Sensors using various analytical techniques are used for on-site chemical warfare agents (CWAs) detection. Among them are devices using well-established techniques such as ion mobility spectrometry (IMS) ^[1] and flame photometry (FP) ^[2] but also analytical techniques based on simple and undemanding sensors. Recently, semiconductor ^[3][4], colorimetric, and fluorescent ^[5][6] sensors have also attracted a lot of interest.

In addition to the above-mentioned techniques, acoustic wave sensors are used for on-site CWA detection ^[7]. As acoustic wave gas sensors meet all requirements for mobility (light weight, low energy consumption) and affordability (low cost, ease of use) well, they are seen as a potential technique for detecting CWA and other hazardous substances. Despite the fact that acoustic wave chemical gas sensors are not a very new issue (the first work on gas sensors with quartz crystal microbalance (QCM) dates from 1964 ^[8] while the first sensor with a surface acoustic wave (SAW) device was described in 1979 ^[9]), work on these devices is still very intensive as evidenced by the number of publications in scientific journals in recent years.

In 2017, a comprehensive review was published on the detection of volatile organic compounds (VOCs) and other gases with SAW sensors ^[10]. Another review from 2022 ^[11] presents achievements in the field of gas sensors based on different types of acoustic wave devices, including QCM. The works cited summarize the achievements in the field of acoustic wave gas sensors for the detection of vapors of various organic compounds and gases. The available scientific literature also includes studies focused solely on the detection of nerve CWAs ^[12].

2. Surface Acoustic Wave and Quartz Crystal Microbalance Devices Used in Gas Sensing

The group of acoustic wave sensors includes several types of piezoelectric devices, among which ware can list quartz crystal microbalances, a variety of surface acoustic wave sensors, and other sensors such as polymorphic sensors with different geometries (cantilever, fork, disk-shaped etc.) ^[13][14]. Sensors of this type are characterized by a very small size, uncomplicated structure, low power consumption, and simple electronic systems, which makes them an ideal candidate for use in portable instruments, in particular for the detection of CWAs and toxic industrial chemicals (TICs) in the field. Regardless of the type of piezoelectric device used, each sensor consists of a transducer with an acoustic wave and a selective coating that interacts with the analyte present in the environment. As a result of this interaction, the parameters of the coating, such as mass, density, stiffness, and electrical conductivity, change. These changes affect the propagation of the acoustic wave in the transducer material to which the sorption layer adheres, changing its speed and attenuation. An ordinary piezoelectric transducer is equipped with a resonant system, which vibrates with a strictly defined frequency depending on the parameters of the acoustic wave. Associated electronic systems convert acoustic wave parameters into electrical quantities. As a result, changes in wave parameters caused by analyte sorption are observed as changes in the resonant frequency, phase, and attenuation in the resonance system. A general diagram of the operation of an acoustic wave chemical gas sensor is shown in Figure 1. There are many types of acoustic wave transducers differing in the type of acoustic wave propagating and the method of its excitation. The most widely used in gas sensing applications are transducers with bulk acoustic wave, Rayleigh and Love-type surface acoustic waves, and plate waves ^[15]. A general scheme of the construction of these transducers and the wave propagating modes are shown in Figure 2. The bulk wave transducers are in the form of a plate made of piezoelectric material with metallic electrodes applied on both sides (Figure 2a). Usually this material is AT-cut quartz, therefore such sensors are called quartz crystal microbalance or quartz microbalance (QMB). Vibrations, excited in the crystal, have the form of a shear, hence the name TSM (transverse shear mode) sensors can also be found. Another type of transducer is a group of SAW devices. In this case, the wave is excited by interdigital electrodes applied on the surface of a piezoelectric substrate. This group includes transducers both with particle displacement oriented perpendicularly (Rayleigh wave, Figure 2b) and parallel (Love wave and surface transversal wave—STW, Figure 2c,d) to the plane of the substrate coated with the sorptive layer. The Love-type SAW propagates within an additional, thin layer of solid material (guiding layer) applied on the piezoelectric substrate. In turn, STW propagates in the very substrate. Both Love-type SAW and STW can also be used for measurements in a liquid medium due to much lower attenuation than in the case of Rayleigh wave. The third group consists of plate transducers, in which the wave propagates in a thin membrane (Lamb wave, Figure 2e). In addition to the acoustic wave transducers presented in Figure 2, there are also many other transducer geometries and types of acoustic waves ^[14]; however, those listed here constitute the vast majority of structures used in chemical gas sensors. Individual acoustic wave transducers differ in the range of operating frequencies, the ability to work in various environments (liquid and gas), sensitivity to changes in the parameters of the selective coating, and many other properties. A comprehensive description of their operation and application can be found, among others in works ^[14][16][17]. However, regardless of the type of transducer used, the selective coating is a key component of any chemical sensor of this type. It determines the basic parameters of the sensor, such as its sensitivity, selectivity, durability, reversibility, chemical resistance, and dynamic properties.