Bioinspired Luminescent Europium-Based Probe Capable of Discrimination between Ag+ and Cu.

Due to their similar coordination properties, discrimination of Cu+ and Ag+ by water-soluble luminescent probes is challenging. We have synthesized LCC4Eu, an 18 amino acid cyclic peptide bearing a europium complex, which is able to bind one Cu+ or Ag+ ion by the side chains of two methionines, a histidine and a 3-(1-naphthyl)-l-alanine. In this system, the naphthyl moiety establishes a cation-π interaction with these cations. It also acts as an antenna for the sensitization of Eu3+ luminescence. Interestingly, when excited at 280 nm, LCC4Eu behaves as a turn-on probe for Ag+ (+150% Eu emission) and as a turn-off probe for Cu+ (-50% Eu3+ emission). Shifting the excitation wavelength to 305 nm makes the probe responsive to Ag+ (+380% Eu3+ emission) but not to Cu+ or other physiological cations. Thus, LCC4Eu is uniquely capable of discriminating Ag+ from Cu+. A detailed spectroscopic characterization based on steady-state and time-resolved measurements clearly demonstrates that Eu3+ sensitization relies on electronic energy transfer from the naphthalene triplet state to the Eu3+ excited states and that the cation-π interaction lowers the energy of this triplet state by 700 and 2400 cm-1 for Ag+ and Cu+, respectively. Spectroscopic data point to a modulation of the efficiency of the electronic energy transfer caused by the differential red shift of the naphthalene triplet, deciphering the differential luminescence response of LCC4Eu toward Ag+ and Cu+.