Enhanced lateral flow immunoassay using gold nanoparticles loaded with enzymes

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.

Lab-in-a-Vial Rapid Test for Internet of Things-Embedded Point-of-Healthcare Protein Biomarker Detection in Bodily Fluids

Amateurs often struggle with detecting and quantifying protein biomarkers in body fluids due to the high expertise required. This study introduces a Lab-in-a-Vial (LV) rapid diagnostic platform, featuring hydrangea-like platinum nanozymes (PtNH), for rapid, accurate detection and quantification of protein biomarkers on-site within 15 min. This method significantly enhances detection sensitivity for various biomarkers in body fluids, surpassing traditional methods such as enzyme-linked immunosorbent assays (ELISA) and lateral flow assays (LFA) by ≈250 to 1300 times. The LV platform uses a glass vial coated with specific bioreceptors such as antigens or antibodies, enabling rapid in vitro evaluation of disease risk from small fluid samples, similar to a personal ELISA-like point-of-care test (POCT). It overcomes challenges in on-site biomarker detection, allowing both detection and quantification through a portable wireless spectrometer for healthcare internet of things (H-IoT). The platform's effectiveness and adaptability are confirmed using IgG/IgM antibodies from SARS-CoV-2 infected patients and nuclear matrix protein (NMP22) from urothelial carcinoma (UC) patients as biomarkers. These tests demonstrated its accuracy and flexibility. This approach offers vast potential for diverse disease applications, provided that the relevant protein biomarkers in bodily fluids are identified.

Palladium/Platinum/Ruthenium Trimetallic Dendritic Nanozymes Exhibiting Enhanced Peroxidase-like Activity for Signal Amplification of Lateral Flow Immunoassays

Developing ultrasensitive lateral flow immunoassays (LFIAs) has garnered significant attention in the field of point-of-care testing. In this study, a trimetallic dendritic nanozyme (Pd@Pt-Ru) was synthesized through Ru deposition on a Pd@Pt core and utilized to enhancing the sensitivity of LFIAs. Pd@Pt-Ru exhibited a Km value of 5.23 mM for detecting H2O2, which indicates an H2O2 affinity comparable with that of horseradish peroxidase. The Ru surface layer reduces the activation energy barrier, which increases the maximum reaction rate. As a proof of concept, the proposed Pd@Pt-Ru nanozyme was incorporated into LFIAs (A-Pd@Pt-Ru-LFIAs) for detecting human chorionic gonadotropin (hCG). Compared with conventional gold nanoparticle (AuNP)-LFIAs, A-Pd@Pt-Ru-LFIAs demonstrated 250-fold increased sensitivity, thereby enabling a visible detection limit as low as 0.1 IU/L. True positive and negative rates both reached 100%, which renders the proposed Pd@Pt-Ru nanozyme suitable for detecting hCG in clinical samples.

Near-infrared responsive gold nanorods for highly sensitive colorimetric and photothermal lateral flow immuno-detection of SARS-CoV-2

The highly infectious characteristics of coronavirus disease 2019 (COVID-19), which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), highlight the necessity of sensitive and rapid nucleocapsid (N) protein-based antigen testing for early triage and epidemic management. In this study, a colorimetric and photothermal dual-mode lateral flow immunoassay (LFIA) platform for the rapid and sensitive detection of the SARS-CoV-2 N protein was developed based on gold nanorods (GNRs), which possessed tunable local surface plasma resonance (LSPR) absorption peaks from UV-visible to near-infrared (NIR). The LSPR peak was adjusted to match the NIR emission laser 808 nm by controlling the length-to-diameter ratio, which could maximize the photothermal conversion efficiency and achieve photothermal detection signal amplification. Qualitative detection of SARS-CoV-2 N protein was achieved by observing the strip color, and the limit of detection was 2 ng mL-1, while that for photothermal detection was 0.096 ng mL-1. Artificial saliva samples spiked with the N protein were analyzed with the recoveries ranging from 84.38% to 107.72%. The intra-assay and inter-assay coefficients of variation were 6.76% and 10.39%, respectively. We further evaluated the reliability of this platform by detecting 40 clinical samples collected from nasal swabs, and the results matched well with that of nucleic acid detection (87.5%). This method shows great promise in early disease diagnosis and screening.

Development of nucleic acid lateral flow immunoassay for molecular detection of Entamoeba moshkovskii and Entamoeba dispar in stool samples

Entamoeba moshkovskii, recently known as a possible pathogenic amoeba, and the non-pathogenic Entamoeba dispar are morphologically indistinguishable by microscopy. Although PCR was used for differential diagnosis, gel electrophoresis is labor-intensive, time-consuming, and exposed to hazardous elements. In this study, nucleic acid lateral flow immunoassay (NALFIA) was developed to detect E. moshkovskii and E. dispar by post-PCR amplicon analysis. E. moshkovskii primers were labeled with digoxigenin and biotin whereas primers of E. dispar were lebeled with FITC and digoxigenin. The gold nanoparticles were labeled with antibodies corresponding to particular labeling. Based on the established assay, NALFIA could detect as low as 975 fg of E. moshkovskii target DNA (982 parasites or 196 parasites/microliter), and 487.5 fg of E. dispar target DNA (444 parasites or 89 parasites/microliter) without cross-reactivity to other tested intestinal organisms. After testing 91 stool samples, NALFIA was able to detect seven E. moshkovskii (87.5% sensitivity and 100% specificity) and eight E. dispar samples (66.7% sensitivity and 100% specificity) compared to real-time PCR. Interestingly, it detected three mixed infections as real-time PCR. Therefore, it can be a rapid, safe, and effective method for the detection of the emerging pathogens E. moshkovskii and E. dispar in stool samples.

An Artificial Miniaturized Peroxidase for Signal Amplification in Lateral Flow Immunoassays

Signal amplification strategies are widely used for improving the sensitivity of lateral flow immunoassays (LFiAs). Herein, the artificial miniaturized peroxidase Fe(III)-MimochromeVI*a (FeMC6*a), immobilized on gold nanoparticles (AuNPs), is used as a strategy to obtain catalytic signal amplification in sandwich immunoassays on lateral flow strips. The assay scheme uses AuNPs decorated with the mini-peroxidase FeMC6*a and anti-human-IgG as a detection antibody (dAb), for the detection of human-IgG, as a model analyte. Recognition of the analyte by the capture and detection antibodies is first evidenced by the appearance of a red color in the test line (TL), due to the accumulation of AuNPs. Subsequent addition of 3,3',5,5'-tetramethylbenzidine (TMB) induces an increase of the test line color, due to the TMB being converted into an insoluble colored product, catalyzed by FeMC6*a. This work shows that FeMC6*a acts as an efficient catalyst in paper, increasing the sensitivity of an LFiA up to four times with respect to a conventional LFiA. Furthermore, FeMC6*a achieves lower limits of detection that are found in control experiments where it is replaced with horseradish peroxidase (HRP), its natural counterpart. This study represents a significant proof-of-concept for the development of more sensitive LFiAs, for different analytes, based on properly designed artificial metalloenzymes.

