Molecular engineering of Au₂₅(SG)₁₈ nanoclusters at the single cluster level to brighten their NIR-II fluorescence

Au₂₅(SG)₁₈ (SG: glutathione) nanoclusters, characterized by their atomically precise structures, exhibit near-infrared II (NIR-II) fluorescence emission and excellent biocompatibility, making them highly promising for imaging applications. However, their comparatively low photoluminescence quantum yield (QY) in aqueous solutions limits their further development. In this study, taking advantage of the molecular-like properties of Au₂₅(SG)₁₈ nanoclusters, we employ a Schiff base reaction to improve their NIR-II emission for the first time. The formation of a Schiff base chemical bond restricts intramolecular motion of surface ligands on the Au₂₅(SG)₁₈ nanoclusters, reduces the nonradiative rate, and increases the radiative transition rate. Consequently, the luminescence quantum yield of PDA-Au₂₅(SG)₁₈ (PDA: 2,6-pyridinedicarboxaldehyde) nanoclusters is enhanced to 3.26%. Moreover, the reaction between amino and aldehyde groups occurs at the single cluster level, ensuring that these PDA-Au₂₅(SG)₁₈ nanoclusters remain discrete with an ultrasmall size of 2.6 nm, facilitating rapid excretion via the renal system and also showing excellent photostability and biocompatibility.