Polyzwitterion Coatings for Underwater

Aug 27, 2018 - These bioinspired coatings display multifunctional properties such as underwater antioil-adhesion and antifreezing thanks to their high...
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Bioinspired Polydopamine/Polyzwitterion Coatings for Underwater Anti-Oil and -Freezing Surfaces Meng-Qi Ma, Chao Zhang, Ting-Ting Chen, Jing Yang,* Jian-Jun Wang, Jian Ji, and Zhi-Kang Xu*

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MOE Key Laboratory of Macromolecular Synthesis and Functionalization, and Key Laboratory of Adsorption and Separation Materials and Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: Zwitterionic polymers are continually suggested as promising alternatives to tune the surface/interface properties of materials in many fields because of their unique molecular structures. Tremendous efforts have been devoted to immobilizing zwitterionic polymers (polyzwitterions, PZIs) on the material surfaces. However, these efforts usually suffer from cumbersome and time-consuming procedures. Herein we report a one-step strategy to facilely achieve the bioinspired polydopamine/polyzwitterion (PDA/PZI) coatings on various substrates. It requires only 30 min to form PDA/PZI coatings by mixing oxidant, dopamine, and zwitterionic monomers, including carboxybetaine methacrylate (CBMA), sulfobetaine methacrylate (SBMA), and 2-methacryloxyethyl phosphorylcholine (MPC). These bioinspired coatings display multifunctional properties such as underwater antioil-adhesion and antifreezing thanks to their high hydrophilicity and underwater superoleophobicity. The coatings even show the antiadhesion property for crude oil with high viscosity. Therefore, the PDA/PZI-coated meshes are efficient for separating both light oil and crude oil from oil/water mixtures. All these results demonstrate that the one-step strategy is a facile approach to design and exploit the bioinspired PDA/PZI coatings for diverse applications.

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UV irradiation,12 and plasma treatment13). For instance, Jiang and coworkers reported that uniform PSBMA brushes could be constructed onto the initiator-covered gold surfaces via atom transfer radical polymerization.14 These processes not only improve the grafting density, but also provide various choices for different zwitterionic monomers. Nevertheless, they usually require cumbersome operations and lack of substrateindependent versatility. Therefore, it still remains a great challenge to develop a universal and easy-to-implement strategy to attach PZIs to diverse substrate surfaces. Mussel-inspired chemistry has been regarded as the most promising breakthrough to fulfill different requirements of surface modification because of some special features, such as mild reaction condition, surface-adaptive adhesion, and fascinating postfunctionalization accessibility.15 Two cases have been reported: PDA acting as an intermedium layer followed by “grafting-to” or “grafting-from” methods and PDA as the adhesive sites to assist the target PZIs in incorporating

INTRODUCTION Polyzwitterions (PZIs) have been widely found in organism. Their typical structures are comprised of equivalently positive and negative charge groups, therefore, the whole molecule exhibits electric neutrality.1 This unique molecular structure bestows those PZIs with robust water-binding capacity to form hydration layer on their surfaces. Furthermore, there are strong dipole−dipole interactions between the molecular chains.2 On the basis of these distinctive features, PZIs have been designed as diverse functional coatings for a wide range of applications in biomaterials,3 biosensors,4 lubricants,5 energy devices,6 and antifouling surfaces.7 Therefore, it is highly desired to design and exploit a variety of routes to graft or coat PZIs on the material and device surfaces. To date, “grafting-to” and “grafting-from” approaches have been well studied to anchor PZIs onto the material surfaces.8 For example, poly(sulfobetaine methacrylate) (PSBMA) was successfully grafted onto the gold surface.9 However, these “grafting-to” methods usually suffer from a complex process of PZI synthesis/purification, low density of anchored chain, and specific pretreatment for substrate surface.1b In view of the “grafting-from” methods, the primary demand is to immobilize suitable initiators on the material surfaces and then to initiate the polymerization of zwitterionic monomers via different processes (transition-metal catalysis,10 ozone inducement,11 © XXXX American Chemical Society

