Fluoride-Free 2D Niobium Carbide MXenes as Stable and Biocompatible Nanoplatforms for Electrochemical Biosensors with Ultrahigh Sensitivity

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.

Recent Advances in Non-Ti MXenes: Synthesis, Properties, and Novel Applications

One of the most fascinating 2D nanomaterials (NMs) ever found is various members of MXene family. Among them, the titanium-based MXenes, with more than 70% of publication-related investigations, are comparatively well studied, producing fundamental foundation for the 2D MXene family members with flexible properties, familiar with a variety of advanced novel technological applications. Nonetheless, there are still more candidates among transitional metals (TMs) that can function as MXene NMs in ways that go well beyond those that are now recognized. Systematized details of the preparations, characteristics, limitations, significant discoveries, and uses of the novel M-based MXenes (M-MXenes), where M stands for non-Ti TMs (M = Sc, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, and Lu), are given. The exceptional qualities of the 2D non-Ti MXene outperform standard Ti-MXene in several applications. There is many advancement in top-down as well as bottom-up production of MXenes family members, which allows for exact control of the M-characteristics MXene NMs to contain cutting-edge applications. This study offers a systematic evaluation of existing research, covering everything in producing complex M-MXenes from primary limitations to the characterization and selection of their applications in accordance with their novel features. The development of double metal combinations, extension of additional metal candidates beyond group-(III-VI)B family, and subsequent development of the 2D TM carbide/TMs nitride/TM carbonitrides to 2D metal boride family are also included in this overview. The possibilities and further recommendations for the way of non-Ti MXene NMs are in the synthesis of NMs will discuss in detail in this critical evaluation.

Research Progress on Ammonia Sensors Based on Ti3C2Tx MXene at Room Temperature: A Review

Ammonia (NH3) potentially harms human health, the ecosystem, industrial and agricultural production, and other fields. Therefore, the detection of NH3 has broad prospects and important significance. Ti3C2Tx is a common MXene material that is great for detecting NH3 at room temperature because it has a two-dimensional layered structure, a large specific surface area, is easy to functionalize on the surface, is sensitive to gases at room temperature, and is very selective for NH3. This review provides a detailed description of the preparation process as well as recent advances in the development of gas-sensing materials based on Ti3C2Tx MXene for room-temperature NH3 detection. It also analyzes the advantages and disadvantages of various preparation and synthesis methods for Ti3C2Tx MXene's performance. Since the gas-sensitive performance of pure Ti3C2Tx MXene regarding NH3 can be further improved, this review discusses additional composite materials, including metal oxides, conductive polymers, and two-dimensional materials that can be used to improve the sensitivity of pure Ti3C2Tx MXene to NH3. Furthermore, the present state of research on the NH3 sensitivity mechanism of Ti3C2Tx MXene-based sensors is summarized in this study. Finally, this paper analyzes the challenges and future prospects of Ti3C2Tx MXene-based gas-sensitive materials for room-temperature NH3 detection.

Fluorine-Free MXenes: Recent Advances, Synthesis Strategies, and Mechanisms

MXenes, an exceptional class of 2D materials, possess high conductivity, adaptable surface chemistry, mechanical strength, and tunable bandgaps, making them attractive for diverse applications. Unlocking the potential of MXenes requires precise control over synthesis methods and surface functionality. Conventionally, fluorine-based etchants are used in MXenes synthesis, posing both environmental concerns and alterations to surface properties, along with the introduction of certain defects. This prompts the exploration of innovative fluorine-free strategies for MXenes synthesis. This review focuses on environmentally friendly, fluorine-free techniques for MXene synthesis, emphasizing mechanisms and recent breakthroughs in alternative etching strategies. The comprehensive coverage includes electrochemical etching, Lewis acid-driven molten salt etching, alkaline/hydrothermal techniques, chemical vapor deposition (CVD), and recent innovative methods. Fluorine-free MXenes synthesis yields terminations such as ─O, ─OH, ─Cl, etc., influencing surface chemistry and improving their properties. The presence of ─OH groups in NaOH etched MXenes boosts their energy storage, while ─Cl functionality from Lewis acidic salts optimizes electrochemical performance. Fluorine-free methods mitigate adverse effects of ─F terminations on MXene conductivity, improving electronic properties and broadening their applications. In addition to traditional approaches, this review delves into novel fluorine-free methods for tailoring MXenes properties. It comprehensively addresses challenges, opportunities, and future perspectives in fluorine-free MXenes.

2D MXenes and their composites; design, synthesis, and environmental sensing applications

Novel 2D layered MXene materials were first reported in 2011 at Drexel University. MXenes are widely used in multidisciplinary applications due to their anomalous electrical conductivity, high surface area, and chemical, mechanical, and physical properties. This review summarises MXene synthesis and applications in environmental sensing. The first section describes different methods for MXene synthesis, including fluorinated and non-fluorinated methods. MXene's layered structure, surface terminal groups, and the space between layers significantly impact its properties. Different methods to separate different MXene layers are also discussed using various intercalation reagents and commercially synthesized MXene without compromising the environment. This review also explains the effect of MXene's surface functionalization on its characteristics. The second section of the review describes gas and pesticide sensing applications of Mxenes and its composites. Its good conductivity, surface functionalization with negatively charged groups, intrinsic chemical nature, and good mechanical stability make it a prominent material for room temperature sensing of environmental samples, such as polar and nonpolar gases, volatile organic compounds, and pesticides. This review will enhance the young scientists' knowledge of MXene-based materials and stimulate their diversity and hybrid conformation in environmental sensing applications.

Cellular uptake and cytotoxicity of PEGylated MXene nanomaterials mediated by protein corona

A stringent analysis of the biocompatibility of MXene is a necessary condition for assessing the biological risk of MXene. Owing to high surface free energy, MXene is capable of adsorbing a large amount of blood proteins to form MXene-protein corona complexes, however, a comprehensive understanding of the relationship between MXene and cellular physiological systems remains limited. Therefore, we investigated the cellular uptake and cytotoxicity effect of MXene Ti3C2Tx and PEGylation Ti3C2Tx mediated by human serum protein corona in THP-1 cells. It was found that PEGylation can alter the interaction between Ti3C2Tx and serum proteins, inducing a significant transformation in the fingerprint of the protein corona. Following protein corona formation, both Ti3C2Tx and PEGylated Ti3C2Tx predominantly accumulated at lysosomal sites within THP-1 cells. Further analysis revealed that clathrin-mediated endocytosis was the primary mechanism of Ti3C2Tx internalization by THP-1 cells. There was no significant effect on cell viability. However, we found that Ti3C2Tx plays a dual role as both a stimulus and scavenger of ROS within THP-1 cells, influenced by its PEGylation and the formation of a protein corona. This study provides important insights for biocompatibility evaluation and rational design of nanoproducts based on Ti3C2Tx in the future.

