REVIEW ARTICLE ON POLYMERIC NANOPARTICLE Final Work 20240304 Characterization and Application of Nanomaterials 2024, 7(2), 6348. https://doi.org/10.24294/can.v7i2.6348 1 Review Synthesis, technological prospects and applications of MXene in biomedicine, supercapacitors and sensors: A review Nujud Badawi M. 1,* , M. Bhuyan 2,* , Namrata Agrawal 3 , Yogesh Kumar 4 1 University of Hafr Al-Batin College of Science, Hafer Al-Batin 39921, Saudi Arabia 2 Institute of Physics, Sachivalaya marg, Bhubaneswar 751005, India 3 Department of Physics, Swami Shraddhanand College, University of Delhi, New Delhi 110036, India 4 Department of Physics, ARSD College, University of Delhi, Delhi 110021, India * Corresponding authors: Nujud Badawi M., nujuds@uhb.edu.sa; M. Bhuyan, bunuphy@um.edu.my Abstract: MXenes are one of the most important classes of materials discussed worldwide by many researchers of diverse fields for diverse applications in recent years. It is a nanomaterial with a wide range of applications due to its multiple forms and structures with fascinating properties, for example, high surface area and porosity, biocompatibility, ease of fictionalizing with various active chemical moieties, benefit of high metallic conductivity, activated metallic hydroxide sites, and sensitivity to moisture. MXenes have great chances for potential applications in environmental issues, water purification, biological applications, and energy storage devices and sensors. MXenes show great selectivity towards the absorption of heavy metals and a good capability to reduce chemical and biological pollutants present in the water. The present review article critically analyzed advancements in water purification using the adsorption and reduction abilities of MXenes and their composites. The mechanism of various procedures, important challenges, and associated problems using MXene and their composites are discussed in detail. The future research directions can be extracted from this article efficiently and comprehensively. The energy storage issues of rechargeable lithium-ion batteries, batteries other than lithium-ion batteries, and electrochemical capacitors are also discussed in detail. Keywords: 2D materials; MXene; energy storage; batteries; capacitors 1. Introduction In 2011, a new type of two-dimensional substance called MXene was discovered for the first time. The MXene family sparked intense interest in research across many disciplines due to its diverse chemical composition and excellent physical and chemical properties, particularly in the areas of energy storage, the environment, catalysis, and biomedical applications, the latter of which, in particular, is experiencing rapid growth. Since graphene’s exfoliation [1,2], a lot of attention has been paid to other 2D layered materials [3,4] because of their exceptional electrical, mechanical, and optical capabilities. Most studied examples of 2D materials other than graphene include transition metal dichalcogenides [4,5], phosphorene [6,7], and their derivatives. In recent years [6], a family of 2D-layered compounds known as “MXenes” has attracted a great deal of interest from the scientific community due to their unusual structural and electrical properties, which make them applicable to a wide range of fields. MXenes are compounds produced by the chemical delamination of ternary (or quaternary) layered carbides or nitrides. The unique properties of the MXene family CITATION Badawi M. N, Bhuyan M, Agrawal N, Kumar Y. Synthesis, technological prospects and applications of MXene in biomedicine, supercapacitors and sensors: A review. Characterization and Application of Nanomaterials. 2024; 7(2): 6348. https://doi.org/10.24294/can.v7i2.6348 ARTICLE INFO Received: 11 May 2024 Accepted: 27 May 2024 Available online:14 September 2024 COPYRIGHT Copyright © 2024 by author(s). Characterization and Application of Nanomaterialsis published by EnPress Publisher, LLC. This work is licensed under the Creative Commons Attribution (CC BY) license. https://creativecommons.org/licenses/ by/4.0/ Characterization and Application of Nanomaterials 2024, 7(2), 6348. 2 are due to their electrical structures, atomic stacking, synthetic methods, surface terminal groups, and peculiar bonding, which consists of metallic and covalent connections. Despite MXenes rising star power, there is a dearth of comprehensive information on them. Many sensors [2–4], as well as portable and wearable electronic devices [5–7], have emerged to sense and interact with physical worlds to foster the development of a smarter life with the vigorous rise of the internet of things (IoT) and the great improvement of the reliability of wireless communication protocols [1]. Despite this, a reliable power supply [8–10] requires the successful integration of on-chip energy- storage units with novel electronic devices due to the intermittent nature of renewable resources (wind [11], solar [12], tidal [13], and geothermal energy [14]). Conventional micro-batteries have a high energy density [15] and can keep devices running for a while, but they are unable to meet the requirement of rapid power delivery in applications where battery replacement is impractical [16]. Micro- supercapacitors (MSCs) have gained widespread interest as a micro-battery substitute because of their fascinating long lifetime, high power density, and fast charging rate [17]. However, their progress in microelectronics is hampered by their low power and energy density [18]. With the rapid development of biomedicine and the attractive physiochemical properties of two-dimensional materials, many researchers have focused on the biomedical applications of two-dimensional materials, such as hexagonal boron nitrides (h-BN), graphene and its derivatives, transition metal oxides (TMOs), layered double hydroxides (LDHs), and MXenes. MXenes have more functional groups on their surface, allowing for greater versatility in modification than most other 2D materials. MXenes are useful for biomedical applications due to their hydrophilicity, compositional versatility, and the presence of full metal atomic layers. Another advantage is that MXenes can be mass-produced at low cost. Bioimaging, antimicrobials, biosensors, drug delivery, tissue engineering, and a wide range of therapeutics are just some of the ways MXenes have been put to use in the medical field so far (Figure 1a) [19]. Because of their high near-infrared light absorption and conversion efficiency, MXenes have found applications in photothermal therapy for tumour ablation. This form of treatment is highly effective in targeting cancer cells while causing minimal collateral damage to healthy tissue. In addition, bioimaging can be used in real-time to track the tumour’s location as contrast chemicals are administered during cancer treatment. In addition, the modified MXenes can also carry anticancer medications and release pharmaceuticals in a specific manner. Photothermal therapy, chemotherapy, and real-time bioimaging monitoring have considerably increased the success rate of cancer treatment thus far. As a bonus, MXenes are proving to be promising raw materials for developing biocompatible tools for easy, rapid detection of biological events [19]. Characterization and Application of Nanomaterials 2024, 7(2), 6348. 3 Figure 1. (a) Properties and biomedical applications of MXenes [19]; (b) MXenes in a wide range of applications [20]. The purpose of this study was to analyze and remark on the most important basic and technological aspects of MXenes about their structure, electronics, properties, applications, and devices (see Figure 1b) [20]. The history of MXenes’ creation is presented first, followed by examples of MXene production, structure, and characteristics, surface modification schemes, biomedical applications, cytotoxicity, and biocompatibility. We have examined the key features and properties of MXenes. The present status, trends and future possibilities in various fields, such as energy, electronics, biomedicine, etc., have been critically examined. Finally, current challenges and future potential applications of MXene have been discussed. 2. Logic of MXene New 2D transition metal carbides, nitrides, and carbonitrides called MXenes (pronounced “machines”) were first fabricated by wind [11], solar [12], tidal [13], and geothermal energy [14]. Hydrofluoric acid (HF) was used at room temperature to selectively etch the “A” (Al atoms) in layered hexagonal ternary carbide, Ti3AlC2, to produce the first MXene made of 2D titanium carbide (Ti3C2). From the precursor MAX phase (Mn+1AXn), MXenes can be constructed with the general formula Mn+1XnTx (n = 1–3), where M is an early transition metal, X is carbon and/or nitrogen, A is an element from groups 12 to 16, T is the surface termination groups like fluorine (–F), oxygen (=O), chlorine (–Cl), and hydroxyl (–OH), and x is the number of surface functionalities [21]. There is a strong M-X bond and a comparatively weak M-A bond in the MAX phase, where an A layer is sandwiched by octahedral Mn+1Xn [22]. Elements that produce MAX phases (Mn+1AXn) are denoted in bold in Figure 2 [23]. Depending on the n value in MXenes (Mn+1XnTx), the interlayer spacing can be anywhere from 0.1 to 1.0 nm [8]. Characterization and Application of Nanomaterials 2024, 7(2), 6348. 4 Figure 2. Elements in the periodic table that are known to form Mn+1AXnphases. Reprinted with the permission from Ref [23]. 3. Synthesis methodologies of MXene: Procedures and structures About 30 different compositions of MXenes have been described in the literature, all of which have been synthesized from MAX phase precursors, with the addition of two or more transition metals to the M layers as the primary route to do so [20]. In particular, MXenes based on titanium, such as Ti3C2Tx and Ti2CTx, are widely used for ecological purposes [1,17]. Other MXene structures, such as those including nitride and carbonitride, have also found use outside of the realm of carbides [23]. MXenes’ distinct layered structures and 2D morphology make them an ideal candidate for use in composites, which can be utilized to improve the properties of other materials [8]. Preparing a sample for synthesis involves taking a few parameters into account, like safety, cost, procedures, instruments (such as monitors and gauges), and control systems. Etching, exfoliation, and chemical vapour deposition (CVD) are the three most common approaches to MXene production. Etching and exfoliation are examples of top-down processes, while chemical vapour deposition, the template method, and plasma-enhanced pulsed laser deposition are examples of bottom-up techniques. The most popular method for synthesizing 2D MXenes is the top-down etching of ‘A’ from MAX phases (Mn+1AXn), where M is an early transition metal; A is one of numerous elements (Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Tl, or Pb); X is C, N or both; and n is 1–4. Metallic bonds are typical between M and A, but covalent, ionic, or metallic bonds can exist between M and X [19]. As a result, it is difficult to break the M-A connection using mechanical shearing or direct exfoliation. Etchants, such as HF, can, however, etch it selectively. Two processes are required in the synthesis of MXenes [20,24] (Figure 3) [25]: First, selective etching of the A element from a precursor (MAX phases or other layered ceramics); second, delamination to obtain a single-layer MXene. However, the manufacturing of MXenes for energy storage applications has not been thoroughly addressed using these methods. The processes of MXene manufacturing are explained in greater detail below. Each MXene has a mother MX phase from which it is created, where “M” represents an early transition metal, “A” represents a group 13 or 14 element, “X” represents carbon or nitrogen, and “n” ranges from 1 to 3 [21]. Characterization and Application of Nanomaterials 2024, 7(2), 6348. 