Application of MXene in Electrochemical Sensors: A Review

Ultrasensitive electrochemical microRNA-21 detection based on MXene and ATRP photocatalytic strategy

A Ti3C2TxMXene-based biosensor has been developed and the photocatalytic atom transfer radical polymerization (photo ATRP) amplification strategy applied to detect target miRNA-21 (tRNA). Initially, Ti3C2TxMXene nanosheets were synthesized from the Ti3AlC2 MAX precursor via selective aluminum etching. Then, functionalization of Ti3C2TxMXene nanosheets with 3-aminopropyl triethoxysilane (APTES) via silylation reactions to facilitate covalent bonding with hairpin DNA biomolecules specifically designed for tRNA detection. Upon binding with the tRNA, the hairpin DNA liberated the azide (N₃) group, initiating a click reaction to affix to the photo ATRP initiator. Through the ATRP photoreaction, facilitated by an organic photoredox catalyst and light, a significant amount of ferrocenyl methyl methacrylate (FMMA) monomer was immobilized on the electrode. Therefore, the electrochemical signal is amplified. The electrochemical efficacy of the biosensor was assessed using square wave voltammetry (SWV). Under optimized conditions, the biosensor demonstrated remarkable sensitivity in detecting tRNA, with a linear detection range from 0.01 fM to 10 pM and a detection limit of 2.81 aM. The findings elucidate that the developed biosensor, in conjunction with the photo ATRP strategy, offers reproducibility, stability, and increased sensitivity, underscoring its potential applications within the experimental medical sector of the biomolecular industry.

LDH-Based Voltammetric Sensors

Layered double hydroxides (LDHs), also named hydrotalcite-like compounds, are anionic clays with a lamellar structure which have been extensively used in the last two decades as electrode modifiers for the design of electrochemical sensors. These materials can be classified into LDHs containing or not containing redox-active centers. In the former case, a transition metal cation undergoing a reversible redox reaction within a proper potential window is present in the layers, and, therefore, it can act as electron transfer mediator, and electrocatalyze the oxidation of an analyte for which the required overpotential is too high. In the latter case, a negatively charged species acting as a redox mediator can be introduced into the interlayer spaces after exchanging the anion coming from the synthesis, and, again, the material can display electrocatalytic properties. Alternatively, due to the large specific surface area of LDHs, molecules with electroactivity can be adsorbed on their surface. In this review, the most significant electroanalytical applications of LDHs as electrode modifiers for the development of voltammetric sensors are presented, grouping them based on the two types of materials.

Ti3AlC2 MAX Phase Modified Screen-Printed Electrode for the Fabrication of Hydrazine Sensor

Hydrazine is considered a powerful reducing agent and catalyst, showing diverse applications in agricultural industries, toxic degradation research, and wastewater management. Additionally, hydrazine can trigger some specific reactions when combined with suitable oxidants. Due to its highly polar nature, hydrazine can easily dissolve in alcohol, water, and various other polar solvents. Therefore, it can be extensively utilized in different areas of application and industries such as rocketry and various chemical applications. Despite its beneficial properties, hydrazine is unstable, posing significant risk due to its highly toxic nature. It is extremely hazardous to both human health and the environment. It can cause various illnesses and symptoms such as dizziness, temporary blindness, damage to the central nervous system, and even death when inhaled in sufficient quantities. Therefore, it is highly important to monitor the level of hydrazine to prevent its toxic and hazardous effects on human beings and the environment. In the present study, we discuss the simple fabrication of a disposable cost-effective and eco-friendly hydrazine sensor. We used a screen-printed carbon electrode, i.e., SPCE, as a base for the construction of a hydrazine sensor. The Ti3AlC2 MAX has been used as a suitable and efficient electrode material for the fabrication of disposable hydrazine sensors. We modified the active surface of the SPCE using a drop-casting approach. The resulting Ti3AlC2 MAX modified SPCE (Ti3AlC2@SPCE) has been utilized as an efficient and low-cost hydrazine sensor. Cyclic voltammetry, i.e., CV, and linear sweep voltammetry, viz., LSV, was employed as a sensing technique in this study. The optimization of pH and electrode material loading was conducted. The Ti3AlC2@SPCE exhibited excellent sensing performance toward hydrazine oxidation. A reasonable detection limit (0.01 µM) was achieved for hydrazine sensing. The fabricated sensor also demonstrated a reasonable linear range of 1-50 µM. This work provides the design and fabrication of simple disposable Ti3AlC2@SPCE as a suitable electrode for the determination of hydrazine using LSV technology.

Recent advances in photoelectrochemical platforms based on porous materials for environmental pollutant detection

Human health and ecology are seriously threatened by harmful environmental contaminants. It is essential to develop efficient and simple methods for their detection. Environmental pollutants can be detected using photoelectrochemical (PEC) detection technologies. The key ingredient in the PEC sensing system is the photoactive material. Due to the unique characteristics, such as a large surface area, enhanced exposure of active sites, and effective mass capture and diffusion, porous materials have been regarded as ideal sensing materials for the construction of PEC sensors. Extensive efforts have been devoted to the development and modification of PEC sensors based on porous materials. However, a review of the relationship between detection performance and the structure of porous materials is still lacking. In this work, we present an overview of PEC sensors based on porous materials. A number of typical porous materials are introduced separately, and their applications in PEC detection of different types of environmental pollutants are also discussed. More importantly, special attention has been paid to how the porous material's structure affects aspects like sensitivity, selectivity, and detection limits of the associated PEC sensor. In addition, future research perspectives in the area of PEC sensors based on porous materials are presented.