Recent advances in the application of MXenes for neural tissue engineering and regeneration

Transition metal carbides and nitrides (MXenes) are crystal nanomaterials with a number of surface functional groups such as fluorine, hydroxyl, and oxygen, which can be used as carriers for proteins and drugs. MXenes have excellent biocompatibility, electrical conductivity, surface hydrophilicity, mechanical properties and easy surface modification. However, at present, the stability of most MXenes needs to be improved, and more synthesis methods need to be explored. MXenes are good substrates for nerve cell regeneration and nerve reconstruction, which have broad application prospects in the repair of nervous system injury. Regarding the application of MXenes in neuroscience, mainly at the cellular level, the long-term in vivo biosafety and effects also need to be further explored. This review focuses on the progress of using MXenes in nerve regeneration over the last few years; discussing preparation of MXenes and their biocompatibility with different cells as well as the regulation by MXenes of nerve cell regeneration in two-dimensional and three-dimensional environments in vitro. MXenes have great potential in regulating the proliferation, differentiation, and maturation of nerve cells and in promoting regeneration and recovery after nerve injury. In addition, this review also presents the main challenges during optimization processes, such as the preparation of stable MXenes and long-term in vivo biosafety, and further discusses future directions in neural tissue engineering.

Fluoride-Free Synthesis of 2D Titanium Carbide (MXenes) Assisted by scCO2 -Based Ternary Solution

2D MXene-Ti3 C2 Tx holds great promise in various electronic applications, especially for electromagnetic interference (EMI) shielding devices and supercapacitors. Ti3 C2 Tx synthesis typically involves the use of hazardous fluorine-containing chemicals that can result in the formation of inert fluoride functional groups on the surface of Ti3 C2 Tx , severely degrading its properties and posing a threat to the performance of electron transfer among electrical devices. Herein, a supercritical carbon dioxide-based ternary solution (scCO2 /DMSO/HCl) to produce fluoride-free Ti3 C2 Tx in mild conditions (via 0.5 m HCl, 20 MPa, 32 °C) is reported. The fluorine-free Ti3 C2 Tx films electrode presents an excellent gravimetric capacitance of 320 F g-1 at 2 mV s-1 in 1 m H2 SO4 . Besides, it is demonstrated that fluorine-free Ti3 C2 Tx films exhibit outstanding EMI shielding efficiency of 53.12 dB at 2.5 µm thickness. The findings offer a mild and practical approach to producing fluoride-free Ti3 C2 Tx and open opportunities for exploring MXenes' potential applications in various fields.

NiCo2O4/Ti2NbC2 (double MXene) nanohybrid-based non-enzymatic electrochemical biosensor for the detection of glucose in sweat

The non-enzymatic electrochemical sensors are attractive due to their high sensitivity, quick detection, low cost, and simple construction. Hence, in this work, a non-enzymatic biosensor was constructed with NiCo2O4 nanoparticles (~ 82 nm) decorated over Ti2NbC2 nanosheets by an in-situ method. The crystal structure, phase purity, morphology and elemental composition of the synthesized NiCo2O4/Ti2NbC2 nanohybrid was investigated using XRD, Raman and FESEM analysis. The electrocatalytic and electrochemical behaviour of the prepared nanohybrid was investigated using cyclic voltammetry and amperometry analysis. Hybrid of NiCo2O4/Ti2NbC2 produces a biocompatible, electrochemically active surface with enhanced electrical conductivity. The enhanced surface area of NiCo2O4 and superior electrical conductivity of Ti2NbC2 nanosheets helped to develop non-enzymatic electrochemical glucose sensor with enhanced sensitivity (425.6 µA mM-1cm-2), low limit of detection and quick response time that satisfy glucose detection applications. Thus, the developed non-enzymatic electrochemical glucose sensor has excellent electrochemical properties and making it as a strong candidate for the detection of glucose concentration in sweat.

Acoustic platforms meet MXenes - a new paradigm shift in the palette of biomedical applications

The wide applicability of acoustics in the life of mankind spread over health, energy, environment, and others. These acoustic technologies rely on the properties of the materials with which they are made of. However, traditional devices have failed to develop into low-cost, portable devices and need to overcome issues like sensitivity, tunability, and applicability in biological in vivo studies. Nanomaterials, especially 2D materials, have already been proven to produce high optical contrast in photoacoustic applications. One such wonder kid in the materials family is MXenes, which are transition metal carbides, that are nowadays flourishing in the materials world. Recently, it has been demonstrated that MXene nanosheets and quantum dots can be synthesized by acoustic excitations. In addition, MXene can be used as a mechanical sensing material for building piezoresistive sensors to realize sound detection as it produces a sensitive response to pressure and vibration. It has also been demonstrated that MXene nanosheets show high photothermal conversion capability, which can be utilized in cancer treatment and photoacoustic imaging (PAI). In this review, we have rendered the role of acoustics in the palette of MXene, including acoustic synthetic strategies of MXenes, applications such as acoustic sensors, PAI, thermoacoustic devices, sonodynamic therapy, artificial ear drum, and others. The review also discusses the challenges and future prospects of using MXene in acoustic platforms in detail. To the best of our knowledge, this is the first review combining acoustic science in MXene research.

Label-free MXene-assisted field effect transistor for the determination of IL-6 in patients with kidney transplantation infection

A spiral interdigitated MXene-assisted field effect transistor (SiMFETs) was proposed for determination of IL-6 in patients with kidney transplantation infection. Our SiMFETs demonstrated enhanced IL-6 detection range of 10 fg/mL-100 ng/mL due to the combination of optimized transistor's structure and semiconducting nanocomposites. Specifically, on one hand, MXene-based field effect transistor drastically amplified the amperometric signal for determination of IL-6; on the other hand, the multiple spiral structure of interdigitated drain-source architecture improved the transconductance of FET biosensor. The developed SiMFETs biosensor demonstrated satisfactory stability for 2 months, and favorable reproducibility and selectivity against other biochemical interferences. The SiMFETs biosensor exhibited acceptable correlation coefficient (R2=0.955) in quantification of clinical biosamples. The sensor successfully distinguished the infected patients from the health control with enhanced AUC of 0.939 (sensitivity of 91.7%, specificity of 86.7%). Those merits introduced here may pave an alternative strategy for transistor-based biosensor in point-of-care clinic applications.

