Nanoscale PdO Catalyst Functionalized Co3O4 Hollow Nanocages Using MOF Templates for Selective Detection of Acetone Molecules in Exhaled Breath

Co-MOF-derived Co3O4 sensors for efficient 3-octanone biomarker monitoring in wheat mildew

3-octanone has been widely identified as a primary biomarker for mold and insect infestation in wheat. Nevertheless, to date, no chemiresistive sensor based on a metal oxide semiconductor with excellent sensing properties for 3-octanone has been developed. In light of the extensively reported superior efficacy of Co3O4-based sensors for ketone detection, we designed Co3O4 samples with various hierarchical morphologies (hollow sphere, multi-wall sphere, flower-like, and urchin-like), which were determined by regulating solvent and organic ligand. The sensing properties of four as-fabricated Co3O4-based sensors were systematically evaluated, verifying their potential for efficient 3-octanone monitoring. The adsorption behavior of 3-octanone was studied by DFT simulation, and the preferential adsorption of the hydroxyl functional group in 3-octanone at Co sites was verified. Among the four samples, the hollow spherical Co3O4 demonstrated the highest sensitivity for 3-octanone (173.88 ± 5.59@50 ppm), which can be attributed to the larger specific surface area (SSA, 60.164 m2/g), lower energy gap (1.306 eV), and the superior concentration of chemisorbed oxygen (23.8 %). Furthermore, the practical value of this sensor in agricultural product inspection and environmental monitoring applications was validated by testing its response to the complex gases emitted from wheat stored for different periods.

Selectivity in Chemiresistive Gas Sensors: Strategies and Challenges

The demand for highly functional chemical gas sensors has surged due to the increasing awareness of human health to monitor metabolic disorders or noncommunicable diseases, safety measures against harmful greenhouse and/or explosive gases, and determination of food freshness. Over the years of dedicated research, several types of chemiresistive gas sensors have been realized with appreciable sensitivities toward various gases. However, critical issues such as poor selectivity and sluggish response/recovery speeds continue to impede their widespread commercialization. Specifically, the mechanisms behind the selective response of some chemiresistive materials toward specific gas analytes remain unclear. In this review, we discuss state-of-the-art strategies employed to attain gas-selective chemiresistive materials, with particular emphasis on materials design, surface modification or functionalization with catalysts, defect engineering, material structure control, and integration with physical/chemical gas filtration media. The nature of material surface-gas interactions and the supporting mechanisms are elucidated, opening opportunities for optimizing the materials design, fine-tuning the gas sensing performance, and guiding the selection of the most appropriate materials for the accurate detection of specific gases. This review concludes with recommendations for future research directions and potential opportunities for further selectivity improvements.

Vitalizing Perovskite Oxide-Based Acetone Sensors with Metal-Organic Framework-Derived Heterogeneous Oxide Catalysts

Perovskite oxides are promising candidates for chemiresistive-type gas sensors owing to their exceptional thermal and chemical stability during solid-gas reactions. However, perovskites suffer from critical issues such as low surface area and poor surface activity, which negatively influence the sensing characteristics. While metal nanoparticles can be incorporated in perovskites to improve their reactivity, the fundamental incompatibility between catalytic metals and perovskite oxides often leads to substantial structural degradation as well as phase instability. Herein, we overcome this challenge through the introduction of an intermediary phase that forms coherent interfaces with both the perovskite phase and catalyst metals. Specifically, we present the case study of p-type La0.8Ca0.2Fe0.98Pt0.02O3 perovskite, whose hole accumulation layer was modulated by the incorporation of metal-organic framework (MOF)-derived n-type α-Fe2O3 nanoparticles decorated with highly dispersed Pt catalysts. The resulting composite exhibited significantly improved surface activity over the nonmodified La0.8Ca0.2FeO3 perovskite, leading to exceptional chemiresistive sensing performance toward acetone gas (Rg/Ra = 39.8 toward 10 ppm of acetone at 250 °C) with high cross-sensitivity against interfering gases. Importantly, our findings reaffirm the critical influence of interfacial engineering in facilitating surface chemical reactions on perovskite oxides and, by doing so, effectively provide a general synthetic guideline to the design of perovskite-based chemiresistors.

Bimetallic MOFs-Derived Metal Oxides Co3O4/SnO2 Microspheres for Ultrahigh Response n-Butanol Gas Sensors

The construction of p-n heterojunctions is expected to be one of the effective means to improve gas sensitivity. In this research, p-n heterojunctions are successfully constructed by metal oxides derived from metal-organic frameworks (MOFs). MOFs-derived bimetallic Co3O4/SnO2 microspheres are prepared by precipitation. Gas-sensing performance shows that the Co3O4/SnO2 sensor exhibits an extremely high response (Ra/Rg = 641) to 20 ppm of n-butanol at 200 °C, which is 19 times that of pristine SnO2. It can detect n-butanol gas at low concentrations, has good selectivity to alcohol gas, and reduces the interference of benzene gas. The improved gas sensitivity can be attributed to the formation of a stable heterojunction between Co3O4 and SnO2, resulting in a greater resistance change of Co3O4/SnO2. Co3O4/SnO2 inherits the characteristic of high specific surface area of MOFs, which provides abundant sites for the reaction of the target gas and oxygen molecules. Finally, the gas-sensing mechanism of the Co3O4/SnO2-based sensor is discussed in detail.

Uniform Nanocrystal Spatial Distribution-Enhanced SnO2-based Sensor for High-Sensitivity Hydrogen Detection

Hydrogen (H2) is colorless, odorless, and has a wide explosive concentration range (4-75 vol %), making rapid and accurate detection of hydrogen leaks essential. This paper demonstrates a method to modify the spatial distribution of nanocrystals (NCs) by adding surfactants to improve the sensing performance. In order to explore its potential for H2 gas-sensing applications, SnO2, containing different mass percentages of PdCu NCs, was dispersed. The results show that the 0.1 wt % PdCu-SnO2 sensor based on surfactant dispersion performs well, with a response to 0.1 vol % H2 that is 18 times higher than that of the undispersed 0.1 wt % PdCu-SnO2 sensor. The enhanced gas-sensing ability after dispersion can be attributed to the fact that the uniform distribution of NCs generates higher quantum efficiency and exposes more active sites on the carrier surface compared to nonuniform distribution. This study provides a simple, novel, and effective method to improve the sensor response.

