Aptamer-enhanced the Ag(I) ion-3,3′,5,5′-tetramethylbenzdine catalytic system as a novel colorimetric biosensor for ultrasensitive and selective detection of paraquat

A facile and simple colorimetric biosensor was first established for paraquat (PQ) detection based on the aptamer-enhanced oxidation process of 3,3',5,5'-tetramethylbenzidine (TMB) by Ag+. The study confirmed that the interaction of PQ-15 aptamer with Ag+ accelerates the electron transfer from the aptamer-Ag+ complex to dissolved oxygen, which enhances the release of superoxide anion radicals (O2̇-) and facilitates the catalytic oxidation of the chromogenic substrate. PQ-15 aptamer will preferentially bind to PQ molecules, resulting in no further enhancement of the catalytic activity of Ag+. Molecular docking results revealed that the PQ molecules are attached to the stem-loop region of the PQ-15 aptamer through σ-π conjugation interactions. The proposed method is simple that only contains Ag+ and corresponding aptamer. The limit of detection (LOD) of the constructed colorimetric biosensor for PQ detection was determined to be 16.5 μg·L-1, belowing the maximum residue limit in fruits and vegetables set by the EU. Moreover, the colorimetric biosensor showed excellent selectivity and anti-interference properties, which was validated for detecting PQ residues in several typical agricultural and water samples.