A MoS2@Ti3C2Tx MXene hybrid-based electrochemical aptasensor (MEA) for sensitive and rapid detection of Thyroxine

Nanohybrid of antimonene@Ti3C2Tx-based electrochemical aptasensor for lead detection

Lead ions (Pb2+), as one of many common heavy metallic environmental pollutants, can cause serious side-effects and result in chronic poisoning to people's health, so it is highly significant to monitor Pb2+ efficiently and sensitively. Here, we proposed an antimonene@Ti3C2Tx nanohybrid-based electrochemical aptamer sensor (aptasensor) for high sensitive Pb2+ determination. The sensing platform of nanohybrid was synthesized by ultrasonication, possessing the advantages of both antimonene and Ti3C2Tx, which not only can vastly enlarge the sensing signal of the proposed aptasensor, but also greatly simplified its manufacturing flow, because antimonene can strongly interact with aptamer through noncovalently bound. The surface morphology and microarchitecture of the nanohybrid were perused by several methods such as scanning electron microscope (SEM), energy-dispersive X-ray mapping spectroscopy (EDS), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and atomic force microscope (AFM). Under optimal empirical conditions, the proposed aptasensor exhibited a wide linear correlation of the current signals with the logarithm of CPb2+ (Log CPb2+) over the span from 1 × 10-12 to 1 × 10-7 M and provided a trace discernment limit of 3.3 × 10-13 M. Moreover, the constructed aptasensor displayed superior repeatability, great consistency, eminent selectivity, and beneficial reproducibility, implying its extreme potential application for water quality control and the environmental monitoring of Pb2+.

Highly defective graphene quantum dots-doped 1T/2H-MoS2 as an efficient composite catalyst for the hydrogen evolution reaction

We present a new composite catalyst system of highly defective graphene quantum dots (HDGQDs)-doped 1T/2H-MoS2 for efficient hydrogen evolution reactions (HER). The high electrocatalytic activity, represented by an overpotential of 136.9 mV and a Tafel slope of 57.1 mV/decade, is due to improved conductivity, a larger number of active sites in 1T-MoS2 compared to that in 2H-MoS2, and additional defects introduced by HDGQDs. High-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) were used to characterize both the 1T/2H-MoS2 and GQDs components while Fourier-transform infrared spectroscopy (FTIR) was employed to identify the functional groups on the edge and defect sites in the HDGQDs. The morphology of the composite catalyst was also examined by field emission scanning electron microscopy (FESEM). All experimental data demonstrated that each component contributes unique advantages that synergistically lead to the significantly improved electrocatalytic activity for HER in the composite catalyst system.

Few Layer Ti3C2 MXene-Based Label-Free Aptasensor for Ultrasensitive Determination of Chloramphenicol in Milk

Quantitative detection of veterinary drug residues in animal-derived food is of great significance. In this work, a simple and label-free electrochemical aptasensor for the highly sensitive detection of chloramphenicol (CAP) in milk was successfully developed based on a new biosensing method, where the single- or few-layer Ti3C2 MXene nanosheets functionalized via the specific aptamer by self-assembly were used as electrode modifiers for a glassy carbon electrode (aptamer/Ti3C2 MXene/GCE). Differential pulse voltammetry (DPV), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), scanning electron microscopy (SEM), atomic force microscope (AFM), and so on were utilized for electrochemical and morphological characterization. Under the optimized conditions, the constructed aptasensor exhibited excellent performance with a wider linearity to CAP in the range from 10 fM to 1 μM and a low detection limit of 1 fM. Aptamer/Ti3C2 MXene/GCE demonstrated remarkable selectivity over other potentially interfering antibiotics, as well as exceptional reproducibility and stability. In addition, the aptasensor was successfully applied to determine CAP in milk with acceptable recovery values of 96.13% to 108.15% and relative standard deviations below 9%. Therefore, the proposed electrochemical aptasensor is an excellent alternative for determining CAP in food samples.

