Gas sensor is an indispensable part of modern society with wide applications in environmental monitoring, healthcare, food industry, public safety, etc. With the development of sensor technology, wireless communication, smart monitoring terminal, cloud storage/computing technology, and artificial intelligence, smart gas sensors represent the future of gas sensing due to their merits of real-time multifunctional monitoring, early warning function, and intelligent and automated feature. Various electronic and optoelectronic gas sensors have been developed for high-performance smart gas analysis. With the development of smart terminals and the maturity of integrated technology, flexible and wearable gas sensors play an increasing role in gas analysis. This review highlights recent advances of smart gas sensors in diverse applications. The structural components and fundamental principles of electronic and optoelectronic gas sensors are described, and flexible and wearable gas sensor devices are highlighted. Moreover, sensor array with artificial intelligence algorithms and smart gas sensors in “Internet of Things” paradigm are introduced. Finally, the challenges and perspectives of smart gas sensors are discussed regarding the future need of gas sensors for smart city and healthy living.

Multi-target detection is a notable development direction in the field of analytical methods, boasting significant potential for breakthroughs in complex sample detection. Herein, an extended gate field-effect transistor (EGFET) sensor array for ultrasensitive and multi-target detection of antibiotics is developed. To enhance the recognition ability for diverse antibiotics, a dual-recognition strategy based on tailor-made molecularly imprinted polymer (MIP) and aptamer is adopted to fabricate the extended gate electrode (sensing electrode). The dual-recognition strategy-based EGFET exhibits superior sensitivity, selectivity, and stability compared to single recognition probes due to the synergistic effect of the affinities of MIP and specific aptamer. The developed sensor array can detect three types of antibiotics, including ampicillin, amoxicillin, and kanamycin, at the same time with record-low detection limit (fM level) and a detection range spanning six orders of magnitude. The practical detection of antibiotics in various samples (tap water, river water, milk, honey, and artificial urine) further validates the feasibility of the sensor array in practical applications. The key advantages of flexibility, high sensitivity and selectivity, and multifunctionality demonstrate the high potential of EGFET sensor array for simultaneous detection of multiple target molecules.

Conductive metal-organic frameworks (MOFs) have received increasing attention in recent years and present high application potential as sensing elements in electronic sensors. In this study, flexible field-effect transistor (FET) sensors based on conductive MOF, i.e., Ni3(HHTP)2, have been constructed. This Ni3(HHTP)2 sensor has high sensitivity (detection limit of 56 ppb) as well as superior selectivity for NO2 detection at room temperature, which is demonstrated by accurate gas detection in a mixed gas atmosphere. Moreover, by employing six flexible substrates, i.e., polyimide (PI), tape (PET), facemask, paper cup, tablecloth, and take-out bag (textile), we successfully demonstrate the universality of the flexible sensor construction with conductive MOF as sensing film on various substrates. This study of conductive MOF-based flexible electronic sensors offers a new opportunity for a wide range of sensing applications with wearable and portable electronic devices.

Two-dimensional violet phosphorus (VP) has emerged as a new sensing material in various sensing applications due to its unique electrical properties and high stability among allotropes of phosphorus. Currently, the research of the VP-based analysis method is at the early stage. In this work, a VP nanosheet-based field-effect transistor (FET) sensor is reported for the detection of NO2 and N2O gases with extraordinary sensing performance. This sensor can achieve excellent sensitivity of up to ∼50% current change/ppm and a low detection limit of 5.9 ppb and enables the NO2 analysis in various mixed gases. Moreover, this sensor can effectively distinguish between NO2 and N2O gases, which is a big challenge for current FET or chemiresistor gas sensors. The different sensing behaviors of the VP sensor to NO2 and N2O gases have been investigated, and the mechanism study shows that the adsorption energy, bond length of the gas molecule on the VP surface, and the decomposition of N2O led to the differential responses. This work is one of the pioneer studies of VP gas sensors and presents a new sensing method for the discriminative analysis of NO2 and N2O for greenhouse gas emission monitoring and air quality control.