Special Issue: Zwitterionic Interfaces: Concepts and Emerging Applications Received: July 10, 2018 Revised: August 21, 2018 Published: August 27, 2018 A

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Langmuir into the codeposited coatings.16 It is quite obvious that the codeposition method is easy to be implemented. Therefore, it is reasonable to explore these mussel-inspired chemistries, especially those polydopamine (PDA) codeposition approaches, for attaching PZIs to the material surfaces.17 However, the noncovalent interactions between PDA and PZIs result in poor stability of the coatings. Furthermore, PZIs are also requiring multistep synthesis and purification before the codeposition process.18 To address these issues, we recently reported a versatile approach to achieve surface functionalization with dopamine and acrylate monomers via a one-step polymerization and codeposition strategy.19 However, this one-step strategy usually takes too much time (about 8 h). To our delight, we found that the reaction rate was mainly determined by the oxidative polymerization of dopamine. Oxidant like CuSO4/H2O2 can be employed as a trigger to fabricate uniform and stable PDA coatings rapidly in our previous work.20 In combination with the rapid deposition of PDA as well as the one-step polymerization and codeposition protocols, we propose a facile one-step strategy adopting oxidants to accelerate the codeposition process of dopamine with typical zwitterionic monomers including CBMA, MPC and SBMA. The as-constructed coatings are marked as PDA/ PCBMA, PDA/PMPC, and PDA/PSBMA, respectively (Figure 1). It takes only 30 min to complete the whole

nonwoven fabrics were obtained by Mitsubishi (Japan). Crude oil was supplied by Sinopec Group. Polypropylene microfiltration membranes (PPMM, mean pore size ∼ 0.2 μm) were acquired from Membrana GmbH (Germany) and cleaned by acetone overnight to remove adsorbed impurities. Tris(hydroxymethyl) aminomethane (Tris), dopamine hydrochloride (DA), sulfobetaine methacrylate (SBMA), 2-methacryloxyethyl phosphorylcholine (MPC), and 2(dimethylamino)ethyl methacrylate (DMAEMA) were commercially purchased from Sigma-Aldrich. β-Propiolactone and FL-BSA (pI 6.0, 66.72 kDa) were acquired from Macklin (China) and Shanghai Jiahe Biotechnology Co., Ltd. (China), respectively. Oil red O and Nile Red were obtained from Aladdin. Copper(II) sulfate pentahydrate (CuSO4·5H2O), hydrogen peroxide (H2O2, 30%), potassium persulfate (K2S2O8), sodium periodate (NaIO4), ethanol, ethyl ether, acetone, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), sodium chloride, dichloroethane, hexane, sodium hydroxide (NaOH), and hydrochloric acid (HCl, 98%) were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used without further purification. Ultrapure water (18.2 MΩ) from the ELGA Lab Water system (France) was used in all experiments. Synthesis of Carboxybetaine Methacrylate (CBMA). CBMA was synthesized by β-propiolactone and DMAEMA reaction according to a previous report.21 β-Propiolactone (0.87 g) and DMAEMA (1.57 g) were dissolved in 10 mL of anhydrous acetone, respectively. The β-propiolactone solution was added dropwise to DMAEMA solution. The mixture was stirred for 5 h at 4 °C under nitrogen gas protection. The white precipitation that appeared was washed by anhydrous acetone and anhydrous ethyl ether and dried under reduced pressure to get CBMA monomer. PDA/PZI Coatings on Different Substrates. DA (2 mg/mL) and zwitterion monomer (50 mg/mL) were dissolved in Tris buffer solution (pH = 8.5, 50 mM). Then the oxidants were added. The oxidants include CuSO4 (1.25 mg/mL)/H2O2 (0.67 mg/mL), K2S2O8 (1 mg/mL), and NaIO4 (1 mg/mL). The substrates prewetted by ethanol were immersed in the mixed solution for 30 min at 25 °C. Then the PDA/PZI-coated substrates were rinsed with ultrapure water overnight. The PDA/PZI-coated PPMMs were dried in a vacuum oven and the other substrates coated by PDA/PZI were dried by nitrogen gas. In optimum dopamine/SBMA mass ratio experiments, the concentration of SBMA was varied individually while the other parameters were unchanged. Chemical Stability of PDA/PZI Coatings. The PDA/PSBMAcoated silicon wafers triggered by different oxidants were immersed in organic solvents with various polarity as well as strong acid (0.1 M HCl) and alkali (0.1 M NaOH) solutions for 2 h. Then the submerged coatings were rinsed with ultrapure water and dried by nitrogen gas. The elution amount (E, %) was calculated by the following equation:

