Article Cite This: Langmuir 2019, 35, 9152−9160
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Electrostatic Adsorption Behavior of Zwitterionic Copolymers on Negatively Charged Surfaces Sheng-Yao Wang,† Li-Feng Fang,*,‡ and Hideto Matsuyama*,† †
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Center for Membrane and Film Technology, Department of Chemical Science and Engineering, Kobe University, Rokkodaicho 1-1, Nada, Kobe 657-8501, Japan ‡ Engineering Research Center for Membrane and Water Treatment (MOE), Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *
ABSTRACT: To investigate the effect of the surface properties and the coating layer properties on surface modification via electrostatic adsorption, the electrostatic adsorption behavior of zwitterionic copolymers on negatively charged surfaces was studied. A series of positively charged zwitterionic copolymers and a series of negatively charged surfaces, including porous substrates and dense films, were fabricated. The electrostatic adsorption behavior of the zwitterionic copolymers on the negatively charged porous substrates was confirmed using the contact angles and fluorescently labeled protein adsorption experiments. The adsorption behavior of the zwitterionic copolymers on the negatively charged dense films was confirmed using quartz crystal microbalance determination and a fluorescently labeled protein adsorption experiment. The results indicated that a lower charge density on the zwitterionic copolymer brings about a higher adsorption mass on the charged surface, whereas an extremely low charge density on the coating layer results in a lower adsorption mass on the charged surface, due to weak interaction. A high density of the film surface charge is beneficial for surface adsorption, whereas an extremely high density of the film surface charge leads to low surface adsorption due to steric hindrance of the negatively charged sites. This work provides an insight into the best strategy for surface modification via electrostatic adsorption.
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needed.8 Comparatively, the surface coating method is a commonly employed method9,10 due to its ease of operation and facile post-treatment. A series of hydrophilic chemicals with functional groups can be easily deposited on a surface via specific interactions11,12 such as hydrophobic, hydrogen bonding, and electrostatic interactions. Most types of materials carry a net charge; hence, certain substances with a contrary charge can easily adsorb on the surface via electrostatic adsorption processes.13 It can be considered that the charged surface is neutralized after adsorption,14 which is beneficial for the promotion of antifouling properties.15 In our recent work, zwitterionic copolymers with a positive charge were used to modify a negatively charged membrane, and the membrane was then endowed with excellent fouling resistance to both negatively
INTRODUCTION Fouling is the accumulation of unwanted material, including microorganisms, inorganics, and organics, on a solid surface and causes contamination, reduces the performance, and even increases the maintenance cost in various applications such as piping, flow channels, membranes, injection/spray nozzles, ship hulls, and so forth.1,2 As the surface plays a critical role in fouling because of the diversity of surface roughness, surface wettability, and surface charge, surface modification is regarded as an efficient means to reduce foulant accumulation. Two approaches are commonly used to alleviate fouling, including the use of antifouling materials and coating/grafting antifouling materials onto the traditional surface to create a smooth surface with super-low surface energy3,4 or a smooth and hydrophilic surface.5 Moreover, according to the Whitesides rule,6,7 the absence of a net charge is another important factor for reducing surface fouling. The grafting method is complicated and energy-consuming, especially when plasma pretreatment or a radiation source is © 2019 American Chemical Society
Received: March 31, 2019 Revised: June 18, 2019 Published: June 19, 2019 9152
DOI: 10.1021/acs.langmuir.9b00950 Langmuir 2019, 35, 9152−9160
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Langmuir and positively charged foulants.16 This was because the zwitterionic component could resist fouling adsorption owing to the presence of a special hydration layer formed by the zwitterion charge groups,17,18 whereas the positively charged component could weaken the charge density on the surface.16 In a previous work, it has been confirmed that surface coating via electrostatic adsorption greatly promotes the membrane properties16 and that work basically emphasizes the application of the zwitterionic copolymer. However, the detailed mechanism of the electrostatic adsorption behavior of the charged zwitterionic copolymer on the charged surface and zwitterionic copolymer was not systematically studied in that work. And there are still many factors influencing the electrostatic adsorption behavior to investigate, for example, the surface properties and the coating layer properties. In this research, we synthesized positively charged zwitterionic copolymers with different amounts of positive charge, and prepared porous surfaces and dense films with different densities of negative charge to investigate the electrostatic adsorption behavior of the positively charged zwitterionic copolymers on the negatively charged surfaces. Water contact angle measurements, fluorescently labeled protein adsorption measurements, and quartz crystal microbalance (QCM) tests were conducted to confirm the adsorption behavior. This was expected to provide an insight into the best strategy for surface modification via electrostatic adsorption.
