Label-free DNA Y junction for detection of Hg2+ using exonuclease III or graphene oxide-assisted background reduction

A label-free DNA Y junction sensor for amplified detection of Hg 2+ has been demonstrated by using thymine-Hg 2+ -thymine (T-Hg 2+ -T) recognition, cascade toehold-mediated DNA strand displacement and dye staining double-stranded DNA (dsDNA). In our work, assistant DNA could bind with the first hairpin DNA with toehold domains by the help of thymine-Hg 2+ -thymine (T–Hg 2+ –T) base pairs recognition. The second hairpin DNA with toehold domains will then hybridize with the complementary sequences in first hairpin DNA, which again initiates a new toehold domains to open the third hairpin DNA, triggering a cascade strand displacement reaction. Exonuclease III (Exo III) was added to digest the three hairpin DNA with recessed 3′ ends in the absence of Hg 2+ to reduce the background signal. As a comparison, Exo III is replaced by GO to reduce the background signal because single-strand DNA (ssDNA), three hairpin DNAs with intercalated dye could be adsorbed onto the surface of GO and lead to quenched fluorescence. The method, designed in a simple “mix, incubate, and detect” format, can detect concentrations of Hg 2+ as low as 0.031 nM, and exhibit good selectivity and practical application.