Switching the Wettability of Titanium Surfaces through DielsâAlder

Article pubs.acs.org/cm

Switching the Wettability of Titanium Surfaces through Diels−Alder Chemistry William Laure,†,‡,§ Patrice Woisel,*,†,‡,§,∥ and Joel̈ Lyskawa*,†,‡,§ †

Université Lille Nord de France, 59044 Lille, France Unité des Matériaux Et Transformations (UMET, UMR 8207), Equipe Ingénierie des Systèmes Polymères (ISP), 59655 Villeneuve d’Ascq Cedex, France § Fédération Biomatériaux et Dispositifs Médicaux Fonctionnalisés (BDMF/FED 4123), 59655 Villeneuve d’Ascq, France ∥ ENSCL, 59655 Villeneuve d’Ascq, France ‡

S Supporting Information *

ABSTRACT: A versatile approach for the elaboration of switchable titanium platforms through the Diels−Alder/retro Diels−Alder (DA/rDA) reactions is reported. For this purpose, catechol based biomimetic anchors integrating either a furan or a maleimide moiety were synthesized and grafted onto titanium surfaces. The DA cycloaddition reaction was then employed to chemically modify TiO2 surfaces and to tailor their wettability. The thermal reversibility of the Diels−Alder reaction was next exploited to switch the wettability of TiO2 surfaces. As a proof of concept, several probes including a ferrocene derivative, a hydrophobic (fluorinated) and a hydrophilic (poly(ethylene oxide)) oligomers were immobilized onto TiO2 surfaces and interchanged afterward. The course of chemical transformations of TiO2 surfaces as well as the faculty of their wettability to be switched (on demand) were investigated by electrochemical, contact angle, and XPS measurements.

■

original biomimetic approach15,16 for surface modification based on the catechol chemistry. Indeed, owing to their propensity to establish a large panoply of interactions with in particular inorganic substrates, catechol derivatives have emerged as important and versatile building blocks for modifying the interfacial properties of titanium surfaces. In this context, catecholamines such as 3,4-dihydroxy-L-phenylalanine (L-DOPA) and 3-hydroxytyramine (dopamine), two biomolecules integrating both a catechol and a free amino groups, play a pivotal role. Indeed, in an alkaline environment, self-polymerization of dopamine results in the formation of a robust thin adherent polydopamine (PDA) films onto titanium surfaces.17−19 Interestingly, in addition to their strong adherence properties and their ease of use, such films exhibit latent reactivity toward nucleophiles such as amines and thiols that can be further exploited to anchor polymers20−22 or biomolecules23 such as selenocystamine,24 RGD25 (arginylglycylaspartic acid), or VEGF26,27 (vascular endothelial growth factor) peptides onto TiO2 surfaces. On the other hand, the nucleophilicity of the free amino group of DOPA and dopamine has also been extensively exploited for elaborating catechol based anchors28,29 for surface functionalization. In this way, many groups reported on the elaboration of nonfouling titanium surfaces by directly immobilizing catecholic poly-

INTRODUCTION Titanium and its alloys are of considerable importance because of their exceptional mechanical, thermal, and electrical properties allowing their utilization in a wide range of applications including energy storage, microelectronics, aircraft industry, and nanotechnologies.1 In addition, these materials exhibit exceptional biocompatibility properties and are widely used in medicine for the conception of implantable biomaterials2 including dental and orthopedic implants. However, the chemical functionalization of the titanium surfaces is often required to improve their physicochemical and/or biological performances (e.g., cell affinity, osteoconductive, and/or osteoinductive properties, tissues integration, prevent biofilm formation etc.). Several methods have been developed to manipulate the interface properties of titanium substrates including plasma treatment,3 electrodeposition,4 layer-by-layer techniques,5 hydroxyapatite deposition,6 and the covalent attachment of target molecules. On the last point, silanization and phosphonic acid/ester chemistry have long been exploited.7−10 However, multistep organic synthesis involving protecting groups11 is usually required to synthesize such organofunctional anchors. Moreover, harsh conditions11−13 are necessary to conveniently immobilize phosphonic acid/ester derivatives onto titanium surfaces, and some coatings prepared via the silanization way appear unstable in physiological conditions.14 In this context, inspired by the composition of adhesive proteins of mussels (MAPs), Messersmith et al. pioneered an © 2014 American Chemical Society

Received: April 15, 2014 Revised: May 19, 2014 Published: May 20, 2014 3771

dx.doi.org/10.1021/cm501354j | Chem. Mater. 2014, 26, 3771−3780

Chemistry of Materials

Article

Scheme 1. Schematic Illustration of Catechol Based Titanium Platforms with Switchable Wettability

To this end, we have combined the remarkable adhesive properties of the catechol derivatives toward Ti surfaces with the thermoreversibility of the furan-maleimide based DA click reaction. For this purpose, two catechol anchors bearing either a maleimide or a furan unit were immobilized onto Ti surfaces and their propensity to form DA adducts in the presence of an electroactive (Ferrocene, Fc) or a fluorinated (Zonyl) probes containing a complementary function was evaluated by means of electrochemistry, XPS, and contact angle measurements. Next, we have investigated the opportunity to exploit the rDA/ DA sequence in the perspective to switch, on command, the wettability of the Ti surface. More particularly, we have demonstrated that an hydrophobic titanium surface could be converted into an hydrophilic one by simply interchanging Zonyl grafted oligomers by PEG chains.

