Improvement of Gas and Humidity Sensing Properties of Organ-like MXene by Alkaline Treatment

Alkalized MXene/laser-induced graphene-based integrated three-electrode devices for micro-droplet detection of albendazole

BACKGROUND

Albendazole is a commonly-used antiparasitic drug that has been extensively applied in animal husbandry and aquaculture to prevent and treat various parasitic infections. However, the potential residues of albendazole in animal-derived foods and the aquatic environment can pose risks to human health and ecological balance. Hence, the development of a highly sensitive and rapid analytical method for albendazole is of utmost importance to safeguard food safety and preserve the ecological environment. MXene nanosheets (Ti3C2Tx) are emerging as promising precursors for the design of such detection systems. (86) RESULTS: In this work, MXene nanosheets (Ti3C2Tx) were alkalized in NaOH solution to form alkalized Ti3C2Tx (Al-MXene). The treatment caused structural changes such as expanded interlayer spacing, reduced particle size, enhanced Zeta potential, and the formation of amorphous TiO2 on the surface. These modifications gave Al-MXene larger electrochemical active specific area and stronger physical adsorption capacity for albendazole compared to the original MXene nanosheets. When Al-MXene was decorated on the surface of laser-induced graphene (LIG), albendazole was efficiently oxidized due to the unique surface activity of Al-MXene and the abundant three-dimensional porous structure of LIG. An integrated three-electrode device based on the Al-MXene/LIG structure was fabricated for the electrochemical detection of albendazole in micro-droplets for milk sample. The device showed a linear detection range from 10 nM to 1 μM, a sensitivity of 411 μA μmoL-1 cm-2, and a detection limit of 7.5 nM (143) SIGNIFICANCE: The development of the integrated three-electrode device based on Al-MXene/LIG provides a highly sensitive and efficient method for detecting albendazole in milk samples. Moreover, it can also offer guidance for fabricating more innovative electrochemical sensors for the rapid screening of veterinary drug residues. This contribution is of great significance in ensuring food safety and protecting public health. (57).

Nb2CTx/MoSe2 composites for a highly sensitive NH3 gas sensor at room temperature

The detection of ammonia (NH3)gas holds significant importance in both daily life and industrial production. In this study, the Nb2CTx/MoSe2 sensor was synthesized using a one-step hydrothermal method and applied for NH3 detection. The morphology and elemental composition of the composites were analyzed through a series of characterization techniques including XRD, TEM, SEM, and XPS, confirming the successful synthesis of Nb2CTx/MoSe2 composite with the optimal mass ratio. The sensing performance of the sensor for NH3 (0.1-100 ppm) was tested at room temperature (∼25 °C). The results showed that, compared to pure Nb2CTx, the sensor based on Nb2CTx/MoSe2 composite exhibited more stable baseline resistance, a 3.5-fold increase in response to 50 ppm NH3, and a reduction in response/recovery time by 56.4 s/32.1 s. Additionally, the sensor's response to NH3 (1 ppm, 50 ppm, 100 ppm) varied by less than 10 % over 90 days, demonstrating excellent stability. The sensing mechanism of NH3 by Nb2CTx/MoSe2 composite is attributed to the formation of a p-n heterojunction and surface charge transfer at the interface between p-type Nb2CTx and n-type MoSe2. Finally, the superior selectivity mechanism of the composite for NH3 was investigated using first-principles calculations. This work opens a new avenue for exploring the application potential of Nb2CTx MXene-based nanocomposites in NH3 detection.

Wearable Self-Powered Pressure Sensors Based on alk-Ti3C2Tx Regulating Contact Barrier Difference for Noncontact Motion Object Recognition

Wearable self-powered pressure sensors present tremendous potential for object recognition. However, the fluctuated approach speed and distance may compromise the device output, thus affecting the recognition accuracy. Herein, a wearable self-powered pressure sensor with high sensitivity (1.48 V kPa-1), high output (130.5V), and high permeability (259.98 mm s-1) are developed where the contact barrier difference and triboelectric charge density of the triboelectric layer surface are dynamically regulated by modulating the surface groups resulting in a lower dielectric loss and higher output. The sensor leverages the polarity difference between the target object and the sensitive material to generate electrical signals at different speeds and distances through an electrostatic induction mechanism. With the assistance of the Transformer model with a self-attention mechanism, an average recognition accuracy of 94.3% is achieved by acquiring sensing signals at different speeds and distances. Simulating the ability of human vision, enables visually impaired people to acclimate to their surroundings more easily and independently and provides a critical advancement in the assistance of the blind with daily life.

Gas-specific adsorption capability of titanium dioxide and MXene nanocomposite thin films prepared by ultrasonic spray printing using inks oxidized at room temperature

The development of two-dimensional (2D) layered MXene materials has opened new possibilities for gas adsorption applications due to their high specific surface area, tunable surface chemistry, and excellent selectivity towards specific gases. These materials exhibit tremendous potential in adsorption-based gas separation and detection, particularly due to their strong interactions with target gas molecules, making them highly effective in gas removal and detection applications. However, currently available methods for synthesizing these oxidized nanocomposite inks are limited by the typically high temperatures involved. The present work addresses this issue by developing titanium dioxide and MXene (TiO2/Ti3C2 MXene) nanocomposite inks, where TiO2 nanoparticles are formed between the layers of the Ti3C2 MXene via oxidation in solution state under extended exposure in an oxygen rich atmosphere at room temperature. As a proof of concept, gas adsorption studies are conducted by applying the TiO2/Ti3C2 MXene nanocomposite inks onto gold-coated silicon wafers via an ultrasonic spray printing method. The gas adsorption results demonstrate that the TiO2/Ti3C2 MXene nanocomposites possess excellent adsorption selectivity toward methane and butane among alkane gases, and can achieve maximum adsorption capacities of 8.62 and 18.8 cm3/g, respectively. The results of ultrasonic spray printing quality tests conducted for different numbers of printed layers using the proposed TiO2/Ti3C2 MXene nanocomposite ink demonstrate that the film thickness can be regulated by controlling the number of printed layers, and the average thin film thickness remains only 0.313 μm for 10 printed layers. Meanwhile, the average roughness of the films resides between 0.130 μm (3 layers) and 0.220 μm (8 layers), which is uniformly and less than the average roughness of 0.225 μm measured for the original gold-plated silicon wafer. Hence, by employing facile ink-based printing techniques, such as ultrasonic spray printing, thin films with controlled thickness can be fabricated, thereby laying the foundation for their practical applications in environmental monitoring and industrial gas separation.

