pH-Sensitive Near-IR Emitting Dinuclear Ruthenium Complex for Recognition, Two-Photon Luminescent Imaging, and Subcellular Localization of Cancer Cells.

A dinuclear Ru(II) complex of [(bpy)Ru(Hdip)Ru(Hbip)](ClO) {bpy is 2,2'-bipyridine, Hdip is 2-(2,6-di(pyridin-2-yl)-pyridin-4-yl)-1-imidazo[4,5-]-[1,10]phenanthroline, and Hbip is 2,6-bis(imidazole-2-yl)-pyridine} was synthesized and characterized by elemental analysis, mass spectrometry, and H NMR spectroscopy. Spectrophotometric pH titrations in aqueous buffer and in vitro cell experiments indicated the response ability of the complex to pH fluctuations in the physiological pH range (6.0-8.0). The complex was found to be capable of differentiating live HeLa cells from healthy HEK293 cells by selectively accumulating in lysosomes of the HeLa cells. The low cytotoxicity (IC > 100 μM), a large Stokes shift (∼200 nm), strong near-IR emission at ∼700 nm, a relatively long excited state lifetime, high photostability, and solubility make this complex considerably promising in real-time tracking and visualization of lysosomes in live cells. More interestingly, the tumor cell-specific two-photon luminescent imaging properties also endow this Ru complex with potential for applications in high-resolution tumor imaging and luminescence-guided tumor resection.