Biointerfaces and Holography - Dr. Axel Rosenhahn

Our mission

Biofouling on ships^1-2

Our group works on understanding the interaction of marine biofouling organisms with surfaces in order to support the substitution of the majorly toxic anti-fouling coatings for under water applications¹. As the European Comission already banned tributyltin containing coatings and further restrictions for other toxic ingredients (e.g. copper or organic biocides) are negotiated, companies are forced to develop environmentally friendly solutions.

Approach

We pursue a knowledge driven and rationale design approach in which coating properties are correlated with anti-fouling or foul release activity. Therefore we systematically tailor surface properties and investigate the influence of specific surface chemistries and morphologies on the anti-fouling performance. To modify surface energy, hydration, charge, or entropy, self-assembly on the basis of silane or thiol chemistry is used^3-5. Biomacromolecules are additionally coupled to such primer matrices to obtain inert properties⁶. Bioinspired morphologies are prepared by self-assembled structures (e.g. layer by layer application of polyelectrolytes)^7-8.

Changing surface properties by self assembly and bioinspired surface morphologies^1,8

To verify successful application of the coatings, state-of-the-art surface analysis is applied. The chemical composition and film thickness is quantified by photoelectron spectroscopy, spectral ellipsometry and infrared spectroscopy (IRRAS). Wetting properties are characterized by contact angle goniometry. To analyze structure and mechanical properties electron microscopy, atomic force microscopy, and optical and confocal microscopy are applied. Interaction of the surfaces with biomacromolecules, such as proteins, is monitored by spectral ellipsometry, quartz crystal microbalance and surface plasmon resonance (SPR).

Surface colonization and microfluidic adhesion assay

Microfluidic adhesion assay⁹

The effect of the prepared surfaces on the colonization by marine organisms and their adhesion strength is determined in biological assays. In our lab we culture a range of model organisms and quantify the adhesion strength by a microfluidic adhesion assay⁹. The interaction with more complex organisms, especially if naturally harvested material is being used, is carried out in collaboration with marine biologists (e.g. Algae with the Callow lab, Birmingham, UK or Barnacles with the Clare lab, Newcastle, UK).

Recent major findings of our research were the correlation of surface wetting with the settlement of algal spores and the proof of validity of the “Berg limit” in the marine environment³. Furthermore, a variation of chain length in ethylene glycol containing polymers revealed that hydration and entropy of the films play an important role for their inert and adhesion weakening properties^4-5. Interestingly, we frequently observe a correlation between protein resistance and inertness against marine biofouling. Using self-assembled and lithographically produced nano- and microstructures we were able to show that structures with dimensions slightly below the contact area of the microorganisms act repulsive. Interestingly nanostructures favor adhesion but weaken the adhesion strength⁸.

Holographic microscope

Holographic tracking of microorganisms^10-11

Many of the biological assays are only snapshots in time. Understanding the kinetics of surface colonization requires knowledge about the settlement behavior of individual microorganisms. For this application we develop novel coherent imaging techniques such as digital in-line holography which allow tracking swimming microorganisms in four dimensions (three spatial and one time dimension)¹⁰. With the obtained trajectories exploration patterns can be analyzed and coating induced motility changes quantified¹¹. Analysis of the velocity- and angular distributions allow determining interaction distances. We also apply the methods in life and medical sciences (e.g. motility of pathogenic trypanosomes).

Coherent X-ray microscopy

X-ray Scattering endstation HORST and Holography experiment¹²

Another important aspect of our work is to understand the mechanism of adhesion and the ultrastructure of adherent microorganisms. We apply different spectroscopic and imaging techniques and in addition pursue the application of X-rays provided by synchrotron sources and free electron lasers. In the recent years we contributed to the field of x-ray holography, coherent x-ray diffraction imaging (CXDI) and x-ray Ptychography of biological samples^12-16. Nowadays such techniques allow obtaining chemical contrast at a high spatial resolution below 50 nm without lenses. For this work we developed a dedicated holographic scattering endstation (HORST) for coherent microscopy of biological samples at Synchrotrons and Free Electron Laser (FEL) sources. HORST has been successfully commissioned at BESSY in Berlin and FLASH in Hamburg. Synchrotron radiation can easily be tuned and we apply it for resonant imaging at absorption edges of elements¹⁴. Chemical and elemental mapping will in the future be used to determine mechanisms of curing of the adhesive of fouling organisms and to explore to which extend surface properties can be optimized to minimize glue-surface interactions. The scattering endstation HORST also allowed pioneering experiments at FLASH and we were the first group to demonstrate the feasibility of holographic microscopy of biological objects with femtosecond pulses at an FEL¹². The long term, visionary goal is to image biological samples in their natural liquid environment with a single femtosecond pulse of high intensity¹⁷. Even though the object explodes and is destroyed, the pulses are faster than the time required for the atoms to move during the Coulomb explosion, and thus carry only the information of the intact object. By such a single pulse imaging approach, the intrinsic resolution limit set by radiation damage can be overcome.

Funding

We are deeply grateful for the various sources of funding which allow us to carry out our research:

As part of our group is located at the KIT, we can be reached there as well.

References

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