Diffusion of nanoparticles within a semidilute polyelectrolyte solution

We studied the diffusion of charged gold nanoparticles within a semidilute solution of weakly charged polyelectrolyte, polyacrylic acid (PAA) of high molecular weight (M-w = 10(6) g mol(-1)) by using fluorescence correlation spectroscopy (FCS). Nanoparticle size (d) was varied between 5 nm to 40 nm and PAA volume fraction (phi) in water ranged from about 8 phi* to 33 phi*, where phi* is the overlap volume fraction. The reduced diffusion coefficient - defined as -D/D-o, where D is the diffusion coefficient in PAA solution and D-o is that in neat water - has a weak dependence on the particle size. D follows a power law of the form similar to phi(-0.5), which can be explained by a mean-field hydrodynamic theory in porous medium. Additional, rheology measurements showed a zero shear rate viscosity and shear thinning, which are typical of high molecular weight polyelectrolytes.