Potential Controls the Interaction of Liposomes with Octadecanol-Modified Au Electrodes: An in Situ AFM Study

Abstract

The formation of supported lipid bilayers using liposomes requires interaction with the solid surface, rupture of the liposome, and spreading to cover the surface with a lipid bilayer. This can result in a less-than-uniform coating of the solid surface. Presented is a method that uses the electrochemical poration of an adsorbed lipid-like layer on a Au electrode to control the interaction of 100 nm DOPC liposomes. An octadecanol-coated Au-on-mica surface was imaged using tapping-mode AFM during the application of potential in the presence or absence of liposomes. When the substrate potential was made negative enough, defects formed in the adsorbed layer and new taller features were observed. More features were observed and existing features increased in size with time spent at this negative poration potential. The new features were 1.8-2.0 nm higher than the octadecanol-coated gold surface, half the thickness of a DOPC bilayer. These features were not observed in the absence of liposomes when undergoing the same potential perturbation. In the presence of liposomes, the application of a poration potential was needed to initiate the formation of these taller features. Once the applied potential was removed, the features stopped growing and no new regions were observed. The size of these new regions was consistent with the footprint of a flattened 100 nm liposome. It is speculated that the DOPC liposomes were able to interact with the defects and became soluble in the octadecanol, creating a taller region that was limited in size to the liposome that adsorbed and became incorporated. This AFM study confirms previous in situ fluorescence measurements of the same system and illustrates the use of a potential perturbation to control the formation of these regions of increased DOPC content.

Selective Electrochemical Bleaching of the Outer Leaflet of Fluorescently Labeled Giant Liposomes

“…More recently,t he same group combined electrochemical measurements with in situ fluorescence imaging to monitort he poration/absorption/ incorporation process of liposomes (100 nm in diameter), made of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) phospholipids,i nto af luorescent adsorbed monolayer of octadecanol-covered surface. [16] Fluorescent redox probes can be also trapped in biomolecular architectures such as liposomes that are used as biomembrane and protocell models. [17,18] On the other hand, liposomes with diameters of af ew micrometers and above are particularly attractive systems due to their accessibilityt oopticalmicroscopy and various micromanipulation techniques.…”

“…A fluorescent signal is therefore observed only when the adsorbed molecule becomes desorbed and separated from the electrode surface. More recently, the same group combined electrochemical measurements with in situ fluorescence imaging to monitor the poration/absorption/incorporation process of liposomes (100 nm in diameter), made of 1,2‐dioleoyl‐ sn ‐glycero‐3‐phosphocholine (DOPC) phospholipids, into a fluorescent adsorbed monolayer of octadecanol‐covered surface …”