Semiconductor nanocrystals obtained by colloidal chemistry for biological applications

Semiconductor nanoparticles in the quantum confinement regime used as biolabels present many advantages over the other chemical species used as fluorophores. They are composed of 2000-6000 atoms rendering a far greater photostability and allowing for long time bioimaging experiments. In this work we present a synthetic route for the obtention of large quantities of highly fluorescent CdSe and CdTe/CdS core-shell nanocrystals based on aqueous colloidal chemistry. The methodologies were optimized and the systems were characterized by optical spectroscopy, transmission electronic microscopy and X-Ray diffractometry. The fluorescent biolabels were tested in live macrophages.     

Effects of pH and Carbon Source on <Emphasis Type="Italic">Synechococcus</Emphasis> PCC 7002 Cultivation: Biomass and Carbohydrate Production with Different Strategies for pH Control

Synechococcus PCC 7002 is an interesting species in view of industrial production of carbohydrates. The cultivation performances of this species are strongly affected by the pH of the medium, which also influences the carbohydrate accumulation. In this work, different methods of pH control were analyzed, in order to obtain a higher production of both Synechococcus biomass and carbohydrates. To better understand the influence of pH on growth and carbohydrate productivity, manual and automatic pH regulation in CO₂ and bicarbonate system were applied. The pH value of 8.5 resulted the best to achieve both of these goals. From an industrial point of view, an alternative way to maintain the pH practically constant during the entire period of cultivation is the exploitation of the bicarbonate-CO₂ buffer system, with the double aim to maintain the pH in the viability range and also to provide the amount of carbon required by growth. In this condition, a high concentration of biomass (6 g L^?1) and carbohydrate content (around 60 %) were obtained, which are promising in view of a potential use for bioethanol production. The chemical equilibrium of C-N-P species was also evaluated by applying the ionic balance equations, and a relation between the sodium bicarbonate added in the medium and the equilibrium value of pH was discussed.     

Thermodynamics of a quasi-two-dimensional electron gas: Effects of magnetic fields, temperature and finite width

The effects of nonzero temperature, finite thickness and magnetic fields on an electron gas confined in a quasi-two-dimensional (Q2DEG) region are studied. Finite values of the thickness allow the existence of energy bands and give new characteristics to the system. Here we study some thermodynamic and transport properties to get a better understanding of these effects. For electronic densities of the order of 10¹² electrons per cm², and temperatures lower than 4 K, it is found that the chemical potential, free energy, magnetization, magnetic susceptibility and the inverse of Hall conductivity are independent of the thickness up to it reaches a threshold value. Systems with thicknesses a little above of the critical value lose the two-dimensional character, but they do not have the properties of a three-dimensional electron gas.     

The local structure around the Nd impurity incorporated into the Bi4Ge3O12 crystal matrix: An ab initio study

The local structure around the Nd impurity introduced at the Bi site in the Bi₄Ge₃O₁₂ compound is theoretically investigated by density-functional theory based augmented plane wave method. A set of parameters that completely describes the Nd neighborhood is calculated by treating the Nd 4f electrons either as localized or as delocalized. In both cases the local structure was dominantly determined by the Nd departure from the ideal host position along the trigonal axis. Displacement occurs in the direction towards the center of the Oxygen octahedron around the Nd.     

Drying of solids with irregular geometry: numerical study and application using a three-dimensional model

This article proposes a numerical solution for the diffusion equation applied to solids with arbitrary geometry using non-orthogonal structured grids for the boundary condition of the first kind. A transient three-dimensional mathematical formulation written in boundary fitted coordinates and numerical formalism to discretize the diffusion equation by using the finite volume method, including numerical analysis of the computational solution are presented. To validate the proposed solution, the results obtained in this work were compared with well-known numerical solution available in literature and good agreement was observed. In order to verify the potential of the proposed numerical solution, it was applied to describe mass transfer inside ceramic roof tiles during drying. For that, it was used experimental data of the drying kinetics at the following temperatures: 55.6; 69.7; 82.7 and 98.6 °C. An optimization technique using experimental dataset has been presented to estimation of transport properties. The obtained statistical indicators enable to conclude that the numerical solution satisfactorily describes the drying processes.     