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cast onto a clean glass plate using an applicator with the thickness of 200 μm to obtain a liquid polymeric film. The film was subsequently immersed in an aqueous coagulation bath to obtain a porous membrane. Finally, the membrane was gently rinsed by Milli-Q water for subsequent experiments. To control the amount of negative charge on the substrate surface, PES/SPES blend substrates were prepared with different blend ratios. The dope solution compositions are listed in Table 1. The substrate samples were labeled as M15-a-b, where a and b are the weight ratios of PES and SPES, respectively.
Table 1. Dope Solution Compositions for Substrate and Film Fabrication substrates/film
PES (%)
SPES (%)
DMAc (%)
M15-9-1/F15-9-1 M15-8-2/F15-8-2 M15-7-3/F15-7-3 M15-6-4/F15-6-4 M15-5-5/F15-5-5
13.5 12.0 10.5 9.0 7.5
1.5 3.0 4.5 6.0 7.5
85.0 85.0 85.0 85.0 85.0
The dense films were fabricated as follows. The casting solutions were prepared similar to those for substrate fabrication. Each casting solution was cast on a clean glass plate, using the applicator, with a thickness of 200 μm. Thereafter, the glass plate was placed in a vacuum oven for 3 days to remove the solvent and prepare the dense film. The film sample was labeled as F15-c-d, where c and d are the weight ratios of PES and SPES, respectively. The surface morphology of the porous substrates and dense films was observed using field-emission scanning electron microscopy (FESEM; JSF-7500F, JEOL Co. Ltd., Japan). The surface roughness of the samples was characterized using atomic force microscopy (AFM; SPI3800N, Hitachi Ltd., Japan) in the dynamic force microscopy (tapping) mode. The surface chemical compositions were determined using X-ray photoelectron spectroscopy (XPS; JSP-9010MC, JEOL Co. Ltd., Japan) with an Al Kα radiation source (1486.6 eV). The water contact angles were measured using a contact angle goniometer (Drop Master 300, Kyowa Interface Science Co., Japan). Synthesis and Characterization of Zwitterionic Copolymer. Zwitterionic copolymers carrying a positive charge were synthesized using SBMA and MTAC monomers via the radical polymerization method. The procedures were the same as those used in our previous study.16 The copolymers were synthesized using different reaction ratios (Table 2). The compositions of the zwitterionic copolymers were analyzed using nuclear magnetic resonance (NMR; JEOL ECZ400, Japan), with deuterated water as the solvent. The zeta potentials of the copolymers were determined using a zeta potential and particle size analyzer (Otsuka Electronics Co., Ltd., Japan). The zwitterionic copolymers were dissolved in deionized water at a concentration of 1000 ppm. Zwitterionic Copolymer Adsorption on Film. QCM (Q-Sense E1, Meiwafosis Co., Ltd., Japan) analysis with dissipation monitoring was carried out to investigate the adsorption of the positively charged zwitterionic copolymers on the polymer films. The fundamental resonant frequency and diameter of the piezoelectric quartz sensor (QSX 301, Q-Sense Co., Sweden) were 5 MHz and 14 mm, respectively. Before the measurements, the sensor was cleaned for 10 min with an ultraviolet/ozone cleaner (ProCleaner 110, BioForce Nanosciences Co., USA). Then, a 1 wt % PES/SPES polymer solution (in DMAc) was spin-coated onto the golden sensor using a spincoater (Opticoat MS-A100, Mikasa Co., Japan) at a shear rate of 3000 rpm for 1 min. Thereafter, the sensor was dried at 80 °C for 20 min to remove the solvent. The sensor was then placed in a QCM flow chamber. Milli-Q water was introduced into the chamber at 5 μL min−1 for at least 30 min to normalize the equipment and obtain a baseline. Following this, a 1000 ppm zwitterionic-polymer-containing aqueous solution was used as the feed solution. After the adsorption curve became steady, the feed solution was again changed to Milli-Q water to remove excess zwitterionic species. Using the Sauerbrey
EXPERIMENTAL SECTION
