Systematic Study of Exchange Coupling in Core–Shell Fe_3−δO₄@CoO Nanoparticles

Although single magnetic domain nanoparticles are very promising for many applications, size reduction usually results in low magnetic anisotropy and unblocked domain at room temperature, e.g., superparamagnetism. An alternative approach is core–shell nanoparticles featured by exchange bias coupling between ferro­(i)­magnetic [F­(i)­M] and antiferromagnetic (AFM) phases. Although exchange bias coupling has been reported for very diverse core–shell nanoparticles, it is difficult to compare these studies to rationalize the effect of many structural parameters on the magnetic properties. Herein, we report on a systematic study which consists of the modulation of the shell structure and its influence on the exchange bias coupling. A series of Fe_3−δO₄@CoO core–shell nanoparticles has been synthesized by seed-mediated growth based on the thermal decomposition technique. The variation of Co reactant concentration resulted in the modulation of the shell structure for which thickness, crystallinity, and interface with the iron oxide core strongly affect the magnetic properties. The thickest CoO shell and the largest F­(i)­M/AFM interface led to the largest exchange bias coupling. Very high values of coercive field (19 000 Oe) and <i>M</i>_R/<i>M</i>_S ratio (0.86) were obtained. The most stricking results consist of the increase of the coercive field while exchange field vanishes when the CoO thickness decreases: it is ascribed to the diffusion of Co species in the surface layer of iron oxide which generates to some extent cobalt ferrite and induces hard/soft exchange coupling between ferrimagnetic phases

Enhanced Collective Magnetic Properties in 2D Monolayers of Iron Oxide Nanoparticles Favored by Local Order and Local 1D Shape Anisotropy

Magnetic nanoparticle arrays represent a very attractive research field because their collective properties can be efficiently modulated as a function of the structure of the assembly. Nevertheless, understanding the way dipolar interactions influence the intrinsic magnetic properties of nanoparticles still remains a great challenge. In this study, we report on the preparation of 2D assemblies of iron oxide nanoparticles as monolayers deposited onto substrates. Assemblies have been prepared by using the Langmuir–Blodgett technique and the SAM assisted assembling technique combined to CuAAC “click” reaction. These techniques afford to control the formation of well-defined monolayers of nanoparticles on large areas. The LB technique controls local ordering of nanoparticles, while adjusting the kinetics of CuAAC “click” reaction strongly affects the spatial arrangement of nanoparticles in monolayers. Fast kinetics favor disordered assemblies while slow kinetics favor the formation of chain-like structures. Such anisotropic assemblies are induced by dipolar interactions between nanoparticles as no magnetic field is applied and no solvent evaporation is performed. The collective magnetic properties of monolayers are studied as a function of average interparticle distance, local order and local shape anisotropy. We demonstrate that local control on spatial arrangement of nanoparticles in monolayers significantly strengthens dipolar interactions which enhances collective properties and results in possible super ferromagnetic order

Carbon Nanotube Degradation in Macrophages: Live Nanoscale Monitoring and Understanding of Biological Pathway

Despite numerous applications, the cellular-clearance mechanism of multiwalled carbon nanotubes (MWCNTs) has not been clearly established yet. Previous <i>in vitro</i> studies showed the ability of oxidative enzymes to induce nanotube degradation. Interestingly, these enzymes have the common capacity to produce reactive oxygen species (ROS). Here, we combined material and life science approaches for revealing an intracellular way taken by macrophages to degrade carbon nanotubes. We report the <i>in situ</i> monitoring of ROS-mediated MWCNT degradation by liquid-cell transmission electron microscopy. Two degradation mechanisms induced by hydroxyl radicals were extracted from these unseen dynamic nanoscale investigations: a non-site-specific thinning process of the walls and a site-specific transversal drilling process on pre-existing defects of nanotubes. Remarkably, similar ROS-induced structural injuries were observed on MWCNTs after aging into macrophages from 1 to 7 days. Beside unraveling oxidative transformations of MWCNT structure, we elucidated an important, albeit not exclusive, biological pathway for MWCNT degradation in macrophages, involving NOX₂ complex activation, superoxide production, and hydroxyl radical attack, which highlights the critical role of oxidative stress in cellular processing of MWCNTs

Fast Assembling of Magnetic Iron Oxide Nanoparticles by Microwave-Assisted Copper(I) Catalyzed Alkyne–Azide Cycloaddition (CuAAC)

