Single-Molecule Sensing Using Nanopores in Two-Dimensional Transition Metal Carbide (MXene) Membranes

Simultaneous detection of multiple mycotoxins using MXene-based electrochemical aptasensor array and a self-developed multi-channel portable device

In response to the pressing need for highly efficient simultaneous detection of multiple mycotoxins, which are often found co-occurring in food raw materials and feed, an MXene-based electrochemical aptasensor array (MBEAA) was developed. This aptasensor array utilizes high-specificity aptamers as recognition elements, enabling the capture of electrical signal changes in the presence of target mycotoxins. Based on this platform, a multi-channel portable electrochemical device, enabling rapid, cost-effective, and simultaneous detection of aflatoxin B1 (AFB1), ochratoxin A (OTA), and zealenone (ZEN) was further developed. The developed system boasts a wide detection range of 1.0 × 10-1 to 10.0 ng mL-1, with remarkable performance characterized by ultra-low detection limits of 41.2 pg mL-1, 27.6 pg mL-1, and 33.0 pg mL-1 for AFB1, OTA, and ZEN, respectively. Successfully applied in corn samples, this method offers a portable, easy-to-operate, and cost-effective solution for simultaneous multi-mycotoxin detection. Moreover, the application of the self-developed detection system could be expanded for simultaneous detection of many different targets when their specific aptamers or antibodies were available.

Sequence-Dependent Single-Molecule DNA Sensing Using Covalent Organic Framework Nanopores

Enzyme-free single-molecule sequencing has the potential to significantly expand the application of nanopore technology to DNA, proteins, and saccharides. Despite their advantages over biological nanopores and natural suitability for enzyme-free single-molecule sequencing, conventional solid-state nanopores have not yet achieved single-molecule DNA sequencing. The biggest challenge for the accuracy of single-molecule sequencing using solid-state nanopores lies in the precise control of the pore size and conformity. In this study, we fabricated nanopore devices by covering the tip of a quartz nanopipette with ultrathin two-dimensional (2D) covalent organic framework (COF) nanosheets (pore size ≈ 1.1 nm). The size of the periodically arranged nanopores in COF is comparable to that of protein nanopores, and the structure of each COF nanopore is consistent at the atomic scale. The COF nanopore device could roughly distinguish dAMP, dCMP, dGMP, and dTMP. Furthermore, a certain percentage of the current blockades originating from 150 nucleotides model DNA molecules (13.5% for dA50dC50dA50 and 11.1% for dC50dA50dC50) show distinct DNA sequence-specific concave and convex resistive current patterns. The finite element simulation confirmed that the current blockade pattern of a DNA molecule passing through a COF nanopore is dependent on the relative location of the nanopore with respect to the wall of the nanopipette. Our study is a significant step toward single-molecule DNA sequencing by solid-state nanopores.

Hydrophilicity and surface charge modulation of Ti3C2T x MXene based membranes for water desalination

Lamellar membranes obtained by stacking 2D layers possess ample transport pathways due to their intricate network of interlayer gaps. This makes them suitable for molecular separation applications. However, controlling the surface chemistry of the nanochannels within the membrane to tune the desired transport properties of water and ions is challenging. Ti3C2T x has been considered for water desalination because of its hydrophilic surface and negative surface charge. Most of the studies of Ti3C2T x membranes have presented promising salt rejection values in forward osmosis mode, which is less practical for water purification. Here, we investigate two types of reverse osmosis MXene-based lamellar membranes consisting of Ti3C2T x nanosheets hybridized with (i) WS2 nanosheets and (ii) polyvinyl phosphonic acid (PVPA). When hydrophilic and flexible Ti3C2T x nanosheets are interleaved with softer and more hydrophobic WS2 nanosheets in 2 : 1 mass ratio, nano capillaries with Janus chemistry are created with comparable rejection to bare Ti3C2T x membrane and threefold higher permeance values. Further, we find that decorating Ti3C2T x nanosheets with anionic polymers improves salt rejection. Our Ti3C2T x /PVPA composite membranes reject ∼97% of divalent ions and ∼80% of monovalent ions with ∼0.2 Lm-2 h-1 bar-1 of water permeance when tested with brackish water, and exhibit significantly improved chlorine resistance and cost benefits over the commercial Toray membranes.

In-situ and label-free measurement of cytochrome C concentration with a Ti2C-MXene sensitized fiber-optic MZI sensor

BACKGROUND

The concentration of cytochrome C is demonstrated to be an effective indicator of the microbial corrosion strength of metals. Traditional cytochrome C sensor can detect cytochrome C with a low detection limit, but their use is limited by their high cost, cumbersome operation, and susceptibility to malignant environments. In addition, studies on the monitoring of cytochrome C in the field of microbial corrosion has still not been carried out. Therefore, there is a need for a highly sensitive, selective, low-cost, anti-interference, and stable cytochrome C sensor with online monitoring and remote sensing capabilities for in-situ measurement of microbial corrosion strength.

RESULTS

This paper proposed a highly sensitive label-free fiber-optic sensor based on Mach-Zehnder interferometer (MZI) for in-situ measurement of the microbial corrosion marker cytochrome C. Two-dimensional Ti2C-MXene material is uniformly immobilized onto the surface of the sensing area to improve the sensitivity, hydrophilicity, and specific surface area of the sensing area, as well as to facilitate the immobilization of specific sensitive materials. The cytochrome C antibody is modified on the surface of Ti2C-MXene to specifically recognize cytochrome C, whose concentration variation can be measured by monitoring the spectral shift of MZI sensor. Results demonstrate a measurement sensitivity of 1.428 nm/μM for cytochrome C concentrations ranging from 0 to 7.04 μM. The detection limit of the sensor is calculated to be 0.392 μM with remarkable performance, including selectivity, stability, and reliability. Besides, the measurement result of the proposed sensor in real microbial corrosive environment is consistent with that of the ideal environment.

