Enhancing the analytical performance of paper lateral flow assays: From chemistry to engineering

Open and closed microfluidics for biosensing

Biosensing is vital for many areas like disease diagnosis, infectious disease prevention, and point-of-care monitoring. Microfluidics has been evidenced to be a powerful tool for biosensing via integrating biological detection processes into a palm-size chip. Based on the chip structure, microfluidics has two subdivision types: open microfluidics and closed microfluidics, whose operation methods would be diverse. In this review, we summarize fundamentals, liquid control methods, and applications of open and closed microfluidics separately, point out the bottlenecks, and propose potential directions of microfluidics-based biosensing.

Development of a Paper-based Hematocrit Test and a Lateral Flow Assay to Detect Critical Fibrinogen Concentrations Using a Bottom-Up Pyramid Workflow Approach

Fibrinogen is a coagulation factor in human blood and the first one to reach critical levels in major bleeding. Hypofibrinogenemia (a too low fibrinogen concentration in blood) poses great challenges to first responders, clinicians, and healthcare providers since it represents a risk factor for exsanguination and massive transfusion requirements. Thus, the rapid assessment of the fibrinogen concentration at the point of care has gained considerable importance in preventing and managing major blood loss. However, in whole blood measurements, hematocrit variations affect the amount (volume fraction) of plasma that passes the detection zone. In an attempt to accurately determine realistic critical levels of fibrinogen (<1.5 mg/mL) in patients needing immediate treatment and medical interventions, we have developed novel diagnostic systems capable of estimating hematocrit and critical fibrinogen concentrations. A lateral flow assay (LFA) for the detection of fibrinogen has been developed by establishing a workflow employing rapid characterization methods to streamline LFA development. The integration of two detection lines enables (i) the identification of fibrinogen (first line) present in the sample and (ii) the determination of the clinically critical fibrinogen concentrations below 1.5 mg/mL (second line). Furthermore, the paper-based separation of blood cells from plasma provides a semiquantitative estimate of the hematocrit by analyzing the fractions. Initial validation of the point-of-care (PoC) hematocrit test revealed good comparability to a standard laboratory method. The developed diagnostic systems have the ability to accelerate decision-making in cases with major bleeding.

Paper-based microfluidic electro-analytical device (PMED) for magneto-assay automation: Towards generic point-of-care diagnostic devices

Rapid diagnostic tests (RDTs) for point-of-care (POC) testing of infectious diseases are popular because they are easy to use. However, RDTs have limitations such as low sensitivity and qualitative responses that rely on subjective visual interpretation. Additionally, RDTs are made using paper-bound reagents, which leads to batch-to-batch variability, limited storage stability and detection of only the analytes they were designed for. This work presents the development of a versatile technology, based on short magneto-assays and inexpensive paper-based microfluidic electro-analytical devices (PMEDs). PMEDs were produced locally using low-cost equipment, they were stable at room temperature, easy to use, and provided quantitative and objective results. The devices served to detect alternatively a variety of magneto-assays, granting quantitation of streptavidin-HRP, biotinylated HRP and Pasmodium falciparum lactate dehydrogenase (Pf-LDH) in less than 25 min, using either commercial or customized screen-printed electrodes and measurement equipment. Furthermore, Pf-LDH detection in diluted lysed whole blood displayed a linear response between 3 and 25 ng mL-1, detection and quantification limits ranging between 1 and 3 ng mL-1 and 6-12 ng mL-1, respectively, and provided results that correlated with those of the reference ELISA. In short, this technology is versatile, simple, and highly cost-effective, making it perfect for POC testing.

Electrokinetic focusing of SARS-CoV-2 spike protein via ion concentration polarization in a paper-based lateral flow assay

The COVID-19 pandemic highlighted the importance of designing sensitive and selective point-of-care (POC) diagnostic sensors for early and rapid detection of infection. Paper-based lateral flow assays (LFAs) are easy to use, inexpensive, and rapid, but they lack sensitivity. Preconcentration techniques can improve the sensitivity of LFAs by increasing the local concentration of the analyte before detection. Here, ion concentration polarization (ICP) is used to focus the analyte, SARS-CoV-2 Spike protein (S-protein), directly over a test line composed of angiotensin converting enzyme 2 (ACE2) capture probes. ICP is the enrichment and depletion of electrolyte ions at opposing ends of an ion-selective membrane under a voltage bias. The ion depleted zone (IDZ) establishes a steep gradient in electric field strength along its boundary. Enrichment of charged species (such as a biomolecule analyte) occurs at an axial location along this electric field gradient in the presence of a fluid flow that counteracts migration of those species - a phenomenon called ICP focusing. In this paper, running buffer composition and pretreatment solutions for ICP focusing in a paper-based LFA are evaluated, and the method of voltage application for ICP-enrichment is optimized. With a power consumption of 1.8 mW, S-protein is concentrated by a factor of 21-fold, leading to a 2.9-fold increase in the signal from the LFA compared to a LFA without ICP-enrichment. The described ICP-enhanced LFA is significant because the preconcentration strategy is amenable to POC applications and can be applied to existing LFAs for improvement in sensitivity.

Signal Amplification Strategy Design in Nanozyme-Based Biosensors for Highly Sensitive Detection of Trace Biomarkers

Nanozymes show great promise in enhancing disease biomarker sensing by leveraging their physicochemical properties and enzymatic activities. These qualities facilitate signal amplification and matrix effects reduction, thus boosting biomarker sensing performance. In this review, recent studies from the last five years, concentrating on disease biomarker detection improvement through nanozyme-based biosensing are examined. This enhancement primarily involves the modulations of the size, morphology, doping, modification, electromagnetic mechanisms, electron conduction efficiency, and surface plasmon resonance effects of nanozymes for increased sensitivity. In addition, a comprehensive description of the synthesis and tuning strategies employed for nanozymes has been provided. This includes a detailed elucidation of their catalytic mechanisms in alignment with the fundamental principles of enhanced sensing technology, accompanied by the presentation of quantitatively analyzed results. Moreover, the diverse applications of nanozymes in strip sensing, colorimetric sensing, electrochemical sensing, and surface-enhanced Raman scattering have been outlined. Additionally, the limitations, challenges, and corresponding recommendations concerning the application of nanozymes in biosensing have been summarized. Furthermore, insights have been offered into the future development and outlook of nanozymes for biosensing. This review aims to serve not only as a reference for enhancing the sensitivity of nanozyme-based biosensors but also as a catalyst for exploring nanozyme properties and their broader applications in biosensing.

