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

Development of a Phage-Displayed Nanobody-Based Competitive Immunoassay for the Sensitive Detection of Soybean Agglutinin

Soybean agglutinin (SBA) is a primary antinutritional factor in soybeans that can inhibit the growth of humans and mammals, disrupt the intestinal environment, and cause pathological changes. Therefore, detecting and monitoring SBA in foods is essential for safeguarding human health. In this paper, M13 phage-displayed nanobodies against SBA were isolated from a naive nanobody library. An M13 phage-displayed nanobody-based competitive enzyme-linked immunosorbent assay (P-cELISA) was then established for SBA analysis using biotinylated anti-M13 phage antibody (biotin-anti-M13) and streptavidin poly-HRP conjugate (SA-poly-HRP). The biotin-anti-M13@SA-poly-HRP probe can easily amplify the detection signal without the chemical modifications of phage-displayed nanobodies. The established P-cELISA presented a linear detection range of 0.56-250.23 ng/mL and a limit of detection (LOD) of 0.20 ng/mL, which was 12.6-fold more sensitive than the traditional phage-ELISA. Moreover, the developed method showed good specificity for SBA and acceptable recoveries (78.21-121.11%) in spiked wheat flour, albumen powder, and whole milk powder. This study proposes that P-cELISA based on biotin-anti-M13@SA-poly-HRP may provide a convenient and effective strategy for the sensitive detection of SBA.

A facile dual-mode immunosensor based on speckle Ag-doped nanohybrids for ultrasensitive detection of Ochratoxin A

Ochratoxin A (OTA) is a potent carcinogen, and is among the most dangerous mycotoxins in agricultural products. In this study, an ultrasensitive dual-mode immunosensor was developed for naked-eye and fluorescence detection of OTA based on Ag-doped core-shell nanohybrids (Ag@CSNH). Complete antigen-labeled Ag@CSNH (CA-Ag@CSNH) were used as a competitive bind and dual-mode probe. The diffused doping structure of CA-Ag@CSNH provided improved stability, color and fluorescence quencher performance. Antibodies modified magnetic beads were used as a capture probe. The competitive binding between OTA and CA-Ag@CSNH produced both color change and fluorescence quenching. Ultraviolet and fluorescence intensitie correlated linearly with OTA concentration ranges of 0.03-3 ng/mL and 10-10000 pg/mL, and limits of detection of 0.0235 ng/mL and 0.9921 pg/mL, respectively. The practical applicability of proposed strategy was demonstrated by analysis of OTA in spiked corn, soybean and flour samples. This study offers a new insight on multi-mode platforms for various applications.

A novel nanoparticle-based fluorescent sandwich immunoassay for specific detection of Salmonella Typhimurium

The diseases caused by foodborne pathogens have a serious impact on human health and social stability. Conventional detection methods can involve long assay times and complex pretreatment steps, making them unsuitable for rapid, large-scale analysis of food samples. We constructed a novel nano-fluorescence sandwich immunosorbent immunoassay (nano-FSIA) to rapidly detect Salmonella Typhimurium in food, based on strong covalent binding between streptavidin and biotin. We used antibodies coupled to large particle-size fluorescent microspheres as fluorescent probes for direct quantitative analysis of S. typhimurium in milk. The optimized parameters were determined, and specificity and sensitivity were validated in phosphate-buffered saline (PBS) and milk. The results demonstrated a wide dynamic detection range for S. typhimurium (103-108 colony forming units [CFU]/mL), with the limit of detection in PBS and milk at 234 and 346 CFU/mL, respectively. The results of nano-FSIA were consistent with those of plate counts and enzyme-linked immunosorbent assays, providing an effective and promising single-bacterium counting method for the rapid detection of Salmonella.

DNA-bridged double gold nanoparticles-based immunochromatography for dual-mode detection of ochratoxin A

A novel immunochromatography strip (ICS) based on the DNA-bridged double gold nanoparticles was established and evaluated for the dual-mode detection of ochratoxin A (OTA). For this purpose, the streptavidin was coupled with the big size of gold nanoparticle (40 nm, AuNP), the OTA monoclonal antibody and biotinylated DNA-SH were simultaneously immobilized on the small size of AuNP (20 nm), and then the enhanced ICS was self-assembled and evaluated. The dual-mode detection of semi-quantification and quantification had been achieved and performed by the proposed ICS. The LOD for semi-quantification (semi-Q-LOD) was 0.06 ng/mL by the directly naked eye (eightfold enhanced than conventional ICS). By the on-site reader, the LOD for quantification (Q-LOD) was 0.03 ng/mL (threefold enhanced), with the detection range between 0.03 and 1.2 ng/mL. The specificity, reliability, and practicability had been well represented. Furthermore, the OTA-positive results of the enhanced ICS method correlated well with those obtained by the referenced HPLC-MS/MS for the market samples. This study provided a new ICS pattern of semi-quantification and quantification for OTA contamination, which could be used as a valuable reference for improving the ICS technology and enhancing the sensitivity.

