The scientists from the University of Waterloo in Canada, the French National Center for Scientific Research (CNRS), and the Mediterranean University of Tunis have developed a new type of ultra-sensitive gas sensor based on microcantilevers – tiny elastic beams rigidly attached to a substrate at only one end and capable of oscillating like a very thin ruler. These sensors require virtually no energy for operation, as their oscillations are generated automatically by thermal processes, making them suitable for portable devices, environmental monitoring systems, and smart buildings. The researchers not only improved the sensors performance but also effectively “separated the roles”: one type of sensor became particularly sensitive to humidity, while the other became sensitive to volatile organic compounds found in industrial emissions, paints, and even in the air exhaled by a person.
The main problem with most gas sensors is their non-selectivity: their simultaneous reaction to multiple substances. In real-world conditions, this significantly reduces their practical value. The researchers have proposed a different approach: usage of multiple sensors made of different materials, each with its own unique “sense” of the surrounding environment. By comparing their responses, substances can be identified by their characteristic “pattern,” like a fingerprint.
To achieve this, using spatial atomic layer deposition, the scientists created thin films of zinc oxide and aluminum-doped zinc oxide approximately 200 nanometers thick – roughly 500 times thinner than a human hair. These films were then formed into microcantilevers tens of micrometers long.
These sensors operate by changing their oscillation frequency. The microcantilever constantly vibrates, and when gas molecules hit its surface, its mass increases slightly which results in the frequency decrease. This shift can be used for determination of the substance concentration. Furthermore, the new design requires no external power source: the thermal motion of the particles generates automatically the oscillations. This makes the sensors simple and energy-efficient. The scientists also optimized the design, specifically the upper electrode thickness (thereby increasing stability of the oscillations), and therefore, measurement accuracy.
Experiments show that the microcantilever made of pure zinc oxide exhibits exceptional sensitivity to humidity. It is capable of detecting even very small changes in the water vapor content in the air. The same sensor also reacts to organic substances such as acetone, ethanol, and isopropanol. The microcantilever made of zinc oxide with added aluminum behaves differently: it reacts much less strongly to humidity but better detects organic compounds, especially ethanol.
This difference is due to the structure of materials. Zinc oxide has a crystalline structure with a large number of active sites on the surface, to which water molecules readily adhere. On the other hand, the material with added aluminum is amorphous, lacking a clear crystalline structure, and interacts with gases differently. Sensitivity is determined not only by the molecules mass but also by their chemical properties – for example, polarity of their bonds, which influences the strength of their interaction with the sensor surface.
Ultimately, the scientists demonstrated how to create a system consisting of several microcantilevers with different compositions, each responding differently to gases. Combining them on a single chip and analyzing the combined signal, it is possible to accurately determine the composition of even complex gas mixtures, including humid air.
As these sensors require virtually no power to operate, they are suitable for portable devices, environmental monitoring systems, smart buildings, and other applications where compactness and high measurement accuracy are essential.
