Challenges and Solutions in Developing Ultrasensitive Biosensors

An AI-assisted microfluidic paper-based multiplexed surface-enhanced raman scattering (SERS) biosensor with electrophoretic removal and electrical modulation for accurate acute myocardial infarction (AMI) diagnosis and prognosis

SERS detects single molecules with exceptional sensitivity. To counter the issue of selectivity faced by point-of-care, herein, an externally applied electric field that allows electrical modulation and electromigrates unbound SERS tags without multiple washing steps is successfully developed and demonstrated to improve the biosensor's selectivity and sensitivity in multiplexed detection of cTnI, HDL, and LDL in human serum at a low LoD. Ultra-sensitive detectors can detect signals from non-specifically absorbed species, and these species can cover up overlapping analyte peaks, amplifying the effect of non-specific binding. Even though antifouling molecules can prevent non-specific adsorption at the sensor interface, this approach does not completely eliminate it. Our significant findings show that an electrically regulated device can electromigrate non-specifically bound species without cross-reacting with endogenous albumin proteins. Stability, repeatability, and reproducibility were good, with an RSD of 10%. Artificial intelligence was employed to interpret and analyze high-dimensional fingerprint SERS spectra using feature selection and dimensionality reduction for accurate acute myocardial infarction diagnosis and prognosis. These machine learning methods allow quantification of cTnI, HDL, and LDL biomarkers with low RMSE. Machine learning classifiers showed strong AUROC values of 0.950 ± 0.111 and 0.884 ± 0.139 for early and recurrent AMI detection, respectively. A high negative predictive value (NPV) of ≥99% indicates an effective early AMI rule-out. In short, this work demonstrated that a simple, low-cost, electrophoretic modulated biosensor with machine learning can diagnose, rule out, and predict recurring AMI.

GABA (γ-aminobutyric acid) enrichment and detection methods in cereals: Unlocking sustainable health benefits

Gamma-aminobutyric acid (GABA), a non-protein amino acid primarily biosynthesized in cereals, is vital to the human neurological system. As the primary neurotransmitter, it promotes relaxation, inhibits depression and sleeplessness, and regulates synaptic transmission and neuronal development. GABA also protects the liver, kidneys, and intestines, reduces blood pressure, and fights cancer and inflammation. Colorimetric and enzyme-based procedures, biosensors, and High-Performance Liquid Chromatography primarily detect GABA. GABA content is enriched by treatments, like germination and fermentation, suitable and economical methods on industrial scales. Moreover, ultrasonication, abiotic stress, heat, and relative humidity are also utilized for GABA enrichment. With advancing technology and knowledge of the nutrients concerning human health, target-specific nutrition utilizing sustainable ingredients needs exploration. Conclusively, the current article delves into the latest developments in GABA enrichment and detection techniques and GABA's potential health benefits. Further studies are required to understand and refine enrichment procedures to develop GABA-enriched foods with health-promoting qualities.

Color-Coded Traffic Signal Method Combined with Nanodiamond Quantum Sensing for Accurate miRNA Detection

Background noise interferes with the accurate detection of early tumor biomarkers. This study introduces a method that effectively reduces background noise to enhance detection accuracy by combining a color-coded signaling approach with the unique fluorescent properties and room-temperature tunable quantum spin characteristics of fluorescent diamonds (FNDs) with nitrogen-vacancy centers. In this approach, a red signal indicates the presence of the target analyte within the spectral region, a green signal indicates its absence, and a yellow signal indicates the need for further analysis using FNDs' quantum spin properties for optical detection magnetic resonance (ODMR) to distinguish the FND signal from background noise. Preliminary results demonstrate that this method enables the detection of breast cancer-related miRNA-21 and miRNA-96 concentrations as low as 1 fM within a 100 × 100 μm2 area, achieving single-molecule detection capability. This method is suitable for accurate biomarker identification and detection under high-background fluorescence conditions.

Multiple aptamer recognition-based quantum dot lateral flow platform: ultrasensitive point-of-care testing of respiratory infectious diseases

Respiratory infectious diseases spread rapidly and have a wide range of impacts, posing a serious threat to public health security. The development of a sensitive, accurate, and rapid detection method for respiratory viruses is crucial for disease prevention and control. However, existing methods are inadequate in satisfying the demand for accurate and convenient detection simultaneously. Therefore, an ultrasensitive point-of-care testing (POCT) platform based on a multiple aptamer recognition-based quantum dot lateral flow immunoassay (MARQ-LFIA) was developed in this work. This platform consisted of multiple high-affinity aptamers for recognizing different sites on a respiratory infectious virus protein, enhancing the efficiency of virus identification in complex environments. By combining a multiple aptamer recognition strategy with quantum dot fluorescent technique to construct LFIA test strips and pairing them with a high-gain portable fluorescence reader, excellent detection sensitivity and specificity were achieved in the case of coronavirus disease 2019 (COVID-19). The limits of detection were 1.427 pg mL-1 and 1643 U mL-1 towards the nucleocapsid protein and inactivated viruses, respectively, indicating that MARQ-LFIA improved detection sensitivity compared to reported methods. More critically, by testing thirty COVID-19 positive and twenty negative patient samples, the positive detection rate increased from 55.17% to 86.67% compared with commercially similar products. The universality of MARQ-LFIA was also investigated for diagnosing influenza B. We believe that MARQ-LFIA can be a promising POCT tool with potential applications in the areas of public health for the growing demand for precision diagnosis and treatment.

Ag-coated Au nanostar-based Lateral Flow Immunoassay for Highly Sensitive Influenza A virus antibody Detection in Colorimetric and Surface-Enhanced Raman Scattering (SERS) modes

Antibody testing for virus aids diagnosis, promotes vaccination and development, and evaluates antibody treatment efficacy. Hence, it is essential to examine and monitor antibody levels for accurate disease diagnosis and prevention. Lateral Flow Immunoassay (LFIA) is a technique that is known for its simplicity and speed, making it a popular choice for immediate detection. Noble metal nanoparticles are extensively employed in LFIA due to their exceptional colorimetric and Raman properties, which are a result of the LSPR effect. Au nanostars (Au NSs) have excellent SERS properties due to multiple sharp branches and more "hot spots" on the surface, while Ag nanoparticles (Ag NPs) have higher extinction coefficient and better refractive index sensitivity. The electromagnetic field strength on the surface of Ag-Au bimetallic nanomaterial is greatly enhanced, which further enhances the SERS intensity. In this work, we created a core-shell nanoparticle by combining Au NS as the core and Ag as the shell (Au NSMBA@Ag). We then inserted a Raman reporter molecule between the core and shell. Using this, we developed an LFIA platform that can detect influenza A virus antibodies in both colorimetric and Raman modes. The detection limit in colorimetric mode was 0.1 ng/mL, while in Raman mode it was 8.0 pg/mL, making it approximately 12 times more sensitive than the colorimetric mode. Furthermore, the method has shown excellent specificity, stability, and resistance to interference. Hence, this method can be applied to various fields such as environmental monitoring, clinical diagnosis, and food safety, showing great potential for future applications.

