Characterization by Electrochemical and X-ray Photoelectron

Article pubs.acs.org/Langmuir

Characterization by Electrochemical and X‑ray Photoelectron Spectroscopic Measurements and Quantum Chemical Calculations of N‑Containing Functional Groups Introduced onto Glassy Carbon Electrode Surfaces by Electrooxidation of a Carbamate Salt in Aqueous Solutions Aiko Kanazawa,† Takuro Daisaku,† Takeyoshi Okajima,† Shunichi Uchiyama,‡ Susumu Kawauchi,§ and Takeo Ohsaka*,† †

Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259-G1-5 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ‡ Department of Life Science and Green Chemistry, Graduate School of Engineering, Saitama Institute of Technology, 1690 Fusaiji, Fukaya, Saitama 369-0293, Japan § Department of Organic and Polymeric Materials, Graduate School of Engineering, Tokyo Institute of Technology, 2-12-1-E4-6 Oo-okayama, Meguro-ku, Tokyo152-8550, Japan S Supporting Information *

ABSTRACT: The present paper deals with characterization of an aminated glassy carbon electrode (GCE) surface obtained by electrooxidation of ammonium carbamate in its aqueous solution (amination reaction) using electrochemical and XPS methods. From the XPS analysis, it was found that not only the primary amine group (i.e., aniline-like aromatic amine moiety) but also other N-containing functional groups (i.e., the secondary amine-like moieties containing pyrrole-type nitrogen and quaternary amine-like moieties containing graphitic quaternary nitrogen) are introduced onto the GCE surface during the amination reaction. Moreover, the presence of the primary and secondary amine groups was ascertained based on the difference in the reactivity of a Michael reaction-type addition reaction of amine groups introduced onto the GCE surface with quinone compounds having a carbonyl group and a CC double bond (i.e., in this case, 1,2-benzoquinone which is in situ prepared by the electrooxidation of catechol) and on the electrochemical redox response of the introduced benzoquinones. This electrochemical treatment of aminated GCE with catechol led to catechol-grafted aminated GCE which indicated two surface redox couples (i.e., the Ia/Ic and IIa/IIc couples with formal potentials of E0′Ia/Ic = ca. 0.17 V and E0′IIa/IIc = ca. 0.03 V vs Ag|AgCl|KCl(sat.) in phosphate buffer solution (pH 7)). From the electrochemical behavior of catechols grafted onto the maleimide-treated aminated GCE and on the methylamine-treated GCE, it was found that the catechol associated with the primary amine groups gave the IIa/IIc redox peaks, while the catechol bound to the secondary amine groups gave the Ia/Ic redox peaks. Further electrochemical measurements and quantum chemical calculations concluded that the IIa/IIc redox peaks are ascribed to the surface-redox reaction of the 1,2-dihydroxybenzene/1,2-benzoquinone couple, while those of the 1,2dihydroxybenzene/1,2-benzoquinone and the N-(4′-hydroxyphenyl)-p-aminophenol/indophenol couples can be associated with the Ia/Ic redox peaks.

1. INTRODUCTION

properties and develop the potential applications of the resulting electrodes. Recently, Uchiyama and co-workers have found that amino groups can be easily introduced onto the GCE surface by the electrooxidation of carbamate salt in its aqueous solution.14 The

Glassy carbon (GC) has been widely used as an electrode material in many fields of electrochemistry because of its wide potential window and physical and chemical stabilities.1−3 On the other hand, much attention has been continuously paid to the development of the techniques for introducing a variety of functional groups and molecules onto glassy carbon electrode (GCE) surfaces4−13 in order to improve the electrode surface © 2014 American Chemical Society

