Polydopamine Coatings with Nanopores for Versatile Molecular

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Polydopamine Coatings with Nanopores for Versatile Molecular Separation Chao Zhang, Yan Lv, Wen-Ze Qiu, Ai He, and Zhi-Kang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 3, 2017

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ACS Applied Materials & Interfaces

Polydopamine Coatings with Nanopores for Versatile Molecular Separation Chao Zhang†, Yan Lv†, Wen-Ze Qiu, Ai He, and Zhi-Kang Xu∗

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, and Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China †

The two authors contributed equally to this work

ABSTRACT There are increasing demands for highly efficient and multi-functional membranes in various separation processes. Recently, mussel-inspired polydopamine (PDA) has provided a promising way to meet these requirements due to both of the surface-adhesive property and film-forming ability. However, traditional PDA coatings usually suffer from the disadvantages of non-uniform, incompactness and instability, leading to poor molecular separation and service performance. Herein, uniform, compact and robust PDA coatings were fabricated on an ultrafiltration substrate via a reasonable screening of oxidants for the oxidized self-polymerization of dopamine. The as-prepared PDA coatings are nanoporous (0.56 ± 0.04 nm and 0.93 ± 0.04 nm) with a thickness of ~75 nm, endowing the composite membranes with high solute rejection and solvent permeability during molecular separation. They are useful in organic solvent nanofiltration due to their superior structural stability. Moreover, the composite membranes can be applied for nanometer catalyst recycle from 1 ACS Paragon Plus Environment


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organic solvents for the first time, which has much broadened the potential applications of these mussel-inspired coatings for versatile separation processes.

Keywords: mussel-inspired chemistry, polydopamine, composite membrane, molecular separation, nanopores

1. INTRODUCTION Molecular-level separation plays a crucial role in chemistry, energy and environment fields.1 Membrane technology has been regarded as a most promising alternative to conventional separation process due to its efficient, energy-saving, and environmentally friendly features.2-4 Composite membranes have been exploited mostly to meet the growing demands for molecular separation, which is calling for both delicate selective layer structures and strong stability to achieve high performance and long lifespan, especially for practical processes involving some harsh condition such as acid/alkaline solutions or even organic solvents. Unfortunately, it is difficult to achieve this goal with traditional membrane materials and manufacturing techniques. Inspired by the adhesive behavior of proteins in mussels, Messersmith and co-workers reported that dopamine was able to form polymer-like coatings on a variety of substrates by oxidized self-polymerization.5 These mussel-inspired coatings have been widely studied in the field of biomaterial, clean energy and environmental science due to its unparalleled advantages, including universal adhesion, biocompatibility and post-functionalization accessibility.6-9 In particular, the PDA coatings have been extensively applied in numerous membrane separation technologies such as aqueous nanofiltration,10 pervaporative11 and gas separation12. In the most of cases, the PDA particles and/or coatings were used as additives or intermediate layers to elaborately tailor the chemical compositions and physical structure of membrane surfaces to meet different requirements.13-14 On the other hand, the accumulation 2 ACS Paragon Plus Environment

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of PDA nanoaggregates constructs a lot of interspaces in the formed coatings, which is able to be tuned by varying the oxidizing conditions and serves as separation channels.15-17 Tunable structures and chemical compositions are immensely beneficial for separation, which indicates that PDA films show tremendous potential as separation selective layer. However, the separation and service performances of these PDA layers leave much from desirable. For example, the PDA coatings acting as selective layer generally need double deposition, indicating a relatively loose structure under air oxidization. In addition, air-oxidized PDA coatings are not stable ascribed to some non-covalent interactions in the structures,18-19 especially in harsh environment, which immensely limits their application in acid and alkali solutions or even organic solvents. Some efforts have been made to address these issues, such as grafting20-21 or mineralization22 on PDA coatings, co-deposition and cross-linking23-26 of PDA with additional chemicals. It can be seen that these improvements are only achieved via multistep processes, and still cannot fundamentally improve the compactness and stability of PDA coatings. Therefore, it remains a tremendous challenge to fabricate compact and stable PDA coatings with a facile strategy with high performance separation. Up to now, we known that dopamine can self-polymerize and aggregate into PDA nanoparticles via oxidation and the particle size will gradually increase with the oxidation degree.27 In fact, the deposition and aggregation of PDA nanoparticles are two competitive processes as small nanoaggregates (2 ∼ 50 nm) can deposit on the substrates, but bigger ones are prone to precipitate from the solution, as schematically illustrated in Figure 1.28-29 Therefore, it is possible for us to tune the structures of the PDA coatings by controlling the oxidized self-polymerization process. In our previous report, we found that CuSO4/H2O2 is able to trigger the rapid oxidization of dopamine for fast and homogeneous deposition of highly uniform and stable PDA coatings. 30 Here, we report to facilely fabricate the compact and robust PDA coatings as the selective layers in the composite membranes via reasonable 3 ACS Paragon Plus Environment


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screening of the oxidants according to the redox potential of oxidants and the mechanism of dopamine oxidation and PDA deposition. We fabricated PDA coatings with high compactness and adequate thickness by regulating the oxidation process. The as-prepared composite membranes exhibit higher molecular separation performance than any other PDA-based separation layers, along with superior stability under acid and alkaline conditions. These nanoporous PDA coatings are able to be applied in organic solvent nanofiltration for catalyst recycle, tremendously broadening their application fields.

