G.E.S. Brito

Interparticle interactions and surface contribution to the effective anisotropy in biocompatible iron oxide nanoparticles used for contrast agents

We have investigated the dynamic magnetic properties of dextran-coated magnetite (Fe3O4) nanoparticles in the form of (a) particles suspended in a carrier liquid and (b) concentrated powder obtained from lyophilization. The blocking temperature was found to increase from TB=42(2)to52(2)K (@μ0H=10mT) after lyophilization, showing the effects of dipolar interactions in samples with identical size distributions. The temperature dependence of the hyperfine field Bhyp(T) reveals the effects of collective magnetic excitations at low temperature, and allowed us to obtain the magnetic anisotropy energy Ea=3.6×10−21J for noninteracting particles. The obtained values can be understood assuming only magnetocrystalline anisotropy, without any additional contributions from surface, shape, or exchange origin. Moreover, a magnetocrystalline anisotropy constant value K1=10kJ∕m3 was obtained by assuming the cubic phase with easy magnetic direction [111] of the bulk material above the Verwey transition, supporting the idea...

Photoluminescence behavior of the Sm3+ and Tb3+ ions doped into the Gd2(WO4)3 matrix prepared by the Pechini and ceramic methods

The Gd2(WO4)3:RE3+ compounds (where RE3+ = Sm and Tb) were prepared by the Pechini and ceramic methods and characterized by X-ray diffraction and infrared spectroscopy. These rare earth materials present high orange (Sm3+-compound) and green (Tb3+-compound) luminescence intensity under UV radiation. The excitation spectra of these compounds presented broad bands arising from ligand-to-metal charge transfer (O®W and O®RE3+) and narrow bands from 4f-intraconfigurational transitions. The excitation spectra of Tb3+ system also exhibit broad bands attributed to the interconfigurational transition (4f-5d). The emission spectra exhibited the 4G5/2®6HJ (J = 5/2, 7/2, 9/2 and 11/2) and 5D4® 7FJ (J = 0-6) transitions (direct excitation), for the systems doped with Sm3+ and Tb3+, respectively, while a broad band assigned to the LMCT (O®W) is observed when the excitation is monitored on the O®W LMCT state around 270 nm. The experimental intensity parameters hSm and hEu present similar behaviors, suggesting that the Sm3+ ion is in a highly polarizable chemical environment with similar covalent character of the metal-donor atom interaction to the tungstate doped europium system. The cross-relaxation process from the 5D3 to the 5D4 levels of the Gd2(WO4)3:Tb3+ system has been also reported.

Synthesis and characterization of Fe3O4 nanoparticles with perspectives in biomedical applications

Nowadays the use of magnetic nanoparticles (MNP) in medical applications has exceeded expectations. In molecular imaging, MNP based on iron oxide coated with appropriated materials have several applications in vitro and in vivo studies. For applications in nanobiotechnology these MNP must present some characteristics such as size smaller than 100 nanometers, high magnetization values, among others. Therefore the MNP have physical and chemical properties that are specific to certain studies which must be characterized for quality control of the nanostructured material. This study presents the synthesis and characterization of MNP of magnetite (Fe3O4) dispersible in water with perspectives in a wide range of biomedical applications. The characterization of the colloidal suspension based on MNP stated that the average diameter is (12.6±0.2) nm determined by Transmission Electron Microscopy where the MNP have the crystalline phase of magnetite (Fe3O4) that was identified by Diffraction X-ray and confirmed by Mossbauer Spectroscopy. The blocking temperature of (89±1) K, Fe3O4 MNP property, was determined from magnetic measurements based on the Zero Field Cooled and Field Cooled methods. The hysteresis loops were measured at different temperatures below and above blocking temperature. The magnetometry determined that the MNP showed superparamagnetic behavior confirmed by ferromagnetic resonance.

