Room-Temperature, Highly Durable Ti3C2Tx MXene/Graphene Hybrid Fibers for NH3 Gas Sensing

Enhanced room-temperature detection of ultra-low level nitrogen dioxide: Improved sensitivity, selectivity and stability through MXene-modified In2O3 microspheres

To address the increasing demand for wearable sensors, the development of gas sensors with high sensitivity and environmentally friendly power consumption for monitoring NO2 at room temperature (RT) is particularly promising. In this paper, porous In2O3 microspheres are prepared via a hydrothermal method, followed by the incorporation of 2D MXene solution to synthesize In2O3@MXene composites. After characterizing the microstructures and morphology of the In2O3@MXene composites, the influence of MXene on the microstructures and NO2 gas-sensing performance at RT is discussed in detail. The results indicate that a moderate amount of MXene greatly affects the energy band structure, chemisorbed and vacancy oxygen content, and the availability of reactive sites for oxygen and NO2, thereby affecting the gas-sensing performance of the In2O3@MXene sensors. Notably, the In2O3@10MXene sensor exhibits the highest response value of 24.98 to 4 ppm NO2 at RT, which is 5.90 times higher than that of In2O3 sensor (4.23). Furthermore, the In2O3@10MXene sensor still presents a response value of 2.83-500 ppb NO2 under RT, confirming an ultra-low ppb level detection limit to NO2 gas at RT. Additionally, the In2O3@10MXene sensor demonstrates favorable gas selectivity and long-term stability. The incorporation of an appropriate amount of MXene effectively enhances the gas-sensing performance of the In2O3@MXene sensors, attributed to the formation of a Schottky heterojunction, increased surface oxygen, and more reactive sites for oxygen and NO2 from MXene.

Flexible and wearable electrochemical sensors for health and safety monitoring

Environmental safety monitoring is a crucial process that involves continuous and systematic observation and analysis of various pollutants in the environment to ensure its quality and safety. This monitoring encompasses a wide range of areas, including physical indicator monitoring (pertaining to parameters such as temperature, humidity, and wind speed), chemical indicator monitoring (focused on detecting harmful substances in environmental media such as air, water, and soil), and ecosystem monitoring (including biodiversity assessments and judgments on the health status of ecosystems). This review delves deeply into the significant advancements achieved in the field of flexible and wearable electrochemical sensors (FWESs) over the past fifteen years (from 2010 to 2024). It emphasizes the broad application of these sensors in health and environmental safety monitoring, with health monitoring primarily focusing on exhaled breath and sweat, and environmental monitoring covering temperature, humidity, and pollutants in air and water. By seamlessly integrating electrochemical principles, advanced sensor manufacturing technologies, and sensor functionalization, FWESs have opened up new avenues for non-invasive real-time monitoring of human health and environmental safety. This review highlights key developments in sensor structures, including flexible substrates, printed electrodes, and active materials. It also underscores the remarkable progress made in healthcare and environmental monitoring through the utilization of FWES. Despite these promising advancements, this emerging field still faces numerous challenges, such as improving sensor accuracy, enhancing durability, and reducing costs. The review concludes by discussing the future directions in this field, including ongoing research efforts aimed at overcoming these challenges and expanding the applications of FWESs in various sectors.

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.

Ultra-Compact MXene/Alginate/PVA Composite Fibers by Intercalation and Chelation for Enhanced Flame Retardancy and Energy Harvesting

MXene fibers with electro-conductivity and electrochemical properties have drawn growing research interest for its promising applications in wearable electronics, flexible electrodes, and smart textiles. However, producing MXene fibers with high strength keeps challenging because loose MXene sheets are hard to compact tightly due to electrostatic repulsion. Herein, ultra-compact MXene-based fibers are produced by intercalating alginate and polyvinyl alcohol (PVA) layers into MXene nanosheets and chelating via metal ions (i.e., Ca2+). The hydrogen and ionic bond are beneficial to compact MXene nanosheets and decrease the interplanar spacing, which improves the tensile strength. These result in MXene-based fibers with low porosity (0.2 vol%) and a high orientation factor of 0.877 exhibiting high electrical conductivity (1006 S cm‒1). In addition, flame retardancy is enhanced without smoldering owing to the synergistic effect of MXene and metal ions. Moreover, these compact MXene-based fibers with electromagnetic interference shielding, mechanical stability, acid, and alkali-resistant properties, and photo-thermal effect can be achieved for scale production. This strategy paves the way for the continuous production of compact functional fibers, applicable in flame retardant fabric, wireless communication, energy harvesting, and wearable flexible textiles.

Selectivity in Chemiresistive Gas Sensors: Strategies and Challenges

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

Interfacial Confinement Derived High-Strength MXene@Graphene Oxide Core-Shell Fibers for Electromagnetic Wave Regulation, Thermochromic Alerts, and Visible Camouflage

Although electrically conductive Ti₃C₂Tx MXene fibers are promising for wearable electronics, the poor inter-sheet interactions and the random stacking structure of MXene sheets seriously hinder electron transport and load transfer of the fibers. Herein, mechanically strong and electrically conductive MXene@graphene oxide (GO) core-shell fibers are fabricated with a coaxial wet-spinning methodology for electromagnetic wave regulation, thermochromic alerts, and visible camouflage. During the coaxial wet-spinning, the trace-carboxylated GO sheets in the shell align readily because of the spatial confinement of the coaxial needle, while the MXene sheets in the core are progressively oriented and flattened because of the spatial confinement of the GO shell. The positively charged chitosan in the coagulating solution enhances the interfacial interactions between the GO and MXene sheets and facilitates the sheet's orientation inside the fibers. Consequently, the highly aligned core-shell fibers exhibit an ultrahigh tensile strength of 613.7 MPa and an outstanding conductivity of ≈7766 S cm-1. Furthermore, fiber-woven textiles not only offer excellent electromagnetic interference shielding performance but also achieve quantitative regulation of electromagnetic wave transmission by adjusting the angle of the double-layered textiles. The textiles can combine with thermochromic coatings for thermotherapy alerts, visual thermochromic warnings, and visible camouflage.