Electrochemical immunoassay for one-pot detection of thyroxin (T4) and thyroid-stimulating hormone (TSH) using magnetic and Janus nanoparticles

Concurrent measurement of thyroid-stimulating hormone (TSH) and thyroxine (T4) hormones profoundly help clinicians diagnose hyper- and hypothyroidism. This work demonstrates the development of a sandwich-type electrochemical immunoassay using Janus and magnetic nanoparticles for one-pot detection of thyroxine (T4) and thyroid-stimulating hormone (TSH). The signaling probe was developed by preparing Janus cadmium (CdO) and zinc oxide (ZnO) NPs decorated by T4/TSH-specific molecularly imprinted polymers (MIP_T4-CdO and MIP_TSH-ZnO). The capture probe was obtained by coating magnetic Fe₃O₄ NPs with 1,3-Bis(3-carboxy propyl) tetramethyl disiloxane and activating using N-hydroxy succinimide (NHS) and 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC), and finally conjugating with T4/TSH-specific antibodies. To analyze T4 and TSH in actual samples, MIP_T4-CdO and MIP_TSH-ZnO were added to the sample solutions, and after incubation, capture probes (Fe₃O₄-Ab_TSH and Fe₃O₄-Ab_T4) were added. An external magnetic field was used to separate the sandwiched nanosystem, followed by adding a dilute solution of nitric acid (HNO₃) to dissolve CdO and ZnO NPs and free Cd(II) and Zn(II) cations. The concentration of these cations was determined using constant-current potentiometric stripping analysis (cc-PSA) on screen-printed electrodes (SPE) modified with multi-walled carbon nanotubes (MWCNT). The obtained signals for Cd(II) and Zn(II) were proportional to T4 and TSH concentrations. Limits of detection (LOD) for T4 and TSH analyses were respectively 0.02 ng.dL^-1 and 0.0002 µU.mL^-1 with a linear range of 0.05-50 ng.dL^-1 and 0.001-100 µU.mL^-1. The proposed nanosystem's main advantage is the simultaneous detection of T4 and TSH in clinical samples with high sensitivity, selectivity, and stability.

Advancements in nanomaterial-based aptasensors for the detection of emerging organic pollutants in environmental and biological samples

The combination of nanomaterials (NMs) and aptamers into aptasensors enables highly specific and sensitive detection of diverse pollutants. The great potential of aptasensors is recognized for the detection of diverse emerging organic pollutants (EOPs) in different environmental and biological matrices. In addition to high sensitivity and selectivity, NM-based aptasensors have many other advantages such as portability, miniaturization, facile use, and affordability. This work showcases the recent advances achieved in the design and fabrication of NM-based aptasensors for monitoring EOPs (e.g., hormones, phenolic contaminants, pesticides, and pharmaceuticals). On the basis of their sensing mechanisms, the covered aptasensing systems are classified as electrochemical, colorimetric, PEC, fluorescence, SERS, and ECL. Special attention has been paid to the fabrication processes, analytical achievements, and sensing mechanisms of NM-based aptasensors. Further, the practical utility of aptasensing approaches has also been assessed based on their basic performance metrics (e.g., detection limits, sensing ranges, and response times).

Advanced growth of 2D MXene for electrochemical sensors

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

Ultrasensitive and amplification-free detection of SARS-CoV-2 RNA using an electrochemical biosensor powered by CRISPR/Cas13a

This study proposed a CRISPR/Cas13a-powered electrochemical multiplexed biosensor for detecting SARS-CoV-2 RNA strands. Current SARS-CoV-2 diagnostic methods, such as reverse transcription PCR (RT-PCR), are primarily based on nucleic acid amplification (NAA) and reverse transcription (RT) processes, which have been linked to significant issues such as cross-contamination and long turnaround times. Using a CRISPR/Cas13a system integrated onto an electrochemical biosensor, we present a multiplexed and NAA-free strategy for detecting SARS-CoV-2 RNA fragments. SARS-CoV-2 S and Orf1ab genes were detected in both synthetic and clinical samples. The CRISPR/Cas13a-powered biosensor achieved low detection limits of 2.5 and 4.5 ag/µL for the S and Orf1ab genes, respectively, successfully meeting the sensitivity requirement. Furthermore, the biosensor's specificity, simplicity, and universality may position it as a potential rival to RT-PCR.

