Transparent, Flexible, and Conductive 2D Titanium Carbide (MXene) Films with High Volumetric Capacitance

Strong and Tough MXene Bridging-induced Conductive Nacre

Nacre is a classic model, providing an inspiration for fabricating high-performance bulk nanocomposites with the two-dimensional platelets. However, the "brick" of nacre, aragonite platelet, is an ideal building block for making high-performance bulk nanocomposites. Herein, we demonstrated a strong and tough conductive nacre through reassembling aragonite platelets with bridged by MXene nanosheets and hydrogen bonding, not only providing high mechanical properties but also excellent electrical conductivity. The flexural strength and fracture toughness of the obtained conductive nacre reach ~282 MPa and ~6.3 MPa m1/2, which is 1.6 and 1.6 times higher than that of natural nacre, respectively. These properties are attributed to densification and high orientation degree of the conductive nacre, which is effectively induced by the combined interactions of hydrogen bonding and MXene nanosheets bridging. The crack propagations in conductive nacre are effectively inhibited through crack deflection with hydrogen bonding, and MXene nanosheets bridging between aragonite platelets. In addition, our conductive nacre also provides a self-monitoring function for structural damage and offers exceptional electromagnetic interference shielding performance. Our strategy of reassembling the aragonite platelets exfoliated from waste nacre into high-performance artificial nacre, provides an avenue for fabricating high-performance bulk nanocomposites through the sustainable reutilization of shell resources.

Electrochemical Properties of Mo4VC4Tx MXene in Aqueous Electrolytes

M5C4Tx MXenes represent the most recently discovered and least studied subfamily of out-of-plane ordered double transition metal carbides with 11 atomic layers, probably the thickest of all 2D materials. Molybdenum (Mo) and vanadium (V) in Mo4VC4Tx offer multiple oxidation states, making this MXene potentially attractive for electrochemical energy storage applications. Herein, we evaluated the electrochemical properties of Mo4VC4Tx free-standing thin films in acidic, basic, and neutral aqueous electrolytes and observed the highest gravimetric capacitance of 219 F g-1 at 2 mV s-1 in a 3 M H2SO4. Further, we investigated the intercalation states of four different cations (H+, Li+, Na+, and K+) in MXenes through ab initio molecular dynamics (AIMD) simulation and used density functional theory (DFT) calculations to assess the charge storage mechanisms in different electrolytes. These studies show hydrated Li+, Na+, and K+ ions forming an electric double layer (EDL) at the MXene surface as the primary charge storage mechanism. This work shows the promise of Mo4VC4Tx MXene for energy storage in aqueous electrolytes.

Stabilizing Ti3C2Tx MXene flakes in air by removing confined water

MXenes have demonstrated potential for various applications owing to their tunable surface chemistry and metallic conductivity. However, high temperatures can accelerate MXene film oxidation in air. Understanding the mechanisms of MXene oxidation at elevated temperatures, which is still limited, is critical in improving their thermal stability for high-temperature applications. Here, we demonstrate that Ti[Formula: see text]C[Formula: see text]T[Formula: see text] MXene monoflakes have exceptional thermal stability at temperatures up to 600[Formula: see text]C in air, while multiflakes readily oxidize in air at 300[Formula: see text]C. Density functional theory calculations indicate that confined water between Ti[Formula: see text]C[Formula: see text]T[Formula: see text] flakes has higher removal energy than surface water and can thus persist to higher temperatures, leading to oxidation. We demonstrate that the amount of confined water correlates with the degree of oxidation in stacked flakes. Confined water can be fully removed by vacuum annealing Ti[Formula: see text]C[Formula: see text]T[Formula: see text] films at 600[Formula: see text]C, resulting in substantial stability improvement in multiflake films (can withstand 600[Formula: see text]C in air). These findings provide fundamental insights into the kinetics of confined water and its role in Ti[Formula: see text]C[Formula: see text]T[Formula: see text] oxidation. This work enables the use of stable monoflake MXenes in high-temperature applications and provides guidelines for proper vacuum annealing of multiflake films to enhance their stability.

A review of etching methods and applications of two-dimensional MXenes

MXenes have been attracting much attention since their introduction due to their amazing properties such as unique structure, good hydrophilicity, metal-grade electrical conductivity, rich surface chemistry, low ionic diffusion resistance, and excellent mechanical strength. It is noteworthy that different synthesis methods have a great influence on the structure and properties of MXenes. In recent years, some modification strategies of MXenes with unique insights have been developed with the increasing research. In summary, this paper reviews and summarizes the recent research progress of MXenes from the perspective of preparation processes (including hydrofluoric acid direct etching, fluoride/concentrated acid hybrid etching, fluoride melt etching, electrochemical etching, alkali-assisted etching and Lewis acid etching strategies), which can provide valuable guidance for the preparation and application of high-performance MXenes-based materials.

MXene-based all-solid flexible electrochromic microsupercapacitor

With the increasing demand for multifunctional optoelectronic devices, flexible electrochromic energy storage devices are being widely recognized as promising platforms for diverse applications. However, simultaneously achieving high capacitance, fast color switching and large optical modulation range is very challenging. In this study, the MXene-based flexible in-plane microsupercapacitor was fabricated via a mask-assisted spray coating approach. By adding electrochromic ethyl viologen dibromide (EVB) into the electrolyte, the device showed a reversible color change during the charge/discharge process. Due to the high electronic conductivity of the MXene flakes and the fast response kinetics of EVB, the device exhibited a fast coloration/bleaching time of 2.6 s/2.5 s, a large optical contrast of 60%, and exceptional coloration efficiency. In addition, EVB acted as a redox additive to reinforce the energy storage performance; as a result, the working voltage window of the Ti3C2-based symmetric aqueous microsupercapacitor was extended to 1 V. Moreover, the device had a high areal capacitance of 12.5 mF cm-2 with superior flexibility and mechanical stability and showed almost 100% capacitance retention after 100 bending cycles. The as-prepared device has significant potential for a wide range of applications in flexible and wearable electronics, particularly in the fields of camouflage, anticounterfeiting, and displays.

Recent Advances in Non-Ti MXenes: Synthesis, Properties, and Novel Applications

One of the most fascinating 2D nanomaterials (NMs) ever found is various members of MXene family. Among them, the titanium-based MXenes, with more than 70% of publication-related investigations, are comparatively well studied, producing fundamental foundation for the 2D MXene family members with flexible properties, familiar with a variety of advanced novel technological applications. Nonetheless, there are still more candidates among transitional metals (TMs) that can function as MXene NMs in ways that go well beyond those that are now recognized. Systematized details of the preparations, characteristics, limitations, significant discoveries, and uses of the novel M-based MXenes (M-MXenes), where M stands for non-Ti TMs (M = Sc, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, and Lu), are given. The exceptional qualities of the 2D non-Ti MXene outperform standard Ti-MXene in several applications. There is many advancement in top-down as well as bottom-up production of MXenes family members, which allows for exact control of the M-characteristics MXene NMs to contain cutting-edge applications. This study offers a systematic evaluation of existing research, covering everything in producing complex M-MXenes from primary limitations to the characterization and selection of their applications in accordance with their novel features. The development of double metal combinations, extension of additional metal candidates beyond group-(III-VI)B family, and subsequent development of the 2D TM carbide/TMs nitride/TM carbonitrides to 2D metal boride family are also included in this overview. The possibilities and further recommendations for the way of non-Ti MXene NMs are in the synthesis of NMs will discuss in detail in this critical evaluation.

