Label-free colorimetric detection of divalent mercuric ions (Hg2+) based on T-Hg2+-T structure and exonuclease III dual-recycling and G-quadruplex-hemin DNAzyme amplification

Detection of divalent mercury attracts great significance in most biological and environmental systems. In this study, a Thymine-divalent mercuric ions-Thymine (T-Hg2+-T) structure based colorimetric assay with Exonuclease III (Exo III) assisted recycling and G-quadruplex-hemin DNAzyme amplification is put forward for Hg(2+)monitoring. The Hg(2+)can combine with T-T mismatches of oligo-1 to form T-Hg2+-T base pairs, which brings forth a double-helix structure with a blunt terminus. Short mononucleotide fragment (trigger DNA) and Hg(2+)could be released as the double-helix structure in oligo-1 is cleaved by Exo III. The released Hg(2+)can be captured by another oligo-1, and generate the new cycles. At the same time, signal DNA (oligo-2) can hybridize with trigger DNA to form double-stranded DNA with a blunt terminus that can be cleaved by Exo III to accrue the trigger DNA and guanine (G)-quadruplex fragment. The trigger DNA can hybridize with another signal DNA, which implements the dual-recycling amplification. The G-quadruplex fragment can form the G-quadruplex-hemin DNAzymes, which can catalyze the oxidation of 2, 2MODIFIER LETTER PRIME-azino-bis(3-ethylbenzothiazo-line-6-sulfonate) disodium salt (ABTS(2-)) by H2O2. The color change, served as the signal output, can be measured with an ultraviolet spectrophotometer. The proposed assay allows Hg(2+)to be detected in the range of 25 pM to 2000 pM with a detection limit of 7 pM. Moreover, it has been commendably applied to the determination of Hg(2+)in environmental samples and biological samples.