André M. Pereira

Size and surface effects on the magnetic properties of NiO nanoparticles

NiO nanoparticles (NPs) were prepared by a sol-gel process using the citrate route. The sol-gel parameters were tuned to obtain samples with different average particle sizes, ranging from 12 to 70 nm. Magnetic characterization revealed an increase in the blocking temperature with the diameter of the NPs and an increase in the effective magnetic anisotropy (K(eff)) with decreasing particle size. The magnetic moment per particle was calculated for all samples using the susceptibility value at T = 300 K. The number of uncompensated spins per NP was found to be proportional to n (n(S)≡ total number of spins), indicating that they are randomly distributed on the NP surface. For small diameters (<30 nm) the surface anisotropy constant was estimated, using, for NiO NPs, a recent model describing the evolution of K(eff) with particle size. Hysteretic loops performed at low temperatures after field cooling displayed loop shifts (∼6.5 kOe in the field axis and ∼0.18 emu g(-1) vertically), coercive field enhancement (H(C)≈ 4.8 kOe) and training effects for the smaller NPs. The sample with NPs of larger diameters presented magnetic properties close to those of bulk NiO.

Star-shaped magnetite@gold nanoparticles for protein magnetic separation and SERS detection

A novel synthetic methodology for star shaped gold-coated magnetic nanoparticles is reported. The coating is performed in two steps: formation of gold nuclei at the surface of magnetite nanoparticles followed by growth of the gold nuclei into a complete star shaped shell. The star-shaped gold-coated magnetic nanoparticles thus obtained preserve the magnetic properties of the precursor magnetite nanoparticles, e.g. they can be easily separated with a magnet. In addition, the gold coating provides interesting optical properties while simultaneously allowing for biofunctionalization that may be advantageous for biological applications, such as (bio)detection via surface-enhanced Raman spectroscopy (SERS). As a proof-of-concept, a capping agent terminated with a nickel(II)-nitrilotriacetate group showing high affinity for histidine was used to modify the surface of the nanoparticles. The resulting star-shaped nanoparticles were used to selectively capture histidine-tagged maltose-binding protein from a crude cell extract. Finally, the performance of star shaped gold-coated magnetic nanoparticles as SERS platforms was demonstrated through the detection of Raman active dye (Astra Blue).

[VO(acac)2] hybrid catalyst: from complex immobilization onto silica nanoparticles to catalytic application in the epoxidation of geraniol

This work reports the preparation of a novel hybrid nanocatalyst through the immobilization of oxidovanadiumIV acetylacetonate ([VO(acac)2]) onto silica nanoparticles functionalized with 3-aminopropyltriethoxysilane (APTES) and its catalytic application in the allylic epoxidation of geraniol. All the nanomaterials were characterized by chemical analysis, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The parent and amine-functionalized silica nanoparticles were also analyzed by 29Si and 13C solid-state nuclear magnetic resonance and the supported [VO(acac)2] nanomaterial was characterized by magnetic susceptibility measurement. The APTES organosilane was grafted onto the silica nanoparticles with an efficiency of 90%, through mono- and bidentate covalent grafting. The [VO(acac)2] complex was covalently anchored on the surface of the APTES-functionalized silica nanoparticles with 25% of efficiency. Magnetic susceptibility measurements in combination with XPS indicated that the oxidation state of the metal center (VO2+) was preserved upon the immobilization reaction. The catalytic performance of the novel hybrid [VO(acac)2] nanocatalyst was evaluated in the epoxidation of geraniol, at room temperature, using tert-butyl hydroperoxide as an oxygen source. The nanocatalyst presented 100% of substrate conversion, 99% of selectivity towards the 2,3-epoxygeraniol product and was stable upon reuse in further four cycles.

Tailored design of CoxMn1−xFe2O4 nanoferrites: a new route for dual control of size and magnetic properties

This work reports the tailored design of novel mixed ferrite nanoparticles, CoxMn1−xFe2O4 (x = 0, 0.3, 0.7, and 1), through an optimized one-pot aqueous coprecipitation process. The influence of the substitution between Mn(II) and Co(II) and of the alkaline agent, isopropanolamine (MIPA) or NaOH, on the morphological, chemical and magnetic properties of the nanomaterials was investigated. The joint action between chemical substitution and type of alkaline agent allowed a precise tuning of the particle size, magnetic properties and inversion degree of the spinel structure. A wide range of particle dimensions (from 3 to 30 nm) and saturation magnetization (from 57 to 71 emu g−1) was achieved. The increase of Co(II) content from x = 0 to x = 1 led to a systematic decrease of the particle size, regardless of the base type, which could be modelled by an exponential decay function. For each Co:Mn composition, the use of MIPA instead of the traditional NaOH promoted a three times reduction of the particle size and simultaneously switched the magnetic state of the CoxMn1−xFe2O4 nanomaterials from ferromagnetic to superparamagnetic. These results constitute a step forward in the challenging quest for high-performance magnetic nanoprobes by an eco-friendly synthesis route.

Unravelling the effect of interparticle interactions and surface spin canting in γ-Fe2O3@SiO2 superparamagnetic nanoparticles

The present study investigates the magnetic properties of spherical monodispersed maghemite (γ-Fe2O3) nanoparticles coated with multiple silica (SiO2) layers of different thicknesses, forming core-shell multifunctional nanomaterials. This study was performed using a combination of local probe techniques (Mossbauer spectrometry) and magnetization measurements. At room temperature, both techniques confirm the superparamagnetic state of the samples, even after being coated with the SiO2 shells. The zero-field-cooling–field-cooling magnetization curves of the silica-coated γ-Fe2O3 nanomaterials with different shell thicknesses allow the evaluation of the intensity of the interparticle dipole–dipole interactions. We estimate the interparticle energy within the framework of dipolar interaction models and relate it with the hyperfine parameters. We further observe that this dipole–dipole interaction increases the superparamagnetic energy barrier, which largely depends on the interparticle distance. Finally, we c...

