Simultaneous Lateral Flow Immunoassay for Multi-Class Chemical Contaminants in Maize and Peanut with One-Stop Sample Preparation

Polycations as Aptamer-Binding Modulators for Sensitive Fluorescence Anisotropy Assay of Aflatoxin B1

Fluorescence induced by the excitation of a fluorophore with plane-polarized light has a different polarization depending on the size of the fluorophore-containing reagent and the rate of its rotation. Based on this effect, many analytical systems have been implemented in which an analyte contained in a sample and labeled with a fluorophore (usually fluorescein) competes to bind to antibodies. Replacing antibodies in such assays with aptamers, low-cost and stable oligonucleotide receptors, is complicated because binding a fluorophore to them causes a less significant change in the polarization of emissions. This work proposes and characterizes the compounds of the reaction medium that improve analyte binding and reduce the mobility of the aptamer-fluorophore complex, providing a higher analytical signal and a lower detection limit. This study was conducted on aflatoxin B1 (AFB1), a ubiquitous toxicant contaminating foods of plant origins. Eight aptamers specific to AFB1 with the same binding site and different regions stabilizing their structures were compared for affinity, based on which the aptamer with 38 nucleotides in length was selected. The polymers that interact reversibly with oligonucleotides, such as poly-L-lysine and polyethylene glycol, were tested. It was found that they provide the desired reduction in the depolarization of emitted light as well as high concentrations of magnesium cations. In the selected optimal medium, AFB1 detection reached a limit of 1 ng/mL, which was 12 times lower than in the tris buffer commonly used for anti-AFB1 aptamers. The assay time was 30 min. This method is suitable for controlling almond samples according to the maximum permissible levels of their contamination by AFB1. The proposed approach could be applied to improve other aptamer-based analytical systems.

A Fluorescence Resonance Energy Transfer Aptasensor for Aflatoxin B1 Based on Ligand-Induced ssDNA Displacement

In this study, a fluorescence resonance energy transfer (FRET)-based aptasensor for the detection of aflatoxin B1 (AFB1) was designed using a carboxyfluorescein (FAM)-labeled aptamer and short complementary DNA (cDNA) labeled with low molecular quencher RTQ1. The sensing principle was based on the detection of restored FAM-aptamer fluorescence due to the ligand-induced displacement of cDNA in the presence of AFB1, leading to the destruction of the aptamer/cDNA duplex and preventing the convergence of FAM and RTQ1 at the effective FRET distance. Under optimal sensing conditions, a linear correlation was obtained between the fluorescence intensity of the FAM-aptamer and the AFB1 concentration in the range of 2.5-208.3 ng/mL with the detection limit of the assay equal to 0.2 ng/mL. The assay time was 30 min. The proposed FRET aptasensor has been successfully validated by analyzing white wine and corn flour samples, with recovery ranging from 76.7% to 91.9% and 84.0% to 86.5%, respectively. This work demonstrates the possibilities of labeled cDNA as an effective and easily accessible tool for sensitive AFB1 detection. The homogeneous FRET aptasensor is an appropriate choice for contaminant screening in complex matrices.

Chromatographic methods for rapid aflatoxin B1 analysis in food: a review

Aflatoxin B1 (AFB1) is a mycotoxin and is the most carcinogenic of all known chemicals. In view of the AFB1 characteristics of widespread distribution, serious pollution, great harm to humans, and animals and difficult to remove, it is urgent to develop a convenient and sensitive detection method. Moreover, chromatographic test strips (CTSs) are a rapid detection technology that combines labeling technology with chromatography technology. CTSs have been widely used in the fields of environmental monitoring, medical diagnosis, and food safety analysis in recent years. Different from other immune assays, they have the advantages of short measuring time, low cost, high efficiency and no need for professionals to operate. In addition, the introduction of nanomaterials has laid a good foundation for the detection of high sensitivity, high specificity and high efficiency via CTSs. Herein, we tend to comprehensively introduce the applications of chromatographic methods in AFB1 detection and pay attention to the signal detection modes based on nanomaterials in antibody-based immunochromatographic strips (ICSs), such as colorimetric, fluorescent, chemiluminescent, and Raman scattering sensing. Some typical examples are also listed in this review. In the end, we make a summary and put forward prospects for the development of CTSs.

