An ESIPT-based AIE fluorescent probe to visualize mitochondrial hydrogen peroxide and its application in living cells and rheumatoid arthritis.

As a chronic inflammatory disease, rheumatoid arthritis (RA) can cause progressive damage to joints and various organs. Hydrogen peroxide plays a significant role in the pathogenesis and progression of RA and thus serves as a biomarker for diagnosing this disease. Although fluorescent probes have emerged as promising tools for detecting H 2 O 2 , most available ones suffer from the aggregation-caused quenching (ACQ) effect, short-wavelength emission, low sensitivity, and poor water solubility. Herein, a new type of "turn-on" AIE probe based on excited state intramolecular proton transfer (ESIPT) was developed, with phenylboronic acid pinacol ester-appended quinolinium as the H 2 O 2 recognition site, which is in the quenched state due to the twisted intramolecular charge transfer (TICT) effect. The probe HTQ-R exhibits good water solubility, high sensitivity, a low detection limit (210 nM), rapid response ability, and good biocompatibility towards hydrogen peroxide, and has shown the ability to accurately target mitochondria. Furthermore, HTQ-R was successfully used to detect exogenous and endogenous hydrogen peroxide in living cells, which enabled real-time monitoring of H 2 O 2 in RA mice, demonstrating its potential significance in the diagnosis and treatment of RA.