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Functionalization of Glassy Carbon with Diazonium Salts in Ionic Liquids Paolo Actis,† Guilain Caulliez,† Galyna Shul,‡ Marcin Opallo,‡ Michel Mermoux,† Bernadette Marcus,† Rabah Boukherroub,§ and Sabine Szunerits*,†,§ Laboratoire d’Electrochimie et de Physicochimie des Mate´riaux et des Interfaces, CNRS-INPG-UJF, 1130 Rue de la Piscine, BP 75, 38402 St. Martin d’He`res Cedex, France, Institute of Physical Chemistry, Polish Academy of Sciences, Department of Electrode Processes, Ul. Kasprzaka 44/52, 01-224, Warszawa, Poland, and Institut de Recherche Interdisciplinaire (IRI, USR 3078) and Institut d’Electronique, de Microe´lectronique et de Nanotechnologie (IEMN, CNRS-8520), Cite´ Scientifique, AVenue Poincare´ - B.P. 60069, 59652 VilleneuVe d’Ascq, France ReceiVed August 1, 2007. ReVised Manuscript ReceiVed February 27, 2008 The paper reports on the chemical functionalization of glassy carbon electrodes with 4-bromobenzene (4-BBDT) and 4-(4′-nitrophenylazo)benzene diazonium tetrafluoroborate (4-NAB) salts in ionic liquids. The reaction was carried out at room temperature in air without any external electrical bias in either hydrophobic (1-butyl-3-methylimidazolium hexafluorophosphate) or hydrophilic (1-butyl-3-methylimidazolium methyl sulfate) ionic liquids. The resulting surfaces were characterized using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and electrochemical measurements. Electrochemical reduction of the terminal nitro groups allowed the determination of surface coverage and formation of an amine-terminated carbon surfaces. The results were compared to glassy carbon chemically modified in an aqueous solution in the presence of 1% sodium dodecyl sulfate (SDS) with the same diazonium salt. Furthermore, Raman spectroscopy coupled with electrochemical measurements allowed to distinguish between the reduction of -NO2 to -NH2 group and the -NdN- to -NHsNH- bond.
1. Introduction Carbon-based materials have become progressively more important, as their surfaces can be easily modified using chemical or electrochemical techniques. Since the first reports by Save´ant et al.1–3 on the covalent attachment of aryl groups by electrochemical reduction of the corresponding diazonium salt on carbonbased material, this attractive approach has been widely employed and studied in detail.4–20 Electrochemical reduction of diazonium ion derivatives in acetonitrile or acid solutions produces, next * To whom correspondence should be addressed: sabine.szunerits@ lepmi.inpg.fr, Tel: +33 4 76 82 65 52. † CNRS-INPG-UJF. ‡ Polish Academy of Sciences. § IRI, FRE 2963 and IEMN, CNRS-8520. (1) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant, J. M. J. Am. Chem. Soc. 1997, 119, 201. (2) Delamar, M.; De´sarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant, J. M. Carbon 1997, 35, 801. (3) Delamar, M.; Hitmi, R.; Pinson, J.; Save´ant, J. M. J. Am. Chem. Soc. 1992, 114, 5883. (4) Anariba, F.; DuVall, S. H.; McCreery, R. L. Anal. Chem. 2003, 75, 3837– 3844. (5) Blankespoor, R.; Limoge, B.; Schollhorn, B.; Syssa-Magale´, J.-L.; Yazidi, D. Langmuir 2005, 21, 3362. (6) Bourdillon, C.; Delamar, M.; Demaille, C.; Hitmi, R.; Moiroux, J.; Pinson, J. J. Electroanal. Chem. 1992, 336, 113. (7) Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038. (8) Brooksby, P. A.; Downard, A. J. J. Phys. Chem. B 2005, 109, 8791. (9) D’Amours, M.; Be´langer, D. J. Phys. Chem. B 2003, 107, 4811. (10) Downard, A. J. Electroanalysis 2000, 12, 1085. (11) Downard, A. J.; Prince, M. J. Langmuir 2001, 17, 5581. (12) DuVall, S. H.; McCreery, R. L. J. Am. Chem. Soc. 2000, 122, 6759. (13) Itoh, T.; McCreery, R. L. J. Am. Chem. Soc. 2002, 124, 10894. (14) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534. (15) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947–5951. (16) Lee, C.-S.; Baker, S. E.; Marcus, M. S.; Yang, W.; Eriksson, M. A.; Hamers, R. J. Nano Lett. 2004, 4, 1713. (17) Liu, Y.-C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254. (18) Liu, Y.-C.; McCreery, R. L. Anal. Chem. 1997, 69, 2091. (19) Saby, C.; Ortiz, B.; Champagne, G. Y.; Be´langer, D. Langmuir 1997, 13, 6805. (20) Saby, C.; Muret, P. Diam. Relat. Mater. 2005, 11, 851–855.