Occurrence of fungi and mycotoxins in herbal medicines and rapid detection of toxin-producing fungi

Contamination from external hazardous materials may greatly influence the safety and efficacy of herbal medicines. This paper aimed to evaluate the levels of contamination by mycotoxins and toxigenic fungi in herbal medicines and establish a rapid method for detecting toxin-producing fungi. Herein, 62.92%, 36.25%, and 64.17% of herbal medicines were contaminated by aflatoxins (AFs), ochratoxins, and fumonisins, respectively. Aspergillus (43.77%), Fusarium (5.17%), and Cladosporium (4.46%) were the three predominant genera. Spearman's correlation results showed that Aspergillus and Fusarium were significantly and positively correlated with mycotoxin content (R > 0.5, P < 0.05). In addition, 323 fungal strains were isolated from herbal medicines, and 20 species were identified, mainly belonging to Aspergillus and Penicillium. Analysis of potential mycotoxin-producing fungi showed that Aspergillus flavus can produce AFs, and Aspergillus ochraceus and Aspergillus niger can produce ochratoxin A (OTA). Multiplex real-time polymerase chain reaction showed that A. flavus harbored AF synthesis genes (aflR), and A. ochraceus and A. niger harbored OTA synthesis genes (aoksl). With these synthesis genes, 67.07% and 37.20% of 164 herbal medicines were positive for toxigenic genes. Furthermore, an excellent correlation was found between the above gene copies and mycotoxin content (R2 = 0.99). Our results confirmed the high detection rate of mycotoxins in herbal medicines and identified pivotal AF- and OTA-producing fungi. In conclusion, this paper provided the contamination status of fungi and mycotoxins in herbal medicines and established a rapid method for detecting toxigenic fungi.

Research advances of SERS analysis method based on silent region molecules for food safety detection

Food safety is a critical issue that is closely related to people's health and safety. As a simple, rapid, and sensitive detection technique, surface-enhanced Raman scattering (SERS) technology has significant potential for food safety detection. Recently, researchers have shown a growing interest in utilizing silent region molecules for SERS analysis. These molecules exhibit significant Raman scattering peaks in the cellular Raman silent region between 1800 and 2800 cm-1 avoiding overlapping with the SERS spectrum of biological matrices in the range 600-1800 cm-1, which could effectively circumvent matrix effects and improve the SERS accuracy. In this review, the application of silent region molecules-based SERS analytical technique for food safety detection is introduced, detection strategies including label-free detection and labeled detection are discussed, and recent applications of SERS analysis technology based on molecules containing alkyne and nitrile groups, as well as Prussian blue (PB) in the detection of pesticides, mycotoxins, metal ions, and foodborne pathogens are highlighted. This review aims to draw the attention to the silent region molecules-based SERS analytical technique and to provide theoretical support for its further applications in food safety detection.

NANOBODY® Molecule, a Giga Medical Tool in Nanodimensions

Although antibodies remain the most widely used tool for biomedical research, antibody technology is not flawless. Innovative alternatives, such as Nanobody® molecules, were developed to address the shortcomings of conventional antibodies. Nanobody® molecules are antigen-binding variable-domain fragments derived from the heavy-chain-only antibodies of camelids (VHH) and combine the advantageous properties of small molecules and monoclonal antibodies. Nanobody® molecules present a small size (~15 kDa, 4 nm long and 2.5 nm wide), high solubility, stability, specificity, and affinity, ease of cloning, and thermal and chemical resistance. Recombinant production in microorganisms is cost-effective, and VHH are also building blocks for multidomain constructs. These unique features led to numerous applications in fundamental research, diagnostics, and therapy. Nanobody® molecules are employed as biomarker probes and, when fused to radioisotopes or fluorophores, represent ideal non-invasive in vivo imaging agents. They can be used as neutralizing agents, receptor-ligand antagonists, or in targeted vehicle-based drug therapy. As early as 2018, the first Nanobody®, Cablivi (caplacizumab), a single-domain antibody (sdAb) drug developed by French pharmaceutical giant Sanofi for the treatment of adult patients with acquired thrombocytopenic purpura (aTTP), was launched. Nanobody® compounds are ideal tools for further development in clinics for diagnostic and therapeutic purposes.

Recent Advances in Electrochemiluminescence Biosensors for Mycotoxin Assay

Rapid and efficient detection of mycotoxins is of great significance in the field of food safety. In this review, several traditional and commercial detection methods are introduced, such as high-performance liquid chromatography (HPLC), liquid chromatography/mass spectrometry (LC/MS), enzyme-linked immunosorbent assay (ELISA), test strips, etc. Electrochemiluminescence (ECL) biosensors have the advantages of high sensitivity and specificity. The use of ECL biosensors for mycotoxins detection has attracted great attention. According to the recognition mechanisms, ECL biosensors are mainly divided into antibody-based, aptamer-based, and molecular imprinting techniques. In this review, we focus on the recent effects towards the designation of diverse ECL biosensors in mycotoxins assay, mainly including their amplification strategies and working mechanism.

