Platinum nanowires/MXene nanosheets/porous carbon ternary nanocomposites for in situ monitoring of dopamine released from neuronal cells.

Dopamine is an important neurotransmitter in the body and closely related to many neurodegenerative diseases. Therefore, the detection of dopamine is of great significance for the diagnosis and treatment of diseases, screening of drugs and unraveling of relevant pathogenic mechanisms. However, the low concentration of dopamine in the body and the complexity of the matrix make the accurate detection of dopamine challenging. Herein, an electrochemical sensor is constructed based on ternary nanocomposites consisting of one-dimensional Pt nanowires, two-dimensional MXene nanosheets, and three-dimensional porous carbon. The Pt nanowires exhibit excellent catalytic activity due to the abundant grain boundaries and highly undercoordinated atoms; MXene nanosheets not only facilitate the growth of Pt nanowires, but also enhance the electrical conductivity and hydrophilicity; and the porous carbon helps induce significant adsorption of dopamine on the electrode surface. In electrochemical tests, the ternary nanocomposite-based sensor achieves an ultra-sensitive detection of dopamine (S/N = 3) with a low limit of detection (LOD) of 28 nM, satisfactory selectivity and excellent stability. Furthermore, the sensor can be used for the detection of dopamine in serum and in situ monitoring of dopamine release from PC12 cells. Such a highly sensitive nanocomposite sensor can be exploited for in situ monitoring of important neurotransmitters at the cellular level, which is of great significance for related drug screening and mechanistic studies.

MXene nanosheet-derived N, S-codoped graphene quantum dots for ultrasensitive and selective detection of 3-nitro-l-tyrosine in human serum.

3-Nitro-l-tyrosine (3NT) is an oxidative stress metabolite associated with neurodegenerative diseases such as Parkinson's disease and rheumatoid arthritis. In this study, the N, S-co-doped graphene quantum dots (NSGQDs) derived from nitrogen-doped Ti3C2Tx MXene nanosheet via the hydrothermal method in the presence of mercaptosuccinic acid was synthesized as an optical sensing probe to detect 3NT in human serum. Tetramethyl ammonium hydroxide, the nitrogen source and delamination agent, was used to prepare nitrogen-doped MXene nanosheets via one step at room temperature. The as-prepared NSGQDs are uniform with an average size of 1.2 ± 0.6 nm, and can be stable in aqueous solution for at least 90 d to serve as the fluorescence probe. The N atoms in N-MXene reduce the restacking and aggregation of MXene nanosheets, while the sulfur dopant in NSGQDs increases the quantum yield from 6.2 to 12.1 % as well as enhances the selectivity of 3NT over the other 12 interferences via coordination interaction with nitro group in 3NT. A linear range of 0.02-150 µM in PBS and 0.05-200 µM in human serum with a recovery of 97-108 % for 3NT detection is observed. Moreover, the limit of detection can be lowered to 4.2 and 7 nM in PBS and 1 × diluted human serum, respectively. Results obtained clearly indicate the potential application of the N-Ti3C2Tx derived NSGQD for effective detection of 3NT, which can open a window for the synthesis of doped GQDs via 2D MXene materials for ultrasensitive and selective detection of other biometabolites and biomarkers of neurodegenerative diseases in biological fluids.

Flexible and Simply Degradable MXene-Methylcellulose Piezoresistive Sensor for Human Motion Detection.

Flexible pressure sensors are intensively demanded in various fields such as electronic skin, medical and health detection, wearable electronics, etc. MXene is considered an excellent sensing material due to its benign metal conductivity and adjustable interlayer distance. Exhibiting both high sensitivity and long-term stability is currently an urgent pursuit in MXene-based flexible pressure sensors. In this work, high-strength methylcellulose was introduced into the MXene film to increase the interlayer distance of 2D nanosheets and fundamentally overcome the self-stacking problem. Thus, concurrent improvement of the sensing capability and mechanical strength was obtained. By appropriately modulating the ratio of methylcellulose and MXene, the obtained pressure sensor presents a high sensitivity of 19.41 kPa-1 (0.88-24.09 kPa), good stability (10000 cycles), and complete biodegradation in H2O2 solution within 2 days. Besides, the sensor is capable of detecting a wide range of human activities (pulse, gesture, joint movement, etc.) and can precisely recognize spatial pressure distribution, which serves as a good candidate for next-generation wearable electronic devices.

