Label-free fluorescence "turn-on" strategy for mercury (II) detection based on the T-Hg2+-T configuration and the DNA-sensitized luminescence of terbium (III).

A novel label-free fluorescence "turn-on" strategy was developed for the sensitive detection of Hg2+ based on the thymine-Hg2+-thymine (T-Hg2+-T) coordination and the fact that single-stranded DNA (ssDNA) greatly enhances the fluorescence of terbium (III) (Tb3+), but double-stranded DNA (dsDNA) does not. In the absence of Hg2+, the mercury-specific DNA (MSD) hybridized with the corresponding complementary strand (cDNA) to form a double helix structure in solution based on Watson-Crick base pairings, which cannot enhance the fluorescence of Tb3+. In the presence of Hg2+, MSD preferentially bound with Hg2+ to form the T-Hg2+-T complex due to the strong affinity of Hg(II) for the T bases of DNA, thus avoiding the hybridization of the cDNA to MSD. The free cDNA can greatly enhance the emission of Tb3+, leading to a sharp increase in fluorescence intensity. Under the optimized conditions, the increased fluorescence intensity was proportional to the concentration of Hg2+ in the range from 10 to 600 nM, and this method can detect concentrations of Hg2+ as low as 0.24 nM. Moreover, satisfactory results were obtained for the detection of Hg2+ in river water and fish samples, and the results were consistent with those from the atomic fluorescence spectroscopy (AFS). Thus, the fluorescence characteristics of Tb3+ were here used in a "turn on" approach to detect Hg2+, which represents a new opportunity for Hg2+ analysis in the field of environmental monitoring and food safety.