pubs.acs.org/Langmuir © 2009 American Chemical Society
Versatile Transformations of Alkylamine-Derivatized Glassy Carbon Electrodes using Aryl Isocyanates Lasse Tholstrup Nielsen,† Marcel Ceccato,† Allan Hjarbæk Holm,‡ Martin Verner Kristensen,† Steen Uttrup Pedersen,*,† and Kim Daasbjerg*,† † Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus, Denmark, and Interdisciplinary Nanoscience Center (iNANO), Institute of Physics and Astronomy, Ny Munkegade, DK-8000 Aarhus C, Denmark, and ‡Grundfos Management A/S, Poul Due Jensens Vej 7, 8850 Bjerringbro, Denmark
Received May 11, 2009. Revised Manuscript Received August 7, 2009 The reaction between a nucleophilic 4-(2-aminoethyl)phenyl-tethered glassy carbon surface and various parasubstituted aryl isocyanates [ONC-PhX; X = NO2, COPh, Cl, H, and NMe2] has been studied in toluene. It is demonstrated that the nucleophilic addition reaction is relatively fast occurring within two hours while providing an efficient and versatile route for derivatizing alkylamine-functionalized surfaces. An often overlooked issue in surface reactions is the possibility for competing physisorption processes. In such cases, the solution-based reactants become adsorbed to the surface or embedded in the grafted layer rather than chemically bonded to the surface. It is shown that for two of the aryl isocyanates (X = NO2 and COPh) this physical adhesion can be so strong that even prolonged ultrasonic treatment cannot remove the adsorbant. However, a single potential excursion is capable of desorbing most of the physisorbed layers. The isocyanate-based method is also compared with the well-known approaches involving diazonium salts for assembling similar chemical systems directly. It is concluded that the method can be used advantageously not only in cases, where such approaches should fall short, but also if the goal is to achieve better control of the positioning of, e.g., redox active molecules in a well-defined layer with the ultimate goal of obtaining distinct electrochemical responses.
Introduction It is now well-established that aromatic organic layers can be covalently tethered to conducting and semiconducting surfaces by electrochemical reduction of aryldiazonium salts.1-3 Since the first study on the subject was published in the early 1990s4 the procedure has attracted considerable interest and films grafted from numerous substituted aryldiazonium salts have been examined.5-7 Surfaces modified by the aryldiazonium salt procedure have been applied in several research fields, including sensors,8 bioelectrochemistry,9 molecular electronics,10 immobilization of organometallics11 and corrosion protection.12 While electrochemical reduction of aryldiazonium salts presents a convenient way to functionalize a given electrode, subsequent chemical transformations can be required to provide the surface with specific (chemical) properties. In this respect, the covalent attachment going through a substrate-aryl bond offers *Corresponding authors. E-mail:
[email protected] (S.U.P.) and kdaa@ chem.au.dk (K.D.). (1) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429–439. (2) Downard, A. J. Electroanalysis 2000, 12, 1085–1096. (3) McCreery, R. L. Chem. Rev. 2008, 108, 2646–2687. (4) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883–5884. (5) Nguyen, N. H.; Esnault, C.; Gohier, F.; Belanger, D.; Cougnon, C. Langmuir 2009, 25, 3504–3508. (6) Brooksby, P. A.; Downard, A. J. J. Phys. Chem. B 2005, 109, 8791–8798. (7) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201–207. (8) Downard, A. J.; Roddick, A. D.; Bond, A. M. Anal. Chim. Acta 1995, 317, 303–310. (9) Polsky, R; Harper, J. C.; Dirk, S. M.; Arango, D. C.; Wheeler, D. R.; Brozik, S. M. Langmuir 2007, 23, 364–366. (10) McCreery, R. L; Viswanathan, U.; Kalakodimi, R. P.; Nowak, A. L. Faraday Discuss. 2006, 131, 33–43. (11) Swarts, J. C.; Laws, D.; Geiger, W. E. Organometallics 2005, 24, 341–343. (12) Matrab, T.; Chehimi, M. M; Perruchot, C.; Adenier, A.; Guillez, A.; Save, M.; Charleux, B.; Cabet-Deliry, E.; Pinson, J. Langmuir 2005, 21, 4686–4694.
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a great advantage in that it can withstand the quite severe experimental conditions often invoked for accomplishing relatively slow surface reactions. At least this is true for carbon-based substrates due to the high strength of C-C bonds (∼100 kcal mol-1), while the immobilization of aryl groups on gold surfaces via the diazonium salt route seems to result in a less stable modification.13,14 If the surface reactions should turn out to be too inefficient it may be necessary to redesign the synthetic route or to prepare the complete target molecule using multistep solution-based synthesis.15 Relatively little information is available in the literature concerning the reactivity of molecules that have been attached covalently to surfaces using diazonium salt chemistry.16 However, it seems safe to state that grafted surfaces, in general, suffer from long reaction times and that this may impose limitations on the development of target molecular systems. For instance, the nucleophilic substitution reaction between carbon felt modified with a benzyl chloride moiety and nucleophiles such as aryl thiolates required reaction times of up to 108 h at elevated temperature to give a high yield.16 In a similar manner, benzyl chloride modified glassy carbon (GC) electrodes have been treated with bipyridines over 4-5 days to obtain the substitution product.17 In a study conducted by Matrab et al. regarding atom transfer radical polymerization on diazonium grafted iron surfaces the goal was to produce relatively long and (13) Llave, E. D. L.; Ricci, A.; Calvo, E. J.; Scherlis, D. A J. Phys. Chem. C 2008, 112, 17611–17617. (14) Shewchuk, D. M.; McDermott, M. T. Langmuir 2009, 25, 4556–4563. (15) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2003, 126, 370–378. (16) Coulon, E.; Pinson, J.; Bourzat, J. D.; Commercon, A.; Pulicani, J. P. J. Org. Chem. 2002, 67, 8513–8518. (17) Holm, A. H.; Moeller, R.; Vase, K. H.; Dong, M.; Besenbacher, F.; Pedersen, S. U.; Daasbjerg, K. New J. Chem. 2005, 29, 659–666.