Microwave Sensor with Light-Assisted Enhancement Based on Ti3C2Tx MXene: Toward ppb-Level NO2 Detection Limits

Chemiresistive sensors are currently the most popular gas sensors, and metal semiconductor oxides are often used as sensitive materials (SMs). However, their high operating temperature means that more energy is required to maintain normal operation of the SM, resulting in an increase in power consumption of the entire sensing system. In order to solve this problem, a microwave gas sensor embedded with multilayer Ti3C2Tx MXene and split ring resonator (SRR) for nitrogen dioxide (NO2) detection was reported in this work. The sensor takes advantage of the weak coupling between the two SRRs to achieve a highly concentrated electric field and high Q-factor, in which the weak coupling region serves as the sensitive region to avoid damage to the resonator structure caused by the excessive conductivity of Ti3C2Tx. The sensor has good selectivity, a lower detection limit of 2 ppb, with an average detection sensitivity of 98.66 mdB ppm-1 in the range of 2-10,000 ppb at room temperature. Additionally, the effect of different lighting source to the sensor performance is investigated, and the sensor reached the best response (1.54 dB) under blue light. Finally, the mechanism of the enhanced sensitivity is discussed in detail based on the results of simulations and tests. The sensor circuit designed in this work provides a new approach for MGSs and for the first time introduces the photocatalytic pathway into microwave sensors, which will contribute to the optimization of microwave gas sensors in the future.

Two-Dimensional Nanostructured Ti3C2Tx MXene for Ceramic Materials: Preparation and Applications

Ti3C2Tx MXene, a novel two-dimensional transition metal carbide with nanoscale dimensions, has attracted significant attention due to its exceptional structural and performance characteristics. This review comprehensively examines various preparation methods for Ti3C2Tx MXene, including acid etching, acid-salt composite etching, alkali etching, and molten salt etching. It further discusses several strategies for interlayer exfoliation, highlighting the advantages and limitations of each method. The effects of these techniques on the nanostructure, surface functional groups, interlayer spacing, and overall performance of Ti3C2Tx MXene are evaluated. Additionally, this paper explores the diverse applications of Ti3C2Tx MXene in ceramic materials, particularly its role in enhancing mechanical properties, electrical and thermal conductivity, as well as oxidation and corrosion resistance. The primary objective of the review is to provide scientific insights and theoretical guidance for the preparation of Ti3C2Tx MXene and its further research and innovative applications in ceramic materials, advancing the development of high-performance, multifunctional ceramics.

Low Power Gas Sensors: From Structure to Application

Gas sensors are pivotal across industries, encompassing environmental monitoring, industrial safety, and healthcare. Recently, a surge in demand for low power gas sensors has emerged, driven by the huge need for applications in portable devices, wireless sensor networks, and the Internet of things (IoT). The practical realization of a densely interconnected sensor network demands gas sensors to have low power consumption for energy-efficient operation. This Perspective offers a comprehensive overview of the progress of low-power sensors for gas and volatile organic compound detection, with a keen focus on the interplay between sensing materials (including metal oxide semiconductors, metal-organic frameworks, and two-dimensional materials), sensor structures, and power consumption. The main gas sensing mechanisms are discussed, and we delve into the mechanisms for achieving low power consumption including material properties and sensor design. Furthermore, typical applications of low power gas sensors are also presented, including wearable technology, food safety, and environmental monitoring. The review will end by discussing some open questions and ongoing needs.

Layered MXene Films via Self-Assembly

MXene has attracted significant attention as a 2D material family due to its metallic conductivity and abundant surface functional groups and has been extensively studied and applied as bulk materials and microscale thin films. MXene possesses ionizable surfaces and edges, as well as high surface area. Its customizable dispersibility demonstrates unique advantages in self-assembly solution processing. Recent studies have demonstrated the application value of layered MXene films at the nanoscale thickness and the reliance of processing on self-assembly techniques. However, this field currently lacks sufficient attention. Here, the regulatory mechanisms are summarized for the preparation of layered MXene films through self-assembly techniques, as well as introduce their applications. Moreover, the future challenges of large-scale applications of MXene self-assembly techniques are proposed. It is believed that this review would provide a dynamic and promising path for the development of layered MXene self-assembly techniques.

Recent insights on MXene-based architectures for monitoring and sensing of gaseous pollutants: A review

Epidemiology and public health concerns have primarily relied on the accurate control of gas pollutants, requiring highly efficient gas sensor devices for detecting hazardous gases. Despite the dedication of many efforts in this era, the precise, continuous scrutiny of gases remains elusive for appropriate gas selectivity, prompt response and recovery time, proper repeatability, as well as low cost. Accordingly, nanostructured architectural sensing cues have received enormous attention toward versatile detection and sensing procedures. As a representational nanostructure, the MXene family has been widely introduced to tailor and augment sensor patterns by providing large surface area, tunable surface chemistry, superior electrical conductivity, chemical stability, compatibility with flexible substrates, and potential for multifunctionality. Additionally, they could be synthesized in various formations of film and layered designs, fibrous membranes, and gel-like structures, creating synergetic effects that can provide superior gas-sensing performance. Herein, the synthesis and benefits of MXene nanosheets as gas-sensitive materials, in tandem with the past-to-present progress of MXene-based gas sensors in the formation of films, fibrous, and gel-like configurations, are comprehensively reviewed. As an in-depth reference, the present overview could shed light on further advancing gas sensor architectures developed based on MXene structures.

Application of MXene composites for target gas detection in food safety

Food contamination has long plagued agriculture, posing significant health risks to consumers. The use of volatile gases for food safety detection has proven highly effective, with composite gas sensors that leverage the two-dimensional material MXene exhibiting notable advancements in detecting various target gases. This paper reviews the progress of MXene-based composite gas sensors in the detection of food safety-related gases. The review begins by examining MXene material synthesis methods and then presents an overview of techniques aimed at enhancing MXene-based sensor detection capabilities. Recently, advancements in MXene composite gas sensors tailored for food safety gases have been highlighted. Finally, challenges encountered in gas-sensing applications of MXene-based composites are outlined, alongside predictions for their future development, aiming to offer insights for the application and advancement of intelligent gas sensors for target gases in food safety.

Strain and External Electric Field Engineering of S-Terminated MXene on Selective and Sensitive Detection of N-Containing Compound Gases: A Computational Study

Gas sensors are used for monitoring hazardous gases and vapors. With the emergence of S-terminated MXene synthesis, herein, we explore the sensing ability of Ti3C2S2 toward N-containing gases, including NH3, NO, NO2, trimethylamine (TMA), and nicotine (NT), using first-principles calculations and a statistical thermodynamic model. We find that Ti3C2S2 exhibits high selectivity to TMA, NO, and NT with moderate adsorption energies of -0.610, -0.490, and -0.476 eV, respectively, minimizing environmental noise from ambient gases. The electronic structure of Ti3C2S2 subtly alters upon adsorption of TMA, NO, and NT, facilitating detectable signals in the sensing device. However, the adsorption of NO2 and NH3 is less pronounced due to weak physisorption (<0.3 eV). Employing engineering strategies including biaxial strain and an external electric field greatly enhances the selectivity and sensitivity of NO2 (NH3) detection by boosting adsorption strength up to -0.351 eV with ε = 5% (-0.370 eV with |E⃗| = 0.6 V/ Å). In addition, the moderate adsorption energies of the gases result in a suitable recovery time in the range of milliseconds, leading to high reusability of the sensing device. The estimated adsorption densities suggest potential coverage of these N-containing molecules even at low concentrations and room temperature. Computational analysis of the sensing capability of Ti3C2S2 using the nonequilibrium Green's function method indicates that it is a promising gas-sensing material. In addition, mechanical modifications, electric field adjustments, and gate voltage alterations could be used to obtain effective sensing materials for N-containing gas detection.

