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, their excited-state dynamics, particularly long-lived emission, remain a subject of debate. In this work, 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. The decay dynamics of excited-state nanoclusters showed distinct photoluminescence behavior influenced by the specific metal core in the BSA-MNCs. BSA-Au NCs showed short-lived emission followed by a long-lived excited state at room temperature, originating from the core-surface state interactions. These interactions facilitate ligand-to-metal charge transfer states, which enable efficient triplet-state harvesting at room temperature. BSA-Ag NCs, on the other hand, exhibit short-lived emission originating only from the core states, resulting in fluorescence as the relaxation pathway at room temperature, which transitioned to phosphorescence with sub-millisecond lifetimes under cryogenic conditions. The time-resolved luminescence imaging microscopy of both MNCs in HeLa cells indeed confirmed the long-lived emission characteristics in BSA-Au NCs but not in BSA-Ag NCs.