Highly selective detection of methanol over ethanol by a handheld gas sensor

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 Efficient, Dual-Functional Self-Assembled Electrospun Nanofiber Filters for Simultaneous PM Removal and On-Site Eye-Readable Formaldehyde Sensing

Air pollution containing particulate matter (PM) and volatile organic compounds has caused magnificent burdens on individual health and global economy. Although advances in highly efficient or multifunctional nanofiber filters have been achieved, many existing filters can only deal with one type of air pollutant, such as capturing PM or absorbing and detecting toxic gas. Here, highly efficient, dual-functional, self-assembled electrospun nanofiber (SAEN) filters were developed for simultaneous PM removal and onsite eye-readable formaldehyde sensing fabricated on a commercial fabric mask. With the use of an electrolyte solution containing a formaldehyde-sensitive colorimetric agent as a collector during electrospinning, the one-step fabrication of the dual-functional SAEN filter on commercial masks, such as a fabric mask and a daily disposable mask, was achieved. The electrolyte solution also allowed the uniform deposition of electrospun nanofibers, thereby achieving the high efficiency of PM filtration with an increased quality factor up to twice that of commercial masks. The SAEN filter enabled onsite and eye-readable formaldehyde gas detection by changing its color from yellow to red under a 5 ppm concentrated formaldehyde gas atmosphere. The repetitive fabrication and detachment of the SAEN filter on a fabric mask minimized the waste of the mask while maintaining high filtration efficiency by replenishing the SAEN filters and reusing the fabric mask. Given the dual functionality of SAEN filters, this process could provide new insights into designing and developing high performance and dual-functional electrospun nanofiber filters for various applications, including individual protection and indoor purification applications.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s42765-023-00279-3.

In-Memory-Computed Low-Frequency Noise Spectroscopy for Selective Gas Detection Using a Reducible Metal Oxide

Concerns about indoor and outdoor air quality, industrial gas leaks, and medical diagnostics are driving the demand for high-performance gas sensors. Owing to their structural variety and large surface area, reducible metal oxides hold great promise for constructing a gas-sensing system. While many earlier reports have successfully obtained a sufficient response to various types of target gases, the selective detection of target gases remains challenging. In this work, a novel method, low-frequency noise (LFN) spectroscopy is presented, to achieve selective detection using a single FET-type gas sensor. The LFN of the sensor is accurately modeled by considering the charge fluctuation in both the sensing material and the FET channel. Exposure to different target gases produces distinct corner frequencies of the power spectral density that can be used to achieve selective detection. In addition, a 3D vertical-NAND flash array is used with the fast Fourier transform method via in-memory-computing, significantly improving the area and power efficiency rate. The proposed system provides a novel and efficient method capable of selectively detecting a target gas using in-memory-computed LFN spectroscopy and thus paving the way for the further development in gas sensing systems.

Operando monitoring of a room temperature nanocomposite methanol sensor

The sensing of volatile organic compounds by composites containing metal oxide semiconductors is typically explained via adsorption-desorption and surface electrochemical reactions changing the sensor's resistance. The analysis of molecular processes on chemiresistive gas sensors is often based on indirect evidence, whereas in situ or operando studies monitoring the gas/surface interactions enable a direct insight. Here we report a cross-disciplinary approach employing spectroscopy of working sensors to investigate room temperature methanol detection, contrasting well-characterized nanocomposite (TiO2@rGO-NC) and reduced-graphene oxide (rGO) sensors. Methanol interactions with the sensors were examined by (quasi) operando-DRIFTS and in situ-ATR-FTIR spectroscopy, the first paralleled by simultaneous measurements of resistance. The sensing mechanism was also studied by mass spectroscopy (MS), revealing the surface electrochemical reactions. The operando and in situ spectroscopy techniques demonstrated that the sensing mechanism on the nanocomposite relies on the combined effect of methanol reversible physisorption and irreversible chemisorption, sensor modification over time, and electron/O2 depletion-restoration due to a surface electrochemical reaction forming CO2 and H2O.

Rational design of fluorescent chemosensor for Pd2+ based on the formation of cyclopalladated complex

According to the assumption that the formation of C-Pd bond becomes a cyclopalladated complex (CPC), we designed and synthesized two C-N-N pincer ligands of BODIPY appended 2,2'-bipyridine derivatives (BP and BPB). It has been confirmed that the C-Pd bond does exist and plays a crucial role in "on-off" fluorescence behavior. Based on it, a coordination-induced fluorescence quenching sensor for Pd²⁺ was constructed. The results indicated that BP possessed high sensitivity and specificity for Pd²⁺ in solution. The limit of detection (LOD) of BP is determined to be 0.97 nM within a linear range between 1.0 and 50.0 nM, meanwhile, the platinum-group ions demonstrate no interference. The bio-imaging application of BP was investigated and it exhibited a promising vitro test for fluorescent imaging of Pd²⁺ ions in MCF-7 cells. Meanwhile, BPB coated sensor label for Pd²⁺ was set up. The visible color variation was displayed under UV light with increasing concentrations of Pd²⁺. Briefly speaking, fluorescence probes of BP and BPB offer new approaches for Pd²⁺ detection in a lab and on-site test, as well as the vivo imaging. Then, with the aid of (TD)DFT calculation, the internal reason for the optical difference between the two ligands was disclosed. This concept of CPC containing a Pd-C covalent bond provides a promising perspective of coordination fluorescence sensors.

