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

We have developed a fluorescence technique for the detection of Hg ²⁺ and Pb²⁺ ions using polythymine (T₃₃)/ benzothiazolium-4-quinolinium dimer derivative (TOTO-3) and polyguanine (G ₃₃)/terbium ions (Tb³⁺) conjugates, respectively. Hg ²⁺ ions induce T₃₃ to form folded structures, leading to increased fluorescence of the T₃₃/TOTO-3 conjugates. Because Pb ²⁺ 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 Pb ²⁺ 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 Hg ²⁺ and Pb²⁺ ions were 10.0 and 1.0 nM, respectively. Relative to other techniques for the detection of Hg²⁺ and Pb ²⁺ ions in soil and water samples, our present approach is simpler, faster, and more cost-effective.