Nanoprobe Diffusion in Concentrated Polyethylene Glycol Solutions *

We applied fluorescence correlation spectroscopy (FCS) to measure changes of the translational diffusion of several nanoprobes (Alexa488, fluorescein, GFP proteins) dispersed in Polyethylene Glycol (PEG) solutions prepared at different concentrations (up 900 mg/ml). We found that the measured FCS correlations of each nanoprobe showed systematic and uniform shift to longer delay times, indicating slowing down of the diffusion.  Further, the measured FCS correlations could be readily fit with the closed-form expression describing freely diffusing nanoparticles.   Remarkably, changes of the diffusion due to PEG concentration could not be accounted for by the corresponding increase of the bulk viscosity of the PEG solution as would be suggested by the Stokes-Einstein relation. Instead, we fitted the concentration dependence of the diffusion coefficient with a stretched exponential [exp(-acⁿ)] where cdenotes the PEG concentration, nis related to the solvent quality, and ais proportional to the hydrodynamic size of the diffusing nanoprobe.  We determined that nis close to 3/4, indicating that water solvent is good solvent, and that ais proportional to the nanoprobe size.