Antibiotic residues cause increasing concern in environmental ecology and public health, which needs efficient analysis strategy for monitoring and control. In this study, a fast, specific, and ultrasensitive sensor based on field-effect transistor (FET) has been proposed for the detection of ampicillin (AMP). The sensor involves monolayer tungsten disulfide (WS2) nanosheet as the sensing channel, single-stranded DNA (ssDNA) as the sensing probe, and gold nanoparticle (Au NP) as the linker. The WS2/Au/ssDNA FET sensor responds rapidly to AMP in a wide linear detection range (10- 12-10- 6 M) and has low limit of detection (0.556 pM), which meets the permissible standards of AMP in water and food. The sensing mechanism study suggests that the excellent sensor response results from the increased number of negative charges in the Debye length and the consequent accu-mulation of holes in WS2 channel after the addition of AMP. Moreover, satisfactory sensing performance was confirmed in real water samples, indicating the potential application of the proposed method in practical AMP detection. The reported FET sensing strategy provides new insights in antibiotic analysis for risk assessment and control.

MXenes have shown exceptional electrochemical properties and demonstrate great promise in chemiresistive gas analysis applications. However, their sensing applications still face low sensitivity and specificity, slow response, and poor stability among the many challenges. Herein, a novel synthetic approach is reported to produce single-atom Pt (Pt SA)-implanted Ti3C2Tx MXene nanosheets as the sensing channel in field-effect transistor (FET) gas sensors. This is a pioneer study of single-atom catalysts loaded on MXene nanosheets for gas detection, which demonstrates that Pt SA can greatly enhance the sensing performance of pristine Ti3C2Tx. The Pt SA-Ti3C2Tx sensor exhibits high sensitivity and specificity toward ppb level (a low detection limit of 14 ppb) triethylamine (TEA) with good multicycle sensing performance. Moreover, the mechanism study and density functional theory (DFT) simulation show that the chemical sensitization effect and TEA adsorption enhancement from highly catalytic and uniformly distributed Pt SA lead to the enhanced sensing performances. This work presents a new prospect of single-atom catalysts for gas analysis applications, which will promote the development of cutting-edge sensing techniques for gas detection for public health and environment.

Ti3C2Tx MXene recently draws increasing attention in gas sensors, but their sensing performances, e.g., sensi-tivity, specificity, and ambient environment applicability, still needs improvement to meet the real application requirement. Herein, a black phosphorus quantum dots (BP QDs) modified Ti3C2Tx nanosheet (BQ/Ti3C2Tx) prepared by van der Waals self-assembly method is reported and applied as the sensing channel in field-effect transistor (FET) sensor. This sensor shows excellent NO2 detection performance at room temperature in terms of wide linear detection range (50 ppb to 10 ppm), short response time, and high specificity. Moreover, two important factors in practical gas detection, i.e., humidity and temperature, are systematically investigated. An efficient calibration strategy is proposed to eliminate the humidity effect, ensuring the accuracy of NO2 detec-tion. The mechanism study confirms that the enhanced sensitivity and selectivity are due to the increased adsorption energy of BQ/Ti3C2Tx for target gas molecules and the forming of P-O-Ti bond. This study offers new insights in the development of sensitive, selective, and reliable gas sensors with functionalized MXene materials.

Color-coding analysis from chemicals of concern is in great demand, but faces low sensitivity and specificity, low resolution, and complex processing among the many challenges. Here, this work resolves these issues to enable the elusive quantitative detection of a variety of fluoroquinolone (FQ) antibiotics. A fluorescent sensor based on the dual-emitting lanthanide metal-organic frameworks combining Tb3+ and Eu3+ as the luminescent center and 1,3,5-benzenetricarboxylic acid as the ligand is constructed. Due to the different sensitization effects to lanthanide metals and different inherent fluorescence emissions of FQs, the sensor exhibits characteristic color variations towards nine FQ and enables the discriminative detection of multiple antibiotics with self-calibrated signals. For the first time, a polynomial surface fitting process is developed to correlate the coordinates of color-coding map and target concentration for quantitative analysis. Moreover, a smartphone-enabled sensing system is demonstrated for on-site imaging analysis of antibiotics. The demonstrated innovative antibiotic detection and color-coding-based signal processing approach will inform the development of cutting-edge analysis systems for public health and environmental monitoring.

