Hybrid waveguide-surface plasmon polariton modes in a guided-mode resonance grating

We analyze the coupling between surface plasmon polaritons in a metal grating and the guided modes of a dielectric waveguide. Our model structure is a gold wire grating on a slab waveguide made of silicon nitride on silica wafer. The excitation of guided-mode resonances, surface plasmon polariton modes and hybrid waveguide-plasmon modes are observed in numerical simulations. Our experiments verify the existence of the predicted modes. These hybrid modes add significant degrees of freedom in designing structures for plasmonic applications.     

Formation of stable Ag-nanoparticle aggregates induced by dithiol cross-linking

Aggregation of thiol-stabilized silver nanoparticles induced by 1,6-hexane dithiol was studied in situ by dynamic light scattering. The aggregates were observed to reach a finite size in the 100-200 nm range depending on the applied conditions. Growth kinetics were shown to be linked to the dynamics of the thiol-exchange reaction. A model for the aggregation process was developed on the basis of a simple diffusion-kinetic approach assuming an elementary kinetic reaction at the surfaces and a spherical diffusion field surrounding the aggregates. The rate constant for the thiol exchange reaction was found to vary between 0.6 and 4.0 x 10(-4) s(-1), and the activation energy was 46 +/- 10 kJ mol(-1).     

Comparative study of the Kumada, Negishi, Stille, and Suzuki-Miyaura reactions in the synthesis of the indole alkaloids hippadine and pratosine

The total synthesis of hippadine by a tandem metalation/cross-coupling/lactamization strategy was investigated starting from either 7-bromoindole or a 6-halogenated methyl piperonate. The Kumada and Negishi cross-coupling reactions failed to provide any of the desired product. However, the Stille and Suzuki reactions furnished hippadine in low yields starting from the electron-deficient methyl 6-iodo- and 6-bromopiperonate, respectively. Starting from the metalated indole, only the Suzuki reaction occurred, affording hippadine in 67-74% and pratosine in 62% isolated yield.     

Dipeptide analogues containing 4-ethoxy-3-pyrrolin-2-ones

[reaction: see text] Pyrrolidine-2,4-diones (1) are naturally occurring analogues of amino acids. We herein present a facile synthesis of N-acylated, O-alkylated pyrrolin-2-ones (2) in high yield and excellent enantiopurity. Molecular mechanics calculations suggest that the resulting dipeptide analogues adopt a linear, extended conformation.     

Combined synthesis and in situ coating of nanoparticles in the gas phase

Ga₂O₃ was-synthesized by doping a premixed H₂/O₂/Ar flat flame with diluted trimethyl gallium Ga(CH₃)₃ in a low-pressure reactor. The mean particle diameter d _p of the resulting metal oxide was characterized in-situ with a particle mass spectrometer (PMS), and was observed to range between 2.5 nm ≤ d _p ≤ 6.5 nm. XRD results show that the as-synthesized Ga₂O₃ nanoparticles are mostly amorphous, although, a few broad reflexes were observed that indicate the presence of some degree of crystallinity. Thermal annealing of the as-synthesized material at 1000 °C for 5 min yielded β-Ga₂O₃ with a monoclinic structure. UV–VIS measurements indicate strong absorption in the UV range (4.8 eV), which corresponds quite well to the direct band gap of bulk Ga₂O₃. Photoluminescence (PL) measurements of the as-synthesized metal oxide show a broad emission ranging from 350 nm to 600 nm with a maximum at 460 nm. Crystalline β-Ga₂O₃ exhibited stronger luminescence than as-synthesized particles.     

Effect of residual lignin and heteropolysaccharides in nanofibrillar cellulose and nanopaper from wood fibers

Unbleached (UN), oxygen-delignified and fully-bleached (FB) birch fibers with a residual lignin content of ca. 3, 2 and <1 %, respectively, were used to produce nanofibrillated cellulose (NFC) and nanopaper by using an overpressure device. The tensile index, elongation and elastic modulus of nanopaper were compared and the effect of residual cell wall components accessed. Under similar manufacturing conditions, UN NFC produced nanopaper with a density of 0.99 g/cm³, higher than that from FB NFC (0.7 g/cm³). This translated in much lower air permeability in the case of UN nanopaper (1 and 11 mL/min for UN and FB samples, respectively). Fundamentally, these observations are ascribed to the finer fibrils produced during microfluidization of UN fibers compared to those from lower yield counterparts (AFM roughness of 8 and 17 nm and surface areas of 124 and 98 m²/g for NFC from UN and FB fibers, respectively). As a result, values of stress at break and energy absorption of nanopaper from high yield fibers are distinctively higher than those from fully bleached NFC. Interactions of water with the surface and bulk material were affected by the chemical composition and structure of the nanofibrils. While UN nanopaper presented higher water contact angles their sorption capacity (and rate of water absorption) was much higher than those measured for nanopaper from FB NFC. These and other observations provided in this contribution are proposed to be related to the mechanoradical scavenging capacity of lignin in high shear microfluidization and the presence of residual heteropolysaccharides.     

Probing lattice defects in Sr2MgSi2O7:Eu2+,Dy3+

The Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ materials were prepared with a solid state reaction and their microscopic structure (at 295 K only) and luminescence were studied at selected temperatures between 150 and 295 K. Undisturbed Sr crystal planes were common in the TEM images of the undoped Sr₂MgSi₂O₇ material, whereas with Eu²⁺ doping more disturbed planes were observed even in the nanometer scale. With Dy³⁺ co-doping, a large number of small lattice domains created by the discontinuities in the crystal structure was observed. The domains with different orientations seem to be centered around point defects. The decay curves of Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ showed fast (ms scale) persistent luminescence. The intensity of persistent luminescence increased considerably between 200 and 250 K while remaining constant in the ranges of 150–200 and 250–295 K. The changes were used to study the depth of the traps. In general, Dy³⁺ co-doping was found to deepen the traps.     

Essential oil composition of Vismia macrophylla leaves (Guttiferae)

The essential oil from Vismia macrophylla Kunth (Guttiferae) leaves, extracted by hydrodistillation, was analyzed by GC/MS. The oil obtained (yield 0.11%) contained twenty-eight compounds, which were identified from their retention indices and by comparison of their mass spectra with those in the Wiley GC-MS Library data base. The major components were beta-caryophyllene (20.1%), germacrene D (11.6%) and beta-elemene (7.0%).     

Essential oil composition and antibacteral activity of Vismia baccifera fruits collected from Mérida, Venezuela

The essential oil from the fruits of Vismia baccifera Triana & Planch. (Gutttiferae), collected in June 2009, was analyzed by GC/MS. A yield of 0.6% oil was obtained by hydrodistillation. Twenty-seven components were identified by comparison of their mass spectra with those in the Wiley GC-MS Library data base. The major components were trans-cadin-l1,4-diene (36.6%), cis-cadin-1,4-diene (18.8%) and beta-caryophyllene (11.9%). The essential oil showed a broad spectrum of antibacterial activity against the important human pathogenic Gram-positive and Gram-negative bacteria Staphylococcus aureus (ATCC 25923), Enterococcus faecalis (ATCC 29212), Escherichia coli (ATCC 25992), Pseudomonas aeruginosa (ATCC 27853) and Klebsiella pneumoniae (ATCC 23357) with MIC values ranging from 9 to 37 microg/mL.