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Applications of Polymer, Composite, and Coating Materials
Universal Coatings Based on Zwitterionic-Dopamine Copolymer Microgels Mohammad Vatankhah-Varnoosfaderani, Xiaobo Hu, Qiaoxi Li, Hossein Adelnia, Maria Ina, and Sergei S Sheiko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05570 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018
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Universal Coatings Based on Zwitterionic-Dopamine Copolymer Microgels Mohammad Vatankhah-Varnoosfaderani*†§, Xiaobo Hu†§, Qiaoxi Li†§, Hossein Adelnia ‡, Maria Ina†, Sergei S. Sheiko*† †
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27599-3290, USA. ‡ Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran § M.V., X.H, and Q.L. contributed equally to this work.
Keywords: zwitterionic, dopamine, microgels, anti-fouling, anti-fogging, film
Abstract. Multifunctional coatings that adhere to chemically-distinct substrates are vital in many industries, including automotive, aerospace, shipbuilding, construction, petrochemical, biomedical and pharmaceutical. We design well-defined, nearly monodisperse microgels that integrate hydrophobic dopamine methacrylamide (Dopamine-MA) monomers and hydrophilic zwitterionic monomers. The dopamine functionalities operate as both intra-particle crosslinkers and interfacial binders, respectively providing mechanical strength of the coatings and their strong adhesion to different substrates. In tandem, the zwitterionic moieties enable surface hydration to empower anti-fouling and anti-fogging properties. Drop-casting of microgel suspensions in ambient as well as humid environments facilitates rapid film formation and tunable roughness through regulation of crosslinking density and deposition conditions.
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1. INTRODUCTION The development of universal coatings – which anchor to chemically-distinct substrates and perform diverse surface functions – has been pursued intensely in recent years.1-7 Hydrophilic coatings are particularly vital due to their anti-fouling, anti-fogging, and antifrosting properties.8-14 Amongst many hydrophilic materials, zwitterionic (ZW) polymers are distinct due to their unique anti-polyelectrolyte behavior,15 which empowers anti-fouling performance in high salinity aqueous media.16-22 Dopamine-based polymers are similarly distinguished in that they form strong covalent and coordination links to both hydrophilic and hydrophobic surfaces, even when submerged in aqueous media.1, 23-33 So far, hybrid ZW/Dopamine polymer coatings have been prepared by “grafting from” or “graft to” techniques. The “grafting from” approach requires pre-modification of a substrate surface with Dopamine-containing initiators followed by polymerization of ZW-monomers (such as SBMA) under an inert atmosphere17, 34-35. This increases fabrication costs, limits thickness regulation, and may also lead to non-uniform surface coverage. “Grafting to” techniques involve the attachment of already made zwitterionic polymers (SBMA) with Dopamine end groups, which usually results in low grafting density due to the steric hindrance and low concentration of end groups.34, 36-37 Comparing to the “graft to” and “graft from” techniques, the microgel approach possesses advantages due to (i) relative easy and fast deposition techniques to achieve dense coatings, such as dip coating, spin coating, and centrifugal deposition,38-42 and (ii) straight-forward thickness control and preparation of thick coatings by precisely controlling the microgel size.43 and (iii) nearly unlimited potential for incorporation of various functional moieties by either copolymerization or encapsulation.44-45 Herein, we report the synthesis of ZW-Dopamine microgels by dispersion polymerization with different dopamine fractions (5.0 and 10.0 mol%) and crosslinker fractions (0, 1.0, 4.0 mol%). Further, the preparation, surface properties, and possible applications of ZWDopamine microgel films were studied. Significantly, these hybrid ZW-Dopamine microgels 2 ACS Paragon Plus Environment
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enable (i) strong anchoring on metal, glass, and polymer substrates along with (ii) a distinct anti-fouling and anti-fogging surface performance.
2. EXPERIMENTAL SECTION 2.1
Materials.
