In this thesis, the author proposed and developed nanostructured materials based Surface Acoustic Wave (SAW) and conductometric transducers for gas sensing applications. The device fabrication, nanostructured materials synthesis and characterization, as well as their gas sensing performance have been undertaken. The investigated structures are based on two structures: lithium niobate (LiNbO3) and lithium tantalate (LiTaO3). These two substrates were chosen for their high electromechanical coupling coefficient. The conductometric structure is based on langasite (LGS) substrate. LGS was selected because it does not exhibit any phase transition up to its melting point (1470°C). Four types of nanostructured materials were investigated as gas sensing layers, they are: polyaniline, polyvinylpyrrolidone (PVP), graphene and antimony oxide (Sb2O3). The developed nanostructured materials based sensors have high surface to volume ratio, resulting in high sensitivity towards di¤erent gas species. Several synthesis methods were conducted to deposit nanostructured materials on the whole area of SAW based and conductometric transducers. Electropolymerization method was used to synthesize and deposit polyaniline nanofibers on 36° YX LiTaO3 and 64° YX LiNbO3 SAW substrates. By varying several parameters during electropolymerization, the effect on gas sensing properties were investigated. The author also extended her research to successfully develop polyaniline/inorganic nanocomposites based SAW structures for room temperature gas sensing applications. Via electrospinning method, PVP fibres and its composites were successfully deposited on 36° YX LiTaO3 SAW transducers. Again in this method, the author varied several parameters of electrospinning such as distance and concentration, and investigated the effect on gas sensing performance. Graphene-like nano-sheets were synthesized on 36° YX LiTaO3 SAW devices. This material was synthesized by spin-coating graphite oxide (GO) on the substrate and then exposin g the GO to hydrazine to reduce it to graphene. X-ray photoelectron spectroscopy (XPS) and Raman characterizations showed that the reduced GO was not an ideal graphene. This information was required to understand the properties of the deposited graphene and link its properties to the gas sensing properties. Thermal evaporation method was used to grow Sb2O3 nanostructures on LGS conductometric transducers. Using this method, different nanoscale structures such as nanorods and lobe-like shapes were found on the gold interdigitated transducers (IDTs) and LGS substrate. The gas sensing performance of the deposited nanostructured Sb2O3 based LGS conductometric sensors was investigated at elevated temperatures. The gas sensing performance of the investigated nanostructured materials/SAW and conductometric structures provide a way for further investigation to future commerciallization of these types of sensors.

Thesis (Ph.D.)(Chemistry)--University of Pretoria, 2004. / Title from opening screen (viewed March 18th, 2005). Includes summaries in English and Afrikaans. Includes bibliographical references.

Este trabalho descreve o desenvolvimento de um biossensor contendo transdução condutométrica sem contato (C4D, capacitively coupled contactless conductivity detection) e ácido fólico (FA) como biorreceptor em microchip, uma nova alternativa que poderá ser utilizada na determinação do biomarcador tumoral FR-α. Essa espécie exibe interações com FA altamente específicas, com constantes de formação da ordem de 109-1010. Os dispositivos microfluídicos, os quais consistiram de uma lâmina de vidro (integrando os eletrodos), dielétrico (contendo a fase biossensora) e substrato de poli(dimetilsiloxano) (PDMS, incorporando os microcanais), foram fabricados utilizando-se processos de fotolitografia e deposição de filmes finos em fase vapor. Objetivando melhorias nos níveis de detecção da C4D, estudos de sensibilidade com base em parâmetros da curva analítica foram conduzidos alterando-se a natureza do dielétrico e a configuração dos eletrodos. Posteriormente, estudos de caracterização foram realizados para as superfícies modificadas com os intermediários de imobilização; condições reacionais distintas (reagente, concentração, solvente e tempo) foram consideradas. As técnicas de microscopia eletrônica de varredura e espectroscopia de fotoelétrons excitados por raios-X foram usadas, respectivamente, a fim de se verificar a possível formação de aglomerados e permitir determinações qualitativas e quantitativas sobre as composições químicas das superfícies. Como resultado dos experimentos de sensibilidade e caracterização de superfície, adotamos os parâmetros seguintes para os ensaios de interações biomoleculares posteriores: filme de SiO2 como dielétrico, eletrodos seletivos à C4D com formato retangular e orientação antiparalela e monocamadas