abstract: The properties of materials depend heavily on the spatial distribution and connectivity of their constituent parts. This applies equally to materials such as diamond and glasses as it does to biomolecules that are the product of billions of years of evolution. In science, insight is often gained through simple models with characteristics that are the result of the few features that have purposely been retained. Common to all research within in this thesis is the use of network-based models to describe the properties of materials. This work begins with the description of a technique for decoupling boundary effects from intrinsic properties of nanomaterials that maps the atomic distribution of nanomaterials of diverse shape and size but common atomic geometry onto a universal curve. This is followed by an investigation of correlated density fluctuations in the large length scale limit in amorphous materials through the analysis of large continuous random network models. The difficulty of estimating this limit from finite models is overcome by the development of a technique that uses the variance in the number of atoms in finite subregions to perform the extrapolation to large length scales. The technique is applied to models of amorphous silicon and vitreous silica and compared with results from recent experiments. The latter part this work applies network-based models to biological systems. The first application models force-induced protein unfolding as crack propagation on a constraint network consisting of interactions such as hydrogen bonds that cross-link and stabilize a folded polypeptide chain. Unfolding pathways generated by the model are compared with molecular dynamics simulation and experiment for a diverse set of proteins, demonstrating that the model is able to capture not only native state behavior but also partially unfolded intermediates far from the native state. This study concludes with the extension of the latter model in the development of an efficient algorithm for predicting protein structure through the flexible fitting of atomic models to low-resolution cryo-electron microscopy data. By optimizing the fit to synthetic data through directed sampling and context-dependent constraint removal, predictions are made with accuracies within the expected variability of the native state. / Dissertation/Thesis / Ph.D. Physics 2011

There are 12 species of bats that occur in Illinois; 5 of these species can be found hibernating in abandoned mines and caves in southern Illinois. Due to the destruction of their natural hibernacula, caves, many species of bats have found abandoned mines to be suitable replacement habitat. A complex of abandoned underground microcrystalline silica mines in southern Illinois owned by Unimin Specialty Minerals Corporation now provides hibernacula for 5 species of cavernicolous bats: the federally endangered Indiana bat (Myotis sodalis), little brown bat (M. lucifugus), eastern pipistrelle (Perimyotis subflavus), big brown bat (Eptesicus fuscus), and the northern long-eared bat (M. septentrionalis). Within the last 10 years the number of bats using these mines has increased dramatically, especially the Indiana bat which has increased from just over 9,000 to 43,000 hibernating in Magazine Mine. One concern of having so many endangered bats hibernating in one mine is stability. Mines were created relatively recently and are still in the process of settling. Thus, these mines might act as a potential sink, drawing in hibernating bats but potentially collapsing and killing them. Thirteen mines were surveyed for bats and for the amount of spalling that occurred over the 16-month study period from September 2006 to December 2007. Factors that could increase the amount of spalling were quantified, including temperature, moisture, and moisture variability in the material of the walls in the mines, and temperature variability. Number of hibernating bats in the mines was also documented. Data were analyzed with logistic regression. Temperature was a significant predictor of spalling (W2 = 12.76, p = 0.0004) when considered as a univariate variable, as was temperature variation (W2 = 21.89, p = <0.0001). Considering multiple logistic regression analyses, moisture was the best predictor. For the 13 mines surveyed, number of hibernating bats ranged from 0 to 3,755. Whereas all three variables were important at predicting the presence of bats, temperature variation (W2 = 35.98, p =<0.0001) was a better predictor than temperature or moisture. In a multiple logistic regression, temperature (W2 = 46.75, p = < 0.0001) and temperature variation (W2 = 20.56, p = < 0.0001) were better at predicting presence of bats then was moisture. The less variation in temperature the more likely that bats will be present. Because bats prefer stable temperatures and spalling occurs more often at high variability of temperatures and very low temperatures, bats were usually in areas that exhibited little or no spalling.

