Engineering MXenes for electrochemical environmental pollutant sensing

Abstract

Environmental pollutant sensing is essential to the sustainable development of human health and ecosystems. As a category of two-dimensional materials consisting of nitrides and carbides, MXenes have emerged as highly attractive candidates for electrochemical sensing of environmental pollutants, including toxic gases, harmful volatile organic compounds, and biologically relevant components, owing to their strong metallic conductivity, easy customization, abundant surface functional groups and large interlayer spacings. This comprehensive review firstly assesses the environmental pollutant sensing mechanism and modular MXene electrode fabrication methods. Subsequently, the research progress on MXenes is summarized by comparing their performances in environmental pollutant detection. Furthermore, ways to improve the electrochemical stability and selectivity of MXenes using different techniques are further discussed. Finally, challenges faced in this field and prospective directions for future research are suggested by integrating emerging technologies and interdisciplinary approaches. The key objective of this review is to motivate engineers and materials scientists to consider incorporating MXenes into technologies for environmental protection, thereby providing inventive solutions to urgent global issues.

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Environmental significance

Growing environmental pollutants have harmful impacts on human health, necessitating the development of advanced electrochemical sensing technology for the real-time monitoring of pollutants. This is essential to the sustainable development of human society and ecosystems.

1. Introduction

Environmental pollutants pose a multitude of dangers that threaten human health. Air pollution, characterized by emissions of particulate matter such as sulfur dioxide and nitrogen oxides, is a major concern, contributing to respiratory diseases and climate change.^1,2 Water pollution, stemming from contaminants such as heavy metal ions (HMIs), pathogens and toxic substances, has harmful effects on aquatic ecosystems and human health.³ Soil pollution, caused by pesticides and heavy metals, can adversely affect crop growth, break the food chain and further harm biodiversity.^4,5 To address these challenges and effectively manage environmental pollution, it is imperative to recognize the dangers of environmental pollution and implement innovative technologies, especially developing advanced sensing techniques for the real-time monitoring of these pollutants.⁶

Several traditional approaches, including spectroscopy and chromatography, are available for the determination of environmental toxicants. For instance, inorganic cations and anions can be quantitatively determined using approaches such as hydride generation-atomic absorption spectrometry (HG-AAS),⁷ electrothermal atomic absorption spectrometry (ET-AAS),⁸ inductively coupled plasma-optical emission spectrometry (ICP-OES),⁹ surface-enhanced Raman scattering (SERS),¹⁰ inductively coupled plasma-mass spectrometry (ICP-MS),¹¹ hydride generation-atomic fluorescence spectrometry (HG-AFS),¹² spectrophotometry,¹³ and X-ray fluorescence (XRF).¹⁴ Furthermore, organic compounds are evaluated using gas chromatography (GC),¹⁵ gas chromatography-mass spectrometry (GC-MS),¹⁶ liquid chromatography-mass spectrometry (LC-MS),¹⁷ capillary electrophoresis,¹⁸ high-performance liquid chromatography (HPLC),¹⁹ and immunoassay techniques.²⁰ However, these methods have a number of shortcomings such as lengthy sample preparation processes, large instrument size, and specialized staff requirements, which restrict their broader on-site environmental applications.²¹ In addition, some other sensing methods have been developed. Table 1 summarizes the key performance metrics, including sensitivity, selectivity, response time, stability, and fabrication complexity, using fluorescence,^22,23 colorimetric,^24,25 chemiluminescence,²⁶ piezoresistive,²⁷ surface plasmon resonance (SPR)²⁸ and surface-enhanced Raman scattering (SERS)²⁹ sensors. These sensing techniques face challenges with sensitivity owing to their reliance on indirect optical or mechanical signal transduction, often requiring probes to amplify signals.³⁰ Reproducibility is also problematic, especially for methods relying on surface enhancements because of difficulties in precise fabrication and maintaining sensor performance over long-term deployments. Among these sensing techniques, electrochemical sensors are a significant subgroup of chemical sensors with necessary attributes to effectively meet the demands for on-site environmental monitoring in terms of fast response, high sensitivity, good portability, and low cost.^31–33 These advancements endow electrochemical sensors with the following functions: identification of organic pollutants in groundwater,³⁴ quantification of trace metals in natural water bodies,^35,36 surveillance of carcinogens in food,³⁷ and environmental samples.^38,39

Table 1 Advantages and limitations of MXene-based sensors

Sensing type	Advantages	Limitations	Applications
Electrochemical	High sensitivity	Moderate selectivity	Detection of heavy metals (e.g., Pb, Cd, Hg) in water
	Rapid response	Stability issues	Monitoring of organic pollutants (e.g., pesticides, phenols) in soil and air
	Portable setup	Fouling effects	Environmental monitoring of water quality and contamination
	Low cost		Food safety
	Miniaturized design
Fluorescence	High selectivity for organic compounds	Lower sensitivity	Detection of organic pollutants (e.g., PAHs, PCBs) in water
	Multi-analyte ability	Instrumentation needs	Monitoring of biomarkers (e.g., proteins, DNA) in biological samples
	Broad linear ranges	Require labeling	Water quality assessment in environmental monitoring
		Susceptible to interference from background fluorescence
Colorimetric	High sensitivity for heavy metals	Moderate sensitivity	Detection of heavy metals (e.g., Cu, Zn, Fe) in water screening of water pollutants (e.g., nitrate, phosphate) in environmental samples
	Simple instrumentation	Limited quantitative analysis	On-site testing in field applications
	Rapid detection	Subject to color interference from sample matrix
	Low-cost
	Short color development time
(Electro)chemiluminescence	High sensitivity for reactive species	Complexity in instrumentation and operation	Detection of reactive species (e.g., hydrogen peroxide, ozone) in water
	No external power	Limited stability of chemiluminescent reagents	Monitoring of organic pollutants (e.g., pesticides, VOCs) in air
	Low detection limit		Biomedical diagnostics for disease biomarkers
	Real-time monitoring
Piezoresistive	High sensitivity for gas pollutants	Complex fabrication	Detection of gas pollutants (e.g., NO2, CO, SO2) in air
	Multi-analyte quantification	Small analytes needed	Monitoring of volatile organic compounds (e.g., benzene, toluene) in industrial settings
	Label-free	Susceptible to mechanical drift	Industrial process control and leak detection
	Real-time monitoring	Limited to specific analytes
	Compact and portable devices	Requires calibration
Surface plasmon resonance (SPR)	High selectivity for biomolecules	Expensive equipment	Detection of biomolecules (e.g., proteins, DNA) in biological samples
	Label free detection	Bulk optics configurations	Environmental monitoring of water contaminants (e.g., pesticides, pharmaceuticals)
	Real-time monitoring	Specialized fabrication	Biosensing applications in medical diagnostics
		Limited multiplexing
		Limited to certain sample types
		Sensitivity to sample matrix effects
Surface-enhanced Raman scattering (SERS)	High sensitivity and specificity for trace contaminants	Limited reproducibility complexity in data analysis	Detection of trace contaminants (e.g., heavy metals, organic pollutants) in water
	Ultra-sensitive for single molecule		Analysis of complex samples (e.g., environmental samples, biological fluids)
	Molecular fingerprinting capability		Forensic applications for substance identification
	Non-destructiveness		Food safety

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Two-dimensional (2D) materials have been demonstrated to show good promising electrochemical sensing because of mass transfer, fast electron transfer kinetics, large surface area, and outstanding electrocatalytic performance.^40,41 MXenes, a distinctive group of 2D materials containing transition metal nitrides or carbides, have attracted widespread attention in environmental pollutant-related electrochemical sensing due to their exceptional metallic conductivity, abundant active catalytic sites, surface functionalization, surface area, hydrophilicity and biocompatibility.^42,43 With constituents such as Ti, C, or N, along with non-toxic breakdown products (CO₂ and N₂), MXenes have emerged as a focal point in environmental remediation research.⁴⁴ Over 30 MXene materials have been documented since 2011, including Ti₃N₃T_x,⁴⁵ Ti₃C₂T_x,^45,46 Nb₂CT_x,⁴⁶ V₂CT_x,⁴⁷ Zr₃C₂,⁴⁸ and Lu₂CT_x.⁴⁹ Moreover, some MXene materials predicted theoretically on the basis of MAX precursors,⁵⁰ where “M” symbolizes early transition metals, “A” denotes group IIIA or IVA elements, and “X” represents elements such as carbon and nitrogen, while “T_x” signifies surface termination groups such as –O, –F, and –OH, as illustrated in Fig. 1. Notably, MXenes such as Ti₃C₂T_x and Ti₂CT_x based on titanium have been harnessed for environmental purification purposes.⁵¹ Ti₃C₂T_x has widespread applications in various environmental sensing applications, including the electrochemical detection of bromate, phenol, nitrite, pharmaceuticals, and heavy metal ions.⁵²

Fig. 1 Formation of elements of MAX phases M_n+1AX_n.

Although some review papers have already introduced the MXene-based electrochemical sensors for pollutant sensing,^30,53 it is essential to summarize the characteristics of MXene-based electrochemical sensors in detecting pollutants from the sensing behavior of MXenes and electrode fabrication methods. In this review, we first introduce the electrochemical sensing principles of MXenes, sensing mechanism and modular-based MXene electrode fabrication. Next, we summarize the progress of MXene-based materials in relation to their use in electrochemical sensing including the identification of organic agents such as pesticides and pharmaceuticals, and inorganic chemical compounds such as heavy metals and nitrites. Further, we discuss the electrochemical stability of MXenes and their impact on sensing and molecular imprinting techniques to enhance the selectivity of these sensors. Finally, a comprehensive outlook on the present challenges and potential developments in the area of electrochemical sensing with MXenes is provided.

2. Electrochemical sensing principle and modular MXene electrode fabrication

2.1 Electrochemical sensing principle

Electrochemical methods such as voltammetry, amperometry, and impedance spectroscopy offer rapid and sensitive detection, suitable for field deployment and miniaturization. Electrochemical (EC) sensing systems typically include a transducer (usually an electrode), a capture probe and a reporter probe. The transducer converts chemical reactions into measurable electrical signals, influenced by factors such as electrode material and redox reaction kinetics. The working principle and methods of MXenes involving ECs are illustrated in Fig. 2. Redox-active species undergo oxidation or reduction at the electrode surface.⁵⁴

Fig. 2 Working principle of MXene-based electrochemical sensors (E_p = applied potential, I_p = current signal).

2.1.1 Sensing principle for organic molecules. MXene-based electrochemical sensors primarily use voltammetric techniques for sensing organic pollutants. Cyclic voltammetry provides insights into the redox behavior of analytes, as some organic molecules can undergo direct redox reactions at the MXene-modified working electrode surface. During the CV scans, changes in the resulting current response correspond to the analyte's intrinsic redox potential. Other techniques such as differential pulse voltammetry and square wave voltammetry apply distinct voltage pulses or waveforms. Taking phenol as example, reaction processes are given as follows:

MXene → MXene⁺ + e⁻

MXene⁺ + C₆H₅OH → MXene + C₆H₅O˙ + H⁺

MXene + C₆H₅O˙ + H⁺ + e⁻ → MXene + C₆H₅OH

MXene properties promote these redox reactions. Its high conductivity facilitates fast electron transfer kinetics. Abundant surface functional groups such as –OH chemically interact with and pre-concentrate the analytes. Large surface area and porosity maximize accessible reaction sites. The current signals correlate precisely to the concentration of nitrites enabling sensitive detection.

