Ti3C2 MXene nanosheet-based capacitance immunoassay with tyramine-enzyme repeats to detect prostate-specific antigen on interdigitated micro-comb electrode

Capacitive biosensors for label-free and ultrasensitive detection of biomarkers

Capacitive biosensors are label-free capacitors that can detect biomarkers with the outstanding advantages of simplicity, low cost, and ultrahigh sensitivity. A typical capacitive biosensor consists of a bioreceptor and a transducer, where the bioreceptor captures the biomarker to form a bioreceptor/biomarker conjugate and the transducer generates a detectable signal. In general, antibodies, aptamers, or proteins are exploited as the bioreceptor, while various electrodes including carbon electrodes (CEs), gold electrodes (AuEs), or interdigitated electrodes (IDEs) may serve as the transducer. Because the formation of bioreceptor/biomarker conjugates often leads to a change in capacitance, the capacitive signal is then employed for biomarker detection. This review summarizes recent advances in capacitive biosensors for the detection of biomarkers over the last five years. With a focus on the three common types of bioreceptors, i.e., antibodies, aptamers, and proteins, capacitive biosensors using CEs, AuEs, and IDEs as the transducers are discussed in detail. The immobilization of bioreceptors and signal amplification strategies are described to provide a robust overview of capacitive biosensors for biomarker detection. In addition, analytical methods and future prospects are given to support the application of capacitive biosensors.

Alanine aminotransferase electrochemical sensor based on graphene@MXene composite nanomaterials

Graphene@MXene composite nanomaterials were utilized to construct an electrochemical sensor for alanine aminotransferase (ALT) detection. The combination of graphene nanosheets with MXene avoids the self-stacking of MXene and graphene, and broadens the charge transfer channel. In addition, the composite nanomaterial provides increased loading sites for pyruvate oxidase. The principle of ALT detection is a two-step enzymatic reaction. L-Alanine was initially transferred to pyruvate catalyzed by ALT. The formed pyruvate was then oxidized by pyruvate oxidase, generating H2O2. Through the detection of the generated H2O2, ALT activity was measured. The linear range of the sensor to ALT was from 5 to 400 U·L-1 with a detection limit of 0.16 U·L-1 (S/N = 3). For real sample analysis, the spiked recovery test results of ALT in serum samples were between 96.89 and 103.93% with RSD < 5%, confirming the reliability of the sensor testing results and potential clinical application of the sensor.

Biomolecule-functionalized nanoformulations for prostate cancer theranostics

BACKGROUND

Even with the advancement in the areas of cancer nanotechnology, prostate cancer still poses a major threat to men's health. Nanomaterials and nanomaterial-derived theranostic systems have been explored for diagnosis, imaging, and therapy for different types of cancer still, for prostate cancer they have not delivered at full potential because of the limitations like in vivo biocompatibility, immune responses, precise targetability, and therapeutic outcome associated with the nanostructured system.

AIM OF REVIEW

Functionalizing nanomaterials with different biomolecules and bioactive agents provides advantages like specificity towards cancerous tumors, improved circulation time, and modulation of the immune response leading to early diagnosis and targeted delivery of cargo at the site of action.

KEY SCIENTIFIC CONCEPTS OF REVIEW

In this review, we have emphasized the classification and comparison of various nanomaterials based on biofunctionalization strategy and source of biomolecules such that it can be used for possible translation in clinical settings and future developments. This review highlighted the opportunities for embedding highly specific biological targeting moieties (antibody, aptamer, oligonucleotides, biopolymer, peptides, etc.) on nanoparticles which can improve the detection of prostate cancer-associated biomarkers at a very low limit of detection, direct visualization of prostate tumors and lastly for its therapy. Lastly, special emphasis was given to biomimetic nanomaterials which include functionalization with extracellular vesicles, exosomes and viral particles and their application for prostate cancer early detection and drug delivery. The present review paves a new pathway for next-generation biofunctionalized nanomaterials for prostate cancer theranostic application and their possibility in clinical translation.

Advanced growth of 2D MXene for electrochemical sensors

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

Emerging Trends and Recent Progress of MXene as a Promising 2D Material for Point of Care (POC) Diagnostics

Two-dimensional (2D) nanomaterials with chemical and structural diversity have piqued the interest of the scientific community due to their superior photonic, mechanical, electrical, magnetic, and catalytic capabilities that distinguish them from their bulk counterparts. Among these 2D materials, two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides with a general chemical formula of M_n+1X_nT_x (where n = 1-3), together known as MXenes, have gained tremendous popularity and demonstrated competitive performance in biosensing applications. In this review, we focus on the cutting-edge advances in MXene-related biomaterials, with a systematic summary on their design, synthesis, surface engineering approaches, unique properties, and biological properties. We particularly emphasize the property-activity-effect relationship of MXenes at the nano-bio interface. We also discuss the recent trends in the application of MXenes in accelerating the performance of conventional point of care (POC) devices towards more practical approaches as the next generation of POC tools. Finally, we explore in depth the existing problems, challenges, and potential for future improvement of MXene-based materials for POC testing, with the goal of facilitating their early realization of biological applications.

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.

