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Unidirectional solute transfer using a Janus membrane
Journal Pre-proof Unidirectional solute transfer using a Janus membrane Ying Dong, Jing Li, Stig Pedersen-Bjergaard, Chuixiu Huang PII:
S0376-7388(19)33217-X
DOI:
https://doi.org/10.1016/j.memsci.2019.117723
Reference:
MEMSCI 117723
To appear in:
Journal of Membrane Science
Received Date: 16 October 2019 Revised Date:
1 December 2019
Accepted Date: 5 December 2019
Please cite this article as: Y. Dong, J. Li, S. Pedersen-Bjergaard, C. Huang, Unidirectional solute transfer using a Janus membrane, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/ j.memsci.2019.117723. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Graphical Abstract
Unidirectional solute transfer is realized using a Janus membrane that is prepared by integrating underoil superhydrophobic (UOSH) and underwater superoleophobic (UWSO) surfaces. Without or with electric field, solutes pass through the Janus membrane from the UOSH side to the UWSO side, but are blocked from penetrating in the reverse direction, forming a solute transfer “diode”.
Unidirectional solute transfer using a Janus membrane Ying Donga, Jing Lib,*, Stig Pedersen-Bjergaardc,d, Chuixiu Huanga,* a
Department of Forensic Medicine, Huazhong University of Science and Technology,
13 Hangkong Road, Wuhan 430030, China b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, 18 Tianshui Middle Road, Lanzhou 730000, China c
School of Pharmacy, University of Oslo, P.O. Box 1068 Blindern, 0316 Oslo,
Norway d
Faculty of Health and Medical Sciences, School of Pharmaceutical Sciences,
University of Copenhagen, Universitetsparken 2, 2100 Copenhagen, Denmark * Corresponding authors. E-mail addresses: [email protected] (J. Li), [email protected] (C. Huang).
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Abstract: A membrane as a selective barrier of mass transfer is of great importance for applications in analysis, environment, energy, and biology. Unidirectional solute transfer that would improve membrane process flow is a challenge. Here, macroscopic extreme wettability of membranes controlling microscopic solute transfer is presented. A Janus membrane is prepared by integrating underoil superhydrophobic (UOSH) and underwater superoleophobic (UWSO) surfaces. Without or with electric field, solutes pass through the Janus membrane from the UOSH side to the UWSO side, but are impeded from penetrating in the reverse direction. The UWSO surface has a high affinity toward solutes in water compared to the UOSH surface at their interface. The composite membrane realizes unidirectional solute transfer, which can be considered as a solute transfer diode. This study may promote the understanding of the correlation between macroscopic and microscopic interfacial behaviors and facilitate the design of interfacial materials for controllable solute transfer and separation.
Keywords: Janus membrane; superwetting; solute transfer; liquid-phase extraction; electromembrane extraction
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1. Introduction Membranes with distinct features are widely used as important unit operation for separation processes, showing promising potential in a variety of applications such as analysis [1-5]. In liquid-phase extraction (LPE) [6-10] and electromembrane extraction (EME) [11-15], analytes migrate from the agitated samples to organic phase. Once the analytes arrive at the donor phase/organic phase interface, they transfer into the organic phase according to their distribution coefficients or drive of electrical potential. Extracted analytes diffuse across the organic phase and then are released into acceptor solution at the organic phase/acceptor phase interface. Controlling the solute transfer affords opportunities for addressing scientific and practical issues such as the aforementioned pretreatment processes. In addition, organic solvents are directly used as free liquid membrane [16-19] or are supported on an inert porous polymeric membrane (so-called supported liquid membrane (SLM)) [20-23], forming the selective barrier. The effect of wetting property of the supported membrane (solid phase) on the solute transfer at the liquid/liquid interfaces is rarely studied. Interfacial materials with special wettability have opposite affinity toward water, oil, and air [24-30]. Based on this, a series of Janus superwetting membranes have been developed to control the transportation of water and oil droplets as well as bubbles, realizing macroscopic unidirectional phase transfer and separation [31-40]. However, unidirectional solute transfer at the microscopic scale remains a challenge, which is limited by the understanding of the correlation between macroscopic and 3
microscopic interfacial behaviors. In this work, taking advantage of superwettability of membrane, analyte migration in LPE and EME is manipulated. Unidirectional solute transfer is demonstrated using a Janus membrane, which is simply prepared by integrating underoil superhydrophobic (UOSH) and underwater superoleophobic (UWSO) membranes. Without or with electric field, analytes can penetrate through the composite membrane from the UOSH side to the UWSO side, but not in the opposite direction, forming a solute transfer “diode”. Like electron transfer at p–n junction, the unidirectional solute transfer is owing to the competition of solutes between UOSH and UWSO surfaces. This work would provide a guidance for the design of mass transfer “diodes” using Janus interfaces with entirely opposite physiochemical properties.
2. Experimental section 2.1. Materials Polypropylene (PP) membrane 1E with a thickness of about 100 µm and a pore size of about 0.1 µm was purchased from Membrana (Wuppertal, Germany). Phenolphthalein, methyl red, aristolochic acid A, calcein, decanol, and tetramethyl orthosilicate were purchased from Aladdin Bio-Chem Technology Co., Ltd (Shanghai, China). All other chemicals were analytical-grade reagents and used without further purification. Ultrapure water was used throughout this work.
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2.2. Preparation of the modified PP To prepare polydopamine-coated PP, dopamine hydrochloride was dissolved in Tris-HCl (50 mM, pH 8.5). PP was prewetted by methanol and then was immersed into the aqueous solution of dopamine (2 mg/mL). The in-situ polymerization was performed for 10 h at 40 °C. Afterward, the substrate was taken out, rinsed with water, and dried at a drying oven at 40 °C. To further enhance the hydrophilicity, silicification was carried out on the polydopamine-coated PP. Tetramethyl orthosilicate (0.15 mL) was added into 10 mL of HCl aqueous solution (pH 3) under stirring for 15 min, and then was mixed with phosphate buffer solution (0.2 M, pH 6.0) at the equal volume. The polydopamine-coated PP was immersed into this silicification solution at room temperature under stirring for 6 h. Afterward, the modified PP was taken out, rinsed with water, and dried at a drying oven at 40 °C.
2.3. Preparation of the sample solutions Stock solutions of phenolphthalein (1 mg/mL), methyl red (1 mg/mL), calcein (1 mg/mL) and aristolochic acid A (0.2 mg/mL) were prepared by dissolving the analytes in ethanol individually. All these stock solutions were stored at 4 ℃. The sample solutions were obtained by diluting the stock solutions with water.
