The diffusion of charged proteins in liquid-filled nanometer-sized apertures with charged surfaces has been investigated with fluorescence correlation spectroscopy (FCS). Based on a two-dimensional (2D) multicomponent diffusion model, key parameters such as the number of molecules diffusing freely inside the nanochannel or interacting with the surfaces, together with the specific diffusion parameters, could be extracted. Different regimes of diffusion have been observed and described by a model, which takes into account the steric exclusion, the reversible surface adsorption of the biomolecules, and the exclusion-enrichment effect that is due to the charge of the proteins and the ionic strength of the solution. Conditions where the diffusion of proteins through nanoconfined spaces can be of the same magnitude as in the bulk were both predicted and experimentally verified.

With fluorescence correlation spectroscopy (FCS), the diffusion and concentration of proteins (wheat germ agglutinin) in a nanofluidic system are measured on a single molecule level. By increasing the ionic concentration, the Debye length of the electrical double layer (EDL) decreases altering the strongly enhanced surface interactions of charged proteins in nanochannels. A theoretical model of the transport of charged mole- cules in nanofluidics facilitates the design of future nanochannel applications.