Browsing by Subject "dendrimer"
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Item Dendritic and linear polymers for separations(Texas A&M University, 2005-02-17) Gonzalez, Sergio OmarMost new fields in chemistry usually began as a curiosity by the researchers, followed by an intrinsic interest in basic biological, physical and chemical properties of reactions, interactions, structural features, and response to external stimuli by chemical elements and/or chemical compounds. If the ?curiosity? has appealing bio-physico-chemical properties this trend is followed by studies on the possible applications of such new fields. As a result, is it expected that these curiosities develop or give insights into new technologies. The development of the field of dendrimer chemistry is no different. In fact, dendrimer chemistry illustrates this trend fittingly. The research in this dissertation follows a similar trend. First, the synthesis of a melamine-based dendrimer is achieved. The synthesis illustrates the concept of using triazines as building blocks in dendrimer synthesis. The characterization of this molecule was followed by a basic inquiry of the properties that were unique relative to its composition. This dendrimer is compared against a small library of similar dendrimers in a structure-activity relationship (SAR) study. From the basic concept of an SAR, we moved toward more applied studies of these molecules. The grafting of organic molecules onto inorganic supports has had influences in the fields of catalysis, separations, and sensors. We developed protocols for the grafting of melamine-based molecules onto hydroxyl rich surfaces. After extensive characterization using solution and surface analyses, we tested the sequestration abilities of these new materials toward the separation of molecules of environmental importance from water. Following the data collected in these experiments, we moved toward a different type of applied technology. The use of linear polymers for separations instead of dendrimers is more attractive from an engineering perspective. We then used what was learned from the study of the separations performed by dendrimers and applied it to the design of linear polymers. We take advantage of a latent solid phase response to external stimuli to remove the herbicide atrazine from aqueous solution to the limit of detection.Item Elucidating the organic-OMS interface and its implications for heterogeneous catalysts(2012-07-16) Wang, QingqingOrganic ? ordered mesoporous silica (OMS) hybrid materials have attracted great interest due to their potential applications for gas separations, and heterogeneous catalysis. Amine-functionalized OMS materials are active in a variety of base-catalyzed reactions. The key to successfully achieving the desired reactivity is the ability to rationally tether the targeted organic functionality onto the OMS surface. Understanding the organic-inorganic interface is crucial for rational design of heterogeneous catalysts, because the local structure and molecule dynamics are paramount in determining the reactivity of the organic groups attached to the OMS surface. This dissertation focuses on three goals that will lead to a description of the organic-OMS interface and designing hybrid catalysts: 1) Determining the dynamics of organic groups attached to the OMS surface, 2) Catalytic testing to understand how the local structure and dynamics of the organic moiety influence the catalytic properties of organic-OMS catalysts, 3) Designing more active hybrid catalysts by introducing higher loadings of organic group using dendrimer structures. Solid-state NMR is uniquely suited for quantifying dynamics in the milli- to nano-second time scale. Deuterium (2H) NMR is a powerful tool to obtain detailed information about the dynamics or organic molecules. In this study, several simple functional groups isotopically labeled with deuterium have been attached to MCM-41 and SBA-15. The spectra display different molecular motions for different organic moieties. The results have indicated that the interactions between the functional groups and silanol groups on the surface influence the mobility of the organic fragments. Also, the porosity of the solid supports effects dynamics via confinement. The catalytic properties of simple amine groups attached to MCM-41, containing primary, secondary, and tertiary amines have been compared in the Nitroaldol (Henry) reaction. The effects of amine identity, structure, loading, presence of surface silanols, and the substrate topology on the catalytic properties have been investigated. The dramatic decrease of the activity of amine-functionalized MCM-41 by capping the residual silanol groups with hexamethyldisilazane was ascribed to the decrease of the interactions of hydrogen bonding between the amine functional groups and surface silanols. The result was consistent with the changes of the molecular motions shown by 2H NMR measurements. Fabricating OMS hybrid materials with high densities of organic functional groups leads to challenges in realizing uniform, catalytically active sites. Our group has immobilized melamine-based dendrimers on the surfaces of amine-functionalized SBA-15 materials by iterative synthesis procedures. The current studies in this dissertation mainly describe the catalytic properties of these dendrimers on SBA-15 and MCM-41 in the Nitroaldol (Henry) reaction, the transesterification reaction of triglycerides and methanol to synthesize methyl esters, and the cross aldol reaction between