A ratiometric lysosomal pH chemosensor based on fluorescence resonance energy transfer

In this work, we presented a naphthalimide-rhodamine based fluorescence resonance energy transfer system (FRET) NR1 as a ratiometric and intracellular pH probe, in which 1,2,3-triazole was identified as an ideal bridge and biocompatibility. It could selectively monitor pH variations in methanol/HEPES solution at room temperature. When the pH changed from 6.20 to 2.00, both the fluorescence intensities at 580 nm and the intensity ratios, R (I _{580 nm}/I_{538 nm}) increased, which allowed the detection of pH changes by both normal fluorescence and ratiometric fluorescence methods. The observation is consistent with the increased FRET from the 1,8-naphthalimide (donor) to the ring-opened, colored form of rhodamine (acceptor). Moreover, as NR1 is lysosomal with low cytotoxicity, it will be helpful for investigating the pivotal role of H⁺ in a biological context, especially in lysosomes through direct intracellular imaging.