Fully Flexible MXene-based Gas Sensor on Paper for Highly Sensitive Room-Temperature Nitrogen Dioxide Detection

Highly sensitive, anti-freezing and stretchable hydrogels with modified MXene for multifunctional applications

Hydrogels have emerged as ideal materials for sensor fabrication due to their tunable mechanical properties, flexibility, and biocompatibility. However, their high-water content makes them prone to freezing at low temperatures, which compromises their conductivity and mechanical integrity, limiting their use in cold environments. To address this issue, we present a novel hydrogel system consisting of a polyacrylamide (PAM) backbone, a binary solvent system of water and ethylene glycol (EG), and functional materials including polyvinyl alcohol (PVA) and modified MXene. The addition of modified MXene as a conductive filler significantly enhances the hydrogel's conductivity, enabling it to maintain its performance even at low temperatures. Furthermore, the introduction of EG effectively lowers the freezing point of the hydrogel to -36.9 °C, ensuring robust frost resistance. The resulting hydrogel not only shows exceptional low-temperature stability but also exhibits high sensitivity. Moreover, the hydrogel demonstrates rapid responsiveness and high accuracy when being integrated as sensors to detect human motion and ammonia concentrations. These results highlight the potential of MXene-enhanced hydrogels for applications in freshness detection of raw meat and human motion tracking, particularly in cold environments.

Surface engineered gold nanodendrites decorated flexible carbon fiber-based electrochemical sensor platform for sensitive detection of L-Cysteine in serum and urine samples

In this work, highly dispersed gold nanodendrites (Au NDs) decorated flexible carbon fiber electrode (Au NDs@FCF electrode) were fabricated by facile, green, and one-step electrochemical deposition protocol and utilized for the direct electrochemical determination of L-Cysteine (L-Cys). The prepared Au NDs@FCF electrodes were characterized by SEM, HR-TEM, XRD, XPS, CV, and EIS towards the dimensions, surface morphological traits, crystalline nature, chemical composition, and electrochemical catalytic oxidation towards L-Cys and electrochemical active surface area (ECASA) of the Au NDs. The developed Au NDs@FCF electrode demonstrates an enzyme mimics electrocatalytic efficiency towards the oxidation of L-Cys at the operating potential of 0.82 V (vs Ag/AgCl) with a lower experimental detection limit of 0.16 nM, higher sensitivity of ∼50.2 μA μM-1 cm-2, and a wide concentration ranges from 100 to 3000 nM with a correlation coefficient of R2 = 0.996. In addition, the developed Au NDs@FCF electrode has exhibited excellent selectivity with various anti-interferences such as glucose, dopamine, uric acid, Na+, Mg2+, Ca2+, high reproducibility, and repeatability with RSD of 2.3 %. The Au NDs@FCF electrode demonstrates outstanding electrocatalytic oxidation and a rapid sensing response time of ∼3 s. The current Au NDs@FCF electrode achieving the successful detection of L-Cys in practical human serum and urine samples highlights its potential application in biomedical diagnostics. This advancement indicates that the sensor can effectively operate in real-world conditions, offering a valuable tool for medical professionals to monitor L-Cys levels in patients accurately.

Lychee-like Bi2MoO6 Spheres for Highly Sensitive Room-Temperature Phosphine Sensing

The increasing use of phosphine in various industries demands the development of reliable sensors. However, progress in this area has been slow, particularly for room-temperature detection. In this study, bismuth molybdate microspheres with a lychee-like structure (Lyc-Bi2MoO6) were prepared via a one-step solvothermal method, which can be employed for the detection of trace concentrations of phosphine. The solvent used was a mixture of isopropanol and ethylene glycol in a 3:1 ratio. Various characterization techniques and gas sensing performance tests demonstrated that Lyc-Bi2MoO6 is a potential phosphine-sensing material for room-temperature application. Sensing performance tests revealed that Lyc-Bi2MoO6 exhibited an impressive ability to detect trace concentrations of phosphine, with a practical detection limit as low as 150 ppb (response = 8.11), rapid response (around 1 min), and excellent long-term stability (a maximum response attenuation of 9.46% over 10 weeks). Density functional theory calculations further aided in the in-depth analysis and interpretation of the behavior and response mechanism of phosphine on the surface of Lyc-Bi2MoO6.

Ultrasensitive Room-Temperature NO2 Gas Sensor Based on MXene-Cu2O Composites

The development of real-time trace-level NO2 quantification platforms that can be operated at room temperature constitutes a critical advancement for occupational safety and public health monitoring systems. This study demonstrates a room-temperature NO2 sensor using MXene-Cu2O composites prepared via a hydrothermal method. Systematic evaluation of MXene-introduced effects identified the 0.84 wt % MXene-Cu2O composite as optimal, exhibiting 4-fold enhanced sensitivity and shorter response (55 s)/recovery (35 s) time compared to pure Cu2O. Additionally, the sensor exhibits a low detection limit (10 ppb), high selectivity, great reversibility, and long-term stability. The enhanced sensing performance originates from precisely engineered interfacial architectures between MXene and Cu2O, which effectively adjust the charge-transfer behavior through the conduction tunnel in the sensing material. Furthermore, oxygen vacancy engineering creates defect-mediated adsorption centers that promote selective NO2 chemisorption through charge polarization effects. This research offers a novel strategy for designing optimized structures to enhance the sensitivity of MOS-based materials for NO2 gas detection.

Recent advances in 2D MXene-based heterostructures for gas sensing: mechanisms and applications in environmental and biomedical fields

MXenes, a unique class of 2D transition metal carbides, have gained attention for gas sensing applications due to their distinctive properties. Since the synthesis of Ti3C2Tx MXene in 2011, significant progress has been made in using MXenes as chemiresistive sensors. Their layered structure, abundant surface groups, hydrophilicity, tunable conductivity, and excellent thermal properties make MXenes ideal for low-power, flexible, room temperature gas sensors, fostering scalable and reproducible applications in portable devices. This review evaluates the latest advancements in MXene-based gas sensors, beginning with an overview of the elemental compositions, structures, and typical fabrication process of MXenes. We subsequently examine their applications in gas sensing domains, evaluating the proposed mechanisms for detecting common volatile organic compounds such as acetone, formaldehyde, ethanol, ammonia, and nitrogen oxides. To set this apart from similar reviews, our focus centered on the mechanistic interactions between MXene sensing materials and analytes (particularly for chemiresistive gas sensors), leveraging the distinct functionalities of MXene chemistries, which can be finely tuned for specific applications. Ultimately, we examine the current limitations and prospective research avenues concerning the utilization of MXenes in environmental and biomedical applications.