(ethylene glycol) (PEG) prepared from dopamine derivatives.30−34 Surface initiated polymerizations (SIP) were also successfully carried out to elaborate polymer brushes onto titanium surfaces via atom transfer radical polymerization35−37 (ATRP), ring-opening metathesis38 (ROMP), or reversible addition−fragmentation chain transfer (RAFT) procedures.39 Recently, our group also described the preparation of different well-defined dopamine end-functionalized polymers by employing the RAFT procedure and their subsequent immobilization onto titanium surfaces.40 Surprisingly, up to now, only limited attention has been devoted to modification of titanium surfaces by combining the catecholic and the “click” chemistry.41−45 In this way, we pioneered the exploitation of the copper-catalyzed azide−alkyne 1,3-dipolar cycloaddition (CuAAC) to modify the surface properties of titanium materials.46 More particularly, we demonstrated that an alkynyl perfluorinated oligomer (Zonyl) could be efficiently tethered onto an azide-terminated titanium surface from a catechol-azide clickable platform, thereby affording a strong hydrophobic titanium surface. More recently, copolymers containing both alkyne and catechol units have been prepared by Fu et al.47 and grafted onto titanium surfaces via coordination interactions of the catechol moieties. The presence of clickable alkyne groups on the surface was then exploited to immobilize several specific probes and hydrophilic poly(ethylene glycol) chains. The aforementioned catechol based methods have been developed in a view to strongly tether target (macro)molecules onto Ti surfaces. Nevertheless, to meet the ever growing demand in various nano- and biotechnology fields for smart surfaces with dynamically and tailored surface properties, the development of titanium substrates with switchable surface properties, such as wettability for example, is highly needed.48 Among the arsenal of reactions that enable a such surface remodeling, the maleimide-furan Diels−Alder (DA) reaction turns out to be an ideal candidate, since it exhibits, unlike of the plethora of chemical click transformations employed to alter surface properties, a temperature-dependent, dynamic covalent behavior that can be readily exploited to cover/uncover on command various substrates.29,49−53 As a part of our interest in the development of functionalized titanium surfaces, we report here the convenient elaboration of Ti substrates with switchable wettability (Scheme 1).

■

EXPERIMENTAL SECTION

Reagents. All reagents were purchased from Sigma-Aldrich and used without further purification. Analytical Techniques. 1H and 13C NMR spectra were recorded at 25 °C with a Bruker Avance 300 spectrometer. ESI-MS spectra were measured using a Platform II Micromass Apparatus. The number-average molar mass (Mn), the weight-average molar mass (Mw), and the molar mass distributions (Mw/Mn) were determined by size exclusion chromatography (SEC) in tetrahydrofuran at 40 °C with a flow rate of 1 mL·min−1. The number-average molar masses (Mn) and the dispersity (Đ = Mw/Mn), were derived from the refractive index (RI) signal by a calibration curve based on polystyrene (PS) standards from Polymer Standards Service. Electrochemical experiments (cyclic voltammetry) were performed using an Autolab PGSTAT 30 workstation. The experiments were carried out in dry acetonitrile containing 0.1 M of recrystallized tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte or in a phosphate buffer solution adjusted to pH = 7. A three-electrode configuration was used with a titanium disk (1.5 cm diameter) as working and counter electrode. An Ag/AgCl electrode was used as reference. All solutions were purged with nitrogen prior to recording the electrochemical measurements. XPS analyses were performed on a Kratos Axis Ultra DLD system (Kratos Analytical) using a nonmonochromatic Al Kα X-ray source (hυ = 1486.6 eV). The emission voltage and the current of the source were set to 12 kV and 3 mA, respectively. The pressure in the analyzing chamber was maintained at 5.10−9 mbar or lower during 3772



Article

CFcH), 5.08 (s, 2H, CH2O), 5.82 (br, 1H, CH2NH), 6.36 (dd, 1H, CHC(CH2)), 6.40 (dd, 1H, CHCHC), 7.42 (dd, 1H, O CHCH). 13 C NMR (75 MHz, CDCl3). 29.6 (CH2CONH), 31.1 (CH2 COO), 39.0 (CH2NH), 58.4 (CH2O), 68.2 (CFcH), 84.7 (CH2CFc(IV)), 110.6 (CHC(CH2)), 110.7 (CHCHC), 143.4 (OCHCH), 149.2 (CHC(CH2)), 170.7 and 172.7 (COO and CONH). ESI-MS: m/z 418 (M+Na). Preparation of 5. A solution of 3 (0.52 g, 1.7 mmol) in DCM (20 mL) was added dropwise to a solution of aminomethyl ferrocene (0.4 g, 1.9 mmol) and triethylamine (0.26 g, 2.6 mmol) in dry DCM (100 mL). The mixture was next stirred at room temperature overnight. The organic phase was washed with water (3 × 100 mL) and dried over MgSO4. After filtration, the solvent was evaporated and the product was obtained as an orange oil in quantitative yields. 1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 1.32 (m, 2H, CH2CH2CH2CONH), 1.64 (m, 4H, NCH2CH2; CH2CH2CONH), 2.15 (t, 2H, CH2CONH), 3.50 (t, 2H, NCH2), 4.14 (m, 11H, CH2NH ; CFcH), 5.58 (br, 1H, CH2NH), 6.68 (s, 2H, CHCH). 13 C NMR (75 MHz, CDCl3). 25.1 (CH2CH2CONH), 26.3 (CH 2 CH2 CH 2 CONH), 28.3 (NCH2 CH 2 ), 36.5 (CH2CONH), 37.6 (CH2N), 38.9 (NHCH2), 68.7 (CFc H), 84.8 (CH2CFc(IV)), 134.2 (CHCH), 170.9 and 172.0 (CO and CONH). ESl-MS: m/z 431 (M+Na) Preparation of 6. Amino-zonyl derivative 6 was synthesized by reduction of the azido-zonyl derivative with lithium aluminum hydride according to adapted literature procedure55 and was obtained as a brown oil in 58% yield. 1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 2.45 (m, 4H, CF2CH2; CH2NH2), 2.85 (m, 2H, CH2NH2), 3.50−3.70 (m, 26H, CH2CH2O), 3.78 (t, 2H, CF2CH2CH2). IR: 3450 cm−1 (ν NH2), 1650 cm−1 (δ NH), 1150 cm−1 (ν C−O). Preparation of 7. A solution of 2 (0.20 g, 0.69 mmol) in DCM (20 mL) was added dropwise to a solution of 6 (0.5 g, 0.62 mmol) and NEt3 (0.94 g, 0.93 mmol)) in dry DCM (100 mL). The mixture was next stirred at room temperature overnight. The organic phase was washed with NaOH 1 M (3 × 100 mL), water (3 × 100 mL) and dried over MgSO4. After filtration, the solvent was evaporated and the product was obtained as a brown oil in 79% yield. 1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 2.45 (m, 4H, CF2CH2; CH2COO), 2.65 (m, 2H, CH2CONH), 3.40 (m, 2H, CH2NHCO), 3.45 (m, 2H, CH2CH2NHCO), 3.50−3.70 (m, 24H, CH2CH2O), 3.78 (t, 2H, CF2CH2 CH2), 5.10 (s, 2H, CH2OCO), 6.36 (dd, 1H, CHC(CH2)), 6.40 (dd, 1H, CHCHC), 7.42 (dd, 1H, OCHCH). IR: 3330 cm−1 (ν NHCO), 1740 cm−1 (ν COO), 1670 cm−1 (ν CONH), 1150 cm−1 (ν C−O). MnSEC = 1450 g·mol−1, Đ = 1.17. Preparation of 8. A solution of 3 (0.21 g, 0.68 mmol) in DCM (20 mL) was added dropwise to a solution of 6 (0.5 g, 0.62 mmol) and NEt3 (0.94 g, 0.93 mmol) in dry DCM (100 mL). The mixture was next stirred at room temperature overnight. The organic phase was washed with NaOH 1 M (3 × 100 mL) and water (3 × 100 mL) and dried over MgSO4. After filtration, the solvent was evaporated and the product was obtained as a brown oil in 76% yield. 1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 1.40 (m, 2H, CH2CH2CH2CONH), 1.62 (m, 4H, NCH2CH2 ; CH2CH2CONH), 2.20 (t, 2H, CH2CONH), 2.45 (m, 2H, CF2CH2), 3.40 (m, 2H, CH2NH), 3.52 (m, 4H, NCH2 ; CH2CH2NH), 3.55−3.70 (m, 24H, CH2CH2O), 3.78 (t, 2H, CF2CH2CH2), 6.68 (s, 2H, CHCH). IR: 3440 cm−1 (ν NHCO), 1710 cm−1 (ν NCO), 1660 cm−1 (ν CONH), 1150 cm−1 (ν CO). MnSEC = 1500 g·mol−1, Đ = 1.35. Preparation of 9. Poly(ethylene glycol) methyl ether (Mn ≈ 2000 g·mol−1, 3 g, 1.5 mmol), glutaric anhydride (0.17 g, 1.5 mmol) and

