A novel and label-free electrochemical aptasensor based on exonuclease III and G-quadruplex DNAzyme for sensitive and selective detection of metronidazole

A novel and label-free electrochemical aptasensor based on exonuclease III (Exo III) and G-quadruplex DNAzyme has been first designed to detect metronidazole (MNZ) sensitively and selectively in this paper. Capture probe (CP), helper probe (HP) and aptamer (Apt) hybridize to form double-stranded DNA (dsDNA) structure with a blunt 3′-terminus, resulting in the guanine (G)-rich CP being enclosed in the dsDNA, followed by specific digestion of the 3′-terminus by Exo III to expose the G-rich sequence. At this time, CP can bind with the added hemin to form G-quadruplex DNAzyme which catalyzes the redox reaction of 3,3′,5,5′-tetramethylbenzidine (TMB) and H2O2, generating a strong current signal. However, in the presence of MNZ, the binding of MNZ and aptamer breaks the original blunt 3′-terminus of the dsDNA structure, blocking the subsequent series of reactions, and only a weak current signal can be observed. Under the optimal conditions, the aptasensor showed excellent sensitivity and specificity for the detection of MNZ with a wide linear range from 10 pM to 500 nM and a detection limit as low as 3.0 pM (S/N = 3). The aptasensor constructed based on MNZ aptamer can effectively avoid the interference of other nitro compounds and antibiotics to a certain extent. Compared with other specific recognition elements, the aptasensor possesses the advantages of strong specificity, simple preparation and low cost. Moreover, the proposed aptasensor has been successfully applied to the detection of MNZ in tap water and honey samples.