Two-Dimensional MXenes as Next-Generation Nanomaterials for Biosensing and Hydrogen Production

Introduction

Nanomaterials can be categorized, based on their dimensional structure, into zero-, one-, two-, and three-dimensional structures. Two-dimensional (2D) materials consist of one to several atomic layers, and their remarkable electrical and optical characteristics arise from the strong covalent or ionic bonds within the layers, as well as due to the weaker van der Waals forces that exist between them.¹ Graphene marked a significant advancement in science and technology as the first 2D material, due to its exceptional mechanical strength, electronic behavior, and thermal conductivity. Several other 2D materials have been discovered after the successful preparation of graphene, and this trend continues to expand. The collection of 2D layered materials has increased each year with the introduction of numerous new compounds. This extensive array of 2D materials forms a diverse family that includes a broad spectrum of properties, which can be further enhanced by combining other 2D materials with vdW heterostructures. Generally, 2D materials are classified according to their structural characteristics, including graphene and transition metal dichalcogenides (TMDs) such as WS₂, MoS₂, WSe_2, and MoSe₂,² hexagonal boron nitride (h-BN),³ layered double hydroxides (LDHs), metal nitrides/carbides (MXenes),⁴ graphitic carbon nitride (g-C₃N₄), transition metal halides (TMHs) such as PbI₂ and MgBr₂, transition metal oxides such as MnO₂ and MoO₃, perovskite-type oxides such as K₂Ln₂Ti₃O₁₀ and RbLnTa₂O₇⁵ and 2D polymers (Figure 1). Two-dimensional (2D) materials are significant owing to their applications at the nano-and atomic scales. Ultrathin structures possess distinct electronic, magnetic, optical, and catalytic properties that distinguish them from their mass counterparts, thereby providing considerable advantages in various applications. Following exfoliation, the surfaces of these materials become exposed, leading to a significant increase in their surface area, which enhances their physical and chemical activity. Compared with their bulk counterparts, 2D semiconductor materials show significant enhancements in Coulomb interactions between charge carriers and defects, leading to the development of longer-lived excitons.

Figure 1 Various 2D materials. Abbreviations: TMDCS, Transition Metal Dichalcogenides; LDHS, Layered double hydroxides; h-BN, Hexagonal boron nitride; g-C3N4, Graphitic carbon nitride; TMHs, Transition metal halides.

MXenes are 2D materials that are made via selective etching of the corresponding MAX phases of layered transition metal nitrides, carbides, or carbonitrides.⁶ The precursor chemical formula is denoted by M_n+1AX_n, where n” changes from 1 to 4. In this formula, “M” denotes an early d-block transition metal, “A” signifies an element of groups 13 to 15 from the periodic table, and “X” denotes either nitrogen or carbon. Unlike transition-metal dichalcogenides (TMDs) and graphene, which are interconnected by van der Waals forces, MXenes are characterized by a more robust M-A bond, and the M-X bond exhibits covalent, metallic, or ionic characteristics. The chemical etching process that eliminates the A layers depends on the metallic nature of the M-A bond, leading to the formation of MXenes.⁷ MXene is synthesized via a selective etching process that utilizes the chemical formula M_n+1X_nT_x, where T signifies the termination functional groups (such as–F,–OH,–Cl, or–O) and x indicates the quantity of these groups. The presence of these surface termination groups and the chemical etching method creates a hydrophilic nature on the MXene surface. Furthermore, these terminal functional groups play a key role in influencing the ion transport and electrical characteristics of MXene. In 2011, Barsoum and Gogotsi identified Ti₃C₂T_x. They employed hydrofluoric (HF) acid to extract Ti₃C₂T_x from its precursor MAX phase, Ti₃AlC.⁸ To date, over 100 MAX phases have been produced through various combinations of the three constituent components of MAX phases.⁹ Although MXenes show promising characteristics, only about 30 variants have been synthesized and their properties evaluated experimentally. This provides a significant opportunity for the identification of new MXenes with unique properties. MXenes are significant in numerous technological applications owing to their diverse chemical characteristics and extensive potential for surface functionalization. Their roles include energy storage, catalysis, sensing, lubrication, and electromagnetic interference shielding. As an electrode, MXene offers distinct benefits for electrochemical energy storage systems, gathering keen interest as it emerges as a leading-edge electrode material for supercapacitors.¹⁰ The challenges related to the agglomeration and accumulation of MXene materials, along with the impact of their surface functional groups on capacitor performance, significantly hinder the real-world uses of MXenes. To enhance the energy storage capabilities, researchers have explored the combination of MXenes with several materials, such as metal oxides or hydroxides, polymers, and carbon-based substances, taking advantage of the benefits offered by these composite materials. Consequently, a variety of MXene composites with remarkable energy storage performances have been developed.

Figure 2 illustrates the growing research interest in MXenes and their applications in biosensing and hydrogen production based on Scopus-indexed publications from 2017 to 2025 (24 November). Analyses were performed on Scopus during the literature review using the keyword “MXene”. Specifically, searching for this term in the titles, abstracts, and keywords resulted in 19,829 records as of November 24, 2025. The keyword search indicated a significant increase in the number of MXene-related publications from 2017 to the present (Figure 2a). Figure 2b shows the number of publications related to their application in biosensing and hydrogen production. The exponential increase in publications indicates the advancement of MXenes and their critical role in biosensing and hydrogen production.

Figure 2 (a) Trend of research publications on MXenes, (b) Trend of research publications on biosensor and hydrogen production using MXenes.

Currently, the progress of MXene-based biosensors has become a favourable field, especially for applications in health and environmental monitoring.¹¹ MXenes exhibit numerous beneficial properties, including large surface area, high electrical conductivity, exceptional hydrophilicity, and ease of surface modification, making them ideal candidates for the development of biosensors.¹² Additionally, their relevance for biosensing applications is further enhanced by their stability in aqueous environments and biocompatibility.¹³ Among the various MXenes, Ti₃C₂ MXene stands out due to its large surface area, exceptional conductivity, significant hydrophilicity, and remarkable absorption capacity, making it ideal for biosensor applications.¹⁴ MXene-based biosensors have shown great potential in the medical field for detecting different biomarkers, pathogens, and metabolites.¹⁵ Recently highly sensitive and selective sensors have been developed for checking glucose levels,¹⁶ discovering cancer biomarkers,^17,18 and finding infectious agents.^19,20 Tian et al synthesized ferrocene (Fc)-modified HDNA2 N-carboxymethyl chitosan/Mo₂C biosensor to detect hairpin DNA1 and hairpin DNA2 in the presence of miRNA-21.²¹ In addition to having a high affinity to the DNA probe, the linear range of the chitosan/Mo₂C nanocomposite varies from 1.0 nm to 1.0 nM, with a LOD of 0.34 fM Rapid diagnostics and continuous health monitoring are made possible by these sensors, which have the potential to renovate personalized medicine and enable early detection of disease.

The antibacterial properties of MXene have been reported.²² The antibacterial effects of Ti₃C₂T_x MXene suspensions were studied by Rasool et al.²³ Their research revealed that both single-and layered Ti₃C₂T_x flakes in a colloidal solution exhibited enhanced antimicrobial properties against B subtilis and E coli compared to graphene oxide (GO), a well-established antimicrobial substance. Recently, the COVID-19 detecting capabilities of MXenes have been examined and published.²⁴ Additionally, Li et al examined the sensing abilities of MXene against different concentrations of antigens from both 2019-nCoV and the influenza virus using a cost-effective MXene-graphene field-effect transistor (FET) sensor integrated with a microfluidic channel, which captures viruses in solution.²⁵ Recently, research has been conducted on the preparation of amino-silane-functionalized Ti₃C₂ nanosheets using a slightly rigorous layer delamination method.²⁶ The MXene functionalized with amino groups provided a dependable substance for the covalent immobilization of cancer bio-receptors aimed at identifying cancer biomarkers, such as carcinoembryonic antigen. Notable improvements have also been made in the environmental applications of MXene-based biosensors. Scientists have engineered sensors with high sensitivity and specificity to detect pollutants,²⁷ heavy metals,²⁸ and hazardous chemicals in water and air.²⁹ These can improve water and air monitoring, control pollution, and address global challenges related to water and air pollution. The combined use of MXenes and other nanomaterials as well as advanced sensing technologies has led to significant advancements. For example, MXenes combined with aptamers, antibodies, or enzymes have created highly specific and sensitive biosensors.³⁰ Moreover, MXenes have been integrated into electrochemical, optical, and field-effect transistor-based sensors and have shown enhanced detection limits and quicker response times.³¹

