Abstract
The permeability and selectivity of biological and artificial ion channels correlate with the specific hydration structure of single ions. However, fundamental understanding of the effect of ion–ion interaction remains elusive. Here, via non-contact atomic force microscopy measurements, we demonstrate that hydrated alkali metal cations (Na+ and K+) at charged surfaces could come into close contact with each other through partial dehydration and water rearrangement processes, forming one-dimensional chain structures. We prove that the interplay at the nanoscale between the water–ion and water–water interaction can lead to an effective ion–ion attraction overcoming the ionic Coulomb repulsion. The tendency for different ions to become closely packed follows the sequence K+ > Na+ > Li+, which is attributed to their different dehydration energies and charge densities. This work highlights the key role of water molecules in prompting close packing and concerted movement of ions at charged surfaces, which may provide new insights into the mechanism of ion transport under atomic confinement.
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Source data are provided with this paper. All other data that support the findings of this study are available from the corresponding authors upon reasonable request.
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Acknowledgements
We thank L. Jiang, B. Song and F. Pan for inspiring discussions and the computational resources provided by the TianHe-1A, TianHe-2 supercomputer, High-Performance Computing Platform of Peking University. This work was supported by the National Key R&D Program under grants 2021YFA1400500 and 2017YFA0205003, the National Natural Science Foundation of China under grants 11888101, U22A20260, 21725302, 11935002, 21902013 and 92053202, the Strategic Priority Research Program of Chinese Academy of Sciences under grants XDB28000000 and XDB33000000, the China Postdoctoral Science Foundation under grants 2022M720003 and 2023T160011 and the Key R&D Program of Guangdong Province under grant 2020B010189001. Y.J. acknowledges the support by the New Cornerstone Science Foundation through the New Cornerstone Investigator Program and the XPLORER PRIZE.
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Y.J. and E.-G.W. designed and supervised the project. Y.T. and J.H. performed the STM/AFM measurements with R.M., S.Y. and D.G. Y.S., D.C., J.C. and L.-M.X. performed ab initio DFT calculations. Y.X., Y.H. and Y.Q.G. carried out the classical MD simulations. Y.S. carried out the theoretical simulations of the AFM images. M.Z.Z. and K.H.L. prepared the monolayer graphene/Cu(111) sample. Y.T., Y.S., Y.X., J.H., Y.H., J.C., C.S., Y.Q.G., E.-G.W. and Y.J. analysed the data. Y.J., Y.T., Y.S., Y.X., L.-M.X. and Y.Q.G. wrote the paper with input from all other authors. The paper reflects the contributions of all authors.
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Nature Nanotechnology thanks Kai Xiao, Francois Peeters, Taesung Kim and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Supplementary Sections 1–12, Figs. 1–21, Tables 1–11 and refs. 1–9.
Supplementary Video 1
Diffusion of 1D K+–water chains at 77 K on the basis of MD simulations. Three short K+–water chains were placed on the Au surface and diffused in a collective manner. The simulation time is 10 ns. Au, K, H and O atoms are denoted as yellow, purple, white and red spheres, respectively.
Supplementary Video 2
Collision of two short K+–water chains at 50 K on the basis of MD simulations. The initial equivalent kinetic energy of 347.2 meV was assigned to the two short K+–water chains at 10 ps, making them move towards collision. The initial kinetic energy serves to provide the initial motion direction, assisting in overcoming activation energy barriers for the combination between the two chains. The chain recombination dynamics is influenced by the initial conformation of the system as well as the thermal bath. The simulation time is 30 ps. The longer K+–water chain was formed at ~12 ps. Au, K, H and O atoms are denoted as yellow, purple, white and red spheres, respectively.
Supplementary Video 3
MD simulations of K+ and water molecules on Au surface at 100 K. This system contains 125 K+ ions and 375 water molecules. The simulation time is 1 ns with a time step of 1 fs. The lifetime of the chain structure is larger than the simulation duration time of 1 ns. Au, K, H and O atoms are denoted as yellow, purple, white and red spheres, respectively.
Supplementary Video 4
MD simulations of K+ and water molecules on Au surface at 300 K. This system contains 125 K+ ions and 375 water molecules. The simulation time is 1 ns with a time step of 1 fs. The lifetime of the chain structure is up to 15 ps. The chain structure can re-form during the simulation. Au, K, H and O atoms are denoted as yellow, purple, white and red spheres, respectively.
Supplementary Video 5
MD simulations of K+ and water molecules in 2D graphene channel at 300 K. This system contains 100 K+ ions and 300 water molecules. The simulation time is 1 ns with a time step of 1 fs. The top view is shown in the upper part of the video, with the graphene omitted. The bottom part shows the side view. The lifetime of the chain structure is up to 150 ps. C, K, H and O atoms are denoted as grey, purple, white and red spheres, respectively.
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Tian, Y., Song, Y., Xia, Y. et al. Nanoscale one-dimensional close packing of interfacial alkali ions driven by water-mediated attraction. Nat. Nanotechnol. 19, 479–484 (2024). https://doi.org/10.1038/s41565-023-01550-9
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DOI: https://doi.org/10.1038/s41565-023-01550-9
