Ppm Ammonia Research Articles

Temperature-dependent variation in ammonia sensing properties of ZnO-GO nanocomposites

Industrialization has caused significant environmental and health risks due to the emission of hazardous gases like ammonia. This study investigates the temperature-dependent gas sensing properties of a ZnO-GO nanocomposite synthesized via microwave-assisted combustion. The ammonia sensor, tested from 25 °C (room temperature) to 200 °C, showed its highest sensitivity (72 %) at 100°C for 100 ppm ammonia. It achieved a response time of 94 seconds and a recovery time of 58 seconds. These results demonstrate the potential of ZnO-GO nanocomposite sensors in reducing ammonia-related risks in industrial environments.

A highly sensitive and selective ZnO-based ammonia sensor: Fe- doping effect

Abstract This study investigates the gas sensing capabilities of ZnO and Fe-doped ZnO films prepared via a cost-effective spin coating method. The films were characterized optically, structurally, and morphologically. X-ray diffraction revealed good crystal quality with a polycrystalline nature, and wurzite structure, though crystal quality and crystallite size decreased with Fe doping. Optical measurements indicated an increased optical band gap from 3.205 ± 0.002 to 3.220 ± 0.002 eV after Fe doping. Scanning electron microscope images confirmed spherical grainy structures with reduced grain sizes after Fe doping. Gas sensing measurements at room temperature (RT) at an exposure of vapors of various toxic chemicals (acetone, ethanol, propanol, methanol, and ammonia), demonstrated a highly selective nature of ZnO and Fe-ZnO towards the ammonia. The Fe-doping into ZnO improved the ammonia sensing capability of ZnO film. The Fe-ZnO film exhibited a gas response of 388.8 ± 5.5 at 400 ppm ammonia exposure, which was nearly 10 times larger than that of ZnO film with a response/recovery time of 22/51 s, good stability, and good repeatability. The Fe-ZnO film’s higher response is attributed to its smaller grain sizes and surplus of charge carriers after Fe doping which promote the adsorption of extra oxygen ions onto the film’s surface and the subsequent interaction between the adsorbed oxygen ions ammonia molecules. It could detect up to the lower limit of 1 ppm ammonia with a response of 24.2 ± 0.9, which is better than the previous reports. These results reveal the Fe-ZnO film as a viable material for developing a cost-effective and efficient ammonia sensor at room temperature.

Fabrication of TPU-supported Ti3C2Tx/Ag2S/TiO2 electrospun mat: Dual-functional materials for human motion and gas detection

In our work, a dual-functional sensor for detecting ammonia and human motion was realized by fabrication of TPU-Supported Ti3C2Tx/Ag2S/TiO2 electrospun mat. The synthesis of Ag2S/TiO2–doped Ti3C2Tx nanocomposites was achieved through a simple hydrothermal approach, which served as the sensing component. The findings from the microscopic morphology analysis indicate that the nanocomposites were evenly dispersed across the flexible TPU. The performance of the fabricated flexible gas-sensitive and strain sensors was evaluated, the results showed a 25.2 % response to 100 ppm ammonia at room temperature (25 ℃), which was approximately five times higher than that of the pure Ti3C2Tx gas-sensitive materials. When subjected to a strain range of 40 %–60 %, the response was measured as 378.597, exhibiting a notably high level of linearity (linear fitting value of 0.9849). The stability and repeatability of the flexible sensor make it play a significant role in room temperature gas sensing and motion monitoring.

MXene anchored fabric for room-temperature operated ammonia sensing

Low-cost and fast-response fabric sensors are highly desirable for shop floor worker daily monitoring and early trace ammonia leak detection. However, the difficulty is how to establish a high bonding capacity between the active material and the fabric to meet the needs of sensing performance on practical applications. Herein, MXene anchored fabric sensors have been fabricated by spray casting technology and the impacts of the preparation process on sensor performance have been investigated. When the casting concentration of MXene is 10 mg/mL and the fabric is spandex, the ammonia sensor exhibits the best sensing performance with a response time of 0.62 s for 100 ppm ammonia and a low limit of detection of 0.4 ppm. Notably, when the sensor used for detection ammonia in wafer cleaner volatiles, the responsivity was up to 96.61 %. This research offers an efficient route for developing MXene anchored fabric for room-temperature operated ammonia sensing.

