Layer-by-layer (LbL) assembly based on ionic interactions has proven to be a versatile route for surface modification and construction of ultrathin nanocomposites. Covalent LbL assembly based on facile ‘click’ covalent bond formation is an effective alternative, especially for the applications where a more robust ultrathin films or nanocomposites is desired. The subject of this dissertation focuses on the design of three different covalent LbL assemblies and their applications on conductive thin films, superhydrophobic surfaces, and solute responsive surfaces, respectively. Surface modification of PE substrates using covalent LbL assembly with PEI and Gantrez is a successful route to prepare a surface graft. The procedure is relative easy, fast and reproducible. Grafting multiple layers of PEI/Gantrez to the PE powder surface provided excellent coverage and promoted stable LbL film growth and excellent adhesion. This carbon black (CB) coated powder was compression molded into films, and their conductivity was measured, which revealed a percolation threshold below 0.01 wt % CB for the PEI-grafted system. Electrical conductivity of 0.2 S/cm was achieved with only 6 wt % CB, which is exceptional for a CB-filled PE film. Direct amination of MWNTs with PEI is a convenient and simple method leading to highly functionalized product that contains 6-8 % by weight PEI. Superhydrophobic PE films can be formed either from ionic LbL self-assembly of MWNT-NH-PEIs and poly(acrylic acid) or from covalent LbL self-assembly of MWNTNH- PEIs and Gantrez when the final graft is acrylated with octadecanoic acid. While the ionically assembled nanocomposite graft is labile under acid, the covalently assembled graft is more chemically robust. Responsive surfaces with significant, reversible, reproducible wettability changes can be prepared by covalent LbL grafting using PNIPAM-c-PNASI and aminated silica nanoparticles. A 65º ΔΘ value was observed with water vs. 1.4 M Na2SO4. The prepared film shows a high surface roughness of ~300 nm, which contributes to the large solute responsive ΔΘ values. The surfaces are reconfigurable in different solute conditions and that the changes in water contact angle are likely due to combination of change in surface roughness along with swell and intercalation of the solute ions into the PNIPAM surface.

The present thesis focuses on the selection of polymers and methods to fabricate stable and stimuli responsive multilayer self-assembly via layer-by-layer (LbL) approach. The polymers utilized in this study are biodegradable and biocompatible such as hyaluronic acid, chitosan and poly(vinyl alcohol) (PVA). The thesis is comprised of six chapters and a brief discussion on the contents of the individual chapters is given below. Chapter I reviews the LbL self-assembly approach in the context of drug delivery. The various interactions such as electrostatic, hydrogen bonding and covalent bonding involved in preparation of stable multilayer assemblies via LbL approach are discussed. Stimuli responsive behaviour of these multilayer assemblies can be tuned by choosing suitable depositing materials and method. Preparation of hollow microcapsules using LbL approach and its application in drug delivery has also been described in this chapter. Chapter II deals with the LbL assembly of a neutral polymer, poly(vinyl alcholol) (PVA). The negative charge on PVA backbone was induced by physical cross-linking with borax. The PVA-borate can undergo electrostatic interaction with positively charged chitosan in LbL process to form multilayer thin film. The thin film of PVA-borate complex/chitosan was found be responsive towards glucose concentration; disintegration of the multilayer assembly was observed at a high glucose concentration. This finding was rationalized on the basis of strong interaction of glucose with borate ions leading to dissociation of PVA-borate complex and subsequent collapse of the assembly. Thus, this multilayer self-assembly is potent for glucose triggered drug delivery. Chapter III reports the construction of a stable hydrogen bonded multilayer self-assembly based on complementary DNA base pairs (adenine and thymine) interaction. The natural polymer such as chitosan was modified with adenine whereas hyaluronic acid was modified with thymine. These two modified polymers were sequentially deposited on flat substrate and melamine formaldehyde (MF) particles; wherein strong interaction among the DNA base pairs led to the formation of stable assembly without utilizing any external cross-linking agent. The modified polymers are non-cytotoxic as proved from MTT assay. Further the multilayer assembly was used for pH responsive anticancer drug doxorubicin hydrochloride (DOX) release. In Chapter IV, glutaraldehyde mediated LbL self-assembly of single polymer multilayer thin films on flat and colloidal substrate by covalent bonding is described. A comparitive study between the native polymer (chitosan) and adenine modified polymer in the growth of thin film is performed. It is established from the study that the conformation of polymer and the availability of cross-linking points on the polymer play a crucial role in controlling the growth of these multilayer assemblies. Chapter V is divided into two parts (A and B). Part A describes a simple and unique protocol for fabrication of water dispersed chitosan nanoparticles (CH NPs). The method utilized in this work is based on the fast desolvation technique without using any additional stabilizer or any sophisticated instrumental setup. Furthermore, the CH NPs prepared from the mentioned protocol were proved to be cell-viable and are found to be responsive towards pH of the solution. In part B of this chapter, the LbL self-assembly of the responsive CH NPs is fabricated via electrostatic interaction with hyaluronic acid (HA). The growth of the multilayer thin film was found to be linear as function of number of bilayers. The morphology of thin film was characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The microscopic images reveal the uniform film morphology devoid of any phase separation of nanoparticles and polymers. Subsequently, the film was loaded with an anticancer therapeutic, doxorubicin hydrochloride (DOX). The release dynamics of encapsulated drug from the self-assembly are tunable and pH responsive. Chapter VI deals with the general and versatile method for the encapsulation of hydrophobic model drugs into polymeric multilayer assembly by using LbL approach. Electrical charge was induced on the surface of pyrene (uncharged organic substance) using an amphiphilic surfactant, sodium dodecyl sulfate (SDS) by micellar solubilization. The SDS micellar solution of pyrene was utilized to grow LbL multilayer thin film on a planar substrate and colloidal particles along with chitosan as a polycation. The LbL self-assembly of pyrene loaded SDS micelles/chitosan is additionally able to encapsulate hydrophobic or hydrophilic model therapeutics, thus providing an opportunity for dual-drug delivery. The desorption kinetics of the two model drugs from the thin film is found to follow a second order rate model.

