Oriented Bacteriorhodopsin/Polycation Multilayers by Electrostatic Layer-by-Layer Assembly

The purple membrane (PM) containing bacteriorhodopsin (BR) is a promising biomaterial for many potential optical and optoelectronic device applications. Organized, heterogeneous assemblies using polycationic poly(dimethyldiallylammonium chloride) (PDAC) and PM fragments have been successfully constructed by the spontaneous alternating adsorption of PDAC and PM. The fabrication process of the multilayers was followed by UV−vis absorption spectroscopy and ellipsometry. The results indicate that the deposition process is linear and highly reproducible from layer to layer and that a monomolecular film of PM may be obtained in each PDAC/PM bilayer by controlling the adsorption time. Second harmonic generation measurements from the composite films gave a second-order susceptibility χ (2) of 8.1 × 10 - 9 esu and confirm that the PM fragments are arrayed with a high degree of orientation and acentric polar order in the films. Atomic force microscopy images provided the surface morphology of sequential layers of PDAC and PM fragments. Relative PDAC/PM bilayer thicknesses of 55 Å were observed, and the homogeneity of the layers was found to improve as the number of layers increased.