2.1.2 Sensing principle for heavy metal ions. Square wave anodic stripping voltammetry (SWASV) is an electroanalytical technique that offers high sensitivity, low detection limits, and real-time capabilities, making it highly appealing for the detection of low-concentration heavy metal ions.⁵⁵ The technique involves two processes: enrichment and dissolution. Metal ions are initially deposited onto the electrode surface through a negative electrodeposition potential. In the SWASV measurement mode, the deposited metal atoms undergo oxidation and dissolve back into metal ions at a specific potential.⁵⁶ Specific heavy metal ions consistently correspond to fixed peak positions under identical conditions, with variations in peak intensity. This intensity is entirely influenced by the properties of the modified electrode material. Therefore, SWASV underscores the significance of electrode materials possessing high conductivity, reproducibility, renewability, surface chemical inertness, a wide potential window, and low background current.⁵⁷ Taking Cd²⁺ and Pb²⁺ on NH₂–Ti₃C₂T_x as an example,⁵⁸ the sensing mechanism involve two aspects: deposition and anodic stripping. The electrochemical determination process consists of preliminary accumulation, electrochemical reduction, and anodic stripping.

(M²⁺)_solution + NH₂–Ti₃C₂T_x → M²⁺–NH₂–Ti₃C₂T_x

M²⁺–NH₂–Ti₃C₂T_x + 2e⁻ → M⁰ + NH₂–Ti₃C₂T_x

M⁰ + NH₂–Ti₃C₂T_x → (M²⁺)_solution + NH₂–Ti₃C₂T_x + 2e⁻

where M represents Cd²⁺ or Pb²⁺. They are then selectively electrodeposited onto the electrode surface and subsequently oxidized to generate oxidation current.

2.2 Modular MXene-based electrode fabrication

Developing large-scale and modular fabrication approach is essential to the integration of MXene-based electrodes in the electrochemical sensors.⁵⁹ MXene flakes with functionalized surfaces and good dispersity facilitate modular electrode fabrication via solution-based processes. Specifically, stable colloidal dispersions of MXenes enable the use of printing or spray coating methods for electrode fabrication. The common laboratory method for producing MXene films is vacuum-assisted filtration (VAF), but scalability is crucial to ensure consistency among batches and overcome limitations in production scale. The modified VAF method can be used for the scalable production of MXene electrodes. In this context, in 2021, Wang et al.⁶⁰ introduced a silt filtration-inspired method for swiftly preparing MXene films, by combining ion-induced MXene dispersion gelation with vacuum-assisted filtration, as depicted in Fig. 3a. This technique significantly reduces preparation time to seconds, making it the fastest method available. The key factors for rapid preparation include achieving proper gaps between MXene microgel particles and ensuring strong gelation and riveting effects within the microgel structure. The resulting MXene microgel film (MMF) exhibits exceptional mechanical properties, conductivity, and enhanced electrochemical properties compared to conventional MXene films. However, increased pressure notably intensified the restacking occurring between MXene flakes, and due to this, the approachability of electrolyte ions reduces and decreases the utilization rate of active sites, which reduces the performance of assembled MXene films.

Fig. 3 (a) Schematic of the preparation process of the modified MXene film (MMF) via a novel method combining ion-induced gelation of MXene dispersions with vacuum-assisted filtration. (b) Synthesis processes of the LMX, SMX, SUMOFs, and SUMOF/LMX composite inks.⁶¹ Reproduced from ref. 61 with permission from Wiley, Copyright 2023. (c) Schematic of the fabrication process of stretchable and bendable SUMOF/LMX composite films.⁶¹ Reproduced from ref. 61 with permission from Wiley, Copyright 2023. (d) SEM images of Ti₃C₂T_x MXene fibers. (e) Schematic of the wet-spinning process of the Ti₃C₂T_x MXene fiber.⁶²

Another method, known as “spin coating”, is widely employed for fabricating uniform MXene-based films. This method is faster and more efficient than VAF. Initially, the MXene solution and additives undergo intense stirring or ultrasonication to form a homogeneous solution. Subsequently, the homogeneous solution is spread onto a clean substrate under centrifugal force at elevated temperatures or vacuum to allow solvent evaporation. Montazeri et al.⁶³ developed MXene transparent contacts using a spin-coating method and applied them to Ti₃C₂-based MXene photodetectors. The transparent MXene film they produced reduced the trade-off between carrier transmission distance and responsiveness, leading to a four-fold increase in photodetector sensitivity compared to gold-based devices.

Another technique named “blade coating” is a scalable technique used in industrial settings, which offers a solution for producing free-standing films and coatings. In 2020, Zhang et al.⁶⁴ demonstrated a method to produce highly robust and electrically conductive Ti₃C₂T_x MXene films using large flakes and a scalable blade coating process. They synthesized large MXene flakes by selecting appropriate MAX phase and optimizing etching conditions. The liquid crystalline MXenes and suitable dispersion properties allowed for aligned flakes during coating, resulting in films with exceptional tensile strength (∼570 MPa), Young's modulus (∼20.6 GPa), and electrical conductivity (∼15 100 S cm⁻¹ after annealing). In another study, In 2023, Jiang et al.⁶¹ used a scalable blade coating method to fabricate stretchable and bendable small-sized ultrathin/large-sized Ti₃C₂T_x MXene (SUMOF/LMX) composite films, as shown in Fig. 3(b and c). These films feature LMX sheets as a flexible scaffold, enabling the assembly of SUMOFs into an orderly layer structure on an elastic substrate. The resulting composite film demonstrates improved tensile strength (≈ 97 MPa). This study highlights the development of MOF-based films with excellent mechanical, electrical, and energy storage properties, offering promising prospects for commercial applications in wearable electronics.

Furthermore, Kim et al.⁶⁵ introduced a slot-die coating method for large-scale fabrication of Ti₃C₂T_x layers, enabling precise control over coating thickness from nanometers to micrometers. The Ti₃C₂T_x membranes demonstrated outstanding nanofiltration performance, with a water permeance of 190 LMH bar⁻¹ and a molecular weight cutoff of 269 Da, surpassing previous MXene membranes and comparable to other 2D materials. These membranes also exhibited enhanced stability under harsh oxidizing aqueous conditions, maintaining stable performance over 30 days of cross-flow filtration, the longest among 2D material-based membranes. The slot-die coating's continuous, fast, scalable, and precise nature makes industrial-scale production of MXene films feasible in sensors.

In another study in 2020, Lipton et al.⁶⁶ developed a method to create Ti₃C₂T_x MXene freestanding films by drop-casting onto hydrophobic plastic substrates. These films, due to the substrate's hydrophobic nature, can be easily separated, showcasing preferential MXene–MXene interactions over MXene–substrate interactions. The drop-casting (DC) technique allows for the production of large-area (>125 cm²), thick (23.2 mm) films with smooth surfaces (14 nm RMS roughness) and high electrical conductivity (∼7000 S cm⁻¹). By employing a DC technique on common plastic substrates, the study improved Ti₃C₂T_x flake alignment through natural forces during film deposition. Moreover in 2020, Eom et al.⁶² developed a large-scale method for creating pure MXene fibers without additives or binders. They utilized wet-spinning assembly, where MXene dispersion was extruded through a nozzle into a coagulant solution consisting of NH₄Cl (50 g), NH₄OH solution (20 mL), and deionized water (DIW) (1000 mL). The MXene dispersions were extruded at a velocity of 7 mL h⁻¹. By introducing ammonium ions during coagulation, they produced flexible, meter-long MXene fibers with high electrical conductivity (7713 S cm⁻¹). The SEM image of the MXene fiber is presented in Fig. 3d, while the wet-spinning process of the Ti₃C₂T_x MXene fiber is shown in Fig. 3e. These methods are usually only effective for preparing thicker films. Concentrated MXene solutions are required for the coating process because a certain level of viscosity is necessary to maintain the shape of the applied solution on the target substrate. Moreover, any residual coating solution may lead to contamination and damage to the film. Additionally, these techniques inherently involve significant wastage, which can be critical when implemented on an industrial scale. However, despite these numerous efforts, there is still room for improvement to meet commercial standards.

3. MXene-based electrochemical sensors for organic pollutants, pesticides and pharmaceuticals

MXenes exhibit a notable capacity for detecting an extensive array of environmental organic pollutants, especially in water phenolic compounds, introduced into ecosystems through agricultural, industrial, sewage, and pharmaceutical sources, pose substantial health risks owing to their toxic and mutagenic, carcinogenic, and teratogenic properties.⁶⁷

3.1 MXene-based electrochemical sensors for organic pollutants

3.1.1 Ti-based MXenes for organic pollutant detection. In this concern, in 2021 Rasheed et al.⁵² introduced an innovative Pt@Ti₃C₂T_x nanocomposite sensor for detecting the endocrine disruptor bisphenol A (BPA). The Ti₃C₂T_x MXene was selected as the electrode material owing to its large surface area and conductivity. However, pristine Ti₃C₂T_x suffers from instability in the anodic potential range required for BPA oxidation. To overcome this, the authors deposited platinum nanoparticles (PtNPs) on the Ti₃C₂T_x surface via a self-reduction process, as shown in Fig. 4a. This generated an intricately engineered Pt@Ti₃C₂T_x nanocomposite interface. In this nanohybrid, Ti₃C₂T_x serves as a reducing agent and a conductive matrix, whereas Pt nanoparticles enhance stability and electrochemical activity. Fig. 4b presents the TEM image of 10%Pt@Ti₃C₂T_x. Impressively, the sensor demonstrated high sensitivity with a detection limit of 32 nM and a linear range of 50 nM to 5 μM, it also shows good selectivity but it should be further improved. Contrary to the study of Rasheed et al., in another study by Vilian et al.,⁶⁸ a pioneering electrochemical sensor was proposed for the sensitive and selective detection of toxic bisphenol A (BPA). They anchored palladium nanoparticles (PdNPs) onto molybdenum disulfide (MoS₂)-modified titanium carbide via a facile one-step hydrothermal technique. MXenes were selected as scaffold materials due their layered structure, high surface area and conductivity. MoS₂ was integrated to impart additional active sites for electrochemical reactions. PdNPs act as efficient catalysts to facilitate electron transfer dynamics. Screen-printed carbon electrodes (SPCEs) were chosen as the sensing platform owing to their design flexibility and amenability for field applications. The electrodes were tailored by drop casting the synthesized PdNPs–MoS₂–MXene nanocomposite. Interestingly, this PdNPs–MoS₂–MXene/SPCE configuration demonstrated greatly enhanced analytical performance compared to earlier reports. It achieved an impressively low detection limit of 0.12 nM, much below regulatory thresholds. More significantly, an extensive linear range from 5 to 175 nM ensured accurate measurements. While the matrix interference requires reduction, notable selectivity was evident even at elevated concentrations. The intrinsic properties of MoS₂ such as stability, conductivity and band structure make it an ideal candidate. However, noble metal nanoparticles help overcome limitations such as poor active sites and conductivity. Their synergism within the optimized architecture helps improve the efficiency of BPA detection. In our opinion this composite material shows good sensing performance. Therefore, employing these promising nanohybrids in sensing applications would yield outstanding analytical capabilities.