Next-Generation Chemiresistive Wearable Breath Sensors for Non-Invasive Healthcare Monitoring: Advances in Composite and Hybrid Materials

Recently wearable breath sensors have received significant attention in personalized healthcare systems by offering new methods for remote, non-invasive, and continuous monitoring of various health indicators from breath samples without disrupting daily routines. The rising demand for rapid, personalized diagnostics has sparked concerns over electronic waste from short-lived silicon-based devices. To address this issue, the development of flexible and wearable sensors for breath sensing applications is a promising approach. Research highlights the development of different flexible, wearable sensors operating with different operating principles, such as chemiresistive sensors to detect specific target analytes due to their simple design, high sensitivity, selectivity, and reliability. Further, focusing on the non-invasive detection of biomarkers through exhaled breath, chemiresistive wearable sensors offer a comprehensive and environmentally friendly solution. This article presents a comprehensive discussion of the recent advancement in chemiresistive wearable breath sensors for the non-invasive detection of breath biomarkers. The article further emphasizes the intricate development and functioning of the sensor, including the selection criteria for both the flexible substrate and advanced functional materials, including their sensing mechanisms. The review then explores the potential applications of wearable gas sensing systems with specific disease detection, with modern challenges associated with non-invasive breath sensors.

Highly Efficient UV-Activated TiO2/SnO2 Surface Nano-matrix Gas Sensor: Enhancing Stability for ppb-Level NOx Detection at Room Temperature

This study presents a new nanoporous TiO2/SnO2 heterojunction for NOx gas detection by using a two-step sol-gel process. The unique TiO2 and SnO2 nanoheterojunction matrix right on the film surface enables the TiO2 photocatalyst to absorb minimal UV power (3 μW/cm2) and effectively transfer electrons to the SnO2 conduction band. The sensor detects NO and NO2 gases down to 4 ppb (response of 0.6%) and 10 ppb (response of 1.3%) at 1 V at room temperature. It also exhibits a fast recovery time (100 ± 40 s at 500 ppb NOx), an improved response over a wide relative humidity range (10-60%), and a long lifetime over 30 days. The ultralow UV power required can be easily harvested from sunlight, eliminating the need for UV LEDs. XPS and SEM analyses indicated that the unique nanoporous TiO2/SnO2 structure improves sensing performance, with oxygen vacancies playing a critical role in the NOx gas sensing mechanism. This work demonstrated the highly efficient UV catalyst effect in sensors with the surface heterojunction matrix. The low-power ppb-level NOx detection is suitable for environmental monitoring and respiratory disease detection.

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.

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.

Ultra-Sensitive NO2 Detection at Room Temperature Enabled by ZnO@MoO3 Core-Shell Nanocomposite

The sensitive detection of NO2 is crucial for environmental monitoring and improving quality of life. Herein, a ZnO@MoO3 core-shell nanocomposite was fabricated via a simple stepwise solution self-assembly and heat-treatment process. Remarkably, the ZnO@MoO3 sensor exhibited a high response value of 5.4 to 2 ppm NO2 at room temperature. Furthermore, it displayed excellent selectivity against interference gases such as CO2, NH3, methanol, ethylene, and trimethylamine, along with outstanding stability and repeatability under varying humidity conditions. The exceptional sensing performance of the ZnO@MoO3 sensor is attributed to the synergistic effects between ZnO and MoO3, as well as the enhanced electron transfer rate enabled by the heterostructures. This work provides an effective strategy for advancing NO2 sensing capabilities in metal oxide composite sensors.

Trends in conductive MOFs for sensing: A review

Metal-organic frameworks (MOFs) are porous, ordered arrays formed by coordination bonds between organic ligands and metal ions or clusters. The highly tunable properties of the MOF structure and performance make it possible to meet the needs of many applications. Conductive MOFs are essential in the domain of sensing due to their electrical conductivity, porosity, and catalytic properties, offering an effective platform for detection. Numerous sensing devices that utilize conductive MOFs have been created. This text presents a thorough overview of the diverse applications of conductive MOFs within the sensing field. The results of this work provide insights into the properties and synthesis methods of conductive MOFs and the working mechanisms of sensors based on conductive MOFs, which will help to deepen the study of such materials, provide a new vision for the design and production of novel functional materials, and promote the development and application of sensors based on conductive MOFs.

MXene as SERS-Active Substrate: Impact of Intrinsic Properties and Performance Analysis

A new member is incorporated into the SERS active materials family daily as a consequence of advances in materials science. Furthermore, it has been demonstrated that MXenes, which display remarkable physicochemical characteristics, are also encompassed within this family. This Review offers a comprehensive and systematic assessment of the potential of MXene structures in the context of SERS applications. First, the historical development of SERS-active substrates and the evolution of various substrates over time are analyzed. Subsequently, the formation and structural properties of MXene structures were subjected to a comprehensive and detailed examination. The principal objective of this Review is to elucidate the rationale behind the preference for MXene as a SERS-active substrate, given its distinctive physicochemical properties. In this context, while MXene's abundant surface functional groups represent a significant advantage, its high electrical conductivity, suitable flexibility, extensive two-dimensional surface areas, and antibacterial activity also warrant consideration in terms of potential applications. It is emphasized that, for MXene nanolayers to demonstrate optimal performance in SERS applications, a plan should be devised to consider these features. By increasing readers' awareness of using MXene as a SERS active substrate, potential opportunities for future application areas may be created.