2. EXPERIMENTAL SECTION 2.1

Material:

Dopamine

hydrochloride,

hydrogen

tetrachloroaurate

trihydrate,

p-mercaptophenol and sodium borohydride aqueous solution were purchased from Sigma-Aldrich. Polyacrylonitrile (PAN) ultrafiltration substrate (pore size is 40-80 nm, MWCO is 10-30 KDa) was obtained from Shanghai MegaVision Membrane Engineering & Technology Co. Ltd (China). Other reagents, including tris(hydroxymethyl) aminomenthane, hydrogen peroxide, polyethylene glycol (PEG) with different molecular weights, inorganic salts, organic solvents and hydrochloric acid were procured from Sinopharm Chemical Reagent Co., Ltd and used without further purification. Water used in all experiments was deionized and ultrafiltrated to 18.2 MΩ with an ELGA LabWater system (France). 2.2 Deposition of polydopamine (PDA) coatings on substrates: Dopamine hydrochloride (2 mg/mL) was dissolved in Tris buffer solution (pH = 8.5, 50 mM) with different oxidants, including air, CuSO4 (5.3 mM)/H2O2 (26.1 mM) and (NH4)2S2O8 (13.1 mM). Typically, PAN substrates were pre-wetted by ethanol and then immersed in the prepared solution and shaken in an air oscillator at 25 °C. Subsequently, the samples were washed by ultrapure water and dried in a vacuum overnight at 60 °C. The deposition time is 12 h in air and 45 min in the cases of using CuSO4 /H2O2 and (NH4)2S2O8 as oxidants. 2.3 Characterization: UV-vis spectrophotometer (UV 2450, Shimadzu, Japan) was used to 4 ACS Paragon Plus Environment

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measure the UV−vis absorption of solutions, and all solutions (1 mL) were diluted to 3 mL to measure their UV-vis absorbance at different time points. Morphologies of the substrates were investigated by field emission scanning electron microscope (FESEM, Hitachi S4800, Japan). Atom force microscopy (AFM, MultiMode, Vecco, USA) was used to characterize the roughness of the membrane surfaces through the tapping mode. Chemical structures and components of the membrane surfaces were characterized by infrared spectrophotometer (FT-IR/ATR, Nicolet 6700, ThermoFisher, USA) equipped with an ATR accessory (ZnSe crystal, 45°) and X-ray photoelectron (XPS, PerkinElmer, Waltham, MA) with Al Kα excitation radiation (1486.6 eV), respectively. A streaming potential method was employed to detect the charging properties of the membrane surfaces using an electro kinetic analyzer (SurPASS Anton Paar, GmbH, Austria) with 1 mM KCl solution as electrolyte solution. The depth profile of open-volume information near the membrane surfaces was evaluated by positron annihilation spectroscopy (PAS) with a high-purity Ge detector and a

22

Na slow

positron beam source (0.5-20 keV), including positron annihilation lifetime spectroscopy (PALS) and Doppler broadening energy spectroscopy (DBES). For PALS, the mean pore radius was calculated by the following semi-empirical equation (1):

= 1 − ∆ + ∆

(1)

where τ3 and r are the o-Ps lifetime and the membrane pore radius, respectively, and ∆ is an empirical parameter determined by calibration on materials with known free volume sizes (∆ = 0.166 nm). In DBES experiments, the S and R parameters were measured as a function of incident positron energy at room temperature and provide information about the porosity of pores (nm to µm) as a function of the depth from the outer surface. The incident positron energy can be expressed in terms of depth, as given in the following semi-empirical equation (2): 5 ACS Paragon Plus Environment


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E =

×

×

."

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(2)

where Z is depth in nm, ρ represents the density of the target polymeric material in kg/m3, and E+ is the incident positron energy in keV. 2.4 Evaluation of molecular separation performance: A laboratory scale cross-flow flat membrane module was employed to evaluate the separation performance of the composite membranes. The operation pressure, temperature and effective area were 0.6 MPa, 30 ± 1 °C and 7.07 cm2, respectively. Each sample was pre-compacted under 0.7 MPa for 2 h before performance evaluation. Different salts and PEG solutions at concentration of 1000 mg/L were used as feed solution with a fixed cross-flow rate of 30 L/h. The water flux (Fw, L/m2⋅h) and rejection (Re, %) are calculated by the following equations (3) and (4): %

#$ = &×'

(3)

+,

() = *1 − + . × 100% -

(4)

where where Q, S and t represent the volume of permeate (L), the effective membrane area (m2) and the permeation time (h), Cp and Cf are the solute concentrations in permeate and feed, respectively. The concentration of salt solution was detected by an electrical conductivity meter (METTLER TOLEDO, FE30, China). The PEG concentrations were measured by measuring absorbance at 535 nm after iodine complexation with a UV–Vis spectrophotometer. All results presented were repeated at least three times. Organic solvent nanofiltration performance was investigated by a dead-end filtration system with a 3.75 cm2 effective area under 0.3 MPa at 25 °C. Each sample was immersed in certain organic solvent for 24 h to complete equilibration before performance evaluation. The solvent permeance (P, L/m2⋅h) and solute rejection (Re, %) were calculated by the aforementioned equation (3) and (4). 6 ACS Paragon Plus Environment