High Sensitivity Bi2O3/Ti3C2Tx Ammonia Sensor Based on Improved Synthetic MXene Method at Room Temperature.

The MXene Ti3C2Tx was synthesized using hydrofluoric acid and an improved multilayer method in this study. Subsequently, a Bi2O3/Ti3C2Tx composite material was produced through hydrothermal synthesis. This composite boasts a unique layered structure, offering a large surface area that provides numerous contact and reaction sites, facilitating the adsorption of ammonia on its surface. The prepared Bi2O3/Ti3C2Tx-based sensor exhibits excellent sensing performance for ammonia gas, including high responsiveness, good repeatability, and rapid response-recovery time. The sensor's response to 100 ppm ammonia gas is 61%, which is 11.3 times and 1.6 times the response values of the Ti3C2Tx gas sensor and Bi2O3 gas sensor, with response/recovery times of 61 s/164 s at room temperature, respectively. Additionally, the gas sensitivity mechanism of the Bi2O3/Ti3C2Tx-based sensor was analyzed, and the gas sensing response mechanism was proposed. This study shows that the sensor can effectively enhance the accuracy and precision of ammonia detection at room temperature and has a wide range of application scenarios.

Co3O4/CQDs composite for selective ammonia detection in exhaled human breath analysis

Co3O4 was synthesized using electrospinning method and different amounts of carbon quantum dots (CQDs) were subsequently doped into the Co3O4 matrix via grinding, resulting in Co3O4/CQDs composite materials with various ratios. Gas sensing tests revealed that the resulting composite Co3O4/CQDs sensors exhibited excellent performance at a low temperature of 50°C, with outstanding selectivity towards ammonia gas and rapid response and recovery characteristics. Specifically, response and recovery time for 50 ppm ammonia gas were 14 s and 164 s, respectively. The outcomes of the exhalation simulation test utilizing sensors composed of this material demonstrate that the sensor's response to simulated chronic kidney disease breath closely approximates that of pure biological samples (1.08), exceeding the average response value of healthy exhaled breath samples (1.0). This demonstrates the sensor's capability to differentiate between healthy individuals and chronic kidney disease patients despite significant water vapor and carbon dioxide interference. This study provides valuable insights for the development of more efficient ammonia sensors and their role in the early detection of chronic kidney disease.

Ternary TiO2/MoS2/ZnO hetero-nanostructure based multifunctional sensing devices

Novel sensing applications benefit from multifunctional nanomaterials responsive to various external stimuli such as mechanics, electricity, light, humidity, or pollution. While few such materials occur naturally, the careful design of synergized nanomaterials unifies the cross-coupled properties which are weak or absent in single-phase materials. In this study, 2D MoS2 integrated with ultrathin dielectric oxide layers forms hetero-nanostructures with significant impacts on carrier transport. The ternary TiO2/MoS2/ZnO hetero-nanostructures, along with their individual properties, improve the performance of multifunctional sensing devices. The synthesized hetero-nanostructure exhibits a responsivity of up to 16 mA/W to 700 nm light and responds to 5 ppm ammonia gas at room temperature. These enhancements are attributed to interface charge transfer and photogating effects. The ternary TiO2/MoS2/ZnO hetero-nanostructure is compatible with existing semiconductor fabrication technologies, making it feasible to integrate into flexible, lightweight semiconductor devices and circuits. These results may inspire new photodetectors and sensing devices based on two-dimensional (2D) layered materials for IoT applications.

Enhanced sensing properties of optical ammonia sensor based on electrospun fibers containing Eosin-Y and silver nanoparticles

