Ultra technically-simple and sensitive detection for Salmonella Enteritidis by immunochromatographic assay based on gold growth

Transitions in Immunoassay Leading to Next-Generation Lateral Flow Assays and Future Prospects

In the field of clinical testing, the traditional focus has been on the development of large-scale analysis equipment designed to process high volumes of samples with fully automatic and high-sensitivity measurements. However, there has been a growing demand in recent years for the development of analytical reagents tailored to point-of-care testing (POCT), which does not necessitate a specific location or specialized operator. This trend is epitomized using the lateral flow assay (LFA), which became a cornerstone during the 2019 pandemic due to its simplicity, speed of delivering results-within about 10 min from minimal sample concentrations-and user-friendly design. LFAs, with their paper-based construction, combine cost-effectiveness with ease of disposal, addressing both budgetary and environmental concerns comprehensively. Despite their compact size, LFAs encapsulate a wealth of technological ingenuity, embodying years of research and development. Current research is dedicated to further evolving LFA technology, paving the way for the next generation of diagnostic devices. These advancements aim to redefine accessibility, empower individuals, and enhance responsiveness to public health challenges. The future of LFAs, now unfolding, promises even greater integration into routine health management and emergency responses, underscoring their critical role in the evolution of decentralized and patient-centric healthcare solutions. In this review, the historical development of LFA and several of the latest LFA technologies using catalytic amplification, surface-enhanced Raman scattering, heat detection, electron chemical detections, magnetoresistance, and detection of reflected electrons detection are introduced to inspire readers for future research and development.

Recent Developments in Lateral Flow Assays for Salmonella Detection in Food Products: A Review

Salmonellosis is a disease transmitted by contaminated food and is one of the leading causes of infections worldwide, making the early detection of Salmonella of crucial importance for public health. However, current detection methods are laborious and time-consuming, thus impacting the entire food supply chain and leading to production losses and economic sanctions. To mitigate these issues, a number of different biosensors have been developed, including lateral flow assays (LFAs), which have emerged as valuable tools in pathogen detection due to their portability, ease of use, time efficiency, and cost effectiveness. The performance of LFAs has been considerably enhanced by the development of new nanomaterials over the years. In this review, we address the principles and formats of the assay and discuss future prospects and challenges with an emphasis on LFAs developed for the detection of different Salmonella serovars in food.

Gold-Silver Alloy Nanoparticle-Incorporated Pitaya-Type Silica Nanohybrids for Sensitive Competitive Lateral Flow Immunoassay

Although the competitive lateral flow immunoassay (CLFIA) using gold nanoparticles (AuNPs) as labels has been widely adopted for the rapid detection of small molecules, its sensitivity is often constrained by the insufficient colorimetric signal produced by conventional AuNPs labels. Herein, we introduce a new type of intensified colorimetric label, denoted as SAAS, which is engineered by integrating gold-silver alloy nanoparticles (Au-Ag NPs) within a dendritic silica scaffold. These pitaya-type silica nanohybrids combine the advantages of the amplified molar extinction coefficient of alloy units with the signal collective effect of numerous Au-Ag NPs in a singular label. The SAAS-based CLFIA strips not only achieve qualitative screening of aflatoxin B1 (AFB1) at an extraordinarily low concentration of 0.2 ng/mL by the naked eye but also enable precise AFB1 quantification through a smartphone, with a remarkable limit of detection of 0.00314 ng/mL. Moreover, by leveraging SAAS as a quencher, we have delved into transforming the conventional signal-off mode of competitive immunoassay into a signal-on configuration. This innovation led to the development of a fluorescent LFIA that augments interpretative precision and sensitivity. Our study demonstrates the substantial potential of the proposed nanohybrid labels in enhancing the sensitivity of CLFIA for detecting small molecules.

Current status of recombinase polymerase amplification technologies for the detection of pathogenic microorganisms

Rapid detection of pathogenic microorganisms is key to the epidemiologic identification, prevention and control of disease in the field of public health. PCR-based pathogen detection methods have been widely used because they overcome the time-consuming issues encountered in traditional culture-based methods, including the limited detecting window-phase of immunological detection. However, the requirement for precise temperature-controlled thermal cyclers severely limits the application of these methods in resource-limited areas. Recombinase polymerase amplification (RPA) is a new type of nucleic acid amplification technology that can amplify DNA or RNA at a constant temperature. It has the advantages of simple operation, high specificity and sensitivity and a short detection time. In recent years, a number of alternative methods for pathogenic microorganism detection have been developed by combining microfluidic technology with RPA. Through the design of chip structures, optimization of injection modes, and utilization of multiple detection and quantification methods, the integration of pathogen nucleic acid extraction, amplification and detection is achieved, and this approach is suitable for the rapid detection of pathogenic microorganisms in various environments. In this review, we compare different nucleic acid amplification techniques, explain the principle of RPA technology, detection methods, and applications for pathogen microorganism detection and describe future direction of RPA application. These methods increase the ability to rapidly screen pathogenic microorganisms, thus improving the management of infectious diseases in the field of public health.