Figure 1. Schematic illustration of oxidant-triggered one-step codeposition of dopamine with zwitterionic monomers for constructing PDA/PZI coatings on various substrates. Oxidants contain CuSO4/H2O2, K2S2O8 and NaIO4. Substrates include silicon wafer, glass sheet, polycarbonate plate, nonwoven fabrics, stainless steel mesh, and polypropylene microfiltration membrane (PPMM).

E=

(1)

where Ta and Tb represent the thickness (nm) of pristine PDA/ PSBMA coatings and the thickness (nm) of those immersed in different solvents and solutions, respectively. Underwater Antioil-Adhesion Assay of PDA/PZI Coatings. The fully dried PDA and PDA/PZI-coated substrates were prewetted by water for a few seconds before the antioil-adhesion assay. The samples were adhered by oil in the air and then submerged in water. Digital photos were taken during the immersion processes. Oil−Water Separation Assay. The fully dried PDA and PDA/ PMPC-coated stainless steel meshes were prewetted by water for a few seconds and then sandwiched in a homemade filtration apparatus composed of two rubber O-rings and two glass tubes. The oil/water mixtures (v/v = 1/1) were poured onto the meshes and the separation was solely driven by the gravity of the liquid. The hexane and dichloroethane are labeled by oil red O. Digital photos were taken during the separation processes. Antifreezing Assay of PDA/PZI Coatings. The as-prepared PDA and PDA/PZI-coated silicon wafers were deposited in the

construction process, and these resulting PDA/PZI coatings exhibit outstanding underwater antioil-adhesion properties and excellent antifreezing performance. Overall, this method significantly simplifies and enriches the surface modification and further sheds a new light on the practical applications of these bioinspired PDA/PZI coatings.

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Ta − Tb × 100% Ta

EXPERIMENTAL SECTION

Materials. Silicon wafers and glass sheets were purchased as received, washed by piranha solution (98% H2SO4/30% H2O2, v/v = 7/3) for 30 min, rinsed with ultrapure water, and then dried by a stream of nitrogen gas before use. Polycarbonate plates and stainless steel meshes (apertures ∼ 38 μm) were acquired as received. PET B

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Figure 2. (a) Thickness, (b) WCA and OCA-W, (c) FESEM images, and (d) AFM images of PDA and PDA/PZI coatings on silicon wafers fabricated by the one-step codeposition strategy from the mixtures of dopamine (2 mg/mL), zwitterionic monomers (50 mg/mL), CuSO4 (1.25 mg/mL), and H2O2 (0.67 mg/mL) for 30 min at 25 °C. The scale of AFM images is 5 × 5 μm2. The volume of both water drop and oil (dichloroethane) is 2 μL. was determined by total organic carbon analyzer (TOC, GE Sievers InnovOx ES, U.S.A.).