A Colorimetric Aptasensor for Ochratoxin A Detection Based on Tetramethylrhodamine Charge Effect-Assisted Silver Enhancement

A novel colorimetric aptasensor based on charge effect-assisted silver enhancement was developed to detect ochratoxin A (OTA). To achieve this objective, gold nanoparticles (AuNPs), which can catalyze silver reduction and deposition, were used as the carrier of the aptamers tagged with a positively charged tetramethylrhodamine (TAMRA). Due to the mutual attraction of positive and negative charges, the TAMRA attracted and retained the silver lactate around the AuNPs. Thus, the chance of AuNP-catalyzed silver reduction was increased. The charge effect-assisted silver enhancement was verified by tagging different base pair length aptamers with TAMRA. Under optimized conditions, the as-prepared OTA aptasensor had a working range of 1 × 10²-1 × 10⁶ pg mL^-1. The detection limit was as low as 28.18 pg mL^-1. Moreover, the proposed aptasensor has been successfully applied to determine OTA in actual samples with satisfactory results.

Advanced biosensors for mycotoxin detection incorporating miniaturized meters

Advanced biosensors, considered as emerging technologies, are capable of accurate, quantitative and real-time analysis for point-of-care testing (POCT) applications. Moreover, the integrating of miniaturized meters into these advanced biosensors makes them ideally appropriate for portable, sensitive and selective detection of biomolecules. Miniaturized meters including PGMs (personal glucose meters), thermometer, pressuremeter, pH meter, etc. are the most accurate devices and wide availability in the market, exhibiting a promising potential towards detection of small molecule mycotoxins. In this article, we introduce and analyze the recent advancements for sensing of mycotoxins measured by handheld meters since the first report in 2012. Furthermore, limitations and challenges for versatile meters application against mycotoxins in food matrix are highlighted. By overcoming the bottleneck problems, we believe the miniaturized meters-based biosensor platform will provide great possibilities for mycotoxins analysis and launch them to the market.

Nanobody-based magnetic chemiluminescence immunoassay for one-pot detection of ochratoxin A

Ochratoxin A (OTA) contamination seriously threatens food safety and human health and requires sensitive and rapid tools for monitoring. In this study, a convenient enzyme-linked immunosorbent assay based on Avi-labeled nanobody Nb-2G/streptavidin-alkaline phosphatase and magnetic beads (MBS-ELISA) was established for the sensitive detection of OTA, which could be used for one-pot detection without immobilization. After optimization, the 50% inhibitory concentration (IC50) and the lowest limit of detection value of the MBS-ELISA was 1.17 ng/mL and 0.07 ng/mL and the linear range was 248.8 pg/mL-5.28 ng/mL, respectively, which accords with state criteria for food safety. The developed one-step MBS-ELISA was almost 20-times more sensitive than the classic BA-ELISA and could generate results within 15 min, which was significantly less than the classic BA-ELISA at approximately 3 h. The MBS-ELISA indicated good recovery (86.4-114.3%) in spiked sorghum, buckwheat, and mung bean. Thus, MBS-ELISA represents a very promising strategy for the simple, rapid, and accurate detection of OTA and other toxic and hazardous contaminants.

Detection of ochratoxin A using a "turn-on" fluorescence assay based on guanine quenching of the aptamer

Ochratoxin A (OTA) is a common mycotoxin with high carcinogenicity; therefore, it is crucial to establish a simple, rapid, and sensitive method for its detection. In this study, we developed a "turn-on" fluorescence assay for detecting OTA based on guanine quenching of the aptamer. The method uses fluorescein (FAM) fluorophore to label the complementary strand of the OTA aptamer, Fc-DNA. In the absence of OTA, the Fc-DNA hybridizes with the aptamer to form a double strand. Due to the occurrence of photo-induced electron transfer (PET), the FAM fluorescence signal is quenched as the FAM on the Fc-DNA approaches the guanine of the aptamer at the 5' end. When OTA is present, the aptamer binds to it and thus, is unable to hybridize with Fc-DNA to form a double strand; the FAM fluorescence signal is restored as FAM moves away from the guanine of the aptamer. The assay achieved OTA detection at a detection limit of 28.4 nM. The application of the original guanine of the aptamer as the quenching agent helps avoid the complex designing and labeling of the aptamer, which ensures the high affinity of the aptamer for OTA. Meanwhile, this "turn-on" detection mode helps avoid potential false-positive results as in the "turn-off" mode and improves the assay's sensitivity. Additionally, the method has good selectivity and can be used to detect OTA in traditional Chinese medicine. This method provides a simple, low-cost, and rapid method for OTA detection.

