Dual-layered and double-targeted nanogold based lateral flow immunoassay for influenza virus

Rapid antigen test based on gold nanoparticles embedded with highly sensitive surface-enhanced Raman scattering probes for detecting antigen concentrations below the visual detection limit

The COVID-19 pandemic has proven that people expect medically high accuracy, sensitivity, and rapid testing for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) detection. These needs have led to developing various testing methods and platforms to replace reverse transcription polymerase chain reaction (RT-PCR), the golden standard in diagnostics. Rapid antigen testing (RAT) using gold nanoparticles (AuNPs) has been developed to compensate for the shortcomings of RT-PCR. However, RAT still shows less sensitive results because it is judged by sight only. Here, we applied gold-nanobridged nanogap particles (Au-NNPs) to conventional lateral flow assay (LFA) technology for high-sensitivity testing of samples containing antigen concentration below the visual detection limit through surface-enhanced Raman scattering (SERS) signal detection. By using Raman dye-embedded Au-NNPs showing susceptible Raman signals instead of simply using gold nanoparticles, the sensitivity of the conventional RAT has been significantly increased. Therefore, it was possible to increase the sensitivity of RAT combined with simplicity and rapidity of the approach. These developments will lead to characterization applications of bio-nanotechnology and nanoparticles for health and social issues.

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.

Effect of Selenium Nanoparticle Size on IL-6 Detection Sensitivity in a Lateral Flow Device

Sepsis is the body's response to an infection. Existing diagnostic testing equipment is not available in primary care settings and requires long waiting times. Lateral flow devices (LFDs) could be employed in point-of-care (POC) settings for sepsis detection; however, they currently lack the required sensitivity. Herein, LFDs are constructed using 150-310 nm sized selenium nanoparticles (SeNPs) and are compared to commercial 40 nm gold nanoparticles (AuNPs) for the detection of the sepsis biomarker interleukin-6 (IL-6). Both 310 and 150 nm SeNPs reported a lower limit of detection (LOD) than 40 nm AuNPs (0.1 ng/mL compared to 1 ng/mL), although at the cost of test line visual intensity. This is to our knowledge the first use of larger SeNPs (>100 nm) in LFDs and the first comparison of the effect of the size of SeNPs on assay sensitivity in this context. The results herein demonstrate that large SeNPs are viable alternatives to existing commercial labels, with the potential for higher sensitivity than standard 40 nm AuNPs.

Ultrasensitive nano-gold labelled, duplex lateral flow immunochromatographic assay for early detection of sugarcane mosaic viruses

Sugarcane is one of the important food and bioenergy crops, cultivated all over the world except European continent. Like many other crops, sugarcane production and quality are hampered by various plant pathogens, among them viruses that infect systemically and cause severe impact to cane growth. The viruses are efficiently managed by their elimination through tissue culture combined with molecular diagnostics, which could detect virus titre often low at 10-12 g mL-1. To harmonize the virus diagnostics by molecular methods, we established a nanocatalysis-based high sensitive lateral flow immunochromatographic assay (LFIA) simultaneously to detect two major sugarcane viruses associated with mosaic disease in sugarcane. LFIA is known for poor sensitivity and stability with its signalling conjugates. However, we synthesized positively charged Cysteamine-gold nanoparticles and used them to prepare highly stable to sensitive immunoconjugates and as a colourimetric detection label. Further nanogold signal enhancement was performed on LFIA to obtain a high detection sensitivity, which is higher than the conventional immunoassays. The linear detection range of the nano-LIFA was 10-6 to 10-9 g mL-1, and with the signal enhancement, the LOD reached up to 10-12 g ml-1. This research paper provides relative merits and advancement on nano-LFIA for specific detection of sugarcane viruses in sugarcane for the first time.

Advanced Point-of-Care Testing Technologies for Human Acute Respiratory Virus Detection

The ever-growing global threats to human life caused by the human acute respiratory virus (RV) infections have cost billions of lives, created a significant economic burden, and shaped society for centuries. The timely response to emerging RVs could save human lives and reduce the medical care burden. The development of RV detection technologies is essential for potentially preventing RV pandemic and epidemics. However, commonly used detection technologies lack sensitivity, specificity, and speed, thus often failing to provide the rapid turnaround times. To address this problem, new technologies are devised to address the performance inadequacies of the traditional methods. These emerging technologies offer improvements in convenience, speed, flexibility, and portability of point-of-care test (POCT). Herein, recent developments in POCT are comprehensively reviewed for eight typical acute respiratory viruses. This review discusses the challenges and opportunities of various recognition and detection strategies and discusses these according to their detection principles, including nucleic acid amplification, optical POCT, electrochemistry, lateral flow assays, microfluidics, enzyme-linked immunosorbent assays, and microarrays. The importance of limits of detection, throughput, portability, and specificity when testing clinical samples in resource-limited settings is emphasized. Finally, the evaluation of commercial POCT kits for both essential RV diagnosis and clinical-oriented practices is included.

