Gold Nanoclusters are of particular interest due to their many possible applications across a wide range of scientific fields. More specifically, nano-sized gold particles have potential to be used in drug delivery systems, cancer therapy and catalysis. This dissertation focuses on improving our understanding of ligated gold nanoclusters by examining the role of a variety of phosphine based ligands, novel methods to produce monodisperse solutions, and investigating the kinetics of water soluble ligated gold nanoclusters. The addition of ligands to solutions of Au have shown to produce small (< 20 Au atoms) clusters. All nanocluster solutions were prepared in a similar manner. Typically, a gold salt, either Chloro(triphenylphosphine) gold(I) (Au(PPh3)Cl), or Potassium gold (III) chloride (KAuCl4), were dissolved in various solvents. Next, an equal concentration of ligand was added to the solution and stirred until completely dissolved. Finally, all were reduced with 5X the concentration of borane-tert-butylamine (BTBC) after which were sonicated for ~20 minutes. The timing and method of adding the ligands and reducing agent varied depending on the solution and solubility of the ligands. Primarily we used Electrospray Ionization Mass Spectrometry (ESI-MS) and Ultraviolet – Visible Spectroscopy (UV-VIS) for the characterization of samples, however, occasionally Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and X-Ray Photoemission Spectroscopy (XPS) were used. The most recent research took advantage of the size selective nature of an alpha hemolysin (a-HL) nanopore to investigate the kinetics of thiol-ligated Au clusters ~2 nm in size. The relationship between ligand rigidity and solvent polarity and the size and dispersity of Au cluster suspensions was investigated. We observed the formation of stable monodisperse clusters with the shortest ligand, (L3), independent of solvent. With a longer flexible ligand, (L6), we observed primarily Au8-10 cores depending on the ratio of L6/PPh3. All other ligands yielded polydisperse distributions. These dispersions contained clusters with a nuclearity between 8 and 11, for example [Au10(PPh3)9]3+ in LBn and [Au8(PPh3)7]2+ in LBp, were observed in the initial stages, but they were not stable and precipitated out or plated the glass vial. We also observed that the polarity of the solvent did not play a significant role in the formation of MPC’s, however a correlation between the size of the solvent and MPC formation was observed. The growth and evolution of two unique gold structures was also observed via UV-Vis and ESI-MS. Solutions were prepared which contained Potassium gold (III) chloride and PPh2(CH2)3PPh2, i.e., 1,3-bis(diphenylphosphino)propane, denoted by L3, reduced with Borane tert-butylamine complex (BTBC) in a 1:1 diethyl ether:methanol mix, which yielded a stable [Au11(L3)5]+3. Starting with this known Au11 solution, the addition of Mn2+ has shown to lead to the formation of a stable diphosphine ligated Au8 and a new Au14 species. Additionally, we show that the co-reduction of NiCl2 and Au in the absence of the ligand (L3) does give us a simple method for the production of a monodisperse [Au9(PPh3)8]2+ cluster solution and via electroless deposition does give us a potential low temperature pathway to the formation of a AuNi nanoalloy particle.

Monothiolate protected gold nanoclusters with near IR luminescence underwent a five-to-ten fold enhancement of quantum efficiency by heating in the presence of excess thiols. Two monothiolate nanoclusters, mercaptosuccinic acid and tiopronin, were shown to benefit from this procedure. Emission maximum around 700-900 nm is favorable for bioimaging applications due to reduction of background signal from autofluorescence. Dithiolate lipoic acid protected gold nanoclusters with higher near IR quantum efficiency present an interesting candidate for biological imaging due to the difference in hydrophobicity, resistance to quenching by divalent cations and cell growth media, and retained quantum efficiency when coupled to agents such as polyethylene glycol. Intracellular and nuclear internalization of mercaptosuccinic gold nanoclusters demonstrate a potential vector for delivery of nuclear targeting agents. The small size, chemical stability, high luminescence, and potential for targeting various intracellular domains make gold nanoclusters worthwhile for further studies as potential bioimaging probes.

