Langmuir monolayer and Langmuir-Blodgett film studies of an amphiphilic coumaryl crown ether

As a potential compound for use in optical fiber fluorescence sensor for rapid detection of saxitoxin, 4-(monoaza-18-crown-6-methyl)-7-octadecanoylaminocoumarin (ODAC) was synthesized and the interfacial and spectroscopic properties of its Langmuir monolayers and Langmuir-Blodgett (LB) films were studied. The surface pressure- and surface potential-area isotherms were obtained on a pure water subphase. In situ fluorescence of the ODAC monolayer at the air-water interface showed a fluorescence band centered around 425 nm (lambda(ex) = 332 nm) decreasing in intensity with increasing surface pressure. This observation is due to the self-quenching of the ODAC molecules as the surface concentration per unit area increases and aggregates are formed. To reduce the aggregation phenomenon, mixed monolayers of ODAC with C(18)GlyGlyAlaGlyNH(2) peptidolipid (PL) (PL:ODAC, 20:1 and 100:1) were used to dilute the fluorophore molecules at the air-water interface and diminish the self-quenching. LB films of pure ODAC and PL: ODAC mixed monolayers (100:1) were prepared and tested on saxitoxin dissolved in a phosphate buffer (pH 7.4). Each LB film showed fluorescence increase in the presence of saxitoxin.