DOI: 10.1002/asia.201500121
Communication
Membranes
Ultrathin pH-Sensitive Nanoporous Membranes for Superfast SizeSelective Separation Qiu Gen Zhang,* Chao Deng, Rong Rong Liu, Zhen Lin, Hong Mei Li, Ai Mei Zhu, and Qing Lin Liu*[a] Abstract: Stimuli-responsive nanoporous membranes have attracted increasing interest in various fields due to their abrupt changes of permeation/separation in response to the external environment. Here we report ultrathin pH-sensitive nanoporous membranes that are easily fabricated by the self-assembly of poly(acrylic acid) (PAA) in a metal hydroxide nanostrand solution. PAA-adsorbed nanostrands (2.5–5.0 nm) and PAA-CuII nanogels (2.0– 2.5 nm) grow competitively during self-assembly. The PAAadsorbed nanostrands are deposited on a porous support to fabricate ultrathin PAA membranes. The membranes display ultrafast water permeation and good rejection as well as significant pH-sensitivity. The 28 nm-thick membrane has a water flux decrease from 3740 to 1350 L m¢1 h¢1 bar¢1 (pH 2.0 to 7.0) with a sharp decrease at pH 5.0. This newly developed pH-sensitive nanoporous membranes may find a wide range of applications such as controlled release and size- and charge-selective separation.
Stimuli-responsive (“smart”) nanoporous membranes have attracted increasing interest in the past decades for potential applications, such as controlled release, selective separation, chemical sensors, and biosensors.[1–3] They exhibit abrupt changes of permeation/separation in response to external environmental conditions including pH, temperature, ionic strength, electric and magnetic fields, light, and the concentration of chemical species.[4, 5] In particular, pH-sensitivity can be used to control the effective pore diameter by adjusting the pH due to a change in the conformation of polymer chains, giving more choices both for the membranes and the application environment than other external stimuli.[6–8] Numerous works have been devoted to the fabrication of pH-sensitive nanoporous membranes because they are widely used in the [a] Dr. Q. G. Zhang, C. Deng, R. R. Liu, Z. Lin, H. M. Li, Dr. A. M. Zhu, Prof. Q. L. Liu Department of Chemical&Biochemical Engineering College of Chemistry&Chemical Engineering Xiamen University 422 Siming South Road, Xiamen 361005 (China) E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500121. Chem. Asian J. 2015, 10, 1133 – 1137
controlled release of chemicals and drugs, flow regulation, selfcleaning surfaces, and size- and charge-selective filtration.[1, 4, 9–12] These membranes are mainly fabricated by nonsolvent-induced phase separation of a pH-responsive polymer, or graft pH-sensitive polymer chains/brushes to the external surface and/or the pore walls of porous membranes. The resulting membranes usually have a thick separation layer, leading to low permeation rates and too long diffusion channels of small molecules across the membrane. A membrane with a good pH-sensitivity and fast permeation of small molecules is desirable for the controlled release of chemicals and drugs as well as size- and charge-selective separation.[13, 14] Very recently, ultrathin nanoporous membranes with a thickness of tens of nanometers on a macroporous support have been attracting extensive interest in many fields including membrane separation, surface engineering, sensors, biology, and biotechnology.[15–20] They have an ultrahigh permeation flux that is inversely proportional to the membrane thickness, and thus are strongly demanded in various separation processes. Currently, various methods have been successfully developed to fabricate ultrathin nanoporous membranes, such as coating, self-assembly, deposition, and surface grafting.[15, 19, 21–23] However, it is still a great challenge to fabricate polymer nanoporous membranes with a thickness of few-tens of nanometers on a macroporous support. Herein, we report a novel 28 nm-thick cross-linked poly(acrylic acid) (PAA) nanoporous membrane with pore sizes of 10–12 nm. It has good pH-sensitivity and an ultrahigh permeation flux for pressure-driven filtration. PAA is one of the most important pH-sensitive materials and has the most commonly used pH-responsive functional groups, that is, carboxyl groups.[4] Our PAA membrane was fabricated by a facile approach, as shown in Figure 1 a, comprising PAA self-assembly in a copper hydroxide nanostrand solution to form PAA-adsorbed nanostrands, which are filtered to prepare an ultrathin layer (Figure 1 b) on a microfiltration filter. This layer is further cross-linked by irradiation with UV light followed by removal of nanostrands, finally forming the 28 nm-thick PAA nanoporous membrane (Figure 1 c,d). PAA self-assembly is the most important step in the membrane fabrication. The process is performed in a copper hydroxide nanostrand solution by mixing a half volume of a very dilute PAA solution (2.5 to 15 mg mL¢1). The copper hydroxide nanostrands have a width of 2.5 nm and a length of few micrometers with particularly abundant positive charges on their
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Figure 1. Fabrication of ultrathin nanoporous PAA membranes: (a) scheme of the membrane fabrication, (b) a cross-sectional SEM image of the PAA-adsorbed nanostrand membrane prepared from 7.5 mg mL¢1 PAA, (c, d) topview and cross-sectional SEM images of the 28 nm-thick PAA membrane prepared from the sample shown in (b).
