Biomimetic Liquid-Sieving through Covalent Molecular Meshes

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Biomimetic liquid-sieving through covalent molecular meshes Minseon Byeon, Jae-Sung Bae, Seongjin Park, Yun Hee Jang, and Ji-Woong Park Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03884 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 19, 2016

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Chemistry of Materials

Biomimetic liquid-sieving through covalent molecular meshes Minseon Byeon, Jae-Sung Bae, Seongjin Park†, Yun Hee Jang† and Ji-Woong Park* School of Materials Science and Engineering and Research Institute for Solar and Sustainable Energies, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju, 61005, Korea † Current address: Department of Energy Systems Engineering, DGIST, Daegu 42988, Korea *Corresponding author: [email protected]

ABSTRACT. The porin pores of biological cell membranes enable molecules to be sieved out selectively while water molecules traverse the channel in a single file. Imitating this streaming mechanism is a promising way to create artificial liquid-sieving membranes, but ultrathin molecular pores need to be produced in a large membrane format to be functional under high transmembrane pressures. Here we show that a membrane composed of a covalent molecular mesh can filter mixtures of small molecules in a liquid by the porin-like mechanism. Tetrahedral network formers are polymerized layer-by-layer on a nanoporous substrate to yield a thin layer of a covalent molecular network containing an array of molecular meshes grown by a porelimited mechanism. Each of the meshes exhibits high water permeability, estimated to be greater

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than 2,500 Lm-2h-1. Glucose or larger molecules are selectively sieved out while the solvent and solutes smaller than glucose traverse the mesh.

Introduction Aquaporins in biological cell membranes selectively conduct water while rejecting glucoses and larger molecules.1-6 A distinct feature of their pore structure is a single constriction site that forms a barrier to the large molecules.7-10 These pore-forming proteins span the lipid membrane and are organized to form a nanochannel that is made narrow at the one site by polar functional groups facing into the channel, and that sieves solutes larger than the pore width while allowing a continuous flow of liquid molecules in single file.5-7,11 The distance between consecutive water molecules flowing through the constricted site of the porin is similar to the intermolecular distance in the bulk water. Synthetic membranes comprised of pores with porin-like molecular constriction sites are expected to enable high throughput separation of solution mixtures in a highly selective manner.12-17 The molecular pore structures, however, must be formed on a largearea membrane sufficiently robust against high operating transmembrane pressure. A promising structure for a synthetic liquid-sieving membrane is a mesh made of covalent molecular networks. An ultrathin slice of a microporous covalent network18-20 would be in fact a two-dimensional molecular mesh, through which liquid molecules can flow in a single file without being separated by more than their intermolecular distance. (The ultrathin covalent network is termed as a covalent molecular mesh below). However, the smallest thickness of films that have been prepared from microporous network materials21-30 is still several orders larger than the intermolecular distance of common liquids and thus too large for allowing a streamlined flow of liquid molecules through such films. Therefore, microporous network


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materials have been mainly studied for separating gases with disparate molecular diffusivities through a relatively thick matrix.20,31-39 Here we describe the preparation of a covalent molecular mesh membrane that allowed porinlike liquid transport and enabled a highly selective sieving of solutes from a solution. The membrane was fabricated by alternatingly reacting a pair of rigid tetrahedral network formers (NFs), tetrakis(4-aminophenyl)methane (TAPM) and tetrakis(4-isocyanatophenyl) methane (TIPM)40 (Fig. 1a), onto a nanoporous alumina substrate. While the NF molecules yield a conformal film of molecular network when deposited layer-by-layer onto the flat substrate surface in solution, they give a non-uniform film containing an array of ultrathin covalent molecular mesh as deposited on a nanoporous substrate, through which water molecules can be traversed by the single-file flow mechanism of porins.

