CONICET | Buscador de Institutos y Recursos Humanos

Electron paramagnetic resonance (EPR) spectroscopy was used to study the pore filling of macroporous silicon (MPS) with lipid vesicles (liposomes), added with a spin label. MPS samples were obtained by electrochemical anodization of p-type boron doped crystalline silicon wafers, in darkness. The resulting macroporous films have thickness of around 30 μm, and pore sizes of 1.0-1.2 μm. The porous surface was functionalized with tetrabutylammonium hydroxide, in order to turn hydrophilic the inner surface of the pore network by removing the hydrogen terminations. After the surface modification, the pore structure was filled with 100 nm unilamellar lipid vesicles (LUV) of the phospholipid dimyristoyl phosphatidylcholine (DMPC) added with 1% spin label 5-doxyl stearic acid (5-SASL), prepared in aqueous solution, PH 7,4, following usual protocols. EPR spectra were acquired in X band using a microwave power of 3.2 mW and a modulation of 2.5 G. The EPR spectrum of the nitroxide radical is sensitive to the orientation of B relative to the p orbital of the nitroxide, which for 5-SASL is parallel to the hydrocarbon chain. Unlike expected for liposomes, which have spherical symetry, the spectra for B parallel to the pore axis are different from those for B perpendicular to the axis. The spectra could be well simulated with an admixture of the spectrum of LUVs, plus a simulated spectrum corresponding to that of a cylindrical distribution of lipid bilayers. This means that cylindrical lipid bilayers were formed covering the inner surface of the pores, although some lipid vesicles remain inside the pores. A 2D photonic crystal made of MPS can be used as a sensor for changes in the dielectric function of the filling material. We propose the use of such devices to study in detail effects of confinement on DMPC phase transition, which occur near room temperature.