Reversible supramolecular modification of surfaces

Enzymes are biocatalysts widely used in a large number of industrial biotech processes as they offer clear advantages over their chemical counterparts. Indeed, enzymes often show high substrate selectivity along with elevated turnover rates. Enzymatic catalysis usually functions under mild conditions of temperature, pressure and acidity. However, the industrial application of enzymes is often limited by their limited stability under operational conditions. Moreover, due to high water solubility of enzymes it is challenging to confined them in a flow reactor system. In order to circumvent these limitations, we have developed a supramolecular strategy that allows the reversible immobilization of active enzyme-polymer conjugates at the surface of filtration membranes. It is based on multivalent host-guest inclusion interactions between the membrane surface and a soluble enzyme-polymer conjugate. Cyclodextrins (CDs) as "host" molecules are covalently attached at the surface of polyethersulfone membranes and a multivalent water-soluble polymer is synthesized as a "guest" molecule. We demonstrate that while this supramolecular surface modification is stable under operational conditions and allows for efficient bio-catalysis, it can be straightforwardly reverse by competitive host-guest interactions. The first part of this manuscript is dedicated to a literature review on selected topics. As the supramolecular strategy we have developed in the course of this PhD research work is based on the use of cyclodextrins as supramolecular host molecules, the first part of this literature review focuses on the physico-chemical characteristics of this class of macrocycles. A special emphasis is done on their ability to form host-guest multivalent inclusion complexes. In this context, we describe the concept of multivalency and the underpinning essential thermodynamic principles, which can be apply to design controllable, directional, and selective self-assemblies. In the second part of this manuscript, we present our strategy to bio-functionalize polymeric membrane surfaces using multivalent reversible inclusion reactions. In more details, the chemical strategy to introduce CD macrocycles, in a covalent fashion, at the surface of the polymeric material is discussed. The synthesis and characterization of an enzyme-polymer conjugate, possessing multiple chemical functional groups (i.e. adamantyl) able to form inclusion complexes with CDs, is presented. It is demonstrated that this supramolecular strategy could be applied to the reversible immobilization of an active enzyme at the surface of polyethersulfone membranes. A similar strategy is applied to the reversible bio-functionalization of gold surfaces and used to prepare sensor chips for surface plasmon resonance (SPR) experiments. Self-assembled monolayers of CDs derivatives are prepared on the surface of a gold sensor chip. A water-soluble protein-polymer conjugate, possessing multiple adamantyl moieties, is synthesized. The supramolecular reversible binding of this new conjugate on the chemically modified SPR chip is demonstrated. The possibility to use this system for antigen/antibody biosensing experiment is successfully confirmed.

