Wash- and Amplification-Free Digital Immunoassay Based on Single-Particle Motion Analysis

Harnessing Bioluminescent Bacteria to Develop an Enzymatic-free Enzyme-linked immunosorbent assay for the Detection of Clinically Relevant Biomarkers

Enzyme-linked immunosorbent assay (ELISA) is the gold standard technique for measuring protein biomarkers due to its high sensitivity, specificity, and throughput. Despite its success, continuous advancements in ELISA and immunoassay formats are crucial to meet evolving global challenges and to address new analytical needs in diverse applications. To expand the capabilities and applications of immunoassays, we introduce a novel ELISA-like assay that we call Bioluminescent-bacteria-linked immunosorbent assay (BBLISA). BBLISA is an enzyme-free assay that utilizes the inner filter effect between the bioluminescent bacteriaAllivibrio fischeriand metallic nanoparticles (gold nanoparticles and gold iridium oxide nanoflowers) as molecular absorbers. Functionalizing these nanoparticles with antibodies induces their accumulation in wells upon binding to molecular targets, forming the classical immune-sandwich complex. Thanks to their ability to adsorb the light emitted by the bacteria, the nanoparticles can suppress the bioluminescence signal, allowing the rapid quantification of the target. To demonstrate the bioanalytical properties of the novel immunoassay platform, as a proof of principle, we detected two clinically relevant biomarkers (human immunoglobulin G and SARS-CoV-2 nucleoprotein) in human serum, achieving the same sensitivity and precision as the classic ELISA. We believe that BBLISA can be a promising alternative to the standard ELISA techniques, offering potential advancements in biomarker detection and analysis by combining nanomaterials with a low-cost, portable bioluminescent platform.

Advances in single molecule arrays (SIMOA) for ultra-sensitive detection of biomolecules

In the contemporary era of scientific and medical advancements, the accurate and ultra-sensitive detection of proteins, nucleic acids and metabolites plays a pivotal role in disease diagnosis and treatment monitoring. Single-molecule detection technologies play a great role in achieving this goal. In recent years, digital detection methods based on single molecule arrays (SIMOA) have brought groundbreaking contributions to the field of single-molecule detection. By confining the target molecules to femtoliter-sized containers, the SIMOA technology achieves detection sensitivity of attomolar. This review delves into the historical evolution and fundamentals of SIMOA technology, summarizes various approaches to optimize its performance, and describes the applications of SIMOA for the ultrasensitive detection of biomarkers for diseases such as cancer, COVID-19, and neurological disorders, as well as in DNA detection. Currently, some SIMOA technologies have been realized for high-throughput and multiplexed detection. It is believed that SIMOA technology will play a significant role in medical monitoring and disease prevention in the future.

Computer vision-assisted smartphone microscope imaging digital immunosensor based on click chemistry-mediated microsphere counting technology for the detection of aflatoxin B1 in peanuts

Aflatoxin B1 is a carcinogenic contaminant in food or feed, and it poses a serious health risk to humans. Herein, a computer vision-assisted smartphone microscope imaging digital (SMID) immunosensor based on the click chemistry-mediated microsphere counting technology was designed for the detection of aflatoxin B1 in peanuts. In this SMID immunosensor, the modified polystyrene (PS) microspheres were used as the signal probes and were recorded by a smartphone microscopic imaging system after immunoreaction and click chemistry reaction. The number of PS probes is adjusted by aflatoxin B1. The customized computer vision procedure was used to efficiently identify and count the obtained PS probes. This SMID immunosensor enables sensitive detection of aflatoxin B1 with a linear range from 0.001 ng/mL to 500 ng/mL, providing a simple, sensitive, and portable tool for food safety supervision.

Computer Vision-Based Artificial Intelligence-Mediated Encoding-Decoding for Multiplexed Microfluidic Digital Immunoassay

Digital immunoassays with multiplexed capacity, ultrahigh sensitivity, and broad affordability are urgently required in clinical diagnosis, food safety, and environmental monitoring. In this work, a multidimensional digital immunoassay has been developed through microparticle-based encoding and artificial intelligence-based decoding, enabling multiplexed detection with high sensitivity and convenient operation. The information encoded in the features of microspheres, including their size, number, and color, allows for the simultaneous identification and accurate quantification of multiple targets. Computer vision-based artificial intelligence can analyze the microscopy images for information decoding and output identification results visually. Moreover, the optical microscopy imaging can be well integrated with the microfluidic platform, allowing for encoding-decoding through the computer vision-based artificial intelligence. This microfluidic digital immunoassay can simultaneously analyze multiple inflammatory markers and antibiotics within 30 min with high sensitivity and a broad detection range from pg/mL to μg/mL, which holds great promise as an intelligent bioassay for next-generation multiplexed biosensing.

