Article pubs.acs.org/JPCC
Comparison of Canonical versus Silver(I)-Mediated Base-Pairing on Single Molecule Conductance in Polycytosine dsDNA Emily Toomey,† Jimmy Xu,† Simon Vecchioni,‡ Lynn Rothschild,§ Shalom Wind,‡ and Gustavo E. Fernandes*,† †
School of Engineering, Brown University, Providence, Rhode Island 02912, United States Department of Biomedical Engineering, Columbia University, New York, New York 10027, United States § Biospheric Science Branch, NASA Ames Research Center, Moffett Field, California 94035, United States Downloaded via UNIV OF SUNDERLAND on September 24, 2018 at 11:52:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Intercalation of metal ions into double-stranded DNA has recently been proposed as a path to efficient charge transport in DNA wires. Until now, the effect of Ag(I) intercalation between mismatched cytosine nucleobases on the conductance of DNA has not been assessed. Here we use a scanning tunneling microscopy (STM) break-junction technique to evaluate and compare the single molecule conductance of polynucleotide sequences of 11 base pairs in length. The resulting single molecule conductance for Ag(I)−polyC is found to be an order of magnitude greater than the control strand made using canonical Watson−Crick pairing. This finding suggests that Ag(I) intercalation alters the dominant electron transport process from standard π-orbital delocalization common in sequences with multiple stacked guanines to an alternate and ultimately more efficient conduit.
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duplex.10 Despite extensive structural and chemical examination, the impact of replacing G-C pairs with Ag(I)-intercalated C-C mismatches on electrical conduction has not yet been studied. In this work, we probe the effects of Ag(I) incorporation on the DNA single molecule conductance using a scanning tunneling microscope (STM) break-junction system built entirely in-house. First described by Xu and Tao, the breakjunction technique for single-molecule conductivity relies on statistical analysis of thousands of repeated molecular junctions that form between a gold substrate saturated with the molecule and a vertically oscillating gold tip.11 Since then, this method has been used in numerous DNA conductance investigations.12,13 By terminating both ends of the molecule with an Au-binding amine linker, single molecules are able to span the gap that is created each time the gold tip is retracted from the gold substrate surface. As a result, current through the molecule is measured as a steady voltage bias is applied to the substrate. Using this method, thousands of conductance traces were gathered for two different oligonucleotide sequences: one comprised entirely of C-Ag(I)-C base pairs and the other being a G-C pairing, Watson−Crick control. By compiling reoccurring conductance values observed in these traces into frequency histograms, single molecule conductance estimates are obtained for both subjects. The silver-intercalated sample is found to be
INTRODUCTION Interest in DNA-based nanotechnology stems from its linear configuration, alterable binding sites, and reproducible geometry, offering the possibility of applications including use as a cheaply replicable nanowire or a template for precise nanostructures.1 However, experimental evidence of DNA conductivity has been far from consistent due to the difficulty of performing direct electrical measurements on single molecules, leading to inconsistent reported behavior ranging from wide-bandgap semiconducting to Ohmic to superconducting.2−4 A possible way around this is to incorporate metal into the DNA molecule to form an enhanced conduction system that is both predictable and reliable. Previously explored methods include replacing base pair imino protons with metal ions to form an M-DNA complex, palladium reduction and seeding followed by plating on DNA origami structures to build nanofabricated “circuits”, and selectively metalizing specific genes through the incorporation of modified triphosphates.5−7 A particular technique of interest characterized in detail by Tanaka et al. replaces traditional hydrogen-bonded Watson− Crick guanine−cytosine (G-C) base pairs with cytosine “mismatches” (C-C).8 Work by Miyake et al. demonstrated that the gaps created by these opposing pyrimidines selectively incorporate Ag(I) ions at the site of the mismatch, resulting in a metal ion encased in π-stacking nucleobases and a stabilizing phosphate backbone.9 Recent investigation of N3-Ag(I)-N3 linear coordination in RNA duplexes revealed that this binding scheme is structurally similar to canonical Watson−Crick base pairs, and as a result preserves the standard conformation of the © 2016 American Chemical Society
Received: December 7, 2015 Revised: March 12, 2016 Published: March 18, 2016 7804
DOI: 10.1021/acs.jpcc.5b11968 J. Phys. Chem. C 2016, 120, 7804−7809
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The Journal of Physical Chemistry C