A Comparative Review of Graphene and MXene-Based Composites towards Gas Sensing

Graphene and MXenes have emerged as promising materials for gas sensing applications due to their unique properties and superior performance. This review focuses on the fabrication techniques, applications, and sensing mechanisms of graphene and MXene-based composites in gas sensing. Gas sensors are crucial in various fields, including healthcare, environmental monitoring, and industrial safety, for detecting and monitoring gases such as hydrogen sulfide (H2S), nitrogen dioxide (NO2), and ammonia (NH3). Conventional metal oxides like tin oxide (SnO2) and zinc oxide (ZnO) have been widely used, but graphene and MXenes offer enhanced sensitivity, selectivity, and response times. Graphene-based sensors can detect low concentrations of gases like H2S and NH3, while functionalization can improve their gas-specific selectivity. MXenes, a new class of two-dimensional materials, exhibit high electrical conductivity and tunable surface chemistry, making them suitable for selective and sensitive detection of various gases, including VOCs and humidity. Other materials, such as metal-organic frameworks (MOFs) and conducting polymers, have also shown potential in gas sensing applications, which may be doped into graphene and MXene layers to improve the sensitivity of the sensors.

Multilayer Fluorine-Free MoBTx MBene with Hydrophilic Structural-Modulating for the Fabrication of a Low-Resistance and High-Resolution Humidity Sensor

2D transition metal borides (MBenes) with abundant surface terminals hold great promise in molecular sensing applications. However, MBenes from etching with fluorine-containing reagents present inert -fluorine groups on the surface, which hinders their sensing capability. Herein, the multilayer fluorine-free MoBTx MBene (where Tx represents O, OH, and Cl) with hydrophilic structure is prepared by a hydrothermal-assisted hydrochloric acid etching strategy based on guidance from the first-principle calculations. Significantly, the fluorine-free MoBTx-based humidity sensor is fabricated and demonstrates low resistance and excellent humidity performance, achieving a response of 90% to 98%RH and a high resolution of 1%RH at room temperature. By combining the experimental results with the first-principles calculations, the interactions between MoBTx and H2O, including the adsorption and intercalation of H2O, are understood first in depth. Finally, the portable humidity early warning system for real-time monitoring and early warning of infant enuresis and back sweating illustrates its potential for humidity sensing applications. This work not only provides guidance for preparation of fluorine-free MBenes, but also contributes to advancing their exploration in sensing applications.

Research Progress on Ammonia Sensors Based on Ti3C2Tx MXene at Room Temperature: A Review

Ammonia (NH3) potentially harms human health, the ecosystem, industrial and agricultural production, and other fields. Therefore, the detection of NH3 has broad prospects and important significance. Ti3C2Tx is a common MXene material that is great for detecting NH3 at room temperature because it has a two-dimensional layered structure, a large specific surface area, is easy to functionalize on the surface, is sensitive to gases at room temperature, and is very selective for NH3. This review provides a detailed description of the preparation process as well as recent advances in the development of gas-sensing materials based on Ti3C2Tx MXene for room-temperature NH3 detection. It also analyzes the advantages and disadvantages of various preparation and synthesis methods for Ti3C2Tx MXene's performance. Since the gas-sensitive performance of pure Ti3C2Tx MXene regarding NH3 can be further improved, this review discusses additional composite materials, including metal oxides, conductive polymers, and two-dimensional materials that can be used to improve the sensitivity of pure Ti3C2Tx MXene to NH3. Furthermore, the present state of research on the NH3 sensitivity mechanism of Ti3C2Tx MXene-based sensors is summarized in this study. Finally, this paper analyzes the challenges and future prospects of Ti3C2Tx MXene-based gas-sensitive materials for room-temperature NH3 detection.

Advances in 2D Materials Based Gas Sensors for Industrial Machine Olfactory Applications

The escalating development and improvement of gas sensing ability in industrial equipment, or "machine olfactory", propels the evolution of gas sensors toward enhanced sensitivity, selectivity, stability, power efficiency, cost-effectiveness, and longevity. Two-dimensional (2D) materials, distinguished by their atomic-thin profile, expansive specific surface area, remarkable mechanical strength, and surface tunability, hold significant potential for addressing the intricate challenges in gas sensing. However, a comprehensive review of 2D materials-based gas sensors for specific industrial applications is absent. This review delves into the recent advances in this field and highlights the potential applications in industrial machine olfaction. The main content encompasses industrial scenario characteristics, fundamental classification, enhancement methods, underlying mechanisms, and diverse gas sensing applications. Additionally, the challenges associated with transitioning 2D material gas sensors from laboratory development to industrialization and commercialization are addressed, and future-looking viewpoints on the evolution of next-generation intelligent gas sensory systems in the industrial sector are prospected.

α-Fe2O3/TiO2/Ti3C2Tx Nanocomposites for Enhanced Acetone Gas Sensors

Metal oxide semi-conductors are widely applied in various fields due to their low cost, easy processing, and good compatibility with microelectronic technology. In this study, ternary α-Fe2O3/TiO2/Ti3C2Tx nanocomposites were prepared via simple hydrothermal and annealing treatments. The composition, morphology, and crystal structure of the samples were studied using XPS, SEM, EDS, XRD, and multiple other testing methods. The gas-sensing measurement results suggest that the response value (34.66) of the F/M-3 sensor is 3.5 times higher than the pure α-Fe2O3 sensor (9.78) around 100 ppm acetone at 220°C, with a rapid response and recovery time (10/7 s). Furthermore, the sensors have an ultra-low detection limit (0.1 ppm acetone), excellent selectivity, and long-term stability. The improved sensitivity of the composites is mainly attributed to their excellent metal conductivity, the unique two-dimensional layered structure of Ti3C2Tx, and the heterojunction formed between the nanocomposite materials. This research paves a new route for the preparation of MXene derivatives and metal oxide nanocomposites.

Sensitivity-Enhanced, Room-Temperature Detection of NH3 with Alkalized Ti3C2Tx MXene

A layered Ti3C2Tx MXene structure was prepared by etching MAX-phase Ti3AlC2 with hydro-fluoric acid (HF), followed by alkalization in sodium hydroxide (NaOH) solutions of varying concentrations and for varying durations. Compared to sensors utilizing unalkalized Ti3C2Tx, those employing alkalized Ti3C2Tx MXene exhibited enhanced sensitivity for NH3 detection at room temperature and a relative humidity of 40%. Both the concentration of NaOH and duration of alkalization significantly influenced sensor performance. Among the tested conditions, Ti3C2Tx MXene alkalized with a 5 M NaOH solution for 12 h exhibited optimal performance, with high response values of 100.3% and a rapid response/recovery time of 73 s and 38 s, respectively. The improved sensitivity of NH3 detection can be attributed to the heightened NH3 adsorption capability of oxygen-rich terminals obtained through the alkalization treatment. This is consistent with the observed increase in the ratio of oxygen to fluorine atoms on the surface terminations of the alkalization-treated Ti3C2Tx. These findings suggest that the gas-sensing characteristics of Ti3C2Tx MXene can be finely tuned and optimized through a carefully tailored alkalization process, offering a viable approach to realizing high-performance Ti3C2Tx MXene gas sensors, particularly for NH3 sensing applications.

