A near-infrared fluorescent probe derived from naphthalimide for detecting cysteine within lysosomes

Cysteine (Cys) is a small molecule thiol and an important component of proteases in lysosomes. Developing fluorescent probes specifically for detecting lysosomal Cys holds significant importance for elucidating its functions in lysosomal redox homeostasis and protease activity. In this study, a near-infrared fluorescent probe was developed to detect Cys within lysosomes. A near-infrared fluorophore was constructed by conjugating dicyanoisophorone (DCI) with 1,8-naphthalimide. The acrylate unit functioned as the recognition moiety for cysteine, while the fluorophore was modified with morpholine to target lysosomes. When Cys was absent, the developed probe only emitted faint fluorescence, attributed to the inhibition of the intramolecular charge transfer (ICT) process. Upon Cys addition, the ICT process was activated, resulting in fluorescence emission at 688 nm. The linear range of the probe for Cys ranged from 0.2 to 10 μM and the detection limit was 0.036 μM. The near-infrared probe for Cys exhibited strong specificity, high sensitivity, short response time and a wide working pH range. Additionally, the probe exhibited negligible cytotoxicity and had been effectively exploited to determine endogenous and exogenous Cys in lysosomes of A549 cells. It had also been effectively used to detect Cys in zebrafish and mice.