adjusting the z-direction voltage of the μ-Drive controller until the tip barely contacted the substrate. After repeating this process many times, it became clear that this precarious contact proved to be more effective in the reproducible formation of junctions, as opposed to pressing the tip against the substrate for a longer period of time. For each trial, the oscilloscope window was adjusted to only view the decaying section of the waveform, which showed the upward movement of the tip as the junction was broken. Data from each triggered event were sent from the oscilloscope to a computer through a GPIB driver. A custom Matlab script dictated the number of events to be captured before the end of the trial and converted the voltage data set into conductances based on the applied voltage and series resistance of the system. Each series of trials recorded between 1000 and 10 000 conductance versus tip displacement curves, each with 2000 data points per curve. The following data sets were then converted from units of voltage to fractions of G0, where G0 is the quantum unit of conductance (G0 = 2e2/ℏ, where e is the electronic charge and ℏ is the reduced Planck constant), while taking into account the series resistance of the system. Afterward, purely exponential events were filtered out in order to restrict analysis to instances of molecular bridge formation. Curves that began or ended outside of the oscilloscope trigger limit were also discounted. The remaining data set was compiled into a histogram of G/G0 versus counts, and the lowest histogram peaks were analyzed. For each peak, the value of the center bin was compared to the raw conductance curves to ensure that “steps” did appear at these levels and that they were not due to artifacts. In addition to recording the center bin values, the differences between peak centers were also calculated for up to eight of the first peaks in the resulting histogram. These differences were further compared to the steps that appeared in the conductance curves.
an order of magnitude more conductive than the control, and the implications of this difference are discussed in the context of charge transport mechanisms, including π-orbital delocalization.
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METHODS DNA Sample Preparation. Single-stranded oligonucleotides were delivered from Integrated DNA Technologies via dry shipment and suspended in 10 mM MOPS, 100 mM NaNO3 buffer (pH = 7) to a concentration of 100 μM. Oligos were ordered with amine linkers (5′ Amino Modifier C6) added to the terminal 5′-sugar OH group in order to facilitate binding to both the gold tip and substrate. Because of their palindromic design, mismatched duplexes were created by adding twice the volume of the template (20 μL), while control sequences were created by adding 10 μL of both the polyC template and complementary sequence to 100 μL of buffer. A volume of 16 μL of 1 mM AgNO3 was added to achieve a 1.5:1 molar ratio of Ag(I):cytosine mismatch in order to mediate the effects of ion availability on strand alignment in highly mismatched duplexes and depletion by weak associations between the metal ions and the phosphate backbone. The solution was heated at 95 °C for 3 min, cooled to room temperature for 2 h, and stored at 4 °C overnight. To remove excess Ag(I) from solution and the phosphate backbone, precipitation was performed through the addition of 4.1 μL of 100 mM NaCl, after which the solution was filtered through a spin column (GE Healthcare illustra MicroSpin G-25). It should be noted that AgNO3 was added to both the control and mismatched sequences to ensure that any weak binding between Ag(I) ions and the backbones remaining after precipitation would be equal. STM Break-Junction Technique. In this study, the STM tip was created from a gold wire which was cleaned in an ultrasonic bath first with acetone and then with ethanol. The tip was then secured with a custom Teflon brace held with a metal screw in order to reduce both the length of the wire and the amount of exposed metal in the setup, which could pick up noise or give rise to leakage currents. Substrates consisting of 50 nm gold evaporated onto silicon were made in-house and cleaned using the same procedure as with the STM tip. For each trial, 10 μL of the suspended sample was deposited onto a clean substrate and left to dry for 15 min. Excess solution was removed via wicking and dry nitrogen. In order to control the tip’s movement, the clamp was attached to a piezoelectric stage (Newport 562 ser. 01907) and linear actuator (Newport ser. 1681 ESA1330), which was regulated by a μ-Drive controller (Newport ESAC) that oscillated in accordance to a triangular wave sent out by a waveform generator (Agilent 33120). A wave of 0.3 Vpp produced a 9.6 V reading on the μ-Drive controller as a result of its 32:1 gain. The approximated linear response is 0.2 μm/V. A bias voltage of 12−21 mV was applied to the substrate surface (KIKUSUI PAB 32 power supply). For all experiments described here, a PC-ONE patch/whole cell clamp amplifier offering a rise time of at most 20 μs (Dagan Corporation) was employed. To reduce amplifier saturation and excess noise, a 1 MΩ resistor was placed in series with the amplifier and a metal box was placed around the stage and tip for shielding. In order to start a new set of scans, the stage was mechanically adjusted to lower the tip toward the substrate until an event was triggered on the oscilloscope (8-bit, Agilent 54621A), indicating contact. Immediately afterward, the tip was raised slightly to break contact and lowered again by manually
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RESULTS AND DISCUSSION Here we extend the C-C mismatch scheme beyond the single base pair replacements studied previously to examine palindromic polycytosine sequences of 11 base pairs (∼4 nm) in length. As shown in Figure 1, the mismatched sequence
Figure 1. Template and complement sequences used for creating the polycytosine canonical control sequence and the Ag(I)−polyC duplex.