Decorated DNA-Based Scaffolds as Lateral Flow Biosensors

Here we develop Lateral Flow Assays (LFAs) that employ as functional elements DNA-based structures decorated with reporter tags and recognition elements. We have rationally re-engineered tile-based DNA tubular structures that can act as scaffolds and can be decorated with recognition elements of different nature (i.e. antigens, aptamers or proteins) and with orthogonal fluorescent dyes. As a proof-of-principle we have developed sandwich and competitive multiplex lateral flow platforms for the detection of several targets, ranging from small molecules (digoxigenin, Dig and dinitrophenol, DNP), to antibodies (Anti-Dig, Anti-DNP and Anti-MUC1/EGFR bispecific antibodies) and proteins (thrombin). Coupling the advantages of functional DNA-based scaffolds together with the simplicity of LFAs, our approach offers the opportunity to detect a wide range of targets with nanomolar sensitivity and high specificity.

Redox Cycling Amplified Electrochemical Lateral-Flow Immunoassay: Toward Decentralized Sensitive Insulin Detection

While paper-based lateral-flow immunoassays (LFA) offer considerable promise for centralized diagnostic applications, the analytical capability of conventional LFA remains constrained due to the low sensitivity of its common optical detection strategy. To address these issues, we report a simple electrochemical LFA (eLFA) with nanocatalytic redox cycling for decentralized insulin detection. Simultaneous binding of insulin with detection antibodies and capture antibodies through the capillary flow at the LFA platform and signal amplification through the rapid nanocatalytic reduction of [Fe(CN)6]3- (Fe3+) with Au nanoparticles (AuNP) and ammonia-borane (AB), coupled to electrochemical redox cycling reactions involving Fe3+, AuNP, and AB on the carbon working electrode, offer higher sensitivity than conventional colorimetric LFA and enzymatic redox cycling. The resulting integrated eLFA strip allows the detection of low insulin concentrations (LOD = 12 pM) and offers considerable promise for highly sensitive decentralized assays of different biological fluids (saliva and serum) without additional pretreatment or washing steps.

Gold Nanotapes and Nanopinecones in a Quantitative Lateral Flow Assay for the Cancer Biomarker Carcinoembryonic Antigen

Colorectal cancer is the third most common malignancy and the second leading cause of cancer death globally. Multiple studies have linked levels of carcinoembryonic antigen in patient serum to poor disease prognosis. Hence, the ability to detect low levels of carcinoembryonic antigen has applications in earlier disease diagnosis, assessment, and recurrence monitoring. Existing carcinoembryonic antigen detection methods often require multiple reagents, trained operators, or complex procedures. A method alleviating these issues is the lateral flow assay, a paper-based platform that allows the detection and quantification of target analytes in complex mixtures. The tests are rapid, are point-of-care, possess a long shelf life, and can be stored at ambient conditions, making them ideal for use in a range of settings. Although lateral flow assays typically use spherical gold nanoparticles to generate the classic red signal, recent literature has shown that alternate morphologies to spheres can improve the limit of detection. In this work, we report the application of alternative gold nanoparticle morphologies, gold nanotapes (∼35 nm in length) and gold nanopinecones (∼90 nm in diameter), in a lateral flow assay for carcinoembryonic antigen. In a comparative assay, gold nanopinecones exhibited a ∼2× improvement in the limit of detection compared to commercially available spherical gold nanoparticles for the same antibody loading and total gold content, whereas the number of gold nanopinecones in each test was ∼3.2× less. In the fully optimized test, a limit of detection of 14.4 pg/mL was obtained using the gold nanopinecones, representing a 24-fold improvement over the previously reported gold-nanoparticle-based carcinoembryonic antigen lateral flow assay.

Mobile Efficient Diagnostics of Infectious Diseases via On-Chip RT-qPCR: MEDIC-PCR

The COVID-19 outbreak has caused public and global health crises. However, the lack of on-site fast, reliable, sensitive, and low-cost reverse transcription polymerase chain reaction (RT-PCR) testing limits early detection, timely isolation, and epidemic prevention and control. Here, the authors report a rapid mobile efficient diagnostics of infectious diseases via on-chip -RT-quantitative PCR (RT-qPCR): MEDIC-PCR. First, the authors use a roll-to-roll printing process to accomplish low-cost carbon-black-based disposable PCR chips that enable rapid LED-induced photothermal PCR cycles. The MEDIC-PCR can perform RT (3 min), and PCR (9 min) steps. Further, the cohort of 89 COVID-19 and 103 non-COVID-19 patients testing is completed by the MEDIC-PCR to show excellent diagnostic accuracy of 97%, sensitivity of 94%, and specificity of 98%. This MEDIC-PCR can contribute to the preventive global health in the face of a future pandemic.

Lateral flow assay of pathogenic viruses and bacteria in healthcare

Healthcare-associated pathogenic viruses and bacteria can have a serious impact on human health and have attracted widespread global attention. The lateral flow assay is a unidirectional detection based on the binding of a target analyte and a bioreceptor on the device via lateral flow. With incredible advantages over traditional chromatographic methods, such as rapid detection, ease of manufacture and cost effectiveness, these test strips are increasingly considered the ideal form for point-of-care applications. This review explores lateral flow assays for pathogenic viruses and bacteria, with a particular focus on methodologies, device components, construction methods, and applications. We anticipate that this review could provide exciting opportunities for developing new lateral flow devices for pathogens and advance related healthcare applications.