Engineering an Ultrafast Ambient NO2 Gas Sensor Using Cotton-Modified LaFeO3/MXene Composites

This work presents a room-temperature (RT) NO2 gas sensor based on cotton-modified LaFeO3 (CLFO) combined with MXene. LaFeO3 (LFO), CLFO, and CLFO/MXene composites were synthesized via a hydrothermal method. The fabricated sensor, utilizing MXene/CLFO, exhibits a p-type behavior and fully recoverable sensing capabilities for low concentrations of NO2, achieving a higher response of 14.2 times at 5 ppm. The sensor demonstrates excellent performance with a response time of 2.7 s and a recovery time of 6.2 s, along with notable stability. The sensor's sensitivity is attributed to gas interactions on the material's surface, adsorption energy, and charge-transfer mechanisms. Techniques such as in situ FTIR (Fourier transform infrared) spectroscopy, GC-MS (gas chromatography-mass spectroscopy), and near-ambient pressure X-ray photoelectron spectroscopy were employed to verify gas interactions and their byproducts. Additionally, finite-difference time-domain simulations were used to model the electromagnetic field distribution and provide insight into the interaction between NO2 molecules and the sensor surface at the nanoscale. A prototype wireless IoT (Internet of Things)-based NO2 gas leakage detection system was also developed, showcasing the sensor's practical application. This study offers valuable insight into the development of room-temperature NO2 sensors with a low detection limit.

Gas sensing performance of Ti3C2Tx MXene heterojunction structures in greenhouse environments: a mini review

With the continuous advancement of smart greenhouse technologies, digital and information-based environmental monitoring has emerged as a focal point of research. The development of high-performance gas sensors is central to achieving this objective. In recent years, MXene materials have been widely applied in the field of gas sensors due to their excellent ion mobility, favorable hydrophilicity, outstanding electronic conductivity, and unique physicochemical properties. Various MXene heterojunction structures have been synthesized for gas detection. This review aims to summarize the current state of research on Ti3C2Tx-based gas sensors, explore methods for synthesizing different morphologies of Ti3C2Tx heterojunction structures, and evaluate the sensing behaviors of these configurations to fully harness their potential for gas monitoring in greenhouse environments. Additionally, an in-depth analysis of the sensing mechanisms associated with Ti3C2Tx heterojunction structures will be provided, offering theoretical support for future investigations. The findings indicate that Ti3C2Tx-based nanomaterials demonstrate considerable promise as high-performance sensors for gas detection in greenhouse settings. This innovative research not only provides new insights into the development of gas sensor technologies but also serves as an important foundation for the digitization of environmental monitoring.

High-Performance Ammonia QCM Sensor Based on SnO2 Quantum Dots/Ti3C2Tx MXene Composites at Room Temperature

Ammonia (NH3) gas is prevalent in industrial production as a health hazardous gas. Consequently, it is essential to develop a straightforward, reliable, and stable NH3 sensor capable of operating at room temperature. This paper presents an innovative approach to modifying SnO2 colloidal quantum dots (CQDs) on the surface of Ti3C2Tx MXene to form a heterojunction, which introduces a significant number of adsorption sites and enhances the response of the sensor. Zero-dimensional (0D) SnO2 quantum dots and two-dimensional (2D) Ti3C2Tx MXene were prepared by solvothermal and in situ etching methods, respectively. The impact of the mass ratio between two materials on the performance was assessed. The sensor based on 12 wt% Ti3C2Tx MXene/SnO2 composites demonstrates excellent performance in terms of sensitivity and response/recovery speed. Upon exposure to 50 ppm NH3, the frequency shift in the sensor is -1140 Hz, which is 5.6 times larger than that of pure Ti3C2Tx MXene and 2.8 times higher than that of SnO2 CQDs. The response/recovery time of the sensor for 10 ppm NH3 was 36/54 s, respectively. The sensor exhibited a theoretical detection limit of 73 ppb and good repeatability. Furthermore, a stable sensing performance can be maintained after 30 days. The enhanced sensor performance can be attributed to the abundant active sites provided by the accumulation/depletion layer in the Ti3C2Tx/SnO2 heterojunction, which facilitates the adsorption of oxygen molecules. This work promotes the gas sensing application of MXenes and provides a way to improve gas sensing performance.

Dynamic Rare-Earth Metal-Organic Frameworks Based on Molecular Rotor Linkers with Efficient Emissions and Ultrasensitive Optical Sensing Performance

4,4',4″-Triphenylamine tricarboxylate (TPA-COOH) with a distinct molecular rotor structure was reacted with rare-earth (RE) metal ions to obtain seven dynamic RE-based luminescent MOFs (RE-LMOFs) (i.e., emission colors in the blue, yellow-green, red, and near-infrared regions and emission peak wavelengths between 400 and 1600 nm) via the effective transfer of absorbed energy from TPA-COOH to the RE metal ions through the antenna effect. Due to the large energy level difference between RE ions, it was rare in the early days to use the same ligand to construct energy-level matching RE-LMOF homologues with multiple RE metal centers. The uncoordinated oxygen atoms on the molecular rotor linkers in RE-LMOFs provide active sites that can specifically capture highly toxic metal ions and strong oxidative pollutants. The limit of detection (LOD) of RE-LMOF for Al(III) ions is far below the maximum concentration of Al(III) ions in drinking water stipulated by the U.S. Environmental Protection Agency (USEPA) and that for H2O2 is much lower than the H2O2 content in cancer cells, showing excellent application potential for diagnosing early cell cancelation.

Emerging Horizons in Gas Sensing: Exploring the Potential of MXenes and MBenes

This review article is focused on the development and application of two types of emerging two-dimensional (2D) materials, namely MXenes and MBenes, in the field of gas detection. Owing to its excellent electrical conductivity, specific surface area, and tunable surface functionality, MXenes have demonstrated high sensitivity and rapid response in detecting a variety of gases, such as volatile organic compounds (VOCs), nitrogen dioxide (NO2), and ammonia (NH3). MBenes are relatively newly discovered materials with excellent electronic stability and gas adsorption capabilities. Both of these materials demonstrate significant potential in improving sensor selectivity and stability through surface functionalization and heterostructure designs. However, despite the impressive gas detection performance of these materials, challenges remain in achieving long-term stability, cost-effectiveness, and commercialization. We summarize the prominent research insights on these materials and offer an outlook on future research directions and potential applications. With ongoing research and technological advancements, MXenes and MBenes are anticipated to have a substantial impact in critical areas such as environmental monitoring and industrial safety.

