Reasmey P. Tan

Increase of magnetic hyperthermia efficiency due to dipolar interactions in low-anisotropy magnetic nanoparticles: Theoretical and experimental results

When magnetic nanoparticles (MNPs) are single-domain and magnetically independent, their magnetic properties and the conditions to optimize their efficiency in magnetic hyperthermia applications are now well-understood. However, the influence of magnetic interactions on magnetic hyperthermia properties is still unclear. Here, we report hyperthermia and high-frequency hysteresis loop measurements on a model system consisting of MNPs with the same size but a varying anisotropy, which is an interesting way to tune the relative strength of magnetic interactions. A clear correlation between the MNP anisotropy and the squareness of their hysteresis loop in colloidal solution is observed : the larger the anisotropy, the smaller the squareness. Since low anisotropy MNPs display a squareness higher than the one of magnetically independent nanoparticles, magnetic interactions enhance their heating power in this case. Hysteresis loop calculations of independent and coupled MNPs are compared to experimental results. It is shown that the observed features are a natural consequence of the formation of chains and columns of MNPs during hyperthermia experiments: in these structures, when the MNP magnetocristalline anisotropy is small enough to be dominated by magnetic interactions, the hysteresis loop shape tends to be rectangular, which enhance their efficiency. On the contrary, when MNPs do not form chains and columns, magnetic interactions reduces the hysteresis loop squareness and the efficiency of MNPs compared to independent ones. The present work should improve the understanding and interpretation of magnetic hyperthermia experiments.

Nano-electromanipulation of Spin Crossover Nanorods: Towards Switchable Nanoelectronic Devices

The nanoscale manipulation and charge transport properties of the [Fe(Htrz)2(trz)](BF4) spin-crossover compound is demonstrated. Such 1D spin-crossover nanostructures are attractive building blocks for nanoelectronic switching and memory devices.

Targeting a G-Protein-Coupled Receptor Overexpressed in Endocrine Tumors by Magnetic Nanoparticles To Induce Cell Death

Nanotherapy using targeted magnetic nanoparticles grafted with peptidic ligands of receptors overexpressed in cancers is a promising therapeutic strategy. However, nanoconjugation of peptides can dramatically affect their properties with respect to receptor recognition, mechanism of internalization, intracellular trafficking, and fate. Furthermore, investigations are needed to better understand the mechanism whereby application of an alternating magnetic field to cells containing targeted nanoparticles induces cell death. Here, we designed a nanoplatform (termed MG-IONP-DY647) composed of an iron oxide nanocrystal decorated with a ligand of a G-protein coupled receptor, the cholecystokinin-2 receptor (CCK2R) that is overexpressed in several malignant cancers. MG-IONP-DY647 did not stimulate inflammasome of Raw 264.7 macrophages. They recognized cells expressing CCK2R with a high specificity, subsequently internalized via a mechanism involving recruitment of β-arrestins, clathrin-coated pits, and dynamin and were directed to lysosomes. Binding and internalization of MG-IONP-DY647 were dependent on the density of the ligand at the nanoparticle surface and were slowed down relative to free ligand. Trafficking of CCK2R internalized with the nanoparticles was slightly modified relative to CCK2R internalized in response to free ligand. Application of an alternating magnetic field to cells containing MG-IONP-DY647 induced apoptosis and cell death through a lysosomal death pathway, demonstrating that cell death is triggered even though nanoparticles of low thermal power are internalized in minute amounts by the cells. Together with pioneer findings using iron oxide nanoparticles targeting tumoral cells expressing epidermal growth factor receptor, these data represent a solid basis for future studies aiming at establishing the proof-of-concept of nanotherapy of cancers using ligand-grafted magnetic nanoparticles specifically internalized via cell surface receptors.

Magnetism of single-crystalline Co nanorods

We report on the synthesis and preparation of oriented nanomaterials of single crystalline metallic Co nanorods of 6 nm in diameters and two different aspect ratios (7 and 15). They display optimized magnetic properties, with a spontaneous magnetization just below the bulk one, and large coercive fields (up to 6.5 kOe at 300 K) as a result of coherent switching (i.e., Stoner and Wohlfarth mechanism). We measured a strong effective anisotropy very close to the expected value resulting from the sum of shape and magnetocrystalline contributions.

Solution Epitaxial Growth of Cobalt Nanowires on Crystalline Substrates for Data Storage Densities beyond 1 Tbit/in2

The implementation of nano-objects in numerous emerging applications often demands their integration in macroscopic devices. Here we present the bottom-up epitaxial solution growth of high-density arrays of vertical 5 nm diameter single-crystalline metallic cobalt nanowires on wafer-scale crystalline metal surfaces. The nanowires form regular hexagonal arrays on unpatterned metallic films. These hybrid heterostructures present an important perpendicular magnetic anisotropy and pave the way to a high density magnetic recording device, with capacities above 10 Terabits/in(2). This method bypasses the need of assembling and orientating free colloidal nanocrystals on surfaces. Its generalization to other materials opens new perspectives toward many applications.

