Characterization of surface-immobilized layers of intact liposomes

Surface-immobilized liposome layers are of interest for various potential applications such as localized drug delivery, but their characterization is challenging. We have employed an AFM method and fluorescent dye release to analyze anchored liposomes. In addition, we studied whether the liposomes are surface-bound solely via specific interaction (NeutrAvidin/biotin) or whether physisorptive binding also plays a role. Liposomes containing PEG-biotin lipids were affinity bound to NeutrAvidin molecules which had been immobilized onto solid supports via three different hydrogel interlayers. After liposome docking, approaching the surface with a colloid probe mounted onto an AFM cantilever showed considerable compression behavior, consistent with expectation based on intact, deformable liposomes but not lipid bilayers, thus showing that disruption of liposomes did not occur upon immobilization onto these support surfaces. Plastic deformation suggestive of liposome disruption on compression was not observed. The kinetics of fluorescent dye release also demonstrated that intact liposomes had been successfully immobilized onto all three supports. Blocking surface-immobilized NeutrAvidin molecules with excess biotin in solution before exposure to liposomes showed that the docking of liposomes was dependent largely but not exclusively on biotin-NeutrAvidin affinity binding, with evidence for some nonspecific physisorption, as the extent of liposome binding onto blocked NeutrAvidin surfaces was appreciably lower than for unblocked surfaces but not zero. Finally, consecutive addition of further NeutrAvidin and liposome layers enabled fabrication of multilayers, and this was clearly seen in AFM compressibility and fluorescent dye release measurements.