Received: November 20, 2013 Revised: April 17, 2014 Published: April 23, 2014 5297

dx.doi.org/10.1021/la4044667 | Langmuir 2014, 30, 5297−5305

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used as auxiliary and reference electrodes, respectively. Before use, GCEs were pretreated by polishing their surfaces on a polishing pad with 1 and 0.06 μm alumina powders (used in this order) and then by scanning the electrode potential between −0.2 and +0.5 V vs Ag|AgCl| KCl(sat.) in 0.1 M PBS (pH 7) at 0.1 V s −1 until the steady-state cyclic voltammogram (CV) was obtained. 2.3. Amination of GCE Surfaces and Modifications of Aminated GCE. The introduction of N-containing functional groups onto the polished GCE surfaces (i.e., amination of GCE surfaces) was accomplished by the electrooxidation of carbamate salt (0.1 M ammonium carbamate aqueous solution (pH ca. 9)) at +1.0 V for 1 h,14 and the obtained electrode was abbreviated as Am-GCE. Modification of Am-GCE surfaces with maleimide was performed by immersing the Am-GCEs into a 0.1 M maleimide aqueous solution for 90 min, and the resulting electrode was designated as maleimidebonded Am-GCE (Mal/Am-GCE). Further electrochemical modification of Am-GCE and Mal/Am-GCE was performed by using cyclic voltammetry (potential scan range, −0.2 to +0.5 V; number of potential cycles, 20; and potential scan rate, 0.1 V s −1) in 0.1 M PBS (pH 7) containing 1 mM catechol, and the obtained electrodes were designated as catechol-bonded Am-GCE (Cat/Am-GCE) and catechol-bonded Mal/Am-GCE (Cat/Mal/Am-GCE), respectively. 2.4. XPS. XPS measurements were performed using an ESCA 3400 electron spectrometer (Shimadzu, Japan), having a X-ray source (Mg Kα line (1253.6 eV); emission current, 20 mA; and acceleration voltage, 10 kV). The samples for XPS measurements were prepared on a GC disk (diameter 6 mm) electrode. For the analysis of XPS spectra, the peak position of the C 1s core-level XPS spectrum was set at 285.0 eV and used as a reference for other peaks.25 The deconvolution of XPS spectra was done by the least-squares method using the Gaussian−Lorentzian peak shape with the values of the error function: (Σχ2) < 0.2.26 2.5. Quantum Chemical Calculation. All quantum chemical calculations for optimized molecular structures of the most stable conformers regarding catechol moieties grafted onto the GCE surface were carried out using the Gaussian 09 (revision C. 01) program package.27 The structures of reactants (i.e., the possible reduced forms of catechol moieties, R1 and R4 in Figure 4) and products (i.e., the possible oxidized forms of catechol moieties P2, P3, P5, and P6 in Figure 4) in aqueous solution were optimized by the ωB97X-D28 longrange corrected density functional including empirical dispersion with the 6-311+G(d,p) basis set,29,30 where the solvent effect on the structures of the reactants and products was taken into account using the reaction field calculation with the integral equation formalism of the polarizable continuum model (IEFPCM).31

obtained electrode has been termed an aminated GCE (AmGCE). The Am-GCE has some interesting electrocatalytic characteristics for oxygen reduction, hydrogen oxidation, and the redox reactions of inorganic and organic compounds15,16 and can be further modified with ο-benzoquinone having a carbonyl group and a CC double bond, namely, a compound containing an α,β-unsaturated ketone moiety.14 Uchiyama et al. have reported chemical sensors using Am-GCEs19 and biosensors based on the chemical modification of Am-GCEs with enzyme.17,18 They have suggested that amino groups are grafted to a GC surface by oxidative pretreatment in 0.1 M ammonium carbamate solution. In contrast to this suggestion, Wildgoose et al.20 have proposed an alternative explanation involving the electropolymerization of ο-benzoquinone formed by oxidation of catechol and/or the polymerization of οbenzoquinone with ο-quinone-like species formed on the GC surface during the oxidative pretreatment step. Thus, further detailed characterization of the Am-GCE surface seems to be necessary for a complete understanding of the interesting modification of carbon electrodes by the electrolysis of ammonium carbamate. In this study, we characterized the Am-GCE surface in detail by the use of electrochemical methods and X-ray photoelectron spectroscopy (XPS). The N-containing functional groups introduced onto the GCE surface were identified by taking advantage of the differences in their reactivity, with οbenzoquinone having a CC double bond and a carbonyl group as well as by XPS analysis of the GCE surface. οBenzoquinone is easily in situ generated by the electrochemical oxidation of 1,2-dihydroxybenzene and can participate in the subsequent unique chemical reaction, where ο-benzoquinone is well known to form adducts with the primary or secondary amines through a 1,4-conjugate addition reaction similar to a so-called Michael addition reaction.21−23 Quantum chemical calculations suggested possible conformers of the oxidized and reduced forms of the catechol adducts formed. We found that not only primary amine groups but also other N-containing functional groups (or moieties) are introduced onto the GCE surface. Also, we discussed briefly the possible mechanism of the introduction of N-containing functional groups.

