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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
91

Adsorption Studies of Polysaccharides and Phospholipids Onto Cellulose

Du, Xiaosong 18 January 2012 (has links)
Interactions between biomolecules and cellulose films at solid/liquid interfaces was studied by surface plasmon resonance spectroscopy (SPR), quartz crystal microbalance with dissipation monitoring (QCM-D) and in situ atomic force microscopy (AFM) measurements. This dissertation shows the porous character of nanocrystalline cellulose films as the key feature for enhanced adsorption of chemically modified polysaccharides and provides quantitative analysis of polymer supported phospholipid structures as a stable platform for studying membrane-related processes. Smooth cellulose I films were prepared by spincoating cellulose nanocrystal suspensions onto positively charged self-assembled monolayers on gold. The adsorption of pullulan cinnamate (PC) onto cellulose surfaces increased with increasing degree of cinnamate substitution. The interactions between PCs with higher degree of substitution (DS) and porous nanocrystalline cellulose (NC) films presumably generated looped multilayer PC structures that adsorbed more than twice as much onto NC films than onto regenerated cellulose (RC) films. PC chains not only covered the NC surface but also penetrated into the porous film. The porous features of NC film are responsible for the greater adsorption of polymer chains relative to tightly packed RC films. Adsorption of phospholipid vesicles onto RC and NC films was also studied. Aggregates of intact vesicle were observed on NC surfaces with high water content ~ 84 % by mass. Phospholipid patches with smooth features were found to assemble onto RC surfaces with a lower degree of hydration ~ 30 % by mass. Vesicle membrane breakage was triggered by a destabilizing agent, LysoPC. The great mass decrease, and changes in dissipation and degree of hydration for phospholipid structures after exposure to LysoPC corresponded to the transformation from vesicles to layered structures. Initial binding of LysoPC micelles to unruptured vesicles was clearly resolved in SPR, whereas the huge mass decrease associated with bound water hides the initial adsorption of LysoPC onto vesicles in QCM-D experiments. The intitial binding of LysoPC micelles onto vesicle membranes lasted for 200 seconds with a maximal increase of 14 % by mass prior to vesicle collapse. The role of cholesterol in phospholipid interactions with model cellulose surfaces was also considered. Supported vesicle layers over RC surfaces were observed for vesicle membranes containing ≥ 6.3 % by mole cholesterol, whereas phospholipid or phospholipid with lower cholesterol content formed disconnected lipid islands on RC surfaces. Meanwhile, intact vesicles were always observed on NC surfaces for phospholipid/cholesterol blends regardless of the cholesterol content. The intact vesicles on cellulose surfaces were attributed to the ability of cholesterol to accommodate vesicle deformation. These studies showed the impact of mesoscale structure of cellulose films on adsorbates. It sheds light on the role of the lignin-carbonhydrate-complex in plant cell wall structure and will inform the next generation of biomimetic nanocomposites. The designed polymer supported biomimetic membranes provide a perfect platform to develop intact and ruptured protoplast systems for the study of plant cell wall self-assembly. / Ph. D.
92

Organic Self-Assembled Films for Nonlinear Optics: Film Structure, Composition and Kinetics of Film Formation

Garg, Akhilesh 12 September 2008 (has links)
Organic materials exhibiting second-order non-linear optical (NLO) properties are a key to the development of advanced electro-optic (EO) modulators used in fiber-optic communications system. This work addresses the fabrication and characterization of organic materials with NLO properties using a self-assembly approach by alternately dipping a charged substrate into positively and negatively charged polymers to build up layer-by-layer (LbL) films. The effect of solution pH on the formation of LbL films fabricated using the polycation poly(allylamine hydrochloride) (PAH) and the polyanion poly{1-[p-(3–-carboxy-4–-hydroxyphenylazo)benzenesulfonamido]-1,2-ethandiyl} (PCBS) was studied using a quartz crystal microbalance with dissipation (QCM-D) monitoring, ellipsometry, absorbance, and second harmonic generation (SHG) measurements. PCBS has an azo-benzene chromophore side group that, when sufficiently oriented, results in measurable SHG. Films of PAH/PCBS fabricated at neutral pH where both PAH and PCBS are highly charged led to thin bilayers, ~1 nm, with a 1:1 molar ratio of PCBS:PAH. This molar ratio was found to be important for long-range polar ordering of PCBS in these films. Increasing the rate of convection was found to reduce the time required for complete adsorption of the polyion. This can have a significant impact on fabrication of films with high bilayer numbers. A variation of the above technique, which involves adsorbing one of the constituents electrostatically and another covalently, was studied using PAH and a reactive dye, Procion Brown (PB), which has a significantly higher hyperpolarizability than PCBS. It was found that a high pH, ~10.5, was important for achieving covalent attachment of the PB to the underlying PAH films. This resulted in much higher SHG intensities compared to when PB was deposited pH at 8.5-9.5 where the attachment of PB was due to a combination of electrostatic and covalent interactions. QCM-D results for PAH/PB films revealed the presence of a high percentage of unreacted amine groups in the underlying PAH film. A rate constant value for PB attachment step to the underlying PAH was also calculated. To enhance the SHG intensity of these films, silver nanoprisms were synthesized and deposited onto films using physisorption. An enhancement in the SHG intensity was observed for both PAH/PCBS and PAH/PB films. / Ph. D.
93

