Recent Progress in MXenes-Based Materials for Gas Sensors and Photodetectors

Abstract

Recently, a new class of two-dimensional (2D) materials known as MXenes, such as Ti₃C₂T_x, have received significant attention due to their exceptional structural and physiochemical properties. MXenes are widely used in a variety of applications, including sensors, due to their excellent charge transport, high catalytic, and conducive properties, making them superior materials for sensing applications. Sensing technology has attracted significant interest from the scientific community due to its wide range of applications. In particular, gas sensing technology is essential in today’s world due to its vital role in detecting various gases. Gas sensors have an essential role in real-time environmental monitoring health assessment, and the demand for air quality monitoring is driving the gas sensor market forward. Similarly, optical sensors are a related technology that can rapidly detect toxic substances and biomaterials using optical absorption spectroscopy. MXenes are highly desirable for gas and optical sensing applications due to their abundant active sites, metallic conductivity, optical properties, customizable surface chemistry, and exceptional stability. In this review article, we compile recent advancements in the development of gas sensors and optical sensors using MXenes and their composite materials. This review article would be beneficial for researchers working on the development of MXenes-based gas sensors and optical sensors.

1. Introduction

Recently, the enormous utilization of two-dimensional (2D) materials has drawn significant attention to various applications from the scientific community [1,2,3,4]. The optoelectronic applications such as energy storage, generation, sensing, catalysis, and optoelectronics leverage 2D materials due to their fascinating physical and optical properties [5,6,7,8,9,10,11,12]. Since the discovery of graphene, there has been rapid growth in the synthesis and utilization of 2D materials, including boron nitride, perovskite materials, metal halides, and transition metal halides [13,14,15,16,17,18]. Their structural flexibility and wide range of compositions have attracted considerable attention, leading to their use in diverse applications [19,20,21,22,23]. In 2011, a new type of 2D material was discovered by Gogotsi and his research team, which known as MXene [24]. MXenes are generally synthesized via a unique etching synthesis procedure [25]. The synthesis of MXenes involves etching layers from the bulk MAX phase using hydrofluoride (HF) as an etchant. Surface functionalization, achieved through exfoliation and etching during MXene synthesis, introduces hydrophilicity, with further modifications possible through calcination or alkalization for specific applications [26]. The MAX phase consists of early d-block transition metals (M), group 13–15 elements (A), and carbon or nitrogen (X) [27]. It features a closely packed multi-layered structure (M_n+1AX_n layers, with n ranging from 1 to 4) of ternary carbides and nitrides [28]. Although the M-A bonds are strong, the presence of the covalent, metallic, and ionic characters of the M-X bonds facilitate the chemical etching of the A layer during the synthesis of the MXene [24]. The wide variety of stoichiometric compositions of MAX phases allows for the formation of numerous MXenes, offering significant diversity and thermodynamic stability with excellent electrical conductivity [29]. These properties make MXenes promising for extensive research and various applications due to their outstanding electrical conductivity, excellent mechanical stability, large specific surface areas, and good hydrophilicity [30].

In the current era of information sensing, the detection, and analysis of information play vital roles across various fields [31]. Conventional sensors are used to detect chemical reactions, biological interactions, and physical variables [32]. Sensors are the new generation technologies which can significantly detect variations in targeted detection, then convert signals into optical, electrical, or thermal outputs for transmission, processing, and storage [33,34,35]. It is expected that an ideal sensor should be highly sensitive and highly selective, with a low detection limit and fast response time [36,37,38]. The demand for high sensitivity and fast response has driven the scientific community to explore newly synthesized materials for sensor fabrication [39]. MXenes have emerged as suitable candidates due to their large specific surface area and high electrical conductivity [40], which may be attributed to the presence of metal-like properties in the MXene [41], surface functional groups that provide good hydrophilicity [42], and a stable structure that resists oxidation under harsh conditions [43]. MXenes are employed in various optical sensors, including electrochemiluminescence (ECL), surface plasmonic, SERS, and fluorescent sensors. Modifying MXenes with other nanomaterials and functionalization can enhance their properties and expand their applications [44]. Transforming MXenes into nanosheets or quantum dots is particularly advantageous for SERS substrates, fluorescence quenchers, and photoluminescent materials [45,46]. The favorable energy levels of MXene nanosheets, due to their wide absorption, provide an excellent platform for the fabrication of photo–thermal, photo–electrochemical, and optical sensors [47]. MXenes have also shown exceptional promise for optical sensors due to their remarkable optical properties, such as photoluminescence, transmittance, surface-enhanced Raman scattering (SERS), and absorption, motivating their use in sensor design and fabrication [48,49,50]. Based on sensing signals, the most efficient and reasonable MXene-based sensors can be categorized into optical and electrical sensors.

The next generation technological progress in recent years has resulted in the proliferation of smart devices across many aspects of our lives [51]. From smartphones to wearable fitness trackers, the need for innovative and efficient sensing technologies is at an all-time high [52]. Especially in the field of gas sensing, there has been substantial advancement due to the necessity of monitoring air quality, detecting hazardous gases and industrial emissions for healthcare and smart home applications [53,54]. Smart gas sensors mark a significant departure from traditional gas detection techniques [54]. Although conventional gas sensors have been in use for many years, they frequently face challenges regarding sensitivity, cost-effectiveness, data processing abilities, and power efficiency [55]. On the other hand, smart gas sensors guide us towards state-of-the-art technologies and advanced materials, offering numerous benefits and making them indispensable across various applications [56].

Optical sensors have emerged as powerful tools in various fields, offering precise and sensitive detection capabilities [57]. Optical sensors have gained attention, particularly in biosensor fabrication, due to their high selectivity and sensitivity, quick response time, cost-effectiveness, and reproducibility [58]. The ability of the optical sensors to convert changes in light properties into measurable signals has enabled their use in diverse applications such as environmental monitoring, biomedical diagnostics, and industrial process control [59,60]. In the past few years, MXenes either alone or its hybrid composites, have been extensively used for the detection of various biological and chemical analytes using gas and optical sensing technologies. In the present review article, we have summarized the reported articles on the fabrication of gas and optical sensors using MXenes and their composites. We believe that the present review article would be useful for the research scholars and scientists working on the design and fabrication of MXene-based gas and optical sensors.

