Nanoscale materials have great applications in many areas. One of these applications is for manufacturing ultra-compact and efficient sensors for chemical and biological molecule detection. Noble metals, such as gold (Au) and silver (Ag), because of their distinguished optical property"localized surface plasmon resonances (LSPRs) that exhibit low loss, are ideal materials to fabricate these nanoscale plasmonic particles or structures. This work addresses the synthesis, characterization, and sensing applications of Au and Ag nanoparticles (NPs). The progress on certain subjects related to our work"NP synthesis, surface functionalization, Au sphere-film structure and two-photon fluorescence"are reviewed in Chapter 1. We also show the calculation results of LSPRs of Au nanosphere suspensions using Mie theory. The measured extinction spectra of Au nanosphere suspensions agree with the calculated results very well. Chapter 2 is a chapter describing the chemical synthesis of a variety of NPs, such as Ag prisms and cubes, Au spheres, rods, and bipyramids. These experiments involved different synthetic mechanisms and methods which enabled us to prepare NPs with desired shapes and optical properties. To put these NPs into application, it is desirable and sometimes necessary to functionalize their surfaces. In Chapter 3, we present the functionalization of Ag cubes with poly(allylamine hydrochloride) (PAH) and poly(allylamine hydrochloride)-dithiocarbamate (PAH-DTC), which follows our previous work on Au NPs. The purpose of studying Ag instead of Au is to use the stronger plasmonic enhancement in Ag when applied to two-photon imaging applications. However, we found that PAH-DTC shrank the Ag cubes. We also functionalized the cationic hexadecyltrimethylammonium bromide (CTAB)-stabilized Au NRs with anionic poly(sodium 4-styrenesulfonate) (PSS). Coated with the strong polyelectrolyte PSS, the NRs become more manageable and can be stable for over six months and are easily immobilized onto positively charged substrate. We put PSS-functionalized Au NPs into use and studied their adsorption process onto PAH-coated optical fiber tapers by monitoring the transmission light through the fiber. When the diameter of the fiber taper gets smaller, stronger coupling occurred between transmitted light inside the taper and the Au NPs on the taper surface (cylinder). This coupling resulted in a loss of the guided light at the plasmon resonance wavelength of the NPs. By monitoring this loss, we can study the adsorption rate of Au NPs onto the fiber. In Chapter 4, we used Au nanospheres to study the adsorption rate on substrates with different curvatures. We also established a theoretical model to explain this phenomenon for cylindrical surface as well as planar and spherical surfaces. Our results fit well with the theory, which predicts that particle adsorption rates depend strongly on surface geometry, and can exceed the planar surface deposition rate by over two orders of magnitude when the diffusion length of the particle is large compared to the surface curvature. In Chapter 5, we studied the optical properties of Au nanospheres separated from a thick Au film by a polyelectrolyte multilayer (PEM) film assembled from PAH and PSS under specific pH condition. The PEM film undergoes swelling and shrinking when the environmental pH is changed as a result of charging and discharging of the polyelectrolytes. Therefore, the PEM film provides an efficient means to tune the distance between Au spheres and Au film. The extinction peak blue-shifted as much as 100 nm when the pH of the water changed from pH 10 to pH 3 for 100 nm diameter Au spheres on a PEM film assembled at pH 9.5. Our preliminary estimates that the gap between sphere and surface can be as small as a few nm even though the film itself is tens of nm thick when it is not constrained by Au spheres. We studied two-photon excitation fluorescence (TPEF) from Ag triangles in Chapter 6. The triangles were fabricated by nanosphere lithography, which used convective self-assembly to make the nanosphere mask. The LSPRs of the nanotriangles were tuned to be in the 800--900 nm range to match with the Ti:Sapphire pulse laser at 880 nm. We found that certain spots on the fluorescence images gave rise to larger fluorescence intensity than rest of the area. SEM imaging reveals that the unusually bright spots seen on the surface were related to regions where the triangles transformed to spherical particles. The larger intensity is tentatively ascribed to the plasmon resonance of those spherical particles in ~400 nm range. / Ph. D.

