Effect of Surface Hydration on Antifouling Properties of Mixed Charged

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Effect of Surface Hydration on Antifouling Properties of Mixed Charged Polymers Chuan Leng, Hao Huang, Kexin Zhang, Hsiang-Chieh Hung, Yao Xu, Yaoxin Li, Shaoyi Jiang, and Zhan Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00768 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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Effect of Surface Hydration on Antifouling Properties of Mixed Charged Polymers Chuan Lenga&, Hao Huanga&, Kexin Zhanga, Hsiang-Chieh Hungb, Yao Xub, Yaoxin Lia, Shaoyi Jiang*b, Zhan Chen*a a

Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States

b

Department of Chemical Engineering, University of Washington, Seattle, Washington 98195, United

States *Corresponding author (Z.C.): e-mail [email protected]; Fax 1-734-647-4865. *Corresponding author (S.J.): e-mail [email protected]; Fax: 1-206-685-3451. &These authors contributed equally

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Abstract Interfacial water structure on a polymer surface in water (or surface hydration) is related to the antifouling activity of the polymer. Zwitterionic polymer materials exhibit excellent antifouling activity due to their strong surface hydration. It was proposed to replace zwitterionic polymers using mixed charged polymers because it is much easier to prepare mixed charged polymer samples with much lower costs. In this study, using sum frequency generation (SFG) vibrational spectroscopy, we investigated interfacial water structures on mixed charged polymer surfaces in water, and how such structures change while exposing to salt solutions and protein solutions. The 1:1 mixed charged polymer exhibits excellent antifouling property while other mixed charged polymers with different ratios of the positive/negative charges do not. It was found that on the 1:1 mixed charged polymer surface, SFG water signal is dominated by the contribution of the strongly hydrogen bonded water molecules, indicating strong hydration of the polymer surface. The responses of the 1:1 mixed charged polymer surface to salt solutions are similar to those of zwitterionic polymers. Interestingly, exposure to high concentrations of salt solutions leads to stronger hydration of the 1:1 mixed charged polymer surface after replacing the salt solution with water. Protein molecules do not substantially perturb the interfacial water structure on the 1:1 mixed charged polymer surface and do not adsorb to the surface, showing that this mixed charged polymer is an excellent antifouling material. Keywords Sum frequency generation vibrational spectroscopy, mixed charged polymer, zwitterionic polymer, salt effect, protein adsorption


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Introduction Polymers have been widely used as biomedical materials (e.g., various implants) and coatings for marine vessels.1-6 For such materials, it is important for their surfaces to exhibit good anti-fouling properties. Extensive research has been performed to develop anti-fouling polymer materials, such as polyethylene glycols (PEGs), hydrogels, amphiphilic polymers, as well as zwitterionic polymers.7-19 It is believed that a surface with less than 5 ng/cm2 adsorbates is an antifouling or super-low fouling surface.20 Molecular mechanisms of anti-fouling behavior of various materials have been investigated, and it is well accepted that interfacial water structure on the polymer surface plays an important role in determining the anti-fouling performance of the surface.21 Recently, zwitterionic polymers have been shown to exhibit excellent antifouling performance.16-21 Various types of zwitterionic polymers have been synthesized and characterized. A zwitterionic polymer molecule has positive charges and negative charges next to each other, while the total net charge of the molecule is zero. It is believed that the excellent anti-fouling property of zwitterionic polymers is due to the fact that the zwitterionic material surface can strongly bind water molecules – or strong hydration occurs on the zwitterionic material surface. Because of such a strong hydration, other molecules or materials cannot replace water molecules attached to the zwitterionic polymer surface. Therefore the zwitterionic polymer exhibits excellent anti-fouling characters. To understand the molecular mechanisms of anti-fouling performance of polymer materials, it is necessary to investigate the surface structures of polymer materials in aqueous environment in situ. This is extremely difficult for many traditional surface sensitive analytical tools because such tools require high vacuum to operate, while it is impossible to probe an interface involving