High Performance H2S Sensor Based on Ordered Fe2O3/Ti3C2 Nanostructure at Room Temperature

The utilization of a heterogeneous nanojunction design has shown significant enhancements in the gas sensing capabilities of traditional metal oxide gas sensors. In this study, a novel room temperature H2S gas sensor employing Fe2O3 functionalized Ti3C2 MXene as the sensing material has been developed. This sensor exhibits a broad detection range (0.01-500 ppm), low detection limit (10 ppb), and rapid response/recovery times (10 s/15 s), making it ideal for ppb-level H2S detection. The exceptional gas sensitivity of Fe2O3/Ti3C2 composite to H2S can be attributed to several key factors. First, the unique layered frame structure of Fe2O3/Ti3C2 significantly amplifies the surface area of the hybrid material, enhancing the absorption and diffusion capabilities of H2S molecules. Second, the abundance of functional groups (-O, -OH, and -F) on the surface of Ti3C2 MXene nanosheets provides additional active sites for H2S adsorption, The density functional theory calculation confirms that the adsorption energy of the Fe2O3/Ti3C2 composite for H2S (-2.93 eV) is significantly lower than that of pure Fe2O3 (-2.37 eV) and Ti3C2 (-0.2 eV). Lastly, the remarkable metal conductivity of Ti3C2 MXene ensures efficient electron transfer, thereby enhancing overall sensing performance.

A Review of Advanced Sensor Technologies for Aquatic Products Freshness Assessment in Cold Chain Logistics

The evaluation of the upkeep and freshness of aquatic products within the cold chain is crucial due to their perishable nature, which can significantly impact both quality and safety. Conventional methods for assessing freshness in the cold chain have inherent limitations regarding specificity and accuracy, often requiring substantial time and effort. Recently, advanced sensor technologies have been developed for freshness assessment, enabling real-time and non-invasive monitoring via the detection of volatile organic compounds, biochemical markers, and physical properties. The integration of sensor technologies into cold chain logistics enhances the ability to maintain the quality and safety of aquatic products. This review examines the advancements made in multifunctional sensor devices for the freshness assessment of aquatic products in cold chain logistics, as well as the application of pattern recognition algorithms for identification and classification. It begins by outlining the categories of freshness criteria, followed by an exploration of the development of four key sensor devices: electronic noses, electronic tongues, biosensors, and flexible sensors. Furthermore, the review discusses the implementation of advanced pattern recognition algorithms in sensor devices for freshness detection and evaluation. It highlights the current status and future potential of sensor technologies for aquatic products within the cold chain, while also addressing the significant challenges that remain to be overcome.

Real-Time and Wireless Transmission of a Nitrogen-Doped Ti3C2Tx Wearable Gas Sensor for Efficient Detection of Food Spoilage and Ammonia Leakage

With the rising popularity of smart homes, there is an urgent need for devices that can perform real-time online detection of ammonia (NH3) concentrations for food quality measurement. In addition, timely warning is crucial to preventing individual deaths from NH3. However, few studies can realize continuous monitoring of NH3 with high stability and subsequent application validation. Herein, we report on an integrated device equipped with a nitrogen-doped Ti3C2Tx gas sensor that shows great potential in detecting food spoilage and NH3 leakage. The nitrogen doping results in the lattice misalignment of Ti3C2Tx, subsequently realizing effective barrier height modulation and enhanced charge transfer efficiency of nitrogen-doped Ti3C2Tx. Density functional theory calculations confirm the greatly enhanced adsorption of NH3 on nitrogen-doped Ti3C2Tx. Our work can inspire the design of efficient gas sensors for real-time and wireless detection of food spoilage and NH3 leakage.

Quantum Confinement and End-Sealing Effects for Highly Sensitive and Stable Nitrogen Dioxide Detection: Homogeneous Integration of Ti3C2Tx-Based Flexible Gas Sensors

The real-time and room-temperature detection of nitrogen dioxide (NO2) holds significant importance for environmental monitoring. However, the performance of NO2 sensors has been hampered by the trade-off between the high sensitivity and stability of conventional sensitive materials. Here, we present a novel fully flexible paper-based gas sensing structure by combining a homogeneous screen-printed titanium carbide (Ti3C2Tx) MXene-based nonmetallic electrode with a MoS2 quantum dots/Ti3C2Tx (MoS2 QDs/Ti3C2Tx) gas-sensing film. These precisely designed gas sensors demonstrate an improved response value (16.3% at 5 ppm) and a low theoretical detection limit of 12.1 ppb toward NO2, which exhibit a remarkable 3.5-fold increase in sensitivity compared to conventional Au interdigital electrodes. The outstanding performance can be attributed to the integration of the quantum confinement effect of MoS2 QDs and the conductivity of Ti3C2Tx, establishing the main active adsorption sites and enhanced charge transport pathways. Furthermore, an end-sealing effect strategy was applied to decorate the defect sites with naturally oxygen-rich tannic acid and conductive polymer, and the formed hydrogen bonding network at the interface effectively mitigated the oxidative degradation of the Ti3C2Tx-based gas sensors. The exceptional stability has been achieved with only a 1.8% decrease in response over 4 weeks. This work highlights the innovative design of high-performance gas sensing materials and homogeneous gas sensor techniques.

Multiwalled Carbon Nanotube-Templated Nickel Porphyrin Covalent Organic Framework for Pencil-Drawn Noninvasive Respiration Sensors

Paper-integrated configuration with miniaturized functionality represents one of the future main green electronics. In this study, a paper-based respiration sensor was prepared using a multiwalled carbon nanotube-templated nickel porphyrin covalent organic framework (MWCNTs@COFNiP-Ph) as an electrical identification component and pencil-drawn graphite electric circuits as interdigitated electrodes (IDEs). The MWCNTs@COFNiP-Ph not only inherited the high gas sensing performance of porphyrin and the aperture induction effect of COFs but also overcame the shielding effect between phases through the MWCNT template. Furthermore, it possessed highly exposed M-N4 metallic active sites and unique periodic porosity, thereby effectively addressing the key technical issue of room-temperature sensing for the respiration sensor. Meanwhile, the introduction of a pencil-drawing approach on common printing papers facilitates the inexpensive and simple manufacturing of the as-fabricated graphite IDE. Based on the above advantages, the MWCNTs@COFNiP-Ph respiration sensor had the characteristics of wide detection range (1-500 ppm), low detection limit (30 ppb), acceptable flexibility for toluene, and rapid response/recovery time (32 s/116 s). These advancements facilitated the integration of the respiration sensor into surgical masks and clothes with maximum functionality at a minimized size and weight. Moreover, the primary internal mechanism of COFNiP-Ph for this efficient toluene detection was investigated through in situ FTIR spectra, thereby directly elucidating that the chemisorption interaction of oxygen modulated the depletion layers, resulting in alterations in sensor resistance upon exposure to the target gas. The encouraging results revealed the feasibility of employing a paper-sensing system as a wearable platform in green electronics.

