Implantable Nanofluidic Membrane and Smart Electronic System for Drug Release Control

L'abstract è presente nell'allegato / the abstract is in the attachmen

Surface Channeling of Charged and Neutral Beams in Capillary Guides

In this review work, the passage of charged and neutral beams through dielectric capillary guides is described from a uniform point of view of beams channeling in capillaries. The motion of beams into the hollow channels formed by the inner walls of capillaries is mainly determined by multiple small-angle scattering (reflection) and can be described in the approximation of surface channeling. It is shown that the surface interaction potential in the case of micro- and nano-capillaries is actually conditioned by the curvature of the reflecting surface. After presenting the analysis of previously performed studies on X-rays propagation into capillaries, which is valid for thermal neutrons, too, the surface channeling formalism is also developed for charged particle beams, in particular, moving in curved cylindrical capillaries. Alternative theories explaining experimental results on the beams passage through capillaries are based on simple thermodynamic estimates, on various diffusion models, and on the results of direct numerical simulations as well. Our work is the first attempt to explain the effective guiding of a charged beam by a capillary from the general standpoint of quantum mechanics, which made it possible to analytically explore the interaction potential for surface channeling. It is established that, depending on the characteristics of a projectile and a dielectric forming the channel, the interaction potential can be either repulsive or attractive; the limiting values of the potential function for the corresponding cases are determined. It has been demonstrated that the surface channeling behaviour can help in explaining the efficient capillary guiding for radiations and beams

Rapid Prototyping of Microfluidic Devices:Realization of Magnetic Micropumps, Fuel Cells and Protein Preconcentrators

With the growing importance of miniaturized energy applications and the development of micro Total Analysis Systems (μTAS), we have realized microfluidic devices, namely, magnetic micropumps, microfluidic fuel cells and membrane-based protein preconcentrators, all having high application potential in future. The choice of rapid prototyping microfabrication technologies and the selection of affordable materials are important aspects, when thinking of commercialization. Thus, we have employed powder blasting, polymer molding and assembly technologies during devices fabrication throughout the thesis. The first type of microfluidic device that we present is a poly(methyl methacrylate) (PMMA) ball-valve micropump with two different designs of the electromagnetic actuator, as optimized by the finite element method. The integration of a permanent magnet in a flexible polydimethylsiloxane (PDMS) membrane, which is clamped into PMMA structure, is proposed for providing a large stroke of the pumping membrane, making the micropump bubble-tolerant and self-priming Focusing on low power consumption for μTAS integration, another type of magnetic micropump with active valves is realized. It consists of a microfluidic chamber structure in glass that is assembled with a PDMS sheet, which comprises two valving membranes and a central actuation membrane, having each an integrated permanent magnet that is peristaltically actuated by a rotating arc-shaped permanent magnets assembly. A lumped circuit model is developed to predict and describe the frequency-dependent flow rate behavior for this type of pump. Powder blasting and PDMS molding rapid prototyping technologies are employed for realization of these two types of micropumps. Fuel cells with fluid delivery and removal options, having chemical reaction sites and electrode structures that can be realized in a microfluidic format, have high potential for applications. Therefore, microfluidic direct methanol fuel cells with embedded ion- permselective medium are studied and such type of fuel cell is realized by integrating a narrow Nafion strip in a molded elastomeric structure. A mechanical clamping assembly technology enables leakage-free operation and stable performance. The characterization reveals its output power density, using H2O2-based oxidant, is among the high-performance direct methanol fuel cells in microscale. Re-using the technology of the fuel cell chip, with its particular ion-permselective Nafion membrane and assembly method, we also have developed a protein preconcentrator with high purification performance. Our device can preconcentrate negatively charged biomolecules located at the anodic compartment side of the Nafion strip within only a few minutes with a high preconcentration factor. Moreover, a complex microfluidic finite element model is proposed to study and understand the physics of the preconcentration effect. Finally, we conclude the thesis with an outlook on future developments based on our work of the project and on the assembly technologies for microfluidic device integration

