Robust Electrografting on Self-Organized 3D ... - ACS Publications

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Robust Electrografting on Self-Organized 3D Graphene Electrodes Philippe Fortgang,† Teddy Tite,‡,§ Vincent Barnier,∥ Nedjla Zehani,† Chiranjeevi Maddi,‡,§ Florence Lagarde,† Anne-Sophie Loir,‡,§ Nicole Jaffrezic-Renault,† Christophe Donnet,‡,§ Florence Garrelie,*,‡,§ and Carole Chaix*,† †

Institut des Sciences Analytiques, UMR 5280, CNRS, Université de Lyon 1, 5 rue de la Doua, 69100 Villeurbanne, France Université de Lyon, F-69003, Lyon, France § Université de Saint-Étienne, Laboratoire Hubert Curien (UMR 5516 CNRS), 42000 Saint-Étienne, France ∥ Laboratoire Georges Friedel, Ecole Nationale Supérieure des Mines, 42023 Saint-Etienne, France ‡

S Supporting Information *

ABSTRACT: Improving graphene-based electrode fabrication processes and developing robust methods for its functionalization are two key research routes to develop new high-performance electrodes for electrochemical applications. Here, a self-organized three-dimensional (3D) graphene electrode processed by pulsed laser deposition with thermal annealing is reported. This substrate shows great performance in electron transfer kinetics regarding ferrocene redox probes in solution. A robust electrografting strategy for covalently attaching a redox probe onto these graphene electrodes is also reported. The modification protocol consists of a combination of diazonium salt electrografting and click chemistry. An alkyne-terminated phenyl ring is first electrografted onto the self-organized 3D graphene electrode by in situ electrochemical reduction of 4-ethynylphenyl diazonium. Then the ethynylphenyl-modified surface efficiently reacts with the redox probe bearing a terminal azide moiety (2-azidoethyl ferrocene) by means of CuI-catalyzed alkyne−azide cycloaddition. Our modification strategy applied to 3D graphene electrodes was analyzed by means of atomic force microscopy, scanning electron microscopy, Raman spectroscopy, cyclic voltammetry, and X-ray photoelectron spectroscopy (XPS). For XPS chemical surface analysis, special attention was paid to the distribution and chemical state of iron and nitrogen in order to highlight the functionalization of the graphene-based substrate by electrochemically grafting a ferrocene derivative. Dense grafting was observed, offering 4.9 × 10−10 mol cm−2 surface coverage and showing a stable signal over 22 days. The electrografting was performed in the form of multilayers, which offers higher ferrocene loading than a dense monolayer on a flat surface. This work opens highly promising perspectives for the development of self-organized 3D graphene electrodes with various sensing functionalities. KEYWORDS: self-organized 3D graphene, diazonium electrografting, click chemistry, ferrocene, X-ray photoelectron spectroscopy

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INTRODUCTION

monatomic layer called pristine graphene to three-dimensional (3D) graphene structures such as tubes, fibers, networks, and porous 3D graphene films.8,9 Porous 3D graphene film consists of a multilayered assembly of several graphene sheets in a disordered pseudographitic structure. Many studies reported graphene and graphene-based composites as promising electrochemical electrodes, showing better electron transfer kinetics than many traditional and

Graphene is a new form of carbon that offers unique mechanical, thermal, electrical, and optical properties. It is the thinnest, most flexible, and strongest material ever measured.1,2 This past decade, graphene has attracted tremendous attention in the fields of electronics, catalysis, energy storage, sensors and in many more potential applications,3−5 thanks to its high surface area, excellent conductivity, and ease of functionalization and production.6,7 Graphene describes a large variety of carbon materials, depending on the number of layers of graphene and the presence of defects on the graphene sheets.5 Graphene can exist in different forms, extending from a two-dimensional (2D) © 2015 American Chemical Society

Received: November 5, 2015 Accepted: December 28, 2015 Published: December 29, 2015 1424

DOI: 10.1021/acsami.5b10647 ACS Appl. Mater. Interfaces 2016, 8, 1424−1433

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ACS Applied Materials & Interfaces

