Search

The outer cell envelope of eukaryotic cells is called the glycocalyx, which is comprised of complex glycoproteins and glycolipids. Signalling between cells occurs either through substrates dissolved in the extracellular liquid and the glycocalyx or through direct cell-cell-interactions. In both cases, the so-called lectins, proteins which bind in a selective manner to carbohydrate epitopes are involved. One of these lectin-mediated processes is bacterial adhesion. Whether successful interactions between substrate and sugar ligand take place is not only dependent on the type of sugar, but also on the orientation of the carbohydrate ligand on the surface. As the glycocalyx is a highly complex and dynamic system, suitable model systems must be developed for investigation of the orientational and conformational dependence on molecular recognition. A suitable model system can be designer surfaces such as self-assembled monolayers (SAMs) composed of azobenzene based glycoconjugates (photoswitchable glyco-SAMs). After irradiation with light of a certain wavelength, as the result of the isomerisation of the N=N double bond, the orientation of the sugar head group changes drastically. In earlier work it was shown, that the bacterial adhesion to photoswitchable glyco-SAMs can be switched between an “on” and “off” state reversibly depending on the E or Z state of the glycoconjugate. In the first part of this work a photoswitchable azobenzene mannobioside for the functionalization of gold surfaces was synthesised and the E/Z isomerisation was investigated. By employing a mannobioside head group the model system should approximate the natural pattern of the glycocalyx by increasing the molecular complexity. The second part of this work is dealt with the improvement and quantification of the E-Z isomerisation of glyco-SAMs. Therefore, three different approaches, which are based on the latest research results, were followed. As it is known that E/Z isomerisation of azobenzene requires space, the first approach was based on providing the needed space. But instead of employing diluter molecules, as it was done in previous work, the azobenzene derivatives were functionalized with a bulky protection group, which was cleaved after SAM formation. The photoisomerisation of the molecules for these so called “self-diluting” SAMs was investigated in solution and on gold surfaces. In the second approach, molecules with rigid biphenyl backbones were synthesised in Suzuki cross coupling reactions and investigated to build up very densely packed SAMs. SAMs constructed of such molecules showed excellent switching properties, due to their dense packing and the resulting π-π-interactions. For the first time the E/Z isomerisation of glyco-SAMs could be quantified. Following an additional idea, space between the azobenzene derivatives on the gold surface could be provided using molecular tripods for the switching unit.

In this work the growth process of a self-assembled monolayer (SAM) was studied in detail by complementary microscopic and spectroscopic methods. Vapour deposited 4’-nitro-1,1’-biphenyl-4-thiol (NBPT) SAMs on Au(111) were investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) showing a complex polymorphic phase behaviour, which strongly correlates with the density of the NBPT molecules on the surface monitored by X-ray photoelectron spectroscopy (XPS) prior STM scanning in situ. In addition the molecular orientation was measured as a function of the NBPT surface coverage by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) in situ as well. In combination the experimental results provide a good insight into the formation process of a SAM from the very beginning, where the NBPT molecules adsorb selectively outside the bridging regions of the herringbone reconstructed gold surface in a down tilted orientation, the formation of molecular patches and the lifting of the herringbone reconstruction, the genesis of a closed molecular layer in a upward orientation, till the very end, where the intermolecular interactions of the NBPT molecules lead to a reorganization of the Au(111) substrate visible as surface islands of gold step edge height. The influence of two different SAM preparation methods, namely the immersion into a solvent in the chemistry laboratory and the chemical vapour deposition under ultra-high vacuum conditions, was investigated by STM for the high coverage regime of NBPT-SAMs, leading to the result that the observed surface morphologies are predominantly independent of the preparation process, except of the formation of high coverage striped phases, which was only observed for solvent immersed samples. Altogether the results show that the conventional picture of the morphology of a SAM-surface-interface, which is laterally dominated by the surface geometry and its binding sites, must be reviewed for monolayers in the medium to high coverage regime. It also becomes clear that the observable surface height differences cannot be addressed completely to changes in the tilt angles of the molecular backbones. Instead of that it is rather likely that gold adatoms and reorganization processes in the substrate structure are dominantly responsible for the visible surface corrugations. This also reviews the perception of surface phenomena like gold islands on SAM surfaces as defect structures arisen during the SAM formation. This work points out that such effects are driven by the interplay of molecule-molecule and molecule-substrate interactions in the end of the assembly process minimizing the overall energy of the SAM-substrate complex.

