Battery operated preconcentration-assisted lateral flow assay

Lateral flow assays: Progress and evolution of recent trends in point-of-care applications

Paper based point-of-care (PoC) detection platforms applying lateral flow assays (LFAs) have gained paramount approval in the diagnostic domain as well as in environmental applications owing to their ease of utility, low cost, and rapid signal readout. It has centralized the aspect of self-evaluation exhibiting promising potential in the last global pandemic era of Covid-19 implementing rapid management of public health in remote areas. In this perspective, the present review is focused towards landscaping the current framework of LFAs along with integration of components and characteristics for improving the assay by pushing the detection limits. The review highlights the synergistic aspects of assay designing, sample enrichment strategies, novel nanomaterials-based signal transducers, and high-end analytical techniques that contribute significantly towards sensitivity and specificity enhancement. Various recent studies are discussed supporting the innovations in LFA systems that focus upon the accuracy and reliability of rapid PoC testing. The review also provides a comprehensive overview of all the possible difficulties in commercialization of LFAs subjecting its applicability to pathogen surveillance, water and food testing, disease diagnostics, as well as to agriculture and environmental issues.

Direct comparison of colorimetric signal amplification techniques in lateral flow immunoassays

The lateral flow immunoassay (LFIA) is widely adopted for point-of-care testing, but its limit of detection (LoD) falls short of that of laboratory-based immunoassays. Several techniques have been proposed to enhance the LoD of LFIAs using visual colorimetric readouts, yet a direct comparison of the LoDs achieved by these techniques has not been performed. In this work, we measure the LoDs of LFIAs designed for the detection of a malaria protein, PfHRP2, using four different colorimetric signal generation techniques: (i) AuNP(40 nm)-tagged detection antibodies (base case), (ii) AuNP-based enhancement of AuNP(40 nm) signal, (iii) oxidation of chloronaphthol/diaminobenzidine (CN/DAB) using HRP-tagged detection antibodies, and (iv) oxidation of CN/DAB using polyHRP(400)-tagged detection antibodies. The LoDs and the 95% confidence intervals of the LoDs achieved by the 4 techniques were 19.34 (13.37-27.62) ng mL-1, 9.57 (6.76-13.28) ng mL-1, 21.57 (14.26-32.18) ng mL-1, and 6.09 (2.23-13.47) ng mL-1, respectively. Contrary to popular perception, enzymatic signal generation using HRP-tagged detection antibodies did not improve the LoD compared to the base case of AuNP-based signal generation. Further studies revealed that the very high extinction coefficient of gold nanoparticles renders them an excellent choice for colorimetric detection, surpassing the performance of enzymatic signal generation using HRP-tagged antibodies. However, enzymatic signal generation using polyHRP-tagged antibodies improved the LoD compared to the base case. These results show that enzymatic signal amplification should not be a priori assumed to be superior to AuNP-based signal generation; and provide a reference point to LFIA developers to select an appropriate signal generation modality.

Laser-printed paper ELISA and hydroxyapatite immobilization for colorimetric congenital anomalies screening in saliva

BACKGROUND

Alpha-fetoprotein (AFP) is a fetal protein that can indicate congenital anomalies such as Down syndrome and spinal canal blockage when detected at abnormal levels in pregnant women. Current AFP detection methods rely on invasive blood or serum samples, which require sophisticated equipment. From the many solutions proposed, colorimetric paper-based assays excel in point-of-care settings. The concept of paper-based ELISA (p-ELISA) enhances traditional methods, aligning with the ASSURED criteria for diagnostics in resource-limited regions. Despite success in microfluidic paper-based assay devices, laser printing remains underexplored for p-ELISA. Additionally, modifying the paper surface provides an additional layer of sensitivity enhancement.

RESULTS

In this study, we developed a novel laser-printed paper-based ELISA (LP-pELISA) for rapid, sensitive, and noninvasive detection of AFP in saliva samples. The LP-pELISA platform was fabricated by printing hydrophobic barriers on filter paper using a laser printer, followed by depositing hydroxyapatite (HAp) as an immobilization material for the antibodies. The colorimetric detection was achieved using AuNPs functionalized with anti-AFP antibodies and silver nitrate enhancement. The LP-pELISA exhibited a linear response for AFP detection in both buffer and saliva samples over a range of 1.0-800 ng mL-1, with a limit of detection (LOD) reaching 1.0 ng mL-1. The assay also demonstrated good selectivity, repeatability, reproducibility, and stability. The LP-pELISA was further validated by testing spiked human saliva samples, showing its potential for point-of-care diagnosis of congenital disabilities.