Results and Discussion. A possible mechanism for the growth of a molecular mesh on a nanoporous substrate is illustrated in Fig. 1. In this mechanism, when NFs are made to polymerize sequentially onto the substrate with an initial pore diameter Do, the network film thickens in a conformal manner on both the inner pore surface and the top surface of the support. Here, the thickness of the network film would be expected to increase to λ N after repeating the polymerization N cycles, in which the increase in the thickness per polymerization cycle is λ. [A layer-by-layer (LBL) crosslinking polymerization of the TAPM and TIPM NF molecules on a flat, non-porous surface showed that

λ was 2 nm for a full deposition cycle (with a full deposition cycle consisting of one cycle with TAPM and one cycle with TIPM).40] For cylindrical pores, the network film is modelled to grow


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radially from the pore wall, with the pore diameter thus reduced to Do–2λN after N cycles of LBL polymerization. As multiple LBL polymerization cycles are repeated, the pore width would be expected to reach a limiting diameter (D*) below which the pore is too narrow for the NF molecules to enter, with the network layer then no longer growing identically both inside and outside of the pores. In this model of network growth, once D* is reached, the pore interior remains unfilled regardless of the number of LBL cycles. In contrast, the network on the non-porous region of the top surface in contact with the bulk solution should continue to grow by the normal LBL mode. Under this pore-limited condition, the network extends laterally from the edge along the circumference of the pore opening. As a result, a freshly deposited layer on the top would be widened by as much as the size of the NF molecule. The pore openings in this model become constricted with further polymerization cycles and eventually sealed off with the mesh of the covalent molecular net as shown in Fig. 1c-e. The width of covalent molecular mesh formed in the last sealing cycle should be approximately equal to the length of two NF molecules (i.e., about 2 nm). Further deposition of NFs over the sealed covalent mesh would thicken the film by the normal LBL mode as shown in the schematic drawing of Fig. 1d with N=10.


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Figure 1. Preparation of covalent molecular meshes using layer-by-layer cross-linking polymerization of network formers (NFs) on the nanoporous substrate. (a) The chemical structures of the two NFs we used and the proposed sequence of layer-by-layer network formation on a flat surface. (b) An optical photograph and schematic representation of a crosssectional view of the porous alumina substrate, with the scale bar corresponding to 100 nm. (c, d) Schematic drawings of the top and side views, respectively, of the membranes formed at


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polymerization cycle numbers (N) 0, 4, 6, and 10. Do, initial pore diameter of bare alumina; D*, the critical pore diameter below which the NFs could not reach the pore interior; ℓ, the thickness of the film formed above the molecular mesh when the polymerization cycle number exceeded the critical number; λ, the increase in the thickness per polymerization cycle; λN, total thickness of the film grown on the non-porous region. (e) Schematic drawing of the covalent molecular mesh formed at the opening of the nanopore.

The substrate employed in the present work for building a covalent molecular mesh was a 60 µm-thick, asymmetric porous alumina membrane consisting of a 100 nm-thick active layer with pores having an average width (Do) of 20 nm and the bottom layer consisting of vertical cylindrical pores with an average diameter of 200 nm (Fig. 1b). The alumina membrane was treated with 3-aminopropyltriethoxysilane (APTES) to obtain the amine-functionalized surface onto which the TAPM and TIPM NFs were alternatingly reacted. Based on the sizes of these NFs, the diameter of the cylindrical pore would be expected to be reduced by 4 nm per LBL cycle and thus the pore openings of the alumina membrane with an average width of 20 nm would be expected to be sealed on average after 5 cycles. The number of polymerization cycles that completely sealed all of the open pores actually appeared to be greater than 6, which was most likely caused by an irregular pore size and shape of the alumina substrate. For the pores with an oval shape, the short-axis diameter determines the critical cycle number for sealing the pore because the networks growing in the short-axis direction join each other first. Scanning electron microscopy (SEM) images of the membranes produced with different numbers of polymerization cycles showed that the covalent molecular mesh indeed formed (Fig.


