Near-Infrared Fluorescent Probe with pH- and Viscosity-Switchable Performance for the Detection of Thrombi in Live Animals and Organs

Blood viscosity changes and blood clots are high-impact diseases, but the pathogenic mechanisms and detection methods are still limited. Due to the complexity of the cellular microenvironment, viscosity is a key factor in regulating the behavior of mitochondria and lysosomes in cells. Conventional fluorescence probes are highly restrictive for complex viscosity detection in live animals. Therefore, we developed two near-infrared fluorescence probes, QL1 and QL2, with dual responses to the pH and viscosity. Notably, QL2 has two maximum fluorescence emissions at 680 and 750 nm, when excitation by 580 and 700 nm, respectively. QL2 exhibited both a pH and viscosity switchable fluorescence response. The two emission peaks exhibited a reverse change trend: the fluorescence at 680 nm decreased by 90%, and the fluorescence at 750 nm increased by about 5-fold with pH from 2 to 10. Meanwhile, both emission peaks show remarkable fluorescence enhancement toward viscosity change, with 185 and 32 times enhancement, respectively. The sensing mechanism and spectral changes are confirmed by DFT calculations. QL2 was further used for viscosity imaging in live cells, zebrafish, and live animals. Most importantly, QL2 is able to successfully track changes in blood clots in live mice and organs, thus enabling the study of blood clots in cerebral strokes and the underlying pathological mechanisms.