SIGNIFICANCE AND NOVELTY

This is the first instance of achieving in-situ and label-free measurement of cytochrome C by using a fiber-optic MZI sensor, which undoubtedly provides a feasible solution for the effective monitoring of microbial metal corrosion in the environment.

Direct Fabrication of Atomically Defined Pores in MXenes Using Feedback-Driven STEM

Controlled fabrication of nanopores in 2D materials offer the means to create robust membranes needed for ion transport and nanofiltration. Techniques for creating nanopores have relied upon either plasma etching or direct irradiation; however, aberration-corrected scanning transmission electron microscopy (STEM) offers the advantage of combining a sub-Å sized electron beam for atomic manipulation along with atomic resolution imaging. Here, a method for automated nanopore fabrication is utilized with real-time atomic visualization to enhance the mechanistic understanding of beam-induced transformations. Additionally, an electron beam simulation technique, Electron-Beam Simulator (E-BeamSim) is developed to observe the atomic movements and interactions resulting from electron beam irradiation. Using the MXene Ti3C2Tx, the influence of temperature on nanopore fabrication is explored by tracking atomic transformations and find that at room temperature the electron beam irradiation induces random displacement and results in titanium pileups at the nanopore edge, which is confirmed by E-BeamSim. At elevated temperatures, after removal of the surface functional groups and with the increased mobility of atoms results in atomic transformations that lead to the selective removal of atoms layer by layer. This work can lead to the development of defect engineering techniques within functionalized MXene layers and other 2D materials.

Emerging Data Processing Methods for Single-Entity Electrochemistry

Single-entity electrochemistry is a powerful tool that enables the study of electrochemical processes at interfaces and provides insights into the intrinsic chemical and structural heterogeneities of individual entities. Signal processing is a critical aspect of single-entity electrochemical measurements and can be used for data recognition, classification, and interpretation. In this review, we summarize the recent five-year advances in signal processing techniques for single-entity electrochemistry and highlight their importance in obtaining high-quality data and extracting effective features from electrochemical signals, which are generally applicable in single-entity electrochemistry. Moreover, we shed light on electrochemical noise analysis to obtain single-molecule frequency fingerprint spectra that can provide rich information about the ion networks at the interface. By incorporating advanced data analysis tools and artificial intelligence algorithms, single-entity electrochemical measurements would revolutionize the field of single-entity analysis, leading to new fundamental discoveries.

The Role of MXene Surface Terminations on Peptide Transportation in Nanopore Sensing

Nanopores with two-dimensional materials have various advantages in sensing, but the fast translocation of molecules hinders their scale-up applications. In this work, we investigate the influence of -F, -O, and -OH surface terminations on the translocation of peptides through MXene nanopores. We find that the longest dwell time always occurs when peptides pass through the Ti3C2O2 nanopores. This elongated dwell time is induced by the strongest interaction between peptides and the Ti3C2O2 membrane, in which the van der Waals interactions dominate. Compared to the other two MXene nanopores, the braking effect is indicated during the whole translocation process, which evidence the advantage of Ti3C2O2 in nanopore sensing. Our work demonstrates that membrane surface chemistry has a great influence on the translocation of peptides, which can be introduced in the design of nanopores for a better performance.

Deciphering feedback regulation of prostaglandin F2α in blood stasis syndrome using nitrogen-doped porous transition metal carbides

Blood stasis syndrome (BSS) has persistent health risks; however, its pathogenesis remains elusive. This obscurity may result in missed opportunities for early intervention, increased susceptibility to chronic diseases, and reduced accuracy and efficacy of treatments. Metabolomics, employing the matrix-assisted laser desorption/ionization (MALDI) strategy, presents distinct advantages in biomarker discovery and unraveling molecular mechanisms. Nonetheless, the challenge is to develop efficient matrices for high-sensitivity and high-throughput analysis of diverse potential biomarkers in complex biosamples. This work utilized nitrogen-doped porous transition metal carbides and nitrides (NP-MXene) as a MALDI matrix to delve into the molecular mechanisms underlying BSS pathogenesis. Structural optimization yielded heightened peak sensitivity (by 1.49-fold) and increased peak numbers (by 1.16-fold) in clinical biosamples. Validation with animal models and clinical serum biosamples revealed significant differences in metabolic fingerprints between BSS and control groups, achieving an overall diagnostic efficacy of 0.905 (95% CI, 0.76-0.979). Prostaglandin F2α was identified as a potential biomarker (diagnostics efficiency of 0.711, specificity = 0.7, sensitivity = 0.6), and pathway enrichment analysis disclosed disruptions in arachidonic acid metabolism in BSS. This innovative approach not only advances comprehension of BSS pathogenesis, but also provides valuable insights for personalized treatment and diagnostic precision.