Ti3C2Tx MXene and cellulose-based aerogel phase change composite decorated laminated fabric with excellent electro/solar-thermal conversion and high latent heat.

Wearable heaters have attracted growing attention for maintaining a relatively constant temperature of the human body in cold environments with near zero energy consumption. Herein, we developed a multifunctional laminated fabric with fascinating electro/solar-thermal conversion, thermal energy storage and thermal insulation properties. With cotton fabric as the substrate, MXene/polydimethylsiloxane (PDMS) conductive network was decorated on the upper layer, and carbon nanotube (CNT)/cellulose nanofiber (CNF)/paraffin (PA) aerogel phase change composites were assembled on the bottom layer. Attributed to the strong conductivity and light absorption of MXene and the light/thermal response of CNT and PA components, this wearable laminated fabric broke the limitation of intermittent solar photothermal heating, and integrated multiple heating modes to precisely heat the human body. Meanwhile, the low thermal conductivity of aerogel retarded heat loss. The laminated fabric can help people better adapt to a variety of complex and changeable environments such as cold winter, rainy days and nights. This study provides a promising and energy-efficient avenue for the development of all-day personal thermal management fabrics.

Ultrathin MXene nanosheet-based TiO2/CdS heterostructure as a photoelectrochemical sensor for detection of CEA in human serum samples.

To develop highly accurate and ultrasensitive strategies is of great importance for the clinical measurement, in particular, the detection of cancer biomarkers. Herein, we synthesized an ultrasensitive TiO2/MXene/CdS QDs (TiO2/MX/CdS) heterostructure as a photoelectrochemical immunosensor, which favors energy levels matching and fast electron transfer from CdS to TiO2 in the help of ultrathin MXene nanosheet. Dramatic photocurrent quenching can be observed upon incubation of the TiO2/MX/CdS electrode by Cu2+ solution from 96-well microplate, which caused by the formation of CuS and subsequent CuxS (x = 1, 2), reducing the absorption of light and boosting the electron-hole recombination upon irradiation. As a result, the as-prepared biosensor demonstrates a linearly increased photocurrent quenching percentage (Q%) value with CEA concentration ranging from 1 fg/mL to 10 ng/mL, as well as a low detection limit of 0.24 fg/mL. Benefit from its excellent stability, high selectivity and good reproducibility of as-prepared PEC immunosensor, we believe that this proposed strategy might provide new opportunities for clinical diagnosis of CEA and other tumor markers.

Cation-Induced Assembly of Conductive MXene Fibers for Wearable Heater, Wireless Communication, and Stem Cell Differentiation.

Emerging wearable electronics, wireless communication, and tissue engineering require the development of conductive fiber-shaped electrodes and biointerfaces. Ti3C2Tx MXene nanosheets serve as promising building block units for the construction of highly conductive fibers with integrated functionalities, yet a facile and scalable fabrication scheme is highly required. Herein, a cation-induced assembly process is developed for the scalable fabrication of conductive fibers with MXene sheaths and alginate cores (abbreviated as MXene@A). The fabrication scheme of MXene@A fibers includes the fast extrusion of alginate fibers followed by electrostatic assembly of MXene nanosheets, enabling high-speed fiber production. When multiple fabrication parameters are optimized, the MXene@A fibers exhibit a superior electrical conductivity of 1083 S cm-1, which can be integrated as Joule heaters into textiles for wearable thermal management. By triggering reversible de/hydration of alginate cores upon heating, the MXene@A fibers can be repeatedly contracted and generate large contraction stress that is >40 times higher than the ones of mammalian skeletal muscle. Furthermore, the MXene@A springs demonstrate large contraction strains up to 65.5% and are then fabricated into a reconfigurable dipole antenna to wirelessly monitor the surrounding heat sources. In the end, with the biocompatibility of MXene nanosheets, the MXene@A fibers enable the guidance of neural stem/progenitor cells differentiation and the promotion of neurite outgrowth. With a cation-induced assembly process, our multifunctional MXene@A fibers exhibit high scalability for future manufacturing and hold the prospect to inspire other applications.