2D MXene-Based Biosensing: A Review

MXene emerged as decent 2D material and has been exploited for numerous applications in the last decade. The remunerations of the ideal metallic conductivity, optical absorbance, mechanical stability, higher heterogeneous electron transfer rate, and good redox capability have made MXene a potential candidate for biosensing applications. The hydrophilic nature, biocompatibility, antifouling, and anti-toxicity properties have opened avenues for MXene to perform in vitro and in vivo analysis. In this review, the concept, operating principle, detailed mechanism, and characteristic properties are comprehensively assessed and compiled along with breakthroughs in MXene fabrication and conjugation strategies for the development of unique electrochemical and optical biosensors. Further, the current challenges are summarized and suggested future aspects. This review article is believed to shed some light on the development of MXene for biosensing and will open new opportunities for the future advanced translational application of MXene bioassays.

MXene-Based Nucleic Acid Biosensors for Agricultural and Food Systems

MXene is a two-dimensional (2D) nanomaterial that exhibits several superior properties suitable for fabricating biosensors. Likewise, the nucleic acid (NA) in oligomerization forms possesses highly specific biorecognition ability and other features amenable to biosensing. Hence the combined use of MXene and NA is becoming increasingly common in biosensor design and development. In this review, MXene- and NA-based biosensors are discussed in terms of their sensing mechanisms and fabrication details. MXenes are introduced from their definition and synthesis process to their characterization followed by their use in NA-mediated biosensor fabrication. The emphasis is placed on the detection of various targets relevant to agricultural and food systems, including microbial pathogens, chemical toxicants, heavy metals, organic pollutants, etc. Finally, current challenges and future perspectives are presented with an eye toward the development of advanced biosensors with improved detection performance.

Highly selective detection of adenine and guanine by NH2-MIL-53(Fe)/CS/MXene nanocomposites with excellent electrochemical performance

Adenine (A) and guanine (G) are mainly found in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) and play a crucial role in genetic information transfer and protein synthesis. In this study, NH₂-MIL-53(Fe)/CS/MXene nanocomposites were prepared for detecting guanine and adenine. With high specific surface area, excellent water dispersion, and numerous active sites, MXene (transition metal carbides, nitrides, and carbonitrides) provides a good platform for loading primitive metal-organic frameworks (MOFs). At the same time, the problem of poor conductivity and dispersion of MOFs is solved. The electrochemical catalytic oxidation of adenine and guanine of NH₂-MIL-53 (Fe)/CS/MXene nanocomposites was carried out by differential pulse voltammetry (DPV). Operating voltage of DPV: 0.7-0.9 V (vs. Ag/AgCl) for G, 1.0-1.2 V (vs. Ag/AgCl) for A, 0.8 V (vs. Ag/AgCl), and 1.1 V (vs. Ag/AgCl) for G and A. The concentration ranges for detecting A and G were 3-118 μM and 2-120 μM with detection limits of 0.57 μM and 0.17 μM (S/N = 3), respectively. The nanocomposite was used for detecting G and A in herring sperm DNA, and the content of G and A was found to be about 9 and 11 μM; the RSD values were 3.4 and 1.3%, respectively.

Novel Electrochemical Aptasensor Based on Ordered Mesoporous Carbon/2D Ti3C2 MXene as Nanocarrier for Simultaneous Detection of Aminoglycoside Antibiotics in Milk

