Search

A study reporting the first quantitative voltammetry at the air/water interface that delineates the distance-dependent electron hopping dynamics in well-defined Langmuir monolayers of gold quantum dot was conducted. The results demonstrate that the electron-transfer dynamics could be quantified by in situ voltammetry of nanoparticle monolayer whose structure is precisely controlled by Langmuir technique and well-defined linkers.

Gold nanoparticle/multiwall carbon nanotube (AuNP/MWNT) thin films were fabricated by using the layer-by-layer (LBL) assembly technique. The microstructure of the film was characterized by TEM and AFM, which showed that the MWNTs in the AuNP/MWNT LBL films are randomly entangled to form a dense network, in which the AuNPs are intercalated. The plasmonic properties and electrocatalytic activity of these films were studied as a function of number of bilayers. Surface plasmon resonance of the AuNP/MWNT films increased linearly and exhibited a red shift with increasing number of bilayers as a result of increasing sizes of the AuNP agglomerates in the films. The AuNP/MWNT films were electrochemically active toward methanol oxidation reaction in alkaline solution. Linear sweep voltammograms revealed a systematic increase in the methanol oxidation current with increasing number of bilayers. This work offers promise to the development of efficient electrodes based on metal nanoparticle/MWNT nanoassembly for electrochemical energy conversion devices such as alkaline fuel cells and biosensors. [ABSTRACT FROM AUTHOR]

Most proteins are specifically localized in membrane-encapsulated organelles or non-membrane-bound compartments. The subcellular localization of proteins facilitates their functions and integration into functional networks; therefore, protein localization is tightly regulated in diverse biological contexts. However, protein localization has been mainly analyzed through immunohistochemistry or the fractionation of subcellular compartments, each of which has major drawbacks. Immunohistochemistry can examine only a handful of proteins at a time, and fractionation inevitably relies on the lysis of cells, which disrupts native cellular conditions. Recently, an engineered ascorbate peroxidase (APEX)-based proximity labeling technique combined with mass spectrometry was developed, which allows for temporally and spatially resolved proteomic mapping. In the presence of H 2 O 2 , engineered APEX oxidizes biotin-phenols into biotin-phenoxyl radicals, and these short-lived radicals biotinylate electron-rich amino acids within a radius of several nanometers. Biotinylated proteins are subsequently enriched by streptavidin and identified by mass spectrometry. This permits the sensitive and efficient labeling of proximal proteins around locally expressed APEX. Through the targeted expression of APEX in the subcellular region of interest, proteomic profiling of submitochondrial spaces, the outer mitochondrial membrane, the endoplasmic reticulum (ER)-mitochondrial contact, and the ER membrane has been performed. Furthermore, this method has been modified to define interaction networks in the vicinity of target proteins and has also been applied to analyze the spatial transcriptome. In this Perspective, we provide an outline of this newly developed technique and discuss its potential applications to address diverse biological questions.

We report the systematic experimental studies demonstrating that a graphene layer inserted at metal/n-Si(001) interface is efficient to explore interface Fermi-level pinning effect. It is confirmed that an inserted graphene layer prevents atomic interdiffusion to form an atomically abrupt Schottky contact. The Schottky barriers of metal/graphene/n-Si(001) junctions show a very weak dependence on metal work-function, implying that the metal Fermi-level is almost completely pinned at charge neutrality level close to the valence band edge of Si. The atomically impermeable and electronically transparent properties of graphene can be used generally to form an intact Schottky contact for all semiconductors.

The ability to direct bimetallic nanoparticles to express desirable surface composition is a crucial step toward effective heterogeneous catalysis, sensing, and bionanotechnology applications. Here we report surface composition tuning of bimetallic Au-Pt electrocatalysts for carbon monoxide and methanol oxidation reactions. We establish a direct correlation between the surface composition of Au-Pt nanoparticles and their catalytic activities. We find that the intrinsic activities of Au-Pt nanoparticles with the same bulk composition of Au0.5Pt0.5 can be enhanced by orders of magnitude by simply controlling the surface composition. We attribute this enhancement to the weakened CO binding on Pt in discrete Pt or Pt-rich clusters surrounded by surface Au atoms. Our finding demonstrates the importance of surface composition control at the nanoscale in harnessing the true electrocatalytic potential of bimetallic nanoparticles and opens up strategies for the development of highly active bimetallic nanoparticles for electrochemical energy conversion.