Designing a Functional CNT+PB@MXene-Coated Separator for High-Capacity and Long-Life Lithium-Sulfur Batteries.

Separators, as indispensable parts of LSBs (lithium-sulfur batteries), play a cucial role in inhibiting dendrite growth and suppressing the shuttle of lithium polysulfide (LiPSs). Herein, we prepared a functional carbon nanotube (CNT) and Fe-based Prussian blue (PB)@MXene/polypropylene (PP) composite separator using a facile vacuum filtration approach. The CNTs and MXene nanosheets are excellent electronic conductors that can enhance the composite separator electrical conductivity, while Fe-based Prussian blue with a rich pore structure can effectively suppress the migration by providing physical space to anchor soluble LiPSs and retain it as cathode active material. Additionally, MXene nanosheets can be well attached to Fe-based Prussian blue by an electrostatic interaction and contribute to the physical barriers that inhibit the shuttle of long-chain soluble Li2Sn (4 ≤ n ≤ 8). When used as a lithium-sulfur (Li-S) cell membrane with a functional coating layer of CNT+PB@MXene facing the cathode side, the batteries reveal a high initial discharge capacity (1042.6 mAh g-1 at 0.2 C), outstanding rate capability (90% retention of capacity at 1.0 C) and high reversible capacity (674.1 mAh g-1 after 200 cycles at 1.0 V). Of note, separator modification is a feasible method to improve the electrochemical performance of LSBs.

Dual-Mode Flexible Sensor Based on PVDF/MXene Nanosheet/Reduced Graphene Oxide Composites for Electronic Skin.

MXene materials have the metallic conductivity of transition metal carbides. Among them, Ti3C2TX with an accordion structure has great application prospects in the field of wearable devices. However, flexible wearable electronic devices face the problem of single function in practical application. Therefore, it is particularly important to study a flexible sensor with multiple functions for electronic skin. In this work, the near-field electrohydrodynamic printing (NFEP) method was proposed to prepare the composite thin film with a micro/nanofiber structure on the flexible substrate using a solution of poly(vinylidene fluoride)/MXene nanosheet/reduced graphene oxide (PMR) nanocomposites as the printing solution. A dual-mode flexible sensor for electronic skin based on the PMR nanocomposite thin film was fabricated. The flexible sensor had the detection capability of the piezoresistive mode and the piezoelectric mode. In the piezoresistive mode, the sensitivity was 29.27 kPa-1 and the response/recovery time was 36/55 ms. In the piezoelectric mode, the sensitivity was 8.84 kPa-1 and the response time was 18.2 ms. Under the synergy of the dual modes, functions that cannot be achieved by a single mode sensor can be accomplished. In the process of detecting the pressure or deformation of the object, more information is obtained, which broadens the application range of the flexible sensor. The experimental results show that the dual-mode flexible sensor has great potential in human motion monitoring and wearable electronic device applications.

Ultraefficiently Calming Cytokine Storm Using Ti3C2T x MXene.

During the global outbreak of COVID-19 pandemic, "cytokine storm" conditions are regarded as the fatal step resulting in most mortality. Hemoperfusion is widely used to remove cytokines from the blood of severely ill patients to prevent uncontrolled inflammation induced by a cytokine storm. This article discoveres, for the first time, that 2D Ti3C2T x MXene sheet demonstrates an ultrahigh removal capability for typical cytokine interleukin-6. In particular, MXene shows a 13.4 times higher removal efficiency over traditional activated carbon absorbents. Molecular-level investigations reveal that MXene exhibits a strong chemisorption mechanism for immobilizing cytokine interleukin-6 molecules, which is different from activated carbon absorbents. MXene sheet also demonstrates excellent blood compatibility without any deleterious side influence on the composition of human blood. This work can open a new avenue to use MXene sheets as an ultraefficient hemoperfusion absorbent to eliminate the cytokine storm syndrome in treatment of severe COVID-19 patients.

