Jacqueline Barton and her colleagues at the California Institute of Technology in Pasadena (Caltech) have demonstrated how a 34 nm long monolayer of double-stranded DNA on a gold surface, comprising just 100 base pairs, can carry a low current. The discovery is another small step towards the ultimate goal of nanoelectronics but also holds promise for exquisite DNA sensors [Barton et al., Nature Chem (2011) doi:10.1038/NCHEM.98].
The formation of conducting nanowires offers researchers a unique opportunity to study charge transfer in one or two dimensions rather than in the bulk. Such wires also hold promise as novel materials for optoelectronics, energy storage devices, logic circuits and sensors. In this regard, DNA has several inherently useful properties such as long-range order and flexibility. It is also relatively easy to make and can be synthesized automatically with specific sequences and lengths.
However, despite the benefits, there have been drawbacks to using DNA as a nanowire in that no one has yet demonstrated consistent electrical properties. Various teams have shown DNA to be a semiconductor and even a superconductor under different conditions. The Barton team recently investigated charge transfer in a 15 base pair DNA strand, a 15-mer, which they used to bridge a carbon nanotube gap, with promising results.
They have now produced a 100-mer through piece-wise syntheses which can be attached to a gold surface at one end and to which a probe can be attached at the other. 100 base pairs is close to the persistent length of DNA strands in aqueous solution so is a natural length with which to work.
Electrochemical tests demonstrated that charge transfer occurs through the DNA nanowires. Importantly, they found that a single base pair mismatch in the DNA reduces the flow of electrons significantly, which might be exploited in sensors.
"Charge transfer through DNA depends on the stacking of the bases. Any small perturbation in stacking turns off the charge transfer. That's what happens, albeit subtly, with a mismatch," Barton told Materials Today. "That's really the source of some of the conflicting results in the past." As such, Barton believes that the team's DNA wires might best be used as an electronic sensor for mismatches(mutations) and protein binding. "The fact that one mismatch in the 100-mer is enough to turn off charge transfer underscores the point," Barton adds. "We are now working on whether this sensing may be used within the cell by repair proteins to sense DNA damage".
David Bradley