Wearable and flexible electrochemical sensors for sweat analysis: a review

Flexible wearable sweat sensors allow continuous, real-time, noninvasive detection of sweat analytes, provide insight into human physiology at the molecular level, and have received significant attention for their promising applications in personalized health monitoring. Electrochemical sensors are the best choice for wearable sweat sensors due to their high performance, low cost, miniaturization, and wide applicability. Recent developments in soft microfluidics, multiplexed biosensing, energy harvesting devices, and materials have advanced the compatibility of wearable electrochemical sweat-sensing platforms. In this review, we summarize the potential of sweat for medical detection and methods for sweat stimulation and collection. This paper provides an overview of the components of wearable sweat sensors and recent developments in materials and power supply technologies and highlights some typical sensing platforms for different types of analytes. Finally, the paper ends with a discussion of the challenges and a view of the prospective development of this exciting field.

Three-dimensional AgNps@Mxene@PEDOT:PSS composite hybrid foam as a piezoresistive pressure sensor with ultra-broad working range

Piezoresistive pressure sensors are becoming increasingly popular for their applications in human motion detection, wearable electronics, health monitoring, and man-machine interfaces. Sensors with superior sensitivity and a broad range of sensing are desirable for practical implementation. To achieve those, a low-cost, scalable and simple fabrication technique of dip coating Ti₃C₂ (MXene), PEDOT:PSS, and AgNPs onto a melamine foam is proposed. The prepared sensor demonstrated sensitivity of 414.27 kPa^-1 at (4.17-12.98 kPa), 182.52 kPa^-1 at (12.98-94.55 kPa), 317.78 kPa^-1 at (94.55 kPa-1.94 MPa), 164.32 kPa^-1 at (> 1.94 MPa), extraordinaire detecting range 977.6 N and outstanding repeatability. The sensor was successfully applied for the real-time detection of heartbeat pulse, limb movement, human weight and powered an LED. Furthermore, an integrated circuit design with sensors had the ability to identify spatial pressure distribution and visualize it on a pressure map.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10853-022-08012-y.

A carbon dots-enhanced laccase-based electrochemical sensor for highly sensitive detection of dopamine in human serum

Enzyme-based electrochemical sensor possesses a significant advantage in the highly efficient detection of small molecules, however, the poor electron transport efficiency limits their wide application. In this study, taking advantage of the distinct biocatalytic activity of laccase and the excellent electroconductibility of carbon dots, a carbon dots-enhanced laccase-based electrochemical sensor for the detection of dopamine (DA) is established. Thereinto, laccase can specifically recognize DA and promote its electrocatalytic oxidation on the electrode, while, the carbon dots can be used as the immobilization substrate of laccase and enhance its electron transfer efficiency, thus achieving the highly sensitive detection of dopamine. The electrochemical performance of the modified electrode interface is studied by electrochemical impedance spectroscopy and differential pulse voltammetry. As demonstrated, the electrocatalytic activity of the proposed electrochemical sensor for DA is significantly improved and exhibits a low detection limit (0.08 μM) and a wide linear range (0.25 μM-76.81 μM). The excellent selectivity allows the sensor has the capacity for specific discrimination the DA from other interferents. Furthermore, by analyzing the DA in human serum verifies the practicability of this assay in real sample analysis.

Cell Membrane and V2C MXene-Based Electrochemical Immunosensor with Enhanced Antifouling Capability for Detection of CD44

The inactive adsorption and interference of biomolecules in electrochemical biosensors is a topic of intense interest. Directly utilizing native cell membranes to endow electrochemical surfaces with antifouling and biocompatible features is a promising strategy, rather than attempting to synthetically replicate complex biological interface properties. In this study, we present a facial and sensitive sandwich-type antifouling immunoassay through platelet membrane/Au nanoparticle/delaminated V₂C nanosheet (PM/AuNPs/d-V₂C)-modified electrode as the substrate of sensing interface and methylene blue/aminated metal organic framework (MB@NH₂-Fe-MOF-Zn) as an electrochemical signal probe. The biosensor perfectly integrates the high conductivity of AuNPs-loaded V₂C MXene with the excellent loading property of NH₂-Fe-MOF-Zn to improve the electrochemical sensing performance. In addition, the excellent antifouling properties of the homogeneous cell membrane can effectively prevent the non-specific adsorption of model proteins. The obtained antifouling biosensor possesses the capability of ultrasensitive detection of CD44 and CD44-positive cancer cell in complex liquids and exhibits good analytical performance for the analysis of CD44 with a linear range from 0.5 ng/mL to 500 ng/mL. This strategy of developing cell membrane-based biosensing systems with enhanced antifouling capability can be easily expanded to the construction of other complex biosensors, and the advanced biological probes and analytical methods provide a favorable means to accurately quantify biomarkers associated with tumor progression.

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.