was created by annealing the polyC template with itself in the presence of AgNO3, while the control was formed by annealing the polyC template with its complement. Electrospray ionization mass spectroscopy studies of Ag(I) binding in similar sequences has shown that C11-(Ag+)11-C11 is the dominant product of this process, indicating that silver ions in the polyC duplex primarily participate in a 1:1 base pair interaction.14 Amine linkers were chosen based on the work by Venkataraman et al. that found amine linkers to give more electrically stable connections to the Au substrate and tip than sulfhydryls or isonitriles due to a preferential binding to undercoordinated Au atoms on the respective surfaces.15 The presence of guanines in the control sequence is a vital factor in the overall transport process. Because guanine has the lowest ionization energy (∼8.24 eV, experimentally) of all four bases, it has been suggested that it can support higher mobility 7805
DOI: 10.1021/acs.jpcc.5b11968 J. Phys. Chem. C 2016, 120, 7804−7809
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Figure 2. Ag(I)−polyC and control conductance. (a) Diagram of STM break-junction process. After gold tip and substrate make contact, the STM tip retracts and a molecule bound to both gold surfaces spans the gap, allowing current to flow, before a complete break occurs. (b) Scheme of polycytosine control sequence bound to the gold substrate and STM tip by amine linkers. (c) Examples of conductance versus probe separation curves for the polycytosine control sequence. Curves have been shifted on the x-axis to accommodate multiple scans. The red line highlights the step conductance value. (d) Conductance histogram for control sequence (x-axis plotted on a log scale). The vertical red line highlights the peak, while the curved line approximates the baseline signal from tunneling. (e) Envelope of conductance histogram for control sequence (x-axis plotted on a linear scale). (f) Scheme of Ag(I)−polyC sequence bound to the gold substrate and STM tip by amine linkers. (g) Examples of conductance vs probe separation curves for the Ag(I)−polyC sequence. Curves have been shifted on the x-axis to accommodate multiple scans. The red lines highlight the recurring step conductance values. (h) Conductance histogram for Ag(I)−polyC sequence (x-axis plotted on a log scale). (i) Envelope of conductance histogram for Ag(I)−polyC sequence (x-axis plotted on a linear scale).
and delocalization of charge.16 However, because the complement to the polycytosine template strand is polyguanine, stacked multiguanine interactions must be considered as well. Empirically demonstrated reduction in ionization potential with the addition of subsequent guanines may be explained by strong coupling between π-orbitals, delocalizing the holes and further reducing the ionization potential.17 The complete absence of guanines in the Ag(I)-intercalated sample not only implies that the average ionization potential of the bases is higher but also suggests that transport via π-orbitals will be less efficient since the energy required to move an electron will be greater. Consequently, it was assumed that any increase in
single molecule conductivity with the silver-intercalated sample must be due to a transport conduit other than the π-orbital mechanism that is strongest in stacked guanine sequences. Similar computational studies on stacked thymine−Hg(II) base pairs (T-Hg(II)-T) have predicted that the incorporation of adjacent Hg(II) ions critically alters the electrical properties of the overall DNA molecule.18 Since the Hg(II) nucleus is the most electron-deficient part of the base pair, stacking multiple T-Hg(II)-T base pairs allows for the lowest unoccupied molecular orbital (LUMO) of each pair to overlap.19 As a result, the increase in donor−acceptor coupling that is seen in stacked T-Hg(II)-T base pairs in comparison to their canonical 7806
DOI: 10.1021/acs.jpcc.5b11968 J. Phys. Chem. C 2016, 120, 7804−7809
