Article pubs.acs.org/Langmuir
Ion Exchange and Antibiofouling Properties of Poly(ether sulfone) Membranes Prepared by the Surface Immobilization of Brønsted Acidic Ionic Liquids via Double-Click Reactions Zhuan Yi, Cui-Jing Liu, Li-Ping Zhu,* and You-Yi Xu MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China S Supporting Information *
ABSTRACT: Brønsted acidic ionic liquids (BAILs) are unique ionic liquids that display chemical structures similar to zwitterions, and they were typically used as solvents and catalysts. In this work, an imidazolebased BAIL monolayer was fabricated onto poly(ether sulfone) (PES) membranes via surface clicking reactions, and the multifunctionality, including ion exchange and biofouling resistance to proteins and bacteria, was demonstrated, which was believed to be one of few works in which BAIL had been considered to be a novel fouling resistance layer for porous membranes. The successful immobilization of the BAILs onto a membrane surface was confirmed by X-ray photoelectron spectroscopy analysis, contact angle measurement, and ζ potential determination. The results from Raman spectroscopy showed that, as a decisive step prior to zwitterion, the BAIL was deprotonated in aqueous solution, and biofouling resistance to proteins and bacteria was found. However, BAIL displayed ion exchange ability at lower pH, and surface hydrophilicity/hydrophobicity of membranes could be tuned on purpose. Our results have demonstrated that the BAIL grafted onto membranes will not only act as an antibiofouling barrier like zwitterions but also provide a platform for surface chemical tailoring by ion exchange, the property of which will become especially important in acidic solutions where the fouling resistance performances of zwitterions are greatly weakened.
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INTRODUCTION Synthetic membranes are playing an increasingly important role in the field of separation, and polymeric membranes over many other filters are accepted as new, effective, and energy-saving materials that have been found wide applications in seawater desalination, drinking water sterilization, protein separation, blood dialysis, and biosensing.1−3 However, membrane fouling, especially biofouling during filtration and storage, has greatly restricted the widespread application of the polymer membrane as an efficient technology in liquid separation. Biofouling is generally associated with protein adsorption and bacterial adhesion under wet conditions, and it remains a great unsolved challenge, although great efforts have been devoted to dealing with this problem in past decades. As found in previous publications, surface characteristics inducing the roughness, surface hydrophilicity/hydrophobicity, toxicity, and surface charge property are critical factors that have been determined to effect grealty the fouling resistance performance of polymeric membranes.4 Among diverse methods, the strategy of surface modification has been developed as one of the most effective processes to regulate the surface characteristics of separation membranes without deterioration their bulk structures, and polymer membranes with excellent fouling resistance can be facilely fabricated from the method of surface modification. © XXXX American Chemical Society
Zwitterionic materials have attracted much attention in recent years as the next-generation high-performance fouling resistance materials because of their amazing hydration capability and nonfouling characteristics.5−10 Zwitterionic materials carrying both positive and negative charges in the same unit had well mimicked the chemicals that were generally found in animal cytomembranes, and their biocompatibility and fouling resistance become easily understood from the aspect of bionics. They are in general electrically neutral and able to capture a great deal of water molecules by ionic solvation, and well-defined physicochemical properties of zwitterions also meet the requirements suggested by Whitesides for a foulingresistant material.11,12 In addition to being widely used as biomaterials, more and more investigation had indicated that zwitterionic materials were also excellent candidates for fouling resistance coatings for filtration membranes, and great achievements have been realized in this area.13−15 Similar to zwitterionic materials, Brønsted acidic ionic liquids [BAILs (Scheme 1)] are charged chemicals exhibiting unique physiochemical properties, and they are readily synthesized from the protonation of corresponding zwitterions with nearly Received: February 2, 2015 Revised: May 2, 2015
