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 theODACmonolayer at the air-water interface showed a fluorescence band centered around 425 nm (λ_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 C₁₈GlyGlyAlaGlyNH₂ 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.