Multilayer Model of Gold Nanoparticles (AuNPs) and Its Application in the Classical Molecular Dynamics Simulation of Citrate-Capped AuNPs

Studies on the interaction between gold nanoparticles (AuNPs) and functional proteins have been useful in developing diagnostic and therapeutic agents. Such studies require a realistic computational model of AuNPs for successful molecular design works. This study offers a new multilayer model of AuNPs to address the inconsistency between its molecular mechanics' interpretation and AuNP's plasmonic nature. We performed partial charge quantum calculation of AuNPs using Au13 and Au55 models. The result showed that it has partial negative charges on the surface and partial positive charges on the inner part, indicating that the AuNP model should be composed of multiatom types. We tested the partial charge parameters of these gold (Au) atoms in classical molecular dynamics simulation (CMD) of AuNPs. The result showed that our parameters performed better in simulating the adsorption of Na+ and dicarboxy acetone in terms of consistency with surface charge density than the zero charges Au in the interface force field (IFF). We proposed that the multiple-charged AuNP model can be developed further into a simpler four-atom type of Au in a larger AuNP size.

Recent Trends in Lateral Flow Immunoassays with Optical Nanoparticles

Rapid, accurate, and convenient diagnosis is essential for effective disease management. Various detection methods, such as enzyme-linked immunosorbent assay, have been extensively used, with lateral flow immunoassay (LFIA) recently emerging as a major diagnostic tool. Nanoparticles (NPs) with characteristic optical properties are used as probes for LFIA, and researchers have presented various types of optical NPs with modified optical properties. Herein, we review the literature on LFIA with optical NPs for the detection of specific targets in the context of diagnostics.

Development of a colloidal gold immunochromatographic strip with enhanced signal for the detection of bovine parvovirus

Bovine parvovirus (BPV) is a pathogen responsible for respiratory and digestive tract symptoms in calves and abortion and stillbirth in pregnant cows. In this study, we developed a colloidal gold immunochromatographic (GICG) strip with an enhanced signal for detecting BPV according to the double-antibody sandwich principle and an enzyme-based signal amplification system to amplify the signal. This system utilizes horseradish peroxidase reacting with a substrate solution containing 3,3',5,5'-tetramethylbenzidine and dextran sulfate to obtain insoluble blue products on the test and control lines. We optimized different reaction conditions, including the amount of monoclonal antibodies (mAbs), pH of the colloidal gold solution, coating solution, blocking solution, sample pad treatment solution, antibody concentration in the control line, and antibody concentration in the detection line. The sensitivity of the signal-enhanced GICG strip showed that the minimum amount for detecting BPV was 102 TCID50, 10 times higher than that of the traditional GICG strip. The results of the specificity test showed that the signal-enhanced GICG strip had no cross-reactivity with BRV, BVDV, or BRSV. The results of the repeatability test showed that the coefficient of variation between and within batches was less than 5%, showing good repeatability. Moreover, for validation, PCR and the signal-enhanced GICG strip were used to detect 280 clinical bovine fecal samples. The concordance rate compared with PCR was 99.29%. Hence, the developed strip exhibited high sensitivity and specificity for the detection of BPV. Therefore, this strip could be a rapid, convenient, and effective method for the diagnosis of BPV infection in the field.

AuPt nanoalloy with dual functionalities for sensitive detection of HPV16 DNA

Human papillomavirus type 16 (HPV16), one of the high-risk types, is responsible for 53% of cervical cancers. The development of an early diagnostic approach with high sensitivity, low-cost, point-of-care testing (POCT) for HPV16 is urgent. In our work, a novel dual-functional AuPt nanoalloy-based lateral flow nucleic acid biosensor (AuPt nanoalloy-based LFNAB) was established with excellent sensitivity for detecting HPV16 DNA for the first time. The AuPt nanoalloy particles were prepared by a one-step reduction method, which was simple, rapid, and green. The AuPt nanoalloy particles retained the performance of initial Au nanoparticles owing to the catalytic activity enabled by Pt. Such dual-functionalities offered two kinds of detection alternatives (i.e., normal mode and amplification mode, respectively). The former is produced just by the black color from the AuPt nanoalloy material itself, and the latter is more color sensitive from its superior catalytic activity. The optimized AuPt nanoalloy-based LFNAB exhibited satisfactory quantitative ability in detecting the target HPV16 DNA in the range of 5-200 pM with a LOD of 0.8 pM at the "amplification mode". The proposed dual-functional AuPt nanoalloy-based LFNAB displayed great potential and promising opportunity in POCT clinical diagnostics.

Lateral Flow Immunoassay for Proteins

Protein biomarkers are useful for disease diagnosis. Identification thereof using in vitro diagnostics such as lateral flow immunoassays (LFIAs) has attracted considerable attention due to their low cost and ease of use especially in the point of care setting. Current challenges, however, do remain with respect to material selection for each component in the device and the synergistic integration of these components to display detectable signals. This review explores the principle of LFIA for protein biomarkers, device components including biomaterials and labeling methods. Medical applications and commercial status are examined as well. This review highlights critical methodologies in the development of new LFIAs and their role in advancing healthcare worldwide.

3D-printed capillaric ELISA-on-a-chip with aliquoting

Sandwich immunoassays such as the enzyme-linked immunosorbent assay (ELISA) have been miniaturized and performed in a lab-on-a-chip format, but the execution of the multiple assay steps typically requires a computer or complex peripherals. Recently, an ELISA for detecting antibodies was encoded structurally in a chip thanks to the microfluidic chain reaction (Yafia et al. Nature, 2022, 605, 464-469), but the need for precise pipetting and intolerance to commonly used surfactant concentrations limit the potential for broader adoption. Here, we introduce the ELISA-on-a-chip with aliquoting functionality that simplifies chip loading and pipetting, accommodates higher surfactant concentrations, includes barrier channels that delay the contact between solutions and prevent undesired mixing, and that executed a quantitative, high-sensitivity assay for the SARS-CoV-2 nucleocapsid protein in 4×-diluted saliva. Upon loading the chip using disposable pipettes, capillary flow draws each reagent and the sample into a separate volumetric measuring reservoir for detection antibody (70 μL), enzyme conjugate (50 μL), substrate (80 μL), and sample (210 μL), and splits washing buffer into 4 different reservoirs of 40, 40, 60, and 20 μL. The excess volume is autonomously drained via a structurally encoded capillaric aliquoting circuit, creating aliquots with an accuracy of >93%. Next, the user click-connects the assay module, comprising a nitrocellulose membrane with immobilized capture antibodies and a capillary pump, to the chip which triggers the step-by-step, timed flow of all aliquoted solutions to complete the assay in 1.5 h. A colored precipitate forming a line on a nitrocellulose strip serves as an assay readout, and upon digitization, yielded a binding curve with a limit of detection of 54 and 91 pg mL^-1 for buffer and diluted saliva respectively, vastly outperforming rapid tests. The ELISA chip is 3D-printed, modular, adaptable to other targets and assays, and could be used to automate ELISA in the lab; or as a diagnostic test at the point of care with the convenience and form factor of rapid tests while preserving the protocol and performance of central laboratory ELISA.

