Imaging of cysteine in cells and tumor mice using water-soluble near-infrared fluorescence probe with a large Stokes shift

As a vital biological thiol, cysteine (Cys) is closely linked to cancer progression and plays a pivotal role in protecting the body against oxidative stress. Therefore, the development of near-infrared (NIR) fluorescent probes with high sensitivity and selectivity for in vivo detection of Cys is significant. In this study, a water-soluble, NIR "turn-on" fluorescent probe (CQAI) is developed by integrating coumarin derivatives with acrylate moieties. The introduction of N-methylquinolinium into the fluorophore scaffold endows the probe with significantly enhanced water solubility and a remarkable Stokes shift of 255 nm. The fluorescence of CQAI is initially quenched due to the suppression of the intramolecular charge transfer (ICT) process and the activation of the photoinduced electron transfer (PET) mechanism. Upon interaction with Cys, a distinct fluorescence emission at 755 nm is observed. The dual-quenching mechanism of CQAI offers a high imaging contrast, exceptional selectivity (unaffected by potential interferents such as homocysteine, glutathione, and hydrogen sulfide), and excellent sensitivity, with a detection limit as low as 0.14 μM. Owing to its high cellular permeability and low cytotoxicity, CQAI effectively detects both endogenous and exogenous Cys in living cells. Furthermore, it enables the monitoring of Cys levels in tumor-bearing mice, taking advantage of its long-wavelength fluorescence for deep-tissue imaging. The superior performance of CQAI highlights its outstanding potential as a powerful tool for exploring the physiological and pathological roles of Cys in complex biological systems.