An indirect competitive fluorescence assay for ochratoxin A based on molecular beacon

A novel, simple and efficient method based on a molecular beacon (MB) probe was developed to detect ochratoxin A (OTA) in malts, which is a common starting material in the brewery industry. With the critical site for OTA binding in capture aptamer in mind, a MB probe containing 20 bases with a fluorophore-quencher pair at the stem ends was designed and synthesized. In the "off" configuration, the fluorescein (FAM) is internally quenched due to close contact with dabcyl and the fluorescence signal is recovered after hybridization with the capture aptamer at the "on" state. In the presence of OTA, the MB probe competes for binding at the loop region of the aptamer, resulting in a decrease in fluorescence signal. Using this indirect competitive assay, the detection of OTA in malt samples was accomplished for the first time. In addition, the effect of binding affinity of the capture aptamer and OTA on the assay performance was investigated. Under optimal conditions, this method allowed for OTA detection at a linear range of 0.0001-1 mu g mL(-1) (correlation coefficient, R = 0.9920) with superior sensitivity and a detection limit as low as 0.05 ng mL(-1). The sensing system also displayed an excellent selectivity and perfect anti-interference capacity in the matrix. Moreover, the entire process of detection was accomplished in less than 20 min. The recovery from spiked malt samples ranged from 81.0% to 95.2% with RSDs below 5.4%. The performance of our method was further validated by ultra-fast liquid chromatography coupled with tandem mass spectrometry. Compared with similar fluorescence assays, the proposed method is simple, efficient and does not require complicated conjugation steps. Taken together, this novel detection strategy could be a promising tool for hand-held devices used during onsite monitoring of contaminants.