An endoplasmic reticulum-targeting NIR fluorescent probe for viscosity imaging in vitro and vivo

The endoplasmic reticulum (ER), a central organelle, play critical roles in protein synthesis, folding and detoxification. Viscosity within the ER lumen is recognized as an essential physical property for maintaining its normal functions, and its dysregulation has been associated with numerous diseases and aging processes. Thus, detecting change of viscosity was meaningful in ER. In this work, a near-infrared (NIR) fluorescent probe (BEQ-ER) with a classic D-π-A structure is designed to measure viscosity fluctuation in ER relying on twisted intramolecular charge transfer (TICT) mechanism. BEQ-ER exhibited strong fluorescence at 682 nm under conditions of high viscosity due to suppressed intramolecular rotation. Moreover, the image results showed BEQ-ER can precisely target ER with a colocalization coefficient of 0.964, and high viscosity was detected in cancer cells. Importantly, BEQ-ER was shown to selectively illuminate tumor tissues in 4 T1 tumor-bearing mice. Therefore, this work provided a valuable tool for investigating disease mechanisms and progression through real-time monitoring of ER viscosity.