Surface Functionalization of Ti3C2Tx MXene Nanosheets with Catechols: Implication for Colloidal Processing

Removal of nickel ions from industrial wastewater using tms-EDTA-functionalized Ti3C2Tx: Experimental and statistical physics modeling

This study investigates the surface modification of Ti3C2Tx MXene using tms-EDTA (EDTA@MXene) to develop an efficient adsorbent for divalent heavy metal cations, such as Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Zn²⁺, from contaminated water. EDTA@MXene showed significantly enhanced adsorption capacities for these ions compared to pristine MXene. Using nickel ion (Ni²⁺) as a model adsorbate, EDTA@MXene demonstrated remarkable removal efficiency, reaching a maximum adsorption capacity of 249.5 mg/g as compared to the 61.4 mg/g of pristine MXene with fast kinetics and attaining equilibrium within 30 min. The results indicated that Ni²⁺ adsorption followed a pseudo-second-order kinetic model, with equilibrium data fitting both Langmuir and Freundlich isotherm models. As the classical adsorption models remained inconclusive on the underlying adsorption mechanisms, advanced statistical physics models were subsequently applied for deeper investigation. The findings revealed that Ni²⁺ ions adsorbed onto the surface in a non-parallel orientation. The adsorption process was reversible, endothermic, and driven mainly by physical interactions, with higher temperatures favoring greater adsorption capacity. EDTA@MXene demonstrated excellent reusability, maintaining high (>80 %) regeneration efficiency after five regeneration cycles. It also exhibited a high adsorption capacity for Ni²⁺ ions from nickel electroplating wastewater, highlighting its potential for real application in the treatment of metal-contaminated industrial wastewater.

Simultaneous detection of heavy metal ions in food samples using a hypersensitive electrochemical sensor based on APTES-incubated MXene-NH2@CeFe-MOF-NH2

Heavy metal ions (HMIs) pollution has become a significant food safety concern owing to rapid urbanization and industrialization. In this study, a hypersensitive electrochemical sensor based on amino-functionalized MXene and bimetallic atom MOFs nanocomposites (MXene-NH2@CeFe-MOF-NH2) was developed and applied for the detection of HMIs. MXene-NH2@CeFe-MOF-NH2 nanocomposites were obtained by incubating MXene@CeFe-MOF prepared by hydrothermal and self-assembly methods in APTES to endow them with a large amount of surface amine functional groups. The introduction of -NH2 could lead to a coordination effect between the nanocomposites and HMIs, promoting the enrichment efficiency of HMIs on the surface of the working electrode. The sensing platform demonstrated excellent detection performance, with limit of detection (LOD) values of 0.69 nM, 0.95 nM, and 0.33 nM for Cd2+, Pb2+ and Hg2+, respectively, which were far below the maximum residue levels specified by the Chinese standards. Furthermore, the sensor was employed to analyze representative real samples (fish, whole milk, rice, and corn) to simulate an accurate analysis in complex scenarios and successfully detect the three HMIs simultaneously.

Surface Mapping of Functionalized Two-Dimensional Nanosheets: Graphene Oxide and MXene Materials

In this study, we characterized the morphology, composition, and surface properties of individual flakes of graphene oxide and Ti3C2Tx MXene chemically modified with ethylenediamine, dopamine, and (3-aminopropyl) triethoxysilane (APTES). Individual monolayers of modified Ti3C2Tx MXene and graphene oxide nanosheets were deposited using the Langmuir-Blodgett technique. We compared the chemical surface modification of these two-dimensional (2D) flakes by employing advanced atomic force microscopy (AFM) modes, including quantitative nanomechanical (QNM) mode, Kelvin-Probe force microscopy (KPFM), and Nano-IR imaging. This approach reveals the distribution of mechanical, electrical, and chemical properties on individual flakes at the nanoscale. QNM analysis confirms that the flakes exhibited full surface coverage after the chemical modification process. In modified MXene flakes, we observed a decrease in apparent elastic modulus and an increase in adhesion of up to four times after their functionalization. Nano-IR imaging demonstrates that chemical modification uniformity is highest for graphene oxide species, while the complex surface distribution was observed for dopamine-modified MXene flakes, with a difference between the inner flat surface and their edges. KPFM indicates greater uniformity of surface electrical potential in differently modified graphene oxide, while a significant increase in surface potential of MXene flakes is seen when modified with dopamine. We suggest that a combination of the added dielectric layer and different grafting densities across the flakes is responsible for the increased or changes in apparent surface potential. Overall, a combination of AFM probing modes is needed for understanding how these functionalized nanosheets can be integrated into diverse polymer matrices.

