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

Cloete, Jezreel (2017-12)

Thesis (PhD)--Stellenbosch University, 2017.

Thesis

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.

AFRIKAANSE OPSOMMING: Die sintese van hidrofielise goud nanopartikels, deur die gebruik van 'n dendrimer templaat metode om dendrimeer enkapsileerde nanopartikels (DENs) to vorm, in kombinasie met 'n monolaag beskermde groepeering (MBG) metode word oor berig. Dit was bereik deur die DENs in klorofoom te vorm and daarna die nanopartikels te ekstrak na water. Die ekstraksie van die nanopartikels was aangehulp deur 'n wateroplosbaare ligand (1,3,5-triaza-7-phosphaadamantane) wat daarna die partikels met 'n beskermde laag aangebied het om agglomerassie te voorkom. Daarbenewens word die prestasie van die wateroplosbare goud nanopartikels as katalisator in die transformeering van stireen bekryf. Die gebruik van die gekombineerde DENs-MBG metode was aangedryf deur die poging of streng beheer oor die aanvanklike groote van die gesintiseerde nanopartikels (DENs metode) te hê en daarna a manier te skep om die partikel groote te handaaf oor verlengde tyd periodes (weke of maande) deur gebruik van die MBG metode. Dit was beoog om nanopartikel groote te kontroleer deur die Au:dendrimeer verhouding te wissel om so Au atoom groepeeringe van 'n sekere nommer, en dus 'n sekere groote te bekom. Gedeurende ons aanvanklike pogings om 'n Au:dendrimeer verhouding van 55:1 te gebruik, het ons daarin geslaag om nanopartikels te vorm met 'n gemiddelde groote van 1.5 nm ± 0.9 wat die ekwivalent van 'n Au55 atoom groepeering is. Dit het daarom voorgekom dat ons voorgestelde sintetise metode gewalideer is. UV/vis spectroscopie en transmissie elektroon microskopie (TEM) wat gebruik is om die groeie evolusie met tyd van die nanopartikels te monitor het egter aangedui dat die verhouding van goud na dendrimeer nie die enigste faktor was wat 'n rol in die formasie van stabiele nanopartikels speel nie. Gebaseer op informasie van die groote evolusie studies wat ogedoen is, is daar gevind dat wanneer Au:dendrimeer verhoudings wat teoreties atomise groepeerings met 'n toe dop konfigurasie sou vorm, DENs groter dan die spesifieke atomiese gropeering gevorm het. Nanopartikels met grotes nader aan wat verwag is het egter gevorm na ekstraksie van die orgaanise fase na die water fase toe. Aan die teenoorgestelde kant, het verhoudinge wat nie toe dop konfigurasies so vorm nie (e.g. Au31), nie verminder in groote na ekstraksie nie. Die meganistiese paaie wat aangehulp is deur die PTA ligand is dus verduidelik. Gevolglik is daar beam om nanopartikel groote te beheer (met die verdure beaaming om potensieel kataliesise verrigting te beheer). Alhoewel die meganisme vir die vermindering van nanopartikel groote gebluik het om te hou vir elke sintetiese poging, is dit egter gevind dat nanopartikel grote gewissel het. Hierdie waarneming het 'n meer in diepte studie meegebring na die fasiliteering van nanopartikel groote vermindering deur die PTA ligand. Hierdie studie was in staat gestel deur die benut van dinamiese lig strooiing (DLS) toerusting in samewerking met zeta potensiaal bepaaling. Die resultaat was dat 'n meer optimum PTA konsentrasie in die water lag, wat benodig is vir nanopartikel groote vermindering en stabiliseering bedink was. Nietemin, alhoewel die resultate verbeteringe in sintetise herhaaldelikheid aangebring het in terme van nanopartikel groote, was dit nog steeds nie na wense nie. Met verbetering aan sintetiese herhaaldelikheid wat bemoedigend was, maar nog steeds nie heeltemal na wense nie, was die effek van proses kondiesies geëvalueer. Dit was gedoen deur 'n oorspronlike reactor stelsel te gebruik en gelui het na die onthuling dat die teenstrydigheid in partikel groote vir opeenvolgende sintetiese prosedures weens die gebrek aan behoudings and steierwerk eienskape van die dendrimeer was om onbeheerde kernvorming en groeie te voorkom. Gebaseer hierop, is dit gevind dat die byvoeging van die stabiliseerende ligand tesame met die reduseermiddel voor ekstraksie na die water laag, voorkom het dat onbeheerde groeie vertraag is en gelei het na beter herhaaldelikheid in terme van nanopartikel groote. Die vermoë om nou nanopartikel groote te beheer, en om hulle herhaaldelik te vorm, het ons dus in plaas gestel om die waterige goud nanopartikels in die gepoogde bifasiese kataliesiese oksidasie van stireen te evalueer. Verbaasend egter het die katalisator oorweldigende selektiwiteit na polystireen gewys met omskakeling wat ook verbaasend nie verskyn of dit afhanklik van nanopartikel grote was nie. Daarbenewens is di took gevind dat toevoeg middels soos H2O2 en t-butyl-hydroperoksied (TBHP) wat veronderstel was om op te tree as oksideermiddels, in stede die eienskape van die polystireen beinvloed het. Die verskeie aspekte wat gelei het na die waargeneemde katalitiese gedrag is bespreek en 'n algehele meganisme word voorgestel.

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