Poly(vinyl alcohol)-Coated Au Nanoclusters with High Stability and Quantum Yields of Fluorescence for Application in pH Sensing

Metal nanoclusters (NCs) have become one of the most promising of the fluorescent nanomaterials. However, it is particularly challenging to prepare NCs with a high quantum yield (QY). Recently, an innovative approach was revealed to improve the QY by using an aggregation-induced emission (AIE) property of specific gold nanoclusters (AuNCs). In this study, an assembly was formed between poly(vinyl alcohol) (PVA) and adenosine 5′-monophosphate-capped gold nanoclusters (AuNCs@AMP) to improve the QY of AuNCs from 14.2% to 35.4% with an obvious luminescent, aggregate-induced emission enhancement (AIEE) property. The intrinsic formative mechanism was explored in depth, which indicated that a suspended particulate was formed with AuNCs at a low concentration of PVA, pushing the AuNCs to a less polar environment and greatly enhancing the metal–metal interaction between them, leading to a blueshift. When more PVA was present in the solution, abundant hydrogen bond interactions between the amino groups in AMP and the hydroxide radical groups in PVA restricted the intramolecular rotation and vibration of capping ligands and reduced the non-radiative relaxation of the corresponding excited states. Therefore, immensely dynamic interactions of the assembly populate the radiative decay pathways and accelerate the effective electron transfer, subsequently leading to a higher luminescent intensity. Meanwhile, luminescent enhancement membranes were prepared with extremely stabilized optical properties (monitored for at least 20 months) based on the assembly and applied as a novel fluorescent probe of pH-responsiveness. This study is beneficial for the design principles of highly luminescent AuNC materials with excellent stability and understanding the AIEE mechanism.