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.

Multifunctional Fiber Robotics with Low Mechanical Hysteresis for Magnetic Navigation and Inhaled Gas Sensing

Recently, increasing research attention has been directed toward detecting the distribution of hazardous gases in the respiratory system for potential diagnosis and treatment of lung injury. Among various technologies, magnetic fiber robots exhibit great potential for minimally invasive surgery and in situ disease diagnosis. However, integrating magnetic fibers with functionalized sensitive materials remains challenging while preserving the miniaturized fibers' mechanical properties. Herein, we report Ti3C2Tx/TPU/NdFeB fibers prepared by facile wet spinning, spray coating, and magnetization, obtaining fibers with decent strength (4.34 MPa) and low hysteresis while maintaining mechanical robustness and magnetoelectric properties. Such fiber robotics could be magnetically actuated for complex movement, while the surface-coated MXene endowed them with the specific response of 5.2% to 40 ppm of triethylamine gas. Fiber robotics realized magnetically driven omnidirectional steering and navigation for propulsion in tubular environments by combination with nitinol guide wires. Consequently, based on magnetic navigation and the chemiresistive gas response, the proposed fiber robotics could locate the position with the highest level of the triethylamine gas inside a bronchial model and provide information on its distribution. This provides a proof-of-concept demonstration for inhaled hazardous gas detection and minimally invasive robotic surgery by multifunctional fiber robotics.

Representative modeling of biocompatible MXene nanocomposites for next-generation biomedical technologies and healthcare

MXenes are a class of 2D transition metal carbides and nitrides (Mn+1XnT) that have attracted significant interest owing to their remarkable potential in various fields. The unique combination of their excellent electromagnetic, optical, mechanical, and physical properties have extended their applications to the biological realm as well. In particular, their ultra-thin layered structure holds specific promise for diverse biomedical applications. This comprehensive review explores the synthesis methods of MXene composites, alongside the biological and medical design strategies that have been employed for their surface engineering. This review delves into the interplay between these strategies and the resulting properties, biological activities, and unique effects at the nano-bio-interface. Furthermore, the latest advancements in MXene-based biomaterials and medicine are systematically summarized. Further discussion on MXene composites designed for various applications, including biosensors, antimicrobial agents, bioimaging, tissue engineering, and regenerative medicine, are also provided. Finally, with a focus on translating research results into real-world applications, this review addresses the current challenges and exciting future prospects of MXene composite-based biomaterials.

PEDOT:PSS-Facilitated Directionally 3-D Assembled MXene-Based Aerogel for High-Performance Chemoresistive Sensing & Breath Analysis

MXene has garnered growing interest in the field of electrochemistry, thanks to its unique electrical and surface characteristics. Nonetheless, significant challenges persist in realizing its full potential in chemoresistive sensing applications. In this study, a novel unidirectional freeze-casting approach for fabricating a Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-facilitated vertically aligned MXene-based aerogel with enhanced chemoresistive sensing properties was introduced. Firstly, the persistent challenge of poor gelation in MXene was addressed by formulating a nanohybrid of MXene and PEDOT:PSS, which acted as flexible conductive nanobinder. Employing a unique freeze-casting method, MXene flakes interconnected by PEDOT:PSS, were stabilized into a flexible, vertically aligned structure, leading to maximum surface exposure and enhanced robustness. The resulting 3-dimentional (3-D) aerogel exhibited a fast, heightened chemoresistive response of 7 to 50 parts per million (ppm) acetone and expanded the working range to between 10 parts per billion (ppb)-8000 ppm. Interfacial heterostructures formed between MXene and PEDOT:PSS, provided active sites, reduced activation energy, and enhanced selectivity. Modulated MXene bandgap, and its electron mobility further facilitated electron transfer, and enhanced signal strength. The sensor showed excellent biocompatibility and was also successfully employed as a breathalyzing tool, for on-demand alcohol consumption monitoring.

Graphene Functionalization by O2, H2, and Ar Plasma Treatments for Improved NH3 Gas Sensing

Graphene-based materials have gained attention for their promise in various applications owing to their two-dimensional structure. Functionalizing the graphene surface can help realize materials with noble properties. In this study, graphene was functionalized by plasma treatment in O2, H2, and Ar environments, and the effects on the NH3 gas-sensing performance were evaluated. The O2 plasma treatment induced oxidation of the graphene (i.e., graphoxide), while the H2 plasma treatment induced hydrogenation (i.e., graphane). Raman scattering spectroscopy suggested that graphoxide had vacancy-type defects and graphane had sp3-type defects, while Ar-treated graphene had both types of defects. Graphane had the highest sheet resistance followed by graphoxide, Ar-treated graphene, and pristine graphene, which can be attributed to the large bandgap of 3.0 eV for graphane. In contrast, graphoxide had the best NH3 gas-sensing performance, which indicates that NH3 gas interacts more strongly with vacancy-type defects than with sp3-type defects. The results showed that functionalizing the graphene structure generated noble materials with a superior NH3 gas-sensing performance compared with pristine graphene.

