Ultra-small two dimensional MXene nanosheets for selective and sensitive fluorescence detection of Ag+ and Mn2+ ions

Recent Advancements in MXene-Based Biosensors for Health and Environmental Applications-A Review

Owing to their unique physicochemical properties, MXenes have emerged as promising materials for biosensing applications. This review paper comprehensively explores the recent advancements in MXene-based biosensors for health and environmental applications. This review begins with an introduction to MXenes and biosensors, outlining various types of biosensors including electrochemical, enzymatic, optical, and fluorescent-based systems. The synthesis methods and characteristics of MXenes are thoroughly discussed, highlighting the importance of these processes in tailoring MXenes for specific biosensing applications. Particular attention is given to the development of electrochemical MXene-based biosensors, which have shown remarkable sensitivity and selectivity in detecting various analytes. This review then delves into enzymatic MXene-based biosensors, exploring how the integration of MXenes with enzymes enhances sensor performance and expands the range of detectable biomarkers. Optical biosensors based on MXenes are examined, focusing on their mechanisms and applications in both healthcare and environmental monitoring. The potential of fluorescent-based MXene biosensors is also investigated, showcasing their utility in imaging and sensing applications. In addition, MXene-based potential wearable biosensors have been discussed along with the role of MXenes in volatile organic compound (VOC) detection for environmental applications. Finally, this paper concludes with a critical analysis of the current state of MXene-based biosensors and provides insights into future perspectives and challenges in this rapidly evolving field.

The potential of MXene-based materials in fluorescence-based sensing/biosensing of ionic and organic contaminants in environment and food samples: Recent advancements and challenges

MXenes belong to one of the recently developed advanced materials with tremendous potential for diverse sensing applications. To date, various types of MXene-based materials have been developed to generate direct/indirect ultrasensitive sensing signals against various forms of analytes via fluorescence quenching or enhancement. In this work, the fluorescence sensing/biosensing capabilities of the MXene-based materials have been explored and evaluated against a list of ionic/emerging pollutants in environment and food matrices. The suitability of an MXene-based sensing approach is also validated through the assessment of the performance based on the basic quality assurance parameters, e.g., limit of detection (LOD), sensing range, and response time. Accordingly, the best performing MXene-based materials are selected and recommended for the given target(s) to help facilitate their scalable applications under real-world conditions.

Ratiometric fluorescence probe based on carbon dots and p-phenylenediamine-derived nanoparticles for the sensitive detection of manganese ions

A ratiometric fluorescence probe based on carbon quantum dots with 420 nm emission (bCQDs) and a p-phenylenediamine-derived fluorescence probe with 550 nm emission (yprobe) is constructed for the detection of Mn2+. The presence of Mn2+ results in the enhanced absorption band at 400 nm of yprobe, and the fluorescence of yprobe is significantly enhanced based on the chelation-enhanced fluorescence mechanism. The fluorescence of bCQDs is then quenched based on the inner filtration effect. The ratio (I550/I420) linearly increases with the increase of Mn2+ concentration within 2.00 × 10-7-1.50 × 10-6 M, and the limit of detection is 1.76 × 10-9 M. Given the fluorescence color changing from blue to yellow, the visual sensing of Mn2+ is feasible based on bCQDs/yprobe coupled with RGB value analysis. The practicability of the proposed method has been verified in tap water, lake water, and sparkling water beverage, indicating that bCQDs/yprobe has promising application in Mn2+ monitoring.

Rapid Determination of Cr3+ and Mn2+ in Water Using Laser-Induced Breakdown Spectroscopy Combined with Filter Paper Modified with Gold Nanoclusters

Excessive emissions of heavy metals not only cause environmental pollution but also pose a direct threat to human health. Therefore, rapid and accurate detection of heavy metals in the environment is of great significance. Herein, we propose a method based on laser-induced breakdown spectroscopy (LIBS) combined with filter paper modified with bovine serum albumin-protected gold nanoclusters (LIBS-FP-AuNCs) for the rapid and sensitive detection of Cr3+ and Mn2+. The filter paper modified with AuNCs was used to selectively enrich Cr3+ and Mn2+. Combined with the multi-element detection capability of LIBS, this method achieved the simultaneous rapid detection of Cr3+ and Mn2+. Both elements showed linear ranges for concentrations of 10-1000 μg L-1, with limits of detection of 7.5 and 9.0 μg L-1 for Cr3+ and Mn2+, respectively. This method was successfully applied to the determination of Cr3+ and Mn2+ in real water samples, with satisfactory recoveries ranging from 94.6% to 105.1%. This method has potential application in the analysis of heavy metal pollution.

Advances in MXene-based luminescence sensing strategies

MXenes have attracted the attention of many researchers as one of the latest two-dimensional (2D) materials over the last decade. Their great potential for biosensing has also been fully exploited after the discovery of their unique properties such as superior optical properties, excellent hydrophilicity, good thermal stability, excellent mechanical property, high electrical conductivity, biocompatibility, large surface area, and ease of surface functionalization. In the MXene-based luminescence sensing strategy, MXenes typically appear in the form of nanosheets, quantum dots and modified MXene nanocomposites, and they are utilized as different sensing platforms or as a luminescence source. In this review, we focused on the MXene-based luminescence sensing strategies, including fluorescence, electrochemiluminescence and chemiluminescence sensors and the comparison of their performance. Finally, the perspectives of the MXene-based luminescence sensors are discussed.