5 Figure 3. Synthesis of Mxene—selective etching of MAX and non-MAX phases to form multilayered MXenes, followed by delamination (chemical/mechanical) to produce a colloidal suspensionof single-layered MXene [25]. MXenes. In the top-down process, MXenes were synthesized from the bulk by a two-stage procedure: chemical etching and delamination [8,26]. To remove the interleaved layers of ‘A’ atoms from the bulk ceramic, an etchant is utilized to cleave the M-A metallic connection. The aggregated nanosheets are then delaminated via liquid-phase exfoliation [8], tetrapropylammonium hydroxide (TPAOH) intercalation [27,28], or ultrasonication [26]. In Figure 4a [29], a schematic depicting the steps involved in creating MXene is given. To prevent the dangers of fluorine-based etchants, fluorine-free techniques are also used [30]. Bottom-up methods often employ CVD technology, which works upwards from the atomic or molecular level [31]. Both top-down synthesis and bottom-up approaches can be used to create Characterization and Application of Nanomaterials 2024, 7(2), 6348. 6 Figure 4. (a) Fabrication of 2D ultrathin Ti3C2 nanosheets by etching, delamination, and surface modification along with its crystal structure [29]; (b) different structures of MXenes [27]. 4. Simulation-based studies on MXene structures To complement experimental studies and get a more in-depth understanding of MXene structures, computer simulations are frequently used. MXenes typically end with surface groups, including –F, –OH, and –O, following exfoliation from the MAX phase. The usefulness of such structures is determined by the relationships among their features and characteristics [26]. There are three possible configurations of MXenes, all of which correspond to the parent MAX phases and have a single Characterization and Application of Nanomaterials 2024, 7(2), 6348. 7 metal in the M site: M2C, M3C2, and M4C3, where M is an early transition metal, A is a particular element from groups 13 or 14, X is carbon or nitrogen, and n is a number from one to four. The A element must be carefully removed from a MAX phase or other stacked precursor (such as MO2Ga2C). In MAX phases, X atoms fill octahedral sites in an otherwise hexagonally stacked M layer with P63/MMC symmetry. As a result, the A and M elements form metallic bonds. Mn+1Xn layers and MXenes containing ordered double-transition metals, like (Cr2V) C2 and (MO2Ti2) C3, are interwoven throughout the “A” element [29]. Computer modelling has helped disclose the structures of MXenes and has also aided in the discovery of new, stable MXenes. There are six distinct MXene structures: (1) Nb4C3 and Ti2C are examples of mono-M components; (2) (Cr, V)3C2 and (Ti, V)3C2 are examples of solid solutions; (3) ordered out-of-plane two-fold M components have one transition metal (typically Cr and Mo) filling the outer layers and the other the focus layers (Ta and Nb); (4) ordered in-plane two-fold M components have the distinguishing M components arranged in the basal plane, such as in [22,27,29–32]. Structures of different MXenes are shown in Figure 4b. The surface terminations (–F, –OH, and –O) group linked to M atoms give MXene its chemical formula, Mn+1XnTz (1 n 3). Figure 5a [22] depicts the steps required to prepare MXene. A typical schematic diagram is depicted in Figure 5b [33], representing the process of synthesizing MXenes from MAX phases. Characterization and Application of Nanomaterials 2024, 7(2), 6348. 8 Figure 5. (a) MXenes, derived from the parent layered solids MAX phases [22]; (b) the schematic diagram represents the process of synthesizing MXenes from MAX phases. Reproduced with permission from [33]. 5. Applications 5.1. Prospects of MXenes in biological applications The promising results of MXenes in stem cell-based tissue treatments and the material sciences make it reasonable to expect future multi-tasking biomedical therapies to be based on their unique properties [33]. Titanium carbide (Ti3C2) MXene nanofibers were used to create smart biomaterials for tissue engineering and cell culture in one study. Fabrication of hydrophilic composite nanofibers via electrospinning and doping was carried out by Chen et al. [34]. The MXene nanofiber composites’ surface functional groups were considered to foster healthy Characterization and Application of Nanomaterials 2024, 7(2), 6348. 9 cellular environments. Bone marrow-derived mesenchymal stem cells (BMSCs) were used to analyse the biochemically representative characteristics, and the resulting MXene composite nanofibers showed good biocompatibility and significantly improved cellular activity, as well as boosting BMSC differentiation to osteoblasts. Smart biomaterials, which have many desirable biological characteristics, are very useful in tissue engineering and repair procedures [35]. Because of their immunomodulatory properties, which selectively reduced the activation of human CD4+IFN-+ T-lymphocytes while stimulating the expansion of immunosuppressive CD4+CD25+FoxP3+regulatory T-cells in a stimulated lymphocyte population, two- dimensional structures with BMSC biocompatibility could stimulate fibroblasts derived from bone marrow-derived stem cells (BMSCs) for post-injury tissue repair. [35,36]. The incorporation of Ti3C2MXene QDs (quantum dots (MQDs) sensor) into a chitosan hydrogel resulted in the creation of a 3D platform with enhanced physicochemical characteristics, which can facilitate stem cell trafficking and tissue restoration. The resultant composite hydrogel was both injectable and thermos- sensitive, and it showed excellent conductivity [37]. In a similar line, biocompatible Ti3C2Tz-enhanced poly (lactic acid) nanocomposite membranes were created by mediating the interface between the hydrophobic poly (lactic acid) matrix and the Ti3C2Tz nanosheets. These membranes achieved an optimized tensile strength of 72 MPa, which is approximately 33% greater than a pure poly (lactic acid) membrane [37,38]. Therefore, Ti3C2Tz insertion into the membrane might improve its biological characteristics, including osteogenic differentiation, proliferation, and in vitro adhesion of MC3T3-E1 murine preosteoblasts. MXenes’ exceptional properties, which include hydrophilicity, high electronic conductivity, and adsorptive, reductive, and antibacterial properties, make them well-suited for a wide range of ecological uses. The adaptability of MXenes makes them desirable for a wide range of uses. Their chemical stability, ion intercalation, and adjustable bang gaps suggest catalysis and energy storage applications like fuel cells, hydrogen storage, and lithium-ion batteries (LIBs), while their high Young’s modulus, good electrical conductivity, and surface chemistry alteration are appealing for the creation of composites. 5.2. MXenes for environmental and water treatment applications 5.2.1. H2O2 oxidation activated by MXene-based materials Hydrogen peroxide (H2O2) oxidation has attracted much attention in the removal of pollutants [36], where Fe2+ can act as a catalyst to form free radicals [36,39]. 2D MXene has a layered structure and surface electronegativity. Metal particles can be loaded onto the surface and layered surfaces of 2D MXene [24,40]. In addition, 2D MXene can also degrade metal nanoparticles and inhibit the aggregation of metal nanoparticles [41]. Interestingly, metal particle-loaded 2D MXene has a larger surface area and better pore structure than pure 2D MXene. The pores in the composite can serve as microreactors for H2O2 activation. Magnetic nanoscale zero-valent iron particles (nZVI)@Ti3C2 nanosheets could remove 91.1% of ranitidine in 30 min [42]. Therefore, 2D MXene-supported metal particles can be Characterization and Application of Nanomaterials 2024, 7(2), 6348. 10 considered a good catalyst to activate H2O2 to degrade organic pollutants. Wu et al. [43] developed a nanoplatform based on multimodal TMO connecting ultrathin Ti3C2 nanosheets to self-assembled MnFe2O4 nanoparticles using chitosan as a chemical cross-linking agent to form an interfacial Schottky junction, Ti3C2@Chitosan-MnFe2O4(TC@Ch-MFO). This led to an improvement in reactive oxygen species (ROS) production and optimization of biocompatibility. This heterojunction could control the catalysis of H2O2 to produce O2 and deplete excess glutathione levels in the hypoxic tumor microenvironment, so that under near- infrared stimulation, the cyclization was completed by the Fenton reaction. TC@ChMFO also provided a multifunctional nanoplatform for photothermal therapy by introducing an effective photothermal agent, Ti3C2. Additionally, it integrated visualization with T1- and T2-weighted magnetic resonance imaging simultaneously (see Figure 6a) [43]. As the toxicity of TC@ChMFO to normal tissues is negligible, this platform might provide new insights into the development of multimodal synergistic nanoplatforms for biological applications. Figure 6. (a) MXene-based Schottky junctions self-assembled transition metal oxides for near-infrared radiation to modify the tumor microenvironment and enhance CDT/PTT/MRI activation [43]; (b) oxidized MXene co-wrinkled multi scale porous structure [44]. Lee et al. [44] proposed a new method to produce MXene oxidative wrinkled multiscale porous structures through a sequential process involving partial oxidation with H2O2 as agglomeration in acid (see Figure 6b). These processes lead to the Characterization and Application of Nanomaterials 2024, 7(2), 6348. 11 simultaneous formation of mesopores caused by oxidation and macropores due to shrinkage patterns. The specific surface area of oxidatively co-degraded MXene (72 m2/g) was found to be five times that of pure MXene. The efficiency of the performance (307 F/g at 20 mV/s in 1 M H2SO4) and the capacity of the scan (225 F/g at 100 mV/s) were also found to be better than in pure MXene. Additionally, 87.6% of capacity was retained after 6000 cycles. This was due to the structure of porous, oxidized, folded MXene. Here, the diffusion of ions through mesopores was promoted, while folded macropores prevented restacking. The performance and mechanism of Ti3C2 MXene-modified Fe3+/hydrogen peroxide system in dark light and visible light were compared in detail, and a new standard for coating Fe3+ reactions and reductions of MXene was proposed by Xu et al. [45]. Taking Bisphenol A (BPA) as the target pollutant, the degradation effect of BPA in the Fe3+/H2O2 system was improved after adding MXene in the dark. The degradation of BPA in 12.5 min under visible light was ≥95%, which was 6 times higher than in the dark. Fe2+ was identified as a useful element for the activation of H2O2 to form OH. It was found that MXene formed a complex with Fe3+. MXene reacted with Fe3+, breaking the TiC bond and activating the Fe3+/Fe2+ cycle. MXene used photogenerated electrons to promote chemical reactions under light. OH and O2 were found to be the main reactive oxygen species in light, while OH was the main reactive oxygen species in light (see Figure 7a). Thus, MXene could utilize O2 to produce O2 and promote the Fe3+/Fe2+ cycle in light. This work provided a theoretical basis for the combination of visible light catalysis and the higher oxidation process of Ti3C2 MXene. Characterization and Application of Nanomaterials 2024, 7(2), 6348. 12 Figure 7. (a) Ti3C2 MXene promotes Fe3+/H2O2 Fenton oxidation: comparison of the process under dark light and visible light [45]; (b) the fabrication process of the Cu/Cu2O/TiO2-X electrode [46]. To detect H2O2, Li et al. [46] used in-situ produced TiO2-X nanoparticles to modify Cu/Cu2O nanoparticles on Ti3C2 MXene to form heterojunctions via a single- step hydrothermal process. The produced Cu/Cu2O/TiO2-X/Ti3C2(Cu/Cu2O/TT) was found to have a linear range of 28.328 mM, a sensitivity of 312 μA mM−1 cm−2 and detection limit of 0.42 μM. The synergistic effect of Cu/Cu2O nanoparticles and TiO2-X/Ti3C2 heterojunction not only improved the electron transfer and electrocatalytic activity but also increased the strength of the target molecules of the catalyst due to the abundance of extended catalytic sites. Therefore, compared to TiO2X/Ti3C2, the detection limit of Cu/Cu2O/TT was low, its reaction faster, and its sensitivity five times higher. Also, Cu/Cu2O/TT exhibited excellent photoelectrochemical sensing performance in detecting H2O2 with a low detection limit, long-term stability, repeatability, and selectivity. The fabrication process of the Cu/Cu2O/TT electrode followed by Li et al. [46] is shown in Figure 7b. Iron particles deposited on the 2D MXene surface may exhibit multiple reaction zones. 