Theory of Ion Transport and Ion Current Rectification in Nanofluidic Diodes

In this thesis, nanofluidic diodes were studied theoretically using fundamental physics as a basis. A comprehensive theory was constructed for ion current rectification (ICR) in nanofluidic systems, written from an engineering and physics perspective. The primary goal of this work was to clarify the fundamental theory of ICR through the interpretation and consideration of various literature sources on the topic, which often use contradictory definitions and simplifications. New figures were created for this research to more effectively convey and clarify vital concepts such as electric double layers (EDL), and included multiple definitions to compare different theoretical approaches. Lastly, a simulation was written to apply our developed theory by numerically modeling the electric potential profile in a nanofluidic diode of asymmetric ion concentration. The simulation results were interpreted to help visualize the formation of EDL in the system, and to conceptualize the mechanisms producing ICR. Three main types of nanofluidic diodes were identified by their characteristic asymmetries and studied in-depth: asymmetry in fixed wall charge, asymmetry in ion concentration, and asymmetry in channel diameter. Foundational electrostatic physics equations, such as the Poisson-Boltzmann equation and Ohm’s law, were derived and manipulated to produce important equations describing electric potential and ion current conductivity in nanofluidic systems. Several of these – the Debye-Hückel approximation of the Poisson-Boltzmann equation, the Debye screening length equation, and the Grahame equation – were later used in the simulation of electric potential profiles. Building on fundamental concepts, the Poisson-Nernst-Planck (PNP) equations were shown to describe the sources of ion movement in nanofluidics in the form of a self-consistent set of coupled mean-field equations. Utilizing these equations and employing electric potential and ion current conductivity relationships, the three main types of nanofluidic diodes were analyzed to examine their sources of ICR, and each was explained through molecular-level behavioral considerations at different applied voltages. Based on the theory developed to explain ICR, a theoretical causal chain for ICR was identified. To visualize asymmetrical electrostatic impact, which is the foundational requirement for ICR to be present in a nanofluidic system, electric potential profiles were simulated for a nanofluidic diode of asymmetric ion concentration. Using the Grahame equation and the Debye length equation to substitute values into the Debye-Hückel approximation, the electric potential was numerically calculated for the example system in equilibrium, forward bias and reverse bias. The simulation results qualitatively agreed with similar models from the literature which were obtained through PNP and analytical methods. Analysis of our simulation results using the theory we developed revealed the importance of an electric potential well which forms near one opening of the nanochannel. This “trench” causes ion accumulation, which increases that ion’s conductivity. Applying forward voltage bias results in this high conductivity at the ions’ entrance, while reverse bias results in the high conductivity at the ions’ exit. Thus, forward bias is characterized by greater ion flux into the channel than out of it, increasing overall ion concentration in the channel and promoting higher ion current through the system. Reverse bias is characterized by greater ion flux out of the channel than into it, decreasing overall ion concentration in the channel and suppressing ion current through the system. Asymmetry in electrostatic impact is therefore sufficient to explain ion current rectification in nanofluidic diodes, and the simulation results were used to illustrate this theoretical discussion

Ion-track technology based synthesis and characterization of gold and gold alloys nanowires and nanocones

Metallic nanostructures are attracting growing interest because of their potential application in various devices such as batteries, solar cells, or drug delivery systems. This thesis focuses on the synthesis and characterization of three different nanostructures: (1) solid cylindrical AuAg nanowires with controlled composition and size, fabricated by electrodeposition in etched ion-track membranes with cylindrical channels, (2) porous cylindrical Au nanowires attained by selective dealloy-ing of AuAg nanowires, and (3) Au nanocones synthesized by electrodeposition in conical channels. AuAg nanowires with controlled diameter and composition, namely Au, Au40Ag60, Au60Ag40, and Ag were synthesized and characterized. By dealloying these nanowires were converted into porous Au-based nanowires with diameters above and below 100 nm possessing an enhanced surface area. Surface morphology and com-position of the nanostructures before and after dealloying were studied by means of high spatial resolution energy-dispersive X-ray spectroscopy (EDX) in a high-resolution transmission electron microscope (TEM). The results demonstrate surface segregation effects in solid AuAg nanowires that strongly vary with the initial composition. Surface segregation occurs on a time scale of days (< 3 days) inde-pendently of the wire dimensions. After dealloying of Au40Ag60 nanowires, the porous nanowires have a silver content below 10% and ligament size from 5 to 30 nm. Solid and porous wires are particularly attractive for future applications, e.g., in sensorics. The characterization of such small nanostructures regarding, e.g. electrical transport properties, requires suitable contacts. Special designs to contact nanowires by laser lithography as well as by using pre-patterned templates were developed. Gold nanocones with sharp tips down to 50 nm diameter and several microns large bases were fabricated. Given by this special geometry, the nanostructures exhibit a high mechanical stability and are freestanding with an aspect ratio of 500 and above. Stable gold nanocone arrays are attractive for a large range of applications including field emission and as coating for hydrophobic surfaces. In this work, the standard wire deposition process from base to tip was inverted in order to improve the electrical and thermal contact of the nanocones to the substrate. After selective removal of the template, 30 µm long gold nanocones with ~ 50 nm sharp tips were freestanding and vertically aligned. Such structures are highly tunable in terms of cone dimensions and number density. The field emission properties of patterned nanocone arrays, investigated in collaboration with the Bergische Universität Wuppertal, exhibit field enhancement factors between 200 and 1000 as well as a maximum emission current ranging from ~ 1 to 100 μA. The results presented in this thesis emphasize the variety of possibilities that ion-track technology offers in order to tailor dimensions and characteristics of nanostructures

