Self-assembling in surfactant aggregates: An alternative way to the realization of fluorescence chemosensors for Cu(II) ions

The self-assembling of the proper subunits of fluorescence chemosensors within surfactant aggregates in water allows the easy design, realization, and testing of new effective sensing systems. It was shown that a proper ligand and a fluorescent dye, once transferred from bulk water into an inert surfactant aggregate, are kept in such a close proximity that communication between the binding site and the signaling unit effectively occurs, thus avoiding the need for a covalent connection between the sensor's two units. To further simplify the sensing system and get rid of the inert surfactant which somehow implies the dilution of the sensor's active components in the aggregate, we have synthesized a series of lipophilic ligands for Cu(II) based on dipeptides GlyLys and GlyGlu functionalized with n-alkyl chains of different lengths at the N-terminus. These ligands are soluble in water and can form homoaggregates in the absence and in the presence of Cu(II) ions. Lipophilic fluorophores, like 8-anilino-naphthalensulfonic acid or Rhodamine 6G, are effectively bound into the aggregate pseudophase, and the binding of Cu(II) ions to the dipeptide units causes a strong fluorescence quenching. The sensor system is very sensitive to Cu(II) (concentrations in the submicromolar range are detected), is promptly reversible, and no interference is observed due to the presence of many metal ions. The sensitivity of the systems improves by decreasing the ligand concentration and (up to a point) the ligand's cmc.