Elaboration and Characterization of Phospholipid Langmuir-Blodgett Films
In order to model biological membranes,DPPE(1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine) and DPPC(1,2-dihexadecanoyl-sn-glycero-3-phosphocholine) Langmuir-Blodgett (LB) films were deposited on hydrophilic mica and hydrophobic highly ordered pyrolitic graphite (HOPG), and subsequently characterized by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM was used to determine the topography and to monitor the stability of the deposited films in air and under water at varying experimental parameters. Angle-resolved XPS was used to measure the mean thickness and the orientation of the deposit. Monolayer transfer pressure was determined at 40 mN/m with molecular area at 0.42 ± 0.02 nm2 for DPPE and 0.55 ± 0.02 nm2 for DPPC. Nanometric resolution was achieved on DPPE crystal and LB films and allowed the determination of the corresponding lattice constants a = 0.64 ± 0.03 nm, b = 0.91 ± 0.07 nm, and c = 5.6 ± 0.2 nm and a = 0.68 ± 0.02 nm, b = 0.93 ± 0.05 nm, and c = 5.6 ± 0.2 nm, respectively. The DPPE monolayer on mica was measured to be 2.7 ± 0.2 nm thick and the surface was hydrophobic. The reorganization of DPPE bilayer when going from the water to the air was directly observed to form monolayer and trilayer domains. OnHOPG, the outermost layer of DPPE was partly hydrophilic and partly hydrophobic. Under the same conditions of deposition, DPPC formed a mixed structure of flat domains and vesicles. In water, monolayer aliphatic tail ending is less compact and presents a higher friction coefficient than polar heads as measured by lateral force AFM.