High-density volatile organic compound monitoring network for identifying pollution sources

The elusive sources of air pollution have hampered effective control across all sectors, with long-term consequences for the greenhouse effect and human health. Multiple monitoring systems have been highly desired for locating the sources. However, when faced with extensive sources, diverse air environments and meteorological conditions, the low spatiotemporal resolution, poor reliability and high cost of existing monitors were significant obstacles to their applications. Extending our previous demonstration of sensitive and reliable electrochemical sensors, we here present a machine-learning-assisted sensor arrays for monitoring typical volatile organic compounds (VOCs), which shows the consistent response with gas chromatography-mass spectrometry in the actual air environment. As a proof-of-concept, a low-cost and high-resolution VOC network of 152 sets of monitors across ~55 km² of mixed-used land is established in southwest Beijing. Benefiting from the strong reliability, the pollution sources are revealed by the VOC network and supported by the joint mobile sampling of a vehicle-mounted gas chromatography-mass spectrometry system. With the sustained help of the network, the sources polluted by the local industrial facilities, traffic, and restaurants are effectively site-specific abatement by the local authorities and enterprises during the next half-year. Our findings open up a promising path toward more effective tracing of regional pollution sources, as well as accelerate the long-term transformation of industry and cities.

Enhanced Pd/a-WO3 /VO2 Hydrogen Gas Sensor Based on VO2 Phase Transition Layer

The utilization of clean hydrogen energy is becoming more feasible for the sustainable development of this society. Considering the safety issues in the hydrogen production, storage, and utilization, a sensitive hydrogen sensor for reliable detection is essential and highly important. Though various gas sensor devices are developed based on tin oxide, tungsten trioxide, or other oxides, the relatively high working temperature, unsatisfactory response time, and detection limitation still affect the extensive applications. In the current study, an amorphous tungsten trioxide (a-WO₃ ) layer is deposited on a phase-change vanadium dioxide film to fabricate a phase transition controlled Pd/a-WO₃ /VO₂ hydrogen sensor for hydrogen detection. Results show that both the response time and sensitivity of the hydrogen sensor are improved greatly if increasing the working temperature over the transition temperature of VO₂ . Theoretical calculations also reveal that the charge transfer at VO₂ /a-WO₃ interface becomes more pronounced once the VO₂ layer transforms to the metal state, which will affect the migration barrier of H atoms in a-WO₃ layer and thus improve the sensor performance. The current study not only realizes a hydrogen sensor with ultrahigh performance based on VO₂ layer, but also provides some clues for designing other gas sensors with phase-change material.

Highly Selective Wearable Alcohol Homologue Sensors Derived from Pt-Coated Truncated Octahedron Au

Unhealthy alcohol inhalation is among the top 10 causes of preventable death. However, the present alcohol sensors show poor selectivity among alcohol homologues. Herein, Pt-coated truncated octahedron Au (Pt_m@Au_to) as the electrocatalyst for a highly selective electrochemical sensor toward alcohol homologues has been designed. The alcohol sensor is realized by distinguishing the electro-oxidation behavior of methanol (MeOH), ethanol (EtOH), or isopropanol (2-propanol). Intermediates from alcohols are further oxidized to CO₂ by Pt_m@Au_to, resulting in different oxidation peaks in cyclic voltammograms and successful distinction of alcohols. Pt_m@Au_to is then modified on wearable glove-based sensors for monitoring actual alcohol samples (MeOH fuel, vodka, and 2-propanol hand sanitizer), with good mechanical performance and repeatability. The exploration of the Pt_m@Au_to-based wearable alcohol sensor is expected to be suitable for environmental measurement with high selectivity for alcohol homologues or volatile organic compounds.

Rapid and Label-Free Methanol Identification in Alcoholic Beverages Utilizing a Textile Grid Impregnated with Chiral Nematic Liquid Crystals

Methanol contamination of alcoholic drinks can lead to severe health problems for human beings including poisoning, headache, blindness, and even death. Therefore, having access to a simple and inexpensive way for monitoring beverages is vital. Herein, a portable, low cost, and easy to use sensor is fabricated based on the exploitation of chiral nematic liquid crystals (CLCs) and a textile grid for detection of methanol in two distinct alcoholic beverages: red wine and vodka. The working principle of the sensor relies on the reorientation of the liquid crystal molecules upon exposure to the contaminated alcoholic beverages with different concentrations of methanol (0, 2, 4, and 6 wt %) and the changes in the observed colorful textures of the CLCs as well as the intensity of the output light. The proposed sensor is label free and rapid.

A Dual Sensing Platform for Human Exhaled Breath Enabled by Fe-MIL-101-NH2 Metal-Organic Frameworks and its Derived Co/Ni/Fe Trimetallic Oxides

Limited by the insufficient active sites and the interference from breath humidity, designing reliable gas sensing materials with high activity and moisture resistance remains a challenge to analyze human exhaled breath for the translational application of medical diagnostics. Herein, the dual sensing and cooperative diagnosis is achieved by utilizing metal-organic frameworks (MOFs) and its derivative. The Fe-MIL-101-NH₂ serves as the quartz crystal microbalance humidity sensing layer, which exhibits high selectivity and rapid response time (16 s/15 s) to water vapor. Then, the Co²⁺ and Ni²⁺ cations are further co-doped into Fe-MIL-101-NH₂ host to obtain the derived Co/Ni/Fe trimetallic  oxides (CoNiFe-MOS-n). The chemiresistive CoNiFe-MOS-n sensor displays the high sensitivity (560) and good selectivity to acetone, together with a lower original resistance compared with Fe₂ O₃ and NiFe₂ O₄ . Moreover, as a proof-of-concept application, synergistic integration of Fe-MIL-101-NH₂ and derived CoNiFe-MOS-n is carried out. The Fe-MIL-101-NH₂ is applied as moisture sorbent materials, which realize a sensitivity compensation of CoNiFe-MOS-n sensors for the detection of acetone (biomarker gas of diabetes). The findings provide an insight for effective utilization of MOFs and the derived materials to achieve a trace gas detection in exhaled breath analysis.