Ultrasensitive detection of hazardous gas at room temperature enabled by MOF@MXene 0D-2D heterostructure

Achieving high sensitivity in detecting trace concentrations of toxic gases, particularly under room temperature (RT) conditions, remains a significant challenge. Herein, a 0D-2D heterostructure that can detect ppb-level H2S at RT is proposed by self-assembling cobalt-based metal-organic framework (Co-MOF) on Ti3C2Tx MXene. Co-MOFs with high specific surface areas can capture and concentrate target gas molecules, enhancing host-guest interactions and thereby boosting the selectivity and sensitivity. MXene nanosheets with high conductivity enable rapid electron transport at heterointerface, hence efficiently accelerating the reaction kinetics. Thereby, the as-prepared chemiresistive gas sensor based on Co-MOF@MXene 0D-2D heterostructure possessed excellent sensitivity against interfering gases and delivered an excellent response value of 11.1 to 400 ppb H2S at RT. The judicious design of MOF@MXene heterostructure may spur advanced hybrid material systems for superior sensing applications.

Highly Stable Heating Fibers of Ti3C2Tx MXene and Polyacrylonitrile via Synergistic Thermal Annealing

Nanohybrid assemblies provide an effective platform for integrating the intrinsic properties of individual components into microscale fibers. In this study, a novel approach for creating mechanically and environmentally stable MXene fibers through the synergistic assembly of MXene and polyacrylonitrile (PAN), is introduced. Unlike fibers generated via a conventional stabilization process, which relies on air-based stabilization to transform the PAN molecules into ring structures fundamental to carbon fibers, the hybrid fibers are annealed in an Ar atmosphere. This unique approach suggests MXene can serve as an oxygen provider that is essential for stabilizing PAN. As a result, significantly improved interfiber compactness is achieved and the oxidation stability of MXene is enhanced under atmospheric conditions. The resulting fibers exhibit exceptional stability, even after extended exposure to high humidity and elevated temperatures. This highlights the suitability of the thermally annealed MXene-PAN (T-MX-PAN) fibers as robust electric heating elements. Notably, these fibers consistently generate heat over 1800 bending cycles. When integrated into fabrics, they demonstrate the capability to generate sufficient heat for melting ice and rapid evaporation. This study highlights the potential of T-MX-PAN fibers as next-generation wearable heaters and offers valuable insights into advancing wearable technology in demanding environments.

Graphene-Based Fiber Materials for Gas Sensing Applications: State of the Art Review

The importance of gas sensors is apparent as the detection of gases and pollutants is crucial for environmental monitoring and human safety. Gas sensing devices also hold the potential for medical applications as health monitoring and disease diagnostic tools. Gas sensors fabricated from graphene-based fibers present a promising advancement in the field of sensing technology due to their enhanced sensitivity and selectivity. The diverse chemical and mechanical properties of graphene-based fibers-such as high surface area, flexibility, and structural stability-establish them as ideal gas-sensing materials. Most significantly, graphene fibers can be readily tuned to detect a wide range of gases, making them highly versatile in gas-sensing technologies. This review focuses on graphene-based composite fibers for gas sensors, with an emphasis on the preparation processes used to achieve these fibers and the gas sensing mechanisms involved in their sensors. Graphene fiber gas sensors are presented based on the chemical composition of their target gases, with detailed discussions on their sensitivity and performance. This review reveals that graphene-based fibers can be prepared through various methods and can be effectively integrated into gas-sensing devices for a diverse range of applications. By presenting an overview of developments in this field over the past decade, this review highlights the potential of graphene-based fiber sensors and their prospective integration into future technologies.

All-MXene electronic tongue for neurotransmitters detection

Neurotransmitters (NTs) are molecules produced by neurons that act as the body's chemical messengers. Their abnormal levels in the human system have been associated with many disorders and neurodegenerative diseases, which makes the monitoring of NTs fundamentally important. Specifically for clinical analysis and understanding of brain behavior, simultaneous detection of NTs at low levels quickly and reliably is imperative for disease prevention and early diagnosis. However, the methods currently employed are usually invasive or inappropriate for multiple NTs detection. Herein, we developed a MXene-based impedimetric electronic tongue (e-tongue) for sensitive NT monitoring, using Nb2C, Nb4C3, Mo2C, and Mo2Ti2C3 MXenes as sensing units of the e-tongue, and Principal Component Analysis (PCA) as the data treatment method. The high specific surface area, distinct electrical properties, and chemical stability of the MXenes gave rise to high sensitivity and good reproducibility of the sensor array toward NT detection. Specifically, the e-tongue detected and differentiated multiple NTs (acetylcholine, dopamine, glycine, glutamate, histamine, and tyrosine) at concentrations as low as 1 nmol L-1 and quantified NTs present in a mixture. Besides, analyses performed with interferents and actual samples confirmed the system's potential to be used in clinical diagnostics. The results demonstrate that the MXene-based e-tongue is a suitable, rapid, and simple method for NT monitoring with high accuracy and sensitivity.