Published on Web 09/04/2009
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well-defined polymer brushes which required reaction times of several hours.18 While nucleophilic substitution reactions are severely constrained geometrically, addition reactions would seem more suitable for carrying out surface derivatizations. One of the most interesting functionalities involved in addition reactions is the isocyanate group. The high reactivity of isocyanates has previously been utilized for reactions with a self-assembled monolayer of 11-mercapto-1-undecanol on gold.19 Diisocyanates have also been applied in layer-by-layer build-up of polymeric urethane on oxidized silicon substrates primed with (3-aminopropyl)dimethoxysilane.20 However, in this case a reaction time of 4 h was used for each layer deposition. To the best of our knowledge surfaces grafted via diazonium salt chemsitry have not been utilized in reactions with isocyanates. In this work, we use a nucleophilic 4-(2-aminoethyl)phenyltethered GC surface as the one reactant and various parasubstituted phenyl isocyanates [ONC-PhX; X = NO2, COPh, Cl, H, and NMe2] in solution as the other. The nucleophilic surface is obtained from an electroreduction of 4-(2-ammonioethyl)benzenediazonium followed by a deprotonation to form the reactive amine functionality.21 We will demonstrate that the nucleophilic addition reaction, in general, is a versatile way to derivatize amine-terminated electrodes. An often overlooked issue in surface reactions is the existence of competing physisorption processes, in which the solutionbased reactants become adsorbed to the surface or embedded in the grafted layer rather than chemically bonded.22 In certain cases, this physical adhesion can be so strong that even an ultrasonic rinsing will be insufficient to detach it. We have therefore carried out a thorough investigation of the possible contribution from such effects. In addition, we also compare the electrochemical responses of the chemically modified electrodes with those obtained if the electroactive groups had been immobilized directly by electrografting of the corresponding aryldiazonium salts.7 It will be shown that the isocyanate-based method can be used advantageously not only in cases, where such approaches fall short, but also if the goal is to achieve better control of the positioning of e.g. redox active molecules. All analysis has been performed by means of electrochemistry or polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS).
Experimental Section Chemicals. Acetonitrile (MeCN) and N,N-dimethylformamide (DMF) were purchased from Laboratory-Scan and used as received unless otherwise specified. Acetone was obtained from Aldrich (Chromasolv). Water was triple distilled and toluene was freshly distilled over sodium. Tetrabutylammonium hydroxide aqueous solution (55-60%; Fluka), 1-isocyanato-4-nitrobenzene (Aldrich), 4-isocyanatophenyl phenyl methanone (Aldrich), isocyanatobenzene (Ferak), 1-chloro-4-isocyanatobenzene (Fluka), 2-isocyanato-1,4-dimethoxybenzene (Aldrich), 4-isocyanatoN,N-dimethylaniline, and 4-(2-aminoethyl)aniline (Fluka) were obtained from commercial sources. The diazonium salts, 4-nitrobenzenediazonium, 4-methylbenzenediazonium, 4-benzoylbenzenediazonium, and 4-(2-aminoethyl)benzenediazonium tetrafluoroborate were prepared according to published procedures (18) Matrab, T.; Chehimi, M. M.; Perruchot, C.; Adenier, A.; Guillez, A.; Save, M.; Charleux, B.; Cabet-Deliry, E.; Pinson, J. Langmuir 2005, 21, 4686–4694. (19) Persson, H. H. J.; Caseri, W.; Suter, U. W. Langmuir 2001, 17, 3643–3650. (20) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 4655–4661. (21) Griveau, S.; Mercier, D.; Vautrin-Ul, C.; Chausse, A. Electrochem. Commun. 2007, 9, 2768–2773. (22) Lemek, T.; Mazurkiewicz, J.; Stobinski, L.; Lin, H. M.; Tomasik, P. J Nanosci. Nanotechnol. 2007, 9, 3081–3088.
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and purified by precipitation from a MeCN or acetone solution by addition of diethyl ether, filtered, vacuum-dried, and stored at -18 °C.23 The synthesis of 4-(2-[3-(4-nitrophenyl)ureido]ethyl)benzenediazonium tetrafluoroborate is described in the Supporting Information. K4Fe(CN)6 (Merck), K3Fe(CN)6 (Merck), and Ru(NH3)6Cl3 (ABCR) were obtained from commercial sources. Tetrabutylammonium tetrafluoroborate (Bu4NBF4) was synthesized using standard procedures. Prior to use the electrolyte solution, 0.1 M Bu4NBF4/MeCN or 0.1 M Bu4NBF4/DMF, was dried by running it through a column containing Al2O3 (Fluka) which had been dried in vacuum at 450 °C overnight. Electrodes and Instrumentation. Glassy carbon (GC) rods (Sigradur G, HTW, diameter = 1 mm) were embedded in epoxy resin and carefully polished (with sandpaper 180, 500, 1000, 2000) followed by treatment with diamond suspensions (Struers, grain size = 9, 3, 1, and 0.25 μm). Afterward the electrodes were washed thoroughly with triple distilled water and ethanol. Finally the electrodes were sonicated 5 min in ethanol. The gold plates were cleaned with a “piranha” solution which comprises 1:3 (v:v) 30% H2O2:H2SO4 for 2 min (CAUTION! Piranha solution is a vigorous oxidant and should be used with extreme care). Before derivatization, the cleaned plates were rinsed with triple distilled water and ethanol (99.99%) and dried under a stream of argon. The electrochemical data were recorded on either a CH Instruments 660B electrochemical workstation or a CH Instruments 601C electrochemical analyzer. A standard 3-electrode electrochemical setup was employed. All glassware was oven-dried before each experiment. The GC electrode (or gold plate) was employed as working electrode. The auxiliary electrode consisted of a platinum wire. The reference electrode used for aqueous solutions was a saturated calomel electrode (SCE). For nonaqueous solutions an Ag/AgI pseudo reference electrode was employed which was