Surface acoustic wave (SAW) gas sensors are of continuous development interest to researchers due to their sensitivity, short detection time, and reliability. Among the most used materials to achieve the sensitive film of SAW sensors are metal oxide semiconductors, which are highlighted by thermal and chemical stability, by the presence on their surface of free electrons and also by the possibility of being used in different morphologies. For different types of gases, certain metal oxide semiconductors are used, and ZnO is an important representative for this category of materials in the field of sensors. Having a great potential for the development of SAW sensors, the discussion related to the development of the sensitivity of metal oxide semiconductors, especially ZnO, by the synthesis method or by obtaining new materials, is suitable and necessary to have an overview of the latest results in this domain.

The influence of sensitive porous films obtained by pulsed laser deposition (PLD) on the response of surface acoustic wave (SAW) sensors on hydrogen at room temperature (RT) was studied. Monolayer films of TiO₂ and bilayer films of Pd/TiO₂ were deposited on the quartz substrates of SAW sensors. By varying the oxygen and argon pressure in the PLD deposition chamber, different morphologies of the sensitive films were obtained, which were analyzed based on scanning electron microscopy (SEM) images. SAW sensors were realized with different porosity degrees, and these were tested at different hydrogen concentrations. It has been confirmed that the high porosity of the film and the bilayer structure leads to a higher frequency shift and allow the possibility to make tests at lower concentrations. Thus, the best sensor, Pd-1500/TiO₂-600, with the deposition pressure of 600 mTorr for TiO₂ and 1500 mTorr for Pd, had a frequency shift of 1.8 kHz at 2% hydrogen concentration, a sensitivity of 0.10 Hz/ppm and a limit of detection (LOD) of 1210 ppm. SAW sensors based on such porous films allow the detection of hydrogen but also of other gases at RT, and by PLD method such sensitive porous and nanostructured films can be easily developed.

The selectivity of a SAW (surface acoustic wave) sensor, with a Co₃O₄ sensitive thin film for NH₃ (ammonia) and the influence of SnO₂ on its sensitivity, was studied. Thin films were deposited by pulsed laser deposition (PLD) on quartz SAW sensor substrates. Two sensors with different types of sensitive films were developed: a Co₃O₄ thin film sensor (S1) and a SnO₂/Co₃O₄ thin film sensor (S2). The sensitive films were deposited in conditions which ensured a porous structure. The sensors were tested in the presence of three gases: NH₃, methanol, and toluene. The selectivity of Co₃O₄ for NH₃ was determined from the difference in the frequency shifts of the sensor for NH₃ and for VOCs (volatile organic compounds). The positive influence of SnO₂ on the sensitivity of sensor S2 was observed from the lower limit of detection (LOD) of this sensor and from the differences in frequency shifts between sensor S1 and sensor S2.

In this work, surface acoustic wave (SAW) sensors with two types of polymer sensing films, polyethyleneimine (PEI) and polydimethylsiloxane (PDMS), containing embedded ZnO, TiO₂, and WO₃ nanoparticles (NPs) for detecting volatile organic compounds (VOCs) were produced and studied. The NPs were obtained using the pulsed laser ablation method, with the same deposition conditions used for all three materials studied. After incorporation of the NPs into the polymer, the suspension obtained was deposited using the airbrush method onto the quartz substrate of the sensor. Sensors were tested for four types of VOCs: Ethanol, toluene, acetone, and dichloroethane. Those based on PEI-sensitive films showed a superior sensitivity to those with PDMS. It was also found that the sensors with WO₃ NPs had the best results for ethanol, acetone, and dichloroethane. The limit of detection (LOD) of the PEI/WO₃ sensor was 6 ppm for ethanol, 15 ppm for acetone, and 9 ppm for dichloroethane. For toluene, the best response was obtained using the PEI/ZnO sensor, which produced a LOD of 9 ppm.

Developing a controlled method for obtaining hybrid enzymatic-based interfaces for sensing application require the use of a multiuse, reusable sensor. By controlling the interface characteristics in terms of the surface chemistry, thickness, and roughness, a tailored response toward various toxic compounds can be obtained, regarding both materials used as active surfaces and fabrication methods. Herein, we report a preliminary study on using a laser-based method (i.e., matrix-assisted pulsed laser evaporation, or MAPLE) for obtaining active polymeric–enzymatic interfaces as hybrid or layered coatings for detecting toxic vapors. The MAPLE fabrication consisted of the simultaneous alternating evaporation of layers of polyethylenimine (PEI) and acetylcholinesterase (AchE) in order to obtain active surfaces as both hybrid PEI-AchE and a PEI/AchE layered coating, respectively. The deposition processes of the polymer and enzyme were carried out using a double-target system and a Nd:YAG pulsed laser, operating at 0.45 J/cm² fluences with a wavelength of 266 nm and a repetition rate of 10 Hz. Fourier transform infrared spectroscopy revealed no significant changes in the functional groups of both hybrid and layered coatings compared with the initial material. The thickness and roughness, as well as the morphologies of the coatings revealed by atomic force microscopy and scanning electron microscopy showed coatings thicker than two μm that had smooth surfaces and average roughness values below six nm. The sensors were tested with simulants for nerve gases and pesticides containing phosphonate ester groups, namely dimethyl methylphosphonate (DMMP) and diisopropyl methylphosphonate (DIMP), and a different sensitivity was shown to the selected chemical agents for each of the sensors. The best sensitivities for DMMP and DIMP obtained by using a PEI-AchE coated sensor are 65 kHz and 200 kHz, respectively, whereas the best sensitivity when using multilayered interfaces is 30 kHz and 10 KHz for DIMP and DMMP, respectively.

The development of hydrogen sensors has acquired a great interest from researchers for safety in fields such as chemical industry, metallurgy, pharmaceutics or power generation, as well as due to hydrogen’s introduction as fuel in vehicles. Several types of sensors have been developed for hydrogen detection, including resistive, surface acoustic wave, optical or conductometric sensors. The properties of the material of the sensitive area of the sensor are of great importance for establishing its performance. Besides the nature of the material, an important role for its final properties is played by the synthesis method used and the parameters used during the synthesis. The present paper highlights recent results in the field of hydrogen detection, obtained using four of the well-known synthesis and deposition methods: sol-gel, co-precipitation, spin-coating and pulsed laser deposition (PLD). Sensors with very good results have been achieved by these methods, which gives an encouraging perspective for their use in obtaining commercial hydrogen sensors and their application in common areas for society.