analysis. Survey (0−1320 eV) and high resolution (C 1s) 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. IR spectra were recorded on a Spectrum One spectrometer from PerkinElmer coupled with a Zn/Se ATR crystal collecting four sample scans. Contact angle measurements 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 nine times to obtain an average value for the surface. Synthesis. Maleimide derivatives 3 and 12 were purchased from Sigma-Aldrich. Furan derivative 1,54 aminomethyl ferrocene,55 and azido-Zonyl derivative56 were prepared according to literature procedures. Preparation of 2. A suspension of N-hydroxysuccinimide (NHS, 2.6 g, 22.7 mmol) in dry dichloromethane (DCM, 50 mL) was added dropwise to a solution (DCM, 100 mL) containing 1 (3 g, 15.1 mmol) and dicyclohexylcarbodiimide (DCC, 4.7 g, 22.7 mmol) at −5 °C under nitrogen. The reaction was allowed to return to room temperature and was stirred overnight. After filtration, the solvent was evaporated and the crude product was purified by column chromatography (SiO2−DCM/ethyl acetate 5:1). The product was obtained as a white solid in 81% yield. 1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 2.76 (t, 2H, CH2 COON), 2.82 (s, 4H, COCH2CH2CO), 2.95 (t, 2H, CH2CH2COON), 5.10 (s, 2H, CH2O), 6.36 (dd, 1H, CHC(CH2)), 6.42 (dd, 1H, CHCHC), 7.42 (dd, 1H, O CHCH). 13 C NMR (75 MHz, CDCl3). 25.6 (CH2CH2), 26.2 (CH2CH2 COON), 28.6 (CH2CH2COON), 58.6 (CH2O), 110.6 (CHC(CH2)), 111.0 (CHCHC), 143.4 (OCH CH), 149.1 (CHC(CH2)), 167.7 (OCOCH2), 168.9 (CO N), 170.7 (CH2COON). Preparation of DF. A solution of 2 (3.62 g, 12.3 mmol) in anhydrous DCM (100 mL) was added dropwise to a solution containing dopamine hydrochloride (2.79 g, 14.7 mmol) and NEt3 (1.86 g, 18.4 mmol) in methanol (20 mL). The mixture was stirred at room temperature for 48 h. After evaporation of the solvent, the residue was dissolved in 100 mL of DCM, and the organic phase was washed with HCl (0.5 mol·L−1, 3 × 100 mL), water (2 × 100 mL), and dried over MgSO4. After filtration, the solvent was evaporated and the crude product was purified by column chromatography (SiO2− DCM/MeOH 10:1). The product was obtained as a brown viscous oil in 58% yield. 1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 2.42 (t, 2H, CH2 COO), 2.66 (m, 4H, CH2CONH, CH2CH2NH), 3.44 (q, 2H, CH2NH), 5.04 (s, 2H, CH2O), 5.94 (br, 1H, CH2 NH), 6.34 (dd, 1H, CHC(CH2)), 6.39 (dd, 1H, CHCHC), 6.55 (dd, 1H, ArHe), 6.70 (d, 1H, ArHa), 6.79 (d, 1H, ArHd), 7.40 (dd, 1H, OCHCH). 13 C NMR (75 MHz, CDCl3). 29.4 (CH2CONH), 30.8 (CH2 COO), 34.7 (CH2CH2NH), 41.1 (CH2CH2NH), 58.5 (CH2O), 110.7 (CHC(CH2)), 111.0 (CHCHC), 115.2 (Car(d)H), 115.7 (Car(a)H), 120.7 (Car(e)−H), 130.7 (Car(f)), 143.0 (Car(b)OH), 143.5 (OCHCH), 144.0 (Car(c)OH), 149.0 (CHC(CH2)), 172.3 and 172.9 (COO and CONH). ESI-MS: m/z 356 (M+Na). Preparation of 4. A solution of 2 (0.50 g, 1.7 mmol) in DCM (20 mL) was added dropwise to a solution of aminomethyl ferrocene (0.4 g, 1.9 mmol) and NEt3 (0.26 g, 2.6 mmol) in dry DCM (100 mL). The mixture was next stirred at room temperature overnight. The organic phase was washed with water (3 × 100 mL) and dried over MgSO4. After filtration, the solvent was evaporated and the product was obtained as a yellow solid in quantitative yields. 1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 2.47 (t, 2H, CH2 COO), 2.72 (t, 2H, CH2CONH), 4.15 (m, 11H, CH2NH ; 3773



