An addition to the oxoacid family: H2B12(OH)12

The acid H(2)B(12)(OH)(12) can be isolated as a crystalline solid by protonation of the hydroxylated borane anion, B(12)(OH)(12)(2)(-). This acidic compound has low solubility in water, conducts protons in the solid state, and has thermal stability to a temperature of 400 degrees C. The conductivity mechanism is a Grotthuss mechanism with a low activation enthalpy (9-13 kcal/mol). This new acid represents an addition to the class of oxoacids, of which sulfuric and phosphoric acid are the most prominent examples.     

"Nanocasting": using SBA-15 silicas as hard templates to obtain ultrasmall monodispersed gamma-Fe2O3 nanoparticles

This work describes the use of mesoporous SBA-15 silicas as hard templates for the size-controlled synthesis of oxide nanoparticles, with the pores acting as nanoscale reactors. This fundamental work is mainly aimed at understanding unresolved issues concerning the occurrence and size dependence of phase transitions in oxide nanocrystals. Aqueous solutions of Fe(NO3)3*9H2O are deposited inside the pores of SBA-15 silicas with mesopore diameters of 4.3, 6.6, and 9.5 nm. By calcination, the nitrate salt transforms into FeOx oxides. The XRD peaks of nanocrystals are broad and overlapping, resulting in ambiguities attributed to a given allotropic variety of Fe2O3 (alpha, epsilon, or gamma) or Fe3O4. The association of XRD, SAED, and Raman information is necessary to solve these ambiguities. The metastable gamma-Fe2O3 variety is selectively formed at low Fe/Si atomic ratio (ca. 0.20) and when a low calcination temperature is used (773 or 873 K followed by quenching to room temperature once the targeted temperature is reached). The small size dispersion of the patterned nanoparticles, suggested on a local scale by TEM, is confirmed statistically by magnetic measurements. The nanoparticles have a superparamagnetic behavior around room temperature. Their magnetic moments (from 220 to 370 mB), their sizes (from 4.0 to 4.8 nm), and their blocking temperatures (from 36 to 58 K) increase with the silica template mesopore diameter. Their magnetic properties are compared to those of standard gamma-Fe2O3 nanoparticles of similar size, obtained by coprecipitation in water and stabilized by a citrate coating.     

Magneto-orientational properties of ionically stabilized aqueous dispersions of Ni(OH)2 nanoplatelets

Magnetic and orientational behavior of nickel hydroxide nanoplatelets ionically stabilized in a liquid matrix is studied. Under an applied field the platelets orient their faces normal to its direction. For characterization of the individual behavior of dispersed and non-interacting particles three techniques are used: SAXS, SQUID and magneto-optics. Analysis reveals that nickel hydroxide in a platelet phase is paramagnetic with a pronounced anisotropy of the intrinsic susceptibility, the major component of which (in the direction normal to platelet face) exceeds the minor one by about 25%.     

Smart hybrid magnetic self-assembled micelles and hollow capsules

Novel magnetic nano-composites are obtained by the self-assembly in water of polypeptide-based diblock copolymers polybutadiene-b-poly(glutamic acid) combined with hydrophobically modified γ-Fe₂O₃ nanoparticles. These hybrid supramolecular objects are either filled micelles (3-d) or hollow vesicles with a magnetic membrane (2-d), deformation of which under an applied magnetic field has been evidenced. These nanoparticles are also able to respond to stimuli such as pH and ionic strength due to the presence of the polypeptide block, thus forming what we called multi-responsive nanocapsules. These superparamagnetic hybrid self-assemblies offer attractive potentialities in biomedicine and biotechnology due to their dimensions (0.1–0.5 μm) small enough to stay for some time in the blood circulation, and due to the properties brought about by the iron oxide nanoparticles: possible manipulation by an external magnetic field gradient, local heating by a radio-frequency field for cancer radio-therapy, labeling of organs to enhance the contrast in Magnetic Resonance Imaging.     