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION 3.1. Characterization of N-Containing Functional Groups on GCE Surfaces by XPS. Figure 1 shows XPS spectra obtained for the GCEs before and after the electrooxidation of carbamate salt. Hereafter the GCEs obtained before and after the electrooxidation are referred to as untreated GCE and Am-GCE, respectively. The signal due to the N 1s core level was clearly observed for Am-GCE (Figure 1(A)), while this signal did not clearly appear for untreated GCE (Figure 1(B)). In this case, from the calculation of C 1s and N 1s peak areas, the values of atomic ratio N/C % were estimated to be 0.4 and 7.9 for untreated GCE and Am-GCE, respectively. The analysis of N 1s core-level XPS spectra obtained in the narrow binding energy region is very useful for the speculation of functional groups (or moieties) actually introduced onto the GCE surfaces. From the deconvolution analysis of XPS spectra as shown in Figure 1(C), it was found that the N 1s spectrum obtained for Am-GCE is composed of three components and that those three components with peaks at 399.6, 400.6, and 401.4 eV are ascribed to aromatic aminetype (aniline-like) nitrogen, pyrrole-type nitrogen, and graphitic

2.1. Chemicals. Ammonium carbamate, catechol (1,2-dihydroxybenzene), citric acid monohydrate, disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O), sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), and trisodium phosphate dodecahydrate (Na3PO4·12H2O) were purchased from Kanto Chemical Co., Inc., Japan. Methylamine, maleimide, boric acid, and sodium chloride were commercially available from Wako Pure Chemical Industries, Ltd., Japan. All of the reagents were of analytical grade and used without any further purification. Phosphate buffer solution (PBS, 0.1 M (1 M = 1 mol dm−3), pH 7) was prepared from 0.1 M NaH2PO4 and Na2HPO4. Universal buffer solutions (pH range from 2.5 to 8.7) containing 0.5 M NaCl were composed of 50 mM H3BO3−citric acid−Na3PO4 threecomponent solutions.24 All aqueous solutions were prepared using water purified by a Millipore Milli-Q system (Millipore, Japan). 2.2. Electrochemical Equipment and Cells. A model 604A ALS/Chi electrochemical analyzer (Bioanalytical System Inc. (BAS), Japan) was used to perform controlled-potential electrolysis and cyclic voltammetric measurements. All electrochemical experiments were conducted with a three-electrode system at room temperature (25 ± 1 °C) under a N2 gas atmosphere. A glassy carbon disk electrode (GCE, diameter 3.0 mm, BAS, Japan) was used as the working electrode, and a platinum spiral wire and potassium chloride-saturated silver|silver chloride electrode (Ag | AgCl | KCl(sat.), TDA-DKK, Japan) were 5298