Emerging trends in nanomaterial design for the development of point-of-care platforms and practical applications

Nanomaterials and nanotechnology offer promising opportunities in point-of-care (POC) diagnostics and therapeutics due to their unique physical and chemical properties. POC platforms aim to provide rapid and portable diagnostic and therapeutic capabilities at the site of patient care, offering cost-effective solutions. Incorporating nanomaterials with distinct optical, electrical, and magnetic properties can revolutionize the POC industry, significantly enhancing the effectiveness and efficiency of diagnostic and theragnostic devices. By leveraging nanoparticles and nanofibers in POC devices, nanomaterials have the potential to improve the accuracy and speed of diagnostic tests, making them more practical for POC settings. Technological advancements, such as smartphone integration, imagery instruments, and attachments, complement and expand the application scope of POCs, reducing invasiveness by enabling analysis of various matrices like saliva and breath. These integrated testing platforms facilitate procedures without compromising diagnosis quality. This review provides a summary of recent trends in POC technologies utilizing nanomaterials and nanotechnologies for analyzing disease biomarkers. It highlights advances in device development, nanomaterial design, and their applications in POC. Additionally, complementary tools used in POC and nanomaterials are discussed, followed by critical analysis of challenges and future directions for these technologies.

Exploring the potential of paper-based electrokinetic phenomena in PoC biosensing

In order to decentralize health care, the development of point-of-care (PoC) assays has gained significant attention in recent decades. The lateral flow immunoassay (LFIA) has emerged as a promising bioanalytical method due to its low cost and single-step detection process. However, its limited sensitivity and inability to detect disease biomarkers at clinically relevant levels have hindered its application for early diagnosis. This review explores the potential of merging different electrokinetic phenomena into paper-based assays to enhance their analytical performance, offering a versatile and affordable approach for PoC testing. The review exposes the challenges faced in integrating electrokinetic phenomena with paper-based biosensing and concludes by discussing the issues that need to be improved to maximize the potential of this technology for early diagnosis.

μPADs on Centrifugal Microfluidic Discs for Rapid Sample-to-Answer Salivary Diagnostics

A fully integrated device for salivary detection with a sample-in-answer-out fashion is critical for noninvasive point-of-care testing (POCT), especially for the screening of contagious disease infection. Microfluidic paper-based analytical devices (μPADs) have demonstrated their huge potential in POCT due to their low cost and easy adaptation with other components. This study developed a generic POCT platform by integrating a centrifugal microfluidic disc with μPADs to realize sample-to-answer salivary diagnostics. Specifically, a custom centrifugal microfluidic disc integrated with μPADs is fabricated, which demonstrated a high efficiency in saliva treatment. To demonstrate the capability of the integrated device for salivary analysis, the SARS-CoV-2 Nucleocapsid (N) protein, a reliable biomarker for SARS-CoV-2 acute infection, is used as the model analyte. By the chemical treatment of the μPAD surface, and by optimizing the protein immobilization conditions, the on-disc μPADs were able to detect the SARS-CoV-2 N protein down to 10 pg mL-1 with a dynamic range of 10-1000 pg mL-1 and an assay time of 8 min. The integrated device was successfully used for the quantification of the N protein of pseudovirus in saliva with high specificity and demonstrated a comparable performance to the commercial paper lateral flow assay test strips.

Recent Progresses in Optical Biosensors for Interleukin 6 Detection

Interleukin 6 (IL-6) is pleiotropic cytokine with pathological pro-inflammatory effects in various acute, chronic and infectious diseases. It is involved in a variety of biological processes including immune regulation, hematopoiesis, tissue repair, inflammation, oncogenesis, metabolic control, and sleep. Due to its important role as a biomarker of many types of diseases, its detection in small amounts and with high selectivity is of particular importance in medical and biological fields. Laboratory methods including enzyme-linked immunoassays (ELISAs) and chemiluminescent immunoassays (CLIAs) are the most common conventional methods for IL-6 detection. However, these techniques suffer from the complexity of the method, the expensiveness, and the time-consuming process of obtaining the results. In recent years, too many attempts have been conducted to provide simple, rapid, economical, and user-friendly analytical approaches to monitor IL-6. In this regard, biosensors are considered desirable tools for IL-6 detection because of their special features such as high sensitivity, rapid detection time, ease of use, and ease of miniaturization. In this review, current progresses in different types of optical biosensors as the most favorable types of biosensors for the detection of IL-6 are discussed, evaluated, and compared.

New technologies and reagents in lateral flow assay (LFA) designs for enhancing accuracy and sensitivity

Lateral flow assays (LFAs) are a popular method for quick and affordable diagnostic testing because they are easy to use, portable, and user-friendly. However, LFA design has always faced challenges regarding sensitivity, accuracy, and complexity of the operation. By integrating new technologies and reagents, the sensitivity and accuracy of LFAs can be improved while minimizing the complexity and potential for false positives. Surface enhanced Raman spectroscopy (SERS), photoacoustic techniques, fluorescence resonance energy transfer (FRET), and the integration of smartphones and thermal readers can improve LFA accuracy and sensitivity. To ensure reliable and accurate results, careful assay design and validation, appropriate controls, and optimization of assay conditions are necessary. Continued innovation in LFA technology is crucial to improving the reliability and accuracy of rapid diagnostic testing and expanding its applications to various areas, such as food testing, water quality monitoring, and environmental testing.