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

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

Highly sensitive lateral flow immunoassays based on Ag@Au nanoflowers with marine medaka (Oryzias melastigm) vitellogenin as a target analyte

In order to improve the sensitivity of lateral flow immunoassays (LFIAs) for the detection of piscine vitellogenin (Vtg), a well-established biomarker for environmental estrogens, Au coated Ag nanoflowers (Ag@Au NFs) were used as labeling probes to develop a LFIA for marine medaka Vtg. The synthesized Ag@Au NFs with good monodispersity had an average diameter of 44.1 nm and absorbance peak of 524 nm. When the concentration of goat anti-mouse IgG and anti-Vtg polyclonal antibody (anti-Vtg PAbs) were 1.3 and 0.4 mg/mL, respectively, the detection range of the LFIA was 0.19-25 ng/mL, and the visual detection limit was 0.1 ng/mL, which was approximately 80 times lower than that of LFIAs based on other nanoparticles (Au NPs, Ag NPs, Au NFs, and FM). After evaluation of its specificity and robustness, the usefulness of Ag@Au NFs labeled LFIA was validated by measuring Vtg induction in the plasma of marine medaka exposed to bisphenol A, a weak estrogenic chemical. This highly sensitive lateral flow immunoassay could detect Vtg biomarker within 15 min without the need of expensive and complicated instruments, and thus offered an ultrasensitive and robust on-site detection method for estrogenic activity in field environment.

Self-enhancement lateral flow immunoassay for COVID-19 diagnosis

Equipment-free colorimetric-based lateral flow immunoassay (LFIA) is the most convenient and popular tool for various applications, including diagnostic tools requiring high sensitivity for the detection of pathogens. Thus, improvements and developments of LFIA are constantly being reported. Herein, we enriched the sensitivity of LFIA using the gold enhancement principle, emphasizing needlessly complicated apparatus, only one step for the strip test operation, and typical time incubation (15 min) process. Self-enhanced LFIA was then executed for subsequent flows by overlapping the additionally enhanced pad composed of gold ions and reducing agent on the conjugate pad and the sample pad. Self-enhanced LFIA was performed to detect SARS-CoV-2 antigens in saliva. The obtained result depicted that the achieved sensitivity was up to tenfold compared with that of conventional LFIA by visual measurements. The detection limits of self-enhanced LFIA detecting nucleocapsid protein antigens in the saliva sample was 0.50 and 0.10 ng/mL employed by naked eye detection and calibration curve-based calculation, respectively. When the proposed device was applied to 207 human saliva samples, the diagnostic performance presented a 96.10 % sensitivity and 99.23 % specificity. This self-enhanced LFIA could be implemented in large-scale production and demonstrates higher sensitivity with effortless use, which meets the requirements for point-of-care testing and on-field mass screening.

Visual and Super-Sensitive Detection of Maize Chlorotic Mottle Virus by Dot-ELISA and Au Nanoparticle-Based Immunochromatographic Test Strip

Maize chlorotic mottle virus (MCMV) is the only species in the Mahromovirus genus and is often co-infected with one or several viruses of the Potyvirus genus, posing a great threat to the global maize industry. Effective viral integrated management measures are dependent on the timely and proper detection of the causal agent of the disease. In this work, six super-sensitive and specific monoclonal antibodies (mAbs) against MCMV were first prepared using purified MCMV virions as the immunogen. Then, the Dot enzyme-linked immunosorbent assay (Dot-ELISA) was established based on the obtained mAbs, and it can detect MCMV in infected maize leaf crude extracts diluted up to 1:10,240-fold (w/v, g/mL). Furthermore, a rapid and user-friendly Au nanoparticle-based immunochromatographic test strip (AuNP-ICTS) based on paired mAbs 7B12 and 17C4 was created for monitoring MCMV in point-of-care tests, and it can detect the virus in a 25,600-fold dilution (w/v, g/mL) of MCMV-infected maize leaf crude extracts. The whole test process for ICTS was completed in 10 min. Compared with conventional reverse transcription-polymerase chain reaction (RT-PCR), the detection endpoint of both serological methods is higher than that of RT-PCR, especially the Dot-ELISA, which is 12.1 times more sensitive than that of RT-PCR. In addition, the detection results of 20 blinded maize samples by the two serological assays were consistent with those of RT-PCR. Therefore, the newly created Dot-ELISA and AuNP-ICTS exhibit favorable application potential for the detection of MCMV in plant samples.

A review of rapid food safety testing: using lateral flow assay platform to detect foodborne pathogens

The detrimental impact of foodborne pathogens on human health makes food safety a major concern at all levels of production. Conventional methods to detect foodborne pathogens, such as live culture, high-performance liquid chromatography, and molecular techniques, are relatively tedious, time-consuming, laborious, and expensive, which hinders their use for on-site applications. Recurrent outbreaks of foodborne illness have heightened the demand for rapid and simple technologies for detection of foodborne pathogens. Recently, Lateral flow assays (LFA) have drawn attention because of their ability to detect pathogens rapidly, cheaply, and on-site. Here, we reviewed the latest developments in LFAs to detect various foodborne pathogens in food samples, giving special attention to how reporters and labels have improved LFA performance. We also discussed different approaches to improve LFA sensitivity and specificity. Most importantly, due to the lack of studies on LFAs for the detection of viral foodborne pathogens in food samples, we summarized our recent research on developing LFAs for the detection of viral foodborne pathogens. Finally, we highlighted the main challenges for further development of LFA platforms. In summary, with continuing improvements, LFAs may soon offer excellent performance at point-of-care that is competitive with laboratory techniques while retaining a rapid format.