Integrating Ti3C2/MgIn2S4 heterojunction with a controlled release strategy for split-type photoelectrochemical sensing of miRNA-21

The harsh operating conditions and time-consuming fabrication process of the photoelectrode modification process have limited the potential applications of photoelectrochemical (PEC) sensors. To overcome these drawbacks, this study introduced a unique split-type PEC biosensor for microRNA-21 (miRNA-21) detection. Specifically, a Ti₃C₂/MgIn₂S₄ heterojunction was adopted as the photosensitive material, and a target-controlled glucose release system, comprising a multifunctional porphyrin-based metal-organic framework (PCN-224), was used for signal amplification. The Ti₃C₂/MgIn₂S₄ heterojunction effectively separated the photogenerated electrons and holes, and improved the photoelectric conversion efficiency, offering a strong initial photocurrent signal during PEC biosensing. Meanwhile, the porous PCN-224 acted as a nimble nanocontainer that encapsulated glucose using a capture probe (CP). In the presence of miRNA-21, the CP formed a CP-miRNA-21 complex and then detached from PCN-224, controllably releasing the trapped glucose. The oxidization of glucose by glucose oxidase resulted in hydrogen peroxide generation, which acted as a scavenger for the holes generated on the surface of Ti₃C₂/MgIn₂S₄, and significantly enhanced the photocurrent response under visible light irradiation. Finally, the sensor exhibited good performance for miRNA-21 detection with a low detection limit (0.17 fM) and wide linearity range (0.5 fM-1.0 nM). Thus, the proposed Ti₃C₂/MgIn₂S₄-based split-type PEC sensor is a promising tool for sensitive and accurate detection of miRNA-21 and provides an innovative basis for the preparation of other high-performance sensors.

Perspectives and Trends in Advanced MXenes-Based Optical Biosensors for the Recognition of Food Contaminants

Fabricating novel biosensing constructs with high sensitivity and selectivity is highly demanded in food contaminants detection. In this prospect, various nanostructured materials were envisaged to build (bio)sensors with superior sensitivity and selectivity. The desirable biocompatibility, brilliant mechanical strength, ease of surface functionalization, as well as tunable optical and electronic features, portray 2D MXenes as versatile scaffolds for biosensing. In this review, we overviewed the state-of-the-art MXenes-based optical biosensing devices to detect mycotoxins, pesticide residues, antibiotic residues, and food borne-pathogens from foodstuff and environmental matrices. Firstly, the synthesis methods and surface functionalization/modification of MXenes are discussed. Secondly, according to the target analytes, we categorized and presented a detailed account of the newest research progress of MXenes-based optical probes for food contaminants monitoring. The efficiency of all the surveyed probes was assessed on the basis of important factors like response time, detection limit (DL), and sensing range. Lastly, the necessity and requirements for future advances in this emerging MXenes material are also given, followed by challenges and opportunities. We hope that this study will bridge the gap between nanotechnology and food science, offering insights for engineers or scientists in both areas to accelerate the progress of MXenes-based materials for food safety detection.

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.

Current-Volt Biosensing "Cystatin C" on Carbon Nanowired Interdigitated Electrode Surface: A Clinical Marker Analysis for Bulged Aorta

A carbon nanowire-modified surface with interdigitated electrode (IDE) sensing system was introduced to identify abdominal aortic aneurysm biomarker "papain," also known as cysteine protease, used as the capture probe to identify Cystatin C. Papain was immobilized through the covalent integration of amine group on papain and the carboxyl group with carbon nanowire. This papain-modified electrode surface was utilized to detect the different concentrations of Cystatin C (100 pg/mL to 3.2 ng/mL). The interaction between papain and Cystatin C was monitored using a picoammeter, and the response curves were compared. With increasing Cystatin C concentrations, the total current levels were gradually increased with a linear range from 200 pg/mL to 3.2 ng/mL, and the current differences were plotted and the detection limit of Cystatin C was calculated as 200 pg/mL. The averaging of three independent experiments (n = 3) was made with 3δ estimation, and the determination coefficient was y = 1.8477 × 0.7303 and R ² = 0.9878. Furthermore, control experiments with creatinine and gliadin failed to bind the immobilized papain, indicating the specific detection of Cystatin C.

Recent Advances in Electrochemical Sensing of Hydrogen Peroxide (H2O2) Released from Cancer Cells

Cancer is by far the most common cause of death worldwide. There are more than 200 types of cancer known hitherto depending upon the origin and type. Early diagnosis of cancer provides better disease prognosis and the best chance for a cure. This fact prompts world-leading scientists and clinicians to develop techniques for the early detection of cancer. Thus, less morbidity and lower mortality rates are envisioned. The latest advancements in the diagnosis of cancer utilizing nanotechnology have manifested encouraging results. Cancerous cells are well known for their substantial amounts of hydrogen peroxide (H₂O₂). The common methods for the detection of H₂O₂ include colorimetry, titration, chromatography, spectrophotometry, fluorimetry, and chemiluminescence. These methods commonly lack selectivity, sensitivity, and reproducibility and have prolonged analytical time. New biosensors are reported to circumvent these obstacles. The production of detectable amounts of H₂O₂ by cancerous cells has promoted the use of bio- and electrochemical sensors because of their high sensitivity, selectivity, robustness, and miniaturized point-of-care cancer diagnostics. Thus, this review will emphasize the principles, analytical parameters, advantages, and disadvantages of the latest electrochemical biosensors in the detection of H₂O₂. It will provide a summary of the latest technological advancements of biosensors based on potentiometric, impedimetric, amperometric, and voltammetric H₂O₂ detection. Moreover, it will critically describe the classification of biosensors based on the material, nature, conjugation, and carbon-nanocomposite electrodes for rapid and effective detection of H₂O₂, which can be useful in the early detection of cancerous cells.