Development of a Reliable ic-ELISA with a Robust Antimatrix Interference Capability Based on QuEChERS Technology for the Rapid Detection of Zearalenone in Edible and Medical Coix Seeds and Subsequent Risk Assessments

Indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) is an ideal immunoassay method for large-scale screenings to detect mycotoxin contaminants. However, the matrix effect of complicated samples has always been challenging when performing immunoassays, as it leads to false-positive or negative results. In this study, convenient QuEChERS technology combined with optimizing the dilution solvent was ingeniously used to eliminate interference from the sample matrix to greatly improve the detection accuracy, and reliable ic-ELISAs for the two official tolerance levels of 60 and 500 μg/kg were developed to screen zearalenone (ZEN) in edible and medical coix seeds without any further correction. Then, the 122 batches of coix seeds were determined, and the positive rate was up to 97.54%. The contaminated distribution was further analyzed, and risk assessment was subsequently performed for its edible and medical purposes. The findings indicated that consumption of coix seeds with higher ZEN contamination levels may cause adverse health effects for both medical and edible consumption in the adult population; even under the condition of average contamination level, ZEN from coix seeds was the more prominent contributor to the total risk compared to other sources when used as food; thus, effective prevention and control should be an essential topic in the future.

Polydopamine nanoparticle-mediated, click chemistry triggered, microparticle-counting immunosensor for the sensitive detection of ochratoxin A

A rapid and accurate detection method is needed for the quantitation of ochratoxin A in agricultural products due to its high toxicity. A microparticle-counting immunosensor based on polydopamine nanoparticle-mediated click chemistry was established for the highly-sensitive detection of ochratoxin A. Polydopamine nanoparticles with good biocompatibility and a strong metal-chelating ability were synthesized and conjugated with the antibody. The Coupled compounds were then used as an immune carrier to change the Cu²⁺ concentration via an immuno-reaction. Some of the remaining Cu²⁺ ions were reduced to Cu⁺ ions, which caused azide-polystyrene microspheres and alkyne-polystyrene microspheres to aggregate via a Cu⁺ ion-mediated click reaction. Particle counting was used to distinguish changes in the sizes of the polystyrene microspheres from dispersed to aggregated to detect ochratoxin A. It showed a wide linear detection range of 0.5-800 ng/mL, and a detection limit of 0.2 ng/mL. This assay provides an attractive analytical tool for the accurate detection of trace targets in complex samples.

An Immuno-Separated Assay for Ochratoxin Detection Coupled with a Nano-Affinity Cleaning-Up for LC-Confirmation

An immuno-separated assay for ochratoxin A detection coupled with a nano-affinity cleaning up for LC-confirmation was developed. Firstly, ochratoxin A was modified to quantum dot beads for immuno-fluorescent reporters. Secondly, Fe3O4 magnetic nanoparticles were conjugated with protein G for immuno-magnetic adsorbents. The immuno-separation of fluorescent reporters by magnetic adsorbents could be completed by ochratoxin A, so the fluorescent reporters released from the immune complex indicate a linear correlation with the concentration of ochratoxin A. Furthermore, the immuno-separated ochratoxin A can be eluted from magnetic adsorbent for LC-conformation. The optimized assay showed results as follows: the quantitative range of the immuno-separated assay was 0.03-100 ng mL-1 of ochratoxin A. The recoveries for spiked samples ranged from 78.2% to 91.4%, with the relative standard deviation (RSD) being 11.9%~15.3%. Statistical analysis indicated no significant difference between the HPLC-FLD results based on commercial affinity column and by nano-affinity cleaning up.

Development of nanobody-horseradish peroxidase-based sandwich ELISA to detect Salmonella Enteritidis in milk and in vivo colonization in chicken

Background

Salmonella Enteritidis (S. Enteritidis) being one of the most prevalent foodborne pathogens worldwide poses a serious threat to public safety. Prevention of zoonotic infectious disease and controlling the risk of transmission of S. Enteriditidis critically requires the evolution of rapid and sensitive detection methods. The detection methods based on nucleic acid and conventional antibodies are fraught with limitations. Many of these limitations of the conventional antibodies can be circumvented using natural nanobodies which are endowed with characteristics, such as high affinity, thermal stability, easy production, especially higher diversity. This study aimed to select the special nanobodies against S. Enteriditidis for developing an improved nanobody-horseradish peroxidase-based sandwich ELISA to detect S. Enteritidis in the practical sample. The nanobody-horseradish peroxidase fusions can help in eliminating the use of secondary antibodies labeled with horseradish peroxidase, which can reduce the time of the experiment. Moreover, the novel sandwich ELISA developed in this study can be used to detect S. Enteriditidis specifically and rapidly with improved sensitivity.

Results

This study screened four nanobodies from an immunized nanobody library, after four rounds of screening, using the phage display technology. Subsequently, the screened nanobodies were successfully expressed with the prokaryotic and eukaryotic expression systems, respectively. A sandwich ELISA employing the SE-Nb9 and horseradish peroxidase-Nb1 pair to capture and to detect S. Enteritidis, respectively, was developed and found to possess a detection limit of 5 × 10⁴ colony forming units (CFU)/mL. In the established immunoassay, the 8 h-enrichment enabled the detection of up to approximately 10 CFU/mL of S. Enteriditidis in milk samples. Furthermore, we investigated the colonization distribution of S. Enteriditidis in infected chicken using the established assay, showing that the S. Enteriditidis could subsist in almost all parts of the intestinal tract. These results were in agreement with the results obtained from the real-time PCR and plate culture. The liver was specifically identified to be colonized with quite a several S. Enteriditidis, indicating the risk of S. Enteriditidis infection outside of intestinal tract.