Evaluation of covalent coupling strategies for immobilizing ligands on silica colloidal crystal films by optical interferometry

Immobilizing ligands is a crucial part of preparing optical sensors and directly connected to the sensitivity, stability, and other characteristics of sensors. In this work, an ordered porous layer interferometry (OPLI) system that can monitor the covalent coupling process of ligands in real time was developed. Films of silica colloidal crystal (SCC), as optical interference substrates, were surface modified by three different reagents: chloroacetic acid, glutaric anhydride, and carboxymethyl dextran. Staphylococcus aureus protein A (SPA), the ligand, was immobilized on SCC films. The covalent coupling process of SPA and SCC films can be dynamically monitored by the OPLI system. In addition, the three different strategies were evaluated by comparing the efficiency of the sensors prepared by different methods for binding Immunoglobulin G (IgG). The glutaric anhydride-modified sensor offers apparent advantages in terms of bound IgG quantity and affinity. This system provides a simple and intuitive way to determine the efficiency of different covalent coupling strategies. Furthermore, the sensor covalently coupled with SPA also excels in the determination of IgG content in complex systems such as milk. At the same time, the covalent coupling gives the sensor the ability to be stored stably over time.

Colorimetric and photothermal dual-mode lateral flow immunoassay based on Au-Fe3O4 multifunctional nanoparticles for detection of Salmonella typhimurium

Au-Fe3O4 multifunctional nanoparticles (NPs) were synthesized and integrated with lateral flow immunoassay (LFIA) for dual-mode detection of Salmonella typhimurium. The Au-Fe3O4 NPs not only combined excellent local surface plasmon resonance characteristics and superparamagnetic properties, but also exhibited good photothermal effect. In the detection, antibody-conjugated Au-Fe3O4 NPs first captured S. typhimurium from complex matrix, which was then loaded on the LFIA strip and trapped by the T-line. By observing the color bands with the naked eyes, qualitative detection was performed free of instrument. By measuring the photothermal signal, quantification was achieved with a portable infrared thermal camera. The introduction of magnetic separation achieved the enrichment and purification of target bacteria, thus enhancing the detection sensitivity and reducing interference. This dual-mode LFIA achieved a visual detection limit of 5 × 105 CFU/mL and a photothermal detection limit of 5 × 104 CFU/mL. Compared with traditional Au-based LFIA, this dual-mode LFIA increased the detection sensitivity by 2 orders of magnitude and could be directly applied to unprocessed milk sample. Besides, this dual-mode LFIA showed good reproducibility and specificity. The intra-assay and inter-assay variation coefficients were 3.0% and 7.9%, and with this dual-mode LFIA, other bacteria hardly produced distinguishable signals. Thus, the Au-Fe3O4 NPs-based LFIA has potential to increase the efficiency of pandemic prevention and control. Au-Fe3O4 nanoparticle proved to be a promising alternative reporter for LFIA, achieving multifunctions: target purification, target enrichment, visual qualitation, and instrumental quantification, which improved the limitations of traditional LFIA.

Mask assistance to colorimetric sniffers for detection of Covid-19 disease using exhaled breath metabolites

According to World Health Organization reports, large numbers of people around the globe have been infected or died for Covid-19 due to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Researchers are still trying to find a rapid and accurate diagnostic method for revealing infected people by low viral load with the overriding goal of effective diagnostic management. Monitoring the body metabolic changes is known as an effective and inexpensive approach for the evaluation of the infected people. Here, an optical sniffer is introduced to detect exhaled breath metabolites of patients with Covid-19 (60 samples), healthy humans (55 samples), and cured people (15 samples), providing a unique color pattern for differentiation between the studied samples. The sniffer device is installed on a thin face mask, and directly exposed to the exhaled breath stream. The interactions occurring between the volatile compounds and sensing components such as porphyrazines, modified organic dyes, porphyrins, inorganic complexes, and gold nanoparticles allowing for the change of the color, thus being tracked as the sensor responses. The assay accuracy for the differentiation between patient, healthy and cured samples is calculated to be in the range of 80%-84%. The changes in the color of the sensor have a linear correlation with the disease severity and viral load evaluated by rRT-PCR method. Interestingly, comorbidities such as kidney, lung, and diabetes diseases as well as being a smoker may be diagnosed by the proposed method. As a powerful detection device, the breath sniffer can replace the conventional rapid test kits for medical applications.

Colorimetric Systems for the Detection of Bacterial Contamination: Strategy and Applications

Bacterial contamination is a public health concern worldwide causing enormous social and economic losses. For early diagnosis and adequate management to prevent or treat pathogen-related illnesses, extensive effort has been put into the development of pathogenic bacterial detection systems. Colorimetric sensing systems have attracted increasing attention due to their simple and single-site operation, rapid signal readout with the naked eye, ability to operate without external instruments, portability, compact design, and low cost. In this article, recent trends and advances in colorimetric systems for the detection and monitoring of bacterial contamination are reviewed. This article focuses on pathogen detection strategies and technologies based on reaction factors that affect the color change for visual readout. Reactions used in each strategy are introduced by dividing them into the following five categories: external pH change-induced pH indicator reactions, intracellular enzyme-catalyzed chromogenic reactions, enzyme-like nanoparticle (NP)-catalyzed substrate reactions, NP aggregation-based reactions, and NP accumulation-based reactions. Some recently developed colorimetric systems are introduced, and their challenges and strategies to improve the sensing performance are discussed.

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.

