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To solve the requirement of on-site, rapid, and visual detection of copper (II) (Cu 2+ ) in aqueous solution, a turn-off sensor for detecting copper (II) ion was developed based on Cu 2+ -dependent DNAzyme as the recognition element and hybridization chain reaction (HCR)-based horseradish peroxidase (HRP) concatemers as the signal amplifier and the signal report element. The detection unit, which was composed of the immobilized Cu 2+ -dependent DNAzyme coupled with HCR-based HRP concatemers via Waston-Crick base pairing, could catalyze hydrogen peroxide (H 2 O 2 ) via TMB, generating obvious green color and turning yellow after sulfuric acid termination with optical absorption at 450 nm. Upon Cu 2+  addition, the substrate strand of the Cu 2+ -dependent DNAzyme concatenated with the HCR-based HRP complex was irreversibly cleaved, efficiently causing dramatic reduction of the detection signal. Under optimal conditions, the detection signal decreased with the concentration of Cu 2+  in 5 min, exhibiting a linear calibration from 0.05 to 3 μM with a detection limit of 8 nM. The sensor also displayed a high selectivity for Cu 2+  given the specificity and anti-interference of the detection unit, and this system was applicable for monitoring Cu 2+  in real water samples. Generally speaking, the proposed sensor exhibits good potential in environment surveys.

A rapid and visual turn-off sensor for detecting copper (II) ion based on DNAzyme coupled with HCR-based HRP concatemers