nihms97811.pdf

Conductivity of a single DNA duplex bridging a carbon nanotube

gap

XUEFENG GUO

1,2

ALON A. GORODETSKY

JAMES HONE

2,4

JACQUELINE K.

BARTON

3,*

, and

COLIN NUCKOLLS

1,2,*

Department of Chemistry, Columbia University, New York 10027, USA

Center for Electronic Transport in Molecular Nanostructures, Columbia University, New York

10027, USA

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena

California 91125, USA

Department of Mechanical Engineering, Columbia University, New York 10027, USA

Abstract

We describe a general method to integrate DNA strands between single-walled carbon nanotube

electrodes and to measure their electrical properties. We modified DNA sequences with amines on

either the 5

′

terminus or both the 3

′

and 5

′

termini and coupled these to the single-walled carbon

nanotube electrodes through amide linkages, enabling the electrical properties of complementary and

mismatched strands to be measured. Well-matched duplex DNA in the gap between the electrodes

exhibits a resistance on the order of 1 M

. A single GT or CA mismatch in a DNA 15-mer increases

the resistance of the duplex ~300-fold relative to a well-matched one. Certain DNA sequences

oriented within this gap are substrates for

Alu

I, a blunt end restriction enzyme. This enzyme cuts

the DNA and eliminates the conductive path, supporting the supposition that the DNA is in its native

conformation when bridging the ends of the single-walled carbon nanotubes.

Since the elucidation of the double helical structure of DNA, scientists have been fascinated

by the possibility that the stacked aromatic base pairs of DNA may enable charge transport

(CT) over significant distances

–

. The nature of the conductive properties of duplex DNA has

consequently attracted substantial interest. Initial solution experiments featured photoinduced

DNA-mediated CT between well-defined donor and acceptor sites

–

. Long-range CT has

been shown to lead to oxidative damage in DNA over 200 Å away from the bound oxidant

and DNA CT has also been found to be exquisitely sensitive to the integrity of the base-pair

stack and to the coupling of the donors and acceptors with the DNA (ref.

). Furthermore,

DNA CT can be attenuated by a single base mismatch

. Indeed, this sensitivity of DNA CT

to the integrity of the base-pair duplex has prompted both the consideration of roles for DNA

CT within the cell

and the construction of electrochemical DNA-based sensors for

mutations, base lesions and protein binding

Previously, we have described a system for measuring the conductivity of a single molecule

covalently immobilized within a nanotube gap

–

. In this system, gaps are formed in single-

*Correspondence and requests for materials should be addressed to C.N. and J.K.B, e-mail: cn37@columbia.edu; jkbarton@caltech.edu.

Supplementary information accompanies this paper on www.nature.com/naturenanotechnology.

Author contributions

X.G. and A.G. performed the experiments and wrote the manuscript. J.H., J.K.B. and C.N. designed the research and wrote the manuscript.

NIH Public Access

Author Manuscript

Nat Nanotechnol

. Author manuscript; available in PMC 2009 September 21.

Published in final edited form as:

Nat Nanotechnol

. 2008 March ; 3(3): 163–167. doi:10.1038/nnano.2008.4.

NIH-PA Author Manuscript

walled carbon nanotubes (SWNTs) that can be reconnected by one or a few molecules attached

to both sides of the gap through amide bond formation. This strategy allows molecules to be

wired into metal electrodes by means of robust amide linkages, avoiding the problems

commonly associated with suspending DNA between electrodes and forming irreproducible

electrical contacts. Moreover, the devices are sufficiently robust that aqueous environments

can be used. Using this method we have made molecular devices that are able to change their

conductance as a function of pH (ref.

), and others that are sensitive to the binding between

protein and substrate

, or that switch their conductance when the bridging molecules are

photoswitched

. Here we describe the first measurements of the conductivity of a single DNA

duplex when it is wired into a carbon electrode through covalent bonds.

Numerous CT measurements on DNA strands bridging two electrodes have also been carried

out in an effort to establish the conductivity of DNA. These measurements have yielded a

remarkably wide range of resistance values (1 to 1 × 10

)

–

. For example, DNA ropes

suspended on a metallic grid were found to behave as a semiconductor with a resistance on the

order of 1 M

(ref.