The production of hydrogen fuel via solar water splitting is an important carbon-neutral strategy for the development of renewable resources and has sparked great interest in the scientific community. Hydrogen production efficiencies for supramolecular photocatalysts of the architecture [{(TL)2Ru(BL)}2RhX2]5+ (BL=bridging ligand, TL=terminal ligand, X=halide) are among the highest reported in deoxygenated organic solvents, but do not function in air-saturated aqueous solution due to quenching of the metal-to-ligand charge transfer (MLCT) excited-state under these conditions. Herein, we report the groundbreaking use of polyelectrolytes to increase efficiency of supramolecular photocatalysts in solar hydrogen production schemes under aqueous aerobic conditions. The new photocatalytic system incorporates poly(4-styrenesulfonate) (PSS) into aqueous solutions containing [{(bpy)2Ru(dpp)}2RhCl2]5+ (bpy = 2,2'-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine). PSS has a profound impact on photocatalyst efficiency, increasing hydrogen production over three times that of deoxygenated aqueous solutions alone. Hydrogen photocatalysis proceeds even under aerobic conditions for PSS containing solutions, an exciting consequence for solar hydrogen production research. Thermodynamics of binding due to intermolecular interactions between Ru,Rh,Ru photocatalysts and polyelectrolytes was probed using isothermal titration calorimetry (ITC). ITC studies reveal the driving forces of aggregate formation, providing new insight into the intermolecular forces that lead to increased photocatalytic efficiency and stability in the presence of water soluble polymers. Synthesis and characterization of a novel supramolecular photocatalyst having hydrophilic terminal ligands are reported. Addition of sulfonated terminal ligands into a Ru,Rh,Ru photocatalyst has a significant impact on the excited-state properties of the complex. The new complex demonstrates increased solubility and hydrogen production efficiency in aqueous solutions. Hydrogen production is observed even under aerobic conditions for the new complex, a stark contrast to the hydrophobic analog in organic solvents. The synthesis, characterization, and electropolymerization of a chromophore-catalyst assembly having vinyl-substituted terminal ligands to create robust water reduction photocatalysts on wide-bandgap semiconductors is reported. The polymeric photocatalysts are expected to show increased stability over a wide pH range and increased photostability compared to chromophore-catalyst assemblies that employ carboxylic or phosphonic acid groups to adsorb the photoreactive catalyst to the metal oxide surface. / Ph. D.

Nanometer thick layers of clay and polymer were formed on mica, silicon, and aluminum 2024-T3 alloy using alternating solutions of positively and negatively charged polymer and clay, respectively. Atomic force microscopy was used to observe the morphology of the composite films on mica and silicon. It was found that solution concentrations of clay above 0.02 weight percent lead to the uncontrolled deposition of clay platelets on the substrate's surface. By using solution concentrations of clay above 0.02 weight percent and ultrasonic agitation together it is possible to deposit a uniform monolayer of clay platelets on a mica substrate in £ 20 seconds. Ultrasonic agitation also produced crude patterns of montmorillonite platelets. Thin films of poly(diallydimethylammonium chloride) (PDDA) were made using concentrations ³ 2 weight percent of PDDA. It was found that the PDDA formed several unusual morphologies. Spherulites of PDDA were observed with AFM and the glass transition temperature of high molecular weight PDDA was measured using differential scanning calorimetry (DSC). Circular regions of positive charge were discovered on silicon wafers provided by three different sources. These areas of charge have never been reported in literature, but can easily be detected by placing wafers into solutions containing negatively or positively charged solutions of clay or polymer, respectively. The exact nature of these charged regions is unknown, but it is hypothesized that impurities on silicon wafers create the circular regions of positive charge. ISAM films made of a polyamide salt and a synthetic clay, Laponite RD®, demonstrated significant corrosion resistance on 2024-T3 Al alloys after 168 hours of salt spray testing. The ISAM films offered corrosion protection only if there was a significant layer of underlying surface oxide present, however. It was found that ISAM deposited films of polyarylic acid (PAA) and polyallylamine hydrochloride (PAH) may offer some corrosion resistance on 2024-T3 Al alloys, but these films' corrosion resistance is severely hampered by the presence of Cl- in the PAH solution. Funding from this project was gratefully received from the Materials Science and Engineering Department at Virginia Tech; Luna Innovations Inc; the American Chemical Society / Petroleum Research Fund #34412-G5 and the Environmental Protection Agency Contract #68-D-00-244. / Master of Science