acetone and 5-hydroxymethylfurfural. The results indicate that the OMS-dendron materials have potential as solid base catalysts for a range of reactions.Item Enhancement of a fluorescent sensor for monitoring glucose concentration in diabetic patients(Texas A&M University, 2007-04-25) Ibey, Bennett LukeThe need for overnight and continuous monitoring of glucose levels in diabetic patients is profound, especially among juveniles. Implantation of a chemical assay which responds optically to changes in glucose concentration shows promise as a technology capable of continuously monitoring blood sugar with little invasion into the body. Previous fluorescent chemical assays, based on the affinity binding reaction between Concanavalin A protein and dextran, performed well but suffered from limited dermal penetration. In this work, a novel replacement for the dextran molecule (glycosylated dendrimer) was fabricated and tested to determine if it would improve the overall response of the sensing chemistry to glucose. Experiments were carried out and it was found that the assay??????s functionality was based on the controlled aggregation of the Con A protein and the modified dendrimer molecule. This new assay proved to be specific to glucose, reversible, and independent of fluorophore dye attached to the protein. This research was furthered by encapsulation of the new assay into a PEG hydrogel which showed response to glucose but, due to leeching, did not perform well under repeated exposures. A new method for encapsulation was proposed based on poration of the hydrogel to create micropores capable of holding the assay chemistry and allowing it to react to incoming glucose, while the surrounding polymer restricted leeching. Preliminary results with previous assays proved the potential of a mannitol based poration procedure, but unforeseen complications in lyophilization of the new sensor assay restricted its completion. Due to instability of Con A in solution, it was hypothesized that the immobilization of it onto the surface of an active substrate would increase its stability overtime as seen in previous works. The immobilization procedure was performed on Con A for both polystyrene spheres and gold (nanoshells and colloid). Both results showed an adequate amount of protein on the surface of the particles, but little binding activity was demonstrated. Overall, the improvements to the sensor chemistry response were notable and the potential for stabilization and enhancement of the response through the use of an active substrate is promising.Item Hybrid Membranes for Light Gas Separations(2012-07-16) Liu, TingMembrane separations provide a potentially attractive technology over conventional processes due to their advantages, such as low capital cost and energy consumption. The goal of this thesis is to design hybrid membranes that facilitate specific gas separations, especially olefin/paraffin separations. This thesis focuses on the designing dendrimer-based hybrid membranes on mesoporous alumina for reverse-selective separations, synthesizing Cu(I)-dendrimer hybrid membrane to facilitate olefin/paraffin separations, particularly ethylene/methane separation, and investigating the influence of solvent, stabilizing ligands on facilitated transport membrane. Reverse-selective gas separations have attracted considerable attention in removing the heavier/larger molecules from gas mixtures. In this study, dendrimer-based chemistry was proved to be an effective method by altering dendrimer structures and generations. G6-PIP, G4-AMP and G3-XDA are capable to fill the alumina mesopores and slight selectivity are observed. Facilitated transport membranes were made to increase the olefin/paraffin selectivity based on their chemical interaction with olefin molecules. Two approaches were explored, the first was to combine facilitator Cu(I) with dendrimer hybrid membrane to increase olefin permeance and olefin/paraffin selectivity simultaneously, and second was to facilitate transport membrane functionality by altering solvents and stabilizing ligands. Promising results were found by these two approaches, which were: 1) olefin/paraffin selectivity slightly increased by introducing facilitator Cu(I), 2) the interaction between Cu(I) and dendrimer functional groups are better known.Item Nanocomposite Membranes for Complex Separations(2010-10-12) Yeu, Seung UkOver the past few decades there has been great interest in exploring alternatives to conventional separation methods due to their high cost and energy requirements. Membranes offer a potentially attractive alternative as they potentially address both of these points. The overarching theme of this dissertation is to design nanocomposite membranes for processes where existing separation schemes are inadequate. This dissertation focuses on three challenges: 1) designing organic-inorganic hybrid membranes for reverse-selective removal of alkanes from light gases, 2) defect-free inorganic nanocomposite membranes that have uniform pores, and 3) nanocomposite membranes for minimizing protein fouling in microfiltration applications. Reverse-selective gas separations that preferentially permeate larger/heavier molecular species based on their greater solubility have attracted considerable recent attention due to both economic and environmental concerns. In this study, dendrimer-ceramic hybrid membranes showed exceptionally high propane/nitrogen selectivities. This result was ascribed to the presence of stable residual solvent that affects the solubility