). However, one group initially found wide bandgap semiconducting

behaviour for DNA duplexes set between two nanoelectrodes using high-voltage electrostatic

trapping

, but later found insulating behaviour for longer strands

. In contrast, in other

research it was found possible to induce superconductivity at low temperatures in dehydrated

DNA bundles on rhenium/carbon electrodes

In general, the variability in the results obtained can be understood by considering the solution

experiments, which show that DNA CT depends sensitively upon the integrity of the base-pair

stack, the absence of damage within the duplex, and the electrical connections to the duplex.

It should be noted that measurements using both conducting atomic force microscopy (AFM)

and scanning tunnelling microscopy (STM)

–

under aqueous conditions have recently

been carried out and show that well-matched DNA exhibits a low resistance (1–10 M

), as

well as an increase in resistance with an intervening base mismatch

. Also, STM

measurements on DNA monolayers have shown effective charge transport for well-matched

DNA oriented by the STM tip

. However, none of these measurements was of a single duplex,

but instead they were carried out for a collection of duplexes on the surface below the AFM

or STM tip. Thus, in the conductivity measurements carried out so far, the integrity of the DNA

was not well established, the connections to the duplex were not well defined, or the

measurement was not definitively of a single DNA duplex.

Fabrication of the cut SWNT devices has been previously described in detail

–

. In brief,

SWNTs were grown using chemical vapour deposition (CVD) on highly doped silicon wafers

with 300 nm of thermally grown silicon oxide on their surface. Metal electrodes consisting of

5 nm of Cr overlaid with 50 nm of Au were deposited through a shadow mask onto the carbon

nanotubes. The silicon wafer serves as a global back gate for the devices. We then spin-cast a

layer of polymethylmethacrylate (PMMA) over the entire device structure and used ultrahigh-

resolution electron beam lithography to open a window in the PMMA. This process exposed

a section of the SWNT only a few nanometres in length, which was excised with an oxygen

ion plasma. The oxidative etching of the carbon nanotube generated carboxylic acid

functionalities on both sides of the gap (Fig. 1a), which can be bridged with amine-terminated

molecules.

We reconnected the carbon nanotube gap with single DNA molecules terminated with amines

using a two-step strategy. First, the freshly cut carbon nanotubes were immersed in a buffer

solution containing standard amide coupling and activating agents (Sulfo-NHS, EDCI). Then,

the activated carbon nanotube termini were reacted with amine-modified DNA to covalently

bridge the gap with a single molecule. Given that the cross-sectional area of duplex DNA (~3

) is comparable to that of the SWNTs grown here, it is unlikely that more than one DNA

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duplex can fit lengthwise within the gap. We explored two different methods to bridge these

gaps. In one method (Fig. 1b), one end each of the two strands of the DNA duplex are bound

to the SWNT electrodes. In a second method (Fig. 1c), a single strand is bound between the

ends of the SWNT electrodes. The strategy in Fig. 1c is inherently more interesting because it

allows for dehybridization/rehydridization with mismatched strands. The measurements were

carried out under ambient conditions. (See Supplementary Information for experimental details

on DNA synthesis, device reconnection and DNA dehybridization/rehybridization.)

Figure 2 shows representative

–

curves for these two different methods of DNA attachment.

We could not determine any significant difference between the conductance measurements

when using these two connection strategies. In Fig. 2a we used a DNA duplex functionalized

on both strands with an amine at the 5

′

end. In the other case (Fig. 2b), we used a DNA duplex

containing a strand functionalized at both the 5

′

and 3

′

ends. The black curves show the source–

drain current (

) as a function of the gate voltage (

) at a constant source–drain bias of 50

mV for the pristine nanotube. Before cutting of the SWNT, the device in Fig. 2a is a hole

transporting semiconducting device, and the one in Fig. 2b is a metallic device. After cutting

and initial treatment of the gap with coupling agents, the devices show no measurable current

(as indicated by the red curves). The green curves in Fig. 2a,b illustrate the conductance of the

two devices after reconnection with the two amine-modified DNAs.

In both cases shown here, the reconnected carbon nanotube devices recover their original p-

type semiconducting or metallic properties. Note that the gate voltage that can be applied to

the reconnected devices is limited; device breakdown is observed for gate voltages greater than

6 V. Over time, at these higher gate biases, the DNA bridges became poorer and poorer

conductors until, ultimately, the current levels are at the noise level of the measurement (see

Supplementary Information, Figs S1 and S2). It is difficult to determine if this effect could be

due to a hydration layer of water around the devices. Table 1 summarizes the device

characteristics measured in the course of this study for the devices before cutting, after cutting

and after reconnection with amine-terminated DNA sequences. (See Supplementary

Information, Figs S1–S7, for experimental details of the electrical measurements.) Using this

method we obtained 10 working devices out of 370 that were tested.

