Metal-Organic Framework-Functionalized Paper-Based Electrochemical Biosensor for Ultrasensitive Exosome Assay

Flexible carbon fiber cloth supports decorated with cerium metal- organic frameworks and multi-walled carbon nanotubes for simultaneous on-site detection of Cd2+ and Pb2+ in food and water samples

The widespread heavy metal pollution endangers human health; hence, accurate on-site detection and quantification of heavy metal content in the surroundings is a vital step in reversing the harmful effect. Herein, an electrochemical sensor based on flexible cerium metal-organic framework@multi-walled carbon nanotubes/carbon cloth (CeMOF@MWCNTs/CC) was constructed for simultaneous on-site detection of Cd²⁺ and Pb²⁺ in food and water samples. The rich carboxyl groups of MWCNTs provided abundant sites for the adsorption of Cd²⁺ and Pb²⁺, and the mutual conversion of Ce³⁺ and Ce⁴⁺ in CeMOF facilitated the reduction and reoxidation of metal ions. The prepared electrode showed excellent performance in the simultaneous measurement of Cd²⁺ and Pb²⁺, with detection limits of 2.2 ppb and 0.64 ppb, respectively. More importantly, the sensing platform has been successfully used to detect simultaneously Cd²⁺ and Pb²⁺ in grain and water samples, and the detection results were consistent with the standard methods, showing great potential in environmental monitoring and food safety.

"One suction and one extrusion" mode-based wash-free platform for determination of breast cancer cell-derived exosomes

A simple and wash-free POCT platform based on microcapillary was developed, using breast cancer cell-derived exosomes as a model. This method adopted the "one suction and one extrusion" mode. The hybridized complex of epithelial cell adhesion molecule (EpCAM) aptamer and complementary DNA-horseradish peroxidase conjugate (CDNA-HRP) was pre-modified on the microcapillary's inner surface. "One suction" meant inhaling the sample into the functionalized microcapillary. The exosomes could specifically bind with the EpCAM aptamer on the microcapillary's inner wall, and then the CDNA-HRP complex was released. "One extrusion" referred to squeezing the shedding CDNA-HRP into the 3,3',5,5'-tetramethylbenzidine (TMB)/H₂O₂ solution, and then the enzyme-catalyzed reaction would occur to make the solution yellow using sulfuric acid as the terminator. Therefore, exosome detection could be realized. The limit of detection was 2.69 × 10⁴ particles mL^-1 and the signal value had excellent linearity in the concentration range from 2.75 × 10⁴ to 2.75 × 10⁸ particles⋅mL^-1 exosomes. In addition, the wash-free POCT platform also displayed a favorable reproducibility (RSD = 2.9%) in exosome detection. This method could effectively differentiate breast cancer patients from healthy donors. This work provided an easy-to-operate method for detecting cancer-derived exosomes without complex cleaning steps, which is expected to be applied to breast cancer screening.

A visualized sensor based on layered double hydroxides with peroxidase-like activity for sensitive acetylcholinesterase assay

Acetylcholinesterase (AChE) plays a crucial role in biological neurotransmission. The aberrant expression of AChE is associated with various neurodegenerative diseases. Therefore, it is of great significance to develop a simple and highly sensitive AChE analysis platform. Herein, a simple colorimetric sensor was constructed for sensitive detection of AChE based on the peroxidase-like catalytic activity of Ni/Co layered double hydroxides (Ni/Co LDHs). In this sensor, the fabricated Ni/Co LDHs possess high peroxidase-like activity, enabling rapid catalysis of o-phenylenediamine (OPD) to produce yellow oxOPD in the presence of H₂O₂. This peroxidase-like activity of Ni/Co LDHs was found to be effectively inhibited by the presence of AChE. It is speculated that the combination of AChE on the outer surface of Ni/Co LDHs through non-covalent interaction may cover the active sites and hinder their adsorption to the substrates, leading to the failure of OPD oxidation. As a result, the yellow color from oxOPD is related to the AChE concentration, enabling the direct AChE assay in an equipment-free manner. In addition, the fabricated Ni/Co LDHs could be modified on a paper surface to obtain a paper-based analytical device for visualized colorimetric detection of AChE. The as-proposed sensor shows high sensitivity to AChE with a detection limit down to 6.6 μU mL^-1. Therefore, this naked-eye paper-based sensor is capable of on-site and real-time detection of AChE, and has outstanding application prospects in clinical diagnosis and biomedical fields.

Inspired by game theory: Multi-signal output photoelectrochemical point-of-care immunoassay based on target-triggered organic electronic barriers

This work presents an integrated photoelectrochemical, impedance and colorimetric biosensing platform for flexible detection of cancer markers based on the targeted response by combining liposome amplification strategies and target-induced non-in situ formation of electronic barriers as the signal transduction modality on carbon-modified CdS photoanodes. Inspired by game theory, the carbon layer modified CdS hyperbranched structure with low impedance and high photocurrent response was firstly obtained by surface modification of CdS nanomaterials. Through a liposome-mediated enzymatic reaction amplification strategy, a large number of organic electron barriers were formed by a biocatalytic precipitation (BCP) reaction triggered by horseradish peroxidase released from cleaved liposomes after the introduction of the target molecule, thereby increasing the impedance characteristics of the photoanode as well as attenuating the photocurrent. The BCP reaction in the microplate was accompanied by a significant color change, which opened up a new window for point-of-care testing. Taking carcinoembryonic antigen (CEA) as a proof of concept, the multi-signal output sensing platform showed a satisfactory sensitive response to CEA with an optimal linear range of 20 pg mL^-1-100 ng mL^-1. The detection limit was as low as 8.4 pg mL^-1. Meanwhile, with the assistance of a portable smartphone and a miniature electrochemical workstation, the electrical signal obtained was synchronized with the colorimetric signal to correct the actual target concentration in the sample, further reducing the occurrence of false reports. Importantly, this protocol provides a new idea for the sensitive detection of cancer markers and the construction of a multi-signal output platform.