Aggregates-based fluorescence sensing technology for food hazard detection: Principles, improvement strategies, and applications

Aggregates often exhibit modified or completely new properties compared with their molecular elements, making them an extraordinarily advantageous form of materials. The fluorescence signal change characteristics resulting from molecular aggregation endow aggregates with high sensitivity and broad applicability. In molecular aggregates, the photoluminescence properties at the molecular level can be annihilated or elevated, leading to aggregation-causing quenching (ACQ) or aggregation-induced emission (AIE) effects. This change in photoluminescence properties can be intelligently introduced in food hazard detection. Recognition units can combine with the aggregate-based sensor by joining the aggregation process, endowing the sensor with the high specificity of analytes (such as mycotoxins, pathogens, and complex organic molecules). In this review, aggregation mechanisms, structural characteristics of fluorescent materials (including ACQ/AIE-activated), and their applications in food hazard detection (with/without recognition units) are summarized. Because the design of aggregate-based sensors may be influenced by the properties of their components, the sensing mechanisms of different fluorescent materials were described separately. Details of fluorescent materials, including conventional organic dyes, carbon nanomaterials, quantum dots, polymers and polymer-based nanostructures and metal nanoclusters, and recognition units, such as aptamer, antibody, molecular imprinting, and host-guest recognition, are discussed. In addition, future trends of developing aggregate-based fluorescence sensing technology in monitoring food hazards are also proposed.

Perspectives of different colour-emissive nanomaterials in fluorescent ink, LEDs, cell imaging, and sensing of various analytes

In the past 2 decades, multicolour light-emissive nanomaterials have gained significant interest in chemical and biological sciences because of their unique optical properties. These materials have drawn much attention due to their unique characteristics towards various application fields. The development of novel nanomaterials has become the pinpoint for different application areas. In this review, the recent progress in the area of multicolour-emissive nanomaterials is summarized. The different emissions (white, orange, green, red, blue, and multicolour) of nanostructure materials (metal nanoclusters, quantum dots, carbon dots, and rare earth-based nanomaterials) are briefly discussed. The potential applications of different colour-emissive nanomaterials in the development of fluorescent inks, light-emitting diodes, cell imaging, and sensing devices are briefly summarized. Finally, the future perspectives of multicolour-emissive nanomaterials are discussed.

Recent trends in MXene-based material for biomedical applications

MXene is a magical class of 2D nanomaterials and emerging in many applications in diverse fields. Due to the multiple advantageous characteristics of its fundamental components, such as structural, physicochemical, optical, and occasionally even biological characteristics. However, it is limited in the biomedical industry due to poor physiological stability, decomposition rate, and lack of controlled and sustained drug release. These limitations can be overcome when MXene forms composites with other 2D materials. The efficiency of pure MXene in biomedicine is inferior to that of MXene-based composites. The availability of functionality on the exterior part of MXene has a key role in the modification of their surface and their characteristics. This review provides an extensive discussion on the synthesizing of MXene and the role of the surface functionalities on the efficiency of MXene. In addition, a detailed discussion of the biomedical applications of MXene, including antibacterial activity, regenerative medicine, CT scan capability, drug delivery, diagnostics, MRI and biosensing capability. Furthermore, an outline of the future problems and challenges of MXene-based materials for biomedical applications was narrated. Thus, these salient features showcase the potential of MXene-based material and will be a breakthrough in biomedical applications in the near future.

A novel fluorescent nanoprobe based on potassium permanganate-functionalized Ti3C2 QDs for the unique "turn-on" dual detection of Cr3+ and Hg2+ ions

Titanium carbide quantum dots (Ti₃C₂ QDs) were synthesized by ammonia-assisted hydrothermal method. We also synthesized potassium permanganate (KMnO₄)-functionalized Ti₃C₂ QDs (Mn-QDs) by modifying Ti₃C₂ nanosheets with KMnO₄ and then cutting the functional nanosheets into Mn-QDs. The Ti₃C₂ QDs and Mn-QDs were characterized by fluorescence spectroscopy (FL), Fourier transform infrared spectroscopy (FTIR), UV-vis spectrophotometry (UV-vis), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Furthermore, the modified Mn-QDs have strong luminescence ability and good dispersion stability, which can be used for Cr³⁺ and Hg²⁺ double ion detection with enhanced fluorescence specificity. Cr³⁺/Hg²⁺ and negatively charged Mn-QDs are bound together by electrostatic interactions. Meanwhile, the surface of Mn-QDs is rich in functional groups, which interacts with Cr³⁺/Hg²⁺ to modify the surface traps, leading to defect passivation and exhibiting photoluminescence enhancement. For the dynamic quenching produced by the interaction of Mn-QDs with Hg²⁺ within 50 μM, it may be caused by the complex formation of Hg²⁺ trapped by the amino group on the surface of Mn-QDs. The detection limits for Cr³⁺ and Hg²⁺ were 0.80 μM and 0.16 μM, respectively. The recoveries of Cr³⁺ and Hg²⁺ ions in real water samples were 93.79-105.10% and 93.91-102.05%, respectively, by standard addition recovery test. In this work, the application of Mn-QDs in Cr³⁺ and Hg²⁺ ion detection was researched, which opens a new way for its application in the field of detecting heavy metal ions.

