Exchange coupled core–shell nanoparticles present high potential to tune adequately the magnetic properties for specific applications such as nanomedicine or spintronics.

Exchange coupled nanoparticles that combine hard and soft magnetic phases are very promising to enhance the effective magnetic anisotropy while preserving sizes below 20 nm. However, the core−shell structure is usually insufficient to produce rare earth-free ferro(i)magnetic blocked nanoparticles at room temperature. We report on onion-type magnetic nanoparticles prepared by a three-step seed mediated growth based on the thermal decomposition method. The core@shell@shell structure consists of a core and an external shell of Fe 3−δ O 4 separated by an intermediate Co-doped ferrite shell. The double exchange coupling at both core@shell and shell@ shell interfaces results in such an increased of the magnetic anisotropy energy, that onion-type nanoparticles of 16 nm mainly based on iron oxide are blocked at room temperature. We envision that these results are very appealing for potential applications based on permanent magnets.

Nanoparticles which combine several magnetic phases offer wide perspectives for cutting edge applications because of the high modularity of their magnetic properties. Besides the addition of the magnetic characteristics intrinsic to each phase, the interface that results from core-shell and, further, from onion structures leads to synergistic properties such as magnetic exchange coupling. Such a phenomenon is of high interest to overcome the superparamagnetic limit of iron oxide nanoparticles which hampers potential applications such as data storage or sensors. In this manuscript, we report on the design of nanoparticles with an onion-like structure which have been scarcely reported yet. These nanoparticles consist in a Fe3- O4 core covered by a first shell of CoFe2O4 and a second shell of Fe3- O4, e.g. a Fe3- O4@CoFe2O4@Fe3- O4 onion-like structure. They were synthesized by a multi-step seed mediated growth approach which consists to perform three successive thermal decomposition of a metal complexes in a high boiling point solvent (about 300 °C). Although TEM micrographs clearly show the growth of each shell from the iron oxide core, core sizes and shell thicknesses markedly differ from what is suggested by the size increase. We investigated very precisely the structure of nanoparticles in performing high resolution (scanning) TEM imaging and geometrical phase analysis (GPA). The chemical composition and spatial distribution of atoms were studied by electron energy loss spectroscopy (EELS) mapping and spectroscopy. The chemical environment and oxidation state of cations were investigated by Mössbauer spectrometry, soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The combination of these techniques allowed us to estimate the increase of Fe 2+ content in the iron oxide core of the core@shell structure and the increase of the cobalt ferrite shell thickness in the core@shell@shell one, while the iron oxide shell appears to be much thinner than expected. Thus, the modification of the chemical composition as well as the size of the Fe3- O4 core and the thickness of the cobalt ferrite shell have a high impact on the magnetic properties. Furthermore, the growth of the iron oxide shell also markedly modifies the magnetic properties of the core-shell nanoparticles, thus demonstrating the high potential of onion-like nanoparticles for tuning accurately the magnetic properties of nanoparticles according to the desired applications.

In this work, the design of a new generation of functionalized large pore silica nanoparticles is addressed for the specific removal of iron from biological environments. Herein, mesoporous silica with a large pore stellate morphology, denoted STMS, were grafted with the highly specific iron chelating agent desferrioxamine B, DFoB. The challenge of this work was the step by step elaboration of the nanoplatform and the evaluation of its chelating efficiency and selectivity. Hence, the controlled covalent grafting of DFoB specific iron chelator, was successfully achieved ensuring a high grafting rate of chelating ligand of 730 nmol•mg -1 (i.e., 0.85 ligand•nm²). Furthermore, these highly chelating STMS silica were able to capture iron(III) stabilized with nitrilotriacetic acid (NTA) in solution at physiological pH with a fast kinetics (less than 30 minutes). For a stoichiometry 0.85:1 (FeNTA : DFoB), the STMS-DFoB nanoparticles allowed reaching capture capacity and efficiency of 480 nmolFe 3+ /mg SiO 2 and 78%, respectively. Regarding the selectivity features of the removal process, studies were performed with two different media composed of various metal ions: (i) an equimolar solution of various metal cations and (ii) a Barth's buffer mimicking the brain solution composition. In both cases, the chelating STMS-DFoB showed a high selectivity for iron versus other ions at the same (Al 3+ ) or different valency (Na + , K + …). Finally, this work paves the way for new nanosystems for metal overload treatments as well as for future highly chelating nanoplatforms that can be used at the interface between depollution and nanomedecine.

Thanks to important advances in synthesis techniques, a wide collection of bimagnetic core-shell nanoparticles with tunable properties was reported in the literature. Such nanoparticles may combine two phases with different...

Iron oxide nanoparticles were synthesized by an original multistep seed-mediated growth approach. The thermal decomposition of an iron stearate precursor was performed successively up to 5 times to produce nanoparticles with a narrow size distribution from 6.4 to 15.0 nm. The chemical composition and crystal structure of each set of nanoparticles was characterized by TEM, FT-IR, XRD, and Mössbauer spectrometry. Each layer was successively grown at the surface of a pristine Fe3‑δO4 nanoparticle by epitaxial relationship and resulted in a single crystal structure. An intermediate wash after each thermal decomposition step resulted in the surface oxidation of each layer. Therefore, the maghemite phase increased relative to the magnetite phase as the nanoparticle expanded. Finally, the study of the magnetic properties by SQUID magnetometry showed the trend of the magnetic anisotropy energy to increase as a function of the nanoparticle size. In contrast, the coercive field and the magnetization saturation display nonmonotonic variations that may result from the interplay of intrinsic and collective properties.

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