Hydrogen has emerged as a remarkable alternative fuel for the future because of its energy density of 120 MJ/kg, which is far above that of gasoline. Hydrogen has gained significant interest as a sustainable alternative energy source and a green energy carrier. However, compared with other production techniques, electrochemical water splitting has several advantages: fast reaction kinetics, production of high-purity hydrogen, and absence of greenhouse gases. The use of novel materials to improve the efficiency of hydrogen production has increased significantly in recent years. The hydrogen evolution reaction (HER), which greatly increases the efficiency of hydrogen evolution, is made possible by the exceptional electrocatalytic qualities.³² MXenes are especially suitable for use in electrochemical water splitting owing to their exceptional structural and electronic properties.³³ Studies on MXenes have demonstrated their efficiency in various methods of hydrogen production, especially in electrocatalytic reactions, where they have been shown to enhance the hydrogen production rate. Many metals in MXenes exhibit favorable hydrogen adsorption from a catalytic standpoint, encouraging the effective kinetics of hydrogen production. Consequently, the use of these metals in the synthesis of catalysts from MXenes has increased. According to recent studies, MXenes have exceptional catalytic support qualities, and the addition of transition metals such as Ru significantly improves their electrocatalytic performance.³⁴ Furthermore, their efficiency as electrocatalysts is greatly increased by their natural capability to absorb hydrogen and enable proton transfer. These qualities increase the potential of MXenes as cutting-edge materials for sustainable energy solutions by making them ideal for incorporation into renewable energy systems and large-scale hydrogen production.

This review begins by presenting an overview of the characteristics of MXene materials, along with various preparation techniques such as etching with HF and fluorine salts, electrochemical etching, base etching, Lewis acid molten salt etching, and direct synthesis. This review also discusses in detail their applications in photocatalytic hydrogen generation and water electrolysis, highlighting recent developments and challenges in sustainable hydrogen production using MXenes. In previous years, many researchers have thoroughly explored the synthesis, characterization, and applications of MXene materials in their articles. Sinha et al discussed an outline of the applications of electrochemical, gas-absorptive, and piezoresistive sensors.³⁵ The use of MXene materials as biosensor was discussed by Ozcan et al,³⁶ whereas Yoon et al focused on MXene nanocomposite-based electrochemical biosensors.¹⁹ However, there is a scarcity of reviews that specifically concentrate on both biosensing applications and hydrogen production using MXene materials. Furthermore, advancements in the research on the use of MXene in sensors and environmental pollution removal are highlighted. The challenges and potential opportunities associated with MXenes in other fields are discussed at the end of this review.

Synthesis Methods for 2D Materials

The top-down and bottom-up methods are commonly used to produce 2D materials (Figure 3). To separate the layers and create 2D materials, top-down techniques exfoliate the bulk materials using physical or chemical processes. This effectively overcomes the van der Waals forces that hold layers together. Bottom-up approaches concentrate on the direct growth or deposition of two-dimensional materials onto a substrate.

Figure 3 Various synthesis methods of 2D materials.

Bottom-Up Approaches

Chemical Vapor Deposition (CVD) is the primary method employed for bottom-up synthesis of 2D materials. This technique has been utilized to create 2D layers of graphene,³⁷ h-BN,³⁸ and MoS_2.³⁹ One of the key benefits of CVD over mechanical or chemical exfoliation is its ability to produce large-area films, making the process more efficient. For instance, CVD has enabled the roll-to-roll production of films composed of graphene, which is suitable for applications as transparent electrodes. This method is particularly significant when there is no equivalent 3D layered form for 2D materials. Silicene illustrates this fact, as it lacks any 3D layered form; thus, the controlled accumulation of silicon on a substrate such as silver.⁴⁰

Top-Down Approaches

Mechanical Exfoliation

Most layered materials have relatively weak interlayer bonds, such as hydrogen or van der Waals interactions, which facilitate layer separation. These materials facilitate the mechanical exfoliation of 2D layers from 3D crystals by cutting one layer from another. Novoselov was the first to use Scotch tape to peel off graphene layers to separate individual layers from highly oriented pyrolytic graphite.⁴¹ The electronic properties of the individual graphene layers could be investigated using this technique. The rubbing of 3D materials against paper is another mechanical method similar to writing chalk on a blackboard or using a graphite pencil on paper. Mechanical exfoliation has been applied to various layered materials beyond graphene, such as h-BN, TMDs, MoO₃, and hydrated WO₃.⁴²

Chemical Exfoliation

Chemical exfoliation can be used to construct a wide range of 2D materials from 3D layered structures such as graphene, graphene oxide, h-BN, TMDs, metal oxides or hydroxides, and clay. The basic idea of using chemical exfoliation involves disturbing the interactions between the layers using different techniques that can be purely chemical, chemical-thermal, and chemical-mechanical. Most of these techniques are performed in a liquid medium. An example of this interaction is the reaction between graphite and a mixture of HNO₃ and H₂SO₄, in combination with potassium chlorate, resulting in the oxidation of graphite. Subsequently, thermal shock heating the material to 1050°C for 30 seconds decomposes the intercalant, resulting in a significant volume increase that separates the 2D graphene oxide layers.⁴³ Sonication-aided exfoliation can be employed as an alternative of thermal shock to achieve the same effect on the intercalated GO layers.⁴⁴ TMDs can also be exfoliated by sonication in various solvents, such as N-methyl-pyrrolidone or isopropanol. Similarly, graphene sheets can be exfoliated by sonication in water along with a surfactant. Modification of the interlayer composition by a chemical process is required when the bonds between the layers are too strong to break. Heating SiC single crystals under high-vacuum or argon conditions sublimates silicon and creates graphene.

Preparation of 2D MXenes

MXenes are usually prepared by selective etching of atomic layers from the MAX phase, which can consist of various elements such as Al, Si, Ga, and/or Sn, using various techniques. This is possible when the metallic bond M-A in the MAX phase is weaker than the covalent bond M-X, which makes it reasonable to etch A atoms selectively without affecting the covalent M–X bonds. In recent years, much attention has been focused on developing various techniques for MXene preparation, especially economical and energy-efficient techniques. The concepts of clean production and green chemistry should also be considered during the preparation of the MAX phase. To date, MXene has usually been prepared from the MAX phase using various techniques, such as fluoride etching, electrochemical etching, molten salt etching, and direct preparation techniques (Figure 4), which are further explained in this section. Table 1 presents a comparative analysis of various techniques used to synthesize MXenes.

Figure 4 Different synthesis methods of MXenes.

Table 1 Different Etching Methods for the Synthesis of MXenes

Etching Using HF and/or Fluoride Salts

When HF was used as an etchant, the chemical bonds between elements A and M were sufficiently strong to destroy the parent phase material. Mashtalir et al reported the dynamic control of selective aluminum corrosion from Ti₃AlC₂ in a 50% HF solution.⁴⁵ The results showed that the progress of the fast phase transformation from the bulk Ti₃AlC₂ to the Ti₃C₂T_x is favored by increasing the temperature of sinking, extending the time for reaction, and decreasing the size of the initial particles. They further confirmed that single-layered Ti₃C₂T_x MXene can be exfoliated during HF etching. Wang et al also prepared Ti₃C₂T_x MXene from the Ti₃AlC₂ powder using 50% HF at room temperature.⁴⁶ In such a process, -OH or -F groups formed on the surface and edges of the MXene with accordion-like morphology. In addition, HF is an excellent etchant that allows the generation of Ti₃C₂T_x flakes with relatively small lateral dimensions. Although HF etching is a simple and widely applicable method for the preparation of MXenes, the toxicity and safety risks of this process are high, and the resultant MXene may suffer from various types of defects.

