A Pyrene-Substituted Tris(bipyridine)osmium(II) Complex as a

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A pyrene-substituted tris-bipyridine osmium(II) complex as a versatile redox probe for characterizing and functionalizing carbon nanotube- and graphene-based electrodes ALAN LE GOFF, Bertrand Reuillard, and Serge Cosnier Langmuir, Just Accepted Manuscript • DOI: 10.1021/la401712u • Publication Date (Web): 17 Jun 2013 Downloaded from http://pubs.acs.org on June 17, 2013

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A pyrene-substituted tris-bipyridine osmium(II) complex as a versatile redox probe for characterizing and functionalizing carbon nanotube- and graphene-based electrodes Alan Le Goff, Bertrand Reuillard and Serge Cosnier Département de Chimie Moléculaire UMR-5250, ICMG FR-2607, CNRS Université Joseph Fourier, BP-53, 38041 Grenoble, France [email protected]

ABSTRACT. We report the functionalization of nanostructured graphene-based electrode with an original

(bis-(2,2’-bipyridine)

(4,4’-bis(4-pyrenyl-1-ylbutyloxy)-2,2’-bipyridine]

osmium(II)

hexafluorophosphate complex bearing pyrene groups. Graphene oxide (GO) and chemically-reduced graphene oxide (c-RGO) paper electrodes were prepared by flow-directed filtration method. After film transfer via soluble membrane technique, the homogenous and stable GO electrode was electrochemically reduced in water to achieve electrochemically reduced graphene oxide (e-RGO) film on the electrode. The electrochemical properties of GO, c-RGO and e-RGO electrodes were characterized by Scanning Electron Microscopy and electrochemistry. Cyclic voltammetry of Ru(NH3)62+/3+ redox probe underlines the important influence of the RGO preparation method on electrochemical properties. We finally achieved the flexible functionalization of graphene-based electrodes using either supramolecular binding of the Os(II) complex bearing pyrene groups or its ACS Paragon Plus Environment

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electropolymerization via the irreversible oxidation of pyrene. The properties of these functionalized graphene paper electrodes were compared to glassy carbon (GC) and Multi-Walled Carbon Nanotube (MWCNT) electrodes. Thanks to its divalent binding sites, the Os(II) complex constitutes a useful tool to probe the π-extended graphitic surface of RGO and MWCNT films. The Os (II) complex interacts strongly via non covalent π-π interactions, with π-extended graphene planes, thus acting as a marker to quantify the electroactive surface of both MWCNT and RGO electrodes and to illustrate their ease of functionalization.

KEYWORDS (Word Style “BG_Keywords”). Graphene oxide, reduced graphene oxide, polypyrene, carbon nanotubes, osmium complexes, electropolymerization, metallopolymers.

1. Introduction Surface modification of electrodes is a continuous subject of research where one of the objectives is to confer novel properties to an electrode material by combination with molecule having particular electrochemical behavior. One important application of surface functionalization is molecular (bio)electrocatalysis where a transition metal complex having electrocatalytic properties or able to mediate bioelectrocatalytic reactions is attached to an electrode by covalent or non covalent binding. In particular, advanced graphitic nanomaterials, i. e. carbon nanotube and graphene, have an ideal combination of exceptional conductivity, high stability, and high specific surface that make these materials highly suitable for molecular electrochemistry, especially for electrocatalytic applications1. More importantly, their reactivity towards covalent or non covalent functionalization is envisioned for the fabrication of novel redox nanomaterials. Carbon nanotubes (CNTs), tubes of enrolled graphene sheets, have proven to have excellent electrochemical properties as electrode materials. Thanks to a combination of high-three dimensional surface and high conductivity, CNTs constitute an excellent alternative in the design of modified electrodes with high surface concentrations of transition metal complex for electrocatalysis2,3 and bioelectrocatalysis4. Transition metal complex are immobilized on


