Diatom-Mimicking Ultrahigh-Flux Mesoporous Silica Thin Membrane

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A Diatom-Mimicking Ultrahigh-flux Mesoporous Silica Thin Membrane with Straight-through Channels for Selective Protein and Nanoparticle Separations Jingling Yang, Geng-Sheng Lin, Chung-Yuan Mou, and Kuo-Lun Tung Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05295 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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Chemistry of Materials

A Diatom-Mimicking Ultrahigh-flux Mesoporous Silica Thin Membrane with Straight-through Channels for Selective Protein and Nanoparticle Separations Jingling Yang,a Geng-Sheng Lin,b Chung-Yuan Mou,a,* and Kuo-Lun Tungb,c,* a. Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan b. Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan c. Advanced Research Center for Green Materials Science and Technology, National Taiwan University, Taipei 10617, Taiwan ABSTRACT: A nature-inspired mesoporous ultrafiltration membrane with unprecedented permeability and high selectivity for selective protein and nanoparticle separations is reported herein. We describe a diatom-mimicking hierarchically porous membrane that consists of a single layer mesoporous silica thin film (MSTF) with perpendicular pores supported on top of a macroporous anodic aluminum oxide (AAO) membrane (MSTFAAO) for ultrafiltration. The MSTF, with a thickness of ca. 30 nm, uniform vertical straight-through mesoporous channels (pore diameter of 5.9 ± 0.4 nm), super-hydrophilicity, and continuous non-breaking to centimeter range (area of 17.3 cm2), is overlaid on the AAO straight-through membrane, which counterintuitively exhibits both ultrahigh water permeability (1027 ± 20 Lm-2 h-1 bar-1) and an excellent rejection rate (> 99 %) for molecules larger than 7 nm. The water permeability is significantly higher than that of the reported nano-sized channelled membranes. A model equation for calculating the theoretical permeability flux of the vertically nanochanneled membrane is established and compared with experimental results.

techniques such as focused ion, ion etching and electron beams,12 which restrict their further applications. Therefore, methods for facile synthesis of filtration membranes with narrow size distribution and ordered structures are strongly desired.

Introduction Recently, ultrafiltration has received signiﬁcant attention and wide application in separation science and in the chemical and pharmaceutical industries.1,2 Ultrafiltration operates by applying a filter membrane to selectively reject or pass molecules under a specific pressure.3,4 Ultrasmall nanochannels with pore sizes ranging from 1 - 10 nm are especially required for filter membranes to achieve molecule-level separation such as the separation of dye molecules, DNA, proteins and viruses.5,6 An ideal filter membrane should have the following characteristics: high selectivity, large area without defects, straight-through pores with little tortuosity, minimal thickness to maximize permeability and minimize pumping energy, adequate thermal/mechanical stability, and ease of synthesis.7,8 To make such a membrane with all the above characteristics is rather challenging. For traditional filtration membranes, high permeability and high selectivity are practically impossible to achieve simultaneously.9 Meanwhile, the pore size and its distribution are crucial to the size-selective separation performance of the filter membranes.10 Common commercial nanoporous membranes generally exhibit random and tortuous pores with a wide pore size distribution and with great thickness. These membranes thus often give unsatisfactorily low liquid flux and selectivity.9,11 In some recent work, porous inorganic or hybrid organicinorganic materials with a narrow size distribution have been made with a variety of low-throughput fabrication

Nature abounds in creating beautiful ordered porous structures. Natural structures of astonishing ordering and patterns are often correlated with their specific biofunctions.13 These structure-function correlations have extensively inspired the development of biomimetic materials. In the case of diatoms, their hierarchical porous silica shells, referred to as frustules,14 are self-organized by the living organic compartments into light-weight, periodic mesoscale-patterned nanoporous architectures, which are of particular interest in the selective filtration of nutrients for its survival. Specifically, this hierarchically ordered structure of diatom frustules often consists of multilayers of an epitheca and a hypotheca. In mimicking such structure to achieve efficient selective ultrafiltration, we would like to artificially develop a two-layer stacking porous silicate and/or alumina with different pore sizes. Both of them have a uniform periodic pore size distribution.15 However, developing a facile artificial chemical synthesis method that produces the desired biomimetic hierarchical periodic membrane with continuous straight-through mesopores on a large area of macroporous supports is rather challenging.16, 17 1

