A convection-free monolayer trough for fluorescence microscopic monitoring of ultrathin organic films

Abstract

A new circular trough has been developed for fluorescence microscopy of ultrathin organic films at an air-water interface or after adsorption to solid supports. The trough includes a transparent bottom and radial motorized barriers. A novel design for leak-free barriers has been introduced by utilizing Kalrez perfluoroelastomer. Very shallow depths of water (less than 0.5 mm) can be utilized, thus minimizing convection and flow of the monolayer. The entire area of the monolayer is accessible for scanning, thereby permitting tracking of distinct features in the monolayer as the area per molecule is changed. The trough is easily adaptable for mounting on the stage of any inverted or upright microscope, and easily demountable for complete cleaning. The monolayer is deposited on solid supports by a horizontal deposition technique. The special design of the trough permits visualization of the film by optical microscopy throughout the transfer process. Defects in the film can thus be monitored during and prior to the transfer process. With this trough we have observed by fluorescence (1) two-dimensional ordering in dipalmitoylphosphatidylcholine monolayers doped with 1 mole% of the fluorescent lipid probe N-(7-nitro-2,1,3-benzoxadiazol-4-yl) phosphatidylethanolamine and (2) irregular domain structures resulting from the presence of impurities and pressure gradients. The lateral diffusion coefficient of fluorescent lipid probes in the monolayer has been measured by photobleaching recovery at a gas-liquid interface and after transfer to solid supports. The results provide evidence of a major structural modification in the monolayer after the transfer process.