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liquid in high vacuum. We have developed systematic approaches to apply a nonlinear optical spectroscopic technique, sum frequency generation (SFG) vibrational spectroscopy, to elucidate molecular structures of polymer materials in aqueous solutions in situ in real time.21-32 We showed that different polymers could exhibit varied surface restructuring behaviors, and it is feasible to quantitatively compare the surface structures of a polymer material in air and in water.22,23 In addition to the polymer surface structures in aqueous environments, we have also applied SFG to probe the interfacial water structures on polymer surfaces at the polymer/solution interfaces in situ in real time.21, 29-33 Especially, SFG results indicated that on polymers with excellent antifouling performance, such as zwitterionic, PEG, and hydrogel surfaces, strongly hydrogen bonded water molecules were dominant.21 The interfacial water structure or the strength of hydration can be varied at different polymer/solution (with different pH) interfaces, which can be well correlated to the antifouling performance.33 On other polymers which are not antifouling, such as poly(methyl methacrylate) (PMMA) and poly(ethylene terephthalate) (PET), weakly hydrogen bonded water was detected at the polymer/water interface.29 Therefore, we concluded that strong interfacial hydration or the domination of strongly hydrogen bonded water at the interface leads to excellent antifouling performance of a polymer material. Zwitterionic polymers are difficult to synthesize, with high cost. It was reported that mixed charged polymer, a polymer with mixed positive and negative charges (but the positive and negative charges do not need to be next to each other), may possess a similar antifouling behavior as a zwitterionic polymer.34-35 The mixed charged polymers are much easier to prepare with lower cost. In this article, we reported SFG studies on mixed charged polymers and their surface structure and property as a function of mixture composition and solution pH. Their interactions with salt and various proteins have also been probed. Our results demonstrated that the 1:1 mixed charged


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polymer exhibited excellent antifouling activity. Strong hydration on the 1:1 mixed charged polymer surface was detected. Water molecules strongly bound to the 1:1 mixed charged polymer surface; protein molecules did not substantially perturb the strongly bound water on the surface. Similar to the zwitterionic polymers, the excellent antifouling behavior of the 1:1 mixed charged polymer is because of the strong surface hydration. Experimental Section The mixed charged polymer brushes were synthesized on both silica prisms (Altos Photonics, Bozeman, MT) and surface plasmon resonance (SPR) chips via atom transfer radical polymerization (ATRP). A similar method was used to prepare zwitterionic polymers or other polymer brushes reported previously.35-38 The SPR chips were made of a glass slide coated with a titanium film (2 nm), a gold film (48 nm), and an additional titanium film (1 nm) for promoting the adhesion of the SiO2 film, via an electron beam evaporator. The substrates were then coated with 20 nm of SiO2 by plasma enhanced chemical vapor deposition. Before polymer brush preparation, both the silica prisms and SPR chips were grafted with ATRP initiator by silanization. First, both the silica prisms and SPR chips were washed with pure ethanol, cleaned with a UV-ozone cleaner (Jelight, model 42) for 40 min, and washed with water and pure ethanol. The ATRP initiator covered surface was formed by soaking both silica prisms and SPR chips in an anhydrous toluene solution containing 0.1% 2-bromo-2-methyl-N-3[(triethoxysilyl)propyl]propanamide at room temperature for 10 minutes. The surface was then rinsed with anhydrous toluene, followed by ethanol, and dried with a stream of air. For the polymer brush preparation, both initiator covered silica prisms and SPR chips were placed in a reaction tube with Cu(I)Br and purged with nitrogen. In the meantime, solutions of 2,2'-bipyridine (BPY) in methanol and monomers in a 1:1 water:methanol mixture were also


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purged with nitrogen. After 15 minutes, the monomer and BPY solutions were introduced to the reaction tube using a syringe, and the reaction proceeded for 1 hour. The final reaction concentrations were: 1:3 water: methanol solution, 0.05 M Cu(I)Br, 0.1 M BPY, and 1.125 M total monomer. Positively charged [2-(Methacryloyloxy)ethyl]trimethylammonium chloride and negatively charged 3-sulfopropyl methacrylate potassium salt were used as starting monomers with five ratios (2:1, 1.5:1, 1:1, 1:1.5 and 1:2) for mixed charged polymer brush synthesis. After the 1 hour reaction, the samples were removed from the reaction tubes and immersed in PBS overnight to ensure that all of the unreacted monomers and reaction solvents were removed before further analysis. The thickness of brush on SPR chips was determined to be around 20 nm using a multi-wavelength ellipsometer (J. A. Woollam, model alpha-SE). We believe that the mixed charged polymer film thickness on silica prisms should be similar. X-ray photoelectron spectroscopy (XPS) was used to characterize the elemental composition of the mixed charged polymer brushes. The polymers with the same compositions were prepared on silica slides for XPS measurement (Kratos axis ultra XPS, Michigan Center for Materials Characterization). The protein adsorption experiments were performed using an SPR equipment which was developed at the Institute of Photonics and Electronics, Prague, Czech Republic, described previously.35-38 This custom-built SPR is based on the attenuated total reflection method and wavelength modulation. It is equipped with a four-channel flow cell along with a temperature control, and uses a peristaltic pump for delivering sample fluids. SFG theories, experimental details, and data analysis have been extensively reported and will not be repeated.39-58 Our SFG equipment and data collection method were published before.22-