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CNT sidewalls by covalent binding5, electrografting of aryldiazonium salts3,6, electropolymerization of functionalized pyrroles and pyrenes7,8 or the supramolecular binding of pyrene onto MWCNT sidewalls2,6,9. The latter technique takes advantages of supramolecular π-interactions between pyrene πextended system and the external graphene walls of CNTs without damaging the CNT electronic properties and hence the conductivity of CNT deposits. The recent developments in the electrochemistry of graphene and graphene oxide represent a novel alternative for carbon-based electrode materials10–13. The electrochemistry of graphene-based electrodes is a growing subject of research in order to understand the electron transfer kinetics at this type of nanomaterials13–15. On the other hand, different techniques have been investigated for the fabrication of graphene-based electrodes. To circumvent, the disadvantages of the widely-used drop-coating method, Ruoff et al show, as it was demonstrated for CNTs, the control over graphene oxide sheet deposition via vacuum filtration process.16 This technique allows the controlled assembly of graphene oxide into graphene oxide paper. Later, these techniques were also developed for reduced graphene oxide.17 While developments in the functionalization strategies of graphene took advantages of the deep investigations of functionalization of carbon nanotubes18, the surface modification by transition metal complex and it associated electrochemistry are still in its early stages19–22. Among these studies, πstacking interactions between graphene sheets and a Cobalt(II) bisterpyridyl complex bearing three pyrene groups were recently investigated by Abruna et al that demonstrate strong Van der Walls interactions of 38 kJ mol-1 between graphene plane and Co(II) complex21. Osmium(II) complexes arouse a growing interest as redox mediators for enzymatic electrocatalysis in biofuel cells and biosensors4,23. The main reason is their well-defined electrochemical behavior associated with the high stability of the Os(II)/Os(III) trisbipyridyl redox states. Moreover, the potential of the Os(II)/Os(III) redox system can be easily modulated by modification of the surrounding ligands, allowing the complexes to wire both copper enzymes for oxygen reduction and glucose oxidases for glucose oxidation24. As a consequence, their immobilization onto electrodes, in particular carbon


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nanotubes, has been a constant field of research in the design of novel modified electrodes for biofuel cell applications25. In this work, the synthesis of an original [tris-(2,2’-bipyridine) (4,4’-bis(4-pyrenyl-1-ylbutyloxy)-2,2’bipyridine] osmium(II) hexafluorophosphate complex bearing pyrene groups was achieved to further investigate and functionalize the surfaces of advanced carbon electrodes either by supramolecular πstacking interactions or electropolymerization. We describe the surfactant-free preparation of GO, chemically-reduced GO (c-RGO) and MWCNT electrodes by filtration of aqueous dispersion of commercial GO nanosheets and MWCNTs respectively and subsequent transfer of respective nanostructured films on glassy carbon (GC) electrodes. In contrast to conventional procedures, this technique, a modified procedure of Reynolds et al26, has three major advantages: (i) no need of surfactant that could disturb the electrochemical experiments27, (ii) reproducibility and homogeneity of coatings generated on electrode surfaces, (iii) flow-directed assembly of the nanostructured film through vacuum filtration16. Taking into account reduced graphene oxide (GO) electrodes can be elaborated by soft electrochemical reduction of GO films in aqueous28,29 or organic media30, we investigated the electrochemical reduction of GO films to obtain electrochemically-reduced GO (e-RGO) electrodes. Thanks to its two pyrene groups, the Os(II) complex is able to strongly interact with both graphene- and CNT-based electrodes and to undergo oxidative electropolymerization. The idealistic electron transfer kinetics of Os(II)/Os(III) redox system represents a useful tool to investigate the electron transfer properties of these different functionalized nanomaterials.

2. Experimental Acetonitrile (HPLC grade) used for electrochemistry measurements was obtained from Rathburn and used without further modification. Tetrabutylammonium perchlorate (Fluka) [Bu4N]ClO4 (TBAP) was used as supporting electrolyte in organic media. All reagents and chemicals products purchased from Aldrich were of reagent grade quality and used as received unless it is mentioned. NMR spectra were ACS Paragon Plus Environment