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.

Customizable Metal Micromesh Electrode Enabling Flexible Transparent Zn-Ion Hybrid Supercapacitors with High Energy Density

Emerging flexible and wearable electronic products are placing a compelling demand on lightweight transparent energy storage devices. Owing to their distinguishing features of safety, high specific energy, cycling stability, and rapid charge/discharge advantages, Zn-ion hybrid supercapacitors are a current topic of discussion. However, the trade-off for optical transmittance and energy density remains a great challenge. Here, a high-performance Zn-ion hybrid supercapacitor based on the customizable ultrathin (5 µm), ultralight (0.45 mg cm-2), and ultra-transparent (87.6%) Ni micromesh based cathode and Zn micromesh anode with the highest figure of merit (84 843) is proposed. The developed flexible transparent Zn-ion hybrid supercapacitors reveal excellent cycle stability (no decline after 20 000 cycles), high areal energy density (31.69 µWh cm-2), and high power density (512 µW cm-2). In addition, the assembled solid flexible and transparent Zn-ion hybrid supercapacitor with polyacrylamide gel electrolyte shows extraordinary mechanical properties even under extreme bending and twisting operation. Furthermore, the full device displays a high optical transmittance over 55.04% and can be conformally integrated with diverse devices as a flexible transparent power supply. The fabrication technology offers seamless compatibility with industrial manufacturing, making it an ideal model for the advancement of portable and wearable devices.

Conductive MXene/Polymer Composites for Transparent Flexible Supercapacitors

Transparent flexible energy storage devices are limited by the trade-off among flexibility, transparency, and charge storage capability of their electrode materials. Conductive polymers are intrinsically flexible, but limited by small capacitance. Pseudocapacitive MXene provides high capacitance, yet their opaque and brittle nature hinders their flexibility and transparency. Herein, the development of synergistically interacting conductive polymer Ti3C2Tx MXene/PEDOT:PSS composites is reported for transparent flexible all-solid-state supercapacitors, with an outstanding areal capacitance of 3.1 mF cm-2, a high optical transparency of 61.6%, and excellent flexibility and durability. The high capacitance and high transparency of the devices stem from the uniform and thorough blending of PEDOT:PSS and Ti3C2Tx, which is associated with the formation of O─H…O H-bonds in the composites. The conductive MXene/polymer composite electrodes demonstrate a rational means to achieve high-capacity, transparent and flexible supercapacitors in an easy and scalable manner.

Direct Ink Writing of Low-Concentration MXene/Aramid Nanofiber Inks for Tunable Electromagnetic Shielding and Infrared Anticounterfeiting Applications

MXene inks offer a promising avenue for the scalable production and customization of printing electronics. However, simultaneously achieving a low solid content and printability of MXene inks, as well as mechanical flexibility and environmental stability of printed objects, remains a challenge. In this study, we overcame these challenges by employing high-viscosity aramid nanofibers (ANFs) to optimize the rheology of low-concentration MXene inks. The abundant entangled networks and hydrogen bonds formed between MXene and ANF significantly increase the viscosity and yield stress up to 103 Pa·s and 200 Pa, respectively. This optimization allows the use of MXene/ANF (MA) inks at low concentrations in direct ink writing and other high-viscosity processing techniques. The printable MXene/ANF inks with a high conductivity of 883.5 S/cm were used to print shields with customized structures, achieving a tunable electromagnetic interference shielding effectiveness (EMI SE) in the 0.2-48.2 dB range. Furthermore, the MA inks exhibited adjustable infrared (IR) emissivity by changing the ANF ratio combined with printing design, demonstrating the application for infrared anticounterfeiting. Notably, the printed MXene/ANF objects possess outstanding mechanical flexibility and environmental stability, which are attributed to the reinforcement and protection of ANF. Therefore, these findings have significant practical implications as versatile MXene/ANF inks can be used for customizable, scalable, and cost-effective production of flexible printed electronics.

Engineered two-dimensional nanomaterials based diagnostics integrated with internet of medical things (IoMT) for COVID-19

More than four years have passed since an inimitable coronavirus disease (COVID-19) pandemic hit the globe in 2019 after an uncontrolled transmission of the severe acute respiratory syndrome (SARS-CoV-2) infection. The occurrence of this highly contagious respiratory infectious disease led to chaos and mortality all over the world. The peak paradigm shift of the researchers was inclined towards the accurate and rapid detection of diseases. Since 2019, there has been a boost in the diagnostics of COVID-19 via numerous conventional diagnostic tools like RT-PCR, ELISA, etc., and advanced biosensing kits like LFIA, etc. For the same reason, the use of nanotechnology and two-dimensional nanomaterials (2DNMs) has aided in the fabrication of efficient diagnostic tools to combat COVID-19. This article discusses the engineering techniques utilized for fabricating chemically active E2DNMs that are exceptionally thin and irregular. The techniques encompass the introduction of heteroatoms, intercalation of ions, and the design of strain and defects. E2DNMs possess unique characteristics, including a substantial surface area and controllable electrical, optical, and bioactive properties. These characteristics enable the development of sophisticated diagnostic platforms for real-time biosensors with exceptional sensitivity in detecting SARS-CoV-2. Integrating the Internet of Medical Things (IoMT) with these E2DNMs-based advanced diagnostics has led to the development of portable, real-time, scalable, more accurate, and cost-effective SARS-CoV-2 diagnostic platforms. These diagnostic platforms have the potential to revolutionize SARS-CoV-2 diagnosis by making it faster, easier, and more accessible to people worldwide, thus making them ideal for resource-limited settings. These advanced IoMT diagnostic platforms may help with combating SARS-CoV-2 as well as tracking and predicting the spread of future pandemics, ultimately saving lives and mitigating their impact on global health systems.

Unveiling the Role of Termination Groups in Stabilizing MXenes in Contact with Water

MXenes are versatile 2D materials demonstrating outstanding electrochemical and physical properties, but their practical use is limited, because of fast degradation in an aqueous environment. To prevent the degradation of MXenes, it is essential to understand the atomistic details of the reaction and to identify active sites. In this letter, we provided a computational analysis of the degradation processes at the interface between MXene basal planes and water using enhanced sampling ab initio molecular dynamics simulations and symbolic regression analysis. Our results indicate that the reactivity of Ti sites toward the water attack reaction depends on both local coordination and chemical composition of the MXene surfaces. Decreasing the work function of the Ti3C2Tx surfaces and avoiding Ti sites that are loosely anchored to the subsurface (e.g., O-coordinated) can improve surface stability. The developed computational framework can be further used to investigate other possible culprits of the degradation reaction, including the role of defects and edges.