Porphyrin-Cored H-Bonded Organic Polymer Hermetically Decorated MXene toward Broadband Optical Nonlinearities

Two-dimensional MXene (Ti3C2Tx) nanosheets suffer from surface oxidation and limited optical tunability. Herein, we report an aqueous-phase functionalization strategy using porphyrin-cored dopamine-type hydrogen-bonded organic polymers (ppd-HOPs) to simultaneously etch, exfoliate, and encapsulate Ti3C2Tx. Inspired by mussel adhesion mechanisms, ppd-HOPs undergo spontaneous self-polymerization on MXene surfaces, effectively passivating terminal groups (-OH, ═O, -F) against ambient moisture/oxygen while expanding interlayer spacing. Comprehensive characterization (X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy) confirms the formation of hermetic ppd-HOP/Ti3C2Tx nanocomposites (Ti3C2Tx(Por1)y and Ti3C2Tx(Por2)y) with covalently anchored different porphyrin units (Por1 and Por2). The hybrids exhibit exceptional broadband nonlinear optical (NLO) responses spanning 400-1600 nm, attributed to synergistic effects between MXene's plasmonic absorption and porphyrin's excited-state dynamics. Structure-dependent NLO modulation is demonstrated: Ti3C2Tx(Por1)y functionalized with polymerizable ppd-HOP1 (Por1-cored dopamine-type hydrogen-bonded organic polymer) achieves a record reverse saturable absorption (RSA) coefficient (βeff = 587 ± 24.1 cm GW-1 at 800 nm), outperforming Ti3C2Tx(Por2)y (βeff = 396 ± 20.0 cm GW-1 at 650 nm). Ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy indicates that significant red-shifted and broadened linear absorption induced by extended π-conjugation in ppd-HOP1 provides more ladders for two-photon absorption processes, enabling wavelength-selective RSA behavior. This bioinspired surface engineering approach not only improves MXene's environmental stability but also pioneers a general paradigm for tailoring MXene-based optoelectronic materials through rational organic-inorganic hybridization.

MXene Surface Architectonics: Bridging Molecular Design to Multifunctional Applications

This review delves into the surface modification of MXenes, underscoring its pivotal role in improving their diverse physicochemical properties, including tailor MXenes' electrical conductivity, mechanical strength, and wettability. It outlines various surface modification strategies and principles, highlighting their contributions to performance enhancements across diverse applications, including energy storage and conversion, materials mechanics, electronic devices, biomedical sciences, environmental monitoring, and fire-resistant materials. While significant advancements have been made, the review also identifies challenges and future research directions, emphasizing the continued development of innovative materials, methods, and applications to further expand MXenes' utility and potential.

