Eva Natividad

Accuracy of available methods for quantifying the heat power generation of nanoparticles for magnetic hyperthermia

Abstract In magnetic hyperthermia, characterising the specific functionality of magnetic nanoparticle arrangements is essential to plan the therapies by simulating maximum achievable temperatures. This functionality, i.e. the heat power released upon application of an alternating magnetic field, is quantified by means of the specific absorption rate (SAR), also referred to as specific loss power (SLP). Many research groups are currently involved in the SAR/SLP determination of newly synthesised materials by several methods, either magnetic or calorimetric, some of which are affected by important and unquantifiable uncertainties that may turn measurements into rough estimates. This paper reviews all these methods, discussing in particular sources of uncertainties, as well as their possible minimisation. In general, magnetic methods, although accurate, do not operate in the conditions of magnetic hyperthermia. Calorimetric methods do, but the easiest to implement, the initial-slope method in isoperibol conditions, derives inaccuracies coming from the lack of matching between thermal models, experimental set-ups and measuring conditions, while the most accurate, the pulse-heating method in adiabatic conditions, requires more complex set-ups.

Nano-objects for addressing the control of nanoparticle arrangement and performance in magnetic hyperthermia

One current challenge of magnetic hyperthermia is achieving therapeutic effects with a minimal amount of nanoparticles, for which improved heating abilities are continuously pursued. However, it is demonstrated here that the performance of magnetite nanocubes in a colloidal solution is reduced by 84% when they are densely packed in three-dimensional arrangements similar to those found in cell vesicles after nanoparticle internalization. This result highlights the essential role played by the nanoparticle arrangement in heating performance, uncontrolled in applications. A strategy based on the elaboration of nano-objects able to confine nanocubes in a fixed arrangement is thus considered here to improve the level of control. The obtained specific absorption rate results show that nanoworms and nanospheres with fixed one- and two-dimensional nanocube arrangements, respectively, succeed in reducing the loss of heating power upon agglomeration, suggesting a change in the kind of nano-object to be used in magnetic hyperthermia.

Correlation of normal and superconducting transport properties on textured Bi-2212 ceramic thin rods

The electric and thermal properties well above and below Tc of Bi-2212 textured ceramics have been correlated through a careful analysis of the microstructure and the transport measurements. Thin rods with the same Bi-2122 stoichiometry and textured by a laser floating zone technique have been studied with that aim. By changing the growth parameters, it has been possible to produce strong changes in microstructure and critical current density, Jc, with small variations in the thermal conductivity. The existence of phase and composition gradients across the thin rods, which explains the variations of Tc, makes the relation difficult between the normal state resistivity and Jc (77 K). A simple qualitative analysis that takes into account the observed microstructure has been developed to correlate the electric transport properties in the normal and in the superconducting states.

Critical assessment of the nature and properties of Fe(II) triazole-based spin-crossover nanoparticles

The shape and size of nanoparticles of the spin crossover compound [Fe(Htrz)2trz]BF4 obtained by the reverse-micelle method and using as co-surfactants dioctylsulfosuccinate and behenic acid are described through systematic transmission electron microscopy observations. A rod shape is systematically derived, and the rod sizes, in particular, their width, are controllable through the surfactant concentration, although a poor reproducibility is observed and ascribed to uncontrolled parameters in the micelle elaboration and microemulsion formation and ageing. The influence of synthetic parameters and nanoparticle processing on the spin crossover properties of nanoparticles is also reported, as characterized by both calorimetry and magnetic measurements. These unravel original size and environment effects. On the one hand, the hysteresis width of the thermal spin crossover exhibited by raw nanoparticles increases linearly with the rod width, until it reaches a value of 40 K, close to that of the bulk material. A similarly good correlation is found with the nanoparticle volume. On the other hand, the removal of the surfactant from the raw nanoparticles is found to systematically reduce the hysteresis width in a drastic manner, by up to 16 K.

