Metal Oxide-based Sensors Research Articles

AI-Assisted Sensor System for the Acetone and Ethanol Detection Using Commercial Metal Oxide-Based Sensor Arrays and Convolutional Neural Network

In recent decades, the traditional landscape of volatile organic compound (VOC) sensing has adopted a new perspective in enhancing the detection of useful VOCs using data intelligence to extract constructive insights of the sensor behaviour towards multiple gases. In the domain of gas sensing, VOCs such as acetone and ethanol have been widely used in sensor testing due to their closely related chemical properties, which poses a challenge in discrimination. Therefore, this study aims to discriminate acetone from ethanol with the use of readily available commercial metal oxide (MOx) sensors through the implementation of Deep Learning (DL) techniques. The data set obtained after exposing a sensing array comprising various MOx sensors to acetone and ethanol was converted to a time-frequency representation known as a scalogram to train and test a multi-input convolutional neural network (CNN). The results show that training the CNN model on the sensor array data set yields better results than with an individual sensor data set. The findings of this research substantiated the ability of DL models to better capture the dynamic interaction of the sensors with acetone and ethanol, leading to the implication of the DL classifier having the capacity to reject sensor inconsistencies and variations in the responses. This research holds promise for advancing health monitoring and disease detection, as the combination of MOx sensors and DL techniques is expected to make significant future contributions in these areas.

Study on the acetone adsorption mechanism of In2O3/SnO2 heterocomposite fibers

A heterocomposite structure offers significant advantages for enhancing the gas sensing performance of metal oxide-based sensors. In this study, In2O3/SnO2 heterocomposite fibers were fabricated by electrospinning. In2O3/SnO2 heterocomposite fibers show excellent gas sensing performance for acetone. The response of the In2O3/SnO2 sensor is 9.3–100 ppm acetone, markedly outperforming sensors made solely of SnO2 or In2O3. To clarify the improved mechanism, the adsorption behavior of In2O3/SnO2 for acetone was studied based on Density Functional Theory (DFT). The adsorption energy of In2O3/SnO2 for acetone at the optimal adsorption site is 0.89 |eV|, which is 0.18 |eV| and 0.04 |eV| higher than that of SnO2 and In2O3, respectively. Additionally, the electron transfer numbers of acetone to In2O3/SnO2 are also 0.08|e| and 0.05|e| greater than those of SnO2 and In2O3, respectively. The n-n heterojunction of In2O3/SnO2 alters the material's barrier, significantly enhancing carrier concentration and electron transfer. This n-n heterojunction is key to the improved gas sensing performance of the In2O3/SnO2 sensor for acetone. Therefore, In2O3/SnO2 heterocomposite fibers have great application potential in the field of gas sensors.

Willow catkins-assisted synthesis of ZnFe2O4/ZnO hetero-tubes for chemiresistive dibutylamine sensors operated at 130 °C

The chemiresistive sensors have enabled the sensing of diverse organoamines, but the low-temperature detection of highly carcinogenic and combustible dibutylamine (DBA) has not been achieved to date. Herein, based on the concept of waste reuse and sustainable development, the discarded willow catkins were served as biotemplates and soaked into zinc or iron nitrates and their mixed solution. The above precursors were calcined at 500 °C in the air to controllably synthesize two homo-tubes and two ZnFe2O4/ZnO hetero-tubes, all of which are assembled by uniform nanoparticles. Among, the 1.25 at% ZnFe2O4/ZnO hetero-tubes possess uniform mesoporous distribution, high content of adsorbed oxygen species, and catalytic sites of high-valent Fe4+ Lewis acid, thus realizing the first metal oxide-based sensor with excellent trace DBA sensing at low temperature. The 1.25 at% ZnFe2O4/ZnO sensor exhibits high response of 79–100 ppm DBA at low 133 °C, which is 1.4, 1.8 and 2.3 times higher than those of 3.2 at% ZnFe2O4/ZnO, ZnO and ZnFe2O4 tubes, respectively. Furthermore, it also behaves rapid response kinetics (11 s∼10 ppm), good humidity resistance, reliable long-term stability (60 days), and the ability to actually detect prometryn herbicide. In addition, its response mechanism for the enhanced DBA sensing was investigated.