Large-Area 2D-MXene Nanosheet Assemblies Using Langmuir-Schaefer Technique: Wrinkle formation.

The formation of uniform sheets of exfoliated MXene over a large area is important for improving their performance in practical applications. In this study, the Langmuir-Schaefer technique was employed to deposit uniform MXene sheets on a solid substrate and control the morphological structure over a large area. At the liquid-gas interface, MXene flakes were densely compressed into nanosheets with minimal gaps between them at 20 mN/m. Through further compression, the wrinkle morphologies of MXene sheets tend to be perpendicularly aligned to the compression direction. These wrinkle structures were also exhibited when MXene sheets were mixed in equal proportions with graphene oxide sheets. Owing to the close correlation of the morphologies of MXene films with the performance of MXene-based materials, the technique employed in this study can provide a route for applications requiring wrinkled MXene, ranging from nanoelectronic devices to energy storage materials, such as supercapacitors and battery electrodes.

Comprehensive Design of the High-Sulfur-Loading Li-S Battery Based on MXene Nanosheets.

The lithium-sulfur battery is the subject of much recent attention due to the high theoretical energy density, but practical applications are challenged by fast decay owing to polysulfide shuttle and electrode architecture degradation. A comprehensive study of the sulfur host microstructure design and the cell architecture construction based on the MXene phase (Ti3C2Tx nanosheets) is performed, aiming at realize stable cycling performance of Li-S battery with high sulfur areal loading. The interwoven KB@Ti3C2Tx composite formed by self-assembly of MXene and Ktejen black, not only provides superior conductivity and maintains the electrode integrality bearing the volume expansion/shrinkage when used as the sulfur host, but also functions as an interlayer on separator to further retard the polysulfide cross-diffusion that possibly escaped from the cathode. The KB@Ti3C2Tx interlayer is only 0.28 mg cm-2 in areal loading and 3 µm in thickness, which accounts a little contribution to the thick sulfur electrode; thus, the impacts on the energy density is minimal. By coupling the robust KB@Ti3C2Tx cathode and the effective KB@Ti3C2Tx modified separator, a stable Li-S battery with high sulfur areal loading (5.6 mg cm-2) and high areal capacity (6.4 mAh cm-2) at relatively lean electrolyte is achieved.

Bioinspired Interlocked Structure-Induced High Deformability for Two-Dimensional Titanium Carbide (MXene)/Natural Microcapsule-Based Flexible Pressure Sensors.

Achieving high deformability in response to minimal external stimulation while maximizing human-machine interactions is a considerable challenge for wearable and flexible electronics applications. Various natural materials or living organisms consisting of hierarchical or interlocked structures exhibit combinations of properties (e.g., natural elasticity and flexibility) that do not occur in conventional materials. The interlocked epidermal-dermal microbridges in human skin have excellent elastic moduli, which enhance and amplify received tactile signal transport. Herein, we use the sensing mechanisms inspired by human skin to develop Ti3C2/natural microcapsule biocomposite films that are robust and deformable by mimicking the micro/nanoscale structure of human skin-such as the hierarchy, interlocking, and patterning. The interlocked hierarchical structures can be used to create biocomposite films with excellent elastic moduli (0.73 MPa), capable of high deformability in response to various external stimuli, as verified by employing theoretical studies. The flexible sensor with a hierarchical and interlocked structure (24.63 kPa-1) achieves a 9.4-fold increase in pressure sensitivity compared to that of the planar structured Ti3C2-based flexible sensor (2.61 kPa-1). This device also exhibits a rapid response rate (14 ms) and good cycling reproducibility and stability (5000 times). In addition, the flexible pressure device can be used to detect and discriminate signals ranging from finger motion and human pulses to voice recognition.