Fabrication of subnanochannels by metal–organic frameworks

Biological ion channels have prominent rectification and ion selectivity. Recent work published in Nature Materials by Huanting Wang and colleagues discloses that subnanochannels constructed by metal–organic frameworks (MOFSNC) can achieve monovalent metal ion selectivity and permeability with ultrahigh and pH-tunable features. The mechanism of ultra-high ion selectivity may be that the interaction between ions and carboxyl groups substantially reduces the energy barrier of monovalent cations passing through MOFSNC. Biological ion channels have prominent rectification and ion selectivity. Recent work published in Nature Materials by Huanting Wang and colleagues discloses that subnanochannels constructed by metal–organic frameworks (MOFSNC) can achieve monovalent metal ion selectivity and permeability with ultrahigh and pH-tunable features. The mechanism of ultra-high ion selectivity may be that the interaction between ions and carboxyl groups substantially reduces the energy barrier of monovalent cations passing through MOFSNC. Biological ion channels that existed in cell membranes play vital roles in life processes.1Dekker C. Solid-state nanopores.Nat. Nanotechnol. 2007; 2: 209-215Crossref PubMed Scopus (1625) Google Scholar Gating function of ion channels can be regulated between a closed state and an open state toward external stimuli.2Sun Y. Ma J. Zhang F. Zhu F. Mei Y. Liu L. Tian D. Li H. A light-regulated host-guest-based nanochannel system inspired by channelrhodopsins protein.Nat. Commun. 2017; 8: 260Crossref PubMed Scopus (83) Google Scholar Inspired by nature, artificial gates of biomimetic ion channels modified with small molecules have aroused research interest of scholars due to their wide range of application.3Qian T. Zhang H. Li X. Hou J. Zhao C. Gu Q. Wang H. Efficient Gating of Ion Transport in Three-Dimensional Metal-Organic Framework Sub-Nanochannels with Confined Light-Responsive Azobenzene Molecules.Angew. Chem. Int. Ed. Engl. 2020; 59: 13051-13056Crossref PubMed Scopus (34) Google Scholar Nonetheless, the gating effects of the nanochannels modified with small molecules are usually weak for the size of the nanochannel is large, so in the closed state, the small molecule gate cannot completely prevent ion migration.4Denny Jr., M.S. Moreton J.C. Benz L. Cohen S.M. Metal–organic frameworks for membrane-based separations.Nat. Rev. Mater. 2016; 1: 1-17Crossref Scopus (430) Google Scholar Artificial nanochannel materials with ultrahigh ion permeability and atomic-level ion selectivity are desirable for nanofluidic applications.5Lu J. Zhang H. Hou J. Li X. Hu X. Hu Y. Easton C.D. Li Q. Sun C. Thornton A.W. et al.Efficient metal ion sieving in rectifying subnanochannels enabled by metal-organic frameworks.Nat. Mater. 2020; 19: 767-774Crossref PubMed Scopus (151) Google ScholarMOFs (Metal–organic frameworks), composed of metal ions or clusters and organic ligands, is a type of porous materials with highly tailorable framework structures, uniform pore sizes, and high porosity.6Guo W. Tian Y. Jiang L. Asymmetric ion transport through ion-channel-mimetic solid-state nanopores.Acc. Chem. Res. 2013; 46: 2834-2846Crossref PubMed Scopus (315) Google Scholar MOFs have been extensively studied in many fields, such as gas separation,7Rodenas T. Luz I. Prieto G. Seoane B. Miro H. Corma A. Kapteijn F. Llabrés I Xamena F.X. Gascon J. Metal-organic framework nanosheets in polymer composite materials for gas separation.Nat. Mater. 