Self-Healing Superhydrophobic Fluoropolymer Brushes as Highly

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Self-Healing Superhydrophobic Fluoropolymer Brushes as Highly Protein-Repellent Coatings Zhanhua Wang†,‡ and Han Zuilhof*,‡,§ †

Materials innovation institute (M2i), Elektronicaweg 25, P.O. Box 5008, 2600 GA, Delft, The Netherlands Laboratory of Organic Chemistry, Wageningen University, Stippeneng 4, 6708 WE Wageningen, The Netherlands § School of Pharmaceutical Science and Technology, Tianjin University, 92 Weijin Road, Nankai District, Tianjin, P.R. China ‡

S Supporting Information *

ABSTRACT: Superhydrophobic surfaces with micro/nanostructures are widely used to prevent nonspecific adsorption of commercial polymeric and/or biological materials. Herein, a selfhealing superhydrophobic and highly protein-repellent fluoropolymer brush was grafted onto nanostructured silicon by surfaceinitiated atom transfer radical polymerization (ATRP). Both the superhydrophobicity and antifouling properties (as indicated for isolated protein solutions and for 10% blood plasma) are well repaired upon serious chemical degradation (by e.g. air plasma). This brush still maintains excellent superhydrophobicity and good antifouling properties even after 5 damage-repair cycles, which opens a new door to fabricate long-term antifouling coatings on various substrates that can be used in harsh environments.

1. INTRODUCTION Inspiration from natural nonwetting superhydrophobic surfaces with micro/nanostructures,1 such as the leaves of the lotus2 and the compound eyes of insects,3 is widely used to make surfaces that prevent the nonspecific adsorption of polymeric and/or biological materials.4−6 Fabricating nanocomposite structures with fluorinated building blocks that can adhere to a surface or modifying a micro/nanostructured surface with fluorinated materials are the two main strategies to fabricate superhydrophobic surfaces. A drawback of the first method is the weak adhesion force between the building block and the surface. For the latter method, researchers typically like to modify the surface with fluorinated organic monolayers,7,8 often fluorinated silane-based molecules. These very thin layer molecules will maintain the surface roughness during surface modification, which is essential for fabricating superhydrophobic surfaces.9 However, the mechanical stability of these layers is still a big challenge due to fast degradation under physical wear and chemical damage (acid, base), which restricts their practical applications.10 Increasing the thickness of the grafted fluorinated materials by modifying the micro/nanostructured materials with covalently bound fluoropolymer brushes11 will improve the stability dramatically. Meanwhile, the thicker the polymer brush, the more stable and longer lifetime the material. However, a thick polymer brush will decrease the surface roughness, which will reduce the surface hydrophobicity and antifouling characteristics. Therefore, careful modification of the nanostructured surface by a fluoropolymer brush with an optimized thickness12 is required to combine a high stability with long-term superhydrophobic and antifouling properties. Inspired by living organisms that extensively demonstrate self-healing capabilities to synthesize, regenerate, replace or repair tissues,13,14 another approach to improve the stability © XXXX American Chemical Society

and service time of materials is the introduction of self-repairing character,15 so as to minimize wear and tear of the material performance along their lifetime.16−18 Current research interests regarding self-healing materials are gradually moving from restoring mechanical19 and structural20 properties to healing functions, such as anticorrosive,21−23 superhydrophobic,24−30 superoleophobic,31,32 electrical conduction,33−36 antibacterial,37 and antifouling38−41 properties. Recently, maintaining long-lasting superhydrophobicity and antifouling properties using self-healing characteristics preprogrammed in the material draws broad attention. Most studies in this field focus here on the capability to repair the superhydrophobicity, while the chemistry and antifouling function behind it are not investigated in depth.38−41 Most of the stability and fouling problems described above are overcome in a recent study from our lab, by covalently grafting fluoropolymer brushes onto an atomically flat Si(111) surface.12 Brushes made of especially [poly(2-perfluorooctylethyl methacrylate) (PMAF17)] possessed an even higher stability and displayed excellent antifouling properties against a range of polymers in different organic media, even exceeding those of fluorous monolayers.42 Apart from improved antifouling and mechanical stability, we also found that upon damage in harsh environments (high/low pH or UV) the wetting and antifouling properties of the fluoropolymer brush could be repaired many times by a simple heat treatment. This self-healing character will further improve the lifetime of materials that invoke such a fluoropolymer brush. Given the highly promising self-healing and antifouling characteristics of Received: April 6, 2016 Revised: May 16, 2016

