High-Resolution Multiwavelength Surface Plasmon Resonance Spectroscopy for Probing

To date, surface plasmon resonance (SPR) spectroscopy identifies molecules via specific bindings with their ligands immobilized on a surface. We demonstrate here that a high-resolution multiwavelength SPR technique can measure the electronic states of the molecules and thus allow direct identification of the molecules. Using this new capability, we have studied the electronic and conformational differences between the oxidized and reduced states of cytochrome c immobilized on a modified gold electrode. When the wavelength of the incident light is far away from the optical absorption bands of the protein, a ∼0.008°d ecrease in the resonance angle, due to a conformational change, occurs as the protein is switched from the oxidized to reduced states. When the wavelength is tuned to the absorption bands, the resonance angle oscillates at the wavelengths of the absorption peaks, which provides electronic signatures of the protein

Localization and Capacitance Fluctuations in Disordered Au Nano-junctions

Nano-junctions, containing atomic-scale gold contacts between strongly disordered leads, exhibit different transport properties at room temperature and at low temperature. At room temperature, the nano-junctions exhibit conductance quantization effects. At low temperatures, the contacts exhibit Coulomb-Blockade. We show that the differences between the room-temperature and low temperature properties arise from the localization of electronic states in the leads. The charging energy and capacitance of the nano-junctions exhibit strong fluctuations with applied magnetic field at low temperature, as predicted theoretically.Comment: 20 pages 8 figure

Density of states and magnetoconductance of disordered Au point contacts

We report the first low temperature magnetotransport measurements on electrochemically fabricated atomic scale gold nanojunctions. As $T \to 0$, the junctions exhibit nonperturbatively large zero bias anomalies (ZBAs) in their differential conductance. We consider several explanations and find that the ZBAs are consistent with a reduced local density of states (LDOS) in the disordered metal. We suggest that this is a result of Coulomb interactions in a granular metal with moderate intergrain coupling. Magnetoconductance of atomic scale junctions also differs significantly from that of less geometrically constrained devices, and supports this explanation.Comment: 5 pages, 5 figures. Accepted to PRB as Brief Repor

Electrochemical fabrication of atomically thin metallic wires and electrodes separated with molecular-scale gaps

Abstract This article summarizes our recent effort to fabricate electrochemically metallic nanowires and electrodes separated with molecular scale nanogaps. The nanowires were fabricated by etching a small portion of a micron-scale metallic wire supported on a solid substrate. The etching was controlled by continuously monitoring the conductance of the wire. When the thinnest portion of the wire reached the atomic scale, the conductance decreased in a stepwise fashion. By further etching away the last few atoms, a molecular-scale gap between two electrodes was created and the ballistic electron transport through the nanowire was replaced with quantum tunneling. By depositing atoms back, the above processes could be reversed, allowing us to achieve a desired nanowire or gap. The nanowires may be used for chemical sensor applications and the nanogaps may be used to wire small molecules to the outside world for molecular electronics applications

Variant‐specific effects of GBA1 mutations on dopaminergic neuron proteostasis

Glucocerebrosidase 1 (GBA1) mutations are the most important genetic risk factors for Parkinson's disease (PD). Clinically, mild (e.g., p.N370S) and severe (e.g., p.L444P and p.D409H) GBA1 mutations have different PD phenotypes, with differences in age at disease onset, progression, and the severity of motor and non‐motor symptoms. We hypothesize that GBA1 mutations cause the accumulation of α‐synuclein by affecting the cross‐talk between cellular protein degradation mechanisms, leading to neurodegeneration. Accordingly, we tested whether mild and severe GBA1 mutations differentially affect the degradation of α‐synuclein via the ubiquitin–proteasome system (UPS), chaperone‐mediated autophagy (CMA), and macroautophagy and differentially cause accumulation and/or release of α‐synuclein. Our results demonstrate that endoplasmic reticulum (ER) stress and total ubiquitination rates were significantly increased in cells with severe GBA1 mutations. CMA was found to be defective in induced pluripotent stem cell (iPSC)‐derived dopaminergic neurons with mild GBA1 mutations, but not in those with severe GBA1 mutations. When examining macroautophagy, we observed reduced formation of autophagosomes in cells with the N370S and D409H GBA1 mutations and impairments in autophagosome–lysosome fusion in cells with the L444P GBA1 mutation. Accordingly, severe GBA1 mutations were found to trigger the accumulation and release of oligomeric α‐synuclein in iPSC‐derived dopaminergic neurons, primarily as a result of increased ER stress and defective macroautophagy, while mild GBA1 mutations affected CMA, which is mainly responsible for the degradation of the monomeric form of α‐synuclein. Overall, our findings provide new insight into the molecular basis of the clinical variability in PD associated with different GBA1 mutations

Carbon Nanofiber Electrodes and Controlled Nanogaps for Scanning Electrochemical Microscopy Experiments

