Response mechanism for surface acoustic wave gas sensors based on surface-adsorption

Sensing the Future-Frontiers in Biosensors: Exploring Classifications, Principles, and Recent Advances

Biosensors are transforming healthcare by delivering swift, precise, and economical diagnostic solutions. These analytical instruments combine biological indicators with physical transducers to identify and quantify biomarkers, thereby improving illness detection, management, and patient surveillance. Biosensors are widely utilized in healthcare for the diagnosis of chronic and infectious diseases, tailored treatment, and real-time health monitoring. This thorough overview examines several categories of biosensors and their uses in the detection of numerous biomarkers, including glucose, proteins, nucleic acids, and infections. Biosensors are commonly classified based on the type of transducer employed or the specific biorecognition element utilized. This review introduces a novel classification based on substrate morphology, offering a comprehensive perspective on biosensor categorization. Considerable emphasis is placed on the advancement of point-of-care biosensors, facilitating decentralized diagnostics and alleviating the strain on centralized healthcare systems. Recent advancements in nanotechnology have significantly improved the sensitivity, selectivity, and downsizing of biosensors, rendering them more efficient and accessible. The study examines problems such as stability, reproducibility, and regulatory approval that must be addressed to enable the widespread implementation of biosensors in clinical environments. The study examines the amalgamation of biosensors with wearable devices and smartphones, emphasizing the prospects for ongoing health surveillance and individualized medical care. This viewpoint clarifies the distinct types of biosensors and their particular roles, together with recent developments in the "smart biosensor" sector, facilitated by artificial intelligence and the Internet of Medical Things (IoMT). This novel approach seeks to deliver a comprehensive evaluation of the present condition of biosensor technology in healthcare, recent developments, and prospective paths, emphasizing their significance in influencing the future of medical diagnostics and patient care.

A review of piezoelectric MEMS sensors and actuators for gas detection application

Piezoelectric microelectromechanical system (piezo-MEMS)-based mass sensors including the piezoelectric microcantilevers, surface acoustic waves (SAW), quartz crystal microbalance (QCM), piezoelectric micromachined ultrasonic transducer (PMUT), and film bulk acoustic wave resonators (FBAR) are highlighted as suitable candidates for highly sensitive gas detection application. This paper presents the piezo-MEMS gas sensors' characteristics such as their miniaturized structure, the capability of integration with readout circuit, and fabrication feasibility using multiuser technologies. The development of the piezoelectric MEMS gas sensors is investigated for the application of low-level concentration gas molecules detection. In this work, the various types of gas sensors based on piezoelectricity are investigated extensively including their operating principle, besides their material parameters as well as the critical design parameters, the device structures, and their sensing materials including the polymers, carbon, metal-organic framework, and graphene.

A useful quality control using herbal volatiles of Artemisia princeps Pamp. cv. ssajuari (ssajuari-ssuk; Korean mugwort) according to air-drying time by fast gas chromatography with uncoated surface acoustic wave sensor (Electronic zNose)

INTRODUCTION

Korean mugwort has been used as a traditional medicine. Specifically, air-dried Artemisia princeps Pamp. cv. ssajuari (ssajuari-ssuk; Korean mugwort) has been used as moxa in moxibustion therapy. Thus, the evaluation of high quality ssajuari-ssuk containing herbal volatiles is of great interest in clinical therapy and the food industry.

OBJECTIVE

The aim of this study is to determine whether fast gas chromatography with uncoated surface acoustic wave (fast GC/SAW) sensor can be a useful technique for performing quality control using herbal volatiles of ssajuari-ssuk air-dried for different times.

METHODOLOGY

Fast GC/SAW sensor, called "Electronic zNose", is also a headspace sampling method and is an effective simple analytical method with a second unit analysis providing on-line measurements without the need for pretreatment of the sample. Headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME-GC-MS) was employed to confirm the identification of the volatiles and compared to fast GC/SAW sensor.

RESULTS

Artemisia princeps Pamp. cv. ssajuari air-dried for 2 years and 4 months (the third year), containing the highest total amount of herbal volatiles, was superior to the others (the first year, the second year, the fourth year) in quality. Moreover, fast GC/SAW sensor shows a high repeatability (relative standard deviation 0.77% ∼ 6.23%), excellent sensitivity (limit of detection value of 0.47 pg/mL), and good linearity (correlation coefficient r² = 0.997) over the range of nanogram for α-thujone.

