Reversible Tethering of Polymers onto Catechol-Based Titanium

Article pubs.acs.org/Langmuir

Reversible Tethering of Polymers onto Catechol-Based Titanium Platforms William Laure,† David Fournier,† Patrice Woisel,*,†,‡ and Joel̈ Lyskawa*,† †

Université Lille, UMR 8207UMETUnité Matériaux et Transformations, F-59000 Lille, France ENSCL, F-59000 Lille, France

‡

S Supporting Information *

ABSTRACT: In this article, we report on the reversible tethering of end-functionalized polymers onto catechol-based titanium platforms by exploiting the reversible Diels−Alder (DA) cycloaddition reaction. For this purpose, furan and maleimide end-functionalized polymers, prepared via reversible addition− fragmentation chain transfer polymerization, were covalently grafted through a DA reaction onto reactive titanium platforms elaborated from catechol-based anchors incorporating either the furan or the maleimide moiety. As a proof of concept, a hydrophilic poly(oligo(ethylene glycol)acrylate) (POEGA) and a hydrophobic poly(2,2,2-trifluoroethyl acrylate) (PTFEA) were grafted onto titanium surfaces and subsequently detached by exploiting the thermoreversible nature of the DA reaction [i.e., retro Diels−Alder (rDA)]. These polymers were interchanged by performing a second DA reaction, thereby allowing the titanium surface wettability to be switched from hydrophobic to hydrophilic on demand. The grafting of furan/maleimide end-functionalized polymers onto functionalized (maleimide/furan, respectively) catechol-based titanium platforms and the subsequent rDA/DA sequence used for switching the titanium surface were evidenced by attenuated total reflectance−Fourier transform-infrared spectroscopy, X-ray photoelectron spectroscopy, and contact angle measurements.

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INTRODUCTION Titanium and its alloys are one of the most investigated materials because of their substantial useful applications in aviation, submarine, energy storage, microelectronics, or nanotechnologies,1 as a result of their exceptional mechanical, thermal, and electrical properties and their excellent corrosion resistance.2 Furthermore, these materials are commonly used as biomedical devices or implants3 owing to their excellent biocompatibility and lower density compared with other metals.4 Nevertheless, the physicochemical and/or biological performances of these materials depend both on their surface morphology and on their chemical composition. Thus, the control of the interfacial properties of such materials remains of prime importance. In this way, many techniques have been developed to design the chemical compositions of titanium surfaces5−7 such as plasma-induced grafting,8 electrodeposition,9 and the covalent attachment of target molecules10 or polymers.11 In this context, the tethering of polymers onto titanium surfaces has received considerable attention for engineering titanium surface properties such as wettability, specific adsorption, corrosion resistance, and biocompatibility owing to their versatile applications in microelectronics,12 sensors,13 (bio/nano)technologies,14 and biomedical devices.15,16 For this purpose, several “grafting to” and “grafting from” approaches have been reported to strongly anchor © XXXX American Chemical Society

macromolecules onto material surfaces involving, among others, living polymerization techniques including ring-opening metathesis polymerization (ROMP),17 atom transfer radical polymerization (ATRP),18−20 or reversible addition−fragmentation chain transfer (RAFT).11 Moreover, many efforts have been devoted to the development of efficient anchoring groups such as phosphonic acid/ester21 or silane22 to provide stable polymeric films onto titanium surfaces. Recently, inspired by the composition of mussels adhesive proteins, the catecholbased anchor has emerged as a powerful tool for the modification of a wide range of substrates including ceramics, polymers, metals, and metal oxides23−31 and more particularly for modulating the interfacial properties of titanium surfaces.32,33 However, few studies have described the incorporation of the adhesive catechol fragment into polymer backbones for titanium surface functionalization. Interestingly, our group has previously reported on the immobilization of various polymer brushes onto the titanium surface by using the grafting-to strategy from well-defined catechol end-functionalized polymers prepared using the RAFT procedure.34 Various catecholfunctionalized poly(ethylene glycol) architectures were also Received: January 16, 2017 Revised: March 13, 2017 Published: March 14, 2017 A

