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Formation, Removal, and Reformation of Surface Coatings on Various Metal Oxide Surfaces Inspired by Mussel Adhesives Taegon Kang,†,‡,▼,¶ Dongyeop X. Oh,⊥,¶ Jinhwa Heo,†,‡ Han-Koo Lee,● Seunghwan Choy,∥ Craig J. Hawker,*,†,‡,§ and Dong Soo Hwang*,⊥,∥,■ †

Materials Research Laboratory, ‡Materials Department, and §Department of Chemistry, University of California, Santa Barbara, California 93106, United States ⊥ POSTECH Ocean Science and Technology Institute, ∥Division of Integrative Biosciences and Biotechnology, ■School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Republic of Korea ● Pohang Accelerator Laboratory, Pohang, Gyeongbuk 790-784, Republic of Korea ▼ Chemical Research Institute, Samsung Cheil Industries, Inc., Uiwang-si, Gyeonggi-do, 437-711, Republic of Korea S Supporting Information *

ABSTRACT: Mussels survive by strongly attaching to a variety of different surfaces, primarily subsurface rocks composed of metal oxides, through the formation of coordinative interactions driven by protein-based catechol repeating units contained within their adhesive secretions. From a chemistry perspective, catechols are known to form strong and reversible complexes with metal ions or metal oxides, with the binding affinity being dependent on the nature of the metal ion. As a result, catechol binding with metal oxides is reversible and can be broken in the presence of a free metal ion with a higher stability constant. It is proposed to exploit this competitive exchange in the design of a new strategy for the formation, removal, and reformation of surface coatings and selfassembled monolayers (SAM) based on catechols as the adhesive unit. In this study, catechol-functionalized tri(ethylene oxide) (TEO) was synthesized as a removable and recoverable self-assembled monolayer (SAM) for use on oxides surfaces. Attachment and detachment of these catechol derivatives on a variety of surfaces was shown to be reversible and controllable by exploiting the high stability constant of catechol to soluble metal ions, such as Fe(III). This tunable assembly based on catechol binding to metal oxides represents a new concept for reformable coatings with applications in fields ranging from friction/wettability control to biomolecular sensing and antifouling. KEYWORDS: self-assembled monolayer, catechol, DOPA, reversible coating

1. INTRODUCTION Self-assembled monolayers (SAMs) are thin organic films that form spontaneously through strong interactions between a target substrate and surface-active organic molecules.1 The headgroup of the small molecule is designed to have a favorable and specific interaction with the substrate, while the tail group provides a steric barrier, which strongly influences the resulting surface properties. During the last decades, the design of SAMs has drawn considerable attention because of their potential as economical and versatile surface coatings for applications including corrosion protection, friction/wettability control, biomolecular sensing, and antifouling.2−6 Because of their nanometer thickness, a primary challenge with the long-term use of SAMs is surface contamination and the introduction of defects through both chemical and physical damage, all of which lead to performance degradation. Development of a strategy that could be used for either © XXXX American Chemical Society

cleaning or repair of the SAM would therefore be of major importance, especially for applications involving a condensed phase, such as antifouling, low friction, and biomarker sensing.2−8 A number of approaches have been investigated for removal/repair of contaminated SAMs; however, they all have drawbacks such as underlying substrate damage or a complex and inefficient coating method.9−13 For example, lotus leaf-inspired nanostructured SAMs on gold substrates reveal promise as a self-cleaning surface; however, fabrication is complex and only applicable to gold surfaces.12 The chemistry associated with marine mussels provides insight into a possible solution. A critical component of the adhesion system of mussels is the strong and reversible Received: July 30, 2015 Accepted: October 22, 2015

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DOI: 10.1021/acsami.5b06910 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

competitive exchange with Fe(III) ions in solution was also demonstrated (Scheme 1B).

catechol-metal ion coordinative interactions leading to nearcovalent binding strengths with a wide variety of substrates.13−17 For example, a single catechol residue (3,4dihydroxyphenylalanine, DOPA) in mussel adhesive proteins has a log stability constant in excess of 40 (for Fe chelation) at 25 °C and pH 7.5.18−20 Based on these features, catechol building blocks have been introduced as the headgroup for SAMs on various metal oxide surfaces.21−23 In addition, catechol groups are widely known to selectively bind to one metal ion with a higher stability constant than other metal ions.20,24,25 Therefore, we speculated that a catechol-based SAM could be easily removed from a variety of metal oxide surfaces by simply flowing a solution of a specific metal ion, such as Fe(III), which has one of the highest stability constants with catechol, over the SAM.18−20 In designing stable and versatile SAMs based on a catechol building block, the facile oxidation of catechols to the corresponding weakly binding quinones is of major concern. To address this problem, we used triethylsilane protected catechol (SPC) as a starting material since the triethylsilyl group not only protects the catechol from oxidation, but can also be removed under facile conditions in the presence of Lewis acid (e.g., H+ or FeCl3) just prior to SAM formation/ usage (Scheme 1A).26,27 In turn, SPC derivatives can be readily