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Figure 1 Molecular formula of the mixed charged polymer. 23

SFG is a second-order nonlinear optical phenomenon where a signal is generated with a

frequency equaling to the summation of the two incident lights’ frequencies. The SFG selection rule determines that SFG signal can only be generated from a medium with no inversion symmetry. Usually a bulk material is centrosymmetric, while surfaces and interfaces lack inversion symmetry. Therefore, SFG is a surface/interface sensitive technique. For IR-visible sum frequency generation used in this study and many other studies, the SFG signal could be enhanced if the frequency of the incident IR light matches a vibrational transition of the sample molecule. In our experiment, a green laser (532 nm) and a frequency tunable IR laser are the two incident beams. The tunable IR laser could cover certain vibrational frequency range of molecular vibration, therefore SFG signal can be used to characterize chemical structures of surfaces/interfaces at a molecular level. In this work, SFG spectra were collected from the mixed charged polymer/water and mixed charged polymer/solution interfaces. Solutions with different pH values (adjusted by small amount of HCl and NaOH solutions), with different concentrations of NaCl salt solutions, and with different protein solutions (bovine serum albumin or BSA, fibrinogen, and lysozyme) were used in this study. We purchased bovine serum albumin (BSA, 99%), fibrinogen (Type I-S, 65−85%,


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may contain 10% sodium citrate and 15% sodium chloride) and lysozyme (90%) from SigmaAldrich. To collect the SFG signal generated from a polymer/water or polymer/solution interface, a right-angle fused silica prism with a mixed charged polymer coating was used to contact water or different solutions. Two input laser beams as described above were overlapped both spatially and temporally on the interface. In particular, we applied SFG to study three types of proteins mentioned above. For each study, we collected SFG signal from the polymer/water interface first. The water in contact with polymer was then replaced by a protein solution and we collected SFG signal from the polymer/protein solution interface. After that, the protein solution in contact with the polymer was replaced by water for several times, and SFG data was acquired again from the polymer/water interface. All spectra were obtained with ssp (s for the SFG signal, s for the visible input laser and p for the IR input laser) polarization combination.

Results and Discussion As discussed above, our recent research indicates that the antifouling behavior of a polymer material is mediated by its interfacial water structure.21 For several polymer materials with excellent antifouling performance which we investigated, interfacial water molecules are strongly hydrogen bonded, evidenced by a strong O-H stretching signal at 3200 cm-1 in the SFG spectra.21 This shows that strong interfacial hydration ensures an excellent antifouling behavior of polymer materials. We observed this phenomenon from many polymer materials, including zwitterionic polymers and polymers containing polyethylene glycol segments.21


