Due to their similar coordination properties, discrimination of Cu + and Ag + by water-soluble luminescent probes is challenging. We have synthesized LCC4 Eu , an 18 amino acid cyclic peptide bearing a europium complex, that is able to bind one Cu + or Ag + ion by the side chains of two methionines, a histidine and a 3-(1-naphthyl)-Lalanine. In this system, the naphthyl moiety establishes a cation- interaction with these cations. It also acts as an antenna for the sensitization of Eu 3+ luminescence. Interestingly, when excited at 280 nm behaves as a turn-on probe for Ag + (+150 % Eu emission) and as a turn-off probe for Cu + (-50% Eu 3+ emission). Shifting the excitation wavelength to 305 nm makes the probe responsive to Ag + (+380% Eu 3+ emission) but not to Cu + or other physiological cations. Thus, LCC4 Eu is uniquely capable of discriminating Ag + from Cu +. A detailed spectroscopic characterization based on steady state and time-resolved measurements clearly demonstrates that Eu 3+ sensitization relies on electronic energy transfer from the naphthalene triplet state to the Eu 3+ excited states and that the cation- interaction lowers the energy of this triplet state by 700 cm-1 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 LCC4 Eu toward Ag + and Cu + .