DOI: 10.1021/acs.langmuir.7b00160 Langmuir XXXX, XXX, XXX−XXX

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polymerizations was azobisisobutyronitrile (AIBN; >98%), which was recrystallized from ethanol. 4-(2-Hydroxy-ethyl)-10-oxa-4-azatricyclo[5.2.1.02,6]dec-8-ene-3,5-dione 145 and both dopamine-based anchors, namely, dopamine-furan (DF)44 and dopamine-maleimide (DM),46 were synthesized according to the literature. The RAFT agents, 2-(1isobutyl)sulfanylthiocarbonylsulfanyl-2-methylpropionic acid (CTA1) and succinimidyl 2-(1-isobutyl)sulfanylthiocarbonylsulfanyl-2-methylpropanoate (CTA2), were synthesized as previously described.47 Analytical Techniques. NMR Spectroscopy. 1H and 13C NMR spectra were recorded at 25 °C with a Bruker Avance 300 spectrometer. Size Exclusion Chromatography. The characterization of OEGAbased polymers was performed in tetrahydrofuran at 40 °C, with a flow rate of 1 mL·min−1 using a Waters e2695 Separation Module equipped with a refractive index detector (Waters 2414). The characterization of TFEA-based polymers was performed on a Waters 515 HPLC pumpSEC system equipped with a refractive index detector (Waters 2410) using toluene as the mobile phase at 35 °C and with a flow rate of 1 mL·min−1. The samples were separated through Waters Styragel HR1, HR4e, and HR6 columns. The number-average molar masses (Mn) and dispersities (D̵ = Mw/Mn) of both polymers were derived from the refractive index (RI) signal by a calibration curve based on polystyrene (PS) standards from Polymer Standards Service. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) analyses were performed on a Kratos Axis Ultra DLD system (Kratos Analytical) using a nonmonochromatic Al Kα X-ray source (hν = 1486.6 eV). The emission voltage and the current of this source were set to 12 kV and 3 mA, respectively. The vacuum in the analyzing chamber was maintained at 5 × 10−9 mbar or lower during analysis. Survey (0−1320 eV) and high-resolution spectra were recorded at pass energies of 160 and 40 eV, respectively. XPS analyses were performed with a takeoff angle of 90° relative to the sample surface. The core-level spectra were referenced with the Ti 2p binding energy at 458.6 eV. Data treatment and peak fitting procedures were performed using Casa XPS software. Attenuated Total Reflectance−Fourier Transform-Infrared Spectroscopy. Attenuated total reflectance−Fourier transform-infrared spectroscopy (ATR−FTIR) spectra were recorded on a Spectrum One spectrometer from Perkin-Elmer coupled with a Zn/Se ATR crystal collecting 128 scans and with a wavenumber range 4000−400 cm−1. Contact Angle Measurements. Contact angles were evaluated with a Digidrop contact angle meter from GBX Scientific Instruments at room temperature. A water drop was used to measure the contact angle value (θ°). The measurement was repeated 10 times to obtain an average value for the surface. Synthesis. Preparation of CTA3. CTA2 (1 g, 2.9 mmol) and furfurylamine (0.52 mL, 2.9 mmol) were stirred in anhydrous dichloromethane (100 mL) with triethylamine (0.6 mL, 4.3 mmol) at room temperature under a nitrogen atmosphere for 48 h. The organic phase was then washed with water (3 × 50 mL) and dried over MgSO4. The solvent was next evaporated, and the crude product was purified by column chromatography (SiO2/dichloromethane) to give a yellow oil in 51% yield. 1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 0.99 (d, 6H, CH(CH3)2), 1.70 (s, 6H, C(CH3)2−CO), 1.95 (m, 1H, CH2− CH(CH3)2), 3.17 (d, 2H, S−CH2−CH), 4.39 (d, 2H, CH2−NH), 6.20 (dd, 1H, CH−C(CH2)), 6.30 (dd, 1H, O−CHCH), 6.75 (br, 1H, CH2−NH), 7.31 (dd, 1H, O−CHCH). 13 C NMR (75 MHz, CDCl3). 22.1 (CH(CH3)2), 25.8 (C(CH3)2− CO), 27.8 (CH(CH3)2), 37.3 (NH−CH2), 45.3 (S−CH2), 57.0 (C(CH3)2−CO), 107.9 (CHC(CH2)), 110.4 (O−CHCH), 142.7 (O−CHCH), 151.0 (CHC(CH2)), 172.8 (CO), 219.8 (C S). Preparation of CTA4. A solution of 1 (0.85 g, 4.1 mmol) in dry dichloromethane (20 mL) was added dropwise to a 100 mL DCM solution containing CTA1 (1.03 g, 4.1 mmol), dicyclohexylcarbodiimide (DCC; 1.26 g, 6.1 mmol), and 4-dimethylaminopyridine (DMAP; 0.05 g, 0.4 mmol) at −10 °C under nitrogen atmosphere. The mixture was allowed to stir at room temperature overnight. After

used to manipulate titanium surface properties such as wettability and to elaborate nonfouling titanium surfaces.11,35−37 Nevertheless, these aforementioned strategies lead to a permanent tethering of macromolecules onto the titanium substrates and impart, in most cases, a given surface property. With the objective of developing smart titanium surfaces with dynamic and tailored properties, the reversible tethering of macromolecules onto titanium surfaces, therefore displaying switchable properties, is of great interest. To achieve this goal, the Diels−Alder (DA) [4 + 2] cycloaddition turns out to be an ideal candidate as its reversible nature38,39 enables surfaces to be successively covered and uncovered on demand.40−42 In this context, Barner-Kowollik et al. proposed an interesting approach combining the remarkable adhesion properties of catechol derivatives with the thermoreversible DA reaction for the elaboration of bioinspired gold surfaces with switchable properties. 43 In this work, a DA reaction between a polyethylene glycol (PEG) bearing a cyclopentadiene group and a catechol derivative featuring a maleimide unit was exploited to elaborate a catechol end-decorated PEG, which was further immobilized on gold surfaces via the autopolymerization of the catechol moiety in a maritime-based environment. Unfortunately, despite the effectiveness of this grafting strategy for tethering PEG chains onto gold surfaces, the protection of the remaining hydroxyl groups located in the catechol-based polymer films was required to achieve the polymer switching through the rDA/DA sequence, thereby limiting the applicability of this procedure for titanium surfaces, which contain hydroxyl groups. Interestingly, our group has recently reported a robust and straightforward approach based on the maleimide/furan DA cycloaddition permitting various furan or maleimide functionalized small molecules to be grafted onto catechol-based titanium platforms containing the complementary entity.44 In particular, we have shown that the retro Diels−Alder (rDA) reaction could be exploited to conveniently detach model molecules and to recycle titanium platforms, which were further subjected to a second DA reaction. In this article, we propose to extend this approach for reversibly tethering well-defined maleimide or furan endfunctionalized polymers onto titanium-based surfaces and to switch their wettability by exploiting successive DA and rDA reactions. In this way, two trithiocarbonate RAFT agents incorporating either a furan or a maleimide moiety were synthesized and then used to promote RAFT polymerizations of oligo(ethylene glycol)monomethyl ether acrylate (OEGA; Mn = 480 g·mol−1) and 2,2,2-trifluoroethyl acrylate (TFEA). The resultant end-functionalized POEGA and PTFEA polymers were subsequently grafted onto catechol-based titanium platforms elaborated from catechol derivatives containing either a furan or a maleimide fragment, leading to hydrophilic and hydrophobic surfaces, respectively. Interestingly, we have next shown that the hydrophobic titanium surface functionalized with the fluorinated polymer could be converted into a hydrophilic one, only by successively performing an rDA upon heating and then a DA involving the end-functionalized POEGA.