automontadas do reagente 3-aminopropil(trietoxisilano) como intermediário de imobilização de FA. As duas etapas finais do trabalho foram: otimização do tempo de funcionalização com FA (3, 5 e 7 h) e caracterização da fase biossensora, realizada a partir de medidas de C4D e microscopia de força atômica (AFM). Para o primeiro caso, os microchips foram aplicados a um padrão de anticorpo monoclonal específico a FA (α-FA). Os ensaios biomoleculares indicaram uma adsorção efetiva de FA junto à superfície de SiO2 silanizada, sem a ocorrência (ao menos em níveis significativos) de impedimentos estéricos de sua espécie bioativa. Dentre os tempos de funcionalização investigados, 3 h foi aquele que resultou em uma maior sensibilidade do método. Em termos da etapa de caracterização eletroquímica da fase biossensora, seus resultados mostraram haver correlação entre a resposta analítica e as interações FA/α-FA. Em adição, conforme indicaram as medidas de AFM, não houve alterações drásticas na morfologia do substrato (SiO2) em função dos processos de modificação química de superfície. Por fim, o uso da C4D como uma técnica de transdução em biossensores mostrou-se uma alternativa promissora para a análise do biomarcador tumoral FR-α. Dentre outros aspectos, essa plataforma analítica requer uma instrumentação simples, barata e portátil, não apresenta inconvenientes relacionados ao contato eletrodo/solução, dispensa o uso de mediadores redox e permite a determinação simultânea de multianalitos. Neste ínterim, alterações no transdutor devem ser implementadas visando um aumento na sensibilidade do método, o qual representa seu fator limitante principal. / This work describes the development of a biosensor containing capacitively coupled contactless conductivity transduction (C4D) and folic acid (FA) as bioreceptor in microchip, a new alternative that can be used in FR-α tumor biomarker analysis. FR-α exhibits highly specific interactions with FA, showing formation constants of the order of 109-1010. The microfluidic devices consisted of a glass layer (integrating the electrodes), dielectric (containing the biosensor phase), and poly(dimetilsiloxane) substrate (PDMS, incorporating microchannel). The microfabrication stage evolved photolithography processes, metal adsorption via sputtering, and plasma-enhanced vapor film deposition. In order to improve detection levels of C4D, sensitivity studies were conducted by changing the dielectric nature and electrode configuration. Through flow analysis with given electrolyte standards, the limits of detection and quantification were calculated based on analytical curve parameters. Subsequently, researches were performed to characterize the modified surfaces with immobilization intermediate considering reaction conditions distinct (reagent, concentration, solvent, and time). The techniques of scanning electron microscopy and X-ray photoelectron spectroscopy were employed, respectively, aiming to verify the clusters formation and allow qualitative and quantitative determinations about the surfaces chemical composition. From the results of sensitivity experiments and surface characterization, we adopt the following parameters for the biomolecular interactions assays: SiO2 film as dielectric, C4D selective electrodes with rectangular shape and antiparallel orientation, and self-assembled monolayers of 3-aminopropyl(triethoxysilane) as intermediary for immobilization of FA. The two final steps of the work were: optimizing the FA functionalization time (3, 5, and 7 h) and phase biosensor characterization, made from measures of C4D and atomic force microscopy (AFM). For the first case, due to the absence of FR-α standard for purchase, the microchips were applied to FA specific monoclonal antibody (α-FA). The biomolecular assay indicated effective adsorption of FA, without occurrence (at least in significant levels) of steric hindrance of its bioactive specie. Among the investigated times of functionalization, 3 h resulted in a higher sensitivity of the method. In terms of biosensor phase electrochemical characterization stage, their results evidenced correlation between analytical response and FA/α-FA interactions. Additionally, as the AFM measurements showed, drastic changes in the morphology of the substrate (SiO2) with the surface modification processes did not occur. Finally, the use of the C4D as transduction technical in biosensors proved to be a promissory alternative for FR-α tumor biomarker analysis. Among other features, this platform has not drawbacks related to the electrode/solution contact, dispenses the use of redox mediators, allows the simultaneous determination of multianalytes, and employs an instrumentation that is simple, cheap, and portable. Nevertheless, changes in the transducer should be implemented to increase the method sensitivity, which represents its main limiting factor.

