Spectroscopic Studies of Competitive Binding of Thallium and Alkaline-Earth Metal Cations onto Poly(sodium acrylate-co-acrylamide) Tagged with Optical Probes

Competitive binding between Tl + and alkaline-earth metal cations (Mg 2+, Ca 2+, and Ba 2+ ) onto copolymers of sodium acrylate and acrylamide with varying charge densities (0.94 < ξ < 2.5) was investigated by fluorescence and absorption spectroscopy with use of phenanthrene (Phen) and a pH-sensitive merocyanine dye (Mc) as optical reporters labeled on the copolymers. Fluorescence from the Phen label on the copolymer with ξ = 2.5 was efficiently quenched by Tl + because of the condensation of Tl + onto the copolymer, but the quenching was strongly suppressed by the addition of the divalent cations. There are two components in the fluorescence decay of the copolymer with ξ = 2.5 in the presence of Tl +: fast and slow decay components attributed to the quenching by condensed and atmospheric Tl + ions, respectively. The addition of the divalent cations led to a marked decrease in the component of the fast decay, while the lifetime for the fast decay remained unchanged. Conversely, the lifetime and its component for the slow decay increased with increasing concentration of added divalent cations. Both the steady-state and time-dependent fluorescence data indicate that condensed Tl + ions are completely replaced by the added alkaline-earth metal cations even at low concentrations (on the order of 10 -5 M) due to their highly preferential binding on the copolymer. In contrast, for the copolymer with ξ = 0.94 (onto which no Tl + condensation occurs but the divalent cations are condensed to decrease the charge density to ξ = 0.5), the divalent cations showed a much smaller suppressive effect on the fluorescence quenching by Tl + . The highly preferential binding of alkaline-earth metal cations on the copolymers was also shown by spectroscopic behavior of the Mc label. The p K obs for the acid−base equilibrium for the Mc label was decreased by added divalent cations but not affected at all by added Tl +, indicating that Tl + is bound onto the polyanion less intimately than are the alkaline-earth metal cations. There was no significant difference between Tl + and Na + in the effect on the p K obs, although Tl + was shown, in a previous study, to be bound onto the polyanions in preference to Na +; the p K obs does not distinguish the bound Tl + and Na + species, but fluorescence of the Phen label does.