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

A GaN-based light-emitting diode (LED) with an aluminum (Al) reflecting and an SiO2 insulating layers (RILs) deposited on the naturally textured p-GaN surface is fabricated and studied. The use of RIL could enhance the current spreading performance and reduce the photon absorption by the p-pad metal. The textured surface is used to limit the total internal reflection and increase photon scattering. In this paper, effects of the use of an Al RL and/or an SiO2 insulating layer on the performance of GaN-based LEDs are systematically studied and compared in detail. At 20 mA, as compared with a conventional LED with naturally textured (planar) p-GaN surface, the studied device exhibits 12.2% (55.5%) enhancement in light output power. Additionally, a 28.5% (95%) increment of luminous flux is achieved. The studied device also shows 15.6% light intensity improvement of far-filed pattern. Experimentally, although power consumption and junction temperature are slightly increased because of the insertion of RIL structure, these drawbacks could be surpassed by the mentioned optical improvements. Therefore, for conventional GaN-based LEDs, light extraction efficiency could be further improved by the employment of RIL structure.

Effects of the Use of an Aluminum Reflecting and an ${\rm SiO}_{2}$ Insulating Layers (RIL) on the Performance of a GaN-Based Light-Emitting Diode With the Naturally Textured p-GaN Surface

The hydrogen-sensing characteristics of a Pd/AlGaN/GaN heterostructure field-effect transistor (HFET) under a nitrogen ambience are studied in this paper. Good and stable hydrogen-sensing behaviors are obtained over the operating temperature from 30 $^{\circ}{\rm C}$ to 250 $^{\circ}{\rm C}$ . In addition, HFET shows the significant hydrogen-detecting ability under an extremely low hydrogen concentration of 10-ppb ${\rm H}_{2}/{\rm N}_{2}$ . Good transient responses are also observed even at room temperature. In addition, a small and nearly constant value of recovery time $({\approx}{\rm 20}~{\rm s})$ is acquired when the hydrogen concentration is higher than 1-ppm ${\rm H}_{2}/{\rm N}_{2}$ at room temperature. Therefore, the studied device shows a promise for high-performance, high-temperature electronics, microsensors, and microelectromechanical system applications.

Hydrogen-Sensing Properties of a Pd/AlGaN/GaN-Based Field-Effect Transistor Under a Nitrogen Ambience

Superconducting diodes are proposed nonreciprocal circuit elements that should exhibit nondissipative transport in one direction while being resistive in the opposite direction. Multiple examples of such devices have emerged in the past couple of years; however, their efficiency is typically limited, and most of them require a magnetic field to function. Here we present a device that achieves efficiencies approaching 100% while operating at zero field. Our samples consist of a network of three graphene Josephson junctions linked by a common superconducting island, to which we refer as a Josephson triode. The three-terminal nature of the device inherently breaks the inversion symmetry, and the control current applied to one of the contacts breaks the time-reversal symmetry. The triode's utility is demonstrated by rectifying a small (nA scale amplitude) applied square wave. We speculate that devices of this type could be realistically employed in the modern quantum circuits.

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Nonreciprocal Supercurrents in a Field-Free Graphene Josephson Triode.

The interesting ZnO nanoparticles (NPs)-based sensor devices are fabricated on sapphire substrates to study the nitrogen oxide (NO2) gas sensing performance. The postheat treatment process is not used to avoid the undesired agglomeration phenomenon. Different concentrations (0.1 and 0.01 wt%) of ZnO NPs are employed to produce the studied NO2 gas sensors. Experimentally, studied devices demonstrate good NO2 sensing responses $\sim 100$ (at 5 ppm NO2/air, 473 K). Especially, an extremely low sensing limit of 10 ppb NO2/air is obtained for the studied devices. This value is superior to those of previously reported results from other NO2 sensors. The studied sensor devices also show the advantages of low cost, easy fabrication, and compact potentiality to integrate with other high-speed and optoelectronic devices. Therefore, the studied devices give the promise for high-performance NO2 monitoring applications.

Chun-Chia Chen

Papers

Effects of the Use of an Aluminum Reflecting and an ${\rm SiO}_{2}$ Insulating Layers (RIL) on the Performance of a GaN-Based Light-Emitting Diode With the Naturally Textured p-GaN Surface

Hydrogen-Sensing Properties of a Pd/AlGaN/GaN-Based Field-Effect Transistor Under a Nitrogen Ambience

Nonreciprocal Supercurrents in a Field-Free Graphene Josephson Triode.

Nitrogen Oxide (NO 2 ) Gas Sensing Performance of ZnO Nanoparticles (NPs)/Sapphire-Based Sensors

Characteristics of a Pd/AlGaN/GaN Transistor Processed Using the Sensitization, Activation, and Electroless Plating (EP) Approaches