Leveraging nanomaterials for ultrasensitive biosensors in early cancer detection: a review

Cancer remains a major global health challenge with a high mortality rate, as evidenced by the rise in new cases every year. Conventional diagnostic methods like PET scans, MRIs, and biopsies, despite being widely used, suffer from significant drawbacks such as high radiation exposure, difficulty in distinguishing malignant from benign tumors, and invasiveness. Early detection, which is crucial for improving treatment outcomes and survival rates, is hindered by the asymptomatic nature of early-stage cancer and the limitations of current diagnostic tools. Cancer biomarkers, detectable in body fluids, offer valuable diagnostic information, and recent advances in nanotechnology have led to the development of highly sensitive nano-biosensors. This review explores recent advancements (2022-2024) in the field of ultrasensitive nano-biosensors, emphasizing the strategic integration of nanomaterials to enhance sensitivity and accuracy in cancer biomarker detection. It highlights how precise nanomaterial positioning in sensor components like electrodes and bioreceptors enables early cancer diagnosis at low biomarker concentrations. These innovations underscore the transformative potential of nanomaterials in revolutionizing early cancer diagnostics, improving patient care, and enhancing survival outcomes.

Recent Advances in Electrochemiluminescence Visual Biosensing and Bioimaging

Electrochemiluminescence (ECL) is one of the most powerful techniques that meet the needs of analysis and detection in a variety of scenarios, because of its highly analytical sensitivity and excellent spatiotemporal controllability. ECL combined with microscopy (ECLM) offers a promising approach for quantifying and mapping a wide range of analytes. To date, ECLM has been widely used to image biological entities and processes, such as cells, subcellular structures, proteins and membrane transport properties. In this review, we first introduced the mechanisms of several classic ECL systems, then highlighted the progress of visual biosensing and bioimaging by ECLM in the last decade. Finally, the characteristics of ECLM were summarized, as well as some of the current challenges. The future research interests and potential directions for the application of ECLM were also outlooked.

The LOD paradox: When lower isn't always better in biosensor research and development

Biosensor research has long focused on achieving the lowest possible Limits of Detection (LOD), driving significant advances in sensitivity and opening up new possibilities in analysis. However, this intense focus on low LODs may not always meet the practical needs or suit the actual uses of these devices. While technological improvements are impressive, they can sometimes overlook important factors such as detection range, ease of use, and market readiness, which are vital for biosensors to be effective in real-world applications. This review advocates for a balanced approach to biosensor development, emphasizing the need to align technological advancements with practical utility. We delve into various applications, including the detection of cancer biomarkers, pathology-related biomarkers, and illicit drugs, illustrating the critical role of LOD within these contexts. By considering clinical needs and broader design aspects like cost-effectiveness, sustainability, and regulatory compliance, we argue that integrating technical progress with practicality will enhance the impact of biosensors. Such an approach ensures that biosensors are not only technically sound but also widely useable and beneficial in real-world applications. Addressing the diverse analytical parameters alongside user expectations and market demands will likely maximize the real-world impact of biosensors.

Ultrasensitive Single-Molecule Biosensor by Periodic Modulation of Magnetic Particle Motion

Ultrasensitive detection of low-abundance biomarkers by modern single-molecule technologies is critical for better diagnosis of severe diseases, but inevitable nonspecific bindings often cause fluctuations in the single-molecule counting results. Here we present an approach to improve the specificity in a single-molecule immunoassay by translating molecular binding signals into periodic nanomotion of magnetic particles. The sandwiched immunoassay is modified by using a long linker to tether one antibody onto a gold-covered substrate and a magnetic particle with another antibody coated as the reporter. By actively oscillating the particles with alternating magnetic fields, we could reliably identify specific binding through intensity fluctuation in plasmonic images of single particles. As a proof of concept, we demonstrate the detection of IFN-γ at the femtomolar level by the digital counting of specifically bound molecules. This active strategy outperforms existing passive motion-based approaches in sensitivity and speed, paving the way for disease diagnosis with low-abundance biomarkers.

Multiplexed Assay for Small-Molecule Quantification via Photo-cross-linking of Structure Switching Aptamers

There is an unmet need for molecular detection assays that enable the multiplexed quantification of small-molecule analytes. We present xPlex, an assay that combines aptamer switches with ultraviolet-cross-linkable complementary strands to record target-binding events. When the aptamer's small-molecule target is present, the cross-linkable strand is displaced, enabling PCR amplification and detection of the relevant aptamer. In the absence of that target, the aptamer is readily cross-linked to the strand, preventing amplification from happening. The resulting aptamer-specific amplicons can be detected and quantified in a multiplexed fashion using high-throughput sequencing. We demonstrate quantitative performance for a pair of small-molecule analytes, dopamine and glucose, and show that this assay retains good specificity with mixtures of the two molecules at various concentrations. We further show that xPlex can effectively evaluate the specificity of cross-reactive aptamers to a range of different small-molecule analytes. We believe that the xPlex assay format could offer a useful strategy for achieving multiplexed analysis of small-molecule targets in a variety of scenarios.

Overview of the Design and Application of Photothermal Immunoassays

Developing powerful immunoassays for sensitive and real-time detection of targets has always been a challenging task. Due to their advantages of direct readout, controllable sensing, and low background interference, photothermal immunoassays have become a type of new technology that can be used for various applications such as disease diagnosis, environmental monitoring, and food safety. By modification with antibodies, photothermal materials can induce temperature changes by converting light energy into heat, thereby reporting specific target recognition events. This article reviews the design and application of photothermal immunoassays based on different photothermal materials, including noble metal nanomaterials, carbon-based nanomaterials, two-dimensional nanomaterials, metal oxide and sulfide nanomaterials, Prussian blue nanoparticles, small organic molecules, polymers, etc. It pays special attention to the role of photothermal materials and the working principle of various immunoassays. Additionally, the challenges and prospects for future development of photothermal immunoassays are briefly discussed.

Digital One-Step Competitive Detection of a Small Molecule in Synthetic and Environmental Waters

Optical methods for single-molecule analysis hold the promise of accurate, sensitive, and rapid detection of target molecules. Here, we demonstrate the efficiency of such an approach for the competitive detection of small molecules in water. Our biosensing method is based on a combination of a single-DNA biochip for the parallelization of tethered particle motion real-time measurements with antibodies and modified targets as molecular competitors. The antibodies are coupled to the particles tethered to the surface by a long DNA bearing in its middle the molecular competitor bound to the antibodies. Competitive target binding leads to a detectable conformational change of the DNA tethers from looped to unlooped in proportions related to the target concentration. We thus managed to detect fluorescein, chosen as a model of a target molecule, in freshwater of various qualities, from solutions prepared with ultrapure water to more complex matrices such as river water and wastewater treatment plant effluent samples. Similar dose-response curves were obtained under these various conditions in a wide range of concentrations from nanomolar to micromolar with a limit of detection around 2 nM.

Reviewing advances in nanophotonic biosensors

Biosensing, a promising branch of exploiting nanophotonic devices, enables meticulous detection of subwavelength light, which helps to analyze and manipulate light-matter interaction. The improved sensitivity of recent high-quality nanophotonic biosensors has enabled enhanced bioanalytical precision in detection. Considering the potential of nanophotonics in biosensing, this article summarizes recent advances in fabricating nanophotonic and optical biosensors, focusing on their sensing function and capacity. We typically classify these types of biosensors into five categories: phase-driven, resonant dielectric nanostructures, plasmonic nanostructures, surface-enhanced spectroscopies, and evanescent-field, and review the importance of enhancing sensor performance and efficacy by addressing some major concerns in nanophotonic biosensing, such as overcoming the difficulties in controlling biological specimens and lowering their costs for ease of access. We also address the possibility of updating these technologies for immediate implementation and their impact on enhancing safety and health.