This study presents a highly sensitive fluorescence intensity-based gas sensor specifically designed to detect ammonia. Eosin-Y fluorescent dye and silver nanoparticles are incorporated into` cellulose acetate to fabricate fibers membrane using the electrospinning technique, which serves as the foundation for the optical ammonia sensors. Afterward, this sensor is illuminated by a 405 nm LED for monitoring purposes, resulting in changes to both intensity and wavelength. The sensitivity of the optical ammonia sensor is evaluated by comparing the fluorescence intensity recorded for pure nitrogen with that for 1000 ppm ammonia, expressed as the ratio I0/I1000 ppm. The sensor's ability to detect ammonia is evaluated based on the differences in fluorescence intensities between nitrogen and ammonia environments. The proposed optical ammonia gas sensor utilizes Eosin-Y molecules as indicator compounds. Notably, the sensor, constructed from an electrospun fiber membrane, demonstrates an exceptional response of 50.9 at 1000 ppm in an ammonia gas environment at room temperature, representing the highest response achieved to date in an ammonia gas sensor utilizing Eosin-Y molecules. Experimental findings highlight the enhanced sensitivity of the proposed Eosin-Y-containing electrospun fibers compared to traditional Eosin-Y-based dyes for optical ammonia sensing. The development of an optical ammonia sensor utilizing electrospun fibers embedded with fluorescent dye presents a promising opportunity, offering practical applicability due to its cost-effectiveness and straightforward manufacturing process. Finally, the innovative optical ammonia sensor approach can be applied across various fields, including environmental, industrial, and medical applications.

Flower-like nickel oxide nanostructures: Superior ammonia gas sensing and efficient dye removal behavior under UV–visible light illumination

In this study, we fabricated the NiO nanostructures using a straightforward solvothermal technique. The NiO nanoparticles were characterized using various analytical techniques. The FESEM images clearly show the flower-like structure of NiO nanoparticles. These nanostructures were then employed as a sensor to detect the presence of highly toxic ammonia (NH3) gas. The flower-shaped nanostructures of NiO played a vital role in improving sensing performance. Moreover, the NiO sensor displayed rapid response to 100 ppm ammonia at room temperature, with responses/recovery times of 40 s/44 s. This research holds promise as a viable approach for effectively sensing and detecting NH3 gas. The NiO exhibited excellent photocatalytic activity and degradation rates of 80% and 84.75% for Methyl orange (MO) and methylene blue dye (MB) respectively under UV-visible light irradiation. The NiO nanoflower remains unaffected after recycling MB dye degradation. This effective photocatalytic performance might be due to the formation of flower-like morphology. This underscores the potential of NiO as a promising candidate for applications in environmental and healthcare safety applications.

Portable and highly sensitive gas sensor based on CaCl2-Cu NPs-MIL53(Al)–NH2 nanocomposite for breath ammonia analysis

Exhaled gas analysis is an effective and non-invasive method for disease diagnosis. However, due to the complexity of the exhaled gases, this is a challenge for most sensing devices. Herein, CaCl2-Cu NPs-MIL53(Al)–NH2 composites modified with screen-printed electrodes (SPEs) and ionic liquids [BMIM][BF4] were used as electrolytes to construct a novel electrochemical ammonia sensor. CaCl2-Cu NPs-MIL53(Al)–NH2/SPEs possessed excellent performance, exhibiting good linearity in the range of 0.5–500 ppm with a low limit of detection (0.25 ppm) and a fast response time for 10 ppm ammonia (5.4 s). Most importantly, CaCl2-Cu NPs-MIL53(Al)–NH2/SPEs showed excellent resistance to high humidity and interference, which is required for exhaled gas analysis. Furthermore, compared to standard spectrophotometry, this method also can provide satisfactory recovery (101.43–103.98 %), suggesting that this sensor is promising for the detection of exhaled gases in the future.

Fiber Optic Sensor Coated with Multiple Layers of Hexagonal Boron Nitride Nanosheets (BNNS) for the Detection of Volatile Organic Compounds.

Nowadays, volatile organic compound (VOC) detection is imperative to ensure environmental safety in industry and indoor environments, as well as to monitor human health in medical diagnosis. Gas sensors with the best sensor response, selectivity, and stability are in high demand. Simultaneously, the advancement of nanotechnology facilitates novel nanomaterial-based gas sensors with superior sensor characteristics and low power consumption. Recently, boron nitride, a 2D material, has emerged as an excellent candidate for gas sensing and demonstrated exceptional sensing characteristics for new-generation gas sensing devices. Herein, ultrathin porous boron nitride nanosheets (BNNSs) with large lateral sizes were synthesized using a facile synthesis approach, and their material characteristics were investigated utilizing a variety of analytical techniques, including X-ray diffraction, Fourier transform infrared spectroscopy, ultraviolet spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. A BNNS-coated cladding-modified fiber optic sensor (FOS) probe was prepared and employed for VOC (ammonia, ethanol, and acetone) sensing across concentrations varying from 0 to 300 ppm. The BNNSs-coated FOS demonstrated better selectivity toward 300 ppm ammonia, and specifically annealed BNNSs displayed a maximum sensor response of 55% along with a response/recovery times of 15 s/34 s compared to its counterparts. The superior ammonia sensing performances could be attributed to the formation of ultrathin nanosheets and a porous surface with slit-like features in hexagonal boron nitride.