2. MXene Synthesis

Synthesizing MXene follows a process akin to other 2D materials, involving the exfoliation of layers from raw materials. Initially, researchers attempted mechanical exfoliation on MAX phase precursors, akin to the process used for graphite. However, due to the robust M-A bonds inherent in MAX, obtaining pure MXene proved challenging. Consequently, researchers explored suitable etching methods to remove the “A” atom layer from MAX, employing etchants such as concentrated HF or HF containing hydrochloric acid–lithium fluoride salts (LiF/HCl) [61]. Ultrasonic treatment facilitated the etching process for MXene synthesis, with the resultant crude product subjected to washing to eliminate residual substances, followed by vacuum drying. To obtain MXene sheets, further detachment or delamination of the multi-layered MXene was achieved through intercalation, employing organic molecules, cations, or organic bases via ultrasonication and centrifugation processes [62,63]. Intercalation is a crucial step for producing free-standing MXene sheets, yielding an aqueous colloidal solution of MXene. This method, also known as the wet etching method, stands as a well-established top-down approach for MXene synthesis.

The initial synthesis of the first MXene, Ti₃C₂, involved the removal of the Al layer from the MAX Phase (Ti₃AlC₂) through etching with 50% HF acid, as in the reaction:

Ti₃AlC₂ + HF ---------- AlF₃ + Ti₃C₂ + 3/2H₂

This process exploited the weaker Ti-Al bonds compared to the Ti-C bonds, facilitating the detachment of the Al layer and leaving functional groups attached to the MXene surface [62]. The resulting M_n+1X_nT_x MXene exhibited a multilayered structure with strong bonding between layers, attributed to van der Waals forces or H-bonding. The duration of HF etching and the temperature varied, influencing the outcome from hours to days and from room temperature to 50 °C [64,65]. Ti₃C₂, like other MXenes, demonstrated dispersibility in both organic and aqueous solutions, such as propylene carbonate, ethanol, and dimethylformamide (DMF), enabling its incorporation with polymers and nanomaterials through solvent exchange, mixing, or self-assembly [66]. The use of concentrated HF acid as an etchant posed environmental and health risks due to its highly corrosive nature. Consequently, safer alternatives have been developed, such as the HF-forming etchant method, which utilizes hydrolysis of bifluoride salts to produce HF in situ. This method offers advantages over raw HF, including safety, high yield, flexibility, and shorter sonication time [67].

Notably, the combination of LiF–HCl has been employed in synthesizing high-quality MXene sheets of Ti₃C₂ and Mo₂C [68], where Li⁺ ions in the etchant solution facilitate concurrent etching and intercalation [69]. Factors like reaction conditions, composition, etching solution, time, and MAX phase particle size affect MXene yield and quality, with prolonged etching potentially causing structural defects, especially in M₄AX₃ MAX phases, which require harsher etching conditions than other MAX phases like M₃AX₂ [70].

Surface functionalities such as –O, –OH, and –F depend on the synthesis method, with –F and –O functionalities predominating in MXenes produced via the HF etching and LiF/HCl processes, respectively [71]. Beyond the top-down approach, a bottom-up approach involving techniques like plasma-enhanced pulsed laser deposition (PEPLD), template methods, and chemical vapor deposition (CVD) has been recommended for high-quality MXene synthesis. For instance, CVD has been utilized to synthesize ultrathin crystals of transition metal carbides/nitrides (TMC/TMN) [72], while the template method using 2D transition metal oxides facilitated the synthesis of MXenes by ammoniating or carbonizing [73]. Also, this template method was used in development of nanosheets of MoN and other nitrides [74]. PEPLD, employing both ionized and non-ionized CH₄ plasma, has been successful in synthesizing high-quality ultrathin Mo₂C crystals [75].

3. MXenes in Gas Sensors

Gas sensors are attracting significant global attention for their crucial role in accurately detecting various toxic gases, which is essential for monitoring environmental health. Gas sensors are used in various fields such as medical diagnostics, domestic and public security, environmental monitoring, industrial production, and food safety [76,77]. The fundamental function of gas sensors is to convert the composition and concentration of gas into electrical signals, facilitating their detection. Extensive efforts have been made and reported in the literature focusing on the detection of toxic and explosive gases for environmental monitoring. For this, various approaches have been investigated, such as conductive polymers, metal oxide semiconductors (MOS), carbon-based materials, and two-dimensional (2D) materials [78,79]. These sensor materials offer numerous advantages such as low detection limits, rapid response and recovery times, high sensitivity, excellent stability, a simple fabrication process, and low cost. Among these sensor materials, MOS are the most extensively explored materials for gas sensing applications. However, MOS requires external stimuli such as high temperatures for adsorption and desorption of gas molecules or ultraviolet light to alter the conductive properties of sensors. This dependency can lead to increasing power consumption, higher costs, and a greater risk of ignition for flammable gases. Hence, there is a significant focus on conducting extensive research to develop advanced gas sensors with low detection limits that can operate effectively at room temperature.

MXene belongs to a novel class of 2D materials and has emerged as a promising candidate in gas sensing applications [80,81]. When compared with other 2D materials such as graphene and transition metal dichalcogenides (TMDs), MXene is attracting greater attention in the field of gas sensing [82]. Due to its facile synthesis methods and unique properties, MXene has become a central point for scientists, leading to a significant number of articles in the literature as depicted in Figure 1 [63].

Furthermore, the rapid growth of MXene in sensing has surpassed graphene and TMDs. This has positioned MXene as a promising material for gas sensing and it has gained increasing attention due to its remarkable electrical conductivity, hydrophilic surface, large specific surface area, abundant surface functional groups, excellent stretchability, and biocompatibility [83,84,85]. These distinctive characteristics that make MXene well-suited for gas sensing applications may be attributed to the presence of M and X sites within its structure. These sites offer tunable surface functionalities, which promote a high adsorption energy of gases, enhancing their sensitivity and selectivity in gas sensing [86]. In addition to its surface properties, MXene exhibits other versatile functionalities such as robust mechanical strength, exceptional stability, good volumetric capacity, significant zeta potential, and rapid ionic transport (Figure 2). These collective features improve its selectivity for various toxic gases, even in very low signal to noise. These unique properties of MXene have inspired scientists to concentrate on MXene-based materials for gas sensing applications rather than other 2D materials [87].

MXene has been demonstrated to be an extremely sensitive platform for selectively detecting a broad range of gases. Moreover, MXene exhibits a rapid gas sensing response even at room temperature by quickly absorbing gas molecules from the environment. Gas sensors based on MXene have exhibited remarkable gas-sensing applications. Notable examples include Ti₃C₂T_X, Ti₃C₂T_X/ZnO, and Ti₃C₂T_X/Mo₂T_xCo₃O₄@PEI [89,90,91,92,93].