Studies have shown that HF synthesized in situ using a mixture of salts such as LiF, NaF, CaF₂, and HCl or H₂SO₄ possesses etching properties similar to those of pristine HF. This in situ-synthesized HF yielded MXene flakes with a longer size and fewer nanodefects compared to flakes obtained from conventional HF etching. For example, Ghidiu et al obtained Ti₃C₂T_x MXene using LiF and HCl solution-based etching, yielding a lattice constant of around 27–28 nm, compared to 20 nm for Ti₃C₂T_x using pristine HF.⁴⁷ This increase in lattice constant due to corrosion using a LiF+HCl solution has been attributed to an increase in interlayer distance, thereby increasing the electrochemically available surface area and promoting ion diffusion in electrolytes. LiF- and HCl-etched MXenes usually have functional groups of –F, –OH, and –O, but usually have fewer –F terminals than those derived from direct HF. This resulted in improved electrical and catalytic properties. However, studies have shown that Ti₃C₂ flakes derived using a precursor with in situ fluoride salt/HCl etching exhibited better performance in the HER than those derived using direct HF, possibly owing to fewer –F terminations and a favorable layer composition.⁴⁸ Other mild etching agents such as NH₄HF₂ can also provide in situ HF on precursor surfaces without side reactions that are harmful to human health. Li⁺ and NH₄⁺ are also used as intercalation agents to produce Ti₃C₂T_x MXene flakes that are free of Al sublayers with an increased interlayer distance. Adibah et al also investigated how in situ HF and direct HF affect Ti₂C₃ MXene morphology using a precursor of Ti₂AlC₃.⁴⁹

Fluoride-Free (Electrochemical) Etching

Electrochemical etching is a highly effective technique for the fabrication of high-performance 2D Ti₃C₂T_x MXenes. This process can be conducted in fluoride-free electrolytes, resulting in Ti₃C₂T_x that does not contain fluorine terminations. Unlike other chemical etching techniques that use HF or LiF/HCl, the electrochemical etching technique does not use any F ions. The developing MXenes contained only Cl, O, and OH groups. By maintaining a steady voltage, careful etching of the aluminum layers could be accomplished using Cl−ions. A novel electrochemical process was introduced by Feng et al, which employs double aqueous electrolytes for the packing of Ti₃C_2.⁵⁰ Li et al successfully synthesized highly pure multilayer MXene through alkali-assisted hydrothermal methods derived from the Bayer process.⁵¹ The surface modification of MXene was achieved by oxidizing a Ti₃AlC₂ solution with NaOH, followed by dissolution in Al(OH)_4. This process leads to the further oxidation of the inner Al atoms, resulting in the formation of new dehydrated oxide hydroxides (AlO(OH) and simple Al(OH)_3. The Ti layers serve as lattice confinement, inhibiting insoluble compounds from easily reacting with -OH to form soluble Al(OH)₄, which could hinder MXene synthesis. Selective etching of the MAX phase using strong alkali can produce hydrophilic products with fluorine-free termination. Transition metal halides serve as electron acceptors and potentially interact with the A layer of the MAX phase in the molten form. Li et al proposed an innovative technique for etching MAX phases via a direct redox coupling reaction of elemental A and a cation from a Lewis acid molten salt.⁵² Nevertheless, excessive etching of the MAX phase into carbine-derived carbon (CDC) is a problem associated with electrochemical etching. A core-shell representation was suggested to explain the process by which Ti₂AlC was electrochemically etched into Ti₂CT_x and CDC. This model highlights the necessity of carefully balancing the etching parameters to produce MXenes, while preventing overloading. Scientists have made significant efforts to achieve these goals. They successfully developed MXene products with excellent electrochemical properties by utilizing organic materials and deep eutectic solvents (DES) as etching solvents. Ti₃C₂ MXene can be produced in DES through a highly dependable and anhydrous ion thermal process.⁵³

Thermal-aided electrochemical etching was used for the synthesis of Ti₂CT_x, Cr₂CT_x, and V₂CT_x.⁵⁴ The use of dil HCl, as an etchant, combined with mild heating accelerated the etching of the MAX phase. The synthesis of Ti₂CT_x via electrochemical etching occurred via a two-step approach. During 1^st stage, Al atoms were initially extracted from the layered structure owing to the applied voltage because it was easier to break the Ti-Al bond than the Ti-C bond. In the 2^nd stage, both Al and Ti species were entirely removed, resulting in only monolayer carbon atoms. MXenes with various structures can be produced by adjusting temperature, etching time, and electrodes. This process also successfully yields two additional MXenes (V₂C and Cr₂C), which are typically regarded as challenging to synthesize. Previous studies have indicated that V₂C requires HF etching for over two days, with HF concentrations reaching as high as 50%. Thermally assisted electrochemical etching not only introduces a complete method for synthesizing MXenes but also enables the fabrication of those that are difficult to produce. This approach can be used for rapid, easy, and safe preparation of MXenes.

Lewis Acid-Molten Salt Etching Method

In addition to the above-mentioned etching procedures, there are other methods for extracting A-layer atoms from MAX phases using molten salts. After the synthesis of MXenes via molten Lewis acids, Huang et al engaged in comprehensive and systematic research on proposing a method for making MXenes using a general molten salt.⁵⁵ The redox potential of the Lewis acid cations was high enough to oxidize the A-layer atoms in the MAX phase. Several molten salts (CdCl₂, CoCl₂, CuCl₂, FeCl₂, NiCl_2, AgCl) were utilized to etch various MAX phases (Ti₂AlC, Ti₃AlC₂, Ti₃AlCN, Nb₂AlC, Ta₂AlC, Ti₂ZnC, Ti₃ZnC₂) to produce the subsequent MXenes. These findings indicate that, in the conventional MAX phase, metal A can be replaced with late-transition metal halides. This finding significantly broadened the scope of MAX phases as MXene precursors. A method that does not involve HF, and thus Lewis acid etching, is an environmentally friendly approach for the preparation of MXene. Li et al successfully synthesized a Ti₃ZnC₂ MAX phase from Ti₃AlC₂ and Lewis acidic molten salt ZnCl₂ via a displacement reaction at a temperature of 550°C. By increasing the ratio of MAX:ZnCl₂, Ti₃ZnC₂ can be further converted into Ti₃C₂Cl₂ MXene.⁵⁶ Talapin group produced several MXenes with -Cl surface modifications in CdCl₂ molten salt and produced a series of MXenes with bromine (-Br) surface terminations using Lewis acidic CdBr₂, thus broadening the molten salt etching method from chlorides to other halides.⁵⁷ Sajid et al enhanced the method by utilizing a ZnCl₂ molten salt, which led to the production of Ti₃C₂Cl₂ MXenes without any residual fluorine.⁵⁸ This resulted in a clean surface and modified electronic features, ultimately improving the HER activity of these chloride terminated MXenes.

Direct Preparation Method

Researchers have also investigated the application of CVD as a method for preparing MXenes, in contrast to the previously mentioned approach for selective removal of A-layer atoms from the MAX phase via chemical etching. Dmitri V Talapin et al established a CVD technique in which titanium chloride (TiCl₃ or TiCl₄), and an additional source of carbon or nitrogen (such as graphite, CH₄, or N₂) as precursors for MXenes synthesis, including the commonly utilized Ti₂CCl_2.⁵⁹ This innovative direct synthesis technique not only reduces time and eliminates the hazardous waste associated with the etching process but also enhances the efficiency of MXene production, thereby facilitating the advancement of industrial applications for these materials. The hydrothermal method is effective for large-scale production of MXenes featuring diverse designs, as it eliminates the need for exposure to highly hazardous HF. Al layers were created through the MAX phase using an innovative leaching technique that eliminates the use of fluorine hydrothermal treatment. The process involves immersing Ti₃AlC₂ in an aqueous NaOH solution for 100 h, followed by a 15 h hydrothermal process with 1 mol/L H₂SO₄ at 85°C.⁶⁰ Another technique for synthesizing Ti₃C₂T_x includes immersing small Ti₃AlC₂ particles in water containing NH₄F before undergoing the hydrothermal reaction.

Properties of MXenes

MXene possesses abundant oxidation-reduction sites, resulting in improved capacity levels. In the case of Ti₃C₂T_x MXene, the oxidation state of titanium varies continuously owing to the hydration of the oxygen-containing functional groups, which enhances the charge transfer ability of the transition metal in its valence state.⁶¹ The =O functional groups exhibit greater stability compared to the-OH and -F groups because of their ability to share a higher number of electrons with M within the MXene layer. This electron sharing facilitates the interconversion between the =O and -OH end groups during charge/discharge cycles, resulting in an abundance of active sites for redox reactions.⁶² The individual layered assembly of 2D MXene contributes to an increased surface area, which enhances ion intercalation and transport.⁶³ Furthermore, the layered configuration of MXene enables compatibility with various intercalating agents, thereby broadening its electrochemical activity. This improves the pseudocapacitance and cycling stability. The notable energy-storage capabilities of MXene are evident in various devices constructed based on it, showing high energy and power densities.

