Membrane-based lateral flow immunochromatographic strip with nanoparticles as reporters for detection: A review

Coupling hCG-based protease sensors with a commercial pregnancy test strip for simple analyses of protease activities

Proteases play an essential role in many cellular processes, and consequently, abnormalities in their activities are related to various diseases. Methods have been developed to measure the activity of these enzymes, but most involve sophisticated instruments or complicated procedures, which hampers the development of a point-of-care test (POCT). Here, we propose a strategy for developing simple and sensitive methods to analyze protease activity using commercial pregnancy test strips that detect human chorionic gonadotropin (hCG). hCG was engineered to have site-specific conjugated biotin and a peptide sequence, which can be cleaved by a target protease, between hCG and biotin. hCG protein was immobilized on streptavidin-coated beads, resulting in a protease sensor. The hCG-immobilized beads were too large to flow through the membrane of the hCG test strip and yielded only one band in the control line. When the peptide linker was hydrolyzed by the target protease, hCG was released from the beads, and the signal appeared in both the control and test lines. Three protease sensors for matrix metalloproteinase-2, caspase-3, and thrombin were constructed by replacing the protease-cleavable peptide linker. The combination of the protease sensors and a commercial pregnancy strip enabled the specific detection of each protease in the picomolar range, with a 30-min incubation of the hCG-immobilized beads and samples. The modular design of the protease sensor and simple assay procedure will facilitate the development of POCTs for various protease disease markers.

The application of nanoparticles in point-of-care testing (POCT) immunoassays

The Covid-19 pandemic has led to greater recognition of the importance of the fast and timely detection of pathogens. Recent advances in point-of-care testing (POCT) technology have shown promising results for rapid diagnosis. Immunoassays are among the most extensive POCT assays, in which specific labels are used to indicate and amplify the immune signal. Nanoparticles (NPs) are above the rest because of their versatile properties. Much work has been devoted to NPs to find more efficient immunoassays. Herein, we comprehensively describe NP-based immunoassays with a focus on particle species and their specific applications. This review describes immunoassays along with key concepts surrounding their preparation and bioconjugation to show their defining role in immunosensors. The specific mechanisms, microfluidic immunoassays, electrochemical immunoassays (ELCAs), immunochromatographic assays (ICAs), enzyme-linked immunosorbent assays (ELISA), and microarrays are covered herein. For each mechanism, a working explanation of the appropriate background theory and formalism is articulated before examining the biosensing and related point-of-care (POC) utility. Given their maturity, some specific applications using different nanomaterials are discussed in more detail. Finally, we outline future challenges and perspectives to give a brief guideline for the development of appropriate platforms.

CRISPR-based quantum dot nanobead lateral flow assay for facile detection of varicella-zoster virus

Varicella-zoster virus (VZV) infects more than 90% of the population worldwide and has a high incidence of postherpetic neuralgia in elderly patients, seriously affecting their quality of life. Combined with clustered regularly interspaced short palindromic repeats (CRISPR) system, we develop a quantum dot nanobeads (QDNBs) labeled lateral flow assay for VZV detection. Our assay allows the identification of more than 5 copies of VZV genomic DNA in each reaction. The entire process, from sample preparation to obtaining the results, takes less than an hour. In 86 clinical vesicles samples, the test shows 100% concordance with quantitative real-time PCR for VZV detection. Notably, when vesicles are present in specific areas, such as the genitals, our method outperforms clinical diagnosis. Compared to traditional detection methods, only a minute amount of blister fluid is required for accurate detection. Therefore, we anticipate that our method could be translated to clinical applications for specific and rapid VZV detection. KEY POINTS: • CRISPR/Cas12a and quantum dot nanobead-based lateral flow assay achieved 5 copies per reaction for VZV detection • Specific identification of VZV in atypical skin lesions • Results read by the naked eye within one hour.

An integrated colorimetric and photothermal lateral flow immunoassay based on bimetallic Ag-Au urchin-like hollow structures for the sensitive detection of E. coli O157:H7

Lateral flow immunoassay (LFIA) with gold nanoparticles (AuNPs) as the signal reporter is widely used for the rapid detection of food-borne pathogens. However, it is difficult for LFIA to achieve sensitive detection due to the insufficient colorimetric signal brightness of AuNPs. Herein, we developed a bimetallic Ag-Au urchin-like hollow nanospheres (BUHNPs) based on the simple and rapid synthesis through co-reduction and galvanic replacement reactions. The BUHNPs exhibit superb colorimetric signal brightness, strong photothermal signals and high antibody coupling efficiency. The obtained colorimetric and photothermal LFIA (BUHNPs-CM-LFIA and BUHNPs-PT-LFIA) based on BUHNPs were used for the sensitive detection of a pathogenic bacterium (Escherichia coli O157:H7). The limit of detection values of the colorimetric and photothermal quantitative analysis were 2.48 × 10³ and 5.50 × 10² CFU mL^-1, which were approximately 4-fold and 18-fold lower than that of AuNPs-LFIA (9.92 × 10³ CFU mL^-1), respectively. This work suggests that a dual-readout LFIA with BUHNPs as a promising signal reporter can be used to improve the detection performance of LFIA and construct a more accurate and sensitive detection platform.

