Anti-idiotypic nanobody-alkaline phosphatase fusion proteins: Development of a one-step competitive enzyme immunoassay for fumonisin B 1 detection in cereal

Rapid colorimetric sensor for ultrasensitive and highly selective detection of Fumonisin B1 in cereal based on laccase-mimicking activity of silver phosphate nanoparticles

Ag3PO4 nanoparticles (NPs) was prepared through a facile coprecipitation method, and was first found to have excellent laccase-mimicking catalytic activity. The study confirms that Fumonisin B1 (FB1) can effectively hinder the production of superoxide anion (O2-) between Ag3PO4 NPs and dissolved oxygen, and further inhibit laccase-mimicking activity of Ag3PO4 NPs. Thus, a novel rapid colorimetric sensor for FB1 analysis in cereal was first established using laccase-mimicking activity as sensing signal. The absorbance variation of sensing solution is directly related to the amount of FB1, and the color change is further combined with smartphone for quantitively analysis of FB1. The limit of detection (LOD) of the sensor is determined as low as 1.73 μg·L-1, which is far lower than the maximum residue limits (MRLs) of FB1 set by European Commission and US Food and Drug Administration (FDA). The average recovery of 87.8-104.5% for FB1 detection was obtained in cereal.

Overview on the Development of Alkaline-Phosphatase-Linked Optical Immunoassays

The drive to achieve ultrasensitive target detection with exceptional efficiency and accuracy requires the advancement of immunoassays. Optical immunoassays have demonstrated significant potential in clinical diagnosis, food safety, environmental protection, and other fields. Through the innovative and feasible combination of enzyme catalysis and optical immunoassays, notable progress has been made in enhancing analytical performances. Among the kinds of reporter enzymes, alkaline phosphatase (ALP) stands out due to its high catalytic activity, elevated turnover number, and broad substrate specificity, rendering it an excellent candidate for the development of various immunoassays. This review provides a systematic evaluation of the advancements in optical immunoassays by employing ALP as the signal label, encompassing fluorescence, colorimetry, chemiluminescence, and surface-enhanced Raman scattering. Particular emphasis is placed on the fundamental signal amplification strategies employed in ALP-linked immunoassays. Furthermore, this work briefly discusses the proposed solutions and challenges that need to be addressed to further enhance the performances of ALP-linked immunoassays.

Determination of Fumonisin B1 by Aptamer-Based Fluorescence Resonance Energy Transfer

Fumonisin FB is produced by Fusarium moniliforme Sheld, of which FB₁ is the most common and the most toxic. The establishment of a rapid detection method is an important means to prevent and control FB₁ pollution. A highly sensitive fluorescent sensor based on an aptamer for the rapid detection of fumonisin B₁ (FB₁) in corn was established. In this study, 5-carboxyfluorescein (FAM) was labeled on the aptamer of FB₁ (F10). F10 was adsorbed on the surface of graphene oxide (GO) by π-π stacking. The FAM fluorescence signal could be quenched by fluorescence resonance energy transfer between fluorescent molecules and graphene oxide (GO). In the presence of FB₁, the binding efficiency of the aptamer to GO was reduced. Therefore, the content of FB₁ in corn samples was determined by fluorescence measurements of mixed FAM-labeled F10, GO and corn samples. This method had a good linear relationship in an FB₁ concentration range of 0-3000 ng/mL. The equation was y = 0.2576x + 10.98, R² = 0.9936. The limit of detection was 14.42 ng/mL, and the limit of quantification was 43.70 ng/mL. The recovery of a spiked standard in the corn sample was 89.13-102.08%, and the time of detection was 30 min.

Establishment of fumonisin B1 detection method for catalytic fluorescence detection of aptamer-regulated carbon dots

Mycotoxin, common in agricultural products, is a small secondary metabolite with strong toxicity. Fumonisin B₁ (FB₁) is the most common and the most toxic. Establishing a rapid detection method is important for preventing and controlling FB₁ pollution. This study prepared carbon dots (CDs) from 2,2'-dithiosalicylic acid (DTSA). Tetramethylbenzidine (TMB) can be catalyzed to produce fluorescence by CDs, while FB₁ can adhere to the surface of CDs, decreasing fluorescence. Aptamer F10 of FB₁ combines with FB₁ attached to the surface of CDs to restore the catalytic ability of CDs and increase the fluorescence value. This method has good linearity in the FB₁ concentration range from 0 to 1.0 μg mL^-1. The standard curve was Y = -0.2512x + 661.4, R² = 0.9903, the limit of detection (LOD) was 17.67 ng mL^-1 and limit of quantitation (LOQ) was 53.55 ng mL^-1. The recovery of the corn sample was 89.83-98.62%, and the detection time was 30 min.