Interactions of Chitin and Lignin Thin Films with Other Molecules

Yu, Guoqiang 12 October 2021 (has links)
As two of the most abundant natural polymers, chitin and lignin not only play critical roles in fungal and plant cell walls but are also important functional materials and promising feedstocks for a variety of chemicals. This study investigated the interactions of chitin and lignin thin films with several other molecules via a quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). Interactions between chitin and family 18 chitinases are vital for understanding bacterial invasion of fungi and human defense against fungal infection. Regenerated chitin (RChitin) thin films were prepared via chemical conversion and spin-coating. Changes in their mass and viscoelasticity were monitored by a QCM-D in real time during incubation with family 18 chitinases. The optimal temperature for the activity of chitinases on surfaces was lower than bulk solution studies in the literature. Family 18 chitinases showed greater activity on dissolved chitin oligosaccharides while family 19 chitinases showed greater activity on RChitin films, which was attributed to chitin-binding domains in family 19 chitinases. Catechyl lignin (C-lignin) is a promising substrate for lignin valorization. Films of C-lignin were synthesized via adsorbed horseradish peroxidase-catalyzed dehydrogenative polymerization (DHP) of caffeyl alcohol (C-alcohol), and degraded through Fenton chemistry with all processes observed by a QCM-D and AFM. The synthetic rate and yield for C-DHP films was lower than DHP films made from coniferyl alcohol (G-alcohol) and p-coumaryl alcohol (H-alcohol). The C-DHP film underwent complete Fenton mediated degradation in contrast to the G-DHP and H-DHP films regardless of their thicknesses. Conventional lignin suffers from recalcitrance to degradation. Copolymer lignin films were synthesized through surface-initiated copolymerization of C and G or C, G and H monolignols. As the concentration of C-alcohol increased, the percentage degradation of the synthesized DHP copolymer films increased. Almost all the CG-DHP or CGH-DHP films were degraded when the percentage of the C-alcohol in the polymerization feed was ≥ 75% and ≥ 60% for CG-DHP and CGH-DHP, respectively. / Doctor of Philosophy / Natural polymers are widely considered as an alternative to fossil fuels for the production of biofuels, biochemicals, and biomaterials. The features of their biodegradability, biocompatibility, and sustainability can significantly alleviate concerns about environmental pollution and energy security. The surfaces of natural polymers are critical to their properties and applications. This dissertation focuses on the study of interfacial behaviors occurring at two of the most abundant natural polymers, chitin and lignin, via surface analysis techniques, a quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). When an endosymbiont bacteria enter a fungal host, they secrete chitinases to soften and loosen the chitin layer in the fungal cell wall. Small chitin fragments will be released from digestion of the chitin layer of the fungal cell wall by chitinases in humans suffering from fungal infections. In order to fully understand the interactions between the fungal chitin layer and chitinases, a chitin thin film was fabricated to mimic the chitin layer, and the changes of the chitin film in mass, viscoelasticity, and morphology during treatment with family 18 chitinases were studied at various temperatures and pH using a QCM-D and AFM. Family 19 chitinases produced greater degradation of chitin thin films than family 18 chitinases, even though the family 18 chitinases had greater activity in solution. Greater surface activity for family 19 chitinases were attributed to chitin-binding domains in their chemical structure that are absent in family 18 chitinases. Millions of tons of lignin are produced in the lignocellulosic biorefinery and are discarded every year due to their recalcitrance to degradation as a result of their heterogeneous and complex structure. A newly discovered lignin, catechyl lignin (C-lignin), has potential for enhancing degradation on account of its simple linear structure. In this dissertation, C-lignin thin films were synthesized on gold-coated QCM-D sensor surfaces via surface-initiated dehydrogenative polymerization of caffeyl alcohol (C-alcohol). Their enzymatic and chemical degradation was investigated. It was found that the C-lignin films underwent complete chelator-mediated Fenton degradation in contrast to conventional lignin films. Although the C-lignin promises to be an ideal substrate for lignin valorization, its narrow distribution in nature severely limits its wide application. In view of this limitation, some people are trying to incorporate C units into conventional lignin through genetically engineered plants. This dissertation demonstrates the successful copolymerization of C-alcohol with conventional monolignols and the improved degradation of the synthesized C unit-containing copolymer lignin films relative to conventional lignin films. The results are expected to inform the design of lignocellulosic biomass for greater utilization.
94