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Figure 2 Fibrinogen (Fib) and lysozyme (Lys) adsorption results on mixed charged polymer surfaces measured using SPR with 1 mg/mL protein solutions. Mixed charged polymer materials prepared with different ratios of positively charged (with quaternary amine) and negatively charged (with sulfonate) groups with bulk mixing ratios of 2:1, 1.5:1, 1:1, 1:1.5, 1:2 were studied. In this study, we investigated mixed charged polymer materials prepared with different ratios of positively charged (with quaternary amine) and negatively charged (with sulfonate) groups with bulk mixing ratios of 2:1, 1.5:1, 1:1, 1:1.5, 1:2 of the two components. Figure 1 shows the molecular formula of the mixed charged polymers used in this study. Before we examined the surface structures of these mixed charged polymer materials, their antifouling activities were tested using fibrinogen and lysozyme. Under our experimental conditions, fibrinogen and lysozyme are negatively and positively charged respectively. Figure 2 shows the protein adsorption results from surface plasmon resonance (SPR) experiments. Clearly the 1:1 mixed charged polymer exhibits the best antifouling activity. The adsorption amount of fibrinogen (~2 ng/cm2) or lysozyme (~4 ng/cm2) is lower than 5 ng/cm2, which is defined as the “super low” fouling limit. We believe that the 1:1 mixed charged polymer surface is more or less neutral and strongly hydrated, leading to super low fouling or antifouling, which will be studied in more detail below. For other mixed charged polymers, the surface net charge is not zero, therefore the adsorption amount of either negatively charged fibrinogen or positively charged lysozyme could be quite high, as observed. That is, on the 2:1 and 1.5:1 positively charged polymer surfaces, fibrinogen molecules were strongly adsorbed (~70 and ~25 ng/cm2 respectively). On the 1:1.5 and 1:2 negatively charged polymer surfaces, lysozyme molecules were strongly adsorbed (~50 and 100 ng/cm2 respectively). 9 ACS Paragon Plus Environment

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Figure 3 XPS data collected from the samples prepared using polymers with positively charged (with quaternary amine) and negatively charged (with sulfonate) groups with bulk mixing ratios of 2:1, 1.5:1, 1:1, 1:1.5, and 1:2 (from bottom to top). Figure 3 displays the XPS results obtained from the above five mixed charged polymer materials with different ratios of the two components. It is worth mentioning that XPS results were obtained in vacuum, not from the polymer/aqueous solution interface, but we believe that the surface chemical compositions detected from XPS should be related to the surface structures of polymer materials in various chemical environments. The XPS results indicated that the surface ratios of the two polymer components are 1.28±0.04:1.00, 1.14±0.01:1.00, 0.97±0.04:1.00, 0.63±0.01:1 (or 1.00±0.02:1.29), and 0.51±0.06:1 (or 1.00±0.12:1.96), respectively. This indicated that the surface compositions of the mixed charged polymers are not the same as the initial 10 ACS Paragon Plus Environment

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monomer compositions before polymerization, but the trend is the same. That is, a higher amount of the negatively charged polymer than the positively charged polymer in the polymer bulk leads to a higher amount of the negatively charged polymer on the surface. The mixed charged polymer prepared by the same amount of the two components exhibits similar surface coverages of the positive and negative charges. This can well interpret the SPR data presented above. It is worth mentioning that it is difficult to accurately control the perfect structure of the mixed charged polymer with alternating positively and negatively charged groups, since diffusion and propagation rates for each monomer can be different. As a result the brush can be enriched slightly in one of the monomer even 1:1 of the positively and negatively charged reactants were applied in the starting materials. Figure 4 shows the SFG spectra collected from the mixed charged polymer/aqueous solution interfaces at different solution pH values. Clearly, the SFG signal from the 1:1 mixed charged polymer/water interface shows a strong 3200 cm-1 peak and a very weak 3400 cm-1 peak, indicating that the interfacial water molecules are strongly hydrogen bonded, leading to strong hydration of the polymer surface. According to our previous results,21, 33 the strong hydration of an overall neutral surface leads to strong antifouling. Therefore the 1:1 mixed charged polymer material is a good antifouling material, which matches the protein adsorption data reported above. SFG spectra collected from other mixed charged polymer surfaces indicated that on these surfaces in solutions, overall water signals were either weaker or contained substantial contribution from weakly hydrogen bonded water, showing weak interfacial hydration. More importantly, such surfaces are either positively or negatively charged, leading to poor antifouling activity of such mixed charged polymers. Since the adsorption amount of the proteins on the mixed charged polymer surface with the opposite charge (e.g., positive lysozyme on mixed charged polymer with


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more sulfonate groups and fibrinogen on mixed charged polymer with more quaternary amine groups) is large, while on the mixed charged polymer surface with the same charge is small, we believe that the electrostatic effect on charged surfaces dominates the effect of the interfacial hydration for protein adsorption. This research further validates the correlation between the interfacial water structure and the antifouling performance of an overall neutral polymer material. SFG spectra collected from zwitterionic polymer poly(sulfobetaine methacrylate) (pSBMA) in solutions at various pH values21, 33

are similar to those observed from the 1:1 mixed charged polymer/solution interfaces shown in