Highly Sensitive Ammonia Gas Sensors at Room Temperature Based on the Catalytic Mechanism of N, C Coordinated Ni Single-Atom Active Center

Significant challenges are posed by the limitations of gas sensing mechanisms for trace-level detection of ammonia (NH3). In this study, we propose to exploit single-atom catalytic activation and targeted adsorption properties to achieve highly sensitive and selective NH3 gas detection. Specifically, Ni single-atom active sites based on N, C coordination (Ni-N-C) were interfacially confined on the surface of two-dimensional (2D) MXene nanosheets (Ni-N-C/Ti3C2Tx), and a fully flexible gas sensor (MNPE-Ni-N-C/Ti3C2Tx) was integrated. The sensor demonstrates a remarkable response value to 5 ppm NH3 (27.3%), excellent selectivity for NH3, and a low theoretical detection limit of 12.1 ppb. Simulation analysis by density functional calculation reveals that the Ni single-atom center with N, C coordination exhibits specific targeted adsorption properties for NH3. Additionally, its catalytic activation effect effectively reduces the Gibbs free energy of the sensing elemental reaction, while its electronic structure promotes the spill-over effect of reactive oxygen species at the gas-solid interface. The sensor has a dual-channel sensing mechanism of both chemical and electronic sensitization, which facilitates efficient electron transfer to the 2D MXene conductive network, resulting in the formation of the NH3 gas molecule sensing signal. Furthermore, the passivation of MXene edge defects by a conjugated hydrogen bond network enhances the long-term stability of MXene-based electrodes under high humidity conditions. This work achieves highly sensitive room-temperature NH3 gas detection based on the catalytic mechanism of Ni single-atom active center with N, C coordination, which provides a novel gas sensing mechanism for room-temperature trace gas detection research.

MXene/Hydrogel-based bioelectronic nose for the direct evaluation of food spoilage in both liquid and gas-phase environments

Various bioelectronic noses have been recently developed for mimicking human olfactory systems. However, achieving direct monitoring of gas-phase molecules remains a challenge for the development of bioelectronic noses due to the instability of receptor and the limitations of its surrounding microenvironment. Here, we report a MXene/hydrogel-based bioelectronic nose for the sensitive detection of liquid and gaseous hexanal, a signature odorant from spoiled food. In this study, a conducting MXene/hydrogel structure was formed on a sensor via physical adsorption. Then, canine olfactory receptor 5269-embedded nanodiscs (cfOR5269NDs) which could selectively recognize hexanal molecules were embedded in the three-dimensional (3D) MXene/hydrogel structures using glutaraldehyde as a linker. Our MXene/hydrogel-based bioelectronic nose exhibited a high selectivity and sensitivity for monitoring hexanal in both liquid and gas phases. The bioelectronic noses could sensitively detect liquid and gaseous hexanal down to 10-18 M and 6.9 ppm, and they had wide detection ranges of 10-18 - 10-6 M and 6.9-32.9 ppm, respectively. Moreover, our bioelectronic nose allowed us to monitor hexanal levels in fish and milk. In this respect, our MXene/hydrogel-based bioelectronic nose could be a practical strategy for versatile applications such as food spoilage assessments in both liquid and gaseous systems.

Flexible Resistive Gas Sensor Based on Molybdenum Disulfide-Modified Polypyrrole for Trace NO2 Detection

High sensitivity and selectivity and short response and recovery times are important for practical conductive polymer gas sensors. However, poor stability, poor selectivity, and long response times significantly limit the applicability of single-phase conducting polymers, such as polypyrrole (PPy). In this study, PPy/MoS2 composite films were prepared via chemical polymerization and mechanical blending, and flexible thin-film resistive NO2 sensors consisting of copper heating, fluorene polyester insulating, and PPy/MoS2 sensing layers with a silver fork finger electrode were fabricated on a flexible polyimide substrate using a flexible electronic printer. The PPy/MoS2 composite films were characterized using X-ray diffraction, Fourier-transform infrared spectroscopy, and field-emission scanning electron microscopy. A home-built gas sensing test platform was built to determine the resistance changes in the composite thin-film sensor with temperature and gas concentration. The PPy/MoS2 sensor exhibited better sensitivity, selectivity, and stability than a pure PPy sensor. Its response to 50 ppm NO2 was 38% at 150 °C, i.e., 26% higher than that of the pure PPy sensor, and its selectivity and stability were also higher. The greater sensitivity was attributed to p-n heterojunction formation after MoS2 doping and more gas adsorption sites. Thus, PPy/MoS2 composite film sensors have good application prospects.

Advancements in MXene Composite Materials for Wearable Sensors: A Review

In recent years, advancements in the Internet of Things (IoT), manufacturing processes, and material synthesis technologies have positioned flexible sensors as critical components in wearable devices. These developments are propelling wearable technologies based on flexible sensors towards higher intelligence, convenience, superior performance, and biocompatibility. Recently, two-dimensional nanomaterials known as MXenes have garnered extensive attention due to their excellent mechanical properties, outstanding electrical conductivity, large specific surface area, and abundant surface functional groups. These notable attributes confer significant potential on MXenes for applications in strain sensing, pressure measurement, gas detection, etc. Furthermore, polymer substrates such as polydimethylsiloxane (PDMS), polyurethane (PU), and thermoplastic polyurethane (TPU) are extensively utilized as support materials for MXene and its composites due to their light weight, flexibility, and ease of processing, thereby enhancing the overall performance and wearability of the sensors. This paper reviews the latest advancements in MXene and its composites within the domains of strain sensors, pressure sensors, and gas sensors. We present numerous recent case studies of MXene composite material-based wearable sensors and discuss the optimization of materials and structures for MXene composite material-based wearable sensors, offering strategies and methods to enhance the development of MXene composite material-based wearable sensors. Finally, we summarize the current progress of MXene wearable sensors and project future trends and analyses.