Fig. 4 (a) Schematic representation of the sensing mechanism relying on BPA oxidation at the Pt@Ti₃C₂T_x-modified electrode.⁵² Reproduced from ref. 52 with permission from Elsevier, Copyright 2021. (b) TEM image of 10%Pt@Ti₃C₂T_x.⁵² Reproduced from ref. 52 with permission from Elsevier, Copyright 2021. (c) Schematic of the application of alk-Ti₃C₂/N-PC/GCE.⁶⁹ Reproduced from ref. 69 with permission from Elsevier, Copyright 2020. (d) SEM image of alk-Ti₃C₂/N-PC. Reproduced from ref. 69 with permission from Elsevier, Copyright 2020. (e) Schematic of the preparation and application of Nb₂CT_x/Zn–Co-NC/GCE. Reproduced from ref. 70 with permission from Elsevier, Copyright 2021. (f) SEM image of the Nb₂CT_x/Zn–Co-NC composite.⁷⁰ Reproduced from ref. 70 with permission from Elsevier, Copyright 2021. (g) Sensing mechanism of hydrazine. (h) SEM image of 10%Er-Nb₂C. Reproduced from ref. 71 with permission from Elsevier, Copyright 2021. (i) CV curve in the absence and presence of 13.76 mM of hydrazine.⁷¹ Reproduced from ref. 71 with permission from Elsevier, Copyright 2021.

Hydroquinone (HQ) and catechol (CT) are toxic phenolic compounds, which usually co-exist in samples and it is a challenge to detect because both have a nearly similar structure. In 2020, Huang et al.⁶⁹ developed an electrochemical sensor using alkalized-Ti₃C₂/nitrogen-doped porous carbon (N-PC) to detect HQ and CT. The alkalized-Ti₃C₂ exhibited an expanded interlayer spacing and the capability to capture HQ and CT due to hydrogen bond interactions between the OH groups on its surface and those in HQ and CT. Additionally, the N-PC derived from MOF-5-NH₂ featured a unique C–N bond, which attracted the OH groups of HQ and CT. Thus, integrating N-PC as an intercalator into the Ti₃C₂ sheets had a dual effect on sensing HQ and CT, enhancing the sensor's performance, as shown in Fig. 4c, while Fig. 4d presents the SEM image of alk-Ti₃C₂/N-PC. The XPS results indicate the presence of pyridinic-N, pyrrolic-N, and graphitic-N, with pyrrolic-N suggesting an abundance of adsorption sites for phenolic pollutants. This sensor leveraged the excellent electrical conductivity of alk-Ti₃C₂ and the large surface area of N-PC to detect HQ and CT based on their benzenediol/benzoquinone redox reactions. The sensor showed a wide linear range of 0.5–150 μM with low detection limits of 4.8 nM for HQ and 3.1 nM for CT. Furthermore, the sensor showed excellent selectivity, and the relative standard deviation (RSD) was less than ±6.6%.

Hydrazine is a crucial raw material, yet it is also a significant environmental pollutant and a potent carcinogen. Due to its excessive use and mishandling, substantial quantities of hydrazine are released into the environment each year. Consequently, developing a reliable and sensitive method for the detection of hydrazine is of utmost importance. In this regard, Yao et al.⁷² used a Ti₃C₂T_x MXene material as a conductive platform and combined it with ZIF-8, known for its unique electrocatalytic activity, to create a MXene/ZIF-8 nanocomposite. This composite was applied to a glassy carbon electrode (GCE) to develop a sensor for electrochemical hydrazine detection. The synergistic effect of MXene's high electrical conductivity and ZIF-8's electrocatalytic properties significantly enhanced the sensor's electrochemical performance for hydrazine detection, suggesting that the MXene/ZIF-8 composite has great potential for electrochemical sensing applications. The current response increased linearly with hydrazine additions across a wide range from 10 μmol L⁻¹ to 7.7 mmol L⁻¹. Beyond 7.7 mmol L⁻¹, the response deviated from linearity, probably due to the saturation of the electroactive sites on the modified electrode. The limit of detection (LOD) was calculated to be 5.1 μmol L⁻¹.

3.1.2 Nb-based MXenes for organic pollutant detection. Nb-MXene is a new member of MXene family and has great potential in sensing applications. In this regard, Huang et al.⁷⁰ proposed a simple self-assembled method to create a novel heterostructure (MXene/ZIF) composed of Nb₂CT_x and Zn–Co-ZIF-derived bimetallic Zn,Co-embedded N-doped carbon (Zn–Co-NC) nanocages. The Nb₂CT_x/Zn–Co-NC composite-modified electrodes were used for the electrochemical detection of 4-nitrophenol (4-NP), as depicted in Fig. 4e. Although Nb₂CT_x exhibits excellent conductivity and a significant specific surface area, its utility in electrochemistry is restrained by restacking, impeding electron transfer. To mitigate restacking, Nb₂CT_x was adorned with Zn–Co-NC (a bimetallic Zn–Co embedded N-doped carbon), which serves as an interpolator. Furthermore, as MXenes were not entirely enveloped by the Zn–Co-NC nanocage, numerous exposed active surfaces remained available for nitrophenol electrocatalysis, resulting in a high-performance electrochemical sensing platform. Fig. 4f presents the SEM image of the Nb₂CT_x/Zn–Co-NC composite. The sensor exhibited a wide linear range from 1 μM to 500 μM, a low detection limit of 0.070 μM, and a high sensitivity of 4.65 μA μM⁻¹ cm⁻². Additionally, the Nb₂CT_x/Zn–Co-NC sensor demonstrated outstanding selectivity and stability, making Nb₂CT_x promising materials for various electrochemical sensors detecting other toxic organic pollutants. In this context, Gul et al.⁷¹ Synthesized 2D Nb₂C MXenes via wet-chemical etching, as a direct electrode material for electrochemical hydrazine sensing, as depicted in Fig. 4g. Additionally, an erbium (Er)-doped Nb₂C MXene was prepared via a hydrothermal process with weight percentages of 2.5%, 5%, 7.5%, and 10%. The SEM image of 10%Er-Nb₂C is presented in Fig. 4h. Nb₂C provides a large surface area, surface termination groups such as –OH, –O, and –F interact with hydrazine via electrostatic interactions, and Er provides more active sites which result in a high-performance sensing platform for hydrazine. The CV curve in the absence and presence of hydrazine is shown in Fig. 4i. The electrochemical tests using cyclic voltammetry and linear sweep voltammetry showed a much higher sensitivity (276 μA mM⁻¹ cm⁻²) than that of undoped Nb₂C (169 μA mM⁻¹ cm⁻²). The low detection limit of 67 μM was obtained from Er-Nb₂C and 10.08 mM from undoped Nb₂C, and these findings highlight the potential of Er-doped Nb₂C MXene materials for hydrazine sensing applications.

From the above discussion, we conclude that both Ti-based and Nb-based MXenes exhibit high sensitivity and selectivity in detecting various organic pollutants. Ti-based sensors, particularly those enhanced with noble metal nanoparticles such as Pt and Pd, show a superior catalytic activity and stability, leading to lower detection limits and broader linear ranges. Furthermore, Nb-based MXenes, especially when doped with elements such as Er, demonstrate significant improvements in sensitivity and a wide linear detection range. More work is required on other metal MXenes, beyond Ti, to enhance the sensitivity and stability of MXene-based sensors, similar to the focus initially given to Ti-based MXenes. In our opinion, noble metals show good sensing performances with Ti-based MXenes, hence it is necessary to try these metals with other MXenes and also try non-noble metals with MXenes to make them cost-efficient. Overall, MXenes represent a promising platform for developing affordable, high-performance electroanalytical techniques needed for organic pollutant detection. The performances of different MXene-based electrochemical sensors for sensing various organic pollutants are summarized in Table 2.

