“This research is important because people believe that carbon nanotubes will be one of the major future building blocks of molecular electronics due to their small dimensions and excellent electronic properties,” Erez Braun of Technion-Israel Institute of Technology told nanotechweb.org. “You cannot self-assemble a circuit directly with nanotubes since they lack recognition. Our research demonstrates that you can harness biology to self-assemble nanoelectronics.”

A year ago Braun and colleagues developed a “sequence-specific molecular lithography” technique. “Here, we harnessed a basic biological process - homologous recombination that is responsible for mixing genes in cells - to manipulate DNA so that it enables us to create sequence-specific DNA junctions and networks, to coat DNA with metal in a sequence-specific manner and to localize molecular objects on any address on a DNA molecule,” said Braun.

Now, the scientists have built on this technique to assemble a carbon nanotube field-effect transistor. They used a three-strand homologous recombination reaction between a long double-stranded DNA (dsDNA) molecule and a short auxiliary single-stranded DNA (ssDNA). These DNA molecules encoded the information to guide the assembly process: the short ssDNA molecule had a sequence identical to the dsDNA at the desired location of the transistor.

To start the process, the team polymerized RecA - a major protein responsible for genetic recombination in bacteria - onto the ssDNA molecules to form nucleoprotein filaments. Then these nucleoprotein filaments bound to the dsDNA molecules at the designated location, according to the sequence matching between the dsDNA and ssDNA.

Next, the scientists functionalized single-walled carbon nanotubes with the protein streptavidin. This bound to antibodies that attached to the RecA protein, locating the nanotube at the correct address. The team stretched the DNA/nanotube assembly on a passivated oxidized silicon wafer before carrying out a metallization process that coated the DNA molecules with gold. The RecA doubled as a sequence-specific resist, so that the active area of the transistor did not receive a gold coating. What’s more, as the nanotube was longer than the gap caused by the RecA, gold covered the ends of the nanotube, creating contacts to the transistor.

Altogether, Braun and colleagues made 45 devices. Fourteen of the devices acted as field-effect transistors with partial or full gating, and ten devices conducted but could not be gated - probably because they contained metallic rather than semiconducting nanotubes.

“This is basic research so it’s hard to predict applications,” said Braun. “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. The next step is to construct a device on a DNA junction, getting rid of the silicon substrate as a gate for the transistor. Then the road is open for more complex logic circuits.”

The researchers reported their work in Science.