CuO nanostructure-decorated InGaN nanorods for selective H2S gas detection

Establishing a heterostructure is one of the adequate strategies for enhancing device performance and has been explored in sensing, and energy applications. In this study, we constructed a heterostructure through a two-step process involving hydrothermal synthesis of CuO nanostructures and subsequent spin coating on MBE-grown InGaN NRs. We found that the CuO content on the InGaN NRs has a great impact on carrier injection at the heterojunction and thus the H2S gas sensing performance. Popcorn CuO/InGaN NR shows excellent gas sensing performance towards different concentrations of H2S at room temperature. The highest response is up to 35.54% to a H2S concentration of 100 ppm. Even more significantly, this response is further enhanced significantly (123.70%) under 365 nm UV light. In contrast, this composite structure exhibits negligibly low responses to 100 ppm of NO2, H2, CO, and NH3. The heterostructure band model associated with a surface reaction model is manifested to elucidate the sensing mechanism.

Quantum enhanced efficiency and spectral performance of paper-based flexible photodetectors functionalized with two dimensional materials

Flexible photodetectors (PDs) have exotic significance in recent years due to their enchanting potential in future optoelectronics. Moreover, paper-based fabricated PDs with outstanding flexibility unlock new avenues for future wearable electronics. Such PD has captured scientific interest for its efficient photoresponse properties due to the extraordinary assets like significant absorptive efficiency, surface morphology, material composition, affordability, bendability, and biodegradability. Quantum-confined materials harness the unique quantum-enhanced properties and hold immense promise for advancing both fundamental scientific understanding and practical implication. Two-dimensional (2D) materials as quantum materials have been one of the most extensively researched materials owing to their significant light absorption efficiency, increased carrier mobility, and tunable band gaps. In addition, 2D heterostructures can trap charge carriers at their interfaces, leading increase in photocurrent and photoconductivity. This review represents comprehensive discussion on recent developments in such PDs functionalized by 2D materials, highlighting charge transfer mechanism at their interface. This review thoroughly explains the mechanism behind the enhanced performance of quantum materials across a spectrum of figure of merits including external quantum efficiency, detectivity, spectral responsivity, optical gain, response time, and noise equivalent power. The present review studies the intricate mechanisms that reinforce these improvements, shedding light on the intricacies of quantum materials and their significant capabilities. Moreover, a detailed analysis of the technical applicability of paper-based PDs has been discussed with challenges and future trends, providing comprehensive insights into their practical usage in the field of future wearable and portable electronic technologies.

Nitrogen-Based Gas Molecule Adsorption on a ReSe2 Monolayer via Single-Atom Doping: A First-Principles Study

Two-dimensional materials have shown immense promise for gas-sensing applications due to their remarkable surface-to-volume ratios and tunable chemical properties. However, despite their potential, the utilization of ReSe2 as a gas-sensing material for nitrogen-containing molecules, including NO2, NO, and NH3, has remained unexplored. The choice of doping atoms in ReSe2 plays a pivotal role in enhancing the gas adsorption and gas-sensing capabilities. Herein, the adsorption properties of nitrogen-containing gas molecules on metal and non-metal single-atom (Au, Pt, Ni, P, and S)-doped ReSe2 monolayers have been evaluated systematically via ab initio calculations based on density functional theory. The findings strongly suggest that intrinsic ReSe2 has better selectivity toward NO2 than toward NO and NH3. Moreover, our results provide compelling evidence that all of the dopants, with the exception of S, significantly enhance both the adsorption strength and charge transfer between ReSe2 and the investigated molecules. Notably, P-decorated ReSe2 showed the highest adsorption energy for NO2 and NO (-1.93 and -1.52 eV, respectively) with charge transfer above 0.5e, while Ni-decorated ReSe2 exhibited the highest adsorption energy for NH3 (-0.76 eV). In addition, on the basis of transition theory, we found that only Au-ReSe2 and Ni-ReSe2 can serve as reusable chemiresisitve gas sensors for reliable detection of NO and NH3, respectively. Hence, our findings indicate that gas-sensing applications can be significantly improved by utilizing a single-atom-doped ReSe2 monolayer.

Control-Etched Ti3C2Tx MXene Nanosheets for a Low-Voltage-Operating Flexible Memristor for Efficient Neuromorphic Computation

Hardware neural networks with mechanical flexibility are promising next-generation computing systems for smart wearable electronics. Overcoming the challenge of developing a fully synaptic plastic network, we demonstrate a low-operating-voltage PET/ITO/p-MXene/Ag flexible memristor device by controlling the etching of aluminum metal ions in Ti3C2Tx MXene. The presence of a small fraction of Al ions in partially etched MXene (p-Ti3C2Tx) significantly suppresses the operating voltage to 1 V compared to 7 V from fully Al etched MXene (f-Ti3C2Tx)-based devices. Former devices exhibit excellent non-volatile data storage properties, with a robust ∼103 ON/OFF ratio, high endurance of ∼104 cycles, multilevel resistance states, and long data retention measured up to ∼106 s. High mechanical stability up to ∼73° bending angle and environmental robustness are confirmed with consistent switching characteristics under increasing temperature and humid conditions. Furthermore, a p-Ti3C2Tx MXene memristor is employed to mimic the biological synapse by measuring the learning-forgetting pattern for ∼104 cycles as potentiation and depression. Spike time-dependent plasticity (STDP) based on Hebb's Learning rules is also successfully demonstrated. Moreover, a remarkable accuracy of ∼95% in recognizing modified patterns from the National Institute of Standards and Technology (MNIST) data set with just 29 training epochs is achieved in simulation. Ultimately, our findings underscore the potential of MXene-based flexible memristor devices as versatile components for data storage and neuromorphic computing.