One-pot green synthesis of cerium oxide–carbon microspheres and their catalytic ozonation activity

Monodisperse carbon microspheres surrounded by CeO2 nanoparticles were prepared by a one-pot green hydrothermal method. The hybrid microspheres were synthesized using a mixed D(+)-glucose and Ce(NO3)3 aqueous solution with subsequent hydrothermal treatment at 160 °C for 12 h. The as-prepared material was characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) spectroscopy. The results showed that microsphere growth is catalyzed by Ce3+ ions, for the same hydrothermal treatment period the diameters of hybrid microspheres were 5 μm, 25 times higher than for pure carbon spheres. These hybrid materials exhibited excellent catalytic activity in salicylic acid (SA) degradation by ozone. Ozonation studies showed when the CS20Ce microsphere was used as a catalyst, the mineralization rate of AS reached 80% after 60 minutes of treatment. Cerium oxide–carbon microspheres catalytic activity was attributed to the synergic effect of the carbon and CeO2 interface structures that were capable of enhancing radical species generation in the ozonation process.

Synthesis and characterization of silver phosphate/calcium phosphate mixed particles capable of silver nanoparticle formation by photoreduction

Silver phosphate is a semi-conductor sensitive to UV-Vis radiation (<530nm). Exposure to radiation removes electrons from the oxygen valence shell, which are scavenged by silver cations (Ag+), forming metallic silver (Ag0) nanoparticles. The possibility of silver nanoparticle formation in situ by a photoreduction process was the basis for the application of mixed calcium phosphate/silver phosphate particles as remineralizing and antibacterial fillers in resin-based dental materials. Mixed phosphate particles were synthesized, characterized and added to a dimethacrylate resin in 20% or 30% mass fractions to investigate their efficacy as ion-releasing fillers for dental remineralization and antibacterial activity. The formation of metallic silver nanoparticles after exposure to visible radiation from a dental curing unit (peak emission: 470nm) was demonstrated by particle X-ray diffraction and scanning electron microscopy analysis of the composite fractured surface. Calcium and phosphate release from materials containing the mixed particles were similar to those containing pure CaP particles, whereas Streptococcus mutans colonies were reduced by three orders of magnitude in relation to the control, which can be attributed to silver release. As expected, the optical properties of the materials containing mixed phosphate particles were compromised by the presence of silver. Nevertheless, materials containing mixed phosphate particles presented higher fracture strength and elastic modulus than those with pure CaP particles.

Study of the ferrofluid drying process for morphological and nanostructutal characterization

A drying method suitable for the study of the morphological and structural properties of colloidal magnetic systems, including a contrast agent used in Magnetic Resonance Imaging (MRI) is described. We tested three alternative ferrofluid drying methods: drying at 70 ‐ C in nitrogen atmosphere; drying in air at 70 ‐ C; and drying by liophylization using an MRI marker in the form of a colloidal suspension (Endorem T M - Guebert). X-ray diffraction (XRD), and transmission electron microscopy (TEM) were applied to each characterization method. The XRD allowed the observation of the possible physical-chemical changes of the stabilizers and also Fe3O4 present in the system. The morphology and nanoparticles size distribution was analyzed by TEM. Among the drying methods examined in this study, the liophylization has shown to be the more adequate one for the nanoparticles (Fe3O4) morphological study and nanostructural characterization, because the structure of the nanoparticles was maintained the same as in the suspension. The drying procedures performed at 70 ‐ C in the atmospheres of nitrogen and air let to the coalescence and growth of the nanoparticles, as well as some degradation has been noticed in some of the stabilizers.

The structure of Zn1-xMnxIn2Se4crystals grown by chemical vapour phase transport

Single crystals of Zn_(1-x)Mn_xIn_2Se_4 were grown by the chemical vapour phase transport technique (CVT). The Mn concentration was varied from x=1 to x=0.01. Through X-rays powder diffraction patterns and Laue diagrams of single crystals we studied the transformation from the hexagonal structure of MnIn_2Se_4 to the tetragonal structure of ZnIn_2Se_4.