Enhanced ammonia sensing performance based on MXene-Ti3C2Tx multilayer nanoflakes functionalized by tungsten trioxide nanoparticles

Wide-range and high-accuracy wireless sensor with self-humidity compensation for real-time ammonia monitoring

Real-time and accurate biomarker detection is highly desired in point-of-care diagnosis, food freshness monitoring, and hazardous leakage warning. However, achieving such an objective with existing technologies is still challenging. Herein, we demonstrate a wireless inductor-capacitor (LC) chemical sensor based on platinum-doped partially deprotonated-polypyrrole (Pt-PPy+ and PPy0) for real-time and accurate ammonia (NH3) detection. With the chemically wide-range tunability of PPy in conductivity to modulate the impedance, the LC sensor exhibits an up-to-180% improvement in return loss (S11). The Pt-PPy+ and PPy0 shows the p-type semiconductor nature with greatly-manifested adsorption-charge transfer dynamics toward NH3, leading to an unprecedented NH3 sensing range. The S11 and frequency of the Pt-PPy+ and PPy0-based sensor exhibit discriminative response behaviors to humidity and NH3, enabling the without-external-calibration compensation and accurate NH3 detection. A portable system combining the proposed wireless chemical sensor and a handheld instrument is validated, which aids in rationalizing strategies for individuals toward various scenarios.

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.

Highly Sensitive Room-Temperature Detection of Ammonia in the Breath of Kidney Disease Patients Using Fe2Mo3O8/MoO2@MoS2 Nanocomposite Gas Sensor

A novel Fe2Mo3O8/MoO2@MoS2 nanocomposite is synthesized for extremely sensitive detection of NH3 in the breath of kidney disease patients at room temperature. Compared to MoS2, α-Fe2O3/MoS2, and MoO2@MoS2, it shows the optimal gas-sensing performance by optimizing the formation of Fe2Mo3O8 at 900 °C. The annealed Fe2Mo3O8/MoO2@MoS2 nanocomposite (Fe2Mo3O8/MoO2@MoS2-900 °C) sensor demonstrates a remarkably high selectivity of NH3 with a response of 875% to 30 ppm NH3 and an ultralow detection limit of 3.7 ppb. This sensor demonstrates excellent linearity, repeatability, and long-term stability. Furthermore, it effectively differentiates between patients at varying stages of kidney disease through quantitative NH3 measurements. The sensing mechanism is elucidated through the analysis of alterations in X-ray photoelectron spectroscopy (XPS) signals, which is supported by density functional theory (DFT) calculations illustrating the NH3 adsorption and oxidation pathways and their effects on charge transfer, resulting in the conductivity change as the sensing signal. The excellent performance is mainly attributed to the heterojunction among MoS2, MoO2, and Fe2Mo3O8 and the exceptional adsorption and catalytic activity of Fe2Mo3O8/MoO2@MoS2-900 °C for NH3. This research presents a promising new material optimized for detecting NH3 in exhaled breath and a new strategy for the early diagnosis and management of kidney disease.

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.

Multiarray Gas Sensors Using Ternary Combined Ti3C2Tx MXene-Based Nanocomposites

This paper reports chemiresistive multiarray gas sensors through the synthesized ternary nanocomposites, using a one-pot method to integrate two-dimensional MXene (Ti3C2Tx) with Ti-doped WO3 (Ti-WO3/Ti3C2Tx) and Ti3C2Tx with Pd-doped SnO2 (Pd-SnO2/Ti3C2Tx). The gas sensors based on Ti-WO3/Ti3C2Tx and Pd-SnO2/Ti3C2Tx exhibit exceptional sensitivity, particularly in detecting 70% at 1 ppm acetone and 91.1% at 1 ppm of H2S. Notably, our sensors demonstrate a remarkable sensing performance in the low-ppb range for acetone and H2S. Specifically, the Ti-WO3/Ti3C2Tx sensor demonstrates a detection limit of 0.035 ppb for acetone, and the Pd-SnO2/Ti3C2Tx sensor shows 0.116 ppb for H2S. Simultaneous measurements with Ti-WO3/Ti3C2Tx- and Pd-SnO2/Ti3C2Tx-based sensors enable the evaluation of both the concentration and type of unknown target gases, such as acetone or H2S. Furthermore, density functional theory calculations are performed to clarify the role of Ti and Pd doping in enhancing the performance of Ti-WO3/Ti3C2Tx and Pd-SnO2/Ti3C2Tx nanocomposites. Theoretical simulations contribute to a deeper understanding of the doping effects, providing essential insights into the mechanisms underlying the enhanced gas response of the gas sensors. Overall, this work provides valuable insights into the gas-sensing mechanisms and introduces a novel approach for high-performance multiarray gas sensing.

