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The focus of this thesis has been on the effects of segregation in mixtures of amphiphilic molecules. Two different systems were investigated: fluorocarbon-hydrocarbon surfactant mixtures and lipid-surfactant mixtures. In fluorocarbon-hydrocarbon surfactant mixtures the repulsive interactions between the chains can lead to a demixing into different types of coexisting micelles, fluorocarbon rich and hydrocarbon rich. From NMR self-diffusion measurements such a demixing was found to occur in the mixture of the partially fluorinated surfactant HFDePC and C16TAC. We furthermore suggested a demixing also within the micelles to explain 19F-NMR line width data and results from neutron scattering. In lipid-surfactant mixtures, a segregation of the molecules may instead be caused by a difference in the preferred curvature of the lipid and the surfactant residing within the same aggregate. Using a surfactant selective electrode, binding isoterms of four different cationic surfactants (C12TAC, C14TAC, C16TAC and HFDePC) to preformed lipid (GMO) vesicles were determined. Perforated vesicles were observed by cryo-TEM in the mixture with C16TAC. To explain the results from the binding isoterms, the formation of pores in the bilayer was regarded as a cooperative process, similar to micelle formation. The surfactant accumulates at the edges of the pores, and increasing the surfactant concentration results in an increased number of pores with a constant surfactant/lipid ratio at the edges. The lipid-surfactant mixtures were also studied at the solid/solution interface using AFM. An adsorbed mesh structure, a counterpart to the bulk perforated lamellar phase, was observed for the first time.

Bilayers formed by molecules that possess long alkyl hydrophobic tails are ubiquitous in the natural world manifesting both in biological systems as well as in chemistry. The lipid bilayer is an integral feature of cell membranes of living systems with functions that are of critical importance to the life of the cell. Long chain amphiphilic surfactant molecules can be introduced within the interlamellar region of layered inorganic host lattices to form anchored alkyl chainbilayerswithinthegalleries.Theintercalatedbilayerbearsastriking resemblance to lipid bilayers. However, unlike lipid bilayers where individual molecules can undergo lateral diffusion and also flip-flop between layers the anchored bilayer is characterized by the total absence of translational mobility. The degrees of freedom of the alkyl chains of the anchored bilayer are restricted to changes in conformation. This thesis describes a detailed investigation of the anchored bilayer formed by the intercalation of the anionic surfactant dodecyl sulphate (DDS) in a layered solid, Mg-Al Layered Double Hydroxide(Mg-AlLDH) using both experimental measurements and Molecular Dynamics (MD) simulations (Chapter 2). The thesis is organized as two parts. The first (Chapters 2 -4) deals with the anchored bilayer in the condensed phase -the conditions for the formation of the bilayer arrangement of the intercalated surfactant chains and the conformation and dynamics of the alkyl chains of the surfactant in the galleries of the layered solid. The surfactant intercalated Mg-AlLDH-DDS may be delaminated in nonpolar solvents to give colloidal dispersions of individual Mg-Al LDH sheets with the DDS surfactant chains remaining tethered to the inorganic sheets(Scheme 1).The second part of thesis(Chapters 5 -9)describe studies on the dispersions of the Mg-AlLDH-DDS in toluene. A summary of the results of the of the investigations of the anchored bilayer, formed by the intercalation of DDS ions in Mg-Al LDH, in the condensed and the dispersed phases is presented in the concluding chapter(Chapter10). Layered Double Hydroxides(LDH) are insulating lamellar solids of the general chemical formula[M’(1-x) Mx(OH)2], where M’ is a divalent metal ion and M a trivalent ion. Their structure may be derived from that of Brucite, Mg(OH)2, by isomorphous substitution of apart of the Mg2+ by trivalent ions like Al3+ with electrical neutrality maintained by interlamellar exchangeable ions. The studies