Single-Layer Hexagonal Boron Nitride Nanopores as High-Performance Ionic Gradient Power Generators

Atomically thin two-dimensional (2D) materials have emerged as promising candidates for efficient energy harvesting from ionic gradients. However, the exploration of robust 2D atomically thin nanopore membranes, which hold sufficient ionic selectivity and high ion permeability, remains challenging. Here, the single-layer hexagonal boron nitride (hBN) nanopores are demonstrated as various high-performance ion-gradient nanopower harvesters. Benefiting from the ultrathin atomic thickness and large surface charge (also a large Dukhin number), the hBN nanopore can realize fast proton transport while maintaining excellent cation selectivity even in highly acidic environments. Therefore, a single hBN nanopore achieves the pure osmosis-driven proton-gradient power up to ≈3 nW under 1000-fold ionic gradient. In addition, the robustness of hBN membranes in extreme pH conditions allows the ionic gradient power generation from acid-base neutralization. Utilizing 1 m HCl/KOH, the generated power can be promoted to an extraordinarily high level of ≈4.5 nW, over one magnitude higher than all existing ionic gradient power generators. The synergistic effects of ultrathin thickness, large surface charge, and excellent chemical inertness of 2D single-layer hBN render it a promising membrane candidate for harvesting ionic gradient powers, even under extreme pH conditions.

Nanopore DNA sequencing technologies and their applications towards single-molecule proteomics

Sequencing of nucleic acids with nanopores has emerged as a powerful tool offering rapid readout, high accuracy, low cost and portability. This label-free method for sequencing at the single-molecule level is an achievement on its own. However, nanopores also show promise for the technologically even more challenging sequencing of polypeptides, something that could considerably benefit biological discovery, clinical diagnostics and homeland security, as current techniques lack portability and speed. Here we survey the biochemical innovations underpinning commercial and academic nanopore DNA/RNA sequencing techniques, and explore how these advances can fuel developments in future protein sequencing with nanopores.

Effect of surface functionalization on DNA sequencing using MXene-based nanopores

As one of the most promising types of label-free nanopores has great potential for DNA sequencing via fast detection of different DNA bases. As one of the most promising types of label-free nanopores, two-dimensional nanopore materials have been developed over the past two decades. However, how to detect different DNA bases efficiently and accurately is still a challenging problem. In the present work, the translocation of four homogeneous DNA strands (i.e., poly(A)20, poly(C)20, poly(G)20, and poly(T)20) through two-dimensional transition-metal carbide (MXene) membrane nanopores with different surface terminal groups is investigated via all-atom molecular dynamics simulations. Interestingly, it is found that the four types of bases can be distinguished by different ion currents and dwell times when they are transported through the Ti3C2(OH)2 nanopore. This is mainly attributed to the different orientation and position distributions of the bases, the hydrogen bonding inside the MXene nanopore, and the interaction of the ssDNA with the nanopore. The present study enhances the understanding of the interaction between DNA strands and MXene nanopores with different functional groups, which may provide useful guidelines for the design of MXene-based devices for DNA sequencing in the future.

Fabrication of angstrom-scale two-dimensional channels for mass transport

Fluidic channels at atomic scales regulate cellular trafficking and molecular filtration across membranes, and thus play crucial roles in the functioning of living systems. However, constructing synthetic channels experimentally at these scales has been a significant challenge due to the limitations in nanofabrication techniques and the surface roughness of the commonly used materials. Angstrom (Å)-scale slit-like channels overcome such challenges as these are made with precise control over their dimensions and can be used to study the fluidic properties of gases, ions and water at unprecedented scales. Here we provide a detailed fabrication method of the two-dimensional Å-scale channel devices that can be assembled to contain a desired number of channels, a single channel or up to hundreds of channels, made with atomic-scale precision using layered crystals. The procedure includes the fabrication of the substrate, flake, spacer layer, flake transfers, van der Waals assembly and postprocessing. We further explain how to perform molecular transport measurements with the Å-channels to directly probe the intriguing and anomalous phenomena that help shed light on the physics governing ultra-confined transport. The procedure requires a total of 1-2 weeks for the fabrication of the two-dimensional channel device and is suitable for users with prior experience in clean room working environments and nanofabrication.

Functional Janus structured liquids and aerogels

Janus structures have unique properties due to their distinct functionalities on opposing faces, but have yet to be realized with flowing liquids. We demonstrate such Janus liquids with a customizable distribution of nanoparticles (NPs) throughout their structures by joining two aqueous streams of NP dispersions in an apolar liquid. Using this anisotropic integration platform, different magnetic, conductive, or non-responsive NPs can be spatially confined to opposite sides of the original interface using magnetic graphene oxide (mGO)/GO, Ti3C2Tx/GO, or GO suspensions. The resultant Janus liquids can be used as templates for versatile, responsive, and mechanically robust aerogels suitable for piezoresistive sensing, human motion monitoring, and electromagnetic interference (EMI) shielding with a tuned absorption mechanism. The EMI shields outperform their current counterparts in terms of wave absorption, i.e., SET ≈ 51 dB, SER ≈ 0.4 dB, and A = 0.91, due to their high porosity ranging from micro- to macro-scales along with non-interfering magnetic and conductive networks imparted by the Janus architecture.

2D materials towards energy conversion processes in nanofluidics

Hierarchically assembled 2D material membranes are extremely promising platforms for energy conversion processes in nanofluidics. In this perspective, we discuss recent advances in the production of smart 2D material membranes that come close to mimicking biological energy conversion processes and how these efforts translate into the design of water purification systems, artificial photosynthesis, and solar energy conversion devices. As we depict here, 2D material membranes synergistically modulate the intrinsic active sites (nanopores), electron transport, mass transfer, and mechanical and chemical stability aiming at cost-effective and highly efficient smart membranes.