Recent Trends and Advances in Porous Metal-Organic Framework Nanostructures for the Electrochemical and Optical Sensing of Heavy Metals in Water

With the expansion and advancement in agricultural and chemical industries, various toxic heavy metals such as lead, cadmium, mercury, zinc, copper, arsenic etc. are continuously released into the environment. Intake of sources contaminated with such toxic metals leads to various health issues. Keeping the serious effects of these toxic metal ions in view, various organic-inorganic nanomaterials based sensors have been exploited for their detection via optical, electrochemical and colorimetric approaches. Since a chemical sensor works on the principle of interaction between the sensing layer and the analytes, a sensor material with large surface area is required to enable the largest possible interaction with the target molecules and hence the sensitivity of the chemical sensor. However, commonly employed materials such as metal oxides and conducting polymers tend to feature relatively low surface areas, and hence resulting in low sensitivity of the sensor. Metal-Organic Frameworks (MOFs) nanostructures are another category of organic-inorganic materials endowed with large surface area, ultra-high and tunable porosity, post-synthesis modification features, readily available active sites, catalytic activity, and chemical/thermal stability. These properties provide high sensitivity to the MOF based sensors due to the adsorption of large number of target analytes. The current review article focuses on MOFs based optical and electrochemical sensors for the detection of heavy metals.

Femtosecond Laser Ionization Mass Spectrometry for Online Analysis of Human Exhaled Breath

A variety of organic compounds in human exhaled breath were measured online by mass spectrometry using the fifth (206 nm) and fourth (257 nm) harmonic emissions of a femtosecond ytterbium (Yb) laser as the ionization source. Molecular ions were enhanced significantly by means of resonance-enhanced, two-color, two-photon ionization, which was useful for discrimination of analytes against the background. The limit of detection was 0.15 ppm for acetone in air. The concentration of acetone in exhaled breath was determined for three subjects to average 0.31 ppm, which lies within the range of normal healthy subjects and is appreciably lower than the range for patients with diabetes mellitus. Many other constituents, which could be assigned to acetaldehyde, ethanol, isoprene, phenol, octane, ethyl butanoate, indole, octanol, etc., were observed in the exhaled air. Therefore, the present approach shows potential for use in the online analysis of diabetes mellitus and also for the diagnosis of various diseases, such as COVID-19 and cancers.

Nanowire Array Breath Acetone Sensor for Diabetes Monitoring

Diabetic ketoacidosis (DKA) is a life-threatening acute complication of diabetes characterized by the accumulation of ketone bodies in the blood. Breath acetone, a ketone, directly correlates with blood ketones. Therefore, monitoring breath acetone can significantly enhance the safety and efficacy of diabetes care. In this work, the design and fabrication of an InP/Pt/chitosan nanowire array-based chemiresistive acetone sensor is reported. By incorporation of chitosan as a surface-functional layer and a Pt Schottky contact for efficient charge transfer processes and photovoltaic effect, self-powered, highly selective acetone sensing is achieved. The sensor has exhibited an ultra-wide acetone detection range from sub-ppb to >100 000 ppm level at room temperature, covering those in the exhaled breath from healthy individuals (300-800 ppb) to people at high risk of DKA (>75 ppm). The nanowire sensor has also been successfully integrated into a handheld breath testing prototype, the Ketowhistle, which can successfully detect different ranges of acetone concentrations in simulated breath samples. The Ketowhistle demonstrates the immediate potential for non-invasive ketone monitoring for people living with diabetes, in particular for DKA prevention.

Porous materials as effective chemiresistive gas sensors

Chemiresistive gas sensors (CGSs) have revolutionized the field of gas sensing by providing a low-power, low-cost, and highly sensitive means of detecting harmful gases. This technology works by measuring changes in the conductivity of materials when they interact with a testing gas. While semiconducting metal oxides and two-dimensional (2D) materials have been used for CGSs, they suffer from poor selectivity to specific analytes in the presence of interfering gases and require high operating temperatures, resulting in high signal-to-noise ratios. However, nanoporous materials have emerged as a promising alternative for CGSs due to their high specific surface area, unsaturated metal actives, and density of three-dimensional inter-connected conductive and pendant functional groups. Porous materials have demonstrated excellent response and recovery times, remarkable selectivity, and the ability to detect gases at extremely low concentrations. Herein, our central emphasis is on all aspects of CGSs, with a primary focus on the use of porous materials. Further, we discuss the basic sensing mechanisms and parameters, different types of popular sensing materials, and the critical explanations of various mechanisms involved throughout the sensing process. We have provided examples of remarkable performance demonstrated by sensors using these materials. In addition to this, we compare the performance of porous materials with traditional metal-oxide semiconductors (MOSs) and 2D materials. Finally, we discussed future aspects, shortcomings, and scope for improvement in sensing performance, including the use of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous organic polymers (POPs), as well as their hybrid counterparts. Overall, CGSs using porous materials have the potential to address a wide range of applications, including monitoring water quality, detecting harmful chemicals, improving surveillance, preventing natural disasters, and improving healthcare.

In-MIL-68 derived In2O3/Fe2O3 shuttle-like structures with n-n heterojunctions to improve ethanol sensing performance

Metal-organic frameworks (MOFs) have a variety of structures and unique properties that make them suitable for use in gas sensors. Herein, In2O3/Fe2O3 was successfully synthesized using simple solvothermal and impregnation methods. The response to 100 ppm of ethanol gas reached 67.5 at an optimum working temperature of 200 °C, and the response/recovery time was 9 s/236 s. The composite also exhibited excellent selectivity, repeatability, and long-term stability. SEM, TEM, XRD, and XPS were used for the characterization of materials. The excellent sensing performance of the sensors is attributed to the construction of n-n heterojunctions, an increase in oxygen vacancies, and the unique structural characteristics of MOFs. The above experimental results indicate that In-MIL-68-derived In2O3/Fe2O3 is a promising ethanol sensing material.