Pt-Embedded Metal–Organic Frameworks Deriving Pt/ZnO-In2O3 Electrospun Hollow Nanofibers for Enhanced Formaldehyde Gas Sensing

Functionalization by noble metal catalysts and the construction of heterojunctions are two effective methods to enhance the gas sensing performance of metal oxide-based sensors. In this work, we adopt the porous ZIF-8 as a catalyst substrate to encapsulate the ultra-small Pt nanoparticles. The Pt/ZnO-In2O3 hollow nanofibers derived from Pt/ZIF-8 were prepared by a facile electrospinning method. The 25PtZI HNFs sensor possessed a response value of 48.3 to 100 ppm HCHO, 2.7 times higher than the pristine In2O3, along with rapid response/recovery time (5/22 s), and lower theoretical detection limit (74.6 ppb). The improved sensing properties can be attributed to the synergistic effects of electron sensitization effects and catalytic effects of Pt nanoparticles, and the high surface O− absorbing capability of heterojunctions. The present study paves a new way to design high performance formaldehyde gas sensors in practical application.

Experimental evaluation of ZnO nanorods based eco – friendly sensor for melamine detection in milk

PurposeThe purpose of this study is to develop a straightforward method for creating a non – toxic metal oxide sensor capable of detecting melamine in milk at ambient temperature.Design/methodology/approachA low temperature, straightforward solution-based approach, specifically the hydrothermal method was utilized to apply the active sensing layer onto the substrate. Subsequently, analysis of the cyclic voltammetry (CV) profile was conducted to detect the concentration of melamine and determine its sensitivity.FindingsAn experimental analysis was performed on a nontoxic metal oxide-based sensor fabricated for detecting melamine sensitivity using the CV profile. The sensor’s performance was evaluated under three different concentrations of melamine (0.1 mmol, 0.2 mmol and 0.3 mmol). The results demonstrated a satisfactory sensitivity of 0.0297 µAmM−1cm−2 with a response time of 2 s.Originality/valueThe primary innovation of this research lies in the creation of a nontoxic and environmentally friendly sensor. The synthesis method employed featured significantly lower temperatures compared to existing literatures. Furthermore, the sensor achieved enhanced sensitivity along with rapid response times.

Achieving Room-Temperature ppb-Level H2S Detection in a Au-SnO2 Sensor with Low Voltage Enhancement Effect.

Although semiconductor metal oxide-based sensors are promising for gas sensing, low-power and room temperature operation (24 ± 1 °C) remains desirable for practical applications particularly considering the request of energy saving or net zero emission. In this study, we demonstrate a Au/SnO2-based ultrasensitive H2S gas sensor with a limit of detection (LOD) of 2 ppb, operating at very low voltages (0.05 to 0.5 V) at room temperature. The Au/SnO2-based sensor showed approximately 7 times higher response (the ratio of change in the current to initial current) of ∼270% and 4 times faster recovery (126 s) compared to the pure SnO2-based sensor when exposed to 500 ppb H2S gas concentration at 0.5 V operating voltage at relative humidity (RH) 17.5 ± 2.5%. The enhancement can be attributed to the catalytic characteristics of AuNPs, increasing the number of adsorbed oxygen species on sensing material surfaces. Additionally, AuNPs aid in forming flower-petal-like Au/SnO2 nanostructures, offering a larger surface area and more active sites for H2S sensing. Moreover, at low voltage (<1 V), the localized dipoles at the Au/SnO2 interface may further enhance the absorption of polar oxygen molecules and hence promote the reaction between H2S and oxygen species. This low-power, ultrasensitive H2S sensor outperforms high-powered alternatives, making it ideal for environmental, food safety, and healthcare applications.

Operating temperature and ruthenium doping influence on the charge carriers type transition in the ɑ-Fe2O3 sensors upon liquefied petroleum gases detection

Metal oxide-based sensors have been extensively used for environmental monitoring, health, and safety. This work focuses on synthesizing ɑ-Fe2O3 and doping it with Ruthenium (Ru) to study its gas-sensing properties over flammable and hazardous gases. An anomalous behavior was observed during the Liquefied Petroleum Gas (LPG). This Ru-doped ɑ-Fe2O3 showed a charge carrier switching transition from n- to p-type conductivity due to Ru doping and the sensor’s operating temperature. The switching behavior on the Ru-doped samples happened between 3,000 and 4,000 ppm of the LPG concentrations. However, the Ru doping content did not seem to be affecting this transition except to alter the LPG response. The sensors’ operating temperature did alter the switching transition from n- to p-type conductivity. The temperatures varied from 175 to 225 °C. Metal Oxide Semiconductor (MOS) based on α -Fe2O3 nanoparticle doped with ruthenium (Ru- α-Fe2O3) was more selective towards LPG with a gas response of 24.41.