A novel electrochemical sandwich-like immunosensor based on carboxyl Ti3C2Tx MXene and rhodamine b/gold/reduced graphene oxide for Listeria monocytogenes

Listeria monocytogenes (LM) is one of the most common food-borne pathogens and can induce a series of diseases with a high mortality rate to humans; hence, it is very necessary to develop a highly sensitive method for LM detection. Based on this need, a new sandwich-like electrochemical immunosensing platform was developed herein by preparing carboxyl Ti₃C₂T_x MXene (C-Ti₃C₂T_x MXene) as the sensing platform and rhodamine b/gold/reduced graphene oxide (RhB/Au/RGO) as the signal amplifier. The high conductivity and large surface area of C-Ti₃C₂T_x MXene make it a desirable nanomaterial to fix the primary antibody of LM (PAb), while the prepared Au/RGO/RhB nanohybrid is dedicated to assembling the secondary antibody (SAb) of LM, offering an amplified response signal. Through the use of RhB molecule as the signal probe, the experiments showed that the peak currents of RhB increase along with an increase in the concentration of LM from 10 to 10⁵ CFU mL^-1, and an extremely low limit of detection (2 CFU mL^-1) was obtained on the basis of the proposed immunosensing platform after optimizing various conditions. Hence, it is confirmed that the developed sandwich-like immunosensor based on C-Ti₃C₂T_x MXene and RhB/Au/Gr has great application in the detection of LM and other analytes.

A Ti3C2-MXene-functionalized LRSPR biosensor based on sandwich amplification for human IgG detection

Long-range surface plasmon resonance (LRSPR) has demonstrated excellent performance in sensing and detection, due to its higher accuracy and sensitivity compared with conventional surface plasmon resonance (cSPR). In this work, we establish an LRSPR biosensor which employs PDA/Ti₃C₂-MXene/PDA-gold film as a sensing substrate and gold nanoparticles (AuNPs) as enhancers. Ti₃C₂-MXene is an emerging two-dimensional (2D) layered material which is used extensively in immunoassay and biosensing. The sensing substrate comprises two polydopamine (PDA) films between which is sandwiched a Ti₃C₂-MXene film based on a gold film, which provides a large surface area and abundant binding sites to rabbit anti-human IgG (Ab₁). Sandwich amplification is adopted to enhance the sensitivity of the LRSPR biosensor, and AuNPs/staphylococcal protein A (SPA)/mouse anti-human IgG (Ab₂) composites are introduced into the flow cell as enhancers after the immune binding of human IgG to Ab₁. The antigen (human IgG) detection range is 0.075 μg mL^-1 to 40 μg mL^-1, and the limit of detection is almost 20 times lower than that for cSPR biosensors. This novel LRSPR biosensor demonstrates excellent performance in immune sensing over a broad detection range and a low limit of detection. Subsequent modification of the LRSPR sensing platform could be made for extensive application in various biological detection fields.

Capacitive biosensor based on vertically paired electrodes for the detection of SARS-CoV-2

Vertically paired electrodes (VPEs) with multiple electrode pairs were developed for the enhancement of capacitive measurements by optimizing the electrode gap and number of electrode pairs. The electrode was fabricated using a conductive polymer layer of PEDOT:PSS instead of Ag and Pt metal electrodes to increase the VPE fabrication yield because the PEDOT:PSS layer could be effectively etched using a reactive dry etching process. In this study, sensitivity enhancement was realized by decreasing the electrode gap and increasing the number of VPE electrode pairs. Such an increase in sensitivity according to the electrode gap and the number of electrode pairs was estimated using a model analyte for an immunoassay. Additionally, a computer simulation was performed using VPEs with different electrode gaps and numbers of VPE electrode pairs. Finally, VPEs with multiple electrode pairs were applied for SARS-CoV-2 nucleoprotein (NP) detection. The capacitive biosensor based on the VPE with immobilized anti-SARS-CoV-2 NP was applied for the specific detection of SARS-CoV-2 in viral cultures. Using viral cultures of SARS-CoV-2, SARS-CoV, MERS-CoV, and CoV-strain 229E, the limit of detection (LOD) was estimated to satisfy the cutoff value (dilution factor of 1/800) for the medical diagnosis of COVID-19, and the assay results from the capacitive biosensor were compared with commercial rapid kit based on a lateral flow immunoassay.