Impact of Palladium Decoration on the Performance of Co-SmFeO<sub>3</sub> Based Gas Sensor

The enhanced ammonia gas sensing properties of palladium decorated Co-SFO are demonstrated here. Pristine SmFeO3 thick films fabricated by screen printing technique were surface modified with Co by dipping method (dipping time 3 min) and identified as Co-SFO thick films. They showed maximum sensitivity to 50 ppm ammonia at 200 °C. In order to further increase its sensitivity, Co-SFO thick film was dipped into Palladium nitrate solution for 1 min, 2 min and 3 min. Surface morphology of as-prepared thick films was studied by FE-SEM. Formation of PdO phase and its uniform distribution over Co-SFO surface was confirmed from EDAX spectra. Gas sensing results revel that the sensitivity of Pd decorated Co-SFO thick films towards 50 ppm ammonia was increased. Moreover, decrease in operating temperature was also observed. Pd decorated Co-SFO thick film with dipping time 3 min has maximum sensitivity at lower operating temperature. The improved sensitivity at low temperature was attributed to the sensitization of palladium which was discussed in details. Keywords: Co-SFO, Chemical sensitization, Noble metal, Gas Response, Reducing gas.

Porous polyaniline/flower-like hybrid phase MoS2/phosphorus-doped graphene ternary nanocomposite for efficient room temperature ammonia sensors

Ammonia, a ubiquitous gas with diverse industrial applications, demands reliable and cost-effective sensing technologies for monitoring and control. In this context, this study presents the development of a novel ternary nanocomposite comprised of porous polyaniline (PANI), hybrid phase molybdenum disulfide (MoS2), and phosphorus-doped graphene (PGO) for the realization of highly efficient room temperature ammonia sensors. The synergistic combination of these three materials leverages their individual properties, such as high surface area, excellent electrical conductivity, and enhanced catalytic activity, to create a robust sensing platform. The PANI/1 T-2 H MoS2/PGO nanocomposites were synthesized by a combination of solvothermal processing and in-situ polymerization techniques. The morphological and structural characteristics of the PANI/1 T-2 H MoS2/PGO nanocomposites were conducted using advanced analytical techniques, that include, Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Raman spectroscopy, Transmission Electron Microscopy (TEM), and X-ray Photoelectron Spectroscopy (XPS), and Brunauer-Emmett-Teller method (BET). The enhanced surface area of PANI/1 T-2 H MoS2/PGO (54.72 m2/g) compared to PANI (31.8 m2/g) has a positive impact on the sensing characteristics of PANI/1 T-2 H MoS2/PGO. The PANI/1 T-2 H MoS2/PGO nanocomposite sensor has shown sensing response values of ∼1070 %, response time of 12 s, recovery time of 30 s towards 100 ppm of NH3, and detection limit is 0.01 ppm (10 ppb). A highly linear gas response of the PANI/1 T-2 H MoS2/PGO sensor is observed in a range of 10–100 ppm ammonia concentration. The development of the PANI/1 T-2 H MoS2/PGO nanocomposite sensor aims to meet the increasing need for temperature-efficient, cost-effective, and energy-efficient gas sensing technologies that can be used in various fields including environmental monitoring and industrial safety.

Highly sensitive and flexible ammonia sensor based on PEDOT:PSS doped with Lewis acid for wireless food monitoring

