Highly Selective Transport of Alkali Metal Ions by Nanochannels of

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Highly Selective Transport of Alkali Metal Ions by Nano-channels of Polyelectrolyte Threaded MIL-53 Metal Organic Framework Liang Gao, Kwong-Yu Chan, Chi-Ying Vanessa Li, Liangxu Xie, and Joseph F Olorunyomi Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b01211 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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Highly Selective Transport of Alkali Metal Ions by Nano-channels of Polyelectrolyte Threaded MIL-53 Metal Organic Framework

Liang Gao 1,2, Kwong-Yu Chan 1*, Chi-Ying Vanessa Li 1*, Liangxu Xie 1, and Joseph F Olorunyomi 1

1 Department

2 Present

of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong.

Address: School of Light Chemistry and Chemical Engineering, Guangdong

University of Technology, Guangzhou, China.

KEYWORDS: Metal Organic Framework, MOF Membrane, nanochannels, Selective Ion Transport, in situ polymerization.

ABSTRACT Conventional ion-exchange polymeric membranes have limited selectivity due to their nonuniform and unstable structures. The rigid, regular, high porosity of metal organic framework (MOF) generally provides MOF membrane with ion exclusion/sieving effect but lack of electrostatic screening. Here we report for the first time a non-biological highly selective MOF membrane with particles of polyelectrolyte threaded in the nanochannel of metal organic

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framework (polyelectrolyte~MOF) and its selective transport of alkali metal cations. Poly(sodium vinyl sulfonated-co-acrylic acid)~MIL-53(Al) is prepared on anodic aluminium oxide substrate via steps of MOF MIL-53(Al) growth followed by in-situ polymerization. The poly(VS-co-AA)~MIL-53(Al) membrane demonstrates highly specific selectivity in transport of alkali metal cations. Rate of ion transport correlates inversely with the hydrated diameter of the ion reaching a low limiting rate near 0.7 nm hydrated diameter. Charge exclusion is demonstrated with blockage of anion transport under a concentration gradient. The highly uniform porous nanostructure of MOF and ionic function of polyelectrolyte offers the MOF membrane with synergistic selectivity based on exclusion forces of the framework and Coulomb forces from fixed charges of polyelectrolytes in nanochannels.


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Introduction Selective ion transport is critical to biomedical, environmental, and energy technologies1-4 as in the operation of electro-dialysis, desalination, fuel cells, flow batteries and bio-sensors. Conventional ion-exchange polymeric membranes have limited selectivity due to their nonuniform and unstable structures4,5 with resulting leakage of undesirable species. A practical approach to reduce leakage of uncharged species such as methanol is filling the large pores of the polymeric matrix with inorganic nanoparticles of SiO2 and TiO2.6-8 Fundamentally, size and charge selectivity can be achieved with exclusion and coulombic interactions engineered at the molecular level, which is particularly desirable for artificial biological ion channels.9,10 Attempts have been made to use structures with well-defined nano-pores such as carbon nanotubes11 and to functionalize pore wall with charges, e.g. an anionic polymer interior of polymer nanotubules12. The use of rigid and well-defined pore structures such as carbon nanotubes, inorganic or ceramic membranes can provide stable and uniform confinement and exclusion forces, while charges on pore wall can provide attraction of counter-ions and repulsion of co-ions. The overlay of exclusion forces in a porous structure and the distributed fixed charges can provide complementary and synergistic roles for selectivity.

Compared to carbon nanotubes and molecular sieves, metal organic frameworks (MOF) offer additional advantages for its higher porosity and much wider choices of pore structures in nanometre scale.13 Sieving based on exclusion forces of MOF has been exploited for separation of non-charged species.14-16 To achieve ionic functionality, the introduction of fixed charges to metal centres or organic linkers of MOF have been proposed.17 The ionic functionality of these fixed charges is limited since high valency charge will invariably affects stability of the framework. The introduction of polyelectrolyte into MOF with fast ion-exchange has been demonstrated by Gao et al. for anion exchange18 and cation exchange19 functions. The