A novel ratiometric electrochemical sensing platform combined with molecularly imprinted polymer and Fe-MOF-NH2/CNTs-NH2/MXene composite for efficient detection of ofloxacin

BACKGROUND

Ofloxacin (OFL) is often abused in medicine and animal husbandry, which poses a great threat to human health and ecological environment. Therefore, it is necessary to establish efficient method to detect OFL. Electrochemical sensor has attracted widespread attention due to the advantages of low cost and fast response. However, most electrochemical sensors usually use one response signal to detect the target, which makes it sensitive to the variable background noise in the complex environment, resulting in low robustness and selectivity. The ratio detection mode and employing molecularly imprinted polymer (MIP) are two strategies to solve these problems.

RESULTS

A novel molecular imprinting polymer-ratiometric electrochemical sensor (MIP-RECS) based on Fe-MOF-NH2/CNTs-NH2/MXene composite was prepared for the rapid and sensitive detection of OFL. The positively charged Fe-MOF-NH2 and CNTs-NH2 as interlayer spacers were introduced into the negatively charged MXene through a simple electrostatic self-assembly technique, which effectively prevented the agglomeration of MXene and increased the electrocatalytic activity. A glass carbon electrode was modified by the composite and a MIP film was electropolymerized on it using o-phenylenediamine and β-cyclodextrin as bifunctional monomers and OFL as template. Then a MIP-RECS was designed by adding dopamine (DA) into the electrolyte solution as internal reference, and OFL was quantified by the response current ratio of OFL to DA. The current ratio and the concentration of OFL displayed a satisfying linear relationship in the range of 0.1 μM-100 μM, with a limit of detection (LOD) of 13.2 nM.

SIGNIFICANCE

Combining molecular imprinting strategy and ratio strategy, the MIP-RECS has impressive selectivity compared with the non-imprinted polymer-RECS, and has better repeatability and reproducibility than non-ratiometric sensor. The MIP-RECS has high sensitivity and accuracy, which was applied for the detection of OFL in four different brands of milk and was verified by HPLC method with satisfactory results.

Aptamer-based biosensors for the detection of neonicotinoid insecticides in environmental samples: A systematic review

Neonicotinoids, sometimes abbreviated as neonics, represent a class of neuro-active insecticides with chemical similarities to nicotine. Neonicotinoids are the most widely adopted group of insecticides globally since their discovery in the late 1980s. Their physiochemical properties surpass those of previously established insecticides, contributing to their popularity in various sectors such as agriculture and wood treatment. The environmental impact of neonicotinoids, often overlooked, underscores the urgency to develop tools for their detection and understanding of their behavior. Conventional methods for pesticide detection have limitations. Chromatographic techniques are sensitive but expensive, generate waste, and require complex sample preparation. Bioassays lack specificity and accuracy, making them suitable as preliminary tests in conjunction with instrumental methods. Aptamer-based biosensor is recognized as an advantageous tool for neonicotinoids detection due to its rapid response, user-friendly nature, cost-effectiveness, and suitability for on-site detection. This comprehensive review represents the inaugural in-depth analysis of advancements in aptamer-based biosensors targeting neonicotinoids such as imidacloprid, thiamethoxam, clothianidin, acetamiprid, thiacloprid, nitenpyram, and dinotefuran. Additionally, the review offers valuable insights into the critical challenges requiring prompt attention for the successful transition from research to practical field applications.

Current status and progress of PD-L1 detection: guiding immunotherapy for non-small cell lung cancer

Non-small cell lung cancer (NSCLC) is the leading cause of cancer-related deaths and represents a substantial disease burden worldwide. Immune checkpoint inhibitors combined with chemotherapy are the standard first-line therapy for advanced NSCLC without driver mutations. Programmed death-ligand 1 (PD-L1) is currently the only approved immunotherapy marker. PD-L1 detection methods are diverse and have developed rapidly in recent years, such as improved immunohistochemical detection methods, the application of liquid biopsy in PD-L1 detection, genetic testing, radionuclide imaging, and the use of machine learning methods to construct PD-L1 prediction models. This review focuses on the detection methods and challenges of PD-L1 from different sources, and discusses the influencing factors of PD-L1 detection and the value of combined biomarkers. Provide support for clinical screening of immunotherapy-advantage groups and formulation of personalized treatment decisions.

A nanoparticle-assisted signal-enhancement technique for lateral flow immunoassays

Lateral flow immunoassay (LFIA), an affordable and rapid paper-based detection technology, is employed extensively in clinical diagnosis, environmental monitoring, and food safety analysis. The COVID-19 pandemic underscored the validity and adoption of LFIA in performing large-scale clinical and public health testing. The unprecedented demand for prompt diagnostic responses and advances in nanotechnology have fueled the rise of next-generation LFIA technologies. The utilization of nanoparticles to amplify signals represents an innovative approach aimed at augmenting LFIA sensitivity. This review probes the nanoparticle-assisted amplification strategies in LFIA applications to secure low detection limits and expedited response rates. Emphasis is placed on comprehending the correlation between the physicochemical properties of nanoparticles and LFIA performance. Lastly, we shed light on the challenges and opportunities in this prolific field.

Enhancing the performance of porous silicon biosensors: the interplay of nanostructure design and microfluidic integration

This work presents the development and design of aptasensor employing porous silicon (PSi) Fabry‒Pérot thin films that are suitable for use as optical transducers for the detection of lactoferrin (LF), which is a protein biomarker secreted at elevated levels during gastrointestinal (GI) inflammatory disorders such as inflammatory bowel disease and chronic pancreatitis. To overcome the primary limitation associated with PSi biosensors-namely, their relatively poor sensitivity due to issues related to complex mass transfer phenomena and reaction kinetics-we employed two strategic approaches: First, we sought to optimize the porous nanostructure with respect to factors including layer thickness, pore diameter, and capture probe density. Second, we leveraged convection properties by integrating the resulting biosensor into a 3D-printed microfluidic system that also had one of two different micromixer architectures (i.e., staggered herringbone micromixers or microimpellers) embedded. We demonstrated that tailoring the PSi aptasensor significantly improved its performance, achieving a limit of detection (LOD) of 50 nM-which is >1 order of magnitude lower than that achieved using previously-developed biosensors of this type. Moreover, integration into microfluidic systems that incorporated passive and active micromixers further enhanced the aptasensor's sensitivity, achieving an additional reduction in the LOD by yet another order of magnitude. These advancements demonstrate the potential of combining PSi-based optical transducers with microfluidic technology to create sensitive label-free biosensing platforms for the detection of GI inflammatory biomarkers.

Recent advances in the metamaterial and metasurface-based biosensor in the gigahertz, terahertz, and optical frequency domains

Recently, metamaterials and metasurface have gained rapidly increasing attention from researchers due to their extraordinary optical and electrical properties. Metamaterials are described as artificially defined periodic structures exhibiting negative permittivity and permeability simultaneously. Whereas metasurfaces are the 2D analogue of metamaterials in the sense that they have a small but not insignificant depth. Because of their high optical confinement and adjustable optical resonances, these artificially engineered materials appear as a viable photonic platform for biosensing applications. This review paper discusses the recent development of metamaterial and metasurface in biosensing applications based on the gigahertz, terahertz, and optical frequency domains encompassing the whole electromagnetic spectrum. Overlapping features such as material selection, structure, and physical mechanisms were considered during the classification of our biosensing applications. Metamaterials and metasurfaces working in the GHz range provide prospects for better sensing of biological samples, THz frequencies, falling between GHz and optical frequencies, provide unique characteristics for biosensing permitting the exact characterization of molecular vibrations, with an emphasis on molecular identification, label-free analysis, and imaging of biological materials. Optical frequencies on the other hand cover the visible and near-infrared regions, allowing fine regulation of light-matter interactions enabling metamaterials and metasurfaces to offer excellent sensitivity and specificity in biosensing. The outcome of the sensor's sensitivity to an electric or magnetic field and the resonance frequency are, in theory, determined by the frequency domain and features. Finally, the challenges and possible future perspectives in biosensing application areas have been presented that use metamaterials and metasurfaces across diverse frequency domains to improve sensitivity, specificity, and selectivity in biosensing applications.