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Wildgoose and his co-workers. However, it is clear from our XPS results that the electrooxidation of carbamate salt leads to the introduction of N-containing functional groups onto the GCE surface, as also supported by the electrochemical characterization mentioned below. 3.2. Electrochemical Characterization of N-Containing Functional Groups on GCE Surfaces. It is well known that the primary and secondary aromatic amines and the secondary aliphatic amine can easily react with an oxidized form of catechol, 1,2-benzoquinone, which is often in situ prepared by an electrochemical oxidation of catechol through a 1,4conjugate addition reaction, (i.e., a so-called Michael reactiontype addition reaction) to form the corresponding adducts.22,23 The resulting adducts undergo the rearrangement reaction in solution (pH >5) to form electroactive catechol moieties which undergo a two-electron/two-proton redox reaction. The analytical techniques based on this principle have been applied to the spectroscopic determination of 1,2-benzoquinone23 and the electrochemical detection of aniline and its derivatives.21 This procedure can be also applied to the heterogeneous reaction systems. That is to say, if the primary and/or the secondary amine groups are present on GCE surfaces, then such N-containing functional groups on the electrode surface could be identified by observing the electrochemical response of catechol bonded to these surface functional groups. The surface-confined catechol moieties are expected to reveal a twoelectron/two-proton redox response. Figure 2 shows the typical cyclic voltammogram (CV) obtained at the GCE, which was treated by a controlledpotential electrolysis at +1.0 V vs Ag|AgCl|KCl(sat.) in 0.1 M ammonium carbamate aqueous solution for 60 min and then by scanning the electrode potential (potential scan range, −0.2 to +0.5 V; potential scan number, 20 cycles) at 0.1 V s−1 in 0.1 M PBS (pH 7.0) containing 1 mM catechol. The thus-treated GCE is called a Cat/Am-GCE. Two couples of redox peaks, indicated as Ia and Ic and IIa and IIc, were observed at this Cat/Am-GCE with formal potentials (E0′) of (0.17 ± 0.01) and (0.03 ± 0.01) V, suggesting the introduction of the primary and/or the secondary amine groups onto the GCE surface (Scheme 1). In order to clarify that the functional groups introduced onto the Cat/Am-GCE are primary and/or secondary amine groups, the following two experiments were carried out: one is a deactivation of primary amine groups by their reaction with maleimide (Scheme 2),37−39 and the other is an introduction of secondary amine groups, −N(CH3)H, onto the GCE surface through the electrooxidation of methylamine (Scheme S1).7,40 In the first experiment, we tried to protect the primary amine groups selectively on the GCE surface with maleimide by utilizing the difference in reactivity of primary and secondary amine groups with maleimide:37−39 primary amine groups can easily react with maleimide via a Michael-type addition reaction, while the reaction of secondary amine groups with maleimide is extremely slow. The conjugate addition reaction of the maleimide-bonded Am-GCEs (Mal/Am-GCEs in Scheme 2) with 1,2-benzoquinone is inhibited because of a decrease in the nucleophilicity of the nitrogen atom of maleimide, although maleimide has a moiety similar to a secondary amine. In Figure 2, the CV, represented by the dashed line, was obtained at the Am-GCE which was first reacted with maleimide and then with 1,2-benzoquinone. The treatment of Am-GCE with maleimide resulted in a significantly decreased electrochemical response for the IIa/IIc redox couple, keeping the redox response for the

Figure 1. Comparative results of the XPS wide-scan spectra obtained for (A) Am-GCE and (B) untreated GCE surfaces. N 1s core-level XPS spectra obtained for (C) Am-GCE and (D) untreated GCE surfaces. Both electrodes are the same as those as used in (A) and (B). (1) Aromatic amine-type (aniline-like) nitrogen, (2) pyrrole-type nitrogen, and (3) graphitic quaternary nitrogen.

