Near-infrared light-responsive hydrogels via peroxide-decorated MXene-initiated polymerization

Bioactive surface-functionalized MXenes for biomedicine

MXenes, with their good biocompatibility, excellent photovoltaic properties, excellent physicochemical properties, and desirable bioactivity, have broad application prospects in the field of tissue regeneration. MXenes have been used in a wide range of applications including biosensing, bioimaging, tumour/infection therapy, bone regeneration and wound repair. By applying bioactive materials to modify the surface of MXenes, a series of multifunctional MXene-based nanomaterials can be designed for different biomedical applications to achieve better therapeutic effects or more desirable biological functions. This paper reviews the existing studies on MXene-based bioactivities, surface modification strategies and biomedical applications. Finally, the challenges, trends and prospects of MXene nanomaterials are discussed. We expect that more and more well-designed MXene-based biomaterials will have a wider range of biomedical applications, thus providing favourable information for the clinical translation of nanomedicine.

Rapid Photothermal-Responsive Soft Hydrogel Actuator Contained Ti3C2Tx MXene and Laponite Clay with Enhanced Mechanical Properties

Photothermal responsive hydrogels are widely used in bionic soft actuators due to their remote-controlled capabilities and flexibility. However, their weak mechanical properties and limited responsiveness hinder their potential applications. To overcome this, we developed an innovative laponite/MXene/PNIPAm (LxMyPN) nanocomposite hydrogel that is mechanically robust and exhibits excellent photothermally responsive properties based on abundant hydrogen bonds. Notably, laponite clay is used as a co-cross-linking agent to improve the mechanical properties of LxMyPN hydrogel, while MXene nanosheets are added to promote the photothermal responsiveness. The resulting L3M0.4PN nanocomposite hydrogel exhibits enhanced mechanical properties, with a compressive strength of 0.201 MPa, a tensile strength of 90 kPa, and a fracture toughness of 27.25 kJ m-2. In addition, the L3M0.4PN hydrogel displays a deswelling ratio of 73.6% within 60 s and experiences an excellent volume shrinkage of 82.4% under light irradiation. Furthermore, hydrogel actuators with fast response behaviors are constructed and employed as grippers capable of grasping and releasing target objects. Overall, this high-strength and fast-responsive hydrogel actuator is beneficial to paving the way for remote controlled soft robots.

Conductive Peptide-Based MXene Hydrogel as a Piezoresistive Sensor

Wearable pressure sensors have become increasingly popular for personal healthcare and motion detection applications due to recent advances in materials science and functional nanomaterials. In this study, a novel composite hydrogel is presented as a sensitive piezoresistive sensor that can be utilized for various biomedical applications, such as wearable skin patches and integrated artificial skin that can measure pulse and blood pressure, as well as monitor sound as a self-powered microphone. The hydrogel is composed of self-assembled short peptides containing aromatic, positively- or negatively charged amino acids combined with 2D Ti3C2Tz MXene nanosheets. This material is low-cost, facile, reliable, and scalable for large areas while maintaining high sensitivity, a wide detection range, durability, oxidation stability, and biocompatibility. The bioinspired nanostructure, strong mechanical stability, and ease of functionalization make the assembled peptide-based composite MXene-hydrogel a promising and widely applicable material for use in bio-related wearable electronics.

A MXene Hydrogel-Based Versatile Microrobot for Controllable Water Pollution Management

The urgent demand for addressing dye contaminants in water necessitates the development of microrobots that exhibit remote navigation, rapid removal, and molecular identification capabilities. The progress of microrobot development is currently hindered by the scarcity of multifunctional materials. In this study, a plasmonic MXene hydrogel (PM-Gel) is synthesized by combining bimetallic nanocubes and Ti3C2Tx MXene through the rapid gelation of degradable alginate. The hydrogel can efficiently adsorb over 60% of dye contaminants within 2 min, ultimately achieving a removal rate of >90%. Meanwhile, the hydrogel exhibits excellent sensitivity in surface enhanced Raman scattering (SERS) detection, with a limit of detection (LOD) as low as 3.76 am. The properties of the plasmonic hydrogel can be further adjusted for various applications. As a proof-of-concept experiment, thermosensitive polymers and superparamagnetic particles are successfully integrated into this hydrogel to construct a versatile, light-responsive microrobot for dye contaminants. With magnetic and optical actuation, the robot can remotely sample, identify, and remove pollutants in maze-like channels. Moreover, light-driven hydrophilic-hydrophobic switch of the microrobots through photothermal effect can further enhance the adsorption capacity and reduced the dye residue by up to 58%. These findings indicate of a broad application potential in complex real-world environments.