to N2, a highly reactive phenyl radical, which binds irreversibly to carbon surfaces such as glassy carbon (GC), highly ordered pyrolytic graphite (HOPG),1–3,17 carbon fibers and powders,2,21–23 pyrolyzed photoresist films,4 and carbon nanotubes,16,24,25 as well as sp3-hybridized diamond.26–28 The resulting organic films have been characterized using different analysis techniques, including electrochemical (electrochemical impedance spectroscopy and cyclic voltammetry),1–3,7,9–11 spectroscopic (XPS, FTIR, Raman),13,18,29,30 and scanning probe techniques.4,7,14,15 Downard at al. also reported on micro- and nanoscale patterning of carbon surfaces modified with aryl diazonium salts through soft31and scanning probe lithographic approaches.32 While the nature of the bonding between the carbon substrate and the so-obtained organic monolayer appears undoubtedly to be a covalent C-C bond, the reaction mechanism and the grafting surface sites are still debated questions. A faster reaction at plane edges than on basal graphite planes has been put forward,29 but the issue is complicated by multilayer formation for longer grafting times.15 At least five factors appear to influence the functional layer formed through electroreduction of aryl diazonium salts on (21) Bath, B. D.; Martin, H. B.; Wightman, R. M.; Anderson, M. R. Langmuir 2001, 17, 7032. (22) Downard, A. J.; Roddick, A. D.; Bond, A. M. Anal. Chim. Acta 1995, 317, 303. (23) Harnisch, J. A.; Gazda, D. B.; Anderegg, J. W.; Porter, M. D. Anal. Chem. 2001, 73, 3954. (24) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3823. (25) Balasubramanian, K.; Friedrich, M.; Jiang, C.; Fan, Y.; Mews, A.; Burghard, M.; Kern, K. AdV. Mater. 2003, 15, 1515. (26) Kuo, T.-C.; McCreery, R. L.; Swain, G. M. Electrochem. Solid State Lett. 1999, 2, 288–290. (27) Wang, J.; Firestone, M. A.; Auciello, O.; Carlisle, J. A. Langmuir 2004, 20, 11450–11456. (28) Yang, W.; Baker, S. E.; Butler, J. E.; Lee, C.-S.; Russell, J. N.; Shang, L.; Sun, B.; Hamers, R. J. Chem. Mater. 2005, 17, 938–940. (29) Ray, K.; McCreery, R. L. Anal. Chem. 1997, 69, 4680. (30) Anariba, F.; Viswanathan, U.; Bicoan, D. F.; McCreery, R. L. Anal. Chem. 2006, 78, 3104. (31) Downard, A. J.; Garret, D. J.; Tan, E. S. Q. Langmuir 2006, 22, 10739. (32) Brooksby, P. A.; Downard, A. J. Langmuir 2005, 21, 1672.