Endogenous/Exogenous Nanovaccines Synergistically Enhance Dendritic Cell-Mediated Tumor Immunotherapy

Traditional dendritic cell (DC)-mediated immunotherapy is usually suppressed by weak immunogenicity in tumors and generally leads to unsatisfactory outcomes. Synergistic exogenous/endogenous immunogenic activation can provide an alternative strategy for evoking a robust immune response by promoting DC activation. Herein, Ti3 C2 MXene-based nanoplatforms (termed MXP) are prepared with high-efficiency near-infrared photothermal conversion and immunocompetent loading capacity to form endogenous/exogenous nanovaccines. Specifically, the immunogenic cell death of tumor cells induced by the photothermal effects of the MXP can generate endogenous danger signals and antigens release to boost vaccination for DC maturation and antigen cross-presentation. In addition, MXP can deliver model antigen ovalbumin (OVA) and agonists (CpG-ODN) as an exogenous nanovaccine (MXP@OC), which further enhances DC activation. Importantly, the synergistic strategy of photothermal therapy and DC-mediated immunotherapy by MXP significantly eradicates tumors and enhances adaptive immunity. Hence, the present work provides a two-pronged strategy for improving immunogenicity and killing tumor cells to achieve a favorable outcome in tumor patients.

Sensitive determination of prostate-specific antigen with graphene quantum dot-based fluorescence aptasensor using few-layer V2CTx MXene as quencher

A novel fluorescence aptasensor of prostate-specific antigen (PSA) was established using few-layer vanadium carbide (FL-V₂CT_x) nanosheet as a quencher. First, FL-V₂CT_x was prepared by the delamination of multi-layer V₂CT_x (ML-V₂CT_x) with tetramethylammonium hydroxide. The aptamer-carboxyl graphene quantum dots (CGQDs) probe was prepared by combining the aminated PSA aptamer and CGQDs. Then, the aptamer-CGQDs were absorbed onto the surface of FL-V₂CT_x by hydrogen bond interaction, which led to the decrease in fluorescence of aptamer-CGQDs due to photoinduced energy transfer. After addition of PSA, PSA-aptamer-CGQDs complex was released from FL-V₂CT_x. The fluorescence intensity of aptamer-CGQDs-FL-V₂CT_x with PSA was higher than that without PSA. The FL-V₂CT_x-based fluorescence aptasensor provided a PSA detection linear range from 0.1 to 20 ng mL^-1 with detection limit of 0.03 ng mL^-1. The ΔF value of fluorescence intensities for aptamer-CGQDs-FL-V₂CT_x with and without PSA was 5.6, 3.7, 7.7, and 5.4 times of ML-V₂CT_x, few-layer titanium carbide (FL-Ti₃C₂T_x), ML-Ti₃C₂T_x and graphene oxide aptasensors, respectively, indicating the advantage of FL-V₂CT_x. The aptasensor had high selectivity for PSA detection compared with some proteins and tumor markers. This proposed method had convenience and high sensitivity for PSA determination. The determination results of PSA in human serum samples using the aptasensor were consistent with those by chemiluminescent immunoanalysis. The fluorescence aptasensor can be successfully applied for PSA determination in serum samples of prostate cancer patients.

Safe Etching Route of Nb2SnC for the Synthesis of Two-Dimensional Nb2CTx MXene: An Electrode Material with Improved Electrochemical Performance

In this study, low-temperature synthesis of a Nb2SnC non-MAX phase was carried out via solid-state reaction, and a novel approach was introduced to synthesize 2D Nb2CTx MXenes through selective etching of Sn from Nb2SnC using mild phosphoric acid. Our work provides valuable insights into the field of 2D MXenes and their potential for energy storage applications. Various techniques, including XRD, SEM, TEM, EDS, and XPS, were used to characterize the samples and determine their crystal structures and chemical compositions. SEM images revealed a two-dimensional layered structure of Nb2CTx, which is consistent with the expected morphology of MXenes. The synthesized Nb2CTx showed a high specific capacitance of 502.97 Fg-1 at 1 Ag-1, demonstrating its potential for high-performance energy storage applications. The approach used in this study is low-cost and could lead to the development of new energy storage materials. Our study contributes to the field by introducing a unique method to synthesize 2D Nb2CTx MXenes and highlights its potential for practical applications.

Recent Progress in MXene Hydrogel for Wearable Electronics

Recently, hydrogels have attracted great attention because of their unique properties, including stretchability, self-adhesion, transparency, and biocompatibility. They can transmit electrical signals for potential applications in flexible electronics, human-machine interfaces, sensors, actuators, et al. MXene, a newly emerged two-dimensional (2D) nanomaterial, is an ideal candidate for wearable sensors, benefitting from its surface's negatively charged hydrophilic nature, biocompatibility, high specific surface area, facile functionalization, and high metallic conductivity. However, stability has been a limiting factor for MXene-based applications, and fabricating MXene into hydrogels has been proven to significantly improve their stability. The unique and complex gel structure and gelation mechanism of MXene hydrogels require intensive research and engineering at nanoscale. Although the application of MXene-based composites in sensors has been widely studied, the preparation methods and applications of MXene-based hydrogels in wearable electronics is relatively rare. Thus, in order to facilitate the effective evolution of MXene hydrogel sensors, the design strategies, preparation methods, and applications of MXene hydrogels for flexible and wearable electronics are comprehensively discussed and summarized in this work.

Retrospective insights into recent MXene-based catalysts for CO2 electro/photoreduction: how far have we gone?

The electro/photocatalytic CO₂ reduction reaction (CO₂RR) is a long-term avenue toward synthesizing renewable fuels and value-added chemicals, as well as addressing the global energy crisis and environmental challenges. As a result, current research studies have focused on investigating new materials and implementing numerous fabrication approaches to increase the catalytic performances of electro/photocatalysts toward the CO₂RR. MXenes, also known as 2D transition metal carbides, nitrides, and carbonitrides, are intriguing materials with outstanding traits. Since their discovery in 2011, there has been a flurry of interest in MXenes in electrocatalysis and photocatalysis, owing to their several benefits, including high mechanical strength, tunable structure, surface functionality, high specific surface area, and remarkable electrical conductivity. Herein, this review serves as a milestone for the most recent development of MXene-based catalysts for the electrocatalytic and photocatalytic CO₂RR. The overall structure of MXenes is described, followed by a summary of several synthesis pathways classified as top-down and bottom-up approaches, including HF-etching, in situ HF-formation, electrochemical etching, and halogen etching. Additionally, the state-of-the-art development in the field of both the electrocatalytic and photocatalytic CO₂RR is systematically reviewed. Surface termination modulation and heterostructure engineering of MXene-based electro/photocatalysts, and insights into the reaction mechanism for the comprehension of the structure-performance relationship from the CO₂RR via density functional theory (DFT) have been underlined toward activity enhancement. Finally, imperative issues together with future perspectives associated with MXene-based electro/photocatalysts are proposed.