Table 2 MXene-based electrochemical sensors for sensing environmental pollutants

Sensing material	Analyte	Detection method	LOD	Linear range	Ref.
Gr/Ti3C2Tx	BPA	DPV	0.35 μM	1–10 μM	73
		Amperometry	4.08 nM	10–180 nM
Ti3C2-MWCNTs	HQ	DPV	0.0066 μM	2–150 μM	74
	CT		0.0039 μM
Ti3C2F/MnMoO4	CT	DPV	0.0003 μM	5–65 nM	75
	HQ		0.00026 μM
Alk-Ti3C2/N-PC	CT	DPV	0.0031 μM	0.5–150 μM	69
	HQ		0.0048 μM	—
Nb2CTx/Zn–Co-NC	4-Nitrophenol		70 nM	1–500 μM	70
N–Ti3C2/PC	4-Aminophenol	DPV	59 nM	1–150 μM	76
	Acetaminophen		50 nM
MXene/ZIF-8/GCE	Hydrazine	Amperometry	5.1 μmol L^−1	10–7700 μmol L^−1	72
Er-doped Nb2C MXene	Hydrazine	LSV/CV	67 μM		71
Ti3C2/MWCNTs	Ochratoxin A		0.028 μM	0.09–10 μM	77
Ti3C2Tx@AgNC/NH2-MWCNTs	CBZ	DPV	0.0001 μM	0.0003–10 μM	78
ERGO/MXene	CBZ	DPV	0.67 nM	2 nM–10 μM	79
Ti3C2Tx MXene	CBZ	DPV	10.3 nM	0.05–100 μM	80
β-CD-MOF/MXene/CNHs	CBZ	EIS	0.001 μM	0.003–10 μM	81
Mo2C@NiMn-LDH	CBZ	DPV	0.2 nM	0.001–232.14 μM	82
MnO2NWs@Mo2TiC2	Fenitrothion	DPV	1 nmol L^−1	1–20 nmol L^−1	83
Ti3C2Tx-flakes	Methiocarb	DPV	0.19 μg ml^−1	1–60 μg mL^−1	84
	Diethofencarb	DPV	0.46 μg ml^−1	1–55 μg mL^−1
MIP/d-S-Ti3C2Tx/QCM	Chlorpyrifos	CV	0.31 pM	10^−12–10^−10 M	85
Ti3C2Tx–TiO2	Thiabendazole	DPASV	0.1 nM	0.3–100.0 nM	86
TFA@Nb2CTx	RIF	CV	4.8 pM	100 pM–1 μM	87
MXene/DNA/PtNPs/Pd	DA	Amperometry	0.03 μM	0.2–1000 μM	88
MXene-perylene diimide	DA		0.24 μM	100–1000 μM	89
Nb4C3Tx	DA		0.023 μM	—	90
Ti3C2Tx/Nafion	DA	CV	0.003 μM	0.015–10 μM	91
Ti3C2Tx	Acetaminophen	DPV	48 nM	0.25–2000 μM	92
	Isoniazid		64 nM	0.1–4.6 mM
Ti3C2/MWCNTs/Chit	Acetaminophen	DPV	0.28 nM	0.0042–7.1 μM	93
	Ifosfamide		0.31 nM	0.0011–1 μM
	Sumatriptan		0.42 nM	0.0033–61 μM
	Domperidone		0.34 nM	0.0046–7.3 μM
Ti3C2Tx	Adrenaline	Chronoamperometry	0.0095 μM	0.02–10 μM	94
Ti3C2Tx/polypyrrole	Uric acid	DPV	0.15 μM	50–500 μM	95
	DA		0.37 μM	12.5–125 μM
MXene/MWCNTs	Synephrine	LSC/CV	0.167 μM	0.5–70 μM	96
MXene/N-rGO	Adrenaline	DPV	3 nM	10 nM–90 μM	97
Ti3C2/BN-nanocomposite	Sulfadiazine	DPV	3 nM	59–186 μM	98
AuNP@Ti3C2Tx	Folic acid	CV	6.2 nM	0.02–3580 μM	99
	Uric acid		11.5 nM	0.03–1520 μM
Ti3C2Tx	Uric acid	DPV	75 nM	0.5–1500 μM	100
	DA		60 nM	0.5–50 μM
	Ascorbic acid		4.6 μM	100–1000 μM
MXene/CuNPs	Piroxicam	SWV	50 nM	0.1–80 μM	101
Ti3C2/rGO	FZD	DPV	0.002 μM	0.01–111 μM	102
CoVO/Ti3C2Tx	NO2^−	DPV	0.1 μM	0.5–2000 μM	103
Ti3C2Tx–Fe2O3	H2O2	CV	7.46 nM	10–1000 nM	104
Ti3C2Tx–Fe2O3	H2O2	Amperometry	5.0 μM	0.005–5.0 mM	105
Ti3C2Tx/Pt NP	H2O2	Amperometry	0.448 μM	490 μM–53.6 mM	106
Nafion/Hb/Ti3C2	H2O2	Amperometry	0.02 μM	0.1–260 μM	107
TiO2–Ti3C2/Nafion/Hb	H2O2	Amperometry	0.014 μM	0.1–380 μM	108
Ti3C2Tx	H2O2	Chronoamperometry	0.0007 μM	—	109
NiO/Ti3C2Tx	H2O2	DPV	0.34 μM	0.01–4.5 mM	110
CuO–CeO2/Ti3C2Tx	H2O2	Chronoamperometry	1.67 μM	5.0–100 μM	111
AuNPs/Ti3C2Tx	NO2^−	Amperometry	0.14 μM	1–4581 μM	112
AuNPs/Ti3C2Tx-PDDA	NO2^−	Amperometry	59 nM	0.1–13 500 μM	113
Pd–Cu–Mo2C	NO2^−	Amperometry	0.35 nM	5–165 nM	114
Ti3C2/Nafion/Hb	NO2^−	Amperometry	0.12 μM	0.5–11 800 μM	115
Ti3C2Tx	BrO3	DPV	0.041 μM	50 nM–5 μM	116
Ti3C2Tx MXene	H2S	Amperometry	16 nM	0.1–300 μM	117
Cu/Cu2O/TiO2/Ti3C2	H2O2	CV	0.42 μM	0.002–28.328 mM	118
Ti3C2@N–C	Cd^2+	SWASV	0.002 μM	0.1–4 μM	119
	Pb^2+	SWASV	0.0011 μM	0.05–2 μM
Alk-Ti3C2	Pb^2+	SWASV	41 nM	0.1–1.5 μM	120
	Cd^2+		98 nM
	Hg^2+		130 nM
	Cu^2+		32 nM
Nb4C3Tx	Pb^2+	SWASV	0.012	0.025–0.5 μM	121
Fe-MOF/Ti3C2Tx	As^3+	SWASV	0.58 ng L^−1	1–100 ng L^−1	122
H–C3N4/Ti3C2Tx	Cd^2+	SWASV	1 nM	50 nM–1.5 μM	123
	Pb^2+		0.6 nM
Ti3C2Tx/BiNPs	Pb^2+	SWASV	0.0108 μM	0.06–0.6 μM	124
	Cd^2+		0.0124 μM	0.08–0.6 μM
Bi@d-Ti3C2	Cd^2+	SWASV	0.4 μg L^−1	1–20 μg L^−1	125
	Zn^2+		0.5 μg L^−1
	Pb^2+		0.2 μg L^−1
PANI-Ti3C2	Hg^2+	SWASV	0.017 μg L^−1	0.1–20 μg L^−1	126
Ti3C2Tx/MWCNTs	Cu^2+	SWASV	0.1 ppb	10–500 ppb	127
	Zn^2+		1.5 ppb	200–600 ppb
NH2–Ti3C2Tx	Cd^2+	DPASV	0.41 μg L^−1	10–500 μg L^−1	58
	Pb^2+		0.31 μg L^−1

---

3.2 MXene-based electrochemical sensors for pesticides

Pesticide residues pose serious health issues.¹²⁸ Some esters derived from phosphoric acid are known as organophosphate pesticides (OPs), which are widely used across the world to boost the agricultural output. However, their overuse may contaminate ecosystems and constitute a serious risk to human health.^129,130 Consequently, researchers have developed diverse sensors for real-world pesticide detection.

3.2.1 Ti-based MXenes for pesticide detection. Carbendazim (CBZ) is a pesticide utilized to control plant diseases in the growth process of fruits and vegetables. However, its excessive application results in pesticide residues in agricultural produce, posing food safety problems for consumers. Therefore, it is necessary to develop an ultrasensitive sensor for onsite sensing, in this context, in 2019 Wu et al.⁸⁰ highlighted the electrochemical sensing potential of the pristine Ti₃C₂T_x MXene synthesized by a delamination process. They successfully achieved carbendazim redox with minimal overpotentials using a Ti₃C₂T_x-modified GCE. Delamination increases the d-spacing between the MXene layers and reveals the presence of –F and –O groups on the MXene surface. The electrocatalytic activity of Ti₃C₂T_x towards carbendazim is attributed to these surface functional groups. Oxygen-containing groups and fluorination are known to enhance the catalytic and electrocatalytic performance. It is observed that the surface fluorine termination of Ti₃C₂ plays a crucial role for sensing applications. The synthesized sensor showed an LOD of 10.3 nM. Sensor showed no clear influence of interfering species. Similarly, in 2024, Liu et al.¹³¹ introduced a unique Salix leaf-like structural material for highly sensitive detection of CBZ. AuNPs exhibit excellent electrocatalytic activity, while the few-layer Ti₃C₂T_x nanosheets possess metallic conductivity that enhances electronic transport in electrochemical reactions. Additionally, the abundant functional groups on these nanosheets contribute to their electrocatalytic performance, making them advantageous for the detection of CBZ. This composite displayed a Salix leaf-like morphology with dart-like and carambola-like topological structures. Fig. 5a presents the SEM image and morphology of AuNPs@ZIF-L–Ti₃C₂T_x. It exhibited excellent performance in the non-enzymatic detection of CBZ, with a low detection limit (1.2 nM) than Wu et al.'s study and extremely high sensitivity (66 684 μA mM⁻¹ cm⁻²) and selectivity toward CBZ. The DPV response of sensor is depicted in Fig. 5b. The synergistic effect of AuNPs, ZIF-L, and Ti₃C₂T_x in the unique material structure contributed to its high-performance characteristics. The MXene-based nanocomposite has also been employed to detect the widely used pesticide Thiabendazole (TBZ), a benzimidazole fungicide that helps preserve the freshness of fruits and vegetables and control various fungal diseases in crops. In 2023, Zhong et al.⁸⁶ introduced an innovative method for sensing thiabendazole (TBZ) using an indirect signal amplification strategy. They employed anodic stripping voltammetry of Cu²⁺ on a modified electrode made of hierarchical Ti₃C₂T_x–TiO₂ for TBZ sensing. The integration of TiO₂ particles effectively prevents the stacking of the Ti₃C₂T_x multilayered structure, thereby increasing the active surface area for the adsorption and loading of Cu²⁺. This enhancement boosts the electrochemical signal and Cu²⁺ quickly coordinates with TBZ, forming a non-electroactive Cu²⁺–TBZ complex as shown in Fig. 5c. The sensor exhibited outstanding electrochemical properties for TBZ detection, featuring a linear range of 0.3 to 100 nM and an impressively low LOD of 0.1 nM. Fig. 5d presents the SEM image of the Ti₃C₂T_x–TiO composite, while Fig. 5e shows the DPV response of TBZ toward the sensor. Additionally, the sensor demonstrated excellent reproducibility, anti-interference characteristics, and stability, proving its effectiveness in detecting TBZ in water and fruit samples.

Fig. 5 (a) SEM image of AuNPs@ZIF-L–Ti₃C₂T_x. Reproduced from ref. 131 with permission from Elsevier, Copyright 2024. (b) DPV AuNPs@ZIF-L–Ti₃C₂T_x/GCE upon the successive injection of CBZ into 0.1 M PBS/pH 7.0; the inset is 0 μM to 0.20 μM CBZ.¹³¹ Reproduced from ref. 131 with permission from Elsevier, Copyright 2024. (c) Fabrication of Ti₃C₂T_x–TiO₂/GCE and underlying electrochemical detection principle. (d) SEM image of the Ti₃C₂T_x–TiO₂ composite. Reproduced from ref. 86 with permission from Elsevier, Copyright 2023. (e) DPV curves of Ti₃C₂T_x–TiO₂/GCE response to different concentrations of TBZ in 0.1 M PBS (pH 6.0).⁸⁶ Reproduced from ref. 86 with permission from Elsevier, Copyright 2023. (f) Schematic of the preparation of the Mo₂C@NiMn-LDH composite for the electrochemical detection of CBZ. Reproduced from ref. 82 with permission from the American Chemical Society, Copyright 2021. (g) CVs of different concentrations of CBZ at Mo₂C@NiMn-LDH/SPCE.⁸² Reproduced from ref. 82 with permission from the American Chemical Society, Copyright 2021.

3.2.2 Mo-based MXenes for pesticide detection. The Mo-MXene is a new family for the sensing of pesticides in this regard, Joseph et al.⁸² reports the synthesis of molybdenum carbide (Mo₂C) MXenes on 3D Globe Amaranth flower-like NiMn layered double-hydroxide (NiMn-LDH) petal arrays, intercalated with a CO₃²⁻ backbone, using a sustainable and scalable hydrothermal method for the electrochemical detection of carbendazim (CBZ), as shown in Fig. 5f. The incorporation of Mo₂C on the fishnet-like area of the NiMn-LDH surface creates a distinctive layered heterojunction structure that enhances ion penetration, increases the active surface area, and improves the interface electron transfer. This synergy between Mo₂C and NiMn-LDH significantly boosts the catalytic activity of NiMn-LDH towards the oxidation of CBZ. The combined effects of NiMn-LDH and Mo₂C improve the sensor's properties, resulting in a wide dynamic linear response (0.001–232.14 μM), a low detection limit (0.2 nM), high sensitivity (95.71 μA μM⁻¹ cm⁻²), good stability (30 cycles), reproducibility (5 electrodes) and high selectivity. The CV response of different concentrations of CBZ is presented in Fig. 5g. Furthermore, bimetallic MXenes are also making significant strides in sensing applications. For instance, Yola et al.⁸³ developed a novel molecularly imprinted electrochemical sensor using one-dimensional ultrathin manganese oxide nanowires and two-dimensional molybdenum titanium carbide MXenes (MnO₂NWs@Mo₂TiC₂ MXene) for the detection of fenitrothion (FEN). The synthesis of the MnO₂NWs@Mo₂TiC₂ MXene nanocomposite was achieved by hydrothermal and pillaring methods. The sensor construction involved cyclic voltammetry (CV) polymerization with a pyrrole monomer and FEN as the target molecule. The MnO₂ nanowires (MnO₂NWs) are uniformly distributed on the surface of the MXene flakes, forming a 1D/2D heterostructure and preventing MXenes from restacking. The MXene material plays a crucial role in enhancing the electrochemical performance of the sensor by providing superior conductivity, high surface area, and effective interaction with the MnO₂ nanowires to form a synergistic nanocomposite. The sensor exhibited a linear detection range of 1.0 × 10⁻⁹ to 2.0 × 10⁻⁸ mol L⁻¹, with a limit of quantification (LOQ) of 1.0 × 10⁻⁹ mol L⁻¹ and a limit of detection (LOD) of 3.0 × 10–10 mol L⁻¹. The sensor demonstrated high selectivity, stability, and reproducibility. It was successfully applied to detect FEN in white flour samples, achieving results close to 100%. The mechanism of the sensor involves the electrochemical polymerization of pyrrole monomer and fenitrothion target molecule on the surface of the MnO₂NWs@Mo₂TiC₂ MXene nanocomposite. The resulting molecularly imprinted polymer (MIP) retains the imprinted nano-cavities that can selectively capture and detect fenitrothion molecules.

