Bisulfite-triggered AIEgens release from metal-organic cages for bioimaging

Aggregation-induced emission luminogens (AIEgens) exhibit efficient fluorescence in the aggregated state, making them promising candidates for bioimaging applications. However, their inherently hydrophobic structures often lead to uncontrollable aggregation, hindering their delivery to target sites for imaging. Researchers have developed strategies to enhance the water solubility of AIEgens through either structural modification or carrier-based encapsulation. However structural modification methods involve complicated synthesis and macrocycles often show low encapsulation efficiency toward AIEgens. Improving the water solubility of AIEgens for bioimaging requires simple yet effective strategies.

We report a novel supramolecular encapsulation approach utilizing water-soluble metal-organic cages (MOCs) as host carriers for AIEgens. Encapsulation significantly prevented the aggregation of AIEgens, enabling their uniform dispersion in aqueous matrices. The AIEgens@MOC complex exhibited excellent structural stability across broad ranges of pH (5.0-9.0), temperature (10°C-80 °C), and time (over 14 days) prior to bisulfite (HSO₃^-) stimulation, confirming the stable host-guest interaction. Upon stimulation with HSO₃^-, the disassembly of MOC led to the release of AIEgens, which subsequently aggregated and exhibited significantly enhanced fluorescence within 60 s. The new AIEgens@MOC complex enabled the succeeded visualization of abnormal intracellular HSO₃^- fluctuations (>20 μM). Moreover, the low cytotoxicity of AIEgens@MOC facilitated its application as an intracellular imaging probe for HSO₃^-, with the capability of specific lysosomal localization.

We have developed a simple and effective strategy to improve the water solubility of AIEgens for bioimaging of important biomarkers in live cells. This method avoided complex chemical modifications, and enabled efficient detection of HSO₃^- in biological systems. By selecting MOCs with appropriate cavity sizes, this strategy is promising to accommodate various AIEgens in the future, offering a generalizable solution to facilitate their bioimaging applications.