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entrapped polyelectrolyte performs better ion-exchange than its bulk counterpart since individual polyelectrolyte chains are separated from each other and fully accessible to water and counter ions. The combination of individual polyelectrolyte chain residing in a rigid, regular, high porosity framework put forward an excellent option for high selectivity in ionexchange membrane. The framework will provide the desirable exclusion/sieving effect where charges on the flexible polyelectrolyte will provide electrostatic screening. Selective ionexchange based on charges has been demonstrated in NaPSS~MIL-101.19 The first examples of polyelectrolyte threaded MOFs18, 19 have been synthesized as micrometre scale particles and their ion exchange performance evaluated by immersion in a bulk solution. Fabrication of the polyelectrolyte~MOF into a 2D system for application as an ion selective membrane remains challenging and unreported. We show here, the first attempt to synthesize a polyelectrolyte~MOF membrane and its performance as an ion-selective membrane. Highly porous and well-packed poly(sodium vinyl sulfonated-co-acrylic acid)~MIL-53(Al) is synthesized onto a nanoporous anodic aluminium oxide (AAO) substrate. The AAO supported membrane is tested for transport of alkali metal cations under a concentration gradient.

Structure and characterization of poly(sodium vinyl sulfonated-co-acrylic acid)~MIL53(Al)/AAO membrane The fabrication of poly(sodium vinyl sulfonated-co-acrylic acid)~MIL-53(Al) on AAO is shown schematically in Figure. 1 and detailed in Methods in Supporting Information. A layer of MIL-53 is first grown on top of an AAO (diameter=1.1 cm, thickness=0.05 mm) circular disc containing aligned 20 nm pores supplied by Whatman. Sodium vinyl sulfonate (VS) monomer and acrylic acid (mole ratio of 20:1) are impregnated into the MOF layer and allowed to polymerize via thermal activation. The acrylic acid copolymer can enhance the thermal stability of PVS. Carboxylic groups (-COOH) interact with hydroxyl (-OH) groups on


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MIL-53 via hydrogen bonding, which can prevent the leaching of poly(VS-co-AA). As shown in SEM image of Figure. 2a, a well-packed MIL-53 layer on AAO is formed without any cracks. Similarly durable and crack-free MIL-53 coated AAO membrane was reported in a gas permeation study21. The integrity of the membrane after repeated usage was demonstrated with a SEM image in Fig. S16. Inset of Figure. 2a reveals crystalline prisms of MIL-53 identical to those of pristine MIL-53. The as-prepared MIL-53/AAO membrane was heat treated to remove residue tetraphthalic acid with corresponding TGA curves shown in Figure. S1. The MIL-53 layers are initially grown on both sides of AAO. To facilitate ion-transportation, the MIL-53 layer on one side of the AAO was removed by wiping with 1 M NaOH, and the exposed AAO surface has no evidence of MIL-53 in the SEM image of Figure. S2. There is also no evidence of MIL-53 penetration into nanopores of AAO as confirmed by XRD pattern in Figure. S3 after the NaOH treatment. After the synthesis of poly(VS-co-AA) into the MIL-53 channels, the membrane was soaked in water for 1 day and washed in acetone (ESI 1.2). Should there be poly(VS-co-AA) on the surface of MIL-53, it would have been dissolved and washed off. The poly(VS-co-AA)~MIL-53 membrane maintained its ionic selectivity over repeated iontransport experiments, again supporting the polymer to be located inside the MIL-53 channels.

Uniform distribution of poly(VS-co-AA) within the nanochannels of MIL-53 is confirmed by EDX mapping of a cross section of poly(VS-co-AA)~MIL-53 membrane (Figure. S4 and S5). The elemental ratios of Na, S, and C from EDX match the molecular formula of MIL-53 and poly(VS-co-AA). From the C:S ratio determined with the EDX data in Figure. S4, the average loading of poly(VS-co-AA) in MIL-53 is calculated to be ca. 6.7 wt.% (calculations described in ESI). EDX mapping of the membrane cross-section in Figure. S5 confirm that only the top layer of AAO contains MIL-53 and the bulk channels of AAO are still kept empty, free of