SIGNIFICANCE

The LP-pELISA is a noninvasive platform showcasing simplicity, cost-effectiveness, and user-friendliness, utilizing laser printing, hydroxyapatite modification, and saliva samples to efficiently detect AFP. Beyond its application for AFP, this method's versatility extends to other biomarkers, positioning it as a catalyst for the evolution of paper-based biosensors. The LP-pELISA holds promise as a transformative tool for point-of-care diagnostics, fostering advancements in healthcare with its innovative technology.

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.

Advancing diagnostic efficacy using a computer vision-assisted lateral flow assay for influenza and SARS-CoV-2 detection

Lateral flow assays (LFAs) have emerged as indispensable tools for point-of-care testing during the pandemic era. However, the interpretation of results through unassisted visual inspection by untrained individuals poses inherent limitations. In our study, we propose a novel approach that combines computer vision (CV) and lightweight machine learning (ML) to overcome these limitations and significantly enhance the performance of LFAs. By incorporating CV-assisted analysis into the LFA assay, we achieved a remarkable three-fold improvement in analytical sensitivity for detecting Influenza A and for SARS-CoV-2 detection. The obtained R2 values reached approximately 0.95, respectively, demonstrating the effectiveness of our approach. Moreover, the integration of CV techniques with LFAs resulted in a substantial amplification of the colorimetric signal specifically for COVID-19 positive patient samples. Our proposed approach, which incorporates a simple machine learning algorithm, provides substantial enhancements in assay sensitivity, improving diagnostic efficacy and accessibility of point-of-care testing without requiring significant additional resources. Moreover, the simplicity of the machine learning algorithm enables its standalone use on a mobile phone, further enhancing its practicality for point-of-care testing.

A simple agglutination system for rapid antigen detection from large sample volumes with enhanced sensitivity

Lateral flow assays (LFAs) provide a simple and quick option for diagnosis and are widely adopted for point-of-care or at-home tests. However, their sensitivity is often limited. Most LFAs only allow 50 μL samples while various sample types such as saliva could be collected in much larger volumes. Adapting LFAs to accommodate larger sample volumes can improve assay sensitivity by increasing the number of target analytes available for detection. Here, a simple agglutination system comprising biotinylated antibody (Ab) and streptavidin (SA) is presented. The Ab and SA agglutinate into large aggregates due to multiple biotins per Ab and multiple biotin binding sites per SA. Dynamic light scattering (DLS) measurements showed that the agglutinated aggregate could reach a diameter of over 0.5 μm and over 1.5 μm using poly-SA. Through both experiments and Monte Carlo modeling, we found that high valency and equivalent concentrations of the two aggregating components were critical for successful agglutination. The simple agglutination system enables antigen capture from large sample volumes with biotinylated Ab and a swift transition into aggregates that can be collected via filtration. Combining the agglutination system with conventional immunoassays, an agglutination assay is proposed that enables antigen detection from large sample volumes using an in-house 3D-printed device. As a proof-of-concept, we developed an agglutination assay targeting SARS-CoV-2 nucleocapsid antigen for COVID-19 diagnosis from saliva. The assay showed a 10-fold sensitivity enhancement when increasing sample volume from 50 μL to 2 mL, with a final limit of detection (LoD) of 10 pg mL-1 (∼250 fM). The assay was further validated in negative saliva spiked with gamma-irradiated SARS-CoV-2 and showed an LoD of 250 genome copies per μL. The proposed agglutination assay can be easily developed from existing LFAs to facilitate the processing of large sample volumes for improved sensitivity.