2D MXene can facilitate the electrical transfer of magnetic materials. In addition, there are many hydrophilic functional groups (such as OH, COOH, and Characterization and Application of Nanomaterials 2024, 7(2), 6348. 13 CHO) on the surface of MXene/nanoscale zero-valent iron particle nanosheets, designed to promote H2O2 activation [43]. Zhu et al. [47] used Ti3C2TX to develop a sensitive and enzyme-free electrochemical sensing interface to detect H2O2 simply and practically. Prussian blue (PB) was electrochemically deposited on the surface of the glassy carbon electrode (GCE). Chitosan (CS) and MXene were degraded on the PB modified GCE surface. Simple MX/CS/PB/GCE detection interface demonstrated good electrochemical detection performance and good selection for H2O2 with a low limit (4 nM) and a wide linear range from 50 nM to 667 μM. The light-driven magnetic MXene microrobot (MXeBOT) was developed as a moveable stage for the removal and degradation of bisphenol A (BPA) by Dekanovsky et al. [48]. A second control motor composed of embedded Fe2O3 nanoparticles (NPs) for magnetic push assisted the MXeBOT. Grafted bismuth nanoparticles acted as co-catalysts. It was investigated how BPA concentration and the chemical composition of the swimming environment affected the stability and reusability of MXeBOTs. MAXBOT could remove/degrade approximately 60% of BPA in just 10 minutes and approximately 100% in 1 hour. More than 86% of BPA mineralized within 1 h. Using Bi/Fe/MXeBOTs for the photocatalytic degradation of BPA demonstrated a significant advantage in converting BPA into CO2 and H2O [49]. 5.2.2. Other advanced water remediation applications of MXenes MXene and its compounds have also proven effective in other environmental applications such as membranes, capacitive deionization, and anti-bacterial agents. MXene is a promising product for water filtration due to its hydrophilicity, high surface area, and excellent electrical properties [48]. Molecules can be separated based on the size of the surface layer and the interaction with the MXene layer. In recent years, MXenes have been used in a variety of environmental applications for the treatment of contaminated groundwater and municipal wastewater, including desalination, outperforming the most widely used material in all regions [50]. MXene composites can adsorb a variety of organic and inorganic compounds and can undergo Faradaic capacitive deionization (CDI) when used in electrochemical applications. This method overcomes the concentration polarization limit of conventional CDI electrodes, greatly reducing the energy required and providing a solution for low-energy desalination of brackish water. Researchers provide an updated review of MXene and MXene mixtures for water purification and desalination applications. Kinetics and isotherms were examined, and the effects of water composition and activity discussed. Based on the literature review, the application of MXene in CDI, pervaporation desalination, and photothermal desalination have been studied. The impact of water composition and performance on recovery efficiency and long-term use has also been highlighted (see Figure 8a) [51]. Characterization and Application of Nanomaterials 2024, 7(2), 6348. 14 Figure 8. (a) Use of MXenes in water purification and energy-saving desalination [51]; (b) MXenes (2D metal carbides) as nanomaterials for water purification [49]. There is an interest in using MXene and its derivatives in water purification due to their unique properties. MXene has high electrical conductivity and hydrophilicity, as well as outstanding adsorption, reduction, and disinfection properties. In particular, Ti3C2TX and its compounds are widely used in water purification to remove various contaminants such as dyes, radionuclides, and heavy metals. Ihsanullah [49] evaluated the effectiveness of MXene and MXene-based mixtures in removing various pollutants from water, focusing on heavy metals, dyes, and radionuclides. The role of the MXene structure in the contaminant elimination process and the regeneration were reviewed and analyzed (see Figure 8b). The properties of MXene, such as high electricity, hydrophilicity, and catalytic activity, have attracted increasing research attention in environmental remediation and water purification applications [50]. Reports are available on recent advances in the synthesis and use of MXenes as adsorbents, desalination membranes, electrochemical deionization electrodes, and catalysts or antimicrobial agents for water purification and other environmental treatment. The challenges and opportunities have also been highlighted for advanced 2D materials by discussing experimental efforts to investigate MXenes for water use, biocompatibility, and environmental impact. Due to their unique mechanical, chemical, and electrical Characterization and Application of Nanomaterials 2024, 7(2), 6348. 15 properties, many successful experiments have been carried out on the use of MXenes in water purification and environmental remediation applications. However, more efforts are needed to solve the stability, biocompatibility, and reusability issues of MXenes in aqueous environments. MXenes are reported to be used for pollutant adsorption/remediation, photodegradation, membrane separation, etc. The increase in pollutants such as air pollution, organic dyes, chemicals, and pesticides released into the water due to population and the global economy has become the most important health problem in the world [51]. Magnetic MXene nanocomposites have many attractive and successful applications due to their unique properties, ease of production, cost-effective preparation, and excellent capability to degrade water and wastewater pollutants. These compounds offer exciting new opportunities for many applications, such as biosensors, cancer therapy, measurement systems, and especially water purification. Researchers are studying magnetic MXene nanocomposites to remove contaminants from ambient water. The use of magnetic nanomaterials for water treatment, the applications and implications of MXene in degrading water and wastewater pollutants, and the role of magnetic- MXene nanocomposites in water treatment have been studied and discussed (Figure 9a) [52]. Characterization and Application of Nanomaterials 2024, 7(2), 6348. 16 Figure 9. (a) Magnetic-MXene-based nanocomposites for water and wastewater treatment [52]; (b) advanced applications of Mxenes in CDI, solar desalination, ion sieving and pervaporation [53]. The advantages of MXene, such as high surface area, high metal conductivity, easy functionalization, biocompatibility, active metal hydroxide surface, and hydrophilicity, make it a good candidate for energy storage, catalysis, sensors, electronics, and environmental application. Due to their good physical and chemical properties and wide range of chemical properties, MXenes have attracted attention in water purification and desalination applications in recent years. Ihsanullah studied the advances in the synthesis of MXenes and MXene-based compounds for seawater desalination. The desalination potential of MXenes was described in detail, focusing on ion screen membranes, capacitive deionization, and solar desalination. The ion removal process and the recycling potential of MXene were also documented to provide insight into the process [53]. MXene has also emerged as a strong candidate for oil/water separation and photocatalytic environmental remediation applications (see Figure 9b) [53]. The ability to reduce organic molecules and some cations is a unique property that makes Characterization and Application of Nanomaterials 2024, 7(2), 6348. 17 MXene the best choice for future water purification applications [12]. Due to its hydrophilicity, high conductivity, high adsorption capacity for anions and cations, and tunable surface, MXene can also be used as a new electrode material for CDI applications [54]. The solar and evaporative desalination properties of MXene have also been investigated in various studies [55]. The rapid development of the economy causes serious water pollution and poses a threat to the environment. Currently, heterogeneous Fenton oxidation has attracted widespread attention due to its high efficiency and simple operation. Hydrogen peroxide and persulfate are two oxidants used in Fenton-like processes. 5.3. MXene-based battery Significant efforts have been devoted to the development of electronic devices with excellent electrical properties and high energy density for energy storage and conversion. 2D materials show great potential in energy storage due to their unique properties. MXenes have excellent properties when used as electrodes for lithium- based batteries. Tang et al. have discussed the developments in MXene and MXene- based composites in terms of synthesis strategies, morphology engineering, physical/chemical attributes, and their use in lithium-ion batteries and lithium-sulfur batteries [56]. 5.3.1. MXeneforlithiumbatteries Lithium batteries are one of the most common types of batteries because they are the engines of modern electronic devices. Lithium batteries are currently used in mobile phones, computers, small cars, airplanes, and home appliances. Lithium’s high electropositivity and flexibility make it better than hydrogen [57]. Lithium- based cathodes have problems such as short life, low charge, toxicity, unstable electrolyte, high cost, high self-discharge, and poor specific energy. Anodes made of materials such as graphite, lithium, soft carbon, and tin have problems such as dendrites, low energy density, cracking/dissolution, and voltage fluctuations [58]. Therefore, it is necessary and urgent to produce good electrodes for these batteries. MXene is considered an effective material due to its large surface area, electrical conductivity, tunable thermal conductivity, and chemical stability. Rechargeable lithium-ion batteries (LIBs) have provided much-needed applications in electric vehicles and consumer electronics due to their high energy density and reduced storage capacity. In particular, research on LIBs focuses on their applications in small electronic devices by increasing energy density and reducing the footprint during fast charging [56]. MXene can not only produce lithium-ion batteries but also can produce lighter fuel than lithium ion. Deng tested MXene as a LIB anode material and found it good due to its high specific surface area, weak interlayer strength, open structure, and surface functional groups [59]. Since MXene consists of functional groups M (when the substituted metal is), X (C and/or N), and T (functional groups O, OH, and F), it is possible to use many substituents for the treatment. goods. For example, the specific potential of different MXenes follows the order Ti2C < Nb2C < V2C, with both Nb2C and V2C exhibiting higher activity [1]. In addition, the difference between different MXenes is also different. This shows the difference between MXenes for anodes or cathodes. Characterization and Application of Nanomaterials 2024, 7(2), 6348. 