A ratiometric emission NIR-fluorescent probe for sensing and imaging pH changes in live cells

A novel ratiometric emission MR-fluorescence probe N,N-dimethyl-4-((1E, 3E)-4-(3,3-dimethyl-3H-indol-2-yl) buta-1,3-dienyl)benzenamine, namely, DIDBA, is facilely synthesized on the basis of ethylene bridging of indole derivatives and 4-(dimethylamino) cinnamaldehyde. The probe exhibits a remarkable NIR ratiometric fluorescence emission (F-618nm/F-697nm) characteristic with pK(a) 4.5, and the linear response over the pH range of 3.5-5.2. DIDBA also displays favorable features include large Stokes shift of 200 nm under acidic conditions, high sensitivity, good selectivity, fine photostability and reversibility. The probe has low cytotoxicity and excellent cell membrane permeability, and is applied successfully to visualize intracellular pH fluctuations in live cells without influence of autofluorescence and native cellular species in biological systems.