Bioinspired Block Copolymer for Mineralized Nanoporous Membrane

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Bio-Inspired Block Copolymer for Mineralized Nanoporous Membrane Hui-Jun Zhou, Guan-Wen Yang, Yao-Yao Zhang, Zhi-Kang Xu, and Guang-Peng Wu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06521 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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Bio-Inspired Block Copolymer for Mineralized Nanoporous Membrane Hui-Jun Zhou,† Guan-Wen Yang,† Yao-Yao Zhang, Zhi-Kang Xu, and Guang-Peng Wu* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, and Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China †Joint

first author

E-mail: [email protected]

ABSTRACT: Homoporous membranes fabricated by self-assembled block copolymers (BCPs) have gained growing attention as their easy availability of well-ordered nanostructures for precise separation. However, it still remains challenges to improve their mechanical integrity, hydrophilic property and pore functionalities of the existing systems. To this end, we report an organic-mineral composite hybrid nanoporous BCP membrane with attractive superhydrophilicity, mechanical stability and fouling-resistance derived from a bio-inspired block copolymer, poly(propylene carbonate)-block-poly(4-vinylcatechol acetonide) (PPC-b-PVCA). The key advances relate: (1) The PPC minor block is qualified as sacrificial since its quite alkalisensitivity for generating monodisperse nanopores. (2) The PVCA matrix block contains the catechol groups, which enables the formation of inorganic layer via biomineralization process, thus producing an organic-mineral composite nanoporous BCP membrane with attractive superhydrophilicity, mechanical stability and fouling-resistance. A ~200 nm thickness BCP film with

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monodisperse through-pores of 12 nm diameter cylinders oriented perpendicularly to a supporting microfiltration membrane is fabricated by sequential blade-casting, solvent annealing, hydrolysis sacrificial block and biomineralization process. The mechanical stability, high water flow (114 m2 h-1 bar-1), size fractionation of nanoparticles as well as protein anti-adsorption performance make the strategy provided here hold the promise of affording an advance platform for filtration, catalysis and drug delivery. KEYWORDS: block copolymer, nanoporous membrane, self-assembly, biomineralization, antifouling, size separation Membranes with nano-channels and uniform pore-size distribution have gained increasingly attention in the recent past since they enable precise separation for water purification, biopharmaceutical depuration, and electronics processing applications.1-4 Among various alternative technologies for making homoporous membrane, self-assembled block copolymer (BCP) stands out as a platform for developing the next-generation of high-performance membranes, because of its versatile morphologies, adjustable pore size (5~50 nm) and the potential for large-scale fabrication.5-14 In particular, the vertically aligned cylinder-forming BCP template can circumvent the inherent challenges of the classic Mehta and Zydney plot, i.e. the balance between membrane selectivity and permeability, thus enabling to provide membranes with both high permeability and separation performance.15-18 Up to now, most copolymers investigated have a polystyrene (PS) block, and many successful BCPs represented by PS-block-poly(methyl methacrylate) (PS-b-PMMA),19-24 PSblock-poly(4-vinylpyridine) (PS-b-4PVP),25-28 PS-block-poly(2-vinylpyridine) (PS-b-2PVP),29,30 PS-block-polyisoprene (PS-b-PI),25,31 PS-block-poly(ethylene oxide) (PS-b-PEO),32-37 PS-blockpolylactide (PS-b-PLA),37-40 PS-b-poly(acrylic acid) (PS-b-PAA)41,42 have been developed.