Materials. [2-(Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA, Mw = 279.35 g mol−1, Merck Millipore Co., Germany) and [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MTAC, Mw = 207.7 g mol−1, SigmaAldrich Chemical Company, USA) solutions (80 wt %) were used for the zwitterionic copolymer synthesis. Potassium peroxodisulfate (K2S2O8) and sodium hydrogen sulfite (NaHSO3) were purchased from Wako Pure Chemical Industries (Osaka, Japan) and were used to initiate radical polymerization. Polyethersulfone (PES, Mw = 58 000 g mol−1, BASF Co., Germany) and sulfonated PES (SPES, Mw = 141 000 g mol−1, Konishi Chemical Ind. Co., Ltd., Japan) were used for substrate preparation. Their chemical structures are shown in Figure 1. Fluorescein isothiocyanate conjugate bovine serum albumin
Figure 1. Chemical structures of PES and SPES. (FITC-BSA) was purchased from Wako Pure Chemical Industries (Osaka, Japan). Dimethylacetamide (DMAc) and sodium chloride (NaCl) were also purchased from Wako Pure Chemical Industries (Osaka, Japan) and used without further purification. Preparation and Characterization of Porous Substrates and Dense Films. Negatively charged porous substrates were prepared using the nonsolvent induced phase separation (NIPS) method. The polymers were first dissolved into the solvent by stirring in room temperature for 8 h to form a homogeneous solution. The solution was then degassed without stirring for 6 h. After that, the solution was 9153
DOI: 10.1021/acs.langmuir.9b00950 Langmuir 2019, 35, 9152−9160
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Langmuir Table 2. Synthesis Conditions and Fundamental Information on Zwitterionic Copolymers monomers in feed (mmol) copolymer
SBMA (mmol)
MTAC (mmol)
SBMA/MTAC ratio in feed (mol/mol)
SBMA/MTAC ratio in polymer (mol/mol)
yield (%)
zeta potential (mV)
S91 S82 S73 S64 S55 S37
13.2 12.0 11.0 9.6 8.2 5.2
1.8 3.8 4.7 6.3 8.2 12.2
9.00 4.00 2.33 1.50 1.00 0.43
8.39 3.55 2.10 1.33 0.75 0.34
25.0 25.8 25.3 27.1 20.7 23.5
7.58 12.34 44.53 43.57 55.32 65.33
equation (eq 1), the mass of the adsorbed zwitterionic copolymer on the PES/SPES film surface was calculated by varying the frequency of the sensor oscillation during the parallel flow of the zwitterionic copolymer solution at room temperature. Δm = − C
Δf n
(1)
where Δm is the mass of the adsorbed copolymer (ng cm−2), C is the mass sensitivity constant (17.7 ng cm−2 Hz−1 at f = 4.95 MHz), Δf is the frequency variation (Hz), and n is the overtone number (n = 7). Surface Coating Process. The PES/SPES porous substrates and films were immersed into an aqueous solution containing 1000 ppm zwitterionic copolymers at room temperature for 1 min. Thereafter, the sample was gently rinsed thrice with Milli-Q water to remove loosely attached copolymers. Finally, the modified substrates and films were kept in Milli-Q water for subsequent characterizations. Coating Layer Evaluation. The coating layer on the film was evaluated using QCM measurements via a protein adsorption experiment. First, a PES/SPES blend film was coated on the QCM sensor. The film preparation procedure on the QCM sensor is the same as that described in the previous section, “Zwitterionic Copolymer Adsorption on Film”. Then, the zwitterionic copolymer was deposited on the PES/SPES film to prepare a zwitterioniccopolymer-modified film. Thereafter, a phosphate buffer solution (PBS) was injected into the QCM chamber before the BSA adsorption test. After the adsorbed mass curve became stable, the feed solution was changed to 1000 ppm of BSA PBS. After the adsorption experiment was conducted for 60 min, the feed solution was again changed to PBS to remove loosely attached BSA from the surface. The adsorption mass was calculated using eq 1. The coating layers on the substrates and films were also evaluated via fluorescence protein adsorption measurements. A fluorescently labeled protein (FITC-BSA) was used as the model probe to determine the fouling resistance of the substrate/film surface in order to confirm the zwitterionic copolymer adsorption on the surface. The detailed procedure for the protein adhesion measurement is described below. A rectangular substrate sample (∼1 × 2 cm2) was first immersed in PBS (pH 7.4) containing 20 ppm fluorescently labeled protein. Thereafter, the sample was placed in a darkened bioshaker (BR-43FL, TAITEC CORPORATION, Japan) at 25 °C for 10 h, with a shaking rate of 100 rpm. Then, the sample was gently washed thrice with PBS to remove loosely attached proteins. Finally, the sample was observed using confocal laser scanning microscopy (FV1000D, Olympus, Japan), and the fluorescence intensity of the image was analyzed using the ImageJ software.
