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 wit hin surfactant aggregates in water allows the easy design, realization, and testing of new effective sensing systems. It was shown that a proper ligan d and a fluorescent dye, once transferred from bulk water into an inert sur factant aggregate, are kept in such a close proximity that communication be tween the binding site and the signaling unit effectively occurs, thus avoi ding 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 whi ch somehow implies the dilution of the sensor's active components in the ag gregate, we have synthesized a series of lipophilic ligands for Cu(II) base d on dipeptides GlyLys and GlyGlu functionalized with n-alkyl chains of dif ferent lengths at the N-terminus. These ligands are soluble in water and ca n form homoaggregates in the absence and in the presence of Cu(II) ions. Li pophilic fluorophores, like 8-anilino-naphthalensulfonic acid or Rhodamine 6G, are effectively bound into the aggregate pseudophase, and the binding o f Cu(II) ions to the dipeptide units causes a strong fluorescence quenching . The sensor system is very sensitive to Cu(II) (concentrations in the subm icromolar 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.