Post-Assay Chemical Enhancement for Highly Sensitive Lateral Flow Immunoassays: A Critical Review

Lateral flow immunoassay (LFIA) has found a broad application for testing in point-of-care (POC) settings. LFIA is performed using test strips-fully integrated multimembrane assemblies containing all reagents for assay performance. Migration of liquid sample along the test strip initiates the formation of labeled immunocomplexes, which are detected visually or instrumentally. The tradeoff of LFIA's rapidity and user-friendliness is its relatively low sensitivity (high limit of detection), which restricts its applicability for detecting low-abundant targets. An increase in LFIA's sensitivity has attracted many efforts and is often considered one of the primary directions in developing immunochemical POC assays. Post-assay enhancements based on chemical reactions facilitate high sensitivity. In this critical review, we explain the performance of post-assay chemical enhancements, discuss their advantages, limitations, compared limit of detection (LOD) improvements, and required time for the enhancement procedures. We raise concerns about the performance of enhanced LFIA and discuss the bottlenecks in the existing experiments. Finally, we suggest the experimental workflow for step-by-step development and validation of enhanced LFIA. This review summarizes the state-of-art of LFIA with chemical enhancement, offers ways to overcome existing limitations, and discusses future outlooks for highly sensitive testing in POC conditions.

Colloidal gold-based immunochromatographic biosensor for quantitative detection of S100B in serum samples

Traumatic brain injury has become a serious public health problem. Timely detection, diagnosis and treatment of brain injury are closely related to the prognosis of patients, so identification of highly sensitive and specific biochemical markers of brain injury has important clinical value. Currently, the most studied and most promising marker is the protein S100B. In this study, a rapid quantitative biosensor for S100B was established using colloidal gold labeling and double antibody (8C10-6B8) sandwich immunochromatography. The biosensor was capable of quantifying S100B within 15 min, and showed no cross-reactivity with S100A, NSE, GFAP, or PGP9.5. The detection limit was determined to be 4.6 pg mL-1 with a linear range of 0.01-2 ng mL-1. Recovery experiments also indicated that the method had an acceptable accuracy. Moreover, the quantitative colloidal gold assay correlated well with the results of a chemiluminescence immunoassay when testing 40 clinical serum samples. Our developed colloidal gold quantitative immunochromatographic biosensor is a rapid, sensitive, specific and accurate method for the detection of S100B protein in serum, which is useful in the clinic for early diagnosis, as well as assessment of disease progression and prognosis of traumatic brain injury.

Quantitative Point of Care Tests for Timely Diagnosis of Early-Onset Preeclampsia with High Sensitivity and Specificity

Preeclampsia is a heterogeneous and multiorgan cardiovascular disorder of pregnancy. Here, we report the development of a novel strip-based lateral flow assay (LFA) using lanthanide-doped upconversion nanoparticles conjugated to antibodies targeting two different biomarkers for detection of preeclampsia. We first measured circulating plasma FKBPL and CD44 protein concentrations from individuals with early-onset preeclampsia (EOPE), using ELISA. We confirmed that the CD44/FKBPL ratio is reduced in EOPE with a good diagnostic potential. Using our rapid LFA prototypes, we achieved an improved lower limit of detection: 10 pg ml-1 for FKBPL and 15 pg ml-1 for CD44, which is more than one order lower than the standard ELISA method. Using clinical samples, a cut-off value of 1.24 for CD44/FKBPL ratio provided positive predictive value of 100 % and the negative predictive value of 91 %. Our LFA shows promise as a rapid and highly sensitive point-of-care test for preeclampsia.

Spike- and nucleocapsid-based gold colloid assay toward the development of an adhesive bandage for rapid SARS-CoV-2 immune response detection and screening

Immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies are important biomarkers used for the diagnosis and screening of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections in both symptomatic and asymptomatic individuals. These antibodies are highly specific to the spike (S) and nucleocapsid (N) proteins of the SARS-CoV-2 virus. This paper outlines the development steps of a novel hybrid (vertical-lateral-vertical) flow assay in the form of a finger-stick point-of-care device, similar to an adhesive bandage, designed for the timely detection and screening of IgM and IgG immune responses to SARS-CoV-2 infections. The assay, comprising a vertically stacked plasma/serum separation membrane, conjugate pad, and detection (readout) zone, utilizes gold nanoparticles (AuNPs) conjugated with SARS-CoV-2 S and N proteins to effectively capture IgM and IgG antibodies from a pinprick (~15 µL) of blood in just one step and provides results of no immune IgM-/IgG-, early immune IgM+/IgG-, active immune IgM+/IgG+ or immune IgM-/IgG+ in a short amount of time (minutes). The adhesive bandage-like construction is an example of the design of rapid, low-cost, disposable, and easy-to-use tests for large-scale detection and screening in households. Furthermore, the bandage can be easily adjusted and optimized to detect different viral infections as they arise by simply selecting appropriate antigens related to pandemics and outbreaks.

Photoelectrochemical sensors based on paper and their emerging applications in point-of-care testing

Point-of-care testing (POCT) technology is urgently required owing to the prevalence of the Internet of Things and portable electronics. In light of the attractive properties of low background and high sensitivity caused by the complete separation of excitation source and detection signal, the paper-based photoelectrochemical (PEC) sensors, featured with fast in analysis, disposable and environmental-friendly have become one of the most promising strategies in POCT. Therefore, in this review, the latest advances and principal issues in the design and fabrication of portable paper-based PEC sensors for POCT are systematically discussed. Primarily, the flexible electronic devices that can be constructed by paper and the reasons why they can be used in PEC sensors are expounded. Afterwards, the photosensitive materials involved in paper-based PEC sensor and the signal amplification strategies are emphatically introduced. Subsequently, the application of paper-based PEC sensors in medical diagnosis, environmental monitoring and food safety are further discussed. Finally, the main opportunities and challenges of paper-based PEC sensing platforms for POCT are briefly summarized. It provides a distinct perspective for researchers to construct paper-based PEC sensors with portable and cost-effective, hoping to enlighten the fast development of POCT soon after, as well as benefit human society.

