DNA-based sensors know what the nose knows

Can a machine mimic the human—or better yet, the canine—sense of smell? To do so, it would have to not only determine whether a chemical vapor is present in small amounts but also figure out, at least partially, what chemical it is. Carbon nanotubes and other nanomaterials do a good job on the first front. Their small size means that the presence of just a few gas molecules is enough to measurably change their electrical properties. Discriminating among many different molecules requires an array of sensors, each with different response characteristics. In 2005 A. T. Charlie Johnson (University of Pennsylvania), Alan Gelperin (Monell Chemical Senses Center in Philadelphia), and their colleagues began to investigate whether a nanotube sensor decorated with single-stranded DNA might provide the needed specificity to serve as an array element. Now the same researchers have shown that with suitably chosen DNA sequences, they can create sensors that can discriminate between very similar molecules, including organic molecules that are enantiomers, or mirror images, of each other. Human noses can do that, but not many electronic sensors can. The figure shows one pair of enantiomers the researchers were able to distinguish, (+)-limonene (left) and (−)-limonene (right). (Solid and dashed triangles represent chemical bonds that extend above and below the page, respectively.) To us, one smells like lemon–orange; the other smells like sour orange and turpentine. (S. M. Khamis et al., AIP Adv. 2, 022110, 2012.)—Johanna Miller