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EXPERIMENTAL SECTION

Materials. All reagents were provided by Sigma-Aldrich and used as received unless otherwise noted. OEGA (average Mn = 480 g· mol−1) and TFEA (99%) were purified through a silica gel column before use to remove inhibitors. The primary radical source used in all B


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Langmuir Scheme 1. Illustration of the Reversible Tethering of Polymers onto Catechol-Based Titanium Platforms

Scheme 2. Synthesis of Chain Transfer Agents CTA3 and CTA4 and Their Resultant End-Functionalized Polymers

filtration, the solvent was removed under reduced pressure, and the product was recrystallized from a mixture (20:1) of hexane and acetone to furnish a bright yellow solid in 77% yield. 1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 0.99 (d, 6H, CH(CH3)2), 1.65 (s, 6H, C(CH3)2−CO), 1.95 (hept, 1H, CH2− CH(CH3)2), 2.86 (s, 1.7H, CH−CO−N), and 2.90 (s, 0.3H, CH− CO−N), 3.16 (d, 2H, S−CH2−CH), 3.78 (t, 2H, CH2−N), 4.24 (t,

2H, CH2−O), 5.26 (t, 1.7H, CH−O−CH) and 5.29 (t, 0.3H, CH− O−CH), 6.51 (t, 1.7H, CHCH) and 6.53 (t, 0.3H, CHCH). 13 C NMR (75 MHz, CDCl3). 22.1 (CH(CH3)2), 25.1 (C(CH3)2− CO), 27.9 (CH(CH3)2), 37.6 (N−CH2), 45.5 (S−CH2−CH), 47.6 ((CH)2−CO−N), 56.0 (C(CH3)2−CO), 62.2 (CH2−O), 80.9 (CH− O−CH), 136.6 (CHCH), 172.9 (CO−C(CH3)2), 175.9 (CO−N), 221.9 (CS). C


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Figure 1. 1H NMR spectra (CDCl3) of CTA3 (a) and CTA4 (b).

Table 1. Results of RAFT-Mediated Polymerizations Involving OEGA and TFEA Performed in the Presence of CTA3 and CTA4a polymer

[M]0/[CTA]0/[I]0

conversion (%)b

Mn,NMR (g·mol−1)b

Mn,SEC (g·mol−1)c

D̵ c

CTA3-POEGA CTA3-PTFEA CTA4-POEGA CTA4′-POEGA CTA4-PTFEA CTA4′-PTFEA

30/1/0.2 100/1/0.2 30/1/0.2

94.5 74.4 92.5

100/1/0.2

64.0

15 050 11 250 15 400 16 700 10 900 11 200

6650 3600 7350 7160 2650 2450

1.22 1.18 1.21 1.22 1.20 1.22

a Reaction conditions: AIBN as an initiator (I), DMF as a solvent, T = 80 °C. bDetermined by 1H NMR. cDetermined by size exclusion chromatography (SEC) using PS standards.

Typical Procedure for RAFT Polymerizations. In a typical experiment, the monomer (OEGA or TFEA), AIBN, and the chain transfer agent (CTA3 or CTA4) in dimethylformamide (5 mL) were added into a Schlenk tube. The components were deoxygenated by flushing with nitrogen for 45 min. The mixture was then heated to 80 °C. At the end of the polymerization, the residual monomer and impurities were removed by dialysis (MWCO = 6000−8000 g·mol−1) against acetone, and polymers were analyzed by 1H NMR and size exclusion chromatography (SEC). Formation of Maleimide End-Functionalized Polymers CTA4′POEGA and CTA4′-PTFEA. Deprotection of CTA4-POEGA and CTA4-PTFEA was accomplished using an rDA reaction. Typically, polymers were dissolved in 50 mL toluene and refluxed for 48 h in an oil bath. Toluene was then removed under reduced pressure to give the maleimide end-functionalized polymers (denoted CTA4′-polymer), which were purified by dialysis against acetone. Preparation of Functionalized Titanium Platforms (Ti-DM and Ti-DF). Titanium plates (Ø = 1.5 cm, 1.8 cm2) were first treated with an acid-oxidizing solution of concentrated sulfuric acid and hydrogen peroxide (H2SO4/H2O2; 1:1 v/v) for 2 min to generate the corresponding hydroxylated titanium dioxide surface. Titanium plates were thoroughly rinsed three times with water, acetone, and ethanol and dried under nitrogen before functionalization. The pretreated titanium surfaces were then soaked in a solution containing 1 mM functionalized anchor DF or DM in a methanol/water mixture (1:1 v/ v) and were shaken overnight at room temperature. After functionalization, the samples were thoroughly rinsed with the solvent used for the grafting and dried under nitrogen. Diels−Alder Reactions. A concentrated solution of the suitable furan/maleimide end-functionalized polymers in DCM (50 mg in 1 mL DCM) was loaded onto the Ti-DF/Ti-DM functionalized surfaces. The latter were then placed in a ventilated oven at 60 °C for 7 days. After immobilization, the samples were thoroughly rinsed with dichloromethane and dried under a nitrogen flow. Retro Diels−Alder Reactions. rDA reactions were achieved by placing the DA-modified titanium surfaces into refluxing toluene for 14

days. After reaction, samples were rinsed with toluene and dichloromethane and dried under a nitrogen flow.