3.1. Pristine MXene-Based Gas Sensor

Numerous research studies have been published based on pristine Ti₃C₂Tx sensors, demonstrating superior selectivity and sensitivity compared to traditional room temperature gas sensors. Yu and their co-workers explored the gas sensing capabilities of one of the thinnest (Ti₂CO₂) layers of the MXene family towards ammonia sensing [94]. They utilized the non-equilibrium Green’s function method to calculate the current–voltage (I–V) relation for NH₃, indicating a significant change in the I–V curve before and after NH₃ absorption on TiC₂CO₂ due to the distinct transport properties of NH₃. To assess the sensitivity of the prepared sensor, they investigated the adsorption of different gases such as NH₃, H₂, CO, CO₂, CH₄, N₂, NO₂, and O₂ using first principles simulations. It can be seen from Figure 3 that the chemisorption of NH₃ on the Ti₂CO₂ sensor is notably high as compared to other analytes, possibly due to biaxial strain, indicating a weak interaction with other gases. This suggested that detecting ammonia using biaxial strain on a monolayer of Ti₂CO₂ is preferable over other gases. Moreover, the nitrogen atom of ammonia demonstrates a strong binding interaction with the titanium of Ti₂CO₂ as compared to other analytes.

This strong binding is attributed to the positioning of the N atom of NH₃ exactly above the Ti atom, resulting in a high binding energy, as shown in Figure 4a,b [94]. They found that the conductivity of the Ti₂CO₂ sensor was found to be increased upon adsorption of ammonia under the strain, indicating a high sensitivity towards ammonia as compared to other analytes under strain. The promising adsorption or desorption of ammonia on the Ti₂CO₂ sensor is attributed to its high adsorption energy (0.37 eV) and high charge transfer (0.174 electrons). Therefore, the absorption of ammonia is reversible, suggesting that the Ti₂CO₂ sensor is well-suited for ammonia sensing [94].

In addition to the work on the Ti₂CO₂ monolayer, a similar study on ammonia sensing using the Zr₂CO₂ MXene have been reported in the literature [95]. This study demonstrated a systematic investigation of the adsorption/desorption behavior of ammonia on an O-functionalized Zr₂CO₂ MXene with both negative and neutral charges, using first principles simulations. The result indicates a nearly similar adsorption behavior of ammonia on the Zr₂CO₂ MXene, like the Ti₂CO₂ MXene. They demonstrated that the adsorption behavior of ammonia on the Zr₂CO₂ MXene monolayer is highly favorable as compared to other gases, with significant charge transfer. This favorable adsorption is attributed to the large adsorption energy (0.81 eV) and Mulliken charge transfer (0.188). Moreover, ammonia molecules can be efficiently desorbed from the monolayer of the Zr₂CO₂ MXene by injecting electrons from ammonia to the Zr₂CO₂ MXene [95].

They observed that the adsorption behavior of other gases (such as H₂, CH₄, CO, CO₂, N₂, and O₂) on the Zr₂CO₂ sensor surface differs significantly from that of ammonia. This difference arises from variations in adsorption energies and Mulliken charge transfer between ammonia and other gas molecules when interacting with the Zr₂CO₂ sensor surface, as depicted in Figure 5 [95]. Since the charge transfer between gases and sensors is crucial for the gas sensing performance, a Hirshfeld charge analysis was carried out to further validate the results. As illustrated in Figure 5, the Hirshfeld charge transfer between ammonia and the Zr₂CO₂ sensor is found to be 0.223 e, which is larger than the other gases. These results are consistent with the Mulliken charge analysis findings. Hence, O-functionalized Zr₂CO₂ MXenes emerge as a promising candidate for ammonia sensing with high selectivity [95].

Mai et al. investigated the sensing behavior of SO₂ on an O-functionalized M₂CO₂ (M = Sc, Hf, Zr, and Ti) monolayer due to the highly toxic nature of SO₂, which can pose different human health risks [96]. The sensing behavior of SO₂ on the Sc₂CO₂ sensor monolayer was qualitatively investigated using first principles calculations. They used two-probe models to estimate the relationship between I and V before and after the SO₂ adsorption, as shown in Figure 6a. The I–V graph exhibited a significant increase in the current after adsorption of SO₂ on a sensor, as shown in Figure 6b. These findings suggest that the Sc₂CO₂ monolayer is a promising sensor for SO₂ sensing.

In another study, Yang et al. also explored the gas sensing capabilities of the Sc₂CO₂ monolayer for detecting carbon monoxide (CO) and nitrogen oxide (NO) [97]. They observed that the Sc₂CO₂ monolayer exhibited good sensitivity to the NO molecule due to a chemical interaction and a significant charge transfer of 0.303 e, which was further increased by applying external strain. Conversely, the sensitivity towards CO was found to be increased after Mn doping into Sc₂CO₂, attributed to the strong adsorption energy (−0.85 eV) [97]. In 2017, Lee et al. developed a flexible room temperature gas sensor by fabricating a Ti₃C₂T_X nanosheet on a polyimide substrate using a simple drop cast method [98]. This sensor demonstrated effective detection capabilities for various polar gases such as methanol, ethanol, acetone, and ammonia even at concentrations as low as 100 ppm at room temperature. The fabricated sensor exhibited p-type sensing behavior. The fabricated sensor exhibited the dynamic gas response of Ti₃C₂T_X and was found to be 21, 14.3, 11.5, and 7.5% for ammonia, methanol, ethanol, and acetone, respectively, as shown in Figure 7 [98].

In a p-type sensor, most carriers are holes, which shows a reduction due to the transfer of electrons from the gases to the predominating surface terminal groups, resulting in an increase in resistance. The fabricated sensor showed the lowest response to acetone (7.5%), whereas it showed the highest response to ammonia (21%), with a theoretical limit of detection of 9.27 ppm [98]. It can be seen from Figure 8, that the resistance of the Ti₃C₂T_X sensor improved upon the introduction of gaseous molecules and decreased when the gases were removed [98].

A plausible sensing mechanism of the fabricated sensor was proposed focusing on the charge interaction between gaseous molecules and the surface functional groups (–O, –OH) of the Ti₃C₂T_X sensor (Figure 9a). For example, the mechanism elucidating ammonia sensing over the Ti₃C₂T_X sensor is schematically depicted in Figure 9b [98]. The Ti₃C₂T_X sensor exhibited an alteration in the electrical response upon the efficient adsorption or desorption of the gaseous molecule on its surface. Specifically, when a gas molecule like ammonia is absorbed into the terminal groups such as –O⁻ and –OH⁻ of the Ti₃C₂T_X sensor, electrons are generated through Equations (1) and (2). These electrons then combined with the holes present in the Ti₃C₂T_X sensor, indicating electron–hole recombination and subsequently resulting in an increase in the resistance [98].

