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Bipolar amphiphiles belonging to a,w-bis(4-amidinophenoxy)alkanes self-assemble in neutral or alkaline aqueous solution on acid-functionalised surfaces to form pH-switchable layers, with the amphiphiles oriented nearly perpendicular to the substrate. Layer thicknesses estimated by in situ ellipsometry, surface plasmon spectroscopy (SPS) or neutron reflection measurements correlated, within a certain amphiphile size range, with the molecular length of the amphiphile. Above a certain chain length, the characterisation by infrared reflection absorption spectroscopy (IRAS) indicated a crystallinelike order of the layered amphiphiles and a tendancy for bilayer formation was observed. These layers were stable and charge selective towards plasma proteins and charged surfactants.The layer stability was strongly pH dependant and no self-assembly was observed in acidic solutions. By cycling the pH between 9 and 2 rapid disassembly and reassembly of the layers was thus observed. The pH-switchable assembly can be used to restore a chemically selective interface after binding of a target molecule, for enhancement of sensor responses and potentially for optoelectronic or molecular electronic applications.

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Fabrication of micro surface morphology of cell culture substrates is one of the current topics in biomaterials research. Recently we found that regular honeycomb structures with micrometer scale dimensions can be fabricated by simple casting of a dilute solution of amphiphilic polymers on solid substrates. The honeycomb films were applicable to the cell culture substrates, in which cell adhesive parts are much smaller than the lateral dimension of cells. This article describes the fabrication and characterization of thin polymer films which have micrometer scale weblike structure. The weblike structure is fabricated by a two-dimensional emulsion templating. The web-structured films were prepared by casting a water in oil (W/O) emulsion onto surfaces of glass, cleaved mica, and metal-coated glass. The films had lower regularity of surface morphology which indicated the polydisperse size distribution of water microspheres. The average diameter of the holes in the web structure was considerably influenced by the extent of coalescence among the water microspheres, which were dispersed in the emulsion. A surface topographical study of the cast films using atomic force microscopy (AFM) showed that the cast films have web structure with submicrometer height and width. The bottom of the hole was scratched using an AFM tip loaded with a constant force of 100 nN. This way we could measure the thickness of the polymer films to be 7 nm, which is thicker than the basal films of regular honeycomb films. Optical characterization using surface plasmon resonance spectroscopy (SPR-S) and surface plasmon resonance microscopy (SPR-M) gave further evidence for the existence of thin basal films on the web-structured films. Using these optical methods the thickness of the basal film could be estimated without any destruction of the cast films. The SPR-S measurement provided an average thickness of 14 nm for the basal films. Furthermore the SPR-M provided plasmon contrast images of the web-structured films, which revealed that the thickness of the basal films varies for each hole in the web structure. Consequently these surface analyses demonstrate that the cast films from W/O emulsion have a double-layered structure: web structure of aggregated polymer as the major surface morphology and thin films of a polymer as a basal structure which covers solid supports.

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