Herein, a novel electrochemical aptasensor using a broad-spectrum aptamer as a biorecognition element was constructed based on a screen-printed carbon electrode (SPCE) for simultaneous detection of aminoglycoside antibiotics (AAs). The ordered mesoporous carbon (OMC) was firstly modified on 2D Ti₃C₂ MXene. The addition of OMC not only effectively improved the stability of the aptasensor, but also prevented the stacking of Ti₃C₂ sheets, which formed a good current passage for signal amplification. The prepared OMC@Ti₃C₂ MXene functioned as a nanocarrier to accommodate considerable aptamers. In the presence of AAs, the transport of electron charge on SPCE surface was influenced by the bio-chemical reactions of the aptamer and AAs, generating a significant decline in the differential pulse voltammetry (DPV) signals. The proposed aptasensor presented a wide linear range and the detection limit was 3.51 nM. Moreover, the aptasensor, with satisfactory stability, reproducibility and specificity, was successfully employed to detect the multi-residuals of AAs in milk. This work provided a novel strategy for monitoring AAs in milk.

Advances of MXenes; Perspectives on Biomedical Research

The last decade witnessed the emergence of a new family of 2D transition metal carbides and nitrides named MXenes, which quickly gained momentum due to their exceptional electrical, mechanical, optical, and tunable functionalities. These outstanding properties also rendered them attractive materials for biomedical and biosensing applications, including drug delivery systems, antimicrobial applications, tissue engineering, sensor probes, auxiliary agents for photothermal therapy and hyperthermia applications, etc. The hydrophilic nature of MXenes with rich surface functional groups is advantageous for biomedical applications over hydrophobic nanoparticles that may require complicated surface modifications. As an emerging 2D material with numerous phases and endless possible combinations with other 2D materials, 1D materials, nanoparticles, macromolecules, polymers, etc., MXenes opened a vast terra incognita for diverse biomedical applications. Recently, MXene research picked up the pace and resulted in a flood of literature reports with significant advancements in the biomedical field. In this context, this review will discuss the recent advancements, design principles, and working mechanisms of some interesting MXene-based biomedical applications. It also includes major progress, as well as key challenges of various types of MXenes and functional MXenes in conjugation with drug molecules, metallic nanoparticles, polymeric substrates, and other macromolecules. Finally, the future possibilities and challenges of this magnificent material are discussed in detail.

Sensitive electrochemical immunosensor for CYFRA21-1 detection based on AuNPs@MoS2@Ti3C2Tx composites

Cytokeratin fragment antigen 21-1 (CYFRA21-1) is a sensitive marker for detecting non-small cell lung cancer (NSCLC). Ti₃C₂T_x modified by gold nanoparticles (AuNPs) and molybdenum disulfide (MoS₂) were synthesized for the first time to obtain the AuNPs@MoS₂@Ti₃C₂T_x composites, which have large specific surface area and good electrocatalytic properties. A novel electrochemical immunoassay for sensitive detection of CYFRA21-1 was developed by loading a large quantity of secondary antibodies (Ab₂) and toluidine blue (TB) on the surface of the material as signal probe, and Nafion-AuNPs mixture as electrode material. When the electrochemical response value of CYFRA21-1 increased linearly within the concentration range of 0.5 pg mL^-1-50 ng mL^-1, the detection limit can reach as low as 0.03 pg mL^-1. In addition, the experimental results showed that the biosensor had the potential to rapidly detect CYFRA21-1 in the complex samples such as patient serum, and had a broad application prospect in the early diagnosis and monitoring of NSCLC.

MXene-MoS2 heterostructure collaborated with catalyzed hairpin assembly for label-free electrochemical detection of microRNA-21

Abnormal expression of microRNAs is greatly associated with the occurrence of various cancer types, revealing great potential of microRNA as biomarkers for cancer diagnosis and prognosis. Herein, a MXene-MoS₂ heterostructure enhancing electrochemical biosensor coupled with catalytic hairpin assembly (CHA) amplification approach for label-free determination of microRNA-21 (miR-21) was successfully assembled. In particular, the unique micro-nano heterostructure with large specific area and favorable electroconductivity exhibited the ability of excellent confinement effect. Thus, rendered the MXene-MoS₂ heterostructure the ability to trigger more target recycling reaction, giving new vitality to the traditional CHA amplification method. Meanwhile, thionine (Thi) and gold nanoparticles (AuNPs) were anchoring at the surface of MXene-MoS₂ heterostructure, respectively, empowered the sensor the capability of capture probes fixation and miR-21 label-free determination. When numerous electronegative double-stranded DNA generated, the electron transfer was greatly hindered, resulting in signal decrease. Accordingly, the design denoted a broad dynamic range from 100 fM to 100 nM and a detection limit of about 26 fM, comparable or lower than previous reported methods for miR-21 detection. Furthermore, the sensing platform supplied satisfactory selectivity, reproducibility and stability towards the miR-21 detection. The real sample determination also showed a promising performance under clinical circumstance. Finally, from the clinical standpoint, the proposed biosensor is a considerable platform toward early disease detection and monitoring.