A MXene-BiFeO3-ZnO nanocomposite photocatalyst served as a high-performance supercapacitor electrode

MXenes have attracted considerable attention in the field of energy storage and conversion due to their high surface area, excellent electrical conductivity, and ability to intercalate various ions. However, simultaneously achieving high capacitance, rate capability, cycling stability, and mechanical flexibility is a significant challenge for designing MXene-based supercapacitors. In this article, we explored MXene-BiFeO3-ZnO nanocomposites for both photocatalytic and electric double-layer supercapacitor applications. While the BiFeO3-ZnO nanohybrid heterostructure improves the charge separation properties in nanocomposite photocatalysts, it was applied as an interlayer spacer between the MXene layers to prevent the stacking effect of electrodes in the supercapacitor. Furthermore, the optimization of MXene content in the nanocomposite was established by photocatalytic studies on methylene blue dye, which revealed a maximum of 98.72% degradation under direct sunlight with superior stability. The electrochemical studies on the best composition material reveal a maximum areal capacitance (Ccv) of 142.8 mF cm-2, an energy density (E) of 1.65 μW h cm-2, and a capacitive retention of 99.98% after 8000 cycles at 7 μA cm-2. Additionally, the flexible solid-state supercapacitor fabricated with the same material demonstrates an areal capacitance of 47.6 mF cm-2 and a capacitive retention of 66% after 8000 cycles at 7 μA cm-2, with potential for high-performance flexible supercapacitors.

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.

MXenes - Versatile 2D materials for identification of biomarkers and contaminants in large scale environments - A review

Recent years have seen a lot of interest in transition metal carbides/carbonitrides (MXenes), Which is one of newly proliferating two-dimensional (2D) materials.The advantages and applications of synthesizing MXenes-based biosensing systems are interesting. There is an urgent requirement for synthesis of MXenes. Through foliation, physical adsorption, and interface modification,it has been proposed that many biological disorders are related to genetic mutation. Majority of mutations were discovered to be nucleotide mismatches. Consequently, accurate -nucleotide mismatched discrimination is crucial for both diagnosing and treating diseases. To differentiate between such a sensitivealterations in the DNA duplex, several detection methods, particularly Electrochemical-luminescence (ECL) ones, have really been investigated.M_n+1X_nT_x is common name for MXenes, a novel family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, where T stands for interface termination units (i.e. = O, OH, and/or F). These electronic characteristics of MXenes may be changed between conductive to semiconducting due to abundant organometallic chemistry.Solid-state ECL sensors predicated on MXene would provide the facile nucleotide detection and convenience for usage with minimal training, mobility and possibly minimal cost.This study emphasizes upcoming requirements and possibilities in this area while describing the accomplishments achieved in the usage and employing of MXenes in the research and development of facile biomarkerdetection and their significance in designing electrochemical sensors. Opportunities are addressed for creating 2D MXene materials sensors and devices with incorporated biomolecule sensing. MXenes Carry out this process sensors, address the advantages of using MXenes and their variants as detecting materials for gathering different types of data, and attempt to clarify the design principles and operation of related MXene-based sensors, such as nucleotide detection, Single nucleotide detectors, Cancer theranostics, Biosensing capabilities, Gliotoxin detection, SARS-COV-2 nucleocapsid detection, electrochemical sensors, visual sensors, and humidity sensors. Finally, we examine the major issues and prospects for MXene-based materials used in various sensing applications.

Highly Sensitive and Selective DNA Sequencing Device Using Metal Adatom Adsorption on 2D Phosphorene

Two-dimensional (2D) material revolutionarily extends the technique capability of traditional nanopore/nanogap-based DNA sequencing devices. However, challenges associated with DNA sequencing on nanopores still remained in improving the sensitivity and specificity. Herein, by first-principles calculation, we theoretically studied the potential of transition-metal elements (Cr, Fe, Co, Ni, and Au) anchored on monolayer black phosphorene (BP) to act as all-electronic DNA sequencing devices. The spin-polarized band structures appeared in Cr-, Fe-, Co-, and Au-doped BP. Remarkably, the adsorption energy of nucleobases can be significantly enhanced on BP with Co, Fe, and Cr doping, which contribute to the enlarged current signal and lower noise levels. Furthermore, the order of nucleobases in terms of their adsorption energies onto the Cr@BP is C > A > G > T, which exhibits more distinct adsorption energies than Fe@BP or Co@BP. Therefore, Cr-doped BP is more effective to avoid ambiguity in recognizing various bases. We thus envisaged a possibility of a highly sensitive and selective DNA sequencing device based on phosphorene.

Oxygen-vacancy-rich molybdenum carbide MXene nanonetworks for ultrasound-triggered and capturing-enhanced sonocatalytic bacteria eradication

Incurable bacterial infection and intractable multidrug resistance remain critical challenges in public health. A prevalent approach against bacterial infection is phototherapy including photothermal and photodynamic therapy, which is unfortunately limited by low penetration depth of light accompanied with inevitable hyperthermia and phototoxicity damaging healthy tissues. Thus, eco-friendly strategy with biocompatibility and high antimicrobial efficacy against bacteria is urgently desired. Herein, we propose and develop an oxygen-vacancy-rich MoO_xin situ on fluorine-free Mo₂C MXene with unique neural-network-like structure, namely MoO_x@Mo₂C nanonetworks, in which their desirable antibacterial effectiveness originates from bacteria-capturing ability and robust reactive oxygen species (ROS) generation under precise ultrasound (US) irradiation. The high-performance, broad-spectrum microbicidal activity of MoO_x@Mo₂C nanonetworks without damaging normal tissues is validated based on systematic in vitro and in vivo assessments. Additionally, RNA sequencing analysis illuminates that the underlying bactericidal mechanism is attributed to the chaotic homeostasis and disruptive peptide metabolisms on bacteria instigated by MoO_x@Mo₂C nanonetworks under US stimulation. Considering antibacterial efficiency and a high degree of biosafety, we envision that the MoO_x@Mo₂C nanonetworks can serve as a distinct antimicrobial nanosystem to fight against diverse pathogenic bacteria, especially eradicating multidrug-resistant bacteria-induced deep tissue infection.