Quartz Crystal Microbalance with Dissipation Monitoring Applications in Polymer Thin Films Analysis

Liu, Gehui 25 January 2022 (has links)
Natural and synthetic polymers are highly related to people's daily life in every perspective and determine everyone's life quality. This study investigated the interactions between polymer thin films and other molecules, specifically natural polymer films with other components in plant and fungal cell walls, crosslinked thermoplastic films with solvent molecules, as well as commodity thermoplastic films with air and moisture during aging by a powerful surface analysis instrument, a quartz crystal microbalance with dissipation monitoring (QCM-D). The assembly and interactions of glucan and chitin are crucial for understanding the fungal infection mechanism. Adsorption of mixed-linkage glucan (MLG) onto regenerated chitin (RChitin) and cellulose (RC) surfaces were investigated by QCM-D and atomic force microscopy (AFM). MLG was irreversibly adsorbed onto both surfaces and formed soft hydrogel-like layers with viscoelastic properties. This work established a QCM-D method to mimic the assembly of natural polymers in fungal cell walls and provided insight into the interactions of these polymers with chitin and cellulose. Poly(ether imide) (PEI) has poor solvent resistance towards solvents including chloroform, dimethylformamide (DMF), dichloromethane (DCM), and N-methyl pyrrolidone (NMP). Exposure to these solvents severely affects the thermal and mechanical performances of PEI. Therefore, crosslinked PEI (X-PEI) films was prepared from azide-terminated PEI (N₃-PEI-N₃) via a thermal crosslinking reaction. X-PEIs maintain outstanding solvent resistance towards common solvents by swelling ratio tests using QCM-D. Meanwhile, the thermal and mechanical properties of X-PEI were enhanced compared to the original PEI. Photo-oxidation is one of the dominant degradation mechanisms affecting the lifespan of polymers. The effect of photooxidative aging on the physiochemical properties of low-density polyethylene (LDPE) films were investigated using QCM-D, differential scanning calorimetry (DSC), and tensile stress-strain tests. The crystallinity, mechanical properties, and weight loss were correlated to understand the aging behavior. Materials after aging showed higher tensile stress and modulus, with reduced mass and elongation properties. Particularly, the aging-induced damage of polymer chain integrity was first determined by QCM-D through the evolution of mass loss during aging, providing supports to the changes of mechanical properties under aging. / Doctor of Philosophy / Natural polymers and thermoplastics are two major materials that are highly related to modern life. The interactions of these polymers with other molecules are important research topics for people to understand and predict the material properties. This dissertation studied the following three topics using a quartz crystal microbalance with dissipation monitoring (QCM-D): 1) interactions between plant natural polymer films and polymers in fungal cell wall; 2) solvent resistance of crosslinked thermoplastic films; and 3) physiochemical changes during photo-oxidation degradation of thermoplastic films. Pathogenic fungal cells can attack beneficial plant cell hosts by adhering themselves onto the plant cells, followed by penetration and enzymatic degradation of the multilayered plant cell walls until the host is digested. Therefore, the interaction between the components in fungal and plant cell walls is critical to understand pathogenic fungal cell invasion. Adsorption of mixed-linkage glucan (MLG) onto regenerated chitin (RChitin) and cellulose (RC) surfaces was monitored by QCM-D and atomic force microscopy (AFM). An irreversible binding interaction of MLG with chitin and cellulose films and a soft hydrogel-like layer on both surfaces were observed in our work. Poly(ether imide) (PEI) is a high-performance polymer with excellent thermal and mechanical properties. However, the good solubilities in common organic solvents that facilitate reasonable processibility limits its applications in solvent-related domains. Several methods of PEI crosslinking were developed in the literature to improve solvent resistance. This study prepared crosslinked PEI (X-PEI) films from azide-terminated PEI (N₃-PEI-N₃) via a simple thermal crosslinking reaction. X-PEI had better resistance to organic solvents from QCM-D measurements and maintained good thermal and mechanical performances. Photo-oxidation from air and sunlight slowly degrades plastics, shortens their service time, and leads to environmental pollution. This work bridged the gap between molecular integrity and its effect on the overall macroscopic mechanical changes through accurate measurement of the mass loss during degradation using a QCM-D. This work is essential in ensuring polymer design and active environmental protection.
95

