Engineering Nitroxide Functional Surfaces Using ... - ACS Publications

∥Institute for Applied Materials (IAM-ESS) and Karlsruhe Nano Micro Facility (KNMF),. Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz...
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Engineering Nitroxide Functional Surfaces Using Bioinspired Adhesion Hendrik Woehlk,†,‡ Jan Steinkoenig,‡,§ Christiane Lang,† Lukas Michalek,† Vanessa Trouillet,∥ Peter Krolla,⊥ Anja S. Goldmann,†,‡,§ Leonie Barner,†,#,§ James P. Blinco,†,‡ Christopher Barner-Kowollik,*,†,‡,§ and Kathryn E. Fairfull-Smith*,† †

School of Chemistry, Physics and Mechanical Engineering and #Institute for Future Environments, Queensland University of Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia ‡ Macromolecular Architectures, Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76128 Karlsruhe, Germany § Institute for Biological Interfaces (IBG), ∥Institute for Applied Materials (IAM-ESS), Karlsruhe Nano Micro Facility (KNMF), and ⊥ Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany S Supporting Information *

ABSTRACT: We pioneer a versatile surface modification strategy based on mussel-inspired oxidative catecholamine polymerization for the design of nitroxide-containing thin polymer films. A 3,4-dihydroxy-L-phenylalanine (L-DOPA) monomer equipped with a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-derived oxidation-labile hydroxylamine functional group is employed as a universal coating agent to generate polymer scaffolds with persistent radical character. Various types of materials including silicon, titanium, ceramic alumina, and inert poly(tetrafluoroethylene) (PTFE) were successfully coated with poly(DOPA-TEMPO) thin films in a one-step dip-coating procedure under aerobic, slightly alkaline (pH 8.5) conditions. Steadily growing polymer films (∼1.1 nm h−1) were monitored by ellipsometry, and their thicknesses were critically compared with those obtained from atomic force microscopic cross-sectional profiles. The heterogeneous composition of surface-adherent nitroxide scaffolds examined by X-ray photoelectron spectroscopy was correlated to that examined by in-solution polymer analysis via high-resolution electrospray ionization mass spectrometry, revealing oligomeric structures with up to six repeating units, mainly composed of covalently linked dihydroxyindole along the polymer backbone. Critically, the reversible redox-active character of the nitroxide-containing polymer scaffolds was investigated by cyclic voltammetric measurements, revealing a convenient and facile access route to electrochemically active nitroxide polymer coatings with potential application in electronic devices such as organic radical batteries.

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INTRODUCTION The surface functionalization of materials is of pivotal interest as it enables control over the chemical and biological properties at the interface of a substrate. Therefore, precision surface engineering has become a critical research objective in materials sciences, electronics, and applied chemistry with numerous approaches reported in the literature.1−4 However, many of these modification methods entail limitations in terms of their substrate specificity (e.g., thiol-anchoring exclusively for noble metals1 or silane-chemistry for hydroxylated surfaces2) or restrictions in the size and shape of the substrate (from coated nanomaterials to large-scale industrial processes). In addition, elaborate synthetic procedures often accompanied by the need for special equipment (e.g., in lithography or chemical vapor deposition)3,4 are major limiting factors in terms of a widespread, versatile, and facile surface modification route. A © XXXX American Chemical Society

ubiquitous, alternative coating technique that takes inspiration from the unique adhesive system of marine mussels overcomes many of the above-listed coating restrictions. Secreted gluelike mussel-foot proteins (mfps) at the end of mussel’s byssal threads allow these animals to adhere to virtually all types of materials and to survive in rough marine environments. At the molecular level, 3,4-dihydroxyphenylalanine (DOPA)- and lysine-enriched mfps exhibit extraordinarily strong adhesive properties based on catechol anchoring with synergistic effects of the lysine residues.5−7 Various synthetic approaches for mussel-inspired adhesive coatings have been reported8 including mfp-mimicking Received: October 30, 2017 Revised: February 12, 2018 Published: February 14, 2018 A