Graphene-labeled synthetic antigen as a novel probe for enhancing sensitivity and simplicity in lateral flow immunoassay

Lateral flow immunoassay (LFIA), which combines immune-specific recognition properties with sensitive nano-signaling features, has emerged as an excellent tool for point-of-care testing (POCT) in food safety and clinical diagnosis. Exploring novel probes with a simple preparation process, improved signal intensity and good stability is conducive to the development and application of LFIA. Herein, a potent non-antibody probe, graphene-labeled synthetic antigen (G-Ag), was created for LFIA, in which graphene endowed a naked-eye visual colorimetric signal with high sensitivity, and the synthetic antigen competed with the target for binding to the antibody on the test line. During the G-Ag probe manufacturing process, only one simple mixing step was needed because graphene nanosheets presented a strong adsorption capacity toward the protein (BSA) on the synthetic antigen, significantly saving time, labor and cost. Especially, the synthetic antigen forms a fabulous probe element without the need for antibody, and thus the proposed LFIA avoids the destruction of antibody activity, and exhibits excellent sensitivity and stability. After optimization, LFIA was successfully applied to analyze clenbuterol; the lowest visually detectable concentration was 0.1 ng mL^-1, and the probe could be well-applied in pork, mutton, sausage and bacon samples, demonstrating favorable specificity and repeatability. Owing to the advantages of simplicity, non-antibody probe, sensitivity and reliability, G-Ag probe-based LFIA has application potential for small-molecule target monitoring and rapid detection.

A Universal Bacterial Catcher Au-PMBA-Nanocrab-Based Lateral Flow Immunoassay for Rapid Pathogens Detection

In traditional lateral flow immunoassays (LFIA) for pathogens detection, capture antibody (CA) is necessary and usually conjugated to Au nanoparticles (NPs) in order to label the target analyte. However, the acquisition process of the Au-CA nanoprobe is relatively complicated and costly, which will limit the application of LFIA. Herein, p-mercaptophenylboronic acid-modified Au NPs (namely Au-PMBA nanocrabs), were synthesized and applied for a new CA-independent LFIA method. The stable Au-PMBA nanocrabs showed outstanding capability to capture both Gram-negative bacteria and Gram-positive bacteria through covalent bonding. The acquired Au-PMBA-bacteria complexes were dropped onto the strip, and then captured by the detection antibody on the test line (T-line). Take Escherichia coli O157:H7 as an example, the gray value of T-line was proportional to the bacteria concentration and the linear range was 10³-10⁷ cfu·mL^-1. This CA-independent strategy exhibited higher sensitivity than the traditional CA-dependent double antibody sandwich method, because detection limit of the former one was 10³ cfu·mL^-1 only by visual observation, which was reduced by 3 orders of magnitude. Besides, this platform successfully screened E. coli O157:H7 in four food samples with recoveries ranging from 90.25% to 107.25%. This CA-independent LFIA showed great advantages and satisfactory potential for rapid foodborne pathogens detection in real samples.

Highly sensitive multiplex detection of foodborne pathogens using a SERS immunosensor combined with novel covalent organic frameworks based biologic interference-free Raman tags

Rapid and reliable multiplex detection of foodborne pathogens is in great demand for ensuring food safety and preventing foodborne diseases. In this study, we developed a highly sensitive SERS immunosensor for the simultaneous detection of multiple foodborne pathogens. Novel biologic interference-free Raman tags synthesized by using the covalent organic frameworks (COF) TBDP as nanocontainer to load biologic interference-free Raman reporters and specific antibodies for interested targets were used to convert and amplify signals of foodborne pathogens. In addition, lectin functionalized magnetic nanoparticles (MNPs@Con A) which could efficiently bind to the carbohydrate constituents on the surface of pathogens were prepared to capture and isolate multiple pathogens simultaneously. The recognition of the target foodborne pathogen impels the generation of sandwich-like composites of MNPs@Con A/pathogen/TBDP@Raman tags, and these composites could be quickly separated from the sample matrix with the assistance of an external magnet. Besides, a mass of Raman reporters was released by eluting the collected MNPs@Con A/pathogen/TBDP@Raman tags composites. Combined with a portable Raman system, characteristic Raman signals (2271 and 2113 cm^-1) of the occupied reporters located at the biologic interference-free region were observed and used for the simultaneous detection of two different foodborne pathogenic strains. And an equal limit of detection of 10¹ CFU/mL was achieved for each strain. This strategy provides new insight into the application of SERS in the detection of pathogenic bacteria.

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

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

A Polydopamine-Coated Gold Nanoparticles Quenching Quantum Dots-Based Dual-Readout Lateral Flow Immunoassay for Sensitive Detection of Carbendazim in Agriproducts

In this study, a fluorometric and colorimetric dual-readout lateral flow immunoassay (LFIA) using antibody functionalized polydopamine-coated gold nanoparticles (Au@PDAs) as a probe was developed for the detection of carbendazim (CBD). Colloidal gold nanoparticles (AuNPs) were coated with polydopamines (PDA) by the oxidation of dopamine to synthesize Au@PDA nanoparticles. The Au@PDA nanoparticles mediated ZnCdSe/ZnS quantum dots (QDs) fluorescence quenching and recovery, resulting in a reverse fluorescence enhancement detection format of CBD. The CBD detection was obtained by the competition between the CBD and the immobilized antigen for Au@PDAs labelled antibody binding, resulting in a significant fluorescence increase and colorimetry decrease corresponded to the concentration of CBD. Dual readout modes were incorporated into the LFIA using the colorimetry signal under natural light and the fluorescence signal under UV light. The cut-off value in the mode of the colorimetric signal and fluorometric signal for CBD detection was 0.5 μg/mL and 0.0156 μg/mL, respectively. The sensitivity of LFIA of the fluorescence mode was 32 times higher than that of the colorimetry mode. There was negligible cross reactivity obtained by using LFIA for the determination of thiabendazole, benomyl, thiophanate-methyl, and thiophanate-ethyl. Consistent and satisfactory results have been achieved by comparing the results of Au@PDAs-QDs-LFIA and liquid chromatography-tandem mass spectrometry (LC-MS/MS) testing spiked cucumber and strawberry samples, indicating good reliability of the Au@PDAs-QDs-LFIA.

