Reversible Luminescent Nanoswitches Based on Aggregation-Induced Emission Enhancement of Silver Nanoclusters for Luminescence Turn-on Assay of Inorganic Pyrophosphatase Activity

Unique aggregation-induced emission (AIE) property has been found and widely applied in chemo/biosensors for thiolated gold nanoclusters and copper nanoclusters; however, little is known about this property of thiolate-protected silver nanoclusters. In this work, specific aggregation-induced emission enhancement (AIEE) of glutathione-capped silver nanoclusters (AgNCs) was verified via its solid-state luminescence and enhanced emission in poor solvent, three stimuli responsive nanoswitches were constructed based on its AIEE property, and a reliable and sensitive PPase assay was developed via ion-triggered luminescence switch. Glutathione-capped AgNCs from a facile one-pot synthesis were found to possess bright red luminescence and aggregation-induced emission enhancement property. This AIEE feature enables AgNCs in sensitive response to pH and temperature in a reversible way, allowing the two nanoswitches to precisely monitor the change of environmental pH and temperature. Complexation reactions among AgNCs, aluminum cation and PPi were also designed for an ion-triggered luminescence nanoswitch, which allows selective response to aluminum cation or PPi in luminescence. This ion-driven luminescence switch is further utilized to design a novel detection strategy for PPase activity through competitive coordination reactions. Our method illustrates a novel detection strategy mediated by complexation reaction between Al3+ and AgNCs avoiding the involvement of copper cations in the detection, and this developed assay performed well in detection of PPase level in fresh rat serum. This work confirms unique aggregation-induced emission enhancement property of glutathione-capped AgNCs, constructs multiple luminescence switches based on its multistimuli responsive behaviors, and demonstrates an example of Al3+-mediated detection strategy for PPase assay.