On-Site Remote Monitoring System with NIR Signal-Based Detection of Infectious Disease Virus in Opaque Salivary Samples

Infectious disease viruses, such as foot-and-mouth disease virus (FMDV), are highly contagious viruses that cause significant socioeconomic damage upon spreading. Developing an on-site diagnostic tool for early clinical detection and real-time surveillance of FMDV outbreaks is essential to prevent the further spread of the disease. However, early diagnosis of FMDV is still challenging due to the limited sensitivity and time-consuming manual result entry of commercial on-site tests for salivary samples. Here, we report a near-infrared (NIR) signal nanoprobe-based highly accurate detection and remote monitoring system toward FMDVs, which automates the analysis and reporting of diagnosis data. The NIR signal lateral flow immunoassay (LFA) was assembled with a nanoprobe with a stable emission intensity at 800 nm, minimizing the interference signal of opaque salivary samples. We investigated the clinical applicability of the NIR signal LFA at biosafety level 3 (BSL-3) laboratories using 147 opaque salivary samples. The NIR signal LFA achieved a 32-fold lower limit of detection (LOD) than a commercial LFA in detecting live FMDVs, including all isolates occurring in the Republic of Korea during 2010-2017. Our results showed that the NIR signal LFA successfully discriminated the FMDV-positive clinical salivary samples from healthy controls with a sensitivity of 96.9%, specificity of 100.0%, and AUC (area under the receiver operator characteristic curve) value of 0.999. Finally, we substantiated the real-time collection of diagnostic results using a customized portable NIR reader at nine different laboratories of government-certified quarantine institutions for foot-and-mouth disease (FMD).

ELISA array-based immunoassay for multiplex detection of osteoporosis-relevant biomarkers

Aim: This study aimed to develop an ELISA array-based immunoassay for the simultaneous detection of osteoporosis-relevant biomarkers. Methods: ELISA array was constructed and its performance was evaluated using commercial antigens. The validity was further confirmed using traditional ELISA. Results: The constructed ELISA array showed good repeatability. The linear ranges and limits of detection for the four markers mentioned above were 0.01-8 ng/ml (1 pg/ml), 5.0-1000 ng/ml (1 ng/ml), 0.5-400 ng/ml (0.1 ng/ml) and 1-100 ng/ml (1 ng/ml), respectively. The measured concentrations of serum samples by ELISA array showed high correlation with those from traditional ELISA. Conclusion: ELISA array-based immunoassay provided a feasible and productive method to multiplex measure markers for osteoporosis.

Antibody conjugated Au/Ir@Cu/Zn-MOF probe for bacterial lateral flow immunoassay and precise synergistic antibacterial treatment

Staphylococcus aureus is one of the most prevalent threats to public health. Rapid detection with high sensitivity and targeted killing is crucial to curb its spread. Herein, a metal-bearing nanocomposite, consisting of a bimetallic nanoparticle and a metal-organic framework (Au/Ir@Cu/Zn-MOF) was constructed. Upon conjugation with anti-S. aureus antibody, this nanocomposite (Ab-Au/Ir@Cu/Zn-MOF) was exploited for its dual functions, i.e. as a reporting probe in a lateral flow immunoassay and a high efficiency antibacterial reagent. Benefiting from the enrichment of Au/Ir NPs by the Cu/Zn-MOF, the Au/Ir@Cu/Zn-MOF-based lateral flow immunoassay sensor exhibited a visual limit of detection of 10³ CFU/mL, which was100 times more sensitive than Au/Ir-based sensor. Moreover, the Ab-Au/Ir@Cu/Zn-MOF probe possessed synergistic photothermal-chemodynamic bactericidal effect that specifically targeted against S. aureus. Under a co-treatment by H₂O₂ (0.4 mM) and 808 nm near infrared irradiation (1 W/cm², 5 min), complete sterilization of 5 × 10⁵-10⁶ CFU/mL S. aureus was achieved at a nanocomposite concentration as low as 6.25 μg/mL. The superior antibacterial efficiency was attributable to the three-fold properties of the Ab-Au/Ir@Cu/Zn-MOF probe: (1) enhanced multi-enzyme mimicking activities that promote reactive oxygen species generation, (2) high photothermal activity (efficiency of 53.70%), and (3) bacteria targeting ability via the antibody coating. By changing the antibody, this nanocomposite can be tailored to target a wide range of bacteria species, for detection and for precise antibacterial treatment.

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.

Gold-silver alloy hollow nanoshells-based lateral flow immunoassay for colorimetric, photothermal, and SERS tri-mode detection of SARS-CoV-2 neutralizing antibody

Although many approaches have been developed for the quick assessment of SARS-CoV-2 infection, few of them are devoted to the detection of the neutralizing antibody, which is essential for assessing the effectiveness of vaccines. Herein, we developed a tri-mode lateral flow immunoassay (LFIA) platform based on gold-silver alloy hollow nanoshells (Au-Ag HNSs) for the sensitive and accurate quantification of neutralizing antibodies. By tuning the shell-to-core ratio, the surface plasmon resonance (SPR) absorption band of the Au-Ag HNSs is located within the near infrared (NIR) region, endowing them with an excellent photothermal effect under the irradiation of optical maser at 808 nm. Further, the Raman reporter molecule 4-mercaptobenzoic acid (MBA) was immobilized on the gold-silver alloy nanoshell to obtain an enhanced SERS signal. Thus, these Au-Ag HNSs could provide colorimetric, photothermal and SERS signals, with which, tri-mode strips for SARS-CoV-2 neutralizing antibody detection were constructed by competitive immunoassay. Since these three kinds of signals could complement one another, a more accurate detection was achieved. The tri-mode LFIA achieved a quantitative detection with detection limit of 20 ng/mL. Moreover, it also successfully detected the serum samples from 98 vaccinated volunteers with 79 positive results, exhibiting great application value in neutralizing antibody detection.