middle of a sample cell composed of a rubber O-ring and two cover glasses. A water droplet (0.1 μL) was dropped on the coatings and the sample stage was cooled down at a rate of 5 °C/min until the droplet was frozen. For the delay time assay, the water droplet was 0.2 μL. According to the literature, the freezing temperature is determined by a sudden change in transparency when the water droplets form an ice nucleus, and the delay time is defined as the time interval between the time when the substrate reaches a target temperature and the time when the ice nucleus appears.22 Characterization. The molecular weights of the products in the solutions of zwitterionic monomers, zwitterionic monomers/CuSO4/ H2O2, zwitterionic monomers/dopamine, and zwitterionic monomers/dopamine/CuSO4/H2O2 were measured by gel permeation chromatography (GPC, Waters 1524, 2414, U.S.A.). The Fourier transform infrared (FT-IR) spectra were detected by an infrared spectrophotometer (Nicolet 6700, U.S.A.) along with an ATR accessory (ZnSe crystal, 45°). The thickness of the coatings was measured by the spectroscopic ellipsometer (Semilab Sopra, GSE-5E, China) with an incident angle at 70° and light spot size at 360 × 360 μm2. The surface chemical compositions were analyzed by X-ray photoelectron spectroscopy (XPS, PerkinElmer, U.S.A.). The morphologies of the modified surfaces were confirmed by field emission scanning electron microscope (FESEM, Hitachi S4800, Japan). Water contact angle (WCA) and oil contact angle in water (OCA-W) were recorded with a DropMeter A-200 contact angle system (MAIST VisionInspection and Measurement Co. Ltd., China). The surface roughness of the samples was observed by scanning probe microscopy (VEECO, Multimodel, U.S.A.). The oil content in filtrate

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RESULTS AND DISCUSSION

A crucial issue in our one-step strategy is the free radical polymerization of zwitterionic monomers into high-molecularweight PZIs accompanying with the formation of PDA via the oxy-polymerization of dopamine. Taken SBMA as an example, it can be seen that both CuSO4/H2O2 and dopamine can convert this zwitterionic monomer into oligomers with an average molecular weight lower than 4000. However, highmolecular-weight PZIs (M̅ n > 104) can be synthesized when CuSO4/H2O2 and dopamine are simultaneously added into the aqueous solutions of CBMA, MPC, and SBMA (Figure S1 in the Supporting Information). It is probably due to the fact that the reactive oxygen species produced by CuSO4/H2O223 can oxidize dopamine into semiquinone radical species24 to initiate the free radical polymerization of zwitterionic monomers. Previous work has indicated that dopamine is able to be codeposited with PSBMA to form bioinspired PDA/PSBMA coatings.7b Therefore, we suggest that these bioinspired coatings are reasonable to be rapidly and simultaneously constructed via the polymerization of zwitterionic monomers with the codeposition of the CuSO4/H2O2-triggered PDA from their aqueous mixtures. C