Figure 2. Self-assembly of PAA chains in the Cu(OH)2 nanostrand dispersion: (a) scheme of PAA self-assembly, (b) a TEM image of the PAA-adsorbed nanostrands and PAA-CuII nanogels prepared from 10.0 mg mL¢1 PAA, (c, d) the diameter distribution of PAA-adsorbed nanostrands and PAA-CuII nanogels, respectively, as estimated from TEM images.
surface.[24] Negatively charged water-soluble molecules can be adsorbed onto the nanostrand surface by electrostatic interactions. By exploiting this property, PAA chains will adsorb onto the nanostrand surface leading to the formation of a PAA-adsorbed layer (Figure 2 a). At the same time, more PAA chains will adsorb onto the PAA-adsorbed layer owing to the chelation of PAA with copper ions, finally forming PAA-adsorbed nanostrands. Moreover, PAA chains may form nanogels with copper(II) ions due to the chelation between PAA and copper(II).[25, 26] Thus, the PAA-adsorbed nanostrands and PAA-CuII nanogels are possibly formed simultaneously in the self-assembly process. Meanwhile, the stability of the nanostrand with PAA were investigated, which is dependent on the PAA concentration (Figure S1, Supporting Information). Typically, the nanostrand with 7.5 mg mL¢1 PAA is stable for 2 days. After Chem. Asian J. 2015, 10, 1133 – 1137
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2 days, the nanostrand would become too big and aggregate to form floccules owing to the chelation of copper ions. In this work, the nanostrand with PAA after aging for 30 min was used to fabricate the ultrathin PAA membrane. Figure 2 b shows a TEM image of the self-assembled PAA nanomaterials prepared from a 10.0 mg mL¢1 PAA solution (15.0 mg mL¢1 PAA shown in Figure S2). Apparently, PAA-adsorbed nanostrands and PAA-CuII nanogels are observed simultaneously, and uniformly dispersed in the solution. To confirm this, their diameters were estimated from the TEM images (Figure 2 c,d). Obviously, the PAA-adsorbed nanostrands are bigger than the non-adsorbed nanostrands (2.5 nm in diameter),[24] and have an average diameter of about 2.95 nm using a 10.0 mg mL¢1 PAA solution and grow to 4.05 nm (15.0 mg mL¢1 PAA). Unlike the PAA-adsorbed nanostrands, the PAA-CuII nanogels grow slightly with increasing PAA concentration, and have an average diameter of 2.05 nm (10.0 mg mL¢1 PAA) to 2.2 nm (15.0 mg mL¢1 PAA). To study the mechanism of PAA self-assembly, UV/Vis spectrophotometry and SEM were employed to investigate the effect of PAA concentration on PAA self-assembly. Figure S3 shows the UV/Vis spectra of nanostrand solution and its PAA solutions. It is found that the UV/Vis spectrum of the nanostrand solution is almost identical to that of the 2.52 nm-diameter CuC6 nanoparticle solutions, suggesting that the nanostrands have the same diameter of about 2.5 nm.[27] As the PAA concentration increases, the absorption peak of the mixed solution shifts to longer wavelengths, revealing that the PAAadsorbed nanostrands are larger than the non-adsorbed nanostrands and gradually grow in size due to the formation of PAA-adsorbed layer, which is also confirmed by TEM. Surprisingly, the peak of the mixture from the 10.0 mg mL¢1 PAA solution does not shift to the right but to the left. This is because PAA chains mainly absorb onto the nanostrands due to their high surface area and abundant positive charges at a low PAA concentration. However, most of PAA chains will form nanogels with CuII at a high PAA concentration leading to a decrease in the PAA chains adsorbed onto the nanostrands. This result was also observed by SEM, as shown in Figure 3
Figure 3. SEM images of the nanostrands (a) and PAA self-assembled nanomaterials (b–f) prepared from 2.5, 7.5, 10.0, 12.5 and 15.0 mg mL¢1 PAA solutions. The samples were prepared by filtering 8 mL of the solution through a polycarbonate filter. Scale bar, 200 nm.
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Communication and Figure S4. At a low PAA concentration, PAA chains mainly self-assembled on the nanostrand surface to form PAA-adsorbed nanostrands that gradually grow with increasing PAA concentration. When the PAA concentration is above 10.0 mg mL¢1, numerous PAA-CuII nanogels are formed with similar average diameter (2.0–2.5 nm) at a higher PAA concentration. From these results it can be concluded that PAA-adsorbed nanostrands and PAA-CuII nanogels are formed simultaneously, and have a competitive growth in a turning point (10.0 mg mL¢1 PAA). The ultrathin PAA membrane was fabricated by filtrating the PAA-adsorbed nanostrand solution on the porous support followed by UV cross-linking and nanostrand removal. As shown in Figure 3, the membrane morphology of the PAA-adsorbed nanostrands is similar to that of the non-adsorbed nanostrands. The PAA concentration has a great effect on the membrane structure. At a high PAA concentration, the deposition of PAA-CuII nanogels possibly affects the properties of the PAA membrane. Meanwhile, these membranes are thicker than those of non-adsorbed nanostrands, and the thickness gradually increases with the PAA concentration (Figure 1 b, Figure S5). In the membrane fabrication, Norrish I a-cleavage of the carboxylic acid groups occurred during UV-irradiation for 15– 60 min in an air atmosphere. The broken groups can then either recombine, degrade the polymer through b-scission, or form inter- and intra-molecular cross-links (Figure S6).[28] As a result, the insoluble cross-linked PAA network was formed between PAA-adsorbed nanostrands. After that, the nanostrands were dissolved quickly by filtering a dilute aqueous HCl solution to form the ultrathin PAA nanoporous membrane. Finally, the 28 nm-thick PAA membrane was successfully fabricated via the PAA-adsorbed nanostrand membrane from the 7.5 mg mL¢1 PAA solution (Figure 1 c,d, Figure S7). This membrane has a smooth surface and a uniform thickness. Other membranes were also facilely obtained using the PAA solution at different concentrations. From the membrane structure, the ultrathin PAA membrane covered on a porous support should have an ultrahigh water flux in pressure-driven filtration. To evaluate the separation performance of the as-fabricated PAA nanoporous membranes, the pure water fluxes and rejections for 10 nm gold nanoparticles and ferritin (~ 12 nm in diameter) were measured by filtration experiments in a deadend mode. As presented in Table 1, the membranes have high water fluxes and good rejections for ferritin and 10 nm gold
