Metal Oxide Sensors Research Articles

A Linear-Quadratic Model for the Quantification of a Mixture of Two Diluted Gases with a Single Metal Oxide Sensor.

The aim of our work is to quantify two gases (acetone and ethanol) diluted in an air buffer using only a single metal oxide (MOX) sensor. We took advantage of the low selectivity of the MOX sensor, exploiting a dual-temperature mode. Working at two temperatures of the MOX sensitive layer allowed us to obtain diversity in the measures. Two virtual sensors were created to characterize our gas mixture. We presented a linear-quadratic mixture sensing model which was closer to the experimental data. To validate this model and the experimental protocol, we inverted the system of quadratic equations to quantify a mixture of the two gases. The linear-quadratic model was compared to the bilinear model proposed in the literature. We presented an experimental evaluation on mixtures made of a few ppm of acetone and ethanol, and we obtained a precision close to the ppm. This is an important step towards medical applications, particularly in terms of diabetes, to deliver a non-invasive measure with a low-cost device.

Ultra-Low-Power Smart Electronic Nose System Based on Three-Dimensional Tin Oxide Nanotube Arrays.

In this work, we present a high-performance smart electronic nose (E-nose) system consisting of a multiplexed tin oxide (SnO2) nanotube sensor array, read-out circuit, wireless data transmission unit, mobile phone receiver, and data processing application (App). Using the designed nanotube sensor device structure in conjunction with multiple electrode materials, high-sensitivity gas detection and discrimination have been achieved at room temperature, enabling a 1000 times reduction of the sensor's power consumption as compared to a conventional device using thin film SnO2. The experimental results demonstrate that the developed E-nose can identify indoor target gases using a simple vector-matching gas recognition algorithm. In addition, the fabricated E-nose has achieved state-of-the-art sensitivity for H2 and benzene detection at room temperature with metal oxide sensors. Such a smart E-nose system can address the imperative needs for distributed environmental monitoring in smart homes, smart buildings, and smart cities.

Technology review: prototyping platforms for monitoring ambient conditions

The monitoring of ambient conditions in indoor spaces is very essential owing to the amount of time spent indoors. Specifically, the monitoring of air quality is significant because contaminated air affects the health, comfort and productivity of occupants. This research work presents a technology review of prototyping platforms for monitoring ambient conditions in indoor spaces. It involves the research on sensors (for CO2, air quality and ambient conditions), IoT platforms, and novel and commercial prototyping platforms. The ultimate objective of this review is to enable the easy identification, selection and utilisation of the technologies best suited for monitoring ambient conditions in indoor spaces. Following the review, it is recommended to use metal oxide sensors, optical sensors and electrochemical sensors for IAQ monitoring (including NDIR sensors for CO2 monitoring), Raspberry Pi for data processing, ZigBee and Wi-Fi for data communication, and ThingSpeak IoT platform for data storage, analysis and visualisation.

Use of electronic nose to discriminate meats from bulls fed diet with or without flaxseed inclusion and subjected to different aging periods

A metal-oxide sensors array electronic nose (e-nose) was used to discriminate beef loins (Longissimus thoracis) obtained from Piemontese bulls fed without or with flaxseed and subjected to 3 different aging periods (2, 7, 10 days) at 4°C. At 7 days of aging, samples were also assessed for flavor intensity by panelists. A comparison between e-nose and panel assessments was performed subjecting a 7 days e-nose reading on cooked meat to partial least square regression for flavor prediction. The e-nose could not discriminate populations in meat samples, however it could represent a valuable tool in supporting flavor scoring from sensory evaluation.

A Comparative Study on Gas Sensing Performance of Photo-Reduced GO with TiO2 and ZnO