N-(methacryloxypropyl)-N,N-dimethyl-N-(3-sulfopropyl)
ammonium
betaine (SBMA), N,N’-methylenebisacrylamide (MBA), azobisisobutyronitrile (AIBN), sodium
borate
decahydrate,
3,
4-dihydroxyphenethylamine
hydrochloride,
tris
(hydroxymethyl) aminomethane (Tris) and methacrylate anhydrate were purchased from Aldrich and used without further purification. Polyvinylpyrrolidone (40,000 g/mol) was purchased from Merck Chemical. Sodium bicarbonate, methanol, ethanol, and tetrahydrofuran were supplied from Fischer Scientific. 2.2 Synthesis of dopamine methacrylamide monomer (Dopamine-MA). DopamineMA was synthesized according to the literature procedure.46 Saturated aqueous solution of sodium borate decahydrate and sodium bicarbonate were prepared by adding 38 g of sodium borate decahydrate and 8 g of sodium bicarbonate in 250 ml of distilled water. The aqueous solution was bubbled with nitrogen for 20 mins, followed by adding 10 g of 3, 4dihydroxyphenethylamine hydrochloride. A mixture of 9.4 mL of methacrylic anhydride and 25 mL THF was then added dropwise into the aqueous solution by syringe pump with a rate of 1 ml/min. This reaction mixture was kept as moderately basic (pH
8) by dropwise adding
1M NaOH solution. After left stirring for 14 hrs at room temperature under nitrogen, it turned out to be a white slurry-like solution. The solution was then washed twice with 50 mL ethyl acetate and layers were separated. The solid in the aqueous phase was removed by filtration. The filtrate was acidified to pH 2 by 6M HCl and then extracted three times with 50 ml of ethyl acetate. The combined organic phase was dried over MgSO4. The solution volume was reduced to 25 ml with a rotary evaporator and the concentrated solution was precipitated in 250 mL hexane. To maximize crystal yield, the suspension was further cooled to -20 °C. After 3 ACS Paragon Plus Environment
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filtering, the resulting light brown solid was further purified by dissolving in 20 mL ethyl acetate and precipitated in 300 mL hexane. The final solid powder was dried in a vacuum overnight at room temperature. 1H-NMR (400 MHz, DMSO-d6) proved successful synthesis of Dopamine-MA by a chemical shift at 3.25 ppm, which is attributed to the formation of amide linkage between 3,4-dihydroxyphenethylamine hydrochloride and methacrylic anhydride. 2.3. Synthesis of ZW-Dopamine microgels. The reactions were carried out in 100 mL flask. SBMA (2.0 g), Dopamine-MA (83 mg for microgels with 5 mol% Dopamine groups), PVP (0.4 g), and AIBN (15 mg) were dissolved in 30 g of water/ethanol (30/70 w/w) medium. The mixture was vigorously stirred for 10 min, bubbled by nitrogen, and placed in oil bath at 60 °C while still stirring vigorously. After nucleation step, the solution of cross-linker MBA at concentration of 0, 1 or 4 mol% (in reference to SBMA) in 9 g water/ethanol (30/70 w/w) medium, was added dropwise during 2 hrs by syringe pump. The reaction was continued overnight. The resulting dispersions were centrifuged and washed with ethanol several times to remove unreacted monomers and stabilizer. The washed microgels were re-dispersed in the same medium (water/ethanol 30/70 w/w). 2.4. Film formation and film cross-linking process. The dispersion of ZW-Dopamine microgels containing different amounts of MBA cross-linker (0, 1, 4 mol%) dispersed in ethanol/water medium was drop coated onto different substrates, including glass, mica, and gold (30 µL solution of 0.04 g/mL drop on ~ 1 cm2 substrate). To evaluate different stages of film formation process, the coated substrates were first dried under ethanol vapor for 1 hr, letting the good-solvent water evaporate first. Then the substrates were exposed to water vapor in a humidity chamber (60 % RH) for controlled amount of time. A drop of Tris-HCl buffer (pH = 8.5) was added on top of the films, followed by a drop of 1 mg/mL NaIO4 solution to cross-link the films. To keep the film hydrated, they were placed in humidity chamber of 60 % RH and left overnight to reach complete oxidation of dopamine. Various 4 ACS Paragon Plus Environment
