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Surface acoustic wave biosensors are a powerful tool for the study of biomolecular interactions. The modulation of a surface-confined acoustic wave is utilized here for the analysis of surface binding. Phase and amplitude of the wave correspond roughly to mass loading and viscoelastic properties of the surface, respectively. We established a procedure to reconstitute phospholipid and lipopolysaccharide bilayers on the surface of a modified gold sensor chip to study the mode of action of membrane-active peptides. The procedure included the formation of a self-assembled monolayer of 11-mercaptoundecanol, covalent coupling of carboxymethyl-dextran, and subsequent coating with a poly- l-lysine layer. The lipid coverage of the surface is highly reproducible and homogeneous as demonstrated in atomic force micrographs. Ethanol/triton treatment removed the lipids completely, which provided the basis for continuous sequences of independent experiments. The setup was applied to investigate the binding of human cathelicidin-derived peptide LL32, as an example for antimicrobial peptides, to immobilized phosphatidylserine membranes. The peptide-membrane interaction results in a positive phase shift and an increase in amplitude, indicating a mass increase along with a loss in viscosity. This suggests that the bilayer becomes more rigid upon interaction with LL32.

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The applicability of superconducting films in high-performance microwave devices suffers from inhomogeneous growth and local defects. Therefore we have installed a laser scanning system to visualize with spatial resolution the distribution of the microwave fields in 2-in.-diam YBaCuO disk resonators. The TM310 mode was imaged at a loaded quality factor of 8.3×104 via changes of the resonant frequency at 2.21 GHz and 40 K. The field distribution agreed with a magnetic-wall model, indicating at the contour of the disk fringe fields, but without strong edge enhancement. The data also indicated an inhomogeneous effective microwave penetration depth. The spatial resolution is presently limited to about 2 mm by heat diffusion.