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MIL-53 and polymers. The poly(VS-co-AA)~MIL-53 layer is ~600 nm, thinner than other reported MIL-53 membranes of > 8 µm.20, 21 After poly(VS-co-AA) incorporation, the XRD pattern of Poly(VS-co-AA)~MIL-53/AAO (shown in Figure. 2b) is identical to that of MIL-53/AAO and bulk MIL-53 which has a HT phase of MIL-53, as alumina reacts with teraphthalic acid in water22. Local HT phase is formed at the membrane/AAO interface and the HT phase has 8.5 Å ×8.5 Å fully open channels.23 Poly(VS-co-AA)~MIL-53 layer has chemical resistance against acid. The XRD pattern of poly(VS-co-AA)~MIL-53 layer remains unchanged after soaking in 10 mM HCl at room temperature for 7 days. Though breathing effect with a transition to a LT phase of compressed channel structure have been observed in the literature using Al(NO3)3 and other aqueous soluble aluminium source23,25,28, recent studies21,22 show only HT phase when insoluble aluminium source such as aluminium oxide is used as the metal source. The HT phase of AAO synthesized MIL-53 is stable when exposed to water and also temperature variations. As shown in Figure 2b. From the XRD patterns, there is no evidence of breathing effect or presence of LT phase for our MIL-53 membrane even after long immersion in aqueous solution. Furthermore, there are no evidence of a phase change or definite distribution of polymer in the poly(VS-co-AA)~MIL-53 sample as shown in the difference between the polymer filled and unfilled XRD patterns (Fig. S17 of ESI).

To confirm successful polymerization of vinyl sulfonate sodium and acrylic acid within MIL53, the poly(VS-co-AA)~MIL-53/AAO is alkaline etched to release the polymer for 1H nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) analysis. As observed in the NMR spectra (Figure. S6) of poly(VS-co-AA)~MIL-53/AAO, there are several peaks appear in the chemical shift range of 1 to 4 ppm, that can be assigned to the H on the hydrocarbon chain of poly(VS-co-AA).24 For comparison, a bulk poly(VS-co-AA) is prepared


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in solution approach with the same VS/AA ratio of 5:1. Identical peaks are observed in the same range of chemical shift for the bulk poly(VS-co-AA). The small discrepancies in NMR spectra can be attributed to the difference in molecular weight distribution and tacticity of the two samples.25 The number-average molecular weight of poly(VS-co-AA) determined by GPC as shown in Figure. S7 is ca. 5,600. Provided the polymer is present as a fully extended conformation, average length of a polymer inside the channel of MIL-53 is estimated to be ca. 19.7 nm, ensuring the threaded polymer to be locked inside the channels of MIL-53 and does not leached out during ion transport experiments. This length is much smaller than the thickness (600 nm) of poly(VS-co-AA)~MIL-53 active layer, thus there are still extra room to allow transportation of polymer chains and ions. This low M.W. poly(VS-co-AA) should be soluble and would be washed off should the polymer resides on the surface of MIL-53. Further confirmation of sulfonate groups is provided by FTIR spectrum of poly(VS-co-AA)~MIL53/AAO membrane and compared to that of MIL-53/AAO. Typical absorption bands of symmetric SO2 stretching is observed at 1193 cm-1 in the poly(VS-co-AA)~MIL-53/AAO sample, as shown in Figure. S8, while no SO2 stretching peak is observed in MIL-53/AAO membrane. The carboxylic peaks of poly(VS-co-AA) overlap with that of MIL-53, with peaks broadening between 1550 and 1700 cm-1.25 Compared to MIL-53/AAO, the shift of OH bending band of MIL-53 from 994 cm-1 to 1046 cm-1 implies the formation of hydrogen bond between carboxylic groups on poly(VS-co-AA) and OH group on MIL-53 for the poly(VS-coAA)~MIL-53/AAO membrane. Similar red-shift of OH group upon interacting with CO2 has been observed by Feréy et al.26

The poly(VS-co-AA)~MIL-53/AAO membrane is porous. The porosity of MIL-53 and poly(VS-co-AA)~MIL-53 are measured by N2 sorption experiments with results given in Table S1. As shown in Figure. 2c, the pore size determined by the HK model with cylindrical