Recent strategies, progress, and prospects of two-dimensional metal carbides (MXenes) materials in wastewater purification: A review

With the rapid development of industrialization, water pollution directly leads to the serious shortage of fresh water. As reported by the World Water Council, nearly 3.8 billion people will face water scarcity by 2030. Therefore, developing advanced nanomaterials to realize wastewater purification is a major challenge. Two-dimensional (2D) transition metal carbides (MXenes), as the emerging 2D layered nanomaterials, have been investigated for the applications of water purification treatment since first reported in 2011. Over 40 different MXenes have been developed for environmental remediation, and dozens more structures and properties are theoretically predicted. Here, we review the advances from the aspects of synthesis strategies for MXenes, purification mechanism, and their applications in wastewater treatment processes. The major points are 1) the synthesis and modification approaches for MXenes such as multi-layered stacked MXenes and delaminated MXenes 2) a discussion of current water remediation over MXene-based materials, 3) a brief introduction for removal behaviors and deep interaction mechanisms, 4) optimization strategies and key points for boosting the remediation performance of MXenes.

Thermo-Responsive Hydrogels Coupled with Photothermal Agents for Biomedical Applications

Intelligent hydrogels are materials with abilities to change their chemical nature or physical structure in response to external stimuli showing promising potential in multitudinous applications. Especially, photo-thermo coupled responsive hydrogels that are prepared by encapsulating photothermal agents into thermo-responsive hydrogel matrix exhibit more attractive advantages in biomedical applications owing to their spatiotemporal control and precise therapy. This work summarizes the latest progress of the photo-thermo coupled responsive hydrogel in biomedical applications. Three major elements of the photo-thermo coupled responsive hydrogel, i.e., thermo-responsive hydrogel matrix, photothermal agents, and construction methods are introduced. Furthermore, the recent developments of these hydrogels for biomedical applications are described with some selected examples. Finally, the challenges and future perspectives for photo-thermo coupled responsive hydrogels are outlined.

Swift Assembly of Adaptive Thermocell Arrays for Device-Level Healable and Energy-Autonomous Motion Sensors

The evolution of wearable technology has prompted the need for adaptive, self-healable, and energy-autonomous energy devices. This study innovatively addresses this challenge by introducing an MXene-boosted hydrogel electrolyte, which expedites the assembly process of flexible thermocell (TEC) arrays and thus circumvents the complicated fabrication of typical wearable electronics. Our findings underscore the hydrogel electrolyte's superior thermoelectrochemical performance under substantial deformations and repeated self-healing cycles. The resulting hydrogel-based TEC yields a maximum power output of 1032.1 nW under the ΔT of 20 K when being stretched to 500% for 1000 cycles, corresponding to 80% of its initial state; meanwhile, it sustains 1179.1 nW under the ΔT of 20 K even after 60 cut-healing cycles, approximately 92% of its initial state. The as-assembled TEC array exhibits device-level self-healing capability and high adaptability to human body. It is readily applied for touch-based encrypted communication where distinct voltage signals can be converted into alphabet letters; it is also employed as a self-powered sensor to in-situ monitor a variety of body motions for complex human actions. The swift assembly approach, combined with the versatile functionality of the TEC device, paves the way for future advancements in wearable electronics targeting at fitness monitoring and human-machine interfaces.

Endogenous/Exogenous Nanovaccines Synergistically Enhance Dendritic Cell-Mediated Tumor Immunotherapy

Traditional dendritic cell (DC)-mediated immunotherapy is usually suppressed by weak immunogenicity in tumors and generally leads to unsatisfactory outcomes. Synergistic exogenous/endogenous immunogenic activation can provide an alternative strategy for evoking a robust immune response by promoting DC activation. Herein, Ti3 C2 MXene-based nanoplatforms (termed MXP) are prepared with high-efficiency near-infrared photothermal conversion and immunocompetent loading capacity to form endogenous/exogenous nanovaccines. Specifically, the immunogenic cell death of tumor cells induced by the photothermal effects of the MXP can generate endogenous danger signals and antigens release to boost vaccination for DC maturation and antigen cross-presentation. In addition, MXP can deliver model antigen ovalbumin (OVA) and agonists (CpG-ODN) as an exogenous nanovaccine (MXP@OC), which further enhances DC activation. Importantly, the synergistic strategy of photothermal therapy and DC-mediated immunotherapy by MXP significantly eradicates tumors and enhances adaptive immunity. Hence, the present work provides a two-pronged strategy for improving immunogenicity and killing tumor cells to achieve a favorable outcome in tumor patients.

