HANCOCK, N.H. Molecular self-assembly assisted by DNA and protein recognition might provide a route to complex logic and memory circuits built with nanotubes, a project in Haifa, Israel, suggests.
The project at the Technion-Israel Institute of Technology has so far fabricated a field-effect transistor on a silicon substrate consisting of a single-walled carbon nanotube attached to gold electrodes. A DNA "scaffold" was used to precisely align the nanotube on the substrate, and an array of proteins attached to the nanotube provided a mask for building attached gold electrodes.
"The idea was that the same proteins do both the structure and the lithography-for example, protecting DNA regions against metallization," said Erez Braun, one of the researchers on the project. The process used to achieve this single FET could be developed into a parallel reaction that could assemble large numbers of carbon nanotubes at precise locations, along with a self-assembled mask providing electrodes and metal interconnect, the scientists said. But Braun emphasized that practical applications are a long way off and that the current work is only a first step.
The experiment represents an advance in the complexity of biomolecular-driven self-assembly. Nanotechnology re-
searchers have become adept at attaching DNA strands to small-scale structures such as nanoclusters or nanotubes, enabling them to assemble into more complex structures via DNA's lock-and-key bonding. To assemble the FET, the Israeli project employed not only DNA bonding, but the more complex protein-recognition processes used by the immune system to target foreign bodies.
The assembly process takes place in a solution containing the various components of the device. The RecA protein from the bacteria Escherichia coli provided a lock-and-key binding capability with an antibody coated onto a carbon nanotube. The basic scaffold for positioning the nanotube was built from a double-stranded piece of DNA. A single strand of DNA that matched the double strand's nucleotide sequence at a specific position was then coated with the RecA protein.
The coated strand migrates to the position on the double-stranded DNA that matches its particular sequence. At the same time, a single-walled carbon nanotube (SWNT) coated with an antibody to RecA migrates to the site of the single-stranded DNA and binds to it, creating a precisely positioned segment that forms the channel of the FET.
The dual-binding procedure allows parallel placement of carbon nanotubes along the double-stranded DNA molecule, due to the possibility of creating a variety of single-strand DNA segments coded for different positions.
The researchers stretched out the double-stranded DNA segments carrying the carbon nanotubes on a silicon substrate. By immersing the system in a solution containing silver atoms, any DNA that was not covered by the RecA became metallized. The next step was to deposit gold, which binds to any section of the double-stranded DNA that has silver atoms attached, turning those portions of the strand into conducting wires that are directly attached to the DNA strand holding the SWNT.
To test the operation of the molecular FETs, the substrate was used to perform the gating function and electron-beam lithography was used to form contacts to the metallized portions of the DNA strands. Out of 45 devices the team produced from three different batches, 14 FETs were found to function properly. Many revealed a metallic channel, in that they conducted current well but could not be turned off. Others appeared to suffer from poor contacts with the metallic sections of the DNA.
Braun suggested that circuits could be built by adding more side chains to the DNA strands to act as gate contacts. The DNA/protein construction process offers a lot of versatility for building circuits, he said.
"In principle, the information encoded in DNA is enough to determine a structure. People have shown this by using DNA hybridization," Braun explained. "The question of complexity is a serious one; it is not clear how much more biology needs to be introduced in order to go into real complex structures, but that is one of the reasons that this research is so interesting."
While DNA offers a one-dimensional positioning capability, proteins perform lock-and-key bonding in three dimensions. Biological systems use this capability to form 3-D structures and also to perform a wide variety of biological functions.
For example, Braun believes that circuit defects might be corrected with protein-based processes. The question for molecular-electronics designers is how to determine which biological processes will lead to practical circuits.