Browsing by Subject "Dendrimer"
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Item Characterization of dendrimer encapsulated nanoparticles by extended x-ray absorption fine structure and electrochemical methods(2011-12) Myers, Vera Sue; Crooks, Richard M. (Richard McConnell)The small size regime and bulky hydrocarbon exterior of dendrimer encapsulated nanoparticles (DENs) often make characterization of these materials a unique challenge. Here, I report on three studies utilizing the techniques of extended X-ray absorption fine structure (EXAFS) and electrochemistry to probe the properties and behavior of these materials. First, the synthesis and characterization of PdCu bimetallic nanoparticles, and Pd and Cu monometallic nanoparticles, consisting of an average of ~64 atoms is described. The bimetallic nanoparticles were prepared by co-complexation of Pd²⁺ and Cu²⁺ salts to interior functional groups of a dendrimer template followed by chemical reduction to yield DENs. EXAFS spectroscopy indicates that these particles have an alloy structure. This is a rare example of a stable nanoparticle in this size range that consists of one reactive metal and one substantially more noble metal. Second, in-situ electrochemical EXAFS is used to evaluate the structure of Pt DENs during the oxygen reduction reaction (ORR). The DENs contained an average of just 225 atoms each. The results indicate that the Pt coordination number (CN) decreases when the electrode potential is moved to positive values. The results are interpreted in terms of an ordered core, disordered shell model. The structure of the DENs is not significantly impacted by the presence of dioxygen, but other electrogenerated species may have a significant impact on nanoparticle structure. Third, the electrochemical dissolution of Cu DENs is investigated using anodic stripping voltammetry (ASV). The effect of the scan rate and Cu loading on the electrode to the stripping wave is performed. The results indicate a large, positive shift of the stripping potential for the dendrimer-metal composites, but no size-dependent changes to peak position.Item Dendrimer-encapsulated metal nanoparticles: synthesis, characterization, and applications to catalysis(Texas A&M University, 2004-09-30) Niu, YanhuiThe research in this dissertation examines the chemistry and applications of dendrimers in homogeneous catalysis. We examined interactions between dendrimers and charged probe molecules, prepared dendrimer-encapsulated metal nanoparticles in organic solvents, studied size-selectivity of dendrimer-encapsulted catalysts, and designed molecular rulers as in-situ probes to measure the location of dendrimer-encapsulted metal nanoparticles. The intrinsic proton binding constant and a constant that characterizes the strength of electrostatic interactions among occupied binding sites in poly(amidoamine) (PAMAM) dendrimers have been obtained by studying the effect of solution pH on the protonation of the dendrimers. The significant finding is that these two factors are greatly modulated by the unique and hydrophobic microenvironment in the dendrimer interior. Hydrophilic poly(propylene imine) (PPI) dendrimers were modified with various hydrophobic alkyl chains through an amide linkage and were then used as templates for preparing intradendrimer copper nanoclusters. The main driving force for encapsulating metal-ions was found to be the differences in metal-ion solubility between the solvent and the interior of the dendrimer. Nanometer-sized metal particles are synthesized and encapsulated into the interior of dendrimers by first mixing together the dendrimer and metal ion solution and then reducing the composite chemically, and the resulting dendrimer-encapsulated metal nanoparticles can then be used as catalysts. By controlling the packing density on the dendrimer periphery using either different dendrimer generations or dendrimer surface functionalities, it is possible to control access of substrates to the encapsulated catalytic nanoparticle. Molecular rulers consisting of a large molecular "stopper", a reactive probe and a linker were designed as in-situ probes for determining the average distance between the surface of dendrimer-encapsulated palladium nanoparticles and the periphery of their fourth-generation, hydroxyl-terminated PAMAM dendrimer hosts. By doing so, we avoid having to make assumptions about the nanoparticle size and shape. The results suggest that the surface of the encapsulated nanoparticle is situated 0.7 ? 