Since the fortuitous discovery of the existence of quantum size effects on the band structure of colloidal semiconductor nanocrystals, the development of synthetic methods that can form nanoscale crystalline materials of controllable size, shape, and composition has blossomed as an empirical scientific achievement. The fact that the term “recipe” is commonly used within the context of describing these synthetic methods is indicative of the experimentally driven nature of the field. In this respect, the highly attractive photophysical properties of semiconductor nanocrystals—as cheap wavelength tunable and high quantum yield absorbers and emitters of light for various applications in lighting, biological imaging, solar cells, and photocatalysis—has driven much of the interest in these materials. Nevertheless, a more rigorously predictive first-principles-grounded understanding of how the basic processes of nanocrystal formation (nucleation and growth) lead to the formation of semiconductor nanocrystals of desired size and size dispersity remains an elusive practical and fundamental goal in materials chemistry. In this thesis, we describe efforts to directly study these dynamic nucleation and growth processes for lead chalcogenide nanoparticles, in many cases in-situ, using a mixture of X-ray scattering and UV-Vis/NIR spectroscopy. The lack of a rigorously predictive and verified mechanism for nanocrystal formation in solution for many material systems of practical interest is due both to the inherent kinetic complexity of these reactions, as well as the spectroscopic challenge of finding in-situ probes that can reliably monitor nanoscale crystal growth. In particular, required are direct time-resolved structural probes of metastable inorganic amorphous and crystalline intermediates formed under the high temperature inert conditions of nanocrystal synthesis. It is, at the very least, highly challenging to apply many of the standard spectroscopic tools of mechanistic inorganic and organic chemistry such as ¹H NMR spectroscopy, IR vibrational spectroscopy, and mass spectrometry to this task. A notable counterexample is, of course, UV-vis/NIR absorbance and emission spectroscopies, which are of great value to the studies described herein. Nevertheless, to address this relative dearth of conventional spectroscopic probes, here we explore the use of X-ray Total Scattering real space Pair Distribution Function (PDF) analysis and Small Angle X-ray Scattering (SAXS) techniques to directly probe the crystallization process in-situ. Time-resolved measurements of the small angle reciprocal space scattering data allow mapping of the time evolution of the colloidal size and concentration of the crystals during synthesis, while the Fourier transform of scattering data over a wide range of reciprocal space provides direct insight into the local structure. Through this approach, we compare direct observations of these nucleation and growth processes to the widely cited theoretical models of these processes (Classical Nucleation Theory and LaMer “Burst Nucleation”) and find a number of stark differences between these widely cited theories and our experiments. The first two chapters cover the results of these 𝘪𝘯-𝘴𝘪𝘵𝘶 diffraction studies. Chapter 1 focuses on small angle X-ray scattering data collection and modeling. Chapter 2 focuses upon lead sulfide and lead selenide real space PDF analysis of local structural evolution during synthesis. Finally, Chapter 3 discusses a project in which we examine the origins of emergent semiconducting electronic structure in an increasing size series of atomically precise oligomers of [Ru₆C(CO)₁₆]²⁻ bridged by Hg²⁺ and Cd²⁺ atoms. Using an atomically well-defined series of molecules that bridge the small molecule and nanoscale size regimes, we discuss the factors that give rise to controllable semiconductor electronic structure upon assembly into extended periodic structures in solution. In all these projects, we seek to highlight the value of applying concepts of molecular inorganic chemistry—ligand binding models, relative bond strengths, in addition to kinetics and thermodynamics—to explain our observations regarding nanocrystal nucleation and growth. Consideration of the chemistry of nanocrystal formation processes provides a valuable compliment to the physics-based classical models of nucleation and growth that do not explicitly consider the system specific molecular structure and bonding.

