Using conductive-probe AFM to measure current through single
molecules, with applications to biosensors
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Shun Lu , Marcus Kuikka , Navanita Sarma , Peter J. Williams , Connie B. Roth , Yang Li , Hogan Yu ,
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Dipankar Sen , and John Bechhoefer
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Depts. of Physics , Chemistry , Molecular Biology and Biochemistry , Simon Fraser University, Burnaby,
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British Columbia. Dept. of Physics , Acadia University, Wolfville, Nova Scotia
The idea of using DNA (deoxyribonucleic acid) as a molecular wire in the electrical
circuit is quite attractive, as it represents a “biological solution” to the problem of making
nanoscale electrical connections. As a result, the conductivity properties of DNA are
very important and have been the focus of much investigation [1]. Using a method
proposed by X.D.Cui et al [2], we have studied the conductivity of organic molecules by
first preparing an insulating, self-assembled monolayer on a conducting surface. Then,
one end of the molecule of interest is bound to the substrate, through the insulating
layer, and the other end is bound to a conducting nanoparticle. We then make physical
contact to the nanoparticle via a conducting-probe atomic force microscope (cp-AFM).
Our first investigations, following Cui et al., were of a monolayer of octanethiol, attached
to the (111) surface of gold. We then investigated the electrical properties of
octanedithiol, attached to the gold substrate and then to a gold nanoparticle via the AuS bond. We then made reproducible measurements of conductivity through these
molecular circuits. Quantized current-voltage (I-V) curves were observed as integer
multiples of the fundamental curves.
Currently, we are applying the same idea to investigate the conductance of DNA
molecules. Single-stranded DNA (ssDNA) is first bound to the Au (111) surface via a
Au-S bond. Then its complementary strand, bound to a gold nanoparticle, is hybridized
to form a molecule of double-stranded DNA (dsDNA) [3], which electrically connects the
nanoparticle to the substrate. We have measured preliminary I-V curves of this dsDNA
construct. Our ultimate goal is to apply this technique to three-armed DNA “aptamers”
developed in the laboratory of D. Sen. This engineered form of DNA can change its
conformation and hence its electrical conductivity upon binding a specific target
molecule. This leads to the possibility of creating biosensors that target a desired
molecule and give an electrical readout of their presence.
[1] R.G. Endres, D.L. Cox and R.R.P. Singh, Rev. Mod. Phys, 76, No.1 2004
[2]
X.D.
Cui,
et
al.,
Science
294,
571-574
(2001).
[3] C. Nogues, S.R. Cohen, S.S. Daube and R. Naaman, Phys. Chem. Chem. Phys.,
2004, 6, 4459-4466