must be developed. Graphene has been covalently or noncovalently functionalized by polymers, biomolecules, and redoxactive or photochemical active molecules.30 Here we focus on the covalent modification of 3D graphene through aryl diazonium salt chemistry to introduce alkyne functionalities on the graphene electrode surface for further modification through the click reaction. Diazonium salt chemistry has been studied on various advanced carbon materials35 like Glassy Carbon,36 Boron-doped diamond (BDD)37 and graphene.38−41 Modifying carbon electrodes through the reduction of in situ generated diazonium salts by cyclic voltammetry (CV) offers several advantages such as low cost due to commercially available precursors such as 4-ethynylaniline and fast preparation of the electrochemical cell. Electrografting also enables controlled electro-addressing on multielectrode arrays and easy control of grafting efficiency (i.e., surface coverage) from less than one monolayer to several multilayers.42 One drawback of diazonium salt chemistry is that the grafting step requires 4-ethynylaniline to be transformed in situ to 4-ethynylphenyl diazonium by the use of sodium nitrite in acidic conditions. These conditions are too aggressive for sensitive molecules and can lead to their degradation. Mild conditions are imperatively required for grafting biomolecules onto a support, so necessitating a twostep procedure. The CuI-catalyzed alkyne−azide 1,3-dipolar cycloaddition conventionally named click reaction is welladapted to this end.43 Click chemistry provides a highly selective and quantitative reaction, which is well suited for electrode surface functionalization. For instance, Ripert et al. used click chemistry to address an azido-modified ferrocene onto a gold electrode previously functionalized with alkyne functions.44 This strategy was successfully applied to the grafting of azido and ferrocenyl modified oligodeoxyribonucleotides (ODN).45 Yeap et al. reported a two-step grafting strategy on a BDD surface by coupling an alkynyl-ferrocene derivative onto azido-modified BDD.37 It is worth noting that the functionalization, which first includes diazonium salt electrografting and the subsequent click reaction with a molecule of interest, has never been described using 3D textured graphene as the electrode material. In this work, we report robust electrochemical grafting on selforganized 3D graphene electrodes processed from a solid carbon source by pulsed laser deposition followed by thermal annealing. To perform efficient functionalization of the graphene, in situ reduction of 4-ethynylaniline using CV was performed, thus introducing alkyne moieties to the surface, and then click chemistry was involved to tether 2-azidoethyl ferrocene (Fc-Azide) via CuI-catalyzed alkyne−azide cycloaddition. The covalent and robust attachment of ferrocene moieties on 3D graphene was analyzed by scanning electron microscopy (SEM), Raman spectroscopy, atomic force microscopy (AFM), cyclic voltammetry (CV), and, finally, chemical surface analysis using X-ray photoelectron spectroscopy (XPS) which established the quality of the functionalization of the graphene-based substrate by electrochemical grafting of a ferrocene derivative.

advanced carbon electrodes such as graphite, doped diamond, glassy carbon and carbon nanotubes.5−7 Applications as electrodes include for example, supercapacitors, lithium batteries, transparent cells and electrochemical sensors.5,7,10−13 It is clear though that the performance of graphene-based electrochemical devices is strongly dependent on both the chosen synthesis method and the ability to control its interfacial properties, especially its reactivity and surface chemistry. On top of that, studies have shown that functionalization of the graphene surface is not only crucial to open its band gap, but also fundamental to improve its interfacial properties, which is an important step in constructing novel electrochemical electrodes with high performance.14,15 This could be highlighted by the fact that a good electrode requires a balance between defective active sites and electrical conductivity.6,16 Although interesting fundamental studies have been reported,17−20 pristine graphene obtained by micromechanical cleavage is not suitable for electrochemical applications due to its small size, low defect density, and chemical inertness.5,6 Other commonly used graphene fabrication methods include the chemical exfoliation of graphite,21 chemical vapor deposition (CVD) growth,22,23 and chemical, electrochemical, thermal, or photocatalytic reduction of graphene oxide (GO).24 The fabrication of 3D porous and corrugated graphenebased architecture is considered to be one of the most promising ways to obtain high-performance electrodes.10,17,25−28 Compared to graphene, 3D-graphene films or electrodes provide a large specific surface area, fast charge transfer, and low mass transport resistance.10,17,25−28 Three-dimensional graphene electrodes have so far been prepared by CVD using a Ni 3D foam template, thermal annealing of a 3D pyrolyzed porous photoresist film sputtered with Ni, and electrochemical reduction of a concentrated GO dispersion.5,10,25 Despite the strong interest of the CVD graphene technique, the metal etching agents (FeCl3, Fe(NO3)3) used to dissolve the Ni severely contaminate the graphene, thus altering its electrochemical properties.5,29 Also, 3D CVD graphene is defect-free and highly hydrophobic, which makes its surface functionalization difficult.5,26 GO-based electrodes are usually preferred because of their rich chemical structure made of defective active oxygenated sites (hydroxyl, epoxy, and carboxylic groups), which enhance the electrocatalytical activity at the interface.10,14,15 However, reduced graphene oxide (rGO), which is commonly used due to its easy fabrication process, has poor electrical conductivity, which is a serious drawback for its electrochemical application. Studies have demonstrated the potential of enhancing the electrochemical activity of rGO by functionalizing its surface with nanoparticles or by mixing with conductive polymers, which broadened applications to sensors.10,30,31 However, preparation methods are still very challenging, and new methods to fabricate high-performing, stable 3D graphenebased electrodes are urgently required. Recently, the thermal conversion of solid carbon feedstocks (TCSC) to graphene was proposed in order to better control its surface properties.32−34 Studies showed that TCSC graphene has excellent optical and electrical properties.32,33 However, although this emerging approach has become an attractive scientific activity, its electrochemical applications are so far unexplored. Here, we propose to fill this gap by synthesizing large scale 3D graphene by a bottom-up method of pulsed laser deposition and investigating its electrochemical and surface functionalization properties. To elaborate an efficient electrochemical biosensor, a robust chemistry for (bio)-functionalizing the conductive surface