Cette thèse présente une étude des solutions aqueuses de trois complexes de polyélectrolytes (PECs) modèles. Les PECs résultent de la complexation de deux polyélectrolytes linéaires de charge opposée: un polycation (le PDADMA) et trois polyanions de longueur de persistance non électrostatique distinctes: le polystyrene sulfoné (PSS), le poly(α-méthyl styrène sulfoné) (PαMSS) et l’acide hyaluronique (HA). En plus de l’influence de la rigidité intrinsèque des polyanions sur la formation et la structure des PECs, les effets de la force ionique et de la méthode de préparation des solutions aqueuses de PECs ainsi que l’influence de la concentration des solutions binaires de polyélectrolytes initiales ont également été abordés. Nous suggérons que le processus de complexation entre polycations et polyanions en régimes semidilué et concentré est analogue à une gélification. Il y a toutefois une différence entre les deux complexes qui est reliée à la différence de rigidité intrinsèque. / This PhD thesis presents a study of the aqueous solutions of three model polyelectrolyte complexes (PECs). PECs were formed between hydrophilic and highly charged linear macrocations of poly(diallyldimethyl ammonium) (PDADMA) and linear macroanions of distinct intrinsic persistence lengths: sulfonated polystyrene (PSS), sulfonated poly(α-methyl styrene) (PαMSS) and hyaluronate (HA). In addition to the effect of the macroion stiffness on the PEC formation and structure, those of the ionic strength and the way of preparing the PEC aqueous solutions as well as that of the concentration regimes of the initial PE aqueous solutions were also tackled. We suggest the complexation between macrocations and macroanions in the semidilute and concentrated regimes can be described as a universal gelation process. A difference between PDADMA-PSS and PDADMA-HA complexes is related to the primary self-assembling process and is associated with the distinct structural models for PECs.

Denna avhandling presenterar experimentella studier av polyelektrolytadsorption på motsatt laddade ytor, där substrat av både kiseloxid och blekt barrsulfatmassa har använts. Ett huvudsakligt syfte med denna forskning var att karaktärisera konformationen hos adsorberade skikt av katjonisk polyakrylamid (CPAM) i jämförelse med katjonisk dextran (Cdextran) och relatera denna information till inbindningskapaciteten av kolloidal kiselsyra. Ett andra syfte i denna avhandling var att studera kinetiken för sekventiell adsorption av polyamidamin epiklorhydrin (PAE) och karboxymetyl cellulosa (CMC) på massafibrer och att bestämma adsorptionsisotermer för deponering av polyelektrolyter skikt för skikt på massafibrer. Adsorptionen av CPAM på kiseloxidytor studeras med stagnationspunkts-reflektometri och kvartskristalls-mikrogravimetri för att bestämma adsorptionskinetiken och mättnadsadsorptionens beroende av polyelektrolytens laddningstäthet, pH och NaCl koncentration. Konformationen hos adsorberade skikt av CPAM och Cdextran bestämdes både före och efter sekundär tillsats av kolloidal kiselsyra (CS) och adsorptionen av CS kvantifierades också som funktion av yttäckningen av polyelektrolyt. Resultaten indikerar att laddningstätheten hos CPAM kontrollerar den adsorberade mängden på kiseloxidytor vid låga NaCl koncentrationer. Både adsorptionen av CPAM och Cdextran på kiseloxid visades vara effektiv i NaCl koncentrationer upp till 1 M, vilket indikerar ett signifikant bidrag av icke-jonisk interaktion mellan polyelektrolyterna och kiseloxid. Adsorptionen av CS var högre på föradsorberad CPAM än Cdextran. Konformationen hos de adsorberade skikten efter tillsats av CS sågs expandera signifikant för skikt baserade på CPAM medan skikt av Cdextran vid låga salthalter verkade återta sin konformation efter en temporär expansion. I den andra delen av avhandlingen studerades sekventiell adsorption av PAE och CMC på massafibrer. Adsorptionsisotermer skikt för skikt på avkryllad massa visade att PAE adsorberade i större mängd än CMC, både i hänseende av massa och laddning. Adsorptionen av PAE var signifikant långsammare än CMC och adsorptionstiden till 90% av mättnadsadsorptionen bestämdes till 3 respektive 1 minut. Zetapotentialen för kryll bestämdes för adsorption av de två första polyelektrolytskikten och