Covering the bases: Quantum effect may hold promise for low-cost DNA sequencing, sensor ap...(nded assembly at the tunnel junction wit...)

The study made a number of measurements using varying amounts of electrical current through the junction, as the microscope's electrode is moved away from the substrate and the hydrogen bonds uniting base pairs are slowly pulled apart. "What you can see straight away," Lindsay notes, "is that with the bases held together by 3 hydrogen bonds, the curves of falling current go on for a long distance. In those held together with two, they go on for less distance." (see fig. 2)

Electron tunneling is a peculiar property of matter acting over tiny distances at the atomic or subatomic scale. In a classical electric circuit, a gate is either open or closed, permitting or blocking the flow of current. But, as Lindsay explains, "when you start to get two electrodes so close to one another that they are within a few atomic diameters, then the electrons can actually leak from one electrode to the other, because in quantum mechanics, they're not confined." These electrons, which violate classical mechanics as they hop across the tiny junction, are said to "tunnel."

Now, Lindsay's research team has developed a method to identify different DNA base pairs, which could serve as the foundation for a new DNA sequencing technology. "The tunnel current is there as a readout of how long that molecular pair survived in the junction, " Lindsay says. "But it turns out that it's an incredibly nice way of identifying which molecular pair it was." Although quantum tunneling seems exotic, Lindsay points out that the routine leaking of electrons from one atom to another to form a chemical bond is a similar process.

If significant challenges to reading single molecules through such a technique can be overcome, the method holds the potential for inexpensive DNA sequencing, operating at the breakneck pace
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