The mechanism for selecting between metal ions with different charges (or valencies - monovalent is 1+, divalent is 2+) is interesting. The article mentions the analogy to biological systems, and I remember talking to a Professor (sadly now died) some years ago about a similar system in a protein ion channel.

The protein channel has a pore lined with backbone carbonyls (C=O) that are positioned such that they can bind to an ion with its first water shell. If I remember correctly, ions of a different charge (divalent?) have a shell that is too large to be bound properly so passes through the pore less efficiently.

Similar to the hydrogen-bonding of these polydopamine membranes:

> Therefore, it is likely that hydrated multivalent ions were more favored to form hydrogen bonds with hydrogen bonding sites along the membrane channels, compared to monovalent metal ions. Such PDA–ion interaction explains the ion selectivity of PDA membranes toward monovalent ions, consistent with simulations that suggest that divalent ions engaged in hydrogen bonding interactions are required to overcome higher energy barriers for their transport across the membrane