Article

Scheme 2. Synthesis of Dopamine Based DF/DM Anchors and Furan/Maleimide Functionalized Probes

1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 1.98 (q, 2H, CO CH2CH2CH2CO), 2.26 (t, 2H, CH2COO), 2.39 (t, 2H, CH2CONH), 3.38 (m, 5H, CH2OCH3; CH2OCH3), 3.48−3.81 (m, 208H, CH2CH2O), 3.87 (t, 2H, CH2CH2 OCO), 4.22 (t, 2H, CH2OCO) 4.42 (d, 2H, CH2NH), 6.22 (dd, 1H, CHC(CH2)), 6.31 (dd, 1H, CHCHC), 7.34 (dd, 1H, OCHCH). Preparation of Functionalized Titanium Platforms. Titanium plates (Ø = 1.5 cm) were first treated with an acidic oxidizing solution of concentrated sulfuric acid and hydrogen peroxide H2SO4/H2O2 (1:1) for 2 min to generate the corresponding hydroxylated titanium dioxide surface. Titanium plates were thoroughly rinsed with water, acetone, and ethanol and dried under nitrogen before functionalization. The pretreated titanium surfaces were soaked in a solution containing 1 mM of functionalized anchor DF or DM in a MeOH/ water mixture (1:1) and were shaken overnight at room temperature. After grafting, the samples were thoroughly rinsed with the solvent used for the grafting and dried under nitrogen. Diels−Alder Reactions. A concentrated solution (DCM or DMF) of the suitable furan/maleimide derivative was loaded onto the DF/ DM modified titanium surfaces. The latter were then placed in an oven at 60 °C for 7 days. After reaction, the samples were thoroughly rinsed with the solvent used for the reaction, with dichloromethane and dried with nitrogen flow. Retro Diels−Alder reactions. Retro DA reactions were achieved by placing the DA modified titanium surfaces into refluxing toluene for 24 h. After reaction, samples were rinsed with toluene and dichloromethane and dried with nitrogen flow.

NEt3 (0.23 g, 2.25 mmol) were dissolved in dry DCM (100 mL) and the mixture was refluxed overnight. The product was concentrated under vacuum and precipitated in a large excess of diethyl ether (yield = 96%). 1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 1.95 (q, 2H, CO CH2CH2CH2CO), 2.38 (t, 2H, CH2CH2COOH), 2.43 (t, 2H, CH2COO), 3.38 (m, 5H, CH2OCH3 ; CH2O CH3), 3.54 (m, 2H, CH2CH2OCH3), 3.55−3.75 (m, 208H, CH2CH2O), 3.87 (m, 2H, CH2CH2OCO), 4.24 (m, 2H, CH2OCO). Preparation of 10. A suspension of N-hydroxysuccinimide (NHS, 0.065 g, 0.57 mmol) in dry DCM (20 mL) was added dropwise to a solution (DCM, 100 mL) containing 9 (1 g, 0.47 mmol) and dicyclohexylcarbodiimide (DCC, 0.15 g, 0.71 mmol) at −5 °C under nitrogen. The mixture was allowed to return to room temperature and stirred overnight. The organic layer was washed with water (3 × 100 mL) and dried over MgSO4. After filtration, the solvent was evaporated and the product was precipitated in Et2O affording a white solid in 69% yield. 1 H NMR (300 MHz, CDCl3), δ (ppm from TMS). 2.07 (q, 2H, CO CH2CH2CH2CO), 2.50 (t, 2H, CH2COO), 2.71 (t, 2H, CH2COON), 2.84 (s, 4H, COCH2CH2CO), 3.37 (m, 5H, CH2OCH3; CH2OCH3), 3.54 (m, 2H, CH2CH2 OCH3), 3.57−3.78 (m, 208H, CH2CH2O), 3.87 (t, 2H, CH2CH2OCO), 4.24 (t, 2H, CH2OCO). Preparation of 11. A solution of 10 (0.77 g, 0.35 mmol) in dry DCM (100 mL) was added dropwise to a solution (DCM, 20 mL) containing furfurylamine (0.038 g, 0.39 mmol) and NEt3 (0.05 g, 0.49 mmol), and the mixture was stirred at room temperature overnight. The organic phase was washed with water (3 × 100 mL) and dried over MgSO4. After filtration, the product was concentrated under vacuum and precipitated in a large excess of diethyl ether (yield = 81%). MnSEC = 3100 g·mol−1, Đ = 1.02.

■

RESULTS AND DISCUSSION Synthesis. We have first synthesized two dopamine-based anchors incorporating, on one hand, a catechol fragment for chelating surface Ti atoms15,57 and, on the other hand, a furan

3774



Article

Figure 1. Cyclic voltammogram of the Ti-DF surface recorded in phosphate buffer at pH 7 (left) and N 1s XPS core-level spectrum of the Ti-DM surface (right).

