In-situ measurement of phospholipid nanodisk adhesion on a solid substrate using neutron reflectometry and atomic force microscopy

A substrate-supported phospholipid bilayer (SPB) is commonly used as a model bio-membranes on a solid substrate. The most common method to make SPBs is the adsorption followed by rupture of phospholipid vesicles on a substrate surface in contact with the vesicle solution. However, this method may present an applicability issue due to the limitation of combination of the phospholipids and substrates. On the other hand, adhesion of disklike micelles consisting of phospholipid, so-called bicelles, is a hopeful alternative method for making SPBs since they don't need the rupture process. The mechanism of the nanodisk adhesion on a substrate is, however, not fully understood yet. In this study, in-situ observation of the bicelle adhesion was performed using neutron reflectometry (NR), which can shed light on the depth profile of bilayer density at the interface between a substrate and bicelle solution. In addition, atomic force microscopy (AFM) was employed for the topographic imaging and force response measurement of the adhesion layer. These results presented that lamellae of phospholipid bilayers were aligned parallel to a surface of a bare Si substrate, whereas a fragile layer of unoriented phospholipid bilayers was formed on a NH2 coated Si substrate.