The purpose of this thesis is a study of sensing properties of pure, platinum-doped and gold-doped tungsten oxide thin films. Required films were prepared by magnetron sputtering on glass and passivated silicon. Their chemical composition was investigated by XPS. The morphology of the films was measured using AFM and SEM. Sensing response of these films to hydrogen was investigated in the range from 1,000 to 10,000 ppm and temperatures between 100 and 350 řC. It was found that platinum oxides PtO2 and PtO were partially reduced during the sensing tests. The possible encapsulation of the platinum by the tungsten oxide was discovered. The sensitivity of the metal doped films was much higher than the sensitivity of the pure films. In the case of platinum-doped films the sensitivity decreased with increasing temperature in contrast to the gold-doped films where the sensitivity increased. The gold was present in two metallic and oxidized states. The gold was reduced during the reactions with hydrogen. After the sensing tests, the size of crystalline grains increased and the platinum-doped films became coarser.

No description available.

[Introduction, p. 3] The solubilities of the silver halides cannot be determined by the conventional methods of analytical chemistry because they are too sparingly soluble (of the order 1 x 10⁻⁵ to 1 x 10⁻⁶ g . equiv./1. at 25°C.) However, electrical, and to a lesser extent optical methods, are admirably suited because of their greater sensitivity. The conductometric and potentiometric methods are the two most important electrical techniques for the measurement of the solubilities of sparingly soluble salts, and are the ones employed in this research. There are large discrepancies between the published values for the solubility of silver chloride. Results obtained by the conductometric and potentiometric methods disagree. In addition, figures quoted by independent authors using the same method differ by as much as 15%.

The flash method proposed by Parker et al. in the early sixties is one of the most important experimental procedures to determine the thermal properties of homogeneous materials. Because of the versatility of this method, researchers have attempted to extend its usefulness into the realm of composite material. However, some difficulties arise because of the existence of preferential heat paths in heterogeneous materials, especially in fiber-reinforced composites. In order for experimental flash method results to be meaningful a homogeneous temperature front must exist at the back face of the sample, where the measurements are made. In this work, the parameters that render the radial temperature response of a fiber reinforced composite homogeneous at the back face were investigated. According to the literature three criteria must be met for homogeneity to occur: fiber-to-matrix volume ratio must be high; sample axial dimension must be large compared to radial fiber dimension and the thermal contact between the fibers and the matrix must be high. Since the first two criteria are met by most fiber-reinforced composite samples subject to the flash method, attention was concentrated on the third criterion. An inequality that must be met by the contact conductance term to establish homogeneity is proposed and some sample temperature profiles are presented. / Master of Science

This work deals with dual detection of organic and inorganic analytes after separation by capillary zone electrophoresis. In the first part, two types of hydrodynamic sampling are tested. Standard hydrodynamic sampling most often used in laboratory-made electrophoretic apparatus, based on lifting the vessel with the sample, in which the sampling end of the capillary is immersed, and a new method based on increasing the pressure in the sampling vessel without moving the capillary. This sampling procedure minimizes experimenter activity because it is controlled by software. Experimenter only changes vessel containing the sample solution for one with separation electrolyte. The experimental parameters, the sampling time and pressure, are optimized to achieve maximum separation efficiency and adequate detection sensitivity. In the second part of the work, the developed method is tested for the separation of amino-acids in a biological sample (urine).