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3. RESULTS AND DISCUSSION It is accepted that the oxidation condition notably influences the formation, aggregation and deposition of PDA nanoparticles. In our case, three representative oxidants, including air, (NH4)2S2O8 and CuSO4/H2O2, are compared here to evaluate these effects in details. Figure 2 shows that the air oxidation slowly turns the dopamine solution from colorless to black with reaction time increasing to 24 h, which is low efficient for the formation of PDA coatings. Nevertheless, the solutions becomes black very quickly and a large amount of PDA nanoparticles are rapidly formed by the two strong oxidants of (NH4)2S2O8 and CuSO4/H2O2. These nanoparticles aggregate further into large particles and precipite from the solution within 15 min in the case of (NH4)2S2O8, resulting in a gradual decrease of UV-vis absorbance at 420 nm (Figure 2d). The rapid precipitation is unbeneficial for the formation PDA coatings on the substrate surfaces. In the case of CuSO4/H2O2, on the other hand, PDA aggregates begin to precipitate after 45 min (Figure 2c). Therefore, a large amount of nanoparticles can be deposited on the substrate surfaces. Apart from the quantity, the adhesive property of PDA nanoparticles also plays a crucial role for the coating formation.31 It is well known that the phenolic hydroxyl group of catechol is of strong adhesive forces, but the quinones possess no adhesive property.32-33 XPS spectra are used to analyze the C-O ratio in the precipitated PDA powders, which represents the content of phenolic hydroxyl group in the nanoparticles. It can be seen that the adhesive ability follows an order of nanoparticles formed by CuSO4/H2O2 > air > (NH4)2S2O8 (Figure 3 and Table S1 in SI). Therefore, CuSO4/H2O2 is the optimized oxidant for the construction of PDA coatings owing to the appropriate amount and the strong adhesive ability of PDA nanoparticles. The morphologies and thicknesses of PDA coatings on PAN ultrafiltration substrates are also compared (Figure S1 in SI), and the results can be further used to evaluate the influence of different oxidants on PDA deposition and coating formation. It can be seen there are 7 ACS Paragon Plus Environment


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visible PDA aggregates deposited on the substrate surface in the case of air oxidization. The coatings show a roughness of 9.75 nm and a thickness of around 100 nm. On the other hand, it is very interesting that the coatings are highly smooth with a roughness of 1.14 nm and a thickness of 50 nm when it is oxidized by CuSO4/H2O2. It means the PDA nanoparticles can totally cover the substrate surface to form a smooth and integral coating in this case. Nevertheless, PDA coatings cannot be fabricated by (NH4)2S2O8 because the precipitated aggregates are very easily washed away. We propose a mechanism for the formation, aggregation and deposition of PDA nanoparticles under different oxidation conditions and indicate the essential factors for constructing compact and uniform PDA coatings (Figure 4). For air oxidation, dopamine is oxidized into PDA slowly, which gradually forms nanoparticles and deposits on the substrate surface. This slow and gradual process is not suitable to produce a sufficient amount of PDA nanoparticles to cover the substrate surface totally in an acceptable time. The nanoparticles will aggregate into large particles and then increase the thickness and roughness of PDA coatings with relatively long time. In contrast, (NH4)2S2O8 is able to oxidize dopamine rapidly and thus form a large amount of PDA nanoparticles in a very short time. However, these PDA nanoparticles are lack of adhesive ability and easy to grow into large aggregates followed by precipitation from the solution, resulting in poor deposition on the substrate surface. CuSO4/H2O2 can similarly trigger the production of numerous PDA nanoparticles. These particles possess less aggregation, which is able to keep fast and uniform deposition for long time (more than 45 min in our cases), continually depositing on the substrate surface and repairing the defects gradually. Finally, the formed PDA coatings are uniform and compact with adequate thickness. By this taken, the key point to fabricate a compact and defect-free PDA coating is to realize rapid oxidization and homogeneous polymerization of dopamine followed by uniform deposition with opportune time. In other word, it needs to 8 ACS Paragon Plus Environment

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select appropriate oxidant to maximize the uniform deposition and minimize uncontrolled aggregation process to reach the goal of repairing defects on substrate surfaces as many as possible. Positron annihilation lifetime spectra (PALS) are used to analyze the pore size of the PDA coating triggered by CuSO4/H2O2. PALS can directly detect the positronium lifetime, and then we used the semi-empirical equation (detail description is in section 2.3) to accurately calculate the pore size. Figure 5a and Table 1 indicate there are two kinds of the pores with mean size of 0.56 ± 0.04 nm and 0.93 ± 0.04 nm at 0.7 keV positron implantation energies, which are both in the range of nanometers. The doppler broadening energy spectrum (DBES) (Figure 5b and Figure S2 in SI) also demonstrates the coating has an accurate thickness of 75 nm, which is lower than those of the selective layers by typical interfacial polymerization for thin film composite membranes.34-35 Therefore, these PDA coatings with optimized structures are great potential as the selective layers of thin film composite membranes for molecular separation by nanofilitration. Figure 6a demonstrates the PDA-coated thin film composite membranes fabricated by CuSO4/H2O2 possess high water permeation flux (60.8 L/m2⋅h) as well as high rejection for Na2SO4 (∼ 90%) and PEG1000, which are far better than those fabricated by the other two oxidants (Figure S3 in SI). Table 2 compares the nanofiltration performances of different thin film composite membranes with PDA-based coatings as selective layers. It can be seen that our membranes possess the highest water permeation flux as well as high rejection for bivalent anions. Moreover, the surface structures and properties of PDA coatings are well tuned by varying the deposition time. With the increasing deposition time, the coating thickness is elaborately tunable from 15 nm to 53 nm with gradually increased apparent compactness (Figure S4 and S5 in SI). The negative surface charges are also reduced with the increase of deposition time (Figure S6 in SI). During the separation process, the negatively charged membrane surface 9 ACS Paragon Plus Environment