2. EXPERIMENTAL SECTION 2.1. Materials. Unless otherwise noted, ACS reagent grade chemicals and solvents were purchased from Sigma-Aldrich and used without further purification. Tris(pentafluorophenyl)borane, triethylsilane, and tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (TDFOCS, SIT8174.0), and azobis(isobutyronitrile) were purchased from Gelest. Triethylene glycol monomethyl ether, eugenol, and 2,2dimethoxy-2-phenylacetophenone (99%) were purchased from SigmaAldrich. Monofunctional PEG thiol (mPEG-thiol MW: 2k) was purchased from Sigma-Aldrich. Deuterated solvents for NMR were purchased from Cambridge Isotope Laboratories, Inc. Synthesis of SAM materials is described in Supporting Information. 2.2. Instrumentation. 1H and 13C NMR spectra were recorded on Varian 600 or Bruker AC 500 spectrometers as indicated. Chemical shifts are reported in parts per million (ppm) and referenced to the residual signal solvent. The lamp used for irradiation of samples was a UVP Black Ray UV Bench Lamp XX-15L, which emits 365 nm light at 15W. Mass spectral data were collected on a Micromass QTOF2 Quadrupole/Time-of-Flight Tandem mass spectrometer (ESI-MS). Gas chromatography was carried out on a Shimadzu GC-2014 using a flame ionization detector and a Restek column (SHRXI-5MS) for separation. 2.3. Preparation of Metal and Metal Oxide Surface. All Si wafers were cleaned to remove any contamination using acetone, isopropyl alcohol, and deionized water for 3 min and dried with N2. Then, various 500 Å thickness metals (Ti, Al, Fe) were deposited onto cleaned Si water using e-beam evaporator (Temescal ves-2550, System Control Technology, Livermore California) with a 1 Å/s deposition rate. Before deposition of main metal layers, 100 Å Ti layer was deposited as interlayer to enhance adhesion between Si and other metal layers using same e-beam evaporation equipment. The base pressure of e-beam evaporator was under 2 × 10−6 Torr. To make the metal oxide layer, Ti and Al were deposited onto the wafers, followed by O2 plasma treatment (200 W, 300 mT, 5 min). The Fe2O3 layer was prepared by a sputtering system (ATC 2000F, AJA International, Inc., North Scituate, Massachusetts) using a Fe target with 2.5 sccm O2 and 25 sccm Ar under 3 mT. 2.4. Coating and Removal of Catechol Conjugates. Catechol conjugated SAM was coated on metal oxides as follows: Catechol conjugated materials were dissolved in DI water to be 10 mg/mL concentration, and then a solid acid-catalyst (Amberlyst 15) was added in the solution until filling ∼10% of the total volume to deprotect triethylsilane groups. After 3 h, the solution was filtered through Whatman syringe filter with 0.2 μm pores. Several drops of Tris buffer (pH 7.4) was added in the solution to adjust pH to be ∼5. The metal oxide substrates were immersed for 24 h in the catechol conjugated TEO solution. The substrates were taken out from the solutions, washed with methanol and acetone, and blown dry under a stream of nitrogen. The detachment of SAM from a metal oxide was conducted as follows. SAM cleaning solution is 10 mM Bis-tris/HCl buffer (pH ∼5) with Fe(III) concentration of 50 mM. Bis-tris is for improving iron chloride solubility.6 The SAM cleaning solution was sonicated for 1 h and centrifuged for 10 min at 400 rpm just prior to use. SAM substrates were immersed for 12 h in the SAM cleaning solution, washed with distilled water and methanol several times, and blown dry under a stream of nitrogen. NOTE: mortal mechanical treatment for effective chemical reaction e.g. ultrasonication was excluded because it could damage the metal oxide layer. 2.5. Contact Angle Measurement. Contact angle (CA) of a water droplet on the substrates was measured with a surface analyzer (DSA-100, Krüss Co., Germany) using the sessile drop method. The samples were placed on a test stage, and 3 μL of a water drop was introduced onto the surface through a microsyringe. At least three different measurements were performed on different areas of each sample at room temperature.

