Israel Perez

Single-Molecule Lysozyme Dynamics Monitored by an Electronic Circuit

Observing Protein Dynamics Following the dynamics of protein conformational changes over the relatively long periods of time typical of enzyme kinetics can be challenging. Choi et al. (p. 319; see the Perspective by Lu) were able to observe changes in lysozyme conformation, which changes its electrostatic potential, by using a carbon-nanotube field-effect transistor. Slower hydrolysis steps were compared with faster, but unproductive, hinge motion, and changes in lysozyme activity that occur with pH were shown to arise from differences in the relative amount of time spent in processive versus nonprocessive states. Changes in protein conformation can be detected via changes in electrostatic potential with a carbon nanotube transistor. Tethering a single lysozyme molecule to a carbon nanotube field-effect transistor produced a stable, high-bandwidth transducer for protein motion. Electronic monitoring during 10-minute periods extended well beyond the limitations of fluorescence techniques to uncover dynamic disorder within a single molecule and establish lysozyme as a processive enzyme. On average, 100 chemical bonds are processively hydrolyzed, at 15-hertz rates, before lysozyme returns to its nonproductive, 330-hertz hinge motion. Statistical analysis differentiated single-step hinge closure from enzyme opening, which requires two steps. Seven independent time scales governing lysozyme’s activity were observed. The pH dependence of lysozyme activity arises not from changes to its processive kinetics but rather from increasing time spent in either nonproductive rapid motions or an inactive, closed conformation.

Electrode Characteristics of Individual, MnO2 Coated Carbon Nanotubes

Electrode Characteristics of Individual, MnO2 Coated Carbon Nanotubes Brad L. Corso, Israel Perez, and Philip G. Collins Department of Physics and Astronomy, University of California Irvine, Irvine CA 92697 We investigate interfacial charge transfer between a manganese oxide (MnO 2 ) pseudocapacitor material and graphitic carbon supports in the limit where the graphitic carbon is defect free. We achieve this limit experimentally by fabricating model electrodes comprising MnO 2 deposited on single, pristine, isolated single- walled carbon nanotubes. Li ion cyclic voltammetry of the composites gives a specific capacitance in accord with MnO 2 storage capacities, but with kinetics limited by the poor electron transfer properties of defect free carbon. By fitting the data to an equivalent circuit model, we determine the charge transfer resistivity of MnO 2 -carbon interfaces to be 9 x 10 7 Ω-cm when defects are absent, a limiting value for high power cathodes. 1. Introduction Graphitic carbons and manganese oxide (MnO 2 ) are both promising materials for heterogeneous, nanostructured pseudocapacitors because of the synergies between graphite’s high conductance and stability and MnO 2 ’s low cost and high theoretical specific capacitance (1, 2). However, existing carbon-MnO 2 composites do not achieve the full potential of both materials. This paper investigates some of the interfacial properties that can play limiting roles. Specifically, we consider the fundamental charge transfer resistance across the carbon-MnO 2 interface, and the role of carbon defect sites in reducing that resistance. It is well established that defect sites, especially those which contain oxygen, promote more efficient electron transfer by carbon electrodes (3-6). However a quantitative measure of the defect-free case has been historically elusive. High quality carbon systems all contain defects, whether at basal plane edges, grain boundaries, or points. As a result, all graphitic electrodes are ensemble mixtures of electrochemically active defects among basal plane carbon. As described in the recent review article by McCreery (3), precise control over the concentration and chemistry of defects is necessary before the fundamental electron transfer rates of these sites can be established quantitatively. To address this issue, we investigate electrochemical processes on the sidewall of a single carbon nanotube. Using high quality, single-walled carbon nanotubes (SWNTs) that are grown in place without further processing or manipulation, we interrogate carbon electrodes in the limit of defect-free carbon. Our SWNTs are electrically connected in a field effect transistor (FET) geometry, so that they can be used as both electrochemical working electrodes and as FET devices. SWNTs have an electrical conductivity that is particularly sensitive to the presence of defects (7), and this property can be exploited to characterize defects with single site sensitivity (8, 9).

Towards a Carbon Nanotube Intermodulation Product Sensor for Nonlinear Energy Harvesting

It is critically important in designing RF receiver front ends to handle high power jammers and other strong interferers. Instead of blocking incoming energy or dissipating it as heat, we investigate the possibility of redirecting that energy for harvesting and storage. The approach is based on channelizing a high power signal into a previously unknown circuit element which serves as a passive intermodulation device. This intermodulation component must produce a hysteretic current-voltage curve to be useful as an energy harvester. Here we demonstrate a method by which carbon nanotube transistors produce the necessary hysteretic - curves. Such devices can be tailored to the desired frequency by introducing functional groups to the nanotubes. These effects controllably enhance the desired behavior, namely, hysteretic nonlinearity in the transistors’ - characteristic. Combining these components with an RF energy harvester may one day enable the reuse of inbound jamming energy for standard back end radio components.

Microwave focusing with uniaxially symmetric gradient index metamaterials

Previous efforts to create a metamaterial lens in the microwave X band frequency range focused on the development of a device with biaxial symmetry. This allows for focusing solely along the central axis of propagation. For applications involving wave direction or energy diversion, focusing may be required off the central axis. This work explores a metamaterial device with uniaxial symmetry, namely in the direction of propagation. Ray-trace optimization and full-wave finite element simulations contribute to the design of the lens. By changing the placement of the focus, we achieve further control of the focus parameters. While the present work uses coils, the unit cell can consist of any structure or material.

Optical remote sensing and correlation of office equipment functional state and stress levels via power quality disturbances inefficiencies

Non-invasive optical techniques pertaining to the remote sensing of power quality disturbances (PQD) are part of an emerging technology field typically dominated by radio frequency (RF) and invasive-based techniques. Algorithms and methods to analyze and address PQD such as probabilistic neural networks and fully informed particle swarms have been explored in industry and academia. Such methods are tuned to work with RF equipment and electronics in existing power grids. As both commercial and defense assets are heavily power-dependent, understanding electrical transients and failure events using non-invasive detection techniques is crucial. In this paper we correlate power quality empirical models to the observed optical response. We also empirically demonstrate a first-order approach to map household, office and commercial equipment PQD to user functions and stress levels. We employ a physics-based image and signal processing approach, which demonstrates measured non-invasive (remote sensing) techniques to detect and map the base frequency associated with the power source to the various PQD on a calibrated source.