10.1021/la703714a CCC: $40.75 2008 American Chemical Society Published on Web 05/13/2008
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carbon substrates: (i) the electrolysis time, (ii) the applied potential, (iii) the concentration of the diazonium salt, (iv) the nature of diazonium salt, and (v) the type of carbon material used. On the basis of XPS and FTIR data, it was evidenced that the multilayer formation occurs by additional diazonium reduction and the subsequent radical attack of the initial monolayer.15,33 Alternative methods for the preparation of organic monolayers covalently bound to carbon surfaces through stable carbon-carbon bonds with a reduced tendency to form multilayers are considered necessary. The newly reported approaches are based on chemical rather than on electrochemical grafting processes.16,28,34–37 The reactions are carried out in solution phase in the presence of solvents. Recently, Tour and co-workers have developed a solvent-free functionalization of carbon nanotubes based on aryldiazonium reaction.38,39 It consists of in situ generation and decomposition of diazonium salts by the reaction of substituted aniline with alkyl nitrite. Room-temperature ionic liquids (ILs) are widely used as clean solvents and catalysts for green chemistry.40–45 Their properties such as hydrophobicity, hydrophilicity, and viscosity can be tuned by controlling the nature of the counteranion or the alkyl chain on the cation. The chemical stability and toxicity of the ionic liquid also depends on the anion used. While PF6- and BF4- can hydrolyze to HF, methyl sulfate anions have been shown to be more biodegradable.44,45 This new class of salts displays, along with low vapor pressure, thermal stability, high ionic conductivity, and remarkable solubility. These properties have been exploited for the functionalization of carbon materials. Indeed, the use of conventional ionic liquids (ILs) to exfoliate and functionalize single-walled carbon nanotubes with aryldiazonium salts was reported by Tour et al.46 The chemical reaction was conducted at room temperature in the presence of ILs and K2CO3. Independently, Liang et al. have synthesized a new class of stable, conductive, and hydrophobic room-temperature ILs based on diazonium salts. The liquid nature of the diazonium IL was exploited for the modification of carbon substrates by either thermal decomposition or electrochemical reduction of the diazonium ion.47 Herein, we have taken a similar approach for chemical functionalization of glassy carbon electrodes with bromo and nitro modified diazonium salts using conventional ionic liquids under mild conditions. The reaction was performed at room temperature for 1 h in 1-butyl-3-methylimidazolium methyl sulfate (hydrophilic, biocompatible) and in 1-butyl-3-methylimidazolium (33) Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. Langmuir 2005, 21, 280. (34) Pinson, J.; Podvorica, F. Chem. Soc. ReV. 2005, 34, 429. (35) Lud, S. Q.; Steenackers, M.; Jordan, R.; Bruno, P.; Gruen, D. M.; Feulner, P.; Garridao, J. A.; Stutzmann, M. J. Am. Chem. Soc. 2006, 128, 16884. (36) Senyange, S.; Anariba, F.; Bocian, D. F.; McCreery, R. L. Langmuir 2005, 21, 11105. (37) Chen, B.; Flatt, A. K.; Jian, H.; Hudson, J. L.; Tour, J. M. Chem. Mater. 2005, 17, 4832. (38) Dyke, C. A.; Tour, J. M. J. Am. Chem. Soc. 2003, 125, 1156. (39) Dyke, C. A.; Tour, J. M. Chem. Eur. J. 2004, 10, 812. (40) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. Chem. Phys. Chem. 2004, 5, 1107. (41) Roger, R. D.; Seddon, K. R. Ionic Liquids: industrial applications for green chemistry: ACS Symposium; American Chemical Society: Washington DC, 2002. (42) Ohtani, B.; Kim, Y.-H.; Yano, T.; Hashimoto, K.; Fujishima, A.; Uosaki, K. Chem. Lett. 1998, 953, 954. (43) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH: Weinheim, 2003. (44) Ranke, J.; Stolte, S.; Stormann, R.; Arning, J.; Jastorff, B. Chem. ReV. 2007, 107, 2183–2206. (45) Matzke, M.; Stolte, S.; Thiele, K.; Juffernholz, T.; Arning, J.; Ranke, J.; Welz-Biermann, U.; Jastorff, B. Green Chem. 2007, 9, 760. (46) Price, B. K.; Hudson, J. L.; Tour, J. M. J. Am. Chem. Soc. 2005, 127, 14867. (47) Liang, C.; Huang, J.-F.; Li, Z.; Luo, H.; Dai, S. J. Org. Chem. 2006, 586.