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MATERIALS AND METHODS

Chemicals. Ferrocene dimethanol (Sigma-Aldrich); sodium perchlorate anhydrous (Alfa Aesar); sodium nitrite, 97% (Sigma-Aldrich); hydrochloric acid, 97% (Sigma-Aldrich); 4-ethynylaniline, 97% (SigmaAldrich); tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine or TBTA (Sigma-Aldrich); copper(II) sulfate, anhydrous (Sigma-Aldrich); dimethyl sulfoxide (DMSO), 99%; and ethanol, 99%, were commercially available and used without any purification. Aqueous buffers and electrolytes were made with deionized water purified through a Milli-Q 1425


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ACS Applied Materials & Interfaces system (Millipore, Bedford, MA). 2-Azidoethyl ferrocene (Fc-Azide) was synthesized as described by Ripert et al.44 Graphene Substrate Elaboration. A nickel (Neyco, 99.99% purity) thin film of ∼300 nm thick was deposited by thermal evaporation on an n-doped Si substrate. An amorphous carbon (a-C) film ∼40 nm thick was deposited at 780 °C on the nickel film. The a-C film was obtained under high vacuum by ablating a graphite target (99.997% purity) with an excimer laser in a deposition chamber evacuated to a base pressure of about 10−4 Pa. A KrF laser with a wavelength of 248 nm, a pulse duration of 20 ns, a repetition rate of 10 Hz and an energy per pulse of 400 mJ was used for the ablation. The energy density of the pulsed laser was set to 40 J cm−2 and the deposition rate of carbon was 2 nm min−1. After thermal annealing (780 °C, 45 min), the system was cooled naturally to room temperature. Raman Characterization. Micro-Raman experiments were carried out in backscattering configuration at room temperature with He−Cd laser wavelength at 442 nm. The spectral resolution was ∼2 cm−1. The laser beam was focused on the sample with a 100× objective in the visible range. The scattered Raman signal was then measured with an Aramis Jobin Yvon confocal spectrometer equipped with a chargecoupled device (CCD) camera. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) characterization. A scanning electron microscope (SEM-FEG; FEI, NovaNanoSEM), and an atomic force microscope (AFM; Agilent 5500) were also used to characterize the films. X-ray Photoelectron Spectroscopy. Chemical surface analysis was carried out using X-ray photoelectron spectroscopy (XPS). The analysis were performed with a Thermo VG Thetaprobe spectrometer instrument with a focused monochromatic Al Kα source (hν = 1486.68 eV, 400 μm spot size). Photoelectrons were analyzed using a concentric hemispherical analyzer operating in the constant ΔE mode. The energy scale was calibrated with sputter-cleaned pure reference samples of Au, Ag, and Cu so that Au 4f7/2, Ag 3d5/2, and Cu 3p3/2 were positioned at binding energies of respectively 83.98, 386.26, and 932.67 eV. The Fe 2s peak was chosen instead of the more intense Fe 2p peak in order to avoid any misinterpretation due to Cu Auger transitions interfering with Fe 2p. High energy resolution spectra of the N 1s and Fe 2p peaks were recorded with a step size of 0.1 eV and pass energy of 50 eV. This pass energy gives a width of the Ag 3d5/2 peak measured on a sputter clean pure Ag sample of 0.55 eV. Intensity mapping of F 1s, Fe 2s, N 1s, and Cu 2p3/2 was performed at a pass energy of 200 eV thanks to the fast acquisition “snapshot” mode using the energy dispersive axis of the 2D channel-plate detector and automatic x and y micrometer translation of the sample holder stage. Finally, inelastic background analysis was performed recording the N 1s peak over a wide energy range (∼120 eV) with a step size of 1 eV and a pass energy of 300 eV. Electrochemical Materials. All Electrochemical measurements were made in a conventional one compartment-three electrode cell in a Faraday cage with an internal volume of 3 mL (Verre Equipements, Collonges au Mont d′Or, France). The electrochemical cells were hermetically sealed on one side with a planar graphene electrode (active surface of 0.07 cm2; a PTFE ring was used to define the exposed area) used as the working electrode and, on the other side, a planar platinum electrode used as the counter electrode. A KCl saturated calomel electrode (SCE) from Radiometer Analytical (Villeurbanne, France) was used as a reference. Measurements were performed using a BioLogic multichannel potentiostat VMP3 (Bio-Logic Science Instruments, Pont de Claix, France). Electrochemical results were recorded and analyzed using EC-Lab software from Bio-Logic Science Instruments. Electrochemistry in Solution. The performances in electrochemical kinetics of the graphene was tested by cyclic voltammetry (CV) starting from −0.2 V vs SCE to 0.6 V vs SCE repeated three times in an aqueous solution containing 1,1′-ferrocene dimethanol 0.5 mM and NaClO4 0.1M. Only the last cycles were used for data interpretations. Several scan rates were tested from 1 V/s down to 50 mV/s. In Situ Generation of Diazonium Salt and Electrografting of 4-Ethynylphenyl Group on Graphene. Surface functionalization of graphene electrodes was carried out at 4 °C in a solution, degassed with

