Plasma oxidized polyhydroxymethylsiloxane--a new smooth surface for supported lipid bilayer formation

A novel substrate for preparation of supported lipid bilayers (SLBs), smooth at the subnanometer scale and of variable thickness from ten to several hundred nanometers, was developed by surface oxidation of spin-coated poly(hydroxymethylsiloxane) (PHMS) films. The deposited polymeric thin films were modified by a combination of oxygen plasma and thermal treatment (PHMS(ox)), in order to convert the outermost surface layer of the polymer film to a stable SiO(2) film, suitable for SLB formation. The hydrophilic, SiO(2)-like surfaces were characterized by XPS, wetting angle, ellipsometry, and AFM. Lipid bilayers were formed on this surface using the well-known vesicle adsorption-rupture-fusion process, usually performed on glass or vapor-deposited SiO(2). Reproducible formation of homogeneous SLBs of different compositions (POPC, DOEPC, and POPC/DOPS) was demonstrated on the new SiO(2) surface by quartz crystal microbalance with dissipation (QCM-D), surface plasmon resonance (SPR), and optical reflectometry measurements. The SLB formation kinetics on the PHMS(ox)-coated sensors showed very similar characteristics, for all investigated PHMS thicknesses, as on reference sensors coated with vapor-deposited SiO(2). The good adhesive properties of the PHMS to gold allows for the preparation of thin PHMS(ox) layers compatible with SPR. The much smaller roughness at the nanometer scale of the PHMS(ox) surfaces, compared to standard vapor-deposited SiO(2)-coated sensors, makes them advantageous for AFM and optical experiments and promising for patterning. To benefit optical experiments with the PHMS(ox) surfaces, it was also investigated how the PHMS film thickness influences the SPR and reflectometry responses upon SLB formation.