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hexafluorophosphate (hydrophobe, toxic) to yield organic layers covalently attached to the surface through C-C bonds. The results were compared with GC electrodes chemically modified with 4-(4′-nitrophenylazo)benzene diazonium tetrafluoroborate dissolved in 1% sodium dodecyl sulfate (SDS) aqueous solution. Replacing ionic liquids with SDS resulted in comparable surface modifications. The main interest in using ionic liquids instead of SDS for the chemical grafting of diazonium salts to carbonbased interfaces is, however, that the use of ionic liquids will allow the local structuring of carbon-based interfaces using simple spotting processes. Owing to the low vapor pressure of the organic solvent, ionic liquid micro- and nanodropelts (including the dissolved diazonium salt) can be deposited on the carbon interface simply by using cheap microspotters as routinely used in biology. A patterned surface for parallel sensing can be formed. The approach additionally allows the multifunctionalization of the surface. This is rather a challenge and has not been shown with more then two compounds in most cases. Aryl diazonium salts are easily and rapidly prepared from a wide range of aromatic amines in one step. The first step toward such arrays is to ensure that the diazonium chemistry works in ionic liquids, which is the focus in this paper. Photoelectron spectroscopy (XPS), Raman spectroscopy, and cyclic voltammetry were used to characterize the resulting layers. Raman spectroscopy coupled with chronoamperometic measurements at different potentials allowed in addition us to distinguish the reduction of the terminal -NO2 to -NH2 and of the internal -NdN- to -NHsNH- groups. These experiments are rather different from the experiment by McCreery13 who investigated the influence of the electrical field on the organic film. We convert electrochemically the organic layer and use Raman spectroscopy to see the induced electrochemical changes.
2. Experimental Section 2.1. Materials. 1-Butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium methyl sulfate, sodium dodecyl sulfate (SDS), acetonitrile, potassium ferrocyanide(II)trihydrate (K4Fe(CN)6 · 3H2O], 4-bromoaniline, fluoroboric acid, sodium nitrite, potassium chloride (KCl), hexamine ruthenium(III) chloride ([Ru(NH3)6]Cl3), and ethanol were purchased from Aldrich and used without further purification. 4-(4′-Nitrophenylazo)benzene diazonium tetrafluoroborate (4NAB) was prepared according to the previously reported method, dried under vacuum, and stored in the dark.18,48 4-Bromobenzene diazonium tetrafluoroborate (4-BBDT) was synthesized using the following conditions: 4-Bromoaniline (3.44 g, 20.0 mmol) was dissolved in fluoroboric acid (48%, 14.6 g, 80 mmol) and water (20 mL), and the solution was cooled in an ice bath. Sodium nitrite (1.46 g, 21.2 mmol) dissolved in water (4 mL) was added dropwise to the reaction mixture under stirring over a period of an hour. The diazonium salt was collected and washed with fluoroboric acid, methanol, and cold ether and used without any further purification. Samples of heat-treated glassy carbon (GC) were purchased from Carbone Lorraine (France). Thermal treatment has been carried out at about 2500 °C. Before use, the GC electrodes were polished using 0.1 µm diamond paste and thoroughly sonicated to remove any debris. 2.2. Chemical Modification of Glassy Carbon with Diazonium Salts. In SDS. The polished carbon electrode was placed in an aqueous solution of 4-NAB (40 mM) containing 1% SDS for 1 h under shaking. The resulting electrode was first sonicated in acetonitrile and then in water for 10 min each to remove any physisorbed material. In Ionic Liquids. The polished carbon electrode was immersed in 10 mM solution of 4-BBDT or 4-NAB in either 1-butyl-3(48) Ounker, M. F. W.; Strarkely, E. B.; Jenkins, G. L. J. Am. Chem. Soc. 1936, 58, 2308.
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Figure 1. Schematic illustration of the chemical grafting of 4-BBDT (A) and 4-NAB diazonium salts (B) in ionic liquids and subsequent electrochemical reduction of 4-NAB modified interfaces to amine-terminated glassy carbon interfaces.