Pre-clinically evaluated visual lateral flow platform using influenza A and B nucleoprotein as a model and its potential applications

A visual colorimetric rapid screening system based on a lateral flow device for simultaneous detection and differentiation between influenza A and B nucleoprotein as a model was developed. Monoclonal antibodies, specific for either influenza A or B nucleoproteins, were evaluated for their reactivities and were used as targeting ligands. With the best antibody pairs selected, the system exhibited good specificity to both viruses without cross reactivity to other closely related respiratory viruses. Further semi-quantitative analysis using a strip reader revealed that the system is capable of detecting influenza A and B protein content as low as 0.04 and 1 ng per test, respectively, using a sample volume as low as 100 μL, within 10 minutes (R² = 0.9652 and 0.9718). With a performance comparison to the commercial tests, the system demonstrated a four-to-eight-fold higher sensitivity. Pre-clinical evaluation with 101 nasopharyngeal swabs reveals correlated results with a standard molecular approach, with 89% and 83% sensitivity towards influenza A and B viruses, and 100% specificity for both viruses.

Visual colorimetric rapid screening system based on lateral flow device for influenza A and B virus detection as a model and its pre-clinical evaluation.

Management of Coronavirus Disease 2019 (COVID-19) Pandemic: From Diagnosis to Treatment Strategies

Following the emergence of severe acute respiratory syndrome (SARS) in 2002 and the Middle East respiratory syndrome (MERS) in 2012, the world is now combating a third large-scale outbreak caused by a coronavirus, the coronavirus disease 2019 (COVID-19). After the rapid spread of SARS-coronavirus (CoV)-2 (the virus causing COVID-19) from its origin in China, the World Health Organization (WHO) declared a Public Health Emergency of International Concern (PHEIC) on January 30, 2020. From the beginning of the COVID-19 pandemic, a significant number of studies have been conducted to better understand the biology and pathogenesis of the novel coronavirus, and to aid in developing effective treatment regimens, therapeutics, and vaccines. This review focuses on the recent advancements in the rapidly evolving areas of clinical care and management of COVID-19. The emerging strategies for the diagnosis and treatment of this disease are explored, and the development of effective vaccines is reviewed.

Highly sensitive protein detection using recombinant spores and lateral flow immunoassay

Lateral flow immunoassays (LFIs) can be used to detect intact bacteria or spores; when gold nanoparticles (AuNPs) are used as the signal reporters, the detection limits are very low. Spore-based surface display has been widely studied for enzyme immobilization and live-nontoxic oral vaccines. In this study, recombinant spores were used to improve the sensitivity of a LFI. We developed a test kit that combines streptavidin-displayed spores with a LFI assay for rapid protein detection. The recombinant spores served as a signal amplifier and AuNPs were used as the signal reporters. For detection of β-galactosidase, which was used as the model protein, the detection limit was about 10^-15 mol, while that of the conventional LFI is about 10^-12 mol. In both methods, nanogold was used as the colorimetric signal and could be observed with the naked eye. This method improved LFI sensitivity without sacrificing its advantages. Furthermore, enhanced green fluorescent protein (eGFP) was also displayed on the surface of the streptavidin-displayed spores. Without AuNPs, the fluorescent recombinant spores acted as the signal, which could be detected by a fluorescence detector, such as a fluorescence microscope. The detection limit was 10^-16 mol under fluorescence microscopy whose magnification was 25-fold. Therefore, in conclusion, in this proof of concept study, the detection limits of both proposed methods were far superior to those of traditional LFI assay.