calibrated against ferrocenium/ferrocene (Fcþ/ Fc) at the end of each experiment using previous determina0 þ tions of EFc = 0.41 and 0.48 V vs SCE in MeCN and DMF, respectively.24 Electrografting. Electrochemical modification was performed via potentiostatic electrolysis of a 2 mM solution of the diazonium salt in MeCN for 300 s at a potential 200 mV negative to the voltammetric peak potential (determined by an initial cyclic voltammetric sweep at a sweep rate of 0.2 V s-1). After potentiostatic electrolysis another cyclic voltammogram was recorded to ensure that passivation had taken place. Electrodes grafted with 4-(2-ammonioethyl)benzenediazonium tetrafluoroborate were rinsed by sonication for 5 min in a solution of MeCN and a few drops of 55-60% aqueous Bu4NOH solution. The total charge, Q, used for the reduction of surface-confined electroactive groups was obtained by coulombmetric integration of the background-subtracted electrochemical response recorded in cyclic voltammetry at a sweep rate of 2 V s-1. For all substituents but NMe2 a linear background was assumed and manually adjusted under the faradaic peak in accordance with our previous studies.25 For NMe2, the background subtraction was carried out using a fourth degree polynomial to describe the steadily increasing background signal close to the limit of the potential window. However, it is important to note that for filmcoated electrodes there is a large uncertainty associated with establishing an appropriate baseline since the properties of the film change during sweeping. Yu et al. have discussed this issue and concluded that the uncertainty on the determination of Q could be of at least 20%.26 (23) Starkey, E. B. In Organic Syntheses; Wiley & Sons: New York, 1943; Collect. Vol. 2, p 225. (24) Daasbjerg, K.; Pedersen, S. U.; Lund, H. In General Aspects of the Chemistry of Radicals; Alfassi, Z. B., Ed.; Wiley: Chichester, U. K., 1999; p 385. (25) Vase, K. H.; Holm, A. H.; Norrman, K.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2007, 23, 3786–3793. (26) Yu, S. S. C.; Tan, E. S. Q.; Jane, R. T.; Downard, A. J. Langmuir 2007, 23, 11074–11084.
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Scheme 1. Outline of Surface Grafting and Post Modification of a 4-(2-Aminoethyl)phenyl-Tethered Surface with Aryl Isocyanates
The surface coverage, Γ, was calculated using Faraday0 s law, i.e., Γ = Q/nFA, where n is the number of electrons transferred for the redox active group, F is Faraday0 s constant, and A represents the geometric area of the electrode. In MeCN or DMF, we assume that n=1 for the direct reduction of the surface-confined nitrobenzene and benzophenone, as well as for the oxidation of the dimethylaminophenyl unit. For the benzophenone case this may not be completely true considering that the redox activity decreases fast upon sweeping, i.e. the radical ions generated upon reduction may be involved in reactions that induce further electrode reactions. For example, reduction of the benzophenone moiety followed by protonation and further reduction would give n = 2. Although the Γ values listed therefore represent maximum values, we estimate that they are reliable within 20% taking into account that a certain degree of reversibility is observed for the redox couples and that the determination of Γ only showed a weak dependency on the sweep rate. Post Modification. 4-(2-Aminoethyl)-tethered GC and Au electrodes were immersed into a glass tube containing a 20 mM toluene solution of the pertinent isocyanate. Usually full dissolution could be accomplished with the aid of sonication but for the 1-isocyanato-4-nitrobenzene and 4-isocyanato-N,N-dimethylaniline suspensions were formed. The solutions were then allowed to stand on a shake table for a given reaction time at room temperature. The derivatized electrodes were rinsed by ultrasonication for 10 min in DMF and MeCN or acetone, respectively. Physisorption. Freshly polished electrodes were immersed into a 20 mM toluene solution of the pertinent isocyanate. The subsequent treatment followed exactly the same procedure as outlined above for the postmodified electrodes. Electrochemical Impedance Spectroscopy (EIS). The electroactivity of the unmodified and modified GC electrodes were evaluated by EIS in the presence of a redox probe. The specific probes and solutions used were K4Fe(CN)6/K3Fe(CN)6 (5 mM of each; 0.1 M KCl, pH 7 using a phosphate-buffered solution or pH 11.3 adjusted with a 0.1 M KOH solution) or Ru(NH3)6Cl3 (5 mM; 0.1 M KCl, phosphate-buffered solution at pH 7). EIS experiments were performed using the CH 660B electrochemical analyzer at open circuit potential. The frequency range was between 0.05 Hz and 100 kHz with a signal amplitude of 20 mV. The AC impedance fitting program supplied with the equipment was used to analyze the electrochemical impedance data and fitting them to a Randles circuit.
Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS). PM-IRRAS spectra were recorded on a Bio-Rad FTS 65A (Randolph, MA) FTIR spectrometer equipped with an external experiment module with a narrow band mercury-cadmium-telluride detector cooled in 12162 DOI: 10.1021/la901666j
liquid nitrogen. The infrared beam was modulated at 37 kHz between s or p polarization by combining a gold wire polarizer with Hinds zinc-selenide photoelastic (PEM-90/II/ZS37) modulator. The PEM was adjusted, so the s polarization was linear at wavenumbers around 1500 cm-1. The gold substrates were irradiated with an incident grazing angle of 80°. The two signals, Rp þ Rs, and, Rp - Rs, were extracted with a high-pass filter (EG&G model 189) and a lock-in amplifier (SR 810 DSP) and digitized sequentially as 100 spectra of each signal in 30 cycles. The differential surface reflectivity [ΔR/R = (Rp - Rs)/(Rp þ Rs)] spectra were obtained with 2 cm-1 resolution. All spectra were recorded at room temperature in dry atmosphere.