nanoparticles. The PAA concentration has a great influence on the membrane separation performance. As the PAA concentration increases, the water flux first decreases from 1940 to 1350 L m¢2 h¢1 bar¢1, then suddenly rises to 2270 L m¢2 h¢1 bar¢1 (10.0 mg mL¢1 PAA), and finally continually decreases to 1370 L m¢2 h¢1 bar¢1. This phenomenon resulted from the membrane structure. As described above, a self-assembled PAA layer was formed by the assembly of PAA chains on the nanostrand surface and became thicker with increasing PAA concentration, producing a compact PAA film. This explains why the water flux first decreases and rejection increases. However, numerous PAA-CuII nanogels are formed at 10.0 mg mL¢1 PAA, leading to the formation of a highly porous membrane with fast water permeation. Moreover, the nanogels would form a packed layer on the PAA-adsorbed nanostrand layer in the membrane preparation, which made the membrane thicker leading to a decrease in the water fluxes. Meanwhile, this packed layer can be easily cracked to form some defects on the membrane in the fabrication process, resulting in a decrease in rejection (Table 1). The membrane prepared from 7.5 mg mL¢1 PAA has a water flux of 1350 L m¢2 h¢1 bar¢1 and a 10–12 nm cut-off. The UV cross-linking time also has a significant influence on the membrane separation performance (Figure 4 a). The UV cross-linking would make PAA stable in an aqueous solution after the removal of nanostrand templates. The water flux de-
Table 1. Separation performance of the ultrathin PAA membranes at neutral conditions. PAA [mg mL¢1]
Water flux [L m¢2 h¢1 bar¢1]
Rejection [%] 10 nm gold
Ferritin
2.5 5.0 7.5 10.0 12.5 15.0
1940 1530 1350 2270 1690 1370
59.2 65.6 77.9 83.0 69.4 46.8
85.6 88.3 93.5 88.5 83.6 53.4
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Figure 4. Separation performance of the ultrathin PAA nanoporous membranes: (a) the effect of cross-linking time, (b) the pH dependence of the pure water flux.
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Communication creases and rejection increases with the cross-linking time. However, the water flux increases and rejection decreases when the cross-linking proceeded for 60 min. This is because a high cross-linking density leads to a small extension of the PAA chains in an aqueous solution. As a consequence, the highly cross-linked membranes have bigger pores than the lowly cross-linked membranes. Typically, the conditions of 7.5 mg mL¢1 PAA solution and UV cross-linking for 45 min are favorable for the fabrication of an ultrathin PAA membrane with a good separation performance. The resulting 28 nmthick membrane has a good rejection for ferritin. A rejection of 93.5 % was calculated based on the ferritin concentration in the filtrate (and 90.7 % from the concentrate), suggesting that this membrane has a good fouling resistance that is expected in protein separation by ultrafiltration. Furthermore, this membrane is stable in the operation for more than 5 days by using the apparatus of pressure-driven filtration (Figure S8), showing its great potential for industrial processes. PAA is one of the most important pH-sensitive materials and widely used in separation membranes.[1] Here the pure water flux through the ultrathin PAA nanoporous membranes was examined in the pH range of 2.0–7.0 at a pressure difference of 80 kPa. These membranes have a significant pH-sensitivity between pH 4.0–5.0 (Figure 4 b). Typically, for the 28 nm-thick membrane, the flux decreases with pH from 3740 (pH 2.0) to 2910 (pH 4.0), 1640 (pH 5.0) and 1350 L m¢2 h¢1 bar¢1 (pH 7.0) with a sharp decrease at pH 5.0 (1270 L m¢2 h¢1 bar¢1). Carboxyl groups are protonated and hydrophobic interactions dominate at a low pH, leading to volume shrinkage of PAA, whereas they dissociate into carboxylate ions at a high pH leading to the swelling of PAA due to the high charge density in PAA, as shown in Figure 4 b (insets). Based on the configurational change of PAA chains, the membrane pore sizes can be adjusted.[1] Therefore, the as-fabricated ultrathin PAA nanoporous membranes have good separation performances and pH-sensitivity during pressure-driven filtration. In summary, we have reported a facile and flexible route for the fabrication of ultrathin pH-sensitive nanoporous membranes by PAA self-assembly in the copper hydroxide nanostrand solution. PAA-adsorbed nanostrands and PAA-CuII nanogels are formed simultaneously in the self-assembly process. The former gradually grow in diameter ranging from 2.5 to 5.0 nm with the PAA concentration, whereas the latter has a sudden increase in quantity at a turning point (10.0 mg mL¢1 PAA) and has a similar diameter of 2.0–2.5 nm. The PAA-adsorbed nanostrands were used to fabricate ultrathin PAA membranes on a porous support. The resulting membranes have a smooth surface and a uniform ultrathin PAA nanoporous layer. They show ultrafast water permeation and good separation performance during pressure-driven filtration, and particularly have a significant pH-sensitivity between pH 4.0 and 5.0. The newly developed pH-sensitive nanoporous membranes may find a wide range of applications such as controlled release and selective separation.