MOF-Based Chemiresistive Gas Sensors: Toward New Functionalities

The sensing performances of gas sensors must be improved and diversified to enhance quality of life by ensuring health, safety, and convenience. Metal-organic frameworks (MOFs), which exhibit an extremely high surface area, abundant porosity, and unique surface chemistry, provide a promising framework for facilitating gas-sensor innovations. Enhanced understanding of conduction mechanisms of MOFs has facilitated their use as gas-sensing materials, and various types of MOFs have been developed by examining the compositional and morphological dependences and implementing catalyst incorporation and light activation. Owing to their inherent separation and absorption properties and catalytic activity, MOFs are applied as molecular sieves, absorptive filtering layers, and heterogeneous catalysts. In addition, oxide- or carbon-based sensing materials with complex structures or catalytic composites can be derived by the appropriate post-treatment of MOFs. This review discusses the effective techniques to design optimal MOFs, in terms of computational screening and synthesis methods. Moreover, the mechanisms through which the distinctive functionalities of MOFs as sensing materials, heterostructures, and derivatives can be incorporated in gas-sensor applications are presented.

Principles of Design and Synthesis of Metal Derivatives from MOFs

Materials derived from metal-organic frameworks (MOFs) have demonstrated exceptional structural variety and complexity and can be synthesized using low-cost scalable methods. Although the inherent instability and low electrical conductivity of MOFs are largely responsible for their low uptake for catalysis and energy storage, a superior alternative is MOF-derived metal-based derivatives (MDs) as these can retain the complex nanostructures of MOFs while exhibiting stability and electrical conductivities of several orders of magnitude higher. The present work comprehensively reviews MDs in terms of synthesis and their nanostructural design, including oxides, sulfides, phosphides, nitrides, carbides, transition metals, and other minor species. The focal point of the approach is the identification and rationalization of the design parameters that lead to the generation of optimal compositions, structures, nanostructures, and resultant performance parameters. The aim of this approach is to provide an inclusive platform for the strategies to design and process these materials for specific applications. This work is complemented by detailed figures that both summarize the design and processing approaches that have been reported and indicate potential trajectories for development. The work is also supported by comprehensive and up-to-date tabular coverage of the reported studies.

A Multimodal Sensing CMOS Imager Based on Dual-Focus Imaging

Advanced machine intelligence is empowered not only by the ever-increasing computational capability for information processing but also by sensors for collecting multimodal information from complex environments. However, simply assembling different sensors can result in bulky systems and complex data processing. Herein, it is shown that a complementary metal-oxide-semiconductor (CMOS) imager can be transformed into a compact multimodal sensing platform through dual-focus imaging. By combining lens-based and lensless imaging, visual information, chemicals, temperature, and humidity can be detected with the same chip and output as a single image. As a proof of concept, the sensor is equipped on a micro-vehicle, and multimodal environmental sensing and mapping is demonstrated. A multimodal endoscope is also developed, and simultaneous imaging and chemical profiling along a porcine digestive tract is achieved. The multimodal CMOS imager is compact, versatile, and extensible and can be widely applied in microrobots, in vivo medical apparatuses, and other microdevices.

CuMoO4 nanorods-based acetone chemiresistor-enabled non-invasive breathomic-diagnosis of human diabetes and environmental monitoring

A nano-enabled low-trace monitoring system for acetone has the potential to revolutionize breath omics-based non-invasive diagnosis of human diabetes and environmental monitoring technologies. This unprecedented study presents the state-of-the-art facile and economic template-assisted hydrothermal route to fabricate novel CuMoO₄ nanorods for room temperature breath and airborne acetone detection. Physicochemical attribute analysis reveals the formation of crystalline CuMoO₄ nanorods with diameters ranging from 90 to 150 nm, and an optical band gap of approximately 3.87 eV. CuMoO₄ nanorods-based chemiresistor demonstrates excellent acetone monitoring performance, with a sensitivity of approximately 33.85 at a concentration of 125 ppm. Acetone detection is rapid, with a response time of 23 s and fast recovery within 31 s. Furthermore, the chemiresistor exhibits long-term stability and selectivity towards acetone, compared to other interfering volatile organic compounds (VOCs) commonly found in human breath such as ethanol, propanol, formaldehyde, humidity, and ammonia. The linear detection range of acetone from 25 to 125 ppm achieved by the fabricated sensor is well-suited for human breath-based diagnosis of diabetes. This work represents a significant advancement in the field, as it offers a promising alternative to time-consuming and costly invasive biomedical diagnostics, with the potential for application in cleanroom facilities for indoor contamination monitoring. The utilization of CuMoO₄ nanorods as sensing nanoplatform opens new possibilities for the development of nano-enabled, low-trace acetone monitoring technologies for non-invasive diabetes diagnosis and environmental sensing applications.

Advances in Noble Metal-Decorated Metal Oxide Nanomaterials for Chemiresistive Gas Sensors: Overview

Highly sensitive gas sensors with remarkably low detection limits are attractive for diverse practical application fields including real-time environmental monitoring, exhaled breath diagnosis, and food freshness analysis. Among various chemiresistive sensing materials, noble metal-decorated semiconducting metal oxides (SMOs) have currently aroused extensive attention by virtue of the unique electronic and catalytic properties of noble metals. This review highlights the research progress on the designs and applications of different noble metal-decorated SMOs with diverse nanostructures (e.g., nanoparticles, nanowires, nanorods, nanosheets, nanoflowers, and microspheres) for high-performance gas sensors with higher response, faster response/recovery speed, lower operating temperature, and ultra-low detection limits. The key topics include Pt, Pd, Au, other noble metals (e.g., Ag, Ru, and Rh.), and bimetals-decorated SMOs containing ZnO, SnO2, WO3, other SMOs (e.g., In2O3, Fe2O3, and CuO), and heterostructured SMOs. In addition to conventional devices, the innovative applications like photo-assisted room temperature gas sensors and mechanically flexible smart wearable devices are also discussed. Moreover, the relevant mechanisms for the sensing performance improvement caused by noble metal decoration, including the electronic sensitization effect and the chemical sensitization effect, have also been summarized in detail. Finally, major challenges and future perspectives towards noble metal-decorated SMOs-based chemiresistive gas sensors are proposed.

