(217d) Membrane Selectivity for Protein Filtration Probed By Atomic Force Microscopy

Even though synthetic membranes have been around since the early 20^th century, predicting their separation performance remains a substantial challenge. This work shows how atomic force microscopy (AFM) could be used to probe membrane selectivity for bio-separations. We selected 0.2 µm PVDF and PES membranes that were surface functionalized to render them hydrophilic, and that had relatively low and high affinity toward proteins, respectively. These two membranes, characterized by SEM, ATR-FTIR, Z-potential and contact angle, showed similar properties in terms of surface charge and hydrophilicity. Thus, these techniques were unable to predict different membrane selectivity toward biomolecules during filtration. Streptavidin (SA) was selected because of its stability in a wide range of operating conditions, and immobilized onto nano- and micro-particles for filtration experiments and atomic force microscopy measurements, respectively. To gain insight into the interaction forces between particles and membranes, the solution conditions (pH = 4.0, 5.0, 7.4, 9.0 and ionic concentration [salt] = 1, 10, 100 mM), and nanoparticle support material (polystyrene, silica and gold) were varied. AFM was used in force-volume mapping mode: 400 measurements, scanning 20 x 20 µm² surface area for each condition. Based on adhesion forces and protein pulling events, we identified stronger interactions between SA and PES compared with SA and PVDF. These data correlated with results from filtration experiments. This work demonstrates that AFM could be used to screen proteins and operating conditions in order to reduce fouling or increase protein capture.