Nuclease Triggered "Signal-On" and Amplified Fluorescent Sensing of Fumonisin B1 Incorporating Graphene Oxide and Specific Aptamer

Remarkable advancements have been achieved in the development of rapid analytic techniques toward fumonisin B₁ (FB₁) monitoring and even trace levels for food safety in recent years. However, the point-of-care testing for quantitative and accurate FB₁ determination is still challenging. Herein, an innovative aptasensor was established to monitor FB₁ by utilizing graphene oxide (GO) and nuclease-triggered signal enhancement. GO can be utilized as a fluorescence quenching agent toward a fluorophore-modified aptamer, and even as a protectant of the aptamer from nuclease cleavage for subsequent target cycling and signal amplification detection. This proposed sensing strategy exhibited a good linearity for FB₁ determination in the dynamic range from 0.5 to 20 ng mL⁻¹ with a good correlation of R² = 0.995. Its limit of detection was established at 0.15 ng mL⁻¹ (S/N = 3), which was significantly lower than the legal requirements by three orders of magnitude. The interferent study demonstrated that the introduced aptasensor possessed high selectivity for FB₁. Moreover, the aptasensor was successfully applied to the detection of wheat flour samples, and the results were consistent with the classical ELISA method. The rapid response, sensitive and selective analysis, and reliable results of this sensing platform offer a promising opportunity for food mycotoxin control in point-of-care testing.

Indirect signal amplification strategy with a universal probe-based lateral flow immunoassay for the rapid quantitative detection of fumonisin B1

Fumonisin B1 (FB1) is a serious threat to the health of humans and animals. Herein, a lateral flow immunoassay based on universal detection probes (goat anti-mouse IgG@Eu) that could combine with any mouse monoclonal antibody was applied to detect FB1 in corn and feed. Compared with that based on direct monoclonal antibody labeling, this assay maintained bioactivity and saved consumption of monoclonal antibodies with the indirect signal amplification effect. The results indicated that this assay had higher sensitivity with a limit of detection (LOD) of 0.025 and 0.097 ng mL^-1 (0.50 and 1.94 ng g^-1 based on sample weight) in corn and feed, respectively. The detection range was about 1-50 ng mL^-1 (20-1000 ng g^-1 based on sample weight). In addition, the evaluation proved that it had good specificity, accuracy, precision, and applicability, and thus was suitable for the rapid and low-cost detection of fumonisin B1.

Development of a biotinylated nanobody for sensitive detection of aflatoxin B1 in cereal via ELISA

Aflatoxin B₁ (AFB₁) contamination is a severe threat to food safety and human health, and requires continuous monitoring. In this study, we developed a biotin-streptavidin-amplified enzyme-linked immunosorbent assay (BA-ELISA) by using biotinylated nanobody Nb26 and streptavidin-conjugated polymerized horseradish peroxide (SA-PolyHRP) for sensitive and rapid detection of AFB₁ in cereal. Under the optimal condition, the IC₅₀ value of the BA-ELISA was improved to 0.21 ng mL^-1 for AFB₁, satisfying the requirement of detection limit in practical applications. The total assay time of our strategy is reduced to 50 min from 2 h in conventional competitive ELISA. Additionally, the BA-ELISA saves as much as 98% of the antibody in comparison to the previous classic ELISA. Our work also demonstrated an interesting phenomenon that the biotinylated Nb26 achieved better selectivity to AFB₁, which could possibly result from the steric hindrance that interferes reaction between the Nb26 and the AFB₁ analogs. Furthermore, the assay was used to detect AFB₁ in two cereal samples, and the results were in good agreement with that obtained by high performance liquid chromatography. The developed BA-ELISA can be used for routine screening analysis of AFB₁, and offers a promising strategy for measuring low concentrations of food contaminants.

Mimotope-Based Immunoassays for the Rapid Analysis of Mycotoxin: A Review

Mycotoxins are toxic contaminants in foods and feeds that are naturally occurring and largely unavoidable. Determining their contents in these products is essential to protect humans from harm. Immunoassays of mycotoxins have been well-established because they are fast, sensitive, simple, and cost-effective. However, a major limitation of immunoassays is the requirement of toxic mycotoxins as competing antigens, standards, or competing tracers. Mimotopes are peptides or proteins that can specifically bind to antibodies and compete with analytes for binding sites by mimicking antigenic epitopes. They can be employed as substitutes for competing antigens, standards, or competing tracers to avoid use of mycotoxins. This review summarizes the production and functionalization of the two main kinds of mimotopes, mimic peptides and anti-idiotypic antibodies (Ab2), and their applications in rapid analysis of mycotoxins.

Carcinogenesis: Failure of resolution of inflammation?

Inflammation in the tumor microenvironment is a hallmark of cancer and is recognized as a key characteristic of carcinogens. However, the failure of resolution of inflammation in cancer is only recently being understood. Products of arachidonic acid and related fatty acid metabolism called eicosanoids, including prostaglandins, leukotrienes, lipoxins, and epoxyeicosanoids, critically regulate inflammation, as well as its resolution. The resolution of inflammation is now appreciated to be an active biochemical process regulated by endogenous specialized pro-resolving lipid autacoid mediators which combat infections and stimulate tissue repair/regeneration. Environmental and chemical human carcinogens, including aflatoxins, asbestos, nitrosamines, alcohol, and tobacco, induce tumor-promoting inflammation and can disrupt the resolution of inflammation contributing to a devastating global cancer burden. While mechanisms of carcinogenesis have focused on genotoxic activity to induce mutations, nongenotoxic mechanisms such as inflammation and oxidative stress promote genotoxicity, proliferation, and mutations. Moreover, carcinogens initiate oxidative stress to synergize with inflammation and DNA damage to fuel a vicious feedback loop of cell death, tissue damage, and carcinogenesis. In contrast, stimulation of resolution of inflammation may prevent carcinogenesis by clearance of cellular debris via macrophage phagocytosis and inhibition of an eicosanoid/cytokine storm of pro-inflammatory mediators. Controlling the host inflammatory response and its resolution in carcinogen-induced cancers will be critical to reducing carcinogen-induced morbidity and mortality. Here we review the recent evidence that stimulation of resolution of inflammation, including pro-resolution lipid mediators and soluble epoxide hydrolase inhibitors, may be a new chemopreventive approach to prevent carcinogen-induced cancer that should be evaluated in humans.