Figure 2. NMR spectra of synthesized zwitterionic copolymers.
similar reactivity ratio of the methacrylates. The molecular weights of all copolymers were over 6000−8000 Da,16 which met the requirement for surface modification. The zeta potential of the copolymers in solution was measured to evaluate the charge of each copolymer. As shown in Table 2, the zeta potential of S91 was only 7.58 mV, whereas S37 carried the most positive charge. Generally, the zeta potential of the copolymers increased with the MTAC segment content. This indicated that copolymers with different amounts of positive charge were successfully synthesized. Electrostatic Adsorption Behavior on Porous Substrate. Characterization of Porous Substrate. XPS measurements were employed to investigate the surface chemical compositions and surface negative charges. The full-scale spectra for the prepared substrates are shown in Figure S1. This figure shows the same characteristic peaks19 (O 1s, 532 eV; C 1s, 284 eV; and S 2p, 164 eV) for all of the substrates. The surface chemical compositions of the substrates, especially the sulfonic acid (−SO3H) content in the matrix and in the skin layer of the surface, are listed in Table 3. To confirm the accuracy of all XPS measurements, the experimental and theoretical data for pure PES substrate were compared; they are also listed in Table 3. The oxygen, carbon, and sulfur element concentrations detected via XPS are almost the same as the theoretical data. Moreover, the experimental and theoretical ratios of sulfur to carbon (S/C, mol/mol) are almost the same. These results confirm the reliability of the XPS data. The surface chemical compositions, especially for the sulfonic acid group content in the skin layer of the surface, which are representative of the charge density of the substrate, were calculated according to the ratios of sulfur to carbon (S/ C, mol/mol). A higher concentration of the sulfonic acid groups corresponded to a higher charge density of the surface
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RESULTS AND DISCUSSION Characterizations of Zwitterionic Copolymers. The chemical compositions of the zwitterionic copolymers were analyzed using NMR measurements. As shown in Figure 2, each segment of the copolymers is represented in the spectra. Furthermore, the practical and theoretical concentration ratios of the SBMA segment to the MTAC segment were calculated; they are listed in Table 2. Both the ratios for the reactions and in the polymers were found to be almost the same due to the 9154
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Langmuir Table 3. Surface Chemical Compositions and Water Contact Angles of PES/SPES Substrates surface chemical composition (mol %) substrate PES (exp.) PES (theor.) M15-9-1 M15-8-2 M15-7-3 M15-6-4 M15-5-5
C
O
S
S/C (mol/mol)
75.0 75.00
18.8 18.75
6.2 6.25
0.0827 0.0833
73.0 72.2 71.3 71.0 70.7
20.3 20.5 20.7 20.9 20.8
6.7 7.3 7.5 8.1 8.6
0.0918 0.101 0.105 0.114 0.122
sulfonic acid group in matrix (wt %)
sulfonic acid group in skin layer of surface (wt %)
0.938 1.88 2.81 3.75 4.69
3.40 6.88 8.31 11.3 13.6
water contact angle (deg)
63.9 56.0 53.0 37.4 25.0
± ± ± ± ±
2.0 1.3 5.5 6.9 3.0
in the matrix, indicating enhanced hydrophilicity,23 which was well in accordance with the XPS results, as the hydrophilic sulfonic acid groups in SPES were segregated onto the substrate surface during the NIPS process.24 These results showed that porous substrates with different surface charge and hydrophilicity were successfully fabricated. Adsorption Behavior on Porous Substrate. The various substrates were modified with different zwitterionic copolymers via surface adsorption. The surface hydrophilicity and protein adsorption were determined. Two groups of experiments were conducted: one group used the same substrate but different zwitterionic copolymers and the other used different substrates but the same zwitterionic copolymer. The results were as follows. Generally, the water contact angles of the surfaces treated with S73 and S91 were lower compared with that of the original substrate (Figure 4a), indicating improved hydrophilicity; this was because the zwitterionic copolymers contributed additional hydrophilic groups to the substrate surface.16 Furthermore, the S73-modified substrate showed a lower contact angle than the S91-modified substrate. Because of the similar hydrophilicity of polySBMA and polyMTAC,25 the copolymers (i.e., S91, S82, S73, S64, S55, and S37) are thought to have the same hydrophilicity and the improved hydrophilicity on the surface is mainly determined by the adsorption mass. Therefore, the result indicates a higher adsorption mass of S73. On comparison (Figure 4b), the S82-, S73-, S64-, and S55modified substrates showed low contact angles, whereas the S91- and S37-modified substrates showed high contact angles. The lower contact angles indicate better hydrophilicity of the surface, which is attributable to the larger amount of zwitterionic copolymer on the surface. Therefore, the contact
Figure 3. Surface morphology of pure PES and PES/SPES substrates via SEM analysis. Pure PES substrate was used as the control.