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Langmuir dendrimers,9 catechol-grafted polymers,10 and the use of plantderived (poly)phenolic compounds (e.g., tannic acid) as lowcost coating agents11,12 and for the generation of metal− phenolic hybrid networks.13 The most employed monomer precursor for universal polymer coatings is dopamine (3,4dihydroxyphenethylamine), which polymerizes under aerobic, slightly alkaline conditions, resulting in a multifunctional polymer network with mussel-like strong adhesive properties.14,15 The poly(dopamine) thin films can be further functionalized by a host of post-polymerization modification routes16,17 for precise and substrate-independent surface conjugation, generating, for example, bioresponsive or nonfouling interfaces.18,19 Alternatively, a sophisticated, one-step approach in designing tailor-made surfaces is based on using prefunctionalized dopamine derivatives as coating agents.20−22 Employing this approach, the manufacturing of material surfaces equipped with (photo-)degradable23,24 or self-cleaning25 properties has been presented. Moreover, orthogonal ligation anchors have been introduced, e.g., for subsequent light-triggered surface patterning with polymers.26 A polymer class of emerging interest is nitroxide-containing macromolecules possessing side chain unpaired electrons delocalized over the N−O bond.27 Nitroxide-containing polymers exhibit wide applicability in organic radical batteries for high-demanding energy storage materials,28,29 as recyclable catalysts in organic oxidative reactions,30 for the design of complex polymer architectures,31 and as self-reporting systems based on nitroxide-induced fluorescence quenching.32,33 Recently, the wide-ranging potential of nitroxides34−36 has been further expanded, revealing their antibacterial37 and antibiofilm activity, mediating bacterial biofilm dispersal and preventing initial biofilm formation.38,39 Herein, we pioneer the design of a nitroxide-containing polymer scaffold equipped with mussel-like adhesive properties for the facile coating and substrate-independent decoration of various materials with nitroxides. The polymerization of the nitroxide-functionalized catecholamine is carefully investigated in terms of its polymer-adhesive performance, the structural composition of the multifunctional polymer scaffold (on the surface and in solution), and the persistent radical character with electrochemically reversible redox properties. The synthesis of nitroxide polymer thin films based on a simple-tooperate adhesive system critically widens the applicational scope of nitroxide-driven technologies from new materials for electronic devices including organic radical batteries to biomedically relevant antibacterial countermeasures and for biofilm remediation.

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Poly(DOPA-TEMPO) (4) Surface Deposition. During polymerization of monomer 3 (as described above), various substrates were immersed typically for 1−12 h (monitoring the deposition kinetics) as well as for 24 h. After coating, the substrates were thoroughly rinsed with water and dried under a nitrogen stream. All substrates were cleaned by ultrasonication in methanol (15 min) and water (15 min) and dried under nitrogen prior to coating. Mechanistic DOPA-TEMPO (3) Polymerization Study by Electron Paramagnetic Resonance (EPR) and UV−Vis Spectroscopy. Monomer 3 (8.49 mg, 0.02 mmol) was dissolved in 10 mM Tris−HCl buffer (pH 8.5) (20 mL). The 1 mM solution was vigorously stirred for continuous supply of oxygen from the air interface. The reaction was performed in a closed vial to avoid solvent evaporation. During oxidative polymerization, aliquots (50 μL) were taken for EPR spectroscopic analysis. UV−vis spectroscopic aliquots (125 μL) were diluted to 0.05 mM prior to the measurements. EPR spectroscopic radical quantification was referenced to the hydroxylamine oxidation of 4-hydroxy-TEMPO previously converted to the hydroxylamine hydrochloride species. All experiments were run in duplicate. Error bars representing the 1 standard deviation of uncertainty are smaller than the symbols and are not displayed in the figures. NMR Spectroscopy. 1H NMR (600 MHz) and 13C NMR (151 MHz) spectroscopies were performed on a Bruker Avance III HD spectrometer. All samples were recorded in MeOD or dimethylformamide (DMF)-d7, respectively. Chemical shifts are expressed in parts per million (ppm), and coupling constants (J values) are reported in hertz. The δ scale is referenced to the characteristic solvent signals of MeOD at 3.31 ppm or DMF-d7 at 2.75 ppm, respectively. High-Resolution Electrospray Ionization Mass Spectrometry (HR-ESI MS). Mass spectrometric characterization was performed using a LTQ Orbitrap XL Q Exactive Plus mass spectrometer (Thermo Fisher Scientific, San Jose, CA) equipped with an HESI II probe. The instrument was calibrated in the range of m/z 74−1822 using standard calibration solutions (Thermo Scientific) and between m/z 1000 and 6000 with ammonium hexafluorophosphate. The capillary temperature was set to 320 °C, and the S-lens radio frequency level was set to 68.0. All samples were filtered prior to injection. Small molecule characterization was performed in undoped MeOH. The polymer samples were dissolved in tetrahydrofuran (THF)/MeOH 3:2 (v/v) doped with 100 μM NaI with a concentration of 0.05 mg mL−1. For polymer characterization, the dimensionless gas flow rates were set to 10 (sheath gas), 0 (sweep gas), and 0 (aux gas). An insource collision-induced dissociation (CID) energy of 70 eV for ESICID MS and a higher-energy collisional dissociation energy of 35 eV for MS/MS, respectively, were employed. The spectra were recorded with a constant spray voltage in the range of 3.1−3.4 kV. The recorded MS spectra were evaluated using Xcalibur software. Electron Paramagnetic Resonance (EPR) Spectroscopy. EPR spectroscopy was performed on a Magnettech MiniScope MS400 spectrometer at room temperature. Small molecule characterization was performed in dichloromethane. The parameters for the mechanistic polymerization study with the generation of the nitroxide free radical are described above. UV−Visible (UV−Vis) Spectroscopy. UV−vis spectra were recorded on a Shimadzu UV-2700 UV−vis spectrophotometer in Tris−HCl (pH 8.5) buffer at room temperature in the range of 240− 600 nm. A quartz Suprasil cuvette with a 10 mm light path was used. Spectra were baseline-corrected with respect to pure Tris−HCl buffer. Static Water Contact Angle Measurements. Determination of the static water contact angles was performed on a Drop Shape Analyzer, DSA100 (Kruess, Hamburg, Germany). A 5 μL water droplet was placed on the surface according to the sessile-drop method, and the reported values indicate the averages of three measurements. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed using a K-Alpha+ XPS spectrometer (Thermo Fisher Scientific, East Grinstead, U.K.) equipped with a microfocused, monochromated Al Kα X-ray source (400 μm spot size). The kinetic energy of the electrons was measured by a 180° hemispherical energy

