Harsh Bais, PhD

PUBLICATIONS


 

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2017

  • Hasti Amiri, Kenneth L. Shepard, Colin Nuckolls, and Raúl Hernández Sánchez Single-Walled Carbon Nanotubes: Mimics of Biological Ion Channels NanoLetters DOI: DOI: 10.1021/acs.nanolett.6b04967
    Abstract

    Here we report on the ion conductance through individual, small diameter single-walled carbon nanotubes. We find that they are mimics of ion channels found in natural systems. We explore the factors governing the ion selectivity and permeation through single-walled carbon nanotubes by considering an electrostatic mechanism built around a simplified version of the Gouy−Chapman theory. We find that the single-walled carbon nanotubes preferentially transported cations and that the cation permeability is size-dependent. The ionic conductance increases as the absolute hydration enthalpy decreases for monovalent cations with similar solid-state radii, hydrated radii, and bulk mobility. Charge screening experiments using either the addition of cationic or anionic polymers, divalent metal cations, or changes in pH reveal the enormous impact of the negatively charged carboxylates at the entrance of the single-walled carbon nanotubes. These observations were modeled in the low-to-medium concentration range (0.1−2.0 M) by an electrostatic mechanism that mimics the behavior observed in many biological ion channel-forming proteins. Moreover, multi-ion conduction in the high concentration range (>2.0 M) further reinforces the similarity between single-walled carbon nanotubes and protein ion channels.

  • Eyal Aklimi, Student Member, IEEE, Daniel Piedra, Student Member, IEEE, Kevin Tien, Student Member, IEEE, Tomás Palacios, Member, IEEE, and Kenneth L. Shepard, Fellow, IEEE Hybrid CMOS/GaN 40-MHz Maximum 20-V Input DC–DC Multiphase Buck Converter. IEEE Journal of Solid-State Circuits.
    Abstract

    This paper presents a 40-MHz hybrid CMOS/GaN integrated multiphase dc–dc switched-inductor buck converter with a maximum 20-V input voltage. The half-bridge switches are realized using lateral AlGaN/GaN HEMTs, while the drivers and other circuitry are implemented in standard 180-nm CMOS. The interface between the CMOS and GaN dice is achieved through face-to-face bonding, reducing inductive parasitics for the connection to less than 15 pH. A capacitively coupled level shifter provides the gate drive for the high-side GaN switch using 5-V CMOS devices. The converter demonstrates 76% efficiency for 8:1 V conversion and over 60% efficiency for conversion ratios up to 16:1.

  • Jordan Thimot and Kenneth L. Shepard. Wirelessly powered implants. Nature Biomedical Engineering 1, 0051 (2017) | DOI: 10.1038/s41551-017-0051
    Abstract

    Phased-array antennas that conform to body surfaces efficiently transfer electromagnetic energy to miniaturized semiconductor devices implanted in pigs.

  • Sefi Vernick, Scott M. Trocchia, Steven B. Warren, Erik F. Young, Delphine Bouilly, Ruben L. Gonzalez, Colin Nuckolls & Kenneth L. Shepard Electrostatic melting in a single-molecule field-effect transistor with applications in genomic identification. Nature Communications 8, 15450 (2017) | DOI: 10.1038/ncomms15450
    Abstract

    The study of biomolecular interactions at the single-molecule level holds great potential for both basic science and biotechnology applications. Single-molecule studies often rely on fluorescence-based reporting, with signal levels limited by photon emission from single optical reporters. The point-functionalized carbon nanotube transistor, known as the single-molecule field-effect transistor, is a bioelectronics alternative based on intrinsic molecular charge that offers significantly higher signal levels for detection. Such devices are effective for characterizing DNA hybridization kinetics and thermodynamics and enabling emerging applications in genomic identification. In this work, we show that hybridization kinetics can be directly controlled by electrostatic bias applied between the device and the surrounding electrolyte. We perform the first single-molecule experiments demonstrating the use of electrostatics to control molecular binding. Using bias as a proxy for temperature, we demonstrate the feasibility of detecting various concentrations of 20-nt target sequences from the Ebolavirus nucleoprotein gene in a constant-temperature environment.