Highly Sensitive and Selective Real-Time Breath Isoprene Detection using the Gas Reforming Reaction of MOF-Derived Nanoreactors

Real-time breath isoprene sensing provides noninvasive methods for monitoring human metabolism and early diagnosis of cardiovascular diseases. Nonetheless, the stable alkene structure and high humidity of the breath hinder sensitive and selective isoprene detection. In this work, we derived well-defined Co₃O₄@polyoxometalate yolk-shell structures using a metal-organic framework template. The inner space, including highly catalytic Co₃O₄ yolks surrounded by a semipermeable polyoxometalate shell, enables stable isoprene to be reformed to reactive intermediate species by increasing the gas residence time and the reaction with the inner catalyst. This sensor exhibited selective isoprene detection with an extremely high chemiresistive response (180.6) and low detection limit (0.58 ppb). The high sensing performance can be attributed to electronic sensitization and catalytic promotion effects. In addition, the reforming reaction of isoprene is further confirmed by the proton transfer reaction-quadrupole mass spectrometry analysis. The practical feasibility of this sensor in smart healthcare applications is exhibited by monitoring muscle activity during the workout.

Metal-organic framework nanocrystal-derived hollow porous materials: Synthetic strategies and emerging applications

Metal-organic frameworks (MOFs) have garnered multidisciplinary attention due to their structural tailorability, controlled pore size, and physicochemical functions, and their inherent properties can be exploited by applying them as precursors and/or templates for fabricating derived hollow porous nanomaterials. The fascinating, functional properties and applications of MOF-derived hollow porous materials primarily lie in their chemical composition, hollow character, and unique porous structure. Herein, a comprehensive overview of the synthetic strategies and emerging applications of hollow porous materials derived from MOF-based templates and/or precursors is given. Based on the role of MOFs in the preparation of hollow porous materials, the synthetic strategies are described in detail, including (1) MOFs as removable templates, (2) MOF nanocrystals as both self-sacrificing templates and precursors, (3) MOF@secondary-component core-shell composites as precursors, and (4) hollow MOF nanocrystals and their composites as precursors. Subsequently, the applications of these hollow porous materials for chemical catalysis, electrocatalysis, energy storage and conversion, and environmental management are presented. Finally, a perspective on the research challenges and future opportunities and prospects for MOF-derived hollow materials is provided.

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MOFs have garnered multi-disciplinary attention due to their unique inherent properties

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Various synthetic strategies of MOFs-derived hollow porous materials are summarized

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Emerging applications of MOFs-derived hollow porous materials are reviewed

Bio-Based Rigid Polyurethane Foams Modified with C-MOF/MWCNTs and TBPBP as Building Insulation Materials: Synergistic Effect and Corresponding Mechanism for Enhancing Fire and Smoke Safety

Rigid polyurethane foams (RPUFs) as building insulation materials quickly burn and release a lot of heat, smoke, and carbon monoxide, and cause human safety risk and severe environmental pollution. To mitigate these disadvantages, MOF/MWCNTs were fabricated via mixing Cu ions' partly substituted framework of ZIF-67 and MWCNTs, and further calcinated MOF/MWCNTs (C-MOF/MWCTs) was newly generated by calcinating MOF/MWCNTs in air. Then, MOF/MWCNTs and C-MOF/MWCNTs were respectively employed together with a phosphorus-nitrogen-containing reactive flame retardant (TBPBP) to prepare renewable bio-based rigid polyurethane foam, including RPUF-T/MOF/MWCNTs 2 and RPUF-T/C-MOF/MWCNTs 2. The characterization results showed that RPUF-T/C-MOF/MWCNTs 2 had better performance than RPUF-T/MOF/MWCNTs 2 and neat RPUF. Compared to neat RPUF, the compressive strength, limiting oxygen index value, and the mass char residue in cone calorimetry test of RPUF-T/C-MOF/MWCNTs 2, respectively, were increased by 105.93%, 46.35%, and 347.32%; meanwhile, the total heat release rate, total smoke production, total carbon monoxide product, and total carbon dioxide product were reduced by 47.97%, 50.46%, 41.38%, 43.37%, respectively. This study provides a referable method for preparing RPUFs with good physical properties, fire, and smoke safety, which is favorable for human safety and environmental protection as new building insulation materials.

Synthesis of various dimensional metal organic frameworks (MOFs) and their hybrid composites for emerging applications - A review

Metal organic frameworks (MOFs) represent the organic and inorganic hybrid porous materials. MOFs are low dense and highly porous materials which in turn provide large surface area that can accumulate and store numerous molecules within the pores. The pore size may also act as a mesh to separate molecules. The porous nature of MOFs is beneficial for altering the intrinsic properties of the materials. Over the past decade, different types of hybrid MOFs have been reported in combination with polymers, carbon materials, metal nanoparticles, metal oxides, and biomolecules for various applications. MOFs have also been used in the fabrication of electronic devices, sensors, energy storage, gas separation, supercapacitors, drug delivery and environmental clean-up. In this review, the unique structural orientation, exceptional properties and recent applications of MOFs have been discussed in the first section along with their porosity, stability and other influencing factors. In addition, various methods and techniques involved in the synthesis and designing of MOFs such as solvothermal, electrochemical, mechanochemical, ultrasonication and microwave methods are highlighted. In order to understand the scientific feasibility of MOFs in developing new products, various strategies have been applied to obtain different dimensional MOFs (0D, 1D, 2D and 3D) and their composite materials are also been conferred. Finally, the future prospects of MOFs, remaining challenges, research gaps and possible solutions that need to be addressed by advanced experimental design, computational models, simulation techniques and theoretical concepts have been deliberated.