Scheme 1. (A) Synthetic Strategy for Silyl-Protected Catechol-Functionalized Triethylene Oxide Derivatives (TEO) and (B) Graphical Representation of the Formation of a Self-Assembled Monolayer (SAM) of CatecholConjugated TEO on Metal Oxide Surfaces and Removal with FeCl3 Solution

obtained from eugenol as a starting material which allows for secondary functionalization of the terminal alkene by thiol−ene chemistry. For this study, we therefore attached a short triethylene oxide (TEO) moiety to SPC via thiol−ene “click” reaction, and investigated the generation of coatings on metal oxide surfaces (TiO2, Al2O3, and Fe2O3) after catechol activation. Removal and repair of these coating layers by B


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ACS Applied Materials & Interfaces 2.6. X-ray Photoelectron Spectroscopy (XPS) Measurement. X-ray photoelectron spectroscopy (XPS) experiments were performed on a PHI 5800 ESCA System, which was operated at 2 × 10−10 Torr with a monochromatic Al Kα (1486.6 eV) anode (250 W, 10 kV, 27 mA). All spectra were calibrated by adjusting to the C 1s peak (284.6 eV) used as a standard. 2.7. NEXAFS. The NEXAFS experiments were performed at the 4D undulator beamline at the Pohang Accelerator Laboratory (PAL) in Korea. A linearly polarized X-ray beam was irradiated on the sample by varying the incidence angle between surface normal and light polarization (E-vector). The data was simultaneously collected in the partial electron yield (PEY) mode by recording the sample current to ground. The total energy resolution is about 150 meV with a beam size of 0.5 mm × 0.5 mm and the probing depth of ∼50 Å for surfacesensitive measurements. Then the data were normalized to the beam current using a grid located upstream of the beamline. All data were collected at room temperature at a base pressure of ∼10−10 Torr. The spectra was fitted using a nonlinear least-squares routine with Gaussian functions for the π* resonant features, a Gaussian broadened step function for the edge jump, and asymmetrically broadened Gaussian functions for the σ* resonant features. 2.8. Surface Roughness Measurement. Surface roughness of the different metal oxide surfaces due to the presence of the TEO conjugated catechol coating were monitored by atomic force microscopy (AFM, Asylum, Santa Barbara, CA, USA). The surfaces were imaged with a silicon tip operating in the tapping mode in air. 2.9. Cell Attachment Test. A mouse preosteoblast cell line, MC3T3-E1, was cultured in minimal essential medium-alpha (MEMα; Hyclone) supplemented with 10% fetal bovine serum (FBS; Hyclone) and 1% penicillin/streptomycin (Hyclone) at 37 °C in a humidified atmosphere of 5% CO2 and 95% air. The subconfluent cells were detached using 0.25% trypsin-EDTA (Hyclone), and the viable cells were counted by a trypan blue assay. The cells were seeded onto the substrate in a 24-well plate, at a density of 5 × 104 cells per well. To quantify the number of attached cells on each substrate, after 6 h of the cell seeding, the samples were treated with CCK-8 (Dojindo) reagents and incubated for 2 h at 37 °C in a humidified atmosphere of 5% CO2. Aliquots of each sample medium were collected in a 96-well plate to measure the absorbance at 450 nm. All experiments were performed in triplicate.