Application of Paper-Based Microfluidic Analytical Devices (µPAD) in Forensic and Clinical Toxicology: A Review

The need for providing rapid and, possibly, on-the-spot analytical results in the case of intoxication has prompted researchers to develop rapid, sensitive, and cost-effective methods and analytical devices suitable for use in nonspecialized laboratories and at the point of need (PON). In recent years, the technology of paper-based microfluidic analytical devices (μPADs) has undergone rapid development and now provides a feasible, low-cost alternative to traditional rapid tests for detecting harmful compounds. In fact, µPADs have been developed to detect toxic molecules (arsenic, cyanide, ethanol, and nitrite), drugs, and drugs of abuse (benzodiazepines, cathinones, cocaine, fentanyl, ketamine, MDMA, morphine, synthetic cannabinoids, tetrahydrocannabinol, and xylazine), and also psychoactive substances used for drug-facilitated crimes (flunitrazepam, gamma-hydroxybutyric acid (GHB), ketamine, metamizole, midazolam, and scopolamine). The present report critically evaluates the recent developments in paper-based devices, particularly in detection methods, and how these new analytical tools have been tested in forensic and clinical toxicology, also including future perspectives on their application, such as multisensing paper-based devices, microfluidic paper-based separation, and wearable paper-based sensors.

Tumor cell membrane-coated continuous electrochemical sensor for GLUT1 inhibitor screening

Glucose transporter 1 (GLUT1) overexpression in tumor cells is a potential target for drug therapy, but few studies have reported screening GLUT1 inhibitors from natural or synthetic compounds. With current analysis techniques, it is difficult to accurately monitor the GLUT1 inhibitory effect of drug molecules in real-time. We developed a cell membrane-based glucose sensor (CMGS) that integrated a hydrogel electrode with tumor cell membranes to monitor GLUT1 transmembrane transport and screen for GLUT1 inhibitors in traditional Chinese medicines (TCMs). CMGS is compatible with cell membranes of various origins, including different types of tumors and cell lines with GLUT1 expression knocked down by small interfering RNA or small molecules. Based on CMGS continuous monitoring technique, we investigated the glucose transport kinetics of cell membranes with varying levels of GLUT1 expression. We used CMGS to determine the GLUT1-inhibitory effects of drug monomers with similar structures from Scutellaria baicalensis and catechins families. Results were consistent with those of the cellular glucose uptake test and molecular-docking simulation. CMGS could accurately screen drug molecules in TCMs that inhibit GLUT1, providing a new strategy for studying transmembrane protein-receptor interactions.

Amino-functionalized Al-MOF modulated TpTt-COF with dual-emission for fluorescent and optosmart detecting tetracycline in food samples

Tetracycline residues in animal-derived food pose serious harm to human health, making it demanded to develop simple and sensitive method for detecting tetracycline. Herein, a dual-emission synchronous response fluorescence probe is reported based on amino-functionalized Al-MOF modulated TpTt-COF (donate as NMT). NMT exhibits excellent dual-emission at 455 and 575 nm under single excitation. Tetracycline is sensitively detected through quenching the dual-emission of NMT. NMT specifically recognizes tetracycline through intermolecular hydrogen bonding between -OH/-NH₂ of tetracycline and deprotonated O^-/-NH-/CN of NMT. A calibration curve is built between the fluorescence ratio and the tetracycline concentration with a detection limit of 0.014 μmol/L. NMT is employed to detect tetracycline in milk, pork and chicken, and displays satisfactory recoveries of 94.39-105.67%, respectively. The optosmart sensor based on NMT and smartphone has been constructed for visually detecting tetracycline. This method provides routes to construct MOF-COF fluorescence probes and has good prospects in food analysis.

Aptamer-Based Point-of-Care Devices: Emerging Technologies and Integration of Computational Methods

Recent innovations in point-of-care (POC) diagnostic technologies have paved a critical road for the improved application of biomedicine through the deployment of accurate and affordable programs into resource-scarce settings. The utilization of antibodies as a bio-recognition element in POC devices is currently limited due to obstacles associated with cost and production, impeding its widespread adoption. One promising alternative, on the other hand, is aptamer integration, i.e., short sequences of single-stranded DNA and RNA structures. The advantageous properties of these molecules are as follows: small molecular size, amenability to chemical modification, low- or nonimmunogenic characteristics, and their reproducibility within a short generation time. The utilization of these aforementioned features is critical in developing sensitive and portable POC systems. Furthermore, the deficiencies related to past experimental efforts to improve biosensor schematics, including the design of biorecognition elements, can be tackled with the integration of computational tools. These complementary tools enable the prediction of the reliability and functionality of the molecular structure of aptamers. In this review, we have overviewed the usage of aptamers in the development of novel and portable POC devices, in addition to highlighting the insights that simulations and other computational methods can provide into the use of aptamer modeling for POC integration.

One-step multiplex analysis of breast cancer exosomes using an electrochemical strategy assisted by gold nanoparticles

Exosomes, as a non-invasive biomarker, perform an important role in breast cancer screening and prognosis monitoring. However, establishing a simple, sensitive, and reliable exosome analysis technique remains challenging. Herein, a one-step multiplex analysis electrochemical aptasensor based on a multi-probe recognition strategy was constructed to analyze breast cancer exosomes. HER2-positive breast cancer cell (SK-BR-3) exosomes were selected as the model targets and three aptamers including CD63, HER2 and EpCAM aptamers were used as the capture units. Methylene blue (MB) functionalized HER2 aptamer and ferrocene (Fc) functionalized EpCAM aptamer, which were modified on gold nanoparticles (Au NPs), i.e. MB-HER2-Au NPs and Fc-EpCAM-Au NPs, were used as signal units. When the mixture of target exosomes, MB-HER2-Au NPs and Fc-EpCAM-Au NPs were added on the CD63 aptamer modified gold electrode, two Au NPs modified by MB and Fc could be specifically captured on the electrode by the recognition of three aptamers with target exosomes. Then one-step multiplex analysis of exosomes was achieved by detecting two independent electrochemical signals. This strategy can not only distinguish breast cancer exosomes from other exosomes (including normal exosomes and other tumor exosomes) but also HER2-positive breast cancer exosomes and HER2-negative breast cancer exosomes. Besides, it had high sensitivity and can detect SK-BR-3 exosomes with a concentration as low as 3.4 × 10³ particles mL^-1. Crucially, this method can be applicable to the examination of exosomes in complicated samples, which is anticipated to afford assistance for the screening and prognosis of breast cancer.