A Novel Strategy for Rapid Fluorescence Detection of FluB and SARS-CoV-2

Undoubtedly, SARS-CoV-2 has caused an outbreak of pneumonia that evolved into a worldwide pandemic. The confusion of early symptoms of the SARS-CoV-2 infection with other respiratory virus infections made it very difficult to block its spread, leading to the expansion of the outbreak and an unreasonable demand for medical resource allocation. The traditional immunochromatographic test strip (ICTS) can detect one analyte with one sample. Herein, this study presents a novel strategy for the simultaneous rapid detection of FluB/SARS-CoV-2, including quantum dot fluorescent microspheres (QDFM) ICTS and a supporting device. The ICTS could be applied to realize simultaneous detection of FluB and SARS-CoV-2 with one test in a short time. A device supporting FluB/SARS-CoV-2 QDFM ICTS was designed and had the characteristics of being safe, portable, low-cost, relatively stable, and easy to use, ensuring the device could replace the immunofluorescence analyzer in cases where there is no need for quantification. This device does not need to be operated by professional and technical personnel and has commercial application potential.

Application of Au@Pt Nanozyme as Enhancing Label for the Sensitive Lateral Flow Immunoassay of Okadaic Acid

In this study, a lateral flow immunoassay (LFIA) was developed to detect okadaic acid (OA) belonging to the diarrheic shellfish poisoning group of aquatic toxins. Newly obtained anti-OA monoclonal antibodies and bimetallic core@shell Au@Pt nanoparticles were used in the indirect format of the LFIA. Peroxidase-mimicking nanozyme properties of Au@Pt enabled using them to enhance band coloration on the test strips and, consequently, for increasing the LFIA sensitivity. The instrumental limit of detection (LOD), the working range of detectable concentrations, and the visual cutoff of the assay were 0.5, 0.8-6.8, and 10 ng/mL, respectively. The assay duration was 20 min. The rapid and simple sample preparation procedure was applied for seawater, river water, and fish samples. The total duration of the sample pretreatment and LFIA was 25/40 min for water/fish samples, ensuring testing rapidity. The developed test system provides sensitive control of raw materials and food products and can be used to detect OA at all stages of the food industry «from sea to fork» chains.

Lateral flow assays for viruses diagnosis: Up-to-date technology and future prospects

Bacteria, viruses, and parasites are harmful microorganisms that cause infectious diseases. Early detection of diseases is critical to prevent disease transmission and provide epidemic preparedness, as these can cause widespread deaths and public health crises, particularly in resource-limited countries. Lateral flow assay (LFA) systems are simple-to-use, disposable, inexpensive diagnostic devices to test biomarkers in blood and urine samples. Thus, LFA has recently received significant attention, especially during the pandemic. Here, first of all, the design principles and working mechanisms of existing LFA methods are examined. Then, current LFA implementation strategies are presented for communicable disease diagnoses, including COVID-19, zika and dengue, HIV, hepatitis, influenza, malaria, and other pathogens. Furthermore, this review focuses on an overview of current problems and accessible solutions in detecting infectious agents and diseases by LFA, focusing on increasing sensitivity with various detection methods. In addition, future trends in LFA-based diagnostics are envisioned.

Merging microfluidics with luminescence immunoassays for urgent point-of-care diagnostics of COVID-19

The Coronavirus disease 2019 (COVID-19) outbreak has urged the establishment of a global-wide rapid diagnostic system. Current widely-used tests for COVID-19 include nucleic acid assays, immunoassays, and radiological imaging. Immunoassays play an irreplaceable role in rapidly diagnosing COVID-19 and monitoring the patients for the assessment of their severity, risks of the immune storm, and prediction of treatment outcomes. Despite of the enormous needs for immunoassays, the widespread use of traditional immunoassay platforms is still limited by high cost and low automation, which are currently not suitable for point-of-care tests (POCTs). Microfluidic chips with the features of low consumption, high throughput, and integration, provide the potential to enable immunoassays for POCTs, especially in remote areas. Meanwhile, luminescence detection can be merged with immunoassays on microfluidic platforms for their good performance in quantification, sensitivity, and specificity. This review introduces both homogenous and heterogenous luminescence immunoassays with various microfluidic platforms. We also summarize the strengths and weaknesses of the categorized methods, highlighting their recent typical progress. Additionally, different microfluidic platforms are described for comparison. The latest advances in combining luminescence immunoassays with microfluidic platforms for POCTs of COVID-19 are further explained with antigens, antibodies, and related cytokines. Finally, challenges and future perspectives were discussed.