SAM layer on Fe2O3, Al2O3, and TiO2 determined by AFM were 1.86, 2.24, and 1.43 nm, respectively. 3.2. Contact Angle Measurements. Characterization of coating formation was investigated by water contact angle measurements. Control experiments with bare Fe2O3, Al2O3, and TiO2 surfaces showed expected contact angles of ∼58°,31,32 ∼60°,33 34° and ∼38°,34,35 respectively. After coating with the catechol-TEO derivative followed by extensive washing, the contact angles for all functionalized surfaces converged to an approximate value of ∼18−24°, clearly indicating the formation of stable coatings with similar surface properties. Significantly, these contact angles also correspond closely to literature values for TEO-based SAMs which gives an initial indication of the well-defined nature of these monolayers.30,36,37 The minor differences in the water contact angle between different metal oxide surfaces, is likely due to the surface roughness and the extent of packing regularity which is derived from the difference in stability constant for the catechol binding group and the different metal centers combined with the distance between adjacent O atoms in the metal oxides. 3.2. Spectral Evidence. The formation of thin organic layer on metal oxide substrates was characterized using an X-ray photoelectron spectroscopy (XPS) spectroscopy.38 The atomic ratios of metal, carbon, oxygen, and sulfur from the thin layer on the metal oxide substrates was therefore obtained using XPS (Figure S1) with the experimental atomic ratios (C/O/S) on the coated Fe2O3, Al2O3, and TiO2 surfaces being (12:11.7:1), (11.3:7.2:1), and (12.9:8.2:1), respectively. These results are comparable to the theoretical atomic ratio (18:7:1) of catecholconjugated TEO (C/O/S) with the higher oxygen concentration of SAMs being due to oxygen in the metal oxide. The C K-edge X-ray absorption fine structure (NEXAFS) spectra of SAMs on the four different substrates was then examined with a single peak at around 284.9 eV and multiple peaks in the range of 291 to 303 eV, ascribable to the π* CC orbital and σ* orbitals (including C−O at ∼295 eV) being observed in each case (Figure 1).39−41 The CC peaks and C−O peaks from these spectroscopic studies verified the presence of catechol units and TEO chains on the metal oxide substrates.39−41 To investigate the bonding geometry of catechol groups on the metal oxide surface, the average tilted angle (α) of catechol groups was examined using the linear function of the π* CC peak relative intensity vs cos2θ from the NEXAFS data at more than 2 incidence angles (θ). For example, the NEXAFS spectra of the SAM on the Fe2O3 substrate was obtained at 5 different incidence angles and the of CC angular dependence (peak around 284.9 eV) indicates that the catechol units are in an ordered conformation on the Fe2O3 surface with the catechol rings having a predominately vertical orientation (tilt angle of ∼75.8°) (Figure 1A). From similar NEXAFS analysis, the catechol units are assembled on TiO2 and Al2O3 surfaces with average tilted angles of ∼65.2° and ∼62.7°, respectively (Figure 1B and 1C). The difference in the tilt angle of the catechol is likely due to surface roughness, different stability constant of the metal for the catechol, and different distance between the adjacent O atoms of the metal oxides.42−44 The NEXAFS data of the coated Fe2O3, Al2O3, and TiO2 surfaces showed the distinct incidence angle dependency of the π* peak, and the XPS data of the coated Fe2O3, Al2O3, and TiO2 surfaces presented that the most abundant element on the surface was carbon (Figure S1). Therefore, the catechol groups of the TEO catechol molecules are oriented as the “standing up” formation on the metal oxides surfaces (Figure S2). Such

3. RESULTS AND DISCUSSION 3.1. Synthesis of SPC-Conjugated TEO. PEO (poly(ethylene oxide)) has been widely studied for its antifouling properties where its ability to block the adsorption of serum proteins, extracellular matrix molecules, and cell adhesion when coated on a variety of surfaces is derived from a well-defined hydration layer.28−30 As an oligomeric form of PEO, TEO (triethylene oxide) was initially chosen as the tail group of SPCconjugated molecules for SAM studies. SPC-conjugated TEO was successfully synthesized as shown in Scheme 1 with the silyl protected derivative being stable in both solution and the solid-state under normal atmospheric conditions. Significantly, the silyl groups could be removed to give the surface active catechol unit by simple treatment with Amberlyst (a solid-type acid catalyst) and SAM coatings prepared on metal oxides (Fe2O3, Al2O3, and TiO2) by dip-coating from a dilute solution (10 mg/mL) for 24 h. At least 24 h are required for the densely packed TEO-catechol coating. Fortunately, the color the dipping solution after 24 h remained clear, implying minimum amount of the catechol molecules is oxidized. It is probably because the TEO-catechol molecules were absorbed on the metal oxide surfaces in the acidic buffer (pH 5.5); the acidic condition minimizes the catechol oxidation. It is also reported that catechol-based materials are not oxidized at pH 5 for 2 days. The root-mean-square (RMS) roughness of the TEO C


