Asymmetric nanodiffusion.

The asymmetric diffusion through conical nanopores is described by the diffusional model. Diffusion is several times faster; when the concentration gradient points from the wide towards the narrow opening of the cone than in the opposite direction. The asymmetric diffusion appears either when the diffusion coefficient depends on the concentration or when the diffusing substances interact with the channel (i.e., ions moving through channels with charged walls). These results suggest that asymmetric nanopores can act as molecular (ionic) filters which could be used for retrieving the molecules of a given component from solutions in which its concentration fluctuates strongly, and only occasionally attains high values.