Magnetic hyperthermia properties of nanoparticles inside lysosomes using kinetic Monte Carlo simulations: Influence of key parameters and dipolar interactions, and evidence for strong spatial variation of heating power

Understanding the influence of dipolar interactions in magnetic hyperthermia (MH) experiments is of crucial importance for a fine optimization of nanoparticle (NP) heating power. In this study, we use a kinetic Monte-Carlo algorithm to calculate hysteresis loops, so both time and temperature are correctly taken into account. It is demonstrated that this algorithm correctly reproduces the high-frequency hysteresis loop of both superparamagnetic NPs and ferromagnetic ones without any ad-hoc parameters. The algorithm is easily parallelizable so calculation on several processors decreases considerably calculation time. The specific absorption rate (SAR) of NPs dispersed inside spherical lysosomes is studied as a function of several key parameters: volume concentration, applied magnetic field, lysosome size, NP diameter and anisotropy. The influence of these parameters is illustrated and comprehensively explained. In summary, the effect of magnetic interactions is to increase the coercive field, saturation field and hysteresis area of major loops. However, for small amplitude magnetic field such as the ones used in MH, the heating power as function of concentration can increase, decrease or display a bell shape, depending of the relationship between the applied magnetic field and the coercive/saturation fields of the NPs. The hysteresis area is found to be well correlated to the parallel or antiparallel nature of the dipolar field acting on each NP. It is shown that the heating power increases or decreases sharply in the vicinity of the lysosome membrane. The amplitude of variation reaches more than one order of magnitude in certain conditions. Finally, implications of these various findings are discussed in the framework of MH optimization. It is concluded that feedbacks on specific points from biology experiments are required for further advance on the optimization of NPs for MH.

Transport in superlattices of magnetic nanoparticles: coulomb blockade, hysteresis, and switching induced by a magnetic field.

We report on magnetotransport measurements on millimetric super-lattices of Co-Fe nanoparticles surrounded by an organic layer. At low temperature, the transition between the Coulomb blockade and the conductive regime becomes abrupt and hysteretic. The transition between both regime can be induced by a magnetic field, leading to a novel mechanism of magnetoresistance. Between 1.8 and 10 K, high-field magnetoresistance due to magnetic disorder at the surface of the particles is also observed. Below 1.8 K, this magnetoresistance abruptly collapses and a low-field magnetoresistance is observed.

An air-cooled Litz wire coil for measuring the high frequency hysteresis loops of magnetic samples--a useful setup for magnetic hyperthermia applications.

A setup for measuring the high-frequency hysteresis loops of magnetic samples is described. An alternating magnetic field in the range 6-100 kHz with amplitude up to 80 mT is produced by a Litz wire coil. The latter is air-cooled using a forced-air approach so no water flow is required to run the setup. High-frequency hysteresis loops are measured using a system of pick-up coils and numerical integration of signals. Reproducible measurements are obtained in the frequency range of 6-56 kHz. Measurement examples on ferrite cylinders and on iron oxide nanoparticle ferrofluids are shown. Comparison with other measurement methods of the hysteresis loop area (complex susceptibility, quasi-static hysteresis loops, and calorific measurements) is provided and shows the coherency of the results obtained with this setup. This setup is well adapted to the magnetic characterization of colloidal solutions of magnetic nanoparticles for magnetic hyperthermia applications.

Room-Temperature Tunnel Magnetoresistance in Self-Assembled Chemically Synthesized Metallic Iron Nanoparticles

We report on room temperature magnetoresistance in networks of chemically synthesized metallic Fe nanoparticles surrounded by two types of organic barriers. Electrical properties, featuring Coulomb blockade, and magnetotransport measurements show that this magnetoresistance arises from spin-dependent tunnelling, so the organic ligands stabilizing the nanoparticles are efficient spin-conservative tunnel barrier. These results demonstrate the feasibility of an all-chemistry approach for room temperature spintronics.

Stabilizing vortices in interacting nano-objects: a chemical approach.

We report a chemical method to prepare metallic Fe porous nanocubes. The presence of pores embedded inside the cubes was attested by electron tomography. Thanks to electronic holography and micromagnetic simulations, we show that the presence of these defects stabilizes the vortices in assembly of interacting cubes. These results open new perspectives toward magnetic vortex stabilization at relatively low cost for various applications (microelectronics, magnetic recording, or biological applications).