Highly Sensitive Immunochromatographic Detection of Zearalenone Based on Ultrabright Red-Emitted Aggregation-Induced Luminescence Nanoprobes

Highly luminescent nanospheres have been demonstrated in enhancing the sensitivity of lateral flow immunoassay (LFIA) due to their loading numerous luminescent dyes. However, the photoluminescence intensities of existing luminescent nanospheres are limited due to the aggregation-caused quenching effect. Herein, highly luminescent aggregation-induced emission luminogens embedded nanospheres (AIENPs) with red emission were introduced as signal amplification probes of LFIA for quantitative detection of zearalenone (ZEN). Optical properties of red-emitted AIENPs were compared with time-resolved dye-embedded nanoparticles (TRNPs). Results showed that red-emitted AIENPs have stronger photoluminescence intensity on the nitrocellulose membrane and superior environmental tolerance. Additionally, we benchmarked the performance of AIENP-LFIA against TRNP-LFIA using the same set of antibodies, materials, and strip readers. Results showed that AIENP-LFIA exhibits good dynamic linearity with the ZEN concentration from 0.195 to 6.25 ng/mL, with half competitive inhibitory concentration (IC₅₀) and detection of limit (LOD) at 0.78 and 0.11 ng/mL, respectively. The IC₅₀ and LOD are 2.07- and 2.36-fold lower than those of TRNP-LFIA. Encouragingly, the precision, accuracy, specificity, practicality, and reliability of this AIENP-LFIA for ZEN quantitation were further characterized. The results verified that the AIENP-LFIA has good practicability for the rapid, sensitive, specific, and accurate quantitative detection of ZEN in corn samples.

Cell-Free Dot Blot: an Ultra-Low-Cost and Practical Immunoassay Platform for Detection of Anti-SARS-CoV-2 Antibodies in Human and Animal Sera

Since its emergence in late 2019, the coronavirus disease 2019 (COVID-19) pandemic has caused severe disruption to key aspects of human life globally and highlighted the need for timely, adaptive, and accessible pandemic response strategies. Here, we introduce the cell-free dot blot (CFDB) method, a practical and ultra-low-cost immune diagnostic platform capable of rapid response and mass immunity screening for the current and future pandemics. Similar in mechanism to the widely used enzyme-linked immunosorbent assays (ELISAs), our method is novel and advantageous in that (i) it uses linear DNA to produce the target viral antigen fused to a SpyTag peptide in a cell-free expression system without the need for traditional cloning and antigen purification, (ii) it uses SpyCatcher2-Apex2, an Escherichia coli-produced peroxidase conjugate as a universal secondary detection reagent, obviating the need for commercial or sophisticated enzyme conjugates, and (iii) sera are spotted directly on a nitrocellulose membrane, enabling a simple "dipping" mechanism for downstream incubation and washing steps, as opposed to individual processing of wells in a multiwell plate. To demonstrate the utility of our method, we performed CFDB to detect anti-severe acute respiratory syndrome coronavirus 2 nucleocapsid protein antibodies in precharacterized human sera (23 negative and 36 positive for COVID-19) and hamster sera (16 negative and 36 positive for COVID-19), including independent testing at a collaborating laboratory, and we show assay performance comparable to that of conventional ELISAs. At a similar capacity to 96-well plate ELISA kits, one CFDB assay costs only ~$3 USD. We believe that CFDB can become a valuable pandemic response tool for adaptive and accessible sero-surveillance in human and animal populations. IMPORTANCE The recent COVID-19 pandemic has highlighted the need for diagnostic platforms that are rapidly adaptable, affordable, and accessible globally, especially for low-resource settings. To address this need, we describe the development and functional validation of a novel immunoassay technique termed the cell-free dot blot (CFDB) method. Based on the principles of cell-free synthetic biology and alternative dot blotting procedures, our CFDB immunoassay is designed to provide for timely, practical, and low-cost responses to existing and emerging public health threats, such as the COVID-19 pandemic, at a similar throughput and comparable performance as conventional ELISAs. Notably, the molecular detection reagents used in CFDB can be produced rapidly in-house, using established protocols and basic laboratory infrastructure, minimizing reliance on strained commercial reagents. In addition, the materials and imaging instruments required for CFDB are the same as those used for common Western blotting experiments, further expanding the reach of CFDB in decentralized facilities.

Progression of Paper-Based Point-of-Care Testing toward Being an Indispensable Diagnostic Tool in Future Healthcare

Point-of-care (POC) diagnostics in particular focuses on the timely identification of harmful conditions close to the patients' needs. For future healthcare these diagnostics could be an invaluable tool especially in a digitalized or telemedicine-based system. However, while paper-based POC tests, with the most prominent example being the lateral flow assay (LFA), have been especially successful due to their simplicity and timely response, the COVID-19 pandemic highlighted their limitations, such as low sensitivity and ambiguous responses. This perspective discusses strategies that are currently being pursued to evolve such paper-based POC tests toward a superior diagnostic tool that provides high sensitivities, objective result interpretation, and multiplexing options. Here, we pinpoint the challenges with respect to (i) measurability and (ii) public applicability, exemplified with select cases. Furthermore, we highlight promising endeavors focused on (iii) increasing the sensitivity, (iv) multiplexing capability, and (v) objective evaluation to also ready the technology for integration with machine learning into digital diagnostics and telemedicine. The status quo in academic research and industry is outlined, and the likely highly relevant role of paper-based POC tests in future healthcare is suggested.