Affordable on-site COVID-19 test using non-powered preconcentrator

A simple, affordable point of care test (POCT) is necessary for on-site detection of coronavirus disease 2019 (COVID-19). The lateral flow assay (LFA) has great potential for use in POCT mainly because of factors such as low time consumption, low cost, and ease of use. However, it lacks sensitivity and limits of detection (LOD), which are essential for early diagnostics. In this study, we proposed a non-powered preconcentrator (NPP) based on nanoelectrokinetics for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Antigen (Ag) lateral flow assay. The non-powered preconcentrator is composed of glass fiber-based composite paper and ion permselective material, and it can be simply operated by force balancing gravitational, capillary, and depletion-induced forces. The proposed approach helps enrich the SARS-CoV-2 viral nucleocapsid (N) proteins based on a 10-min operation, and it improved the LOD by up to 10-fold. The corresponding virus enrichment, which was evaluated using the reverse-transcriptase polymerase chain reaction (RT-PCR), revealed an improvement in ΔCt values > 3. We successfully demonstrated the enhancement of the NPP-assisted LFA, we extended to applying it to clinical samples. Further, we demonstrated an affordable, easy-to-implement form of LFA by simply designing NPP directly on the LFA buffer tube.

Electrophoresis-Assisted Multilayer Assembly of Nanoparticles for Sensitive Lateral Flow Immunoassay

Lateral flow immunoassay (LFIA) is a rapid, simple, and inexpensive point-of-need method. A major limitation of LFIA is a high limit of detection (LOD), which impacts its diagnostic sensitivity. To overcome this limitation, we introduce a signal-enhancement procedure that is performed after completing LFIA and involves controllably moving biotin- and streptavidin-functionalized gold nanoparticles by electrophoresis. The nanoparticles link to immunocomplexes forming multilayer aggregates on the test strip, thus, enhancing the signal. Here, we demonstrate lowering the LOD of hepatitis B surface antigen from approximately 8 to 0.12 ng mL^-1 , making it clinically acceptable. Testing 118 clinical samples for hepatitis B showed that signal enhancement increased the diagnostic sensitivity of LFIA from 73 % to 98 % while not affecting its 95 % specificity. Electrophoresis-driven enhancement of LFIA is universal (antigen-independent), takes two minutes, and can be performed by an untrained person.

Toward Next Generation Lateral Flow Assays: Integration of Nanomaterials

Lateral flow assays (LFAs) are currently the most used point-of-care sensors for both diagnostic (e.g., pregnancy test, COVID-19 monitoring) and environmental (e.g., pesticides and bacterial monitoring) applications. Although the core of LFA technology was developed several decades ago, in recent years the integration of novel nanomaterials as signal transducers or receptor immobilization platforms has brought improved analytical capabilities. In this Review, we present how nanomaterial-based LFAs can address the inherent challenges of point-of-care (PoC) diagnostics such as sensitivity enhancement, lowering of detection limits, multiplexing, and quantification of analytes in complex samples. Specifically, we highlight the strategies that can synergistically solve the limitations of current LFAs and that have proven commercial feasibility. Finally, we discuss the barriers toward commercialization and the next generation of LFAs.

Quantum Dot-Based Lateral Flow Immunoassay as Point-of-Care Testing for Infectious Diseases: A Narrative Review of Its Principle and Performance

Infectious diseases are the world’s greatest killers, accounting for millions of deaths worldwide annually, especially in low-income countries. As the risk of emerging infectious diseases is increasing, it is critical to rapidly diagnose infections in the early stages and prevent further transmission. However, current detection strategies are time-consuming and have exhibited low sensitivity. Numerous studies revealed the advantages of point-of-care testing, such as those which are rapid, user-friendly and have high sensitivity and specificity, and can be performed at a patient’s bedside. The Lateral Flow Immunoassay (LFIA) is the most popular diagnostic assay that fulfills the POCT standards. However, conventional AuNPs-LFIAs are moderately sensitive, meaning that rapid detection remains a challenge. Here, we review quantum dot (QDs)-based LFIA for highly sensitive rapid diagnosis of infectious diseases. We briefly describe the principles of LFIA, strategies for applying QDs to enhance sensitivity, and the published performance of the QD-LFIA tested against several infectious diseases.