Conclusions

This newly developed a sandwich ELISA that used the SE-Nb9 as capture antibody and horseradish peroxidase-Nb1 to detect S. Enteriditidis in the spike milk sample and to analyze the colonization distribution of S. Enteriditidis in the infected chicken. These results demonstrated that the developed assay is to be applicable for detecting S. Enteriditidis in the spiked milk in the rapid, specific, and sensitive way. Meanwhile, the developed assay can analyze the colonization distribution of S. Enteriditidis in the challenged chicken to indicate it as a promising tool for monitoring S. Enteriditidis in poultry products. Importantly, the SE-Nb1-vHRP as detection antibody can directly bind S. Enteritidis captured by SE-Nb9, reducing the use of commercial secondary antibodies and shortening the detection time. In short, the developed sandwich ELISA ushers great prospects for monitoring S. Enteritidis in food safety control and further commercial production.

Graphic Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01376-y.

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.

Multivalent nanobody-biotin amplified enzyme-linked immunosorbent assay for the environmental detection of 3-phenoxybenzoic acid

The 3-phenoxybenzoic acid (3-PBA) metabolized from pyrethroids is more toxic and has a longer half-life to degradation in a natural environment compared to its parent compounds. Few reports have focused on the environmental detection of 3-PBA. In this study, anti-3-PBA nanobodies in trivalent form (Nb3) were biotinylated. A sensitive enzyme-linked immunosorbent assay (ELISA) based on the combination of Nb3-biotin and streptavidin-horseradish peroxidase (SA-HRP) was developed for the environmental detection of 3-PBA. After optimization, the ELISA showed a half-maximum signal inhibition concentration (IC₅₀) of 0.39 ng mL^-1 in phosphate-buffered saline (pH 7, 20% MeOH) and a limit of detection (LOD) of 0.02 ng mL^-1, which was more sensitive than the parent Nb-based ELISAs with IC₅₀ and LOD values of 1.4 ng mL^-1 and 0.1 ng mL^-1, respectively. The Nb3-biotin amplified assay showed negligible cross-reactivity with its structural analogues (<0.1%). The average recoveries of 3-PBA from spiked canal water and soil samples ranged from 86.54-109.25% at 0.5-50 ng mL^-1 (or ng g^-1 (dw)). The 3-PBA residues in canal water and soil samples determined using this assay were in the ranges <LOD-1.46 ng mL^-1 and <LOD-4.35 ng g^-1 (dw), respectively, which correlated well with the results obtained using liquid chromatography tandem mass spectrometry. The results suggest that multivalent Nb-biotin-SA amplified ELISA is a promising tool for the routine environmental screening of 3-PBA.

Aptasensors for mycotoxins in foods: Recent advances and future trends

Mycotoxin contamination in foods has posed serious threat to public health and raised worldwide concern. The development of simple, rapid, facile, and cost-effective methods for mycotoxin detection is of urgent need. Aptamer-based sensors, abbreviated as aptasensors, with excellent recognition capacity to a wide variety of mycotoxins have attracted ever-increasing interest of researchers because of their simple fabrication, rapid response, high sensitivity, low cost, and easy adaptability for in situ measurement. The past few decades have witnessed the rapid advances of aptasensors for mycotoxin detection in foods. Therefore, this review first summarizes the reported aptamer sequences specific for mycotoxins. Then, the recent 5-year advancements in various newly developed aptasensors, which, according to the signal output mode, are divided into electrochemical, optical and photoelectrochemical categories, for mycotoxin detection are comprehensively discussed. A special attention is taken on their strengths and limitations in real-world application. Finally, the current challenges and future perspectives for developing novel highly reliable aptasensors for mycotoxin detection are highlighted, which is expected to provide powerful references for their thorough research and extended applications. Owing to their unique advantages, aptasensors display a fascinating prospect in food field for safety inspection and risk assessment.

Enzyme cascade-amplified immunoassay based on the nanobody-alkaline phosphatase fusion and MnO2 nanosheets for the detection of ochratoxin A in coffee

Ochratoxin A (OTA) is a common food contaminant with multiple toxicities and thus rapid and accurate detection of OTA is indispensable to minimize the threat of OTA to public health. Herein a novel enzyme cascade-amplified immunoassay (ECAIA) based on the mutated nanobody-alkaline phosphatase fusion (mNb-AP) and MnO2 nanosheets was established for detecting OTA in coffee. The detection principle is that the dual functional mNb-AP could specifically recognize OTA and dephosphorylate the ascorbic acid-2-phosphate (AAP) into ascorbic acid (AA), and the MnO2 nanosheets mimicking the oxidase could be reduced by AA into Mn2+ and catalyze the 3,3',5,5'-tetramethyl benzidine into blue oxidized product for quantification. Using the optimal conditions, the ECAIA could be finished within 132.5 min and shows a limit of detection of 3.38 ng mL-1 (IC10) with an IC50 of 7.65 ng mL-1 and a linear range (IC20-IC80) of 4.55-12.85 ng mL-1. The ECAIA is highly selective for OTA. Good recovery rates (84.3-113%) with a relative standard deviation of 1.3-3% were obtained and confirmed by high performance liquid chromatography with a fluorescence detector. The developed ECAIA was demonstrated to be a useful tool for the detection of OTA in coffee which provides a reference for the analysis of other toxic small molecules.