Supramolecular structures of polypeptides

Daniela Silvano

The formation of supramolecular structures is a central issue in bio- and nanotechnology and therein plays an important role for many novel developments such as biocompatible materials for integrating living cells or coating for implantable sensing devices as well as for designing smart molecular sensor surfaces for miniaturized bioanalytical techniques. The main focus of this thesis is the investigation of polypeptide layers on surfaces, in particular the formation of self-assembled monolayers (SAMs). SAMs made of long hydrocarbon chains or aromatic molecules comprising surface active and other functional groups have been deeply characterized over decades. Surprisingly, peptide SAMs did not meet the same strong interest despite their great potential in several areas. Natural (20) and artificial amino acids with different physical and chemical properties offer a plethora of novel possibilities to design coating of solid surfaces. In fact their combination in simple oligopeptide sequences provides a huge number of molecular components for the formation of SAMs. Control of SAM features such as density, flexibility, adaptability, interface properties, by tailoring the amino acid sequence opens a new route to manifold applications ranging from fundamental research to nano- and biotechnology. In this thesis different and complementary techniques have been used to investigate the self-assembling ability of polypeptides on surfaces with different physical and chemical properties. Infrared spectroscopy and circular dichroism allowed the determination of the peptide secondary structure on surfaces and in solution. Surface plasmon resonance provided the optical thicknesses of peptide SAMs on gold whereas contact angle measurements have been used to characterize their interfacial properties. In the first part of the thesis, a series of hydrophobic peptides (Ac-Cys-Alax-CONH2, x=3-6) have been self assembled on gold via the thiol group present in the cysteine. The main interest was whether these peptides formed densely packed SAMs with a defined secondary structure within the layer and how the properties of such SAMs depended on the length of the polypeptide backbone. The second part concerns the study of the interactions of an amphipathic helical peptide with various functionalized substrates. The aim was to investigate whether a proper design of the peptide could be used to predetermine its secondary structure on the substrate and whether the orientation of the peptide on the surface could be directly controlled via certain parameters of the surrounding medium. Since the hydrophilic residues of the peptide are histidines, a particular effort was spent in characterizing the peptide adsorption on substrates exposing nitrilotriacetic acid (NTA) moieties. Once established that the adsorption process did not alter its helical structure, the peptide was adsorbed on a substrate exposing two different functional groups: NTA, that could interact with the histidine residues, and methyl groups, that could interact with the hydrophobic ones. The goal was to bind the peptide in different orientation to the same substrate, exploiting the particular spatial distribution of its residues. The third part of this thesis concern the formation of nano- and micro-sized fibrils through molecular self-assembly of simple oligopeptides and protected amino acids. Such systems were chosen as models to investigate the role of the different driving forces in the formation of fibrillar structures. The aromatic protecting group being fixed, it could be shown that the protected amino acids formed fibrils with different sizes and morphologies, or did not form fibrils at all, depending on how strongly their side chains interact. The possibility of forming fibrils by using protected amino acids is of major importance not only in bio-related field but also in organic nanotechnology.

EPFL2008

Growth and characterization of specific self-assembled supramolecular architectures at crystal surfaces

Marta Canas Ventura

In the framework of this thesis self-assembled supramolecular architectures of several molecular species on crystal metal surfaces are characterized. These investigations combine results obtained by means of scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), X-Ray Photoelectron Spectroscopy (XPS), and Near-Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS). The interest in controlling the formation of specific arrangements of supramolecular ensembles at surfaces is motivated to a great extent by the prospect to design molecular devices and nanomaterials with customized properties. The goal is a "spontaneous" creation of targeted, well-ordered molecular architectures, which shall be achieved by taking advantage of the molecule's propensity to self-assemble. Control means, in this context, predictability. This term address the accuracy of the prediction of the structures resulting from molecular self-assembly for systems exhibiting different characteristics. Here, system stands for a particular combination of a single-crystal surface and adsorbed molecular species. In a first part, the focus is given to the impact of a Pd(111) surface exhibiting high chemical activity. Due to surface induced deprotonation two resulting chemical states are found for the adsorbed terephthalic acid species. Deprotonation of the carboxyl moieties is observed to affect the topology of the molecular assemblies even though hydrogen bonds (H-bonds) are responsible for the stabilization of assemblies of both, intact and deprotonated molecular species. Therefore, the selective fabrication of a single specific molecular structure becomes a challenge when the surface is prone to induce chemical reactions. Secondly, a molecular species exhibiting conformational flexibility is studied on a largely inert Au(111) surface. It is observed that the characteristic conformational flexibility is also active on the surface, and the molecule is found to adopt different conformations depending on the locally most favorable bonding motif. In particular, phenyl groups may undergo a rotation such that one-dimensional chains can be stabilized by means of intermolecular H-bonds. This phenomenon is very interesting from an adsorbate-substrate interaction point of view, but it limits the predictability of supramolecular structures formed upon self-assembly. In a third step, a comparative study of the self-assembly of three closely related molecular species highlights the impact of concurrent competing intermolecular interactions. Molecules exhibiting an increasing number of possible interaction channels are found to assemble into an increasing number of distinct supramolecular architectures. The most flexible molecular species allowing for different hydrogen-bonding patterns as well as dipolar coupling self-assembles into various structurally complex architectures. Structural predictability, however, is low since the formation of one or the other assembly appears to depend on small variations in sample preparation conditions and on poorly controllable local surface properties. Finally, a system where the resulting structures fully agree with predictions evidences the possibility to control the formation of targeted specific supramolecular architectures. The formation of well ordered bimolecular ribbons and wires over extended length scales is accomplished by the rational choice of both, complementary building blocks for the selective formation of three-fold H-bonds and a nanostructured surface to guide the molecular self-assembly. Furthermore, the impact of H-bond formation on the electronic structure of self-assembled molecular building blocks is probed by STS measurements. The experimental data reveal shifts in the electronic levels depending on the H-bond configuration and on the local characteristics of the underlying surface. In order to improve the precision of such a study, single molecules, homo- and heteromolecular dimers as well as trimers are successfully isolated and assembled by molecular manipulation with the STM tip. Further STS studies on such artificially assembled structures are suggested.