This research focuses on the microwave properties of nanoparticles for use as contrast and hyperthermia agents. Currently, visible light is used for irradiation of nanoparticles as hyperthermia agents. Additionally, visible/Near-infrared light is used for photoacoustic tomography (PAT) imaging. Compared to optical wavelengths, frequencies in microwave range transmit through tissue with high penetration depth . Thus, deep cancerous cells and malignant tissue may be treated and imaged. These nanoparticles could enable the use of a hybrid microwave/acoustic technique known as thermoacoustic tomography. Here, quantitative measurements of the heat generation in super paramagnetic iron oxide nanoparticle (SPIONs), gold nanoparticles (AuNPs), and gold nanoclusters (AuNCs) induced by microwave energy at 3 GHz, are presented and compared. Based on our experiments, SPIONs are the most efficient nanoparticles for microwave heating. Very high concentrations of SPIONs are able to convert microwave energy into heat about 22° C more than DI-water. AuNPs, which support plasmon resonances, do not provide heat under microwave irradiation as predicted by our computational analysis based on Mie Theory. AuNCs are a new form of ultra-small (<2.5 nm) AuNPs which do not support plasmonic resonances and have supra-molecular properties such as sub-conduction band transitions. Interestingly, AuNCs have the potential to absorb microwave energy and may provide an alternative to SPIONs. These nanoparticles had not yet been studied before in this frequency region. In addition, the absorption coefficient of nanoparticles were calculated using complex permittivity data from a dip probe kit and a Vector Network Analyzer (VNA) in a broad band range from 500 MHZ to 10 GHz. This method allows identification of best frequency region with highest penetration depth. In the last step, the nanoparticles with different concentrations were tested as exogenous contrast agents in a Thermoacoustic Tomography (TAT) system. TAT utilizes the penetration depth of microwave energy while producing high resolution images through acoustic waves. The addition of an exogenous contrast agent improves image quality by more effectively converting microwave energy to heat. The experiment reveals that the time resolved thermoacoustic signal (TA) from SPIONs is stronger than AuNPs and AuNCs and thus, the image contrast produced by SPIONs is stronger than the two other aforementioned nanoparticles.

During the past few years, gold nanoparticles (AuNPs) have received considerable attention in many fields due to their optical properties, photothermal effect and biocompatibility. AuNPs, particularly AuNCs and AuNRs, exhibit great potential in diagnostics and imaging. In the present study, AuNCs were used to selectively image and quantify intracellular antioxidants. It was reported by Chen et al. that the strong fluorescence of AuNCs is quenched by highly reactive oxygen species (hROS). Most of applications depend on fluorescence quenching, however, for our project we designed turn-on fluorescent sensors using AuNCs that sense antioxidants. In the presence of antioxidants, AuNCs fluorescence switch on, while in the absence of antioxidants their fluorescence immediately turn off due to hROS effect. AuNRs were also used for cellular imaging in which AuNRs were conjugated to Cy3-labelled molecular beacon (MB) DNA. Next, the previous complex was loaded in two different strains of magnetotactic bacteria (MTB). MTB were used as a targeted delivery vehicle in which magnetosomes direct the movement of bacteria. The DNA sequence was specific to a certain sequence in mitochondria. The exposure of MTB to an alternating magnetic field (AMF) leads to the increase of temperature inside the bacteria, which destruct the cell wall, and hence, bacterial payload is released. When MD-DNA hybrid with the target sequence, AuNR and Cy3 separate from each other, the fluorescence of the Cy3 is restored.

Gold nanoclusters possess both theoretical and practical importance in the development of ultrasensitive biosensors based on surface-enhanced Raman spectroscopy (SERS). Manipulation of gold nanoclusters in a predictable and reproducible manner for the application of refined biochemical analysis still remains challenging. In this study, high-purity gold nanoclusters are isolated via a simple method based on density gradient centrifugation. Three distinct bands including monomers, small aggregates (2-4 nanospheres), and large aggregates (>5 nanospheres) can be separated via density gradient centrifugation. The isolated gold nanoclusters greatly enhance the Raman intensity of the trapped dye molecules such that single nanocluster detection is feasible. To develop a gold nanoparticle-based biosensor for influenza virus, effort was also made to modify recognition moieties such as aptamers to gold nanoparticles via distinct approaches. The increase of hydraulic diameter and the shift of optical absorbance spectrum indicate the success of surface modification to gold nanoparticles. / Master of Science