Artificial intelligence-assisted colorimetric lateral flow immunoassay for sensitive and quantitative detection of COVID-19 neutralizing antibody

Currently, vaccination is the most effective medical measure to improve group immunity and prevent the rapid spread of COVID-19. Since the individual difference of vaccine effectiveness is inevitable, it is necessary to evaluate the vaccine effectiveness of every vaccinated person to ensure the appearance of herd immunity. Here, we developed an artificial intelligent (AI)-assisted colorimetric polydopamine nanoparticle (PDA)-based lateral flow immunoassay (LFIA) platform for the sensitive and accurate quantification of neutralizing antibodies produced from vaccinations. The platform integrates PDA-based LFIA and a smartphone-based reader to test the neutralizing antibodies in serum, where an AI algorithm is also developed to accurately and quantitatively analyze the results. The developed platform achieved a quantitative detection with 160 ng/mL of detection limit and 625-10000 ng/mL of detection range. Moreover, it also successfully detected totally 50 clinical serum samples, revealing a great consistency with the commercial ELISA kit. Comparing with commercial gold nanoparticle-based LFIA, our PDA-based LFIA platform showed more accurate quantification ability for the clinical serum. Therefore, we envision that the AI-assisted PDA-based LFIA platform with sensitive and accurate quantification ability is of great significance for large-scale evaluation of vaccine effectiveness and other point-of-care immunoassays.

Triple immunochromatographic test system for detection of priority aquatic toxins in water and fish

Aquatic toxins are a group of toxic compounds produced by several types of freshwater and marine algae and cyanobacteria and transported through the food chains of water bodies. Potential contamination of aquaculture products (raw and processed fish and seafood) with aquatic toxins requires the use of efficient screening methods for their control. In this study, a multiplex immunochromatographic test system for the simultaneous detection of three aquatic toxins-phycotoxins domoic acid (DA) and okadaic acid (OA), and cyanotoxin microcystin-LR (MC-LR)-is for the first time developed. For this, a competitive indirect immunochromatographic analysis (ICA) based on gold-labeled secondary antibodies was carried out. The LODs/cutoffs/working ranges of the ICA were 0.05/0.3/0.07-0.29, 1.3/100/3.2-58.2, and 0.1/2.0/0.2-1.1 ng/mL for MC-LR, DA, and OA, respectively. The assay duration was 18 min. The developed test system was used to analyze water samples from natural sources (salt and fresh water) and fish samples. For sample preparation of water, simple dilution with a buffer was proposed; for fish samples, methanol-water extraction was utilized. It was demonstrated that the triple LFIA specifically detected target aquatic toxins with recoveries of 85.0-121.5%. The developed multiplex LFIA can be considered a promising analytical solution for the rapid, easy, and sensitive control of water and food safety.

Double Immunochromatographic Test System for Sensitive Detection of Phycotoxins Domoic Acid and Okadaic Acid in Seawater and Seafood

In this investigation, a double immunochromatographic analysis (ICA) of two relevant phycotoxins, domoic acid (DA) and okadaic acid (OA), was developed for the first time. The ICA was performed in the indirect competitive format using gold nanoparticles conjugated with anti-species antibodies. Under optimal conditions, the instrumental detection limits/cutoffs for simultaneous detection of DA and OA were 1.2/100 and 0.1/2.5 ng/mL, respectively. The time of the assay was 18 min. The ICA was applied to test seawater and a large panel of seafood, including mussels, tiger shrimps, octopuses, whelks, crabs, and scallops. The proposed simple sample preparation method for seafood takes only 20 min. For seawater, a dilution by buffer was implemented. The assay recoveries varied from 80.8% to 124.5%. The competitive potential of the proposed technique as a tool to control natural water and seafood samples is determined by its simplicity, rapidity, and sensitivity.

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

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

A highly sensitive photothermal immunochromatographic sensor for detection of aflatoxin B1 based on Cu2-xSe-Au nanoparticles

In the study, Cu_2-xSe-Au nanoparticles (CSA) with a photothermal conversion efficiency of 60.78 % at 808 nm were applied to the construction of thermal analysis immunochromatographic test strips for the highly sensitive quantitative detection of aflatoxin B₁ (AFB₁) in grain. The CSA was coupled with the AFB₁ antibody to form a photothermal sensor probe by physical adsorption. The constructed immunosensor exhibited high sensitivity and a wide linear range from 0.01 to 10 μg/L in PBS. The detection limits of 0.00842 μg/L based on the thermal analysis was significantly improved by 11.88-fold compared with colorimetric results. No cross-reaction with the other mycotoxins was found except for aflatoxin B₂, aflatoxin M₁, aflatoxin G₁ and aflatoxin G₂. Applied to analysize grain sample, the method achieved the detection of AFB₁ ranging from 0.16 to 160 μg/kg.

