Theses and Dissertations

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Open Access
A clonable selenium nanoparticle in action: high resolution localization of FtsZ using electron tomography
(Colorado State University. Libraries, 2021) Borgognoni, Kanda, author; Ackerson, Christopher J., advisor; Neilson, James, committee member; Kennan, Alan J., committee member; Tsunoda, Susan, committee member
A meaningful understanding of biochemistry requires that we understand the function of proteins, which is heavily dependent on their structure and location within an organism. As the Resolution Revolution of cryo-electron microscopy gains unprecedented ground largely due to the recent development of commercially available direct electron detectors, energy filters, and high-end computation, thousands of protein structures have been solved at atomic or near-atomic resolution, with the highest resolution structure to date being solved at 1.2 Å. A major challenge that has limited the broad use of cryo-electron tomography (cryo-ET) is locating a protein of interest in an organism, as no commercially available high-contrast markers which can be generated in vivo exist. Herein, we present a breakthrough study which aims to solve this problem by synthesizing high contrast metal nanoparticles labeling desired proteins in situ. We isolated a Glutathione Reductase-like Metalloid Reductase (GRLMR), which can reduce selenite and selenate into selenium nanoparticles (SeNPs), from Pseudomonas moraviensis stanleyae found in the roots of a Se hyperaccumulator Stanleya pinnata, or Desert Princes' Plume. A recombinant variant, denoted as a clonable Selenium NanoParticle (cSeNP), was fused to filamentous temperature sensitive protein Z (FtsZ), and the chimera was expressed in vivo using a T7 expression system in model organism E. coli for a proof-of-concept study. Because the SeNPs biogenically produced are amorphous, they exist in a quasistable state and are composed of polymeric Sen in the form of chains and rings that are constantly breaking and reforming. To stabilize the particles during cellular preservation ex aqua, a disproportionation-like reaction can be done either in vivo or as a post-fixation step to form crystalline metal selenide (MSe) NPs that can withstand the processing liquids used. Thereafter, electron tomography was used to acquire a tilt series that was reconstructed into a tomogram and segmented using IMOD, generating a model representing MSeNPs labeling FtsZ filaments. As such, we have demonstrated the potential of using cSeNP as a high resolution marker for cryo-ET. While our study relied on traditional preservation and embedment techniques, we anticipate that for cells preserved via vitrification, cloned SeNPs can be used without subsequent transformation to MSeNPs, as the amorphous particles are stable in aqueous media. Prospectively, we expect that clonable nanoparticle technology will revolutionize cryo-ET, allowing us to localize proteins in vivo at high resolution while maintaining organism viability through metal immobilization. Furthermore, this technique can be expanded to other imaging modalities, such as light microscopy and X-ray tomography, through the discovery and engineering of other clonable nanoparticles.
Open Access
Applications of superatom theory in metal cluster chemistry
(Colorado State University. Libraries, 2016) Tofanelli, Marcus A., author; Ackerson, Christopher J., advisor; Prieto, Amy L., committee member; Shores, Mathew, committee member; Farmer, Delphine, committee member; Roberts, Jacob, committee member
One of the largest modern scientific debates is understanding the size dependent properties of a metal. While much effort has been performed on understanding metal particles from the top down to much less work has been accomplished from the bottom up. This has lead to a great deal of interest in metal clusters. Metal clusters containing 20 to 200 metal atoms are similar yet strikingly different to both to normal coordination chemistry and continuous bulk systems, therefore neither a classical understanding for bulk or molecular systems appears to be appropriate. Superatom theory has emerged as a useful concept for describing the properties of a metal cluster in this size range. In this model a new set of ‘superatomic’ orbitals arises from the valence electrons of all the metals in a cluster. From superatom theory the properties of a metal cluster, such as stability, ionization energy, reactivity, and magnetism, should depend on valence of the superatomic orbitals, similar