Biomimetic KcsA channels with ultra-selective K+ transport for monovalent ion sieving

Abstract

Ultra-selective and fast transport of K⁺ are of significance for water desalination, energy conversion, and separation processes, but current bottleneck of achieving high-efficiency and exquisite transport is attributed to the competition from ions of similar dimensions and same valence through nanochannel communities. Here, inspired by biological KcsA channels, we report biomimetic charged porous subnanometer cages that enable ultra-selective K⁺ transport. For nanometer to subnanometer scales, conically structured double-helix columns exhibit typical asymmetric transport behaviors and conduct rapid K⁺ with a transport rate of 94.4 mmol m^-2 h^-1, resulting in the K⁺/Li⁺ and K⁺/Na⁺ selectivity ratios of 363 and 31, respectively. Experiments and simulations indicate that these results stem from the synergistic effects of cation-π and electrostatic interactions, which impose a higher energy barrier for Li⁺ and Na⁺ and lead to selective K⁺ transport. Our findings provide an effective methodology for creating in vitro biomimetic devices with high-performance K⁺ ion sieving.

Fig. 1. Design and synthesis of the biomimetic K⁺ channels.

a KcsA K⁺ channels embedded in the plasma membrane (left panel) inspired our biomimetic K⁺ channels composed of CPOS units (middle panel). The empty channels with a cross section of 5.3 × 6.8 Ų (right panel) provide multifunctional channels for ion transport. The green arrows indicate the exposed −SO₃⁻ ribbons (C, gray; N, blue; O, red; S, yellow; H, white). b Schematic showing the preparation and incorporation of the CPOS into conical transmembrane nanochannels. A single conical nanochannel (−COO⁻ wall) was activated using ethanediamine molecules, which provide a functionalized inner surface on the post-track-etching polyimide (PI) membrane (step 1). Subsequently, in situ growth occurred in which the nanoparticle seeds, namely CPOS (see Supplementary Methods for details), were assembled from the tip to the base of the nanochannel (step 2).

Fig. 2. Characteristics of the functionalized nanochannel.

a SEM image of the base and tip profiles of the asymmetrical nanochannel. The nanocone has an average inner base and tip diameter of 750 nm and 75 nm, respectively (Supplementary Methods). b SEM image showing the in situ synthesis of the CPOS material, which is full of the nanocone due to the initial growth from the tip to the base. c PXRD pattern of CPOS, demonstrating their successful embedding within the confined nanochannel. This further confirmed two different locations on the surface (black and orange spots), as indicated in the XRD profiles. d The CO₂ adsorption-desorption isotherms of the CPOS exhibit type-I isotherm with a sharp uptake at low pressures (P < 0.10). e Pore size distributions of the CPOS, showing the size variation for the nanometer-to-subnanometer channels. f Surface zeta electric potential of the doubly helical columns, which have net negative charge.

Fig. 3. Asymmetric ion transport behaviors of the biometric K⁺ channels.

a I‒V curves of the conical nanochannel before (orange) and after (blue) CPOS growth, respectively, measured in 0.1 M solutions (pH 6.8). Inset, the corresponding ion rectification, as calculated by $∣I_{-}∣/∣I_{+}∣$. Error bars denote the standard deviation. b Bar plot summarizing the rectification ratios of monovalent (K⁺, Na⁺, and Li⁺) and bivalent metal ions (Mg²⁺ and Ca²⁺) through the modified nanochannel in 0.1 and 0.01 M solutions, respectively. Error bars exhibit the standard deviation. c Transmembrane ion conductance as a function of the solution concentration. The experimental conductance decreases nonlinearly with the decrease in KCl concentration, indicating the existence of surface-charge-governed ion transport. The black, dashed fit line (R² = 0.995) shows the remarkable deviation from the bulk value when the concentration is <0.1 M. Error bars represent standard deviation (n = 11). d Cycling performance of the biomimetic K⁺ channels. The asymmetric ionic transport properties (top) of the conical nanochannel were investigated under a symmetric voltage of ±1 V (bottom). e Three stochastic cycles (namely cycles 2, 7, and 11), with the calculated rectification ratios showing the stable asymmetric ionic transport.

Fig. 4. Mechanism of selective K⁺ transport.

High-resolution Li 1s (a), Na 1s (b), and K 2p (c) XPS spectra for the CPOS material. Samples were treated using corresponding halide solutions and then washed thoroughly. The obvious peaks with binding energies of 55.8 eV (Li 1s), 1072.1 eV (Na 1s), and 293.6 eV (K 2p) confirmed the interactions between the metal ions and the CPOS material, which were not observed in pristine CPOS samples (Supplementary Fig. 8). d Partial ¹H NMR spectra (400 MHz, 298 K) of R in the presence of various metal ions showing the proton resonance shifts for Li⁺, Na⁺, and K⁺, with TMS as an internal standard set at 0 ppm. The addition of metal ions led to upfield shifts with respect to the reference peak (black), with the extent of the shift indicating the corresponding interactions. e FTIR spectra of the CPOS (black) with Li⁺ (green), Na⁺ (purple), and K⁺ (pink). The characteristic peaks at 3550.5, 1690.2, 1188.3, and 545.1 cm^‒1 represent distortions and transformations due to the interactions with metal ions. f Temperature dependence of the ion conductivity for of Li⁺ (green square), Na⁺ (purple circle), and K⁺ (pink hexagon). Solid symbols represent experimental data; dashed lines represent curve-fitting results. g Cation transference number through the negatively charged CPOS pores. h Mobility ratio of metal ions (cations/Cl^‒) as a function of cation radii. i Mobility of metal ions through the CPOS pores. The net negative charge of the channels facilitates cation transport.

Fig. 5. Biomimetic K⁺ ion channels with ultrahigh K⁺/Li⁺ and K⁺/Na⁺ selectivity.

a Theoretical RDFs of steady-state ion–oxygen and ion–hydrogen distances in the CPOS pores (Supplementary Fig. 11). b Hydrogen and oxygen atomic density profiles along the z-axis for Li⁺ (top panel), Na⁺ (middle panel), and K⁺ (bottom panel) around the screwing cavity. c Experimental ion transport rate as a function of the hydration ionic diameter and hydration free energy, respectively. The transport rate through the CPOS pores was measured using ternary ion mixtures as feed solutions and an applied voltage of ‒1 V (Supplementary Fig. 13). The green and pink dashed lines represent the fitting results for the transport rate and hydration free energy data, respectively. d Comparison of the ion concentration in the permeation compartment (left). The feed solutions include a ternary ion mixture of LiCl, NaCl, and KCl, while the permeation compartment was filled with deionized water (see Methods and Supplementary Methods for details). As a result, ion selectivity in the CPOS channels was obtained by calculating the ion concentrations ratios in the permeation compartment (right). e The K⁺/Li⁺ selectivity was performed for five cycles by conducting independent measurements. Results showed that the selectivity remained ultrahigh and that stable monovalent ionic separation was maintained. f Comparison of the K⁺/Na⁺, K⁺/Li⁺, and Na⁺/Li⁺ ion selectivity performance reported using nanopore/channels or membranes (Supplementary Table 2).