Effect of surface ligand density on near-infrared electrochemiluminescence of glutathione-stabilized Au nanoclusters

Here, we report the crucial role of surface ligand density in electrochemiluminescence (ECL) of water-soluble Au NCs stabilized with glutathione ligands. The Au NCs, having four distinct surface ligand densities, displayed unique ECL features that were dependent on the surface ligand density of the NCs, similar to their fluorescence properties. The Au NCs displayed dual-wavelength ECL emissions at ∼620 nm and ∼820 nm. The ECL properties were significantly affected by the surface ligand density of the NCs, with lower densities resulting in enhanced ECL emission, particularly in the near-infrared region. This enhancement surpassed the near-infrared ECL emissions observed in other known ECL-active Au NCs, resulting in approximately a 9-fold improvement compared to well-established Au NCs stabilized with bovine serum albumin (BSA). In addition, the dual-emission features of the Au NCs stabilized with glutathione were attributed to the presence of two Au-S emission motifs related to different Au-S bonding environments resulting from varied surface ligand densities. These findings revealed the critical role of the surface ligand density in modulating not only the fluorescence of the Au NCs but also the ECL of NCs.