A highly specific competitive direct enzyme immunoassay for sterigmatocystin as a tool for rapid immunochemotaxonomic differentiation of mycotoxigenicAspergillusspecies

Enzyme immunoassays for the detection of mycotoxins in plant-based milk alternatives: pitfalls and limitations

Plant-based milk alternatives (PBMAs) are a potential source of mycotoxin uptake. To ensure food safety, simple and rapid testing methods of PBMAs for mycotoxins are therefore required. This study investigated the applicability of enzyme immunoassay (EIA) methods for direct testing of PBMAs without sample extraction. Mycotoxin analyses included aflatoxin B₁ (AFB₁), sterigmatocystin (STC), ochratoxin A (OTA), deoxynivalenol (DON), and T-2/HT-2-toxin (T-2/HT-2). It was found that the PBMA matrix negatively affected the EIA to varying degrees, thus affecting the reliability of the results. A dilution of PBMAs of at least 1:8 was necessary to overcome matrix interference. This resulted in calculated detection limits of 0.4 µg/L (AFB₁), 2 µg/L (STC), 0.08 µg/L (OTA), 16 µg/L (DON), and 0.4 µg/L (T-2/HT-2). After analysis of 54 PBMA products from German retail stores, positive results in at least one test system were obtained for 23 samples. However, most positive results were near the calculated detection limit. Control analyses of selected samples by LC–MS/MS for AFB₁, STC, and OTA qualitatively confirmed the presence of trace amounts of STC in some samples, but quantitative agreement was poor. It was concluded that the high diversity of ingredients used in PBMAs led to a highly variable degree of sample matrix interference even in a 1:8 dilution. Since the use of higher dilutions conflicts with the need to achieve low detection limits, the application of EIA for routine mycotoxin analysis in PBMA for mycotoxins requires further study on the development of a feasible sample preparation method.

Development of a polyclonal antibody-based indirect competitive ELISA for determination of sterigmatocystin in wheat and corn flours

Sterigmatocystin (STC) is a toxic secondary metabolite produced by more than 50 fungal species, including Aspergillus flavus, A. parasiticus, A. nidulans, and A. versicolor. The Joint FAO/WHO Expert Committee on Food Additives concluded that sterigmatocystin is genotoxic and carcinogenic with the critical effect determined to be carcinogenicity. The present study describes a simple method to prepare hapten and immunogens in order to generate polyclonal antibodies against this metabolite. We developed a sensitive and specific polyclonal antibody-based competitive indirect enzyme-linked immunosorbent assay (ciELISA) for monitoring STC in wheat and corn flours without the need for derivatisation of STC or clean-up of samples by immunoaffinity chromatography for quantification. The half inhibitory concentration (IC₅₀) of the established method was 4.52 ± 0.81 ng mL^-1, with the limit of detection (IC₁₀) being 0.19 ± 0.04 ng mL^-1 in wheat and corn flour matrices with the coefficient of variation of less than 22%.The assay was very specific to STC and showed no cross-reactivity with its analogue structures. The ELISA allowed for up to 5% methanol without significant influence on the IC₅₀ value. Validation of the assay was performed by spiking STC into a blank flour matrix and the recoveries were in the range of 75.3 % to 104.5 % with a coefficient of variation less than 15%. A small retail survey was conducted by purchasing wheat (n = 8) and corn flours (n = 2) from local grocery stores. All of these retail samples were negative for STC using the developed ELISA method and were confirmed by LC-MS/MS. We demonstrated a rapid, simple, and reliable method for screening STC in wheat and corn flours.

Preparation and Evaluation of Core⁻Shell Magnetic Molecularly Imprinted Polymers for Solid-Phase Extraction and Determination of Sterigmatocystin in Food

Magnetic molecularly imprinted polymers (MMIPs), combination of outstanding magnetism with specific selective binding capability for target molecules, have proven to be attractive in separation science and bio-applications. Herein, we proposed the core⁻shell magnetic molecularly imprinted polymers for food analysis, employing the Fe₃O₄ particles prepared by co-precipitation protocol as the magnetic core and MMIP film onto the silica layer as the recognition and adsorption of target analytes. The obtained MMIPs materials have been fully characterized by scanning electron microscope (SEM), Fourier transform infrared spectrometer (FT-IR), vibrating sample magnetometer (VSM), and re-binding experiments. Under the optimal conditions, the fabricated Fe₃O₄@MIPs demonstrated fast adsorption equilibrium, a highly improved imprinting capacity, and excellent specificity to target sterigmatocystin (ST), which have been successfully applied as highly efficient solid-phase extraction materials followed by high-performance liquid chromatography (HPLC) analysis. The MMIP-based solid phase extraction (SPE) method gave linear response in the range of 0.05⁻5.0 mg·L-1 with a detection limit of 9.1 µg·L-1. Finally, the proposed method was used for the selective isolation and enrichment of ST in food samples with recoveries in the range 80.6⁻88.7% and the relative standard deviation (RSD) <5.6%.

Upconversion Nanophosphor-Involved Molecularly Imprinted Fluorescent Polymers for Sensitive and Specific Recognition of Sterigmatocystin

Originated from the bottom-up synthetic strategy, molecularly imprinted polymers (MIPs) possess the inherent ability of selective and specific recognition and binding of the target analytes, with their structural cavities that can match the target molecules in respect to size, shape, and functional groups. Herein, based on the high selectivity of MIPs and the fluorescence properties of the β-NaYF₄:Yb3+, Er3+ upconversion nanoparticles, MIPs with both specificity and fluorescent signals are fabricated to recognize trace sterigmatocystin (ST) with high selectivity and sensitivity. The structure analogue of ST, 1,8-dihydroxyanthraquinone (DT), was employed as the template molecule, acrylamide as the functional monomer, 3-methacryloyloxypropyltrimethoxysilane as the crosslinking agent, and a new molecular imprinting technique of non-aqueous sol-gel method is used to synthesize a molecularly imprinted material with high selectivity to ST. Under optimal conditions, the fluorescence enhancement of fluorescent MIPs increased as the concentration of ST increased. In the range of 0.05⁻1.0 mg L-1, fluorescence enhancement and the concentration showed a good linear relationship with a detection limit of 0.013 mg L-1. Real sample analysis achieved the recoveries of 83.8⁻88.8% (RSD 5.1%) for rice, 82.1⁻87.5% (RSD 4.6%) for maize, and 80.6⁻89.2% (RSD 3.0%) for soybeans, respectively, revealing the feasibility of the developed method.