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has a strong electrostatic repulsion to divalent anion (SO42-), and thus high rejection determines by the Donnan effect. Additionally, Mg2+ has a higher charge density than Na+, which enhances shielding effect for THE membrane surface charges. Therefore, the rejection of Na2SO4 is higher than MgSO4. In view of monovalent anion, the steric hindrance plays a dominant role on rejection performance. The hydrated radius of Mg2+ (0.346 nm) is much higher than that of Na+ (0.183 nm), resulting in a high rejection for MgCl2. Therefore, the membranes follow the rejection order of Na2SO4 > MgSO4 > MgCl2 > NaCl (Figure 6b). The chemical and structural stabilities are very important for the practical application of the composite membranes. Their separation performances were investigated in detail under acid or alkali conditions for a long operation time. Aqueous solutions with pH value from 3 to 11 have almost no influence on the performances of the PDA-coated membranes, which is also proved by the unchanged structures from FESEM images (Figure S7 and S8 in SI). When pH < 3 or pH >11, there appears some visible pores on the membrane surfaces, resulting in a sharp drop of salt rejection. Figure 6c and 6d show that the salt rejection and water permeation flux change slightly during the ten immersion cycles at pH = 3 and pH = 11, indicating acceptable chemical stability for practical separation. Additionally, the water permeation flux keeps at 60 L/m2⋅h along with high salt rejection of 90% during 120 h continuous filtration, demonstrating superior structural stability. This stability is partially due to the residual copper ions in the PDA selective skin layer, because there are strong chelation interactions between PDA and Cu2+.30 However, these copper ions may be a potential risk for drinking water. Therefore, their concentration was detected in the filtrate during the long-term continuous filtration experiment (Figure 6f). It can be seen that Cu2+ concentration is lower than 2 µg/L, which is one thousandth of standard value for safe drinking water (2 mg/L).

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Apart from water purification, nanofiltration membranes are also of significant importance for the separation and recycle of organic solvents, reactants, products and catalysts in industries.36-39 We measured the permeation properties of our PDA-coated membranes with a series of typical solvent. Figure 7 shows that the relatively nonpolar heptane has the lowest permeance, and that of the methanol is the highest (3.93 L/m2⋅h). Permeance of pure organic solvent is not only related with the molecular polarity but also the liquid viscosity as well as the molar diameter of the molecule, and accords with the following equation:2 1 = 2 × 3

45 8 6 × 7

where P, K, δp, η and d is the permeance of pure organic solvent, the proportionality constant for selective skin layer, the solubility parameter, the solvent viscosity and the molar diameter of solvent, respectively. In view of methanol and nonpolar heptane, methanol has a higher solubility parameter (δp) and smaller molar diameter (d) (Table S2 in SI), resulting in a better solvent permeance. It is well known that physical aging and compaction remain a great challenge for organic solvent nanofiltration,40 and the instability of membrane structure will result in a significant decrease in permeance. Therefore, the change of methanol permeance during long-term operation was used to evaluate the stability of our membrane structures (Figure S9 in SI). It is worth noting that the permeance decreases only 9.7 % and finally maintains a steady value after continuous filtration of 200 min, demonstrating the prepared PDA-coated composite membranes, including the support layer, the selective layer and the interfaces between them are very stable in methanol. There are no visible changes in their morphologies after the composite membranes were treated by heptane, methanol, acetone and tetrahydrofuran (Figure S10 in SI). However, there are some cracks appearing on the membrane surfaces when strong polar solvents, such as DMF and DMSO, were filtrated for 6 11 ACS Paragon Plus Environment


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hours. (Figure S11 in SI). It can be ascribed to the good solubility of polyacrylonitrile in the polar solvents (Figure S12 in SI). During the filtration processes, the polyacrylonitrile substrate is gradually solved in the polar solvents, which may induce a stress effect and then result in some cracks to the PDA selective layers. Anyway, our PDA selective layers are tremendously stable in typical organic solvents. It is also known that nanoparticles have been widely employed as catalyst for various chemical reactions in industry, but its separation and recovery from the organic reaction system is always faced with tough challenge, especially sub-nanometer-sized particles.41-44 The most common method is the immobilization of nanometer catalysts, which in turn worsens catalytic activity due to the reduction of catalytic sites. As a consequence, it calls for a facile, efficient and time-saving separation technology. To our delight, our PDA-coated composite membranes possess nanopores and superior stability under various organic solvents. Herein, we used Au nanoparticles with the size ranging from 1.58 nm to 5.4 nm to evaluate its capability for recycling nanometer-sized particles from organic solvent. Figure 7b shows that the permeate turns colorless from deep brown of the feed, and the DLS signal of Au nanoparticles totally disappears after filtration. It can be quantitatively calculated from UV-vis spectra that the rejection of Au nanoparticles reaches as high as 97.5% (Figure S13 in SI). All these results indicate the PDA-coated composite membranes have great potentials in organic solvent nanofiltration for nanometer catalyst recycle.

