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.