Engineering CuMOF in TiO2 Nanochannels as Flexible Gas Sensor for High-Performance NO Detection at Room Temperature

As a marker molecule in respiratory gases for the pulmonary disease asthma, nitric oxide (NO) has attracted much attention for real-time gas monitoring. However, low sensitivity, poor selectivity, and high operating temperature limit the practical applications of metal oxide semiconductor (MOS) based chemiresistor gas sensors. Herein, by deliberately introducing metal-organic frameworks (MOFs) in free-standing TiO₂ nanochannels (NCs), a chemiresistor gas sensor with excellent detection ability and outstanding selective traits is developed for sensing NO at room temperature (RT). The precisely engineered Cu(II)-based MOF Cu-TCA (H₃TCA = tricarboxytriphenyl amine) induces more active surface in the NCs, causing the buildup of CuTCA/TiO₂ p-n heterojunctions that improve the sensing response at RT just via a simple UV irradiation (λ = 365 nm). Importantly, the specialized reductive reaction of Cu(II) by NO enables a remarkable selectivity toward NO analysis. Owing to the synergistic large active surface and chemical sensitization effects from Cu-TCA, the resulting Cu-TCA/TiO₂ NCs show outstanding sensing performance; i.e., the response ((R_gas - R_air)/R_air) reaches 124% at 50 ppm of NO with a detection limit of 140 ppb at RT. In addition, the response time decreases to 25.6% if the system is subjected to UV irradiation. The as-formed sensing membrane is also demonstrated to be practically effective for flexible and wearable sensing devices for quantitative NO analysis. This study facilitates the use of MOFs to achieve synergistically enhanced selectivity and sensitivity to develop high-performance gas sensors.

Tuning the sensing responses towards room-temperature hypersensitive methanol gas sensor using exfoliated graphene-enhanced ZnO quantum dot nanostructures

A suitable and non-invasive methanol sensor workable in ambient temperature conditions with a high response has gained wide interest to prevent detrimental consequences for industrial workers from its low-level intoxication. In this work, we present a tunable and highly responsive ppb-level methanol gas sensor device working at room temperature via a bottom-up synthetic approach using exfoliated graphene sheet (EGs) and ZnO quantum dots (QDs) on an aluminum anodic oxide (AAO) template. It is verified that EGs-supported AAO with a vertical electrode configuration enabled high and fast-responsive methanol sensing. Moreover, the hydroxyl and carboxyl groups of the high surface area EGs and ZnO QDs with a 3.37 eV bandgap efficiently absorbing UV light led to 56 times high response due to the enhanced polarization on the sensor surface compared to non-UV-radiated EGs/AAO at 800 ppb of methanol. The optimal resonance frequency of methanol is determined to be 100 kHz, which could detect methanol with high response of 2.65% at 100 ppm. The limit of detection (LOD) concentration is obtained at 2 ppb level. This study demonstrates the potential of UV-assisted ZnO, EGs, and AAO-based capacitance sensor material for rapidly detecting hazardous gaseous light organic molecules at ambient conditions, and the overall approach can be easily expanded to a novel non-invasive monitoring strategy for light and hazardous volatile organic exposures.

Advances in functional guest materials for resistive gas sensors

Resistive gas sensors are considered promising candidates for gas detection, benefiting from their small size, ease of fabrication and operation convenience. The development history, performance index, device type and common host materials (metal oxide semiconductors, conductive polymers, carbon-based materials and transition metal dichalcogenides) of resistive gas sensors are firstly reviewed. This review systematically summarizes the functions, functional mechanisms, features and applications of seven kinds of guest materials (noble metals, metal heteroatoms, metal oxides, metal-organic frameworks, transition metal dichalcogenides, polymers, and multiple guest materials) used for the modification and optimization of the host materials. The introduction of guest materials enables synergistic effects and complementary advantages, introduces catalytic sites, constructs heterojunctions, promotes charge transfer, improves carrier transport, or introduces protective/sieving/enrichment layers, thereby effectively improving the sensitivity, selectivity and stability of the gas sensors. The perspectives and challenges regarding the host-guest hybrid materials-based gas sensors are also discussed.

Design of an Artificial Opal/Photonic Crystal Interface for Alcohol Intoxication Assessment: Capillary Condensation in Pores and Photonic Materials Work Together

Alcohol intoxication has a dangerous effect on human health and is often associated with a risk of catastrophic injuries and alcohol-related crimes. A demand to address this problem adheres to the design of new sensor systems for the real-time monitoring of exhaled breath. We introduce a new sensor system based on a porous hydrophilic layer of submicron silica particles (SiO₂ SMPs) placed on a one-dimensional photonic crystal made of Ta₂O₅/SiO₂ dielectric layers whose operation relies on detecting changes in the position of surface wave resonance during capillary condensation in pores. To make the active layer of SiO₂ SMPs, we examine the influence of electrostatic interactions of media, particles, and the surface of the crystal influenced by buoyancy, gravity force, and Stokes drag force in the frame of the dip-coating preparation method. We evaluate the sensing performance toward biomarkers such as acetone, ammonia, ethanol, and isopropanol and test sensor system capabilities for alcohol intoxication assessment. We have found this sensor to respond to all tested analytes in a broad range of concentrations. By processing the sensor signals by principal component analysis, we selectively determined the analytes. We demonstrated the excellent performance of our device for alcohol intoxication assessment in real-time.

Nanocatalytic Interface to Decode the Phytovolatile Language for Latent Crop Diagnosis in Future Farms

Crop diseases cause the release of volatiles. Here, the use of an SnO₂-based chemoresistive sensor for early diagnosis has been attempted. Ionone is one of the signature volatiles released by the enzymatic and nonenzymatic cleavage of carotene at the latent stage of some biotic stresses. To our knowledge, this is the first attempt at sensing volatiles with multiple oxidation sites, i.e., ionone (4 oxidation sites), from the phytovolatile library, to derive stronger signals at minimum concentrations. Further, the sensitivity was enhanced on an interdigitated electrode by the addition of platinum as the dopant for a favorable space charge layer and for surface island formation for reactive interface sites. The mechanistic influence of oxygen vacancy formation was studied through detailed density functional theory (DFT) calculations and reactive oxygen-assisted enhanced binding through X-ray photoelectron spectroscopy (XPS) analysis.

