Thiazole-based ratiometric fluorescence pH probe with large Stokes shift for intracellular imaging  

    A ratiometric fluorescent pH probe 4-((1E, 3E)-4-(benzo[d]thiazol-2-yl)buta-1, 3-dienyl)-N, Ndimethylbenzenamine (BTDB) was synthesized via ethylene bridging of thiazol and cinnamaldehyde for extremely acidic sensing. BTDB exhibits ratiometric fluorescence emission (I425 nm/I595 nm) characteristics with pKa of 2.34 and a large Stokes shift of 177 nm under acidic conditions. The linear response to pH is in the range of 2.3–4.0. Quantum chemical calculations with the B3LYP exchange functional theory demonstrated that the ratiometric response of the probe to acidic pH was due to H+ binding with the N of cinnamaldehyde and the induced decreases of the intramolecular charger transfer (ICT) process. The as-prepared probe displayed excellent cell membrane permeability was further applied successfully to monitor pH fluctuations in live cells with excellent lysosomal targeting ability, and extreme acidity in Escherichia coli cells.