A comprehensive review on advanced synthesis approaches of MXenes and their dual role in photocatalytic degradation and antimicrobial applications

3.1 Hydrothermal method

The hydrothermal method is highly effective in producing MXenes on a large scale that can have various designs, addressing the dangers associated with using HF vapor where workers are concerned. Besides, it has no negative implications for the environment as well. ¹⁷ The MAX technology integrated the advanced leaching technique, which does not use fluorine hydrothermal treatment for the formation of aluminium layers. This method employs a heterogeneous reaction system in which aqueous solutions are brought to temperatures higher than the boiling point of water in an environment of high pressure in a vessel known as an autoclave containing precursors. ¹⁸ Temperature, solution pH, and pressure are used in synergy to affect the size, morphology, properties, and shape of QDs. Standard synthesis conditions include a temperature range of 100–180 °C and pH of 6–9 because the reaction time depends on these factors. It has already been pointed out that regulation of hydrothermal conditions enables the control of the material size, properties, and thickness. ¹⁹

Xue et al. demonstrated that the size, thickness, and characteristics of MXene are affected by the reaction temperature by synthesising water-soluble Ti₃C₂ MXene quantum dots (MQDs) using a hydrothermal technique. The average particle diameters and thicknesses at 100, 120, and 150 °C were 2.9, 3.7, and 6.2 nm, and 0.99, 0.91, and 0.89 nm, respectively, respectively. At 100 °C, MXene exhibited a new structure with a measurable d-spacing value. A fusion structure consisting of a TiO₂ surface and a CTi core was seen at 120 °C, while 150 °C produced an amorphous MXene due to extensive Ti etching (Figure 3a–g). ¹⁸⁷ In a one-pot process, a simple and efficient hydrothermal method was employed to synthesize MoS₂-functionalized titanium carbide materials (Ti₃C₂–MoS₂ composites). Ti₃C₂ was first prepared by etching Al atoms from Ti₃AlC₂ using hydrofluoric acid. Then, Ti₃C₂–MoS₂ composites were fabricated through the hydrothermal reaction of Ti₃C₂ with sodium molybdate and thiourea. This synthesis approach highlights the integration of MoS₂ into the MXene structure, optimizing its properties for subsequent applications. The Ti₃C₂–MoS₂ composites successfully removed the organic pesticide paraquat. ²¹ Thirumal et al. described the preparation of antimony nanoneedle decorated-MXene nanosheet hybrid materials through a hydrothermal process for the degradation of hazardous pollutants such as Methylene Blue and Rhodamine B. The preparation involved mixing 0.5 g MXene with 50 ml DI water, adding 0.25 g Sb powder and stirring at room temperature with ultrasonic for 60 min. The obtained mixture was then hydrothermally treated at 150 °C for 12 h to form the black sponge-like powder which was labeled as MX@Sb–H. ²² Finding suitable substitutes for fluoric acid in MXene production is an active area of study at the moment. Making adsorbent materials from MXene using practical and efficient current methods is critical. Chemical stability, optimal specific surface distribution, and functional group management on surfaces all need these techniques. These programs aim to tackle various environmental problems. ^{23 , 24}

Figure 3:

Morphological analysis of Hydrothermally synthesized MQDs (a) Schematic presentation of preparation of MQDs . (b, e) Microscopy pictures of MQD-100 taken using TEM, and HRTEM . (c, f ) Images of MQD-120 captured using TEM, and HRTEM, . pictures of MQD-150 taken using TEM, and HRTEM and AFM (d, g). Insets of (e)–(g) represent matching Fourier transform patterns of the MQDs. ¹⁸⁷  

3.2 Chemical vapor deposition method

Chemical vapor deposition is widely used to prepare films, heterostructures, and various functional devices, and it can be used as a method for MXene synthesis. Despite the use of CVD in synthesizing transition-metal nitrides and carbide like Mo₂N, TiC_x, and Mo₂Cnits use in the synthesis of MXenes is quite limited. This method leads to the formation of MXenes with different geometrical features, open surfaces, and edge planes reconstructed with catalytic sites. ²⁵ Wang et al. synthesized Mo₂C thin sheets using CVD, depositing targets and using radio frequency magnetrons to deposit material onto a quartz wafer. Thickness of 4.3–4.7 nm of MXene was achieved by preparing the sample at an annealing temperature of 600 °C and depositing it at a pressure of 6 × 10⁻⁴ Pa. The existence of Mo–O and C–O bonds, which fluctuate according to the target material’s oxidation state, was detected by X-ray photoelectron spectroscopy research. ²⁶ In another study, the synthesis of two-dimensional Mo₂C MXene was performed via the CVD technique. Molten copper dissolves molybdenum above 1,085 °C, which encourages the formation of α-Mo₂C crystals devoid of defects. This occurs as molecular oxygen reacts with carbon atoms released from methane decomposition at melting point. The formation of α-Mo₂C crystals devoid of defects is enhanced as the temperature rises beyond 1,085 °C, when molybdenum dissolves in molten copper. ²⁷

Gavillet et al. reported the synthesis of Ti₂CCl₂ MXene via CVD. During sythesis the gaseous reagents react on titanium surfaces to create the carpet like material (Figure 4A). When MXene carpet grows thicker, reagent gas access to the reaction zone is reduced, and growth is self-limited. The uniform MXene carpet, the development of bulges, and the transformation into spherical MXene vesicles (Figure 4B–D). After the growth stage, the vesicles broke free from the substrate, as shown in Figure 4E–G. During an extended CVD process, titanium metal was entirely transformed into vesicles. A TEM examination found that the Ti₂CCl₂ sheets extended outward from the center to form layers perpendicular to the vesicle surface. A microscope device produced multiple images of the fracture vesicle but also examined single vesicles with an instrument named FIB to find a clear space in the center. During the MXene carpet growth stage, the TiC_x nucleus below started to buckle and form small TiC_x crystallites around the vesicle center. ²⁸

Figure 4:

Schematic diagram for preparation MXenes via CVD. (A, B). An illustration of the chemical vapour deposition procedures. (B) CVD-Ti₂CCl₂ and CVD-Ti₂CCl₂ XRD patterns and Rietveld refinement outcomes. (C) A comparison is made between the Raman spectra of a typical material and CVD-Ti₂CCl₂ and CVD-Ti₂CCl₂ MXenes. (D) SEM pictures of CVD-Ti₂CCl₂ taken from the front and the side. (E) CVD-Ti₂NCl₂ elemental distribution and high-resolution HAADF images. ²⁹

Scientists have discovered that Ti₂CCl₂ MXene is most effectively formed between 850 and 950 °C. At temperatures more than 1,000 °C, TiC_x emerged as the primary byproduct. The Ti₂CCl₂ phase became apparent after 2 h of heating to 950 °C, and the finished products showed a very consistent Ti₂CCl₂ to TiC_x ratio. Here, it took up to 10 days for a response, which was 2 h longer than usual. It is fair to wonder what role, if any, kinetics or thermodynamics had in the growth of MXene. To synthesize MXene phase, reactions of TiC_x with Ti and TiCl₃ or TiCl₄ failed. After being heated to 950 °C for an extended period of time, pure MS-Ti₂CCl₂ partially transformed into TiC_x. Based on these findings, we concluded that Ti₂CCl₂ had a higher kinetic preference and was a product that could compete with TiC_x. ^{9 , 29}

3.3 Selective etching method

It is common practice to carefully remove layers from parent MAX phases while producing MXenes. ³⁰ Fluoride ion-containing acidic solutions have found widespread use as etchants for this specific application. ²⁹ A combination of powders of the MAX phase and an HF acid based on water is mixed and allowed to rest at ambient temperature for a while to produce the by-product. Consequently, the metallic connections between the MX layers are replaced by weak couplings of surface terminations such as hydroxyl, fluoride, or oxygen on the ML-MXene surface. After centrifugation and filtration separate the liquid and solid components, the mixture is washed with deionised water to maintain a pH range of 4–6. In this method, MXene is synthesised using a very small number of FL layers. A flat MXene is what we term an MXene with less than five layers. ³¹ In cases when the pH of the MXene solution lowers, such as when the pH of Ti₃C₂T_x hits about one, the flakes of MXene may become wrinkled. ^{32 , 33} Since Mo₂Ga₂C is the first step in preparing Mo₂CT_x MXenes by the removal of Ga layers, it might also serve as a source for MXenes. ³³