Figure 4. Therefore the antifouling activity of the 1:1 mixed charged polymer could be similar to that of the zwitterionic polymer. A small difference between the SFG spectra collected from the 1:1 mixed charged polymer/water and pSBMA/water interfaces21,33 is that the spectrum from the 1:1 polymer/water interface has a weak signal (shoulder) at ~3400 cm-1 from the weakly hydrogen bonded water. This is because the negatively charged SO3- group binds water more strongly than the positively charged N(CH3)3+ group. In the mixed charged polymer surfaces,

Figure 4 SFG spectra collected from the different mixed charged polymer/aqueous solution interfaces at different pH values of the solutions.

negatively charged and positively charged groups are not extremely homogeneously distributed, 12 ACS Paragon Plus Environment

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therefore some water molecules may only interact with the N(CH3)3+ groups, and weakly hydrogen bonded water molecules could be detected. However, for zwitterionic pSBMA, two different charged groups are connected to each other and water molecules could interact with both groups and therefore only form strong hydrogen bonds, leading to a single strongly hydrogen bonded water signal. It is interesting to observe that the interfacial water structure on the 1:1 mixed charged polymer surface did not vary in the pH range of 5 to 9, which is similar to that of the zwitterionic polymer pSBMA, but different from poly(carboxybetaine acrylamide)s.33 When pH is in the range of 5−9, the sulfonate group cannot be protonated, and the charge balance of the 1:1 mixed charged polymer does not change. Therefore in this pH range, the acidity in the solution would not change the antifouling performance of the 1:1 mixed charged polymer.


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Figure 5 SFG spectra collected from the 1:1 mixed charged polymer/water and pSBMA/water interfaces. SFG spectra collected from interfaces between 1:1 mixed charged polymer or pSBMA and NaCl solutions with different concentrations. For the polymer materials designed for use in marine environments, it is necessary to study the interfacial water structure between polymer surface and seawater. Because seawater has a high salt content which may affect interfacial hydrogen bonding, we examined how salt in the aqueous solution in contact with zwitterionic or mixed charged polymers may influence polymer interfacial hydration. Figure 5 shows the interfacial water signal change at different experimental conditions for 1:1 mixed charged polymer and zwitterionic polymer pSBMA. Similar (but not identical) SFG 14 ACS Paragon Plus Environment

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spectra were detected in the O-H stretching region from the pSBMA/water and 1:1 mixed charged polymer/water interfaces. A small signal at 3400 cm-1 (from weakly hydrogen bonded water) was observed from the 1:1 mixed charged polymer/water interface, while this signal was not detected from the pSBMA/water interface, as we discussed above. Then the water in contact with pSBMA or 1:1 mixed charged polymer was replaced with a 0.02 M NaCl solution, the SFG signal collected from the pSBMA/solution interface is much weaker than that from the 1:1 mixed charged polymer/solution interface, indicating that NaCl molecules disrupted the interfacial water order on the pSBMA surface more than the mixed charged polymer surface. This shows that the 1:1 mixed charged polymer surface has a stronger resistance against salt surface hydration disruption. We believe that the 1:1 mixed charged polymer should have better antifouling activity under this experimental condition, which will be tested in the future. After exposure to this salt solution, the 0.02 M NaCl solution was replaced with water and SFG spectra were collected from both the pSBMA/water and 1:1 mixed charged polymer/water interfaces. Water signal on both surfaces recovered, showing that both surfaces are strongly hydrated again in water. After that, water in contact with pSBMA and 1:1 mixed charged polymer was replaced by a 0.1 M NaCl solution. Again, the SFG signal collected from the pSBMA/solution interface is much weaker than that from the 1:1 mixed charged polymer/solution interface, indicating that NaCl molecules strongly disrupted the interfacial water order on the pSBMA surface under this experimental condition as well. This result confirmed that the 1:1 mixed charged polymer surface has a stronger resistance against salt surface hydration disruption. Then the 0.1 M NaCl solution was replaced with water and SFG spectra were collected from both the pSBMA/water and 1:1 mixed charged polymer/water interfaces. Different from what was presented above, h Here the SFG signal collected from the pSBMA/water interface recovered, but


