Fluorescence detection of mercury(II) and lead(II) ions using aptamer/reporter conjugates

We have developed a fluorescence technique for the detection of Hg2+ and Pb2+ ions using polythymine (T33)/benzothiazolium-4-quinolinium dimer derivative (TOTO-3) and polyguanine (G33)/terbium ions (Tb3+) conjugates, respectively. Hg2+ ions induce T33 to form folded structures, leading to increased fluorescence of the T33/TOTO-3 conjugates. Because Pb2+ ions compete with Tb3+ ions to form complexes with G33, the extent of formation of the G33–Tb3+ complexes decreases upon increasing the Pb2+ concentration, leading to decreased fluorescence at 545 nm when excited at 290 nm. To minimize interference from Hg2+ ions during the detection of Pb2+ ions, we conducted two-step fluorescence measurements; prior to addition of the G33/Tb3+ probe, we recorded the fluorescence of a mixture of the T33/TOTO-3 conjugates and Hg2+ ions. The fluorescence signal obtained was linear with respect to the Hg2+ concentration over the range 25.0–500 nM (R2 = 0.99); for Pb2+ ions, it was linear over the range 3.0–50 nM (R2 = 0.98). The limits of detection (at a signal-to-noise ratio of 3) for Hg2+ and Pb2+ ions were 10.0 and 1.0 nM, respectively. Relative to other techniques for the detection of Hg2+ and Pb2+ ions in soil and water samples, our present approach is simpler, faster, and more cost-effective.