Rational design of NiO/Ti3C2Tx nanocomposites with enhanced triethylamine sensing performance

MXenes with excellent conductivity and abundant surface functional groups have displayed great advantages as platforms for sensing materials. NiO also has drawn much attention for gas detection due to its unique merits of excellent catalytic activity. Herein, NiO nanoparticles are incorporated with multilayer Ti3C2Tx-MXene to develop excellent triethylamine sensors. Due to the larger specific surface area and formed p-p heterojunctions, the response of the NiO/Ti3C2Tx gas sensor is endowed with a response value of 950% to 50 ppm triethylamine gas and is much higher than that of the pristine NiO sensor. Moreover, the NiO/Ti3C2Tx sensor displays a fast response time of 8 s (50 ppm triethylamine), excellent reproducibility, and reliable long-term stability. This study proves that NiO/Ti3C2Tx sensors have potential for the effective detection of triethylamine gas.

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.

Selective Detection of NH3 Gas by Ti3C2Tx Sensors with the PVDF-ZIF-67 Overlayer at Room Temperature

In the realm of NH3 gas-sensing applications, the electrically conductive nature of Ti3C2Tx MXene, adorned with surface terminations such as -O and -OH groups, renders it a compelling material. However, the inherent challenges of atmospheric instability and selectivity in the presence of gas mixtures have prompted the exploration of innovative solutions. This work introduces a strategic solution through the deposition of a mixed-matrix membrane (MMM) composed of poly(vinylidene fluoride) (PVDF) as the matrix and zeolitic imidazolate framework-67 (ZIF-67) as the filler. This composite membrane acts as a selective filter, permitting the passage of a specific gas, namely NH3. Leveraging the hydrophobic and chemically inert nature of PVDF, the MMM enhances the atmospheric stability of Ti3C2Tx by impeding water molecules from interacting with the MXene. Furthermore, ZIF-67 is selective to NH3 gas via acid-base interactions within the zeolite group and selective pore size. The Ti3C2Tx sensor embedded in the MMM filter exhibits a modest 1.3% change in the sensing response to 25 ppm of NH3 gas compared to the response without the filter. This result underscores the filter's effectiveness in conferring selectivity and diffusivity, particularly at 35% relative humidity (RH) and 25 °C. Crucially, the hydrophobic attributes of PVDF impart heightened stability to the Ti3C2Tx sensor even amidst varying RH conditions. These results not only demonstrate effective NH3 detection but also highlight the sensor's adaptability to diverse environmental conditions, offering promising prospects for practical applications.

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.

Enhanced NO2 Sensitivity of Vertically Stacked van der Waals Heterostructure Gas Sensor and Its Remarkable Electric and Mechanical Tunability

Nanodevices based on van der Waals heterostructures have been predicted, and shown, to have unprecedented operational principles and functionalities that hold promise for highly sensitive and selective gas sensors with rapid response times and minimal power consumption. In this study, we fabricated gas sensors based on vertical MoS2/WS2 van der Waals heterostructures and investigated their gas sensing capabilities. Compared with individual MoS2 or WS2 gas sensors, the MoS2/WS2 van der Waals heterostructure gas sensors are shown to have enhanced sensitivity, faster response times, rapid recovery, and a notable selectivity, especially toward NO2. In combination with a theoretical model, we show that it is important to take into account created trapped states (flat bands) induced by the adsorption of gas molecules, which capture charges and alter the inherent built-in potential of van der Waals heterostructure gas sensors. Additionally, we note that the performance of these MoS2/WS2 heterostructure gas sensors could be further enhanced using electrical gating and mechanical strain. Our findings highlight the importance of understanding the effects of altered built-in potentials arising from gas molecule adsorption induced flat bands, thus offering a way to enhance the gas sensing performance of van der Waals heterostructure gas sensors.

ZnO-Au@ZIF-8 core-shell nanorod arrays for ppb-level NO2 detection

ZnO-Au@ZIF-8 core-shell heterostructures were prepared by ZIF-8 encapsulation of sacrificial ZnO-Au nanorods. Because of the catalytic activity of the Au nanoparticles and the sieving effects of the ZIF-8, the ZnO-Au@ZIF-8 heterostructures showed an outstanding response of 1.8 to 5 ppb NO2, and exhibited higher selectivity, stability, anti-humidity and fast response and recovery properties. The combination of the gas-selective catalytic activity of noble metals with the MOF filter used in this work can be easily extended to synthesize other types of MOS@MOF sensors, opening a new avenue for the detection of hazardous gases.

Constructed MXene matrix composites as sensing material and applications thereof: A review

Most studies on MXene matrix composites for sensor development have primarily focused on synthesis and application. Nevertheless, there is currently a lack of research on how the introduction of different materials affects the sensing properties of these composites. The rapid development of MXene has raised intriguing questions about improving sensor performance by combining MXene with other materials such as polymers, metals and inorganic non-metals. This review will concentrate on the construction of MXene-based composites and explore ways to enhance their sensor applications. Specifically, this review describes why the introduction of materials to the system brings the advantage of low concentration and high sensitivity assays, as well as the MXene-based frameworks that have been recently investigated. Lastly, in order to capture the current trend of MXene-based composites in sensor applications and identify promising research directions, this review will critically evaluate the potential applications of newly developed MXene systems.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Robust and Multifunctional Ti3 C2 Tx /Modified Sawdust Composite Paper for Electromagnetic Interference Shielding and Wearable Thermal Management

Robust, ultrathin, and environmental-friendliness papers that synergize high-efficiency electromagnetic interference (EMI) shielding, personal thermal management, and wearable heaters are essential for next-generation smart wearable devices. Herein, MXene nanocomposite paper with a nacre-like structure for EMI shielding and electrothermal/photothermal conversion is fabricated by vacuum filtration of Ti3 C2 Tx MXene and modified sawdust. The hydrogen bonding and highly oriented structure enhance the mechanical properties of the modified sawdust/MXene composite paper (SM paper). The SM paper with 50 wt% MXene content shows a strength of 23 MPa and a toughness of 13 MJ·M-3 . The conductivity of the SM paper is 10 195 S·m-1 , resulting in an EMI shielding effectiveness (SE) of 67.9 dB and a specific SE value (SSE/t) of 8486 dB·cm2 ·g-1 . In addition, the SM paper exhibits excellent thermal management performance including high light/electro-to-thermal conversion, rapid Joule heating and photothermal response, and sufficient heating stability. Notably, the SM paper exhibits low infrared emissivity and distinguished infrared stealth performance, camouflaging a high-temperature heater surface of 147-81 °C. The SM-based e-skin achieves visualization of Joule heating and realizes human motions monitoring. This work presents a new strategy for designing MXene-based wearable devices with great EMI shielding, artificial intelligence, and thermal management applications.