MATERIALS AND METHODS

All employed materials and experimental procedures for the synthesis of monomer 3 as well as NMR spectroscopic data are reported in the Supporting Information. Polymerization of DOPA-2,2,6,6-Tetramethylpiperidine-1oxyl (TEMPO). Compound 3 (529 mg, 1.25 mmol) was dissolved in 10 mM Tris buffer (151 mg, 1.25 mmol in 125 mL of H2O) previously adjusted to pH 8.5 using 1 M HCl. The 10 mM solution of 3 was vigorously stirred in an open glass vial and exposed to air. After 72 h, the precipitate was isolated by centrifugation and decantation. The solid was washed with water (3 × 30 mL), followed by centrifugation and decantation. The product, poly(DOPA-TEMPO) (4), was lyophilized, yielding a yellow-brown solid (78 mg), which was analyzed by high-resolution electrospray ionization mass spectrometry (HR-ESI MS). B

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Scheme 1. (a) Synthetic Route for the Preparation of the DOPA-Based Monomer Carrying a TEMPO Moiety (3); 4-AminoTEMPO was Attached to Catechol- and Amino-Protected L-DOPA (1) in an EDC-Mediated Amide Coupling Reaction; Subsequent Cleavage of the tert-Butyldimethylsilyl and Boc Protecting Groups Using an Aqueous/Methanolic Solution of Hydrogen Chloride Yielded Monomer 3 as the Corresponding Hydroxylamine Dihydrochloride; (b) Oxidative Polymerization of 3 and Surface Immobilization onto Various Substrates were Performed in a Tris−HCl Buffer (pH 8.5) under Aerobic Conditionsa

a

Various substrates were coated typically for 24 h. Cyclic Voltammetry (CV). Electrochemical analysis was conducted on a BioLogic VSP potentiostat using a standard threeelectrode setup with a glassy carbon or platinum working electrode (3 mm diameter, BASi, West Lafayette) together with an Ag/AgCl reference electrode and a platinum counter electrode. The working electrodes were immersed in a 10 mM DOPA-TEMPO (3) solution (0.1 mmol, 42 mg) in 10 mL of Tris−HCl buffer (pH 8.5) and coated for 1, 4, and 7 days with further monomer (0.05 mmol) addition after each CV measurement. The coated electrodes were thoroughly rinsed with water before electrochemical analysis. CV measurements of coated working electrodes were performed in a 0.1 M aqueous NaCl solution (previously purged with N2 for 15 min) at room temperature. Five cycles were recorded in the range of 0−1.0 V employing a scan rate of 100 mV s−1.

analyzer operated in the constant analyzer energy mode at 50 eV pass energy for elemental spectra. The K-Alpha+ charge compensation system was employed during analysis using electrons of 8 eV energy and low-energy argon ions to prevent any localized charge build-up. The XPS spectra were acquired and evaluated using Thermo Avantage software.40 The high-resolution spectra were fitted with one or more Voigt profiles (binding energy (BE) uncertainty: ±0.2 eV). The Scofield sensitivity factors were applied for quantification.41 All spectra were referenced to the C 1s (C−C, C−H) peak at BE 285.0 eV and controlled by means of well-known photoelectron peaks of metallic elements. Atomic Force Microscopy (AFM). AFM imaging of coated silicon substrates was performed on a Multimode 2 Atomic Force Microscope (Digital Instruments, Santa Barbara). A HQ:NSC18/Al BS (MikroMasch, Sofia, Bulgaria) AFM cantilever (typical resonant frequency of 75 kHz and force constant of 2.8 N m−1) was employed in alternating current mode for the determination of the polymer film height profiles. The polymer film was beforehand partially removed, and the underlying Si substrate was set as a zero line. The surface roughness was determined using an NT-MDT Solver Pro atomic force microscope. A ContGD-G (BudgetSensors, Sofia, Bulgaria) AFM cantilever (typical resonant frequency of 13 kHz and force constant of 0.2 N m−1) was employed in contact mode (set point of 2 V deflection). The root-mean-square (RMS) surface roughness was determined over a 20 × 20 μm2 area, and the error bars indicate the standard error of five measurements. Spectroscopic Ellipsometry (SE). Determination of the polymer film thicknesses deposited on silicon substrates was performed on a J. A. Woollam M-2000UI Ellipsometer in the wavelength range of 245− 1690 nm and at angles of incidences of 65, 70, and 75°. Two independent kinetic studies were conducted, both ran in duplicate. The polymer film thickness was determined at least at two different spots on the surface. The data was fitted and evaluated using CompleteEASE software. A Cauchy model was applied for the polymer surface deposition (with A = 1.45, B = 0.01, surface roughness excluded) on Si/SiOx substrates. The SiOx interlayer was determined prior to polymerization. Error bars represent 1 standard deviation of uncertainty.