Potential of nanobiosensor in sustainable agriculture: the state-of-art

A rapid surge in world population leads to an increase in worldwide demand for agricultural products. Nanotechnology and its applications in agriculture have appeared as a boon to civilization with enormous potential in transforming conventional farming practices into redefined farming activities. Low-cost portable nanobiosensors are the most effective diagnostic tool for the rapid on-site assessment of plant and soil health including plant biotic and abiotic stress level, nutritional status, presence of hazardous chemicals in soil, etc. to maintain proper farming and crop productivity. Nanobiosensors detect physiological signals and convert them into standardized detectable signals. In order to achieve a reliable sensing analysis, nanoparticles can aid in signal amplification and sensor sensitivity by lowering the detection limit. The high selectivity and sensitivity of nanobiosensors enable early detection and management of targeted abnormalities. This study identifies the types of nanobiosensors according to the target application in agriculture sector.

A Novel Lateral Flow Immunochromatographic Assay for Rapid and Simultaneous Detection of Aflatoxin B1 and Zearalenone in Food and Feed Samples Based on Highly Sensitive and Specific Monoclonal Antibodies

Simultaneous aflatoxin (AFB1) and zearalenone (ZEN) contamination in agro-products have become widespread globally and have a toxic superposition effect. In the present study, we describe a highly sensitive and specific dual lateral flow immunochromatographic assay (dual test strip) for rapid and simultaneous detection of AFB1 and ZEN in food and feed samples based on respective monoclonal antibodies (mAbs). Two immunogens AFB1-BSA (an AFB1 and bovine serum albumin (BSA) conjugate) and ZEN-BSA (a ZEN and BSA conjugate) were synthesized in oximation active ester (OAE) and amino glutaraldehyde (AGA). The molecular binding ratio of AFB1:BSA was 8.64:1, and that of ZEN:BSA was 17.2:1, identified by high-resolution mass spectrometry (HRMS) and an ultraviolet spectrometer (UV). The hybridoma cell lines 2A11, 2F6, and 3G2 for AFB1 and 2B6, 4D9 for ZEN were filtered by an indirect non-competitive enzyme-linked immunosorbent assay (inELISA) and an indirect competitive enzyme-linked immunosorbent assay (icELISA), respectively. As AFB1 mAb 2A11 and ZEN mAb 2B6 had the lowest 50% inhibitive concentration (IC50) and cross-reactivity (CR), they were selected for subsequent experiments. By systematically optimizing the preparation condition of gold nanoparticles (AuNPs), AuNPs-labeled mAbs, and detection condition, the visual limit of detection (LOD) of the dual test strip was 1.0 μg/L for AFB1 and 5.0 μg/L for ZEN, whereas that of the test strip reader was 0.23 μg/L for AFB1 and 1.53 μg/L for ZEN. The high reproducibility and stability of the dual test were verified using mycotoxin-spiked samples. The dual test strips were highly specific and sensitive for AFB1 and ZEN, which were validated using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Thus, the proposed AFB1 and ZEN dual test strip is suitable for rapid and simultaneous detection of AFB1 and ZEN contamination in food and feed samples.

Development of a Lateral Flow Strip with a Positive Readout for the On-Site Detection of Aflatoxin B1