and vice versa. As shown in Table 3, the sulfonic acid group content in the skin layer of the surface is much higher than that in the matrix, and increases with the SPES content in the blend system. This phenomenon is mainly caused by the surface segregation of the amphiphilic copolymer in the blend system during the phase separation process.20−22 SEM and AFM were conducted to observe the topography of the PES/SPES substrates (Figures 3 and S2). The results show that the PES/SPES substrates are smoother and denser than the pure PES substrate. The surface roughness slightly decreases with an increase in the SPES content in the substrate, and the root mean square (RMS) roughness for all substrates is over 1.5 nm (Table S1). Surface hydrophilicity is another important property of surfaces. In this research, water contact angle measurements were carried out to evaluate the hydrophilicity of the surfaces (Table 3). The water contact angle gradually decreased with an increase in the SPES content
Figure 4. Water contact angles on substrates. (a) S91- and S73-modified PES/SPES substrates (M15-9-1, M15-8-2, M15-7-3, M15-6-4, and M155-5). (b) Different zwitterionic-copolymer (S91, S82, S73, S64, S55, and S37)-modified M15-7-3 substrates. 9155
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Figure 5. Fluorescently labeled protein adsorption results for zwitterionic-copolymer-modified substrates. (a,c) Effect of substrates on adsorption of FITC-BSA on copolymer-modified PES/SPES substrates. (b,d) Effect of zwitterionic copolymer on adsorption of FITC-BSA on copolymermodified PES/SPES substrate (PES/SPES = 7/3). The scale bar in fluorescence images is 200 μm. The relative fluorescence intensities for the substrates were calculated using ImageJ. The label S91-M15-9-1 indicates the PES/SPES substrate (PES/SPES = 9/1) modified with zwitterionic copolymer S91.
charge on the porous surface,16 for example, S64 > S55 > S37. Another reason is the stability of the copolymer on the charged surface because the interaction between the copolymer and substrate originates from charge attraction. The low density of positive charge on S91 results in a weak interaction between the copolymer and the substrate, leading to poor stability of the copolymer on the substrate; this will be supported by the QCM result. A protein adsorption experiment was conducted to confirm the adsorption behavior of the positively charged copolymers on the porous surfaces. FITC-BSA, which is a negatively charged protein, was used as the model foulant. The light green color in the images indicates a high protein adsorption mass and severe surface fouling, whereas the dark/dark green color indicates a low BSA adsorption mass on the surface. Generally, a surface with improved hydrophilicity exhibits lower adsorption of a foulant. It should be noted that the sulfonic acid groups in the PES/SPES substrates confer not only a negative charge but also hydrophilicity and antifouling properties.26−28 Furthermore, upon the addition of SPES into the PES substrate, the substrate becomes smoother (Figure S2 and Table S1), which is also beneficial for fouling resistance.29,30 Therefore, as shown in row 1 of Figure 5a,
Table 4. Surface Chemical Compositions and Contact Angles of Pure PES and PES/SPES Films
film PES (exp.) PES (theor.) F15-9-1 F15-8-2 F15-7-3 F15-6-4 F15-5-5
sulfonic acid group in matrix S/C (mol/mol) (wt %)
sulfonic acid group in skin layer of surface (wt %)
0.0903
RMS roughness (nm)
contact angle (deg)
0.77
80.7 ± 1.7
0.56 0.63 0.84 0.89 0.71
78.1 77.2 71.0 67.6 65.3
0.0833 0.0864 0.0938 0.100 0.107 0.106
0.938 1.88 2.81 3.75 4.69
1.28 4.16 6.72 9.09 8.59
± ± ± ± ±
1.5 1.8 1.1 0.7 0.8
angle data confirm that more S82, S73, S64, and S55 are attached on the substrates than the other copolymers (e.g., S91 and S37). These phenomena can be explained as follows. One reason is similar to that explained in our previous article: more zwitterionic copolymers with a lower density of positive charge were consumed to neutralize the same density of negative 9156
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Figure 6. Zwitterionic copolymer adsorption curves for pure PES film and PES/SPES films via QCM analysis. (a) Adsorption mass of identical zwitterionic copolymer (S73, 1000 ppm) on pure PES film and different PES/SPES films (PES/SPES = 9/1, 7/3, 6/4, and 5/5). (b) Adsorption mass of different zwitterionic copolymers (S55, S64, S73, and S91) on same PES/SPES film (PES/SPES = 7/3). For the pure PES film, S73 was used as the model copolymer. The concentration of the zwitterionic copolymers was 1000 ppm.