Operando Single Particle Catalysis - Combining a Nanoreactor and Plasmonic Nanospectroscopy

Heterogeneous catalysis is an important cornerstone of modern society with strong ties to the development of sustainable sources of energy and products. Catalysts are typically realized as supported metal nanoparticles that offer active sites that can accelerate chemical reactions by providing energetically more favorable reaction paths. Despite their broad use, the scrutiny of catalysts under realistic application conditions, such as high pressure and temperature, is a major experimental challenge. This difficulty is further amplified by the complexity present in real catalysts, often consisting of large ensembles of nanoparticles that all are unique. Furthermore, reactors used in catalysis studies often give rise to ill-defined reaction conditions in terms of catalyst distribution, reactant concentration and temperature. To mitigate these challenges, techniques are being developed to enable studies of catalytic nanoparticles under relevant operation conditions, so-called operando techniques. In this context, down-sized chemical reactors can be utilized to achieve precise control of both the catalyst, and the operating conditions. In this thesis, I have performed in situ studies of chemical reactions in/on nanoparticles by utilizing plasmonic nanospectroscopy based on the localized surface plasmon resonance (LSPR) phenomenon. The resonance condition for LSPR depends on both nanoparticle properties (size, shape, material) and the surrounding medium, which makes it possible to determine the physical and chemical state of individual nanoparticles optically. The LSPR response was used to study the oxidation of Cu nanoparticles, revealing the complex nature of nanoparticle oxidation kinetics, as well as particle specific oxidation mechanisms. Furthermore, a nanoreactor platform was developed and used in combination with plasmonic nanospectroscopy to perform operando characterization of individual Cu and Pt catalyst nanoparticles during CO oxidation. The obtained results illustrate how the oxidation of Cu results in catalyst deactivation and how reactant gradients formed inside the catalyst bed strongly affects the state of the catalyst, and thus its activity. Moreover, the nanoreactor enabled operando characterization of catalyst beds comprising 1000 well defined nanoparticles that could be individually addressed

Mesoporous film architectures and step gradient formation

The development of novel devices along with technological progress of our time requires miniaturization and compartmentalization, and with this nanotechnology. For example, future innovative solutions are urgently pursued for life-inspired sustainable water and energy management. Nanoscale pores and channels, as one field within nanotechnology, hold great potential in mimicking the outstanding transport phenomena of their biological paragons. Such transport properties, as observed in biological pores and channels, originate from complex architectures and are influenced by pore geometry, surface charge distribution, chemical composition, and wettability. However, desired transport properties in advanced applications require enhanced control of surface functionalization in nanoscale pores and channels along with nanoporous material architecture design. In this regard, mesoporous silica thin films represent suitable model materials for nanoporous material architecture design providing ordered nanoscale pores and nanoscale film thicknesses. In this work, mesoporous silica thin films were investigated to create mesoporous step gradient architectures with respect to pore size, surface wettability, and surface charge. This work was divided into three main sections: i) generating a material library allowing step gradient design, ii) the fabrication of mesoporous architectures, and iii) (nano)local polymer placement into such multilayer architectures. To create mesoporous architectures, a material library was built in the first place. Thereby, the ionic pore accessibility of hydrophilic mesoporous silica thin films was investigated in dependence of preparation parameters, i.e. the template removal. Hydrophobic mesoporous silica thin films with tunable surface wettability were developed using co-condensation of tetraethylorthosilicate and methylated silica precursors resulting in mesoporous (organo)silica thin films. As a side note, an enhanced chemical stability in basic environment was observed for hydrophobic thin films. To replace petro-based templating macromolecules, hydroxypropyl cellulose was successfully applied as bio-based structure directing template for the generation of mesoporous silica thin films with permselective ionic pore accessibility. Mesoporous step gradient architectures were prepared by applying the developed material library combining mesoporous layers with orthogonal properties. Examples are: the fabrication of hydrophilic pore size step gradients, and the combination of layers with different wettability. Interestingly, investigation of mesoporous wettability step gradient films with respect to the ionic pore accessibility in dependence of the hydrophobic top layer’s thickness showed an overcoming of the hydrophobic layer through electrostatic attraction of the hydrophilic bottom layer in case of the thinnest hydrophobic top layer. Regarding local polymer placement, multilayer step gradient mesoporous film formation turned out to be advantageous, too. For example, the layer-selective polymer functionalization of hydrophilic double layered mesoporous silica thin films was achieved by predisposition of a single layer, followed by selective iniferter binding. Layer-selective polymer grafting was achieved resulting in step gradients with charge density control. To further investigate the limits of polymer placement in mesoporous film architectures, plasmonic metal nanoparticles were incorporated into mesoporous silica thin films. These particles served as nanoscopic plasmonic light source and were combined with photopolymerizations. Investigation of the prepared mesoporous composite materials allowed precise placement of the nanoparticles in mesoporous silica thin films with tunable density. The concept of nanolocal polymer placement using plasmonic metal nanoparticles in combination with photopolymerization was demonstrated for two distinct polymerization approaches. Due to the sensitivity of the nanoparticle’s surface plasmons on the surrounding refractive index, such mesoporous composite materials further demonstrated application as sensing unit allowing to detect local refractive index changes, e.g. in consequence of nanolocal polymer placement