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DOI: 10.1021/acs.langmuir.6b01318 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Scheme 1a

a (A) Schematic representation of Si nanowire fabrication, initiator immobilization and modification of the nanostructures with a fluoropolymer brush. (B) Schematic illustration of the protein adhesion behavior on the freshly prepared, damaged, and repaired fluoropolymer brush modified nanostructured silicon surface.

this highly stable fluoropolymer brush, more detailed work on the factors like molecular architecture,43 degree of fluorination, and brush structure governing this behavior is of significant interest. The current study focuses on the potential of modification of the surface structure to further extend these antifouling and self-healing characteristics. To this aim, a PMAF17 fluoropolymer brush was covalently bound onto a nanostructured silicon surface by surface-initiated atom transfer radical polymerization (ATRP), and a series of fouling studies was performed onto this. Next to polymers, protein fouling was selected as the fouling study objective, because surfaces are easily fouled by proteins in aqueous environments,44,45 which leads to a wide range of unwanted phenomena in areas ranging from biosensing46 to marine fouling.47,48 As core experiments unmodified silicon, flat and nanostructured PMAF17 brushes (see Scheme 1A) were immersed into bovine serum albuminphosphate buffered saline (1 mg/mL; BSA-PBS) solution and in 10% cow blood plasma. After taking them out and a gentle wash step, the presence of any adhered proteins was investigated by changes in the static water contact angle (CA) and via X-ray photoelectron spectroscopy (XPS) signals that are characteristic for protein fouling (e.g., N 1s). In order to investigate the self-healing antifouling properties of the nanostructured PMAF17 brush, the freshly prepared fluoropolymer brushes were purposely damaged by air plasma, and subsequently heated to 120 °C for 30 min. After that the freshly prepared, damaged, and heated fluoropolymer brushes were also dipped into the BSA-PBS solution and blood plasma, taken out, washed, and analyzed (Scheme 1B). Like the protein antifouling, also the self-healing character of all these fluoropolymer brushes was systematically studied by static water CA and XPS. Finally, we briefly discuss the potential of

such self-repairing brushes in terms of the observed antifouling characteristics after five damage-repair cycles (Scheme 1B).

2. EXPERIMENTAL SECTION Materials. Bovine serum albumin (heat shock fraction, pH 7.4, ≥ 98%), (3-aminopropyl)triethoxysilane (APTES) (99%), silver nitrate (99%), hydrofluoric acid (48 wt % in H2O, ≥99.99% purity, on trace metals basis), α,α,α-trifluorotoluene (TFT) (anhydrous), dichloromethane (CH2Cl2, laboratory reagent grade), acetone (semiconductor grade VLSI PURANAL), 4,4′-dinonyl-2,2′-bipyridine (dNbpy), αbromoisobutyryl bromide, triethylamine (HPLC, ≥99.5%), copper(I) bromide (Cu(I)Br), and Fluorinert FC-40 were received from SigmaAldrich and used without further purification. 2-Perfluorooctylethyl methacrylate (MAF17) was purchased from Sigma-Aldrich and passed through basic alumina column before using. Phosphate-buffered saline (PBS; pH 7.4) was obtained from Fluka and used as received. Silicon wafers, with a 0.2° miscut angle along the ⟨112⟩ plane, were (111)oriented, n-type, phosphorus-doped and with a specific resistance of 1−10 Ω cm−1, as purchased from Siltronix (France). Surface Etching. Si(111) wafers were cut into 1 × 1 cm2 pieces. The surfaces were sonicated for 5 min in pure acetone and subsequently transferred into the etching solution (AgNO3, 0.01 M and HF, 4.6 M). The etching was kept at 50 °C for 1 h, taken out, washed by water, and dried by argon. Surface Functionalization. A 25 mL vial containing 20 mL of toluene and a freshly etched silicon surface was flushed using a stream of dry argon for 5 min. A volume of 0.4 mL of APTES was added into the solution. The inert atmosphere was maintained by a continuous flow of argon for 5 min and followed by heating at 80 °C for 6 h. The resulting functionalized surface was washed using toluene several times, sonicated for 5 min in CH2Cl2, and dried with a stream of argon. Initiator Immobilization. A 25 mL vial containing 1 mL of CH2Cl2 and a clean amine-terminated surface was flushed using a stream of dry argon for 5 min. Initiator α-bromoisobutyryl bromide (2 mL) and triethylamine (5 drops) were added into the vial. The inert B