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2). Top-view images (Fig. 2,a-d, f, and g) showed evidently that the pore openings narrowed with increasing N. Open pores were visible on the membranes with N ≤ 5, whereas they were absent on the membrane with N ≥ 6. Cross-sectional SEM images evidently showed that the cylindrical cavities underneath the top layer were left unfilled in all samples. A conformal morphology of the network films coated on the porous substrate was revealed when the alumina substrate was etched away with sodium hydroxide (Fig. 2e). The thickness of the coating at N = 10 grown on the top of membrane was about 20 nm (Fig. 2g), confirming a linear growth of layer by 2 nm per deposition cycle even on the rough porous substrate. A TEM image (Fig. 2h) of an ultrathin cross-section of the composite membrane with N=6 showed vertical alumina walls coated with thin urea network layers that appeared relatively dark when stained with RuO4. The roughness of the coating on the porous surface, estimated by atomic force microscopy(AFM) shown in Figure S1, appeared to become reduced by planarization with LBL cycles over sealed pores. The roughness of the network film on the flat surface was reported to be below 2 nm.40 All of these data were consistent with the network on the nanoporous support having grown according to the pore-limited mechanism proposed in Fig. 1. Intriguingly, the molecular meshes were successfully generated despite the irregular size and shape of the substrate pores.


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Figure 2. Structural characterization of covalent molecular mesh membranes formed by applying different numbers of LBL polymerization cycles. (a-d) Top-view SEM images of the membranes deposited by applying 0, 4, 5, and 6 polymerization cycles. (e) An SEM image of the residue obtained after selectively etching the alumina from the 6-cycle-deposited membrane with 3M NaOH. (f) A low-magnification, top-view SEM image of the 6-cycle-deposited membrane. (g) The top-view SEM image of the membrane deposited by applying 10 polymerization cycles, i.e., above the critical number. The thickness of this film was measured to be about 20 nm. (h) A TEM image of a 100 nm-thick, microtomed cross-section of the 6-cycle-deposited membrane stained with RuO4. The urea network layer appeared darker. Insets in a-d and g are side views of the membranes.


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The formation of the molecular mesh was further evidenced by the variation of water flux through the membranes with increasing the deposition cycle number, N (Fig. 3). In Fig. 3a is shown how the pore diameter and the film thickness change with N. The steady state flux of water decreased steeply as the number of deposition cycles was increased from 0 to 5, as predicted by the Poiseuille law (flux being proportional to the square of pore size).41 When N was increased to greater than 5, the flux trend line deviated from the Poiseuille dependence and instead exhibited an inverse linear relationship with N. Considering that the film thickness increases 2 nm per deposition cycle for N larger than 5,40 the result may be accounted for by the general relationship between flux and thickness in transport through a dense membrane (Fig. S2b). The change in the relationship between the flux and N is a strong indication of the transition from transport occurring through the pores to transport occurring via diffusion when the membrane pores became sealed with the molecular network. Our hypothesis is supported further by the dependence of the flux on transmembrane pressure, as shown in Fig. 3c. For the membrane prepared with N smaller than 5, the water flux increased in proportion to the operating pressure as expected by Poiseuille pore flow model. In contrast no change of flux was observed upon increasing the pressure when the nanopores were completely sealed with the network by increasing N above 5. The data indicated that the membrane prepared by applying six deposition cycles was similar to the anticipated model of a covalent molecular mesh shown in Fig. 1. The permeability of water through the net formed by applying 6 LBL cycles measured at different temperatures exhibited an Arrhenius dependence (Fig. S3), with an activation barrier estimated to be about 3.31 kcal/mol. The activation barrier was much smaller than those (4.3~7.2 kcal/mol) generally observed for reverse osmosis or nanofiltration membranes.42 It was also smaller than the