Partitioning-Induced Isolation of Analyte and Analysis via Multiscaled Aqueous Two-Phase System

Most fluorescence-based bioanalytical applications need labeling of analytes. Conventional labeling requires washing to remove the excess fluorescent labels and reduce the noise signals. These pretreatments are labor intensive and need specialized equipment, hindering portable applications in resource-limited areas. Herein, we use the aqueous two-phase system (ATPS) to realize the partitioning-induced isolation of labeled analytes from background signals without extra processing steps. ATPS is formed by mixing two polymers at sufficiently high concentrations. ATPS-based isolation is driven by intrinsic affinity differences between analytes and excess labels. To demonstrate the partitioning-induced isolation and analysis, fluorescein isothiocyanate (FITC) is selected as the interfering fluorophore, and a monoclonal antibody (IgG) is used as the analyte. To optimize ATPS compositions, different molecular weights and mass fractions of polyethylene glycol (PEG) and dextran and different phosphate-buffered saline (PBS) concentrations are investigated. Various operational scales of our approach are demonstrated, suggesting its compatibility with various bioanalytical applications. In centimeter-scale ATPS, the optimized distribution ratios of IgG and FITC are 91.682 and 0.998 using PEG 6000 Da and dextran 10,000 Da in 10 mM PBS. In millimeter-scale ATPS, the analyte is enriched to 6.067 fold using 15 wt % PEG 35,000 Da and 5 wt % dextran 500,000 Da in 10 mM PBS. In microscale ATPS, analyte dilutions are isolated into picoliter droplets, and the measured fluorescence intensities linearly correlated with the analyte concentrations (R² = 0.982).

Robust Detection of Cancer Markers in Human Serums Using All-Dielectric Metasurface Biosensors

One of the most significant characteristics, which biosensors are supposed to satisfy, is robustness against abundant molecules coexisting with target biomolecules. In clinical diagnoses and biosensing, blood, plasma, and serum are used daily as samples. In this study, we conducted a series of experiments to examine the robustness of all-dielectric metasurface biosensors, which comprise pairs of a highly fluorescence-enhancing silicon nanopellet array and a transparent microfluidic chip. The metasurface biosensors were shown to have high performance in detecting various targets from nucleic acids to proteins, such as antigens and antibodies. The present results show almost four-order wide dynamic ranges from 0.16 ng/mL to 1 μg/mL for prostate-specific antigen (PSA) and from 2 pg/mL to 25 ng/mL for carcinoembryonic antigen (CEA). The ranges include clinical criteria for PSA, 4 ng/mL and CEA, 5 ng/mL. To date, a systematic demonstration of robustness has not been reported regarding the metasurface biosensors. In detecting cancer markers of PSA and CEA in human serums, we demonstrate that the metasurface biosensors are robust enough in a wide target concentrations, including the clinical diagnosis criteria.

Digital detection of proteins

This paper reviews methods for detecting proteins based on molecular digitization, i.e., the isolation and detection of single protein molecules or singulated ensembles of protein molecules. The single molecule resolution of these methods has resulted in significant improvements in the sensitivity of immunoassays beyond what was possible using traditional "analog" methods: the sensitivity of some digital immunoassays approach those of methods for measuring nucleic acids, such as the polymerase chain reaction (PCR). The greater sensitivity of digital protein detection has resulted in immuno-diagnostics with high potential societal impact, e.g., the early diagnosis and therapeutic intervention of Alzheimer's Disease. In this review, we will first provide the motivation for developing digital protein detection methods given the limitations in the sensitivity of analog methods. We will describe the paradigm shift catalyzed by single molecule detection, and will describe in detail one digital approach - which we call digital bead assays (DBA) - based on the capture and labeling of proteins on beads, identifying "on" and "off" beads, and quantification using Poisson statistics. DBA based on the single molecule array (Simoa) technology have sensitivities down to attomolar concentrations, equating to ∼10 proteins in a 200 μL sample. We will describe the concept behind DBA, the different single molecule labels used, the ways of analyzing beads (imaging of arrays and flow), the binding reagents and substrates used, and integration of these technologies into fully automated and miniaturized systems. We provide an overview of emerging approaches to digital protein detection, including those based on digital detection of nucleic acids labels, single nanoparticle detection, measurements using nanopores, and methods that exploit the kinetics of single molecule binding. We outline the initial impact of digital protein detection on clinical measurements, highlighting the importance of customized assay development and translational clinical research. We highlight the use of DBA in the measurement of neurological protein biomarkers in blood, and how these higher sensitivity methods are changing the diagnosis and treatment of neurological diseases. We conclude by summarizing the status of digital protein detection and suggest how the lab-on-a-chip community might drive future innovations in this field.

Surface modification for improving immunoassay sensitivity

Immunoassays are widely performed in many fields such as biomarker discovery, proteomics, drug development, and clinical diagnosis. There is a growing need for high sensitivity of immunoassays to detect low abundance analytes. As a result, great effort has been made to improve the quality of surfaces, on which the immunoassay is performed. In this review article, we summarize the recent progress in surface modification strategies for improving the sensitivity of immunoassays. The surface modification strategies can be categorized into two groups: antifouling coatings to reduce background noise and nanostructured surfaces to amplify the signals. The first part of the review summarizes the common antifouling coating techniques to prevent nonspecific binding and reduce background noise. The techniques include hydrophilic polymer based self-assembled monomers, polymer brushes, and surface attached hydrogels, and omniphobicity based perfluorinated surfaces. In the second part, some common nanostructured surfaces to amplify the specific detection signals are introduced, including nanoparticle functionalized surfaces, two dimensional (2D) nanoarrays, and 2D nanomaterial coatings. The third part discusses the surface modification techniques for digital immunoassays. In the end, the challenges and the future perspectives of the surface modification techniques for immunoassays are presented.