Antibody engineering-driven controllable chemiluminescence resonance energy transfer for immunoassay with tunable dynamic range

The donor-acceptor distance is a critical factor for the occurrence of chemiluminescent resonance energy transfer (CRET). We herein evaluate the donor-acceptor distance and transfer efficiency of CRET immunoassays of a series of donors which contain different sized antibody fragments, intact monoclonal antibody (IgG), antigen binding fragment (Fab), and single chain fragment antibody (scFv). Core/multishell quantum dots were used as the acceptor in three CRET systems. IgG is the maximum antibody fragment, leading to the longest donor-acceptor distance and the lowest transfer efficiency. Donors with Fab and scFv show significantly decreased distance and increased transfer efficiency. These results suggest an inverse correlation between donor size and transfer efficiency and can be used to provide guidance for the construction of controllable CRET. By combining the controllable CRET with immunoassay, we further develop a tunable sulfamethazine analytical system. Three different sized donors based CRET immunoassay show a markedly different sensitivity and dynamic range. Such adjustable detection provides greater flexibility for contaminant detection in different foodstuffs with different residue limits. This work not only illustrates the effect of donor-acceptor distance on regulating the energy transfer efficiency of CRET system, but also provides a guideline for the construction of a tunable immunoassay.

NanoLuc luciferase as a suitable fusion partner of recombinant antibody fragments for developing sensitive luminescent immunoassays

Enzyme-linked immunosorbent assays (ELISAs) are essential for monitoring various biomarkers. Competitive and noncompetitive (sandwich) assay formats are used to determine hapten and macromolecule levels, respectively. Both formats require more sensitive detection of reporter enzymes for greater assay sensitivities. We previously reported the utility of wild-type Gaussia luciferase (wtGLuc) as a fusion partner with antibody single-chain Fv fragments (scFvs) for developing sensitive luminescent ELISAs. Here, we evaluated utility of NanoLuc luciferase (NLuc), a recently developed luciferase, as fusion partner with scFvs from the view of comparison with wtGLuc and a mutant of alkaline phosphatase (ALP'). Thyroxine (T₄) and T₄-labeled albumin were chosen as model haptenic and macromolecular antigens, respectively. An in-house-prepared anti-T₄ scFv was fused with NLuc, wtGLuc, or ALP'. The scFv-NLuc fusion protein showed 47-fold and 29-fold lower limit of detection [LOD; 59 zmol (per assay)] than the wtGLuc- and ALP'-fusions, respectively. In a competitive T₄ ELISA, the NLuc-fusion showed 9.3- and 6.3-fold lower LOD, (0.67 pg) than the wtGLuc- and ALP'-fusions, respectively, with a higher specificity in clinical applications. A typical colorimetric ELISA using a peroxidase-labeled second antibody showed 70-fold higher LOD than NLuc-based ELISA. Another advantage of the NLuc-fusion was shown in the sandwich assays; the LOD of T₄-labeled albumin (5.0 fmol) was >6-fold lower than that of the other luminescent ELISAs. In an additional sandwich assay developed to count bacteriophage particles, NLuc enabled more sensitive determination than wtGLuc, whereas ALP' showed nearly equivalent performance. Its slowest alteration rate for light intensity after starting the enzyme reaction should enable robust batch-by-batch assay operations. Thus, we concluded that scFv-NLuc fusions serve as suitable probes in various types of immunoassays and may facilitate higher sensitivities with practical specificities.

Toxicant substitutes in immunological assays for mycotoxins detection: A mini review

Recurring mycotoxins contamination has posedaseriousthreatto food safety worldwide. Competitive immunoassays are widely used techniques for high-throughput mycotoxins detection in agricultural products and foods. However, the inevitable introduction of mycotoxin conjugates produced by chemical conjugation usually results in complicated by-products, large batch errors and threats to operators and environment. Biologically derived surrogates of mycotoxin conjugates or mycotoxin standards are renewable immunoreagents. They can serve the same function as the responding counterparts in the immunoassays. The substitute-based immunoassays exhibit satisfactory sensitivity, pose less health threats to operators and environment, and contribute to the standardization of immunoassays for mycotoxins. This review focuses on the current applications of substitute-based immunoassays, clarifies their underlying mechanisms and provides a careful comparison. Challenges and future prospects are discussed.

Application of phage-display developed antibody and antigen substitutes in immunoassays for small molecule contaminants analysis: A mini-review

Toxic small molecule contaminants (SMCs) residues in food threaten human health. Immunoassays are popular and simple techniques for SMCs analysis. However, immunoassays based on polyclonal antibodies, monoclonal antibodies and chemosynthetic antigens have some defects, such as complicated preparation of antibodies, risk of toxic haptens using for antigen chemosynthesis and so on. Phage-display technique has been proven to be an attractive alternative approach to producing antibody and antigen substitutes of SMCs, and opened up new realms for developing immunoassays of SMCs. These substitutes contain five types, including anti-idiotypic recombinant antibody (AIdA), anti-immune complex peptide (AIcP), anti-immune complex recombinant antibody (AIcA) and anti-SMC recombinant antibody (anti-SMC RAb). In this review, the principle of immunoassays based on the five types of substitutes, as well as their application and advantages are summarized and discussed.