Selbstorganisierte Monoschichten (SAMs) von Butanthiolat (C4) auf einkristallinen Au(111) Oberflächen wurden durch Adsorption aus der Lösung hergestellt. Das thermisch aktivierte Desorptionsverhalten der C4 Molek\"ule vom Gold Substrat wurde qualitativ mittels thermischen Desorptionsmessungen (TDM) untersucht und die Desorptionstemperatur wurde zu T_Des = 473 K bestimmt. So ließen sich durch thermische Behandlung unterhalb von T_Des nach der Deposition, Proben mit sehr guter Oberflächenqualitäat herstellen. Durch Beugungsexperimente mit langsamen Elektronen (LEED) konnten unterschiedliche Adsorptionsgeometrien der C4 Moleküle auf Au(111) nachgewiesen werden: Das ex situ Verfahren zur Probenpräparation liefert Adschichten in pxr3 und p(4x4) Phasen. Die dicht gepackte (r3xr3)R30 Phase konnte nicht beobachtet werden. Der Fokus der Spektroskopie von C4/Au liegt auf den Proben in der pxr3 Phase im Zwei-Photonen Photoelektronen Spektroskopie (2PPE) Modus. Diese Spektren zeigen deutliche Signaturen von zwei unbesetzten Resonanzzuständen bei Energien von E-E_F = 3,7 eV und 3,9 eV. Der Zustand niedrigerer Energie ist die charakteristische sigma*-Resonanz der Au-S Bindung zwischen Thiolat und Gold-Substrat. Die Energie des anderen Resonanzzustands stimmt gut mit der eines Grenzflächenzustands überein, der bereits für SAMs eines anderen Alkanthiolats auf Au(111) in einer dicht gepackten Phase gefunden wurde. Darüber hinaus liefern die 2PPE Daten Hinweise darauf, dass die hohe Qualität der ex situ hergestellten SAMs die Bildung von Bildpotentialzuständen unterstützt. Die reversible photo- und thermisch aktivierte Isomerisation des molekularen Schalters 3-(4-(4-Hexyl-phenylazo)-phenoxy)-propan-1-thiol (ABT), der durch Selbstorganisation aus Lösung auf Au(111) deponiert wurde, wurde mittels laserbasierter Photoelektronenspektroskopie untersucht. Änderungen des molekularen Dipolmoments, das jeweils für das trans- und das cis-Isomer von ABT einen charakteristischen Wert hat, wurden über Veränderungen der Austrittsarbeit beobachtet, die über die Verschiebung der Photoemissionsschwellenenergie zugänglich ist. Eine quantitative Analyse der Daten zeigt, dass der Anteil der Moleküle innerhalb der dicht gepackten Monoschicht, der einen Schaltvorgang ausführt, in der Größenordnung von 1% liegt. Dieses Ergebnis deutet darauf hin, dass nur an Defekten des Substrats und der Monoschicht die sterischen und elektronischen Behinderungen der Isomerisationsreaktion in einer dicht gepackten Monoschicht überwunden werden können. Untersuchungen des Zinn-Phthalocyanin (SnPc) Moleküls von Wang et al. haben gezeigt, dass bei Adsorption auf Ag(111) Substraten, die nichtplanare Form dieser Moleküle als Grundlage für einen molekularen Schalter genutzt werden kann. Die Position des zentralen Zinnatoms kann mit der Spitze eines Rastertunnelmikroskops von der tin-up in die tin-down Position geschaltet werden. Untersuchungen von Submono- und Monoschichten der Moleküle, die unter UHV-Bedingungen auf die Ag(111) Substrate aufgebracht wurden, mit LEED ergaben je nach Schichtdicke und thermischer Aktivierung verschiedene Adsorptionsgeometrien. Mit Photoelektronen Spektroskopie konnten die Energien der HOMO- und