Abstract: Naphthalene and pyrene singly and doubly labeled polyelectrolytes were prepared by radical copolymerization of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) from 5% to 99% (molar fraction) and N, N-dimethylacrylamide with the label monomers. Fluorescence nonradiative energy transfer (NRET) was measured in NaCl and CaCl₂aqueous solutions up to the concentration of 6×10^-3mol/L. Intra- and inter-molecular NRET determined with doubly labeled samples and mixtures of singly labeled samples indicated that owing to the salt screening effect, increasing salt concentration enhanced the NRET and the enhancement of calcium cation was stronger than that of sodium cation. The normalized NRET E_n, defined as the NRET in salt solution relative to that in water for a given sample, showed that intramolecular E_ndecreased with the increase of charge density F_AMPSin the samples with a maximum at F_AMPS≈0. 45. In contrast, the intermodular E_nincreased with the increase of F_AMPS, suggesting that there were inhomogeneous intermolecular aggregates in polyelectrolyte solutions due to the condensed coun-terions even at a concentrations lower than the overlap threshold c^*.

Key words: Polyelectrolyte, Nonradiative energy transfer, Salt effect, Fluorescence label, Counterion con-densation