Integration of on-chip lysis and paper-based sensor for rapid detection of viral and exosomal RNAs

In recent years, paper-based nucleic acid sensors have been demonstrated for the ability to detect DNA and RNA molecules extracted from viruses and bacteria. In clinical samples, these nucleic acids are mostly encapsulated in lipid membranes and need to be released before being analyzed using paper-based sensors. For the nucleic acid amplification tests (NAATs), it is also desirable to remove the interfering molecules that can inhibit the nucleic acid amplification. To achieve a field deployable NAAT, we report a portable sensor system that combines the thermolysis and paper-based NAATs to detect target RNA molecules carried by viral and exosomal nanoparticles. The sensor cartridge includes a lysis chamber with a pressure-controlled diaphragm valve, paper flow channels, and three paper-based NAAT reaction chambers to extract, transport, and detect nucleic acids respectively. A compact instrument was prototyped to automate the assay, collect fluorescence images of the nucleic acid amplification, and generate amplification curves for NAATs. The pump-free and paper-based sensor achieved quantitative analysis of influenza A virus (IAV) RNA and exosome microRNA within 1 h, with the lowest detect concentration of 10⁴ TCID₅₀/mL and 10⁶ EV/mL for IAV and exosome, respectively. Owing to the advantages of easy storage, simple operation, and low cost, such as system has great potential to be used as a point-of-care test for in-field diagnosis of viral and bacterial infections.

Recent advances in electrochemical nanomaterial-based aptasensors for the detection of cancer biomarkers

New technologies have provided suitable tools for rapid diagnosis of cancer which can reduce treatment costs and even increase patients' survival rates. Recently, the development of electrochemical aptamer-based nanobiosensors has raised great hopes for early, sensitive, selective, and low-cost cancer diagnosis. Here, we reviewed the flagged recent research (2021-2023) developed as a series of biosensors equipped with nanomaterials and aptamer sequences (nanoaptasensors) to diagnose/prognosis of various types of cancers. Equipping these aptasensors with nanomaterials and using advanced biomolecular technologies have provided specified biosensing interfaces for more optimal and reliable detection of cancer biomarkers. The primary intention of this review was to present and categorize the latest innovations used in the design of these diagnostic tools, including the hottest surface modifications and assembly of sensing bioplatforms considering diagnostic mechanisms. The main classification is based on applying various nanomaterials and sub-classifications considered based on the type of analyte and other vital features. This review may help design subsequent electrochemical aptasensors. Likewise, the up-to-date status, remaining limitations, and possible paths for translating aptasensors to clinical cancer assay tools can be clarified.

Recent advancements of nanomodified electrodes - Towards point-of-care detection of cardiac biomarkers

The increasing number of deaths from cardiovascular diseases has become a substantial concern in both developed and underdeveloped countries. Rapid and on-site monitoring of this disease is urgently important to control, prevent and make awareness of public health. Recently, a lot of focus has been placed on nanomaterials and modify these nanomaterials have been explored to detect cardiac biomarkers. By implementing biosensors that are modified with novel recognition elements and more stable nanomaterials, the use of electrochemistry for point-of-care devices is more realistic every day. This review focuses on the current state of nanomaterials conjugated biorecognition elements (enzyme integrated with nanomaterials, antibody conjugated nanomaterials and aptamer conjugated nanomaterials) for electrochemical cardiovascular disease detection. Specifically, a lot of attention has been given to the trends toward more stable biosensors that have increased the potential to be used as point-of-care devices for the detection of cardiac biomarkers due to their high stability and specificity. Moreover, the recent progress on biomolecule-free electrochemical nanosensors for cardiovascular disease detection has been considered. At last, the possibility and drawbacks of some of these techniques for point-of-care cardiac device development in the future have been discussed.

Bottom-Up Signal Boosting with Fractal Nanostructuring and Primer Exchange Reaction for Ultrasensitive Detection of Cancerous Exosomes

Exosomes are emerging as promising biomarkers for cancer diagnosis, yet sensitive and accurate quantification of tumor-derived exosomes remains a challenge. Here, we report an ultrasensitive and specific exosome sensor (NPExo) that initially leverages hierarchical nanostructuring array and primer exchange reaction (PER) for quantitation of cancerous exosomes. This NPExo uses a high-curvature nanostructuring array (bottom) fabricated by single-step electrodeposition to enhance capturing of the target exosomes. The immuno-captured exosome thus provides abundant membrane sites to insert numerous cholesterol-DNA probes with a density much higher than that by immune pairing, which further allows PER-based DNA extension to assemble enzyme concatemers (up) for signal amplification. Such a bottom-up signal-boosting design imparts NPExo with ultrahigh sensitivity up to 75 particles/mL (i.e., <1 exosome per 10 μL) and a broad dynamic range spanning 6 orders of magnitude. Furthermore, our sensor allows monitoring subtle exosomal phenotypic transition and shows high accuracy in discrimination of liver cancer patients from healthy donors via blood samples, suggesting the great potential of NPExo as a promising tool in clinical diagnostics.

A colorimetric and photothermal dual-mode biosensing platform based on nanozyme-functionalized flower-like DNA structures for tumor-derived exosome detection

Tumor-derived exosomes can be served as a kind of promising biomarkers for early diagnosis of cancers. Herein, a colorimetric/photothermal dual-mode exosomes sensing platform is developed for human breast cancer cell (MCF-7)-derived exosomes based on encapsulation of 3,3',5,5'-tetramethylbenzidine-loaded graphene quantum dot nanozymes (TMB-GQDzymes) into DNA flowers (DFs) via rolling circle amplification (RCA). To achieve specific detection, EpCAM aptamer for MCF-7 cell-derived exosomes is immobilized on the well plate, while the complementary sequence of another CD63 aptamer is designed into the circular template to obtain abundant capture probes. Benefitting from the dual-aptamer recognition strategy, a sandwich structure of EpCAM aptamer/exosomes/TMB-GQDzymes@DFs is formed, in which the GQDzymes can catalyze the oxidation of TMB in the presence of H₂O₂. The resulting products of TMB oxidation (oxTMB) can induce not only the absorption changes but also a near-infrared (NIR) laser-driven photothermal effect, achieving dual-mode detection of exosomes with the limit of detection (LOD) of 1027 particles/μL (colorimetry) and 2170 particles/μL (photothermal detection), respectively. In addition, this sensing platform has demonstrated excellent performance to well distinguish breast cancer patients from healthy individuals in serum samples analysis. Overall, the proposed dual-readout biosensor opens promising prospects for exosome detection in biological study and clinical applications.