Mercury is one of the leading chemicals of concern and receives much attention in environmental safety. It is of great necessity to develop advanced Hg2+ analysis method for rapid detection and monitoring. Field-effect transistor (FET) sensor, an emerging electronic sensor, has received great attention in environmental analysis since it has unique advantages in achieving rapid analysis of chemicals. Herein, an FET sensor is constructed with Ti3C2Tx MXene as the channel material to detect Hg2+ in water. The sensor displays rapid and selective response to Hg2+. Moreover, the sensor achieves satisfactory performance in Hg2+ detection in high salinity environment (1 M NaCl), which benefits its applications in real water analysis. Based on the investigation of sensing mechanism, the strong response of Ti3C2Tx MXene FET sensor to Hg2+ is due to the adsorption and reduction of Hg2+ to Hg+ on the Ti3C2Tx surface. This reported label-free Ti3C2Tx MXene platform can detect Hg2+ in high salinity environment with high specificity, which has significant application potential for on-site monitoring and risk assessment of Hg2+ in aqueous systems.

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 Ti3C2Tx 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.

Room temperature gas sensing performance of FET sensors with graphene, black phosphorous, and MoS2 is investigated, which depends on the intrinsic electronic properties of the 2D channel.

Novel hypercross-linked β-cyclodextrin nanospheres are rapidly synthesized with 4-amino-6-hydroxy-2-mercaptopyrimidine as a cross-linker within 5 min at a low temperature of 60 °C in the open water phase, thereby radically simplifying and accelerating the conventional time-consuming and environmentally harsh synthesis of cyclodextrin-containing polymers. The nanospheres demonstrate excellent and controllable performance as alkali-responsive nanocarriers and selective adsorbents for antibiotics.

Silver (Ag) is a widely used heavy metal, and its oxidation state (Ag+) causes serious harm to organisms after bioaccumulation and biomagnification, posing urgent demand for the rapid, efficient, and simply operated Ag+ detection techniques. In this work, a fast, portable, and label-free Ag+ detection sensor based on a Ti3C2Tx MXene field-effect transistor (FET) is reported. The Ti3C2Tx MXene works as the sensing element in the FET sensor, which shows excellent sensing performance, i.e., fast response (few seconds) and good sensitivity and selectivity to Ag+ without any detection label or probe. Utilizing the visual photograph, transmission electron microscopy image, and Ag elemental mapping analysis, the sensing mechanism of the label-free Ti3C2Tx MXene FET sensor is demonstrated to be the in situ reduction of Ag+ and the formation of Ag nanoparticles (AgNPs). Moreover, Ag+ detection in real samples shows that the proposed FET devices have satisfactory sensing capability for Ag+ in tap water and river water. This study puts forward a novel FET strategy for Ag+ detection in aqueous systems, which is of essential and inspiring meaning for motivating the potential applications of MXene-based sensor devices in analytical applications and the realization of on-site environmental monitoring.

MXenes, a group of emerging two-dimensional (2D) transition metal carbides or nitrides, have attracted wide interest due to their unique structures and properties. Their stability and applicability in different media especially in an alkaline environment are directly associated with their potential applications and are not yet explored. Herein, a field-effect transistor (FET) is fabricated with single/double-layer Ti3C2Tx MXene. The Ti3C2Tx FET indicator shows a fast (∼1 s), sensitive, and selective response to alkali. Moreover, the device can work even in a high-salinity (2 M NaCl) environment, suggesting its high anti-interference ability for alkali in a high-ionic-strength environment. Using an in situ morphological image evolution study, it is demonstrated that the response signal results from alkali-induced denaturation of Ti3C2Tx nanosheets. The Ti3C2Tx-based alkali FET indicator and systematic evaluation on alkali-induced structure evolution of Ti3C2Tx provide essential insights into MXene-based FETs and future applications of MXene in alkaline environments.

