Multimodal Ochratoxin A-Aptasensor Using 3′-FAM-Enhanced Exonuclease I Tool and Magnetic Microbead Carrier

Thanks to its preparatory ease, close affinity, and low cost, the aptasensor can serve as a promising substitute for antibody-dependent biosensors. However, the available aptasensors are mostly subject to a single-mode readout and the interference of unbound aptamers in solution and non-target-induced transition events. Herein, we proposed a multimodal aptasensor for multimode detection of ochratoxin A (OTA) with cross-validation using the 3′-6-carboxyfluorescein (FAM)-enhanced exonuclease I (Exo I) tool and magnetic microbead carrier. Specifically, the 3′-FAM-labeled aptamer/biotinylated-cDNA hybrids were immobilized onto streptavidin-magnetic microbeads via streptavidin–biotin interaction. With the presence of OTA, an antiparallel G-quadruplex conformation was formed, protecting the 3′-FAM labels from Exo I digestion, and then anti-FAM-horseradish peroxidase (HRP) was bound via specific antigen-antibody affinity; for the aptamers without the protection of OTA, the distal ssDNA was hydrolyzed from 3′ → 5′, releasing 3′-FAM labels to the solution. Therefore, the OTA was detected by analyzing the "signal-off" fluorescence of the supernatant and two "signal-on" signals in electrochemistry and colorimetry through the detection of the coating magnetic microbeads in HRP's substrate. The results showed that the 3′-FAM labels increased the activity of Exo I, producing a low background due to a more thorough digestion of unbound aptamers. The proposed multimodal aptasensor successfully detected the OTA in actual samples. This work first provides a novel strategy for the development of aptasensors with Exo I and 3′-FAM labels, broadening the application of aptamer in the multimode detection of small molecules.