Architectured design of superparamagnetic Fe3O4 nanoparticles for application as MRI contrast agents: mastering size and magnetism for enhanced relaxivity

This work reports the mastered design of novel water-dispersible superparamagnetic iron oxide nanomaterials with enhanced magnetic properties and reduced size. A straightforward cost-effective aqueous coprecipitation route was developed, based on the use of three new coprecipitation agents: the polydentate bases diethanolamine, triethanolamine and triisopropanolamine. Through the selection of these alkanolamines which presented different complexing properties, an improvement of the surface spin order could be achieved upon the reduction of the nanomaterial dimensions (from 8.7 to 3.8 nm) owing to the complexation of the polydentate bases with the subcoordinated iron cations on the particle surface. In particular, the alkanolamine with the highest chelating ability (triethanolamine) led to the nanomaterial with the smallest size and the thinnest magnetic dead layer. In order to evaluate the importance of the dual control of size and magnetism, the relaxometric properties of the nanomaterials were investigated, whereby maximum values of transverse relaxivity r2 of 300.30 and 253.92 mM-1 s-1 at 25 and 37 °C, respectively (at 20 MHz) were achieved, making these nanomaterials potential T2-weighted MRI contrast agents. Moreover, these values were significantly higher than those reported for commercial T2 contrast agents and other iron oxides with identical dimensions. Hence, we were able to demonstrate that the r2 enhancement cannot only be achieved by an increase of particle/cluster size, but also through the precise control of the surface magnetic properties while constraining the nanomaterial dimensions. These achievements open up new perspectives on the mastered design of magnetic nanoprobes, overcoming the limitations related to the deleterious effect of size reduction.

Gd5(Si,Ge)4 thin film displaying large magnetocaloric and strain effects due to magnetostructural transition

Magnetic refrigeration based on the magnetocaloric effect is one of the best alternatives to compete with vapor-compression technology. Despite being already in its technology transfer stage, there is still room for optimization, namely, on the magnetic responses of the magnetocaloric material. In parallel, the demand for different magnetostrictive materials has been greatly enhanced due to the wide and innovative range of technologies that emerged in the last years (from structural evaluation to straintronics fields). In particular, the Gd5(SixGe1−x)4 compounds are a family of well-known alloys that present both giant magnetocaloric and colossal magnetostriction effects. Despite their remarkable properties, very few reports have been dedicated to the nanostructuring of these materials: here, we report a ∼800u2009nm Gd5Si2.7Ge1.3 thin film. The magnetic and structural investigation revealed that the film undergoes a first order magnetostructural transition and as a consequence exhibits large magnetocaloric ef...

Highly Active Ruthenium Supported on Magnetically-Recyclable Chitosan-Based Nanocatalyst for Nitroarenes Reduction

A Ru supported on a magnetically separable chitosan‐based nanomaterial (Mn@CS@Ru) was prepared by wet impregnation based on ionic gelation using sodium tripolyphosphate as a cross‐linking agent. The ionic gelation of chitosan leads to a supporting matrix to promote the embedding of manganese(II) ferrite and Ru nanoparticles (NPs) by electrostatic interactions. The effects of the formulation and method parameters on the fabrication process were investigated, and the resulting as‐prepared Mn@CS@Ru nanocatalyst was characterized. The catalytic activity of the Mn@CS@Ru nanomaterial was evaluated in the reduction of 4‐nitrophenol (4‐NP) and 4‐nitroaniline (4‐NA) in the presence of sodium borohydride as a reducing agent at room temperature. The turnover frequency values in the reduction of 4‐NP and 4‐NA were 273.9 and 336.5u2005min−1, respectively, which were attributed to the very small size of the hybrid nanomaterial (32.0±2.8u2005nm with 3.9±0.1u2005nm Ru NPs) that provided a large surface‐area‐to‐volume ratio for the chemical reaction. Furthermore, the hybrid nanocatalyst was recovered easily by magnetic separation after the catalytic reaction and could be reused in at least 10 cycles without a loss of catalytic activity, which confirms its high stability. The present route is a new approach to synthesize highly active magnetic heterogeneous catalysts for the reduction of nitroarenes based on metallic NPs with easy accessibility, excellent activity, and convenient recovery.

Scanning Hall Probe Imaging of LaFe13-xSix

Magnetocaloric materials with a Curie temperature near room temperature are of interest for application in high-efficiency solid state cooling. There are several promising families of materials including the LaFe13-xSix system which offers large magnetocaloric entropy change, low magnetic and thermal hysteresis, and tunability of the metamagnetic transition by introduction of interstitial hydrogen or partial substitution on the La or Fe sites. There is a large amount of literature on the properties and mechanism of the magnetocaloric effect in this material system, and more recently our group and several other groups have discussed the origins of the dynamics of the metamagnetic transition and its relation to magnetic hysteresis. Nevertheless, although extremely informative in other systems, there has been little spatially resolved information concerning the nature of the magnetic transition in this system. Here we use scanning Hall probe imaging to study LaFe13-xSix polycrystalline samples with x=1.2 prepared by induction melting to resolved the local static and dynamic magnetic properties. We find that the local properties of the magnetic transition are governed by chemical inhomogeneity rather that demagnetization effects associated with sample geometry.

Spontaneous magnetization above

In the present work, spontaneous magnetization is observed in the inverse magnetic susceptibility of La