A ratiometric fluorescent probe for the detection of peroxynitrite in environmental and plant systems

Peroxynitrite (ONOO^-) plays a dual role in plant stress responses and environmental redox processes. However, its real-time, in situ detection remains challenging. Here, we report a novel ratiometric fluorescent probe based on a carbazole-quinolinium scaffold. Owing to the intramolecular charge transfer (ICT) effect, this probe displays intense red fluorescence (λ_em = 645 nm). Upon reaction with ONOO^-, the conjugated double bond undergoes oxidative cleavage, leading to disruption of the π-conjugated system and consequent attenuation of the ICT effect. As a result, the red emission is substantially quenched, while pronounced blue fluorescence emerges (λ_em = 460 nm). This distinct fluorescence shift enables the probe to serve as a robust ratiometric platform for the convenient and quantitative detection of ONOO^-. It exhibits a low detection limit of 0.039 µM, excellent selectivity over various interfering species and functions effectively across a broad pH range. Crucially, the probe successfully demonstrates practical applications in detecting ONOO^- spiked into environmental water and soil samples, fabricating portable test strips for convenient on-site analysis, and achieving in situ imaging in plant tissues, effectively circumventing the issue of blue autofluorescence. This work provides a versatile and reliable tool for monitoring ONOO^- dynamics in both environmental and plant systems.