Metal Oxide-Based Sensors for Ecological Monitoring: Progress and Perspectives

This paper aims to provide a large coverage of recent developments regarding environmental monitoring using metal oxide-based sensors. Particular attention is given to the detection of gases such as H2, COx, SOx, NOx, and CH4. The developments and analyses of the design of sensors and types of metal oxide sensing materials are emphasized. The sensing mechanisms and peculiarities of metal oxides used in chemoresistive sensors are provided. The main parameters that affect the sensitivity and selectivity of metal oxide sensors are indicated and their significance to the sensor signal is analyzed. Modern data processing algorithms, employed to optimize the measurement process and processing of the sensor signal, are considered. The existing sensor arrays/e-nose systems for environmental monitoring are summarized, and future prospects and challenges encountered with metal oxide-based sensor arrays are highlighted.

In-situ sodium chloride template synthesis of γ-WO3 hollow cubes for high-sensing and dual-selective hydrazine/dibutylamine detection at different temperatures

The fabrication of a chemiresistive sensor with dual-selectivity to highly toxic and explosive N2H4/dibutylamine (DBA) remains challenging due to no report. To this aim, we synthesized uniform (NH4)0.33WO3 cuboid precursors through optimized solvothermal reaction and in-situ formed NaCl template. Subsequently, the precursors were calcined in air to transform into γ-WO3 hollow cubes (WO3-HC) assembled from near-spherical nanoparticles, and the effect of calcination temperature on microstructures and gas-sensing properties was investigated. Among, WO3-HC-5 obtained by calcining precursor at 500 °C has broad pore channels, large specific surface area and abundant oxygen vacancy defects. Under the catalytic synergy of surface W6+ Lewis acid with dangling bonds, WO3-HC-5 sensor realizes the bifunctional monitoring of N2H4 and DBA. It presents large response value (S = 606.6) to 100 ppm N2H4 at 50 °C and a good response (S = 76.7) to the same concentration of DBA at 217 °C, and has fast response rate, low practical detection limit, satisfactory stability and humidity resistance. WO3-HC-5 represents the first metal oxide-based sensor with dual-selectivity towards aforementioned harmful gases at different operating temperatures. Moreover, the bifunctional mechanism has been characterized and analyzed in detail.

Natural catkins-derived single-crystal WO3 tubes rich in oxygen vacancies for high response and rapid detection of hydrazine at near room temperature

It is urgent to improve the comprehensive detection ability of chemoresistive gas sensors for highly toxic and explosive N2H4 due to less report. To this aim, we simply immersed discarded catkins in ammonium metatungstate aqueous solution to obtain a biotemplate precursor, which was then calcinated in air to controllably replicate two biomorphic WO3 materials by cross-linked nanoparticles. Amongst, single crystal WO3-6 tubes calcinated at 600 °C show good overall sensing performances towards trace N2H4 for the first time. At near room temperature (50 °C), WO3-6 sensor presents large response value (S = 120) and short response time (12 s) to 10 ppm N2H4, along with low practical detection limit (5 ppb). The above four important indexes are significantly better than those of currently reported metal oxide-based sensors. Meanwhile, it also exhibits good selectivity, satisfactory reproducibility, stability and moisture resistance. Such enhanced N2H4 sensing mechanism mainly roots from the synergy of its unique tubular structure (e.g. broad pores, oxygen vacancy defects), catalysis of W6+ ions with dangling bonds, as well as surface adsorbed oxygen species.