Well-defined Au/ZnO nanoparticle composites were prepared by modifying ZnO with preformed Au nanoparticles protected with bifunctional glutathione ligand. In this approach, the Au nanoparticles were highly monodisperse and their loading on ZnO surface could be precisely controlled by the anchoring conditions. Steady-state and time-resolved photoluminescence of the composites revealed the ability of the Au nanoparticles to efficiently extract conduction band electrons from the photoexcited ZnO. The composites exhibited strongly enhanced photocatalytic activity without requiring thermal activation process in degrading organic substrates in both oxidative and reductive pathways. A clear correlation between the photocatalytic activity and the Au loading was found for both oxidative and reductive photocatalytic reactions. These results demonstrate that thiolate-protected AuNPs can significantly enhance the charge separation by extracting electrons from the photoexcited ZnO and consequently improve the photocatalytic activity of the composites.

Recently one-dimensonal (1-D) Pt nanostructures have shown greatly enhanced intrinsic oxygen reduction reaction (ORR) activity (ORR kinetic current normalized to Pt surface area) and/or improved durability relative to conventional supported Pt catalysts. In this study, we report a simple synthetic route to create Pt-covered multiwall carbon nanotubes (Pt NPs/MWNTs) as promising 1-D Pt nanostructured catalysts for ORR in proton exchange membrane fuel cells (PEMFCs). The average ORR intrinsic activity of Pt NPs/MWNTs is ∼0.95 mA/cm(2) Pt at 0.9 ViR-corrected versus reversible hydrogen electrode (RHE), ∼3-fold higher than a commercial catalyst -46 wt % Pt/C (Tanaka Kikinzoku Kogyo) in 0.1 M HClO4 at room temperature. More significantly, the mass activity of Pt NPs/MWNTs measured (∼0.48 A/mgPt at 0.9 ViR-corrected vs RHE) is higher than other 1-D nanostructured catalysts and TKK catalysts. The enhanced intrinsic activity of 1-D Pt NPs/MWNTs could be attributed to the weak chemical adsorption energy of OHads-species on the surface Pt NPs covering MWNTs.

Multiwall carbon nanotube (MWNT)/manganese oxide (MnO2) nanocomposite ultrathin film electrodes have been created via redox deposition of MnO2 on layer-by-layer (LbL)-assembled MWNT films. We demonstrate that these LbL-assembled MWNT (LbL-MWNT)/MnO2 thin films consist of a uniform coating of nanosized MnO2 on the MWNT network structure using SEM and TEM, which is a promising structure for electrochemical capacitor applications. LbL-MWNT/MnO2 electrodes yield a significantly higher volumetric capacitance of 246 F/cm3 with good capacity retention up to 1000 mV/s due to rapid transport of electrons and ions within the electrodes. The electrodes are generated with two simple aqueous deposition processes: the layer-by-layer assembly of MWNTs followed by redox deposition of MnO2 at ambient conditions, thus providing a straightforward approach to the fabrication of high-power and -energy electrochemical capacitors with precise control of electrode thickness at nanometer scales.

We report a systematic investigation of the optically excited vibrations in monolayer-protected gold clusters capped with hexane thiolate as a function of the particle size in the range of 1.1-4 nm. The vibrations were excited and monitored in transient absorption experiments involving 50 fs light pulses. For small quantum-sized clusters (< or =2.2 nm), the frequency of these vibrations has been found to be independent of cluster size, while for larger clusters (3 and 4 nm), we did not observe detectable optically excited vibrations in this regime. Possible mechanisms of excitation and detection of the vibrations in nanoclusters in the course of the transient absorption are discussed. The results of the current investigation support a displacive excitation mechanism associated with the presence of finite optical energy gap in the quantum-sized nanoclusters. Observed vibrations provide a new valuable diagnostic tool for the investigations of quantum size effects and structural studies in metal nanoclusters.

We present a systematic study of optical properties of a series of hexanethiolate-capped Au clusters of varying sizes using femtosecond transient absorption, time-resolved fluorescence, and two-photon absorption cross-sectional measurements. An abrupt change in optical properties and their trends has been found at the 2.2 nm size. Displacively excited vibrations with a period of 450 fs have been detected in the transient absorption signal for smaller clusters < or = 2.2 nm. These results strongly suggest an emerging optical gap between the highest occupied and lowest unoccupied orbitals in the narrow size range at 2.2 nm.