Second near-infrared fluorescent dye for lateral flow immunoassays rapid detection of influenza A/B virus

Sensitive and rapid diagnostic point of care testing (POCT) system is of great significance to prevent and control human virus infection. Here reported an immunochromatographic strip technology. The second near-infrared (NIR-II) fluorescent dye encapsulated into polystyrene (PS) nanoparticles, was integrated into a lateral flow assay platform to achieve excellent detection of influenza A/B. This surface-functionalized and mono-dispersed PS nanoparticles has been conjugated with influenza nucleoprotein monoclonal antibody as targets for influenza antigen-detection. This assay achieved the detection limit of 0.015 ng/mL for influenza A nucleoprotein and 4.3*10^-5 HAU/mL (10^2.08 TCID₅₀/mL) influenza A virus (influenza B: 0.037 ng/mL, 9.7*10^-7 HAU/mL (10^0.43 TCID₅₀/mL)). Compared with an Au-based lateral flow test strip, the strip's sensitivity is about 16-fold higher than it. Strip detection properties remain stable for 6 months under 4 °C to 30 °C storage. The assay's intra assay variation is 5.14% and the inter assay variation is 7.74%. Other potential endogenous and exogenous interfering substances (whole blood, nasal mucin, saliva, antipyretics, antihistamines and neuraminidase inhibitors) showed negative results, which verified the excellent specificity of this method. This assay was successfully applied to the POCT quantitative detection of influenza A/B virus, the sensitivity to influenza A and B viruses was 70% and 87.5% respectively, and the specificity was 100%. Therefore, these microspheres can be used as an effective material for rapid POCT detection in clinical specimens.

Multifunctional Au@Pt@Ag NPs with color-photothermal-Raman properties for multimodal lateral flow immunoassay

Multimodal lateral flow immunoassay (LFIA) has displayed its potential to improve practicability and elasticity of point-of-care testing. Herein, multifunctional core-shell-shell Au@Pt@Ag NPs loaded with dual-layer Raman reporter molecules of 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) with a characteristic combination of color-photothermal-Raman performance were constructed for colorimetric LFIA (CM-LFIA), photothermal LFIA (PT-LFIA) and surface-enhanced Raman scattering-based LFIA (SERS-LFIA), respectively. The highly specific nanoprobes, being obtained through the combination of the resulted dual-layer DTNB modified Au@Pt@Ag NPs with the antibody, were triumphantly utilized in exploring multimodal LFIA with one visual qualitative and two optional quantitative modes with excellent sensing sensitivity. Under optimal conditions, the limit of detection (LOD) for the model hazardous analyte dehydroepiandrosterone (DHEA) were 1.0 ng mL^-1 for CM-LFIA, 0.42 ng mL^-1 for PT-LFIA, and 0.013 ng mL^-1 for SERS-LFIA, three of which were over 100-fold, 200-fold and 7 000-fold more sensitive than conventional visual AuNPs-based LFIA, respectively. In addition, the quantitative PT-LFIA and SERS-LFIA sensors worked well in spiked real samples with acceptable recoveries of 96.2 - 106.7% and 98.2 - 105.2%, respectively. This assay demonstrated that the developed multimodal LFIA had a great potential to be a powerful tool for accurate tracing hazardous analytes in complex samples.

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.

Fast and ultrafast thermal contrast amplification of gold nanoparticle-based immunoassays

For highly sensitive point-of-care (POC) diagnostics, we explored the limit of thermal contrast amplification (TCA) reading of gold nanoparticles (GNPs/mm²) at test regions in immunoassays. More specifically, we built and compared fast (minute scale) and ultrafast (seconds scale) TCA setups using continuous-wave (CW) and ms pulsed lasers, respectively. TCA improved the limit of detection (LoD) for silica-core gold nanoshells (GNSs) preloaded in nitrocellulose (NC) membrane as model lateral flow immunoassays (LFAs) by 10- to 20-fold over visual reading. While the ultrafast TCA led to higher thermal signals, this came with a twofold loss in LoD vs. fast TCA primarily due to noise within the infrared sensor and a necessity to limit power to avoid burning. To allow higher laser power, and therefore amplification fold, we also explored transparent glass coverslip substrate as a model microfluidic immunoassay (MIA). We found the ultrafast TCA reading of GNS-coated coverslips achieved a maximal signal amplification (57-fold) over visual reading of model LFAs. Therefore, ultrafast TCA-MIA is promising for ultrasensitive and ultrafast diagnostics. Further advantages of using TCA in MIA vs. LFA could include lower sample volume, multiplexed tests, higher throughput, and fast reading. In summary, TCA technology is able to enhance the sensitivity and speed of reading GNPs (GNPs/mm²) within both LFAs and MIAs.

Lateral flow assay applied to pesticides detection: recent trends and progress

Devices based on lateral flow assay (LFA) have been gaining more and more space in the detection market mainly due to their simplicity, speed, and low cost. These devices have excellent sensing format versatility and make these strips an ideal choice for field applications. The COVID-19 pandemic boosted the democratization of this method as a "point of care testing" (POCT), and the trend is that these devices become protagonists for the monitoring of pesticides in the environment. However, designing LFA devices for detecting and monitoring pesticides in the environment is still a challenge. This is because analytes are small molecules and have only one antigenic determinant, which makes it difficult to apply direct immunoassays. Furthermore, most LFA devices provide only qualitative or semi-quantitative results and have a limited number of applications in multi-residue analysis. Here, we present the state of the art on the use of LFA in the environmental monitoring of pesticides. Based on well-documented results, we review all available LFA formats and strategies for pesticide detection, which may have important implications for the future of monitoring pesticides in the environment. The main advances, challenges, and perspectives of these devices for a direction in this field of study are also presented.