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Langmuir The mass ratio of dopamine to zwitterionic monomer in the aqueous mixture has a great impact on the codeposited thickness, the surface wettability and the surface morphology of these bioinspired PDA/PZI coatings. Taken SBMA as an example (Figures S2 and S3 in the Supporting Information), the optimum mass ratio of dopamine/SBMA is around 1:25 (see detailed discussion in the Supporting Information) using CuSO4/H2O2 as an oxidant. On the basis of this ratio, various PDA/PZI coatings, including PDA/PCBMA, PDA/PMPC, and PDA/PSBMA, have been facilely and rapidly constructed on silicon wafers from mixtures of dopamine and corresponding zwitterionic monomers using CuSO4/H2O2 as a trigger. It should be noted that 30 min is enough to fabricate these coatings with thickness more than 25 nm (Figure 2a), demonstrating that it is a simple and fast strategy to immobilize PZIs on various substrate surfaces (Table S1 in the Supporting Information). Nevertheless, the deposition efficiency of PDA is slightly suppressed by PZIs because the excellent hydrophilicity of PZIs makes PDA/PZI aggregates tend to be dispersed in solution rather than deposited onto the substrate surfaces. Therefore, the thickness of the PDA/PZI coatings is always less than that of the PDA ones. The chemical compositions of these coatings were analyzed in details by XPS spectra (Table S2 and Figure S5 in the Supporting Information). The results indicate that only a small amount of PZIs have been incorporated into the coatings, and they follow a decreased sequence of PMPC > PSBMA > PCBMA. Owing to the participation of PZIs, the as-prepared PDA/PZI coatings show high hydrophilicity in the air and superoleophobicity under water (Figure 2b). The WCA is 42° on the PDA coatings, whereas all the PDA/PZI coatings show a WCA lower than 15°. On the other hand, the PDA/PZI coatings have a higher OCA-W between 164° and 170° compared with that of 148° on the PDA coatings. This surface wettability is derived from both the coating compositions and the coating morphology. Figure 2c displays that plenty of nanoparticles exhibit on the PDA/PZI coatings. It indicates that the codeposition reduces the coating uniformity and thus their roughness is increased relatively compared to the PDA coatings (Figure 2d). It is probably because that the small PDA/PZI aggregates are prone to be dispersed in the deposition solution because of their high hydrophilicity as mentioned above, while the large ones tend to be deposited onto the substrate surfaces driven by gravity, resulting in increased roughness of PDA/PZI coatings. All these results demonstrate that PZIs can be facilely anchored to the substrate surfaces with the assistance of CuSO4/H2O2 via musselinspired chemistry. Except for CuSO4/H2O2, two typical oxidants such as K2S2O8 and NaIO4 were also employed to facilitate the codeposition of dopamine with zwitterionic monomers. Taking the PDA/PSBMA as an example, the thickness of codeposition coatings triggered by K2S2O8 and NaIO4 can reach up to 19 and 16 nm in only 30 min, respectively. The corresponding WCAs are all lower than 15°, indicating a high hydrophilicity (Figure 3a and 3b). This demonstrates that the one-step codeposition strategy can attach hydrophilic polyzwitterions to the substrate surfaces with the assistance of various oxidants. In addition, the chemical stability of three oxidant-triggered PDA/PSBMA coatings was evaluated under diverse conditions (Figure 3c). Organic solvents (acetone, DMF, DMSO), acidic solution (0.1 M HCl), and basic solution (0.1 M NaOH) were used as typical mediums. For a given medium, it is found that

Figure 3. (a) Thickness and (b) WCA of PDA/PSBMA coatings on silicon wafers triggered by CuSO4/H2O2, K2S2O8, and NaIO4, respectively. The one-step codeposition strategy was conducted for 30 min at 25 °C from the mixtures of dopamine (2 mg/mL), SBMA (50 mg/mL), and different oxidants: CuSO4 (1.25 mg/mL)/H2O2 (0.67 mg/mL), K2S2O8 (1 mg/mL), and NaIO4 (1 mg/mL). (c) Elution amount of the PDA/PSBMA coatings immersed in different organic solvents and aqueous solutions for 2 h.

the elution amount for the sample triggered by CuSO4/H2O2 is always lower than that triggered by K2S2O8 and NaIO4, suggesting the best chemical stability resisting organic solvents with diverse polarity, strong acidic and basic solutions. The reason could be ascribed to the residual copper ions in the CuSO4/H2O2-triggered PDA/PSBMA coatings which indeed act as cross-linking sites to chelate the amine and imine groups with enhanced stability of the codepostion coatings.20 Therefore, CuSO4/H2O2 has proved to be a desired choice for fabricating PDA/PZI coatings with high stability. It is very interesting that the oil droplets are hardly attached onto the PDA/PZI coatings underwater even when they are deformed by pressing during the measurements of OCA-W. No visible oil residuals are left on the PDA/PZI coating surfaces when the syringe is retracted (Figure 4). On the other hand, serious deformation of oil droplet can be seen when the syringe is lifted from the PDA coatings, and finally the oil droplet remains on the coating surface. This oil adhesion property was further evaluated on a stage with a tilt angle of 1°. The oil droplets slide off from the PDA/PMPC, PDA/PSBMA, and PDA/PCBMA coatings in 7, 9, and 13 s, respectively (Figure S6 in the Supporting Information), demonstrating better underwater superoleophobicity property and underwater antioil-adhesion performance than other surfaces reported in literatures (Table S3 in the Supporting Information). The underwater antioil-adhesion property is attributed to the remarkable hydration capacity of PZIs.25 It is well-known that PZIs can bind water molecules tightly and form a dense hydration layer on the PDA/PZI coatings, preventing the oil droplets from directly contacting the coating surfaces. In contrast, the oil droplets are easy to pin on the PDA coatings D

