A strategy for constructing novel red emitting fluorescent probes for neutrophil elastase tracking based on self-immolative linker and TICT effect

Neutrophil elastase (NE) is a crucial biomarker for the diagnosis of lung related diseases. Until now, most of the basic structure of the reported NE fluorescent probes is connected by the amide bond between the pentafluoropropionyl recognition group and the fluorophore containing an amino group, which extremely limited the application of the fluorophores containing hydroxyl groups. To address this limitation, we have rationally constructed a red emitting NE fluorescent probe, namely SNARF-NE, which consists of a hydroxyl containing rhodamine derivative SNARF-OH and pentafluoropropionyl, connected by a self-immolative linker. As we expected, SNARF-NE was successfully used to detect NE activity in a red emitting fluorescence turn-on model along with favorable sensitivity and ultrahigh selectivity. Furthermore, in order to further improve the response ability of SNARF-NE, a novel fluorophore TSNARF-OH has been redesigned by restricting the TICT effect of SNARF-OH to construct the NE fluorescent probe TSNARF-NE. When used to detect NE levels, as we expected, TSNARF-NE exhibited faster response time, higher quantum yield, longer fluorescence lifetime, lower detection limit and higher catalytic efficiency than SNARF-NE. Most importantly, TSNARF-NE has shown an ability to image and track endogenous NE levels in lung cancer cells and zebrafish model much faster compare with SNARF-NE. All the experimental results clearly demonstrate that the utilization of self-immolative linker and TICT blocking methods is highly valuable for designing a wide range of NE fluorescent probes with outstanding optical properties.