D.Phil. / Clean water (i.e. water that is free of toxic chemicals and pathogens) is essential to human health and in South Africa the demand is fast exceeding the supply. The prevalence of toxic contaminants in water remains a huge challenge for water supplying companies and municipalities. However, the presently used water treatment technologies either fail to remove these contaminants to acceptable levels or they transform them into more toxic substances (e.g., DBPs). Nanocatalysts, especially TiO2 (titania) have a proven potential to treat ‘difficult-to-remove’ contaminants and hence are expected to play an important role in solving many serious environmental and pollution challenges. In this study TiO2 nanocatalysts were used for the degradation of Rhodamine B dye both under UV and visible light irradiation. Two phases of titania, i.e. anatase and rutile phases, were compared for the degradation of Rhodamine B under UV light irradiation. The anatase titania was found to be more photocatalytically active for the degradation of Rhodamine B than the rutile phase. It completely degraded 100 mg ℓ–1 (100 mℓ) of Rhodamine B within 270 min and was two times more photocatalytically active than the rutile phase (Kapp of 0.017 min–1 compared to 0.0089 min–1). To extend the band edge of the titania nanocatalysts towards visible-light, TiO2 was doped with metal ions (Ag, Co, Ni and Pd). These metal-ion-doped titania nanocatalysts were photocatalytically active under visible-light illumination. The Pd-doped titania had the highest photodegradation efficiencies, followed by Ag-doped and Co-doped, while Ni-doped had the lowest. The optimum metal-ion loading percentage was found to be at 0.4%, with the exception of Co-doped titania as it had the highest efficiencies at 1% loadings. The free and metal-ion-doped titania nanocatalysts were embedded on carbon-covered alumina (CCA) supports. The CCA-supported TiO2 nanocatalysts were more photocatalytically active under visible light illumination than they were under UV-light irradiation. The CCA-supported metal-ion-doped titania nanocatalysts were more photocatalytically active under visible light than their unsupported counterparts. The CCA-supported Pd-TiO2 nanocatalysts were the most active while CCA-supported Ni-TiO2 catalysts were the least active. The improved photocatalytic activities observed were as a result of increased surface areas of the CCA-supported nanocatalysts. Also, supporting the nanocatalysts did not destroy the anatase phase of the titania while doping with metal ions and supporting on CCAs resulted in decreased band gap energies, hence the visible-light photocatalytic activities. Finally, the metal-ion-doped titania nanocatalysts were supported on glass slides using the layer-by-layer thin film self-assembly technique. This was to overcome the aggregation and post treatment problems associated with the use of TiO2 in suspension form. PAH and PSS were the polyelectrolytes used. These metal-ion-doped titania thin films were highly porous and strongly adhered by the polyelectrolytes onto the glass slides. The thin films were photocatalytically active for the degradation of Rhodamine B under visible light irradiation. The photocatalytic degradation efficiencies observed were similar for all four metal-ions (i.e. Ag, Co, Ni and Pd) with average degradation of 30%, 50%, 70% and 90% for 5 catalysts (5 glass slides) of 1, 3, 5 and 10 bi-layers, respectively, after 330 min. Although, these were less active than the suspended titania nanocatalysts, this study proved as a stepping stone towards large scale use of titania nanocatalysts using solar energy as the irradiation source. Also, catalyst reusability studies were performed and the PAH/PSS m-TiO2 thin films were found to be highly stable over the five cycles it was tested for.

Thesis advisor: Chia-kuang Frank Tsung / Pt-based alloy and core-shell nanoparticles have been intensively studied to regulate its size and shape. It has known that these nanoparticles show enhanced catalytic activity in various important fields such as heterogeneous catalysis, and electrochemical energy storage including fuel cells and metal-air batteries. Here, we report a facile hydrothermal synthesis of sub-10 nm PdPt alloy and sub-20 nm Au@PdPt core-shell structures. By using a mild reducing agent in aqueous solution, metal precursors are co-reduced. Specific gases are introduced during the synthesis to optimize the reaction conditions. The PdPt alloy and Au@PdPt core-shell nanostructures were characterized and confirmed by TEM, HRTEM, EDS, ICP-OES and XRD. The resulting PdPt and Au@PdPt particles are monodispersed single crystalline and octahedral shape enclosed by (111) facets. The electrocatalytic activity for the oxidation of formic acid was tested. It was found that the catalytic activity toward the formic acid oxidation of Au@PdPt core-shell particles were much higher than those of PdPt alloy particles. In addition, Pt-rich compositions were the most active in both PdPt alloy and Au@PdPt core-shell nanoparticles. Further studies on thinner alloy-shell core-shell nanoparticles reveal that there is a volcano-curve relationship between the lattice strain strength related to alloy-shell thickness and the catalytic performance. It is proposed that there are three key parameters that can determine the catalytic activity: the alloy composition, the presence of the gold core, and the thickness of alloy-shell. / Thesis (MS) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