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These elaborate BCPs have shown the ability to fabricate nanoporous membranes by selectively etching one block after self-assembly, by performing non-solvent induced phase separation, or by confined swelling-induced pore generation. By using these BCPs and fabrication methods, polymer films with well-ordered nanopores, uniform distribution and high porosity were obtained. Although these creative and elaborated BCPs have enabled the advancement of numerous nanoporous membranes, the polystyrene-based block copolymers typically suffer from several practical constraints that impede their widespread applications. The first concern is the mechanical integrity of these membranes because the stiffness and strength of polystyrene matrix hardly be comparable to the commercial polymer membranes such as polypropylene (PP), polyether sulfone (PES) and poly(vinylidene fluoride) (PVDF). Another drawback is the poor wettability of the PS-based membranes arising from their intrinsic hydrophobicity, which frequently cause severe membrane fouling on the surface. In such situations, high maintenance cost and increased energy consumption were encountered in the process of separation. Construction of organic-inorganic hybrid composite membranes has been demonstrated as a promising strategy to solve the issues that mentioned above because the inorganic material can provide hydrophilicity while maintaining rigidity of the membranes. Many surface modification approaches, including electro/electroless deposition of Au nanoparticles,28,43 chemical reduction growth of Ag nanoparticles,44 combination of metal-organic frameworks45 as well as coassembly of TiO2 nanoparticles,46 were demonstrated to solve these problems in recent years. By combining sequential infiltration synthesis (SIS) and thermal-induced assembly of PMMA majority PS-b-PMMA material,47 Nealey and co-workers fabricated nanoporous alumina ultrafiltration membrane with high wettability and stability to circumvent the above issues.20 However, this method needs delicate floating the fragile block copolymer thin film in water and


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transferring it onto a porous support, rendering scale-up of these membranes unlikely. Therefore, BCP-based nanoporous membrane with improved structural stability and surface wettability via practicable fabrication process is still a major challenge for advanced performance. Mussel-inspired chemistry is an emerging tool for the material surface modification and functionalization.48,49,50 One of the most representative example is 3,4-dihydroxyphenylalanine (DOPA) because of its chemical versatility of catechol (1,2-dihydroxyphenyl) groups. By using which, robust adhesive attraction on almost all of the material surface were demonstrated.51 By incorporating catechol into synthetic materials, these synthetic materials have shown in particular for constructing organic-inorganic composite membranes to improve their hydrophilicity, fouling resistance as well as the mechanical integrity.49,52 Very recently, we discovered that polystyreneb-poly(propylene carbonate) (PS-b-PPC) material was a qualified candidate to replace the frequently-used PS-b-PMMA for the high resolution lithography via the directed self-assembly (DSA). The main advantage of PS-b-PPC material is its strong phase separation performance (i.e. high Flory−Huggins interaction parameter) for obtaining perpendicular sub-10 nm features using thermal treatment.53,54 More importantly, the aliphatic PPC domain demonstrates higher degradability than PMMA under plasma etching and higher sensitivity than PLA under alkaline condition, indicating its great potential for making nanoporous devices and materials.53,55 Herein,

a

bio-inspired

block

copolymer

poly(propylene

carbonate)-block-poly(4-

vinylcatechol acetonide) (PPC-b-PVCA) is synthesized with an aim to fabricating the organicinorganic hybrid nanoporous membrane. This particular block copolymer is designed for following reasons: i) the alkali-sensitive PPC block can be easily removed under dilute base condition for nanopore formation; ii) the acetonide protecting groups on PVCA can be simply cleavaged under mild acidic chemical environment generating well-structured poly(vinyl


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catechol)s, where a facile biomineralization process could be well performed to provide a membrane with attractive thermal, chemical, mechanical stability and fouling-resistance. The facile preparation in combination with their high wettability and mechanical integrity render our materials can be utilized as a versatile platform for precise nanoscale separation.

A

O

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(bdi)-Zn-BSTP

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cd

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Tg PPC: 38 oC

Tg PVCA: 128 oC

Log Intensity

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PPC-b-PVCA

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VCA

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CO2

DSC (mW/mg)

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G

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√3q 2q

√7q

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Figure 1. Synthesis and characterization of PPC-b-PVCA block copolymer. (A) synthetic pathway for PPC-b-PVCA by using (B) (bdi)-Zn-BSTP catalyst; (C) the 1H-NMR (CDCl3, 400 MHz), (D) GPC curve, polystyrene standard using THF as eluent, (E) DSC and (F) SAXS data of the PVCA-b-PPC sample with a Mn of 32 kg/mol and PDI = 1.12; (G) the top-down SEM image of a 50 nm PVCA-b-PPC film on silicon wafer via solvent annealing in CHCl3 for 30 min.


