A parallel G-quadruplex-selective luminescent probe for the detection of nanomolar calcium(II) ion

A parallel G-quadruplex-selective iridium(III) complex has been synthesized and employed as a luminescent probe in a label-free G-quadruplex-based detection assay for Ca²⁺ ions in aqueous solution. In this assay, a guanine-rich oligonucleotide (G4, 5'-G₄T₄G₄-3') initially exists in an antiparallel G-quadruplex conformation, resulting in a low luminescence signal. Upon incubation with Ca²⁺ ions, the antiparallel G-quadruplex is induced into a parallel G-quadruplex conformation, which greatly enhances the luminescence emission of the iridium(III) probe. This method was highly sensitive for Ca²⁺ ions with a limit of detection in the nanomolar range, and was selective for Ca²⁺ over other metal ions.