Table 5. Adsorption Mass of Zwitterionic Copolymers on Pure PES Film and PES/SPES Films (PES/SPES = 7/3) and That of BSA on Modified Films adsorption mass (ng cm−2) copolymer ID pure PES S91 S73 S64 S55
b
zwitterionic copolymer 136 790 1003 1019 834
± ± ± ± ±
50 20 155 170 40
BSAa 236 225 83 83 220
± ± ± ± ±
15 22 14 11 19
a
The BSA adsorption experiment was conducted for 60 min. bS73 was used as the model copolymer. Figure 8. BSA adsorption behavior on PES/SPES films (PES/SPES = 7/3) coated with different zwitterionic copolymers (S91, S73, S64, and S55). Pure PES film was used as the control. BSA was dissolved in PBS, and its concentration was 1000 ppm. The BSA adsorption experiment was conducted for 60 min.
the fluorescence intensity of the images gradually decreases with an increase in the SPES content in the substrate, which is in accordance with the previous results.27,28 The images comparing the different substrates modified with the same copolymer show a similar order of the original substrates in row 1 (Figures 5a and S3). The substrates, especially the hydrophobic surfaces (e.g., M15-9-1, M15-8-2, and M15-7-3), become much darker compared with the original surfaces, indicating greater hydrophilicity, which well agrees with the contact angle data (Figure 4a). As for M15-6-4 and M15-5-5, because of the hydrophilic surfaces of the original substrates, the modified surfaces also show a low relative fluorescence intensity. Another interesting phenomenon to be noted is that the S73- and S64-modified substrates are much darker than the S91-, S55-, and S37-modified substrates (Figure 5b,d). In our previous work, S73 was the best candidate for substrate
modification among S73, S55, and S37 because the highest adsorption mass of S73, as well as the hydrophilic component, were adsorbed on the substrate, according to the QCM results.16 According to the theory of that work, the S91modified substrate should show the best performance because the substrate would adsorb the most S91 to balance the negative charge on the surface. However, the current results indicate that the S91-modified substrate does not show the best antifouling properties among all samples, and it is as poor as the S37-modified substrate, which well agrees with the contact angle measurement data. The strong fluorescence
Figure 7. Summary of electrostatic adsorption behavior of positively charged zwitterionic copolymer on negatively charged surface. 9157
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SPES films. For the PES/SPES film with PES/SPES = 9/1, the low adsorption mass is attributed to the small amount of sulfonic acid groups on the film surface. The adsorption mass for the PES/SPES films with PES/ SPES = 5/5 and 6/4 is also much lower than that on the PES/ SPES film with PES/SPES = 7/3. One possible reason is the steric hindrance of the negatively charged sites. According to the XPS data, there is 8.58 wt % sulfonic acid in the skin layer of the surface of F15-5-5 and 9.09 wt % sulfonic acid in the surface of F15-6-4, which are much larger than those in F15-91 and F15-7-3. Unlike that of the porous surface, the surface roughness of the film was extremely low (Table 4). Therefore, the adsorbed copolymer was uniformly distributed on the film surface. Although a high density of negative charge on the film results in a stronger interaction with the copolymer, the initially attached copolymers will significantly influence the subsequently attaching copolymers. On one hand, the initially attached copolymers would cover the neighboring negatively charged sites and screen the intrinsic surface charge, which will weaken the electrostatic interaction and reduce further adsorption of identical copolymers. On the other hand, the initially attached copolymers provide a surface with a positive charge, which would reject the approach of identical copolymers due to the electrostatic repulsion effect. However, this phenomenon cannot be elucidated using the zwitterionic copolymer adsorption on the porous substrate via water contact angle measurements and protein adsorption experiments. The adsorption behavior of the different zwitterionic copolymers on the same substrate is illustrated in