nitrogen, of 0.1 M HCl containing 40 mM NaNO2 and 2 mM 4-ethynylaniline. 4-Ethynylaniline was first dissolved in HCl 0.1 M and NaNO2 was added just before grafting. The electrochemical cell is also degassed with nitrogen before grafting. The electrochemical grafting of the in situ generated diazonium salt was performed by CV starting from 0.4 V vs SCE to −0.8 V vs SCE with a scan rate of 0.1 V/s repeated three times. After functionalization, the electrochemical cells containing all the electrodes were thoroughly rinsed with Milli-Q water and ethanol and kept 2 h in each solvent to ensure the removal of any adsorbed species. Click Reaction of Ferrocene-azide with 4-EthynylphenylModified Graphene. The 4-ethynylphenyl-functionalized graphene electrodes were treated for 16 h at ambient temperature with a DMSO/ H2O 1:1 solution containing 1.5 mM of Fc-Azide, 5 mM of sodium ascorbate, 1 mM of TBTA, and 2.5 mM of copper(II) sulfate. Copper(II) sulfate was added just before the reaction was started. After functionalization, the electrochemical cells containing all the electrodes were thoroughly rinsed with Milli-Q water and ethanol, and the electrode was kept (or immersed) 16 h in water to ensure complete elimination of any physisorbed species. Electrochemical Characterization of Functionalized SelfOrganized 3D Graphene Electrodes. The graphene electrodes were electrochemically characterized by CV starting from −0.2 V vs SCE to 0.8 V vs SCE repeated three times in an aqueous solution containing NaClO4 0.1 M as the support electrolyte without any redox species in solution. Several scan rates were tested from 1 V s−1 down to 50 mV s−1. Only the last cycles were used for data interpretation. The amount of grafted ferrocene was calculated by integrating the oxidation and reduction peaks, the average of both values being used to determine the amount of grafted ferrocene. A stability test was performed by repeating the CV characterization at 0.1 V s−1. During storage time, the electrode was kept at ambient temperature and in wet conditions. The electrolyte was renewed before each CV measurement.

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RESULTS AND DISCUSSION Elaboration of Graphene-based Material and Its Characterization. Figure 1 illustrates the fabrication of the

Figure 1. Scheme of self-organized multilayer graphene fabrication by pulsed laser deposition.

electrode. A Ni thin film was deposited onto a silicon substrate by thermal evaporation, followed by the deposition of a thin carbon film under high vacuum by pulsed laser deposition (Figure 1). After in situ thermal annealing at 780 °C, the system was cooled down naturally to room temperature to form multilayer textured graphene on nickel-silicide (Figure 1). The texturing of the surface is explained through a diffusion mechanism of Ni atoms into the Si substrate during heating with the concomitant formation of transition metal-silicides (Figure S1). By using this bottom-up approach, the electrodes can be used directly, without the need to transfer them to a suitable substrate. The morphology of the sample was characterized using SEM. Important corrugation can be observed on the surface of the sample (Figure 2a). High-magnification images reveal the presence of a rough 3D macroporous architecture (Figure 2b), providing high specific surface area. The roughness was evaluated to be around 57 nm, using Gwyddion AFM software (Figure S2). Further structural information was obtained by Raman spectroscopy 1426


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Figure 3. Cyclic voltammograms of a 3D self-organized graphene of 0.5 mM ferrocene dimethanol solution in 0.1 M NaClO4 aqueous electrolyte. The two vertical dotted lines are to help read the theoretical ΔEp value of 59 mV.