In-situ/operando characterization of FeOx-based chemiresistive sensor of acetone vapours by X-ray absorption spectroscopy

Metal oxide thin films are archetypal active elements employed in chemiresistive gas sensors. Most existing metal oxide-based sensors operate at elevated temperatures; however, the sensing mechanism has generally been deduced considering the room-temperature properties of metal oxide compounds. We report an in-operando X-ray absorption spectroscopy study on a chemiresistive sensor of acetone vapours; namely, the chemical structure of iron-oxide nanoparticles (NPs) exposed to trace concentrations of acetone vapours under varying temperature conditions was examined. Our results show that the iron-oxide NPs, identified as maghemite/magnetite (γ-Fe2O3/Fe3O4) compound, partially reduced with temperature. Likewise, the iron oxide NPs were partially reduced upon exposure to trace concentrations of acetone vapour (at constant temperature). The latter observation suggests that acetone molecules are chemically adsorbed on iron oxide. The chemical changes are discussed in terms of the conductive response of a consubstantial chemiresistive sensor. Acetone chemisorption requires a revision of the commonly accepted sensing mechanism based on the physical adsorption of acetone molecules.

Temperature-dependent NO2/diisopropylamine sensor based on SnO2 squama-wrapped tubes derived from waste bamboo shoot skin

The development of a conductometric sensor with dual-selectivity remains challenging because most sensors were only reported for sensing a given harmful gas. Accordingly, waste bamboo shoot skin (BSS) was selected as biotemplate and simply immersed in SnCl4 aqueous solution. Then, the obtained BSS precursor was calcined at 600 °C to reproduce biomorphic SnO2-6 squama-wrapped tubes, which achieved temperature-controlled dual-selectivity for two different gases. At a low temperature of 92 °C, SnO2-6 sensor has a super-high response (S = 1300) towards 10 ppm NO2, being obviously larger than most reported metal oxide-based sensors. Also it exhibits a good response (S = 46) towards 100 ppm diisopropylamine (DIPA) at 170 °C for the first time. Such excellent dual-sensing capability mainly relates to the synergism of uniform mesopore distribution, relatively large specific surface area and rich oxygen vacancy defects involved in SnO2-6 squama-wrapped tubes, which not only helps to accelerate rapid gas transport, but also efficiently exposes more active sites, increases the contents of adsorbed oxygen species and promotes surface chemical reactions. Meanwhile, the sensor has reversible response-recovery characteristic, good stability and humidity resistance to both gases. In addition, the dual-functional sensing mechanism was also investigated.

Facile engineering of metal–organic framework derived SnO2-ZnO composite based gas sensor toward superior acetone sensing performance

Aiming to achieve high-sensitivity disease diagnosis, it is of great significance to continuously improve the sensing performance of metal oxide based sensors. The morphology and component of metal oxides can be rationally engineered by using metal–organic frameworks as highly versatile platforms. Herein, the bimetallic Sn/Zn-ZIF-8 is synthesized by self-assembly, and then SnO2-ZnO hollow nanoparticles are obtained by using bimetallic Sn/Zn-ZIF-8 as precursor. The obtained SnO2-ZnO composite maintained the dodecahedral structure of Sn/Zn-ZIF-8, and in which SnO2 is orthogonal due to the framework-induced effect. Based on the unique structure and composition of SnO2-ZnO, 6% SnO2-ZnO exhibits excellent selectivity, high response, low detection limit (0.82 ppb) and good response linearity between response and concentration of acetone, showing the potential of the practical application in clinical diagnosis of diabetic patients. In-depth discussions reveal that the excellent sensing performance can be attributed to the n-n heterostructure of SnO2-ZnO that is favorable for electron transport, the larger adsorption energy to acetone that results from orthogonal SnO2, and the abundant active reaction sites provided by the dodecahedral mesoporous structure. This synthetic strategy may provide an effective method for the facile engineering of high-performances metal oxide-based sensors toward practical health assessment.