Effect of a charge layer on the plasmon-polariton dispersion curve in doped semiconductor superlattices

We consider the effects of a charge layer (accumulation and depletion layers) on the spectrum of collective plasmon-polariton modes of a semiconductor superlattice consisting of alternating mediaA andB, where mediumA is an-doped semiconductor and mediumB an insulator. The effect of the charge layer is taken into account assuming a linear position dependence for the free-carrier concentration and for the dielectric function, within mediumA. Applications are made considering mediumA asn-type semiconductor and mediumB being vacuum. We compare our results with those obtained in the abscence of the charge layer and we discuss their differences.     

Supercritical fluid chromatography with photodiode array detection for pesticide analysis in papaya and avocado samples

To improve the analysis of pesticides in complex food matrices with economic importance, alternative chromatographic techniques, such as supercritical fluid chromatography, can be used. Supercritical fluid chromatography has barely been applied for pesticide analysis in food matrices. In this paper, an analytical method using supercritical fluid chromatography coupled to a photodiode array detection has been established for the first time for the quantification of pesticides in papaya and avocado. The extraction of methyl parathion, atrazine, ametryn, carbofuran, and carbaryl was performed through the quick, easy, cheap, effective, rugged, and safe methodology. The method was validated using papaya and avocado samples. For papaya, the correlation coefficient values were higher than 0.99; limits of detection and quantification ranged from 130–380 and 220–640 μg/kg, respectively; recovery values ranged from 72.8–94.6%; precision was lower than 3%. For avocado, limit of detection values were ?450 μg/kg; precision was lower than 11%; recoveries ranged from 50.0–94.2%. Method feasibility was tested for lime, banana, mango, and melon samples. Our results demonstrate that the proposed method is applicable to methyl parathion, atrazine, ametryn, and carbaryl, toxics pesticides used worldwide. The methodology presented in this work could be applicable to other fruits.     

Effects of Ultraviolet Radiation (UVA+UVB) and Copper on the Morphology,Ultrastructural Organization and Physiological Responses of the Red Alga Pterocladiella capillacea

The effect of ultraviolet (UV) radiation and copper (Cu) on apical segments of Pterocladiella capillacea was examined under two different conditions of radiation, PAR (control) and PAR+UVA+UVB (PAR+UVAB), and three copper concentrations, ranging from 0 (control) to 0.62, 1.25 and 2.50 μm . Algae were exposed in vitro to photosynthetically active radiation (PAR) at 70 μmol photons m^?2 s^?1, PAR + UVB at 0.35 W m^?2 and PAR +UVA at 0.70 W m^?2 during a 12‐h photocycle for 3 h each day for 7 days. The effects of radiation and copper on growth rates, content of photosynthetic pigments and photosynthetic performance were analyzed. In addition, samples were processed for light and transmission electron microscopy. The content of photosynthetic pigments decreased after exposure to radiation and Cu. Compared with PAR radiation and copper treatments modified the kinetics patterns of the photosynthesis/irradiance curve. The treatments also caused changes in the ultrastructure of cortical and subcortical cells, including increased cell wall thickness and accumulation of plastoglobuli, as well as changes in the organization of chloroplasts. The results indicate that the synergistic interaction between UV radiation and Cu in P. capillacea, led to the failure of protective mechanisms and causing more drastic changes and cellular imbalances.     

Influence of substrate temperature and N2/Ar flow ratio on the stoichiometry,structure and hardness of TaNx coatings deposited by DC reactive sputtering

The effect of substrate temperature and N2/Ar flow ratio on the stoichiometry, structure and hardness of TaN_x coatings prepared on (111) Si substrates by DC reactive sputtering was investigated. For the structural, chemical and morphological analysis, X‐ray diffraction (XRD), Auger electron scanning and atomic force microscopy were respectively used. Hardness values of thin films were determined using the work of indentation model from nanoindentation measurements. TaN stoichiometric coatings were obtained for samples deposited at room temperature. The stoichiometric TaN phase was not obtained by increasing the temperature up to 773 K, even when increasing the N₂/Ar flow ratio. Even when a saturation in nitrogen content was achieved, nitrogen vacancies are still present in those samples. For coatings prepared at 773 K and low N₂/Ar flow ratio, a phase mixture between TaN_x and cubic α‐Ta was observed, while a cubic structure δ‐TaN was formed by increasing the N₂/Ar flow ratio. A maximum in hardness and (38 GPa) was obtained for the sample deposited at 773 K and a N₂/Ar flow ratio of 0.2, which presented a δ‐TaN cubic crystalline structure and a roughness value of 1.6 nm. Copyright © 2015 John Wiley & Sons, Ltd.