Recent advances using MXenes in biomedical applications

An MXene is a novel two-dimensional transition metal carbide or nitride, with a typical formula of Mn+1XnTx (M = transition metals, X = carbon or nitrogen, and T = functional groups). MXenes have found wide application in biomedicine and biosensing, owing to their high biocompatibility, abundant reactive surface groups, good conductivity, and photothermal properties. Applications include photo- and electrochemical sensors, energy storage, and electronics. This review will highlight recent applications of MXene and MXene-derived materials in drug delivery, tissue engineering, antimicrobial activity, and biosensors (optical and electrochemical). We further elaborate on recent developments in utilizing MXenes for photothermal cancer therapy, and we explore multimodal treatments, including the integration of chemotherapeutic agents or magnetic nanoparticles for enhanced therapeutic efficacy. The high surface area and reactivity of MXenes provide an interface to respond to the changes in the environment, allowing MXene-based drug carriers to respond to changes in pH, reactive oxygen species (ROS), and electrical signals for controlled release applications. Furthermore, the conductivity of MXene enables it to provide electrical stimulation for cultured cells and endows it with photocatalytic capabilities that can be used in antibiotic applications. Wearable and in situ sensors incorporating MXenes are also included. Major challenges and future development directions of MXenes in biomedical applications are also discussed. The remarkable properties of MXenes will undoubtedly lead to their increasing use in the applications discussed here, as well as many others.

A two-step electrochemical biosensor based on Tetrazyme for the detection of fibrin

In this study, an electrochemical biosensor was constructed for the detection of fibrin, specifically by a simple two-step approach, with a novel artificial enzyme (Tetrazyme) based on the DNA tetrahedral framework as signal probe. The multichannel screen-printed electrode with the activated surface cannot only remove some biological impurities, but also serve as a carrier to immobilize a large number of antigen proteins. The DNA tetrahedral nanostructure was employed to ensure the high sensitivity of the probe for biological analysis. The hemin was chimeric into the G-quadruplex to constitute the complex with peroxidase catalytic activity (hemin/G4-DNAzyme), subsequently, Tetrazyme was formed through combining of this complex and DNA tetrahedral nucleic acid framework. The artificial enzyme signal probe formed by the covalent combination of the homing peptide (Cys-Arg-Glu-Lys-Ala, CREKA), which is the aptamer of fibrin and the new artificial enzyme is fixed on the surface of the multichannel carbon electrode by CREKA-specific recognition, so as to realize the sensitive detection of fibrin. The feasibility of sensing platform was validated by cyclic voltammetry (CV) and amperometric i-t curve (IT) methods. Effects of Tetrazyme concentration, CREKA concentrations and hybridization time on the sensor were explored. Under the best optimal conditions of 0.6 μmol/L Tetrazyme, 80 μmol/L CREKA, and 2.5 h reaction time, the immunosensor had two linear detection ranges, 10-40 nmol/L, with linear regression equation Y = 0.01487X - 0.011 (R2 = 0.992), and 50-100 nmol/L, with linear regression equation Y = 0.00137X + 0.6405 (R2 = 0.998), the detection limit was 9.4 nmol/L, S/N ≥ 3. The biosensor could provide a new method with great potential for the detection of fibrin with good selectivity, stability, and reproducibility.

An electrochemical sensor founded on heterogeneous MXene & MOF composite for tanshinol sensing

A new kind of electrochemical sensor based on the MXene & MOF composite-modified carbon cloth was prepared firstly by self-assembly through hydrogen bonds, and then by air-annealing process for detection. The preparation processing introduced chemical bonds between MXene and MOF, which remarkably enhanced the electron transfer ability. Accordingly, combing the unique features of MXene and MOF themselves, the novel electrochemical sensor exhibited exceptional performance to detect tanshinol. Via differential pulse voltammetry, we could obtain a linear tanshinol concentration range of 0.08-8 μM and the limit of detection is 0.034 μM. Furthermore, this developed electrochemical sensor could determine concentrations of tanshinol in real Chinese herbal samples, confirming its practicability and reliability.

Screen-printed electrode designed with MXene/doped-polyindole and MWCNT/doped-polyindole for chronoamperometric enzymatic glucose sensor

The enzymatic glucose sensors as modified by MXene-dPIn and MWCNT-dPIn on a screen-printed carbon electrode (SPCE) were investigated. Herein, MXene was molybdenum carbide (Mo3C2) which has never been utilized and reported for glucose sensors. The biopolymer type to support the enzyme immobilization was examined and compared between chitosan (CHI) and κ-carrageenan (κC). MWCNT-dPIn obviously showed a larger electroactive surface area, lower charge transfer resistance and higher redox current than Mo3C2-dPIn, indicating that MWCNT-dPIn is superior to Mo3C2-dPIn. For the chitosan-based sensors, the sensitivity value of CHI-GOD/Mo3C2-dPIn is 3.53 μA mM-1 cm-2 in the linear range of 2.5-10 mM with the calculated LOD of 1.57 mM. The sensitivity value of CHI-GOD/MWCNT-dPIn is 18.85 μA mM-1 cm-2 in the linear range of 0.5-25 mM with the calculated LOD of 0.115 mM. For the κ-carrageenan based sensors, κC-GOD/MWCNT-dPIn exhibits the sensitivity of 15.80 μA mM-1 cm-2 and the widest linear range from 0.1 to 50 mM with the calculated LOD of 0.03 mM. The presently fabricated sensors exhibit excellent reproducibility, good selectivity, high stability, and disposal use. The fabricated glucose sensors are potential as practical glucose sensors as the detectable glucose ranges well cover the glucose levels found in blood, urine, and sweat for both healthy people and diabetic patients.