Results and Discussion A general reaction scheme is presented in Scheme 1 for the systems studied in this work. The first step involves the reductive electrografting of 4-(2-ammonioethyl)benzenediazonium tetrafluoroborate (1Hþ) on a GC surface (GC-1Hþ).21 Upon deprotonation the corresponding nucleophilic amine-functionalized surface is generated (GC-1). Note that this two-step procedure to form GC-1 is necessary since a preparation of compound 1 containing both the -NH2 and -N2þ groups would be precluded due to its instability. The reactivity of GC-1 is tested toward various aryl isocyanates having both electronwithdrawing and -donating para substituents. These films are denoted GC-10 PhX, in which X = NO2, COPh, Cl, H, or NMe2 while the prime on 1 is designating the fact that the terminal amine group on 1 has been converted into an urea functionality in its reaction with the isocyanate group on the para-substituted benzene ring. For the analysis of GC-10 -PhX, direct electrochemical detection can be employed for X = NO2, COPh, and NMe2. Indirect electrochemical characterization using redox probes in cyclic voltammetry or impedance spectroscopy can be carried out for any of the substituents but it is only presented for X = H and Cl herein. In addition, PM-IRRAS analysis has been performed on GC-10 PhNO2. First a detailed description of the individual steps in Scheme 1 is provided. Electrografting To Form GC-1Hþ. Figure 1 shows the cyclic voltammogram obtained at a freshly polished GC electrode in a 2 mM solution of 1Hþ in MeCN along with the voltammogram obtained at GC-1Hþ prepared by means of a potentiostatic electrolysis for 300 s at Epc - 200 mV (Epc = -0.64 V vs SCE), where Epc denotes the cathodic peak potential on the initial sweep. The reduction wave seen for the bare electrode corresponds to the irreversible reduction of 1Hþ, while the featureless voltammogram Langmuir 2009, 25(20), 12160–12168
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Figure 1. Cyclic voltammograms of a 2 mM 1Hþ solution
in MeCN obtained at a sweep rate of 0.2 V s-1 (a) at a freshly polished GC electrode and (b) after potentiostatic electrolysis at Epc - 200 mV for 300 s.
seen for GC-1Hþ shows that formation of an insulating film on the electrode surface is the result of the electrolysis.27 The reason for this is that the reduction of the aryldiazonium salt leads to the generation of aryl radicals, either through the intermediacy of shortlived aryldiazenyl radicals28 or directly because of a concerted electron transfer/nitrogen expulsion process.29 The high reactivity of the aryl radicals as well as the fact that they are generated close to the electrode surface ensures that a considerable fraction reacts with the surface through covalent C-C or O-C bonds. The latter type may be formed because of the many oxygen-containing functionalities always present on a GC surface.30,31 In addition, fast SH homolytic substitution reactions on already-immobilized aryl groups will usually take place31,32 to form a couple-of-nanometer thick multilayered film at the end. It might also be mentioned that recent approaches involving specifically designed grafting agents have accomplished the goal of generating near-monolayer film structures.33-35 Analysis of the GC-1, GC-1Hþ, and GC-10 -PhX (X = H and Cl) Films Using Redox Probes and Impedance Spectroscopy. A common methodology used for analyzing the properties of assembled films is to measure the charge transfer properties of a redox probe.36 Figure 2 shows the cyclic voltammograms of the redox probe Fe(CN)63- at pH 7 recorded before and after grafting of 1Hþ. For the bare GC electrode as well as GC-1Hþ, the quasi-reversible behavior of the ferricyanide system is evident, suggesting that the electron transfer is essentially unaffected by the presence of the grafted layer. This effect may be explained by the fact that the positively charged ammonium group gives rise to a favorable electrostatic interaction with the negatively charged Fe(CN)63-.21,37 (27) Downard, A. J. Langmuir 2000, 16, 9680–9682. (28) Daasbjerg, K; Sehested, K. J. Phys. Chem. A 2002, 106, 11098–11106. (29) Andriux, C. P.; Pinson, J. J. Am. Chem. Soc. 2003, 125, 14801–14806. (30) McCreery, R. L. in Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, p 258. (31) Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Langmuir 2005, 21, 280–286. (32) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534–6540. (33) Nielsen, L. T.; Vase, K. H.; Dong, M.; Besenbacher, F.; Pedersen, S. U.; Daasbjerg, K. J. Am. Chem. Soc. 2007, 129, 1888–1889. (34) Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. J. Am. Chem. Soc. 2008, 130, 8576–8577. (35) Malmos, K.; Dong, M.; Pillai, S.; Kingshott, P.; Besenbacher, F.; Pedersen, S. U.; Daasbjerg, K. J. Am. Chem. Soc. 2009, 131, 4928–4936. (36) Chen, P.; McCreery, R. L. Anal. Chem. 1996, 68, 3958–3965. (37) Breton, T.; Belanger, D. Langmuir 2008, 24, 8711–8718.
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Figure 2. Cyclic voltammograms recorded of 2 mM Fe(CN)63- in phosphate buffer (0.1 M H2PO4-/HPO42-, pH 7) at a sweep rate of 0.1 V s-1. Voltammograms are obtained at (a) a bare GC electrode before and (b) after grafting, i.e., GC-1Hþ, (c) GC-1Hþ after having been immersed in a solution of isocyanatobenzene, and (d) GC-1 after having been immersed in a solution of isocyanatobenzene, i.e., GC-10 -PhH. Insets show Nyquist plots recorded of 5 mM Fe(CN)63-/ Fe(CN)64- for GC-10 -PhH and GC-1Hþ.