2NH₃ + 3O⁻  =   N₂ + 3H₂O + 3e⁻

(1)

NH₃ + OH⁻   =   NH₂ + H₂O + e⁻

(2)

Following the initial demonstration of the Ti₃C₂T_X gas sensor, Kim and co-workers have advanced their research by developing an ultrahigh chemo resistive gas sensor based on Ti₃C₂T_X, aiming for the precise detection of volatile organic compounds (VOCs) [99]. Their experiment design involved fabricating the Ti₃C₂T_X sensor as a metallic channel and conducting a comparative analysis with BP, RGO, and MoS₂ for sensing various VOCs at room temperature. Utilizing the HF etching method, Kim and his team synthesized the Ti₃C₂T_X sensor by replacing Al in the Ti₃AlC₂ powder through the LiF/HCl route. The resulting Ti₃C₂T_X flakes exhibited high conductivity and were fully terminated with functional groups such as –OH, –O, and –F, as shown in Figure 10.

The fabricated –OH-functionalized Ti₃C₂(OH)₂ sensor demonstrated a very low detection limit of 50–100 ppb for VOCs at room temperature. This interesting combination resulted in highly attractive gas detection capabilities with an ultrahigh signal-to-noise ratio (SNR) when exposed to 100 ppm of ethanol, acetone, propanol, CO₂, NO₂, SO₂, and ammonia in real-time, as shown in Figure 11a. Among the different gases tested, ethanol exhibited the highest response of 1.7%, followed by propanol, and ammonia, whereas the sensor’s response decreased towards some acidic gases like CO₂, NO₂, and SO₂, as shown in Figure 11a. The observed results suggest that Ti₃C₂T_X exhibits high selectivity towards VOCs compared to acidic gases, attributed to its –OH group. This group plays a crucial role by forming hydrogen bonds with VOCs rather than in acidic gases. Additionally, the team compared the sensing performance of the prepared Ti₃C₂T_X sensor with other 2D materials such as BP, MoS₂, and reduced graphene oxide (RGO). Figure 11b illustrates the real-time gas response of BP, MoS₂, RGO, and Ti₃C₂T_X sensors towards 100 ppm of ethanol, acetone, and ammonia.

The gas sensing performance of MXenes have been observed to be influenced by precursor materials such as carbon sources, atoms, and the lateral dimension of the sheets. To investigate these effects, Christoper et al. utilized three commonly used carbon sources, graphite, TiC, and carbon lampblack, to synthesize Ti₃AlC₂ [100]. The synthesis of the Ti₃AlC₂ MAX phase was conducted at high temperature (1650 °C) for two hours using three different starting carbon sources, resulting in diverse morphologies and properties due to the distinct pathways, as shown in Figure 12a. The MAX phase synthesized using graphite as the carbon source exhibiting a flake size of up to 19.2 μm, while the MAX phase produced by the TiC precursor showed a flake size of 9.3 μm. Additionally, the MAX phase prepared using amorphous carbon lampblack showed a flake size of 2.4 μm, as shown in Figure 12b.

They investigate the sensing performance of different MAX phases towards 100 ppm ethanol and acetone and 5 ppm of ammonia at room temperature [100]. Interestingly, the MAX phase obtained from smaller-sized flakes derived from amorphous carbon lampblack exhibited a lower gas response as compared to the MAX phase derived from graphite and TiC precursors, as shown in Figure 12c. In another study, Wu et al. developed a sensor based on a single layer Ti₃C₂ MXene for the selective detection of ammonia at room temperature [89]. To prevent the formation of Li-ion residue on Ti₃C₂, they employed a different etching method, namely NaF + HCl, in contrast to the previously reported LiF + HCl etching route. This approach resulted in a clean surface, which facilitates the fast adsorption of ammonia due to the increased available area. After synthesis, they prepared a sensor by coating the surface of ceramic tubes with a colloidal suspension of a single layer of the Ti₃C₂ MXene for gas detection. The sensor was then exposed to various gases at a concentration of 500 ppm, such as CH₄, H₂O, NH₃, H₂S, NO, ethanol, methanol, and acetone. The Ti₃C₂ MXene-based sensor showed high selectivity for ammonia compared to other gases [89]. Tai and co-workers reported the synthesis of a Ti₃C₂Tx/γ-PGA gas sensor for NO₂ detection [101]. In their study, they demonstrated that modifying Ti₃C₂Tx with γ-poly(L-glutamic acid) (γ-PGA) significantly enhanced its gas sensing performance. This improvement is attributed to the unique synergistic interaction between γ-PGA and Ti₃C₂Tx, and the blocking effect facilitated by water molecules as well as increased effective adsorption of gas molecules. Additionally, their sensor exhibited fast response and recovery times, along with excellent stability. In another work, Tai’s group developed a PANI/Nb₂CTx sensor for ammonia detection under humid conditions [90]. The fabricated PANI/Nb₂CTx sensor showed excellent gas sensing performance for ammonia concentrations as low as 0.1–10 ppm, with a detection limit of 20 ppb at 87.1% relative humidity. This superior performance is attributed to the various hydrogen bonds formed between polyaniline (PANI) and Nb₂CTx, resulting in superior selectivity, high sensitivity, and good long-term reliability, Furthermore, Tai’s group also developed a V₂CTx MXene sensor for NO₂ detection [102].

Table 1. Gas sensing response of selected sensors based on different MXene structures at room temperature.