Emergence of MXene and MXene-Polymer Hybrid Membranes as Future- Environmental Remediation Strategies

The continuous deterioration of the environment due to extensive industrialization and urbanization has raised the requirement to devise high-performance environmental remediation technologies. Membrane technologies, primarily based on conventional polymers, are the most commercialized air, water, solid, and radiation-based environmental remediation strategies. Low stability at high temperatures, swelling in organic contaminants, and poor selectivity are the fundamental issues associated with polymeric membranes restricting their scalable viability. Polymer-metal-carbides and nitrides (MXenes) hybrid membranes possess remarkable physicochemical attributes, including strong mechanical endurance, high mechanical flexibility, superior adsorptive behavior, and selective permeability, due to multi-interactions between polymers and MXene's surface functionalities. This review articulates the state-of-the-art MXene-polymer hybrid membranes, emphasizing its fabrication routes, enhanced physicochemical properties, and improved adsorptive behavior. It comprehensively summarizes the utilization of MXene-polymer hybrid membranes for environmental remediation applications, including water purification, desalination, ion-separation, gas separation and detection, containment adsorption, and electromagnetic and nuclear radiation shielding. Furthermore, the review highlights the associated bottlenecks of MXene-Polymer hybrid-membranes and its possible alternate solutions to meet industrial requirements. Discussed are opportunities and prospects related to MXene-polymer membrane to devise intelligent and next-generation environmental remediation strategies with the integration of modern age technologies of internet-of-things, artificial intelligence, machine-learning, 5G-communication and cloud-computing are elucidated.

MXene catalytic amplification-fluorescence/absorption dimode aptamer sensor for the detection of trace Pb2+ in milk

Lead ion (Pb²⁺) is a toxic heavy metal, which is very harmful to organisms. Therefore, the establishment of a rapid, simple, and sensitive method is of great significance to food safety and human health. It was found that MXeneTi₃C₂ nanosheet (NS) has a strong catalytic effect on the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) via H₂O₂ to form the oxidized product (TMB_OX); it has a strong fluorescence peak at 415 nm and an absorption (Abs) peak at 295 nm. The aptamer of Pb²⁺ (Apt_pb) can be adsorbed on the surface of an NS to form MXene-Apt conjugates, which reduces its catalytic active sites and inhibits its catalytic activity. When the target Pb²⁺ is added, it specifically binds with Apt_pb to release MXene NSs to enhance the dimode signals. Therefore, a new MXene catalytic fluorescence/absorption dimode aptamer biosenering platform was fabricated for the determination of trace Pb²⁺ in milk and water samples, with the fluorescence assay linear range (LR) of 5.0 × 10⁻²-2.0 nmol/L.

New Horizons for MXenes in Biosensing Applications

Over the last few decades, biosensors have made significant advances in detecting non-invasive biomarkers of disease-related body fluid substances with high sensitivity, high accuracy, low cost and ease in operation. Among various two-dimensional (2D) materials, MXenes have attracted widespread interest due to their unique surface properties, as well as mechanical, optical, electrical and biocompatible properties, and have been applied in various fields, particularly in the preparation of biosensors, which play a critical role. Here, we systematically introduce the application of MXenes in electrochemical, optical and other bioanalytical methods in recent years. Finally, we summarise and discuss problems in the field of biosensing and possible future directions of MXenes. We hope to provide an outlook on MXenes applications in biosensing and to stimulate broader interests and research in MXenes across different disciplines.

A Review on MXene Synthesis, Stability, and Photocatalytic Applications

Photocatalytic water splitting, CO₂ reduction, and pollutant degradation have emerged as promising strategies to remedy the existing environmental and energy crises. However, grafting of expensive and less abundant noble-metal cocatalysts on photocatalyst materials is a mandatory practice to achieve enhanced photocatalytic performance owing to the ability of the cocatalysts to extract electrons efficiently from the photocatalyst and enable rapid/enhanced catalytic reaction. Hence, developing highly efficient, inexpensive, and noble-metal-free cocatalysts composed of earth-abundant elements is considered as a noteworthy step toward considering photocatalysis as a more economical strategy. Recently, MXenes (two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides) have shown huge potential as alternatives for noble-metal cocatalysts. MXenes have several excellent properties, including atomically thin 2D morphology, metallic electrical conductivity, hydrophilic surface, and high specific surface area. In addition, they exhibit Gibbs free energy of intermediate H atom adsorption as close to zero and less than that of a commercial Pt-based cocatalyst, a Fermi level position above the H₂ generation potential, and an excellent ability to capture and activate CO₂ molecules. Therefore, there is a growing interest in MXene-based photocatalyst materials for various photocatalytic events. In this review, we focus on the recent advances in the synthesis of MXenes with 2D and 0D morphologies, the stability of MXenes, and MXene-based photocatalysts for H₂ evolution, CO₂ reduction, and pollutant degradation. The existing challenges and the possible future directions to enhance the photocatalytic performance of MXene-based photocatalysts are also discussed.