Automated and ultrasensitive point-of-care glycoprotein detection using boronate-affinity enhanced organic electrochemical transistor patch

Quantifying trace glycoproteins in biofluids requires ultrasensitive components, but feedback is not available in the current portable platforms of point-of-care (POC) diagnosis technologies. A compact and ultrasensitive bioelectrochemical patch was based on boronate-affinity amplified organic electrochemical transistors (BAAOECTs) for POC use was developed to overcome this dilemma. Benefit from the cascading signal enhancement deriving from boronate-affinity targeting multiple regions of glycoprotein and OECTs' inherent signal amplification capability, the BAAOECTs achieved a detection limit of 300 aM within 25 min, displaying about 3 orders of magnitude improvement in sensitivity compared with the commercial electrochemical luminescence (ECL) kit. By using a microfluidic chip, a microcontroller module, and a wireless sensing system, the testing workflows of the above patch was automated, allowing for running the sample-to-answer pipeline even in a resource-limited environment. The reliability of such portable biosensing platform is well recognized in clinical diagnostic applications of heart failure. Overall, the remarkable enhanced sensitivity and automated workflow of BAAOECTs biosensing platform provide a prospective and generalized design policy for expanding the POC diagnosis capabilities of glycoproteins.

Recent Progress in Nanoparticles Based Sensors for the Detection of Mercury (II) Ions in Environmental and Biological Samples

To maintain a green and sustainable environment for human beings, rapid detection of potentially toxic heavy metals like mercury (Hg(II)) has attracted great attention. Recently, sensors have been designed which can selectively detect Hg(II) over other common available cations and give a naked eye or fluorometric response. In the last two decades, the trend is shifting from bulky organic chemosensors toward nanoparticles due to their rapid response, low cost, eco-friendly and easy synthesis. In this review, promising nanoparticles-based sensors for Hg(II) detection are discussed. The nano-sensors are functionalized with nucleotide or other suitable materials which coordinate with Hg(II) ions and give clear color or fluorescence change. The operational mechanisms are discussed focusing on its four basic types. The nanoparticles-based sensors are even able to detect Hg in three different oxidation states (Hg(II), Hg(I) and Hg(0)). Recently, the trend has been shifted from ordinary nanoparticles to magnetic nanoparticles to simultaneously detect and remove Hg(II) ions from environmental samples. Furthermore, the nano-sensors for Hg(II) are compared with each other and with the reported organic chemosensors.

Surface plasmon resonance-enhanced photoelectrochemical immunoassay with Cu-doped porous Bi2WO6 nanosheets

The development of rapid screening sensing platforms to improve pre-screening mechanisms in community healthcare is necessary to meet the significant need for portable testing in biomarker diagnostics. Here, we designed a portable smartphone-based photoelectrochemical (PEC) immunoassay for carcinoembryonic antigen (CEA) detection using Cu-doped ultrathin porous Bi2WO6 (CuBWO) nanosheets as the photoactive material. The CuBWO nanosheets exhibit a fast photocurrent response and excellent electrical transmission rate under UV light due to their surface plasmon resonance effect (SPR). The method uses glucose oxidase-labeled secondary antibody as a signal indicator for sandwich-type immune conjugation. In the presence of the target CEA, the electrons and holes generated at the surface of the photo-excited ultrathin porous CuBWO were rapidly consumed by the production of H2O2 from glucose oxidase oxidizing glucose, resulting in a weakened photocurrent signal. The photocurrent intensity increased logarithmically and linearly with increasing CEA concentration (0.02-50 ng mL-1), with a detection limit of 15.0 pg mL-1 (S/N = 3). The system provides a broader idea for inferring the electron-hole transport mechanism in ultrathin porous nanosheet layer materials and developing efficient PEC sensors.

The combination of DNA nanostructures and materials for highly sensitive electrochemical detection

Due to the wide range of electrochemical devices available, DNA nanostructures and material-based technologies have been greatly broadened. They have been actively used to create a variety of beautiful nanostructures owing to their unmatched programmability. Currently, a variety of electrochemical devices have been used for rapid sensing of biomolecules and other diagnostic applications. Here, we provide a brief overview of recent advances in DNA-based biomolecular assays. Biosensing platform such as electrochemical biosensor, nanopore biosensor, and field-effect transistor biosensors (FET), which are equipped with aptamer, DNA walker, DNAzyme, DNA origami, and nanomaterials, has been developed for amplification detection. Under the optimal conditions, the proposed biosensor has good amplification detection performance. Further, we discussed the challenges of detection strategies in clinical applications and offered the prospect of this field.

Nanopore Blockade Sensors for Quantitative Analysis Using an Optical Nanopore Assay

Nanopore sensing is a popular biosensing strategy that is being explored for the quantitative analysis of biomarkers. With low concentrations of analytes, nanopore sensors face challenges related to slow response times and selectivity. Here, we demonstrate an approach to rapidly detect species at ultralow concentrations using an optical nanopore blockade sensor for quantitative detection of the protein vascular endothelial growth factor (VEGF). This sensor relies on monitoring fluorescent polystyrene nanoparticles blocking nanopores in a nanopore array of 676 nanopores. The fluorescent signal is read out using a wide-field fluorescence microscope. Nonspecific blockade events are then distinguished from specific blockade events based on the ability to pull the particles out of the pore using an applied electric field. This allows the detection of VEGF at sub-picomolar concentration in less than 15 min.

PEI-Mediated Assembly of Fe3O4 onto SiO2-Encapsulated CsPbBr3 for Highly Sensitive Fluorescent Lateral Flow Immunoassay

The conventional lateral flow immunoassay (LFIA) method using colloidal gold nanoparticles (Au NPs) as labeling agents faces two inherent limitations, including restricted sensitivity and poor quantitative capability, which impede early viral infection detection. Herein, we designed and synthesized CsPbBr3 perovskite quantum dot-based composite nanoparticles, CsPbBr3@SiO2@Fe3O4 (CSF), which integrated fluorescence detection and magnetic enrichment properties into LFIA technology and achieved rapid, sensitive, and convenient quantitative detection of the SARS-CoV-2 virus N protein. In this study, CsPbBr3 served as a high-quantum-yield fluorescent signaling probe, while SiO2 significantly enhanced the stability and biomodifiability of CsPbBr3. Importantly, the SiO2 shell shows relatively low absorption or scattering toward fluorescence, maintaining a quantum yield of up to 74.4% in CsPbBr3@SiO2. Assembly of Fe3O4 nanoparticles mediated by PEI further enhanced the method's sensitivity and reduced matrix interference through magnetic enrichment. Consequently, the method achieved a fluorescent detection range of 1 × 102 to 5 × 106 pg·mL-1 after magnetic enrichment, with a limit of detection (LOD) of 58.8 pg·mL-1, representing a 13.3-fold improvement compared to nonenriched samples (7.58 × 102 pg·mL-1) and a 2-orders-of-magnitude improvement over commercial colloidal gold kits. Furthermore, the method exhibited 80% positive and 100% negative detection rates in clinical samples. This approach holds promise for on-site diagnosis, home-based quantitative tests, and disease procession evaluation.