Recent Advances in Nanomaterials for the Treatment of Acute Kidney Injury

Acute kidney injury (AKI) is a serious clinical syndrome with high morbidity, elevated mortality, and poor prognosis, commonly considered a "sword of Damocles" for hospitalized patients, especially those in intensive care units. Oxidative stress, inflammation, and apoptosis, caused by the excessive production of reactive oxygen species (ROS), play a key role in AKI progression. Hence, the investigation of effective and safe antioxidants and inflammatory regulators to scavenge overexpressed ROS and regulate excessive inflammation has become a promising therapeutic option. However, the unique physiological structure and complex pathological alterations in the kidneys render traditional therapies ineffective, impeding the residence and efficacy of most antioxidant and anti-inflammatory small molecule drugs within the renal milieu. Recently, nanotherapeutic interventions have emerged as a promising and prospective strategy for AKI, overcoming traditional treatment dilemmas through alterations in size, shape, charge, and surface modifications. This Review succinctly summarizes the latest advancements in nanotherapeutic approaches for AKI, encompassing nanozymes, ROS scavenger nanomaterials, MSC-EVs, and nanomaterials loaded with antioxidants and inflammatory regulator. Following this, strategies aimed at enhancing biocompatibility and kidney targeting are introduced. Furthermore, a brief discussion on the current challenges and future prospects in this research field is presented, providing a comprehensive overview of the evolving landscape of nanotherapeutic interventions for AKI.

3D grape string-like heterostructures enable high-efficiency sodium ion capture in Ti3C2Tx MXene/fungi-derived carbon nanoribbon hybrids

2D transition metal carbides and carbonitrides (MXenes) have emerged as promising electrode materials for electrochemistry ion capture but always suffer from severe layer-restacking and irreversible oxidation that restrains their electrochemical performance. Here we design a dual strategy of microstructure tailoring and heterostructure construction to synthesize a unique 3D grape string-like heterostructure consisting of Ti3C2Tx MXene hollow microspheres wrapped by fungi-derived N-doping carbon nanoribbons (denoted as GMNC). The 3D grape string-like heterostructure effectively avoids the aggregation of Ti3C2Tx MXene sheets and enhances the stability of MXenes, providing abundant active sites for ion capture, and an interconnected conductive bionic nanofiber network for high-rate electron transport. In consequence, GMNC achieves a superior adsorption capacity for sodium ions (Na+) in capacitive deionization (CDI) (162.37 mg gNaCl-1) with an ultra-high instantaneous adsorption rate (30.52 mg g-1 min-1) at an applied voltage of 1.6 V and satisfactory cycle stability over 100 cycles, which is a strong performer among the state-of-the-art values for MXene-based CDI electrodes. In addition, in situ electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) measurement combined with density functional theory (DFT) reveals the mechanisms of the Na+ capture process in the GMNC heterostructure. This work opens a new avenue for designing high-performance MXenes with a 3D hierarchical heterostructure for advanced electrochemical applications.

MXene-Based Elastomer Mimetic Stretchable Sensors: Design, Properties, and Applications

Flexible sensors based on MXene-polymer composites are highly prospective for next-generation wearable electronics used in human-machine interfaces. One of the motivating factors behind the progress of flexible sensors is the steady arrival of new conductive materials. MXenes, a new family of 2D nanomaterials, have been drawing attention since the last decade due to their high electronic conductivity, processability, mechanical robustness and chemical tunability. In this review, we encompass the fabrication of MXene-based polymeric nanocomposites, their structure-property relationship, and applications in the flexible sensor domain. Moreover, our discussion is not only limited to sensor design, their mechanism, and various modes of sensing platform, but also their future perspective and market throughout the world. With our article, we intend to fortify the bond between flexible matrices and MXenes thus promoting the swift advancement of flexible MXene-sensors for wearable technologies.

Architectural design of anode materials for superior alkali-ion (Li/Na/K) batteries storage

Developing high-performance anode materials remains a significant challenge for clean energy storage systems. Herein, we investigated the (MXene/MoSe2@C) heterostructure hybrid nanostructure as a superior anode material for application in lithium, sodium, and potassium ion batteries (LIBs, SIBs, and PIBs). Moreover, the anode structure's stability was examined via the open-source Large-scale atomic/molecular massively Parallel Simulator code. Our results indicated that the migration of SIBs toward the anode material is significantly greater than other ions during charge and discharge cycles. Therefore, SIBs systems can be competitive with PIBs and LIBs systems. In addition, the average values of the potential energies for the anode materials/ions complexes are about ~ - 713.65, ~  - 2030.41, and ~ - 912.36 kcal mol-1 in systems LIBs, SIBs, and PIBs, respectively. This study provides a rational design strategy to develop high-performance anode materials in SIBs/PIBs/LIBs systems, which can be developed for other transition metal chalcogenide-based composites as a superior anode of alkali metal ion battery storage systems.

Pristine Ti3C2Tx MXene Enables Flexible and Transparent Electrochemical Sensors

In the era of the internet of things, there exists a pressing need for technologies that meet the stringent demands of wearable, self-powered, and seamlessly integrated devices. Current approaches to developing MXene-based electrochemical sensors involve either rigid or opaque components, limiting their use in niche applications. This study investigates the potential of pristine Ti3C2Tx electrodes for flexible and transparent electrochemical sensing, achieved through an exploration of how material characteristics (flake size, flake orientation, film geometry, and uniformity) impact the electrochemical activity of the outer sphere redox probe ruthenium hexamine using cyclic voltammetry. The optimized electrode made of stacked large Ti3C2Tx flakes demonstrated excellent reproducibility and resistance to bending conditions, suggesting their use for reliable, robust, and flexible sensors. Reducing electrode thickness resulted in an amplified faradaic-to-capacitance signal, which is advantageous for this application. This led to the deposition of transparent thin Ti3C2Tx films, which maintained their best performance up to 73% transparency. These findings underscore its promise for high-performance, tailored sensors, marking a significant stride in advancing MXene utilization in next-generation electrochemical sensing technologies. The results encourage the analytical electrochemistry field to take advantage of the unique properties that pristine Ti3C2Tx electrodes can provide in sensing through more parametric studies.

Programming Anisotropic Functionality of 3D Microdenticles by Staggered-Overlapped and Multilayered Microarchitectures

Natural sharkskin features staggered-overlapped and multilayered architectures of riblet-textured anisotropic microdenticles, exhibiting drag reduction and providing a flexible yet strong armor. However, the artificial fabrication of three-dimensional (3D) sharkskin with these unique functionalities and mechanical integrity is a challenge using conventional techniques. In this study, it is reported on the facile microfabrication of multilayered 3D sharkskin through the magnetic actuation of polymeric composites and subsequent chemical shape fixation by casting thin polymeric films. The fabricated hydrophobic sharkskin, with geometric symmetry breaking, achieves anisotropic drag reduction in frontal and backward flow directions against the riblet-textured microdenticles. For mechanical integrity, hard-on-soft multilayered mechanical properties are realized by coating the polymeric sharkskin with thin layers of zinc oxide and platinum, which have higher hardness and recovery behaviors than the polymer. This multilayered hard-on-soft sharkskin exhibits friction anisotropy, mechanical robustness, and structural recovery. Furthermore, coating the MXene nanosheets provides the fabricated sharkskin with a low electrical resistance of ≈5.3 Ω, which leads to high Joule heating (≈229.9 °C at 2.75 V). The proposed magnetomechanical actuation-assisted microfabrication strategy is expected to facilitate the development of devices requiring multifunctional microtextures.

