No Assembly Required: DNA brings carbon nanotube circuits in line

By harnessing the binding patterns of DNA and proteins, researchers have devised an efficient way of making carbon-nanotube transistors. The new technique offers the possibility of assembling nanotubes into complex circuits that could eventually yield computer chips that are faster and more powerful than those available today.

Chip manufacturers are forever trying to squeeze more and more components onto a single chip. The smallest features on today’s microchips are about 100 nanometers in width, whereas a carbon nanotube is only about 1 nanometer wide.

Current techniques for assembling carbon nanotubes on chips aren’t yet up to the job of making complex circuits. Using an atomic force microscope, for example, researchers can mechanically place carbon nanotubes, one by one, on a chip. Looking through a microscope, researchers find each nanotube and delicately bring it in line with an electrical contact. “It’s like using tweezers,” says Erez Braun of the Technion-Israel Institute of Technology in Haifa.

Braun and his colleagues took advantage of the binding features of proteins and DNA, which automatically combine in specific, predictable ways and assemble into highly organized structures. Because these materials are on the same scale as nanotubes are, researchers are increasingly using this self-assembling strategy of biology to build nanoscale devices.

“Self-assembly allows us to make complex circuits without human intervention,” says Braun. “The information encoded in the molecules can determine the configuration of a circuit.”

To fabricate their transistor, Braun and his colleagues first worked with components in a test tube to devise a way of attaching carbon nanotubes to precise locations on a DNA scaffold. The researchers affixed a bacterial protein to a synthetic piece of single-stranded DNA. When this was mixed with a longer fragment of double-stranded DNA, the two DNA fragments bound where their sequences matched. Next, the researchers decorated nanotubes with several molecules of a second protein.

The scientists used an intervening molecular complex that binds to both of the proteins, thereby bringing together the nanotubes and the DNA molecules.

When the Technion team flowed the test-tube solution over a silicon chip, the DNA-nanotube structures stuck to the chip surface. Finally, the team coated the exposed ends of each DNA fragment with gold contacts, making a simple circuit.

To see whether these devices worked as transistors, with the chip’s

silicon base functioning as the gate, the researchers applied different voltages. At one voltage, current flowed through the nanotubes; at another voltage, no current flowed. Those two states are equivalent to the 1 and 0 elements of digital computation. “That’s how the logic in your computer works,” says Braun. The scientists describe their work in the Nov. 21 Science.

Thomas LaBean of Duke University in Durham, N.C., characterizes the Technion achievement as the “latest advance in a very promising line of research.” He and his colleagues are also fabricating DNA scaffolds to use in assembling carbon-nanotube circuits. However, instead of starting with linear strands of DNA, LaBean and his colleagues are fabricating more-complex, two- and three-dimensional DNA scaffolds to serve as their starting material.

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