An indole-carbazole-based ratiometric emission pH fluorescent probe for imaging extreme acidity

A ratiometric emission fluorescent probe 1,1-dimethyl-2-(2-(N-ethyl-carbazol-3-yl)-vinyl)-1H-benzo[e]indole (CzBI) is facilely synthesized via the ethylene bridging of carbazoaldehyde and benzoindole. The probe exhibits a remarkable emission ratio (F-605 (nm)/F-523 nm) enhancement with a pK(a) value of 2.44, and the linear response over the extremely acidic pH range of 1.50-3.60. CzBI also displays a large Stokes shift of 100 nm under extremely acidic conditions, high sensitivity and excellent reversibility, which are favorable for intracellular acidic pH imaging. In addition, the probe has excellent cell membrane permeability and is applied successfully to visualize intracellular pH fluctuations in live cells and, further monitor extremely acidic pH changes in E. coli cells without influence of autofluorescence and native cellular species in biological systems. (C) 2015 Elsevier B.V. All rights reserved.