The electrochemical behavior of electrodes made by sealing carbon nanofibers in glass or with electrophoretic paint has been studied by scanning electrochemical microscopy (SECM). Because of their small electroactive surface area, conical geometry with a low aspect ratio and high overpotential for proton and oxygen reduction, carbon nanofiber (CNF) electrodes are promising candidates for producing electrode nanogaps, imaging with high spatial resolution and for the electrodeposition of single metal nanoparticles (e.g., Pt, Pd) for studies as electrocatalysts. By using the feedback mode of the SECM, a CNF tip can produce a gap that is smaller than 20 nm from a platinum disk. Similarly, the SECM used in a tip-collection substrate-generation mode, which subsequently shows a feedback interaction at short distances, makes it possible to detect a single CNF by another CNF and then to form a nanometer gap between the two electrodes. This approach was used to image vertically aligned CNF arrays. This method is useful in the detection in a homogeneous solution of short-lifetime intermediates, which can be electrochemically generated at one electrode and collected at the second at distances that are equivalent to a nanosecond time scale. A nanogap is an electrode arrangement in which two collinear electrodes are separated by a gap of nanometer dimensions. There have been a number of studies involving nanogaps, most of these involving studies of the electronic transport properties of single molecules, which bridge the gap (e.g., DNA or other macromolecules). 1,2 Nanogaps are usually prepared with fixed dimensions, e.g., by mechanically produced break junctions, 3 break junctions formed by electromigration, 4 electrodeposition, 5 carbon nanotube extracted lithography, 6 direct e-beam lithography, 7 and conventional microscale fabrication techniques such as optical lithography, electron-beam evaporation, and liftoff. 8 In most cases, the exact gap dimensions are uncontrolled, although there have been reports of the formation of controlled nanogaps of fixed dimensions by several different approaches. 9-11 Our group has been interested in nanogaps whose dimensions are continuously variable, for example, in connection with studies of single-molecule electrochemistry 12 and in studies of rapid homogeneous reactions coupled to electron-transfer reactions at electrodes. 13 Generally, kinetic studies of electrochemically generated, unstable species can be achieved if the species lifetime is of the order of the diffusion time across the gap, d 2 /2D, where d is the gap separation and D is the diffusion coefficient. To increase the range of systems that can be studied by scanning electrochemical microscopy (SECM) to chemical species with lifetimes in the microsecond and nanosecond region, one must develop experimental strategies for decreasing d to the nanometer level, i.e., to a nanogap. This can be achieved by using extremely small probes (UMEs) that can approach a substrate or another electrode to very small distances. With glass-insulated disk electrodes, of the type most frequently used in SECM, one is limited by the RG value of the electrode, i.e., the ratio of the radius of glass insulating shroud to that of the metallic disk electrode. Another problem associated with disk UMEs is that the exposed electroactive part is sometimes recessed slightly beneath the insulating layer. While this has only a minor effect during micorometer-scale measurements, any feedback-based approach to a nanometer distance often results in the insulator touching the substrate and blocking any further movement of the recessed electroactive disk. A useful approach to solving this problem is to use conical electrodes that can approach the substrate or another similar electrode to very small d values without colliding. Conical UMEs have been previously studied by a combination of scanning electron microscopy (SEM), steady-state voltammetry

Angle-Tunable Enhanced Infrared Reflection Absorption Spectroscopy via Grating-Coupled Surface Plasmon Resonance

Surface enhanced infrared absorption (SEIRA) spectroscopy is an attractive method for increasing the prominence of vibrational modes in infrared spectroscopy. To date, the majority of reports associated with SEIRA utilize localized surface plasmon resonance from metal nanoparticles to enhance electromagnetic fields in the region of analytes. Limited work has been performed using propagating surface plasmons as a method for SEIRA excitation. In this report, we demonstrate angle-tunable enhancement of vibrational stretching modes associated with a thin poly(methyl methacrylate) (PMMA) film that is coupled to a silver-coated diffraction grating. Gratings are fabricated using laser interference lithography to achieve precise surface periodicities, which can be used to generate surface plasmons that overlap with specific vibrational modes in the polymer film. Infrared reflection absorption spectra are presented for both bare silver and PMMA-coated silver gratings at a range of angles and polarization states. In addition, spectra were obtained with the grating direction oriented perpendicular and parallel to the infrared source in order to isolate plasmon enhancement effects. Optical simulations using the rigorous coupled-wave analysis method were used to identify the origin of the plasmon-induced enhancement. Angle-dependent absorption measurements achieved signal enhancements of more than 10-times the signal in the absence of the plasmon.This article is from Analytical Chemistry86 (2014): 2610-2617, doi:10.1021/ac4038398. Posted with permission.</p

Pure and mixed chlorophyll a Langmuir and Langmuir-Blodgett films. Structure, electrical and optical properties

Photosynthetic pigments, which undergo electron transfer reactions, are specifically associated with the D1-D2-Cytb559 protein complex. A particular aggregated state of in vivo chlorophyll a (Chl a) is responsible for the high-energy conversion efficiency. The Langmuir- Blodgett (L-B) technique allows preparation of systems that potentially mimic the packing of Chl a in vivo. Water has a pronounced effect on the optical and electrical properties of chl a L-B films, and is a key element in the formation of dimers with (anti) parallel transition moments of the constituent monomers. The incorporation of lipid makes the transport of photogenerated charge carriers more difficult. In the case of Chl a-Cytochrome c mixtures, keto groups of chl a play a major role in the pigment-protein association

Nanoscale visualization of microcrystalline chlorophyll a investigated with scanning tunneling microscopy

We have studied the topography of microcrystalline chlorophyll a films, that have been electrodeposited onto gold electrodes, using scanning tunneling microscopy (STM). The STM revealed that the films possess a polycrystalline structure with small grain sizes of 50 to 100 nm in diameter and a very rough surface topography with a surface roughness of 5 to 10 nm over surface areas of 1 × 1 μm. The surface structure correlates with results from an earlier photocurrent study of microcrystalline chlorophyll a