Revisiting Protein Protected Gold and Silver Nanoclusters: Excited State Dynamics and Long-Lived Emission

Owing to their intriguing photophysical properties, protein-functionalized metal nanoclusters (MNCs) have become benchmark nanomaterials in various fields including electronics, optics, energy, sensing, and biomedicine. However, the excited state dynamics of these MNCs, especially the long-lived emission, remain a debatable topic. Here, we investigated the excited-state dynamics of bovine serum albumin (BSA) functionalized Ag and Au nanoclusters (BSA-Ag NCs and BSA-Au NCs) and provided new insights into their long-lived emission. Our results showed completely different excited state dynamics in these MNCs despite their closely similar number of atoms in the BSA-caged metal core. BSA-Au NCs showed short-lived emission followed by a long-lived excited state at room temperature, originating from the core and surface states, respectively. In contrast, the BSA-Ag NCs showed short-lived emission originating only from the core states. The long-lived emission of BSA-Au NCs was attributed to ligand-to-metal charge transfer (LMCT) facilitated by the presence of strong metal-sulfur (M-S) bonds. The time-resolved luminescence imaging (TRLI) microscopy of both MNCs in HeLa cells indeed confirmed the long-lived emission characteristics in BSA-Au NCs but not in BSA-Ag NCs.