4. CONCLUSION In summary, we provide a facile and efficient method to fabricate compact and robust PDA coatings via accurately regulating oxidants to tailor the oxidized self-polymerization of dopamine and PDA deposition. The CuSO4/H2O2-triggered PDA coatings with negative charges on ultrafiltration substrates show nanopores (0.56 ± 0.04 nm and 0.93 ± 0.04 nm) with a thickness of only ~75 nm. Therefore, the as-prepared composite membranes exhibit 12 ACS Paragon Plus Environment

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not only excellent molecular rejection performance for PEG 1000 and NaSO4 (both > 90%) but also high water permeation flux of 60 L/m2⋅h, which is much better than that of conventional PDA-based nanofiltration membranes. In addition, the PDA-coated composite membranes show robust stability in organic solvents, which enables them to be firstly applied for nanometer catalyst recycle from organic solvents. These novel mussel-inspired coatings for versatile molecular separation processes will open a new door for their future practical applications. ASSOCIATED CONTENT Supporting Information. Surface and cross sectional SEM images of the composite membranes, AFM images and zeta potentials of the composite membranes, PEG rejection as a function of molecular weight of the composite membranes, surface morphologies and separation performances of the composite membranes under acid/alkaline solutions and organic solvents, UV-vis spectra of feed and permeate methanol solutions containing Au NPs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support is acknowledged to the National Natural Science Foundation of China (Grant no. 21534009), and the Zhejiang Provincial Natural Science Foundation of China (Grant no. LZ15E030001). The authors thank Prof. Kueir-Rarn Lee from Chung Yuan University (Taiwan) for the analysis of positron annihilation spectroscopy. REFERENCES



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Entry to Superhydrophilic-Superoleophobic Coatings. Chem. Mater. 2016, 28, 4697-4705. (17) Hong, S. H.; Hong, S.; Ryou, M. H.; Choi, J. W.; Kang, S. M.; Lee, H. Sprayable Ultrafast Polydopamine Surface Modifications. Adv. Mater. Interfaces 2016, 3, 1500857. (18) Hong, S.; Na, Y. S.; Choi, S.; Song, I. T.; Kim, W. Y.; Lee, H. Non-Covalent Self-Assembly and Covalent Polymerization Co-Contribute to Polydopamine Formation. Adv. Funct. Mater. 2012, 22, 4711-4717. (19) Wei, H. L.; Ren, J.; Han, B.; Xu, L.; Han, L. L.; Jia, L. Y. Stability of Polydopamine and Poly(DOPA) Melanin-Like Films on the Surface of Polymer Membranes under Strongly Acidic and Alkaline Conditions. Colloids Surf., B 2013, 110, 22-28. (20) Zhang, R. N.; Su, Y. L.; Zhao, X. T.; Li, Y. F.; Zhao, J. J.; Jiang, Z. Y. A Novel Positively Charged Composite Nanofiltration Membrane Prepared by Bio-Inspired Adhesion of Polydopamine and Surface Grafting of Poly(ethylene imine). J. Membr. Sci. 2014, 470, 9-17. (21) Du, Y.; Lv, Y.; Qiu, W. Z.; Wu, J.; Xu, Z. K. Nanofiltration Membranes with Narrowed Pore Size Distribution via Pore Wall Modification. Chem. Commun. 2016, 52, 8589-8592. (22) Lv, Y.; Yang, H. C.; Liang, H. Q.; Wan, L. S.; Xu, Z. K. Novel Nanofiltration Membrane with Ultrathin Zirconia Film as Selective Layer. J. Membr. Sci. 2016, 500, 265-271. (23) Lv, Y.; Yang, H. C.; Liang, H. Q.; Wan, L. S.; Xu, Z. K. Nanofiltration Membranes via Co-Deposition of Polydopamine/Polyethylenimine Followed by Cross-Linking. J. Membr. Sci. 2015, 476, 50-58. (24) Li, M. M.; Xu, J.; Chang, C. Y.; Feng, C. C.; Zhang, L. L.; Tang, Y. Y.; Gao, C. J. Bioinspired Fabrication of Composite Nanofiltration Membrane Based on the Formation of DA/PEI Layer Followed by Cross-Linking. J. Membr. Sci. 2014, 459, 62-71. (25) Du, Y.; Qiu, W.-Z.; Lv, Y.; Wu, J.; Xu, Z. K. Nanofiltration Membranes with Narrow Pore Size Distribution via Contra-Diffusion-Induced Mussel-Inspired Chemistry. ACS Appl. Mater. Interfaces 2016, 8, 29696-29704. (26) Lv, Y.; Du, Y.; Qiu, W. Z.; Xu, Z. K. Nanocomposite Membranes via the Codeposition of Polydopamine/Polyethylenimine with Silica Nanoparticles for Enhanced Mechanical Strength and High Water Permeability. ACS Appl. Mater. Interfaces 2017, 9, 2966-2972. (27) Kim, H. W.; McCloskey, B. D.; Choi, T. H.; Lee, C.; Kim, M. J.; Freeman, B. D.; Park, H. B. Oxygen Concentration Control of Dopamine-Induced High Uniformity Surface Coating Chemistry. ACS Appl. Mater. Interfaces 2013, 5, 233-238. (28)Bernsmann, F.; Ponche, A.; Ringwald, C.; Hemmerle, J.; Raya, J.; Bechinger, B.; Voegel, J. C.; Schaaf, P.; Ball, V. Characterization of Dopamine-Melanin Growth on Silicon Oxide. J. Phys. Chem. C 2009, 113, 8234-8242. (29) Jiang, J.; Zhu, L.; Zhu, L.; Zhu, B.; Xu, Y. Surface Characteristics of a Self-Polymerized Dopamine Coating Deposited on Hydrophobic Polymer Films. Langmuir 2011, 27, 14180-14187. (30) Zhang, C.; Ou, Y.; Lei, W. X.; Wan, L. S.; Ji, J.; Xu, Z. K. CuSO4/H2O2-Induced Rapid Deposition of Polydopamine Coatings with High Uniformity and Enhanced Stability. Angew. Chem., Int. Ed. 2016, 55, 3054-3057. 15 ACS Paragon Plus Environment