Heating ability of cobalt ferrite nanoparticles showing dynamic and interaction effects

The heating ability of magnetic nanoparticles (MNPs) intended for magnetic hyperthermia is quantified by means of the specific absorption rate (SAR), also referred to as specific loss power. This quantification is mainly performed on ferrofluids, even though the SAR values so obtained are often not representative of the nanoparticle performance inside tissues (solid matrices). In this study, the SAR of cobalt ferrite MNPs with mean crystallite diameters of 5.5 nm and 7.4 nm, functionalized or not, and dispersed in liquid or solid media, was determined in the 180–300 K temperature range. Higher SAR values were systematically obtained for samples in liquid media. On the one hand, heat capacity data together with zero-field-cooled and field-cooled magnetization curves allowed correlation of these results with ferrofluid dynamics originating from viscosity changes in samples dispersed in diethylene glycol. On the other hand, the higher degree of agglomeration attained by the functionalized MNPs after immobilization in paraffin wax seemed responsible for the decrease in SAR values and the shift in blocking temperature. In sum, MNPs spatial arrangements acquired after ferrofluid injection in magnetic hyperthermia should be taken into account to predict SAR values during therapies.

Radial changes in the microstructure of LFZ-textured Bi-2212 thin rods induced by stoichiometry modifications

Three different Bi-2212 powders, with stoichiometric deviations both in the Bi and Sr content as well as in the Bi/Sr ratio, have been used as precursors in the processing of laser floating zone-textured thin rods. The developed microstructure in the as-grown and annealed rods has been studied by ICP analysis, scanning electron microscopy and energy dispersive spectroscopy microanalysis. The marked differences in phase distribution, composition and concentration found among the rods have been correlated with the initial stoichiometry. Finally, the effect of these microstructural inhomogeneities on the critical temperature, critical current density at 77 K and pinning behaviour of the rods is discussed.

A multifunctional magnetic material under pressure

Fe(II)(Metz)6](Fe(III)Br4)2 (Metz = 1-methyltetrazole) is one of the rare systems combining spin-crossover and long-range magnetic ordering. A joint neutron and X-ray diffraction and magnetometry study allows determining its collinear antiferromagnetic structure, and shows an increase of the Néel temperature from 2.4 K at ambient pressure, to 3.9 K at 0.95 GPa. Applied pressure also enables a full high-spin to low-spin switch at ambient temperature.

Correlation of radial inhomogeneties and critical current at 77 K in LFZ Bi-2212 textured thin rods

Abstract The fabrication of thin Bi-2212 rods by laser floating zone techniques gives high critical currents, Ic, but strong radial inhomogeneities in microstructure and physical properties. By changing the precursor stoichiometry or the processing parameters, we have succeeded in improving the homogeneity and the Ic values. Towards the centre of the rods, the superconducting material tends to have lower Tc values associated to higher Bi content in the Bi-2212 superconducting grains. At 77 K, this gradient of Bi originates strong variations of critical current density across the rod.

AC susceptibility as a tool to probe the dipolar interaction in magnetic nanoparticles

Abstract The dipolar interaction is known to substantially affect the properties of magnetic nanoparticles. This is particularly important when the particles are kept in a fluid suspension or packed within nano-carriers. In addition to its usual long-range nature, in these cases the dipolar interaction may also induce the formation of clusters of particles, thereby strongly modifying their magnetic anisotropies. In this paper we show how AC susceptibility may be used to obtain information regarding the influence of the dipolar interaction in a sample. We develop a model which includes both aspects of the dipolar interaction and may be fitted directly to the susceptibility data. The usual long-range nature of the interaction is implemented using a mean-field approximation, whereas the particle-particle aggregation is modeled using a distribution of anisotropy constants. The model is then applied to two samples studied at different concentrations. One consists of spherical magnetite nanoparticles dispersed in oil and the other of cubic magnetite nanoparticles embedded on polymeric nanospheres. We also introduce a simple technique to address the presence of the dipolar interaction in a given sample, based on the height of the AC susceptibility peaks for different driving frequencies.

Multiple-length-scale small-angle X-ray scattering analysis on maghemite nanocomposites

Small-angle X-ray scattering (SAXS) analysis has been performed on maghemite–poly(4-vinylpyridine) nanocomposites prepared by in situ precipitation from iron–polymer coordination compounds. According to electron microscopy observations, the nanocomposites contain isolated spherical particles with a narrow size distribution, uniformly distributed throughout the polymer matrix. The scattering intensity of nanocomposites has relevant contributions from both the polymer and the nanocomposites, showing features characteristic of multiscale structured systems, namely two power laws and a Guinier regime. The data have been analysed in terms of Beaucages unified approach and it is found that the maghemite particle size increases with the iron/polymer weight ratio used in the preparation of the nanocomposites. SAXS curves also feature a bump that was analysed as arising from a second particle population or from interactions. Magnetization and transmission electron microscopy results give arguments favouring the latter interpretation. It is found that the maghemite particle sizes vary linearly with the iron weight ratio used in the preparation of the nanocomposites.