Ensemble effects in the coupling of acetylene to benzene on a bimetallic surface: A study with Pd{111}/Au

Acetylene coupling to benzene on the Pd(lll) surface is greatly enhanced by the presence of catalytically inert Au atoms. LEED and Auger spectroscopy show that progressive annealing of Au overlayers on Pd(lll) leads to the formation of a series of random surface alloys with continuously varying composition. Cyclization activity is a strong function of surface composition-the most efficient catalyst corresponds to a surface of composition similar to 85% Pd. CO TPD and HREELS data show that acetylene cyclization activity is not correlated with the availability of singleton Pd atoms, nor just with the presence of 3-fold pure Pd sites-the preferred chemisorption site for C2H2 on Pd{111}. The data can be quantitatively rationalized in terms of a simple model in which catalytic activity is dominated by Pd6Au and Pd-7 surface ensembles, allowance being made for the known degree to which pure Pd{111} decomposes the reactant and product molecules.

Resonant Rayleigh light scattering response of individual Au nanoparticles to antigen--antibody interaction

A proof-of-concept study was reported on analysis of antigen–antibody recognition based on resonant Rayleigh scattering response of single Au nanoparticles in an imaging chamber. As benefited by a traditional dark-field microscope and a spectrograph, individual Au nanoparticles (30 nm) were observed with high signal-to-noise ratio and they were effectively utilized to monitor changes in refractive index induced by specific binding of the adsorbates. Using PSA antigen as a model, a LSPR ?max shift of about 2.85 nm was recorded for a molecular binding corresponding to 0.1 pg ml-1 of the protein biomarker. This result successfully demonstrates a non-labeling detection system for proteins as well as thousands of different chemical or biological species, and it possesses a great potential as a sensitive, on-chip and multiplexing detection.

Au Nanoparticles for Applications in Analysis of Cellular and Biomolecular Recognitions

Au nanoparticles (AuNPs) have attracted a great interest in fabrication of various biosensor systems for analysis of cellular and biomolecular recognitions. In conjunction with vast conjugation chemistry available, the materials are easily coupled with biomolecules such as nucleic acids, antigens or antibodies in order to achieve their many potential applications as ligand carriers or transducing platforms for preparation, detection and quantification purposes. Furthermore, the nanoparticles possess easily tuned and unique optical/ physical/ chemical characteristics, and high surface areas, making them ideal candidates to this end. In this topic, sensing mechanisms based on localized surface plasmon resonance (LSPR), particle aggregation, catalytic property, and Fluorescence Resonance Energy Transfer (FRET) of AuNPs as well as barcoding technologies including DNA biobarcodes will be discussed.

The ultrasensitive detection of prostate cancer biomarker based on resonant Rayleigh light-scattering response of single Au nanoparticle

A proof-of-concept study was reported on analysis of antigen-antibody recognition based on resonant Rayleigh scattering response of single Au nanoparticles on a microimaging chamber. As benefited by a traditional dark-field microscope and a spectrograph, tiny 30 nm Au nanoparticles were effectively used as nanosensors to monitor changes in refractive index induced by every single binding of the adsorbates. The individual Au nanoparticles were observed with very high signal-to-noise ratio, and a LSPR ?max shift of about 2.5 nm accounting for the detection of PSA antigen with concentration as low as 0.1 pg ml-1 was recorded. This resulted in the successful demonstration of a non-labelling detection system for proteins as well as thousands of different chemical or biological species with possibility of miniaturization and multiplexing scheme.

One-Pot, High-Yield Synthesis of Convex Au Octahedra by H2O2 - Mediated Process In Solution Phase

A simple, non-seeding and high-yield synthesis of convex gold octahedra with size of ca. 50 nm in aqueous solution is described. The octahedral nanoparticles were systematically prepared by reduction of HAuCl4 using ascorbic acid (AA) in the presence of cetyltrimethylammonium bromide (CTAB) as the stabilizing surfactant while concentrations of Au3+ were fixed. The synthesizing process is especially different to other wet synthesis of metallic nanoparticles because it is mediated by H2O2. Mechanism of the H2O2 – mediated process will be described in details. The gold octahedra were shown to be single crystals with all 8 faces belonging to {111} family. Moreover, the single crystalline particles also showed attractive optical properties towards LSPR that should find uses as labels for microscopic imaging, materials for colorimetric biosensings, or nanosensor developments.

Highly Sensitive, Uniform, and Reproducible Surface-Enhanced Raman Spectroscopy from Hollow Au-Ag Alloy Nanourchins

A hierarchical nanoparticle strategy to simultaneously gain super Raman signal amplification, high uniformity, and reproducibility is presented. Using hollow Au-Ag alloy nanourchins, an ultrahigh sensitivity, e.g., down to 1 fM concentrations for DEHP molecule is obtained. A small standard deviation of <10% is achieved by simply dropping and evaporating sub-100 nm nanourchins onto a substrate.

Theory of phonon dissipation in the conduction of stressed Au nanowires

Recent experiments on Au break junctions [Phys. Rev. Lett. 88 (2002) 216803] have characterized the nonlinear conductance of stretched short Au nanowires. They reveal in the voltage range 10-20 meV the signatures of dissipation effects, likely due to phonons in the nanowire, reducing the conductance below the quantized value of 2e(2)/h. We present here a theory, based on a model tight-binding Hamiltonian and on non-equilibrium Green's function techniques, which accounts for the main features of the experiment. The theory helps in revealing details of the experiment which need to be addressed with a more realistic, less idealized, theoretical framework. (C) 2004 Elsevier B.V. All rights reserved.

