Plasmonic nanostructures have been proposed as useful materials for photon harvesting applications. However, the mechanisms by which energy transfer occurs across interfaces formed between plasmonic materials and their environment are under debate. A commonly invoked mechanism is indirect hot charge carrier transfer, where hot carriers are generated in the plasmonic material by nonradiatve plasmon decay, followed by transfer of these carriers to interfacial species in a sequential process. Alternatively, chemical interface damping has been reported to allow direct interaction between surface plasmons and interfacial species electronic states. Here we provide evidence from experiment and theory that for plasmon-mediated catalytic O 2 dissociation on Ag plasmonic nanoparticles, the direct interaction of O 2 molecules with surface plasmon near-fields was responsible for observed photocatalysis. These results offer important mechanistic insights for the design of plasmonic materials that maximize efficiency for promoting catalytic small molecule activation using photon fluxes.

Monolithic hierarchical nanoporous gold disks, 500 nm in diameter, 75 nm in thickness and 3.5 nm in pore radius, have been fabricated by hybrid processes. A surface-enhanced Raman scattering enhancement factor of at least 10(8) has been obtained on individual disks using benzenethiol self-assembled monolayer with 785 nm laser excitation.

Plasmonic metal nanostructures have shown great potential in sensing, photovoltaics, imaging and biomedicine, principally due to the enhancement of local electric field by light-excited surface plasmons, i.e., collective oscillation of conduction band electrons. Thin films of nanoporous gold have received a great deal of interest due to the unique 3-dimensional bicontinuous nanostructures with high specific surface area. However, in the form of semi-infinite thin films, nanoporous gold exhibits weak plasmonic extinction and little tunability in the plasmon resonance, because the pore size is much smaller than the wavelength of light. Here we show that by making nanoporous gold in the form of disks of sub-wavelength diameter and sub-100 nm thickness, these limitations can be overcome. Nanoporous gold disks not only possess large specific surface area but also high-density, internal plasmonic "hot-spots" with impressive electric field enhancement, which greatly promotes plasmon-matter interactions as evidenced by spectral shifts in the surface plasmon resonance. In addition, the plasmonic resonance of nanoporous gold disks can be easily tuned from 900 to 1850 nm by changing the disk diameter from 300 to 700 nm. Furthermore, nanoporous gold disks can be fabricated as either bound on a surface or as non-aggregating colloidal suspension with high stability.

No abstract

Photon illumination of metal nanoparticle catalysts can promote reaction rate and selectivity through transient charge transfer to adsorbed species. Here we demonstrate that illumination of 2 nm diameter Pt nanoparticle catalysts with pulsed visible light enhances time-averaged rates of H2 production via methanol decomposition compared with static illumination. Based on CO temperature-programmed desorption, in-situ FTIR, and kinetic measurements, we propose that pulsed illumination promotes reaction rates compared to static illumination by oscillating the binding energy of surface intermediates at frequencies that are in resonance with reaction kinetics. We also show that the impact of light is chemically specific, influencing some elementary step energetics more than others. Our results suggest that using light pulses to dynamically control the energetics of elementary steps on catalytic surfaces may enable higher activity or selectivity than is possible with static illumination or dictated by linear free energy scaling relations.

We evaluate the performance of line-scan Raman microscopy (LSRM), a versatile label-free technique, for high-throughput chemical imaging of cell population. We provide detailed design and configuration of a home-built LSRM system developed in our laboratory. By exploiting parallel acquisition, the LSRM system achieves a significant throughput advantage over conventional point-scan Raman microscopy by projecting a laser line onto the sample and imaging the Raman scattered light from the entire line using a grating spectrograph and a charge-coupled device (CCD) camera. Two-dimensional chemical maps can be generated by scanning the projected line in the transverse direction. The resolution in the x and y direction has been characterized to be ~600-800 nm for 785 nm laser excitation. Our system enables rapid classification of microparticles with similar shape, size, and refractive index based on their chemical composition. An equivalent imaging throughput of 100 microparticles/s for 1 μm polystyrene beads has been achieved. We demonstrate the application of LSRM to imaging bacterial spores by identifying endogenous calcium dipicolinate. We also demonstrate that LSRM enables the study of intact microalgal cells at the colonial level and the identification of intra- and extracellular chemical constituents and metabolites, such as chlorophyll, carotenoids, lipids, and hydrocarbons. We conclude that LSRM can be an effective and practical tool for obtaining endogenous microscopic chemical and molecular information from cell population.

We present label-free, in situ monitoring of individual DNA hybridization in microfluidics. By immobilizing molecular sentinel probes on nanoporous gold disks, we demonstrate sensitivity approaching the single-molecule limit via surface-enhanced Raman scattering which provides robust signals without photobleaching for more than an hour. We further demonstrate that a target concentration as low as 20 pM can be detected within 10 min under diffusion-limited transport.

We present a microfluidic surface-enhanced Raman scattering (SERS) sensor for rapid and label-free biomolecular detection. Our sensor design mitigates a common limiting factor in microfluidic SERS sensors that utilize integrated nanostructures: low-efficiency transport of biomolecules to nanostructured surface which adversely impacts sensitivity. Our strategy is to increase the total usable nanostructured surface area, which provides more adsorption sites for biomolecules. Specifically, a nanoporous gold disk (NPGD) array, a highly effective SERS substrate, has been monolithically integrated inside a microfluidic chip. Individual NPGD is known to feature an order of magnitude larger surface area than its projected disk area. The increased surface area arises from nanoscale pores and ligaments three-dimensionally distributed in the NPGD, which manifest themselves as high-density SERS hot-spots. High-density NPGD arrays further guarantee large coverage of these hot-spots on the microchannel floor. The sensor performance has been demonstrated using Rhodamine 6G to quantify spatial uniformity and determine the shortest detection time. Next, the sensor is applied to detect two biomolecules, dopamine and urea, with unprecedented detection limit and speed compared to other existing microfluidic SERS sensors. The sensor holds great promise in point-of-care applications for various biomolecular detections.