methylimidazolium hexafluorophosphate or 1-butyl-3-methylimidazolium methyl sulfate and left for 1 h under shaking. The electrode was then sonicated in acetonitrile and water for 10 min each to remove any physisorbed material and ionic liquid. Electrochemical Reduction. Electrochemical reductions were performed in an aqueous 0.1 M KCl solution (10% EtOH/H2O) by either cycling the modified BDD five times between 0.0 and -1.5 V vs SCE (used for the determination of surface coverage) or applying increasingly negative potentials (-0.8 to -1.4 V vs SCE) for 1 min (for Raman studies). 2.3. Instrumentation. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) measurements were performed with an XR3E2 spectrometer from Vacuum Generators. In a UHV chamber (10-10 mbar), samples were irradiated with a monochromatic Mg KR X-ray source (1253.6 eV). The ejected electrons were collected by a hemispherical analyzer at constant pass energy of 30 eV. The energy scale was calibrated with the Cu2p3/2, Cu3p, and Au4f7/2 lines characterized by binding energies of 932.7, 75.2, and 84.0 eV, respectively. To correct for any charging effect, all the binding energies referred to the C1s line of the carbon contamination at 285.0 eV. Analysis was carried out at an angle of 90° between the sample and the analyzer. Electrochemical Measurements. Cyclic voltammetric measurements were performed using an Autolab 30 (Eco Chimie, The Netherlands). The carbon electrode (A ≈ 0.13 cm2) was sealed against the bottom of a single compartment electrochemical cell by means of a rubber O-ring. The electrical contact was made to a copper plate through the bottom of the substrate. The counter electrode was a platinum wire and the reference electrode a SCE. An aqueous solution of Fe(CN)64- (10 mM) in KCl (0.1 M) as well as Ru(NH3)63+ (10 mM) in KCl (0.1 M) was used for the electrochemical studies. The electrochemical responses were fitted to theoretical working curves using DigiSim 3.03. Raman Spectroscopy. Raman measurements were performed using a Renishaw’s InVia Raman spectrometer. Spectra were recorded with the green line of an Ar-ion laser (λ ) 514.53 nm) using a 50× (NA ) 0.95) objective. The acquisition time was 10 s with an output laser power of 5 mW.
3. Results and Discussion 3.1. Chemical Grafting of 4-Bromobenzene (4-BBDT) on Glassy Carbon. The surface of GC was modified through a spontaneous electron transfer between the electrode and the diazonium salt by immersion in a solution of 4-BBDT in 1-butyl3-methylimidazolium hexafluoroposphate or 1-butyl-3-methylimidazolium methyl sulfate for 1 h at room temperature (Figure 1A). The resulting surface was thoroughly washed using a series of solvents (chloroform, ethanol, water) in an ultrasonic bath to
Figure 2. XPS survey spectra of GC electrode before (a) and after chemical functionalization with 4-BBDT (b) and 4-NAB molecules (c) in ILs; after electrochemical reduction of surface (c) in 0.1 M KCl/ 10%EtOH; 5 scans, scan rate ) 50 mV s-1 (d).
remove eventually physisorbed diazonium salts and the highviscosity ionic liquid. X-ray photoelectron spectroscopy was used to evaluate the chemical composition of the modified GC surfaces and to follow the changes of the chemical bonding associated with transformations occurring on the surface. Figure 2a displays the XPS survey spectrum of the GC surface before chemical functionalization. It displays signals due to C1s at 285 eV and O1s at 531 eV. The latter peak is a result from surface oxidation.49 High-resolution XPS spectrum of the C1s of the glassy carbon before functionalization exhibits two signals due to C1s from the bulk (C-H, C-C) and from the surface C-O features at 284.7 and 285.9 eV, respectively (Figure 3A). After modification of the GC electrode with 4-BBDT, an additional peak at 70.9 eV due to Br3d was observed (Figure 2b). The absence of a peak at ∼402 eV from N1s of the diazonium molecule and at ∼400 eV of the ionic liquids is a further indication that the diazonium salt has reacted and that the ionic liquid is completely removed from the surface. It is in accordance with a covalent linking of the 4-BBDT molecules on the surface and not physisorption.17,26 The XPS spectra are independent of the ionic liquid used and show that both hydrophilic and hydrophobic (49) Yumitori, S. J. Mater. Sci. 2000, 35, 139.
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Figure 4. High-resolution XPS spectra of the N1s of 4-NAB-terminated GC electrode before (A) and after electrochemical reduction in 0.1 M KCl/10%EtOH; 5 scans, scan rate ) 50 mV s-1 (B).