18 Additionally, work groups often have different characteristics; for example, bare MXene is magnetic, while active MXene is a semiconductor material. They may also interfere with lithium absorption and transport [54]. Nonnatural functional groups (e.g., chlorine) may be beneficial to MXene by hardening the surface and reducing the interaction of natural OH and F functional groups [55]. Even if the “M” stoichiometry is the same type and prepared by the manufacturing process, the capacity of the lithium-ion battery will be directly affected due to the difference in surface properties. Generally speaking, Ti2CTx has a higher surface energy than Ti3C2TX, which tends to reduce the lattice parameter k and thus inhibit the addition of lithium ions [59]. Lithium-ion MXene/metal electrodes: The electrode material must have excellent electrical conductivity and be able to withstand high temperatures without cracking or breaking. Nowadays, metal electrodes such as titanium, aluminum, magnesium, copper, silver, zinc, and platinum face many problems. Corrosion, cracking, and low energy density are associated with intrinsic metallic electrodes. Due to the excellent properties of MXene, some researchers have mixed metal electrodes with MXene to improve its electrochemical behavior. Mxene has a significant problem that affects its performance, such as the aggregation problem of MXene flakes [60]. A simple and effective way to solve this problem is to provide MXene hybrid composites produced with suitable materials such as silicon, tin, different nanocarbon fillers, metal oxides, phosphorus, metal sulfides, and double-layer oxides. The MXene surface is modified in many aspects, including decoration with silicon- and tin-based nanoparticles, bonding the network with carbon nanofillers, and creating heterostructures between graphene and MXene layers. These hybrid materials have high ion energy, high lithium-ion emissivity, and conductivity, in addition to extraordinary capacity. According to the current situation, lithium-ion battery electrode materials should help in improving the electrical properties of batteries, making them more efficient than before. MXene- based hybrid nanostructured materials provide insight into future challenges and guide in finding new materials to be used in future energy applications. Since the discovery and development of graphene [60], 2D materials and the study of their properties have attracted widespread attention in materials science [34]. In recent years, 2D transition metal carbides, nitrides, and carbonitrides (MXenes for short) have been rapidly developed since the discovery of Ti3C2 in 2011 [35]. MXene is prepared by selectively etching individual atomic layers (called MAX phases) in a precursor layer [60]. The general structure of the MAX phase is Mn+1AXn (n = 1, 2, 3), where M represents the early transition metal (M = Ti, Sr, V, Cr, Ta, Nb, Zr, Mo, or Hf), and A represents group sp elements (IIIA or IVA only), X represents C or N, and both [39,61]. The MAX phase can be said to be a symbiotic structure in which tightly packed planar atom layers and hexagonal MX layers are stacked alternately. Figure 10 shows the classical configuration of the MAX system (using Ti2AlC as an example) [62]. In this structure, MX bonds are mostly ionic/covalent bonds, and all MA bonds are metal bonds [38]. Therefore, the MX bond is stronger than the MA bond and enables the removal of the A layer from the lamellar layer. However, Characterization and Application of Nanomaterials 2024, 7(2), 6348. 19 unlike the weak van der Waals interlayer interactions between graphite and transition metal dichalcogenides, the MA bond is very strong, resulting in detachment of the MX layer from the MAX strength; this is not easily achieved by direct cleavage or ultrasonication techniques. Due to the different properties and strengths of M-X and MA bonds, it is possible to select the appropriate chemical in the process. This leaves a stable chemical structure of the volume near Mn+1XnTx (T, F, OH, O, etc.), representing the functional group on the surface, near MXene (Figure 10a) [62]. The interactions between MXene layers are mainly hydrogen bonds and van der Waals bonds. Water, cations, DMSO, TBAOH, etc. are added to different parts of MXene. After sonication, the MXene layer becomes a suspension. Figure 10. Schematic illustration for the formation of Ti2CTx MXene from Ti2AlC phase. (a) Reproduced with permission [61]; (b) plate-to-layer Bi2MoO6/MXene- heterostructured anode for lithium-ion batteries [62]. Due to the low electrical conductivity of Bi2MoO6 and the large volume expansion/contraction during charging and discharging, appropriate modification is essential to solve these problems. To overcome the issues, Zhang et al. [62] prepared the plate-to-layer Bi2MoO6/Ti3C2Tx (MXene) heterostructure by electrostatically assembling positively charged Bi2MoO6 nanosheets onto negatively charged MXene nanosheets. MXene nanosheets in heterostructure provided a highly conductive substrate to support and anchor Bi2MoO6 nanosheets, thereby increasing electrical conductivity and structure stability (see Figure 10b). When the mass fraction of Characterization and Application of Nanomaterials 2024, 7(2), 6348. 20 MXene was optimized to 30%, the Bi2MoO6/MXene heterostructure exhibited a specific capacity of 692 mAh g−1 at 100 mA g−1 after 200 cycles. A 99.6% Coulomb efficiency was achieved with 545.1 mAhg-1 at 1 Ag−1 after 1000 cycles. The research resulted not only in providing high-performance lithium materials but also presented an idea to create heterostructures by modifying various metal oxides of MXene nanosheets and using them as lithium-ion batteries [62]. Zou et al. [63] synthesized a MXene/Ag composite product (Ti3C2(OH)0.8F1.2) by directly reducing AgNO3 aqueous solution in the presence of MXene. The results demonstrated that the reduction of Ag in MXene solutions was associated with the presence of low-valence Ti. The recovery capacity at 1 ℃ was 310 mAhg−1, at 10 ℃ it was 260 mAhg−1 and the recovery capacity at 50 ℃ was 150 mAhg−1. Moreover, the composite could withstand more than 5000 cycles at 1 to 50 ℃ without decay (see Figure 11a). Figure 11. (a) Synthesis of MXene/Ag composites for high-speed lithium storage with ultra-long cycle life [63]; (b) schematic diagram of the preparation process of MXene@SiNPs@NC foam composites [64]. Metal-based anode materials such as Sb, Sn, and Bi have received great attention in battery applications due to their high electrical conductivity and high energy density. However, the large volume expansion during the alloying process makes the work very fast. Tian et al. synthesized Sb, Sn, and Bi on MXene sheets via a simple one-step electrodeposition process in a green ethylene glycol system. A strong, flexible, self-contained, and binder-free anode for potassium-ion batteries was prepared. The coating ensured a short diffusion for potassium ions and provided a buffer for the volume change during potassiumization/depotassiumization. Characterization and Application of Nanomaterials 2024, 7(2), 6348. 21 Efficient and flexible MXene materials could then be used as current generators, providing wide electrical paths to facilitate the transport of electricity while cycling and adapting to changing volumes. MXene@Sb resulted in a high reversible capacity of 516.8 mAhg−1, a peak value of 270 mAhg−1 at 500 mA g−1, with the fading of only 0.042% per cycle [64]. Lithium-ion MXene/silicon electrode. Organo-silicon electrodes are one of the best energy storage electrodes due to their unique capacity, non-toxicity, and raw materials. However, silicon expands in volume during cycling, resulting in poor electrochemical cycling performance [65]. The volume expansion that occurs during lithiation/delithiation is the cause of cracking and breakage in the electrode. The combination of silicon and MXene can be used to produce excellent electrochemical electrodes for lithium-ion batteries. The improved cycling performance of silicon/MXene electrodes can be attributed to the excellent thermal stability of MXene. Titanium-based MXene (Ti3C2TX) nanosheets have become useful materials for low-voltage electronics (Si) due to their unique charge (lithium) storage capacity and rich abundance of silicon (Si). Si/Ti3C2TX composite material has been shown to have a wide and stable cycle time, good performance, and high economic value. To better understand and improve the electrochemical performance of composites, a review by Jiang et al. focused on the electrochemical processes occurring in Si/MXene composites [65]. Silicon (Si) is one of the most promising materials for lithium-ion batteries (LIB) due to its unique properties. However, low transmission efficiency and large voltage fluctuations block the development of silicon-based anodes. Zhang et al. [66] created a 3D structure containing silicon nanoparticles (SiNPs) anchored on nitrogen-doped carbon (NC) foam and coated with an MXene layer (MXene@SiNPs@NC foam) (see Figure 11b), which could act as a self-standing Si-based anode for high- performance LIBs. The design of the NC foam was based on the conductivity principle, and the MXene-based layer provided the advantage of electrons/ion diffusion while allowing the anode to adapt to large changes in Si volume during lithiation/desalination. The independent MXene@SiNPs@NC foam electrode had a capacity (1658 mAhg−1 after 100 cycles at 0.1 ℃) and a residual capacity (857 mAhg−1 after 500 cycles at 0.5 ℃). Moreover, the battery developed entirely using MXene@SiNPs@NC foam //NCM111 exhibited a high gravimetric energy density (433 Wh kg−1). This MXene@SiNPs@NC foam anode demonstrated good electrical properties. Silicon (Si) is considered one of the candidates that can replace graphite in anodes. Lithium-ion batteries store greater energy and, therefore, improve returns. However, the high mechanical stress caused by its high volumetric variation during charge and discharge cycles, added to its low electrical conductivity, has impeded its wide use. For this reason, silicon-based composites are studied to find out their commercial viability. MXene Ti3C2 is a two-dimensional material whose good mechanical resistance and conductivity can contribute to solving the problems of Si anodes. The results revealed that the addition of Ti3C2 particles to electrodes could reach 80% and 89% of their theoretical capacity when Ti3C2 represents 20% and 40% of the mass of the active electrode material, respectively, compared to the 56% achieved by the pure Si electrode. This improvement is explained by a reduction in Characterization and Application of Nanomaterials 2024, 7(2), 6348. 22 the resistance to charge transfer observed in the EIS results. Finally, the electrode with 20% by weight of Ti3C2 (640 mAhg−1) obtained the best specific capacity after 100 charge and discharge cycles than obtained by the pure Si electrode (572 mAhg−1) [67]. Flexible pressure sensors are one of the most important sensors in electronic skin (e-skin), robotics, and medicine. However, sensors are still complex and expensive due to power consumption, unreliable operation, and manufacturing processes. Ti3C2TX is the most studied MXene in the field of pressure sensing, with good mechanical and electrical properties, excellent hydrophilicity, and flexibility. It increases the sensitivity of pressure sensors and the efficiency of the electrode process and continues to use pressure measurements in various areas, such as electrical skin elasticity. The MXene can be used in pressure sensors, which include piezoresistive, capacitive, piezoelectric, triboelectric, and potentiometric switching technologies. Integration of various devices is also possible [68]. 5.3.2. MXene promoting beyond lithium battery The lithium-based battery is undoubtedly the most studied and analyzed battery in scientific literature. However, lithium is not abundant in the earth’s crust and will soon suffer the same fate as the overconsumption of fossil fuels, forcing us to look beyond lithium. Fortunately, research on other metal-ion batteries (sodium, potassium, aluminum, magnesium, zinc, etc.) has rapidly increased, and much has been borrowed from research on lithium-ion batteries. Although their theoretical capacity is lower than that of lithium [69], they can reduce the cost of lithium resources by serving as another energy store. The lithium-ion battery has been a leader in the electronics industry and research and development for nearly two decades. Due to concerns about the cost and future availability of lithium, sodium- ion batteries (SIBs) and other new technologies are emerging as candidates for sustainable energy. Research in this technology is increasing, with a focus on developing new cathode and anode materials that can improve cycle stability, operating costs, and energy. 