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Experimental Section Fabrication of ultrathin PAA membranes: The copper hydroxide nanostrand solution was prepared by the approach reported previously.[23] A 1 mm aqueous 2-aminoethanol solution was mixed with an equal volume of a 4 mm copper nitrate solution followed by standing at 20 8C for 7 days to form copper hydroxide nanostrands. Subsequently, 8 mL of the nanostrand solution was quickly mixed with 4 mL of a very dilute PAA solution (2.5–15.0 mg mL¢1, Mw 1800) with pH ~ 6.8 and aged for 30 min, forming a PAA-adsorbed nanostrand solution. Then, 8 mL of the resulting solution was filtered on a 200 nm-pore polycarbonate (Whatman) filter placed on a glass filter holder at a suction pressure of 80 kPa, and subsequently cross-linked using a hand-held EF-180/FA UV lamp (Spectronics Corporation). After that, the nanostrands were dissolved by filtering an aqueous HCl solution of pH 2, finally forming an ultrathin PAA nanoporous membrane. Filtration experiments: Ultrafiltration experiments were performed using a glass filter holder with a membrane area of 2.84 cm2 at a suction vacuum pressure of 80 kPa. Pure water flux (J, L m¢2 h¢1 bar¢1) of the PAA membrane was measured by filtering 100 mL of water across the membrane and calculated by using the following equation: J ¼ V=ðAtpÞ
ð1Þ
where V is the volume of the water filtered (L), A is the effective membrane filtration area (m2), t is the filtration time (h), and p is the suction pressure across the membrane (bar). To evaluate membrane separation properties, the rejection of ferritin (from equine spleen, Sigma–Aldrich) and 10 nm gold nanoparticles (British Biocell International) was measured by filtering their solution across the membrane. The feed, the filtrate and the concentrate were characterized by using a UV-vis spectrophotometer. The rejection (R, %) was calculated by R ¼ ð1¢C p =C f Þ 100 %
ð2Þ
where Cf and Cp are the concentrations of ferritin or 10 nm gold nanoparticles in the feed and the permeate, respectively. Characterization: the structure of the PAA-adsorbed nanostrands was characterized by TEM (JEM-1400, 100 kV, Japan). The samples were prepared by dropping their dispersion on a copper grid coated with a carbon film. Meanwhile, the morphology of the membranes and PAA-adsorbed nanostrands was observed by SEM (ZEISS SIGMA, Germany). The samples for cross-sectional observation were prepared by freeze-fracturing in liquid nitrogen. All of the samples were coated with a ~ 5 nm thick platinum layer using a JFC-1600 auto-fine coater before observation to improve their conductivity.
Acknowledgements The research was supported by National Nature Science Foundation of China Grant (No. 21306155), the research fund for the Doctoral Program of Higher Education (No. 20120121120013) and the Fundamental Research Funds for the Central Universities (No. 2012121029).
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Communication Keywords: metal hydroxide nanostrands · pH-sensitivity · selfassembly · separation membranes · ultrathin films [1] C. Zhao, S. Nie, M. Tang, S. Sun, Prog. Polym. Sci. 2011, 36, 1499. [2] S. Ganta, H. Devalapally, A. Shahiwala, M. Amiji, J. Controlled Release 2008, 126, 187. [3] H. H. Himstedt, H. Du, K. M. Marshall, S. R. Wickramasinghe, X. Qian, Ind. Eng. Chem. Res. 2013, 52, 9259. [4] D. Wandera, S. R. Wickramasinghe, S. M. Husson, J. Membr. Sci. 2010, 357, 6. [5] E. S. Gil, S. M. Hudson, Prog. Polym. Sci. 2004, 29, 1173. [6] J. Hendri, A. Hiroki, Y. Maekawa, M. Yoshida, R. Katakai, Radiat. Phys. Chem. 2001, 60, 617. [7] C. Song, W. Shi, H. Jiang, J. T, D. Ge, J. Membr. Sci. 2011, 372, 340. [8] K. Kontturi, S. Maf¦, J. A. Manzanares, B. L. Svarfvar, P. Viinikka, Macromolecules 1996, 29, 5740. [9] M. Homayoonfal, M. R. Mehrnia, Sep. Purif. Technol. 2014, 130, 74. [10] J. I. Clodt, V. Filiz, S. Rangou, K. Buhr, C. Abetz, D. Hçche, J. Hahn, A. Jung, V. Abetz, Adv. Funct. Mater. 2013, 23, 731. [11] L. Ferro, O. Scialdone, A. Galia, J. Supercrit. Fluids 2012, 66, 241. [12] Z. Wang, X. Yao, Y. Wang, J. Mater. Chem. 2012, 22, 20542. [13] C. C. Striemer, T. R. Gaborski, J. L. McGrath, P. M. Fauchet, Nature 2007, 445, 749. [14] D. L. Gin, R. D. Noble, Science 2011, 332, 674. [15] S. Karan, S. Samitsu, X. Peng, K. Kurashima, I. Ichinose, Science 2012, 335, 444.