A path-choice-based biosensor to detect the activity of the alkaline phosphatase as the switch

Alkaline phosphatase (ALP), which converts the phosphate group (-PO₄) in the substrate to the hydroxyl group (-OH), is a useful tool in the biological analysis, a good indicator of dissolved inorganic phosphorus levels and an important biomarker for several diseases. In conventional designs for ALP detection, both the interferent with a -PO₄ and the target with a -OH will go into the sensing path and give out the undesired background and the desired signal respectively. This limited the sensitivity of the method and required the complicated design to achieve a satisfying limit of detection (LOD) of ALP. Here, we provided a new sensing strategy for ALP detection design. We designed a path-choice-based biosensor with two DNA tracks in which ALP works as the switch to guide the reaction path of lambda exonuclease (λ exo). The path-choice character enlarged the difference between signal and background by separating the interferent removing path and the target sensing path. The substrate preference of ALP and λ exo was studied to optimize the structure of DNA tracks. The path-choice-based biosensor achieved simple, fast (30 min), sensitive (LOD 0.014 U L^-1) and selective detection of the activity of ALP. The method has been applied to detect the activity of ALP in cell lysates, which shows the potential application in ALP-related biological research.

Competitive HRP-Linked Colorimetric Aptasensor for the Detection of Fumonisin B1 in Food based on Dual Biotin-Streptavidin Interaction

Fumonisin B1 (FB1) is the most prevalent and toxic form among fumonisin homologues which are produced by fusarium species and it contaminates various types of food products, posing serious health hazards for humans and animals. In this work, a colorimetric assay for the detection of FB1 has been developed based on competitive horseradish peroxidase (HRP)-linked aptamer and dual biotin-streptavidin interaction. In short, a biotinylated aptamer of FB1 was immobilized on the microplate by biotin-streptavidin binding; the complementary strand (csDNA) of the aptamer was ligated with HRP by biotin-streptavidin binding again to form a csDNA-HRP sensing probe, competing with FB1 to bind to the aptamer. The color change can be observed after the addition of chromogenic and stop solution, thereby realizing the visual detection of FB1. Under optimal conditions, good linearity was observed within the concentration range of 0.5 to 300 ng/mL, with a detection of limit of 0.3 ng/mL. This assay is further validated by spike recovery tests towards beer and corn samples, it provides a simple, sensitive and reliable method for the screening of FB1 in food samples and may be potentially used as an alternative to conventional assays.

An Electrochemical Fiveplex Biochip Assay Based on Anti-Idiotypic Antibodies for Fast On-Site Detection of Bioterrorism Relevant Low Molecular Weight Toxins

Modern threats of bioterrorism force the need for multiple detection of biothreat agents to determine the presence or absence of such agents in suspicious samples. Here, we present a rapid electrochemical fiveplex biochip screening assay for detection of the bioterrorism relevant low molecular weight toxins saxitoxin, microcystin-LR, T-2 toxin, roridin A and aflatoxin B1 relying on anti-idiotypic antibodies as epitope-mimicking reagents. The proposed method avoids the use of potentially harmful toxin-protein conjugates usually mandatory for competitive immunoassays. The biochip is processed and analyzed on the automated and portable detection platform pBDi within 13.4 min. The fiveplex biochip assay revealed toxin group specificity to multiple congeners. Limits of detection were 1.2 ng/mL, 1.5 ng/mL, 0.4 ng/mL, 0.5 ng/mL and 0.6 ng/mL for saxitoxin, microcystin-LR, T-2 toxin, roridin A or aflatoxin B1, respectively. The robustness of the fiveplex biochip for real samples was demonstrated by detecting saxitoxin, microcystin-LR, HT-2 toxin, roridin A and aflatoxin B1 in contaminated human blood serum without elaborate sample preparation. Recovery rates were between 52–115% covering a wide concentration range. Thus, the developed robust fiveplex biochip assay can be used on-site to quickly detect one or multiple low molecular weight toxins in a single run.

Development of Real-Time Immuno-PCR Based on Phage Displayed an Anti-Idiotypic Nanobody for Quantitative Determination of Citrinin in Monascus

Citrinin (CIT) is a mycotoxin that has been detected in agricultural products, feedstuff, and Monascus products. At present, research has been performed to develop methods for CIT detection, mainly through TLC, HPLC, biosensor, and immunoassay. The immunoassay method is popular with researchers because of its speed, economy, simplicity, and ease of control. However, mycotoxins are inevitably introduced during the determination. Immunoassays require the use of toxins coupled to carrier proteins or enzymes to make competitive antigens. In this study, anti-idiotypic nanobody X27 as CIT mimetic antigen was used as non-toxic surrogate reagents in immunoassay. Therefore, the X27-based real-time immuno-PCR (rtIPCR) method had been established after optimal experiments of annealing temperature and amplification efficiency of real-time PCR, concentration of coating antibody, phage X27, and methyl alcohol. The IC₅₀ value of the established method in the present study is 9.86 ± 2.52 ng/mL, which is nearly equivalent to the traditional phage ELISA method. However, the linear range is of 0.1-1000 ng/mL, which has been broadened 10-fold compared to the phage ELISA method. Besides, the X27-based rtIPCR method has no cross-reactivity to the common mycotoxins, like aflatoxin B₁ (AFB₁), deoxynivalenol (DON), ochratoxin A (OTA), and zearalenone (ZEN). The method has also been applied to the determination of CIT in rice flour and flour samples, and the recovery was found to be in the range of 90.0-104.6% and 75.8-110.0% respectively. There was no significant difference in the results between the rtIPCR and UPLC-MS. The anti-idiotypic nanobody as a non-toxic surrogate of CIT makes rtIPCR a promising method for actual CIT analysis in Monascus products.

