Forget surveillance cameras – what if our world were filled with always-on electronic noses tuned to detect all manner of contraband at minute concentrations? Drugs, explosives, air pollution, all of them could be monitored continuously, creating a "smell portrait" of the entire planet, and making it virtually impossible to conceal the trace odour of many legal and illegal substances.
That's the vision of scientists who work on electronic noses, and now, thanks to the silkmoth, the world's most sensitive electronic nose for explosives was recently demonstrated in the lab. One thousand times more sensitive than comparable devices, it consists of a tiny, vibrating cantilever coated in a forest of titanium dioxide nanotubes that were inspired by similar structures on the antennae of silkmoths.
Male silkmoths can detect females at distances of 5-10 kilometres (3-6 miles), and can register as few as one or two molecules of their pheromone scent. Five years ago, Denis Spitzer of the defence-focused Institut Franco-Allemand de Recherches de Saint-Louis heard about the super-sensitive feats of the silkmoth and wondered whether this could be reproduced by a machine.
One of the keys to the moth's performance is that its antennae are covered with a carpet of tiny sensilla, or microscopic fibres. These sensilla give the moth's detection apparatus much greater surface area than they would otherwise have, which is important for detecting very small concentrations of volatile compounds. These sensilla inspired Spitzer to upgrade an existing solution to "smelling" compounds with silicon.
Cantilevers – which, under high magnification, look like tiny diving boards – are a tried and true method for detecting target substances in air. Coated with a substance to which target molecules will attach themselves selectively, driven by a piezoelectric actuator, these tiny silicon rectangles vibrate at a high frequency, and when a molecule of the target substance sticks to them, the frequency of their vibration changes.
By covering a typical cantilever with titanium dioxide nanotubes, Spitzer and his team were able to increase its surface area 100-fold, and multiply its sensitivity by a factor of 1,000.
"For me, this is bioinspiration, not biomimicry," says Spitzer. The moth detects molecules through a "lock and key" system in which individual molecules are caught by smell sensor receptors, which then activate neurons connected to the animal's brain. Spitzer's system works by a very different mechanism, but this insight from the physical structure of the moth's antennae proved to be a crucial one.
Currently, this electronic nose can detect traces of TNT and related compounds as low as parts per million or less, because the titanium dioxide coating likes to bond with molecules of these explosives. Spitzer estimates it will be 2-4 years before this system will make it into the real world, but he already has plans to test it outside the lab. Future research efforts will be aimed at making the device even more sensitive, as well as making it selective for a host of other compounds, including drugs and pollutants.
Ultimately, notes Spitzer, this sensor's real competition will be the drug and bomb-sniffing dogs employed by law enforcement the world over. Presently, no electronic system is more sensitive than a well-trained sniffer dog, but Spitzer thinks that he can create something that will be as sensitive as these animals, while overcoming their one shortfall: after an hour, a dog's nose becomes saturated with the target substance, making them less useful.
"This kind of detector can work continuously," says Spitzer. "The principle is that it's reusable. You just heat the system and the TNT goes away."
Ultra-sensitive, continuously operating, and as cheap to mass-produce as current silicon microchips, that's the sort of device that will transform the surveillance state into one which can see, hear – and smell.