Sensitivity enhancement in lateral flow assays: a systems perspective

Lateral flow assays (LFAs) are rapid, inexpensive, easy-to-manufacture and -use tests widely employed in medical and environmental applications, particularly in low resource settings. Historically, LFAs have been stigmatized as having limited sensitivity. However, as their global usage expands, extensive research has demonstrated that it is possible to substantially improve LFA sensitivity without sacrificing their advantages. In this critical review, we have compiled state-of-the-art approaches to LFA sensitivity enhancement. Moreover, we have organized and evaluated these approaches from a system-level perspective, as we have observed that the advantages and disadvantages of each approach have arisen from the integrated and tightly interconnected chemical, physical, and optical properties of LFAs.

Magnetic SERS Strip for Sensitive and Simultaneous Detection of Respiratory Viruses

Rapid and early diagnosis of respiratory viruses is key to preventing infections from spreading and guiding treatments. Here, we developed a sensitive and quantitative surface-enhanced Raman scattering-based lateral flow immunoassay (SERS-based LFIA) strip for simultaneous detection of influenza A H1N1 virus and human adenovirus (HAdV) by using Fe₃O₄@Ag nanoparticles as magnetic SERS nanotags. The new type of Fe₃O₄@Ag magnetic tags, which were conjugated with dual-layer Raman dye molecules and target virus-capture antibodies, performs the following functions: specific recognition and magnetic enrichment of target viruses in the solution and SERS detection of the viruses on the strip. Based on this strategy, the magnetic SERS strip can directly be used for real biological samples without any sample pretreatment steps. The limits of detection for H1N1 and HAdV were 50 and 10 pfu/mL, respectively, which were 2000 times more sensitive than those from the standard colloidal gold strip method. Moreover, the proposed strip is easy to operate, rapid, stable, and can achieve high throughput and is thus a potential tool for early detection of virus infection.

How to Improve Sensitivity of Sandwich Lateral Flow Immunoassay for Corpuscular Antigens on the Example of Potato Virus Y?

A simple approach was proposed to decrease the detection limit of sandwich lateral flow immunoassay (LFIA) by changing the conditions for binding between a polyvalent antigen and a conjugate of gold nanoparticles (GNPs) with antibodies. In this study, the potato virus Y (PVY) was used as the polyvalent antigen, which affects economically important plants in the Solanaceae family. The obtained polyclonal antibodies that are specific to PVY were characterized using a sandwich enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR). For LFIA, the antibodies were conjugated with GNPs with a diameter of 17.4 ± 1.0 nm. We conducted LFIAs using GNP conjugates in a dried state on the test strip and after pre-incubation with a sample. Pre-incubating the GNP conjugates and sample for 30 s was found to decrease the detection limit by 60-fold from 330 ng∙mL^-1 to 5.4 ng∙mL^-1 in comparison with conventional LFIA. The developed method was successfully tested for its ability to detect PVY in infected and uninfected potato leaves. The quantitative results of the proposed LFIA with pre-incubation were confirmed by ELISA, and resulted in a correlation coefficient of 0.891. The proposed approach is rapid, simple, and preserves the main advantages of LFIA as a non-laboratory diagnostic method.

Detecting Biothreat Agents: From Current Diagnostics to Developing Sensor Technologies

Although a fundamental understanding of the pathogenicity of most biothreat agents has been elucidated and available treatments have increased substantially over the past decades, they still represent a significant public health threat in this age of (bio)terrorism, indiscriminate warfare, pollution, climate change, unchecked population growth, and globalization. The key step to almost all prevention, protection, prophylaxis, post-exposure treatment, and mitigation of any bioagent is early detection. Here, we review available methods for detecting bioagents including pathogenic bacteria and viruses along with their toxins. An introduction placing this subject in the historical context of previous naturally occurring outbreaks and efforts to weaponize selected agents is first provided along with definitions and relevant considerations. An overview of the detection technologies that find use in this endeavor along with how they provide data or transduce signal within a sensing configuration follows. Current "gold" standards for biothreat detection/diagnostics along with a listing of relevant FDA approved in vitro diagnostic devices is then discussed to provide an overview of the current state of the art. Given the 2014 outbreak of Ebola virus in Western Africa and the recent 2016 spread of Zika virus in the Americas, discussion of what constitutes a public health emergency and how new in vitro diagnostic devices are authorized for emergency use in the U.S. are also included. The majority of the Review is then subdivided around the sensing of bacterial, viral, and toxin biothreats with each including an overview of the major agents in that class, a detailed cross-section of different sensing methods in development based on assay format or analytical technique, and some discussion of related microfluidic lab-on-a-chip/point-of-care devices. Finally, an outlook is given on how this field will develop from the perspective of the biosensing technology itself and the new emerging threats they may face.

