DNA reveals another electronic property

Not yet possible in any useful sense, DNA has the potential to be the backbone for molecular electronic circuits – electronics at the smallest scale – and towards this scientist are hunting through its structure and testing modifications to find useful non-linear electrical characteristics and switching behaviours.

TITECH DNA zipper electronicsAt the Tokyo Institute of Technology, a team has connected a short DNA strand – 90 nucleotides long, called a ’90-mer’ (like polymer) – in an unusual way and found both conductivity, a non-linear characteristic and a self-repairing nature.

TITECH DNA zipper electronicsThe experiment involves a scanning tunnelling microscope (STM) with a gold probe tip, a gold substrate and the DNA strand.

According to the university, the usual way to look at such strands is to catch one long-ways between the microscope probe tip and the substrate, and this reveals that resistance increases with length until nothing much can be measured.

What the Tokyo team did was to measure across the strand instead of along it.

They made one end of the strand ‘sticky’ by bonding sulphur atoms separately each of the single strands that make up the double-stranded helix of the DNA 90-mer.

As sulphur bonds to gold, the sulphur-tipped 90-mers stuck to the substrate by one end (above left).

TITECH DNA zipper electronicsBy bringing the gold STM tip close to the attached DNA, they could sometimes pick up the end of just one of the single strands of the 90-mer, leaving the other single-strand stuck to the substrate (diagram right). This let the researchers measure the electrical characteristics across a mid-length DNA strand for the first time.

Conductivity was found to be high, revealed by theoretical modelling to be due to delocalised π electrons that move freely around the molecule, according to the researchers.

TITECH DNA zipper electronicsAs they pulled the probe away from the substrate, they found a kink in the distance-conductance curve – sadly hard to see in the graph provided by the university (left) as thousands of measurements are superimposed. The combined kinks form the dark patch just below G/G0=0.002  conductivity and above ~0.1nm displacement.

“The single plateau and subsequent conductance decay in the traces indicate that these plateaus are attributed to the single-molecule junction that contains DNA,” according to the team in a paper published in Nature Communications.

This and the high conductivity is what leads the researchers, who now have a full theoretical grip on the processes, to conclude that this is useful knowledge for future DNA circuit designers to exploit.

TITECH DNA zipper electronicsThe experiments also revealed that the DNA strand would unravel if the ends were pulled, and then spontaneous re-connect without residual damage when the probe descended again – the 90-mer displayed some odd characteristics while this happens, described in the Nature paper along with a possible mechanism.

Lastly, if the substrate was coated with one type of ‘half-DNA’ strand and the partner half-strands were deposited on the probe tip, 90-mer DNA spontaneously formed when the probe was moved close to the substrate.

These last two characteristics suggest robust self-assembly should future circuits need it.

Full details can be read without payment in the Nature Communications paper ‘Single-molecule junction spontaneously restored by DNA zipper‘.

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