Article pubs.acs.org/Langmuir
Cite This: Langmuir 2018, 34, 2397−2402
Ultrafast Tailoring of Carbon Surfaces via Electrochemically Attached Triazolinediones William Laure,† Kevin De Bruycker,‡ Pieter Espeel,‡ David Fournier,† Patrice Woisel,† Filip E. Du Prez,*,‡ and Joel̈ Lyskawa*,† †
Université de Lille, CNRS, ENSCL, UMR 8207UMETUnité Matériaux Et Transformations, Ingénierie des Systèmes Polymères (ISP) Team, F-59000 Lille, France ‡ Polymer Chemistry Research Group, Centre of Macromolecular Chemistry (CMaC), Department of Organic and Macromolecular Chemistry, Ghent University, Krijgslaan 281 S4, B-9000 Ghent, Belgium S Supporting Information *
ABSTRACT: The straightforward coupling between a triazolinedione (TAD) unit and citronellyl derivatives via an Alder-ene reaction has been exploited to tailor the physicochemical surface properties of glassy carbon (GC) surfaces in an ultrafast and additive-free manner. For this purpose, we first covalently grafted a TAD precursor onto GC via electrochemical reduction of an in situ generated diazonium salt, which was then electrochemically oxidized into the desired GC-bonded TAD unit. A kinetic study of the modification of this reactive layer with an electroactive ferrocene probe proved that a complete functionalization was obtained in merely 1 minute. Further modification experiments with a fluorinated probe demonstrated that the surface properties can be swiftly tailored on demand. The different modification steps, as well as the efficiency of this strategy, were investigated by electrochemistry, contact angle goniometry, and X-ray photoelectron spectroscopy analysis.
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INTRODUCTION
metals, metal oxides, alloys, semiconductors, ceramics, and polymers.9 While the aforementioned grafting techniques are versatile and widely applicable, the attachment of various target molecules also requires the synthesis of derivatives with surface-reactive moieties. Therefore, two-step modification procedures were developed, in which the substrate is first functionalized with a reactive moiety, followed by the immobilization of the target molecules or polymers, for example, using the well-known copper-catalyzed azide−alkyne cycloaddition (CuAAC)10 or reversible Diels−Alder cycloaddition.11 Nevertheless, despite the efficiency and “click” character of such coupling reactions, they have a major drawback as they are time-consuming. Indeed, the slow kinetics of these processes can take from several hours to a few days, which limits the scope of possible applications. Consequently, decreasing the reaction time is of great interest to surface
Surface functionalization has attracted considerable interest in the past decade, with the aim of designing surfaces with tailored physicochemical properties, such as wettability, corrosion resistance, or adhesion, or to improve their biocompatibility and/or biological performances (e.g., cell affinity, tissues integration, or prevention of biofilm formation). Consequently, several methods have been developed to modify surfaces, including plasma treatment,1 electrodeposition,2 layer-by-layer techniques,3 and the immobilization of self-assembled monolayers.4 Alternatively, the covalent attachment of organic molecules or polymers using silane,5 phosphonic acid/ester,6 or the bioinspired catechol chemistry,7 for example, was proven to be an efficient strategy to tailor the physicochemical surface properties,8 although the immobilization step remains highly time consuming. Finally, the electrochemical reduction of a diazonium salt represents a simple and powerful method to graft thin organic films onto conductive surfaces, allowing the covalent attachment of a wide range of functionalized aryl groups onto various surfaces including carbon, diamond, © 2018 American Chemical Society
Received: September 25, 2017 Revised: December 20, 2017 Published: January 22, 2018 2397
DOI: 10.1021/acs.langmuir.7b03363 Langmuir 2018, 34, 2397−2402
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Langmuir
