G-quadruplex DNAzyme-based Hg 2+ and cysteine sensors utilizing Hg 2+-mediated oligonucleotide switching

Abstract

A simple and sensitive colorimetric Hg 2+ detection method is reported, based on the Hg 2+-mediated structural switch of an unlabeled oligonucleotide strand. In the absence of Hg 2+, the oligonucleotide strand forms a stem-loop. A G-rich sequence in the strand is partially caged in the stem-loop structure and cannot fold into a G-quadruplex. In the presence of Hg 2+, T–Hg 2+–T coordination chemistry leads to the formation of another stem-loop structure and the release of the G-rich sequence. The released sequence folds into a G-quadruplex, which binds hemin to form catalytically active G-quadruplex DNAzymes. This is detected as an absorbance increase in a H 2O 2–2,2′-azinobis(3-ethylbenzothiozoline)-6-sulfonic acid (ABTS) reaction system using UV–vis absorption spectroscopy. This simple colorimetric sensor can detect aqueous Hg 2+ at concentrations as low as 9.2 nM with high selectivity. Based on the strong binding interaction between Hg 2+ and the sulfur-containing amino acid cysteine (Cys), and the competition between Cys and a oligonucleotide for Hg 2+, the proposed Hg 2+-sensing system can be further exploited as a Cys-sensing method. The method has a detection limit for Cys of 19 nM.