Specific lateral flow detection of isothermal nucleic acid amplicons for accurate point-of-care testing

For point-of-care testing (POCT), coupling isothermal nucleic acid amplification schemes (e.g., recombinase polymerase amplification, RPA) with lateral flow assay (LFA) readout is an ideal platform, since such integration offers both high sensitivity and deployability. However, isothermal schemes typically suffers from non-specific amplification, which is difficult to be differentiated by LFA and thus results in false-positives. Here, we proposed an accurate POCT platform by specific recognition of target amplicons with peptide nucleic acid (PNA, assisted by T7 Exonuclease), which could be directly plugged into the existing RPA kits and commercial LFA test strips. With SARS-CoV-2 as the model, the proposed method (RPA-TeaPNA-LFA) efficiently eliminated the false-positives, exhibiting a lowest detection concentration of 6.7 copies/μL of RNA and 90 copies/μL of virus. Using dual-gene (orf1ab and N genes of SARS-CoV-2) as the targets, RPA-TeaPNA-LFA offered a high specificity (100%) and sensitivity (RT-PCR Ct < 31, 100%; Ct < 40, 71.4%), and is valuable for on-site screening or self-testing during isolation. In addition, the dual test lines in the test strips were successfully explored for simultaneous detection of SARS-CoV-2 and H1N1, showing great potential in response to future pathogen-based pandemics.

Cu2+@NMOFs-to-bimetallic CuFe PBA transformation: An instant catalyst with oxidase-mimicking activity for highly sensitive impedimetric biosensor

In this work, a facile impedance biosensor was constructed for sensitive assaying of miRNA-10b based on the Cu²⁺ modified NH₂-metal organic frameworks (NMOF@Cu²⁺) coupling with a three-dimensional (3D) DNA walker signal amplification strategy. Specifically, abundant Cu²⁺ can adhere to the MOF via the coordination reaction between NH₂ and Cu²⁺, which can be applied as a skeleton to produce CuFe Prussian blue analogue@NMOF (CuFe PBA@NMOF) just in time. Meanwhile, the carboxyl group, which is rich in the organic ligands of the NMOF, can be used to assemble DNA strands (complementary strand, CS) (CS-NMOF@Cu²⁺) for biorecognition reaction. With the introduction of the target, a 3D DNA walker was triggered to shear out large amounts of assistant strands (AS), which were then anchored on the surface of GCE. Afterward, CS-NMOF@Cu²⁺ can be assembled on the GCE by hybridization with AS. Eventually, abundant CuFe PBA@NMOF were generated in situ on the electrode with the help of K₃[Fe(CN)₆], which can catalyze the 4-chloro-1-naphthol (4-CN) precipitation without H₂O₂, thereby increasing the resistance of the platform. Under the optimal conditions, the EIS biosensor presents reliable analytical performance in a wide linear range from 0.8 pM to 250 pM with a low detection limit of 0.5 pM.

A Versatile Design-Enabled Analysis of Circulating Extracellular Vesicles in Disease Diagnosis

Circulating extracellular vesicles (EVs) are considered as potential biomarkers for treatment and diagnosis of many diseases. Most of the existing methods for the EV analysis only have a single function and thus reveal limited information carried by EVs. Herein, a phosphatidylserine-targeting peptide-facilitated design that enables the versatile analysis of circulating EVs for varying requirement is proposed. In the design, DNA probes are inserted into the EV membrane through hydrophobic interactions, and accelerate the removal of protective shielding from DNA-gated metal-organic framework, thereby releasing a large number of methylene blue molecules to amplify the electrochemical signal. Electrochemical results demonstrate equally high sensitivities toward the quantification of EVs derived from different cell sources using an indiscriminative DNA probe. More importantly, the probe can be endowed with extended function for more accurate classification of cell-specific features through the identification of specific EV biomarkers, and demonstrates the potential use in the diagnosis of cardiovascular in a principle-of-proof study for clinical application. Therefore, the method provides a versatile design for the identification of EV features, and may address the needs of clinical diagnosis in the future.

Detection of rare CTCs by electrochemical biosensor built on quaternary PdPtCuRu nanospheres with mesoporous architectures

Circulating tumor cells (CTCs) are promising liquid biopsy biomarkers for early cancer detection and anti-cancer therapy evaluation. The ultra-low abundance of CTCs in blood samples requires highly sensitive and accurate detection ways. In this study, we propose the design of a dual-recognition electrochemical biosensor to improve both the specificity and signal response. PdPtCuRu mesoporous nanospheres (PdPtCuRu MNSs) with excellent three dimensions (3D) nanopore structures were synthesized by one-pot method and connected to mucin 1 (MUC1) aptamer to serve as signal amplification probe. Besides, superconductive carbon black, Ketjen Black (KB), and gold nanoparticles (AuNPs) modified organometallic frame (CeMOF-Au) were combined to work as signal transducer. The characteristic branching structure of KB provides abundant contact points to load CeMOF-Au to heighten the interface electron transfer rate. In addition, AuNPs were reduced on the surface of CeMOF, which could effectively bind the capture antibody and further enhance the conductivity. Under the optimized condition, the limit of detection (LOD) of the as-constructed biosensor was less than 10 cells mL^-1 for model A549 cells, and showed good specificity and accuracy in spiked serum samples. We envision the as-proposed electrochemical biosensor would alternate as a useful tool for the clinical detection of CTCs for cancer diagnosis.

Detection of SARS-CoV-2 in clinical and environmental samples using highly sensitive reduced graphene oxide (rGO)-based biosensor

Quantitative RT-PCR (qRT-PCR) is the most commonly used diagnostic tool for SARS-CoV-2 detection during the COVID-19 pandemic. Despite its sensitivity and accuracy, qRT-PCR is a time-consuming method that requires expensive laboratories with highly trained personnel. In this work, on-site detection of SARS-CoV-2 in municipal wastewater was investigated for the first time. The wastewater was unprocessed and did not require any prefiltration, prior spiking with virus, or viral concentration in order to be suitable for use with the biosensor. The prototype reported here is a reduced graphene oxide (rGO)-based biosensor for rapid, sensitive and selective detection of SARS-CoV-2. The biosensor achieved a limit of detection (LOD) of 0.5 fg/mL in phosphate-buffered saline (PBS) and exhibited specificity when exposed to various analytes. The response time was measured to be around 240 ms. To further explore the capabilities of the biosensor in real clinical and municipal wastewater samples, three different tests were performed to determine the presence or absence of the virus: (i) qRT-PCR, (ii) a rapid antigen-based commercially available test (COVID-19 Test Strips), and (iii) the biosensor constructed and reported here. Taken together, our results demonstrate that a biosensor that can detect SARS-CoV-2 in clinical samples as well as unfiltered and unprocessed municipal wastewater is feasible.