In contrast to MAX phases, which only have one GaA layer, Mo₂Ga₂C contains two. Zr₃C₂T_x MXene’s etching process has an advantage over conventional etching techniques as it eliminates the Al and Al₃C₃ layers. The primary building block of this MXene is Zr₃Al₃C₅. The production of MXenes is feasible by high-temperature the MAX phase’s etching. Experiments on the first nitride-based MXene synthesised using this method were conducted in 2016. The aluminium layer of Ti₄AlN₃ powder was eroded in 550 °C in a gaseous state made using a combination of molten salts that included 59 % fluoride of potassium, 29 % fluoride of lithium, and 12 % fluoride of sodium. ³⁴ To further exfoliate the powder, tetrabutylammonium hydroxide was utilised, resulting in 35]. A minimum of 20 MXenes have been produced by the selective removal of atomic layers by chemical etching after the introduction of carbide, nitride, and carbo-nitride agents. Etchants that employ a broad range of ions and those that use water-soluble salts are the two most common kinds, such as fluorine ions. ³⁶ To begin, in MAX systems, you must first separate the MXenes from the rest of the molecule by cleaving the M-A bonds. How well this method works is dependent on how long the corrosion takes and how much the liquid is stirred.

Synthesis processes have a considerable impact on Ti₃C₂T_x’s chemical composition, electrical conductivity, surface terminations, ateral dimensions, flaws, and etching efficiency. The etching process of Ti₃C₂T_x MXene has been the focus of much scientific investigation since its discovery in 2011, in particular. Fluoride etching, etching with fluoride-based salts, and etching without fluoride were among the etchants studied for the manufacture of Ti₃C₂T_x MXene. ^{37 , 38 , 39 , 40} When it comes to fluoride-based salt etching, the etchant is usually a mixture of LiF and NH₄HF₂. A synergistic combination of etching and fluoride ion intercalation is achieved by this method. It is possible to increase the spacing between MXene layers by addition of lithium ions to them, which would make it easier to transfer ions between them and make them more resistant to cycling-induced volume changes. When there are more intercalated ions, the pseudocapacitive behaviour improves, leading to increased capacitance and better energy storage capabilities. In fluoride-free procedures, etchants such aqueous acid solutions (e.g., HCl, H₂SO₄) or other chemicals (e.g., LiCl/AlCl₃ combination) may be employed to avoid the needs for critical fluoride compounds. Byproducts of these processes could be MXenes with different surface chemistry and structural characteristics than MXenes etched with fluoride. Without fluorine termination, the material may have less hydrophobic properties, slower charge-transfer kinetics, and less reactive surface.

Electrochemical characteristics of MXenes etched without fluoride is affected by various etchants. ^{41 , 42} A one-step gas-phase dry etching method that does not use solvents was proposed as “Gas-Phase Selective Etching.” This technique, as shown by Zhu et al., used halogen and hydrogen halide gases, which have a high oxidation potential, to extract A-group elements (Al, Sn, Si, etc.) from MAX phases. ⁴³

3.4 Sublimation method

Targeted experiments using molten cryolite at around 960 °C were able to remove silicon layers from Ti₃SiC₂. Sublimation of indium sheets from Ti₂InC in a vacuum at around 800 °C was also used to synthesise TiC_x. ^{59 , 60 , 61} A three-dimensional cubic structure, distinct from their two-dimensional ancestors, is imparted to the resultant carbides by treatment variables such gas composition and temperature. Transition metal carbides may only be able to preserve at temperatures below 800 °C their non-stoichiometric structure, according to their phase diagram. ⁶¹ Because MXenes have the ability to form three-dimensional structures, it is imperative that all synthesis and heat treatment procedures be carried out at temperatures lower than these limits. A plethora of methods for producing 2D metal nitrides have been developed. Synthesising 2-dimensional nitrides using the ammoniation of 2D hexagonal oxides was a previous method. This method also yields two more two-dimensional nitrides, W₂N and V₂N. ^{62 , 63}

4.1 Impact of surface modification on MXene characteristics

The total mechanical qualities are significantly impacted by the insufficient strength of the interface’s interaction that results from the incompatibility of MXenes with hydrophobic polymers. The restricted diversity of hydrophilic functional groups makes it affects the performance criteria for certain domains of use. As an example, the inefficiency of electrical energy storage may be attributed to opposing electric terminal groups (−F, –OH) that prevent the electrolyte ions from moving and the Li⁺ and Na⁺ ions from passing through. This drastically lowers the battery’s energy retention capacity. ⁶⁴ The surface modification of MXene is therefore crucial for controlling MXene interface. ⁶⁵

Surface modification of MXenes significantly enhances their properties for photocatalytic applications, particularly in the degradation of organic pollutants and hydrogen production. MXenes, a family of 2D transition metal carbides and nitrides, possess unique properties such as high electrical conductivity, large surface area, and tunable surface chemistry, which make them ideal candidates for photocatalysis. ^{66 , 67} The incorporation of functional groups like –OH, –O, and –F on MXene surfaces plays a crucial role in modulating their catalytic performance. For instance, oxygen-functionalized Ti₃C₂T_x MXene, when combined with gold nanoparticles, shows enhanced electron transfer capabilities due to electronic metal-support interaction, thereby improving catalytic activity. ⁶⁶ Similarly, the presence of hydroxyl and fluoro terminations on Ti₄N₃ nitride MXene influences its hydrogen evolution and oxygen reduction reactions, although the overall catalytic performance remains largely unaffected by the variation in surface coverage of these groups. ⁶⁸

In photocatalysis, MXenes can be used as co-catalysts to improve the efficiency of semiconductor photocatalysts by enhancing charge separation and light absorption. For instance, the formation of heterostructures with MXenes, such as ZnO/MXene hybrids, has been shown to significantly increase the photodegradation rate of organic pollutants like methylene blue due to improved charge transfer and increased surface area. ⁶⁹ Additionally, MXene-based composites, such as those combined with cellulose acetate or other semiconductors, have demonstrated enhanced photocatalytic degradation of contaminants, attributed to their ability to form Schottky junctions and heterostructures that facilitate efficient charge carrier separation. ^{70 , 71} The incorporation of MXenes in photocatalytic systems also aids in reducing the recombination of photogenerated charge carriers, thereby improving the overall photocatalytic efficiency. ⁷² Furthermore, the use of surfactants like CTAB can modulate the interface assembly between MXenes and semiconductors, preventing restacking and enhancing charge transport, which is crucial for efficient solar-to-chemical conversion. ⁷³ Overall, surface modifications of MXenes play a critical role in optimizing their photocatalytic properties, offering pathways for developing advanced materials for environmental and energy applications. ⁷⁴

4.2 Catalytic performance of MXene and its influences

In addition to its great electrical conductivity, MXenes has the desirable properties of charge separation and transfer. Photocatalytic processes may find MXenes to be an appropriate catalyst because of these properties. Some potential applications include photocatalysis for photodecomposition for hydrogen production, photodegradation for the degradation of organic contaminants and carbon dioxide reduction. Many optimisation approaches have been used to improve the effectiveness of MXenes, functional material catalysts, because to their extensive application in catalysis.

MXenes, particularly in the form of two-dimensional materials are showing great potential candidates for enhancing photocatalytic performance due to their exceptional physicochemical properties. These properties include a tunable bandgap, large surface area, and high electrical conductivity have ability to form Schottky junctions and heterostructures, which are crucial for improving charge separation and reducing electron-hole recombination rates in photocatalytic processes. ⁶⁷ The incorporation of MXenes as co-catalysts with semiconductors, such as ZnO and NiTiO₃, has been shown to significantly enhance photocatalytic efficiency. For instance, the formation of ZnO/MXene hybrids resulted in a 17.8-fold increase in the photodegradation rate of methylene blue compared to ZnO alone, attributed to the increased surface area and improved charge transfer facilitated by MXene. ⁶⁹ Similarly, the creation of Schottky heterojunctions with Ti₃C₂ MXene and NiTiO₃ nano2rods improved the degradation efficiency of dyes like malachite green by enhancing electron transport and reducing recombination. ⁷⁵ MXenes also exhibit excellent stability and functional group versatility, which are critical for long-term photocatalytic applications, such as the degradation of pharmaceuticals and organic contaminants in wastewater. ^{75 , 76} Furthermore, MXene-based photocatalytic membranes combine separation and degradation processes, offering a robust solution for wastewater treatment. ⁷⁷ The ability to engineer surface terminations and create heterostructures with MXenes further enhances their photocatalytic activity, making them suitable for applications in hydrogen evolution and CO₂ reduction. ⁷⁸ Overall, the unique properties of MXenes, including their structural stability, conductivity, and functionalization potential, make them highly effective in improving the catalytic performance of photocatalysts across various environmental and energy applications.