Thesis (Ph.D. (Physics)) --University of Limpopo, 2011 / The importance of gold for scientific uses is of fundamental importance to research and technology developments. The bulk gold shows reluctance to participate in chemical reactions, the effect which has been corrected by the change in the size towards nanoclusters. It is therefore imperative that the structure of gold nanomaterials is understood for better applications in catalysis and other developments. Molecular dynamics and the density functional theory have proven to be good tools in computational material science and have thus been used to greater lengths. Molecular dynamics simulations on different gold nanoclusters and nanotubes were successfully carried out at different thermodynamic conditions. The effect of size on the melting of materials was duly tested and our results to some extend agree with what has already been reported. Gold nanoclusters show melting below the bulk and the melting temperatures increase with cluster size. However, the Au55 cluster shows different results in that it melts above the bulk due to structural reconstruction. The structure of the clusters changes from spherical shapes to tetragonal or face centred cubic (fcc) structures. Gold nanotubes show no resistance to temperature and different configurations are obtained in different ensembles. Single wall nanotubes form spherical clusters in the NVT while the NPT conditions give patches of clusters at elevated temperatures. The multi wall nanotubes also form spherical clusters in the NVT but fcc structures are obtained in the NPT Berendsen ensemble towards melting. Ab initio calculations in DMOL3 code on different gold nanoclusters show the stability of the clusters to increase with size and the Au3 and Au8 clusters contain the most stable structures. The Au-Au bond length in the dimer was obtained to within reasonable agreement with experiments and other theoretical works. Doping of the clusters further improved their stability although different impurities give different observations. The QMERA code calculations show that a gold atom on top of the surface causes slanting of the outer MD layers. The morphology of the quantum atoms also changes as compared to the neutral surface and the results are compared by the DMOL3 code which confirms the QMERA results. / Mintek, and the National Research Foundation

Ce projet concerne la synthèse et caractérisation de nouveaux matériaux composites à base de nanoclusteurs de thiolates d’or Au25(SR)18 (tGNCs), supportés sur divers polymères de coordination (MOFs), ainsi que sur ZrO2. L’activité catalytique de ces matériaux a été évaluée sur la transformation de différents substrats. Les tGNCs sont des matériaux atomiquement bien définis et connus pour être actifs dans des réactions d’oxydation. Les nanoparticules de MOFs sont des matériaux pouvant servir de support pour des tGNCs avec de bonnes dispersions. Certains MOFs sont connus pour avoir des propriétés acides et peuvent être actifs en catalyse. Parmi eux, MIL-101 (Cr), UiO-66 (Zr) et ZIF-8 (Zn) on été choisis en raison de leur propriétés acides et/ou de stabilité thermique. La synergie entre les tGNCs et les MOFs a été évaluée à travers la conversion catalytique de différents substrats tels le glucose, le fructose, l’alcool benzylique et le furfural, impliquant des étapes nécessitant un caractère acide et/ou oxydant. Globalement, il n’a pas été observé d’impact de la présence d’or sur la réactivité de ces substrats, et les tendences catalytiques sont celles obtenues avec les MOFs seuls. Cela est certainement dû à la stabilité thermique non suffisante des MOFs qui prévient une calcination efficace des tNGCs. Lorsque ces clusters sont déposés sur ZrO2, il a été possible de les calciner à différentes températures pour étudier l’effet du ligand et de la taille de particules, pour des réactions d’oxydation en phase liquide. Ainsi, il a été montré par exemple que la température de calcination a un impact significatif sur le comportement catalytique de ces composites, qui ont donné de bonnes activités pour l’oxydation de l’alcool benzylique en benzaldéhyde dans le toluène et en conditions douces, et pour l’esterification oxydante du furfural en furoate de méthyle / This research project reports the synthesis and characterization of new composite materials based on Au25(SR)18 thiolate gold nanoclusters (tGNCs), supported over a range of metal organic frameworks (MOFs), and ZrO2. The synthesized composite materials were tested for catalytic transformations of various substrates. tGNCs are atomically well defined materials known to be active in oxidation reactions. MOFs nanoparticles are materials suitable for high dispersion of tGNCs. Some MOFs are known to have acidity and can be active as catalysts. Among them, MIL-101 (Cr), UiO-66 (Zr) and ZIF-8 (Zn) were chosen due to their acidic and/or thermal stability properties. The synergy between tGNCs and MOFs has been tested through catalytic conversions of different substrates like glucose, fructose, benzylalcohol and furfural, involving steps requiring acidic and oxidative features. Globally, no impact of the presence of Au clusters was observed, and the composite materials showed the same catalytic trends as those obtained with the MOFs alone. This is mainly due to the not sufficient thermal stability of the MOFs that prevents efficient calcination of the tGNCs. In contrast, when deposited on ZrO2 it was possible to calcine Au25(SG)18 nanoclusters at different temperatures to study the ligand and particle size effects in liquid phase oxidation reactions. For example, the calcination temperature had a significant impact on the catalytic behaviour of this composite materials, which showed good activity for the oxidation of benzyl alcohol into benzaldehyde in toluene under mild conditions, and of furfural oxidative esterification into methyl-2-furoate