Nanomaterials for Molecular Detection and Analysis of Extracellular Vesicles

Extracellular vesicles (EVs) have emerged as a novel resource of biomarkers for cancer and certain other diseases. Probing EVs in body fluids has become of major interest in the past decade in the development of a new-generation liquid biopsy for cancer diagnosis and monitoring. However, sensitive and specific molecular detection and analysis are challenging, due to the small size of EVs, low amount of antigens on individual EVs, and the complex biofluid matrix. Nanomaterials have been widely used in the technological development of protein and nucleic acid-based EV detection and analysis, owing to the unique structure and functional properties of materials at the nanometer scale. In this review, we summarize various nanomaterial-based analytical technologies for molecular EV detection and analysis. We discuss these technologies based on the major types of nanomaterials, including plasmonic, fluorescent, magnetic, organic, carbon-based, and certain other nanostructures. For each type of nanomaterial, functional properties are briefly described, followed by the applications of the nanomaterials for EV biomarker detection, profiling, and analysis in terms of detection mechanisms.

Practical tips and new trends in electrochemical biosensing of cancer-related extracellular vesicles

To tackle cancer and provide prompt diagnoses and prognoses, the constantly evolving biosensing field is continuously on the lookout for novel markers that can be non-invasively analysed. Extracellular vesicles (EVs) may represent a promising biomarker that also works as a source of biomarkers. The augmented cellular activity of cancerous cells leads to the production of higher numbers of EVs, which can give direct information on the disease due to the presence of general and cancer-specific surface-tethered molecules. Moreover, the intravesicular space is enriched with other molecules that can considerably help in the early detection of neoplasia. Even though EV-targeted research has indubitably received broad attention lately, there still is a wide lack of practical and effective quantitative procedures due to difficulties in pre-analytical and analytical phases. This review aims at providing an exhaustive outline of the recent progress in EV detection using electrochemical and photoelectrochemical biosensors, with a focus on handling approaches and trends in the selection of bioreceptors and molecular targets related to EVs that might guide researchers that are approaching such an unstandardised field.

Dark-Field Imaging Monitoring of Adenosine Triphosphate in Live Cells by Au NBPs@ZIF-8 Nanoprobes

Monitoring the fluctuation of adenosine triphosphate (ATP) level in living cells could promote the understanding of metabolic pathways and cell biology. Here, we proposed a highly sensitive, selective, and biocompatible nanoprobe with core-shell structure, namely Au NBPs@ZIF-8 composed by gold nanobipyramids (Au NBPs) and zeolitic imidazolate framework-8 (ZIF-8), for monitoring intracellular ATP level fluctuation in living cells. Because the coordination between ATP and Zn²⁺ (the metal node of ZIF-8) was much stronger than that between 2-methylimidazole and Zn²⁺, which caused the decomposition of the ZIF-8 shell and the exposure of Au NBPs in the presence of ATP, it led to the change of the localized surface plasmon resonance scattering properties of nanoprobes under dark-field microscopy. Tricolor (RGB) analysis showed that R/G value had a good linear relationship with the ATP concentrations in the range of 10 μM to 4 mM (R² = 0.999) with a detection limit of 5.28 μM. This ATP sensing platform also exhibited excellent selectivity in complex intracellular interfering substances. Besides, we realized intracellular ATP real-time imaging in HeLa cells and observed the ATP level fluctuation under dark-field microscopy. The method mentioned here could be further applied for delivery of therapeutics for biomedical applications.

A Paper-Based Electrochemical Sensor Based on PtNP/COFTFPB-DHzDS@rGO for Sensitive Detection of Furazolidone

Herein, a paper-based electrochemical sensor based on PtNP/COF_TFPB-DHzDS@rGO was developed for the sensitive detection of furazolidone. A cluster-like covalent organic framework (COF_TFPB-DHzDS) was successfully grown on the surface of amino-functional reduced graphene oxide (rGO-NH₂) to avoid serious self-aggregation, which was further loaded with platinum nanoparticles (PtNPs) with high catalytic activity as nanozyme to obtain PtNP/COF_TFPB-DHzDS@rGO nanocomposites. The morphology of PtNP/COF_TFPB-DHzDS@rGO nanocomposites was characterized, and the results showed that the smooth rGO surface became extremely rough after the modification of COF_TFPB-DHzDS. Meanwhile, ultra-small PtNPs with sizes of around 1 nm were precisely anchored on COF_TFPB-DHzDS to maintain their excellent catalytic activity. The conventional electrodes were used to detect furazolidone and showed a detection limit as low as 5 nM and a linear range from 15 nM to 110 μM. In contrast, the detection limit for the paper-based electrode was 0.23 μM, and the linear range was 0.69-110 μM. The results showed that the paper-based electrode can be used to detect furazolidone. This sensor is a potential candidate for the detection of furazolidone residue in human serum and fish samples.

Electrochemical nano-sensing interface for exosomes analysis and cancer diagnosis

Exosomes are a class of the nanosized extracellular vesicles, which have emerged as representative liquid biopsy biomarkers. To date, the electrochemical nanosensors are of great significance in the exosome detection with the advantages of easy operation, high accuracy and reliable repeatability. Especially, the growing field of nano interface has provided the electrochemical sensing platforms for the accurate exosomes analysis. The incorporation of multiple nanomaterials can take advantages and synergistic properties of functional units. So, based on the integration of with nanomaterial-based signal transduction and specific biorecognition, the nano-sensing interface provides excellent electrochemical features owing to rapid mass transport and excellent conductivity. The nano-sensing interface with a wide variety of morphologies and structure also provides the large active surface area for the immobilization of bio-capturing agents. Furthermore, through the design of nanostructured electrode array, the efficiency of transducer can be greatly improved. It should be noticed that the elaboration of a proper sensor requires the profound knowledge of the nano-sensing interface. Therefore, this article presents a review of the recent advance in exosomes detection based on the electrochemical nano-sensing interface, including electrochemical analysis principles, exosome sensing mechanisms, nano-interface construction strategies, as well as the typical diagnosis application. In particular, the article is focused on the exploration of the various electrochemical sensing performance of nano-interface in the exosome detection. We have also prospected the future trend and challenge of the electrochemical nano-sensing interface for exosomes analysis in clinical cancer diagnosis.