Topological barrier to Cas12a activation by circular DNA nanostructures facilitates autocatalysis and transforms DNA/RNA sensing

Control of CRISPR/Cas12a trans-cleavage is crucial for biosensor development. Here, we show that small circular DNA nanostructures which partially match guide RNA sequences only minimally activate Cas12a ribonucleoproteins. However, linearizing these structures restores activation. Building on this finding, an Autocatalytic Cas12a Circular DNA Amplification Reaction (AutoCAR) system is established which allows a single nucleic acid target to activate multiple ribonucleoproteins, and greatly increases the achievable reporter cleavage rates per target. A rate-equation-based model explains the observed near-exponential rate trends. Autocatalysis is also sustained with DNA nanostructures modified with fluorophore-quencher pairs achieving 1 aM level (<1 copy/μL) DNA detection (106 times improvement), without additional amplification, within 15 min, at room temperature. The detection range is tuneable, spanning 3 to 11 orders of magnitude. We demonstrate 1 aM level detection of SNP mutations in circulating tumor DNA from blood plasma, genomic DNA (H. Pylori) and RNA (SARS-CoV-2) without reverse transcription as well as colorimetric lateral flow tests of cancer mutations with ~100 aM sensitivity.

Nanoplasmonic biosensors for multicolor visual analysis of acetylcholinesterase activity and drug inhibitor screening in point-of-care testing

The monitoring of acetylcholinesterase (AChE) activity and the screening of its inhibitors are significance of the diagnosis and drug therapy of nervous diseases. A metal ions-mediated signal amplification strategy was developed for the highly sensitive and multicolor assay of AChE activity and visually screening its drug inhibitors. After the specific reaction between AChE and acetylthiocholine (ATCh), the hydrolysis product thiocholine (TCh) can directly and decompose the α-FeOOH nanorods (NRs) to release amounts of Fe2+, which was regarded as Fenton reagent to efficiently catalyze H2O2 to produce ·OH. Then, the as-formed ·OH can further largely shorten the gold nanobipyramids (Au NBPs), generating a series of palpable color variations. The linear range for AChE activity was 0.01-500.0 U/L with the limit of detection as low as 0.0074 U/L. The vivid visual effects could be easily distinguished for the multicolor assay of AChE activity by naked eye in visible light. To achieve the point-of-care testing, Au NBPs were further assembled on polymeric electrospun nanofibrous films (ENFs) surface as test strips for the easy-to-use test of AChE activity by RGB values with a smartphone. Fascinatingly, this proposed strategy can be used for the visual screening AChE inhibitors or non-inhibitors. Comparing with the clinical drugs (rivastigmine tartrate, and donepezil), some natural alkaloids such as evodiamine, caffeine, camptothecin, and berberine hydrochloride were selected as inhibitor modes to confirm the drug screening capability of this method. This proposed strategy may have great potential in the other disease-related enzymatic biomarkers assay and the rapid screening of drug therapy.

Classification of Molecular Binding Traces for Dynamic Single-Molecule Sensing

Interference from nonspecific binding imposes a fundamental limit in the sensitivity of biosensors that is dependent on the affinity and specificity of the available sensing probes. The dynamic single-molecule sensing (DSMS) strategy allows ultrasensitive detection of biomarkers at the femtomolar level by identifying specific binding according to molecular binding traces. However, the accuracy in classifying binding traces is not sufficient from separate features, such as the bound lifetime. Here, we establish a DSMS workflow to improve the sensitivity and linearity by classifying molecular binding traces in surface plasmon resonance microscopy with multiple kinetic features. The improvement is achieved by correlation analysis to select key features of binding traces, followed by unsupervised k-clustering. The results show that this unsupervised classification approach improves the sensitivity and linearity in microRNA (hsa-miR155-5p, hsa-miR21-5p, and hsa-miR362-5p) detection to achieve a limit of detection at the subfemtomolar level.

Continuous Monitoring Biosensing Mediated by Single-Molecule Plasmon-Enhanced Fluorescence in Complex Matrices

Continuous detection of critical markers directly at the point of interest and in undiluted biological fluids represents the next fundamental step in biosensing. The goal of realizing such a platform is utterly challenging because it requires a reversible biosensor that enables the tracking of pico- to nanomolar molecular concentrations over long time spans in a compact device. Here we describe a sensing method based on plasmon-enhanced fluorescence capable of single-molecule detection of unlabeled analyte by employing biofunctionalized gold nanoparticles. The very strong plasmon-enhanced fluorescence signals allow for single-molecule sensing in unaltered biological media, while the use of low-affinity interactions ensures the continuous tracking of increasing and decreasing analyte concentrations with picomolar sensitivity. We demonstrate the use of a sandwich assay for a DNA cancer marker with a limit of detection of picomolar and a time response of 10 min. The enhanced single-molecule signals will allow for miniaturization into a small and cheap platform with multiplexing capability for application in point-of-care diagnostics, monitoring of industrial processes, and safe keeping of the environment.

Amplification-Free Digital Immunoassay down to the Attomolar Level by Synergistic Sedimentation of Brownian Motion Suppression and Dehydration Transfer

Amplification-free digital immunoassays (DIAs) typically utilize optical nanoparticles to enhance single immunocomplex molecule detection. The efficiency and uniformity of transferring the nanoparticles from a bulk solution to a solid surface determine the limit of detection (LOD) and the accuracy of DIAs. Previous methods suffer from issues like low efficiency, nonuniform distribution, and particle aggregation. Here, we present a novel technique named synergistic sedimentation of Brownian motion suppression and dehydration transfer (SynSed) for nanoparticles using water-soluble polymers. The efficiency of transferring quantum dots (QDs) was increased from 10.7 to 91.4%, and the variation in QD distribution was restricted to 8.8%. By incorporating SynSed into DIAs, we achieved a remarkable reduction in the LOD (down to 3.9 aM) for carcinoembryonic antigen and expanded the dynamic range to cover 3 orders of magnitude in concentration, ranging from 0.01 to 10 fM. DIAs enhanced with SynSed possess ultrahigh sensitivity, advanced accuracy, and specificity, offering a great premise in early disease diagnostics, risk stratification, and treatment response monitoring.

Dual-mode colorimetric and fluorescence biosensors for the detection of foodborne bacteria

Due to the growing demand for detection technologies, there has been significant interest in the development of integrated dual-modal sensing technologies, which involve combining two signal transduction channels into a single technique, particularly in the context of food safety. The integration of two detection signals not only improves diagnostic performance by reducing assumptions, but also enhances diagnostic functions with increased application flexibility, improved accuracy, and a wider detection linear range. The top two output signals for emerging dual-modal probes are fluorescent and colorimetric, due to their exceptional advantages for real-time sensitive sensing and point-of-care applications. With the rapid progress of nanotechnology and material chemistry, the integrated colorimetric/fluorimetric dual-mode systems show immense potential in sensing foodborne pathogenic bacteria. In this comprehensive review, we present a detailed summary of various colorimetric and fluorimetric dual-modal sensing methods, with a focus on their application in detecting foodborne bacteria. We thoroughly examine the sensing methodologies and the underlying principles of the signal transduction systems, and also discuss the challenges and future prospects for advancing research in this field.

