Acoustic microsensors are commonly known as high-resolution mass-sensitive devices. This is a restricted view in many chemical and biosensor applications, especially in liquids. Sensitivity to non-gravimetric effects is a challenging feature of acoustic sensors. In this review we give an overview of recent developments in resonant sensors including micromachined devices and also list recent activity relating to the (bio)chemical interface of acoustic sensors. Major results from theoretical analysis of quartz crystal resonators, descriptive for all acoustic microsensors are summarized, and non-gravimetric contributions to the sensor signal from viscoelasticity and interfacial effects are discussed. We finally conclude with some future perspectives.

Acoustic microsensors—the challenge behind microgravimetry

In recent years, interest in the development of highly sensitive acoustic wave devices as biosensor platforms has grown. A considerable amount of research has been conducted on AT-cut quartz resonators both in thickness excitation and in lateral excitation configurations. In this report, we demonstrate the fabrication of a ZnO solidly mounted resonator operated in thickness shear mode (TSM) using lateral field excitation of the piezoelectric film. Theoretical Christoffel equation calculations are provided to explore the conditions for excitation of a TSM wave in c-axis-oriented ZnO through lateral excitation. The existence of a TSM wave is verified through the comparison of theoretical and experimentally obtained acoustic velocity values from frequency versus thickness measurements and water loading of the resonators. A major strength of this design is that it incorporates a simple eight-layer, single-mask fabrication process compatible with existing integrated circuit fabrication processes and can be eas...

Lateral field excitation of thickness shear mode waves in a thin film ZnO solidly mounted resonator

Nonspecific-adsorption behavior of polyethylenglycol and bovine serum albumin studied by 55-MHz wireless-electrodeless quartz crystal microbalance.

AT-cut quartz crystals have been driven with the so-called lateral field excitation. When applying a voltage to the two electrodes placed on one crystal surface and separated by a small gap an electric field is generated, which is mainly confined in the lateral direction. Extraordinary changes can be observed in the conductance spectrum when applying a liquid to the other (bare) surface of the crystal. In contrast to traditional quartz crystal sensors, these changes must be attributed to electrical properties of the adjacent medium. It is suggested that a redistribution of the exciting electric field from the lateral toward the thickness direction occurs. The assumption is supported by simulations which allow insights into the piezoelectric excitation and transduction mechanism of the acoustic device under varied electrical boundary conditions at the bare surface as well as the shear displacement patterns involved. Results from 10 MHz plano–plano and 6 MHz plano–convex sensors which have been exposed to liquids of varying permittivity show the strong dependence of the sensor response on liquid permittivity which overlies the known dependence on density–viscosity.

https://iopscience.iop.org/article/10.1088/0957-0233/19/5/055201/pdf

Quartz crystal resonator sensors under lateral field excitation—a theoretical and experimental analysis

The most popular bulk acoustic wave (BAW) sensor is the quartz crystal microbalance (QCM), which has electrodes on both the top and bottom surfaces of an AT-cut quartz wafer. In the QCM, the exciting electric field is primarily perpendicular to the crystal surface, resulting in a thickness field excitation (TFE) of a resonant temperature compensated transverse shear mode (TSM). The TSM, however, can also be excited by lateral field excitation (LFE) in which electrodes are placed on one side of the wafer leaving a bare sensing surface exposed directly to a liquid or a chemi/bio selective layer allowing the detection of both mechanical and electrical property changes caused by a target analyte. The use of LFE sensors has motivated an investigation to identify other piezoelectric crystal orientations that can support temperature-compensated TSMs and operate efficiently at high frequencies resulting in increased sensitivity. In this work, theoretical search and experimental measurements are performed to identify the existence of high-frequency temperature-compensated TSMs in LiTaO3. Prototype LFE LiTaO3 sensors were fabricated and found to operate at frequencies in excess of 1 GHz and sensitively detect viscosity, conductivity, and dielectric constant changes in liquids.

A lateral-field-excited LiTaO 3 high-frequency bulk acoustic wave sensor

Lateral field excited (LFE) AT-cut quartz acoustic wave sensors in which the electrodes are located on the reference surface have been fabricated and tested in liquid environments. The sensing surface, which is opposite to the reference surface, is free allowing the electric field of the thickness shear mode (TSM) to penetrate into the liquid. This results in increased sensitivity to both mechanical and electrical property changes of the liquid. In the present paper, several 5-MHz LFE sensors with a range of electrode spacings were exposed to liquid environments in which the viscosity, relative permittivity, and conductivity were varied. The LFE sensors demonstrate sensitivity to viscosity that is more than twice that obtained for the standard quartz crystal microbalance (QCM), and sensitivity to relative permittivity and conductivity about 1.5 times that of the QCM sensors with modified electrodes. The present results clearly indicate that the LFE sensors may have a wide range of liquid phase applications in which sensitivity is crucial.

A lateral field excited liquid acoustic wave sensor

Although organophosphate pesticides have been widely used in agriculture to control pests, they can have adverse effects on the environment and also enter the food chain causing adverse health effects in humans. Therefore, a need exists to develop a low-cost, robust, sensitive, and portable sensor for organophosphate pesticides. A lateral field excited (LFE) acoustic wave sensor has been developed in which the sensing surface is bare and the external excitation is applied to the reference surface. The LFE sensor is sensitive to both mechanical and electrical property changes in the monitored solution. In the present work, the LFE sensor is used as the sensing platform to detect the organophosphate pesticide, phosmet (C11H12NO4PS2). A polyepichlorohydrin (PECH) polymer film is applied to the sensing surface to detect phosmet in solution. The results show that the PECH-coated LFE sensor displays a nearly linear response to the addition of phosmet with a slope of 20 Hz/ppm and detection limit in the ppb range.

Pesticide detection using a lateral field excited acoustic wave sensor

Bacteria have brought great harm to the public, especially after the emergence of multidrug-resistant bacteria. This has rendered traditional antibiotic therapy ineffective. In recent years, hyperthermia has offered new treatments to remove bacteria. Microwaves (MW) are a component of the electromagnetic spectrum and can rapidly heat materials. Taking advantage of this characteristic of MW, related studies have shown that both thermal and non-thermal effects of MW can inactivate various bacteria. Even though the understanding of MW in the field of bacteria is not sufficient for widespread use at present, MW has performed well in dealing with microorganisms and controlling infection. This review will focus on the application of MW in bacteria and discuss the advantages, prospects and challenges of using MW in the bacterial field.

Microwaves, a potential treatment for bacteria: A review

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A lateral field excited liquid acoustic wave sensor

Pesticide detection using a lateral field excited acoustic wave sensor

Microwaves, a potential treatment for bacteria: A review