A Sensitive Immunoaffinity Column-Linked Indirect Competitive ELISA for Ochratoxin A in Cereal and Oil Products Based on a New Monoclonal Antibody

Rational design of molecularly imprinted electrochemical sensor based on Nb2C-MWCNTs heterostructures for highly sensitive and selective detection of Ochratoxin a

Developing an efficient method for screening Ochratoxin A (OTA) in agriculture products is vital to ensure food safety and human health. However, the complex food matrix seriously affects the sensitivity and accuracy. To address this issue, we designed a novel molecularly imprinted polymer (MIP) electrochemical sensor based on multiwalled carbon nanotube-modified niobium carbide (Nb2C-MWCNTs) with the aid of the density functional theory (DFT). In this design, a glassy carbon electrode (GCE) was first modified by Nb2C-MWCNTs heterostructure. Afterward, the MIP layer was prepared, with ortho-toluidine as a functional monomer selected via DFT and OTA acting as a template on the surface of Nb2C-MWCNTs/GCE using in-situ electropolymerization. Electrochemical tests and physical characterization revealed that Nb2C-MWCNTs improved the sensor's active surface area and electron transmission capacity. Nb2C-MWCNTs had a good synergistic effect on MIP, endowing the sensor with high sensitivity and specific recognition of OTA in complex food matrix systems. The MIP sensor showed a wide linear range from 0.04 to 10.0 μM with a limit of detection (LOD) of 3.6 nM. Moreover, it presented good repeatability and stability for its highly antifouling effect on OTA. In real sample analysis, the recoveries, ranging from 89.77% to 103.70%, agreed well with the results obtained by HPLC methods, suggesting the sensor has good accuracy and high potential in practical applications.

An Extremely Highly Sensitive ELISA in pg mL-1 Level Based on a Newly Produced Monoclonal Antibody for the Detection of Ochratoxin A in Food Samples

In this study, an extremely highly sensitive enzyme-linked immunosorbent assay (ELISA) based on a newly produced monoclonal antibody (mAb) for the detection of ochratoxin A (OTA) in food samples was developed. OTA-Bovine serum albumin (BSA) conjugate was prepared and used as the immunogen for the production of the mAb. Among four hybridoma clones (8B10, 5C2, 9B7, and 5E11), the antibody from 8B10 displayed the highest affinity recognition for OTA. Based on the mAb (8B10), the IC50 and LOD of the ELISA for OTA were 34.8 pg mL-1 and 1.5 pg mL-1, respectively, which was 1.53~147 times lower than those in published ELISAs, indicating the ultra-sensitivity of our assay. There was no cross-reactivity of the mAb with the other four mycotoxins (AFB1, ZEN, DON, and T-2). Due to the high similarity in molecular structures among OTA, ochratoxin B (OTB), and ochratoxin C (OTC), the CR values of the mAb with OTB and OTC were 96.67% and 22.02%, respectively. Taking this advantage, the ELISA may be able to evaluate total ochratoxin levels in food samples. The recoveries of the ELISA for OTA in spiked samples (corn, wheat, and feed) were 96.5-110.8%, 89.5-94.4%, and 91.8-113.3%; and the RSDs were 5.2-13.6%, 8.2-13.0%, and 7.7-13.7% (n = 3), respectively. The spiked food samples (corn) were measured by ELISA and HPLC-FLD simultaneously. A good correlation between ELISA (x) and HPLC-FLD (y) with the linear regression equation of y = 0.918x - 0.034 (R2 = 0.985, n = 5) was obtained. These results demonstrated that the newly produced mAb-based ELISA was a feasible and ultra-sensitive analytical method for the detection of OTA in food samples.

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

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

Development of an Ultrasensitive and Rapid Fluorescence Polarization Immunoassay for Ochratoxin A in Rice