Rapid detection of phycotoxin domoic acid in seawater and seafood based on the developed lateral flow immunoassay

A lateral flow immunoassay (LFIA) of phycotoxin domoic acid (DA) contaminating seawater and marine organisms was developed in this investigation. Nine clones of monoclonal antibodies against DA were produced and characterized. The test system was implemented in the indirect competitive format, where gold nanoparticles as a marker were conjugated with secondary antibodies. The developed test system allows for the detection of DA with a cutoff of 60 ng mL^-1 and an instrumental detection limit of 1.4 ng mL^-1 within 15 min. The LFIA was applied to detect DA in seawater, mussels, shrimps, and octopuses. A simple method of seafood sample preparation was proposed. The entire analytical cycle, from obtaining a sample to the estimation of final results, takes only 30 min. The assay recoveries ranged from 88.5% to 124%. The developed analytical method is a promising solution for rapid on-site monitoring of marine toxicants in water and food throughout the farm-to-fork chain.

Immunoprofiling of Severity and Stage of Bacterial Infectious Diseases by Ultrabright Fluorescent Nanosphere-Based Dyad Test Strips

Bacterial infectious diseases are common clinical diseases that seriously threaten human health, especially in countries and regions with poor environmental hygiene. Due to the lack of characteristic clinical symptoms and signs, it is a challenge to distinguish a bacterial infection from other infections, leading to misdiagnosis and antibiotic overuse. Therefore, there is an urgent need to develop a specific method for detection of bacterial infections. Herein, utilizing ultrabright fluorescent nanospheres (FNs) as reporters, immunochromatographic dyad test strips are developed for the early detection of bacterial infections and distinction of different stages of bacterial infectious diseases in clinical samples. C-reactive protein (CRP) and heparin-binding protein (HBP) are quantified and assayed because their levels in plasma are varied dynamically and asynchronously during the progression of the disease. The detection limits of CRP and HBP can reach as low as 0.51 and 0.65 ng/mL, respectively, due to the superior fluorescence intensity of each FN, which is 570 times stronger than that of a single quantum dot. The assay procedure can be achieved in 22 min, fully meeting the needs of rapid and ultrasensitive detection in the field. This constructed strip has been successfully used to profile the stage and severity of bacterial infections by monitoring the levels of CRP and HBP in human plasma samples, showing great potential as a point-of-care biosensor for clinical diagnosis. In addition to bacterial infections, the developed ultrabright FN-based point-of-care testing can be readily expanded for rapid, quantitative, and ultrasensitive detection of other trace substances in complex systems.

Latest Trends in Lateral Flow Immunoassay (LFIA) Detection Labels and Conjugation Process

LFIA is one of the most successful analytical methods for various target molecules detection. As a recent example, LFIA tests have played an important role in mitigating the effects of the global pandemic with SARS-COV-2, due to their ability to rapidly detect infected individuals and stop further spreading of the virus. For this reason, researchers around the world have done tremendous efforts to improve their sensibility and specificity. The development of LFIA has many sensitive steps, but some of the most important ones are choosing the proper labeling probes, the functionalization method and the conjugation process. There are a series of labeling probes described in the specialized literature, such as gold nanoparticles (GNP), latex particles (LP), magnetic nanoparticles (MNP), quantum dots (QDs) and more recently carbon, silica and europium nanoparticles. The current review aims to present some of the most recent and promising methods for the functionalization of the labeling probes and the conjugation with biomolecules, such as antibodies and antigens. The last chapter is dedicated to a selection of conjugation protocols, applicable to various types of nanoparticles (GNPs, QDs, magnetic nanoparticles, carbon nanoparticles, silica and europium nanoparticles).

Magnetic Nanoclusters Increase the Sensitivity of Lateral Flow Immunoassays for Protein Detection: Application to Pneumolysin as a Biomarker for Streptococcus pneumoniae

Lateral flow immunoassays for detecting biomarkers in body fluids are simple, quick, inexpensive point-of-care tests widely used in disease surveillance, such as during the coronavirus disease 2019 (COVID-19) pandemic. Improvements in sensitivity would increase their utility in healthcare, food safety, and environmental control. Recently, biofunctional magnetic nanoclusters have been used to selectively label target proteins, which allows their detection and quantification with a magneto-inductive sensor. This type of detector is easily integrated with the lateral flow immunoassay format. Pneumolysin is a cholesterol-dependent cytolysin and one of the most important protein virulence factors of pneumonia produced by Streptococcus pneumoniae. It is recognized as an important biomarker for diagnosis in urine samples. Pneumonia is the infectious disease that causes the most deaths globally, especially among children under five years and adults over 65 years, most of them in low- and middle-income countries. There especially, a rapid diagnostic urine test for pneumococcal pneumonia with high sensitivity and specificity would be helpful in primary care. In this work, a lateral flow immunoassay with magnetic nanoclusters conjugated to anti-pneumolysin antibodies was combined with two strategies to increase the technique's performance. First, magnetic concentration of the protein before the immunoassay was followed by quantification by means of a mobile telephone camera, and the inductive sensor resulted in detection limits as low as 0.57 ng (telephone camera) and 0.24 ng (inductive sensor) of pneumolysin per milliliter. Second, magnetic relocation of the particles within the test strip after the immunoassay was completed increased the detected signal by 20%. Such results obtained with portable devices are promising when compared to non-portable conventional pneumolysin detection techniques such as enzyme-linked immunosorbent assays. The combination and optimization of these approaches would have excellent application in point-of-care biodetection to reduce antibiotic misuse, hospitalizations, and deaths from community-acquired pneumonia.