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RESULTS AND DISCUSSION Synthesis of the Furan/Maleimide End-Functionalized Chain Transfer Agent. Two new RAFT agents containing a furan (CTA3) or a maleimide (CTA4) moiety were designed to ensure the grafting of end-functionalized polymers onto complementary catechol-based titanium platforms featuring maleimide (Ti-DM) and furan (Ti-DF) units, respectively, through DA reactions (Scheme 1). The synthetic routes of the RAFT agents CTA3 and CTA4 are described in Scheme 2. Briefly, the furan-based RAFT agent CTA3 was prepared by coupling the hydroxysuccinimide activated ester of the 2-(1-isobutyl)sulfanylthiocarbonylsulfanyl-2-methylpropionic acid (CTA2) with the furfurylamine. To prevent side reactions during polymerizations, the maleimide group of the RAFT agent CTA4 was protected in the form of a furan-based cycloadduct.48 CTA4 was thus obtained by reacting 2-(1-isobutyl)sulfanylthiocarbonylsulfanyl-2-methylpropionic acid (CTA1) with alcohol 1 in the presence of DCC and DMAP. The structure of RAFT agents was investigated using 1H NMR (Figure 1) and 13C NMR (see Figure S1). Both 1H NMR spectra of CTA3 and CTA4 revealed the presence of the characteristic signals of the isobutylsulfanylthiocarbonylsulfanyl [S−(CS)−S−iBu] moiety [1.0−3.2 ppm (a−d)] and those of the furan [6.2 (f), 6.3 (g), and 7.3 ppm (h)] or the protected maleimide fragment [2.9 (g), 5.3 (h), and 6.5 ppm (i)]. Furthermore, 13C NMR spectra confirmed the structure of RAFT agents owing to the presence of chemical shifts at 220, 172.8, and 172.9 ppm ascribed to the thiocarbonyl fragment, D


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Figure 2. 1H NMR spectrum (CDCl3) (a) and SEC trace (b) of CTA3-POEGA.

Figure 3. 1H NMR (CDCl3) spectra of CTA4-PTFEA (a) and CTA4′-PTFEA obtained after deprotection through the rDA reaction. (b) SEC traces of CTA4-PTFEA (c) and CTA4′-PTFEA (d).

through the DA reaction, both functionalized CTA3 and CTA4 were used to promote RAFT polymerizations of two different monomers (Scheme 2). Poly(oligo(ethylene glycol)methyl ether acrylate) (POEGA) was first chosen for its pronounced

the amide group of CTA3, and the ester group of CTA4 (see Supporting Information S1), respectively. RAFT Polymerizations Using CTA3 and CTA4. With the aim of modifying the interfacial properties of titanium surfaces E


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Figure 4. XPS survey spectra of the Ti-DF platforms before and after the DA reaction with the end-functionalized polymers (a). ATR−FTIR spectra of the Ti-DF platforms modified with the end-functionalized polymers (b). 1800−2400 cm−1 region corresponding to the Zn/Se crystal adsorption band was removed for clearness.

hydrophilic character,49 whereas the polymerization of TFEA leads to very hydrophobic polymers.50 The polymerizations of both polymers were initiated by AIBN at 80 °C using dimethylformamide as the solvent. Details of the experimental conditions and results of RAFT polymerizations of OEGA and TFEA are provided in Table 1. POEGA and PTFEA functionalized polymers (furan and protected maleimide) were obtained with low dispersity values (D̵ < 1.22) and symmetrical SEC traces (see Figures S3 and S4). Characterizations of Polymers. The structure of polymers was first elucidated by 1H NMR spectroscopy. For instance, the 1H NMR spectrum of the CTA3-POEGA (Figure 2a) displayed the characteristic resonances of the repeating ethylene oxide unit [3.4 ppm (i), from 3.5 to 3.8 ppm (h) and 4.2 ppm (g)] and those belonging to the furan moiety at 6.2 (k), 6.3 (l). and 7.4 ppm (m) (zoomed-in part of Figure 2a). Moreover, by taking into account the integration of these protons and those of the methyl terminated groups (i) from the repeated OEGA unit, the molecular weight of the polymer could be calculated (Mn,NMR = 15 050 g·mol−1). In the case of CTA4-PTFEA polymer, the 1H NMR spectrum (Figure 3a) evidenced the presence of the characteristic protons of the PTFEA backbone [1.5−2.6 ppm (e,d) and 4.5 ppm (g)] and those of the oxa-norbornene end group at 6.5 (l), 5.3 (k), and 2.9 ppm (j). The integration of these signals allowed the determination of Mn,NMR of 10 900 g·mol−1 for CTA4-PTFEA. Generation of Maleimide End-Functionalized Polymers Using the Retro Diels−Alder Reaction. Before immobilization of the maleimide end-functionalized polymers onto the Ti-DF platform, CTA4-based polymers were subjected to an rDA reaction to generate the reactive maleimide unit. To this end, CTA4 polymers were refluxed in toluene for 48 h; afterward, the resultant polymers, referred as “CTA4′polymers,” were analyzed by 1H NMR spectroscopy and SEC (Figure 3). As depicted in Figure 3b, 1H NMR spectrum of the CTA4′-PTFEA clearly indicates the complete disappearance of the proton resonances of the bridgehead bicyclic structure and the appearance of a single signal at 6.75 ppm (j) attributed to the maleimide fragment. In addition, no change in the SEC traces was observed (Figure 3c,d) before and after the deprotection step, suggesting that no degradation occurred during the thermal process (see Table 1).