Aflatoxin B₁ is one of the contamination indicators for food safety monitoring. The rapid and effective assessment and determination of AFB₁ in food is of great importance to dietary safety. The lateral flow assay shows advantages in its simplicity, and rapidity, and provides a visual readout, while the available lateral flow assay for AFB₁ requires a competitive format that produces readings inversely proportional to the AFB₁ concentration, which is counterintuitive and may lead to a potential misinterpretation of the results. Herein, we developed a positive readout aptamer-based lateral flow strip (Apt-strip) for the detection of AFB₁. This Apt-strip relies on the competition between AFB₁ and fluorescein-labeled complementary DNA strands (FAM-cDNA) for affinity binding to limited aptamers against AFB₁ (AFB₁-Apt). In the absence of AFB₁, AFB₁-Apt hybridizes with FAM-cDNA. No signal at the T-line of the Apt-strip was observed. In contrast, AFB₁-Apt binds to AFB₁ in the sample, and then a part of the FAM-cDNA is hybridized with the free AFB₁-Apt, at which time the other unreacted FAM-cDNA is captured by A35-Apt on the T-line. The signal was observed. This method achieved fast detection of AFB₁ with a detection limit (DL) of 0.1 ng/mL, positive readout, and increased sensitivity.

Visual detection of aflatoxin B1 based on specific aptamer recognition combining with triple amplification strategy

A highly sensitive and specific visual detection method for aflatoxin B1 (AFB1) based on the target specificity of aptamer, rolling circle amplification (RCA) and enzyme catalysis biological amplification effect has been established. In this work, AFB1 aptamer immobilized on the surface of magnetic beads (MB) serves as a molecular recognition probe. In the absence of AFB1, the aptamer and auxiliary linking probe (LP) maintain a double stranded state due to partial base pair complementarities. By contrast, in the presence of AFB1, the aptamer preferentially binds to AFB1 specifically, and the LP later restores to a single stranded state. Subsequently, the RCA reaction is triggered by above-mentioned single stranded LP to generate long DNA strands, which are employed to capture amounts of signal probes (SP) and horse radish peroxidases (HRP). Finally, amounts of HRP catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂ and leads to a dramatic color change of the solution from colorlessness to deep blue as a signal indicator, obtaining a high sensitivity, high specificity and visual detection of AFB1. Under optimal conditions, a good linear detection range (0.5-40 pg·mL^-1) was achieved, and the limit of detection (LOD) was 0.13 pg·mL^-1. Besides, the proposed aptasensor showed excellent specificity for AFB1 compared with five other mycotoxins. More than that, all reactions occur on the surface of the magnetic beads, which not only facilitates the detection operation process including the efficient isolation and collection of AFB1 from sample matrix, but also gets better selectivity and stronger resistibility to target analyte in complex sample matrix, adequately indicating its potential application in AFB1 practical detection.

Recent Progress in Rapid Determination of Mycotoxins Based on Emerging Biorecognition Molecules: A Review

Mycotoxins are secondary metabolites produced by fungal species, which pose significant risk to humans and livestock. The mycotoxins which are produced from Aspergillus, Penicillium, and Fusarium are considered most important and therefore regulated in food- and feedstuffs. Analyses are predominantly performed by official laboratory methods in centralized labs by expert technicians. There is an urgent demand for new low-cost, easy-to-use, and portable analytical devices for rapid on-site determination. Most significant advances were realized in the field bioanalytical techniques based on molecular recognition. This review aims to discuss recent progress in the generation of native biomolecules and new bioinspired materials towards mycotoxins for the development of reliable bioreceptor-based analytical methods. After brief presentation of basic knowledge regarding characteristics of most important mycotoxins, the generation, benefits, and limitations of present and emerging biorecognition molecules, such as polyclonal (pAb), monoclonal (mAb), recombinant antibodies (rAb), aptamers, short peptides, and molecularly imprinted polymers (MIPs), are discussed. Hereinafter, the use of binders in different areas of application, including sample preparation, microplate- and tube-based assays, lateral flow devices, and biosensors, is highlighted. Special focus, on a global scale, is placed on commercial availability of single receptor molecules, test-kits, and biosensor platforms using multiplexed bead-based suspension assays and planar biochip arrays. Future outlook is given with special emphasis on new challenges, such as increasing use of rAb based on synthetic and naïve antibody libraries to renounce animal immunization, multiple-analyte test-kits and high-throughput multiplexing, and determination of masked mycotoxins, including stereoisomeric degradation products.