The above-mentioned results reported in the literature shows that the Ti-MXene has been more extensively studied and used in pesticide detection. There is a growing body of literature exploring the electrochemical sensing potential of the Ti-MXene and its composites for various analytes including pesticides. Mo-MXenes, although less extensively studied in the context of pesticide sensing, have shown promising results and have been successfully employed in the development of sensors for OP detection. In summary, both Mo-MXenes and Ti-MXenes have demonstrated potential for pesticide sensing. Ti-MXenes have been more widely studied and integrated into various sensor designs, while Mo-MXene shows promise in terms of its material properties and performance in pesticide detection. Overall, future research in Mo-MXene-based electrochemical sensors for pesticide detection should focus on further improving the detection limits, expanding the range of detectable pesticides and enhancing the sensors' performance in complex sample matrices. Additionally, efforts should be made to explore the scalability and practicality of these sensors for real-world applications, including field testing and integration into monitoring systems. Continued advancements in MXene-based sensor technology hold great potential for addressing pesticide contamination issues and ensuring food safety.

3.3 MXene-based electrochemical sensors for pharmaceutical detection

Pharmaceutical pollutants can lead to antibiotic resistance, hormone disruption and undesired physiological effects. The rising pharmaceutical use due to population growth makes these pollutants, often overlooked by authorities, a potential major societal concern.^132,133 The detection of the concentration of specific pharmaceutical contaminants in the environment is a challenge due to their trace levels.^133–136

3.3.1 Ti-based MXenes for pharmaceutical detection. Furazolidone (FZD), a type of nitrofuran derivative medication, is extensively used as an antibiotic and can lead to environmental contamination.^137–139 Its extensive use as a supplement in animal feed as well as veterinary and human medication directly endangers the ecology and public health. Consequently, regulations have been established to limit or prohibit FZD presence in various foods. Given this context, a portable and affordable technology for detecting FZD residues in food and water is essential. In 2021, Rajakumaran et al.¹⁰² designed and synthesized electrochemical sensors based on TiC/reduced-graphene oxide (rGO) composites by the probe-sonication method. The resulting TiC/rGO composite-modified SPCE exhibited excellent electrocatalytic activity for detecting FZD, which is illustrated in Fig. 6a, while Fig. 6b presents the SEM image of the TiC/rGO composite. The use of rGO nanosheets as spacers and the distinctive re-stacking configuration of TiC particles enhance the composite's stability, leading to improved electrical conductivity. It shows a wide linear range (0.01 to 111 μM) and low LOD (2 nM), coupled with good sensitivity (19.63 μA μM⁻¹ cm⁻²). The DPV response of TiC/rGO/SPCE toward FZD and the Nyquist plot for different modified electrodes are shown in Fig. 6c and d respectively. The TiC/rGO composite has a BET surface area of 49.23 m² g⁻¹, which is about 7.5 times larger than that of pristine TiC. The loading of MXenes on GO nanosheets is a challenge because of numerous functional groups on MXenes and GO, which causes a strong repulsion between adjacent nanosheets. To address this issue, Wei et al.¹⁴⁰ introduced poly(diallyldimethylammonium chloride) (PDDA), a cationic polymer, which serves as a cross linker to bridge MXenes and GO nanosheets. They developed a sensor using a composite of PDDA-assembled graphene oxide (GO) and MXenes (GM-P) on screen-printed carbon electrodes (SPCEs) for detecting FZD, as shown in Fig. 6e, while the SEM images of GM-P are presented in Fig. 6f. The integration of GO with MXenes prevents significant stacking of MXenes while preserving its structure. XRD analysis reveals that the spacing between MXene layers increased after assembly with GO using PDDA, and no TiO₂ peak was detected as illustrated in Fig. 6g, indicating that PDDA effectively prevents the oxidation of MXene. An ultralow detection limit of 1.22 nM and a sensitivity of 12.65 μA μM⁻¹ cm⁻² were observed, and the sensor also shows remarkable selectivity even after using 5-folds of interfering species. The resulting GM-P/SPCE, derived from an electrochemical reduction method, exhibited excellent conductivity, large specific surface area, and a synergistic effect of rGO and MXenes, enhancing the electrochemical properties of the sensor.

Fig. 6 (a) Illustration of the TiC/rGO mechanism towards FZD. Reproduced from ref. 102 with permission from Elsevier, Copyright 2021. (b) FESEM image of the TiC/RGO composite. Reproduced from ref. 102 with permission from Elsevier, Copyright 2021. (c) DPV response of TiC/RGO/SPCE towards FZD detection. Reproduced from ref. 102 with permission from Elsevier, Copyright 2021. (d) Nyquist plot for bare, TiC and TiC/SPCE electrodes.¹⁰² Reproduced from ref. 102 with permission from Elsevier, Copyright 2021. (e) Schematic of the synthesis process of MXenes and graphene nanosheets self-assembled into GM-P composites under low-temperature ultrasound with the assistance of PDDA for electrochemical detection of FZD. (f) SEM images of GM-P. Reproduced from ref. 140 with permission from Elsevier, Copyright 2024. (g) XRD patterns of the MXene (Ti₃C₂T_x), graphene oxide, and GM-P samples.¹⁴⁰ Reproduced from ref. 140 with permission from Elsevier, Copyright 2024. (h) Schematic of the synthesis process of Nb₂CT_x@MoS₂. (i) SEM image of Nb₂CT_x@MoS₂.¹⁴¹ Reproduced from ref. 141 with permission from Elsevier, Copyright 2023. (j) Diagram showing the electrochemical detection of the anti-tuberculosis medicine RIF using TFA-modified Nb₂CT_x MXenes.⁸⁷ Reproduced from ref. 87 with permission from Elsevier, Copyright 2023.

Given the rapidly expanding applications of MXenes, they can also be utilized to detect dopamine (DA), a widely known peripheral vasostimulant used in the treatment of low blood pressure, slow heart rate, and cardiac arrest. In this regard, Shahzad et al.⁹¹ (2019) introduced simple Ti₃C₂T_x/Nafion-based electrochemical sensor for the detection of DA. It is observed that the negative surface functional groups of MXenes are helpful for DA sensing, and additional Nafion promotes the interaction of DA with electrode surface, which results in a better electrocatalytic response. Sensors show good sensitivity with a low limit of detection of 3 nM and as well as good selectivity.

Acetaminophen (ACOP) and isoniazid (INZ) are pharmaceutical compounds that can act as environmental pollutants when they enter water bodies through improper disposal or wastewater discharge. Acetaminophen, a common pain reliever, and isoniazid, an antibiotic for tuberculosis, can pose risks to aquatic ecosystems and human health due to their persistence and bioactivity. Therefore, it is necessary to detect these pollutants to mitigate their environmental impact and protect public health. Therefore, in 2019 Zhang et al.⁹² developed a sensitive electrochemical sensor based on MXene-modified SPE. Different Surface functional groups on MXenes provide multiple binding sites for ACOP and INZ. MXene's high conductivity and large surface area facilitate electron transfer, improving the oxidation process. The use of 0.1 M H₂SO₄ as the electrolyte ensures optimal conditions for the electrooxidation of both drugs, as ACOP is deprotonated and repelled by the negatively charged MXene surface at higher pH levels. The detection limits for ACOP and INZ are 0.048 μM and 0.064 mM, respectively, indicating high sensitivity.

3.3.2 Nb-based MXenes for pharmaceutical detection. The exceptional properties of Nb-MXenes, such as Nb₂CT_x and Nb₄C₃T_x, make them highly promising for electrochemical sensing applications. These materials exhibit excellent electrochemical stability and significant activity, positioning them as effective platforms for detecting pharmaceutical residues in aqueous media. In 2023, Rasheed's group¹⁴¹ synthesized a heterostructure comprising MXene and MoS₂ (Nb₂CT_x@MoS₂) for highly sensitive and selective detection of DA. The composite was prepared via a simple hydrothermal method and used to modify carbon cloth (CC), as depicted in Fig. 6h. The Nb₂CT_x@MoS₂-modified CC demonstrated linear detection of DA from 1 fM to 100 μM with a remarkably low limit of detection (LOD) of 0.23 fM, the lowest reported for DA detection using MXene-based composites. The SEM image of Nb₂CT_x@MoS₂ is presented in Fig. 6i. Moreover, the sensor exhibited excellent selectivity against interferents such as uric acid and ascorbic acid, along with good repeatability, cycling stability, and storage stability. These results highlight the potential of this composite for flexible electrochemical sensors enabling highly sensitive and selective determination of DA. Similarly, Rasheed's group¹⁴² developed another novel composite ReS₂–Nb₂CT_xvia a hydrothermal method for the electrochemical detection of dipyridamole (DIPY), a crucial medication for cardiovascular diseases and cancer treatment. The ReS₂ nanoparticles were distributed on the surface of Nb₂CT_x and partial oxidation of Nb₂CT_x is also observed during the hydrothermal synthesis process, which enhanced the activity of material. The high stability of ReS₂–Nb₂CT_x was also observed. The developed sensor showed excellent selectivity, achieving an LOD of 28 pM and a linear detection range of 100 pM to 1 μM for DIPY. Moreover, the newly developed sensor demonstrated remarkable selectivity for DIPY, even when other interfering substances such as oxalic acid, AA, citric acid, and glucose were present at concentrations ten times higher than that of DIPY.

Nb-based MXenes show significant potential in sensing pharmaceutical residues. To expand their applicability, Ankitha et al.⁸⁷ (2023) synthesized an exceptionally stable Nb₂CT_x MXene modified with trifluoroacetic acid (TFA) for the electrochemical detection of rifampicin (RIF), an anti-tuberculosis drug. An increase in the d-spacing of Nb₂CT_x was observed, suggesting the intercalation of TFA between the layers of Nb₂CT_x nanosheets. This intercalation led to increased conductivity and high electron transfer kinetics towards RIF analyte molecules. Furthermore, the oxidation of Nb₂CT_x is also observed. When applied to a carbon cloth (CC) substrate, as illustrated in Fig. 6j, the 0.3TFA@Nb₂CT_x modification demonstrated the ability to detect RIF concentrations linearly within the range of 100 pM to 1 μM, achieving an outstandingly low detection limit of 4.8 pM. This LOD enhancement is a significant advancement in comparison to the existing literature. The sensor shows excellent selectivity towards RIF even in the presence of 10 folds concentration of interferents.