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adhesion. The underwater antioil-adhesion property was further evaluated by different oils. Both light oil (hexane) and heavy oil (crude oil) can immediately levitate off the prewetted PDA/PZI-coated PPMMs when they are immersed into water, and no visible oil residuals are left on the membrane surfaces (Figure S8 in the Supporting Information). The oil can be thoroughly removed by water, proved by the absence of luminescence of Nile Red under the UV light, whereas the PDA ones show obvious luminescence indicating a serious oil fouling (Figure S9 in the Supporting Information). These results depict that PZIs can efficiently endow the bioinspired coatings with remarkable underwater antioiladhesion performance for both light and heavy oils. This material-independent underwater antioil-adhesion property of PDA/PZIs has practical significance in various fields, such as underwater anticrude-oil-adhesion for oil tanker,26 petrochemical industry,27 and oil/water separation.28 The antioil-adhesion of such PDA/PZI coatings is promising to prompt an effective separation for oil/water mixtures which are frequently produced in oil spill accidents.29 The PDA/ PMPC-coated stainless steel meshes were used to separate the mixtures of crude oil and water. Water permeates through the meshes driven by gravity but the crude oil is retained (Figure S10a in the Supporting Information). With increasing the volume of mixtures, the oil content in the filtrate is still no more than 2.0 ppm, indicating an efficient separation of crude oil and water (Figure S11 in the Supporting Information). Although the retained oil gradually covers the PDA/PMPCcoated meshes during the separation process, the meshes can be easily cleaned by water and reused (Figure S10b in the Supporting Information) because of their excellent underwater anticrude-oil-adhesion property. These PDA/PMPC-coated meshes are also useful to separate light oil from water (Figure S12b in the Supporting Information). In contrast, the PDAcoated stainless steel meshes are permeated by both water and oil (Figures S10c and S12a in the Supporting Information), showing no separation function for the oil/water mixtures. It is gradually recognized that antifreezing is a significant property for practical applications under the outdoor and lowtemperature environments. The superior hydration capability of PZIs can also make them expected candidates for constructing antifreezing coatings. The antifreezing performance of PDA/PZI coatings was evaluated by detecting the freezing temperature and the delay time. Figure 6a shows that the freezing temperatures of PDA/PMPC, PDA/PSBMA, and PDA/PCBMA surfaces are −22.39 ± 1.42, −20.56 ± 1.22, and −19.65 ± 1.30 °C, which are 2.98, 1.15, and 0.24 °C lower than that of the PDA-coated surfaces, respectively, indicating that the PZIs can suppress the formation of ice to some extent. The delay times of PDA/PMPC, PDA/PSBMA, and PDA/ PCBMA coatings are 439.5 ± 44.7, 148.4 ± 31.2, and 29.5 ± 13.6 s, which are 419.4, 128.3, and 9.4 s longer than that of the PDA ones, respectively. The antifreezing capacity of these coatings follows a sequence of PDA < PDA/PCBMA < PDA/ PSBMA < PDA/PMPC, which is in accordance with the codeposition amount of the PZIs aforementioned (Table S2 in the Supporting Information). In principle, the rate of ice formation is strongly controlled by the fraction of ice-like interfacial water.22 It is prone to form ice when there is a large fraction of ice-like interfacial water on the surface, resulting in high freezing temperature and short delay time. In our cases, the PZIs are able to attract water molecules tightly by the electrostatic interaction, and these captured water molecules

Figure 4. Series of OCA-W images taken when 2 μL oil (dichloroethane) droplets were pressed, retracted, and left on the PDA and PDA/PZI coatings fabricated on silicon wafers by the onestep codeposition strategy from mixtures of dopamine (2 mg/mL), zwitterionic monomers (50 mg/mL), CuSO4 (1.25 mg/mL), and H2O2 (0.67 mg/mL) for 30 min at 25 °C. The arrow indicates the direction of syringe movement.