resultaten tydde på att kryllmaterialet omladdade inom en minut efter tillsatserna av polyelektrolyt. Reflektometriförsök inom sekventiell adsorption av PAE och CMC på kiseloxid antydde att den låga molekylviktsfraktionen av PAE störde uppbyggnaden av polyelektrolyt-multiskikten. / This thesis presents experimental studies of polyelectrolyte adsorption on oppositely charged surfaces, where substrates of both silica and bleached softwood kraft pulp were used. A major aim of this research was to characterise the conformation of adsorbed layers of cationic polyacrylamide (CPAM), in comparison to cationic dextran (Cdextran), and relate this information to the binding capacity of colloidal silica. A second aim in this thesis was to study the kinetics of the sequential adsorption of polyamide epichlorohydrine (PAE) and carboxymethyl cellulose (CMC) on pulp fibres, and to determine the adsorption isotherms for the layer-by-layer deposition of polyelectrolytes on pulp fibres. The adsorption of CPAM on silica surfaces was studied using stagnation point adsorption reflectometry and quartz crystal microgravimetry to determine its adsorption kinetics as well as the dependencies of polyelectrolyte charge densities, pH, and NaCl concentration on saturation adsorption. The conformation of adsorbed layers of CPAM and Cdextran, analysed in terms of amount of water and layer thickness, was determined both before and after the secondary adsorption of colloidal silica (CS), and the adsorption of CS was also quantified as a function of the surface coverage of the polyelectrolyte. Results indicate that the charge density of CPAM controlled the amount of the polyelectrolyte adsorbed on silica surfaces at low NaCl concentrations. The adsorption of both CPAM and Cdextran on silica was shown to be effective at up to 1 M NaCl concentrations, which indicates that non-ionic interactions between the polyelectrolytes and silica contribute significantly. CS adsorption was higher on pre-adsorbed CPAM than on Cdextran. The conformation of the adsorbed layer after CS addition was seen to expand significantly in CPAM-based layers, while the Cdextran layer appeared to restore its conformation after a temporary expansion at low salt concentrations. In the second part of the thesis, the sequential adsorption of PAE and CMC on pulp fibres was determined using the polyelectrolyte titration technique. Layer-by-layer adsorption isotherms derived on fractionated pulp showed that PAE adsorbed in higher amounts than CMC did, both in terms of adsorbed mass and adsorbed charge. The adsorption of PAE was significantly slower compared to CMC, and the adsorption times required to reach 90% of the saturation adsorption were 3 and 1 min, respectively. The zeta potential of pulp fines was determined for the adsorption of the two first polyelectrolyte layers, and data indicated that the fines recharge within one minute after the polyelectrolyte additions. Reflectometry experiments regarding the sequential adsorption of PAE and CMC on silica indicated that the low-molecular-weight fraction of PAE disturbed the formation of polyelectrolyte multilayers. / QC 20101112

Neste trabalho, aplicamos técnicas de óptica não-linear de segunda ordem, em particular a Espectroscopia por Geração de Soma de Freqüências (SFG) e a Geração de Segundo Harmônico (SHG), para investigar a adsorção e o ordenamento molecular de filmes automontados de polieletrólitos variando-se os parâmetros que influenciam a adsorção (pH, força iônica, densidade de carga), além de investigar a estabilidade térmica e possíveis transições de fases nesses filmes multicamadas como função do pH das soluções e o do número de camadas. A espectroscopia SFG de filmes dos polieletrólitos poly(allylamine hydrochloride) (PAH) e poly(styrene sulfonate) (PSS) permitiu acompanhar a adsorção das camadas pelo monitoramento do campo elétrico na interface (substrato+filme adsorvido)/solução. As medidas de fase do sinal SFG confirmaram a supercompensação de cargas em todos os valores de pH estudados, exceto a pH 12, onde a adsorção foi possível devido a interações eletrostáticas locais. Os filmes secos mostraram grande ordenamento e homogeneidade quando a secagem era espontânea (sem a ação de jato de N2), independentemente do pH. Nestes filmes, observou-se pela primeira vez que as camadas adsorvidas podem alterar o ordenamento e a conformação das cadeias previamente adsorvidas. A espectroscopia SFG também apontou para a redução da densidade de carga e do ordenamento molecular quando a força iônica era aumentada, devido ao efeito de blindagem eletrostática. Os filmes tornaram-se mais inomogêneos e desordenados em virtude da compensação extrínseca. O