Insulating Behavior ofλ-DNA on the Micron Scale

Abstract

We have investigated the electrical conductivity of λ-DNA using DNA covalently bonded to Au electrodes. Thiol-modified dTTP was incorporated into the 'sticky' ends of bacteriophage λ-DNA using DNA polymerase. Two-probe measurements on such molecules provide a hard lower bound for the resistivity ρ > 10 6 Ωcm at bias potentials up to 20 volts, in conflict with recent claims of moderate to high conductivity. By direct imaging, we show that the molecules are present after the measurements. We stress the importance of eliminating salt residues in these measurements.The question whether DNA is electrically conducting has generated broad interest. The initial spurt of interest arose in photoexcitation experiments which were interpreted in terms of long-range electron transfer [1]. In the past few years, there have been upwards of 20 papers reporting the results of more direct electrical measurements ranging from contactless meaurements at microwave frequencies to DC measurements. A distressingly wide range of conductivity values -from ρ < 10 −4 Ωcm to ρ > 10 6 Ωcm -has been reported [2,3,4,5,6]. Proximity-induced superconductivity in DNA has also been claimed [7]. Recently, local polarization measurements by 'electrostatic force microscopy' have been used to show that λ-DNA is insulating [8,9]. We note, however, that the force-microscopy experiments probe conductivity at relatively weak bias potentials.In many of the DC measurements, contact with the metal electrodes (usually Au) was achieved by laying down the molecules directly on the electrodes. Although expedient, this approach raises several concerns. It is very difficult to prove that the DNA molecule is in direct physical contact with the electrodes. Even if contact is attained, the weak physical adhesion between DNA and Au may produce an insulating contact and possibly account for the wide variation in reported resistivities [10]. A recent experiment on octanedithiol [11] has shown that deliberate chemical bonding between organic molecules and metal electrodes is a pre-requisite for achieving reproducible conductivity results. Thus a better approach would be to achieve direct chemical binding between the open ends of λ-DNA and Au. The bonds should be strong enough to withstand shear forces in a flow, and should survive the measurement process. A second concern is the shunting effect of buffer residue. Because of its finite conductance, the buffer salts which coat the electrodes and substrate produce a spurious conductance signal. Hence adequate salt removal is important. We report the results of experiments performed along these lines. Our results show that λ-DNA is a good insulator up to bias potentials of 20 volts.Chemical binding between organic molecules and Au is usually achieved by the Au-thiol (SH) chemical bond [12]. Commercially available oligonucleotides modified to incorporate the thiol group usually have carbon-chain spacers (C3 or C6) between the thiol group and DNA [3,6,13], which may present barriers to electron transfer. To avoid the spacer pro...

Probing biomechanical properties with a centrifugal force quartz crystal microbalance

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Initial interactions in electromagnetic field‐induced biosynthesis

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DNA-based thermoelectric devices: A theoretical prospective

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“…12 Recent transport experiments have shown that chemical bonding between DNA and metal electrodes is a prerequisite for achieving reproducible conductivity results. 17,[20][21][22][23][24] In a previous work, we considered the role of contact effects in a polyGACT single strand connected to metallic leads at both ends. In general, interference effects between the DNA molecular bands and the electronic structure of the leads at the metal-DNA interface degrades the transmission, as expected.…”