Microheater Integrated Nanotube Array Gas Sensor for Parts-Per-Trillion Level Gas Detection and Single Sensor-Based Gas Discrimination

Real-time monitoring of health threatening gases for chemical safety and human health protection requires detection and discrimination of trace gases with proper gas sensors. In many applications, costly, bulky, and power-hungry devices, normally employing optical gas sensors and electrochemical gas sensors, are used for this purpose. Using a single miniature low-power semiconductor gas sensor to achieve this goal is hardly possible, mostly due to its selectivity issue. Herein, we report a dual-mode microheater integrated nanotube array gas sensor (MINA sensor). The MINA sensor can detect hydrogen, acetone, toluene, and formaldehyde with the lowest measured limits of detection (LODs) as 40 parts-per-trillion (ppt) and the theoretical LODs of ∼7 ppt, under the continuous heating (CH) mode, owing to the nanotubular architecture with large sensing area and excellent surface catalytic activity. Intriguingly, unlike the conventional electronic noses that use arrays of gas sensors for gas discrimination, we discovered that when driven by the pulse heating (PH) mode, a single MINA sensor possesses discrimination capability of multiple gases through a transient feature extraction method. These above features of our MINA sensors make them highly attractive for distributed low-power sensor networks and battery-powered mobile sensing systems for chemical/environmental safety and healthcare applications.

Sensors for Volatile Organic Compounds

This paper provides an overview of recent developments in the field of volatile organic compound (VOC) sensors, which are finding uses in healthcare, safety, environmental monitoring, food and agriculture, oil industry, and other fields. It starts by briefly explaining the basics of VOC sensing and reviewing the currently available and quickly progressing VOC sensing approaches. It then discusses the main trends in materials' design with special attention to nanostructuring and nanohybridization. Emerging sensing materials and strategies are highlighted and their involvement in the different types of sensing technologies is discussed, including optical, electrical, and gravimetric sensors. The review also provides detailed discussions about the main limitations of the field and offers potential solutions. The status of the field and suggestions of promising directions for future development are summarized.

In Situ Exsolution Catalyst: An Innovative Approach to Develop Highly Selective and Sensitive Gas Sensors

The gas sensing characteristics of oxide semiconductors can be enhanced by loading noble metal or metal oxide catalysts. The uniform distribution of nanoscale catalysts with high thermal stability over the sensing materials is essential for sensors operating at elevated temperatures. An in situ exsolution process, which can be applied to catalysts, batteries, and sensors, provides a facile synthetic route for developing second-phase nanoparticles with uniform distribution, excellent thermochemical stability, and strong adhesion to the mother phase. In this study, we investigated the effect of Co-exsolved nanoparticles on the gas sensing characteristics of La_0.43Ca_0.37Co_0.06Ti_0.94O_3-d (LCCoT). The amount and size of the Co-exsolved nanoparticles on the surface of the perovskite mother phase were adjusted depending on the reduction temperature of the exsolution process. The LCCoT with Co-exsolved nanoparticles prepared by reduction at 700 °C exhibited a response (resistance ratio) of 116.3 to 5 ppm ethanol at 350 °C, which was 10-fold higher than the response of a sensor without exsolution. The high gas response was attributed to the catalytic effect promoted by the uniformly distributed Co-exsolved nanoparticles and the formation of p-n junctions on the sensing surface during reduction. Additionally, we demonstrated the catalytic effect of Co-exsolved nanoparticles using a proton transfer reaction-quadrupole mass spectrometer. By controlling the amount and distribution of exsolved nanoparticles on semiconductor chemiresistors, a new pathway for designing high-performance gas sensors with enhanced thermal stability can be achieved.

Highly Selective Mixed Potential Methanol Gas Sensor Based on a Ce0.8Gd0.2O1.95 Solid Electrolyte and Au Sensing Electrode

A Ce_0.8Gd_0.2O_1.95-based mixed potential type sensor attached with a commercially available Au paste sensing electrode material was fabricated to detect methanol. The optimum working temperature of the sensor was 545 °C, and the response value to 100 ppm methanol was -53 mV. The selectivity of the sensor was poor. The addition of a 4A molecular sieve filter layer and the method of pattern recognition were combined to improve it. Only gas molecules smaller than the pore diameter of the 4A molecular sieve were able to pass through the zeolite channel, and the selectivity coefficient of the sensor to methanol was improved by adding the filter layer. Meanwhile, there was an obvious distinction between the response and recovery times of the sensor toward methanol, ethanol, acetone, n-butanol, and n-pentanol. Next, the pattern recognition method was adopted. The relationship between the response value and the logarithm of gas concentration and the relationship between the maximum rate of the response process and the gas concentration were plotted separately. By comprehensively considering the two characteristic parameters of the response value and the maximum value of the differential response signal, the purpose of qualitative identification of gas types and quantitative analysis of gas concentrations was hopefully achieved.

Anchoring Platinum Clusters onto Oxygen Vacancy-Modified In2O3 for Ultraefficient, Low-Temperature, Highly Sensitive, and Stable Detection of Formaldehyde

To avoid carcinogenicity, formaldehyde gas, currently being only detected at higher operating temperatures, should be selectively detected in time with ppb concentration sensitivity in a room-temperature indoor environment. This is achieved in this work through introducing oxygen vacancies and Pt clusters on the surface of In₂O₃ to reduce the optimal operating temperature from 120 to 40 °C. Previous studies have shown that only water participates in the competitive adsorption on the sensor surface. Here, we experimentally confirm that the adsorbed water on the fabricated sensor surface is consumed via a chemical reaction due to the strong interaction between the oxygen vacancies and Pt clusters. Therefore, the long-term stability of formaldehyde gas detection is improved. The results of theoretical calculations in this work reveal that the excellent formaldehyde gas detection of Pt/In₂O_3-x originates from the electron enrichment due to the surface oxygen vacancies and the molecular adsorption and activation ability of Pt clusters on the surface. The developed Pt/In₂O_3-x sensor has potential use in the ultraefficient, low-temperature, highly sensitive, and stable detection of indoor formaldehyde at an operating temperature as low as room temperature.