Esta dissertação objetiva investigar a evolução geomorfológica dos ambientes proglaciais (geleiras Ecology, Sphinx, Baranowski, Tower e Windy) da margem leste do campo de gelo Warszawa (62°12’0” S - 58°30’0” W) na Ilha Rei George, Antártica, entre 1956 - 2017. Dados obtidos por sensores remotos, como Aster GDEM2, imagens Sentinel-2 (2017) e WorldView-2 (2014), possibilitaram o mapeamento geomorfológico das formas de relevo de mesoescala proglaciais. A variação frontal e de área total das geleiras foi estimada e mapeada com dados de imagens de satélite (Sentinel-2 de 2017 e WorldView-2 de 2014) e vetores de variação frontal de 1956, 1979, 1988 e 2000. Evidenciou-se um contínuo processo de retração nas geleiras Ecology, Sphinx, Baranowski, Tower e Windy, com perda total de área de 33%, 25%, 37%, 71% e 30%, respectivamente, no período. A geleira Windy apresentou mudanças recentes em seu término de maré para terrestre, formando uma área proglacial com a exposição de formas de relevo deposicionais glaciais. Estimou-se um aumento das áreas livres de gelo de 6,3km² no período entre 1956-2017. Em resposta ao processo de retração glacial, há exposição de formas de relevo nos ambientes livres de gelo na área de estudo com a formação da rede de drenagem fluvioglacial e glaciolacustre e a formação de ambientes proglaciais com exposição de morainas laterais, frontais, latero-frontais e eskers. As áreas livres de gelo recentes são suscetíveis a processos de retrabalhamento por ação gravitacional, eólica e pluvial. Foram determinados três estágios de evolução do ambiente proglacial das geleiras associadas às feições geomorfológicas geradas em cada fase. Como mudança ambiental detectada está a sucessão de ambientes proglaciais, paraglaciais e periglaciais. / This dissertation aims to investigate the proglacial geomorphological evolution (Ecology, Sphinx, Baranowski, Tower and Windy glaciers) in the western sector of Warszawa Ice Cap (62°12’0” S 58° 30’ 0” W), Admiralty Bay coast, King George Island, Antarctica between 1956 and 2017. Data obtained by remote sensors, such as Aster GDEM2, Sentinel-2 (2017) and WorldView-2 (2014) images, were applied in geomorphological mapping of the proglacial mesoscale landforms. Glacial retreat and fluctuations of glaciers areas were estimated (using Sentinel-2 images, acquired in 2017, WorldView-2 data, acquired in 2014, and outline data of the 1956, 1979, 1988 and 2000). The results evidenced the continuous retreat processes in period for Ecology, Sphinx, Baranowski, Tower and Windy glaciers, with of 33%, 25%, 37%, 71% and 30% of the area lost, respectively. The Windy glacier changed of outlet glacier for land terminus conditions in latest decades, with a recent proglacial environment development. In response to glacial retreat processes there is a glacial landforms exposition in new ice-free environments in the study area. Was recorded an increase of the 6.3km² in ice-free land areas between 1956 and 2017 in in the western sector of Warszawa Ice Cap. As response the retreat processes there are the development of the glacio-fluvial drainage network, glacio-lacustrine landforms and the formation of proglacial ice marginal environments with lateral moraines, lateral-frontal moraines, frontal moraines, eskers. Recent ice-free areas are susceptible to gravitational, wind and pluvial reworking process. Three stages of evolution of the proglacial environments of the glaciers associated with the geomorphological features formed in each phase and environmental changes in response the glacier retreat was determined. Was detected the succession of proglacial for paraglacial and periglacial environments as evidence of the environmental change.