Quantitative performance of digital ELISA for the highly sensitive quantification of viral proteins and influenza virus

A single-molecule assay (SiMoA) using a digital enzyme-linked immunosorbent assay (ELISA) has been attracting attention as a promising method that can detect viruses with ultra-high sensitivity. However, the quantitative application of digital ELISA has not been adequately reported. Therefore, in this study, we first evaluated the linearity and sensitivity of digital ELISA using a Certified Reference Material of C-reactive protein (NMIJ CRM 6201-c) as a quality control material. Next, we originally screened those antibody pair that are suitable for detecting recombinant viral proteins of influenza A virus, nucleoprotein (NP), and hemagglutinin (HA), and established the measurement system. Under optimized conditions, the limit of detection (LOD) of NP and HA was 0.59 fM and 0.99 fM, and the coefficient of determination, R², was 0.9998 and 0.9979, respectively. Two subtypes of influenza virus, A/Puerto Rico/8/1934 (H1N1) [PR8] and A/Panama/2007/99 (H3N2) [Pan99], were also quantified under established conditions, and the LOD of PR8 was 3.1 × 10² PFU/mL on targeting NP and 7.4 × 10² PFU/mL on targeting HA. The LOD of Pan99 was 5.3 × 10² PFU/mL on targeting NP. The specificity and robustness of the recombinant viral protein and influenza virus measurements using digital ELISA were also evaluated. Our measurement system showed enough specificity to discriminate the viral subtypes properly and showed sufficient inter- and intra-assay variations for both measurements of recombinant viral proteins and viruses, except for NP-targeting virus measurement.

Magnetically localized and wash-free fluorescence immunoassay (MLFIA): proof of concept and clinical applications

Immunoassays are used for many applications in various markets, from clinical diagnostics to the food industry, generally relying on gold-standard ELISAs that are sensitive, robust, and cheap but also time-consuming and labour intensive. As an alternative, we propose here the magnetically localized and wash-free fluorescence immunoassay (MLFIA): a no-wash assay to directly measure a biomolecule concentration, without mixing nor washing steps. To do so, a fluorescence no-wash measurement is performed to generate a detectable signal. It consists of a differential measurement between the fluorescence of fluorophores bound to magnetic nanoparticles specifically captured by micro-magnets against the residual background fluorescence of unbound fluorophores. Targeted biomolecules (antibodies or antigens) are locally concentrated on micro-magnet lines, with the number of captured biomolecules quantitatively measured without any washing step. The performance of the MLFIA platform is assessed and its use is demonstrated with several biological models as well as clinical blood samples for HIV, HCV and HBV detection, with benchmarking to standard analyzers of healthcare laboratories. Thus, we demonstrated for the first time the versatility of the innovative MLFIA platform. We highlighted promising performances with the successful quantitative detection of various targets (antigens and antibodies), in different biological samples (serum and plasma), for different clinical tests (HCV, HBV, HIV).

Artificial Intelligence-Assisted Digital Immunoassay Based on a Programmable-Particle-Decoding Technique for Multitarget Ultrasensitive Detection

The development of a multitarget ultrasensitive immunoassay is significant to fields such as medical research, clinical diagnosis, and food safety inspection. In this study, an artificial intelligence (AI)-assisted programmable-particle-decoding technique (APT)-based digital immunoassay system was developed to perform multitarget ultrasensitive detection. Multitarget was encoded by programmable polystyrene (PS) microspheres with different characteristics (particle size and number), and subsequent visible signals were recorded under an optical microscope after the immune reaction. The resultant images were further analyzed using a customized, AI-based computer vision technique to decode the intrinsic properties of polystyrene microspheres and to reveal the types and concentrations of targets. Our strategy has successfully detected multiple inflammatory markers in clinical serum and antibiotics with a broad detection range from pg/mL to μg/mL without extra signal amplification and conversion. An AI-based digital immunoassay system exhibits great potential to be used for the next generation of multitarget detection in disease screening for candidate patients.

Biosensors: Electrochemical Devices-General Concepts and Performance

This review provides a general overview of different biosensors, mostly concentrating on electrochemical analytical devices, while briefly explaining general approaches to various kinds of biosensors, their construction and performance. A discussion on how all required components of biosensors are brought together to perform analytical work is offered. Different signal-transducing mechanisms are discussed, particularly addressing the immobilization of biomolecular components in the vicinity of a transducer interface and their functional integration with electronic devices. The review is mostly addressing general concepts of the biosensing processes rather than specific modern achievements in the area.

Continuous biomarker monitoring with single molecule resolution by measuring free particle motion

There is a need for sensing technologies that can continuously monitor concentration levels of critical biomolecules in applications such as patient care, fundamental biological research, biotechnology and food industry, as well as the environment. However, it is fundamentally difficult to develop measurement technologies that are not only sensitive and specific, but also allow monitoring over a broad concentration range and over long timespans. Here we describe a continuous biomolecular sensing methodology based on the free diffusion of biofunctionalized particles hovering over a sensor surface. The method records digital events due to single-molecule interactions and enables biomarker monitoring at picomolar to micromolar concentrations without consuming any reagents. We demonstrate the affinity-based sensing methodology for DNA-based sandwich and competition assays, and for an antibody-based cortisol assay. Additionally, the sensor can be dried, facilitating storage over weeks while maintaining its sensitivity. We foresee that this will enable the development of continuous monitoring sensors for applications in fundamental research, for studies on organs on a chip, for the monitoring of patients in critical care, and for the monitoring of industrial processes and bioreactors as well as ecological systems.