A nanomaterial-free and thionine labeling-based lateral flow immunoassay for rapid and visual detection of the transgenic CP4-EPSPS protein

Nanomaterial-based lateral flow immunoassays (LFIAs) have been widely used for the on-site detection of genetically modified components. However, the practical applications are often limited by the complex matrix, such as in red samples. In this study, a thionine (Thi) labeling-based LFIA was developed for the first time to detect CP4-EPSPS protein. The optimal labeling concentration of Thi was 0.5 mg/mL, and the antibody could be rapidly coupled to Thi in 10 min. The visual limit of detection (vLOD) levels for transgenic soybean, sugar beet, and cotton containing the CP4-EPSPS protein reached 0.05%, 0.1%, and 0.1%, respectively, and had no interference from other proteins. After storage at 4 °C for three months, the LFIA sensitivity remained unchanged and showed good stability. This method could be used to screen and detect a variety of transgenic crops containing the CP4-EPSPS protein, and the results were consistent with the current standard assay. This study pioneered the development of an immunochromatographic method using Thi as a marker and applied it to the detection of the CP4-EPSPS protein in herbicide-tolerant transgenic crops. This provides a new method for the rapid immunoassay of Thi as a dye and has good prospects for practical application.

Simultaneous quantitative analysis of Listeria monocytogenes and Staphylococcus aureus based on antibiotic-introduced lateral flow immunoassay

Food poisoning caused by microorganisms has caused widespread concern. Herein, a highly sensitive on-site screening test strip for the detection of different pathogenic microorganisms (Listeria monocytogenes and Staphylococcus aureus) was designed. In this analysis platform, colloidal gold-coupled vancomycin was used as a signal unit to label Gram-positive bacteria, and highly sensitive polyclonal antibodies were used as recognition molecules to capture these specific strains. Compared with the traditional dual-antibody sandwich model, this new type of antibiotic-pathogen-antibody sandwich model is low-cost and can simultaneously detect multiple microorganisms. Under optimal conditions, this strategy showed satisfactory sensitivity and a wide linear range (L. monocy and S. aure could be directly assayed within linear ranges of 5 × 10⁴ to 10⁷ and 5 × 10² to 10⁷ CFU mL^-1, and the visual detection limits were 10⁵ and 10³ CFU mL^-1, respectively). The analytical performance and practicability of this sensor system have been further studied. This developed biosensor was applied to bacteria-contaminated water, milk and broth with satisfactory results. All of these attractive characteristics make the assay possess potential applications in food safety, medical diagnosis and environmental monitoring.

Nanomaterial application in bio/sensors for the detection of infectious diseases

Infectious diseases are a potential risk for public health and the global economy. Fast and accurate detection of the pathogens that cause these infections is important to avoid the transmission of the diseases. Conventional methods for the detection of these microorganisms are time-consuming, costly, and not applicable for on-site monitoring. Biosensors can provide a fast, reliable, and point of care diagnostic. Nanomaterials, due to their outstanding electrical, chemical, and optical features, have become key players in the area of biosensors. This review will cover different nanomaterials that employed in electrochemical, optical, and instrumental biosensors for infectious disease diagnosis and how these contributed to enhancing the sensitivity and rapidity of the various sensing platforms. Examples of nanomaterial synthesis methods as well as a comprehensive description of their properties are explained. Moreover, when available, comparative data, in the presence and absence of the nanomaterials, have been reported to further highlight how the usage of nanomaterials enhances the performances of the sensor.

An Enhanced Lateral Flow Assay Based on Aptamer-Magnetic Separation and Multifold AuNPs for Ultrasensitive Detection of Salmonella Typhimurium in Milk

In this paper, a novel and ultrasensitive lateral flow assay (LFA) based on aptamer-magnetic separation, and multifold Au nanoparticles (AuNPs) was developed for visual detecting Salmonella enterica ser. Typhimurium (S. Typhimurium). The method realized magnetic enrichment and signal transduction via magnetic separation and achieved signal amplification through hybridizing AuNPs-capture probes and AuNPs-amplification probes to form multifold AuNPs. Two different thiolated single-strand DNA (ssDNA) on the AuNPs-capture probe played different roles. One was combined with the AuNPs-amplification probe on the conjugate pad to achieve enhanced signals. The other was connected to transduction ssDNA1 released by aptamer-magnetic capture of S. Typhimurium, and captured by the T-line, forming a positive signal. This method had an excellent linear relationship ranging from 8.6 × 102 CFU/mL to 8.6 × 107 CFU/mL with the limit of detection (LOD) as low as 8.6 × 100 CFU/mL in pure culture. In actual samples, the visual LOD was 4.1 × 102 CFU/mL, which did not carry out nucleic acid amplification and pre-enrichment, increasing three orders of magnitudes than unenhanced assays with single-dose AuNPs and no magnetic separation. Furthermore, the system showed high specificity, having no reaction with other nontarget strains. This visual signal amplificated system would be a potential platform for ultrasensitive monitoring S. Typhimurium in milk samples.