Grafting of End-Functionalized Polymers onto Titanium Platforms through the Diels−Alder Reaction. The previously described catechol-based anchors DF and DM (Scheme 1) were first used to elaborate Ti-DF- and Ti-DMreactive titanium platforms, respectively. To this end, bare titanium plates were soaked into a 1 mM MeOH/water solution of DF or DM overnight under stirring at room temperature. After washing with methanol and drying, the TiDM and Ti-DF platforms were subjected to DA reactions in the presence of the complementary furan and maleimide endfunctionalized polymers (CTA3 polymers and CTA4′ polymers), respectively. For this purpose, concentrated solutions of functionalized polymers were placed onto titanium platforms and heated at 60 °C for 7 days. It is worth noting that no grafting occurred on titanium surfaces when DA reactions were achieved with polymers lacking in the complementary reactive unit. XPS Investigations. The grafting of maleimide and furan end-functionalized polymers onto Ti-DF and Ti-DM platforms was first investigated using XPS. First, as depicted in Figures 4a, S2, and S5, the grafting of maleimide- and furan-based catechol anchors onto the titanium surface was revealed by a significant decrease in the titanium content and a substantial increase in the C 1s and N 1s signals (amide component at 400.3 eV) compared with the bare titanium surface.17 After completion of the DA reaction with end-functionalized polymers, a further decrease in the titanium content was clearly observed on both Ti-DF and Ti-DM platforms, whereas sulfur was detected in accordance with the immobilization of RAFT-promoted polymers on surfaces (see Table S2). In particular, the inspection of the C 1s core-level spectra of the titanium platforms modified with the end-functionalized POEGA (Figure 6b) revealed the characteristic C−C, C−H (284.8 eV), and C−O (286.1 eV) components of the ethylene oxide repeating unit.36,43,51 Furthermore, in the case of titanium platforms modified with CTA3/4′-PTFEA (Figures 4a and S7), an intense signal at 689 eV was detected on the survey spectra, which was attributed to the presence of fluorine on the titanium surface, thereby corroborating the grafting of the fluorinated polymer onto the titanium platform through the DA cycloaddition. Finally, from the attenuation of the titanium signals,52,53 film thicknesses of 1 to 2 nm could be estimated (calculation in Supporting Information S8). F


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Figure 5. Evolution of Ti-DM contact angles after DA1 reaction with CTA3-PTFEA and subsequent rDA and DA2 cycloaddition with CTA3POEGA.

Figure 6. XPS survey spectra of Ti-DM platforms throughout the DA1/rDA/DA2 sequence (a). C 1s core-level spectrum of the Ti-DM surface after DA1 achieved with CTA3-POEGA (top) and C 1s core level spectrum of Ti-DM after the DA1/rDA sequence and subsequent DA2 achieved with CTA3-POEGA (bottom) (b).

ATR−FTIR. Further evidence of the titanium functionalization was obtained by infrared spectroscopy using the attenuated total reflectance (ATR) technique. For example, the ATR− FTIR spectra of Ti-DF platforms subjected to the DA cycloaddition in the presence of maleimide end-functionalized polymers are displayed in Figure 4b. POEGA-modified Ti-DF surfaces exhibit the characteristic absorption bands at 1000− 1200 cm−1 and 1730 cm−1 ascribed to ether bonds and carbonyl ester, respectively, belonging to the polymer backbone. On the other hand, the grafting of PTFEA polymers onto Ti-DF platforms was evidenced by the presence of the characteristic absorption band of the carbonyl group at 1750 cm−1 and the C−F stretching band at 1280 cm−1.50,54 Contact Angle Measurements. The impact of the grafting of POEGA and PTFEA end-functionalized polymers onto the titanium platforms was investigated thanks to static water contact angle measurements (Figures 5 and S6). As

expected, the grafting of end-functionalized POEGA onto TiDM and Ti-DF platforms leads to slightly more hydrophilic titanium surfaces. Indeed, although Ti-DM and Ti-DF surfaces displayed contact angles of 78° and 68°, respectively, these values decreased to 54° and 52°, respectively, after the grafting of the POEGA and are in good accordance with previously reported values.17,51 Interestingly, after the grafting of the endfunctionalized PTFEA onto titanium platforms, the titanium surfaces became highly hydrophobic as the contact angles largely increased to 113° for Ti-DF and 118° for Ti-DM. Reversible Tethering of Polymer Chains onto the Catechol-Based Titanium Surfaces. We next envisioned to detach on demand maleimide or furan end-functionalized polymers from the catechol-based titanium platforms by exploiting the rDA and subsequently to attach another type of polymer via a second DA (DA2). As a proof of concept, the hydrophobic titanium surfaces functionalized with the fluoriG


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to a plethora of smart titanium surfaces with controlled interfacial properties, paving the way for promising applications in electronics, sensors, biotechnologies, and nanotechnologies.

nated PTFEA (contact angles of 118° and 113° for Ti-DM and Ti-DF, respectively) were subjected to the rDA reaction by refluxing the titanium plates in toluene. After the rDA reaction, the contact angles decreased to almost the same values measured for catechol-based titanium surfaces [i.e., 76° and 73° for recycled Ti-DF and Ti-DM (Figures 5 and S6)], suggesting in both cases the removal of the hydrophobic polymers from the substrates. This was further evidenced by XPS investigations (Figures 6a and S2). Whereas F 1s is largely detected on Ti-DM functionalized with PTFEA, the inspection of XPS survey spectrum and atomic quantifications of recycled Ti-DM surface (after rDA) revealed a dramatic decrease in F 1s contents accompanied by an increase in the titanium ratio, thereby suggesting the elimination of PTFEA from the titanium surface. Interestingly, despite some fluorine traces (1−3%) present on the surface after rDA, the subsequent DA reaction (DA2) with the corresponding end-functionalized POEGA led to more hydrophilic surfaces because the contact angle decreased to 57° for Ti-DF and 58° for Ti-DM.36 It is noteworthy that these latter values are close to those measured through a direct DA functionalization of titanium platforms with the same POEGA (52° and 54° for Ti-DF and Ti-DM, respectively), further corroborating the efficiency of the rDA/ DA2 sequence for the reversible tethering of polymers onto catechol-based titanium surfaces. These results were confirmed by XPS investigations. A decrease in the titanium ratio (see Figure S2) was first observed after functionalization with CTA3-POEGA, suggesting the grafting of the hydrophilic polymer onto the Ti-DM surface via DA2. Furthermore, the examination of the C 1s core-level spectra of these surfaces revealed (Figure 6b, lower graph) the presence of the characteristic C−C, C−H (284.9 eV), and C− O (286.2 eV) components of the oligoethylene glycol chains similar to those observed in the case of the direct functionalization of Ti-DM with CTA3-POEGA via DA1 (Figure 6a, upper graph) in accordance with previous works dealing with POEGA grafting.36,43,51 Therefore, both contact angle and XPS measurements clearly demonstrated the reversible tethering of the polymer onto catechol-based titanium surfaces by using the DA1/rDA/DA2 sequence.

