Microscopy
and Analysis - September 2009
Measuring the Electron Transport Properties
of DNA Molecules
Sidney
R. Cohen, Claude Nogues and Ron Naaman, Weizmann Institute
of Science, Rehovot, Israel
Introduction
Deoxyribonucleic acid (DNA) is a nucleic acid that contains
the genetic instructions used in the development and functioning
of all known living organisms and some viruses. The main role
of DNA molecules is the long-term storage of information. DNA
nanotechnology uses the unique molecular-recognition properties
of DNA to create self-assembling branched DNA complexes with
useful properties. Precisely this self-recognition property
has been proposed as a possible approach to molecular electronics:
proposals for using DNA to create a self-assembled electronic
circuit using this unique self-recognition property, whereby
DNA spontaneously organizes into a pre-designed circuit pattern
have been forwarded [1]. Such circuitry would require that DNA
possesses sufficient electronic conductivity. Furthermore, in
natural functioning of DNA, response to and repair of radiation
damage likely involves some form of charge transport across
significant lengths of the DNA molecules.
Until recently, marked controversy existed regarding the
conductive properties of DNA, with experimental results spanning
the full range from DNA being a superconductor to a complete
insulator. Recent research at the Weizmann Institute of Science
has shed new light on the electrical transport properties
of DNA, focusing on the mechanism by which short oligomers
of DNA mediate current flow along individual strands [2,3,4].
DNA Measurement Challenges
Much of the discrepancy between different laboratories arose
from the unique structure of DNA. Even relatively short DNA
oligomers, such as those used in this work (26 base pairs)
can relax into varied uncontrolled conformations when attached
to a solid surface. Another critical issue is formation of
a well-characterized electrical contact to the termini of
the molecule. The force applied between AFM tip and DNA must
be limited so that the delicate molecular structure is not
destroyed.
The method developed in our lab addressed these considerations.
In order to control the DNA conformation, and prevent unpredictable
interactions with the surface, single strands of DNA were
chemically bound to a flat gold electrode surface through
a gold-thiol covalent bond. This formed the bottom electrical
contact. These bound strands were interspersed in a surrounding
matrix of a shorter alkanethiol, which prevented them from
lying prone on the surface. Separately, a gold nanosphere
(GNP) 10 nm in diameter was attached to the complementary
strand of this chain. A solution containing this modified
strand was then incubated with the modified electrode, so
that due to the complementarity, the two strands "hybridized",
resulting in a double-stranded (ds) DNA bound to a bottom
gold electrode with the GNP forming the upper electric contact
(linking of the two single strands, aided by genetic similarity
between corresponding DNA sequences).
This preparation method is a fundamental aspect of this research
model, and solves the issues of indeterminate configuration
of DNA and poorly-defined electrical contact. Furthermore,
using this protocol, several controls could be made. Electrical
response of the tip could be checked "in-line" with
the measurements on the DNA molecules by observing tunneling
current through the short thiol molecules.
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Figure 1: Schematic of the measuring scheme.
Single strand DNA was absorbed onto an underlying gold
electrode through a thiol linkage. The complementary
strand of the DNA was modified by a terminal GNP, and
this complex was allowed to hybridize with the bound
complementary strand. These complexes were clearly visible
in the AFM micrographs due to the protrusions of the
GNPs. AFM current measurements were made by placing
the AFM tip into contact with the top of the GNP and
sweeping the bias between tip and substrate while monitoring
current. Reprinted from [2].