Figure 2. Cyclic voltammograms (CVs) obtained at Cat/Am-GCE (solid line) and at Am-GCE, which was first reacted with maleimide and then with 1,2-benzoquinone (dashed line), in 0.1 M phosphate buffer solution (PBS, pH 7). The dotted line shows the background response obtained in 0.1 M PBS (pH 7) before the electrochemical treatment for binding catechol moieties. Potential scan rate, 0.1 V s −1.

quaternary nitrogen corresponding to highly coordinated nitrogen atoms substituting for inner carbon atoms within the graphene sheet, respectively.32−36 In addition, the total area of the N 1s peak region obtained for the Am-GCE surface consisted of 29.2% aromatic amine-type N, 30.2% pyrrole-type N, and 40.6% graphitic quaternary N. On the other hand, as shown in Figure 1(D), the N 1s spectrum observed for untreated GCE, the intensity of which is about 1/10 of that observed for Am-GCE, is deconvoluted into two components which are ascribed to pyrrole-type nitrogen at 400.3 eV and graphitic quaternary nitrogen at 401.4 eV. Those two kinds of nitrogen would be derived from the GCE material itself. Wildgoose and his co-workers have conducted experiments similar to those that Uchiyama et al. did and have reported that the N 1s core-level XPS spectrum observed at 402.1 eV is ascribed to the nitrogen of an amide group of the carbamate anion and/or an ammonium ion as its countercation from carbamate salts adsorbed onto GCE surfaces.20 Therefore, our obtained results were completely different from those by 5299


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Scheme 1. Schematic Illustration Showing the Electrochemical Treatments of the Aminated GCE (Am-GCE) in PBS (pH 7) Containing Catechol

Scheme 2. Schematic Illustration Showing the Chemical Reactions of the Primary (A) and Secondary (B) Amine Groups of AmGCE with Maleimide and the Subsequent Electrochemical Treatment in PBS (pH 7) Containing Catechol

to be approximately 1.9 × 10−10 and 7.3 × 10−11 mol cm−2, respectively, while after the treatment of Am-GCE with maleimide those of the primary and secondary amine groups on Cat/Mal/Am-GCE were estimated to be ca. 7.7 × 10−11 and ca. 7.5 × 10−11 mol cm−2, respectively. Here it should be noted that the similar redox responses were also observed even without the grafting of ammonium carbamate as previously reported,20 but the surface coverages of probable surfaceconfined electroactive species (Figure S1) were about 1/25 to 1/3 of those estimated above for the primary and secondary amine groups. In the second experiment to identify the surface-confined amine groups, secondary amine groups were introduced onto the GCE surface by a controlled-potential electrolysis at +1.0 V in 0.1 M methylamine aqueous solution7,40,41 (Scheme S1). That is, this electrolysis produced a methylamine (i.e., H(CH3)N-secondary amine groups)-modified GCE (MetAmGCE) surface. From the XPS measurement of the MetAmGCE surface (Figure 3(A),(B)), the introduction of the N-

Ia/Ic couple almost constant. Thus, this indicates that the IIa/ IIc couple corresponds to the redox reaction of the catechol moieties bonded to the primary amine groups on the Mal/AmGCE surface, while the Ia/Ic redox response is ascribed to that of the catechol moieties bonded to the secondary amine groups (Scheme 2). As readily seen from the comparison of the CVs in Figure 2 which were obtained before and after repeating the potential cycle at 0.1 V s−1 between −0.2 and +0.5 V vs Ag| AgCl|KCl(sat.) 20 times in 0.1 M PBS containing 1 mM catechol, this electrochemical treatment for binding catechol moieties to the primary and secondary amines on the GC electrode surface, which could have been introduced by the electrooxidation of ammonium carbamate, led to a significant change in the electrical double layer charging current, probably reflecting the physicochemical change of the electrode surfaces by the introduction of quinone moieties. Therefore, from the amounts of charge for the IIa/IIc and Ia/Ic redox responses shown in Figure 2, the surface coverages of the primary and secondary amine groups on the Cat/Am-GCE were estimated 5300


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Figure 3. XPS spectra of (A) the wide-scan region and (B) the N 1s region obtained for MetAm-modified GCE: (1) aromatic amine-type (aniline-like) nitrogen, (2) pyrrole-type nitrogen, and (3) graphitic quaternary nitrogen. (C) CV obtained for Cat/MetAm-GCE in 0.1 M PBS (pH 7). The dotted line shows the background response obtained in 0.1 M PBS (pH 7) before the electrochemical treatment for binding catechol moieties. Potential scan rate, 0.1 V s −1.