Multi-Interface Engineering of MXenes for Self-Powered Wearable Devices

Self-powered wearable devices with integrated energy supply module and sensitive sensors have significantly blossomed for continuous monitoring of human activity and the surrounding environment in healthcare sectors. The emerging of MXene-based materials has brought research upsurge in the fields of energy and electronics, owing to their excellent electrochemical performance, large surface area, superior mechanical performance, and tunable interfacial properties, where their performance can be further boosted via multi-interface engineering. Herein, a comprehensive review of recent progress in MXenes for self-powered wearable devices is discussed from the aspects of multi-interface engineering. The fundamental properties of MXenes including electronic, mechanical, optical, and thermal characteristics are discussed in detail. Different from previous review works on MXenes, multi-interface engineering of MXenes from termination regulation to surface modification and their impact on the performance of materials and energy storage/conversion devices are summarized. Based on the interfacial manipulation strategies, potential applications of MXene-based self-powered wearable devices are outlined. Finally, proposals and perspectives are provided on the current challenges and future directions in MXene-based self-powered wearable devices.

MXene-based fibers: Preparation, applications, and prospects

With the vigorous development and huge demand for portable wearable devices, wearable electronics based on functional fibers continue to emerge in a wide range of energy storage, motion monitoring, disease prevention, electromagnetic interference (EMI) shielding, etc. MXene, as an emerging two-dimensional inorganic compound, has shown great potential in functional fiber manufacturing and has attracted much research attention due to its own good mechanical properties, high electrical conductivity, excellent electrochemical properties and favorable processability. Herein, this paper reviews recent advances of MXene-based fibers. Speaking to MXene dispersions, the properties of MXene dispersions including dispersion stability, rheological properties and liquid crystalline properties are highlighted. The preparation techniques used to produce MXene-based fibers and application progress regarding MXene-based fibers into supercapacitors, sensors, EMI shielding and Joule heaters are summarized. Challenges and prospects surrounding the development of MXene-based fibers are proposed in future. This review aims to provide processing guidelines for MXene-based fiber manufacturing, thereby achieving more possibilities of MXene-based fibers in advanced applications with a view to injecting more vitality into the field of smart wearables.

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.

In Situ Growth of COF/PVA-Carrageenan Hydrogel Using the Impregnation Method for the Purpose of Highly Sensitive Ammonia Detection

Flexible ammonia (NH3) gas sensors have gained increasing attention for their potential in medical diagnostics and health monitoring, as they serve as a biomarker for kidney disease. Utilizing the pre-designable and porous properties of covalent organic frameworks (COFs) is an innovative way to address the demand for high-performance NH3 sensing. However, COF particles frequently encounter aggregation, low conductivity, and mechanical rigidity, reducing the effectiveness of portable NH3 detection. To overcome these challenges, we propose a practical approach using polyvinyl alcohol-carrageenan (κPVA) as a template for in the situ growth of two-dimensional COF film and particles to produce a flexible hydrogel gas sensor (COF/κPVA). The synergistic effect of COF and κPVA enhances the gas sensing, water retention, and mechanical properties. The COF/κPVA hydrogel shows a 54.4% response to 1 ppm NH3 with a root mean square error of less than 5% and full recovery compared to the low response and no recovery of bare κPVA. Owing to the dual effects of the COF film and the particles anchoring the water molecules, the COF/κPVA hydrogel remained stable after 70 h in atmospheric conditions, in contrast, the bare κPVA hydrogel was completely dehydrated. Our work might pave the way for highly sensitive hydrogel gas sensors, which have intriguing applications in flexible electronic devices for gas sensing.

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.

MXene Key Composites: A New Arena for Gas Sensors

With the development of science and technology, the scale of industrial production continues to grow, and the types and quantities of gas raw materials used in industrial production and produced during the production process are also constantly increasing. These gases include flammable and explosive gases, and even contain toxic gases. Therefore, it is very important and necessary for gas sensors to detect and monitor these gases quickly and accurately. In recent years, a new two-dimensional material called MXene has attracted widespread attention in various applications. Their abundant surface functional groups and sites, excellent current conductivity, tunable surface chemistry, and outstanding stability make them promising for gas sensor applications. Since the birth of MXene materials, researchers have utilized the efficient and convenient solution etching preparation, high flexibility, and easily functionalize MXene with other materials to prepare composites for gas sensing. This has opened a new chapter in high-performance gas sensing materials and provided a new approach for advanced sensor research. However, previous reviews on MXene-based composite materials in gas sensing only focused on the performance of gas sensing, without systematically explaining the gas sensing mechanisms generated by different gases, as well as summarizing and predicting the advantages and disadvantages of MXene-based composite materials. This article reviews the latest progress in the application of MXene-based composite materials in gas sensing. Firstly, a brief summary was given of the commonly used methods for preparing gas sensing device structures, followed by an introduction to the key attributes of MXene related to gas sensing performance. This article focuses on the performance of MXene-based composite materials used for gas sensing, such as MXene/graphene, MXene/Metal oxide, MXene/Transition metal sulfides (TMDs), MXene/Metal-organic framework (MOF), MXene/Polymer. It summarizes the advantages and disadvantages of MXene composite materials with different composites and discusses the possible gas sensing mechanisms of MXene-based composite materials for different gases. Finally, future directions and inroads of MXenes-based composites in gas sensing are presented and discussed.