Enhanced mass transport in graphene nanofluidic channels

Enhanced mass transport in carbon-based nanoscale conduits (e.g. carbon nanotubes, graphene nanochannels/capillaries, graphene/graphene oxide membranes) has attracted tremendous interest over the last decade due to its significant implications for water desalination/purification, nanofiltration, electronic cooling, battery/fuel cells, and lab-on-a-chip. Further development of carbon-based nanoscale conduits for practical applications relies on understanding fundamental mechanisms of transport through individual conduits, which have not been well studied due to challenges in fabrication and measurement. In this thesis, the construction of two-dimensional planar graphene nanochannel devices and the studies of enhanced water and ion transport inside the graphene nanochannels are reported for the first time. The graphene nanochannels are fabricated by conformally covering high-quality graphene on the surfaces of silica nanochannels. A new fabrication scheme consisting of graphene wet transfer, graphene patterning and vacuum anodic bonding is developed to create such graphene nanochannels with heights ranging from 24 to 124 nm. Using these nanochannels and a new hybrid nanochannel based capillary flow measurement technique, we successfully measured the hydraulic resistance (water permeability) of single graphene nanochannels. Our results demonstrate that the frictionless surface of graphene induces a boundary slip and enhances water flow inside the graphene nanochannel. The measured slip length of graphene in the graphene nanochannels poses a median value around 16 nm, albeit with a large variation from 0 to 200 nm regardless of the channel height. The small-yet-widely-varying values of the graphene slip length are attributed to the surface charge of graphene and the interaction between graphene and underneath silica substrate, which are in good agreement with the prediction of our molecular dynamics (MD) simulation. In addition, we also investigated enhanced ion transport inside the graphene nanochannels. Higher electroosmotic conductance at low electrolyte concentrations (10-6 M~10-2 M) is observed in graphene nanochannels when compared with silica nanochannels with the same geometry. Our results suggest that the enhanced electroosmotic flow is also due to the boundary slip at the graphene/electrolyte interface. Besides, our analysis shows that the surface charge on the graphene, originating from the dissociation of oxygen-containing functional groups, is crucial to the enhanced electroosmotic flow inside nanochannels

Combining Nanoplasmonics and Nanofluidics for Single Particle Catalysis

Nanoparticles are, due to their large exposed surface area, widely used in the field of heterogeneous catalysis where they accelerate and steer chemical reactions. Although catalysis has been known about for centuries, the scrutiny of catalysts under realistic application conditions is still a major challenge. This difficulty originates from the fact that real catalyst materials are very complex, often consisting of large ensembles of nanoparticles that all are unique. Furthermore, the typically used macroscopic reactors in catalysis studies gives rise to locally, at the level of the active site, ill-defined reactant concentrations and diffusion limitations.To overcome these limitations, on one hand, techniques are being developed that are sensitive enough to probe individual catalytic particles and that at the same time can operate under realistic reaction conditions. On the other hand, strategies to more carefully control the amount and structure of catalyst material, as well as to precisely control mass transport to and from the active catalyst, are being investigated by scaling down the size of the used chemical reactor. To further push the limit of downsizing, in this thesis, I present a miniaturized reactor platform based on nanofluidic channels that have been carefully decorated with catalytic nanoparticles, and that is integrated with plasmonic nanospectroscopy readout. This optical technique relies on the nanoscale phenomenon known as the Localized Surface Plasmon Resonance (LSPR) and enables the study of individual metal nanoparticles in operando by means of dark-field scattering spectroscopy.As the first step in this development, we constructed a nanofluidic device with integrated plasmonic nanoparticles to detect minute changes in the liquid flowing through the channels, as well as molecules binding to the nanoparticles. As the second step, we developed the nanofluidic system with an integrated heater and to facilitate gas flow through the nanochannels with the possibility to connect to a mass spectrometer for on-line product analysis. This system was then successfully used to correlate activity with surface and bulk oxidation state changes taking place on individual catalytic Cu and Pt nanoparticles during CO oxidation, measured by means of plasmonic nanospectroscopy. To this end, in a separate study, I also employed the plasmonic approach to study the oxidation process of Cu nanoparticles both experimentally and by electrodynamics simulations