The sensor can detect H2S gas with a concentration as low as 0.5 ppm. The frequency shift of sensor is derived from the change of mass loading on films. The responses became much faster and stronger with the increase of RH.

Surface acoustic wave (SAW)-based formaldehyde gas sensor using bi-layer nanofilms of bacterial cellulose (BC) and polyethyleneimine (PEI) was developed on an ST-cut quartz substrate using sol-gel and spin coating processes. BC nanofilms significantly improve the sensitivity of PEI films to formaldehyde gas, and reduces response and recovery times. The BC films have superfine filamentary and fibrous network structures, which provide a large number of attachment sites for the PEI particles. Measurement results obtained using in situ diffuse reflectance Fourier transform infrared spectroscopy showed that the primary amino groups of PEI strongly adsorb formaldehyde molecules through nucleophilic reactions, thus resulting in a negative frequency shift of the SAW sensor due to the mass loading effect. In addition, experimental results showed that the frequency shifts of the SAW devices are determined by thickness of PEI film, concentration of formaldehyde and relative humidity. The PEI/BC sensor coated with three layers of PEI as the sensing layer showed the optimal sensing performance, which had a frequency shift of 35.6 kHz for 10 ppm formaldehyde gas, measured at room temperature and 30% RH. The sensor also showed good selectivity and stability, with a low limit of detection down to 100 ppb.

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A Love mode surface acoustic wave (SAW) humidity sensor based on bacterial cellulose (BC) coated ST-cut quartz was developed in this study. The BC film is composed of ultrafine interwoven fibers to form a highly porous network, and its surface contains a large amount of hydroxyl groups, which significantly improve the adsorption capability of SAW sensing layer for water molecules. This results in significant mass loading effects and enhanced responsivity of the SAW sensor. The resonant frequency of the sensor changes linearly with RH at lower relative humidity (RH) values (e.g., RH30%), but when RH80%, an exponential increase in frequency shift as a function of RH is obtained due to the enhanced mass loading effect. A frequency shift of 89.8 kHz was measured using a sensor with a BC film with a thickness of 148 nm thick when the RH was increased from 30% to 93%. The frequency of the sensor can be fully shifted back to the original reading when the RH was returned back to 30%, with the response and recovery times of 12 s and 5 s, respectively.The SAW sensor also exhibits good short-term repeatability and long-term stability for humidity sensing.

A highly sensitive surface acoustic wave (SAW) ultraviolet (UV) detector operated at room temperature was developed using a sensing layer of zinc oxide (ZnO) nanorods (NRs) grown on ST-cut quartz using a hydrothermal method. Under illumination of the UV light with a wavelength of 365 nm and an intensity of 6 μW cm-2 , the resonant frequency of the SAW UV sensor based on ZnO NRs/ZnO nanofilm structure was decreased by ~200 Hz, mainly due to electroacoustic effect. NR structures enhance the sensitivity of the sensor. The estimated enhancement in the sensitivity based on the experimental results is ~3.5 folds compared to a device with only a thin nanolayer ZnO film. Meanwhile, density of ZnO NRs over the ZnO seed layer was also investigated and an optimum design was proposed.