or a maleimide moiety capable of undergoing reversible Diels− Alder cycloaddition reactions. The dopamine-furan (DF) and dopamine-maleimide (DM)43 (see Supporting Information S1) anchors were prepared by reacting dopamine with hydroxysuccinimide esters 2 that can be conveniently elaborated from acid 154 and 3, respectively (Scheme 2). The structure of the DF anchor was confirmed by 1H NMR, 13 C NMR, and ESI mass spectroscopy (see Supporting Information S2). 1H NMR spectra recorded in CDCl3 confirm the presence of the characteristic signals of the catechol unit (6.5 to 7 ppm) and those of the furan fragment between 6.3 and 7.4 ppm. Moreover, the 13C NMR spectrum of DF reveals chemical shifts at 172.3 and 172.9 ppm corresponding to the two carbonyl groups of the DF structure. Immobilization of DF and DM Anchors onto Ti Surfaces. In a first step, the titanium surfaces were treated with an acidic oxidizing solution of concentrated sulfuric acid and hydrogen peroxide (H2SO4/H2O2 1:1) in order to generate the corresponding hydroxylated titanium dioxide surface.8 These pretreated surfaces were then soaked in a MeOH/ water (1:1) solution containing 1 mM of DF or DM and stirred overnight at room temperature to afford, after washing with MeOH and drying, the titanium functionalization platforms TiDF and Ti-DM, respectively (Scheme 1). The electrochemical properties of the catechol fragment were exploited to prove the grafting of the DF and the DM anchors onto the titanium surfaces. Indeed, cyclic voltammograms (CV) of the DF (Figure 1) and DM (see Supporting Information S3 and S4) modified titanium surfaces gave rise to the irreversible two electron oxidation wave (at 1 V vs Ag/AgCl) corresponding to the two-step oxidation of the catechol unit.39,46 A surface coverage of Γ = 9.8 × 1015 molecules per cm2 for Ti-DF and 1.0 × 1016 molecules per cm2 for Ti-DM was calculated according to Γ = Q/nFA, where n is the number of electrons exchanged (n = 2) during measurements, F is the Faraday constant, A is the surface area (Ø= 1.5 cm), and Q is the charge obtained by integration of the anodic peak area. These electrochemical results highlight the high grafting density obtained from the dopamine derivative anchors and are in accordance with previous studies dealing with the preparation of catechol-modified titanium surfaces.46

The grafting of DF and DM anchors onto titanium surfaces was further investigated by XPS measurements. XPS survey spectra of the modified titanium surfaces showed a substantial increase of the C 1s and N 1s signals compared to unmodified titanium surface while a significant decrease of the Ti content was observed (see Supporting Information S5). Moreover, in both cases, the N 1s core level spectra could be deconvoluated into one component at 400.3 eV attributed to the amide functions58 of the DF/DM anchors (Figure 1 right and see Supporting Information S4). Diels−Alder Reactions (DA). As a proof of concept, TiDM and Ti-DF platforms were first subjected to Diels−Alder (DA1) in the presence of ferrocene derivatives 4 and 5, respectively, that act here as electroactive probes (Scheme 3). For this purpose, concentrated solutions of 4 or 5 were placed onto Ti-DM and Ti-DF platforms, respectively, and heated at 60 °C for 7 days. The grafting of ferrocene derivatives onto titanium platforms through DA reaction was revealed by cyclic voltammetry (Figure 2 left and see Supporting Information S6). Indeed, the characteristic reversible wave of the ferrocene probe corresponding to its oxidation into ferricinium was observed at 0.49 V (vs Ag/AgCl). Moreover, the linear evolution in current with scan rate for both oxidation and reduction waves and the constant values of redox potentials vs scan rates further confirmed the covalent attachment of the ferrocene unit onto the titanium surface (Figure 2 right; see Supporting Information S6). By integration of the ferrocene signal (n = 1), a surface coverage of Γ = 2.83 × 1014 molecules per cm2 could be calculated for the Ti-DF functionalized surface thereby indicating a high grafting density of the materials. It is noteworthy that the grafting density is comparable with that obtained through the CuAAC46 click reaction. Nevertheless, we can also notice that the ferrocene surface coverage obtained from the Ti-DM platform (see Supporting Information S6) is two times lower (Γ = 1.46 × 1014 molecules·cm−2) than that estimated after the DA reaction from the Ti-DF surface. This is likely to be due to the partial degradation of the maleimide functionality during the grafting process.50 Finally, the immobilization of the electroactive probe onto the titanium platforms was further evidenced by XPS investigations. Fe 2p XPS core level spectrum of Ti functionalized surfaces displayed 3775



Article

Scheme 3. Reversible Functionalization of the Ti-DF Platform via DA/rDA Reactions

the characteristic peaks of iron 2p from ferrocene at 707.9 eV (2p3/2) and 720.7 eV (2p1/2) (Figure 3 left and see Supporting Information S7) thus corroborating the anchorage of the electroactive probe onto the titanium materials. Retro Diels−Alder Reactions (rDA). The thermoreversible character of the Diels−Alder reaction was then exploited to detach, upon heating, the electroactive ferrocene unit from the Ti-DF and Ti-DM platforms. To this end, ferrocene-grafted plates were subjected to rDA reaction in refluxing toluene for 24h. The removal of the Fc units from Ti-DF and Ti-DM surfaces was first proven by cyclic voltammetry. Indeed, CVs of titanium plates recorded after rDA reactions clearly revealed the disappearance of the characteristic wave of the ferrocene probe (blue curve in Figure 2 and red curve in Supporting Information S6 for Ti-DF and Ti-DM surfaces, respectively). Furthermore, XPS investigations confirm the removal of electroactive moieties from surfaces as any iron component was observed in the Fe 2p core level spectra after rDA

cycloreversions (Figure 3 left and see Supporting Information S7). Second Diels−Alder (DA2). We next hypothesized that the recycled Ti-DF and Ti-DM platforms could be reused to perform a second DA cycloaddition reaction. To confirm this hypothesis, the recycled Ti-DM and Ti-DF platforms were treated with hydrophobic fluorinated oligomers 7 and 8, respectively, under the same conditions as those described for the first DA reaction. End-functionalized oligomers59 7 and 8 were synthesized from amino-zonyl 655 by coupling reaction with derivatives 2 and 3, respectively. The course of the DA2 reaction was investigated by XPS analysis. XPS surveys of functionalized titanium surfaces exhibited a F 1s component at 689.4 eV attributed to presence of the hydrophobic oligomer onto the surface (Figure 3 right and see Supporting Information S7), thereby demonstrating the faculty of Ti-DF and Ti-DM platforms to be refunctionalized through a rDA/ DA2 sequence. Finally, the efficiency of the second DA reaction performed on Ti-DF and Ti-DM platforms was investigated by 3776



Article

Figure 2. (Left) Cyclic voltammograms of the Ti-DF surface after the DA reaction with 5 (red) and the subsequent rDA in refluxing toluene (blue), recorded in acetonitrile with TBAPF6 0.1 M as electrolyte. (Right) Evolution of the peak current as a function of scan rate for anodic (red) and cathodic waves (black) for the ferrocene-grafted Ti-DF surface.