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Langmuir atmosphere was maintained by a continuous flow of argon for 5 min and followed by shaking at 80 rpm at 25 °C for 2 h. The resulting initiator-functionalized surface was washed using CH2Cl2 several times, sonicated for 5 min in CH2Cl2, and dried with a stream of argon. Preparation of the PMAF17 Brush. A single-neck 15 mL flask was filled with initiator immobilized nanostructured Si(111) surface, 1 mL MAF17, 1 mL α,α,α-TFT and 25 mg dNbpy. The mixture was put into liquid nitrogen, after the mixture was completely frozen, 5 mg CuBr was added. The flask was connected with vacuum line and depressurized immediately to 1 mbar, then the frozen mixture was slowed thawed, after which the flask was filled again with argon. After repeating this freeze−pump−thaw cycle another, then the Ar-filled system was cooled in liquid nitrogen, placed under vacuum, and allowed to slowly warm up to room temperature. The reaction mixture (in closed flask) was then brought to 110 °C for a defined reaction time to obtain optimized thicknesses of the resulting polymer brushes. This procedure yielded faster and more reproducible brush thicknesses than performing the reaction under argon, as we reported previously.11 The polymerization was stopped after a fixed time by exposing the reaction mixture to air. The mixture were diluted with 5 mL TFT and kept in 110 °C for 30 min, then taken out and dried by argon, followed by sonication by FC-40 and TFT for 30 and 10 min, respectively, to remove any physisorbed reaction components, and dried under a stream of argon. In the end, the surface was put in the vacuum overnight to eliminate any left solvent. Protein Adsorption Behavior onto the Nanostructured Fluoropolymer Brush. Clean and well-characterized surfaces were used for fouling studies. For all experiments the concentration of fouling protein (BSA) and blood plasma were 1 mg/mL and 10 wt % in PBS solution (pH = 7.4). The well-cleaned surface was immersed into the protein solution for 1 h, then taken out, washed with PBS for 3 times, rinsed, and dried with argon stream followed by putting into vacuum for 2 h. Unmodified silicon and flat fluoropolymer brush was used as reference in this polymer absorption survey. Contact Angle Measurements. The static water contact angle measurements were conducted using a Krüss DSA 100 contact angle goniometer having an automated drop dispenser and an image/video capture system. The static contact angles were measured at three different places on a modified surface by dispensing three small droplets (9.0 μL volume of deionized water) with the help of an automated drop dispenser. Advancing and receding angles were measured as the droplet volume of 1 μL was continuously increased to 11 μL and decreased back to 1 μL again, at 10 μL/min and monitored by video recording, to estimate contact angle hysteresis. The tangent 1 fitting model was implemented for contact angle measurements with an accuracy of ±2°. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectra at ambient temperature were obtained using a JPS-9200 photoelectron spectrometer (JEOL, Japan) for all 1 × 1 cm2 samples used in the study of the antifouling experiment. A monochromatic Al Kα X-ray source (hν = 1486.7 eV, 12 kV and 20 mA) with an analyzer pass energy of 10 eV was used. A base pressure of 3 × 10−7 Torr was maintained in the XPS chamber during measurements and the spectra were collected at room temperature. The X-ray incidence angle and the electron acceptance angle was 10° to the surface normal. The takeoff angle φ (angle between sample and detector) of 80° is defined to a precision of 1°. The intensity of the XPS core-level electron was measured as the peak area after standard background subtraction according to the linear procedure. All XPS spectra were evaluated using the Casa XPS software (version 2.3.15). The symmetrical GL (30) line shape was employed, which consists of a Gaussian (70%) and a Lorentzian (30%) component. The full width at half-maximum of each component was constrained to 1.0−1.1 eV. The relative areas of each component peak were fixed by the stoichiometry of the main hydrocarbon peak, which was assigned to aliphatic carbon (CH2) with a binding energy of 285.0 eV.