efeito da força iônica foi o de reduzir a densidade de carga e assim o ordenamento molecular, mesmo se a secagem fosse espontânea. A técnica de SHG foi aplicada a filmes automontados de PAH e PS-119 para confirmar sua estabilidade térmica em função do pH e do número de camadas, além de verificar o ordenamento molecular antes e após o tratamento térmico. Os resultados mostraram que os filmes não são termicamente estáveis: o sinal é completamente destruído à temperatura de 150oC aproximadamente, ao contrário do que relata a literatura. As medidas SHG também confirmaram a isotropia dos filmes no plano das amostras, independente do pH ou do número de camadas. Comparando-se o sinal SHG antes e depois do aquecimento, comprovou-se que após o resfriamento lento, o sinal era restituído a quase o mesmo valor que antes, mostrando que o processo de desordem térmica é reversível. Entretanto, nenhuma transição de fase foi observada, pois a redução do sinal SHG foi lenta e gradual, sem nenhuma variação brusca que caracterizasse uma transição vítrea. Enfim, nossas medidas do sinal SHG em função do número de camadas também discordaram dos resultados da literatura, pois a susceptibilidade de segunda ordem não cresceu linearmente com o número de bicamadas. Isso indica que as moléculas não adsorvem com mesmo ordenamento em cada bicamada. Desse modo, podemos concluir que as técnicas SFG e SHG fornecem informação a nível microscópico que podem levar ao aprimoramento das aplicações dos filmes automontados, e que seriam difíceis de obter com técnicas tradicionais. / In this study, we have applied second-order nonlinear optical techniques, in particular Sum-Frequency Generation (SFG) and Second-Harmonic Generation (SHG), to investigate the adsorption and the molecular ordering of self-assembled polyelectrolyte films varying the parameters which are relevant to polyelectrolyte adsorption (pH, ionic strength, charge density), besides investigating the thermal stability and possible phase transistions in these multilayer films as function of pH of the solutions and of the number of layers. SFG spectroscopy of films fabricated with the polyelectrolytes PAH and PSS allowed us to monitor the adsorption of each layer by the electric field at the (substrate+adsorbed film)/solution interface. Phase-measurements of the SFG signal confirmed that charge supercompensation occurred at all pH values investigated, except at pH 12, where the adsorption was possible by local electrostatic interactions. Dry films have shown great order and homogeneity if the drying was spontaneous (without blow-drying with N2), independently of pH. In these films, it was observed for the first time that layer adsorption can modify the order and the conformation of previously adsorbed chains. SFG spectroscopy also pointed to the reduction of the charge density and of the molecular order if the ionic strength was increased, due to the electrostatic screening effect. The films were more inhomogeneous and disordered due to extrinsic charge compensation. The effect of the ionic strength was to reduce the charge density and the molecular order, even if the drying were slow (spontaneous). The SHG technique was applied to LBL films of PAH and PS-119 to confirm their thermal stability as a function of pH of the solutions and the number of layers, besides comparing the molecular order before and after the thermal treatment. The results have shown that the films are not thermally stable, with the SHG signal nearly vanished if the temperature of 150 oC is reached, in contrast of what is reported in the literature. SHG measurements have also confirmed that the films are isotropic in the plan of the samples, independent of pH or the number of layers. Comparing the SHG signal before and after heating, it was found that the SHG signal was considerably reduced at high temperatures, but after slow cooling it was recovered to almost the same value as before heating, showing that the thermal disorder is reversible. However, no phase transistion was observed, since the SHG signal reduction was slow and gradual, without any sudden change that would characterize a glass transition. At last, our SHG measurements as a function of the number of layers also disagreed with results in the literature, therefore the second order susceptibility did not grow linearly with the number of bilayers. This indicates that the molecules do not adsorb with same order in each bilayer. Therefore, we can conclude that SFG and SHG techniques provide information on the film arrangement at the microscopic level which could be difficult to get with traditional techniques and could also lead to the improvement of applications of LBL films.