to a normal atom. However superatom theory has largely been used to describe the high stabilities of metal clusters with completed electronic configurations. Thus many features of superatom theory have remained largely untested and the extent that the superatom model truly applies has remained in question for many years. Over the past decade increases in synthetic and analytical techniques have allowed for the isolation of a series of stable monodisperse gold thiolate monolayer protected clusters (MPCs) containing from 10 to 500 gold atoms. The wide range in sizes and high stability of gold thiolate clusters provides an instrumental system for understanding superatom theory and the transition from molecular-like cluster to bulk-like system. In the first part of this thesis the effects of the superatomic valence is investigated under superatomic assumptions. Au25(SR)18 (where SR= any thiolate) can be synthesized in 3 different oxidation states without any major distortions to the geometry of the cluster, thus it is possible to test 3 different superatomic configurations for a single cluster. These studies show that the superatom model correctly predicts changes observed in the stability, absorption spectrum, crystal structures, and magnetic susceptibility for each charge state of Au25(SR)18. In addition, the superatom model is shown to also apply to the isoelectronic PdAu24(SR)18 superatomic cluster. This work is discussed in Chapters 2, 3, and 4. The second part of this thesis focuses on the transition from superatomic metal clusters to metal nanoparticles. Au144(SR)60 is studied in order to understand this transition. Although the plasmon is not immediately apparent through linear absorption spectroscopy, a plasmonic feature is observed in transient absorption spectroscopy. This observation in combination with the absence of a HOMO-LUMO gap suggests that Au144(SR)60 can be treated with bulk assumptions. However Au144(SR)60 shows quantized behavior and powder x-ray diffraction reveals that symmetry of the metal core does not represent what is observed in the bulk. Au144(SR)60 appears to show both superatomic and bulk behavior making it an instrumental tool for understanding the transition from superatomic to bulk behavior. This work is discussed in Chapters 2, 5, and 6.
Open Access
Bioengineering of cloneable inorganic nanoparticles
(Colorado State University. Libraries, 2023) Hendricks, Alexander Ryo, author; Ackerson, Christopher J., advisor; Chung, Jean, committee member; McNally, Andy, committee member; Cohen, Bob, committee member
When a defined protein/peptide (or combinations thereof) control and define the synthesis of an inorganic nanoparticle, the result is a cloneable NanoParticle (cNP). This is because the protein sequence/structure/function is encoded in DNA, and therefore the physicochemical properties of the nanoparticle are also encoded in DNA. Thus the cloneable nanoparticle paradigm can be considered as an extension of the central dogma of molecular biology (e.g. DNA -> mRNA -> Protein -> cNP); modifications to the DNA encoding a cNP can modify the resulting properties of the cNP. The DNA encoding a cNP can be recombinantly transferred into any organism. Ideally, this enables recombinant production of cNPs with the same defined physiochemical properties. Such cNPs are of primary interest for applications in biological imaging as clonable contrast agents. The advancement of cNPs for broader and more rigorous applications in imaging (and elsewhere) requires further development through multidisciplinary approaches. Described in This Thesis is a bioengineering approach to improve the cNP platform through development of the enzymes responsible for nanoparticle formation.In the first chapter, background and significance is given to provide rationale behind the cNP platform. Among all the modalities of biological imaging, there is no 'one-size-fits-all' solution. Biological fluorescence microscopy (FM) and electron microscopy (EM) are the preferred methods of choice when imaging at cellular levels. Although the relatively recent advent of fluorescent proteins and super-resolution microscopy have ushered major scientific breakthroughs, FM is resolution-limited: only the cellular components which are labeled by fluorophores can be resolved – everything else in a cell (~99% of components) is imaged with low resolution owing to the diffraction limit of light. Biological EM comparatively can image widefield cells at atomic-level resolution yet lacks an analogous toolset to fluorescent proteins. cNPs