Colorimetric aptasensor for sensitive detection of quinclorac based on exonuclease III-assisted cyclic release of phosphorodiamidate morpholino oligomer mimic enzyme strategy

Abstract

Here, a colorimetric aptasensor was constructed for sensitively detecting quinclorac (QNC), a common herbicide. The aptasensor involved a novel amplification strategy and a classical strand displacement strategy. The amplification strategy, termed exonuclease III (Exo III)-assisted cyclic release of phosphorodiamidate morpholino oligomer (PMO) mimic enzyme strategy, was developed based on two new findings on PMO: 1) DNA hybridized with PMO could resist Exo III digestion; 2) a designed G-rich PMO (named P2) could bind to hemin to form a G-quadruplex PMOzyme with peroxidase-like activity. In this strategy, a designed DNA-PMO duplex (D1-P1) completely hybridized with DNA2 (D2) in the other designed DNA-PMO duplex (D2-P2) to trigger D2 degradation by Exo III and cyclic release of P2. After that, the hemin-binding P2 catalyzed colorless tetra-methyl benzidine (TMB) to blue TMB⁺. The cycle process was performed at high Exo III concentrations without strict control and with constant background signals. In that case, the developed strategy was sensitive, efficient, easy to operate, reliable, and ultralow background. Meanwhile, a QNC aptamer was used to develop the strand displacement strategy based on magnetic beads. The colorimetric aptasensor was sensitive and selective for QNC detection with a detection limit of 7.1 ng mL^-1. It was successfully applied to detect QNC in soil and river water with good recovery rates (92-98%) and a relative standard deviation (n = 3) <5%. The success of this study could provide a general reference strategy for developing sensitive aptasensors and other nucleic acid-related sensors.

Keywords: Aptamer; Colorimetric aptasensor; Exonuclease III-assisted recycling amplification; Mimic enzyme; Phosphorodiamidate morpholino oligomer; Quinclorac.