2015; 14: 48-55Crossref PubMed Scopus (1438) Google Scholar catalyzes,8Guo Y. Ying Y. Mao Y. Peng X. Chen B. Polystyrene sulfonate threaded through a metal–organic framework membrane for fast and selective lithium-ion separation.Angew. Chem. Int. Ed. Engl. 2016; 55: 15120-15124Crossref PubMed Scopus (196) Google Scholar sensor technology,9Zhang H. Hou J. Hu Y. Wang P. Qu R. Jiang L. Liu J.Z. Freeman B.D. Hill A.J. Wang H. Ultrafast selective transport of alkali metal ions in metal organic frameworks with subnanometer pores.Sci. Adv. 2018; 4: eaaq0066Crossref PubMed Scopus (223) Google Scholar and drug delivery.10Wang J. Wan J. Yang N. Li Q. Wang D. Hollow multishell structures exercise temporal–spatial ordering and dynamic smart behavior.Nat. Rev. Chem. 2020; 4: 159-168Crossref Scopus (97) Google Scholar In breakthrough work recently published in Nature Materials, Huanting Wang and colleagues demonstrated the assembly of three-dimensional (3D) porous UiO-66-(COOH)2 into 1D polymer nanochannels to construct MOF-based subnanochannels with an ångström-porous structures for achieving metal ion sieving.Compared with 1D and 2D channel materials, UiO-66 MOFs endowed with well-defined 3D channels consisting of ~6-Å-sized windows and ~8‒11-Å-sized cavities. Additionally, UiO-66 MOFs have abundant functional groups imparted the framework to act as biomimetic ion filters. The authors adopted a facilitated interfacial growth strategy to introduce UiO-66-(COOH)2 crystals into artificial nanochannels to fabricate UiO-66-(COOH)2-SNCs. The authors successfully regulated the reaction of H4BTEC (1,2,4,5-benzenetetracarboxylic acid) molecules and Zr4+ ions at the tip region of the PET-NC via counter-diffusion (Figure 1A). Consequently, the seeded MOF nanocrystals can facilitate the nanoconfined nucleation and growth of UiO-66-(COOH)2 (Figure 1B). SEM images of the functional nanochannels indicate that MOF crystals were intergrown in the wall of PET-NC (Figure 1C).The authors studied the ion transport properties of hybrid Asy-MOFSNC membranes and found that the monovalent chloride of Asy-MOFSNC became more asymmetric compared with the I‒V curve of PET-NC (Figure 1D). This behavior results from the Asy-MOFSNC exhibiting a greater asymmetry in both pore size and surface charge distribution, due to the negatively charged and ångström-porous MOF structure in the tip region. The pH-responsive property of the Asy-MOFSNC are presented in Figure 1E. With the increase of pH, the conductivity of the KCl of Asy-MOFSNC increased sharply, while the conductivity of MgCl2 declined dramatically. Correspondingly, the K+/Mg2+ selectivity of the MOFSNC increased a hundred times. Based on above results, the Asy-MOFSNC demonstrated pH-tunable, ultrahigh K+/Mg2+ selectivity. The authors further investigated the dependence of ion conductance on electrolyte concentration by varying the concentrations of KCl, LiCl and MgCl2. The conductance order measured at the same concentrations was K+> Li+ > Mg2+ (Figure 1E). Compared with ion separation results by artificial channels previously reported, the binary K+/Mg2+ and Na+/Mg2+ selectivities were up to 822.7 and 336.7, respectively (Figure 1F).Subsequently, the authors put forward the mechanism of metal ion selectivity in UiO-66-(COOH)2-SNC (Figure 1G). When passing the functional nanochannels, the metal ions are liable to be partially dehydrated and interact with carboxylic groups on the MOF framework, leading to different ion conductivities, mobilities and concentrations in the MOFSNC. Additionally, divalent metal ions exhibit a stronger binding affinity with carboxylic groups on the MOF window than monovalent ions. Further, the authors also calculated the potential of mean force profile of cation migration along with the cross of the MOF window, and the results indicated that the energy barrier for Mg2+ across the MOF window was five times higher than that of K+.Huanting Wang et al. have successfully prepared the MOFSNCs with a hybrid heterostructure to achieve artificial multifunctional ion channels, exhibiting pH-tunable, ultrahigh monovalent metal ion selectivity, and permeability. Two key factors of the MOFs: the dehydration–rehydration effect and interactions between ions and the flexible carboxyl ligands are for ultraselective and ultrafast metal ion transport in MOFSNCs. This study has offered a new method on the development of multifunctional ion channel membranes, such as efficient power generation and ion separation. Biological ion channels that existed in cell membranes play vital roles in life processes.1Dekker C. Solid-state nanopores.Nat. Nanotechnol. 