Recent advances in gold nanoparticle-based lateral flow immunoassay for the detection of bacterial infection

Diagnosis of bacterial infections (BI) is becoming an increasingly difficult task in clinical practice due to their high prevalence and frequency, as well as the growth of antibiotic resistance worldwide. World Health Organization (WHO) reported antibiotic resistance is a major public health problem. BI becomes difficult or impossible to treat when the bacteria acquire immunity against antibiotics. Thus, there is a need for a quick and accurate technique to detect infection. Lateral flow immunoassay (LFIA) is an ideal technique for point-of-care testing of a disease or pathological changes inside the human body. In recent years, several LFIA based strips are being used for the detection of BI by targeting specific analytes which may range from the causative bacterium, whole-cell, DNA, or biomarker. Numerous nanoparticles like lipid-based nanoparticles, polymeric nanoparticles, and inorganic nanoparticles such as quantum dots, magnetic, ceramic, and metallic nanoparticles (copper, silver gold, iron) are widely being used in the advanced treatment of BI. Out of these gold nanoparticle (AuNPs), is being used for detection BI more effectively than other nanoparticles due to their surface functionalization, extraordinary chemical stability, biorecognition, and signal amplification properties and help to improve in conjugation with capture antibodies, and act as a color marker with unique optical properties on LFIA strips. Herein, a review that provides an overview of the principle of LFIA, how LFIA based strip is developed, and how it is helpful to detect a specific biomarker for bedside detection of the BI.

Unveiling the Potential Role of Nanozymes in Combating the COVID-19 Outbreak

The current coronavirus disease 2019 (COVID-19) outbreak is considered as one of the biggest public health challenges and medical emergencies of the century. A global health emergency demands an urgent development of rapid diagnostic tools and advanced therapeutics for the mitigation of COVID-19. To cope with the current crisis, nanotechnology offers a number of approaches based on abundance and versatile functioning. Despite major developments in early diagnostics and control of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), there is still a need to find effective nanomaterials with low cost, high stability and easy use. Nanozymes are nanomaterials with innate enzyme-like characteristics and exhibit great potential for various biomedical applications such as disease diagnosis and anti-viral agents. Overall the potential and contribution of nanozymes in the fight against SARS-CoV-2 infection i.e., rapid detection, inhibition of the virus at various stages, and effective vaccine development strategies, is not fully explored. This paper discusses the utility and potential of nanozymes from the perspective of COVID-19. Moreover, future research directions and potential applications of nanozymes are highlighted to overcome the challenges related to early diagnosis and therapeutics development for the SARS-CoV-2. We anticipate the current perspective will play an effective role in the existing response to the COVID-19 crisis.

A Pt-Ir nanocube amplified lateral flow immunoassay for dehydroepiandrosterone

The traditional gold-nanoparticle-based lateral flow immunoassay (LFIA) cannot satisfy the requirements for the sensitive detection of dehydroepiandrosterone (DHEA) in human urine. To enhance the sensitivity of the LFIA, platinum-iridium nanocubes (Pt-Ir NCs) with high catalytic efficiency and stability were synthesized and labelled with polyclonal antibody (pAb) to form a pAb-Pt-Ir probe. For the detection of DHEA, a novel LFIA with Pt-Ir NCs as an optical label and an enhanced LFIA in which the peroxidase-like activity of the Pt-Ir NCs was triggered by the introduction of the chromogenic substrate 3-amino-9-ethyl-carbazole (AEC) were developed and compared with a LFIA with platinum nanocubes (PtNCs) as an optical label. The visual limit of detection was 0.5 ng mL-1 for Pt-Ir-LFIA and 0.05 ng mL-1 for AEC-enhanced Pt-Ir-LFIA, in comparison to 100 ng mL-1 for PtNCs-LFIA and 50 ng mL-1 for AEC-enhanced PtNCs-LFIA. The average recoveries from spiked urine samples ranged from 90.8% to 110.4%, with a coefficient of variation below 12.6%, suggesting the accuracy and reliability of our developed immunoassay. Achieving excellent sensitivity, specificity, and reproducibility, Pt-Ir-LFIA provided a promising platform for monitoring DHEA.

Graphite-like carbon nitride-laden gold nanoparticles as signal amplification label for highly sensitive lateral flow immunoassay of 17β-estradiol

Conventional gold nanoparticles-based lateral flow immunoassays (AuNPs-LFIA) lack sensitivity. In this work, we developed a graphite-like carbon nitride-laden AuNPs (g-C₃N₄@Au) assisted LFIA to improve sensitivity for 17β-estradiol (E2) in foods. g-C₃N₄ nanosheets were applied as carriers, because of their excellent chemical stability, large surface areas and low-cost, loading large numbers of AuNPs to amplify signals and improve the overall response of g-C₃N₄@Au-based LFIA (g-C₃N₄@Au-LFIA). The lowest visual limit of detection (vLOD) of E2 is 0.5 ng mL^-1 for g-C₃N₄@Au-LFIA, which exhibit a significantly three-fold improved analytical performance compared with that of AuNPs-LFIA. Additionally, this method was successfully used to the detection of E2 in four spiked food samples, offering a great potential of the g-C₃N₄@Au based LFIA for its application in food products.