Phase Behavior and Rheological Properties of Size-Fractionated MXene (Ti3C2Tx) Dispersions

Understanding the dispersion behavior of MXenes is interesting from a fundamental colloid science perspective and critical to enabling the fluid-phase manufacturing of MXene devices with controlled microstructures and properties. However, the polydispersity, irregular shape, and charged surfaces of MXenes result in a complex phase behavior that is difficult to predict through theoretical calculations. As two-dimensional (2D) nanomaterials, MXenes can form lyotropic liquid crystal phases, gels, and aggregates. This work aims to elucidate the effects of MXene (Ti3C2Tx) sheet size on their phase behavior and associated rheological properties. Aqueous dispersions of large sheets with an average lateral dimension of 3.0 μm, small sheets with an average lateral dimension of 0.3 μm, and a bimodal mixture of the two sizes were investigated by using cross-polarized optical microscopy and rheology. At low concentrations, the large MXene dispersions exhibited lyotropic liquid crystal behavior and extended aligned textures, but increasing concentration resulted in the formation of dense flocs. Dispersions of small sheets formed small birefringent domains with increasing concentration but lacked long-range ordering. A bimodal mixture of these sizes enabled the formation of liquid crystalline phases with extended aligned textures with less floc formation. These results provide insights into using polydispersity to tune dispersion microstructure and rheological properties that can be applied to designing dispersions for fluid-phase manufacturing methods, such as direct ink writing.

The Need for Smart Materials in an Expanding Smart World: MXene-Based Wearable Electronics and Their Advantageous Applications

As a result of the transformation of inflexible electronic structures into flexible and stretchy devices, wearable electronics now provide great advantages in a variety of fields, including mobile healthcare sensing and monitoring, human-machine interfaces, portable energy storage and harvesting, and more. Because of their enriched surface functionalities, large surface area, and high electrical conductivity, transition metal nitrides and carbides (also known as MXenes) have recently come to be extensively considered as a group of functioning two-dimensional nanomaterials as well as exceptional fundamental elements for forming flexible electronics devices. This Review discusses the most recent advancements that have been made in the field of MXene-enabled flexible electronics for wearable electronics. The emphasis is placed on extensively established nonstructural features in order to highlight some MXene-enabled electrical devices that were constructed on a nanometric scale. These attributes include devices configured in three dimensions: printed materials, bioinspired structures, and textile and planar substrates. In addition, sample applications in electromagnetic interference (EMI) shielding, energy, healthcare, and humanoid control of machinery illustrate the exceptional development of these nanodevices. The increasing potential of MXene nanoparticles as a new area in next-generation wearable electronic technologies is projected in this Review. The design challenges associated with these electronic devices are also discussed, and possible solutions are presented.

Electrochemical Performance of Biocompatible TiC Films Deposited through Nonreactive RF Magnetron Sputtering for Neural Interfacing

The efficacy of neural electrode stimulation and recording hinges significantly on the choice of a neural electrode interface material. Transition metal carbides (TMCs), particularly titanium carbide (TiC), have demonstrated exceptional chemical stability and high electrical conductivity. Yet, the fabrication of TiC thin films and their potential application as neural electrode interfaces remains relatively unexplored. Herein, we present a systematic examination of TiC thin films synthesized through nonreactive radio frequency (RF) magnetron sputtering. TiC films were optimized toward high areal capacitance, low impedance, and stable electrochemical cyclability. We varied the RF power and deposition pressure to pinpoint the optimal properties, focusing on the deposition rate, surface roughness, crystallinity, and elemental composition to achieve high areal capacitance and low impedance. The best-performing TiC film showed an areal capacitance of 475 μF/cm2 with a capacitance retention of 93% after 5000 cycles. In addition, the electrochemical performance of the optimum film under varying scanning rates demonstrated a stable electrochemical performance even under dynamic and fast-changing stimulation conditions. Furthermore, the in vitro cell culture for 3 weeks revealed excellent biocompatibility, promoting cell growth compared with a control substrate. This work presents a novel contribution, highlighting the potential of sputtered TiC thin films as robust neural electrode interface materials.

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

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

Recent Advances and Strategies in MXene-Based Electrodes for Supercapacitors: Applications, Challenges and Future Prospects

With the growing demand for technologies to sustain high energy consumption, supercapacitors are gaining prominence as efficient energy storage solutions beyond conventional batteries. MXene-based electrodes have gained recognition as a promising material for supercapacitor applications because of their superior electrical conductivity, extensive surface area, and chemical stability. This review provides a comprehensive analysis of the recent progress and strategies in the development of MXene-based electrodes for supercapacitors. It covers various synthesis methods, characterization techniques, and performance parameters of these electrodes. The review also highlights the current challenges and limitations, including scalability and stability issues, and suggests potential solutions. The future outlooks and directions for further research in this field are also discussed, including the creation of new synthesis methods and the exploration of novel applications. The aim of the review is to offer a current and up-to-date understanding of the state-of-the-art in MXene-based electrodes for supercapacitors and to stimulate further research in the field.

Discovery of fast and stable proton storage in bulk hexagonal molybdenum oxide

Ionic and electronic transport in electrodes is crucial for electrochemical energy storage technology. To optimize the transport pathway of ions and electrons, electrode materials are minimized to nanometer-sized dimensions, leading to problems of volumetric performance, stability, cost, and pollution. Here we find that a bulk hexagonal molybdenum oxide with unconventional ion channels can store large amounts of protons at a high rate even if its particle size is tens of micrometers. The diffusion-free proton transport kinetics based on hydrogen bonding topochemistry is demonstrated in hexagonal molybdenum oxide whose proton conductivity is several orders of magnitude higher than traditional orthorhombic molybdenum oxide. In situ X-ray diffraction and theoretical calculation reveal that the structural self-optimization in the first discharge effectively promotes the reversible intercalation/de-intercalation of subsequent protons. The open crystal structure, suitable proton channels, and negligible volume strain enable rapid and stable proton transport and storage, resulting in extremely high volumetric capacitance (~1750 F cm-3), excellent rate performance, and ultralong cycle life (>10,000 cycles). The discovery of unconventional materials and mechanisms that enable proton storage of micrometer-sized particles in seconds boosts the development of fast-charging energy storage systems and high-power practical applications.

Graphene-like emerging 2D materials: recent progress, challenges and future outlook

Owing to the unique physical and chemical properties of 2D materials and the great success of graphene in various applications, the scientific community has been influenced to explore a new class of graphene-like 2D materials for next-generation technological applications. Consequently, many alternative layered and non-layered 2D materials, including h-BN, TMDs, and MXenes, have been synthesized recently for applications related to the 4th industrial revolution. In this review, recent progress in state-of-the-art research on 2D materials, including their synthesis routes, characterization and application-oriented properties, has been highlighted. The evolving applications of 2D materials in the areas of electronics, optoelectronics, spintronic devices, sensors, high-performance and transparent electrodes, energy conversion and storage, electromagnetic interference shielding, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nanocomposites are discussed. In particular, the state-of-the-art applications, challenges, and outlook of every class of 2D material are also presented as concluding remarks to guide this fast-progressing class of 2D materials beyond graphene for scientific research into next-generation materials.