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RESULTS AND DISCUSSION Our design of versatile and substrate-independent nitroxidecontaining adhesive polymer scaffolds for surface functionalization is based on a mussel-derived adhesive system using polymerizable catecholamines. Initially, the synthesis of a catecholamine monomer carrying a 2,2,6,6-tetramethylpiperidin-1-ol (reduced form of TEMPO) side chain functional group is introduced. The oxidative polymerization of DOPATEMPO (3) concomitant with polymer surface deposition and oxidation to the free nitroxide radical was performed under mild aqueous conditions in the presence of oxygen. Various materials with different physicochemical properties were covered with a thin nitroxide-containing polymer film in a one-step dip-coating procedure. The chemical surface composition was carefully examined by X-ray photoelectron spectroscopy (XPS), and the polymer deposition kinetics was studied by spectroscopic ellipsometry (SE) and critically compared with height profiles obtained by atomic force microscopy (AFM). Moreover, the multifunctional polymer structures were analyzed via an in-depth high-resolution electrospray ionization C

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vertically immersed in the coating solution to reduce polymer particle sedimentation. The standard coating time was 24 h, whereas the polymer deposition kinetic studies were performed in the initial phase of polymerization (1−12 h). Furthermore, different electrode materials, particularly glassy carbon and platinum working electrodes, were coated for subsequent electrochemical analysis of the adherent nitroxide polymers. The first indication of a successful poly(DOPA-TEMPO) thin film (4) surface coverage was confirmed by static water contact angle measurements (Figure S6) with similar contact angles after coating regardless of the underlying material properties. Most impressively, bare PTFE (118.6°) showed a significantly increased wettability when coated with polymer 4 (56.9°). XPS Characterization of Versatile Poly(DOPA-TEMPO) Coatings. The chemical composition of deposited thin poly(DOPA-TEMPO) (4) films and the adhesive performances depending on the substrate’s properties were investigated by XPS. Successful polymer film coverage was observed for all employed materials (silicon, titanium, alumina, and PTFE), indicated by a decreasing intensity of the substrate-specific signals (Si 2p, Ti 2p, Al 2p, and F 1s) after 12 and 24 h of coating time, respectively (Figure 1), and consequently

mass spectrometric (HR-ESI MS) study. The oxidation-driven polymerization pathway was carefully assessed with regard to the nitroxide radical stability in a tandem electron paramagnetic resonance (EPR) and UV−visible (UV−vis) spectroscopic experiment. Finally, the reversible redox-active character of the nitroxide coatings was investigated by cyclic voltammetry (CV). Preparation of the DOPA-TEMPO Coating Agent. 3,4Dihydroxy-L-phenylalanine (L-DOPA) was selected as a precursor molecule carrying an additional carboxylic acid moiety for targeted nitroxide attachment via an amide linkage with 4-amino-TEMPO (Scheme 1a). Following an earlier procedure,42 4-amino-TEMPO was ligated to catechol- and amino-protected L -DOPA (1) in an 1-ethyl-3-(3dimethylaminopropyl)carbodiimide (EDC)-mediated amide coupling reaction. Subsequent deprotection of intermediate 2 was performed under acidic conditions with an aqueous/ methanolic solution of hydrogen chloride. Furthermore, the acidic conditions induced a nitroxyl radical disproportionation, generating the protonated hydroxylamine species as well as the N-oxoammonium cation. The latter species was rapidly reduced in the presence of methanol,43 and consequently, monomer DOPA-TEMPO (3) was exclusively isolated as the corresponding hydroxylamine dihydrochloride salt with excellent water solubility properties for subsequent aqueous polymerization and surface coatings. The hydroxylamine hydrochloride form of the TEMPO moiety is prone to spontaneous oxidation to the nitroxide free radical in aqueous aerobic systems44 (refer to Section 2.6) and thus no procedure to oxidize the hydroxylamine to the corresponding nitroxide was conducted with the adjacent oxidation-sensitive catechol moiety. The conversion to the TEMPO hydroxylamine hydrochloride moiety of monomer 3 facilitates its structural identification by 1H and 13C NMR spectroscopy (Figures S1−S3) as the presence of persistent radicals typically impedes NMR spectroscopic characterization owing to paramagnetic signal broadening (e.g., as evident in the 1 H and 13C NMR spectra of intermediate 2 in Figures S4 and S5). The appearance of characteristic 1H NMR signals at 12.05 and 12.85 ppm, respectively, clearly underpins the formation of the hydroxyammonium salt of compound 3 (Figure S2).43 The synthetic pathway toward monomer 3 was further supported by HR-MS with m/z(exp) 701.4231 [M + Na]+ for intermediate 2 (m/z(theo) 701.4226) and m/z(exp) 352.2231 [M − HCl − Cl]+ for monomer 3 (m/z(theo) 352.2232). Oxidative Polymerization and Surface Deposition. The polymerization of tailor-made catecholamine 3 carrying a TEMPO functional group, present as a hydroxylamine, was performed under mild conditions at ambient temperature in the presence of oxygen (Scheme 1b). Upon addition of monomer 3 to an aqueous Tris−HCl buffer system adjusted to pH 8.5, the polymerization progress was visually followed as a rapid color change from yellow to dark orange within the first hours. As the reaction progressed, an increasing turbidity associated with polymer particle formation was observed when left overnight under vigorous stirring and exposed to air. To exemplify the universal approach of this substrate-independent nitroxide surface functionalization technique, various materials were selected as coating objects, namely, titanium, alumina, and poly(tetrafluoroethylene) (PTFE) as representatives for metals, ceramics, and inert polymer composites, respectively. Silicon wafers were selected as model substrates for in-depth surface characterization. The polymer surface deposition was carried out under vigorous stirring and exposed to air to ensure a continuous supply of oxygen. The above-noted substrates were