5.1 Cationic dyes

A lot of research has focused on MXenes’ capacity to break down dyes. The significant advancements made in degrading cationic dyes utilising MXene-based heterostructures are the primary emphasis of this section. Ti₃C₂ MXenes may reduce the strength of the –N=N-bond in azodyes and are therefore more appealing to cationic dyes due to their anionic surface charge. One common area of interest in cationic dye research is the use of MXene-based heterostructures for the photocatalytic degradation of methylene blue and rhodamine B. Using calcination, Qiaoran Liu and colleagues synthesised Ti₃C₂@TiO₂. They went one step farther by adding CdS as a third ingredient. ⁸² Both MB and RhB were completely degraded by the CdS@Ti₃C₂@TiO₂ combination. This phenomena might be explained by the charge transfer bridging function of the Ti₃C₂ MXene layers and the existence of a Schottky junction between Ti₃C₂ and TiO₂. Because this connection limits the flow of electrons back to TiO₂, charge carrier recombination is reduced. Studying how RhB dye degrades was the focus of Ding et al. ⁹¹ A ternary heterojunction catalyst made of TiO₂@Ti₃C₂/g-C₃N₄ was fabricated using g-C₃N₄. With its O/OH⁻ and TiO₂ terminals, Ti₃C₂ MXene has remarkable electron transport characteristics, and g-C₃N₄ efficiently absorbs light and helps electrons move to the conduction band. To improve the active surface area and pore size, it was suggested that n-type Schottky heterojunctions be formed by g-C₃N₄ and TiO₂, and that n–n heterojunctions be formed by TiO₂ and Ti₃C₂ MXene. Efficient separation of charge carriers was made possible by the presence of the necessary heterojunctions. The possibility of ZnO/MXene composites for the degradation of various hues has been the subject of much investigation. Hoai Ta et al. recently created and published a novel ZnO/Ti₃C₂T_x MXene material that resembles rice crust. This compound was best synthesised by calcination. According to their research, this molecule has the potential to break down 99.8 % of the dye RhB in only 70 min. ⁹² A decrease in the recombination rate and an increase in the creation of charge carriers as a result of the development of a Schottky junction are two possible explanations for the improved efficiency of ZnO/Ti₃C₂T_x. The band gap of this material is about 1.6 eV smaller than that of ZnO.

Scientists have looked at potential heterojunction-forming alternatives to MXene in an effort to improve dye degradation. Bismuth oxyhalides play a significant role in photocatalysis due to their two-dimensional plate-like shape and outstanding electrical and optical properties. However, the large energy gaps imposed by these compounds limit their possible applications. Several heterostructures based on bismuth oxyhalides have been investigated by researchers in an effort to circumvent this restriction. Photocatalytic degradation of RhB was shown by Liu et al. ⁹⁸ synthesised Ti₃C₂/BiOCl/TiO₂ heterojunctions. ⁹³ Titanium dioxide and pure bismuth oxychloride may be the only substances can absorb ultraviolet light because of their large energy gaps. The catalyst’s capacity to absorb visible light was improved by the intercalation of MXene. Active species trapping experiments proved that h⁺ and ·OH mostly enhance RhB breakdown. A bismuth oxyhalide hybrid composite, BiOBr/Ti₃C₂, was synthesised by Liu et al. ⁹⁹ using a straightforward reflux process. ⁹⁴ The efficiency of the photocatalytic reaction was assessed by measuring the rate of RhB degradation. With an efficiency of 99.8 %, our system outperformed earlier models in RhB degradation. ⁹⁵ Improved migration of photoinduced charge carriers was made possible by a clear heterojunction involving the BiOBr nanosheet and the TiO₂ nanoparticles.

The electrostatic self-assembly approach was used to create NiMnO₃/NiMn₂O₄–Ti₃C₂T_x MXene nanocomposites. The rhombohedral/cubic structure of NiMnO₃/NiMn₂O₄ in the nanocomposites was confirmed using XRD analysis. Microscopy and scanning electron microscopy demonstrate that the multilayer Ti₃C₂T_x MXene sheets are decorated with NiMnO₃/NiMn₂O₄ nanoparticles. Through FESEM analysis, we estimated the average diameter of NiMnO₃/NiMn₂O₄ nanoparticles to be 87 nm and the interlayer spacing of Ti₃C₂T_x sheets to be 172 nm. In a photo-degradation experiment using methylene blue, rhodamine B, and methyl orange as target pollutants, the NiMnO₃/NiMn₂O₄ − Ti₃C₂T_x MXene (20 wt%) exhibited the most superior performance. The MB dye was entirely destroyed in the presence of 20 wt% NiMnO₃/NiMn₂O₄ – Ti₃C₂T_x. The degradation of mixed dyes was examined in four distinct combinations: MB + MO (80 % and 92 % efficiency), MB + RhB + MO (75 %, 78 %, and 96 % efficiency), and MB + RhB + MO + MO. The efficiency degradation was achieved with MB + RhB at 90 % and 97 %, and MO⁺RhB at 60 % and 72 %. The participation of active radicals ·O₂⁻ and ·OH in the photocatalytic process was further confirmed by the scavenger test. Additional verification of the radical participation was supplied by the deterioration process. Ti₃C₂T_x (20 wt%) combined with NiMnO₃ and NiMn₂O₄. Despite five cycles, the nanocomposite demonstrated adequate repeatability. ⁹⁶

Spinel ferrites are crucial in catalysis due to their exceptional electrical characteristics, biocompatibility, and durability. The “M” stands for divalent metal ions like Ni²⁺, Co²⁺, Cu²⁺, Zn²⁺, or Fe²⁺. To study MB deterioration, researchers have used certain nanohybrids, such as CuFeO₃/Ti₃C₂ and NiFe₂O₄/Ti₃C₂. A Schottky heterojunction, which forms between Ti₃C₂ and ferrites, may be responsible for the enhanced degrading efficiency. As a result of the Schottky barrier preventing the reverse flow of h⁺, h⁺ accumulates in MXene. As a means of reducing RhB and separating Cr(VI) , ^{97 , 98 , 99} A ternary system including lamellar α-Fe₂O₃/ZnFe₂O₄@Ti₃C₂ was created by Zhang et al. The photocatalytic performance of 10 wt% α-Fe₂O₃/ZnFe₂O₄@Ti₃C₂ was greatly enhanced by the excellent dispersion of these materials on the Ti₃C₂ MXene surface. Robust electrical contacts were observed at the interface of Ti₃C₂ MXene and α-Fe₂O₃/ZnFe₂O₄, which improved efficiency by separating photogenerated charge carriers.

