New nano-noses

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Our sensory organs have been facing competition for some time now from their electronic counterparts. Indeed, our smartphones could already incorporate inexpensive, mass-market-ready sensor chips to do our “sniffing” at the fish counter.

Ten million olfactory cells with four hundred different olfactory receptors generate signal patterns that our brains interpret as the corresponding odors. These highly sensitive sensory organs took millions of years to evolve. Scientists at Karlsruhe Institute of Technology KIT (the Karlsruhe Institute of Technology) didn’t need quite so long to develop an electronic olfactory apparatus for their “smelldect” project. The principle is just the same: A chemical signal is converted into an electrical signal. The very first e-noses were developed in 1982 at the University of Warwick, already comprising three metal oxide sensors. The “sniffers” on the market today are based on a wide variety of technologies. The best known applications are air quality sensors in vehicles: In tunnels or extremely dense traffic they switch in-car ventilation to recirculation mode if they detect high concentrations of carbon monoxide, nitrogen oxides or hydrocarbons.

Electronic noses in smartphones

Electronic noses
The electronic nose is small enough to be used in smartphones. (Image: Martin Sommer, KIT).

KIT’s electronic noses are aimed at a wider range of applications. In future, we can expect low-cost, mass-market-ready devices, for example, to prevent cable fires in electrical appliances, or in smartphones to assist shoppers at the fish counter.

This is all achieved by an array of individual sensors containing tin dioxide nanowires that react to the various molecules in the ambient air with characteristic changes in resistance. Together, the array generates an odor-specific signal pattern, which the odor sensor recognizes within seconds from those the chip has previously been “taught”.

The process is initiated by a light-emitting diode integrated in the sensor housing that irradiates the nanowires with UV light. This has the effect of decreasing the tin dioxide’s originally very high electrical resistance to a value at which tiny changes due to the molecules become detectable. When the sensor then perceives an odor, the resistance decreases. When the odor dissipates, the original condition is quickly restored. The “nose” is then ready again for a new measurement.

Flexible through training

The sensor chip can be trained for numerous different odors, hence the possible applications also cover a very wide range. They include, for example, monitoring the air in a room, operating as a fire alarm or checking foodstuffs. They would even be suitable as a functional nose for a robot. Since a given smell is not always the same – a rose smells differently in sunshine than in rain – existing electronic noses have to be trained for specific purposes, but these are universally selectable.

About the project

The project partners JVI Electronics and FireEater support KIT with industrial production and distribution. The two companies and KIT already used electronic noses as the basis for intelligent fire detectors in the 2015 EU project “SmokeSense”. They detect gases resulting from smoldering or combustion and provide a reliable analysis of the burning material.

 

Electronica noses (Image: Amadeus Bramsiepe, KIT).

Electronic noses consist of a sensor array and a mechanism for pattern recognition. (Image: Amadeus Bramsiepe, KIT).