are proposed as a multimodal, uniform, and precise means of clonable contrast for biological EM (and other modalities) analogous to fluorescent proteins. In the second chapter, a tellurium reductase is isolated and characterized from screened environmental bacterial cultures collected throughout the Colorado Mineral Belt. A strain of Rhodococcus erythropolis PR4 was found to be highly resistant to a broad range of metal(loid) species at toxic concentrations – notably 4.5 mM TeO32− determined by broth microdilution. Through screening of cell lysate in the presence of metal(loid) substrates, a mycothione reductase was characterized as a Te-specialized enzyme which reduces Te preferentially over Se. This is a surprising finding on the basis of reduction potentials for the two substrates. The standard reduction of potential for the reaction TeO32− + 3 H2O + 4e− ← → Te + 6 OH− is −0.57 V vs Hydrogen. The corresponding reduction of SeO32− is −0.366 V. Thus, SeO32− is the preferred substrate for reduction in the absence of a mechanism for substrate selectivity. We hypothesize that the R. erythropolis mycothione reductase may form the basis of a cloneable tellurium nanoparticle (cTeNP). In the third chapter, metal(loid) reductase substrate specificity is developed through directed evolution of a glutathione reductase-like metalloid reductase (GRLMR). The native substrate of GRLMR is selenodiglutathione (GS-Se-SG), where zerovalent selenium nanoparticles are formed in the presence of NADPH. Error prone polymerase chain reaction was used to create a library of ~100,000 GRLMR variants. The library was expressed in Escherichia coli with 50 mM SeO32− to select a GRLMR variant with 2 mutations. One mutation (a D to E) appears to be silent, whereas the other (L to H) resides within 5Å of the active site. Compared to the GRLMR parent enzyme, the evolved enzyme became less capable of reducing reduced glutathione (GSSG) and GS-Se-SG in favor of SeO32−. The evolved enzyme also gained an ability to reduce SeO42−. We have described this enzyme as a selenium reductase (SeR). This is the first known instance of the substrate specificity profile of a metal(loid) reductase changing as a result of directed evolution. In the fourth chapter, the cNP concept is discussed in greater detail. The cNP synthesis paradigm is loosely defined as a system of ligands, reductants, and inorganic cations – where ligands are peptides, reductants are enzyme-cofactor pairs, and inorganic cations are dietary or supplemented metal(loid) ions. This modular platform is adaptable to a wide variety of metal(loid)/enzyme/peptide systems. The story of the creation of a cloneable Se nanoparticle (cSeNP) is also retraced. Briefly, a bacterial endophyte Pseudomonas moraviensis subsp. Stanleyae was found to be capable of efficient selenium reduction under aerobic conditions. Continued characterization led to the discovery of GRLMR which unraveled the cellular mechanism for reducing SeO32−. The enzyme can endow host cells with selenium resistance through nanoparticle formation when cloned. GRLMR was further modified through the fusion of a selenium nanoparticle-binding peptide which improved overall kinetic rates, nanoparticle retention, and nanoparticle uniformity. In the fifth chapter, preliminary work is described which may enable further development of the next generation of cNPs through reduced enzyme mass/mericity and 'multicolored' nanoparticles. Work is described which investigates the plasticity of GRLMR towards reducing other metals such as bismuth. Fluorescence assisted cell sorting (FACS) was used to determine if the relative quantity of intracellular metal(loid) nanoparticles can be differentiated, which is hypothesized to correlate to relative metal(loid) reductase activity. Whereas selenium content could be discerned between active and inactive GRLMR-expressing bacteria, relative bismuth content has yet to be analogously discerned. On the other hand, work was done towards rationally designing a monomeric GRLMR; there are ongoing efforts to use machine learning to graft the active site of GRLMR into a different monomeric template. Finally, a Muchor racemosus cytochrome b5 reductase (Cb5R) was identified in the literature which may serve as an ideal candidate to develop more minimalistic cSeNPs. Initial work has revealed that the enzyme is particularly resistant to soluble expression, which may hinder its ability to function as a clonable contrast agent. However, ongoing work is being done to 'supercharge' the enzyme to enable more facile expression.
