Structural Engineering of Naphthalimide Fluorophore Enables Target-Activated NIR Emission for Switch-On Cysteine...

Cysteine (Cys), as one of the essential amino acids in humans, plays critical roles in numerous physiological processes, and its abnormal levels are closely associated with various diseases. Therefore, rapid and sensitive detection of Cys is of great significance for early disease diagnosis and mechanistic studies. In this work, we developed a novel near-infrared (NIR) fluorescent "turn-on" probe, NA-XL, featuring a large Stokes shift (177 nm) for highly efficient detection of the biothiol Cys. The probe incorporates a naphthalimide fluorophore and an acrylate group that serves as both the Cys recognition site and fluorescence quencher. The fluorescence quenching of NA-XL is predominantly governed by strong low-frequency vibrations. Upon selective reaction with Cys, this vibration-mediated quenching process is effectively suppressed, leading to a pronounced NIR fluorescence emission at 729 nm. The probe demonstrates excellent selectivity and sensitivity, achieving a low detection limit (LOD) of 10.7 nM. Furthermore, NA-XL exhibits outstanding responsiveness to both exogenous and endogenous Cys in cellular imaging, enabling real-time monitoring of Cys dynamics. These findings highlight its potential for biomedical research applications.