Emitter-Quencher Pair of Single Atomic Co Sites and Monolayer Titanium Carbide MXenes for Luminol Chemiluminescent Reactions

A facile, one-step doping protocol was adopted to synthesize Co single atomic site catalysts (SASCs) in UiO-66 metal-organic frameworks. In view of highly uniform active sites of Co-O₆ moieties, the SASCs specifically contribute to catalyzing the generation of a large amount of singlet oxygen instead of superoxide or hydroxyl radicals, which endows Co SASCs with a the remarkable enhancement effect (∼3775 times) on luminol chemiluminescent (CL) emission. Interestingly, monolayer titanium carbide MXenes can drastically quench the CL signal of the Co SASC-boosted luminol reaction by ∼94.6% as highly efficient luminescent absorbents. Furthermore, the emitter-quencher pair of Co SASCs and titanium carbide MXenes was successfully adopted to develop an immunoassay method for cardiac troponin I (cTnI) on an immunochromatographic test strip platform. With a sandwich immunoreaction mode, a titanium carbide MXene-labeled cTnI tracer antibody was captured on the test line of a test strip, which significantly inhibited the CL response of the Co SACs-boosted luminol system. The dynamic range for quantitating cTnI is 1.0-100 pg mL^-1, with a detection limit of 0.33 pg mL^-1 (3σ). The test strip was successfully used to detect cTnI in human serum samples collected from cardiopathy patients. This proof-of-principle work manifests both the CL enhancement of SASCs and the quenching behavior of MXenes, which shows the thrilling prospects of combinational usage of the two functionalized nanomaterials for tracking biological recognition events.

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.

Diagnostics of SARS-CoV-2 infection using electrical impedance spectroscopy with an immunosensor to detect the spike protein

Mass testing for the diagnostics of COVID-19 has been hampered in many countries owing to the high cost of the methodologies to detect genetic material of SARS-CoV-2. In this paper, we report on a low-cost immunosensor capable of detecting the spike protein of SARS-CoV-2, including in samples of inactivated virus. Detection is performed with electrical impedance spectroscopy using an immunosensor that contains a monolayer film of carboxymethyl chitosan as matrix, coated with an active layer of antibodies specific to the spike protein. In addition to a low limit of detection of 0.179 fg/mL within an almost linear behavior from 10⁻²⁰ g/mL to 10⁻¹⁴ g/mL, the immunosensor was highly selective. For the samples with the spike protein could be distinguished in multidimensional projection plots from samples with other biomarkers and analytes that could be interfering species for healthy and infected patients. The excellent analytical performance of the immunosensors was validated with the distinction between control samples and those containing inactivated SARS-CoV-2 at different concentrations. The mechanism behind the immunosensor performance is the specific antibody-protein interaction, as confirmed with the changes induced in C–H stretching and protein bands in polarization-modulated infrared reflection absorption spectra (PM-IRRAS). Because impedance spectroscopy measurements can be made with low-cost portable instruments, the immunosensor proposed here can be applied in point-of-care diagnostics for mass testing even in places with limited resources.

Nano-bio interfaces effect of two-dimensional nanomaterials and their applications in cancer immunotherapy

The field of two-dimensional (2D) nanomaterial-based cancer immunotherapy combines research from multiple subdisciplines of material science, nano-chemistry, in particular nano-biological interactions, immunology, and medicinal chemistry. Most importantly, the "biological identity" of nanomaterials governed by bio-molecular corona in terms of bimolecular types, relative abundance, and conformation at the nanomaterial surface is now believed to influence blood circulation time, bio-distribution, immune response, cellular uptake, and intracellular trafficking. A better understanding of nano-bio interactions can improve utilization of 2D nano-architectures for cancer immunotherapy and immunotheranostics, allowing them to be adapted or modified to treat other immune dysregulation syndromes including autoimmune diseases or inflammation, infection, tissue regeneration, and transplantation. The manuscript reviews the biological interactions and immunotherapeutic applications of 2D nanomaterials, including understanding their interactions with biological molecules of the immune system, summarizes and prospects the applications of 2D nanomaterials in cancer immunotherapy.

Ultra pH-sensitive detection of total and free prostate-specific antigen using electrochemical aptasensor based on reduced graphene oxide/gold nanoparticles emphasis on TiO2/carbon quantum dots as a redox probe

The development of a rapid, sensitive, and straightforward detection method of prostate-specific antigen (PSA) is indispensable for the early diagnosis of prostate cancer (PCa). This work relates an electrochemical method using functionalized single-stranded DNA aptamer to diagnose PCa and benign prostate hyperplasia. The sensing platform relies on PSA recognition by aptamer/Au/GO-nanohybrid-modified glassy carbon electrode. Besides ferrocyanide TiO2/carbon quantum dots (CQDs) probe is used to investigate the effect of nanoparticle-containing electrolyte. Optimization of incubation time of aptamer/Au/GO-nanohybrid and volume fraction of nafion were done using Design Expert 10 software reporting 42.4 h and 0.095% V/V, respectively. In ferrocyanide medium, PSA detection as low as 3, 2.96, and 0.85 ng mL-1 was achieved with a dynamic range from 0.5 to 7 ng ml-1, in accord with clinical values, using cyclic voltammetry, square wave voltammetry, and electrochemical impedance spectroscopy, respectively. Moreover, this sensor exhibited conspicuous performance in TiO2/CQDs-containing medium with different pH values of 5.4 and 8 to distinguish total PSA and free PSA, resulting in very low limit of detections, 0.028, and 0.007 ng ml-1, respectively. The results manifested the proposed system as a forthcoming sensor in a clinical and point of care analysis of PSA.

