Diego Muraca

Red-green emitting and superparamagnetic nanomarkers containing Fe3O4 functionalized with calixarene and rare earth complexes.

The design of bifunctional magnetic luminescent nanomaterials containing Fe3O4 functionalized with rare earth ion complexes of calixarene and β-diketonate ligands is reported. Their preparation is accessible through a facile one-pot method. These novel Fe3O4@calix-Eu(TTA) (TTA = thenoyltrifluoroacetonate) and Fe3O4@calix-Tb(ACAC) (ACAC = acetylacetonate) magnetic luminescent nanomaterials show interesting superparamagnetic and photonic properties. The magnetic properties (M-H and ZFC/FC measurements) at temperatures of 5 and 300 K were explored to investigate the extent of coating and the crystallinity effect on the saturation magnetization values and blocking temperatures. Even though magnetite is a strong luminescence quencher, the coating of the Fe3O4 nanoparticles with synthetically functionalized rare earth complexes has overcome this difficulty. The intramolecular energy transfer from the T1 excited triplet states of TTA and ACAC ligands to the emitting levels of Eu(3+) and Tb(3+) in the nanomaterials and emission efficiencies are presented and discussed, as well as the structural conclusions from the values of the 4f-4f intensity parameters in the case of the Eu(3+) ion. These novel nanomaterials may act as the emitting layer for the red and green light for magnetic light-converting molecular devices (MLCMDs).

Compact Ag@Fe3O4 core-shell nanoparticles by means of single-step thermal decomposition reaction.

A temperature pause introduced in a simple single-step thermal decomposition of iron, with the presence of silver seeds formed in the same reaction mixture, gives rise to novel compact heterostructures: brick-like Ag@Fe3O4 core-shell nanoparticles. This novel method is relatively easy to implement, and could contribute to overcome the challenge of obtaining a multifunctional heteroparticle in which a noble metal is surrounded by magnetite. Structural analyses of the samples show 4 nm silver nanoparticles wrapped within compact cubic external structures of Fe oxide, with curious rectangular shape. The magnetic properties indicate a near superparamagnetic like behavior with a weak hysteresis at room temperature. The value of the anisotropy involved makes these particles candidates to potential applications in nanomedicine.

Quasi-static magnetic measurements to predict specific absorption rates in magnetic fluid hyperthermia experiments

In this work, the issue on whether dynamic magnetic properties of polydispersed magnetic colloids modeled using physical magnitudes derived from quasi-static magnetic measurement can be extrapolated to analyze specific absorption rate data acquired at high amplitudes and frequencies of excitation fields is addressed. To this end, we have analyzed two colloids of magnetite nanoparticles coated with oleic acid and chitosan in water displaying, under a radiofrequency field, high and low specific heat power release. Both colloids are alike in terms of liquid carrier, surfactant and magnetic phase composition but differ on the nanoparticle structuring. The colloid displaying low specific dissipation consists of spaced magnetic nanoparticles of mean size around 4.8 nm inside a large chitosan particle of 52.5 nm. The one displaying high specific dissipation consists of clusters of magnetic nanoparticles of mean size around 9.7 nm inside a chitosan particle of 48.6 nm. The experimental evaluation of Neel and Brown relaxation times (∼10−10 s and 10−4 s, respectively) indicate that the nanoparticles in both colloids magnetically relax by Neel mechanism. The isothermal magnetization curves analysis for this mechanism show that the magnetic nanoparticles behave in the interacting superparamagnetic regime. The specific absorption rates were determined calorimetrically at 260 kHz and up to 52 kA/m and were well modeled within linear response theory using the anisotropy density energy retrieved from quasi-static magnetic measurement, validating their use to predict heating ability of a given polydispersed particle suspension. Our findings provide new insight in the validity of quasi-static magnetic characterization to analyze the high frequency behavior of polydispersed colloids within the framework of the linear response and Wohlfarth theories and indicate that dipolar interactions play a key role being their strength larger for the colloid displaying higher dissipation, i.e., improving the heating efficiency of the nanoparticles for magnetic fluid hyperthermia.