Analytical methods to determine and sense heavy metal pollutants using MXene and MXene-based composites: Mechanistic prophecy into sensing properties

The presence of heavy metal ions in the biosphere is of grave concern, as these are toxic and impact living organisms. Lack of pure drinking water can spread waterborne diseases like cholera, dysentery, typhoid, etc. The presence of heavy metals like arsenic and radioactive materials can cause cancer. The detection and removal of these heavy metals are important for sustaining life. Herein, MXene comes to the rescue as a crucial and potential material, which can sense and adsorb heavy metal ions. Developed in 2011, MXenes are an emerging class of 2D nanomaterials that are appropriate substitutes for existing heavy metal ions sensing materials and have shown excellent efficiency due to their better hydrophilicity, capacity of transportation of electrons, functionalization, and a great variety in compositions as compared to the other nanomaterials properties. This work gives an insight into the chemistry and synthesis of MXenes for further utilization as a sensor in heavy metal ions toxicity and underlines the key future challenges to knowing the full prospective of MXenes in environmental systems.

An overview of MXene-Based nanomaterials and their potential applications towards hazardous pollutant adsorption

With the massive development of industrialization, multiple ecological contaminants in gaseous, liquid, and solid forms are vented into habitats, which is currently at the forefront of worldwide attention. Because of the possible damage to public health and eco-diversity, high-efficiency clearance of these environmental contaminants is a serious concern. Improved nanomaterials (NMs) could perform a significant part in the exclusion of contaminants from the atmosphere. MXenes, a class of two-dimensional (2D) compounds that have got tremendous consideration from researchers for a broad array of applications in a variety of industries and are viewed as a potential route for innovative solutions to identify and prevent a variety of obstreperous hazardous pollutants from environmental compartments due to their exceptional innate physicochemical and mechanical features, including high specific surface area, physiological interoperability, sturdy electrodynamics, and elevated wettability. This paper discusses the recent progress in MXene-based nanomaterials' applications such as environmental remediation, with a focus on their adsorption-reduction characteristics. The removal of heavy metals, dyes, and radionuclides by MXenes and MXene-based nanomaterials is depicted in detail, with the adsorption mechanism and regeneration potential highlighted. Finally, suggestions for future research are provided to ensure that MXenes and MXene-based nanomaterials are synthesized and applied more effectively.

Synthesis, Toxicity Assessment, Environmental and Biomedical Applications of MXenes: A Review

MXenes are a family of two-dimensional (2D) composite materials based on transition metal carbides, nitrides and carbonitrides that have been attracting attention since 2011. Combination of electrical and mechanical properties with hydrophilicity makes them promising materials for biomedical applications. This review briefly discusses methods for the synthesis of MXenes, their potential applications in medicine, ranging from sensors and antibacterial agents to targeted drug delivery, cancer photo/chemotherapy, tissue engineering, bioimaging, and environmental applications such as sensors and adsorbents. We focus on in vitro and in vivo toxicity and possible mechanisms. We discuss the toxicity analogies of MXenes and other 2D materials such as graphene, mentioning the greater biocompatibility of MXenes. We identify existing barriers that hinder the formation of objective knowledge about the toxicity of MXenes. The most important of these barriers are the differences in the methods of synthesis of MXenes, their composition and structure, including the level of oxidation, the number of layers and flake size; functionalization, test concentrations, duration of exposure, and individual characteristics of biological test objects Finally, we discuss key areas for further research that need to involve new methods of nanotoxicology, including predictive computational methods. Such studies will bring closer the prospect of widespread industrial production and safe use of MXene-based products.

Fluorescent-based nanosensors for selective detection of a wide range of biological macromolecules: A comprehensive review

Thanks to their unique attributes, such as good sensitivity, selectivity, high surface-to-volume ratio, and versatile optical and electronic properties, fluorescent-based bioprobes have been used to create highly sensitive nanobiosensors to detect various biological and chemical agents. These sensors are superior to other analytical instrumentation techniques like gas chromatography, high-performance liquid chromatography, and capillary electrophoresis for being biodegradable, eco-friendly, and more economical, operational, and cost-effective. Moreover, several reports have also highlighted their application in the early detection of biomarkers associated with drug-induced organ damage such as liver, kidney, or lungs. In the present work, we comprehensively overviewed the electrochemical sensors that employ nanomaterials (nanoparticles/colloids or quantum dots, carbon dots, or nanoscaled metal-organic frameworks, etc.) to detect a variety of biological macromolecules based on fluorescent emission spectra. In addition, the most important mechanisms and methods to sense amino acids, protein, peptides, enzymes, carbohydrates, neurotransmitters, nucleic acids, vitamins, ions, metals, and electrolytes, blood gases, drugs (i.e., anti-inflammatory agents and antibiotics), toxins, alkaloids, antioxidants, cancer biomarkers, urinary metabolites (i.e., urea, uric acid, and creatinine), and pathogenic microorganisms were outlined and compared in terms of their selectivity and sensitivity. Altogether, the small dimensions and capability of these nanosensors for sensitive, label-free, real-time sensing of chemical, biological, and pharmaceutical agents could be used in array-based screening and in-vitro or in-vivo diagnostics. Although fluorescent nanoprobes are widely applied in determining biological macromolecules, unfortunately, they present many challenges and limitations. Efforts must be made to minimize such limitations in utilizing such nanobiosensors with an emphasis on their commercial developments. We believe that the current review can foster the wider incorporation of nanomedicine and will be of particular interest to researchers working on fluorescence technology, material chemistry, coordination polymers, and related research areas.