Two Detection Modes of Nanoslit Sensing Based on Planar Heterostructure of Graphene/Hexagonal Boron Nitride

Solid-state nanopore sequencing is now confronted with problems of stochastic pore clogging and too fast speed during the DNA permeation through a nanopore, although this technique is revolutionary with long readability and high efficiency. These two problems are related to controlling molecular transportation during sequencing. To control the DNA motion and identify the four bases, we propose nanoslit sensing based on the planar heterostructure of two-dimensional graphene and hexagonal boron nitride. Molecular dynamics simulations are performed on investigating the motion of DNA molecules on the heterostructure with a nanoslit sensor. Results show that the DNA molecules are confined within the hexagonal boron nitride (HBN) domain of the heterostructure. And the confinement effects of the heterostructure can be optimized by tailoring the stripe length. Besides, there are two ways of DNA permeation through nanoslits: the DNA can cross or translocate the nanoslit under applied voltages along the y and z directions. The two detection modes are named cross-slit and trans-slit, respectively. In both modes, the ionic current drops can be observed when the nanoslit is occupied by the DNA. And the ionic currents and dwell times can be simultaneously detected to identify the four different DNA bases. This study can shed light on the sensing mechanism based on the nanoslit sensor of a planar heterostructure and provide theoretical guidance on designing devices controlling molecular transportation during nanopore sequencing.

High durability and stability of 2D nanofluidic devices for long-term single-molecule sensing

Nanopores in two-dimensional (2D) membranes hold immense potential in single-molecule sensing, osmotic power generation, and information storage. Recent advances in 2D nanopores, especially on single-layer MoS2, focus on the scalable growth and manufacturing of nanopore devices. However, there still remains a bottleneck in controlling the nanopore stability in atomically thin membranes. Here, we evaluate the major factors responsible for the instability of the monolayer MoS2 nanopores. We identify chemical oxidation and delamination of monolayers from their underlying substrates as the major reasons for the instability of MoS2 nanopores. Surface modification of the substrate and reducing the oxygen from the measurement solution improves nanopore stability and dramatically increases their shelf-life. Understanding nanopore growth and stability can provide insights into controlling the pore size, shape and can enable long-term measurements with a high signal-to-noise ratio and engineering durable nanopore devices.

Latest advances on MXenes in biomedical research and health care

The unique combination of physical and chemical properties of MXenes has propelled a growing number of applications in biomedicine and healthcare. The expanding library of MXenes with tunable properties is paving the way for high-performance, application-specific MXene-based sensing and therapeutic platforms. In this article, we highlight the emerging biomedical applications of MXenes with specific emphasis on bioelectronics, biosensors, tissue engineering, and therapeutics. We present examples of MXenes and their composites enabling novel technological platforms and therapeutic strategies, and elucidate potential avenues for further developments. Finally, we discuss the materials, manufacturing, and regulatory challenges that need to be synergistically addressed for the clinical translation of MXene-based biomedical technologies.

Graphical abstract

Advances in 2D MXenes-based materials for water purification and disinfection: Synthesis approaches and photocatalytic mechanistic pathways

MXenes two-dimensional materials have recently excited researchers' curiosity for various industrial applications. MXenes are promising materials for environmental remediation technologies to sense and mitigate various intractable hazardous pollutants from the atmosphere due to their inherent mechanical and physicochemical properties, such as high surface area, increased hydrophilicity, high conductivity, changing band gaps, and robust electrochemistry. This review discusses the versatile applications of MXenes and MXene-based nanocomposites in various environmental remediation processes. A brief description of synthetic procedures of MXenes nanocomposites and their different properties are highlighted. Afterward, the photocatalytic abilities of MXene-based nanocomposites for degrading organic pollutants, removal of heavy metals, and inactivation of microorganisms are discussed. In addition, the role of MXenes anti-corrosion support in the lifetime of some semiconductors was addressed. Current challenges and future perspectives toward the application of MXene materials for environmental remediation and energy production are summarized for plausible real-world use.

High-Voltage Biomolecular Sensing Using a Bacteriophage Portal Protein Covalently Immobilized within a Solid-State Nanopore

The application of nanopores as label-free, single-molecule biosensors for electrical or optical probing of structural features in biomolecules has been widely explored. While biological nanopores (membrane proteins and bacteriophage portal proteins) and solid-state nanopores (thin films and two-dimensional materials) have been extensively employed, the third class of nanopores known as hybrid nanopores, where an artificial membrane substitutes the organic support membrane of proteins, has been only sparsely studied due to challenges in implementation. G20c portal protein contains a natural DNA pore that is used by viruses for filling their capsid with viral genomic DNA. We have previously developed a lipid-free hybrid nanopore by "corking" the G20c portal protein into a SiN_x nanopore. Herein, we demonstrate that through chemical functionalization of the synthetic nanopore, covalent linkage between the solid-state pore and the G20c portal protein considerably improves the hybrid pore stability, lifetime, and voltage resilience. Moreover, we demonstrate electric-field-driven and motor protein-mediated transport of DNA molecules through this hybrid nanopore. Our integrated protein/solid-state device can serve as a robust and durable framework for sensing and sequencing at high voltages, potentially providing higher resolution, higher signal-to-noise ratio, and higher throughput compared to the more conventional membrane-embedded protein platforms.