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Inspired by the diatom frustules structure, in this study, we developed a three dimensional (3D) dual-layered structure with a single-layered vertical mesoporous silica thin film (MSTF) supported on a periodic macroporous anodic aluminium oxide (AAO) straight-through membrane (MSTFAAO) of centimeter scale by a feasible polymer interlayer strategy. For a demonstration of ultrafiltration, the biomimetic MSTFAAO membrane with perpendicular straight-through mesopores was employed as a filter for size exclusion filtration of proteins and quantum dots. The biomimetic membrane combines the advantages of high selectivity, defect-free large area (area of 17.3 cm2), outstanding permeability under low pressure (minimizing the pumping energy), super-hydrophilicity, thermal stability and adequate chemical stability for operation in both aqueous and organic solutions. Moreover, a model equation for calculating the theoretical permeability flux of the vertically nanochanneled membrane was established.

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cleaning, a dual-layer biomimetic MSTFAAO membrane composed of a single-layer MSTF with perpendicular mesopore channels standing on a macroporous AAO membrane substrate was obtained. Characterization. Top-view and edge-view micrographs and energy-dispersive X-ray (EDX) mapping were taken on a field emission scanning electron microscope (SEM) (Hitachi S-4800) operated at an accelerating voltages of 5 kV. The sample was baked at 80°C overnight prior to SEM imaging. Samples for transmission electron microscopy (TEM) were milled using a focused ion beam instrument (FIB, JEOL JIB-4500 and FEI Nova 200 Dual Beam). Before FIB milling, a Pt layer with a thickness of ca. 60 nm was plated onto the surface of the sample to increase the sample conductivity. To protect the morphology of the sample, an amorphous carbon layer with a thickness of ca. 1 μm was coated onto the sample. FIB was operated with an accelerating voltage of 5-30 kV to cut the membrane into slice samples, which were lifted out for TEM study. TEM was performed with a Field Emission Transmission Electron Microscope (JEOL, JEM-2100F) at 200 kV. Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) was conducted with a grazing-incidence geometry (Nano-Viewer, Rigaku), equipped with a two-dimensional area detector (Rigaku, 100K PILATUS) and a generator (31 kW mm-2, rotating anode X-ray source with a Cu Kα radiation of λ = 0.154 nm). The scattering vector, q (q = 4π (sinθ)/λ), and the scattering angles (θ) in these patterns were calibrated using silver behenate (CAS number 2489-05-6).18 The samples were mounted on a z-axis goniometer with an incident angle of 0.2°. Contact angle measurement was performed with a Contact Angle Goniometer (FTA125). The zeta potential and size of proteins were analysed by dynamic light scattering (Zetasizer Nano S). The thermal stability of MSTFAAO was analyzed by thermogravimetric analysis instrument (TGA/DSC1, Mettler-Toledo).

Experimental Section Synthesis of a biomimetic MSTFAAO membrane by the polymer interlayer method. First, polystyrene (PS) was used to coat a smooth surface layer on the macropores of anodic aluminium (AAO) oxide membranes for subsequent growth of a mesoporous silica thin film (MSTF). PS (2.0 g, MW 260,000) was dissolved in 20.0 g toluene at 50°C for 1 h, and then 0.25 wt% photo-initiator (2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone) was dissolved in the above solutions. Then, 700 µL mixed PS solutions were used to spin-coat the AAO membranes (4.7 cm in diameter, situated on a 5 × 5 cm2 glass sheet) at 2000 rpm for 30 s, and then immediate cross-linking of the polymers on the AAO surface was performed under UV light irradiation for 5 min. The spin-coating and cross-linking procedures were repeated three times. Subsequent curing of the PS on AAO supports at 100 °C for 1 h resulted in the as-prepared membrane labelled PS/AAO.