Microfluidic Paper-Based Preconcentration and Retrieval for Rapid Ribonucleic Acid Biomarker Detection and Visualization

Microfluidic paper-based analytical devices (μPADs) have attracted significant attention in the field of point-of-care (POC) diagnostics. However, the heterogeneous structure of the paper often impairs the limit of detection (LOD) for low-abundance targets when those targets are directly analyzed. One viable solution to bypass this limitation is to elevate the target concentration above the LOD on-site to reach a valid readout. Here, we developed a 3D μPADs preconcentrator (3D-μP²) to increase sample concentration by electrokinetic trapping and demonstrated its application in increasing the LOD of a downstream colorimetric assay. The three-dimensional (3D) structure of this device was composed of a loading pad, a vertical fluid path formed by stacked absorbent pads, and an ion-selective membrane of PEDOT:PSS. This novel design facilitates fast preconcentration, high capacity in sample processing, and easy target retrieval. The concentration of an exemplary target, a single-stranded DNA sequence, was increased up to 170-fold within 80 s. The LOD of the colorimetric assay to verify the DNA target was increased 3 orders of magnitude with a preconcentrated sample compared to the control. The device and its analysis equipment used in this study were all cheap and portable. Thus, the 3D-μP² can be a powerful POC tool for sample pretreatment in resource-limited areas.

Lateral flow assays for hormone detection

Endocrine diseases are the fifth most common cause of death and have a considerable impact on society given that they induce long-term morbidity in patients. For many decades, the measurement of hormones has been of great interest since this can be used to diagnose a plethora of pathological conditions. As a result, the endocrine testing market has experienced exponential growth. Several techniques have been utilised for the detection of hormones; however, they are expensive, laborious and require specialist training. Conversely, lateral flow assays (LFAs) are cheap (<£1) and rapid (<5 min) devices. LFAs typically rely on biochemical interactions between antibodies and antigens to produce coloured signals proportional to analyte concentrations, which can be visually inspected. Given their simplicity, LFAs are now considered the most attractive point-of-care device in medicine. However, the measurement of hormones in biofluids using LFAs faces many challenges including (i) the necessity for sensitive detection methods, (ii) the need for multiplexed devices for the confirmation of a diagnosis, and (iii) difficulties in sample preparation and pre-concentration. As such, most hormone LFAs remain in the research phase, and the few that have been commercialised require further optimisation before they can be employed for routine use. This review summarises the basic principles underlying lateral flow technology and provides an overview of recent advances, challenges, and potential solutions for the detection of hormone biomarkers via LFAs. Finally, hormone LFA kits available on the market are presented, with a look towards future developments and trends in the field.

Osmotic Processor for Enabling Sensitive and Rapid Biomarker Detection via Lateral Flow Assays

Urine is an attractive biospecimen for in vitro diagnostics, and urine-based lateral flow assays are low-cost devices suitable for point-of-care testing, particularly in low-resource settings. However, some of the lateral flow assays exhibit limited diagnostic utility because the urinary biomarker concentration is significantly lower than the assay detection limit, which compromises the sensitivity. To address the challenge, we developed an osmotic processor that statically and spontaneously concentrated biomarkers. The specimen in the device interfaces with the aqueous polymer solution via a dialysis membrane. The polymer solution induces an osmotic pressure difference that extracts water from the specimen, while the membrane retains the biomarkers. The evaluation demonstrated that osmosis induced by various water-soluble polymers efficiently extracted water from the specimens, ca. 5–15 ml/h. The osmotic processor concentrated the specimens to improve the lateral flow assays’ detection limits for the model analytes—human chorionic gonadotropin and SARS-CoV-2 nucleocapsid protein. After the treatment via the osmotic processor, the lateral flow assays detected the corresponding biomarkers in the concentrated specimens. The test band intensities of the assays with the concentrated specimens were very similar to the reference assays with 100-fold concentrations. The mass spectrometry analysis estimated the SARS-CoV-2 nucleocapsid protein concentration increased ca. 200-fold after the osmosis. With its simplicity and flexibility, this device demonstrates a great potential to be utilized in conjunction with the existing lateral flow assays for enabling highly sensitive detection of dilute target analytes in urine.