Dynamic and Wearable Electro-responsive Hydrogel with Robust Mechanical Properties for Drug Release

Electro-responsive dynamic hydrogels, which possess robust mechanical properties and precise spatiotemporal resolution, have a wide range of applications in biomedicine and energy science. However, it is still challenging to design and prepare electro-responsive hydrogels (ERHs) which have all of these properties. Here, we report one such class of ERHs with these features, based on the direct current voltage (DCV)-induced rearrangement of sodium dodecyl sulfate (SDS) micelles, where the rearrangement can tune the hydrogel networks that are originally maintained by the SDS micelle-assisted hydrophobic interactions. An enlarged mesh size is demonstrated for these ERHs after DCV treatment. Given the unique structure and properties of these ERHs, hydrophobic cargo (thiostrepton) has been incorporated into the hydrogels and is released upon DCV loading. Additionally, these hydrogels are highly stretchable (>6000%) and tough (507 J/m²), showing robust mechanical properties. Moreover, these hydrogels have a high spatiotemporal resolution. As the cross-links within our ERHs are enabled by the non-covalent (i.e., hydrophobic) interactions, these hydrogels are self-healing and malleable. Considering the robust mechanical properties, precise spatiotemporal resolution, dynamic nature (e.g., injectable and self-healing), and on-demand drug delivery ability, this class of ERHs will be of great interest in the fields of wearable bioelectronics and smart drug delivery systems.

A novel electrochemical biosensor based on signal amplification of Au HFGNs/PnBA-MXene nanocomposite for the detection of miRNA-122 as a biomarker of breast cancer

Cancer is one of the leading causes of death worldwide and a crisis for global health. Breast cancer is the second most common cancer globally. In the perusal, a novel electrochemical biosensor amplified with hierarchical flower-like gold, poly (n-butyl acrylate), and MXene (AuHFGNs/PnBA-MXene) nanocomposite and activated by highly special antisense ssDNA (single-stranded DNA) provide a promising alternative for miRNA-122 detection as a biomarker of breast cancer. The biosensor presented a detection limit of 0.0035 aM (S/N = 3) with a linear range from 0.01 aM to 10 nM. The platform was tried on 20 breast cancer miRNAs extracted from actual serum specimens (10 positives and 10 negatives). Founded on the quantitatively obtained outcomes and statistic analysis (t-test, box-graph, receiver performance characteristic curve, and cut-off amount), the biosensor showed a meaningful discrepancy between the native and positive groups with 100% specificity and 100% sensitivity. While, RT-qPCR showed less specificity and sensitivity (70% specificity, 100% sensitivity) than the proposed biosensor. To assess the quantitative capacity and biosensor detection limit for clinical tests, the biosensor diagnosis performance for continually diluted miRNA extracted from patients was compared to that gained by RT-qPCR results, indicating that the biosensor detection limit was lower than RT-qPCR. ssDNA/AuHFGN/PnBA-MXene/GCE displayed little cross-reaction with other sequences and also showed desirable stability, reproducibility, and specificity and stayed stable until 32 days. As a result, the designed biosensor can perform as a hopeful method for diagnosis applications.

MXene-Based Porous Monoliths

In the past decade, a thriving family of 2D nanomaterials, transition-metal carbides/nitrides (MXenes), have garnered tremendous interest due to its intriguing physical/chemical properties, structural features, and versatile functionality. Integrating these 2D nanosheets into 3D monoliths offers an exciting and powerful platform for translating their fundamental advantages into practical applications. Introducing internal pores, such as isotropic pores and aligned channels, within the monoliths can not only address the restacking of MXenes, but also afford a series of novel and, in some cases, unique structural merits to advance the utility of the MXene-based materials. Here, a brief overview of the development of MXene-based porous monoliths, in terms of the types of microstructures, is provided, focusing on the pore design and how the porous microstructure affects the application performance.