Flux measurements and size exclusion ultrafiltration. Pressure-driven separation was carried out using a dead-end filtration device under a pressure of 7 kPa at room temperature. The effective permeation area of the filter was 12.6 cm2. Pure water flux (Jw, in L m-2 h-1 bar-1) was used to evaluate the permeation performance of the membranes. The flux values reported here are the average results of three parallel experiments conducted with three membrane coupons. The Jw was determined according to the following equation:

Growth of a single-layer MSTF on the top surface of the PS/AAO membrane was conducted in an oil-in-water emulsion as our previously reported.18 First, we prepared the oil-in-water emulsion by mixing cetyltrimethylammonium bromide (CTAB) (0.965 g), ethanol (30.0 g) and decane (3 mL) at 50°C. Then, the PS/AAO membrane was immersed in the microemulsion, followed by the introduction of NH3 aqueous solution (7.5 g, 35.5 wt%) and tetraethyl orthosilicate (TEOS)/ethanol solution (8.35 mL, 20 % by volumes) under stirring at 50°C for 1 h. Then, the reaction was aged at 50°C for 20 h without stirring. The molar ratios of CTAB : H2O : NH3 : decane : ethanol : TEOS were set to 1:8400:90:5.8:250:2.8.18 The synthesized MSTF/PS/AAO membrane was rinsed with ethanol and then calcined in an air atmosphere by ramping the temperature from 30°C to 400°C at a rate of 1°C/min, which was maintained for 30 min; followed by UV ozone cleaning for 30 min. Calcination and the ozone cleaning were used to remove residual PS and surfactant. After removing the surfactant templates and PS by calcination and UV ozone

Jw =

V A×t

(1)

where A corresponds to effective filtration area of membrane (m2), and V denotes the total water volume (L) collected during time t (h). Pure water permeability (P) was calculated according to the following equation: P=

V A × t ×ΔP

(2)

where ΔP represents the transmembrane pressure (bar).19 2


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Figure 1. (a) Synthesis and (b) construction of diatom-mimicking mesoporous MSTFAAO filter membrane.

Ultrafiltration of proteins and quantum dots was performed using the dead-end filtration device as mentioned above under the same permeation conditions. Ten milliliters of a solution were placed into a filtration device equipped with a membrane filter. The solution that permeated through the thin membrane was collected. The concentration changes in the protein solutions before and after the filtration were determined by UV-vis absorption spectroscopy (Hitachi U-3310). The separation performance was evaluated based on the amount of the protein present in the permeate and the original (feed) solution. The feed solutions of bovine serum albumin (BSA), ovalbumin (OVA), lysozyme (LYZ) and cytochrome c (Cyt c) were prepared at concentrations of 800 ppm, 500 ppm, 300 ppm and 120 ppm, respectively. The ultrafiltration of quantum dots (CdSe@ZnS, with a particle size of ca. 10.5 nm and an initial solution concentration of 3.1×10-1 ppm, and PbS, with a particle size of ca. 3.0 nm and an initial solution concentration of 1.0 ppm) was also tested. The concentration of quantum dots in the feed and the permeate was determined by fluorescence (FL) spectrometry and UV−vis spectrometry, respectively. The rejection rate was calculated using the following equation: Cp

R= (1- ) ×100 % Cf

tule to achieve efficient selective ultrafiltration, we designed the dual-layer MSTFAAO membrane that is composed of a highly ordered mesoporous silica thin film (MSTF) contact standing on an anodic aluminium oxide (AAO) membrane with periodic straight-through macropores. In the synthesis of the MSTF, a substrate with a flat and smooth top surface is essential. Therefore, the macroporous AAO was first spin-coated with cross-linked polystyrene (PS) to form a smooth top surface, denoted PS/AAO (Figure 1). Then, growth of MSTF onto the smooth surface of PS/AAO by immersing the PS/AAO membrane into an oil-in-water emulsion consisting of CTAB, ethanol and decane.18 Subsequently, the PS layer was removed by annealing the MSTF/PS/AAO in air by a dynamic heating process, followed by UV ozone cleaning. The diatom-mimicking MSTFAAO membrane was then obtained. Meanwhile, the inorganic composition gave this biomimetic MSTFAAO membrane good thermostability (Figure S1). Details of the preparation methods are provided in the Experimental Section. The surface texture and pore structure of bare AAO and biomimetic MSTFAAO membranes were characterized by optical microscopy and scanning electron microscopy (SEM). Centimeter-sized thin film of mesoporous silica in biomimetic MSTFAAO can be routinely prepared with optical uniformity. The contrast topview SEM images (Figure 2a and 2b) of the bare AAO membrane and the MSTFAAO membrane confirm that the continuous periodic nature of the mesoporous silica thin film of MSTFAAO shows no apparent defects or breaks. A cloudy appearance of biomimetic MSTFAAO (Figure 2b1 and 2b2) under SEM compared with the bare AAO indicates a uniform coating of MSTF on AAO. The precise elemental mappings and energy-dispersive X-ray (EDX) spectrum in Figure S2 demonstrate that elemental Si is uniformly distributed over the entire surface of the MSTFAAO, while elemental Al is derived from the AAO support. A magnified top-view SEM image (Figure 2b3) reveals a thin layer of MSTF with periodic hexagonally ar-

