No description available.

[pt] O objetivo da pesquisa foi dedicado à preparação de novos materiais híbridos à base de sílica contendo sistemas moleculares ou nanométricos de carbono com sistema de elétrons pi conjugados, imobilizados covalentemente para posterior aplicações como adsorventes e eletrodos para pré-concentração e determinação de compostos aromáticos. Deste modo, propôs-se o desenvolvimento das metodologias para as imobilizações covalentes de antraceno (Ant), pontos quânticos de óxido de grafeno (GOQDs) e nanopontos de carbono (CDs) na superfície da sílica (SiO2). Estes compostos são constituídos de sistemas conjugados de elétrons pi e, portanto, podem formar interação intramolecular de empilhamento pi–pi com compostos aromáticos. Nanocompósito de GOQDs imobilizados foi preparado através da interação covalente de fragmentos nanoméricos de óxido de grafeno (GO). Três tipos de SiO2-Ant foram obtidos através de abordagem de montagem da superfície pela reação de alquilação condensação de sílica modificada com grupos aminopropila. Estes materiais podem ter diferentes arranjos de ligantes de antraceno na superfície da sílica e, portanto, podem apresentar diferentes propriedades doadoras de elétrons-pi. Nanocompósitos de sílica gel com CDs imobilizados foram preparados por duas abordagens diferentes. Na primeira, os CDs foram obtidos a partir do ácido cítrico como material de partida e posteriormente imobilizados em sílica. O segundo método consistiu no uso de sílica gel porosa como nanorreator para a preparação de CDs. Este método é simples e robusto, o qual permite um maior controle do tamanho e da composição do nanomaterial. Além disso, os efeitos da modificação da sílica, tamanho dos poros, tempo de síntese, estabilidade hidrolítica, tamanho das nanopartículas de carbono e rendimento quântico da fluorescência foram avaliados. Foi demonstrado que a carbonização do gel aminosilica com ácido cítrico incorporado resultou na formação de CDs não imobilizados e CDs imobilizados. Devido à forte fixação na superfície, os CDs podem ser facilmente purificados através de simples lavagem do nanocompósito com água. Os C-pontos preparados apresentaram fotoluminescência independente de excitação a 445 nm com rendimento quântico até 80 porcento, o que os tornam atraentes para aplicação bioanalítica. O nanocompósito imobilizado com GOQDs foi preparado por ligações covalentes dos fragmentos de GO em nanoescala. A composição da camada superficial de todos adsorventes foi determinada a partir de análises elementares e termogravimétricas, fotoelétrons de raios X, ressonância magnética nuclear no estado sólido, Raman, FTIR e espectroscopia de fluorescência. As propriedades texturais dos adsorventes foram determinadas, bem como a cinética de imobilização do ligante, o grau de transformação da superfície e a estabilidade hidrolítica dos grupos enxertados, o efeito da porosidade da sílica no grau de conversão do ligante. As propriedades de adsorção dos materiais foram avaliados pela extração em fáse sólida nos modos dinâmico e estático para os compostos modelos azul de metileno e antraceno em várias fases móveis aquosas-orgânicas e orgânicas, na presença de componentes interferentes. Os nanomateriais obtidos apresentaram maior afinidade para compostos aromáticos do que os cartuchos comerciais C18 para extração em fase sólida (SPE) de fases orgânicas móveis que permitem a separação em uma etapa dos hidrocarbonetos aromáticos policíclicos (PAHs) na matriz com alta concentração de ácidos graxos. Ademais, sílica gel com GOQDs imobilizados foi usado para análise eletroquímica de antibióticos e hormônios selecionados. O eletrodo demonstrou atividade eletrocatalítica em relação ao estriol (ET), dietilestilbestrol (DES), sulfametoxazol (SMZ) e trimetoprim (TMP) que possibilitaram determinar esses analitos com até 0.009 μmol L(-1) (ET). / [en] The objective of the present research was devoted to the preparation of new silica-based hybrid materials having covalently immobilized carbon-containing molecular and nanometric systems with delocalized pi-electrons that can be used as adsorbents and electrodes for preconcentration and determination of aromatic compounds. For this purpose, methodologies for covalent immobilization of anthracene (Ant), graphene oxide quantum dotes (GOQDs) and carbon nanodots (CDs) on silica surface (SiO2) have been developed. These compounds have a conjugated system of pi-electrons and thus can form intramolecular pi–pi stacking interaction with aromatic compounds. The nanocomposite with immobilized GOQDs was prepared by covalent immobilization of nanoscale graphene oxide (GO) fragments. Three sets of SiO2-Ant were obtained using reaction surface assembling reactions by alkylation and condensation of silica with immobilized primary amines. These materials can have different arrangements of anthracene ligands on silica surface and so pi-donor properties. Silica gel nanocomposites with immobilized CDs were prepared by two different approaches. For the first one, the CDs were obtained from the molecular precursor and then immobilized on silica. For the second method, CDs were prepared by nanoreactor approach directly in silica gel pores. The second method is facile and robust and also allows better control of the particle size and composition. The effects of silica gel modification, pore size, synthesis time, hydrolytic stability, carbon nanoparticle size and quantum yield of the fluorescence were studied. It was demonstrated that the carbonization of aminosilica gel with embedded citric acid resulted in the formation of unbound CDs and immobilized CDs. Due to firm attachment to the surface CDs can be easily separated from low-molecular-weight impurities and CDs by simple rinsing of the nanocomposite with water. As prepared CDs demonstrate excitation-independent photoluminescence at 445 nm with quantum yield up to 80 percent that makes them attractive for bioanalytical application. The composition of the surface layer of the adsorbents was determined from elemental and thermogravimetric analysis, X-ray photoelectron, solid nuclear magnetic resonance, Raman, FTIR, and fluorescent spectroscopy. The textural properties of the adsorbents were determined as well as ligand immobilization kinetics, the degree of surface transformation and hydrolytic stability of the grafted groups, the effect of silica porosity on ligand conversion degree. The adsorption properties of the materials were studied in dynamic and static solid-phase extraction (SPE) modes for the model compounds methylene blue and anthracene in various aqueous-organic and organic mobile phases, in the presence of interfering components. It was demonstrated that studied materials have a higher affinity towards aromatic compounds than commercial C18 SPE cartridges in organic mobile phases that allow one-step separation of polycyclic aromatic hydrocarbons in the matrix with a high concentration of fatty acids. Also, silica-immobilized GOQDs was used for electrochemical analysis of selected antibiotics and hormones. The electrode demonstrated electrocatalytic activity towards estriol (ET), diethylstilbestrol (DES), sulfamethoxazole (SMZ), and trimethoprim (TMP) that made it possible to determine these analytes with up to 9 nmol L(-1) (ET).

