Physically Self-Assembled Monolayers (PSAMs) of Lecithin Lipids at Hydrophilic Silicon Oxide Interfaces

Abstract

A new method of making physically self-assembled monolayers (PSAMs) on hydrophilic solid surfaces is presented. This method uses a mixture of a nonpolar solvent, such as hexane, and a strong polar solvent, such as ethanol, to dissolve the lipids. The deposition of two lecithin lipids, dipalmitoylphosphatidylcholine (DPPC) and dilauroylphosphatidylcholine (DLPC), has been studied. These lipids physically self-assemble, or adsorb, onto hydrophilic silicon oxide/silicon surfaces when such surfaces are in contact with the lipid solution. The adsorbed layers were probed with ex-situ attenuated total reflection infrared (ATR-IR) spectroscopy, ellipsometry, contact angle measurements, and atomic force microscopy (AFM). The thicknesses of the adsorbed monolayers are about 2.8 +/- 0.2 nm for DPPC and 2.0 +/- 0.2 nm for DLPC, as determined by ellipsometry and AFM. Smooth, uniform monolayers of controlled surface density are formed. The surface density of adsorbed layers is comparable to those of close-packed lipid monolayers, as calculated from the ellipsometry and ATR-IR results. Producing controlled-thickness monolayers has applications in boundary lubrication, biomaterials, sensor technologies, and electronics. The method can be used for depositing many biological surfactants or lipids without the need to modify these surfactants chemically to form chemical bonds with the surfaces, as required by the usual chemical SAMs. Moreover, the new method has several advantages compared to the Langmuir-Blodgett (LB) method.

Surface modification of glass plates and silica particles by phospholipid adsorption

“…Curiously enough, the hydrophobized glass surface still exhibits significant acid-base interactions (Fig. 8) which means that the n-tetradecane layer is probably not uniform but forms nanodroplets thus leaving patches of bare glass surface [18]. If the layer were compact, both c À S and c þ S interactions should be negligible, which is not the case.…”

“…In the presence of a solid substrate a variety of configurations can be envisaged, depending on the pH of the medium (controlling in turn the charge of the particles and the amount of positive and negative groups ionized in the headgroup), and also on the hydrophobicity/ hydrophilicity of the surface. This balance determines to what extent the non-polar tails are horizontally or vertically arranged with the molecules polar heads directed either towards or away from the solid surface [18].…”

“…Investigation appears less abundant in the case of adsorption from solution of phospholipids onto solid surfaces (see, however [5,11,15,18,[28][29][30][31][32][33]). Jackson et al [29] studied adsorption of phospholipids onto glass beads from aqueous dispersions of vesicles formed from DPPC and its mixtures with phosphatidylinositol (PI) at different temperatures.…”

“…This explains the significant adsorption in spite of the fact that DPPC solubility in water is extremely low ((10 À6 wt.%), and a chain-melting transition temperature is 41°C, above which the fluid multilamellar liposomes form. Below this temperature the liposomes are ''frozen" [18]. A procedure for obtaining physically self-assembled monolayers (PSAMs) on hydrophilic Si/SiO 2 solid surface was presented by Phang and Franses [18].…”

“…Below this temperature the liposomes are ''frozen" [18]. A procedure for obtaining physically self-assembled monolayers (PSAMs) on hydrophilic Si/SiO 2 solid surface was presented by Phang and Franses [18]. They used non-polar hexane, or its mixture with polar ethanol to dissolve the phospholipid (DPPC or DLPC).…”