Single Molecule Microscopy Informs the Design of Membrane Absorbers

Membranes are used in industrial and pharmaceutical separations due to their practical geometry and increased surface area which provide better performance compared to traditional resin-based materials. Porous membranes coated with covalently bound polymers are used to bind analytes and separate mixtures based on specific forces. However, mass transport in these systems is typically modeled and studied empirically at the ensemble level, which convolves the contributions of transport through the porous support, through the polymer, and heterogeneous adsorption at the interface. Ensemble results complicate analysis and obscure heterogeneity, hindering the design of polymers for high-precision separations of similar-sized and -charged species, such as lanthanides and actinides. In this work, we apply super-resolution fluorescence microscopy to study transport behavior at polymer-modified membrane interfaces at the single-molecule level. We develop a platform for imaging micron-thin polymer films using two different grafting methods. We utilize UV-initiated polymerization and Activators Generated by Electron Transfer – Atom Transfer Radical Polymerization (AGET-ATRP) to obtain different surface grafting densities and geometries to compare design and performance. Nanoscale imaging is achieved through Total Internal Reflection (TIRF) microscopy, and we track the diffusion of anionic dye as a model analyte. We observe heterogenous diffusion on UV-grafted polymer and membrane surfaces, comparably more homogenous behavior on AGET-ATRP grafted polymer, as well as rare, long-lasting adsorption events at the interface. We hope these studies inform the design of membrane absorbers for separations, as well as promote the use of single-molecule methods to aid in the study of bulk phenomena.