reported in this thesis are on an Mg-Al LDH,Mg(1−x)Alx(OH)2, x ranging from 0.17 to 0.37. Dodecyl sulphate surfactant ions have been ion-exchange intercalated in Mg-AlLDH (Chapter 3). By varying the Mg-Al ratio, differing packing densities of the surfactant chains in the interlamellar space of the Mg-Al LDH-DDS are realized. At high packing densities the alkyl chains of the intercalated dodecyl sulphate ions anchored on opposing Mg-Al LDH sheets are arranged as bilayers while at lower packing densities the surfactant chains form a monolayer with the chains oriented flat in the galleries. This composition driven monolayer to bilayer transformation in the surfactant intercalated Mg-AlLDH-DDS is also reproduced by MD simulations. The simulations also indicate that there are profound differences in the factors that decide the arrangement of the surfactant chains. In the bilayer arrangement it is dispersive van der Waals interactions between the chains in opposing layers that is responsible for the cohesive energy of the solid whereas at lower packing densities, where a monolayer arrangement is favored, Coulomb interactions between the positively charged Mg-Al LDH sheets and the negatively charged head-group of the DDS anion dominate. The conformation and dynamics of the alkyl chains of the intercalated surfactant chains in both the monolayer and bilayer arrangements as well as the effect of packing density on these parameters is reported in Chapter 4. The conformation was studied using spectroscopic techniques, infra-red, Raman and 13C Nuclear Magnetic Resonance (NMR) while the dynamics by Variable Contact Time Cross Polarization Magic Angle Spinning(VCT -CPMAS) and2DWidelineSeparation(2DWiSe)NMR techniques. The results showed the expected trends; the concentration of gauche defects and the dynamics of the chains increase with decreasing packing density. There is, however a sharp increase in the gauche concentration and conformational mobilities of the intercalated surfactant chains associated with the bilayer to monolayer transformation. The results of the MD simulations, too, reflect these trends. The second part of thesis describes the delamination of the intercalated anchored bilayer (Mg-AlLDH-DDS) in non-polar solvents. Delamination results in a colloidal dispersion Of the anchored bilayer, isolated Mg-AlLDH sheets with the DDS chains tethered to them, as neutral nanosheets of nanometer thickness and micron size. With increasing concentration of the anchored bilayers in the solvent a gel state is realized. The sol to gel transformation of the dispersions of the anchored bilayer in toluene has been investi-gated. Frequency dependent rheology measurements (Chapter6) were used to investigate the visco-elastic properties of the dispersions and Small Angle X-ray Scattering (SAXS) measurements(Chapter 7) to understand the structure and shape of the nanosheets. The rheology experiments showed that the dispersions irrespective of their concentrations showed shear thinning. The SAXS results indicate a tactoid structure of the dispersions as well as in the gel phase. At higher concentrations, the X-ray scattering curves indicated that the layers stack loosely with an interlamellar space of ~ 39 Å , a value much larger than the interlayer lattice spacing of solid Mg-AlLDH-DDS( ~ 27 Å). The nature of interactions between solvent molecules and the anchored DDS chains were probed by 1Hand 2H NMR measurements(Chapter8). A clear association between the toluene molecules and the alkyl chains of the anchored surfactant was observed. 2D NOESY experiments established that there are toluene molecules in close proximity that interact with the methyl tail of the anchored surfactant. NMR measurements were also able to distinguish two types of solvent molecules based on their widely differing mobilities. MD simulations(Chapter9)of the dispersed anchored bilayer are able to reproduce the essential features of the experimental observations including the formation of a loosely bound lamellar structure. It also provides an explanation on how the spectroscopic observation of motional heterogeneity gives rise to the viscoelastic properties of the dispersed anchored bilayer.