Figure 3. High-resolution XPS spectra of the C1s of GC electrode before (A) and after (B) chemical modification with 4-BBDT molecules in ionic liquids.
ionic liquids result in the covalent linking of diazonium salts. High-resolution XPS spectrum of the C1s of the chemically modified electrode with 4-BBDT in ionic liquids displays signals at 284.8, 286.3, and 288.73 eV due to carbon from the bulk, C-Br, and C-O features (Figure 3B). 3.2. Chemical Grafting of 4-(4′-Nitrophenylazo)benzene Diazonium Tetrafluoroborate (4-NAB) on Glassy Carbon. Having verified the success of the grafting process using 4-bromobenzene diazonium salt, the surface of GC was modified in the same way using NAB (Figure 1B). Figure 2c displays the XPS survey spectrum of the 4-NAB-modified GC surface. It shows next to C1s additional peaks due to O1s and N1s. Highresolution XPS spectrum of the C1s displays three signals at 284.6, 285.12, and 286.6 eV due to carbon from the bulk, C-N, and C-O features, respectively. High-resolution spectrum XPS of the N1s shows two signals at 401 and 406 eV due to -NdNand -NO2, respectively (Figure 4A), consistent with the chemical composition of the grafted NAB molecules. 3.3. Evaluation of Barrier Properties of 4-NAB-Modified Glassy Carbon. Cyclic voltammetry was used to evaluate the electrochemical barrier properties of the 4-nitroazobenzene layer. Figure 5 shows cyclic voltammetric i-E curves in aqueous solutions of 10 mM Fe(CN)64- and Ru(NH3)63+/2+ in 0.1 M KCl for GC electrodes before and after derivatization with 4-NAB molecules in ILs. The i-E curve of the nonderivatized GC
electrode in the presence of Fe(CN)64- shows a well-defined wave with a k0app ) 0.0013 cm s-1 (Figure 5A). After chemical modification, the i-E curve not only changes shape, but also shows an attenuation of the peak currents reflecting a strongly suppressing electron-transfer reaction of Fe(CN)64-. The behavior is characteristic of barriers with high surface coverage and low concentration of defect sites. The NO2-terminated layer is thus a highly efficient barrier for the redox probe.19,50 In the case of an outer-sphere redox mediator like Ru(NH3)62+, which is not sensitive to the surface state of the electrode, only a small decrease of the apparent rate constant to k0app ) 0.001 cm s-1 was observed (Figure 5B). The difference in the electrochemical behavior of the redox probes might be due to the fact that the positively charged Ru(NH3)62+ more easily penetrates the organic layer than the negatively charged Fe(CN)64-. A similar behavior has been observed on acid-terminated layers.19 3.4. Electrochemical Reduction of NAB-Modified GC. An aqueous solution (0.1 M KCl containing 10% EtOH) was used to electrochemically reduce the -NO2 groups terminating the 4-NAB-terminated GC. Amine groups are formed in a multielectron and multiproton pathway according to the reaction scheme described below.1
R-NO2 + e- f R-NO2•+
-
(1)
R-NO2 + 4H + 4e f R-NHOH + H2O
(2)
R-NHOH + 2H+ + 2e- f R-NH2 + H2O
(3)
RsNdNsR + 2H+ + 2e- f R-NH-NH-R
(4)
(50) Evans, N. I.; Gilicincki, A. G. J. Phys. Chem. 1992, 96, 2528.