The creation of a two-dimensional photocatalyst based on MXene using in situ polymerisation. Figure 5a shows the synthesis of Ti₂CT_x MXene, which was achieved by etching Ti₂AlC with HF and then functionalising it with glycidyl methacrylate. Afterwards, a conducting polymer thin film with a micrometer-scale thickness was created by combining Ti₂C@PGMA with PEDOT: PSS, which stands for poly(3,4-ethylenedioxythiophene). The resulting film was Ti₂C@GMA/PEDOT: PSS. By incorporating the conducting polymer into the MXene substrate, the photocatalytic efficiency of the system for degrading dyes was enhanced. Extensive testing using BET, FTIR, XRD, SEM, EDX, and XPS confirmed its good qualities. In 60 min under visible light, the photocatalyst degraded 10 mL of 10 mg/L dye solutions with 94.9 % efficiency for Rhodamine B and 98.6 % efficiency for Congo Red. Acidic conditions were optimal for degradation, with the cationic dye Rhodamine B outperforming the anionic dye Congo Red. The influence of dye concentration and solution pH on photocatalytic degradation was methodically examined, highlighting the superior efficacy of Ti₂CT_x MXene-based materials in wastewater treatment (Figure 5b). This work illustrated their considerable potential as efficient photocatalysts powered by visible light for reducing environmental pollution. ¹⁰⁰

Figure 5:

The creation of a two-dimensional MXene based photocatalyst via in situ polymerisation and its photocatalytic action (a) A schematic illustration of the synthesis approach for Ti₂C-based composites, designed to enhance photocatalytic activity by the amalgamation of glycidyl methacrylate (GMA) with poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS). (b) Proposed mechanism for the photocatalytic degradation of dye by the Ti₂C@GMA/PEDOT: PSS composite under ultraviolet light exposure. ¹⁰⁰

The degradation of organic dyes, such methylene blue and rhodamine B, was achieved by Kumar et al. by a photocatalytic method employing Titanium Carbide MXene, a new kind of two-dimensional transition metal carbide. Titanium Aluminium Carbide in its bulk phase was exfoliated to produce Ti₃C₂T_x MXene, a layered structure with hexagonal lattice symmetry. The transition from the bulk MAX phase, which showed a (002) plane at 9.2°, to Ti₃C₂T_x MXene, which showed a (002) plane at 8.88°, is further supported by the XRD spectrum. This confirms that the Al layer was effectively removed from the MAX phase. The TEM scans showed that the Ti₃C₂T_x nanosheets were thin, sheet-shaped, and transparent, with dimensions between 70 and 150 nm. After being treated with powerful hydrofluoric acid, the thin sheets did not exhibit any holes or faults, which confirmed that the sheets were really stacked Ti₃C₂T_x. When it comes to breaking down MB, RhB, and mixtures of the two, the manufactured Ti₃C₂T_x MXene shows remarkable photocatalytic effectiveness. In comparison to RhB dye’s 98.9 % degradation rate after 30 min, MB dye’s efficiency was 99.32 %. After 45 min, the MB mixture had a degradation efficiency of 98.9 % and the RhB mixture of 99.75 %, according to the experiments conducted to evaluate the degradation of a combination of MB and RhB dyes under UV light. It was found that the reaction rate constant (k) for MB, RhB, and their combinations was 0.0215 min⁻¹, for RhB it was 0.0058 min⁻¹, for one mixture it was 0.0020 min⁻¹, and for another mixture it was 0.009 min⁻¹, respectively. ¹⁰¹

5.2 Anionic dyes

MXene-based materials have shown significant promise in the degradation of anionic dyes through various mechanisms, including photocatalytic degradation and adsorption. The MXene/ZnS/chitosan-cellulose composite (MX/ZnS/CC) exemplifies this by utilizing a Schottky heterostructure to enhance photocarrier separation and improve photocatalytic performance, achieving a synergistic removal capacity of up to 5.63 g/g for anionic dyes like methyl orange through electrostatic interactions and radical generation. ¹⁰² Similarly, Ti₃C₂T_x MXene decorated with NiMnO₃/NiMn₂O₄ nanoparticles has demonstrated effective photocatalytic degradation of mixed dyes, including anionic dyes, with degradation efficiencies of 72 % for MO, facilitated by the transfer of photo-generated electrons and the involvement of reactive species such as ·OH and ·O₂⁻ radicals. ¹⁰³ The incorporation of Mn₂O₃ nanorods into Ti₃C₂T_x MXene sheets also enhances photocatalytic activity, achieving 82 % degradation efficiency for MO, with hydroxyl and superoxide radicals playing a crucial role in the process. ¹⁰⁴ Furthermore, MXene membranes modified with chloride salts, such as MgCl₂, have shown improved separation performance for anionic dyes like congo red, achieving nearly 100 % rejection due to enhanced interactions between the dye molecules and the membrane. ¹⁰⁵ The use of MXene in combination with ultrafiltration systems has also been explored, where MXene-UF systems demonstrated high selectivity and retention for anionic dyes through electrostatic interactions, although performance varied with pH and the presence of background ions. ¹⁰⁶ These studies collectively highlight the versatility and effectiveness of MXene-based materials in the degradation and removal of anionic dyes from aqueous solutions, leveraging their unique structural and chemical properties to enhance catalytic and separation processes.

The removal of anionic dyes was achieved using a composite material consisting of MXene, ZnS, and chitosan-cellulose material, which worked in tandem with photocatalytic degradation and adsorption. To improve the separation efficiency of photocarriers and photocatalytic performance, MXene was used as a cocatalyst to build a Schottky heterostructure with ZnS. In addition to its primary function as a dye adsorbent, the chitosan-cellulose composite material contributed to the enhancement of material stability and the production of free radicals, which facilitated the breakdown of the dye. The MX/ZnS/CC composite’s chemical and physical characteristics were examined in depth using a battery of tests. The composite of MX, ZnS, and CC showed strong adsorption capabilities for anionic dyes, with a capacity of 1.29 g/g, and great photodegradation and adsorption synergy, with a combined removal capacity of 5.63 g/g. Compared to pure MXene, ZnS, chitosan-cellulose material, and MXene/ZnS, the optical and electrical characteristics of the MX/ZnS/CC composite were superior, and the composite exhibited a stronger synergistic removal ability (Figure 6a–e). The dye synergistic elimination percentage improved by up to 309 % after compounding. The MX/ZnS/CC composite is primarily used to remove anionic dyes from water, with a 100 % success rate at 50 mg/L dye, by adsorbing them via electrostatic interactions and then catalyzing the formation of ·O₂⁻, h⁺, and ·OH to breakdown the dyes. ¹⁰²

Figure 6:

Comparison of dye removal efficiency between MXene, ZnS, chitosan-cellulose material, and the MXene/ZnS composite.(a)–(c) Dye removal capability and C/C0 curves for MX/ZnS/CC composite under various pH settings, as measured by adsorption and photocatalytic degradation. (d) The dye adsorption capability on the MX/ZnS/CC composite over a range of pH values. (e) The zeta potential of varied pH settings for the MX/ZnS/CC composite. ¹⁰²

5.3 MXene’s photocatalytic capabilities to filter wastewater and water for organic pollutants

Dyeing is a common practice in many sectors due to the wide variety of items that may be coloured using these potent chemical ingredients. Because of how commonplace they are, synthetic dyes are always in demand. Potential technical and industrial users include the food processing, paper & pulp, leather, textile, and cosmetics industries. Some hues are known to be carcinogenic, poisonous, and even irritating to the eyes and skin. ^{107 , 108} More and more research suggests that some synthetic colours might cause allergic responses and perhaps cancer. Before synthetic dyes were invented, natural colours were widely employed; these pigments came from plants, animals, and minerals. Excessive colouration in water reduces the amount of light that can be used for effective photosynthesis, which in turn hinders the process. ^{109 , 110 , 111}

To create Ti₃C₂ nanosheets, Miao et al. used HF etching to remove the Al layer from Ti₃AlC₂, a parent material. ¹¹² Making a TiO₂/Ti₃C₂T_x nanocomposite is the next step after synthesising Ti₃C₂ nanosheets. Using the in situ solvothermal approach, a very efficient TiO₂/Ti₃C₂T_x photocatalyst was synthesised. Under illumination of 300 W Hg, the synthesised photocatalyst removes 90 % of BPA in under 2 h. The degrading efficiency of the synthesised photocatalyst is greatly affected by two main factors: the introduction of oxygen vacancies and the existence of active hydroxyl and superoxide radicals in the interlayer of the TiO₂/Ti₃C₂T_x photocatalyst. Adding oxygen vacancies to the TiO₂/Ti₃C₂T_x material significantly improved its photocatalytic performance. However, when oxygen vacancies were present, the Schottky barrier was lower.