3. RESULTS AND DISCUSSION Our strategy to prepare self-healing superhydrophobic and antifouling fluoropolymer brushes is shown in Scheme 1A. First of all, a Si surface was etched in an AgNO3/HF solution for 60 min at 50 °C to obtain a Si nanowire array with an SiOx layer on top (Figure 1A).49 Second, the nanostructured surface was

Figure 1. SEM images of the freshly etched silicon (A), after initiator immobilization (B), and ATRP (C) at 45° view. C

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Figure 2. Static water contact angle of unmodified Si (A, A1), flat (B, B1), and nanostructured (C, C1) PMAF17 brushes, and of nanostructured PMAF17 brushes after damage (D, D1), after repair (E, E1), and after five damage−repair cycles (F, F1) before (A−F) and after (A1−F1) immersing into BSA-PBS solution for 1 h. Static water contact angle of flat (G) and nanostructured (H) PMAF17 brushes, and of repaired (H1) and after five damage−repair cycles (H2) after immersing into 10 wt % cow blood plasma.

Figure 3. XPS N 1s narrow scan of unmodified silicon (A), flat (B), and nanostructured (C) PMAF17 brushes, and of nanostructured PMAF17 brushes after damage (D), after repair (E), and after five damage−repair cycles (F) before (black curves) and after (blue curves) immersion into BSA-PBS solution for 1 h, and of nanostructured PMAF17 brushes (G), after repair (H), and after five damage-repair cycles (I) after immersion into 10% cow blood plasma.

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Figure 4. XPS C 1s narrow scan of unmodified silicon (A, A1), flat (B, B1), and nanostructured (C, C1) PMAF17 brushes, and of nanostructured PMAF17 brushes after damage (D, D1), after repair (E, E1), and after five damage−repair cycles (F, F1) before (A−F) and after (A1−F1) immersing into BSA solution for 1 h.

functionalized by an amine group-terminated silane (APTES), followed by immobilization of an ATRP initiator through the reaction between the amine group and bromoisobutyryl bromide. Finally, the PMAF17 brush was grafted onto the initiator-coated Si nanowire surface, analogous to our previous work for flat Si.11 The PMAF17 brush was chosen for its high hydrophobicity and surface-reregulating properties. Moreover, dense polymer brushes with controlled thicknesses can be easily obtained for this brush, which we deemed essential for fabricating well-defined superhydrophobic surfaces and achieving protein antifouling. A reaction time of 2 h yielded a 75 nm PMAF17 brush, as determined in our previous work on flat Si surfaces.12 After initiator immobilization and polymerization, the single nanowires aggregate and forms disordered nanowire clusters (Figure 1B and C). Water contact angle measurements (CA) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the fluoropolymer brush. The static,