MXene	Analyte	Conc. (ppm)	Response	References
Ti2CO2	Ammonia	100		[94]
Ti3C2TX	NH3, Methanol, Ethanol, Acetone	100	0.21 0.143 0.115 0.075	[98]
Ti3C2TX	NH3, Propanol, Ethanol, Acetone, NO2, SO2, CO2	100	0.8 0.88 1.7 0.97 0.25 0.2 0.1	[99]
Ti3C2TX	NO2	5	4.5%	[103]
Ti3C2TX	Ethanol, Ammonia, Acetone	100 5 10	0.16 0.23 0.62	[100]
Ti3C2TX	Acetone, Ethanol, Methanol, Ammonia, TCM, Water, NO2	10 10 10 10 10,000 10,000 10	1.4 1.7 2.2 0.7 0.1 0.5 0.9	[104]
Ti3C2TX	Ethanol, Acetaldehyde, Formaldehyde, Methanol, Methane, NO2, Ammonia	100	2% 3% 5% 2% 4% 10% 28.87%	[105]
Ti3C2TX	Ethanol CO2	0.1%, 1%	35%, 0.5%	[106]
Ti3C2TX	CH4, H2S, H2O, Ethanol, Methanol, Acetone, Ammonia NO	500	0.5% 0.16% 0.37% 1.5% 0.2% 0.3% 6.13% 0.4%	[89]
V2CTX	Acetone, Methane, H2, H2S	100	0.0226 0.0167 0.2435 0.005	[107]
V4C3TX	Acetone	100	2.5	[108]
Mo2CTX	Toluene	100	2.65%	[109]
α-Mo2C	NO2, Ammonia, Acetone, Propanal, Ethanol, Hexane, Toluene	5 5 1000	15% 4% 3% 4% 6% 1% 2%	[110]

presents a summary of the gas sensing performance of different MXene-based sensors measured at room temperature.

3.2. Hybrid MXene-Based Gas Sensor

Significant work has been published for gas sensing utilizing pristine MXene-based sensors. However, their sensing abilities are still far away from the desired standards. The chemical and physical properties of pristine MXene, as well as its structure, greatly influence sensing performance. Therefore, strategies for improving gas sensing performance were realized by changing the carbon source of the MAX phase precursors, reducing the flake size, and increasing defect concentration. Further enhancement can be achieved by hybridizing pristine MXene with other materials to create synergistic effects between them. Designing hybrid composites offers a promising approach for improving sensitivity, selectivity, response/recovery times, and stability compared to pristine MXene, since MXene’s higher specific surface area and other advantageous properties make it suitable for integration with a diverse array of materials. These include metal oxides semiconductors, TMDs, 2D materials (graphene), and organic polymers. Such hybridization not only improves their complexity but also enhances functionality, offering a wider spectrum of applications.

A lot of the published literature has focused on MOS due to their outstanding selectivity, sensitivity, and stability along with their ease of production and responsiveness to a diverse range of gases. Despite their merits, MOS typically needs some external stimuli, such as high temperature or UV light for activation, leading to increased costs, elevated power consumption, and risks of ignition with flammable gases. The combination of MOS with MXenes presents various advantages. MOS exhibit exceptional gas adsorption and catalytic characterization capabilities, while MXenes enhances electrical conductivity and offer a vast surface area for efficient gas adsorption. This combination enhances sensitivity and selectivity towards different gasses. Tai et al. synthesized the MXene-based composite gas sensor (TiO₂/Ti₃C₂T_X) using in situ growth of TiO₂ nanowires on the Ti₃C₂ surface [111]. The designed composite sensor exhibits high sensitivity (1.63 times) and low response/recovery times (0.62/0.52) to 10 ppm ammonia at room temperature as compared to pristine MXene (Ti₃C₂T_X) [111]. In a separate study, He et al. used the hydrothermal approach to design an MXene decorated with SnO₂ nanoparticles [112]. The resulting MXene/SnO₂ composite heterojunction exhibited remarkable sensitivity and selectivity towards ammonia with a detection limit of 0.5 ppm at room temperature (Figure 13a). Moreover, the sensor exhibited rapid response and recovery times, both under 30 s, along with excellent stability (Figure 13b). Comparative analysis revealed that the sensor on the composite heterojunction outperformed pristine MXene by 40%, demonstrating n-type behavior [112]. The higher performance is attributed to an increased number of electrons on the SnO₂ surface due to difference in the Fermi levels of the MXene and SnO₂. Additionally, the composite structure provided selective adsorption abilities, further enhancing its functionality (Figure 13c,d). Wang et al. fabricated a composite comprising MXene (Ti₃C₂T_X) and WO₃ for NO₂ sensing using the electrospinning method [113]. Their Ti₃C₂T_X/WO₃ composite sensor exhibited excellent gas sensing activity towards different concentrations of NO₂ gas at room temperature (Figure 13e). The remarkable gas sensing response can be attributed to the higher electrical conductivity of both Ti₃C₂T_X and WO₃ materials in air. Moreover, there was an accumulation of a greater number of electrons on the WO₃ surface due to the transfer of electrons from Ti₃C₂T_X to WO₃, driven by the difference in Fermi levels (Figure 13f) [113]. Comparative analysis indicated that the gas sensing response of the composite sensor was 15 times higher than other reported chemiresistive-based sensors, such as MXene/WS₂ hybrids for 2 ppm NO₂ and Ti₃C₂T_X/SnS₂ for 1000 ppm NO₂ sensing [114,115].

The coupling of MXene with other 2D materials can offer a new platform for functionalizing hybrid materials. These hybrids can provide higher surface areas for gas adsorption, enhanced charge transfer capabilities, and improved electrical conductivity. Combining MXene with materials like graphene and other transition metal dichalcogenides provides distinct advantages, leading to superior sensing performance.

Tai and his co-workers also reported the synthesis of a hydride composite of MXene (Mo₂TiC₂Tx) with MoS₂ for NO₂ detection. Their study demonstrated that the Mo₂TiC₂Tx/MoS₂ gas sensor exhibited a superior response to NO₂, along with excellent selectivity against various interfering gases [116]. In another investigation, Tai’s group reported the sensing of NO₂ using a V₂CTx/SnS₂ composite sensor at room temperature [117]. They demonstrated that the fabricated sensor exhibited NO₂ sensitivity 581.6 times higher than that of pristine V₂CTx. Lee et al. reported the development of a metal binder-free Ti₃C₂Tx/GO hybrid fiber for ammonia detection [93]. They demonstrated that the fabricated sensor exhibits an improved ammonia sensing response (ΔR/R₀ = 6.77%) at ambient temperature as compared to MXene and graphene alone. This improvement is attributed to the synergistic impact of the electrical properties and gas adsorption capabilities of MXene and graphene. In other study, Tran et al. synthesized an rGO/Ti₃C₂Tx hybrid sensor using a simple ultrasonication method for NO₂ detection [118]. They showed that the developed sensor exhibited a sharp, large concentration-dependent, and cyclic response to NO₂ gas at room temperature. Moreover, the heterostructure showed excellent response to other toxic gases such as methane and toluene, outperforming other hybrid composites such as MoS₂/Ti₃C₂T_x and g-C₃N₄/Ti₃C₂T_x, which were synthesized for comparison purposes [118].