To create the TiO₂/g-C₃N₄ photocatalyst, Wu et al. used MXene Ti₃C₂ and using in situ calcination as a single step. ¹¹³ Transmission electron microscopy, X-ray diffraction, Brunauer-Emmett-Teller surface area analysis, X-ray photoelectron spectroscopy, and other characterisation techniques have been used to study the TiO₂/g-C₃N₄ photocatalyst. Using these methods, we investigated the photocatalyst’s morphology, chemical make-up, and photoelectrochemical capabilities. The photodegradation efficiency of PBA is outperformed by graphene, pure TiO₂, and g-C₃N₄ by the newly produced TiO₂/g-C₃N₄ photocatalyst. The exceptional performance of the MXene-based photocatalyst is due to its large surface area (26.41 m² g⁻¹) and small pore volume (0.135 cm³ g⁻¹). In addition, the photocatalyst’s surface-attached functional groups make photodegradation of TiO₂/g-C₃N₄ easier. Growing pores provide more surface area and volume, which in turn creates more photon-absorbing zones.

Grzegórska et al. demonstrated the first synthesis of a hybrid photocatalyst combining Zn/Ti layered double hydroxide with MXene – Ti₃C₂. This catalyst was then used to photocatalytically degrade acetaminophen and ibuprofen, two drugs that are ubiquitous in nature and have a tendency to accumulate in aquatic ecosystems. Researchers looked examined how the photocatalytic activity of an LDH/MXene composite changed with different concentrations of MXene (0.5 wt%, 2.5 wt%, and 5 wt%). In terms of photocatalytic activity, the LDH/MXene composite with 2.5 wt% MXene degraded acetaminophen (100 % within 40 min) and ibuprofen (99.7 %) within 60 min. The mineralization of acetaminophen and ibuprofen was also shown to be improved for the composite material. The removal efficiency of both medicines was unaffected when interfering ions (Na⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻) were introduced to the model seawater. The produced composite material was proven to have high stability and reusability by the photocatalytic experiment conducted in the four consecutive cycles and by FTIR, TEM, and XPS studies conducted after the photodegradation process. The purpose of doing the trapping experiment was to determine how different reactive oxidizing species affected the photocatalytic process. It was shown that ·O₂⁻ was mostly responsible for the photocatalytic breakdown of acetaminophen, while ·OH and h⁺ primarily impacted the breakdown of ibuprofen. The potential mechanism for pharmaceutical degradation was lastly suggested based on the outcomes of the Mott-Schottky analysis, bandgap calculation, and ROS trapping experiment. This study shows that LDH/MXene composites may be used to treat medicines, which is both unique and feasible. It suggests that MXene is heavily involved in electron-hole separation, leading to substantial photocatalytic activity ¹¹⁴ (Figure 7).

Figure 7:

Efficiency of acetaminophen degradation (C/C₀) (a), TOC reduction (%) for ACT (b), efficiency of ibuprofen degradation (C/C₀) (c) TOC reduction (%) for IBP (d) Zn/Ti LDH and Zn/Ti LDH-Ti₃C₂ photocatalysts. ¹¹⁴

5.4 Degradation of heavy metal pollutants

An elemental class with a wide range of chemical and biological impacts is heavy metals. Heavy metals are recognised as environmental toxins due to the harm they due to many living organisms, including people, animals, and plants. The poisoning of soil with heavy metals may be attributed to both natural processes and human activity. Anthropogenic activities, including farming, mining, and manufacturing, have significantly increased the concentrations of heavy metals in terrestrial ecosystems. Because of their innate tenacity, heavy metals tend to build up in plants and soil. ¹¹⁵ Because they impede gas exchange, nutrient absorption, and photosynthesis, toxic metals stunt plant development, dry matter buildup, and productivity. A nanocomposite based on Bi₂MoO₆ and Ti₃C₂ was synthesised on MXene by Zhao et al. using a one-step hydrothermal procedure. ¹¹⁶ A photocatalytic material that can degrade Cr⁶⁺ pollutants into less dangerous forms was produced by this synthesis. These contaminants are often present in water and wastewater. The exceptional photocatalytic activity of the Bi₂MoO₆/Ti₃C₂ nanocomposite was shown by means of several characterisation methods, such as TEM, HRTEM, SEM, XRD, EDS, BET, PL, EPR, DRS,and EIS in comparison to pure Bi₂MoO₆. Total photodegradation efficacy is shown by the synthetic MXene photocatalyst after 60 min of exposure to visible light. Research shows that compared to pure Bi₂MoO₆, the photodegradation efficacy of the Bi₂MoO₆/Ti₃C₂ photocatalyst is 11.2 times higher. This finding opens up new avenues for researching cost-effective and efficient photocatalysts. Sun et al. used electrostatic self-assembly in 2021 to create a novel photocatalyst based on MXene. ¹¹⁷

A new composite membrane made of MXene and poly-melamine-formaldehyde was described by Zhang et al. This membrane uses PMF particles as spacers, and its –NH₂ groups and hydroxyl groups work together to enhance pollutant adsorption. Glutaraldehyde crosslinking prevents the membrane from enlarging. With an 83.7 % PMF particle loading, the MXene/PMF composite membrane shows an outstanding capacity to adsorb water and a water permeability of 381.2 L m⁻² h⁻¹ bar⁻¹, which is 405 % higher than the MXene membrane. Compared to MXene membranes, the removal rates of Zn²⁺, Pb²⁺, phenol, and crystal violet in static adsorption are 20–100 percent higher, reaching 96.2 %, 91.7 %, 99.1 %, and 96.4 % correspondingly. The membrane reaches a breakthrough volume of 75 mL (about 8,500 times the membrane volume) for a 2 ppm p-nitrophenol solution and 350 mL (approximately 39,800 times the membrane volume) for methyl blue solution in dynamic adsorption, while the saturation values are 1,500 mL and 5,000 mL, respectively. Even after four cycles of adsorption and desorption, the membrane clearance rate remains above 90 %. The composite membrane developed in this study is very effective at filtering out contaminants from wastewater ¹¹⁸ (Figure 8).

Figure 8:

SEM pictures of MAX’s dense layered structure (a) and Ti₃C₂T_x powder’s accordion-like structure (b). Image depicting the distribution of MXene nanosheets in a composite membrane with PMF-0.2 (d) and the Tyndall effect (c). SEM pictures (e) and atomic force microscopy (f) of the MXene/PMF-0.2 composite membrane’s surface. Cross-sectional scanning electron micrographs of composite membranes made of MXene and PMF-0.2 (g). Membrane XRD patterns of MAX, MXene, and MXene/PMF-0.2 composites (h). TGA and DSC analysis of MXene/PMF-0.2 composite membranes in a nitrogen environment, starting at ambient temperature and increasing to 800 °C at a rate of 10 °C per minute (i). ¹¹⁸

Parasnis et al. presented a hybrid microfiltration method for the effective remediation of aqueous solutions containing high quantities of lead (Pb(II)) in their research. The technique made use of dry mycelium membranes that were covered with two-dimensional layers of Ti₃C₂T_x-MXene. An electrochemical deposition approach was used to cover the individual hyphal fibres of the prefabricated mycelium membrane with a uniform coating of Ti₃C₂T_x-MXene, allowing for the fabrication of these hybrid Ti₃C₂T_x-MXene/mycelium membranes. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to identify the best ECD settings for increasing Pb(II) uptake. Using immersion-based Pb(II) remediation studies (no flow), MyMX membranes showed fast sorption kinetics over an initial Pb(II) concentration range of 60–1,500 ppm in both single-ion and co-ion solutions, and an outstanding Pb(II) removal efficiency (>87 % to 99 %). As shown by infrared and X-ray photoelectron spectroscopies, the improved Pb(II) sorption was ascribed to electrostatic interactions and surface complexation aided by hyphal surface proteins and Ti₃C₂T_x-MXene functional groups. When compared to other microporous membranes used for heavy metal remediation, MyMX membranes outperformed them with a Pb(II) sorption capacity of around 1,347 mg/g and a high permeation rate of 51,800 L m⁻² bar⁻¹ h⁻¹ at 1,500 ppm Pb(II). One major step forward in water purification technology was the hybrid MyMX membrane, which included biomaterials ¹¹⁹ (Figure 9).

Figure 9:

Mechanism of lead removel. ¹¹⁹

Table 1 displays the new nanocomposites made of MXene that are being developed to degrade organic contaminants and heavy metals.

