Synthesis and antibacterial activity of silver nanoparticles with different sizes

Silver nanoparticles with different sizes (7, 29, and 89 nm mean values) were synthesized using gallic acid in an aqueous chemical reduction method. The nanoparticles were characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray diffraction (XRD), and ultraviolet–visible (UV–Vis) absorption spectroscopy; the antibacterial activity was assessed using the standard microdilution method, determining the minimum inhibitory concentration (MIC) according to the National Committee for Clinical Laboratory Standards. From the microscopies studies (TEM) we observed that silver nanoparticles have spherical (7 and 29 nm) and pseudospherical shape (89 nm) with a narrow size distribution. The sizes of the silver nanoparticles were controlled by varying some experimental conditions. It was found that the antibacterial activity of the nanoparticles varies when their size diminishes.     

Gerbes and Duality

We describe a global approach to the study of duality transformations between antisymmetric fields with transitions and argue that the natural geometrical setting for the approach is that of gerbes; these objects are mathematical constructions generalizing U(1) bundles and are similarly classified by quantized charges. We address the duality maps in terms of the potentials rather than on their field strengths and show the quantum equivalence between dual theories which in turn allows a rigorous proof of a generalized Dirac quantization condition on the couplings. Our approach needs the introduction of an auxiliary form satisfying a global constraint which in the case of 1-form potentials coincides with the quantization of the magnetic flux. We apply our global approach to refine the proof of the duality equivalence between the d=11 supermembrane and d=10 IIA Dirichlet supermembrane.     

Guest-Editorial

Editorial Introduction

Guest-Editorial     

Adsorption of n-alkanes on plasma-oxidized high-strength carbon fibers

The objective of this work was to characterize the degree of heterogeneity brought about by oxygen plasma treatment of carbon fibers by studying its effects on the adsorption of n-alkanes. Untreated and unsized high-strength carbon fibers were subjected to oxygen plasma treatments with different degrees of severity. A sample of the same material oxidized following a standard industrial method was also studied for comparison. Adsorption of C5-C10n-alkanes at 303-353 K was measured by inverse gas chromatography (IGC). Elution peaks were symmetrical for the fresh and industrially oxidized samples; however, a large extent of asymmetry was observed for the plasma-treated fibers. Differences in surface heterogeneity were quantified in terms of several adsorption thermodynamic magnitudes. Differential heats of adsorption exhibited values similar to those corresponding to the probe-basal plane interaction. The dispersive component of the surface tension of the solids increased clearly upon plasma oxidation, the increase being systematic according to the severity of plasma treatment. It can be concluded that plasma oxidation generates high-surface-energy sites responsible for trapping of n-alkane molecules, this effect being more marked as the chain length increases. The possibility of this effect being associated to creation of micropores was ruled out on the basis of volumetric CO2 adsorption experiments and IGC measurements at finite dilution. Scanning tunneling microscopy observations allowed us to establish a possible connection between fiber surface nanostructure and IGC results. The sites accessible to n-alkane molecules in the industrially oxidized sample seem to be highly disordered, thus leading to a weaker interaction with the adsorbate.     

Novel acid-catalyzed rearrangement of tetrahydro-1,2,3,4-tetrazines: unexpected formation of glycosazones

The present contribution discloses a simple and unexpected acid-catalyzed cleavage of tetrahydrotetrazines leading to 1,2-bis(hydrazones). Incorporation of a chiral fragment derived from carbohydrates enables the rapid preparation of glycosazones, a family of compounds employed by Emil Fischer to elucidate the configuration of sugars. In addition, a mechanistic proposal accounts for experimental observations.     

Enhancement by polydispersity of the biaxial nematic phase in a mixture of hard rods and plates

The phase diagram of a polydisperse mixture of uniaxial rodlike and platelike hard parallelepipeds is determined for aspect ratios kappa=5 and 15. All particles have equal volume, and polydispersity is introduced in a highly symmetric way. The corresponding binary mixture is known to have a biaxial phase for kappa=15, but to be unstable against demixing into two uniaxial nematics for kappa=5. The phase diagram for kappa=15 is qualitatively similar to that of the binary mixture, regardless of the amount of polydispersity, while for kappa=5 a sufficient amount of polydispersity stabilizes the biaxial phase. This provides clues for designing an experiment to observe this long searched biaxial phase.     

Charge transfer and dissociation in collisions of metal clusters with atoms

We present a combined theoretical and experimental study of charge transfer and dissociation in collisions of slow Li31(2+) clusters with Cs atoms. We provide a direct quantitative comparison between theory and experiment and show that good agreement is found only when the exact experimental time of flight and initial cluster temperature are taken into account in the theoretical modeling. We demonstrate the validity of the simple physical image that consists in explaining evaporation as resulting from a collisional energy deposit due to cluster electronic excitation during charge transfer.     

Analytical applications of membrane extraction in chromatography and electrophoresis

An overview of the analytical applications of membrane-based systems for sample enrichment in chromatography and capillary electrophoresis is presented. A brief introduction to the different types of membranes and the main forces related to the transport through them is also given.     

Experimental design methodologies to optimize monobutyltin chloride determination by hydride generation gas phase molecular absorption spectrometry

A hydride generation gas phase molecular absorption spectrometry (HG-GPMAS) method for the determination of butyltin compound is optimized by experimental design. This method is based on the conversion of the butyltin chloride into gaseous monobutyltin hydride by adding a sodium tetrahydroborate (III) solution. The hydride generated is collected in a liquid nitrogen cryogenic trap. This is revolatilized, driven to the quartz flow cell and measured with GPMAS with diode array detection. A Plackett-Burmann design is used for the study of the factors that influence the absorption signal. The optimization of the parameters affecting the production and collection of the monobutyltin hydride is achieved using a central composite design. Partial least square (PLS), multiple linear regression (MLR) and univariate calibration are applied to the spectra obtained. The quality parameters (detection limits and precision) for the butyltin chloride are reported. An interference study is made.