π-Electronic Coassembled Microflake Sensors with Förster Resonance Energy Transfer Enhanced Discrimination of Methanol and Ethanol

In the field of fluorescence-based gas sensing, it is very difficult to realize the distinction of the molecules with similar chemical properties and slight structural differences (e.g., methanol and ethanol). Herein, we fabricated coassemblies of energy-donor molecule 1 (M1) and energy-acceptor molecule 2 (M2) with different molar ratios. These materials can selectively differentiate methanol and ethanol by regulating the distance of exciton migration of donor M1 by embedding energy-acceptor M2. More importantly, methanol can also be detected from the mixture vapors of methanol and ethanol. These results provide a new approach for developing fluorescence sensors that are highly sensitive to molecules with very small difference in the chemical structures.

Oxide/ZIF-8 Hybrid Nanofiber Yarns: Heightened Surface Activity for Exceptional Chemiresistive Sensing

Though highly promising as powerful gas sensors, oxide semiconductor chemiresistors have low surface reactivity, which limits their selectivity, sensitivity, and reaction kinetics, particularly at room temperature (RT) operation. It is proposed that a hybrid design involving the nanostructuring of oxides and passivation with selective gas filtration layers can potentially overcome the issues with surface activity. Herein, unique bi-stacked heterogeneous layers are introduced; that is, nanostructured oxides covered by conformal nanoporous gas filters, on ultrahigh-density nanofiber (NF) yarns via sputter deposition with indium tin oxide (ITO) and subsequent self-assembly of zeolitic imidazolate framework (ZIF-8) nanocrystals. The NF yarn composed of ZIF-8-coated ITO films can offer heightened surface activity at RT because of high porosity, large surface area, and effective screening of interfering gases. As a case study, the hybrid sensor demonstrated remarkable sensing performances characterized by high NO selectivity, fast response/recovery kinetics (>60-fold improvement), and large responses (12.8-fold improvement @ 1 ppm) in comparison with pristine yarn@ITO, especially under highly humid conditions. Molecular modeling reveals an increased penetration ratio of NO over O₂ to the ITO surface, indicating that NO oxidation is reliably prevented and that the secondary adsorption sites provided by the ZIF-8 facilitate the adsorption/desorption of NO, both to and from ITO.

Water-Selective Nanostructured Dehumidifiers for Molecular Sensing Spaces

Humidity and moisture effects, frequently called water poisoning, in surroundings are inevitable for various molecular sensing devices, strongly affecting their sensing characteristics. Here, we demonstrate a water-selective nanostructured dehumidifier composed of ZnO/TiO₂/CaCl₂ core-shell heterostructured nanowires for molecular sensing spaces. The fabricated nanostructured dehumidifier is highly water-selective without detrimental adsorptions of various volatile organic compound molecules and can be repeatedly operated. The thermally controllable and reversible dehydration process of CaCl₂·nH₂O thin nanolayers on hydrophilic ZnO/TiO₂ nanowire surfaces plays a vital role in such water-selective and repeatable dehumidifying operations. Furthermore, the limitation of detection for sensing acetone and nonanal molecules in the presence of moisture (relative humidity ∼ 90%) was improved more than 20 times using nanocomposite sensors by operating the developed nanostructured dehumidifier. Thus, the proposed water-selective nanostructured dehumidifier offers a rational strategy and platform to overcome water poisoning issues for various molecular and gas sensors.

Differentiable detection of ethanol/methanol in biological fluids using prompt graphene-based electrochemical nanosensor coupled with catalytic complex of nickel oxide/8-hydroxyquinoline

Serious health hazards of volatile organic compounds such as methanol and ethanol for living species and their adverse effects on the environment raised a global requirement for developing a portable, precise, and sensitive detection platform capable of simultaneous and differentiable detection of alcohols in aquatic biological and non-biological fluids. Each year, methanol toxicity causes serious healthcare problems and leads to high mortalities in developing countries. Hence, designing and developing a practical nanosensor for diagnostic applications and environmental monitoring is crucial. Herein, we have addressed this demand by fabricating a portable, ultra-sensitive, and precise nanosensor capable of simultaneous and differentiable detection of methanol and ethanol in any aquatic specimen in about 1 min. The nanosensor is composed of the integrated graphene oxide (GO) flakes with the catalytic complex of NiOx and 8-hydroxyquinoline (8HQ) capable of identification of methanol and ethanol with an analytical sensitivity/detection limit of 30.66 μA(μmol/mL)-1.cm-2/6.87 nmol mL-1 and 118.99 μA(μmol/mL)-1.cm-2/1.80 nmol mL-1 using voltammetric assays between the linear range of 0.014-0.01 μmol mL-1 and 0.83-0.58 μmol mL-1, respectively. The outcome of the assessments exhibited the favorable capability of the prepared nanosensor for precise/prompt detection of alcohols in blood specimens and showed an ideal correlation with the outcome of the gold standard.

H2S sensing under various humidity conditions with Ag nanoparticle functionalized Ti3C2Tx MXene field-effect transistors

Despite the critical need to monitor H₂S, a hazardous gas, in environmental and medical settings, there are currently no reliable methods for rapid and sufficiently discriminative H₂S detection in real-world humid environments. Herein, targeted hybridizing of Ti₃C₂T_x MXene with Ag nanoparticles on a field-effect transistor (FET) platform has led to a step change in MXene sensing performance down to ppb levels, and enabled the very high selectivity and fast response/recovery time under room temperature for H₂S detection in humid conditions. For the first time, we present a novel relative humidity (RH) self-calibration strategy for the accurate detection of H₂S. This strategy can eliminate the influence of humidity and enables the accurate quantitative detection of gas in the total RH range. We further elucidate that the superior H₂S sensing performance is attributed to the electron and chemical sensitization effects. This study opens new avenues for the development of high-performance MXene-based sensors and offers a viable approach for addressing real-world humidity effect for gas sensors generally.