Development of a biotin-streptavidin-amplified nanobody-based ELISA for ochratoxin A in cereal

A biotin-streptavidin-amplified enzyme-linked immunosorbent assay using a biotinylated nanobody (BA-Nb ELISA) was developed to detect ochratoxin A (OTA) in cereal. The limit of detection (LOD) of the BA-Nb ELISA, which equals to 10% maximal inhibitory concentration, was 0.011 ng/mL for OTA in buffer, and the sensitivity was approximately improved by one order of magnitude compared with the traditional Nb ELISA (LOD = 0.112 ng/mL). Under optimal conditions, the developed assay could be accomplished in 40 min with maximal inhibitory concentration of 0.138 ng/mL and the linear detection range of 0.034-0.460 ng/mL. The average recovery rate of the BA-Nb ELISA ranged from 92.8% to 114%, and the relative standard deviation was in the range of 2.04-9.85%. The developed BA-Nb ELISA was validated by liquid chromatography-tandem mass spectrometry (LC-MS/MS), and the results indicated the reliability of BA-Nb ELISA for the detection of OTA in cereal.

Development of a one-step immunoassay for triazophos using camel single-domain antibody-alkaline phosphatase fusion protein

Triazophos is mainly used in Asian and African countries for the control of insects in agricultural production. Camelid variable domains of heavy-chain antibodies (VHHs) show great promise in monitoring environmental chemicals such as pesticides. To improve the rate of success in the generation of VHHs against triazophos, genes specifically encoding VHH fragments from the unique allotype IgG3a of an immunized Camelus bactrianus were amplified by using a pair of novel primers and introduced to construct a diverse VHH library. Five out of seven isolated positive clones, including the VHH T1 with the highest affinity to triazophos, were derived from the allotype IgG3a. A one-step enzyme-linked immunosorbent assay (ELISA) using VHH T1 genetically fused with alkaline phosphatase (AP) had a half-maximum inhibition concentration of 6.6 ng/mL for triazophos. This assay showed negligible cross-reactivity with a list of important organophosphate pesticides (< 0.1%). The average recoveries of triazophos from water, soil, and apple samples determined by the one-step ELISA ranged from 83 to 108%, having a good correlation with those by a gas chromatography mass spectrometry (R² = 0.99). The VHH-AP fusion protein shows potential for the analysis of triazophos in various matrices.

Electrochemical Biochip Assays Based on Anti-idiotypic Antibodies for Rapid and Automated On-Site Detection of Low Molecular Weight Toxins

Phycotoxins and mycotoxins, such as paralytic shellfish poisoning toxins, type A trichothecenes, and aflatoxins are among the most toxic low molecular weight toxins associated with human poisoning incidents through the consumption of naturally contaminated food. Therefore, there is an utmost need for rapid and sensitive on-site detection systems. Herein, an electrochemical biochip for fast detection of saxitoxin, T-2 toxin as well as aflatoxin M1 and their corresponding congeners, respectively, using a portable and fully automated detection platform (pBDi, portable BioDetector integrated) was developed. Toxin analysis is facilitated upon the biochip via an indirect competitive immunoassay using toxin-specific antibodies combined with anti-idiotypic antibodies. The developed biochips enable detection in the low ng/mL-range within 17 min. Moreover, the assays cover a wide linear working range of 2–3 orders of magnitude above the limit of detection with an inter-chip coefficient of variation lower than 15%. The broad specificity of the employed antibodies which react with a large number of congeners within the respective toxin group allows efficient screening of contaminated samples for the presence of these low molecular weight toxins. With respect to the analysis of human urine samples, we focused here on the detection of saxitoxin, HT-2 toxin, and aflatoxin M1, all known as biomarkers of acute toxin exposure. Overall, it was proved that the developed biochip assays can be used to rapidly and reliably identify severe intoxications caused by these low molecular weight toxins.

Nanobody-Alkaline Phosphatase Fusion Protein-Based Enzyme-Linked Immunosorbent Assay for One-Step Detection of Ochratoxin A in Rice

Ochratoxin A (OTA) has become one a focus of public concern because of its multiple toxic effects and widespread contamination. To monitor OTA in rice, a sensitive, selective, and one-step enzyme-linked immunosorbent assay (ELISA) using a nanobody-alkaline phosphatase fusion protein (Nb28-AP) was developed. The Nb28-AP was produced by auto-induction expression and retained an intact antigen-binding capacity and enzymatic activity. It exhibited high thermal stability and organic solvent tolerance. Under the optimal conditions, the developed assay for OTA could be finished in 20 min with a half maximal inhibitory concentration of 0.57 ng mL^-1 and a limit of detection of 0.059 ng mL^-1, which was 1.1 times and 2.7 times lower than that of the unfused Nb28-based ELISA. The Nb28-AP exhibited a low cross-reactivity (CR) with ochratoxin B (0.92%) and ochratoxin C (6.2%), and an ignorable CR (<0.10%) with other mycotoxins. The developed Nb-AP-based one-step ELISA was validated and compared with a liquid chromatography-tandem mass spectrometry method. The results show the reliability of Nb-AP-based one-step ELISA for the detection of OTA in rice.