In the View of Electrons Transfer and Energy Conversion: The Antimicrobial Activity and Cytotoxicity of Metal-Based Nanomaterials and Their Applications

The global pandemic and excessive use of antibiotics have raised concerns about environmental health, and efforts are being made to develop alternative bactericidal agents for disinfection. Metal-based nanomaterials and their derivatives have emerged as promising candidates for antibacterial agents due to their broad-spectrum antibacterial activity, environmental friendliness, and excellent biocompatibility. However, the reported antibacterial mechanisms of these materials are complex and lack a comprehensive understanding from a coherent perspective. To address this issue, a new perspective is proposed in this review to demonstrate the toxic mechanisms and antibacterial activities of metal-based nanomaterials in terms of energy conversion and electron transfer. First, the antimicrobial mechanisms of different metal-based nanomaterials are discussed, and advanced research progresses are summarized. Then, the biological intelligence applications of these materials, such as biomedical implants, stimuli-responsive electronic devices, and biological monitoring, are concluded based on trappable electrical signals from electron transfer. Finally, current improvement strategies, future challenges, and possible resolutions are outlined to provide new insights into understanding the antimicrobial behaviors of metal-based materials and offer valuable inspiration and instructional suggestions for building future intelligent environmental health.

Approach to Significantly Enhancing the Electrochromic Performance of PANi by In Situ Electrodeposition of the PANi@MXene Composite Film

Electrochromic materials (ECMs) are capable of reversibly adjusting their transmittance or reflectance properties in response to changes in the external biasing voltages. In this study, we enhanced the electrochromic and electrochemical properties of polyaniline (PANi) effectively through the incorporation of MXene Ti2CTx using an in situ composite strategy. This improvement in the electrochromic and electrochemical properties observed can be attributed to the intermolecular forces between the aniline group of PANi and the terminal groups of MXene Ti2CTx sheets. The presence of hydrogen bonds between the PANi monomers and the MXene sheets was confirmed through theoretical calculations and photoluminescence results, which effectively improved the composite interfaces. Additionally, the PANi@MXene composite films were successfully prepared through a simple one-step in situ polymerization process, as verified by SEM and XPS characterization. The electrochemical studies revealed enhanced electronic conductivity, a high ion diffusion coefficient, and a narrow energy redox gap, all contributing to the excellent electrochemical properties observed. Overall, our results demonstrate that the MXene Ti2CTx composition effectively enhances the electrochromic performance of PANi. The PANi@MXene composite films exhibited a high optical modulation range, rapid switching response time, good thermal radiation regulation, and excellent operational stability. This composite strategy significantly improves the performance and practical applicability of ECMs.

Facile and Affordable Design of MXene-Co3 O4 -Based Nanocomposites for Detection of Hydrogen Peroxide in Cancer Cells: Toward Portable Tool for Cancer Management

Hydrogen peroxide (H2 O2 ) is a primary reactive oxygen species (ROS) that can act as a chemical signal in developing and progressing serious and life-threatening diseases like cancer. Due to the stressful nature of H2 O2 , there is an urgent need to develop sensitive analytical approaches to be applied to various biological matrices. Herein, a portable point-of-care electrochemical system based on MXene-Co3 O4 nanocomposites to detect H2 O2 in different cancer cell-lines is presented. The developed sensor is affordable, disposable, and highly selective for H2 O2 detection. This approach achieves a dynamic linear range of 75 µm with a LOD of 0.5 µm and a LOQ of 1.6 µm. To improve the practical application, the level of ROS is evaluated both in cancer cell lines MDA-MB-231 and DU145, respectively, to breast and prostate cancers, and in healthy HaCat cells. Moreover, the same cancer cells are treated with transforming growth factor-β1, and MXene-Co3 O4 modified strip is capable to monitorROS variation. The results are satisfactory compared with the cellular ROS fluorescent assay based on DCFH/DCFH-DA. These results open new perspectives for real-time monitoring of cancer progression and the efficacy of the therapy.

A review on solution- and vapor-responsive sensors for the detection of phthalates

Phthalic acid esters, largely referred to as phthalates, are today acknowledged as important pollutants used in the manufacture of polyvinyl chloride (PVC)-based plastics, whose use extends to almost every aspect of modern life. The risk of exposure to phthalates is particularly relevant as high concentrations are regularly found in drinking water, food-contact materials and medical devices, motivating an immense body of research devoted to methods for their detection in liquid samples. Conversely, phthalate vapors have only recently been acknowledged as potentially important atmospheric pollutants and as early fire indicators; additionally, deposition of these vapors can pose significant problems to the proper functioning of spacecraft and diverse on-board devices, leading to major space agencies recognizing the need of developing vapor-responsive phthalate sensors. In this manuscript we present a literature survey on solution- and vapor-responsive sensors and analytical assays for the detection of phthalates, providing a detailed analysis of a vast array of analytical data to offer a clear idea on the analytical performance (limits of detection and quantification, linear range) and advantages provided by each class of sensor covered in this review (electrochemical, optical and vapor-responsive) in the context of their potential real-life applications; the manuscript also gives detailed fundamental information on the various physicochemical responses exploited by these sensors and assays that could potentially be harnessed by new researchers entering the field.