Figure 4. ωB97X-D/6-311+G(d,p) optimized molecular structures of the most stable conformers of (A) redox couple IIa/IIc (the possible reduced form is R1, and the possible oxidized forms are P2 and P3) and (B) redox couple Ia/Ic (the possible reduced form is R4, and the possible oxidized forms are P5 and P6) in aqueous solution using IEFPCM. White, gray, blue, and red spheres indicate hydrogen, carbon, nitrogen, and oxygen atoms, respectively.

Scheme 3. Surface Redox Reactions Speculated for Redox Couples IIa/IIc (A) and Ia/Ic (B) on the Cat/Am-GCE Surface in Aqueous Solutions

in Figure 3(B), it was found that the total area of the N 1s peak region obtained for the MetAm-GCE surface consisted of 32.6% aromatic amine-type N (399.4 eV), 38.8% pyrrole-type N (400.5 eV), and 28.6% graphitic quaternary N (401.4 eV). In this case, it is likely that both H(CH3)N− and H2N− groups, which form during the one-electron oxidation of methylamine, contribute to the aromatic amine-type N signal at 399.4 eV.40,42 The electrochemical treatment of the MetAm-GCE with catechol provided catechol-bonded MetAm-GCE (Cat/ MetAm-GCE). Figure 3(C) shows a typical CV obtained for Cat/MetAm-GCE in 0.1 M PBS (pH 7.0). We can see a welldefined redox response with E0′ = ca. 0.16 V, and from the comparison of Figures 2 and 3(C), we can say that the Ia/Ic redox response with E0′ = (0.17 ± 0.01) V shown in Figure 2 corresponds to the redox reaction of the catechol moieties bound to the secondary amine groups, the surface coverage of which on Cat/MetAm-GCE was estimated to be ca. 4.1 × 10−11 mol cm−2. As mentioned above, it is also noted in this case that the electrical double layer charging current is different before and after the electrochemical treatment for binding catechol moieties. Both of these experiments thus clarify that the primary and secondary amine groups are introduced onto the GCE surface

containing functional groups on the GCE surface was confirmed. In this case, from the calculation of C 1s and N 1s peak areas, the value of atomic ratio N/C % was estimated to be 9.4. From the peak-fitting analysis of XPS spectra as shown 5301


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Scheme 4. Possible Reaction Sequences during the Electrooxidation of Carbamic Acid

couples gave slopes of −55 and −60 mV/pH unit, respectively, in agreement with that (−59 mV/pH) expected for a twoelectron, two-proton redox process (Figure S2). Figure 4(A) and Table S1 show the results of quantum chemical calculation regarding conformers R1, P2, and P3, which are models suitable for the IIa/IIc redox couple (Scheme 3(A)). The values of the relative Gibbs free energy, ΔG(P2+2H) − R1 for the reaction (R1 → P2 + 2H) and ΔG(P3+2H) − R1 for the reaction (R1 → P3 + 2H), were estimated to be 131.22 and 136.47 kcal mol−1, respectively. In this case, the difference (Δ(ΔG) = ΔG (P3+2H) − R1 − ΔG(P2+2H) − R1) between ΔG(P3+2H) − R1 and ΔG(P2+2H) − R1 was 5.25 kcal mol−1. This indicates that conformer P2 is more stable than conformer P3 and that a catechol/1,2benzoquinone-type redox reaction is expected for the IIa/IIc redox couple, i.e., the reaction P2 ⇄ R1 shown in Scheme 3(A). On the other hand, Figure 4(B) and Table S1 show the results of the quantum chemical calculation regarding conformers R4, P5, and P6, which are the models suitable for the Ia/Ic redox couple (Scheme 3(B)). From Table S1, the optimized structure of P5 and P6 seems to be the structure of 1,2-benzoquinone. In this case, however, it is difficult to

