Water-soluble gold nanoclusters prepared by protein-ligand interaction as fluorescent probe for real-time assay of pyrophosphatase activity

• Water-soluble gold nanoclusters are prepared by protein-ligand interaction.
• The Au0@AuI-MPA core-shell nanoclusters have hydrophobic interactions with BSA.
• Diverse water-soluble AuNCs can be obtained by tuning the proteins and thiolates.
• A sensing system is established for the detection of pyrophosphatase.

This paper reports a new and facile method for the synthesis of water-soluble thiolate-protected AuNCs via protein-ligand interaction. Using 3-mercaptopropionic acid (MPA) as a model ligand and bovine serum albumin (BSA) as a model protein, water-soluble AuNCs (BSA/MPA-AuNCs) with intense orange-yellow fluorescent emission (quantum yield=16%) are obtained. Results show that AuNCs produced with this method have hydrophobic interactions with BSA. The synthetic strategy is then successfully extended to produce water-soluble AuNCs protected by other thiolates. Moreover, a sensitive and eco-friendly sensing system is established for detection of the activity of inorganic pyrophosphatase (PPase), which relies on the selective coordination of Fe3+with BSA/MPA-AuNCs, the higher affinity between pyrophosphate (PPi) and Fe3+, and the hydrolysis of PPi by PPase. A good linearity between the fluorescence intensity and PPase activity within the range from 0.1 to 3 U/L is found, with a detection limit down to 0.07 U/L. Additionally, the fluorescent assay developed here is utilized to assay the PPase activity in real biological samples and as well as to evaluate PPase inhibitor, illustrating the great potential for biological analysis.