2D materials are promising in many energy-related applications, especially energy storage, due to their ability to transport ions between layers and over large areas, enhanced ion adsorption, and rapid surface redox reactions [70]. Tang et al.’s [70] potassium metal batteries are considered attractive alternatives to lithium-ion batteries. However, uncontrollable dendrite growth of potassium metal anodes limits their practical applications. Functionalization of the potassium anode has been reported to be achieved by encapsulating the metal in a titanium-free, nitrogen-containing MXene/carbon nanotube freestanding scaffold. High and fast electrical conduction in the scaffold helps reduce density and current flow during coating/stripping. Additionally, theoretical calculations and experimental studies confirmed that the “potassium-loving” form of MXene can cause potassium nucleation and add potassium during exposure to stabilize the products. As a result, the produced potassium metal anode exhibits dendrite-free morphology, high Coulomb efficiency, and a long lifetime during the plating/stripping process. Such anodes also improve the electrochemical performance of potassium-sulfur batteries compared to bare metal anodes. The potential of Ti2N monolayers and their Ti2NT2 derivatives (T = O, F, and OH) as Characterization and Application of Nanomaterials 2024, 7(2), 6348. 23 anode materials for lithium-ion and ultra-lithium-ion batteries were investigated by first-principle calculations. The single layer exposed and removed is a very good metal material. The diffusion barriers of the bare Ti2N monolayer were approximately Li+ 21.5 meV, Na+ 14.0 meV, K+ 7.0 meV, Mg2+ 75.9 meV, and Ca2+ 38.0 meV. The 2D functional groups of Ti2NT2 increased the conduction effect by approximately 1-fold. The calculated capacity of single cations of Ti2N and Ti2NT2 was close to conventional graphite anodes in lithium-ion batteries. In comparison, the Mg2+ capacity of Ti2N and Ti2NT2 exceeded 2000 mAhg-1 due to the two- electron reaction and multilayer adsorption of Mg2+. A comparison of the electrical properties of Ti2N and Ti2C showed that Ti2N is a better material than Ti2C due to its sensitivity to various cations [24] (see Figure 12a). The heavy reliance on LIBs has led to growing concerns about the sustainability of lithium and alternative metals and ethical issues in mining. Developing alternative energy storage technologies beyond lithium has become an important part of the world’s energy research portfolio [71]. Other technologies, from electric transportation to renewable energy to large-scale energy storage, play an important role in developing energy storage for the future. Considering their chemical and economic benefits, potassium ion batteries (PIBs) are promising and are becoming strong competitors to LIBs and SIBs in many forms. However, many people do not understand the process by which potassium is deposited in materials and how it differs from lithium and sodium, and the material liquid interface chemistry in PIBs is also not fully understood. Therefore, there are still some important problems in the commercialization of PIB technology. 5.4. Supercapacitor Supercapacitors have attracted significant attention as promising successors to traditional energy storage devices due to several key advantages like higher power density, faster charge-discharge cycles, and greater energy density over batteries and conventional dielectric capacitors [1,22]. There are two main types of supercapacitors based on their working principles: electrochemical double-layer capacitors (EDLC), pseudocapacitors, and hybrid capacitors. EDLCs store energy by the adsorption/desorption of ions on the surface through the electrical double layer, whereas pseudocapacitors store energy through surface redox reactions [4]. The energy density (E) of supercapacitors is governed by the formula E = CV2/2, where C is capacitance and V is voltage. Therefore, enhancing capacitance and voltage window results in higher energy density supercapacitors. Voltage remains relatively constant for a specific electrolyte, so increasing capacitance is crucial for overall capacitor performance improvement [2,22]. As a result, there is significant research interest in enhancing capacitance to improve supercapacitor performance. MXene exhibits significant potential as an electrode material for batteries and supercapacitors, due to its remarkable electrochemical properties resulting from the presence of transition metal carbide/nitride components and its high electrical conductivity [28]. MXenes typically originate from the MAX phase, which follows the general formula Mn+1AXn. Here, M represents an early transition metal, A is a group of 13 or 14 elements, and X consists of C and/or N; n is generally 1–3. MXene Characterization and Application of Nanomaterials 2024, 7(2), 6348. 24 formation occurs through the chemical etching of the A layer within the MAX phase using fluoride ion-containing solutions such as hydrofluoric acid (HF) [11]. Following the etching process, the configuration of MXene may exhibit significant variations, depending on the specific post-treatment method employed. After selectively etching the A layer with HF, the resultant powder manifests a multi- layered MXene structure [72]. In this structure, the atoms of the A layer, predominantly Al or Si, are substituted by terminal groups such as –O, –OH, and/or –F, leading to a phase denoted as Mn+1XnTx. Otherwise, when intercalants like Li+ ions are used instead of etchants, sonication yields a colloidal phase comprising few- layered or monolayer-exfoliated MXene nanosheets. These exfoliated MXene nanosheets reveal a significantly expanded surface area due to the removal of the A layer spacing. Generally, the use of alkali metal ions as intercalants results in an enlargement of the surface area by at least a factor of two compared to that of the multilayered MXene [33,34,73]. Upon the exfoliation of MXene nanosheets, stacked layers will probably form in MXene films, driven by impromptu attractive interactions such as van der Waals forces and hydrogen bonding [36]. MXene films offer the benefits of flexibility, large conductivity, and surface hydrophilicity, making them favorable for enhanced electrochemical processes. Consequently, free- standing films based on MXene have been widely employed as active materials in supercapacitors and various electrochemical electrodes [37–38,74]. While MXene films possess numerous advantages, their applications are limited due to the lack of regulated film-forming processes [75]. While monolayer MXene nanosheets allow for the optimization of theoretical surface area and electrochemical surface activity, processes primarily involving layer-stacking film formation compromise the intrinsic properties anticipated from monolayers, thus affecting the performance of electrochemical applications [22,76]. Couly et al. proposed a simple approach to fabricate asymmetric micro- supercapacitors using MXene. These micro-supercapacitors are modifiable, free of binder, and current-collector-free. The method involved employing a customized mask and an expandable spray coating technique on a flexible and transparent substrate [77]. The electrode comprised titanium carbide MXene (Ti3C2Tx) and reduced graphene oxide, both possessing a 2D layered structure that facilitated rapid ion diffusion within the interdigitated electrode framework. This asymmetric micro- supercapacitor operated within a 1 V voltage window and retained 97% of its initial capacitance after ten thousand cycles. Furthermore, it demonstrated an energy density of 8.6 mW hcm−3 at a power density of 0.2 W cm−3. Moreover, these micro- supercapacitors exhibited remarkable flexibility under mechanical bending. By leveraging the capacity of Ti3C2Tx—MXene electrodes to operate at negative potentials in aqueous electrolytes, it was demonstrated that employing Ti3C2Tx as a negative electrode and reduced graphene oxide as a positive electrode in asymmetric configurations represented a viable approach to enhance both the energy and power densities of micro-supercapacitors. Rakhi et al. [78] developed 2D Ti2CTx MXene nanosheets by selectively etching the Al layer from the Ti2AlC MAX phase, utilizing HF. The hexagonal symmetry of the parent Ti2AlC MAX phase was preserved in the MXene sheets. An extensive investigation was conducted to evaluate the influence of various post-etch Characterization and Application of Nanomaterials 2024, 7(2), 6348. 25 annealing atmospheres, including argon (Ar), nitrogen (N2), a combination of nitrogen and hydrogen (N2/H2), and air, on both the structure and electrochemical characteristics of the MXene nanosheets. It was found that the MXene sheets displayed changes in structure, morphology, and electrochemical behavior following annealing in air compared to the HF-treated MAX phase. In contrast, samples subjected to annealing in Ar, N2, and N2/H2 atmospheres maintained their initial morphology. A substantial enhancement was observed in the supercapacitor performance of the annealed samples, particularly in those annealed in Ar, N2, and N2/H2 atmospheres. In a symmetric two-electrode setup, the MXene sample annealed in an N2/H2 atmosphere exhibited superior capacitive performance, boasting a specific capacitance of 51 Fg−1 at 1 Ag−1 and an impressive rate performance of 86%. This notable enhancement in electrochemical performance post-annealing was attributed to several factors, including the increased carbon content, decreased fluorine content on the surface, and the preservation of the original two-dimensional layered morphology, ensuring optimal access of the aqueous electrolyte to the electrodes. Wen et al. [79] introduced a novel electrode material for supercapacitors, nitrogen-doped two-dimensional MXene (N-Ti3C2Tx), through annealing of Ti3C2Tx in ammonia post-etching. Through precise control of annealing temperatures within the range of 200 ℃ to 700 ℃, the concentrations of nitrogen in N-Ti3C2Tx materials were made ranging from 1.7% to 20.7%. The incorporation of nitrogen as a heteroatom into the Ti3C2Tx structure led to an expansion of the c-lattice parameter in MXene sheets, from 1.92 nm in Ti3C2Tx to 2.46 nm in N-doped counterparts after treatment with ammonia at 200 ℃. The resultant doped MXene materials exhibited significantly enhanced electrochemical capacitance of 192 F/g in 1 M H2SO4 and 82 F/g in 1 M MgSO4 electrolyte, under optimized conditions. These values represented a substantial rise in the capacitances, as observed for undoped Ti3C2Tx materials, which recorded 34 F/g in 1 M H2SO4 and 52 F/g in 1 M MgSO4 (see Figure 12b). Characterization and Application of Nanomaterials 2024, 7(2), 6348. 26 Figure 12. (a) First calculations of Ti2N and Ti2NT2 (T=O, F, OH) monolayers as anode materials for lithium-ion batteries and other batteries [24]; (b) nitrogen-doped Ti3C2Tx MXene electrodes for high-performance supercapacitors [79]. 5.5. Biomedical applications Levitt et al. [80] fabricated free-standing Ti3C2Tx MXene/carbon nanofiber electrodes by electrospinning to combine Ti3C2Tx MXene flakes with polyacrylonitrile (PAN), followed by carbonization. Through this approach, delaminated MXene flakes could be incorporated within carbon nanofibers, resulting in fiber mats that could be employed as electrodes. Notably, these composite electrodes exhibited superior sturdiness compared to their coated counterparts. In the spinning dope, MXene flakes were introduced into PAN solutions at a weight ratio of 2:1, resulting in fiber mats containing MXene up to 35 wt%. The resulting composite electrodes demonstrated an impressive areal capacitance of up to 205 mF cm−2 at 50 mV s−1, nearly three times higher than that of pure carbonized PAN nanofibers, which registered at 70 mF cm−2 under the same conditions. Furthermore, in comparison to electrospun nanofibers spray-coated with Ti3C2Tx, these composite fibers exhibited a twofold increase in areal capacitance at 10 mV s−1 (refer to Figure 13a). This versatile technique holds promise beyond energy storage applications, as it can be extended to generate MXene composite fibers using a variety of polymers. Such applications may include filtration, adsorption, and electrocatalysis, where fibers with high aspect ratios, accessible surface areas, and porosity are highly desirable. 5.6. MXene as gas sensors Kim et al. [81] introduced chemical sensors with exceptional gas-sensing Characterization and Application of Nanomaterials 2024, 7(2), 6348. 