Development of toxic gas sensor from anthocyanin-embedded polycaprolactone-co-polylactic acid nanofibrous mat

The colorless ammonia gas has been a significant intermediate in the industrial sector. However, prolonged exposure to ammonia causes harmful effects to organs or even death. Herein, an environmentally friendly solid-state ammonia sensor was developed utilizing colorimetric polycaprolactone-co-polylactic acid nanofibrous membrane. Pomegranate (Punica granatum L.) peel contains anthocyanin (ACN) as a naturally occurring spectroscopic probe. A mordant (potassium aluminum sulfate) is used to immobilize the anthocyanin direct dyestuff inside nanofibers, generating mordant/anthocyanin (M/ACN) coordinated complex nanoparticles. When exposed to ammonia, the color change of anthocyanin-encapsulated polycaprolactone-co-polylactic acid nanofibrous membrane from purple to transparent was examined by absorbance spectra and CIE Lab color parameters. With a quick colorimetric shift, the polycaprolactone-co-polylactic acid fabric exhibits a detection limit of 5-150 mg/L. The absorbance spectra showed a hypsochromic shift when exposed to ammonia, displaying an absorption shift from 559 nm to 391 nm with an isosbestic point of 448 nm. Scanning electron microscopy (SEM) images revealed that the polycaprolactone-co-polylactic acid nanofibers had a diameter of 75-125 nm, whereas transmission electron microscopy (TEM) images revealed that M/ACN nanoparticles exhibited diameters of 10-20 nm.

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.

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.

Density functional theory studies of Ti3C2TxMXene nanosheets decorated with Au for sensing SF6/N2nitrogen-containing decomposition gases

SF6/N2mixture is an alternative gas of SF6, which is already used in electrical equipment. When a malfunction occurs , SF6/N2will decompose and further react with trace water and oxygen to produce nitrogen-containing gases such as NO, NO2, N2O and NF3. It is necessary to monitor these gases to ensure the safe operation of the equipment. This paper is based on density functional theory (DFT), the nanomaterial Ti3C2Txdoped with Au atom was selected as sensing material. The result shows that Au/Ti3C2Txhas larger adsorption energy when NO and NO2adsorbed on the surface, the stable structures were conformed more easily with NO and NO2compared with N2O and NF3. The density of states analysis and the frontier molecule orbital analysis reveal more change of the system before and after NO and NO2adsorption, suggesting the material showed good sensitivity performance to NO and NO2. Thus, Au/Ti3C2Txis considered to have the potential for sensing NO and NO2.

Tailored mesoporous γ-WO3 nanoplates: unraveling their potential for highly sensitive NH3 detection and efficient photocatalysis

Herein, the monoclinic phase of tungsten oxide (γ-WO3) was successfully obtained after annealing hydrothermally synthesised WO3 powder at 500 °C. As per the result obtained from the N2 adsorption-desorption isotherm, the material has been identified as mesoporous with a specific surface area of 3.71 m2 g-1 from BET (Brunauer-Emmett-Teller) analysis. Moreover, the average pore size (49.52 nm) and volume (0.050 cm3 g-1) were also determined by the BJH (Barrett-Joyner-Halenda) method. FE-SEM (field emission scanning electron microscopy) and HR-TEM (high resolution transmission electron microscopy) have confirmed the formation of nanoplates with an average diameter of approximately 274 nm. Raman spectroscopy has shown peaks at the lower wavenumber region (270 cm-1 and 326 cm-1) and the higher wavenumber region (713 cm-1 and 806 cm-1) for O-W-O bending modes and stretching modes, respectively. The combined effect of relative humidity (RH-11%-RH-95%-RH-11%) and NH3 (150 ppm, 300 ppm, 450 ppm, 600 ppm, 700 ppm, and 800 ppm) was investigated in this reported work. The synthesised γ-WO3 has shown highly responsive behaviour for humidity of 96.5% (RH-11%-95%) and NH3 sensing (under humidity) of 97.4% (RH-11%-95% with 800 ppm NH3). The response and recovery time were calculated as 15 s and 52 s, and 16 s and 54 s for humidity, and NH3 under humidity, respectively. The experimental findings demonstrated that the resistance of the sensor depends on the concentration of NH3 and humidity. Moreover, γ-WO3 has been investigated as a promising catalyst for the dye degradation of methylene blue (MB) with a degradation efficiency of 72.82% and methyl orange (MO) with a degradation efficiency of 53.84% under visible light exposure. This dye degradation occurred within 160 min in the presence of a catalyst under visible light irradiation.