Biotemplate-assisted synthesis of CuO hierarchical tubes for highly chemiresistive detection of dimethylamine at room temperature

The fabrication of low-temperature metal oxide-based sensors with highly sensitive to carcinogenic organic-amines remains challenging. Herein, a biomorphic CuO-5 material was synthesized by simply immersing mulberry twig slices in copper nitrate solution and calcining the precursors in air at 500 ℃. Its multistage tubes are assembled from well-crystallized nanoparticles, and possess uniform mesopores and abundant oxygen vacancies. These microstructural factors facilitate the rapid diffusion of gas molecules to sensing layer, and improve the conductivity of CuO material, as well as the content of active sites and surface adsorbed oxygen species. In particular, under the synergy of adsorption and catalysis of the suspended bonds for surface Cu2+ ions, this material achieves the high sensing and selective detection of trace dimethylamine (DMA) at room temperature for the first time. At 25 °C, CuO-5 sensor reaches a response value of 35.9–50 ppm DMA, which is 8.3 times larger than that of template-free CuO-0 sample. Meanwhile, the sensor also has good response-recovery reproducibility, low detection limit, and satisfactory moisture resistance and stability. In addition, the mechanism for enhanced sensing of DMA at room temperature is discussed in detail.

A simple, sensitive, label-free electrochemical immunosensor based on the chitosan-coated silver/cerium oxide (CS@Ag/CeO2) nanocomposites for the detection of alpha-fetoprotein (AFP)

Metal oxide-based sensors have the benefit of inexpensive, quick response, and high sensitivity in detecting specific biological species. In this article, a simple electrochemical immunosensor was fabricated using antibody-chitosan coated silver/cerium oxide (Ab-CS@Ag/CeO2) nanocomposites on a gold electrode for sensitive alpha-fetoprotein (AFP) diagnosis in human serum samples. Successfully synthesis of AFP antibody-CS@Ag/CeO2 conjugates was confirmed through Fourier transform infrared spectra of the prototype. The amine coupling bond chemistry was then used to immobilize the resultant conjugate on a gold electrode surface. It was observed that the interaction of the synthesized Ab-CS@Ag/CeO2 nanocomposites with AFP prevented an electron transfer and reduced the voltammetric Fe(CN)6 3−/4− peak current, which was proportional to the amount of AFP. The linear ranges of AFP concentration were found from 10−12–10−6 g.ml−1. The limit of detection was calculated using the calibration curve and came out to be 0.57 pg.ml−1. The designed label-free immunosensor successfully detected AFP in human serum samples. As a result, the resulting immunosensor is a promising sensor plate form for AFP detection and could be used in clinical bioanalysis.

Design Synthesis of ZnO Tube Bundles Rich in Oxygen Vacancies Assembled by Nanorods/Quasi-Nanospheres for Enhanced Sensing to NO/NO2

How to in situ construct homo-nanostructures assembled from diverse nanobuilding blocks and achieve high sensing of NO/NO2 gases is still challenging. To this end, two biomorphic ZnO materials cross-linked by nanorods and quasi-nanospheres were obtained through facile zinc nitrate solution infiltration and calcination using kitchen garbage of a discarded loofah sponge as the biotemplate. Among the tube bundles obtained by calcination at 600 °C, ZnO-6 possesses good crystallinity, small-sized mesopore distribution, and large specific surface area, as well as abundant oxygen vacancies. The synergy of the aforementioned microstructural features can improve electron transport efficiency, promote fast gas diffusion, increase the content of adsorbed oxygen species, and expose more surface active sites, thereby efficiently improving its sensing properties toward nitrogen oxides. At 92 °C, the ZnO-6 sensor simultaneously achieves large response values from 100 to 10 ppm of NO and 342 to 10 ppm of NO2, and their recovery times are reduced to 12 and 15 s, respectively. These indicators have apparent superiority over most reported corresponding metal oxide-based sensors. In addition, the sensor can apply the detection of trace NOx in real environments.

Biotemplate synthesis of NiO/ZnO tubes rich in oxygen vacancies for enhanced sensing detection of hydrazine at low temperature

The development of conductometric sensors for high sensing and selective detection of very toxic and explosive hydrazine gas remains urgent challenge. Herein, waste and reproducible willow catkins were chosen as biotemplate and simply immersed into mixed Ni(NO3)2/Zn(NO3)2 solution with different Ni/Zn ratios. Afterwards, the isolated precursors were calcined at 500 ℃ in air to generate three biomorphic NiO/ZnO hetero-tubes, which were all replicated from small-size nanoparticles with good crystallinity. Amongst, 6.1 wt% NiO/ZnO composite achieves high sensing and rapid detection of trace N2H4. At 92 °C, its response value (S = 443) to 100 ppm N2H4 is significantly higher than those of 3.7 wt% NiO/ZnO (S = 113) and 7.2 wt% NiO/ZnO (S = 210). Especially, its response value is the highest among reported metal oxide-based sensors. Meanwhile, it also has short response time (6 s), low practical detection limit (10 ppb) along with satisfactory stability and moisture tolerance. Such well over-all sensing performance roots from the synergism of mesoporous hetero-tubes assembled by small-size nanoparticles, proper tube thickness, high concentration of Ni3+ Lewis acid, as well as rich oxygen vacancy defects. Moreover, the mechanism for enhanced N2H4 sensing performance was also discussed in detail.