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activation barrier (4.3 kcal/mol) for self-diffusion of water.43 The activation barrier for the passage of water through our molecular mesh was determined to be only slightly larger than the barrier (3.0 kcal/mol) for passage of water through the human red cell AQP1.1 This barrier has been regarded to be related to the hydrogen bonding between a water molecule and the pore surface. We postulate that the transport of water through the covalent molecular mesh was also governed by hydrogen bonding, most likely between water and urea. The flux of water through the molecular mesh membrane (formed at N=6) was about 25 Lm-2h1

. As shown in Fig. 3b, the water flux through the membrane at N=8 was reduced to about 20%

that at N=6. The flux through the mesh at N=7, which should be only 2 nm thicker than that at N=6, decreased about 50 %. This result indicates that a molecular mesh with a thickness exceeding 4 nm (that corresponds to the thickness by two LBL cycles) contributes negligibly to the permeation of water. These considerations make it reasonable to assume the average thickness of molecular mesh formed by 6 LBL cycles to be 2 nm which is the thickness corresponding to one full LBL cycle with the NF pair. With an approximation that the last cycle of LBL deposition yields a 4 nm-wide circular mesh suspended over each nanopore, the flux through each molecular mesh, regardless of the operating pressure, may be estimated to be greater than 2,500 Lm-2h-1. We tested the molecular mesh membranes for transport of various organic solvents as shown in Figure S9. It was found that non-polar solvents such as hexane or toluene does not traverse the membrane, whereas, other polar organic solvents including methanol, ethanol, acetonitrile, acetone, THF, and DMF showed moderate to high fluxes. Steady fluxes of organic solvents for more than 20 h under high pressure of 10 bar suggest the robustness of molecular mesh to organic chemicals.


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Figure 3. Water transport through the composite membranes formed by the pore-limited LBL network deposition. (a) The change of the pore diameter (D), the total thickness (λN) of LBLdeposited network on the non-porous region, and the thickness (l) of active molecular mesh formed over each pore, as a function of the LBL cycle number (N). (D, λN, and l are designated in Inset). (b) and (d) Water flux at 10 bar through the membranes prepared with different N. (c) Flux vs operating pressure for the membranes deposited with the network with N=0 (bare alumina, dark squares) and N=4 (red circles).


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Figure 4. Molecular sieving of aqueous solutions through the covalent molecular mesh. (a) Percent of different solutes rejected by the molecular mesh membranes made by applying different numbers of polymerization cycles (N). Aqueous solutions of 5000 ppm glucose, 0.5 mM Ts-γ-CD, and 500 ppm MgSO4 were passed through the membrane at 10 bar. (b) Percent rejections of various solutes plotted against their average sizes when passed through the membrane prepared with an N of 5, 6, and 7, respectively. The molecular species tested (and their sizes) were water (0.2 nm), sodium ion (0.37 nm), magnesium ion (0.43 nm), glucose (0.7 nm), and Ts-γ-CD (1.5 nm). (c) HPLC chromatograms of the mixed urea/glucose solution in feed and its permeate through a molecular mesh membrane (with N=6). The feed solution contained 0.5 wt/v% urea and 0.5 wt/v% glucose in water.