Figure 1. XPS quantification of residual substrate-characteristic peaks (atom %) of silicon (Si 2p), titanium (Ti 2p), alumina (Al 2p), and PTFE (F 1s) substrates after coating with poly(DOPA-TEMPO) (4) for 12 and 24 h, respectively.

increasing C 1s, N 1s, and O 1s peaks related to poly(DOPA-TEMPO) (4). XPS quantification of the residual substrate-characteristic peaks leads to concentrations of 0.8 atom % titanium, 2.6 atom % aluminum, and 3.5 atom % fluorine after 12 h of polymerization, which suggests an evenly growing polymer film irrespective of the underlying material’s properties. The residual substrate-specific peaks could be further reduced to less than 1 atom % for the above-mentioned materials when coated for 24 h, indicating polymer film thicknesses close to 10 nm. However, XPS analysis of coated Si substrates showed a slower polymer film growth with 16.2 atom % silicon after 12 h and 3.2 atom % after 24 h of coating time. This observation is most likely attributed to the smooth surface topography of the employed Si wafers with an RMS surface roughness of 0.5 nm over a 20 × 20 μm2 area determined by AFM. A comparison of the smooth silicon surface with the surface topographies of the other employed bare substrates (e.g., 31.9 nm surface roughness for titanium, refer to Figure S7a) suggests that the initial polymer attachment and polymer grip of compound 4 are critically influenced by the topographical properties of the underlying substrate. This assumption is in further agreement with the good polymer surface coverage on the chemically inert yet rough PTFE substrate, which appears to be in the range of titanium and alumina as determined by XPS. D

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Figure 2. XPS characterization of poly(DOPA-TEMPO) (4) deposited on Si for 24 h. (a) C 1s high-resolution XPS spectrum with polymer-derived C−C and C−H (at 285.0 eV), C−O and C−N (286.3 eV), and OC−N (288.3 eV) peaks. (b) Corresponding N 1s high-resolution XPS spectrum deconvoluted into four components. The dominant peak (at 400.1 eV) is attributed to the amide linkage and various amine species derived from cyclized and open-chain DOPA moieties along the polymer backbone. The TEMPO-derived nitrogen species appear at higher binding energies, including N−O (401.6 eV) and higher oxidized species (403.1 and 405.9 eV).

Figure 3. Comparative kinetic study of poly(DOPA-TEMPO) (4) film formation on silicon substrates. (a) Polymer film thicknesses determined by ellipsometry between 1 and 12 h of coating time. (b) Selected AFM height profile of surface 4.1 after 12 h of polymerization with the averaged polymer film thickness of 14.3 nm (excluding peaks ≥18 nm derived from polymer particle sedimentation). (c) Corresponding AFM image of surface 4.1.

In-depth C 1s and N 1s XPS characterizations revealed identical polymer surface compositions irrespective of the underlying material properties. The high-resolution C 1s XPS spectrum exemplarily shown for coated silicon (24 h) in Figure 2a can be deconvoluted into three main components corresponding to C−C and C−H (at 285.0 eV, 35.6 atom %), C−O and C−N (at 286.3 eV, 26.1 atom %), and OC−N (at 288.3 eV, 5.8 atom %).45 Other functional groups such as oquinones or pyrrolecarboxylic acid moieties, which have been proposed to appear in unfunctionalized poly(dopamine) structures, are not evidenced in the C 1s spectrum.46,47

The corresponding high-resolution N 1s XPS spectrum depicted in Figure 2b can be deconvoluted into four components with a dominant peak at 400.1 eV (6.8 atom %) and three minor nitrogen species at higher binding energies. The main component corresponds to N-containing diverse polymer backbone structures (e.g., uncyclized catecholamines and cyclized (indole) units) as well as to the amide functional group connecting the TEMPO side chains. The nitroxide free radical was identified to appear at 401.6 eV (1.6 atom %), which was supported by XPS reference measurements of a TEMPO-containing polymer analogue to poly(DOPATEMPO) (4) with a characteristic main peak at 401.5 eV E