EPFL2008

Fabrication of Functional Silicon Surfaces using Metallic Nanostructures Produced by Block Copolymer Self-Assembly

Li Wang

Self-assembly is the autonomous organization of components into patterns or structures without human intervention. At the molecular level, self-assembly usually involves non-covalent interactions of various natures, such as van der Waals, electrostatic and hydrophobic interactions, hydrogen and coordination bonds. Thus, this process provides one solution to the fabrication of ordered aggregates from components with typical sizes in the submicron scale, which is a key to applications in nanotechnology. The work presented in this thesis aims at producing hybrid and functional nanostructured silicon surfaces using self-assembled metallic nanostructures. As self-assembling tool, we use polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) copolymers, which are amphiphilic molecules able to form well-defined nanostructures and to guide the positioning of metallic components into periodic arrangements. Various approaches were explored to reach this goal. First, self-assembled monolayers of PS-b-P2VP copolymer micelles were used as nanoreactors for the chemical synthesis of metal nanostructures of tunable dimensions and morphologies. This was achieved via a post-loading method, where the micelle cores are impregnated with metal salts after their deposition on silicon surfaces. Gold, silver and iron oxide nanoparticle (NP) arrays of controlled sizes and periodicities were produced. We also exploited the responsive properties of PS-b-P2VP micelles to create ordered arrays of gold nanorings having feature sizes below 100 nm. Surface plasmon resonance properties were studied and showed a significant change of optical properties between the gold particle and nanoring morphologies. Then, we focused our efforts on the controlled positioning of preformed gold NPs using PS-b-P2VP micelles as guiding templates. Two methods were developed. In the first, hierarchical positioning of NPs was achieved in one step during the spontaneous micelle phase inversion into nanoring structures. In the second, we used block copolymer-assisted lithography for producing periodic amino-silane nanopatterns, which then acted as electrostatic traps for the selective immobilization of gold NPs in aqueous solution. For both approaches, we showed the formation of ordered arrangements of individual gold NPs or clusters on silicon surfaces, whose morphologies, densities and periodicities could be tuned by simply varying the initial block copolymer molecular weights or deposition conditions. In a last part, we demonstrated the fabrication of functional silicon surfaces with high aspect ratio and controlled morphologies using self-assembled metallic patterns. Using a simple and efficient process called metal-assisted chemical etching (MAC-Etch), both highly porous silicon surfaces and ultra long silicon nanowires were produced. In particular, we reported for the first time the possible fabrication of free-standing nanoporous silicon membranes by this method and demonstrated the effective transport of molecules across the pores. Finally, the catalytic properties of self-assembled gold NPs were evaluated to grow vertically aligned silicon nanowires in a Low Pressure Chemical Vapor Deposition (LPCVD) reactor via a Vapor-Liquid-Solid (VLS) growth mechanism.

EPFL2011

Supramolecular structures of polypeptides

Daniela Silvano

EPFL2008

Growth and characterization of specific self-assembled supramolecular architectures at crystal surfaces

Marta Canas Ventura

EPFL2008

Fabrication of Functional Silicon Surfaces using Metallic Nanostructures Produced by Block Copolymer Self-Assembly

Li Wang

EPFL2011