LUMO-Orbitale bestimmt werden, in sehr guter Übereinstimmung mit Ergebnissen aus Rastertunnel-Spektroskopie. Allerdings konnte ein stabiles photoinduziertes Schalten nicht nachgewiesen werden. Self-assembled monolayers (SAMs) of butanethiolate (C4) on single crystalline Au(111) surfaces were prepared by adsorption from solution. The thermally activated desorption behaviour of the C4 molecules from the gold substrate was examined by qualitative thermal desorption measurements (TDM), through this a desorption temperature T_Des = 473 K could be determined. With this knowledge, it was possible to produce samples of very good surface quality, by thermal treatment T_Sample < T_Des following the actual deposition process. Low energy electron diffraction (LEED) experiments yielded different adsorption geometries of C4 molecules on Au(111). The ex situ preparation scheme produced samples in pxr3 and p(4x4) phases, the high coverage (r3xr3)R30 phase was not observed. The focus of the spectroscopy of C4/Au was on the samples of the pxr3 phase using two-photon photoemission spectroscopy (2PPE). The spectra revealed clear signatures of two unoccupied resonance states at energies E-E_F = 3.7 eV and 3.9 eV. The low-energy state was assigned to the characteristic sigma*-resonance associated with the Au-S bond of the thiolate. The energy of the other resonance state agreed well with an interface state reported before for SAMs of an other alkanethiol on Au(111) in a densely packed phase. Furthermore the 2PPE data indicated that the high quality of the ex situ prepared SAMs supported the formation of image potential states. The reversible photo- and thermally activated isomerization of the molecular switch 3-(4-(4-Hexyl-phenylazo)-phenoxy)-propane-1-thiol (ABT), deposited by self-assembly from solution on Au(111), was examined using laser-based photoelectron spectroscopy. Differences in the molecular dipole moment characteristic for the trans and the cis isomer of ABT were observed via changes in the sample work function, accessible by detection of the threshold energy for photoemission. A quantitative analysis of our data showed that the fraction of molecules within the densely packed monolayer that undergoes a switching process is of the order of 1%. This result indicates the relevance of substrate and film defects required to overcome the steric and electronic hindrance of the isomerization reaction in a densely packed monolayer. Investigations of the tin-phthalocyanin (SnPc) molecule by Wang et al. have demonstrated that after adsorption on Ag(111), the nonplanar shape of the SnPc molecule can be exploited as a basic principle for a molecular switch. The position of the center tin atom can be switched from a tin-up to a tin-down position, using the tip of a scanning tunneling microscope. LEED measurements of UHV deposited adlayers of different coverages of SnPc molecules, found different adsorption geometries, depending on layer thickness and thermal activation. Photoelectron spectroscopy could determine the energy of HOMO and LUMO orbitals in very good agreement with results from scanning tunneling spectroscopy. However, stable photoinduced switching could not be demonstrated.

