Rational design of an ultrabright quinolinium-fused rhodamine turn-on fluorescent probe for highly sensitive detection of SO2 derivatives: Applications in food safety and bioimaging.

Sulfur dioxide (SO2) is an essential signaling molecule involved in various physiological processes within living organisms. Bisulfite (HSO3-) possesses antioxidant, antimicrobial, and preservative properties, making it a common food additive. However, elevated levels of SO2 or excessive HSO3- intake can lead to a range of diseases, highlighting the importance of detecting SO2 and its derivatives (HSO3-/SO32-). This study presents a quinolinium-fused rhodamine fluorogenic probe (RQB-R) for ultrafast, highly selective, and sensitive detection of HSO3-. The probe operates via a dual-response mechanism, exhibiting a visible color change and a transition from nonemissive to intense red fluorescence upon interaction with HSO3-. The detection mechanism involves a 1,4-nucleophilic addition reaction of HSO3- at the 4-position of the quinolinium unit, which bypasses the photoinduced electron-transfer fluorescence quenching pathway and activates the intramolecular charge transfer mechanism, thereby enhancing fluorescence emission. Practical applications of the RQB-R probe include rapid quantification of HSO3- levels in sugar samples and integration into smartphone-assisted detection platforms. This method demonstrates excellent biocompatibility and enables visualization of both exogenous and endogenous HSO3- within MCF-7 cells, with a specific focus on targeting mitochondria.