Enzyme-Linked DNA Displacement (ELIDIS) Assay for Ultrasensitive Electrochemical Detection of Antibodies

Here we report the development of a method for the electrochemical ultrasensitive detection of antibodies that couples the programmability and versatility of DNA-based systems with the sensitivity provided by enzymatic amplification. The platform, termed Enzyme-Linked DNA Displacement (ELIDIS), is based on the use of antigen-DNA conjugates that, upon the bivalent binding of a specific target antibody, induce the release of an enzyme-DNA hybrid strand from a preformed duplex. Such enzyme-DNA hybrid strand can then be electrochemically detected with a disposable electrode with high sensitivity. We applied ELIDIS to demonstrate the sensitive (limit of detection in the picomolar range), specific and multiplexed detection of five different antibodies including three clinically relevant ones. ELIDIS is also rapid (it only requires two reaction steps), works well in complex media (serum) and is cost-effective. A direct comparison with a commercial ELISA kit for the detection of Cetuximab demonstrates the promising features of ELIDIS as a point-of-care platform for antibodies detection.

Confirming Silent Translocation through Nanopores with Simultaneous Single-Molecule Fluorescence and Single-Channel Electrical Recordings

Most of what is known concerning the luminal passage of materials through nanopores arises from electrical measurements. Whether nanopores are biological, solid-state, synthetic, hybrid, glass-capillary-based, or protein ion channels in cells and tissues, characteristic signatures embedded in the flow of ionic current are foundational to understanding functional behavior. In contrast, this work describes passage through a nanopore that occurs without producing an electrical signature. We refer to the phenomenon as "silent translocation." By definition, silent translocations are invisible to the standard tools of electrophysiology and fundamentally require a simultaneous ancillary measurement technique for positive identification. As a result, this phenomenon has been largely unexplored in the literature. Here, we report on a derivative of Cyanine 5 (sCy5a) that passes through the α-hemolysin (αHL) nanopore silently. Simultaneously acquired single-molecule fluorescence and single-channel electrical recordings from bilayers formed over a closed microcavity demonstrate that translocation does indeed take place, albeit infrequently. We report observations of silent translocation as a function of time, dye concentration, and nanopore population in the bilayer. Lastly, measurement of the translocation rate as a function of applied potential permits estimation of an effective energy barrier for transport through the pore as well as the effective charge on the dye, all in the absence of an information-containing electrical signature.

Solid-State Nanopore Sensors with Enhanced Sensitivity through Nucleic Acid Amplification

Solid-state nanopores have wide applications in DNA sequencing, energy conversion and storage, seawater desalination, sensors, and reactors due to their high stability, controllable geometry, and a variety of pore-forming materials. Solid-state nanopore sensors can be used for qualitative and quantitative analyses of ions, small molecules, proteins, and nucleic acids. The combination of nucleic acid amplification and solid-state nanopores to achieve trace detection of analytes is gradually attracting attention. This review outlines nucleic acid amplification strategies for enhancing the sensitivity of solid-state nanopore sensors by summarizing the articles published in the past 10 years. The future development prospects and challenges of nucleic acid amplification in solid-state nanopore sensors are discussed. This review helps readers better understand the field of solid-state nanopore sensors. We believe that solid-state nanopore sensors will break through the bottleneck of traditional detection and become a powerful single-molecule detection platform.

A simple electrochemical strategy for the detection of the cancer marker CA19-9 by signal amplification using copper organic framework nanocomposite

In this work, label-free electrochemical immunosensing of the cancer biomarker carbohydrate antigen 19-9 (CA19-9) is reported using [Fe(CN)6]3-/4- as a signal probe and a copper organic framework (Cu-BTC) nanocomposite for the amplification of the signal. The immunosensor was fabricated by the following process. First, the Cu-BTC nanomaterial with a larger surface area and good biocompatibility was synthesized to improve the dispersion of gold nanoparticles (Au NPs). Then, nitrogen-doped graphene (N-GR) was combined with Cu-BTC to form the nanocomposite. The synthesized Cu-BTC@N-GR@AuNPs@CS nanocomposite was employed to modify the surface of the immunosensor to accelerate the electron transfer rate and improve the immobilization amount of CA19-9 antibodies (Ab). Various techniques, including TEM, SEM and XPS were used to characterize Cu-BTC and nanocomposites. Differential pulse voltammetry (DPV) was used to measure the electrochemical response of the immunosensor in [Fe(CN)6]3-/4-. The signal intensity of the immunosensor was linearly changed upon increasing the concentration of CA19-9 antigen from 10 μU mL-1 to 100 U mL-1, and a detection limit of 4.2 μU mL-1 was achieved. Furthermore, the immunosensor showed good stability, reproducibility and specificity, indicating its potential application in clinical analysis.

Bacterial metabolism-triggered-chemiluminescence-based point-of-care testing platform for sensitive detection and photothermal inactivation of Staphylococcus aureus

Post-operative pathogenic infections in liver transplantation seriously threaten human health. It is essential to develop novel methods for the highly sensitive and rapid detection of Staphylococcus aureus (S. aureus). Interestingly, the combination of the property of bacteria to secrete hydrogen peroxidase, bacterial metabolism-triggered-chemiluminescence (CL)-based bioassays can be as a candidate point-of-care testing (POCT) for the detection of S. aureus against the CL substrate Luminol and hydrogen peroxide without excitation light sources. Here, a CL-based strategy with stable and visualized CL intensity was fabricated according to a hybrid biomimetic enzyme of copper-Hemin metal-organic framework, which enhances the biological enzyme activity while improving the stability and sensitivity of the assay. By further integrating S. aureus-specific capture and one-step separation of the antibody-modified Fe3O4 NPs (Fe3O4 NPs@Ab), the portable device integrated smartphone enables CL-based POCT for specific detection of S. aureus in the range of 101-106 CFU/mL with a limit of detection as low as 1 CFU/mL. Specifically, S. aureus can be eliminated after detection with high antibacterial efficiency due to the excellent photothermal properties of Fe3O4 NPs@Ab. The developed multifunctional platform has the advantages of simplicity of operation and low cost, indicating great potential in clinical applications.

Long-Term In Vivo Molecular Monitoring Using Aptamer-Graphene Microtransistors

Long-term, real-time molecular monitoring in complex biological environments is critical for our ability to understand, prevent, diagnose, and manage human diseases. Aptamer-based electrochemical biosensors possess the promise due to their generalizability and a high degree of selectivity. Nevertheless, the operation of existing aptamer-based biosensors in vivo is limited to a few hours. Here, we report a first-generation long-term in vivo molecular monitoring platform, named aptamer-graphene microtransistors (AGMs). The AGM incorporates a layer of pyrene-(polyethylene glycol)5-alcohol and DNase inhibitor-doped polyacrylamide hydrogel coating to reduce biofouling and aptamer degradation. As a demonstration of function and generalizability, the AGM achieves the detection of biomolecules such as dopamine and serotonin in undiluted whole blood at 37 °C for 11 days. Furthermore, the AGM successfully captures optically evoked dopamine release in vivo in mice for over one week and demonstrates the capability to monitor behaviorally-induced endogenous dopamine release even after eight days of implantation in freely moving mice. The results reported in this work establish the potential for chronic aptamer-based molecular monitoring platforms, and thus serve as a new benchmark for molecular monitoring using aptamer-based technology.