Nanoelectrokinetic-assisted lateral flow assay for COVID-19 antibody test

A lateral flow assay (LFA) platform is a powerful tool for point-of-care testing (POCT), especially for self-testing. Although the LFA platform provides a simple and disposable tool for Coronavirus disease of 2019 (COVID-19) antigen (Ag) and antibody (Ab) screening tests, the lower sensitivity for low virus titers has been a bottleneck for practical applications. Herein, we report the combination of a microfluidic paper-based nanoelectrokinetic (NEK) preconcentrator and an LFA platform for enhancing the sensitivity and limit of detection (LOD). Biomarkers were electrokinetically preconcentrated onto a specific layer using the NEK preconcentrator, which was then coupled with LFA diagnostic devices for enhanced performance. Using this nanoelectrokinetic-assisted LFA (NEK-LFA) platform for self-testing, the severe acute respiratory syndrome coronavirus 2 Immunoglobulin G (SARS-CoV-2 IgG) sample was preconcentrated from serum samples. After preconcentration, the LOD of the LFA was enhanced by 32-fold, with an increase in analytical sensitivity (16.4%), which may offer a new opportunity for POCT and self-testing, especially in the COVID-19 pandemic and endemic global context.

Digital microfluidics-like manipulation of electrokinetically preconcentrated bioparticle plugs in continuous-flow

Herein, we demonstrate digital microfluidics-like manipulations of preconcentrated biomolecule plugs within a continuous flow that is different from the commonly known digital microfluidics involving discrete (i.e. droplets) media. This is realized using one- and two-dimensional arrays of individually addressable ion-permselective membranes with interconnecting microfluidic channels. The location of powered electrodes, dictates which of the membranes are active and generates either enrichment/depletion diffusion layers, which, in turn, control the location of the preconcentrated plug. An array of such powered membranes enables formation of multiple preconcentrated plugs of the same biosample as well as of preconcentrated plugs of multiple biosample types introduced via different inlets in a selective manner. Moreover, digital-microfluidics operations such as up-down and left-right translation, merging, and splitting, can be realized, but on preconcentrated biomolecule plugs instead of on discrete droplets. This technology, based on nanoscale electrokinetics of ion transport through permselective medium, opens future opportunities for smart and programmable digital-like manipulations of preconcentrated biological particle plugs for various on-chip biological applications.

Ultrasensitive Competitive Lateral Flow Immunoassay with Visual Semiquantitative Inspection and Flexible Quantification Capabilities

Antibiotics abuse has caused various problems threatening human health and ecological environment. Monitoring antibiotics residual levels is of great significance, yet still challenging for quantitative point-of-need testing with high-sensitivity and visual capability. Here we developed a competitive lateral flow immunoassay (CLFIA) platform with flexible readout for enrofloxacin (ENR), a regularly added antibiotic. To overcome the limitation of low sensitivity of traditional colloidal gold-based CLFIA, the three-dimensionally assembled gold nanoparticles (AuNPs) within dendritic silica scaffold were fabricated as signal reporters. The assembly structure effectively retained the intrinsic absorption features of hydrophobic AuNPs and greatly enhanced the light extinction ability of a single label for signal amplification. The obtained CLFIA strips can not only achieve qualitative screening of ENR at a very low concentration by naked eye (cutoff value: 0.125 ng/mL), but also enable ultrasensitive quantification of ENR by an optical scanner (limit of detection: 0.00195 ng/mL) or a smartphone (limit of detection: 0.0078 ng/mL). Moreover, to elaborate the visual inspection degree of CLFIA against traditional yes/no interpretation, a novel multirange gradient CLFIA strip was prepared for visually semiquantitative identification of ENR with four concentration ranges. The novel CLFIA platform demonstrated sensitive, specific, and reliable determination of ENR with flexible signal readout and provides a potential and invigorating pathway to point-of-need immunoassay of antibiotics.

Paper-Based Biosensors: Frontiers in Point-of-Care Detection of COVID-19 Disease

This review summarizes the state of the art of paper-based biosensors (PBBs) for coronavirus disease 2019 (COVID-19) detection. Three categories of PBB are currently being been used for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) diagnostics, namely for viral gene, viral antigen and antibody detection. The characteristics, the analytical performance, the advantages and drawbacks of each type of biosensor are highlighted and compared with traditional methods. It is hoped that this review will be useful for scientists for the development of novel PBB platforms with enhanced performance for helping to contain the COVID-19 outbreak, by allowing early diagnosis at the point of care (POC).