MXenes exhibit extraordinary conductivity, and their composition includes various transition metals such as Ti and Mo, which are known for their high conductivity. These metals create conductive pathways within the MXene structure, facilitating electron flow and enhancing the overall conductivity of the material.⁶⁴ MXene allows for greater electron availability because they are two-dimensional (2D) material with a large specific surface area. This enables electrons to move freely across the 2D plane, minimizing electron losses and thereby enhancing conductivity. The structure of MXene significantly influences its conductivity; single-layer and larger flakes exhibit superior interactions compared with multilayered and smaller flakes. Furthermore, in contrast to other 2D materials like metal sulfides, hydroxides or graphene, MXene possesses a variety of functional groups, including -OH and -F. Modification of the functional groups would represent a means to tune its electronic structures and charge transfer properties, thus impacting its conductivity performance.⁶⁵ The combination of transition metals and functional groups makes MXene an outstanding conductive material with numerous potential applications. The remarkable conductivity of MXene enables rapid electron flow, which in turn enables the development of high-energy-density supercapacitors.

MXene exhibits significant hydrophilicity because of the abundant–OH and -F functional groups present in its layered structures.⁶⁶ This composition results in a highly polar MXene surface. When in contact with water, these polar functional groups are capable of forming hydrogen bonds with H₂O, enhancing the attractive forces between MXene and H₂O.⁶⁷ The hydrophilicity of MXene can be enhanced owing to its inherent features, including a high specific surface area and porous structure. This characteristic provides MXene with remarkable dispersion, stability, and potential for application in water.

MXene is composed of multiple layers of 2D nanosheets stacked via relaxed bonding. Additionally, by intercalating various materials such as polymers or liquids, the layered structure of MXene can be significantly enhanced, leading to the formation of composite materials and improved flexibility. The mechanical characteristics of MXenes are affected by various factors, including their structural composition, dimensions, defects, incomplete edges of nanosheets, and surface terminal groups. The surface terminal groups play an important role in enhancing the mechanical properties of MXenes. For instance, the presence of =O terminal groups strongly enhances the interlayer interactions, which improves the shear and strain resistance performance. By utilizing the contact surface area, the interaction with the external environment can be maximized, leading to reduced stress concentration. This enhancement allows for greater displacement and deformation capacity which improves the flexibility and durability of MXenes and mitigates the effects of outside vibrations on the inner structure of capacitors.⁶⁸ Furthermore, research conducted by Guo et al indicates that the introduction of =O terminal groups under both uniaxial and biaxial conditions resulted in an increase in the tensile range of Ti₂C to 28% and 20%, respectively. This finding suggests that surface terminal groups effectively inhibit the collapse of Ti₂C.⁶⁹

Applications of MXenes

Applications of 2D MXenes as Biosensors

Owing to their special qualities, MXenes have received considerable interest for biosensing applications (Figure 5). MXenes can interact with biomolecules via hydrogen bonding, van der Waals interactions, electrostatic interactions, or ligand binding owing to their hydrophilic surface groups (–OH, =O, and–F). MXenes are exceptional candidates for use as carriers in biosensor device applications because of this feature.⁷⁰ Different MXene compositions have shown non-cytotoxicity and biocompatibility, further confirming their potential for biomedical applications.

Figure 5 Applications of MXenes as Biosensors.

Sensor for Biomolecules

Small biomolecules, such as glucose, ascorbic acid, cholesterol, and neurotransmitters, such as dopamine, glutamate, and uric acid, are important for daily human activities and the body’s resistance to illness. Therefore, in order to effectively monitor human health, it is essential to quickly and easily identify these biomolecules.⁷¹ Conventional methods of biomolecule monitoring include colorimetry, spectrometry, and HPLC. However, these techniques have certain drawbacks, including lengthy processing times, expensive reagents, complicated extraction procedures, and high training requirements. Biosensors have recently become popular for the detection of biomolecules because of their high sensitivity, selectivity, compatibility, stability, and low cost. Biosensors are fabricated using a variety of metal nanoparticles, metal oxides, metal carbides/nitride nanoparticles, or nanosheets to enhance their performance. Among these nanomaterials, MXene provides a large number of efficient surface sites that are suitable for immobilizing bioreceptors and improving the selectivity, stability, sensitivity, and accuracy of biosensors.

Ti₃C₂ was synthesized by Xu et al using the hydrothermal method to identify intracellular glutathione (GSH), a vital biomarker for both health and illness.⁷² Due to surface flaws and quantum confinement effects, the Ti₃C₂ displayed a blue emission. In the presence of GSH, Förster resonance energy transfer (FRET) occurs from Ti₃C₂ to GSH over a broad concentration range (1–100 μM), making it possible to detect GSH concentrations as low as 0.02 μM GSH-functionalized Ti₃C₂ has also been reported to be a significant biomarker for uric acid.⁷³ In which the uricase enzyme first oxidizes uric acid into allantoin and H₂O₂, and the horseradish peroxidase enzyme oxidizes o-phenylenediamine (OPD) to yellow-colored 2,3-diaminophenazine (oxOPD). The oxOPD product was emitted at 568 nm, and GSH@Ti₃C₂ was efficiently emitted at 425 nm. The addition of uric acid to the composite of GSH@Ti₃C₂ and oxOPD leads to FRET, which causes the 568 nm emission to increase and GSH@Ti₃C₂ 425 nm emission to decrease. This sensing nanoplatform allowed uric acid to be detected within a concentration range of 1.2–75 μM with a detection limit of 125 nM.

Red-emitting carbon dots (RCDs) and Ti₃C₂ MXene nanosheets were used by Zhu et al to develop a glucose sensor.⁷⁴ Ti₃C₂ nanosheets successfully reduced the fluorescence intensity of RCDs by more than 96% (Figure 6a and b), which is then restored upon interaction with glucose. To prevent IFE from RCDs to Ti₃C₂ MXene nanosheets, glucose oxidase aids in the oxidation of glucose, which produces H₂O₂ and oxidizes Ti₃C₂ to Ti (OH)_4. To investigate simple, sensitive, and effective methods for cholesterol measurement, a Ti₃C₂T_x MXene-based enzymatic biosensor for cholesterol measurement was developed by Xia et al.⁷⁵ Using chitosan and MXene as supporting materials, they immobilized the cholesterol oxidase (ChO_x) enzyme onto the electrode surface to construct a biosensor (Figure 6c). Shahzad et al introduced an electrochemical biosensor based on MXene technology for detecting dopamine (DA).⁷⁶ This biosensor provides a broad detection range for DA (0.015–10 mM) and has a limit of detection (LOD) of 3 nM. Table 2 summarizes MXene-based biosensors designed to detect biomolecules.

Figure 6 (a) Reduction of fluorescence intensity of RCDs on addition of Ti3C2 MXene, (b) quenching efficiency of different 2D materials Reprinted from74 with permission, and (c) synthesis of the Chit/ChOx/Ti3C2Tx/GCE Reprinted from75 with permission.

Table 2 The Detection of Biomolecules and Pathogens by MXenes Based Biosensors

Sensor for Pathogens

Pandemics are primarily caused by pathogenic bacteria and viruses. The initial detection of these viruses and pathogenic bacteria is crucial for accurate diagnosis and minimizing disease fatality, as well as for effective control of the spread of these pathogens. Owing to its exceptional electrical conductivity and optical properties, MXene is an ideal option for the application of biosensors in pathogenic bacteria and viruses.⁸⁴ MXene offers a large number of effective sites for immobilizing bioreceptors on the surface, which improves the sensitivity, selectivity, and accuracy of the biosensor.

The global healthcare system faces several challenges owing to the serious health impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. An effective biosensing technique was developed by a research team to detect viruses using an amino-functionalized probe DNA. The immobilization of NH₂-pDNA on the Ti₃C₂T_x MXene nanosheet-modified SPCE has been shown successfully by Bharti et al.⁷⁹ The novel technique developed by the researchers enabled the result within 12 min with a high level of sensitivity in the following range (0.1 pM to 1 μM), with a detection limit of 0.004 pM. The results obtained in the spiked serum samples displayed equal sensitivity, with a detection limit of 0.003 pM and a linear range of 1 pM to 1 μM. This result is significant because it was obtained in a real-world trial setup, indicating its high potential for real-world applications.