Ionic liquid decorated MXene/Poly (N-isopropylacrylamide) composite hydrogel with high strength, chemical stability and strong adsorption

Organic phenolic pollutants in industrial wastewater cause severe environmental pollution and physiological damage. Poly (N-isopropylacrylamide) (PNIPAM) hydrogels generally have poor mechanical strength and are also intrinsically frangible, limiting their widespread applications in wastewater treatment. Combining them with 2-dimensional materials can also only improve the mechanical properties of hydrogels. Here, we report a high-strength, chemical stability and strong adsorption MXene/poly (N-isopropylacrylamide) (PNIPAM) thermosensitive composite hydrogel for efficient removal of phenolic pollutants from industrial wastewater. Ionic liquids (ILs) were grafted onto the surface of MXenes and introduced into NIPAM monomer solution to obtain composite hydrogels by in-situ polymerization for improved mechanical strength and adsorption capacity of the composite hydrogel. Compared with the MXene/PNIPAM composite hydrogel, the introduction of ILs simultaneously improves the mechanical and adsorption properties of the composite hydrogel. The ILs bind to the surface of MXene flakes through electrostatic interactions, which improved the thermal stability and oxidation resistance of MXenes while maintaining its good dispersion. Using 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) modified MXene (MXene-EMIMBF4) did not change significantly were observed after aging for 45 days. As-prepared composite hydrogels demonstrated excellent mechanical properties, reusability, and high adsorption capacity for p-Nitrophenol (4-NP). The MXene-EMIMBF4/PNIPAM hydrogel could recover after ten 95% strain compression cycles under the synergistic effect of chemical bonding and electrostatic attraction. Its maximum adsorption capacity for 4-NP was 200.29 mg g^-1 at room temperature, and the adsorption capacity maintained at ∼90% of its initial value after five adsorption cycles, which was related to the introduction of EMIMBF4 to form a denser network structure. The adsorption data followed the pseudo-second-order kinetics and Freundlich models.

Recent Advances in Targeted Nanotherapies for Ischemic Stroke

Ischemic stroke (IS) is a severe neurological disease caused by the narrowing or occlusion of cerebral blood vessels and is known for high morbidity, disability, and mortality rates. Clinically available treatments of stroke include the surgical removal of the thrombus and thrombolysis with tissue fibrinogen activator. Pharmaceuticals targeting IS are uncommon, and the development of new therapies is hindered by the low bioavailability and stability of many drugs. Nanomedicine provides new opportunities for the development of novel neuroprotective and thrombolytic strategies for the diagnosis and treatment of IS. Numerous nanotherapeutics with different physicochemical properties are currently being developed to facilitate drug delivery by accumulation and controlled release and to improve their restorative properties. In this review, we discuss recent developments in IS therapy, including assisted drug delivery and targeting, neuroprotection through regulation of the neuron environment, and sources of endogenous biomimetic specific targeting. In addition, we discuss the role and neurotoxic effects of inorganic metal nanoparticles in IS therapy. This study provides a theoretical basis for the utilization of nano-IS therapies that may contribute to the development of new strategies for a range of embolic diseases.

Silver-decorated MXene nanosheets as a radical initiator for polymerization and multifunctional hydrogels

Silver nanoparticle-decorated multilayered titanium carbide MXene (Ag/Ti₃C₂T_x) itself is capable of initiating the polymerization of a variety of acrylic monomers, due to it being able to generate hydroxyl radicals via the pseudo-Fenton reaction. Furthermore, double-network hydrogel Ag/Ti₃C₂T_x@gelatin/PAAm is synthesized by a one-pot procedure and displays a good near-infrared light-triggered shape-memory performance.

Thermal Properties of MXenes and Relevant Applications

The properties and applications of MXenes (a family of layered transition metal carbides, nitrides, and carbonitrides) have aroused enormous research interests for a decade since the successful synthesis of few-layer transition metal carbides in 2011. Though MXenes, as the building blocks, have already been applied in various fields (such as wearable electronics) owing to the distinctive optical, mechanical and electrical properties, their thermal stability and intrinsic thermal properties were less thoroughly investigated compared to other characteristics in early reports. The pioneering theoretical prediction of the thermoelectric nature of MXenes was performed in 2013 while the first experiment-based report concerning the degradation behavior of the 2D structure at elevated temperatures in a controlled atmosphere was published in 2015, followed by numerous discoveries regarding the thermal properties of MXenes. Herein, after a brief description of the synthesis, this Review summarized the latest insights into the thermal stability and thermophysical properties of MXenes, and further associated these unique properties with relevant applications by multiple examples. Finally, current hurdles and challenges in this field were provided along with some advices on potential research directions in the future.