<p>DNA nanotechnology has provided programming construction of various nanostructures at nanometer-level precision over the last three decades. DNA self-assembly is usually implemented by annealing process in bulk solution. In recent several years, a new method thrives by fabricating two-dimensional (2D) nanostructures on solid surfaces. My researches mainly focus on this field, surface-assisted DNA assembly driven by base stacking. I have developed methods to fabricate DNA 2D networks via isothermal assembly on mica surfaces. I have further explored the applications to realize quasicrystal fabrication and nanoparticles (NPs) patterning.</p><p><br></p> <p>In this dissertation, I have developed a strategy to assemble DNA structures with 1 or 2 pair(s) of blunt ends. Such weak interactions cannot hold DNA motifs together in solution. However, with DNA-surface attractions, DNA motifs can assemble into large nanostructures on solid surface. Further studies reveal that the DNA-surface attractions can be controlled by the variety and concentration of cation in the bulk solution. Moreover, DNA nanostructures can be fabricated at very low motif concentrations, at which traditional solution assembly cannot render large nanostructures. Finally, assembly time course is also studied to reveal a superfast process for surface-assisted method compared with solution assembly.</p><p><br></p> <p>Based on this approach, I have extended my research scope from 1D to 2D structures assembled from various DNA motifs. In my studies, I have successfully realized conformational change regulated by DNA-surface interaction and steric effect. By introduction of DNA duplex “bridges” and unpaired nucleotide (nt) spacers, we can control the flexibility/rigidity of DNA nanomotifs, which helps to fabricate more delicate dodecagonal quasicrystals. The key point is to design the length of spacers. For 6-point-star motif, a rigid structure is required so that only 1-nt spacers are added. On the other hand, 3-nt spacers are incorporated to enable an inter-branch angle change from 60° to 90° for a more flexible 5-point-star motif. By tuning the ratio of 5 and 6 -point-star motifs in solution, we can obtain 2D networks from snub square tiling, dodecagonal tiling, a mixture of dodecagonal tiling and triangular tiling, and triangular tiling.</p><p><br></p> Finally, I have explored the applications of my assembly method for patterning NPs. Tetragonal and hexagonal DNA 2D networks have been fabricated on mica surfaces and served as templates. Then modify the surfaces with positively-charged “glues”, <i>e.g.</i> poly-L-lysine (PLL) or Ni²⁺. After that, various NPs have been patterned into designated lattices, including individual DNA nanomotifs, gold NPs (AuNPs), proteins, and silica complexes. Observed NP lattices and fast Fourier Transform (FFT) patterns have demonstrated the DNA networks’ patterning effect on NPs.

In recent years, DNA nanotechnology has emerged as one of the most powerful strategies for bottom-up construction of nanomaterials. Due to the high programmability of DNA molecules, their self-assembly can be rationally designed. Engineered 3D DNA crystals, as critical products from the design of DNA self-assembly, have been proposed as the structural scaffolds for organizing nano-objects into three-dimensional, macroscopic devices. However, for such applications, many obstacles need to be overcome, including the crystal stability, the characterization methodology, the revision of crystal designs as well as the modulation of crystallization kinetics. My PhD research focuses on solving these problems for engineered 3D DNA crystals to pave the way for their downstream applications.<br>In this thesis, I started by enhancing the stability of engineered 3D DNA crystals. I developed a highly efficient post-assembly modification approach to stabilize DNA crystals. Enzymatic ligation was performed inside the crystal lattice, which was designed to covalently link the sticky ends at the crystal contacts. After ligation, the crystal became a covalently bonded 3D network of DNA motifs. I investigated the stability of ligated DNA crystals under a wide range of solution conditions. Experimental data revealed that ligated DNA crystals had significantly increased stability. With these highly stabilized DNA crystals, we then demonstrated their applications in biocatalysis and protein encapsulation as examples.<br>I also established electron microscope imaging characterization methods for engineered 3D DNA crystals. For crystals from large-size DNA motifs, they are difficult to study by X-ray crystallography because of their limited diffraction resolutions to no better than 10 Å. Therefore, a direct imaging method by TEM was set up. DNA crystals were either crushed or controlled to grow into microcrystals for TEM imaging. To validate the imaging results, we compared the TEM images with predicted models of the crystal lattice. With the advance in crystal characterization, DNA crystals of varying pore size between 5~20 nm were designed, assembled, and validated by TEM imaging.<br>The post-assembly ligation was further developed to prepare a series of new materials derived from engineered 3D DNA crystals, which were inaccessible otherwise. With the directional and spatial control of ligation in DNA crystal, I prepared new DNA-based materials including DNA microtubes, complex-architecture crystals, and an unprecedented reversibly expandable, self-healing DNA crystal. The integration of weak and strong interactions in crystals enabled a lot of new opportunities for DNA crystal engineering.<br>In the final chapter, I investigated the effect of 5’-phosphorylation on DNA crystallization kinetics. I found that phosphorylation significantly enhanced the crystallization kinetics, possibly by strengthening the sticky-ended cohesion. Therefore, DNA crystals can be obtained at much lower ionic strength after phosphorylation. I also applied the result to controling the morphology of DNA crystals by tuning the crystallization kinetics along different crystallographic axes. Together with previously methods to slow down DNA crystallization, the ability to tune DNA crystallization kinetics in both ways is essential for DNA crystal engineering.