Epoxy-based vitrimeric semi-interpenetrating network/MXene nanocomposites for hydrogen gas barrier applications

Herein, we report MXene-filled epoxy-based vitrimeric nanocomposites featuring a semi-interpenetrating network (S-IPN) to develop a hydrogen gas (H2) barrier coating with self-healing characteristics for compressed H2 storage applications. The reversible epoxy network was formed by synthesizing linear epoxy chains with pendent bis-hydroxyl groups using amino diol, which were then crosslinked with 1,4-benzenediboronic acid to generate dynamic boronic ester linkages. To achieve the S-IPN-type molecular arrangement, the epoxy chains were in situ crosslinked in the presence of poly(ethylene-co-vinyl alcohol) (EVOH), giving rise to a self-healing network (EEP) with a healing efficiency of 87%. Into the S-IPN vitrimer (EEP), a 2D platelet-type nanofiller MXene was incorporated to introduce a tortuous path for H2 gas diffusion along with improved mechanical properties. The nanocomposite coating was applied to nylon 6 liner material, which is conventionally used in all-composite H2 storage vessels. The application of a 2 wt% MXene/EEP nanocomposite coating showed a permeability coefficient of 0.062 cm3 mm m-2 d-1 atm-1 exhibiting ∼96% reduction in gas permeability compared to uncoated nylon 6. The same nanocomposite exhibited a healing efficiency of 79%. Increasing the MXene loading to 10 wt% further reduced the permeability coefficient to 0.002 cm3 mm m-2 d-1 atm-1; however, the healing efficiency decreased due to restricted chain mobility. In essence, the current work highlights the potential of vitrimeric S-IPN nanocomposite coatings for H2 gas-barrier applications, enhancing safety and performance.

Evolution of Surface Chemistry in Two-Dimensional MXenes: From Mixed to Tunable Uniform Terminations

Surface chemistry of MXenes is of great interest as the terminations can define the intrinsic properties of this family of materials. The diverse and tunable terminations also distinguish MXenes from many other 2D materials. Conventional fluoride-containing reagents etching approaches resulted in MXenes with mixed fluoro-, oxo-, and hydroxyl surface groups. The relatively strong chemical bonding of MXenes' surface metal atoms with oxygen and fluorine makes post-synthetic covalent surface modifications of such MXenes unfavorable. In this minireview, we focus on the recent advances in MXenes with uniform surface terminations. Unconventional methods, including Lewis acidic molten salt etching (LAMS) and bottom-up direct synthesis, have been proven successful in producing halide-terminated MXenes. These synthetic strategies have opened new possibilities for MXenes because weaker surface chemical bonds in halide-terminated MXenes facilitate post-synthetic covalent surface modifications. Both computational and experimental results on surface termination-dependent properties are summarized and discussed. Finally, we offer our perspective on the opportunities and challenges in this exciting research field.

Advancements in MXene Composite Materials for Wearable Sensors: A Review

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

Glycine-Ti3C2Tx Hybrid Material Improves the Electrochemical Corrosion Resistance of a Water-Borne Epoxy Coating

Improving the dispersibility and compatibility of nanomaterials in water-borne epoxy resins is an important means to improve the protection ability and corrosion resistance of coatings. In this study, glycine-functionalized Ti3C2Tx (GT) was used to prepare an epoxy composite coating. The results of Fourier transform infrared spectroscopy and X-ray diffraction showed that glycine was successfully modified. The scanning electron microscopy and transmission electron microscopy results showed that the aggregation of Ti3C2Tx was alleviated. Electrochemical impedance spectroscopy test results show that, after 60 days of immersion, GT coating still shows the best protection performance, and the composite coating |Z|f = 0.01 Hz is 3 orders of magnitude higher than that of the pure epoxy coating. This is mainly because, after adding glycine, the -COOH group on the surface of glycine binds to the -OH group on the surface of Ti3C2Tx, improving the aggregation of Ti3C2Tx itself. At the same time, the -NH group of glycine can also participate in the curing reaction of epoxy resin to strengthen the bonding strength between the coating and the metal. The good dispersion of GT in epoxy resin makes it fill the pores and holes left by epoxy resin curing and strengthen the corrosion resistance. The easy availability and green properties of glycine provide a simple and environmentally friendly method for the modification of Ti3C2Tx.

