A ratiometric pH probe for intracellular pH imaging

A ratiometric fluorescence pH probe 2-(N-ethylcarbazole-3-vinyl)-benzothiazole ( ECBT ) is facilely developed by ethylene bridging of N-ethyl-3-carbazolecarboxaldehyde and benzothiazole, which is a useful fluorescent probe for strongly acidic pH sensing. The pH titration indicates remarkable fluorescence enhancement effect and an emission red-shift of 87 nm with pH decreasing from 7.00 to 1.00. Furthermore, the solution color changes from light blue to orange under a UV lamp illumination. A p K a value of 1.62 and a good linearity in the pH range of 1.23–2.10 indicate that the probe can be used in quantitative fluorescence imaging under strongly acidic condition. In addition, ECBT exhibits high selectivity and sensitivity, good photostability and excellent reversibility for pH sensing. The probe is further applied successfully to imaging extreme acidity in Escherichia coli cells without influence of autofluorescence in biological systems.