(Nano)tag-antibody conjugates in rapid tests

Antibodies (Abs) are naturally derived materials with favorable affinity, selectivity, and fast binding kinetics to the respective antigens, which enables their application as promising recognition elements in the development of various types of biosensors/bioassays, especially in rapid tests. These tests are low-cost and easy-to-use biosensing devices with broad applications including medical or veterinary diagnostics, environmental monitoring and industrial usages such as safety and quality analysis in food, providing on-site quick monitoring of various analytes, making it possible to save analysis costs and time. To reach such features, the conjugation of Abs with various nanomaterials (NMs) as tags is necessary, which range from conventional gold nanoparticles to other nanoparticles recently introduced, where magnetic, plasmonic, photoluminescent, or multi-modal properties play a critical role in the overall performance of the analytical device. In this context, to preserve the Ab affinity and provide a rapid response with long-term storage capability, the use of efficient bio-conjugation techniques is critical. Thanks to their prominent role in rapid tests, many studies have been devoted to the design and development of Abs-NMs conjugates with various chemistries including passive adsorption, covalent coupling, and affinity interactions. In this review, we present the state-of-the-art techniques allowing various Ab-NM conjugates with a special focus on the efficiency of the developed probes to be employed in in vitro rapid tests. Challenges and future perspectives on the development of Ab-conjugated nanotags in rapid diagnostic tests are highlighted along with a survey of the progress in commercially available Ab-NM conjugates.

An innovative prussian blue nanocubes decomposition-assisted signal amplification strategy suitable for competitive lateral flow immunoassay to sensitively detect aflatoxin B1

A prussian blue nanocubes decomposition-assisted signal amplification strategy for competitive lateral flow immunoassay (PBNCD-SALFIA) was innovatively proposed to analyze aflatoxin B₁ (AFB₁) in foodstuffs. The signal was amplified on account of the PBNCs can degrade and fade under alkaline condition, which could weak the color of the test- and control-lines, thereby improving the sensitivity of the sensor. The strategy was technically-simple, just using NaOH as an amplifier can realize signal amplification in the competition assay. Under optimized conditions, the enhanced strip exhibited excellent specificity and sensitivity in AFB₁ monitoring with a detection limit of 23 pg/mL, which was approximately 4- and 8-folds lower than those of PBNCs-LFIA (90 pg/mL) and conventional gold nanoparticles-LFIA (175 pg/mL), respectively. Taking the advantage of the color-fading, this platform revealing the lowest detectable concentration of 0.184, 0.368 and 0.184 μg/kg for AFB₁ in corn, peanut and pumpkin seed within 58 min, separately, showing reliable results.

Gold nanoparticle-decorated metal organic frameworks on immunochromatographic assay for human chorionic gonadotropin detection

Gold nanoparticle-decorated metal organic frameworks (MOF@AuNPs) with significantly enhanced color signal intensity were synthesized through in situ growth of AuNPs on the MOF skeleton. The resultant MOF@AuNP nanocomposites were characterized with 16.7-fold higher absorbance than conventional 40 nm AuNPs (AuNP₄₀). Thus, for the first time, we applied it as a signal amplification label to improve the immunochromatographic assay (ICA) of human chorionic gonadotropin (HCG). The detection limit of our enhanced ICA was 1.69 mIU/mL, which is ca. 10.6-fold improvement in sensitivity compared to traditional AuNP₄₀-ICA. The recoveries of this MOF@AuNPs-ICA ranged from 86.03 to 119.22%, with coefficients of variation of 3.05 to 13.74%. The reliability and practicability were further validated by the clinically used chemiluminescence immunoassay method. Given their excellent signal amplification ability, the proposed MOF@AuNPs could serve as an ideal ICA label for rapid and sensitive detection of disease biomarkers. Graphical abstract.

Dual sensitivity enhancement in gold nanoparticle-based lateral flow immunoassay for visual detection of carcinoembryonic antigen

In this work, we presented the development of cost-effective dual sensitivity enhancement in gold nanoparticle-based lateral flow test strip for detection of carcinoembryonic antigen. On the one hand, we employed protein G as a host matrix for oriented immobilization of antibodies within the nitrocellulose membrane. On the other hand, we utilized gold enhancement approach to visualize the final signals effectively. Primary examinations revealed that the smaller sized nanoparticles have greater signal enhancement compared to bigger ones. So, mono-dispersed gold nanoparticles with average diameters of 11.40 ± 1.40 nm were utilized as tags. The measurement of fluorescent intensity of FITC-tagged secondary antibody attached to the polyclonal antibody, in the presence/absence of protein G as a host matrix on microplate wells, showed the successful oriented immobilization of antibodies via the host matrix. The FESEM images confirmed the attachment and growth of nanoparticles within the porous nitrocellulose membrane, after gold enhancement. Finally, under the optimized conditions, the developed strip could quantify the standard values of target within 2-50 ng/mL range with a limit of detection of 0.35 ng/mL. This strategy enabled the reduction of antibody consumption from a conventional amount of 0.6 µg/strip down to 0.012 µg/strip. The serum samples containing carcinoembryonic antigen were also successfully analyzed by the developed strip with a visual detection limit of 10 ng/mL, which confirms favorable characteristics of the developed test strip for point-of-care applications.