To create an MXene-based biosensor for virus detection, Chen et al developed a DNA-functionalized chemoresistive biosensor designed for the selective and efficient detection of the nucleocapsid gene of SARS-CoV-2.⁸⁰ This study involved the construction of a biosensor by noncovalently loading a probe DNA molecule onto 2D Ti₃C₂T_x MXene. After the SARS‑CoV‑2-N gene hybridized with the ssDNA probe on the MXene surface, the conductance of the sensing channel increased; however, no signal was observed for non-complementary targets (SARS-CoV-1 N and MERS-CoV N genes), indicating the high specificity of the biosensor. Figure 7 shows how the DNA-functionalized MXene sensor can detect the SARS-CoV-2 N gene in saliva with a limit of detection (LOD) of less than 105 copies/mL.

Figure 7 (a) Detection of SER-CoV-2 N gene by ssDNA/Ti3C2Tx sensor, (b) Real-time response, (c) response versus concentration plot, and (d) selectivity test of ssDNA/Ti3C2Tx sensor Reprinted from80 with permission.

Recently, a group of researchers has shown interest in creating MXene-based diagnostic probes for the detection of pathogenic bacteria. Zhang et al reported, for the first time, an MXene-based biosensing approach for Mycobacterium tuberculosis detection.⁸⁵ For developing the biosensor zirconium-linked Ti₃C₂ was used, and peptide nucleic acid (PNA) was used as the capture probe for targeting complementary biomarkers. This hybridized M tuberculosis-specific target biomarker was immobilized on the Au surface of the electrode, and accordingly, the conductance of this electrode was improved owing to the interaction between MXene and the target biomarkers. The advanced biosensor showed a high level of efficiency in detecting S aureus, E coli, M smegmatis, P aeruginosa, and the BCG vaccine.

Sensor for Food Contaminants

Various impurities, including gliotoxins, mycotoxins, endotoxins, exosomes, microorganisms, antibiotics, heavy metals, and pesticides are present in agricultural products. Mycotoxins can cause several health issues, including cancer and immune deficiency, which threaten both humans and animals. Similarly, another mycotoxin containing sulfur is gliotoxin, which belongs to a class of naturally occurring peptides generally known as piperazine-2,5-dione. It is regarded as one of the most toxic metabolites produced by certain species of fungi, such as Aspergillus fumigatus, and can cause significant environmental issues. A precise identification approach is essential to ensure food safety against gliotoxins and mycotoxins. Various traditional techniques, such as gas chromatography (GC), high-performance liquid chromatography (HPLC), and mass spectrometry, have been employed to detect food contaminants. However, these conventional methods have limitations, including the need for multiple assay steps, skilled personnel, and laborious pretreatments. Thus, there is a great need to develop fast, innovative, easy-to-use, sensitive, and stable detection methods to effectively identify mycotoxin contaminants. Therefore, MXene-based biosensors have been extensively investigated for the identification of foodborne contaminants in the past decade.^86–89 Recently, researchers have concentrated on creating MXene-based biosensors to screen food contaminants. Aflatoxin (AF), which is found in various natural agricultural products, is a highly toxic mycotoxin known for its mutagenic, carcinogenic, and teratogenic properties.⁹⁰ The maximum permissible concentration of AFB1 in cereal products is 2.00 ppb, which is part of the total AF content of 4.0 ppb. Wu et al designed an SERS aptasensor specifically for AFB1 detection in agricultural foods.⁹¹ The AFB1 aptamer was modified with Au nanoparticles (NPs), followed by the application of 4,40-Vinylenedipyridine as a trigger (Figure 8).AuNP/MXene SERS sensor was used to detect AFB1 in food products. Experimental findings indicated a LOD of 0.6 pg m/L and a linear concentration range of 1×10⁻³–100 ng m/L. The formation of extensive SERS hot spots resulted from the hydrogen bonding between MXene and the aptamer.

Figure 8 The AuNPs/Mxene SERS sensor for the detection of AFB1 contaminant in food i) Initiation: Start of formation of dimer, ii) Termination: End of formation of dimer.88

Researchers have been interested in detecting T-2 toxins using nanomaterials such as CNTs, Au NPs, upconversion nanoparticles (UCNPs), and magnetic nanoparticles (MNPs). The complementary DNA (cDNA) was combined with MNPs, while the T-2 aptamer was paired with UCNPs to form a hybrid aptamer.⁹² It has been noticed that with a decrease in the amount of T-2 antibodies, there was a decline in the electrochemical signal. The experimental findings demonstrated a strong response within the concentration range of 0.1–100 ng m/L for T-2 toxin. The recovered percentages obtained from the real samples using the developed sensor were found to be in the range of 95.97 to 104.00%. This result indicates that the proposed sensor has potential efficiency for the detection of T-2 toxins in food products.

Gliotoxin is another harmful metabolite found in some fungal species, such as aspergillus fumigatus, which poses significant risks to human and animal health. Wang et al examined a modified MXene-based DNA electrochemical biosensor for gliotoxin detection.⁹³ A chemical reaction involving the phosphate group of DNA and Ti was used to functionalize MXene nanosheets with tetrahedral DNA (TDNs). These modified DNA probes improved electron transfer between the electrochemical species and the electrode surface. Under optimized conditions, the proposed electrochemical sensor exhibited a broad linear range of 5–10 nM and detection limit of 5 pM. The MXene-based electrochemical biosensor was evaluated using human serum with a satisfactory recovery of 96.3–115%.

Sensor for Environmental Pollutants

MXene nanostructures have recently gained significant attention for the detection of hazardous materials.⁹⁴ Liu et al were the first to report the detection of NaNO₂ using a MXene Ti₃C₂ composite.⁹⁵ This study demonstrated that Ti₃C₂ enabled electron flow in Hb, and provided a biosensor working within an extensive linear range of 0.5–11,800 µM for NaNO₂ detection with a detection limit of 0.12 µM. Organophosphate pesticides (OPs) are environmental pollutants frequently released into the atmosphere and that infiltrate agricultural operations. Song et al developed an innovative biosensor, AChEChit/Ti₃C₂/Au NPs/MnO₂/Mn₃O₄/GCE, for detecting OPs.⁹⁶ Under optimal conditions, this biosensor achieved impressive results and can detect at a low limit of 1.34×10⁻¹³ M. Additionally, the biosensor’s capability to sense methamidophos in real samples was confirmed, exhibiting excellent recovery rates ranging from 95.2% to 101.3%.

Various MXene-based optical biosensors have been discussed in the literature for the detection of various metal ions through different mechanisms. Guan et al synthesized N-and P–modified Ti₃C₂, which showed a photoluminescence quantum yield (PLQY) of 20.1%.⁹⁷ These N/P@MQDs were utilized for the detection of Cu²⁺ ions. The photoluminescence (PL) intensity of the N/P@MQDs linearly decreased owing to FRET when Cu²⁺ ions were added in the range of 2–100 μM and 250–5000 μM, thus permitting the sensitive detection of Cu²⁺ at 2 μM. Ti₃C₂T_x MQDs were also employed for the detection of Fe³⁺ ions.⁹² The Ti₃C₂T_x MQDs exhibited white, blue, and blue PL when prepared by the solvothermal method using ethanol, DMSO, and DMF, respectively. The PL QY (4.1, 10.7, and 6.9%) and the average size of the Ti₃C₂T_x MQDs were also affected by the solvent selection. A range of ions, including Fe³⁺, Fe²⁺, Cu²⁺, Ni²⁺, Co²⁺, Mn²⁺, Ag⁺, Al³⁺, Cd²⁺, Mg²⁺, Hg²⁺, Zn²⁺, and Pb²⁺, were used to evaluate the metal ion selectivity of the Ti₃C₂T_x MQDs. The Ti₃C₂T_x MQDs probe was found to detect Fe³⁺ within the linear ranges of 5–470 and 510–750 μM, with a minimum detection limit of 2 μM. Yang et al described Nb₂C MQDs for the detection of Fe³⁺ ions.⁹⁸ The PL intensity of the Nb₂C MQDs exhibited a linear quenching as the concentration of Fe³⁺ increased from 0 to 300 μM. This can be explained by the coordination relations between OH and –COOH surface groups on the Nb₂C MQDs and the Fe³⁺ ions. Rathi et al used water-soluble cationic surfactant cetyltrimethylammonium bromide (CTAB) during the exfoliation stage of Nb₂CT_x.⁹⁹ The modification with surfactant CTAB has shown a three times improvement in the NO₂ gas sensor. The gas sensor response increased from 0.543 ppm^–1 to 1.686 ppm^–1 upon modification with CTAB.