Fabrication of graphene oxide-based pretreatment filter and Electrochemical-CRISPR biosensor for the field-ready cyanobacteria monitoring system

Microcystis aeruginosa (M. aeruginosa) cause the eutrophication of lakes and rivers. To effectively control the overgrowth of M. aeruginosa, a suitable measurement method should be required in the aquatic fields. To address this, we developed a field-ready cyanobacterial pretreatment device and an electrochemical clustered regularly interspaced short palindromic repeats (EC-CRISPR) biosensor. The cyanobacterial pretreatment device consists of a syringe, glass bead, and graphene oxide (GO) bead. Then, the M. aeruginosa dissolved in the freshwater sample was added to fabricated filter. After filtration, the purified gene was loaded onto a CRISPR-based electrochemical biosensor chip to detect M. aeruginosa gene fragments. The biosensor was composed of CRISPR/Cpf1 protein conjugated with MXene on an Au microgap electrode (AuMGE) integrated into a printed circuit board (PCB). This AuMGE/PCB system maximizes the signal-to-noise ratio, which controls the working and counter electrode areas requiring only 3 μL samples to obtain high reliability. Using the extracted M. aeruginosa gene with a pre-treatment filter, the CRISPR biosensor showed a limit of detection of 0.089 pg/μl in fresh water. Moreover, selectivity test and matrix condition test carried out using the EC-CRISPR biosensor. These handheld pre-treatment kit and biosensors can enable field-ready detection of CyanoHABs.

Versatile MXene composite probe-mediated homogeneous electrochemiluminescence biosensor with integrated signal transduction and near-infrared modulation strategy for concanavalin A detection

Based on the highly specific interaction between concanavalin A (Con A) and glucose (Glu), a competitive electrochemiluminescence (ECL) biosensor was constructed for ultrasensitive detection of Con A. Nanocomposites with excellent electrocatalytic and photothermal properties were obtained by covalently bonding zinc oxide quantum dots (ZnO QDs) to vanadium carbide MXene (V2C MXene) surfaces. The modification of ZnO QDs hinders the aggregation of V2C MXene and increases the catalytic activity of oxygen reduction reaction, thus amplifying the luminol cathodic emission. In addition, the excellent photothermal performance of the V2C MXene-ZnO QDs can convert light energy into heat energy under the irradiation of 808 nm near infrared laser, thus increasing the temperature of the reaction system and accelerating the electron transfer process to realize the synergistic amplified homogeneous ECL system. This innovative work not only enriches the fundamental research on multifunctional MXene nanomaterials for biosensing, but also provides an effective strategy for ECL signal amplification.

Electrostatic Self-Assembly of MXene on Ruthenium Dioxide-Modified Carbon Cloth for Electrochemical Detection of Kaempferol

A superior composite material consisting of MXene and ruthenium dioxide-modified carbon cloth is synthesized by pulsed laser deposition and electrostatic self-assembly, which is further utilized to construct a class of novel electrochemical (EC) sensors for kaempferol (KA) detection. The carbon-cloth-based electrodes modified by ruthenium dioxide and then MXene are characterized by X-ray diffraction, scanning electron microscope, and X-ray photoemission spectroscopy. The EC process on the modified electrodes is analyzed by cyclic voltammetry, EC impedance spectroscopy, and differential pulse voltammetry. It is found that positively charged RuO2 not only possesses the remarkable electrical conductivity and electrocatalysis activity but also hampers the restacking of MXene, which accordingly enhances the exposure of the active surface area and greatly boosts the electrocatalysis activity of the entire composite. Consequently, this newly developed composite-based EC sensor exhibits a high sensitivity, selectivity, and remarkable stability to detect KA with two linear ranges of 0.06-1 and 1-15 µM. The inferred limit of detection is 0.039 µM via differential pulse voltammetry. More importantly, this novel EC sensor is found to be applicable for detecting KA in practical traditional Chinese medicines.

Recent Advances in Two-Dimensional MXene-Based Electrochemical Biosensors for Sweat Analysis

Sweat, a biofluid secreted naturally from the eccrine glands of the human body, is rich in several electrolytes, metabolites, biomolecules, and even xenobiotics that enter the body through other means. Recent studies indicate a high correlation between the analytes' concentrations in the sweat and the blood, opening up sweat as a medium for disease diagnosis and other general health monitoring applications. However, low concentration of analytes in sweat is a significant limitation, requiring high-performing sensors for this application. Electrochemical sensors, due to their high sensitivity, low cost, and miniaturization, play a crucial role in realizing the potential of sweat as a key sensing medium. MXenes, recently developed anisotropic two-dimensional atomic-layered nanomaterials composed of early transition metal carbides or nitrides, are currently being explored as a material of choice for electrochemical sensors. Their large surface area, tunable electrical properties, excellent mechanical strength, good dispersibility, and biocompatibility make them attractive for bio-electrochemical sensing platforms. This review presents the recent progress made in MXene-based bio-electrochemical sensors such as wearable, implantable, and microfluidic sensors and their applications in disease diagnosis and developing point-of-care sensing platforms. Finally, the paper discusses the challenges and limitations of MXenes as a material of choice in bio-electrochemical sensors and future perspectives on this exciting material for sweat-sensing applications.