Toward Smart Sensing by MXene

The Internet of Things era has promoted enormous research on sensors, communications, data fusion, and actuators. Among them, sensors are a prerequisite for acquiring the environmental information for delivering to an artificial data center to make decisions. The MXene-based sensors have aroused tremendous interest because of their extraordinary performances. In this review, the electrical, electronic, and optical properties of MXenes are first introduced. Next, the MXene-based sensors are discussed according to the sensing mechanisms such as electronic, electrochemical, and optical methods. Initially, biosensors are introduced based on chemiresistors and field-effect transistors. Besides, the wearable pressure sensor is demonstrated with piezoresistive devices. Third, the electrochemical methods include amperometry and electrochemiluminescence as examples. In addition, the optical approaches refer to surface plasmonic resonance and fluorescence resonance energy transfer. Moreover, the prospects are delivered of multimodal data fusion toward complicated human-like senses. Eventually, future opportunities for MXene research are conveyed in the new material discovery, structure design, and proof-of-concept devices.

Advanced growth of 2D MXene for electrochemical sensors

Over the last few years, electroanalysis has made significant advancements, particularly in developing electrochemical sensors. Electrochemical sensors generally include emerging Photoelectrochemical and Electrochemiluminescence sensors, which combine optical techniques and traditional electrochemical bio/non-biosensors. Numerous EC-detecting methods have also been designed for commercial applications to detect biological and non-biological markers for various diseases. Analytical applications have recently focused significantly on one of the novel nanomaterials, the MXene. This material is being extensively investigated for applications in electrochemical sensors due to its unique mechanical, electronic, optical, active functional groups and thermal characteristics. This study extensively discusses the salient features of MXene-based electrochemical sensors, photoelectrochemical sensors, enzyme-based biosensors, immunosensors, aptasensors, electrochemiluminescence sensors, and electrochemical non-biosensors. In addition, their performance in detecting various substances and contaminants is thoroughly discussed. Furthermore, the challenges and prospects the MXene-based electrochemical sensors are elaborated.

Emerging MXene-Based Flexible Tactile Sensors for Health Monitoring and Haptic Perception

Due to their potential applications in physiological monitoring, diagnosis, human prosthetics, haptic perception, and human-machine interaction, flexible tactile sensors have attracted wide research interest in recent years. Thanks to the advances in material engineering, high performance flexible tactile sensors have been obtained. Among the representative pressure sensing materials, 2D layered nanomaterials have many properties that are superior to those of bulk nanomaterials and are more suitable for high performance flexible sensors. As a class of 2D inorganic compounds in materials science, MXene has excellent electrical, mechanical, and biological compatibility. MXene-based composites have proven to be promising candidates for flexible tactile sensors due to their excellent stretchability and metallic conductivity. Therefore, great efforts have been devoted to the development of MXene-based composites for flexible sensor applications. In this paper, the controllable preparation and characterization of MXene are introduced. Then, the recent progresses on fabrication strategies, operating mechanisms, and device performance of MXene composite-based flexible tactile sensors, including flexible piezoresistive sensors, capacitive sensors, piezoelectric sensors, triboelectric sensors are reviewed. After that, the applications of MXene material-based flexible electronics in human motion monitoring, healthcare, prosthetics, and artificial intelligence are discussed. Finally, the challenges and perspectives for MXene-based tactile sensors are summarized.

Chemical scissor-mediated structural editing of layered transition metal carbides

Intercalated layered materials offer distinctive properties and serve as precursors for important two-dimensional (2D) materials. However, intercalation of non-van der Waals structures, which can expand the family of 2D materials, is difficult. We report a structural editing protocol for layered carbides (MAX phases) and their 2D derivatives (MXenes). Gap-opening and species-intercalating stages were respectively mediated by chemical scissors and intercalants, which created a large family of MAX phases with unconventional elements and structures, as well as MXenes with versatile terminals. The removal of terminals in MXenes with metal scissors and then the stitching of 2D carbide nanosheets with atom intercalation leads to the reconstruction of MAX phases and a family of metal-intercalated 2D carbides, both of which may drive advances in fields ranging from energy to printed electronics.

Immunoengineered MXene nanosystem for mitigation of alloantigen presentation and prevention of transplant vasculopathy

MXenes are an emerging class of nanomaterials with significant potential for applications in nanomedicine. Amongst MXene technologies, titanium carbide (Ti₃C₂T_x) nanomaterials are the most mature and have received significant attention to tackle longstanding clinical challenges due to its tailored physical and material properties. Cardiac allograft vasculopathy is an aggressive form of atherosclerosis and a major cause of mortality among patients with heart transplants. Blood vessel endothelial cells (ECs) stimulate alloreactive T-lymphocytes to result in sustained inflammation. Herein, we report the first application of Ti₃C₂T_x MXene nanosheets for prevention of allograft vasculopathy. MXene nanosheets interacted with human ECs and downregulated the expression of genes involved in alloantigen presentation, and consequently reduced the activation of allogeneic lymphocytes. RNA-Seq analysis of lymphocytes showed that treatment with MXene downregulated genes responsible for transplant-induced T-cell activation, cell-mediated rejection, and development of allograft vasculopathy. In an in vivo rat model of allograft vasculopathy, treatment with MXene reduced lymphocyte infiltration and preserved medial smooth muscle cell integrity within transplanted aortic allografts. These findings highlight the potential of Ti₃C₂T_x MXene in treatment of allograft vasculopathy and inflammatory diseases.

Application Prospects of MXenes Materials Modifications for Sensors

The first two-dimensional (2D) substance sparked a boom in research since this type of material showed potential promise for applications in field sensors. A class of 2D transition metal nitrides, carbides, and carbonitrides are referred to as MXenes. Following the 2011 synthesis of Ti3C2 from Ti3AlC2, much research has been published. Since these materials have several advantages over conventional 2D materials, they have been extensively researched, synthesized, and studied by many research organizations. To give readers a general understanding of these well-liked materials, this review examines the structures of MXenes, discusses various synthesis procedures, and analyzes physicochemistry properties, particularly optical, electronic, structural, and mechanical properties. The focus of this review is the analysis of modern advancements in the development of MXene-based sensors, including electrochemical sensors, gas sensors, biosensors, optical sensors, and wearable sensors. Finally, the opportunities and challenges for further study on the creation of MXenes-based sensors are discussed.