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DOI: 10.1021/acs.langmuir.5b00420 Langmuir XXXX, XXX, XXX−XXX
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Transformation between BAIL and zwitterion is reversible. It is found from previous works that some well-defined zwitterions can be synthesized from BAILs in the presence of organic or inorganic bases.23,24 Just as many organic acids, free acid groups in BAIL can be ionized under aqueous conditions (namely deprotonation), which means that the dissociation of the free acid groups from BAIL in aqueous solution will produce the same chemical that will be also found in the presence of bases. This facile transformation endows BAIL with additional performance more than catalysts and solvents that have been previously defined in the bulk state. Moreover, the degree of ionization of the free acid substitutes can be regulated by external conditions, and the reversible transition between zwitterion and BAIL is controllable. As expected, the controlled dissociation of the acid substitutes may bring BAIL some unique properties that integrate the well-defined characteristics of an ionic liquid and zwitterion. To the best of our knowledge, compared with zwitterion, BAIL has not been widely considered as a fouling resistance material for synthetic membranes before, and any method for tethering BAIL onto membranes has not been published. To incorporate BAIL onto polymeric membranes, we tried to tether polyBAIL brushes on a membrane surface via surfaceinitiated “living” free radical polymerization that had proven their effectiveness in surface functional design. However, this attempt failed, and BAIL brushes could not grow from a membrane surface because of the low reactivity of imidazoletype monomers as well as the disturbance from free acid groups.25,26 As an alternative method, click chemistry is employed herein to link BAIL monomers to a poly(ether sulfone) (PES) ultrafiltration (UF) membrane that had been seeded with reactive ene groups. The preparation of a PES membrane with reactive ene groups and immobilization of a BAIL monolayer on a membrane surface via two-steps click reactions that are shown in Scheme 2. Typically, a pre-designed PES polymer containing pendant ene groups was first
Scheme 1. Chemical Structures of BAIL with Imidazole Cations and Carboxylic Acid Substitutes (A) as Well as the Corresponding Zwitterionic Compound (B)
100% atom efficiency.16 In the bulk state, BAIL displays characteristics identical to those of common ionic liquids, and it is known as zwitterionic molten salt (ZMS) in some of the literature, which serves as strong evidence that BAIL has shared close relationships with both zwitterions and ionic liquids.17 Despite the similar chemical structure of BAIL and zwitterion, their applications are entirely different. Unlike common neutral ionic liquid, BAIL can be dually used as the solvent and catalyst in typical acid-promoted organic reactions,16 and free acid substitutes in BAIL are interpreted as the functional groups that are responsible for their catalytic activity.18,19 However, this property has never been defined for zwitterions. More interestingly, functional BAILs with giant hydrophobic substitutes will assemble into amazing structures that display a selective permeability to various ions, and they are potentially new candidates for nanofiltration material.20 Anisotropic proton-conductive materials as well as a three-dimensional water nanosheet can be also fabricated from the self-assembly of functional BAIL.21,22 Although the application of BAIL is obviously different from that of zwitterion, considering the similarity in structure between BAIL and zwitterion, it is reasonable to assume that BAIL may be a novel foulingresistant material like zwitterion under proper conditions, and the potential application of BAIL as fouling resistance material is a new area that has not been defined for BAIL to the best of our knowledge.
