Low-Dimensional Assemblies of Magnetic MnFe2O4 Nanoparticles and Direct In Vitro Measurements of Enhanced Heating Driven by Dipolar Interactions: Implications for Magnetic Hyperthermia

Magnetic fluid hyperthermia (MFH), the procedure of raising the temperature of tumor cells using magnetic nanoparticles (MNPs) as heating agents, has proven successful in treating some types of cancer. However, the low heating power generated under physiological conditions makes it necessary a high local concentration of MNPs at tumor sites. Here, we report how the in vitro heating power of magnetically soft MnFe2O4 nanoparticles can be enhanced by intracellular low-dimensional clusters through a strategy that includes: (a) the design of the MNPs to retain Neel magnetic relaxation in high-viscosity media, and (b) culturing MNP-loaded cells under magnetic fields to produce elongated intracellular agglomerates. Our direct in vitro measurements demonstrated that the specific loss power (SLP) of elongated agglomerates (SLP = 576 +/- 33 W/g) induced by culturing BV2 cells in situ under a dc magnetic field was increased by a factor of 2 compared to the SLP = 305 +/- 25 W/g measured in aggregates freely formed within cells. A numerical mean-field model that included dipolar interactions quantitatively reproduced the SLPs of these clusters both in phantoms and in vitro, suggesting that it captures the relevant mechanisms behind power losses under high-viscosity conditions. These results indicate that in situ assembling of MNPs into low-dimensional structures is a sound possible way to improve the heating performance in MFH

Exchange bias of MnFe2O4@γFe2O3 and CoFe2O4@γFe2O3 core/shell nanoparticles

AbstractWe compare here exchange bias (EB) properties of chemically synthesized core–shell nanoparticles (NPs), based either on a core of soft ferrite (MnFe2O4) or hard ferrite (CoFe2O4) protected by a maghemite shell (γ-Fe2O3). These NPs dispersed in acidic solutions are electrostatically stabilized, yielding to stable colloidal dispersions with a strong interparticle repulsion and negligible dipolar interactions in the probed range of temperatures. Field cooled (FC) magnetic hysteresis loops of non-textured frozen dispersions (with magnetic anisotropy axis of NPs distributed at random) and those of a powder based on the same NPs present a shift along the H-axis, expressing the coupling between the spin-ordered cores and the disordered surface layer of the NPs. The bias field is found to present a maximum, larger for NPs based on harder ferrite core. It is obtained for a cooling field of the order of one half of the anisotropy field, which is much larger for the CoFe2O4 cores than for MnFe2O4 ones. In powders, particles are in contact leading to an interparticle exchange which is not present in the dilute solutions where exchange bias properties are only due to an intraparticle exchange between core and surface. The thermal dependence of the bias field is well described by a reduced exponential behavior with a characteristic freezing temperature of about 8K

Thermodiffusion of repulsive charged nanoparticles – the interplay between single-particle and thermoelectric contributions

International audienceThermodiffusion of different ferrite nanoparticles (NPs), ∼10 nm in diameter, is explored in tailor-made aqueous dispersions stabilized by electrostatic interparticle interactions. In the dispersions, electrosteric repulsion is the dominant force, which is tuned by an osmotic-stress technique, i.e. controlling of osmotic pressure Π, pH and ionic strength. It is then possible to map Π and the NPs’ osmotic compressibility χ in the dispersion with a Carnahan–Starling formalism of effective hard spheres (larger than the NPs’ core). The NPs are here dispersed with two different surface ionic species, either at pH ∼ 2 or 7, leading to a surface charge, either positive or negative. Their Ludwig–Soret $S_{T}$ coefficient together with their mass diffusion $D_{m}$ coefficient are determined experimentally by forced Rayleigh scattering. All probed NPs display a thermophilic behavior ($S_{T}$ < 0) regardless of the ionic species used to cover the surface. We determine the NPs’ Eastman entropy of transfer and the Seebeck (thermoelectric) contribution to the measured Ludwig–Soret coefficient in these ionic dispersions. The NPs’ Eastman entropy of transfer $Ŝ_{NP}$ is interpreted through the electrostatic and hydration contributions of the ionic shell surrounding the NPs

Thermoelectricity and thermodiffusion in charged colloids

International audienceThe Seebeck and Soret coefficients of ionically stabilized suspension of maghemite nanoparticles in dimethyl sulfoxide are experimentally studied as a function of nanoparticle volume fraction. In the presence of a temperature gradient, the charged colloidal nanoparticles experience both thermal drift due to their interactions with the solvent and electric forces proportional to the internal thermoelectric field. The resulting thermodiffusion of nanoparticles is observed through forced Rayleigh scattering measurements, while the thermoelectric field is accessed through voltage measurements in a thermocell. Both techniques provide independent estimates of nanoparticle’s entropy of transfer as high as 82 meV K$_{−1}$. Such a property may be used to improve the thermoelectric coefficients in liquid thermocells