Figure 2. SEM images of multilayer graphene at (a) 400× and (b) 60 000× magnification. (c) Raman mapping of 2D/G intensity ratio from the area shown in panel a, and (d) typical Raman spectra from the spots marked with correspondingly colored cross in panel c.

quasi-reversible electron transfer with a heterogeneous standard rate constant k° value equal to 3.5 × 10−2 cm s−1. This value is quite similar to the value of k° in the literature53 measured by scanning electrochemical microscopy on single-layer graphene electrodes with ferrocene methanol. This result indicates that ferrocene is adapted as a future redox probe for graphene, showing faster electron transfer than ferrocyanide (k°(Fe(CN)63−) ≈ 10−4 to 10−3 cm2 s−1) and hexaamineruthenium (k°(Ru(NH3)63+) ≈ 10−5 to 10−4 cm2 s−1) and a comparable rate constant to hexachloroiodate (k°(IrCl62−) ≈ 2 × 10−2 to 5 × 10−2 cm2 s−1).19 Therefore, ferrocene was then chosen as a redox probe to control graphene functionalization. Electrochemical Grafting of 3D Graphene Electrode and Its Characterization. As shown in Figure 4, the modification of 3D graphene with a ferrocene derivative was performed in two steps. The first step (step 1) consisted in modifying the self-organized 3D graphene with alkyne function by electrografting a 4-ethynyl phenyl diazonium salt under reduction potential applied through CV. The diazonium derivative was generated in situ from the 4-ethynyl aniline by adding hydrochloric acid and sodium nitrite to the solution. Voltammograms in Figure S4 show typical grafting behavior with diazonium salts. The first scan (in blue) is characteristic of diazonium salt reduction. The negative peak at −0.153 V vs SCE corresponds to 4-ethynylphenyl diazonium salt reduction at the electrode to form radicals that binds covalently to carbon atoms on the self-organized 3D graphene surface.38,54 The subsequent two cycles show the passivation of the electrode by the grafted layer that blocks access to the surface of the diazonium salts.54 This electrochemical behavior is characteristic and indicates that the graphene electrode surface was well functionalized with 4-ethynylphenyl groups. Similar results with 4-ethynylaniline were obtained on glassy carbon and pyrolytic graphite electrodes by Limoges and co-workers.36 The second step (step 2, Figure 4) is a CuI-catalyzed Huisgen 1,3-dipolar cycloaddition between the 4-ethynylphenyl grafted electrode and Fc-Azide. This redox probe was chosen as a model to optimize the experimental conditions of grafting and to prove that our strategy is an easy method for 3D graphene functionalization. The attachment of the ferrocene group after electrografting and click reaction was studied using CV at various scan rates (Figure 5). The bell-shaped voltammogram at a slow scan rate (Figure 5) is a qualitative indication that the ferrocene is tethered to the electrode. The analysis of peak currents as a function of

analyses. Figure 2c,d shows typical Raman mapping of the 2D/G intensity ratio from all the area shown in Figure 2a, and typical Raman spectra at different spots of the sample, respectively. The intensity of D peak at ∼1361 cm−1 is relatively high with respect to G peak intensity, which indicates a disordered sp2 carbon structure. From the integrated intensity ratio ID/IG,46 we estimate the average size of the sp2 domain to be ca. 7 nm, which corresponds to a defect density of 0.6 × 1012 cm−2. Interestingly, the presence of a single symmetric 2D mode at ∼2728 cm−1 with a 2D/G intensity ratio of ∼0.6 indicates the formation of turbostratic multilayer graphene.34,46,47 The formation of multilayer graphene was observed over the whole surface of the sample with good homogeneity, as confirmed by Raman mapping (Figure 2c). Surface texture is an important issue when the main interest is to investigate the electrochemical properties and functionalization of materials.5,6,17,48,49 In particular, it is worth noting that defects such as chemically active sites and microscale roughness could improve the electrochemical performance of electrodes.5,6,49 The self-organized 3D structure of the rough electrodes is relevant for fast mass and electron transport kinetics, owing to the combination of 3D porous structures and the excellent intrinsic properties of graphene. The electrochemical properties of 3D graphene electrodes, as described previously, were studied by CV. The electron transfer of a ferrocene dimethanol (Fc(CH2OH)2) redox probe on graphene exhibited a reversible process (ΔEp = 59 mV and Ip/Ia = 1) for scan rates below 20 mV/s for our 3D textured graphene. For higher scan rates, electron transfer kinetics becomes quasi-reversible (ΔEp > 59 mV and Ip/Ia = 1) (for more details, see Figure 3 and Table S1). To evaluate electron transfer kinetics on the self-organized 3D graphene, we calculated the heterogeneous standard rate constant of the electron transfer k°. The Nicholson method50,51 was applied using 6.4 × 10−6 cm2 s−1 as the value of the diffusion coefficient of Fc(CH2OH)2, as reported in the literature,52 and assuming that the electron transfer coefficient (α) was equal to 0.5. Full details of the Nicholson method can be found in the Supporting Information (Table S2 and Figure S3). For scan rates beyond 20 mV s−1, 3D graphene electrodes showed a 1427