Scheme 2. Illustration of the Fabrication of Membranes with Ene Groups and the Following Thiol-ene Click Reaction
B
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lithium bis(trifluoromethane sulfonimide) (TFSILi) [the pH values of solutions were adjusted to 1 to enhance the anion exchange (see Ion Exchange Characteristics of BAIL in Solution)]. Anion exchange was conducted at room temperature for 0.5 h, and the resultant membrane was taken out and washed thoroughly with deionized water. Wet membranes were finally dried in a vacuum oven at the room temperature for further characterization. Raman Spectroscopy. Raman spectra of the BAIL monomers were recorded using a Bruker IF S66 FRA-106 Fourier transform spectrometer with a Raman attachment. The excitation source was a 1064 nm near-infrared Nd:YAG laser with a nominal power of 500 mW, and the visible laser light (green at 514.5 nm or red at 784 nm) will lead a strong fluorescence. FT-Raman spectra were collected over the range from 3500 cm−1 (Stokes) to 400 cm−1 with a resolution of 4 cm−1 from 64 scans. The samples were prepared by dissolving BAIL monomers into ultrapure water with a weight concentration of 40 wt %, and rather weak Raman absorption will be found if concentration of BAIL in solution is lower than this critical value. Surface Characterization. The surface compositions of asprepared membranes were analyzed by X-ray photoelectron spectroscopy (XPS) (PHI 5000 C ESCA system, PHI Co.) with an electron takeoff angle of 60° relative to the sample. Contact angle measurement was conducted with an OCA20 contact angle system (Dataphysics Instruments, GmbH) in the sessile mode. The reported values are averaged from five determinations conducted at random locations. ζ Potential Determination. The ζ potential of membranes before and after surface modification was determined via a streaming potential method on the electro-kinetic analyzer (SurPASS Anton Paar, GmbH). The pH of the solution was autotitrated from 3 to 10 and measurements were performed at a constant temperature of 25 °C. The ζ potentials were calculated according to the Helmholtz− Smoluchowski equation that has been described in the literature.30,31 Protein Adsorption Evaluation. The confocal laser scanning microscope was used to image the adsorption of fluorescein isothiocyanate (FITC)-labeled protein on a membrane surface. The bovine serum albumin (BSA) was labeled by fluorescein (FITC-I) in a phosphate-buffered solution (PBS, pH 7.4) following a reported procedure,15 and labeled BSA was then diluted to a concentration of 20 μg/mL for subsequent usage. In a typical procedure of protein adsorption, clean membrane samples with an area of 10 cm2 were immersed in the FITC-labeled BSA solution (10 mL) and kept at 25 °C for 8 h in darkness. After the adsorption, the membrane was taken out, washed gently with PBS, and observed with an Olympus BX51 fluorescence microscope with the pinhole of 1000 μm at a wavelength of 488 nm. The power, amplification factor, and diameter of the laser remained unchanged for the parallel samples. Quantitative protein adsorptions were determined with an aid of an UV−vis spectrophotometer, with the absorption of the FTIC-1 as an internal standard for labeled proteins. The amounts of free proteins reserved in solution were calculated from an intensity decrease in UV absorption at a wavelength of 490 nm. The adsorption of lysozyme (Ly) on membranes was obtained following procedures identical to those applied to BSA. Bacterial Adhesion. Two typical bacterial species, Staphylococcus epidermidis (ATCC catalog no. 12228) and Escherichia coli (ATCC catalog no. 11775), were used in the bacterial adhesion experiments. In a typical process, a membrane sample with an area of 2 cm × 2.5 cm was placed in a particular medium inoculated with 7.6 × 107 colonyforming units (CFU)/mL of bacteria for 24 h at 37 °C while being constantly swirled. After a predetermined period, the membrane was taken out and rinsed with fresh PBS to remove the free bacteria. Then the membrane was treated using a 3.0 wt % glutaraldehyde solution for 3 h at 10 °C. The fixed bacteria were finally observed with a scanning electron microscope (SEM). The bacteria in solution were counted after culturing the live bacteria for 24 h at 37 °C. The antibacterial efficiency was assessed according to the liveability of the bacteria in solution.
synthesized and blended into PES membranes. The BAIL monomers carrying ene groups were then connected to a membrane surface via the sequential thiol-ene click reaction using 1,3-dimercaptopropane (DCP) or α,ω-dimercapto poly(ethylene glycol) (DPEG) as the spacer, and highly effective dimethylphenylphosphine (Me2PPh) was used as the nucleophilic catalyst. The hydrophilicity/hydrophobicity, antibiofouling properties, surface ζ potential, and ion exchange performance of the modified PES membranes were investigated in detail. We will demonstrate in this work that a novel and functional surface exhibiting both fouling resistance and tunable hydrophilicity can be fabricated when BAIL is considered as a new upper layer for porous membranes.