Strategies for the Voltammetric Detection of Loop-Mediated Isothermal Amplification

Loop-mediated isothermal amplification (LAMP) is rapidly developing into an important tool for the point-of-use detection of pathogens for both clinical and environmental samples, largely due to its sensitivity, rapidity, and adaptability to portable devices. Many methods are used to monitor LAMP, but not all are amenable to point-of-use applications. Common methods such as fluorescence often require bulky equipment, whereas colorimetric and turbidimetric methods can lack sensitivity. Electrochemical biosensors are becoming increasingly important for these applications due to their potential for low cost, high sensitivity, and capacity for miniaturization into integrated devices. This review provides an overview of the use of voltammetric sensors for monitoring LAMP, with a specific focus on how electroactive species are used to interface between the biochemical products of the LAMP reaction and the voltammetric sensor. Various strategies for the voltammetric detection of DNA amplicons as well as pyrophosphate and protons released during LAMP are presented, ranging from direct DNA binding by electroactive species to the creative use of pyrophosphate-detecting aptamers and pH-sensitive oligonucleotide structures. Hurdles for adapting these devices to point-of-use applications are also discussed.

Au/Fe3O4-based nanozymes with peroxidase-like activity integrated in immunochromatographic strips for highly-sensitive biomarker detection

Because of their simplicity, rapidity, and cost-effectiveness, immunochromatographic strips (ICTs) have been widely used as an effective tool in various fields. However, typical strips for the preliminary screening suffer from limited detection sensitivity, particularly in biomarker detection with trace concentration. Herein, to tackle this challenge, we integrated homemade gold-decorated Fe₃O₄ nanoparticles (Au/Fe₃O₄ NPs) with flexible strips, exploring the excellent peroxidase-like activity of this labeled material, and then enhancing the detection sensitivity via signal amplification. The limit of detection (LOD) of the strips is as low as 0.05 mIU mL^-1 when human chorionic gonadotropin (hCG) is as a biomarker model, which is 500 times lower than that of the traditional color-based strip. Overall, our results demonstrated the potential for Au/Fe₃O₄ NP based-ICTs for the rapid detection of the biomarker in an instrument-free and point-of-care testing format.

Detection of Glucose Based on Noble Metal Nanozymes: Mechanism, Activity Regulation, and Enantioselective Recognition

Glucose monitoring is essential to evaluate the degree of glucose metabolism disorders. The enzymatic determination has been the most widely used method in glucose detection because of its high efficiency, accuracy, and sensitivity. Noble metal nanomaterials (NMs, i.e., Au, Ag, Pt, and Pd), inheriting their excellent electronic, optical, and enzyme-like properties, are classified as noble metal nanozymes (NMNZs). As the NMNZs are often involved in two series of reactions, the oxidation of glucose and the chromogenic reaction of peroxide, here the chemical mechanism by employing NMNZs with glucose oxidase (GOx) and peroxidase (POD) mimicking activities is briefly summarized first. Subsequently, the regulation strategies of the GOx-like, POD-like and tandem enzyme-like activities of NMNZs are presented in detail, including the materials, size, morphology, composition, and the reaction condition of the representative NMs. In addition, in order to further mimic the enantioselectivity of enzyme, the design of NMNZs with enantioselective recognition of d-glucose and l-glucose by using different chiral compounds (DNA, amino acids, and cyclodextrins) and molecular imprinting is further described in this review. Finally, the feasible solutions to the existing challenges and a vision for future development possibilities are discussed.

METCAM Is a Potential Biomarker for Predicting the Malignant Propensity of and as a Therapeutic Target for Prostate Cancer

Prostate cancer is the second leading cause of cancer-related death worldwide. This is because it is still unknown why indolent prostate cancer becomes an aggressive one, though many risk factors for this type of cancer have been suggested. Currently, many diagnostic markers have been suggested for predicting malignant prostatic carcinoma cancer; however, only a few, such as PSA (prostate-specific antigen), Prostate Health Index (PHI), and PCA3, have been approved by the FDA. However, each biomarker has its merits as well as shortcomings. The serum PSA test is incapable of differentiating prostate cancer from BPH and also has an about 25% false-positive prediction rate for the malignant status of cancer. The PHI test has the potential to replace the PSA test for the discrimination of BPH from prostate cancer and for the prediction of high-grade cancer avoiding unnecessary biopsies; however, the free form of PSA is unstable and expensive. PCA3 is not associated with locally advanced disease and is limited in terms of its prediction of aggressive cancer. Currently, several urine biomarkers have shown high potential in terms of being used to replace circulating biomarkers, which require a more invasive method of sample collection, such as via serum. Currently, the combined multiple tumor biomarkers may turn out to be a major trend in the diagnosis and assessment of the treatment effectiveness of prostate cancer. Thus, there is still a need to search for more novel biomarkers to develop a perfect cocktail, which consists of multiple biomarkers, in order to predict malignant prostate cancer and follow the efficacy of the treatment. We have discovered that METCAM, a cell adhesion molecule in the Ig-like superfamily, has great potential regarding its use as a biomarker for differentiating prostate cancer from BPH, predicting the malignant propensity of prostate cancer at the early premalignant stage, and differentiating indolent prostate cancers from aggressive cancers. Since METCAM has also been shown to be able to initiate the spread of prostate cancer cell lines to multiple organs, we suggest that it may be used as a therapeutic target for the clinical treatment of patients with malignant prostate cancer.

