Enzyme-Free Colorimetric Detection of Cu²⁺ by Utilizing Target-Triggered DNAzymes and Toehold-Mediated DNA Strand...

A new enzyme-free system for colorimetric Cu²⁺ detection, which relies on target-triggered DNAzymes and toehold-mediated DNA strand-displacement circuits, is described. The system employs a DNAzyme designed to undergo self-cleavage in the presence of Cu²⁺ and release a catalyst strand that triggers a sequential toehold-mediated strand displacement reaction. This event leads to the release of a split G-quadruplex DNAzyme strand that is initially caged and inactivated by a blocker strand. A fuel strand is further incorporated for the recycling of the catalyst strand to promote another toehold-mediated strand displacement event, which consequently produces a large number of active split G-quadruplex DNAzymes. By employing this design principle, target Cu²⁺ was very successfully identified with a detection limit of 1.31 nm based on the distinct colorimetric signal developed by the oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid promoted by the peroxidase mimicking activity of the released G-quadruplex DNAzymes. Finally, the practical capability of this sensing system was very successfully demonstrated by its use to reliably determine Cu²⁺ in tap water.