Label-free hairpin DNA-scaffolded silver nanoclusters for fluorescent detection of Hg²⁺ using exonuclease III-assisted target recycling amplification.

A new label-free DNA sensing protocol was designed for fluorescent detection of mercury(II) (Hg(2+)), coupling hairpin DNA-scaffolded silver nanocluster (DNA-AgNC) with exonuclease III-assisted target recycling amplification. The assay was carried out through target-induced conformational change of hairpin DNA, while the signal derived from the formed silver nanoclusters on hairpin DNA probes. Initially, target Hg(2+) was specifically coordinated with thymine-thymine (T-T) mismatches to form an intact hairpin DNA. Then, the newly formed hairpin DNA was digested through exonuclease III from blunt 3' termini and restrained at 3' protruding terminus, thus resulting in the release of target Hg(2+) from hairpin DNA. The liberated target Hg(2+) initiated the next cycling, thereby causing the conformational change of numerous hairpin probes from the stem-loop DNA structure to single-stranded DNA. Under the optimal conditions, the fluorescent intensity of the as-produced DNA-AgNCs decreased with the increasing Hg(2+) concentration within a dynamic range from 0.1 nM to 10nM with a detection limit (LOD) of 24 pM. Moreover, the low-cost fluorescent sensing system exhibited high reproducibility and good specificity, thus representing an optional sensing platform for rapid screening of Hg(2+) in environmental water samples.