regimes depend on dipolar
strength, temperature, particle size, aggregate shape and length,
magnetization, and magnetic anisotropy (together with axes arrangement).
We demonstrate through a detailed nanoparticle characterization (including
the temperature dependence of magnetization and anisotropy) that the
collective relaxation is responsible for the heat generation of magnetic
nanostructures. We believe that our findings and our approach to study
the collective magnetic relaxation open new perspectives for designing
more efficient magnetic nanocarriers for hyperthermia and explain
superferromagnetism as a collective blocked regime.
Isometric and anisometric iron oxide magnetic nanoparticles, synthesized via an eco-friendly route, present modulated heating efficiency for magnetic hyperthermia applications.
This study investigated the fabrication of spherical gold shelled maghemite nanoparticles for use in magnetic hyperthermia (MHT) assays. A maghemite core (14 ± 3 nm) was used to fabricate two samples with different gold thicknesses, which presented gold (g)/maghemite (m) content ratios of 0.0376 and 0.0752. The samples were tested in MHT assays (temperature versus time) with varying frequencies (100–650 kHz) and field amplitudes (9–25 mT). The asymptotic temperatures (T∞) of the aqueous suspensions (40 mg Fe/mL) were found to be in the range of 59–77 °C (naked maghemite), 44–58 °C (g/m=0.0376) and 33–51 °C (g/m=0.0752). The MHT data revealed that T∞ could be successful controlled using the gold thickness and cover the range for cell apoptosis, thereby providing a new strategy for the safe use of MHT in practice. The highest SAR (specific absorption rate) value was achieved (75 kW/kg) using the thinner gold shell layer (334 kHz, 17 mT) and was roughly twenty times bigger than the best SAR value that has been reported for similar structures. Moreover, the time that was required to achieve T∞ could be modeled by changing the thermal conductivity of the shell layer and/or the shape/size of the structure. The MHT assays were pioneeringly modeled using a derived equation that was analytically identical to the Box–Lucas method (which was reported as phenomenological).
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