Devices were also reconnected with mismatched DNA, as it has been shown in a variety of

experiments that single-base mismatches dramatically attenuate CT. The DNA duplexes and

mismatches explored here are shown in Fig. 3a. The mismatched devices were found to have

higher resistance than corresponding devices reconnected with well-matched DNA. These

results could not be compared quantitatively with those on the well-matched duplex, because

different devices were fabricated to test the different duplexes. A device was therefore first

reconnected with well-matched DNA duplexes functionalized with the amines on the 5

′

and 3

′

termini of one strand, and then the duplex was dehybridized using a 1:1 solution of formamide

and deionized water at 30 °C and rehybridized with different complements (Fig. 3a–c).

Figure 3b shows the corresponding current–voltage curves for this sequence of experiments,

and Fig. 3c shows the current at

−

3 V for this sequence at a constant source drain bias of

50 mV. Rehybridization with the complement so as to generate a CA mismatch reduced the

current significantly and yielded an increase in the on-state resistance of nearly 300-fold from

0.5 M

to 155 M

(Fig. 3c). Replacing the complement featuring a CA mismatch with a

complement featuring a GT mismatch yielded no changes in the device characteristics.

However, the original on-state resistance and nanoamp current levels could then be recovered

by replacing the GT mismatched complement with the original well-matched sequence.

Importantly, the device could be taken through multiple dehybridization/rehybridization

cycles, as shown in Fig. 3b,c.

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As further confirmation that CT in the carbon nanotube gap is DNA-mediated, we reconnected

separate devices first with DNA featuring a GT mismatch or DNA featuring a CA mismatch

(see Supplementary Information, Figs S3 and S4). Dehybridization of the mismatched DNA

and replacement with well-matched DNA yielded an increase in the current and a decrease in

the on-state resistance in both instances. It is important to note that the thermodynamically

stable GT mismatch produced an effect that is identical to that found with the

thermodynamically destabilizing CA mismatch. As we have found in solution experiments,

the attenuation in DNA CT seen with mismatches does not correlate with thermodynamic

stability of the duplex

. Ultrafast spectroscopic experiments indicate that DNA CT depends

upon the sequence-dependent dynamics of DNA (ref.

). Certainly, the changes observed here

in the electrical characteristics of the device with mismatches cannot be due to poorer stability

of the DNA.

Although the mismatch experiments provide strong evidence that the observed signals do not

result from ionic conduction from the DNA molecules, as an additional control, newly cut

devices were subjected to the same reconnection conditions but with the DNA excluded. After

removal from the solution and rinsing, all of the devices treated in this manner remained at

open circuit with no measurable current.

Devices were also reconnected with single-stranded DNA featuring amines at both the 5

′

and

′

ends but without its complement. Although carbon nanotube gaps could be bridged with the

single-stranded DNA, the resulting devices were found to be highly unstable (see

Supplementary Information, Fig. S5). After three voltage cycles, the current passing through

single-stranded DNA degraded to open-circuit levels. Such instability may result from voltage-

induced oxidation of the exposed nucleobases and was not observed with duplex DNA.

Additional control experiments were performed to determine if non-specific absorption of

DNA could be responsible for the conduction changes during dehybridization/rehybridization.

Devices were partially cut with a shorter oxygen plasma treatment before being taken through

the sequential steps of reconnection and exchanges from matched to mismatched sequences.

In essence, the SWNT is only nicked, not cut completely through, so the electrical connection

is maintained. The devices treated in this way displayed little change in either the resistance

or threshold voltage (see Supplementary Information, Fig. S6).

As a final test that the duplex DNA within the gap adopts a native conformation under the

conditions of the experiment, we used

Alu

I, a blunt end restriction enzyme, to cut the DNA

(Fig. 4).

Alu

I only cuts DNA that is in its native conformation. Devices were reconnected with

duplex DNA containing the restriction sequence 5

′

-AGCT-3

′

. The device was subsequently

incubated with

Alu

I, resulting in a concomitant decrease in the current to the noise limits of

the measurement. As another control, a device reconnected with a nearly identical sequence

that featured the sequence 5

′

-AGTC-3

′

in place of the restriction site was incubated with

Alu

I. In this instance, no significant change was observed in the electrical characteristics of the

device (see Supplementary Information, Fig. S7). These data support the observation of a

sequence-specific restriction event. The enzyme is able to cleave its target sequence, yielding

no detectable current in the device. Under the experimental conditions presented, then, the

DNA duplex is intact, and the results suggest that it adopts a native conformation.