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Figure 4. Illustration of the two-step functionalization of a 3D graphene electrode: (Step 1) Electrochemical reduction through cyclic voltammetry of in situ generated diazonium salts for covalent immobilization of 4-ethynylphenyl moieties; (Step 2) CuI-catalyzed Huisgen 1,3-dipolar cycloaddition between the immobilized alkyne functions and the ferrocene derivatives bearing the corresponding azide group; CuI was obtained by reducing CuII with ascorbic acid.

modified by similar techniques using aryl diazonium salts and click chemistry to link ferrocene to the electrode. This high amount of grafted molecules presents the advantage of higher sensitivity for further sensing applications. The heterogeneous rate constant (kET) between the grafted ferrocene and the electrode was calculated using the Laviron analysis56 by plotting E°′ − Ep vs ln (υ) (Supporting Information Figure S5). A kET of 0.4 s−1 was determined, which is quite slow compared to the kET obtained by Liu et al. on glassy carbon modified with a ferrocene probe57 via diazonium chemistry. Nevertheless, in this work the ferrocene was linked to the aryl groups through a peptide bond instead of using click chemistry. The obtained kET could be explained by the multilayer grafting (see X-ray Photoelectron Spectroscopy section) which increases the distance of redox probes from the electrode.58 The decrease of graphene conduction properties may be also envisaged. This effect is induced by the diazonium grafting strategy that changes the hybridization of some carbon atoms from sp2 to sp3. This introduces a band gap opening and generates insulating and semiconducting regions in our functionalized 3D self-organized graphene.38 To conclude, we obtained high loading of the ferrocene redox probe with a stable signal over time, with quite slow electron transfer kinetics of the grafted redox probe. This last property is probably due to the grafting reaction. SEM and Raman spectroscopy were performed before and after functionalization (Figure S8). No structural changes on the 3D graphene electrode were observed, which is a strong indication that the structural and intrinsic properties inside the 3D multilayer graphene are preserved after grafting. In order to confirm that our functionalization steps could be used for the electroaddressing of probes on a multielectrode array, control experiments were performed. As shown in Figure 6, the CV of a bare self-organized 3D graphene electrode and a control experiment, where no potential was applied during step 1 (Figure 4), are similar and exhibit no ferrocene peak after removing any physisorbed species. This result confirms the selectivity of the method which allows one electrode from a multielectrode device to be specifically

Figure 5. Cyclic voltammograms of a ferrocene-functionalized 3D graphene electrode in 0.1 M NaClO4 aqueous electrolyte at various scan rates from 0.05 to 1 V s−1. (Inset) Faradaic peak current as a function of scan rate.

scan rate showed a linear relationship which indicated that ferrocene was bound to the electrode (inset Figure 5). The high signal stability over time (20% loss of ferrocene signal over a period of 22 days) (Figures S6 and S7) was another strong indication of covalent grafting of ferrocene at the electrode. The ferrocene coverage (Γ in mol cm−2) on the graphene electrode was estimated from the charge average (Q) of the anodic and cathodic peaks on the voltammograms and by assuming a one-electron transfer in accordance with the following relationship (eq 1):55 Γ=

Q nFS

(1)

in which n = 1 is the number of electrons involved during the redox event, F is the Faraday constant, and S is the area of the exposed graphene electrode (0.07 cm2). The ferrocene coverage was estimated to be 4.9 × 10−10 mol cm−2. This is twice the amount grafted on BDD electrodes37 and 1.5 higher than on glassy carbon electrodes36 1428