Ochratoxin A (OTA) is a known food contaminant that affects a wide range of food and agricultural products. The presence of this fungal metabolite in foods poses a threat to human health. Therefore, various detection and quantification methods have been developed to determine its presence in foods. Herein, we describe a rapid and ultrasensitive tracer-based fluorescence polarization immunoassay (FPIA) for the detection of OTA in rice samples. Four fluorescent tracers OTA-fluorescein thiocarbamoyl ethylenediamine (EDF), OTA-fluorescein thiocarbamoyl butane diamine (BDF), OTA-amino-methyl fluorescein (AMF), and OTA-fluorescein thiocarbamoyl hexame (HDF) with fluorescence polarization values (δFP = FPbind-FPfree) of 5, 100, 207, and 80 mP, respectively, were synthesized. The tracer with the highest δFP value (OTA-AMF) was selected and further optimized for the development of an ultrasensitive FPIA with a detection range of 0.03-0.78 ng/mL. A mean recovery of 70.0% to 110.0% was obtained from spiked rice samples with a relative standard deviation of equal to or less than 20%. Good correlations (r² = 0.9966) were observed between OTA levels in contaminated rice samples obtained by the FPIA method and high-performance liquid chromatography (HPLC) as a reference method. The rapidity of the method was confirmed by analyzing ten rice samples that were analyzed within 25 min, on average. The sensitivity, accuracy, and rapidity of the method show that it is suitable for screening and quantification of OTA in food samples without the cumbersome pre-analytical steps required in other mycotoxin detection methods.

Change of Amino Acid Residues in Idiotypic Nanobodies Enhanced the Sensitivity of Competitive Enzyme Immunoassay for Mycotoxin Ochratoxin A in Cereals

Anti-idiotypic nanobodies, usually expressed by gene engineering protocol, has been shown as a nontoxic coating antigen for toxic compound immunoassays. We here focused on how to increase immunoassay sensitivity by changing the nanobody’s primary sequence. In the experiments, two anti-idiotype nanobodies against monoclonal antibody 1H2, which is specific to ochratoxin A, were obtained and named as nontoxic coating antigen 1 (NCA1) and nontoxic coating antigen 2 (NCA2). Three differences between the nanobodies were discovered. First, there are six amino acid residues (AAR) of changes in the complementarity determining region (CDR), which compose the antigen-binding site. One of them locates in CDR1 (I–L), two of them in CDR2 (G–D, E–K), and three of them in CDR3 (Y–H, Y–W). Second, the affinity constant of NCA1 was tested as 1.20 × 10⁸ L mol⁻¹, which is about 4 times lower than that of NCA2 (5.36 × 10⁸ L mol⁻¹). Third, the sensitivity (50% inhibition concentration) of NCA1 for OTA was shown as 0.052 ng mL⁻¹, which was 3.5 times lower than that of nontoxic coating antigen 2 (0.015 ng mL⁻¹). The results indicate that the AAR changes in CDR of the anti-idiotypic nanobodies, from nonpolar to polar, increasing the affinity constant may enhance the immunoassay sensitivity. In addition, by using the nontoxic coating antigen 2 to substitute the routine synthetic toxic antigen, we established an eco-friendly and green enzyme-linked immunosorbent assay (ELISA) method for rapid detection of ochratoxin A in cereals. The half-maximal inhibitory concentration (IC₅₀) of optimized ELISA was 0.017 ng mL⁻¹ with a limit of detection (LOD) of 0.003 ng mL⁻¹. The optimized immunoassay showed that the average recoveries of spiked corn, rice, and wheat were between 80% and 114.8%, with the relative standard deviation (RSD) ranging from 3.1–12.3%. Therefore, we provided not only basic knowledge on how to improve the structure of anti-idiotype nanobody for increasing assay sensitivity, but also an available eco-friendly ELISA for ochratoxin A in cereals.

Development of ELISA and Lateral Flow Immunoassays for Ochratoxins (OTA and OTB) Detection Based on Monoclonal Antibody

Ochratoxins were important secondary metabolites secreted by fungi, and OTA and OTB are mainly significant mycotoxin, having toxic effects on humans and animals. Therefore, it is important to establish a rapid, sensitive, and precise method for ochratoxins detection and quantification in real samples. In this study, a stable monoclonal antibody (mAb) that recognizing both OTA and OTB toxins was employed for the establishment of indirect competitive ELISA (ic-ELISA), colloidal gold nanoparticles (CGNs), and nanoflowers gold strips (AuNFs) for detection of ochratoxins in real samples. A 6E5 hybridoma cell line stable secreting mAb against both OTA and OTB toxins was obtained by fusion of splenocytes with myeloma SP2/0 cells. The 6E5 mAb had a high affinity (3.7 × 10⁸ L/mol) to OTA, and also showed similar binding activity to OTB. The optimized ic-ELISA resulted in a linear range of 0.06–0.6 ng/mL for ochratoxins (OTA and OTB) detection. The IC50 was 0.2 ng/mL and the limit of detection (LOD) was 0.03 ng/mL. The mean recovery rate from the spiked samples was 89.315 ± 2.257%, with a coefficient variation of 2.182%. The result from lateral flow immunoassays indicated that the LOD of CGNs and AuNFs were 5 and 1 μg/mL, respectively. All these results indicated that the developed ic-ELISA, CGNs, and AuNFs in this study could be used for the analysis of the residual of ochratoxins (OTA and OTB) in food and agricultural products.