Photoluminescent Molecules and Materials as Diagnostic Reporters in Lateral Flow Assays

The lateral flow assay (LFA) is a point-of-care diagnostic test commonly available in an over-the-counter format because of its simplicity, speed, low cost, and portability. The reporter particles in these assays are among their most significant components because they perform the diagnostic readout and dictate the test's sensitivity. Today, gold nanoparticles are frequently used as reporters, but recent work focusing on photoluminescent-based reporter technologies has pushed LFAs to better performance. These efforts have focused specifically on reporters made of organic fluorophores, quantum dots, lanthanide chelates, persistent luminescent phosphors, and upconversion phosphors. In most cases, photoluminescent reporters show enhanced sensitivity compared to conventional gold nanoparticle-based assays. Here, we examine the advantages and disadvantages of these different reporters and highlight their potential benefits in LFAs. Our assessment shows that photoluminescent-based LFAs can not only reach lower detection limits than LFAs with traditional reporters, but they also can be capable of quantitative and multiplex analyte detection. As a result, the photoluminescent reporters make LFAs well-suited for medical diagnostics, the food and agricultural industry, and environmental testing.

One-step time-resolved fluorescence microsphere immunochromatographic test strip for quantitative and simultaneous detection of DON and ZEN

Deoxynivalenol (DON) and zearalenone (ZEN) are mycotoxins that contaminate a wide range of grains and crops. In this study, a one-step time-resolved single-channel immunochromatographic test strip based on europium ion polystyrene fluorescence microspheres was first developed for sensitive and quantitative detection of DON and ZEN. The concentration of the artificial antigen and the mass ratio of the monoclonal antibody to fluorescent microspheres for conjugation were optimized to simplify the sample addition process during immunochromatographic assay and improve the on-site detection efficiency. The limits of detection (LOD) of the single-channel immunochromatographic test strip for DON and ZEN detection were 0.17 and 0.54 μg/L, respectively. Meanwhile, the dual-channel immunochromatographic test strip was designed to simultaneously detect DON and ZEN, with LODs of 0.24 and 0.69 μg/L achieved for DON and ZEN, respectively. The developed test strips also yielded recovery results consistent with that obtained by LC-MS/MS for DON and ZEN detection in real samples of wheat and corn flour, confirming the practicability and reliability of the test strip. The developed immunochromatographic test strips realize quick and sensitive detection of DON and ZEN, exhibiting potential for broad applications in the point-of-care testing platform of multiple mycotoxins in agricultural products. Graphic abstract.

Development of Time-Resolved Fluorescence Immunochromatographic Assays for Simultaneously Detecting Tylosin and Tilmicosin in Milk in Group-Screening Manner

Tylosin and tilmicosin (T&T) residues in livestock products have received extensive attention from consumers. Time-resolved fluorescence immunochromatographic assay (TRFICA), as a fast, efficient and sensitive immunoassay method, has played an increasingly important role in the food safety field. Therefore, herein a quantitative and visual TRFICA was established for simultaneously detecting T&T in milk in a group-screening manner. Under the optimal conditions, the standard curve range of developed TRFICA based on the T&T was 1.87~7.47 ng/mL, and the half-maximal inhibition concentrations (IC₅₀) were 4.06 ng/mL and 3.74 ng/mL, respectively. The limits of detection (LOD) of the TRFICA method were from 1.72 ng/mL to 1.39 ng/mL, and the visual cut-off values were 31.25 ng/mL and 62.50 ng/mL for T&T in milk, respectively. Moreover, the stability experiments showed that the strips could be stored at 4 °C for more than 6 months, the total detection time was less than 13 min, and the cross-reactivities (CRs) with related compounds were less than 0.1%, which concluded that the developed TRFICA method could be used in real milk sample detection.