An Ultra-Thin MXene Film for Multimodal Sensing of Neuroelectrical Signals with Artifacts Removal

Neuroelectrical signals transmitted onto the skin tend to decay to an extremely weak level, making them highly susceptible to interference from the environment and body movement. Meanwhile, for comprehensively understanding cognitive nerve conduction, multimodal sensing of neural signals, such as magnetic resonance imaging (MRI) and functional near-infrared spectroscopy (fNIRS), is highly required. Previous metal or polymer conductors cannot either provide a seamless on-skin feature for accurate sensing of neuroelectrical signals or be compatible with multimodal imaging techniques without opto- and magnet- artifacts. Herein, a ≈20 nm thick MXene film that is able to simultaneously detect electrophysiological signals and perform imaging by MRI and fNIRS with high fidelity is reported. The ultrathin film is made of crosslinked Ti3 C2 Tx film via poly (3,4-ethylene dioxythiophene): polystyrene sulfonate (PEDOT: PSS), showing a record high electroconductivity and transparency combination (11 000 S cm-1 @89%). Among them, PEDOT: PSS not only plays a cross-linking role to stabilize MXene film but also shortens the interlayer distance for effective charge transfer and high transparency. Thus, it can achieve a low interfacial impedance with skin or neural surfaces for accurate recording of electrophysiological signals with low motion artifacts. Besides, the high transparency originating from the ultrathin feature leads to good compatibility with fNIRS and MRI without optical and magnetic artifacts, enabling multimodal cognitive neural monitoring during prolonged use.

Are MXenes suitable for soft multifunctional composites?

MXenes are a family of two-dimensional (2D) nanomaterials known for their high electrical and thermal conductivity, as well as high aspect ratios. Recent research has focused on dispersing MXenes within compliant polymer matrices, aiming to create flexible and stretchable composites that harness MXenes' exceptional conductivity and aspect ratios. Experimental findings demonstrate the potential of MXene polymer composites (MXPCs) as flexible electrical, thermal conductors, and high dielectric materials, with promising applications in soft matter engineered systems. However, the 2D structure of MXene inclusions and their relatively large elastic modulus can impart increased stiffness to the polymer matrix, posing limitations on the mechanical flexibility of these functional materials. Here, we introduce a modeling platform to predict the mechanics and functionality of MXene elastomer composites and assess their suitability as soft multifunctional materials. Our investigation primarily focuses on understanding the influence of MXenes' size, layered structure, and percolation arrangements on the effective properties of the resulting composites. Through our model, we successfully determined the elastic modulus, thermal conductivity, and dielectric constant of MXene elastomer composites, and our results exhibit strong agreement with those obtained through finite element analysis. By utilizing this framework, we can theoretically identify the necessary microstructures of MXenes and guide the experiments, enabling the creation of MXPCs with the desired synergistic mechanical and functional properties.

Laser-Induced MXene-Functionalized Graphene Nanoarchitectonics-Based Microsupercapacitor for Health Monitoring Application

Microsupercapacitors (micro-SCs) with mechanical flexibility have the potential to complement or even replace microbatteries in the portable electronics sector, particularly for portable biomonitoring devices. The real-time biomonitoring of the human body's physical status using lightweight, flexible, and wearable micro-SCs is important to consider, but the main limitation is, however, the low energy density of micro-SCs as compared to microbatteries. Here using a temporally and spatially controlled picosecond pulsed laser, we developed high-energy-density micro-SCs integrated with a force sensing device to monitor a human body's radial artery pulses. The photochemically synthesized spherical laser-induced MXene (Ti3C2Tx)-derived oxide nanoparticles uniformly attached to laser-induced graphene (LIG) act as active electrode materials for micro-SCs. The molecular dynamics simulations and detailed spectroscopic analysis reveal the synergistic interfacial interaction mechanism of Ti-O-C covalent bonding between MXene and LIG. The incorporation of MXene nanosheets improves the graphene sheet alignment and ion transport while minimizing self-restacking. Furthermore, the micro-SCs based on a nano-MXene-LIG hybrid demonstrate high mechanical flexibility, durability, ultrahigh energy density (21.16 × 10-3 mWh cm-2), and excellent capacitance (∼100 mF cm-2 @ 10 mV s-1) with long cycle life (91% retention after 10 000 cycles). Such a single-step roll-to-roll highly reproducible manufacturing technique using a picosecond pulsed laser to induce MXene-derived spherical oxide nanoparticles (size of quantum dots) attached uniformly to laser-induced graphene for biomedical device fabrication is expected to find a wide range of applications.

Flexible Transparent Conductive Electrodes: Unveiling Growth Mechanisms, Material Dimensions, Fabrication Methods, and Design Strategies

Flexible transparent conductive electrodes (FTCEs) constitute an indispensable component in state-of-the-art electronic devices, such as wearable flexible sensors, flexible displays, artificial skin, and biomedical devices, etc. This review paper offers a comprehensive overview of the fabrication techniques, growth modes, material dimensions, design, and their impacts on FTCEs fabrication. The growth modes, such as the "Stranski-Krastanov growth," "Frank-van der Merwe growth," and "Volmer-Weber growth" modes provide flexibility in fabricating FTCEs. Application of different materials including 0D, 1D, 2D, polymer composites, conductive oxides, and hybrid materials in FTCE fabrication, emphasizing their suitability in flexible devices are discussed. This review also delves into the design strategies of FTCEs, including microgrids, nanotroughs, nanomesh, nanowires network, and "kirigami"-inspired patterns, etc. The pros and cons associated with these materials and designs are also addressed appropriately. Considerations such as trade-offs between electrical conductivity and optical transparency or "figure of merit (FoM)," "strain engineering," "work function," and "haze" are also discussed briefly. Finally, this review outlines the challenges and opportunities in the current and future development of FTCEs for flexible electronics, including the improved trade-offs between optoelectronic parameters, novel materials development, mechanical stability, reproducibility, scalability, and durability enhancement, safety, biocompatibility, etc.

An ultrasensitive photo-driven self-powered aptasensor for microcystin-RR assay based on ZnIn2S4/Ti3C2 MXenes integrated with a matching capacitor for multiple signal amplification

A photo-driven self-powered aptasensor was constructed based on a matching capacitor and the ZnIn2S4/Ti3C2 heterojunction as the photoanode and Cu2O as the photocathode in a dual-photoelectrode sensing matrix for multiple signal amplification for the ultrasensitive detection of microcystin-RR (MC-RR). The introduction of Ti3C2 MXene nanosheets on the photoanode surface can not only accelerate the transfer and separation of photoinduced electron/hole pairs, thus enhancing the output signal of the photo-driven self-powered system, but also provide a larger specific surface area for the immobilization of the bio-recognition unit aptamer. More importantly, for a portable and miniaturized device, a micro-workstation with the size of a universal serial bus (USB) disk and a novel short-circuit current access was proposed to capture the instantaneous output electrical signal for real-time data tracking. In such a way, a sensitivity of 2.7 mA pM-1 was achieved when the matching capacitor was integrated into the self-powered system, which was 22 times that without a capacitor. After the interaction between MC-RR and the corresponding aptamer, a 'signal-off' detection configuration was formed via the steric hindrance effect. Therefore, such a multiple signal amplification system realized the ultrasensitive and selective determination of MC-RR successfully. Under optimal conditions, the linear range of the self-powered aptasensor was 0.1 to 100 pM and the detection limit was 0.033 pM (S/N = 3). The aptasensor was applied to the detection of MC-RR in fish, exhibiting good reproducibility (≈3.88%), paving the way for detecting microcystins in real-life samples.