2.4. LPE and EME equipment and procedure A H-shape cell system was used as the LPE and EME equipment. The H-shape 5
cell was composed of two half cells which acted as the sample and acceptor compartment, and each of them had a tubular side connector. Original PP with UOSH and the modified PP with UWSO were prewetted by decanol and water, respectively. Then, original PP and the modified PP were sandwiched by an O ring and were used to compartmentalize the two sides joined via the tubular connectors and a clamp. After the H-shape cell was assembled, 10 mL of the sample solution was poured into the sample compartment (the left cell), and the acceptor solution was 10 mL of 0.01 M NaOH aqueous solution. The LPE process was initiated by magnetic stirring at a speed of 800 rpm. For the EME process, cathode and anode, made of platinum wires (d = 0.5 mm), were introduced into the donor and acceptor phases, respectively. The electrodes were connected to an electrophoresis apparatus power (DYY-2C, Beijing Liuyi Biotechnology CO. LTD., China). The voltage was set as 50 V. The EME process was initiated by magnetic stirring at a speed of 800 rpm. After the default extraction time, the donor and acceptor phases in LPE and EME were collected individually and subsequently analyzed by using a UV-vis spectrophotometer (UV-2600, Shimadzu).
2.5. Competitive extraction equipment and procedure A three-cell system was applied in the competitive extraction test. The middle cell that acted as the sample compartment had two tubular side connectors linked with two acceptor compartments, respectively. Original PP and the modified PP were sandwiched between the sample and acceptor compartments by an O ring and a clamp. 6
The left acceptor phase contacted with the UOSH side of the composite membrane. On the contrary, the UWSO side of another composite membrane was fixed facing the right acceptor phase. The volume of the donor phase was 15 mL and the acceptor phases were 10 mL of 0.01 M NaOH aqueous solution. The extraction process was conducted under magnetic stirring at a speed of 800 rpm. For the EME process, two cathodes were introduced into the donor phase, and two anodes were applied in the left and right acceptor phases, respectively.
2.6. Characterization The surface structures were analyzed on a field emission scanning electron microscope (SEM, FEI Quanta 650FEG). The samples were pretreated by Au-sputtered specimens to increase surface conductivity. The corresponding voltage and current were 30 kV and 10 µA, respectively. The element distribution maps were obtained using energy-dispersive X-ray spectroscopy (EDS) accessory. The surface chemical compositions were further detected by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi). The three-dimensional profiles of membranes were observed by an atomic force microscope (NT-MDT Prima, Bruker Dimension Edge). The contact angles were measured on a JC2000D contact angle system (Zhongchen Digital Equipment Co., Ltd. Shanghai, China). About 5 µL droplets were used to measure the contact angles. The average contact angle values were got by measuring the samples at five different positions. A mobile phone (Huawei Mate 9) was used to record the extraction process. The absorbance of the 7
donor and acceptor phases during the extraction process was determined by using a UV-vis spectrophotometer (UV-2600, Shimadzu). A spectrofluorometer (F-97 Pro, Shanghai Lengguang Technology Co., Ltd. China) was used to measure the fluorescence intensity of the calcein donor and acceptor phases before and after LPE. The excitation wavelength was 474 nm. An inverted fluorescence microscope (IX-71, OLYMPUS) was employed to observe the fluorescence of the used membranes after LPE.
3. Results and discussion Fig. 1 shows the process used to prepare the Janus superwetting membrane for unidirectional solute transfer. PP membrane was modified by polydopamine and SiO2 using the mussel-inspired method and then silicification [41-43]. Original PP with smooth and clean surface has low oxygen content (Fig. 2a,b), which is confirmed by EDS and XPS measurements (Figs. S1, S2 and Table S1). The surface structure and composition induce hydrophobicity in air (Fig. 2c). The water contact angle of original PP is 144.2±1.9°. Due to low surface tension of oil, the stable oil layer on the PP surface can extremely repel water droplet in oil. The underoil water contact angle is 169.6±2.0°. After in-situ oxidation polymerization and silicification, the coating makes the PP surface rough (Fig. 2d,e) and the oxygen content of the modified PP is greatly increased (Figs. S3-S6), resulting in superhydrophilicity and UWSO (Figs. 2f and S9). Once water droplet contacts with the surface of the modified PP, the water contact angle is less than 10° and is rapidly decreased to 0° in 1/3 s (Movie S1). The 8
underwater oil contact angle of the modified PP is 166.8±2.2°. Figs. 2g,h and S7 show the cross-section SEM images of the original and modified PP. It is found that both original PP and the modified PP have the thickness of about 100 µm. Besides, the high porosity can be also demonstrated by the AFM observation (Fig. S8). From the corresponding cross-section Si distribution map (Fig. 2i), the entire PP membrane is successfully modified to become very hydrophilic. To test the dynamic adhesive behavior, a water droplet (5 µL) and an oil droplet (CCl4, 5 µL) move horizontally and vertically relative to original PP in oil and the modified PP in water (Figs. S10, S11 and Movies S2-S5), respectively. Under a pressure, the water and oil droplets move through the whole surfaces with ease. At vertical direction, the distorted water and oil droplets can easily recover the spherical shape without any residual after getting away the surfaces. The outstanding antiadhesive property further verifies UOSH of original PP and UWSO of the modified PP. To achieve unidirectional solute transfer, the prepared UOSH and UWSO membranes were assembled into a twin-layer composite membrane (Fig. 1b). Notably, the surface modification does not change the pore size of PP (about 0.1 µm), which is not responsible for the controllable solute transfer.
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Fig. 1. Schematic illustration of preparation of the Janus membrane for unidirectional solute transfer.
Fig. 2. (a, b) SEM images of original PP. (c) Photographs of water droplet in air and in oil (hexane) on the surface of original PP. (d, e) SEM images of the modified PP. (f) Photographs of water droplet in air and oil droplet (CCl4) in water on the surface of
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the modified PP. In air, water droplet contacts with the surface of the modified PP at once. (g, h) Cross-section SEM images of the modified PP. (i) The corresponding cross-section Si distribution map.
