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Construction of nanofluidic devices with an ultimate ion selectivity analogue to biological ion channels has been of great interest for their versatile applications in energy harvesting and conversion, mineral extraction, and ion separation. Herein, we report a three-dimensional (3D) sub-1 nm nanofluidic device to achieve high monovalent metal ion selectivity and conductivity. The 3D nanofluidic channel is constructed by assembly of a carboxyl-functionalized metal–organic framework (MOF, UiO-66-COOH) crystals with subnanometer pores into an ethanediamine-functionalized polymer nanochannel via a nanoconfined interfacial growth method. The 3D UiO-66-COOH nanofluidic channel achieves an ultrahigh K⁺/Mg²⁺ selectivity up to 1554.9, and the corresponding K⁺ conductivity is one to three orders of magnitude higher than that in bulk. Drift-diffusion experiments of the nanofluidic channel further reveal an …