A Ti3C2 MXene-integrated near-infrared-responsive multifunctional porous scaffold for infected bone defect repair

Infected bone defect repair has long been a major challenge in orthopedic surgery. Apart from bacterial contamination, excessive generation of reactive oxygen species (ROS), and lack of osteogenesis ability also threaten the defect repair process. However, few strategies have been proposed to address these issues simultaneously. Herein, we designed and fabricated a near-infrared (NIR)-responsive, hierarchically porous scaffold to address these limitations in a synergetic manner. In this design, polymethyl methacrylate (PMMA) and polyethyleneimine (PEI) were used to fabricate the porous PMMA/PEI scaffolds via the anti-solvent vapor-induced phase separation (VIPS) process. Then, Ti3C2 MXenes were anchored on the scaffolds through the dopamine-assisted co-deposition process to obtain the PMMA/PEI/polydopamine (PDA)/MXene scaffolds. Under NIR laser irradiation, the scaffolds were able to kill bacteria through the direct contact-killing and synergetic photothermal effect of Ti3C2 MXenes and PDA. Moreover, MXenes and PDA also endowed the scaffolds with excellent ROS-scavenging capacity and satisfying osteogenesis ability. Our experimental results also confirmed that the PMMA/PEI/PDA/MXene scaffolds significantly promoted new bone formation in an infected mandibular defect model. We believe that our study provides new insights into the treatment of infected bone defects.

Radiation-Induced Surface Modification of MXene with Ionic Liquid to Improve Electrochemical Properties and Chemical Stability

For the first time, an ionic liquid was grafted onto Ti3C2Tx MXene interlayers (MXene-g-IL) using a radiation technique. The IL was tightly immobilized on the surface of MXene nanosheets via chemical linkage, which exhibited excellent specific capacitance (160 F g-1 at 5 mV s-1) and improved structural stability (maintaining the sheet-like structure for 180 days). The facile, efficient, and scalable synthetic strategy derived from the radiation technique can open a new avenue for covalent functionalization of MXene-based materials and promote their further application.

Grafting Behavior of Amine Ligands for Surface Modification of MXene

Surface modification to improve the oxidation stability and dispersibility of MXene in diverse organic media is a facile strategy for broadening its application. Among the various ligands that can be grafted on the MXene surface, oleylamine (OAm), with amine functionalities, is an advantageous candidate owing to its strong interactions and commercial viability. OAms are grafted onto MXene through covalent bonds induced by nucleophilic reactions and H bonds in liquid interface reactions at room temperature. In addition, this grafting behavior of the ligand was characterized by a reduction in the slope with an increase in the ligand concentration (C_l), confirming that the OAms were grafted via Langmuir-like behavior, and the monolayer of OAms was developed via two distinct steps (I: lying-down phase; II: ordered monolayer). MXene nanosheets modified by OAm (OAm-MX) are highly dispersible in a wide range of organic solvents owing to the alkyl chain of the OAms, which induces hydrophobic properties on the surface of MXene. The OAm-MX dispersion exhibits outstanding oxidation and dispersion stability and remarkable coating performance on a wide range of substrates owing to their excellent solution processability. Therefore, this study provides fundamental insights into the adsorption behavior and interaction between amine ligands and MXene nanosheets for the surface chemistry of MXene.

