Counterion Binding Dynamics Of A Polyelectrolyte

The dynamics of adsorption and desorption of counterions on a polyelectrolyte chain in aqueous solution is studied at a single molecular level. The investigation is through the process of fluorescence resonance energy transfer (FRET) between a fluorescence donor chemically attached to the chain end of a model polyanion (sodium polystyrene sulfonate, NaPSS) and a positively charged fluorescence acceptor (Atto 610), which also serves as the counterion probe in the solution. Dual-color fluorescence correlation spectroscopy (DC-FCS) is used and the results provide information on the rate constant for the counterion probe to adsorb onto and desorb from the charged main chain and multiple affecting factors. The desorption rate is constantly higher than the adsorption rate, making an overall balance towards desorption and leaving the main chain effectively charged. The adsorption rate increases linearly with the concentration of the counterion probes due to the reduced entropy penalty for adsorption, while the desorption rate remains unchanged. A lower desorption rate is found with the higher molecular weight sample and its adsorption rate shows a smaller change with the concentration, due to the less predominant entropy effect because of the higher surrounding counterion concentrations. Experiments with elevated salt concentrations (NaCl) demonstrate the adsorption competition between the sodium ion and the fluorescent probe and that the high salt level promotes more adsorption, making the chain less charged. Such a process induces a contraction of the chain from its original expanded conformation.