Electronic DNA promises biomolecular semiconductor

Deoxyguanosine, one of DNA's four nucleosides, could soon form a cheap, efficient and easily made semiconductor in photodetectors, according to Italian researchers. They fabricated experimental photodetectors by placing a droplet of deoxyguanosine (DG) dissolved in chloroform at the juncture of two gold electrodes, just 120 nanometers apart. As the chloroform evaporated, the DG molecules assembled themselves into a semiconducting ribbon-like crystal.

"By using only a few milligrams of DG material, it is possible to construct more than 1000 devices," said Ross Rinaldi, team leader and professor of physics at the National Nanotechnology Laboratory of the National Institute for the Physics of Matter (INFM) in Lecce. The small amount required means materials cost at least two orders of magnitude less than those for inorganic semiconductors, grown epitaxially, which are currently used to build commercial photodetectors, he tells BioMedNet News.

Furthermore, the DG-based metal-semiconductor-metal (MSM) photodetectors are about twice as sensitive to light as commercially available detectors, writes Rinaldi in the current issue of Applied Physics Letters, published yesterday.

Scientists have toyed with creating molecular electronics since the 1970s; by the mid 1990s, they had started to incorporate biomolecules in semiconductors.

Building electronic devices on a molecular scale enables researchers to create extremely dense logic and memory circuits such as those used in molecular diodes, switches, photodetectors and rectifiers.

But manipulating DG molecules remains a challenge, concedes Rinaldi. He and his colleagues are trying to double the length of the DG portion of the photodetectors to a quarter of a micrometer. At this size, conventional semiconductor fabrication techniques could more easily duplicate the material and a modified inkjet printer nozzle could be used to deposit its chloroform solution between the two electrodes of a semiconductor.

Another of his concerns is the solution's concentration, which determines whether the DG assembles itself. Without the ribbon-like crystal formation, the molecules are non-conductive.

Semiconductors, used to produce electronic devices such as diodes, transistors and computer memory chips, allow a limited amount of electron movement, depending on the crystal structure of the material used for assembly. Impurities in elements used in semiconductors enhance the conduction of electricity by either adding free electrons or creating electron deficiencies - the process of adding these impurities is called doping.

Nevertheless, Rinaldi is confident that the outstanding semiconducting properties of biomolecular compounds such as DG could soon lead to the replacement of traditional semiconductors in some electronic devices at least.

"All the basic functions of digital electronics, like rectification, amplification and storage, could soon rely on a few molecules, or even on a single molecule," he concluded.

Such enthusiasm is shared by A.T. Charlie Johnson, associate professor of physics and astronomy at the University of Pennsylvania, but with qualifications. "There is a lot of controversy right now about how electricity is conducted in DNA and other biomolecules, and this work strongly supports the view that DG, DNA, and other relatives are semiconductors with a large energy gap," he told BioMedNet News this afternoon.

But as usual in applied science, says Johnson, the devil is in the details, and there are many details that need to be worked out before this is a viable technology.

"They claim the sensitivity [of the experimental photodetectors] is competitive with existing technologies, but there are many other important issues like speed, dark current, and noise level that need to be resolved," he said. "Finally, there are critical materials issues, such as the need for the DG to assemble in a very ordered manner, that might complicate device fabrication."

Johnson added: "But it will be an interesting research direction for some time."