DNAzyme Amplified Aptasensing Platform for Ochratoxin A Detection Using a Personal Glucose Meter

Aptamer-based sensors have emerged as a major platform for detecting small-molecular targets, because aptamers can be selected to bind these small molecules with higher affinity and selectivity than other receptors such as antibodies. However, portable, accurate, sensitive, and affordable detection of these targets remains a challenge. In this work, we developed an aptasensing platform incorporating magnetic beads and a DNAzyme for signal amplification, resulting in high sensitivity. The biosensing platform was constructed by conjugating a biotin-labeled aptamer probe of small-molecular targets such as toxins and a biotin-labeled substrate strand on magnetic beads, and the DNAzyme strand hybridized with the aptamer probe to block the substrate cleavage activity. The specific binding of the small-molecular target by the aptamer probe can replace the DNAzyme strand and then induce the hybridization between the DNAzyme strand and substrate strand, and the iterative signal amplification reaction of hydrolysis and cleavage of the substrate chain occurs in the presence of a metal ion cofactor. Using invertase to label the substrate strand, the detection of small molecules of the toxin is successfully transformed into the measurement of glucose, and the sensitive analysis of small molecules such as toxins can be realized by using the household portable glucose meter as a readout. This platform is shown to detect ochratoxin, a common toxin in food, with a linear detection range of 5 orders of magnitude, a low detection limit of 0.88 pg/mL, and good selectivity. The platform is easy to operate and can be used as a potential choice for quantitative analysis of small molecules, at home or under point-of-care settings. Moreover, by changing and designing the aptamer probe and the arm of DNAzyme strand, it can be used for the analysis of other analytes.

A sensitive sandwich ELISA using a modified biotin-streptavidin amplified system for histamine detection in fish, prawn and crab

Histamine poisoning from seafood is a significant public health and safety concern. To detect histamine sensitively and accurately, a novel competitive sandwich immunoassay using a modified biotin-streptavidin system coupling with polylysine was developed. Using this strategy, a sandwich ELISA with an IC₅₀ value of 112.8 ng mL^-1 and a broad linear range of 11.7-1500 ng mL^-1 with a correlation coefficient of 0.9942 was validated. Without any sample derivatization procedure, the recovery of histamine ranged from 80.19% to 108.3% with a coefficient of variation of 1.43-11.7% in tuna, prawn and crab. The sandwich ELISA had a detectionlimit of 5.86 ng mL^-1, which was 15-fold lower than an indirect competitive ELISA (ic-ELISA). This simple, sensitive and accurate method can be applied to detect histamine in routine seafood samples.

Molecular Aspects of Mycotoxins-A Serious Problem for Human Health

Mycotoxins are toxic fungal secondary metabolities formed by a variety of fungi (moulds) species. Hundreds of potentially toxic mycotoxins have been already identified and are considered a serious problem in agriculture, animal husbandry, and public health. A large number of food-related products and beverages are yearly contaminated by mycotoxins, resulting in economic welfare losses. Mycotoxin indoor environment contamination is a global problem especially in less technologically developed countries. There is an ongoing effort in prevention of mould growth in the field and decontamination of contaminated food and feed in order to protect human and animal health. It should be emphasized that the mycotoxins production by fungi (moulds) species is unavoidable and that they are more toxic than pesticides. Human and animals are exposed to mycotoxin via food, inhalation, or contact which can result in many building-related illnesses including kidney and neurological diseases and cancer. In this review, we described in detail the molecular aspects of main representatives of mycotoxins, which are serious problems for global health, such as aflatoxins, ochratoxin A, T-2 toxin, deoxynivalenol, patulin, and zearalenone.

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.

Simultaneous detection of zearalenone, citrinin, and ochratoxin A in pepper by capillary zone electrophoresis

In the present study, a simple and fast method for simultaneous detection of zearalenone, citrinin, and ochratoxin A utilising capillary zone electrophoresis with an ultraviolet detector was developed. The optimised approach was validated and applied using pepper samples. The proposed method yielded satisfactory linearity between the signal and the mycotoxin concentration in the range of 1.5-150 μg/kg for zearalenone, 4.5-150 μg/kg for citrinin, and 0.8-150 μg/kg for ochratoxin A. The limits of detection for these mycotoxins ranged from 0.3 to 1.5 μg/kg. The corresponding intra- and inter-day precisions were less than 3.5 % and 4.1 %, respectively. Moreover, the matrix effect was also assessed and the result was compared using the capillary zone electrophoresis and high-performance liquid chromatography methods. The developed approach could be used for simultaneous detection of zearalenone, citrinin, and ochratoxin A in pepper.

