Reexamination of the monolayer properties of a fluorescein amphiphile in Langmuir and Langmuir-Schaefer films

We herein report a reexamination of Langmuir and Langmuir-Schaefer films of (5-octadecanoylamino)-fluorescein (ODFL) prepared from three different spr eading solvents: ethanol (EtOH), CHCl3/EtOH (9:1, v/v), and CHCl3/trifluoro acetic acid (9:1, v/v). It was found that the limiting molecular area of th e ODFL monolayer is strongly spreading solvent-dependent, ranging from 15 t o 64 Angstrom (2)molecule(-1). These results strongly disgree with reported results from Tsukanova et al. (Langmuir 2000, 16, 1167) of the same molecu le by using ethanol as spreading solvent. Our study also shows that this fl uorescein amphiphile significantly aggregates at the air-water interface. T he UV-vis spectra of the monolayer are redshifted about 20 nm compared to t hat in the solution. Fluorescence quenching was observed from both a pure O DFL monolayer and a monolayer of ODFL mixed with stearic acid. However, flu orescence emission spectra were readily observed from mixed monolayers of O DFL with three synthetic peptide lipid molecules. This study shows that one should be extremely cautious when mixing fluorescence lipids as probes for Langmuir and Langmuir-Blodgett film investigations.