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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.7b00160. 13 C NMR spectra of CTA3 and CTA4, 1H NMR spectra of the corresponding end-functionalized polymers, and their SEC traces. XPS spectra, ATR−FTIR, and contact angle measurements of the whole functionalization process achieved with both titanium platforms (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.W.). *E-mail: [email protected] (J.L.). ORCID

David Fournier: 0000-0002-6791-6131 Patrice Woisel: 0000-0001-5860-5107 Joël Lyskawa: 0000-0003-2990-2561 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche, Région Hauts-de-France and FEDER supported and funded this work.

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REFERENCES

(1) Kestler, H.; Clemens, H. Production, Processing and Application of γ(TiAl)-Based Alloys. In Titanium and Titanium Alloys; Wiley-VCH Verlag GmbH & Co. KGaA, 2005; pp 351−392. (2) Zhecheva, A.; Sha, W.; Malinov, S.; Long, A. Enhancing the microstructure and properties of titanium alloys through nitriding and other surface engineering methods. Surf. Coat. Technol. 2005, 200, 2192−2207. (3) Liu, X.; Chu, P. K.; Ding, C. Surface modification of titanium, titanium alloys, and related materials for biomedical applications. Mater. Sci. Eng., R 2004, 47, 49−121. (4) Brunette, D. M.; Tengvall, P.; Textor, M.; Thomsen, P. Titanium in Medicine: Material Science, Engineering, Biological Responses and Medical Applications; Springer: Berlin, 2001. (5) Rodríguez-Cano, A.; Cintas, P.; Fernández-Calderón, M.-C.; Pacha-Olivenza, M.-Á .; Crespo, L.; Saldaña, L.; Vilaboa, N.; GonzálezMartín, M.-L.; Babiano, R. Controlled silanization−amination reactions on the Ti6Al4V surface for biomedical applications. Colloids Surf., B 2013, 106, 248−257. (6) Li, G.; Yang, P.; Liao, Y.; Huang, N. Tailoring of the Titanium Surface by Immobilization of Heparin/Fibronectin Complexes for Improving Blood Compatibility and Endothelialization: An in Vitro Study. Biomacromolecules 2011, 12, 1155−1168. (7) Gawalt, E. S.; Avaltroni, M. J.; Danahy, M. P.; Silverman, B. M.; Hanson, E. L.; Midwood, K. S.; Schwarzbauer, J. E.; Schwartz, J. Bonding Organics to Ti Alloys: Facilitating Human Osteoblast Attachment and Spreading on Surgical Implant Materials. Langmuir 2003, 19, 200−204. (8) Ong, J. L.; Carnes, D. L.; Bessho, K. Evaluation of titanium plasma-sprayed and plasma-sprayed hydroxyapatite implants in vivo. Biomaterials 2004, 25, 4601−4606.

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CONCLUSIONS In this work, furan and maleimide end-functionalized polymers were prepared by RAFT polymerization from furan and furanprotected maleimide RAFT agents. These CTAs were then utilized to promote the polymerization of oligo(ethylene glycol)monomethyl ether acrylate and 2,2,2-trifluoroethyl acrylate. These well-defined polymers (as demonstrated by NMR and SEC characterizations) were next immobilized onto maleimide and furan catechol-based titanium platforms through the DA [4 + 2] cycloaddition. Successful immobilization of these polymers onto the titanium substrates was evidenced by XPS, ATR−FTIR, and contact angle measurements. Interestingly, the thermoreversible character of the DA reaction was then exploited to detach polymers from the surfaces, allowing the recycling of the catechol-based titanium platforms. This strategy was used to provide highly hydrophobic PTFEAgrafted titanium surfaces, which may be converted, on demand, into a hydrophilic titanium surface through the rDA and a subsequent DA reaction achieved with POEGA. The versatility of this strategy based on the reversible tethering of polymers onto catechol-based titanium platforms may facilitate the access H


