1 Wat 'oudjes':
DNA is a semiconductor
Apr 1, 1999
physicsworld.com/cws/article/news/1999/apr/01/dna-is-a-semiconductor
DNA molecules - the building blocks of life - can conduct electricity as efficiently as a good semiconductor according to two Swiss physicists. Hans-Werner Fink and Christian Schönenberger from the University of Basel say that DNA's electrical properties make it "ideally suited for the construction of mesoscopic electronic devices" (Nature 398 407).
Fink and Schönenberger made their measurements by placing discrete amounts of DNA molecules in a water-based solution. A tiny droplet of the solution was then placed onto a gold-coated carbon foil. Blotting paper was then used to remove most of the water on the device, causing some of the individual DNA molecules to span the holes in the foil. According to their calculations, these strands coalesce into a DNA 'rope' 2 microns in diameter.
By using a low-energy coherent electron beam from a LEEP microscope, Fink and Schönenberger were able to visualise the DNA strands without damaging the molecules. Next they used a mechanical tip to break one end of the DNA rope away from the foil. The tip was then used to create a small measurable voltage between this end of the rope and the foil.
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Superconductivity: it's in the genes
Jan 12, 2001
physicsworld.com/cws/article/news/2001/jan/12/superconductivity-its-in-the-genes
In the quest for ever-smaller electronic devices, scientists have long dreamt of building circuits up atom by atom. But finding molecules capable of conducting electric current has not been easy. Now, Alik Kasumov of the Laboratoire de Physique des Solides in France and co-workers have shown that DNA molecules act as ohmic conductors above 1 K and that below this temperature they can superconduct (A Y Kasumov et al 2001 Science 291 280).
Following on from the discovery that carbon nanotubes can act as electrical wires, Kasumov showed two years ago that these rolled up sheets of graphite atoms lose their resistance when connected to superconductors. Now Kasumov has shown that this is also true for DNA by connecting double-stranded DNA molecules to rhenium and carbon superconducting electrodes 0.5 µm apart. By cooling the electrodes to below their superconducting transition temperatures, the researchers observed so-called 'proximity induced' superconductivity in the DNA.
Evidence for electrical conductivity in DNA molecules has been inconclusive until now. Optical experiments have shown that a transfer of charge may be possible in such molecules. But the message from transport measurements has been mixed: some have indicated that DNA could be a conductor while others suggested that DNA is an insulator. Kasumov and colleagues have found that above 1 K, the resistance per molecule is less than 100 kilo-ohm, a figure that varies weakly with temperature and is an order of magnitude lower than previous measurements. Even at very low temperatures, the researchers found that DNA molecules can conduct ohmically over distances of a few hundred nanometres.
However, the physical mechanism responsible for conduction in DNA remains unclear and it is possible that the contacts act as strong dopants of electrons or holes. The researchers add that conductivity measurements could in turn help biologists to look for particular sequences of base pairs within DNA molecules.
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DNA induced to superconductivity
By Kimberly Patch
www.trnmag.com/Stories/020701/DNA_induced_to_superconductivity_020701.html
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
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Conduction seen in DNA backbone
Dec 17, 2007
physicsworld.com/cws/article/news/2007/dec/17/conduction-seen-in-dna-backbone
Physicists in Japan have gained important new insights into how DNA might behave as an electrical conductor. Their discovery could help provide a better understanding of the role that conduction plays in how living cells detect and repair damaged DNA and could ultimately lead to strands of DNA being used in “molecular electronics” technologies of the future.
Biophysicists are keen to understand how electrons are conducted in DNA because conduction is thought to be an important mechanism by which enzymes recognize damaged DNA that, if not repaired, could lead to cancer. Some scientists also believe that conduction through DNA could protect the genomes of some organisms by transmitting the damage caused by oxidizing chemicals to certain locations on chromosomes where the damage causes the least harm.
Tiny electronic circuits
A better understanding of conduction could also lead to the engineering of new forms of DNA with properties more suited to electronic applications. DNA is an attractive building block for tiny electronic circuits because of its ability to assemble into complex interconnected patterns that would be required for assembling circuit components.
Not long after the double-stranded structure of DNA was revealed by Watson and Crick in 1953, scientists suspected that the molecule it might support electrical conduction. This is because the bases in the middle of the double helix stack in a way reminiscent of graphite – which is an excellent conductor. At about the same time, the physicist Leon Brillouin suggested that the DNA backbone – the long strands that support the bases and give DNA its structure – rather than the bases, might support conduction because of its periodic structure.
While the conductive properties of DNA have been studied using a wide range of techniques, most experiments have focused on understanding conduction in terms base stacking and have yielded conflicting results. Alternative or complementary conduction mechanisms – such as Brillouin’s backbone conduction – have been largely ignored.
Now, Tetsuhiro Sekiguchi of the Japan Atomic Energy Agency and Hiromi Ikeura-Sekiguchi at the AIST research centre are the first to measure how electrons move through the DNA backbone using a technique called resonant Auger spectroscopy ( (Phys. Rev Lett. 99 228102 ).
Spectator Auger decay
The team directed a beam of X-rays onto DNA to excite electrons from phosphorus atoms in the backbone of the molecule. If these electrons remain near to the site of their excitation, other electrons with a specific energy distribution indicative of “spectator Auger decay” will be emitted from the sample. However, if excited electrons are able to conduct along the backbone, the emitted electrons will have an energy distribution associated with "normal Auger decay."
By comparing the relative intensities of Auger electrons with spectator and normal decays, the team could determine the time that it takes for an electron to move away from a phosphorous atom and take part in conduction – called the delocalization time.
What they found is that electrons in the backbone delocalize in less than one femtosecond (10-15) in wet DNA. These results imply that electron movement occurs a thousand times faster in the DNA backbone than in the bases stacked in the core.
This first observation of conduction along the backbone could help reconcile the seemingly contradictory results of the many base-stacking studies of conduction. Indeed, these latest results suggest that focusing on the interplay between electron transport through the backbone and the stacked bases could be crucial to understanding DNA conduction.
About the author
André Brown is doing a PhD in biophysics at the University of Pennsylvania