A homogeneous enzyme-free ratiometric immunoassay for the determination of C-peptide

In this study, a homogeneous enzyme-free ratiometric (HOMO- EF-RA) immunoassay was developed for the sensitive detection of C-peptide. In the immunoassay, there have been a miscible detection system by mixing with the fluorescent quantum dots conjugated antigen (QD-Ag conjugates) and the dylight dye conjugated antibody (DL-Ab conjugates). When connecting between Ag-QD conjugate and Ab-DL conjugate by specific recognition, the system emitted fluorescence resonant energy transfer (FRET). The target C-peptide can inhibit the connection and FRET formation between QD-Ag conjugates and DL-Ab conjugates, thus changing the dual fluorescence. By measuring the ratio dual fluorescence changes of the system, the content of C-peptide was evaluated without any enzyme used and multiple incubation and washing steps. This immunoassay realized the highly sensitive (as low as 0.12 ng mL^-1), selective and rapid (as less as 6 min) detection of C-peptide. Furthermore, the the simple and convenient immunoassay was applied successfully to the determination of C-peptide in real serum samples.

Single-Nanoparticle Differential Immunoassay for Multiplexed Gastric Cancer Biomarker Monitoring

Precision medicine demands the best application of multiple unambiguous biomarkers to bring uniform decisions in disease prognosis. The remarkable development of heterogeneous immunoassay greatly promotes precision medicine when combined with the biomarker combination strategy. Nevertheless, the cumbersome washing steps in heterogeneous immunoassay have inevitably compromised the accuracy because of the sample losses and nature change of the matrix, challenging the further exploration of a more facile and lower limit-of-detection analysis. The new methodologies with high throughputs and specificity are never out of date to provide simultaneous evaluations and uniform decisions on multiple analytes through a simple process. Herein, we propose a new wash-free immunoassay, named differential assay, for multiplexed biomarker monitoring. The method is based on counting the number difference of unbound nanoparticle tags before and after immunoreactions from a solid support (i.e., magnetic microsphere) by single-particle inductively coupled plasma mass spectrometry (sp-ICP-MS), discarding the tedious washing steps. We primarily explore the proof-of-concept proposal within two types (sandwich and competitive assay), demonstrating the good feasibility for further facile clinical practice. To provide efficient multiplexed evaluations, we synthesized PtNPs with four diameters and screened the most suitable size for efficient differential immunoassay. The wash-free strategy was successfully utilized in simultaneous serological biomarker (CA724, CA199, and CEA) evaluation, with results in good accordance with those measured by the clinical routine method. Potentially, the proposed differential bioassay can be regarded as a more facile and valuable tool in malignancy prognosis and cancer recurrence monitoring.

A microfluidic chip using Au@SiO2 array-based highly SERS-active substrates for ultrasensitive detection of dual cervical cancer-related biomarkers

In this work, a microfluidic chip using Au@SiO₂ array-based highly active SERS substrates was developed for quantitative detection of squamous cell carcinoma antigen (SCCA) and carcinoembryonic antigen (CEA) associated with cervical cancer. The chip consisted of six functional units with pump-free design, enabling parallel detection of multiple samples in an automatic manner without external pumps and improving the portability. Ag nanocubes (AgNCs) were labeled with Raman reporters and coupled with antibodies (labeling) to prepare SERS tags, while the Au nanoparticle-modified SiO₂ microsphere (Au@SiO₂) array was conjugated with antibodies (coating) to generate the highly SERS-active capturing substrate. In the presence of target biomarkers, they were captured by SERS tags and capturing substrate, resulting in the formation of "sandwich" structures which were trapped in the detection chamber. As the immune reaction proceeded, a large number of "hot spots" were generated by the proximity of the Au@SiO₂ array substrate and AgNCs, greatly amplifying SERS signals. With this chip, the limits of detection of the SCCA and CEA levels in human serum were estimated to be as low as 0.45 pg mL^-1 and 0.36 pg mL^-1, respectively. Furthermore, the good selectivity and reproducibility of this chip were confirmed. Finally, clinical serum samples were analyzed by this chip, and the outcomes were consistent with those of enzyme-linked immunosorbent assay (ELISA). Thus, the proposed microfluidic chip can be potentially applied for the clinical diagnosis of cervical cancer.

Recent Advances in Digital Biosensing Technology

Digital biosensing assays demonstrate remarkable advantages over conventional biosensing systems because of their ability to achieve single-molecule detection and absolute quantification. Unlike traditional low-abundance biomarking screening, digital-based biosensing systems reduce sample volumes significantly to the fL-nL level, which vastly reduces overall reagent consumption, improves reaction time and throughput, and enables high sensitivity and single target detection. This review presents the current technology for compartmentalizing reactions and their applications in detecting proteins and nucleic acids. We also analyze existing challenges and future opportunities associated with digital biosensing and research opportunities for developing integrated digital biosensing systems.

Future of Digital Assays to Resolve Clinical Heterogeneity of Single Extracellular Vesicles

Extracellular vesicles (EVs) are complex lipid membrane vehicles with variable expressions of molecular cargo, composed of diverse subpopulations that participate in the intercellular signaling of biological responses in disease. EV-based liquid biopsies demonstrate invaluable clinical potential for overhauling current practices of disease management. Yet, EV heterogeneity is a major needle-in-a-haystack challenge to translate their use into clinical practice. In this review, existing digital assays will be discussed to analyze EVs at a single vesicle resolution, and future opportunities to optimize the throughput, multiplexing, and sensitivity of current digital EV assays will be highlighted. Furthermore, this review will outline the challenges and opportunities that impact the clinical translation of single EV technologies for disease diagnostics and treatment monitoring.

