Label-free colorimetric detection of aqueous mercury ion (Hg2+) using Hg2+-modulated G-quadruplex-based DNAzymes

Mercury ion (Hg(2+)) is able to specifically bind to the thymine-thymine (T-T) base pair in a DNA duplex, thus providing a rationale for DNA-based selective detection of Hg(2+) with various means. In this work, we for the first time utilize the Hg(2+)-mediated T-T base pair to modulate the proper folding of G-quadruplex DNAs and inhibit the DNAzyme activity, thereby pioneering a facile approach to sense Hg(2+) with colorimetry. Two bimolecular DNA G-quadruplexes containing many T residues are adopted here, which function well in low- and high-salt conditions, respectively. These G-quadruplex DNAs are able to bind hemin to form the peroxidase-like DNAzymes in the folded state. Upon addition of Hg(2+), the proper folding of G-quadruplex DNAs is inhibited due to the formation of T-Hg(2+)-T complex. This is reflected by the notable change of the Soret band of hemin when investigated by using UV-vis absorption spectroscopy. As a result of Hg(2+) inhibition, a sharp decrease in the catalytic activity toward the H(2)O(2)-mediated oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)diammonium salt (ABTS) is observed, accompanied by a change in solution color. Through this approach, aqueous Hg(2+) can be detected at 50 nM (10 ppb) with colorimetry in a facile way, with high selectivity against other metal ions. These results indicate our introduced label-free method for colorimetric Hg(2+) detection is simple, quantitative, sensitive, and highly selective.