What tunes the structural anisotropy of magnetic fluids under a magnetic field?

In the present study, the structure of monophasic ionic magnetic fluids under a static magnetic field is explored. In these aqueous electrostatically stabilized ferrofluids, we vary both the isotropic interparticle interactions and the anisotropic dipolar magnetic interaction by tuning the ionic strength and the size of the nanoparticles. Small angle neutron scattering measurements carried out on nanoparticles dispersed in light water exhibit miscellaneous 2D nuclear patterns under a magnetic field with various q-dependent anisotropies. In this nondeuterated solvent where the magnetic scattering is negligible, this anisotropy originates from an anisotropy of the structure of the dispersions. Both the low q region and the peak of the structure factor can be anisotropic. On the scale of the interparticle distance, the structure is better defined in the direction perpendicular to the field. In the thermodynamic limit (q-->0), the model previously described in ref 10 matches the data without any fitting parameters: the interparticle interaction is more repulsive in the direction parallel to the magnetic field. At low q, the amplitude of the anisotropy of the pattern is governed by the ratio of two interaction parameters: the reduced parameter of the anisotropic magnetic dipolar interaction, gamma/Phi, over the isotropic interaction parameter, , in zero field, which is proportional to the second virial coefficient.     

Liquid-liquid phase-transfer of magnetic nanoparticles in organic solvents

We report a novel route for the preparation of well-defined colloidal dispersions of magnetic nanoparticles stabilized by steric repulsion in organic solvents. The usual methods standardly lead to the surfaction of multiparticle aggregates, incompatible with our long-term aim of studying and modeling the influence of magnetic dipolar interactions in colloidal dispersions which are free of aggregates, all other interactions being perfectly defined. A new and reproducible method based on a surfactant-mediated liquid-liquid phase transfer of individually dispersed gamma-Fe(2)O(3) nanoparticles from an aqueous colloidal dispersion to an organic phase is developed. The choice of the reagent and the preparation techniques is discussed. Among several solvent/surfactant pairs, the cyclohexane/dimethyldidodecylammonium bromide (DDAB) system is found to fulfill the colloidal stability criterion: aggregation does not appear, even upon aging. A complete transfer of isolated particles is observed above a threshold in DDAB concentration. The nanoparticle surface is then fully covered with adsorbed DDAB molecules, each surfactant head occupying a surface of 0.57+/-0.05 nm(2). The volume fraction of the cyclohexane-based organosols is easily tunable up to a volume fraction of 12% by modifying the volume ratio of the organic and of the aqueous phases during the liquid-liquid phase transfer.     

Experimental evidence for violation of the fluctuation-dissipation theorem in a superspin glass

We present the experimental observation of the fluctuation-dissipation theorem violation in an assembly of interacting magnetic nanoparticles in the low temperature superspin-glass phase. The magnetic noise is measured with a two-dimension electron gas Hall probe and compared to the out of phase ac susceptibility of the same ferrofluid. For "intermediate" aging times of the order of 1 h, the ratio of the effective temperature T(eff) to the bath temperature T grows from 1 to 6.5 when T is lowered from T(g) to 0.3 T(g), regardless of the noise frequency. These values are comparable to those measured in an atomic spin glass as well as those calculated for a Heisenberg spin glass.     

Dynamics of paramagnetic nanostructured rods under rotating field

The dynamical rotational behavior of magnetic nanostructured rods based on the auto-association of maghemite nanoparticles and block-copolymers is probed by optical microscopy under rotating fields in a simple liquid. The re-orientation of the rods by a field rotated by 90° is first studied. The measured relaxation is characteristic of paramagnetic objects. Under a stationary rotating field, a synchronous rotational regime is observed at low field frequency. Above a frequency threshold which scales as H², the dynamics becomes asynchronous with back-and-forth rotations. These behaviors are well predicted by the presented model.