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geometry is ca. 0.87 nm, which is in agreement with earlier reference21,22 and close to the 0.85 nm value from crystal structure of MIL-53 and sufficiently large to encapsulate VS and AA monomers. The pores size decreases to 0.80 nm after loading of poly VS-co-AA with a corresponding pore volume of 0.34 mL/g compared to 1.1 mL/g of MIL-53. A widen pore size distribution for poly(VS-co-AA)~MIL-53 is consistent with the gradual uptake in the initial stage of the isotherm can be explained by the presence of occupied and unoccupied channels. Since there are possible issues of issues of closed end capillaries,29 differential site-site interactions, and hierarchical filling of channels30 in the case of poly(VS-co-AA)~MIL-53, we have also performed the classical HK analyses which assumes a slit-pore geometry with results shown in ESI Fig. S18. Pore volume after polymer loaded decreases from 0.59 mL/g to 0.14 mL/g. Thus, maximum water capacity could be estimated to be 13.8 wt.% (based on the poly(VS-co-AA)~MIL-53 layer). The decrease in pore volume of MIL-53 suggest a 23% filling of MIL-53 channel voids, with a residue volume of 76% that are accessible to nitrogen. Since poly(VS-co-AA) is a much larger molecule than nitrogen, it cannot fill all the volume that is accessible to nitrogen due to imperfect fitting at the molecular level. Computer simulation results suggest a 31% dead volume in the MIL-53 that cannot be accessed by the polymer.

From the TGA (Figure. S9) and the estimated loading of poly(VS-co-AA) by EDX (Figure. S5), we can estimate the ion-exchange capacity of each piece of poly(VS-co-AA)~MIL-53 to be around 0.42 µmol/cm2 (Details on the calculation as in SI). MIL-53 layer is thermally stable until ca. 500oC.

Permselectivity of poly(sodium vinyl sulfonated-co-acrylic acid)~MIL-53(Al)/AAO membrane


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Permselectivity of the poly(VS-co-AS)~MIL-53/AAO membrane is studied with a twochamber cell as shown in Figure. S13 and illustrated schematically in Figure. 3a, and the experimental procedure is detailed in Supporting Information. No external electric field is applied and ion transport is driven by concentration gradient between the two chambers. While different pairs of left chamber electrolyte and right chamber electrolyte are studied, the left chamber is filled initially with protons at a higher concentration than that in the right chamber. Proton flux from left to right is monitored together with corresponding changes of other ions in either chamber. For convenience, we denote the left chamber as the proton feeding chamber (PFC) and the right chamber the proton receiving chamber (PRC). The studies are made at dilute concentrations of 1 to 5 mM. At these low concentrations, the Debye screen length for a monovalent ion at room temperature is 3 to 10 nm, which is much larger than the pore size of MIL-53. Cation selectivity is studied with 5 mM initial HCl concentration in the left chamber and 5 mM chloride of an alkali metal cation in the right chamber, as shown in Figure. 3a. Anion transport is absent since experiments are conducted with identical anions at the same concentrations in both chambers and the cation exchange polyelectrolyte-MOF membrane should reject anion passage. As shown in Figure. 3b concentrations of H+ in the receiving (right) compartment and K+ in the feeding (left) compartment increase almost identically with time. In the absence of anion exchange due to identical Cl- concentrations on either side, the fluxes of H+ and K+ must be equal at all times to maintain electroneutrality. Hence the increase in [H+] in the right chamber is equal that of [K+] in the left chamber. The small difference between [H+] and [K+] in Figure. 3b is within error bars of pH measurement and ex situ ICP-OES determination of [K+]. The diffusivity of proton is an order of magnitude higher than those of alkali metal cations. Ion transport in both directions is therefore limited by diffusion of the metal cation. The time profiles of each ion concentration can be used to quantify diffusion rate of