Sensitive Autocatalytic Hybridization Circuit for Reliable In Situ Intracellular Polynucleotide Kinase Imaging

Exploring the characteristic functions of polynucleotide kinase (PNK) could substantially promote the elucidation of PNK-related mechanistic pathways. Yet, the sensitive and reliable detection of intracellular PNK still presents a challenging goal. Herein, we propose a simple autocatalytic hybridization circuit (AHC) for in situ intracellular imaging of PNK with high reliability. The AHC amplifier consists of two mutually activated hybridization chain reaction (HCR) modules for magnified signal transduction. The PNK is transduced into initiator I by phosphorylation and cleavage of mediator H_p. Initiator I activates the initial HCR-1 module, leading to the formation of long dsDNA nanowires that carry numerous initiator T. Then, T-initiated feedback HCR-2 module generates branched products that contain plentiful initiator I, thus realizing an autocatalytic HCR amplification reaction. Simultaneously, the HCR-2 module is also assembled as a versatile signal transduction unit for generating the amplified readout. Based on the mutually sustained accumulation of two initiators for the reciprocal activation of two reaction modules, continuous signal amplification and assembly of high-molecular-weight copolymers endow the AHC system with high sensitivity and robustness for the PNK assay. Moreover, the PNK-sensing AHC system achieves reliable imaging of intracellular PNK, thus showing great potential to decipher the correlation between PNK and related diseases.

Design and Application of Electrochemical Sensors with Metal-Organic Frameworks as the Electrode Materials or Signal Tags

Metal-organic frameworks (MOFs) with fascinating chemical and physical properties have attracted immense interest from researchers regarding the construction of electrochemical sensors. In this work, we review the most recent advancements of MOF-based electrochemical sensors for the detection of electroactive small molecules and biological macromolecules (e.g., DNA, proteins, and enzymes). The types and functions of MOF-based nanomaterials in terms of the design of electrochemical sensors are also discussed. Furthermore, the limitations and challenges of MOF-based electrochemical sensing devices are explored. This work should be invaluable for the development of MOF-based advanced sensing platforms.

Label-free fluorescence aptasensor for the detection of patulin using target-induced DNA gates and TCPP/BDC-NH2 mixed ligands functionalized Zr-MOF systems

Patulin (PAT) is an unsaturated lactone mycotoxin primarily produced by Penicillium expansum and Aspergillus clavatus. Given the potential health risks and economic losses associated with PAT, the rapid detection of PAT using fluorescent aptasensors is of significant importance in evaluating food safety. However, it easily increases the cost and complexity caused by signal labeling. We combined TCPP/BDC-NH₂ mixed ligands functionalized Zr metal-organic frameworks (Zr-MOF_mix) and terminated three-stranded DNA gates (ttsDNA gates) to fabricate a label-free fluorescent aptasensor for PAT detection. The Zr-MOF_mix system was synthesized via a one-pot strategy and could be used to address the problem of pore size limitation and increase the loading amounts of dyes. TtsDNA gate was integrated into the Zr-MOF_mix system to control the release of dyes, exhibiting a high signal-to-background ratio. The single-stranded aptamer region in ttsDNA gate situated away from the surface of the Zr-MOF_mix, resulting in a natural release of dyes in the absence of PAT. While binding to PAT resulted in target-induced conformational changes that helped form the hairpin structure of the aptamer. This structure hindered the release of dyes from the pores of Zr-MOF_mix, thus reducing the fluorescence signals intensity. The stimuli-responsive DNA-gated material provides a platform for PAT analysis under conditions of a low limit of detection (0.871 pg/mL). Furthermore, the excellent specificity and anti-interference of the fluorescent aptasensor make the system suitable for the analysis of apple juice samples. This label-free strategy is cheaper and simper compared with labeled detection, especially for the development of multi-target-detection.

Recent Advances on the Metal-Organic Frameworks-Based Biosensing Methods for Cancer Biomarkers Detection

Sensitive and selective detection of cancer biomarkers is crucial for early diagnosis and treatment of cancer, one of the most dangerous diseases in the world. Metal-organic frameworks (MOFs), a class of hybrid porous materials fabricated through the assembly of metal ions/clusters and organic ligands, have attracted increasing attention in the sensing of cancer biomarkers, due to the advantages of adjustable size, high porosity, large surface area and ease of modification. MOFs have been utilized to not only fabricate active sensing interfaces but also arouse a variety of measurable signals. Several representative analytical technologies have been applied in MOF-based biosensing strategies to ensure high detection sensitivity toward cancer biomarkers, such as fluorescence, electrochemistry, electrochemiluminescence, photochemistry and colorimetric methods. In this review, we summarized recent advances on MOFs-based biosensing strategies for the detection of cancer biomarkers in recent three years based on the categories of metal nodes, and aimed to provide valuable references for the development of innovative biosensing platform for the purpose of clinical diagnosis.

Introduction of Nanomaterials to Biosensors for Exosome Detection: Case Study for Cancer Analysis

Exosomes have been gaining attention for early cancer diagnosis owing to their biological functions in cells. Several studies have reported the relevance of exosomes in various diseases, including pancreatic cancer, retroperitoneal fibrosis, obesity, neurodegenerative diseases, and atherosclerosis. Particularly, exosomes are regarded as biomarkers for cancer diagnosis and can be detected in biofluids, such as saliva, urine, peritoneal fluid, and blood. Thus, exosomes are advantageous for cancer liquid biopsies as they overcome the current limitations of cancer tissue biopsies. Several studies have reported methods for exosome isolation, and analysis for cancer diagnosis. However, further clinical trials are still required to determine accurate exosome concentration quantification methods. Recently, various biosensors have been developed to detect exosomal biomarkers, including tumor-derived exosomes, nucleic acids, and proteins. Among these, the exact quantification of tumor-derived exosomes is a serious obstacle to the clinical use of liquid biopsies. Precise detection of exosome concentration is difficult because it requires clinical sample pretreatment. To solve this problem, the use of the nanobiohybrid material-based biosensor provides improved sensitivity and selectivity. The present review will discuss recent progress in exosome biosensors consisting of nanomaterials and biomaterial hybrids for electrochemical, electrical, and optical-based biosensors.

