Microwave-Mediated Synthesis of Near-Infrared-Emitting Silver Ion-Modified Gold Nanoclusters for Ratiometric Sensing of Hydrosulfide in Environmental Water and Hydrogen Sulfide in Live Cells

Protein-stabilized gold nanoclusters (AuNCs) are intensively used in nanoscale biological systems and biosensors. However, protein-stabilized AuNCs that exhibit an emission peak at the first near-infrared (NIR) window (700-900 nm) have rarely been explored. Herein, we present a rapid microwave synthetic approach for developing NIR-emissive AuNCs (named NIR750-AuNCs) through pH-mediated NaBH4 reduction of HAuCl4 in the presence of lysozyme as a template. The NIR750-AuNCs exhibited emission peaks at 750 nm with luminescence lifetimes of 1.0 mu s and quantum yields (QY) of 4.9%. The incorporation of Ag(I) into NIR750-AuNCs [named NIR750-AuNCs@Ag(I)] efficiently enhanced their QY (13.7%) and luminescence lifetime (1.9 mu s) due to the coordination of the Ag(I) ions with electron-rich residues of the lysozyme shell that mediates the ligand-to-metal-metal charge transfer (LMMCT) process. Additionally, this coordination reaction enables NIR750-AuNCs@Ag(I) to exhibit excellent resistance to photobleaching. The lysozyme shell allowed the conjugation of NIR750-AuNCs@Ag(I) with FITC molecules. As a result, the as-made FITC/NIR750-AuNCs@Ag(I) display two well-resolved emission peaks at 525 and 750 nm with almost equal intensities. These outstanding features allowed the use of FITC/NIR750-AuNCs@Ag(I) for ratiometric sensing of 5-25 mu M NaHS in environmental samples with an excellent reproducibility (relative standard deviation of intensity ratio (<2.5%), satisfactory selectivity, and acceptable recovery (94.4%-101.9%). The sensing mechanism of FITC/NIR750-AuNCs@Ag(I) is attributable to HS-- triggered removal of Ag(I) from the electron-rich residues of the lysozyme shell. Also, FITC/NIR750-AuNCs@Ag(I) were implemented for ratiometric imaging of exogenous and endogenous H2S in live cells. This work demonstrates the synthesis of protein-stabilized AuNCs with an emission maximum of 750 nm, paving the road to designing NIR-emitting AuNCs.