Development of a Nanobody-AviTag Fusion Protein and Its Application in a Streptavidin-Biotin-Amplified Enzyme-Linked Immunosorbent Assay for Ochratoxin A in Cereal

Ochratoxin A (OTA) is a common food contaminant that threatens consumers' safety and health. A sensitive and selective biotin-streptavidin-amplified enzyme-linked immunosorbent assay (BA-ELISA) for OTA using a nanobody-AviTag fusion protein (Nb-AviTag) was developed in this study. The prokaryotic expression vector Nb28-AviTag-pAC6 for Nb-AviTag was constructed, followed by transformation to the AVB101 cells for antibody expression and in vivo biotinylation. The purified Nb28-AviTag was used to establish the BA-ELISA and the procedures for this Nb-AviTag-based BA-ELISA were optimized. The Nb-AviTag-based BA-ELISA exhibited the half maximal inhibitory concentration (IC₅₀) of 0.14 ng mL^-1 and the limit of detection (LOD = IC₁₀) of 0.028 ng mL^-1 for OTA basing on the optimized experiment parameters. The assay sensitivity was improved 4.6 times and 4.3 times compared to Nb-based ELISA, respectively. This method had LODs of 1.4 μg kg^-1 in barley, 0.56 μg kg^-1 in oats, and 0.84 μg kg^-1 in rice for OTA. The average recovery percent was in a range of 84-137%, and the relative standard derivation percent ranged from 0.64% to 7.8%. The content of OTA in contaminated cereal samples was determined by both the developed Nb-AviTag-based method and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The results demonstrated that the Nb-AviTag was a robust and promising bioreceptor in highly sensitive detection of OTA and other low molecular weight compounds using BA system.

VHH Antibodies: Reagents for Mycotoxin Detection in Food Products

Mycotoxins are the toxic secondary metabolites produced by fungi and they are a worldwide public health concern. A VHH antibody (or nanobody) is the smallest antigen binding entity and is produced by heavy chain only antibodies. Compared with conventional antibodies, VHH antibodies overcome many pitfalls typically encountered in clinical therapeutics and immunodiagnostics. Likewise, VHH antibodies are particularly useful for monitoring mycotoxins in food and feedstuffs, as they are easily genetic engineered and have superior stability. In this review, we summarize the efforts to produce anti-mycotoxins VHH antibodies and associated assays, presenting VHH as a potential tool in mycotoxin analysis.

Phage-Mediated Competitive Chemiluminescent Immunoassay for Detecting Cry1Ab Toxin by Using an Anti-Idiotypic Camel Nanobody

Cry toxins have been widely used in genetically modified organisms for pest control, raising public concern regarding their effects on the natural environment and food safety. In this work, a phage-mediated competitive chemiluminescent immunoassay (c-CLIA) was developed for determination of Cry1Ab toxin using anti-idiotypic camel nanobodies. By extracting RNA from camels' peripheral blood lymphocytes, a naive phage-displayed nanobody library was established. Using anti-Cry1Ab toxin monoclonal antibodies (mAbs) against the library for anti-idiotypic antibody screening, four anti-idiotypic nanobodies were selected and confirmed to be specific for anti-Cry1Ab mAb binding. Thereafter, a c-CLIA was developed for detection of Cry1Ab toxin based on anti-idiotypic camel nanobodies and employed for sample testing. The results revealed a half-inhibition concentration of developed assay to be 42.68 ± 2.54 ng/mL, in the linear range of 10.49-307.1 ng/mL. The established method is highly specific for Cry1Ab recognition, with negligible cross-reactivity for other Cry toxins. For spiked cereal samples, the recoveries of Cry1Ab toxin ranged from 77.4% to 127%, with coefficient of variation of less than 9%. This study demonstrated that the competitive format based on phage-displayed anti-idiotypic nanobodies can provide an alternative strategy for Cry toxin detection.

Development of a phage chemiluminescent enzyme immunoassay with high sensitivity for the determination of imidaclothiz in agricultural and environmental samples

In this study, we isolated six phage-displayed peptides by biopanning phage-displayed peptide libraries on an immobilized anti-imidaclothiz monoclonal antibody. After analyzing the relative sensitivity of the individual phage-displayed peptides, we subsequently developed and optimized both a phage enzyme immunoassay (P-ELISA) and a phage chemiluminescent enzyme immunoassay (P-CLEIA) to improve the sensitivity and linear range of imidaclothiz assays. The P-CLEIA (50% inhibition concentration (IC₅₀) of 0.86ngmL^-1, linear range of 0.13-5.84ngmL^-1) was more sensitive and had a wider linear range compared to the P-ELISA (IC₅₀ of 1.45ngmL^-1, linear range of 0.55-3.82ngmL^-1). Besides, the sensitivities of the P-ELISA and P-CLEIA were increased by >4-fold and 8-fold, respectively as compared to homologous immunoassays developed using the same monoclonal antibody. Neither method had significant cross-reactivity with the analogues of imidaclothiz except for imidacloprid. Recoveries of the P-ELISA and P-CLEIA for imidaclothiz in paddy water, soil, cabbage, rice, apple, pakchoi, pear and tomato samples were 72.3-101.3% and 73.9-102.6%, respectively. The P-ELISA and P-CLEIA detected imidaclothiz in the authentic samples, and showed good correlation with results obtained from high-performance liquid chromatography (HPLC).