Towards Greener and More Sustainable Synthesis of MXenes: A Review

The unique properties of MXenes have been deemed to be of significant interest in various emerging applications. However, MXenes provide a major drawback involving environmentally harmful and toxic substances for its general fabrication in large-scale production and employing a high-temperature solid-state reaction followed by selective etching. Meanwhile, how MXenes are synthesized is essential in directing their end uses. Therefore, making strategic approaches to synthesize greener, safer, more sustainable, and more environmentally friendly MXenes is imperative to commercialize at a competitive price. With increasing reports of green synthesis that promote advanced technologies and non-toxic agents, it is critical to compile, summarize, and synthesize the latest development of the green-related technology of MXenes. We review the recent progress of greener, safer, and more sustainable MXene synthesis with a focus on the fundamental synthetic process, the mechanism, and the general advantages, and the emphasis on the MXene properties inherited from such green synthesis techniques. The emerging use of the so-called green MXenes in energy conversion and storage, environmental remediation, and biomedical applications is presented. Finally, the remaining challenges and prospects of greener MXene synthesis are discussed.

2D Conductive Covalent Organic Frameworks with Abundant Carbonyl Groups for Electrochemical Sensing

Due to their permanent porosity, robust chemical stability, and tunable structure, covalent organic frameworks (COFs) are very attractive in the application of energy storage, catalysis, sorption, and sensing. However, the very low conductivity of COFs severely restricts their application in electrochemical sensing. Here, an aza-fused π-conjugated COFs with abundant carbonyl groups (COF1) was synthesized and deployed as electrode materials in electrochemical sensing for the first time. The current response of the acetylcholinesterase biosensor based on COF1 increases three times when compared to the electrode without COF1. The effects of carbonyl groups on signal enhancement were proved in depth by a series of characterization and comparison experiments with the prepared COF2 without carbonyl groups. The results demonstrated that exposed carbonyl active sites of COF1 can promote the effective immobilization and bioactivity preserving of enzyme molecules and contribute to the enrichment of analytes. Together with the good conductivity of COF1 derived from a fully extended 2D aromatized π-conjugated system, all of which improve the biosensor performance. The COF1-based biosensor exhibited fast response speed, high sensitivity, good selectivity and practicability, and robust stability for organophosphorus pesticide detection and proved to be a promising tool for the rapid and onsite detection of organophosphorus pesticides in the environment.

Multiplexed detection of SARS-CoV-2 based on upconversion luminescence nanoprobe/MXene biosensing platform for COVID-19 point-of-care diagnostics

Multiplexed detection is essential in biomedical sciences since it is more efficient and accurate than single-analyte detection. For an accurate early diagnosis of COVID-19, a multiplexed detection strategy is required to avoid false negatives with the existing gold standard assay. Nb₂CT_x nanosheets were found to efficiently quench the fluorescence emission of lanthanide-doped upconversion luminescence nanoparticles at wavelengths ranging from visible to near-infrared spectrum. Using this broad-spectrum quencher, we developed a label-free FRET-based biosensor for rapid and accurate detection of SARS-CoV-2 RNA. To target ORF and N genes, two types of oligo-modified lanthanide-doped upconversion nanoparticles can be used simultaneously to identify-two sites in one assay via upconversion fluorescence enhancement intensity measurement with detection limits of 15 pM and 914 pM, respectively. Moreover, with multisite cross-validation, this multiplexed and sensitive biosensor is capable of simultaneous and multicolor analysis of two gene fragments of SARS-CoV-2 Omicron variant within minutes in a single homogeneous solution, which significantly improves the detection efficiency. The diagnosis result via our assay is consistent with the PCR result, demonstrating its application in the rapid and accurate screening of multiple genes of SARS-CoV-2 and other infectious diseases.

Tunable Resistive Switching in 2D MXene Ti3C2 Nanosheets for Non-Volatile Memory and Neuromorphic Computing

An artificial synapse is essential for neuromorphic computing which has been expected to overcome the bottleneck of the traditional von-Neumann system. Memristors can work as an artificial synapse owing to their tunable non-volatile resistance states which offer the capabilities of information storage, processing, and computing. In this work, memristors based on two-dimensional (2D) MXene Ti₃C₂ nanosheets sandwiched by Pt electrodes are investigated in terms of resistive switching (RS) characteristics, synaptic functions, and neuromorphic computing. Digital and analog RS behaviors are found to coexist depending on the magnitude of operation voltage. Digital RS behaviors with two resistance states possessing a large switching ratio exceeding 10³ can be achieved under a high operation voltage. Analog RS behaviors with a series of resistance states exhibiting a gradual change can be observed at a relatively low operation voltage. Furthermore, artificial synapses can be implemented based on the memristors with the basic synaptic functions, such as long-term plasticity of long-term potentiation and depression and short-term plasticity of the paired-pulse facilitation and depression. Moreover, the "learning-forgetting" experience is successfully emulated based on the artificial synapses. Also, more importantly, the artificial synapses can construct an artificial neural network to implement image recognition. The coexistence of digital and analog RS behaviors in the 2D Ti₃C₂ nanosheets suggests the potential applications in non-volatile memory and neuromorphic computing, which is expected to facilitate simplifying the manufacturing complexity for complex neutral systems where analog and digital switching is essential for information storage and processing.

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.

Two-dimensional Ti3C2Tx MXene promotes electrophysiological maturation of neural circuits

BACKGROUND

The ideal neural interface or scaffold for stem cell therapy shall have good biocompatibility promoting survival, maturation and integration of neural stem cells (NSCs) in targeted brain regions. The unique electrical, hydrophilic and surface-modifiable properties of Ti₃C₂T_x MXene make it an attractive substrate, but little is known about how it interacts with NSCs during development and maturation.

RESULTS

In this study, we cultured NSCs on Ti₃C₂T_x MXene and examined its effects on morphological and electrophysiological properties of NSC-derived neurons. With a combination of immunostaining and patch-clamp recording, we found that Ti₃C₂T_x MXene promotes NSCs differentiation and neurite growth, increases voltage-gated current of Ca²⁺ but not Na⁺ or K⁺ in matured neurons, boosts their spiking without changing their passive membrane properties, and enhances synaptic transmission between them.

CONCLUSIONS

These results expand our understanding of interaction between Ti₃C₂T_x MXene and NSCs and provide a critical line of evidence for using Ti₃C₂T_x MXene in neural interface or scaffold in stem cell therapy.