High-Speed Rail (HSR) as a fast, reliable and environmentally friendly mode of transportation has received a lot of attention in recent decades. The International Union of Railways reported that there are more than 18600 miles of HSR in operation and about 1.6 billion passengers per year are carried by them. Although there are plans for HSR in many states including Florida, the United States, however, is still hesitant to develop its own HSR network. One of the main barriers to developing high-speed rail is excessive vibration propagation to the media which may cause annoyance to people who live in the track neighborhood. Train induced vibration also contributes to track settlement, developing track flaws, and increasing life cycle cost of track and supporting structures. The aim of this research is to address this problem by conducting a comprehensive investigation into track dynamics. For this purpose, three-dimensional mass-spring-damper models of vehicle, track and supporting structures were developed and matrices of mass, stiffness, and damping of each subsystem were formed. The response of the whole system was, then, determined by coupling the subsystems using Hertz contact theory. The differential equations of the coupled system were solved by the Newmark integration method and the results including vertical and lateral displacements and forces were presented in the time domain. Since the purpose of this dissertation is to quantify the effect of track and vehicle condition on vibration level, rail defects were also taken into account and rail random irregularities for the vertical profile, Gauge, alignment and cross level (super elevation) were incorporated into a numerical solution. The results of the study show the effect of track and vehicle parameters on the response of the vehicle, track, and substructures. Since Florida and some other states in the United States are very prone to hurricanes, an investigation was conducted into the effect of wind speed on vehicle stability. For this purpose, a curved beam was modeled to consider the influence of track curvature, cant deficiency, wind speed and train speed simultaneously. The results from the study show the maximum allowable values of train speed and axle load for different wind speeds. The findings can be used to decide under what circumstances there is a risk of vehicle overturning and how to avoid it.

abstract: In mesoscopic physics, conductance fluctuations are a quantum interference phenomenon that comes from the phase interference of electron wave functions scattered by the impurity disorder. During the past few decades, conductance fluctuations have been studied in various materials including metals, semiconductors and graphene. Since the patterns of conductance fluctuations is related to the distributions and configurations of the impurity scatterers, each sample has its unique pattern of fluctuations, which is considered as a sample fingerprint. Thus, research on conductance fluctuations attracts attention worldwide for its importance in both fundamental physics and potential technical applications. Since early experimental measurements of conductance fluctuations showed that the amplitudes of the fluctuations are on order of a universal value (e2/h), theorists proposed the hypothesis of ergodicity, e.g. the amplitudes of the conductance fluctuations by varying impurity configurations is the same as that from varying the Fermi energy or varying the magnetic field. They also proposed the principle of universality; e.g., that the observed fluctuations would appear the same in all materials. Recently, transport experiments in graphene reveal a deviation of fluctuation amplitudes from those expected from ergodicity. Thus, in my thesis work, I have carried out numerical research on the conductance fluctuations in GaAs nanowires and graphene nanoribbons in order to examine whether or not the theoretical principles of universality and ergodicity hold. Finite difference methods are employed to study the conductance fluctuations in GaAs nanowires, but an atomic basis tight-binding model is used in calculations of graphene nanoribbons. Both short-range disorder and long-range disorder are considered in the simulations of graphene. A stabilized recursive scattering matrix technique is used to calculate the conductance. In particular, the dependence of the observed fluctuations on the amplitude of the disorder has been investigated. Finally, the root-mean-square values of the amplitude of conductance fluctuations are calculated as a basis with which to draw the appropriate conclusions. The results for Fermi energy sweeps and magnetic field sweeps are compared and effects of magnetic fields on the conductance fluctuations of Fermi energy sweeps are discussed for both GaAs nanowires and graphene nanoribbons. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2015