Enzyme-based digital bioassay technology - key strategies and future perspectives

Digital bioassays based on single-molecule enzyme reactions represent a new class of bioanalytical methods that enable the highly sensitive detection of biomolecules in a quantitative manner. Since the first reports of these methods in the 2000s, there has been significant growth in this new bioanalytical strategy. The principal strategy of this method is to compartmentalize target molecules in micron-sized reactors at the single-molecule level and count the number of microreactors showing positive signals originating from the target molecule. A representative application of digital bioassay is the digital enzyme-linked immunosorbent assay (ELISA). Owing to their versatility, various types of digital ELISAs have been actively developed. In addition, some disease markers and viruses possess catalytic activity, and digital bioassays for such enzymes and viruses have, thus, been developed. Currently, with the emergence of new microreactor technologies, the targets of this methodology are expanding from simple enzymes to more complex systems, such as membrane transporters and cell-free gene expression. In addition, multiplex or multiparametric digital bioassays have been developed to assess precisely the heterogeneities in sample molecules/systems that are obscured by ensemble measurements. In this review, we first introduce the basic concepts of digital bioassays and introduce a range of digital bioassays. Finally, we discuss the perspectives of new classes of digital bioassays and emerging fields based on digital bioassay technology.

Digital plasmonic nanobubble detection for rapid and ultrasensitive virus diagnostics

Rapid and sensitive diagnostics of infectious diseases is an urgent and unmet need as evidenced by the COVID-19 pandemic. Here, we report a strategy, based on DIgitAl plasMONic nanobubble Detection (DIAMOND), to address this need. Plasmonic nanobubbles are transient vapor bubbles generated by laser heating of plasmonic nanoparticles (NPs) and allow single-NP detection. Using gold NPs as labels and an optofluidic setup, we demonstrate that DIAMOND achieves compartment-free digital counting and works on homogeneous immunoassays without separation and amplification steps. DIAMOND allows specific detection of respiratory syncytial virus spiked in nasal swab samples and achieves a detection limit of ~100 PFU/mL (equivalent to 1 RNA copy/µL), which is competitive with digital isothermal amplification for virus detection. Therefore, DIAMOND has the advantages including one-step and single-NP detection, direct sensing of intact viruses at room temperature, and no complex liquid handling, and is a platform technology for rapid and ultrasensitive diagnostics.

Self-assembled multiprotein nanostructures with enhanced stability and signal amplification capability for sensitive fluorogenic immunoassays

Fundamentally improving the sensing sensitivity of immunoassay remains a huge challenge, which limited further critical applications. Herein we designed a new immunoprobe by integrating biometric unit (antibody) and signal amplification element (enzyme) to form urease-antibody-CaHPO₄ hybrid nanoflower (UAhNF) via the biomineralization process. The dual-functional UAhNF enhances the stability of urease in NaCl (10 mmol L^-1) and high temperature (60 °C), and also maintains the ability of antibody recognition, fitting greatly well with the need for immunosensor. Using imidacloprid as a model target, the fixed coating antigens are competed with imidacloprid to capture primary antibodies, and the secondary antibody of UAhNF was linked to construct the competitive-type fluorogenic immunoassays. An in-situ etching process of copper nanoparticles initiated by urease is integrated with UAhNF-based immune response for further improving the detection sensitivity. The proposed immunosensor possessed a 50% inhibition concentration value of 0.72 ng mL^-1, which is 30-fold lower than conventional enzyme-linked immunosorbent assay. This presented approach provided a versatile sensing tool by varying building blocks, making it practically functional for a variety of bioassay applications.

Modified ELISA for Ultrasensitive Diagnosis

An enzyme-linked immunosorbent assay (ELISA) can be used for quantitative measurement of proteins, and improving the detection sensitivity to the ultrasensitive level would facilitate the diagnosis of various diseases. In the present review article, we first define the term 'ultrasensitive'. We follow this with a survey and discussion of the current literature regarding modified ELISA methods with ultrasensitive detection and their application for diagnosis. Finally, we introduce our own newly devised system for ultrasensitive ELISA combined with thionicotinamide adenine dinucleotide cycling and its application for the diagnosis of infectious diseases and lifestyle-related diseases. The aim of the present article is to expand the application of ultrasensitive ELISAs in the medical and biological fields.

Plasmonic Au-Metal Oxide Nanocomposites for High-Temperature and Harsh Environment Sensing Applications

Noble metal nanoparticles like Au have long been admired for their brilliant colour, significantly influenced by plasmon resonance. When embedded in metal oxides, they exhibit unique properties which make them an excellent choice for sensing in high-temperature and harsh environment atmospheres. In this review, the various morphologies of Au nanoparticles (AuNPs) used in combination with metal oxides for sensing gases at temperatures greater than 300 °C are discussed. Theoretical discussions on the plasmon resonance properties of AuNPs as well as computational techniques like finite difference time domain (FDTD), are often used for understanding and correlating their extinction spectra and are briefed initially. The sensing properties of AuNPs embedded on a metal oxide matrix (such as TiO₂ , SiO₂ , NiO etc) for quantifying multiple analytes are then elucidated. The effect of high temperature as well as gas environments including corrosive atmospheres on such nanocomposites, and the different approaches to comprehend them are presented. Finally, techniques and methods to improve on the challenges associated with the realization and integration such Au-metal oxide plasmonic nanostructures for applications such as combustion monitoring, fuel cells, and other applications are discussed.

