A profluorescent ratiometric probe for intracellular pH imaging.

A ratiometric pH probe composed of a fluorescein moiety and an ionic near-infrared-emitting phosphorescent cyclometalated iridium(III) complex bis(6-(benzo[b]thien-2-yl)phenanthridinato)(4-(3-carboxypropyl)-4'-methyl-2,2'-bipyridine)iridium(III) was synthesized. With good cell permeability, the probe demonstrated a linear ratiometric response to the pH variation in the physiological range in HeLa cell assay.