Esta dissertação objetiva investigar a evolução geomorfológica dos ambientes proglaciais (geleiras Ecology, Sphinx, Baranowski, Tower e Windy) da margem leste do campo de gelo Warszawa (62°12’0” S - 58°30’0” W) na Ilha Rei George, Antártica, entre 1956 - 2017. Dados obtidos por sensores remotos, como Aster GDEM2, imagens Sentinel-2 (2017) e WorldView-2 (2014), possibilitaram o mapeamento geomorfológico das formas de relevo de mesoescala proglaciais. A variação frontal e de área total das geleiras foi estimada e mapeada com dados de imagens de satélite (Sentinel-2 de 2017 e WorldView-2 de 2014) e vetores de variação frontal de 1956, 1979, 1988 e 2000. Evidenciou-se um contínuo processo de retração nas geleiras Ecology, Sphinx, Baranowski, Tower e Windy, com perda total de área de 33%, 25%, 37%, 71% e 30%, respectivamente, no período. A geleira Windy apresentou mudanças recentes em seu término de maré para terrestre, formando uma área proglacial com a exposição de formas de relevo deposicionais glaciais. Estimou-se um aumento das áreas livres de gelo de 6,3km² no período entre 1956-2017. Em resposta ao processo de retração glacial, há exposição de formas de relevo nos ambientes livres de gelo na área de estudo com a formação da rede de drenagem fluvioglacial e glaciolacustre e a formação de ambientes proglaciais com exposição de morainas laterais, frontais, latero-frontais e eskers. As áreas livres de gelo recentes são suscetíveis a processos de retrabalhamento por ação gravitacional, eólica e pluvial. Foram determinados três estágios de evolução do ambiente proglacial das geleiras associadas às feições geomorfológicas geradas em cada fase. Como mudança ambiental detectada está a sucessão de ambientes proglaciais, paraglaciais e periglaciais. / This dissertation aims to investigate the proglacial geomorphological evolution (Ecology, Sphinx, Baranowski, Tower and Windy glaciers) in the western sector of Warszawa Ice Cap (62°12’0” S 58° 30’ 0” W), Admiralty Bay coast, King George Island, Antarctica between 1956 and 2017. Data obtained by remote sensors, such as Aster GDEM2, Sentinel-2 (2017) and WorldView-2 (2014) images, were applied in geomorphological mapping of the proglacial mesoscale landforms. Glacial retreat and fluctuations of glaciers areas were estimated (using Sentinel-2 images, acquired in 2017, WorldView-2 data, acquired in 2014, and outline data of the 1956, 1979, 1988 and 2000). The results evidenced the continuous retreat processes in period for Ecology, Sphinx, Baranowski, Tower and Windy glaciers, with of 33%, 25%, 37%, 71% and 30% of the area lost, respectively. The Windy glacier changed of outlet glacier for land terminus conditions in latest decades, with a recent proglacial environment development. In response to glacial retreat processes there is a glacial landforms exposition in new ice-free environments in the study area. Was recorded an increase of the 6.3km² in ice-free land areas between 1956 and 2017 in in the western sector of Warszawa Ice Cap. As response the retreat processes there are the development of the glacio-fluvial drainage network, glacio-lacustrine landforms and the formation of proglacial ice marginal environments with lateral moraines, lateral-frontal moraines, frontal moraines, eskers. Recent ice-free areas are susceptible to gravitational, wind and pluvial reworking process. Three stages of evolution of the proglacial environments of the glaciers associated with the geomorphological features formed in each phase and environmental changes in response the glacier retreat was determined. Was detected the succession of proglacial for paraglacial and periglacial environments as evidence of the environmental change.

