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).  The measured FCS correlations of each nanoprobe showed a 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.   The concentration dependence of the nanoprobe diffusion 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 data with a stretched exponential [exp(-acⁿ)] where c denotes the PEG concentration, n is related to the solvent quality, and a is proportional to the hydrodynamic size of the diffusing nanoprobe.  We determined that n is close to 3/4, indicating that water solvent is good solvent and that a is proportional to the nanoprobe size.