Handheld Device for Selective Benzene Sensing over Toluene and Xylene

More than 1 million workers are exposed routinely to carcinogenic benzene, contained in various consumer products (e.g., gasoline, rubbers, and dyes) and released from combustion of organics (e.g., tobacco). Despite strict limits (e.g., 50 parts per billion (ppb) in the European Union), routine monitoring of benzene is rarely done since low-cost sensors lack accuracy. This work presents a compact, battery-driven device that detects benzene in gas mixtures with unprecedented selectivity (>200) over inorganics, ketones, aldehydes, alcohols, and even challenging toluene and xylene. This can be attributed to strong Lewis acid sites on a packed bed of catalytic WO3 nanoparticles that prescreen a chemoresistive Pd/SnO2 sensor. That way, benzene is detected down to 13 ppb with superior robustness to relative humidity (RH, 10-80%), fulfilling the strictest legal limits. As proof of concept, benzene is quantified in indoor air in good agreement (R2 ≥ 0.94) with mass spectrometry. This device is readily applicable for personal exposure assessment and can assist the implementation of low-emission zones for sustainable environments.

Pt Single Atom-Induced Activation Energy and Adsorption Enhancement for an Ultrasensitive ppb-Level Methanol Gas Sensor

As an important organic chemical raw material, methanol is used in various industries but is harmful to human health. Developing an effective and accurate detection device for methanol is an urgent need. Herein, we demonstrate a novel gas-sensing material with a Pt single atom supported on a porous Ag-LaFeO₃@ZnO core-shell sphere (Ag-LaFeO₃@ZnO-Pt) with a high specific surface area (192.08 m²·g^-1). Based on this, the surface activity of the Ag-LaFeO₃@ZnO-Pt gas sensor is enhanced obviously, which improved the working temperature and detection limit for methanol gas. Consequently, this sensor possesses an ultrahigh sensitivity of 453.02 for 5 ppm methanol gas at a working temperature of 86 °C and maintains a high sensitivity of 21.25 even at a concentration as low as 62 ppb. The sensitivity of Ag-LaFeO₃@ZnO-Pt to methanol gas is increased by 6.69 times compared with the Ag-LaFeO₃@ZnO core-shell sphere (Ag-LaFeO₃@ZnO). Additionally, the minimum detection limit is found to be 3.27 ppb. Detailed theoretical calculations revealed that the unoccupied 5d state of Pt single atoms increases the adsorption and activation energy of methanol and oxygen, which facilities methanol gas-sensing performance. This work will provide a novel strategy to design high-performance gas-sensing materials.

Selectivity in trace gas sensing: recent developments, challenges, and future perspectives

Selectivity is one of the most crucial figures of merit in trace gas sensing, and thus a comprehensive assessment is necessary to have a clear picture of sensitivity, selectivity, and their interrelations in terms of quantitative and qualitative views.

2D Molybdenum Carbide MXenes for Enhanced Selective Detection of Humidity in Air

2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo2 CTx flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub-µm MXene multilayer for different organic vapors and humidity at 101 -104  ppm in dry air is studied. Reasonably, the low-noise resistance signal allows the detection of H2 O down to 10 ppm. Moreover, humidity suppresses the response of Mo2 CTx to organic analytes due to the blocking of adsorption active sites. By measuring the impedance of MXene layers as a function of ac frequency in the 10-2 -106  Hz range, it is shown that operation principle of the sensor is dominated by resistance change rather than capacitance variations. The sensor transfer function allows to conclude that the Mo2 CTx chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2 eV. Density functional theory calculations, elucidating the Mo2 C surface interaction with organic analytes and H2 O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance.

Discriminating BTX Molecules by the Nonselective Metal Oxide Sensor-Based Smart Sensing System

Discriminating structurally similar volatile organic compounds (VOCs) molecules, such as benzene, toluene, and three xylene isomers (BTX), remains a significant challenge, especially, for metal oxide semiconductor (MOS) sensors, in which selectivity is a long-standing challenge. Recent progress indicates that temperature modulation of a single MOS sensor offers a powerful route in extracting the features of adsorbed gas analytes than conventional isothermal operation. Herein, a rectangular heating waveform is applied on NiO-, WO₃-, and SnO₂-based sensors to gradually activate the specific gas/oxide interfacial redox reaction and generate rich (electrical) features of adsorbed BTX molecules. Upon several signal preprocessing steps, the intrinsic feature of BTX molecules can be extracted by the linear discrimination analysis (LDA) or convolutional neural network (CNN) analysis. The combination of three distinct MOS sensors noticeably benefits the recognition accuracy (with a reduced number of training iterations). Finally, a prototype of a smart BTX recognition system (including sensing electronics, sensors, Wi-Fi module, UI, PC, etc.) based on temperature modulation has been explored, which enables a prompt, accurate, and stable identification of xylene isomers in the ambient air background and raises the hope of innovating the future advanced machine olfactory system.

Combination of Ceramic Laser Micromachining and Printed Technology as a Way for Rapid Prototyping Semiconductor Gas Sensors

The work describes a fast and flexible micro/nano fabrication and manufacturing method for ceramic Micro-electromechanical systems (MEMS)sensors. Rapid prototyping techniques are demonstrated for metal oxide sensor fabrication in the form of a complete MEMS device, which could be used as a compact miniaturized surface mount devices package. Ceramic MEMS were fabricated by the laser micromilling of already pre-sintered monolithic materials. It has been demonstrated that it is possible to deposit metallization and sensor films by thick-film and thin-film methods on the manufactured ceramic product. The results of functional tests of such manufactured sensors are presented, demonstrating their full suitability for gas sensing application and indicating that the obtained parameters are at a level comparable to those of industrial produced sensors. Results of design and optimization principles of applied methods for micro- and nanosystems are discussed with regard to future, wider application in semiconductor gas sensors prototyping.