of hydrocarbon species. Mesoporous silica-ceramic nanocomposite membranes have been fabricated to provide defectless mesoporous membranes. As mesoporous silica is iteratively synthesized in the ceramic macropores, the coating method and the surfactant removal step significantly affected permeance and selectivity. It was also shown that support layers can cause a lower selectivity than Knudsen limit. Membrane fouling which results from deposition and nonspecific adsorption of proteins on the membrane surface is irreversible in nature, and results in a significant decrease in the membrane performance. To address this problem, two approaches were explored: 1) control of the surface chemistry tethering alumina membranes with organic components and 2) development of a novel photocatalytic membrane that exhibits hydrophilicity and can be easily regenerated. Both approaches can offer a viable route to the synthesis of attractive membranes, in that 1) the density of protein-resistant organic groups such as PEG is controllable by changing scaffolds or synthesis conditions and 2) the photocatalytic nanocomposite membranes can open the way for a new regeneration method that is environmentally benign.Item Synthesis and characterization of covalently-linked dendrimer bioconjugates and the non-covalent self-assembly of streptavidin-based megamers(Texas A&M University, 2005-02-17) McLean, Megan ElizabethThis work details the attachment of dendrimers to proteins, peptides and single stranded DNA (ssDNA). Dendrimers based on melamine satisfy many of the synthetic demands in the field of bioconjugate chemistry including: monodispersity, synthetic flexibility and scalability. The solution-phase syntheses of both ssDNA-dendrimer and peptide-dendrimer bioconjugates is described, and thorough characterization by matrix-assisted laser desorption ionization/ time-of-flight (MALDI-TOF) mass spectrometry, UV-vis spectroscopy, fluorescence spectroscopy, and polyacrylamide gel electrophoresis is discussed. Non-covalent DNA-dendrimer complexes have been shown to facilitate antisense gene delivery, but are vulnerable to dissociation and subsequent enzymatic degradation within the cell. In an effort to prepare biocompatible antisense agents capable of effectively shielding ssDNA from intracellular nuclease digestion, disulfide-linked ssDNA-dendrimers were prepared and rigorously characterized to rule out the possibility of an electrostatic-based interaction. Hybridization assays were performed to determine if the covalently-attached dendrimer affected the ability of the attached ssDNA strand to anneal with a complementary sequence to form double-stranded DNA (dsDNA)-dendrimers. Results indicate that ssDNA-dendrimer conjugates readily anneal to complementary ssDNA strands either in solution or attached to gold surfaces. Nuclease digestions of conjugates in solution suggested that enzymatic manipulation of dsDNA-dendrimers is possible, offering promise for DNA-based computation and other fields of DNA-nanotechnology. Much larger bioconjugates consisting of dendrimers, proteins and peptides were prepared with the goal of obtaining molecular weights sufficient for enhanced permeability and retention (EPR) in tumors. While the dendrimer provides the advantages of a purely synthetic route for drug delivery, the protein portion of the bioconjugate provides a monodisperse, macromolecular scaffold for the non-covalent self-assembly of the dendrimers. The strategy presented herein is based on the strong interaction between biotin and the 60 kD tetrameric protein streptavidin. Each monomer of streptavidin is capable of binding 1 biotin molecule, thus when biotin functionalized peptide-dendrimers are added to streptavidin they bind to form a cluster of dendrimers, or a megamer. The biotinylated peptides that link the dendrimers to the streptavidin core provide a way to actively target specific cell types for drug delivery. Megamer formation through the addition of tetrameric streptavidin was successful as indicated by MALDI-TOF, UV-vis titration and gel electrophoresis assays.Item Synthesis and characterization of melamine-based dendrimers with potential biological applications(2009-05-15) Crampton, Hannah LouiseThe convergent strategy towards dendrimer synthesis is well-suited to generate macromolecules with a diverse periphery, at the expense of time and effort, while the divergent strategy has historically been effective at yielding higher generation dendrimers, although they are often plagued by impurities. Both the convergent and divergent routes were applied to the synthesis of melamine-based dendrimers, offering a comparison of the routes within a system. Generation-1 dendrons heterogeneously functionalized with Boc-protected amines and hydrazones were synthesized convergently and coupled to a generation-1 tris(piperazine) core to yield a generation-2 dendrimer bearing 18 Boc-amines and three hydrazones. Although the yield for the final coupling step was rather low (56%), the yields for all intermediate steps were quite high. Attempts toward obtaining a generation-3 dendrimer through this route were unsuccessful due presumably to steric hindrance. The materials obtained showed no impurities in their 1H and 13C NMR and mass spectra, although several chromatographic purifications were necessary throughout the synthesis. A divergent strategy based on addition of a dichlorotriazine monomer to polyamine cores was used to synthesize dendrimers of generations 1-5. All intermediates and dendrimers were either purified by precipitation, or did not need purification. 