Single-microbead space-confined digital quantification strategy (SMSDQ) for counting microRNAs at the single-molecule level

Quantification of microRNAs (miRNAs) at the single-molecule level is of great significance for clinical diagnostics and biomedical research. The challenges lie in the limits to transforming single-molecule measurements into quantitative signals. To address these limits, here, we report a new approach called a Single Microbead-based Space-confined Digital Quantification (SMSDQ) to measure individual miRNA molecules by counting gold nanoparticles (AuNPs) with localized surface plasmon resonance (LSPR) light-scattering imaging. One miRNA target hybridizes with the alkynyl-modified capture DNA probe immobilized on a microbead (60 μm) and the azide-modified report DNA probe anchored on AuNP (50 nm), respectively. Through the click reaction between the alkynyl and azide group, a single microbead can covalently link the AuNPs in the confined space within the view of the microscope. By digitally counting the light-scattering spots of AuNPs, we demonstrated the proposed approach with single-molecule detection sensitivity and high specificity of single-base discrimination. Taking the advantages of ultrahigh sensitivity, specificity, and the digital detection manner, the approach is suitable for evaluating cell heterogeneity and small variations of miRNA expression and has been successfully applied to direct quantification of miRNAs in one-tenth single-cell lysates and serum samples without RNA-isolated and nucleic acid amplification steps.

Advances in field-effect biosensors towards point-of-use

The future of medical diagnostics calls for portable biosensors at the point of care, aiming to improve healthcare by reducing costs, improving access, and increasing quality-what is called the 'triple aim'. Developing point-of-care sensors that provide high sensitivity, detect multiple analytes, and provide real time measurements can expand access to medical diagnostics for all. Field-effect transistor (FET)-based biosensors have several advantages, including ultrahigh sensitivity, label-free and amplification-free detection, reduced cost and complexity, portability, and large-scale multiplexing. They can also be integrated into wearable or implantable devices and provide continuous, real-time monitoring of analytesin vivo, enabling early detection of biomarkers for disease diagnosis and management. This review analyzes advances in the sensitivity, parallelization, and reusability of FET biosensors, benchmarks the limit of detection of the state of the art, and discusses the challenges and opportunities of FET biosensors for future healthcare applications.

Development and Analytical Evaluation of a Point-of-Care Electrochemical Biosensor for Rapid and Accurate SARS-CoV-2 Detection

The COVID-19 pandemic has underscored the critical need for rapid and accurate screening and diagnostic methods for potential respiratory viruses. Existing COVID-19 diagnostic approaches face limitations either in terms of turnaround time or accuracy. In this study, we present an electrochemical biosensor that offers nearly instantaneous and precise SARS-CoV-2 detection, suitable for point-of-care and environmental monitoring applications. The biosensor employs a stapled hACE-2 N-terminal alpha helix peptide to functionalize an in situ grown polypyrrole conductive polymer on a nitrocellulose membrane backbone through a chemical process. We assessed the biosensor's analytical performance using heat-inactivated omicron and delta variants of the SARS-CoV-2 virus in artificial saliva (AS) and nasal swab (NS) samples diluted in a strong ionic solution, as well as clinical specimens with known Ct values. Virus identification was achieved through electrochemical impedance spectroscopy (EIS) and frequency analyses. The assay demonstrated a limit of detection (LoD) of 40 TCID50/mL, with 95% sensitivity and 100% specificity. Notably, the biosensor exhibited no cross-reactivity when tested against the influenza virus. The entire testing process using the biosensor takes less than a minute. In summary, our biosensor exhibits promising potential in the battle against pandemic respiratory viruses, offering a platform for the development of rapid, compact, portable, and point-of-care devices capable of multiplexing various viruses. The biosensor has the capacity to significantly bolster our readiness and response to future viral outbreaks.

Advancements in Biosensors Based on the Assembles of Small Organic Molecules and Peptides

Over the past few decades, molecular self-assembly has witnessed tremendous progress in a variety of biosensing and biomedical applications. In particular, self-assembled nanostructures of small organic molecules and peptides with intriguing characteristics (e.g., structure tailoring, facile processability, and excellent biocompatibility) have shown outstanding potential in the development of various biosensors. In this review, we introduced the unique properties of self-assembled nanostructures with small organic molecules and peptides for biosensing applications. We first discussed the applications of such nanostructures in electrochemical biosensors as electrode supports for enzymes and cells and as signal labels with a large number of electroactive units for signal amplification. Secondly, the utilization of fluorescent nanomaterials by self-assembled dyes or peptides was introduced. Thereinto, typical examples based on target-responsive aggregation-induced emission and decomposition-induced fluorescent enhancement were discussed. Finally, the applications of self-assembled nanomaterials in the colorimetric assays were summarized. We also briefly addressed the challenges and future prospects of biosensors based on self-assembled nanostructures.

Dual Signal Amplification by Urease Catalysis and Silver Nanoparticles for Ultrasensitive Colorimetric Detection of Nucleic Acids

Signal amplification techniques are highly desirable for the analysis of low-level targets that are closely related with diseases and the monitoring of important biological processes. However, it is still challenging to achieve this goal in a facile and economical way. Herein, we developed a novel dual signal amplification strategy by combining urease catalysis with the release of Ag⁺ from silver nanoparticles (AgNPs). This strategy was used for quantifying a DNA sequence (HIV-1) related with human immunodeficiency virus (HIV). DNA target HIV-1 hybridizes with the capture DNA probe on magnetic beads and the reporter DNA probe on AgNPs, forming a sandwich complex. The captured AgNPs are then transformed into numerous Ag⁺ ions that inactivate numerous ureases. Without catalytic production of ammonia from urea, the substrate solution shows a low pH 5.8 that will increase otherwise. The pH change is monitored by a pH indicator (phenol red), which allows for colorimetric detection. The proposed approach is sensitive, easy to use, economic, and universal, exhibiting a low detection limit of 9.7 fM (i.e., 1.94 attomoles) and a dynamic linear range of 4 orders for HIV-1 sequence detection.

Bi-functional antibody-CRISPR/Cas12a ribonucleoprotein conjugate for improved immunoassay performance

Protein conjugates are commonly used in biochemistry, including diagnostic platforms such as antibody-based immunoassays. Antibodies can be bound to a variety of molecules creating conjugates with desirable functions, particularly for imaging and signal amplification. Cas12a is a recently discovered programable nuclease with the remarkable capability to amplify assay signals due to its trans-cleavage property. In this study, we directly conjugated antibody with Cas12a/gRNA ribonucleoprotein without the loss of function in either constituent. The conjugated antibody was suitable for immunoassays and the conjugated Cas12a was capable of amplifying the signal produced in an immunosensor without the need to change the original assay protocol. We applied the bi-functional antibody-Cas12a/gRNA conjugate to successfully detect two different types of targets, a whole pathogenic microorganism, Cryptosporidium, and a small protein, cytokine IFN-γ, with sensitivity reaching one single microorganism per sample and 10 fg/mL for IFN-γ, respectively. With simple substitution of the antibody conjugated with the Cas12a/gRNA RNP, this approach can potentially be applied to increase sensitivity of a variety of immunoassays for a broad range of different analytes.