Recently, much interest has been placed on wearable electronics, for example wearable gas sensor. With wearability, gas sensor will be more pragmatic to be utilized for everyman to monitor health conditions by checking their breath and/or by protecting themselves ubiquitously from hazardous gases. [1] To improve the performance of wearable gas sensor, essential functions of gas sensor such as sensitivity, selectivity, and stability should be more developed as adapting the conditions to be wearable. Among many approaches to enhance these functions, incorporating two nanomaterials have been adapted in metal oxide sensor (MOS) due to their synergistic effect on gas performance. i.e. SnO2/ZnO composite. [2] When contacting two different types of materials, heterojunction is created at the interface by transferring charge carriers, resulting in enhanced sensing performance. For low temperature operation, two-dimensional (2D) materials such as graphene oxide (GO) has been introduced as a sensing material. [3] Due to its unique characteristics, GO has successfully demonstrated room temperature gas sensing to ammonia and NO2. In a similar manner of MOS, hybrid GO/metal oxide composites have been attempted to enhance sensing performance. By hybridizing two different materials, synergistic effect on heterojunction increased gas selectivity, and tailored morphology of the composite enhanced gas response. [4] To increase conductivity of the composite at room temperature, chemical or thermal reduction process typically accompanies. However, chemical reduction using toxic chemical such as hydrazine can have the issue to apply on wearable sensor due to their potential danger to human. Also, thermal reduction would not be compatible to wearable fabrics having low thermal resistance. [5] In this study, graphene oxide (GO) nanosheets in ethanol were photo-reduced with an aid of UV irradiation in which TiO2 or ZnO was added as typical photocatalyst. Room temperature gas sensing was conducted toward various VOC gases before and after photoreduction, and comparative results depending on photocatalyst werer studied. Due to the effect of heterostructure of GO and TiO2 (ZnO), the composite can identify ethanol, methanol, and acetone at room temperature. After photo-reduction, the resistance of the composite was much reduced as showing stable initial resistance, and gas sensing behavior was reversed to p-type owing to the reduction of GO. Relying on the peculiarity of the photocatalysts, initial resistance and gas response were changed, which suggests the worth of comparative study of photocatalyst. The color change of the composite was clearly observed after UV irradiation. The morphologies and structure of the composites were examined by SEM and XRD, and the comparative study on photocatalyst were implemented by UV-vis, XPS, and gas sensing results.

(Invited) System-on-Chip Sensor Integration in Advanced CMOS Technology

The growing demand for the integration of several functionalities on a single device has peaked with the rise of the Internet of Things. We are technologically near to having sensors in portable and wearable technologies, a feat made possible by the integration of sensor fabrication with CMOS manufacturing. In this paper we address semiconductor metal oxide sensors, which have the potential of becoming a universal sensor. We describe recent capabilities to model relevant materials and processes for these emerging devices and present optimized designs based on those analysis. In this review we discuss the modeling of sensor fabrication, followed by electro-thermo-mechanical analyses, which are essential to estimating stress build-up and sensor lifetimes. We further address the recent advances in understanding the metal oxide layer, which can be treated similarly to a semiconductor transistor, where ionosorption of gas ions creates a surface potential, changing the conducting behavior of the thin film.

New Digital Metal-Oxide (MOx) Sensor Platform.

The application of metal oxide gas sensors in Internet of Things (IoT) devices and mobile platforms like wearables and mobile phones offers new opportunities for sensing applications. Metal-oxide (MOx) sensors are promising candidates for such applications, thanks to the scientific progresses achieved in recent years. For the widespread application of MOx sensors, viable commercial offerings are required. In this publication, the authors show that with the new Sensirion Gas Platform (SGP) a milestone in the commercial application of MOx technology has been reached. The architecture of the new platform and its performance in selected applications are presented.

Sniffing Entrapped Humans with Sensor Arrays.

Earthquakes are lethal natural disasters frequently burying people alive under collapsed buildings. Tracking entrapped humans from their unique volatile chemical signature with hand-held devices would accelerate urban search and rescue (USaR) efforts. Here, a pilot study is presented with compact and orthogonal sensor arrays to detect the breath- and skin-emitted metabolic tracers acetone, ammonia, isoprene, CO2, and relative humidity (RH), all together serving as sign of life. It consists of three nanostructured metal-oxide sensors (Si-doped WO3, Si-doped MoO3, and Ti-doped ZnO), each specifically tailored at the nanoscale for highly sensitive and selective tracer detection along with commercial CO2 and humidity sensors. When tested on humans enclosed in plethysmography chambers to simulate entrapment, this sensor array rapidly detected sub-ppm acetone, ammonia, and isoprene concentrations with high accuracies (19, 21, and 3 ppb, respectively) and precision, unprecedented by portable sensors but required for USaR. These results were in good agreement (Pearson’s correlation coefficients ≥0.9) with benchtop selective reagent ionization time-of-flight mass spectrometry (SRI-TOF-MS). As a result, an inexpensive sensor array is presented that can be integrated readily into hand-held or even drone-carried detectors for first responders to rapidly screen affected terrain.

