Iridium Complex with Specific Intercalation in the G-Quadruplex: A Phosphorescence and Electrochemiluminescence Dual-Mode Homogeneous Biosensor for Enzyme-Free and Label-Free Detection of MicroRNA

Design and exploitation of high-property and multifunctional emitters for achieving immobilization-free, label-free, enzyme-free, and dual-mode determination of microRNA (miRNA) with high sensitivity and selectivity are particularly urgent but remain a huge challenge. Herein, for the first time, we reported an Ir3+-based N-heterocyclic complex, namely, IrPyPt, ascertained its phosphorescence and electrochemiluminescence (ECL) dual-emission behavior in homogeneous solution, and discovered its turn-up phosphorescence and signal-off ECL response to G-quadruplex DNA over other DNAs. Inspired by this, IrPyPt was pioneered as an emitter to combine nucleic acid probes (ON1 and ON2) for developing a phosphorescence and ECL dual-mode homogeneous biosensor, which was applied to label-free, enzyme-free, highly sensitive, and credible analysis of miRNA-21 used as a target analyte. miRNA-21 triggered the hybridization chain reaction of ON1 and ON2 to generate abundant G-quadruplexes, which caught IrPyPt to yield IrPyPt@G-quadruplex, in which the intramolecular rotary motion of IrPyPt was inhibited and IrPyPt was unable to diffuse to the electrode, contributing to a highly boosted phosphorescence emission and a sharply declined ECL emission. With this design, dual-mode analysis of miRNA-21 was achieved with limits of detection down to 1.40 and 0.18 pM, respectively. Furthermore, the dual-mode biosensor was triumphantly applied to determine the concentration of miRNA-21 in cancer cell lysates and thus provided a great potential alternative for miRNA-related diseases' early and accurate diagnosis.