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(31) Ding, Y.; Weng, L. T.; Yang, M.; Yang, Z.; Lu, X.; Huang, N.; Leng, Y. Insights into the Aggregation/Deposition and Structure of a Polydopamine Film. Langmuir 2014, 30, 12258-12269. (32) Anderson, T. H.; Yu, J.; Estrada, A.; Hammer, M. U.; Waite, J. H.; Israelachvili, J. N. The Contribution of DOPA to Substrate-Peptide Adhesion and Internal Cohesion of Mussel-Inspired Synthetic Peptide Films. Adv. Funct. Mater. 2010, 20, 4196-4205. (33) Lee, H.; Scherer, N. F.; Messersmith, P. B. Single-Molecule Mechanics of Mussel Adhesion. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 12999-13003. (34) Chen, H. M.; Hung, W. S.; Lo, C. H.; Huang, S. H.; Cheng, M. L.; Liu, G.; Lee, K. R.; Lai, J. Y.; Sun, Y. M.; Hu, C. C.; Suzuki, R.; Ohdaira, T.; Oshima, N.; Jean, Y. C. Free-Volume Depth Profile of Polymeric Membranes Studied by Positron Annihilation Spectroscopy: Layer Structure from Interfacial Polymerization. Macromolecules 2007, 40 (21), 7542-7557. (35) Peng, J. M.; Su, Y. L.; Chen, W. J.; Zhao, X. T.; Jiang, Z. Y.; Dong, Y. A.; Zhang, Y.; Liu, J. Z.; Cao, X. Z. Polyamide Nanofiltration Membrane With High Separation Performance Prepared by EDC/NHS Mediated Interfacial Polymerization. J. Membr. Sci. 2013, 427, 92-100. (36) Vandezande, P.; Gevers, L. E. M.; Vankelecom, I. F. J. Solvent Resistant Nanofiltration: Separating on a Molecular Level. Chem. Soc. Rev.2008, 37, 365-405. (37) Karan, S.; Samitsu, S.; Peng, X.; Kurashima, K.; Ichinose, I. Ultrafast Viscous Permeation of Organic Solvents Through Diamond-Like Carbon Nanosheets. Science 2012, 335, 444-447. (38) Sorribas, S.; Gorgojo, P.; Tellez, C.; Coronas, J.; Livingston, A. G. High Flux Thin Film Nanocomposite Membranes Based on Metal-Organic Frameworks for Organic Solvent Nanofiltration. J. Am. Chem. Soc. 2013, 135, 15201-15208. (39) Zhang, H.; Mao, H.; Wang, J.; Ding, R.; Du, Z.; Liu, J.; Cao, S. Mineralization-Inspired Preparation of Composite Membranes with Polyethyleneimine-Nanoparticle Hybrid Active Layer for Solvent Resistant Nanofiltration. J. Membr. Sci. 2014, 470, 70-79. (40) Gorgojo, P.; Karan, S.; Wong, H. C.; Jimenez-Solomon, M. F.; Cabral, J. T.; Livingston, A. G. Ultrathin Polymer Films with Intrinsic Microporosity: Anomalous Solvent Permeation and High Flux Membranes. Adv. Funct. Mater. 2014, 24, 4729-4737. (41) Corma, A.; Garcia, H. Supported Gold Nanoparticles as Catalysts for Organic Reactions. Chem. Soc. Rev. 2008, 37, 2096-2126. (42) Stevens, P. D.; Fan, J.; Gardimalla, H. M.; Yen, M.; Gao, Y. Superparamagnetic Nanoparticle-Supported Catalysis of Suzuki Cross-Coupling Reactions. Org. Lett. 2005, 7, 2085-2088. (43) Huang, L.; Chen, J.; Gao, T.; Zhang, M.; Li, Y.; Dai, L.; Qu, L.; Shi, G. Reduced Graphene Oxide Membranes for Ultrafast Organic Solvent Nanofiltration. Adv. Mater. 2016, 28, 8669-8674. (44) Deng, C.; Zhang, Q. G.; Han, G. L.; Gong, Y.; Zhu, A. M.; Liu, Q. L. Ultrathin Self-Assembled Anionic Polymer Membranes for Superfast Size-Selective Separation. Nanoscale 2013, 5, 11028-11034.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions Figure 1. Schematic diagram of PDA aggregation and deposition processes. “×” represents the PDA particles are too large to tightly deposit on to the substrate surface. Figure 2. (a) Photographs of dopamine solutions with different oxidants. (b-d) Time-dependence of UV-vis absorbance at 420 nm for various diluted dopamine solutions with different oxidants: air, CuSO4 (5.3 mM)/H2O2 (26.1 mM), and (NH4)2S2O8 (13.1 mM). Figure 3. High-resolution XPS spectra of O1s in PDA powders oxidized by different oxidants: (a) Air, (b) CuSO4 (5.3 mM)/H2O2 (26.1 mM) and (c) (NH4)2S2O8 (13.1 mM). Figure 4. Schematic diagram of PDA deposition processes on the substrate surface oxidized by three different oxidants. “×” represents the PDA particles are not able to deposit onto the substrate surface. Figure 5. (a) o-Ps lifetime distribution curves for the PDA-coated thin film composite membranes fabricated by CuSO4/H2O2 with different positron implantation energies. PDF is probability density function. (b) S parameters as a function of positron incident energy and depth for the PDA-coated thin film composite membranes fabricated by CuSO4/H2O2. Figure 6. (a) Nanofiltration performances of the PDA-coated membranes oxidized by different oxidants: (A) air, (B) (NH4)2S2O8 and (C) CuSO4/H2O2. (b) Effects of inorganic salts on the nanofiltration performance of the PDA-coated membranes oxidized by CuSO4/H2O2. (c) Nanofiltration performance of the PDA-coated membranes treated by aqueous solution under pH = 3 with different immersion cycles. (d) Nanofiltration performance of the PDA-coated membranes treated by aqueous solution under pH = 11 with different immersion cycles. Immersion time for each cycle is 12 h. F/F0 and R/R0 represent the variation of water permeation flux and salt rejection, respectively. (e) Structure stability of the PDA-coated membranes with different operation times (Na2SO4 solution as feed). (f) Concentration of Cu2+ in the filtrate with different operation times. All solute concentrations are 1000 mg/L. Figure 7. (a) Plot of solvent permeance against the combined solvent property (viscosity, molar diameter, and solubility parameter) for PDA-coated membranes oxidized by CuSO4/H2O2. (b) Photographs and (c) the size distribution of Au nanoparticles in the feed and the permeate methanol solutions. 17 ACS Paragon Plus Environment