Designing State-of-the-Art Gas Sensors: From Fundamentals to Applications

Gas sensors are crucial in environmental monitoring, industrial safety, and medical diagnostics. Due to the rising demand for precise and reliable gas detection, there is a rising demand for cutting-edge gas sensors that possess exceptional sensitivity, selectivity, and stability. Due to their tunable electrical properties, high-density surface-active sites, and significant surface-to-volume ratio, nanomaterials have been extensively investigated in this regard. The traditional gas sensors utilize homogeneous material for sensing where the adsorbed surface oxygen species play a vital role in their sensing activity. However, their performance for selective gas sensing is still unsatisfactory because the employed high temperature leads to the poor stability. The heterostructures nanomaterials can easily tune sensing performance and their different energy band structures, work functions, charge carrier concentration and polarity, and interfacial band alignments can be precisely designed for high-performance selective gas sensing at low temperature. In this review article, we discuss in detail the fundamentals of semiconductor gas sensing along with their mechanisms. Further, we highlight the existed challenges in semiconductor gas sensing. In addition, we review the recent advancements in semiconductor gas sensor design for applications from different perspective. Finally, the conclusion and future perspectives for improvement of the gas sensing performance are discussed.

MXene-Based Chemo-Sensors and Other Sensing Devices

MXenes have received worldwide attention across various scientific and technological fields since the first report of the synthesis of Ti3C2 nanostructures in 2011. The unique characteristics of MXenes, such as superior mechanical strength and flexibility, liquid-phase processability, tunable surface functionality, high electrical conductivity, and the ability to customize their properties, have led to the widespread development and exploration of their applications in energy storage, electronics, biomedicine, catalysis, and environmental technologies. The significant growth in publications related to MXenes over the past decade highlights the extensive research interest in this material. One area that has a great potential for improvement through the integration of MXenes is sensor design. Strain sensors, temperature sensors, pressure sensors, biosensors (both optical and electrochemical), gas sensors, and environmental pollution sensors targeted at volatile organic compounds (VOCs) could all gain numerous improvements from the inclusion of MXenes. This report delves into the current research landscape, exploring the advancements in MXene-based chemo-sensor technologies and examining potential future applications across diverse sensor types.

Efficient Self-Assembly Preparation of 3D Carbon-Supported Ti3 C2 Tx Hollow Spheres for High-Performance Potassium Ion Batteries

MXenes are considered a promising negative electrode material for potassium ion batteries (PIBs) in view of their low potassium ion diffusion barrier and excellent electrical conductivity. However, the stacking phenomenon in practical applications severely reduces their active surface and leads to slow K+ diffusion. Herein, a facile composite template method is proposed to construct stacking-resistance 3D carbon-supported Ti3 C2 Tx (3D-C@Ti3 C2 Tx ) hollow spheres. Due to the unique structure, when used as a negative electrode material, as-prepared 3D-C@Ti3 C2 Tx hollow spheres show not only improved rate capability with 160.4 mAh g-1 at 100 mA g-1 and 133.7 mAh g-1 at 500 mA g-1 , but also stable cycling performance with 142.5 mAh g-1 specific capacity remained at 2 A g-1 after 4200 cycles. Furthermore, the full cells with 3D-C@Ti3 C2 Tx anode can operate stably for 1000 cycles at 100 mA g-1 . Moreover, the linear fit analysis demonstrates that 3D-C@Ti3 C2 Tx hollow spheres have a fast and stable capacitive potassium storage mechanism. This method is simple and easy to implement, which provide a feasible path to solve the stacking problem of 2D materials.

Recent Advances in Nanostructured Materials for Application as Gas Sensors

Many different industries, including the pharmaceutical, medical engineering, clinical diagnostic, public safety, and food monitoring industries, use gas sensors. The inherent qualities of nanomaterials, such as their capacity to chemically or physically adsorb gas, and their great ratio of surface to volume make them excellent candidates for use in gas sensing technology. Additionally, the nanomaterial-based gas sensors have excellent selectivity, reproducibility, durability, and cost-effectiveness. This Review article offers a summary of the research on gas sensor devices based on nanomaterials of various sizes. The numerous nanomaterial-based gas sensors, their manufacturing procedures and sensing mechanisms, and most recent advancements are all covered in detail. In addition, evaluations and comparisons of the key characteristics of gas sensing systems made from various dimensional nanomaterials were done.

Tailoring MXene Thickness and Functionalization for Enhanced Room-Temperature Trace NO2 Sensing

In this study, precise control over the thickness and termination of Ti3C2TX MXene flakes is achieved to enhance their electrical properties, environmental stability, and gas-sensing performance. Utilizing a hybrid method involving high-pressure processing, stirring, and immiscible solutions, sub-100 nm MXene flake thickness is achieved within the MXene film on the Si-wafer. Functionalization control is achieved by defunctionalizing MXene at 650 °C under vacuum and H2 gas in a CVD furnace, followed by refunctionalization with iodine and bromine vaporization from a bubbler attached to the CVD. Notably, the introduction of iodine, which has a larger atomic size, lower electronegativity, reduce shielding effect, and lower hydrophilicity (contact angle: 99°), profoundly affecting MXene. It improves the surface area (36.2 cm2 g-1), oxidation stability in aqueous/ambient environments (21 days/80 days), and film conductivity (749 S m-1). Additionally, it significantly enhances the gas-sensing performance, including the sensitivity (0.1119 Ω ppm-1), response (0.2% and 23% to 50 ppb and 200 ppm NO2), and response/recovery times (90/100 s). The reduced shielding effect of the -I-terminals and the metallic characteristics of MXene enhance the selectivity of I-MXene toward NO2. This approach paves the way for the development of stable and high-performance gas-sensing two-dimensional materials with promising prospects for future studies.

"Visualization" Gas-Gas Sensors Based on High Performance Novel MXenes Materials

The detection of toxic, harmful, explosive, and volatile gases cannot be separated from gas sensors, and gas sensors are also used to monitor the greenhouse effect and air pollution. However, existing gas sensors remain with many drawbacks, such as lower sensitivity, lower selectivity, and unstable room temperature detection. Thus, there is an imperative need to find more suitable sensing materials. The emergence of a new 2D layered material MXenes has brought dawn to solve this problem. The multiple advantages of MXenes, namely high specific surface area, enriched terminal functionality groups, hydrophilicity, and good electrical conductivity, make them among the most prolific gas-sensing materials. Therefore, this review paper describes the current main synthesis methods of MXenes materials, and focuses on summarizing and organizing the latest research results of MXenes in gas sensing applications. It also introduces the possible gas sensing mechanisms of MXenes materials on NH3 , NO2 , CH3 , and volatile organic compounds (VOCs). In conclusion, it provides insight into the problems and upcoming challenges of MXenes materials for gas sensing.