ZIF-X (8, 67) based nanostructures for gas-sensing applications

ZIF-8 and ZIF-67 are the most investigated zeolitic imidazolate frameworks (ZIFs) materials that have aroused enormous scientific interests in numerous areas of application including electrochemistry, gas storage, separation, and sensors by reason of their fascinating structural properties. Recently, there is a rapidly growing demand for chemical gas sensors for the detection of various analytes in widespread applications including environmental pollution monitoring, clinical analysis, wastewater analysis, industrial applications, food quality, consumer products, and automobiles. In general, the key to the development of superior gas sensors is exploring innovative sensing materials. ZIF-X (8, 67) based nanostructures have demonstrated great potential as ideal sensing materials for high-performance sensing applications. In this review, the general properties and applications of ZIF-X (8, 67) including gas storage and gas adsorption are first summarized, and then the recent progress of ZIF-X (8, 67) based nanostructures for gas-sensing applications and the structure-property correlations are summarized and analyzed.

Defective ZnO Nanoflowers Decorated by Ultra-Fine Pd Clusters for Low-Concentration CH4 Sensing: Controllable Preparation and Sensing Mechanism Analysis

To detect low concentration of CH4 is indeed meaningful in industrial manufacturing, such as the petrochemical industry and natural gas catalysis, but it is not easy to detect low concentration of CH4 due to its high symmetrical and stable structure. In this work, defect-rich ZnO1−x nanoflowers (NFs) were synthesized by a two-step route so as to obtain defect-enhanced gas-sensing performance, namely hydrothermal synthesis followed by H2 treatment. In order to achieve low-concentration detection of CH4, the ultra-thin Pd clusters’ (Cs, diameter about 1–2 nm) sensitizer was synthesized and decorated onto the surface of ZnO1−x NFs. It is found that Pd Cs-2/ZnO1−x gas sensors show enhanced gas-sensing properties to CH4, even at ppm concentration level. At its optimal working temperature of 260 °C, the gas response to 50 ppm CH4 can reach 5.0 with good gas selectivity; the response and recovery time is only 16.2 and 13.8 s, respectively. In the Results, we discussed the CH4 gas-sensing mechanism deeply. Overall, it is very necessary to detect low-concentration methane safely. It is possible for further safe detection of low-concentration methane gas in the future.

Multilayered Mesoporous Composite Nanostructures for Highly Sensitive Label-Free Quantification of Cardiac Troponin-I

Cardiac troponin-I (cTnI) is a well-known biomarker for the diagnosis and control of acute myocardial infarction in clinical practice. To improve the accuracy and reliability of cTnI electrochemical immunosensors, we propose a multilayer nanostructure consisting of Fe₃O₄-COOH labeled anti-cTnI monoclonal antibody (Fe₃O₄-COOH-Ab₁) and anti-cTnI polyclonal antibody (Ab₂) conjugated on Au-Ag nanoparticles (NPs) decorated on a metal–organic framework (Au-Ag@ZIF-67-Ab₂). In this design, Fe₃O₄-COOH was used for separation of cTnI in specimens and signal amplification, hierarchical porous ZIF-67 extremely enhanced the specific surface area, and Au-Ag NPs synergically promoted the conductivity and sensitivity. They were additionally employed as an immobilization platform to enhance antibody loading. Electron microscopy images indicated that Ag-Au NPs with an average diameter of 1.9 ± 0.5 nm were uniformly decorated on plate-like ZIF-67 particles (with average size of 690 nm) without any agglomeration. Several electrochemical assays were implemented to precisely evaluate the immunosensor performance. The square wave voltammetry technique exhibited the best performance with a sensitivity of 0.98 mA mL cm⁻² ng⁻¹ and a detection limit of 0.047 pg mL⁻¹ in the linear range of 0.04 to 8 ng mL⁻¹.

Gas sensing based on metal-organic frameworks: Concepts, functions, and developments

Effective detection of pollutant gases is vital for protection of natural environment and human health. There is an increasing demand for sensing devices that are equipped with high sensitivity, fast response/recovery speed, and remarkable selectivity. Particularly, attention is given to the designability of sensing materials with porous structures. Among diverse kinds of porous materials, metal-organic frameworks (MOFs) exhibit high porosity, high degree of crystallinity and exceptional chemical activity. Their strong host-guest interactions with guest molecules facilitate the application of MOFs in adsorption, catalysis and sensing systems. In particular, the tailorable framework/composition and potential for post-synthetic modification of MOFs endow them with widely promising application in gas sensing devices. In this review, we outlined the fundamental aspects and applications of MOFs for gas sensors, and discussed various techniques of monitoring gases based on MOFs as functional materials. Insights and perspectives for further challenges faced by MOFs are discussed in the end.

Online Accurate Detection of Breath Acetone Using Metal Oxide Semiconductor Gas Sensor and Diffusive Gas Separation

Breath acetone (BrAce) level is an indicator of lipid oxidation rate, which is crucial for evaluating the status of ketoacidosis, ketogenic diet, and fat burning during exercise. Despite its usefulness, detecting BrAce accurately is challenging because exhaled breath contains an enormous variety of compounds. Although many sensors and devices have been developed for BrAce measurement, most of them were tested with only synthetic or spiked breath samples, and few can detect low concentration BrAce in an online manner, which is critical for extending application areas and the wide acceptance of the technology. Here, we show that online detection of BrAce can be achieved using a metal oxide semiconductor acetone sensor. The high accuracy measurement of low concentration BrAce was enabled by separating major interference gases utilizing their large diffusion coefficients, and the accuracy is further improved by the correction of humidity effect. We anticipate that the approach can push BrAce measurement closer to being useful for various applications.