Recent advances in paper-based preconcentrators by utilizing ion concentration polarization

One of the most cited limitations of biochemical detection is its poor sensitivity, owing to the relatively high complexity of micro-samples. Moreover, some samples cannot be easily self-replicated and their abundance cannot be increased through traditional technologies. Therefore, the preconcentration of low-abundance samples is a key requirement for microfluidic biological analysis. In recent years, the ion-concentration polarization phenomenon has aroused widespread interest in the application of microfluidic technology. In addition, paper-based materials are readily available, easy to modify, and exhibit good hydrophilicity. The study of the ion-concentration polarization preconcentration of micro-samples in paper-based microfluidic chips is of considerable significance. In this review, we discuss the development and applications of ion-concentration polarization paper-based preconcentrator in the past 5 years, with emphasis on key progresses in chip fabrication and performance optimization under different conditions. The current needs and development prospects in this field have also been discussed.

Simplified formats of modern biosensors: 60 years of using immunochromatographic test systems in laboratory diagnostics

Immunochromatographic test systems known to foreign laboratory diagnostic experts as lateral flow immunoassay (LFIA) are simplified tape formats of modern biosensors. For 60 years, they have been widely used for the rapid detection of target molecules (ligands) in biosubstrates and the diagnosis of many diseases and conditions. The growing popularity of these test systems for providing medical care or diagnostics in developing countries, medical facilities, in emergency situations, as well as for individual home use by patients while monitoring their health are the main factors contributing to the continuous development and improvement of these methods, the emergence of a new generation of formats. The attractiveness and popularity of these fast, easy-to-use, inexpensive and portable diagnostic tools is associated primarily with their high analytical sensitivity and specificity, as well as the ease of interpretation of the results. These qualities have passed the test of time, and today LFIA test systems are fully consistent with the modern world concept of «point-of-care testing», finding wide application not only in medicine, but also in ecology, veterinary medicine, and agriculture. This review will highlight the modern principles of designing the most widely used formats of immunochromatographic test systems for clinical laboratory diagnostics, summarize the main advantages and disadvantages of the method, as well as current achievements and prospects of LFIA technology. Modern innovations aimed at improving the analytical characteristics of LFIA technology are interesting, promising and can bring additional benefits to immunochromatographic platforms that have gained popularity and attractiveness for six decades.

Recent advances in sensitivity enhancement for lateral flow assay

Conventional lateral flow assay (LFA) is typically performed by observing the color changes in the test lines by naked eyes, which achieves considerable commercial success and has a significant impact on the fields of food safety, environment monitoring, disease diagnosis, and other applications. However, this qualitative detection method is not very suitable for low levels of disease biomarkers' detection. Although many nanomaterials are used as new labels for LFA, additional readers limit their application to some extent. Fortunately, a lot of work has been done for improving the sensitivity of LFA. In this review, currently reported LFA sensitivity enhancement methods with an objective evaluation are summarized, such as sample pretreatment, the change of flow rate, and label evolution, and future development direction and challenges of LFAs are discussed.

Rapid point-of-care testing methods/devices for meat species identification: A review

The authentication of animal species is an important issue due to an increasing trend of adulteration and mislabeling of animal species in processed meat products. Polymerase chain reaction is the most sensitive and specific technique for nucleic acid-based animal species detection. However, it is a time-consuming technique that requires costly thermocyclers and sophisticated labs. In recent times, there is a need of on-site detection by point-of-care (POC) testing methods and devices under low-resource settings. These POC devices must be affordable, sensitive, specific, user-friendly, rapid and robust, equipment free, and delivered to the end users. POC devices should also confirm the concept of micro total analysis system. This review discusses POC testing methods and devices that have been developed for meat species identification. Recent developments in lateral flow assay-based devices for the identification of animal species in meat products are also reviewed. Advancements in increasing the efficiency of lateral flow detection are also discussed.

Six decades of lateral flow immunoassay: from determining metabolic markers to diagnosing COVID-19

