Article pubs.acs.org/cm
Fabrication of Water-Soluble, Green-Emitting Gold Nanoclusters with a 65% Photoluminescence Quantum Yield via Host−Guest Recognition Hao-Hua Deng,†,‡ Xiao-Qiong Shi,†,‡ Fei-Fei Wang,†,‡ Hua-Ping Peng,†,‡ Ai-Lin Liu,†,‡ Xing-Hua Xia,§ and Wei Chen*,†,‡ †
Department of Pharmaceutical Analysis, Fujian Medical University, Fuzhou 350004, China Higher Educational Key Laboratory for Nano Biomedical Technology of Fujian Province, Fujian Medical University, Fuzhou 350004, China § State Key Laboratory of Analytical Chemistry for Life Science and Collaborative Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ‡
S Supporting Information *
ABSTRACT: 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, 1H 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.
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ligand shell.30 Over the past few years, it has become clear that the origin of NC PL cannot be simply ascribed to the metal core but also is closely associated with the ligands capped on the surface of NCs.31−33 Some studies have suggested that by tuning the surface ligand configuration, the luminescence from metal NCs can be dramatically enhanced. In this respect, solvent- or cation-induced aggregation has been reported to be a promising strategy.34−39 In the aggregation state, intramolecular rotation of the ligands is greatly restricted, which blocks energy loss and remarkably enhances emission efficiency. However, such an aggregation-induced enhancement strategy generally yields inhomogeneous emitters with poor stability. Moreover, the large size of architectures seriously limits their potential applications in sensing and labeling, which usually need good dispersity.
INTRODUCTION
Few-atom metal nanoclusters (NCs) have attracted a tremendous amount of interest because they are appealing materials for both fundamental research and practical application.1−16 They possess size-dependent physical and chemical properties, which are substantially different from those of larger nanoparticles and bulk metals.17−20 One of the most fascinating features is the emergence of photoluminescence (PL). Compared with commonly used organic dye molecules, semiconductor quantum dots, and fluorescent proteins, luminescent metal NCs are superior in terms of their ultrasmall size, low toxicity, excellent biocompatibility, good photostability, facile synthesis, and cheapness.21−29 However, the quantum yields (QYs) of the most currently reported metal NCs are very low (usually