Ultrasensitive lateral flow immunoassay for staphylococcal enterotoxin B using nanosized fluorescent metal-organic frameworks

Owing to their outstanding optical properties and superior physical/chemical stability, dye-doped fluorescent nanoparticles (NPs) are growing exponentially as signal labels of immunochromatographic lateral flow immunoassay (LFA) for the detection of various analytes. However, the key challenge in the design of these fluorescent NPs is to confine the fluorophores inside NPs at extreme concentrations, at which dyes tend to self-quench resulting from the formation of non-fluorescent aggregates. Looking for other advantageous nanomaterials, we propose for the first time the use of a nanosized fluorescent metal-organic framework (nanoMOF) in LFA for the detection of staphylococcal enterotoxin B (SEB) as a model analyte. Featured by the chromophore assembly, the nanoMOF exhibited a high dye loading (∼60%) and strong fluorescence intensity, which was due to the reduced self-quenching of dyes in a variety of MOF matrices. The strong green fluorescence intensity of the nanoMOF gives a high contrast against the background of the strips and the sensitivity reflected by photoluminescence was improved by the enhanced antenna effect. Furthermore, due to the high surface area for antibody stemming, the limit of detection (LOD) of the MOF based LFA for SEB detection was as low as 0.025 ng mL^-1. The compatibility of the MOF based LFA with dairy samples and its stability under long-term storage conditions were also demonstrated. The integration of a nanoMOF into LFA to detect toxins could inspire the utilization of such nanomaterial-based labels in similar immunochromatographic testing methods to improve their performance.

Lateral Flow Immunoassays for Detecting Viral Infectious Antigens and Antibodies

Abundant immunological assays currently exist for detecting pathogens and identifying infected individuals, making detection of diseases at early stages integral to preventing their spread, together with the consequent emergence of global health crises. Lateral flow immunoassay (LFIA) is a test characterized by simplicity, low cost, and quick results. Furthermore, LFIA testing does not need well-trained individuals or laboratory settings. Therefore, it has been serving as an attractive tool that has been extensively used during the ongoing COVID-19 pandemic. Here, the LFIA strip's available formats, reporter systems, components, and preparation are discussed. Moreover, this review provides an overview of the current LFIAs in detecting infectious viral antigens and humoral responses to viral infections.

Point-of-care CRISPR/Cas biosensing technology: A promising tool for preventing the possible COVID-19 resurgence caused by contaminated cold-chain food and packaging

The ongoing coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused great public health concern and has been a global threat due to its high transmissibility and morbidity. Although the SARS-CoV-2 transmission mainly relies on the person-to-person route through the respiratory droplets, the possible transmission through the contaminated cold-chain food and packaging to humans has raised widespread concerns. This review discussed the possibility of SARS-CoV-2 transmission via the contaminated cold-chain food and packaging by tracing the occurrence, the survival of SARS-CoV-2 in the contaminated cold-chain food and packaging, as well as the transmission and outbreaks related to the contaminated cold-chain food and packaging. Rapid, accurate, and reliable diagnostics of SARS-CoV-2 is of great importance for preventing and controlling the COVID-19 resurgence. Therefore, we summarized the recent advances on the emerging clustered regularly interspaced short palindromic repeats (CRISPR)/Cas system-based biosensing technology that is promising and powerful for preventing the possible COVID-19 resurgence caused by the contaminated cold-chain food and packaging during the COVID-19 pandemic, including CRISPR/Cas system-based biosensors and their integration with portable devices (e.g., smartphone, lateral flow assays, microfluidic chips, and nanopores). Impressively, this review not only provided an insight on the possibility of SARS-CoV-2 transmission through the food supply chain, but also proposed the future opportunities and challenges on the development of CRISPR/Cas system-based detection methods for the diagnosis of SARS-CoV-2.

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.

Diagnostic Approaches For COVID-19: Lessons Learned and the Path Forward

Coronavirus disease 2019 (COVID-19) is a transmitted respiratory disease caused by the infection of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Although humankind has experienced several outbreaks of infectious diseases, the COVID-19 pandemic has the highest rate of infection and has had high levels of social and economic repercussions. The current COVID-19 pandemic has highlighted the limitations of existing virological tests, which have failed to be adopted at a rate to properly slow the rapid spread of SARS-CoV-2. Pandemic preparedness has developed as a focus of many governments around the world in the event of a future outbreak. Despite the largely widespread availability of vaccines, the importance of testing has not diminished to monitor the evolution of the virus and the resulting stages of the pandemic. Therefore, developing diagnostic technology that serves as a line of defense has become imperative. In particular, that test should satisfy three criteria to be widely adopted: simplicity, economic feasibility, and accessibility. At the heart of it all, it must enable early diagnosis in the course of infection to reduce spread. However, diagnostic manufacturers need guidance on the optimal characteristics of a virological test to ensure pandemic preparedness and to aid in the effective treatment of viral infections. Nanomaterials are a decisive element in developing COVID-19 diagnostic kits as well as a key contributor to enhance the performance of existing tests. Our objective is to develop a profile of the criteria that should be available in a platform as the target product. In this work, virus detection tests were evaluated from the perspective of the COVID-19 pandemic, and then we generalized the requirements to develop a target product profile for a platform for virus detection.

Preparation of Monoclonal Antibody against Deoxynivalenol and Development of Immunoassays

Fusarium toxins are the largest group of mycotoxins, which contain more than 140 known secondary metabolites of fungi. Deoxynivalenol (DON) is one of the most important compounds of this class due to its high toxicity and its potential to harm mankind and animals and a widespread contaminant of agricultural commodities, such as wheat, corn, barley, oats, bread, and biscuits. Herein, a hybridoma cell 8G2 secreting mAb against DON was produced by fusing the splenocytes with a tumor cell line Sp2/0. The obtained mAb had a high affinity (2.39 × 10⁹ L/mol) to DON. An indirect competitive Enzyme-Linked Immunosorbent Assay (ic-ELISA) showed that the linear range for DON detection was 3.125–25 μg/mL, and the minimum inhibitory concentration (IC₅₀) was 18.125 μg/mL with a limit of detection (LOD) of 7.875 μg/mL. A colloidal gold nanoparticle (AuNP) with 20 nm in diameter was synthesized for on-site detection of DON within 10 min with vLOD of 20 μg/mL. To improve the limit of detection, the gold nanoflower (AuNF) with a larger size (75 nm) was used to develop the AuNF-based strip with vLOD of 6.67 μg/mL. Compared to the vLOD of a convectional AuNP-based strip, the AuNF-based strip was three times lower. Herein, three immunoassay methods (ic-ELISA and AuNP/AuNF-based strips) were successfully developed, and these methods could be applied for the DON detection in agricultural products.