Das stäbchenförmige Bakterium Escherichia coli teilt sich in zwei gleich große Tochterzellen. Dies ist nur möglich, wenn sich die Zelle in der Mitte teilt. Bei E. coli wird die Zellteilung durch den Zusammenschluss der FtsZ-Proteine an der Membran zum Z-Ring eingeleitet. Topologische Regulierung des Z-Ringes erfolgt durch räumlich-zeitliche Oszillationen von Min-Proteinen zwischen den beiden Zellpolen. MinC, MinD und MinE binden an und lösen sich von der Membran unter Hydrolyse von ATP und in antagonistischer Art und Weise, was zu einer alternierenden Ansammlung von MinC und MinD an den Zellpolen führt. Gemittelt über die Zeit ergibt sich somit ein MinD-Verteilungsprofil, das maximale Konzentration an den Zellpolen und ein Minimum in der Zellmitte aufweist. MinC bindet an MinD und folgt somit seiner Verteilung. Der Zusammenschluss von FtsZ-Proteinen wird durch MinC unterbunden, und somit kann sich der Z-ring nur an einer Position herausbilden, die ein Minimum an MinC aufweist - der Zellmitte. Das Min-system wurde in der Vergangenheit auch mit einem in-vitro-Ansatz untersucht, indem Min-Proteine in künstliche, aufliegende Lipiddoppelschichten (supported lipid bilayers, SLB) rekonstitutiert wurden. Dabei bildeten die Min-Proteine kein oszillierendes Muster aus, sondern organisierten sich vielmehr in parallelen und propagierenden Wellen (Loose, 2008, Science, 320). In diesen in-vitro-Experimenten war das Membransubstrat wesentlich größer als die Wellenlänge der Min-Proteinwellen. In vivo hingegen ist die Länge der Zelle in der gleichen Größenordnung wie die charakteristische Länge des Oszillationsmusters der Min-Proteine. Daher war es das Ziel dieser Arbeit, den Einfluß einer beschränkten Fläche und geometrischer Formgebung der künstlichen Lipiddoppelschichten auf die Wellenpropagation der Min-Protein zu untersuchen. Flächige Beschränkung künstlicher Membranen erfolgte durch Mikrostrukturtechnologie. Deckglässchen wurden mit einer Goldschicht und mikroskopischen Aussparungen unterschiedlicher geometrischer Formen strukturiert. Funktionale SLBs bildeten sich nur auf Glasflächen ohne Goldbeschichtung aus. Nach der Rekonstitution der Min-Proteine, organisierten sich diese auf den Membranstücken in parallele Wellen. Dabei bestimmte die flächige Beschränkung der künstlichen Membranen die Ausbreitungsrichtung der Min-Proteinwellen. Min-Proteinwellen konnten entlang gekrümmter Membranstreifen, in Ring- und sogar in Slalomstrukturen geleitet werden. In geraden, länglichen Strukturen richteten sich die Wellen entlang der längsten Achse aus. Kopplung von Proteinwellen auf räumlich getrennten Membranstücken in Abhängigkeit des Abstandes und des sogenannten Molecular Crowdings in der wässrigen Lösung konnte ebenfalls beobachtet werden. Diese Kopplung ist ein Indiz für inhomogene Proteinverteilungen in der Lösung oberhalb der Membran. Desweiteren konnten Min-Proteinwellen auch in diversen dreidimensionalen künstlichen Membranen rekonstitituiert werden. Im Wildtyp von E. coli ähneln die Min-Proteindynamiken der einer Oszillation mit einer charakteristischen Länge von 5 µm. Auf SLBs, bilden Min-Proteine Wellen mit einer Wellenlänge aus, die ca. zehnmal größer ist als in vivo. Dieser Unterschied zwischen der in-vivo- und der in-vitro-Welt wurde untersucht und diskutiert. In vitro konnte die Wellenlänge um 50 % durch Erhöhung des Molecular Crowding in der Lösung sowie um 33 % durch Temperaturerhöhung verkleinert werden. Das oszillierende Muster könnte dahingegen eine Folge der Kompartimentierung sein. Erste Versuche, das Min-System in geschlossene Membrankompartimente zu rekonstitutieren, wurden getestet. / Escherichia coli, a rod-like bacterium, divides by binary fission. Cell division into two daughter cells of equal size requires that fission takes place at a midcell position. In E. coli, cell division is initiated by assembly of the FtsZ-proteins at the inner membrane to the Z-ring. Topological regulation of the Z-ring is achieved by spatiotemporal pole-to-pole oscillations of Min-proteins. MinC, MinD and MinE bind to and detach from - under hydrolysis of ATP - the membrane in an antagonistic manner leading to an alternating accumulation of MinC and MinD at the cell poles. Averaged over time, the distribution profile of MinD exhibits maximal concentration at the cell poles and a minimum at the cell center. MinC binds to MinD and thus follows its distribution. FtsZ assembly is inhibited by MinC and therefore the Z-ring can only form at a cell position low in MinC - at the cell center. In the past, the Min-system was also investigated in an in vitro approach by reconstitution of Min-proteins into a supported lipid bilayer (SLB). Here, Min-proteins did not self-organize into an oscillatory pattern but into parallel and propagating