Advances in MXene-Based Electrochemical (Bio)Sensors for Neurotransmitter Detection

Neurotransmitters are chemical messengers that play an important role in the nervous system's control of the body's physiological state and behaviour. Abnormal levels of neurotransmitters are closely associated with some mental disorders. Therefore, accurate analysis of neurotransmitters is of great clinical importance. Electrochemical sensors have shown bright application prospects in the detection of neurotransmitters. In recent years, MXene has been increasingly used to prepare electrode materials for fabricating electrochemical neurotransmitter sensors due to its excellent physicochemical properties. This paper systematically introduces the advances in MXene-based electrochemical (bio)sensors for the detection of neurotransmitters (including dopamine, serotonin, epinephrine, norepinephrine, tyrosine, NO, and H₂S), with a focus on their strategies for improving the electrochemical properties of MXene-based electrode materials, and provides the current challenges and future prospects for MXene-based electrochemical neurotransmitter sensors.

Process Controlled Ruthenium on 2D Engineered V-MXene via Atomic Layer Deposition for Human Healthcare Monitoring

In searching for unique and unexplored 2D materials, the authors try to investigate for the very first time the use of delaminated V-MXene coupled with precious metal ruthenium (Ru) through atomic layer deposition (ALD) for various contact and noncontact mode of real-time temperature sensing applications at the human-machine interface. The novel delaminated V-MXene (DM-V₂ CT_x ) engineered ruthenium-ALD (Ru-ALD) temperature sensor demonstrates a competitive sensing performance of 1.11% °C^-1 as of only V-MXene of 0.42% °C^-1 . A nearly threefold increase in sensing and reversibility performance linked to the highly ordered few-layered V-MXene and selective, well-controlled Ru atomic doping by ALD for the successful formation of Ru@DM-V₂ CT_X heterostructure. The advanced heterostructure formation, the mechanism, and the role of Ru have been comprehensively investigated by ultra-high-resolution transmission/scanning transmission electron microscopies coupled with next-generation spherical aberration correction technology and fast, accurate elemental mapping quantifications, also by ultraviolet photoelectron spectroscopy. To the knowledge, this work is the first to use the novel, optimally processed V-MXene over conventionally used Ti-MXene and its surface-internal structure engineering by Ru-ALD process-based temperature-sensing devices function and operational demonstrations. The current work could potentially motivate the development of multifunctional, future, next-generation, safe, personal healthcare electronic devices by the industrially scalable ALD technique.

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.

MXenes Antibacterial Properties and Applications: A Review and Perspective

The mutations of bacteria due to the excessive use of antibiotics, and generation of antibiotic-resistant bacteria have made the development of new antibacterial compounds a necessity. MXenes have emerged as biocompatible transition metal carbide structures with extensive biomedical applications. This is related to the MXenes' unique combination of properties, including multifarious elemental compositions, 2D-layered structure, large surface area, abundant surface terminations, and excellent photothermal and photoelectronic properties. The focus of this review is the antibacterial application of MXenes, which has attracted the attention of researchers since 2016. A quick overview of the synthesis strategies of MXenes is provided and then summarizes the effect of various factors (including structural properties, optical properties, surface charges, flake size, and dispersibility) on the biocidal activity of MXenes. The main mechanisms for deactivating bacteria by MXenes are discussed in detail including rupturing of the bacterial membrane by sharp edges of MXenes nanoflakes, generating the reactive oxygen species (ROS), and photothermal deactivating of bacteria. Hybridization of MXenes with other organic and inorganic materials can result in materials with improved biocidal activities for different applications such as wound dressings and water purification. Finally, the challenges and perspectives of MXene nanomaterials as biocidal agents are presented.

Recent Developments in the Design and Fabrication of Electrochemical Biosensors Using Functional Materials and Molecules

Electrochemical biosensors are superior technologies that are used to detect or sense biologically and environmentally significant analytes in a laboratory environment, or even in the form of portable handheld or wearable electronics. Recently, imprinted and implantable biosensors are emerging as point-of-care devices, which monitor the target analytes in a continuous environment and alert the intended users to anomalies. The stability and performance of the developed biosensor depend on the nature and properties of the electrode material or the platform on which the biosensor is constructed. Therefore, the biosensor platform plays an integral role in the effectiveness of the developed biosensor. Enormous effort has been dedicated to the rational design of the electrode material and to fabrication strategies for improving the performance of developed biosensors. Every year, in the search for multifarious electrode materials, thousands of new biosensor platforms are reported. Moreover, in order to construct an effectual biosensor, the researcher should familiarize themself with the sensible strategies behind electrode fabrication. Thus, we intend to shed light on various strategies and methodologies utilized in the design and fabrication of electrochemical biosensors that facilitate sensitive and selective detection of significant analytes. Furthermore, this review highlights the advantages of various electrode materials and the correlation between immobilized biomolecules and modified surfaces.

