Breaking the Photostability and pH Limitation of Halo‐Fluoresceins through Chitosan Conjugation

Halo-fluoresceins are widely used in cell and tissue staining, intracellular sensing, and photodynamic therapy, but their notorious photo-instability and pH dependence restrict their applications, especially in long-term visible light exposure and acidic environments. To overcome these limitations, here a strategy is proposed of conjugating chitosan with the carboxyl group of halo-fluorescein (CS-halofluorescein). The cross-linked polymer chains and the hydrogen-bonding networks of chitosan help shielding out ¹ O₂ from direct attacking the encapsulated halo-fluoresceins, leading to a two orders of magnitude lower photobleaching rate. Meanwhile, the condensation of primary amines of chitosan with the carboxyl group on halo-fluorescein blocks the pH-dependent intramolecular spirocyclization, leading to pH-inert fluorescein derivatives. The greatly improved photostability and pH inertness of CS-halofluoresceins can be harvested for aerobic photoredox synthesis and photodynamic bacteria inactivation in extremely acidic media. Moreover, food additive nature of chitosan and erythrosine (TIF) and excellent film-forming property of chitosan allow coating-based light-assisted preservation of perishable fruits, leading to appreciably extended shelf life of fruits (e.g., perishable strawberry, rt: > 3 days; 4 °C: > 5 days).