Room temperature NH3 gas sensor based on In(OH)3/Ti3C2Tx nanocomposites

Industrialization and agricultural demand have both improved human life and led to environmental contamination. Especially the discharge of a lot of poisonous and harmful gases, including ammonia, ammonia pollution has become a pressing problem. High concentrations of ammonia can pose significant threats to both the environment and human health. Therefore, accurate monitoring and detection of ammonia gas are crucial. To address this challenge, we have developed an ammonia gas sensor using In(OH)3/Ti3C2Tx nanocomposites through an in-situ electrostatic self-assembly process. This sensor was thoroughly characterized using advanced techniques like XRD, XPS, BET, and TEM. In our tests, the I/M-2 sensor exhibited remarkable performance, achieving a 16.8% response to 100 ppm NH3 at room temperature, which is a 3.5-fold improvement over the pure Ti3C2Tx MXene sensor. Moreover, it provides swift response time (20 s), high response to low NH3 concentrations (≤ 10 ppm), and excellent long-term stability (30 days). These exceptional characteristics indicate the immense potential of our In(OH)3/Ti3C2Tx gas sensor in ammonia detection.

Architectural design and affecting factors of MXene-based textronics for real-world application

Today, textile-based wearable electronic devices (textronics) have been developed by taking advantage of nanotechnology and textile substrates. Textile substrates offer flexibility, air permeability, breathability, and wearability, whereas, using nanomaterials offers numerous functional properties, like electrical conductivity, hydrophobicity, touch sensitivity, self-healing properties, joule heating properties, and many more. For these reasons, textronics have been extensively used in many applications. Recently, new emerging two-dimensional (2D) transition metal carbide and nitride, known as MXene, nanomaterials have been highly considered for developing textronics because the surface functional groups and hydrophilicity of MXene nanoflakes allow the facile fabrication of MXene-based textronics. In addition, MXene nanosheets possess excellent electroconductivity and mechanical properties as well as large surface area, which also give numerous opportunities to develop novel functional MXene/textile-based wearable electronic devices. Therefore, this review summarizes the recent advancements in the architectural design of MXene-based textronics, like fiber, yarn, and fabric. Regarding the fabrication of MXene/textile composites, numerous factors affect the functional properties (e.g. fabric structure, MXene size, etc.). All the crucial affecting parameters, which should be chosen carefully during the fabrication process, are critically discussed here. Next, the recent applications of MXene-based textronics in supercapacitors, thermotherapy, and sensors are elaborately delineated. Finally, the existing challenges and future scopes associated with the development of MXene-based textronics are presented.

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.

Fully Transient 3D Origami Paper-Based Ammonia Gas Sensor Obtained by Facile MXene Spray Coating

Developing high-performance chemiresistive gas sensors with mechanical compliance for environmental or health-related biomarker monitoring has recently drawn increasing research attention. Among them, two-dimensional MXene materials hold great potential for room-temperature hazardous gas (e.g., NH3) monitoring regardless of the complicated fabrication process, insufficient 2D/3D flexibilities, and poor environmental sustainability. Herein, a Ti3C2Tx MXene/gelatin ink was developed for patterning electrodes through a facile spray coating. Particularly, the patterned Ti3C2Tx-based coating exhibited good adhesion on the paper substrate against repeated peeling-off and excellent mechanical flexibility against 1000 cyclic stretching. The porous morphology of the coating facilitated the NH3 sensing ability. As a result, the 2D kirigami-shaped NH3 sensor exhibited a good response of 7% to 50 ppm of NH3 with detectable concentrations ranging from 5-500 ppm, decent selectivity over interferences, etc., which could be well-maintained even at 50% stretched state. In addition, with the help of mechanically guided compressive buckling, 3D mesostructured MXene origamis could be obtained, holding promise for detecting the coming direction and height distribution of hazardous gas, e.g., the NH3. More importantly, the as-fabricated MXene/gelatin origami paper could be fully degraded in PBS/H2O2/cellulase solution within 19 days, demonstrating its potential as a high-performance, shape morphable, and environmentally friendly wearable gas sensor.

2D nanomaterials for realization of flexible and wearable gas sensors: A review

Gas sensors are extensively employed for monitoring and detection of hazardous gases and vapors. Many of them are produced on rigid substrates, but flexible and wearable gas sensors are needed for intriguing usage including the internet of things (IoT) and medical devices. The materials with the greatest potential for the fabrication of flexible and wearable gas sensing devices are two-dimensional (2D) semiconducting nanomaterials, which consist of graphene and its substitutes, transition metal dichalcogenides, and MXenes. These types of materials have good mechanical flexibility, high charge carrier mobility, a large area of surface, an abundance of defects and dangling bonds, and, in certain instances adequate transparency and ease of synthesis. In this review, we have addressed the different 2D nonmaterial properties for gas sensing in the context of fabrication of flexible/wearable gas sensors. We have discussed the sensing performance of flexible/wearable gas sensors in various forms such as pristine, composite and noble metal decorated. We believe that content of this review paper is greatly useful for the researchers working in the research area of fabrication of flexible/wearable gas sensors.

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.

Powering the Future: Unleashing the Potential of MXene-Based Dual-Functional Photoactive Cathodes in Photo-Rechargeable Zinc-Ion Capacitor

Dual-functional photo-rechargeable (photo-R) energy storage devices, which acquire stored energy from solar energy harvesting, are being developed to battle the current energy crisis. In this study, these findings on the photo-driven characteristics of MXene-based photocathodes in photo-R zinc-ion capacitors (ZICs) are presented. Along with the pristine Ti3 C2 Tx MXene, tellurium/Ti3 C2 Tx (Te/Ti3 C2 Tx ) hybrid nanostructure is synthesized via facile chemical vapor transport technique to examine them for photocathodes in ZICs. Interestingly, the evaluated self-powered photodetector devices using MXene-based samples revealed a pyro-phototronic behavior introduced into the samples, with higher desirability observed in Te/Ti3 C2 Tx . The photo-R ZICs results exhibited a capacitance enhancement of 50.86% for Te/Ti3 C2 Tx at two scan rates of 5 and 10 mV s-1 under illumination, compared to dark conditions. In contrast, a capacitance enhancement of 30.20% is obtained for the pristine Ti3 C2 Tx at only a 5 mV s-1 scan rate. Furthermore, both samples achieved photo-charging voltage responses of ≈960 mV, and photoconversion efficiencies of 0.01% (for Te/ Ti3 C2 Tx ) and 0.07% (for Ti3 C2 Tx ). These characteristics in MXene-based single photo-R ZICs are significant and considerable with the distinguished integrated photo-R supercapacitors with solar cells, or coupled energy-harvesting and energy-storing devices reported recently in the literature.