(3)

where Cf (mg/L) and Cp (mg/L) correspond to the solute concentration of feed solution and permeate, respectively.19

Results Synthesis and characterization of biomimetic MSTFAAO membrane. Diatom frustules have been used extensively in water filtration for micron-sized exclusions.15 The diatom frustules, basically composed of silicate and/or alumina, consist of multilayers of an epitheca and a hypotheca. The two layers typically overlap one another with different pore sizes, while both of the pores are uniform periodic macropores.14,15 Aiming to perfectly mimic the hierarchically periodic porous structure of the diatom frus3



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Figure 3. (a1, a2) TEM images of diatom-mimicking MSTFAAO microtomed specimen coated with Pt and carbon protection layer prepared by focused ion beam (FIB). (a3, a4 and a5) High-magnification TEM images of MSTFAAO from the area outlined by the circle in panel (a2). (b) Droplet profile on the MSTFAAO membrane surface. (c) 2D-GISAXS scattering profile, (d) 1D intensity profile plotted against qy for the GISAXS pattern of the thin MSTF membranes.

side-view SEM images of the biomimetic MSTFAAO in Figure 2d reveal the ultrathin smooth layer of MSTF on top of the AAO surface, with the MSTF having an ultrathin uniform thickness of ~ 30 nm and AAO having uniform perpendicular channels with a length of 54 μm (Figure S3a). The uniform hierarchical porous structure of MSTFAAO is exactly like that of the diatom frustule (Figure 2e and 2f). These results verify that the structure of the as-fabricated MSTFAAO membrane is diatom frustule-like.

Figure 2. (a1) Low-magnification top-view SEM image and (a2) high-magnification SEM image of bare AAO. (b1) Low-magnification top-view SEM image and (b2,b3) high-magnification top-view SEM images with FFT pattern of MSTFAAO. (c) Photograph of diatom-mimicking MSTFAAO membrane. (d) Cross-sectional SEM images of MSTFAAO. (e) Top-view SEM image of diatom frustule. (f) Schematic of the morphologic structure of diatom-mimicking MSTFAAO.

To further characterize the side-view of the biomimetic MSTFAAO membranes, transmission electron microscopy (TEM) was used. Figure 3a shows the cross-section of the MSTFAAO membrane after focused ion beam (FIB) milling. To increase the sample conductivity and protect the morphology of the sample during FIB milling, a Pt layer and carbon layer were coated onto the sample before FIB milling. As shown in Figure 3a1 and 3a2, the macroporous AAO, with a uniform vertical channel structure, was fully covered with an ultrathin and smooth layer of MSTF (thickness of ca. 30 nm) without any visible cracks. More importantly, the single-layer mesoporous channels of the MSTF are perpendicularly aligned throughout the cross-

ranged nanopores covering both the walls and pores of the AAO membrane. Significantly, the MSTF is a translucent single layer with extremely uniform and small thickness through which the pore structure of the underlying AAO substrate can be observed clearly. The MSTF has an average pore size of 5.9 ± 0.4 nm, and the AAO has a pore size of ca. 147 nm. The two dimension (2D) hexagonal packing diffraction pattern of the space group p6mm of MSTF is revealed by the fast Fourier transform (FFT) pattern (Figure 2b3).18 The photo image in Figure 2c shows a round MSTFAAO membrane with a diameter of 4.7 cm. The

4


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Figure 4. (a) Schematic showing the solution diffusion through the membranes. (b) Permeation flux of the MSTFAAO membrane compared with the permeation flux of other ultrafiltration membranes.21-28 (c) UV-vis absorption spectra before and after a solution of mixed Cyt c and BSA was filtered through the MSTFAAO nanofilter device. (d) Fluorescence spectra before and after CdSe@ZnS quantum dots were filtered through the nanofilter device. (e) UV-vis absorption spectra before and after the PbS quantum dots were filtered through the nanofilter device.