0.2 nm from the surface of the dendrimer.Item Dendrimer-encapsulated nanoparticles : synthetic methods and characterization including extended X-ray absorption-fine structure(2010-12) Weir, Michael Glen; Crooks, Richard M. (Richard McConnell); Bard, Allen J.; Frenkel, Anatoly I.; Henkelman, Graeme; Johnston, Keith; Willets, KatherineThis work describes the synthesis of dendrimer-encapsulated nanoparticles (DENs) and the expansion of the characterization ability for these materials. The dendrimer-template method for the synthesis of nanoparticles allows precise control over the size, composition and structure of nanoparticles in the 40-250 atom range. In this size regime, the surface structure of the nanoparticles dominates their catalytic properties. The long term goal of this research is to correlate the structure of these nanoparticles to their catalytic activity, improving the ability to predict superior catalysts a priori. As a prerequisite for this analysis, the precise structure of the catalytically active nanoparticle must be determined. Characterization of nanoparticles in the 1-2 nm region is significantly more difficult than more commonly used nanoparticles of 3-5 nm diameter or larger. Typical characterization of these nanoparticles involves UV-vis spectroscopy for Mie absorbance and transmission electron microscopy for size analysis. This work involves the use of extended X-ray absorption-fine structure (EXAFS) to determine the local structure of the nanoparticles. For monometallic Pt DENs, EXAFS was combined with UV-vis, TEM, X-ray photoelectron spectroscopy (XPS) and electrochemistry to determine that the Pt system is not simply nanoparticles but a more complex, bimodal state. EXAFS has also been used to differentiate between different bimetallic structures. For PdAu DENs, there are two synthetic methods used. When both metals are reduced simultaneously, the resulting nanoparticles have a quasi-random alloy structure. These nanoparticles were then extracted from the dendrimer into an organic solvent by use of alkanethiols. The extraction process changed the alloy structure into Au-core/Pd-shell. When Pd and Au were reduced in sequence, the DENs were formed as a Au-core/Pd-shell material, regardless of the order of the reduction of the metals. The Au-core/Pd-shell structure was also present after extraction. In addition to structural analysis to determine the result of different synthetic methods, EXAFS was also used in situ to measure the structure of Pt DENs during the oxidation of absorbed CO. These in situ measurements are important for determining the structure of the actual catalyst rather than the precursor nanoparticle. In this case, the Pt DENs changed from a bimodal distribution into fully reduced nanoparticles by the application of a reducing potential. The binding of CO to the Pt DENs and subsequent oxidation did not cause measurable agglomeration of the nanoparticles. This reduction of the Pt system by electrochemical means was also explored as a synthetic method. The Pt-dendrimer complex was placed on a TEM grid for electrochemical treatment. A potential step was shown to reduce some of the Pt-dendrimer complexes into Pt nanoparticles of the expected size. However, most of the complexes were not reduced. Therefore, only the standard chemical reduction followed by electrochemical treatment is sufficient to fully reduce the nanoparticle samples. This work has explored additional synthetic methods for the synthesis of monometallic and bimetallic DENs. The use of EXAFS, as well as other advanced characterization techniques, has advanced knowledge of the structure of various DENs. Both the characterization toolset and the synthetic methods will provide a basis for investigations of catalytically active materials.Item Dendrimers as drug and gene delivery vectors : a self consistent field theory study(2013-08) Lewis, Thomas Wade Stakesby; Ganesan, VenkatThis research focuses on the modeling of dendrimer molecules for their application as delivery vectors within drug and gene therapy systems. We examine how the architecture and composition of dendrimers affect their drug and gene binding efficacies along with their interactions with anionic bilayers. We specifically focus on how the weakly basic nature of dendrimer monomers and the addition of neutral grafts to dendrimer surface groups affect their interactions with drugs, linear polyelectrolytes, and bilayers. By using polymer self-consistent field theory (SCFT) to model such systems, we develop a computationally efficient means to provide physical insights into these systems, which are intended to guide dendrimer design for delivery applications.We study the conformational properties of weakly basic (annealed) polyelectrolyte dendrimers by developing a SCFT model that explicitly accounts for the acid-base equilibrium reaction of the weakly basic monomers. We specifically focus on the role of local counterion concentration upon the charge and conformations of the annealed polyelectrolyte dendrimers. We compare our results to existing polymer scaling theories and develop a strong stretching theory for the dendrimer molecules.We extend the previous study to model the interactions between weakly basic dendrimers and weakly acidic, hydrophobic drug molecules. We specifically examine the effects of excluded volume, electrostatic, and enthalpic interactions on the binding efficacies between dendrimers and drugs under a variety of dendrimer generations, solution