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The oscillating cantilevers as detecting element

The oscillating cantilevers as detecting element
For a new generation of artificial noses, we investigate the use of so-called cantilevers. A cantilever is a small-scale version of a springboard in a swimming pool. The springboard and the cantilever can vibrate up and down with a certain frequency. The resonance frequency of a cantilever is the frequency for which the amplitude of the oscillation is largest. It depends on the length and the stiffness of the board and the weight of the swimmer standing on the end of the board. The cantilever can be used as a mass sensor by making it sensitive for the adhesion of gas molecules. When the molecule binds to the cantilever, the total mass of the cantilever changes and with that, the resonance frequency.
Measuring the change of the resonance frequency gives a measure of the mass and, thus, the number of molecules that adhere to the cantilever. This is the basic sensing mechanism. Further on, we will see that we can also use the change of the stiffness of the cantilever by gas absorption as a sensing mechanism. An important reason for using oscillating cantilevers as sensor is that they can operate very well in gas environment. The amplitude of the cantilever oscillation is not damped very much by the gas and favors sensitivity. We will see that by making the cantilever very small and thin, it becomes more and more sensitive for small mass changes. This way, cantilevers can be made extremely sensitive.
Over the last decade, the sensitivity has changed from picograms down to yoctograms. A yoctogram is about the mass of a single VOC molecule. However, these sensors are operated in ideal lab conditions and not under practical circumstances. Luckily, in practice, one does not need the ultimate sensitivity since there are sufficient VOC molecules available in the exhaled air. There are several features that have to be added to a cantilever to make it a useful sensor. The cantilever has to be made sensitive to specific gas. This is done by depositing a specific chemical layer on top of the cantilever to which only one type of gas adheres. This is called functionalization.
The cantilever has to be brought into oscillation so as you want to measure changes in the oscillation frequency. For this, we use a thin piezoelectric layer on top of the cantilever. The frequency change of voltage signal can be measured by standard electronics with very high accuracy. The piezoelectric layer used for driving the cantilever oscillation is also used to create a voltage signal from the oscillating cantilever. By incorporating cantilever, drive, and sensor on the same sensor area, a very compact transducer is obtained that translates to the adhesion of a specific VOC molecule directly into an electronic signal. By using an area of cantilevers, which are sensitive to different VOCs, the basic building block of an electronic nose is constructed.

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.

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