Recent advances in multilayer self-assembly have enabled the precise construction of nanocomposite ultrathin films on a variety of substrates, from large-area planar surfaces to nanoparticles. As a result, a wide range of physico-chemical properties may be represented by selecting from an array of surface preparations, molecules, assembly conditions, and post-assembly treatments. Such multilayer nanofilm assemblies are particularly attractive for use as specialized membranes for selective transport, which have many applications for separations, sensors, and drug delivery systems. In this work, nanocomposite ultrathin films built with layer-by-layer (LbL) self-assembly methods have been applied to surface modification to control interfacial behavior, including diffusion, anti-fouling, and biomimetic membranes. Transport and interfacial properties of nanocomposite membranes constructed using LbL self-assembly with synthetic and/or bio-polymers were characterized, and permeability values of clinically relevant small molecules through the nanofilms were determined. Correlations between permeability and film properties were also examined. Nanofilm coatings around 100nm thickness decreased diffusion coefficients of glucose up to five orders of magnitude, and were found to greatly affect enzymatic glucose sensor responses. Surface modification on top of the nanofilms with poly(ethylene glycol) provided anti-fouling effects. However, weak-weak polyelectrolyte multilayers (PEMs) should not be used to control transport due to their susceptibility under normal physiological conditions. Natural/biological polymers also provided multilayer film structures at the specific conditions, but their transport-limiting properties were not significant compared to synthetic PEMs. Even when covalently crosslinked, biological PEMs did not reduce the permeability of a small molecule. Finally, the predicting model of projecting analyte permeation through multi-phase nanocomposite films comprised with known diffusion coefficients was theoretically and experimentally evaluated. The modeling was matched reasonably well to experimental data. The outcomes will be the key knowledge or engineering principles to support future efforts in research and development. It is anticipated that the system developed for determining transport properties will provide a general platform for assessing new candidate materials. The theory developed will be useful in estimating transport properties of novel nanocomposite materials that may be interesting in a broad array of chemical and biological systems, from analytical separations to implantable biomedical applications, and will provide useful design rules for materials and fabrication process selection.

Islet transplantation has emerged as a promising cell-based therapy for the treatment of diabetes, but its clinical efficacy remains limited by deleterious host responses that underlie islet destruction. In this dissertation, we describe the assembly of cell surface-supported thin films that confer molecular-level control over the composition and biophysicochemical properties of the islet surface with implications for improving islet engraftment. Specifically, the process of layer-by-layer (LbL) polymer self assembly was employed to generate nanothin films of diverse architecture with tunable properties directly on the extracellular surface of individual islets. Importantly, these studies are the first to report in vivo survival and function of nanoencapsulated cells, and have helped establish a conceptual framework for translating the diverse applications of LbL films to cellular interfaces. Additionally, through proper design of film constituents, coatings displaying ligands and bioorthogonally reactive handles may be generated, providing a modular strategy for incorporating exogenously derived regulators of host responses alongside native constituents of the islet surface. Towards this end, a strategy was developed to tether thrombomodulin to the islet surface in a site-specific manner, thereby facilitating local generation of the powerful anti-inflammatory agent, activated protein C. Collectively, this work offers novel biomolecular strategies for cell surface engineering with broad biomedical and biotechnological applications in cell-based therapeutics and beyond.