A eletrodinâmica clássica estocástica pode ser entendida como sendo a teoria clássica de Maxwell, onde se inclui um novo elemento da realidade física: As flutuações eletromagnéticas de ponto zero. Sob esse enfoque, estudamos a interação de um \"ensemble\", de osciladores harmônicos carregados com a radiação térmica e de ponto zero (atérmica). Incluímos os efeitos de dissipação através da força de reação da radiação. Além disso estudamos também a excitação do oscilador por uma força determinística com dependência temporal arbitrária. Nossa análise estatística do sistema físico é baseada na solução exata da equação de Fokker-Planck adequada ao problema. Obtém-se a evolução temporal, no espaço de fase, para uma dada distribuição inicial que caracteriza um \"ensemble\" de osciladores forçados que apresentam estados excitados na forma de estados coerentes e estados coerentes comprimidos e pulsantes. A comparação direta com a formulação quântica do mesmo problema nos faz reconhecer que é possível obter da física clássica alguns resultados antes só obtidos pela teoria quântica. Identificamos na radiação de ponto zero o ingrediente que torna possível entender a estabilidade do estado fundamental e o princípio de incerteza. / Classical stochastic electrodynamics may be understood as classical electrodynamics theory, when a new element of physical reality is included: The zero point electrodynamics fluctuations. Under this approach, we study the interaction of a charged harmonic oscillator with the thermal radiation and zero point radiation. We include the effect of dissipation by the radiation reaction force. We also study the excitation of this oscillator by a deterministic force with arbitrary temporal dependence. Our statistical analysis of the physical system is based on the exact solution of the appropriate Fokker-Planck equation. We get the temporal evolution in phase space for a given initial distribution that characterizes one \"ensemble\" of forced oscillators that presents excited states in the coherent and squeezed states form. A direct comparison with the quantum formulation of the same problem make us recognize that it is possible to get some results from classical physics which were accomplished previously only by quantum theory. We identify in the zero point radiation the ingredient that makes it possible to understand the stability of the fundamental state and the uncertainty principle.

FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico / FlutuaÃÃes temporais como as originadas de variaÃÃes sazonais e perturbaÃÃes climÃticas, possuem funÃÃo importante na manutenÃÃo da estrutura e da diversidade nas comunidades biÃticas. A HipÃtese do DistÃrbio IntermediÃrio explica que as caracterÃsticas gerais das comunidades e a dinÃmica das populaÃÃes, especialmente a riqueza de espÃcies, pode sofrer alteraÃÃes diante de distÃrbios nos ecossistemas. Ambientes aquÃticos estÃo sujeitos à variabilidade temporal elevada, como resultado de interaÃÃes entre parÃmetros fÃsicos, quÃmicos e biolÃgicos. Consequentemente, a composiÃÃo de espÃcies e abundÃncia relativa das populaÃÃes e comunidades està sujeita a rearranjos frequentes. Este estudo teve como objetivo avaliar as condiÃÃes trÃficas do reservatÃrio de General Sampaio, que està situado no estado do CearÃ, semiÃrido do Brasil. Utilizou o Ãndice de Estado TrÃfico (TSI) proposto por Toledo et al. (1983), e o relacionou à abundÃncia relativa, à dominÃncia de fitoplÃncton e à variaÃÃo anual da pluviosidade. Foram obtidos dados climÃticos, fÃsicos, para a transparÃncia do disco de Secchi, fÃsforo total, ortofosfato, nitrato, nitrito, amÃnia, nitrogÃnio total e clorofila a, em paralelo com a amostragem de fitoplÃncton para identificaÃÃo e cÃlculo de contagem do nÃmero de indivÃduos componentes das espÃcies. Houve variabilidade estacional das variÃveis que foram analisadas para os perÃodos climÃticos chuvoso e seco. CondiÃÃes trÃficas indicaram a presenÃa de condiÃÃes oligotrÃficas à eutrÃficas durante o perÃodo analisado. Esta variaÃÃo promoveu condiÃÃes favorÃveis para um padrÃo de dominÃncia do grupo Chlorophyceae, durante as estaÃÃes chuvosas, e de cianofÃceas durante as estaÃÃes secas. Assim, pode-se argumentar que estado trÃfico e abundÃncia relativa das espÃcies de fitoplÃncton estÃo relacionados com as flutuaÃÃes sazonais na hidrologia do sistema