MOF-based surface tailoring the near-infrared luminescence property of gold nanoclusters for ratiometric fluorescence sensing of acetylcholinesterase

Ultrasmall gold nanoclusters (AuNCs) with distinct photoluminescence properties are promising fluorophores for optical applications. However, it remains a great challenge to precisely modulate their near-infrared (NIR) fluorescence properties via a simple yet efficient strategy. Herein, we report a new, surface tailoring strategy for effectively modulating the NIR fluorescence property of AuNCs by harnessing the attractive property of metal -organic frameworks. With zeolite imidazolate frameworks-8 (ZIF-8) as the example, we showed that highly fluorescent AuNCs can be in situ encapsulated into ZIF-8 via a one-pot reaction, yielding AuNCs@ZIF-8 com-posites with enhanced NIR fluorescence intensity. Particularly, AuNCs@ZIF-8 composites exhibit an emission maximum at 830 nm, which is 190 nm longer than the emission wavelength of AuNCs without the ZIF-8 encapsulation. Mechanism studies based on fluorescence decay spectra and X-ray photoelectron spectroscopy revealed that ZIF-8 mainly poses a significant effect on the ligand-metal charge transfer process of AuNCs due to the confinement effect. By incorporating Rhodamine B (RhB) into NIR-emitting AuNCs@ZIF-8 composites, we established a ratiometric fluorescent sensing platform for the detection of acetylcholinesterase (AChE) based on the turn-on fluorescence response of AuNCs towards thiolates. The AuNCs@ZIF-8/RhB system exhibits a high sensitivity with a detection limit of 0.4 mU/mL and good specificity towards common biological interferences. Moreover, the present ratiometric fluorescence assay shows good recovery for AChE detection in human serum, suggesting its potential application for real sample analysis. Finally, the fluorescence response mechanism of both AuNCs@ZIF-8 and AuNCs towards AChE is elucidated by fluorescence lifetime analysis and density func-tional theory (DFT) calculation. This study demonstrates the feasibility of modulating the NIR fluorescence property of AuNCs by MOF-based surface tailoring strategy towards enhanced biosensing application.