The above-mentioned results reported in the literature indicate that MXene materials, especially Ti- and Nb-based MXenes, have shown great potential as electrochemical sensing platforms for detecting various pharmaceutical pollutants. Their exceptional properties such as large surface area and adjustable surface functionalization make them suitable for electroanalytical applications. Sensors based on MXene/composite materials have enabled low detection limits, wide linear ranges and high sensitivities for different pharmaceutical targets. Doping/intercalation with conductive nanomaterials or metals is an effective strategy to further improve the electrochemical performance by enhancing the active sites, charge transfer and mass transport. However, to advance this field, future research should focus on broadening the analytical applications to enable multi-class detection of diverse pharmaceuticals simultaneously. Exploring synergistic combinations of new MXene variants (e.g., V₂C and Nb₂C) with catalytic nanomaterials can lead to even more efficient sensors. Additionally, developing flexible and wearable sensor platforms that integrate MXenes will be crucial for real-time in situ and in vivo monitoring, paving the way for innovative applications in health and environmental monitoring.

4. MXene-based electrochemical sensors for inorganic pollutant detection

Some inorganic compounds such as noxious gases and heavy metal ions (HMIs) emanate from diverse sources.^143–145 In this section, we highlight the accomplishments of electrochemical sensors using MXenes and their composites in the detection of inorganic toxins, encompassing HMI, hazardous gases, nitrates, and other forms of inorganic contaminants.

4.1 MXene-based electrochemical sensors for nitrite and hydrogen peroxide detection

Nitrite is one of the inorganic environmental contaminants that might have a negative impact on the ecological environment due to its extensive adoption as a nitrogen fertilizer in agriculture and an industrial food additive. Elevated nitrite levels within the human body can interact with oxygenated hemoglobin, resulting in the formation of methemoglobin. This altered hemoglobin variant exhibits diminished capability for oxygen transportation in the bloodstream. Additionally, it can lead to the production of cancer causing N-nitrosamines in human beings, which could have negative health effects.¹⁴⁶ According to available reports,¹⁴⁷ it has been documented that the ingestion of nitrite in quantities ranging from 0.3 to 0.5 grams has been associated with instances of poisoning, while a higher dosage of approximately 3 grams has been linked to cases resulting in fatality.^148,149 The World Health Organization's standards state that there should be no more than 3 mg L⁻¹ of nitrite in drinking water, hence it is crucial to precisely measure the amount of nitrite present in the environment.¹⁴⁷

4.1.1 Ti-based MXenes for nitrite and hydrogen peroxide detection. Research has clearly indicated that conventional electrodes require a large overpotential for the electrochemical oxidation of nitrites. To overcome this issue, Zou et al.¹¹² developed a nitrite sensing platform using functionalized 2D titanium carbide (Ti₃C₂T_x) combined with gold nanoparticles (AuNPs). The electrochemical characterization of the synthesized composite revealed high electrocatalytic activity and a large electroactive area due to the synergistic effect of MXene and AuNPs. Ti₃C₂T_x provided a large surface area to evenly distribute the AuNPs, preventing their aggregation and interference with stacking by interposing between MXene layers. The AuNPs catalyzed the oxidation of nitrites, enabling their sensitive and selective electrochemical determination by differential pulse voltammetry (DPV). This platform achieved an ultra-low detection limit of 0.14 μM and demonstrated remarkable selectivity against various compounds including NaNO₃, Na₂CO₃, NaH₂PO₄, NaCl, NaF, Na₂HPO₄, (NH₄)₂S₂O₈, and different concentrations of Co(NO₃)₂, Cu(NO₃)₂, and FeCl₂. The introduction of AuNPs on Ti₃C₂T_x significantly enhanced the electrode surface's conductivity, resulting in high sensitivity. In another work, Feng et al.¹⁵⁰ established an electrochemical sensor for nitrite detection using a one-pot green synthesis approach. This approach involved the use of xylan-based carbon quantum dots (CQDs) that were coated with gold nanoparticles (Au@CQDs). A sensing platform using Au@CQDs–MXene/GCE is shown in Fig. 7a, while the TEM image of Au@CQDs–MXene is presented in Fig. 7b. The results indicated that the interlayer spacing of MXene increases with the loading of CQDs and further expands with the addition of Au@CQDs. The excellent conductivity of AuNPs mitigates the negative impact of oxygen-containing functional groups on MXenes. The sensor showed exceptional selectivity, stability, and high sensitivity with an LOD of 0.078 μM (S/N = 3). Fig. 7c presents the DPV response of sensors towards nitrates. Moreover, recently in 2023, Zhuang et al.¹⁰³ introduced an electrochemical sensor designed for nitrate detection. This innovative sensor combined a 2D Ti₃C₂T_x MXene sheet with a three-dimensional (3D) urchin-like Co(VO₃)₂(H₂O)₄ (CoVO) structure. CoVO exhibits excellent catalytic activity towards NO₂⁻, while the Ti₃C₂T_x material boasts high electrical conductivity, reducing barriers and facilitating electron transport. The resulting electrochemical sensor exhibited substantial linearity in detecting nitrate concentrations ranging from 0.5 to 2000 μM, accompanied by a low LOD of 0.1 μM. The composite takes advantage of synergistic effects, fast electron transfer and high catalytic activity for the sensitive detection of nitrite. The above-mentioned results reported in the literature show the importance of MXene's extraordinary properties for nitrate sensing and also show remarkable selectivity towards nitrate.

Fig. 7 (a) Development of the nitrite detection platform using Au@CQDs–MXene/GCE. Reproduced from ref. 150 with permission from Elsevier, Copyright 2022. (b) TEM images of Au@CQDs–MXene. Reproduced from ref. 150 with permission from Elsevier, Copyright 2022. (c) DPV response of the Au@CQDs–MXene modified GCE in 0.1 M PBS (pH 7.0) with different concentrations of nitrite (0 μM–3.2 mM); the inset shows the peaks from 0 μM to 100 μM.¹⁵⁰ Reproduced from ref. 150 with permission from Elsevier, Copyright 2022. (d) SEM images of CuO–CeO₂/MXene. Reproduced from ref. 111 with permission from Elsevier, Copyright 2022. (e) Assembly of the electrochemical H₂O₂ sensor using CuO–CeO₂. Reproduced from ref. 111 with permission from Elsevier, Copyright 2022. (f) The current responses of CuO–CeO₂/MXene/GCE for different concentrations of H₂O₂.¹¹¹ Reproduced from ref. 111 with permission from Elsevier, Copyright 2022. (g) Schematic of the construction of the Nb₂C@MWCNTs-STAB/GCE sensor. (h) SEM image of Nb₂C@MWCNTs-STAB. Reproduced from ref. 151 with permission from Elsevier, Copyright 2022. (i) DPV responses of the Nb₂C@MWCNTs-STAB/GCE sensor in NO₂⁻ solutions of different concentrations.¹⁵¹ Reproduced from ref. 151 with permission from Elsevier, Copyright 2022.

On another note, hydrogen peroxide (H₂O₂) is an extensively used oxidant in several sectors including food processing, paper manufacture, wastewater treatment, textiles, and chemicals.¹⁵² It is also employed for disinfection in the food industry, which can lead to H₂O₂ residues in food. Excessive H₂O₂ poses health risks including cancer, diabetes, and Alzheimer's disease.¹⁵³ Therefore, it is essential to create credible methods for the precise detection of H₂O₂ in food and blood samples.^154,155 In 2021, Cheng et al.¹⁵⁵ introduced a distinctive electrochemical H₂O₂ sensor by fusing an ultra-thin MXene nanosheet with a three-dimensional flower-like Cu-MOF. Cu-MOF displayed high electrocatalytic activity for H₂O₂ reduction. The introduction of MXenes significantly enhanced the sensor's current response to H₂O₂ by improving the electrical conductivity and accelerating electron transfer rates. Additionally, MXene modification on the electrode surface facilitated the immobilization of Cu-MOF via electrostatic interaction, using MXene's large surface area and negative surface groups. The sensor exhibits an extensive linear range spanning from 1 mol L⁻¹ to 6.12 mmol L⁻¹ and an estimated limit of detection of 0.35 mol L⁻¹ for H₂O₂ detection, utilizing chronoamperometry at a detection potential of 0.35 V. Following this research in 2022 Zhou et al.¹¹¹ introduced an electrochemical sensor designed for continuous H₂O₂ sensing, relying on CuO–CeO₂/Ti₃C₂T_x. The SEM image of CuO–CeO₂/Ti₃C₂T_x is illustrated in Fig. 7d. The CuO–CeO₂ nanocomposites synthesized by a sol–gel method served as catalysts, facilitating the process of H₂O₂ electrolysis on the electrode surface and amplifying the reduction current for H₂O₂ sensing, as shown in Fig. 7e. Electron-rich –F functional groups are present on the surface of MXenes, and CuO–CeO₂ binds to the MXene surface, forming a nanoparticle layer. This layer facilitates the exposure of more reaction sites for H₂O₂. In essence, MXenes enhance the interaction with CuO–CeO₂, promoting the efficient utilization of H₂O₂. It exhibited a strong linear relationship between 5 and 100 μM for H₂O₂ detection, and the current response is shown in Fig. 7f, boasting a low LOD 1.67 μM.

4.1.2 Nb-based MXenes for nitrite and hydrogen peroxide detection. Nb-MXenes can also be used for the detection of nitrate in this regard, Chen et al.¹⁵¹ developed a highly sensitive electrochemical sensor for the detection of nitrite by functionalizing niobium carbide MXene-intercalated multi-walled carbon nanotubes (Nb₂C@MWCNTs) with stearyl trimethyl ammonium bromide (STAB), as depicted in Fig. 7g. The SEM image in Fig. 7h shows that multi-walled carbon nanotubes (MWCNTs) have been integrated into hierarchical Nb₂C structures to enhance the spacing and expose more active sites. The synergistic interaction between MXenes and MWCNTs resulted in a sensing platform with exceptional metallic conductivity, a large specific surface area, and outstanding electronic conductivity. STAB was then introduced to neutralize the negative surface charge on Nb₂C@MWCNTs, enhancing the electrostatic attraction towards the anionic nitrite. The sensor demonstrated remarkable analytical performance, including wide linear detection ranges from 0.1 to 100 μM and 100 to 2000 μM, as well as an impressive limit of detection of 0.022 μM for nitrites. The DPV response of Nb₂C@MWCNTs/GCE at different concentrations of NO₂⁻ is depicted in Fig. 7i. The sensor also exhibited good selectivity, repeatability, reproducibility and long-term stability. This work presents an effective strategy for developing sensitive electrochemical sensors by functionalizing conductive MXenes with surfactants. The Nb-MXene nanocomposite has also been used for the detection of H₂O₂. In this regard, Arjun et al.¹⁵⁶ modified carbon cloth (CC) with a composite of Nb₂CT_x MXene and Prussian blue (PB) for electrochemical H₂O₂ detection. Nb₂CT_x was first drop-cast onto CC, followed by PB deposition using chronoamperometry at 0.7 V, resulting in uniform PB coverage on Nb₂CT_x. The high electrical conductivity of the Nb₂CT_x MXene facilitates efficient electron transfer kinetics at the electrode surface. Prussian blue (PB) exhibits an enzyme-mimetic property and affinity towards reactive oxygen species such as H₂O₂, enabling its electrocatalytic reduction. The Nb₂CT_x/PB480-modified CC detected H₂O₂ with linear ranges from 1 μM to 10 μM and 10 μM to 100 μM, achieving a detection limit of 200 nM. The sensor exhibited high selectivity for H₂O₂, even in the presence of interfering substances such as dopamine, ascorbic acid, uric acid, and sodium chloride. Additionally, it demonstrated good storage stability, retaining 97% of its activity after one week. This sensor is suitable for detecting H₂O₂ in milk, as the buffer pH used in the study matches the pH of milk samples.