Quantification of Tumor Protein Biomarkers from Lung Patient Serum Using Nanoimpact Electrochemistry

Protein quantification with high throughput and high sensitivity is essential in the early diagnosis and elucidation of molecular mechanisms for many diseases. Conventional approaches for protein assay often suffer from high costs, long analysis time, and insufficient sensitivity. The recently emerged nanoimpact electrochemistry (NIE), as a contrast, allows in situ detection of analytes one at a time with simplicity, fast response, high throughput, and the potential of reducing the detection limits down to the single entity level. Herein, we propose a NIE-enabled electrochemical immunoassay using silver nanoparticles (AgNPs) as labels for the detection of CYFRA21-1, a typical protein marker for lung carcinoma. This strategy is based on the measurement of the impact frequency and the charge intensity of the electrochemical oxidation of individual AgNPs before and after they are modified with anti-CYFRA21-1 and in turn immunocomplexed with CYFRA21-1. Both the frequency and intensity modes of single-nanoparticle electrochemistry correlate well with each other, resulting in a self-validated immunoassay that provides linear ranges of two orders of magnitude and a limit of detection of 0.1 ng/mL for CYFRA21-1 analysis. The proposed immunoassay also exhibits excellent specificity when challenged with other possible interfering proteins. In addition, the CYFRA21-1 content is validated by a conventional, well-known enzyme-linked immunosorbent assay and successfully quantified in a diluted healthy serum with a satisfactory recovery. Moreover, CYFRA21-1 detection in serum samples of lung cancer patients is successfully demonstrated, suggesting the feasibility of the NIE-based immunoassay in clinically relevant diagnosis. To the best of our knowledge, this is the first report to construct NIE-based electrochemical immunoassays for the specific detection of tumor protein biomarkers.

Elucidation and control of low and high active populations of alkaline phosphatase molecules for quantitative digital bioassay

Alkaline phosphatase (ALP), a homo‐dimeric enzyme has been widely used in various bioassays as disease markers and enzyme probes. Recent advancements of digital bioassay revolutionized ALP‐based diagnostic assays as seen in rapid growth of digital ELISA and the emerging multiplex profiling of single‐molecule ALP isomers. However, the intrinsic heterogeneity found among ALP molecules hampers the ALP‐based quantitative digital bioassays. This study aims quantitative analysis of single‐molecule activities of ALP from Escherichia coli and reveals the static heterogeneity in catalytic activity of ALP with two distinct populations: half‐active and fully‐active portions. Digital assays with serial buffer exchange uncovered single‐molecule Michaelis–Menten kinetics of ALP; half‐active molecules have halved values of the catalytic turnover rate, k_cat, and the rate constant of productive binding, k_on, of the fully active molecules. These findings suggest that half‐active ALP molecules are heterogenic dimers composed of inactive and active monomer units, while fully active ALP molecules comprise two active units. Static heterogeneity was also observed for ALP with other origins: calf intestine or shrimp, showing how the findings can be generalized across species. Cell‐free expression of ALP with disulfide bond enhancer and spiked zinc ion resulted in homogenous population of ALP of full activity, implying that inactive monomer units of ALP are deficient in correct disulfide bond formation and zinc ion coordination. These findings provide basis for further study on molecular mechanism and biogenesis of ALP, and also offer the way to prepare homogenous and active populations of ALP for highly quantitative and sensitive bioassays with ALP.

An overview of proteomic methods for the study of 'cytokine storms'

Introduction: The cytokine storm is a form of excessive systemic inflammatory reaction triggered by a myriad of factors that may lead to multi-organ failure, and finally to death. The cytokine storm can occur in a number of infectious and noninfectious diseases including COVID-19, sepsis, ebola, avian influenza, and graft versus host disease, or during the severe inflammatory response syndrome.Area covered: This review mainly focuses on the most common and well-known methods of protein studies (PAGE, SDS-PAGE, and high- performance liquid chromatography). It also discusses other modern technologies in proteomics like mass spectrometry, soft ionization techniques, cytometric bead assays, and the next generation of microarrays that have been used to get an in-depth understanding of the pathomechanisms involved during the cytokine storm.Expert opinion: Overactivation of leukocytes drives the production and secretion of inflammatory cytokines fueling the cytokine storm. These events lead to a systemic hyper-inflammation, circulatory collapse and shock, and finally to multiorgan failure. Therefore, monitoring the patient's systemic cytokine levels with proteomic technologies that are redundant, economical, and require minimal sample volume for real-time assessment might help in a better clinical evaluation and management of critically ill patients.

Single-Particle Counting Based on Digital Plasmonic Nanobubble Detection for Rapid and Ultrasensitive Diagnostics

Rapid and sensitive diagnostics of infectious diseases is an urgent and unmet need as evidenced by the COVID-19 pandemic. Here we report a novel strategy, based on DIgitAl plasMONic nanobubble Detection (DIAMOND), to address these gaps. Plasmonic nanobubbles are transient vapor bubbles generated by laser heating of plasmonic nanoparticles and allow single-particle detection. Using gold nanoparticles labels and an optofluidic setup, we demonstrate that DIAMOND achieves a compartment-free digital counting and works on homogeneous assays without separation and amplification steps. When applied to the respiratory syncytial virus diagnostics, DIAMOND is 150 times more sensitive than commercial lateral flow assays and completes measurements within 2 minutes. Our method opens new possibilities to develop single-particle digital detection methods and facilitate rapid and ultrasensitive diagnostics.