through the electrooxidation of ammonium carbamate in its aqueous solution and also that the IIa/IIc redox peaks observed at ca. 0.03 V in Figure 2 are ascribed to the redox reaction of catechol moieties bound to the primary amine groups on the Am-GCE surface, while the Ia/Ic redox peaks observed at ca. 0.17 V correspond to the electrochemical response of catechol moieties bound to the secondary amine groups. Here, it should be mentioned that the amide group of the carbamate anion and the ammonium ion from carbamate salts adsorbed on the GCE surface, which were, as mentioned in section 3.1, considered to be the origin of the N 1s core-level XPS spectrum observed at 402.1 eV by Wildgoose et al.,20 do not give the abovementioned results expected from both of these experiments. As shown in Scheme 3, two different mechanims are considered for the redox reactions of the surface-confined catechol moieties: one corresponds to the redox reaction of a catechol/1,2-benzoquinone couple, and the other corresponds to that of an N-(4′-hydroxyphenyl)-p-aminophenol/indophenol couple. These two couples of catechol/1,2-benzoquinone and N-(4′-hydroxyphenyl)-p-aminophenol/indophenol are well known to undergo a two-electron, two-proton redox reaction.43 Actually, the plots of E0′ vs pH for the Ia/Ic and IIa/IIc redox 5302


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catechol moieties grafted onto the GCE surface were optimized by quantum chemical calculations. In addition, a possible mechanism for the introduction of N-containing functional groups onto the GCE surface by the electrooxidation of carbamic acid was discussed briefly.

determine the more stable conformer between P5 and P6 as the oxidized form of R4 because conformers P5 and P6 can be resonating conformers with respect to each other. Thus, the redox reaction for the Ia/Ic couple was found to be the reaction between R4 and resonating conformers of P5 and P6. In addition, the calculation of the redox potential from the ΔG value in each redox reaction showed that the redox potential for reaction P5 (or P6) ⇄ R4 is more positive than that for reaction P2 ⇄ R1, which is in agreement with the experimental results. 3.3. Possible Mechanisms of Introduction of NContaining Functional Groups. A carbamic acid undergoes a one-electron oxidation at the GCE (Scheme 4(a) and Figure S3) to form a radical species (HṄ COO−). The radical species attacks the defects on the GCE,11−13,44 and an abstraction reaction of hydrogen takes place, which leads to the formation of a new radical species (site) on the GCE surface. The subsequent coupling reaction between this radical site and another radical species of carbamic acid produces the primary amine group with CO2 elimination (Scheme 4(b)). Oxygencontaining functional groups such as carbonyl groups are considered to be formed on the GCE surface under the present condition of a constant-potential electrolysis of carbamic acid (i.e., 1.0 V vs Ag|AgCl|KCl(sat.), 1 h).3 The existence of defects on the graphite basal plane (e.g., single vacancy V1 (5−9)44) may also lead to the introduction of oxygen-containing functional groups (e.g., carbonyl group).3 Thus, H2NCOO− reacts with active sites generated by the introduction of a carbonyl group to produce the primary and secondary amine groups (Scheme 4(c)). The quaternary amine groups can also be expected to be formed, typically as shown in Scheme 4(d) (i.e., (i) the electrooxidation of single-vacancy V1 (5−9)44 to produce carbonyl groups, (ii) the reaction of H2NCOO− with the carbon of one carbonyl group followed by the elimination of CO2 and OH− to form the imine group, (iii) the elimination of H2O to form the tertiary amine group, and (iv) finally the electrooxidation with OH− elimination). Here, it should be noted that the yield of grafting is very low, probably due to the dimerization of electrogenerated radical species HṄ COO− to produce hydrazine and finally N2 (Scheme 4(e)).45

