An off-on quinoxaline-based fluorescent probe for lighting up biothiols in living cells and zebrafish

A novel fused four-ring quinoxaline fluorophore and a maleimide receptor were firstly integrated into one molecule to develop a novel, sensitive and highly effective probe for biothiols detection. The fluorescence of quinoxaline fluorophore was quenched by the maleimide group via the photoinduced electron transfer (PET) mechanism. After the Michael addition of the biothiols towards maleimide, the PET process was blocked and the fluorescence restored. The above sensing mechanisms were confirmed by the mass spectroscopy and explained by density functional theory (DFT) calculations, ensuring QX9M as a highly selective probe towards biothiols over other amino acids. Moreover, the probe featured a good linearity range and a low detection limit to Cys, Hcy and GSH. For the practical applications, the probe can be used to monitor the biothiols not only in living cells but also in zebrafish with an off-on process.