Characterization of the surfaces generated by liposome binding to the modified dextran matrix of a surface plasmon resonance sensor chip

The dextran matrix of a surface plasmon resonance (SPR) sensor chip modifie d with hydrophobic residues (BIAcore sensor chip L1) provides an ideal subs trate for liposome adsorption. Liposomes of different lipid compositions ar e captured on the sensor chips by inserting these residues into the liposom e membrane, thereby generating stable lipid surfaces. To gain a more detail ed understanding of these surfaces, and to prove whether the liposomes stay on the matrix as single particles or form a continuous lipid layer by lipo some fusion, we have investigated these materials, using atomic force micro scopy (AFM) and fluorescence microscopy, Force measurements with AFM probes functionalized with bovine serum albumin (BSA) were employed to recognize liposome adsorption. Analysis of the maximal adhesive force and adhesion en ergy reveals a stronger interaction between BSA and the dextran matrix comp ared to the lipid-covered surfaces. Images generated using BSA-coated AFM t ips indicated a complete and homogeneous coverage of the surface by phospho lipid. Single liposomes could not be detected even at lower lipid concentra tions, indicating that the liposomes fuse and form a lipid bilayer on the d extran matrix. Experiments with fluorescently labeled liposomes concurred w ith the AFM studies. Surfaces incubated with liposomes loaded with TRITC-la beled dextran showed no fluorescence, indicating a complete release of the encapsulated dye. In contrast, surfaces incubated with liposomes containing a fluorescently labeled lipid showed fluorescence. (C) 2000 Academic Press .