No description available.

One of the main problems encountered in chemical analysis operations in the field is collecting sufficient sample from the source and transferring that sample to the measurement instrument for fast separation and identification. I have been involved in developing a field-portable gas chromatography-mass spectrometry (GC-MS) system with solid phase microextraction (SPME) sampling for point detection of chemical agents. The objective is to minimize the analysis time between sampling and detection of a potential chemical threat. SPME offers a convenient means for sampling gaseous and liquid samples, concentrating the analytes, and transferring the analytes to the injection port of a GC system for separation and identification. GC-MS has advantages of high efficiency, speed, and applicability for field analysis. Work was done to optimize the SPME fiber coating, capillary column dimensions, and GC operating conditions to provide complete analysis within 3 minutes. Since isothermal operation of the GC was a prior requirement, many components in the chromatograms were unresolved. Therefore, a peak de-convolution algorithm was applied to allow for identification and quantitation of poorly resolved and often completely obscured trace components. Details of the instrumentation and optimization of operating conditions are described in this thesis.

Crystalline films of vanadium dioxide were obtained through thermal annealing of amorphous vanadium dioxide thin films sputtered on silicon dioxide. An annealing process was found that yielded polycrystalline vanadium dioxide thin films, semi-continuous thin films, and films of isolated single-crystal particles. Orientation Imaging Microscopy (OIM) was used to characterize and study the phase and the orientation of the vanadium dioxide crystals obtained, as well as to diferentiate them from other vanadium oxide stoichiometries that may have formed during the annealing process. There was no evidence of any other vanadium oxides present in the prepared samples. Indexing of the crystals for the orientation study was performed with the Kikuchi patterns for the tetragonal phase of vanadium dioxide, since it was observed that the Kikuchi patterns for the monoclinic and tetragonal phases of vanadium dioxide are indistinguishable by OIM. It was found that a particle size of 100 nm was in the lower limit of particles that could be reliably characterized with this technique. It was also found that all VO2 crystals large enough to be indexed by OIM had a preferred orientation with the C axis of the tetragonal phase parallel to the plane of the specimen.

