C Chad Harrell

University of Florida, Gainesville, Florida, United States

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Publications (11)108.05 Total impact

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    ABSTRACT: In this paper, we describe resistive-pulse sensing of two large DNAs, a single-stranded phage DNA (7250 bases) and a double-stranded plasmid DNA (6600 base pairs), using a conically shaped nanopore in a track-etched polycarbonate membrane as the sensing element. The conically shaped nanopore had a small-diameter (tip) opening of 40 nm and a large-diameter (base) opening of 1.5 microm. The DNAs were detected using the resistive-pulse, sometimes called stochastic sensing, method. This entails applying a transmembrane potential difference and monitoring the resulting ion current flowing through the nanopore. The phage DNA was driven electrophoretically through the nanopore (from tip to base), and these translocation events were observed as transient blocks in the ion current. We found that the frequency of these current-block events scales linearly with the concentration of the DNA and with the magnitude of the applied transmembrane potential. Increasing the applied transmembrane potential also led to a decrease in the duration of the current-block events. We also analyzed current-block events for the double-stranded plasmid DNA. However, because this DNA is too large to enter the tip opening of the nanopore, it could not translocate the pore. As a result, much shorter duration current-block events were observed, which we postulate are associated with bumping of the double-stranded DNA against the tip opening.
    No preview · Article · Jan 2007 · Langmuir
  • C Chad Harrell · Zuzanna S Siwy · Charles R Martin
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    ABSTRACT: Preparation of conical nanopore membranes by controlling the nanopore shape was investigated. It was found that the cone angle of conical nanopores prepared by anisotropic etching of heavy ion-tracked polymer films can be systematically varied by varying the transmembrane potential difference applied during pore etching. The diameter of the base opening was found to be increasing with the magnitude of the applied potential difference. The results suggested that larger base openings can be obtained at higher transmembrane potentials.
    No preview · Article · Mar 2006 · Small
  • C. Chad Harrell · Zuzanna S. Siwy · Charles R. Martin