No description available.

Organische Solarzellen bieten die Aussicht auf eine ökologische und zugleich ökonomische Energiequelle. Nachteile des Konzepts liegen in der z.T. geringen Stabilität der für Absorption und Ladungstransport verwendeten Moleküle und einer unvollständigen Ausnutzung des Sonnenspektrums. Zur Verbesserung beider Merkmale werden in dieser Arbeit einzelne organische Bestandteile durch anorganische Materialien mit hoher Stabilität und breiten Absorptionsbanden ersetzt. Insbesondere werden als Absorber kolloidale Quantenpunkte (QP) verwendet, denen aufgrund nicht-linearer und durch Größeneffekte steuerbarer optischer Eigenschaften in der Photovoltaik der dritten Generation großes Interesse gilt. Dazu werden dünne anorganisch-organische Filme mit einem Verfahren hergestellt, das auf Wechselwirkungen zwischen Partikeln in Lösung und geladenen Oberflächen beruht (electrostatic layer-by-layer self-assembly). TiO2-Nanokristalle als Elektronenleiter, kolloidale CdTe- und CdSe-QP als Absorber und konjugierte Polymere als Lochleiter werden in die Filme integriert und diese als aktive Schichten in photovoltaischen Zellen verwendet. Die Struktur der Filme wird zunächst mittels AFM, SEM, XPS sowie durch eine Beladung mit organischen Farbstoffen untersucht. Sie weisen Porosität auf einer Skala von Nanometern sowie eine kontrollierbare Dicke und Mikrostruktur auf. Darauf aufbauend werden durch weitere lösungsbasierte Prozessschritte photovoltaische Zellen gefertigt und Zusammenhänge zwischen Struktur und Zellenleistung elektronisch und spektroskopisch untersucht. Einflussfaktoren der Zelleffizienz wie die Ladungsträgererzeugung und interne Widerstände können so bestimmt und die Effizienz von CdSe-QP als Sensibilisatoren nachgewiesen werden. Die Arbeit demonstriert die Eignung der gewählten Methoden und Zelldesigns zur Herstellung von photovoltaischen Zellen und eröffnet neue Ansätze für die Entwicklung und Fertigung insbesondere auf QP basierender Zellen. / Organic solar cells offer the prospect of a both ecological and economical energy source. Drawbacks of the concept are low stabilities of the molecules used for absorption and charge transport and an incomplete utilization of the solar spectrum. In order to improve both these characteristics, individual organic components are replaced by inorganic materials with a high stability and broad absorption bands in this work. In particular, colloidal quantum dots (QDs) are used as absorbers, the non-linear and size controllable optical properties of which are attracting great interest in third generation photovoltaics. For this application, inorganic/organic thin films are produced with a method based on interactions between particles in solution and charged surfaces (electrostatic layer-by-layer self-assembly). TiO2-nanocrystals as electron conductors, colloidal CdTe- and CdSe-QDs as absorbers and conjugated polymers as hole conductors are integrated into the films, which are used as active layers in photovoltaic cells. The structure of the films is investigated by AFM, SEM, XPS and by loading the films with organic dye molecules. The films show porosity on a nanometer scale as well as a controllable thickness and microstructure. Complemented by further solution based processing steps, photovoltaic cells are manufactured and correlations between the structure and performance of the cells are investigated both electronically and spectroscopically. Individual factors that determine the cell efficiency, such as carrier generation and internal resistances, are determined and the efficiency of CdSe-QDs as sensitizers is demonstrated. This work proves the suitability of the chosen methods and cell designs for manufacturing photovoltaic cells and opens up new approaches for the development and manufacture of in particular QD-based solar cells.

The electrostatic layer-by-layer (LbL) self-assembly of polyelectrolyte’s has shown applications in thin film coatings, micro patterning, nano-bioreactors and capsules for drug delivery. The film architecture can be precisely designed and controlled to nanometer scale precision with a range from 5 nm to a few microns. Both in vitro and in vivo studies indicate potential applications in biology, pharmaceutics, medicine, and other biomedical areas. This thesis work focused on the design and development of protocols to fabricate polyelectrolyte multi-layer patterns on a variety of substrates such as glass, metals and plastics such as acrylic and polycarbonate. The micro-scale polyelectrolyte patterns have applications in the creation of DNA, protein or cell based microarrays. This work also demonstrated the use of polyelectrolyte multi-layers in the enhancement of fluorescence signals from fluorophore-tagged molecules captured within the multi-layers. In-situ measurements using Fiber Bragg Gratings were carried out to study the kinetics of adsorption and desorption of polyelectrolytes participating in the layer buildup process under different process environmental conditions.