Trivalent cations switch the selectivity in nanopores.

In this letter, we study the effect of cation charge on anion selectivity in the pore using grand canonical Monte Carlo simulations. The mechanism of anion selectivity inside nanopores was found to be primarily a consequence of the screening of negative charges by the cations. In the case of monovalent cations, screening was not very effective and anions were rejected. We found an 'off-state' at high pH and an 'on-state' at low pH. When there are divalent cations, screening is good and there is no rejection of the anion. The concentration of anions at high pH is similar to that at low pH. The system is always in an 'on-state'. Trivalent cations show an inverse selectivity mechanism: at high pH the concentration is higher than at low pH, i.e., the pore is in the 'on-state' at high pH and in the 'off-state' at low pH.