Nanochitin/MXene Composite Coated on Quartz Crystal Microbalance for Humidity Sensing

MXenes, as a typical graphene-like material, excels in the realm of humidity sensing owing to its two-dimensional layer structure, high electrical conductivity, tunable chemical properties, hydrophilicity, and large specific surface area. This study proposed a quartz crystal microbalance (QCM) humidity sensor using a nanochitin/Ti3C2Tx MXene composite as a humidity-sensing material. The morphology, nanostructure, and elemental composition of nanochitin, Ti3C2Tx MXene, and nanochitin/Ti3C2Tx MXene composite materials were characterized using transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. Compared to the pure Ti3C2Tx MXene-coated QCM humidity sensor, the nanochitin/Ti3C2Tx MXene-coated QCM humidity sensor exhibited a higher sensitivity (20.54 Hz/%RH) in the humidity range of 11.3% to 97.3%. The nanochitin/Ti3C2Tx Mxene-coated QCM humidity sensor also demonstrated low humidity hysteresis (2.12%RH), very fast response/recovery times (4.4/4.1 s), a high quality factor (37 k), and excellent repeatability and sustained stability over time. Eventually, a bimodal exponential kinetics adsorption model was utilized for the analysis of the response mechanism of the nanochitin/Ti3C2Tx MXene composite material-based QCM humidity sensor. This study provides new ideas for optimizing the moisture-sensitive performance of MXene-based QCM humidity sensors.

Alkalized Cellulose Nanofiber-Interweaved PEDOT:PSS Thin-Film Sensors via Layer-by-Layer Spraying Assembly for Ultrafast Molecular Ammonia Detection

As a typical representative of conductive polymers (CPs), poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS) is intensively employed for chemiresistive ammonia (NH3) sensing on account of its favorable aqueous solubility, benign environmental stability, and outstanding room-temperature conductivity; however, it is severely plagued by low sensitivity and sluggish reaction kinetics. To circumvent these limitations, the guest-alkalized cellulose nanofibers (AC) were introduced into the host PEDOT:PSS matrix by the layer-by-layer spraying assembly method (LBLSA) in this work. The componential proportion-optimized PEDOT:PSS/AC/PEDOT:PSS (P/AC/P) sensor delivered a large sensitivity of 20.2%/ppm within 0.1-3 ppm of NH3 at 21 °C@26% RH, an experimental limit of detection (LoD) as low as 30 ppb, a high response of 18.1%, and a short response/recovery times (4.8/4.0 s) toward 1 ppm of NH3, which ranked among the best cases thus far. Also, excellent repeatability and long-term stability and selectivity were demonstrated. Meanwhile, the flexible P/AC/P sensors worked well under various bending angles and bending times. This work combines a green material system and a facile film deposition method to overcome the liquid dispersion incompatibility when preparing a multicomponent mixture for swift trace NH3 detection. The universality and extensibility of this methodology endow a broad prospect in the field of future wearable optoelectronic systems.

Room Temperature Chemiresistive Gas Sensors Based on 2D MXenes

Owing to their large surface area, two-dimensional (2D) semiconducting nanomaterials have been extensively studied for gas-sensing applications in recent years. In particular, the possibility of operating at room temperature (RT) is desirable for 2D gas sensors because it significantly reduces the power consumption of the sensing device. Furthermore, RT gas sensors are among the first choices for the development of flexible and wearable devices. In this review, we focus on the 2D MXenes used for the realization of RT gas sensors. Hence, pristine, doped, decorated, and composites of MXenes with other semiconductors for gas sensing are discussed. Two-dimensional MXene nanomaterials are discussed, with greater emphasis on the sensing mechanism. MXenes with the ability to work at RT have great potential for practical applications such as flexible and/or wearable gas sensors.

Gas-Sensing Mechanisms and Performances of MXenes and MXene-Based Heterostructures

MXenes are a class of 2D transition-metal carbides, nitrides, and carbonitrides with exceptional properties, including substantial electrical and thermal conductivities, outstanding mechanical strength, and a considerable surface area, rendering them an appealing choice for gas sensors. This manuscript provides a comprehensive analysis of heterostructures based on MXenes employed in gas-sensing applications and focuses on addressing the limited understanding of the sensor mechanisms of MXene-based heterostructures while highlighting their potential to enhance gas-sensing performance. The manuscript begins with a broad overview of gas-sensing mechanisms in both pristine materials and MXene-based heterostructures. Subsequently, it explores various features of MXene-based heterostructures, including their composites with other materials and their prospects for gas-sensing applications. Additionally, the manuscript evaluates different engineering strategies for MXenes and compares their advantages to other materials while discussing the limitations of current state-of-the-art sensors. Ultimately, this review seeks to foster collaboration and knowledge exchange within the field, facilitating the development of high-performance gas sensors based on MXenes.

Room-temperature chemiresistive ammonia sensors based on 2D MXenes and their hybrids: recent developments and future prospects

Detection of ammonia (NH3) gas at room temperature is essential in a variety of sectors, including pollution monitoring, commercial safety and medical services, etc. Two-dimensional (2D) materials have emerged as fascinating candidates for gas-sensing applications due to their distinct properties. MXenes, a type of 2D transition metal carbides/nitrides/carbonotrides, have drawn the interest of researchers due to their high conductivity, large surface area, and changing surface chemistry. The review begins by describing the NH3 gas-detecting methods of 2D materials and then concentrates on MXene-based sensors, emphasising the benefits that MXenes provide in this context. The study also explains the prime factors involved in evaluating sensor performance, which include sensor response, sensitivity, selectivity, stability, charge transfer values, adsorption energy and response/recovery times. Subsequently, the review covers two main categories: pristine/intercalated MXenes and MXene-based hybrid materials. The review investigates the approaches for improving the sensing characteristics of pristine and intercalated MXenes by introducing MXene hybrids like MXene-metal oxide hybrids, MXene-transition metal dichalcogenides hybrid, MXene-other 2D materials hybrid, MXene-polymers and other hybrids and other MXene-derived materials. In summary, this review offers a thorough overview of current advancements and potential applications for room-temperature ammonia sensors based on 2D MXenes and their hybrids. In order to pave the way for future improvements in MXene-based gas-sensing technology for room temperature ammonia detection, the study concludes by outlining potential future scope and conclusions.

Recent developments in 2D MXene-based materials for next generation room temperature NO2 gas sensors

MXenes with distinctive structures, good electrical conductivity and abundant functional groups have shown great potential in the fabrication of high performance gas sensors. Since the sensing mechanism of MXene-based gas sensors often involves a surface-dominant process, they can work at room temperature. In this regard, a significant amount of research has been carried out on MXene-based room temperature gas sensors and they can be viewed as one of the possible materials for NO2 sensing applications in the future. In this review, we focus on the most recent research and improvements in pure MXenes and their nanocomposites for NO2 gas sensing applications. First, we have explored the mechanisms involved in MXenes for NO2 gas sensing. Following that, other ways to tune the MXene sensing performance are investigated, including nanocomposite formation with metal oxides, polymers, and other 2D materials. A comparative analysis of the RT NO2 sensor performance based on MXenes and their hybrids is provided. We also discuss the major challenges of using MXene-related materials and the areas that can further advance in the future for the development of high-performance room temperature NO2 gas sensors.