Hydrogen Production Using MXene

Hydrogen Production Using MXenes-Based Electrocatalysts

The growth of hydrogen energy is a practical solution to the current energy and environmental problems. An essential technique for hydrogen production via water splitting. MXene-based HER catalysts have attracted increasing interest in recent years. As HER electrocatalysts, MXenes and MXene-based composites exhibit encouraging developments as possible substitutes for Pt-based catalysts. 2D MXenes with -O/-OH groups showed metallic characteristics and confirmed their remarkable charge transfer capabilities.¹⁰⁰ The interaction between 2D MXenes and H was facilitated by these surface oxygen atoms. Further investigation revealed that the Heyrovsky mechanism was followed by HER for O-terminated MXenes. In terms of efficiency, the O-terminated surface outperformed the OH- and F-terminated surfaces. These findings show that terminated surface modifications can be beneficial for adjusting the catalytic activity of 2D MXenes in the HER.

Jiang et al used ultrathin O-functionalized Ti₃C₂ MXenes to create an efficient HER electrocatalyst.¹⁰¹ Hydrogen adsorption kinetics were hindered by the F-functionalized MXenes on the basal plane, and adversely affected the HER. Ti₃C₂T_x was dispersed in a KOH solution to create Ti₃C₂O_x, which reduced the F-termination through OH groups.¹⁰² Ti₃C₂(OH)_x was further calcined at 450°C in an Ar atmosphere, which caused a dehydration reaction, and the OH groups were converted to O-terminal groups. Ti₃C₂O_x nanosheets have been demonstrated to be excellent HER electrocatalysts. Compared to Ti₃C₂(OH)_x, at a current density of 10 mA cm⁻² Ti₃C₂T_x-450 exhibited an overpotential of 190 mV. Furthermore, the Tafel slope of 60.7 mV dec⁻¹ is attributed to the highly active O sites on the basal plane of Ti₃C₂O_x. In conclusion, the terminal groups on the basal plane affect the catalytic efficiency of MXenes in the HER.

The HER activity of MXenes can be improved by confining them to single metal atoms. The stability of a single-atom catalyst in the 2D material lattice can be achieved through the formation of strong covalent bonds.¹⁰³ Using the model material Ti₃CNO₂, the influence of different metal atoms (Fe, Zn, Ir, Re, Os, and Rh) was examined in HER.¹⁰⁴ Additionally, Li et al demonstrated that the modification of MXenes with transition metal (TM) could enhance their catalytic performance.¹⁰⁵ The theoretical understandings of the catalytic mechanisms support the design and use of TM-doped MXenes in HER. Ultrathin 2D Ti_3−xC₂T_y MXenes were used to produce Pt₁/Ti_3−xC₂T_y, a stable single-atom catalyst with a high reduction capacity and many Ti defect vacancies.¹⁰⁶ A single Pt atom MXene (Mo₂TiC₂T_x-Pt_SA) catalyst was formed by Zhang et al.¹⁰⁴ Double transition metal MXenes (Mo₂TiC₂T_x) were first exfoliated electrochemically, and then single Pt atoms were immobilized in the Mo vacancies (Figure 9). The Mo₂TiC₂T_x-Pt_SA performed better than pristine Mo₂TiC₂T_x and Mo₂TiC₂T_x-V_Mo and achieved an overpotential of 30 mV at 10 mA cm⁻². Because of the strong interaction between the Pt atoms and Mo₂TiC₂T_x, Mo₂TiC₂T_x-Pt_SA exhibited exceptional stability as an HER electrocatalyst. Single-site Co-substituted 2D molybdenum carbide can be obtained from a Co-substituted Mo₂GaC MAX precursor.¹⁰⁷ At 10 mA cm⁻² Mo₂CT_x:Co showed better activity than Mo₂CT_x with an overpotential of 180 mV. According to the DFT results, the co-substituted Mo₂CT_x improved the HER kinetics by increasing hydrogen adsorption on the MXene surface.

Figure 9 (a) Synthesis of Mo2TiC2Tx-PtSA, (b) Polarization curve during HER Reprinted from108 with permission.

According to DFT calculations, hydrogen can be captured by the sides of MXene-containing metals and by C or N atoms. These species can act as fast-moving reaction sites for the evolution of hydrogen. The creation of particular nanostructures is a creative way to improve the HER activity of MXene. MXene nanoribbons or nanodots can be prepared using a variety of synthetic techniques, such as ball-milling¹⁰⁹ and hydrothermal treatment.¹¹⁰ HF etching can also be used to produce Ti₃C₂T_x nanofibers (NFs) from hydrolyzed Ti₃AlC_2.¹¹¹ With an overpotential of 169 mV at 10 mA cm⁻², the resultant Ti₃C₂T_x NFs performed better than Ti₃C₂T_x flakes, which had an overpotential of 385 mV. Furthermore, the specific surface area and number of active sites of MXene NFs were far greater than those of MXene flakes. Consequently, the exceptionally high HER performance of the Ti₃C₂T_x NFs can be attributed primarily to the enhancement of the specific surface area and the large number of active sites.

The combination of MXenes with various active materials, including chalcogenides,¹¹² layered hydroxides,¹¹³ phosphides,¹¹⁴ metal nanoparticles,¹¹⁵ and metal-free black phosphorus,¹¹⁶ has shown enhanced HER activity. Following freezing with liquid nitrogen and subsequent annealing, a hierarchical MoS₂/Ti₃C₂T_x structure was achieved.¹¹⁷ The rapid freezing process facilitated the rolling of the Ti₃C₂T_x nanowires and the formation of vertically aligned MoS₂ microcrystals. The MoS₂/Ti₃C₂T_x composite offered numerous active sites for electrocatalytic processes and enhanced the charge transfer. The catalytic performance was excellent, with an initial overpotential of 30 mV and low overpotential of 168 mV at 10 mA cm⁻². Furthermore, the exchange current density was improved more than 25 times. By placing several carbon-coated MoS₂ nanocrystals on carbon-stabilized Ti₃C₂ MXenes, a new composite material MoS₂/Ti₃C₂T_x@C was prepared.¹¹⁸ The resulting nanocrystal material exhibited exceptional HER activity and constancy in acidic environments. In addition to MoS₂, other transition metal chalcogenides, such as NiS₂,¹¹⁹ VS₂,¹²⁰ and MoSe₂,¹²¹ in combination with MXenes can improve HER activity. MXene-supported metal nanoparticles (NPs) can significantly enhance HER activity. Li et al described the presence of Pt₃Ti NPs on Ti₃C₂T_x MXenes, where Pt interacted with Ti through in-situ co-reduction.¹²² As the temperature increased, Pt atoms were converted to intermetallic compounds, as observed from the in-situ X-ray absorption spectra Pt/Ti₃C₂T_x-550 exhibited excellent performance, and a mere 32.7 mV potential was required at 10 mA cm⁻². Wang et al developed a new composite, FeNi@Mo₂TiC₂T_x @Ni foam (NF), by incorporating Fe²⁺ ions and subsequent in situ coupling with surface Ni atoms on nickel foam.¹²³ FeNi@Mo₂TiC₂T_x@NF displayed high HER activity with an overpotential of 165 mV at 10 mA cm⁻², which is attributed to the synergistic effect of Mo₂TiC₂T_x and FeNi nanoalloys.

MXenes-Based Photocatalysts for Hydrogen Production

Remarkable electrical conductivity, high surface area, and variable surface functionality are some of the key factors contributing to the promising activity of MXene-based materials in photocatalytic hydrogen evolution reactions. These properties can improve the charge separation and electron flow in the photocatalysts. After combining with semiconductors, such as TiO₂, g-C₃N₄, or metal sulfides, MXenes promote the effective absorption of light and electron flow. Additionally, strong interfacial bonding with other photocatalytic materials is made possible by their surface functional groups, which increase photocatalytic activity. Table 3 presents various 2D MXene-based photocatalytic systems and their hydrogen production activities.