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types of substrates were tested, such as mica, glass, and gold. After NaIO4 treatment, all the coatings were washed with water spray for at least 5 times and then dried under N2 flow. 2.5. Characterization. A drop of the diluted dispersion was placed onto a filter with 200 nm pore diameter and critical point drying in carbon dioxide medium. The samples were sputter coated with gold and then examined by SEM (Philips XL30) at acceleration voltage of 20 kV. The average hydrodynamic diameter and polydispersity index (PDI) were determined by Dynamic Light Scattering (DLS, Nano ZS, Malvern Instruments) at angle of 173 by using He-Ne laser (4 mW) operated at 633 nm. Atomic Force Microscopy (AFM) (Nanoscope II, Digital Instruments) was used to investigate surface morphology at PeakForce QNM mode. Fluorescent protein fouling images were taken using Zeiss CLSM 700 Confocal Laser Scanning Microscope. 3. RESULTS AND DISCUSSIONS 3.1 Preparation and characterization of ZW-Dopamine microgels. Incorporation of water-insoluble co-monomers such as Dopamine-MA within hydrophilic zwitterionic microgels is a challenging task for synthesis. The previously reported inverse-emulsion polymerization of pure ZW microgels is not well suitable for copolymerization of waterinsoluble monomers.47 To resolve the problem, we develop a one-pot dispersion polymerization of [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA) and dopamine methacrylamide (Dopamine-MA) to synthesize ZW-Dopamine microgels. The critical point for successful dispersion polymerization is the selection of solvent, which must be good solvent for all of the used monomers and anti-solvent for the resulting oligomers and polymers. As shown in Figure 1a, both ZW and Dopamine-MA monomers are dissolved in ethanol/water (70/30, wt/wt%). When polymerization starts, the produced oligomers precipitated to form nucleus for the particle growth. Subsequent polymerization results in the growth of microgels followed by their stabilization by polyvinylpyrrolidone (PVP). In addition to the solubility issue, two problems were resolved to 5 ACS Paragon Plus Environment
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ensure stability of the reagents. First, in order to prevent oxidation of the catechol group in the dopamine monomer by persulfate-type initiators, we used 2, 2’-azobis(isobutyronitrile) (AIBN) as initiator. Second, since the crosslinker (N,N’-methylenebisacrylamide (MBA)) has higher reactivity and good solubility in both ethanol and water, which will impede the initial nucleation process, the crosslinkers (if any) were injected after nucleation step in a drop-wise way. This combination of carefully balanced formulations and an accurately timed sequence of initiation, polymerization, and crosslinking reactions enables preparation of well-defined, nearly monodisperse microgel particles. Below, we briefly describe the preparation procedure, while more details can be found in the experimental section. First, SBMA, Dopamine-MA, PVP, and AIBN were dissolved in a mixture solvent of ethanol/H2O (70/30 wt/wt%). After degassing with N2 for 30 min, polymerization started by placing the reaction vial in an oil bath of 60 oC. For preparation of cross-linked microgels, when the reaction system turned to milky, a solution of MBA was added in a drop-wise way for 2 hrs by syringe pump. While crosslinked particles are inherently stabilized by both PVP and crosslinking, using only PVP was sufficient for stabilization of uncross-linked microgels in solution during polymerization. However, upon solvent evaporation during film formation, the lack of crosslinking facilitated particle coalescence. The reaction
continued for 18 hrs under gentle agitation. The presence of Dopa and ZW moieties in the microgels was confirmed by UV-Vis and FTIR spectra (Figure S1, supporting information). This system has two control parameters: Dopamine-MA fractions (5 mol% and 10 mol%) and MBA fractions (0 mol%, 1 mol% and 4 mol%). Figures 1b-e display SEM and DLS results of ZW-Dopamine (95:5 mole ratio) microgels with different crosslinker fractions (0 mol%, 1 mol% and 4 mol%). All microgels are nearly monodispersed. For example, microgels prepared with a crosslinker fraction of 4 mol% demonstrate a hydrodynamic diameter of Dh= 800 nm and dispersity of Ð = 1.002 (RSD = 0.043) (Figure 1e). DLS results show no significant difference in dispersity of microgels with different crosslinker density. However, 6 ACS Paragon Plus Environment
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The incorporation of crosslinker has two counter effects on the size of microgels. During the polymerization, the increase of crosslinker helps to produce larger particles due to greater incorporation of growing oligomers/monomers to particles. However, higher fraction of crosslinker will result in lower swelling ratio, which results in smaller size. These counteraction factors yields smallest microgels that are prepared using an intermediated crosslinker fraction of 1 mol% (Figure 1e). Monodispersed microgels with 10 mol% DopaMA and 1 mol% MBA were also prepared (Figure S2, supporting information).