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substrate. Recent studies on stretch dynamics of DNA suggest sample damage may occur over time as a result of rupture of terminal hydrogen bonds, citing that the ends of the duplex are 70 times softer than the middle, yet subject to most of the shear force.13 The jagged appearance of the Ag(I)−polyC steps may indicate higher sensitivity to this stretch-induced damage, potentially leading to displacement of the terminal ion or fragmented molecules of lower conductivity that produce lower range steps consequently contributing to a higher baseline. Such breakage may explain why areas of the substrate appeared to be “worn down” with no molecular steps appearing after many thousands of impacts by the tip. Additionally, the redundancy of the polyC sequence allows it to form a variety of possible configurations, including a slipped-base duplex. In this configuration, strand misalignment produces an overhang, creating a solvent-accessible gap between the amine linker and the conductance pathway of the electrodes that leaves the system susceptible to noise. Thus, the higher baseline and noisier signal of the Ag(I)−polyC traces are potential consequences of the sequence’s repetitive design. Similar measurements by Xu et al. on native DNA find that the 10 bp sequence used in their study conducts at 7.5 × 10−5G0 and that conductance decays with sequence length.23 Despite differences in experimental setup, it is remarkable that the 11 bp control duplex presented here has a conductance of the same order at roughly 6.8 × 10−5G0 (Figure 2d). Charge transport is likely slightly lower in the work by Xu et al. as they utilize a sequence with two A-T pairs at the center. As mentioned previously, it has been shown that stacked G-C pairs have a lower resistance than other sequence geometries, so it is reasonable that an 11 bp sequence composed of consecutive GC bases would perform similarly to a shorter strand with an AT conductance bottleneck. Though the tests performed by Xu et al. are done through an aqueous layer of buffer salts, the similarity in results between our two groups suggests that the control sequence in this study provides a reliable baseline for DNA conductance. This supports the conclusion that the enhanced charge transport in silver-paired strandsbehavior characteristic of much shorter strandsis not a system artifact; rather, it represents a significant finding specific to corefunctionalized DNA. Given this control baseline, the order of magnitude difference between the metalized sequence and the canonical control indicates a departure from charge transport dominated by πorbital delocalization, as the control sequence is expected to utilize this conductance pathway more efficiently yet has a lower measured conductivity. This observation is also consistent with expectations stemming from the charge transport models of T-Hg(II)-T base pairs mentioned above, signaling that the enhanced conductivity may be due to improved mobility of excess electrons rather than hole coupling. However, more research is needed to determine the precise nature of the transport mechanism at hand.
control sequences is attributed to their impact on excess electron transfer rather than enhanced hole coupling, although these predictions have not yet been experimentally confirmed.18 Studies on the crystal structure of T-Hg(II)-T base pairs have also shown that the short distance between adjacent Hg(II) ions facilitates metallophilic attractions; these attractions vertically compress the double helix, pulling stacked ions closer together.20 While it is yet to be experimentally determined whether C-Ag(I)-C base pairs possess similar characteristics, quantum mechanical/molecular mechanical (QM/MM) modeling of these base pairs has estimated distances between neighboring Ag(I) ions to be small enough to support argentophilic, weak-binding interactions, potentially altering charge transport and conduction pathways within the molecule.21 Figure 2a illustrates the general break junction procedure used for conductivity measurement, beginning with the vertically oscillating tip in contact with the charged substrate so that current flows between the two electrodes. Without the formation of a molecular bridge, the conductivity decreases exponentially with distance as the tip moves away from the gold surface, as expected from basic tunneling.22 However, when a DNA molecule bridges the junction, a small amount of current flows through the molecule and disrupts the exponential decay. By recording thousands of conductance versus distance traces, it is possible to relate reoccurring conductance steps to the conductivity of one or more molecules spanning