Variable surface transport modalities on functionalized nylon films revealed with single molecule spectroscopy

Functionalization of polymer films with ion exchange ligands is a common method for creating surfaces optimized for separations and purification. Surfaces are typically evaluated for their ability to retain target molecules, but this retention encompasses a variety of physical and chemical processes. In this work we use single molecule fluorescence microscopy to investigate two ion exchange ligands that enhance surface binding of their respective target proteins. Single molecule tracking reveals that in addition to increasing the rate of surface interaction, functionalization can also increase the surface mobility of the target molecules resulting in large areas of the membrane being explored during adsorption, likely due to hopping of the protein molecules to adjacent binding sites. Hopping was only observed for one of the ligands and not the other. The enhanced mobility was found to be proportional to the UV exposure time during ligand grafting, which suggests that the hopping scales with the grafted polymer chain length. Functionalization of separation membranes with ion-exchange ligands allows control of the surface mobility of protein molecules facilitating optimized membrane design.