Electronic nose can identify different mint scents

Researchers at the Karlsruhe Institute of Technology (KIT) have developed an electronic nose (E-nose) with an artificial sense of smell, which can be used to achieve a reliable, quick and objective discrimination of mint scents. Members of KIT’s Light Technology Institute (LTI), Botanical Institute, Institute of Functional Interfaces (IFG) and Institute of Microstructure Technology (IMT) developed and tested the device with six different mint species, with the results published in the journal Chemosensors.

Different varieties of mint produce species-specific scents, which are used in industrial applications such as the production of perfumes. Industrial quality control of mint oil is subject to strict legal regulations in order to prevent adulteration, but the current process is time-consuming and requires a great deal of expertise.

In the development of their E-nose, the interdisciplinary research team was guided by the biological model: the olfactory cells, which in humans transmit information to the brain via electrical pulses, were replaced by 12 special QCM (quartz crystal microbalance) sensors. These sensors consist of two electrodes equipped with a quartz crystal. Such components can also be found in mobile phones, as they enable highly accurate mobile phone frequencies at low cost.

“The mint scents are deposited on the surface of the sensors. This changes their resonant frequency and we get a reaction to the respective scent,” said Professor Christof Wöll from the IFG.

Scents consist of organic molecules in different compositions. To enable the new sensors to absorb these molecules, the IFG researchers used 12 specific sensor materials, including metal–organic frameworks (MOFs) developed at the IFG.

“These materials are highly porous and particularly well suited for sensor applications because they can absorb many molecules like a sponge,” Prof Wöll said. “By combining the sensors with the different materials, we create what might be called a neural network.”

The scientists tested the electronic nose with six different mint species — including classic peppermint, horsemint and catnip. Prof Wöll explained, “We use different machine learning methods to train the sensors so that they can create the fingerprint of the respective scent from the collected data and thus distinguish the scents.” After each scent sample, the E-nose is flushed with carbon dioxide for about half an hour to allow the sensors to regenerate.

The results showed that the electronic nose with its QCM sensors can assign mint scents with a high specificity to a particular species, making it is a user-friendly, reliable and cost-effective alternative to conventional methods such as mass spectrometry — and a suitable tool for applications ranging from pharmaceutical quality control to the monitoring of mint oil as an environmentally friendly bioherbicide. Further development will focus on sensors that regenerate more quickly for picking up scents again. The IFG researchers will meanwhile continue to concentrate on MOF materials in order to adapt them for other applications, such as artificial olfactory perception for medical diagnostics.

Image caption: The combination of sensors and materials simulates the artificial sense of smell. Image credit: Amadeus Bramsiepe, KIT.

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