For conventional LPE, phenolphthalein (10 mg/L) is extracted from neutral aqueous solution (Fig. S14), across SLM (decanol supported on original PP), and then migrates into acceptor phase (pH 12 aqueous solution). The introduction of a superhydrophilic membrane at the donor phase/SLM and SLM/the acceptor phase interfaces drastically influences the analyte migration. Unidirectional solute transfer is realized using the prepared Janus membrane. As shown in Figs. 3a,b and S15, when the UOSH and UWSO surfaces are fixed facing the donor and acceptor phases (labelled as UOSH/UWSO), respectively, the color of the acceptor phase gradually deepens. It is indicated that phenolphthalein penetrates through UOSH/UWSO, resulting in the “forward direction” of the solute transfer “diode”. In contrast, if the composite membrane is inverted (labelled as UWSO/UOSH) and the sample solution contacts with the UWSO side, phenolphthalein migration is restrained. After 90-min LPE, the acceptor phase is still colorless. This is considered as the “reverse direction” of the solute transfer “diode”.
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Fig. 3. (a) Phenolphthalein (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively. (b) Phenolphthalein (10 mg/L) is blocked from the UWSO side. (c) Methyl red (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively, and a voltage of 50 V is used as the drive. (d) Methyl red (10 mg/L) is blocked from the UWSO side in spite of the presence of 50 V.
Under a voltage of 50 V, in EME, the Janus membrane is also available to the unidirectional solute transfer. An electric field gradient would facilitate the analyte migration. When UOSH/UWSO is used, methyl red (10 mg/L) in the donor phase is gradually decreased, leading to a light color (Figs. 3c and S16). Meanwhile, the
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acceptor phase is changed from colorless to yellow, indicating the “forward direction” transfer of methyl red. In contrast, in spite of the presence of 50 V, methyl red cannot migrate to the acceptor phase that is still colorless after 90-min EME (Fig. 3d), showing the “reverse direction” blocking of UWSO/UOSH. In addition, water electrolysis causes pH change of the donor phase, displaying the color change. After 90-min LPE and EME, the acceptor phase for UWSO/UOSH has very low absorbance whereas the absorbance of the acceptor phase for UOSH/UWSO is greatly increased (Figs. 4 and S17). Note that the absorbance of the donor phase for UWSO/UOSH is slightly decreased during the extraction.
Fig. 4. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of LPE time. Phenolphthalein (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) The absorbance at 428 nm of the donor and acceptor phases as a function of EME time. Methyl red (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes under 50 V.
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Competitive extraction test was performed to further demonstrate the unidirectional solute transfer using the Janus superwetting membrane. As shown in Figs. 5a and S19, phenolphthalein (10 mg/L) aqueous solution links with two acceptor phases. The left acceptor phase contacts with the UOSH side of the composite membrane. On the contrary, the UWSO side of another composite membrane is fixed facing the right acceptor phase. Due to the unidirectional solute transfer, only the right acceptor phase becomes red after 90-min competitive LPE. Moreover, a parallel voltage of 50 V was applied to the competitive EME (Figs. 5b and S20). The color of the donor phase containing methyl red (10 mg/L) gradually turns light. Under electric drive, methyl red selectively passes through UOSH/UWSO and migrates to the right acceptor phase that shows yellow after 90-min competitive EME. In contrast, the left acceptor phase is still colorless. During the competitive LPE and EME, the absorbance of the left acceptor phase is fairly low whereas the right acceptor phase displays a rising absorbance (Figs. 6 and S21). The design of membrane process flow using the solute transfer “diode” availably regulates the migration direction of analytes.
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Fig. 5. (a) Phenolphthalein (10 mg/L) and (b) methyl red (10 mg/L) are competitively extracted using the UWSO/UOSH (left) and UOSH/UWSO (right) membranes.
Fig. 6. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of competitive LPE time. (b) The absorbance at 428 nm of the acceptor phase as a function of competitive EME time. Phenolphthalein (10 mg/L, a) and methyl red (10 mg/L, b) are competitively extracted using the UWSO/UOSH and UOSH/UWSO membranes.
LPE and EME are widely used as pretreatment techniques for extracting analytes such as drugs and toxicants from complex matrices. Exposure to aristolochic acid is related with a high incidence of uroepithelial tumorigenesis, and is associated with urothelial cancer [44,45]. Using LPE, aristolochic acid A cannot spontaneously migrate into the acceptor phase under the concentration gradient (Fig. S22). If the electric field gradient is implemented, analytes with negative charge can favorably diffuse across SLM toward the positive electrode in the acceptor phase (the pKa of
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aristolochic acid A is about 3). As shown in Figs. S23 and S24, under a voltage of 50 V, aristolochic acid A (10 mg/L) penetrates through UOSH/UWSO into the acceptor phase but is obstructed from UWSO/UOSH, resulting in the great difference of absorbance of the acceptor phases for UOSH/UWSO and UWSO/UOSH after 90-min EME. The unidirectional solute transfer using the Janus membrane is applied for the practical pretreatment. To reveal the mechanism of the unidirectional solute transfer using the Janus superwetting membrane, calcein was used as a fluorescence probe to investigate the controllable analyte migration. As shown in Fig. S25, the yellow green of the donor phase containing calcein (5 mg/L) for both UOSH/UWSO and UWSO/UOSH fades after 90-min LPE. However, only the acceptor phase for UOSH/UWSO is colored and has strong characteristic absorption and fluorescence peaks (Figs. 7a, S26 and S27). In contrast, both the absorbance at 490 nm and fluorescence intensity at 515 nm of the acceptor phase for UWSO/UOSH are fairly low. Note that the absorbance of the donor phase for UWSO/UOSH is obviously decreased. After 90-min LPE, fluorescence microscope was used to observe the distribution of the fluorescence probe at the interface of UWSO and UOSH. For the “forward direction”, calcein migrates from the donor phase to the oil-wetted UOSH membrane that shows strong green fluorescence (Fig. 7b). The calcein extracted by the oil supported on the UOSH membrane can transfer through the interface of the oil-wetted UOSH and water-wetted UWSO surfaces, and then rapidly diffuses into the acceptor phase, resulting in a very weak fluorescence of the UWSO membrane. For the “reverse 16
direction”, calcein arrives at the UWSO surface and is gradually enriched due to its superhydrophilic property and high porosity, leading to the decreased absorbance of the donor phase (Figs. 4a and S23). Nevertheless, the interface of the water-wetted UWSO and oil-wetted UOSH surfaces provides a selective barrier to prevent the enriched calcein from permeating. Contrary to UOSH/UWSO, the UWSO membrane has obvious green fluorescence whereas the UOSH membrane is dark (Fig. 7b). A diode that conducts current primarily in one direction is commonly made of n-type and p-type semiconductors. The former contains negative charge carriers (electrons). When a sufficiently high electrical potential is applied to the p side, electrons can flow through the p-n junction to the electron-rich side. When the potential is applied to the n side, the junction does not allow the flow of electrons in the reverse direction. Similarly, the interface of the water-wetted UWSO and oil-wetted UOSH surfaces plays an important role in the unidirectional solute transfer (Fig. 7c). When concentration gradient or electric field gradient is applied to the UOSH side, solutes dissolved in water can pass through the interface to the water-loving side. In the reverse direction, because the interaction between solutes and the UWSO membrane is strong, the interface does not allow the permeating of solutes. In other words, the UWSO surface dominates in the competition of solutes at the interface over the UOSH surface, forming a solute transfer “diode”.