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on competitive exchange of catechol binding groups. As shown in Scheme 1B, initial formation of the catechol coating is through deprotection of the triethylsilyl units followed by binding of the in situ generated catechol units to the metal oxide surface, Removal of the resulting TEO SAM could be accomplished by exposure of the catechol coating to a solution of FeCl3. The increased stability constant and redox potential of catechol chelation to Fe(III) when compared to catechol binding to oxide surfaces leads to removal of the TEO SAM.19,47,48 The removal of the TEO SAM was verified by the NEXFAS spectra of the washed metal oxide surfaces (Figure S4). On the exposure of the freshly regenerated oxide surface to the TEO derivative, reformation of the TEO SAM was observed. This cycling of attachment and detachment for the TEO SAMs was evaluated by a static contact angle test (Figure 2A, B, C). After they were washed with free Fe(III), contact angles returned to the initial values for the metal oxide surfaces, confirming that the FeCl3-mediated washing detached the

Figure 1. Angle-dependent X-ray absorption near edge structure (NEXAFS) spectra of catechol-TEO coated (A) Fe2O3, (B) Al2O3, and (C) TiO2 surfaces. The ion beams were incident on the x−z plane at different angles (θ). The peak around 285.9 eV is associated with the π orbitals of a C atom (π*) and the broad peak near 295 eV corresponds to transitions to σ orbitals (σ*). (A,i) The π* intensity as a function of cos2 θ (Fe2O3 surface) is well fitted into (A,ii) a straight line.

orientation of catechol groups are hardly found in a poorly packed monolayer structure and a multilayer structure.45,46 This results suggest that the TEO catechol coating on the Fe2O3 is a well-packed monolayer.45,46 Such tilt angles and comparison with the literature for SAMs suggest the packing density of the TEO-catechols in the range of 2−4 molecules/nm2.10,15,41 We also tried to coat SiO2 with TEO SAM but the TEO catechol molecules were poorly absorbed on the SiO2 surface and did not form a SAM, probably because of the unstable catechol−Si bond (Figure S3).26 3.3. Formation, Removal, and Reformation of Surface Coatings. In many applications where surface properties are important, such as antifouling and biomarker sensing, a removable and reformable surface coating is a desirable feature as contamination of the SAM deteriorates the performance of the original coating. To address this challenge, we developed a novel strategy for the removal of a contaminated SAM and the recovery of a new SAM on a single metal oxide surface based

Figure 2. Static water contact angle of catechol-TEO coated (A) Fe2O3, (B) Al2O3, and (C) TiO2. (D) Static water contact angle of catechol-TEO coated Al2O3 after washing with solutions of varying Fe(III) concentration. (D) Each single value was determined using a freshly prepared TEO catechol-coated surface. (A−D) Each value represents the mean of three analyses and its standard deviation. D


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process. The thickness of the long chain PEO coating was approximately >5 nm estimated by the NEXAFS data (see Supporting Information).54,55 MC3T3 cells were seeded on the sample surfaces and incubated in the cell culture medium for 6 h at 37 °C. After that the number of the remaining cells on the samples was counted.56 In all cases, the PEO-coated surfaces substantially inhibited cell attachment when compared to the corresponding metal oxide surfaces with a significant reduction in the number of adhered MC3T3 cells being observed. Removal of the PEO-catechol layer by washing with FeCl3 solution results in the generation of a surface for which the number of attached cells is essentially the same as for the bare oxide surface. The formation and removal of the PEO-catechol coating on the oxide substrates was then observed to be directly mirrored by the removal and fouling of the surfaces by viable MC3T3 cells (Figure 3). This clearly illustrates the control of surface properties and the tunability of antifouling performance through the formation, removal, and reformation of PEO coatings. In addition, the result shows that the PEO catechol coating was maintained within the incubation in the physiological environment (at pH ∼7.5 for 6 h). This surface coating recyclability would be useful for in vitro biomedical applications. For examples, healthcare deviceassociated infection (HDAIs) is one of the most adverse events in high-technology healthcare device such as endoscope, robot-assisted surgery machine, ultrasonic imaging, and dialysis facility due to the nonspecific fouling (contamination) on the surface of the healthcare device.57,58 Therefore, proper and frequent cleaning and sterilization of the healthcare facilities are extremely important to prevent the extrinsic infections. However, most biomolecules have a tendency to adsorb nonspecifically onto surfaces/interfaces of the expensive healthcare facilities including endoscopes, surgical robot, and dialysis, and it is difficult to remove completely the contaminated molecules from the expensive healthcare facility after a one-time application to one patient. The catechol-based coating on these devices can be readily removed after use, and then the catechol-based coating can be newly regenerated on the surface of the devices prior to use. The recyclability and price competitiveness of the coating based on the silyl protected catechol chemistry here would be an ideal approach to coat the metal oxide surface of the expensive healthcare device and biochip.