6.1 Pure MXene

Pure MXene coatings have the potential to be antimicrobial. A Ti₃C₂T_x coating was created by Huang et al. using anodic electrophoretic deposition on top of untreated titanium. ¹⁴¹ According to the findings of the experiments, the coating has strong bonding strength and is very hydrophilic, making it difficult for Staphylococcus aureus and MRSA to attach and producing less effective biomembranes. Hence, it effectively shields the area around the implant from infection. In their study, Rasool and colleagues evaluated the effectiveness of Ti₃C₂T_x on polyvinylidene fluoride membranes for wastewater treatment. ¹⁴² The results showed that Ti₃C₂T_x significantly improved the membranes’ ability to draw water. In addition, the growth of Escherichia coli was inhibited by 67 % and Bacillus subtilis by 73 % when exposed to the Ti₃C₂T_x/PVDF membranes.

The permeate that passed through the Ti₃C₂T_x/PVDF membranes showed no signs of bacterial growth. Environmental factors also have a significant role in determining membrane efficiency. Older membranes were more effective than new ones in suppressing E. coli and B. subtilis by more than 99 %. This finding paves the way for the development of antimicrobial membranes that are very efficient in treating wastewater. For their study, Diedkova et al. coated polycaprolactone nanofibers with MXene. ¹⁴³ Studying bacterial adhesion and biofilm formation was the focus of the project. According to the findings, bacterial adhesion to PCL membranes was reduced, and E. coli and Pseudomonas aeruginosa growth was prevented by the MXene coatings. By incorporating Ti₃C₂T_x into polyetheretherketone, Du et al. improved orthopedic implants’ osseointegration and antibacterial capabilities. ¹⁴⁴ Thanks to the inclusion of Ti₃C₂T_x, the composites may now be used for photothermal sterilization and in vivo osseointegration. When subjected to near-infrared light, they achieve a sterilization rate of 100 %. Incorporating Nb₂C onto titanium plates allowed Yang et al. to synthesize Nb₂C@TP. ¹⁴⁵ In addition to destroying preexisting biofilms, the results showed that Nb₂C@TP inhibited bacterial motility via thermal damage when subjected to near-infrared light. Because of this, biofilms were unable to develop and spread, leading to a 99 % reduction in microbes. At the same time as it stimulates angiogenesis and tissue regeneration, Nb₂C@TP may reduce the production of reactive oxygen species and excessive inflammatory responses, providing a possible remedy for infections caused by medical implants. To that end, MXene has great potential as a material with biological uses.In conclusion, coatings or films made of pure MXene are possible to produce. Bioimplant materials may be effectively infused with it due to its exceptional hydrophilicity, which boosts their antibacterial capabilities via preventing the attachment of bacteria and the formation of biofilms. As a result, MXene is considered a promising biomedical material.

Ti₃C₂ flakes can attach to or insert into bacterial cells due to their sharp edges. Upon exposure to NIR light (808 nm), Ti₃C₂ flakes absorb the irradiation energy, significantly increasing their temperature. This high temperature accelerates the destruction of bacterial structures, leading to cell death (Figure 10a). To study the thermal transfer from Ti₃C₂ to surrounding bacteria (E. coli and S. aureus), transient absorption was measured. The pump light was set at 780 nm (the Ti₃C₂ absorption peak), creating a non-equilibrium excited state, followed by a probe light that recorded the decay rates. The results showed that S. aureus slowed down Ti₃C₂’s excitation dynamics, implying that bacteria could absorb thermal energy directly from Ti₃C₂ flakes under light irradiation (Figure 10b). SEM images indicated that bacteria without treatment or with 20-min light exposure alone remained intact. After exposure to Ti₃C₂ alone for 20 min, some bacterial cells contacted Ti₃C₂ flakes, but without light, the bacteria remained unharmed. However, when Ti₃C₂ was combined with light exposure, bacterial cell structures were destroyed (Figure 10c). TEM images clearly showed Ti₃C₂ flakes in contact with bacterial cells, with destruction of the bacterial outer layers and some flakes inserted into the bacterial cell. This demonstrated that Ti₃C₂ flakes could penetrate and disrupt bacterial cell structures upon light exposure (Figure 10d). The GSH oxidation assay showed no significant GSH loss after 20-min exposure to Ti₃C₂ and/or light, suggesting no substantial generation of superoxide radicals (O₂²⁻), ruling out oxidative stress as the primary antibacterial mechanism (Figure 10e). ¹⁴⁶

Figure 10:

Antibacterial properties of MXene under illumination. (a) Schematic representation of the antibacterial mechanisms of MXene under illumination. (b) Pump-probe spectroscopy of a Ti₃C₂ and Staphylococcus aureus combination, as well as Ti₃C₂ in aqueous solution. Scanning electron microscopy pictures of S. aureus subjected to different treatment settings. (c) The green tint denotes MXene, whereas the blue color signifies S. aureus (d) Transmission electron microscopy picture of S. aureus after exposure to Ti₃C₂ under illumination. (e) The arrows indicate the obliterated outer membrane of the bacterial cell (red arrow) and the region where Ti₃C₂ penetrates or interacts with the cell (blue arrow). (f) GSH depletion assay under diverse circumstances. ¹⁴⁶

6.2 Metal-doped MXene

Nanoparticles made of metals and metal oxides, such as zinc (Zn), copper (Cu), and silver (Ag), have antibacterial capabilities that are second to none and have several uses. According to research, ^{147 , 148} oxidative stress, protein malfunction, and membrane degradation are the mechanisms by which metal and metal oxide nanoparticles kill bacteria.

Silver nanoparticles are very effective against a wide range of germs. Unfortunately, the environmental instability of Ag⁺ and the cytotoxic effects of excess silver, which may accumulate in living organisms, limit its use in biomedical settings. ^{149 , 150} Therefore, the quest for a suitable medium to control the release of Ag⁺ ions while reducing their toxicity has been ongoing. In their 2023 study, Qin et al. produced MTX/Ag by first intercalating MXene into montmorillonite using ultrasonic waves and simultaneously reducing and immobilizing silver nanoparticles. This structure successfully immobilizes Ag and inhibits the harmful reaction caused by Ag leakage.

Furthermore, regulated release of Ag is achieved by the photothermal effect of near-infrared stimulation on mesoscale Xene. Thanks to the production of reactive oxygen species, photothermal treatment, and the presence of Ag⁺ ions, MTX/Ag exhibits excellent and long-lasting antibacterial action. With MTX/Ag, the inhibition rate is more than 99 %. In addition, when subjected to high humidity, the control group showed signs of mold development on the third day, whereas the MTX/Ag-NIR group showed no signs of mold infection. The filter paper, which came into direct contact with the wood powder, remained clear of infection due to the contamination of silver. This finding has uncovered a new path for creating wood anti-mold coatings. As a PTT substance, Ag₂S is quite effective. Unfortunately, the photocatalytic activity is hindered by its narrow-forbidden band of 0.9 eV, which accelerates the combination of photogenerated electrons and holes in Ag₂S. This limits its usefulness in biomedical settings. Wu et al. synthesized Ag₂S/Ti₃C₂ and developed a material named Ti₃C₂T_x to tackle this difficulty. ¹⁵¹ Ti₃C₂ enhanced the composite’s ability to separate the charge carriers created by light, leading to enhanced photocatalytic activity and the production of reactive oxygen species. A remarkable inhibition rate of 99.99 % was achieved by combining photothermal treatment and photodynamic therapy using Ag₂S/Ti₃C₂. This significantly improved the healing of infected wounds in mice.

Ti₃C₂ and Au NPs were used as the building components for self-assembling biomaterials. After describing the materials, Ti₃C₂/Au NPs were tested for their antibacterial and anti-biofilm capabilities against Staphylococcus aureus (S. aureus). The groups treated with light had fewer bacterial colonies than those without light, with the Ti₃C₂/Au NPs + Light group having the smallest number of colonies (Figure 11A and B). Hemolysin production was assessed using the media from lysed rabbit red blood cells (Figure 11C and D). While antibiotic treatments focus on bacterial infections, resistance often develops due to factors such as antibiotic resistance genes and adaptive metabolic activity. Crystal violet staining revealed that Ti₃C₂/Au NPs reduced biofilm formation by 85 % compared to other groups (Figure 11E and F), showing superior antibacterial properties. Live/dead staining confirmed increased bacterial death in the Ti₃C₂/Au NPs + Light group (Figure 11G and H). SEM analysis revealed that most bacteria were dead, with cell walls showing damage in the Ti₃C₂/Au NPs + Light group (Figure 11I), while other groups exhibited normal bacterial morphology. ¹⁵² The Ti₃C₂/Au NPs self-delivery system presented in this study may provide novel materials and procedures for anti-infectious wound therapies.