Prior to any reactivity studies we wanted to confirm our expectation that the more nucleophilic GC-1 film would possess a higher reactivity than GC-1Hþ. The same GC-1Hþ electrode produced from the electrografting of 1Hþ (and further sonicated in MeCN with a drop of conc. HCl added to ensure that all amino groups would be protonated) was therefore immersed into a toluene solution of isocyanatobenzene for 1 h followed by careful rinsing. Afterward the electron transfer properties toward Fe(CN)63- were tested once again. This voltammogram included in Figure 2 (curve c) is almost identical to the one recorded before the electrode was immersed into the reaction solution implying that no surface reaction or adsorption had taken place. Subsequently the electrode was deprotonated (by sonication in a solution of MeCN and a drop of aqueous Bu4NOH) to form GC-1 which was immersed into the isocyanatobenzene solution also for 1 h. The electrode was carefully rinsed and the redox behavior of Fe(CN)63- was investigated. The cyclic voltammogram (Figure 2, curve d) recorded becomes essentially featureless. Note that no measurement in this series at pH 7 could be carried out on the GC-1 film itself as the surface bound amine group will be protonated.37 Nevertheless, we believe such a complete blocking of the charge transfer reaction should be explained by the occurrence of a successful reaction, in which GC-10 -PhH has been formed (Scheme 1). In the same manner we were able to show that GC-10 -PhCl could be successfully formed (Figures 1S and 2S, Supporting Information).38,39 To quantify the blocking abilities of the various electrodes while at the same time including GC-1 in the study, electrochemical impedance spectroscopy (EIS) was employed for determining the charge transfer resistance, RCT, at pH 7, where GC-1Hþ is the most stable form, and at pH 11.3, where GC-1 is the stable one.37 In addition to Fe(CN)63- we used Ru(NH3)63þ as redox probe (Figure 2S, Supporting Information) to elucidate (38) This being said there may also be an effect on the voltammograms of external redox probes originating from permeability changes induced by solvent or ions.39 To ensure that this effect has no consequence for the overall interpretation we also reacted 4-isocyanato-N,N-dimethylaniline with GC-1Hþ and GC-1 electrodes and only in the later case was it possible to detect directly the dimethylaniline functionality in cyclic voltammetry (Supporting Information, Figure 3S). (39) Cruickshank, A. C.; Tan, E. S. Q.; Brooksby, P. A.; Downard, A. J. Electrochem. Commun. 2007, 9, 1456–1462.
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Table 1. Charge Transfer Resistance, RCT, Measured by EIS at pH 7 and 11.3 for Various Electrodes using the Redox Probes Fe(CN)63-/Fe(CN)64- and Ru(NH3)63þ RCT (Ω cm2) at pH 7
RCT (Ω cm2) at pH 11.3
film
Fe(CN)63-
Ru(NH3)63þ
Fe(CN)63-
GCa GC-1 GC-1Hþ GC-10 -PhH GCphysb
1.60 103
5.50 102
1.90 103 3.30 105 4.40 103
7.60 104 2.90 103 6.50 102
8.10 103 2.65 105 3.90 105
a Bare electrode. b Bare electrode onto which isocyanatobenzene has been physisorbed.
charge effects. In the inset of Figure 2 Nyquist plots for GC-10 PhH and GC-1 are shown with additional examples being available in the Supporting Information (Figures 4S-6S). The values of RCT derived from such plots are collected in Table 1. The results present a compelling quantification of the blocking experiments for Fe(CN)63- at pH 7, in that RCT is much higher for GC-10 -PhH than GC-1Hþ. In fact, the latter has a value close to that of a bare electrode which can be attributed to the Coulombic interactions between the ammonium groups and Fe(CN)63-. As expected the opposite effect with the finding of a comparatively much higher value of RCT for GC-1Hþ comes true if Ru(NH3)63þ is used as redox probe. In addition, upon increasing pH from 7 to 11.3 in the Fe(CN)63- solution, thereby transforming GC-1Hþ to the neutral GC-1 film, RCT increases by more than a factor of 100 for GC-1 to become almost the same as for GC-10 -PhH; no pH effect is seen for the latter. The similar blocking abilities of the multilayered GC-1 and GC-10 -PhH indicate that the additional phenyl layer formed in the chemical addition reaction is rather loosely structured. At this point we may also note that RCT for a bare electrode is largely unaffected of the electrode being exposed to a solution of isocyanatobenzene, showing that physisorption of isocyanatobenzene (GCphys) is unfavorable and therefore should not interfere with the chemical reaction that forms GC-10 Ph (see below for further discussion). Analysis of the GC-10 -PhX (X = NO2, COPh, and NMe2) Films Using Cyclic Voltammetry and PM-IRRAS. For GC-10 -PhCl and GC-10 -PhH, very limited if any information would be obtainable from direct electrochemical measurements. In contrast, for the remaining GC-10 -PhX electrodes, the pendant urea-bonded functionality is electroactive and can be detected directly electrochemically. In the case of X = NO2, complementary PM-IRRAS measurements were also carried out, exploiting the strong and characteristic IR signals of the nitro group. The general conditions already outlined for the postmodification step with a 1 h reaction time were employed in all cases. X = NO2. In Figure 3, several successive voltammograms of a GC-10 -PhNO2 electrode recorded in a pure electrolyte solution are shown.40 For GC-10 -PhNO2, both the reduction wave of the nitrophenyl group and the reoxidation wave of the corresponding radical anion are evident, although continued potential cycling clearly deactivates the redox couple. The surface coverage Γ determined from an integration of the electrochemical signal on the first sweep is 2.9 10-10 mol cm-2 which, however, can be increased to ∼6 10-10 mol cm-2 using reaction times g2 h in the preparation of the film (see below). It should also be noted that the redox active moiety will be positioned at least a couple of nanometers from the electrode surface because of the multilayered nature of the GC-1 film. (40) It should be noted that a corresponding cyclic voltammogram of GC-1 itself is completely featureless.
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Figure 3. Successive cyclic voltammograms recorded of GC-10 PhNO2 in 0.1 M Bu4NBF4/MeCN at a sweep rate of 2 V s-1.