Surface Functionalized MXenes for Wastewater Treatment-A Comprehensive Review

Over 80% of wastewater worldwide is released into the environment without proper treatment. Whilst environmental pollution continues to intensify due to the increase in the number of polluting industries, conventional techniques employed to clean the environment are poorly effective and are expensive. MXenes are a new class of 2D materials that have received a lot of attention for an extensive range of applications due to their tuneable interlayer spacing and tailorable surface chemistry. Several MXene-based nanomaterials with remarkable properties have been proposed, synthesized, and used in environmental remediation applications. In this work, a comprehensive review of the state-of-the-art research progress on the promising potential of surface functionalized MXenes as photocatalysts, adsorbents, and membranes for wastewater treatment is presented. The sources, composition, and effects of wastewater on human health and the environment are displayed. Furthermore, the synthesis, surface functionalization, and characterization techniques of merit used in the study of MXenes are discussed, detailing the effects of a range of factors (e.g., PH, temperature, precursor, etc.) on the synthesis, surface functionalization, and performance of the resulting MXenes. Finally, the limits of MXenes and MXene-based materials as well as their potential future research directions, especially for wastewater treatment applications are highlighted.

Minimally invasive nanomedicine: nanotechnology in photo-/ultrasound-/radiation-/magnetism-mediated therapy and imaging

Traditional treatments such as chemotherapy and surgery usually cause severe side effects and excruciating pain. The emergence of nanomedicines and minimally invasive therapies (MITs) has brought hope to patients with malignant diseases. Especially, minimally invasive nanomedicines (MINs), which combine the advantages of nanomedicines and MITs, can effectively target pathological cells/tissues/organs to improve the bioavailability of drugs, minimize side effects and achieve painless treatment with a small incision or no incision, thereby acquiring good therapeutic effects. In this review, we provide a comprehensive review of the research status and challenges of MINs, which generally refers to the medical applications of nanotechnology in photo-/ultrasound-/radiation-/magnetism-mediated therapy and imaging. Additionally, we also discuss their combined application in various fields including cancers, cardiovascular diseases, tissue engineering, neuro-functional diseases, and infectious diseases. The prospects, and potential bench-to-bedside translation of MINs are also presented in this review. We expect that this review can inspire the broad interest for a wide range of readers working in the fields of interdisciplinary subjects including (but not limited to) chemistry, nanomedicine, bioengineering, nanotechnology, materials science, pharmacology, and biomedicine.

Functionalization of MXene-based nanomaterials for the treatment of micropollutants in aquatic system: A review

The increased industrialization and urbanization generate a larger quantity of effluent that is discharged into the environment regularly. Based on the effluent composition produced from various industries, the number of hazardous substances such as heavy metals, hydrocarbons, volatile organic compounds, organic chemicals, microorganisms introduced into the aquatic systems vary. The conventional wastewater treatment systems do not meet the effluent standards before discharge and require a different treatment system before reuse. Adsorption is an eco-friendly technique that uses selective adsorbents to remove hazardous pollutants even at microscale levels. MXene, a 2-Dimensional nanomaterial with resplendent properties like conductivity, hydrophilicity, stability, and functionalized surface characteristics, is found as a potential candidate for pollutant removal systems. This review discusses the fabrication, characterization, and application of MXene based nanoparticles to remove many pollutants in water treatment systems. The improvement in surface properties and adsorption capacity of MXene based NPs, when modified using different modification agents, has also been discussed. Their feasibility in terms of economic and environmental aspects has been evaluated to understand their scope for practical application in large-scale industries. The challenges towards the synthesis and toxicity's importance have been discussed, with the appropriate recommendations.

Visible light triggered controlled formation of rapidly self-healing hydrogels based on thiol-disulfide exchange

The properties of stimuli-responsive hydrogels can be tailored under various external stimuli, but it is difficult to realize the customized adjustment of hydrogel properties since the crosslinking degree in the gelation process is intractable. Here, a visible light triggered thiol-disulfide exchange reaction was applied for constructing disulfide-crosslinked hydrogels from P(EMA-SS-PEG), a poly(ethylene glycol) grafted poly(ethyl methacrylate) derivative with a disulfide linkage as the grafting point. This photochemical method provides mild gelation conditions to handily regulate the morphology, mechanical properties, swelling ratio, and degradation rate of hydrogels by simply varying the irradiation time. Based on this strategy, these disulfide-crosslinked hydrogel coatings showed rapid self-healing in 10 min under ambient conditions, which was dependent on the width of the scratch, temperature, and humidity. Notably, spraying water on these coatings could significantly accelerate the self-healing process of large scratches (360 μm) at room temperature with a self-healing time of 1 hour, enabling the practical application of hydrogel coatings in a natural environment.