Figure 11:

The addition of various therapies causes Staphylococcus aureus to enter various stages. (A–B) Photographs of the colonies and statistical data. (C–D) Hemostasin activity and statistical data shown in a photograph. (E–F) statistical data and digital pictures of biofilm development. (G–H) Here are several scanning electron micrographs of S. aureus. ¹⁵²

Classical gold nanoparticles have no antibacterial effects. On the other hand, ultra-small gold nanoclusters (AuNCs) with core sizes less than 2 nm may penetrate bacteria and cause ROS buildup. This has potent antibacterial effects because it interferes with the metabolism of the bacteria, leading to their eventual demise. ¹⁵³ In their work, Zheng et al. achieved a synergistic antibacterial effect with an antibacterial rate exceeding 98 % by effectively attaching AuNCs to the surface of MXene nanosheets. ¹⁵⁴ Microscopically thin MXene nanosheets may cross bacterial cell walls. In addition, crumpled MXene-AuNCs structures were effectively used to prevent biofilm growth. Crumpled structures are less likely to attract bacteria due to their hydrophobic properties, and they may contain a higher concentration of biocides, making them more effective in killing germs.

Cuprous oxide is an inexpensive biocide with several applications in antimicrobials. However, severe photocorrosion results from an overabundance of photogenerated electrons and holes inside Cu₂O crystals, which shortens the period of Cu₂O-induced reactive oxygen species production. Also, the Cu²⁺ that is released breaks down the structure of the Cu₂O semiconductor, which means that Cu₂O can’t make ROS anymore. Therefore, the Cu₂O sterilization system must carefully include effective catalysts. The Cu₂O/MXene nanosheets were created by Wang et al. by combining the conductivity of MXene with the semiconductor properties of Cu₂O. ¹⁵⁵ According to the results, the localized surface plasmon resonance (LSPR) phenomenon allows MXene to enhance the efficiency of electron-hole pair separation in Cu₂O, as well as to generate more reactive oxygen species and a stronger electric field strength |E|, both of which contribute to the efficient elimination of bacteria. Cu and Cu²⁺ may also damage bacteria by denaturing their DNA, a known bacterial killer. A bacterial suppression effectiveness of 95.59 % against S. aureus and 97.04 % against P. aeruginosa was achieved by combining Cu with MXene, which had a synergistic influence. These values exceeded what could be accomplished with only MXene and Cu₂O.

When exposed to near-infrared light, light-responsive materials rapidly destroy bacteria by producing reactive oxygen species and thermotherapy. However, starting these effects with visible light would be more practical and economical, especially for treating wounds. ¹⁵⁶ Because of their fast transfer of photogenerated carriers, low inclination to form electron-hole complexes, and great capacity to absorb visible light, porphyrins are often used as photosensitizers or photocatalysts. Using the hydrothermal method, Cheng et al. synthesized ZnTCPP/Ti₃C₂T_x. ¹⁵⁷ Reactive oxygen species were synthesized by the ZnTCPP/Ti₃C₂T_x group when visible light was applied, while the Ti₃C₂T_x group produced very little ROS. Antimicrobial investigations in vitro showed that bacterial viability was unaffected by darkness. The ZnTCPP/Ti₃C₂T_x combination showed a 99.86 % inhibition rate against S. aureus and a 99.92 % inhibition rate against E. coli after being exposed to visible light for 10 min. In comparison to the Ti₃C₂T_x group, these rates were much greater. In addition, the results showed that the enhanced photocatalytic activity of ZnTCPP/Ti₃C₂T_x may hasten wound healing.

Reactive oxygen species are difficult for MXene to scavenge and cannot remove an excess of ROS close to the lesion. Reactive oxygen species may impede wound healing and cause pain for patients when they pile up, damaging oxidation and severe inflammatory responses. To overcome this drawback, Zheng et al. synthesized injectable hydrogel scaffolds with various functions by combining chemically cross-linked hydrogels with anti-inflammatory nanoparticles CeO₂ and Ti₃C₂T_x. ¹⁵⁸ By lowering oxidative stress and providing oxygen, FOM significantly protected L929 cells inside the wound microenvironment, according to in vitro experiments. In addition, FOM showed outstanding antibacterial properties, completely halting the development of E. coli, S. aureus, and MRSA.

Additionally, a mouse model infected with MRSA was used to evaluate the wound healing and bactericidal effects of FOM. FOM is essential in killing MRSA bacteria, promoting cell proliferation, increasing the number of new blood vessel formations, and helping epithelial tissue renewal. Wounds infected with MRSA may also heal much more quickly after using FOM.

6.3 Polymer-modified MXene

Polymers are modified to overcome the material’s restrictions, neutralize the nanoparticles’ surface charge, and stop them from clumping together. ¹⁵⁹ Limitations on MXene’s photothermal effects and practical applications are caused by its intrinsic tendency to self-aggregate and form protein corona under physiological settings. Hydrogels MX-CS were synthesized by Dong et al. by mixing a Ti₃C₂T_x suspension with a CS solution that was acidic. ¹⁶⁰ This boosted photothermal impact and elevated anti-MRSA activity (>99 %), causing MRSA cells and MXene to aggregate at high temperatures inside the hydrogel. Scattered MXene@PDA particles improved the scaffolds’ electrical conductivity and thermal resistance. ¹⁶¹ The HPEM scaffolds may significantly reduce the time it takes for wounds to heal and for skin to regenerate after an infection with MRSA. This is accomplished using their potent anti-infective capabilities (they have a 99.03 % success rate in deactivating MRSA), which they do by encouraging cell proliferation, the production of new blood vessels, and granulation tissue. A bio-ink was made by Nie et al. by combining Ti₃C₂T_x, GelMA, Alg, and β-TCP along with sodium alginate. ¹⁶² Afterward, composite hydrogel scaffolds were made using 3D printing technology. Scaffolds treated with near-infrared light inhibited bacterial growth and killed any bacteria that had previously adhered to them. Its efficacy in treating sick mandible bone defects was shown by the bactericidal rate, which reached 98 %. As they broke down and released MXene into the environment, the scaffolds maintained their anti-infective properties.

Drawing inspiration from muscles and ligaments’ flexible and robust features, Li et al. used targeted freezing-assisted salting to develop MXene@PVA hydrogels. ¹⁶³ These hydrogels were remarkable for several reasons, including their high water content, extensive swelling capability, and powerful water attraction. They were also biocompatible, had great mechanical qualities, and converted light into heat efficiently. According to the controlled laboratory antimicrobial tests, the MXene-PVA hydrogel effectively prevented S. aureus and E. coli growth by 95.5 % and 98.3 %, respectively. Because MXene is electrically conductive, the hydrogel may set up a system for cellular communication. Cell growth is stimulated, gene expression is upregulated, and electrical impulses are amplified by this network. So, it significantly speeds up the healing process of infected wounds. Within an interpenetrating polymer network composed of GA-modified collagen and poly (acrylic acid), Zhang et al. integrated Ti₃C₂T_x. Next, they used in situ polymerization to create GCol-MX-PAA multifunctional hydrogels; their goal was to make the hydrogels more biocompatible and enhance their adhesive capabilities. ¹⁶⁴ The hydrogel underwent modifications to improve its mechanical properties, biocompatibility, and adhesion. Furthermore, PTT’s synergistic effect demonstrated its exceptional antibacterial efficacy, with a bacterial suppression rate of 95 %.

Although MXene is quickly expelled from wounds, it may cause further damage when it comes into contact with tissues and stays there as nanosheets. ¹⁶⁵ To reduce the likelihood of MXene nanosheets coming into direct touch with tissue, nanofibers may be embedded with MXene. A flexible bandage material was created by Mayerberger et al. by modifying electrostatically spun CS nanofibers with Ti₃C₂T_x. ¹⁶⁶ This dressing effectively reduces E. coli bacteria by 95 % by physically damaging their membranes. Furthermore, it is an excellent choice for wound dressings.