Monolayer MoS2 (ML-MoS2) with various polymorphic phases attracts growing interests for device applications in recent years. Herein, a field-effect transistor (FET) gas sensor is developed on the basis of monolayer MoS2 with a heterophase of a 1T metallic phase and a 2H semiconducting phase. Lithium-exfoliated MoS2 nanosheets own a monolayer structure with rich active sites for gas adsorption. With thermal annealing from 50 to 300 °C, the initial lithium-exfoliated 1T-phase MoS2 gradually transforms into the 2H phase, during which the 1T and 2H heterophases can be modulated. The 1T/2H heterophase MoS2 shows p-type semiconducting properties and prominent adsorption capability for NO2 molecules. The highest response is observed for 100 °C annealed MoS2 of a 40% 1T phase and a 60% 2H phase, which shows a sensitivity up to 25% toward 2 ppm NO2 at room temperature in a very short time (10 s) and a lower limit of detection down to 25 ppb. This study demonstrates that the gas detection capability of ML-MoS2 could be boosted with the heterophase construction, which brings new insights into transition-metal dichalcogenide gas sensors.

Mercury ions (Hg2+) are the dominant mercury species in water, and have severe toxic effects on human health and the ecosystem. Due to the limitations of traditional analytical methods for mercury determination, e.g., complex pretreatment, costly instruments, and being unsuitable for low-level online sensing, numerous efforts have been made to develop rapid, low-cost, and portable sensors for Hg2+ determination. The excellent characteristics of graphene, i.e., a unique two-dimensional structure, large surface area, high conductivity and carrier mobility, and good optical and electrochemical properties, lead to intense studies on graphene-based Hg2+ sensors with desirable performance and broad application prospects. The scope of this review is recent advances in graphene Hg2+ sensors with a special focus on optical, electrochemical, and electronic sensors. The working principles and advantages of representative graphene Hg2+ sensors are introduced and compared with traditional methods. Moreover, challenges as well as potential solutions and research directions of mercury sensors are discussed, which will guide future research of Hg2+ sensors in biomedical and environmental monitoring applications.

Although aptamer has been demonstrated as an important probe for antibiotic determination, the selective sensing of different antibiotics is still a challenge due to their structure similarities and wide folding degrees of aptamer. Herein, a field-effect transistor using MoS2 nanosheet as the channel and an aptamer DNA (APT) with its configuration shaped by a complementary strand DNA (CS) is employed for kanamycin (KAN) determination. This probe structure contributes to an enhanced selectivity and reliability with reduced device-to-device variations. This MoS2/APT/CS sensor shows time-dependent performance in antibiotic sensing. Prolonged detection time (20 s-300 s) leads to an enhanced sensitivity (1.85-4.43 N-1) and a lower limit of detection (1.06-0.66 nM), while a shorter detection time leads to a broader linear working range. A new sensing mechanism relying on charge release from probe is proposed, which is based on the "replacement reaction" between KAN and APT-CS. This sensor exhibits an extremely high selectivity (selectivity coefficient of 12.8) to kanamycin over other antibiotics including streptomycin, tobramycin, amoxicillin, ciprofloxacin and chloramphenicol. This work demonstrates the merits of probe engineering in label-free antibiotic detection with FET sensor, which presents significant promises in sensitive and selective chemical and biological sensing.

Increasing demand for timely and accurate environmental pollution monitoring and control requires new sensing techniques with outstanding performance, i.e., high sensitivity, high selectivity, and reliability. Metal–organic frameworks (MOFs), also known as porous coordination polymers, are a fascinating class of highly ordered crystalline coordination polymers formed by the coordination of metal ions/clusters and organic bridging linkers/ligands. Owing to their unique structures and properties, i.e., high surface area, tailorable pore size, high density of active sites, and high catalytic activity, various MOF-based sensing platforms have been reported for environmental contaminant detection including anions, heavy metal ions, organic compounds, and gases. In this review, recent progress in MOF-based environmental sensors is introduced with a focus on optical, electrochemical, and field-effect transistor sensors. The sensors have shown unique and promising performance in water and gas contaminant sensing. Moreover, by incorporation with other functional materials, MOF-based composites can greatly improve the sensor performance. The current limitations and future directions of MOF-based sensors are also discussed.