In this study, we designed a series of experiments to determine the factors governing ion permeation through individual carbon nanotubes (CNTs) less than 1.5 nm in diameter and 20 µm in length. We then rationalize the experimental results by using a model, which is drawn from previous literature on protein ion channels and is centered around a simplified version of the Gouy-Chapman theory of electrical double layer. Lastly, we experimentally demonstrate and discuss the general similarities in ion permeation characteristics between CNTs and biological ion-selective pores. The role of many potential factors influencing the ion transport is assessed by taking two experimental approaches: (1) studying the effect of electrolyte concentration and composition on channel conductance and reversal potential, and (2) examining a second type of nanochannel as a parallel ion conduction pathway within the same device architecture and measurement set-up, which we refer to as leakage devices. This helps to differentiate the effect of CNT on ionic transport from any other possible source. Taken together, these two experimental methods provide strong evidence that the electrostatic potential arising from ionized carboxyl groups at the nanopore entrance has a significant effect on ionic permeation in a manner consistent with a simple electrostatic mechanism.

<p>Peptides are a fundamental building-block of living systems and play crucial roles at both functional and structural level. Therefore, they have attracted increased attention as a platform to design and engineer new self-assembled systems that span the nano-to-meso scales. The rules of peptide design and folding enable the construction of suitable building-blocks to develop soft materials for biomaterial applications. Herein we present the use of the alpha-helical secondary structure to create two distinct structural motifs, namely coiled-coils and helical bundles. These peptide components can differ in size and incorporate a host of different functional moieties, the effects of which are described through their hierarchical assembly. </p> <p>First, we describe the self-assembly of coiled coil oligomers (trimer and tetramer) of the GCN4 leucine zipper peptide. The trimeric coiled coil was modified with varying number of aromatic groups (one to three) along each helical backbone, to facilitate higher order assemblies into banded nano- to micron-sized structures, the formation of which could be controlled reversibly as a function of pH. In addition, the electrostatic and aromatic interactions of the peptide material were harnessed for non-covalent binding of small drug molecules, followed by their subsequent pH-triggered release. Furthermore, these nanostructures are compatible with MCF-7 breast cancer cells, making them suitable drug-delivery agents for chemotherapeutics. In the absence of aromatic modifications, the coiled-coil trimer assembles into higher-order nanotubes that can be harnessed for selective encapsulation of high molecular weight biomolecules. With an increase in oligomerization from three to four, along with a single aromatic group modification on each helix, the tetrameric coiled-coil mutant successfully demonstrates a metal-assisted two-tier structural assembly into microbarrels and spheres.</p> <p>Second, we present the higher-order assembly of short tetrameric and pentameric helical bundle proteins, covalently stabilized by a belt of disulfide bridges, with metal-binding ligands at each helix termini. The addition of metals like Zn(II) and Cu(II) promote the assembly of the bundles into a 3D globular matrix, which upon thermal annealing transforms into microspheres. Additionally, these microspheres also demonstrate the metal-assisted inclusion of His-tagged fluorophores. Thus, peptide-based materials can be constructed by self-assembly of alpha-helical building blocks into systems with sophisticated, diverse morphologies and dynamic chemical properties, that can be further modulated to enhance performance for medical applications. </p>

Thesis (Ph.D.) -- Mississippi State University. Dave C. Swalm School of Chemical Engineering.. / Title from title screen. Includes bibliographical references.