Edge phonons in black phosphorus

Black phosphorus has recently emerged as a new layered crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and behaviour of phonons near different black phosphorus edges are experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations. Polarized Raman results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements.

Influence of size-corrected bound-electron contribution on nanometric silver dielectric function. Sizing through optical extinction spectroscopy

The study of metal nanoparticles (NPs) is of great interest due to their ability to enhance optical fields on the nanometric scale, which makes them interesting for various applications in several fields of science and technology. In particular, their optical properties depend on the dielectric function of the metal, its size, shape and surrounding environment.This work analyses the contributions of free and bound electrons to the complex dielectric function of spherical silver NPs and their influence on the optical extinction spectra. The contribution of free electrons is usually corrected for particle size under 10?nm, introducing a modification of the damping constant to account for the extra collisions with the particles boundary.For the contribution of bound electrons, we considered the interband transitions from the d-band to the conduction band including the size dependence of the electronic density states for radii below 2?nm. Bearing in mind these specific modifications, it was possible to determine optical and band energy parameters by fitting the bulk complex dielectric function. The results obtained from the optimum fit are: Kbulk?=?2???1024 (coefficient for bound-electron contribution), Eg?=?1.91?eV (gap energy), EF?=?4.12?eV (Fermi energy), and ?b?=?1.5???1014?Hz (damping constant for bound electrons).Based on this size-dependent dielectric function, extinction spectra of silver particles in the nanometric?subnanometric radius range can be calculated using Mies theory, and its size behaviour analysed. These studies are applied to fit experimental extinction spectrum of very small spherical particles fabricated by fs laser ablation of a solid target in water. From the fitting, the structure and size distribution of core radius and shell thickness of the colloidal suspension could be determined. The spectroscopic results suggest that the colloidal suspension is composed by two types of structures: bare core and core?shell. The former is composed by Ag, while the latter is composed by two species: silver?silver oxide (Ag?Ag2O) and hollow silver (air?Ag) particles. High-resolution transmission microscopy and atomic force microscopy analysis performed on the dried suspension agree with the sizing obtained by optical extinction spectroscopy, showing that the latter is a very good complementary technique to standard microscopy methods.

Surface and interface interplay on the oxidizing temperature of iron oxide and Au–iron oxide core–shell nanoparticles

This article presents the effect of oxidation temperature on shape anisotropy, phase purity and growth of core–shell heterostructures and consequently their effect on structure–property relationships. Iron oxide and Au–iron oxide nanocomposites were synthesized by a thermal decomposition method by passing pure oxygen at different temperatures (125–250 °C). The prepared nanoparticles were surface functionalized by organic molecules; the presence of the organic canopy prevented both direct particle contact as well as further oxidation, resulting in the stability of the nanoparticles. We have observed a systematic improvement in the core and shell shape through tuning the reaction time as well as the oxidizing temperatures. Spherical and spherical triangular shaped core–shell structures have been obtained at an optimum oxidation temperature of 125 °C and 150 °C for 30 minutes. However, further increase in the temperature as well as oxidation time results in core–shell structure amendment and results in fully grown core–shell heterostructures. As stability and ageing issues limit the use of nanoparticles in applications, to ensure the stability of the prepared iron oxide nanoparticles we performed XRD analysis after more than a year and they remained intact showing no ageing effect. Specific absorption rate values useful for magnetic fluid hyperthermia were obtained for two samples on the basis of detailed characterization using X-ray diffraction, high-resolution transmission electron microscopy, Mossbauer spectroscopy, and dc-magnetization experiments.

