Resonator-Based Ammonia Sensing with Chitosan Film

The characteristics of a radial electric field excited resonator loaded with a film of chitosan acetate in ammonia were measured as follows. In this series of measurements with the resonator inside the chamber, the thermostat was not used. In this case, the temperature in the room was continuously measured by an electronic thermometer. The camera with the resonator and the film was connected to the measuring port of the impedance analyzer E4990A, and a series of measurements were carried out with the camera lid open in the pointed above frequency range. Then, a container with a 10% aqueous solution of ammonia with a volume of 6 mL was placed inside the chamber and the chamber lid was closed. The free evaporation of ammonia into the air atmosphere of the chamber began. At the same time, there was a continuous automatic measurement and recording of the real and imaginary parts of the electrical impedance in the range of 1–2001 kHz; each measurement took about 120 s. These measurements were repeated for 7 h; then, the chamber lid was opened, the container with ammonia solution was removed, and the measurements continued for another 1 h. The temperature of the chamber during the entire measurement cycle varied in the range of 26–27 °C. As a result of this series of measurements, loaded resonator spectra were obtained at different concentrations of ammonia in the air, which caused changes in the mechanical and electrical properties of the film and led to a minor change in the resonant frequencies of the loaded resonator (Figure 5). The dependence of the concentration of ammonia in the air on time is discussed in detail in [8 (link)]. In the presented work, we did not set out to determine the dependence of the conductivity of the chitosan film on the concentration of ammonia in the air, but simply used this effect to smoothly change the conductivity of a thin layer on the resonator surface.
The next day (18 h after the end of the main experiment), a control measurement was carried out, which showed that the frequency dependencies completely restored their original appearance, i.e., the chitosan acetate film restored its properties.