G-quadruplex DNAzyme-based Hg2+ and cysteine sensors utilizing Hg2+-mediated oligonucleotide switching

A simple and sensitive colorimetric Hg2+ detection method is reported, based on the Hg2+-mediated structural switch of an unlabeled oligonucleotide strand. In the absence of Hg2+, 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 Hg2+,T-Hg2+-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 H2O2-2,2'-azinobis(3-ethylbenzothiozoline)-6-sulfonic acid (ABTS) reaction system using UV-vis absorption spectroscopy. This simple calorimetric sensor can detect aqueous Hg2+ at concentrations as low as 9.2 nM with high selectivity. Based on the strong binding interaction between Hg2+ and the sulfur-containing amino acid cysteine (Cys), and the competition between Cys and a oligonucleotide for Hg2+, the proposed Hg2+-sensing system can be further exploited as a Cys-sensing method. The method has a detection limit for Cys of 19 nM. (C) 2011 Elsevier B.V. All rights reserved.