2007; 2: 209-215Crossref PubMed Scopus (1625) Google Scholar Gating function of ion channels can be regulated between a closed state and an open state toward external stimuli.2Sun Y. Ma J. Zhang F. Zhu F. Mei Y. Liu L. Tian D. Li H. A light-regulated host-guest-based nanochannel system inspired by channelrhodopsins protein.Nat. Commun. 2017; 8: 260Crossref PubMed Scopus (83) Google Scholar Inspired by nature, artificial gates of biomimetic ion channels modified with small molecules have aroused research interest of scholars due to their wide range of application.3Qian T. Zhang H. Li X. Hou J. Zhao C. Gu Q. Wang H. Efficient Gating of Ion Transport in Three-Dimensional Metal-Organic Framework Sub-Nanochannels with Confined Light-Responsive Azobenzene Molecules.Angew. Chem. Int. Ed. Engl. 2020; 59: 13051-13056Crossref PubMed Scopus (34) Google Scholar Nonetheless, the gating effects of the nanochannels modified with small molecules are usually weak for the size of the nanochannel is large, so in the closed state, the small molecule gate cannot completely prevent ion migration.4Denny Jr., M.S. Moreton J.C. Benz L. Cohen S.M. Metal–organic frameworks for membrane-based separations.Nat. Rev. Mater. 2016; 1: 1-17Crossref Scopus (430) Google Scholar Artificial nanochannel materials with ultrahigh ion permeability and atomic-level ion selectivity are desirable for nanofluidic applications.5Lu J. Zhang H. Hou J. Li X. Hu X. Hu Y. Easton C.D. Li Q. Sun C. Thornton A.W. et al.Efficient metal ion sieving in rectifying subnanochannels enabled by metal-organic frameworks.Nat. Mater. 2020; 19: 767-774Crossref PubMed Scopus (151) Google Scholar MOFs (Metal–organic frameworks), composed of metal ions or clusters and organic ligands, is a type of porous materials with highly tailorable framework structures, uniform pore sizes, and high porosity.6Guo W. Tian Y. Jiang L. Asymmetric ion transport through ion-channel-mimetic solid-state nanopores.Acc. Chem. Res. 2013; 46: 2834-2846Crossref PubMed Scopus (315) Google Scholar MOFs have been extensively studied in many fields, such as gas separation,7Rodenas T. Luz I. Prieto G. Seoane B. Miro H. Corma A. Kapteijn F. Llabrés I Xamena F.X. Gascon J. Metal-organic framework nanosheets in polymer composite materials for gas separation.Nat. Mater. 2015; 14: 48-55Crossref PubMed Scopus (1438) Google Scholar catalyzes,8Guo Y. Ying Y. Mao Y. Peng X. Chen B. Polystyrene sulfonate threaded through a metal–organic framework membrane for fast and selective lithium-ion separation.Angew. Chem. Int. Ed. Engl. 2016; 55: 15120-15124Crossref PubMed Scopus (196) Google Scholar sensor technology,9Zhang H. Hou J. Hu Y. Wang P. Qu R. Jiang L. Liu J.Z. Freeman B.D. Hill A.J. Wang H. Ultrafast selective transport of alkali metal ions in metal organic frameworks with subnanometer pores.Sci. Adv. 2018; 4: eaaq0066Crossref PubMed Scopus (223) Google Scholar and drug delivery.10Wang J. Wan J. Yang N. Li Q. Wang D. Hollow multishell structures exercise temporal–spatial ordering and dynamic smart behavior.Nat. Rev. Chem. 2020; 4: 159-168Crossref Scopus (97) Google Scholar In breakthrough work recently published in Nature Materials, Huanting Wang and colleagues demonstrated the assembly of three-dimensional (3D) porous UiO-66-(COOH)2 into 1D polymer nanochannels to