Mitochondria-targeting probes with large Stokes shift for detecting Amyloid-β and cellular viscosity changes

For effective in vivo applications, imaging probes must exhibit sufficient tissue penetration depth, high sensitivity, and specificity. Increasing evidence suggests that pathological accumulation of Aβ results in elevated mitochondrial viscosity. To achieve red-shifted absorption and emission characteristics of small-molecule theranostic agents and to enhance their mitochondrial targeting efficiency, a series of M-series probes (M13 ∼ M15) was rationally designed based on the previously reported Q-series compounds. Using compound Q16 as the parent structure, the M series probes retained the electron-donating dimethylamino group while replacing the benzene ring with a quinoline moiety. This modification was intended to enhance the intramolecular charge transfer (ICT) effect of the "D-π-A" system, thereby red-shifting the fluorescence emission wavelength and expanding the Stokes shift. The enhanced push-pull effect induced a redshift in the emission wavelength of probe M13 to 806 nm in DMSO, resulting in a Stokes shift of 266 nm. This large Stokes shift effectively minimizes the overlap between excitation and emission wavelengths, thereby reducing self-quenching effects. Building on this, the interactions between M-series probes and Aβ aggregates were further explored. The probes exhibited the expected fluorescence characteristics and displayed varying degrees of response upon binding with Aβ aggregates. To enable a more precise early diagnosis, M13, M14, and M15 were evaluated for their ability to monitor changes in mitochondrial viscosity and their mitochondrial targeting efficiency. The results demonstrated that the M-series fluorescent probes could effectively monitor variations in mitochondrial viscosity in cells. All three probes demonstrated strong mitochondrial targeting in HeLa cells, with M14 achieving a high colocalization coefficient of 0.89 when compared with a commercial mitochondrial dye. These findings highlight the potential application of M-series probes in the early diagnosis and treatment of Alzheimer's disease (AD).