MXene materials have shown great potential as effective sensing platforms for nitrite and hydrogen peroxide due to their unique properties such as high conductivity, tunable surface chemistry and layered structure. For nitrite sensing, sensors based on Ti- and Nb-MXenes have demonstrated low detection limits, wide linear ranges and good selectivity. Functionalization strategies such as doping and composite formation have further improved the sensor performance. Regarding hydrogen peroxide sensing, noble metal–MXene and metal oxide–MXene composites have enabled sensitive detection. The synergistic effects between different components enhance active sites, electron transfer and anti-fouling ability. However, there is still room for improvement in some aspects. Future work could focus on exploring new MXene variants and their combinations with additional catalysts. The fabrication of hierarchical 3D structures and flexible devices would expand the practical applications.

4.2 MXene-based electrochemical sensors for heavy metals ion detection

Heavy metal contamination has long been recognized as one of the most widespread and important environmental issues in the natural environment. Heavy metal contamination is known for its non-biological decay and high cytotoxicity to organisms, which have a serious hazard to the living environment.¹⁵⁷ Heavy metals such as lead (Pb), mercury (Hg), cadmium (Cd), chromium (Cr), iron (Fe), copper (Cu), arsenic (As), and manganese (Mn) are discharged from various industries and remain persistent in the environment for extended periods. After release, these metal ions enter water bodies, make their way to the surface water, and pose serious risks to aquatic life and human health. As a result, researchers and environmental scientists are very concerned with the identification and removal of these dangerous contaminants.^158,159 In this context, the MXene is highly effective for the electrochemical detection of heavy metals due to their advantageous properties.

4.2.1 Ti-based MXenes for heavy metal ion detection. Over the years, titanium (Ti) has been employed as a transition metal in titanium carbide MXenes due to its high metallic conductivity and abundant surface terminations, which enable them to act as powerful superconductors. Thus, it has been proved to be an important material for heavy metal sensing. In this context, in 2017, Zhu et al.¹²⁰ developed a 2D accordion-like alk-Ti₃C₂, created through acid etching and alkaline intercalation treatment for simultaneous electrochemical detection of multiple heavy metal ions (Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺) by square wave anodic stripping voltammetry. Under optimized conditions, the alk-Ti₃C₂ platform exhibited remarkable selectivity and sensitivity, with detection limits of 0.098 μM for Cd²⁺, 0.041 μM for Pb²⁺, 0.032 μM for Cu²⁺, and 0.130 μM for Hg²⁺ surpassing many previously reported values. The alk-Ti₃C₂ MXene material was synthesized by selectively etching Ti₃AlC₂ MAX phase followed by alkaline intercalation treatment using KOH. This insertion of K⁺ ions between Ti₃C₂ layers increased the interlayer spacing and altered the surface chemistry, resulting in improved electron transfer properties and enhanced heavy metal uptake, as illustrated in Fig. 8a. XRD results also indicated the increase in interlayer spacing, as shown in Fig. 8b. All four species showed well-separated potential peaks during simultaneous detection and intermetallic interference was also observed, as depicted in Fig. 8c. Following this work, in 2022, Xia et al.¹⁶⁰ synthesized N and P co-doped Ti₃C₂T_x MXene nanoribbons (N,P-Ti₃C₂T_x) by a hydrothermal method for the detection of Cu²⁺ and Hg²⁺, as shown in Fig. 8d. The TEM image of N,P-Ti₃C₂T_x is illustrated in Fig. 8e. Operating under optimal conditions, this sensor demonstrated low detection limits of 1.8 nM and 0.29 nM for Cd²⁺ and Hg²⁺ respectively, and it shows wide linear ranges of 0.02–10.0 μM and 0.005–2.5 μM for Cu²⁺ and Hg²⁺ respectively. The doping of nitrogen and phosphorus enhances the electrical properties of the material. Additionally, the high oxygen content (34.12%) indicates the presence of electronegative functional groups on the MXene surface, facilitating the efficient adsorption of Cu²⁺ and Hg²⁺via electrostatic interactions. Recently in another study, Dong et al.¹⁶¹ developed a novel electrochemical sensing platform for the simultaneous detection of Cd²⁺ and Pb²⁺ using a composite of MXene@rGO aerogel doped with UiO-66-NH₂ as illustrated in Fig. 8f. The MXene@rGO composite enhances conductivity by accelerating electron transport, while UiO-66-NH₂ provides amino groups for binding heavy metal ions. It is observed that aniline or protonated aniline (PhNH₂/PhNH₃⁺) present in the composite facilitates redox processes on the electrode surface. This sensor can independently and simultaneously detect Cd²⁺ and Pb²⁺ with detection limits of 0.46 ppb and 0.40 ppb, respectively. Well-separated stripping peaks were observed for Cd²⁺ and Pb²⁺ during simultaneous detection, indicating no interference between the ions, and also showed good selectivity. From the above-mentioned results reported in the literature, we concluded that Ti₃C₂T_x MXene-based electrochemical sensors offer promising results for HMI sensing due to their ability to undergo in situ redox reactions with HMIs and strong interactions with negatively charged functional groups on their surface. To enhance the benefits and stability, researchers prefer using multilayer composites, enabling selective detection of heavy metal ions. The emergence of two-dimensional MXene nanomaterials has opened up new possibilities for advanced electrochemical sensing platforms, given their unique properties such as hydrophilicity, conductivity, and surface terminations.

Fig. 8 (a) Structural illustrations of the parental Ti₃AlC₂ MAX phase and the resulting MXene before and after alkalization. Reproduced from ref. 120 with permission from Elsevier, Copyright 2017. (b) XRD patterns of the synthesized Ti₃AlC₂, Ti₃C₂ and alk-Ti₃C₂. Reproduced from ref. 120 with permission from Elsevier, Copyright 2017. (c) SWASV response of the alk-Ti₃C₂-modified GCE for the simultaneous analysis of Cd(II), Pb(II), Cu(II) and Hg(II) over a concentration range of 0.1–1.5 μM.¹²⁰ Reproduced from ref. 120 with permission from Elsevier, Copyright 2017. (d) Schematic of the synthesis of N,P-Ti₃C₂T_xR and electrochemical sensing of Cu²⁺ and Hg²⁺. Reproduced from ref. 160 with permission from Elsevier, Copyright 2022. (e) TEM image of N,P-Ti₃C₂T_x.¹⁶⁰ Reproduced from ref. 160 with permission from Elsevier, Copyright 2022. (f) Schematic of the construction process of the UiO-66-NH₂-MXene@rGO/GCE sensor electrode and simultaneous detection of Cd²⁺ and Pb²⁺.¹⁶¹ Reproduced from ref. 161 with permission from Elsevier, Copyright 2024. (g) Schematic of the construction of the Nb₄C₃T_x sensor and detection towards Pb²⁺. (h) XRD pattern of ML-Nb₂CT_x, DL-Nb₂CT_x, ML-Nb₄C₃T_x, and DL-Nb₄C₃T_x. (i) SEM image of ML-Nb₂CT_x. (j) SEM image of ML-Nb₄C₃T_x.¹²¹

4.2.2 Nb-based MXenes for heavy metal ion detection. Niobium carbide-based MXenes, particularly Nb₄C₃T_x, have emerged as a new class of materials for heavy metal detection due to their exceptional electrochemical properties and unique structural characteristics. In this context, Rasheed et al.¹²¹ developed a Nb₄C₃T_x-modified glassy carbon electrode for the electrochemical detection of Pb²⁺ ions in aqueous solutions, as depicted in Fig. 8g. The developed sensor achieves a detection limit of 12 nM at an applied potential of approximately −0.6 V. Moreover, it exhibits excellent selectivity in the presence of Cu²⁺ and Cd²⁺ ions. The XRD (Fig. 8h) and SEM (Fig. 8i and j) measurements reveal that the large interlayer spacing and higher c lattice parameter of Nb₄C₃T_x, compared to Nb₂CT_x, enable the adsorption of a greater amount of Pb²⁺ between the sheets. Additionally, the superior conductivity of Nb₄C₃T_x enhances the electrochemical response at the electrode surface. A comparative analysis was performed to assess the analytical performance of Nb₂CT_x against previously discussed modified Ti₃C₂T_x, and alkaline-intercalated Ti₃C₂T_x shows remarkable performance toward HMI. Nb₄C₃T_x has a larger interlayer spacing of 14.85 Å compared to Ti₃C₂T_x. This favors faster adsorption and intercalation of ions, enhancing sensitivity. Furthermore, Rasheed et al.¹⁶² developed an electrochemical aptasensor for the highly selective and sensitive detection of Pb²⁺ using a lead-specific DNA oligonucleotide as the molecular recognition element on a gold nanoparticles/Nb₄C₃T_x MXene (Au@ Nb₄C₃T_x)-modified electrode. The thiol-modified lead-binding DNA was conjugated to Au@Nb₄C₃T_x through an Au–S bond. Gold nanoparticles (AuNPs) were formed on the surface of Nb₄C₃T_xvia in situ reduction. The strong deposition of AuNPs on Nb₄C₃T_x is driven by the negatively charged surface of Nb₄C₃T_x. The stability of the AuNPs on the Nb₄C₃T_x surface was tested by vigorously shaking the suspension in deionized water for five days. The sensor shows high selectivity and sensitivity with a detection limit of 4 nM and a linear range of 10 nM to 5 μM. This study highlights the potential of Nb₄C₃T_x as a robust platform for immobilizing DNA oligonucleotides for various environmental and biomedical sensing applications. The above-mentioned results reported in the literature show extraordinary potential of Nb₄C₃T_x over Ti₃C₂T_x. In conclusion, Nb₄C₃T_x provides additional active sites for heavy metal coordination compared to Ti-based MXenes, potentially improving adsorption capacity. Studies also showed that incorporating nanoparticles such as transition metal oxides (TMOs) onto the surface of MXene effectively expanded the interlayer spacing, resulting in a larger electroactive surface area conducive to enhanced detection of heavy metal ions. Furthermore, nanoscale MXenes show good performance in sensing as smaller flake sizes allow better electrolyte accessibility to more active sites, potentially enhancing the electrochemical performance of Ti₃C₂T_x film electrodes due to the high surface area-to-volume ratio compared to large-sized MXenes.¹⁶³ The reduced particle size facilitates rapid diffusion of target analytes into and out of the sensing interface, improving reaction kinetics and reducing response times. Small MXene particles can also be more uniformly dispersed within sensor matrices such as polymers and composites, ensuring consistent and homogeneous sensing performance. As the MXene size decreases towards 1D or 0D nanostructures, its flexibility increases, potentially introducing new sensing mechanisms. These nanostructures enable the miniaturization of sensors into portable, flexible, implantable, or paper-based formats for point-of-care or in situ detection. Moreover, the increased fraction of surface atoms available for functionalization enhances chemical selectivity by allowing precise control over surface groups and moieties.¹⁶⁴ To fully leverage these advantages, future work can focus on fabricating well-ordered composite and hybrid structures integrating nano-MXenes with other nanomaterials. This may synergistically enhance the sensor performance via multi-functional integration. Selective surface modification of nano-MXenes also holds potential to facilitate highly sensitive detection of specific target analytes such as disease biomarkers, explosives, and toxic gases at trace levels relevant to real-world application. Further research should focus on the exploration of composition variants and phased MXenes synthesized from diverse MAX precursors. The reproducibility and stability are enhanced through novel surface coatings/intercalation strategies. The efficacies of different MXene-based electrochemical sensors for detecting various heavy metals are summarized in Table 2.