MXene (Ti3C2Tx)-Embedded Nanocomposite Hydrogels for Biomedical Applications: A Review

Polymeric nanocomposites have been outstanding functional materials and have garnered immense attention as sustainable materials to address multi-disciplinary problems. MXenes have emerged as a newer class of 2D materials that produce metallic conductivity upon interaction with hydrophilic species, and their delamination affords monolayer nanoplatelets of a thickness of about one nm and a side size in the micrometer range. Delaminated MXene has a high aspect ratio, making it an alluring nanofiller for multifunctional polymer nanocomposites. Herein, we have classified and discussed the structure, properties and application of major polysaccharide-based electroactive hydrogels (hyaluronic acid (HA), alginate sodium (SA), chitosan (CS) and cellulose) in biomedical applications, starting with the brief historical account of MXene's development followed by successive discussions on the synthesis methods, structures and properties of nanocomposites encompassing polysaccharides and MXenes, including their biomedical applications, cytotoxicity and biocompatibility aspects. Finally, the MXenes and their utility in the biomedical arena is deliberated with an eye on potential opportunities and challenges anticipated for them in the future, thus promoting their multifaceted applications.

Preparation Strategies and Applications of MXene-Polymer Composites: A Review

As a new member of the 2D material family, MXene integrates high metallic conductivity and hydrophilic property simultaneously. It shows tremendous potential in fields of energy storage, sensing, electromagnetic shielding, and so forth. Due to the abundant surface functional groups, the physical and chemical properties of MXene can be tuned by the formation of MXene-polymer composites. The introduction of polymers can expand the interlayer spacing, reduce the distance of ion/electron transport, improve the surface hydrophilicity, and thus guide the assembly of MXene-polymer structures. Herein, the preparation strategies of MXene-polymer composites including physical mixing, surface modification, such as anchoring through TiN and Ti-O-C bonds, bonding through esterification, grafting functional groups through TiOSi/TiOP bonds, photograft reaction, as well as in situ polymerization are highlighted. In addition, the possible mechanisms for each strategy are explained. Furthermore, the applications of MXene-polymer composites obtained by different preparation strategies are summarized. Finally, perspectives and challenges are presented for the designs of MXene-polymer composites.

Functional Group Modification and Bonding Characteristics of Ti3 C2 MXene-Organic Composites from First-Principles Calculations

The unique physical structure and abundant surface functional groups of MXene make the grafted organic molecules exhibit specific electrical and optical properties. This work reports the results of first-principles calculations to investigate the composite systems formed by different organic molecular monomers, namely acrylic acid (AA), acrylamide (AM), 1-aziridineethanol (1-AD) and glucose, and Ti₃ C₂ MXene saturated with different functional groups, namely -OH, -O and -F. The results show that the interaction between organic molecules and the MXene surface depends on the type of functional groups of the organic molecules, while the strength of the interaction is determined by the type of surface functional groups and the number of hydrogen bonds. The bare Ti₃ C₂ and Ti₃ C₂ (OH)₂ can readily form strong chemical and hydrogen bonds with AA and AM molecules, leading to strong adsorption energy and a large amount of charge transfer, while the interaction between organic molecules and MXene saturated by -F or -O groups mainly exhibits physical interactions, accompanied by low adsorption energy and a small amount of charge transfer. This research provides theoretical guidance for the synthesis of high-performance MXene organic composite systems.

Application of Redox-Responsive Hydrogels Based on 2,2,6,6-Tetramethyl-1-Piperidinyloxy Methacrylate and Oligo(Ethyleneglycol) Methacrylate in Controlled Release and Catalysis

Hydrogels have reached momentum due to their potential application in a variety of fields including their ability to deliver active molecules upon application of a specific chemical or physical stimulus and to act as easily recyclable catalysts in a green chemistry approach. In this paper, we demonstrate that the same redox-responsive hydrogels based on polymer networks containing 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) stable nitroxide radicals and oligoethylene glycol methyl ether methacrylate (OEGMA) can be successfully used either for the electrochemically triggered release of aspirin or as catalysts for the oxidation of primary alcohols into aldehydes. For the first application, we take the opportunity of the positive charges present on the oxoammonium groups of oxidized TEMPO to encapsulate negatively charged aspirin molecules. The further electrochemical reduction of oxoammonium groups into nitroxide radicals triggers the release of aspirin molecules. For the second application, our hydrogels are swelled with benzylic alcohol and tert-butyl nitrite as co-catalyst and the temperature is raised to 50 °C to start the oxidation reaction. Interestingly enough, benzaldehyde is not miscible with our hydrogels and phase-separate on top of them allowing the easy recovery of the reaction product and the recyclability of the hydrogel catalyst.

