A Realistic Diffusion Model for Ultrathin Polyelectrolyte Films

In order to obtain information on transport processes in ultrathin films of polyelectrolytes, we studied the time dependence of the fluorescence of a dye (FITC) attached to a polymer and embedded in a defined, variable depth inside the film by total internal reflection fluorescence (TIRF) spectroscopy. Adding a rhodamine solution to the outer aqueous phase, we observe FITC quenching due to energy transfer and from this determine surface concentration and permeability of rhodamine. We derive a diffusion coefficient on the order of 10-15 cm2/s. This value can be manipulated by changing the film structure. So, the diffusion coefficient varies by nearly 2 orders of magnitude going along the surface normal. Studying the paramagnetic quenching by the smaller spin label TEMPOL, we observe that the diffusion coefficient is more than a factor of 200 larger for this molecule.