Sébastien Lachaize

A Simple Chemical Route toward Monodisperse Iron Carbide Nanoparticles Displaying Tunable Magnetic and Unprecedented Hyperthermia Properties

We report a tunable organometallic synthesis of monodisperse iron carbide and core/shell iron/iron carbide nanoparticles displaying a high magnetization and good air-stability. This process based on the decomposition of Fe(CO)(5) on Fe(0) seeds allows the control of the amount of carbon diffused and therefore the tuning of nanoparticles magnetic anisotropy. This results in unprecedented hyperthermia properties at moderate magnetic fields, in the range of medical treatments.

Iron Nanoparticle Growth in Organic Superstructures

A tunable synthesis of iron nanoparticles (NPs) based on the decomposition of {Fe[N(SiMe(3))(2)](2)}(2) in the presence of organic superstructures composed of palmitic acid and hexadecylamine is reported. Control of the size (from 1.5 to 27 nm) and shape (spheres, cubes, or stars) of the NPs has been achieved. An environment-dependent growth model is proposed on the basis of results obtained for the NP morphology under various conditions and a complete Mossbauer study of the colloid composition at different reacting stages. It involves (i) an anisotropic growth process inside organic superstructures, leading to monocrystalline cubic NPs, and (ii) isotropic growth outside these superstructures, yielding polycrystalline spherical NPs.

Magnetic configurations of 30 nm iron nanocubes studied by electron holography.

Ferromagnetic nanomaterials exhibit unique magnetic properties common to materials with dimensions approaching the atomic scale and have potential applications in magnetic data storage. Technological applications, however, require that the detailed magnetic behaviors and configurations of individual and interacting magnetic nano-objects be clarified. We determined the magnetic remnant configurations in single crystalline 30 nm Fe nanocubes and groups of nanocubes using off-axis electron holography in a transmission electron microscope. Our measurements on an isolated cube reveal a vortex state whose core size has been determined. Two neighboring nanocubes with adjacent {100} surfaces exhibit a ferromagnetic dipolar coupling, while similar magnetic interactions between four cubes in a square arrangement induce a bending of the magnetic induction, i.e., a magnetic flux closure state. The various configurations were successfully simulated by micromagnetic calculations.

Magnetic hyperthermia in single-domain monodisperse FeCo nanoparticles: Evidences for Stoner–Wohlfarth behavior and large losses

We report on hyperthermia measurements on a colloidal solution of 14.2±1.5 nm monodisperse FeCo nanoparticles (NPs). Losses as a function of the magnetic field display a sharp increase followed by a plateau, which is what is expected for losses of ferromagnetic single-domain NPs. The frequency dependence of the coercive field is deduced from hyperthermia measurement and is in quantitative agreement with a simple model of noninteracting NPs. The measured losses (1.5 mJ/g) compare to the highest of the literature, although the saturation magnetization of the NPs is well below the bulk one.

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.

Large specific absorption rates in the magnetic hyperthermia properties of metallic iron nanocubes

We report on the magnetic hyperthermia properties of chemically synthesized ferromagnetic 11 and 16 nm Fe(0) nanoparticles of cubic shape displaying the saturation magnetization of bulk iron. The specific absorption rate measured on 16 nm nanocubes is 1690±160 W/g at 300 kHz and 66 mT. This corresponds to specific losses-per-cycle of 5.6 mJ/g, largely exceeding the ones reported in other systems. A way to quantify the degree of optimization of any system with respect to hyperthermia applications is proposed. Applied here, this method shows that our nanoparticles are not fully optimized, probably due to the strong influence of magnetic interactions on their magnetic response. Once protected from oxidation and further optimized, such nano-objects could constitute efficient magnetic cores for biomedical applications requiring very large heating power.

Ultrasmall iron nanoparticles: Effect of size reduction on anisotropy and magnetization

Stable iron nanoparticles have been synthesised by the decomposition of {Fe(N[Si(CH3)3]2)2}2 under dihydrogen pressure. Those conditions lead to a system of monodisperse and metallic nanoparticles which diameter is less than 2nm and stabilized by HN[Si(CH3)3]2. The magnetization is found to be MS=1.92μB∕at., i.e., 10% lower than the bulk value. The Mossbauer spectrum is fitted by two contributions of metallic iron. The magnetic anisotropy energy constant increases up to 5.2×105J∕m3, i.e., ten times the bulk one.

Use of long chain amine as a reducing agent for the synthesis of high quality monodisperse iron(0) nanoparticles

This article reports the synthesis of iron(0) nanoparticles at moderate temperature—from 120 °C to 150 °C—using the reduction of the organometallic iron(II) precursor {Fe[N(SiMe3)2]2}2 by hexadecylamine (HDA) in the absence of dihydrogen (H2). The nanoparticles are monodisperse in size and self-assemble into 2D super-lattices suitable for transport measurements. The nanoparticles are stabilized in mesitylene by a mixture of HDA and hexadecylammonium chloride (HDA·HCl). The resulting truncated single-crystalline nanocubes have a narrow size distribution and a high magnetization close to the bulk value. The products are characterized by transmission electronic microscopy (TEM and HRTEM), SQUID measurements, Mossbauer and Infra-Red spectroscopies. Fe(II) reduction is accompanied by oxidation of amines into imines which was detected as a by-product. This reduction occurs at 120 °C and above. The temperature, in conjunction with the reaction time, allows for a fine control of the nano-objects final size. The latter can also be tuned with the HDA·HCl concentration. Finally, this one-pot synthesis produces high-quality magnetic nanoparticles with mean sizes in the range 6 to 10 nm depending on the conditions.

Complex Nano-objects Displaying Both Magnetic and Catalytic Properties: A Proof of Concept for Magnetically Induced Heterogeneous Catalysis

Addition of Co2(Co)9 and Ru3(CO)12 on preformed monodisperse iron(0) nanoparticles (Fe(0) NPs) at 150 °C under H2 leads to monodisperse core-shell Fe@FeCo NPs and to a thin discontinuous Ru(0) layer supported on the initial Fe(0) NPs. The new complex NPs were studied by state-of-the-art transmission electron microscopy techniques as well as X-ray diffraction, Mössbauer spectroscopy, and magnetic measurements. These particles display large heating powers (SAR) when placed in an alternating magnetic field. The combination of magnetic and surface catalytic properties of these novel objects were used to demonstrate a new concept: the possibility of performing Fischer-Tropsch syntheses by heating the catalytic nanoparticles with an external alternating magnetic field.

New generation of magnetic and luminescent nanoparticles for in vivo real-time imaging

A new generation of optimized contrast agents is emerging, based on metallic nanoparticles (NPs) and semiconductor nanocrystals for, respectively, magnetic resonance imaging (MRI) and near-infrared (NIR) fluorescent imaging techniques. Compared with established contrast agents, such as iron oxide NPs or organic dyes, these NPs benefit from several advantages: their magnetic and optical properties can be tuned through size, shape and composition engineering, their efficiency can exceed by several orders of magnitude that of contrast agents clinically used, their surface can be modified to incorporate specific targeting agents and antifolding polymers to increase blood circulation time and tumour recognition, and they can possibly be integrated in complex architecture to yield multi-modal imaging agents. In this review, we will report the materials of choice based on the understanding of the basic physics of NIR and MRI techniques and their corresponding syntheses as NPs. Surface engineering, water transfer and specific targeting will be highlighted prior to their first use for in vivo real-time imaging. Highly efficient NPs that are safer and target specific are likely to enter clinical application in a near future.