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EXPERIMENTAL SECTION
Materials. PES with pendant reactive amino groups (PES-NH2) was synthesized following the procedure described in our previous work.27 PES resin (A100, REDAL) with a number-average molecular weight of 26000 g/mol was supplied by Solvay Chemicals. Commercially available N,N-dimethylacetamide (DMAc) was used as a solvent for membrane fabrication. α,ω-Dimercapto poly(ethylene glycol) (Mn = 400 g/mol) was synthesized using poly(ethylene glycol) (PEG400) and 3,3′-dithiodipropionic acid as the starting materials according to the process described in ref 28. Two BAILs containing ene groups (shown in Scheme 1), 1-vinylimidazole-3-carboxypropyl bromide (n = 1; BAIL-C3) and 1-vinylimidazole-3-carboxyvaleric bromide (n = 3; BAIL-C5), were synthesized via a well-defined quaterization reaction.29 The synthesis process and the detailed characterizations of monomers are shown in the Supporting Information (S1 and S3−S5). Dimethylphenylphosphine (Me2PPh), 1,3-dimercaptopropane (DCP), and other reagents were all purchased from Sigma-Aldrich and used directly without further purification. Fabrication of Blend Membranes with Reactive Ene Groups. The PES polymer with pendant ene groups was synthesized through the amidation between PES-NH2 and methacrylol chloride, as described in the Supporting Information, S2. Then the blend membrane of PES and PES carrying ene groups was prepared following the conventional immersion−precipitation phase inversion process as reported in ref 27. PEG400 was used as a pore-forming agent and pure water as a coagulant. The composition of the casting solution for membrane preparation is shown in Table 1.
Table 1. Compositions of Casting Solution for the Preparation of Blend Membranes PES (wt %)
PES with ene groups (wt %)
PEG400 (wt %)
DMAc (wt %)
15.3
2.7
8.0
74.0
Protocols for Surface Clicking Reactions. As shown in Scheme 2, BAIL monomers were anchored to the as-fabricated PES membrane via a sequential thiol-ene click reaction using the dimethylphenylphosphine (Me2PPh) as a catalyst. In a typical procedure, a PES membrane with the size of 5 cm × 5 cm (∼100 mg) was immersed in a solution containing 75 mL of anhydrous acetonitrile and a predetermined DCP or DPEG. The oxygen in the reaction vial was removed by nitrogen charging for 60 min. The first clicking reaction lasted for 6.5 h after the addition of 100 μL of Me2PPh using a syringe. Afterward, 1.5 g of BAIL monomers was added, and the second clicking reaction was kept for an additional 6.5 h. The reaction was terminated by transferring the membrane to fresh acetonitrile, and unreacted monomers were washed away. The mole ratio of BAIL to DCP (or DPEG) was set constant at 1.5:1.0. Compared to reactive ene groups on the membrane surface, the spacer and BAIL monomer in the reaction solution were excessive and the surface clicking was enhanced. Surface Anion Exchange. Ion exchange was conducted on BAILimmobilized membranes in a salt solution containing 30 mg/mL C
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Figure 1. XPS wide scan spectra (left) and the high-resolution N 1s spectra (right) of the investigated membranes DCP, DPEG-C3, DCP-C5 and DPEG-C5.