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Langmuir (9) Yang, B.; Uchida, M.; Kim, H.-M.; Zhang, X.; Kokubo, T. Preparation of bioactive titanium metal via anodic oxidation treatment. Biomaterials 2004, 25, 1003−1010. (10) Rammelt, S.; Illert, T.; Bierbaum, S.; Scharnweber, D.; Zwipp, H.; Schneiders, W. Coating of titanium implants with collagen, RGD peptide and chondroitin sulfate. Biomaterials 2006, 27, 5561−5571. (11) Wach, J.-Y.; Malisova, B.; Bonazzi, S.; Tosatti, S.; Textor, M.; Zürcher, S.; Gademann, K. Protein-Resistant Surfaces through Mild Dopamine Surface Functionalization. Chem.Eur. J. 2008, 14, 10579−10584. (12) Li, L.; Hu, W.; Chi, L.; Fuchs, H. Polymer Brush and Inorganic Oxide Hybrid Nanodielectrics for High Performance Organic Transistors. J. Phys. Chem. B 2010, 114, 5315−5319. (13) Bilgic, T.; Klok, H.-A. Surface-initiated controlled radical polymerization enhanced DNA biosensing. Eur. Polym. J. 2015, 62, 281−293. (14) Ayres, N. Polymer brushes: Applications in biomaterials and nanotechnology. Polym. Chem. 2010, 1, 769−777. (15) Vasconcelos, D. M.; Falentin-Daudré, C.; Blanquaert, D.; Thomas, D.; Granja, P. L.; Migonney, V. Role of protein environment and bioactive polymer grafting in the S. epidermidis response to titanium alloy for biomedical applications. Mater. Sci. Eng., C 2014, 45, 176−183. (16) Sobocinski, J.; Laure, W.; Taha, M.; Courcot, E.; Chai, F.; Simon, N.; Addad, A.; Martel, B.; Haulon, S.; Woisel, P.; Blanchemain, N.; Lyskawa, J. Mussel Inspired Coating of a Biocompatible Cyclodextrin Based Polymer onto CoCr Vascular Stents. ACS Appl. Mater. Interfaces 2014, 6, 3575−3586. (17) Ye, Q.; Wang, X.; Li, S.; Zhou, F. Surface-Initiated RingOpening Metathesis Polymerization of Pentadecafluorooctyl-5-norbornene-2-carboxylate from Variable Substrates Modified with Sticky Biomimic Initiator. Macromolecules 2010, 43, 5554−5560. (18) Liu, J.; Ye, Q.; Yu, B.; Wang, X.; Zhou, F. Contact printing a biomimetic catecholic monolayer on a variety of surfaces and derivation reaction. Chem. Commun. 2012, 48, 398−400. (19) Fan, X.; Lin, L.; Messersmith, P. B. Cell Fouling Resistance of Polymer Brushes Grafted from Ti Substrates by Surface-Initiated Polymerization: Effect of Ethylene Glycol Side Chain Length. Biomacromolecules 2006, 7, 2443−2448. (20) Wang, X.; Ye, Q.; Gao, T.; Liu, J.; Zhou, F. Self-Assembly of Catecholic Macroinitiator on Various Substrates and Surface-Initiated Polymerization. Langmuir 2012, 28, 2574−2581. (21) Viornery, C.; Chevolot, Y.; Léonard, D.; Aronsson, B.-O.; Péchy, P.; Mathieu, H. J.; Descouts, P.; Grätzel, M. Surface Modification of Titanium with Phosphonic Acid To Improve Bone Bonding: Characterization by XPS and ToF-SIMS. Langmuir 2002, 18, 2582− 2589. (22) Porté-Durrieu, M. C.; Guillemot, F.; Pallu, S.; Labrugère, C.; Brouillaud, B.; Bareille, R.; Amédée, J.; Barthe, N.; Dard, M.; Baquey, C. Cyclo-(DfKRG) peptide grafting onto Ti−6Al−4V: Physical characterization and interest towards human osteoprogenitor cells adhesion. Biomaterials 2004, 25, 4837−4846. (23) Ye, Q.; Wang, X.; Hu, H.; Wang, D.; Li, S.; Zhou, F. Polyelectrolyte Brush Templated Multiple Loading of Pd Nanoparticles onto TiO2 Nanowires via Regenerative Counterion Exchange−Reduction. J. Phys. Chem. C 2009, 113, 7677−7683. (24) Comas, H.; Laporte, V.; Borcard, F.; Miéville, P.; Juillerat, F. K.; Caporini, M. A.; Gonzenbach, U. T.; Juillerat-Jeanneret, L.; GerberLemaire, S. Surface Functionalization of Alumina Ceramic Foams with Organic Ligands. ACS Appl. Mater. Interfaces 2012, 4, 573−576. (25) Xu, L. Q.; Pranantyo, D.; Liu, J. B.; Neoh, K.-G.; Kang, E.-T.; Ng, Y. X.; Teo, S. L.-M.; Fu, G. D. Layer-by-layer deposition of antifouling coatings on stainless steel via catechol-amine reaction. RSC Adv. 2014, 4, 32335−32344. (26) Lee, H.; Lee, K. D.; Pyo, K. B.; Park, S. Y.; Lee, H. CatecholGrafted Poly(ethylene glycol) for PEGylation on Versatile Substrates. Langmuir 2010, 26, 3790−3793.