Characterization of wax valving and μPIV analysis of microscale flow in paper-fluidic devices for improved modeling and design

Paper-fluidic devices are a popular platform for point-of-care diagnostics due to their low cost, ease of use, and equipment-free detection of target molecules. They are limited, however, by their lack of sensitivity and inability to incorporate more complex processes, such as nucleic acid amplification or enzymatic signal enhancement. To address these limitations, various valves have previously been implemented in paper-fluidic devices to control fluid obstruction and release. However, incorporation of valves into new devices is a highly iterative, time-intensive process due to limited experimental data describing the microscale flow that drives the biophysical reactions in the assay. In this paper, we tested and modeled different geometries of thermally actuated valves to investigate how they can be more easily implemented in an LFIA with precise control of actuation time, flow rate, and flow pattern. We demonstrate that bulk flow measurements alone cannot estimate the highly variable microscale properties and effects on LFIA signal development. To further quantify the microfluidic properties of paper-fluidic devices, micro-particle image velocimetry was used to quantify fluorescent nanoparticle flow through the membranes and demonstrated divergent properties from bulk flow that may explain additional variability in LFIA signal generation. Altogether, we demonstrate that a more robust characterization of paper-fluidic devices can permit fine-tuning of parameters for precise automation of multi-step assays and inform analytical models for more efficient design.

Rapid Detection of Neisseria gonorrhoeae Genomic DNA Using Gold Nanoprobes Which Target the Gonococcal DNA Uptake Sequence

The rapid spread of antimicrobial resistant Neisseria gonorrhoeae continues to pose a serious threat to global health. To successfully treat and control gonococcal infections, rapid diagnosis is critical. Currently, nucleic acid amplification tests are the recommended diagnostic, however, these are both technically demanding and time consuming, making them unsuitable for resource-poor clinics. Consequently, there is a substantial need for an affordable, point-of-care diagnostic to use in these settings. In this study, DNA-functionalised gold nanoparticles (gold nanoprobes), with the ability to specifically detect the DNA Uptake Sequence (DUS) of Neisseria gonorrhoeae, were prepared. Using complementary annealing, the gold nanoprobes were shown to hybridise to genomic gonococcal DNA, causing a significant shift in their salt stability. By exploiting the shift in nanoprobe stability under the presence of target DNA, a solution-based colorimetric diagnostic for gonococcal DNA was prepared. Detection of purified genomic DNA was achieved in under 30 minutes, with a detection limit of 15.0 ng. Significantly, testing with DNA extracted from an off-target control organism suggested specificity for Neisseria. These results highlight the potential of DUS-specific gold nanoprobes in the rapid point-of-care diagnosis of gonococcal infections.

Vanadium Disulfide Nanosheet Boosts Optical Signal Brightness as a Superior Enzyme Label to Improve the Sensitivity of Lateral Flow Immunoassay

The color-enzyme lateral flow immunoassay (LFIA) has attracted widespread attention to expand the detection range and improve sensitivity via amplifying the color signal after catalyzing the substrate. As a kind of layered transition-metal dichalcogenide (TMD), the vanadium disulfide nanosheet (VS₂NS) possesses superior peroxidase-like catalytic activity. Here, a VS₂NS was applied as an enzyme label in the LFIA to detect 17β-estradiol (E2). Compared to natural horseradish peroxidase, the VS₂NS expresses a more prominent enzyme catalytic performance, stability, and adsorption ability. Under optimal conditions, the calculated limit of detection (cLOD) of the VS₂NS-based LFIA is 0.065 ng mL^-1 for E2, which is sixfold lower than that of the optimized colloidal nanoparticle-based LFIA (cLOD = 0.406 ng mL^-1). Besides, the detection linear range of the VS₂NS-based LFIA can be widened by 1.5 times after the catalytic reaction. Moreover, the VS₂NS-based LFIA exhibits excellent practicability in real sample detection. Simultaneously, this study helps open up the application of the VS₂NS in the trace analysis of LFIAs, which can broaden TMDs' scope of application and better show their properties of color enzymes.

Influenza A, Influenza B, and SARS-CoV-2 Similarities and Differences – A Focus on Diagnosis

In late December 2019, the first cases of viral pneumonia caused by an unidentified pathogen were reported in China. Two years later, SARS-CoV-2 was responsible for almost 450 million cases, claiming more than 6 million lives. The COVID-19 pandemic strained the limits of healthcare systems all across the world. Identifying viral RNA through real-time reverse transcription-polymerase chain reaction remains the gold standard in diagnosing SARS-CoV-2 infection. However, equipment cost, availability, and the need for trained personnel limited testing capacity. Through an unprecedented research effort, new diagnostic techniques such as rapid diagnostic testing, isothermal amplification techniques, and next-generation sequencing were developed, enabling accurate and accessible diagnosis. Influenza viruses are responsible for seasonal outbreaks infecting up to a quarter of the human population worldwide. Influenza and SARS-CoV-2 present with flu-like symptoms, making the differential diagnosis challenging solely on clinical presentation. Healthcare systems are likely to be faced with overlapping SARS-CoV-2 and Influenza outbreaks. This review aims to present the similarities and differences of both infections while focusing on the diagnosis. We discuss the clinical presentation of Influenza and SARS-CoV-2 and techniques available for diagnosis. Furthermore, we summarize available data regarding the multiplex diagnostic assay of both viral infections.

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.

Simple manipulation of enzyme-linked immunosorbent assay (ELISA) using an automated microfluidic interface

Among lateral flow immunoassay (LFIA) platforms, enzyme-based LFIAs provide signal amplification to improve sensitivity. However, most enzyme-based LFIAs require multiple timed steps, complicating their utility in point-of-care testing (POCT). Here, we report a microfluidic interface for LFIAs that automates sample, buffer, and reagent addition, greatly simplifying operation while achieving the high analytical stringency associated with more complex assays. The microfluidic interface also maintains the low cost and small footprint of standard LFIAs. The platform is fabricated from a combination of polyester film, double-sided adhesive tape, and nitrocellulose, and fits in the palm of your hand. All reagents are dried on the nitrocellulose to facilitate sequential reagent delivery, and the sample is used as the wash buffer to minimize steps. After the sample addition, a user simply waits 15 min for a colorimetric result. This manuscript discusses the development and optimization of the channel geometry to achieve a simple step enzyme amplified immunoassay. As a proof-of-concept target, we selected lipoarabinomannan (LAM), a WHO identified urinary biomarker of active tuberculosis, to demonstrate the device feasibility and reliability. The results revealed that the device successfully detected LAM in phosphate buffer (PBS) as well as spiked urine samples within 15 min after sample loading. The minimum concentration of color change was achieved at 25 ng mL-1.