Die markierungsfreie Detektion von molekularen Wechselwirkungen mittels Feldeffekt-basierter Sensoren ist eine vielversprechende Strategie zur Entwicklung einer neuen Generati-on von Biochips mit direkter elektrischer Auslesung und somit geeignet für schnelle, einfache und kostengünstige Analysen. In dieser Arbeit wurde als Transducer eine kapazitive Elektrolyt-Isolator-Silizium- (EIS) Struktur zur markierungsfreien elektrischen Detektion geladener Makromoleküle anhand ihrer intrinsischen Ladung verwendet. Als Modellsystem für die Untersuchung der im EIS-Sensor durch die Ausbildung „planarer“ bzw. „brush“-ähnlicher Molekülschichten induzierten Effekte wurden Polyelektrolyt-Multischichten (PEM) bzw. DNA-Moleküle verwendet. Die Adsorption der positiv und negativ geladenen Polyelektrolyt-Schichten an die Sensor-Oberfläche, sowie der Einfluss der Polyelektrolyt-Konzentration, der Ionenstärke und der Art des Elektrolyten auf das EIS-Signal wurden elektrochemisch untersucht. Zusätzlich wurde die Ausbildung der PEM physikalisch unter Verwendung eines Rasterkraftmikroskopes und der Ellipsometrie charakterisiert. Basierend auf Silizium-Isolator-Silizium-Strukturen wurde zum ersten Mal ein Mikroarray mit „Nanoplate“ EIS-Sensoren entwickelt, die alle auf einem einzigen Chip integriert waren. Dies ermöglicht mittels differenzieller Messanordnungen eine verlässliche Detektion der DNA-Hybridisierung bzw. -Denaturierung. Die Eigenschaften des Biosensors wurden durch Verwendung von Gold-Nanopartikeln für die Immobilisierung der DNA auf der Sensorober-fläche sowie durch eine niedrige Salzkonzentration im Messpuffer entscheidend verbessert. Die Ergebnisse dieser neuen Vorgehensweise wurden mittels Fluoreszenz-Mikroskopie vali-diert. Darüber hinaus wurde ein elektrostatisches Modell für einen EIS-Sensor mit einer „planaren“ und einen weiteren, mit „brush“-ähnlicher Molekularschicht entwickelt. Das Modell prognos-tiziert eine starke Abhängigkeit der Sensorsignalstärke von der Elektrolytkonzentration, der Ladungsdichte auf der Oberfläche und dem Abstand zwischen geladener Schicht und Sensor-oberfläche. Die Prognosen stimmten durchweg gut mit den experimentellen Ergebnissen überein. / Label-free detection of molecular interactions utilizing field-effect devices is one of the most attractive approaches for a new generation of biochips with direct electrical readout for a fast, simple and cost-effective analysis. In this study, a capacitive electrolyte-insulator-semiconductor (EIS) structure was used as transducer for the label-free electrical detection of charged macromolecules via their intrinsic charge. Polyelectrolyte multilayers (PEM) and DNA molecules were utilized as model systems to study the charge effects induced in EIS sensors by the formation of “planar”- and “brush”-like molecular layers, respectively. The layer-by-layer adsorption of positively and negatively charged polyelectrolyte (PE) layers onto the sensor surface as well as the influence of PE concentration, ionic strength and type of the applied electrolyte on the EIS sensor signal was electrochemically studied. In addition, the PEMs build-up was physically characterized using atomic force microscopy, scanning electron microscopy and ellipsometry. An array of on-chip integrated nanoplate EIS sensors based on a silicon-on-insulator structure was developed for the first time, enabling the reliable detection of DNA hybridiza-tion/denaturation in a differential measurement setup. Enhanced DNA biosensor characteris-tics were achieved by the immobilization of DNA molecules on the sensor surface via Au-nanoparticles and used low-concentrated buffer solution for the measurements. The results of this novel approach were validated by means of the fluorescence microscopy method. Furthermore, an electrostatic model for an EIS sensor modified with “planar”- and “brush”-like molecular layers was developed. The model predicts a strong dependence of the sensor signal on the electrolyte concentration, surface charge density and the distance between the charged layer and the sensor surface. This is consistently agreeing with the experimental re-sults.