Development of an Anti-Idiotypic VHH Antibody and Toxin-Free Enzyme Immunoassay for Ochratoxin A in Cereals

Enzyme-linked immunosorbent assay (ELISA) test kits have been widely used for the determination of mycotoxins in agricultural products and foods, however, this test uses toxin standards with high toxicity and carcinogenicity that seriously threaten human health. In this work, the anti-idiotypic nanobody VHH 2-24 was first developed and then, using it as a surrogate standard, a toxin-free enzyme immunoassay for ochratoxin A (OTA) was established. The IC₅₀ value of the VHH 2-24 surrogate standard-based ELISA was 0.097 µg/mL, with a linear range of 0.027–0.653 µg/mL. The average recoveries were tested by spike-and-recovery experiments, and ranged from 81.8% to 105.0%. The accuracy of the developed ELISA for detecting OTA was further verified by using the high performance liquid chromatography (HPLC) method, and an excellent correlation was observed. In summary, the toxin-free ELISA established in this study proves the latent use of the anti-idiotypic VHH as a surrogate calibrator for other mycotoxins and highly toxic small molecule analysis to improve assay properties for highly sensitive analyte determination in agricultural products.

Development of a new format of competitive immunochromatographic assay using secondary antibody-europium nanoparticle conjugates for ultrasensitive and quantitative determination of ochratoxin A

In this study, to enhance the sensitivity of conventional immunochromatographic assay (ICA) two strategies including using a high sensitive label and changing the assay format, were simultaneously applied to develop an ultrasensitive format of ICA. In new format, primary monoclonal antibody against ochratoxin A (OTA) was used without any labeling, and a secondary polycolonal antibody was labeled with europium fluorescent nanoparticles (EuNPs). Detection was performed in a single step by inserting the test strip into a microtube containing all reagents. The results were obtained within 12 min and read by a portable fluorescent strip reader. Salient features of the new format of ICA compared with conventional format include: (1) A 100-fold decrease in limit of detection (LOD) due to application of two amplification strategy; (2) Reduction in expensive monoclonal antibody consumption. The established method was evaluated for the quantitative determination of OTA with LOD as low as 0.4 pg mL^-1.

Development of an immunochromatographic assay for the specific detection of Bacillus thuringiensis (Bt) Cry1Ab toxin

Cry1Ab has been widely used in genetically modified (GM) crops and its amino acid sequence had high identity to Cry1Ac toxin. Existing nanogold immunochromatographic strips cannot distinguish Cry1Ab from Cry1Ac toxin. In this study, a rapid (5-6 min), qualitative nanogold immunochromatographic strip was successfully developed for the specific detection of Cry1Ab toxin. The assay was based on double antibody sandwich format with the visual detection limit (vLOD) of 0.1 μg mL^-1. The results of immunochromatographic assay were all positive validated against the DAS-ELISA (recoveries between 109.6 and 111.8%). In addition, 10%, 5% and 0% error probability results were found in 20 times repeated tests for Cry1Ab concentration of 0.1, 0.2, 0.5 and 1 μg mL^-1, respectively, demonstrating the reproducibility of the test strip. Furthermore, the test strip could be stored for 3 months under dry conditions without significant loss of sensitivity. Furthermore, the practical sample analysis results showed that the test strip was able to detect the presence of Cry1Ab in GM materials containing as low as 0.5% MON 810 Bt maize which indicated the practical value of the test strip. To our knowledge, this is the first report on the detection of Cry1Ab by immunochromatographic assay without interference from Cry1Ac toxin.

Novel haptens and monoclonal antibodies with subnanomolar affinity for a classical analytical target, ochratoxin A

Ochratoxin A is a potent toxic fungal metabolite whose undesirable presence in food commodities constitutes a problem of public health, so it is strictly regulated and controlled. For the first time, two derivatives of ochratoxin A (OTAb and OTAd) functionalized through positions other than the native carboxyl group of the mycotoxin, have been synthesized in order to better mimic, during the immunization process, the steric and conformational properties of the target analyte. Additionally, two conventional haptens making use of that native carboxyl group for protein coupling (OTAe and OTAf) were also prepared as controls for the purpose of comparison. The immunological performance in rabbits of protein conjugates based on OTAb and OTAd overcome that of conjugates employing OTAe and OTAf as haptens. After immunization of mice with OTAb and OTAd conjugates, a collection of high-affinity monoclonal antibodies to ochratoxin A was generated. In particular, one of those antibodies, the so-called OTAb#311, is very likely the best antibody produced so far in terms of selectivity and affinity to ochratoxin A.