Table 3 Photocatalytic Hydrogen Production Activity by Various MXenes Based Catalysts

To increase the efficiency of hydrogen production, Chen et al created a CdS NS@Ti₃C₂ MXene composite using a one-step solvothermal method.¹³⁹ CdS NS@Ti₃C₂ exhibited the best photocatalytic performance and achieved a hydrogen evolution rate of 1.73 mmol g⁻¹ h⁻¹, which is almost five times higher than that of CdS nanosheet. UV–vis DRS analysis of the composite material indicated improved light absorption owing to the addition of Ti₃C₂ MXene. Photoluminescence (PL) and time-resolved PL analyses showed that the carrier lifetime of CdS NS@Ti₃C₂-5 was longer than that of CdS NS. The apparent quantum efficiency (AQE) showed a maximum value of 9.56% when irradiated with light at 370 nm. This result highlights the collaborative effect of 2D CdS and Ti₃C₂ MXene, which enhances the effective transfer of photogenerated charges and significantly increases hydrogen production in the absence of metal co-catalysts. Similarly, Huang et al developed a Ti₃C₂T_x MXene/CdS (CMX) composite through the in situ growth of CdS nanosheets on exfoliated Ti₃C₂T_x via a solvothermal approach.¹²⁴ The optimized catalyst (CM3, containing 3% MXene) exhibited a hydrogen production rate of 28.7 mmol g⁻¹ h⁻¹ under visible light irradiation. This heterojunction showed 4.2 times higher activity compared to pure CdS (6.9 mmol g⁻¹ h⁻¹) and 26 times higher activity relative to the Pt-loaded CdS (11.2 mmol g⁻¹ h⁻¹). Moreover, the CM3 catalyst exhibited excellent recyclability over four consecutive cycles. This enhanced reactivity is due to synergies, improved light absorbability, Schottky-type charge separation, and solvothermal effects.

Modification of surface terminations greatly enhances photocatalytic hydrogen evolution using MXene-based catalysts. Ran et al developed a CdS/Ti₃C₂ photocatalyst using a hydrothermal method to substitute expensive Pt with more abundant co-catalysts for hydrogen production through a photocatalytic process.¹²⁵ The surface of Ti₃C₂ was modified by introducing –O and –OH groups to prepare a CdS/Ti₃C₂ heterojunction photocatalyst. It showed an impressive hydrogen production of 14,342 μmol g⁻¹ h⁻¹ under visible-light irradiation, which is higher than that of the 2.5 wt% Pt-loaded CdS. During the hydrothermal process, the-F terminations of Ti₃C₂ were substituted by -OH and -O groups. This modification increased the number of active sites -O functional groups and enhanced the efficiency of MXenes for the HER.

Wang et al explain the synthesis of a 2D/3D CdS nanoflowers/Ti₃C₂ MXene heterostructures and its photocatalytic hydrogen evolution efficiency.¹⁴⁰ Utilizing 0.25 M Na₂S and 0.35 M Na₂SO₃ in water as the sacrificial solution, the hydrogen generation rate of 15 wt% Ti₃C₂/CdS under visible light irradiation reached at 88.162 mmol g⁻¹ h⁻¹. Integrated lamellar Ti₃C₂ MXene enhances the separation efficiency of photogenerated carriers and improves light absorption owing to the integration of the 2D and 3D structures. This composite promoted swift electron flow from the CdS to the Ti₃C₂ layers, which minimized charge recombination. Xiao and Zhang prepared a 1D/2D CdS/Ti₃C₂ composite through the growth of CdS nanorods on Ti₃C_2.¹⁴¹ This nanocomposite, containing 20 mg of MXene, demonstrated notable photocatalytic hydrogen generation of 2407 μmol g⁻¹ h⁻¹ under visible light irradiation. This enhanced photoactivity is due to the distinctive 2D structure, high electrical conductivity, and superior light-harvesting ability of Ti₃C_2. In a similar manner, Ding et al employed a series of electrostatic self-assembly processes and a solvothermal technique to produce 2D/2D CdS/Ti₃C₂T_x composites and examined their photocatalytic hydrogen evolution.¹⁴² The resulting photocatalyst achieved a hydrogen evolution rate of 3226 μmol g⁻¹ h⁻¹ when lactic acid was used as the sacrificial chemical (Figure 10).

Figure 10 (a) Preparation of CdS/Ti3C2Tx composites, (b) Mechanism for H2 generation, and Photocatalytic activity for H2 generation (c and d) Rainbow arrow: Visible light, Dashed arrow: Transition of electron from VB to CB Reprinted from126 with permission.

The work presented by Huang et al illustrated the in situ formation of a 1D CdS/2D Nb₂CT_x MXene Schottky heterojunction to enhance photocatalytic hydrogen generation.¹²⁶ In this respect, the solvothermal method was adopted to ensure strong interfacial contact between CdS nanorods and Nb₂CT_x MXene nanosheets. This arrangement promotes efficient charge separation, which is essential for the photocatalytic processes. The optimized CdS/Nb₂CT_x composite showed a hydrogen production rate of 5040 μmol g⁻¹ h⁻¹ when exposed to visible light irradiation. This heterojunction shows an enhancement of about 4.3 times in hydrogen production activity over pristine CdS nanosheets. This enhancement in performance is due to the formation of a Schottky heterojunction between CdS and Nb₂CT_x. In addition, Nb₂CT_x MXene controls the agglomeration of the CdS nanorods, thus maintaining a high surface area that promotes catalysis.

Sherryna et al explored a V₂C MXene/g-C₃N₄ nanocomposite as an effective photocatalyst for hydrogen evolution under visible-light irradiation.¹²⁷ The optimal composite containing 15 wt% vanadium carbide (V₂C) exhibited a hydrogen production rate of 360 mmol g⁻¹ h⁻¹, almost four times greater than that of g-C₃N_4. The strong interaction between the two surfaces and the Schottky barrier formed at the V₂C/g-C₃N₄ interface facilitated charge separation and delayed recombination. The group tested various sacrificial solvents, including methanol, glycerol, and Triethanolamine (TEOA), for their effects on the photocatalytic activity. Methanol, which acts as a sacrificial reagent, produces the maximum amount of hydrogen, followed by TEOA. This observation indicates that sacrificial reagents can significantly enhance catalytic activity and directly influence the photoredox reaction as h+ scavengers.

Li et al developed g-C₃N₄@Ti₃C₂ quantum dots (QDs) using a self-assembly method and examined their photocatalytic hydrogen generation capabilities.¹²⁸ An ideal photocatalyst with 100 mL Ti₃C₂ MXene QDs exhibited a remarkable photocatalytic hydrogen generation rate of 5111.8 μmol g⁻¹ h⁻¹. This remarkable performance was due to the enhanced specific surface area of g-C₃N₄, increase in the number of active sites, and enhanced electronic conductivity of the composite material. Wong et al developed a crystalline g-C₃N₄/Ti₃C₂T_x MXene by integrating Ti₃C₂T_x MXene in crystalline g-C₃N₄ through a combined salt-assisted and freeze-drying method to enhance the photocatalytic production of H_2.¹⁴³ The ideal photocatalyst g-C₃N₄/Ti₃C₂T_x/Pt (0.5 wt% Ti₃C₂T_x) achieved a H₂ generation rate of 2651.93 μmol g⁻¹ h⁻¹. The authors explained that the enhanced performance is due to the synergistic effect of the strongly crystalline phase of g-C₃N₄, which promotes rapid charge mobility, along with the strong role of the dual co-catalysts Ti₃C₂T_x and Pt, which enhances the charge separation efficiency and produces numerous active sites. In general, g-C₃N₄/Ti₃C₂T_x composites without co-catalyst Pt exhibited lower photocatalytic activities than those of the g-C₃N₄-MXene system with Pt. For example, g-C₃N₄/p-Ti₃C₂T_x synthesized by Kang et al¹²⁹ showed a hydrogen generation rate of 982.2 μmol g⁻¹ h^−1, whereas a higher photocatalytic hydrogen evolution rate of 1948 μmol g⁻¹ h⁻¹ was reported for g-C₃N₄/Ti₃C₂T_x/Pt synthesized by Liu et al.¹³⁰

Li et al described the synthesis of an innovative photocatalyst composed of Ti₃C₂ MXene, MoS_2, and TiO₂ nanosheets (Figure 11).¹³¹ This ternary heterostructure showed a remarkable improvement in photocatalytic hydrogen generation under simulated sunlight irradiation compared with its individual and binary analogs. This improvement was due to the effective charge pair separation and transport resulting from the closely packed 2D–2D interface, high surface area, and more active sites from TiO_2. A further increase in the hydrogen evolution rate of 6425.30 μmol h⁻¹ g⁻¹ was demonstrated by the ideal composite (Ti₃C₂@TiO₂@MoS₂ with 15 wt% MoS₂) compared to TiO₂ and the composite Ti₃C₂@TiO_2. This study outlined the synergistic effect of the three constituents, where Ti₃C₂ acted as an electron pool, MoS₂ served as a co-catalyst that reduced the overpotential, and TiO₂ was the primary light absorber.