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RESULTS AND DISCUSSION The synthetic procedure and characterization of bio-inspired PPC-b-PVCA are shown in Figure 1. We utilized β-diiminate zinc ((bdi)-Zn-BSTP) (Figures 1A and 1B) as the catalyst because of its abilities of controlled zinc-coordinated epoxides/CO2 copolymerization and living reversible addition-fragmentation chain transfer (RAFT) polymerization. The design and characterization of this catalyst were demonstrated in our previous study.56 During the reaction, the initiating group on the zinc complex, i.e. 3-(benzylthiocarbonothioylthio)propionate (BSTP), enabled the controlled alternating copolymerization of CO2 and PO, and produced a poly(propylene carbonate) (PPC) polymer end-capped with BSTP group. In subsequent, the BSTP group at the end of PPC (PPC-BSTP) allowed polymerization of 4-vinylcatechol acetonide (VCA) via living radical polymerization, and produced well-defined PVCA-b-PPC copolymers in an effective one-pot strategy. A combination of 1H NMR spectroscopy (Figure 1C), 13C NMR spectroscopy (Figure S1 in Supporting Information) and unimodal gel permeation chromatography (GPC, Figure 1D) with narrow polydispersity coefficient demonstrated the successful preparation of the predesigned PPC-b-PVCA block copolymer. In this study, a PPC-bPVCA sample with total molecular weight of ~32 kg/mol, ~20 wt % of PPC, and a polydispersity index of 1.12, was targeted and used. The occurrence of microphase separation occurred since two glass transition temperatures were clear observed. One Tg at ~35 °C came from PPC block, another Tg ∼130 °C was of PVCA block (Figure 1E). A hexagonally packed cylinder morphology in the powder state was characterized by small-angle X-ray scattering (SAXS). The PPC block formed the cores in PVCA matrix with a domain spacing values (L0) of


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26.8 nm (Figure 1F), which was calculated by the peaks at principal reflection q, 3 q, √4q and √7q. To fabricate BCP separation membranes, the prerequisite is formation of assembled perpendicular-oriented cylinders under which high density and directed pathway through the film could be obtained. By carefully selecting and screening annealing conditions, we identified that solvent annealing of PPC-b-PVCA in chloroform can effectively deliver the desired perpendicular structures. Figure 1G gives representative top-down scanning electron microscopy (SEM) images for a 50 nm thin films on native oxide silicon wafer after annealing in chloroform (CHCl3) for 0.5 h at 25 °C. The formation of perpendicular PPC cylinders indicated CHCl3 is a nonpreferential solvent for both PVCA and PPC block to obtain perpendicularly oriented nanoporous through the film thickness under this annealing condition. The comprised domain spacing (~25.2 nm) compared with the bulk sample (~26.8 nm) was attributed both to the solvent dilution effect and to the temperature dependence of segment−segment interaction parameter for PVCA and PPC.57


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Scheme 1. Schematic illustration of the fabrication procedure for mineralized nanoporous membrane. (a) A ~200 nm thickness of PVCA-b-PPC thin film was blade-coated onto the surface of water-prefilled PES membrane and (b) subsequently solvent-annealed in CHCl3 vapour atmosphere to form a cylindrical morphology that oriented perpendicularly to the substrate; (c) Porous channels were generated by hydrolyzing the PPC blocks to 1,2-propanediol and carbon dioxide under alkaline condition; d) The surface ZrO2 mineralized composite membrane was fabricated through chelating Zr4+ in acidic zirconium sulfate solution via the catechol groups that released by taking off the acetonide groups on the main PVCA chain.

With the well characterized PPC-b-PVCA polymer and the assembly condition in hand, the fabrication process of the PPC-b-PVCA derived organic-inorganic composite membrane on PES supporting membrane is provided in Scheme 1, where four basic steps including BCP thin film casting, structure refining via solvent anneal, removing of the PPC domain and surface mineralization are involved. Initially, a piece of commercial PES membrane with an average


