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

Sulfur dioxide (SO) is an essential signaling molecule involved in various physiological processes within living organisms. Bisulfite (HSO) possesses antioxidant, antimicrobial, and preservative properties, making it a common food additive. However, elevated levels of SO or excessive HSO intake can lead to a range of diseases, highlighting the importance of detecting SO and its derivatives (HSO/SO). This study presents a quinolinium-fused rhodamine fluorogenic probe (RQB-R) for ultrafast, highly selective, and sensitive detection of HSO. 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 HSO. The detection mechanism involves a 1,4-nucleophilic addition reaction of HSO 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 HSO levels in sugar samples and integration into smartphone-assisted detection platforms. This method demonstrates excellent biocompatibility and enables visualization of both exogenous and endogenous HSO within MCF-7 cells, with a specific focus on targeting mitochondria.