The two-photon absorption properties of Au25 cluster has been investigated with the aid of two-photon excited fluorescence in the communication wavelength region with a cross-section of 2700 GM at 1290 nm. Additional visible fluorescence has been discovered for small gold clusters which is two-photon allowed (after excitation at 800 nm), and the absolute cross-section has been determined for gold clusters with number of gold atoms varying from 25 to all the way up to 2406 using one and two-photon excited time-resolved fluorescence upconversion measurements. Record high TPA cross-sections have been measured for quantum sized clusters making them suitable for two-photon imaging as well as other applications such as optical power limiting and lithography.

A facile preparative route to alkanethiolate (Cn) Au38 nanoparticles, where n = 4, 6, 8, 10, and 12, is described. Subnanometer-sized nanoparticles are initially produced by a modified Brust synthesis, which undergo core-size evolution upon removal of reaction impurities that have served as additional protecting layers. C4-C12 Au38 nanoparticles are prepared in approximately 300 mg quantities by the selective removal of reaction impurities with dimethyl sulfoxide. The prepared nanoparticles are 1.1-1.2 nm in core size, and all exhibit optical and electrochemical characteristics of Au38 nanoparticles. Voltammetry of these Au38 nanoparticles reveals that the energy gap between the first one-electron oxidation and the first reduction is rather insensitive to the ligand employed. By contrast, the energy gaps between the first and second oxidations and between the second and third oxidations are ligand-dependent; both substantially increase with ligand thickness. The charging energetics of alkanethiolate-coated Au38 nanoparticles can thus be described as a sum of electron addition energies and the discrete electronic energy levels of the Au38 core.

We characterize the movement of carbon black particles in inhomogeneous, two-dimensional dc electric fields. Motivated by display applications, the particles are suspended in a nonpolar solvent doped with a charge control agent. The two-dimensional fields are generated between strip electrodes on a glass slide spaced 120 microm apart with field strengths up to 10(4) V/m. Such fields are insufficient to drive either electrohydrodynamic instabilities or natural convection due to ohmic heating, but they move the particles between the electrodes in about 30 s. In the center region between the strip electrodes, the particles move by electrophoresis; that is, the particle velocity is proportional to the electric field. However, when imposing a constant-potential or constant-current boundary condition at the electrodes to derive the electrical field, the electrophoretic mobility calculated from the measured particle velocities is outside the range of mobilities predicted from the theory of O'Brien and White. Near the electrodes the particles either speed up or slow down, depending on the polarity of the electrode, and these changes in velocity cannot be explained simply by electrophoresis in a spatially varying electric field. We suggest that this anomalous motion arises from electrohydrodynamic flows originating from the interaction between the space charge of the polarized layers above the electrodes and the electric field. Approximate calculations indicate such flows could be sufficiently strong to explain the anomalous trajectories near the edges of the electrodes.

Electrical current versus potential relationships were measured for solutions of dodecane containing the charge control agent poly(isobutylene succinimide) (PIBS) at various concentrations. Both one-dimensional (parallel planar electrodes) and two-dimensional (strip electrodes) fields were studied. The initial current was proportional to the applied voltage for both electrode configurations. Using the initial decay rate of the current (t < 0.5 s) in the planar electrode cell and the Gouy-Chapman model for electrode polarization, we determined the diffusion coefficient of the charge carriers (micelles) in the solution, from which we calculated their effective radius to be 10 nm. The constancy of the carrier radius over a 7-fold change in PIBS concentration, along with the proportionality between conductivity and concentration, supports the hypothesis that the charged species result from the interactions between two micelles. The experimentally determined geometric factor (cell constant) relating current to applied potential at time zero for the strip electrode cell agrees with the value predicted from the solution of Laplace's equation for the electrical potential in this system. The intermediate-time (0.5-3.0 s) decay rate of current was faster than predicted from the classical Gouy-Chapman theory of the double layer, possibly because of volume fraction effects in the double layer. The very long-time (minutes to hours) residual current that we observed is not explained, but we suspect that some charge transfer across the electrode must have occurred because there was insufficient ion capacity (i.e., amount of PIBS) in the solution to account for the total charge transferred through the cell.