Improvement in sensitivity for lateral flow immunoassay of ferritin using novel device design based on gold-enhanced gold nanoparticles

This work introduces a low-cost adhesive tape combined with a hydroxylamine/polyvinyl alcohol/polyethylene oxide (HA/PVA/PEO) blend film to fabricate novel devices for improving sensitivity of gold nanoparticle (AuNP)-based lateral flow immunoassays (LFIAs) via two platforms: (1) LFIA device with integrated gold enhancement and (2) LFIA device with two independent sample inlets. The detection of ferritin has been used for proof-of-concept. The adhesive tape inserted in the devices assists to separate two solutions independently flowing from two different inlets toward a nitrocellulose membrane. On-device gold enhancement was achieved by the enlargement of AuNPs via the catalytic reaction of KAuCl₄ and HA using the HA/PVA/PEO blend film easily prepared via a solution-casting technique, which could delay the flow of HA released from the film for 180s and improve storage stability of the device. Under optimal conditions evaluated by naked eyes, the gold enhancement (LOD = 0.5 ng/mL) and double-sample inlet (LOD = 2 ng/mL) devices exhibited 20-fold and fivefold higher sensitivity respectively than a conventional device, verifying the sensitivity improvement. Furthermore, the proposed device was successfully detected ferritin in human serum samples within 10 min via naked-eye observation, exhibiting rapidity and simplicity of use, and the capability to perform on-site assays.

Paper-microfluidic signal-enhanced immunoassays

Over the past decade, paper-based microfluidic devices have become popular for their simplicity and ability to conduct diagnostic tests at a low cost. An important class of diagnostic assays that paper-based analytical devices have been used for is immunoassays. The lateral flow immunoassay (LFIA), of which the home pregnancy test is the most prominent example, has been one of the most commercially successful membrane-based diagnostic tests. Yet, the analytical sensitivity of LFIAs is lower than the corresponding laboratory technique called ELISA (enzyme-linked immunoassay). As a consequence, traditional LFIAs fail to deliver on the promise of bedside diagnostic testing for many applications. Recognizing this shortcoming, several new developments have been made by researchers to enhance the sensitivity of membrane-based immunoassays. In this chapter, we present the various strategies that have been employed to this end. In the end, we present a brief SWOT analysis to guide future work in this area.

Ball pen writing-without-ink: a truly simple and accessible method for sensitivity enhancement in lateral flow assays

Lateral flow assays (LFAs), a popular point-of-care testing platform, have found widespread applications from laboratory to clinics. However, LFA-based testing is still subject to limited detection sensitivity, especially for classical gold nanoparticle-based LFAs. Inspired by traditional pen-based writing technologies, we developed a ball pen writing-without-ink method to amplify the detection signal of LFAs through controlling fluid flow rate. An enhancement of detection sensitivity by two times was obtained. Since the underlying mechanism of this method to improve detection sensitivity is to control the flow rate of the liquid on paper, it may be suitable for most paper-based platforms.

Spectral image contrast-based flow digital nanoplasmon-metry for ultrasensitive antibody detection

Background

Gold nanoparticles (AuNPs) have been widely used in local surface plasmon resonance (LSPR) immunoassays for biomolecule sensing, which is primarily based on two conventional methods: absorption spectra analysis and colorimetry. The low figure of merit (FoM) of the LSPR and high-concentration AuNP requirement restrict their limit of detection (LOD), which is approximately ng to μg mL⁻¹ in antibody detection if there is no other signal or analyte amplification. Improvements in sensitivity have been slow in recent for a long time, and pushing the boundary of the current LOD is a great challenge of current LSPR immunoassays in biosensing.

Results

In this work, we developed spectral image contrast-based flow digital nanoplasmon-metry (Flow DiNM) to push the LOD boundary. Comparing the scattering image brightness of AuNPs in two neighboring wavelength bands near the LSPR peak, the peak shift signal is strongly amplified and quickly detected. Introducing digital analysis, the Flow DiNM provides an ultrahigh signal-to-noise ratio and has a lower sample volume requirement. Compared to the conventional analog LSPR immunoassay, Flow DiNM for anti-BSA detection in pure samples has an LOD as low as 1 pg mL⁻¹ within only a 15-min detection time and 500 μL sample volume. Antibody assays against spike proteins of SARS-CoV-2 in artificial saliva that contained various proteins were also conducted to validate the detection of Flow DiNM in complicated samples. Flow DiNM shows significant discrimination in detection with an LOD of 10 pg mL⁻¹ and a broad dynamic detection range of five orders of magnitude.

Conclusion

Together with the quick readout time and simple operation, this work clearly demonstrated the high sensitivity and selectivity of the developed Flow DiNM in rapid antibody detection. Spectral image contrast and digital analysis further provide a new generation of LSPR immunoassay with AuNPs.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-01188-6.

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.

High sensitivity and rapid detection of hepatitis B virus DNA using lateral flow biosensors based on Au@Pt nanorods in the absence of hydrogen peroxide

Au@Pt nanorods with enhanced oxidase-like activity were designed and used as signal probes to construct LFBs for the high sensitivity detection of hepatitis B virus DNA (HBV-DNA).

Lateral Flow Immunoassay with Quantum-Dot-Embedded Silica Nanoparticles for Prostate-Specific Antigen Detection

Prostate cancer can be detected early by testing the presence of prostate-specific antigen (PSA) in the blood. Lateral flow immunoassay (LFIA) has been used because it is cost effective and easy to use and also has a rapid sample-to-answer process. Quantum dots (QDs) with very bright fluorescence have been previously used to improve the detection sensitivity of LFIAs. In the current study, a highly sensitive LFIA kit was devised using QD-embedded silica nanoparticles. In the present study, only a smartphone and a computer software program, ImageJ, were used, because the developed system had high sensitivity by using very bright nanoprobes. The limit of PSA detection of the developed LFIA system was 0.138 ng/mL. The area under the curve of this system was calculated as 0.852. The system did not show any false-negative result when 47 human serum samples were analyzed; it only detected PSA and did not detect alpha-fetoprotein and newborn calf serum in the samples. Additionally, fluorescence was maintained on the strip for 10 d after the test. With its high sensitivity and convenience, the devised LFIA kit can be used for the diagnosis of prostate cancer.