Figure 3. (Left) Fe 2p XPS core level spectra of the Ti-DF surface functionalized with the ferrocene derivative 5 before (red) and after the rDA (blue).(Right) XPS survey spectra of Ferrocene functionalized Ti-DF plate after rDA (blue), and after DA2 reaction with Zonyl oligomer 8 (green).

hydrophobic titanium surfaces were prepared from Ti-DM and Ti-DF platforms by DA reaction with the suitable Zonyl derivatives 7 and 8 (Figure 4 and see Supporting Information S9). The impact of titanium surfaces modification on the wettability was studied thanks to static water contact angle measurements (Figure 4 and see Supporting Information S9 for Ti-DF and Ti-DM platforms, respectively). The unmodified titanium surfaces exhibited a contact angle of 23 (±3°), which is in accordance with the presence of hydrophilic hydroxyl groups on the titanium surface. After modification of the titanium surface with DF and DM anchors, the surfaces became more hydrophobic (68° ± 3 for Ti-DF and 78 ± 4 for Ti-DM). These values increased to 102° (±4) and 94 (±5) for Ti-DF and Ti-DM, respectively, after DA1 reaction with Zonyl derivatives 8 and 7 that was in agreement with the grafting of the hydrophobic oligomers onto surfaces.46 After recycling of the surface through rDA and subsequent DA2 reaction with 11 and the commercially available hydrophilic poly(ethylene oxide) oligomer 12 (Mn ≈ 2000 g·mol−1), the contact angle dramatically decreased to 20° (±3) and 27° (±3) for Ti-DM and Ti-DF, respectively. It is noteworthy that these contact angle values are almost identical to those measured via the

using ferrocene as electrochemical probe (see Supporting Information S8). For this purpose, Ti-DM and Ti-DF Zonylfunctionalized surfaces were treated with 4 and 5, respectively, after sustaining a rDA reaction. Integration of the electrochemical signal of the ferrocene leads to a high surface coverage of Γ = 2.10 × 1014 molecules·cm−2 and 1.14 × 1014 molecules· cm−2 for Ti-DF and Ti-DM surfaces, respectively. However, it should be point out that these values are slightly lower than those determined after DA1. This is likely due to the fact that the rDA reaction is not fully quantitative in this case (see remaining fluorine component in Figure 5 after DA1). Nevertheless, functionalization rates obtained with Ti-DM platforms appears two times lower than those estimated for TiDF surfaces thus further corroborating the potential degradation of the DM anchors during the grafting process.50 Switching the Titanium Surface Wettability. Following these interesting results, we next investigated the opportunity to switch the wettability of the titanium materials. More particularly, as depicted in Figure 4 and Supporting Information S9, we envisioned to transform, through a rDA/ DA2 sequence, hydrophobic titatinum surfaces coated with Zonyl into hydrophilic substrates grafted by PEG chains. The 3777



Article

Figure 4. Changes of water drop profile of the Ti-DF surface after subsequent DA1 reaction with 8, rDA (in boiling toluene) and DA2 cycloaddition with 12.

Figure 5. (Left) XPS survey spectra of the Zonyl-grafted Ti-DF surface (green) and after subsequent rDA (blue) and DA2 reactions with 12 (black). (Right) C 1s core level spectra of the Zonyl-grafted Ti-DF surface (top) and after the rDA/DA2 sequence (down) with 12.

direct functionalization (e.g., DA1) of the Ti-DM (29° ± 3) and Ti-DF (35° ± 3) platforms with the poly(ethylene oxide) oligomers 11 and 12, respectively. Further evidence of the surface modification was obtained by XPS investigations (Figure 5 and see Supporting Information S10). XPS survey spectra of the hydrophobic surfaces were characterized by the F 1s peak at 689.3 eV attributed to Zonyl based derivatives. Deconvolution of the XPS C 1s core-level

spectra clearly shows the presence of the CF2 and CF3 groups of the Zonyl oligomer (at 291.9 and 294.3 eV respectively60), as well as the C−C,C−H (284.9 eV) and the C−O (286.5 eV) components of the PEO block.50,61 After the recycling of the titanium surfaces through rDA and the subsequent DA2, one can observe the disappearance of the fluorine characteristic peak of Zonyl at 689.3 eV and the concomitant increase of the C 1s and O 1s components attributed to the hydrophilic PEO 3778