Low Temperature Step Annealing Synthesis of the Ti2AlN MAX Phase to Fabricate MXene Quantum Dots

We present the synthesis of the Ti2AlN MAX phase using two-step annealing at temperatures of 600 °C and 1100 °C, the lowest synthesis temperatures reported so far. After the successful synthesis of the Ti2AlN MAX phase, two-dimensional Ti2N MXene was prepared through wet chemical etching and further fragmented into light emitting MXene quantum dots (MQDs) with a size of 3.2 nm by hydrothermal method. Our MQDs displayed a 6.9% quantum yield at a 310 nm wavelength of excitation, suggesting promising nanophotonic applications.

Bovine serum albumin functionalized blue emitting Ti3 C2 MXene Quantum Dots as a sensitive fluorescence probe for Fe3+ ions detection and its toxicity analysis

In the present work, an improved class of protein functionalized fluorescent 2D Ti₃ C₂ MXene quantum dots (MXene QDs) was prepared by hydrothermal method. Exfoliated 2D Ti₃ C₂ sheets is used as the starting precursor and transport protein bovine serum albumin (BSA) is used to functionalize the MXene QDs. BSA functionalized MXene QDs exhibited excellent photophysical property and stability at various physiological parameters. The HR-TEM analysis showed that the BSA@MXene QDs are quasi-spherical in shape with a size of ~2 nm. The fluorescence intensity of BSA@MXene QDs was selectively quenched in the presence of Fe³⁺ ions. The mechanism of fluorescence quenching was further substantiated using time resolved fluorescence and Stern-Volmer analysis. The sensing assay showed a linear response within the concentration range of 0-150 μM of Fe³⁺ ions with excellent limit of detection. BSA@MXene QDs probe showed good selectivity toward ferric ions even in the presence of other potential interferences. The practical applicability of BSA@MXene QDs was further tested in real samples for Fe³⁺ ions quantification and the sensor had good recovery rates. The cytotoxicity of the BSA@MXene QDs toward the human glioblastoma cells assay revealed that BSA@MXene QDs are biocompatible at lower doses possesses significant cytotoxicity.

MXenes in photomedicine: Advances and prospects

MXenes and their related nanocomposites with superior physicochemical properties such as high surface area, ease of synthesis and functionalization, high drug loading capacity, collective therapy potentials, pH-triggered drug release behavior,...

A Simple Approach to MXene Micropatterning from Molecularly Driven Assembly

Here, a micropatterning strategy is demonstrated to achieve stable and selective MXene adsorption through the molecularly driven assembly. MXene flakes were assembled by strong interaction with a silicon substrate, which was functionalized by microcontact printing (μCP) to create an active surface. A clear micropattern was observed by scanning electron microscopy showing uniform coverage of MXene flakes. Atomic force microscopy revealed a pattern thickness of around 50 nm, much thinner than the patterns obtained by direct μCP. The obtained micropattern presents good stability against rinsing and sonication. X-ray photoelectron spectroscopy shows that this stability can be attributed to strong covalent bonding between MXene and active molecules on a silicon substrate. The sheet resistance of the as-formed MXene layer was measured at around 154.67 (Ω/□), which is lower than those of other published techniques with a similar thickness of around 50 nm. This method can achieve a well-defined MXene pattern around the sub-100 μm scale without requiring prior MXene surface modification. Therefore, MXene can retain its intrinsic surface property, allowing further molecule adsorption as a sensing platform. Moreover, this patterning technique does not require complicated control of ink preparation and offers possible application on a substrate of any geometry with few layers of thickness.

Electrochemical and optical biosensors based on multifunctional MXene nanoplatforms: Progress and prospects

Two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides (MXene) have emerged as a rising family of atomic layered nanomaterials which undergoes intensive investigations in interdisciplinary applications. The large surface-to-volume ratio, excellent mechanical strength, desirable biocompatibility, along with tunable electronic and optical properties, render 2D MXenes exceptional attractive as versatile nanoplatforms for biosensing. Herein, advanced progress and novel paradigms of MXene-based biosensors are reviewed, focusing on the combination of MXenes with various detection techniques that promotes target recognition and signal transducing. Regarding the nature of transducing signals, MXene-based biosensors are categorized into two groups where MXenes serve as electrical platforms or optical platforms, respectively. The merits of MXenes are critically compared with other 2D materials to illustrate the distinctive advantages of MXenes in biosensing, while challenges such as environmental vulnerability was discussed to guide the sensor design. Facing with the rapid development of wearable electronics and internet of medical things, as well as escalating demanding in precision medicine, perspectives are provided to elucidate the potential of MXenes in propelling advances in these trending biomedical applications.

