Ultrathin silver‐coated gold nanoparticles as suitable substrate for surface‐enhanced Raman scattering

Surface‐enhanced Raman scattering (SERS) is an extremely powerful tool for the analysis of the composition of bimetallic nanoparticle (BNP) surfaces because of the different adsorption schemes adopted by several molecules on different metals, such as Au and Ag. The preparation of BNPs normally implies a change in the plasmonic properties of the core metal. However, for technological applications it could be interesting to synthesize core–shell structures preserving these original plasmonic properties. In this work, we present a facile method for coating colloidal gold nanoparticles (NPs) in solution with a very thin shell of silver. The resulting bimetallic Au@Ag system maintains the optical properties of gold but shows the chemical surface affinity of silver. The effectiveness of the coating method, as well as the progressive silver enrichment of the outermost part of the Au NPs, has been monitored through the SERS spectra of several species (chloride, luteolin, thiophenol and lucigenin), which show different behaviors on gold and silver surfaces. A growth mechanism of the Ag shell is proposed on the basis of the spectroscopic and microscopic data consisting in the formation and deposit of Ag clusters on the Au NP surface. Copyright © 2009 John Wiley & Sons, Ltd.     

Kidney exchange simulation and optimization

One of the challenges in a kidney exchange program (KEP) is to choose policies that ensure an effective and fair management of all participating patients. In order to understand the implications of different policies of patient allocation and pool management, decision makers should be supported by a simulation tool capable of tackling realistic exchange pools and modeling their dynamic behavior. In this paper, we propose a KEP simulator that takes into consideration the wide typology of actors found in practice (incompatible pairs, altruistic donors, and compatible pairs) and handles different matching policies. Additionally, it includes the possibility of evaluating the impact of positive crossmatch of a selected transplant, and of dropouts, in a dynamic environment. Results are compared to those obtained with a complete information model, with knowledge of future events, which provides an upper bound to the objective values. Final results show that shorter time intervals between matches lead to higher number of effective transplants and to shorter waiting times for patients. Furthermore, the inclusion of compatible pairs is essential to match pairs of specific patient–donor blood type. In particular, O-blood type patients benefit greatly from this inclusion.     

A spectral algorithm for large-scale systems of nonlinear monotone equations

A derivative-free iterative scheme that uses the residual vector as search direction for solving large-scale systems of nonlinear monotone equations is presented. It is closely related to two recently proposed spectral residual methods for nonlinear systems which use a nonmonotone line-search globalization strategy and a step-size based on the Barzilai-Borwein choice. The global convergence analysis is presented. In order to study the numerical behavior of the algorithm, it is included an extensive series of numerical experiments. Our computational experiments show that the new algorithm is computationally efficient.     

Compositional and electrical properties of Cr,Nb, Cr/Nb,CrNbN, and CrN/NbN multilayers grown using the d.c. magnetron sputtering technique

Cr, Nb, Cr/Nb, CrN_x, NbN_x, CrNbN, and (CrN/NbN)_n structures were produced on Si and glass substrates, using the d.c. magnetron sputtering technique. Compositional analysis, based on binding energies of Cr, Nb, and N, was carried out by means of X-ray photoelectron spectroscopy (XPS). Through Auger electron spectroscopy (AES), depth profiles were obtained, allowing to demonstrate the multilayers production. Surface morphological characteristics, as roughness and grain size, were evaluated by atomic force microscopy (AFM), revealing very smooth surfaces, that is a consequence of the deposition parameters used in the synthetization experiments. Finally, for different configurations, conductivity measurements were carried out, revealing the influence of nitrogen content and temperature on electron transport. It was found that substoichiometric nitrides (CrN_0.35 and NbN_0.12) exhibited the highest conductivity, because the nitrogen atoms act as donor of electrons.     

2,4‐Dibromo‐3‐methoxy‐6‐nitrobenz­aldeyde: effect of substituents on ring geometry

In the title compound, C₈H₅Br₂NO₄, the endocyclic angles of the ring deviate significantly from the ideal value of 120°. The substituents deviate from the plane of the ring, with large twist angles for the aldehyde, nitro and methoxy groups. The geometry of the mol­ecule in the crystal is compared with that of the isolated mol­ecule, as given by a self‐consistent field molecular‐orbital Hartree–Fock calculation. Only weak hydrogen bonds of the C—H?Br and C—H?O types are present in the crystal structure.     

Use of hydrogen peroxide to achieve interference-free stripping voltammetric determination of copper at the bismuth-film electrode

In this work, a new approach is presented to allow interference-free determination of Cu (II) by stripping voltammetry using the bismuth-film electrode. The addition of hydrogen peroxide to the electroanalytical cell has promoted complete resolution between re-dissolution peaks of Bi (III) and Cu (II). The absence of interference could be evaluated by the correlation coefficient (r > 0.99) between Cu (II) concentration and its shifted current peak (at +212 mV) while achieving a slightly fluctuation of the bismuth current peak at −180 mV. Studies were performed aiming towards the optimum conditions for trace determination of Cu (II) using hydrogen peroxide. The methodology was applied to a real sample (sugarcane spirits) and the results were compared to those from graphite furnace atomic absorption spectrometry. The analytical parameters of merit and the results of the analysis indicated that the analytical methodology could be readily used for trace determination of Cu (II).     

Suzuki-Miyaura cross-coupling reactions of aryl tellurides with potassium aryltrifluoroborate salts

[reaction: see text] Palladium(0)-catalyzed cross-coupling between potassium aryltrifluoroborate salts and aryl tellurides proceeds readily to afford the desired biaryls in good to excellent yield. The reaction seems to be unaffected by the presence of electron-withdrawing or electron-donating substituents in both the potassium aryltrifluoroborate salts and aryl tellurides partners. Biaryls containing a variety of functional groups can be prepared. A chemoselectivity study was also carried out using aryl tellurides bearing halogen atoms in the same compound. In addition, this new version of the Suzuki-Miyaura cross-coupling reaction was monitored by electrospray ionization mass spectrometry where some reaction intermediates were detected and analyzed.     

Glycine enolates: the effect of formation of iminium ions to simple ketones on alpha-amino carbon acidity and a comparison with pyridoxal iminium ions

Equilibrium constants in D2O were determined by 1H NMR analyses for formation of imines/iminium ions from addition of glycine methyl ester to acetone and from addition of glycine to phenylglyoxylate. First-order rate constants, also determined by 1H NMR, are reported for deuterium exchange between solvent D2O and the alpha-amino carbon of glycine methyl ester and glycine in the presence of increasing concentrations of ketone and Br?nsted bases. These rate and equilibrium data were used to calculate second-order rate constants for deprotonation by DO- and by Br?nsted bases of the alpha-imino carbon of the ketone adducts. Formation of the iminium ion between acetone and glycine methyl ester and between phenylglyoxylate and glycine is estimated to cause 7 unit and 15 unit decreases, respectively, in the pKa's of 21 and 29 for deprotonation of the parent carbon acids. The effect of formation of iminium ions to phenylglyoxylate and to 5'-deoxypyridoxal (DPL) [Toth, K.; Richard, J. P. J. Am. Chem. Soc. 2007, 129, 3013-3021] on the carbon acidity of glycine is similar. However, DPL is a much better catalyst than phenylglyoxylate of deprotonation of glycine, because of the exceptionally large thermodynamic driving force for conversion of the amino acid and DPL to the reactive iminium ion.