Mes travaux de recherche concernent la synthèse et la caractérisation des nanoclusters stabilisés pardes ligands thiolés, et plus particulièrement des nanoclusters d’or et d’argent. L’étude de ces clusterspar des méthodes de caractérisation optique en solution, séparation sur le gel d’électrophorèse et laspectrométrie de masse à haute résolution a permis de sonder quelques propriétés structurelles etélectroniques. L’originalité de ce travail de thèse vient, du couplage d’un spectromètre de masse avecun laser accordable en longueur d’onde UV/Visible pour étudier les propriétés optiques de clusters enphase gazeuse. En effet, ce travail a permis d’obtenir le premier spectre sur des mesures optiques deces espèces.Une partie de ce travail est consacrée à la synthèse et la caractérisation des nanoclusters d’or stabiliséspar les glutathions allant de Au4(SG) 4 à Au25(SG) 18. Les expériences de fragmentation induite parcollision sont réalisées sur les clusters afin de sonder leurs structures. Nous avons développé unenouvelle méthode basée sur l’analyse des structures isotopiques expérimentales pour déterminer lenombre d’électrons accommodés le coeur métallique.Ces études expérimentales ont été étendues à d’autres systèmes tels que les clusters d’argent stabiliséspar les glutathions. Deux méthodes de synthèse des clusters Agx(SG)y ont été développées aulaboratoire. Dans le cadre de ce travail, pour la première fois, nous avons pu déterminer la stichométriede ces clusters Ag31(SG) 19 et Ag15(SG) 11. / My research works concerns the synthesis and characterization of nanoclusters stabilized by thiolates(SG), particularly the gold and silver nanoclusters. The study of these clusters by opticalcharacterization techniques in solution, separation by gel electrophoresis and high-resolution massspectrometry has allowed to probe some structural and electronic proprieties. The origin of this workcome from experimental setup coupled mass spectrometry to UV/Visible optical parametric oscillatortunable laser for understanding the optical proprieties of the clusters in gas phase. In fact, this workhas allowed to get the first optical gas phase spectrum of this species. The first part of this work is dedicated to the synthesis and characterization of glutathione stabilizedgold nanoclusters with size between Au4 (SG) 4 and Au25 (SG) 18. Collision induced dissociationexperiments are performed to probe some structural proprieties of the clusters. We have developednew method based on the analysis of the experimental isotopic distribution allowed to find the numberof electrons bear in metallic core.These experimental studies have been extended to other systems such as glutathione stabilized silvernanoclusters. Two synthesis methods Agx(SG)y have been developed in the laboratory. For the firsttime, we have found the stoicheiometry of these clusters as Ag31 (SG) 19 and Ag15 (SG) 11.