Open Access
Cloneable' nanoparticles: identification and utilization of metal reducing enzymes as biological electron microscopy tags
(Colorado State University. Libraries, 2019) Butz, Zachary J., author; Ackerson, Christopher J., advisor; Nelson, James R., committee member; Snow, Christopher D., committee member; Santangelo, Thomas J., committee member
The ability to image individual proteins in biological systems has yet to be realized. The identification and utilization of 'cloneable' nanoparticles (cNP), i.e. genetically encoded tags capable of forming in situ inorganic nanoparticles from soluble inorganic precursors is the focus of this dissertation. The long-term goal of this project is to produce GFP analogues that can then be used in electron microscopy, light microscopy, and correlative light/electron microscopy. The first chapter of this dissertation explores a metal reducing enzyme capable for converting soluble inorganic materials to insoluble (nano)particulates. Glutathione reductase-like metalloid reductase, GRLMR, was first isolated from Pseudomonas moraviensis stanleyae and characterized. GRLMR was identified as not only being able to reduce the precursor selenodiglutathione to produce Se⁰ nanoparticles but was also capable of increasing a host cells resistance to 10-fold that of the cell sans GRLMR. The structure of the enzyme was then predicted using Phyre² and related to other glutathione reductases to determine possible residues important for its inherent activity. In the second chapter a dodecapeptide was identified using phage display for its ability to bind to Se⁰ nanoparticles produced by GRLMR. Fusing this peptide to the C-terminus of GRLMR resulted in unexpected enzymes characteristics. Only when concatenated to GRLMR, the Se0 binding peptide conveyed increased size control of nanoparticle product over a wide range of substrate not seen with GRLMR alone. The peptide facilitated greater affinity between the enzyme and the nanoparticle product as well. Finally, presence of the peptide on GRLMR was also able to increase the enzyme's kinetics for precursor reduction. Raman spectroscopy was used to characterize which residues on the peptide were responsible for the interaction between the peptide and the nanoparticle surface. The third chapter explores the application of GRLMR as a cNP. A cNP tag containing two concatenated copies of GRLMR and two Se⁰ binding peptides was constructed and fused to the polymerizing protein FtsZ for expression and studies in native activity. Variants of the tagged FtsZ were isolated and studied in vitro or observed in vivo. In vitro studies resulted in filaments decorated with Se⁰ nanoparticles that could be observed with and without formal staining with uranyl acetate. Images resulting from in vivo studies indicated that both the tag and FtsZ were able to function to produce filaments within cells of high contrast. The fourth chapter isolates and characterizes a Te-reducing enzyme identified from screening environmental isolates collected throughout the Colorado Mineral Belt. A specific isolate, R. erythropolis PR4 possessed resistance to a broad range of metal and metalloid species. Specifically, R. erythropolis grew exceptionally well in up to 4.5 mM of TeO3²⁻ determined by broth microdilution. The lysate from the bacteria was also incubated in different metals and metalloids to identify any proteins with metal reductase activity. Mycothione reductase, a glutathione reductase analogue was characterized with Te-reductase activity. Mycothione reductase was then isolated and characterized and could form Te⁰ nanoparticles and bundled fibers. Although mycothione reductase was able to reduce SeO3²⁻, when the enzyme was incubated with TeO3²⁻ and an excess of SeO3²⁻ the resulting particulate had a mole ratio in favor of Te.
Open Access
Fundamental insights into the alloy miscibility and surface chemistry of metal nanoclusters
(Colorado State University. Libraries, 2022) Anderson, Ian David, author; Ackerson, Christopher J., advisor; Shores, Matthew P., committee member; Van Orden, Alan, committee member; Prenni, Jessica E., committee member
The fascinating and varied properties of metals have captured people's imaginations long before the advent of modern chemistry. Basic metallurgy, dating as far back as the fourth millennium BC, remains one of the most consequential processes in human history. Today we enjoy an effective mastery over metals in their continuous bulk state, complete with alloy phase diagrams which describe properties as a function of temperature and percent composition. The coordination chemistry of single-metal complexes is similarly well-studied, initiated by the pioneering work of Alfred Werner in 1893. Size-dependent properties found at these two extremes (continuous bulk versus discrete molecular) have facilitated a myriad of applications in nearly every aspect of society through the development of unique materials. Between bulk metals and coordination complexes exists a new and rapidly growing area of chemistry concerned with clusters containing several to hundreds of metal atoms. Although there are commonalities shared with both molecular and bulk systems, these clusters also exhibit notable behavioral differences which can often not be explained through simple classical interpretations. The challenge of working with these species has been considerably eased within the past fifteen years from advancements in synthesis and characterization, in particular for monolayer-protected clusters (MPCs) of gold. These MPCs can be synthesized to precise monodispersity and are therefore defined by a molecular formula instead of the more general average size and dispersity used to define larger (typically > 3 nm) colloidal nanoparticles. Minor adjustments to the nuclearity, metal atom identity, or surface