A novel near-infrared fluorescent probe with large stokes shifts for sensing extreme acidity and its application in bioimaging.

In this work, we reported a novel near-infrared (NIR) fluorescent probe RQNN with large Stokes shift (98 nm) for monitoring pH under extremely acidic conditions. For the preparation of this probe, a 1,4-diethylpiperazine moiety was introduced in rhodamine scaffold to tune the electron-donating character, and an o-phenylenediamine was introduced in spironolactone to provide larger steric hindrance. The deprotonated-protonated equilibrium between RQNN, RQNN-H and RQNN-H were evaluated in different pH by absorption and emission spectra. As expected, RQNN exhibited lower pk values (pk1 = 4.83, pk2 = 2.99), indicating that the probe can be used in extremely acidic pH. Moreover, RQNN possessed highly selective response to H over essential metal ions and biologically related redox molecules, high photo-stability, rapid response time, and excellent reversibility. Importantly, the probe had excellent cell membrane permeability and was further applied successfully to monitor pH fluctuations in live cells.