the gap between electrodes. These recurring steps are often discovered by analyzing peaks in histograms that count the frequency of recorded conductance steps and their corresponding events. Using this technique, histograms were obtained for both the control and Ag(I)-intercalated DNA molecules. Figure 2d shows the histogram obtained from thousands of conductance curves for the polyC control. As indicated by the dashed line, a prominent peak is seen at roughly 0.68 × 10−4G0, with a less distinct peak at 1.1 × 10−4G0. Although the spacing between these values would appear to suggest a single molecule conductance of about (4−5) × 10−5G0, the absence of a peak implies that either it has been overwhelmed by the baseline or that no significant peak exists. In order to validate the presence of the histogram peak in Figure 2d, it was necessary to examine individual conductance curves for corresponding molecular steps. Figure 2c shows sample traces in relation to the histogram peak conductance of 6.8 × 10−5G0. The high frequency of corresponding molecular steps confirms that the peaks are indeed a result of reoccurring conductance values. Figure 2h displays the histogram obtained from thousands of conductance curves for the Ag(I)-polyC duplex, revealing a peak around 1 × 10−3G0. While this peak is verified by examining corresponding molecular steps in individual traces, there were a notable number of curves that also showed plateaus at roughly half of this value (see Figure 2g). Because of these clear, reoccurring molecular steps as well as a resolvable peak at twice its value, potentially signifying a configuration of two molecules in parallel, 0.5 × 10−3G0 is the best estimate of the single molecule conductance for the Ag(I)−polyC sequence. The absence of a peak at this level could be caused by a variety of reasons, including an overwhelming baseline signal. Comparing Figure 2c to Figure 2g, there is a notable difference between the stable appearance of the control steps and the jaggedness of the Ag(I)−polyC plateaus, implying a disparity in the stability of their interactions with the tip and
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CONCLUSIONS In summary, we have investigated the change in conductance that occurs by replacing canonical C-G pairs with Ag(I)intercalated cytosine mismatches. Using the STM breakjunction technique, we resolved thousands of molecular steps in sample traces and estimated the single molecule conductance of the canonical control sequence to be on the order of 10−5G0, while the single molecule conductance of the Ag(I)−polyC sequence was an order of magnitude higher at 10−4G0. These 7807
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(3) Yamahata, C.; Takekawa, T.; Kumemura, M.; Hosogi, M.; Hashiguchi, G.; Collard, D.; Fujita, H. Electrical and Mechanical Characteristics of DNA Bundles Revealed by Silicon Nanotweezers. SolidState Sensors, Actuators and Microsystems Conference, Lyon, France, June 10−14, 2007; Transducers, 2007; pp 395−398. (4) Kasumov, A. Y.; Kociak, M.; Guéron, S.; Reulet, B.; Volkov, V. T.; Klinov, D. V.; Bouchiat, H. Proximity-Induced Superconductivity in DNA. Science 2001, 291, 280−282. (5) Rakitin, A.; Aich, P.; Papadopoulos, C.; Kobzar, Y.; Vedeneev, A. S.; Lee, J. S.; Xu, J. M. Metallic Conduction through Engineered DNA: DNA Nanoelectronic Building Blocks. Phys. Rev. Lett. 2001, 86, 3670− 3673. (6) Geng, Y.; Liu, J.; Pound, E.; Gyawali, S.; Harb, J. N.; Woolley, A. T. Rapid Metallization of Lambda DNA and DNA Origami Using a Pd Seeding Method. J. Mater. Chem. 2011, 21, 12126. (7) Burley, G. A.; Gierlich, J.; Mofid, M. R.; Nir, H.; Tal, S.; Eichen, Y.; Carell, T. Directed DNA Metallization. J. Am. Chem. Soc. 2006, 128, 1398−1399. (8) Tanaka, K.; Yamada, Y.; Shionoya, M. Formation of Silver(I)Mediated DNA Duplex and Triplex through an Alternative Base Pair of Pyridine Nucleobases. J. Am. Chem. Soc. 2002, 124, 8802−8803. (9) Miyake, Y.; Togashi, H.; Tashiro, M.; Yamaguchi, H.; Oda, S.; Kudo, M.; Tanaka, Y.; Kondo, Y.; Sawa, R.; Fujimoto, T.; et al. MercuryII-Mediated Formation of Thymine−HgII−Thymine Base Pairs in DNA Duplexes. J. Am. Chem. Soc. 2006, 128, 2172−2173. (10) Kondo, J.; Yamada, T.; Hirose, C.; Okamoto, I.; Tanaka, Y.; Ono, A. Crystal Structure of Metallo DNA Duplex Containing Consecutive Watson−Crick-like T−HgII−T Base