The sensing element of this customized apparatus is a multifinger device, formed by interdigitated electrodes (Au, Au/Cr, Nb, NbN, Ag, Al with 20-40 μm spacing) patterned on SiO2/Si substrates. A back gate contact (patent) is prepared directly on the Si substrate, and the sensing platform is placed in a chamber equipped with flow-meters, vacuum/pressure meters, units for controlling the gate voltage, resistivity and heating. The automated control of the sensor state is facilitated by a computer interface, designed specifically for the management of working parameters and data acquisition, and a microcontroller is used for data elaboration and transmission by an appropriate communication protocol. Using this system, gas measurements can be performed from as low as 10 ppb, with a good degree of sensitivity. The system is able to read signals from many elements simultaneously (up to 13), which therefore enables the development of sensing platforms formed by arrays of different sensors within a single device.
Quartz Crystal Nano Balance (QCN)
The quartz crystal nano-balance (QCN) is a gas detection system that has been designed and constructed using engineering solutions that enable the probing of extremely low detection limits. The circuit oscillator comprises of a 4Mhz AT-cut quartz crystal, characterized by an almost zero frequency drift with respect to temperature, in the range 18-30°C. The quartz resonator, coated by the active sensing material, is sandwiched between a pair of electrodes and housed in a small sensor chamber (1cm3). The circuit has been assembled using a high quality Pierce oscillator that ensures very stable oscillations during measurements. Following gas adsorption the mass variations (Δm) of the layer are calculated via the Sauerbrey relation:
<Δf = - [(2f 2)/ ( A (ρ G))1/2)] Δm
An advantage of the QCN over alternative systems is that the high counter frequency allows for the monitoring of very small mass variations, with a sensitivity of a little as 4 nanograms/ Hertz. Complete and automated control of the gas flow system and the state of the sensor has also been realized, through the use of a computer interface with dedicate software for data acquisition and signal management. This control system assures the optimization of the sensor response, while also facilitating the controlled heating of the device.