Infectious germs and oxidative stress from reactive oxygen species slow wound healing, seriously threatening human health. By working together, Riaz and colleagues synthesized LPFEG-Mxene, a multipurpose chiral supramolecular composite hydrogel system, by combining Mxene with LPFEG. ¹⁶⁷ The hydrogel system showed remarkable photothermal antioxidant efficacy against E. coli, P. aeruginosa, and S. aureus, in addition to its broad ROS scavenging capabilities. Its intense photothermal antibacterial action further enhanced its potential for use in antimicrobial coatings and the cure of infected wounds. A hydrogel was created by Li et al. to heal diabetic lesions. ¹⁶⁸ Encapsulated with Ti₃C₂T_x and oxyhemoglobin/hydrogen, the hydrogel was made by mixing hyaluronic acid-graft-dopamine and polydopamine. Near-infrared light may be converted into heat by MXene. Eliminating microorganisms and reactive oxygen species helps keep cellular redox equilibrium and oxidative stress to a minimum. In addition to improving MXene’s antibacterial and anti-oxidant properties, covering it with PDA makes it easier for the nanosheets to attach to the hydrogel. The dressing can efficiently provide oxygen since HbO₂ is also present. By eliminating reactive oxygen species, eliminating bacteria, and promoting the growth of new blood vessels (angiogenesis), oxygenation significantly sped up the healing process of the infected diabetic lesion.

Zeng et al. fabricated MXene-PDA, which is a self-assembled layer of polydopamine on top of MXene nanosheets, and MXene-PEIS, which is a zwitterionic polymer functionalized with PEIS, by adding PEIS to the PDA layer via the Michael addition process. By combining the antifouling effects of the zwitterionic polymer PEIS with the MXene nanosheets, the as-prepared MXene-PEIS shows remarkable antifouling performance. The antibacterial rate was enhanced by 88 %, and the microalgae adhesion density was reduced by 98 %. Figure 12a shows that blank self-polishing coatings have a discernible antimicrobial impact, with an E. coli rate of around 45 % and a S. aureus rate of 56 %. Typical microalgae (Dunaliella and Porphyridium) adhering to SP, MXene/SP, and MXene-PEIS/SP coatings show decreasing densities, as seen in Figure 12b. Because of the complementary properties of MXene, PEIS, and self-polishing coatings, the findings demonstrate that MXene-PEIS/SP coatings have antifouling solid capabilities.

Figure 12:

Antibacterial/antifouling performance MXene-PEIS composite (a) Synthetic scheme (b) the effectiveness of SP, MXene/SP, and MXene-PEIS/SP as antibacterial agents against Escherichia coli and Staphylococcus aureus; (c) the statistical outcomes of the study.

6.4 Drug-doped MXene

Many researchers are interested in using MXene as a drug delivery system because of its large surface area and remarkable photothermal conversion characteristics. ^{169 , 170} Nanoribbon fibers responsive to temperature and near-infrared light were synthesized by Jin et al. from MXene. ¹⁷¹ Moreover, these fibers are vitamin E rich and may release it in a controlled manner. The researchers used the electrostatic spinning method to create nanofibers from a mixture of MXene and PAN-PVP. Nanofibers with improved wetting and diffusion characteristics were coated with a P copolymer. The addition of MXene to the nanofibers made them very effective against bacteria and light and had remarkable photothermal characteristics. By using near-infrared light, the thermally responsive properties of MXene were triggered, resulting in the polymer-coated interface relaxing and releasing vitamin E. Results from the cellular evaluation showed that T-RMFs-treated cells were more capable of attaching and dividing into new cells than control cells, indicating that these cells had an extraordinary capacity to mend wounds.

A nano-complex was synthesized by Zheng et al. (2022) by combining the antibiotic ciprofloxacin (Cip) with Ti₃C₂T_x. ¹⁷² Afterwards, they created Cip – Ti₃C₂TSG, a temperature-sensitive injectable hydrogel. In a controlled laboratory setting and a living organism, researchers found that near-infrared light sped up the release of the antibiotic Cip, leading to the rapid and efficient photothermal and chemical elimination of bacteria. Nanosheets of Ti₃C₂T_x directly disrupted the bacterial cell membrane, making it easier for Cip to enter the cells. Furthermore, the regrowth of germs after the regulated release of Cip prevented photothermal treatment. Xu et al. combined MXene, amoxicillin, and polyvinyl alcohol to create an electrostatically spun nanofiber membrane that exhibited antibacterial properties. The original aim of developing this membrane was to combat infections that manifest in wounds. ¹⁷³ To fight bacterial infection at the site of the lesion, MXene stimulates the synthesis of AMX by converting near-infrared radiation into heat energy.

Antimicrobial efficacy against E. coli and S. aureus was 96.1 % and 99.1 %, respectively, when subjected to near-infrared radiation. The MAP nanofiber membrane significantly accelerated wound healing in infected mice when subjected to laser radiation, according to experimental trials performed on live beings. Using MXene’s photothermal conversion capacity, Sun et al. increased adenosine discharge under near-infrared irradiation, thereby maintaining the activation signal at the injury site, in a different study. Wound healing and new blood vessel production were facilitated by its use in a therapeutic animal model. The composite system (MXene-TK-DOX@PDA nanoparticles) created by Zhang et al. is pH and ROS-sensitive. ^{174 , 175} Covalent bonding is used in this technology to bind medications to nanocarriers based on MXene. This method enhances the quantity of medicine that can be loaded, decreases the likelihood of unpleasant side effects, and delays the drug’s release. Photothermal conversion efficiency is further improved by coating MXene-based nanoplatforms with pH-responsive polydopamine. Five hours later, the in vitro antimicrobial tests showed that both E. coli and B. subtilis had been ultimately killed.

This work presents a novel MXene-doped composite microneedle patch exhibiting superior mechanical strength, photothermal antibacterial capabilities, and reactive oxygen species elimination qualities for treating infected wounds. Upon insertion of the MN tips with MXene nanosheets into the skin’s cuticle, they rapidly disintegrate, releasing the nanomaterials into the subcutaneous infection site. This research created a full-thickness cutaneous wound model infected with MRSA and followed the method in Figure 13a to assess the MN patches’ wound healing effects. In vitro experiments showed 2MHMN had the best cell migration. In addition, the 2MHMN NIR(+) group showed a moderate surface temperature under NIR light (Figure 13b), which did not harm the surrounding skin tissue. The 2MHMN was used for in vivo investigations. Animal trials were placed into four groups: control, HAMN, 2MHMN, and 2MHMN NIR(+). Photographs of rats’ infected wounds on days 0, 1, 3, 5, 7, 9, and 12 (Figure 13c) showed wound area variations in each group. 2MHMN and 2MHMN NIR(+) groups exhibited lower wound areas at the same time as other groups, suggesting improved wound healing (Figure 13d). In 3 days, the 2MHMN NIR(+) group healed 50 % of the wound, followed by the 2MHMN group, whereas the HAMN and control groups took over 5 days. Each group’s wound area was compared (Figure 13e). ¹⁷⁶

Figure 13:

Examination of in vivo wound healing infected with MRSA. (a) A detailed plan of the experiments conducted on animals is needed. (b) The 2MHMN NIR(+) group’s wounds were imaged using real-time infrared thermal imaging at 0, 0.5, 1, 3, and 5 min after NIR laser irradiation. (c) On days 0, 1, 3, 5, 7, 9, and 12, there are representative images and (d) depictions of the wound contours in various groups. Image scale: 1 cm. for each group on days 1, 3, 5, 7, 9, and 12; (e) average wound area analysis.

The MXene-doped MNs have exhibited remarkable wound-healing capabilities in an MRSA-infected wound model, attributable to their synergistic antibacterial and antioxidant properties, including enhanced re-epithelialization, collagen deposition, angiogenesis, and suppression of pro-inflammatory factor expression. Consequently, the multifunctional MXene-doped MN patches provide an exceptional option for treating wounds afflicted by drug-resistant bacteria in clinical settings. Table 2 shows comaprsion of MXene based materials for antimicrobial activity.