Specific Coordination between Zr-MOF and Phosphate-Terminated DNA Coupled with Strand Displacement for the Construction of Reusable and Ultrasensitive Aptasensor

Electrochemical aptasensors involved in chemical labeling are often single-use and sensitivity-limited because the probes are commonly single-point labeled and irreversible. In this work, the specific coordination between Zr⁴⁺ and phosphate group (-PO₄^3-) was employed to construct a new aptasensor that is highly sensitive and reusable, using Ochratoxin A (OTA) as the test model. The OTA binding aptamer (OBA) was hybridized with the thiolated supporting sequence (TSS) immobilized on the surface of a gold electrode. The UiO-66 with a formula of [Zr₆O₄(OH)₄(BDC)₆], one of the class of Zr metal-organic frameworks (MOFs), was then particularly grafted on the terminal of OBA through the specific coordination between Zr⁴⁺ and 5'-PO₄^3-, i.e., the Zr-O-P coordination bond. Similarly, as much as the 5'-PO₄^3- and 3'-methylene blue dual-labeled sequences (DLS) were further assembled on UiO-66 due to the large surface area of MOF and rich active sites of Zr⁴⁺. Owing to the specific coordination for signal amplification, the developed aptasensor shows greatly enhanced sensitivity. A wide detection range from 0.1 fM to 2.0 μM and an ultralow detection limit of 0.079 fM (S/N = 3) for OTA were obtained. Additionally, the TSS can rehybridize with a new OBA to regenerate the aptasensor but without complicated pretreatments, enabling a aptasensor that is readily reusable for OTA detection. The aptasensor was successfully applied for OTA detection in the red wine samples, demonstrating a promising prospect for food safety monitoring.

Green Enzyme-Linked Immunosorbent Assay Based on the Single-Stranded Binding Protein-Assisted Aptamer for the Detection of Mycotoxin

In this work, a green enzyme-linked immunosorbent assay (ELISA) based on the single-stranded binding protein (SSB)-assisted aptamer was designed for biosensing applications. Combined with the biotin-streptavidin (SA) system and the high catalytic activity of horseradish peroxidase (HRP), this SSB-assisted aptamer sensor was applied for the detection of aflatoxin B1, ochratoxin A, and zearalenone. In this novel ELISA, mycotoxin-protein conjugations were replaced by SSB to avoid the hazard of mycotoxin, whereas antibodies were replaced by aptamer to avoid the complex and tedious preparation of antibodies. In the absence of target mycotoxins, SSB can bind the aptamer-biotin specifically. Detection was performed using the strong combination of biotin and SA after adding SA-HRP and substrate/chromogen solution, thereby resulting in a strong yellow color signal. In the presence of target mycotoxins, the aptamer-biotin cannot bind to the SSB, thereby leading to a weak yellow color signal. Under optimal conditions, the designed method was successfully applied for the determination of real sample and exhibited high specificity and low limits of detection in corn (112 ng L^-1 for aflatoxin B1, 319 ng L^-1 for ochratoxin A, and 377 ng L^-1 for zearalenone). The green ELISA may also be extended to the detection of other biohazardous targets by changing the aptamer.

Recent Advances in Ochratoxin A Electrochemical Biosensors: Recognition Elements, Sensitization Technologies, and Their Applications

Ochratoxin A (OTA) is a class of mycotoxin that are mainly produced by Aspergillus and Penicillium and widely found in plant origin food. OTA-contaminated foods can cause serious harm to animals and humans, while high stability of OTA makes it difficult to remove in conventional food processing. Thus, sensitive and rapid detection of OTA undoubtedly plays an important role in OTA prevention and control. In this paper, the conventional and novel methods of OTA at home and abroad are summarized and compared. The latest research progress and related applications of novel OTA electrochemical biosensors are mainly described with a new perspective. We innovatively divided the recognition element into single and combined recognition elements. Specifically, signal amplification technologies applied to the OTA electrochemical aptasensor are proposed. Furthermore, summary of the current limitations and future challenges in OTA analysis is included, which provide reference for the further research and applications.

Ultrasensitive and rapid detection of ochratoxin A in agro-products by a nanobody-mediated FRET-based immunosensor

Ochratoxin A (OTA) is a major concern for public health and the rapid detection of trace OTA in food is always a challenge. To minimize OTA exposure to consumers, a nanobody (Nb)-mediated förster resonance energy transfer (FRET)-based immunosensor using quantum dots (Nb-FRET immunosensor) was proposed for ultrasensitive, single-step and competitive detection of OTA in agro-products at present work. QDs of two sizes were covalently labeled with OTA and Nb, acting as the energy donor and acceptor, respectively. The free OTA competed with the donor to bind to acceptor, thus the FRET efficiency increased with the decrease of OTA concentration. The single-step assay could be finished in 5 min with a limit of detection of 5 pg/mL, which was attributed to the small size of Nb for shortening the effective FRET distance and improving the FRET efficiency. The Nb-FRET immunosensor exhibited high selectivity for OTA. Moreover, acceptable accuracy and precision were obtained in the analysis of cereals and confirmed by the liquid chromatography-tandem mass spectrometry. Thus the developed Nb-FRET immunosensor was demonstrated to be an efficient tool for ultrasensitive and rapid detection of OTA in cereals and provides a detection model for other toxic small molecules in food and environment.

