Combination of microslit electrokinetic measurements and Reflectometric Interference Spectroscopy (RIfS)
Structure and function of bio(macro)molecules are determined by electrostatic forces depending on their ionization in aqueous electrolyte solutions. Furthermore, adsorption and supramolecular assembly of biopolymers at interfaces in general involves a complex interplay of long-range electrostatic forces as well as van-der-Waals- and hydrophobic interactions. Revealing the underlying principles is of highest importance in the application of biomedical materials in demanding products and technologies such as immunosorbent assays or bioactive cell scaffolds. To address the related need for the advanced experimental evaluation of surface-confined biopolymers in the presence of aqueous electrolytes we combined two label-free in situ methods, microslit electrokinetic measurements of electrical charge densities and the Reflectometric Interference Spectroscopy for the simultaneous determination of the optical thickness of interfacial layers. As compared to separate measurements this advanced approach permits to obtain complementary information about interfacial processes under identical and well-defined experimental conditions and to conclude on correlations between (i) charge and structure/conformation of biopolymers and (ii) charge and adsorption, desorption, and orientation of biopolymers at interfaces.
Figure: Left: Principle of streaming potential/current measurements combined with Reflectometric Interference Spectroscopy. The charge of the polymer film is determined by streaming potential and streaming current measurements via non-polarisable electrodes positioned at the inlet and outlet of the channel. Structural variations of the polymer layer as well as adsorption or desorption processes are followed simultaneously through evaluation of the interference pattern resulting from the interference of the partial beams I1 and I2. (The relative dimensions in the scheme are not in scale.) Right: Adsorption of fibrinogen (FGN) at poly(octadecene-alt-maleic acid) films at different protein solution concentrations studied by the combination of streaming current measurements and Refelctometric Interference Spectroscopy. Both, the streaming current vs. pressure gradient and the optical layer thickness immediately respond to the variation of the protein solution concentration. While the optical layer thickness correlates with the surface concentration of adsorbed FGN the streaming current vs. pressure gradient reflects the variation of the interfacial charge during the adsorption process. The protein solution concentration was adjusted at t = 0 min in the reservoir system of the Microslit Electrokinetic Set-up.
R. Zimmermann, T. Osaki, T. Kratzmüller, G. Gauglitz, S. S. Dukhin, C. Werner Analytical Chemistry 2006, 78, 5851-5857.
R. Zimmermann, T. Osaki, G. Gauglitz, C. Werner Biointerphases 2007, 2, 159-164.