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Figure 1. Schematic diagram of PDA aggregation and deposition processes. “×” represents the PDA particles are too large to tightly deposit on to the substrate surface.


1.0

Absorbance

(b) 0.8

0.6

0.4

0.2

Dopamine + air 0.0 0

5

10

15

20

25

Time (h) (c)

2.0

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6

1.2

0.8

Dopamine + CuSO4/H2O2

0.4

0.0 0

10

20

30

40

50

60

Time (min)

(d)

2.0

Absorbance

Page 19 of 27

1.6

1.2

Precipitation

0.8

Dopamine + (NH4)2S2O8

0.4

0.0 0

10

20

30

40

50

60

Time (min)

Figure 2. (a) Photographs of dopamine solutions with different oxidants. (b-d) Time-dependence of UV-vis absorbance at 420 nm for various diluted dopamine solutions with different oxidants: air, CuSO4 (5.3 mM)/H2O2 (26.1 mM), and (NH4)2S2O8 (13.1 mM).



(a)

(b) O1s

O1s

(c) O1s

C=O (59.8%) C-O (40.2%)

540

537

534

531

528

525

Binding Energy (eV)

Intensity (a.u.)

Intensity (a.u.)

C=O (59.2%)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

C-O (40.8%)

540

537

534

531

528

525

Binding Energy (eV)

C=O (61.4%)

C-O(38.6%)

540

537

534

531

528

525

Binding Energy (eV)

Figure 3. High-resolution XPS spectra of O1s in PDA powders oxidized by different oxidants: (a) Air, (b) CuSO4 (5.3 mM)/H2O2 (26.1 mM) and (c) (NH4)2S2O8 (13.1 mM).


Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Schematic diagram of PDA deposition processes on the substrate surface oxidized by three different oxidants. “×” represents the PDA particles are not able to deposit onto the substrate surface.



Mean Depth (nm) 0.07

0.505

(a) 0.500

5 KeV

0.05

S parameter

0.06

PDF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

0.04 0.03

1.5 KeV

0.02

0

41 126 242 382 544

Supporting layer

(b)

0.495

Transition layer

0.490

PDA layer

0.485

0.01 0.480

0.00 0.475

1

2

3

4

5

6

7

o-Ps lifetime (ns)

8

0

1

2

3

4

5

6

Positron Incident Energy (keV)

Figure 5. (a) o-Ps lifetime distribution curves for the PDA-coated thin film composite membranes fabricated by CuSO4/H2O2 with different positron implantation energies. PDF is probability density function. (b) S parameters as a function of positron incident energy and depth for the PDA-coated thin film composite membranes fabricated by CuSO4/H2O2.


