A turn-on fluorescent probe for selective detection of peroxynitrite via electrophilic addition mechanism and cellular imaging

Peroxynitrite (ONOO-), a key pathological mediator, demands fast and reliable detection. Herein, we report TADBI, a turn-on fluorescent probe based on triphenylamine-benzo indole framework. TADBI shows a rapid (< 50 s) colorimetric change from violet to colourless, and a strong fluorescence enhancement at 440 nm upon ONOO- sensing, with a low detection limit of 71.8 nM. DFT studies reveal that ONOO- - induced disruption of π- conjugation switches the probe from a non-emissive locally excited state to an ICT state, supressing non-radiative decay enabling fluorescence enhancement. The large Stokes shift reduces excitation-emission overlap and re-absorption, thereby minimizing self-absorption, enhancing signal-to-noise ratio, and clearer biological imaging. Mechanistic studies using ¹H NMR and HRMS revealed that ONOO- undergoes electrophilic addition to the C = C bond, rather than oxidative cleavage, and activating fluorescence. Live-cell imaging in HeLa cells confirm efficient and selective detection of both exogenous and endogenous ONOO-, highlighting TADBI as a promising probe for real-time biological imaging.