27 characteristics through the synthesis of Ti3C2Tx (see Figure 13b). These sensors exhibited remarkable selectivity towards hydrogen-bonding gases compared to acidic gases. Also, an empirical Limit of Detection (LOD) of 50 ppb was achieved and a theoretical LOD reaching the sub-ppb level for Volatile Organic Compounds (VOC), making the lowest detection limits among gas sensors utilizing 2D materials and functioning at room temperature without any pretreatment. Furthermore, the signal- to-noise ratio (SNR) of Ti3C2Tx sensors surpassed that of all other 2D materials by up to two orders of magnitude. The remarkable sensitivity of Ti3C2Tx sensors could be attributed to both the metallic conductivity of the core channels and the robust adsorption energy of the surface functional groups (Figure 13b) [82]. Figure 13. (a) Electrospun MXene/carbon nanofibers as supercapacitor electrodes [81]; (b)Schematic representation of Ti3C2Tx films [82]. 5.7. Sensors in biomedical applications The distinctive 2D layered structure and remarkable array of physical and chemical attributes exhibited by MXenes, such as hydrophilic nature, biocompatibility, adeptness in light-to-heat conversion, and mechanical resilience, make them promising candidates across diverse biomedical domains. These applications encompass sensor technologies, bioimaging modalities, tissue engineering approaches, and advanced drug delivery systems. Biosensors Upon their discovery, MXenes swiftly ascended as a profoundly promising category of 2D materials. Their notable characteristics, including a substantial specific surface area, elevated conductivity, near-infrared absorption, and ease of functionalization, make them exceptionally well-suited for biomedical and Characterization and Application of Nanomaterials 2024, 7(2), 6348. 28 environmental applications (Tables 1 and 2) [83]. In in-vitro experiments, MXenes exhibit toxicity against bacterial and animal cells, primarily due to mechanisms such as oxidative stress and the mechanical damage inflicted on cell membranes by the sharp edges of the nanosheets (Figure 14a). Figure 14. (a) Schematic representation of cytotoxic effects of MXene on bacterial cells [83]; (b) Schematic representation of (a) In vivo computed tomography Imaging of the Ta4C3 MXene composite (b) Disintegration of MnOx components from MnOx/Ta4C3 under a specific tumor microenvironment for contrast-enhanced T1-weighted magnetic resonance imaging (c) In vivo photoacoustic imaging [84]. Table 1. Biosensors for medicinal use [83]. Sensor Composition Detectable Analyte Sensor Type Sensor Efficiency MXene-Ti3C2 Hemoglobin Electrochemical biosensors detection limit of 20 nM MXene-Ti3C2 Hemoglobin Electrochemical biosensor linear range of 0.5–11,800 μM, detection limit of 0.12 μM GOx/Au/Ti3C2Tx- MXene/Nafion/GCE Glucose Electrochemical biosensor (amperometric) detection limit of 5.9 μM Ti3C2-MXene functionalized with aminosilane Carcinoembryonic antigen (CEA) Electrochemical biosensor linear detection range of 0.0001–2000 ng mL−1 with sensitivity of 37.9 µA ng−1mL cm−2 per decade Ti3C2 Human papillomavirus (HPV) Optical biosensor detection limit of 100 pM Ti3C2Tx MXene and phosphomolybdic acid embedded with polypyrrole Osteopontin Aptamer biosensor 0.98 μg/L Ti3C2Tx/PtNP modified GCE Ascorbic acid, dopamine, uric acid, acetaminophen Electrochemical biosensor nM level OH-terminated Ti3C2 Label-free single- nucleotide in human urine Electrochemiluminescence biosensor Detection limit of 5 nM Characterization and Application of Nanomaterials 2024, 7(2), 6348. 29 Table 1. (Continued). Table 2. Biosensors for environmental use [83]. Sensor Composition Detectable Analyte Sensor Type Sensor Efficiency Mo2Ti2AlC3/MWCNT Bisphenol A Electrochemical biosensor (amperometric) 0.01–8.50 μM MXene-Ti3C2 Tyrosinase Electrochemical biosensor linear range from 0.05 to 15.5 μM L−1, detection limit of 12 nM L−1 MXene-Ti3C2 Ag+and Mn2+ Optical sensors Range of 0.1–40 μM for Ag+, detection limits of 9.7 nM; 0.5–60 μM for Mn2+ions, Ti3C2Tx (MXene)-modified glassy carbon electrode BrO3− Electrochemical biosensor linear response from 50 nM to 5 μM, detection limit of 41 nM Hydroxyl terminated alk- Ti3C2 modified GCE Cd(II), Pb(II), Cu(II) and Hg(II) Electrochemical biosensor Detection limit of 0.098, 0.041, 0.032 and 0.130 μM for Cd(II), Pb(II), Cu(II) and Hg(II), respectively AChE-Chit/Ti3C2-MXene/Au NPs/MnO2/Mn3O4/GCE Organophosphorus pesticides Electrochemical biosensor concentration range (10−12–10−6M), limit of detection (1.34 × 10−13M) AChE/CS-Ti3C2Tx/GCE Organophosphorous pesticides (malathion) Electrochemical biosensor concentration range of 10−14–10−8M, limit of detection 0.3 × 10−14M AChE/Ag@Ti3C2Tx Organophosphorous pesticides (malathion) Electrochemical biosensor concentration range of 10−14–10−8 M Ti3C2Tx carbamate pesticides (methiocarb and diethofencarb) Electrochemical biosensor detection limits were 0.19 μg mL−1 and 0.46 μg mL−1 for methiocarb and diethofencarb respectively CdS/MXene-NH2/ZnSnO3 Cd2+, perfluorohexane Photoelectrochemical biosensor linear range of 0.008–100 nM, detection limit of 4.21 pM MnMoO4-MXene-GCE Hydroquinone, catechol electrochemical biosensor linear response from 5 nM to 65 nM, detection limit of 0.26 nM for Hydroquinone and 0.30 nM for catechol Sensor Composition Detectable Analyte Sensor Type Sensor Efficiency Ti3C2Tx/Prussian blue Glucose and lactate in sweat Electrochemical biosensor (amperometric) Sensitivities of 35.3 µA mm−1cm−2 for glucose and 11.4 µA mm−1cm−2 for lactate MXene-Ti3C2Tx incorporated with a dialysis microfluidic chip Urea, uric acid, and creatinine Electrochemical biosensor MXene-Ti3C2Tx modified screen-printed electrode Acetaminophen (ACOP), isoniazid (INZ) Electrochemical biosensor Linear ranges from 0.25 to 2000 μM for ACOP and 0.1–4.6 mM for INZ. The detection limits of ACOP and INZ were 0.048 μM and 0.064 mM Ti3C2Tx/ZIF-8 HIV-1 protein Electrochemical biosensor detection limit 0.3 fM Au/Ti3C2T/HB5 HER2-positive cancer cells Electrochemical cytosensor linear range of 102–106 cells/mL, detection limit of 47 cells/mL Chit/ChOx/Ti3C2Tx Cholesterol Electrochemical biosensor concentration of cholesterol ranging from 0.3 to 4.5 nM, detection limit of 0.11 nM, sensitivity of 132.66 μA nM−1cm−2 Ti3C2Tx MXene/LBG/PDMS Cortisol Electrochemical impedimetric immunosensor linearity 0.01–100 nM, detection limit 88 pM PEI-Ru@Ti3C2@AuNPs SARS-CoV-2 RdRp gene Electrochemiluminescent biosensor Detection limit of 12.8 aM ZnO/Ti3C2 Glucose Electrochemical enzymatic biosensor Sensitivity 29 μA mM−1cm−2, limit of detection ≈ 17 μM, linear detection range 0.05–0.7 mM) Characterization and Application of Nanomaterials 2024, 7(2), 6348. 30 The interest in early detection of cancer has grown in the context of bioimaging and biosensing, owing to their capacity to unveil intricate cellular processes and a diverse array of diagnostic indicators. Ti3C2 is the most extensively researched MXene nanostructure in applications related to bioimaging and biosensing, while Ta4C3 MXene is commonly employed for multimodal imaging. The presence of tantalum (Ta) in the Ta4C3 MXene, characterized by a high atomic number and a notable X-ray attenuation coefficient, enables its utilization in CT imaging without requiring supplementary contrast agents. This streamlines the setting up of multimodal imaging. Regarding biosensing, platforms utilizing MXenes have been developed for various techniques, including electrochemistry, fluorescence, surface plasmon resonance, surface-enhanced Raman scattering, and colorimetric and chemiluminescence-based biosensing [84] (see Figure 14b). Surface modification and functionalization are pivotal in augmenting the characteristics of materials based on MXene. Following surface modification, MXene-based materials have been propelled into the spotlight and have found widespread application in various fields, such as biomedicine, energy, and the environment. Specifically, this includes biosensing, drug delivery systems, bioimaging, photothermal therapy, antibacterial agents, and theragnostic nanoplatforms within the realm of biomedical applications [85]. 5.8. Flexible pressure sensor MXenes have found application in different sensing scenarios because of their adjustable surface functionalities. Sensors based on MXenes have proven effective in detecting a range of pollutants, exhibiting a lower detection limit and impressive selectivity/sensitivity [85,86]. The domains of electronic skin, human-machine interaction, and health monitoring demand pressure sensors that are both flexible and highly sensitive. However, most microstructure designs employed for creating pressure sensors with high performance involve intricate preparation processes. Yin et al. [87] obtained a 3D porous structure of MXene/polyaniline (PANI) foam through the application of a steam-induced foaming method. Utilizing this structure, a flexible piezoresistive sensor was manufactured. It demonstrated remarkable characteristics, including high sensitivity (690.91 kPa−1 quick response and recovery time of 106/95 ms, and excellent fatigue resistance (10,000 cycles). The pressure sensor, derived from MXene/PANI foam, could rapidly perceive subtle pressure changes. It was found suitable for applications in human activity and supervision of health [86,87]. As composite materials find extensive use in aeronautic structures, research on composite material repair methods is becoming increasingly comprehensive. However, owing to the unique nature of the composite materials, the visual efficacy of repairs and the service condition of the composite repaired structure is challenging, necessitating highly sensitive piezoresistive strain sensors. Wang et al. [88] introduced an innovative MXene film sensor technology capable of real-time and in-situ health monitoring of composite structures. The proposed method enabled the creation of MXene thin films with exceptional properties. MXene sensors were produced and organized into arrays and integrated into the composite repair structure to monitor strain across various points. The piezoresistive reaction of the MXene Characterization and Application of Nanomaterials 2024, 7(2), 6348. 31 sensor was acquired, and the primary operational mechanism was elucidated. The strategic arrangement of MXene sensors in the repaired structure enhanced the monitoring process, offering a more thorough understanding of the failure mechanism. This monitoring approach, utilizing MXene sensors, can be used for investigating the health status of composite repair structures under both tensile and compression conditions [88] (see Figure 15). Figure 15. Monitoring of repaired composite structure [88]. Sen mechanism of MXene sensor and sensor array arrangement of composite material repair structure. Despite possessing notable properties such as a large surface area, biocompatibility, metallic conductivity, hydrophilicity and tunable size, the application of MXene in biomedical settings is constrained by its limited stability in physiological environments, absence of sustained and controlled drug release, and low biodegradability. These challenges have prompted consideration for the adoption of MXene/polymer nanocomposites. The presence of functional groups on the MXene surface allows for polymer functionalization. Hence, these MXene nanocomposites functionalized with polymers showcase notable features such as high photothermal conversion efficiency, selectivity, responsiveness to stimuli, electron sensitivity, enhanced antibacterial properties, and more. These characteristics make them suitable for use in biomedical applications, including photothermal therapy, drug delivery, diagnostic imaging, biosensing, bone regeneration, and antibacterial activities [89] (see Figure 16). Characterization and Application of Nanomaterials 2024, 7(2), 6348. 32 Figure 16. MXene-Polymer composites in biomedical applications [89]. 5.9. Drug delivery system The healing of chronic wounds is a fundamental and significant concern in medical and healthcare domains, and in recent times, hydrogel systems have emerged as promising carriers for drug delivery in the context of wound management, which is responsive to stimuli. Yang et al. [90] introduced a hydrogel system based on 2D MXene, designed for effective drug delivery with photo- and magnetic-responsive capabilities, specifically tailored for the treatment of deep chronic wounds. The smart and responsive drug delivery system using MXene consisted of magnetic colloids enveloped by MXene and dual-network hydrogels made from poly (N-isopropyl acrylamide) and alginate. It exhibited versatile responsiveness and precise drug release mechanisms, effectively mitigating the toxic side effects of drugs and enhancing the wound healing process. The real-world efficacy of the MXene-based hydrogel drug delivery system was validated through its application in treating full-thickness cutaneous wounds and subcutaneous infected wounds in a rat model, highlighting its substantial potential in clinical wound healing management and other relevant biomedical domains. 