Jacek Lipkowski

AFM studies of solid-supported lipid bilayers formed at a Au(111) electrode surface using vesicle fusion and a combination of Langmuir-Blodgett and Langmuir-Schaefer techniques.

Atomic force microscopy (AFM) has been used to characterize the formation of a phospholipid bilayer composed of 1,2-dimyristyl-sn-glycero-3-phosphocholine (DMPC) at a Au(111) electrode surface. The bilayer was formed by one of two methods: fusion of lamellar vesicles or by the combination of Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) deposition. Results indicate that phospholipid vesicles rapidly adsorb and fuse to form a film at the electrode surface. The resulting film undergoes a very slow structural transformation until a characteristic corrugated phase is formed. Force-distance curve measurements reveal that the thickness of the corrugated phase is consistent with the thickness of a bilayer lipid membrane. The formation of the corrugated phase may be explained by considering the elastic properties of the film and taking into account spontaneous curvature induced by the asymmetric environment of the bilayer, in which one side faces the gold substrate and the other side faces the solution. The effect of temperature and electrode potential on the stability of the corrugated phase has also been described.

In situ IR reflectance absorption spectroscopy studies of pyridine adsorption at the Au(110) electrode surface

In situ subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been employed to study the adsorption of pyridine at the Au(110) electrode surface. The IR spectra provide direct spectroscopic evidence that pyridine molecules adsorb, bonded through the nitrogen atom to the Au(110) surface, in the whole range of electrode potentials. IR spectroscopy was further used to study the effect of the electrode potential on the tilt angle (angle between the C2 axis of the molecule and the direction normal to the surface) of the adsorbed molecules. The IR data consistently show that the tilt angle decreases progressively with increasing electrode potential (charge on the metal). The change of the tilt angle may be described in terms of the electric field–dipole interaction and is caused by the dielectric saturation phenomenon. At low charge density, where the field is small, the film of N-bonded pyridine molecules is less rigid and the molecules exhibit a waving motion. This motion is restrained significantly in the presence of a large electrostatic field (high charge density at the metal). The monolayer of adsorbed molecules can be considered as being effectively frozen at sufficiently high charge densities.

Electrochemical and PM-IRRAS a glycolipid-containing biomimetic membrane prepared using Langmuir-Blodgett/Langmuir-Schaefer deposition.

Differential capacitance, chronocoulometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to characterize the structure and orientation of a DMPC + cholesterol + GM 1 (60:30:10 mol %) bilayer supported at a Au(111) electrode surface prepared using combined Langmuir-Blodgett/Langmuir-Schaefer (LB/LS) deposition. The electrochemical measurements indicate that the incorporation of ganglioside GM 1 into the membrane significantly improves the quality of the bilayer, reflected in the very low capacitance value of approximately 0.8 microF cm (-2). An analysis of the infrared data suggests that the incorporation of the glycolipid into the membrane changes both the orientation of the lipid acyl chains in the membrane and the hydration of the membrane, particularly with respect to the interfacial region of the lipids.

Simulation of SNIFTIRS experiments

We present a simulation of subtractively normalized interfacial Fourier transform infrared spectra (SNIFTIRS) for the case of pyridine adsorbed at the Au(111) electrode. From experimental data and SNIFTIRS simulation the tilt angles can be evaluated as a function of the electrode potential for pyridine molecules adsorbed on terrace and defect sites, providing a detailed view of the site and potential dependent molecular orientation.

SHG Studies on Halide Adsorption at Au(111) Electrodes

Second-order optical nonlinear spectroscopy such as Second Harmonic Generation (SHG) is inherently sensitive to interfacial regions. Using our recently developed method of Interference SHG Anisotropy (ISHGA), the anisotropy of the SHG field E(2ω, ϕ)∝ ∑ n=0 m k n cos[n (ϕ+ζn)] = A+B cos(ϕ+α)+... can be measured (i.e., the variation of the 2ω-field generated during the rotation of the sample around the surface normal). This field can be separated into its various sources, denoted as A, B,..., permitting us to achieve information on the geometric and electronic structure of the interface and their changes upon potential and adsorption.