CONCLUSION

Therefore, the fast GC/SAW sensor can be a useful analytical method for the discrimination and quality control of volatiles of herbal and medicinal plants according to air-drying time, as it provides a simple and second unit analysis.

A review on two-dimensional materials for chemiresistive- and FET-type gas sensors

Two-dimensional (2D) materials have shown great potential for gas sensing applications due to their large specific surface areas and strong surface activities. In addition to the commonly reported chemiresistive-type gas sensors, field-effect transistor (FET)-type gas sensors have attracted increased attention due to their miniaturized size, low power consumption, and good compatibility with CMOS technology. In this review, we aim to discuss the recent developments in chemiresistive- and FET-type gas sensors based on 2D materials, including graphene, transition metal dichalcogenides, MXenes, black phosphorene, and other layered materials. Firstly, the device structure and the corresponding fabrication process of the two types of sensors are given, and then the advantages and disadvantages are also discussed. Secondly, the effects of intrinsic and extrinsic factors on the sensing performance of 2D material-based chemiresistive and FET-type gas sensors are also detailed. Subsequently, the current gas-sensing applications of 2D material-based chemiresistive- and FET-type gas sensors are systematically presented. Finally, the future prospects of 2D materials in chemiresistive- and FET-type gas sensing applications as well as the current existing problems are pointed out, which could be helpful for the development of 2D material-based gas sensors with better sensing performance to meet the requirements for practical application.

FEM Analysis of Sezawa Mode SAW Sensor for VOC Based on CMOS Compatible AlN/SiO₂/Si Multilayer Structure

A Finite Element Method (FEM) simulation study is conducted, aiming to scrutinize the sensitivity of Sezawa wave mode in a multilayer AlN/SiO₂/Si Surface Acoustic Wave (SAW) sensor to low concentrations of Volatile Organic Compounds (VOCs), that is, trichloromethane, trichloroethylene, carbon tetrachloride and tetrachloroethene. A Complimentary Metal-Oxide Semiconductor (CMOS) compatible AlN/SiO₂/Si based multilayer SAW resonator structure is taken into account for this purpose. In this study, first, the influence of AlN and SiO₂ layers’ thicknesses over phase velocities and electromechanical coupling coefficients (k²) of two SAW modes (i.e., Rayleigh and Sezawa) is analyzed and the optimal thicknesses of AlN and SiO₂ layers are opted for best propagation characteristics. Next, the study is further extended to analyze the mass loading effect on resonance frequencies of SAW modes by coating a thin Polyisobutylene (PIB) polymer film over the AlN surface. Finally, the sensitivity of the two SAW modes is examined for VOCs. This study concluded that the sensitivity of Sezawa wave mode for 1 ppm of selected volatile organic gases is twice that of the Rayleigh wave mode.

Rapid Monitoring of Pharmacological Volatiles of Night-Flowering Evening-Primrose According to Flower Opening or Closing by Fast Gas Chromatography/Surface Acoustic Wave Sensor (Electronic zNose)

INTRODUCTION

Aroma is important in night-flowering species, as visually they can not be observed well. Thus, the analysis of the volatiles of evening-primrose is of great interest in biological fields and therapy. Furthermore, the analysis of volatiles demands rapid and simple procedure, because volatiles decompose.

OBJECTIVE

The aim of this study is to show the rapid monitoring of the volatiles of evening-primrose according to the flowering or closing by fast gas chromatography/surface acoustic wave GC/SAW. Moreover, calibration according to the sensor temperature of the GC/SAW was performed, achieving a high reproducibility and excellent sensitivity.

METHODOLOGY

GC/SAW is an effective analytical method that provides on-line measurements without pretreatment of sample. Headspace solid-phase micro-extraction coupled to gas chromatography mass spectrometry (HS-SPME-GC-MS) and dynamic headspace trapping and extraction with GC-MS were employed to confirm the identification of the volatiles of evening-primrose compared to GC/SAW.

RESULTS

Linalool was found to be the dominant component, comprising 96.4-25.2% of the total amount, according to the opening or closing. Interestingly, the amount of indole also varied according to the opening or closing (3.0-0.0%) such as linalool. Also, while the sensitivity increased with the reduction in the sensor temperature of the GC/SAW, the reproducibility showed a tendency to decrease.

CONCLUSION

The results showed that flower opening is related to the volatiles emission, which is pharmacological and plant defensive. GC/SAW can be a useful analytical method for the rapid monitoring of volatiles of evening-primrose according to the opening or closing as it provides second unit analysis, as well as simple, and aroma pattern recognition. Copyright © 2017 John Wiley & Sons, Ltd.

