Twisted Intramolecular Charge Shuttle (TICS) Enables Ultrafast Ratiometric Sensing and Imaging of Nitrite

Ratiometric fluorescence sensing offers superior interference resistance and self-calibration, yet designing these probes remains challenging due to a lack of generalizable mechanisms. Herein, we introduce a novel, mechanism-driven strategy based on the twisted intramolecular charge shuttle (TICS) process to achieve ultrafast, sensitive nitrite (NO₂ ^-) ratiometric detection. By strategically installing an o-phenylenediamine (OPD) fragment at the reactive meso-position of a pyronin fluorophore, we successfully induced and modulated the TICS process through a specific reaction with NO₂ ^-. This generates a robust ratiometric response with a remarkably low detection limit (6.6 nM) and an ultrafast response time of 18 s-two orders of magnitude faster than the classical Griess method. To demonstrate practical utility, we incorporated the probe into a portable device for rapid, visual on-site quantification of nitrite in food and water. Furthermore, the probe successfully imaged endogenous NO₂ ^- dynamics in living plants, revealing a direct correlation between nitrite fluctuations and environmental stress. This study establishes TICS as a powerful platform for probe design, marking a significant transition from empirical dye screening toward rational, mechanism-driven molecular engineering.