Sensing with MXenes: Progress and Prospects

Various fields of study consider MXene a revolutionary 2D material. Particularly in the field of sensors, the metal-like high electrical conductivity and large surface area of MXenes are desirable characteristics as an alternative sensor material that can transcend the boundaries of existing sensor technology. This critical review provides a comprehensive overview of recent advances in MXene-based sensor technology and a roadmap for commercializing MXene-based sensors. The existing sensors are systematically categorized as chemical, biological, and physical sensors. Each category is then classified into various subcategories depending on the electrical, electrochemical, structural, or optical sensing mechanism, which are the four fundamental working mechanisms of sensors. Representative structural and electrical approaches for boosting the performance of each category are presented. Finally, factors that hinder commercializing MXene-based sensors are discussed, and several breakthroughs in realizing commercially available MXene-based sensors are suggested. This review provides broad insights pertaining to previous and existing MXene-based sensor technology and perspectives on the future generation of low-cost, high-performance, and multimodal sensors for soft-electronics applications.

Phenanthroline Derivative Fluorescent Probe for Rapid and Sensitive Detection of Silver(I)

In the present work, a phenanthroline derivative (2-(2-methoxyphenyl)-4-phenyl-1,10-phenanthroline, MPP), as a fluorescent probe, was synthesized to realize a rapid, simple and sensitive detection of silver(I). The detection conditions of Ag⁺ were optimized. This fluorescent probe has the advantages of a fast reaction time, a wide pH applicable range, and a low detection limit, exhibiting a good linear response between the fluorescence intensity and the concentration in the range of 0.05 - 1.5 μmol/L for Ag⁺. The detection limit is as low as 3.38 × 10^-8 mol/L (S/N = 3). This probe had been used to detect Ag⁺ in real samples, and the recovery efficiency of spiked Ag⁺ had been also tested. The recovery efficiency is satisfactory, ranging from 92.0 to 105.4%. Therefore, this fluorescent probe should provide a new choice for the quantitative detection of silver ions in environmental water samples.

Label-Free, Fast Response, and Simply Operated Silver Ion Detection with a Ti3C2Tx MXene Field-Effect Transistor

Silver (Ag) is a widely used heavy metal, and its oxidation state (Ag⁺) causes serious harm to organisms after bioaccumulation and biomagnification, posing urgent demand for the rapid, efficient, and simply operated Ag⁺ detection techniques. In this work, a fast, portable, and label-free Ag⁺ detection sensor based on a Ti₃C₂T_x MXene field-effect transistor (FET) is reported. The Ti₃C₂T_x MXene works as the sensing element in the FET sensor, which shows excellent sensing performance, i.e., fast response (few seconds) and good sensitivity and selectivity to Ag⁺ without any detection label or probe. Utilizing the visual photograph, transmission electron microscopy image, and Ag elemental mapping analysis, the sensing mechanism of the label-free Ti₃C₂T_x MXene FET sensor is demonstrated to be the in situ reduction of Ag⁺ and the formation of Ag nanoparticles (AgNPs). Moreover, Ag⁺ detection in real samples shows that the proposed FET devices have satisfactory sensing capability for Ag⁺ in tap water and river water. This study puts forward a novel FET strategy for Ag⁺ detection in aqueous systems, which is of essential and inspiring meaning for motivating the potential applications of MXene-based sensor devices in analytical applications and the realization of on-site environmental monitoring.

Potential environmental applications of MXenes: A critical review

Various environmental pollutants (e.g., air, water and solid pollutants) are discharged into environments with the rapid development of industrializations, which is presently at the forefront of global attention. The high efficient removal of these environmental pollutants is of important concern due to their potential threat to human health and eco-diversity. Advanced nanomaterials may play an important role in the elimination of pollutants from environmental media. MXenes as the new intriguing class of graphene-like 2D transition metal carbides and/or carbonitrides have been widely used in energy storage, environmental remediation benefitting from exceptional structural properties such as highly active sites, high chemical stability, hydrophilicity, large interlayer spacing, huge specific surface area, superior sorption-reduction capacity. However, the comprehensive investigation concerning the removal of various environmental pollutants on MXenes is yet not available up to date. In this review, we summarized the synthesis and properties of MXenes to demonstrate the key roles in ameliorating their adsorption performance; then the recent advances and achievements in environmental application of MXenes on the removal of gases, organics, heavy metals and radionuclides were comprehensively reviewed in details; Finally, the formidable challenges and further perspectives regarding utilizing MXene in environmental remediation were proposed. Hopefully, this review can provide the useful information for environmental scientists and material engineers on designing versatile MXenes in actual environmental applications.

