Amphiphilic anthracyl crown ether. A Langmuir and Langmuir-Schaefer films study

As a potential material for use in optical fiber fluorescence sensors for rapid detection of saxitoxin, 9-(monoaza-18-crown-6-methyl)-10-hexadecylanthracene (CAC₁₆) was synthesized, and the interfacial and spectroscopic properties of the Langmuir monolayers and Langmuir-Schaefer films of CAC₁₆ were studied. The surface pressure-area and surface potential-area isotherms of CAC₁₆ on different subphases were obtained. An increased limiting molecular area was observed on a pH 2 subphase. In situ fluorescence emission spectra (λ_ex = 366 nm) of the CAC₁₆ monolayer showed a broad fluorescence band on a pH 2 subphase but none on a pure water subphase. Mixed monolayers of CAC₁₆/C₂₀ (arachidic acid) on a pure water subphase showed an increased fluorescence emission intensity of anthracene with an increase in the proportion of C₂₀. This suggests that the low fluorescence activity of the pure CAC₁₆ monolayer could be caused by self-quenching due to the high concentration of CAC₁₆ at the interface. In a mixed monolayer, C₂₀ acts as a two-dimensional solvent to dilute CAC₁₆ and diminish the self-quenching, thus recovering the fluorescence activity of CAC₁₆. When CAC₁₆ was mixed with C₁₈OH (stearyl alcohol), the monolayers showed no fluorescence signal, regardless of the C₁₈OH content. Analysis of the surface pressure-area isotherms showed that CAC₁₆ is not miscible with C₁₈OH. Langmuir-Schaefer films of CAC₁₆/C₂₀ showed better anthracene emission spectra than the monolayers at the air-water interface.