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Figure 4. (a) Expanded region of ESI(−)-CID (70 eV) mass spectrum of poly(DOPA-TEMPO) (4) in THF/MeOH 3:2 (v/v) doped with 100 μM NaI depicted from m/z 950−1480. The selected region exemplarily shows the trimer and tetramer profiles of polymer 4 with single-charged molecular ions or deprotonated hydroxylamine ions (○), monohydrochloride adducts (□), and dihydrochloride adducts (Δ). (b) Proposed repeating unit of ○ present as a homotrimer at m/z(exp) 1034.4985 (m/z(theo) 1034.4982) and homotetramer at m/z(exp) 1378.6597 (m/z(theo) 1378.6592) composed of covalently linked dihydroxyindole units along the polymer backbone and free radical TEMPO moieties in the side chains. For further detailed MS characterization, refer to the Supporting Information (Figures S9−S11).

attributed to N−O (refer to Figure S8). The two other Ncontaining components at higher binding energies (403.1 eV, 0.7 atom % and 405.9 eV, 0.2 atom %) are presumably higher oxidized nitrogen species. The latter peak has been referred to the overoxidized N-oxoammonium cation.48 Deposition Kinetics of Poly(DOPA-TEMPO) Surface Coverage. The engineering of catecholic polymer films including the coating speed is known to be dependent on various preparation conditions including the pH, type of oxidant, and monomer concentration.49,50 In the present study, the surface deposition kinetics of poly(DOPA-TEMPO) (4) was monitored within the first 12 h of polymerization at pH 8.5 in the presence of air and a 10 mM concentration of monomer 3. A continuously growing polymer film on silicon substrates was observed in the initial stages of polymerization with an estimated growth rate close to 1.1 nm h−1 determined by spectroscopic ellipsometry (SE) as an average after 12 h of coating time (Figure 3a). The growth rate was slower than the literature-reported growth rates for nonfunctionalized dopamine polymerization (∼3.6 nm h−1)51 and other poly(phenolic) systems12 employing similar coating conditions, most likely resulting from steric and electronic effects of the amide-linked, additional TEMPO group adjacent to the polymerizable catecholamine functionality impeding rapid polymer formation. Furthermore, the number of reactive sites for intermolecular multilateral monomer coupling reactions was reduced as one cross-linking point is already occupied by the attached TEMPO functionality. The presence of the persistent nitroxide radical may also interfere with the oxidative catecholamine polymerization.52 AFM height profiles were recorded to support complete surface coverage of the poly(DOPA-TEMPO) (4) films and to critically evaluate the coating thicknesses obtained by SE. A selected AFM height profile (surface 4.1) of a 12 h coated Si substrate is depicted in Figure 3b. The averaged poly(DOPATEMPO) film thickness of 14.3 nm (excluding peaks ≥18 nm derived from polymer particle sedimentation) is in excellent agreement with the corresponding film thickness of 14.7 nm

obtained by ellipsometry (value shown in Figure 3a). The same observation is valid for surface 4.2 (AFM image and height profile are shown in Figure S7b,c) with observed polymer film thicknesses of 11.6 nm (SE) and 10.1 nm (AFM), respectively, after 12 h of polymerization. In addition, high surface roughness and adherent polymer particles were visualized in Figure 3c, which are inherent for this class of bioinspired polymer coating.8,12 A comparison of the surface roughness of bare silicon (0.5 nm) and polymer-coated analogues showed a significant increase of the surface roughness (28.9 nm after coating). The same trend was also apparent from other investigated materials such as titanium or alumina. However, due to the inherent surface roughness of the starting materials, the impact of the polymer coating on the surface roughness was rather small (Figure S7a). AFM was unsuitable to determine the roughness of PTFE as a substrate (see Supporting Information). HR-ESI Mass Spectrometric Elucidation of Poly(DOPATEMPO) Structures. The structural elucidation of polymerized catecholamine derivatives, e.g., poly(DOPA-TEMPO) (4), remains challenging because of the overall structural complexity of these dynamic and disperse polymer systems including various mechanistic polymerization scenarios proposed in the literature46,53−55 and their limited characterization access caused by their inherent insolubility. In addition, the persistent radical character of polymer 4 impedes structural characterization by NMR spectroscopy because of radical-induced paramagnetic signal broadening. Thus, high-resolution mass spectrometry was exploited as a powerful characterization technique for further structural insights into the heterogeneous composition of poly(DOPA-TEMPO) (4) with radical character.42,56 The structural examination of poly(DOPA-TEMPO) (4) was carried out using an Orbitrap spectrometer with an electrospray ion source in combination with an in-source collision-induced dissociation (CID) fragmentation technique. Employing the negative ion mode and a collision energy of 70 F

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Figure 5. (a) Selected UV−vis spectra recorded during oxidative polymerization of DOPA-TEMPO (3) (1 mM in Tris−HCl buffer (pH 8.5), diluted by factor 20 for UV−vis spectroscopic analysis). The initial spectrum (0 day) was obtained shortly after dissolving monomer 3 in Tris−HCl buffer (pH 8.5). (b) Time-dependent absorbances at λmax = 282 → 269 nm (●) indicating the polymerization progress and at λ = 333 nm (■) implying o-quinoic reactive intermediates. (c) Selected first-derivative EPR spectra monitoring the nitroxide radical formation during aerobic polymerization of DOPA-TEMPO (3) initially present as a hydroxylamine hydrochloride species. (d) Corresponding time-dependent doubleintegrated EPR radical quantification (■) in comparison to the nitroxide radical formation of the hydroxylamine hydrochloride salt of HO−TEMPO serving as a reference (□).