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RESULTS AND DISCUSSION Membranes Surface Chemistry. XPS was used to inspect the surface compositions of the PES membranes modified by DCP, DPEG-C3, DCP-C5, and DPEG-C5. The DCP showed herein represents the modified membrane tethered with DCP after the first thiol-ene click reaction. Sample DPEG-C3 denotes the membrane immobilized with BAIL-C3 using DPEG as the spacer by sequential thiol-ene click reactions, and two other membranes can be interpreted similarly. The XPS determination is conducted with a takeoff angle of 60°, and it corresponds to a detecting depth of 3−5 nm in the outer surface; evolution of surface composition will become attainable in this range if the clicking reaction has been conducted successfully. The whole spectra shown in Figure 1 (left) demonstrate this explanation, and the intensity of N 1s that appeared in the spectra displays an obvious increase after BAIL immobilization. Quantitative analysis indicates that the elemental fraction of N 1s in DPEG-C3, DCP-C5, and DPEGC5 increased to 8.2, 14.7, and 15.3%, respectively. However, the control sample with only DCP on the surface shows a much lower nitrogen concentration of ∼7.1%. Because the binding energy of N 1s from imidazole cations is distinctly different from that of uncharged amide bonds, the N 1s peak is used as the internal standard to analyze the grafting yield of BAIL on membranes. The high-resolution spectra for N 1s are shown in Figure 1 (right). For the control membrane DCP, the N 1s peak observed at around 399.5 eV can be assigned to the uncharged N of amide bonds from seeded PES with pendant ene groups, and this signal will be found in all other membranes because BAIL monomers are tethered from these reactive sites. After BAIL-C3 or BAIL-C5 was grafted, the integrated N 1s peak can be fitted into two smaller peaks that correspond to the uncharged N (399.5 eV) and charged N+ (402.5 eV), respectively.32 From the quantitative analysis of the fitted spectra, it can be calculated that mole fractions of charged N+ from imidazole cations in the whole N element are 39.6, 61.6, and 65.8% for membranes labeled as DPEG-C3, DCP-C5, and DPEG-C5, respectively. Obviously, these fitted results demonstrate again that BAIL has been immobilized successfully onto PES membranes via the designed clicking reaction.
Theoretically, the amounts of BAIL immobilized onto the membrane surface will be equivalent to that of active ene groups if ene groups on the membrane surface are 100% activated, and mole fraction of N+ in the whole N element on the BAIL-immobilized membrane surface will reach 66.7% under this ideal condition. The determined N+ mole fraction (65.8%) in sample DPEG-C5 is very close to the theoretical value of 66.7%, and the results clearly indicate that nearly 100% activation of the ene groups on the PES membrane surface has been achieved on this membrane, perhaps because of excessive feeds of the reactive monomers as well as the high catalytic activity of Me2PPh.33 As a result, a well-defined BAIL monolayer for C5 monomers has been fabricated on the PES membranes by the designed sequential thiol-ene clicking reactions. The grafting yield of BAIL-C3 much lower than that of BAIL-C5 indicates that the BAIL-C3 monolayer has not yet been achieved. However, the successful fabrication of a BAIL-C5 monolayer on a membrane surface is still the first example in which thiol-ene click chemistry has been applied to the surface immobilization of BAIL monomers. Under the same reaction conditions, the detected grafting yield of BAIL-C3 that is lower than that of BAIL-C5 can be interpreted as follows: stronger dissociation of -COOH in BAIL-C3 than in BAIL-C5 will lead stronger side reaction, and the ideal clicking process has been disturbed. Actually, the Me2PPh is commonly used as a highly efficient catalyst for the thiol-ene clicking reaction, and it takes advantage over organic amines as the catalyst for the RCOOH participated click reaction because it is less basic but more nucleophilic than amines of an identical substitution pattern.34 Under the same conditions, the catalyst of isopropamide leads to a much lower grafting yield and poorer hydrophilicity that is readily determined by contact angle measurement (not shown). This means a well-defined BAIL monolayer for BAIL-C3 will be fabricated only with an alternative catalyst that is more suitable for the BAIL-C3 monomer. However, the successful fabrication of the BAIL-C5 monolayer for either spacer within the setting time period indicates the Me2PPh is suitable for the BAIL-C5 monomer. As described in the literature, the thiol-ene clicking reaction for small organic molecules catalyzed by Me2PPh generally reaches completion in minutes. Our reaction time setting for surface D