section (Figure 3a3, 3a4 and 3a5 and Figure S3b). Furthermore, surface wettability is crucial for filter membranes. As Figure 3b shows, the measured water contact angle is 8.8°, indicating that the biomimetic MSTFAAO membrane is hydrophilic with excellent wettability, which enables rapid diffusion of water molecules through the filter membrane.3

indicates the highly ordered vertical nanochannels of mesoporous thin films over a large area. Protein and Quantum Dot Separations. The separation ability of the MSTFAAO membrane was evaluated using a dead-end filtration device (Figure s4). For tests with pure water, a ultrahigh-flux permeate flux (1027 ± 20 L m-2 h-1 bar-1) was determined; this value is significantly higher than the permeability of commercial ultrafiltration membranes (1~5 L m-2 h-1 bar-1)21, 22 and precede most of the reported inorganic and polymer membranes, e.g., amine functioned SBA-15,21 polysulfone (PSf UF),22,23 PS-b-PAA,24 carbon nanotube membranes (CNT),25 ZSM-5,26 PET,27 and Al2O3-TiO228 (Figure 4a and b). Significantly, the MSTFAAO membrane achieves such a high permeability (1027 ± 20 L m-2 h-1 bar-1) under the low pressure of 0.07 bar, which means that less pumping energy is demanded during filtration. Furthermore, the pure water flux of the MSTFAAO membrane under high temperature (70 °C) was also tested. The measured permeation value is 1614 ±

To confirm the macro-uniformity of the vertical mesopore channels over the entire MSTF membrane, the membrane was further examined by grazing-incidence smallangle X-ray scattering (GISAXS) (Figure 3c and 3d). Three prominent spots on the left and right x-axis of the grazingincidence X-ray beam are shown in the GISAXS pattern,and there is no structure along the z-direction. The narrow (100), (110) and (200) peaks indicate a highly ordered 2D hexagonal packing of vertical pores of the MSTF,20 with a d-spacing of 6.87 nm, equivalent to a poreto-pore center distance of 7.94 nm, which is consistent with the SEM results (Figure 2b) of an average pore size of 5.9 ± 0.4 nm and pore wall sizes of 2.0 ± 0.2 nm. This result 5



30 L m-2 h-1 bar-1. The result is higher than the permeation data (1027 ± 20 L m-2 h-1 bar-1) conducted at room temperature, which due to the viscosity of water decreases as the temperature increases. We suggest that the high permeate flux of this biomimetic membrane is attributed to the uniformly periodic diatom-mimicking structure with highly ordered vertical mesopore and the super-hydrophilic surface that minimize the flow resistance (Figure 4a).

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Table 1. Ultrafiltration results of various proteins using MSTFAAO and bare AAO membrane, respectively.

To demonstrate the molecular-sieving ability of the biomimetic MSTFAAO membranes, four proteins with different molecular weights (MWs) and sizes were selected, including bovine serum albumin (BSA) (67 kDa and size 7 nm), ovalbumin (OVA) (43 kDa and size 5 nm), lysozyme (LYZ) (14 kDa and size 3 nm), and cytochrome c (Cyt c) (12 kDa and size 2 nm). The prepared protein solutions are without aggregation as shown by the particle size distribution from dynamic light scattering (Figure S5). The concentration changes in the protein solutions before and after filtration were determined by UV-vis absorption spectroscopy after standard calibration curves were established (Figure S6). To avoid the influence of surface charge adsorption between the protein and the membrane filter, the pH of the feed solutions was adjusted to pH 4 to maintain the uncharged state of the MSTFAAO membrane (zeta potential of MSTFAAO and proteins are shown in Figure S7). The filtration results are shown in Table 1. MSTFAAO achieved 99.6 % rejection of BSA, 57.1% rejection of OVA, 29.1 % rejection of LYZ and 10.4 % rejection of Cyt c (Figure S8). The results indicate that more than 70.9 % of the LYZ and 89.6 % of the Cyt c passed through the biomimetic MSTFAAO membrane, while BSA was completely rejected. The results demonstrate that the biomimetic MSTFAAO membrane exhibits excellent selectivity for the separation of proteins with different MWs.

