Since their development, surface acoustic wave (SAW) devices have attracted much research attention due to their unique functional characteristics, which make them appropriate for the detection of chemical species. The scientific community has directed its efforts toward the development and integration of new materials as sensing elements in SAW sensor technology with a large area of applications, such as for example the detection of volatile organic compounds, warfare chemicals, or food spoilage, just to name a few. Thin films play an important role and are essential as recognition elements in sensor structures due to their wide range of capabilities. In addition, other requisites are the development and application of new thin film deposition techniques as well as the possibility to tune the size and properties of the materials. This review article surveys the latest progress in engineered complex materials, i.e., polymers or functionalized carbonaceous materials, for applications as recognizing elements in miniaturized SAW sensors. It starts with an overview of chemoselective polymers and the synthesis of functionalized carbon nanotubes and graphene, which is followed by surveys of various coating technologies and routes for SAW sensors. Different coating techniques for SAW sensors are highlighted, which provides new approaches and perspective to meet the challenges of sensitive and selective gas sensing.

The objective of this work is the application of laser-induced forward transfer (LIFT) for the fabrication of chemiresistor sensors. The receiver substrate is an array with metal electrodes and the active materials placed by LIFT are single-walled carbon nanotubes (SWCNT). The functionality of such sensors depends on the geometry of the active material onto the metallic electrodes. First the best geometry for the sensing materials and electrodes was determined, including the optimization of the process parameters for printing uniform pixels of SWCNT onto the sensor electrodes. The sensors were characterized in terms of their sensing characteristics, i.e., upon exposure to ammonia, proving the feasibility of LIFT.

Nanostructured tungsten trioxide (WO ₃ ) thin films are deposited by pulsed laser deposition (PLD) and radio-frequency (RF) assisted PLD onto interdigitated sensor structures. Structural characterization by x-ray diffraction and Raman spectroscopy shows the WO ₃ films are polycrystalline, with a pure monoclinic phase for the PLD grown films. The as-fabricated WO ₃ sensors are tested for ammonia (NH ₃ ) detection, by measuring the electrical response to NH ₃ at different temperatures. Sensors based on WO ₃ deposited by RF-PLD do not show any response to NH ₃ . In contrast, sensors fabricated by PLD operating at 100 °C and 200 °C show a slow recovery time whilst at 300 °C, these sensors are highly sensitive in the low ppm range with a recovery time in the range of a few seconds. The microstructure of the films is suggested to explain their excellent electrical response. Columnar WO ₃ thin films are obtained by both deposition methods. However, the WO ₃ films grown by PLD are porous, (which may allow NH ₃ molecules to diffuse through the film) whereas RF-PLD films are dense. Our results highlight that WO ₃ thin films deposited by PLD can be applied for the fabrication of gas sensors with a performance level required for industrial applications.

Ammonia is one of the most frequently produced chemicals in the world, and thus, reliable measurements of different NH₃ concentrations are critical for a variety of industries, among which are the agricultural and healthcare sectors. The currently available technologies for the detection of NH₃ provide accurate identification; however, they are limited by size, portability, and fabrication cost. Therefore, in this work, we report the laser-induced forward transfer (LIFT) of single-walled carbon nanotubes (SWCNTs) decorated with tin oxide nanoparticles (SnO₂ NPs), which act as sensitive materials in chemiresistive NH₃ sensors. We demonstrate that the LIFT-fabricated sensors can detect NH₃ at room temperature and have a response time of 13 s (for 25 ppm NH₃). In addition, the laser-fabricated sensors are fully reversible when exposed to multiple cycles of NH₃ and have an excellent theoretical limit of detection of 24 ppt.