A universal, portable, and ultra-sensitive pipet immunoassay platform for deoxynivalenol detection based on dopamine self-polymerization-mediated bioconjugation and signal amplification

Deoxynivalenol (DON) is highly toxic to the environment and human health. It is important to detect DON with ultra-high sensitivity, ease of operation, and low cost. Inspired by the excellent stability and biocompatibility of polydopamine, a universal, portable and ultra-sensitive pipet immunoassay platform was reported for DON detection based on dopamine self-polymerization (polydopamine coating and polydopamine nanoparticles). The polydopamine coating acted as an effective strategy for biomolecule immobilization on the pipet to improve the coating efficiency that significantly reduced the required concentration of biomolecules. Performing the ELISA in pipets saved nearly 67% of the antigen amount and 83% of the antibody amount, which reduced the detection cost and simplified the experimental steps. The dual signal amplification in this pipet immunoassay enabled ultra-high sensitivity. Polydopamine nanoparticles acted as the enrichment carrier of horseradish peroxidase-goat anti-mouse IgG for the first-round signal amplification, followed by the tyramine-mediated loading of streptavidin-HRP for the second-round signal amplification. The dual-enriched HRP catalyzed the color-developing substrate to achieve highly sensitive colorimetric DON detection with a limit of detection of 0.435 ng/mL.

Nucleic acid isothermal amplification-based soft nanoarchitectonics as an emerging electrochemical biosensing platform

The emergence of nucleic acid isothermal amplification strategies based on soft nanoarchitectonics offers a new dimension to the traditional electrochemical technique, particularly because of its flexibility, high efficiency, and increased sensitivity for analytical applications. Various DNA/RNA isothermal amplification strategies have been developed for the design and fabrication of new electrochemical biosensors for efficient and important biomolecular detection. Herein, we provide an overview of recent efforts in this research field and the strategies for signal-amplified sensing systems, with their biological applications, current challenges and prospects in this promising new area.

An electrochemiluminescence immunosensor based on ABEI-GO-AgNPs as a double-amplified luminophore for the ultra-sensitive detection of prostate-specific antigen

A sandwich electrochemiluminescence (ECL) immunosensor based on an N-(4-aminobutyl)-N-ethylisoluminol-graphene oxide-Ag nanoparticle (ABEI-GO-AgNPs) complex and cysteine silver nanowires (AgCysNWs) was prepared to detect prostate-specific antigen (PSA). Our results showed that an ECL signal probe, ABEI-GO-AgNPs, with an ultrahigh specific surface area, favorable catalytic properties, and electrical conductivity was prepared by a one-step synthesis method. ABEI-GO-AgNPs with good biocompatibility immobilized secondary antibody (Ab₂) via AgN bonds. Furthermore, AgCysNWs containing many -COOH groups were prepared and used to enrich primary antibody (Ab₁), which could be used as an affinity probe for the selective capture of PSA. Lastly, through layer-by-layer assembly, we established an ECL immunosensing platform for the sensitive detection of PSA. Under the optimized conditions, the designed ECL immunosensor showed promising sensitivity and selectivity for the detection of PSA in the linear range of 5.5 × 10^-7-5.5 ng/mL, with a detection limit of 1.2 × 10^-7 ng/mL. The constructed ECL sensing platform possessed good specificity, reproducibility, and stability and could detect PSA in actual human serum samples.

An Autocatalytic DNA Circuit Based on Hybridization Chain Assembly for Intracellular Imaging of Polynucleotide Kinase

Polynucleotide kinase (PNK) plays an essential role in various cellular events by regulating phosphorylation processes, and abnormal homeostasis of PNK could cause many human diseases. Herein, we proposed an autocatalytic hybridization system (AHS) through the elaborate integration of hybridization chain assembly (HCA) and catalytic DNA assembly (CDA) that enables a highly efficient positive feedback amplification. The PNK-targeting AHS biosensor is composed of three modules: a recognition module, an HCA amplification module, and a CDA autocatalytic module. In the presence of PNK, the recognition module could transform the PNK input into an exposed nucleic acid initiator (I). Then the initiator strand I could trigger the autonomous HCA process in the amplification module, and the resulted HCA products could reassemble the split CDA trigger strand T, subsequently inducing the CDA process in the autocatalytic module to form abundant DNA duplex products. Consequently, the embedded initiator strand I was liberated from the CDA duplex product to autonomously trigger the new rounds of HCA circuit. The rational integration and cooperative cross-activation between the HCA and CDA module could prominently accelerate the reaction and realize the exponential amplification efficiency by initiator regeneration. As a result, the self-sustainable AHS amplifier could implement the sensitive detection of PNK in vitro and in biological samples and further fulfill accurate monitoring of the intracellular PNK activity and the effective screening of PNK inhibitors. This work paves a way for exploiting highly efficient artificial DNA circuits to analyze low-abundance biomarkers, holding great potential in biochemical research and clinical diagnosis.

Molecular Characterization of Exosomes for Subtype-Based Diagnosis of Breast Cancer

Breast cancer is very heterogeneous and the most frequently diagnosed cancer worldwide, and precise therapy targeting specific subtypes may improve the survival rates of breast cancer patients. In this study, we designed a biomimetic vesicle by camouflaging catalytic DNA machinery with a breast cancer cell membrane, which enabled the molecular classification of circulating exosomes for subtype-based diagnosis through homotypic recognition. In addition, the vesicles specifically targeted and fused with breast cancer exosomes with phenotypic homology and manipulated the DNA machinery to amplify electrochemical signaling using exosomal RNA as an endogenous trigger. The biomimetic vesicles prepared with MCF-7 cancer cell-derived membranes were shown to recognize estrogen receptor-positive breast cancer exosomes and exhibited a low detection limit of 557 particles mL^-1 with microRNA-375 used as the endogenous biomarker. Furthermore, the biomimetic vesicles prepared with MDA-MB-231 cancer cell-derived membranes displayed satisfactory performance in a homotypic analysis of triple-negative breast cancer exosomes with a potential therapeutic target, PD-L1 mRNA, used as the endogenous biomarker. Most importantly, cross-validation experiments confirmed the high accuracy and selectivity of this homotypic recognition-driven analysis for molecular subtyping of breast cancer. When applied to clinical samples of breast cancer patients, the vesicles demonstrated feasibility and reliability for evaluating the molecular features of cancer cell-derived exosomes and enabled stage-specific monitoring of breast cancer patients because the electrochemical signals showed a positive correlation with disease progression. Therefore, this work may provide new ideas for the precise diagnosis and personalized treatment of breast cancer patients throughout the whole disease process.