Universal CRISPR/Cas12a-associated aptasensor suitable for rapid detection of small proteins with a plate reader

With the discovery of the collateral cleavage activity, CRISPR/Cas12a has recently been identified as a key enabling approach in novel DNA biosensor development. Despite its remarkable success in nucleic acid detection, realizing a universal CRISPR/Cas biosensing system for non-nucleic acid targets remains challenging, particularly at extremely high sensitivity ranges for analyte concentrations lower than the pM level. DNA aptamers can be designed to bind to a range of specific target molecules, such as proteins, small molecules, and cells, with high affinity and specificity through configuration changes. Here, by harnessing its diverse analyte-binding ability and also redirecting the specific DNA-cutting activity of Cas12a to selected aptamers, a simple, sensitive, and universal biosensing platform has been established, termed CRISPR/Cas and aptamer-mediated extra-sensitive assay (CAMERA). With simple modifications to the aptamer and guiding RNA of Cas12a RNP, CAMERA demonstrated 100 fM sensitivity for targeting small proteins, such as IFN-γ and insulin, with less than 1.5-h detection time. Compared with the gold-standard ELISA, CAMERA achieved higher sensitivity and a shorter detection time while retaining ELISA's simple setup. By replacing the antibody with an aptamer, CAMERA also achieved improved thermal stability, allowing to eliminate the requirement for cold storage. CAMERA shows potential to be used as a replacement for conventional ELISA for a variety of diagnostics but with no significant changes for the experimental setup.

Sandwich-type electrochemical immunosensor based on CuFe2O4-Pd for cardiac troponin I detection

A sandwich-type electrochemical immunosensor was designed by highly efficient catalytic cycle amplification strategy of CuFe₂O₄-Pd for sensitive detection of cardiac troponin I. CuFe₂O₄ with coupled variable valence metal elements exhibited favorable catalytic performance through bidirectional cycling of Fe²⁺/Fe³⁺ and Cu⁺/Cu²⁺ redox pairs. More importantly, Cu⁺ acted as the intermediate product of the catalytic reaction, promoted the regeneration of Fe²⁺ and ensured the continuous recycling occurrence of the double redox pairs, and significantly amplified the current signal response. Pd nanoparticles (Pd NPs) loaded on the surface of amino-functionalized CuFe₂O₄ (CuFe₂O₄-NH₂) served as electrochemical mediators to capture labeled antibodies (Ab₂), and also as co-catalysts of CuFe₂O₄ to further enhance the catalytic efficiency, thus improving the sensitivity of the electrochemical immunosensor. Under the optimal experimental conditions, the linear range was 0.001 ~ 100 ng/mL, and the detection limit was 1.91 fg/mL. The electrochemical immunosensor has excellent analytical performance, giving a new impetus for the sensitive detection of cTnI.

Selection and truncation of aptamers as fluorescence sensing platforms for selective and sensitive detection of nitrofurazone

Nitrofurazone (NFZ) is an antibiotic banned in many countries, as its residue seriously harms the human body. Herein, anti-NFZ aptamers were selected and identified based on the magnetic bead SELEX technique using a ssDNA library with a full length of 90 nucleotides (nt). Five full sequence candidate aptamers (NFZ8, NFZ24, NFZ28, NFZ34, and NFZ70) were obtained by secondary structure analysis. We optimized the entire sequence to obtain a truncated aptamer, a 16 nt sequence (NFZ8-1:5'-GTTCTATTGAAAAAAC-3') that showed the highest affinity for NFZ (K_d = 76.11 nM). The binding site of NFZ and aptamer NFZ8-1 was found to be "GAA" by molecular docking. In addition, utilizing the most special truncated aptamer NFZ8-1 as the identification probe, a graphene oxide fluorescent aptasensor is an innovative for the detection of NFZ residue that showed a wide linear reach from 1.25 to 160 ng/mL and a low limit of detection of 1.13 ng/mL. In the actual water environment sample detection, the recovery rate ranged from 95.21 to 113.66%, and the coefficient of variation ranged from 3.53 to 11.24%. These results demonstrate that the NFZ-truncated aptamer applied to the aptasensor provides a novel methodology for recognizing NFZ residues.

Machine Learning Color Feature Analysis of a High Throughput Nanoparticle Conjugate Sensing Assay

Plasmonic nanoparticles are finding applications within the single molecule sensing field in a "dimer" format, where interaction of the target with hairpin DNA causes a decrease in the interparticle distance, leading to a localized surface plasmon resonance shift. While this shift may be detected using spectroscopy, achieving statistical relevance requires the measurement of thousands of nanoparticle dimers and the timescales required for spectroscopic analysis are incompatible with point-of-care devices. However, using dark-field imaging of the dimer structures, simultaneous digital analysis of the plasmonic resonance shift after target interaction of thousands of dimer structures may be achieved in minutes. The main challenge of this digital analysis on the single-molecule scale was the occurrence of false signals caused by non-specifically bound clusters of nanoparticles. This effect may be reduced by digitally separating dimers from other nanoconjugate types. Variation in image intensity was observed to have a discernible impact on the color analysis of the nanoconjugate constructs and thus the accuracy of the digital separation. Color spaces wherein intensity may be uncoupled from the color information (hue, saturation, and value (HSV) and luminance, a* vector, and b* vector (LAB) were contrasted to a color space which cannot uncouple intensity (RGB) to train a classifier algorithm. Each classifier algorithm was validated to determine which color space produced the most accurate digital separation of the nanoconjugate types. The LAB-based learning classifier demonstrated the highest accuracy for digitally separating nanoparticles. Using this classifier, nanoparticle conjugates were monitored for their plasmonic color shift after interaction with a synthetic RNA target, resulting in a platform with a highly accurate yes/no response with a true positive rate of 88% and a true negative rate of 100%. The sensor response of tested single stranded RNA (ssRNA) samples was well above control responses for target concentrations in the range of 10 aM-1 pM.

Design, Rationalization, and Automation of a Catalytic Sensing Mechanism for Homogeneous SERS Biosensors

The current pandemic has shown that we need sensitive and deployable diagnostic technologies. Surface-enhanced Raman scattering (SERS) sensors can be an ideal solution for developing such advanced point-of-need (PON) diagnostic tests. Homogeneous (reagentless) SERS sensors work by directly responding to the target without any processing step, making them capable for simple one-pot assays, but their limitation is the achievable sensitivity, insufficient compared to what is needed for sensing of viral biomarkers. Noncovalent DNA catalysis mechanisms have been recently exploited for catalytic amplification in SERS assays. These advances used catalytic hairpin assembly (CHA) and other DNA self-assembly processes to develop sensing mechanisms with improved sensitivities. However, these mechanisms have not been used in OFF-to-ON homogeneous sensors, and they often target the same biomarker, likely due to the complexity of the mechanism design. There is still a strong need for a catalytic SERS sensor with a homogeneous mechanism and a rationalization of the catalytic sensing mechanism to translate this sensing strategy to different targets and applications. We developed and investigated a homogeneous SERS sensing mechanism that uses catalytic amplification based on DNA self-assembly. We systematically investigated the role of three domains in the fuel strand (internal loop, stem, and toehold), which drives the catalytic mechanism. The thermodynamic parameters determined in our studies were used to build an algorithm for automated design of catalytic sensors that we validated on target sequences associated with malaria and SARS-CoV-2 strains. With our mechanism, we were able to achieve an amplification level of 20-fold for conventional DNA and of 36-fold using locked nucleic acids (LNAs), with corresponding improvements observed in the sensor limit of detection (LOD). We also show a single-base sequence specificity for a sensor targeting a sequence associated with the omicron variant, tested against a delta variant target. This work on catalytic amplification of homogeneous SERS sensors has the potential to enable the use of this sensing modality in new applications, such as infectious disease surveillance, by improving the LOD while conserving the sensor's homogeneous character.