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Fig. 7. (a) Fluorescence spectra of the acceptor phase after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) Fluorescence microscopy images of the used UOSH and UWSO surfaces after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO (left) and UWSO/UOSH (right) membranes. (c) Schematic illustration of the mechanism of the unidirectional solute transfer.
4. Conclusions In summary, macroscopic extreme wettability has been developed to microscopic controllable solute transfer. A Janus membrane has been designed and prepared to perform unidirectional solute transfer for sample pretreatment, which is based on the
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integration of the UOSH and UWSO surfaces to a composite membrane. In LPE and EME, analytes pass through the composite membrane from the UOSH side to the UWSO side, but are impeded from penetrating in the reverse direction. Similar to electron transfer through p-n junction to the electron-rich side, unidirectional solute transfer is owing to the dominant affinity of the UWSO surface toward solutes dissolved in water, forming a solute transfer “diode”. The design will be extended to other mass transfer “diodes” using Janus interfaces with entirely opposite physiochemical properties, in which one side has an affinity toward the mass rather than another side. We believe that the Janus superwetting membrane will be widely used to control the solute transfer to improve membrane process flow. This discovery will advance the development of interfacial materials with macroscopic special wettability in microscopic interfacial applications such as extraction, water purification, and exchange membrane.
Acknowledgements This work was supported financially by the National Nature Science Foundation of China (51602236, 21876055, and 81801875) and the Fundamental Research Funds for the Central Universities in China (2017KFYXJJ021).
Appendix A. Supplementary Information Supplementary data associated with this article can be found in the online version at 19
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Figure Captions Fig. 1. Schematic illustration of preparation of the Janus membrane for unidirectional solute transfer. Fig. 2. (a, b) SEM images of original PP. (c) Photographs of water droplet in air and in oil (hexane) on the surface of original PP. (d, e) SEM images of the modified PP. (f) Photographs of water droplet in air and oil droplet (CCl4) in water on the surface of the modified PP. In air, water droplet contacts with the surface of the modified PP at once. (g, h) Cross-section SEM images of the modified PP. (i) The corresponding cross-section Si distribution map. Fig. 3. (a) Phenolphthalein (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively. (b) Phenolphthalein (10 mg/L) is blocked from the UWSO side. (c) Methyl red (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively, and a voltage of 50 V is used as the drive. (d) Methyl red (10 mg/L) is blocked from the UWSO side in spite of the presence of 50 V. Fig. 4. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of LPE time. Phenolphthalein (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) The absorbance at 428 nm of the donor and acceptor phases as a function of EME time. Methyl red (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes under 50 V. Fig. 5. (a) Phenolphthalein (10 mg/L) and (b) methyl red (10 mg/L) are competitively extracted using the UWSO/UOSH (left) and UOSH/UWSO (right) membranes. Fig. 6. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of competitive LPE time. (b) The absorbance at 428 nm of the acceptor phase as a function of competitive EME time.
Phenolphthalein (10 mg/L, a) and methyl red (10 mg/L, b) are competitively extracted using the UWSO/UOSH and UOSH/UWSO membranes. Fig. 7. (a) Fluorescence spectra of the acceptor phase after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) Fluorescence microscopy images of the used UOSH and UWSO surfaces after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO (left) and UWSO/UOSH (right) membranes. (c) Schematic illustration of the mechanism of the unidirectional solute transfer.
Fig. 1. Schematic illustration of preparation of the Janus membrane for unidirectional solute transfer.
Fig. 2. (a, b) SEM images of original PP. (c) Photographs of water droplet in air and in oil (hexane) on the surface of original PP. (d, e) SEM images of the modified PP. (f) Photographs of water droplet in air and oil droplet (CCl4) in water on the surface of the modified PP. In air, water droplet contacts with the surface of the modified PP at once. (g, h) Cross-section SEM images of the modified PP. (i) The corresponding cross-section Si distribution map.
Fig. 3. (a) Phenolphthalein (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively. (b) Phenolphthalein (10 mg/L) is blocked from the UWSO side. (c) Methyl red (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively, and a voltage of 50 V is used as the drive. (d) Methyl red (10 mg/L) is blocked from the UWSO side in spite of the presence of 50 V.
Fig. 4. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of LPE time. Phenolphthalein (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) The absorbance at 428 nm of the donor and acceptor phases as a function of EME time. Methyl red (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes under 50 V.
Fig. 5. (a) Phenolphthalein (10 mg/L) and (b) methyl red (10 mg/L) are competitively extracted using the UWSO/UOSH (left) and UOSH/UWSO (right) membranes.
Fig. 6. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of competitive LPE time. (b) The absorbance at 428 nm of the acceptor phase as a function of competitive EME time. Phenolphthalein (10 mg/L, a) and methyl red (10 mg/L, b) are competitively extracted using the UWSO/UOSH and UOSH/UWSO membranes.
Fig. 7. (a) Fluorescence spectra of the acceptor phase after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) Fluorescence microscopy images of the used UOSH and UWSO surfaces after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO (left) and UWSO/UOSH (right) membranes. (c) Schematic illustration of the mechanism of the unidirectional solute transfer.
Highlights Interfacial materials are prepared for microscopic solute transfer. Underoil superhydrophobic and underwater superoleophobic membranes are integrated. Unidirectional solute transfer is achieved using the Janus membrane. The solute transfer “diode” proceeds through the competitive affinity toward solutes. The design will be extended to other mass transfer “diodes”.
Conflict of Interest The authors declare no competing financial interest.
Author Statement Manuscript ID: MEMSCI_2019_3036 Title: Unidirectional solute transfer using a Janus membrane Journal: Journal of Membrane Science
All authors have made substantial contributions to the work reported in the manuscript, including those who proposed the conception or design of the work, performed the experiments and analyzed the data, and co-wrote the manuscript. We have approved the final version to be published. The work described is original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part.