From quenched to unquenched orbital magnetic moment on metallic@oxide nanoparticles: dc magnetic properties and electronic correlation

In this study, the correlation between magnetic, structure, and electronic properties of Ag@Fe3O4 hetero nanostructures are presented. These nanostructures were prepared using a two-step new chemical approach. Three different nanoparticle systems with different Ag concentrations have been prepared and characterized using high resolution transmission electron microscopy, dc magnetization (magnetization and coercive field as a function of temperature), X-ray absorption near edge spectroscopy, and magnetic circular dichroism studies (XMCD). From the correlation between XMCD and dc magnetic measurements (Verwey transition) the presence of non-stoichiometric magnetite in Ag@Fe3O4 nanoparticle systems was confirmed. From the spin and orbital contribution to the total magnetic moment, we conclude that the sample with less Ag seeds particle concentration presents a non-quenched orbital contribution. These phenomena were analyzed based on the actual models and correlated with dc magnetic properties. From these, we conclude that the enhancement on the orbital contribution increases the spin orbital interaction, also increasing the magnetocrystalline anisotropy reflected on the dc magnetic properties.

Universal behavior of the coefficients of the continuous equation in competitive growth models.

The competitive growth models (CGM) involving only one kind of particles, are a mixture of two processes, one with probability p and the other with probability 1-p. The p dependence produce crossovers between two different regimes. We demonstrate that the coefficients of the continuous equation, describing their universality classes, are quadratic in p (or 1-p ). We show that the origin of such dependence is the existence of two different average time rates. Thus, the quadratic p dependence is a universal behavior of all the (CGM). We derive analytically the continuous equations for two CGM, in 1+1 dimensions, from the microscopic rules using a regularization procedure. We propose generalized scalings that reproduce the scaling behavior in each regime. In order to verify the analytic results and the scalings, we perform numerical integrations of the derived analytical equations. The results are in excellent agreement with those of the microscopic CGM presented here and with the proposed scalings.

Optical and Magnetic Properties of Fe Nanoparticles Fabricated by Femtosecond Laser Ablation in Organic and Inorganic Solvents

Magnetic nanoparticles have attracted much interest due to their broad applications in biomedicine and pollutant remediation. In this work, the optical, magnetic, and structural characteristics of colloids produced by ultrashort pulsed laser ablation of a solid Fe target were studied in four different media: HPLC water, an aqueous solution of trisodium citrate, acetone, and ethanol. Optical extinction spectroscopy revealed an absorption band in the UV region for all, in contrast to the results obtained with nanosecond lasers. Micro-Raman spectroscopy showed that the samples are heterogeneous in their composition, with hematite, maghemite, and magnetite nanoparticles in all four solvents. Similar results were obtained by electron diffraction, which also found α-Fe. Magnetic properties were studied by vibrating-sample magnetometry, and showed nanoparticles in the superparamagnetic state. Under certain experimental conditions, submicrometer-sized iron oxide nanoparticles agglomerate into fractal patterns that show self-similar properties. Self-assembled annular structures on the nanometer scale were also observed and are reported for the first time.

Magnetic nanoparticles of Ni/NiO nanostructured in film form synthesized by dead organic matrix of yeast

An innovative sustainable protocol of nanobiotechnology has been developed to synthesize Ni/NiO magnetic nanoparticles, nanostructured in film form, through a dead organic matrix of the yeast Rhodotorula mucilaginosa, which was isolated from the Amazon region. It is a synergistic strategy that utilizes green technology, thus minimizing environmental impact and reducing costs. The best conditions for the adsorption of the metal through the dead organic matrix and subsequent synthesis of the nanoparticles were monitored by analyzing the biosorption of nickel by the yeast. The structural characteristics of the film-forming nanoparticles were investigated via high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy (FTIR). The magnetic properties of the nanoparticles produced by the dead organic matrix were determined in a superconducting quantum interference device (SQUID). Results indicate that the Ni/NiO nanoparticles are mainly spherical, with an average size of 5.5 nm, present magnetic properties, synthesized extracellularly and involve the proteins of the yeast, which probably confer organization in the film form. Such natural bioprocess suggests a rational protocol strategy as a template for the industrial-scale synthesis of magnetic nanoparticles of metals from the dead organic matrix of yeast and also provides a possible green system of nanobioremediation of metals from wastewater.