Harnessing protein sensing ability of electrochemical biosensors via a controlled peptide receptor-electrode interface

BACKGROUND

Cathepsin B, a cysteine protease, is considered a potential biomarker for early diagnosis of cancer and inflammatory bowel diseases. Therefore, more feasible and effective diagnostic method may be beneficial for monitoring of cancer or related diseases.

RESULTS

A phage-display library was biopanned against biotinylated cathepsin B to identify a high-affinity peptide with the sequence WDMWPSMDWKAE. The identified peptide-displaying phage clones and phage-free synthetic peptides were characterized using enzyme-linked immunosorbent assays (ELISAs) and electrochemical analyses (impedance spectroscopy, cyclic voltammetry, and square wave voltammetry). Feasibilities of phage-on-a-sensor, peptide-on-a-sensor, and peptide-on-a-AuNPs/MXene sensor were evaluated. The limit of detection and binding affinity values of the peptide-on-a-AuNPs/MXene sensor interface were two to four times lower than those of the two other sensors, indicating that the peptide-on-a-AuNPs/MXene sensor is more specific for cathepsin B (good recovery (86-102%) and %RSD (< 11%) with clinical samples, and can distinguish different stages of Crohn's disease. Furthermore, the concentration of cathepsin B measured by our sensor showed a good correlation with those estimated by the commercially available ELISA kit.

CONCLUSION

In summary, screening and rational design of high-affinity peptides specific to cathepsin B for developing peptide-based electrochemical biosensors is reported for the first time. This study could promote the development of alternative antibody-free detection methods for clinical assays to test inflammatory bowel disease and other diseases.

Nanobiosensors Design Using 2D Materials: Implementation in Infectious and Fatal Disease Diagnosis

Nanobiosensors are devices that utilize a very small probe and any form of electrical, optical, or magnetic technology to detect and analyze a biochemical or biological process. With an increasing population today, nanobiosensors have become the broadly used electroanalytical tools for the timely detection of many infectious (dengue, hepatitis, tuberculosis, leukemia, etc.) and other fatal diseases, such as prostate cancer, breast cancer, etc., at their early stage. Compared to classical or traditional analytical methods, nanobiosensors have significant benefits, including low detection limit, high selectivity and sensitivity, shorter analysis duration, easier portability, biocompatibility, and ease of miniaturization for on-site monitoring. Very similar to biosensors, nanobiosensors can also be classified in numerous ways, either depending on biological molecules, such as enzymes, antibodies, and aptamer, or by working principles, such as optical and electrochemical. Various nanobiosensors, such as cyclic voltametric, amperometric, impedimetric, etc., have been discussed for the timely monitoring of the infectious and fatal diseases at their early stage. Nanobiosensors performance and efficiency can be enhanced by using a variety of engineered nanostructures, which include nanotubes, nanoparticles, nanopores, self-adhesive monolayers, nanowires, and nanocomposites. Here, this mini review recaps the application of two-dimensional (2D) materials, especially graphitic carbon nitride (g-C₃N₄), graphene oxide, black phosphorous, and MXenes, for the construction of the nanobiosensors and their application for the diagnosis of various infectious diseases at very early stage.

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.

Chemically Exfoliated Titanium Carbide MXene for Highly Sensitive Electrochemical Sensors for Detection of 4-Nitrophenols in Drinking Water

Soil and water contamination by numerous pollutants has been increasingly posing threats to food, water, agriculture, and human health. Using novel nanoscale materials to develop rapid electrochemical sensors is very promising due to the discovery of a number of new two-dimensional (2D) electronic materials. Of particular importance is 2D transition-metal carbide MXene that has been shown to possess transformative properties pertaining to its physical, chemical, and environmental characteristics, leading to their potential sensor applications. Designing electrochemical sensors using MXene has the potential to pave the way for monitoring environmental pollutants. Here, a stacked layer of chemically exfoliated MXene (Ti₃C₂T _x ) was demonstrated as an electrochemical sensor for detection of 4-nitrophenol (4-NP) with high sensitivity and a low limit of detection. Successful selective exfoliation of the MAX (Ti₃AlC₂) phase of the material by chemical etching without oxidation is shown to be the key to achieving higher sensitivity and a lower detection limit. In the optimal conditions, the proposed MXene sensor electrodes were capable of detecting 4-NP in a broad concentration range from 500 nM to 100 μM with a good linear sensing range (regression fit, R = 0.995). The higher sensitivity and notable limit of detection reached about 16.35 μA μM^-1 cm^-2 and 42 nM/L, respectively, with good reproducibility and repeatability. The real-time application of the proposed MXene sensor electrodes was confirmed by testing in tap water samples with excellent recoveries of 95-99%.