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R-NHOH f R-NO + 2H+ + 2e-
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(5)
Figure 6 shows consecutive cyclic voltammograms of 4-NABmodified GC when immersed in an aqueous 0.1 M KCl solution containing 10% EtOH. The general electrochemical behavior of the glassy carbon interface modified in hydrophilic (Figure 6A) and hydrophobic (Figure 6B) ionic liquids and in an aqueous solution containing 1% SDS (Figure 6C) is comparable, showing that the layers formed have similar electrochemical characteristics. Reduction waves at about Ep1 ) -0.59 VSCE (most likely due to oxygen reduction; sample potential as on unmodified GC) and Ep2 ) -1.00 VSCE (reduction of -NO2) are clearly seen in all three cases, which decreased significantly in the second and third scans. This indicates that nearly all the electroactive -NO2 groups are reduced. However, the reduction of -NO2 to -NH2 is incomplete: the presence of a reversible couple at about E0 ) -0.32 VSCE, assigned to the hydroxyaminophenyl/nitrosophenyl interconversion (reaction 5).7 The main difference in the electrochemical reductive scans of the GC substrates modified in different ways with the same organic molecule is the ratio between the reduction waves Ep1/Ep2. The charge under Ep1 is similar in all cases, while the charge under Ep2 is higher for interfaces modified with hydrophilic IL and SDS compared to hydrophobic IL. This could be an indication that hydrophilic ILs form more compact modified layers. It is common practice to use the charge under the -NO2 reduction peak to determine the surface coverage of interfaces modified with diazonium salts.29 However, it has been recently pointed out by Combellas and Downard that this method is not accurate as the thickness of the organic layer increases. The integration of the -NO2 wave never exceeds 1 ML.8 The
Figure 6. Electrochemical reduction of NAB-terminated GC substrates prepared in hydrophilic IL (A), hydrophobic IL (B), and in aqueous solution containing 1% SDS (C): solution, 0.1 M KCl/water/10%EtOH; scan rate ) 50 mV s-1, black line (first scan), gray line (second scan), and dotted gray (third scan), dotted black line in A: unmodified GC substrate.
Figure 5. Cyclic voltammograms of GC electrode before (black line) and after chemical derivatization with NAB (gray line) in ILs using Fe(CN)64- (10 mM) (A) and Ru(NH3)63+ (B) redox couples in KCl (0.1 M); scan rate: 50 mV s-1.
determined surface coverage for 4-NAB-terminated GC being Γ ) (4.23 ( 1.6) × 10-10 mol cm-2 [Γ ) (2.54 ( 0.97) × 1014 molecules cm-2] using the hydrophilic 1-butyl-3-methylimidazolium methyl sulfate ionic liquid and Γ ) (4.88 ( 1.2) × 10-10 mol cm-2 [Γ ) (2.93 ( 0.49) × 1014 molecules cm-2] in the case of the hydrophobic ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) has to be thus interpreted within this approximation. XPS Analysis. Figure 2d shows the XPS survey spectrum of the electrochemically reduced 4-NAB-terminated GC electrode. It displays similar features to that of the 4-NAB-terminated GC substrate with the disappearance of the N1s component at higher binding energy (406 eV). The high-resolution spectrum of the N1s (Figure 4B) shows a broad signal centered at 401 eV due to -NdN-, -NH2, and/or -NO groups. The absence of the higher
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Figure 7. Raman spectra of GC electrode (a), NAB powder (b), GC chemically modified with NAB using ILs (c), GC chemically functionalized with NAB in 1% SDS aqueous solution (d), and GC modified with NAB using ILs after subtracting peaks due to GC (e). The Raman acquisition time was 10 s with an output laser power of 5 mW. Table 1. Raman Frequencies and Assignment for Solid NAB and NAB-Terminated GCa solid NAB
NAB-GC
assignment
836, w 921, w 1007, w 1104, m 1123, s 1135, w 1178, m 1240, w 1337, m 1399, s 1444, s 1472 1586, m
851, w 917, w 1007, m 1103, m 1123, w 1136, s 1183, m 1232, w 1336, s 1398, s 1447, s
NO2 bend CH bend ring deformation NO2 stretch NO2 stretch phenyl-N stretch C-H bend C-C stretch NO2 stretch NdN stretch NdN stretch phenyl-N-N stretch and ring deformation phenyl CdC stretch
a
1592, s
w, weak; m, medium; s, strong.