DNAzyme-Functionalized Nanomaterials: Recent Preparation, Current Applications, and Future Challenges

DNAzyme-nanomaterial bioconjugates are a popular hybrid and have received major attention for diverse biomedical applications, such as bioimaging, biosensor development, cancer therapy, and drug delivery. Therefore, significant efforts are made to develop different strategies for the preparation of inorganic and organic nanoparticles (NPs) with specific morphologies and properties. DNAzymes functionalized with metal-organic frameworks (MOFs), gold nanoparticles (AuNPs), graphene oxide (GO), and molybdenum disulfide (MoS₂ ) are introduced and summarized in detail in this review. Moreover, the focus is on representative examples of applications of DNAzyme-nanomaterials over recent years, especially in bioimaging, biosensing, phototherapy, and stimulation response delivery in living systems, with their several advantages and drawbacks. Finally, the perspective regarding the future directions of research addressing these challenges is also discussed and highlighted.

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.

Nano-functionalized paper-based IoT enabled devices for point-of-care testing: a review

Over the last few years, the microfluidics phenomenon coupled with the Internet of Things (IoT) using innovative nano-functional materials has been recognized as a sustainable and economical tool for point-of-care testing (POCT) of various pathogens influencing human health. The sensors based on these phenomena aim to be designed for cost-effectiveness, make it handy, environment-friendly, and get an accurate, easy, and rapid response. Considering the burgeoning importance of analytical devices in the healthcare domain, this review paper is based on the gist of sensing aspects of the microfabricated paper-based analytical devices (μPADs). The article discusses the various used design methodologies and fabrication approaches and elucidates the recently reported surface modification strategies, detection mechanisms viz., colorimetric, electrochemical, fluorescence, electrochemiluminescence, etc. In a nutshell, this article summarizes the state-of-the-art research work carried out over the nano functionalized paper-based analytical devices and associated challenges/solutions in the point of care testing domain.

Probing polymer brushes with electrochemical impedance spectroscopy: a mini review

Polymer brushes are frequently used as surface-tethered antifouling layers in biosensors to improve sensor surface-analyte recognition in the presence of abundant non-target molecules in complex biological samples by suppressing nonspecific interactions. However, because brushes are complex systems highly responsive to changes in their surrounding environment, studying their properties remains a challenge. Electrochemical impedance spectroscopy (EIS) is an emerging method in this context. In this mini review, we aim to elucidate the potential of EIS for investigating the physicochemical properties and structural aspects of polymer brushes. The application of EIS in brush-based biosensors is also discussed. Most common principles employed in these biosensors are presented, as well as interpretation of EIS data obtained in such setups. Overall, we demonstrate that the EIS-polymer brush pairing has a considerable potential for providing new insights into brush functionalities and designing highly sensitive and specific biosensors.

Advanced Nanomaterials for Preparedness Against (Re-)Emerging Viral Diseases

While the coronavirus disease (COVID-19) accounts for the current global pandemic, the emergence of other unknown pathogens, named "Disease X," remains a serious concern in the future. Emerging or re-emerging pathogens continue to pose significant challenges to global public health. In response, the scientific community has been urged to create advanced platform technologies to meet the ever-increasing needs presented by these devastating diseases with pandemic potential. This review aims to bring new insights to allow for the application of advanced nanomaterials in future diagnostics, vaccines, and antiviral therapies, thereby addressing the challenges associated with the current preparedness strategies in clinical settings against viruses. The application of nanomaterials has advanced medicine and provided cutting-edge solutions for unmet needs. Herein, an overview of the currently available nanotechnologies is presented, highlighting the significant features that enable them to control infectious diseases, and identifying the challenges that remain to be addressed for the commercial production of nano-based products is presented. Finally, to conclude, the development of a nanomaterial-based system using a "One Health" approach is suggested. This strategy would require a transdisciplinary collaboration and communication between all stakeholders throughout the entire process spanning across research and development, as well as the preclinical, clinical, and manufacturing phases.

Using airflow-driven, evaporative gradients to improve sensitivity and fluid control in colorimetric paper-based assays

Microfluidic paper-based analytical devices (μPADs) are foundational devices for point-of-care testing, yet suffer from limitations in regards to their sensitivity and capability in handling complex assays. Here, we demonstrate an airflow-based, evaporative method that is capable of manipulating fluid flows within paper membranes to offer new functionalities for multistep delivery of reagents and improve the sensitivity of μPADs by 100-1000 times. This method applies an air-jet to a pre-wetted membrane, generating an evaporative gradient such that any solutes become enriched underneath the air-jet spot. By controlling the lateral position of this spot, the solutes in the paper strip are enriched and follow the air jet trajectory, driving the reactions and enhancing visualization for colorimetric readout in multistep assays. The technique has been successfully applied to drive the sequential delivery in multistep immunoassays as well as improve sensitivity for colorimetric detection assays for nucleic acids and proteins via loop-mediated isothermal amplification (LAMP) and ELISA. For colorimetric LAMP detection of the COVID-19 genome, enrichment of the solution on paper could enhance the contrast of the dye in order to more clearly distinguish between the positive and negative results to achieve a sensitivity of 3 copies of SARS-Cov-2 RNAs. For ELISA, enrichment of the oxidized TMB substrate yielded a sensitivity increase of two-to-three orders of magnitude when compared to non-enriched samples - having a limit of detection of around 200 fM for IgG. Therefore, this enrichment method represents a simple process that can be easily integrated into existing detection assays for controlling fluid flows and improving detection of biomarkers on paper.

On-Site Detection of Carcinoembryonic Antigen in Human Serum

Real-time connectivity and employment of sustainable materials empowers point-of-care diagnostics with the capability to send clinically relevant data to health care providers even in low-resource settings. In this study, we developed an advantageous kit for the on-site detection of carcinoembryonic antigen (CEA) in human serum. CEA sensing was performed using cellulose-based lateral flow strips, and colorimetric signals were read, processed, and measured using a smartphone-based system. The corresponding immunoreaction was reported by polydopamine-modified gold nanoparticles in order to boost the signal intensity and improve the surface blocking and signal-to-noise relationship, thereby enhancing detection sensitivity when compared with bare gold nanoparticles (up to 20-fold in terms of visual limit of detection). Such lateral flow strips showed a linear range from 0.05 to 50 ng/mL, with a visual limit of detection of 0.05 ng/mL and an assay time of 15 min. Twenty-six clinical samples were also tested using the proposed kit and compared with the gold standard of immunoassays (enzyme linked immunosorbent assay), demonstrating an excellent correlation (R = 0.99). This approach can potentially be utilized for the monitoring of cancer treatment, particularly at locations far from centralized laboratory facilities.