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recorded on a Bruker AVANCE 400 operating at 400.0 MHz for 1H. ESI mass spectra were recorded with a Bruker APEX-Qe ESI FT-ICR mass spectrometer. UV-visble spectra were recorded with a carry 1 spectrophotometer with a quartz cuvette (1 cm depth). 2.1. Electrochemistry measurements The electrochemical experiments performed in MeCN were carried out in a three-electrode electrochemical cell under dry argon atmosphere and in a glove box ([O2] 95 %,) were obtained from Nanocyl. Graphene oxide and chemically-reduced graphene oxide were obtained from Nanoinnova Technologies. Carbon nanomaterials were used as received without any purification step. MWCNT GO and RGO films were prepared using a modified procedure from Wu et al26 and Dikin et al.16 MWCNTs and GO (10 mg) were dispersed in pure water (250 mL) and sonicated during 30 min. The solution was carefully decanted overnight and the remaining transparent supernatant (100 mL) was then filtered over cellulose nitrate filter (Sartorius, 0.45 µm, ∅ 3.5 cm) resulting in the deposition of MWCNTs and GO films. The obtained membrane was deposited on a GC electrode (surface area 0.07 cm-2) and carefully dissolved by several washings with acetone. According to a previously reported electrochemical procedure28, e-RGO electrodes were prepared by electrochemical reduction of GO electrodes performed by cyclic voltammetry in 0.1M Na2SO4 (10 cycles at 100 mV.s–1 between 0 and 1.5V vs SCE (figure S1).

Functionalization of electrodes with Os(II) complex For the supramolecular binding of Os(II) complex, electrodes were incubated for 30 min in a MeCN solution containing 0.4mM of Os(II) complex and thoroughly washed with MeCN and acetone. For the electropolymerization experiments, all electrodes were used as working electrodes in a threeelectrode cell in the presence of Os(II) complex (0.2mM) in degassed and anhydrous MeCN 0.1M ACS Paragon Plus Environment

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Bu4NClO4 (TBAP) solution. Electropolymerization was performed through oxidation of pyrenes either using controlled potential electrolysis at 0.9V vs Ag/AgNO3 or cyclic voltammetry (recording 20 cycles at 100 mV.s–1 between 0 and 1.1V vs Ag/AgNO3). The electrodes are then rinsed with MeCN and acetone.

3. Results and discussions 3.1. Fabrication and electrochemistry in water of GO, c-RGO and e-RGO electrodes GO and RGO electrodes were prepared in several steps. First, commercial and well-characterized GO and RGO nanosheets (Nanoinnova) were dispersed in water by 30min-ultrasonication and filtrated onto a cellulose ester membrane. This allows the formation of a homogeneous bucky paper having characteristic brown and black colors for GO and RGO film respectively (figure 1).

Figure 1. (A) SEM Micrograph of a GO electrode (insert) SEM micrograph of a cross section of the GO film ; (B) SEM Micrograph of a c-RGO electrode.


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The GO and RGO electrodes were finally obtained after transferring the respective film onto a GC electrode by dissolution of the cellulose ester membrane. These electrodes were characterized by SEM. According to SEM micrographs from figure 1, this fabrication process gives access to flow-directed assembly of graphene oxide nanosheets into a homogenous and stable GO “bucky” paper16 (figure 1A). Considering the high hydrophobicity of RGO nanosheets, the RGO film has a very different morphology: The RGO flakes are heterogeneously oriented and form high specific surface graphitic material thanks the crumpled morphology of RGO flakes. e-RGO electrodes were subsequently produced by electrochemical reduction in 0.1 M Na2SO4 as reported by Y. Shao et al28 (figure S1). Figure S1 shows the successive reduction scans performed at GO electrodes. The capacitive current increase after each scan arises from the high electroactive area of the as-formed nanographite surface and from the intercalation of electrolyte during the reduction of GO, preventing from stacking of graphene nanosheets28. This technique allows the electroreduction of GO surface oxides and the fabrication of e-RGO electrodes having high capacitance and enhanced electron transfer kinetics28. SEM images shows the conservation of the orientation of previously-deposited GO nanosheets. The electrochemical properties of GO and e-RGO electrodes were investigated by employing a redox probe having well-characterized outer-sphere electron transfer mechanism, Ru(NH3)62+/3+ (figure 2).


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Figure 2. Cyclic voltammetry of a 1mM solution of Ru(NH3)2+/3+ at (grey line) GC, (red line) GO, (blue line) e-RGO and ( black line) c-RGO electrodes in 0.1M KCl (v = 20mV s-1).