O estudo da variação da composição atmosférica contribui para a compreensão dos efeitos das emissões antrópicas sobre a troposfera. A análise química da água de chuva assume importância neste contexto, uma vez que a precipitação úmida constitui um excelente meio de remoção de muitos poluentes da atmosfera. O aumento de acidez das chuvas, em várias partes do planeta, tem sido associado, predominantemente, à presença de ácidos fortes como o sulfúrico e nítrico, e ácidos orgânicos como acético e fórmico, originados pela oxidação de compostos de enxofre, nitrogênio e carbono, respectivamente. Dando continuidade a pesquisas anteriores sobre deposição úmida realizadas no IQ-USP, no presente trabalho buscou-se extrair mais informações de titulações potenciométricas e condutométricas de água de chuva, mediante aperfeiçoamento dos métodos de cálculo, com recursos quimiométricos modernos, de modo a complementar as medidas de pH e condutividade convencionais. As concentrações de espécies ácidas ou básicas nas amostras de água de chuva são da ordem de 10-5 mol L-1, muito abaixo das concentrações empregadas na prática cotidiana das titulações potenciométricas (tipicamente, no intervalo de 10-1 a 10-3 mol L-1). Dessa forma, propôs-se o uso de um método de regressão não linear (RNL) multiparamétrica, baseado no método de Levenberg-Marquardt, para a análise de dados potenciométricos (TP-RNL) após procedimento de transposição de coordenadas das curvas de titulação. Ao lado das determinações das concentrações dos analitos, pôde-se estimar ou refinar as constantes de protonação das bases de Brönsted presentes. Para tanto, procedeu-se ao ajuste por RNL das concentrações e/ou das constantes de protonação das bases presentes, de modo a minimizar o quadrado dos desvios entre os valores experimentais e os calculados em função do titulante adicionado, recorrendo a uma equação geral baseada no número médio de protonação de cada uma das bases. Ficou demonstrada a possibilidade de se determinar a soma de ácidos fortes completamente dissociados, soma de ácidos fracos com pK próximo a 4 (principalmente ácidos acético e fórmico), HCO3-/H2CO3 e a soma de bases fracas com pK ao redor de 9 (essencialmente NH3/NH4+). Durante as titulações, a facilidade em se coletar medidas de condutância simultaneamente às de pH levou a elaboração de um método sofisticado de análise de curvas de titulação condutométrica. Para tanto, desenvolveu-se uma equação de RNL baseada na contribuição das espécies em cada valor de pH e na condutância iônica medida (TC-RNL). A equação de TC-RNL é alimentada por dados de pH em função do titulante adicionado, oriundos de titulações potenciométricas. Dessa forma, foi possível caracterizar diretamente amônio e bicarbonato, além do conteúdo iônico total que não participa dos equilíbrios ácidos/base existentes no sistema. Adicionalmente, foi possível separar a contribuição do ácido acético da do fórmico, tarefa impossível por TP-RNL, ao menos nas baixas concentrações encontradas na chuva. Nas mesmas amostras de chuva tituladas foi procedida a análise por eletroforese capilar de zona com detecção condutométrica sem contato (CZE-CCD), conforme procedimento descrito na literatura, estimando-se os cátions e ânions majoritários. Num aperfeiçoamento do método para ânions, viabilizou-se a determinação simultânea HCO3-. Para validar os resultados obtidos por CZE-CCD, recorreu-se à cromatografia de íons. As concentrações de vários analitos, dependendo das amostras de chuva, avizinham-se do limite de quantificação das técnicas TP-RNL, TC-RNL e CZE-CCD. Mesmo assim, para as amostras de chuva analisadas, em geral, houve concordância satisfatória entre os valores estimados pelas três técnicas para espécies agrupadas da seguinte forma: ácidos fortes, ácidos fracos, amônio e bicarbonato. / Ivestigation of the variability of atmospheric chemical composition is an important tool for the evaluation and comprehension of anthropic emissions effects.Rainwater analysis has great importance in this context because wet deposition represents an efficient removal route of many atmospheric pollutants. The acidification of rainwater over wide regions of the planet is associated with the increased presence of strong acids, like sulfuric and nitric ones, and organic acids, such as acetic and formic, mainly originated from oxidative reactions od sulfur, nitrogen and carbon compounds in the atmosphere. In the context of a broad research line at IQ-USP devoted to the study of wet deposition and associated atmospheric chemistry, this dissertation advances the subject of using moderm chemometric methods to extract reliable analytical information from potentiometric and conductometric titrations of rainwater. The concentrations of acidic and alkaline species in rainwater are typically below 5 x 10-5 mol L-1, out of the reach of conventional potentiometric titrations (widely applied in the range of 