Figure 5b and summarized in Table 5. The PES/SPES film with PES/ SPES = 7/3 was used as the negatively charged substrate and was spin-coated on the QCM sensor. The zwitterionic copolymers, S73 and S64 showed the highest adsorption mass of ∼1000 ng cm−2 on the film (Table 5). The explanation for this is similar to that in our previous report.16 As the constant amount of negative charge on the surface would consume the same amount of contrary charge to neutralize the initial surface, a larger amount of the zwitterionic copolymer with a lower density of positive charge (i.e., S73 and S64) is adsorbed on the film. It is worth noting that when the zwitterionic copolymer carries a lower density of positive charge, for example, S91, the adsorption mass is lower than that of S73 and S64, which is in accordance with the adsorption results for the porous surfaces (Figures 4 and 5). The detailed reasons are as follows. Because of the low density of positive charge on S91 (i.e., 7.58 eV), which is much lower than those of the other copolymers, the electrostatic interaction between the charged surface and copolymer is weak, leading to unsteady adsorption and a lower adsorption mass. To compare the different adsorption mechanism for S91- and S55-modified surfaces, the desorption experiments were conducted and the results are shown in Figure S6. It indicated that the stability of the S91-modified surface is much worse than that of the S55-modified surface when rinsed by 0.01 M NaCl solution. The detailed adsorption behavior of this copolymer on the charged surface is summarized in Figure 7. The BSA adsorption on the zwitterionic copolymers coating the films was also evaluated using QCM tests. Different zwitterionic copolymers were applied to modify identical films (Figure 8 and Table 5). The BSA adsorption mass of the films (PES/SPES = 7/3) coated with copolymer S73 and S64 is
intensity for the S91-modified substrate implies that the surface is hydrophobic, indicating a low adsorption mass of S91. Based on the above discussion, the electrostatic adsorption behavior on the porous substrates can be qualitatively described using the surface hydrophilicity and fluorescently labeled protein adsorption data. However, the quantitative analysis of the adsorption behavior should be conducted in the future. Electrostatic Adsorption Behavior on Dense Film. Characterization of Film. To further evaluate and quantify the adsorption behavior of the zwitterionic copolymers on the negatively charged surfaces, dense films were also applied. The surface chemical compositions and surface roughness of the surfaces were analyzed using XPS (Figure S4, Tables 4 and S2) and AFM (Figure S5), respectively. The surface hydrophilicity was determined using the water contact angle (Table 4). It was found that the concentration of sulfonic acid groups in the skin layer was higher than that in the matrix, and it increased with the SPES content in the blend system. Surface segregation was also observed during the solvent evaporation process. This was mainly caused by the evaporation of the polar organic solvent (i.e., DMAc) from the interior of the liquid film toward the surface, which facilitated the movement of the polar groups (i.e., sulfonic acid groups) together with the solvent evaporation. Another reason was the water vapor in the air during film formation. However, the concentration of sulfonic acid groups in the skin layers of the films was lower than that in the porous substrates and was ascribed to the drag effect by water during phase separation.31,32 The surface roughness of the films was much lower than that of the porous substrates, that is, the RMS roughness was lower than 0.9 nm. The effective surface area was much lower than that of the porous substrates (Table S3). Moreover, the water contact angle was higher than that on the porous substrates. On one hand, the sulfonic acid group content in the skin layers of the films is lower than that on the porous substrates. On the other hand, the smoother surfaces indicate a higher contact angle.33−35 These results showed that various films with different surface charge and surface hydrophilicity were fabricated. Adsorption on Dense Film. QCM measurements were conducted to accurately detect the copolymer adsorption on the charged films. The adsorption behavior of the same zwitterionic copolymer on surfaces with different negative charge amounts