Neste trabalho, aplicamos técnicas de óptica não-linear de segunda ordem, em particular a Espectroscopia por Geração de Soma de Freqüências (SFG) e a Geração de Segundo Harmônico (SHG), para investigar a adsorção e o ordenamento molecular de filmes automontados de polieletrólitos variando-se os parâmetros que influenciam a adsorção (pH, força iônica, densidade de carga), além de investigar a estabilidade térmica e possíveis transições de fases nesses filmes multicamadas como função do pH das soluções e o do número de camadas. A espectroscopia SFG de filmes dos polieletrólitos poly(allylamine hydrochloride) (PAH) e poly(styrene sulfonate) (PSS) permitiu acompanhar a adsorção das camadas pelo monitoramento do campo elétrico na interface (substrato+filme adsorvido)/solução. As medidas de fase do sinal SFG confirmaram a supercompensação de cargas em todos os valores de pH estudados, exceto a pH 12, onde a adsorção foi possível devido a interações eletrostáticas locais. Os filmes secos mostraram grande ordenamento e homogeneidade quando a secagem era espontânea (sem a ação de jato de N2), independentemente do pH. Nestes filmes, observou-se pela primeira vez que as camadas adsorvidas podem alterar o ordenamento e a conformação das cadeias previamente adsorvidas. A espectroscopia SFG também apontou para a redução da densidade de carga e do ordenamento molecular quando a força iônica era aumentada, devido ao efeito de blindagem eletrostática. Os filmes tornaram-se mais inomogêneos e desordenados em virtude da compensação extrínseca. O efeito da força iônica foi o de reduzir a densidade de carga e assim o ordenamento molecular, mesmo se a secagem fosse espontânea. A técnica de SHG foi aplicada a filmes automontados de PAH e PS-119 para confirmar sua estabilidade térmica em função do pH e do número de camadas, além de verificar o ordenamento molecular antes e após o tratamento térmico. Os resultados mostraram que os filmes não são termicamente estáveis: o sinal é completamente destruído à temperatura de 150oC aproximadamente, ao contrário do que relata a literatura. As medidas SHG também confirmaram a isotropia dos filmes no plano das amostras, independente do pH ou do número de camadas. Comparando-se o sinal SHG antes e depois do aquecimento, comprovou-se que após o resfriamento lento, o sinal era restituído a quase o mesmo valor que antes, mostrando que o processo de desordem térmica é reversível. Entretanto, nenhuma transição de fase foi observada, pois a redução do sinal SHG foi lenta e gradual, sem nenhuma variação brusca que caracterizasse uma transição vítrea. Enfim, nossas medidas do sinal SHG em função do número de camadas também discordaram dos resultados da literatura, pois a susceptibilidade de segunda ordem não cresceu linearmente com o número de bicamadas. Isso indica que as moléculas não adsorvem com mesmo ordenamento em cada bicamada. Desse modo, podemos concluir que as técnicas SFG e SHG fornecem informação a nível microscópico que podem levar ao aprimoramento das aplicações dos filmes automontados, e que seriam difíceis de obter com técnicas tradicionais. / In this study, we have applied second-order nonlinear optical techniques, in particular Sum-Frequency Generation (SFG) and Second-Harmonic Generation (SHG), to investigate the adsorption and the molecular ordering of self-assembled polyelectrolyte films varying the parameters which are relevant to polyelectrolyte adsorption (pH, ionic strength, charge density), besides investigating the thermal stability and possible phase transistions in these multilayer films as function of pH of the solutions and of the number of layers. SFG spectroscopy of films fabricated with the polyelectrolytes PAH and PSS allowed us to monitor the adsorption of each layer by the electric field at the (substrate+adsorbed film)/solution interface. Phase-measurements of the SFG signal confirmed that charge supercompensation occurred at all pH values investigated, except at pH 12, where the adsorption was possible by local electrostatic interactions. Dry films have shown great order and homogeneity if the drying was spontaneous (without blow-drying with N2), independently of pH. In these films, it was observed for the first time that layer adsorption can modify the order and the conformation of previously adsorbed chains. SFG spectroscopy also pointed to the reduction of the charge density and of the molecular order if the ionic strength was increased, due to the electrostatic screening effect. The films were more inhomogeneous and disordered due