The persistent activity of Jupiter-family comets at 3-7 AU

We present an analysis of comet activity based on the Spitzer Space Telescope component of the Survey of the Ensemble Physical Properties of Cometary Nuclei. We show that the survey is well suited to measuring the activity of Jupiter-family comets at 3-7 AU from the Sun. Dust was detected in 33 of 89 targets (37 ± 6%), and we conclude that 21 comets (24 ± 5%) have morphologies that suggest ongoing or recent cometary activity. Our dust detections are sensitivity limited, therefore our measured activity rate is necessarily a lower limit. All comets with small perihelion distances (q <1.8 AU) are inactive in our survey, and the active comets in our sample are strongly biased to post-perihelion epochs. We introduce the quantity ɛfρ, intended to be a thermal emission counterpart to the often reported Afρ, and find that the comets with large perihelion distances likely have greater dust production rates than other comets in our survey at 3-7 AU from the Sun, indicating a bias in the discovered Jupiter-family comet population. By examining the orbital history of our survey sample, we suggest that comets perturbed to smaller perihelion distances in the past 150 yr are more likely to be active, but more study on this effect is needed.

DEPENDENCE OF LIGHT OUTPUT FROM NOMINALLY SMOOTH AL-OX-AU TUNNEL-JUNCTIONS ON ELECTRODE MORPHOLOGY

The light output from nominally smooth Al-Ox-Au tunnel junctions is observed to be substantially independent of the deposition rate of the Au film electrode. Films deposited quickly (2 nm s-1) and those deposited slowly (0.16 nm s-1) have similar spectral dependences and intensities. (This is in contrast to roughened films where those deposited quickly give out less light, especially towards the blue end of the spectrum.) The behaviour can be interpreted in terms of the ratio l(ph)/l(em) where l(ph) and l(em) are the mean free paths of surface plasmons between external photon emissions and internal electromagnetic absorptions respectively. Once l(ph)/l(em) exceeds 100, as it does on smooth films, grain size has little further effect on the spectral shape of the light output. In fast-deposited films there are two compensating effects on the output intensity: grain boundary scattering decreases it and greater surface roughness increases it.

CHARACTERIZATION OF THE AU SURFACE-STRUCTURE OF AL-L-AU LIGHT-EMITTING TUNNEL-JUNCTIONS

Visible light is emitted from the Au-air interface of Al-I-Au thin-film tunnel junctions (deposited over a thin layer of CaF2 on glass) as a result of the decay of surface plasmon polaritons (SPPs). We show the surface topography of such a Au film and relate its large-scale features to the outcoupling of fast SPP's to photons. The absence of short-scale roughness features is explained by thier disappearance through surface diffusion. To confirm this a controlled sequence of 5-nm, 20-ms scanning tunneling microscope (STM) W tip crashes has been used to produce indentations 3 nm deep with a lateral dimension of 5-7 nm on a Au crystal in air at room temperature. Four sequences of indentations were drawn in the form of a square box. Right from the start, feature decay is observed and over a period of 2 h a succession of images shows that the structure disappears into the background as a result of surface diffusion. The surface diffusion constant is estimated to be 10(-18) cm2 s-1. The lack of light output via slow mode SPPs is an inevitable consequence of surface annealing.

A density functional theory study of hydrogen dissociation and diffusion at the perimeter sites of Au/TiO2

Reactivity of supported gold catalysts is a hot topic in catalysis for many years. This communication reports an investigation on the dissociation of molecular hydrogen at the perimeter sites of Au/TiO2 and the spillover of hydrogen atoms from the gold to the support using density functional theory calculations. It is found that the heterolytic dissociation is favoured in comparison with homolytic dissociation of molecular hydrogen at the perimeter sites. However, the surface oxygen of the rutile TiO2(110) surface at these sites can be readily passivated by the formed OH, suggesting that further dissociation of molecular hydrogen may occur at pure gold sites.

Acrolein hydrogenation on Pt(211) and Au(211) surfaces:a density functional theory study

Partial hydrogenation of acrolein, the simplest alpha, beta-unsaturated aldehyde, is not only a model system to understand the selectivity in heterogeneous catalysis, but also technologically an important reaction. In this work, the reaction on Pt(211) and Au(211) surfaces is thoroughly investigated using density functional theory calculations. The formation routes of three partial hydrogenation products, namely propenol, propanal and enol, on both metals are studied. It is found that the pathway to produce enol is kinetically favoured on Pt while on Au the route of forming propenol is preferred. Our calculations also show that the propanal formation follows an indirect pathway on Pt(211). An energy decomposition method to analyze the barrier is utilized to understand the selectivities at Pt(211) and Au(211), which reveals that the interaction energies between the reactants involved in the transition states play a key role in determining the selectivity difference.

Examining the redox and formate mechanisms for water-gas shift reaction on Au/CeO2 using density functional theory

We perform DFT calculations to investigate the redox and formate mechanisms of water-gas-shift (WGS) reaction on Au/CeO2 catalysts. In the redox mechanism, we analyze all the key elementary steps and find that the OH cleavage is the key step. Three possible pathways of OH cleavage are calculated: (1) OHad '' + *'--> H-ad' + O-ad"; (2) H-ad' + OHad '' --> H-2(g) + O-ad '' + *'; and (3) OHad" + OHad '' --> 2O(ad '') + H-2(g) (*': the free adsorption sites on the oxides; ad': adsorption on the metal; ad": adsorption on the oxide, respectively). In the formate mechanism, we identify all the possible pathways for the formation and decomposition of surface formates in the WGS reaction. It is found that there is a shortcoming in the redox and formate mechanisms which is related to surface oxygen reproduction. Four possible pathways for producing surface oxygen are studied, and all the barriers of the four pathways are more than 1 eV. Our results suggest that the processes to reproduce surface oxygen in the reaction circle are not kinetically easy. (C) 2008 Elsevier B.V. All rights reserved.