Technologies based on lateral flow immunoassay (LFIA), known in some countries of the world as immunochromatographic tests, have been successfully used for the last six decades in diagnostics of many diseases and conditions as they allow rapid detection of molecular ligands in biosubstrates. The popularity of these diagnostic platforms is constantly increasing in healthcare facilities, particularly those facing limited budgets and time, as well as in household use for individual health monitoring. The advantages of these low-cost devices over modern laboratory-based analyzers come from their availability, opportunity of rapid detection, and ease of use. The attractiveness of these portable diagnostic tools is associated primarily with their high analytical sensitivity and specificity, as well as with the easy visual readout of results. These qualities explain the growing popularity of LFIA in developing countries, when applied at small hospitals, in emergency situations where screening and monitoring health condition is crucially important, and as well as for self-testing of patients. These tools have passed the test of time, and now LFIA test systems are fully consistent with the world's modern concept of ‘point-of-care testing’, finding a wide range of applications not only in human medicine, but also in ecology, veterinary medicine, and agriculture. The extensive opportunities provided by LFIA contribute to the continuous development and improvement of this technology and to the creation of new-generation formats. This review will highlight the modern principles of design of the most widely used formats of test-systems for clinical laboratory diagnostics, summarize the main advantages and disadvantages of the method, as well as the current achievements and prospects of the LFIA technology. The latest innovations are aimed at improving the analytical performance of LFIA platforms for the diagnosis of bacterial and viral infections, including COVID-19.

Improvement in Detection Limit for Lateral Flow Assay of Biomacromolecules by Test-Zone Pre-enrichment

Lateral flow assay (LFA) is one of the most prevalent commercially available techniques for point-of-care tests due to its simplicity, celerity, low cost and robust operation. However, conventional colorimetric LFAs have inferior limits of detection (LODs) compared to sophisticated laboratory-based assays. Here, we report a simple strategy of test-zone pre-enrichment to improve the LOD of LFA by loading samples before the conjugate pad assembly. The developed method enables visual LODs of miR-210 mimic and human chorionic gonadotropin protein, to be improved by 10–100 fold compared with a conventional LFA setup without introducing any additional instrument and reagent except for phosphate running buffer, while no obvious difference occurred for Aflatoxin B1 (AFB1). It takes about 6–8 min to enrich every 50 μL of sample diluted with phosphate running buffer, therefore we can get visual results within 20 min. We identified a parameter by modeling the entire process, the concentration of probe-analyte conjugate at test zone when signaling unit being loaded, to be important for the improvement of visual limit of detection. In addition, the test-zone pre-enrichment did not impair the selectivity when miR-210 mimic was adopted as target. Integrated with other optimization, amplification and modification of LFAs, the developed test-zone pre-enrichment method can be applied to further improve LOD of LFAs.

Enhanced paper-based ELISA for simultaneous EVs/exosome isolation and detection using streptavidin agarose-based immobilization

EVs/exosomes are considered as the next generation of biomarkers, including for liquid biopsies. Consequently, the quantification of EVs/exosomes is crucial for facilitating EV/exosome research and applications. Paper-based enzyme-linked immunosorbent assay (p-ELISA) is a portable diagnostic system with low cost that is simple and easy to use; however, it shows low sensitivity and linearity. In this study, we develop p-ELISA for targeting EVs/exosomes by using streptavidin agarose resin-based immobilization (SARBI). This method reduces assay preparation times, provides strong binding, and retains good sensitivity and linearity. The time required for the total assay, including preparation steps and surface immobilization, was shortened to ∼2 h. We evaluated SARBI p-ELISA systems with/without CD63 capture Ab and then with fetal bovine serum (FBS) and EVs/exosome-depleted fetal bovine serum (dFBS). The results provide evidence supporting the selective capture ability of SARBI p-ELISA. We obtain semiquantitative p-ELISA results using an exosome standard (ES) and human serum (HS), with R2 values of 0.95 and 0.92, respectively.

Automated, Universal, and Mass-Producible Paper-Based Lateral Flow Biosensing Platform for High-Performance Point-of-Care Testing