The deactivation of the redox couple is most likely due to the protonation of the radical anion of the nitrophenyl group formed on the reductive sweep by the residual water always present in the electrolyte solution. In addition, in a detailed analysis given below we will reveal that the propensity of having physisorption of 1-isocyanato-4-nitrobenzene on GC-1 electrodes is low, although it does take place on bare GC electrodes. Hence, a desorption process cannot be the origin of the deactivation seen. Concerning the specific kinetic features of the signal decay they will be treated in a forthcoming publication.41 The nitrophenyl group having easily identifiable IR absorption bands signals offers a good opportunity to complement the electrochemical analysis with recordings of PM-IRRAS spectra. PM-IRRAS is a high-sensitive surface technique which can be used in the characterization of thin films on various surfaces. In this study we used Au plates which are also amendable for grafting with diazonium salts.42 In order to ease the assignment of the bands in the PMIRRAS spectrum of Au-10 -PhNO2 we synthesized the 4-(2[3-(4-nitrophenyl)-ureido]-ethyl)-benzenediazonium salt (i.e., 10 -PhNO2) and recorded its KBr IR-spectrum (Figure 7S, Supporting Information). This diazonium salt was further electrografted to an Au plate to form Au-10 -PhNO2 directly [denoted Au-(10 -PhNO2)direct]. The PM-IRRAS spectra for Au-10 -PhNO2 and Au-(10 PhNO2)direct are shown in Figure 4. Most notably, in both spectra, distinct signals from the NdO stretch at 1332 (sym) and 1511 cm-1 (asym) of the nitro group confirm that the intended chemical surface modification has taken place. The additional weak bands at 1342 and 1304 cm-1 could originate from the urea moiety which in solution has a band at 1360-1300 cm-1 due to N-C-N stretch (asym).43 Similar bands are also found in the IRspectrum of 10 -PhNO2 (Figure 7S, Supporting Information). In the PM-IRRAS spectrum of Au-(10 -PhNO2)direct the band present at 1557 cm-1 is assigned to the urea N-H bending (1585-1515 cm-1 in solution). The same band is observed at 1553 cm-1 for the parent diazonium analog (Figure 7S, Supporting Information). Interestingly, for Au-10 -PhNO2 this band is weaker which may, however, stem from a different orientation of the molecules in the layer. (41) Nielsen, L. T.; Iruthayaraj, J.; Ceccato, M.; Hinge, M.; Pedersen, S. U.; Daasbjerg, K. Manuscript in preparation. (42) Bernard, M.-C.; Chausse, A.; Cabet-Deliry, E.; Chehimi, M. M.; Pinson, J.; Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2003, 15, 3450–3462. (43) All infrared frequency ranges listed originate from: Socrates, G. In Infrared and Raman Characteristic Group Frequencies; Wiley: Chichester, U.K., 2001.
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Figure 4. PM-IRRAS spectra of Au-10 -PhNO2 (---) and Au-(10 PhNO2)direct (;).
The bands at 1600 and 1612 cm-1 present in both PM-IRRAS spectra (and in Figure 7S) are associated with the CdC stretch from the benzene ring (1625-1590 cm-1 in solution). The fact that the 1612 cm-1 band of the spectrum of Au-10 -PhNO2 is relatively stronger than that of Au-(10 -PhNO2)direct indicates that there also may be a contribution from H-N-H bending in a primary amine (1650-1580 cm-1 in solution) in the former case. This assignment is further confirmed by carrying out deuterium exchange reactions on Au-1 (Figure 8S, Supporting Information) containing the pendant amino group, in that this particular band diminishes considerably. Hence, this would suggest that some of the amino groups (most likely inner-layer groups) in Au-1 do not react with the isocyanate and therefore is not transformed into the corresponding urea functionality in Au-10 -PhNO2, at least not under the reaction conditions used herein. The broad band at 1680 cm-1 in the spectra of both Au-10 PhNO2 and Au-(10 -PhNO2)direct has a contribution from the carbonyl stretch of the urea moiety (1705-1635 cm-1 in solution). In comparison, the IR spectrum of 10 -PhNO2 (Figure 7S, Supporting Information) has an intense and well-defined band at 1696 cm-1. Yet, considering that the 1680 cm-1 band is broad and that even Au-1 has a signal in this region we conclude that the band cannot exclusively be assigned to the urea group. At this point, we are not able to provide a definitive answer as to the origin of this additional contribution, but most likely it stems from the stretch of some kind of carbonyl group which apparently is introduced already during the electrografting of 1Hþ on Au. While in this manner having substantiated the validity of the proposed chemical assembling scheme, the next step would be to compare the outcome of this approach with the direct electrochemical approach, i.e., electrografting of the 4-nitrobenzenediazonium salt under potentiostatic conditions. Figure 5 shows the cyclic voltammogram of such a film assembled on a GC surface. The voltammogram pertains to the second cycle as the first cycle giving small and irreproducible signals is required for reorganizing the assembled film and opening up the structure to the electrolyte/solvent system. The direct aryl radical-based electrografting leads to the formation of a multilayered film consisting of oligomeric-like structures of nitrophenyl in addition to nitroazobenzenes.31,44,45 Because of the multilayer formation Γ becomes as high as (44) Saby, C.; Ortiz, B.; Champagne, G. Y.; Belanger, D. Langmuir 1997, 13, 6805–6813. (45) Doppelt, P.; Hallais, G.; Pinson, J.; Podvorica, F.; Verneyre, S. Chem. Mater. 2007, 19, 4570–4575.
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Figure 5. Cyclic voltammogram recorded of a 4-nitrobenzenediazonium-grafted GC electrode (obtained by potentiostatic electrolysis at Ep,c - 200 mV) in 0.1 M Bu4NBF4/MeCN at a sweep rate of 2 V s-1.
Figure 6. Successive voltammograms of GC-10 -PhCOPh recorded in 0.1 M Bu4NBF4/DMF at a sweep rate of 2 V s-1.
1.4 10-9 mol cm-2, but the electrochemical response is broad, containing contributions from several species. The herein presented isocyanate procedure for introducing nitrophenyl groups is cleaner in the sense that a better defined layer is formed, in which neighbor-neighbor interactions between the redox groups are comparatively less important because of the lower surface coverage. The price to pay is a half as high surface coverage and a more time-consuming film preparation procedure. X = COPh. With 4-isocyanatophenyl phenyl methanone as reagent another electrochemically active one-electron pendant redox group can be introduced using the general procedure outlined in Scheme 1. In Figure 6 the recording of successive voltammograms of the GC-10 -PhCOPh electrode is shown. First of all, it is evident that while the first reductive sweep contains several peaks the expected one-electron redox system pertaining to the benzophenone moiety with a midwave potential of -1.9 V vs SCE becomes the only one upon further sweeping.46 At the same time the signal decreases and on the sixth scan the electrochemical signal has almost vanished. A plausible explanation for the additional waves seen on the first sweep is that they originate from physisorbed 4-isocyanatophenyl phenyl methanone. This assignment is substantiated by (46) The potential of the surface attached benzophenone is reasonably close to that of the freely diffusion counterpart (=-1.7 V vs SCE).