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

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

Interfacial Assembly of Ti3 C2 Tx /ZnIn2 S4 Heterojunction for High-Performance Photodetectors

Two-dimensional (2D) materials have emerged as prospective candidates for electronics and optoelectronics applications as they can be easily fabricated through liquid exfoliation and used to fabricate various structures by further subsequent processing methods in addition to their extraordinary and unique optoelectronic properties. Herein, the Ti3 C2 Tx /ZIS heterostructure with nanometer-thick Ti3 C2 Tx -MXene and ZnIn2 S4 (ZIS) films is fabricated by successive interfacial assembly of liquid exfoliated 2D MXene and ZnIn2 S4 nanoflakes. Benefiting from the superior light-harvesting capability and low dark current of ZnIn2 S4 , the limited absorbance, large scattering coefficient, and high dark current disadvantages of MXene are ameliorated. Meanwhile, the separation and transport of photogenerated carriers in ZnIn2 S4 are improved due to the excellent electrical conductivity of Ti3 C2 Tx nanoflakes. As a result, the as-prepared Ti3 C2 Tx /ZIS heterostructure photodetector has excellent optoelectronic characteristics in terms of a high responsivity of 1.04 mA W-1 , a large specific detectivity up to 1 × 1011  Jones, a huge on/off ratio at around 105 , and an ultralow dark current at ≈10-12  A. This work demonstrates a convenient method to construct heterostructured photodetectors by liquid exfoliated 2D nanoflakes, the as-fabricated Ti3 C2 Tx /ZIS heterostructured photodetectors show promising application potential for low-cost, reliable, and high-performance photodetectors.

Which 2D Material is Better for DNA Detection: Graphene, MoS2, or MXene?

Despite much research on characterizing 2D materials for DNA detection with nanopore technology, a thorough comparison between the performance of different 2D materials is currently lacking. In this work, using extensive molecular dynamics simulations, we compare nanoporous graphene, MoS₂ and titanium carbide MXene (Ti₃C₂) for their DNA detection performance and sensitivity. The ionic current and residence time of DNA are characterized in each nanoporous materials by performing hundreds of simulations. We devised two statistical measures including the Kolmogorov-Smirnov test and the absolute pairwise difference to compare the performance of nanopores. We found that graphene nanopore is the most sensitive membrane for distinguishing DNA bases. The MoS₂ is capable of distinguishing the A and T bases from the C and G bases better than graphene and MXene. Physisorption and the orientation of DNA in nanopores are further investigated to provide molecular insight into the performance characteristics of different nanopores.

Ionic Liquid Decelerates Single-Stranded DNA Transport through Molybdenum Disulfide Nanopores

Nanopores in two-dimensional (2D) materials have emerged to offer in principle necessary spatial resolution for high-throughput DNA sequencing. However, their fidelity is severely limited by the fast DNA translocation. A recent experiment indicates that introducing ionic liquids could slow down DNA translocation in a MoS₂ nanopore. However, the corresponding in-depth molecular mechanism underlying the experimental findings is not fully understood, which is crucial for the future improvement of rational DNA translocation control. Here, we computationally investigate and then experimentally identify the effect of BmimCl ionic liquid on the retardation of ssDNA translocation through a single-layer MoS₂ nanopore. Our all-atom molecular dynamics simulations demonstrate that the strong interaction between Bmim⁺ and ssDNA offers a considerable dragging force to decelerate the electrophoretic motion of ssDNA in the BmimCl solution. Moreover, we show that Bmim⁺ ions exhibit preferential binding on the sulfur edges of the nanopore. These Bmim⁺ in the pore region can not only act as a steric blockage but also form π-π stackings with nucleobases, which provide a further restriction on the ssDNA motion. Therefore, our molecular dynamics simulation investigations deepen the understanding of the critical role of ionic liquid in DNA translocation through a nanopore from a molecular landscape, which may benefit practical implementations of ionic liquids in nanopore sequencing.

Fabrication of solid-state nanopores

Nanopores are valuable single-molecule sensing tools that have been widely applied to the detection of DNA, RNA, proteins, viruses, glycans, etc. The prominent sensing platform is helping to improve our health-related quality of life and accelerate the rapid realization of precision medicine. Solid-state nanopores have made rapid progress in the past decades due to their flexible size, structure and compatibility with semiconductor fabrication processes. With the development of semiconductor fabrication techniques, materials science and surface chemistry, nanopore preparation and modification technologies have made great breakthroughs. To date, various solid-state nanopore materials, processing technologies, and modification methods are available to us. In the review, we outline the recent advances in nanopores fabrication and analyze the virtues and limitations of various membrane materials and nanopores drilling techniques.