One-pot in-situ hydrothermal synthesis of ternary In2S3/Nb2O5/Nb2C Schottky/S-scheme integrated heterojunction for efficient photocatalytic hydrogen production

MXene-derived photocatalysts continue to fascinate the research community in developing photo-driven green and sustainable fuel production. However, the efficiency of MXene-derived photocatalyst is still low due to the wide bandgap and high recombination rate of photo-excited charge carriers. Here, we have synthesized the Nb₂C MXene-derived ternary photocatalyst via one-pot in-situ hydrothermal method for photocatalytic hydrogen (H₂) evolution. The partial oxidation of Nb₂C MXene into Nb₂O₅ nanorods and coupling with In₂S₃ nanoparticles via in-situ chemical anchoring were the key factors toward high efficiency and long-term stability during photocatalytic H₂ evolution. The optimized ternary photocatalyst composite manifested the highest H₂ evolution efficiency at 68.8 µmol g^-1 h^-1, which was 11 and 7.5 times higher than the Nb₂O₅/Nb₂C (NNC) and pure In₂S₃ photocatalyst, respectively. Moreover, the photocatalytic stability of the optimized ternary photocatalyst composite was analyzed for five consecutive cycles, and above 87% activity retention was observed even after the fifth cycle without any obvious decline. The separation efficiency of photoexcited charge carriers could be attributed to the synergic effect of the In₂S₃/Nb₂O₅ heterojunction and the redox reactions at different sites of the composite. More importantly, the participation of Schottky junction and S-scheme heterojunction charge transfer for the obtained novel ternary photocatalyst was evaluated via ultraviolet photoelectron spectroscopy (UPS) and electron paramagnetic resonance (EPR). This research will provide additional insight into the extended potential of MXene-derived ternary photocatalysts towards efficient H₂ production to meet future global energy demands.

In Situ Electrosynthesis of MAX-Derived Electrocatalysts for Superior Hydrogen Evolution Reaction

MAX phases are frequently dominated as precursors for the preparation of the star material MXene, but less eye-dazzling by their own potential applications. In this work, the electrocatalytic hydrogen evolution reaction (HER) activity of MAX phase is investigated. The MAX-derived electrocatalysts are prepared by a two-step in situ electrosynthesis process, an electrochemical etching step followed by an electrochemical deposition step. First, a Mo₂ TiAlC₂ MAX phase is electrochemically etched in 0.5 m H₂ SO₄ electrolyte. Just several hours, electrochemical dealloy etching of Mo₂ TiAlC₂ MAX powders by applying anode current can acquire a moderated HER performance, outperforming most of reported pure MXene. It is speculated that in situ superficially architecting endogenous MAX/amorphous carbide (MAC) improves its intrinsic catalytic activity. Subsequently, highly active metallic Pt nanoparticles immobilized on MAC (MAC@Pt) shows a transcendental overpotential of 40 mV versus RHE in 0.5 m H₂ SO₄ and 79 mV in 1.0 m KOH at the current density of 10 mA cm^-2 without iR correction. Ultrahigh mass activity of MAC@Pt (1.5 A mg_pt ^-1 ) at 100 mV overpotential is also achieved, 29-folds than those of commercial PtC catalysts.

Advances and emerging challenges in MXenes and their nanocomposites for biosensing applications

Two-dimensional materials have unique properties and their better functionality has created new paradigms in the field of sensing. Over the past decade, a new family of 2D materials known as MXenes has emerged as a promising material for numerous applications, including biosensing. Their metallic conductivity, rich surface chemistry, hydrophilicity, good biocompatibility, and high anchoring capacity for biomaterials make them an attractive candidate to detect a variety of analytes. Despite such notable properties, there are certain limitations associated with them. This review aims to present a detailed survey of MXene's synthesis; in particular, their superiority in the field of biosensing as compared to other 2D materials is addressed. Their low oxidative stability is still an open challenge, and recent investigations on MXene's oxidation are summarized. The hexagonal stacking network of MXenes acts as a distinctive matrix to load nanoparticles, and the embedded nanoparticles can bind an excess number of biomolecules (e.g., antibodies) thereby improving biosensor performance. We will also discuss the synthesis and corresponding performance of MXenes nanocomposites with noble metal nanoparticles and magnetic nanoparticles. Furthermore, Nb and Ti2C-based MXenes, and Ti3C2-MXene sandwich immunoassays are also reviewed in view of their importance. Different aspects and challenges associated with MXenes (from their synthesis to final applications) and the future perspectives described give new directions to fabricate novel biosensors.

Abundance and Metabolism Disruptions of Intratumoral Microbiota by Chemical and Physical Actions Unfreeze Tumor Treatment Resistance

Intratumoral or intestinal microbiota correlates with tumorigenesis and progression, and microbiota regulation for reinforcing various anti-tumor approaches is of significant importance, which, however, suffers from no precise regulation method and unclear underlying mechanism. Herein, a microbiome metabolism-engineered phototherapy strategy is established, wherein Nb2 C/Au nanocomposite and the corresponding phototherapy are harnessed to realize "chemical" and "physical" bacterial regulations. Flora analysis and mass spectrometry (MS) and metabonomics combined tests demonstrate that the synergistic microbiota regulations can alter the abundance, diversity of intratumoral microbiome, and disrupt metabolic pathways of microbiome and tumor microenvironment, wherein the differential singling pathways and biosynthetic necessities or metabolites that can affect tumor progression are identified. As well, anti-TNFα is introduced to unite with bacterial regulation to synergistically mitigate bacterial-induced inflammation, which, along with the metabolism disruptions of intratumoral microbiota and tumor microenvironment, unfreezes tumor resistance and harvests significantly-intensified phototherapy-based anti-tumor outcomes against 4T1 and CT26 tumors. The clear underlying principles of microbiome-regulated tumorigenesis and the established microbiome metabolism regulation method provide distinctive insights into tumor therapy, and can be also extended to other gut microbiome-associated lesions interference.

MXene-based electrochemical and biosensing platforms to detect toxic elements and pesticides pollutants from environmental matrices

Fabricating new biosensing constructs with high selectivity and sensitivity is the most needed environmental detection tool. In this context, several nanostructured materials have been envisaged to construct biosensors to achieve superior selectivity and sensitivity. Among them, MXene is regarded as the most promising to develop biosensors due to its fascinating attributes, like high surface area, excellent thermal resistance, good hydrophilicity, unique layered topology, high electrical conductivity, and environmentally-friendlier properties. MXenes-based materials have emerged as a prospective for catalysis, energy storage, electronics, and environmental sensing and remediation applications thanks to the above-mentioned exceptional characteristics. This review elaborates on the contemporary and state-of-the-art advancements in MXene-based electrochemical and biosensing tools to detect toxic elements, pharmaceutically active residues, and pesticide contaminants from environmental matrices. At first, the surface functionalization/modification of MXenes is discussed. Afterwards, a particular focus has been devoted to exploiting MXene to construct electrochemical (bio) sensors to detect various environmentally-related pollutants. Lastly, current challenges in this arena accompanied by potential solutions and directions are also outlined.