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Figure 7. Successive voltammograms recorded of a 4-benzoylbenzenediazonium-grafted GC electrode (obtained by potentiostatic electrolysis at Ep,c - 200 mV) in 0.1 M Bu4NBF4/DMF at a sweep rate of 2 V s-1.
the observation that for a bare GC electrode immersed in a solution of 20 mM 4-isocyanatophenyl phenyl methanone for 1 h and rinsed by sonication, a subsequent cyclic voltammogram recorded in a pure electrolyte solution exhibits a two-wave signal around -2 V vs SCE (Figure 9S, Supporting Information). The decay of the signal assigned to the covalently attached benzophenone in GC-10 -PhCOPh upon sweeping is attributed to the protonation of the surfaced-attached benzophenone upon reduction. This process is notably faster than the corresponding for GC-10 -PhNO2 because of a higher basicity of the radical anion of GC-10 -PhCOPh as deduced from the substantially more negative reduction potential of GC-10 -PhCOPh compared with GC-10 PhNO2. A direct electrografting of a GC electrode with the 4-benzoylbenzenediazonium salt was attempted but as illustrated in Figure 7 the voltammograms of the modified electrode showed ill-defined waves exhibiting several peaks. Presumably, this may once again be attributed to the uncontrolled radical chemistry going on during the grafting process, potentially leading to the formation of various chemical structures. Thus, the benzophenone redox system provides another illustrative example, where the isocyanate approach can be used advantageously if the formation of a well-defined redox system is the goal. X = NMe2. To evaluate the influence of having a strongly electron-donating group present on the isocyanate reagent we turned our attention toward 4-isocyanato-N,N-dimethylaniline. From an electrochemical point of view this group is also highly suitable for analysis since it is detectable by electrochemical oxidation in cyclic voltammetry as shown by the voltammogram recorded for GC-10 -PhNMe2 (Figure 8). On the reverse reductive sweep a wave pertaining to the reduction of the radical cation of GC-10 -PhNMe2 is evident. The oxidation peak potential (=0.5 V vs SCE) is reasonable in line with that of the solution counterpart, N,N-dimethylaniline in MeCN (=0.71 V vs SCE).47 The redox pair possesses a relatively high stability in that it is able to withstand several voltammetric sweeps at a sweep rate of 2 V s-1. The large peak separation is indicative of a relatively slow charge transfer process which again is due to the multilayered nature of GC-1 used in the isocyanate reaction. In addition, the voltammograms reveal no indications of adsorption phenomena which is further substantiated by the observation that bare electrodes after being immersed into a solution of 4-isocyanato(47) Seo, E. T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy, D. W.; Adams, R. N. J. Am. Chem. Soc. 1966, 88, 3498–3503.
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Figure 8. Successive voltammograms of GC-10 -PhNMe2 recorded in 0.1 M Bu4NBF4/MeCN at a sweep rate of 2 V s-1.
N,N-dimethylaniline and then transferred to a pure electrolyte solution for cyclic voltammetric analysis show no significant signals. Several experiments were carried out to immobilize the N,Ndimethylaniline functionality through electrografting of the 4-(dimethylamino)benzenediazonium salt, both by controlledpotential electrolysis which should produce a multilayer and by individual cyclic voltammetric scans which should give a much thinner layer. In the best cases, it was possible to obtain a relatively well-defined signal (Figure 10S, Supporting Information) as also reported in literature for the immobilization of N,N-diethylaniline.48 However, several of the attempts resulted in the formation of film that would show a completely irreversible signal only or no signals at all. We noted that the presence of supporting electrolyte (i.e., 0.1 M Bu4NBF4), in general, had a favorable influence on the formation of electroactive films. Still, the conclusion remains, that if a higher reproducibility is a request the isocyanate route is recommendable. Physisorption. The EIS analysis of the GC-10 -PhX electrodes indicated that there were no physisorption in the case of X = H and Cl. On the other hand for X = COPh cyclic voltammetry (Figure 6) gave strong evidence of the presence of physically adsorbed species which were resistant toward ultrasonic treatment but not electrochemical reductive sweeping. For X = NO2, the situation is somewhat more dubious since molecular moieties containing this strong electron-withdrawing group tend to appear at the same potential, no matter if they are covalently attached or not. For this reason and considering that the nitrophenyl group is one of the most widely studied redox systems, we decided to carry out a thorough study of adsorption phenomena of 1-isocyanato-4-nitrobenzene. For comparative purposes 4-isocyanato-N,N-dimethylaniline that shows only little adsorption tendency was included as reference system. Four different electrode types, i.e. a bare GC electrode, GC1Hþ, GC-10 -PhH, and a 4-methylbenzene-grafted electrode (GC-PhMe) prepared using electrochemical reduction of 4-methylbenzenediazonium salt were immersed into a toluene solution of 1-isocyanato-4-nitrobenzene for 1 h followed by the normal rinsing procedure, including sonication for 10 min in both DMF and MeCN or acetone. The electrochemical activity of the electrodes was then investigated in a pure electrolyte solution. While voltammetric waves were evident for the bare electrode, only small (48) Floch, F. L.; Simonato, J.-P.; Bidan, G. Electrochim. Acta 2009, 54, 3078– 3085.