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membranes with only spacers on them. It is worth noting that the BAIL-C5-immobilized membranes display smaller contact angles in comparison with those of the corresponding BAIL-C3-immobilized membranes. This phenomenon can be assigned to the higher grafting yield of BAIL-C5 than that of BAIL-C3, as verified from the XPS results. In addition, the dynamic contact angle results, including the advancing and receding angles, also proved that better hydrophilicity was achieved when BAIL monomers were immobilized, demonstrating again the successful tethering of BAIL onto membranes. More importantly, it is found that the BAIL monolayer on the membrane surface displays good stability, and hydrophilicity changes slightly for membranes either exposed in air or treated with hot water; covalent tethering of BAIL on membranes can be interpreted as an important reason for this observation (Supporting Information, S6). Actually, the stability of BAIL on a membrane surface will have a great impact on its long-term application, and modification will become inefficient if BAIL is easily lost from membranes. TFSILi, which carries a large hydrophobic anion, is employed as an anion exchange agent, and surface anion exchange is conducted to demonstrate the intactness of the chemical property of the BAIL tethered on a membrane surface. The water contact angles of the anion-exchanged membranes were measured, and the results are shown in Figure 3. It can be found that the contact angles for all the investigated membranes subject to ion exchange have increased to more than 70°, indicating that the hydrophobicity has been increased obviously after this facile operation. This phenomenon proves once again the successful surface immobilization of BAIL by double thiol-ene clicking reactions. More importantly, the surface-immobilized BAILs hold ion exchange ability as observed in their solution states, which actually provides a useful method for regulating the surface hydrophilicity/ hydrophobicity of the membrane by BAIL immobilization and further ion exchange, and hydrophobic low-surface energy anions containing F are suitable candidates when fouling and release performance is anticipated. It is worth noting that the surface ion exchange can be conducted with but not limited to TFSI−, and these hydrophobic anions, including TFSI−, will show quantitative ion exchange at a fast rate in aqueous solutions. The surface chemistry of the membrane suffering from anion exchange was determined by XPS, and the wide scan is shown in Figure 4. Compared to the membrane before the surface ion exchange, the F element (BE = 689 eV) was noticeably detected on the modified membrane when surface anion exchange was accomplished. This result shows that the TFSI− anions have been successfully incorporated onto the BAIL-
clicking reaction is much longer and can be decreased to improve the efficiency and to obtain different grafting yields. It is noteworthy that the permeability and rejection of the modified membranes remain almost unchanged after the surface clicking reaction (Supporting Information, S6), indicating that the surface clicking reaction is an effective method to bring membrane new surface chemistry without deterioration of pore structure, and the reason can be assigned to the mild reaction condition of the click chemistry and the monolayering of BAIL on the surface. The intact surface structure of membranes before and after modification has been confirmed by SEM observation (Supporting Information, S7). Membranes Surface Hydrophilicity/Hydrophobicity. The successful immobilization of BAIL onto membranes was further analyzed by water contact angle measurements, and the static and dynamic contact angles are shown in Figure 2. The
Figure 2. Water contact angles of the as-prepared PES membrane without any modification (control), the spacer-tethered PES membranes (DCP and DPEG), and the BAIL-immobilized PES membranes (DCP-C3, DCP-C5, DPEG-C3, and DPEG-C5).