Nanomaterial-mediated paper-based biosensors for colorimetric pathogen detection

The pervasive spread of infectious diseases and pandemics, such as the 2019 coronavirus disease (COVID-19), are becoming increasingly serious and urgent threats to human health. Preventing the spread of such diseases prioritizes the development of sensing devices that can rapidly, selectively, and reliably detect pathogens at minimal cost. Paper-based analytical devices (PADs) are promising tools that satisfy those criteria. Numerous paper-based biosensors have been established that rival conventional pathogen detection methods. Among them, colorimetric strategies are promising since results can be interpreted by eye, and are simple to operate, which is advantageous for point-of-care testing (POCT). Particularly, the application of nanomaterials on paper-based biosensors has become important as these materials are capable of converting signals from pathogens through unique mechanisms to yield an amplified colorimetric readout. To highlight the research progress on using nanomaterials in colorimetric paper-based biosensor for pathogen detection, we discuss the sensing mechanisms of how they work, structural and analytical characteristics of the devices, and representative recent applications. Current challenges and future directions of using PADs and nanomaterial-mediated strategies are also discussed.

Antibiotic and mammal IgG based lateral flow assay for simple and sensitive detection of Staphylococcus aureus

Lateral flow assay (LFA), performed with simple devices and short detection time, is popular in field applications. Herein, a novel sandwich type-based LFA was constructed for high sensitivity and selectivity detection of Staphylococcus aureus (S. aureus). Vancomycin-immobilized gold nanoparticles (VAN-Au NPs) were utilized as the first identifier to capture S. aureus and the specificity was guaranteed by the second recognition agent of pig immunoglobulin G (IgG). In addition, gold growth was adopted for signal amplification to further improve the detection sensitivity. S. aureus could be directly assayed by this LFA within the concentration range of 1.0 × 10³-1.0 × 10⁸ cfu mL^-1 with a detection limit of 1.0 × 10³ cfu mL^-1. Furthermore, the novel sandwich LFA realized S. aureus detection in food samples with admissible recoveries and established a rapid, simple, cost-effective and sensitive platform, could meet the demand for on-site testing of S. aureus.

Antimicrobial effects of three herbs (Brassica juncea, Forsythia suspensa, and Inula britannica) on membrane permeability and apoptosis in Salmonella

AIMS

This study aimed synergistic effects of three herbs in Salmonella via increased membrane permeability and apoptosis.

METHODS AND RESULTS

Using high-performance liquid chromatography, four types of phenylethyl glycosides and a lignan were detected in the herb mixture (Brassica juncea, Forsythia suspensa, and Inula britannica). During treatment with the herb mixture (1×, 2×, or 4× the MIC), viable cells decreased to 1·87 log CFU per ml (Salmonella Gallinarum) and 2·33 log CFU per ml (Salmonella Enteritidis) after 12 h of incubation according to inhibition of tricarboxylic acid cycle (P < 0·01). In addition, N-phenyl-1-naphthylamine uptake increased from 229·00 to 249·67 AU in S. Gallinarum and from 232·00 to 250·67 AU in S. Enteritidis (P < 0·05), whereas membrane potential decreased from 8855·00 to 3763·25 AU and from 8703·67 to 4300·38 AU, respectively. Apoptotic Salmonella cells were observed by confocal laser scanning microscopy and flow cytometry. Transmission electron microscopy observations with negative staining showed protein leakage from damaged Salmonella.

CONCLUSIONS

These results showed the synergistic effect of the three herbs against avian pathogenic Salmonella induced by membrane damage and apoptosis.

SIGNIFICANCE AND IMPACT OF THE STUDY

Salmonella causes enormous economic losses in the poultry industry. These results indicated that potency of natural antimicrobial agents due to apoptosis in Salmonella.

Dramatically Enhanced Immunochromatographic Assay Using Cascade Signal Amplification for Ultrasensitive Detection of Escherichia coli O157:H7 in Milk

The conventional colloidal gold immunochromatographic assay (AuNP-ICA) cannot meet the requirements for the rapid and sensitive detection of Escherichia coli (E. coli) O157:H7 because of its poor sensitivity. Herein, a novel two-step cascade signal amplification strategy that integrates in situ gold growth and nanozyme-mediated catalytic deposition was proposed to enhance the detection sensitivity of conventional AuNP-ICA dramatically. The enhanced strip displayed ultrahigh sensitivity in E. coli O157:H7 detection and had a detection limit of 1.25 × 10¹ CFU/mL, which is approximately 400-fold lower than that of traditional AuNP-ICA (5 × 10³ CFU/mL). The amplified strip had no background signal in the T-line zone in the absence of E. coli O157:H7 even after one round of cascade signal amplification. The enhanced strip demonstrated excellent selectivity against E. coli O157:H7 with a negligible cross-reaction to nine other common pathogens. Intra-assays and interassays showed that the improved strip has acceptable accuracy and precision for determining E. coli O157:H7. The average recoveries in a real milk sample ranged from 87.33 to 112.15%, and the coefficients of variation were less than 10%. The enhanced strip also showed great potential in detecting a single E. coli O157:H7 cell in a 75 μL solution.

Recent advances in high-sensitivity detection methods for paper-based lateral-flow assay

Paper-based lateral-flow assays (LFAs) have achieved considerable commercial success and continue to have a significant impact on medical diagnostics and environmental monitoring. Conventional LFAs are typically performed by examining the color changes in the test bands by naked eye. However, for critical biochemical markers that are present in extremely small amounts in the clinical specimens, this readout method is not quantitative, and does not provide sufficient sensitivity or suitable detection limit for a reliable assay. Diverse technologies for high-sensitivity LFA detection have been developed and commercialization efforts are underway. In this review, we aim to provide a critical and objective overview of the recent progress in high-sensitivity LFA detection technologies, which involve the exploitation of the physical and chemical responses of transducing particles. The features and biomedical applications of the technologies, along with future prospects and challenges, are also discussed.