In this work, a fluorescent sensing strategy was developed for the detection of mercury(II) ions (Hg2+) in aqueous solution with excellent sensitivity and selectivity using a target-induced DNAzyme cascade with catalytic and molecular beacons (CAMB). In order to construct the biosensor, a Mg2+-dependent DNAzyme was elaborately designed and artificially split into two separate oligonucleotide fragments. In the presence of Hg2+, the specific thymine–Hg2+–thymine (T–Hg2+–T) interaction induced the two fragments to produce the activated Mg2+-dependent DNAzyme, which would hybridize with a hairpin-structured MB substrate to form the CAMB system. Eventually, each target-induced activated DNAzyme could catalyze the cleavage of many MB substrates through true enzymatic multiple turnovers. This would significantly enhance the sensitivity of the Hg2+ sensing system and push the detection limit down to 0.2 nM within a 20 min assay time, much lower than those of most previously reported fluorescence assays. Owning to the strong coordination of Hg2+ to the T–T mismatched pairs, this proposed sensing system exhibited excellent selectivity for Hg2+ detection, even in the presence of 100 times of other interferential metal ions. Furthermore, the applicability of the biosensor for Hg2+ detection in river water samples was demonstrated with satisfactory results. These advantages endow the sensing strategy with a great potential for the simple, rapid, sensitive, and specific detection of Hg2+ from a wide range of real samples.
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