Moreover, the formation of heterojunctions between MXene and polymers enhances mechanical flexibility and durability. Consequently, composite materials of MXene with polyaniline (PANI), Polypyrrole (PPy), Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT-PSS), Polyethylenimine (PEI), etc., are attracting significant attention in sensor applications due to their high sensitivity and the low cost of polymers [119,120,121,122].

Li et al. developed a flexible chemiresistive gas sensor for ammonia detection based on a PANI/Ti₃C₂Tx hybrid film [119]. They reported that the sensor exhibited a low detection limit, high sensitivity, excellent repeatability, and good stability in air. These attributes were attributed to the Schottky junction and improved protonation degree of PANI in the hybrid composite. Similarly, Shen and co-workers synthesized a hybrid nanocomposite of PPy and MXene using a simple in situ polymerization approach for ammonia sensing [120]. They found that the PPy/Mxene-12 sensor demonstrated excellent response to 100 ppm ammonia at room temperature, which was 2.38 and 6.38 times higher than that of PPy and MXene, respectively. Additionally, the sensor showed excellent selectivity with a low detection limit. Chu and coworkers synthesized a PEDOT:PSS/MXene hybrid composite sensor using a dip coating technique for ammonia sensing [121]. The fabricated sensor shows a strong response to 100 ppm ammonia at room temperature, with fast response and recovery times of 116 and 40 s, respectively. They attributed the improved sensing performance to the synergistic effect between the PEDOT:PSS polymer and the Ti₃C₂Tx MXene. In another study, Zhou et al. synthesized the nitrogen-doped Ti₃C₂Tx MXene/polyethyleneimine composite films decorated with reduced graphene oxide nanosheets for CO₂ sensing [122]. They reported that the fabricated ternary sensor with a PEI loading of 0.001 mg/mL exhibited reversible and superior performance compared to other counterparts at room temperature. Moreover, the sensor demonstrated favorable repeatability, selectivity, a low detection limit of 8 ppm, and long-term stability.

Table 2. Gas sensing responses of selected sensors based on hybrids of MXenes at room temperature.

MXene Hybrid	Analyte	Conc. (ppm)	Response	References
Ti3C2Tx/TiO2	NO2	10	1.9%	[123]
Ti3C2TX/CuO	Toluene	50	11.4%	[124]
Ti3C2TX/WSe2	Ethanol	40	12%	[125]
Ti3C2TX/ZnO	NO2	100	41.9%	[126]
Ti3C2TX/SnO-SnO2	Acetone	100	12.1%	[126]
Ti3C2TX/ZnSnO3	Formaldehyde	100	194.7%	[127]
Ti3C2TX/In2O3	Methanol	5	29.6%	[128]
Ti3C2TX/α-Fe2O3	Acetone	5	16.6%	[129]
Ti3C2TX/Graphene fibers	Ammonia	100	6.77%	[129]
Nb2CTx/PANI	Ammonia	100	1.19%	[90]
Ti3C2TX/Poly glutamic acid	NO	50	1127.3%	[90]
Ti3C2Tx/WS2	NO	100	76.9%	[115]
Nb2CTx/PANI	Ammonia	100	301.31%	[130]

shows the reported performance of gas sensors using MXenes [90,115,123,124,125,126,127,128,129,130].

4. MXene-Based Optical Sensors

MXene-based materials have emerged as one of the most captivating materials within the realm of 2D materials, owing to their distinctive array of properties. These include facile surface functionalization, high conductivity, outstanding hydrophilicity, extensive surface area, biocompatibility, and superior optical characteristics, alongside its promising potential for employment in biosensing applications. MXenes exhibit versatility in their forms, ranging from quantum dots to composite structures and nanosheets, achieved through various synthesis methods and appropriate etching techniques. Over the past few years, MXene has garnered attention for its application in wearable sensors, electrochemical sensors, and optical biosensors.

MXenes have rapidly emerged as a prominent category within the realm of 2D materials, owing to their exceptional mechanical, electronic, and electrical properties. The interplay between the structural and electronic characteristics of MXenes is intimately tied to their optical behavior. Specifically, transitions between electronic bands can significantly enhance the optical response at specific wavelengths. Investigating optical properties involves a thorough examination of factors such as the lattice structure of MXenes, surface modifications, and elemental compositions. Various research studies are published to explore the physicochemical properties (optical properties, including reflectivity, transmittance, plasmon resonance, and absorption) of the MXenes and their hybrid composite materials. The electronic band structure, particularly the energy bandgap, plays a pivotal role in shaping these optical characteristics. For instance, MXenes with higher electronegative terminations, such as –O, tend to exhibit wider bandgaps compared to terminations like –F and –OH [48]. Thus, it is expected that MXenes with wide band gaps can be used for photocatalytic applications under UV light irradiation. In contrast, MXenes with narrow band gaps can be utilized as visible light photocatalysts for water treatment and photocatalytic hydrogen evolution reactions.

Theoretical approaches, notably density functional theory (DFT), have been instrumental in predicting MXenes’ optical response. Initial DFT investigations focused on determining the band structure and density of states for various MXene compositions, such as Ti₂N, Ti₃C₂, TiC₂, and Ti₃N₂ [131]. These studies revealed favorable energy bandgaps within the visible light range, suggesting potential applications in photocatalysis and light-driven processes [132]. Experimental findings complement theoretical predictions, demonstrating MXenes’ remarkable optical properties. For example, delaminated Ti₃C₂T_x films and Ti₃C₂T_x intercalated with NH₄HF₂ structures exhibit high transmittance levels, reaching up to 77% in the visible region and even 90% extending into the infrared spectrum [42,133]. Moreover, the optical response of MXenes varies with the number of layers, and optimization strategies, including thickness variation and ion intercalation, have been explored to optimize transmittance [28,133]. Studies have also highlighted the reversible changes in transmittance observed during electrochemical intercalation processes, particularly within the UV–visible range. Additionally, variations in flake size impact the optical absorption spectra of Ti₃C₂T_x, further emphasizing the nuanced relationship between MXene structure and optical behavior [134]. The reduction in size of MXene flakes correlates with a decrease in absorbance values at 237 nm, whereas larger flakes exhibit higher absorbance at 375 nm. This suggests a decrease in absorption within the visible range as flake size diminishes. However, despite this optical trend, smaller flakes demonstrate superior electrochemical performance with higher capacitance, likely due to enhanced electrolyte accessibility to active sites, thus conflicting with optical properties [135]. Surface functionalization, such as with –F, –O, and –OH groups, could passivate MXene layers, offering a means to control both electronic and optical properties [136]. Computational studies utilizing DFT have been employed to observe the effects of surface termination groups (–F, –O, and –OH) on the optical properties of Ti₃C₂T₂ [137]. In the visible range, O-functionalized MXene exhibits higher absorption and reflectivity compared to unfunctionalized MXene, while –F- and –OH-functionalized MXene display lower absorption. Conversely, in the UV range, all –F-, –O-, and –OH-functionalized MXenes demonstrate enhanced absorption [137]. Similarly, in Ti₃C₂T₂ and Ti₂CT₂, –O termination shows superior light absorption in the UV range compared to –F and –OH terminations, possibly attributed to the formation of states near the Fermi energy level [137,138]. Optical sensors have garnered significant attention due to their diverse applications ranging from food processing control, drug discovery, environmental monitoring, therapeutics, and clinical diagnostics. Transduction elements in sensing systems leverage phenomena such as light scattering, fluorescence/phosphorescence, light absorption, Raman scattering, and surface plasmon resonance (SPR). With the burgeoning growth of nanotechnology, 2D materials like graphene and MXenes have witnessed remarkable utilization in sensing applications owing to their unique properties including colorimetry, photoluminescence, electrochemiluminescence (ECL), surface-enhanced Raman scattering (SERS), and SPR. Various forms of MXenes, including quantum dots and nanosheets, are elucidated herein alongside their diverse applications [138].

