Analysis of fluorescence correlation spectroscopy & fluorescence anisotropy measurements of fluorescent nanoprobes in linear polymer solutions
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Abstract
We combine fluorescence correlation spectroscopy (FCS) , fluorescence anisotropy (FA) and fluorescence imaging microscopy methods to measure the rotational diffusion and the translational diffusion of fluorophores mixed in non-fluorescent –hence “invisible”- aqueous Linear polymers solutions under thermal fluctuations.
We measured changes of the emission spectrum, the lifetime, and the apparent rotational and translational diffusion coefficients of the fluorophores with systematic increase of the polymer concentration up to 1200 mg/ml for PEG and up to 120 mg/ml for PVA, at room temperature. The spectrum and the lifetime appear to be insignificantly altered by the polymer solutions. The fluorescence correlation functions can be readily fit with the expression describing normal particle diffusion. We then determine changes of the apparent rotational and translational diffusion coefficients with systematic increase of polymer concentration. Notably, the changes cannot be accounted for by the corresponding changes of the bulk viscosity of the linear polymer solutions as would be suggested by the Stokes-Einstein relations for both diffusion coefficients.
Instead, we analyze the data with the entropic model proposed by de-Gennes and fit each set of data with a stretched exponential [exp(-αcn)] with n being related to the quality of the solvent.
The fits yield n-value close to 3/4, suggesting a good behavior of the host polymer-water system. Moreover, the α-value for translation is similar to that of rotation, indicating similar local entropic effects on the rotation and translation, which is predicted by the model.