Hg2+-mediated stabilization of G-triplex based molecular beacon for label-free fluorescence detection of Hg2+, reduced glutathione, and glutathione reductase activity.

G-triplexes have been reported recently with the similar function to G-quadruplex that can combine with thioflavin T (ThT) and emit strong fluorescence but easier to be controlled and excited. In this work, we report an Hg2+-mediated stabilization of G-triplex based functional molecular beacon (G3TMB) sensing system for the label-free detection of Hg2+, reduced glutathione (GSH), and glutathione reductase (GR) activity. In the presence of Hg2+, the extended G-triplex sequence containing the "T" bases can form a stable hairpin structure due to the strong interactions of "T-Hg2+-T", resulting in the locking of G-tracts in the stem of the G3TMB effectively. However, the hairpin structure of the G3TMB can be opened by the introduction of GSH through the stronger "GSH-Hg2+" interaction. Therefore, by employing the fact that GR can catalyze the reduction of oxidized glutathione (GSSG) into GSH, this concept can be applied to fluorescence "off-on" detection of GR activity, with a linear range of 0.02-30 mU/mL and detection limit of 0.01 mU/mL. This work may expand a new perspective of G-triplex based functional molecular beacon as the label-free fluorescent probes in the detection of small biomolecule and enzyme activity.