173 research outputs found
Density independent feeding success of malaria vectors (Diptera: Culicidae) in Tanzania
The possibility of density dependent feeding success in the African malaria vectors Anopheles gambiae Giles (sensu lato) and A. funestus Giles was investigated by examining the gonotrophic status of mosquitoes from 423 collections made in a Tanzanian village. Up to 500 mosquitoes were caught in any single 35 minute indoor resting collection. Estimates of the total numbers of mosquitoes resting were made by removal sampling. These indicated that the efficiency of resting collections decreased with increasing mosquito population. Of 1904 mosquito blood meals tested by ELISA, 95% were of human origin. There was no decrease in the proportion of mosquitoes fully fed at high population densities and the only demonstrable avoiding action by hosts in response to high densities of mosquitoes was the increased use of bednets during the wet season. The late biting cycle of these mosquitoes precludes many other density-dependent host defence mechanisms because the hosts are asleep at the time of attack. African malaria vectors have evolved with man and perhaps their evasion of density-dependent host responses has reinforced their preference for human bloo
Structure and Magnetism of Neutral and Anionic Palladium Clusters
The properties of neutral and anionic Pd_N clusters were investigated with
spin-density-functional calculations. The ground state structures are
three-dimensional for N>3 and they are magnetic with a spin-triplet for 2<=N<=7
and a spin nonet for N=13 neutral clusters. Structural- and spin-isomers were
determined and an anomalous increase of the magnetic moment with temperature is
predicted for a Pd_7 ensemble. Vertical electron detachment and ionization
energies were calculated and the former agree well with measured values for
anionic Pd_N clusters.Comment: 5 pages, 3 figures, fig. 2 in color, accepted to Phys. Rev. Lett.
(2001
Aromaticity in a Surface Deposited Cluster: Pd on TiO (110)
We report the presence of \sigma-aromaticity in a surface deposited cluster,
Pd on TiO (110). In the gas phase, Pd adopts a tetrahedral
structure. However, surface binding promotes a flat, \sigma-aromatic cluster.
This is the first time aromaticity is found in surface deposited clusters.
Systems of this type emerge as a promising class of catalyst, and so
realization of aromaticity in them may help to rationalize their reactivity and
catalytic properties, as a function of cluster size and composition.Comment: 4 pages, 3 figure
Density independent feeding success of malaria vectors (Diptera: Culicidae) in Tanzania
Gold nanoparticles supported on magnesium oxide for CO oxidation
Au was loaded (1 wt%) on a commercial MgO support by three different methods: double impregnation, liquid-phase reductive deposition and ultrasonication. Samples were characterised by adsorption of N2 at -96°C, temperature-programmed reduction, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. Upon loading with Au, MgO changed into Mg(OH)2 (the hydroxide was most likely formed by reaction with water, in which the gold precursor was dissolved). The size range for gold nanoparticles was 2-12 nm for the DIM method and 3-15 nm for LPRD and US. The average size of gold particles was 5.4 nm for DIM and larger than 6.5 for the other methods. CO oxidation was used as a test reaction to compare the catalytic activity. The best results were obtained with the DIM method, followed by LPRD and US. This can be explained in terms of the nanoparticle size, well known to determine the catalytic activity of gold catalysts
Chemically-synthesised, atomically-precise gold clusters deposited and activated on titania
Synchrotron XPS was used to investigate a series of chemically-synthesised, atomically-precise gold clusters Au(n)(PPh₃)(y) (n = 8, 9, 11 and 101, with y depending on cluster size) immobilized on titania nanoparticles. The gold clusters were washed with toluene at 100 °C or calcined at 200 °C to remove the organic ligand. From the position of the Au 4f₇/₂ peak it is concluded that cluster size is not altered through the deposition. From the analysis of the phosphorous spectra, it can be concluded that the applied heat treatment removes the organic ligands. Washing and calcination leads to partial oxidation and partial agglomeration of the clusters. Oxidation of the clusters is most likely due to the interaction of the cluster core with the oxygen of the titania surface after removal of ligands. The position of the Au 4f₇/₂ peak indicates that the size of the agglomerated clusters is still smaller than that of Au₁₀₁.David P. Anderson, Jason F. Alvino, Alexander Gentleman, Hassan Al Qahtani, Lars Thomsen, Matthew I. J. Polson, Gregory F. Metha, Vladimir B. Golovko and Gunther G. Andersso
Advanced photocatalysts: Pinning single atom co-catalysts on titania nanotubes
Single atom (SA) catalysis, over the last 10 years, has become a forefront in heterogeneous catalysis, electrocatalysis, and most recently also in photocatalysis. Most crucial when engineering a SA catalyst/support system is the creation of defined anchoring points on the support surface to stabilize reactive SA sites. Here, a so far unexplored but evidently very effective approach to trap and stabilize SAs on a broadly used photocatalyst platform is introduced. In self-organized anodic TiO2 nanotubes, a high degree of stress is incorporated in the amorphous oxide during nanotube growth. During crystallization (by thermal annealing), this leads to a high density of Ti3+-O-v, surface defects that are hardly present in other common titania nanostructures (as nanoparticles). These defects are highly effective for SA iridium trapping. Thus a SA-Ir photocatalyst with a higher photocatalytic activity than for any classic co-catalyst arrangement on the semiconductive substrate is obtained. Hence, a tool for SA trapping on titania-based back-contacted platforms is provided for wide application in electrochemistry and photoelectrochemistry. Moreover, it is shown that stably trapped SAs provide virtually all photocatalytic reactivity, with turnover frequencies in the order of 4 x 10(6) h(-1) in spite of representing only a small fraction of the initially loaded SAs.Web of Science3130art. no. 210284
Surface Deposition and Imaging of Large Ag Clusters Formed in He Droplets
The utility of a continuous beam of He droplets for the assembly and surface
deposition of Ag clusters, ~ 300 - 6 000, is studied with transmission
electron microscopy. Images of the clusters on amorphous carbon substrates
obtained at short deposition times have provided for a measure of the size
distribution of the metal clusters. The average sizes of the deposited clusters
are in good agreement with an energy balance based estimate of Ag cluster
growth in He droplets. Measurements of the deposition rate indicate that upon
impact with the surface the He-embedded cluster is attached with high
probability. The stability of the deposited clusters on the substrate is
discussed.Comment: 24 pages, 5 figure
Defining the Borders of Uniform International Contract Law: The CISG and Remedies for Innocent, Negligent or Fraudulent Misrepresentation
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