1H NMR spectroscopy indicated that reactions were complete up to G4-NH2 by integration, and mass spectroscopy confirmed that assignment. HPLC and GPC of Gn-Cl dendrimers showed sharp peaks for G1-G3, but G4-Cl appeared to have a small amount of impurities that are similar in size and polarity to the fully-substituted dendrimer. The G1-G3 dendrimers were confidently assigned as pure by conventional organic chemistry standards, but the assignment of purity to higher generations remained tentative. A G1-Cl dendrimer was functionalized with imidazole, and then deprotected and PEGylated with PEG5000 to yield a water soluble dendrimer. The imidazole-capped, Boc-protected dendrimer and the deprotected dendrimer were characterized by 1H and 13C NMR spectroscopy and mass spectrometry. The degree of PEGylation on the PEGylated material could not be definitively ascertained; however, the material is capable of solubilizing very hydrophobic Zn-phthalocyanines in water.Item Synthesis and electrochemical characterization of highly monodisperse dendrimer-templated monolayer protected clusters(Texas A&M University, 2006-04-12) Kim, Yong-GuWe described the synthesis of multilayer organic thin films prepared by sequential vapor-phase coupling of monomers. The reactions were carried out at room temperature and atmospheric pressure. Films prepared using up to six sequential coupling reactions are reported. Homobifunctionalized monomers, such as hexamethylenediamine, react primarily via a single endgroup rather than cross coupling to the reactive surface via both reactive groups. We synthesized bifunctionalized polyamidoamine (PAMAM) dendrimers having both quaternary ammonium groups and primary amines on their periphery were prepared. The high positive charge on the surface of these dendrimers prevents agglomeration, and the unquanternized amine groups provide a reactive handle for immobilizing the dendrimer-encapsulated nanoparticles onto surfaces. We prepared highly monodisperse, 1-2 nm diameter Au nanoparticles using bifunctionalized PAMAM dendrimers as templates. The synthesis is carried out in water, takes less than 30 min, and requires no subsequent purification. The high monodispersity is a function of the template synthesis, which avoids size variations arising from random nucleation and growth phenomena, and the use of magic number equivalent ratios of AuCl4-/dendrimer. We investigated the electrochemical properties of Au, Pd and PdAu monolayer-protected clusters (MPCs), prepared by dendrimer-templating and subsequent extraction, are described. Purification of the extracted Au, Pd and PdAu nanoparticles was not required to obtain well-defined differential pulse voltammetry peaks arising from quantized double-layer charging. The calculated sizes of the nanoparticles were essentially identical to those determined from the electrochemical data. The capacitance of the particles was independent of the composition of core metal.Item The design and evaluation of triazine dendrimers for gene delivery(2011-02-22) Mintzer, Meredith AnnThe interest in using gene therapy to target a variety of both inherited and acquired diseases has intensified over the last two decades. Because free DNA is easily degraded by serum nucleases in the bloodstream, the need for developing carrier systems that can compact and protect the DNA was quickly realized. Viral vector systems were some of the earliest carriers used, primarily because of the ease with which such systems can infect host cells. However, difficulties experienced when using viral vectors, including immunogenicity and the potential for genetic recombinations, forced researchers to design alternative delivery strategies. Non-viral vectors offer one alternative to overcome this dilemma. In addition to avoiding the biological problems experienced using viral carriers, non-viral vectors also offer the potential for large-scale production. Dendrimers are one non-viral carrier that has shown appreciable ability to deliver DNA into cells, a process called transfection. In the past, triazine dendrimers have shown biocompatibility, and the ability to synthesize these structures to contain cationic charges on the surface makes these structures potentially suitable for transfection studies. In this study, a small library of triazine dendrimers was synthesized in an attempt to understand how variations to both the periphery and core of triazine dendrimers affect the transfection efficiency of these dendriplexes. In the first subset of structures, a common core was used and various peripheral groups were appended to the dendrimer surface. The physicochemical and biological data, obtained in collaboration with Thomas Kissel at Philipps-Universitat Marburg, showed that the surface groups have a notable affect on transfection efficiency. Dendrimers with a higher amine number and neutral surface groups show high DNA binding affinity and higher transfection efficiency. In the second subset of dendrimers, variations to the core showed that transfection efficiency is improved both by increasing generation number and dendrimer flexibility. With this data in hand, triazine dendrimers with both higher generation number and higher flexibility have been synthesized. Two different triazine linker groups, trimethylene-dipiperidine and polyglycoldiamine, have been used. These structures will be evaluated to determine if increasing both flexibility and generation number together can further improve transfection efficiency.