M5X4: A Family of MXenes

MXenes are two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides typically synthesized from layered MAX-phase precursors. With over 50 experimentally reported MXenes and a near-infinite number of possible chemistries, MXenes make up the fastest-growing family of 2D materials. They offer a wide range of properties, which can be altered by their chemistry (M, X) and the number of metal layers in the structure, ranging from two in M2XTx to five in M5X4Tx. Only one M5X4 MXene, Mo4VC4, has been reported. Herein, we report the synthesis and characterization of two M5AX4 mixed transition metal MAX phases, Ti2.5Ta2.5AlC4 and Ti2.675Nb2.325AlC4, and their successful topochemical transformation into Ti2.5Ta2.5C4Tx and Ti2.675Nb2.325C4Tx MXenes. The resulting MXenes were delaminated into single-layer flakes, analyzed structurally, and characterized for their thermal and optical properties. This establishes a family of M5AX4 MAX phases and their corresponding MXenes. These materials were experimentally produced based on guidance from theoretical predictions, leading to more exciting applications for MXenes.

Transparent Electronics for Wearable Electronics Application

Recent advancements in wearable electronics offer seamless integration with the human body for extracting various biophysical and biochemical information for real-time health monitoring, clinical diagnostics, and augmented reality. Enormous efforts have been dedicated to imparting stretchability/flexibility and softness to electronic devices through materials science and structural modifications that enable stable and comfortable integration of these devices with the curvilinear and soft human body. However, the optical properties of these devices are still in the early stages of consideration. By incorporating transparency, visual information from interfacing biological systems can be preserved and utilized for comprehensive clinical diagnosis with image analysis techniques. Additionally, transparency provides optical imperceptibility, alleviating reluctance to wear the device on exposed skin. This review discusses the recent advancement of transparent wearable electronics in a comprehensive way that includes materials, processing, devices, and applications. Materials for transparent wearable electronics are discussed regarding their characteristics, synthesis, and engineering strategies for property enhancements. We also examine bridging techniques for stable integration with the soft human body. Building blocks for wearable electronic systems, including sensors, energy devices, actuators, and displays, are discussed with their mechanisms and performances. Lastly, we summarize the potential applications and conclude with the remaining challenges and prospects.

Carbonized Syndiotactic Polystyrene/Carbon Nanotube/MXene Hybrid Aerogels with Egg-Box Structure: A Platform for Electromagnetic Interference Shielding and Solar Thermal Energy Management

Functional materials for electromagnetic interference (EMI) shielding are a consistently hot topic in the booming communication engineering, proceeding the development that tends to the multifunctional EMI shielding materials. Herein, a series of carbonized syndiotactic polystyrene/carbon nanotube/MXene (CsPS/CNT/MXene) hybrid aerogels were fabricated for EMI shielding and solar thermal energy conversion purposes. To fabricate the hybrid aerogels, a porous CNT/MXene framework was initially prepared using freeze-casting. Subsequently, sPS was infused into the porous structure, followed by hyper-cross-linking and carbonization of sPS under an inert atmosphere. The resulting aerogels exhibited a distinctive egg-box structure, comprising numerous nanofibrous carbon microspheres embedded within the lamellar framework. The mass ratio between CNT and MXene was regulated to identify an optimum aerogel, that is, the CCM-4-6, which exhibited impressive properties including Young's compression modulus of 0.67 MPa, a water contact angle of 137.6 ± 4.1°, a specific surface area of 110 m2 g-1, an electrical conductivity of 43.0 S m-1, and an EMI SE value of 40 dB. Meanwhile, phase-change composites were fabricated through encapsulating paraffin wax within the hybrid aerogels. For the CCM-4-6 aerogel, a noteworthy encapsulation ratio was achieved at about 76.7%, along with remarkable latent heat, good thermal reliability, and commendable solar thermal energy conversion capacity. This study presents a facile route to prepare multifunctional EMI shielding materials.

Can MXene be the Effective Nanomaterial Family for the Membrane and Adsorption Technologies to Reach a Sustainable Green World?

Environmental pollution has intensified and accelerated due to a steady increase in the number of industries, and exploring methods to remove hazardous contaminants, which can be typically divided into inorganic and organic compounds, have become inevitable. Therefore, the development of efficacious technology for the separation processes is of paramount importance to ensure the environmental remediation. Membrane and adsorption technologies garnered attention, especially with the use of novel and high performing nanomaterials, which provide a target-specific solution. Specifically, widespread use of MXene nanomaterials in membrane and adsorption technologies has emerged due to their intriguing characteristics, combined with outstanding separation performance. In this review, we demonstrated the intrinsic properties of the MXene family for several separation applications, namely, gas separation, solvent dehydration, dye removal, separation of oil-in-water emulsions, heavy metal ion removal, removal of radionuclides, desalination, and other prominent separation applications. We highlighted the recent advancements used to tune separation potential of the MXene family such as the manipulation of surface chemistry, delamination or intercalation methods, and fabrication of composite or nanocomposite materials. Moreover, we focused on the aspects of stability, fouling, regenerability, and swelling, which deserve special attention when the MXene family is implemented in membrane and adsorption-based separation applications.

Electrochemistry of Titanium Carbide MXenes in Supercapacitor

Novel electrode materials are always explored to achieve better performance of supercapacitors. Titanium carbide MXenes, Ti3 C2 Tx , are one of the very promising candidates for electrode materials in supercapacitors due to their unique structural and ion storage properties as 2D materials. Their large specific surface area, adjustable functionalized surface terminals, high electrical conductivities, hydrophilicity, and high Faradaic capacitance, also known widely but confusingly as pseudocapacitance, are highly desirable for making high-performance electrodes with increased dis-/charging rates and capacities. Herein, some selective electrochemical considerations of Ti3 C2 Tx MXenes for uses in supercapacitors are critically reviewed and assessed, aiming at a better fundamental understanding of the electrochemical basics and processes in Ti3 C2 Tx MXene-based electrode materials for supercapacitor applications.

Micro-Cup Architecture for Printing and Coating Asymmetric 2d-Material-Based Solid-State Supercapacitors

High energy density micro-supercapacitors (MSCs) are in high demand for miniaturized electronics and microsystems. Research efforts today focus on materials development, applied in the planar interdigitated, symmetric electrode architecture. A novel "cup & core" device architecture that allows for printing of asymmetric devices without the need of accurately positioning the second finger electrode here have been introduced. The bottom electrode is either produced by laser ablation of a blade-coated graphene layer or directly screen-printed with graphene inks to create grids with high aspect ratio walls forming an array of "micro-cups". A quasi-solid-state ionic liquid electrolyte is spray-deposited on the walls; the top electrode material -MXene inks- is then spray-coated to fill the cup structure. The architecture combines the advantages of interdigitated electrodes for facilitated ion-diffusion, which is critical for 2D-material-based energy storage systems by providing vertical interfaces with the layer-by-layer processing of the sandwich geometry. Compared to flat reference devices, volumetric capacitance of printed "micro-cups" MSC increased considerably, while the time constant decreased (by 58%). Importantly, the high energy density (3.99 µWh cm-2 ) of the "micro-cups" MSC is also superior to other reported MXene and graphene-based MSCs.