We can now place the values found here in the proper context to establish a range for the

conductivity of a single, intact DNA duplex. Our measurements place the resistance of well-

matched DNA duplexes with ~6 nm length in the range of 0.1–5 M

(Table 1). For comparison,

based on the bulk c-axis resistance, highly oriented pyrolytic graphite (HOPG) with similar

dimensions should also have a resistance of ~1 M

(ref.

). We estimate this value by

substituting a HOPG stack of equivalent diameter for the double-stranded DNA. Thus it appears

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that DNA, in its well-matched and well-stacked duplex form, behaves electrically much like

an array of stacked aromatic graphite planes. Importantly, just as we have seen in solution, the

presence of intervening mismatches attenuates DNA-mediated CT. This attenuation leads to a

~300-fold increase in resistance. Such an increase in the resistance of mismatched DNA is

consistent with previously reported STM measurements

–

. Also, it should be noted that

even within our own measurements, the covalent

-bonded linkages at the termini of the DNA

duplex must also decrease the conductivity observed versus the conductivity expected with

coupling directly into the base-pair stack. Therefore, the values obtained represent the upper

limits of the resistance of the DNA

-stack.

In conclusion, we have outlined a method to integrate a single DNA duplex within an electrical

device. The DNA molecules are covalently wired into electrical circuits through robust amide

linkages that are stable over a wide range of chemistries and conditions. The experiments

presented here illustrate the ability of DNA to mediate CT over significant distances and allow

for the direct measurement of the resistance of a single well-matched DNA molecule. It is

perhaps not surprising that DNA, if in its native conformation, and containing a stack of

aromatic heterocycles in its core, resembles the aromatic stacked planes of graphite with respect

to electrical characteristics. However, significantly, CT through a DNA assembly is sensitive

to perturbations that arise in the base-pair stack. As illustrated directly here, as well as in

solution experiments, single base mismatches attenuate CT through DNA. A molecular

stacked array in solution is necessarily less robust than a solid-state conductive material. Thus,

although the sensitivity of DNA CT to perturbations in stacking suggests that DNA may not

be appropriate to serve as a robust wire for nanoelectronic circuits, DNA molecules bridging

nanodevices can surely serve as uniquely powerful reporters to transduce biochemical events

into electrical signals at the single-molecule level.

Supplementary Material

Refer to Web version on PubMed Central for supplementary material.

Acknowledgments

We acknowledge primary financial support from the Nanoscale Science and Engineering Initiative of the National

Science Foundation (NSF) under NSF award number (CHE-0117752 and CHE-0641523) and by the New York State

Office of Science, Technology, and Academic Research (NYSTAR) and the NSF NIRT Award (ECCS-0707748).

C.N. acknowledges a NSF CAREER award (no. DMR-02-37860). J.K.B. thanks the National Institutes of Health

(NIH) (JKB-GM61077) for their financial support of this work.

References

1. Eley DD, Spivey DI. Semiconductivity of organic substances: Nucleic acid in dry state. Trans Faraday

Soc 1962;58:411–415.

2. O’Neill, MA.; Barton, JK. Sequence-Dependent DNA Dynamics: The Regulator of DNA-Mediated

Charge Transport in Charge Transfer in DNA. Wagenknecht, HA., editor. Springer-Verlag; Weinheim,

Germany: 2005. p. 27-75.

3. Schuster GB. Long-range charge transfer in DNA I & II. Top Curr Chem 2004;236

4. Giese B. Long-distance electron transfer through DNA. Ann Rev Biochem 2002;71:51–70. [PubMed:

12045090]

5. Delaney S, Barton JK. Long-range DNA charge transport. J Org Chem 2003;68:6475–6483. [PubMed:

12919006]

6. Murphy CJ, et al. Long-range photoinduced electron transfer through a DNA helix. Science

1993;262:1025–1029. [PubMed: 7802858]

7. Lewis FD, et al. Distance-dependent electron transfer in DNA hairpins. Science 1997;277:673–676.

[PubMed: 9235887]

GUO et al.

Page 5

Nat Nanotechnol

. Author manuscript; available in PMC 2009 September 21.