Ammonia serves as a crucial indicator of food spoilage, but current sensors lack sufficient sensitivity, stability, selectivity, and automation for practical on-site and real-time monitoring. A highly sensitive and flexible ammonia sensor based on PEDOT:PSS doped with Lewis acid (FeCl3 solution) for wireless food monitoring is proposed in this study to solve the above limitations. The intercalation effect caused by the diffusion of Fe3+ and hydrolyzed H+ in the film coordinating with SO3− groups present in PSS chains leads to an increased distance between adjacent PEDOT-rich domains. It enhances ammonia sensitivity by providing additional ionic carriers and carrier transport paths within the PEDOT:PSS molecule. The results demonstrated a significant enhancement in the sensitivity of PEDOT:PSS sensor doped with FeCl3, exhibiting a response value of about 25 % at 25 ℃ for 1 ppm NH3, with an impressive detection limit of 0.23 ppm and quantification limit of 0.78 ppm (1–400 ppm). Moreover, it showcases acceptable repeatability both at 1 ppm (RSD = 4.30 %) and 100 ppm ammonia (RSD = 3.61 %), along with the notable selectivity and long-term stability. XPS, Raman analysis, and the density function theory (DFT) calculations further elucidated the sensing mechanism of the sensor towards ammonia. Additionally, a portable wireless detection device was designed, which was equipped with the Bluetooth communication and the micro-controller system to detect the decay process of fresh shrimp, realizing wirelessly real-time monitoring of food freshness through a smart phone. It’s worth noting that the innovative sensing technology holds great promise for food quality monitoring.

An ammonia-sensitive fluorescence sensor based on polyvinyl alcohol-graphene quantum dots/halloysite nanotubes hybrid film for monitoring fish freshness

A fluorescent hybrid film composed of nitrogen-doped graphene quantum dots (N-GQDs) loaded on halloysite nanotubes (HNTs) (N-GQDs/HNTs nanocomposite) as a sensitive element and polyvinyl alcohol (PVA) as a film-forming matrix was designed for freshness detection. The PVA-N-GQDs/HNTs hybrid film exhibited significantly enhanced fluorescence attributed to the loading of N-GQDs onto the surface of HNTs through electrostatic interactions and hydrogen bonding, effectively reducing their aggregation. The fluorescence of the hybrid film could be quenched by ammonia via photoinduced electron transfer (PET), with good linearity in the range of 20 ppm to 500 ppm ammonia and a limit of detection (LOD) of 0.63 ppm. In addition, the hybrid film was applied to monitor the freshness of seawater fish and freshwater fish stored at refrigeration and room temperature to evaluate the practicality of this approach. The developed hybrid film showed promise for nondestructive and on-site monitoring of fish spoilage.

Engineering hierarchical heterostructure material based on multiwalled carbon nanotubes/SnO2 nanorod-flowers for high-efficient ammonia detection

The rational manufacturing of multi-component materials with rich heterostructure is emerging as a possible strategy to develop improved carbon nanotube-based gas sensors. Herein, hierarchical heterostructure of multiwalled carbon nanotubes/SnO2 nanorod-flowers (HMCNS) was synthesized by a facile water-bathing combined hydrothermal approach for highly sensitive ammonia detection. Exploration in the microstructure reveals that hierarchical flowers-like SnO2 consisted of nanorods demonstrates a hollow geometry for wrapping multiwalled carbon nanotubes, featuring a unique three-dimensional (3D) hierarchical architecture. Gas sensor based on SnO2@4.5%CNTs hierarchical heterostructure exhibits highly enhanced ammonia-sensing performances, including ultra-high response of 1650 toward 20 ppm ammonia, good selectivity, 250 ppb level detection, and superior long-term stability. The improvement in ammonia sensing capability could be attributed to the peculiar hierarchical architecture and the creation of Mott-Schottky heterostructure which has been strongly validated by electrochemical research. This work will provide a concise and effective avenue to construct hierarchical composite with Mott-Schottky heterostructure for high-performance ammonia gas sensor.

Improved ammonia sensitivity and broadband photodetection by using PBTTT-C14/WS2-QDs nano-heterojunction based thin film transistor