advancing and receding water CA and sliding angle of the freshly prepared PMAF17 brush were 152° (Figure 2C), 156°, 148°, and 6° (Supporting Information Video S1), respectively. This observation illustrates that the PMAF17-modified nanostructured surface is superhydrophobic.50,51 Obvious peaks ascribed to carbon, oxygen, and fluorine atoms can be observed in the XPS spectra (see Figure S1) indeed confirm that the fluoropolymer brush was successfully grafted onto the surface. To study the antifouling potential, freshly prepared nanostructured PMAF17 brushes were immersed into a BSA-PBS solution for 1 h, taken out, washed and analyzed. Unmodified Si and flat PMAF17 brushes are used as a reference here. After this fouling study, the static water CA of the surface is still 152° (Figure 2C1), which means that no significant amount of BSA was adsorbed onto the PMAF17-modified nanostructured surface. Even after immersing the surface for 1 h into the 10% cow blood plasma sample, the CA only dropped two E

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Figure 5. (A) Water static contact angle, advancing contact angle, and receding contact angle changes of the nanostructured PMAF17 brush between plasma for 30 s and 120 °C for 2 h. (B) Increased thickness of BSA and blood plasma fouling on different surfaces determined by XPS. FFPB and NFPB denote flat and nanostructured fluoropolymer brush, respectively.

where d = thickness of fouling layer, λFp = attenuation length of F 1s photoelectrons in the polymer brushes (2.05 nm),55 and φ = takeoff angle between the surface and the detector (in this experiment: φ = 80°). The fouling-induced increased thickness on unmodified Si is ∼1.0 nm, and this is 5 times higher than that on the PMAF17-modified flat silicon. Again, in contrast, on the nanostructured PMAF17 brush surface, no increase was found for BSA (Figure 5B) (d = 0.00 (±0.01) nm), while for the blood plasma experiments d = 0.05 (±0.03) nm. Although for such extremely thin adlayers the experimental uncertainty is relatively large and 0.05 nm is more or less an upper limit, an estimate for the absorbed amount of protein of ∼7 (±5) ng/ cm2 can be made, assuming a density of proteins of 1.4 g/ cm3.56 These observations indicate that modifying the flat Si with our PMAF17 brush decreases the protein adsorption significantly, but above all that the PMAF17-modified nanostructured Si surface will almost completely prevent protein from adsorbing onto it. As reported in our previous work,12 fluoropolymer brushes with 30−100 nm thickness display excellent self-healing properties. In order to check the self-healing properties of nanostructured PMAF17 brushes, the brush was first damaged by air plasma for 30 s and then repaired by heating at 120 °C for 30 min. The static water CA decreases to from 152° to 135° after air plasma damage, but then fully recovers to the original value after the brief heat treatment (Figure 2C−E), illustrating the superhydrophobicity can be repaired. In fact, this damage− repair cycle can be repeated at least five times, and all indicators of the hydrophobicity (static, advancing and receding water CA, and the sliding angle (see Video S2)) recover to the original values even after five damage−repair cycles (Figure 5A). We hypothesized that above the Tg of the brush polymer, the polymer chain might display sufficient mobility to reorient itself and reform an optimal surface. The driving force for this process should then be the low surface energy of the fluorinated materials, which should cause undamaged fluorinated tails to come to the top of the surface during heating and repair the hydrophobicity and antifouling character of the surface. Compared with the self-healing process of the flat PMAF17 brush, the nanostructured PMAF17 brush can also be repaired in a shorter time (120 versus 30 min). We attribute this to the larger interbrush space on the nanostructured surface, which allows the brush segment to more readily reorient itself during the heat treatment, which results in faster self-healing. In addition, the increased space makes it also possible for segments buried deeply into the brush to come to the top of the surface to replace any damage inflicted there. As a result,