Development of novel complementary metal-oxide semiconductor-based colorimetric sensors for rapid detection of industrially important gases

A sensitive and low-cost colorimetric sensor array was developed to detect industrially important gases. We selected several target gases and indicators, and then used a printing method to fabricate an array to read the color value and to analyze the data. Using the printing method, various indicators can be produced in a uniform array on a desired substrate in a short time and at a low-cost. Four gases (ammonia, methylamine, trimethylamine, and hydrazine) and ten indicators were used to confirm the ability of the gas detection system. The color change of the dyes in the array was extracted in real time using an array reader with an integrated complementary metal-oxide semiconductor (CMOS) image sensor. The developed gas detection system successfully detected gases using an array reader, which provided data for sensitive color changes in real time using the CMOS image sensor. This mechanism could be applied industrially for detecting toxic gases because the developed sensor is highly sensitive and inexpensive.

Multi-unit calibration rejects inherent device variability of chemical sensor arrays

Inherent sensor variability limits mass-production applications for metal oxide (MOX) gas sensor arrays because calibration for replicas of a sensor array needs to be performed individually. Recently, calibration transfer strategies have been proposed to alleviate calibration costs of new replicas, but they still require the acquisition of transfer samples. In this work, we present calibration models that can be extended to uncalibrated replicas of sensor arrays without acquiring new samples, i.e., general or global calibration models. The developed methodology consists in including multiple replicas of a sensor array in the calibration process such that sensor variability is rejected by the general model. Our approach was tested using replicas of a MOX sensor array in the classification task of six gases and synthetic air, presented at different background humidity and concentration levels. Results showed that direct transfer of individual calibration models provides poor classification accuracy. However, we also found that general calibration models kept predictive performance when were applied directly to new copies of the sensor array. Moreover, we explored, through feature selection, whether particular combinations of sensors and operating temperatures can provide predictive performances equivalent to the calibration model with the complete array, favoring thereby the existence of more robust calibration models.

On the Temporal Stability of Analyte Recognition with an E-Nose Based on a Metal Oxide Sensor Array in Practical Applications.

The paper deals with a functional instability of electronic nose (e-nose) units which significantly limits their real-life applications. Here we demonstrate how to approach this issue with example of an e-nose based on a metal oxide sensor array developed at the Karlsruhe Institute of Technology (Germany). We consider the instability of e-nose operation at different time scales ranging from minutes to many years. To test the e-nose we employ open-air and headspace sampling of analyte odors. The multivariate recognition algorithm to process the multisensor array signals is based on the linear discriminant analysis method. Accounting for the received results, we argue that the stability of device operation is mostly affected by accidental changes in the ambient air composition. To overcome instabilities, we introduce the add-training procedure which is found to successfully manage both the temporal changes of ambient and the drift of multisensor array properties, even long-term. The method can be easily implemented in practical applications of e-noses and improve prospects for device marketing.

Laser Micromilling Technology as a Key for Rapid Prototyping SMD ceramic MEMS devices

The flexible laser micromilling technology for ceramic MEMS producing of microhotplate in the surface mounted device (SMD) package for the metal oxide (MOX) gas sensors is describing. There are discusses technological and economic aspects of small-scale production of gas MOX sensors in comparison with classical clean room technologies using for mass production MEMS devices. The main technical factors affecting on using MOX sensors in various applications are presented. Current results demonstrate that using described technology possible to manufacturing all parts of MOX gas sensor in the SMD form-factor SOT-23 package type.

Volatile Aroma Sensing Performance of High Quality Pristine Graphene, obtained via a facile Exfoliation technique

Instead of traditional inspection judgment of human sensory analyst (which is inconsistent and irreproducible) and cost intensive instrumental techniques like, HPLC, GCMS, LCMS, NMR, etc. (which also required skilled man power), here we develop laboratory synthesized pristine graphene based sensor device for tea quality discrimination by sensing the volatile aroma compounds, responsible for its quality. Other than metal oxide sensors which involves high operational temperature, we approach this graphene based sensor is working under ambient atmosphere. The sensing performance is visualized by principal component analysis, which shows that the sensor is capable to well recognize the tea aroma compounds and thus can be potentially applicable as futuristic tea sensor. Very facile and time resolved technique has also been accounted for graphene synthesis.

