Dr. Ralf Zimmermann, researcher
Teresa Büttner, PhD student
Felix Schrön, PhD student
Susanne Bartsch, technician
Nelly Rein, technician

Understanding the electrostatic interactions of polymers in aqueous environments is one of the keys for the development of advanced biofunctional materials. To provide answers to fundamental questions on the mechanisms of the charge formation at bio-interfaces as well as to clarify interrelations between charge and structure of polymer materials we develop and apply complementary analytical techniques in combination with tailored models for interfacial charge formation and simulations.

Furthermore, strategies for unravelling the mechanisms of molecular transport in soft materials as well as new diagnostic principles are established.

Advanced electrokinetic methods

Electrokinetic and surface conductivity measurements provide valuable information about the charging and structure of polymers at interfaces. In order to use the versatile options of this approach, we developed the Microslit Electrokinetic Set-up. The instrument was further extended by implementing Reflectometric Interference Spectroscopy for the simultaneous determination of the optical layer thickness of polymer films and ATR-FTIR spectroscopy for assessing secondary structure variations of immobilized biopolymers as influenced by variations of the interfacial electrical charge.

Charging by unsymmetrical water ion adsorption

Charging of solid-liquid interfaces is an ubiquitous phenomenon of particular importance in surface and materials science. On many solid surfaces the charging is caused by ionization of functional groups, e.g., carboxyl or amino groups. However, also for surfaces without ionisable surface groups charging is observed in aqueous environments.

The phenomenon is often attributed to the unsymmetrical adsorption of hydroxide and hydronium ions. Despite of numerous theoretical and experimental studies, very little is known on the interaction of hydroxide and hydronium ions with water-hydrophobic interfaces. To address this question, we systematically perform electrokinetic measurements at various material surfaces.

Charge and structure at soft interfaces

Soft polymer coatings are widely applied in biomaterials science as they allow for tailoring of surface properties and implementation of advanced functional features into traditional materials. The optimization of the coating performance in biomedical and technical applications necessarily requires the measurement, analysis, and understanding of their physico-chemical properties.

We apply streaming current, surface conductivity and swelling measurements in combination with tailored models for the charging and electrokinetics at soft surfaces for a comprehensive characterization of the film charge and structure.

Ion-specific phenomena at bilayer lipid membranes

Cell membranes, the interfacial boundary the between intra- and extracellular space are complex systems and involved in many cell functions such
as signalling, protein sorting or apoptosis. At the MBC we use supported bilayer lipid membranes as model system that allow for a characterization
of cellular membranes in simpler and well defined environments and to study membrane protein functions.

Methods like electrokinetic measurements, fluorescence microscopy, and ATR-FTIR spectroscopy are applied to study the influence of charge
formation on the membrane formation, fluidity, structure, and stability.

Molecular transport in hydrogels

Biohybrid hydrogels consiting of electrically neutral synthetic polymers and highly anionic glycosaminoglycans offer exciting options for
regenerative therapies as they allow for the electrostatic conjugation of various growth factors. We work on the establishment of analytical
methods and models that provide insights into correlations between the molecular transport and the hydrogel composition and structure. These
 models are further used to predict the spatially and timely distribution of morphogens in cell culture experiments.

Microfluidics

Microchannel flows constitute most part of the transport process in various Lab-On-Chip devices. At the micro- and nanometer size of structures in
these systems, transport processes are significantly affected by interfacial phenomena such as electrokinetic effects and surface roughness.
As the fabrication of chips with fine channels come into realize, people demand detailed understanding how material properties and chip design influence the transport in microchannels. At the MBC we apply analytical and theoretical approaches to better understand electrosurface phenomena in
microfluidics.

• Zimmermann, R.; Duval; J.F.L.; Werner, C.:
  Probing biointerfaces: electrokinetics. In: Biointerfaces: Where Material meets Biology/ed. by D.W. Hutmacher and W. Chrzanowski. RSC Press 2015, Chapter 3, 55-73
• Zimmermann, R.; Romeis, D.; Bihannic, I.; Chohen Stuart, M.; Sommer, J.-U.; Werner, C.; Duval, J.F.L.:
  Electrokinetic as an alternative to neutron reflectivity for evaluation of segment density distribution in PEO brushes. Soft Matter 10 (2014) 7804-7809
• Zimmermann, R.; Dukhin, S.S.; Werner, C.; Duval, J.F.L.:
  On the use of electrokinetics for unravelling charging and structure of soft planar polymer films. Current Opinion in Colloid & Interface Science 18 (2013) 83-92
• Zimmermann, R.; Bartsch, S.; Freudenberg, U.; Werner, C.:
  Electrokinetic analysis to reveal composition and structure of biohybrid hydrogels. Analytical Chemistry 84 (2012) 9592-9595
• Zimmermann, R.; Küttner, D.; Renner, L.; Kaufmann, M.; Werner, C.:
  Fluidity modulation of phospholipid bilayers by electrolyte ions: Insights from fluorescence microscopy and microslit electrokinetic experiments. Journal of Physical Chemistry / A: Molecules, spectroscopy, kinetics,environment & general theory 116 (2012) 6519-6525
• Duval, J. F. L.; Küttner, D.; Werner, C.; Zimmermann, R.:
  Electrohydrodynamics of soft polyelectrolyte multilayers: point of zero-streaming current. Langmuir 27 (2011) 10739-10752
• Duval, J. F. L.; Küttner, D.; Nitschke, M.; Werner, C.; Zimmermann, R.:
  Interrelations between charging, structure and electrokinetics of nanometric polyelectrolyte films. Journal of Colloid and Interface Science 362 (2011) 439-449
• Zimmermann, R.; Kuckling, D.; Kaufmann, M.; Werner, C.; Duval, J. F. L.:
  Electrokinetics of a poly(N-isopropylacrylamide-co-carboxyacrylamide) soft thin-film. Evidence of segment distribution in the swollen state. Langmuir 26 (2010) 18169-18181
• Zimmermann, R.; Freudenberg, U.; Schweiß, R.; Küttner, D.; Werner, C.:
  Hydroxide and hydronium ion adsorption - A survey. Current Opinion in Colloid and Interface Science 15 (2010) 196-202
• Zimmermann, R.; Osaki, T.; Gauglitz, G.; Werner, C.:
  Combined microslit electrokinetic measurements and reflectometric interference spectroscopy to study protein adsorption processes. Biointerphases 2 (2007) 159-164

• Microslit Electrokinetic Set-Up (MES): in-house developed instrument to analyze the electrical charging of interfaces and infer on the structural features of polymer films
• Quarz Crystal Microbalance (Q-Sense E4)
• Spectroscopic Ellipsometer M-2000 (J. A. Woollam Co., Inc.)
• VIS spectrometer Specol 1100 (Analytic Jena) for Reflectometric interference Spectoscopy (RlfS) at interfaces (can be optionally used in combination with the MES)
• FTIR spectrometer Equinox 55 (Bruker) with SBSR-Unit (OTISPEC) for ATR-FTIR spetroscopy at
• Video-based optical contact angle meter OCA 30 (Dataphysics)