S10 and Table S1, indicating the presence of incorporated oquinoic structures along the polymer backbone. ESI tandem mass spectrometry (MS/MS) additionally supports the existence of a polymer scaffold (Figure S11 and Table S2). A distinct fragmentation pattern of a former pentamer species into mainly trimer and dimer structures was observed, accompanied by multiple losses of nitroxyl-adjacent methyl radicals most likely forming the nitrone species during (−)ESI.57 However, a precise structural assessment is challenging because of the heterogeneous character of the polymer backbone repeating units in combination with varying redox states and appearances of the TEMPO moiety, leading to innumerable isomeric (and thus isobaric) structures of poly(DOPA-TEMPO) (4). An in-depth HR-ESI MS structural elucidation of a poly(DOPA-TEMPO) analogue system with 1methoxy-2,2,6,6-tetramethylpiperidine functional groups has been previously reported by our team, revealing detailed insights into tailor-made functional poly(catecholamine) structures with the incorporation of various identifiable building blocks.42

eV, oligomers with up to six repeating units were identified as single-charged species (overview spectra are depicted in Figure S9). The most abundant peaks within the oligomer profiles refer to uncoordinated ionized species (○), monohydrochloride adducts (□), and dihydrochloride conjugates (Δ). An expanded ESI MS region depicted in Figure 4a exemplifies the trimer and tetramer profiles of polymer 4. Structural elucidation reveals that the oligomer structures were predominantly composed of cyclized, indolelike structures along the polymer backbone. The peak at m/z(exp) 1034.4985 can be assigned to a homotrimer with consistently incorporated and covalently linked dihydroxyindole (DHI) units forming the polymer scaffold and with free radical TEMPO moieties in the side chains (m/z(theo) 1034.4982). An analogous observation holds for the corresponding homotetramer with four covalently linked DHI−TEMPO units at m/z(exp) 1378.6597 (m/ z(theo) 1378.6592), which further strengthens the identification of the DHI unit as a key component along the polymer backbone as it is in poly(dopamine).14 Further proposed structures for detected tetramer species are depicted in Figure G

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Figure 6. (a) Cyclic voltammograms after five cycles (recorded in 0.1 M aqueous NaCl, scan rate 100 mV s−1) of poly(DOPA-TEMPO) (4) thin films deposited on a glassy carbon working electrode recorded after 1, 4, and 7 days of coating time. (b) Corresponding schematic representation of reversible nitroxide oxidation to the N-oxoammonium cation and subsequent reduction to the free radical.

Monitoring the Oxidative Polymerization and Nitroxide Radical Formation. A tandem UV−vis and EPR spectroscopic study was performed to gain further mechanistic insights into the oxidation-driven catecholamine polymerization of monomer 3 and to elucidate the impact of reactive oxidative polymerization intermediates on the nitroxide radical formation and stability (derived from hydroxylamine oxidation under the polymerization conditions). Under diluted polymerization conditions (for better spectroscopic quantification),49 a gradual color change from colorless via yellow to orange and later brown was observed without any visible polymer particle formation during aerobic polymerization (Figure S12). The initial UV−vis spectrum of monomer 3 (Figure 5, red line, 0 day, immediately recorded after dissolving 3 in Tris−HCl buffer) showed a characteristic peak at λmax = 282 nm, which is unambiguously derived from the catechol moiety.25,58 With proceeding polymerization time, a second peak emerged, progressively increasing in intensity and climaxing at λmax = 333 nm after 9 days (yellow line), whereas the catecholassigned peak at λ = 282 nm slowly broadened and shifted to shorter wavelengths within the same time period. The emerging peak at longer wavelengths accompanied by the formation of a broad shoulder at around λ = 450 nm most likely indicates catechol oxidation toward o-quinoic species, which have been identified as reactive intermediates in dopamine polymerizations absorbing at longer wavelengths.25,58 Interestingly, with the increasing polymerization time (from 9 days onward, green and purple lines), the peak at λmax = 333 nm steadily diminished again and converted to a broad shoulder after 25 days accompanied by a brown colorization of the solution. This decline supports the hypothesis that oquinoic structures are exclusively formed as reactive intermediates initiating the polymerization and are further reconverted to catecholic aromatic systems, e.g., to DHI-like structures. However, the initial catechol-derived UV absorption, which steadily shifted toward shorter wavelengths (λmax = 269 nm after 25 days), cannot be assigned to a specific structure (e.g., the DHI-like structures) as poly(catecholamine)s are heterogeneously composed dynamic systems and further complex supramolecular assembled oligomer structures. Figure