History of Digital Detectors in Intraoral Radiography

The first digital sensor used in intraoral radiography, RadioVisioGraphy was introduced in 1987. It was based on the activity of a charged-Coupled Device (CCD) sensor. Later on, other digital sensors became commercially available, including the Digora featuring an imaging plate coated with photostimulable phosphor (PSP) in 1994 and the complementary metal-oxide semiconductorbased sensor, CDR active pixel sensor, in 1998. These technologies have recently undergone considerable improvement, and several dental clinics have switched from film-based systems to digital imaging. In the early days, digital sensors were considered inferior to film in clinical diagnostics, but according to recent research results, there is no longer any significant difference because of the improvement in performance. However, our latest research using a new evaluation method and a precise phantom model indicated some differences between the performance of CCD and PSP systems and further differences between PSP systems of different function and performance.

Evaluation and Field Calibration of a Low-Cost Ozone Monitor at a Regulatory Urban Monitoring Station.

The performance of a low cost ozone monitor (Aeroqual Series 500 portable gas monitors using a metal oxide sensor for ozone; model OZL) was assessed under field conditions. Ten ozone monitors were calibrated under clean-air laboratory conditions and controlled ozone concentrations of 5 to 100 ppb. Good linearity and response were obtained relative to a research-grade ozone monitor. One monitor was co-located at a regulatory air quality monitoring station that uses a U.S. federal equivalent method (FEM) ozone analyzer. Raw data from the Aeroqual monitor collected over 4 months (June-October) at a 10-minute time-resolution, showed good agreement (r2=0.83) with the FEM values but with an overestimation of ~12%. Data were averaged to different time resolutions; 1 h time averaged concentrations showed the best fit with the FEM results (r2=0.87). Data analyses suggested the potential of interferences due to temperature, relative humidity, nitrogen oxides, and volatile organic compounds. Correction models using temperature, humidity, and nitrogen dioxide (NO2) were tested to relate the monitor concentrations to the FEM values. Temperature and humidity were two readily available variables. The model (#3) that added NO2 did not provide a substantial improvement in the fit. Thus, the models with only temperature and humidity can be easily developed by any user. The best model explained 91% of the variance and showed statistically significant improvement of the goodness of fits as well as decreased influence of the interfering variables on the diurnal and weekly patterns. The correction models were also able to lower the effect of seasonal temperature changes, allowing the use of the monitors over long-term sampling campaigns. Thus, the Aeroqual ozone monitor can return "FEM-like" concentrations after appropriate corrections. Data provided by a network of monitors could provide intra-urban spatial variations in ozone concentrations and provide more accurate human exposure assessments by reducing exposure misclassification.

Review—System-on-Chip SMO Gas Sensor Integration in Advanced CMOS Technology

The growing demand for the integration of functionalities on a single device is peaking with the rise of IoT. We are near to having multiple sensors in portable and wearable technologies, made possible through integration of sensor fabrication with mature CMOS manufacturing. In this paper we address semiconductor metal oxide sensors, which have the potential to become a universal sensor since they can be used in many emerging applications. This review concentrates on the gas sensing capabilities of the sensor and summarizes achievements in modeling relevant materials and processes for these emerging devices. Recent advances in sensor fabrication and the modeling thereof are further discussed, followed by a description of the essential electro-thermal-mechanical analyses, employed to estimate the devices' mechanical reliability. We further address advances made in understanding the sensing layer, which can be modeled similar to a transistor, where instead of a gate contact, the ionosorped gas ions create a surface potential, changing the film's conduction. Due to the intricate nature of the porous sensing films and the reception-transduction mechanism, many added complexities must be addressed. The importance of a thorough understanding of the electro-thermal-mechanical problem and how it links to the operation of the sensing film is thereby highlighted.

Defect-original room-temperature hydrogen sensing of MoO3 nanoribbon: Experimental and theoretical studies

Rapid development in the hydrogen energy sector inspires improvement in the sensing properties of low-temperature hydrogen sensors. Both effective experimental researches and simulated calculations are highly beneficial in understanding the sensing mechanism of metal-oxide hydrogen sensors and enhancing their sensing capabilities. In this work, we prepare ultra-long orthorhombic MoO3 nanoribbons using a hydrothermal method. Hydrogen sensors based on single MoO3 are assembled under an optical microscope through a simple process. The concentration of Mo5+ in the nanoribbons can be adjusted by annealing in different atmospheres. The nanoribbon annealed in H2 atmosphere with Mo5+ concentration of ∼25% presents a higher sensor response of 11.23 towards 1000 ppm H2 than those annealed in a vacuum or an O2 atmosphere. Our gas-sensing testing results for different background atmospheres reveal that the oxygen species are closely related to the hydrogen sensing performance. The calculated researches show that pure hydrogen molecules cannot be adsorbed on MoO3 (010) surfaces both with and without defects. Oxygen molecules can capture the electrons from the MoO3 material to form chemisorbed species on the MoO3 (010) surface with terminal oxygen vacancies. The injected H2 gas can react with the pre-adsorbed O2− to form H2O, releasing electrons back to the sensing material. Our theoretical results are in good agreement with those of the experimental investigation.