Figure 1: a) Dispersion copolymeization of [2-(methacryloyloxy)ethyl] dimethyl-(3sulfopropyl)ammonium hydroxide (SBMA) and Dopamine methacrylamide (Dopamine-MA). The reactions were carried out in ethanol/water 70/30 wt/wt% medium at 60 °C, using 2, 2‘azobis(isobutyronitrile) (AIBN) (15 mg) and polyvinylpyrrolidone (PVP) (0.4g) as initiator and stabilizer, respectively. b-d) SEM images of ZW-Dopamine (95/5 mole ratio) microgels with a crosslinker fraction of b) 0 mol%, c) 1 mol%, and d) 4 mol%. e) Dynamic light scattering of ZWDopamine (95/5 mole ratio) microgels with indicated crosslinker fractions. The slightly different size
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for SEM and DLS measurements are due to different sample preparation. For SEM, microgels in ethanol were filtered through a membrane with 200 nm pores and then dried on top of the membrane by supercritical CO2 drying technique. The DLS measurements were carried out in methanol. The scale bar is 5 µm.
3.2. Film formation. For coating preparation, purified ZW-Dopamine microgels were redispersed in ethanol/water solvent. The utilization of the ethanol/water mixture as a dispersion medium not only enables a better spreading onto different substrates, but also greatly facilitates the film formation. Similar to other latex coatings,48-50 formation of ZW-Dopamine microgel films advances in three consecutive stages: solvent evaporation, particle deformation upon spreading, and particle coalescence due to interpenetration of polymer strands (Figure 2a). To obtain solid coatings, all three stages should occur sequentially and while the coalescence process requires sufficient mobility of polymer chains at the particle surface. In this regard, it is important to emphasize again that water and ethanol act as good solvent and anti-solvent for ZW-based polymers, respectively. Under ambient conditions (T = 23 °C, relative humidity (RH) = 30%), quick evaporation of ethanol results in an increase of water fraction inside particles, which enhances swelling, deformation, and coalescence of microgel particles resulting in dense and smooth films (Route 1 in Figure 2a). The evaporation sequence can be inverted by depositing films in an ethanol-saturated atmosphere, which favors water vaporization. Due to the decreasing water fraction – good solvent for ZWpolymers, the particles vitrify and hence maintain their spherical shape yielding corrugated coatings (Route 2 in Figure 2a). However, film spreading can be reactivated by increasing humidity (Route 3 in Figure 2a). As such, dense coatings of variable thickness and surface roughness can be prepared by quenching and reactivation of the spreading and coalescence processes. This gives a facile handle for accurate control of the surface morphology of the ZW-Dopamine coatings. However, for preparation of smooth coatings, it is sufficient to use simple casting (Route 1) without vapor annealing.
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All of the afore-mentioned scenarios were documented experimentally with atomic force microscopy (AFM). A simple drop-casting method was used to prepare micron-thick films on different substrates (mica, glass, metals, polymers) from dilute (0.04 g/mL on ~ 1 cm2 substrate) aqueous dispersions. Figure 2b-e show AFM height micrographs of microgels with a crosslinker fraction of 0, 1.0, 4.0 mol% captured at different stages of the film formation process on a mica substrate. Initially, all of the studied microgels display the same film morphology of densely packed spherical particles after being incubated in an ethanolsaturated atmosphere for 1 hr (Figure 2b and Figure S3 (supporting information)), followed by progressively decreasing roughness (Figure 2c-e) under controlled environmental conditions discussed below. Naturally, the highly crosslinked (4 mol%) particles reveal the largest corrugation, which was also evidenced by strong light scattering (Figure S4, Supporting Information).
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Figure 2. a) Schematic representation of procedures of film formation under ambient environment (route 1), ethanol-saturated medium (route 2) and then water saturated medium (route 3). b) AFM height micrographs of dense films of ZW-Dopamine microgels (95/5 mole ratio) with a crosslinker fraction of 4 mol% as cast in ambient environment and after 1 hour in EtOH vapors. Under ethanol vapor, the particles shrink, yet remain spherical and densely packed. The scale bar is 1 µm. c-e) Upon exposure to water vapor in a humidity chamber (~ 60 % RH) for different amount of time (indicated at the upper-right corner), the particles undergo deformation and coalescence, resulting in smoother surface. The corresponding AFM images correspond to ZW-Dopamine microgels with different crosslinker fractions 0 mol% (c), 1.0 mol% (d), and 4.0 mol% (e) after exposure to 60% RH. Height profiles were measured along the dashed lines. f) Relative roughness was determined from AFM height profiles averaged for > 100 sample points.
