'Electronic nose' recognises a variety of scents

Scientists at Karlsruhe Institute of Technology (KIT) have developed a sensor that can be taught different scents, sniffing out potential hazards — such as smouldering cables or spoilt food — earlier than a human being.

The human nose consists of about 10 million olfactory cells with about 400 different olfactory receptors. These receptors perceive the scents and generate a specific signal pattern; the brain allocates the signal pattern to a certain smell.

“We use the biological nose as a model,” said Dr Martin Sommer, who coordinates the smelldect project at KIT’s Institute of Microstructure Technology. “In our electronic nose, nanofibres react to complex gas mixtures — ie, scents — and also generate signal patterns, on the basis of which the sensor identifies the scents.”

The objective of smelldect is to develop a low-cost olfactory sensor suitable for mass production and everyday use. Measuring only a few centimetres in size, the ‘nose’ consists of a sensor chip equipped with nanowires made of tin dioxide on many individual sensors. The chip calculates specific signal patterns from the resistance changes of the individual sensors. These depend on the molecules in ambient air and differ for different scents. If a specific pattern has been taught to the chip before, the sensor can identify the scent within seconds.

The electronic nose is a sensor chip on which nanowires react with scents, ie, complex gas mixtures. Image credit: Martin Sommer, KIT.

To start the process, the researchers use a light-emitting diode that is integrated in the sensor housing and irradiates the nanowires with UV light. As a result, the initially very high electrical resistance of tin dioxide decreases, such that changes of resistance caused by molecules responsible for the smell and attached to the tin dioxide surface can be detected.

“When the sensor perceives a smell, the resistance decreases further,” said Dr Sommer. “If a smell disappears, the electrical resistance increases again to the initial level and the nose is ready for other measurements.”

The sensor chip can learn a number of different scents and, hence, can be used for various purposes; for example, in the household for the control of ambient air or as a smoke alarm, during shopping to find out how fresh the fish or meat is, or as the nose of a robot.

“The difficulty consists in the fact that a scent does not always remain the same,” Dr Sommer noted. “For instance, the smell of a rose in the sun differs from that of a rose in rain. Currently, we are training the electronic nose for specific uses which can be chosen universally however.

“In the future, the electronic nose might be incorporated into all electric devices in order to prevent cable fires. Or it can be used in smartphones. When going shopping, everybody might be accompanied by his/her own highly sensitive electronic nose.”

KIT is supported by project partners JVI Elektronik and FireEater, who in 2015 helped the institute develop a smart fire alarm based on an electronic nose. It detects low-temperature carbonisation and combustion gases and allows for a reliable analysis to identify the burning material.

Top image caption: The electronic nose, like the human nose, perceives complex gas mixtures — ie, scents — and can recognise them based on specific signal patterns. Image credit: Amadeus Bramsiepe, KIT.