Luminescent sensor for Cd2+, Hg2+ and Ag+ in water based on a sulphur-containing receptor: quantitative binding-softness relationship

A water-soluble, high-output fluorescent sensor, based on a lumazine ligand with a thiophene substituent for Cd2+, Hg2+ and Ag+ metal ions, is reported. The sensor displays fluorescence enhancement upon Cd2+ binding (log beta=2.79 +/- 0.08) and fluorescence quenching by chelating with Ag+ and Hg2+ (log beta = 4.31 +/- 0.15 and 5.42 +/- 0.1, respectively). The mechanism of quenching is static and occurs by formation of a ground-state non-fluorescent complex followed by rapid intersystem crossing. The value of the Stern-Volmer quenching rate constant (k(q)) by Ag+ ions is close to 6.71 x 10(12) Mol/L/s at 298 K. The thermodynamic parameters (triangle G, triangle H and triangle S) were also evaluated and indicated that the complexation process is spontaneous, exothermic and entropically favourable. The quantitative linear relationship between the softness values of Klopman (sigma(K)) or Ahrland (sigma(A)) and the experimental binding constants (beta) being in the order of Hg2+ > Ag+ > Cd2+ suggests that soft-soft interactions are the key for the observed sensitivity and selectivity in the presence of other metal ions, such as: Pb2+, Ni2+, Mn2+, Cu2+, Co2+, Zn2+ and Mg2+ ions. Copyright (c) 2008 John Wiley & Sons, Ltd.