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The molecular sieving performance was further measured by passing aqueous solutions of cyclodextrin, glucose, magnesium sulfate (MgSO4) and sodium chloride (NaCl) through the molecular mesh formed by applying different values of N (Fig. 4). The percent of glucose molecules rejected by the membranes was 0% for N < 6, 90% for N = 6, and 95-100% for N > 6. Cyclodextrin was rejected nearly completely for N ≥ 6 but not at all for N ≤ 5. The lack of rejection of cyclodextrin at N=5 suggested that the membrane formed by 5 LBL cycles had pores much larger than the size of cyclodextrin (1.5 nm). In the meantime, magnesium sulfate passed through the 6-cycle-deposited membrane but was rejected nearly completely by the membranes with N ≥7, which displayed very low permeability. This result is consistent with the pure water permeation data in Fig. 3 that showed ineffectiveness for liquid permeation of the covalent mesh formed with N above 7. The molecular meshes are likely comprised of ring structures (Fig. 5) formed by connection between two or more NFs. The molecular weight cut-off (MWCO) of this mesh was observed to be smaller than the size of glucose, i.e., about 7 Å (Fig.4b). But this size is less than the size of rings that NFs can in principle form if these rings were not twisted. This discrepancy was accounted for by the structural models of the rings consisting of more than 4 tetraphenylmethyl repeating units: these rings were found to be twisted due to the formation of hydrogen-bonding between urea groups (See supporting materials for details). The distribution of pore sizes and shapes of the support on which the molecular meshes form are important factors influencing the structure and performance of the molecular mesh membrane. A few extra polymerization cycles were in general needed to eliminate pinholes in the support with pore size distribution. When the molecular meshes were formed on a different alumina membrane with an average pore diameter of 18 nm and with a relatively uniform distribution,


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and these pores were sealed with 4 LBL polymerization cycles (Fig. S6 and S7). Our porelimited deposition of molecular network for preparing covalent molecular meshes may be applicable to different types of porous substrates that can be fabricated using advanced lithography or self-assembly techniques.

Figure 5. Models of ring structures constituting the molecular meshes. (a) Schematic depiction of a ring showing the structural formula of the constituent repeating tetraphenylmethyl-based unit linked by two urea bonds. Note that two other urea bonds emanating from the real tetraphenylmethyl node of the network were excluded for simplification. Calculated models of


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rings consisting of (b) 2, (c) 3, and (d) 4 units. The dotted line in (d) indicates hydrogen bonding between urea groups.

Conclusion We report that a membrane composed of a covalent molecular mesh can filter mixtures of small molecules in a liquid by the porin-like mechanism. Tetrahedral network formers were polymerized layer-by-layer on a nanoporous substrate to yield an ultra-thin layer of a covalent molecular network containing an array of molecular meshes. The network-forming molecules did not enter the nanopores narrower than a critical size during the solution-deposition process, resulting in the formation of a non-uniform layer containing an array of ultrathin covalent molecular meshes. We demonstrate that the resultant covalent molecular meshes exhibit high flux transport of water with highly selective separation power for the sub-nm-size solutes.

Methods Materials. Anhydrous N,N-dimethylformamide (DMF, 99.8%, Aldrich), toluene (99.8%, Aldrich)

and

3-aminopropyltriethoxysilane

(APTES,

99%,

Aldrich),

1,8-

diazabicyclo[5,4,0]undec-7-ene (DBU, 98%, Aldrich), porous alumina supports (Whatman and Synkera, USA), (D)-glucose (>99.5%, Aldrich), magnesium sulfate (MgSO4, >99.5%, Aldrich), urea (Aldrich), mono-6-O-(p-toluenesulfonyl)-γ-cyclodextrine (Ts-γ-CD, >90%, TCI), ruthenium tetroxide (RuO4, 0.5% aqueous solution in water, Acros organics) were purchased and used as received. Anhydrous tetrahydrofuran (THF) was freshly distilled under nitrogen from Na/bezophenone ketyl radical prior to use. Tetrakis(4-aminophenyl)methane (TAPM), tetrakis(4isocyanatophenyl)methane (TIPM) was synthesized by the reported method.40