Methods for fast, simple detection of biomarkers to detect and differentiate closely related Bacillus endospores including Bacillus anthracis (BA), Bacillus thuringiensis (BT), Bacillus atrophaeus (BG), and Bacillus cereus (BC) using thermochemolysis and methylation (TCM), coiled wire filament (CWF), solid phase micro extraction (SPME) and gas chromatography-mass spectrometry (GC-MS) were developed. The main biomarkers detected and used for differentiation include dipicolinic acid methyl ester (DPAME), fatty acid methyl esters (FAMEs), 3-methyl-2-butenoic acid methyl ester (3M2BAME), 2-butenoic acid methyl ester (2BAME), and several methylated sugars. TCM of endospores was performed based on hydrolysis and methylation at elevated temperature after the endospores were mixed with sulfuric acid (H2SO4) with or without addition of tetramethylammonium hydroxide (TMAH) in methanol (MeOH). TCM products were then introduced into a heated GC injector port using a coiled wire filament (CWF) or solid phase microextraction (SPME) for detection and differentiation of the endospores by GC-MS.The CWF, which consisted of a tiny platinum helical wire coil attached to a retractable plunger that moved the coil in and out of a syringe needle housing, allowed for sampling to be accomplished by dipping the CWF in an endospore sample suspension, evaporating the suspension liquid, and then introducing the CWF into the injection port to enable on-line TCM. New SPME techniques, including half-half extraction, coated-needle extraction (CNE), and a new home-made polymer coated needle were used to speed up solid phase micro extraction of biomarkers produced from TCM. These simplified the detection of anthrose and other biomarkers. TCM with a CWF and TCM with SPME produced high intensity profiles of DPAME, FAMEs, 2BAME, 3M2BAME and methylated sugars. While the presence of DPAME can be used for the general detection of endospores (Bacillus and Clostridium) and the presence of 3M2BAME for the detection of BA, specific saturated and unsaturated C15, C16, and C17 fatty acid methyl esters and methylated sugars provide additional information for differentiating various Bacillus species grown at different temperatures and in different media. DPAME was detected in samples containing as few as 2,500 and 6,000 endospores using TCM-CWF with and without a concentration step, respectively. GC-MS peak area percent reproducibility for FAMEs using TCM and CWF varied from 3 to 13% (RSD). Better than 97% correct predictability of Bacillus species identity was obtained from a blind experiment consisting of 145 samples using DPAME and specific FAMEs. Conventional SPME and a modified form of "in-needle" extraction allow for detection of the biomarkers in less than 35 min. The detection limits with SPME sample introduction injection were approximately 5 x 103 endospores.Using these approaches, differentiation of Bacillus endospores and other biological agents grown under different conditions were based on the following characteristics: (1) presence of DPAME and specific FAMEs (iso or anteiso C15:0 and iso or anteiso C17:0) in Bacillus endospores, (2) unique presence of 3M2BAME (anthrose by-product) in BA, (3) absence of 2-butenoic acid methyl ester in BG, and (4) presence and absence of specific methylated monosaccharides in various Bacillus species. Clostridium endospores and non-sporulating bacteria, such as Yersinia pestis (YP) and Francisella tularensis (FT) could also be easily distinguished from Bacillus endospores based on the presence and absence of several specific sugar derivatives and fatty acid methyl esters (FAMEs), such as iso or anteiso C15:0 and iso or anteiso C17:0, and > C18 FAMEs which were simultaneously produced during TCM.

This dissertation focuses on the development of analytical methodology for the analysis of polycyclic aromatic hydrocarbons (PAHs) in water samples. Chemical analysis of PAHs is of great environmental and toxicological importance. Many of them are highly suspect as etiological agents in human cancer. Among the hundreds of PAHs present in the environment, the U.S. Environmental Protection Agency (EPA) lists sixteen as "Consent Decree" priority pollutants. Their routine monitoring in environmental samples is recommended to prevent human contamination risks. A primary route of human exposure to PAHs is the ingestion of contaminated water. The rather low PAH concentrations in water samples make the analysis of the sixteen priority pollutants particularly challenging. Current EPA methodology follows the classical pattern of sample extraction and chromatographic analysis. The method of choice for PAHs extraction and pre-concentration is solid-phase extraction (SPE). PAHs determination is carried out via high-performance liquid chromatography (HPLC) or gas chromatography/mass spectrometry (GC/MS). When HPLC is applied to highly complex samples, EPA recommends the use of GC/MS to verify compound identification and to check peak-purity of HPLC fractions. Although EPA methodology provides reliable data, the routine monitoring of numerous samples via fast, cost effective and environmentally friendly methods remains an analytical challenge. Typically, 1 L of water is processed through the SPE device in approximately 1 h. The rather large water volume and long sample processing time are recommended to reach detectable concentrations and quantitative removal of PAHs from water samples. Chromatographic elution times of 30 - 60 min are typical and standards must be run periodically to verify retention times. If concentrations of targeted PAHs are found to lie outside the detector's response range, the sample must be diluted (or concentrated), and the process repeated. In order to prevent environmental risks and human contamination, the routine monitoring of the sixteen EPA-PAHs is not sufficient anymore. Recent toxicological studies attribute a significant portion of the biological activity of PAH contaminated samples to the presence of high molecular weight (HMW) PAHs, i.e. PAHs with MW ≥ 300. Because the carcinogenic properties of HMW-PAHs differ significantly from isomer to isomer, it is of paramount importance to determine the most toxic isomers even if they are present at much lower concentrations than their less toxic isomers. Unfortunately, established methodology cannot always meet the challenge of specifically analyzing HMW-PAHs at the low concentration levels of environmental samples. The main problems that confront classic methodology arise from the relatively low concentration levels and the large number of structural isomers with very similar elution times and similar, possibly even virtually identical, fragmentation patterns. This dissertation summarizes significant improvements on various fronts. Its first original component deals with the unambiguous determination of four HMW-PAHs via laser-excited time-resolved Shpol'skii spectroscopy (LETRSS) without previous chromatographic separation. The second original component is the improvement of a relatively new PAH extraction method - solid-phase nanoextraction (SPNE) - which uses gold nanoparticles as extracting material for PAHs. The advantages of the improved SPNE procedure are demonstrated for the analysis of EPA-PAHs and HMW-PAHs in water samples via GC/MS and LETRSS, respectively.