waves (Loose, 2008, Science, 320). In this in vitro assay, the membrane substrate was infinitely large compared to the wavelength. However, in vivo, the cell length is on the same order of magnitude as the respective length scale of the oscillatory pattern of Min-proteins. Therefore, we wished to investigate the effect of lateral confinement and geometric structuring of artificial lipid bilayers on the Min-protein wave propagation. Lateral confinement of artificial membranes was achieved by microfabrication technology. Glass slides were patterned by a gold coating with microscopic windows of different geometries, and functional SLBs were only formed on uncoated areas. Upon reconstitution, Min-proteins organized into parallel waves on the geometric membrane patches. Confinement of the artificial membranes determined the direction of propagation of Min-protein waves. Min-protein waves could be guided along curved membrane stripes, in rings and even along slalom-geometries. In elongated membrane structures, the protein waves always propagate along the longest axis. Coupling of protein waves across spatially separated membrane patches was observed, dependent on gap size and level of molecular crowding of the aqueous media above the bilayer. This indicates the existence of an inhomogeneous and dynamic protein gradient in the solution above the membrane. Furthermore, reconstitution of Min-protein waves in various three-dimensional artificial membranes was achieved. In wild-type E. coli, Min-protein dynamics resemble that of an oscillation with a characteristic length scale of 5 µm. On supported lipid bilayers, Min-proteins self-organize into waves with a wavelength approximately 10-fold larger than in vivo. These discrepancies between the in vivo and in vitro world were investigated and discussed. In vitro, the wavelength could be decreased by a factor of 50 % by increase of the molecular crowding in solution and by 33 % through temperature increase. The oscillatory pattern is thought to be a consequence of compartmentalization and first attempts to encapsulate the Min-system in closed bilayer compartments are presented.

Imaging mass spectrometry (MS) analysis allows scientists the ability to obtain spatial and chemical information of analytes on a wide variety of surfaces. The ability to image biological analytes is an important tool in many areas of life science research, including: the ability to map pharmaceutical drugs in targeted tissue, to spatially determine the expression profile of specific proteins in healthy vs. diseased tissue states, and to rapidly interrogate biomolecular microarrays. However, there are several avenues for improving the imaging MS experiment for biological samples. Three significant directions this work addresses include: (1) reducing chemical noise and increasing analyte identification by developing sample preparation methodologies, (2) improving the analytical figures of merit (i.e., spatial resolution, analysis time) by implementing a spatially dynamic optical system, and (3) increasing both mass spectral resolution and ion detection sensitivity by modifying a commercial time-of-flight (TOF) MS. Firstly, sample methodology schemes presented in these studies consist of obtaining both ?top-down? and ?bottom-up? information. In that, both intact mass and peptide mass fingerprinting data can be obtained to increase protein identification. This sample methodology was optimized on protein microarrays in preparation for bio tissue analysis. Other work consists of optimizing novel sample preparation strategies for hydrated solid-supported lipid bilayer studies. Sample methods incorporating nanomaterials for laser desorption/ionization illustrate the ability to perform selective ionization of specific analytes. Specifically, our results suggest that silver nanoparticles facilitate the selective ionization of olefin containing species (e.g., steroids, vitamins). Secondly, an advanced optical design incorporating a spatially dynamic optical scheme allows for laser beam expansion, homogenization, collimation, shaping, and imaging. This spatially dynamic optical system allows user defined beam shapes, decreases analysis times associated with mechanical movement of the sample stage, and is capable of increasing the MS limits of detection by simultaneously irradiating multiple spots. Lastly, new data acquisition strategies (multiple anode detection schemes) were incorporated into a commercial time-of-flight mass spectrometer to increase both sensitivity and resolution in a matrix assisted laser desorption/ionization mass spectrometer. The utility of this technique can be applied to many different samples, where high mass spectral resolution allows for increased mass measurement accuracy.