Biomimetic two-dimensional nanozymes: synthesis, hybridization, functional tailoring, and biosensor applications

Biological enzymes play important roles in mediating the biological reactions in vitro and in vivo due to their high catalytic activity, strong bioactivity, and high specificity; however, they have also some disadvantages such as high cost, low environmental stability, weak reusability, and difficult production. To overcome these shortcomings, functional nanomaterials including metallic nanoparticles, single atoms, metal oxides, alloys, and others have been utilized as nanozymes to mimic the properties and functions of natural enzymes. Due to the development of the synthesis and applications of two-dimensional (2D) materials, 2D nanomaterials have shown high potential to be used as novel nanozymes in biosensing, bioimaging, therapy, logic gates, and environmental remediation due to their unique physical, chemical, biological, and electronic properties. In this work, we summarize recent advances in the preparation and functionalization, as well as biosensor and immunoassay applications of various 2D material-based nanozymes. To achieve this aim, first we demonstrate the preparation strategies of 2D nanozymes such as chemical reduction, templated synthesis, chemical exfoliation, calcination, electrochemical deposition, hydrothermal synthesis, and many others. Meanwhile, the structure and properties of the 2D nanozymes prepared by conjugating 2D materials with nanoparticles, metal oxides, biomolecules, polymers, ions, and 2D heteromaterials are introduced and discussed in detail. Then, the applications of the prepared 2D nanozymes in colorimetric, electrochemical, fluorescent, and electrochemiluminescent sensors are demonstrated.

Ti3C2TX MXene for Sensing Applications: Recent Progress, Design Principles, and Future Perspectives

Sensors are becoming increasingly significant in our daily life because of the rapid development in electronic and information technologies, including Internet of Things, wearable electronics, home automation, intelligent industry, etc. There is no doubt that their performances are primarily determined by the sensing materials. Among all potential candidates, layered nanomaterials with two-dimensional (2D) planar structure have numerous superior properties to their bulk counterparts which are suitable for building various high-performance sensors. As an emerging 2D material, MXenes possess several advantageous features of adjustable surface properties, tunable bandgap, and excellent mechanical strength, making them attractive in various applications. Herein, we particularly focus on the recent research progress in MXene-based sensors, discuss the merits of MXenes and their derivatives as sensing materials for collecting various signals, and try to elucidate the design principles and working mechanisms of the corresponding MXene-based sensors, including strain/stress sensors, gas sensors, electrochemical sensors, optical sensors, and humidity sensors. In the end, we analyze the main challenges and future outlook of MXene-based materials in sensor applications.

Preparation and properties of a dual-function cellulose nanofiber-based bionic biosensor for detecting silver ions and acetylcholinesterase

A dual-function cellulose nanofiber (CNF)-based bionic biosensor with good biocompatibility was developed for detecting Ag⁺ and acetylcholinesterase (AChE) by grafting deoxyribonucleic acid (DNA) onto CNF. The Ag⁺ ions captured by the biosensor acted as recognition sites for the detection of AChE. The CNF-based bionic biosensor (CNF-DNA) could detect Ag⁺ concentrations as low as 10^-6 nM in the presence of interference metal ions (Hg²⁺, Ba²⁺, Cd²⁺, Mg²⁺, Mn²⁺, Pb²⁺, and Zn²⁺). DNA-template silver nanoclusters (DNA-AgNCs) were formed on the surface of CNF-DNA during the detection of Ag⁺ (CNF-DNA-AgNCs). This new strategy yielded CNF-DNA-AgNCs through the adsorption of Ag⁺ ions onto the cytosine base of the single-stranded DNA in CNF-DNA without the use of any additional reducer. Meanwhile, the CNF-DNA-AgNCs exhibited excellent sensitivity and selectivity for trace levels (0.053 mU/mL) of AChE in the presence of interference reagents. The novel strategy proposed in this paper may establish a foundation for further research on DNA-template AgNCs for developing biosensors and biomarkers for in vivo and in vitro detection.

Recent Advances in MXene Nanocomposite-Based Biosensors

The development of advanced biosensors with high sensitivity and selectivity is one of the most demanded concerns in the field of biosensors. To meet this requirement, up until now, numerous nanomaterials have been introduced to develop biosensors for achieving high sensitivity and selectivity. Among the latest nanomaterials attracting attention, MXene is one of the best materials for the development of biosensors because of its various superior properties. MXenes are two-dimensional inorganic compounds with few atomic layers that possess excellent characteristics including high conductivity and superior fluorescent, optical, and plasmonic properties. In this review, advanced biosensors developed on the basis of the MXene nanocomposite are discussed with the selective overview of recently reported studies. For this, introduction of the MXene including the definition, synthesis methods, and its properties are discussed. Next, MXene-based electrochemical biosensors and MXene-based fluorescent/optical biosensors are provided, which are developed on the basis of the exceptional properties of the MXene nanocomposite. This review will suggest the direction for use of the Mxene nanocomposite to develop advanced biosensors with high sensitivity and selectivity.

