The oscillating cantilevers as detecting element

At the University of Twente we have chosen oscillating cantilevers as sensing elements to measure the concentration of VOCs in the exhaled air.

A cantilever is a miniature version of a springboard in a swimming pool, that vibrates up and down with a certain frequency determined by the mass, dimensions and stiffness of the material it is made of. When VOC molecules adhere to the cantilever the mass (and possibly the stiffness) change and with that the frequency of the oscillation. Changes in frequencies can be measured very accurately with standard electronics. Thus by measuring the frequency shift one can infer the total mass of the molecules adhered to the cantilever.

To make the cantilever into a useful sensor we have to add several features. First we need to bring the cantilever into oscillation. For that we use a so-called piezoelectric thin film capacitor on top of the cantilever. This material can deform under an applied AC voltage and bends the cantilever, bringing it into oscillation. The same piezoelectric capacitor is used to measure the frequency shift. Further the cantilever is covered with a functionalization layer to which ideally only one specific VOC type can adhere.

Cantilever, the piezoelectric layer and the functionalization layer together form the sensor that translates the adhesion of a particular VOC into a voltage signal, suitable for further processing, using electronics.

This may all sound still fairly abstract. In the next steps we will explain in much more detail how we can make cantilevers work as an electronic nose.