    No preview · Article · Feb 2006 · Small
  • C Chad Harrell · Punit Kohli · Zuzanna Siwy · Charles R Martin
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    ABSTRACT: There is considerable interest in developing chemical devices that mimic the function of biological ion channels. We recently described such a device, which consisted of a single conically shaped gold nanotube embedded within a polymeric membrane. This device mimicked one of the key functions of voltage-gated ion channels: the ability to strongly rectify the ionic current flowing through it. The data obtained were interpreted using a simple electrostatic model. While the details are still being debated, it is clear that ion-current-rectification in biological ion channels is more complicated and involves physical movement of an ionically charged portion of the channel in response to a change in the transmembrane potential. We report here artificial ion channels that rectify the ion current flowing through them via this "electromechanical" mechanism. These artificial channels are also based on conical gold nanotubes, but with the critical electromechanical response provided by single-stranded DNA molecules attached to the nanotube walls.
    No preview · Article · Jan 2005 · Journal of the American Chemical Society
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    ABSTRACT: Stochastic electrophoretic capture of individual nanometer-scale particles at the small opening of a conically shaped nanopore in a synthetic membrane is described. Particle capture is sensed using a scanning electrochemical microscope (SECM) to measure the decrease in the transport rate of a redox-active molecule through the pore. The SECM tip is positioned at the larger backside opening of pore and used to amperometrically monitor the transport rate prior, during, and after particle capture. Following capture, the particle is released by electrophoretically driving it out of the pore opening and back into the solution. The capture and release method is demonstrated by detection of charged polystyrene spheres (43-150-nm diameter) using a polycarbonate membrane with conically shaped pores, the small opening of the pore having a diameter of 60 nm. The inverse of the time to capture polystyrene spheres increases with particle concentration over the range 10(8)-10(10) particles/mL. Selective detection based on nanoparticle charge and size is also demonstrated. A quantitative theoretical description of the rate of particle capture is presented, and the physical mechanism of particle capture, based on the balance of electrostatic and entropic forces, is considered.
    No preview · Article · Nov 2004 · Analytical Chemistry
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    ABSTRACT: Ion channels are protein pores that span cell membranes and open and close in response to stimuli like changes in the transmembrane potential, binding of a ligand, or mechanical stress. When open, ions pass through the pore, and hence across the cell membrane, and when closed, ion-transport is precluded. Hence, these channels are nanodevices that have a current-rectification function. There is intense research effort aimed at understanding the molecular-level mechanism for this function. One approach for elucidating the mechanism is to construct a simple abiotic system that mimics this function and to use the mechanistic details of this mimic as a guide to understand the more complex biological channel. We describe here such an abiotic mimic: a synthetic membrane that contains a single conical gold nanotube. The advantage of this mimic is that the surface charge and chemistry of the nanotube wall can be varied, at will, by judicious choice of electrolyte or by thiol chemisorption. This has allowed us to make conical Au nanotubes that rectify the ion current and, just as importantly, to definitively elucidate the mechanism of this function.
    Preview · Article · Oct 2004 · Journal of the American Chemical Society
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    ABSTRACT: We describe synthetic membranes in which the molecular recognition chemistry used to accomplish selective permeation is DNA hybridization. These membranes contain template-synthesized gold nanotubes with inside diameters of 12 nanometers, and a “transporter” DNA-hairpin molecule is attached to the inside walls of these nanotubes. These DNA-functionalized nanotube membranes selectively recognize and transport the DNA strand that is complementary to the transporter strand, relative to DNA strands that are not complementary to the transporter. Under optimal conditions, single-base mismatch transport selectivity can be obtained.
    No preview · Article · Sep 2004 · Science
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    ABSTRACT: Electroless deposition of gold on the pore walls of polycarbonate templates is currently the best known method for controlling inside diameters of template-synthesized nanotubes. It would be very useful to have alternative template-based synthetic chemistries that yield nanotubes composed of other materials, but which still allow for precise control over the nanotube wall thickness and i.d. A film-formation process that is based on layer-by-layer deposition of the film-forming material along the pore walls of the template membrane provides this desired alternative synthetic chemistry. We describe here the use of Mallouk's alpha,omega-diorganophosphonate/Zr layer-by-layer film-forming method for preparing nanotubes within the pores of alumina template membranes. We have found that this method allows accurate, quantitative, and predictable control over the wall thickness, and thus i.d., of the layered nanotubes obtained.
    No preview · Article · Jun 2004 · Journal of the American Chemical Society
  • Naichao Li · Shufang Yu · C Chad Harrell · Charles R Martin
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    ABSTRACT: We have been investigating applications of nanopore membranes in analytical chemistry-specifically in membrane-based bioseparations, in electroanalytical chemistry, and in the development of new approaches to biosensor design. Membranes that have conically shaped pores (as opposed to the more conventional cylindrical shape) may offer some advantages for these applications. We describe here a simple plasma-etch method that converts cylindrical nanopores in track-etched polymeric membranes into conically shaped pores. This method allows for control of the shape of the resulting conical nanopores. For example, the plasma-etched pores may be cylindrical through most of the membrane thickness blossoming into cones at one face of the membrane (trumpet-shaped), or they may be nearly perfect cones. The key advantage of the conical pore shape is a dramatic enhancement in the rate of transport through the membrane, relative to an analogous cylindrical pore membrane. We demonstrate this here by measuring the ionic resistances of the plasma-etched conical pore membranes.
    No preview · Article · May 2004 · Analytical Chemistry
  • C Chad Harrell · Sang Bok Lee · Charles R Martin
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    ABSTRACT: There is increasing interest in investigating transport and electrochemical phenomena in synthetic membrane samples that contain a single pore of nanoscopic diameter. Approaches used to date for preparing such single-nanopore membranes include microfabrication-based methods, the track-etch method, and a method based on the incorporation of a single fullerene nanotube within a synthetic membrane. We describe here an alternative approach that we believe is easier and more accessible than the previously described methods. This method is based on a very low pore density track-etch membrane obtained from commercial sources. Fluorescence microscopy is used to identify and isolate a single nanopore in this membrane. Membrane samples containing single nanopores with diameters as small as 30 nm have been prepared. Furthermore, we show here that an electroless plating method can be used to deposit a gold nanotube within the single nanopore, and this provides a route for further decreasing the inside diameter of the pore. A single-nanotube membrane with an electrochemically determined inside diameter of approximately 2 nm was prepared and evaluated.
    No preview · Article · Jan 2004 · Analytical Chemistry
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    ABSTRACT: Mother Nature has created tiny, nanometer sized transport channels that are highly selective for given ions and can be controlled by an applied electric field. These ion channels are the principal nanodevices that mediate communication of a cell with other cells via ion transport, and enable concerted functioning of a living organism. This fundamental principal has motivated us to investigate the transport and electrochemical phenomena in synthetic analogues of ion channels based on single nanopore membranes. We have fabricated a voltage responsive synthetic single nanotube in the form of a conical shaped pore within a polymer membrane. The conical pore was prepared by using the track-etching technique. The shape of the pore consists of a large opening diameter of 5 µm and a small opening diameter of 60 nm (figure 1). We have shown that these conical pores rectify in a manner analogous to voltage-gated ion channels. Also, we have shown that such pores can be electroless gold plated to form a corresponding gold nanotube, and that this provides a route for systematically and reproducible changing the pore diameter to molecular dimensions. Also, this approach allows one to fine-tune the surface functionality of the nanotubes using rich gold-thiol chemistry. We show here that a single conical pore coated with gold having a small pore diameter of 40 nm shows a non-rectifying response. However, the I-V response of the single nanotube can be easily tuned by modifying the surface of the nanotube with ss-DNA (figure 2). We will show that by modifying the gold nanotube membranes with ssDNA a non-linear current voltage curve is obtained. Also, we will present and discuss the tuning of the rectifying properties of the nanotube membranes by ssDNA modification. As we will show in this presentation, these single ssDNA modified gold nanotube membranes constitute an extraordinarily powerful system for understanding voltage rectification in synthetic voltage-gated ion channels. Figure 1: Nanopore geometry within the polycarbonate membrane. Electroless gold nanowire was deposited within the single nanopore membrane to show the pore shape. Transmembrane Potential (mV) i (nA) Au plated 15_Bases 30_Bases 45_Bases Figure 2: I-V characteristics of ss-DNA modified gold nanotube membrane.
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