Ultrasensitive dual-quenching electrochemiluminescence immunosensor for prostate specific antigen detection based on graphitic carbon nitride quantum dots as an emitter

Early monitoring of prostate-specific antigen (PSA) is crucial in diagnosis and proactive treatment of prostate disease. Herein, a dual-quenching ternary ECL immunosensor was designed for PSA detection based on graphitic carbon nitride quantum dots (g-CNQDs, as an emitter), potassium persulfate (K₂S₂O₈, as a coreactant), and silver nanoparticles doped multilayer Ti₃C₂ MXene hybrids (Ag@TCM, as a coreaction accelerator). First, Ag@TCM was immobilized on the surface of a glassy carbon electrode, then g-CNQDs was further adsorbed on Ag@TCM to acquire a higher initial ECL signal at a potential window from - 1.3 to 0.0 V (vs. Ag/AgCl). Ag@TCM not only acted as the coreaction accelerator, but also as a matrix to load enormous g-CNQDs and prostate-specific capture antibody via Ag-N bond. Meanwhile, prostate-specific detection antibody was marked by gold nanoparticles modified manganese dioxide as a dual-quenching probe (Ab2- Au@MnO₂). When Ab2-Au@MnO₂ was introduced into the ternary ECL system via sandwiched immuno-reaction, the high-sensitive detection of PSA was achieved by the dual-quenching effect, caused by the resonant energy transfer from g-CNQDs (energy donor) to Au@MnO₂ (energy acceptor). As a result, this ECL immunosensor showed a good dynamic concentration range from 10 fg·mL^-1 to 100 ng·mL^-1 with a detection limit of 6.9 fg·mL^-1 for PSA detection. The dual-quenching ECL strategy presented high stability and good specificity to open up a new pathway for ultrasensitive immunoassay.

Recent advances in techniques for fabrication and characterization of nanogap biosensors: A review

Nanogap biosensors have fascinated researchers due to their excellent electrical properties. Nanogap biosensors comprise three arrays of electrodes that form nanometer-size gaps. The sensing gaps have become the major building blocks of several sensing applications, including bio- and chemosensors. One of the advantages of nanogap biosensors is that they can be fabricated in nanoscale size for various downstream applications. Several studies have been conducted on nanogap biosensors, and nanogap biosensors exhibit potential material properties. The possibilities of combining these unique properties with a nanoscale-gapped device and electrical detection systems allow excellent and potential prospects in biomolecular detection. However, their fabrication is challenging as the gap is becoming smaller. It includes high-cost, low-yield, and surface phenomena to move a step closer to the routine fabrications. This review summarizes different feasible techniques in the fabrication of nanogap electrodes, such as preparation by self-assembly with both conventional and nonconventional approaches. This review also presents a comprehensive analysis of the fabrication, potential applications, history, and the current status of nanogap biosensors with a special focus on nanogap-mediated bio- and chemical sonsors.

Sensitive pH-responsive point-of-care electrochemical immunoassay for influenza A (H1N1) virus using glucose oxidase-functionalized Ti3C2-MXene nanosheets

Influenza A (H1N1) virus is a serious health threat and potential leading cause of death around the world during the processes of immunity and inflammation. Herein a sensitive pH-responsive point-of-care (POC) electrochemical immunoassay was designed for the quantitative monitoring of H1N1 influenza virus using glucose oxidase (GOx) and secondary antibody-functionalized Ti₃C₂-MXene nanosheets. The assay was carried out on the basis of the sandwich-type immunoreaction in the capture antibody-coated microplate. Two-dimensional (2D) Ti₃C₂-MXene nanosheets with a large surface area could efficiently enhance the loading amount of GOx molecules, thereby resulting in the signal amplification. Accompanying the formed immunocomplexes, labeled GOx molecules catalyzed glucose into gluconic acid and hydrogen peroxide. The generated gluconic acid caused a pH change of the detection solution, which was quantitatively determined on a handheld pH meter. Two labeling strategies with and without Ti₃C₂-MXene nanosheets were investigated to determine the target H1N1 influenza virus, and improved properties were acquired with the Ti₃C₂-MXene-labeled system. Under optimum conditions, the Ti₃C₂-MXene-based immunoassay gave good dynamic responses toward the target H1N1 influenza virus from 0.01 μg mL^-1 to 100 μg mL^-1 with a detection limit of 1.3 ng mL^-1. Good reproducibility, high specificity, and acceptable stability were also achieved in the analysis of the target H1N1 influenza virus. Significantly, measurements of the H1N1 influenza virus from clinical human samples were demonstrated to further confirm the method reliability and accuracy of the Ti₃C₂-MXene-based electrochemical immunoassay. Importantly, such a pH-meter-based immunoassay can be suitable for use in point-of-care applications and opens new opportunities for diagnostics.

