A dicyanoisophorone-based, near-infrared, lysosome-targeting pH sensor with an extremely large Stokes shift.

Intracellular pH plays an important role in various biological processes; abnormal pH changes in the intracellular compartment leads to the production of free radicals, the disruption of membrane contractility, inappropriate apoptosis, and necrosis, resulting in serious illness. Although fluorescent probes have widely been used to detect pH levels owing to their high sensitivity and specificity, there is still a demand for near-infrared (NIR) fluorescent probes with high Stokes shift. Here, a NIR fluorescent probe, PipDC, comprising N-ethyl piperazine (response unit) and naphthyl dicyanoisophorone (fluorophore), was designed for pH sensing. The probe has an extremely large Stokes shift (290 nm), and its fluorescence intensity at 730 nm sharply increases when the environment changes from basic to acidic owing to the protonation of piperazine, which results in the quenching of the photoinduced electron transfer effect. It exhibited a specific response to acidic microenvironments regardless of other interfering substances. In addition, PipDC operates well in the lysosome environment in living cells and displays an off-on fluorescence response with pH alterations. Together, these results suggest that PipDC is a promising fluorescent probe for intracellular pH sensing.