Ionic liquids : The solid-liquid interface and surface forces

Hjalmarsson, Nicklas January 2016 (has links)
Ionic liquids (ILs) present new approaches for controlling interactions at the solid-liquid interface. ILs are defined as liquids consisting of bulky and asymmetric ions, with a melting point below 373 K. Owing to their amphiphilic character they are powerful solvents but also possess other interesting properties. For example, ILs can self-assemble and are attracted to surfaces due to their charged nature. As a result, they are capable of forming nanostructures both in bulk and at interfaces. This thesis describes how the solid-IL interface responds to external influences such as elevated temperatures, the addition of salt and polarisation. An improved understanding of how these factors govern the surface composition can provide tools for tuning systems to specific applications such as friction. Normal and friction forces are measured for ethylammonium nitrate (EAN) immersed between a mica surface and a silica probe, at different temperatures or salt concentrations. The results demonstrate that an increase in temperature or low concentrations of added salt only induce small changes in the interfacial structure and that the boundary layer properties remain intact. In contrast, at sufficiently large salt concentrations the smaller lithium ion prevails and the surface composition changes. The interfacial layer of a similar IL is also investigated upon the addition of salt and the results reveal that lithium ions affect the surface composition differently depending on the ion structure of the IL. This demonstrates that the surface selectivity strongly depends on the ion chemistry. Remarkably, a repulsive double layer force manifests itself for EAN at 393 K, which is not observed for lower temperatures. This indicates a temperature dependent change in EAN’s microscopic association behaviour and has general implications for how ILs are perceived. A new method is developed based on a quartz crystal microbalance to investigate how the surface compositions of ILs respond to polarisation. The approach demonstrates that interfacial layers of both a neat IL and an IL dissolved in oil can be controlled using potentials of different magnitudes and signs. Furthermore, the method enables two independent approaches for monitoring the charges during polarisation which can be used to quantify the surface composition. The technique also provides information on ion kinetics and surface selectivity. This work contributes to the fundamental understanding of the solid-IL interface and demonstrates that the surface composition of ILs can be controlled and monitored using different approaches. / Jonvätskor möjliggör nya tillvägagångssätt för att kontrollera interaktioner vid gränsskiktet mellan fasta ytor och vätskor. Jonvätskor definieras som vätskor som består av stora och asymmetriska joner med en smältpunkt under 373 K. På grund av sin amfifila karaktär är de starka lösningsmedel men har också andra intressanta egenskaper. Jonvätskor kan till exempel självorganisera sig och attraheras till ytor på grund av sin laddning. En följd av detta är att de bildar nanostrukturer både i bulk och på ytor. Denna avhandling beskriver hur gränsskiktet mellan fasta ytor och jonvätskor svarar på yttre påverkan såsom en ökning i temperatur, tillsättning av ett salt samt polarisering. En ökad förståelse för hur dessa faktorer styr ytkompositionen av jonvätskor kan bidra med verktyg för att kontrollera system till specifika applikationer såsom friktion. Normala- och friktionskrafter mäts för etylammonium nitrat (EAN) mellan en glimmeryta och en kolloidprob vid olika temperaturer eller saltkoncentrationer. Resultaten visar att en ökning av temperatur eller låga koncentrationer av tillsatt salt bara marginellt framkallar ändringar i strukturen på gränsytan och att det adsorberade lagret förblir intakt. När saltkoncentrationen emellertid var tillräckligt hög får den mindre litiumjonen överhanden och ytsammansättningen ändras. Ytlagret av en liknande jonvätska undersöks också vid tillsättning av salt och resultaten avslöjar att litiumjoner påverkar ytsammansättningen annorlunda beroende på jonstrukturen av jonvätskan. Detta visar att ytselektiviteten starkt beror på jonkemin. En repulsiv dubbellagerkraft yttrar sig anmärkningsvärt för EAN vid 393 K vilket inte observeras vid lägre temperaturer. Detta indikerar en ändring i EANs mikroskopiska sammansättningsbeteende och har generella återverkningar för hur jonvätskor uppfattas. En ny metod har utvecklats baserad på en kvartskristall mikrovåg för att undersöka hur ytsammansättningen av jonvätskor reagerar på polarisering. Denna metod visar att det adsorberade lagret av både en ren jonvätska och en jonvätska löst i olja kan kontrolleras genom att applicera spänningar med olika tecken och storlekar. Dessutom möjliggör metoden två oberoende tillvägagångssätt för att övervaka laddningarna under polarisering vilket kan användas för att kvantifiera ytsammansättningen. Tekniken ger också information om jonkinetik och ytselektivitet. Detta arbete bidrar till den grundläggande förståelsen av gränsskiktet mellan fasta ytor och jonvätskor och visar att ytsammansättningen av jonvätskor kan kontrolleras och övervakas med olika tillvägagångssätt. / <p>QC 20160518</p>
96