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pore size of 0.2 μm was used as supporting substrate, which was filled with water to preclude the BCP solutions penetrating into the underlying pores.38,58 A 2 wt % PVCA-b-PPC toluene solution was then blade-casted onto the supporting membrane, and form a composite membrane with a ~200 nm thickness of BCP layer on the top of the PES membrane (Scheme 1a). Then the membrane was moved to an oven preheated to 130 oC and kept for 10 min to evaporate the water and solvent, followed by another 2-hours-heating at 80 oC to solid the composite membrane. Afterwards, the bilayer composite membrane was solvent annealed in CHCl3 to assemble perpendicularly oriented cylinders (Scheme 1b). To generate porous channels in the coated BCP layer, the composite membrane was immersed in a 4M NaOH methanol (60% by volume) aqueous solution at 80 oC for 1 h (Scheme 1c). Under alkaline condition, the hydroxide (-OH) attacked the carbonyl group and hydrolyzed the propylene carbonate unit into CO2 and 1, 2-propane diol.55 CO2 directly escaped in gaseous, while the 1, 2-propane diol was dissolved in methanol solution, thus generating the nanoporous thin films in one step. It should be parenthetically noted here that the carbonate units on PPC block are more sensitive to the ester groups (such as polylactic acid),55 which indicates the potential advantage for membrane fabrication compared with the well-studied other block copolymers, e.g., PS-b-PLA. The successful removal of PPC domain from the ordered PPC-bPVCA thin films was demonstrated by attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR) measurements, as shown in Figure 2. For the original PPC-b-PVCA thin film, the characteristic carbonyl stretch signal from the PPC was clearly observed at 1740 cm-1 (Figure 2A). The disappearance of this characteristic peak after alkali treatment demonstrated complete removal of the PPC block occurred (Figure 2B). It should be noted here that during the hydrolysis stage, the well-ordered surface topography was without any


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destruction, which affords us an opportunity to develop a library of organic-inorganic composite membranes with high-performance, as shown in the following part.

A

O n

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O

O

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n

O

2500

O m

O

2000

1500

-1

1000

Wavenumber(cm ) Figure 2. ATR-FTIR monitoring the fabrication process for the mineralized nanoporous ultrafiltration membrane. (A) the annealed bi-layered composite membrane in CHCl3; (B) the composite membrane after removal of the PPC domain by immersed in a mixed solution of 4M NaOH solution and methanol (MeOH/water = 40:60 v/v) at 80 oC for 1 h. The absorbance at 1740 cm−1 is the characteristic carbonyl stretch signal of the PPC block.

After removal of the PPC domain, the nanoporous membrane was immersed into Zr(SO4)2/HCl solution, where two kinds of reactions occurred simultaneously on the surface of the reserved porous PVCA matrix block (Scheme 1d). The acid-sensitive acetonide groups on the PVCA were taken off to deliver the desired catechol groups, which could chelate Zr4+ from the aqueous solution and generate an ultrathin ZrO2 layer via the hydrolysis process of zirconium


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sulfate (Zr(SO4)2). The reaction process for the growth of ZrO2 via hydrolysis of zirconium sulfate is provided in equations (1)-(3).59 2[Zr(SO4)2]n + 3nH2O → 4n[Zr2(OH)3(SO4)4]3 - + 3nH +

(1)

[Zr2(OH)3(SO4)4]3 - + 3OH - → 4Zr2(OH)6SO4 + 3SO42 -

(2)

Zr2(OH)6SO4 + 2OH - → 2ZrO2 + SO42 - + 4H2O

(3)

Figure 3. (A) XPS spectra of the composite membrane with different mineralization time in 1 wt % Zr(SO4)2 in 1M HCl aqueous solution at 80 °C. The EDX images of the surface (B) and cross section (C) of the composite membrane with the distribution of Zr element (brown point) after 1h mineralization. The BCP layer is marked in dash line in C.

Since the deprotection of acetonide groups and the bio-mineralization of ZrO2 coating were integrated into one step, the reaction conditions (i.e., temperature, concentration, pH and reaction time) needed to be optimized. Strong acid concentration benefits the removal of the acetonide protection group,60 but the high concentration of H+ will frustrate the hydrolysis of Zr(SO4)2 for growth of ZrO2. In addition, a high temperature and long reaction time could promote the hydrolysis of Zr(SO4)2 and lead to a thicker mineral coating. However, this may result in a situation where the pores would be blocked by the ZrO2 layer. During exploratory investigations into the deprotection of acetonide groups and hydrolysis process of zirconium sulfate, we found