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[16] Y. Han, Z. Xu, C. Gao, Adv. Funct. Mater. 2013, 23, 3693. [17] X. Peng, J. Jin, Y. Nakamura, T. Ohno, I. Ichinose, Nat. Nanotechnol. 2009, 4, 353. [18] Q. Wang, S. Samitsu, I. Ichinose, Adv. Mater. 2011, 23, 2004. [19] H. Ma, K. Yoon, L. Rong, M. Shokralla, A. Kopot, X. Wang, D. Fang, B. S. Hsiao, B. Chu, Ind. Eng. Chem. Res. 2010, 49, 11978. [20] H. Ma, K. Yoon, L. Rong, Y. Mao, Z. Mo, D. Fang, Z. Hollander, J. Gaiteri, B. S. Hsiao, B. Chu, J. Mater. Chem. 2010, 20, 4692. [21] Q. Zhang, S. Ghosh, S. Samitsu, X. Peng, I. Ichinose, J. Mater. Chem. 2011, 21, 1684. [22] M. Nagale, B. Y. Kim, M. L. Bruening, J. Am. Chem. Soc. 2000, 122, 11670. [23] C. Deng, Q. G. Zhang, G. L. Han, Y. Gong, A. M. Zhu, Q. L. Liu, Nanoscale 2013, 5, 11028. [24] S. Karan, Q. Wang, S. Samitsu, Y. Fujii, I. Ichinose, J. Membr. Sci. 2013, 448, 270. [25] F. T. Wall, S. J. Gill, J. Phys. Chem. 1954, 58, 1128. [26] P. Schuetz, F. Caruso, Adv. Funct. Mater. 2003, 13, 929. [27] S. Chen, J. M. Sommers, J. Phys. Chem. B 2001, 105, 8816. [28] A. Gestos, P. G. Whitten, G. M. Spinks, G. G. Wallace, Soft Matter 2010, 6, 1045.
Received: February 4, 2015 Revised: February 27, 2015 Published online on March 17, 2015
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Membranes
Ultrathin pH-Sensitive Nanoporous Membranes for Superfast SizeSelective Separation Qiu Gen Zhang,* Chao Deng, Rong Rong Liu, Zhen Lin, Hong Mei Li, Ai Mei Zhu, and Qing Lin Liu*[a] Abstract: Stimuli-responsive nanoporous membranes have attracted increasing interest in various fields due to their abrupt changes of permeation/separation in response to the external environment. Here we report ultrathin pH-sensitive nanoporous membranes that are easily fabricated by the self-assembly of poly(acrylic acid) (PAA) in a metal hydroxide nanostrand solution. PAA-adsorbed nanostrands (2.5–5.0 nm) and PAA-CuII nanogels (2.0– 2.5 nm) grow competitively during self-assembly. The PAAadsorbed nanostrands are deposited on a porous support to fabricate ultrathin PAA membranes. The membranes display ultrafast water permeation and good rejection as well as significant pH-sensitivity. The 28 nm-thick membrane has a water flux decrease from 3740 to 1350 L m¢1 h¢1 bar¢1 (pH 2.0 to 7.0) with a sharp decrease at pH 5.0. This newly developed pH-sensitive nanoporous membranes may find a wide range of applications such as controlled release and size- and charge-selective separation.
Stimuli-responsive (“smart”) nanoporous membranes have attracted increasing interest in the past decades for potential applications, such as controlled release, selective separation, chemical sensors, and biosensors.[1–3] They exhibit abrupt changes of permeation/separation in response to external environmental conditions including pH, temperature, ionic strength, electric and magnetic fields, light, and the concentration of chemical species.[4, 5] In particular, pH-sensitivity can be used to control the effective pore diameter by adjusting the pH due to a change in the conformation of polymer chains, giving more choices both for the membranes and the application environment than other external stimuli.[6–8] Numerous works have been devoted to the fabrication of pH-sensitive nanoporous membranes because they are widely used in the [a] Dr. Q. G. Zhang, C. Deng, R. R. Liu, Z. Lin, H. M. Li, Dr. A. M. Zhu, Prof. Q. L. Liu Department of Chemical&Biochemical Engineering College of Chemistry&Chemical Engineering Xiamen University 422 Siming South Road, Xiamen 361005 (China) E-mail: [email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500121. Chem. Asian J. 2015, 10, 1133 – 1137
controlled release of chemicals and drugs, flow regulation, selfcleaning surfaces, and size- and charge-selective filtration.[1, 4, 9–12] These membranes are mainly fabricated by nonsolvent-induced phase separation of a pH-responsive polymer, or graft pH-sensitive polymer chains/brushes to the external surface and/or the pore walls of porous membranes. The resulting membranes usually have a thick separation layer, leading to low permeation rates and too long diffusion channels of small molecules across the membrane. A membrane with a good pH-sensitivity and fast permeation of small molecules is desirable for the controlled release of chemicals and drugs as well as size- and charge-selective separation.[13, 14] Very recently, ultrathin nanoporous membranes with a thickness of tens of nanometers on a macroporous support have been attracting extensive interest in many fields including membrane separation, surface engineering, sensors, biology, and biotechnology.[15–20] They have an ultrahigh permeation flux that is inversely proportional to the membrane thickness, and thus are strongly demanded in various separation processes. Currently, various methods have been successfully developed to fabricate ultrathin nanoporous membranes, such as coating, self-assembly, deposition, and surface grafting.[15, 19, 21–23] However, it is still a great challenge to fabricate polymer nanoporous membranes with a thickness of few-tens of nanometers on a macroporous support. Herein, we report a novel 28 nm-thick cross-linked poly(acrylic acid) (PAA) nanoporous membrane with pore sizes of 10–12 nm. It has good pH-sensitivity and an ultrahigh permeation flux for pressure-driven filtration. PAA is one of the most important pH-sensitive materials and has the most commonly used pH-responsive functional groups, that is, carboxyl groups.[4] Our PAA membrane was fabricated by a facile approach, as shown in Figure 1 a, comprising PAA self-assembly in a copper hydroxide nanostrand solution to form PAA-adsorbed nanostrands, which are filtered to prepare an ultrathin layer (Figure 1 b) on a microfiltration filter. This layer is further cross-linked by irradiation with UV light followed by removal of nanostrands, finally forming the 28 nm-thick PAA nanoporous membrane (Figure 1 c,d). PAA self-assembly is the most important step in the membrane fabrication. The process is performed in a copper hydroxide nanostrand solution by mixing a half volume of a very dilute PAA solution (2.5 to 15 mg mL¢1). The copper hydroxide nanostrands have a width of 2.5 nm and a length of few micrometers with particularly abundant positive charges on their
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Figure 1. Fabrication of ultrathin nanoporous PAA membranes: (a) scheme of the membrane fabrication, (b) a cross-sectional SEM image of the PAA-adsorbed nanostrand membrane prepared from 7.5 mg mL¢1 PAA, (c, d) topview and cross-sectional SEM images of the 28 nm-thick PAA membrane prepared from the sample shown in (b).