NIH-PA Author Manuscript

8. Hall DB, Holmlin RE, Barton JK. Oxidative DNA damage through long-range electron transfer. Nature

1996;382:731–735. [PubMed: 8751447]

9. Dandliker PJ, Holmlin RE, Barton JK. Oxidative thymine dimer repair in the DNA helix. Science

1997;275:1465–1468. [PubMed: 9045609]

10. Kelley SO, Barton JK. Electron transfer between bases in double helical DNA. Science 1999;283:375–

381. [PubMed: 9888851]

11. Gasper SM, Schuster GB. Photoinduced electron transfer to anthraquinones linked to duplex DNA:

the effect of gaps and traps on long-range radical cation migration. J Am Chem Soc 1997;119:12762–

12771.

12. Giese B, Amaudrut J, Kohler AK, Spormann M, Wesselly S. Direct observation of hole transfer

through DNA by hopping between adenine bases and by tunnelling. Nature 2001;412:318–320.

[PubMed: 11460159]

13. Wan C, Fiebig T, Kelley SO, Treadway CR, Barton JK. Femtosecond dynamics of DNA-mediated

electron transfer. Proc Natl Acad Sci USA 1999;96:6014–6019. [PubMed: 10339533]

14. Nunez ME, Hall DB, Barton JK. Long-range oxidative damage to DNA: Effects of distance and

sequence. Chem Biol 1999;6:85–97. [PubMed: 10021416]

15. Henderson PT, Jones D, Hampikian G, Kan Y, Schuster GB. Long-distance charge transport in duplex

DNA: The phonon-assisted polaron-like hopping mechanism. Proc Natl Acad Sci USA

1999;96:8353–8358. [PubMed: 10411879]

16. Kelley SO, Homlin RE, Stemp EDA, Barton JK. Photoinduced electron transfer in ethidium-modified

DNA duplexes: Dependence on distance and base stacking. J Am Chem Soc 1997;119:9861–9870.

17. Boal AK, et al. DNA-bound redox activity of DNA repair glycosylases containing [4Fe-4S] clusters.

Biochemistry 2005;44:8397–8407. [PubMed: 15938629]

18. Nunez ME, Holmquist GP, Barton JK. Evidence for DNA charge transport in the nucleus.

Biochemistry 2001;40:12465–12471. [PubMed: 11601969]

19. Boon EM, Salas JE, Barton JK. An electrical probe of DNA–protein interactions on DNA-modified

surfaces. Nature Biotechnol 2002;20:282–286. [PubMed: 11875430]

20. Boon EM, et al. Mutation detection by electrocatalysis at DNA-modified electrodes. Nature

Biotechnol 2000;18:1096–1100. [PubMed: 11017050]

21. Guo X, et al. Covalently bridging gaps in single-walled carbon nanotubes with conducting molecules.

Science 2006;311:356–359. [PubMed: 16424333]

22. Guo X, et al. Single-molecule devices as scaffolding for multicomponent nanostructure assembly.

Nano Lett 2007;7:1119–1122. [PubMed: 17394373]

23. Guo X, et al. Chemoresponsive monolayer transistors. Proc Natl Acad Sci USA 2006;103:11452–

11456. [PubMed: 16855049]

24. Whalley AC, Steigerwald ML, Guo X, Nuckolls C. Reversible switching in molecular electronic

devices. J Am Chem Soc 2007;129:12590–12591. [PubMed: 17902658]

25. Fink HW, Schonenberger C. Electrical conduction through DNA molecules. Nature 1999;398:407–

410. [PubMed: 10201370]

26. Porath D, Bezryadin A, de Vries S, Dekker C. Direct measurement of electrical transport through

DNA molecules. Nature 2000;403:635–638. [PubMed: 10688194]

27. Storm AJ, van Noort J, de Vries S, Dekker CS. Insulating behavior for DNA molecules between

nanoelectrodes at the 100 nm length scale. Appl Phys Lett 2001;79:3881–3883.

28. Kasumov AY, et al. Proximity-induced superconductivity in DNA. Science 2000;291:280–282.

[PubMed: 11209072]

29. Gohen H, Nogues C, Naaman R, Porath D. Direct measurement of electrical transport through single

DNA molecules of complex sequence. Proc Natl Acad Sci USA 2005;102:11589–11593. [PubMed:

16087871]

30. van Zalinge H, et al. Variable-temperature measurements of the single-molecule conductance of

double-stranded DNA. Angew Chem Int Edn 2006;45:5499–5502.

31. Hihath J, Xu B, Zhang P, Tao N. Study of single-nucleotide polymorphisms by means of electrical

conductance measurements. Proc Natl Acad Sci USA 2005;102:16979–16983. [PubMed: 16284253]

GUO et al.

