Investigation On Synthetic Mechanism Of Gold Thiolate And The Effect Of Surfactant On The Polymer-GNPs System

Since the synthesis of mono-dispersed gold nanoparticles (GNPs) by Brust-Schiffrin method (BSM), much attention has been focused on the gold nanoparticles for their optical, electronic, catalytic and biomedical properties and small size effect, which have been widely used in dye, catalyst, electronics and medicine. Studying the preparation mechanism of gold nanoparticles supplies clues on how to control the properties of resulting GNPs. In the process to study the synthetic mechanism of GNPs by Brust-Schiffrin two phase method, the polymeric gold thiolates [Au(Ⅰ)SR]n which were prepared by precipitating from the intermediate solutions by BSM have attracted much interests for their luminescent property. On the other hand, gold thiolates can also be used to get novel GNPs by thermolysis or other method.Although many reports have been involved in the mechanism of the preparation of GNPs, few ones concerned on the thorough and systemetic study on the preparation process of [Au(Ⅰ)SR]n and the way to control their composition are still undefined. To study the preparation process of gold thiolates clearly, more characterization methods should be engaged in. IR and1H NMR are popularly used to study the process, but many new techniques are needed to be developed to explore the composition of the solid precipitates of gold thiolates.In this paper, a series gold thiolates capped with benzyl mercaptan,[Au(Ⅰ) S C7H7]n, were prepared by precipitating from the intermediate solutions during the synthesis of GNPs by Brust-Schiffrin two phase method. The process of preparation of the precursor species before precipitation was monitored by UV and solution1H NMR. Gold (Ⅰ) thiolates precipitated from these solutions were characterized by1H solid-state nuclear magnetic resonance spectroscopy (1H SSNMR) under fast magic angle spinning (MAS), Raman spectroscopy and thermogravimetric analysis (TGA). To study how to control the formation of gold thiolates, both1-octadecanethiol and benzyl mercaptan were used to form gold thiolates with mixed composition.Tg (glass transition temperature)-confinement effect has been studied by a great amount of reports in literature with no general consensus revealed. Here, we characterized the surfactant on thiolated polystyrene stabilized gold nanoparticles (PS-S-GNPs) by1H solid-state nuclear magnetic resonance (1H SSNMR) spectroscopy and investigated the effect of surfactant on Tg of PS-S-confined on GNPs. Two different types of PS-S-GNPs were synthesized by two-phase method with surfactant tetraoctylammonium bromide (TOAB) and by one-phase method without surfactant. Then1H SSNMR under fast magic angle spinning (MAS) was utilized to characterize residual TOAB at the interface and the mobility of the adsorbed species on GNPs. The result of dipolar filtered1H SSNMR showed that the mobility of PS-S-reduced when the polymeric chains were confined on the surface of GNPs. Finally, differential scanning calorimeter (DSC) showed that Tg of PS-S-on GNPs without surfactant increased while Tg of PS-S-GNPs with surfactant TOAB reduced. Our results clearly demonstrate the importance of the synthetic methods, and are of great significance to get further insight into Tg-confinement effect.