The Synthesis and characterization ofhydrophilic gold nanoparticles via a DEN-MPC method and their evaluation as biphasic catalysts

[摘要] ENGLISH ABSTRACT: The synthesis of hydrophilic gold nanoparticles, by using a dendrimer template method to produce dendrimer encapsulated nanoparticles (DENs), in combination with a monolayer protected cluster (MPC) method is reported on. This was achieved by forming the DENs in chloroform and then extracting the nanoparticles into water. The extraction of the nanoparticles was facilitated by a water-soluble ligand (1,3,5-triaza-7-phosphaadamantane) which subsequently also provided the particles with a protective layer to limit agglomeration. In addition, the catalytic performance in the transformation of styrene of the water-soluble gold nanoparticles is described.The use of the combined DENs-MPC method was driven by an attempt to tightly control the initial size of the synthesized nanoparticles (DENs method) and afterwards provide a means of maintaining the particle size over an extended period of time (weeks or months) using the MPC method. It was aimed to control nanoparticle size by means of varying the Au:dendrimer ratio in order to produce Au atom clusters of a certain number and thus nanoparticles of a certain size. During our initial synthetic attempts utilizing a Au:dendrimer ratio of 55:1, we managed to synthesize nanoparticles with an average size of 1.5 nm ± 0.9 which is the equivalent of an Au55 atom cluster and thus appeared to validate our proposed synthesis method. However, studies using UV/vis spectroscopy and transmission electron microscopy (TEM) in an attempt to monitor the growth evolution of the nanoparticles over time revealed that the gold to dendrimer ratio was not the only factor that plays a role in the formation of stable nanoparticles. Based on the information gained from the size evolution studies, it was found that when Au:dendrimer ratios were used which would theoretically produce specific atomic cluster sizes possessing closed shell configurations (such as Au55 and Au13), DENs with sizes larger than the specific cluster size would form. However, upon extraction into the aqueous phase, the particles would reduce in size closer to the expected atomic cluster size. Conversely, Au:dendrimer ratios which would not result in closed shell configurations (e.g. Au31), did not exhibit this reduction in size upon extraction from the organic into the aqueous phase. The mechanistic pathway, which was hypothesized to be facilitated by the 1,3,5-triaza-7-phosphaadamantane (PTA) ligand, is therefore elucidated.It was subsequently aimed to tailor nanoparticle size (with the further aim of potentially tailoring catalytic performance). However, although the mechanism of size reduction upon extraction of the nanoparticles from the organic into the aqueous layer appeared to hold for each synthetic attempt, the extent of nanoparticle size reduction was found to be variable for successive synthetic procedures. This observation prompted a more in-depth study into the exact nature of the facilitation of gold nanoparticle size reduction by the water-soluble PTA ligand. This study was enabled by the utilization of dynamic light scattering (DLS) equipment in conjunction with nanoparticle zeta potential determination, and resulted in devising a more optimum PTA concentration in the aqueous layer required for nanoparticle size reduction and stabilization. In addition to this, the re-evaluation of the gold salt reduction procedure further improved on achieving nanoparticle size and shape reproducibility. However, although these results yielded improvements in synthesis repeatability in terms of nanoparticle size, it was still not satisfactory.With improvements in synthetic repeatability showing promise, yet still not being satisfactory, the effect of process conditions such as stirring and manner of reagent addition were evaluated. This was done utilizing a custom made nanoparticle synthesis reactor set-up and led to the revelation that the inconsistency in size produced for successive synthetic procedures was due to the lack of retention and scaffolding properties of the dendrimer in preventing uncontrolled nucleation and nanoparticle growth. Based on this it was found that addition of the stabilizing ligand along with the reducing agent before extraction into the aqueous layer proved to arrest any uncontrolled growth and resulted in addressing the synthetic inconsistency in terms of nanoparticle size.The ability to now tailor nanoparticle size, and produce them consistently, therefore enabled the evaluation of the synthesized aqueous gold nanoparticles in the attempted biphasic catalytic oxidation of styrene. Interestingly however, the catalyst showed overwhelming selectivity toward polystyrene with conversion surprisingly also not appearing to be dependent on nanoparticle size. In addition, it was found that additives such as H2O2 and t-butyl-hydroperoxide (TBHP) which were intended to act as oxidizing agents, instead influenced polystyrene properties. The various aspects resulting in the observed catalytic behavior is discussed and an overall mechanism proposed.

[发布机构] Stellenbosch University