Anatomy and Physiology in Health and Illness

Continuous, real-time monitoring of the level of pH and free chlorine in drinking water is of great importance to public health. However, it is challenging when conventional analytical instruments, such as bulky pH electrodes and expensive free chlorine meters, are used. These instruments have slow response, are difficult to use, prone to interference from operators, and require frequent maintenance. In contrast, microfabricated electrochemical sensors are cheaper, smaller in size, and highly sensitive. Therefore, these sensors are desirable for online monitoring of pH and free chlorine in water. In this review, we discuss different physical configurations of microfabricated sensors. These configurations include potentiometric electrodes, ion-sensitive field-effect transistors, and chemo-resistors/transistors for electrochemical pH sensing. Also, we identified that micro-amperometric sensors are the dominant ones used for free chlorine sensing. We summarized and compared the structure, operation/sensing mechanism, applicable materials, and performance parameters in terms of sensitivity, sensing range, response time and stability of each type of sensor. We observed that novel sensor structures fabricated by solution processing and operated by smart sensing methodologies may be used for developing pH and free chlorine sensors with high performance and low cost. Finally, we highlighted the importance of the concurrent design of materials, fabrication processes, and electronics for future sensors.

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Microfabricated electrochemical pH and free chlorine sensors for water quality monitoring: recent advances and research challenges

Ultra thin NiO nanosheets for high performance hydrogen gas sensor device

This paper presents an overview of semiconductor materials used in gas sensors, their technology, design, and application. Semiconductor materials include metal oxides, conducting polymers, carbon nanotubes, and 2D materials. Metal oxides are most often the first choice due to their ease of fabrication, low cost, high sensitivity, and stability. Some of their disadvantages are low selectivity and high operating temperature. Conducting polymers have the advantage of a low operating temperature and can detect many organic vapors. They are flexible but affected by humidity. Carbon nanotubes are chemically and mechanically stable and are sensitive towards NO and NH3, but need dopants or modifications to sense other gases. Graphene, transition metal chalcogenides, boron nitride, transition metal carbides/nitrides, metal organic frameworks, and metal oxide nanosheets as 2D materials represent gas-sensing materials of the future, especially in medical devices, such as breath sensing. This overview covers the most used semiconducting materials in gas sensing, their synthesis methods and morphology, especially oxide nanostructures, heterostructures, and 2D materials, as well as sensor technology and design, application in advance electronic circuits and systems, and research challenges from the perspective of emerging technologies.

https://www.mdpi.com/1424-8220/20/22/6694/pdf

Semiconductor Gas Sensors: Materials, Technology, Design, and Application

Improved selectivity and low concentration hydrogen gas sensor application of Pd sensitized heterojunction n-ZnO/p-NiO nanostructures

An interesting ammonia gas sensor based on a p-type NiO thin film, prepared by a radio frequency sputtering process, is studied and demonstrated. As compared with conventional n-type metal-oxide sensors, the studied device shows comparable and good sensing performance, including a high-sensing response ratio of 289%, a lower response time of 31 s, and a lower recovery time of 78 s, under an introduced 1000 ppm NH3/air gas at 250 °C and 350 °C, respectively. In addition, the studied sensor device exhibits a lower detection limit (<5 ppm NH3/air) at 250 °C. Consequently, based on these advantages and inherent benefits of low cost, chemical stability, and easy fabrication, etc., the studied NiO thin-film sensor shows the promise for high-performance ammonia gas sensing applications.

On the Ammonia Gas Sensing Performance of a RF Sputtered NiO Thin-Film Sensor

Hydrogen sensing performance of a nickel oxide (NiO) thin film-based device

An ammonia sensor based on a Pt/AlGaN/GaN Schottky diode, fabricated by the electroless plating (EP) technique, has been studied in this work. The studied sensor device shows a significant sensing response under an extremely low ammonia concentration of 10 ppb NH 3 /air at 115 °C. As exposed to a 1000 ppm NH 3 /air gas, a high sensing response of 16.22 with a response (recovery) time of 8.73 (2.04) min is obtained. Even at room temperature (25 °C), the studied sensor exhibits good ammonia sensing performance with a sensing response of 2.68 at 1000 ppm NH 3 /air and a low detection limit of 1 ppm NH 3 /air. Based on the excellent sensing performance and inherent advantages of low-power consumption and low-temperature operation, the studied sensor device provides the promise for high-performance ammonia sensing applications.

Study of an electroless plating (EP)-based Pt/AlGaN/GaN Schottky diode-type ammonia sensor

A GaN-based ion sensitive field-effect transistor (ISFET) prepared by a hydrogen peroxide (H 2 O 2 ) treatment is fabricated and studied. A 3-nm-thick Ga x O y layer formed by an immersion in H 2 O 2 solution is examined and confirmed by EDS and XPS analyses. Experimentally, the studied pH-ISFET presents a higher voltage sensitivity (54.88 mV/pH), a higher current sensitivity (−56.09 μA/pH mm), a lower drift rate (1.41 μA/h mm), an extremely low hysteresis (0.4 mV), and a lower voltage decay rate (−0.14 mV/pH day) after 28 days. Moreover, insignificant interference effects from Na + and K + ions were observed. Thus, the studied GaN-based ISFET utilizing an H 2 O 2 treatment promises to fabricate high-performance pH sensing applications.

On a GaN-based ion sensitive field-effect transistor (ISFET) with a hydrogen peroxide surface treatment

In this work, enhanced hydrogen sensing characteristics of a GaN-based Schottky diode-type sensor with a GaOx layer are studied and demonstrated. A thin GaOx layer inserted in Pd/GaN interface is oxidized by the immersion in an H2O2 solution at room temperature. Experimentally, a significantly high hydrogen sensing response of 1.8 × 105 and a large Schottky barrier height variation ratio of 33.1% are found upon exposure to a 1% H2/air gas at 300 K. In addition, a very low detection limit of 0.1 ppm H2/air at 300 K is obtained. These improved properties could be attributed to the effective dissociation of hydrogen molecules and rougher Pd surface caused by the presence of the GaOx layer. The response (recovery) time constant of 13.3 (23.6) s is obtained upon exposure to a 1% H2/air gas at 300 K. The related hydrogen adsorption analysis of the proposed device is also studied and demonstrated.

Chun-Chia Chen

Papers

On the Ammonia Gas Sensing Performance of a RF Sputtered NiO Thin-Film Sensor

Hydrogen sensing performance of a nickel oxide (NiO) thin film-based device

Study of an electroless plating (EP)-based Pt/AlGaN/GaN Schottky diode-type ammonia sensor

On a GaN-based ion sensitive field-effect transistor (ISFET) with a hydrogen peroxide surface treatment

Enhancement of hydrogen sensing performance of a GaN-based Schottky diode with a hydrogen peroxide surface treatment