Stepwise Ligand-Exchange Strategy for the High-Yield Synthesis of Water-Soluble, Metal-Doped MAg₂₄ Nanoclusters

Water-soluble gold nanoclusters (NCs) with diverse nuclearities and protecting ligands have been extensively investigated as luminescent probes for biosensing and bioimaging. In contrast, the development of water-soluble silver NCs has progressed far more slowly, primarily because synthetic routes for producing stable Ag NCs with a broad range of protecting ligands are lacking. Herein, we present an efficient, high-yield stepwise ligand-exchange strategy that addresses this challenge, enabling the preparation of water-soluble, metal-doped MAg₂₄(SR)₁₈ NCs (M = Pt, Pd, Au, or Ag; SR = thiolate) in isolated yields exceeding 80%. The method leverages MAg₁₄ intermediates stabilized by thiolate and phosphine ligands with sharply contrasting lability, a key feature that provides distinctly different exchange rates and preserves the MAg₁₂ icosahedral core throughout the transformation. Beginning with preformed MAg₂₄(DMBT)₁₈, where DMBT denotes 2,4-dimethylbenzenethiolate, a PPh₃-for-DMBT exchange generates mixed-ligand MAg₁₄(DMBT)₆(PPh₃)₈ intermediates that undergo sequential substitution with water-soluble thiols to yield water-soluble, metal-doped MAg₂₄ NCs. This strategy is compatible with a wide variety of ligands─including 4-mercaptobenzoic acid, 6-mercaptohexanoic acid, captopril, and l-glutathione─and supports diverse metal dopants without loss of yield. Photoluminescence (PL) studies reveal dopant- and ligand-dependent PL quantum yields ranging from 0.2 to 12.2%, with excited-state dynamics indicating that nonradiative decay decreases exponentially with increasing PL peak energy in accordance with the energy-gap law. These findings offer design guidelines for creating highly emissive, water-soluble Ag-based NCs via the rational choice of metal dopants and protecting ligands.