5 × 10−9 mbar or lower during 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 CasaXPS software. Analyses were performed on GC surfaces with a diameter of 1.5 cm from HTW (Germany). 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 ten times to obtain an average value for the surface. Analyses were performed on GC surfaces with a diameter of 1.5 cm from HTW (Germany). Synthesis. Citronellyl-Modified Ferrocene 2. A solution of dicyclohexylcarbodiimide (DCC, 2.476 g, 12 mmol) in dry dichloromethane (DCM, 15 mL) was added dropwise to an ice-cooled mixture of ferrocene carboxylic acid (2.3 g, 10 mmol), citronellol (2.74 mL, 15 mmol), and 4-dimethylaminopyridine (122 mg, 1 mmol) in dry DCM (25 mL). The brown, turbid reaction mixture was stirred over the weekend at room temperature. After filtration, the solvent was evaporated and the crude product was purified by column chromatography (SiO2−hexane/EtOAc 100/0 to 95/5). The compound was then isolated as a brown oil in 65% yield. 1 H NMR (300 MHz, CDCl3): δ (ppm from TMS) 0.99 (d, 3H, CH3−CH), 1.26 (m, 1H, CH−CH3), 1.37−1.84 (m, 2H, CH−CH2− CH2 and m, 2H, CH2−CH2−O), 1.62 (s, 3H, CH3−C(CH)−CH3), 1.70 (s, 3H, CH3−C(CH)−CH3), 2.03 (m, 2H, C(CH)−CH2), 4.19 (s, 5H, CFcH), 4.25 (t, 2H, CH2−O−CO), 4.38 (t, 2H, CFcH− (CFcH)2−CFcH), 4.80 (t, 2H, CFcH−CFc−CFcH), 5.13 (m, 1H, CH3−C(CH)−CH3). 13 C NMR (300 MHz, CDCl3): δ (ppm from TMS) 17.7 (CH3− C(CH)−CH3), 19.5 (CH3−CH), 25.5 (C(CH)−CH2), 25.7 (CH3− C(CH)−CH3), 29.7 (CH−CH3), 35.8 (CH2−CH2−O), 37.1 (CH− CH2−CH2), 62.7 (CH2−O), 69.7 (CFc−H), 70.1 (CFcH−CFc− CFcH), 71.2 (CFcH−(CFcH)2−CFcH), 71.6 (CFc(IV)), 124.6 (C CH), 131.4 (CCH), 171.8 (CO). Citronellyl-Modified Zonyl 3. A suspension of N-hydroxysuccinimide (NHS, 4.06 g, 35.3 mmol) in dry DCM was added dropwise to a solution (200 mL) containing citronellic acid (4 g, 23.5 mmol) and DCC (7.27 g, 35.3 mmol) at −5 °C under nitrogen. The reaction was allowed to return to room temperature and was stirred overnight. After filtration, the organic phase was washed three times with water (H2O, 3 × 100 mL) and dried over MgSO4. After filtration, the solvent was evaporated and the crude product was purified by column chromatography (SiO2−DCM). The product was obtained in 62% yield. 1 H NMR (300 MHz, CDCl3): δ (ppm from TMS) 1.06 (d, 3H, CH3−CH), 1.28−1.52 (m, 2H, CH2−CH(CH3)), 1.61 (s, 3H, CH3− C(CH)−CH3), 1.68 (s, 3H, CH3−C(CH)−CH3), 1.95−2.14 (m, 3H, C(CH)−CH2 and CH−CH3), 2.4 and 2.62 (dd, 2H, CH2−CO−O), 2.83 (s, 4H, CO−CH2−CH2−CO), 5.09 (m, 1H, CH3−C(CH)− CH3). 13 C NMR (300 MHz, CDCl3): δ (ppm from TMS) 17.7 (CH3− C(CH)), 19.4 (CH3−CH), 25.3 (C(CH)−CH2), 25.6 (CO−CH2− CH2−CO), 25.7 (CH3−C(CH)), 30.2 (CH3−CH), 36.5 (CH2− CH(CH3)), 38.2 (CH2−CO−O), 123.9 (C(CH)−CH2), 131.9 (C(CH)−CH2), 168.0 (CH2−CO−O), 169.1 (CO−CH2−CH2− CO). A solution of the NHS-functionalized compound (0.26 mg, 0.95 mmol) in anhydrous DCM (80 mL) was added dropwise to a solution containing amino Zonyl derivative11b (0.70 g, 0.87 mmol) and NEt3 (0.13 g, 1.3 mmol) in dry DCM (20 mL). The mixture was stirred at room temperature overnight. The organic phase was washed with 1 M NaOH (3 × 100 mL) and water (3 × 100 mL) and dried over MgSO4. After filtration, the solvent was removed under reduced pressure and the product was obtained as a brown viscous oil in 44% yield. 1 H NMR (300 MHz, CDCl3): δ (ppm from TMS) 0.93 (d, 3H, CH3−CH), 1.10−1.44 (m, 2H, CH2−CH(CH3)), 1.60 (s, 3H, CH3−
scientists for the development of novel functional materials, surfaces, and nanoparticles. Triazolinedione (TAD) compounds12 were already demonstrated to have a quite high reactivity toward both conjugated dienes and isolated alkenes in Diels−Alder or Alder-ene reactions. Most often, these reactions are complete within seconds to minutes, proceed at room temperature in an equimolar way without the need for a catalyst, and thus meet the criteria of a click reaction. Consequently, TADs were exploited to produce block copolymers,13 cross-linked plant-oilbased materials,14 and also covalently linked layer-by-layer assemblies onto surfaces.15 Surprisingly, despite the attractive character of the TAD chemistry in the fast functionalization of materials, no example could be found in literature in which TAD was immobilized onto surfaces for the subsequent attachment of target molecules. In this context, we report on a facile and highly efficient strategy, using two subsequent electrochemical reactions, to obtain a TAD-functionalized substrate that can be further modified to tailor the surface properties in an ultrafast manner. First, an urazole moiety was covalently attached onto glassy carbon (GC) surfaces via the electrochemical reduction of an in situ-generated diazonium salt. Subsequent electrochemical oxidation readily transformed the urazole into the desired TAD. Finally, these TAD groups were modified with citronellyl-based probes via the ultrafast Alder-ene click reaction. Indeed, these citronellyl residues originate from citronellal, a readily available, naturally occurring terpene showing a high reactivity toward TADs.12 While a ferrocene probe was used to investigate the kinetics of the modification procedure, a fluorinated probe was applied to tailor the surface properties. The different steps of the modification, as well as the efficiency of the overall strategy, were investigated by electrochemistry, contact angle goniometry, and X-ray photoelectron spectroscopy (XPS) analysis.