A Study on AIN Film-Based SAW Attenuation in Liquids and Their Potential as Liquid Ethanol Sensors

In this paper, we report attenuation characteristics of aluminum nitride (AIN) film-based surface acoustic waves (SAWs) in liquids and their potential as liquid ethanol sensors. An AIN film-based SAW resonator was fabricated for liquid sensing application. The fabricated SAW device had a Rayleigh wave mode at a resonant frequency of 147.1 MHz and a low temperature coefficient of frequency (TCF) of −21.7 ppm/K. The signal attenuation in the transmission line of the SAW device was presented when ethanol (ETH) droplets and deionized water (DIW) with different concentrations and volume (0.2–1 µL) were dropped on the sensing area respectively. The attenuation of SAW as a function of time and liquid position was investigated. Residues left on the wave propagation path resulted in a frequency shift of the SAW device after liquid evaporation. For ETH, there was a 49 kHz frequency shift caused by a large amount of residues, while the frequency shift of DIW was not distinct, on account of a clean surface. The linear relationship between evaporation rate and ethanol concentration was demonstrated. The evaporation rate of ethanol droplets showed good consistency, and the evaporation time variation was less than 5% at each concentration level. Therefore, the proposed SAW device had great potentials to determine ethanol concentrations based on evaporation rate.

A Low Noise CMOS Readout Based on a Polymer-Coated SAW Array for Miniature Electronic Nose

An electronic nose (E-Nose) is one of the applications for surface acoustic wave (SAW) sensors. In this paper, we present a low-noise complementary metal-oxide-semiconductor (CMOS) readout application-specific integrated circuit (ASIC) based on an SAW sensor array for achieving a miniature E-Nose. The center frequency of the SAW sensors was measured to be approximately 114 MHz. Because of interference between the sensors, we designed a low-noise CMOS frequency readout circuit to enable the SAW sensor to obtain frequency variation. The proposed circuit was fabricated in Taiwan Semiconductor Manufacturing Company (TSMC) 0.18 μm 1P6M CMOS process technology. The total chip size was nearly 1203 × 1203 μm². The chip was operated at a supply voltage of 1 V for a digital circuit and 1.8 V for an analog circuit. The least measurable difference between frequencies was 4 Hz. The detection limit of the system, when estimated using methanol and ethanol, was 0.1 ppm. Their linearity was in the range of 0.1 to 26,000 ppm. The power consumption levels of the analog and digital circuits were 1.742 mW and 761 μW, respectively.

An Effective Quality Control of Pharmacologically Active Volatiles of Houttuynia cordata Thunb by Fast Gas Chromatography-Surface Acoustic Wave Sensor

Fast gas chromatography-surface acoustic wave sensor (GC/SAW) has been applied for the detection of the pharmacological volatiles emanated from Houttuynia cordata Thunb which is from South Korea. H. cordata Thunb with unpleasant and fishy odors shows a variety of pharmacological activities such as anti-microbial, anti-inflammatory, anti-cancer, and insect repellent. The aim of this study is to show a novel quality control by GC/SAW methodology for the discrimination of the three different parts of the plant such as leaves, aerial stems, and underground stems for H. cordata Thunb. Sixteen compounds were identified. β-Myrcene, cis-ocimene and decanal are the dominant volatiles for leaves (71.0%) and aerial stems (50.1%). While, monoterpenes (74.6%) are the dominant volatiles for underground stems. 2-Undecanone (1.3%) and lauraldehyde (3.5%) were found to be the characteristic components for leaves. Each part of the plant has its own characteristic fragrance pattern owing to its individual chemical compositions. Moreover, its individual characteristic fragrance patterns are conducive to discrimination of the three different parts of the plant. Consequently, fast GC/SAW can be a useful analytical method for quality control of the different parts of the plant with pharmacological volatiles as it provides second unit analysis, a simple and fragrant pattern recognition.

The development of love wave-based humidity sensors incorporating multiple layers

A Love wave humidity sensor is developed by using a multilayer structure consisting of PVA/SiO2 layers on an ST-90°X quartz substrate. The theoretical result shows that the sensor with such a two-layer structure can achieve a higher sensitivity and a smaller loss than the structures with a single polymer layer. Comparative experiments are performed for the sensor incorporating PVA/SiO2 layers and the sensor incorporating a PVA layer. The experimental results agree well with the theoretical predication.