chemistry of gold MPCs have been shown to induce extensive changes in their observed properties and overall stability. Complete regiochemical control over both the metal core composition and surface ligand environment is therefore of immediate interest. This goal is especially important for potential applications in catalysis, electronics, biolabeling, energy conversion/storage, and theranostics. The work described herein covers two overarching themes: i) examining the alloying ability of gold MPCs with various late transition metals, and ii) an investigation of MPC surface chemistry through the introduction of multidentate ligands. Synthesis and analysis of the classically-immiscible rhodium-gold system using Au25(SR)18 as a template offers a fresh perspective of alloy gold MPCs containing metals with an open d-shell, alongside an updated framework for understanding MPC stability. Acetylide-for-thiolate, thiolate-for-acetylide, and intercluster exchange between acetylide- and thiolate-protected gold MPCs reveal lability which cannot be adequately rationalized through traditional MPC ligand exchange arguments. The first example of a thiolated gold MPC co-protected by several oxygen-containing diglyme ligands is described, which exhibits enhanced thermal stability as a result of the robust gold-diglyme, thiolate-diglyme, and diglyme-diglyme interactions. A straightforward synthetic pathway to fully dithiolate-protected gold MPCs is also described, as well as a post-synthetic ligand exchange study showcasing their resistance against incoming monodentate thiol exchange. Lastly we provide a series of vignettes detailing our efforts towards the synthesis of various MPCs using metals such as osmium, iridium, and bismuth. Overall these studies afford fundamental advancements in the understanding of soluble, air-stable metal nanoclusters and open up new opportunities for their applications.
Open Access
Fundamental research into gold nanocluster properties
(Colorado State University. Libraries, 2021) Window, Phillip S., author; Ackerson, Christopher J., advisor; Neilson, James R., committee member; Kennan, Alan J., committee member; Peersen, Olve, committee member
Gold materials are popular for research into many applications with their interesting properties, such as magnetism, bio-inactivity, and other size-dependent properties. As the size of the gold material decreases from a bulk material to the nanoscale, new properties are introduced moving through different size regimes. As the particle size reaches the 2-3 nm range and move into the quantum-confined particle range, the most interesting particle changes occur and gold nanomaterials have extremely interesting research potential. These materials exist between the bulk and molecular systems and have similar properties to both; however, they are different enough from both of these to have their own unique application possibilities. Some properties of gold nanoclusters can be attributed more to the core or more to the ligand layer of the nanocluster. Certain properties, like electronics and magnetism, are due to the superatomic electron count and electronic structure from the core and depend on the number of gold atoms in the nanocluster. Extensive research has been done on investigating and altering these properties in small nanoclusters, however, larger nanoclusters have hardly been studied as they can be more difficult to work with. Within this work is investigated the magnetism and thus electronic structure of Au102(SPh)44 and Au133(tBBT)52 in different oxidation states. Paramagnetism up to two unpaired electrons is observed with both these nanoclusters through solution phase magnetic studies. Through this, electronic structure information has been obtained to elucidate the behavior of unique superatomic 1G and 1H orbitals. Looking at the outside of a nanocluster structure, interactions of nanoclusters with other nanoclusters, molecules, surfaces, and solvents are all due to the ligand layer of the nanocluster. Investigations of the ligand layer have been performed extensively through many techniques. However, further studies are always helpful since controlling the ligand layer is essential for functionalization for potential applications. Within this work is investigated the interactions of Au25(SR)18 with other Au25 nanoclusters in both solution and solid phase, as well as ligand exchange reactions of Au133(tBBT)52. Studies on Au25(SR)18 within solution include investigations of a supramolecular assembly, or supercluster, formed solely of the nanocluster itself with control over its growth and size. Studies on Au25(SR)18 within the solid-phase include controlled crystallization techniques that result in different solid-phase structures with previously unseen properties. Ligand exchange studies have also been expanded from small nanocluster materials only in previously published studies to the large nanocluster, Au133(tBBT)52. Within this dissertation, some of the first empirical studies into the oxidation state- dependent properties of large gold nanoclusters, Au102(SPh)44 and Au133(tBBT)52, were performed. This betters the field's understanding of how many unpaired electron spins these large gold nanocluster can sustain at room temperature and further elucidates the behavior of superatomic electronic structure and behavior based on electron count. Furthermore, this dissertation presents the first investigations into the formation of supramolecular assemblies of gold nanocluster as recyclable materials, and more interactions of gold nanoclusters based on ligand layer interactions through polymorphism studies and ligand exchange studies. These investigations all help understand how to control the ligand layer for future applications of gold nanoclusters and nanoparticles, from molecular to bulk materials.