Pairs. Angew. Chem., Int. Ed. 2014, 53, 2385−2388. (11) Xu, B.; Tao, N. J. Measurement of Single-Molecule Resistance by Repeated Formation of Molecular Junctions. Science 2003, 301, 1221−1223. (12) Liu, S.; Clever, G. H.; Takezawa, Y.; Kaneko, M.; Tanaka, K.; Guo, X.; Shionoya, M. Direct Conductance Measurement of Individual Metallo-DNA Duplexes within Single-Molecule Break Junctions. Angew. Chem., Int. Ed. 2011, 50, 8886−88890. (13) Bruot, C.; Xiang, L.; Palma, J. L.; Tao, N. Effect of Mechanical Stretching on DNA Conductance. ACS Nano 2015, 9, 88−94. (14) Swasey, S. M.; Leal, L. E.; Lopez-Acevedo, O.; Pavlovich, J.; Gwinn, E. G. Silver(I) as DNA Glue: Ag+-mediated Guanine Pairing Revealed by Removing Watson-Crick Constraints. Sci. Rep. 2015, 5, 10163. (15) Venkataraman, L.; Klare, J. E.; Tam, I. W.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Single-Molecule Circuits with Well-Defined Molecular Conductance. Nano Lett. 2006, 6, 458−462. (16) Hush, N. S.; Cheung, A. S. Ionization Potentials and Donor Properties of Nucleic Acid Bases and Related Compounds. Chem. Phys. Lett. 1975, 34, 11−13. (17) Xiang, L.; Palma, J. L.; Bruot, C.; Mujica, V.; Ratner, M. A.; Tao, N. Intermediate Tunnelling−Hopping Regime in DNA Charge Transport. Nat. Chem. 2015, 7, 221−226. (18) Voityuk, A. A. Electronic Coupling Mediated by Stacked [Thymine-Hg-Thymine] Base Pairs. J. Phys. Chem. B 2006, 110, 21010−21013. (19) Uchiyama, T.; Miura, T.; Takeuchi, H.; Dairaku, T.; Komuro, T.; Kawamura, T.; Kondo, Y.; Benda, L.; Sychrovsky, V.; Bour, P.; et al. Raman Spectroscopic Detection of the T-HgII-T Base Pair and the Ionic Characteristics of Mercury. Nucleic Acids Res. 2012, 40, 5766−5774. (20) Kondo, J.; Yamada, T.; Hirose, C.; Okamoto, I.; Tanaka, Y.; Ono, A. Crystal Structure of Metallo DNA Duplex Containing Consecutive Watson-Crick-like T-HgII-T Base Pairs. Angew. Chem., Int. Ed. 2014, 53, 2385−2388. (21) Kumbhar, S.; Johannsen, S.; Sigel, R. K. O.; Waller, M. P.; Müller, J. A QM/MM Refinement of an Experimental DNA Structure with Metal-Mediated Base Pairs. J. Inorg. Biochem. 2013, 127, 203− 210.
observations suggest that electrical conduction in the polycytosine Ag(I)-intercalated sequence is fundamentally altered by the insertion of silver ions into the molecule and is dominated by a process other than π-orbital delocalization, as this mechanism has been found to be most efficient in stacked guanine bases. This supports the hypothesis of a conduction bridge through the chain of silver ions. Jaggedness of molecular steps as well as a high baseline signal in the Ag(I)−polyC measurements potentially suggest sensitivity to stretch-induced shear force, polynucleobase misalignment, and end-frayinginduced terminal ion loss and thus highlights the need for future exploration of the electrode−molecule dynamics. Nevertheless, these results pave the way for subsequent determination of the conduction mechanism at hand in the modified sequences and offer hope that properties of DNAbased electronic devices may be manipulated via site-specific incorporation of metal ions for enhanced electron mobility in nanoscale components.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11968. Denaturation profile of Ag(I)−polyC after annealing and precipitation as well as additional sample scans showing the 0.5 × 10−3G0 conductance step for the Ag(I)−polyC (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel (401) 863-2447; e-mail
[email protected] (G.E.F.). Present Address
E.T.: Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, Cambridge MA 02141. Funding
We gratefully acknowledge the support of U.S. Army Research Laboratory (ARL) Contract W911NF-14-2-0075, the NSF Partnerships for Innovation: Building Innovation Capacity program award 1430007, AOARD Grant 13RSZ086 (134121), the NASA Ames Research Center Investment Fund, a NASA Space Technology Research Fellowship to S.V., the Rhode Island Space Grant, and the Karen T. Romer Undergraduate Research and “Teaching Award at Brown University.” We also acknowledge support from the NSF Partnerships for Innovation: Building Innovation Capacity program award 1430007 and from AOARD Grant 13RSZ086 (134121). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Prof. Jacob Rosenstein for his help with signal amplification and noise reduction. REFERENCES
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