Ab Initio Study of Hydrostable Metal-Organic Frameworks for Postsynthetic Modification and Tuning toward Practical Applications

Metal-organic frameworks (MOFs), a subclass of nanoporous coordination polymers, have emerged as one of the most promising next-generation materials. The postsynthetic modification method, a strategy that provides tunability and control of these materials, plays an important role in enhancing its properties and functionalities. However, knowing adjustments which leads to a desired structure-function a priori remains a challenge. In this comprehensive study, the intermolecular interactions between 21 industrially important gases and a hydrostable STAM-17-OEt MOF were investigated using density functional theory. Substitutions on its 5-ethoxy isophthalate linker included two classes of chemical groups, electron-donating (-NH₂, -OH, and -CH₃) and electron-withdrawing (-CN, -COOH, and -F), as well as the effect of mono-, di-, and tri-substitutions. This resulted in 651 unique MOF-gas complexes. The adsorption energies at the ground state and room temperature, bond lengths, adsorption geometry, natural bond orbital analysis of the electric structure, HOMO-LUMO interactions, and the predicted zwitterionic properties are presented and discussed. This study provides a viable strategy for the functionalization, which leads to the strongest affinity for each gas, an insight into the role of different chemical groups in adsorbing various gas molecules, and identifies synthetic routes for moderating the gas adsorption capacity and reducing water adsorption. Recommendations for various applications are discussed. A custom Python script to assess and visualize the hypothetical separation of two equal gas mixtures of interest is provided. The methodology presented here provides new opportunities to expand the chemical space and physical properties of STAM-17-OEt and advances the development of other hydrostable MOFs.

Enhanced NO2 gas-sensing performance by core-shell SnO2/ZIF-8 nanospheres

Timely detection of harmful, poisonous and air pollutant gases is of vital importance to the protection of human beings from exposure to rigorous gases. The development of gas-sensing devices based on sphere-like porous SnO₂/ZIF-8 nanocomposites is required to overcome this challenge. Nanostructures with high surface area, more porosity and hollow interior provide plenty of active cites for high responses in metal oxide gas sensors. The engineered gas sensors have excellent sensing sensitivity (164), rapid response and recovery times (60, 45 s), and favorable selectivity for NO₂ gases under 300 °C. Consequently, NO₂ gas sensors based on core-shell SnO₂/ZIF-8 nanospheres are regarded viable capacity industrial applicants.

The Synergistic Properties and Gas Sensing Performance of Functionalized Graphene-Based Sensors

The detection of toxic gases has long been a priority in industrial manufacturing, environmental monitoring, medical diagnosis, and national defense. The importance of gas sensing is not only of high benefit to such industries but also to the daily lives of people. Graphene-based gas sensors have elicited a lot of interest recently, due to the excellent physical properties of graphene and its derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO). Graphene oxide and rGO have been shown to offer large surface areas that extend their active sites for adsorbing gas molecules, thereby improving the sensitivity of the sensor. There are several literature reports on the promising functionalization of GO and rGO surfaces with metal oxide, for enhanced performance with regard to selectivity and sensitivity in gas sensing. These synthetic and functionalization methods provide the ideal combination/s required for enhanced gas sensors. In this review, the functionalization of graphene, synthesis of heterostructured nanohybrids, and the assessment of their collaborative performance towards gas-sensing applications are discussed.

Metal-organic frameworks for advanced transducer based gas sensors: review and perspectives

The development of gas sensing devices to detect environmentally toxic, hazardous, and volatile organic compounds (VOCs) has witnessed a surge of immense interest over the past few decades, motivated mainly by the significant progress in technological advancements in the gas sensing field. A great deal of research has been dedicated to developing robust, cost-effective, and miniaturized gas sensing platforms with high efficiency. Compared to conventional metal-oxide based gas sensing materials, metal-organic frameworks (MOFs) have garnered tremendous attention in a variety of fields, including the gas sensing field, due to their fascinating features such as high adsorption sites for gas molecules, high porosity, tunable morphologies, structural diversities, and ability of room temperature (RT) sensing. This review summarizes the current advancement in various pristine MOF materials and their composites for different electrical transducer-based gas sensing applications. The review begins with a discussion on the overview of gas sensors, the significance of MOFs, and their scope in the gas sensing field. Next, gas sensing applications are divided into four categories based on different advanced transducers: chemiresistive, capacitive, quartz crystal microbalance (QCM), and organic field-effect transistor (OFET) based gas sensors. Their fundamental concepts, gas sensing ability towards various gases, sensing mechanisms, and their advantages and disadvantages are discussed. Finally, this review is concluded with a summary, existing challenges, and future perspectives.

Emerging Theranostic Nanomaterials in Diabetes and Its Complications

Diabetes mellitus (DM) refers to a group of metabolic disorders that are characterized by hyperglycemia. Oral subcutaneously administered antidiabetic drugs such as insulin, glipalamide, and metformin can temporarily balance blood sugar levels, however, long-term administration of these therapies is associated with undesirable side effects on the kidney and liver. In addition, due to overproduction of reactive oxygen species and hyperglycemia-induced macrovascular system damage, diabetics have an increased risk of complications. Fortunately, recent advances in nanomaterials have provided new opportunities for diabetes therapy and diagnosis. This review provides a panoramic overview of the current nanomaterials for the detection of diabetic biomarkers and diabetes treatment. Apart from diabetic sensing mechanisms and antidiabetic activities, the applications of these bioengineered nanoparticles for preventing several diabetic complications are elucidated. This review provides an overall perspective in this field, including current challenges and future trends, which may be helpful in informing the development of novel nanomaterials with new functions and properties for diabetes diagnosis and therapy.

