Surface acoustic wave (SAW) technology provides a sensitive platform for sensing chemicals in gaseous and fluidic states with the inherent advantages of passive and wireless operation. In this review, we provide a general overview on the fundamental aspects and some major advances of Rayleigh wave-based SAW sensors in sensing chemicals in a gaseous phase. In particular, we review the progress in general understanding of the SAW chemical sensing mechanism, optimization of the sensor characteristics, and the development of the sensors operational at different conditions. Based on previous publications, we suggest some appropriate sensing approaches for particular applications and identify new opportunities and needs for additional research in this area moving into the future.

Corrosion has been a great concern in the oil and natural gas industry costing billions of dollars annually in the U.S. The ability to monitor corrosion online before structural integrity is compromised can have a significant impact on preventing catastrophic events resulting from corrosion. This article critically reviews conventional corrosion sensors and emerging sensor technologies in terms of sensing principles, sensor designs, advantages, and limitations. Conventional corrosion sensors encompass corrosion coupons, electrical resistance probes, electrochemical sensors, ultrasonic testing sensors, magnetic flux leakage sensors, electromagnetic sensors, and in-line inspection tools. Emerging sensor technologies highlight optical fiber sensors (point, quasi-distributed, distributed) and passive wireless sensors such as passive radio-frequency identification sensors and surface acoustic wave sensors. Emerging sensors show great potential in continuous real-time in-situ monitoring of oil and natural gas infrastructure. Distributed chemical sensing is emphasized based on recent studies as a promising method to detect early corrosion onset and monitor corrosive environments for corrosion mitigation management. Additionally, challenges are discussed including durability and stability in extreme and harsh conditions such as high temperature high pressure in subsurface wellbores.

The integration of nanoporous materials such as metal organic frameworks (MOFs) with sensitive transducers can result in robust sensing platforms for monitoring gases and chemical vapors for a range of applications. Here, we report on an integration of the zeolitic imidazolate framework – 8 (ZIF-8) MOF with surface acoustic wave (SAW) and thickness shear mode quartz crystal microbalance (QCM) devices to monitor carbon dioxide (CO₂) and methane (CH₄) under ambient conditions. The MOF was directly coated on the Y-Z LiNbO₃ SAW delay lines (operating frequency, f₀ = 436 MHz) and AT-cut quartz TSM resonators (resonant frequency, f₀ = 9 MHz) and the devices were tested for various gases in N₂ under ambient conditions. The devices were able to detect the changes in CO₂ or CH₄ concentrations with relatively higher sensitivity to CO₂, which was due to its higher adsorption potential and heavier molecular weight. The sensors showed full reversibility and repeatability which were attributed to the physisorption of the gases into the MOF and high stability of the devices. Both types of sensors showed linear responses relative to changes in the binary gas compositions thereby allowing to construct calibration curves which correlated well with the expected mass changes in the sorbent layer based on mixed-gas gravimetric adsorption isotherms measured on bulk samples. For 200 nm thick films, the SAW sensitivities to CO₂ and CH₄ were 1.44 × 10⁻⁶/vol% and 8 × 10⁻⁸/vol%, respectively, against the QCM sensitivities 0.24 × 10⁻⁶/vol% and 1 × 10⁻⁸/vol%, respectively, which were evaluated as the fractional change in the signal. The SAW sensors were also evaluated for 100 nm–300 nm thick films, the sensitivities of which were found to increase with the thickness due to the increased number of pores for the adsorption of a larger amount of gases. In addition, the MOF-coated SAW delay lines had a good response in wireless mode, demonstrating their potential to operate remotely for the detection of the gases at emission sites across the energy infrastructure.

A small DC magnetic field can induce an enormous response in the impedance of a soft magnetic conductor in various forms of wire, ribbon, and thin film. Also known as the giant magnetoimpedance (GMI) effect, this phenomenon forms the basis for the development of high-performance magnetic biosensors with magnetic field sensitivity down to the picoTesla regime at room temperature. Over the past decade, some state-of-the-art prototypes have become available for trial tests due to continuous efforts to improve the sensitivity of GMI biosensors for the ultrasensitive detection of biological entities and biomagnetic field detection of human activities through the use of magnetic nanoparticles as biomarkers. In this review, we highlight recent advances in the development of GMI biosensors and review medical devices for applications in biomedical diagnostics and healthcare monitoring, including real-time monitoring of respiratory motion in COVID-19 patients at various stages. We also discuss exciting research opportunities and existing challenges that will stimulate further study into ultrasensitive magnetic biosensors and healthcare monitors based on the GMI effect.

Wireless and passive surface acoustic wave (SAW) devices with nanoporous metal-organic framework (MOF) sensing layers are attractive gas sensors for applications in many fields such as energy industries and air pollution control. Here, we report on enhancing the sensitivity and detection limit of zeolitic imidazolate framework-8 (ZIF-8) MOF-coated SAW reflective delay line mass sensors by increasing the operating frequency for sensitive detection of carbon dioxide (CO ₂ ) and methane (CH ₄ ) at ambient conditions. In particular, we show at least four times higher sensitivity of an 860 MHz (4- $\mu \text{m}$ periodicity) SAW reflective delay line coated with a 240 nm thick ZIF-8 compared to the sensitivity of a 430 MHz (8- $\mu \text{m}$ periodicity) otherwise identical sensor device to the targeted gases. The detection limits of the higher frequency wireless devices for CO ₂ and CH ₄ were estimated to be 0.91 vol-% and 7.01 vol-%, respectively. The enhanced sensitivity for higher frequency devices is explained in terms of the frequency dependent acoustic wave energy confinement.

Magnetic anisotropy plays an important role in determining the magnetic functionality of thin film based electronic devices. We present here, the first systematic study of the correlation between magnetoresistance (MR) response in organic spin valves (OSVs) and magnetic anisotropy of the bottom ferromagnetic electrode over a wide temperature range (10 K–350 K). The magnetic anisotropy of a La0.67Sr0.33MnO3 (LSMO) film epitaxially grown on a SrTiO3 (STO) substrate was manipulated by reducing film thickness from 200 nm to 20 nm. Substrate-induced compressive strain was shown to drastically increase the bulk in-plane magnetic anisotropy when the LSMO became thinner. In contrast, the MR response of LSMO/OSC/Co OSVs for many organic semiconductors (OSCs) does not depend on either the in-plane magnetic anisotropy of the LSMO electrodes or their bulk magnetization. All the studied OSV devices show a similar temperature dependence of MR, indicating a similar temperature-dependent spinterface effect irrespective of LSMO thickness, resulting from the orbital hybridization of carriers at the OSC/LSMO interface