10-1 and 10-3 mol L-1). Thus, multiparametric non-linear regression based on Levenberg-Marquardt method combined with coordinates transposition to reduce iterative calculation is proposed for the analysis of potentiometric data (PT-NLR). General equations based on the average protonation number of each base are used to describe the system. Bases concentrations and/or protonation constants are adjusted by NLR in order to minimize the sum-of-squares of the residuals between experimental and calculated values of the titration curves. The data analysis was tuned to provide four parameters as output: the sum of strong acids, the sum of weak acids with pK around 4.2 (mainly acetic and formic acids), the sum of HCO3-/H2co3 and the sum of weak bases with pK near 9 (essentially NH3/NH4+). To extract more analytical information from the acid/base titrations, conductance meassurements were made simultaneously with the pH readings, and a sophisticated data analysis method was envisioned. The pH data and available protonation constants for bases included in the model are used to compute the distribution of the species. The available conductivity of each species of the model is introduced in a general equation where the only unknowns are the total concentrations of the bases and of electrolyte not involved in acid/base equilibria. All these concentrations are adjusted by NLR on the conductance data, resulting in the first successful conductometric titration method with non-linear regression analysis, CT-NLR. The great advantage over PT-NLR was the capability to distinguish the contributions of acetic and formic acids in rainwater (an impossible task in PT-NLR method), but total strong acids, ammonium and bicarbonate are determined as well, beside the total ionic content of species not involved acid-base equilibria. Rainwater samples were also analysed by capillary zone electriphoresis equipment with contactless conductometric detection (CZE-CCD), as decribed in literature, regarding the major cations and anions. An improved CE method for anions was developed to afford the simultaneous determination of HCO3-. Validation of results was made with the reference method of ion chromatography. Although in many samples some analytes are low and close to the quantification limits od CZE-CCD, PT-NLR and CT-NLR, satisfactory agreement was achieved between values obtained by the three techniques for strong acids, weak acids, ammonium and bicarbonate.

O estudo da variação da composição atmosférica contribui para a compreensão dos efeitos das emissões antrópicas sobre a troposfera. A análise química da água de chuva assume importância neste contexto, uma vez que a precipitação úmida constitui um excelente meio de remoção de muitos poluentes da atmosfera. O aumento de acidez das chuvas, em várias partes do planeta, tem sido associado, predominantemente, à presença de ácidos fortes como o sulfúrico e nítrico, e ácidos orgânicos como acético e fórmico, originados pela oxidação de compostos de enxofre, nitrogênio e carbono, respectivamente. Dando continuidade a pesquisas anteriores sobre deposição úmida realizadas no IQ-USP, no presente trabalho buscou-se extrair mais informações de titulações potenciométricas e condutométricas de água de chuva, mediante aperfeiçoamento dos métodos de cálculo, com recursos quimiométricos modernos, de modo a complementar as medidas de pH e condutividade convencionais. As concentrações de espécies ácidas ou básicas nas amostras de água de chuva são da ordem de 10-5 mol L-1, muito abaixo das concentrações empregadas na prática cotidiana das titulações potenciométricas (tipicamente, no intervalo de 10-1 a 10-3 mol L-1). Dessa forma, propôs-se o uso de um método de regressão não linear (RNL) multiparamétrica, baseado no método de Levenberg-Marquardt, para a análise de dados potenciométricos (TP-RNL) após procedimento de transposição de coordenadas das curvas de titulação. Ao lado das determinações das concentrações dos analitos, pôde-se estimar ou refinar as constantes de protonação das bases de Brönsted presentes. Para tanto, procedeu-se ao ajuste por RNL das concentrações e/ou das constantes de protonação das bases presentes, de modo a minimizar o quadrado dos desvios entre os valores experimentais e os calculados em função do titulante adicionado, recorrendo a uma equação geral baseada no número médio de protonação de cada uma das bases. Ficou demonstrada a possibilidade de se determinar a soma de ácidos fortes completamente dissociados, soma de ácidos fracos com pK próximo a 4 (principalmente ácidos acético e fórmico), HCO3-/H2CO3 e a soma de bases fracas com pK ao redor de 9 (essencialmente NH3/NH4+). Durante as titulações, a facilidade em se coletar medidas de condutância simultaneamente às de pH levou a elaboração de um método sofisticado de análise de curvas de titulação condutométrica. Para tanto, desenvolveu-se uma equação de RNL baseada