Page 6

Nat Nanotechnol

. Author manuscript; available in PMC 2009 September 21.

NIH-PA Author Manuscript

32. Wierzbinski E, Arndt J, Hammond W, Slowinski K.

In situ

electrochemical distance tunneling

spectroscopy of ds-DNA molecules. Langmuir 2006;22:2426–2429. [PubMed: 16519433]

33. Ceres DM, Barton JK.

In situ

scanning tunneling microscopy of DNA-modified gold surfaces: bias

and mismatch dependence. J Am Chem Soc 2003;125:14964–14965. [PubMed: 14653712]

34. Bhattacharya PK, Barton JK. The influence of intervening mismatches on long-range guanine

oxidation in DNA duplexes. J Am Chem Soc 2001;123:8649–8656. [PubMed: 11535068]

35. O’Neill MA, Becker HC, Wan C, Barton JK, Zewail AH. Ultrafast dynamics in DNA-mediated

electron transfer: base gating and the role of temperature. Angew Chem Int Edn 2003;42:5896–5900.

36. Tsang DZ, Dresselhaus MS. The c-axis electrical conductivity of kish graphite. Carbon 1976;14:43–

46.

GUO et al.

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. Author manuscript; available in PMC 2009 September 21.

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Figure 1. A method to cut and functionalize individual SWNTs with DNA strands

, Functionalized point contacts made through the oxidative cutting of a SWNT wired into a

device.

, Bridging by functionalization of both strands with amine functionality.

, Bridging

by functionalization of one strand with amines on either end.

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Figure 2. Device characteristics for individual SWNTs connected with DNA

a,b

, Source–drain current versus

at a constant source –drain voltage (50 mV) before cutting

(black curve: 1), after cutting (red curve: 2) and after connection with the DNA sequence shown

(green curve: 3), for a semiconducting SWNT device (

) and a metallic SWNT device (

Guanine, G; cytosine, C; adenine, A; thymine, T.

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Figure 3. Mismatches have a large effect on DNA conductance

, Replacing well-matched (WM) duplexes with CA and GT mismatches.

, Source–drain

current versus

at a constant source–drain voltage (50 mV) for a SWNT device taken through

the sequence 1 through 6. The current levels for points 2, 3, 5 and 6 are ~300 times lower.

Source–drain current at

−

3 V at a constant source–drain voltage (50 mV) for the sequence

1 through 6.

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Figure 4. Enzymes can be used to cleave the DNA between the ends of the SWNTs

Source–drain current versus

at a constant source–drain voltage (50 mV) for a metallic

SWNT device after cutting and reconnection with the shown DNA sequence before (green

curve: 1) and after reaction with

Alu

1 (red curve: 2).

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Table 1

Summary of the resistance values obtained before cutting and after reconnection.

Before cutting

After reconnection

Carbon nanotube type

Effective resistance

)

DNA sequence

Type of linkage

†

Effective resistance

)

DNA conductance (

)

Semiconducting

0.65

Well matched

′

amine

2.5

1.4 × 10

−

Semiconducting

1.3

Well matched

′

amine

2.8

1.7 × 10

−

Semiconducting

0.90

CA mismatch

′

amine

1.5 × 10

−

Semiconducting

0.48

Well matched

′

& 3

′

amine

3.3

9.2 × 10

−

Metallic

0.23

Well matched

′

& 3

′

amine

0.5

8.6 × 10

−

Metallic

0.23

CA mismatch

′

& 3

′

amine

155.0

1.7 × 10

−

Metallic

0.23

GT mismatch

′

& 3

′

amine

111.0

2.3 × 10

−

Metallic

0.20

CA mismatch

′

& 3

′

amine

3.9 × 10

−

Semiconducting

0.24

GT mismatch

′

& 3

′

amine

8.5 × 10

−

Metallic

0.52

Well matched

(

Alu

′

& 3

′

amine

7.3 × 10

−

Semiconducting

1.5

Single-stranded

′

& 3

′

amine

3.0

1.7 × 10

−

Resistance values were calculated using a gate bias of

−

4 V and a source–drain bias of

−

50 mV.

†

The 5

′

amine linkage corresponds to a –OCONH–(CH)

–NH

linker on the 5

′

ends of both strands. The 3

′

and 5

′

amine linkage corresponds to a –P

–(CH

)

–NH

linker on both the 3

′

and 5

′

ends

of one strand. See Supplementary Information, Figs S2 and S3 for the sequences used.

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