Autocatalytic DNA circuit for Hg2+ detection with high sensitivity and selectivity based on exonuclease III and G-quadruplex DNAzyme

Utilizing G-quadruplex as the signal report probe, an ultrasensitive and label-free autocatalytic DNA circuit for Hg2+ detection on the basis of exonuclease III (Exo III)-assisted cascade signal amplification has been proposed. In the absence of Hg2+, the hairpin A and the DNA1 cannot hybridize due to the thymine-thymine (T-T) mismatches. Therefore, hairpin probes with the 3′-protruding terminus can be resistant to Exo III digestion, preventing the G-rich sequence to be released. In the presence of Hg2+, the combination of the DNA1 with the 3′ end-extruding hairpin A via T-Hg2+-T coordination chemistry triggers the digestion reaction of Exo III, leading to the release of the DNA1 and the sequence with domains c, d, and e. Both of the DNA1 and the sequence with domains c, d, and e can combine with other hairpin probes and activate another round of the cleavage reaction. The produced G-rich sequence can form G-quadruplex structure by binding with N-Methyl mesoporphyrin IX (NMM). The biosensor exhibits excellent selectivity and high sensitivity for Hg2+. The linear range of this biosensor is from 10 fM to 100 nM, and the linear equation can be expressed as: F610 = 1.3 × 105 Lg C + 7.40 × 104 (R2 = 0.998), in which F610 is the fluorescence intensity at 610 nm, C represents the Hg2+ concentrations, and Lg is the logarithm of 10. The detection limit is 10 fM. The biosensor is robust and can be applied to the detection of Hg2+ in water samples. By substituting the target-recognition elements, this sensing system can also be used for the detection of other metal ions.