Figure 11 (a) Synthesis of Ti3C2@TiO2@MoS2, (b) Photocatalytic hydrogen production with time, (c) Comparison of photocatalytic hydrogen production rate of different composites Reprinted from131 with permission.

To increase the efficiency of photocatalytic hydrogen evolution using Ti₃C₂ MXene-based materials, a new PtO/Ti₃C₂/TiO₂ ternary composite was synthesized using an in-situ oxidation and photodeposition process by Yang et al.¹³² Firstly, Ti₃C₂ MXene was oxidized thermally to form a Ti₃C₂/TiO₂ heterojunction, and then PtO nanodots were deposited to improve catalytic performance. Among the prepared materials, the PtO/Ti₃C₂/TiO₂-600 demonstrated the highest hydrogen generation compared to Ti₃C₂/TiO₂-600 without PtO. Sun et al prepared a highly effective and stable CuZnInS/Ti₃C₂ catalyst for hydrogen production.¹³³ The improvement in catalytic activity is attributed to the effective inhibition of charge-carrier recombination due to the presence of MXene. The application of co-catalysts is a well-established approach for enhancing the activity of photocatalysts. M Shao et al reported that MoS₂/Mo₂C acted as a highly effective co-catalyst for photocatalytic hydrogen generation.¹⁴⁴ The CdS-based catalyst, which was modified by integrating MoS₂/Mo₂C, has significant results in photocatalytic hydrogen evolution and showed a remarkably high rate of 34,000 μmol g⁻¹h⁻¹, which is nearly 112 times higher than that of pure CdS.

Density Functional Theory (DFT) Modeling of HER Activity in MXenes

DFT modeling is significantly important for predicting the HER activity of MXenes by studying the influence of terminal surface functional groups on electronic structure and adsorption characteristics. The Gibbs free energy of hydrogen adsorption (ΔG_H*) is commonly applied as an activity descriptor for the hydrogen evolution reaction.¹⁴⁵

ΔG_H* = ΔE_ads + ΔE_ZPE + TΔS

where ΔE_ads and ΔE_ZPE indicate the adsorption energy and zero-point energy correction based on DFT calculations, T and ΔS represent the temperature and the entropy change. Near-zero values for this parameter ensure optimized HER activity. DFT predictions always show that the terminated surfaces containing –O and –OH groups affect the d-band center of transition metals present in MXenes. As a result, favorable hydrogen binding is observed compared to MXenes terminated by -F groups, which decrease the HER activity by exhibiting unfavorable electronic interactions.¹⁴⁶ Predicted DFT values for terminated surfaces containing varying amounts of ΔG_H* are in good agreement with experimentally measured values for obtained overpotentials and Tafel slopes on MXene surfaces.¹⁴⁶ Thus, DFT modeling serves as a vital bridge between theory and experiment for balanced MXene-based HER catalyst design.

Stability of MXenes-Based Materials

MXenes have drawn rapidly growing interest in biosensing and catalysis for their remarkable properties, but their poor oxidation stability remains a critical limitation to their practical implementation in aqueous and ambient environments.¹⁴⁷ In water and moist air, MXenes undergo gradual oxidation, forming TiO₂, amorphous carbon, or gaseous byproducts, which reduces the conductivity, structural integrity, and catalytic activity.^148,149 Recent studies have shown that such oxidation is highly site-specific, starting preferentially at flake edges, defect-rich regions, and undercoordinated Ti sites where water and dissolved oxygen readily adsorb. Lipatov et al observed that the oxidation of MXene initiates at the edges, where there are completely uncovered areas that are exposed to oxidizing substances.¹⁴⁹ Hydrolysis-driven pathways have increasingly been recognized as dominant under ambient conditions, often offsetting the role of molecular oxygen alone.^150,151 For the first time, the water-induced degradation of MXenes was studied by Wu et al in 2022.¹⁵¹ They performed a first-principle molecular dynamics (FPMD) simulation, and the results indicated that the interaction of water molecules with the basal plane of the MXene happens simultaneously with the breaking of the bonds between titanium and the MXene sheets. Water molecules irreversibly adsorbed on the surface of Ti atoms, deprotonated it and pullout Ti atoms. To mitigate degradation, several stabilization strategies have been developed, including antioxidant capping using ascorbates or polyphenols that covered surface Ti atoms and suppress water attack, as well as edge passivation via polyanions or surfactants.¹⁵² Inert or low-temperature storage, high-concentration dispersions, and controlled pH environments further extend MXene shelf-life. These advances are essential for improving MXene biocompatibility, preserving catalytic longevity, and enabling consistent performance in real-world aqueous applications.

Conclusion and Future Perspective

MXenes have seen major breakthroughs owing to their uniqueness in being as two-dimensional material, high hydrophilicity, high electrical conductivity, large potential for surface modification, high malleability, and ion insertion. With scientists investigating further into finding new uses for MXene in different domains of science, major advancements have been made in the development of supercapacitors for holding chemical energy. This review article aims to cover the structure, properties, preparation methods, and applications of MXene-based composites in developing biosensors and hydrogen production, covering recent studies on various composites of MXene. Presently, the major methods of MXene preparation are through etching, which can be either acidic or alkaline, and have mostly focused on HF etching. This method presents a major hazard with regard to the use of highly corrodible substances such as waste acids and bases. These traditional methods, besides being harmful to the environment, involve low mass production and incur major costs. Although direct preparation methods involve advancements in finding ways to synthesize MXenes without releasing toxic substances, there is still a major need for advancements in their production efficiency. Hence, there is a major urgency to develop a novel method of MXene preparation, which would help in scaling up their manufacturing.

Currently, MXenes are considered some of the most promising materials owing to their numerous applications, including environmental monitoring, biosensing, and health monitoring. They are especially well suited for biosensing because of their remarkable physicochemical characteristics, including excellent electrical conductivity, biocompatibility, and surface modifications. In this review, a critical overview of the recent advances in MXene-based biosensors is presented. Even with this promising development, issues that need to be considered include enhancement in large-scale production, reproducibility, and biocompatibility of MXene-based biosensors with reduced toxicity and ensuring their long-term stability. A major challenge is the scalability of MXene-based biosensors for industrial manufacturing. Further studies are needed to enhance the reproducibility of these sensors in practical applications and ensure consistency across batches. In this review, recent developments in the use of MXenes as electrocatalysts for HER are discussed. Although MXene-based electrocatalysts have been greatly improved in recent decades, it is still challenging to produce highly active electrocatalysts that can outperform Pt-based materials and be commercialized. These materials offer excellent solar energy absorption and conversion capabilities for the production of hydrogen. The overall photocatalytic performance of MXenes increases with enhanced light absorption, effective charge carrier separation, and enhanced surface reactions upon coupling with other semiconductors.

Recent advancements in MXene-based nanomaterials have shown great potential in the design of photocatalytic semiconductors; however, the journey to establish stable nanocatalysts is long. Although these nanomaterials hold incredible promise, they must undergo deep ecotoxicological assessments and life-cycle evaluations before further applications. Future studies related to kinetics and thermodynamic control technologies involved in the synthesis of MXene-based photocatalysts should be conducted. With the high photocatalytic efficiency of the removal of organic pollutants in water, it is possible to expect that MXene-based composite photocatalysts will also successfully serve in the case of air pollutants, such as volatile organic compounds, NO, SO₂, H₂S, NO₂, and various exhaust emissions.