A surface acoustic wave nitric oxide sensor

A dual delay line surface acoustic wave (SAW) device employing a ruthenium-doped tungsten trioxide (WO₃:Ru) film as a sensing element for nitric oxide (NO) is presented. Exposure of this semiconducting metal oxide (SMO) film to NO results in oxidation of the film, decreasing the film´s carrier concentration and, therefore, its conductivity. This decrease in film conductivity causes a corresponding increase in the SAW velocity. Therefore, when the device is configured as a dual delay line oscillator, the frequency of the sensing channel becomes a sensitive measure of NO concentration. Responses of this sensor to NO concentrations in ppb to low ppm levels in air along with higher concentrations of possible interferent gases are presented. The sensor response features are analyzed in terms of response time, recovery time, minimum detection limit, saturation detection limit and linearity. Conclusions are drawn and future improvements to the sensor are suggested