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and negligible signals were observable for the GC-1Hþ, GC-10 -PhH, and GC-PhMe electrodes. The reproducibility of the measurements at the bare electrode was rather low but, in general, a two-peak wave (Figure 11S, Supporting Information) with peak potentials close to that of GC-10 -PhNO2 was observed. We ascribe this electrode dependency as well as the two-peak nature to the presence of several distinct adsorption sites on the inhomogeneous GC material. At the same time it may be concluded that the substituent effect on the redox potential from the weakly electron-withdrawing isocyanate group (or any other related derivatives) is small in accordance with expectation. Essentially, no electrochemical signals were observable on the second and subsequent sweeps implying that most of the immobilized layer desorbs on the first sweep. In principle, even covalent bonding of isocyanates might occur through OH groups present in oxidized parts of the GC surface but if that should be the case such bonding is apparently broken as a result of the sweeping. In summary, the propensity of 1-isocyanato-4-nitrobenzene to physisorb at organic films is strongly dependent on the exact nature of the films as evidenced by the lack of electrochemical signals in the cases of GC-1Hþ, GC-10 -PhH, and GC-PhMe.49 A possible interpretation of this phenomenon is that a substituent on the phenyl ring sterically inhibits the 1-isocyanato-4-nitrobenzene molecules from optimizing the π-π interactions with the organic film. Considering that the GC-1 electrodes structurally are much more alike GC-1Hþ and GC-PhMe than the bare electrode it would be unlikely that physically adsorbed species to any large extent should be present on GC-10 -PhNO2. Moreover, if physisorption had occurred one would have expected as for the GC-10 -PhCOPh system (Figure 6) to see a much larger decline of the electrochemical signal going from the first to the second cycle (Figure 3) because of the desorption occurring on the first reductive sweep. Reaction Times. To obtain an understanding of the timedependency of the chemisorption processes we prepared a number of GC-10 -PhNO2, GC-10 -PhCOPh, and GC-10 -PhNMe2 electrodes for varying immersion (or reaction) times, timmersion. Voltammograms were recorded for each set of electrodes and the surface coverage was extracted from an integration of the wave on the first sweep. These results are displayed in Figure 9 in terms of Γ vs timmersion plots. Before discussing the data, we would like to emphazise that the electrochemical measurements provide only the density of solvated redox active sites and not necessarily the total number of sites. Usually, the electroactive units will be located in the outer layers or along pinholes with solvent accessibility, whereas groups located in the inner layers being in a more hydrophobic environment may be invisible to the electrochemical characterization.6,41 For the two-step chemically prepared GC-10 -PhX film, the PhX groups would be expected to be located primarily at the solvent accessible sites as this would be a requirement for being formed at all. Thus, we believe that the determination of the surface coverage for the particular kind of films studied herein should provide a good measure of the total amount of redox active sites.50 For all the three series we find that the reactions are completed within two hours resulting in a final coverage of (5-8) 10-10 mol cm-2 of chemisorbed species. It may be noted that this (49) Notably, for a GC-PhH electrode prepared through electrografting of benzenediazonium salt, some physisorption usually takes place. (50) As suggested by a reviewer we carried out an experiment involving a thicker than usual GC-1 film (produced at an electrolysis potential 600 mV more negative than usual, i.e. Ep,c - 800 mV). This had only a modest effect on the surface density, indicating that the derivatization reaction indeed proceeds mainly in the outer part.
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Figure 9. Plot of surface coverages, Γ, obtained from integration of the voltammetric signals against the immersion time, timmersion, for GC-10 -PhNO2 (2, measured for 5, 60, 120, 1440, and 2880 min), GC-10 -PhCOPh (O, measured for 5, 60, 150, and 2880 min), and GC-10 -PhNMe2 (0, measured for 0.75, 5, 1000, and 3820 min).
value compares well with the (4-10) 10-10 mol cm-2 obtained for the nucleophilic acyl substitution reaction between 4-nitrobenzoyl chloride and an aniline film formed by reducing an already-grafted nitrophenyl-containing film using various reducing methods.22 As elucidated above GC-10 -PhCOPh is the only of the modified electrodes, at which physisorption may occur concomitantly with the chemical reaction, so in this case cyclic voltammetric studies of a bare GC electrode that had been immersed into a solution of 4-isocyanatophenyl phenyl methanone were carried out (Figure 9S, Supporting Information). The coverage was determined to be ∼1 10-10 mol cm-2, independent of reaction time. This shows that physisorption as expected is a very fast process but also that its contribution to the coverage determined for GC-10 -PhCOPh will be small. This result is nicely in line with the fact that the signal from the physisorbed species in terms of the “bumps” seen on the first reductive sweep in Figure 6 is small. Finally, it is noteworthy that there appears to be essentially no substituent effect in the reactivity going from the electron-withdrawing to -donating substituents. Hence, we may conclude that nucleophilic addition reactions between a surface attached alkylamine and aryl isocyanates can take place rather swiftly at GC surfaces and be used advantageously for extending the length and modifying the structural properties of surface-attached molecular structures.
Summary The reaction between a nucleophilic 4-(2-aminoethyl)phenyltethered glassy carbon surface and various aryl isocyanates, OCN-PhX, having para substituents going from a strongly electron-withdrawing nitro group to a potent electron-donating dimethylamino group has been studied in toluene. It is demonstrated that a surface-attached alkylamine group, unless kept deactivated in its protonated ammonium form, can be involved in nucleophilic addition reactions that are completed within two hours. The study also shows that physisorption of aryl isocyanates may be a competing process for X = NO2 and COPh and that the physical adhesion may be so strong that it is able to withstand ultrasonication. However, potential sweeping is sufficiently effective for desorbing the physisorbed layer. Electrografting using appropriately designed diazonium salts would allow assembly of similar systems while at the same time providing higher surface coverages. However, since these DOI: 10.1021/la901666j
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radical-based methods are highly uncontrollable they usually give rise to the formation of multilayers consisting of oligomeric-like structures of the target system. The advantage in this respect of using a chemical reaction in the assembling process is that a welldefined and, in essence, a close-to a single layer of the target molecule can be generated in a distance of a couple of nanometers from the surface. For redox active molecules this is of particular importance since it leads to sharper and more well-defined electrochemical signals. Acknowledgment. We acknowledge Research Director Jens Hinke, SP Group, for fruitful discussions and Dr. Mogens Hinge for development of the software for background subtraction.
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Financial support from the Danish Agency for Science, Technology, and Innovation, the Danish National Research Foundation, and the iNANO School is greatly appreciated. Supporting Information Available: Text discussing synthetic procedures with schemes showing the reactions, blocking experiments, EIS measurement, PM-IRRAS spectra, electrochemical detection of physisorbed isocyanates, electrochemical responses of grafted N,N-dimethylaniline including figures showing cyclic voltammograms, Nyquist plots, and KBr IR, and PM-IRRAS spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.
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