MXene-Based Composites as Nanozymes in Biomedicine: A Perspective

MXene-based nanozymes have garnered considerable attention because of their potential environmental and biomedical applications. These materials encompass alluring and manageable catalytic performances and physicochemical features, which make them suitable as (bio)sensors with high selectivity/sensitivity and efficiency. MXene-based structures with suitable electrical conductivity, biocompatibility, large surface area, optical/magnetic properties, and thermal/mechanical features can be applied in designing innovative nanozymes with area-dependent electrocatalytic performances. Despite the advances made, there is still a long way to deploy MXene-based nanozymes, especially in medical and healthcare applications; limitations pertaining the peroxidase-like activity and sensitivity/selectivity may restrict further practical applications of pristine MXenes. Thus, developing an efficient surface engineering tactic is still required to fabricate multifunctional MXene-based nanozymes with excellent activity. To obtain MXene-based nanozymes with unique physicochemical features and high stability, some crucial steps such as hybridization and modification ought to be performed. Notably, (nano)toxicological and long-term biosafety analyses along with clinical translation studies still need to be comprehensively addressed. Although very limited reports exist pertaining to the biomedical potentials of MXene-based nanozymes, the future explorations should transition toward the extensive research and detailed analyses to realize additional potentials of these structures in biomedicine with a focus on clinical and industrial aspects. In this perspective, therapeutic, diagnostic, and theranostic applications of MXene-based nanozymes are deliberated with a focus on future perspectives toward more successful clinical translational studies. The current state-of-the-art biomedical advances in the use of MXene-based nanozymes, as well as their developmental challenges and future prospects are also highlighted. In view of the fascinating properties of MXene-based nanozymes, these materials can open significant new opportunities in the future of bio- and nanomedicine.

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.

Biomedical Applications of an Ultra-Sensitive Surface Plasmon Resonance Biosensor Based on Smart MXene Quantum Dots (SMQDs)

In today’s world, the use of biosensors occupies a special place in a variety of fields such as agriculture and industry. New biosensor technologies can identify biological compounds accurately and quickly. One of these technologies is the phenomenon of surface plasmon resonance (SPR) in the development of biosensors based on their optical properties, which allow for very sensitive and specific measurements of biomolecules without time delay. Therefore, various nanomaterials have been introduced for the development of SPR biosensors to achieve a high degree of selectivity and sensitivity. The diagnosis of deadly diseases such as cancer depends on the use of nanotechnology. Smart MXene quantum dots (SMQDs), a new class of nanomaterials that are developing at a rapid pace, are perfect for the development of SPR biosensors due to their many advantageous properties. Moreover, SMQDs are two-dimensional (2D) inorganic segments with a limited number of atomic layers that exhibit excellent properties such as high conductivity, plasmonic, and optical properties. Therefore, SMQDs, with their unique properties, are promising contenders for biomedicine, including cancer diagnosis/treatment, biological sensing/imaging, antigen detection, etc. In this review, SPR biosensors based on SMQDs applied in biomedical applications are discussed. To achieve this goal, an introduction to SPR, SPR biosensors, and SMQDs (including their structure, surface functional groups, synthesis, and properties) is given first; then, the fabrication of hybrid nanoparticles (NPs) based on SMQDs and the biomedical applications of SMQDs are discussed. In the next step, SPR biosensors based on SMQDs and advanced 2D SMQDs-based nanobiosensors as ultrasensitive detection tools are presented. This review proposes the use of SMQDs for the improvement of SPR biosensors with high selectivity and sensitivity for biomedical applications.

Recent Advanced in MXene Research toward Biosensor Development

MXene is a rapidly emerging group of two-dimensional (2D) multifunctional nanomaterials, drawing huge attention from researchers of a broad scientific field. Reporting the synthesis of MXene was the following breakthrough in 2D materials following the discovery of graphene. MXene is considered the most recent developments of materials, including transition metal carbonitrides, nitrides, and carbides synthesized by etching or mechanical-based exfoliation of selective MAX phases. MXene has a plethora of prodigious properties such as unique interlayer spacing, high ion and electron transport, large surface area, excellent thermal and electrical conductivity, exceptional volumetric capacitance, thermal shock, and oxidation resistance, easily machinable and inherently hydrophilic, and biocompatibility. Owing to the abundance of tailorable surface function groups, these properties can be further enhanced by surface functionalization with covalent and non-covalent modifications via numerous surface functionalization methods. Therefore, MXene finds their way to a plethora of applications in numerous fields including catalysis, membrane separation, energy storage, sensing, and biomedicine. Here, the focus is on reviewing the structure, synthesis techniques, and functionalization methods of MXene. Furthermore, MXene-based detection platforms in different sensing applications are survived. Great attention is given to reviewing the applications of MXene in the detection of biomolecules, pathogenic bacteria and viruses, cancer biomarkers food contaminants and mycotoxins, and hazardous pollutants. Lastly, the future perspective of MXene-based biosensors as a next-generation diagnostics tool is discussed. Crucial visions are introduced for materials science and sensing communities to better route while investigating the potential of MXene for creating innovative detection mechanisms.

MXene-Chitosan Composites and Their Biomedical Potentials

Today, MXenes with fascinating electronic, thermal, optical, and mechanical features have been broadly studied for biomedical applications, such as drug/gene delivery, photothermal/photodynamic therapy, antimicrobials/antivirals, sensing, tissue engineering, and regenerative medicine. In this context, various MXene-polymer composites have been designed to improve the characteristics such as physiological stability, sustained/controlled release behaviors, biodegradability, biocompatibility, selectivity/sensitivity, and functionality. Chitosan with advantages of ease of modification, biodegradability, antibacterial activities, non-toxicity, and biocompatibility can be considered as attractive materials for designing hybridized composites together with MXenes. These hybrid composites ought to be further explored for biomedical applications because of their unique properties such as high photothermal conversion efficiency, improved stability, selectivity/sensitivity, stimuli-responsiveness behaviors, and superior antibacterial features. These unique structural, functional, and biological attributes indicate that MXene-chitosan composites are attractive alternatives in biomedical engineering. However, several crucial aspects regarding the surface functionalization/modification, hybridization, nanotoxicological analyses, long-term biosafety assessments, biocompatibility, in vitro/in vivo evaluations, identification of optimization conditions, implementation of environmentally-benign synthesis techniques, and clinical translation studies are still need to be examined by researchers. Although very limited studies have revealed the great potentials of MXene-chitosan hybrids in biomedicine, the next steps should be toward the extensive research and detailed analyses in optimizing their properties and improving their functionality with a clinical and industrial outlook. Herein, recent developments in the use of MXene-chitosan composites with biomedical potentials are deliberated, with a focus on important challenges and future perspectives. In view of the fascinating properties and multifunctionality of MXene-chitosan composites, these hybrid materials can open significant new opportunities in the future for bio- and nano-medicine arena.