Recent progress in energy, environment, and electronic applications of MXene nanomaterials

Since the discovery of graphene, two-dimensional (2D) materials have gained widespread attention, owing to their appealing properties for various technological applications. Etched from their parent MAX phases, MXene is a newly emerged 2D material that was first reported in 2011. Since then, a lot of theoretical and experimental work has been done on more than 30 MXene structures for various applications. Given this, in the present review, we have tried to cover the multidisciplinary aspects of MXene including its structures, synthesis methods, and electronic, mechanical, optoelectronic, and magnetic properties. From an application point of view, we explore MXene-based supercapacitors, gas sensors, strain sensors, biosensors, electromagnetic interference shielding, microwave absorption, memristors, and artificial synaptic devices. Also, the impact of MXene-based materials on the characteristics of respective applications is systematically explored. This review provides the current status of MXene nanomaterials for various applications and possible future developments in this field.

Recent Advances in Two-Dimensional MXene-Based Electrochemical Biosensors for Sweat Analysis

Sweat, a biofluid secreted naturally from the eccrine glands of the human body, is rich in several electrolytes, metabolites, biomolecules, and even xenobiotics that enter the body through other means. Recent studies indicate a high correlation between the analytes' concentrations in the sweat and the blood, opening up sweat as a medium for disease diagnosis and other general health monitoring applications. However, low concentration of analytes in sweat is a significant limitation, requiring high-performing sensors for this application. Electrochemical sensors, due to their high sensitivity, low cost, and miniaturization, play a crucial role in realizing the potential of sweat as a key sensing medium. MXenes, recently developed anisotropic two-dimensional atomic-layered nanomaterials composed of early transition metal carbides or nitrides, are currently being explored as a material of choice for electrochemical sensors. Their large surface area, tunable electrical properties, excellent mechanical strength, good dispersibility, and biocompatibility make them attractive for bio-electrochemical sensing platforms. This review presents the recent progress made in MXene-based bio-electrochemical sensors such as wearable, implantable, and microfluidic sensors and their applications in disease diagnosis and developing point-of-care sensing platforms. Finally, the paper discusses the challenges and limitations of MXenes as a material of choice in bio-electrochemical sensors and future perspectives on this exciting material for sweat-sensing applications.

Progress and challenges of emerging MXene based materials for thermoelectric applications

To realize sustainable development, more and more countries forwarded carbon neutrality goal. Accordingly, improving the utilization efficiency of traditional fossil fuel is an effective strategy for this great goal. Keeping this in mind, developing thermoelectric devices to recover waste heat energy resulted in the consumption process of fuel is demonstrated to be promising. High performance thermoelectric devices require advanced materials. MXenes are a kind of 2D materials with a layered structure, which demonstrate excellent thermoelectric performance owing to their unique physical, mechanical, and chemical properties. Also, substantial achievement has been gained during the past few years in synthesizing MXene based materials for thermoelectric devices. In this review, the mainstream synthetic routes of MXene from etching MAX were summarized. Significantly, the current state and challenges of research on improving the performance of MXene based thermoelectrics are explored, including pristine MXene and MXene based composites.

Recent Advancements in Physiological, Biochemical, and Multimodal Sensors Based on Flexible Substrates: Strategies, Technologies, and Integrations

Flexible wearable devices have been widely used in biomedical applications, the Internet of Things, and other fields, attracting the attention of many researchers. The physiological and biochemical information on the human body reflects various health states, providing essential data for human health examination and personalized medical treatment. Meanwhile, physiological and biochemical information reveals the moving state and position of the human body, and it is the data basis for realizing human-computer interactions. Flexible wearable physiological and biochemical sensors provide real-time, human-friendly monitoring because of their light weight, wearability, and high flexibility. This paper reviews the latest advancements, strategies, and technologies of flexibly wearable physiological and biochemical sensors (pressure, strain, humidity, saliva, sweat, and tears). Next, we systematically summarize the integration principles of flexible physiological and biochemical sensors with the current research progress. Finally, important directions and challenges of physiological, biochemical, and multimodal sensors are proposed to realize their potential applications for human movement, health monitoring, and personalized medicine.

Recent Progress in Emerging Novel MXenes Based Materials and their Fascinating Sensing Applications

Early transition metals based 2D carbides, nitrides and carbonitrides nanomaterials are known as MXenes, a novel and extensive new class of 2D materials family. Since the first accidently synthesis based discovery of Ti₃ C₂ in 2011, more than 50 additional compositions have been experimentally reported, including at least eight distinct synthesis methods and also more than 100 stoichiometries are theoretically studied. Due to its distinctive surface chemistry, graphene like shape, metallic conductivity, high hydrophilicity, outstanding mechanical and thermal properties, redox capacity and affordable with mass-produced nature, this diverse MXenes are of tremendous scientific and technological significance. In this review, first we'll come across the MXene based nanomaterials possible synthesis methods, their advantages, limitations and future suggestions, new chemistry related to their selected properties and potential sensing applications, which will help us to explain why this family is growing very fast as compared to other 2D families. Secondly, problems that help to further improve commercialization of the MXene nanomaterials based sensors are examined, and many advances in the commercializing of the MXene nanomaterials based sensors are proposed. At the end, we'll go through the current challenges, limitations and future suggestions.

Skin-Interfaced Superhydrophobic Insensible Sweat Sensors for Evaluating Body Thermoregulation and Skin Barrier Functions

Monitoring sweat rate is vital for estimating sweat loss and accurately measuring biomarkers of interest. Although various optical or electrical sensors have been developed to monitor the sensible sweat rate, the quantification of the insensible sweat rate that is directly related to body thermoregulation and skin barrier functions still remains a challenge. This work introduces a superhydrophobic sweat sensor based on a polyacrylate sodium/MXene composite sandwiched between two superhydrophobic textile layers to continuously measure sweat vapor from insensible sweat with high sensitivity and rapid response. The superhydrophobic textile on a holey thin substrate with reduced stiffness and excellent breathability allows the permeation of sweat vapor, while preventing the sensor from being affected by the external water droplets and internal sensible sweat. Integrating the insensible sweat sensor with a flexible wireless communication and powering module further yields a standalone sensing system to continuously monitor insensible sweat rates at different body locations for diverse application scenarios. Proof-of-concept demonstrations on human subjects showcase the feasibility to continuously evaluate the body's thermoregulation and skin barrier functions for the assessment of thermal comfort, disease conditions, and nervous system activity. The results presented in this work also provide a low-cost device platform to detect other health-relevant biomarkers in the sweat (vapor) as the next-generation sweat sensor for smart healthcare and personalized medicine.

Atomic Plasma Grafting: Precise Control of Functional Groups on Ti3C2Tx MXene for Room Temperature Gas Sensors

Gas sensing properties of two-dimensional (2D) materials are derived from charge transfer between the analyte and surface functional groups. However, for sensing films consisting of 2D Ti₃C₂T_x MXene nanosheets, the precise control of surface functional groups for achieving optimal gas sensing performance and the associate mechanism are still far from well understood. Herein, we present a functional group engineering strategy based on plasma exposure for optimizing the gas sensing performance of Ti₃C₂T_x MXene. For performance assessment and sensing mechanism elucidation, we synthesize few-layered Ti₃C₂T_x MXene through liquid exfoliation and then graft functional groups via in situ plasma treatment. Functionalized Ti₃C₂T_x MXene with large amounts of -O functional groups shows NO₂ sensing properties that are unprecedented among MXene-based gas sensors. Density functional theory (DFT) calculations reveal that -O functional groups are associated with increased NO₂ adsorption energy, thereby enhancing charge transport. The -O functionalized Ti₃C₂T_x sensor shows a record-breaking response of 13.8% toward 10 ppm NO₂, good selectivity, and long-term stability at room temperature. The proposed technique is also capable of improving selectivity, a well-known challenge in chemoresistive gas sensing. This work paves the way to the possibility of using plasma grafting for precise functionalization of MXene surfaces toward practical realization of electronic devices.