A non-contact porous composite fiber paper-based humidity sensor for wearable breathing and skin humidity monitoring

Flexible sensors capable of continuously monitoring surface moisture and respiratory status play a crucial role in human health monitoring.

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.

Paper Supercapacitor Developed Using a Manganese Dioxide/Carbon Black Composite and a Water Hyacinth Cellulose Nanofiber-Based Bilayer Separator

Flexible and green energy storage devices have a wide range of applications in prospective electronics and connected devices. In this study, a new eco-friendly bilayer separator and primary and secondary paper supercapacitors based on manganese dioxide (MnO2)/carbon black (CB) are developed. The bilayer separator is prepared via a two-step fabrication process involving freeze-thawing and nonsolvent-induced phase separation. The prepared bilayer separator exhibits superior porosity of 46%, wettability of 46.5°, and electrolyte uptake of 194% when compared with a Celgard 2320 trilayer separator (39%, 55.58°, and 110%). Moreover, lower bulk resistance yields a higher ionic conductivity of 0.52 mS cm-1 in comparison to 0.22 mS cm-1 for the Celgard separator. Furthermore, the bilayer separator exhibits improved mean efficiency of 0.44% and higher specific discharge capacitance of 13.53%. The anodic and cathodic electrodes are coated on a paper substrate using MnO2/CB and zinc metal-loaded CB composites. The paper supercapacitor demonstrates a high specific capacitance of 34.1 mF cm-2 and energy and power density of 1.70 μWh cm-2 and 204.8 μW cm-2 at 500 μA, respectively. In summary, the concept of an eco-friendly bilayer cellulose separator with paper-based supercapacitors offers an environmentally friendly alternative to traditional energy storage devices.

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.

A Review on Electrospun Nanofiber Composites for an Efficient Electrochemical Sensor Applications

The present review article discusses the elementary concepts of the sensor mechanism and various types of materials used for sensor applications. The electrospinning method is the most comfortable method to prepare the device-like structure by means of forming from the fiber structure. Though there are various materials available for sensors, the important factor is to incorporate the functional group on the surface of the materials. The post-modification sanction enhances the efficiency of the sensor materials. This article also describes the various types of materials applied to chemical and biosensor applications. The chemical sensor parts include acetone, ethanol, ammonia, and CO2, H2O2, and NO2 molecules; meanwhile, the biosensor takes on glucose, uric acid, and cholesterol molecules. The above materials have to be sensed for a healthier lifestyle for humans and other living organisms. The prescribed review articles give a detailed report on the Electrospun materials for sensor applications.

ZnO-Loaded Graphene for NO2 Gas Sensing

This paper investigates the effect of decorating graphene with zinc oxide (ZnO) nanoparticles (NPs) for the detection of NO₂. In this regard, two graphene sensors with different ZnO loadings of 5 wt.% and 20 wt.% were prepared, and their responses towards NO₂ at room temperature and different conditions were compared. The experimental results demonstrate that the graphene loaded with 5 wt.% ZnO NPs (G95/5) shows better performance at detecting low concentrations of the target gas than the one loaded with 20 wt.% ZnO NPs (G80/20). Moreover, measurements under dry and humid conditions of the G95/5 sensor revealed that the material is very sensitive to ambient moisture, showing an almost eight-fold increase in NO₂ sensitivity when the background changes from dry to 70% relative humidity. Regarding sensor selectivity, it presents a significant selectivity towards NO₂ compared to other gas compounds.

3D Janus structure MXene/cellulose nanofibers/luffa aerogels with superb mechanical strength and high-efficiency desalination for solar-driven interfacial evaporation

Interfacial solar steam generation (ISSG) is considered to be an attractive technique to address the water shortage. However, developing a sustainable thermal management, salt rejection, and excellent mechanical strength ISSG device for long-term stability desalination is still a challenge. Herein, a biomass ISSG device with superb mechanical properties was prepared by introducing a luffa sponge as the skeleton and constructing the MXene/cellulose nanofibers (CNFs) aerogels via freeze-drying. The Janus MXene-decorated CNFs/luffa (JMCL) aerogels integrated the multifunction of fast water transport, good thermal management, and efficient photothermal conversion in a single module, to achieve high-efficiency desalination. 3D Janus structure endowed the JMCL aerogel with opposite wettability, which is feasible to construct the localized photothermal generation and self-floating. The mechanical strength of JMCL aerogels is 437 times that of MXene/CNFs aerogels. The JMCL aerogels delivered a water evaporation rate of 1.40 kg m^-2h^-1 and an efficiency of 91.20% under 1 sun illumination. The excellent salt resistance during 24 h working and long-term solar vapor generation of up to 28 days were achieved. The multifunctional JMCL aerogels with 3D Janus structure offer new insights for developing good durability and eco-friendly biopolymer-based steam generators.

Field Effect Transistor Gas Sensors Based on Mechanically Exfoliated Van der Waals Materials

The high surface-to-volume ratio and flatness of mechanically exfoliated van der Waals (vdW) layered materials make them an ideal platform to investigate the Langmuir absorption model. In this work, we fabricated field effect transistor gas sensors, based on a variety of mechanically exfoliated vdW materials, and investigated their electrical field-dependent gas sensing properties. The good agreement between the experimentally extracted intrinsic parameters, such as equilibrium constant and adsorption energy, and theoretically predicted values suggests validity of the Langmuir absorption model for vdW materials. Moreover, we show that the device sensing behavior depends crucially on the availability of carriers, and giant sensitivities and strong selectivity can be achieved at the sensitivity singularity. Finally, we demonstrate that such features provide a fingerprint for different gases to quickly detect and differentiate between low concentrations of mixed hazardous gases using sensor arrays.