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the SFG signal collected from the 1:1 mixed charged polymer/water interface increased substantially compared to that collected from the original 1:1 mixed charged polymer/water interface. We believe that the 1:1 mixed charged polymer surface treated with a 0.1 M NaCl solution can enhance the surface hydration, which is likely due to the trap of the ions in the polymer brush which makes the polymer brush more ordered, leading to more ordered water molecules at the interface. Water in contact with pSBMA or 1:1 mixed charged polymer was then replaced by a 0.5 M NaCl. In this case, SFG signals collected from the pSBMA/solution and 1:1 mixed charged polymer/solution interfaces were both weak. However, after the 0.5 M NaCl solutions were replaced by water, SFG spectrum collected from the pSBMA/water interface recovered, while the signal from the 1:1 mixed charged polymer/water interface substantially increased again, showing that hydration on the 1:1 mixed charged polymer surface can be enhanced by treating the surface with a 0.5 M NaCl solution. The exposure to a salt solution influences both the zwitterionic polymer and the 1:1 mixed charged polymer surface hydration. Interestingly, the exposure of the 1:1 mixed charged polymer to 0.1 and 0.5M salt solutions enhanced the surface hydration after returning the 1:1 mixed charged polymer to water over a period of time. After 1 hour, the water signal returned to the original intensity, indicating that the hydration increase was temporary. We believe that this is because salts diffused out from the polymer brush into water after 1 hour exposing to pure water. As presented above, at the 1:1 mixed charged polymer/water interface, strongly hydrogen bonded interfacial water molecules dominated. Here, we examined whether various protein molecules could influence the interfacial water structure on the 1:1 mixed charged polymer surface. As we published previously,31 both a zwitterionic polymer brush (e.g. pSBMA) and an oligomer ethylene glycol brush (pOEGMA) are excellent antifouling coatings, strongly resisting protein


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adsorptions. On pSBMA and pOEGMA surfaces in water, water molecules form strong hydrogen bonds, indicated by the dominating strong water signal at 3200 cm-1 in the SFG spectra detected from the polymer/water interfaces. However, when water in contact with pSBMA or pOEGMA was replaced by a protein solution, different results were obtained.31 Protein molecules could not change the interfacial water structure on pSBMA, evidenced by the identical SFG water signals in the spectra collected from the pSBMA/water and pSBMA/protein solution interfaces.31 Various types of protein molecules such as bovine serum albumin (BSA), fibrinogen, and lysozyme were used in the study. Differently the interfacial water structure on the pOEGMA surface could be affected by protein molecules, shown by the different water SFG signals observed at the pOEGMA/water and pOEGMA/protein solution interfaces.31 Interestingly, the results are markedly different at different pOEGMA/protein solution interfaces.31 Here we examined the interfacial water behavior at the 1:1 mixed charged polymer/water interfaces before and after replacing water with various protein solutions. The protein solution concentration for all the samples studied here was 1 mg/mL. Similar to the zwitterionic polymer pSBMA/water interface, SFG signal detected from the 1:1 mixed charged polymer/water interface is dominated by a peak at 3200 cm-1, contributed by strongly hydrogen bonded water at the interface (Figure 6). After water was replaced by a BSA solution, SFG water spectral features do not vary, and the intensity was only slightly changed, showing that BSA only slightly perturb the interfacial water structure on the 1:1 mixed charged polymer surface. The perturbation is much smaller compared to that of BSA on the pOEGMA interfacial water structure studied previously. When the BSA solution was replaced by water, the SFG spectrum from the 1:1 mixed charged polymer/water interface was similar to that collected from the polymer/water interface before the protein contact, indicating that BSA was not adsorbed onto the 1:1 mixed charged polymer surface.