Advancements in Testing Strategies for COVID-19

The SARS-CoV-2 coronavirus, also known as the disease-causing agent for COVID-19, is a virulent pathogen that may infect people and certain animals. The global spread of COVID-19 and its emerging variation necessitates the development of rapid, reliable, simple, and low-cost diagnostic tools. Many methodologies and devices have been developed for the highly sensitive, selective, cost-effective, and rapid diagnosis of COVID-19. This review organizes the diagnosis platforms into four groups: imaging, molecular-based detection, serological testing, and biosensors. Each platform's principle, advancement, utilization, and challenges for monitoring SARS-CoV-2 are discussed in detail. In addition, an overview of the impact of variants on detection, commercially available kits, and readout signal analysis has been presented. This review will expand our understanding of developing advanced diagnostic approaches to evolve into susceptible, precise, and reproducible technologies to combat any future outbreak.

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

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

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

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

The helper oligonucleotides enable detection of folded single-stranded DNA by lateral flow immunoassay after HCR signal amplification

A combination of Hybridization Chain Reaction (HCR) and Lateral Flow Immunoassay (LFIA) is an attractive strategy for a simple signal amplification DNA/RNA detection. The present study aimed to report a strategy used to solve a problem encountered when the target DNA contained folded secondary structure during HCR, enabling HCR hairpin probes to easily access the target site. The 24-nt conserved sequence within 3'-UTR, present only in dengue virus genome but not in other species, is an ideal target to use as a probe binding site for pan-dengue virus detection. Thus, the 105-nt target containing the 24-nt target sequence was chosen as a target with secondary structures. The 24-nucleotide (nt) synthetic target DNA successfully induced HCR reaction within 5 min at room temperature. However, the HCR detection of the 105-nt synthetic target DNA with secondary structures was problematic. The probe hybridization was prevented by the secondary structures of the target, resulting in a failure to generate HCR product. To solve this problem, two helper oligonucleotides (helper1 and helper2) were designed to linearize the folded structure of the 105-nt target through strand-displacement mechanism, allowing the HCR hairpin probes to easily access the target site. The HCR product with the labeled helper oligonucleotides and the labeled probes were successfully detected by LFIA. With this strategy, the combination of the helper-enhanced HCR and LFIA exhibited a limit of detection (LOD) in a nanomolar range of the 105-nt DENV synthetic target DNA. Our study demonstrated that signal amplification by the combination of HCR and LFIA could successfully detect the target DNA with secondary structure, but not target RNA with secondary structure. In summary, this work provided a proof of concept of two main issues including probe hybridization enhancement by helper oligonucleotide for the target with complicated secondary structure and the advantage of a combination of labeled helper and HCR probes design for LFIA to overcome the false positive result from HCR probe leakage. Our findings on the use of helper oligonucleotides may be beneficial for the development of other isothermal amplification, since the secondary structure of the target is one of the major obstacles among hybridization-based methods.

Design of a Signal-Amplified Aptamer-Based Lateral Flow Test Strip for the Rapid Detection of Ochratoxin A in Red Wine

In order to improve the weak optical performance of gold nanoparticles and realize the signal amplification of lateral flow chromatography test strips, individual gold nanoparticles (AuNPs) were aggregated into gold nanoparticle aggregates through functional groups around polyamidoamine (PAMAM) dendrimers. A signal-amplified aptamer-based lateral flow chromatography test strip was constructed for the rapid determination of ochratoxin A (OTA). Under optimal conditions, the visual detection limit of this test strip was 0.4 ng mL⁻¹ and the semi-quantitative limit of detection (LOD) was 0.04 ng mL⁻¹. Compared with other traditional aptamer lateral flow chromatography test strips, its sensitivity was improved about five times. The whole test could be completed within 15 min. The aptamer-based strip was applied to the detection of OTA in red wine; the average recoveries ranged from 93% to 105.8% with the relative standard deviation (RSD) varying from 3% to 8%, indicating that the test strip may be a potentially effective tool for the on-site detection of OTA.

Incorporation of engineered nanoparticles of biochar and fly ash against bacterial leaf spot of pepper

In agriculture, the search for higher net profit is the main challenge in the economy of the producers and nano biochar attracts increasing interest in recent years due to its unique environmental behavior and increasing the productivity of plants by inducing resistance against phytopathogens. The effect of rice straw biochar and fly ash nanoparticles (RSBNPs and FNPs, respectively) in combination with compost soil on bacterial leaf spot of pepper caused by Xanthomonascampestris pv. vesicatoria was investigated both in vitro and in vivo. The application of nanoparticles as soil amendment significantly improved the chili pepper plant growth. However, RSBNPs were more effective in enhancing the above and belowground plant biomass production. Moreover, both RSBNPs and FNPs, significantly reduced (30.5 and 22.5%, respectively), while RSBNPs had shown in vitro growth inhibition of X.campestris pv. vesicatoria by more than 50%. The X-ray diffractometry of RSBNPs and FNPs highlighted the unique composition of nano forms which possibly contributed in enhancing the plant defence against invading X.campestris pv. vesicatoria. Based on our findings, it is suggested that biochar and fly ash nanoparticles can be used for reclaiming the problem soil and enhance crop productivity depending upon the nature of the soil and the pathosystem under investigation.