Double-enhanced lateral flow immunoassay for potato virus X based on a combination of magnetic and gold nanoparticles

This study presents the joint use of magnetic nanoparticles (MNPs) and gold nanoparticles (GNPs) for double enhancement in a lateral flow immunoassay (LFIA). The study realizes two types of enhancement: (1) increasing the concentration of analytes in the samples using conjugates of MNPs with specific antibodies and (2) increasing the visibility of the label through MNP aggregation caused by GNPs. The proposed strategy was implemented using a LFIA for potato virus X (PVX), a significant potato pathogen. MNPs conjugated with biotinylated antibodies specific to PVX and GNPs conjugated with streptavidin were synthesized and characterized. The LFIAs with and without the proposed enhancements were compared. The double-enhanced LFIA achieved the highest sensitivity, equal to 0.25 ng mL^-1 and 32 times more sensitivity than the non-enhanced LFIA (detection limit: 8 ng mL^-1). LFIAs using one of the types of amplification (magnetic concentration without GNPs-causing aggregation or MNP aggregation without the concentration stage) showed intermediate levels of sensitivity. The double-enhanced LFIA was successfully used for PVX detection in potato leaves. The results for PVX detection in the infected plants were similar for the double-enhanced LFIA developed and the conventional LFIA based on the GNP conjugates; however, the new system provided significant coloring enhancement. This study confirmed that a simple combination of MNPs and GNPs has great potential for high-sensitivity detection and could possibly be adopted for LFIAs of other compounds.

Dual dye-loaded Au@Ag coupled to a lateral flow immunoassay for the accurate and sensitive detection of Mycoplasma pneumoniae infection

We present an attractive model of surface-enhanced Raman scattering-based lateral flow immunoassay (SERS-LFIA) for the sensitive and accurate detection of Mycoplasma pneumoniae (MP) infection in human serum. The SERS-LFIA strip uses Au@Ag nanoparticles (Au@Ag NPs) loaded with two layers of Raman dye 5,5′-dithiobis-(2-nitrobenzoic acid) (DTNB) as SERS tags. The advantages of the dual dye-loaded SERS tags (Au/DTNB@Ag/DTNB) are the high sensitivity and the bioconjugation flexibility of the detection antibody. As determined from our SERS-LFIA strip, human IgM was quantified by monitoring the SERS signal on the test line. The limit of detection for human IgM was 0.1 ng mL⁻¹, which was 100 times more sensitive than that by using the colorimetric method. Our assay results for 20 MP-specific IgM positive serum specimens showed 100% accuracy and detection rate, whereas the parallel enzyme-linked immunosorbent assay only showed 85% detection rate. The SERS-LFIA strip also exhibited high specificity and potential clinical applications. Therefore, our SERS-based LFIA strip has strong potential for practical applications in the sensitive and rapid detection of MP.

Schematic illustration of quantitative detection of human IgM using SERS-based lateral flow immunoassay.

Recent advances in nanoparticle-based lateral flow immunoassay as a point-of-care diagnostic tool for infectious agents and diseases

Recent advances in lateral flow immunoassay-based devices as a point-of-care analytical tool for the detection of infectious diseases are reviewed.

Gold nanocage-based lateral flow immunoassay for immunoglobulin G

The authors describe a gold nanocage-based lateral flow strip biosensor (LFSB) for low-cost and sensitive detection of IgG. This protein was used as a model analyte to demonstrate the proof-of-concept. The method combines the unique optical properties of gold nanocages (GNCs) with highly efficient chromatographic separation. A sandwich-type of immunoreactions occurs on the GNC-based LFSB which has the attractive features of avoiding multiple incubation, separation, and washing steps. The captured GNCs on the purple test zone and control zone of the biosensor are producing characteristic purple bands, and this enables IgG even to be visually detected. Quantitatation was accomplished by reading the intensities of the bands with a portable strip reader. The LFSB fabrication and assay parameters were optimized. The biosensor displays a linear response in the 0.5 to 50 ng·mL^-1 IgG concentration range, and it has a 15 min assay time. The detection limit is 0.1 ng·mL^-1 of IgG, which is 2.5 times lower than that when using a gold nanoparticle-based LFSB. In our perception, this assay has a wide potential for the detection of other proteins and species for which respective antibodies are available.