Pt Decorated Ti3C2Tx MXene with NIR-II Light Amplified Nanozyme Catalytic Activity for Efficient Phototheranostics

The clinical application of photothermal therapy (PTT) is severely limited by the tissue penetration depth of excitation light, and enzyme therapy is hampered by its low therapeutic efficiency. As a two-dimensional ultrathin nanosheet with high absorbance in the near-infrared-II (NIR-II) region, the titanium carbide (Ti₃C₂) nanosheet can be used as a substrate to anchor functional components, like nanozymes and nanodrugs. Here, we decorate Pt artificial nanozymes on the Ti₃C₂ nanosheets to synthesize Ti-based MXene nanocomposites (Ti₃C₂T_x-Pt-PEG). In the tumor microenvironment, the Pt nanoparticles exhibit peroxidase-like (POD-like) activity, which can in situ catalyze hydrogen peroxide to generate hydroxyl radicals (^•OH) to induce cell apoptosis and necrosis. Meanwhile, the composite shows a desirable photothermal effect upon NIR-II light irradiation with a low power density (0.75 W cm^-2). Especially, the POD-like activity is significantly enhanced by the elevated temperature arising from the photothermal effect of Ti₃C₂T_x. Therefore, satisfactory synergistic PTT/enzyme therapy has been accomplished, accompanied by an applicable photoacoustic imaging capability to monitor and guide the therapeutic process. This work may provide an approach for hyperthermia-amplified nanozyme catalytic therapy, especially based on metal catalysts and MXene nanocomposites.

Ultrathin graphdiyne nanosheets confining Cu quantum dots as robust electrocatalyst for biosensing featuring remarkably enhanced activity and stability

There is an urgent need for developing electrochemical biosensor based on the acetylcholinesterase (AChE) inhibition to real-time analysis of organophosphorus pesticides (OPs), but it is suffered from the sluggish electrode kinetics and high oxidation potential toward signal species. Herein, a nanocomposite of ultrafine Cu quantum dots (QD) uniformly loaded on three-dimensional ultrathin graphdiyne (GDY) nanosheets (denoted as Cu@GDY) was synthesized via a one-step strategy, which showing high-density of active sites with persistent stability. Then an AChE biosensor based on Cu@GDY was fabricated to detect OPs, and the results revealed that the Cu@GDY nanocomposite can significantly amplifies electrochemical signal and reduces the oxidation potential for OPs. The strong interaction between active site of Cu@GDY and thiocholine signal species caused rapid analyte aggregation and decreased the reaction activation energy of thiocholine electro-oxidation. Benefiting from the excellent catalytic activity of Cu@GDY nanocomposite and reasonable regulation of enzyme inhibition kinetics, the biosensor achieved rapid and sensitive detection of OPs with a detection limit of 1 μg L^-1 for paraoxon. Furthermore, the biosensor demonstrated great reproducibility, good stability and high recovery rate for OPs detection in real samples. Cu@GDY based sensor also displayed high catalytic activities and good selectivity to the non-enzymatic detection of glucose in alkaline medium. Cu@GDY offers a versatile and promising platform for sensors and biosensors featuring remarkably enhanced activity and stability, and can be applied to many other fields as desirable electrocatalyst.

2D metal carbides and nitrides (MXenes) for sensors and biosensors

MXenes are layered two-dimensional (2D) materials discovered in 2011 (Ti₃C₂X) and are otherwise called 2D transition metal carbides, carbonitrides, and nitrides. These 2D layered materials have been in the limelight for a decade due to their interesting properties such as large surface area, high ion transport, biocompatibility, and low diffusion barrier. Therefore, MXenes are widely preferred by researchers for applications in electronics, sensing, biosensing, electrocatalysis, super-capacitors and fuel cells. There are a number of methods available for the bulk synthesis of MXene-based nanomaterials. In addition, the possibility of structural modification as required and its outstanding surface chemistry offer a fascinating interface for the development of novel biosensors. In this review, we specifically discuss important MXene synthesis routes. Moreover, critical parameters such as surface functionalization that can dictate the mechanical, electronic, magnetic, and optical properties of MXenes are also discussed. Following this, methods available for the surface functionalization and modification strategies of MXenes are also discussed. Furthermore, the emergence of gas, electrochemical, and optical biosensors based on MXenes since its first report is discussed in detail. Finally, future directions of MXenes biosensors for critical applications are discussed.

Recent advances in flexible and wearable chemo- and bio-sensors based on two-dimensional transition metal carbides and nitrides (MXenes)

Due to their excellent hydrophilicity, outstanding conductivity, unique structures, and physicochemical properties, MXenes have become a potential candidate material for flexible and wearable chemo- and bio-sensors.

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.

Redox Host-Guest Nanosensors Installed with DNA Gatekeepers for Immobilization-Free and Ratiometric Electrochemical Detection of miRNA

Electrochemical nanosensors by integrating functional nucleic acids and nanomaterials hold a great promise in the fast detection of biomarkers, yet the current systems possess limitations on the accessibility of target-probe and probe-electrode interactions and the repeatability of detection. Herein, a host-guest assembly strategy is developed to build redox nanosensors for an immobilization-free and ratiometric electrochemical detection system. Specifically, electroactive molecule (E_m ) guests are loaded in porous hosts of polydopamine nanoparticles (MPDA) to act as dual-signal redox reporters. Hybrid DNA probes of G-quadruplex and a single-stranded anchor DNA are installed as gatekeepers for sealing the mesopores. Thereby, miRNA triggered E_m release by strand displacement reactions and the homogeneous transportation of the hosts/guests to the electrode facilitate the generation of reference signal/response signal at different potentials. Concomitantly applied NIR irradiation boosts the electron transfer from MPDA to the electrode and results in a tenfold increase in the reference signal. Finally, the sensing system through the differential pulse voltammetry method achieves a highly repeatable detection (relative standard deviation 3.8%) of miRNA with a lower detection limit (362 × 10^-15 m). This attractive system paves the way for rational designs of advanced electrochemical biosensors and smart diagnosis.

