Conformation-Dependent Exonuclease III Activity Mediated by Metal Ions Reshuffling on Thymine-Rich DNA Duplexes for an Ultrasensitive Electrochemical Method for Hg²⁺ Detection

Hg(2+) is known to bind very strongly with T-T mismatches in DNA duplexes to form T-Hg(2+)-T base pairs, the structure of which is stabilized by covalent N-Hg bonds and exhibits bonding strength higher than hydrogen bonds. In this work, we exploit exonuclease III (Exo III) activity on DNA hybrids containing T-Hg(2+)-T base pairs and our experiments show that Hg(2+) ions could intentionally trigger the activity of Exo III toward a designed thymine-rich DNA oligonucleotide (e-T-rich probe) by the conformational change of the probe. Our sensing strategy utilizes this conformation-dependent activity of Exo III, which is controlled through the cyclical shuffling of Hg(2+) ions between the solution phase and the solid DNA hybrid. This interesting attribute has led to the development of an ultrasensitive detection platform for Hg(2+) ions with a detection limit of 0.2 nM and a total assay time within minutes. This simple detection strategy could be used for the detection of other metal ions which exhibit specific interactions with natural or synthetic bases.