Chemical functionalization and modification of Au clusters

Abstract

Atom-precise water-soluble amine-terminated Au nanoclusters (NCs) with various core sizes have been synthesized by a new single-phase synthetic method. The formed NCs are narrow-sized and the core size and size distribution can be controlled by the reducing agent concentration, with the formation of smaller NC core diameters at higher concentrations of the reducing agent. Furthermore, based on UV-Vis spectroscopy, three absorption peaks at around 690, 440, and 390 nm were observed at 0.30 M of the reducing agent, which are spectroscopic fingerprints of Au25 NCs and are strongly suggestive of the formation of fluorenylmethyloxycarbonyl (Fmoc)-glycine (Gly)-cystamine (CSA)-protected Au25(SR)18 NCs. The reactivity of surface amine functional groups of the resulting amine-terminated Au NCs was investigated by the Michael addition reaction of the primary amines with methyl acrylate. Based on the UV-Vis spectra and Transmission Electron Microscopy (TEM) images, the size of the NCs remained unchanged during the synthesis and after the functionalization reaction which suggests the high stability of these particles. The functionalization of the ligand was confirmed by Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR) and the NC size was characterized by TEM. In the second part of this study, two synthetic methods, direct synthesis and postfunctionalization (divergent synthesis) have been used to produce Au cluster-cored dendrimers. The ester-terminated Au cluster-cored dendrimers formed by direct synthesis are stable and resistant to aggregation in solution and in the presence of an excess of reducing agent. In contrast, amine-terminated cluster-cored dendrimers undergo aggregation in solution over time due to the high reactivity of the surface, which makes them unstable and limits their applications. The divergent method involves repeated two-reaction sequences on Gly-CSA Au NCs, consisting of Michael addition and subsequent amidation by ethylenediamine, and produces stable amine-terminated Au cluster-cored dendrimers with no change in core size after the reaction based on TEM images. Therefore, various amine and ester-terminated Au cluster-cored dendrimers with various dendron generations and core sizes can be formed using the divergent strategy. Direct synthesis does not allow for control of cluster size, while the divergent method does not give fully uniform dendrons at higher generations. Finally, the catalytic activity of these Au cluster-cored dendrimers has been studied for the reduction of 4-nitrophenol.