degrees to 150° (Figure 2H). We rationalize this excellent antifouling by the combination of the superhydrophobicity and the smoothness of this surface that is apparent from the small sliding angle50 (6°) and the low contact angle hysteresis (8°),52 both of which are also favorable for releasing any weakly adhered fouling proteins.53,54 In contrast, on the flat PMAF17 brush modified surface the static water CA decreases from 118° to 109° upon interaction with BSA and to 94° with blood plasma, illustrating that some protein adsorption happens on this surface (Figure 2B, B1, and G). The heaviest protein fouling was observed on unmodified Si: in this case no change of water CA was observed, because both the protein-fouled and unmodified Si surfaces are highly hydrophilic, but XPS data clearly indicate this: the N 1s peak around 400 eV, which is exclusively observed after protein adsorption, was in the case of BSA adsorption only seen for unmodified Si and for the flat PMAF17 brush after the exposure to protein, but not for the PMAF17-grafted nanostructured surface (Figure 3A−C). In addition, for the blood plasma experiments only a tiny N 1s signal was observed for the PMAF17-grafted nanostructured surface (Figure 3G). These observations demonstrate that the PMAF17-modified nanostructured surface possesses excellent antifouling properties against both isolated proteins but also to complex mixtures. Moreover, the N 1s peak on unmodified Si is much stronger than that found on the flat PMAF17 brush, which indicates that the flat PMAF17-modified surface also reduces the protein adsorption. This antifouling behavior is also confirmed from XPS C 1s narrow scans. After immersing unmodified silicon into a BSA-PBS solution for 1 h, obviously increased signals for carbon from C−N and CO bonds are observed (Figure 4A and A1), confirming strong protein adhesion. On the flat PMAF17 modified surface, the C-F/allcarbon ratio was decreased from 58% to 52%, demonstrating some protein was adsorbed onto this surface. In contrast, on the PMAF17-grafted nanostructured surface the C-F/all-carbon ratio remained unchanged (Figure 4C and C1), indicating the absence of protein adsorption. Quantitative confirmation of these antifouling results can also be obtained by XPS, which displays an attenuation of any substrate signals (here: those of the brush) due to the adsorption of an overlayer (here: fouling proteins). The increased thickness of the fluoropolymer brush by the fouling protein can be quantified by utilizing the increased C/F ratio, since the protein we studied here does not contain any fluorine atoms, using ⎛ C⎞ d p = λpF sin φ ln⎜1 + ⎟ ⎝ F⎠ F

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Langmuir even after five damage-repair cycles (air plasma for in total 150 s), the surface superhydrophobicity can still be fully repaired. We hypothesized that above the glass transition temperature of the brush (here: 40 °C) the polymer chain might display sufficient mobility to reorient itself and reform an optimal surface. The low surface energy of the fluorinated materials provides the driving force for undamaged fluorinated tails buried below the brush layer to come to the top of the surface during heating, and thus repair the superhydrophobicity. The chemistry involved in this damage−repair process was studied by XPS. As stated above, upon air plasma-induced damage, the C−F/all-carbon ratio decreases from 58% to 52% (Figure 4C and D). Moreover, about 4% increase was observed for the C O/all-carbon ratio, which illustrates that both loss of fluorous side chains and oxidation reactions take place during the air plasma step. However, upon heating at 120 °C for 30 min, the C−F/all-carbon and CO/all-carbon ratios almost recover to the original value (57% and 7%), confirming that the surface chemical composition was regenerated (Figure 4E). Even after five damage−repair cycles, the C−F/all-carbon and CO/allcarbon ratios still maintain at 55% and 8%, respectively (Figure 4F), which indicates that even upon five damage-repair cycles the surface composition of PMAF17-modified nanostructured surfaces can largely be repaired. Next, the antifouling property of the freshly prepared, damaged and repaired nanostructured PMAF17 brush was investigated. As discussed above, the freshly prepared nanostructured fluoropolymer brush possesses superior antifouling properties. For the damaged polymer brush, the static water CA decreases from 135° to 109° after immersion into BSA-PBS solution for 1 h (Figure 2D and D1), which indicates that heavy protein fouling happens. In contrast, only a 1° decrease of static water CA was observed for the repaired nanostructured PMAF17 brush (Figure 2E and E1), demonstrating the recovery of the antifouling properties of the repaired nanostructured fluoropolymer brush. Even after five damage−repair cycles, only a 4° decrease of the static water CA was obtained after the fouling study (Figure 2F and F1), further confirming well repaired antifouling characteristics. In addition, with the blood plasma only 4° and 8° decrease were observed for one damage-repair and five damage-repair cycles (Figure 2H1 and H2). Some protein adsorption does happen on the five-times damaged-repaired brush. This is because water will induce the gradually accumulated hydrophilic groups that result from the plasma damage (150 s) to slowly come to the top of the surfaces in aqueous media, which decreases the surface hydrophobicity and leads to some protein adsorption. This self-healing antifouling behavior was also confirmed by XPS results. An obvious nitrogen peak was found on the damaged nanostructured PMAF17 brush after fouling with BSA (Figure 3D, blue curve), while only very little was observed on the repaired nanostructured fluoropolymer brush for both BSA (Figure 3E, blue curve) and blood plasma (Figure 3H). Even after five damage−repair cycles, the intensity of the nitrogen peak was still much lower than that on the damaged one for both BSA (Figure 3F, blue curve) and blood plasma (Figure 3I). Further proof of this repaired antifouling property can also be obtained from the XPS carbon spectra. The ratio of C−C and C−O/C−N increases dramatically after the fouling experiments for the damaged PMAF17 brush (Figure 4D and D1), indicating heavy protein adsorption. In contrast, for the repaired PMAF17 brush, no change was found upon dipping into the BSA solution (Figure 4E and E1), illustrating the