High-Voltage MXene-Based Supercapacitors: Present Status and Future Perspectives

As an emerging class of 2D materials, MXene exhibits broad prospects in the field of supercapacitors (SCs). However, the working voltage of MXene-based SCs is relatively limited (typically ≤ 0.6 V) due to the oxidation of MXene electrode and the decomposition of electrolyte, ultimately leading to low energy density of the device. To solve this issue, high-voltage MXene-based electrodes and corresponding matchable electrolytes are developed urgently to extend the voltage window of MXene-based SCs. Herein, a comprehensive overview and systematic discussion regarding the effects of electrolytes (aqueous, organic, and ionic liquid electrolytes), asymmetric device configuration, and material modification on the operating voltage of MXene-based SCs, is presented. A deep dive is taken into the latest advances in electrolyte design, structure regulation, and high-voltage mechanism of MXene-based SCs. Last, the future perspectives on high-voltage MXene-based SCs and their possible development directions are outlined and discussed in depth, providing new insights for the rational design and realization of advanced next-generation MXene-based electrodes and high-voltage electrolytes.

Microfluidic-Based Continuous Fabrication of Ultrathin Hydrogel Films with Controllable Thickness

Ultrathin hydrogel films composed of cross-linked polymer networks swollen by water, with soft and moisturized features similar to biological tissue, play a vital role in flexible biosensors and wearable electronics. However, achieving efficient and continuous fabrication of such films remains a challenge. Here, we present a microfluidic-based strategy for the continuous fabrication of free-standing ultrathin hydrogel films by using laminar flow, which can be precisely controlled in the micrometer scale. Compared with conventional methods, the microfluidic-based method shows advantages in producing hydrogel films with a high homogeneity as well as maintaining the structural integrity, without the need of supporting substrates and sophisticated equipment. This strategy allows the precise control over the thickness of the hydrogel films ranging from 15 ± 0.2 to 39 ± 0.5 μm, by adjusting the height of the microfluidic channels, with predictable opportunities for scaling up. Therefore, our strategy provides a facile route to produce advanced thin polymer films in a universal, steerable, and scalable manner and will promote the applications of thin polymer films in biosensors and wearable electronics.

A Study on Optimal Indium Tin Oxide Thickness as Transparent Conductive Electrodes for Near-Ultraviolet Light-Emitting Diodes

This research study thoroughly examines the optimal thickness of indium tin oxide (ITO), a transparent electrode, for near-ultraviolet (NUV) light-emitting diodes (LEDs) based on InGaN/AlGaInN materials. A range of ITO thicknesses from 30 to 170 nm is investigated, and annealing processes are performed to determine the most favorable figure of merit (FOM) by balancing transmittance and sheet resistance in the NUV region. Among the films of different thicknesses, an ITO film measuring 110 nm, annealed at 550 °C for 1 min, demonstrates the highest FOM. This film exhibits notable characteristics, including 89.0% transmittance at 385 nm, a sheet resistance of 131 Ω/□, and a contact resistance of 3.1 × 10^-3 Ω·cm². Comparing the performance of NUV LEDs using ITO films of various thicknesses (30, 50, 70, 90, 130, 150, and 170 nm), it is observed that the NUV LED employing ITO with a thickness of 110 nm achieves a maximum 48% increase in light output power at 50 mA while maintaining the same forward voltage at 20 mA.

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

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

A review on accelerated development of skin-like MXene electrodes: from experimental to machine learning

Foreshadowing future needs has catapulted the progress of skin-like electronic devices for human-machine interactions. These devices possess human skin-like properties such as stretchability, self-healability, transparency, biocompatibility, and wearability. This review highlights the recent progress in a promising material, MXenes, to realize soft, deformable, skin-like electrodes. Various structural designs, fabrication strategies, and rational guidelines adopted to realize MXene-based skin-like electrodes are outlined. We explicitly discussed machine learning-based material informatics to understand and predict the properties of MXenes. Finally, an outlook on the existing challenges and the future roadmap to realize soft skin-like MXene electrodes to facilitate technological advances in the next-generation human-machine interactions has been described.

Piezoresistive MXene/Silk fibroin nanocomposite hydrogel for accelerating bone regeneration by Re-establishing electrical microenvironment

The electrical microenvironment plays an important role in bone repair. However, the underlying mechanism by which electrical stimulation (ES) promotes bone regeneration remains unclear, limiting the design of bone microenvironment–specific electroactive materials. Herein, by simple co-incubation in aqueous suspensions at physiological temperatures, biocompatible regenerated silk fibroin (RSF) is found to assemble into nanofibrils with a β-sheet structure on MXene nanosheets, which has been reported to inhibit the restacking and oxidation of MXene. An electroactive hydrogel based on RSF and bioencapsulated MXene is thus prepared to promote efficient bone regeneration. This MXene/RSF hydrogel also acts as a piezoresistive pressure transducer, which can potentially be utilized to monitor the electrophysiological microenvironment. RNA sequencing is performed to explore the underlying mechanisms, which can activate Ca²⁺/CALM signaling in favor of the direct osteogenesis process. ES is found to facilitate indirect osteogenesis by promoting the polarization of M2 macrophages, as well as stimulating the neogenesis and migration of endotheliocytes. Consistent improvements in bone regeneration and angiogenesis are observed with MXene/RSF hydrogels under ES in vivo. Collectively, the MXene/RSF hydrogel provides a distinctive and promising strategy for promoting direct osteogenesis, regulating immune microenvironment and neovascularization under ES, leading to re-establish electrical microenvironment for bone regeneration.

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MXene nanosheets could direct the selective growth of silk nanofibrils.

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Prepared MXene/RSF hydrogel exhibited good conductivity and sensing ability.

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The electroactive hydrogel could promote osteogenic differentiation of BMSCs by activating the Ca²⁺/CALM signaling pathway.

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The conductive system created an osteoblast–macrophage–endotheliocyte virtuous circle for bone microenvironment.

Emerging MXene-Based Flexible Tactile Sensors for Health Monitoring and Haptic Perception

Due to their potential applications in physiological monitoring, diagnosis, human prosthetics, haptic perception, and human-machine interaction, flexible tactile sensors have attracted wide research interest in recent years. Thanks to the advances in material engineering, high performance flexible tactile sensors have been obtained. Among the representative pressure sensing materials, 2D layered nanomaterials have many properties that are superior to those of bulk nanomaterials and are more suitable for high performance flexible sensors. As a class of 2D inorganic compounds in materials science, MXene has excellent electrical, mechanical, and biological compatibility. MXene-based composites have proven to be promising candidates for flexible tactile sensors due to their excellent stretchability and metallic conductivity. Therefore, great efforts have been devoted to the development of MXene-based composites for flexible sensor applications. In this paper, the controllable preparation and characterization of MXene are introduced. Then, the recent progresses on fabrication strategies, operating mechanisms, and device performance of MXene composite-based flexible tactile sensors, including flexible piezoresistive sensors, capacitive sensors, piezoelectric sensors, triboelectric sensors are reviewed. After that, the applications of MXene material-based flexible electronics in human motion monitoring, healthcare, prosthetics, and artificial intelligence are discussed. Finally, the challenges and perspectives for MXene-based tactile sensors are summarized.

