Ion-induced aggregation of conjugated polyelectrolytes studied by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is applied to investigate aggregation behavior of an anionic conjugated polyelectrolyte (CPE) featuring branched carboxylate groups (PPE-(d)CO2) upon the addition of different metal ions. FCS results reveal that monovalent metal ions (Na(+), K(+)) bind to the CPE chain but do not induce aggregation, and divalent metal ions (Ca(2+), Cu(2+)) bind and induce the formation of small aggregates. Iron cations (Fe(2+) and Fe(3+)) bind strongly and induce the formation of large CPE aggregates. This is believed to be because of the propensity of these metals to bind to six ligands and thus bridge two of the triacid units on adjacent PPE-(d)CO2 chains. A comparison between the Stern-Volmer (SV) fluorescence quenching and the relative diffusion time change from FCS demonstrates that the most efficient SV quenching is observed for the ions that give rise to large increases in FCS diffusion time, underscoring the importance of ion-induced aggregation in the amplified quenching effect.