Triple Enhancement for Sensitive Immunochromatographic Assay: A Case Study for Human Fatty Acid-Binding Protein Detection

The work considers a combination of three enhancing approaches for immunochromatographic assay (ICA) and the integration of their impacts into changes of the limit of detection (LOD). Human fatty acid binding protein (FABP), an early biomarker of acute myocardial infarction, was the target analyte. Starting from the common ICA protocol with an LOD equal to 11.2 ng/mL, three approaches were realized: (1) replacement of spherical gold nanoparticles with gold nanoflowers having a branched surface (20-fold lowering the LOD); (2) enhanced labeling of immune complexes via nanoparticle aggregates (15-fold lowering); (3) in-situ growth of bound nanoparticles by reduction of gold salts (3-fold lowering). Single and combined implementations of these approaches have been studied. It has been shown that the LOD decrease for combined approaches is close to the multiplied contribution of each of them. The final LOD for FABP was 0.05 ng/mL, which is 220 times lower than the LOD for the common ICA protocol. The efficiency of the enhanced ICA with three combined approaches was confirmed by testing human serum samples for FABP presence and content. The development presents a new efficient technique for rapid sensitive detection of FABP for medical diagnostics. Moreover, the demonstrated multiple enhancements could be applied for various demanded analytes.

Multiplexed lateral flow assay integrated with orthogonal CRISPR-Cas system for SARS-CoV-2 detection

The development of field-deployable detection platform amenable for multiplexed genes testing will significantly improve the efficiency and reliability during point-of-care testing (POCT) applications. In this regard, an orthogonal CRISPR-Cas-mediated multiplexed lateral flow assay (designated as OC-MLFA) is proposed for SARS-CoV-2 genome detection. Taking the advantage of activation and cleavage preferences between Cas12a and Cas13a, orthogonal (two-independent-channel signal readout) CRISPR-Cas system is investigated. Lateral flow strips with two target lines are designed to accommodate the orthogonal CRISPR system. The interference between Cas12a and Cas13a channels can be effectively eliminated via the elaborate nucleic acids and lateral flow strips design. The high preamplification efficiency from reverse transcription recombinase polymerase amplification (RT-RPA) and Cas enzyme mediated trans-cleavage process bring the sensitivity of our OC-MLFA method to 10 copies per test (30 μL). Nasopharyngeal swab clinical samples with different cycle threshold (Ct) values according to the RT-PCR method were analyzed with the proposed OC-MLFA, during which 76 out of 76 detection accuracy was obtained. Featured with the multiplexed genes detection simultaneously in one reaction and colorimetric readout through single strip, the OC-MLFA we proposed herein ensures great accuracy and efficiency, which endows promising field-deployable POCT application feasibility.

Magnetic Bead Handling Using a Paper-Based Device for Quantitative Point-of-Care Testing

Microfluidic paper-based analytical devices (μPADs) have been extensively proposed as ideal tools for point-of-care (POC) testing with minimal user training and technical requirements. However, most μPADs use dried bioreagents, which complicate production, reduce device reproducibility and stability, and require transport and storage under temperature and humidity-controlled conditions. In this work, we propose a μPAD produced using an affordable craft-cutter and stored at room temperature, which is used to partially automate a single-step colorimetric magneto-immunoassay. As a proof-of-concept, the μPAD has been applied to the quantitative detection of Plasmodium falciparum lactate dehydrogenase (Pf-LDH), a biomarker of malaria infection. In this system, detection is based on a single-step magneto-immunoassay that consists of a single 5-min incubation of the lysed blood sample with immuno-modified magnetic beads (MB), detection antibody, and an enzymatic signal amplifier (Poly-HRP). This mixture is then transferred to a single-piece paper device where, after on-chip MB magnetic concentration and washing, signal generation is achieved by adding a chromogenic enzyme substrate. The colorimetric readout is achieved by the naked eye or using a smartphone camera and free software for image analysis. This μPAD afforded quantitative Pf-LDH detection in <15 min, with a detection limit of 6.25 ng mL−1 when the result was interpreted by the naked eye and 1.4 ng mL−1 when analysed using the smartphone imaging system. Moreover, the study of a battery of clinical samples revealed concentrations of Pf-LDH that correlated with those provided by the reference ELISA and with better sensitivity than a commercial rapid diagnostic test (RDT). These results demonstrate that magneto-immunoassays can be partly automated by employing a μPAD, achieving a level of handling that approaches the requirements of POC testing.

Digital Quantification Method for Sensitive Point-of-Care Detection of Salivary Uric Acid Using Smartphone-Assisted μPADs

Uric acid (UA) is an important biomarker for many diseases. A sensitive point-of-care (POC) testing platform is designed for the digital quantification of salivary UA based on a colorimetric reaction on an easy-to-build smartphone-assisted microfluidic paper-based analytical device (SμPAD). UA levels are quantified according to the color intensity of Prussian blue on the SμPAD with the aid of a MATLAB code or a smartphone APP. A color correction method is specifically applied to exclude the light effect. Together with the engineering design of SμPADs, the background calibration function with the APP increases the UA sensitivity by 100-fold to reach 0.1 ppm with a linear range of 0.1-200 ppm. The assay time is less than 10 min. SμPADs demonstrate a correlation of 0.97 with a commercial UA kit for the detection of salivary UA in clinical samples. SμPADs provide a sensitive, fast, affordable, and reliable tool for the noninvasive POC quantification of salivary UA for early diagnosis of abnormal UA level-associated health conditions.

