A ratiometric lysosomal pH probe based on the imidazo[1,5-a]pyridine–rhodamine FRET and ICT system

Abstract

A new pH-activatable ratiometric fluorescent probe (RhMP) has been developed, in which an imidazo[1,5-a]pyridine fluorophore as a fluorescence resonance energy transfer (FRET) donor linked to a rhodamine B fluorophore as a FRET acceptor. The simultaneous fluorescence intensity enhancement of imidazo[1,5-a]pyridine and rhodamine B moieties along with the decrease of pH value should result from the integration of ICT and FRET processes. It's the first time a ratiometric fluorescent probe based on FRET system using imidazo[1,5-a]pyridine derivative as a fluorophore donor has been reported. At pK_a = 4.96, the fluorescence intensity ratio (I_476.5/I_577.5) of the probe displayed excellent pH-dependent performance and responded linearly to minor pH changes in the range of 4.0–5.8. The probe exhibited excellent selectivity among different metal cations and brilliant reversibility. In addition, RhMP has low cytotoxicity and has been successfully applied in HeLa cells. The fluorescence microscopic images demonstrated this probe could image weak acid pH changes of the lysosome in live cells.