and remain pinned even when the coated substrate is vertically placed (Figure S7 in the Supporting Information). The underwater antioil-adhesion property was compared in detail using the PDA/PMPC coatings fabricated on various commercial substrates, including silicon wafer, glass sheet, polycarbonate plate, nonwoven fabrics, stainless steel mesh, and PPMM. It can be seen from Figure 5 that the crude oil spontaneously escapes from the coated surfaces when the PDA/PMPC-coated substrates are submerged in water. The PDA/PMPC-coated substrates turn to be clean immediately while the PDA-coated ones show heavy underwater crude-oil-

Figure 5. Photos for the underwater anticrude-oil-adhesion property of the PDA/PMPC coatings. The coatings were fabricated on different substrates by the one-step codeposition strategy from the mixtures of dopamine (2 mg/mL), MPC (50 mg/mL), CuSO4 (1.25 mg/mL), and H2O2 (0.67 mg/mL) for 30 min at 25 °C. E

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Figure 6. Freezing temperature and delay time of (a) PDA and PDA/PZI coatings and (b) pristine and salt-treated PDA/PSBMA coatings on silicon wafers. The target temperature of the delay time assay is −18 °C. (c) Schematic illustration of PDA/PZI coatings with hydration layer and ion-rich interfacial layer after saline solution treatment.

zwitterionic monomers for constructing bioinspired coatings on diverse substrates in just 30 min. The as-fabricated PDA/ PZI coatings exhibit excellent underwater antioil-adhesion performance for efficient oil/water separation, even for the high viscosity crude oil. Besides, the antifreezing property is also achieved due to the outstanding hydration capability of PZIs. We believe this strategy possesses significant advantages over the existing ones, injecting new vitality to the development of bioinspired coatings for surface modification.

are very hard to escape and to form ice-like water. Therefore, the stronger hydration capacity PZI owns, the better its antifreezing properties are. According to previous reports, each PMPC, PSBMA, and PCBMA repeating unit can capture 23− 24,30 7.86, and 6.3231 water molecules, respectively. These data demonstrate the hydration capability obeys the following sequence: PMPC > PSBMA > PCBMA, which is also in agreement with the antifreezing performance of the PDA/PZI coatings aforementioned. These results indicate that the antifreezing capacity of these coatings is related to both the codeposition amount and hydration ability of PZIs. To further elucidate the antifreezing property mentioned above, we used inorganic salt to weaken the hydration capacity of PZIs. Figure 6b shows that the freezing temperature goes up from −22.39 ± 1.49 to −18.85 ± 1.18 °C after the PDA/ PSBMA coatings are immersed in saline solution (NaCl). The water droplets on the coating surfaces turn into ice within 118.2 ± 33.5 s, which is close to one-third delay time of the pristine PDA/PSBMA coatings (300.2 ± 65.2 s). After treated by the saline solution, the zwitterionic parts of the PDA/ PSBMA coatings will adsorb Cl− and Na+ correspondingly to form an ion-rich interfacial layer32 (Figure 6c), undermining the hydration capacity of PSBMA, which can be characterized by the changes of surface wettability. It can be seen that the WCA of the salt-treated PDA/PSBMA coatings increases from 12.4° to 28.3° (Figure S13 in the Supporting Information). Thus, less water molecules are attached to PZIs and it is easy for water molecules to form ice-like interfacial water, leading to a discount of antifreezing performance. These results in turn give a strong proof that the hydration capability of PZIs is the key to their antifreezing performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02320.

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Supporting figures, tables, and references (DOCX).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jian-Jun Wang: 0000-0002-1704-9922 Jian Ji: 0000-0001-9870-4038 Zhi-Kang Xu: 0000-0002-2261-7162 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support is acknowledged to the National Natural Science Foundation of China (Grant No. 21534009) and the 111 Project (Grant No. B16042).

CONCLUSION In summary, we present an oxidant-triggered one-step approach to realize the codeposition of dopamine with F

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DOI: 10.1021/acs.langmuir.8b02320 Langmuir XXXX, XXX, XXX−XXX