Volker Scheumann

Sensitivity studies for specific binding reactions using the biotin/streptavidin system by evanescent optical methods

The combination of various evanescent optical methods such as surface plasmon spectroscopy, waveguide mode spectroscopy and an integrated optical Mach-Zehnder-interferometer are used to characterize biotinylated self-assembled monolayers as well as the binding of streptavidin to these labels. The aim of designing a highly specific and sensitive, re-usable affinity sensor for antigens on the basis of an integrated optical Mach-Zehnder interferometer is based on a proper understanding of the characteristics of the entire binding matrix architecture. Therefore, a variety of biotin-derivatives immobilized in a monolayer are investigated with respect to their affinity to streptavidin and the possibility to remove the steptavidin layer specifically. The density of the streptavidin layer as well as the optical constants of the involved molecules are measured. Finally the integrated optical Mach-Zehnder interferometer is tested with respect to the sensitivity to an antigen-antibody binding reaction. An attempt to further increase the sensitivity by simultaneous detection of a fluorescence signal failed due to bleaching effects.

Probing the Electrochemical Deposition and/or Desorption of Self-Assembled and Electropolymerizable Organic Thin Films by Surface Plasmon Spectroscopy and Atomic Force Microscopy

Abstract The combination of surface plasmon spectroscopy (SPS) and atomic force microscopy (AFM) with electrochemical methods (cyclic voltammetry, potential step) is used to (i) probe the film/substrate interaction in alkanethiol monolayers formed on gold surfaces and (ii) monitor the electrochemically driven deposition of organic molecules onto a metal surface. The reductive desorption of alkanethiols at single crystal and polycrystalline gold surfaces was investigated by SPS and AFM. These experiments demonstrate the possibility of desorbing a self-assembled monolayer at a well-defined potential with all the consequences for selective (re)-functionalization. The self-assembly of alkanethiols on gold under potential control was also monitored by SPS. The results show that the surface derivatization of gold electrodes can be actively controlled by the manipulation of the electrode potential. Finally, the immobilization of biotin on gold surfaces has been carried out by the electropolymerization of a water-soluble, biotinylated derivative of 3-hydroxyphenylacetic acid. The molecular recognition of the biotinylated polyphenol film by the bacterial protein streptavidin was monitored by SPS. The packing density of the biotin labels in the polymer film leads to a very fast diffusion-controlled docking of the streptavidin to the surface. These studies clearly prove the usefulness of electrochemically controlled deposition to produce ultrathin film organic surfaces with specific function.

Selective Nucleation and Area-Selective OMCVD of Gold on Patterned Self-Assembled Organic Monolayers Studied by AFM and XPS: A Comparison of OMCVD and PVD

We demonstrate the area selective organometallic chemical vapor deposition (OMCVD) of ultrathin gold layers, using (trimethylphosphine)methylgold [(CH 3 ) 3 P]AuCH 3 as the gold source, onto self-assembled monolayers (SAMs) of ω-functionalized, long chain alkanethiols on gold and silver coated silicon and mica samples. The dependence on the reaction time of surface coverage with gold nano-clusters is analyzed by atomic force microscopy (AFM). X-ray photoelectron spectroscopy (XPS) shows that nucleation and growth occur only on thiol functionalized surfaces and not on methyl or hydroxyl functionalized SAMs. The selectivity of the growth is completely lost if gold is deposited by thermal evaporation of the pure metal, as shown in direct comparison with the OMCVD method employing mixed SAMs of different surface reactivity (-SH vs. -CH 3 ) that were patterned by microcontact printing.

Selective ultrathin gold deposition by organometallic chemical vapor deposition onto organic self-assembled monolayers (SAMs)

We demonstrate a selective deposition of ultrathin gold layers via OMCVD (organometallic chemical vapor deposition) onto self-assembled dithiols. Dithiols have been self-assembled to produce a thiolated surface. The gold layer deposited from a gold precursor, present in the vapor around the sample, is bound to the exposed thiol groups. We demonstrate that it is possible to deposit gold only onto the areas where the binding thiol groups are located, and investigate the growth process with spontaneous desorption time-of-flight mass spectrometry, Rutherford backscattering spectroscopy, atomic absorption spectroscopy and atomic force microscopy.

Shape-persistent macrocycles: Building blocks for complex organic and polymeric architectures

Alkyl- and Oligostyrene substituents were attached to shape-persistent macrocycles based on a phenyl-ethynyl backbone. In good solvents for both the rigid core and the flexible corona no aggregation occurred. Whereas, addition of a solvent that selectively solubilizes the corona induced a solvophobic aggregation. For alkyl substituted rings the experimental data were described by a monomer-dimer equilibrium. In contrast, the oligostyrene substituted rings formed more expanded aggregates which were investigated by scattering and by imaging methods. The superstructures are consequently described as hollow supramolecular cylindrical brushes.