Cascade-Enhanced Lateral Flow Immunoassay for Sensitive Detection of Okadaic Acid in Seawater, Fish, and Seafood

In this investigation, a new approach for developing a sensitive lateral flow immunoassay (LFIA) was proposed for the detection of the hazardous marine toxin okadaic acid (OA). It is based on the indirect format with anti-species antibodies labeled by gold nanoparticles (AuNPs) and cascade signal amplification. The latter is performed by first passing a mixture of anti-OA antibodies and a tested sample along the immunochromatographic test strip and then performing several cycles of the interaction of anti-species antibodies conjugated with AuNPs with free antibodies, which bind to anti-species antibodies but are not specific to the target analyte. As a result, branched aggregates are formed, due to which the colorimetric signal intensification occurs. The developed test system enabled the detection of OA with an instrumental detection limit of 30 pg/mL and a cutoff of 1 ng/mL, which exceeds these characteristics in the LFIA without amplification by 7 and 2 times, respectively. The OA recoveries from seawater, fish, and seafood varied from 76.9% to 126%. The test system may be required for point-of-care monitoring of samples for phycotoxin contamination; the developed principle of signal amplification can be used in cases where highly sensitive detection of trace amounts of a contaminant is required.

Diagnosis of disease relevant nucleic acid biomarkers with off-the-shelf devices

Changes in the level of nucleic acids in blood may be correlated with some clinical disorders like cancer, stroke, trauma and autoimmune diseases, and thus, nucleic acids can serve as potential biomarkers for pathological processes. The requirement of technical equipment and operator expertise in effective information readout of modern molecular diagnostic technologies significantly restricted application outside clinical laboratories. The ability to detect nucleic acid biomarkers with off-the-shelf devices, which have the advantages of portability, simplicity, low cost and short response time, is critical to provide a prompt clinical result in circumstances where the laboratory instruments are not available. This review throws light on the current strategies and challenges for nucleic acid diagnosis with commercial portable devices, indicating the future prospect of portable diagnostic devices and making a great difference in improving the healthcare and disease surveillance in resource-limited areas.

Expanded detection range of lateral flow immunoassay endowed with a third-stage amplifier indirect probe

Here, a third-stage amplifier indirect probe (TsAIP) based lateral flow immunoassay (LFIA) was proposed to detect furazolidone (FZD) with Prussian blue nanoparticles (PBNPs) as carrier to label the goat anti-mouse antibody-horseradish peroxidase conjugation [GAMA(HRP)]. In this strategy, owing to the fact that one monoclonal antibody (mAb) can combine several GAMA molecules simultaneously, the indirect probe can generate primary signal amplification, then realize second-stage amplification attributing to PBNPs, and finally achieve third-stage amplification because of the conjugated HRP. The TsAIP-based LFIA shows improved performance for FZD metabolite derivative with a detection limit of 1 ng mL^-1. The detection range is expanded about 2-fold compared with the original outcome. Besides, the proposed sensor could be successfully applied in food samples. This method provides a platform for broadening the detection range and application of PBNPs based LFIAs.

Plasmon color-preserved gold nanoparticle clusters for high sensitivity detection of SARS-CoV-2 based on lateral flow immunoassay

Lateral flow immunoassays (LFI) have shown great promise for point-of-care (POC) sensing applications, however, its clinical translation is often hindered by insufficient sensitivity for early detection of diseases, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This is mainly due to weak absorption signals of single gold nanoparticles (AuNPs). Here, we developed AuNP clusters that maintain the red color of isolated individual AuNPs, but increase the colorimetric readout to improve the detection sensitivity. The plasmon color-preserved (PLASCOP) AuNP clusters is simply made by mixing streptavidin-coated AuNP core with satellite AuNPs coated with biotinylated antibodies. The biotinylated antibody-streptavidin linker forms a gap size over 15 nm to avoid plasmon coupling between AuNPs, thus maintaining the plasmonic color while increasing the overall light absorption. LFI sensing using PLASCOP AuNP clusters composed of 40 nm AuNPs showed a high detection sensitivity for SARS-CoV-2 nucleocapsid proteins with a limit of detection (LOD) of 0.038 ng mL⁻¹, which was 23.8- and 5.9-times lower value than that of single 15 nm and 40 nm AuNP conjugates, respectively. The PLASCOP AuNP clusters-based LFI sensing also shows good specificity for SARS-CoV-2 nucleocapsid proteins from other influenza and coronaviruses. In a clinical feasibility test, we demonstrated that SARS-CoV-2 particles spiked in human saliva could be detected with an LOD of 54 TCID₅₀ mL⁻¹. The developed PLASCOP AuNP clusters are promising colorimetric sensing reporters that present improved sensitivity in LFI sensing for broad POC sensing applications beyond SARS-CoV-2 detection.

Enhancement of the Detection Performance of Paper-Based Analytical Devices by Nanomaterials

Paper-based analytical devices (PADs), including lateral flow assays (LFAs), dipstick assays and microfluidic PADs (μPADs), have a great impact on the healthcare realm and environmental monitoring. This is especially evident in developing countries because PADs-based point-of-care testing (POCT) enables to rapidly determine various (bio)chemical analytes in a miniaturized, cost-effective and user-friendly manner. Low sensitivity and poor specificity are the main bottlenecks associated with PADs, which limit the entry of PADs into the real-life applications. The application of nanomaterials in PADs is showing great improvement in their detection performance in terms of sensitivity, selectivity and accuracy since the nanomaterials have unique physicochemical properties. In this review, the research progress on the nanomaterial-based PADs is summarized by highlighting representative recent publications. We mainly focus on the detection principles, the sensing mechanisms of how they work and applications in disease diagnosis, environmental monitoring and food safety management. In addition, the limitations and challenges associated with the development of nanomaterial-based PADs are discussed, and further directions in this research field are proposed.

