Authors: Jason D. Slinker, Natalie B. Muren, Sara E. Renfrew and Jacqueline K. Barton*
Affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California
In the pursuit of functional nanoscale circuits, many researchers are trying to identify reliable methods to assemble components on such a small scale. Although much work has been devoted to utilizing DNA to assemble molecular wires such as carbon nanotubes, relatively few papers have demonstrated DNA’s potential use as a nanowire. In a recent paper in Nature Chemistry, the Barton group from Cal Tech revealed charge transport in DNA over 34 nm, which surpasses most reports on molecular wires thus far.
DNA is a very viable candidate for nanoscale circuitry because it is easily synthesized and has a very precise, controllable length. Additionally, it is easily modified at both the 3’- and 5’-termini with an array of functionalities. The 100-mer duplexes in this paper were modified with a Nile Blue redox probe on one end, and on the other end a thiol linker to easily attach the DNA to gold. By using cyclic voltammetry and looking at the integrated cathodic and anodic peak areas, the researchers were able to ascertain electrochemical characteristics of the DNA between the Nile Blue redox probe and a gold electrode.
To prove that the charge was actually passing through the DNA, they compared the electrochemistry of well-matched DNA duplexes to those that contained a single mismatched base pair as well as fully mismatched strands. Even a single mismatched base pair will disrupt the pi-stacking of the base pairs through which the charge transfer occurs, and indeed, the researchers observed an attenuation of charge transport by a factor of two when a mismatch was introduced to the duplex.
By comparing these results to their previous studies with a Nile Blue-modified 17-mer duplex, they obtained similar electron transfer rates, which is impressive and convincing given that their 100-mer duplex is almost six times the length. In addition, they conclude that the rate-limiting step of the charge transport is not that which occurs between the base pairs, but rather the “electron tunneling through the alkanethiol linker.”
Unfortunately, DNA does not solve all of the problems of nanocircuitry because DNA is after all fragile, requiring a proper buffer. Additionally, as the nanocircuits become more complex and structurally demanding, the charge transport’s sensitivity to slight structural variations become a larger problem. Nevertheless, a DNA nanowire would be a top candidate for biosensing that requires a physiological environment.