5.10. MXene in cancer treatment Typical approaches to cancer treatment encompass surgical procedures, chemotherapy, and radiotherapy involving the use of anti-cancer medications. Recently, MXenes have generated considerable attention for their potential use as drug carriers. This is attributed to their planar structure, abundant surface functional groups, biocompatibility, and negatively charged surface. Even though nanosized Ti3C2, the pioneering MXene for drug delivery vehicles, exhibits an enhanced permeability and retention effect that allows it to accumulate at the tumor site, surface modification is essential. This is because it tends to restack under physiological conditions. The tumor microenvironment, which typically has a lower pH compared to normal tissues, offers an opportunity for controlled drug release, as the presence of H+ ions can disrupt the electrostatic interaction between the drug and MXene. Additionally, controlled drug release can be achieved through near-infrared radiation stimulation [91]. Characterization and Application of Nanomaterials 2024, 7(2), 6348. 33 Micro- and nanosystems incorporating MXene can be utilized for precise administration of anticancer drugs or therapeutic agents, offering the benefits of minimal toxicity and excellent biocompatibility. Confirming the effective uptake of therapeutic chemicals from the bloodstream to the intended site presents a significant challenge in systemic disposition. The vascular endothelial cell, serving as a barrier between organs and blood, emerges as a crucial focal point in this regard. By incorporating MXene into composites and subsequently functionalizing or modifying them with appropriate functional groups or agents, their loading capacity, biocompatibility, and bioavailability can be enhanced [92]. MXene-based micro- and nanosystems, following surface functionalization or modification with bioactive or biocompatible agents, have been employed for targeted delivery of anticancer drugs or therapeutic agents in cancer therapy (Figure 17) [92]. Figure 17. MXene with potential biomedical applications [92]. Liu et al. [93] described the creation of a drug delivery platform for the efficient loading of the chemotherapeutic doxorubicin (DOX) (see Figure 18). This platform exhibited dual drug release modes, responding to both near-infrared laser stimulation and variations in pH. Due to the surface modification involving thiol polyethene glycol aldehyde chains (SH-PEG-CHO) linked to the MXene through gold nanoparticles, the MXene@Au-PEG-DOX system exhibited favorable photothermal stability, biocompatibility, and tissue compatibility. Furthermore, using the effective photothermal conversion capabilities of both gold particles and MXene, it demonstrated a synergistic approach to combine photothermal ablation with chemotherapy to treat tumors. The enhancement of MXene through surface modification involving Au and PEG served not only to enhance the photothermal Characterization and Application of Nanomaterials 2024, 7(2), 6348. 34 stability of the drug delivery system and the biocompatibility of the nanocomposites but also to regulate drug release through the formation of Schiff base bonds. Consequently, this, in turn, led to improved targeted therapeutic effects. Figure 18. (a) Illustrative diagram of fabrication of MXene and MXene@Au nanosheets; (b) modification of MXene@Au surface using thiol polyethylene glycol aldehyde chains (SH-PEG-CHO), and DOX loading; (c) illustrative representation of MXene-based drug delivery system for near-infrared laser-triggered and pH- responsive drug release in tumor tissue [93]. A drug carrier with targeting capabilities for folate receptors was developed by Liu et al. [93], leveraging the plentiful surface functional groups and outstanding biocompatibility of MXenes. The drug carrier exhibited a drug-loading capacity of up to 69.9%, along with an extended drug release duration of up to 48 h. The findings indicated a substantial release of DOX in a pH 4.5 PBS solution. In comparison to the free drug, MXenes-FA-SP@DOX demonstrated increased cell inhibition and prolonged drug efficacy at lower concentrations (below 10 μg mg−1). This drug delivery system can be used for the treatment of malignant tumors. Dong et al. [94] synthesized drug-loaded MXene/agarose hydrogel (MXene@Hydrogel). Initially, superior photothermal conversion efficiency and stability were achieved by preparing two-dimensional MXene nanosheets. Subsequently, these MXene nanosheets, along with the therapeutic drug, were integrated into a low-melting-point agarose hydrogel network to form the drug-loaded MXene/agarose hydrogel (MXene@Hydrogel). Incorporating a low concentration of MXene (20 ppm) facilitated rapid heating of the MXene@Hydrogel to 60 ℃ under near-infrared (NIR) irradiation, prompting its melting and subsequent release of encapsulated drugs. Control over drug release kinetics and on/off functionality could be easily Characterization and Application of Nanomaterials 2024, 7(2), 6348. 35 modulated by adjusting agarose concentration, MXene concentration, light intensity, and exposure duration. Moreover, doxorubicin, an anticancer drug, maintained its efficacy upon release from the MXene@Hydrogel network under NIR irradiation. With its outstanding biocompatibility, this NIR-responsive MXene@Hydrogel represents a promising strategy for developing a smart hydrogel-based drug delivery system tailored for localized cancer therapy. A nanoparticle system (MXene-TK-DOX@PDA) sensitive to reactive oxygen species (ROS) was engineered for efficient drug delivery in chemotherapy and applications in antibacterial therapy by Zhang et al. [95] (See Figure 19). The surface of (3-aminopropyl) triethoxysilane (APTES)-functionalized MXene served as a platform for conjugating DOX via a ROS-cleavable diacetyl thioketal (TK) linkage. Following this, the MXene nanosheets were coated with pH-responsive polydopamine (PDA) to serve as a gatekeeper. This incorporation of PDA enhanced the biocompatibility and stability of the MXene-TK-DOX@PDA nanoparticles. The ultrathin planar structure of MXene-TK-DOX@PDA nanoparticles, with a compact lateral size of around 180 nm, showcased exceptional photothermal conversion efficiency, superior stability in photothermal applications, and a notable extinction coefficient of 23.3 Lg−1cm−1 at 808 nm. The synthesized nanoparticles demonstrated effective release of DOX, responsive to both ROS and pH, attributed to the cleavage of the thioketal linker in MXene-TK-DOX@PDA. Furthermore, the MXene-TK- DOX@PDA nanoparticles exhibited potent antibacterial effects against both Gram- negative Escherichia coli (E. coli) and gram-positive Bacillus subtilis (B. subtilis) within a 5-hour timeframe. Figure 19. MXene-based nanoparticles as drug delivery carriers [95]. Using a layer-by-layer approach, a drug delivery system with pH/near-infrared responsiveness was created by Wu et al. [96]. This system is comprised of hollow hydroxyapatite (HAP), chitosan (CS)/hyaluronic acid (HA) multilayers, gold nanorods (AuNRs), and MXene. Multilayers of chitosan/hyaluronic acid were applied to the hollow hydroxyapatite (HAP) surface to mitigate the abrupt release of DOX during the initial delivery phase. Integration of MXenes and gold nanorods Characterization and Application of Nanomaterials 2024, 7(2), 6348. 36 (AuNRs) into the hybrid matrix notably enhanced the photothermal conversion efficiency of the microcapsules. The exceptional pH-/NIR-responsive drug delivery properties of HAP/CS/HA/MXene/AuNRs microcapsules were demonstrated through the disruption of electrostatic forces among chitosan/hyaluronic acid multilayers, the dissolution of HAP under acidic conditions, and the synergistically enhanced photothermal effect between MXene and AuNRs. Liu et al. [97] developed a novel catalyst heterostructure (MXene/CoNWs) activated by NIR radiation, comprising 2D MXene and 1D cobalt nanowires (CoNWs), for combating bacteria without the use of drugs (see Figure 20). The incorporated CoNWs swiftly captured photogenerated electrons from Ti3C2 MXene nanosheets, thus mitigating the recombination of electron-hole pairs and enhancing carrier transfer under 808 nm NIR illumination, leading to increased reactive oxygen species (ROS) production. Moreover, the 2D/1D heterostructure exhibited a significantly amplified photothermal effect due to synergistic interactions between plasmonic CoNWs and the MXene semiconductor. Coatings of this heterostructure on orthopaedic implants demonstrated antibacterial efficacy exceeding 90% against both Gram-positive and Gram-negative bacteria within a mere 20-minute exposure period. Additionally, in vitro assessments indicated acceptable cytocompatibility of MXene/CoNWs heterojunctions. Figure 20. Schematic representation of MXene/CoWNs activation and action on bacteria [97]. 5.11. Recent advanced progress in biomedical applications of MXenes Owing to the captivating physicochemical attributes of MXenes, these materials and their blends have been engineered for diverse biomedical applications. Ti3C2Tx was the first MXene to be identified, and it soon emerged as the most extensively researched MXenes. It demonstrated outstanding catalytic characteristics in the field Characterization and Application of Nanomaterials 2024, 7(2), 6348. 37 of photocatalytic Hydrogen Evolution reaction. Nevertheless, the characteristics of Ti3C2Tx were significantly influenced by its surface functional groups and associated materials. In particular, Ti3C2Tx terminated with oxygen displayed the most optimal catalytic activity. Its efficacy could also be enhanced by combining it with other photoactive materials like TiO2, ZnO, MoS2, WS2, CdS, and graphitic carbon nitride. These composite materials not only enhanced light absorption but also improved charge separation and active sites, thereby enhancing the overall performance of Ti3C2Tx under UV-visible light irradiation [98] (See Figure 21). Koyappayil et al. [99] modified Ti3C2Tx MXene nanosheets by incorporating β-hydroxybutyrate dehydrogenase to create a biosensor for amperometric detection of β- hydroxybutyrate. MXene demonstrated its excellence as an immobilization matrix, exhibiting strong compatibility with the enzyme β-hydroxybutyrate dehydrogenase. The biosensor utilizing MXene, working optimally at a potential of −0.35 V (vs. Ag/AgCl), exhibited a broad linear detection range (0.36 to 17.9 mM), a sensitivity of 0.480 μA mM−1 cm−2, and a low detection limit (45 μM). Successful application of this biosensor was demonstrated in accurately determining β-hydroxybutyrate levels in real serum samples, even when spiked with additional compounds. Figure 21. Titanium Carbide MXene in hydrogen production [98]. 5.12. Cytocompatibility and cytotoxicity evaluations of MXenes In contemporary biomedical research, MXene materials are increasingly employed. Zhang et al. [100] conducted experiments to assess the osteogenic potential of Ti3C2Tx films, crucial for bone tissue engineering. Results from cellular studies revealed strong compatibility with cells and a propensity to stimulate osteogenic differentiation in vitro. Furthermore, the implantation of MXene films into rat subcutaneous and calvarial defect sites showcased remarkable biocompatibility, osteoinductive properties, and the ability to facilitate in vivo bone regeneration. Examining the potential risks associated with Ti3C2Tx MXene exposure to the circulatory system, Huang et al. [101] investigated its cytocompatibility with red blood cells (RBCs) and human umbilical vein endothelial cells (HUVECs). Results indicated excellent compatibility even at high concentrations, with minimal hemolysis observed. Even at a high concentration of 200 μg/mL, Ti3C2Tx induced minimal hemolysis at 0.8%, whereas GO at the same treatment concentration resulted in a considerably higher hemolysis rate of 50.8%. The structures of RBCs were left undisturbed. The presence of fully covered surface-terminating –O and – OH groups in Ti3C2Tx resulted in a highly hydrophilic surface, impeding its influx Characterization and Application of Nanomaterials 2024, 7(2), 6348. 