Figure 2. Self-assembly of PAA chains in the Cu(OH)2 nanostrand dispersion: (a) scheme of PAA self-assembly, (b) a TEM image of the PAA-adsorbed nanostrands and PAA-CuII nanogels prepared from 10.0 mg mL¢1 PAA, (c, d) the diameter distribution of PAA-adsorbed nanostrands and PAA-CuII nanogels, respectively, as estimated from TEM images.
surface.[24] Negatively charged water-soluble molecules can be adsorbed onto the nanostrand surface by electrostatic interactions. By exploiting this property, PAA chains will adsorb onto the nanostrand surface leading to the formation of a PAA-adsorbed layer (Figure 2 a). At the same time, more PAA chains will adsorb onto the PAA-adsorbed layer owing to the chelation of PAA with copper ions, finally forming PAA-adsorbed nanostrands. Moreover, PAA chains may form nanogels with copper(II) ions due to the chelation between PAA and copper(II).[25, 26] Thus, the PAA-adsorbed nanostrands and PAA-CuII nanogels are possibly formed simultaneously in the self-assembly process. Meanwhile, the stability of the nanostrand with PAA were investigated, which is dependent on the PAA concentration (Figure S1, Supporting Information). Typically, the nanostrand with 7.5 mg mL¢1 PAA is stable for 2 days. After Chem. Asian J. 2015, 10, 1133 – 1137
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2 days, the nanostrand would become too big and aggregate to form floccules owing to the chelation of copper ions. In this work, the nanostrand with PAA after aging for 30 min was used to fabricate the ultrathin PAA membrane. Figure 2 b shows a TEM image of the self-assembled PAA nanomaterials prepared from a 10.0 mg mL¢1 PAA solution (15.0 mg mL¢1 PAA shown in Figure S2). Apparently, PAA-adsorbed nanostrands and PAA-CuII nanogels are observed simultaneously, and uniformly dispersed in the solution. To confirm this, their diameters were estimated from the TEM images (Figure 2 c,d). Obviously, the PAA-adsorbed nanostrands are bigger than the non-adsorbed nanostrands (2.5 nm in diameter),[24] and have an average diameter of about 2.95 nm using a 10.0 mg mL¢1 PAA solution and grow to 4.05 nm (15.0 mg mL¢1 PAA). Unlike the PAA-adsorbed nanostrands, the PAA-CuII nanogels grow slightly with increasing PAA concentration, and have an average diameter of 2.05 nm (10.0 mg mL¢1 PAA) to 2.2 nm (15.0 mg mL¢1 PAA). To study the mechanism of PAA self-assembly, UV/Vis spectrophotometry and SEM were employed to investigate the effect of PAA concentration on PAA self-assembly. Figure S3 shows the UV/Vis spectra of nanostrand solution and its PAA solutions. It is found that the UV/Vis spectrum of the nanostrand solution is almost identical to that of the 2.52 nm-diameter CuC6 nanoparticle solutions, suggesting that the nanostrands have the same diameter of about 2.5 nm.[27] As the PAA concentration increases, the absorption peak of the mixed solution shifts to longer wavelengths, revealing that the PAAadsorbed nanostrands are larger than the non-adsorbed nanostrands and gradually grow in size due to the formation of PAA-adsorbed layer, which is also confirmed by TEM. Surprisingly, the peak of the mixture from the 10.0 mg mL¢1 PAA solution does not shift to the right but to the left. This is because PAA chains mainly absorb onto the nanostrands due to their high surface area and abundant positive charges at a low PAA concentration. However, most of PAA chains will form nanogels with CuII at a high PAA concentration leading to a decrease in the PAA chains adsorbed onto the nanostrands. This result was also observed by SEM, as shown in Figure 3
Figure 3. SEM images of the nanostrands (a) and PAA self-assembled nanomaterials (b–f) prepared from 2.5, 7.5, 10.0, 12.5 and 15.0 mg mL¢1 PAA solutions. The samples were prepared by filtering 8 mL of the solution through a polycarbonate filter. Scale bar, 200 nm.
