DNA induced to superconductivity

By Kimberly Patch, Technology Research News

DNA has already proved itself most useful as the basis of life on earth and is showing promise for massively parallel computing in a test tube. More controversial is its potential role as a material that conducts electricity.

There have been several attempts to test the electrical conductivity of DNA molecules, and the results are mixed. In one of the latest efforts, a research group from France and Russia has shown that DNA can conduct electricity and even becomes a proximity-induced superconductor when its metal contacts become superconducting at very low temperatures.

The measurements, though preliminary, show promise for using DNA in sensing applications and eventually in building nanoscale electrical circuits.

The researchers used double-stranded, six-micron-long DNA molecules as connectors between rhenium and carbon electrodes.

The tricky part in getting the DNA to conduct was finding contacts that effectively funnel the electricity through the DNA, said Helene Bouchiat, director of research at the in the French National Center for Scientific Research. "We had made some tries with pure gold contacts with no success. We used carbon as a top layer hoping that it [would] promote chemical bonding between the molecule in the contact," she said.

If the researchers results are correct, DNA could easily be used to conduct electricity, said Danny Porath, a physicist at the Center for Nanoscience and Nanotechnology at Tel Aviv University. "The conduction properties described here are by far better than those found in previous experiments and beyond expectations of many people in the field. If this is correct that means that DNA is indeed an incredible candidate for molecular electronics," he said.

The researchers built the structures on stable, freshly cleaved mica substrates. The first layer was a two-nanometer-thick layer of rhenium. Then came a two-nanometer-thick layer of DNA molecules, which was combed into one direction using the flow of the solution. The top layer was a forest of individual carbon fibers up to 40 nanometers tall, according to Bouchiat.

The thickness of the rhenium layer was carefully controlled in order to minimize kinks in the DNA molecules at the edges of the metallic pads. Keeping kinks out of the DNA is a key to providing good conduction from the contacts through the DNA molecules, according to Bouchait.

The researchers calculations showed that 100 to 200 DNA molecules bridged the two electrodes in their samples. In several samples they destroyed some of the DNA in order to get structures that contained from 3 to 40 combed DNA molecules.

The researchers flowed electricity between the electrodes through the DNA in order to measure the resistance of the DNA.

The DNA provided an average resistance of about 300 kilohm per DNA molecule, although the actual number is likely lower because all the combed molecules were not necessarily in contact with the electrodes, according to Bouchait. For comparison, the resistance of metallic, single-walled nanotubes is typically 100 kilohm and the resistance of semiconducting nanotubes is one megaohm or higher.

The experiments showed that the molecules can conduct electricity over distances of a few hundred nanometers even at very low temperatures. The researchers also found that the resistance of DNA dropped considerably when the electrodes became superconducting at one degree Kelvin. Zero Kelvin is absolute zero, or -273 degrees Celsius.

Superconductivity occurs when electrons moving through a material face no resistance. The electrons become coherent in the quantum mechanical sense, meaning they behave as though they are a single wave.

The resistance of the DNA samples increased steadily as the temperature decreased until the temperature fell below the superconducting temperature of the contacts. At this point the resistance of the samples that had 30 and 40 combed DNA molecules decreased substantially. These transition changes showed that there was proximity-induced superconductivity in the DNA molecules themselves, according to Bouchait.

It has historically proven difficult to make DNA conduct electricity, which makes many researchers cautious about these results. The results are "very surprising, but very important if correct," said Porath.

According to researchers who have found conductivity in DNA, the important parameters are the contacts and the structure of the DNA. "There's no question that the connections are critical. I really think much of the variability of results [in] looking at DNA conductivity depends upon the variability in making connections," said Jacqueline Barton, a chemistry professor at the California Institute of Technology.

The exact order of the four types of base pairs that make up the DNA molecule also factor into the way DNA conducts electricity, said Barton. "We found from our solution studies the charge transport through DNA is very sensitive to base pair stacking and structure. It depends upon the overlap of the DNA base pairs."

If DNA were successfully harnessed as a conductor, self assembling networks of DNA could potentially be used eventually to build nanoscale electronic circuits. "This is one of the solutions for the prediction of Moore's Law that claims that we're heading towards the end of the conventional microelectronics," said Porath. "Possible replacements are systems that are made of building blocks and use self-assembly. The DNA, if conducting, would be a very good candidate for this purpose due to its... self-assembly properties... and large toolbox provided by enzymes," he said.

Using DNA for electronic circuits is a far-off goal, however. It is too early in the research to say whether this would even be possible, said Bouchiat.

Research applications like using conductivity to sense different types of DNA, however, could become practical within a few years, according to Bouchait.

Because the conduction in DNA is so sensitive to the order of its bases, it could be used as a way to sense various sequences, said Barton. "It provides a fundamentally new way to achieve sensitive... mutation analysis," said Barton.

Bouchiat's research colleagues were A. Yu Kasumov, M. Kociak, S. Gueron and B. Reulet from the CNRS, and V. T. Volkov and D.V. Klinov of the Russian Academy of Sciences. They published the research in the January 12, 2001 issue of Science. The research was funded by the CNRS and The Russian Academy of Sciences.

Timeline:   < 3 years; many years
Funding:   University
TRN Categories:  MicroElectroMechanical Systems (MEMS)
Story Type:   News
Related Elements:  Technical paper, "Proximity Induced Superconductivity in DNA," Science, January 12, 2001


February 7, 2001

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