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of N 1s peak intensity, shown in Figure 7b, confirms the presence of nitrogen. A line-scan of Fe 2s and Cu 2p3/2 peak intensities across this area was also performed. The Fe 2s peak was chosen instead of the more intense Fe 2p peak in order to avoid any misinterpretation because of the presence of Cu Auger transitions interfering with Fe 2p. These line-scans presented in Figure 7c unambiguously indicate that Fe and Cu were detected only in the exposed area. These elements can be associated with ferrocene moieties and residual traces of catalyst, respectively. The chemical states of Fe and N were investigated by performing high energy resolution spectra of N 1s and Fe 2p peaks. The N 1s peak spectra of Fc-Azide deposited on a silicon wafer and of a 4-ethynylphenyl modified graphene sample after the CuI-catalyzed click reaction with Fc-Azide are presented in Figure 8. Each experimental spectrum was fitted using synthetic components consisting of Gaussian−Lorentzian product functions with a mixing ratio of 30% for which the position in binding energy, the full width at half-maximum and the proportion are presented in Figure 8. In the case of a Fc-Azide moiety deposited on a silicon wafer, three components, which are assigned to −NN+N− at 400.9 eV, −NN+N− at 402 eV and −NN+N− at 403 eV, were used to fit the N 1s peak. The component at 403 eV, which corresponds to the central electron deficient N atom in the azido group as supported by XPS data from the literature,59 is a good indicator of the Fc-Azide physisorption. This component does not appear in the N 1s spectrum of the 4-ethynylphenyl modified graphene sample after the CuI-catalyzed click reaction, as illustrated in Figure 8b. After the click reaction, the Fc-Azide moiety is covalently bounded with no evidence of physisorption, leaving two components in the N 1s signals associated with N−NN at 400.7 eV and N−NN at 401.7 eV in the triazole ring. These components are separated by 1 eV which is in accordance with the densityfunctional theory calculation and XPS data reported on sp2 N atoms in a metal-free phtalocyanine aromatic compound.60 Moreover, the ratio of component area is close to the 1:2 expected ratio in such a structure after cycloaddition of azides with the 4-ethynylphenyl modified graphene surface. These XPS results confirm that a covalent bound links Fc-Azide and the 4-ethynylphenyl modified 3D graphene electrode via the formation of a 1,2,3-triazole ring, as demonstrated through electrochemical data. Further analysis of the high energy resolution spectra of the Fe 2p peak was performed (Supporting Information) in order to control the ferrocene signature. One can conclude that the iron signature can only be assigned to ferrocene attached to the 3D graphene electrode and excludes any ferrocene decomplexation phenomena or iron oxide at the electrode surface. When the Fe3+/Fe2+ ratio between the physisorbed Fc-Azide on silicon wafer and ferrocene functionalized 3D graphene electrode are compared, a significant difference in their oxidation state is apparent. Fe2+ is predominant on the physisorbed silicon (Figure S9) and this is consistent with the initial chemical state of iron in Fc-Azide. On the ferrocene functionalized 3D graphene electrode, the ferrocene complex presents a ratio of Fe3+ and Fe2+ close to 0.5 indicating an oxidation process. The explanation for this effect has been described by Zanoni et al.:61 it is due to electrochemical aging which can occur after an increasing number of voltammetric cycles. The oxidation state Fe3+ originated from the substrate-assisted redox process where the overall +1 charge of the surface complex is neutralized by the presence of surface O− groups.

Figure 6. Voltammograms in 0.1 M NaClO4 at 0.1 V s−1 of a 3D graphene: (red) bare 3D graphene before electrografting and click reaction procedure, (black) Fc-modified self-organized 3D graphene performed by electrografting and click reaction procedure, and (blue) control experiment that consists of carrying out the two modification procedures without applying any potential during Step 1 (Figure 4).

modified by applying a reduction potential to the electrode. The sequential modification of each electrode with different probes can be envisioned. To highlight the functionalization of the graphene-based substrate by electrochemical grafting of a ferrocene derivative, we performed chemical surface analyses using X-ray photoelectron spectroscopy. Chemical mapping of F, Fe, N, and Cu were first carried out as preliminary analyses to localize and characterize the area of 3D graphene electrode exposed inside the electrochemical cell. The fluorine signal was followed to visualize the location of the polytetrafluoroethylene (PTFE) ring, defining the exposed area, while the presence of iron, nitrogen and copper are indicators of the surface functionalization. The results of chemical mapping are illustrated in Figure 7. The mapping

Figure 7. Surface chemical mapping of F, Fe, N and Cu: (a) mapping of the intensity of F 1s peak allowing the position of the PTFE ring to be localized, (b) mapping of the intensity of N 1s indicating that the click reaction occurred at the center of the exposed area, and (c) line-scan of the Cu 2p3/2 and Fe 2s peak intensities across the exposed area showing the presence of ferrocene moieties and residual traces of catalyst.

of F 1s peak intensity (Figure 7a) distinctly shows rings due to the contact of the PTFE seal with the surface of the sample. The analysis highlights three rings corresponding to three different functionalized zones of the graphene-based substrate. The click reaction successfully occurred in the exposed area as the mapping 1429


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Figure 8. XPS narrow scans of N 1s for (a) Fc-Azide moieties deposited on a silicon wafer and (b) Fc-Azide linked to a 4-ethynylphenyl modified 3D graphene electrode. Open circles are experimental curves. The fit (bold line) using synthetic components whose position “E”, full width at halfmaximum (fwhm), and proportion in the N 1s peak “%” are presented at the top right corner of each N 1s spectrum.