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LBL cross-linking polymerization. The substrate (nanoporous alumina) was cleaned by treating with air-plasma (Harrick Plasma Cleaner PDC-32G) for 15 min and then immersed in 2 w/v % APTES solution in toluene for 2h under a nitrogen atmosphere. Before starting the LBL cycle, baths of the TIPM and TAPM monomers were dissolved in THF in concentration of 0.2 w/v %. DBU was added to each monomer solution in a concentration of 2.7 mM as a catalyst. The substrate with the amine-terminated surface was first immersed into the TIPM bath for 1h to deposit an isocyanate-saturated layer. Long immersion time was used to ensure fully saturated functionalization of the APTES coated surface with TIPM. After this first cycle, immersion time of the substrate in both monomer baths was set identically for 20s. The substrate was taken out from the monomer bath and then cleaned successively by dipping and shaking in three baths of pure solvents: DMF, DMF, and THF. The isocyanate-terminated substrate was then immersed in TAPM bath for 20s and the substrate was washed again with pure solvents. After repeating desired LBL deposition cycles, the substrate was cleaned and dried overnight at room temperature under reduced pressure. Scanning electron microscopy (SEM). SEM was performed on a JOEL JSM-6700F scanning electron microscope. The membrane samples were immersed in liquid nitrogen and fractured. The samples were dried in vacuum overnight and then stained by 0.5wt% aqueous RuO4 solution for 30min. The samples were then sputter-coated with about 3 nm of platinum before imaging. The membrane specimen whose alumina support was removed was prepared by immersing the network-deposited membranes in a 3M aqueous NaOH solution for 2 h. Transmission electron microscopy (TEM). Ultrathin sections (100 nm) of the networkdeposited membrane (as embedded in epoxy resin) were obtained using PT-XL PowerTome ultramicrotomes (Boeckeler Instruments, USA) equipped with a diamond knife (Drukker,


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Netherlands). Sections for TEM analysis were collected on 200 lines/inch square mesh copper grids, and stained with 0.5 % aqueous RuO4 solution (Aldrich, 95%) for 30 min. TEM images were obtained on a JEOL JEM-2100F transmission electron microscope operating at 200 kV. Liquid permeation measurements. The fluxes of water and feed solution (containing MgSO4, glucose, Ts-γ-CD) through the composite membranes were measured using a SterlitechTM HP4750 dead-end filtration stirred cell (Kent, USA) equipped with a home-made membrane holder. The volume capacity of the stirred cell was 300 ml. All membranes were soaked for 2 h in pure solvent prior to flux measurement. The feed solution was stirred at a rate of 300 rpm using a Teflon coated magnetic stir bar which provides agitation to reduce concentration polarization or cake formation typical of dead-end filtration. The cell was pressurized using nitrogen gas connected to a pressure-regulated cylinder. Permeation through the membranes was determined by measuring volumetric flux at a pressure of 10 bar. Concentration of the solutes was measured by the TDS meter (TDS6 Acorn series, OAKION®) for MgSO4, HPLC (2695 separation module, Waters) with RI detector (410 differential refractometer, Waters) using Carbohydrate Analysis column (Waters) and glucometer (SD Codefree, SD Biosensor) for glucose, UV-vis spectrophotometer (Lambda 20, Perkin Elmer) for

Ts-γ-CD (λmax at 263nm). Molecular formula, mass, and size of the solutes are shown in Figure S1.

ASSOCIATED CONTENTS Supporting Information. Molecular characteristics of the solutes, water flux dependency on pore diameter (D) or film thickness (ℓ), activation energy of water transport through 6 cycledeposited-membrane, estimation of the flux per molecular mesh, flux and rejection data,


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conformational model of the ring structure in the urea network, and the preparation and test of molecular mesh membrane formed on an alumina membrane with different pore size and distribution. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Prof. Ji-Woong Park, E-mail: [email protected]

Author Contributions JWP conceived and run the project. MB performed membrane fabrications and characterization and measurements and other experiments. JWP and MB analyzed data. JSB synthesized monomers. SP and YHJ performed molecular modelling and YHJ and JWP interpreted its result. JWP an MB wrote the manuscript. All authors have given approval to the final version of the manuscript. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This research was supported by the National Research Foundation (NRF) of Korea grant funded by the Korea government (NRF-2015R1A2A1A15052067), the Korea Agency for


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Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (16FIP-B116951-01) and the research program funded by the GIST Research Institute (GRI). YHJ acknowledges a financial support of the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (2014M1A8A1049267).

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