DNA assembles nanotube transistor

Nanotechnology is all about making machines and materials molecule-by-molecule. Such precision promises to enable microscopic machines, faster electronics, and materials that harbor new properties.

Because it is difficult and tedious to manually put atoms and molecules in place, researchers are looking for ways to cause materials to self-assemble. Self-assembly is an especially attractive concept because it has the potential to be quick, relatively easy, and very inexpensive.

One way to make things assemble automatically is to coax nature's self-assembly molecule -- DNA -- to assemble into templates that can in turn cause other molecules to line up in all the right places.

Researchers from the Technion-Israel Institute of Technology have brought the idea a large step forward by demonstrating a DNA-template self-assembly process that makes transistors in a test tube using an assortment of raw ingredients: carbon nanotubes, silver, gold, and four types of protein molecules.

The process could eventually be used to make many types of materials, molecular machines and electronics, and even entire computers.

DNA is made up of four bases -- adenine, cytosine, guanine and thymine -- attached to a sugar-phosphate backbone. In cells, two strands of DNA zip together into the familiar double helix when their bases line up -- adenine connects to thymine and cytosine to guanine -- and sequences of bases act as templates to build proteins. Nanotubes are rolled-up sheets of carbon atoms that form naturally in soot and can be smaller than one nanometer in diameter, or 75,000 times narrower than a human hair.

Researchers have been able to make artificial DNA molecules that have tailor-made sequences of bases for some time. The key to using this type of DNA as a template for tiny components and new materials is finding ways to connect nonbiological materials like metal and carbon nanotubes to specific sequences of DNA bases. "Combining DNA, proteins, metal particles and carbon nanotubes and a test tube is not easy since these materials are alien to each other," said Erez Braun, a professor of physics at the Technicon-Israel Institute of Technology.

The researchers accomplished this by co-opting the natural antibody process. Antibodies connect to specific proteins that make up the outside cell walls of pathogens like bacteria in order to capture and dispose of the bacteria.

The researchers' process self-assembles a transistor in several steps. First, the researchers coax a long double strand of DNA and a short single strand to position the nanotube.

The short single strand is coated with a protein from an E. coli bacteria that connects to a target span of 500 bases on the double strand. The span measures about 250 nanometers, or 250 millionths of a millimeter. An antibody to the bacteria protein then binds to the protein, followed by a second antibody that binds to the first one. Finally, a carbon nanotube that has been coated with a second type of protein binds to the second antibody, connecting the nanotube along the target sequence of the double strand of DNA.

The DNA-nanotube assembly is then stretched out on a silicon wafer, where the E. coli protein carries out a second job as a resist, or shield.

When a solution of silver is mixed with the DNA, silver molecules attach only to those segments of DNA that are unprotected by the protein. This sets up the second step of the wire-building process. When the researchers add suspended gold particles and electrify the solution, gold deposits around the silver clusters to form gold wires on both sides of the nanotube.

These gold wires are the source and drain electrodes of a transistor. The nanotube forms the transistor's semiconducting channel, and the silicon surface acts as a gate electrode, which controls the flow of current running through the device to turn it on or off.

"We harnessed a basic biological process... responsible for mixing genes in cells... to create sequence-specific DNA junctions and networks, to coat DNA with metal in a sequence-specific manner and to [position] molecular objects on [a specific] address in a DNA molecule," said Braun.

The demonstration "is a very significant [advance] in developing the technology for assembling carbon nanotube-based devices," said Deepak Srivastava, a senior scientist and technical lead in computational nanotechnology at the NASA Ames Research Center. "People have always talked about using wet chemistry for assembling molecular electronic components into precise locations," he said. "This is a first proof of the principal."

The research is novel because it uses biological molecular recognition techniques to assemble synthetic building blocks, said Srivastava. The technique could eventually be used in a next generation of electronics and in other applications that require nanoscale molecular components to assemble into complex system-level architectures -- like embedded sensors, molecular machines and nano-manufacturing applications, he said.

The researchers' next step is to construct a device on a DNA junction, said Braun. This would involve getting rid of the silicon substrate that acts as a gate for the current prototype transistor. Once this is possible, "the road is open for self-assembling more complex logic circuits," he said.

Today's computer chips are largely made up of transistors arranged into circuits that carry out the basic logic of computing. Researchers are working to make transistors smaller in order to speed computing; smaller components are faster because electrical signals have less distance to travel. Self-assembly processes could eventually prove less expensive than today's silicon manufacturing techniques.

It is not clear how long it will take before the self-assembly process can be used to manufacture components, said Braun. "It's hard to predict applications," he said. "A lot needs to be done before it becomes technology, but it's a good step forward since self-assembly of carbon nanotube devices opens many possibilities for electronics and diagnostics."

Braun's research colleagues were Kinneret Keren, Rotem S. Berman, Evgeny Buchstab, and Uri Savon. The work appeared in the November 21, 2003 issue of Science. The research was funded by the Israeli Science Foundation, the Techinion-Israel Institute of Technology, and the Clore Foundation.

Timeline:   Unknown
Funding:   Government; Private; University
TRN Categories:  Nanotechnology; Integrated Circuits
Story Type:   News
Related Elements:  Technical paper, "DNA-Templated Carbon Nanotube Field-Effect Transistor," Science, November 21, 2003