Paper-based lateral flow assays (LFAs) are among the most widely used biosensing platforms for point-of-care testing (POCT). However, the conventional colloidal gold label of LFAs show low sensitivity and limited quantitative capacity. Alternatively, the use of enzyme/chemical reaction-based signal amplification with structural modifications has enhanced analytical capacity but requires multiple user interventions as a trade-off, increasing complexity, test imprecision, and time. These platforms are also difficult to manufacture, limiting their practical applications. In this study, within the current LFA production framework, we developed a highly sensitive, automated, universal, and manufacturable LFA biosensing platform by (i) incorporating gold nanoparticles into a polymer-networked peroxidase with an antibody as a new scheme for enhanced enzyme conjugation and (ii) integrating a mass-producible and time-programmable amplification part based on a water-swellable polymer for automating the sequential reactions in the immunoassay and signal amplification, without compromising performance, simplicity, and production feasibility. We applied this platform to evaluate cardiac troponin I (cTnI), a gold-standard biomarker for myocardial infarction diagnosis. Quantitative analysis of cTnI in clinical setting remains limited to the laboratory-based high-end and costly standard equipment. Coupled with an enzyme-catalyzed chemiluminescence method, this platform enables automated, cost-effective (0.66 USD per test), and high-performance testing of human cTnI in serum samples within 20 min with a detection range of 6 orders of magnitude, detection limit of 0.84 pg mL^-1 (595-fold higher than conventional cTnI-LFA), and a coefficient of variation of 2.9-8.5%, which are comparable to the standard equipment and acceptable for clinical use. Moreover, cTnI analysis results using clinical serum/plasma samples revealed a strong correlation (R² = 0.991) with contemporary standard equipment, demonstrating the practical application of this platform for high-performance POCT.

Magnetic Focus Lateral Flow Sensor for Detection of Cervical Cancer Biomarkers

We report on a magnetic focus lateral flow biosensor (mLFS) for ultrasensitive detection of protein biomarkers in a practical format. With valosin-containing protein as a target protein, we show that the developed mLFS concept could detect as low as 25 fg/mL with magnetic focus to enhance target capture efficiency to deliver a 10⁶-fold improvement in sensitivity compared to that of conventional lateral flow (LF) systems. The conceptualized strategy utilizes a simple magnet placed beneath the three-dimensional printed LF device to concentrate the targets at the signal zone without any additional instrumentation. In addition, protein mixtures extracted from the tissue of cervical cancer patients was also utilized to validate the sensor. To investigate the effect of magnetic focus on sensitivity, surface-enhanced Raman spectroscopy and dark-field imaging was utilized to characterize the distribution and movement of Fe₃O₄ core-Au shell nanoprobes in a model LF strip. Our experiments show that the magnetic focus results in an increased interaction time between the magnetic probe-labeled targets and the capture antibody, yielding a higher capture efficiency, allowing for ultrasensitive detection of the target not possible before with LF. The proposed mLFS can be utilized to detect a range of trace protein biomarkers for early diagnosis and can be combined with diverse pretreatment and signal amplification steps to query complex samples.

Emerging paper microfluidic devices

Paper has unique advantages over other materials, including low cost, flexibility, porosity, and self-driven liquid pumping, thus making it widely used in various fields in biology, chemistry, physics and materials science.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2016-2018)

One of the most cited limitations of capillary and microchip electrophoresis is the poor sensitivity. This review continues to update this series of biannual reviews, first published in Electrophoresis in 2007, on developments in the field of online/in-line concentration methods in capillaries and microchips, covering the period July 2016-June 2018. It includes developments in the field of stacking, covering all methods from field-amplified sample stacking and large-volume sample stacking, through to isotachophoresis, dynamic pH junction, and sweeping. Attention is also given to online or in-line extraction methods that have been used for electrophoresis.

A colorimetric paper-based analytical device coupled with hollow fiber membrane liquid phase microextraction (HF-LPME) for highly sensitive detection of hexavalent chromium in water samples

A simple and highly sensitive procedure based on the combination of hollow fiber membrane liquid phase microextraction and a microfluidic paper-based analytical device (µPAD) was developed for pre-concentration and determination of hexavalent chromium in water samples. The hexavalent chromium was pre-concentrated using the HF-LPME technique via ion exchange or a coupled transport process through a supported ionic liquid (Aliquat 336) prior to colorimetric detection with diphenylcarbazide on the µPAD. The violet colour could be seen by the naked eye. Images from the µPADs were scanned using a commercial desktop scanner at 600 dpi resolution. ImageJ software was used for quantitative analysis by measuring the intensity values at green colour channel since it gave the best sensitivity among the RGB colour. Under optimal conditions, the calibration curve was linear in the range 10-90 µg L^-1, with a limit of detection of 3 µg L^-1. The developed method was successfully applied to determine the level of hexavalent chromium spiked into natural water samples at the parts-per-billion (ppb) level, and the results were in good agreement with those obtained using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The developed method was able to improve the detection limit of the conventional µPAD, and was expected to be used for the effective analysis of hexavalent chromium in natural water.