deste reservatÃrio, que reflete uma caracterÃstica exclusiva de regiÃes semiÃridas. / Temporal fluctuations as those derived from seasonal variations and disturbances, may have an important role in maintaining the structure and diversity of biota. The Intermediate Disturbance Hypothesis states that the general characteristics of communities and population dynamics, especially the species richness may change the face of disturbances in ecosystems. Aquatic environments are subject to high temporal variability as a result of interactions between physical, chemical, and biological parameters. Consequently, species composition and relative abundance of populations and communities are subject to frequent rearrangements. This study aimed to evaluate the trophic conditions of the General Sampaio Reservoir, which is situated in the semiarid Cearà State of Brazil. We used the Trophic State Index (TSI) proposed by Toledo et al. (1983), and related it to species diversity, the relative abundance of phytoplankton and the variation of annual rainfall. We obtained climatic and physics data, Secchi disc transparency, total phosphorus, orthophosphate, nitrate, nitrite, ammonia, total nitrogen and chlorophyll a, in parallel to the sampling of phytoplankton for species identification and the calculation of the number counts. There was seasonal variability in the variables that were analyzed for the rainy versus dry climatic periods. Trophic conditions indicated the presence of oligotrophic to eutrophic conditions during the research in this reservoir. This variation facilitated favorable conditions for Chlorophyceae dominance during the rains periods, and for cyanobacteria dominance during the dry periods. Thus, it may be argued that the trophic state and relative abundance of phytoplankton is related to seasonal fluctuations in the hydrology of the reservoir system, which reflects a unique feature of semiarid regions.

FundaÃÃo de Amparo à Pesquisa do Estado do Cearà / Pacientes com doenÃa de Parkinson (DP) desenvolvem freqÃentemente flutuaÃÃes motoras com perda de beneficio terapÃutico. A levodopa permanece como agente dopaminÃrgico mais efetivo, entretanto, apÃs alguns anos de tratamento observa-se geralmente flutuaÃÃes motoras e discinesias. A ocorrÃncia destas complicaÃÃes à responsÃvel por incapacidade e constituem desafio terapÃutico. PublicaÃÃes recentes tÃm sugerido que a amantadina, inicialmente prescrita como uma substÃncia com atividade antiviral, poderia reduzir as discinesias em pacientes com DP. A disfunÃÃo orofarÃngea tambÃm à freqÃentemente observada em pacientes com DP e à posteriormente uma causa de aspiraÃÃo silenciosa que conduz a pneumonia e eventualmente morte. Neste estudo, avaliamos o efeito da amantadina nas discinesias e flutuaÃÃes motoras induzidas por levodopa em pacientes com DP. Utilizamos desenho duplo-cego, randomizado e controlado por placebo. Durante o estudo foram randomizados 20 pacientes, tendo 16 completado o protocolo conforme o planejado. No grupo tratado com amantadina, foi observada reduÃÃo da duraÃÃo da discinesia (p=0.037). NÃo foram relatadas, nem observadas reaÃÃes adversas apÃs uso da amantadina. Neste estudo observou-se uma reduÃÃo das dicinesias nos pacientes tratados com amantadina. Foram avaliados 15 pacientes com DP e portadores de discinesia induzida por levodopa, 12 pacientes com DP sem discinesias, e 7 controles. Os pacientes com DP e discinesia apresentaram eficiÃncia de deglutiÃÃo semelhante ao grupo controle. A eficiÃncia da deglutiÃÃo foi significativamente menor no grupo de pacientes com DP sem discinesia do que no grupo controle (p=0,02). EpisÃdios de aspiraÃÃo foram observados em apenas um paciente. De acordo com este estudo, a presenÃa de discinesia nÃo foi associada a piora da funÃÃo orofarÃngea dos pacientes com DP, o que reforÃa o conceito que outros sistemas diferentes do dopaminÃrgico possam estar envolvidos na gÃnese da disfunÃÃo orofarÃngea na DP / Patients presenting with ParkinsonÂs disease (PD) frequently develop motor fluctuations and loss of therapeutic benefit. Levodopa is the most effective dopaminergic agent, however, after some years of treatment motor fluctuations and dyskinesias are commonly observed. These complications are a source of disability and a therapeutic challenge. Previous studies have shown that amantadine, initially