Manufacture and application of RuO2 solid-state metal-oxide pH sensor to common beverages

A new reproducible solid-state metal-oxide pH sensor for beverage quality monitoring is developed and characterised. The working electrode of the developed pH sensor is based on the use of laser-etched sputter-deposited RuO2 on Al2O3 substrate, modified with thin layers of sputter-deposited Ta2O5 and drop-cast Nafion for minimisation of redox interference. The reference electrode is manufactured by further modifying a working electrode with a porous polyvinyl butyral layer loaded with fumed SiO2. The developed pH sensor shows excellent performance when applied to a selection of beverage samples, with a measured accuracy within 0.08 pH of a commercial glass pH sensor.

Assessment of a combined gas chromatography mass spectrometer sensor system for detecting biologically relevant volatile compounds

There have been a number of studies in which metal oxide sensors (MOS) have replaced conventional analytical detectors in gas chromatography systems. However, despite the use of these instruments in a range of applications including breath research the sensor responses (i.e. resistance changes w.r.t. concentration of VCs) remain largely unreported. This paper addresses that issue by comparing the response of a metal oxide sensor directly with a mass spectrometer (MS), whereby both detectors are interfaced to the same GC column using an s-swafer. It was demonstrated that the sensitivity of an in-house fabricated ZnO/SnO2 thick film MOS was superior to a modern MS for the detection of a wide range of volatile compounds (VCs) of different functionalities and masses. Better techniques for detection and quantification of these VCs is valuable, as many of these compounds are commonly reported throughout the scientific literature. This is also the first published report of a combined GC-MS sensor system. These two different detector technologies when combined, should enhance discriminatory abilities to aid disease diagnoses using volatiles from e.g. breath, and bodily fluids. Twenty-nine chemical standards have been tested using solid phase micro-extraction; 25 of these compounds are found on human breath. In all but two instances the sensor exhibited the same or superior limit of detection compared to the MS. Twelve stool samples from healthy participants were analysed; the sensor detected, on average 1.6 peaks more per sample than the MS. Similarly, analysing the headspace of E. coli broth cultures the sensor detected 6.9 more peaks per sample versus the MS. This greater sensitivity is primarily a function of the superior limits of detection of the metal oxide sensor. This shows that systems based on the combination of chromatography systems with solid state sensors shows promise for a range of applications.

Unmanned Aerial Systems for Monitoring Trace Tropospheric Gases

The emission of greenhouse gases (GHGs) has changed the composition of the atmosphere during the Anthropocene. Accurately documenting the sources and magnitude of GHGs emission is an important undertaking for discriminating the contributions of different processes to radiative forcing. Currently there is no mobile platform that is able to quantify trace gases at altitudes <100 m above ground level that can achieve spatiotemporal resolution on the order of meters and seconds. Unmanned aerial systems (UASs) can be deployed on-site in minutes and can support the payloads necessary to quantify trace gases. Therefore, current efforts combine the use of UASs available on the civilian market with inexpensively designed analytical systems for monitoring atmospheric trace gases. In this context, this perspective introduces the most relevant classes of UASs available and evaluates their suitability to operate three kinds of detectors for atmospheric trace gases. The three subsets of UASs discussed are: (1) micro aerial vehicles (MAVs); (2) vertical take-off and landing (VTOL); and, (3) low-altitude short endurance (LASE) systems. The trace gas detectors evaluated are first the vertical cavity surface emitting laser (VCSEL), which is an infrared laser-absorption technique; second two types of metal-oxide semiconductor sensors; and, third a modified catalytic type sensor. UASs with wingspans under 3 m that can carry up to 5 kg a few hundred meters high for at least 30 min provide the best cost and convenience compromise for sensors deployment. Future efforts should be focused on the calibration and validation of lightweight analytical systems mounted on UASs for quantifying trace atmospheric gases. In conclusion, UASs offer new and exciting opportunities to study atmospheric composition and its effect on weather patterns and climate change.