As mentioned above, the film roughness can be controlled without changing cross-linking density. For this purpose, the evaporation process was shifted to promote evaporation of water prior to ethanol (Figure 2b). With the absence water as the good solvent, particles shrink and 10 ACS Paragon Plus Environment
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maintain their distinct shape. To reactivate the spreading process, the cast films were placed in a humidity chamber (60% RH), providing water to facilitate particles swelling and deformation. The subsequent coalescence of neighboring particles results in solid films of controlled roughness depending on crosslink density (Figures 2c-e). We also studied films from microgels with a higher Dopamine fraction. For example, Figure S2 (Supporting Information) shows flat films prepared of microgels containing 10 mol% Dopamine and 1 mol% MBA. Catechols are readily oxidized to form reactive quinones, which further transform into covalent crosslinks.51 Therefore, after achieving a desired film thickness and morphology, the coatings are promptly stabilized by treating the film in a weak basic Tris-HCl buffer solution (pH = 8.5) with a strong oxidant (NaIO4). NaIO4 is one of the most commonly used chemical oxidants for catechols. As shown in Figure 3a, the oxidization of catechols results in covalent links both between neighboring particles within a film (designated by short dashed lines on the right-hand side of the panel). As an evidence of the happening of crosslinking between microgels, we observed formation of a microgel network upon adding oxidant agent (NaIO4) to the PBS solution of microgel particles (0.15 g/ml) (Figure S5). Oxidization and postcrosslinking of the catecholic groups was also monitored by UV-Vis and FTIR spectroscopy (Figure S1, Supporting Information) using protocols described elsewhere.29, 51 Moreover, it is reasonable to assume that microgels have strong adhesion to substrates due to the formation of covalent and noncovalent interactions between dopamine and substrates.1, 23 3.3. Contact angle, anti-fogging, and anti-frosting of films. The quinone-anchored and cross-linked coatings were further explored with regard of potential applications. Figures 3b,c display the results of the contact angle measurements. As shown in Figure 3b, coating with ZW-Dopamine microgels (5 mol% Dopa-MA and a fraction of 0 mol% MBA) results in a significant decrease of the contact angle of a brine solution from 40±1o (on a bare glass) to 12±1°, which indicates a more hydrophilic composition of the coating. Significantly, the 11 ACS Paragon Plus Environment
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contact angle of the brine solution is lower than that of pure water (25±1°) due to the antipolyelectrolyte properties of ZW-based polymers. As mentioned above, the Dopamine functionality enables strong adhesion of the synthesized ZW-Dopamine microgels to different substrates. Figure 3c summarizes the results of the water contact angle measurements on different uncoated and coated substrates, including metal (Al), inorganic non-metals (SiO2 and glass), and polymers (polytetrafluoroethylene (PTFE), polystyrene (PS) and polyethylene terephthalate (PET)). Regardless of the nature of the substrate, the ZW-Dopamine coated substrates became more hydrophilic which was evidenced by a significant decrease of the contact angle against water. The similar equilibrium contact angles for all the coated substrates suggest that microgels form uniform and stable coatings on chemically different substrates. The coating stability is ascribed to strong adhesion which is validated by unchanging contact angle values after long equilibration and thorough washing of coated substrates with water jet.
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Figure 3. a) Oxidation of catechol groups to quinone (in presence of oxidant agent Tris-HCl buffer solution with pH=8) enables both inter-particle crosslinking between microgel particles and the substrate as shown by short dashed lines. After film formation, tris-buffer solution (pH 8.5) was dropped on top of the substrate and left overnight to fully crosslink Dopamine-MA groups in a humidity chamber of 60 % RH. b) Contact angle of brine (10 wt% of NaCl) and pure water on uncoated and ZW-Dopamine-coated glass was measured after 600s equilibrium. c) Contact angle of pure water was measured on different substrates (as indicated) before and after coating with ZWDopamine (95/5 mol/mol%) microgels containing 0 mol% MBA. The contact angle was measured 600s after film deposition to reach equilibrium. d) Anti-fogging performance due to superhydrophilicity. Glass slides were partially coated with Zw-Dopamine (95/5 mol/mol%) microgel films with different crosslinker fractions as indicated, using the previously-described procedures. After cooling to -20 °C, the slides were taken out and exposed to ambient conditions (23 °C, 30% RH). The coated part remained clear, while the uncoated part fogged up within 30 seconds. e,f) Anti-frosting performance. The uncoated and coated glass slides with microgels contain 0, 1.0 and 4.0 mol% crosslinker were exposed to 85% RH for 3 hours followed by quick cooling to -20 oC for 2 hours to
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freeze the absorbed water on the slides (e) and then they were exposed to the ambient conditions for 1 min (f). The ice on the coated slides was melted while the ice remains on the uncoated slide.