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Communication and Figure S4. At a low PAA concentration, PAA chains mainly self-assembled on the nanostrand surface to form PAA-adsorbed nanostrands that gradually grow with increasing PAA concentration. When the PAA concentration is above 10.0 mg mL¢1, numerous PAA-CuII nanogels are formed with similar average diameter (2.0–2.5 nm) at a higher PAA concentration. From these results it can be concluded that PAA-adsorbed nanostrands and PAA-CuII nanogels are formed simultaneously, and have a competitive growth in a turning point (10.0 mg mL¢1 PAA). The ultrathin PAA membrane was fabricated by filtrating the PAA-adsorbed nanostrand solution on the porous support followed by UV cross-linking and nanostrand removal. As shown in Figure 3, the membrane morphology of the PAA-adsorbed nanostrands is similar to that of the non-adsorbed nanostrands. The PAA concentration has a great effect on the membrane structure. At a high PAA concentration, the deposition of PAA-CuII nanogels possibly affects the properties of the PAA membrane. Meanwhile, these membranes are thicker than those of non-adsorbed nanostrands, and the thickness gradually increases with the PAA concentration (Figure 1 b, Figure S5). In the membrane fabrication, Norrish I a-cleavage of the carboxylic acid groups occurred during UV-irradiation for 15– 60 min in an air atmosphere. The broken groups can then either recombine, degrade the polymer through b-scission, or form inter- and intra-molecular cross-links (Figure S6).[28] As a result, the insoluble cross-linked PAA network was formed between PAA-adsorbed nanostrands. After that, the nanostrands were dissolved quickly by filtering a dilute aqueous HCl solution to form the ultrathin PAA nanoporous membrane. Finally, the 28 nm-thick PAA membrane was successfully fabricated via the PAA-adsorbed nanostrand membrane from the 7.5 mg mL¢1 PAA solution (Figure 1 c,d, Figure S7). This membrane has a smooth surface and a uniform thickness. Other membranes were also facilely obtained using the PAA solution at different concentrations. From the membrane structure, the ultrathin PAA membrane covered on a porous support should have an ultrahigh water flux in pressure-driven filtration. To evaluate the separation performance of the as-fabricated PAA nanoporous membranes, the pure water fluxes and rejections for 10 nm gold nanoparticles and ferritin (~ 12 nm in diameter) were measured by filtration experiments in a deadend mode. As presented in Table 1, the membranes have high water fluxes and good rejections for ferritin and 10 nm gold
nanoparticles. The PAA concentration has a great influence on the membrane separation performance. As the PAA concentration increases, the water flux first decreases from 1940 to 1350 L m¢2 h¢1 bar¢1, then suddenly rises to 2270 L m¢2 h¢1 bar¢1 (10.0 mg mL¢1 PAA), and finally continually decreases to 1370 L m¢2 h¢1 bar¢1. This phenomenon resulted from the membrane structure. As described above, a self-assembled PAA layer was formed by the assembly of PAA chains on the nanostrand surface and became thicker with increasing PAA concentration, producing a compact PAA film. This explains why the water flux first decreases and rejection increases. However, numerous PAA-CuII nanogels are formed at 10.0 mg mL¢1 PAA, leading to the formation of a highly porous membrane with fast water permeation. Moreover, the nanogels would form a packed layer on the PAA-adsorbed nanostrand layer in the membrane preparation, which made the membrane thicker leading to a decrease in the water fluxes. Meanwhile, this packed layer can be easily cracked to form some defects on the membrane in the fabrication process, resulting in a decrease in rejection (Table 1). The membrane prepared from 7.5 mg mL¢1 PAA has a water flux of 1350 L m¢2 h¢1 bar¢1 and a 10–12 nm cut-off. The UV cross-linking time also has a significant influence on the membrane separation performance (Figure 4 a). The UV cross-linking would make PAA stable in an aqueous solution after the removal of nanostrand templates. The water flux de-
Table 1. Separation performance of the ultrathin PAA membranes at neutral conditions. PAA [mg mL¢1]
Water flux [L m¢2 h¢1 bar¢1]
Rejection [%] 10 nm gold
Ferritin
2.5 5.0 7.5 10.0 12.5 15.0
1940 1530 1350 2270 1690 1370
59.2 65.6 77.9 83.0 69.4 46.8
85.6 88.3 93.5 88.5 83.6 53.4
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Figure 4. Separation performance of the ultrathin PAA nanoporous membranes: (a) the effect of cross-linking time, (b) the pH dependence of the pure water flux.
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Communication creases and rejection increases with the cross-linking time. However, the water flux increases and rejection decreases when the cross-linking proceeded for 60 min. This is because a high cross-linking density leads to a small extension of the PAA chains in an aqueous solution. As a consequence, the highly cross-linked membranes have bigger pores than the lowly cross-linked membranes. Typically, the conditions of 7.5 mg mL¢1 PAA solution and UV cross-linking for 45 min are favorable for the fabrication of an ultrathin PAA membrane with a good separation performance. The resulting 28 nmthick membrane has a good rejection for ferritin. A rejection of 93.5 % was calculated based on the ferritin concentration in the filtrate (and 90.7 % from the concentrate), suggesting that this membrane has a good fouling resistance that is expected in protein separation by ultrafiltration. Furthermore, this membrane is stable in the operation for more than 5 days by using the apparatus of pressure-driven filtration (Figure S8), showing its great potential for industrial processes. PAA is one of the most important pH-sensitive materials and widely used in separation membranes.[1] Here the pure water flux through the ultrathin PAA nanoporous membranes was examined in the pH range of 2.0–7.0 at a pressure difference of 80 kPa. These membranes have a significant pH-sensitivity between pH 4.0–5.0 (Figure 4 b). Typically, for the 28 nm-thick membrane, the flux decreases with pH from 3740 (pH 2.0) to 2910 (pH 4.0), 1640 (pH 5.0) and 1350 L m¢2 h¢1 bar¢1 (pH 7.0) with a sharp decrease at pH 5.0 (1270 L m¢2 h¢1 bar¢1). Carboxyl groups are protonated and hydrophobic interactions dominate at a low pH, leading to volume shrinkage of PAA, whereas they dissociate into carboxylate ions at a high pH leading to the swelling of PAA due to the high charge density in PAA, as shown in Figure 4 b (insets). Based on the configurational change of PAA chains, the membrane pore sizes can be adjusted.[1] Therefore, the as-fabricated ultrathin PAA nanoporous membranes have good separation performances and pH-sensitivity during pressure-driven filtration. In summary, we have reported a facile and flexible route for the fabrication of ultrathin pH-sensitive nanoporous membranes by PAA self-assembly in the copper hydroxide nanostrand solution. PAA-adsorbed nanostrands and PAA-CuII nanogels are formed simultaneously in the self-assembly process. The former gradually grow in diameter ranging from 2.5 to 5.0 nm with the PAA concentration, whereas the latter has a sudden increase in quantity at a turning point (10.0 mg mL¢1 PAA) and has a similar diameter of 2.0–2.5 nm. The PAA-adsorbed nanostrands were used to fabricate ultrathin PAA membranes on a porous support. The resulting membranes have a smooth surface and a uniform ultrathin PAA nanoporous layer. They show ultrafast water permeation and good separation performance during pressure-driven filtration, and particularly have a significant pH-sensitivity between pH 4.0 and 5.0. The newly developed pH-sensitive nanoporous membranes may find a wide range of applications such as controlled release and selective separation.