construct MOF-based subnanochannels with an ångström-porous structures for achieving metal ion sieving. Compared with 1D and 2D channel materials, UiO-66 MOFs endowed with well-defined 3D channels consisting of ~6-Å-sized windows and ~8‒11-Å-sized cavities. Additionally, UiO-66 MOFs have abundant functional groups imparted the framework to act as biomimetic ion filters. The authors adopted a facilitated interfacial growth strategy to introduce UiO-66-(COOH)2 crystals into artificial nanochannels to fabricate UiO-66-(COOH)2-SNCs. The authors successfully regulated the reaction of H4BTEC (1,2,4,5-benzenetetracarboxylic acid) molecules and Zr4+ ions at the tip region of the PET-NC via counter-diffusion (Figure 1A). Consequently, the seeded MOF nanocrystals can facilitate the nanoconfined nucleation and growth of UiO-66-(COOH)2 (Figure 1B). SEM images of the functional nanochannels indicate that MOF crystals were intergrown in the wall of PET-NC (Figure 1C). The authors studied the ion transport properties of hybrid Asy-MOFSNC membranes and found that the monovalent chloride of Asy-MOFSNC became more asymmetric compared with the I‒V curve of PET-NC (Figure 1D). This behavior results from the Asy-MOFSNC exhibiting a greater asymmetry in both pore size and surface charge distribution, due to the negatively charged and ångström-porous MOF structure in the tip region. The pH-responsive property of the Asy-MOFSNC are presented in Figure 1E. With the increase of pH, the conductivity of the KCl of Asy-MOFSNC increased sharply, while the conductivity of MgCl2 declined dramatically. Correspondingly, the K+/Mg2+ selectivity of the MOFSNC increased a hundred times. Based on above results, the Asy-MOFSNC demonstrated pH-tunable, ultrahigh K+/Mg2+ selectivity. The authors further investigated the dependence of ion conductance on electrolyte concentration by varying the concentrations of KCl, LiCl and MgCl2. The conductance order measured at the same concentrations was K+> Li+ > Mg2+ (Figure 1E). Compared with ion separation results by artificial channels previously reported, the binary K+/Mg2+ and Na+/Mg2+ selectivities were up to 822.7 and 336.7, respectively (Figure 1F). Subsequently, the authors put forward the mechanism of metal ion selectivity in UiO-66-(COOH)2-SNC (Figure 1G). When passing the functional nanochannels, the metal ions are liable to be partially dehydrated and interact with carboxylic groups on the MOF framework, leading to different ion conductivities, mobilities and concentrations in the MOFSNC. Additionally, divalent metal ions exhibit a stronger binding affinity with carboxylic groups on the MOF window than monovalent ions. Further, the authors also calculated the potential of mean force profile of cation migration along with the cross of the MOF window, and the results indicated that the energy barrier for Mg2+ across the MOF window was five times higher than that of K+. Huanting Wang et al. have successfully prepared the MOFSNCs with a hybrid heterostructure to achieve artificial multifunctional ion channels, exhibiting pH-tunable, ultrahigh monovalent metal ion selectivity, and permeability. Two key factors of the MOFs: the dehydration–rehydration effect and interactions between ions and the flexible carboxyl ligands are for ultraselective and ultrafast metal ion transport in MOFSNCs. This study has offered a new method on the development of multifunctional ion channel membranes, such as efficient power generation and ion separation. This study was financially supported by the National Natural Science Foundation of China ( 22061018 and 21702079 ), the startup funding from South-Central University for Nationalities ( YZZ19005 ), and the Open Project of Hubei Key Laboratory of Wudang Local Chinese Medicine Research ( Hubei University of Medicine ) ( WDCM2019003 ).