to extrinsic charge compensation. The effect of the ionic strength was to reduce the charge density and the molecular order, even if the drying were slow (spontaneous). The SHG technique was applied to LBL films of PAH and PS-119 to confirm their thermal stability as a function of pH of the solutions and the number of layers, besides comparing the molecular order before and after the thermal treatment. The results have shown that the films are not thermally stable, with the SHG signal nearly vanished if the temperature of 150 oC is reached, in contrast of what is reported in the literature. SHG measurements have also confirmed that the films are isotropic in the plan of the samples, independent of pH or the number of layers. Comparing the SHG signal before and after heating, it was found that the SHG signal was considerably reduced at high temperatures, but after slow cooling it was recovered to almost the same value as before heating, showing that the thermal disorder is reversible. However, no phase transistion was observed, since the SHG signal reduction was slow and gradual, without any sudden change that would characterize a glass transition. At last, our SHG measurements as a function of the number of layers also disagreed with results in the literature, therefore the second order susceptibility did not grow linearly with the number of bilayers. This indicates that the molecules do not adsorb with same order in each bilayer. Therefore, we can conclude that SFG and SHG techniques provide information on the film arrangement at the microscopic level which could be difficult to get with traditional techniques and could also lead to the improvement of applications of LBL films.

L'oxydation spontanée de la dopamine en solution légèrement basique a été étudiée sur la base de la publication de Lee [Science, 318:426-430, 2007], et le produit de la réaction a été identifié comme de la mélanine. La capacité de la mélanine de lier des groupements amines de façon covalente a été confirmée par la quantification des sites de liaison correspondants. En outre il est possible de rédisperser des agrégats de mélanine dans des solutions fortement basiques. Les grains de mélanine ainsi obtenus ont été utilisés pour construire des films multicouches avec le poly(diallyldimethyl ammonium) (PDADMA). Différentes méthodes d'oxydation de la dopamine pour former des films de mélanine à l'interface solide-liquide ont été développées. Toutes les méthodes mènent à la formation de films continus de mélanine ayant des morphologies de surface similaires. Elles deviennent imperméables à des sondes électrochimiques à partir d'une épaisseur de l'ordre de 10 nm. Une plus grande perméabilité à des sondes chargées positivement ou neutres qu'à des sondes négatives a été confirmée. L'adsorption de protéines à des revêtements de mélanine a été expliquée par une combinaison d'interactions électrostatiques et covalentes. Pour arriver à cette explication le potentiel zêta de dépôts de mélanine a été mesuré en fonction du pH. La formation de la mélanine dans des films multicouches de poly(L-lysine) (PLL) et de hyaluronate (HA) a été étudiée: la mélanine est capable de remplir des films (PLL-HA)n de manière homogène, et les composés ainsi obtenus peuvent être détachés de leurs substrats comme membranes autosupportées préparées par une méthode biomimétique sous conditions douces.

<p>The aims of this study are to first immobilize positively charged gold nanorods to negatively charged cell culture surfaces. Second, to use polyelectrolytes for controlling the distance between gold nanorods and fluorophores. This is used to optimally determine the distance, of which maximum fluorescence enhancement is achieved, between gold nanorods and fluorophores. In order to approach these aims, we use UV/VIS absorption spectroscopy, fluorescence spectroscopy, atomic force microscopy, and ellipsometry. The results show that we could control the immobilization of gold nanorods on plastic microwell plates and create reproducible polyelectrolyte layers, in order to control the distance between the gold nanorods and fluorophores. In addition, the localized surface plasmon resonance wavelength red shifted as the PELs increased. In conclusion, we found that the maximum fluorescence enhancement of the fluorophores (Cy7) is about 2.3 times at a fluorophores-nanoparticles separation of approximately 9-12 nm. This work contributes some research information towards the design of optical biochip platforms based on plasmon-enhanced fluorescence.</p>