Article

(6) Moroni, A.; Caja, V.; Egger, E.; Wolf, F. G.; Trinchese, L.; Rollo, G.; Chao, E. Y. Porous Titanium Implants with and without Hydroxyapatite Coating. In Bioceramics and the Human Body; Ravaglioli, A.; Krajewski, A., Eds.; Springer: The Netherlands, 1992; pp 141−147. (7) Bauer, S.; Schmuki, P.; von der Mark, K.; Park, J. Prog. Mater. Sci. 2013, 58 (3), 261−326. (8) Porté-Durrieu, M. C.; Guillemot, F.; Pallu, S.; Labrugère, C.; Brouillaud, B.; Bareille, R.; Amédée, J.; Barthe, N.; Dard, M.; Baquey, C. Biomaterials 2004, 25 (19), 4837−4846. (9) Viornery, C.; Chevolot, Y.; Léonard, D.; Aronsson, B.-O.; Péchy, P.; Mathieu, H. J.; Descouts, P.; Grätzel, M. Langmuir 2002, 18 (7), 2582−2589. (10) Zoulalian, V.; Monge, S.; Zürcher, S.; Textor, M.; Robin, J. J.; Tosatti, S. J. Phys. Chem. B 2006, 110 (51), 25603−25605. (11) Adden, N.; Gamble, L. J.; Castner, D. G.; Hoffmann, A.; Gross, G.; Menzel, H. Langmuir 2006, 22 (19), 8197−8204. (12) Tchoul, M. N.; Fillery, S. P.; Koerner, H.; Drummy, L. F.; Oyerokun, F. T.; Mirau, P. A.; Durstock, M. F.; Vaia, R. A. Chem. Mater. 2010, 22 (5), 1749−1759. (13) White, M. A.; Maliakal, A.; Turro, N. J.; Koberstein, J. Macromol. Rapid Commun. 2008, 29 (18), 1544−1548. (14) Adden, N.; Gamble, L. J.; Castner, D. G.; Hoffmann, A.; Gross, G.; Menzel, H. Biomacromolecules 2006, 7 (9), 2552−2559. (15) Lee, H.; Lee, B. P.; Messersmith, P. B. Nature 2007, 448 (7151), 338−341. (16) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Science 2007, 318 (5849), 426−430. (17) Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W. Langmuir 2012, 28, 6428−6435. (18) Hong, S.; Na, Y. S.; Choi, S.; Song, I. T.; Kim, W. Y.; Lee, H. Adv. Funct. Mater. 2012, 22 (22), 4711−4717. (19) Della Vecchia, N. F.; Avolio, R.; Alfè, M.; Errico, M. E.; Napolitano, A.; d’Ischia, M. Adv. Funct. Mater. 2013, 23, 1331−1340. (20) Luo, R.; Tang, L.; Wang, J.; Zhao, Y.; Tu, Q.; Weng, Y.; Shen, R.; Huang, N. Colloids Surf., B 2013, 106 (0), 66−73. (21) Karkhanechi, H.; Takagi, R.; Matsuyama, H. Desalination 2014, 337 (0), 23−30. (22) Chien, H.-W.; Tsai, W.-B. Acta Biomater. 2012, 8 (10), 3678− 3686. (23) Lee, H.; Rho, J.; Messersmith, P. B. Adv. Mater. 2009, 21 (4), 431−434. (24) Weng, Y.; Song, Q.; Zhou, Y.; Zhang, L.; Wang, J.; Chen, J.; Leng, Y.; Li, S.; Huang, N. Biomaterials 2011, 32, 1253−1263. (25) Chien, C. Y.; Tsai, W. B. ACS Appl. Mater. Interfaces 2013, 5, 6975−6983. (26) Poh, C. K.; Shi, Z.; Lim, T. Y.; Neoh, K. G.; Wang, W. Biomaterials 2010, 31, 1578−1585. (27) Kang, J.; Sakuragi, M.; Shibata, A.; Abe, H.; Kitajima, T.; Tada, S.; Mizutani, M.; Ohmori, H.; Ayame, H.; Son, T. I.; Aigaki, T.; Ito, Y. Mater. Sci. Eng., C 2012, 32, 2552−2561. (28) Zeng, C.; Seino, H.; Ren, J.; Hatanaka, K.; Yoshie, N. Macromolecules 2013, 46 (5), 1794−1802. (29) Engel, T.; Kickelbick, G. Chem. Mater. 2012, 25 (2), 149−157. (30) Dalsin, J. L.; Hu, B.-H.; Lee, B. P.; Messersmith, P. B. J. Am. Chem. Soc. 2003, 125, 4253−4258. (31) Dalsin, J. L.; Hu, B.-H.; Lee, B. P.; Messersmith, P. B. J. Am. Chem. Soc. 2003, 125 (14), 4253−4258. (32) Wach, J.-Y.; Malisova, B.; Bonazzi, S.; Tosatti, S.; Textor, M.; Zürcher, S.; Gademann, K. Chem.Eur. J. 2008, 14 (34), 10579− 10584. (33) Zhang, F.; Liu, S.; Zhang, Y.; Chi, Z.; Xu, J.; Wei, Y. J. Mater. Chem. 2012, 22 (33), 17159−17166. (34) Mizrahi, B.; Khoo, X.; Chiang, H. H.; Sher, K. J.; Feldman, R. G.; Lee, J.-J.; Irusta, S.; Kohane, D. S. Langmuir 2013, 29 (32), 10087− 10094. (35) Gao, C.; Li, G.; Xue, H.; Yang, W.; Zhang, F.; Jiang, S. Biomaterials 2010, 31 (7), 1486−1492.

oligomer. Additionally, the C 1s core level spectrum of the hydrophilic titanium surface exhibits the C 1s components attributed to the grafted poly(ethylene glycol)50,61 while characteristic components from the fluorinated oligomer almost disappear. Thus, both XPS and contact angle measurements clearly demonstrate the ability of the new Ti-DF and Ti-DM platforms to have their wettability switched, on demand through the DA/rDA/DA2 sequence. Moreover, despite the fact that DA reactions on Ti-DM were found to be less efficient than on Ti-DF, similar results in terms of wettability were obtained starting from both titanium platforms.

■

CONCLUSIONS In conclusion, we have designed and prepared two versatile titanium platforms by immobilizing onto TiO2 surfaces catechol based anchors integrating either a maleimide or a furan unit. The Diels−Alder cycloaddition reaction was then exploited to conveniently graft various (macro)molecules onto titanium surfaces and to modify their interface properties. The ability to switch, on command, the titanium wettability through the retro Diels−Alder/Diels−Alder cycloaddition reactions was proven by contact angle measurements, electrochemical and XPS investigations. As wettability plays a key role in many nanotechnological and biomedical applications, our findings may open up new avenues for external stimuli-responsive smart titanium surfaces.

■

ASSOCIATED CONTENT

S Supporting Information *

1

H, 13C NMR and ESI MS spectra of DF/DM anchors. Characterization spectra (CV, XPS, and contact angle measurements) of the whole functionalization process achieved with the two titanium platforms. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected]. *Email: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Region Nord-Pas De Calais for their financial support. The authors thank Arnaud Beaurain from the “Plateforme régionale d’analyse des surfaces” for his assistance in XPS analyses. Francine Cazier from Université du Littoral Côte d’Opale is acknowledged for her assistance and valuable discussions in mass spectroscopy analyses.