Automatic Frequency Control of Microwave Radiation Sources

Payne, Bobby D. 08 1900 (has links)
Resonant cavity controlled klystron frequency stabilization circuits and quartz-crystal oscillator frequency stabilization circuits were investigated for reflex klystrons operating at frequencies in the X-band range. The crystal oscillator circuit employed achieved better than 2 parts in 10 in frequency stability. A test of the functional properties of the frequency standard was made using the Stark effect in molecules.
97

Design and Study of Collagen-Tethered LL37 for Chronic Wound Healing

Lozeau, Lindsay Dawn 23 January 2018 (has links)
As society draws closer to the post-antibiotic era and the pipeline for alternatives dries, there is an urgent need for the development of novel antimicrobial therapies that do not promote bacterial resistance, particularly for immunocompromised chronic wound patients. Antimicrobial peptides (AMPs), including human-derived LL37, show considerable promise as broad spectrum alternatives that also have wound healing properties; however, few have been clinically implemented as novel antimicrobials due to their cytotoxicity stemming from a poor understanding of their mechanisms and low stability in vivo. It has been suggested that tethering, or attaching AMPs onto surfaces, is a viable strategy of delivering bioactive AMPs to surfaces while reducing cytotoxicity and improving stability. Thus, we designed new chimeric versions of LL37 with collagen-binding domains (CBD), derived from collagenase (cCBD-LL37) and fibronectin (fCBD-LL37) for non-covalent tethering onto collagen, a prevalent biopolymer in commercially available wound dressings and scaffolds. Our overall hypothesis was that CBDs would mediate stable tethering of broadly active, non-cytotoxic CBD-LL37 onto collagen-based scaffolds. We first studied the loading, release and bioactivities (e.g. antimicrobial activity and cytotoxicity) of each CBD-LL37 on commercially available 100% collagen type I PURACOL® wound scaffolds. We found that both cCBD-LL37 and fCBD-LL37 bound highly to collagen, were active against relevant wound pathogens, demonstrated stable activity after 14 days of release, and were not cytotoxic to human fibroblasts. The addition of different CBDs onto LL37 also markedly altered their soluble bioactivities. Using similar methods, we then studied the loading, release and bioactivity of each CBD-LL37 on a commercially available FIBRACOL® wound scaffolds, comprised of 90% collagen type I and 10% calcium alginate biopolymers. We found that both cCBD-LL37 and fCBD-LL37 also bound highly to and retained on collagen for 14 days, but were only active against Gram-negative P. aeruginosa. This suggested that the presence other biopolymers in addition to collagen, which is common among commercial wound dressings, could cause significant differences in binding, retention and bioactivities of CBD-LL37. To better understand how CBD modification affected CBD-LL37 structure leading to different bioactivities, we studied the CBD sequence-, peptide structure-, concentration-, time-, and bilayer composition-dependent interactions of soluble CBD-LL37 and compared these findings with the properties of unmodified LL37. Using Molecular Dynamics (MD) simulations, circular dichroism (CD) spectroscopy, quartz crystal microbalance with dissipation (QCM-D), and fluorescent bilayer imaging we determined the structural basis behind CBD alterations in bioactivities. MD and CD, in addition to other intrinsic CBD properties (helicity, amphiphilicity, charge) we hypothesized that cCBD-LL37 utilized similar mechanisms as unmodified LL37 while fCBD-LL37 demonstrated based primarily on surface adsorption. We used QCM-D and Voigt-Kelvin viscoelastic modeling to determine the time- and concentration-dependent interactions of unmodified LL37 with model mammalian lipid bilayers, the mechanisms of which are still controversial in literature despite being widely studied. These results were used to propose a model for the interaction mechanism of LL37 with zwitterionic bilayers that aligned with its bioactive concentrations. LL37 adsorbed at concentrations where it is immunomodulatory until reaching a threshold which corresponded with its antimicrobial concentrations. The threshold was correlated to lipid bilayer saturation, after which LL37 formed transmembrane pores. We observed collapse of the bilayer into a rigid proteolipid film at concentrations higher than the reported cytotoxic threshold of LL37. The mechanistic and structural information for each CBD-LL37 and unmodified LL37 provided a baseline for QCM-D and Voigt-Kelvin viscoelastic modeling to further elucidate the time-, concentration-, lipid composition- and CBD sequence-dependent basis behind the observed bioactivities of cCBD-LL37 and fCBD-LL37. We found that similar to LL37, cCBD-LL37 demonstrated pore formation mechanisms likely due to their similar charges, structural content and amphiphilicity. fCBD-LL37 demonstrated time-dependent, adsorption-based mechanism likely due to its anchoring aromatic residues, low charge, and low amphiphilicity. Knowledge gained from this study allowed mechanistic predictions of two newly designed hypothetical CBD-LL37 peptides. Results from this study contribute to a better understanding of a new class of antimicrobial, non-cytotoxic therapies based on collagen-tethered CBD-LL37, bringing it closer to clinical implementation in chronic wound applications and demonstrate the viability of biopolymer tethering as a platform toward using AMPs to quench the resistance crisis.
98