State-of-the-Art Research on Chemiresistive Gas Sensors in Korea: Emphasis on the Achievements of the Research Labs of Professors Hyoun Woo Kim and Sang Sub Kim

This review presents the results of cutting-edge research on chemiresistive gas sensors in Korea with a focus on the research activities of the laboratories of Professors Sang Sub Kim and Hyoun Woo Kim. The advances in the synthesis techniques and various strategies to enhance the gas-sensing performances of metal-oxide-, sulfide-, and polymer-based nanomaterials are described. In particular, the gas-sensing characteristics of different types of sensors reported in recent years, including core-shell, self-heated, irradiated, flexible, Si-based, glass, and metal-organic framework sensors, have been reviewed. The most crucial achievements include the optimization of shell thickness in core-shell gas sensors, decrease in applied voltage in self-heated gas sensors to less than 5 V, optimization of irradiation dose to achieve the highest response to gases, and the design of selective and highly flexible gas sensors-based WS2 nanosheets. The underlying sensing mechanisms are discussed in detail. In summary, this review provides an overview of the chemiresistive gas-sensing research activities led by the corresponding authors of this manuscript.

Application of Metal-Organic Framework-Based Composites for Gas Sensing and Effects of Synthesis Strategies on Gas-Sensitive Performance

Gas sensing materials, such as semiconducting metal oxides (SMOx), carbon-based materials, and polymers have been studied in recent years. Among of them, SMOx-based gas sensors have higher operating temperatures; sensors crafted from carbon-based materials have poor selectivity for gases and longer response times; and polymer gas sensors have poor stability and selectivity, so it is necessary to develop high-performance gas sensors. As a porous material constructed from inorganic nodes and multidentate organic bridging linkers, the metal-organic framework (MOF) shows viable applications in gas sensors due to its inherent large specific surface area and high porosity. Thus, compounding sensor materials with MOFs can create a synergistic effect. Many studies have been conducted on composite MOFs with three materials to control the synergistic effects to improve gas sensing performance. Therefore, this review summarizes the application of MOFs in sensor materials and emphasizes the synthesis progress of MOF composites. The challenges and development prospects of MOF-based composites are also discussed.

Gas sensing materials roadmap

Gas sensor technology is widely utilized in various areas ranging from home security, environment and air pollution, to industrial production. It also hold great promise in non-invasive exhaled breath detection and an essential device in future internet of things. The past decade has witnessed giant advance in both fundamental research and industrial development of gas sensors, yet current efforts are being explored to achieve better selectivity, higher sensitivity and lower power consumption. The sensing layer in gas sensors have attracted dominant attention in the past research. In addition to the conventional metal oxide semiconductors, emerging nanocomposites and graphene-like two-dimensional materials also have drawn considerable research interest. This inspires us to organize this comprehensive 2020 gas sensing materials roadmap to discuss the current status, state-of-the-art progress, and present and future challenges in various materials that is potentially useful for gas sensors.

General Strategy to Fabricate Porous Co-Based Bimetallic Metal Oxide Nanosheets for High-Performance CO Sensing

Two-dimensional (2D) porous bimetallic oxide nanosheets are attractive for high-performance gas sensing because of their porous structures, high surface areas, and cooperative effects. Nevertheless, it is still a huge challenge to synthesize these nanomaterials. Herein, we report a general strategy to fabricate porous cobalt-based bimetallic oxide nanosheets (Co-M-O NSs, M = Cu, Mn, Ni, and Zn) with an adjustable Co/M ratio and the homogeneous composition using metal-organic framework (MOF) nanosheets as precursors. The obtained Co-M-O NS possesses the porous nanosheet structure and ultrahigh specific surface areas (146.4-220.7 m² g^-1), which enhance the adsorption of CO molecules, support the transport of electrons, and expose abundant active sites for CO-sensing reaction. As a result, the Co-M-O NS exhibited excellent sensing performances including high response, low working temperature, fast response-recovery, good selectivity and stability, and ppb-level detection limitation toward CO. In particular, the Co-Mn-O NS showed the highest response of 264% to 100 ppm CO at low temperature (175 °C). We propose that the excellent sensing performance is ascribed to the specific porous nanosheet structure, the relatively highly active Co³⁺ ratio resulting from cation substitution, and large amounts of chemisorbed oxygen species on the surface. Such a general strategy can also be introduced to design noble-metal-free bimetallic metal oxide nanosheets for gas sensing, catalysis, and other energy-related fields.

Investigation of doping effects of different noble metals for ethanol gas sensors based on mesoporous In2O3

Elaborating the sensitization effects of different noble metals on In₂O₃has great significance in providing an optimum method to improve ethanol sensing performance. In this study, long-range ordered mesoporous In₂O₃has been fabricated through replicating the structure of SBA-15. Different noble metals (Au, Ag, Pt and Pd) with the same doping amount (1 at%) have been introduced by anin situdoping routine. The results of the gas sensing investigation indicate that the gas responses towards ethanol can be obviously increased by doping different noble metals. In particular, the best sensing performance towards ethanol detection can be achieved through Pd doping, and the sensors based on Pd-doped In₂O₃not only possess the highest response (39.0-100 ppm ethanol) but also have the shortest response and recovery times at the optimal operating temperature of 250 °C. The sensing mechanism of noble metal doped materials can be attributed to the synergetic effect combining 'catalysis' and 'electronic and chemical sensitization' of noble metals. In particular, the chemical state of the noble metal also has a great influence on the gas sensing mechanism. A detailed explanation of the enhancement of gas sensing performance through noble metal doping is presented in the gas sensing mechanism part of the manuscript.

Metal Oxide Based Heterojunctions for Gas Sensors: A Review

The construction of heterojunctions has been widely applied to improve the gas sensing performance of composites composed of nanostructured metal oxides. This review summarises the recent progress on assembly methods and gas sensing behaviours of sensors based on nanostructured metal oxide heterojunctions. Various methods, including the hydrothermal method, electrospinning and chemical vapour deposition, have been successfully employed to establish metal oxide heterojunctions in the sensing materials. The sensors composed with the built nanostructured heterojunctions were found to show enhanced gas sensing performance with higher sensor responses and shorter response times to the targeted reducing or oxidising gases compare with those of the pure metal oxides. Moreover, the enhanced gas sensing mechanisms of the metal oxide-based heterojunctions to the reducing or oxidising gases are also discussed, with the main emphasis on the important role of the potential barrier on the accumulation layer.