As a result of the enhanced hydrophilicity, the surface demonstrates anti-fogging and antifrosting behavior (Figure 3d-f). As water contact angle decreases, surface hydration prevent further condensation of water. We cooled the coated glass slides at -20 °C for 1 h, and then transferred them to ambient lab conditions (23 °C, 30 % RH). The uncoated glass surface fogged up quickly within 30 s exposure to humid air, whereas the coated surface maintained high clarity, demonstrating an excellent anti-fogging behavior (Figure 3d). For anti-frosting test, a Zw-Dopamine-coated glass substrate was exposed to a relative humidity 85% for 3h, and then the temperature was quickly cooled down to -20 oC and kept for 2 h to observe the frost formation. As shown in Figure 3e, the ZW-Dopamine coated glass exhibit less frost than the uncoated glass. After exposing to ambient condition for 1 min, the ice on the coated slides melted quickly while the ice on the uncoated slide remained (Figure 3f and Movie S1 (Supporting Information)). Substrates coated with lower crosslinked microgels show stronger anti-fogging and anti-frosting performance due to the better surface coverage. From Figure 3d and 3f, we can also see that the microgel coated glasses demonstrate high transparency, which is very important for applications like anti-fogging. 3.4. Anti-fouling of film. To test the anti-fouling performance, a protein adsorption assay was carried out. Namely, a partially coated gold substrate was soaked in 0.1 mg/mL fluorescein isothiocyanate labeled bovine serum albumin PBS solution. After gentle washing with PBS, the substrates were examined under confocal microscopy. As shown in Figure 4 all of the coated substrates showed low/non-fouling of bovine serum albumin protein, compared to the corresponding uncoated substrates. In Figure S6 (Supporting Information), we showed the anti-fouling properties of coated glass, aluminum and polystyrene film those coated with Zw-Dopamine microgels after exposed with fluorescein isothiocyanate labeled BSA solution (0.3 mg/ml PBS). 14 ACS Paragon Plus Environment
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Figure 4. (a-c) Confocal microscopy of anti-fouling performance. Fluorescent-tagged bovine serum albumin (BSA) in PBS solution (0.1 mg/mL) was used to test the anti-fouling performance of the coatings. Gold substrates were partially coated with Zw-Dopamine (95/5 mol/mol%) microgel films with different crosslinker fractions as indicated and incubated with BSA-solution overnight. The coated side showed nearly no stain of the fluorescent proteins, whereas the uncoated bare gold surface showed many protein aggregations. The scale bar is 100 µm. (d) Average fluorescence intensity of samples exposed to fluorescently tagged BSA protein.
4. CONCLUSIONS In conclusion, well-defined narrowly-dispersed zwitterionic-dopamine microgels were synthesized and readily applied to a wide spectrum of substrates resulting in their hydrophilic surface modification. The resulting coatings show a dense structure of tunable roughness, depending on both the cross-link density and preparation conditions. To tune the roughness without manipulating the crosslink density, film formation process was carried out under controlled vapor pressures of water (good solvent) and ethanol (anti-solvent for ZwDopamine microgels). To fix a desired surface roughness, post-cross-linking was conducted through oxidizing catecholic groups of Dopamine-MA using Tris buffer. As long as complete surface coverage is achieved, the ZW-dopa films exhibit distinct anti-fouling and anti-fogging performances. Even though further increase of the film thickness is not required for property enhancement, the thickness can be readily tuned by changing the microgel size, concentration of microgels in the dispersions, and deposition strategies.39,
41, 43, 52
This method offers
promising advantages over traditional hydrophilic surface modifications via pSBMA grafting, including but not limited to convenient processing, low cost, high surface coverage efficiency, and superior hydration properties.
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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional experimental results are included in the support information, AFM images of films with different microgels, appearance of coated glasses, oxidation induced gelation of microgel solution, UV-vis and FTIR of oxidated catechol groups, confocal micrographs of coated and uncoated substrates after incubation with fluorescent-tagged BSA, and so forth. AUTHOR INFORMATION Corresponding Authors *(M.V.) E-mail:
[email protected] *(S.S.S.) E-mail:
[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We acknowledge financial support from the National Science Foundation DMR 1407645 and Halliburton Company.
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