An Overview for the Nanoparticles-Based Quantitative Lateral Flow Assay

The development of the lateral flow assay (LFA) has received much attention in both academia and industry because of their broad applications to food safety, environmental monitoring, clinical diagnosis, and so forth. The user friendliness, low cost, and easy operation are the most attractive advantages of the LFA. In recent years, quantitative detection has become another focus of LFA development. Here, the most recent studies of quantitative LFAs are reviewed. First, the principles and corresponding formats of quantitative LFAs are introduced. In the biomaterial and nanomaterial sections, the detection, capture, and signal amplification biomolecules and the optical, fluorescent, luminescent, and magnetic labels used in LFAs are described. The invention of dedicated strip readers has drawn further interest in exploiting the better performance of LFAs. Therefore, next, the development of dedicated reader devices is described and the usefulness and specifications of these devices for LFAs are discussed. Finally, the applications of LFAs in the detection of metal ions, biotoxins, pathogenic microorganisms, veterinary drugs, and pesticides in the fields of food safety and environmental health and the detection of nucleic acids, biomarkers, and viruses in clinical analyses are summarized.

Sensitive Colorimetric Detection of Interleukin-6 via Lateral Flow Assay Incorporated Silver Amplification Method

Interleukin-6 (IL-6) is a pro/anti-inflammatory cytokine, the quantitative detection of which has been extensively considered for diagnosis of inflammatory associated diseases. However, there has not yet been a reliable, low-cost, and user-friendly platform developed for point-of-care (POC) detection of IL-6, which will eliminate the conventional costly, time-consuming, and complex assays. In this work, we developed a lateral flow assay for colorimetric detection of IL-6, using anti-IL-6 antibodies conjugated to gold nanoparticles (AuNPs) as the detection probes. Silver amplification technique was incorporated with the newly developed assay in order to enhance the obtained colorimetric signals, allowing sensitive detection of IL-6 in human serum in the desired physiological ranges (i.e., 5–1000 pg/mL). A limit of detection of 5 pg/mL could be achieved for IL-6 detection in serum with the amplification step which was not achievable in the standard assay. The corresponding specificity and reproducibility tests were all preformed to confirm the reliability of this assay for quantitative measurement of IL-6 in a POC manner.

Comparative Study of Four Coloured Nanoparticle Labels in Lateral Flow Immunoassay

The detection limit of lateral flow immunoassay (LFIA) is largely determined by the properties of the label used. We compared four nanoparticle labels differing in their chemical composition and colour: (1) gold nanoparticles (Au NPs), red; (2) Au-core/Pt-shell nanoparticles (Au@Pt NPs), black; (3) latex nanoparticles (LPs), green; and (4) magnetic nanoparticles (MPs), brown. The comparison was carried out using one target analyte—Erwinia amylovora, the causal bacterial agent of fire blight. All nanoparticles were conjugated with antibodies through methods that provide maximum functional coverage like physical adsorption (Au NPs, Au@Pt NPs) and covalent bonding (LPs, MPs). All conjugates demonstrated the same ability to bind with E. amylovora through enzyme-linked immunosorbent assay where optical properties of the nanoparticles do not determine the registered signal. However, half-maximal binding was achieved at different numbers of nanoparticles because they differ in size. All conjugates based on four nanoparticle labels were used for lateral flow assays. As a result, Au@Pt NPs provided the minimal detection limit that corresponded to 10³ CFU/mL. Au NPs and LPs detected 10⁴ CFU/mL, and MPs detected 10⁵ CFU/mL. The results highlight that simply choosing a coloured label can significantly affect the detection limit of LFIA.

Multiplexed Profiling of Extracellular Vesicles for Biomarker Development

Extracellular vesicles (EVs) are cell-derived membranous particles that play a crucial role in molecular trafficking, intercellular transport and the egress of unwanted proteins. They have been implicated in many diseases including cancer and neurodegeneration. EVs are detected in all bodily fluids, and their protein and nucleic acid content offers a means of assessing the status of the cells from which they originated. As such, they provide opportunities in biomarker discovery for diagnosis, prognosis or the stratification of diseases as well as an objective monitoring of therapies. The simultaneous assaying of multiple EV-derived markers will be required for an impactful practical application, and multiplexing platforms have evolved with the potential to achieve this. Herein, we provide a comprehensive overview of the currently available multiplexing platforms for EV analysis, with a primary focus on miniaturized and integrated devices that offer potential step changes in analytical power, throughput and consistency.

Point-of-care COVID-19 diagnostics powered by lateral flow assay

Since its first discovery in December 2019, the global coronavirus disease 2019 (COVID-19) pandemic caused by the novel coronavirus (SARS-CoV-2) has been posing a serious threat to human life and health. Diagnostic testing is critical for the control and management of the COVID-19 pandemic. In particular, diagnostic testing at the point of care (POC) has been widely accepted as part of the post restriction COVID-19 control strategy. Lateral flow assay (LFA) is a popular POC diagnostic platform that plays an important role in controlling the COVID-19 pandemic in industrialized countries and resource-limited settings. Numerous pioneering studies on the design and development of diverse LFA-based diagnostic technologies for the rapid diagnosis of COVID-19 have been done and reported by researchers. Hundreds of LFA-based diagnostic prototypes have sprung up, some of which have been developed into commercial test kits for the rapid diagnosis of COVID-19. In this review, we summarize the crucial role of rapid diagnostic tests using LFA in targeting SARS-CoV-2-specific RNA, antibodies, antigens, and whole virus. Then, we discuss the design principle and working mechanisms of these available LFA methods, emphasizing their clinical diagnostic efficiency. Ultimately, we elaborate the challenges of current LFA diagnostics for COVID-19 and highlight the need for continuous improvement in rapid diagnostic tests.