Jing Li 29/11/2019
S0376-7388(19)33217-X
DOI:
https://doi.org/10.1016/j.memsci.2019.117723
Reference:
MEMSCI 117723
To appear in:
Journal of Membrane Science
Received Date: 16 October 2019 Revised Date:
1 December 2019
Accepted Date: 5 December 2019
Please cite this article as: Y. Dong, J. Li, S. Pedersen-Bjergaard, C. Huang, Unidirectional solute transfer using a Janus membrane, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/ j.memsci.2019.117723. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Graphical Abstract
Unidirectional solute transfer is realized using a Janus membrane that is prepared by integrating underoil superhydrophobic (UOSH) and underwater superoleophobic (UWSO) surfaces. Without or with electric field, solutes pass through the Janus membrane from the UOSH side to the UWSO side, but are blocked from penetrating in the reverse direction, forming a solute transfer “diode”.
Unidirectional solute transfer using a Janus membrane Ying Donga, Jing Lib,*, Stig Pedersen-Bjergaardc,d, Chuixiu Huanga,* a
Department of Forensic Medicine, Huazhong University of Science and Technology,
13 Hangkong Road, Wuhan 430030, China b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, 18 Tianshui Middle Road, Lanzhou 730000, China c
School of Pharmacy, University of Oslo, P.O. Box 1068 Blindern, 0316 Oslo,
Norway d
Faculty of Health and Medical Sciences, School of Pharmaceutical Sciences,
University of Copenhagen, Universitetsparken 2, 2100 Copenhagen, Denmark * Corresponding authors. E-mail addresses: [email protected] (J. Li), [email protected] (C. Huang).
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Abstract: A membrane as a selective barrier of mass transfer is of great importance for applications in analysis, environment, energy, and biology. Unidirectional solute transfer that would improve membrane process flow is a challenge. Here, macroscopic extreme wettability of membranes controlling microscopic solute transfer is presented. A Janus membrane is prepared by integrating underoil superhydrophobic (UOSH) and underwater superoleophobic (UWSO) surfaces. Without or with electric field, solutes pass through the Janus membrane from the UOSH side to the UWSO side, but are impeded from penetrating in the reverse direction. The UWSO surface has a high affinity toward solutes in water compared to the UOSH surface at their interface. The composite membrane realizes unidirectional solute transfer, which can be considered as a solute transfer diode. This study may promote the understanding of the correlation between macroscopic and microscopic interfacial behaviors and facilitate the design of interfacial materials for controllable solute transfer and separation.
Keywords: Janus membrane; superwetting; solute transfer; liquid-phase extraction; electromembrane extraction
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1. Introduction Membranes with distinct features are widely used as important unit operation for separation processes, showing promising potential in a variety of applications such as analysis [1-5]. In liquid-phase extraction (LPE) [6-10] and electromembrane extraction (EME) [11-15], analytes migrate from the agitated samples to organic phase. Once the analytes arrive at the donor phase/organic phase interface, they transfer into the organic phase according to their distribution coefficients or drive of electrical potential. Extracted analytes diffuse across the organic phase and then are released into acceptor solution at the organic phase/acceptor phase interface. Controlling the solute transfer affords opportunities for addressing scientific and practical issues such as the aforementioned pretreatment processes. In addition, organic solvents are directly used as free liquid membrane [16-19] or are supported on an inert porous polymeric membrane (so-called supported liquid membrane (SLM)) [20-23], forming the selective barrier. The effect of wetting property of the supported membrane (solid phase) on the solute transfer at the liquid/liquid interfaces is rarely studied. Interfacial materials with special wettability have opposite affinity toward water, oil, and air [24-30]. Based on this, a series of Janus superwetting membranes have been developed to control the transportation of water and oil droplets as well as bubbles, realizing macroscopic unidirectional phase transfer and separation [31-40]. However, unidirectional solute transfer at the microscopic scale remains a challenge, which is limited by the understanding of the correlation between macroscopic and 3
microscopic interfacial behaviors. In this work, taking advantage of superwettability of membrane, analyte migration in LPE and EME is manipulated. Unidirectional solute transfer is demonstrated using a Janus membrane, which is simply prepared by integrating underoil superhydrophobic (UOSH) and underwater superoleophobic (UWSO) membranes. Without or with electric field, analytes can penetrate through the composite membrane from the UOSH side to the UWSO side, but not in the opposite direction, forming a solute transfer “diode”. Like electron transfer at p–n junction, the unidirectional solute transfer is owing to the competition of solutes between UOSH and UWSO surfaces. This work would provide a guidance for the design of mass transfer “diodes” using Janus interfaces with entirely opposite physiochemical properties.
2. Experimental section 2.1. Materials Polypropylene (PP) membrane 1E with a thickness of about 100 µm and a pore size of about 0.1 µm was purchased from Membrana (Wuppertal, Germany). Phenolphthalein, methyl red, aristolochic acid A, calcein, decanol, and tetramethyl orthosilicate were purchased from Aladdin Bio-Chem Technology Co., Ltd (Shanghai, China). All other chemicals were analytical-grade reagents and used without further purification. Ultrapure water was used throughout this work.
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2.2. Preparation of the modified PP To prepare polydopamine-coated PP, dopamine hydrochloride was dissolved in Tris-HCl (50 mM, pH 8.5). PP was prewetted by methanol and then was immersed into the aqueous solution of dopamine (2 mg/mL). The in-situ polymerization was performed for 10 h at 40 °C. Afterward, the substrate was taken out, rinsed with water, and dried at a drying oven at 40 °C. To further enhance the hydrophilicity, silicification was carried out on the polydopamine-coated PP. Tetramethyl orthosilicate (0.15 mL) was added into 10 mL of HCl aqueous solution (pH 3) under stirring for 15 min, and then was mixed with phosphate buffer solution (0.2 M, pH 6.0) at the equal volume. The polydopamine-coated PP was immersed into this silicification solution at room temperature under stirring for 6 h. Afterward, the modified PP was taken out, rinsed with water, and dried at a drying oven at 40 °C.
2.3. Preparation of the sample solutions Stock solutions of phenolphthalein (1 mg/mL), methyl red (1 mg/mL), calcein (1 mg/mL) and aristolochic acid A (0.2 mg/mL) were prepared by dissolving the analytes in ethanol individually. All these stock solutions were stored at 4 ℃. The sample solutions were obtained by diluting the stock solutions with water.