100

100

60 100 40

50

20

0

(c)

F/F0

80

60 60

40

40

20

20

0

0

C

B

Water flux Rejection

0

A

(b)

Na2SO4 MgSO4 MgCl2

NaCl

Salts R/R0

(d)

F/F0

R/R0

1.0

1.0

1.0

0.8

0.8

0.8

0.8

F/F0

0.6

0.6

0.4

0.4

0.4

0.2

0.2

0.2

0.0

0.0

0.6

0.6

0.4 0.2 0.0

0.0 0 1 2 3 4 5 6 7 8 9 10

80

60

60

40

40

2

80

Water flux 20

20

Rejection

(e)

0

0 0

20

40

60

80

100 120

Operation time (h)

Immersion cycles

3

(f)

2+

100

Rejection (%)

Immersion cycles

Concentration of Cu (mg/L)

0 1 2 3 4 5 6 7 8 9 10 100

1.2

F/F0

1.0

R/R0

1.2

1.2

1.2

Rejection (%)

150

80

Rejection (%) 2 Water flux (L/m ⋅ h)

80

2

Water flux (L/m ⋅ h)

(a)

Water flux Rejection

200

Water flux (L/m ⋅ h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


R/R0

Page 23 of 27

2

Maximum concentration for drinking water 1

0 0

40

80

120

160

200

240

Operation time (h)

Figure 6. (a) Nanofiltration performances of the PDA-coated membranes oxidized by different oxidants: (A) air, (B) (NH4)2S2O8 and (C) CuSO4/H2O2. (b) Effects of inorganic salts on the nanofiltration performance of the PDA-coated membranes oxidized by CuSO4/H2O2. (c) Nanofiltration performance of the PDA-coated membranes treated by aqueous solution under pH = 3 with different immersion cycles. (d) Nanofiltration performance of the PDA-coated membranes treated by aqueous solution under pH = 11 with different immersion cycles. Immersion time for each cycle is 12 h. F/F0 and R/R0 represent the variation of water permeation flux and salt rejection, respectively. (e) Structure stability of the PDA-coated membranes with different operation times (Na2SO4 solution as feed). (f) Concentration of Cu2+ in the filtrate with different operation times. All solute concentrations are 1000 mg/L. 23 ACS Paragon Plus Environment


4.0 3.5 3.0 2.5 2.0

(a)

7. Methanol 6. Acetone 5. Tetrahydrofuran 4. Ethanol 3. Isopropanol 2. Butanol 1. Heptane 5 4 Slope

3 1.5

1

6

0.023

Feed

(c)

30

7

Intensity (%)

2

Permeance (L/h⋅m )

4.5

~~~~

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

Permeate

20

10

2

2

R = 0.95 0

1.0 0

20

40

-1

60

-2

80 -3

100

δp⋅η ⋅dm (×10 )

0

1

2

3

4

5

6

7

Particle diameter (nm)

Figure 7. (a) Plot of solvent permeance against the combined solvent property (viscosity, molar diameter, and solubility parameter) for PDA-coated membranes oxidized by CuSO4/H2O2. (b) Photographs and (c) the size distribution of Au nanoparticles in the feed and the permeate methanol solutions.


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Table 1. Positron lifetime results including lifetime, intensity and calculated pore radius for the PDA-coated thin film composite membranes fabricated by CuSO4/H2O2 with different positron implantation energies.

keV

τ3-1 (ns)

I3-1

d1-1 (Å)

τ3-2 (ns)

I3-2

d2-1 (Å)

0.7

1.92±0.03

11.97±0.08

5.56±0.04

4.81±0.07

4.05±0.51

9.32±0.04

5

2.40±0.02

15.97±0.13

6.40±0.02

6.04±0.09

6.37±0.71

10.42±0.04

*τ, I and d (2r) are the o-Ps lifetime, intensity and the membrane pore size, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

Table 2. Nanofiltration performance of different composite membranes with PDA-based coatings as selective layers. PDA coating as selective layer

Coating time (h)

Zeta potential (mV)

Rejection molecule

Rejection (%)

Water flux (L/m2⋅h⋅ba r)

Ref.

PDA layer via double deposition

40

5.5

CaCl2

68.7

11.7

[10]

PDA layer followed by PEI grafting

9

12.1

MgCl2

73.7

7.2

[20]

PDA layer followed by PEI grafting and cross-linking

16.3

/

MgCl2

89.3

5.5

[24]

Co-deposited PDA/PEI layer followed by cross-linking

4.7

6.5

MgCl2

92.0

1.7

[23]

Co-deposited PDA/PEI layer followed by SMPS grafting

15

-9.5

NaSO4

96.2

3.3

PDA/PEI layer via Contra-Diffusi on method

2.3

5

MgCl

95

5

[25]

PDA layer triggered by CuSO4/H2O2

0.75

-14.5

NaSO4

90.0

10.1

This work

PEI: Poly(ethylene imine);

[21]

SMPS: Sodium 3-mercapto-1-propanesulfonate


Page 27 of 27

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Figure for ToC


Polydopamine Coatings with Nanopores for Versatile Molecular

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