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Made available in DSpace on 2014-12-17T15:14:47Z (GMT). No. of bitstreams: 1 MarioAAM.pdf: 4836888 bytes, checksum: 433efaa92f7e444a6422049d8ebf9384 (MD5) Previous issue date: 2008-12-05 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / We have developed a theoretical study of magnetic bilayers composed by a ferromagnetic film grown in direct contact on an antiferromagnetic one. We have investigated the interface effects in this systems due to the interfilms coupling. We describe the interface effects by a Heisenberg like coupling with an additional unidirectional anisotropy. In the first approach we assume that the magnetic layers are thick enough to be described by the bulk parameters and they are coupled through the interaction between the magnetic moments located at the interface. We use this approach to calculate the modified dynamical response of each material. We use the magnetic permeability of the layers (with corrections introduced by interface interactions) to obtain a correlation between the interface characteristics and the physical behavior of the magnetic excitations propagating in the system. In the second model, we calculated an effective susceptibility of the system considering a nearly microscopical approach. The dynamic response obtained by this approach was used to study the modifications in the spectrum of the polaritons and its consequences on the attenuated total reflection (ATR). In addition, we have calculated the oblique reflectivity. We compare our result with those obtained for the dispersion relation of the magnetostatic modes in these systems / Realizamos um estudo te?rico de bicamadas acopladas, constitu?das de um filme ferromagn?tico (F) crescido em contato direto sobre um antiferromagn?tico (AF). Investigamos os efeitos de interface nestes sistemas decorrentes do acoplamento inter-filmes. Modelamos o acoplamento atrav?s de uma intera??o tipo Heisenberg acrescida de uma anisotropia unidirecional. Em nosso estudo consideramos duas abordagens distintas: Na primeira, desenvolvemos um modelo fenomenol?gico, o qual considera que os filmes s?o espessos o bastante para serem descritos pelos seus par?metros de volume, e que est?o acoplados atrav?s da intera??o entre os momentos magn?ticos vizinhos da interface. Este modelo permitiu o c?lculo das permeabilidades magn?ticas dos filmes, modificadas pela intera??o entre estes objetos. Usamos estes resultados para estudar os modos magnetost?ticos que se propagam no sistema. O comportamento da frequ?ncia destes modos com a dire??o de propaga??o da componente do vetor de onda paralela na superf?cie, foi utilizado para investigar as modifica??es geradas pelos efeitos de interface; Na segunda abordagem, desenvolvemos o c?lculo anal?tico da suscetibilidade m?dia do sistema, utilizando um modelo quase microsc?pico. Analisamos o resultado da resposta din?mica atrav?s do c?lculo dos pol?ritons e da reflex?o total atenuada (ATR). Adicionalmente, calculamos a refletividade direta do sistema para o caso de uma radia??o incidente com dire??o arbitr?ria, a qual est? relacionada com os modos magnetost?ticos do sistema

Made available in DSpace on 2014-12-17T15:15:04Z (GMT). No. of bitstreams: 1 MarioAM.pdf: 1150858 bytes, checksum: 72544ace95c9eefd2f93d90922e6d6ec (MD5) Previous issue date: 2007-03-21 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The magnetic order of bylayers composed by a ferromagnetic film (F) coupled with an antiferromagnetic film (AF) is studied. Piles of coupled monolayers describe the films and the interfilm coupling is described by an exchange interaction between the magnetic moments at the interface. The F has a cubic anisotropy while the AF has a uniaxial anisotropy. We analyze the effects of an external do magnetic field applied parallel to the interface. We consider the intralayer coupling is strong enough to keep parallel all moments of the monolayer an then they are described by one vector proportional to the magnetization of the layer. The interlayer coupling is represented by an exchange interaction between these vectors. The magnetic energy of the system is the sum of the exchange. Anisotropy and Zeeman energies and the equilibrium configuration is one that gives the absolute minimum of the total energy. The magnetization of the system is calculated and the influence of the external do field combined with the interfilm coupling and the unidirectional anisotropy is studied. Special attention is given to the region near of the transition fields. The torque equation is used to study dynamical behavior of these systems. We consider small oscillations around the equilibrium position and we negleet nonlinear terms to obtain the natural frequencies of the system. The dependence of the frequencies with the external do field and their behavior in the phase transition region is analized / O ordenamento de bicamadas magn?ticas compostas por um filme ferromagn?tico (F) fino acoplado com um filme antiferromagn?tico (AF) ? estudado, considerando que os filmes podem ser descritos por uma pilha de camadas monoat?micas (infinitas) acopladas. O acoplamento entre os filmes ? representado pela intera??o entre os spins das monocamadas da interface. Consideramos que o filme F tem uma anitropia c?bica e os momentos do filme AF sentem a presen?a de uma anisotropia uniaxial. Adicionalmente, os spins que comp?em as camadas da interface sentem a presen?a de uma anisotropia unidirecional. Os efeitos de um capo magn?tico est?tico aplicado paralelo ao plano da interface s?o analisados. Consideramos que a intera??o de troca entre os momentos magn?ticos de uma mesma monocamada ? suficientemente forte para mant?