5. Electrochemical stability of MXenes

The electrochemical stability of MXenes is crucial for unlocking their full potential in electrochemical applications. MXenes exhibit inherent electrochemical instability, which arises from factors such as reactive surface terminations, residual impurities, morphology-related defects, and unfavorable electrode potentials. Reactive surface terminations, particularly –OH groups, are prone to oxidation reactions during potential cycling, leading to surface degradation. Additionally, residual etching byproducts or contaminants left on MXene surfaces can catalyze unwanted side reactions, accelerating degradation over time.¹⁶⁵ Furthermore, structural defects, poor crystallinity, and small particle sizes increase the number of reactive sites vulnerable to attack, compromising overall stability.¹⁶⁶ Regarding the potential window range, MXenes show redox activity within a narrow potential window of −1 to 1 V vs. Ag/AgCl in water. Thus, operating MXenes beyond their stable potential window or exposing them to harsh environments would exacerbate corrosion processes, further compromising stability. Mechanical stresses from repeated ion intercalation/deintercalation during cycling can also contribute to instability.^167,168 This instability not only affects sensor sensitivity over time due to surface area and redox activity loss but also leads to poor reproducibility and a short sensor lifespan, necessitating frequent replacement. In addition, surface changes during degradation make the accurate quantification of analyte concentrations a challenge. Enhancing MXene's electrochemical stability is vital for improving the sensor performance and reliability.

Some strategies can be employed to enhance MXene's electrochemical stability. Surface modification is highly effective. For example, the fluorination of –OH sites introduces stronger –F termination layers or passivates surfaces with polymers or oxides and composition tuning is achieved through alloying or doping with stable elements. Moreover, surface coatings, electrolyte engineering, potential window selection, material densification, and ambient control can all contribute to stabilizing MXene nanomaterials and ensuring reliable performance in electrochemical applications. Zhu et al.¹⁶⁹ intercalated polyoxometalate (POM) into an MXene (Ti₃C₂T_x) layer for the first time by using cetyltrimethylammonium (CTA⁺) cations. This CTAMX-PW12 hybrid exhibited superior electrochemical stability, retaining 85% of its initial capacitance after 5000 cycles compared to other hybrids that became inactive after 500 cycles. Redox reactions in CTAMX-PW12 remained diffusion-controlled, indicating the successful intercalation of POMs. Furthermore, Hao et al.¹⁷⁰ introduced a new method by bridging a Ti₃C₂T_x MXene with tannic acid (TA) molecules, which was found to significantly enhance its electrochemical stability, oxidation resistance, and structural integrity. Cycling tests indicated that the MXene@TA electrode retained 96.1% capacitance after 10 000 cycles at 5 A g⁻¹, whereas pure MXenes experienced a sharp drop after just 1500 cycles. TA bridging significantly improved the electrochemical cycling stability, rate performance, and retention of MXene materials in both half-cell and full-device configurations.

6. Molecular imprinting techniques for increasing the selectivity of MXenes

Essentially, the selectivity of MXenes is moderate because of its open adsorption sites for target molecules. Therefore, how to increase the selectivity of MXene is crucial to realize the real sensing applications of MXene-based electrodes. Molecularly imprinted polymers (MIPs), also known as “plastic antibodies”, are synthetic recognition elements designed to mimic natural receptors,¹⁷¹ because they can create selective binding sites for target analytes via a process similar to the lock and key principle.¹⁷² MIPs have been integrated with MXenes to enhance the electrode responses in specific electrochemical reactions.¹⁷³ The formation of MXene–MIPs involves several steps. Initially, a suspension of MXenes is dropped onto the working electrode surface and allowed to dry. Subsequently, the electrode is immersed in a solution containing both the monomer and the template molecule. Following this, electrochemical parameters are adjusted, and electro-polymerization is initiated using the CV technique. This polymerization method effectively regulates the thickness and porosity of the films. In this section, some electrochemical studies employing MXene–MIP composites are discussed. For example, Wang and colleagues¹⁷⁴ developed an electrochemical sensor for detecting ascorbic acid (AA) by modifying a glassy carbon electrode with MXenes and an imprinted poly-(o-phenylenediamine) (o-PD) polymer. They electro-polymerized o-PD onto the MXene-modified electrode to create an imprinted film that provides specific binding sites for AA. The electro-polymerization time affects the thickness of MIP films, and thicker films of MIP provide more imprinting sites to improve the selectivity. However, if polymer is too thick, the AA template molecules cannot be removed. The polymer thickness can be improved by the number of scan cycles. A recent study by Shi et al.¹⁷⁵ has introduced a novel MXene fiber-based electrochemical sensor with MIPs. The MXene fibers provided superior electrical conductivity, while the MIP layer allowed simultaneous recognition and quantification of hydrocortisone (HC) with high selectivity. By increasing the concentration of HC, the current decreases because the HC molecules occupied the holes of the MIPs and restricted the electron transfer. Following the above-mentioned work, Ma et al.¹⁷⁶ developed a molecular imprinting sensor using an MXene/NH₂-CNTs substrate material, achieving high sensitivity and selectivity. The MIP layer's cavity network allowed simultaneous identification and quantification of fisetin, ensuring selectivity. After electro-polymerization on MXene/NH₂-CNTs/GCE, a smooth and uniform ppy film was observed. The extraction of fisetin molecules resulted in increased surface porosity, indicating the successful removal of template molecules. Comparatively, the surface of NIP/MXene/NH₂-CNTs/GCE appeared hazier and denser, while MIP/GCE exhibited rough surfaces, suggesting specific imprinted sites. These observations confirm the successful coating of molecularly imprinted cavities on the MXene/NH₂-CNTs nanocomposite surface. In another study, Lu and co-workers¹⁷⁷ created a new dual-signal ratiometric MIP biosensor for the electrochemical detection of CC using MXene/Fe@Ti-MOF-NH₂ nanomaterials. The electro-polymerization method synthesized a conductive pTHi built-in signal film and MIP film, increasing the recognition sites and enabling dual-signal amplification.

Although MIP–MXene electrochemical sensors have great potential but there are still many challenges that need to be addressed. MIPs are employed for sensing small molecules such as disease biomarkers and amino acids, but when applying on large molecules, there is a huge gap between selectivity and applicability. Thus, more intense research is needed to focus on the feasibility of MIP on large molecules. Furthermore, many factors such as the molar ratio of template to monomer, scan rate and cycles of polymerization, elution time, pH of electro-polymerization solution and incubation time have influences on MIP-based electrochemical sensor performance. These factors should also be fully considered for better sensing performances.

7. Conclusion and future perspectives

MXenes possess inherent qualities including substantial specific surface area, high electrical conductivity, notable redox capacities and important electrocatalytic and electrochemical properties, making them highly appealing for environmental pollutant sensors. When coupled with recognition elements or composited with supportive nanomaterials, MXenes have demonstrated excellent performance in identifying various toxins. For instance, in environmental monitoring, MXenes are ideal for detecting pollutants such as heavy metals and organic contaminants in water and air due to their high sensitivity and rapid response times. In industrial safety, they are useful for detecting hazardous gases and chemicals, ensuring workplace safety through continuous monitoring. However, several challenges remain yet to be addressed for their full commercialization.

Certain challenges persist in the pursuit of MXene-based electrochemical sensor applications, including fabrication approaches and mechanism study. Various alternative etching agents including LiF/HCl, molten ZnCl₂ and electrochemical techniques have been suggested as substitutes for HF etchants. However, the results obtained from these alternatives give rise to significant inquiries. Therefore, the pursuit of safer, ecologically sustainable and economically efficient techniques for large-scale manufacturing is of utmost importance. Innovative experimental approaches and conditions are necessary to address the issues for the synthesis of MXenes. Scale-up processes such as blade coating or slot-die printing holding appeal for industrial production need focus because the integration of MXenes into in-line sensing platforms for continuous monitoring is based on these approaches. The microfabrication approach for MXene warrants exploration. The operational mechanism of MXenes as environmental pollutant sensors, particularly the interaction between MXene-based substrates and pollutants for achieving sensor selectivity, is not fully understood. Interactions between the MXene surface and contaminant molecules are crucial for sensor selectivity. To facilitate practical applications, it is essential to comprehensively understand and accurately interpret the signal mechanism in MXene-based resistance sensors using in situ characterization technologies. Additionally, gaining a deeper insight into the structure–property relationship of MXenes will greatly contribute to advancing our understanding of their sensing mechanism. Selectivity also merits improvement, such as via molecular imprinting which displays early success but requires streamlining. Portability and miniaturization are essential for on-site applications. Advancing fabrication of flexible, freestanding MXene films and sensors integrated with microfluidics could transform deployment. It should be noted that when MXenes are subjected to air or aqueous solutions, they undergo rapid oxidation, resulting in the formation of transition metal oxides such as TiO₂. This oxidative process poses a challenge for flexible sensors that depend on the inherent high conductivity of MXenes, negatively impacting their performances, while surface modifications, protective coatings, or novel synthesis techniques are crucial for improving sensor longevity and performance. It is essential to focus more on addressing this challenge to ensure the practical viability of MXene-based sensors in various applications. Moreover, exploring the synthesis of new MXene compositions and structures can lead to improved sensing capabilities. For instance, developing MXenes with different transition metal compositions (e.g., Nb, V, and Zr) can introduce unique catalytic properties and selectivity.

Cost-effectiveness is a crucial consideration in sensor technology. Our analysis reveals that MXene-based electrochemical sensors demonstrate significant cost-effectiveness compared to alternative methods, as highlighted in Table 1. With lower material costs, streamlined fabrication processes, and reduced operational expenses, MXene-based sensors emerge as economically feasible solutions for diverse sensing applications. Additionally, their inherent stability and scalability for mass production enhance long-term reliability and affordability. These findings highlights the potential of MXene-based electrochemical sensors as a cost-effective and practical choice in the field of sensor technology we also electrode recycle is another important issue. The concept of a recyclable electrode aligns with the principles of sustainability and cost-effectiveness, aiming to reduce waste and promote the efficient use of resources in the development and application of electrochemical sensors. It should be paid more attention to develop effective strategies to enhance the recyclability of MXene electrodes in electrochemical sensors. Moreover, optimizing the regeneration process and ensuring efficient restoration of the electrode electrochemical properties after each use may be an efficient approach. By integrating improved regeneration techniques and exploring eco-friendly synthesis methods, MXenes would offer immense potential for innovative advancements across various fields such as electronics, energy, environmental science, and healthcare, positioning them as transformative materials in modern technology and research landscapes.

Data availability

No primary research results, software or code has been included and no new data were generated or analysed as part of this review.

Conflicts of interest

The authors report no declarations of interest.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Key Research and Development Program of China Sino-Austrian intergovernmental industry-university-research cooperation project (Project No. 2022YFE0117000), the Fundamental Research Funds for the Central Universities (HUST: YCJJ202203008), National Natural Science Foundation of China (52171069 and 22005109), and the Open Project Program of Hubei Key Laboratory of Materials Chemistry and Service Failure (2020MCF02).

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