Near-infrared probes based on the phenanthridinium for dynamic monitoring of viscosity fluctuations during mitophagy

Abnormal changes in the mitochondrial microenvironment can cause a variety of diseases. Consequently, based on the twisted intramolecular charge transfer (TICT) theory, three hemicyanine fluorescent probes 1a-c were designed and developed by linking the phenanthridine group with different electron donor groups for the dynamic detection of mitochondrial viscosity in this paper. They have the advantages of near-infrared emission, excellent pH stability and viscosity sensitivity. Optical experiments showed that the maximum absorption and emission wavelengths of the probes in different solvents located at 411 − 529 nm and 623 − 684 nm with large Stokes shifts of 133 – 230 nm. In addition, when the viscosity increased from 0.89 cP to 856 cP, their fluorescence intensities in glycerol enhanced by 115-fold, 83-fold and 303-fold compared with water, respectively. The fluorescence intensities of probes showed excellent fit to the Forster Hoffmann equation, and exhibited nearly OFF-ON response to viscosity. Moreover, the probe 1a with good photostability and low cytotoxicity could target mitochondria. It was verified that 1a also could detect variation in mitochondrial viscosity during autophagy by modelling starvation and drug-induced (rapamycin and nystatin) in HeLa cells. Therefore, this study provides a useful probe for the analysis of mitophagy process in HeLa cells by the viscosity variation of mitochondria.