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.

Adsorption Behavior of Dissolved Gas Molecules in Transformer Oil on Rh Modified GeSe Monolayer

Dissolved gas analysis in transformer oil is useful for detecting early transformer failures. The research on gas sensors for monitoring dissolved gas in transformer oil has attracted wide attention from academia and industry. In this study, Rh-doped GeSe monolayers were used as gas sensing materials based on the density functional theory (DFT). The potential of the Rh-GeSe monolayer as a gas sensor was evaluated by calculating the geometric structure, adsorption distance (dsub/gas), binding energy (Eb), adsorption energy (Eads), transfer charge (ΔQ), the density of states (DOS), band structure, electron localization function (ELF), charge difference density (CDD), and sensitivity (S) of Rh-GeSe monolayer with eight gas molecules (SO2, C2H2, NO2, H2, CH4, CO2, H2S, and CO). The results show that the Rh-GeSe monolayer has a prominent response to SO2, C2H2, and NO2 gas molecules and has great potential to become an excellent gas sensor. This study provides a theoretical basis for the application of Rh-GeSe monolayer in the field of gas sensing and provides a new way for the development of other gas sensors.

Boron-rich hybrid BCN nanoribbons for highly ambient uptake of H2S, HF, NH3, CO, CO2 toxic gases

Nanomaterials-based gas sensors are widely applied for the monitoring and fast detection of hazardous gases owing to their sensitivity and selectivity. Hydrogen sulfide (H2S), hydrogen fluoride (HF), ammonia (NH3), and carbon monoxide/dioxide (CO/CO2) produced from petroleum fields, sewage, mines, and gasoline are harmful for both human life and environment. With an increase in the emission of these toxic compounds, their real-time monitoring and efficient adsorbent application and storage are very necessary. To this end, we investigated the adsorption characteristic and sensitivity factor of these five toxic gases on armchair and zigzag hybrid boron-carbon-nitride (BCN) nanoribbons with/without boron-rich (B-rich) defects using first principle calculation, where 25%, 33%, and 50% carbon concentration were considered. Our findings reveal that B-rich nanoribbons have strong adsorption energy, charge transfer, and structural deformation owing to the double acceptor of B-rich defects. Moreover, the zigzag and armchair forms of these hybrid BCN nanoribbons show physical adsorption, altering their band gap and phase transition after adsorbing these toxic gases, where B-rich nanoribbons possess high sensitivity to NH3 and CO among other gases. Furthermore, B-rich hybrid nanoribbons have higher CO2 adsorption energy than the standard free energy of CO2 at room temperature. This study suggests that hybrid BCN nanoribbons and B-rich defected structures can be good candidates for the uptake and storage of toxic gases, helping experimental groups to design efficient ambient gas sensors.

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.

An Overview of Electrochemical Sensors Based on Transition Metal Carbides and Oxides: Synthesis and Applications

Sensors play vital roles in industry and healthcare due to the significance of controlling the presence of different substances in industrial processes, human organs, and the environment. Electrochemical sensors have gained more attention recently than conventional sensors, including optical fibers, chromatography devices, and chemiresistors, due to their better versatility, higher sensitivity and selectivity, and lower complexity. Herein, we review transition metal carbides (TMCs) and transition metal oxides (TMOs) as outstanding materials for electrochemical sensors. We navigate through the fabrication processes of TMCs and TMOs and reveal the relationships among their synthesis processes, morphological structures, and sensing performance. The state-of-the-art biological, gas, and hydrogen peroxide electrochemical sensors based on TMCs and TMOs are reviewed, and potential challenges in the field are suggested. This review can help others to understand recent advancements in electrochemical sensors based on transition metal oxides and carbides.

Fiber Crossbars: An Emerging Architecture of Smart Electronic Textiles

Smart wearables have a significant impact on people's daily lives, enabling personalized motion monitoring, realizing the Internet of Things, and even reshaping the next generation of telemedicine systems. Fiber crossbars (FCs), constructed by crossing two fibers, have become an emerging architecture among the accessible structures of state-of-the-art smart electronic textiles. The mechanical, chemical, and electrical interactions between crossing fibers result in extensive functionalities, leading to the significant development of innovative electronic textiles employing FCs as their basic units. This review provides a timely and comprehensive overview of the structure designs, material selections, and assembly techniques of FC-based devices. The recent advances in FC-based devices are summarized, including multipurpose sensing, multiple-mode computing, high-resolution display, high-efficient power supply, and large-scale textile systems. Finally, current challenges, potential solutions, and future perspectives for FC-based systems are discussed for their further development in scale-up production and commercial applications.