Detecção piezelétrica não-gravimétrica de boro empregando cristal de quartzo com eletrodo separado e superfície quimicamente modificada / Non-gravimetric piezoelectric detection of boron by using quartz crystal with electrode separated and chemically modified surface

Jesus, Dosil Pereira de 27 August 2003 (has links)
Cristais piezelétricos de quartzo (PQC\'s) com configuração de eletrodo separado e superfície quimicamente modificada foram utilizados para obtenção de um sensor para boro. Uma cela de detecção foi construída, de maneira que um eletrodo de aço inoxidável era mantido a uma pequena distância (0,3 mm) da superfície do PQC, da qual foi removido o eletrodo de ouro original. Foi verificado,por espectroscopia de impedância, medidas de freqüência de oscilação e simulações de espectros de impedância do circuito equivalente da cela, que as propriedades físico-químicas (condutividade, permissividade, densidade e viscosidade) das soluções influenciam de modo diferenciado as freqüências características da cela (ressonância e oscilação). Com o intuito de tomar a superfície sem eletrodo do PQC seletiva a boro, foram experimentadas várias estratégias de modificação química, as quais tinham por objetivo imobilizar polióis (manitol e N-metilglucamina) através de reações de silanização ou deposição de polímeros modificados. Os melhores resultados foram obtidos com o recobrimento do PQC com um filme fino (0,1 a 0,5 &#181;m) de um polímero resultante da modificação da poliepicloridrina com N-metilglucamina (NG). O sensor para boro foi avaliado em um sistema de análises por injeção em fluxo (FIA), em que a solução transportadora era uma solução de etilenodiaminotetraacetato disódico (EDTA) 50 mmol/L (pH 8,5). Observou-se que o mecanismo de resposta desse sensor era predominantemente não-gravimétrico, devido a um aumento de rigidez do filme de recobrimento, causado pela complexação do boro pelos grupos NG ancorados ao polímero. Além disso, o sensor apresenta uma característica bastante interessante que é a retenção do boro, o que possibilita uma diminuição do limite de detecção (LD) em FIA, por meio do aumento do volume de amostra injetado. Para remover o boro do filme sensor, era injetada uma solução de manitol 1 mol/L. Para volumes injetados de 160 e 1000 &#181;L de solução de borato foram alcançados LD\'s de 2 e 0,3 /-lmol/L, respectivamente. É possível detectar até 3 ng do analito presentes no volume de amostra injetado. O efeito de vários prováveis íons interferentes (Mg 2+, Ca2+, Al3+, Cr3+, Mn2+, Fé3+, Co2+, Cu2+, Ni2+, Zn2+, Sn2+, Pb2+, HPO42-, SiO32-, F-, Cl-, SO42-, NO3-, CH3COO-, CO32-, VO3-, Sb(OH)6- AsO4-, AsO3-, SeO3-, SeO4- e Ge(OH) 62-)foram avaliados, porém apenas o íon germanato apresentou significativa interferência, tanto que o sensor também foi considerado adequado para a determinação dessa espécie. A adição de sulfeto à amostra eliminou completamente a interferência do germânio. A não interferência dos íons metálicos é devida à complexação dos mesmos por EDTA presente na solução transportadora. Foi constatado que a presença de polióis, em alta concentração, na amostra afeta a resposta do sensor, uma vez que esses compostos competem com o filme sensor pelo analito. Os resultados obtidos com o sensor encorajaram a construção de um equipamento automatizado de FIA, o qual foi empregado com sucesso na determinação de boro em amostras de águas minerais, sucos de uva, vinagre, vinho e amostra certificada de água de rio. / Piezoelectric quartz crystals (PQC\'s) with electrode-separated configuration and chemically modified surface were used to develop a boron sensor. A detection cell was built in such a way that a stainless-steel electrode was held at a short distance (0.3 mm) from the PQC surface, whose original gold electrode was removed. By using impedance spectroscopy, oscillation frequency measurements, and simulations of impedance spectra of the cell equivalent circuit, it was observed that the physico-chemical properties (conductivity, permissivity, density and viscosity) of the solution affect in different ways the characteristic frequencies of the cell (resonance and oscillation). In order to make the electrodeless PQC surface selective to boron, several chemical modification strategies were tested with the aim of anchoring poliols (manitol and N-metilglucamine) via silanization reactions or deposition of modified polymers. Best results were obtained with PQC coated with a thin film (0.1 to 0.5 &#181;m) of a polymer, which was the product of the polyepichlorohydrin modification with N-methylglucamine (NG). The sensor was evaluated in a flow injection analysis system (FIA), which used a carrier solution containing disodic ethylenediaminetetraacetate (EDTA) 50 mmol/L (pH 8.5). It was observed that the mechanism of detection of this sensor is predominantly nongravimetric, owing to an increase in the stiffness of the covering film, because of the complex formation involving boron and the NG groups, which were incorporated in the polymer. Moreover, the sensor shows an interesting characteristic that is the boron retention, which allows decreasing the limit of detection (LOD) of the FIA method by increasing the injected sample volume. Boron is eluted from the sensor film with 1 mol/L manitol solution. For 160 and 1000 &#181;L volumes of borate solution, the LOD\'s were 2 and 0.3 &#181;mol/L, respectively. It is possible to detect down to 3 ng of boron in the injected sample volume. The effect of several ions (Mg 2+, Ca2+, Al3+, Cr3+, Mn2+, Fé3+, Co2+, Cu2+, Ni2+, Zn2+, Sn2+, Pb2+, HPO42-, SiO32-, F-, Cl-, SO42-, NO3-, CH3COO-, CO32-, VO3-, Sb(OH)6-\' AsO4-, AsO3-, SeO3-, SeO4- e Ge(OH) 62-) was evaluated. Only the germanate ion showed a significant interference, thus, the sensor can also be suitable for this species. However, this interference can be eliminated by the addition of sulfide to the sample. The non-interference of the metal ions is due to the complex formation with EDTA present in the carrier solution. Polyols at high concentration in the samples affect the sensor response, because these compounds compete with the sensor film for the analyte. An automated FIA equipment was built and successfully applied for determination of boron in samples such as mineral water, grape juice, vinegar, wine, and a standard reference material (river water).
99

