Label-free detection of Hg2+ based on Hg2+-triggered toehold binding, Exonuclease III assisted target recycling and hybridization chain reaction

We constructed a fluorometric and sensitive analysis of mercury (II) based on toehold-mediated strand displacement reaction, exonuclease III (Exo III) assisted target recycling, hybridization chain reaction (HCR), fluorescence intercalator SYBR Green I (SG) and grapheme oxide (GO). Firstly, toehold-mediated strand displacement reaction was constructed by using thymine-Hg 2+ -thymine (T-Hg 2+ -T) recognition mechanism between the helper hairpin DNA and assisted DNA. And then, the Exo III disintegrated the double-stranded DNA (dsDNA) by catalyzing the gradual removal of mononucleotides from 3′-hydroxyl terminus of dsDNA with a blunt 3′ terminus. It resulted in the target and assisted DNA recycling and released a trigger strand DNA at the same time. Finally, the trigger strand DNA initiated the hybridization chain reaction (HCR), forming long double helices, which achieved a secondary amplification due to the continuous target recycling, trigger strand DNA release and HCR. The number of T-T mismatches, the concentration of SG, Exo III and GO, the incubation time of Exo III were all optimized. Under the optimized condition, the limit of detection for Hg 2+ can be down to 7.37 pM with a linear range from 0 to 1.5   nM. This sensor exhibits high selectivity against other metal ions, and it works well for real samples.