Impermeable Graphene Skin Increases the Heating Efficiency and Stability of an MXene Heating Element

A Joule heater made of emerging 2D nanosheets, i.e., MXene, has the advantage of low-voltage operation with stable heat generation owing to its highly conductive and uniformly layered structure. However, the self-heated MXene sheets easily get oxidized in warm and moist environments, which limits their intrinsic heating efficiencies. Herein, an ultrathin graphene skin is introduced as a surface-regulative coating on MXene to enhance its oxidative stability and Joule heating efficiency. The skin layer is deposited on MXene using a scalable solution-phased layer-by-layer assembly process without deteriorating the excellent electrical conductivity of the MXene. The graphene skin comprises narrow and hydrophobic channels, which results in ≈70 times higher water impermeability of the hybrid film of graphene and MXene (GMX) than that of the pristine MXene. A complementary electrochemical analysis confirms that the graphene skin facilitates longer-lasting protection than conventional polymer coatings owing to its tortuous pathways. In addition, the sp2 planar carbon surface with a low heat loss coefficient improves the heating efficiency of the GMX, indicating that this strategy is promising for developing adaptive heating materials with a tractable voltage range and high Joule heating efficiency.

Surface nitrided MXene sheets with outstanding electroconductivity and oxidation stability

Two-dimensional Ti3C2Tx MXenes are promising candidates for a wide range of film- or fiber-based devices owing to their solution processability, high electrical conductivity, and versatile surface chemistry. The surface terminal groups (Tx) of MXenes can be removed to increase their inherent electrical performance and ensure chemical stability. Therefore, understanding the chemical evolution during the removal of the terminal groups is crucial for guiding the production, processing, and application of MXenes. Herein, we investigate the effect of chemical modification on the electron-transfer behavior during the removal of the terminal groups by annealing Ti3C2Tx MXene single sheets under argon (Ar-MXene) and ammonia gas (NH3-MXene) conditions. Annealing in ammonia gas results in surface nitridation of MXenes and preserves the electron-abundant Ti3C2 structure, whereas annealing MXene single sheets in Ar gas results in the oxidation of the titanium layers. The surface-nitrided MXene film exhibits an electrical conductivity two times higher than that of the Ar-MXene film. The oxidation stability is quantified by calculating the oxidation rate constants for severe reactions with H2O2. The surface-nitrided MXene is 13 times more stable than Ar-MXene. The investigation of MXene single sheets provides fundamental insights that are valuable for designing electrically conductive and chemically stable MXenes.

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.

2D Materials Beyond Post-AI Era: Smart Fibers, Soft Robotics, and Single Atom Catalysts

Recent consecutive discoveries of various 2D materials have triggered significant scientific and technological interests owing to their exceptional material properties, originally stemming from 2D confined geometry. Ever-expanding library of 2D materials can provide ideal solutions to critical challenges facing in current technological trend of the fourth industrial revolution. Moreover, chemical modification of 2D materials to customize their physical/chemical properties can satisfy the broad spectrum of different specific requirements across diverse application areas. This review focuses on three particular emerging application areas of 2D materials: smart fibers, soft robotics, and single atom catalysts (SACs), which hold immense potentials for academic and technological advancements in the post-artificial intelligence (AI) era. Smart fibers showcase unconventional functionalities including healthcare/environmental monitoring, energy storage/harvesting, and antipathogenic protection in the forms of wearable fibers and textiles. Soft robotics aligns with future trend to overcome longstanding limitations of hard-material based mechanics by introducing soft actuators and sensors. SACs are widely useful in energy storage/conversion and environmental management, principally contributing to low carbon footprint for sustainable post-AI era. Significance and unique values of 2D materials in these emerging applications are highlighted, where the research group has devoted research efforts for more than a decade.

Recent Advances in MXene-Based Fibers: Fabrication, Performance, and Application

Two-dimensional transition metal carbide/nitrides (MXenes) have recently received extensive attention due to their diverse material types and versatile structures, large-scale production, and excellent properties. MXene sheets possess abundant hydrophilic functional groups on their surface, which enable them to be assembled into macroscopic fibers or compounded with other functional materials to produce composite fibers. This review aims to provide a comprehensive analysis of MXene fibers in terms of their fabrication, structure, properties, and recent applications as flexible and wearable electronics. The review will discuss the principles of different methods used to synthesize MXene fibers and analyze the characteristics of the as-synthesized fibers, with a particular focus on the wet spinning method. The fundamental relationships between the microstructure of MXene fibers and their resulting mechanical and electrical properties will be explored. Furthermore, the review will elaborate on the progress made in MXene-based fibers in the rapidly growing field of wearable electronics applications, provide insights into future development of MXene fiber materials and propose solutions to the challenges facing practical applications.

MXene Fibers for Flexible and Wearable Electronics: Recent Progress and Future Perspectives

With the impetus of flexible electronics and micro-nano fabrication technology, the human demand for flexible intelligent wearable devices is on an upsurge. In recent years, new functional fibers have undergone rapid development and emerged as an indispensable carrier of flexible wearable e-textiles. However, to achieve their functional applications and durability, new functional fibers must possess good electrical and mechanical properties. As an emerging two-dimensional material, MXenes have attracted immense attention for their high electrical conductivity, mechanical strength, specific surface area, adjustable surface properties, and exceptional processability. As such, MXenes have become an ideal candidate for the primary functional component of functional fibers. This paper presents a comprehensive review of research progress on MXene-based fibers in the construction of flexible wearable electronic textiles. Firstly, we briefly outline the preparation methods of MXenes materials. Next, we summarize the processing types of MXene-based fibers and highlight their performance parameters. Lastly, we summarize the primary application scenarios of MXene-based fibers and anticipate the future development of flexible wearable e-textiles.