Cristais piezelétricos de quartzo com eletrodos separados e superfície modificada como sensores em fase líquida / Piezoelectric quartz crystals with electrodes separated and modified surface as sensors in liquid media

Jesus, Dosil Pereira de 10 August 1999 (has links)
Esta dissertação descreve a construção e avaliação de um detector para fase líquida utilizando cristal piezelétrico de quartzo (PQC) (3,579545MHz) com a configuração de eletrodos separados. A cela de detecção possui dois eletrodos de latão, que foram posicionados a uma pequena distância (cerca de alguns décimos de milímetros) da superfície do cristal, cujos eletrodos de prata originais foram removidos. Um dos espaços existentes entre os eletrodos e o PQC é preenchido com ar e o outro com um líquido, o qual pode fluir pela cela. Os eletrodos são conectados a um oscilador, cuja freqüência de oscilação é medida através de um freqüencímetro interfaceado a um microcomputador. O detector foi caracterizado, no que se refere à influência da condutividade, densidade, viscosidade e vazão da solução, que flui pela cela, na freqüência de oscilação do PQC. Foi estudado também, com o auxílio de um analisador de impedâncias, o comportamento do módulo e do ângulo de fase da impedância elétrica oferecida pela cela de detecção para a variação das propriedades reológicas, mencionadas anteriormente, da solução. Para testar o arranjo como sensor gravimétrico em um sistema de análise por injeção em fluxo (FIA), foram desenvolvidos dois métodos de silanização da superfície do cristal, um envolvendo o reagente trimetilclorosilano (TCS) e outro utilizando o composto N-(&#946;-aminoetil)-&#947;-aminopropiltrimetoxisilano (AEAPTS). Os PQCs silanizados com o TCS foram testados para a detecção de compostos orgânicos (clorofórmio e acetona), enquanto os outros cristais, silanizados com AEAPTS, foram utilizados para detecção de íons metálicos. Para esses últimos PQCs notou-se uma boa sensibilidade para íons Cu2+, com os quais conseguiu-se um límite de detecção de aproximadamente 20&#181;M. / This work describes the construction and evaluation of a liquid-phase detector using a piezoelectric quartz crystal (PQC) (3,579545 MHz) with separated electrodes. The detection cell has two brass electrodes, which were placed at a small distance (a tenth of millimeters) from the crystal surface, whose original silver electrodes were removed. One of the gaps between the crystal and the electrodes is filled with air, while the other one is filled with a liquid, which flows through the cell. The electrodes are connected to an oscillator, whose oscillation frequency is measured by a frequency counter interfaced to a microcomputer. The influence of the conductivity, density, viscosity and flow rate of the solution on the PQC oscillation frequency was determined. The behavior of the impedance of the cell and its phase angle as a function of the previously mentioned reologic properties ofthe solution were also studied using an impedance analyzer. Two methods for the silanization of the crystal surface, using trimethylchlorosilane (TCS) and N-(&#946;-aminoethyl)-&#947;-aminopropyltrimethoxysilane (AEAPTS), were developed to test the assembly as a gravimetric sensor in a flow injection analysis system. The TCS-silanized PQCs were tested for the detection of organic compounds (chloroform and acetone), while the AEAPTS-silanized PQC were used for the detection of metallíc íons. These last PQC showed a good sensibility to Cu2+ ions, for which a limit of detection of about 20&#181;M was obtained.
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Dispositivos para cristais piezelétricos de quartzo com eletrodos separados para estudos de processos físicos e químicos de superfícies e filmes depositados / Separated electrodes piezelectric quartz crystal devices for studies ogf physical and chemical process and film coated

