Near-infrared hybrid rhodol dyes (probes A and B) for sensitive ratiometric visualization of pH changes were prepared by incorporating hemicyanine dyes into traditional rhodol dyes. This approach was based on π-conjugation changes involving a rhodol hydroxyl group as a spiropyran switch upon pH changes. Electronic spectra of probes A-2 and B-2 contain sharp absorption peaks at 535 nm and fluorescence peaks at 558 nm with similar π-conjugation and a closed spiropyran form at a basic pH of 10.2. However, acidic pH conditions break down the hemiaminal ether groups, leading to indolenium moieties and significantly extending the π-conjugation within the rhodol fluorophores, resulting in additional near-infrared emissions for probes A-1 and B-1. As a result, probes A and B exhibit gradual decreases of the absorption peaks at 535 nm and gradual increases in absorption peaks at 609 and 622 nm upon transition from basic to acidic pH, respectively. Both probes display ratiometric fluorescence sensing responses to pH downgrades from 10.2 to 3.6 with visible fluorescence decreases at 558 nm, as well as corresponding increases of the near-infrared fluorescence peaks at 688 and 698 nm, respectively. They exhibit fast, sensitive, and selective fluorescence responses with clearly defined ratiometric features to pH changes and show low cytotoxicity and excellent cell permeability. Our probes were successfully applied to ratiometrically detect pH changes in mitochondria, D. melanogaster first-instar larvae, and to visualize the mitophagy process caused by either cell nutrient starvation or drug treatment.
Keywords: mitochondria; near-infrared fluorescence; pH; ratiometric imaging; rhodol dye; spiropyran switch.