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individual metal cations by Eq. S2. As shown in Figure. 3c, the measured permeability of ions through the polyelectrolyte~MOF membrane has an inverse correlation with their hydrated diameters. The hydrated diameters of the alkali metal cations21 are close to the pore size of MIL-53 at 0.85 nm. Although ion transport may take place with stripping of hydration shell in sub-nanometre pores, results shown in Figure. 3b suggest transport selectivity due to a hydrated ion. Control experiment of AAO membrane (ESI) shows a several orders increase in ion transport compared to polymer~MIL-53/AAO membrane, indicating the polymer~MIL-53 layer to be dictating ion transport and selectivity. On the other hand, the other control experiment of MIL-53/AAO membrane without poly(VS-co-AA) shows zero ion transport due to exclusion forces of molecular channels and absence of charge sites in the MOF. Replacing the single ion solution in Figure 3(a) with a 2.5 mM CsCl + 2.5 mM (nBu)4NCl mixed solution shows ion exchange dominated by Cs+ transport rate which is about 8 times that of (Bu)4N+ as shown in ESI Fig. S21, confirming the selectivity indicated in Figure 3(c). Molecular Dynamics simulation on the interaction of alkali ion (Li+, Na+, K+ and Cs+) in the presence of MOF and poly-VS-co-AA threaded MOF in water There are factors that support the transport of a hydrated ion: i) MIL-53 is hydrophilic and ii) at the dilute concentration, the Debye length is much larger than the channel width of MIL-53. To quantitatively analyse these factors, molecular dynamics simulations are carried out (details shown in Supplementary information and Figure. 4). Each alkali ion is subjected to different water environments: in bulk water; in water filled MIL-53; in water filled MIL-53 with polyVS-co-AA threaded inside its channels and its empty neighbouring channel (Figure. 4). In Figure. S15, the radial distribution functions of water indicate strength of hydration decreases in the order of Li+ > Na+> K+ > Cs+. This order remains unchanged whether water is in the bulk, inside a MIL-53 pore, inside a MIL-53 pore threaded with (VS-co-AA)3. As listed in Table 1,


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the radial distribution functions of water away from a cation have higher peak values inside MIL-53 compared to bulk water. This hydration around cation is further strengthened when MIL-53 is threaded with (VS-co-AA)3. Hence, ion selectivity of poly(VS-co-AA)~MIL53/AAO membrane can be analysed with consideration of an hydrated ion. The (n-Bu)4N+ cation with a diameter of 0.99 nm does not hydrate and its selectivity or exclusion is based on exclusion forces. Among the cations tested, the minimum flux is observed for (n-Bu)4N+ at 0.26 mmol m-2h-1. Thus the sieving action of poly(VS-co-AA)~MIL-53/AAO membrane is demonstrated. For Cs+ with a hydrated diameter of 0.66 nm, the flux is 1.2 mmol m-2h-1.

Since the loading of poly(VS-co-AA) in MIL-53 is only 6.7 wt%, it is likely that cation is transported via an empty MIL-53 channel not occupied with polyelectrolyte. At this loading, however, the polymer chains are distributed on average with alternate threaded channels and unthreaded channels. Figure. S15 and Table 1 include MD simulations results of cation hydration in an unthreaded MIL-53 pore next to a threaded pore. The same order of hydration Li+ > Na+> K+ > Cs+ in MOF with presence of (VS-co-AA)3 is shown, indicating that the fixed charge of (VS-co-AA)3 can extend its Debye length into a neighbouring unoccupied channel. In addition to selective ion transport of alkali metal cations, exclusion of anions based on Coulomb forces can be demonstrated by experiments with anion gradients across the poly(VSco-AA)~MIL-53/AAO membrane. The ion transport experiments are repeated with different anions in the proton feeding chamber and proton receiving chamber, as shown in Figure. S14. In each test, anions concentrations remain unchanged >24 h in both chambers while similar cation exchange are observed as before. The anions used Cl- and NO3- have small diameters < 0.7 nm. They are excluded due to like charge repulsion. The ion transport experiments demonstrate that small cations with little hydration, like K+ and Cs+, can readily undergo ion-


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exchange with proton. On the other hand, ion flux dramatically decreases when the hydrated diameter of cation increases to 0.7 nm while anions of different sizes are all excluded. The combination of uniform microporous framework of MIL-53 and the sulfonate anions of poly(VS-co-AA) provides exclusion of anions and sieving of cations according to their hydrated diameter. The size selectivity of alkaline metal cations is approaching that of biological ion channels. In principle, higher selectivity of ion transport can be provided by a higher loading of poly(VS-co-AA) into the framework. But at the same time, transport rate will be lowered due to increased vdw exclusion and reduced porosity. An optimal loading can be determined by the trade-off of selectivity and transport rate. The minimum loading required for selective transport also depends on ionic strength of the solution and the resulting Debye length. Donnan exclusion will be enabled for a low concentration electrolyte even at low polyelectrolyte loading in the framework. Although the poly(VS-co-AA)~MIL53/AAO membrane cannot be applied in alkaline media, the preliminary results suggest possible applications for low pH and neutral aqueous solutions. Exploration to biomedical context with pH 7.4 should be conducted in future. Given the diverse structures of available MOF frameworks, threading polyelectrolyte within the nanochannels of MOF would provide a versatile class of polyelectrolyte~MOF membranes to cater precise separation and recognition of different ions in aqueous or aprotic media.