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After water in contact with the 1:1 mixed charged polymer was replaced by a fibrinogen solution, the SFG signal from the 1:1 mixed charged polymer/fibrinogen solution interface was similar to that collected from the 1:1 mixed charged polymer/water interface. Further, the SFG signal did not change after we replaced fibrinogen solution with water. This result clearly shows that fibrinogen did not substantially interact with the strong hydrated layer or have a noticeable adsorption on the 1:1 mixed charged polymer surface either, similar to BSA. When water in contact with the 1:1 mixed charged polymer was replaced by a lysozyme solution, the SFG signal slightly changed (Figure 6). A shoulder at around 3300 cm-1 appeared, contributed by the N-H stretching mode of lysozyme molecules at the interface. When the lysozyme solution was replaced by water, the SFG spectrum collected from the 1:1 mixed charged polymer/water interface recovered, but not completely. We believe that lysozyme molecules could weakly perturb the interfacial water structure on a 1:1 mixed charged polymer surface, but they did not strongly adsorb. When the lysozyme solution was replaced by water, most of the molecules left the interface, but still some lysozyme molecules stayed in the interfacial region; therefore the SFG water signal was not totally recovered. The association of lysozyme to the 1:1 mixed charged polymer is likely because the 1:1 mixed charged polymer does not have the exact same amount of positively and negatively charged polymers. As our XPS data showed, the ratio of the positively charged and negatively charged components is 0.97:1 in the 1:1 mixed charged polymer. Likely this polymer is slightly negatively charged, while lysozyme is positively charged under our experimental condition, therefore lysozyme can interact with the 1:1 mixed charged polymer surface more strongly than other proteins. This also explains why slightly more lysozyme adsorption amount than fibrinogen on the 1:1 mixed charged polymer surface was observed using SPR (Figure 2). In summary, all the proteins did not substantially adsorb to the 1:1 mixed charged polymer surface, so the 1:1 mixed


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charged polymer possesses good antifouling properties. Moreover, the influence of proteins on the interfacial water structure on the 1:1 mixed charged polymer surface is much weaker than that of pOEGMA. As reported previously, proteins could strongly perturb the interfacial water on pOEGMA surface, but they did not adsorb.31

Figure 6 SFG spectra collected from 1:1 mixed charged polymer/water (a1), 1:1 mixed charged polymer/BSA solution (a2), 1:1 mixed charged polymer/water again (a3) interfaces; 1:1 mixed charged polymer/water (b1), 1:1 mixed charged polymer/fibrinogen solution (b2), 1:1 mixed charged polymer/water again (b3) interfaces; 1:1 mixed charged polymer/water (c1), 1:1 mixed charged polymer/lysozyme solution (c2), and 1:1 mixed charged polymer/water again (c3) interfaces.

Conclusion A mixed charged polymer with a net negative or positive charge exhibits poor antifouling performance, because it could strongly adsorb molecules with opposite charge. Additionally, it 19 ACS Paragon Plus Environment

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was found that the interfacial water on such a polymer/water interface either has a low ordering structure or contains substantial amount of weakly hydrogen bonded water species. The 1:1 mixed charged polymer has an overall neutral property. Its surface is strongly hydrated, and the interfacial water molecules are mainly strongly hydrogen bonded. Similar to zwitterionic polymers, BSA and fibrinogen do not substantially perturb the interfacial water on the 1:1 mixed charged polymer surface, because such water molecules are strongly hydrogen bonded. Lysozyme could slightly perturb the interfacial water, but they do not considerably adsorb to the 1:1 mixed charged polymer. The antifouling activity of the 1:1 mixed charged polymer is due to the strong hydration of its surface, and foulants cannot replace interfacial water to stick to the 1:1 mixed charged polymer surface. The 1:1 mixed charged polymer has similar behavior upon exposure to salt solution to zwitterionic polymers. Interestingly, exposure to high concentrations of salt solutions (0.1 and 0.5 M) enhanced surface hydration of the 1:1 mixed charged polymers. It was found that a surface neutral zeta potential for a zwitterionic sulfobetaine system was achieved by adding slightly enriched quaternary ammonium monomer to the system.59 We believe that this should be true for the mixed charged polymer materials as well. Here the 1:1 mixed charged polymer system may not be the optimized material for antifouling. In the future it is necessary to study surface zeta potential, antifouling behavior, and interfacial structure of mixed charged polymer systems with lightly varied positively charged group : negatively charged group ratio to obtain the mixed charged polymers with optimized function. With excellent antifouling performance of the 1:1 mixed charged polymer and much lower cost compared to the zwitterion polymers, we believe that the mixed charged polymers will have wide applications for antifouling.


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Effect of Surface Hydration on Antifouling Properties of Mixed Charged

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