ene-seeded PES blend membrane was used as a control sample, and its static contact angle was ∼80°. However, the contact angles decrease obviously to ∼70° when spacers were immobilized (for both the DCP and DPEG). Under the catalysis of Me2PPh, the second “thiol-ene” click reaction was conducted and the BAIL monomer was further tailed to the spacers that had been immobilized in the first “thiol-ene” click reaction. As shown in Figure 2, the contact angle of the BAILimmobilized membranes (DCP-C3, DCP-C5, DPEG-C3, and DPEG-C5) decreased further compared to those of the
Figure 3. Static water contact angles of membranes DCP-C5, DPEG-C3, and DPEG-C5 before and after anion exchange with TFSILi. E
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Figure 4. XPS wide scans of the membrane before (bottom) and after (above) ion exchange. The membrane functionalized with DPEG-C3 serves as a representative example.
with TFSI− anions to generate oil-like hydrophobic ionic liquid in aqueous solution. This observation shows that the BAILs have been partially but not completely transformed into zwitterions in an aqueous environment, considering that welldefined ion exchange is impossible for zwitterion because of the covalent bonding of the cation and anion in zwitterion.35 Interestingly, after the pH of the BAIL solution had been adjusted to neutral (7), the water-insoluble oil-like liquid was not produced. This result can be explained by the enhanced dissociation of carboxyl groups at a pH higher than the pKa (the pKa of BAIL-C3 is ∼3.5, and that of BAIL-C5 is ∼4.4) and the possible transition of BAIL into zwitterions.24 As the pH of the aqueous solution containing BAIL was adjusted back to an acidic value (pH 1.0), the oil-like liquid was generated again. Obviously, these results indicate that the chemical state of BAIL in aqueous solution is pH-dependent, and the possible transitions between BAIL and zwitterion is summarized in Figure 5. To further explore the chemical characteristics of BAIL monomers in aqueous solution, their Raman−IR spectra were collected and are shown in Figure 6. The characteristic scatterings at 1342, 1373, and 1425 cm−1 can be ascribed to the signals from imidazole cations that have been well-defined, and the vibrations at 2897, 2933, 2980, 3079, and 3105 cm−1 are attributed to the absorptions from -CH2- and -CHCH2 groups.36,37 These signals verify the chemical structures of
immobilized PES membrane during anion exchange. Generally, ion exchange is a characteristic of ionic liquids (i.e., BAIL), but the zwitterion does not show this property. The successful anion exchange confirmed by XPS has proven that the intrinsic chemical property of the BAIL tethered onto the membrane is reserved. However, unlike well-defined ionic liquids having no active acid groups, BAIL will transit into zwitterions at a pH higher than the pKa, and the performance of BAIL in solutions with higher pH values is totally different. Chemical Transitions between BAIL and Zwitterion. As discussed above, the anion exchange ability of BAIL offers an alternative method for designing and modulating the surface chemistry of the PES membrane by surface ion exchange. To improve our understanding of the anion exchange characteristics of BAIL, the performance of free BAIL in aqueous solution was investigated. As described in the Introduction Section, the dissociation of acid groups in BAIL is thought to be a key step that induces the transformation of BAIL into zwitterion basing on the opposite transition from zwitterion to BAIL.16 It is determined that the pH values of aqueous solutions containing BAIL-C3 and BAIL-C5 were 1.9 and 3.2, respectively (10 mmol/L), which clearly indicates that the BAIL in aqueous solutions will dissociate. As shown in Figure 5, the BAIL dissolved in water solutions are still able to combine
Figure 5. Reversible transition between BAIL and zwitterions in aqueous solutions (above) as well as the viewable generation of the water-insoluble liquids after an ion exchange (bottom).
