Exonuclease III assisted and label-free detection of mercury ion based on toehold strand displacement amplification strategy

A label-free, exonuclease III (Exo III) assisted and cost-effective Hg2+ fluorometric assay based on a strand displacement amplification strategy was developed with high sensitivity and selectivity. Thymine–thymine (T–T) mismatches between the toehold domains in hairpin DNA and assistant DNA can serve as specific recognition elements for Hg2+ binding with the help of T–Hg2+–T base pairs to initiate a toehold-mediated strand displacement reaction. Exo III provides a reduced background signal, and the method is label-free using an intercalator, Sybr green I (SG). The number of T–T mismatches, the concentration of SG and Exo III, the incubation temperature and time were all optimized. Under the optimized condition, the limit of detection for Hg2+ can be as low as 9.42 pM with a linear range from 0.01 to 1 nM. Moreover, this proposed strategy shows excellent selectivity, and was successfully applied for the detection of Hg2+ in laboratory tap water and Yanjing lake water samples in our university.