repaired PMAF17 possesses the same excellent protein repellent property as the freshly prepared one. Even after five damage−repair cycles, the ratio of C−C and C−O/C−N increases by 6% and 2% (Figure 4F and F1), which is still much lower than that on the damaged one. Quantifying the protein adsorption amount by XPS results also confirm these results (Figure 5B). The increased thickness on the repaired PMAF17 was around 0.05 nm for BSA, and 0.10 nm for blood plasma, both of which are much lower than that found on the damaged ones (0.41 and 0.56 nm, respectively). For the nanostructured PMAF17 brush that underwent five damage−repair cycles, the increased thickness by protein fouling on it is about 0.19 nm for BSA and 0.22 nm for blood plasma, i.e., in both cases, ≥50% lower than that on the damaged PMAF17. All of these observations indicate that the repaired PMAF17 brush possesses excellent self-healing antifouling properties against both isolated proteins and highly complex protein mixtures as occurring in blood.

4. CONCLUSIONS Fluoropolymer brushes that are grafted onto a nanostructured silicon surface by surface-initiated ATRP can combine very good antifouling and self-healing properties. In addition, a repairable superhydrophobicity (static water CA = 152°) was observed, as also obvious from the repairable small (6°) water sliding angle and low CA hysteresis (8°). The self-healing properties are governed by maintaining a very low surface energy, which induces in air reorientation of the brush chains to bring fluorinated tails to the outside of the brush. These brushes maintain excellent superhydrophobicity and good antifouling properties even after five damage−repair cycles, which opens a new door to prepare long-term antifouling coatings on various substrates that can be used in harsh environments. Such further studies are currently ongoing in our lab.

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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.6b01318. Video which shows how we measure the sliding angle of PMAF17 brush (AVI) Video which shows how we measure the sliding angle of PMAF17 brush after five damage-repair cycles (AVI) XPS wide scan of PMAF17 brush-modified nanostructured Si surface (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +31 317 482361. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was carried out under Project Number M62.4.12453 in the framework of the Research Program of the Materials innovation institute (M2i) (www.m2i.nl). The authors also thank Dr. Marc van den Berg (Océ), Esther van Andel, Dr. Sidharam Pujari, and Dr. Maarten Smulders (WUR) for helpful discussions. G

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DOI: 10.1021/acs.langmuir.6b01318 Langmuir XXXX, XXX, XXX−XXX