was first investigated. The surface charge was controlled by changing the blend ratio of PES/SPES to 9/1, 7/ 3, 6/4, and 5/5. S73 was chosen as the model copolymer to evaluate the adsorption behavior. According to Figure 6a, the PES/SPES film with PES/SPES = 7/3 shows the highest adsorption mass of the copolymer, ∼1000 ng cm−2, whereas the others show much lower. The adsorption mass on the pure PES film is similar to that on the PES/SPES film with PES/ SPES = 9/1 and low (∼100 ng cm−2), whereas that on the PES/SPES films with PES/SPES = 5/5 and 6/4 is ∼200−300 ng cm−2. It should be noted that the pure PES film also adsorbs a little amount of the copolymer. On one hand, the hydrophobic PES film and the hydrophobic segment in the copolymer (e.g., ester bond and methyl group) exhibit a hydrophobic interaction.36 On the other hand, the PES surface carries a weak negative charge in a neutral environment,37,38 which leads to the adsorption of the positively charged copolymer. However, the adsorption mass is much lower than those on most of the PES/ 9158
DOI: 10.1021/acs.langmuir.9b00950 Langmuir 2019, 35, 9152−9160
Langmuir lower than that of the film coated with the other copolymers and pure PES film. These results well agree with the adsorption amounts of the zwitterionic copolymer, that is, when a larger amount of zwitterionic copolymer was adsorbed on the film, the antifouling properties of the film was better. For example, films coated with copolymers S73 and S64 show a high copolymer adsorption mass (∼1000 ng cm−2) and low BSA adsorption mass (∼83 ng cm−2). This can be considered as additional strong evidence of the adsorption behavior of the zwitterionic copolymers on the negatively charged surfaces.
CONCLUSIONS In this work, the electrostatic adsorption behavior of positively charged zwitterionic copolymers on negatively charged surfaces was systematically investigated using water contact angle measurements, QCM tests, and fluorescently labeled protein adsorption experiments. The results indicate that with an increase in the charge density of the surface, the adsorption mass of the coating layer first increases and then decreases. A high surface charge is beneficial for surface adsorption; however, an extremely high charge density of the surface leads to a low surface adsorption due to the steric hindrance of the charged sites. Moreover, with an increase in the charge density of the coating layer material, the adsorption mass on the identical surface first increases and then decreases. A high charge density of the coating layer helps to strengthen the interaction between the coating layer and the surface; however, for an extremely high charge density of the coating layer, a low surface adsorption mass is necessary to neutralize the surface charge. This work provides an insight into the best strategy for surface modification via electrostatic adsorption.
ACKNOWLEDGMENTS
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00950. Wide-scan XPS spectra of PES/SPES blend substrates; topology of pure PES and PES/SPES substrates via AFM analysis; more fluorescence images for zwitterionic-copolymer-modified substrates; wide-scan spectra of pure PES and PES/SPES blend films; surface roughness of pure PES and PES/SPES blend films; stability of zwitterionic copolymer S55 and S91 on the PES/SPES film (PES/SPES = 7/3); arithmetic average surface roughness (Ra), RMS roughness, and surface average pore size of each membrane; surface chemical compositions of pure PES and PES/SPES films; and specific surface area of films and substrates via AFM measurements (PDF)
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The authors gratefully thank the financial support through the Grants-in-Aid from the Special Coordination Funds for Promoting Science and Technology, Creation of Innovation Centers for Advanced Interdisciplinary Research Areas (Innovative BioProduction, Kobe) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and the National Natural Science Foundation of China (21805240).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (L.-F.F.). *E-mail:
[email protected] (H.M.). ORCID
Hideto Matsuyama: 0000-0003-2468-4905 Notes
The authors declare no competing financial interest. 9159
DOI: 10.1021/acs.langmuir.9b00950 Langmuir 2019, 35, 9152−9160
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DOI: 10.1021/acs.langmuir.9b00950 Langmuir 2019, 35, 9152−9160