Enhancing the Sensitivity of Lateral Flow Immunoassay by Magnetic Enrichment Using Multifunctional Nanocomposite Probes

For lateral flow immunoassay (LFIA), it is an important challenge to enhance the detection sensitivity to the same level as polymerase chain reaction or enzyme-linked immunosorbent assay to make LFIA pervasive in the field of on-site environmental analysis. We recently demonstrated that the LFIA sensitivity is dramatically enhanced by using Pt-nanoparticle-latex nanocomposite beads (Pt-P2VPs) as probes for the detection of the influenza A (H1N1) antigen compared with using conventional Au colloids as probes. Here, to further enhance the LFIA sensitivity using Pt-P2VPs, superparamagnetic iron oxide nanoparticles (SPIONs) were chemically conjugated to Pt-P2VPs (Pt-P2VP@SPION) to give them magnetic separation capability (enrichment and/or purification). To investigate the effect of magnetic enrichment on the LFIA sensitivity in a sandwich format, the C-reactive protein (CRP) was chosen as a model analyte and anti-CRP antibody (CRPAb)-conjugated Pt-P2VP@SPION (Pt-P2VP@SPION-CRPAb) beads were used as probes. The visual limit of detection (LOD) of LFIA was successfully lowered by increasing the magnetic enrichment factor φ. The minimum LOD under the present experimental conditions was 0.08 ng/mL for φ = 40, which is 26-fold lower than that of the standard Au-nanoparticle-based LFIA. In theory, the LOD can be unlimitedly decreased by just increasing φ. However, the times required for both the antigen-antibody binding reaction and magnetic separation dramatically increase with φ. We also propose solutions to overcome this drawback.

Development of a microarray lateral flow strip test using a luminescent organic compound for multiplex detection of five mycotoxins

While lateral flow immunoassay (LFIA) is a simple technique that offers a rapid, robust, user friendly, and point-of-care test, its capacity for multiplex detection is rather limited. This study therefore combined the multiplexity of microarray technique and the simple and rapid characteristics of LFIA to enable simultaneous and quantitative detection of five mycotoxins, namely aflatoxin B1 (AFB1), deoxynivalenol (DON), fumonisin B1 (FUMB1), T-2 toxin (T-2), and zearalenone (ZON). In addition, we have synthesized a novel extra-large Stokes shift and strong fluorescence organic compound to be used as a reporter molecule which can be detected under UV light without light filter requirement. Many parameters including microarray spotting buffer, blocking buffer, and concentrations of mycotoxin antibodies were optimized for the microarray LFIA (μLFIA) construction. With the optimal conditions, the μLFIA could accurately and quantitatively detect multiple mycotoxins at the same time. The limits of detection of AFB1, DON, FUMB1, T-2, and ZON were 1.3, 0.5, 0.4, 0.4, and 0.9 ppb, respectively. The recoveries of these five mycotoxins were 70.7%-119.5% and 80.4%-124.8% for intra-assay and inter-assay, respectively. Combining the advantages of the novel reporter molecule and the multiplex capability of μLFIA test, this system could simultaneously detect multiple mycotoxins in one sample with high specificity and high sensitivity. Moreover, this system presents a promising affordable point-of-care platform to detect other analytes as well.

Immunoassays for rapid mycotoxin detection: state of the art

The widespread presence of mycotoxins in nature not only poses a huge health risk to people in terms of food but also causes incalculable losses to the agricultural economy. As a rapidly developing technology in recent years, the mycotoxin immunoassay technology has approached or even surpassed the traditional chromatography technology in some aspects. Using this approach, the lateral flow immunoassay (LFIA) has attracted the interest of researchers due to its user-friendly operation, short time consumption, little interference, low cost, and ability to process a large number of samples at the same time. This paper provides an overview of the immunogens commonly used for mycotoxins, the development of antibodies, and the use of gold nanoparticles, quantum dots, carbon nanoparticles, enzymes, and fluorescent microsphere labeling materials for the construction of LFIAs to improve detection sensitivity. The analytical performance, detection substrates, detection limits or detection ranges of LFIA for mycotoxins have been listed in recent years. Finally, we describe the future outlook for the field, predicting that portable mobile detection devices and simultaneous quantitative detection of multiple mycotoxins is one of the important directions for future development.