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that 1 wt % Zr(SO4)2 in 1M HCl aqueous solution under the temperature of 80 °C for 90 min was the optimum conditions, under which the desired catechol groups were smoothly delivered to coordinate the Zr4+, giving a dense, smooth and thin ZrO2 layer on the membrane surface without blocking the nanoporous. This reaction pathway, i.e., deprotection of acetonide groups and subsequent in-situ biomineralization of ZrO2 coating were evidenced by a series of characterizations. Figure 3A shows the XPS results of the membrane before and after immersion into the acidified Zr(SO4)2 solution. Compared with the composite membrane before mineralization (black line), the mineralized membrane exhibited new binding energy peaks at 180 eV, 330 eV and 345 eV, which were ascribed to Zr 3p1, Zr 3p5 and Zr 3d5, respectively. As the mineralization time became longer, both Zr and O peak intensity of the resulting membrane increased. The O/C ratio increased from 0.18 for the non-mineralized membrane to 0.30 for the membrane after mineralization 30 min, and 0.45 for the membrane after mineralization 90 min respectively (Table S1 in Supporting Information). In the XPS spectra, The strengthened intensity of zirconium and oxygen over time substantiated the appearance of ZrO2 coating layer. It should be mentioned that increasing reaction time doubtlessly improved the growth of ZrO2 layer, however, the pore size distribution gradually became wider on the time of the mineralization and eventually resulted in pore blocking (Figure S2 in Supporting Information). Because of the improved permeability to acidified Zr(SO4)2 solution of the catechol groups covered membrane porous, we postulated that the ZrO2-mineralization process take placed throughout the entire membrane. To probe this hypothesis, the energy dispersive X-ray spectroscopy (EDX) was performed. The uniformly decorated by Zr element on the membrane surface was shown in the top-down SEM image in Figure 3B. From the sectional view (Figure 3C), the Zr element shown as the orange spots was uniformly distributed into the BCP film


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substrate with a thickness of about 200 nm, while a comparatively small amount diffused into the PES membrane substrate. The SEM image of the corresponding EDX image was provided in Figure S3 in Supporting Information. It was therefore substantiated that as a result of the full covered catechol groups in the membrane porous, the mineralization process occurred throughout the entire membrane, thus providing a well ZrO2-wrapped organic-inorganic composite membrane.

Figure 4. (a) The photography of a 5 cm diameter Zr-mineralized composite membrane after removing the minor PPC domain and mineralization at 80 oC for 90 min in 1M HCl with 1 wt % Zr(SO4)2 solution. SEM images of the surfaces (b, c) and cross-sections (d, e) of the composite membrane with different magnifications. Platinum-sputtering for the cross-session sample was performed before observation with SEM.

Figure 4a gives a whole view of the prepared composite membrane with a diameter of 5 cm after removal of the minor PPC domain and mineralization of ZrO2 at 80 oC for 90 min in 1M


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HCl with 1 wt % Zr(SO4)2 solution. The average thickness of the top layer as shown in the crosssection SEM images was approximately 200 nm. Figure 4b-c show SEM images of the top-view at two different magnifications, from which the hexagonal pore structure at the surface was well preserved after removal of PPC domains and mineralization without any pore blocking. The average pore sizes on the film surface were 12.5 ± 0.6 nm after the BCP film immersed into the mineralizing solution for 1.5 h. It is clear that hexagonally packed cylinders of pores exists homogeneously throughout the whole area (Figures 4b-c). Figures 4d-e show the cross-sections of the membrane at different magnifications. It should be mentioned that platinum-sputtering for the cross-session sample was performed before observation with SEM. Figure 4e clearly demonstrates the perpendicular cylindrical pores across the entire membrane thickness. In this case, membranes with high connectivity and direct pathways would be fabricated, as shown in the following part.

Figure 5. a) Dynamic water contact angle and b) water flux of the non-mineralized (black) and ZrO2-modified composite membranes (orange).

