Supramolecules-Guided Synthesis of Brightly Near-Infrared Au22 Nanoclusters with Aggregation-Induced Emission for Bioimaging

Protein-protected gold nanoclusters with emission in the near-infrared wavelength range have been widely considered for applications in biomedical fields. However, their quantum yield (QY) remains low, thus limiting their practical applications. Herein, a novel strategy to synthesize bovine serum albumin-encapsulated gold nanoclusters (BSA-AuNCs) with QY of 23% is developed. Assembled coordination polymers (supramolecules) of Au(I)-BSA complexes are initially formed because of the intermolecular forces between BSA ligands. The forces are easily controlled by pH level during the reaction, leading to significant change in the photoluminescence of BSA-AuNCs. By regulating the pH and reaction temperature, Au(0)@Au(I) core-shell structured BSA-AuNCs are fabricated in 2 h. Importantly, such AuNCs are in a rigidified state with high Au(I) content in the shell, offering an explanation for their high luminescence character. Further increasing QY to 29% is achieved by confining BSA-AuNCs into a cationic polymer, poly(allylamine) hydrochloride (AuNCs@PAH). Enhanced cellular uptake and improved sensitivity of AuNCs@PAH to glutathione compared to BSA-AuNCs is demonstrated. These findings may give insights into the synergistic effect of pH level and reaction temperature on the properties of protein-encapsulated AuNCs and provide a possible way for up-scaled fabrication of brighter AuNCs protected by other protein templates.