Matthias Lessel

Subsurface influence on the structure of protein adsorbates as revealed by in situ X-ray reflectivity.

The adsorption process of proteins to surfaces is governed by the mutual interactions among proteins, the solution, and the substrate. Interactions arising from the substrate are usually attributed to the uppermost atomic layer. This actual surface defines the surface chemistry and hence steric and electrostatic interactions. For a comprehensive understanding, however, the interactions arising from the bulk material also have to be considered. Our protein adsorption experiments with globular proteins (α-amylase, bovine serum albumin, and lysozyme) clearly reveal the influence of the subsurface material via van der Waals forces. Here, a set of functionalized silicon wafers enables a distinction between the effects of surface chemistry and the subsurface composition of the substrate. Whereas the surface chemistry controls whether the individual proteins are denatured, the strength of the van der Waals forces affects the final layer density and hence the adsorbed amount of proteins. The results imply that van der Waals forces mainly influence surface processes, which govern the structure formation of the protein adsorbates, such as surface diffusion and spreading.

Nanosurgery of cells and chromosomes using near-infrared twelve-femtosecond laser pulses

Abstract. Laser-assisted surgery based on multiphoton absorption of near-infrared laser light has great potential for high precision surgery at various depths within the cells and tissues. Clinical applications include refractive surgery (fs-LASIK). The non-contact laser method also supports contamination-free cell nanosurgery. In this paper we describe usage of an ultrashort femtosecond laser scanning microscope for sub-100 nm surgery of human cells and metaphase chromosomes. A mode-locked 85 MHz Ti:Sapphire laser with an M-shaped ultrabroad band spectrum (maxima: 770  nm/830  nm) and an in situ pulse duration at the target ranging from 12 fs up to 3 ps was employed. The effects of laser nanoprocessing in cells and chromosomes have been quantified by atomic force microscopy. These studies demonstrate the potential of extreme ultrashort femtosecond laser pulses at low mean milliwatt powers for sub-100 nm surgery of cells and cellular organelles.

Nanofluidics of thin polymer films: Linking the slip boundary condition at solid–liquid interfaces to macroscopic pattern formation and microscopic interfacial properties

If a thin liquid film is not stable, different rupture mechanisms can be observed causing characteristic film morphologies: spinodal dewetting and dewetting by nucleation of holes. This rupturing entails liquid flow and opens new possibilities to study microscopic phenomena. Here we use this process of dewetting to gain insight on the slip boundary condition at the solid-liquid interface. Having established hydrodynamic models that allow for the determination of the slip length in a dewetting experiment based on nucleation, we move on to the quantification and molecular description of slip effects in various systems. For the late stage of the dewetting process involving the Rayleigh-Plateau instability, several distinct droplet patterns can be observed. We describe the importance of slip in determining what pattern may be found. In order to control the slip length, we use polymeric liquids on different hydrophobic coatings of silicon wafers. We find that subtle changes in the coating can lead to large changes in the slip length. Thus, we gain insight into the question of how the structure of the substrate affects the slip length.

Sliding fluids: Dewetting experiments reveal the solid/liquid boundary condition

Nanoscale liquid polymer films are ideal candidates to probe the solid/liquid boundary condition: Prepared on hydrophobized Si wafer, the films are not stable, they dewet. The dewetting induces a flow without applying an external force. Probing the dynamics of the dewetting film and the morphology of the liquid front, we can deduce the slip length. A variation of the type of hydrophobic coating (silane or Teflon®) of the Si wafer enables us to tune the boundary condition from a no-slip to a nearly full-slip condition. For a short introduction to the topic, we offer a phenomenological approach and supply multimedia files.

Increasing the wear resistance by interstitial alloying with boron via chemical vapor deposition.

The wear resistance of a Rh(111) surface can be strongly increased by interstitial alloying with boron atoms via chemical vapor deposition of trimethylborate [B(OCH3)3] at moderate temperatures of about 800 K. The fragmentation of the precursor results in single boron atoms that are incorporated in the fcc lattice of the substrate, as displayed by X-ray photoelectron diffraction. The penetration depth of the boron atoms is in the range of at least 100 nm with the boron distribution displaying a nearly homogeneous depth profile, as examined by combined X-ray photoelectron spectroscopy and Ar ion etching experiments. Compared to the bare Rh(111) surface, the wear resistance of the boron-doped Rh surface is increased to about 400%, as probed by the scratching experiments with atomic force microscopy. The presented synthesis route provides an easy method for case hardening of micro- or nanoelectromechanical devices (MEMS and NEMS, respectively) at moderate temperatures.