The dynamic water contact angle (WCA) experiments further confirmed our pre-engineered modification of the membrane (Figure 5a). An initial WCA of ~84° was clearly observed for the non-modified ones with barely changes even after a measurement period of 5 min, implying the


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hydrophobic properties of the PVCA surface. For the ZrO2-modified one, in contrast, the water drop quickly permeated into the membrane in 50 s, and displayed a contact angle of 8°. The superior hydrophilicity enabled high water flux, which was plotted in Figure 5b. The flux of the mineralized membrane was about 114 ± 4 L m-2 h-1 bar-1 at a pressure of 1.2 bar, which is 5 times higher than the non-ZrO2-mineralized membrane. The superior hydrophilicity as well as the much higher water flux is resulted from the fully-covered, ultrathin, and uniform zirconia layer on the surface of the porous matrix.

a) 0.60

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1

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Figure 6. a) The filtration performance of the ZrO2-mineralized composite membrane to separate ~ 20 nm gold nanoparticles. b) and c) TEM images of the feed and filtrate SiO2 solutions with two monodisperse sizes of ~5 nm and ~25 nm. Insets are the corresponding DLS results.

By taking advantage of the relatively narrow pore size distribution in the BCP layer, two experiments were performed for the size fractionation of nanoparticles. In the first experiment, an aqueous solution of Au particles with the size of ~20 nm was used to test the size-sieving capability of the membrane. As shown in Figure 6a, the filtrate is colorless without measureable UV–vis spectra, suggesting the membrane exhibits a sharp cutoff retention of the 20 nm particles. In the second experiment, a solution of two monodisperse SiO2 nanoparticles with size


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of ~5 and ~25 nm was used as filtrate as characterized by the TEM and DLS in Figure 6b. The TEM image and DLS analysis of the filtrate were provided in Figure 6c, and indicated that nanoparticles with diameter smaller than 12 nm could pass through the membrane, which agrees well with the pore sizes shown in Figure 4.

Figure 7. SEM images of (A) none-mineralized and (B) ZrO2-minerallized nanoporous membranes after exposure to protein solution for 2 h.

In addition to the precise separation performance, the ZrO2-mineralized composite membrane exhibited excellent antifouling property. SEM images in Figure 7 show the nonmineralized and the ZrO2 modified composite membranes after exposure to a 1 mg mL-1 bull serum albumin (BSA) protein solution. Severe pores blocking by proteins adsorption via hydrophobic interactions were clearly observed for the non-modified membrane. By contrast, protein aggregation and membrane fouling were not detected on the ZrO2-minerallized membrane because of the improved hydrophilicity, as previously mentioned. Lastly, the chemical stability was also studied. The sample was immersed into aqueous solutions with different acid and alkali circumstance. By using EDX, ignorable changes of the Zr/C ratios was detected when our ZrO2-minerallized membrane rinsed in alkaline solution or/and acidic solution


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(data not shown), which illuminates the promising for long-term operation stability of our membranes. CONCLUSIONS In summary, a bio-inspired BCP PPC-b-PVCA is synthesized as a template for fabricating well-ordered organic-inorganic hybrid nanoporous membranes. The membrane manufacturing process is quite simple since 1) the alkali-sensitive PPC block is easily removed under dilute base condition for nanopore formation; 2) the PVCA can deliver the bio-inspired catechol groups, from where an ultrathin ZrO2 layer via biomineralization process generates simultaneously. Based on the design, an organic-inorganic composite nanoporous BCP membrane with super-hydrophilicity, mechanical stability, as well as fouling-resistance was produced. The uniform nanopores with high densities in our mineralized composite membranes could conduct accurate size separations. The facile preparation, sharp size discrimination, in combination with their high wettability and mechanical integrity renders our materials hold the promise of affording an advance platform for filtration, catalysis and drug delivery.

MATERIALS AND METHODS Materials. Propylene epoxide (Sigma-Aldrich) was distilled by CaH2 and stored in a N2-filled glovebox. Toluene was distilled from sodium/benzophenone under N2. The research grade carbon dioxide (99.999%) was purified and dried by using two tandem steel columns that packed with 4 Å molecular sieves. The columns had been dried over 200 oC under vacuum. Polyether sulfone (PES) microfiltration membranes (mean pore size 0.22 μm, thickness 120 μm) were procured from Shanghai Mega Vision Membrane Engineering and Technology Co., Ltd. (China). Gold nanoparticles and silicon dioxide nanoparticles were purchased from Beijing DK nano Co.,