Colorimetric Sensor for Online Accurate Detection of Breath Acetone

Breath acetone (BrAce) is a validated biomarker of lipid oxidation and has been extensively studied for many applications, such as monitoring ketoacidosis in diabetes, guiding ketogenic diet, and measuring fat burning during exercise. Although many sensors have been reported for BrAce measurement, most of the contributions tested only synthetic or spiked breath samples, because of the complex components of human breath. Here, we show that online accurate detection of BrAce can be achieved using a colorimetric sensor. The high selectivity is enabled by the specific reaction between acetone and hydroxylamine sulfate, and the sensor has a high agreement with a reference instrument in ketosis monitoring. We anticipate that the colorimetric acetone sensor can be applied to various health-related applications.

Non-Magnetic Bimetallic MOF-Derived Porous Carbon-Wrapped TiO2/ZrTiO4 Composites for Efficient Electromagnetic Wave Absorption

Non-magnetic bimetallic MOF-derived porous carbon-wrapped TiO₂/ZrTiO₄ composites are firstly used for efficient electromagnetic wave absorption. The electromagnetic wave absorption mechanisms including enhanced interfacial polarization and essential conductivity are intensively discussed. Modern communication technologies put forward higher requirements for electromagnetic wave (EMW) absorption materials. Metal-organic framework (MOF) derivatives have been widely concerned with its diverse advantages. To break the mindset of magnetic-derivative design, and make up the shortage of monometallic non-magnetic derivatives, we first try non-magnetic bimetallic MOFs derivatives to achieve efficient EMW absorption. The porous carbon-wrapped TiO₂/ZrTiO₄ composites derived from PCN-415 (TiZr-MOFs) are qualified with a minimum reflection loss of - 67.8 dB (2.16 mm, 13.0 GHz), and a maximum effective absorption bandwidth of 5.9 GHz (2.70 mm). Through in-depth discussions, the synergy of enhanced interfacial polarization and other attenuation mechanisms in the composites is revealed. Therefore, this work confirms the huge potentials of non-magnetic bimetallic MOFs derivatives in EMW absorption applications.

Bimetal oxide CuO/Co3O4 derived from Cu ions partly-substituted framework of ZIF-67 for toluene catalytic oxidation

A MOF-templated method is developed to prepare bimetal oxide CuO/Co₃O₄ by in situ pyrolysis of Cu²⁺ partly-substituted ZIF-67 precursor. The physicochemical properties of CuO/Co₃O₄ are studied by various characterizations such as X-ray diffraction, Raman analysis, transmission electron microscope, scanning electron microscope, N₂ adsorption-desorption measurement, X-ray photoelectron spectroscope, O₂ temperature-programmed desorption, H₂ temperature-programmed reduction, etc. Comparison with CuO, Co₃O₄ and Mix-CuO/Co₃O₄, 90 % of both toluene conversion and mineralization over CuO/Co₃O₄ are fulfilled at around 229 °C under the condition of 1000 ppm toluene and weight hour space velocity =20,000 mL/(g h), which is promoted more than 40 °C. The better catalytic performance of CuO/Co₃O₄ attributes to high mutual dispersion of two oxides, porous structure, lower temperature reducibility, abundant lattice defects, more active oxygen species, higher Co³⁺/Co²⁺ and O_latt/O_ads molar ratios. Meanwhile, CuO/Co₃O₄ exhibits a better catalytic stability at different conversions and a good tolerance to 10 vol.% of water vapour. The investigation of temperature-dependent active oxygen species and in-situ DRIFTS results reveal that toluene oxidation on CuO/Co₃O₄ obeys Mars van Krevelen mechanism.

Inorganic-Diverse Nanostructured Materials for Volatile Organic Compound Sensing

Environmental pollution related to volatile organic compounds (VOCs) has become a global issue which attracts intensive work towards their controlling and monitoring. To this direction various regulations and research towards VOCs detection have been laid down and conducted by many countries. Distinct devices are proposed to monitor the VOCs pollution. Among them, chemiresistor devices comprised of inorganic-semiconducting materials with diverse nanostructures are most attractive because they are cost-effective and eco-friendly. These diverse nanostructured materials-based devices are usually made up of nanoparticles, nanowires/rods, nanocrystals, nanotubes, nanocages, nanocubes, nanocomposites, etc. They can be employed in monitoring the VOCs present in the reliable sources. This review outlines the device-based VOC detection using diverse semiconducting-nanostructured materials and covers more than 340 references that have been published since 2016.

Rational Design of Semiconductor-Based Chemiresistors and their Libraries for Next-Generation Artificial Olfaction

Artificial olfaction based on gas sensor arrays aims to substitute for, support, and surpass human olfaction. Like mammalian olfaction, a larger number of sensors and more signal processing are crucial for strengthening artificial olfaction. Due to rapid progress in computing capabilities and machine-learning algorithms, on-demand high-performance artificial olfaction that can eclipse human olfaction becomes inevitable once diverse and versatile gas sensing materials are provided. Here, rational strategies to design a myriad of different semiconductor-based chemiresistors and to grow gas sensing libraries enough to identify a wide range of odors and gases are reviewed, discussed, and suggested. Key approaches include the use of p-type oxide semiconductors, multinary perovskite and spinel oxides, carbon-based materials, metal chalcogenides, their heterostructures, as well as heterocomposites as distinctive sensing materials, the utilization of bilayer sensor design, the design of robust sensing materials, and the high-throughput screening of sensing materials. In addition, the state-of-the-art and key issues in the implementation of electronic noses are discussed. Finally, a perspective on chemiresistive sensing materials for next-generation artificial olfaction is provided.