na contribuição das espécies em cada valor de pH e na condutância iônica medida (TC-RNL). A equação de TC-RNL é alimentada por dados de pH em função do titulante adicionado, oriundos de titulações potenciométricas. Dessa forma, foi possível caracterizar diretamente amônio e bicarbonato, além do conteúdo iônico total que não participa dos equilíbrios ácidos/base existentes no sistema. Adicionalmente, foi possível separar a contribuição do ácido acético da do fórmico, tarefa impossível por TP-RNL, ao menos nas baixas concentrações encontradas na chuva. Nas mesmas amostras de chuva tituladas foi procedida a análise por eletroforese capilar de zona com detecção condutométrica sem contato (CZE-CCD), conforme procedimento descrito na literatura, estimando-se os cátions e ânions majoritários. Num aperfeiçoamento do método para ânions, viabilizou-se a determinação simultânea HCO3-. Para validar os resultados obtidos por CZE-CCD, recorreu-se à cromatografia de íons. As concentrações de vários analitos, dependendo das amostras de chuva, avizinham-se do limite de quantificação das técnicas TP-RNL, TC-RNL e CZE-CCD. Mesmo assim, para as amostras de chuva analisadas, em geral, houve concordância satisfatória entre os valores estimados pelas três técnicas para espécies agrupadas da seguinte forma: ácidos fortes, ácidos fracos, amônio e bicarbonato. / Ivestigation of the variability of atmospheric chemical composition is an important tool for the evaluation and comprehension of anthropic emissions effects.Rainwater analysis has great importance in this context because wet deposition represents an efficient removal route of many atmospheric pollutants. The acidification of rainwater over wide regions of the planet is associated with the increased presence of strong acids, like sulfuric and nitric ones, and organic acids, such as acetic and formic, mainly originated from oxidative reactions od sulfur, nitrogen and carbon compounds in the atmosphere. In the context of a broad research line at IQ-USP devoted to the study of wet deposition and associated atmospheric chemistry, this dissertation advances the subject of using moderm chemometric methods to extract reliable analytical information from potentiometric and conductometric titrations of rainwater. The concentrations of acidic and alkaline species in rainwater are typically below 5 x 10-5 mol L-1, out of the reach of conventional potentiometric titrations (widely applied in the range of 10-1 and 10-3 mol L-1). Thus, multiparametric non-linear regression based on Levenberg-Marquardt method combined with coordinates transposition to reduce iterative calculation is proposed for the analysis of potentiometric data (PT-NLR). General equations based on the average protonation number of each base are used to describe the system. Bases concentrations and/or protonation constants are adjusted by NLR in order to minimize the sum-of-squares of the residuals between experimental and calculated values of the titration curves. The data analysis was tuned to provide four parameters as output: the sum of strong acids, the sum of weak acids with pK around 4.2 (mainly acetic and formic acids), the sum of HCO3-/H2co3 and the sum of weak bases with pK near 9 (essentially NH3/NH4+). To extract more analytical information from the acid/base titrations, conductance meassurements were made simultaneously with the pH readings, and a sophisticated data analysis method was envisioned. The pH data and available protonation constants for bases included in the model are used to compute the distribution of the species. The available conductivity of each species of the model is introduced in a general equation where the only unknowns are the total concentrations of the bases and of electrolyte not involved in acid/base equilibria. All these concentrations are adjusted by NLR on the conductance data, resulting in the first successful conductometric titration method with non-linear regression analysis, CT-NLR. The great advantage over PT-NLR was the capability to distinguish the contributions of acetic and formic acids in rainwater (an impossible task in PT-NLR method), but total strong acids, ammonium and bicarbonate are determined as well, beside the total ionic content of species not involved acid-base equilibria. Rainwater samples were also analysed by capillary zone electriphoresis equipment with contactless conductometric detection (CZE-CCD), as decribed in literature, regarding the major cations and anions. An improved CE method for anions was developed to afford the simultaneous determination of HCO3-. Validation of results was made with the reference method of ion chromatography. Although in many samples some analytes are low and close to the quantification limits od CZE-CCD, PT-NLR and CT-NLR, satisfactory agreement was achieved between values obtained by the three techniques for strong acids, weak acids, ammonium and bicarbonate.