Figure 6. Raman spectra of BAIL-C3 and BAIL-C5 at various pH values. F
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Figure 7. SEM images of bacterial adhesion (first and second rows) and fluorescent optical images of protein adsorption (third and fourth rows) on the modified PES membranes.
inspected at lower pH values (pH < pKa). It is believed that BAILs with this unique property will provide porous membranes with some special characteristics that are different from those of either the well-defied ionic liquid or zwitterions. In the following section, the antibacterial and protein-resistant performances of the modified PES membranes are investigated, and the results are shown in Figure 7. From the SEM images shown in the first and second rows of Figure 7, it can be found that of many Gram-negative bacteria, E. coli adhered to the control sample with only DCP tethered on the surface, and the morphologies of the bacteria remain intact, indicating the low antibacterial efficiency of the DCP spacer. After the BAIL-C5 was introduced, the adhesion and propagation of E. coli on the membrane surface were greatly restrained. Similar phenomena are also observed in the culture of the Gram-positive bacterium S. aureus on the membrane surface. These results demonstrate that the membranes modified by BAIL exhibit excellent antiadhesion and antibacterial ability. Although PEG is reported to be an outstanding fouling resistance material, it is found that only PEG with molecular weights higher than 1000 Da will show acceptable antibacterial performance (not adhesion), and the PEG spacer applied in our work is much smaller than the critical length.39 Therefore, the antiadhesion performance of the BAIL-modified PES membranes is reasonably ascribed to the immobilized BAIL layer, rather than the linked PEG spacer. This result is readily observed for the membrane modified by DPEG-C3 and DPEG-C5. The antibacterial performance can be learned from two related aspects: one is preventing the adhesion of bacteria on the surface, and the other is killing the bacteria in contact with
BAIL-C3 and BAIL-C5 monomers. From the spectra of BAILC3 at pH 7 and 1.9, it can be found that a strong peak at ∼1658 cm−1 and a weak peak at ∼1715 cm−1 are simultaneously inspected, which can be ascribed to the deprotonated and protonated carboxyl groups, respectively.38 This result indicates the coexistence of the deprotonated -COO− and the protonated -COOH in the BAIL-C3 monomer when it is dissolved in water. However, the peak at ∼1715 cm −1 disappears in the spectrum that is recorded at pH 10, indicating that the -COOH groups in BAIL-C3 were completely dissociated into -COO− at basic conditions, in which the ion exchange has not been inspected. Moreover, under all three pH conditions, the intensity from -COO− is much stronger than that of the COOH, demonstrating that BAIL dissociates easily when it is dissolved in water. Only 40% yield of oil-like ionic liquid precipitated at the bottom of vials demonstrates this explanation. It is worth noting that the interactions between Br− and imidazole cation can be detected in the bulk state.24 However, similar signals have not been determined in an aqueous solution, indicating the ionization or dissociation of Br− from BAIL in aqueous solution. Actually, the chemical transition of BAIL into zwitterions detected in aqueous solutions inspires us that the BAIL immobilized on the PES membrane surface will show characteristics similar to those of zwitterions considering most of the filtration membranes will be applied in aqueous solution with a pH close to 7, under which conditions the BAIL is mostly deprotonated. Antibiofouling Performance. At higher pH values (pH > pKa), the deprotonation of the carboxyl groups will lead to a useful transition from BAIL into zwitterion, and BAIL will display a property totally different from that of the performance G
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Langmuir the surface. By counting the surviving E. coil and S. aureus in the culture solutions, we find that membranes with DPEG-C3, DCP-C5, and DPEG-C5 on the surface did not show a strong capability to kill bacteria (see the results in Table 2 and
determination, and the results are shown in Figure 8. It can be seen that the control sample, the DCP-grafted PES membrane
Table 2. Protein Adsorption and Antibacterial Performance of Modified Membranes sample controla DCP DPEG DPEG-C3 DCP-C5 DPEG-C5
BSA (μg/cm2) 19.1 17.3 14.8 9.3 7.6 5.9
± ± ± ± ± ±
0.7 0.9 0.6 0.3 0.5 1.1
Ly (μg/cm2)
E. coli (%)
± ± ± ± ± ±
44.6 69.1 10.3 77.6 87.4 97.2
14.8 12.1 10.9 8.7 6.3 4.4
0.2 0.5 1.3 0.9 1.2 0.7
killed E. coli (cells/cm2)
S. aureus (%)
× × × × × ×
10.7 23.1