Cytocompatibility of Ti3C2Tx MXene with Red Blood Cells and Human Umbilical Vein Endothelial Cells and the Underlying Mechanisms

Two-dimensional (2D) nanomaterials have been widely used in biomedical applications because of their biocompatibility. Considering the high risk of exposure of the circulatory system to Ti3C2Tx, we studied the cytocompatibility of Ti3C2Tx MXene with red blood cells (RBCs) and human umbilical vein endothelial cells (HUVECs) and showed that Ti3C2Tx had excellent compatibility with the two cell lines. Ti3C2Tx at a concentration as high as 200 μg/mL caused a negligible percent hemolysis of 0.8%. By contrast, at the same treatment concentration, graphene oxide (GO) caused a high percent hemolysis of 50.8%. Scanning electron microscopy revealed that RBC structures remained intact in the Ti3C2Tx treatment group, whereas those in the GO group completely deformed, sunk, and shrunk, which resulted in the release of cell contents. This difference can be largely ascribed to the distinct surficial properties of the two nanosheets. In specific, the fully covered surface-terminating -O and -OH groups leading to Ti3C2Tx had a very hydrophilic surface, thereby hindering its penetration into the highly hydrophobic interior of the cell membrane. However, the strong direct van der Waals attractions coordinated with hydrophobic interactions between the unoxidized regions of GO and the lipid hydrophobic tails can still damage the integrity of the cell membranes. In addition, the sharp and keen-edged corners of GO may also facilitate its relatively strong cell membrane damage effects than Ti3C2Tx. Thus, the excellent cell membrane compatibility of Ti3C2Tx nanosheets and their ultraweak capacity to provoke excessive ROS generation endowed them with much better compatibility with HUVECs than GO nanosheets. These results indicate that Ti3C2Tx has much better cytocompatibility than GO and provide a valuable reference for the future biomedical applications of Ti3C2Tx.

Flexible Triboelectric Tactile Sensor Based on a Robust MXene/Leather Film for Human-Machine Interaction

With the rapid development of Internet of Things (IoT) technology in recent years, self-actuated sensor systems without an external power supply such as flexible triboelectric nanogenerator (TENG)-based strain sensors have received wide attention due to their simple structure and self-powered active sensing properties. However, to satisfy the practical applications of human wearable biointegration, flexible TENGs impose higher requirements for establishing a balance between material flexibility and good electrical properties. In this work, the strength of the MXene/substrate interface was greatly improved by utilizing leather with a unique surface structure as the substrate material, resulting in a mechanically strong and electrically conductive MXene film. Due to the natural fiber structure of the leather surface, the surface of the MXene film with a rough structure was obtained, which improved the electrical output performance of the TENG. The electrode output voltage of MXene film on leather based on single-electrode TENG can reach 199.56 V and the maximum output power density can reach 0.469 mW/cm². Combined with laser-assisted technology, the efficient array preparation of MXene and graphene was achieved and applied to various human-machine interface (HMI) applications.

Solution-Processed Flexible Transparent Electrodes for Printable Electronics

Flexible transparent electrodes (FTEs) have been widely witnessed in various printable electronic devices, especially those involving light. So far, solution processes have demonstrated increasing advantages in preparing FTEs not only in their mild operation conditions and high-throughput but also in the diversity in micropatterning conductive nanomaterials into networks. For the FTEs, both high transparency and high conductivity are desirable, which therefore create requirements for the conductive network by considering the trade-off relationship between the coverage and the micropatterns of the network. In addition, the conductive networks also affect the flexibility of FTEs due to the deformation during bending/stretching. Consequently, solution processes capable of micropatterning conductive nanomaterials including nanoparticles, nanowires/polymers, and graphene/MXene play a crucial role in determining the performance of FTEs. Here, we reviewed recent research progress on solution-processed FTEs, including the solution processes, the solution-processable conductive nanomaterials and the substrates for making FTEs, and applications of FTEs in flexible electronics. Finally, we proposed several perspective outlooks of the FTEs, which aim at not only the enhanced performance but also the performances in extreme conditions and in integration. We believe that the review would offer inspiration for developing functional FTEs.

Rational Design of Ti3C2Tx MXene Inks for Conductive, Transparent Films

Transparent conductive electrodes (TCEs) with a high figure of merit (FOM_e, defined as the ratio of transmittance to sheet resistance) are crucial for transparent electronic devices, such as touch screens, micro-supercapacitors, and transparent antennas. Two-dimensional (2D) titanium carbide (Ti₃C₂T_x), known as MXene, possesses metallic conductivity and a hydrophilic surface, suggesting dispersion stability of MXenes in aqueous media allowing the fabrication of MXene-based TCEs by solution processing. However, achieving high FOM_e MXene TCEs has been hindered mainly due to the low intrinsic conductivity caused by percolation problems. Here, we have managed to resolve these problems by (1) using large-sized Ti₃C₂T_x flakes (∼12.2 μm) to reduce interflake resistance and (2) constructing compact microstructures by blade coating. Consequently, excellent optoelectronic properties have been achieved in the blade-coated Ti₃C₂T_x films, i.e., a DC conductivity of 19 325 S cm^-1 at transmittances of 83.4% (≈6.7 nm) was obtained. We also demonstrate the applications of Ti₃C₂T_x TCEs in transparent Joule heaters and the field of supercapacitors, showing an outstanding Joule heating effect and high rate response, respectively, suggesting enormous potential applications in flexible, transparent electronic devices.

A heterostructure of a 2D bimetallic metal-organic framework assembled on an MXene for high-performance supercapacitors

Two-dimensional (2D) MXenes (transition metal carbide or carbonitride) and metal-organic frameworks (MOFs) have emerged as appealing electrode materials for supercapacitors due to the advantages of each material and a 2D structure. However, a solitary MXene or MOF suffers from either inadequate redox reactive sites or low electronic conductivity and instability. Here, NiCo-MOF/MXene heterostructures are fabricated by assembling ultrathin 2D bimetallic NiCo-MOF nanosheets on exfoliated MXene nanosheets by a simple room-temperature ultrasonic method. The 2D/2D NiCo-MOF/MXene heterostructures combine the advantages of a MOF, MXene and hierarchical structure, i.e. a large surface area, a highly electrically conductive network, rapid ion diffusion and structural stability. As a result, the optimal NiCo-MOF/M₁₀ electrode exhibits a highly improved capacitance (1176.8 F g^-1vs. 653.4 F g^-1) and cycle life (72.5% vs. 50.5%), compared with the pristine NiCo-MOF. Moreover, a two-electrode cell using NiCo-MOF/M₁₀ as the cathode shows outstanding energy storage capability. This study provides an opportunity to enhance energy storage by designing 2D heterostructures.