MXenes: Synthesis, Optical Properties, and Applications in Ultrafast Photonics

Recently, 2D materials are in great demand for various applications such as optical devices, supercapacitors, sensors, and biomedicine. MXenes as a kind of novel 2D material have attracted considerable research interest due to their outstanding mechanical, thermal, electrical, and optical properties. Especially, the excellent nonlinear optical response enables them to be potential candidates for the applications in ultrafast photonics. Here, a review of MXenes synthesis, optical properties, and applications in ultrafast lasers is presented. First, aqueous acid etching and chemical vapor deposition methods for preparing MXenes are introduced, in which the storage stability and challenges of the existing synthesis techniques are also discussed. Then, the optical properties of MXenes are discussed specifically, including plasmonic properties, optical detection, photothermal effects, and ultrafast dynamics. Furthermore, the typical ultrafast pulsed lasers enabled by MXene-based saturable absorbers operated at different wavelength regions are summarized. Finally, a summary and outlook on the development of MXenes is presented in the perspectives section.

Ti3C2Tx MXene-Activated Fast Gelation of Stretchable and Self-Healing Hydrogels: A Molecular Approach

MXene-based hydrogels, a flourishing family of soft materials, have recently emerged as promising candidates for stretchable electronics. Despite recent progress, most works use MXenes as conductive nanofillers. Herein, by tuning the molecular interactions between MXene nanosheets and other constituents within the hydrogels, we demonstrate Ti₃C₃T_x MXene can act as a versatile cross-linker to activate the fast gelation of a wide range of hydrogels, starting from various monomer- and polymer-based precursors. The gelation behavior varies significantly across hydrogels. In general, the fast gelation mechanism is attributed to the easier generation of free radicals with the help of Ti₃C₂T_x MXene and the presence of multiscale molecular interactions between MXene and polymers. The use of MXene as a dynamic cross-linker leads to superior mechanical properties, adhesion, and self-healing ability. Owing to the inherent photothermal behavior of Ti₃C₃T_x and the heterogeneous phase-transforming features of polymers, a polymer-MXene hydrogel is demonstrated to exhibit distinctive thermosensation-based actuation upon near-infrared illumination, accompanied by rapid shape transformation.

Effective removal of U(VI) and Eu(III) by carboxyl functionalized MXene nanosheets

With the development of nuclear power, the negative environmental impact such as radioactive pollution has become an urgent issue to impede the utilization of nuclear energy. The construction of promising organic-inorganic hybrid materials is considered as an effective strategy for environmental remediation of radioactive contamination. In this work, two-dimensional transition metal carbide (MXene), an emerging inorganic layered material, has been successfully modified by carboxyl terminated aryl diazonium salt to both enhance its chelating ability to radionuclides and improve its water stability. The carboxyl functionalized Ti₃C₂T_x MXene (TCCH) shows excellent removal ability for U(VI) and Eu(III), evidenced by ultrafast adsorption kinetics (3 min), high maximum adsorption capacities (344.8 mg/g for U and 97.1 mg/g for Eu) and high removal percentage of radionuclides from artificial groundwater (> 90%). The adsorption of U(VI) and Eu(III) on TCCH are in good accord with the Langmuir adsorption isotherm model and the pseudo-second-order kinetic model. Ionic strength experiments, X-ray photoelectron spectroscopy (XPS) and Extended X-ray absorption fine structure (EXAFS) analyses were conducted to assess the detailed adsorption mechanism. The results reveal that the adsorption of U(VI) on TCCH follows an inner-sphere configuration, whereas the adsorption of Eu(III) is determined by both inner-sphere complexation and electrostatic interaction.

Ti3C2Tx MXene-Based Light-Responsive Hydrogel Composite for Bendable Bilayer Photoactuator

Soft actuators based on hydrogel materials, which can convert light energy directly into mechanical energy, are of the utmost importance, especially with enhancements in device development. However, the hunt for specific photothermal nanomaterials with distinct performance remains challenging. In this study, we successfully fabricated a bilayer hydrogel actuator consisting of an active photothermal layer from incorporated Ti₃C₂T_x MXene in poly(N-isopropylacrylamide) p(NIPAm)hydrogel structure and a passive layer from the N-(2-hydroxylethylpropyl)acrylamide (HEAA) hydrogel structure. The uniform and effective incorporation of MXene into the NIPAm hydrogel structures were characterized by a battery of techniques. The light responsive swelling properties of the MXene-embedded NIPAm-based hydrogel demonstrated fully reversible and repeatable behavior in the light on–off regime for up to ten consecutive cycles. The effect of MXene loading, the shape of the actuator, and the light source effects on the bilayer NIPAm-HEAA hydrogel structure were investigated. The bilayer hydrogel with MXene loading of 0.3% in the NIPAm hydrogel exhibited a 200% change of the bending angle in terms of its bidirectional shape/volume after 100 s exposure to white light at an intensity of 70 mW cm⁻². Additionally, the bending behavior under real sunlight was evaluated, showing the material’s potential applicability in practical environments.

