High-depth fluorescence imaging using a two-photon FRET system for mitochondrial pH in live cells and tissues

We developed a fluorescent pH probe (1) capable of two-photon excitation and far-visible-emission based on FRET, composed of naphthalimide–piperazine–rhodamine. It exhibited a pH-dependent reversible and fast ratiometric fluorescence change in the rhodamine emission. Probe 1 was applied to image the pH perturbations of mitochondria in living cells and tissues.