■

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: Wenheim, 2005; pp 351−392. (2) Vörös, J.; Wieland, M.; Ruiz-Taylor, L.; Textor, M.; Brunette, D. Characterization of Titanium Surfaces. In Titanium in Medicine; Springer: Berlin/Heidelberg, 2001; pp 87−144. (3) Ong, J. L.; Carnes, D. L.; Bessho, K. Biomaterials 2004, 25 (19), 4601−4606. (4) Yang, B.; Uchida, M.; Kim, H.-M.; Zhang, X.; Kokubo, T. Biomaterials 2004, 25 (6), 1003−1010. (5) Hammond, P. T. Mater. Today 2012, 15 (5), 196−206. 3779



Article

(36) Ye, Q.; Wang, X.; Hu, H.; Wang, D.; Li, S.; Zhou, F. J. Phys. Chem. C 2009, 113 (18), 7677−7683. (37) Fan, X.; Lin, L.; Messersmith, P. B. Compos. Sci. Technol. 2006, 66 (9), 1198−1204. (38) Ye, Q.; Wang, X.; Li, S.; Zhou, F. Macromolecules 2010, 43 (13), 5554−5560. (39) Liu, J.; Yang, W.; Zareie, H. M.; Gooding, J. J.; Davis, T. P. Macromolecules 2009, 42 (8), 2931−2939. (40) Zobrist, C. D.; Sobocinski, J.; Lyskawa, J. l.; Fournier, D.; Miri, V. R.; Traisnel, M.; Jimenez, M.; Woisel, P. Macromolecules 2011, 44 (15), 5883−5892. (41) Spadavecchia, J.; Perumal, R.; Barras, A.; Lyskawa, J.; Woisel, P.; Laure, W.; Pradier, C.-M.; Boukherroub, R.; Szunerits, S. Analyst 2014, 139 (1), 157−164. (42) Kaminska, I.; Qi, W.; Barras, A.; Sobczak, J.; Niedziolka-Jonsson, J.; Woisel, P.; Lyskawa, J.; Laure, W.; Opallo, M.; Li, M.; Boukherroub, R.; Szunerits, S. Chem. Eur. J. 2013, 19 (26), 8673−8678. (43) 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. Nanoscale 2013, 5 (7), 2692−2702. (44) Kaminska, I.; Das, M. R.; Coffinier, Y.; Niedziolka-Jonsson, J.; Sobczak, J.; Woisel, P.; Lyskawa, J.; Opallo, M.; Boukherroub, R.; Szunerits, S. ACS Appl. Mater. Interfaces 2012, 4 (2), 1016−1020. (45) Barras, A.; Lyskawa, J.; Szunerits, S.; Woisel, P.; Boukherroub, R. Langmuir 2011, 27 (20), 12451−12457. (46) Watson, M. A.; Lyskawa, J. L.; Zobrist, C. D.; Fournier, D.; Jimenez, M.; Traisnel, M.; Gengembre, L. O.; Woisel, P. Langmuir 2010, 26 (20), 15920−15924. (47) Xu, L. Q.; Jiang, H.; Neoh, K.-G.; Kang, E.-T.; Fu, G. D. Polym. Chem. 2012, 3 (4), 920−927. (48) Lai, Y.; Lin, L.; Pan, F.; Huang, J.; Song, R.; Huang, Y.; Lin, C.; Fuchs, H.; Chi, L. Small 2013, 9 (17), 2945−2953. (49) Zydziak, N.; Yameen, B.; Barner-Kowollik, C. Polym. Chem. 2013, 4 (15), 4072−4086. (50) Preuss, C. M.; Goldmann, A. S.; Trouillet, V.; Walther, A.; Barner-Kowollik, C. Macromol. Rapid Commun. 2013, 34 (8), 640− 644. (51) Urakami, H.; Yilmaz, A. G.; Osiceanu, P.; Yagci, Y.; Vilela, F.; Titirici, M.-M. Macromol. Rapid Commun. 2013, 34, 1080−1084. (52) Tischer, T.; Claus, T. K.; Bruns, M.; Trouillet, V.; Linkert, K.; Rodriguez-Emmenegger, C.; Goldmann, A. S.; Perrier, S.; Börner, H. G.; Barner-Kowollik, C. Biomacromolecules 2013, 14 (12), 4340−4350. (53) Blinco, J. P.; Trouillet, V.; Bruns, M.; Gerstel, P.; Gliemann, H.; Barner-Kowollik, C. Adv. Mater. 2011, 23 (38), 4435−4439. (54) Yamashiro, M.; Inoue, K.; Iji, M. Polym. J. 2008, 40 (7), 657− 662. (55) Laquièvre, A.; Allaway, N. S.; Lyskawa, J.; Woisel, P.; Lefebvre, J.-M.; Fournier, D. Macromol. Rapid Commun. 2012, 33 (9), 848−855. (56) Fournier, D.; Du Prez, F. Macromolecules 2008, 41 (13), 4622− 4630. (57) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Science 2007, 318, 426−430. (58) Graf, N.; Yegen, E.; Gross, T.; Lippitz, A.; Weigel, W.; Krakert, S.; Terfort, A.; Unger, W. E. S. Surf. Sci. 2009, 603 (18), 2849−2860. (59) Perrier, S.; Jackson, S. G.; Haddleton, D. M.; Améduri, B.; Boutevin, B. Macromolecules 2003, 36 (24), 9042−9049. (60) Järn, M.; Heikkilä, M.; Lindén, M. Langmuir 2008, 24 (19), 10625−10628. (61) Fan, X.; Lin, L.; Dalsin, J. L.; Messersmith, P. B. J. Am. Chem. Soc. 2005, 127 (45), 15843−15847.

3780


Switching the Wettability of Titanium Surfaces through DielsâAlder

Recommend Documents

Switching the Wettability of Titanium Surfaces through DielsâAlder