A bio-inspired injectable hydrogel as a cell platform for real-time glycaemic regulation

Frequent subcutaneous insulin injection and islet transplantation are promising therapeutic options for type 1 diabetes mellitus. However, poor patient compliance, insufficient appropriate islet β cell donors and body immune rejection limit their clinical applications. The design of a platform capable of encapsulating insulin-secreting cells and achieving real-time blood glucose regulation, is a so far unmet need. Herein, inspired by the natural processes of regulating blood glucose in pancreatic islet β cells, we developed a poly(N-isopropylacrylamide-co-dextran-maleic acid-co-3-acrylamidophenylboronic acid) (P(AAPBA-Dex-NIPAM)) hydrogel as a cell platform with glucose responsiveness and thermo-responsiveness for the therapy of diabetes. This platform showed good biocompatibility against insulin-secreting cells and presented glucose-dependent insulin release behaviour. The bioinspired P(AAPBA₆-Dex-NIPAM₆₄) hydrogel had a positive effect on real-time glycaemic regulation, as observed by intraperitoneal glucose tolerance tests. The non-fasting blood glucose of diabetic rats was restored to a normal level during the period of treatment. Additionally, the inflammatory response did not occur after administration of the platform. Collectively, we expected that the bio-mimetic platform combined with an insulin-secreting capability could be a new diabetic treatment strategy.

Recent progress and advances in the environmental applications of MXene related materials

MXenes are a new type of two-dimensional (2D) transition metal carbide or carbonitride material with a 2D structure similar to graphene. The general formula of MXenes is M_n+1X_nT_x, in which M is an early transition metal element, X represents carbon, nitrogen and boron, and T is a surface oxygen-containing or fluorine-containing group. These novel 2D materials possess a unique 2D layered structure, large specific surface area, good conductivity, stability, and mechanical properties. Benefitting from these properties, MXenes have received increasing attention and emerged as new substrate materials for exploration of various applications including, energy storage and conversion, photothermal treatment, drug delivery, environmental adsorption and catalytic degradation. The progress on various applications of MXene-based materials has been reviewed; while only a few of them covered environmental remediation, surface modification of MXenes has never been highlighted. In this review, we highlight recent advances and achievements in surface modification and environmental applications (such as environmental adsorption and catalytic degradation) of MXene-based materials. The current studies on the biocompatibility and toxicity of MXenes and related materials are summarized in the following sections. The challenges and future directions of the environmental applications of MXene-based materials are also discussed and highlighted.

MXene hydrogels: fundamentals and applications

Hydrogels have recently garnered tremendous interest due to their potential application in soft electronics, human-machine interfaces, sensors, actuators, and flexible energy storage. Benefiting from their impressive combination of hydrophilicity, metallic conductivity, high aspect ratio morphology, and widely tuneable properties, when two-dimensional (2D) transition metal carbides/nitrides (MXenes) are incorporated into hydrogel systems, they offer exciting and versatile platforms for the design of MXene-based soft materials with tunable application-specific properties. The intriguing and, in some cases, unique properties of MXene hydrogels are governed by complex gel structures and gelation mechanisms, which require in-depth investigation and engineering at the nanoscale. On the other hand, the formulation of MXenes into hydrogels can significantly increase the stability of MXenes, which is often the limiting factor for many MXene-based applications. Moreover, through simple treatments, derivatives of MXene hydrogels, such as aerogels, can be obtained, further expanding their versatility. This tutorial review intends to show the enormous potential of MXene hydrogels in expanding the application range of both hydrogels and MXenes, as well as increasing the performance of MXene-based devices. We elucidate the existing structures of various MXene-containing hydrogel systems along with their gelation mechanisms and the interconnecting driving forces. We then discuss their distinctive properties stemming from the integration of MXenes into hydrogels, which have revealed an enhanced performance, compared to either MXenes or hydrogels alone, in many applications (energy storage/harvesting, biomedicine, catalysis, electromagnetic interference shielding, and sensing).