Fabrication of Water-Soluble, Green-Emitting Gold Nanoclusters with a 65% Photoluminescence Quantum Yield via Host-Guest Recognition

Metal nanoclusters (NCs) as a new type of fluorescent material have been extensively explored because of their attractive set of features such as their ultrafine size, low toxicity, and excellent photostability. However, little progress has been made in producing water-soluble, homogeneous, and ultrabright metal NCs. In this study, gold NCs (AuNCs) with a photoluminescence quantum yield (QY) as high as 65% are synthesized in water through a simple blending route. Weak emission is observed from the 6-aza-2-thiothymine-protected AuNCs (ATT-AuNCs); however, the fluorescence intensity can be prominently enhanced by introducing L-arginine (Arg) into the capping layer. The fluorescence enhancement mechanism is systematically investigated by the measurements of ultraviolet-visible absorption spectroscopy, photoluminescence spectroscopy, fluorescence lifetime spectroscopy, transmission electron microscopy, dynamic light scattering, X-ray photoelectron spectroscopy, H-1 nuclear magnetic resonance, and calculations from density functional theory, with results isolating the vital role of the ligand shell and ruling out the effect of the gold core. The supramolecular host-guest assemblies formed between ATT capped on the gold core and the guanidine group of Arg make the capping ligands of ATT rigid. Subsequently, the intramolecular vibration and rotation of ATT are greatly suppressed, which reduce the nonradiative relaxation of excited states and, as a result, predominantly increase the luminescence QY of ATT-AuNCs. Further experiments demonstrate that a small change in guanidine substituents can arouse obvious changes in the photoluminescence features of NCs. We envision that this work will substantively contribute to the process of developing efficient synthetic routes to high-quality metal NCs.