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EXPERIMENTAL SECTION
Materials. All reagents were provided by Sigma-Aldrich and used as received unless otherwise noted. 4-(4-Aminophenyl)urazole 1 was synthesized according to the literature.16 In Situ Electrochemical Reduction of the Diazonium Salt. In situ production and subsequent electrochemical reduction of the diazonium salt from 4-(4-aminophenyl)urazole (1) were carried out in a 5 mM acidic solution (0.5 M HCl) of 1 containing 1 equiv of sodium nitrite. The mixture was deaerated by bubbling an inert gas through the solution. Alder-ene Click Coupling on GC-Surfaces. Functionalization of TAD-modified electrodes was achieved by dipping the GC surfaces into a 1 mM solution of 2 or 3 in acetonitrile for 5 min at room temperature. The surfaces were then thoroughly rinsed with acetonitrile and dried with a nitrogen flow. Analytical Techniques. Electrochemical Experiments (Cyclic Voltammetry). Electrochemical experiments were performed using an Autolab PGSTAT30 workstation. The experiments were carried out in dry acetonitrile containing 0.1 M of recrystallized tetrabutylammonium hexafluorophosphate (TBAPF6) as an electrolyte or in a phosphate buffer solution adjusted to pH = 7. A three-electrode configuration was used with a GC (3 mm diameter) working electrode, and a platinum rod as a counter electrode. An Ag/AgCl electrode was used as the reference. All solutions were purged with nitrogen prior to recording the electrochemical measurements. 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 pressure in the analyzing chamber was maintained at 2398
DOI: 10.1021/acs.langmuir.7b03363 Langmuir 2018, 34, 2397−2402
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Scheme 1. Electrochemical Deposition of the Urazole Derivative 1 and Its Subsequent Oxidation into TAD Allowing the Ultrafast Tailoring of Carbon Surfaces via Alder-ene Reactions
Figure 1. (a) CV of the GC electrode monitored after the addition of 1 equiv of NaNO2 in the cell containing 1, first and subsequent scans (conditions: 5 mM of 1 in 0.5 M HCl, scan rate = 100 mV/s vs Ag/AgCl). (b) CV of the GC electrode modified by diazonium salt of 1 compared to the CV of phenylurazole in solution (conditions: phosphate buffer at pH = 3 and scan rates = 100 mV/s vs Ag/AgCl). C(CH)−CH3), 1.68 (s, 3H, CH3−C(CH)−CH3), 1.88−2.07 (m, 3H, C(CH)−CH2 and CH−CH3), 2.32−2.60 (m, 4H, CH2−CO−NH and CH2−CF2), 3.46 (t, 2H, CH2−NH−CO), 3.55 (t, 2H, CH2−CH2− NH−CO), 3.58−3.74 (m, 26H, CH2−CH2−O), 3.78 (t, CH2−CH2− CF2), 5.09 (m, 1H, CH3−C(CH)−CH3).