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Experimental Section Fabrication of ultrathin PAA membranes: The copper hydroxide nanostrand solution was prepared by the approach reported previously.[23] A 1 mm aqueous 2-aminoethanol solution was mixed with an equal volume of a 4 mm copper nitrate solution followed by standing at 20 8C for 7 days to form copper hydroxide nanostrands. Subsequently, 8 mL of the nanostrand solution was quickly mixed with 4 mL of a very dilute PAA solution (2.5–15.0 mg mL¢1, Mw 1800) with pH ~ 6.8 and aged for 30 min, forming a PAA-adsorbed nanostrand solution. Then, 8 mL of the resulting solution was filtered on a 200 nm-pore polycarbonate (Whatman) filter placed on a glass filter holder at a suction pressure of 80 kPa, and subsequently cross-linked using a hand-held EF-180/FA UV lamp (Spectronics Corporation). After that, the nanostrands were dissolved by filtering an aqueous HCl solution of pH 2, finally forming an ultrathin PAA nanoporous membrane. Filtration experiments: Ultrafiltration experiments were performed using a glass filter holder with a membrane area of 2.84 cm2 at a suction vacuum pressure of 80 kPa. Pure water flux (J, L m¢2 h¢1 bar¢1) of the PAA membrane was measured by filtering 100 mL of water across the membrane and calculated by using the following equation: J ¼ V=ðAtpÞ
ð1Þ
where V is the volume of the water filtered (L), A is the effective membrane filtration area (m2), t is the filtration time (h), and p is the suction pressure across the membrane (bar). To evaluate membrane separation properties, the rejection of ferritin (from equine spleen, Sigma–Aldrich) and 10 nm gold nanoparticles (British Biocell International) was measured by filtering their solution across the membrane. The feed, the filtrate and the concentrate were characterized by using a UV-vis spectrophotometer. The rejection (R, %) was calculated by R ¼ ð1¢C p =C f Þ 100 %
ð2Þ
where Cf and Cp are the concentrations of ferritin or 10 nm gold nanoparticles in the feed and the permeate, respectively. Characterization: the structure of the PAA-adsorbed nanostrands was characterized by TEM (JEM-1400, 100 kV, Japan). The samples were prepared by dropping their dispersion on a copper grid coated with a carbon film. Meanwhile, the morphology of the membranes and PAA-adsorbed nanostrands was observed by SEM (ZEISS SIGMA, Germany). The samples for cross-sectional observation were prepared by freeze-fracturing in liquid nitrogen. All of the samples were coated with a ~ 5 nm thick platinum layer using a JFC-1600 auto-fine coater before observation to improve their conductivity.
Acknowledgements The research was supported by National Nature Science Foundation of China Grant (No. 21306155), the research fund for the Doctoral Program of Higher Education (No. 20120121120013) and the Fundamental Research Funds for the Central Universities (No. 2012121029).
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[16] Y. Han, Z. Xu, C. Gao, Adv. Funct. Mater. 2013, 23, 3693. [17] X. Peng, J. Jin, Y. Nakamura, T. Ohno, I. Ichinose, Nat. Nanotechnol. 2009, 4, 353. [18] Q. Wang, S. Samitsu, I. Ichinose, Adv. Mater. 2011, 23, 2004. [19] H. Ma, K. Yoon, L. Rong, M. Shokralla, A. Kopot, X. Wang, D. Fang, B. S. Hsiao, B. Chu, Ind. Eng. Chem. Res. 2010, 49, 11978. [20] H. Ma, K. Yoon, L. Rong, Y. Mao, Z. Mo, D. Fang, Z. Hollander, J. Gaiteri, B. S. Hsiao, B. Chu, J. Mater. Chem. 2010, 20, 4692. [21] Q. Zhang, S. Ghosh, S. Samitsu, X. Peng, I. Ichinose, J. Mater. Chem. 2011, 21, 1684. [22] M. Nagale, B. Y. Kim, M. L. Bruening, J. Am. Chem. Soc. 2000, 122, 11670. [23] C. Deng, Q. G. Zhang, G. L. Han, Y. Gong, A. M. Zhu, Q. L. Liu, Nanoscale 2013, 5, 11028. [24] S. Karan, Q. Wang, S. Samitsu, Y. Fujii, I. Ichinose, J. Membr. Sci. 2013, 448, 270. [25] F. T. Wall, S. J. Gill, J. Phys. Chem. 1954, 58, 1128. [26] P. Schuetz, F. Caruso, Adv. Funct. Mater. 2003, 13, 929. [27] S. Chen, J. M. Sommers, J. Phys. Chem. B 2001, 105, 8816. [28] A. Gestos, P. G. Whitten, G. M. Spinks, G. G. Wallace, Soft Matter 2010, 6, 1045.
Received: February 4, 2015 Revised: February 27, 2015 Published online on March 17, 2015
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