38 into the inherently hydrophobic interior of the cell membrane (Figure 22). The remarkable compatibility of Ti3C2Tx nanosheets with cell membranes and their minimal capacity to induce excessive reactive oxygen species (ROS) generation make them compatible with HUVECs. Figure 22. Cytocompatibility of Ti3C2Tx MXene with red blood cells and human umbilical vein endothelial cells and the underlying mechanisms [101]. In the realm of flexible electronics and biomedical devices, MXene fibers hold promise due to their superior conductivity. Usman et al. [102] investigated the incorporation of silk fibroin biopolymer into MXene formulations to enhance fiber properties. The addition of silk fibroin led to improved durability, with fibres enduring up to 1 hour of high-frequency sonication. Also, fibers containing approximately 5 wt% silk fibroin demonstrated intriguing characteristics, such as high conductivity (approximately 3700 S cm−1), elevated volumetric capacitance (approximately 910 F cm−3), and non-cytotoxicity towards THP−1 monocytic cells. For the development of bioimaging tools, Neubertova et al. [103] suggested covalent functionalization of Ti3C2T flakes using the chelating agent diethylenetriaminepentaacetic acid (DTPA), followed by complexation with Gd3+ ions. This established functionalization technique endowed the inherently diamagnetic Ti3C2T flakes with a paramagnetic response, facilitating their utilization in T1-weighted magnetic resonance (MR) imaging. Additionally, a noticeable relationship between magnetic relaxation time and flake concentration was observed, enabling the estimation of the distribution of flakes in space. The covalent decoration approach applied to MXene resulted in safeguarding its surface against oxidation in phosphate buffered saline and blood serum while enhancing cytocompatibility simultaneously. Furthermore, the chelation of Gd3+ ions demonstrated superior prevention of leakage compared to electrostatic chemisorption. A high photothermal conversion efficiency of MXene-Gd was demonstrated, suggesting potential applications in photothermal therapy. Yang et al. [104] highlighted the emergence of phototherapy as a promising strategy for combating bacterial infections without the associated risk of antibiotic resistance. However, the presence of endogenous antioxidative glutathione (GSH) in bacteria poses a challenge, as it hampers the desired antibacterial effects of externally generated reactive oxygen species (ROS) during phototherapy. A quad- Characterization and Application of Nanomaterials 2024, 7(2), 6348. 39 channel synergistic antibacterial nanoplatform composed of Ti3C2 MXene/MoS2 (MM) 2D bio-heterojunctions (2D bio-HJs) was fabricated. This platform exhibited photothermal, photodynamic, peroxidase-like (POD-like), and glutathione oxidase- like properties. Upon exposure to NIR laser, the 2D bio-HJs induced localized heating and elevated extracellular ROS levels, resulting in bacterial inactivation. Also, the Mo4+ ions released from the platform penetrated the bacterial membrane, triggering intracellular ROS generation and depletion of intracellular GSH, further contributing to bacterial eradication. The 2D bio-HJs produced localized heating and elevated extracellular ROS levels upon NIR laser irradiation, leading to bacterial deactivation. Simultaneously, Mo4+ ions easily penetrated the ruptured bacterial membrane, triggering intracellular ROS and depleting intracellular GSH. In the “ROS Hurricane”, bacteria faced substantial destruction from internal and external sources. However, when supplemented with fibroblast growth factor-21 (FGF21), 2D bio-hybrid junctions (bio-HJs) demonstrated favorable compatibility with cells and enhanced cell movement in vitro. Scheibe et al. [105] emphasized the necessity of conducting an in vitro examination to assess the impact of various types of MXenes and their precursors on human cell lines within the framework of nanomaterial safety. To this end, a range of multi-layered, few-layered, and single- layered Ti3C2Tx, alongside TiC, Ti2AlC, and Ti3AlC2, were synthesized. Following the trends in nanomaterial safety assessment, thorough characterization encompassing morphology, size purity, and surface charge was conducted (Figure 23). Subsequently, various biological responses (including cytotoxicity, membrane permeability, reactive oxygen species generation, and mechanical stress) induced by MXenes, TiC, and parental MAX phases were investigated and compared using human fibroblasts (MSU1.1) and cervical cancer cells (HeLa) as model systems, reflecting differing tumorigenic properties. The findings indicated that exposure to elevated concentrations (≥400 μg/mL) of TiC, Ti2AlC, and Ti3AlC2 particles, sized < 44 μm, could pose significant harm, eliciting pronounced cytotoxic effects through oxidative and mechanical stress mechanisms. Conversely, all forms of Ti3C2Tx were deemed safe for MSU1.1 cells, exhibiting only minor cytotoxic tendencies at the highest concentration levels. Furthermore, the observed cytotoxic behaviors were contingent upon cell type, with greater susceptibilities observed in cancer-derived cells. Figure 23. Cytotoxicity assessment of Ti-Al-C based MAX phases and Ti3C2Tx MXenes on human fibroblasts and cervical cancer cells [105]. Characterization and Application of Nanomaterials 2024, 7(2), 6348. 40 5.13. MXene-based materials for tissue engineering 2D MXenes are being used in the fields of tissue engineering and regenerative medicine. Also, because of their good electrical behavior, MXenes are used for neural tissue regeneration. Establishing artificial conduits to facilitate the growth of regenerating nerves and guide them through damaged areas is imperative for neuronal restoration. Despite the historical use of various natural and synthetic polymers in neural tissue engineering, certain drawbacks persist [106]. For instance, while poly (3,4-ethylene dioxythiophene) exhibits commendable biocompatibility and sufficient electrical and chemical stability, its proclivity towards chronic inflammation and limited degradability render it an unsuitable candidate. Presently, MXenes are being incorporated as additives into polymeric nanofibers, fostering the growth of supportive cells and endowing them with sustained neurotrophic properties [107]. To validate the efficacy of MXene-modified polylactic acid nanofibers in facilitating nerve-guiding path formation, an experimental study was conducted. Integration of MXenes notably augmented the electrical conductivity of the material. The resultant current flow in the modified nanofiber membrane remained below 150 mA under an applied voltage of 5.5 V, although higher potentials induced dielectric behavior in the fabricated nanofibers. These findings underscore the potential of MXenes in conferring electroconductivity to polymer membranes for neural tissue engineering applications. Notably, the presence of polylactic acid membranes containing immobilized MXenes did not exhibit adverse effects on cellular viability, as evidenced by resazurin reduction assays. However, the outcomes were partially satisfactory due to challenges associated with MXene immobilization and handling during cell culture. Furthermore, the composite material demonstrated promising anti-adhesive properties against bacterial colonization, potentially disrupting cellular morphology and membrane integrity. 6. Challenges and future prospects The numerous studies on MXenes and scientific reports, conferences, and articles are growing day by day, and the research in the right direction may address all challenges and issues. The possible right direction to the current state-of-art can open up new opportunities for innovations and developments of MXene-based devices. The current cutting-edge research on water-related issues can address the real problems, and the remarkable properties can make these efforts successful [3,9– 13,73]. MXenes are mostly designed using the top-down approach, while very few reports on the bottom-up approaches are available in the literature [11,72]. Serious efforts are needed to explore the hidden potential in the bottom-up approaches to the synthesis of MXenes, to improve the characteristic properties of the MXenes, and to obtain desirable forms and structures of MXenes. After almost a decade of discovery of MXenes, this remarkable progress has been observed in the many fields and aspects related to MXenes, for example, synthesis, structural patterns, more applicability, and the discovery of new MXene compounds. But still, there are challenges left. The major challenges remain for MXenes and the application of MXene-based materials. Very few MAX phases have been experimentally exfoliated Characterization and Application of Nanomaterials 2024, 7(2), 6348. 41 to date, while a few dozen possibilities are predicted. Theoretically, the MXenes can be the best possible cathode for high energy density 3458 Wh kg−1 Li-air batteries, but there is no successful report regarding this aspect [108,109]. Recently, Zn-air batteries have been successfully reported with promising performance, which proves the potential of MXenes [106,107]. Most of the reports are about coin-cell-based devices; it is just to see the actual potential of the material, while the pouch-type or large assembly of cells is needed to be tested for futuristic flexible, printable, or wearable applications of MXene-based batteries and supercapacitors. The performance of MXene-based devices with solid electrolytes needs to be tested to confirm the possibility of flexible and wearable MXene-based devices [61,110]. MXenes are proposed to be the future materials for absorption of toxic pollutants, but they need to be exploited fully [111]. For example, one of the promising MXene KxMeO2-based electrodes for batteries is under investigation, while the existing report shows lower operating potential and lower energy density in comparison to graphite/LiMn2O4-based LIBs [112] (Figure 24). Figure 24. (a) Average operating potential versus gravimetric capacity plot for selected layered TMOs for PIBs; (b) typical charge/discharge curve for P2-type KxCoO2Xu [112]. The operating potential and capacity values reported for the various battery cells are derived from the potentiated state in half cells. While calculating the energy densities of full cells, graphite (with a capacity of 279 mAhg−1 at 0.25 V vs. K/K+) is Characterization and Application of Nanomaterials 2024, 7(2), 6348. 42 employed as the negative electrode material in conjunction with various positive electrode materials. The key latest developments in MXene and MXene-based materials and devices are presented. Some key challenges and issues related to MXene-based materials for various applications need to be taken up [112]. 7. Conclusion MXene and MXene-based materials can be synthesized through various approaches and methodologies tailored to meet some specific properties and structural specification requirements. The exponential growth in the research articles and expansion of research groups around the world suggests expected tremendous improvement in the synthesis techniques soon. The specific properties of MXenes and MXene-based materials, e.g., specific surface area, chemical stability, hydrophilicity, thermal conductivity, and environmental compatibility, can be modified and customized according to the needs of applications. Water purification, absorption of impurities, metal removal, and degradation of impurities can be successfully done using MXenes-based materials in a better way. MXenes exhibit promising potential in diverse realms, including environmental remediation, biomedical applications, batteries, supercapacitors, and other advanced engineering fields. MXenes demonstrate excellent biocompatibility in biomedical applications. 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