Nanobody Technology for Mycotoxin Detection in the Field of Food Safety: Current Status and Prospects

Mycotoxins, which are toxic, carcinogenic, and/or teratogenic, have posed a threat to food safety and public health. Sensitive and effective determination technologies for mycotoxin surveillance are required. Immunoassays have been regarded as useful supplements to chromatographic techniques. However, conventional antibodies involved in immunoassays are difficult to be expressed recombinantly and are susceptible to harsh environments. Nanobodies (or VHH antibodies) are antigen-binding sites of the heavy-chain antibodies produced from Camelidae. They are found to be expressed easily in prokaryotic or eukaryotic expression systems, more robust in extreme conditions, and facile to be used as surrogates for artificial antigens. These properties make them the promising and environmentally friendly immunoreagents in the next generation of immunoassays. This review briefly describes the latest developments in the area of nanobodies used in mycotoxin detection. Moreover, by integrating the introduction of the principle of nanobodies production and the critical assessment of their performance, this paper also proposes the prospect of nanobodies in the field of food safety in the foreseeable future.

T-2 Toxin/HT-2 Toxin and Ochratoxin A ELISAs Development and In-House Validation in Food in Accordance with the Commission Regulation (EU) No 519/2014

T-2 toxin/HT-2 toxin (T-2/HT-2) and ochratoxin A (OTA) are mycotoxins that can contaminate a variety of agricultural commodities. To protect consumers' health, indicative limits for T-2/HT-2 and maximum limits for OTA have been set by the European Commission, requiring food business operators and controlling agencies to conduct routine checks for the presence of these harmful contaminants. Screening methods are increasingly used for monitoring purposes. Due to the demand for new and improved screening tools, two individual detection methods, T-2/HT-2 and OTA enzyme-linked immunosorbent assays (ELISAs), were developed in this study. The T-2/HT-2 ELISA was based on a T-2 monoclonal antibody with an IC50 (50% inhibitory concentration) of 0.28 ng/mL and 125% cross-reactivity with HT-2. As regards the OTA ELISA, a new sensitive monoclonal antibody specific to OTA with an IC50 of 0.13 ng/mL was produced. Both developed ELISA tests were then validated in agricultural commodities in accordance with the new performance criteria guidelines for the validation of screening methods for mycotoxins included in Commission Regulation (EU) No 519/2014. The T-2/HT-2 ELISA was demonstrated to be suitable for the detection of T-2/HT-2 in cereals and baby food at and above the screening target concentration (STC) of 12.5 μg/kg and 7.5 μg/kg, respectively. The OTA ELISA was shown to be applicable for the detection of OTA in cereals, coffee, cocoa and wine at and above the STC of 2 μg/kg, 2.5 μg/kg, 2.5 μg/kg and 0.4 ng/mL, respectively. The accuracy of both ELISAs was further confirmed by analysing proficiency test and reference samples. The developed methods can be used for sensitive and high-throughput screening for the presence of T-2/HT-2 and OTA in agricultural commodities.

Determination of Aflatoxin M1 and Chloramphenicol in Milk Based on Background Fluorescence Quenching Immunochromatographic Assay

Harsh demanding has been exposed on the concentration of aflatoxin M1 (AFM1) and chloramphenicol (CAP) in milk. In this study, we developed a new method based on background fluorescence quenching immunochromatographic assay (bFQICA) to detect AFM1 and CAP in milk. The detection limit for AFM1 was 0.0009 ng/mL, while that for the CAP was 0.0008 ng/mL. The assay variability was determined with 3 AFM1 standards (i.e., 0.25 ng/mL, 0.5 ng/mL, and 1.0 ng/mL), and the actual detection value was 0.2497, 0.5329, and 1.0941, respectively. For the assay variability of 3 CAP standards (i.e., 0.10 ng/mL, 0.30 ng/mL, and 0.50 ng/mL), the actual detection value was 0.0996, 0.3096, and 0.4905, respectively. The recovery rate of AFM1 was 99.7%-101.7%, while that for CAP was 95.3%-97.6%. For the test stability, AFM1 and CAP showed satisfactory test stability even at month 5. Compared with the sensitivity of liquid chromatography-mass spectrometry (LC-MS) method, no statistical difference was noticed in results of the bFQICA. Our method is convenient for the detection of AFM1 and CAP in milk with a test duration of about 8 minutes. Additionally, an internal WiFi facility is provided in the system allowing for quick connection and storage in the intelligent cell phone.