Proteins

MW (kDa)

Size (nm)

MSTFAAO rejection (%)

AAO rejection (%)

BSA

67

7

99.6

29.4

OVA

43

5

57.1

3.1

LYZ

14

3

29.1

2.6

Cyt c

12

2

10.4

0.6

charged, and LYZ is slightly positively charged (Figure S7 and S9); thus, LYZ was rejected at a rejection rate of 89.8 %, while 29.1 % rejection was achieved at pH 4 (the surface of MSTFAAO is uncharged at pH 4). The nanoporous biomimetic MSTFAAO membrane displays exceptional size and charge selectivity. To verify the size exclusion ability of inorganic nanoparticles, the ultrafiltration of different quantum dots by diatom-mimicking MSTFAAO membrane was investigated. Two inorganic quantum dots, CdSe@ZnS (with a particle size of ca. 10.5 nm) and PbS (with a particle size of ca. 3.0 nm) were chosen (Figure S10 and S11). As shown in Figure 4d and 4e, the membrane can reject the CdSe@ZnS quantum dots with a rejection rate of 99.0 %, while most of the PbS quantum dots can pass through the membrane with a rejection rate of 29.4 %. Generally, for the traditional filtration membranes, higher water permeability invariably leads to lower selectivity.29 Counterintuitively, our diatommimicking MSTFAAO membranes maintained both high water permeability and outstanding selectivity, in contrast to the behavior of traditional membranes. Meanwhile, the inorganic composition renders the diatom-mimicking MSTFAAO membrane own good thermostability (Figure S1). These results confirm that the prepared diatom-mimicking MSTFAAO membrane can be used as an effective size-selective nanofilter for both inorganic nanoparticles and biomolecule filtration at the macroscale due to its uniform periodic vertical mesopore channels and centimeter size.

Next, we used the biomimetic MSTFAAO membrane to separate a two-protein mixture with different molecular sizes. A mixture of BSA (7 nm, 67 kDa) and Cyt c (2 nm, 12 kDa) was applied for ultrafiltration (Figure 4c). After the protein solutions were mixed, two characteristic peaks at 280 and 416 nm are shown in the UV-vis spectra (Figure 4c), which corresponds to BSA and Cyt c, respectively. However, only the Cyt c peak at 416 nm can be observed in the UV-vis spectrum of the permeate solution filtered by the MSTFAAO membrane, while the peak of BSA almost completely disappears (Figure 4c). We calculated that 89.4 % of the Cyt c passed through the MSTFAAO membrane, while 99.7 % of the BSA was rejected. Because the MSTFAAO membrane has a uniform pore size of 5.9 ± 0.4 nm and vertical straight-through channels, the proteins with a molecular size larger than 5.9 ± 0.4 nm can be blocked. Smaller proteins can pass through the membrane, thus making size sieving possible. Furthermore, the charge-selective adsorption of MSTFAAO membranes was investigated by adjusting the pH value of the feed solution. At pH 7, the surface of MSTF is negatively

Theoretical Calculation of the Permeation. Many recent reports on membranes have focused on for ultrafiltration.21,22 Our diatom-mimicking MSTFAAO membrane, in addition to its excellent permeability and selective cutoff behavior, possesses the additional great advantage of its amenability to theoretical analyses of fluid flow because it has uniform and periodic perpendicular pores. Unlike other membranes, where the pore size and orientation are widely distributed, this periodic-patterned material would greatly help in the understanding of nanoflow in the pores. Here, we attempt to calculate the theoretical flow rates according to the Hagen- Poiseuille equation as follows:

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Table 2. Textural properties and permeability of MSTF, MSTFAAO and bare AAO membranes, respectively. Sample

Porosity (%)

Pore (nm)

AAO

42

147.0

MSTF

49

MSTFAAO

-

size

Pore (nm)

Experimental

54000

2124

2067 ± 20

5.9

30

7187

-

-

-

1247

1027 ± 20

πd ΔP

(such as bacteria) exclusion due to their interparticle macropores, our centimeter-sized MSTFAAO film actually employs the mesopore for separation of nano-sized objects. We are actually mimicking the fine effect of the sizeexclusion function of the diatom shell over a large scale. We suggest that the high permeate flux and excellent molecular-sieving ability in this diatom-mimicking membrane can be attributed to: (i) the uniformly periodic duallayer structure in which mesoporous MSTF acts as a highly efficient size-selective layer, and macroporous AAO acts as a highly porous straight-through support layer, (ii) the highly ordered vertical straight-through pores with minimized flow resistance, and (iii) uniform large scale nonbreaking and super-hydrophilic surface.

