| Scientists at the Technion-Israel Institute of Technology have harnessed the power of DNA to create a self-assembling nanoscale transistor. The research, published in the Nov. 21, 2003 issue of Science, is a crucial step in the development of nanoscale devices, and is viewed as a major step towards developing nanoscale electronics. The advance points to a possible future method of creating the smallest-ever molecule-sized circuits. Erez Braun, lead scientist on the project and associate professor in the Faculty Physics at the Technion, says science has been intrigued with the idea of using biology to build electronic transistors that assemble without human manipulation. However, until now, demonstrating it in the lab has remained elusive. "This paper shows you can start with DNA proteins and molecular biology and construct an electronic device," he said. It is significant that the transistor self-assembles instead of having to be painstakingly put together molecule by molecule, a slow and inefficient way of constructing these tiny devices. "Erez Braun and his colleague Uri Sivan are some of the few pioneers in this field," said Horst Stormer, professor in Columbia University's Departments of Physics and Applied Physics and scientific director of the Nano Science and Engineering Centers. "This is outstanding research in the area that matters most in nanotechnology: self-assembly." To get the transistors to self-assemble, the Technion research team attached a carbon nanotube - known for its extraordinary electronic properties - onto a specific site on a DNA strand, and then made metal nanowires out of DNA molecules at each end of the nanotube. The device is a transistor that can be switched on and off by applying voltage to it. The carbon nanotubes used in the experiment are only one nanometer, or a billionth of a meter, across. In computing technology, as scientists reach the limits of working with silicon, carbon nanotubes are widely recognized as the next step in squeezing an increasing number of transistors onto a chip, vastly increasing computer speed and memory. Braun emphasized that computers are only one application; these transistors may, for example, enable the creation of any number of devices in future applications, such as tiny sensors to perform diagnostic tests in healthcare. Though transistors made from carbon nanotubes have already been built, those required labor-intensive fabrication. The goal is to have these nanocircuits self-assemble, enabling large-scale manufacturing of nanoscale electronics. DNA, according to Braun, is a natural place to look for a tool to create these circuits. "But while DNA by itself is a very good self-assembling building block, it doesn't conduct electrical current," he noted. To overcome these challenges, the researchers manipulated strands of DNA to add bacteria protein to a segment of the DNA. They then added certain protein molecules to the test tube, along with protein-coated carbon nanotubes. These proteins naturally bond together, causing the carbon nanotube to bind to the DNA strand at the bacteria protein. The new technique takes advantage of a biological process known as recombination, where a segment of DNA is swapped out for an almost identical piece. The cell uses recombination to repair damaged DNA and to swap genes. By attaching a nanotube to the protein, the nanotube moves to an exact location along the DNA strand. "The DNA serves as a scaffold, a template that will determine where the carbon nanotubes will sit," Braun said. "That's the beauty of using biology." Finally, they created tiny metal nanowires by coating DNA molecules with gold. In this step, the bacteria protein served another purpose: it prevented the metal from coating the bacteria-coated DNA segment, creating extending gold nanowires only at the ends of the DNA strand. The goal, Braun explained, was to create a circuit. However, "at this point, the carbon nanotube is located on a segment of DNA, with metal nanowires at either end. Theoretically, one challenge here would be to encourage the nanotube to line up parallel to the DNA strand, meet the nanowires at either end, and thus make a circuit. "There are some points where nature smiles upon you, and this was one of those points," Braun continued. "Carbon nanotubes are naturally rigid structures, and the protein coating makes the DNA strand rigid as well. The two rigid rods will align parallel to each other, thus making an ideal DNA-nanotube construct." "In a nutshell, what this does is create a self-assembling carbon nanotube circuit," he concluded. Scientists controlled the creation of transistors by regulating voltage to the substrate. Out of 45 nanoscale devices created in three batches, almost a third emerged as self-assembled transistors. Braun added, however, that while this research demonstrates the feasibility of harnessing biology as a framework to construct electronics, creating working electronics from self-assembling carbon nanotube transistors is still in the future. Braun conducted the research with colleagues Kinneret Keren, Rotem S. Berman, Evgeny Buchstab, and Uri Sivan |