Room Temperature Chemiresistive Gas Sensors Based on 2D MXenes

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

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

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

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

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

Porous polyoxotungstate/MXene hybrid films allowing for visualization of the energy storage status in high-performance electrochromic supercapacitors

Electrochromic supercapacitors (ECSCs) have recently received growing attention for potential smart energy storage components in intelligent electronics. However, in the development of ECSCs, the design and assembly of high-performance electrode materials remain ongoing challenges. In this study, Ti₃C₂T_x MXene and polyoxotungstate (P₂W₁₈) were deposited on TiO₂ nanowires to construct a unique three-dimensional (3D) porous hybrid film, NW@MXene/P₂W₁₈, via a convenient layer-by-layer self-assembly approach. The 3D porous structure of the nanocomposite reduced the aggregation and stacking of Ti₃C₂T_x MXene nanosheets during self-assembly, leading to the formation of unobstructed ion diffusion channels and interfacial charge transfer between adjacent layers, resulting in a good electrochemical performance. Compared to the tightly packed structure, the porous hybrid film demonstrated an enhanced electrochromic energy storage performance with a higher areal capacitance (i.e., 19.0 mF cm^-2 at a current density of 0.6 mA cm^-2), in addition to a high cycling stability (i.e., 90.7% retention rate after 2000 cycles), and an excellent color rendering efficiency. Subsequently, an asymmetric ECSC was fabricated using an NW@MXene/P₂W₁₈ film as the cathode and a TiO₂ nanowire film as the anode. This ECSC exhibited a high areal capacitance of 4.0 mF cm^-2 at a current density of 0.1 mA cm^-2 with a wide operating window of 4.5 V, whilst also achieving high-speed color switching between olive green and dark blue during the charge/discharge processes, ultimately offering new avenues for the development of electrochromic energy storage electrode materials and the design of novel 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 Ti₃C₂T_x and Ti₂CT_x 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.

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

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

Cu2O/Ti3C2Txnanocomposites for detection of triethylamine gas at room temperature

Triethylamine gas is one of the harmful volatile organic compounds for human health and the ecological environment. Therefore, in order to prevent the detrimental effects of triethylamine gas, it has greatly requirement to be accurately detected. Unfortunately, Cu₂O has a low triethylamine gas response and slow recovery. Because of this, we prepared Cu₂O/Ti₃C₂T_xnanocomposites by a facile ultrasonication technique. Cu₂O is uniformly dispersed on the surface and interlayers of multilayer Ti₃C₂T_xto form a stable hybrid heterostructure. The optimized Cu₂O/Ti₃C₂T_xnanocomposite sensor's response to 10 ppm triethylamine at room temperature is 181.6% (∣R_g-R_a∣/R_a × 100%). It is 3.5 times higher than the original Cu₂O nanospheres (52.1%). Moreover, due to the characteristics of high carrier migration rate and excellent conductivity of Ti₃C₂T_x, the response recovery rate (1062 s/74 s) of Cu₂O/Ti₃C₂T_xcomposites is greatly improved than pristine Cu₂O (3169 s/293 s). In addition, Cu₂O/Ti₃C₂T_xnanocomposites sensor also shows excellent repeatability, outstanding selectivity, and long-term stability. Thus, the Cu₂O/Ti₃C₂T_xnanocomposites sensor has broad application prospects for detecting triethylamine gas at room temperature.

pH-Responsive Liquid Marbles Based on Dihydroxystearic Acid

Herein, we report pH-responsive liquid marbles stabilized by 9,10-dihydroxystearic acid (DHSA). The particle morphology and the pH-responsive behavior of the liquid marbles were investigated. The rolling time during the preparation of liquid marbles has a great influence on the thickness of powder adsorption and the stability of the marbles. Compared with the liquid marbles stabilized by other fatty acids (e.g., stearic acid and docosoic acid), the liquid marbles prepared by DHSA have a much higher mechanical robustness. The increase in the number of hydroxyl groups on the carbon chain of fatty acids improves the mechanical robustness of the liquid marbles. Such liquid marbles immediately disintegrated on the surface of an alkaline solution or after exposure to NH₃ gas, which extends their applications in the NH₃ sensor and chemical reactions.