Figure 9. (a) Determination of the in-depth profile of nitrogen of the graphene-based sample, electrochemically grafted with ferrocene, the inelastic background being modeled with QUASES software (details of the procedure used are presented in the Supporting Information). (b) Schematic illustration of the possible distribution of 4-ethynylphenyl moieties at different depths for the click reaction of ferrocene molecules, explaining the in-depth profile of nitrogen.

method is not sensitive to roughness, unlike conventional angular XPS analysis. Details of the analysis are presented in the Supporting Information. The results of the analysis are presented

To determine the depth distribution of the grafted ferrocene in one or several layers, we analyzed the N 1s spectrum using the Tougaard inelastic electron background analysis62,63 because this 1430


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ACS Applied Materials & Interfaces in Figure 9. The best fit (Figure 9a) is obtained for a distribution of nitrogen corresponding to a layer of 1.7 nm buried at a depth of 1 nm. The estimated thickness of this layer suggests multilayer grafting of ferrocene molecules, assuming that one monolayer is 0.68 nm thick. Furthermore, the distance between the carbon on the top of the ferrocene complex and the nitrogen in the triazole ring is assessed to be between 0.67 and 0.94 nm depending on the nitrogen atom chosen, which is in accordance with the burying depth. These theoretical calculations are obtained from known interatomic bond lengths and molecule geometry. As shown in Figure 9b, the building of a structure with more than one monolayer probably occurred during the first step of the electrochemical reduction of diazonium salts for the covalent immobilization of 4-ethynylphenyl molecules, leaving a distribution of sites at different depths for the click reaction of ferrocene molecules. To summarize, XPS analyses not only confirm the electrochemical data about the covalent bonding of ferrocene via triazole linkage but also give some indications about the aging process occurring in the ferrocene functionalized 3D graphene electrode. In-depth XPS analyses highlight a nitrogen distribution within a thickness of 1.7 nm which can be explained by a multilayer electrografting process.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: fl[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Agence Nationale de la Recherche (ANR) and the University of Lyon for financial support of the POLCARB project (Projet d’Avenir Lyon St-Etienne (ANR-11IDEX-0007-02)). Lyon Ingénierie Projet was very helpful for reporting. We also warmly thank Mohamad Hijazi for synthesizing the Fc-Azide and Stephanie Reynaud and Nadège Ollier for SEM facilities.

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CONCLUSIONS The development of original self-organized 3D graphene electrodes from a solid-state carbon source by pulsed laser deposition method with thermal annealing was reported. Cyclic voltammetry revealed the excellent performances in electrochemical kinetics of this carbonaceous substrate. Electron transfer kinetics obtained with a solution of Fc(CH2OH)2 showed a quasi-reversible process. We also demonstrated the successful and robust attachment of ethynyl aryl groups on the surface of the sample, paving the way for the specific attachment of molecules bearing an azide function by using the click reaction. The method was applied to ferrocene-azide in order to model the grafting of redox molecules on this kind of substrate. The electrochemical response of the functionalized electrode was studied. We confirmed that our addressing method was efficiently controlled by electrochemistry, showing a high loading of ferrocene and a stable electrochemical response of the electrode over 22 days. The grafting was unequivocally confirmed by AFM, SEM, electrochemistry, and XPS. The quantity of grafted molecules was higher than on other often used substrates such as glassy carbon or BDD due to a multilayer grafting structure, as confirmed by in-depth XPS measurements. High loading of recognition elements on the electrode will thus be possible. This method allows electrochemically controlled functionalization for addressing probes on a multielectrode device. This work opens highly promising perspectives for the development of self-organized 3D graphene electrodes with various sensing functionalities and may be applied to fragile sensing objects such as biomolecules or living systems.

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for kET determination in diffusionless electrochemical systems, stability of the ferrocene modified 3D graphene electrode, high energy resolution spectra of Fe 2p peak, XPS inelastic electron background analysis of the in-depth distribution of nitrogen. (PDF)

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ABBREVIATIONS AFM, atomic force microscopy BDD, Boron-doped diamond CV, cyclic voltammetry CVD, chemical vapor deposition fwhm, full width at half-maximum GO, graphene oxide ODN, oligodeoxyribonucleotides PTFE, polytetrafluoroethylene rGO, reduced graphene oxide SEM, scanning electron microscopy TCSC, thermal conversion of solid carbon feedstocks XPS, X-ray photoelectron spectroscopy 3D, three-dimensional REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10647. Characterization of the bare self-organized 3D electrodes, Nicholson Method for k° determination in diffusive system, electrochemical grafting of 4-ethynylaniline on self-organized 3D graphene electrodes, Laviron analysis 1431


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