Transduction of Analytical Signals by Supramolecular Assemblies of Amphiphiles Containing Heterogeneously Distributed Fluorophores

We report on the development and characterization of supramolecular assemblies of amphiphiles which are able to transduce changes of pH into optical signals. A series of assemblies have been investigated, including vesicular bilayer lipid membranes, monolayer films of fatty acids or phospholipids at the air-water interface, and covalently immobilized alkylsilane monolayers. In all cases, these membrane assemblies were formed from mixtures of species having zwitterionic and ionizable headgroups. In addition, a small amount of the fluorescently labeled phospholipid nitrobenzoxadiazole phosphatidylethanolamine (1 - 3%) was incorporated into the membranes. This resulted in systems which were able to transduce alterations of pH into changes in fluorescence intensity. Investigations into the physical mechanism responsible for alterations of fluorescence intensity from such membranes suggested that the nitro-benzoxadiazole phosphatidylethanolamine (NBD-PE) signal was based on changes of self-quenching of the probe which occurred when the physical and electrostatic characteristics of the membranes were altered. Useful signals were obtained only when microscopic phase separation existed within the membranes, indicating that careful design of the membrane was required to optimize the signal generated.