One-Step, DNA-Programmed, and Flash Synthesis of Anisotropic Noble Metal Nanostructures on MXene

Precise morphological control over anisotropic noble metal nanoparticles (ANPs) is one of the key issues in the nano-research field owing to their unique optoelectronic, magnetic, mechanical, and catalytic properties. Although nanostructures fabricated by the directed assembly of adsorbate have been widely demonstrated recently, facile yet universal synthesis of nanocrystal with tunable morphologies, green templates, no seeds, and high yield remains challenging. Herein, we develop a versatile method, allowing for the rapid, one-step, seedless, surfactant-free synthesis of a noble metal nanostructure with tunable anisotropy on MXene in a sequence-dependent manner through a single-DNA molecular regulator. Based on the mild reducibility of MXene and the selective affinity of the DNA to the specific facets in the crystals, oriented aggregations and the growth of ANPs (Au, Pt, Pd) can be achieved and the resulting asymmetric morphology from polyhedrons, or flowers, or nanoplates to dendrites is observed. The ability to align such ANPs on the MXene surface is expected to lead to improved photothermal effect and surface-enhanced Raman scattering. Furthermore, our work makes the fabrication of the ANPs or ANP-MXene heterostructure easier, stimulating further explorations of physical, chemical, and biological applications.

2D MXene Nanomaterials for Versatile Biomedical Applications: Current Trends and Future Prospects

Research on 2D nanomaterials is still in its early stages. Most studies have focused on elucidating the unique properties of the materials, whereas only few reports have described the biomedical applications of 2D nanomaterials. Recently, important questions about the interaction of 2D MXene nanomaterials with biological components have been raised. 2D MXenes are monolayer atomic nanosheets derived from MAX phase ceramics. As a new type of inorganic nanosystems, they are being widely used in biology and biomedicine. This review introduces the latest developments in 2D MXenes for the most advanced biomedical applications, including preparation and surface modification strategies, treatment modes, drug delivery, antibacterial activity, bioimaging, sensing, and biocompatibility. Besides, this review also discusses the current development trends and prospects of 2D inorganic nanosheets for further clinical applications. These emerging 2D inorganic MXenes will play an important role in next-generation cancer treatments.

Facile Atomic-Level Tuning of Reactive Metal-Support Interactions in the Pt QDs@ HF-Free MXene Heterostructure for Accelerating pH-Universal Hydrogen Evolution Reaction

Supported metallic nanoparticles render highly tunable physical and chemical properties to mixed-dimensionality materials in electrocatalysts. However, some supports are susceptible to being dissolved in acidic solution or are unstable in ambient air. The development of high-performance catalysts has been facing the major hurdles of the sluggish activity in alkaline solution and requesting high energy to stabilize the nanoparticles on their supports, challenging the pH-universality and the applicability of the supported metallic nanoparticles. Here, a one-step strategy is proposed to modulate the growth of Pt quantum dots (QDs) on HF-free MXene under atomic-level by a low-temperature metal-support interaction reaction. By controllable tailoring in the morphology and strain induced by terminations, Pt (111) QDs with a sub-nanoscale size of 1.15 nm are grown as 0D/1D heterostructure to overcome the restrictions of employing reduction gas and high annealing temperature. The catalyst exhibits a low overpotential of 33.3 mV for acidic solution, while 65.1 mV for alkaline solution at a specific current density of 10 mA cm-2 . This study not only paves a scalable pathway to developing cost-efficient catalysts in moderate conditions, but also demonstrates an effective surface modulation strategy for 0D/1D heterostructures.

Surface plasmon resonance aptasensor based on niobium carbide MXene quantum dots for nucleocapsid of SARS-CoV-2 detection

A novel label-free surface plasmon resonance (SPR) aptasensor has been constructed for the detection of N-gene of SARS-CoV-2 by using thiol-modified niobium carbide MXene quantum dots (Nb2C-SH QDs) as the bioplatform for anchoring N-gene-targeted aptamer. In the presence of SARS-CoV-2 N-gene, the immobilized aptamer strands changed their conformation to specifically bind with N-gene. It thus increased the contact area or enlarged the distance between aptamer and the SPR chip, resulting in a change of the SPR signal irradiated by the laser (He-Ne) with the wavelength (λ) of 633 nm. Nb2C QDs were derived from Nb2C MXene nanosheets via a solvothermal method, followed by functionalization with octadecanethiol through a self-assembling method. Subsequently, the gold chip for SPR measurements was modified with Nb2C-SH QDs via covalent binding of the Au-S bond also by self-assembling interaction. Nb2C-SH QDs not only resulted in high bioaffinity toward aptamer but also enhanced the SPR response. Thus, the Nb2C-SH QD-based SPR aptasensor had low limit of detection (LOD) of 4.9 pg mL-1 toward N-gene within the concentration range 0.05 to 100 ng mL-1. The sensor also showed excellent selectivity in the presence of various respiratory viruses and proteins in human serum and high stability. Moreover, the Nb2C-SH QD-based SPR aptasensor displayed a vast practical application for the qualitative analysis of N-gene from different samples, including seawater, seafood, and human serum. Thus, this work can provide a deep insight into the construction of the aptasensor for detecting SARS-CoV-2 in complex environments. A novel label-free surface plasmon resonance aptasensor has been constructed to detect sensitively and selectively the N-gene of SARS-CoV-2 by using thiol-modified niobium carbide MXene quantum dots as the scaffold to anchor the N-gene-targeted aptamer.

Preparation and Application of 2D MXene-Based Gas Sensors: A Review

Since MXene (a two-dimensional material) was discovered in 2011, it has been favored in all aspects due to its rich surface functional groups, large specific surface area, high conductivity, large porosity, rich organic bonds, and high hydrophilicity. In this paper, the preparation of MXene is introduced first. HF etching was the first etching method for MXene; however, HF is corrosive, resulting in the development of the in situ HF method (fluoride + HCl). Due to the harmful effects of fluorine terminal on the performance of MXene, a fluorine-free preparation method was developed. The increase in interlayer spacing brought about by adding an intercalator can affect MXene’s performance. The usual preparation methods render MXene inevitably agglomerate and the resulting yields are insufficient. Many new preparation methods were researched in order to solve the problems of agglomeration and yield. Secondly, the application of MXene-based materials in gas sensors was discussed. MXene is often regarded as a flexible gas sensor, and the detection of ppb-level acetone at room temperature was observed for the first time. After the formation of composite materials, the increasing interlayer spacing and the specific surface area increased the number of active sites of gas adsorption and the gas sensitivity performance improved. Moreover, this paper discusses the gas-sensing mechanism of MXene. The gas-sensing mechanism of metallic MXene is affected by the expansion of the lamellae and will be doped with H2O and oxygen during the etching process in order to become a p-type semiconductor. A p-n heterojunction and a Schottky barrier forms due to combinations with other semiconductors; thus, the gas sensitivities of composite materials are regulated and controlled by them. Although there are only several reports on the application of MXene materials to gas sensors, MXene and its composite materials are expected to become materials that can effectively detect gases at room temperature, especially for the detection of NH3 and VOC gas. Finally, the challenges and opportunities of MXene as a gas sensor are discussed.