An ambient temperature broadband photo-detector and ammonia (NH3) gas sensor devices are developed with PBTTT-C14/WS2-QDs heterojunction thin film. This composite thin film of PBTTT-C14 conjugate polymer and hydrothermally synthesized WS2-QDs have been deposited by high yield and cost-effective ‘floating film transfer method’ (FTM), which works as a channel (p-type) of the thin film transistor (TFT). To determine the NH3 sensing behavior of the TFT, different concentrations of analyte (0.5–20 ppm) have been exposed on the channel of the TFT, which shows the response of ∼ 50 % at 10 ppm ammonia concentration that is significantly higher than PBTTT-C14 only TFT. This improvement is made possible by the NH3-responsive depleted layer of polymer/QDs heterojunction, which varies widely in the presence of ammonia. As a consequence, the ON/OFF ratio, carrier mobility and threshold voltage (Vth) of the device changes in a wider range. Besides, this TFT based gas sensor is also highly selective to the other interfering gases. In addition to NH3 gas sensitivity, This TFT also shows very high photosensitivity under white light illumination that enhances ‘ON’ and ‘OFF’ current of the device by ∼ 2.2 and 70 times, respectively under one sun white light illumination, whereas Vth of the device is shifted by ∼ 22 V. Besides, the device shows quite fast response/ recovery time under NH3 gas exposure and light illumination.

A portable electrochemical room-temperature sensor based on flower-like structure UIO-66-NH2@MoS2 composite for ammonia detection

Due to the toxicity and corrosiveness of the ammonia gas, low-cost, portable, and accurate methods of ammonia detection are of great importance in environmental testing and disease diagnosis. In this study, flower-like UIO-66-NH2@MoS2 composite modified screen-printed electrodes (SPEs) were prepared by hydrothermal treatment using ionic liquids [BMIM][BF4] as electrolytes. The UIO-66-NH2@MoS2/SPEs were then combined with a miniature electrochemical workstation to construct a novel electrochemical ammonia sensor. The experimental results showed that UIO-66-NH2@MoS2 had excellent adsorption capacity for ammonia, exhibiting good linearity in the range of 5–500 ppm with a low limit of detection (2.1 ppm) and a fast response time for 100 ppm ammonia (13 s). This sensor also possessed excellent stability, reproducibility, and anti-interference ability. The synthesized UIO-66-NH2@MoS2 nanocomposite was demonstrated to be an excellent candidate for the construction of high-performance ammonia sensors for various applications.

Nitrogen-doped carbon nano-onions/polypyrrole nanocomposite based low-cost flexible sensor for room temperature ammonia detection

One of the frontier research areas in the field of gas sensing is high-performance room temperature-based novel sensing materials, and new family of low-cost and eco-friendly carbon nanomaterials with a unique structure has attracted significant attention. In this work, we propose a novel low-cost flexible room temperature ammonia gas sensor based on nitrogen-doped carbon nano-onions/polypyrrole (NCNO-PPy) composite material mounted low-cost membrane substrate was synthesized by combining hydrothermal and in-situ chemical polymerization methods. The proposed flexible sensor revealed high sensing performance when employed as the sensing material for ammonia detection at room temperature. The NCNO-PPy ammonia sensor exhibited 17.32% response for 100 ppm ammonia concentration with a low response time of 26 s. The NCNO-PPy based flexible sensor displays high selectivity, good repeatability, and long-term durability with 1 ppm as the lower detection limit. The proposed flexible sensor also demonstrated remarkable mechanical robustness under extreme bending conditions, i.e., up to 90° bending angle and 500 bending cycles. This enhanced sensing performance can be related to the potential bonding and synergistic interaction between nitrogen-doped CNOs and PPy, the formation of defects from nitrogen doping, and the presence of high reactive sites on the surface of NCNO-PPy composites. Additionally, the computational study was performed on optimized NCNO-PPy nanocomposite for both with and without NH3 interaction. A deeper understanding of the sensing phenomena was proposed by the computation of several electronic characteristics, such as band gap, electron affinity, and ionization potential, for the optimized composite.

Enhanced room temperature ammonia sensing using Ti2VC2Tx double-transition-metal MXenes: experimental and theoretical insights

Double-transition-metal (DTM) MXenes are potentially multifunctional materials with greater compositional and structural tunability than mono-transition-metal (MTM) MXenes, enabling controllable formation of defects and controllable tuning of material properties. Herein, we investigate Ti2VC2Tx's ammonia gas sensing via experiments and theoretical calculations. Ti2VC2Tx exhibited a notable response, exceeding 25 %, to 100 ppm ammonia at room temperature with rapid response/recovery times of 4 s/16.2 s. Furthermore, the Ti2VC2Tx sensor shows exceptional ammonia selectivity, attributed to closer adsorption distances and higher adsorption energies of ammonia molecules. This study broadens the application area of DTM MXenes and further demonstrates the potential of MXene materials for gas sensing.