|
We labeled poly(vinyl) alcohol (PVA, Mw ≈ 85 kDa) linear chains with 5-([4,6-Dichlorotriazin-2-yl]amino) fluorescein hydrochloride and measured their translational diffusion within similar non-fluorescent PVA solutions using fluorescence correlation spectroscopy (FCS). We found that the measured correlation functions could be readily fit with an expression derived for a freely diffusive nanoprobe, allowing us to determine changes of apparent diffusion coefficients with changes of PVA concentration. The data indicate slowing down of the diffusion of the fluorescent PVA as the surrounding PVA concentration is increased. However, the changes of the diffusion of the labeled chains cannot be accounted for by corresponding changes of solution viscosity. Instead, we use an entropic-based model suggested by de Gennes and his collaborators, and fit the data with a stretched exponential [exp(-αcn )] with c denoting the PVA concentration, n being related to the solvent quality, and α being a prefactor that is predicted to depend on probe size. We determined n = 0.75, suggesting that the host solvent, water, is a good solvent for the PVA as this value is close to the theoretical value n = 3/4. This result is similar to other measurements for different nanoprobes such as rhodamine 6G (n = 0.77) and phycoerythrin (n = 0.84), as previously reported (Michelman-Ribeiro, A., et al., Biomacromolecules 8, 1595-1600 (2007)). Further, we tested the α-dependence on probe size by collecting FCS data for various other nanoprobes, and found a systematic linear increase with size for globular nanoprobes which, however, differs from that of the PVA chains. That is, the hydrodynamic diameter of the non-globular, linear PVA chains appears not to be the determinant size for their diffusion within the polymeric PVA solution, indicating possible structural reconfiguration of the flexible, moving chains. These results are consistent with the proposed entropic-based model devised by de Gennes et al., and demonstrate how diffusing nanoprobes –globular or linear- can probe the quality of a host polymeric system.
|
