A benzimidazole-based highly selective colorimetric and far-red fluorometric pH sensor for intracellular imaging

A turn-on fluorescent probe (E)-4-(2-(1H-benzo[d]imidazol-2-yl)vinyl)-N,N-diphenylaniline (BVD) was facilely synthesized via the ethylene bridging of triphenylamine and benzimidazole. The probe exhibited strong emission enhancement in the far-red fluorescence region at 605 nm with a pKa value of 4.14 and linear response to the acidity range of 4.52–3.37. BVD displayed a large Stokes shift of 105 nm under acidic conditions, high sensitivity, quick response, excellent reversibility and good photostability, which are favorable for monitoring the pH variation in real time. More importantly, BVD had excellent cell membrane permeability and was applied successfully to visualize intracellular pH fluctuations in living cells without the influence of autofluorescence.