π-Conjugated Copper Phthalocyanine Nanoparticles as Highly Sensitive Sensor for Colorimetric Detection of Biomarkers

Though numerous nanomaterials with enzyme-like activities have been utilized as probes and sensors for detecting biological molecules, it is still challenging to construct highly sensitive detectors for biomarkers using polymeric materials. Benefiting from the π-d delocalization effect of electrons, excellent metal-chelating property, high electron transferability, and good chemical stability of π-conjugated phthalocyanine, the design of the copper phthalocyanine-based conjugated polymer nanoparticles (Cu-PcCP NPs) as a colorimetric sensor for a variety of biomarkers is reported. The Cu-PcCP NPs are synthesized through a simple microwave-assisted polymerization, and their chemical structures are thoroughly characterized. The colorimetric results of Cu-PcCP NPs demonstrate excellent peroxidase-like detecting activity and also great substrate selectivity than most of the reported Cu-based nanomaterials. The Cu-PcCP NPs can achieve a detection limit of 4.88 μM for the H₂ O₂ , 4.27 μM for the L-cysteine, and 21.10 μM for the glucose via a cascade catalytic system, which shows comparable detecting sensitivity as that of many earlier reported enzyme-like nanomaterials. Moreover, Cu-PcCP NPs present remarkable resistance to harsh conditions, including high temperature, low pH, and excessive salts. These highly specific π-conjugated copper-phthalocyanine nanoparticles not only overcome the current limitation of polymeric material-based sensors but also provide a new direction for designing next-generation enzyme-like nanomaterial-based colorimetric biosensors.

Highly Efficient Isolation and Sensitive Detection of Small Extracellular Vesicles Using a Paper-Based Device

Small extracellular vesicles (sEVs) play important roles in mediating intercellular communication and regulating biological processes. Facile sEV isolation is the essential and preliminary issue for their function investigation and downstream biomedical applications, while the traditional methods are challenged by tedious procedures, low purity, low yield, and potential damage. In this work, we developed an sEV isolation paper-based device (sEV-IsoPD) based on a three-dimensional (3D) paper chip, which is composed of a porous membrane for size exclusion and a metal-organic framework (MOF)/antibody-modified paper for immunoaffinity capture. In combination with a peristaltic pump-driven flow system, the sEV-IsoPD can efficiently isolate EV from cell culture medium and serum. Compared with the ultracentrifugation method, sEV-IsoPD exhibited a 5.1 times higher yield (1.7 × 10⁹ mL^-1), 1.6 times higher purity (1.6 × 10¹¹ mg^-1), and 7.5 times higher recovery (77.3%) with only 8.3% of the time (30 min) and 1.0% of the instrument cost ($710). Moreover, sEV concentration can be visually detected in a quantitative manner with this paper-based device with a linear range from 3.0 × 10⁶ to 3.0 × 10¹⁰ mL^-1 and a detection limit of 2.2 × 10⁶ mL^-1. The sEV-IsoPD provides an efficient and practical approach for the rapid isolation and visible detection of sEVs, which are promising for the preparation of sEVs and diagnosis of disease.

Programmable DNA-Fueled Electrochemical Analysis of Lung Cancer Exosomes

Molecular diagnostics devoted to discover and monitor new biomarkers is gaining increasing attention in clinical diagnosis. In this work, a programmable DNA-fueled electrochemical analysis strategy is designed for the determination of an emerging biomarker in lung cancer, PD-L1-expressing exosomes. Specifically, PD-L1-expressing exosomes are first enriched onto magnetic beads functionalized with PD-L1 antibody and are able to interact with cholesterol-modified hairpin templates. Then, programmable DNA synthesis starts from the hairpin template-triggered primer exchange reaction and generates a large number of extension products to activate the trans-cleavage activity of CRISPR-Cas12a. After that, CRISPR-Cas12a-catalyzed random cleavage boosts the degradation of methylene blue-labeled signaling strands, so electro-active methylene blue molecules can be enriched onto a cucurbit[7]uril-modified electrode for quantitative determination. Our method demonstrates high sensitivity and specificity toward electrochemical analysis of PD-L1-expressing exosomes in the range from 10³ to 10⁹ particles mL^-1 with a low detection limit of 708 particles mL^-1. When applied to clinical samples, our method reveals an elevated level of circulating PD-L1-expressing exosomes in lung cancer patients, especially for those at the advanced stages. Therefore, our method may provide new insight into liquid biopsy for better implementation of immunotherapy in lung cancer in the future.

Rapid On-Chip Isolation of Cancer-Associated Exosomes and Combined Analysis of Exosomes and Exosomal Proteins

Exosomes are lipid bilayer extracellular vesicles secreted by various types of cells and inherit abundant molecular information from parental cells. Tumor-derived exosomes have been widely recognized as noninvasive biomarkers for early cancer diagnosis and surveillance, but the separation of intact exosomes and detection of exosomal proteins remain challenging. Herein, we proposed a microfluidic chip for specific exosome isolation, integrated with sensitive quantification by a novel PTCDI-aptamer signal switch strategy. To enhance the capture efficiency, an alternating drop-shaped micropillar array was designed to assist the capture of tumor-derived exosomes by Tim4-modified magnetic beads (Tim4 beads) on the chip. Following capture, a chelating agent can easily elute intact exosomes which were further used for profiling exosomal surface proteins by the multiplexed fluorescence turn-on approach. Profiting from the efficient on-chip enrichment of the Tim4 beads and superior fluorescence signal transduction strategy, the detection limit of the analysis platform for HepG2 exosomes is as low as 8.69 × 10³ particles/mL with a wide linear range spanning 6 orders of magnitude. Meanwhile, the proposed platform could recognize subtle changes in protein levels on the exosomal surface from various cell lines. More importantly, this strategy is successfully applied to analyze exosomes in human serum to distinguish liver cancer patients from healthy individuals. Combined analysis of different types of biomarkers on the exosomal membrane surface can greatly improve the accuracy of cancer type identification and disease monitoring. We hope that this convenient, rapid, and sensitive platform may become a powerful tool in the field of exosome analysis and early cancer screening.