2.4. LPE and EME equipment and procedure A H-shape cell system was used as the LPE and EME equipment. The H-shape 5
cell was composed of two half cells which acted as the sample and acceptor compartment, and each of them had a tubular side connector. Original PP with UOSH and the modified PP with UWSO were prewetted by decanol and water, respectively. Then, original PP and the modified PP were sandwiched by an O ring and were used to compartmentalize the two sides joined via the tubular connectors and a clamp. After the H-shape cell was assembled, 10 mL of the sample solution was poured into the sample compartment (the left cell), and the acceptor solution was 10 mL of 0.01 M NaOH aqueous solution. The LPE process was initiated by magnetic stirring at a speed of 800 rpm. For the EME process, cathode and anode, made of platinum wires (d = 0.5 mm), were introduced into the donor and acceptor phases, respectively. The electrodes were connected to an electrophoresis apparatus power (DYY-2C, Beijing Liuyi Biotechnology CO. LTD., China). The voltage was set as 50 V. The EME process was initiated by magnetic stirring at a speed of 800 rpm. After the default extraction time, the donor and acceptor phases in LPE and EME were collected individually and subsequently analyzed by using a UV-vis spectrophotometer (UV-2600, Shimadzu).
2.5. Competitive extraction equipment and procedure A three-cell system was applied in the competitive extraction test. The middle cell that acted as the sample compartment had two tubular side connectors linked with two acceptor compartments, respectively. Original PP and the modified PP were sandwiched between the sample and acceptor compartments by an O ring and a clamp. 6
The left acceptor phase contacted with the UOSH side of the composite membrane. On the contrary, the UWSO side of another composite membrane was fixed facing the right acceptor phase. The volume of the donor phase was 15 mL and the acceptor phases were 10 mL of 0.01 M NaOH aqueous solution. The extraction process was conducted under magnetic stirring at a speed of 800 rpm. For the EME process, two cathodes were introduced into the donor phase, and two anodes were applied in the left and right acceptor phases, respectively.
2.6. Characterization The surface structures were analyzed on a field emission scanning electron microscope (SEM, FEI Quanta 650FEG). The samples were pretreated by Au-sputtered specimens to increase surface conductivity. The corresponding voltage and current were 30 kV and 10 µA, respectively. The element distribution maps were obtained using energy-dispersive X-ray spectroscopy (EDS) accessory. The surface chemical compositions were further detected by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi). The three-dimensional profiles of membranes were observed by an atomic force microscope (NT-MDT Prima, Bruker Dimension Edge). The contact angles were measured on a JC2000D contact angle system (Zhongchen Digital Equipment Co., Ltd. Shanghai, China). About 5 µL droplets were used to measure the contact angles. The average contact angle values were got by measuring the samples at five different positions. A mobile phone (Huawei Mate 9) was used to record the extraction process. The absorbance of the 7
donor and acceptor phases during the extraction process was determined by using a UV-vis spectrophotometer (UV-2600, Shimadzu). A spectrofluorometer (F-97 Pro, Shanghai Lengguang Technology Co., Ltd. China) was used to measure the fluorescence intensity of the calcein donor and acceptor phases before and after LPE. The excitation wavelength was 474 nm. An inverted fluorescence microscope (IX-71, OLYMPUS) was employed to observe the fluorescence of the used membranes after LPE.
3. Results and discussion Fig. 1 shows the process used to prepare the Janus superwetting membrane for unidirectional solute transfer. PP membrane was modified by polydopamine and SiO2 using the mussel-inspired method and then silicification [41-43]. Original PP with smooth and clean surface has low oxygen content (Fig. 2a,b), which is confirmed by EDS and XPS measurements (Figs. S1, S2 and Table S1). The surface structure and composition induce hydrophobicity in air (Fig. 2c). The water contact angle of original PP is 144.2±1.9°. Due to low surface tension of oil, the stable oil layer on the PP surface can extremely repel water droplet in oil. The underoil water contact angle is 169.6±2.0°. After in-situ oxidation polymerization and silicification, the coating makes the PP surface rough (Fig. 2d,e) and the oxygen content of the modified PP is greatly increased (Figs. S3-S6), resulting in superhydrophilicity and UWSO (Figs. 2f and S9). Once water droplet contacts with the surface of the modified PP, the water contact angle is less than 10° and is rapidly decreased to 0° in 1/3 s (Movie S1). The 8
underwater oil contact angle of the modified PP is 166.8±2.2°. Figs. 2g,h and S7 show the cross-section SEM images of the original and modified PP. It is found that both original PP and the modified PP have the thickness of about 100 µm. Besides, the high porosity can be also demonstrated by the AFM observation (Fig. S8). From the corresponding cross-section Si distribution map (Fig. 2i), the entire PP membrane is successfully modified to become very hydrophilic. To test the dynamic adhesive behavior, a water droplet (5 µL) and an oil droplet (CCl4, 5 µL) move horizontally and vertically relative to original PP in oil and the modified PP in water (Figs. S10, S11 and Movies S2-S5), respectively. Under a pressure, the water and oil droplets move through the whole surfaces with ease. At vertical direction, the distorted water and oil droplets can easily recover the spherical shape without any residual after getting away the surfaces. The outstanding antiadhesive property further verifies UOSH of original PP and UWSO of the modified PP. To achieve unidirectional solute transfer, the prepared UOSH and UWSO membranes were assembled into a twin-layer composite membrane (Fig. 1b). Notably, the surface modification does not change the pore size of PP (about 0.1 µm), which is not responsible for the controllable solute transfer.
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Fig. 1. Schematic illustration of preparation of the Janus membrane for unidirectional solute transfer.
Fig. 2. (a, b) SEM images of original PP. (c) Photographs of water droplet in air and in oil (hexane) on the surface of original PP. (d, e) SEM images of the modified PP. (f) Photographs of water droplet in air and oil droplet (CCl4) in water on the surface of
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the modified PP. In air, water droplet contacts with the surface of the modified PP at once. (g, h) Cross-section SEM images of the modified PP. (i) The corresponding cross-section Si distribution map.
For conventional LPE, phenolphthalein (10 mg/L) is extracted from neutral aqueous solution (Fig. S14), across SLM (decanol supported on original PP), and then migrates into acceptor phase (pH 12 aqueous solution). The introduction of a superhydrophilic membrane at the donor phase/SLM and SLM/the acceptor phase interfaces drastically influences the analyte migration. Unidirectional solute transfer is realized using the prepared Janus membrane. As shown in Figs. 3a,b and S15, when the UOSH and UWSO surfaces are fixed facing the donor and acceptor phases (labelled as UOSH/UWSO), respectively, the color of the acceptor phase gradually deepens. It is indicated that phenolphthalein penetrates through UOSH/UWSO, resulting in the “forward direction” of the solute transfer “diode”. In contrast, if the composite membrane is inverted (labelled as UWSO/UOSH) and the sample solution contacts with the UWSO side, phenolphthalein migration is restrained. After 90-min LPE, the acceptor phase is still colorless. This is considered as the “reverse direction” of the solute transfer “diode”.
