Vibrational characterization of a planar-supported model bilayer system utilizing surface-enhanced Raman scattering (SERS) and infrared reflection-absorption spectroscopy (IRRAS)

Abstract A combined spectroscopic method utilizing surface-enhanced Raman scattering (SERS) and infrared reflection–absorption spectroscopy (IRRAS) has been employed to study the structural complexities of a model bilayer system. These two vibrational spectroscopic techniques offer complementary information, and when used in concert, present an extremely powerful tool for the structural elucidation of ultrathin monomolecular assemblies. The success of this analysis approach is based on the use of a unique metal film planar-support that has a total thickness that is amenable for IRRAS analysis while providing an ideal surface conducive for surface-enhanced Raman scattering experiments. Therefore, the same thin film system can be monitored by both techniques. In this study, these metal planar-supports, composed of optimized vapor-deposited Ag films, were prepared and monitored following a procedure previously developed [J. Phys. Chem. B, 106, 2002, 8747]. Monitoring these films is essential to ensure a reproducible rough surface morphology that gives rise to the greatest surface enhancements for SERS. The model bilayer systems studied are comprised of (1) an inner monolayer composed of long hydrocarbon chain alkanethiols, and (2) an outer monolayer composed of the phospholipid, 1,2-[d62]-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC-d62). Two types of monomolecular architecture techniques are employed to form these bilayers—self-assembly and Langmuir–Blodgett transfer. The first step in the preparation of these films involves the self-assembly of the alkanethiol onto the Ag surface. The formation of this stable chemisorbed monolayer produces a hydrophobic surface that acts as the driving force for the organization of the lipid monolayer. The hydrophobic surface comprised of the long alkyl chains of the alkanethiol forces the lipid to transfer alkyl chains down leaving the polar headgroup portion of the lipid molecule exposed. Deuteration of one of the phospholipid layer is performed to allow each monomolecular layer of the bilayer to be spectroscopically observed separately. The Raman experiments utilize a unique fiber-optic interface that provides high collection efficiency and sensitivity for monolayer samples. From the dual spectroscopic analysis performed on this model bilayer system, it appears that the inner 1-dodecanethiol monolayer undergoes a slight disordering upon addition of the outer phospholipid monolayer. This disordering is observed with spectral intensity changes for the ν(C–S) gauche (G) and trans (T) bands at 633 and 705 cm−1, respectively, the ν(C–C) stretches between 1060 and 1130 cm−1, and the vibrations associated with the terminal methyl group, ν(CH3), based on the SERS and IRRAS analysis. Even though the inner layer does undergo this slight conformational change, it appears to maintain its structure as an overall ordered assembly. The outer DPPC-d62 monolayer was transferred in the gel state to the alkanethiol surface and maintains its high degree of conformational order after the completion of the transfer. Spectral data as well as calculated spectral band intensity ratios are presented that show the power of this dual spectroscopic approach to the structural determination of a model bilayer system.