Development of fluorescent film sensors for the detection of divalent copper

Monolayers of several peptide lipids at air-water and air-solid interfaces were prepared using Langmuir and Langmuir-Blodgett (LB) film techniques, and tested as fluorescent sensors for copper ions in aqueous phase. In one method, both the ionophore and the fluorophore were in the same molecule (lipid A), so intramolecular interaction was responsible for the fluorescence quenching of monolayers of this lipid. In the other method, ionophore and fluorophore were located on two different molecules (lipids B and C) so the intramolecular coupling does not exist; instead the fluorescence quenching was realized by a through-space interaction mechanism. Several experimental techniques, including π-A isotherm, epifluorescence microscopy, and absorption and emission spectroscopies were used to study the different characteristics of copper ion effect on the properties of the lipid monolayers. Additionally, the fluorescence quenching properties of the Langmuir monolayers were found to be transferred to the one-layer LB films. On LB films, the fluorescence response presented a clear selectivity for copper ions in comparison with several other transition metal ions. Further, an excellent reversibility was observed: the fluorescence was switched OFF by immersing the solid substrate in copper ion solution and ON by washing with HCl solution. The intermolecular approach used here seems to be a very flexible and general method to design surface-oriented fluorescent sensors to meet different analytic purposes.