One Sentence Summary

Single-particle digital plasmonic nanobubble detection allows rapid and ultrasensitive detection of viruses in a one-step homogeneous assay.

A digital protein microarray for COVID-19 cytokine storm monitoring

Despite widespread concern regarding cytokine storms leading to severe morbidity in COVID-19, rapid cytokine assays are not routinely available for monitoring critically ill patients. We report the clinical application of a digital protein microarray platform for rapid multiplex quantification of cytokines from critically ill COVID-19 patients admitted to the intensive care unit (ICU) at the University of Michigan Hospital. The platform comprises two low-cost modules: (i) a semi-automated fluidic dispensing/mixing module that can be operated inside a biosafety cabinet to minimize the exposure of the technician to the virus infection and (ii) a 12-12-15 inch compact fluorescence optical scanner for the potential near-bedside readout. The platform enabled daily cytokine analysis in clinical practice with high sensitivity (<0.4 pg mL-1), inter-assay repeatability (∼10% CV), and rapid operation providing feedback on the progress of therapy within 4 hours. This test allowed us to perform serial monitoring of two critically ill patients with respiratory failure and to support immunomodulatory therapy using the selective cytopheretic device (SCD). We also observed clear interleukin-6 (IL-6) elevations after receiving tocilizumab (IL-6 inhibitor) while significant cytokine profile variability exists across all critically ill COVID-19 patients and to discover a weak correlation between IL-6 to clinical biomarkers, such as ferritin and C-reactive protein (CRP). Our data revealed large subject-to-subject variability in patients' response to COVID-19, reaffirming the need for a personalized strategy guided by rapid cytokine assays.

A critical review: Recent advances in "digital" biomolecule detection with single copy sensitivity

Detection of a single biomolecule, ranging from nucleic acids, proteins, viruses to bacteria, is of paramount importance in various fields including biology, environment, food and agriculture industry, public health, and medicine. With the understanding of the biological functions of these biomolecules (or bioparticles) and their impacts on public health, environmental pollution, and food safety, advanced detection techniques are unprecedentedly demanded for their early and/or sensitive detection. In this critical review, a series of elegant research about digital detection of biomolecules with potential single copy sensitivity is reviewed and summarized with the focus on the design principle and the innovation of how to accomplish the “digital” detection concept. Starting with a brief introduction on the importance of digital detection, recent advances in “digital” biomolecule detection with single copy sensitivity are grouped and discussed based on the difference of signal reporting systems, including surrogate signal development for “digital” detection, direct visualization for “digital” detection, and nucleic acid amplification enabled “digital” detection. Interdisciplinary combination and integration of different cutting-edge techniques are also discussed with details. The review is closed with the conclusion and future trends.

Multiparameter single-particle motion analysis for homogeneous digital immunoassay

Digital homogeneous non-enzymatic immunosorbent assay (digital Ho-Non ELISA) is a new class of digital immunoassay. In this paper, we developed a multiparameter single-particle motion analysis method for a highly sensitive digital Ho-Non ELISA.

Development of fully automated and ultrasensitive assays for urinary adiponectin and their application as novel biomarkers for diabetic kidney disease

Glomerular filtration rate (GFR) and urinary albumin excretion rate (UAER) are used to diagnose and classify the severity of chronic kidney disease. Total adiponectin (T-AN) and high molecular weight adiponectin (H-AN) assays were developed using the fully automated immunoassay system, HI-1000 and their significance over conventional biomarkers were investigated. The T-AN and H-AN assays had high reproducibility, good linearity, and sufficient sensitivity to detect trace amounts of adiponectin in the urine. Urine samples after gel filtration were analyzed for the presence of different molecular isoforms. Low molecular weight (LMW) forms and monomers were the major components (93%) of adiponectin in the urine from a diabetic patient with normoalbuminuria. Urine from a microalbuminuria patient contained both high molecular weight (HMW) (11%) and middle molecular weight (MMW) (28%) adiponectin, although the LMW level was still high (52%). The amount of HMW (32%) and MMW (42%) were more abundant than that of LMW (24%) in a diabetic patient with macroalbuminuria. T-AN (r = − 0.43) and H-AN (r = − 0.38) levels showed higher correlation with estimated GFR (eGFR) than UAER (r = − 0.23). Urinary levels of both T-AN and H-AN negatively correlated with renal function in diabetic patients and they may serve as new biomarkers for diabetic kidney disease.

Monodisperse Liposomes with Femtoliter Volume Enable Quantitative Digital Bioassays of Membrane Transporters and Cell-Free Gene Expression

Digital bioassays have emerged as a new category of bioanalysis. However, digital bioassays for membrane transporter proteins have not been well established yet despite high demands in molecular physiology and molecular pharmacology due to the lack of biologically functional monodisperse liposomes with femtoliter volumes. Here, we established a simple and robust method to produce femtoliter-sized liposomes (femto-liposomes). We prepared 10⁶ monodispersed water-in-oil droplets stabilized by a lipid monolayer using a polyethylene glycol-coated femtoliter reactor array device. Droplets were subjected to the optimized emulsion transfer process for femto-liposome production. Liposomes were monodispersed (coefficient of variation = 5-15%) and had suitable diameter (0.6-5.3 μm) and uniform volumes of subfemtoliter or a few femtoliters; thus, they were termed uniform femto-liposomes. The unilamellarity of uniform femto-liposomes allowed quantitative single-molecule analysis of passive and active transporter proteins: α-hemolysin and F_oF₁-ATPase. Digital gene expression in uniform femto-liposomes (cell-free transcription and translation from single DNA molecules) was also demonstrated, showing the versatility of digital assays for membrane transporter proteins and cell-free synthetic biology.