4.1. Colorimetric Sensors

Notably, colorimetric sensors, renowned for their simplicity, cost-effectiveness, and high selectivity/sensitivity, have capitalized on MXene nanocomposites for sensor fabrication. Based on the visualization of a nano-plasmonic strategy, Wang et al. developed Ti₃C₂ MXenes for label-free and visual sensing of silver ions, exploiting the reductive and adsorptive properties of silver towards MXene to form plasmonic silver nanoparticles conducive to silver sensing [139].

In another study, Li et al. [140] proposed the sensing properties of the Ti₃C₂/CuS nanocomposites for the determination of cholesterol. An SEM image of the synthesized Ti₃C₂/CuS nanocomposites is shown in Figure 14a. It can be seen that the Ti₃C₂ MXene is attached to the CuS. The synthetic process and probable oxidation of cholesterol is displayed in Figure 14b. Ti₃C₂/CuS nanocomposites have demonstrated peroxidase-like activity for cholesterol detection via a colorimetric method, achieving a LOD of 1.9 μM [140].

The colorimetric change from colorless to blue, catalyzed by the nanocomposite in the presence of hydrogen peroxide and 3,3,5,5-tetramethylbenzidine, correlates with cholesterol concentration in the solution. The Ti₃C₂/CuS nanocomposites are also highly selective towards the detection of cholesterol, as shown in Figure 14c. Furthermore, a 2D heterojunction hybrid combining a Ni-Fe-layered double hydroxide and Ti₃C₂T_x MXene nanosheets has been designed for colorimetric sensing of glutathione in human serum [141]. This hybrid enhances electron transfer rates and offers increased surface area for ion interaction, thereby exhibiting heightened peroxidase-like catalytic activity.

Additionally, Prussian blue-functionalized Ti₃C₂T_x composites have been utilized for colorimetric sensing of pesticides and H₂O₂ [142]. The immobilization of acetylcholinesterase on Prussian blue-functionalized Ti₃C₂T_x-modified electrodes enables the electrochemical sensing of malathion. Similarly, thrombin detection in blood samples has been achieved based on the peroxidase mimicking the nanozyme activity of Ti₃C₂T_x MXenes, wherein adsorption of ssDNA on Ti₃C₂ nanosheets enhances the catalytic activity for sensing, while the presence of thrombin leads to aptamer detection on the nanosheet surface, accompanied by a decrease in catalytic activity [143].

4.2. Surface Plasmon Resonance (SPR) Sensors

Surface plasmon resonance (SPR) technology has revolutionized sensing approaches in various fields such as food safety, energy storage, optics, and medical diagnostics [144]. Due to their rapid response and high sensitivity, MXene-based SPR sensors have garnered significant attention from researchers. Xu et al. proposed a theoretical design for an MXene-based SPR sensor, combining transition metal dichalcogenides with the Ti₃C₂ MXene [145]. They achieved higher refractive index sensitivity in aqueous solutions using a five-layer WS₂ and monolayer Ti₃C₂T_x MXene setup excited at a wavelength of 633 nm. This setup demonstrated enhanced sensitivity compared to conventional bare Au-based SPR sensors. Moreover, Srivastava et al. [146] demonstrated the theoretical possibility of designing SPR sensors by combining transition metal dichalcogenides, Ti₃C₂ MXene, black phosphorus, and Au. The authors recorded the highest sensitivity using a single layer of the Ti₃C₂T_x MXene nanomaterial, with a penetration depth 1.52 times higher than conventional Au sensors.

In the development of fiber optic-based sensors, graphene-like materials have been used instead of transition metal dichalcogenides. An SPR sensor based on Au/graphene/Ti₃C₂ was developed for volatile organic compound detection [147]. The combination of metals and varying numbers of MXene and graphene layers has proven effective in sensor development, with the refractive index of the sensor varying according to solvent variation. Additionally, an SPR biosensor employing black phosphorus and Ti₃C₂T_x layers has been developed for gas and biosensing applications [148]. The thickness and atomic layer numbers of MXene are crucial as they interact with specific biomolecules, providing a platform for SPR-based biosensing. Wu et al. [149] synthesized Au-decorated Ti₃C₂ MXene nanosheets and its formation was confirmed by the XRD technique. The authors found that the Ti₃C₂ MXene and Ti₃C₂ MXene/AuNPs were successfully formed, as shown in Figure 15a.

Furthermore, an ultrasensitive SPR biosensor for the detection of carcinoembryonic antigen (CEA) was developed as shown in Figure 15b. The SPR spectra of CEA in the presence of different concentrations (2 × 10⁻¹⁶–2 × 10⁻⁸ M) is presented in Figure 15c. It is clearly shown that the proposed sensor has good sensing activity with a decent linear range. The proposed biosensor exhibited an LOD of 0.07 fM. In another approach, CEA detection based on an SPR sensor was achieved by immobilizing amine-functionalized Ti₃C₂-AuNPs-SPA with polyclonal anti-CEA antibodies, achieving a LOD of 0.15 fM [150]. This new approach for detecting biomolecules in complex mixtures has demonstrated good selectivity, as evidenced by the high recoveries of spiked blank serum samples.

