Fluorescence correlation spectroscopy studies of diffusion of a weak polyelectrolyte in aqueous solutions

We apply fluorescent correlation spectroscopy (FCS) to investigate solution dynamics of a synthetic polyelectrolyte, i.e., a weak polycarboxylic acid in aqueous solutions. The technique brings single molecule sensitivity and molecular specificity to dynamic measurements of polyelectrolyte solutions. Translational diffusion of Alexa-labeled poly(methacrylic acid), PMAA^*, chains was studied in very dilute, $10−4 mg/ml,$ solutions as a function of solution $pH$ and ionic strength. The observed changes in diffusion coefficients were consistent with about twofold expansion of PMAA^* coils when $pH$ was changed from 5 to 8, and with chain contraction for alkaline metal ion concentrations from 0.01 to 0.1 M. The dependence of the hydrodynamic size of PMAA^* chains on the counterion type followed the sequence: $Li+>Na+≈Cs+>K+.$ The dependence of translational diffusion on polyacid concentration was weak at the low concentration limit, but chain motions were significantly slower at higher polymer concentrations when PMAA chains overlapped. Finally, measurements of dynamics of PMAA^* chains in “salt-free” solutions showed that self-diffusion of PMAA^* chains significantly slowed down when PMAA concentration was increased, probably reflecting the sensitivity of PMAA^* translational motions to the onset of interchain domain formation. These results illustrate the utility of the FCS technique for studying hydrodynamic sizes of polyelectrolyte coils in response to variation in solution $pH$ or concentration of salt and polyelectrolytes. They also suggest that FCS will be a promising technique for selective observation of the dynamics of polyelectrolyte components in complex polymer mixtures.