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Smoothed dV/dw (cm³/g·nm)

c) 800 3 -1

Volume Adsorbed (cm g )

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600

MIL-53 (Al) 400

PVS-AA-MIL-53(Al) 200

0

0.0

0.2

0.4

0.6

p/po

0.8

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0.87 2.5 2.0 1.5

Poly(VS-co-AA)~ MIL-53 MIL-53

1.0 0.5 0.80

0.0

1.0

1

2

3

Pore Width (nm)

Figure. 2. (a) SEM images of poly(VS-co-AA)~MIL-53/AAO membrane (scale bar: 10 µm) (inset): high magnification (top view, scale bar: 200 nm); (b) XRD patterns of AAO substrate, simulated MIL-53, MIL-53/AAO membrane, poly(VS-co-AA)~MIL-53/AAO membrane and poly(VS-co-AA)~MIL-53/AAO membrane after soaking in 10 mM HCl for 7 days. (c) N2 isotherm of MIL-53 and poly(VS-co-AS)~MIL-53 and their respective pore size distribution analysed by the H-K model assuming a cylindrical pore.


(c) 2.0 +

K

-1 -2

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Ion Flux / mmol m h

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1.5 +

Cs

1.0 Na+

Li+

0.5

(n-Bu)4N+

0.0 0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

Hydrated Diameter / nm

Figure. 3. (a) Set-up of ion transport experiment. The poly(VS-co-AA)~MIL53/AAO is placed in the middle of the flange connecting the two chambers, with HCl on the left chamber and KCl in the right chamber. Protons transport from left to right, while K+ ions transport from right to left. For each run, the anions in both chambers are kept the same. (b) Proton concentration change at receiving (right) chamber and potassium concentration at proton feeding chamber (left chamber). (c) Proton fluxes of the corresponding cations in the receiving cell. The hydrated diameters are obtained from Ref.[26]


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Table 1. The value of the first peak in radial distribution functions (RDF) of water away from an X ion (where X is Li+, Na+, K+, or Cs+) (Figure. S15) in i) bulk water, ii) a channel of MIL53, iii) an empty channel next to a channel of MIL-53 threaded with poly(VS-co-AA), iv) a channel of MIL-53 threaded with poly(VS-co-AA), corresponding to the four cases schematically shown in Figure. 4(a-d).

Ion

Bulk water

A channel of

Empty channel next to

A channel of

MIL-53

poly(VS-co-AA)~MIL-53

poly(VS-co-AA)~MIL-53

Li+

14.0

41.5

33.2

43.7

Na+

8.6

23.8

16.0

25.3

K+

5.3

15.0

7.9

15.8

Cs+

3.4

7.9

4.5

8.2

ASSOCIATED CONTENT

Supporting information. Details of methods of synthesis, materials characterizations, molecular dynamics calculations, Figures S1-S121, and Tables S1, can be found in Supporting Information.

AUTHOR INFORMATION

Corresponding Author * K.Y.C. (email: [email protected])


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Author Contributions

The manuscript was written through contributions of all authors. All authors have

given approval to the final version of the manuscript.

Funding Sources Hong Kong Research Grants Council via General Research Fund (GRF 17301415) and NSFCRGC Joint Research Scheme (N_HKU728/17); NSFC (Grant No. 51603046) and Guangzhou Science and Technology Program (201804010243).

Acknowledgements This work is financially supported by Hong Kong Research Grants Council via General Research Fund (GRF 17301415) and NSFC-RGC Joint Research Scheme (N_HKU728/17). L.G. also gratefully acknowledges the financial support from NSFC (Grant No. 51603046) and Guangzhou Science and Technology Program (201804010243). The computations were performed using The University of Hong Kong’s Information Technology Services (ITS) research computing facilities that are supported in part by the Hong Kong UGC Special Equipment Grant (SEG HKU09). Electron microscopy work were performed in the Electron Microscopy Unit (EMU) of The University of Hong Kong and the authors thank Mr. Frankie Chan (EMU) for his assistance.


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Highly Selective Transport of Alkali Metal Ions by Nanochannels of

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