The insulating properties of GO films were confirmed by the negligible current recorded at GO electrode in a 1 mM solution of Ru(NH3)6Cl3. e-RGO electrodes exhibits high capacitive and faradaic currents compared to GO electrodes. These features confirms that the electrochemical reduction of GO electrodes lead to the recovery of high conductivity between reduced GO nanosheets while increasing electron transfer properties of e-RGO film compared to GO. This is consistent with the removal of oxides on the surface of multi-layered GO nanosheets and the formation of e-RGO28–30. In contrast to GO electrodes, well-defined redox systems, with ΔEp = 150mV and 80 mV, were respectively observed for e-RGO and RGO electrodes. CV with ΔEp larger than idealistic 56.4mV32 is consistent with the electrochemical properties of graphene-based electrodes recently investigated by C. E. Banks et al13. They showed that graphene-based electrode exhibited both edge plane and basal plane sites at the surface of the electrode (ΔEp = 122mV) that was responsible for a mixed electrochemical behaviour between basal and edge plane pyrolytic graphite. This is the reason why e-RGO electrodes exhibit high ΔEp values since basal planes might be statistically favored by the horizontally-oriented deposition of GO nanosheets, as observed on figure 1A. e-RGO would therefore behave as oriented pyrolytic graphite. On the contrary, c-RGO electrodes ACS Paragon Plus Environment

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exhibits electron transfer kinetics similar to those of GC electrodes (ΔE = 80mV) since edge planes sites are much more observed on the surface of the graphitic electrode (figure 1B). Another reason for this lower value may be the porosity of c-RGO film (illustrated by SEM images from figure 1). The latter could be at the origin of occurrence of a thin layer process. As a consequence, semi-infinite planar diffusion model is not sufficient to describe electron transfer processes at these porous electrodes. These effects have been already investigated at carbon nanotube-33 and graphene-based electrodes34. Both RGO and e-RGO exhibit high capacitive response, which is characteristic of graphene-based electrodes. RGO have especially four-time higher capacitive response compared to e-RGO and 16-time higher compared to GC electrode, according to CV scans performed at 10mV s-1. The electrochemistry of these electrodes was further investigated in non aqueous media.

3.2. Reactivity towards [bis-(2,2’-bipyridine) (4,4’-bis(4-pyrenyl-1-ylbutyloxy)-2,2’bipyridine] osmium(II) hexafluorophosphate complex. We further investigate the functionalization of GO and RGO electrodes using a trisbypridyl osmium complex bearing two pyrene groups. [bis-(2,2’-bipyridine) (4,4’-bis(4-pyrenyl-1-ylbutyloxy)-2,2’bipyridine] osmium(II) hexafluorophosphate complex was prepared by refluxing [Os(bpy)2Cl2] with previously synthesized 4,4’-bis(4-pyrenyl-1-ylbutyloxy)-2,2’-bipyridine9. The complex was fully characterized by 1H and 13C NMR, mass spectroscopy and UV-visible spectrophotometry. Thanks to the two pyrene groups, this Os(II) complex is able to strongly interact via π-π nteractions with graphene surfaces of GO, e-RGO but also MWCNTs. This complex was therefore used to interrogate and functionalize the surfaces of these advanced carbon electrodes. Figure 3 presents a schematic representation of the organometallic complex interacting with graphene.


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Figure 3. Schematic representation of the interactions between Os(II)-pyrene and graphene ; Cyclic voltammograms in MeCN-TBAP (0.1 M) of e-RGO, MWCNT and c-RGO electrode functionalized by supramolecular π-stacking of Os(II) complex in MeCN 0.1M TBAP (v = 100 mV s-1).

We compared GO and e-RGO electrodes with MWCNT electrode. The MWCNT electrode was prepared by the technique developed by Reynolds et al26, very similar to the preparation of GO electrodes16. A dispersion of MWCNTs in water is filtrated and the as-formed MWCNT film is transferred on GC electrode. On GC electrode, the Os(II) complex presents a well-defined monoelectronic reversible peak system at E1/2ox = 0.39V and three redox couples at E1/2red= -1.60, -1.80 and -2.12V corresponding to the successive one-electron reduction of the three bipyridine ligands (figure S1). The reduction of bipyridine ligands was not further investigated at these electrodes. An irreversible oxidation is observed at Epox = 0.8V and corresponds to the irreversible oxidation of the two pyrene groups The ability of these different graphitic carbon electrodes for developing π-π interactions was examined by transferring the electrodes in an Os(II)-free solution after incubating the electrodes for 30 minutes in a Os(II) 1mM solution. After four washing steps, the presence of the monoelectronic redox peak system at E1/2ox = 0.35V confirms the immobilization of Os(II) complex on all graphene-based


