Growth of Graphene Nanoflakes/h-BN Heterostructures

2D materials such as graphene, hexagonal boron nitride (h-BN), or transition metal dichalcogenides, and their heterostacks are gaining increasing interest because of their extraordinary properties, which can range from superconductivity to large charge carrier mobilities. In this paper, the electronic and structural modifications of h-BN on Rh(111) are investigated by the intercalation of carbon forming graphene nanoflakes between the h-BN and the Rh(111) surface. The carbon atoms-natural impurities in Rh bulk crystals-diffuse to the surface during the h-BN growth and segregate there during cooling. The graphene nanoflakes are present at particular sites under the wires of the h-BN nanomesh leading to an altered appearance of the Moire pattern and modified electronic and chemical properties. Thus, a novel fabrication route of graphene nanoflakes located in a heterostack between Rh(111) and h-BN is shown by steering the segregation of carbon impurities at the rhodium surface.

Functional polymer brushes via surface-initiated controlled radical polymerizations

Stefano Tugulu

The term polymer brush refers to a well defined arrangement of polymer chains, which are tethered with one end to an interface and usually the surface of a solid substrate. Due to steric repulsion the polymer chains furthermore adopt a defined, stretched chain conformation, which significantly differs from the random walk conformation of free polymer chains in solution or in conventionally solution casted polymer coatings. From a structural point of view polymer brushes can therefore be regarded as the most defined type of polymer coating that can currently be realized. Especially the combination of surface-initiated polymerizations (SIP) with modern, controlled polymerization methods thereby allows tailoring the structure of the polymer brushes almost on the molecular level. Specifically precise control over the thickness, composition, chain architecture, grafting density and via the use of lithographic techniques also over the topography of the brushes can be achieved. This high degree of control over the structural and implicated (physico-)chemical properties of the brush allows at the same time tailoring and fine tuning the interfacial properties of the brush. This thesis describes the systematic exploitation of well-defined polymer brushes as functional coatings with fine-tuned physicochemical properties to direct and control the interaction with and the response of specific biological, bioanalytical and even reactive chemical environments. In detail the use of surface-initiated atom transfer radical polymerization (SI-ATRP) for the fabrication of functional polymer brushes will be presented. These brushes were used as highly defined coatings in various applications ranging from biomedical and bioanalytical applications to biomimetic materials fabrication (Figure 1). Figure 1 Numerous synthetic approaches for the preparation of polymer brushes have been described in the literature. Chapter 2 will therefore present a summary of the most important types of surface-initiated polymerizations, focusing especially on controlled SIP methods and SI-ATRP. Chapter 3 describes a chemoselective and specific immobilization strategy to decorate intrinsically bioinert polymer brushes with proteins of interest in a defined orientation and density. Specifically the resulting substrates represent attractive candidates for the realization of protein microarrays. In view of this application protein-small molecule and protein-protein interactions as well as posttranslational modifications were analyzed within a feasibility study. Due to their well-defined structure polymer brushes represent also attractive candidates for the realization of molecularly defined cell adhesion substrates for tissue engineering or as highly defined biophysical model system to analyze cell adhesion and mechanics. Chapter 4 describes the decoration of intrinsically bioinert polymer brushes with small peptide ligands with defined density to induce integrin specific cell adhesion on the brushes. Peptide functionalized polymer brushes could be readily endothelialized and adhering cell layers have shown to preserve their homeostatic ability to respond to an applied mechanical stimulus in a physiological manner. In Chapter 5 the limitations of high density polymer brushes in biomedical applications were assessed by analyzing their stability under in vitro cell culture conditions. Intrinsically bioinert high density polymer brushes were found to degraft from glass or silicon substrates upon swelling in good solvents, which lead to deterioration of the bioinert properties of the substrate. The degrafting of the brushes might be connected to swelling induced mechanical activation of chemical bonds, which link the brushes to the substrate. Chapter 6 describes the transfer of the developed SIP approach onto flexible PDMS substrates by exploiting the formation of interpenetrating networks of ATRP-initiator siloxane and PDMS. The modification of the PDMS substrates with hydrophilic brushes effectively reduces the unspecific protein adsorption on the substrates. This approach might therefore represent an attractive and facile route for the surface modification of miniaturized PDMS devices for biomedical or bioanalytical applications. In Chapter 7 the systematic development of a reliable and robust protocol for the aqueous SI-ATRP of sodium methacrylate is presented, which gives access to poly(methacrylic acid) (PMAA) brushes with defined molecular weight and grafting density. The resulting brushes were assessed for their ability to direct the mineralization of calcium carbonate. Chapter 8 describes the use of photolithographically patterned poly(methacrylic acid) brushes as biomimetic, acidic macromolecular matrix to fabricate microstructured calcite thin films that are an exact 3D replica of the PMAA brush. The presented strategy relies on three key elements: (i) the use of photolithographic techniques to prepare microstructured PMAA brushes; (ii) the ability of PMAA brushes to stabilize amorphous calcium carbonate (ACC) and (iii) the possibility to convert the metastable ACC phase into a polycrystalline calcite film via a thermal treatment.

EPFL2007

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

Electrophoretic deposition of multi-scale structured TiO2 surfaces

The long term fixation of biomedical implants constitutes an issue in the clinical praxis. Implant fixation can be improved by functionalizing its surface. In fact, the tissue response around an implant is conditioned by its surface morphological and chemical properties. Coatings for implants (orthopedic and dental) function at the bone-implant fixation interface. As proved by research activities in the biomedical field, coatings with topographic features in the nano- and micro-range are promising in terms of enhanced osteointegration, thus improved implant fixation. As for surface chemistry, titanium dioxide (TiO2) is an estabilished material for biomedical implants thanks to its outstanding physical and chemical properties such as high corrosion resistance and good biocompatibility. Electrophoretic deposition (EPD) is a simple and versatile coating technique. According to literature, EPD is an emerging potential coating technique for biomedical implants. In this work, the use of EPD in the preparation of TiO2 coatings characterized by a controlled multi-scale structured surface was addressed. For this aim, EPD was combined with a new procedure of particle functionalization and with the use of a fugitive spacer (polystyrene, PS) for the sintering treatment. A control and better understanding of the EPD process were achieved through a preliminary parametric study on EPD of TiO2 nanoparticles and PS microbeads. Particle concentration was found to play a key role in the EPD deposit formation of TiO2 nanoparticles (cathodic) and PS microbeads (anodic). A threshold value of particle concentration was identified, below which no deposit growth occurred. An interpretation of this finding was provided on the base of a new formulation of the EPD deposition mechanism. The results of the particle concentration study contributed to the preparation of suspensions with TiO2-PS composite particles, which could be used for cathodic EPD. Composite TiO2-PS particles with TiO2 nanoparticles covering PS microbeads were obtained in a newly developed wet colloidal process based on heterocoagulation and the use of polyelectrolytes. The resulting TiO2-PS particles were cathodically deposited on Ti6Al4V substrates. A sintering post-treatment caused the burning out of the PS beads and the densification of the TiO2 nanoparticles. The desired coatings with controlled micro- and nano-topography were achieved. The investigation on the role of particle concentration in EPD and the new interpretation of the deposition mechanism bring a deeper understanding of the EPD process. The achievement of TiO2 coatings with controlled morphology in the micro- and nano-range prepared by cathodic EPD on Ti6Al4V substrates is a contribution to the ongoing research on biomedical implants. Mechanical tests and surface analysis evaluating wettability and in vitro cell behavior with the achieved coatings are relevant topics for further research.

EPFL2009