(27) Faure, E.; Falentin-Daudré, C.; Jérôme, C.; Lyskawa, J.; Fournier, D.; Woisel, P.; Detrembleur, C. Catechols as versatile platforms in polymer chemistry. Prog. Polym. Sci. 2013, 38, 236−270. (28) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-inspired surface chemistry for multifunctional coatings. Science 2007, 318, 426−430. (29) Dalsin, J. L.; Hu, B.-H.; Lee, B. P.; Messersmith, P. B. Mussel Adhesive Protein Mimetic Polymers for the Preparation of Nonfouling Surfaces. J. Am. Chem. Soc. 2003, 125, 4253−4258. (30) Fan, X.; Lin, L.; Messersmith, P. B. Surface-initiated polymerization from TiO2 nanoparticle surfaces through a biomimetic initiator: A new route toward polymer−matrix nanocomposites. Compos. Sci. Technol. 2006, 66, 1198−1204. (31) Liu, J.; Yang, W.; Zareie, H. M.; Gooding, J. J.; Davis, T. P. pHDetachable Polymer Brushes Formed Using Titanium−Diol Coordination Chemistry and Living Radical Polymerization (RAFT). Macromolecules 2009, 42, 2931−2939. (32) Xu, L. Q.; Jiang, H.; Neoh, K.-G.; Kang, E.-T.; Fu, G. D. Poly(dopamine acrylamide)-co-poly(propargyl acrylamide)-modified titanium surfaces for ‘click’ functionalization. Polym. Chem. 2012, 3, 920−927. (33) Zhang, F.; Liu, S.; Zhang, Y.; Chi, Z.; Xu, J.; Wei, Y. A facile approach to surface modification on versatile substrates for biological applications. J. Mater. Chem. 2012, 22, 17159−17166. (34) Zobrist, C.; Sobocinski, J.; Lyskawa, J.; Fournier, D.; Miri, V.; Traisnel, M.; Jimenez, M.; Woisel, P. Functionalization of Titanium Surfaces with Polymer Brushes Prepared from a Biomimetic RAFT Agent. Macromolecules 2011, 44, 5883−5892. (35) Malisova, B.; Tosatti, S.; Textor, M.; Gademann, K.; Zürcher, S. Poly(ethylene glycol) Adlayers Immobilized to Metal Oxide Substrates Through Catechol Derivatives: Influence of Assembly Conditions on Formation and Stability. Langmuir 2010, 26, 4018−4026. (36) Fan, X.; Lin, L.; Dalsin, J. L.; Messersmith, P. B. Biomimetic Anchor for Surface-Initiated Polymerization from Metal Substrates. J. Am. Chem. Soc. 2005, 127, 15843−15847. (37) Mizrahi, B.; Khoo, X.; Chiang, H. H.; Sher, K. J.; Feldman, R. G.; Lee, J.-J.; Irusta, S.; Kohane, D. S. Long-Lasting Antifouling Coating from Multi-Armed Polymer. Langmuir 2013, 29, 10087− 10094. (38) Sanyal, A. Diels−Alder Cycloaddition-Cycloreversion: A Powerful Combo in Materials Design. Macromol. Chem. Phys. 2010, 211, 1417−1425. (39) Gevrek, T. N.; Ozdeslik, R. N.; Sahin, G. S.; Yesilbag, G.; Mutlu, S.; Sanyal, A. Functionalization of Reactive Polymeric Coatings via Diels−Alder Reaction Using Microcontact Printing. Macromol. Chem. Phys. 2012, 213, 166−172. (40) Blinco, J. P.; Trouillet, V.; Bruns, M.; Gerstel, P.; Gliemann, H.; Barner-Kowollik, C. Dynamic Covalent Chemistry on Surfaces Employing Highly Reactive Cyclopentadienyl Moieties. Adv. Mater. 2011, 23, 4435−4439. (41) Preuss, C. M.; Zieger, M. M.; Rodriguez-Emmenegger, C.; Zydziak, N.; Trouillet, V.; Goldmann, A. S.; Barner-Kowollik, C. Fusing Catechol-Driven Surface Anchoring with Rapid Hetero Diels− Alder Ligation. ACS Macro Lett. 2014, 3, 1169−1173. (42) Urakami, H.; Yilmaz, A. G.; Osiceanu, P.; Yagci, Y.; Vilela, F.; Titirici, M.-M. Facile Polymer Functionalization of HydrothermalCarbonization-Derived Carbons. Macromol. Rapid Commun. 2013, 34, 1080−1084. (43) Preuss, C. M.; Goldmann, A. S.; Trouillet, V.; Walther, A.; Barner-Kowollik, C. Biomimetic Dopamine-Diels−Alder Switches. Macromol. Rapid Commun. 2013, 34, 640−644. (44) Laure, W.; Woisel, P.; Lyskawa, J. Switching the Wettability of Titanium Surfaces through Diels−Alder Chemistry. Chem. Mater. 2014, 26, 3771−3780. (45) Heath, W. H.; Palmieri, F.; Adams, J. R.; Long, B. K.; Chute, J.; Holcombe, T. W.; Zieren, S.; Truitt, M. J.; White, J. L.; Willson, C. G. Degradable Cross-Linkers and Strippable Imaging Materials for Stepand-Flash Imprint Lithography. Macromolecules 2008, 41, 719−726. I


Article

Langmuir (46) Mazur, M.; Barras, A.; Kuncser, V.; Galatanu, A.; Zaitzev, V.; Turcheniuk, K. V.; Woisel, P.; Lyskawa, J.; Laure, W.; Siriwardena, A.; Boukherroub, R.; Szunerits, S. Iron oxide magnetic nanoparticles with versatile surface functions based on dopamine anchors. Nanoscale 2013, 5, 2692−2702. (47) Qiu, X.-P.; Tanaka, F.; Winnik, F. M. Temperature-Induced Phase Transition of Well-Defined Cyclic Poly(Nisopropylacrylamide)s in Aqueous Solution. Macromolecules 2007, 40, 7069−7071. (48) Heredia, K. L.; Grover, G. N.; Tao, L.; Maynard, H. D. Synthesis of Heterotelechelic Polymers for Conjugation of Two Different Proteins. Macromolecules 2009, 42, 2360−2367. (49) Lutz, J.-F. Polymerization of oligo(ethylene glycol) (meth)acrylates: Toward new generations of smart biocompatible materials. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3459−3470. (50) Le, D. V.; Kendrick, M. M.; O’Rear, E. A. Admicellar Polymerization and Characterization of Thin Poly(2,2,2-trifluoroethyl acrylate) Film on Aluminum Alloys for In-Crevice Corrosion Control. Langmuir 2004, 20, 7802−7810. (51) Zhang, F.; Xu, F. J.; Kang, E. T.; Neoh, K. G. Modification of Titanium via Surface-Initiated Atom Transfer Radical Polymerization (ATRP). Ind. Eng. Chem. Res. 2006, 45, 3067−3073. (52) Laforgue, A.; Addou, T.; Bélanger, D. Characterization of the Deposition of Organic Molecules at the Surface of Gold by the Electrochemical Reduction of Aryldiazonium Cations. Langmuir 2005, 21, 6855−6865. (53) Li, Y.; Zhao, M.; Wang, J.; Liu, K.; Cai, C. Biofunctionalization of a “Clickable” Organic Layer Photochemically Grafted on Titanium Substrates. Langmuir 2011, 27, 4848−4856. (54) Merino, E. G.; Atlas, S.; Raihane, M.; Belfkira, A.; Lahcini, M.; Hult, A.; Dionísio, M.; Correia, N. T. Molecular dynamics of poly(ATRIF) homopolymer and poly(AN-co-ATRIF) copolymer investigated by dielectric relaxation spectroscopy. Eur. Polym. J. 2011, 47, 1429−1446.

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Reversible Tethering of Polymers onto Catechol-Based Titanium

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