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electrodes by supramolecular π-stacking interaction (figure 3). The immobilization of the redox specie was further assessed by linear dependence of anodic and cathodic peak current towards scan rates. Surface concentrations were calculated by integration of the charge under the oxidation peak. The number of equivalent layers of Os(II) complex on the different electrodes was estimated by using the hard sphere model, taking into account that one monolayer of closely-packed metal trisbipyridyl complex would correspond to ca 8 x 10-11 mol cm-2 35. Important surface concentrations were measured for c-RGO and MWCNT electrodes in the same order of magnitude (70 and 56 Os(II) layers respectively). This mainly arises from the high specific surface of both nanomaterials, 180m2 g-1 for MWCNTs and 100m2 g-1 for c-RGO. Furthermore, this also reflects the expected strong interactions between graphene planes or sidewalls with pyrene groups. GC electrode showed no adsorption properties and e-RGO surfaces showed poor adsorption properties towards Os(II) complex. On e-RGO electrodes, the estimation of surface concentrations indicates that the complex covers approximately 20% of the surface of the electrode. This suggests inability to intercalate between graphene sheets. This also underlines a HOPG-like behavior for e-RGO electrodes. Upon reduction of GO film, the removal of oxides induces stronger interactions between as-formed e-RGO nanosheets, preventing the complex to intercalate between graphene nanosheets.

3.3. Electropolymerization and formation of an OsII polypyrene film Since pyrene groups undergo oxidative electropolymerization, we also investigate the electrosynthesis of a poly-[Os(II)-pyrene] on the different electrodes. The irreversible oxidation of pyrene is observed at 0.8V. Figure 4 shows the electropolymerization of Os(II) complex performed by successive scans above the oxidation of pyrenes in MeCN.


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Figure 4. Electrochemical polymerization of Os(II) complex (0.2 mM) in MeCN + 0.1 M TBAP by repeatedly scanning the potential (20 scans) from -0.2 to 0.9V on GC, e-RGO, c-RGO and MWCNT electrode (v = 100mV s-1).

At all electrodes, the formation of poly-Os(II)-pyrene film is corroborated by the increase of the current intensity for the monoelectronic oxidation of Os(II) while more and more metallopolymer precipitates on the electrode. In the case of c-RGO and MWCNT, the current increase after each scan is higher than those recorded on GC and e-RGO electrodes. After transfer of the modified electrodes in solution free of monomer, the presence of the Os(II)/Os(III) redox couple confirms the formation of insoluble Os(II) oligomers on the electrode surface (figure 5). In addition, this redox system exhibits a linear dependence of peak currents towards scan rate, which is characteristic of immobilized redox species.


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Figure 5. (up) Cyclic voltammograms in MeCN-TBAP (0.1 M) of e-RGO, c-RGO, and MWCNT electrode functionalized by (straight line) poly-[Os(II)-pyrene] and (dashed line) supramolecular π stacking of Os(II) complex in MeCN 0.1M TBAP (v = 100 mV s-1), (down) Graph of the number of Os(II) complex layers immobilized by (white) π-stacking interactions and (gray) electropolymerization on GC, e-RGO, c-RGO and MWCNT electrodes. The higher amount of immobilized Os complex by electropolymerization than by π-staking procedure, corroborates the fact that π-staking interactions favor the immobilization of a pure monolayer compared to electropolymerization which leads to the accumulation of several layers.


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Polymerization yields were estimated from chronoamperometric experiments. e-RGO and GC electrodes exhibit low polymerization yields, 1 and 4% respectively, compared to c-RGO and MWCNT electrodes, 18 and 30% respectively. GO and MWCNT electrodes show increased interactions with the Os(II) complex, as shown previously, that favors the electrodeposition of the metallopolymer. As previously demonstrated for π-stacking interactions, we summarized the number of layers of complex immobilized at the different carbon electrodes in the graph from figure 5. In the case of the polymer deposited on MWCNTs, SEM images performed before and after electrodeposition unambiguously confirmed the efficient formation of the poly-[Os(II)-pyrene] metallopolymer covering the surface of MWCNT walls (diameter 10 nm) with average thickness of 3 to 5 nm (figure 6Cand D). On the contrary, the thin polymer layer was difficult to observe on the planar eRGO surface or on the crumpled morphology of c-RGO (figure 6A and B).