DNA Coil Dynamics and Hydrodynamic Gating of Pressure-Biased Nanopores

Nanopores are ideally suited for the analysis of long DNA fragments including chromosomal DNA and synthetic DNA with applications in genome sequencing and DNA data storage, respectively. Hydrodynamic fluid flow has been shown to slow down DNA transit time within the pore, however other influences of hydrodynamic forces have yet to be explored. In this report, a broad analysis of pressure-biased nanopores and the impact of hydrodynamics on DNA transit time, capture rate, current blockade depth, and DNA folding are conducted. Using a 10 nm pore, it is shown that hydrodynamic flow inhibits the early stages of linearization of DNA and produces predominately folded events which are initiated by folded DNA (2-strands) entering the pore. Furthermore, utilizing larger pores (30 nm) leads to unique DNA gating behavior in which DNA events can be switched on and off with the application of pressure. A computational model, based on combining electrophoretic drift velocities with fluid velocities, accurately predicts the pore size required to observe DNA gating. Hydrodynamic fluid flow generated by a pressure bias, or potentially more generally by other mechanisms like electroosmotic flow, is shown to have significant effects on DNA sensing and can be useful for DNA sensing technologies.

Optimizing the sensitivity and resolution of hyaluronan analysis with solid-state nanopores

Hyaluronan (HA) is an essential carbohydrate in vertebrates that is a potentially robust bioindicator due to its critical roles in diverse physiological functions in health and disease. The intricate size-dependent function that exists for HA and its low abundance in most biological fluids have highlighted the need for sensitive technologies to provide accurate and quantitative assessments of polysaccharide molecular weight and concentration. We have demonstrated that solid state (SS-) nanopore technology can be exploited for this purpose, given its molecular sensitivity and analytical capacity, but there remains a need to further understand the impacts of experimental variables on the SS-nanopore signal for optimal interpretation of results. Here, we use model quasi-monodisperse HA polymers to determine the dependence of HA signal characteristics on a range of SS-nanopore measurement conditions, including applied voltage, pore diameter, and ionic buffer asymmetry. Our results identify important factors for improving the signal-to-noise ratio, resolution, and sensitivity of HA analysis with SS-nanopores.

Ionically Active MXene Nanopore Actuators

Reversible electrochemical intercalation of cations into the interlayer space of 2D materials induces tunable physical and chemical properties in them. In MXenes, a large class of recently developed 2D carbides and nitrides, low intercalation energy, high storage capacitance, and reversible intercalation of various cations have led to their improved performance in sensing and energy storage applications. Herein, a coupled nanopore-actuator system where an ultrathin free-standing MXene film serves as a nanopore support membrane and ionically active actuator is reported. In this system, the contactless MXene membrane in the electric field affects the cation movement in the field through their (de)intercalation between individual MXene flakes. This results in reversible swelling and contraction of the membrane monitored by ionic conductance through the nanopore. This unique nanopore coupled to a mechanical actuation system could provide new insights into designing single-molecule biosensing platforms at the nanoscale.

Single Molecule DNA Analysis Based on Atomic-Controllable Nanopores in Covalent Organic Frameworks

We explored the application of two-dimensional covalent organic frameworks (2D COFs) in single molecule DNA analysis. Two ultrathin COF nanosheets were exfoliated with pore sizes of 1.1 nm (COF-1.1) and 1.3 nm (COF-1.3) and covered closely on a quartz nanopipette with an orifice of 20 ± 5 nm. COF nanopores exhibited high size selectivity for fluorescent dyes and DNA molecules. The transport of long (calf thymus DNA) and short (DNA-80) DNA molecules through the COF nanopores was studied. Because of the strong interaction between DNA bases and the organic backbones of COFs, the DNA-80 was transported through the COF-1.1 nanopore at a speed of 270 μs/base, which is the slowest speed ever observed compared with 2D inorganic nanomaterials. This study shows that the COF nanosheet can work individually as a nanopore monomer with controllable pore size like its biological counterparts.

Porous Ti3C2Tx MXene Membranes for Highly Efficient Salinity Gradient Energy Harvesting

Extracting osmotic energy through nanoporous membranes is an efficient way to harvest renewable and sustainable energy using the salinity gradient between seawater and river water. Despite recent advances of nanopore-based membranes, which have revitalized the prospect of blue energy, their energy conversion is hampered by nanomembrane issues such as high internal resistance or low selectivity. Herein, we report a lamellar-structured membrane made of nanoporous Ti3C2Tx MXene sheets, exhibiting simultaneous enhancement in permeability and ion selectivity beyond their inherent trade-off. The perforated nanopores formed by facile H2SO4 oxidation of the sheets act as a network of cation channels that interconnects interplanar nanocapillaries throughout the lamellar membrane. The constructed internal nanopores lower the energy barrier for cation passage, thereby boosting the preferential ion diffusion across the membrane. A maximum output power density of the nanoporous Ti3C2Tx MXene membranes reaches up to 17.5 W·m-2 under a 100-fold KCl gradient at neutral pH and room temperature, which is as high as by 38% compared to that of the pristine membrane. The membrane design strategy employing the nanoporous two-dimensional sheets provides a promising approach for ion exchange, osmotic energy extraction, and other nanofluidic applications.