pOH conditions, drug sizes, and Bjerrum length values.We study the role of neutral dendrimer grafts on the conformations and drug binding efficacies of dendrimers. We then elucidate how the observed conformational changes affect the charge of the dendrimers. Furthermore, we examine how the presence of grafts affects the steric, electrostatic, and hydrophobic interactions between the drugs and dendrimers under a variety of solution conditions. We compare our results with the binding efficacies observed for non-grafted dendrimers to delineate the conditions under which the grafted dendrimers are better suited as drug hosts.We include semi-flexible, anionic linear polyelectrolyte (LPE) molecules in our grafted dendrimer SCFT framework to model the interactions between dendrimers and negatively charged genetic materials. Specifically, we examine how neutral dendrimer grafts, LPE stiffness, and solution pOH affect the interactions between dendrimers and LPEs. We then use our SCFT potential fields as input into Monte Carlo simulations in order to determine the dendrimer-LPE potentials of mean force and the resulting loop and tail statistics of the dendrimer-adsorbed LPE chains.We incorporate a negatively charged bilayer into our grafted dendrimer SCFT framework to model dendrimer interactions with a cellular membrane. We specifically examine the role of dendrimer grafting length, solution pH, and membrane tension on such interactions. By comparing our results with SCFT calculations of fixed dendrimer conformations and hard sphere nanoparticles in the presence of membranes, we delineate the role of dendrimer flexibility and porosity on the interactions between dendrimers and anionic bilayers.Item Design, synthesis, and evaluation of dendrimers based on melamine as drug delivery vehicles(2009-05-15) Lim, Jong DooA variety of dendrimers based on melamine are designed, synthesized, and evaluated for drug delivery systems. The synthesis of a dendrimer, including multiple copies of four orthogonally reactive groups, is described. The three groups on the surface are nucleophilic and include four free hydroxyl groups, four tert-butyldiphenylsilyl (TBDPS) ether groups, and sixteen amines masked as tert-butoxycarbonyl (BOC) groups. The core of the dendrimer displays two electrophilic monochlorotriazines. The dendrimer above is further manipulated for in vivo biodistribution: incorporation of the reporting groups Bolton-Hunter and DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid); PEGylation for biocompatibility and size tuning. In preliminary biodistribution studies, dendrimers circulate in the blood for a longer time as the molecular weight increases, which is important to passively target tumor tissues via the EPR effect. Also, high uptake by the tumor tissues was observed in mice bearing prostate cancer xenografts. A drug delivery vehicle for the anticancer agent paclitaxel is described. This drug delivery vehicle contains sixteen molecules of paclitaxel via acid-labile ester linkage, two Bolton-Hunter groups, and sixteen monochlorotriazine groups for PEGylation. The in vitro drug release studies shows faster release of paclitaxel at lower pH in PBS.Item Methods for the syntheses of compositionally diverse dendrimers(Texas A&M University, 2005-11-01) Steffensen, Mackay BagleyDendrimers are a unique class of macromolecules that present perfect branching on a molecular scale. The pattern of branching at the atomic scale is compared to the branching of trees, from whence dendrimers get their name. Dendrimers have been attractive synthetic targets for the past twenty years. The methods and building blocks used in the synthesis of dendrimers vary, but molecules of this class of polymeric materials all possess symmetrical branching emanating from the core. At each branch point the number of groups increases exponentially. Efforts directed toward the synthesis of dendrimers presenting multiple functional groups at the surface and within the dendrimer structure are described. Methods are described which provide access to dendrimers in a one-pot per generation fashion, with triazines as the common moiety. Chemoselective routes utilize the temperature dependant substitution of cyanuric chloride to construct dendrimers, obviating the use of protected monomers or the need to manipulate functional groups during the synthesis. These methods are atom economical, as the only by-products are HCl and a base to scavenge it. The methods are efficient, with typical isolated yields of product in the middle to high ninety percent range, often on a multi-gram scale. Methods are described for conducting three separate reactions in a single pot. Specific emphasis is placed on structural control of the interior and surface groups of the dendrimers. The synthesis of a G3 dendrimer of layered composition is described. The use of a different difunctional linkage group for each generation of dendrimer growth produced a G3 dendrimer with layered composition without the