catechol-conjugated TEO from the surface through a competitive exchange reaction driven by the higher stability constant, redox potential, and unsatisfied valences of the Fe(III) when compared to the oxide surfaces.19,47−49 In case of the Fe2O3 surface, the free Fe3+ ions in the washing buffer has higher stability constant for catechol than the Fe in the Fe2O3 surfaces.50,51 Therefore, the catechol more preferably binds to Fe3+ ions in the buffer than Fe3+ ions in the Fe2O3 surface. The versatile nature of this approach can be seen with the repeated cycling experiments showing SAM attachment and detachment on a variety of substrates. Contact angle values were consistently changed with the coating and removal of the SAM, though the attachment and detachment efficiency of TEO SAMs gradually decreased with the repetitive recovery and removal of the SAM, which may be associated with minor amounts of irreversible oxidation and adsorption of catechol molecules on the metal oxide substrates (Figure 3).49 To investigate the influence of Fe(III)

Figure 3. Relative number of MC3T3 viable cells attached to four different Al2O3 substrates: bare substrate, freshly prepared PEO catechol coated substrate, PEO catechol coating-removed substrate, and second round PEO catechol coated substrate. The light absorption intensity (at 450 nm) of the colorimetric assay (CCK-8 assay) is directly proportional to the number of the viable cells.56 Each value represents the mean of three analyses and its standard deviation.

concentration on SAM removal, the contact angle change for a series of TEO SAM-coated on Al2O3 substrates was studied. At very low Fe(III) concentrations, no reaction is observed and the catechol-based coating is stable. Significantly, increasing the Fe(III) concentration resulted in a gradual increase in contact angle illustrating an ability to tune surface properties and coating stability (Figure 2D). At Fe(III) concentrations of >5 mM, the contact angle was comparable to that for the bare Al2O3 substrate and is consistent with essentially complete removal of the TEO SAM. As a control experiment and to demonstrate the critical role of the high stability constant (∼25−44) for the associated Fe(III)-catechol complex in removal of the catechol-mediated SAM, the use of metal ions with lower stability constant were examined. For these experiments, Zn(II) and Ca(II) solutions which have stability constants for the corresponding catechol complexes of ∼8−15 were shown to be ineffective in removal of the SAM coating, even at high concentrations (50 mM) (Figure S5).18,19,52,53 3.4. Antifouling Property of Surface Coatings. To further demonstrate the utility of this strategy for control of surface properties and applications such as antifouling, cell attachment tests were conducted. For these studies, low molecular weight tri(ethylene glycol) SAMs do not offer significant antifouling performance.30 As a result, the TEO units were replaced with PEO chains (2000 amu) and initial deposition of the PEO-catechol derivatives on a variety of oxide surfaces again shows the facile and robust nature of this

4. CONCLUSIONS In this study, we have demonstrated the modular synthesis of surface active materials from eugenol as a readily available starting material coupled with the assembly of these systems on a variety of metal oxide surfaces. By exploiting the strong binding affinity of the resulting catechol headgroup, these coatings were shown to be stable under standard conditions. Moreover, these catechol-based SAMs could be easily removed by competitive exchange with FeCl3 leading to the development of a new strategy for the formation, removal and reformation of surface coatings. The cycle of formation and removal could be repeated numerous times leading to accurate control and tuning of surface properties and antifouling performance. The synthetic versatility of these catechol-based systems and their ability to bind to different substrates will enable this concept for reformable coatings to be widely applied for functional coatings especially for in vitro biomedical devices that require frequent cleaning and sterilization. E


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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/acsami.5b06910. Supporting data (Figures S1−S8) and experimental details for XPS, NEXAFS, and cell attachment test (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ¶

T.K. and D.X.O. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Research Foundation of Korea Grant funded by the Korean Government (NRF-C1ABA0012011-0029960 & NRF-2014R1A2A2A01006724 & NRF-2013Fostering Core Leaders of the Future Basic Science Program), and the Marine Biotechnology program (Marine Biomaterials Research Center) funded by the Ministry of Oceans and Fisheries, Korea (D11013214H480000110). The NEXAFS experiments at the PLSII were supported in part by MEST and POSTECH. We also thank the Institute for Collaborative Biotechnologies through the U.S. Army Research Office (Contract W911NF-09-D-0001 for financial support and the MRSEC program of the National Science Foundation (DMR 1121053) for access to shared experimental facilities.

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