Analysis of 2D nanomaterial BC3 for COVID-19 biomarker ethyl butyrate sensor

Ethyl butyrate (EB) was identified in recent research as a prominent biomarker of COVID-19, as concentrations of EB were higher in exhaled breath of COVID-19 patients. Electronic sensitivities of pristine, Al- and Si-doped BC₃ nanosheets to the EB molecule were investigated in this study using density functional theory. It is found that the pure BC₃ was ineffective in sensing EB due to low adsorption energy and sensitivity. Aluminum- and silicon-doped BC₃ nanosheets were effective in forming a strong interaction with EB and were also sensitive. Our calculations show that the band gaps of the Al-doped and Si-doped BC₃ sheets were significantly decreased upon EB adsorption, which increased the electrical conductance of the sheets and the sensitivity. However, Si-doped BC₃ had a recovery time of almost 22 hours, making it less potent than Al-doped BC₃, which had a recovery time of just 7.7 minutes. The shorter recovery time of the Al-doped BC₃ sheet is due to its moderate adsorption energy of 25.8 kcal mol^-1. These results can help facilitate the development of an EB biosensor for COVID-19 testing and other similar applications.

Synergistic Integration of Chemo-Resistive and SERS Sensing for Label-Free Multiplex Gas Detection

Practical sensing applications such as real-time safety alerts and clinical diagnoses require sensor devices to differentiate between various target molecules with high sensitivity and selectivity, yet conventional devices such as oxide-based chemo-resistive sensors and metal-based surface-enhanced Raman spectroscopy (SERS) sensors usually do not satisfy such requirements. Here, a label-free, chemo-resistive/SERS multimodal sensor based on a systematically assembled 3D cross-point multifunctional nanoarchitecture (3D-CMA), which has unusually strong enhancements in both "chemo-resistive" and "SERS" sensing characteristics is introduced. 3D-CMA combines several sensing mechanisms and sensing elements via 3D integration of semiconducting SnO2 nanowire frameworks and dual-functioning Au metallic nanoparticles. It is shown that the multimodal sensor can successfully estimate mixed-gas compositions selectively and quantitatively at the sub-100 ppm level, even for mixtures of gaseous aromatic compounds (nitrobenzene and toluene) with very similar molecular structures. This is enabled by combined chemo-resistive and SERS multimodal sensing providing complementary information.

Microcellular sensing media with ternary transparency states for fast and intuitive identification of unknown liquids

Rapid, accurate, and intuitive detection of unknown liquids is greatly important for various fields such as food and drink safety, management of chemical hazards, manufacturing process monitoring, and so on. Here, we demonstrate a highly responsive and selective transparency-switching medium for on-site, visual identification of various liquids. The light scattering–based sensing medium, which is designed to be composed of polymeric interphase voids and hollow nanoparticles, provides an extremely large transmittance window (>95%) with outstanding selectivity and versatility. This sensing medium features ternary transparency states (transparent, semitransparent, and opaque) when immersed in liquids depending on liquid-polymer interactions and diffusion kinetics. Several different types of these transparency-changing media can be configured into an arrayed platform to discriminate a wide variety of liquids and also quantify their mixing ratios. The outstanding versatility and user friendliness of the sensing platform allow the development of a practical tool for discrimination of diverse organic liquids.

Fluorometric trace methanol detection in ethanol and isopropanol in a water medium for application in alcoholic beverages and hand sanitizers

Detection of methanol (MeOH) in an ethanol (EtOH)/isopropanol (ⁱPrOH) medium containing water is crucial to recognize MeOH poisoning in alcoholic beverages and hand sanitizers. Although chemical sensing methods are very sensitive and easy to perform, the chemical similarities between the alcohols make MeOH detection very challenging particularly in the presence of water. Herein, the fluorometric detection of a trace amount of MeOH in EtOH/ⁱPrOH in the presence of water using alcohol coordinated Al(iii)-complexes of an aldehydic phenol ligand containing a dangling pyrazole unit is described. The presence of MeOH in the EtOH/ⁱPrOH causes a change of the complex geometry from tetrahedral (Td) to octahedral (Oh) due to the replacement of the coordinated EtOH/ⁱPrOH by MeOH molecules. The Td-complex exhibited fluorescence but the Oh-species did not, because of the intramolecular photo-induced electron transfer (PET). By interacting the Oh species with water, its one MeOH coordination is replaced by a water molecule followed by the proton transfer from the water to pyrazole-N which generates strong fluorescence by inhibiting the PET. In contrast, the water interaction dissociates the Td-complex to exhibit fluorescence quenching. The water induced reversal of the fluorescence response from the decrease to increase between the absence and presence of MeOH is utilized to detect MeOH in an EtOH/ⁱPrOH medium containing water with a sensitivity of ∼0.03–0.06% (v/v). The presence of water effected the MeOH detection and allows the estimation of the MeOH contamination in alcoholic beverages and hand sanitizers containing large amounts of water.

Detection of MeOH in EtOH/ⁱPrOH and commercial samples based on water induced reversal of fluorescence response from the decrease to increase between the absence and presence of MeOH.