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.

Recent Developments and Future Perspective on Electrochemical Glucose Sensors Based on 2D Materials

Diabetes is a health disorder that necessitates constant blood glucose monitoring. The industry is always interested in creating novel glucose sensor devices because of the great demand for low-cost, quick, and precise means of monitoring blood glucose levels. Electrochemical glucose sensors, among others, have been developed and are now frequently used in clinical research. Nonetheless, despite the substantial obstacles, these electrochemical glucose sensors face numerous challenges. Because of their excellent stability, vast surface area, and low cost, various types of 2D materials have been employed to produce enzymatic and nonenzymatic glucose sensing applications. This review article looks at both enzymatic and nonenzymatic glucose sensors made from 2D materials. On the other hand, we concentrated on discussing the complexities of many significant papers addressing the construction of sensors and the usage of prepared sensors so that readers might grasp the concepts underlying such devices and related detection strategies. We also discuss several tuning approaches for improving electrochemical glucose sensor performance, as well as current breakthroughs and future plans in wearable and flexible electrochemical glucose sensors based on 2D materials as well as photoelectrochemical sensors.

Recent Advances of MXene-Based Electrochemical Immunosensors

Electrochemical immunosensors are the largest class of affinity biosensing devices with strong practicability. In recent years, MXenes have become hotspot materials of electrochemical biosensors for their excellent properties, including large specific surface area, good electrical conductivity, high hydrophilicity and rich functional groups. In this review, we firstly introduce the composition and structure of MXenes, as well as their properties relevant to the construction of biosensors. Then, we summarize the recent advances of MXenes-based electrochemical immunosensors, focusing on the roles of MXenes in various electrochemical immunosensors. Finally, we analyze current problems of MXenes-based electrochemical immunosensors and propose an outlook for this research field.

Electrochemical Sensing of Glucose Using Glucose Oxidase/PEDOT:4-Sulfocalix [4]arene/MXene Composite Modified Electrode

Glucose is one of the most important monosaccharides found in the food, as a part of more complex structures, which is a primary energy source for the brain and body. Thus, the monitoring of glucose concentration is more important in food and biological samples in order to maintain a healthy lifestyle. Herein, an electrochemical glucose biosensor was fabricated by immobilization of glucose oxidase (GOX) onto poly(3,4-ethylenedioxythiophene):4-sulfocalix [4]arene (PEDOT:SCX)/MXene modified electrode. For this purpose, firstly, PEDOT was synthesized in the presence of SCX (counterion) by the chemical oxidative method. Secondly, MXene (a 2D layered material) was synthesized by using a high-temperature furnace under a nitrogen atmosphere. After that, PEDOT:SCX/MXene (1:1) dispersion was prepared by ultrasonication which was later utilized to prepare PEDOT:SCX/MXene hybrid film. A successful formation of PEDOT:SCX/MXene film was confirmed by HR-SEM, Fourier transform infrared (FT-IR), and Raman spectroscopies. Due to the biocompatibility nature, successful immobilization of GOX was carried out onto chitosan modified PEDOT:SCX/MXene/GCE. Moreover, the electrochemical properties of PEDOT:SCX/MXene/GOX/GCE was studied through cyclic voltammetry and amperometry methods. Interestingly, a stable redox peak of FAD-GOX was observed at a formal potential of –0.435 V on PEDOT:SCX/MXene/GOX/GCE which indicated a direct electron transfer between the enzyme and the electrode surface. PEDOT:SCX/MXene/GOX/GCE also exhibited a linear response against glucose concentrations in the linear range from 0.5 to 8 mM. The effect of pH, sensors reproducibility, and repeatability of the PEDOT:SCX/MXene/GOX/GCE sensor were studied. Finally, this new biosensor was successfully applied to detect glucose in commercial fruit juice sample with satisfactory recovery.

Preparation of a disposable electrochemiluminescence sensor chip based on an MXene-loaded ruthenium luminescent agent and its application in the detection of carcinoembryonic antigens

We developed a novel type of disposable ECL sensor chip for CEA detection in serum samples of healthy humans and cancer patients. The disposable ECL sensor chip has many advantages including convenience, rapid detection, low cost, and easy mass production.

MXene-modified molecularly imprinted polymer as an artificial bio-recognition platform for efficient electrochemical sensing: progress and perspectives

The development of efficient electrochemical sensors of exceptional features, molecularly imprinted polymers (MIPs) have been extensively utilized due to their great vitality as an alternative to bio-recognition elements. MIPs as...

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,...

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.