PM IRRAS Investigation of Thin Silica Films Deposited on Gold. Part 1. Theory and Proof of Concept

International audiencePolarization modulation infrared reflection absorption spectroscopy (PM IRRAS) was successfully used for the first time to characterize an optically transparent thin oxide film. SiO2 layers of 7 nm thickness were synthesized by plasma enhanced chemical vapor deposition (PECVD) on 200 nm thick gold covered glass slides. Despite the fact that silica is transparent and absorptive to IR radiation when deposited in the form of thin films on a gold surface, it preserves the high metallic reflectivity for the IR light. At grazing angles of incidence of the IR beam, the enhancement of the normal component of the electric field at the interface is comparable to that of Au alone. In addition, the analysis of the structure of a 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid monolayer deposited using the Langmuir−Blodgett technique is demonstrated

Polarization Modulation Infrared Reflection Absorption Spectroscopy Investigations of Thin Silica Films Deposited on Gold. 2. Structural Analysis of a 1,2-Dimyristoyl-<i>sn</i>-glycero-3-phosphocholine Bilayer

International audienceIn this paper we report on the structural analysis of bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) using polarization modulation infrared reflection absorption spectroscopy (PM IRRAS). The lipid bilayers were formed on SiO2|Au and Au surfaces using the Langmuir−Blodgett and Langmuir−Schaeffer techniques. As we showed in part 1 (Zawisza, I.; Wittstock, G.; Boukherroub, R.; Szunertis, S. Langmuir 2007, 23, 9303−9309), SiO2 layers of 7 nm thickness, synthesized by plasma-enhanced chemical vapor deposition on 200 nm thick gold covered glass slides, allow PM IRRAS investigations. Only minor changes in the order and structure of the lipid bilayer are observed when deposited on SiO2|Au and Au surfaces. The choline moiety in the leaflet directed toward the SiO2 surface exists in trans conformation and shows a tilt of 28° with the surface normal of the CN bond. On the silica surface in the second leaflet directed toward air and in two layers deposited on the Au surface, trans and gauche isomers of the choline moiety are present and the tilt of the CN bond increases to 55° with respect to the surface normal. The order and molecular orientation in the DMPC bilayers on SiO2 and Au surfaces are not affected by time. The analysis of the phosphate stretching mode on the Au surface shows slight dehydration of this group and reorientation of the phosphate moiety

Application of Thin Titanium/Titanium Oxide Layers Deposited on Gold for Infrared Reflection Absorption Spectroscopy: Structural Studies of Lipid Bilayers

International audienceUltrathin titanium layers when deposited on the surface of gold can be successfully applied for infrared reflection absorption spectroscopy (IRRAS) investigations. It was shown that the reflectivity, the phase shift, and the mean square electric field of the p- and s-polarized IR radiation in up to 20 nm thick titanium layers covered with a 3−4 nm thick layer of native oxide are comparable to those of the air/gold interface. The surface selection rule is fulfilled. Thus, qualitative and quantitative analysis of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers transferred in liquid expanded (LE) and liquid condensed (LC) states can be performed. Differences are found in the hydration state and molecular arrangement of the two investigated bilayers. In the DMPC bilayer in the LE state, the C−N bond in the positively charged choline moiety is inclined by ~70° toward the surface of the negatively charged titanium substrate. In the phosphate moiety, the in-plane vector of the O−P−O group makes a small angle of ~15° to the surface normal. This open structure of the lipid molecule corresponds to the B crystal structure of the DMPC molecule and provides space for strong hydration of the polar headgroup. In the DMPC bilayer in the LC state, the intermolecular distances are reduced; the C−N bond of the choline group makes a smaller angle to the surface normal, and the in-plane vector of the O−P−O group in the phosphate moiety displays a larger tilt. The degree of hydration is reduced. The arrangement of the polar headgroup region corresponds to the A crystal structure of the DMPC molecule