Abstract: ABC triblock copolymers self-organize into a wide variety of supramolecular structures in the bulk. However, their associative behavior in selective solvents has scarcely been studied. Within the search for new stimuli-responsive supramolecular architectures, our attention focused on a pH-responsive polystyrene-b-poly(2-vinylpyridine)-b-poly(ethylene oxide) (PS-b-P2VP-b-PEO) triblock copolymer. In addition to the synthesis of monodisperse spherical core-shell-corona (CSC) micelles, the reversibility and the cooperativity of the response to pH variations were studied, morphological transitions were induced and multi-responsive micellar gels were prepared. The micellization mechanism, the structure, the responsiveness and the internal organization of these new nanomaterials were investigated using a combination of transmission electronic microscopy, atomic force microscopy, light scattering, small-angle neutron and X-ray scattering, nuclear magnetic resonance and rheology. Finally, efforts were geared towards potential applications. The ability of PS-b-P2VP-b-PEO CSC micelles to encapsulate and release hydrophobic species was probed and gold nanoparticles were successfully synthesized within the P2VP layer of spherical and cylindrical micelles, which acted as nanoreactors./Résumé : Les copolymères triséquencés ABC sauto-organisent et forment une large gamme de structures supramoléculaires en phase solide. Cependant, peu détudes portent sur leur comportement associatif induit par des solvants sélectifs. Dans le cadre de la recherche de nouvelles architectures supramoléculaires sensibles aux stimuli externes, nous avons entrepris létude dun copolymère triséquencé sensible au pH : polystyrène-b-poly(2-vinylpyridine)-b-poly(oxyde déthylène). Outre la synthèse de micelles sphériques de type CSC, le caractère réversible et coopératif de la réponse au pH a été étudié, ainsi que linduction de transitions morphologiques et la préparation de gels micellaires sensibles à la température et au pH. Le mécanisme de micellisation, les paramètres structuraux, la sensibilité aux stimuli ainsi que lorganisation interne de ces nouveaux nanomatériaux ont été étudiés par une combinaison de microscopies électronique à transmission et à force atomique, diffusion lumineuse, diffusion de neutrons et rayons X aux petits angles, résonance magnétique nucléaire et rhéologie. Enfin, des applications ont été envisagées : la capacité des micelles CSC à encapsuler et libérer des composés hydrophobes a été testée et des nanoparticules dor ont été synthétisées avec succès au sein de ces nanoréacteurs, cest-à-dire dans la couche de P2VP des micelles sphériques et cylindriques.

Graphene has great potential for energy storage and conversion applications due to its outstanding electrical conductivity, large surface area and chemical stability. However, the pristine graphene offers unsatisfactory performance as a result of several intrinsic limitations such as aggregation and inertness. The functionalization of graphene is considered as a powerful way to modify the physical and chemical properties of graphene, and improve the material performance, which unfortunately still being preliminary and need further knowledge on controllable functionalization methods and the structure-property relationships. This thesis aims to provide in-depth understanding on these aspects. We firstly explored oxygen-functionalized graphene for supercapacitor electrodes. A mild solvothermal method was developed for graphene preparation from the reduction of graphene oxide; the solvent-dependent reduction kinetics is an interesting finding in this method that could be attributed to the solvent-graphene oxide interactions. Using the solvothermal method, oxygen-functionalized graphene with controlled density of oxygen functional groups was prepared by tuning the reduction time. The oxygen-containing groups, primarily phenols and quinones, reduce the graphene aggregation, improve the wetting properties and introduce the pseudocapacitance. Consequently, excellent supercapacitive performance was achieved. Nitrogen-doped graphene was synthesized by the pyrolysis of graphene oxide with nitrogen-containing molecules and used as an electrocatalyst for oxygen reduction reactions. We achieved the structural control of the nitrogen-doped graphene, mainly the content of graphitic nitrogen, by manipulating the pyrolysis temperature and the structure of nitrogen-containing molecules; these experiments help understand the evolution of the bonding configurations of nitrogen dopants during pyrolysis. Superior catalytic activity of the prepared nitrogen-doped graphene was found, due to the enriched content of graphitic nitrogen that is most active for the oxygen reduction reaction. Moreover, we demonstrated a facile strategy of producing superhydrophobic octadecylamine-functionalized graphite oxide films. The long hydrocarbon chain in octadecylamine reduces the surface energy of the graphene oxide film, resulting in a high water contact angle and low hysteresis. The reaction mechanism and the effect of hydrocarbon chain length were systematically investigated. In addition to the researches on graphene-based materials, some results on advanced carbon nanomaterials and polymer composites for electronic packaging will also be discussed as appendix to the thesis. These include carbon nanotube-based capacitive deionizer and gas sensor, and hexagonal boron nitride-epoxy composites for high thermal conductivity underfill.