Synergizing Functional Nanomaterials with Aptamers Based on Electrochemical Strategies for Pesticide Detection: Current Status and Perspectives

Owing to the high toxicity and large-scale use of pesticides, it is imperative to develop selective, sensitive, portable, and convenient sensors for rapid monitoring of pesticide. Therefore, the electrochemical detection platform offers a promising analytical approach since it is easy to operate, economical, efficient, and user-friendly. Meanwhile, with advances in functional nanomaterials and aptamer selection technologies, numerous sensitivity-enhancement techniques alongside a widespread range of smart nanomaterials have been merged to construct novel aptamer probes to use in the biosensing field. Hence, this study intends to highlight recent development and promising applications on the functional nanomaterials with aptamers for pesticides detection based on electrochemical strategies. We also reviewed the current novel aptamer-functionalized microdevices for the portability of pesticides sensors. Furthermore, the major challenges and future prospects in this field are also discussed to provide ideas for further research.

Electrochemical Biosensor Based on HRP/Ti3C2/Nafion Film for Determination of Hydrogen Peroxide in Serum Samples of Patients with Acute Myocardial Infarction

Hydrogen peroxide (H₂O₂) has been reported to mediate a variety of physiological and pathological processes in living systems. In this work, a biosensor for determination of H₂O₂ was prepared by using an HRP/Ti₃C₂/Nafion film-modified glassy carbon electrode (GCE). Ti₃C₂ nanosheets with remarkable conductivity and high specific surface area were chosen as carriers for HRP. Moreover, this biosensor modified with HRP has a specific catalytic effect on H₂O₂. The difference in peak current could reflect the quantitative change of H₂O₂. The linear range of the biosensor is 5-8000 μM, and the detection limit is 1 μM (S/N = 3). This biosensor was used to detect H₂O₂ in clinical serum samples of normal controls and patients with acute myocardial infarction (AMI) before and after percutaneous coronary intervention (PCI). The results showed that the difference between normal controls and patients is significant (P < 0.05), as well as the difference for patients before and after PCI (P < 0.01), but no significant difference existed between postoperative patients and normal controls. This biosensor has the advantages of simple preparation, high sensitivity, and quick detection, showing potential application in clinical diagnosis.

Recent advances in DNA walker machines and their applications coupled with signal amplification strategies: A critical review

DNA walkers, a type of dynamic nanomachines, have become the subject of burgeoning research in the field of biology. These walkers are powered by driving forces based on strand displacement reactions, protein enzyme/DNAzyme reactions and conformational transitions. With the unique properties of high directionality, flexibility and efficiency, DNA walkers move progressively and autonomously along multiple dimensional tracks, offering abundant and promising applications in biosensing, material assembly and synthesis, and early cancer diagnosis. Notably, DNA walkers identified as signal amplifiers can be combined with various amplification approaches to enhance signal transduction and amplify biosensor sensing signals. Herein, we systematically and comprehensively review the walking principles of various DNA walkers and the recent progress on multiple dimensional tracks by presenting representative examples and an insightful discussion. We also summarized and categorized the diverse signal amplification strategies with which DNA walkers have coupled. Finally, we outline the challenges and future trends of DNA walker machines in emerging analytical fields.

Measurement of Electric Double Layer Capacitance Using Dielectrophoresis-Based Particle Manipulation

An electric double layer (EDL) generally exists at the interface between a conductive electrode and its adjacent liquid electrolyte. Accurate measurement of the capacitance of EDL is requisite but a great challenge due to the complexity of its variation mechanism correlated with the magnitude and frequency of applied signals and the difficulty in measuring the inner layer potentials across the EDL. Herein, a novel dielectrophoresis (DEP)-based approach is proposed to measure the capacitance of an EDL at a microelectrode/electrolyte interface. The measurement is achieved by employing DEP manipulation to micro polystyrene (PS) spheres suspended in a liquid electrolyte and determining the capacitance of EDL on the microelectrodes from the moving velocities of spheres. This method allows measurement of the capacitances of EDL under alternating current (AC) signals with different magnitudes and frequencies, so that the capacitance change with the magnitude and frequency of the applied signal can be characterized. The method avoids the impedance interference from the liquid electrolyte, external measuring systems, and other crosstalks, enabling an accurate measurement of double layer capacitance. In addition, the inner layer potentials across EDL under different magnitudes and frequencies of applied signals are comprehensively investigated, which facilitates an understanding of the ion behavior at the interfacial boundary that governs external observations of electrochemical reactions. The accurate measurement of the capacitance of EDL is of significance to explore the mechanism of interfacial functioning of electrochemical and bioelectrical devices and systems.

Sensitive silica-alumina modified capacitive non-Faradaic glucose sensor for gestational diabetes

A highly sensitive silica-alumina (Si-Al)-modified capacitive non-Faradaic glucose biosensor was introduced to monitor gestational diabetes. Glucose oxidase (GOx) was attached to the Si-Al electrode surface as the probe through amine-modification followed by glutaraldehyde premixed GOx as aldehyde-amine chemistry. This Si-Al (∼50 nm) modified electrode surface has increased the current flow upon binding of GOx with glucose. Capacitance values were increased by increasing the glucose concentrations. A mean capacitance value was plotted and the detection limit was found as 0.03 mg/mL with the regression coefficient value, R² = 0.9782 [y = 0.8391x + 1.338] on the linear range between 0.03 and 1 mg/mL. Further, a biofouling experiment with fructose and galactose did not increase the capacitance, indicating the specific glucose detection. This Si-Al-modified capacitance sensor detects a lower level of glucose presence and helps in monitoring gestational diabetes.