4.3. Surface-Enhanced Raman Scattering

Raman spectroscopy has emerged as a powerful analytical tool for the easy identification and interpretation of trace amounts of chemicals due to their unique vibrational characteristics. When light of a specific wavelength interacts with the surface of materials, it produces oscillations, generating enhanced signals known as surface-enhanced Raman scattering (SERS) signals [151]. MXenes provide a flexible environment as SERS substrates, owing to their hydrophilic nature, optical properties, and large surface area, making them promising for SERS with excellent stability and selectivity. Several MXene-based SERS approaches have been reported for sensing pollutants such as pesticides and dyes. For instance, a hybrid composite of the Ti₃C₂T_x MXene with silver, gold, and platinum nanoparticles has been used in SERS-based sensors for the detection of methylene blue [152]. Furthermore, the Ti₃C₂T_x MXene has been individually explored as a SERS substrate for the effective sensing of organic dyes like Rhodamine 6G [153]. SERS signals in MXenes are observed due to inter-band transitions of vacant energy states, followed by charge transfer. Similarly, SERS signals of two-dimensional transition metal nitrides (Ti₂N) have been verified by fabricating them on silicon, paper, and glass-based SERS substrates. A paper-based Ti₂N MXene-based SERS substrate has been utilized for the detection of trace-level explosives [154]. In another approach, Au nanorods immobilized on the Ti₃C₂T_x MXene were synthesized for the detection of organic pollutants such as Rhodamine 6G, malachite green, and crystal violet on SERS-based sensors. The Au nanorods distribute uniformly on MXene via strong electrostatic interactions, exhibiting high sensitivity and good reproducibility. The synergic connection between the charge transfer of MXene and resonance Raman scattering enables the detection of single molecules of pollutants [155].

Zheng et al. [156] synthesized a SERS aptasensor based on an internal standard approach for the sensitive detection of Ochratoxin A. Au–Ag Janus nanoparticles accumulate with MXene via H-bonding, and chelation interactions occur between MXene and OTA aptamers. In the presence of OTA, the Au–Ag Janus nanoparticles separate from MXene due to the formation of the OTA/aptamers complex, leading to Raman signal generation. Medetalibeyoglu et al. [157] developed an ultra-sensitive carcinoembryonic antigen sensor using Fe₃O₄@Au nanoparticles functionalized with MXene as a SERS substrate.

Figure 16a,b show the SEM picture of the d-Ti₃C₂T_X MXene and Fe₃O₄ NPs@Au NPs/d-Ti₃C₂T_X MXene, respectively. It is clearly seen that the Fe₃O₄ NPs@Au NPs are strongly attached to the surface of the d-Ti₃C₂T_X MXene. Furthermore, the authors investigated the sensing activities of the proposed material and found that the fabricated sensor is selective towards the determination of CEA, as shown in Figure 16c. The interesting LOD of 0.033 pg mL⁻¹ was reported for the above proposed sensor. It is clear that the Fe₃O₄ NPs@Au NPs/d-Ti₃C₂T_X MXene has a selective nature for the sensing of CEA with decent LOD.

Table 3. Utilization of MXenes as optical sensors for the detection of various biomolecules and hazardous materials.

Material	Type of Sensor	Detection	Reference
Ti3C2 MXenes	Colorimetric	Ag+	[139]
Ti3C2/CuS nanocomposites	Colorimetric	Cholesterol	[140]
Ti3C2Tx MXene	Colorimetric	Glutathione	[141]
Ti3C2Tx composites	Colorimetric	Pesticides and H2O2	[142]
Ti3C2Tx	Colorimetric	Thrombin	[143]
Au/graphene/Ti3C2	SPR Sensor	Organic compound	[147]
Black phosphorus and Ti3C2Tx	SPR Sensor	Gas and biosensing	[148]
Ti3C2-AuNPs-SPA	SPR Sensor	CEA	[149]
Ti3C2Tx hybrid composite	SERS	Methylene blue	[152]
Ti3C2Tx	SERS	Rhodamine 6G	[153]
Ti2N MXene	SERS	Explosives	[154]
Au nanorod-immobilized Ti3C2Tx	SERS	Rhodamine 6G, malachite green, and crystal violet	[155]
Fe3O4@Au nanoparticle-functionalized MXene	SERS	Carcinoembryonic antigen	[157]

summarizes the utilization of MXene and its composite materials in various optical sensing applications.

5. Conclusions and Future Perspectives

In conclusion, this review article has summarized the recent advancements in the fabrication of gas sensors and optical sensors using MXenes. MXenes exhibit excellent properties that make them promising candidates for fabricating gas sensors and biosensors. Numerous gas sensors have been demonstrating outstanding performance in terms of detection limit. MXenes are perfect for the selective and precise detection of different gases at room temperature due to their high surface area, excellent conductivity, and adaptable surface chemistry. Moreover, composites of MXenes significantly enhance the specific surface area, expand interlayer spacing, and introduce new active sites for gas adsorption. Therefore, modifying MXenes with different materials can improve gas sensing performance, including selectivity, gas response and recovery times, as well as detection limit. This highlights the synergetic effects of the MXene and its composites. These properties underscore MXenes’ desirability in the development of high-performance sensing devices. Similarly, various studies have highlighted the potential of MXenes in the fabrication of optical sensors. While MXenes and their hybrid materials have been utilized in gas and optical sensors construction, their application in detecting toxic gases through gas sensing technology is noteworthy. MXene-based sensors and their composite derivatives are currently in the early stages of development. Although approximately 20 types of MXenes have been synthesized, only a few, particularly Ti₃C₂T_x, have been extensively explored for sensing applications. However, there are some drawbacks that need to be addressed to expand their gas sensing capabilities. These include the low hydrophilicity in multilayer MXenes, high metallic conductivity, low flexibility, and challenges with decomposition and oxidation stability. Additionally, they also exhibit slow response and recovery times, attributed to sluggish kinetics at low temperatures. To overcome these challenges, significant efforts are required to enhance the sensing performance. This can be achieved through optimizing flake size and morphology, choosing better surface termination, and exploring the fabrication of hybrid materials. Moreover, to expand the application of MXenes in various real-time applications, reducing production costs and focusing on computational predictions, along with generating experimental data, are crucial steps for future studies. Research should advance beyond individual evaluations of MXene sensors’ selectivity and sensitivity for particular gases or VOCs. There should be shifted towards more focused applications such as human health monitoring, non-invasive disease diagnosis, and the detection of hazardous and toxic gases in real-world environments.