Nanopores in Graphene and Other 2D Materials: A Decade's Journey toward Sequencing

Nanopore techniques offer a low-cost, label-free, and high-throughput platform that could be used in single-molecule biosensing and in particular DNA sequencing. Since 2010, graphene and other two-dimensional (2D) materials have attracted considerable attention as membranes for producing nanopore devices, owing to their subnanometer thickness that can in theory provide the highest possible spatial resolution of detection. Moreover, 2D materials can be electrically conductive, which potentially enables alternative measurement schemes relying on the transverse current across the membrane material itself and thereby extends the technical capability of traditional ionic current-based nanopore devices. In this review, we discuss key advances in experimental and computational research into DNA sensing with nanopores built from 2D materials, focusing on both the ionic current and transverse current measurement schemes. Challenges associated with the development of 2D material nanopores toward DNA sequencing are further analyzed, concentrating on lowering the noise levels, slowing down DNA translocation, and inhibiting DNA fluctuations inside the pores. Finally, we overview future directions of research that may expedite the emergence of proof-of-concept DNA sequencing with 2D material nanopores.

Solid-state and polymer nanopores for protein sensing: A review

In two decades, the solid state and polymer nanopores became attractive method for the protein sensing with high specificity and sensitivity. They also allow the characterization of conformational changes, unfolding, assembly and aggregation as well the following of enzymatic reaction. This review aims to provide an overview of the protein sensing regarding the technique of detection: the resistive pulse and ionic diodes. For each strategy, we report the most significant achievement regarding the detection of peptides and protein as well as the conformational change, protein-protein assembly and aggregation process. We discuss the limitations and the recent strategies to improve the nanopore resolution and accuracy. A focus is done about concomitant problematic such as protein adsorption and nanopore lifetime.

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.

Two-Dimensional Transition Metal Carbides and Nitrides (MXenes) for Water Purification and Antibacterial Applications

Two-dimensional (2D) materials such as graphene, graphene oxide (GO), metal carbides and nitrides (MXenes), transition metal dichalcogenides (TMDS), boron nitride (BN), and layered double hydroxide (LDH) metal-organic frameworks (MOFs) have been widely investigated as potential candidates in various separation applications because of their high mechanical strength, large surface area, ideal chemical and thermal stability, simplicity, ease of functionalization, environmental comparability, and good antibacterial performance. Recently, MXene as a new member of the 2D polymer family has attracted significant attention in water purification, desalination, gas separation, antibacterial, and antifouling applications. Herein, we review the most recent progress in the fabrication, preparation, and modification methods of MXene-based lamellar membranes with the emphasis on applications for water purification and desalination. Moreover, the antibacterial properties of MXene-based membranes show a significant potential for commercial use in water purification. Thus, this review provides a directional guide for future development in this emerging technology.

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.

Nanoconfinement Effect for Signal Amplification in Electrochemical Analysis and Sensing

Owing to the urgent need for electrochemical analysis and sensing of trace target molecules in various fields such as medical diagnosis, agriculture and food safety, and environmental monitoring, signal amplification is key to promoting analysis and sensing performance. The nanoconfinement effect, derived from nanoconfined spaces and interfaces with sizes approaching those of target molecules, has witnessed rapid development for ultra-sensitive analyzing and sensing. In this review, the two main types of nanoconfinement systems - confined nanochannels and planes - are assessed and recent progress is highlighted. The merits of each nanoconfinement system, the nanoconfinement effect mechanisms, and applications for electrochemical analysis and sensing are summarized and discussed. This review aims to help deepen the understanding of nanoconfinement devices and their effects in order to develop new analysis and sensing applications for researchers in various fields.

Challenges and Opportunities for Clustered Regularly Interspaced Short Palindromic Repeats Based Molecular Biosensing

Clustered regularly interspaced short palindromic repeats, CRISPR, has recently emerged as a powerful molecular biosensing tool for nucleic acids and other biomarkers due to its unique properties such as collateral cleavage nature, room temperature reaction conditions, and high target-recognition specificity. Numerous platforms have been developed to leverage the CRISPR assay for ultrasensitive biosensing applications. However, to be considered as a new gold standard, several key challenges for CRISPR molecular biosensing must be addressed. In this paper, we briefly review the history of biosensors, followed by the current status of nucleic acid-based detection methods. We then discuss the current challenges pertaining to CRISPR-based nucleic acid detection, followed by the recent breakthroughs addressing these challenges. We focus upon future advancements required to enable rapid, simple, sensitive, specific, multiplexed, amplification-free, and shelf-stable CRISPR-based molecular biosensors.

Deep Learning-Enhanced Nanopore Sensing of Single-Nanoparticle Translocation Dynamics

Noise is ubiquitous in real space that hinders detection of minute yet important signals in electrical sensors. Here, the authors report on a deep learning approach for denoising ionic current in resistive pulse sensing. Electrophoretically-driven translocation motions of single-nanoparticles in a nano-corrugated nanopore are detected. The noise is reduced by a convolutional auto-encoding neural network, designed to iteratively compare and minimize differences between a pair of waveforms via a gradient descent optimization. This denoising in a high-dimensional feature space is demonstrated to allow detection of the corrugation-derived wavy signals that cannot be identified in the raw curves nor after digital processing in frequency domains under the given noise floor, thereby enabled in-situ tracking to electrokinetic analysis of fast-moving single- and double-nanoparticles. The ability of the unlabeled learning to remove noise without compromising temporal resolution may be useful in solid-state nanopore sensing of protein structure and polynucleotide sequence.

The world of two-dimensional carbides and nitrides (MXenes)

A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) includes structures with three, five, seven, or nine layers of atoms in an ordered or solid solution form. Dozens of MXene compositions have been produced, resulting in MXenes with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, sensing, and medicine. Here we present a forward-looking review of the field of MXenes. We discuss the challenges to be addressed and outline research directions that will deepen the fundamental understanding of the properties of MXenes and enable their hybridization with other 2D materials in various emerging technologies.