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Fig. 3. (a) Phenolphthalein (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively. (b) Phenolphthalein (10 mg/L) is blocked from the UWSO side. (c) Methyl red (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively, and a voltage of 50 V is used as the drive. (d) Methyl red (10 mg/L) is blocked from the UWSO side in spite of the presence of 50 V.
Under a voltage of 50 V, in EME, the Janus membrane is also available to the unidirectional solute transfer. An electric field gradient would facilitate the analyte migration. When UOSH/UWSO is used, methyl red (10 mg/L) in the donor phase is gradually decreased, leading to a light color (Figs. 3c and S16). Meanwhile, the
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acceptor phase is changed from colorless to yellow, indicating the “forward direction” transfer of methyl red. In contrast, in spite of the presence of 50 V, methyl red cannot migrate to the acceptor phase that is still colorless after 90-min EME (Fig. 3d), showing the “reverse direction” blocking of UWSO/UOSH. In addition, water electrolysis causes pH change of the donor phase, displaying the color change. After 90-min LPE and EME, the acceptor phase for UWSO/UOSH has very low absorbance whereas the absorbance of the acceptor phase for UOSH/UWSO is greatly increased (Figs. 4 and S17). Note that the absorbance of the donor phase for UWSO/UOSH is slightly decreased during the extraction.
Fig. 4. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of LPE time. Phenolphthalein (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) The absorbance at 428 nm of the donor and acceptor phases as a function of EME time. Methyl red (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes under 50 V.
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Competitive extraction test was performed to further demonstrate the unidirectional solute transfer using the Janus superwetting membrane. As shown in Figs. 5a and S19, phenolphthalein (10 mg/L) aqueous solution links with two acceptor phases. The left acceptor phase contacts with the UOSH side of the composite membrane. On the contrary, the UWSO side of another composite membrane is fixed facing the right acceptor phase. Due to the unidirectional solute transfer, only the right acceptor phase becomes red after 90-min competitive LPE. Moreover, a parallel voltage of 50 V was applied to the competitive EME (Figs. 5b and S20). The color of the donor phase containing methyl red (10 mg/L) gradually turns light. Under electric drive, methyl red selectively passes through UOSH/UWSO and migrates to the right acceptor phase that shows yellow after 90-min competitive EME. In contrast, the left acceptor phase is still colorless. During the competitive LPE and EME, the absorbance of the left acceptor phase is fairly low whereas the right acceptor phase displays a rising absorbance (Figs. 6 and S21). The design of membrane process flow using the solute transfer “diode” availably regulates the migration direction of analytes.
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Fig. 5. (a) Phenolphthalein (10 mg/L) and (b) methyl red (10 mg/L) are competitively extracted using the UWSO/UOSH (left) and UOSH/UWSO (right) membranes.
Fig. 6. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of competitive LPE time. (b) The absorbance at 428 nm of the acceptor phase as a function of competitive EME time. Phenolphthalein (10 mg/L, a) and methyl red (10 mg/L, b) are competitively extracted using the UWSO/UOSH and UOSH/UWSO membranes.
LPE and EME are widely used as pretreatment techniques for extracting analytes such as drugs and toxicants from complex matrices. Exposure to aristolochic acid is related with a high incidence of uroepithelial tumorigenesis, and is associated with urothelial cancer [44,45]. Using LPE, aristolochic acid A cannot spontaneously migrate into the acceptor phase under the concentration gradient (Fig. S22). If the electric field gradient is implemented, analytes with negative charge can favorably diffuse across SLM toward the positive electrode in the acceptor phase (the pKa of
15
aristolochic acid A is about 3). As shown in Figs. S23 and S24, under a voltage of 50 V, aristolochic acid A (10 mg/L) penetrates through UOSH/UWSO into the acceptor phase but is obstructed from UWSO/UOSH, resulting in the great difference of absorbance of the acceptor phases for UOSH/UWSO and UWSO/UOSH after 90-min EME. The unidirectional solute transfer using the Janus membrane is applied for the practical pretreatment. To reveal the mechanism of the unidirectional solute transfer using the Janus superwetting membrane, calcein was used as a fluorescence probe to investigate the controllable analyte migration. As shown in Fig. S25, the yellow green of the donor phase containing calcein (5 mg/L) for both UOSH/UWSO and UWSO/UOSH fades after 90-min LPE. However, only the acceptor phase for UOSH/UWSO is colored and has strong characteristic absorption and fluorescence peaks (Figs. 7a, S26 and S27). In contrast, both the absorbance at 490 nm and fluorescence intensity at 515 nm of the acceptor phase for UWSO/UOSH are fairly low. Note that the absorbance of the donor phase for UWSO/UOSH is obviously decreased. After 90-min LPE, fluorescence microscope was used to observe the distribution of the fluorescence probe at the interface of UWSO and UOSH. For the “forward direction”, calcein migrates from the donor phase to the oil-wetted UOSH membrane that shows strong green fluorescence (Fig. 7b). The calcein extracted by the oil supported on the UOSH membrane can transfer through the interface of the oil-wetted UOSH and water-wetted UWSO surfaces, and then rapidly diffuses into the acceptor phase, resulting in a very weak fluorescence of the UWSO membrane. For the “reverse 16
direction”, calcein arrives at the UWSO surface and is gradually enriched due to its superhydrophilic property and high porosity, leading to the decreased absorbance of the donor phase (Figs. 4a and S23). Nevertheless, the interface of the water-wetted UWSO and oil-wetted UOSH surfaces provides a selective barrier to prevent the enriched calcein from permeating. Contrary to UOSH/UWSO, the UWSO membrane has obvious green fluorescence whereas the UOSH membrane is dark (Fig. 7b). A diode that conducts current primarily in one direction is commonly made of n-type and p-type semiconductors. The former contains negative charge carriers (electrons). When a sufficiently high electrical potential is applied to the p side, electrons can flow through the p-n junction to the electron-rich side. When the potential is applied to the n side, the junction does not allow the flow of electrons in the reverse direction. Similarly, the interface of the water-wetted UWSO and oil-wetted UOSH surfaces plays an important role in the unidirectional solute transfer (Fig. 7c). When concentration gradient or electric field gradient is applied to the UOSH side, solutes dissolved in water can pass through the interface to the water-loving side. In the reverse direction, because the interaction between solutes and the UWSO membrane is strong, the interface does not allow the permeating of solutes. In other words, the UWSO surface dominates in the competition of solutes at the interface over the UOSH surface, forming a solute transfer “diode”.