A Customized Microfluidic Paper-Based Platform for Colorimetric Immunosensing: Demonstrated via hCG Assay for Pregnancy Test

Over the past decades, paper-based lateral flow immunoassays (LFIAs) have been extensively developed for rapid, facile, and low-cost detection of a wide array of target analytes in a point-of-care manner. Conventional home pregnancy tests are the most significant example of LFAs, which detect elevated concentrations of human chorionic gonadotrophin (hCG) in body fluids to identify early pregnancy. In this work, we have upgraded these platforms to a higher version by developing a customized microfluidic paper-based analytical device (μPAD), as the new generation of paper-based point-of-care platforms, for colorimetric immunosensing. This will offer a cost-efficient and environmentally friendly alternative platform for paper-based immunosensing, eliminating the need for nitrocellulose (NC) membrane as the substrate material. The performance of the developed platform is demonstrated by detection of hCG (as a model case) in urine samples and subsequently indicating positive or negative pregnancy. A dual-functional silane-based composite was used to treat filter paper in order to enhance the colorimetric signal intensity in the detection zones of μPADs. In addition, microfluidic pathways were designed in a manner to provide the desired regulated fluid flow, generating sufficient incubation time (delays) at the designated detection zones, and consequently enhancing the obtained signal intensity. The presented approaches allow to overcome the existing limitations of μPADs in immunosensing and will broaden their applicability to a wider range of assays. Although, the application of the developed hCG μPAD assay is mainly in qualitative (i.e., positive or negative) detection of pregnancy, the semi-quantitative measurement of hCG was also investigated, indicating the viability of this assay for sensitive detection of the target hCG analyte within the related physiological range (i.e., 10-500 ng/mL) with a LOD value down to 10 ng/mL.

Smartphone-Enabled Personalized Diagnostics: Current Status and Future Prospects

Smartphones are becoming increasingly versatile thanks to the wide variety of sensor and actuator systems packed in them. Mobile devices today go well beyond their original purpose as communication devices, and this enables important new applications, ranging from augmented reality to the Internet of Things. Personalized diagnostics is one of the areas where mobile devices can have the greatest impact. Hitherto, the camera and communication abilities of these devices have been barely exploited for point of care (POC) purposes. This short review covers the recent evolution of mobile devices in the area of POC diagnostics and puts forward some ideas that may facilitate the development of more advanced applications and devices in the area of personalized diagnostics. With this purpose, the potential exploitation of wireless power and actuation of sensors and biosensors using near field communication (NFC), the use of the screen as a light source for actuation and spectroscopic analysis, using the haptic module to enhance mass transport in micro volumes, and the use of magnetic sensors are discussed.

Paper and Other Fibrous Materials-A Complete Platform for Biosensing Applications

Paper-based analytical devices (PADs) and Electrospun Fiber-Based Biosensors (EFBs) have aroused the interest of the academy and industry due to their affordability, sensitivity, ease of use, robustness, being equipment-free, and deliverability to end-users. These features make them suitable to face the need for point-of-care (POC) diagnostics, monitoring, environmental, and quality food control applications. Our work introduces new and experienced researchers in the field to a practical guide for fibrous-based biosensors fabrication with insight into the chemical and physical interaction of fibrous materials with a wide variety of materials for functionalization and biofunctionalization purposes. This research also allows readers to compare classical and novel materials, fabrication techniques, immobilization methods, signal transduction, and readout. Moreover, the examined classical and alternative mathematical models provide a powerful tool for bioanalytical device designing for the multiple steps required in biosensing platforms. Finally, we aimed this research to comprise the current state of PADs and EFBs research and their future direction to offer the reader a full insight on this topic.

Nanobiotechnology as a platform for the diagnosis of COVID-19: a review

A sensitive method for diagnosing coronavirus disease 2019 (COVID-19) is highly required to fight the current and future global health threats due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV 2). However, most of the current methods exhibited high false‐negative rates, resulting in patient misdiagnosis and impeding early treatment. Nanoparticles show promising performance and great potential to serve as a platform for diagnosing viral infection in a short time and with high sensitivity. This review highlighted the potential of nanoparticles as platforms for the diagnosis of COVID-19. Nanoparticles such as gold nanoparticles, magnetic nanoparticles, and graphene (G) were applied to detect SARS-CoV 2. They have been used for molecular-based diagnosis methods and serological methods. Nanoparticles improved specificity and shorten the time required for the diagnosis. They may be implemented into small devices that facilitate the self-diagnosis at home or in places such as airports and shops. Nanoparticles-based methods can be used for the analysis of virus-contaminated samples from a patient, surface, and air. The advantages and challenges were discussed to introduce useful information for designing a sensitive, fast, and low-cost diagnostic method. This review aims to present a helpful survey for the lesson learned from handling this outbreak to prepare ourself for future  pandemic.

Development of a quantum dot nanobead-based fluorescent strip immunosensor for on-site detection of aflatoxin B1 in lotus seeds

Owing to the serious threat of aflatoxin B₁ (AFB₁) to public health, development of a reliable method for accurate determination of it is extremely necessary and urgent. In this study, a simple, rapid and highly-sensitive quantum dot nanobeads (QBs) based lateral flow fluorescent strip immunosensor was developed for on-site detection of AFB₁ in edible and medicinal lotus seeds. Carboxylated QBs were used as the fluorescent markers to prepare the fluorescent probe through coupling QBs with anti-AFB1 antibodies. Bovine serum albumin (BSA)-AFB₁ antigens and goat anti-mouse IgG antibodies were coated on the nitrocellulose (NC) membrane to prepare the test (T) and control (C) lines, respectively. Qualitative analysis of AFB₁ was realized by naked eye, and the quantitative determination was achieved with a portable strip reader. Results showed that the newly-developed test strip sensor could achieve rapid detection of AFB₁ within 15 min, allowing a limit of detection (LOD) of 1 ng/mL (2 μg/kg) and a linear range of 1-19 ng/mL (2-38 μg/kg). Recovery rates from the fortified lotus seeds with low, medium and high spiking concentrations (2.5, 5 and 10 μg/kg) ranged from 94.0% to 116.0% with relative standard deviations less than 10%. All the results were confirmed by a standard LC-MS/MS method. The QBs-based fluorescent strip immunosensor with high sensitivity, easy operation, and low cost provided a preferred solution for rapid, on-site screening and highly-sensitive quantitation of AFB₁ in a large number of lotus seed samples.