Gold-based nanoplatform for a rapid lateral flow immunochromatographic test assay for gluten detection

Background

Gluten, a food allergen, is available in foods derived from wheat, rye and barley. It damages the small intestine and causes celiac disease. Herein, we designed a rapid immunochromatographic lateral flow test assay for detecting the gluten contents of raw materials. In this rapid test, the presence of gluten was screened through the capturing of gliadin (a toxic component of gluten) by two identical gliadin monoclonal antibodies. One of the antibodies was immobilized on the membrane in the test zone as a capture reagent. The other antibody was labeled with gold nanoparticles (AuNPs) as a detector reagent.

Results

Gold nanoparticles with a size of about 20 nm were synthesized and conjugated to the gliadin monoclonal antibodies. The detection limit of the experimental assay was 20 ppm and positive results were visualized after 15 min using only 40 μL of the extracted sample for each test. Analysis of different flour samples identified the best sensitivity and specificity of the lateral flow test strip (LFTS).

Conclusion

The experimental LFTS is an easy-to-use and rapid method for the screening of gluten level in raw materials. The LFTS may be employed to ensure the safety of foods.

Glucose and Ethanol Detection with an Affinity-Switchable Lateral Flow Assay

The lateral flow assay (LFA) is one of the most successful analytical platforms for rapid on-site detection of target substances. This type of assay has been used in many rapid diagnoses, for example, pregnancy tests and infectious disease prevention. However, applications of LFAs for very small molecules remain a demanding challenge due to the problem of obtaining the corresponding binding partners to form sandwich complexes. In this paper, we report an affinity-switchable (AS) LFA (ASLFA) for the rapid and selective detection of hydrogen peroxide (H₂O₂), glucose, and ethanol in blood serum and urine samples. Unlike classical LFAs, which rely on the "always on" interaction between the antigen and the antibody, the working principle of ASLFA is based on the gold nanoparticle-conjugated AS biotin probe Au@H₂O₂-ASB, which can be activated by H₂O₂ for binding with the streptavidin (SA) protein. In the presence of glucose and ethanol, glucose oxidase and alcohol oxidase can react with the substrate to generate H₂O₂ and thereby activate Au@H₂O₂-ASB for binding with SA. Therefore, this ASLFA approach can be an alternative for classical glucose and ethanol detection methods in a wide variety of samples, where simple and rapid on-site detection is essential.

Nanomaterial-based aptamer biosensors for ochratoxin A detection: a review

Ochratoxin A (OTA) is a widely distributed mycotoxin that often contaminates food, grains and animal feed. It poses a serious threat to human health because of its high toxicity and persistence. Therefore, the development of an inexpensive, highly sensitive, accurate and rapid method for OTA detection is imperative. In recent years, various nanomaterials used in the establishment of aptasensors have attracted great attention due to their large surface-to-volume ratio, good stability and facile preparation. This review summarizes the development of nanomaterial-based aptasensors for OTA determination and sample treatment over the past 5 years. The nanomaterials used in OTA aptasensors include metal, carbon, luminescent, magnetic and other nanomaterials. Finally, the limitations and future challenges in the development of nanomaterial-based OTA aptasensors are reviewed and discussed.

Recent advances in nanoremediation: Carving sustainable solution to clean-up polluted agriculture soils

Agriculture is one of the foremost significant human activities, which symbolizes the key source for food, fuel and fibers. This activity results in a lot of ecological harms particularly with the excessive usage of chemical fertilizers and pesticides. Different agricultural practices have remained industrialized to advance food production, due to the growth in the world population and to meet the food demand through the routine use of more effective fertilizers and pesticides. Soil is intensely embellished by environmental contamination and it can be stated as "universal incline." Soil pollution usually occurs from sewage wastes, accidental discharges or as byproducts of chemical residues of unrestrained production of numerous materials. Soil pollution with hazardous materials alters the physical, chemical, and biological properties, causing undesirable changes in soil fertility and ecosystem. Engineered nanomaterials offer various solutions for remediation of contaminated soils. Engineered nanomaterial-enable technologies are able to prevent the uncontrolled release of harmful materials into the environment along with capabilities to combat soil and groundwater borne pollutants. Currently, nanobiotechnology signifies a hopeful attitude to advance agronomic production and remediate polluted soils. Studies have outlined the way of nanomaterial applications to restore the eminence of the environment and assist the detection of polluted sites, along with potential remedies. This review focuses on the latest developments in agricultural nanobiotechnology and the tools developed to combat soil or land and or terrestrial pollution, as well as the benefits of using these tools to increase soil fertility and reduce potential toxicity.

Improved Strategies for CRISPR-Cas12-based Nucleic Acids Detection

The COVID-19 pandemic has brought great challenges to traditional nucleic acid detection technology. Thus, it is urgent to develop a more simple and efficient nucleic acid detection technology. CRISPR-Cas12 has signal amplification ability, high sensitivity and high nucleic acid recognition specificity, so it is considered as a nucleic acid detection tool with broad development prospects and high application value. This review paper discusses recent advances in CRISPR-Cas12-based nucleic acid detection, with an emphasis on the new research methods and means to improve the nucleic acid detection capability of CRISPR-Cas12. Strategies for improving sensitivity, optimization of integrated detection, development of simplified detection mode and improvement of quantitative detection capabilities are included. Finally, the future development of CRISPR-Cas12-based nucleic acids detection is prospected.