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The surfaces were imaged and electrically probed using conductive
probe atomic force microscopy using an NT-MDT P47 AFM [5]
with a MikroMasch NSC36/Ti-Pt tip [7]. A schematic of this
technique is shown in Figure 1. In order to avoid damage
to the delicate monolayers, imaging was performed in the dynamic
(semi-contact) mode. ds-DNA was clearly recognized by the
round features representing the gold nanoparticles. Through
a custom-designed protocol controlled by the macro-script
language of the AFM [6], after locating such a construct in
dynamic (also known as semi-contact) mode, the tip was placed
directly above it. Then, the conductive AFM tip was lowered
vertically into contact with the complex, at a prescribed
minimal force (see Figure 2) and current-voltage characteristics
were measured. This experimental protocol allowed several
controls to be performed in a single scan, comparing measurements
on the GNP-DNA complex, the ss-DNA monolayer and on the short
alkanethiol matrix. Thus, it was verified that the single
strand does not conduct whereas the double strand does. The
success of this procedure required stringent environmental
conditions: no air currents; good vibration isolation (achieved
by using Minus K negative stiffness vibration isolation [8]);
and drift compensation. The drift compensation was also controlled
through a custom script, using a peak-finding algorithm
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Figure 2: Typical AFM force curve. The
gray bar shows the range of forces chosen for contact
force during I/V measurements, placed in the attractive
force regime so that no compressive force was applied
on the DNA strands. Reprinted from [4]
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Electron Transport Properties of
DNA
DNA is comprised of a sequence of bases. Hybridization of
the strands demands that this sequence correspond to a combination
of specifically paired "base-pairs". The type and
sequence of these base pairs determines the electrical properties:
the guanine-cytosine (GC) pair has a lower ionization potential
than the thymine-adenine (TA) pair. Thus, the electrical properties
may be expected to depend on the relative compositions of
these different base pairs. Indeed, the I-V measurements showed
just such a difference, as displayed in Figure 3.
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The Figure 3: Typical I/V
curves for the 3 different GC-content double strands.
Insert: calculated dI/dV curve indicating the calculation
of the apparent bandgap. Reprinted from [4].
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Four quantities were derived from such curves: (1) The resistance
at low voltage bias; (2) The voltage threshold above which
current rapidly increases; (3) The effective resistance at
high bias; and (4) The maximum current density at 1.5 V bias
(presuming all current travels through one ds-DNA chain. In
fact, geometrical considerations show that up to three DNA
double strands could fit below each GNP so this figure is
a maximum, and could be 3 times smaller). The results are
displayed in Table I. As is apparent, for increasing
GC content, the resistance decreases at both high and low
bias, voltage threshold drops, and current density rises.
At low bias, the current density increases by 1 order of magnitude
for each increment in GC content.
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GC content
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Voltage
threshold(a)
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Resistivity(b)(Ohms)
| V | < 1 V
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Current density at
1.5V c nA/nm2
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Resistivity (Ohms)
| V | > 1 V
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0
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2.6 +/- 0.09 V
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1414±192
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0.02 +/- 0.01
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539±149
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8
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1.9 +/- 0.10 V
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605±166
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0.18 +/- 0.05
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54±37
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14
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0.9 +/- 0.12 V
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29±12
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1.72 +/- 0.29
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0.4±0.16
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Table 1: Summary
of current-voltage characteristics. All values reported as mean
+/- standard error. (a) Determined from dI/dV curves as indicated
in Figure 3. (b) Determined from best fit slope of Current vs.
Voltage over indicated range. (c) Calculated presuming all current
over a given nanoparticle passes through one ds-DNA.
Conclusions
Two possibilities have been considered for electron flow through
a DNA molecule. One is a tunneling process, where the electron
effectively shoots through the molecule without localized interactions
over the internal structure of the molecule. The other is a
hopping process, where the electron actually resides for small
periods of time in certain positions along the molecule. The
relatively large current densities observed here are not consistent
with hopping, and indicate that the electrons interact with
a delocalized electronic structure extending over the (backbone)
of the complex while passing through the DNA. Therefore, the
observed "bandgap" does not represent the total density
of states, but rather the density of these delocalized states.
References
1. Braun, E., Eichen, Y., Sivan, U. et al. Nature
1998, 391, 775.
2. Nogues, C., Cohen, S.R., Daube, S.S., Naaman, R. Phys. Chem.
Chem. Phys. 2004, 6, 4459.
3. Cohen, H., Nogues, C., Naaman, R., Porath, D. Proc. Natl.
Acal. Sci. U.S.A. 2005, 102, 11589.
4. Nogues, C., Cohen S.R., Daube, S., Apter, N., Naaman, R.
J. Phys. Chem. B 2006, 110, 8910.
5. NT-MDT, Zelenograd, Russia
6. Power Script, NT-MDT, Zelenograd, Russia.
7. MikroMasch, Tallinn, Estonia
8. Minus K Technology Inc., CA, USA.
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