A highly sensitive piezoresistive sensor based on MXenes and polyvinyl butyral with a wide detection limit and low power consumption

As a new class of two-dimensional transition-metal carbides and carbonitrides, MXenes have been widely used in energy storage, sensing, catalysis, electromagnetic interference shielding and other fields. It is a challenge to simultaneously realize a sensor with extremely high sensitivity, wide detection limits, low power consumption and good mechanical stability. In this work, taking advantage of the high conductivity of MXenes and the porous structure of polyvinyl butyral, a highly sensitive piezoresistive sensor was fabricated. The fabricated MXene/PVB-based sensor exhibits high sensitivity and reliability with a factor of ∼11.9 kPa^-1, ∼1.15 kPa^-1 and ∼0.20 kPa^-1 in the ranges of 31.2 Pa-312 Pa, 312 Pa-62.4 kPa and 62.4 kPa-1248.4 kPa, respectively. The sensor has a wide detection range (∼31.2 Pa to ∼2.205 MPa), low detection limit (6.8 Pa), low detection voltage (0.1 mV), low power consumption (∼3.6 × 10^-10 W), fast response time (∼110 ms) and good mechanical stability (over 10 000 maximum-pressure cycles). Moreover, it is demonstrated that the sensor can detect subtle bending and release activities of humans, including arterial pulses and voice signals, which makes it potentially suitable to be used as a wide detection range, highly sensitive and low power consumption piezoresistive sensor. This work provides a new avenue to expand the application of MXene-based flexible pressure sensors with a wide sensing range and ultra-low power consumption.

MXenes: Are they emerging materials for analytical chemistry applications? - A review

MXenes are an emerging class of 2D materials that exhibit unique properties of high conductivity and hydrophilicity. They can be easily functionalized with other materials due to the abundance of surface terminated functionalities. The versatile chemistry of MXenes allows fine-tuning their properties for different analytical chemistry applications such as electrochemical and optical sensing. MXenes may also be useful adsorbents for analytical extractions due to their exceptional surface chemistry, high surface areas, and ease of functionalization as per the nature of the target compounds. The features of the MXenes that can make them excellent materials for analytical applications are listed and critically appraised. The emerging applications of MXenes in electrochemical and optical sensing are discussed with the pertinent examples. The potential of MXene-based sorbents for analytical extractions is highlighted based on the current literature that describes their applications in adsorptive removal and environmental remediation. In the end, limitations, challenges, and future opportunities are briefly presented.

Simple hydrothermal approach for synthesis of fluorescent molybdenum disulfide quantum dots: Sensing of Cr3+ ion and cellular imaging

Nowadays, fluorescent molybdenum disulfide quantum dots (MoS₂ QDs) have proven to be potential candidates in the sensing and bioimaging areas owing to their exceptional intrinsic characteristics. Here, a simple hydrothermal strategy was explored for the preparation of MoS₂ QDs using ammonium heptamolybdate and 6-mercaptopurine (6-MP) as precursors. The emission peak of MoS₂ QDs was significantly quenched in the presence Cr³⁺ ion due to the selective surface chemistry on the surfaces of MoS₂ QDs. The designed fluorescent MoS₂ QDs showed a linear fluorescence quenching response with increasing concentration of Cr³⁺ ion (0.1-10 μM), allowing to detect Cr³⁺ ion even at 0.08 μM. This fluorescent MoS₂ QDs were utilized for the quantification of Cr³⁺ ion in real samples (water and biological samples). Interestingly, the synthesized MoS₂ QDs exhibited negligible cytotoxicity on NRK cells and acted as good candidates for imaging of Trichoderma viride fungal cells.

Application of Chemically Exfoliated Boron Nitride Nanosheets Doped with Co to Remove Organic Pollutants Rapidly from Textile Water

Two-dimensional layered materials doped with transition metals exhibit enhanced magnetization and improved catalytic stability during water treatment leading to potential environmental applications across several industrial sectors. In the present study, cobalt (Co)-doped boron nitride nanosheets (BN-NS) were explored for such an application. Chemical exfoliation process was used to exfoliate BN-NS and the hydrothermal route was adopted to incorporate Co dopant in various concentrations (e.g., 2.5, 5, 7.5, and 10 wt%). X-ray diffraction (XRD) study indicated that crystallinity improved upon doping with the formation of a hexagonal phase of the synthesized material. Selected area electron diffraction (SAED) confirmed enhanced crystallinity, which corroborates XRD results. Interlayer spacing was evaluated through a high-resolution transmission electron microscope (HR-TEM) equipped with Gatan digital micrograph software. Compositional and functional group analysis was undertaken with energy dispersive X-ray (EDS) and Fourier transform infrared (FTIR) spectroscopy, respectively. Field emission scanning electron microscope (FE-SEM) and HR-TEM were utilized to probe surface morphologies of prepared samples. Bonding modes in the sample were identified through Raman analysis. Optical properties were examined using UV-vis spectroscopy. Photoluminescence spectra were acquired to estimate the separation and recombination of excitons. Magnetic properties were studied by means of hysteresis loop acquired using VSM measurements. Methylene blue dye was degraded with as-prepared host and doped nanosheets used as catalysts and investigated through absorption spectra ranging from 250 to 800 nm. The experimental results of this study indicate that Co-doped BN-NS showed enhanced magnetic properties and can be used to degrade dyes present as an effluent in industrial wastewater.