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Ltd. (China). Other reagents, such as 2, 2'-azobis (2-methylpropionitrile) (AIBN), zirconium sulfate, hydrochloric acid, sodium hydroxide and bovine serum albumin (BSA, pI 4.8, 67 kDa) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used without any purification. Phosphate buffered saline (PBS) with ionic strength 10 mM at pH 7.4 was used, which was prepared by using analytical-grade chemicals and ultrapure water. The water was purified by ELGA Lab Water system (France). β-Diiminate zinc ((bdi)-Zn-BSTP) catalyst was prepared as described in our previous work.56 4-vinylcatechol acetonide (4-VCA) was synthesized according to published procedure.59 Synthesis of PPC-b-PVCA. In a glovebox, (bdi)-Zn–BSTP (80 mg, 0.1 mmol) and propylene epoxide (0.58 g, 10 mmol), toluene (1 mL) and a magnetic stir bar were placed in a 50 mL autoclave. The autoclave was pressurized to 3.0 MPa of CO2 and allowed to stir at 25 oC for 12 h. After the CO2 pressure was slowly released, a certain amount of 4-VCA (3.00 g, 17 mmol), AIBN (0.2 equivalent to the catalyst), and tetrahydrofuran (2 mL) were then added into the reaction system. The polymerization continued at 85 °C for additional 48 h. The crude block copolymer was purified by repetitively dissolving in THF and precipitating in diethyl ether. Membrane Preparation. PES membrane with an average pores size of 220 nm was used as supporting substrate, which was immersed in deionized water for about 5 min, enabling the pores filled with water to prevent BCP solutions penetrating into the underlying pores. A 2 wt % toluene solution of PPC-b-PVCA sample with total molecular weight of ~32 kg mol-1 (~20 wt % of PPC, PDI = 1.12) was then blade-coated onto the water-prefilled PES substrate, giving a composite membrane with a ~200 nm thickness of BCP layer that densely formed on the top of the PES supporting membrane. Then the membrane was moved to an oven preheated to 130 oC and kept for 10 min to evaporate the water and solvent, followed by another 2-hours-heating at


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80 oC to solid the composite membrane. After that, the bilayer composite membrane was placed in a sealed 500 mL chamber along with a small vial containing 5 mL CHCl3 to allow the completion of microphase separation by a slow evaporation of the solvent at room temperature. The microphase-separated film with perpendicularly oriented PPC cylinders was sequentially immersed in the 4M NaOH solution (MeOH/water = 60/40) at 80 °C for 1 h to alkali-hydrolyze PPC domains, then rinsed in deionized water and dried at 40 oC in a vacuum oven. The asprepared membrane was immersed into a 20 mL HCl-acidified 1 wt % Zr(SO4)2 solution under 80 °C for mineralization. After a designed time, the obtained membrane was throughly washed by deionized water and dried in a vacuum oven at 40 oC for use. NMR experiments. NMR-H1 spectrum (400 MHz) and NMR-C13 (100 MHz) were carried out in deuterated chloroform on a Bruker DPX 400 MHz type spectrometer. For 1H NMR, their peak frequencies were referenced versus TMS shifts at 0 ppm; For 13C NMR, their peak frequencies were referenced versus chloroform-d at 77.0 ppm. Gel permeation chromatography (GPC). A Waters Chromatography was used to get the information of polymer molecular weight. The system was equipped with a three-size-exclusion column set of Polymer Laboratories, a 1515 pump, and a differential refractometer (model 2414). THF was used for the system (run at 35 oC). The THF solvent with 1.00 mL min-1 flow rate was performed. A known concentration ~5 mg mL-1 of polymer solutions and a volume of 50 μL for each sample was used. Thermogravimetric analysis (TGA). TGA measurements were carried with a TG Q50 Thermogravimetric Analyzer. A ~5 mg amount of sample was heated from 50 to 850 °C, 10 °C/ min under the protection of N2.