Efficient nickel or copper oxides decorated graphene-polyaniline interface for application in selective methanol sensing

Graphene sheets decorated with nickel or copper oxides that were anchored on polyaniline (denoted as PANI-graphene/NiO and PANI-graphene/CuO) were prepared by a simple, easy to-control electrochemical method and applied as novel materials for sensitive and selective methanol sensing. The fabricated sensors exhibited good electrocatalytic activity, appropriate dynamic linear range (20-1300 mM), sensitivity (0.2-1.5 μA mM^-1 cm^-2) and excellent selectivity towards methanol. It should be highlighted from the selectivity tests that no significant interference was observed from ethanol and other alcohols. To our best knowledge, using inexpensive but efficient transition metals like Ni, Cu instead of Pt, Pd and their composites with PANI, graphene would be scientifically novel and practically feasible approach for sensor fabrication that could be potentially used to identify methanol adulteration in counterfeit alcoholic beverages.

Exclusive and ultrasensitive detection of formaldehyde at room temperature using a flexible and monolithic chemiresistive sensor

Formaldehyde, a probable carcinogen, is a ubiquitous indoor pollutant, but its highly selective detection has been a long-standing challenge. Herein, a chemiresistive sensor that can detect ppb-level formaldehyde in an exclusive manner at room temperature is designed. The TiO₂ sensor exhibits under UV illumination highly selective detection of formaldehyde and ethanol with negligible cross-responses to other indoor pollutants. The coating of a mixed matrix membrane (MMM) composed of zeolitic imidazole framework (ZIF-7) nanoparticles and polymers on TiO₂ sensing films removed ethanol interference completely by molecular sieving, enabling an ultrahigh selectivity (response ratio > 50) and response (resistance ratio > 1,100) to 5 ppm formaldehyde at room temperature. Furthermore, a monolithic and flexible sensor is fabricated successfully using a TiO₂ film sandwiched between a flexible polyethylene terephthalate substrate and MMM overlayer. Our work provides a strategy to achieve exclusive selectivity and high response to formaldehyde, demonstrating the promising potential of flexible gas sensors for indoor air monitoring.

Formaldehyde, a probable carcinogen, is a ubiquitous indoor pollutant, but its ultraselective detection has been a long-standing challenge. Here, the authors develop a chemiresistive sensor that can detect ppb-level formaldehyde in an exclusive manner at room temperature.

Carbonized polymer dots activated hierarchical tungsten oxide for efficient and stable triethylamine sensor

Hierarchical metal oxide semiconductors present great potential in detecting toxic and hazardous gases with special emphasis on the regulation of their structures and compositions to advance sensor performance. Herein, marine polysaccharide derived carbonized polymer dots (CPDs) are presented to activate hierarchical tungsten oxide (WO₃) as efficient and stable triethylamine sensor. Owing to the promoted receptor and transducer function of the oxide/polymer/carbon heterostructure, the CPDs/WO₃ sensor exhibits extraordinary sensing characteristics for triethylamine detection, including higher response (4.3 times), faster response/recovery (4.3 times/2.1 times), lower operating temperature (30 °C) and lower detection limit (2.4 times) as compared with hierarchical WO₃ sensor, which are also superior to most of the previous reports related to triethylamine detection. Importantly, the adsorption-desorption kinetic of WO₃ is found to be enhanced by 67 times after introducing CPDs, mainly derived from abundant slit-like channels for gas diffuse, desirable defect feature as reactive sites, and favorable 0D-2D interface for charge transfer and transport. This work not only establishes an alternative strategy for promoting metal oxide semiconductor gas sensors but also provides a fundamental understanding of CPDs in gas-sensing field.

Biochemical Methanol Gas Sensor (MeOH Bio-Sniffer) for Non-Invasive Assessment of Intestinal Flora from Breath Methanol

Methanol (MeOH) in exhaled breath has potential for non-invasive assessment of intestinal flora. In this study, we have developed a biochemical gas sensor (bio-sniffer) for MeOH in the gas phase using fluorometry and a cascade reaction with two enzymes, alcohol oxidase (AOD) and formaldehyde dehydrogenase (FALDH). In the cascade reaction, oxidation of MeOH was initially catalyzed by AOD to produce formaldehyde, and then this formaldehyde was successively oxidized via FALDH catalysis together with reduction of oxidized form of β-nicotinamide adenine dinucleotide (NAD+). As a result of the cascade reaction, reduced form of NAD (NADH) was produced, and MeOH vapor was measured by detecting autofluorescence of NADH. In the development of the MeOH bio-sniffer, three conditions were optimized: selecting a suitable FALDH for better discrimination of MeOH from ethanol in the cascade reaction; buffer pH that maximizes the cascade reaction; and materials and methods to prevent leaking of NAD+ solution from an AOD-FALDH membrane. The dynamic range of the constructed MeOH bio-sniffer was 0.32-20 ppm, which encompassed the MeOH concentration in exhaled breath of healthy people. The measurement of exhaled breath of a healthy subject showed a similar sensorgram to the standard MeOH vapor. These results suggest that the MeOH bio-sniffer exploiting the cascade reaction will become a powerful tool for the non-invasive intestinal flora testing.

A versatile logic detector and fluorescent film based on Eu-based MOF for swift detection of formaldehyde in solutions and gas phase

Due to the huge threat of formaldehyde (FA) on human beings, the development of chemical sensors for swift detection of FA in solutions and gas phase is highly anticipated. In this paper, a versatile logic detector and a portable fluorescent film based on small-scaled Eu-based MOF were applied successfully to detect FA in solutions and gas phase, respectively. For FA in aqueous solution, the design of logic detector will efficiently identify FA in different concentration ranges: when the FA concentration are 0-500 ppb, 500-1000 ppb and >1000 ppb, the output signals of logic detector are the concentration level of FA ("L", "H" and "VH"), and accompanied by red, purple and blue signal lamps to remind, respectively. For FA in the air, the color of rigid film sensor will gradually change from red to blue with the increase of FA under UV lamp, and the detection limit of gaseous FA is 11.8 ppb. Through the preparation of logic devices and fluorescent films, Eu-based MOF realized swift detection of FA in solutions and gas phase, which will be very helpful to improve the human response level to FA from different emission sources.