Application of Titanium Carbide MXenes in Chemiresistive Gas Sensors

The titanium carbide MXenes currently attract an extreme amount of interest from the material science community due to their promising functional properties arising from the two-dimensionality of these layered structures. In particular, the interaction between MXene and gaseous molecules, even at the physisorption level, yields a substantial shift in electrical parameters, which makes it possible to design gas sensors working at RT as a prerequisite to low-powered detection units. Herein, we consider to review such sensors, primarily based on Ti3C2Tx and Ti2CTx crystals as the most studied ones to date, delivering a chemiresistive type of signal. We analyze the ways reported in the literature to modify these 2D nanomaterials for (i) detecting various analyte gases, (ii) improving stability and sensitivity, (iii) reducing response/recovery times, and (iv) advancing a sensitivity to atmospheric humidity. The most powerful approach based on designing hetero-layers of MXenes with other crystals is discussed with regard to employing semiconductor metal oxides and chalcogenides, noble metal nanoparticles, carbon materials (graphene and nanotubes), and polymeric components. The current concepts on the detection mechanisms of MXenes and their hetero-composites are considered, and the background reasons for improving gas-sensing functionality in the hetero-composite when compared with pristine MXenes are classified. We formulate state-of-the-art advances and challenges in the field while proposing some possible solutions, in particular via employing a multisensor array paradigm.

Wearable chemical sensors based on 2D materials for healthcare applications

Chemical sensors worn on the body could make possible the continuous, noninvasive, and accurate monitoring of vital human signals, which is necessary for remote health monitoring and telemedicine. Attractive for creating high-performance, wearable chemical sensors are atomically thin materials with intriguing physical features, abundant chemistry, and high surface-to-volume ratios. These advantages allow for appropriate material-analyte interactions, resulting in a high level of sensitivity even at trace analyte concentrations. Previous review articles covered the material and device elements of 2D material-based wearable devices extensively. In contrast, little research has addressed the existing state, future outlook, and promise of 2D materials for wearable chemical sensors. We provide an overview of recent advances in 2D-material-based wearable chemical sensors to overcome this deficiency. The structure design, manufacturing techniques, and mechanisms of 2D material-based wearable chemical sensors will be evaluated, as well as their applicability in human health monitoring. Importantly, we present a thorough review of the current state of the art and the technological gaps that would enable the future design and nanomanufacturing of 2D materials and wearable chemical sensors. Finally, we explore the challenges and opportunities associated with designing and implementing 2D wearable chemical sensors.

MXene/SnS2 Heterojunction for Detecting Sub-ppm NH3 at Room Temperature

Detection of ultralow concentrations of ammonia is very important in many applications such as fishing, poultry, agriculture, industry, biomedicine, and clinical diagnosis. However, detecting sub-ppm NH₃ remains a challenge for chemiresistive-type gas sensors. Two-dimensional (2D) materials display tremendous potential for effective gas detectors that can be used in these applications. The as-developed MXene/SnS₂ heterojunction-based chemiresistive-type sensor presents superior gas-sensing performance toward sub-ppm ammonia at room temperature. The sensor can detect NH₃ concentrations down to 10 ppb at room temperature. It also displays excellent long-term stability, with a decline in the response at ∼3.4% for 20 days. The developed sensor also displays good selectivity toward NH₃ relative to some potential interferents, such as HCHO, C₂H₅OH, CH₃OH, C₃H₆O, benzene, and NO₂. The measured in situ diffuse-reflectance infrared Fourier transform (DRIFT) spectra confirm that the products of nitric oxides during the chemical reactions occurred at the surface of MXene/SnS₂. Density functional theory (DFT) based on the first principles was implemented to compute the adsorption ability of NH₃ at the surface of the MXene/SnS₂ heterostructure. This indicates that the enhancement in the sensing properties of the MXene/SnS₂ heterostructure-based chemosensor could be ascribed to the stronger NH₃ adsorption, better catalytical activity, and more effective charge transfer bestowed by the formed heterostructure and the electron-redistribution-assisted stronger extraction of electrons from the sensing material.

Functional two-dimensional MXenes as cancer theranostic agents

Recently, MXenes, as a kind of two-dimensional (2D) layered materials with exceptional performance, have become the research hotspots owing to their unique structural, electronic, and chemical properties. They have potential applications in electrochemical storage, photocatalysis, and biosensors. Furthermore, they have certain characteristics such as large surface area, favorable biocompatibility, and ideal mechanical properties, which can expand their applications in biomedical fields, especially in cancer therapy. To date, several researchers have explored the applications of MXenes in tumor elimination, which exhibited other fantastic properties of those 2D MXenes, such as efficient in vivo photothermal ablation, low phototoxicity, high biocompatibility, etc. In this review, the structures, properties, modifications, and preparation methods are introduced respectively. More importantly, the multifunctional platforms for cancer therapy based on MXenes nanosheets (NSs) are reviewed in detail, including single-modality and combined-modality cancer therapy. Finally, the prospects and challenges of MXenes are prospected and discussed. STATEMENT OF SIGNIFICANCE: In this review, the structures, properties, modifications, and preparation methods of MXenes nanomaterials are introduced, respectively. In addition, the preparation conditions and morphological characterizations of some common MXenes for therapeutic platforms are also summarized. More importantly, the practical applications of MXenes-based nanosheets are reviewed in detail, including drug delivery, biosensing, bioimaging, and multifunctional tumor therapy platforms. Finally, the future prospects and challenges of MXenes are prospected and discussed.

Highly selective NH3 gas sensor based on Co(OH)2/Ti3C2T x nanocomposites operating at room temperature

Ammonia (NH3) is a common air pollutant and is a biomarker for kidney disease. Therefore, the preparation of ammonia gas sensors with high sensitivity, good selectivity and low operating temperature is of great importance for health protection. Using the in situ electrostatic self-assembly approach, a chemoresistive gas sensor based on Co(OH)2/Ti3C2T x hybrid material was created in this study. The prepared samples were characterized by XRD, XPS, TEM, BET and other testing methods for structure, surface topography and elements. These samples were fabricated into sensors, and the gas sensing properties of the materials were investigated under different test conditions. The results show that the gas response value of the C/M-2 sensor is up to about 14.7%/100 ppm, which is three times the response value of the sensor made of pure MXene to NH3. In addition, the Co(OH)2/Ti3C2T x hybrid sensors exhibit excellent repeatability, high sensitivity under low concentration (less than 5 ppm), fast response/recovery time (29 s/49 s) and long-time stability, which indicates their promising utility in the IoT field.