Die Untersuchung von Proteinen stellt besondere Herausforderungen an die Entwicklung von sogenannten Biochips dar. Hierfür sind funktionalisierte Mikro- und Nanometer Strukturoberflächen mit sehr hoher Auflösung erforderlich. Ziel ist die Immobilisierung von Biomolekülen an wohl definierten funktionellen Positionen, um eine weitere detaillierte Analyse zu ermöglichen. Die Verwendung von Selbstaggregierenden Monolagen (SAMs) ist für die oben genannten Ziele eine ideale Plattform. Durch diese Monolagen lassen sich chemische und physikalische Eigenschaften der Oberflächen sehr genau kontrollieren. Sie dienen als Bindeglied zwischen organischen und anorganischen Umgebungen. Durch verschiedene lithographische Techniken können auf diesen Oberflächen lateral definierte chemische modifizierte Strukturen erzeugt werden. In der vorliegenden Arbeit wurde die laterale Strukturierung selbstaggregierender Monolagen (SAMs) mittels EUV Interferenz Lithographie (EUV-IL) untersucht. Dabei zeigt sich, dass die direkte Strukturierung der Nitrobiphenyl-Thiol SAMs durch Vernetzung die gleichen chemischen und physikalischen Eigenschaften aufweisen wie Elektronenvernetzte Proben. Die Methode der EUV-IL eignet sich somit besonders gut für die Fabrikation von großflächigen Vorlagen wie z.B. für Biochips. Die Kombination aus SAM und EUV Lithographie basierter Technik besitzt ein enormes technisches Potential. Es lassen sich hierbei Nanosiebe oder durch Anbindung von multivalenten NTA-Derivaten an die chemisch funktionalisierte SAM mit NTA / His-tag Wechselwirkung Biochips realisieren. Diese NTA-Chelator Proteinbiochips erlauben eine spezifische, hochaffine, umkehrbare und langfristige Immobilisierung His-markierter Proteine. Deren Verwendung ist in der Untersuchung bzw. Analyse von unterschiedlichen Proteineigenschaften auf Oberflächen sehr hilfreich. Die Herstellung und Anwendung einer Vielzahl von physikalischen, chemischen und biologischen Strukturen wird hierdurch ermöglicht. Konkret konnten verschiedene Herstellungsmethoden zur Realisierung eines Biochips demonstriert werden. Außerdem können mit dem Ansatz aus SAM und EUV-IL Nanosiebe produziert werden. Dabei handelt es sich um Membrane mit einer Schichtdickenstärke und Perforation im Nanoskala Bereich. In dieser Arbeit werden definierte Porenöffnungen bis zu ~30nm gezeigt. Mögliche praktische Anwendungen sind z.B. Filter.

Subject of this work was the electrochemical metallization of organic layers on Au(111)- and n-Si(111)-electrodes. By self-assembly of thiols on Au(111) and grafting of 1-alkenes on n-Si(111):H omega-functionalized SAMs were prepared. The SAM molecules served as immobilized ligands whose end groups acted as active sites for the complexation of metal ions at the solution side of the interface. The separately conducted electrochemical reduction of the coordinatively bound metal ions in a metal-ion free electrolyte yielded metallic islands of monoatomic height atop of the SAMs of whose zero-valent state was confirmed by XPS. The aggregation of islands implies the leaving of the complexation sites and surface diffusion. By this process the donor atoms not covered by metal islands became again available for complexation. The retrieve of vacant coordination sites after each reduction allowed an increase of the metal coverage beyond the amount predetermined by the number of coordination sites up to Theta = 0.9 by consecutive cycles of complexation and reduction which were accompanied by a transition from Frank-van der Merwe to Volmer-Weber growth mode. The modification of the n-Si(111):H surface by covalently attached organic molecules was done by grafting of 4-vinylpyridine via thermally activated hydrosilylation. The modified n-Si(111) surface showed clearly altered properties in the CV like a shift of Efb and a strong inhibition of the electrodeposition of Cu from electrolyte solution. STM investigations revealed islands lacking any long range order whose dimensions of 35×12 Å at a height of approximately 2 Å suggest the surface polymerization of five molecules each in horizontal orientation. After complexation of the Mez+ the electrochemical reduction in the CVs was superimposed by HER whereby only XPS investigations could verify the coordination of the metals. XPS confirmed the presence of 4-VPy via the signals of N 1s and C 1s core levels as well as of metallic species after complexation.