Simultaneous detection of small molecules, proteins and microRNAs using single molecule arrays

Biological samples such as blood, urine, cerebrospinal fluid and saliva contain a large variety of proteins, nucleic acids, and small molecules. These molecules can serve as potential biomarkers of disease and therefore, it is desirable to simultaneously detect multiple biomarkers in one sample. Current detection techniques suffer from various limitations including low analytical sensitivity and complex sample processing. In this work, we present an ultrasensitive method for simultaneous detection of small molecules, proteins and microRNAs using single molecule arrays (Simoa). Dye-encoded beads modified with specific capture probes were used to quantify each analyte. Multiplex competitive Simoa assays were established for simultaneous detection of cortisol and prostaglandin E2. In addition, competitive and sandwich immunoassays were combined with a direct nucleic acid hybridization assay for simultaneous detection of cortisol, interleukin 6 and microRNA 141. The multi-analyte Simoa assay shows high sensitivity and specificity, which provides a powerful tool for the analysis of many different samples.

Advances in Optical Single-Molecule Detection: En Route to Supersensitive Bioaffinity Assays

The ability to detect low concentrations of analytes and in particular low-abundance biomarkers is of fundamental importance, e.g., for early-stage disease diagnosis. The prospect of reaching the ultimate limit of detection has driven the development of single-molecule bioaffinity assays. While many review articles have highlighted the potentials of single-molecule technologies for analytical and diagnostic applications, these technologies are not as widespread in real-world applications as one should expect. This Review provides a theoretical background on single-molecule-or better digital-assays to critically assess their potential compared to traditional analog assays. Selected examples from the literature include bioaffinity assays for the detection of biomolecules such as proteins, nucleic acids, and viruses. The structure of the Review highlights the versatility of optical single-molecule labeling techniques, including enzymatic amplification, molecular labels, and innovative nanomaterials.

Multiplexed homogeneous digital immunoassay based on single-particle motion analysis

Homogeneous digital immunoassay is a powerful analytical method for highly sensitive protein biomarker detection with a simple protocol. However, it has not been multiplexed yet. In this study, we developed a multiplexed homogeneous digital immunoassay based on single-particle motion analysis (digital homogeneous non-enzyme-linked immunosorbent assay, digital Ho-Non ELISA). In this assay, multiple target antigen molecules react with the optical subpopulation of magnetic nanobeads labeled with fluorescent dyes and capture antigen-specific antibodies. Then, these beads are magnetically pulled into femtoliter-sized reactors. The surface of these reactors is modified with multiple detection antibodies specific to each antigen by molecular tethers. Each antigen on the particles reacts with the detection antibodies anchored to the surface of the reactors. Magnetic force enhances the efficiency of bead encapsulation in the reactors, and subsequent physical compartmentalization of beads enhances the binding efficiency of the antigen-antibody reaction. The tethered beads show characteristic Brownian motion distinct from free diffusion or non-specific binding of the antigen-free beads. The color of the beads is attributed to target-identification, and the number of tethered beads is attributed to the concentration of the specific target. We measured two biomarkers (PSA and IL6) as model targets by multiplexed digital Ho-Non ELISA. Our method showed higher sensitivity compared to previous digital Ho-Non ELISA and could detect multiple targets simultaneously with the same performance as in single-plex detection. This new strategy has the potential to open a new avenue for next-generation multiplexed immunoassays in in vitro diagnostics.

A Digital Protein Microarray for COVID-19 Cytokine Storm Monitoring

Despite widespread concern for cytokine storms leading to severe morbidity in COVID-19, rapid cytokine assays are not routinely available for monitoring critically ill patients. We report the clinical application of a machine learning-based digital protein microarray platform for rapid multiplex quantification of cytokines from critically ill COVID-19 patients admitted to the intensive care unit (ICU) at the University of Michigan Hospital. The platform comprises two low-cost modules: (i) a semi-automated fluidic dispensing/mixing module that can be operated inside a biosafety cabinet to minimize the exposure of technician to the virus infection and (ii) a 12-12-15 inch compact fluorescence optical scanner for the potential near-bedside readout. The platform enabled daily cytokine analysis in clinical practice with high sensitivity (<0.4pg/mL), inter-assay repeatability (~10% CV), and near-real-time operation with a 10 min assay incubation. A cytokine profiling test with the platform allowed us to observe clear interleukin-6 (IL-6) elevations after receiving tocilizumab (IL-6 inhibitor) while significant cytokine profile variability exists across all critically ill COVID-19 patients and to discover a weak correlation between IL-6 to clinical biomarkers, such as Ferritin and CRP. Our data revealed large subject-to-subject variability in a patient's response to anti-inflammatory treatment for COVID-19, reaffirming the need for a personalized strategy guided by rapid cytokine assays.