prescribed as an anti-viral substance, potentially reduce dyskinesia in PD patients. Oropharyngeal dysfunction are also frequently seen in PD patients. The latter is a cause of unsuspected aspiration leading to pneumonia and eventually death. Presently, we evaluated the effects of amantadine on daytime fluctuations and dyskinesia in PD. We performed a double-blind, randomized, placebo-controlled study. Twenty PD cases were randomized and 16 finished the study. In those treated with amantadine, a reduction of the duration of dyskinesia was observed (P=0,037). In this study, adverse drug reactions were not registered. We conclude that a reduction of dyskinesia was associated with the use of amantadine in PD patients. We have also studied the oropharyngeal function through videofluorocopy. We evaluated 15 patients with PD presenting dyskinesia, 12 cases with PD without dyskinesia and 7 age-matched control. Patients with PD and dyskinesias had oropharyngeal swallowing efficiency similar to control. Patients with PD and without dyskinesia had a significant reduction of oropharyngeal swallowing efficiency in relation to control (P=0,02). Silent aspiration was observed in only one case. According to this study, dyskinesia was not associated with worsening of oropharyngeal function in PD which reinforce the concept that other systems different from dopamine may be involved in the genesis of oropharyngeal dysfunction in PD.

Nesta tese foi estudado o espalhamento Raman intensificado pela superfície (SERS) em regime de detecção de uma molécula em eletrodo de prata ativado por ciclos de oxidação e redução. Neste regime, de baixas concentrações, são observadas intensas flutuações de intensidade SERS as quais foram controladas neste substrato pela aplicação de potencial ao eletrodo, o que foi associado a alterações na concentração de moléculas adsorvidas na superfície do eletrodo. Além da dependência com o potencial aplicado, foram estudadas através de simulações Monte Carlo, a contribuição nestas flutuações da constante de adsorção das moléculas, do número de \"hot spots\" (regiões de altas intensificações SERS) e do tipo de \"hot spot\" (em termos de eficiência para detecção de espectros de uma molécula). Através destas simulações foram verificadas flutuações de intensidade muito semelhantes às observadas experimentalmente. Além das flutuações de intensidade foram também observadas flutuações de intensidades relativas, como por exemplo, das relações de intensidades anti-Stokes/Stokes, as quais foram interpretadas segundo um modelo de ressonância, através do qual foi possível estimar as energias de ressonância nos \"hot spots\". Alguns dos resultados indicaram a contribuição de ressonâncias finas, as quais foram interpretadas como resultado de interferências entre ressonâncias de plasmon de superfície. Interferências como estas foram demonstradas através de simulações pelo método DDA (\"Discrete Dipole appoximation\") em modelos simples de \"hot spots\" formados por nanobastões de Au. / In this thesis it was studied surface-enhanced Raman scattering (SERS) at single-molecule detection on Ag electrode activated by oxidation and reduction cycles. At this low concentration limit it was observed strong SERS intensity fluctuations that were controlled by the applied potential to the electrode and this control was associated to changes in surface concentration of adsorbed molecules. Furthermore, it was studied through Monte Carlo simulations the influence of adsorption constant, number of \"hot spots\" (regions of high SERS enhancements) and type of \"hot pot\" (in terms of efficiency for single-molecule detection). With such simulations, it was verified fluctuations of SERS intensities very similar to experimental observations. Besides absolute intensity fluctuations, we also observed fluctuations of relative intensities as, for instance, the. anti-Stokes to Stokes intensity ratios. These fluctuations were interpreted according to a resonance model, which made possible the estimative of resonance energies at the SERS \"hot spots\". Some of these results indicated the existence of sharp resonances that were interpreted as a result of interferences among surface plasmon resonances, which were demonstrated through DDA (Discrete Dipole Approximation) simulations in simple models of \"hot spots\" formed by Au nanorods