Metal oxide semiconductor gas sensors are utilised in a variety of different roles and industries. They are relatively inexpensive compared to other sensing technologies, robust, lightweight, long lasting and benefit from high material sensitivity and quick response times. They have been used extensively to measure and monitor trace amounts of environmentally important gases such as carbon monoxide and nitrogen dioxide. In this review the nature of the gas response and how it is fundamentally linked to surface structure is explored. Synthetic routes to metal oxide semiconductor gas sensors are also discussed and related to their affect on surface structure. An overview of important contributions and recent advances are discussed for the use of metal oxide semiconductor sensors for the detection of a variety of gases—CO, NOx, NH3 and the particularly challenging case of CO2. Finally a description of recent advances in work completed at University College London is presented including the use of selective zeolites layers, new perovskite type materials and an innovative chemical vapour deposition approach to film deposition.

https://www.mdpi.com/1424-8220/10/6/5469/pdf

Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring

Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review

Implantable medical devices (IMDs) have experienced a rapid progress in recent years to the advancement of state-of-the-art medical practices. However, the majority of this equipment requires external power sources like batteries to operate, which may restrict their application for in vivo situations. Furthermore, these external batteries of the IMDs need to be changed at times by surgical processes once expired, causing bodily and psychological annoyance to patients and rising healthcare financial burdens. Currently, harvesting biomechanical energy in vivo is considered as one of the most crucial energy-based technologies to ensure sustainable operation of implanted medical devices. This review aims to highlight recent improvements in implantable triboelectric nanogenerators (iTENG) and implantable piezoelectric nanogenerators (iPENG) to drive self-powered, wireless healthcare systems. Furthermore, their potential applications in cardiac monitoring, pacemaker energizing, nerve-cell stimulating, orthodontic treatment and real-time biomedical monitoring by scavenging the biomechanical power within the human body, such as heart beating, blood flowing, breathing, muscle stretching and continuous vibration of the lung are summarized and presented. Finally, a few crucial problems which significantly affect the output performance of iTENGs and iPENGs under in vivo environments are addressed.

Recent Advances in Nanogenerator-Driven Self-Powered Implantable Biomedical Devices

Al-doped zinc oxide (ZnO) thin films were prepared by chemical spray pyrolysis technique. The dopant concentration (Al/Zn at%) varied from 0 to 1.5 at%. Structural analysis of the films reveals that all the films are of polycrystalline zinc oxide in nature, possessing hexagonal wurtzite structure with (0 0 2) preferred orientation. The lattice constants calculated from the most prominent peaks are found to be in good agreement with the ICDD reference pattern: zinc oxide, 01-080-0074 (a = 3.2535 A and c = 5.2151 A). The sensing properties of the films towards methanol vapour are investigated for various concentrations of methanol in air at different operating temperatures in the range 200–350 °C. It is observed that compared to the undoped ZnO film, Al-doped films show higher sensitivity to methanol vapour. Among all the Al-doped films studied, the 0.5 at% Al-doped ZnO film shows the maximum response (∼44%) at 275 °C to 500 ppm of methanol vapour in air. Further, the films show fast response and recovery to methanol vapour at higher operating temperatures. The methanol-sensing mechanism of the film has been explained.

Al-doped ZnO thin films as methanol sensors

The research and development of new electronic-nose applications in the biomedical field has accelerated at a phenomenal rate over the past 25 years. Many innovative e-nose technologies have provided solutions and applications to a wide variety of complex biomedical and healthcare problems. The purposes of this review are to present a comprehensive analysis of past and recent biomedical research findings and developments of electronic-nose sensor technologies, and to identify current and future potential e-nose applications that will continue to advance the effectiveness and efficiency of biomedical treatments and healthcare services for many years. An abundance of electronic-nose applications has been developed for a variety of healthcare sectors including diagnostics, immunology, pathology, patient recovery, pharmacology, physical therapy, physiology, preventative medicine, remote healthcare, and wound and graft healing. Specific biomedical e-nose applications range from uses in biochemical testing, blood-compatibility evaluations, disease diagnoses, and drug delivery to monitoring of metabolic levels, organ dysfunctions, and patient conditions through telemedicine. This paper summarizes the major electronic-nose technologies developed for healthcare and biomedical applications since the late 1980s when electronic aroma detection technologies were first recognized to be potentially useful in providing effective solutions to problems in the healthcare industry.

https://www.mdpi.com/1424-8220/11/1/1105/pdf

Advances in Electronic-Nose Technologies Developed for Biomedical Applications

Thin films of indium tin oxide (In2O3+SnO2) (ITO) were grown on alumina substrate using the direct evaporation method. The sensing characteristics of these films to methanol vapors were studied at room temperature. The effects of different concentrations of tin oxide in indium oxide and film thickness on sensitivity were studied. The effects of thin layers of noble metals—Au, Ag, Pt and Cu on top of ITO films were studied. A novel approach of stimulated recrystallization of ITO film by depositing ITO film over ultra thin layer of Cu stimulator was used to improve the sensitivity, selectivity and minimum detection limit of methanol sensors operating at room temperature.

Indium tin oxide (ITO) thin film gas sensor for detection of methanol at room temperature

Indium Tin Oxide thin film gas sensors for detection of ethanol vapours

Benzene being a major pollutant and a potential human carcinogen, development of a sensitive and selective sensor for its detection is the need of the hour In the present work, the studies on the development of indium tin oxide thin film sensor for detection of benzene are presented Thin films of indium tin oxide (ITO) were deposited using thermal evaporation technique, and, under the optimized fabrication conditions, sets of sensors were fabricated Their response to benzene vapours was studied The sensing mechanism explaining the redox reactions on the surface of ITO has been proposed The films were coated with thin layers of metals and oxides to enhance the performance for the sensitive and selective detection of benzene Cross sensitivity of benzene with respect to toluene was studied and selectivity was obtained with a two layered structure of Cr/ITO

Development of ITO thin film sensor for detection of benzene

Preparation and characterization of indium tin oxide thin films for their application as gas sensors

Indium tin oxide (ITO) polycrystalline thin films grown on alumina substrates by the thermal evaporation technique are used for fabricating gas sensors to detect carbon tetrachloride (CCl4). The electrical conductance of the sensors appears to increase and decrease on exposure to gaseous CCl4, depending on the operating temperature. The thin-film gas sensors with a thickness of about 100 nm shows a maximum sensitivity when operated at 448 K for various concentrations of CCl4. A tentative mechanism is proposed for the role of CCl4 gas in increasing and decreasing the conductance of ITO thin-film gas sensors with changes of operating temperature.

V.S. Vaishnav

Papers

Indium tin oxide (ITO) thin film gas sensor for detection of methanol at room temperature

Indium Tin Oxide thin film gas sensors for detection of ethanol vapours

Development of ITO thin film sensor for detection of benzene

Preparation and characterization of indium tin oxide thin films for their application as gas sensors

Fabrication of carbon tetrachloride gas sensors using indium tin oxide thin films