Stepped electrode designs of TSM langasite resonators for high-temperature applications.

In this paper, we report the simulation and fabrication of thickness-shear mode langasite resonators with stepped elliptical electrode designs to investigate their effects on energy trapping and suppression of spurious modes at elevated temperatures. Finite element analysis was conducted to analyze the design of a stepped elliptical electrode on a contoured langasite crystal. Based on the simulation findings, langasite resonators with stepped electrodes were fabricated, and their displacement profiles and frequency-temperature properties were characterized using network analysis and laser Doppler vibrometry. Results demonstrate improved frequency separation between the resonant and spurious modes, and enhanced spurious mode suppression at both room and higher temperatures, suggesting that stepped elliptical electrode designs can effectively enhance the sensing performance of langasite resonators.

Quartz crystal microbalance-based biosensing of hepatitis B antigen using a molecularly imprinted polydopamine film.

Quartz crystal microbalance (QCM)-based biosensors are highly attractive as rapid diagnostic devices for detecting infectious diseases. However, the fabrication of QCM-based biosensors often involves tedious processes due to the poor stability of the biological recognition elements. In this work, the simple self-polymerisation of dopamine was used to functionalise the QCM crystal surface with a molecularly imprinted polydopamine (MIPDA) sensing film for detecting the hepatitis B core antigen (HBcAg), a serological biomarker of hepatitis B. Recognition cavities that complemented the size and shape of HBcAg were observed on the QCM crystal surface after functionalisation with the MIPDA film. The MIPDA-QCM biosensor showed a selective affinity for HBcAg, recording frequency responses up to 7.8 folds larger towards HBcAg compared to human serum albumin at the same analyte concentrations. The biosensor response was enhanced by using the optimal concentrations of 10 mg mL-1 of dopamine and 1 mg mL-1 of template for MIPDA film formation, resulting in a low detection limit (0.88 µg mL-1) that enables the detection of clinically relevant titres of HBcAg. The detection process could be completed within 10 min after sample loading without additional steps for signal amplification, highlighting the practical advantages of the MIPDA-QCM biosensor for point-of-care detection of hepatitis B.

Evaluation of Ru-Ti Electrode-Based TSM Langasite Resonators for High-Temperature Applications.

High-temperature (HT) properties of a thickness-shear mode (TSM) langasite resonator with Ru-Ti electrodes are reported for the first time. Resonators with 300 nm Ru and 15 nm Ti films as the primary and adhesive electrode layers, respectively, were investigated and compared against those with Au-Cr and Au-Ti electrodes. HT stability of the fabricated samples under continuous excitation were examined up to 750 °C by monitoring their morphological changes, sheet resistance, resonance parameters, and their equivalent circuit elements. Results indicate that for Ru-Ti electrodes, a polycrystalline RuO2 cover layer was formed on the surface of Ru, which protected the underlying layer from further oxidation. Consequently, the electrical and motional resistances of the Ru-Ti sample experienced the least change post-annealing, which was also reflected in its ability to retain the highest Q -factor after heat treatment. Ru-Ti-based resonator also exhibited comparable performance to other samples in terms of resonant frequency shifts and second-order temperature coefficients, further strengthening the position of Ru as a suitable alternative to other electrode materials.

Optimal Design of TSM Langasite Resonator for High-Temperature Applications: A Review.

In this review article, we address two vital design considerations that govern the high-temperature operation of a thickness-shear mode langasite resonator: 1) electrode design and 2) electrode material. Optimal electrode designs to mitigate unwanted spurious modes and achieve a high Q-factor for fundamental and higher overtone modes have been discussed in great detail. Governing equations that determine the size, shape, and orientation of these electrodes have also been presented. In addition, the suitability of six platinum-group metals as electrode materials for high-temperature resonators have been assessed and summarized. Furthermore, the adhesion to the substrate, electrical conductivity, thermal stability, and various other temperature-dependent properties of these metals have been discussed. Finally, several combinations and operating ranges of these electrode materials have been thoroughly evaluated.

Quartz crystal microbalance-based biosensors as rapid diagnostic devices for infectious diseases.

Infectious diseases are the ever-present threats to public health and the global economy. Accurate and timely diagnosis is crucial to impede the progression of a disease and break the chain of transmission. Conventional diagnostic techniques are typically time-consuming and costly, making them inefficient for early diagnosis of infections and inconvenient for use at the point of care. Developments of sensitive, rapid, and affordable diagnostic methods are necessary to improve the clinical management of infectious diseases. Quartz crystal microbalance (QCM) systems have emerged as a robust biosensing platform due to their label-free mechanism, which allows the detection and quantification of a wide range of biomolecules. The high sensitivity and short detection time offered by QCM-based biosensors are attractive for the early detection of infections and the routine monitoring of disease progression. Herein, the strategies employed in QCM-based biosensors for the detection of infectious diseases are extensively reviewed, with a focus on prevalent diseases for which improved diagnostic techniques are in high demand. The challenges to the clinical application of QCM-based biosensors are highlighted, along with an outline of the future scope of research in QCM-based diagnostics.

A Langasite Crystal Microbalance Coated with Graphene Oxide-Platinum Nanocomposite as a Volatile Organic Compound Sensor: Detection and Discrimination Characteristics.

We propose a novel langasite crystal microbalance (LCM) sensor with a graphene-based sensing medium to detect and discriminate volatile organic compounds (VOCs) at room temperature. A thin film of graphene oxide embedded with Pt nanostructures (GO-Pt nanocomposite) was deposited on the electrode surface of the LCM, a thickness-shear acoustic wave resonator. Ethyl acetate, acetic acid, and ethanol were chosen as typical VOCs for this study. Sensitivity and selectivity of coated LCM were investigated for different concentrations of the VOCs by analysing the resonant properties of the sensor. When exposed to VOCs, a negative shift in series resonance frequency was observed due to the mass loading of VOC molecules. Simultaneously, changes in equivalent resistance and parallel resonance frequency of the sensor were also observed due to the interaction of VOCs with charge carriers on the GO-Pt nanocomposite film surface. This dual measurement of both series and parallel resonance frequencies allowed for detection and discrimination of VOCs. Moreover, the high thermal stability of langasite makes the proposed sensor suitable even for harsh environmental conditions.

Modeling and Electrode Design Optimizations of Plano-Plano Langasite Crystal Resonator.

The 3-D finite-element model (FEM) of a Y-cut plano-plano langasite crystal thickness shear mode (TSM) resonator is presented, and Mindlin's theory is used to investigate the optimal electrode shapes and sizes for langasite crystal resonator. Circular and elliptical electrodes of various arc lengths are studied to identify the most optimal electrode design configuration in order to achieve TSM vibration free from any anharmonic modes. Simulation results show that resonators with elliptical electrodes have noticeably better suppression of spurious modes compared to that of circular electrodes. Moreover, spurious mode suppression is accomplished for multiple electrode sizes for the same shape, which greatly differs from Mindlin's theory. Hence, three optimized designs are shortlisted and their mass loading sensitivities are investigated. Circular and elliptical electrodes of the same area show similar responses to added mass, indicating that elliptical electrodes have no apparent advantage over circular electrode in mass sensing applications.