(4)

128μL

where q refers to the volumetric flow rate of the laminar flow through a circular channel of diameter d, ΔP is the transmembrane pressure,  is the viscosity of water (8.9×10-4 Pa·s at 25°C), and L represents channel length. For a unit area with N channels and a porosity of , the flux J is as follows: 2

J=N×q=

εd ΔP 32μL

(5)

In our case, for the AAO membrane, porosity and average pore size were calculated by ImageJ (Table 2). Equation (5) gives a permeability of J/P = 2124 L m-2 h-1 bar-1, which is in fairly good agreement with the experimentally measured value of 2067 L ± 20 m-2 h-1 bar-1 for AAO.

Conclusions

The dual-layer MSTFAAO membrane behavior can be modelled by the sequential flow of the Darcy model as follows: J/P =

1 μ(RAAO +RMSTF /εAAO )

Permeability (L m-2 h-1 bar-1) Calculated

4

q=

length

In summary, we have successfully developed a 3D diatom-mimicking filter membrane for ultrafiltration. The biomimetic filter is composed of a single-layered contactstanding MSTF with uniform perpendicular mesopore channels on a periodic macroporous AAO substrate. The highly ordered biomimetic MSTFAAO membrane exhibited excellent hydrophilicity (CA = 8.8°), continuously nonbreaking to centimeter range (area of 17.3 cm2) and ultrahigh-flux water permeability up to 1027 ± 20 L m-2 h-1 bar-1. The permeability is significantly higher than that of most ultrafiltration membranes reported thus far in the scientific literature. The biomimetic MSTFAAO membrane exhibited a superior molecular-sieving ability in both protein separation and quantum dot ultrafiltration, where a rejection rate as high as 99 % was obtained for molecular-sieving separation of protein mixtures. The excellent size exclusion performance, coupled with the outstanding permeability and thermal stability, renders this diatom-mimicking MSTFAAO membrane a potential nanofilter. In addition to size exclusion separation, our membrane material can be easily applied to protein concentration and change of solvent, which are important daily operations in biochemical laboratories.

(6)

(Equation (6) is derived in the Supporting Information) with the flow resistance R = P/(J). Using Equation (4) and (5), one can determine R for AAO and MSTF as shown in the Supporting Information. The AAO factor in equation (6) accounts for the fraction of MSTF nanochannels that are blocked by the wall of AAO when MSTF binds together with AAO in MSTFAAO. Equation (6) gives a calculated permeability of 1247 L m-2 h-1 bar-1, while the experimentally determined value is 1027 ± 20 L m-2 h-1 bar-1 for MSTFAAO (Table 2). The agreement is fair. However, one can reasonably expect the deviation because (a) some dead-end pores are present within AAO, (b) the end effect,30 (c) the nonuniformity of pores, and other factors. A more detailed understanding of the fluid flow in biomimetic MSTFAAO can be obtained by numerical simulation of the NavierStokes equation for the model, which is outside the scope of this work.

Discussion Overall, the 3D dual-layer diatom-mimicking MSTFAAO membranes exhibit excellent size sieving for different-sized proteins in aqueous solution and quantum dots in toluene solution, while possessing outstanding water permeability under low pressure. The performance precedes most of the reported inorganic and polymer membranes (Figure 4b). While natural diatomaceous earth has been extensively used in water filtration for micron-sized

ASSOCIATED CONTENT Supporting Information include theoretical calculations of the permeation, Thermal gravity analysis, EDX spectrum, SEM images, Size distribution curves, Zeta potentials results, TEM

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images is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author * Chung-Yuan Mou. Email: [email protected] * Kuo-Lun Tung. Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have approved the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Taiwan University, the Ministry of Science and Technology, Taiwan (104-2221-E-002176-MY3 and 107-3017-F-002-001), and the “Advanced Research Center of Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006). Jingling Yang was supported by a postdoctoral fellowship from the Ministry of Science and Technology (MOST). We thank Ching-Yen Lin and Ya-Yun Yang at the Instrument Centre of National Taiwan University for assistance with SEM, Chia-Ying Chien and SuJen Ji of the Ministry of Science and Technology (National Taiwan University) for assistance with the focused ion beam instrument (FIB) and high-magnification TEM experiments.

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Diatom-Mimicking Ultrahigh-Flux Mesoporous Silica Thin Membrane

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