Quantum effects are key to DNA electronics

The patterns of DNA base pairs, which code our genes, can also be used to tune the flow of electricity, American researchers have found. 

The work could one day lead to DNA-based electronics, made of components much tinier than those squeezed on to current silicon-based computer chips.

In Nature Chemistry today, a team led by Nongjian Tao at Arizona State University describe how the pattern of bases (A, C, G or T) can make electricity flow as smoothly through DNA as through a metal wire, or in hopping steps more like semiconductors.

The insight relies on exploiting the quantum nature of the electron – hence determining whether electrons act like waves or particles as they move through a DNA strand.

For decades, engineers have been cramming more and more circuit elements on to computer chips by making the individual components, such as the transistor “switches”, smaller.

The problem is, this shrinkage can’t go on forever. Eventually, you reach a limit where neighbouring transistors interfere with one another. To go beyond silicon, scientists are looking towards making electronics from individual molecules.

And DNA is one of the most promising molecules for building electronics, being first discovered by nature 3.8-odd billion years ago.

The key advantages of DNA are its stability and the way it can be programmed to assemble into predesigned structures. In 2009, for example, scientists used DNA as a kind of circuit board for assembling electronic components six nanometres apart – much tinier than current silicon processing allows.

DNA also has interesting electrical conductivity, although its measured properties have varied in a somewhat mystifying way.

In 2014, for example, scientists developed DNA-based wires showing it can conduct electricity a bit like metals.

And in April this year, researchers at the University of Georgia made the world’s smallest diode (one of the fundamental building blocks of computing) from a strand of DNA just 11 base pairs long. In this case, DNA acts more like a semiconductor.

The new work provides an understanding as to why a DNA molecule can have such different properties (metallic or semiconducting). The trick, just like our genome, is in the pattern of ACGT bases along a DNA chain.

Each base pair has a different electrical conductivity, but creating conductive DNA is not simply a case of inserting a long sequence of the most conductive base. Instead, Tao and his team realised they needed to create patterns of bases that shared electrons with one another – similar to the way electrons are shared by the atoms in a metal. For example, alternating series of five guanine (G) bases created the best electrical conductivity.

While being shared across bases, electrons can move easily through the DNA by quantum tunnelling (the weird quantum effect the allows particles to “walk through walls”).

But when DNA is coded in other arrangements, the electrons move more like particles, hopping along the strand.

“Think of trying to get across a river,” says Limin Xiang, who is a co-author on the work. “You can either walk across quickly on a bridge or try to hop from one rock to another.”

The team validated their theory by tethering strands of DNA between a pair of two gold electrodes, and measuring their resistance to a small current.

Using this insight, scientists could design circuits based on DNA with different molecules acting as different circuit elements (wires, diodes, resistors and so on).

So far, the team has only tested strands up to 16 base pairs long. Whether they can build robust, long lasting devices remains to be seen.

Molecular electronics are unlikely to replace semiconductor devices any time soon. But the cool thing about molecules is they can do a range of jobs that go beyond what semiconductors can do, such as sensing the chemical environment.

Designer DNA could form the backbone of new kinds of specialist devices, such as electronic noses. And that’s nothing to sniff at.

For a primer on quantum mechanics, see Quantum physics for the terminally confused

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