Carbon nanotag based visual detection of influenza A virus by a lateral flow immunoassay

The authors report on a rapid and direct visual test for the detection of influenza A virus using a carbon nanotag based lateral flow assay. Carbon nanoparticles in the form of nanostrings are acting as reporters. As carbon nanotags accumulate in the test zone due to formation of an antibody-antigen-carbon nanotag antibody complex, and this allows for the direct visualization of the analytical signal. Under optimal conditions, influenza A virus can be determined in allantoic fluid inoculated with the virus with a limit of detection of 350 TCID₅₀.mL⁻¹ (i.e., the 50% tissue culture infectious dose). No interference is found for several other tested proteins, and for closely related viruses. Cell lysates containing different seasonal strains of influenza A viruses (including the H1N1 and H3N2 strains) collected from clinical samples were analyzed. It is demonstrated that the method can detect both influenza A viruses without interference by biological matrices. In our perception, this method has a wide potential in that it may be extended to a generally applicable platform for rapid diagnosis influenza A viruses.

Graphical abstract

Aptamer-based 'point-of-care testing'

Aptamers are single-stranded oligonucleotides that can be artificially generated by a method called Systematic evolution of ligands by exponential enrichment (SELEX). The generated aptamers have been assessed for high-performance sensing applications due to their appealing characteristics. With either aptamers alone or complementing with antibodies, several high sensitive and portable sensors have been demonstrated for use in 'point-of-care testing'. Due to their high suitability and flexibility, aptamers are conjugated with nanostructures and utilized in field applications. Moreover, aptamers are more amenable to chemical modifications, making them capable of utilization with most developed sensors. In this overview, we discuss novel, portable, and aptamer-based sensing strategies that are suitable for 'point-of-care testing'.

Surface-enhanced Raman scattering based lateral flow immunochromatographic assay for sensitive influenza detection

A novel sensitive SERS-lateral flow immunochromatographic integration system using Raman active molecule-coated gold nanostar as reporters for influenza virus detection is reported. Qualitative and quantitative SERS signal detection can be achieved.

Extrinsic surface-enhanced Raman scattering detection of influenza A virus enhanced by two-dimensional gold@silver core–shell nanoparticle arrays

A surface enhanced Raman scattering (SERS) based biosensor using a direct immunoassay platform was demonstrated for influenza A detection. The sensitivity was improved ~4 times by using a well-tuned Au@Ag 2D array instead of a flat Au film.

Silver and gold enhancement methods for lateral flow immunoassays

Sensitivity is the main concern at the development of rapid test by lateral flow immunoassays. On the other hand, low limits of detection are often required at medical diagnostics and other field of analysis. To overcome this drawback, several enhancement protocols have been described. In this paper, we have selected different silver enhancement methods and one dual gold conjugation, and we critically compared the amplification produced when applied to a gold-nanoparticle based lateral flow immunoassay for the detection of prostate specific antigen (PSA). The highest amplification was obtained by using an immersion method based on a solution of silver nitrate and hydroquinone/citrate buffer in proportion 1:1. Under these conditions, the system is capable of detecting PSA within 20 min at levels as low as 0.1 ng/mL, with a 3-fold sensitivity improvement.

Rapid detection and quantification of the marine toxic algae, Alexandrium minutum, using a super-paramagnetic immunochromatographic strip test

The dinoflagellates of Alexandrium genus are known to be producers of paralytic shellfish toxins that regularly impact the shellfish aquaculture industry and fisheries. Accurate detection of Alexandrium including Alexandrium minutum is crucial for environmental monitoring and sanitary issues. In this study, we firstly developed a quantitative lateral flow immunoassay (LFIA) using super-paramagnetic nanobeads for A. minutum whole cells. This dipstick assay relies on two distinct monoclonal antibodies used in a sandwich format and directed against surface antigens of this organism. No sample preparation is required. Either frozen or live cells can be detected and quantified. The specificity and sensitivity are assessed by using phytoplankton culture and field samples spiked with a known amount of cultured A. minutum cells. This LFIA is shown to be highly specific for A. minutum and able to detect reproducibly 10(5)cells/L within 30min. The test is applied to environmental samples already characterized by light microscopy counting. No significant difference is observed between the cell densities obtained by these two methods. This handy super-paramagnetic lateral flow immnunoassay biosensor can greatly assist water quality monitoring programs as well as ecological research.