-los alinhados em uma mesma dire??o. Cada camada monoat?mica ? representada por um ?nico vetor (vetor magnetiza??o) proporcional ao momento magn?tico total da monocamada, e a intera??o entre estes vetores ? proporcional a intera??o efetiva entre as monocamadas. A energia magn?tica do sistema ? escrita como a soma das energias de troca, anisotropia e Zeeman, e as configura??es de equil?brio s?o obtidas atrav?s da minimiza??o da energia. A magnetiza??o do sistema ? calculada em fun??o do campo magn?tico dc aplicado e o efeito do meesmo, combinando com a intera??o interfilmes e anisotropia unidirecional, ? investrigado. A aten??o especial ? dada ?s regi?es de campo pr?ximas as transi??es de fase. Usamos a equa??o do torque para estudae o comportamento din?mico desses sistemas. Considerando que os vetores magnetiza??o oscilam em torno da posi??o de equil?brio, com amplitudes suficientemente pequenas para que os efeitos n?o lineares possam ser desprezados, calculamos as "freq??ncias naturais" do sistema dando especial aten??o as duas de menos valor. A depend?ncia das freq??ncias com a intensidade do campo ? determinada e o comportamento nas regi?es de transi??o de fase ? analisado

Biossensores que empregam dispositivos de efeitos de campo podem ser obtidos em diversas arquiteturas, incluindo dispositivos Eletrólito-Isolante-Semicondutor (EIS), que são capacitores em que o eletrodo metálico é substituído por um filme e uma solução. Medindo-se a capacitância em função do potencial aplicado, é possível detectar variações de pH oriundas de reações ou interações entre o filme e o analito. Nesta dissertação, sensores foram produzidos com a adsorção de filmes automontados de dendrímero (PAMAM) e nanotubos de carbono (SWNT) num chip. Medidas de espectroscopia de impedância foram realizadas para investigar o crescimento de cada bicamada do filme automontado, e os dados foram analisados com circuitos equivalentes que continham uma capacitância de dupla camada, um elemento de fase constante e uma capacitância para a região de depleção. Para o chip, os melhores ajustes foram obtidos na frequência de 2 kHz, em que a concentração de dopantes foi 6,6x1020 m-3 para o chip com isolante de SiO2 e de 1,1x1021 m-3 para o chip com isolante de SiO2/Ta2O5. O potencial de banda plana foi -0,2 V e -0,06V, respectivamente. Para os chips recobertos com os filmes de PAMAM/SWNT, observamos que a região de depleção é causada pelas cargas positivas do PAMAM. Com relação às implicações para biossensores, verificamos que o desempenho ótimo deve ser obtido com 3 bicamadas de PAMAM/SWNT. Isso pode explicar a observação empírica na literatura de que existe uma espessura ideal dos filmes para um desempenho otimizado. / Biosensors based on field effect devices can be produced with several architectures, including Electrolyte-Insulator-Semiconductor (EIS) devices, which are capacitors where conventional metal electrodes are replaced by a sensing layer and an electrolyte solution. By measuring the capacitance as a function of the bias voltage, it is possible to detect pH changes that may originate from reactions or interactions between the film in the sensing unit and the analyte. In this study sensors were obtained by adsorbing layer-by-layer (LbL) films made with dendrimers (PAMAM) and carbon nanotubes (SWNT) on a semiconductor chip. Impedance spectroscopy measurements were performed to monitor the growth of each bilayer in the LbL film, whose data were analyzed with equivalent circuits containing a double-layer capacitance, a constant phase element and a capacitance for the depletion region. The results for the semiconductor chip could be best fitted for a frequency of 2 kHz, where the doping concentration was 6.6 x1020 m-3 for the insulating SiO2 layer and 1.1 x1021 m-3 for the SiO2/Ta2O5 layer. The flat band voltage was -0.2 V and -0.06 V, respectively. In the analysis of the chip coated with different numbers of PAMAM/SWNT bilayers, we found that the depletion region appears as a contribution from the positive charges in the PAMAM layer. With regard to implications for biosensors, we found that optimized performance should be reached with three PAMAM/SWNT bilayers, which may explain the empirical finding in the literature that an ideal thickness exists for enhanced performance.