AFM Measurements of DNA Molecule Electron Transport Properties
Jim McMahon Nanotechnology writer for Zebra Communications, P.O. Box 940968, Simi Valley, CA 93094-0968
jim.mcmahon@
zebracom.net
Introduction Deoxyribonucleic acid (DNA) has been considered as a
possibility for molecular electronics. Because DNA is able to recognize other molecules—other strands of DNA—and because it binds together with similar DNA strands in a very unique way, scientists have suggested the possibility of using DNA as an electronic circuit without having to build in any other circuitry. Te DNA would bind with other similar DNA strands that it recognizes and then use the connecting properties of the DNA to create a self-assembled biological wire for electrical conduction. Until recently, uncertainty existed about whether DNA could conduct at all, and if it could, how well it could conduct. Scientific speculations ranged from DNA being a superconductor to a complete insulator. Recent research, however, by Dr. Sidney R. Cohen in collabo- ration with Dr. Ron Naaman and Dr. Claude Nogues of the Weizmann Institute of Science, Scanned Probe Microscopy Unit, in Rehovot, Israel, aided by the enabling technologies of ultra-high-resolution microscopy and negative-stiffness vibration isolation, has shed new light on the electrical transport properties of DNA, focusing on the capacity of single molecules of DNA to transport current along individual strands.
DNA Measurement Challenges DNA is a nucleic acid that contains the genetic instructions
used in the development and functioning of all known living organisms and some viruses. Te 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. To measure the electronic properties of DNA, Dr. Cohen
and his staff needed to connect an electrode to the end of a DNA molecule, which is only a few nanometers in length, using an AFM (atomic force microscope) [1, 2]. One difficulty in measuring something this small is ensuring that a good electrical contact is made to the molecule—the researcher wants to measure the electrical properties of the molecule, not the quality of the connection. To facilitate this bio-molecular connection, the lab attached a bio-link, a gold electrode, to a single strand of DNA, and then attached a very small gold ball (10 to 20 nanometers in size) to a complementary DNA strand, aſter which these two strands were hybridized (linking of the two single strands, aided by genetic similarity between corresponding DNA sequences) as shown in Figure 1. If the strands are complementary, their matching cousin on the other strand will form a double-strand. Single strands of DNA do not conduct electricity. Te double-strand does conduct for certain configurations.
20 DNA molecules are very easily destroyed. Hooking up
these gold connectors and balls at the nano level without tearing them off or burning them out is quite challenging (Figure 2). Tis preparation method, developed by Dr. Nogues, is critical and somewhat time-consuming but is a fundamental aspect of this research model. Using an AFM, with the DNA double-strand displayed on a flat surface, the researchers could then locate the gold ball, put the AFM tip on top of the ball, flow a current through the double-strand, and view the current voltage characteristics (Figure 3).
Electron Transport Properties of DNA Dr. Cohen explains, “Tere are two possibilities when we
talk about electrons flowing through a DNA molecule. We can break it down into two different kinds of electron transport. One is called a ‘tunneling process,’ where the electron effectively shoots through the molecule without caring too much about the internal structure of the molecule. Te other is called a ‘hopping process,’ where the electron actually resides for small periods of time in certain positions along the molecule. In this case the electron will be affected by temper- ature. DNA consists of a sequence of base pairs. We found that variations in both the sequence and the composition of a strand’s base pairs can also affect the progress of electron
Figure 1: Schematic of the measuring system. DNA oligomer is attached to a gold electrode below and hybridizes with a DNA attached to a gold nanopar- ticle, which then forms the upper electrode for the double-stranded DNA. Current is measured by applying a bias between upper and lower electrodes with placement controlled by the AFM tip. Modified from a figure in [1].
doi:10.1017/S1551929510000830
www.microscopy-today.com • 2010 September
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