5b summarizes the UV−vis spectroscopic time-dependent monitoring of DOPA-TEMPO (3) polymerization. The absorption maxima, λmax = 282 → 269 nm (●), indicate the overall progress of the polymerization, reaching a plateau after 17 days, whereas at λ = 333 nm (■), the formation of reactive o-quinoic intermediates can be monitored, which then further undergo oxidative cross-coupling reactions, for example, via the benzo moiety to yield catecholic poly(DOPA-TEMPO) (4). It is noteworthy that TEMPO-derived UV−vis spectroscopic interferences can be excluded as proven by control measurements showing only low absorption in the high-energy UV range (Figure S12). Simultaneously, the generation of nitroxide free radicals derived from hydroxylamine oxidations was followed by EPR spectroscopy, showing the typical hyperfine tripletlike structures derived from the nitroxide radical (Figure 5c). A retarded radical release during DOPA-TEMPO (3) polymerization (Figure 5d, ■) was observed compared to that for the reference system (the oxidation of the 4-hydroxy-TEMPOderived hydroxylamine species, □). A plateau was reached after 5 days, suggesting predominant oxidation processes of the catecholamine moiety at this stage (as indicated in Figure 5b), which hamper nitroxide radical formation. After 5 days, however, the nitroxide radical liberation continued, which was expressed in EPR spectroscopic peak broadening, as shown in Figure 5c (blue → yellow line). The EPR spectroscopic peak broadening induced by close spatial proximity of the nitroxide radicals indicates DOPA-TEMPO oligomer formation (e.g., dimerization) caused by high concentrations of o-quinoic structures.59 After 9 days and at the maximum of oxidized reactive intermediates detected by UV−vis spectroscopy (Figure 5b), a continuous formation of radicals was observed by EPR spectroscopy, which reached a plateau after 17 days. Overall, more than 85% of poly(DOPA-TEMPO) (4) radicals were generated from hydroxylamine oxidation, as determined by EPR spectroscopy. Investigating the Reversible Redox Potential of Nitroxide Thin Films. The electrochemical redox behavior of the nitroxides incorporated into poly(DOPA-TEMPO) (4) films was evidenced by cyclic voltammetry. Thin adherent H

DOI: 10.1021/acs.langmuir.7b03755 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir nitroxide polymer films were exemplarily deposited on a glassy carbon and a platinum working electrode, respectively, showing our versatile coating approach applicable for nitroxide decoration of electrode materials. As depicted for the glassy carbon electrode in Figure 6, the polymeric TEMPO radicals underwent reversible one-electron oxidation, forming the Noxoammonium cations, followed by reduction to the free radical species.60 A higher nitroxide density at the electrode’s interface was achieved by extending the coating time and further addition of monomer 3 to the coating solution, which resulted in continuously increased current values. Furthermore, repetitive cyclic voltammetry measurements (five redox cycles) confirmed a good cycling stability of the nitroxide-containing polymers. Similar observations were made for the coated platinum working electrode, as shown in Figure S13.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.E.F.-S., L.B., and C.B.-K. acknowledge financial support from the Australian Research Council (Discovery Project DP150100234) and the Queensland University of Technology (QUT). K.E.F.-S. acknowledges funding from an Australian Research Council Future Fellowship (FT140100746). C.B.-K. acknowledges funding by an Australian Research Council Laureate Fellowship as well as continuous funding from the Karlsruhe Institute of Technology (KIT) in the context of the BIFTM program of the Helmholtz association. J.S.’s studies are funded by a Landesgraduierten scholarship by the state of Baden-Wuerttemberg. The K-Alpha+ instrument was financially supported by the Federal Ministry of Economics and Technology on the basis of a decision by the German Bundestag. Some of the data reported in this article were obtained at the Central Analytical Research Facility (CARF) operated by the Institute for Future Environments (QUT). Access to CARF is supported by generous funding from the Science and Engineering Faculty (QUT).

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CONCLUSIONS A sophisticated and versatile avenue for the generation of nitroxide functional surfaces is reported. On the basis of bioinspired catecholamine polymerization of a nitroxidefunctionalized coating agent, various materials were successfully covered with thin nitroxide-containing polymer films. Careful investigations of the adhesive performances and structural composition of poly(DOPA-TEMPO) (4) were conducted using in-depth surface characterization techniques such as X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and high-resolution electrospray ionization mass spectrometry (HR-ESI MS) as a structural access route. The disperse, heterogeneous polymer composition with identified dihydroxyindole-like key structures was found to exhibit strong heteromultivalent binding strengths to even inert surfaces such as PTFE. The reversible redox-active character of the nitroxide functional groups confirmed by cyclic voltammetry (CV) emphasizes their use as an electrochemically active polymer material with versatile adhesive properties. Furthermore, our simple and substrate-independent nitroxide surface decoration technique may be employed for biomedical applications requiring materials with antimicrobial properties.

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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.7b03755.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], christopher. [email protected] (C.B.-K.). *E-mail: [email protected] (K.E.F.-S.). ORCID

Anja S. Goldmann: 0000-0002-1597-2836 Leonie Barner: 0000-0002-6034-0942 James P. Blinco: 0000-0003-0092-2040 Christopher Barner-Kowollik: 0000-0002-6745-0570 Kathryn E. Fairfull-Smith: 0000-0002-9412-632X I

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DOI: 10.1021/acs.langmuir.7b03755 Langmuir XXXX, XXX, XXX−XXX