A Step Toward ESIPT-Based Mitochondrial Probe That Responds to ATP Level

An excited state intramolecular proton transfer (ESIPT) probe with a benzoindolium terminal group has been synthesized, whose fluorescence shows large Stokes' shift (Δλ≈ 250 nm) and good fluorescence quantum yield (λ_em≈ 715 nm, φ_fl≈0.2 in CH₂Cl₂). Spectroscopic studies suggests that the probe was also involved in a minor equilibrium Ar-OH (λ_em≈ 715 nm) ↔ Ar-O^- (λ_em~ 610 nm) + H⁺, resulting from deprotonation of phenolic proton. This made it possible for two channel responses. When being used to stain biological cells, the probe exhibited excellent selectivity toward intracellular mitochondria, but giving unusually strong emission from ~600 nm. And NIR emission was only observable when the cellular ATP production was inhibited. The study thus illustrated a unique reaction-based probe for detecting ATP in the intracellular organelle.