Temperature-controlled dual-selectivity nitric oxide/acetone sensor constructed from mesoporous SnO2 tubes doped by biomass-derived graphitic carbon

How to achieve good dual-functional selectivity of metal oxide-based sensors to harmful gases remains challenge due to few relevant reports. To this purpose, we selected waste willow catkins as structure-oriented template and carbon source to controllably prepare biomorphic GC/SnO2-500 sensing material by air calcination of 500 °C. Its tubular morphology is cross-linked by 3.51 wt% biomass-derived graphitic carbon (GC) and small-size nanoparticles. It has relatively large specific surface area, uniform mesopore distribution and rich oxygen vacancies. The synergistic effect of above positive factors promotes the rapid gas transport to sensing layer, and also increases the reactive sites and its resistance modulation ability, thus achieving good dual-functional selectivity. At 92 °C, GC/SnO2-500 sensor displays large response value of 802 to 10 ppm NO, and exhibits a good response towards 100 ppm acetone at 217 °C (S = 22). Both of the indexes are better than those of most corresponding gas sensors reported presently. Meanwhile, the sensor has short response time, satisfactory long-term stability and humidity tolerance. Moreover, portable temperature-dependent dual-functional gas-sensing mechanism is also discussed.

Paper-Based Hydrogen Sensors Using Ultrathin Palladium Nanowires

Hydrogen (H2), as a chemical energy carrier, is a cleaner alternative to conventional fossil fuels with zero carbon emission and high energy density. The development of fast, low-cost, and sensitive H2 detection systems is important for the widespread adoption of H2 technologies. Paper is an environment-friendly, porous, and flexible material with great potential for use in sustainable electronics. Here, we report a paper-based sensor for room-temperature H2 detection using ultrathin palladium nanowires (PdNWs). To elucidate the sensing mechanism, we compare the performance of polycrystalline and quasi-single-crystalline PdNWs. The polycrystalline PdNWs showed a response of 4.3% to 1 vol % H2 with response and recovery times of 4.9 and 10.6 s, while quasi-single-crystalline PdNWs showed a response of 8% to 1 vol % H2 with response and recovery times of 9.3 and 13.0 s, respectively. The polycrystalline PdNWs show excellent selectivity, stability, and sensitivity, with a limit of detection of 10 ppm H2 in air. The fast response of ultrathin polycrystalline PdNW paper-based sensors arises from the synergistic effects of their ultrasmall diameter, high-index surface facets, strain-coupled grain boundaries, and porous paper substrate. This paper-based sensor is one of the fastest chemiresistive H2 sensors reported and is potentially orders of magnitude less expensive than current state-of-the-art H2-sensing solutions. This brings low-cost, room-temperature chemiresistive H2 sensing closer to the performance of ultrafast optical sensors and high-temperature metal oxide-based sensors.

The influence of advanced materials on the analytical performance of semiconductor-based gas sensors.

Chemiresistive gas sensors are metal oxide-based sensors that have received significant attention in different fields. Ambient gas sensors are especially important in the fabrication of wearable probes for the real-time detection of biomarkers in human body samples. Usually, room temperature sensors are affordable due to their low power consumption, resulting in simple instrumentation and maintenance. To fabricate versatile gas sensors, i.e. sensitive, selective, ambient temperature operating gas sensors, and improve the sensing performance of the traditionally used sensor, new materials play an important role. In other words, new advanced materials are essential for designing and fabricating new gas sensors. Hence, in this review, the application and impact of new advanced materials in the fabrication of reliable gas sensors are discussed in detail. Special emphasis is given to the effect of new materials in the fabrication of room-temperature operating systems. Finally, future research outlook and possible challenges that may be encountered by reliable gas sensors are also explained.