A Review on MXene Synthesis, Stability, and Photocatalytic Applications

Photocatalytic water splitting, CO₂ reduction, and pollutant degradation have emerged as promising strategies to remedy the existing environmental and energy crises. However, grafting of expensive and less abundant noble-metal cocatalysts on photocatalyst materials is a mandatory practice to achieve enhanced photocatalytic performance owing to the ability of the cocatalysts to extract electrons efficiently from the photocatalyst and enable rapid/enhanced catalytic reaction. Hence, developing highly efficient, inexpensive, and noble-metal-free cocatalysts composed of earth-abundant elements is considered as a noteworthy step toward considering photocatalysis as a more economical strategy. Recently, MXenes (two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides) have shown huge potential as alternatives for noble-metal cocatalysts. MXenes have several excellent properties, including atomically thin 2D morphology, metallic electrical conductivity, hydrophilic surface, and high specific surface area. In addition, they exhibit Gibbs free energy of intermediate H atom adsorption as close to zero and less than that of a commercial Pt-based cocatalyst, a Fermi level position above the H₂ generation potential, and an excellent ability to capture and activate CO₂ molecules. Therefore, there is a growing interest in MXene-based photocatalyst materials for various photocatalytic events. In this review, we focus on the recent advances in the synthesis of MXenes with 2D and 0D morphologies, the stability of MXenes, and MXene-based photocatalysts for H₂ evolution, CO₂ reduction, and pollutant degradation. The existing challenges and the possible future directions to enhance the photocatalytic performance of MXene-based photocatalysts are also discussed.

Mildly Peeling Off and Encapsulating Large MXene Nanosheets with Rigid Biologic Fibrils for Synchronization of Solar Evaporation and Energy Harvest

Efficient and nondestructive liquid exfoliation of MXene with large lateral size has drawn growing research interest due to its outstanding properties and diverse potential applications. The conventional sonication method, though enabling a high production yield of MXene nanosheets, broke them down into submicrometric sizes or even quantum dots, and thus sacrificed their size-dependent properties, chemical stability, and wide applications. Herein, rigid biological nanofibrils in combination of mild manual shake were found to be capable of peeling off MXene nanosheets by attaching on MXene surfaces and localizing the shear force. With comparison to sonication, this efficient and nondestructive exfoliation approach produced the MXene nanosheets with the lateral size up to 4-6 μm and a comparable yield of 64% within 2 h. The resultant MXene nanosheets were encapsulated with these biological fibrils, and thus enabled super colloidal and chemical stability. A steam generation efficiency of ∼86% and a high evaporation rate of 3.3 kg m^-2 h^-1 were achieved on their aerogels under 1-Sun irradiation at ∼25 °C. An evaporation rate of 0.5 kg m^-2 h^-1 still maintained even at the atmospheric temperature of -5 °C. More importantly, an electricity generation up to ∼350 mV also accompanied this solar evaporation under equivalent 5-Sun irradiation. Thus, this fibrous strategy not only provides an efficient and nondestructive exfoliation method of MXene, but also promises synchronization of solar-thermal evaporation and energy harvest.

Covalent Surface Modification of Ti3C2Tx MXene with Chemically Active Polymeric Ligands Producing Highly Conductive and Ordered Microstructure Films

As interest continues to grow in Ti₃C₂T_x and other related MXenes, advancement in methods of manipulation of their surface functional groups beyond synthesis-based surface terminations (T_x: -F, -OH, and ═O) can provide mechanisms to enhance solution processability as well as produce improved solid-state device architectures and coatings. Here, we report a chemically important surface modification approach in which "solvent-like" polymers, polyethylene glycol carboxylic acid (PEG6-COOH), are covalently attached onto MXenes via esterification chemistry. Surface modification of Ti₃C₂T_x with PEG6-COOH with large ligand loading (up to 14% by mass) greatly enhances dispersibility in a wide range of nonpolar organic solvents (e.g., 2.88 mg/mL in chloroform) without oxidation of Ti₃C₂T_x two-dimensional flakes or changes in the structure ordering. Furthermore, cooperative interactions between polymer chains improve the nanoscale assembly of uniform microstructures of stacked MXene-PEG6 flakes into ordered thin films with excellent electrical conductivity (∼16,200 S·cm^-1). Most importantly, our covalent surface modification approach with ω-functionalized PEG6 ligands (ω-PEG6-COOH, where ω: -NH₂, -N₃, -CH═CH₂) allows for control over the degree of functionalization (incorporation of valency) of MXene. We believe that installing valency onto MXenes through short, ion conducting PEG ligands without compromising MXenes' features such as solution processability, structural stability, and electrical conductivity further enhance MXenes surface chemistry tunability and performance and widens their applications.