Rapid, multiplexed, and nucleic acid amplification-free detection of SARS-CoV-2 RNA using an electrochemical biosensor

Considering the worldwide health crisis associated with highly contagious severe respiratory disease of COVID-19 outbreak, the development of multiplexed, simple and rapid diagnostic platforms to detect severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is in high demand. Here, a nucleic acid amplification-free electrochemical biosensor based on four-way junction (4-WJ) hybridization is presented for the detection of SARS-CoV-2. To form a 4-WJ structure, a Universal DNA-Hairpin (UDH) probe is hybridized with two adaptor strands and a SARS-CoV-2 RNA target. One of the adaptor strands is functionalized with a redox mediator that can be detected using an electrochemical biosensor. The biosensor could simultaneously detect 5.0 and 6.8 ag/μL of S and Orf1ab genes, respectively, within 1 h. The biosensor was evaluated with 21 clinical samples (16 positive and 5 negative). The results revealed a satisfactory agreement with qRT-PCR. In conclusion, this biosensor has the potential to be used as an on-site, real-time diagnostic test for COVID-19.

In-situ fabrication of few-layered MoS2 wrapped on TiO2-decorated MXene as anode material for durable lithium-ion storage

Rational construction of hybrid materials integrating the collective virtues of individual building blocks has spurred significant interest in electrode materials for energy storage. Herein, a smart hybrid was fabricated via in-situ assembling of the few-layered MoS₂ (f-MoS₂) coated on the multi-layered Ti₃C₂ MXene decorated with the TiO₂ nanoparticles by the scalable hydrothermal and annealing approaches. In the unique architecture, the multi-layered Ti₃C₂ with the expanded interspaces as the conductive backbone can facilitate the electron transport, provide adequate space to facilitate the infiltration of organic electrolyte into the interior of electrode, and inhibit the aggregation of MoS₂ nanosheets, while the f-MoS₂ with enlarged interlayer can be beneficial for the lithium-ion diffusion and prevent the multi-layered Ti₃C₂from restacking. Moreover, the TiO₂ decorated on the Ti₃C₂ can effectively inhibit the instability of long-chain lithium polysulfides dissolved in organic electrolyte to improve the cycling stability. Thanks to the synergistic effect of the building blocks, the Ti₃C₂/TiO₂@f-MoS₂ hybrid employed as lithium storage anode delivers an extraordinary endurable ability with a high storage capacity of 403.1 mA h g^-1 after 1200 cycles at 2 A g^-1. Importantly, the smart hybridization strategy in this work paves an efficient way to explore the high-performance MXene-based hybrid materials in energy storage fields.

Progress and Insights in the Application of MXenes as New 2D Nano-Materials Suitable for Biosensors and Biofuel Cell Design

Recent progress in the application of new 2D-materials-MXenes-in the design of biosensors, biofuel cells and bioelectronics is overviewed and some advances in this area are foreseen. Recent developments in the formation of a relatively new class of 2D metallically conducting MXenes opens a new avenue for the design of conducting composites with metallic conductivity and advanced sensing properties. Advantageous properties of MXenes suitable for biosensing applications are discussed. Frontiers and new insights in the area of application of MXenes in sensorics, biosensorics and in the design of some wearable electronic devices are outlined. Some disadvantages and challenges in the application of MXene based structures are critically discussed.

Recent Advances in MXene Nanocomposite-Based Biosensors

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