binding energy signal at 406 eV clearly indicates a complete transformation of -NO2. However, the XPS spectra give no indication for the presence of -NdN- or -NHsNH- in the layer. Raman Analysis with and without External Bias. One would additionally expect to see the electrochemical characteristics of the azobenzene group (reaction 4). A solution of azobenzene (10 mM) in acetonitrile shows a first reversible reduction peak with E0 ) -1.14 VSCE and an irreversible reduction peak with Ep ) -2.28 VSCE (data not shown). This corresponds to radical anion and subsequent dianion formation, which is readily protonated. Downward et al. have shown that the wave could not be observed for some reason, which is not clear.8 However, slow electrode kinetics could lead to the reduction of azobenzene to hydrobenzene simultaneously with the reduction of the nitro group at Ep2 ) -1.00 VSCE. Raman spectroscopy in combination with electrochemistry was used in the following to better understand this complex situation. Figure 7a shows the Raman spectrum of a polished GC electrode displaying characteristic Raman bands at 1351 and 1593 cm-1 for sp2 carbon. Figure 7b displays the Raman spectrum of solid 4-NAB. The NO2 stretching bands are seen between 1100-1340 cm-1; the NdN bands are located at 1136, 1398, and 1448 cm-1, and at higher frequencies, the CdC stretching bands of the phenyl ring are observed (1590 cm-1). Table 1 summarizes the Raman characteristics of solid NAB. The Raman spectra recorded on the GC electrode chemically modified with 4-NAB in ILs (Figure 7c) and in 1% SDS (Figure 7d) show the
Actis et al.
Figure 8. Differential Raman spectra of NAB-terminated GC as a function of applied potential. The experiment was performed in an aqueous solution of 0.1 M KCl/10%EtOH; the different redox potentials were applied for 1 min, after which Raman spectra were recorded. The Raman acquisition time was 10 s with an output laser power of 5 mW.
same characteristic Raman signatures as the solid 4-NAB (Table 1) and are in agreement with the values reported by McCreery.13,18,30 Indeed, he reported that intense Raman spectra can be obtained for some specific bonded molecules like NAB. The strong signal arises from the electronic conjugation between graphene planes of the glassy carbon electrode and the aromatic monolayer through the covalent grafting bond.17,18 As Raman can ambiguously distinguish between -NO2 and -NdN- groups, it was in addition used in connection with chronoamperometry to understand the electrochemical behavior of the -NdN- function on GC in aqueous solutions. McCrerry had already investigated spectral Raman changes when a 4-NAB-terminated GC electrode immersed in acetonitrile was negatively biased and examined the effect of the generated electrical field on the spectral characteristic.13 The effect of the electrical field on the 4-NAB layer is not investigated here, but rather the change of the chemical composition of the modified GC interface after electrical bias. Figure 8 shows the change in the Raman spectra of 4-NABterminated GC depending on the applied potential bias. The Raman bands of -NO2 at 1103 cm-1 and 1336 cm-1 are conserved up to a potential of about -1.1 V vs SCE, and thereafter, the signal is entirely lost. The -NdN- vibration bands are decreasing significantly at potentials more negative then 1.1 V/Ag/AgCl. This is due to the reduction of the azobenzene group to its radical anion and further dianion (reaction 4), leading to the Raman signal loss of the 4-NAB layer (loss of the conjugated π system). This observation allows the determination of the optimal reduction potential for the selective conversion of -NO2 into -NH2 groups.
Conclusion We have shown that covalent grafting of aryldiazonium salts in conventional hydrophobic or hydrophilic ionic liquids on glassy carbon interfaces is feasible. The spectroscopic and electrochemical characteristics and surface coverage of the functionalized surfaces are comparable to GC surfaces using aqueous solution in the presence of 1% SDS. Raman investigations in connection with chronoamperometry helped to understand the complex electrochemical behavior of this interface. While both chemical approaches (SDS-based and IL-based) result in surface layers with similar characteristics, the main interest in using ILs is the possibility of patterning
Functionalization of Glassy Carbon with Diazonium Salts in Ionic Liquids
carbon-based interfaces with multifunctional groups. Indeed, the local modification of carbon-based material with a variety of different functional diazonium salts (Br, F, Cl, NO2, COOH, etc.) could be achieved be depositing locally ionic liquid droplets, where the respective diazoinum salt is dissolved. This is currently under investigation in our group. This approach introduces new opportunities for chemical functionalization of carbon-based interfaces toward applications in the field of bio- and nanotechnology.
Langmuir, Vol. 24, No. 12, 2008 6333
Acknowledgment. The Centre National de la Recherche Scientifique (CNRS) and the Agence Nationale de la Recherche (ANR) are gratefully acknowledged for financial support. We would like to thank Berthome´ Gre´gory for help in recording XPS spectra as well as Philip Hamoumoud for preliminary investigations. Galyna Shul and Marcin Opallo acknowledge financial help through the ECO-NET program. LA703714A