use of protecting groups or functional group interconversions. A G3 dendrimer was synthesized presenting five different functionalities at the periphery on a 10 gram scale, resulting in approximately 70% overall yield. The peripheral groups are composed of orthogonal functionality, which can be independently and selectively unmasked or manipulated in the presence of the other functionality. The syntheses of dendrimers incorporating the short linker hydrazine produce materials with interesting physical properties as well as a low ratio of carbon to nitrogen. The use of dendrimers in the construction of novel macromolecular constructs is also described.Item NMR studies of the conformation of a triazine dendrimer and the synthesis of a platinated triazine dendrimer(2009-05-15) Moreno, Karlos XavierA general picture of dendrimer conformation has appeared through studies of various dendrimer systems. Though the studies define some conformational abilities of a dendrimer, the studies are only able to examine one portion of the general picture. NMR studies of a generation three melamine dendrimer with unique NMR signatures from core to periphery describes most, if not all, of the general concepts of dendrimers in one system. A generation three melamine dendrimer was synthesized by a convergent route using diamines identified from competition reactions towards a monochlorotriazine. The cyclic monoamines surveyed displayed a relative reactivity range of 40x, expanding the previously identified series to a range of 320x. Azetidine is 40x more reactive than the cyclic, nine-membered ring (C8H17N), and 320x more reactive than benzyl amine. Sterics and pKa values explain the differences in reactivity of the cyclic monoamines. Differences in the nucleophilicity of the amine groups consisting of 2-aminoazetidine, 2-aminopyrrolidine, and 4-aminopiperidine are 180x, 70x and 20x, respectively. One-dimensional NMR spectra of the exchangeable NH region show that the dendrimer supports a rich rotamer population. Observations of the data show that the rotamer populations change from a preferred extended conformation to a more closed conformation, indicative of sterics being a driving force of conformational architecture. Variable temperature NOESY studies show that the peripheral groups backfold into the interior of the dendrimer in DMSO- d6. The backfolding can be removed by changing the solvent to either CDCl3 or CD3OD. Variable temperature (VT) coefficients measured for the exchangeable NH protons implies that solvent may be excluded from the interior of the dendrimer. Proton relaxation studies provide evidence that the dendrimer tumbles slowly in solution, and the periphery moves more freely than the interior. Synthesis towards the attachment of carboplatin-like peripheral groups on a generation three dendrimer was unsuccessful. A diethyl malonate unit was attached to the periphery of the dendrimer followed by capping with 4-aminomethylpiperidine. Hydrolysis of the esters and treatment with activated platinum led to a black precipitate product. Two alternate routes of achieving the desired platinated dendrimer are described.Item Synthesis and application of melamine-based dendrimer/sba-15 hybrid materials(Texas A&M University, 2007-09-17) Lunn, Jonathan DavidPorous inorganic materials that can be used in applications such as catalysis and separations have been intensely studied due to their potential stability, ease of recovery and high surface areas. Organic-inorganic hybrid materials meet these criteria by exploiting the physical robustness of porous inorganic materials and the chemical functionality of organic materials. While amorphous oxides are widely used industrially as inorganic supports, disordered pore structures make them difficult to characterize. Ordered-mesoporous-silica (OMS), such as SBA-15, that have ordered pores structures simplify characterization and are useful models for studying hybrid materials. Dendrimers, once appropriately functionalized, have high densities of uniformly distributed functional groups. In this thesis, melamine-based dendrimer/SBA-15 hybrids were synthesized and characterized using a wide range of analytical techniques. This thesis shows that the porosity can be independently altered by varying 1) the dendrimer generation, 2) the linker molecule, and 3) the surface amine loading. Cu(II) sequestration results demonstrate that the peripheral functional groups of the dendrimer are readily accessible for binding. This thesis also describes preliminary work toward preparing an enantioselective catalyst using L-proline supported on OMS. This work includes the synthesis and testing of three dendrimer-like proline derivatives. Future work in this area is outlined and will include synthesizing catalysts based on the dendrimer hybrid system previously described. Solution dendrimer syntheses will also be performed to create solution-based catalysts.Item Synthesis, Characterization, and Applications of a Melamine Based Dendrimer