Self-Assembled and Langmuir-Blodgett Organic Thin Films As Functional Materials
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Self-Assembled and Langmuir-Blodgett Organic Thin Films As Functional Materials Deborah H. Charych and Mark D. Bednarski
Introduction Biological membranes provide numerous functions for the survival of cells, ranging from molecular recognition to signal transduction and energy conversion. The spacial organization of proteins, enzymes, glycoproteins, and glycolipids in the membrane is provided by the lipid bilayer matrix. The lipids of the membrane are small molecules which have the common characteristic of having both a hydrophilic and a hydrophobic moiety. In aqueous media, they are entropically driven to selforganize in bimolecular sheets (see the article by Alper in this issue). These sheets, in addition to providing a fluid matrix for a wide variety of biomolecules, also serve to impede the flow of molecules across the membrane. The idea of using the biological membrane as a model system for organizing molecules forms the basis of the relatively new and multidisciplinary field of surface and materials chemistry. Systems based on this model are constructed primarily by two types of wet chemical methods of film deposition: (1) self-organization on a surface from a solution of randomly oriented molecules1-2 (often referred to as self-assembled monolayers or SAM in the literature), and (2) by the building up of monolayer or multilayer assemblies using Langmuir-Blodgett techniques (LB films).13'4 These well-defined surfaces have been applied in chemical sensing,56 tribology,710 MRS BULLETIN/NOVEMBER 1992
photolithography1114 non-linear optics,1516 and implant surface modification.1718 In addition, these surfaces provide model systems for fundamental studies in electron transfer dynamics,19 membrane lateral diffusion,20 and molecular recognition.21,22 In this report, we will discuss how such membrane-based organized assemblies are constructed, and describe their applications as functional materials in five areas of research: electron beam lithography, lithographically resolved chemical synthesis on surfaces, lubrication, coatings for implant materials, and chemical sensing devices. Complete reviews on the technological applications of SAM and LB films can also be found in References 1-4.
Formation of Molecular Assemblies Langmuir-Blodgett and Self-Assembly Methods The Langmuir-Blodgett (LB) technique was first reported by Katherine Blodgett in 1935.3 LB films are formed by spreading a solution of the film-forming molecule onto the clean water surface contained in a Langmuir trough (Figure 1). On evaporation of the spreading solvent (usually chloroform or hexane), the insoluble film on the water surface can be compressed by a moveable barrier which rides on the Langmuir trough, Figure la, b. The position of the barrier determines the surface concentration (molecules/area) of the
lipid, which generally determines the molecular phase. Depending on the chemical nature of the lipid, it is possible to observe gaseous, liquid, and solid phases of the monolayer. Using a surface balance, simultaneous recording of the surf
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