surface was first investigated by monitoring the CV of the urazole-functionalized electrode in a phosphate buffer at pH equal to 3 (Figure 1b). The corresponding CV exhibits an anodic peak at 0.63 V vs Ag/AgCl, attributed to the quasireversible oxidation of the urazole moiety, which is similar to the redox behavior of the phenylurazole in the same conditions. The grafting was further confirmed by XPS. Figure 2b shows the survey spectra and atomic quantifications of GC electrodes before and after modification. After electrochemical grafting, the contribution of carbon at 285.0 eV decreased, whereas those of nitrogen at 401.2 eV and oxygen at 532.4 eV increased significantly, as expected from the structure of the immobilized urazole moiety. Generation of the Triazolinedione-Reactive Layer. Urazoles are typically converted into TADs with oxidants such as DABCO-bromine, N-bromosuccinimide, or gaseous N2O4.12 However, because the TAD moieties in this work are surface-bound, we expected an inefficient oxidation, especially with the heterogeneous oxidants. Interestingly, Varmaghani et al. reported that a TAD can also be generated in situ via the electrochemical oxidation of the corresponding urazole.20 Although this strategy was solely used for the generation of sulfonamide derivatives, we explored this elegant oxidation procedure for the production of TAD-functional substrates. For this purpose, the urazole-functionalized electrode was subjected to 10 oxidative cycles from 0.3 to 1.7 V vs Ag/AgCl (t < 5 min). Acetonitrile was used as a solvent to prevent potential side-
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RESULTS AND DISCUSSION The TAD-reactive moiety was immobilized in two steps onto a GC surface via electrochemical reduction of a diazonium salt (Scheme 1).17 Considering the high reactivity of the TAD unit and to prevent side-reactions, we employed 4-(4aminophenyl)urazole 1, that is, a TAD precursor, for grafting.16 Urazole Grafting. The direct immobilization of urazole 1 onto the GC surface was achieved in one step through the electrochemical reduction of the corresponding diazonium salt, which is generated in situ from an aqueous 1 mM HCl (0.5 M) solution of 1 upon addition of 1 equiv of sodium nitrite in the electrochemical cell.18 The corresponding cyclic voltammogram exhibits the characteristic broad peak that corresponds to the irreversible reduction of the diazonium salt and is assigned to the formation of the aryl radical, which then attaches to the GC surface (Figure 1a). In addition, the following cyclic voltammetry deposition cycles show no further reduction peaks, while a decrease in current was observed, which results from the passivation of the GC electrode due to the formation of the grafted layer.19 The grafting of the urazole group onto the GC 2399
DOI: 10.1021/acs.langmuir.7b03363 Langmuir 2018, 34, 2397−2402
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Figure 2. (a) CV of GC electrode during the oxidation of the urazole into TAD, first and subsequent scan (conditions: acetonitrile TBAPF6 0.1 M, v = 100 mV/s). (b) XPS spectrum of the GC electrode before (lower) and after functionalization with urazole (middle) and finally after subsequent electro-oxidation and functionalization with fluorinated probe 3 (upper).
Figure 3. (a) Cyclic voltammogram of the GC surface after coupling reaction with the ferrocene probe 2. (b) Kinetic studies, evaluation of surface coverage as a function of reaction time (conditions: acetonitrile TBAPF6 0.1 M, v = 100 mV/s).
exhibits the characteristic and well-defined oxidation−reduction wave of the ferrocene probe at 0.7 V vs Ag/AgCl.11b Furthermore, the linear increase in current with scan rate (see Figure S3) on the one hand, and constant values of redox potentials vs scan rates on the other hand, further confirmed the covalent attachment of the ferrocene unit onto the GC electrode. The integration of the ferrocene signal (n = 1) reveals a surface coverage (Γ) of 1.05 × 1014 molecules/cm2, thereby indicating a high grafting density of the materials, as already stated for CuAAC21 or Diels−Alder reactions.11b Finally, the kinetics of the Alder-ene reaction were assessed by integrating the anodic peak of the ferrocene signal at different times to estimate the surface coverage. Figure 3b demonstrates that the reaction is very fast, as the coupling was already complete in merely 1 minute. To the best of our knowledge, there is no example in the literature reporting postfunctionalization of surfaces in such an ultrafast, straightforward, and efficient manner.22 Grafting of a Fluorinated Oligomer. Finally, with the aim to modulate the physico-chemical surface properties of GC, Zonyl21 derivative 3 (Scheme 1) was synthesized, containing a
reactions of the generated TAD unit in aqueous media. As depicted in Figure 2a, the corresponding CVs clearly display the disappearance of the anodic and the cathodic waves assigned to the oxidation−reduction events of the urazole moiety, which proves the conversion of urazole into TAD. Grafting of the Electroactive Ferrocene Probe. As a proof of concept, the reactive TAD-functionalized layer was subjected to an Alder-ene coupling reaction with the citronellylbearing electroactive ferrocene probe 2 (Scheme 1). The substituted double bond of the citronellyl group is an excellent reagent for Alder-ene reactions with TAD substrates.12 The straightforward functionalization of the TAD layer was reached within 5 min at ambient temperature by simply dipping the electrode into a 1 mM solution of 2 in acetonitrile. It is worth noting that this reaction does not require any metal/ligand catalyst, additional photoinitiator, or dye sensitizer. Moreover, no reaction occurs if a reference experiment is performed with the urazole-functionalized surface, thereby demonstrating the key role of the TAD unit (see Figures S1 and S2). The main asset of this experiment lies in the fact that the reaction can be easily evaluated by electrochemical measurements (Figure 3a). As expected, the CV corresponding to the modified GC surface 2400
DOI: 10.1021/acs.langmuir.7b03363 Langmuir 2018, 34, 2397−2402
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from the corresponding aniline, and its subsequent electrochemical oxidation into the reactive TAD moiety. The modification of the TAD-bearing surfaces via an ultrafast (