Detection of ochratoxin A in beer samples with a label-free monolithically integrated optoelectronic biosensor

An optical biosensor for label-free detection of ochratoxin A (OTA) in beer samples is presented. The biosensor consists of an array of ten Mach-Zehnder interferometers (MZIs) monolithically integrated along with their respective broad-band silicon light sources on the same Si chip (37mm²). The chip was transformed to biosensor by functionalizing the MZIs sensing arms with an OTA-ovalbumin conjugate. OTA determination was performed by pumping over the chip mixtures of calibrators or samples with anti-OTA antibody following a competitive immunoassay format. An external miniaturized spectrometer was employed to continuously record the transmission spectra of each interferometer. Spectral shifts obtained due to immunoreaction were transformed to phase shifts through Discrete Fourier Transform. The assay had a detection limit of 2.0ng/ml and a dynamic range 4.0-100ng/ml in beer samples, recoveries ranging from 90.6 to 116%, and intra- and inter-assay coefficients of variation of 9% and 14%, respectively. The results obtained with the sensor using OTA-spiked beer samples spiked were in good agreement with those obtained by an ELISA developed using the same antibody. The good analytical performance of the biosensor and the small size of the proposed chip provide for the development of a portable instrument for point-of-need determinations.

Quantitative determination of major capsaicinoids in serum by ELISA and time-resolved fluorescent immunoassay based on monoclonal antibodies

To monitor capsaicinoids in serum on-site, three new monoclonal antibodies (mAbs) were firstly proposed using a conjugate of 4-[(4-hydroxy-3-methoxybenzyl) amino]-4-oxobutanoic acid as the immunogen. Among them, the YQQD8 mAb showed the highest sensitivity and cross-reactivity to major capsaicinoids, such as capsaicin, dihydrocapsaicin and N-vanillylnonanamide. A competitive indirect enzyme-linked immunosorbent assay (icELISA) and a time-resolved fluorescent immunochromatographic assay (TRFICA) were established based on this mAb. The linear range was 1.1-27.0ngmL(-1) for icELISA and 1.9-62.5ngmL(-1) for TRFICA and the limit of detection (LOD) of TRFICA was 1.5ngmL(-1). To decrease the interference of sample components and increase accuracy, serum samples were diluted four times before assays. As a result, the linear range of serum samples was 4.6-107.9ngmL(-1) for icELISA and 7.6-250.0ngmL(-1) for TRFICA. Both icELISA and TRFICA showed good recoveries (91.0-112.8% for icELISA and 87.6-111.5% for TRFICA) and concordant results in spiked experiments. Overall, this is the first report of immunoassay based on the mAbs for quantitative determination of major capsaicinoids, and the results demonstrate that both methods can meet the demands of rapid on-site assay for capsaicinoids in serum samples.

Ultrasensitive and quantitative gold nanoparticle-based immunochromatographic assay for detection of ochratoxin A in agro-products

In most cases of mycotoxin detection, quantitation is critical while immunochromatographic strip tests are qualitative in nature. Moreover, the sensitivity of this technique is questioned. In order to overcome these limitations, an ultrasensitive and quantitative immunochromatographic assay (ICA) for rapid and sensitive quantitation of ochratoxin A (OTA) was developed. The assay was based on a competitive format and its sensitivity was improved by using a sensitive and selective OTA monoclonal antibody (OTA-mAb). The visible ICA results were obtained within 15 min, and in addition to visual examination, they were read by the rapid color intensity portable strip reader. The visual and computational detection limits (vLOD and cLOD, respectively) for ochratoxin A were 0.2 and 0.25 ng mL(-1), respectively. These values were lower than those reported by previous studies in a range 5-2500 folds. For validation, contaminated samples including wheat, maize, rice and soybean were assayed by ICA and a standard high performance liquid chromatography (HPLC). The results were in good agreement for both ICA and HPLC methods. The average recoveries of the HPLC were in the range 72-120% while the ICA values were from 76 to 104%, confirming the accuracy and sensitivity of this method.

Specific antibody-induced fluorescence quenching for the development of a directly applicable and label-free immunoassay

A simple and label-free immunoassay was proposed based on the specific antibody-analyte immune binding reaction induced by fluorescence quenching of the analyte.