Double ion-exchange reaction-based photoelectrochemical immunoassay for sensitive detection of prostate-specific antigen

This work developed a double ion-exchange reaction-based photoelectrochemical (PEC) immunoassay with the split-type detection mode for sensitive detection of prostate-specific antigen (PSA, used as a model). The nanocomposite of cadmium sulfide and nickel sulfide (CdS@NiS nanocomposite), as the photoactive material, was rapidly synthesized by two-step hydrothermal treatment. In the presence of target PSA, the cupric oxide nanoparticle (CuO NP) labeled detection antibody was introduced into the detection system by sandwich immunoreaction and the copper (Cu²⁺) ions was released from CuO nanoparticles by acid to participate in double ion-exchange reaction. The double ion-exchange reaction on the photoelectric sensing interface between Cu²⁺ and CdS@NiS nanocomposites formed the weak photoactive material Cu_xS (x = 1, 2) to reduce the photocurrent. Under optimal conditions, the double ion-exchange reaction-based PEC immunoassay exhibited good photocurrent responses toward target PSA within the dynamic working range from 0.01 ng mL^-1 to 50 ng mL^-1 at a low limit of detection (LOD) of 2.9 pg mL^-1. Besides, our work could achieve good reproducibility and high specificity under the split-type detection mode. Compared with human PSA ELISA kit, the accuracy obtained by our strategy was satisfactory. Importantly, this Cu²⁺-activated double ion-exchange reaction-based PEC immunoassay provides a promising platform for the detection of biomarkers.

DNAzyme concatemer-catalyzed precipitation on an interdigitated micro-comb electrode for capacitance immunosensing of interleukin-6 with rolling circle amplification

A novel capacitance immunosensor based on DNAzyme concatemer-amplified signal-generation tags was developed for the sensitive detection of interleukin-6 (IL-6) on an interdigitated micro-comb electrode.

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.

MXenes-Based Bioanalytical Sensors: Design, Characterization, and Applications

MXenes are recently developed 2D layered nanomaterials that provide unique capabilities for bioanalytical applications. These include high metallic conductivity, large surface area, hydrophilicity, high ion transport properties, low diffusion barrier, biocompatibility, and ease of surface functionalization. MXenes are composed of transition metal carbides, nitrides, or carbonitrides and have a general formula Mn+1Xn, where M is an early transition metal while X is carbon and/or nitrogen. Due to their unique features, MXenes have attracted significant attention in fields such as clean energy production, electronics, fuel cells, supercapacitors, and catalysis. Their composition and layered structure make MXenes attractive for biosensing applications. The high conductivity allows these materials to be used in the design of electrochemical biosensors and the multilayered configuration makes them an efficient immobilization matrix for the retention of activity of the immobilized biomolecules. These properties are applicable to many biosensing systems and applications. This review describes the progress made on the use and application of MXenes in the development of electrochemical and optical biosensors and highlights future needs and opportunities in this field. In particular, opportunities for developing wearable sensors and systems with integrated biomolecule recognition are highlighted.

Efficient BiVO4 photoanode decorated with Ti3C2TX MXene for enhanced photoelectrochemical sensing of Hg(II) ion

A highly sensitive photoelectrochemical (PEC) sensing platform was constructed for Hg²⁺ determination based on the Schottky heterojunction between an emerging 2D material Ti₃C₂T_X MXene and a promising semiconductor material BiVO₄. Through simply spin-coating the single-layer Ti₃C₂T_X onto the surface of BiVO₄ film, the modified electrode exhibited significantly enhanced PEC activity. However, the boost in photocurrent could be noticeably suppressed due to the consumption of hole-scavenging agents (reduced glutathione) by the added Hg²⁺. Owing to the selective decrease in the photocurrent with the addition of Hg²⁺, the PEC sensor based on BiVO₄/Ti₃C₂T_X displayed a wide linear range from 1 pM to 2 nM with the limit of detection down to 1 pM. Moreover, the PEC sensor also exhibited satisfactory accuracy and repeatability in practical sample water, the Yangtze River water, demonstrating the great potential for monitoring heavy metal ions in natural water resources.

Algae Extraction Controllable Delamination of Vanadium Carbide Nanosheets with Enhanced Near-Infrared Photothermal Performance

The two-dimensional (2D) vanadium carbide (V₂ C) MXene has shown great potential as a photothermal agent (PTA) for photothermal therapy (PTT). However, the use of V₂ C in PTT is limited by the harsh synthesis condition and low photothermal conversion efficiency (PTCE). Herein, we report a completely different green delamination method using algae extraction to intercalate and delaminate V₂ AlC to produce mass V₂ C nanosheets (NSs) with a high yield (90 %). The resulting V₂ C NSs demonstrated good structural integrity and remarkably high absorption in near infrared (NIR) region with a PTCE as high as 48 %. Systemic in vitro and in vivo studies demonstrate that the V₂ C NSs can serve as efficient PTA for photoacoustic (PA) and magnetic resonance imaging (MRI)-guided PTT of cancer. This work provides a cost-effective, environment-friendly, and high-yielding disassembly approach of MAX, opening a new avenue to develop MXenes with desirable properties for a myriad of applications.