An Open Sandwich Immunochromatography for Non-competitive Detection of Small Antigens

Immunochromatography assay is an easy and rapid on-site detection method. However, conventional sandwich immunochromatographies using two antibodies can only detect target molecules above a threshold size. Small molecules below 1000 in molecular weight are usually detected using competitive immunoassay. However, competitive immunoassay is not suitable for visual detection of low concentration samples. Based on the principles of open sandwich immunoassay, which detects small molecules via interchain interaction of separated variable region fragments (V_H and V_L) from a single antibody, we developed non-competitive open sandwich immunochromatography. Bone Gla protein (BGP)-C7, a peptide containing the seven C-terminal amino acids of human osteocalcin, was selected as the target. By using V_L fragments fixed on a nitrocellulose membrane, and colored cellulose bead-labeled V_H fragments, we specifically detected 10 ng/mL of BGP-C7. This is the first report of open sandwich immunochromatography, which is an easy and rapid method for on-site, signal-on detection of small molecules.

Developments in mycotoxin analysis: an update for 2019-2020

This review summarises developments on the analysis of various matrices for mycotoxins published in the period from mid-2019 to mid-2020. Notable developments in all aspects of mycotoxin analysis, from sampling and quality assurance/quality control of analytical results, to the various detection and quantitation technologies ranging from single mycotoxin biosensors to comprehensive instrumental methods are presented and discussed. Aside from sampling and quality control, discussion of this past year’s developments is organised by detection and quantitation technology and covers chromatography with targeted or non-targeted high resolution mass spectrometry, tandem mass spectrometry, detection other than mass spectrometry, biosensors, as well as assays that use alternatives to antibodies. This critical review aims to briefly present the most important recent developments and trends in mycotoxin determination as well as to address limitations of the presented methodologies.

Rapid determination of aflatoxin B1 by an automated immunomagnetic bead purification sample pretreatment method combined with high-performance liquid chromatography

We aimed to establish an automated versatile sample preconcentration method based on the modified immunomagnetic beads, which was utilized to enrich for aflatoxin B1 from the matrices. The critical main parameters affecting the extraction efficiency, such as usage amount of immunomagnetic beads, reaction time, elution time, and blending way were investigated. Under the optimized conditions, the content of aflatoxin B1 was analyzed by high-performance liquid chromatography, the mobile phase consists of water-acetonitrile-methanol (42:18:10, v/v/v), and fluorescence detection was performed with excitation and emission wavelengths at 360 and 440 nm, respectively. Moreover, the performance of preconcentration method was compared with the conventional method based on the immunoaffinity column. The accuracy of two clean-up methods was within the error range. In addition, the stability and recyclability of the immunomagnetic beads was studied by recycling them five times. The results for the respective analysis in various samples demonstrated that the developed extraction platform provides a promising approach that is simple, rapid, sensitive, and easy to use.

Time-Resolved Fluorescence Immunochromatography Assay (TRFICA) for Aflatoxin: Aiming at Increasing Strip Method Sensitivity

Aflatoxin is the most harmful mycotoxin that is ubiquitous in foods and agro-products. Because of its high toxicity, maximum admissible levels of aflatoxins (AF) is regulated worldwide, and monitoring of their occurrence in several commodities is mandatory for assuring food safety and consumers' health. Considering that the strip method is very simple and convenient for users, in order to enhance strip assay's sensitivity, a lot of time-resolved fluorescence immunochromatography assays (TRFICAs) were developed recently with increasing several times of assay sensitivity compared with traditional gold nanoparticle-based strip assay (GNP-SA). This review briefly describes the newly developed TRFICA for aflatoxin determination, including TRFICA for aflatoxin B1 (AFB1) detection, TRFICA for aflatoxin M1 (AFM1) detection, TRFICA for total aflatoxins (AFB1 + B2 + G1 + G2) detection and the latest identification-nanobody-based TRFICA for aflatoxin detection. The application of TRFICA for aflatoxin detection in different agro-products is also concluded in this review. Reasonably, TRFICA has been becoming one of the most important tool for monitoring aflatoxin in foods and agro-products.

Improved magnetic lateral flow assays with optimized nanotags for point-of-use inductive biosensing

Magnetic nanotags in lateral flow immunoassay for inductive biosensing: clusters of superparamagnetic critical size optimize the reading out.