Controllable Dual Band in Photoluminescence and Electrochemiluminescence of Gold Nanoclusters for Bioimaging and Sensing

To elucidate the luminescence mechanism of gold nanoclusters (Au NCs) with a controllable dual band, which is suitable for applications to bioimaging and chemical sensing, photoluminescence (PL) and electrochemiluminescence (ECL) of Au NCs are investigated with varying ligand densities. Despite the similar sizes of Au NCs, the dual band of orange-red and near-infrared in PL and ECL depends on the ligand density on the surface of the NCs. Besides, the intensities of the two bands in PL change with excitation energy, while those in ECL change with electric potential, suggesting that the two bands are attributable to two chromophoric motifs with distinct characters of Au–S bonds. Au(I)–S motifs with charge transfer characteristics exhibit noticeable pH-dependence, while Au(0)–S motifs are mainly correlated to Au cores bound to ligands. The time profiles of the two bands are independent of relative intensities, indicating that energy transfer is not primarily responsible for the relative intensities despite the coexistence of the two motifs. Accordingly, unique features of luminescence are ascribed to the excitation of the two motifs, as well as the density of the two motifs, influenced by ligand density, suggesting molecule-like electronic transitions in Au NCs. We propose a mechanism to improve the controllability of Au NCs, which includes the ligand density and excitation energy, to tune the color and efficiency of luminescence for a wide range of applications.