Tuning thermal and electrical properties of MXenes via dehydration

Recently, MXenes (a class of two-dimensional transition metal carbides) have attracted great attention in various applications such as humidity sensors, owing to their unique electrical and thermal properties. However, previous studies of MXenes mostly focus on their humidity-sensing characteristics such as the mechanical response, and only few reports on their electrical and thermal response are available. Herein, we present novel transient electrothermal experiments to demonstrate that a transition from a negative to a positive resistance-temperature relationship can take place when the MXene sample becomes fully dehydrated. This surprising and unusual phenomenon was elucidated through non-equilibrium molecular dynamics simulations and attributed to water absorption/desorption onto the chemically active MXene surface. A linear relationship was also found between electrical/thermal properties and environmental humidity, which could be related to water adsorption on the surface of the MXene sensor. We further decomposed the total measured thermal conductivity and found that phonons were the dominant thermal carriers in the MXene sample. The main breakthrough of this work is the discovery of the unusual resistance-temperature relationship, which should be applicable to the design of MXene-based sensors for various applications.

Humidity-sensitive chemoelectric flexible sensors based on metal-air redox reaction for health management

Numerous studies have shown flexible electronics play important roles in health management. The way of power supply is always an essential factor of devices and self-powered ones are very attractive because of the fabrication easiness, usage comfort and aesthetics of the system. In this work, based on the metal-air redox reaction, which is usually used in designing metal-air batteries, we design a self-powered chemoelectric humidity sensor where a silk fibroin (SF) and LiBr gel matrix containing parallel aligned graphene oxide (GO) flakes serve as the electrolyte. The abundant hydrophilic groups in GO/SF and the hygroscopicity of LiBr lead to tight dependence of the output current on the humidity, enabling the sensor high sensitivity (0.09 μA/s/1%), fast response (1.05 s) and quick recovery (0.80 s). As proofs of concept, we design an all-in-one respiratory monitoring-diagnosing-treatment system and a non-contact human-machine interface, demonstrating the applications of the chemoelectric humidity sensor in health management.

Ti3C2Tx MXene/urchin-like PANI hollow nanosphere composite for high performance flexible ammonia gas sensor

Ammonia (NH₃) has been used as a typical indicator to monitor food spoilage, human health, and air quality. However, the development of flexible NH₃ sensors with high response, excellent selectivity and low cost remains a huge challenge. Herein, a high performance NH₃ sensor based on Ti₃C₂T_x MXene nanosheet/urchin-like PANI hollow nanosphere composite (MP) was fabricated through template method and in situ polymerization. The NH₃ sensor is fabricated with no high cost electrodes through directly depositing this composite on flexible polyethylene terephthalate (PET) during polymerization. This optimized MP film sensor exhibits high response of 3.70 to 10 ppm NH₃ at room temperature, which is 4.74-fold in comparison with urchin-like PANI hollow nanosphere (u-PANI). It also shows excellent selectivity, good repeatability, satisfactory flexibility, air stability and low detection limit of 30 ppb. The effective morphology control and heterojunction construction of MP composite are responsible for superior sensing performance. Moreover, the application of this film sensor in the monitoring of the spoilage process of fresh pork is demonstrated. This study offers a new strategy for fabricating high performance flexible room-temperature NH₃ sensors, which may be scale fabrication and application in daily life.

A review on MXene and its nanocomposites for the detection of toxic inorganic gases

In the search of the viable candidate for the sensing of pollutant gases, two-dimensional (2D) material transition metal carbides (MXenes) have attracted immense attention due to their outstanding physical and chemical properties for sensing purposes. The formation of unique 2D layered structure with high conductivity, large mechanical strength, and high adsorption properties furnish their strong interactions with gaseous molecules, which holds a promising place for developing ideal gas sensing devices. This review looks at recent achievements in diversified MXenes, with a focus gaining on in-depth understanding of MXene-based materials in room temperature inorganic gas sensors through both theoretical and experimental studies. In the first part of the review, the properties and advantages of sensing material (MXene) in comparison with other 2D materials are discussed. In the second part, the unique advantages of chemiresistive based sensors and the demerits of other detection methods are summarized in detail. This section is followed by the unique structural design of MXene bases materials for improving the sensing performance towards detection of inorganic gases. The interaction between MXene and the adsorbed gases on its surface is discussed, with a possible sensing mechanism. Finally, an overview of the current progress and opportunities for the demand of MXene is emphasized and perspectives for future improvement of the design of MXene in gas sensors are highlighted. Therefore, this review highlights the opportunities and the advancement in 2D material-based gas sensors which could provide a new avenue for rapid detection of toxic gases in the environment.

Development of Highly Sensitive and Humidity Independent Room Temeprature NO2 Gas Sensor Using Two Dimensional Ti3C2Tx Nanosheets and One Dimensional WO3 Nanorods Nanocomposite

Room temperature gas sensors have been widely explored in gas sensor technology for real-time applications. However, humidity has found to affect the room temperature sensing and the sensor life, necessitating the development of novel sensing materials with high sensitivity and stability under humid conditions at room temperature. In this work, the room temperature sensing performance of a Ti₃C₂T_x decorated, WO₃ nanorods based nanocomposite has been investigated. The hydrothermally synthesized WO₃/Ti₃C₂T_x nanocomposite has been investigated for structural, morphological, and electrical studies using X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and Brunanuer-Emmett-Teller techniques. The WO₃/Ti₃C₂T_x sensors have been found to be highly selective to NO₂ at room temperature and exhibit much higher sensitivity in comparison to pristine WO₃ nanorods. Furthermore, sodium l-ascorbate treated Ti₃C₂T_x sheets in WO₃/Ti₃C₂T_x enhanced the stability and reversibility of the sensor toward NO₂ even under variable humidity conditions (0-99% relative humidity). This study shows the potential room temperature sensing application of a WO₃/Ti₃C₂T_x nanocomposite-based sensor for detecting NO₂ at sub-ppb level. Further, a plausible sensing mechanism based on WO₃/Ti₃C₂T_x nanocomposite has been proposed to explain the improved sensing characteristics.

MXenes Thin Films: From Fabrication to Their Applications

Two-dimensional MXenes possessed exceptional physiochemical properties such as high electrical conductivity (20,000 Scm-1), flexibility, mechanical strength (570 MPa), and hydrophilic surface functionalities that have been widely explored for energy storage, sensing, and catalysis applications. Recently, the fabrication of MXenes thin films has attracted significant attention toward electronic devices and sensor applications. This review summarizes the exciting features of MXene thin film fabrication methods such as vacuum-assisted filtration (VAF), electrodeposition techniques, spin coating, spray coating, dip-coating methods, and other physical/chemical vapor deposition methods. Furthermore, a comparison between different methods available for synthesizing a variety of MXenes films was discussed in detail. This review further summarizes fundamental aspects and advances of MXenes thin films in solar cells, batteries, electromagnetic interference shielding, sensing, etc., to date. Finally, the challenges and opportunities in terms of future research, development, and applications of MXenes-based films are discussed. A comprehensive understanding of these competitive features and challenges shall provide guidelines and inspiration for further growth in MXenes-based functional thin films and contribute to the advances in MXenes technology.