Two dimensional (2D) nanoparticles (NP) assemblies have become very attractive due to their original collective properties, which can be modulated as a function of the nanostructure. Beyond precise control on nanostructure and easy way to perform, fast assembling processes are highly desirable to develop efficient and popular strategies to prepare systems with tunable collective properties. In this article, we report on the highly efficient and fast 2D assembling of iron oxide nanoparticles on a self-assembled monolayer (SAM) of organic molecules by the microwave (MW)-assisted copper­(I) catalyzed alkyne–azide cycloaddition (CuAAC) click reaction. Microwave irradiation favors a dramatic enhancement of the assembling reaction, which was completed with maximum density in NPs within one hour, much faster than the conventional CuAAC click reactions that require up to 48 h. Moreover, the MW-assisted click reaction presents the great advantage to preserve specific reactions between alkyne and azide groups at SAM and NP surfaces, respectively, and also to avoid undesired reactions. To the best of our knowledge, this is the first time this approach is performed to nanoparticles assembled on surfaces

Dr. Rómulo Santana Aguilar. Este 2009 se cumplen 35 años de su sensible fallecimiento

En recuerdo a su ilustre memoria citamos la nota necrológica escrita por J. VlLA VALENTI en Barcelona, diciembre 1975  

High Exchange Bias in Fe_3−δO₄@CoO Core Shell Nanoparticles Synthesized by a One-Pot Seed-Mediated Growth Method

Core–shell nanoparticles (NPs), which consist in a ferrimagnetic (FIM)/antiferromagnetic (AFM) interface and result in exchange bias coupling, became recently of primary importance in the field of magnetic nanoparticles. The enhancement of some applications such as hyperthermia or magnetic storage media based on the miniaturization of devices is correlated to the size reduction of NPs, which results in the decrease of the magnetocrystalline anisotropy and of the blocking temperature. We present here the synthesis of Fe_3−δO₄@CoO core–shell NPs by a one-pot seed-mediated growth process based on the thermal decomposition of metal complexes at high temperature. A 2 nm thick CoO shell was grown homogeneously from the starting Fe_3−δO₄ core surface. The Fe_3−δO₄@CoO core–shell NP structure has been deeply investigated by performing XRD and advanced techniques based on TEM such as EELS and EFTEM. The high quality of the core–shell interface resulted in the large exchange bias coupling at 5 K (<i>H</i>_E ≈ 4.1 kOe) between the FIM and the AFM components. In comparison to starting Fe_3−δO₄ NPs, the dramatic enhancement of the magnetic properties such as a high coercive field (at 5 K, <i>H</i>_C ≈ 15 kOe) were measured. Furthermore, the core–shell structure resulted in the enhancement of the magnetocrystalline anisotropy and the increase of the blocking temperature to 293 K

Design of Covalently Functionalized Carbon Nanotubes Filled with Metal Oxide Nanoparticles for Imaging, Therapy, and Magnetic Manipulation

Nanocomposites combining multiple functionalities in one single nano-object hold great promise for biomedical applications. In this work, carbon nanotubes (CNTs) were filled with ferrite nanoparticles (NPs) to develop the magnetic manipulation of the nanotubes and their theranostic applications. The challenges were both the filling of CNTs with a high amount of magnetic NPs and their functionalization to form biocompatible water suspensions. We propose here a filling process using CNTs as nanoreactors for high-yield <i>in situ</i> growth of ferrite NPs into the inner carbon cavity. At first, NPs were formed inside the nanotubes by thermal decomposition of an iron stearate precursor. A second filling step was then performed with iron or cobalt stearate precursors to enhance the encapsulation yield and block the formed NPs inside the tubes. Water suspensions were then obtained by addition of amino groups <i>via</i> the covalent functionalization of the external surface of the nanotubes. Microstructural and magnetic characterizations confirmed the confinement of NPs into the anisotropic structure of CNTs making them suitable for magnetic manipulations and MRI detection. Interactions of highly water-dispersible CNTs with tumor cells could be modulated by magnetic fields without toxicity, allowing control of their orientation within the cell and inducing submicron magnetic stirring. The magnetic properties were also used to quantify CNTs cellular uptake by measuring the cell magnetophoretic mobility. Finally, the photothermal ablation of tumor cells could be enhanced by magnetic stimulus, harnessing the hybrid properties of NP loaded-CNTs