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

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

On the Evolution of Pt Nanoparticles on Few-Layer Graphene Supports in the High-Temperature Range

Controlling the size, dispersion, and shape of nanoparticles (NPs) in the high-temperature range is a key topic for the development of new technologies with applications in the particular fields of catalysis and energy storage. In this article, we present an approach combining in situ transmission electron microscopy (TEM), electron tomography (ET), and molecular dynamics (MD) calculations for assessing the evolution of Pt NPs deposited onto few-layer graphene supports. Spherical Pt NPs with average sizes of 2 nm located preferentially at the support topographical defects (e.g., steps and edges) diffuse and coalesce along these defects, such that, after annealing to 700 °C, the nanoparticles were located exclusively here. Their dispersion remained significant; only the particle size distribution changed from mono- to bimodal. This statistical variation is discussed herein by reviewing fundamental issues such as the NP–support interaction and NP faceting, diffusion, and subsequent sintering in the high-temperature range. Fundamental MD simulations are reported here as reinforcements of the experimental findings and as a means to provide deeper insight into the phenomenological issues behind the behavior of the system investigated

On the Evolution of Pt Nanoparticles on Few-Layer Graphene Supports in the High-Temperature Range

Controlling the size, dispersion, and shape of nanoparticles (NPs) in the high-temperature range is a key topic for the development of new technologies with applications in the particular fields of catalysis and energy storage. In this article, we present an approach combining in situ transmission electron microscopy (TEM), electron tomography (ET), and molecular dynamics (MD) calculations for assessing the evolution of Pt NPs deposited onto few-layer graphene supports. Spherical Pt NPs with average sizes of 2 nm located preferentially at the support topographical defects (e.g., steps and edges) diffuse and coalesce along these defects, such that, after annealing to 700 °C, the nanoparticles were located exclusively here. Their dispersion remained significant; only the particle size distribution changed from mono- to bimodal. This statistical variation is discussed herein by reviewing fundamental issues such as the NP–support interaction and NP faceting, diffusion, and subsequent sintering in the high-temperature range. Fundamental MD simulations are reported here as reinforcements of the experimental findings and as a means to provide deeper insight into the phenomenological issues behind the behavior of the system investigated

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