"Chemical transistor" effect in two-dimensional amine-functionalized graphene oxide membranes

Selective membranes with preferential transport of specific substances have attracted wide research interests over the past years for ion extraction and wastewater treatment. Graphene oxide membrane with 2D transport channels exhibits fast water transport and effective sieving effect, but its ability of ion-ion selectivity, especially for those with similar size and charge, is relatively limited. Here, we developed a layer-by-layer structure with NH₃⁺ binding sites based on multilayered graphene oxide and polyethylenimine membrane and focused on the permeation mechanism of four anions: Cl^-, NO₃^-, SO₄^2-, and PO₄^3-. The membrane shows excellent selectivity for Cl^-. Although Cl^- and NO₃^- have similar size and charge, the permeation of Cl^- is four times faster than that of NO₃^- due to their distinct structure configurations and hydration properties. These Cl^--selective binding sites exhibit ultrahigh selectivity of Cl^- to SO₄^2- and PO₄^3- in both binary and quaternary anion mixtures, which are 11 and 39 (binary) and 12 and 17 (quaternary) respectively. The density functional theory calculations reveal that Cl^- has a lower energy barrier to overcome when hopping through the binding sites. The permeation of the impermeable ions could also be adjusted by the addition of Cl^-, showing the chemical transistor effect. The combination of graphene oxide and polyethylenimine demonstrates a successful strategy to construct selective membranes based on the interactions between ions and the transport channel for the agriculture industry and wastewater treatment.