Nanobody-alkaline phosphatase fusion-mediated phosphate-triggered fluorescence immunoassay for ochratoxin a detection

Ochratoxin A (OTA) is a kind of mycotoxin that seriously harms the health of humans and animals. In this study, a nanobody-alkaline phosphatase fusion-mediated phosphate-triggered fluorescence immunoassay (Nb-AP-mediated PT-FIA) was developed for detecting OTA. Based on the constructed phosphate-triggered fluorescence sensing system for Nb-AP and the optimal working conditions, the Nb-AP-mediated PT-FIA has a half maximal inhibition concentration (IC₅₀) of 0.46 ng/mL, a limit of detection (IC₁₀) of 0.12 ng/mL, and a linear range (IC_20-80) of 0.2-1.26 ng/mL, respectively. The recovery experiment indicated acceptable accuracy and precision of the Nb-AP-mediated PT-FIA, and the results were validated by high performance liquid chromatography with fluorescence detector. Thus this proposed method is applicable to sensitive, rapid, and low-cost detection of OTA and other toxic analytes with low molecular weight in food and environment.

An immunoassay for ochratoxin A using tetramethylrhodamine-labeled ochratoxin A as a probe based on a binding-induced change in fluorescence intensity

In an immunoassay, ochratoxin A (OTA) competitively displaces the bound tetramethylrhodamine (TMR)-OTA fluorescent probe from the antibody, causing a decrease in fluorescence.

A double-recognized aptamer-molecularly imprinted monolithic column for high-specificity recognition of ochratoxin A

In this study, a double-recognized aptamer-molecularly imprinted monolithic column (Apt-MIP monolithic column) was prepared by introducing both aptamer and MIP to reduce non-specific adsorption. Its preparation parameters such as the time of photo-initiation, the dosage of photo-initiator and the concentration of aptamer were investigated in detail. The recovery ratios of ochratoxin A (OTA) to ochratoxin B (OTB) on Apt-MIP monolithic column, Apt monolithic column and MIP monolithic column were 116.1, 40.8 and 69, respectively. Even if the concentration of OTB was 10 times that of OTA, the recovery of OTB was only about 2.9%. Applied to beer samples, the prepared Apt-MIP monolithic column drastically resisted background adsorption and the high-specificity recognition for OTA was obtained with the recoveries of 95.5-105.9%. This work provided a simple and effective method to selectively identify OTA from complex samples.

Nanobody-based fluorescence resonance energy transfer immunoassay for noncompetitive and simultaneous detection of ochratoxin a and ochratoxin B

A noncompetitive and homogeneous fluorescence resonance energy transfer (FRET) immunoassay was developed using a nanobody (Nb) for highly sensitive and simultaneous detection of ochratoxin A (OTA) and ochratoxin B (OTB). The promoted intrinsic fluorescence (λ_ex: 280 nm) of tryptophan residues (donor) in Nb can excite the fluorescence of OTA and OTB (acceptor) for detection (λ_em: 430 nm). Using optimal conditions, the limits of detection of the Nb-based FRET immunoassay were 0.06 and 0.12 ng/mL for OTA and OTB, respectively. Minimal cross reactivity was detected for several analogues of OTA and OTB as well as nonspecific proteins and antibodies. Acceptable accuracy and precision were obtained in the spike and recovery study, and the results correlated well with those by HPLC. These results demonstrated that the developed method could be a useful tool for noncompetitive, homogeneous, and simultaneous detection of OTA and OTB as well as other environmental analytes with similar fluorescence properties.

Advances in biosensors for the detection of ochratoxin A: Bio-receptors, nanomaterials, and their applications

Ochratoxin A (OTA) is a class of mycotoxin mainly produced by the genera Aspergillus and Penicillium. OTA can cause various forms of kidney, liver and brain diseases in both humans and animals although trace amount of OTA is normally present in food. Therefore, development of fast and sensitive detection technique is essential for accurate diagnosis of OTA. Currently, the most commonly used detection methods are enzyme-linked immune sorbent assays (ELISA) and chromatographic techniques. These techniques are sensitive but time consuming, and require expensive equipment, highly trained operators, as well as extensive preparation steps. These drawbacks limit their wide application in OTA detection. On the contrary, biosensors hold a great potential for OTA detection at for both research and industry because they are less expensive, rapid, sensitive, specific, simple and portable. This paper aims to provide an extensive overview on biosensors for OTA detection by highlighting the main biosensing recognition elements for OTA, the most commonly used nanomaterials for fabricating the sensing interface, and their applications in different read-out types of biosensors. Current challenges and future perspectives are discussed as well.