Bioinspired recognition elements for mycotoxin sensors

Mycotoxins are low molecular weight molecules produced as secondary metabolites by filamentous fungi that can be found as natural contaminants in many foods and feeds. These toxins have been shown to have adverse effects on both human and animal health, and are the cause of significant economic losses worldwide. Sensors for mycotoxin analysis have traditionally applied elements of biological origin for the selective recognition purposes. However, since the 1970s there has been an exponential growth in the use of genetically engineered or synthetic biomimetic recognition elements that allow some of the limitations associated with the use of natural receptors for the analyses of these toxins to be circumvented. This review provides an overview of recent advances in the application of bioinspired recognition elements, including recombinant antibodies, peptides, aptamers, and molecularly imprinted polymers, to the development of sensors for mycotoxins based on different transduction elements. Graphical abstract Novel analytical methods based on bioinspired recognition elements, such as recombinant antibodies, peptides, aptamers, and molecularly imprinted polymers, can improve the detection of mycotoxins and provide better tools than their natural counterparts to ensure food safety.

Development of an Indirect Competitive Enzyme-Linked Immunosorbent Assay for Glycocholic Acid Based on Chicken Single-Chain Variable Fragment Antibodies

Glycocholic acid (GCA) is an important metabolite of bile acids, whose urine levels are expected to be a specific diagnostic biomarker for hepatocellular carcinoma (HCC). A high-throughput immunoassay for determination of GCA would be of significant advantage and useful for primary diagnosis, surveillance, and early detection of HCC. Single-chain variable fragment (scFv) antibodies have several desirable characteristics and are an attractive alternative to traditional antibodies for the immunoassay. Because chicken antibodies possess single heavy and light variable functional domains, they are an ideal framework for simplified generation of recombinant antibodies for GCA detection. However, chicken scFvs have rarely been used to detect GCA. In this study, a scFv library was generated from chickens immunized with a GCA hapten coupled to bovine serum albumin (BSA), and anti-GCA scFvs were isolated by a phage-displayed method. Compared to the homologous coating antigen, use of a heterologous coating antigen resulted in about an 85-fold improvement in sensitivity of the immunoassay. This assay, under optimized conditions, had a linear range of 0.02-0.18 μg/mL, with an IC50 of 0.06 μg/mL. The assay showed negligible cross-reactivity with various related bile acids, except for taurocholic acid. The detection of GCA from spiked human urine samples ranged from 86.7% to 123.3%. These results, combined with the advantages of scFv antibodies, indicated that a chicken scFv-based enzyme-linked immunosorbent assay is a suitable method for high-throughput screening of GCA in human urine.

Recent Advances in Mycotoxin Determination for Food Monitoring via Microchip

Mycotoxins are one of the main factors impacting food safety. Mycotoxin contamination has threatened the health of humans and animals. Conventional methods for the detection of mycotoxins are gas chromatography (GC) or liquid chromatography (LC) coupled with mass spectrometry (MS), or enzyme-linked immunosorbent assay (ELISA). However, all these methods are time-consuming, require large-scale instruments and skilled technicians, and consume large amounts of hazardous regents and solvents. Interestingly, a microchip requires less sample consumption and short analysis time, and can realize the integration, miniaturization, and high-throughput detection of the samples. Hence, the application of a microchip for the detection of mycotoxins can make up for the deficiency of the conventional detection methods. This review focuses on the application of a microchip to detect mycotoxins in foods. The toxicities of mycotoxins and the materials of the microchip are firstly summarized in turn. Then the application of a microchip that integrates various kinds of detection methods (optical, electrochemical, photo-electrochemical, and label-free detection) to detect mycotoxins is reviewed in detail. Finally, challenges and future research directions in the development of a microchip to detect mycotoxins are previewed.

Colloidal gold-McAb probe-based rapid immunoassay strip for simultaneous detection of fumonisins in maize

BACKGROUND

Fumonisins are a kind of toxic and carcinogenic mycotoxin. A rapid immunochromatographic test strip has been developed for simultaneous detection of fumonisin B₁ , B₂ and B₃ (FB₁ , FB₂ and FB₃ ) in maize based on colloidal gold-labelled monoclonal antibody (McAb) against FB₁ probe.

RESULTS

The anti-FB₁ McAb (2E11-H3) was produced through immunisation and cell fusion, and identified as high affinity, specificity and sensitivity. The cross-reaction ratios with fumonisin B₂ and B₃ were accordingly 385% and 72.4%, while none with other analogues. The colloid gold-labelled anti-FB₁ McAb probe was successfully prepared and used for establishing the immunochromatographic strip. The test strip showed high sensitivity and specificity, the IC₅₀ for FB₁ was 58.08 ng mL^-1 , LOD was 11.24 ng mL^-1 , calculated from standard curve. Moreover, the test strip exhibited high cross-reactivity with FB₂ and FB₃ , and could be applied to the simultaneous detection of FBs (FB₁ :FB₂ :FB₃ = 12:4:1) in maize sample with high accuracy and precision. The average recoveries of FBs in maize ranged from 90.42% to 95.29%, and CVs were 1.25-3.77%. The results of the test strip for FBs samples showed good correlation with high-performance liquid chromatography analysis.