with Twelve Cysteine Groups on the Periphery(2010-07-14) Vittur, Brandon M.A potential drug delivery vehicle based on a melamine dendrimer with twelve free thiols on the periphery for constructing bio-labile disulfides has been synthesized. Under ideal conditions for the native chemical ligation reaction, attempts for attaching the cell penetrating peptide TAT, via native chemical ligation proved difficult due to the low solubility of the dendrimer. A camptothecin derivative containing a reactive disulfide was prepared for disulfide exchange with the melamine dendrimer. Up to 7 exchange reactions were achieved as determined by mass spectroscopy. NMR and mass spectroscopy was used to characterize all of the intermediates. Capping groups to replace the hydrophobic piperidine with more water-soluble groups to aid the ligation reaction and optimization of the disulfide exchange step to give 12 substitutions have been proposed for future studies. The end target is a peptide dendrimer containing a cell penetrating peptide to mediate endocytosis and a bio-labile linker connecting an anti-tumor drug to the dendrimer, which would ultimately be released inside the cancerous cell.Item Underpotential deposition as a synthetic and characterization tool for core@shell dendrimer-encapsulated nanoparticles(2011-08) Carino, Emily V.; Crooks, Richard M. (Richard McConnell); Bard, Allen J.; Stevenson, Keith J.; Henkelman, Graeme; Mullins, C. BuddieThe synthesis and characterization of Pt core/ Cu shell (Pt@Cu) dendrimer-encapsulated nanoparticles (DENs) having full and partial Cu shells deposited via electrochemical underpotential deposition (UPD) is described. Pt DENs containing averages of 55, 147, and 225 Pt atoms immobilized on glassy carbon electrodes served as the substrate for the UPD of a Cu monolayer. This results in formation of Pt@Cu DENs. Evidence for this conclusion is based on results from the analysis of cyclic voltammograms (CVs) for the UPD and stripping of Cu on Pt DENs, and from experiments showing that the Pt core DENs catalyze the hydrogen evolution reaction before Cu UPD, but that after Cu UPD this reaction is inhibited. Results obtained by in-situ electrochemical X-ray absorption spectroscopy (XAS) confirm the core@shell structure. Calculations from density functional theory (DFT) show that the first portion of the Cu shell deposits onto the (100) facets, while Cu deposits lastly onto the (111) facets. The DFT-calculated energies for Cu deposition on the individual facets are in good agreement with the peaks observed in the CVs of the Cu UPD on the Pt DENs. Finally, structural analysis of Pt DENs having just partial Cu shells by in-situ XAS is consistent with the DFT-calculated model, confirming that the Cu partial shell selectively decorates the (100) facets. These results are of considerable significance because site-selective Cu deposition has not previously been shown on nanoparticles as small as DENs. In summary, the application of UPD as a synthetic route and characterization tool for core@shell DENs having well defined structures is established. A study of the degradation mechanism and degradation products of Pd DENs is provided as well. These DENs consisted of an average of 147 atoms per dendrimer. Elemental analysis and UV-vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in air-saturated solutions, less oxidation in N₂-saturated solution, and no detectable oxidation when the DENs are in contact with H₂. Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd²⁺-complexing ligands such as chloride. For the air- and N₂-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy. Calculations from density functional theory (DFT) show that the first portion of the Cu shell deposits onto the (100) facets, while Cu deposits lastly onto the (111) facets. The DFT-calculated energies for Cu deposition on the individual facets are in good agreement with the peaks observed in the CVs of the Cu UPD on the Pt DENs. Finally, structural analysis of Pt DENs having just partial Cu shells by in-situ XAS is consistent with the DFT-calculated model, confirming that the Cu partial shell selectively decorates the (100) facets. These results are of considerable significance because site-selective Cu deposition has not previously been shown on nanoparticles as small as DENs. In summary, the application of UPD as a synthetic route and characterization tool for core@shell DENs having well defined structures is established. A study of the degradation mechanism and degradation products of Pd DENs is provided as well. These DENs consisted of an average of 147 atoms per dendrimer. Elemental analysis and UV-vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in air-saturated solutions, less oxidation in N2-saturated solution, and no detectable oxidation when the DENs are in contact with H2. Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd2+-complexing ligands such as chloride. For the air- and N2-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy.