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David Bradley ISSUE #32
July 2003

Odor sniffers

Tracking down smells that even the most sensitive human nose cannot pick up is now possible, thanks to an inexpensive sensor devised by scientists at the University of Bonn. The sensor can work around the clock picking up the first signs of unpleasant odors or dangerous gases in sewage plants, refuse tips, farms, or industrial plants.

The Bonn stinkometer is based on a lab-on-a-chip device that can distinguish between the original and a cheap imitation perfume, for instance, even when they smell so similar that even an olfactory genius would have to admit defeat. ''In the nose there are more than a thousand different receptor types for different odor molecules,'' explains biophysicist Peter Boeker of the Bonn Institute of Agricultural Technology. ''Most of them are very specialised and only react to a few substances. Our sensor has only six, which, however, in each case they cover a broad spectrum of volatile chemical compounds.'' The researchers describe their device not as an "artificial nose" but a "chemical eye"!

Tim Hamacher and Peter Boeker developed the chemosensor???
Tim Hamacher and Peter Boeker (right)
developed the chemosensor

Six round gold chips, arranged in a circle, are responsible for the perception of different odors and the ''chemical eye'' fits into a space not much bigger than a credit card. On each gold receptor there is a different wafer-thin layer of a 'sticky substance', the 'gas chromatographic' phase, which traps odor molecules from the air. Each sticky layer has a preference for different sizes and shapes of molecules. Tiny quartz crystals vibrate the gold chips at their resonant frequency. Should a molecule stick temporarily to one of the chips, its frequency of vibration falls, which is readily detected by the accompanying circuitry. ''We can measure this change,'' Boeker explains, ''and we can do this so precisely that each receptor can even detect amounts of one billionth of a gram.'' Quartz microscales like these have been around for a long time; however, as odor detectors they are only suitable in combination with each other.


The sensor is not as sensitive as the human nose yet but it can detect amazingly small differences between fake and genuine perfume something even an expert human sniffer cannot always do.
The sensor is not as sensitive as the human nose
yet but it can detect amazingly small differences
between fake and genuine perfume something
even an expert human sniffer cannot always do.

Each type of sticky layer has its preferences and an odor changes the resonating frequency of several quartz microscales in its own characteristic way. ''With our six sets of scales we can therefore measure not only six smells, but hundreds or even thousands,'' Boeker adds.

The scientists see applications in any fields in which unpleasant or even dangerous emissions may arise, but where surveillance by human "odor watchdogs" would be impractical or simply too expensive. The device costs just ? 15,000.

However, while the device can distinguish between very similar smells, its sensitivity is much lower than the human nose. In order to perceive the smell of a grapefruit, our sense of smell only requires just two molecules, the chemical eye needs hundreds if not thousands to stick to its gold chips.