CONCLUSION

The immunochromatographic test strip could be employed in the rapid simultaneous detection of FB₁ , FB₂ and FB₃ in maize samples on-site. © 2016 Society of Chemical Industry.

A Simple and Specific Noncompetitive ELISA Method for HT-2 Toxin Detection

We developed an HT-2 toxin-specific simple ELISA format with a positive read-out. The assay is based on an anti-immune complex (IC) scFv antibody fragment, which is genetically fused with alkaline phosphatase (AP). The anti-IC antibody specifically recognizes the IC between a primary anti-HT-2 toxin Fab fragment and an HT-2 toxin molecule. In the IC ELISA format, the sample is added together with the scFv-AP antibody to the ELISA plate coated with the primary antibody. After 15 min of incubation and a washing step, the ELISA response is read. A competitive ELISA including only the primary antibody recognizes both HT-2 and T-2 toxins. The anti-IC antibody makes the assay specific for HT-2 toxin, and the IC ELISA is over 10 times more sensitive compared to the competitive assay. Three different naturally contaminated matrices: wheat, barley and oats, were used to evaluate the assay performance with real samples. The corresponding limits of detection were 0.3 ng/mL (13 µg/kg), 0.1 ng/mL (4 µg/kg) and 0.3 ng/mL (16 µg/kg), respectively. The IC ELISA can be used for screening HT-2 toxin specifically and in relevant concentration ranges from all three tested grain matrices.

Developments in mycotoxin analysis: an update for 2015-2016

This review summarises developments in the determination of mycotoxins over a period between mid-2015 and mid-2016. Analytical methods to determine aflatoxins, Alternaria toxins, ergot alkaloids, fumonisins, ochratoxins, patulin, trichothecenes and zearalenone are covered in individual sections. Advances in proper sampling strategies are discussed in a dedicated section, as are methods used to analyse botanicals and spices and newly developed liquid chromatography mass spectrometry based multi-mycotoxin methods. This critical review aims to briefly discuss the most important recent developments and trends in mycotoxin determination as well as to address limitations of presented methodologies.

Nanobody-based enzyme immunoassay for ochratoxin A in cereal with high resistance to matrix interference

A sensitive indirect competitive nanobody-based enzyme linked immunosorbent assay (Nb-ELISA) for ochratoxin A (OTA) with high resistance to cereal matrix interference was developed. Nanobodies against OTA (Nb15, Nb28, Nb32, Nb36) were expressed in E. coli cells and their thermal stabilities were compared with that of an OTA-specific monoclonal antibody 6H8. All nanobodies could still retain their antigen-binding activity after exposure to temperature 95°C for 5min or to 90°C for 75min. Nb28 that exhibited the highest sensitivity in ELISA was selected for further research. An indirect competitive ELISA based on Nb28 was developed for OTA, with an IC₅₀ of 0.64ng/mL and a linear range (IC₂₀-IC₈₀) of 0.27-1.47ng/mL. Cereal samples were analyzed following a 2.5 fold dilution of sample extracts, showing the good resistance to matrix interference of the Nb-ELSIA. The recovery of spiked cereal samples (rice, oats, barley) ranged from 80% to 105% and the Nb-ELISA results of OTA content in naturally contamined samples were in good agreement with those determined by a commercial ELISA kit. The results indicated the reliablity of nanobody as a promising immunoassay reagent for detection of mycotoxins in food matrix and its potential in biosensor development.

Alkaline phosphatase-fused repebody as a new format of immuno-reagent for an immunoassay

Enzyme-linked immunoassays based on an antibody-antigen interaction are widely used in biological and medical sciences. However, the conjugation of an enzyme to antibodies needs an additional chemical process, usually resulting in randomly cross-linked molecules and a loss of the binding affinity and enzyme activity. Herein, we present the development of an alkaline phosphatase-fused repebody as a new format of immuno-reagent for immunoassays. A repebody specifically binding to human TNF-α (hTNF-α) was selected through a phage display, and its binding affinity was increased up to 49 nM using a modular engineering approach. A monomeric alkaline phosphatase (mAP), which was previously isolated from a metagenome library, was genetically fused to the repebody as a signal generator, and the resulting repebody-mAP fusion protein was used for direct and sandwich immunoassays of hTNF-α. We demonstrate the utility and potential of the repebody-mAP fusion protein as an immuno-reagent by showing the sensitivity of 216 pg mL^-1 for hTNF-α in a sandwich immunoassay. Furthermore, this repebody-mAP fusion protein enabled the detection of hTNF-α spiked in a serum-supplemented medium with high accuracy and reproducibility. It is thus expected that a mAP-fused repebody can be broadly used as an immuno-reagent in immunoassays.

An ultrasensitive colorimetric strategy for protein O-GlcNAcylation detection via copper deposition-enabled nonenzymatic signal amplification

An ultrasensitive colorimetric method for protein O-GlcNAcylation analysis is developed using AuNP-catalyzed copper deposition as a nonenzymatic signal amplification strategy.