Neves, Carlos Antonio 02 October 2000 (has links)
Este trabalho descreve o desenvolvimento de dispositivos usando cristais piezelétricos de quartzo com eletrodos separados (ESPC) que, através do monitoramento da freqüência de oscilação e do espectro de impedância, permitem estudar modificações superficiais do cristal. Para o monitoramento de freqüência de oscilação, usou-se um circuito oscilador TTL seguido por um freqüencímetro e por uma interface conectada a um microcomputador. Para o controle desta interface, foi desenvolvida uma biblioteca de ligação dinâmica e um programa em HP-VEE 4.0. Os espectros de impedância foram obtidos via programas que controlam um analisador de impedâncias via protocolo GPIB. O primeiro dispositivo presta-se a estudos de filmes depositados (DFD) e é feito em acrílico com eletrodos em latão posicionados a décimos de milímetros do cristal. Sua parte superior possui uma camisa de água para a termostatização. O segundo, semelhante ao primeiro, permite a modificação de superfície (DMS) de ESPC através da introdução de reagentes na fase gasosa. Os dois dispositivos foram caracterizados e o primeiro mostrou boa linearidade entre as massas depositadas e aquelas estimadas pela equação de Sauerbrey. O DFD foi utilizado para monitoramento em tempo não real da modificação superficial do cristal por N-[3-(trimetoxisilil)propil]-1,2-etanodiamina seguida por reação com ácido iodoacético. Os resultados mostraram evidências de aumento de massa e mudança nas propriedades viscoelásticas do filme. O DMS foi utilizado para monitoramento em tempo real da modificação superficial com trimetilclorosilano (TMCS) e dimetildiclorosilano (DMDCS). Com o TMCS, devido à formação de uma monocamada, não foi possível monitorar variações significativas de freqüência de oscilação. Com o DMDCS foi possível verificar a formação de poli(dimetilsiloxano). / This dissertation describes the development of devices based on separated-electrode piezoelectric quartz crystals (ESPC) to study surface modifications. This is accomplished by monitoring the oscillation frequency and impedance spectrum of the crystal in the cell. A TTL oscillator coupled to a frequency counter and a microcomputer interface is used for monitoring the oscillation frequency. A dynamic link library and HP-VEE 4.0 programs were developed to control the interface. Other programs were also developed to acquire the impedance spectra from a spectrum analyzer by using the GPIB protocol. The first device (DFD) is intended to study deposited films. It is made in Plexiglas, with brass electrodes positioned at a few tenths of millimeters from the crystal surface, and a thermostatic water jacket. The second device (DMS) is similar to the first one, but allows the use of corrosive reagents in the gas phase for surface modification. The performance of both devices was evaluated and the results showed good agreement between the deposited mass and the one predicted by Sauerbrey equation. The DFD was used for offline monitoring of the crystal surface modification by N-[3-(triethoxysilyl)propyl]-1,2-ethanediamine followed by iodoacetic acid. The results show evidences of mass and viscoelastic variations of the film. The DMS was used for on-line monitoring during surface modification with trimethylchlorosilane (TMCS) and dimethyldichlorosilane (DMDCS). Due to the monolayer formed by TMCS, no significant frequency variation could be observed. On the other hand, the polymerization of DMDCS could be monitored.

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