Open Access
Glyme-synthesized nanomaterials
(Colorado State University. Libraries, 2021) Armstrong, James, author; Ackerson, Christopher J., advisor; Prieto, Amy, committee member; Kennan, Alan, committee member; Basaraba, Randall, committee member
Nanomaterials include materials with at least one dimension in the nanometer range. These materials include nanoparticles, quantum dots, thin films, self-assembled materials, supramolecular materials and more. Nanoscience is an intriguing field for cutting edge research for energy, biology, medicine, optical and other applications. Coinage-metal (Au, Ag, Cu) nanomaterials are particularly of interest for the stability of nanoparticles synthesized with these metals. These metals can also be utilized to produce supramolecular assemblies, e.g. Hydrogels. In particular, this dissertation will cover four projects involving coinage metal nanomaterials. Chapter 2 discusses the ligand-exchange of a gold-thiolate nanocluster synthesized in diglyme, while chapters 3-5 investigate a unique supramolecular assembly of coinage-metal thiolates using glymes as antisolvent. Chapter 3 explores the underlying makeup of these amorphous assemblies, while chapters 4 and 5 investigate the application of this supramolecular assembly for additive manufacturing applications and antimicrobial applications, respectively. All of these products are linked through the synthesis and characterization of nanomaterials, which require the use of glymes (1,2-dme, diglyme, triglyme, etc.) as a necessary synthetic solvent or antisolvent. Nanoclusters are small, atomically precise nanoparticles with a metal core and a passivating layer of organic ligands. Coinage metal nanoclusters are studied for their stability, especially gold nanoclusters, allowing for long-term studies of properties and applications, as well as post-synthetic modifications. Precise control over ligand shell composition, particularly of mixed ligand layers is desired for control over nanocluster functionality. Supramolecular materials build bulk properties through noncovalent interactions. Self-assembled supramolecular materials utilize small molecules which assemble into larger secondary and tertiary structures. These materials are of interest for a broad range of applications like additive manufacturing and biological applications. The motivation behind this work was to explore nanomaterials which results from a glyme based synthesis. Gold nanocluster synthesis in diglyme is found to produce a stable gold-thiolate nanocluster with a single glyme ligand. The precision of a single-unique ligand could lead to further enhancements in nanocluster functionality in the future. Addition of glyme to a coinage-metal thiolate solution results in the rapid precipitation of a rigid supramolecular assembly. The resultant metallogel exhibits properties unique from similar materials without the use of glyme in synthesis. The metallogel is composed of oligomers reminiscent of nanoparticle precursors; as such, metallogel-nanoparticle composites are readily synthesized.
Open Access
Interrogating reactions of gold nanoclusters: insights into catalysis and the Brust-Schiffrin synthesis
(Colorado State University. Libraries, 2017) Dreier, Timothy Andrew, author; Ackerson, Christopher J., advisor; Kennan, Alan J., committee member; Henry, Charles, committee member; Peebles, Christie, committee member
Over the past several decades, interest in the synthesis and behavior of atomically precise gold nanoclusters has gained substantial momentum. Herein, both catalytic behavior and synthetic mechanisms are explored using techniques more typically applied to organic chemistry. In the case of catalysis, Au25(SR)18 has emerged as a well-studied model system. In an effort to investigate their potential as intact, homogeneous, unsupported catalysts, we have discovered that Au25(SR)18 clusters are not stable in oxidizing conditions reported for catalytic styrene oxidation. Further investigation suggests that the active catalytic species is an Au(I) species resulting from oxidative decomposition of the starting gold cluster. Equally important to chemical behavior is an understanding of the reaction dynamics during the synthesis of atomically precise clusters. Because the Brust-Schiffrin method is the standard procedure by which gold nanoclusters are synthesized, the role of oxygen in it has been investigated for both organic and aqueous systems. In either case, it is clear obtaining the desired product depends on a radically mediated etching step. These results give new insight into how the Brust-Schiffrin method might be modified to further synthesis of uniquely interesting nanocluster systems.
