An ultrasensitive conformation-dependent colorimetric assay has been developed for the detection of mercury(II) ions. It is based
on the use of exonuclease III (Exo III)-assisted target recycling and gold nanoparticles (AuNPs). In the absence of Hg(II), the
hairpin-shaped DNA probe (H-DNA) binds to AuNPs and stabilizes them in solutions of high ionic strength. In the presence of
Hg(II), on the other hand, the sticky termini of the H-DNA form a rigid DNA duplex stem with a blunt 3′-terminus. Thus, Exo III
is activated as a biocatalyst for selective and stepwise removal of mononucleotides from the 3′-terminus of the H-DNA. As a
result, Hg(II) is released from the T−Hg(II)−T complexes. The guanine-rich sequences released from the H-DNA are then selfassembled with potassium ion to form a stable G-quadruplex conformation. In solutions of high ionic strength, this results in
aggregation of AuNPs and a color change from red to blue which can be seen with bare eyes. The method is highly sensitive and
selective. It has a linear response in the 10 pM to 100 nM Hg(II) concentration range, and the detection limit is as low as 3.2 pM
(at an S/N ratio of 3). The relative standard deviation at a level of 0.5 nM of Hg(II) is 4.9% (for n = 10). The method was applied
to the detection of Hg(II) in spiked environment water samples, with recoveries ranging from 92% to 106%.

• An ultrasensitive conformation-dependent colorimetric probe for the detection of mercury(II) using exonuclease III-assisted target recycling and gold nanoparticles