Diese Arbeit befasst sich mit dem Wachstum von Dünnschichtstrukturen, die zur Herstellung eines Spin-selektiven Schottky-Barrier-Tunneltransistors (SS-SBTT) erforderlich sind. Das Bauelement basiert auf dem Transport von Ladungsträgern durch eine dünne halbleitende (SC) Schicht, die zwei ferromagnetische (FM) Kontakte trennt. Daher müssen hochqualitative und gitterangepasste vertikale FM/SC/FM-Trilayer gezüchtet werden, was aufgrund der inkompatiblen Kristallisationsenergien zwischen SC und Metallen eine experimentelle Herausforderung darstellt. Das Problem wurde mit einem Festphasenepitaxie-Ansatz gelöst, bei dem eine dünne amorphe Ge-Schicht (4-8 nm) durch Ausglühen über Fe3Si auf GaAs(001)-Substraten kristallisiert wird. Langsame Glühgeschwindigkeiten bis zu einer Temperatur von 260°C konnten ein neues gitterangepasstes Polymorph von FeGe2 erzeugen, über das ein zweites Fe3Si mittels Molekularstrahlepitaxie gezüchtet werden könnte. SQUID-Magnetometermessungen zeigen, dass die dreischichtigen Proben in antiparallele Magnetisierungszustände versetzt werden können. Vertikale Spin-Ventil-Bauelemente, die mit verschiedenen Trilayern hergestellt wurden, wurden verwendet, um zu demonstrieren, dass der Ladungstransport über die Heteroübergänge spinselektiv ist und bei Raumtemperatur einen Magnetowiderstand von höchstens 0,3% aufweist. Der Effekt nimmt bei niedrigen Temperaturen ab, was mit einem ferromagnetischen Übergang in der FeGe2-Schicht korreliert. Durch TEM- und XRD-Experimente konnte festgestellt werden, dass das neue FeGe2-Polymorph die Raumgruppe P4mm aufweist und bis zu 17% Si-Atome als Ersatz für Ge-Stellen enthält. Die Isolierung von FeGe2 war möglich, indem das Verhältnis von Fe-, Si- und Ge-Atomen so eingestellt wurde, dass die richtige Stöchiometrie bei vollständiger Durchmischung erreicht wurde. Anhand von FeGe2-Dünnschichten wurde ein zunehmender spezifischer Widerstand bei niedriger Temperatur und ein semi-metallischer Charakter beobachtet. / This thesis discusses the growth of thin film structures required to fabricate a Spin-Selective Schottky Barrier Tunnel transistor (SS-SBTT). The device relies on charge carriers being transported through a thin semiconducting (SC) layer separating two ferromagnetic (FM) contacts. Thus, high quality and lattice-matched FM/SC/FM vertical trilayers must be grown, which is experimentally challenging due to incompatible crystallization energies between SC and metals. The problem was solved using a solid-phase epitaxy approach, whereby a thin amorphous layer of Ge (4-8 nm) is crystallized by annealing over Fe3Si on GaAs(001) substrates. Slow annealing rates up to a temperature of 260°C could produce a lattice-matched Ge-rich compound, over which a second Fe3Si could be grown my molecular-beam epitaxy. The compound obtained during annealing is a new layered polymorph of FeGe2. SQUID magnetometry measurements indicate that the trilayer samples can be placed in states of antiparallel magnetization. Vertical spin valve devices created using various trilayers were used to demonstrate that charge transport is spin-selective across the heterojunctions, showing a magnetoresistance of at most 0.3% at room temperature. The effect decreases at low temperature, correlating with a ferromagnetic transition in the FeGe2 layer. TEM and XRD experiments could determine that the new FeGe2 polymorph has a space group P4mm, containing up to 17% Si atoms substituting Ge sites. Isolating FeGe2 was possible by tuning the proportion Fe, Si and Ge atoms required to obtain the right stoichiometry upon full intermixing. Hall bars fabricated on FeGe2 thin films were used to observe an increasing resistivity at low temperature and semimetallic character.

No description available.

No description available.