Open Access
The dynamic nature of ligand layers on gold nanoclusters
(Colorado State University. Libraries, 2020) Hosier, Christopher Allen, author; Ackerson, Christopher J., advisor; Kennan, Alan J., committee member; Henry, Chuck, committee member; Kipper, Matthew, committee member
Gold nanoclusters have been heavily investigated over the last few decades for their potential use in sensing, imaging, energy conversion, and catalytic applications. The development of methodology that allows for controlled functionalization of the surface ligand layer in these compounds is of particular interest due to the role of ligands in determining a large number of cluster properties. One of the fundamental ways of tailoring the ligand layer is the use of ligand exchange reactions. Despite the synthetic utility that ligand exchange reactions afford, a significant number of unanswered challenges currently limits the scope and control that can be obtained with these reactions. While a large variety of ligand types have been used to protect nanocluster surfaces, the majority of reported ligand exchange reactions revolve around chalcogenate-for-chalcogenate exchange. Site-selectivity in these reactions is limited to kinetic phenomenon, and the role of intercluster exchange largely remains a mystery. Additionally, recent works suggest that changes in ligand orientation can impact bulk material properties. In this thesis, we seek to address these challenges by reporting new exchange methodology, probing the evolution of exchanged ligand layers over time, investigating the stability of ligand layers in reaction conditions, and exploring the impact of ligand orientation on nanocluster behavior and reactivity. By addressing these questions and challenges, we seek to move closer to the goal of developing methodology that can be easily and reliably used to tailor gold nanoclusters for directed applications.
Open Access
The use of coordinating solvents in gold cluster synthesis
(Colorado State University. Libraries, 2016) Compel, W. Scott, author; Ackerson, Christopher J., advisor; Finke, Richard, committee member; Krapf, Diego, committee member; Prieto, Amy, committee member; Reynolds, Melissa, committee member
Monolayer-protected clusters (MPCs) are nanoparticles ca. 1-3 nm in diameter composed of a metal core and an organic monolayer shell. In this size range MPCs are larger than metal-ligand complexes but too small to exhibit a surface plasmon resonance. The electronic structures of particles in this size regime resemble discrete molecular orbital energy levels as opposed to the band-like behavior observed in larger, plasmonic nanoparticles. MPCs are composed of ten to a few hundred atoms and can be characterized as simple chemical compounds with discrete molecular formulae as opposed to average particle diameters. In these systems, addition or removal of a single metal atom profoundly affects stability and observed properties. This phenomenon gives rise to an exceptionally diverse class of materials with seemingly endless potential evolving from minute compositional changes. Thiolate-protected gold clusters are exemplary MPCs due to their intrinsic high stability that allows for long-term studies and post-synthetic modification. These clusters exhibit unique physiochemical properties that allow for potential applications in electronics, catalysis, biomedicine, and sensing. The past two decades since their discovery brought about a significant body of research regarding the origin of Au cluster properties and total structure elucidation. However, modern approaches for Au cluster synthesis produce polydisperse mixtures of clusters that must undergo extensive postreaction ripening or fractionalization to obtain a pure, single product. New synthetic approaches for monodisperse Au clusters in high yield must be developed before their applications may be realized. The motivation behind this work was to explore the issue of polydispersity in Au cluster synthesis. Through combinatorial screening of synthetic co-solvent systems we find that synthesis in coordinating solvents (i.e., glymes) greatly enhances the monodispersity of Au cluster products. During synthesis, glyme chelates the metal in the metallopolymer precursor and modifies the surface of the resulting particle, resulting in a new series of metastable Au clusters. The synthetic methods presented herein result in pure, single products in high yield. The surface modification brought about by diglyme potentially renders the clusters available for single-ligand functionalization to tailor cluster properties for desired functionality. The products are evaluated for biomedical and sensing applications.

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