Fast and selective fluoride ion conduction in sub-1-nanometer metal-organic framework channels

Abstract

Biological fluoride ion channels are sub-1-nanometer protein pores with ultrahigh F^- conductivity and selectivity over other halogen ions. Developing synthetic F^- channels with biological-level selectivity is highly desirable for ion separations such as water defluoridation, but it remains a great challenge. Here we report synthetic F^- channels fabricated from zirconium-based metal-organic frameworks (MOFs), UiO-66-X (X = H, NH₂, and N⁺(CH₃)₃). These MOFs are comprised of nanometer-sized cavities connected by sub-1-nanometer-sized windows and have specific F^- binding sites along the channels, sharing some features of biological F^- channels. UiO-66-X channels consistently show ultrahigh F^- conductivity up to ~10 S m^-1, and ultrahigh F^-/Cl^- selectivity, from ~13 to ~240. Molecular dynamics simulations reveal that the ultrahigh F^- conductivity and selectivity can be ascribed mainly to the high F^- concentration in the UiO-66 channels, arising from specific interactions between F^- ions and F^- binding sites in the MOF channels.

Fig. 1

Bioinspired design of synthetic MOF channels for fluoride ion conduction. a Schematic of a biological fluoride (F⁻) ion channel with an angstrom-sized region as F⁻ selectivity filter and nanometer-sized vestibule and outlet for selective, ultrafast F⁻ transport. b Schematics of bioinspired artificial zirconium-based UiO-66-X (X = H, NH₂, and N⁺(CH₃)₃) MOF channels with sub-1-nanometer crystalline pores for selective and ultrafast F⁻ transport. Sub-1-nanometer MOF channels consist of angstrom-sized triangular windows (~6 Å in diameter) for ion sieving and nanometer-sized octahedral cavities (~11 Å in diameter) for ultrafast ion conduction

Fig. 2

Fabrication and characterization of PET-UiO-66-X nanochannels. a Schematic of a single bullet-shaped nanochannel and SEM images of the channel tip, cross section (the scale bar is 500 nm) and base. Average tip diameter is 36.3 ± 5.6 nm, while average base diameter is 328.3 ± 35.2 nm. The PET nanochannel surface possesses BDC linkers for in-situ growth of UiO-66-X MOFs. b Schematic of a PET-UiO-66-X nanochannel and SEM images of the nanochannel tip, cross section (the scale bar is 500 nm) and base (SEM images of PET-UiO-66-NH₂ are shown as an example). Schematic of UiO-66-X crystal structure along the PET-nanochannel surface. c N₂ adsorption/desorption isotherms of UiO-66, UiO-66-NH₂, and UiO-66-N⁺(CH₃)₃. d Pore size distributions of UiO-66-X MOFs. The distributions for UiO-66-NH₂ and UiO-66-N⁺(CH₃)₃ are displayed vertically for ease of viewing. Pore size decreases as MOFs functional group size increases

Fig. 3

Ionic transport properties of PET nanochannels and sub-1-nanometer MOF channels. a Symmetric I‒V curves of a bullet-shaped PET-nanochannel observed in 1.0 M KF and KCl solutions, respectively. b Schematics of hydrated ions transport in a PET-nanochannel. Because d_Channel ≫ d_H-ion, ions are hydrated in the PET-nanochannel. The diameter of hydrated F⁻ is larger than that of hydrated Cl⁻, so hydrated F⁻ ions conduct slower than hydrated Cl⁻ ions in the PET-nanochannel. c–e I–V curves of PET-UiO-66-X nanochannels measured in 1.0 M KF and KCl solutions, respectively. Compared with UiO-66 and UiO-66-NH₂ channels, UiO-66-N⁺(CH₃)₃ channels exhibit the highest F⁻ conductance and selectivity. Error bars represent the standard deviation of three measurements of a sample. f Schematic illustrations of dehydrated ions passing through sub-1-nanometer PET-UiO-66-X nanochannels. Ions are dehydrated in MOF channels because d_H-ion > d_Window > d_Ion. The angstrom-sized MOF pore windows with specific F⁻ binding zirconium sites and functional NH₂ and N⁺(CH₃)₃ groups serve as F⁻ selectivity filter for ion sieving, so dehydrated F⁻ ions conduct faster than Cl⁻ in PET-UiO-66-X nanochannels. g, h Top view of UiO-66-X pore windows. UiO-66-N⁺(CH₃)₃ channels have the smallest window size and strongest F⁻ specific interactions due to the N⁺(CH₃)₃ groups compared with UiO-66 and UiO-66-NH₂ channels

