In this work, a sensitive coating based on Langmuir–Blodgett (LB) films containing monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) with an immobilized glucose oxidase (GOx) enzyme was created. The immobilization of the enzyme in the LB film occurred during the formation of the monolayer. The effect of the immobilization of GOx enzyme molecules on the surface properties of a Langmuir DPPE monolayer was investigated. The sensory properties of the resulting LB DPPE film with an immobilized GOx enzyme in a glucose solution of various concentrations were studied. It has shown that the immobilization of GOx enzyme molecules into the LB DPPE film leads to a rising LB film conductivity with an increasing glucose concentration. Such an effect made it possible to conclude that acoustic methods can be used to determine the concentration of glucose molecules in an aqueous solution. It was found that for an aqueous glucose solution in the concentration range from 0 to 0.8 mg/mL the phase response of the acoustic mode at a frequency of 42.7 MHz has a linear form, and its maximum change is 55°. The maximum change in the insertion loss for this mode was 18 dB for a glucose concentration in the working solution of 0.4 mg/mL. The range of glucose concentrations measured using this method, from 0 to 0.9 mg/mL, corresponds to the corresponding range in the blood. The possibility of changing the conductivity range of a glucose solution depending on the concentration of the GOx enzyme in the LB film will make it possible to develop glucose sensors for higher concentrations. Such technological sensors would be in demand in the food and pharmaceutical industries. The developed technology can become the basis for creating a new generation of acoustoelectronic biosensors in the case of using other enzymatic reactions.

Properties of the Langmuir-Blodgett (LB) films of arachidic and stearic acids, versus the amount of the films’ monolayers were studied and applied for chloroform vapor detection with acoustoelectric high-frequency SAW sensors, based on an AT quartz two-port Rayleigh type SAW resonator (414 MHz) and ST-X quartz SAW delay line (157.5 MHz). Using both devices, it was confirmed that the film with 17 monolayers of stearic acid deposited on the surface of the SAW delay line at a surface pressure of 30 mN/m in the solid phase has the best sensitivity towards chloroform vapors, compared with the same films with other numbers of monolayers. For the SAW resonator sensing using slightly longer arachidic acid molecules, the optimum performance was reached with 17 LB film layers due to a sharper decrease in the Q-factor with mass loading. To understand the background of the result, Atomic Force Microscopy (AFM) in intermittent contact mode was used to study the morphology of the films, depending on the number of monolayers. The presence of the advanced morphology of the film surface with a maximal average roughness (9.3 nm) and surface area (29.7 µm²) was found only for 17-monolayer film. The effects of the chloroform vapors on the amplitude and the phase of the acoustic signal for both SAW devices at 20 °C were measured and compared with those for toluene and ethanol vapors; the largest responses were detected for chloroform vapor. For the film with an optimal number of monolayers, the largest amplitude response was measured for the resonator-based device. Conversely, the largest change in the acoustic phase produced by chloroform adsorption was measured for delay-line configuration. Finally, it was established that the gas responses for both devices coated with the LB films are completely restored 60 s after chamber cleaning with dry air.

An important technical task is to develop methods for recording the phase transitions of water to ice. At present, many sensors based on various types of acoustic waves are suggested for solving this challenge. This paper focuses on the theoretical and experimental study of the effect of water-to-ice phase transition on the properties of Lamb and quasi shear horizontal (QSH) acoustic waves of a higher order propagating in different directions in piezoelectric plates with strong anisotropy. Y-cut LiNbO₃, 128Y-cut LiNbO₃, and 36Y-cut LiTaO₃ plates with a thickness of 500 μm and 350 μm were used as piezoelectric substrates. It was shown that the amplitude of the waves under study can decrease, increase, or remain relatively stable due to the water-to-ice phase transition, depending on the propagation direction and mode order. The greatest decrease in amplitude (42.1 dB) due to glaciation occurred for Lamb waves with a frequency of 40.53 MHz and propagating in the YX+30° LiNbO₃ plate. The smallest change in the amplitude (0.9 dB) due to glaciation was observed for QSH waves at 56.5 MHz propagating in the YX+60° LiNbO₃ plate. Additionally, it was also found that, in the YX+30° LiNbO₃ plate, the water-to-ice transition results in the complete absorption of all acoustic waves within the specified frequency range (10–60 MHz), with the exception of one. The phase velocities, electromechanical coupling coefficients, elastic polarizations, and attenuation of the waves under study were calculated. The structures “air–piezoelectric plate–air”, “air–piezoelectric plate–liquid”, and “air–piezoelectric plate–ice” were considered. The results obtained can be used to develop methods for detecting ice formation and measuring its parameters.

The gas-sensitive properties of the forward SH1 and backward A1 acoustic waves in the YX lithium niobate plate loaded with a biofilm based on the mycelium of higher fungi are considered. The possibility of using an acoustoelectronic device based on a single interdigital transducer (IDT) as a gas-sensitive element is shown. In the absence of a sensor coating on the surface of the piezoelectric plate, the S11 parameter of the above waves does not change when exposed to water vapor, ethanol and acetone in the range from 7 to 20 MHz. When a sensor film based on the mycelium of basidiomycetes is applied to the side of the plate free from the IDT, the S11 parameter of the backward wave A1 changes depending on the film thickness. In this case, the value of this parameter for the forward wave SH1 remains unchanged. The magnitude of the change in the S11 parameter when exposed to saturated vapors of the test water sample for the forward wave SH1 is approximately twice as high as for the backward wave A1. However, the lower detection limit of water vapor for the backward wave is 4.32 g/m3, while for the forward wave it is only 7.59 g/m3.

Рассмотрены газочувствительные свойства прямой SH1 и обратной A1 акустических волн в пластине YX ниобата лития, нагруженной биопленкой на основе мицелия высших грибов. Показана возможность использования акустоэлектронного устройства на основе одиночного встречно-штыревого преобразователя (ВШП) в качестве газочувствительного элемента. При отсутствии сенсорного покрытия на поверхности пьезоэлектрической пластины параметр S11 вышеуказанных волн не меняется при воздействии паров воды, этанола и ацетона в диапазоне от 7 до 20 МГц. При нанесении сенсорной пленки на основе мицелия базидиальных грибов на сторону пластины свободную от ВШП параметр S11 обратной волны A1 меняется в зависимости от толщины пленки. При этом величина данного параметра для прямой волны SH1 остается неизменным. Величина изменения параметра S11 при воздействии насыщенных паров тестовой пробы воды для прямой волны SH1 примерно в два раза выше, чем для обратной A1. Однако, нижний предел обнаружения паров воды для обратной волны 4.32 г/м3, в то время как для прямой волны он составляет лишь 7.59 г/м3.

The effect of the construction features of an acousto-electronic device based on waves in plate on its sensory properties was investigated. The contribution of each side of the plate or its parts (the area between the IDT on the front side of the plate, on the back side of the plate, etc.) to the magnitude of the sensor response when exposed to water vapor or liquid was estimated. In the absence of a sensor coating on the surface of the piezoelectric, the front side demonstrates the greatest sensitivity due to the interaction of water vapor with the electrode system. In the case of isolation of the electrode system, there is no response to the effect of water vapor on the surface of the delay line. When a sensor coating based on higher Fungi metabolites is formed in the space between the IDT, both on the front and back sides of the delay line, the back side demonstrates the greatest sensitivity to water vapor. When placing a cell with liquid on the surface of the delay line, it is shown that most acoustic modes are more sensitive to the application of water to the full area of the back side of the plate.