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Polydopamine film which contains amine groups and phenolic hydroxyl groups exhibits fully reversible, pH-switchable permselectivity for both cationic and anionic redox-active probe molecules. At high pH the film has a net negative charge that excludes anions but passes cations; at low pH it is positively charged and excludes cations but passes anions.
Yu et al. (Fri,) studied this question.