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.

Nacre-Inspired Strong MXene/Cellulose Fiber with Superior Supercapacitive Performance via Synergizing the Interfacial Bonding and Interlayer Spacing

MXene fibers are promising candidates for weaveable and wearable energy storage devices because of their good electrical conductivity and high theoretical capacitance. Herein, we propose a nacre-inspired strategy for simultaneously improving the mechanical strength, volumetric capacitance, and rate performance of MXene-based fibers through synergizing the interfacial interaction and interlayer spacing between Ti₃C₂T_X nanosheets. The optimized hybrid fibers (M-CMC-1.0%) with 99 wt % MXene loading exhibit an improved tensile strength of ∼81 MPa and a high specific capacitance of 885.0 F cm^-3 at 1 A cm^-3 together with an outstanding rate performance of 83.6% retention at 10 A cm^-3 (740.0 F cm^-3). As a consequence, the fiber supercapacitor (FSC) based on the M-CMC-1.0% hybrid delivers an output capacitance of 199.5 F cm^-3, a power density of 1186.9 mW cm^-3, and an energy density of 17.7 mWh cm^-3, respectively, implying its promising applications as portable energy storage devices for future wearable electronics.

Boosting Interfacial Polarization Through Heterointerface Engineering in MXene/Graphene Intercalated-Based Microspheres for Electromagnetic Wave Absorption

Multi-layer 2D material assemblies provide a great number of interfaces beneficial for electromagnetic wave absorption. However, avoiding agglomeration and achieving layer-by-layer ordered intercalation remain challenging. Here, 3D reduced graphene oxide (rGO)/MXene/TiO2/Fe2C lightweight porous microspheres with periodical intercalated structures and pronounced interfacial effects were constructed by spray-freeze-drying and microwave irradiation based on the Maxwell-Wagner effect. Such approach reinforced interfacial effects via defects introduction, porous skeleton, multi-layer assembly and multi-component system, leading to synergistic loss mechanisms. The abundant 2D/2D/0D/0D intercalated heterojunctions in the microspheres provide a high density of polarization charges while generating abundant polarization sites, resulting in boosted interfacial polarization, which is verified by CST Microwave Studio simulations. By precisely tuning the 2D nanosheets intercalation in the heterostructures, both the polarization loss and impedance matching improve significantly. At a low filler loading of 5 wt%, the polarization loss rate exceeds 70%, and a minimum reflection loss (RLmin) of -67.4 dB can be achieved. Moreover, radar cross-section simulations further confirm the attenuation ability of the optimized porous microspheres. These results not only provide novel insights into understanding and enhancing interfacial effects, but also constitute an attractive platform for implementing heterointerface engineering based on customized 2D hierarchical architectures.

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.

Toward Smart Sensing by MXene

The Internet of Things era has promoted enormous research on sensors, communications, data fusion, and actuators. Among them, sensors are a prerequisite for acquiring the environmental information for delivering to an artificial data center to make decisions. The MXene-based sensors have aroused tremendous interest because of their extraordinary performances. In this review, the electrical, electronic, and optical properties of MXenes are first introduced. Next, the MXene-based sensors are discussed according to the sensing mechanisms such as electronic, electrochemical, and optical methods. Initially, biosensors are introduced based on chemiresistors and field-effect transistors. Besides, the wearable pressure sensor is demonstrated with piezoresistive devices. Third, the electrochemical methods include amperometry and electrochemiluminescence as examples. In addition, the optical approaches refer to surface plasmonic resonance and fluorescence resonance energy transfer. Moreover, the prospects are delivered of multimodal data fusion toward complicated human-like senses. Eventually, future opportunities for MXene research are conveyed in the new material discovery, structure design, and proof-of-concept devices.

Enhanced ammonia sensing response based on Pt-decorated Ti3C2Tx/TiO2composite at room temperature

Herein, we report a Pt-decorated Ti₃C₂T_x/TiO₂gas sensor for the enhanced NH₃sensing response at room temperature. Firstly, the TiO₂nanosheets (NSs) arein situgrown onto the two-dimensional (2D) Ti₃C₂T_xby hydrothermal treatment. Similar to Ti₃C₂T_xsensor, the Ti₃C₂T_x/TiO₂sensor has a positive resistance variation upon exposure to NH₃, but with slight enhancement in response. However, after the loading of Pt nanoparticles (NPs), the Pt-Ti₃C₂T_x/TiO₂sensor shows a negative response with significantly improved NH₃sensing performance. The shift in response direction indicates that the dominant sensing mechanism has changed under the sensitization effect of Pt NPs. At room temperature, the response of Pt-Ti₃C₂T_x/TiO₂gas sensor to 100 ppm NH₃is about 45.5%, which is 13.8- and 10.8- times higher than those of Ti₃C₂T_xand Ti₃C₂T_x/TiO₂gas sensors, respectively. The experimental detection limit of the Pt-Ti₃C₂T_x/TiO₂gas sensor to detect NH₃is 10 ppm, and the corresponding response is 10.0%. In addition, the Pt-Ti₃C₂T_x/TiO₂gas sensor shows the fast response/recovery speed (23/34 s to 100 ppm NH₃), high selectivity and good stability. Considering both the response value and the response direction, the corresponding gas-sensing mechanism is also deeply discussed. This work is expected to shed a new light on the development of noble metals decorated MXene-metal oxide gas sensors.

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.