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Differential scanning calorimetry (DSC). A NETZSCH DSC 214 Polyma instrument (NETZSCH, Germany) was performed for DSC measurements. The instrument was equipped with a mechanical refrigeration (IC70 intracooler). The heating/cooling rate is 10 K/min ranged from -20 to 160 °C. An indium standard was used to calibrate the temperature and heat flow. The third heating scan at midpoint was taken to calculate Tg. Small angle X-ray scattering (SAXS). SAXS was measured on a custom-built beamline of France XENOCS company with a X-ray wavelength of 0.154 nm. An acquisition time was 600 s. Attenuated total reflection Fourier transform infrared spectrometer (ATR-FTIR). A Nicolet 6700 spectrometer with an ATR accessory (ZnSe crystal, 45°) was used to collect ATRFTIR spectra. For each spectrum, a 32 scans was taken at a nominal resolution of 2 cm−1. X-ray photoelectron spectrometer (XPS). The surface chemistries were revealed by XPS using Al Kα excitation radiation at 1486.6 eV (PerkinElmer, USA). The signals were collected with a survey depth of 5–10 nm from 0 to 1000 eV. Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectrometer (EDX). A FESEM (Hitachi, S4800, Japan) with a voltage of 3.0 Kv was used to characterize the surface morphologies of membranes. EDX with a voltage of 15.0 kV was utilized to analyze the element distribution (Hitachi, S4800, Japan). In order to avoid charging problem, all the samples were protected by sputtered with a thin film of platinum under argon atmosphere (30 s). Dynamic water contact angles (DWCA). The DWCA were detected at room temperature using a DropMeter A-200 contact angle system from MAIST Vision Inspection & Measurement Co., Ltd., China. An automatic piston syringe with 2 μL of water was dropped onto the separator


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ACS Nano

surface, which was automatic photographed. A circle-fitting algorithm was performed to calculate the DWCA. Transmission electron microscopy (TEM). A Hitachi 7700 TEM, with an acceleration voltage of 120 kV was used for TEM images. A carbon-coated copper grid was used to support samples. 10 min later, a strip of filter paper was used to blot away excess solution. Allow samples to airdry before observation under room temperature. Dynamic light scattering (DLS). A Malvern Zetasizer Nano ZS90 DLS was performed for the measurement. The instrument was equipped with a He–Ne laser with an angle of 173o at wavelength of 633 nm. Water flux and filtration tests. The permeate flux was measured by a dead-end filtration apparatus (Millipore) under 0.12 MPa. The filtration performance of mineralized membrane was tested by a dead-end filtration apparatus with an effective membrane area of 4.9 cm2. Gold nanoparticles and silicon dioxide nanoparticles were then used as the feed solution, respectively. Measurement of antifouling property. Under slight stirring, one gram of BSA was dissolved in 1 L phosphate buffered saline, which was kept for 24 h to remove air bubbles. A dead-end filtration equipment was used to test the dynamic protein filtration and to evaluate the antifouling property of the non-mineralized and mineralized membranes.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on ACS Publications website at DOI: 10.1021/acsnano.xxxxxx.

13C

NMR spectrum of the block polymer, SEM image of composite

membrane mineralized for 2 h, the corresponding cross section SEM image of the EDX results in


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Figure 3C of the composite membrane after 1h mineralization, and a table of elemental compositions calculated by XPS analysis. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Yao-Yao Zhang: 0000-0002-8031-9139 Zhi-Kang Xu: 0000-0002-2261-7162 Guang-Peng Wu: 0000-0001-8935-964X Author Contributions †H.J.Z.

and G.W.Y. contributed equally for joint first author. H.J.Z. performed the membrane

fabrication, characterization and water treatment experiments. G.W.Y and Y.Y.Z. synthesized the VCA monomer, catalyst and block copolymer. Z.K.X and G.P.W. directed the study, designed experiments, and wrote the manuscript. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENTS Financial support is acknowledged to the Fundamental Research Funds for the Central Universities (2018QNA4056), National Natural Science Foundation of China (Grant 21674090), Qianjiang Talent-D Foundation (QJD1702025) and the “Hundred Talents Program” of Zhejiang University from China. Conflict of Interest The authors declare no competing financial interests. REFERENCES


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Table of Contents Graphic

OH HO Zr O O Zr O O

OH HO Zr O O Zr O O Zr O O

O O Zr O O Zr HO OH

OH O Zr OH O Zr O O

O O Zr O O Zr OH HO

Mussel-Inspired Block Copolymer Membrane


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Bioinspired Block Copolymer for Mineralized Nanoporous Membrane

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