17
Fig. 7. (a) Fluorescence spectra of the acceptor phase after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) Fluorescence microscopy images of the used UOSH and UWSO surfaces after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO (left) and UWSO/UOSH (right) membranes. (c) Schematic illustration of the mechanism of the unidirectional solute transfer.
4. Conclusions In summary, macroscopic extreme wettability has been developed to microscopic controllable solute transfer. A Janus membrane has been designed and prepared to perform unidirectional solute transfer for sample pretreatment, which is based on the
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integration of the UOSH and UWSO surfaces to a composite membrane. In LPE and EME, analytes pass through the composite membrane from the UOSH side to the UWSO side, but are impeded from penetrating in the reverse direction. Similar to electron transfer through p-n junction to the electron-rich side, unidirectional solute transfer is owing to the dominant affinity of the UWSO surface toward solutes dissolved in water, forming a solute transfer “diode”. The design will be extended to other mass transfer “diodes” using Janus interfaces with entirely opposite physiochemical properties, in which one side has an affinity toward the mass rather than another side. We believe that the Janus superwetting membrane will be widely used to control the solute transfer to improve membrane process flow. This discovery will advance the development of interfacial materials with macroscopic special wettability in microscopic interfacial applications such as extraction, water purification, and exchange membrane.
Acknowledgements This work was supported financially by the National Nature Science Foundation of China (51602236, 21876055, and 81801875) and the Fundamental Research Funds for the Central Universities in China (2017KFYXJJ021).
Appendix A. Supplementary Information Supplementary data associated with this article can be found in the online version at 19
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Figure Captions Fig. 1. Schematic illustration of preparation of the Janus membrane for unidirectional solute transfer. Fig. 2. (a, b) SEM images of original PP. (c) Photographs of water droplet in air and in oil (hexane) on the surface of original PP. (d, e) SEM images of the modified PP. (f) Photographs of water droplet in air and oil droplet (CCl4) in water on the surface of the modified PP. In air, water droplet contacts with the surface of the modified PP at once. (g, h) Cross-section SEM images of the modified PP. (i) The corresponding cross-section Si distribution map. Fig. 3. (a) Phenolphthalein (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively. (b) Phenolphthalein (10 mg/L) is blocked from the UWSO side. (c) Methyl red (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively, and a voltage of 50 V is used as the drive. (d) Methyl red (10 mg/L) is blocked from the UWSO side in spite of the presence of 50 V. Fig. 4. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of LPE time. Phenolphthalein (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) The absorbance at 428 nm of the donor and acceptor phases as a function of EME time. Methyl red (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes under 50 V. Fig. 5. (a) Phenolphthalein (10 mg/L) and (b) methyl red (10 mg/L) are competitively extracted using the UWSO/UOSH (left) and UOSH/UWSO (right) membranes. Fig. 6. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of competitive LPE time. (b) The absorbance at 428 nm of the acceptor phase as a function of competitive EME time.
Phenolphthalein (10 mg/L, a) and methyl red (10 mg/L, b) are competitively extracted using the UWSO/UOSH and UOSH/UWSO membranes. Fig. 7. (a) Fluorescence spectra of the acceptor phase after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) Fluorescence microscopy images of the used UOSH and UWSO surfaces after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO (left) and UWSO/UOSH (right) membranes. (c) Schematic illustration of the mechanism of the unidirectional solute transfer.
Fig. 1. Schematic illustration of preparation of the Janus membrane for unidirectional solute transfer.
Fig. 2. (a, b) SEM images of original PP. (c) Photographs of water droplet in air and in oil (hexane) on the surface of original PP. (d, e) SEM images of the modified PP. (f) Photographs of water droplet in air and oil droplet (CCl4) in water on the surface of the modified PP. In air, water droplet contacts with the surface of the modified PP at once. (g, h) Cross-section SEM images of the modified PP. (i) The corresponding cross-section Si distribution map.
Fig. 3. (a) Phenolphthalein (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively. (b) Phenolphthalein (10 mg/L) is blocked from the UWSO side. (c) Methyl red (10 mg/L) easily passes through the composite membrane from the UOSH side to the UWSO side. The UOSH and UWSO surfaces are fixed facing the donor and acceptor phases, respectively, and a voltage of 50 V is used as the drive. (d) Methyl red (10 mg/L) is blocked from the UWSO side in spite of the presence of 50 V.
Fig. 4. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of LPE time. Phenolphthalein (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) The absorbance at 428 nm of the donor and acceptor phases as a function of EME time. Methyl red (10 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes under 50 V.
Fig. 5. (a) Phenolphthalein (10 mg/L) and (b) methyl red (10 mg/L) are competitively extracted using the UWSO/UOSH (left) and UOSH/UWSO (right) membranes.
Fig. 6. (a) The absorbance at 230 nm of the donor phase and the absorbance at 552 nm of the acceptor phase as a function of competitive LPE time. (b) The absorbance at 428 nm of the acceptor phase as a function of competitive EME time. Phenolphthalein (10 mg/L, a) and methyl red (10 mg/L, b) are competitively extracted using the UWSO/UOSH and UOSH/UWSO membranes.
Fig. 7. (a) Fluorescence spectra of the acceptor phase after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO and UWSO/UOSH membranes. (b) Fluorescence microscopy images of the used UOSH and UWSO surfaces after 90-min LPE. Calcein (5 mg/L) is extracted using the UOSH/UWSO (left) and UWSO/UOSH (right) membranes. (c) Schematic illustration of the mechanism of the unidirectional solute transfer.
Highlights Interfacial materials are prepared for microscopic solute transfer. Underoil superhydrophobic and underwater superoleophobic membranes are integrated. Unidirectional solute transfer is achieved using the Janus membrane. The solute transfer “diode” proceeds through the competitive affinity toward solutes. The design will be extended to other mass transfer “diodes”.
Conflict of Interest The authors declare no competing financial interest.
Author Statement Manuscript ID: MEMSCI_2019_3036 Title: Unidirectional solute transfer using a Janus membrane Journal: Journal of Membrane Science
All authors have made substantial contributions to the work reported in the manuscript, including those who proposed the conception or design of the work, performed the experiments and analyzed the data, and co-wrote the manuscript. We have approved the final version to be published. The work described is original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part.
Jing Li 29/11/2019