Figure 6. (A) SEM Micrograph of a e-RGO electrode ; (B) c-RGO and MWCNT electrode (C) before and (D) after electrodeposition of the poly(Os(II)-pyrene.


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4. Conclusion These results underline the facile fabrication of both MWCNT and RGO electrodes using a soluble membrane technique and the flexible use of functionalized-pyrene molecules to modify carbon-based nanostructured materials by self-assembly of π-extended molecules and immobilization of redox species. Electrochemical reduction of GO electrodes produces RGO electrodes with very different electrochemical behavior compared to c-RGO electrodes, arising from different interactions between graphene nanosheets during film formation. The Os complex served as an excellent redox probe to characterize RGO surfaces in organic media showing the important contribution of π-stacking on the electrochemical properties of the different kinds of advanced carbon electrodes. By comparing the electrochemistry at these three different nanostructured carbon-based electrodes, two major observations can be made : (i) electrochemical reduction of GO films restores conductivity of graphene-based electrodes and greatly improves electron transfer kinetics; (ii) both MWCNT and c-RGO electrodes combine high electroactive area with strong interactions with pyrene-functionalized molecules and polymers. Pyrene groups revealed to be a versatile way of functionalization of nanostructured graphitic carbon electrodes either by supramolecular binding or electropolymerization. These functionalization techniques open ways in the fabrication of redox nanomaterials for bioelectrochemical applications. In this matter, graphene-based electrodes can be a promising alternative in terms of electrochemical properties, high electroactive surface and ease of functionalization.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. l) ACKNOWLEDGMENT The authors would like to acknowledge the platform ‘functionalization of surfaces and transduction’ of the scientific structure ‘Nanobio’ for providing facilities. The Région Rhones-Alpes is acknowledged for the PhD funding of B. Reuillard.

Supporting Information Available: Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES (1) McCreery, R. L. Advanced Carbon Electrode Materials for Molecular Electrochemistry. Chem. Rev. 2008, 108, 2646–2687. (2) Tran, P. D.; Le Goff, A.; Heidkamp, J.; Jousselme, B.; Guillet, N.; Palacin, S.; Dau, H.; Fontecave, M.; Artero, V. Noncovalent Modification of Carbon Nanotubes with Pyrene-Functionalized Nickel Complexes: Carbon Monoxide Tolerant Catalysts for Hydrogen Evolution and Uptake. Ang. Chem. Int. Ed. 2011, 50, 1371–1374. (3) Le Goff, A.; Artero, V.; Jousselme, B.; Tran, P. D.; Guillet, N.; Metaye, R.; Fihri, A.; Palacin, S.; Fontecave, M. From Hydrogenases to Noble Metal-Free Catalytic Nanomaterials for H2 Production and Uptake. Science 2009, 326, 1384–1387. (4) Holzinger, M.; Le Goff, A.; Cosnier, S. Carbon nanotube/enzyme biofuel cells. Electrochim. Acta.2012, 82, 179-190. (5) Callegari, A.; Cosnier, S.; Marcaccio, M.; Paolucci, D.; Paolucci, F.; Georgakilas, V.; Tagmatarchis, N.; Vazquez, E.; Prato, M. Functionalised single wall carbon nanotubes/polypyrrole composites for the preparation of amperometric glucose biosensors. J. Mater. Chem. 2004, 14, 807– 810. (6) Le Goff, A.; Moggia, F.; Debou, N.; Jegou, P.; Artero, V.; Fontecave, M.; Jousselme, B.; Palacin, S. Facile and tunable functionalization of carbon nanotube electrodes with ferrocene by covalent coupling and π-stacking interactions and their relevance to glucose bio-sensing. J. Electroanal. Chem. 2010, 641, 57–63.


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A Pyrene-Substituted Tris(bipyridine)osmium(II) Complex as a

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