Control of external pH and ionic strength is used to separate proteins with surface-modified, nanoporous polycarbonate track etched (PCTE) membranes. The porous PCTE membranes were modified with monolayers of self-assembled thiols (HSC10H20COOH) on electroless gold. The hydraulic radius of the pores in the surface-modified membranes was 8.7 nm. Two proteins of nearly identical molecular weight, bovine serum albumin (BSA) and bovine hemoglobin (BHb), were used as the permeants. The fluxes of BSA and BHb through the membranes show maximum values at the isoelectric points (pI) of the proteins. At pH values above and below the pI, charge interactions between the proteins, their counterions, and the pore surface leads to a decrease in flux. The imposition of a difference in ionic strength across the membrane causes osmotic flow and leads to a significant increase in the protein fluxes and an enhancement of the selectivity of BSA over BHb. In protein separation experiments, the BSA and BHb fluxes are nearly 3 times larger than those observed with no ionic strength difference.

We investigated the effect of external pH, ionic strength, and applied voltage to control the transport of ions through porous PCTE/Au/acid thiol membranes with pore radii of 4.0-5.9 nm. Electroless gold was deposited on the inside surfaces of the nanopores of the membrane. The thiols 11-mercaptoundecanoic acid (HSC10H20COOH) and 3-mercapto-1-propanesulfonic acid (HSC3H6SO3H) were used to form self-assembled monolayers (SAMs) on the gold surfaces of the pores. The ionic permeant through the pores of the membranes was the organic ion benzene sulfonate (BS). The transport of benzene sulfonate in porous membranes modified with SAMs of the HSC10H20COOH weak acid thiol was highly affected by external variation of the pH; the BS flux could be changed by a factor of 100. However, transport of benzene sulfonate in membranes modified with SAMs of the HSC3H6SO3H strong acid thiol was influenced less by external pH than transport in membranes modified with SAMs of the HSC10H20COOH weak acid thiol. Increasing the ionic strength of the bulk solution caused the flux of benzene sulfonate to increase due to the decrease of the Debye length. Application of a voltage to the membrane has a significant influence on the benzene sulfonate flux through the porous membrane. Controlling the pH and the applied potential can lead to flux changes as large as a factor of 170.