2D SnSe2 nanoflakes decorated with 1D ZnO nanowires for ppb-level NO2 detection at room temperature

The detection of air pollutant nitrogen dioxide (NO₂) is of great importance arising from its great harm to the ecological environment and human health. However, the detection range of most NO₂ sensors is ppm-level, and it is still challenging to achieve lower concentration (ppb-level) NO₂ detection. Herein, 2D tin diselenide nanoflakes decorated with 1D zinc oxide nanowires (SnSe₂/ZnO) heterojunctions were first reported by facile hydrothermal and ultra-sonication methods. The response of the fabricated SnSe₂/ZnO sensor enhances 3.41 times on average compared with that of pure SnSe₂ sensor to 50-150 ppb NO₂ with a high detection sensitivity (22.57 ppm^-1) at room temperature. In addition, the SnSe₂/ZnO sensor has complete recovery, negligible cross-sensitivity, and small relative standard deviation (6.98%) during the 1 month sensing test, which can meet the requirements for NO₂ detection in environmental monitoring. The enhanced NO₂ sensing performance can be attributed to the n-n heterojunction constructed between SnSe₂ and ZnO. The as-prepared sensor based on SnSe₂/ZnO hybrid significantly promotes the development of the low detection limit of the NO₂ sensor at room temperature.

Recent progress in the fabrication of flexible materials for wearable sensors

Wearable sensors have been widely studied because of their small size, light weight, and potential for the noninvasive tracking and monitoring of human physiological information. Wearable flexible sensors generally consist of two parts: a flexible substrate in contact with the skin and a signal processing module. At present, wearable electronics cover many fields, such as machinery, physics, chemistry, materials science, and biomedicine. The design concept and selection of materials are very important to the function of a sensor. In this review, we summarize the latest developments in flexible materials for wearable sensors, including developments in flexible materials, electrode materials, and new flexible biodegradable materials, and describe the important role of innovation in material and sensor design in the development of wearable flexible sensors. Strategies and challenges related to the improvement of the performances of wearable flexible sensors, as well as the development prospects of wearable devices based on flexible materials, are also discussed.

Recent Progress of Nanostructured Sensing Materials from 0D to 3D: Overview of Structure-Property-Application Relationship for Gas Sensors

Along with the progress of nanoscience and nanotechnology, nanomaterials with attractive structural and functional properties have gained more attention than ever before, especially in the field of electronic sensors. In recent years, the gas sensing devices have made great achievement and also created wide application prospects, which leads to a new wave of research for designing advanced sensing materials. There is no doubt that the characteristics are highly governed by the sensitive layers. For this reason, important advances for the outstanding, novel sensing materials with different dimensional structures including 0D, 1D, 2D, and 3D are reported and summarized systematically. The sensing materials cover noble metals, metal oxide semiconductors, carbon nanomaterials, metal dichalcogenides, g-C₃ N₄ , MXenes, and complex composites. Discussion is also extended to the relation between sensing performances and their structure, electronic properties, and surface chemistry. In addition, some gas sensing related applications are also highlighted, including environment monitoring, breath analysis, food quality and safety, and flexible wearable electronics, from current situation and the facing challenges to the future research perspectives.

Preparation and Application of 2D MXene-Based Gas Sensors: A Review

Since MXene (a two-dimensional material) was discovered in 2011, it has been favored in all aspects due to its rich surface functional groups, large specific surface area, high conductivity, large porosity, rich organic bonds, and high hydrophilicity. In this paper, the preparation of MXene is introduced first. HF etching was the first etching method for MXene; however, HF is corrosive, resulting in the development of the in situ HF method (fluoride + HCl). Due to the harmful effects of fluorine terminal on the performance of MXene, a fluorine-free preparation method was developed. The increase in interlayer spacing brought about by adding an intercalator can affect MXene’s performance. The usual preparation methods render MXene inevitably agglomerate and the resulting yields are insufficient. Many new preparation methods were researched in order to solve the problems of agglomeration and yield. Secondly, the application of MXene-based materials in gas sensors was discussed. MXene is often regarded as a flexible gas sensor, and the detection of ppb-level acetone at room temperature was observed for the first time. After the formation of composite materials, the increasing interlayer spacing and the specific surface area increased the number of active sites of gas adsorption and the gas sensitivity performance improved. Moreover, this paper discusses the gas-sensing mechanism of MXene. The gas-sensing mechanism of metallic MXene is affected by the expansion of the lamellae and will be doped with H2O and oxygen during the etching process in order to become a p-type semiconductor. A p-n heterojunction and a Schottky barrier forms due to combinations with other semiconductors; thus, the gas sensitivities of composite materials are regulated and controlled by them. Although there are only several reports on the application of MXene materials to gas sensors, MXene and its composite materials are expected to become materials that can effectively detect gases at room temperature, especially for the detection of NH3 and VOC gas. Finally, the challenges and opportunities of MXene as a gas sensor are discussed.