Label-free detection of sub-nanomolar lead(II) ions in aqueous solution using a metal-based luminescent switch-on probe.

A label-free oligonucleotide-based luminescent switch-on assay has been developed for the selective detection of sub-nanomolar Pb(2+) ions in aqueous solution and real water samples. An iridium(III) complex was employed as a G-quadruplex specific luminescent probe and a guanine rich DNA (PS2.M, 5'-GTG(3)TAG(3)CG(3)T(2)G(2)-3') was employed as recognition unit for Pb(2+) ions. The PS2.M exists in a single-stranded conformation in the absence of Pb(2+) ions, and the weak binding of the iridium(III) probe to ssDNA results in a weak luminescence signal. Upon binding to Pb(2+) ions, the single-stranded DNA sequence (PS2.M) is induced into a G-quadruplex conformation, which greatly enhances the luminescence emission of the iridium(III) probe. The assay can detect Pb(2+) ions in aqueous media with a limit of detection of 600 pM. It also exhibits good selectivity for Pb(2+) ions over other heavy metal ions. Furthermore, the application of the assay for the detection of Pb(2+) ions in spiked river water samples has been demonstrated.