Fig. 4

MD simulations of ion transport in UiO-66 channels. a The simulation cartoon shows UiO-66 cavities filled with water molecules (sky blue spheres), and they are connected via narrow windows. For clarity, UiO-66 is shown as a green wireframe. K⁺ and F⁻ ions are represented by red and dark blue spheres, respectively. b The mobility of Cl⁻ and F⁻ ions in UiO-66. F⁻ mobility is presented as a function of the strength of F–Zr LJ potential at 0.0457 M and 3.748 M, respectively. At 3.748 M, F⁻ mobility is enhanced by around 10 times compared with that at 0.0457 M. Error bars represent the standard deviation of calculations of 5 samples. c, e, g Radial distribution function of water molecules around anions sitting at cavity center and window center. Cl⁻ ions have a weaker second hydration shell compared with F⁻ ions at 0.0457 M when sitting at cavity center (c, e). However, at 3.748 M, F⁻ ions have a relative smaller hydration shell as part of water molecules within second shell were shared with neighbored F⁻ ions (g). d, f, h The simulation cartoon shows the arrangement of water molecules around anions sitting at cavity center (left) and window center (right), corresponding to c, e and g, respectively. Water molecules are red (O) and white (H), and ions (F⁻ or Cl⁻) are dark blue. F⁻ ions have two hydration layers at cavity center, and the second hydration shell should peel off at window center at 0.0457 M (d). Cl⁻ ions do not have a distinct second hydration shell (f), thus, smaller dehydration energy would be required for transport through windows compared with F⁻ ions at 0.0457 M. At 3.748 M, water molecules of F⁻ second hydration shell are shared with neighbored F⁻ ions (h)

Fig. 5

Influence of concentration and pH on F⁻, Cl⁻ conductivity and F⁻/Cl⁻ selectivity. a, b F⁻ and Cl⁻ conductivities in PET-UiO-66-X nanochannels as a function of ion concentration. F⁻ conductivities in UiO-66-X nanochannels are much higher than those in bulk solution, while Cl⁻ conductivities in UiO-66-X nanochannels are much lower than those in bulk solution. c F⁻/Cl⁻ selectivity of PET-UiO-66-X nanochannels as a function of ion concentration. Among the MOFs considered, the UiO-66-N⁺(CH₃)₃ nanochannel exhibits the highest F⁻/Cl⁻ selectivity (~240). d F⁻/Cl⁻ selectivity of PET-UiO-66-X nanochannels measured in 0.1 M KF and KCl solutions at different pH values. Error bars represent the standard deviation of three measurements of a sample

Fig. 6

F⁻ selective properties of PET-UiO-66-X nanochannels. a Ionic conductivities of a PET-UiO-66-NH₂ nanochannel decrease as dehydrated anion diameters increase. b Average ion selectivity as a function of dehydrated anion diameter ratio (Selectivity = $k_{{\mathrm{F}}^{-}}$/k_Anion, diameter ratio = d_Anion/$d_{{\mathrm{F}}^{-}}$). Error bars represent the standard deviation of independent measurement of three samples. c, d Systematic comparison of ionic conductivities and selectivities of PET-UiO-66, PET-UiO-66-NH₂, and PET-UiO-66-N⁺(CH₃)₃ nanochannels. The PET-UiO-66-N⁺(CH₃)₃ nanochannel exhibits the highest F⁻ conductivity and selectivity. All data were obtained using 0.1  M electrolyte solutions at a pH value of 5.7. Error bars represent the standard deviation of three measurements of a sample