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ASSOCIATED CONTENT

* Supporting Information S

Cyclic voltammograms obtained at the bare GC electrode in PBS before and after electrochemical treatment with catechol. pH dependence of formal potentials in CVs obtained at Cat/ Am-GCE. Steady-state voltammogram and typical log[I/(Ilim − I)] vs E plot obtained for electrooxidation of ammonium carbamate. Electrooxidation of methylamine and amination at GCE and subsequent electrochemical treatment of methylamine-modified GCE. Typical bond lengths related to the most stable conformers of P2, P3, and P5 (or P6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-45-924-5404. Fax: +81-45-924-5489. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Mr. Tomoaki Yoshida is acknowledged for his help with the electrooxidation of carbamic acid. The numerical calculations were carried out on the TSUBAME2.5 supercomputer at the Tokyo Institute of Technology, Tokyo, Japan, and on the supercomputer at the Research Center for Computational Science, Okazaki, Japan.

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REFERENCES

(1) Van Der Linden, W. E.; Dieker, J. W. Glassy Carbon as Electrode Material in Electroanalytical Chemistry. Anal. Chim. Acta 1980, 119, 1−24. (2) Kamau, G. N. Surface Preparation of Glassy Carbon Electrodes. Anal. Chim. Acta 1988, 207, 1−16. (3) McCreery, R. L. Carbon Electrodes: Structural Effects on Electron Transfer Kinetics. In Electroanalytical Chemistry: A Series of Advances; Bard, A. J., Ed.; Marcel Dekker, Inc.: New York, 1991; Vol. 17, pp 221−374. (4) Fujihira, M.; Fukuda, Y.; Osa, T. Organo-modified Carbon Electrode. III. Modification via Nitration. Rev. Polarogr. (Kyoto) 1977, 23, 30. (5) Mazur, S.; Matusinovic, T.; Cammann, K. Organic Reactions of Oxide-free Carbon Surfaces, An Electroactive Derivative. J. Am. Chem. Soc. 1977, 99, 3888−3890. (6) Oyama, N.; Brown, A. P.; Anson, F. C. Introduction of Amine Functional Groups on Graphite Electrode Surfaces and Their Use in the Attachment of Ruthenium(II) to the Electrode Surface. J. Electroanal. Chem. 1978, 87, 435−441. (7) Barbier, B.; Pinson, J.; Desarmot, G.; Sanchez, M. Electrochemical Bonding of Amines to Carbon Fiber Surfaces toward Improved Carbon-epoxy Composites. J. Electrochem. Soc. 1990, 137, 1757−1764. (8) Delamar, M.; Hitmi, R.; Pinson, J.; Savéant, J. M. Covalent Modification of Carbon Surfaces by Grafting of Functionalized Aryl Radicals Produced from Electrochemical Reduction of Diazonium Salts. J. Am. Chem. Soc. 1992, 114, 5883−5584.

4. CONCLUSIONS We have characterized in detail the surface of a glassy carbon electrode (GCE), which is aminated by the electrooxidation of ammonium carbamate in its aqueous solution,14 through XPS analysis and the unique chemical reactions of primary and secondary amine groups with maleimide and/or 1,2-benzoquinone. Nitrogen-containing functional groups (or moieties), introduced onto the GCE surface by this electrooxidation, were identified using an XPS technique such as aromatic amine-type N (i.e., an aniline-like primary amine group with a binding energy of 399.6 eV), pyrrole-type N (i.e., a pyrrole-like secondary amine group with a binding energy of 400.6 eV), and a graphitic quaternary N (binding energy = 401.4 eV)containing functional groups (or moieties). The primary and secondary amine groups on the GCE surface reacted with 1,2benzoquinone (in situ electrogenerated from catechol) to introduce the catechol moieties onto it. The resulting catecholgrafted GCE showed two redox peaks at E0′ = ca. 0.03 and ca. 0.17 V vs Ag|AgCl|KCl(sat.) in PBS (pH 7.0) which are ascribed to the redox reactions of catechol moieties bound to the primary and secondary amine groups on the GCE surface, respectively. Probable structures of stable conformers of the 5303


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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on May 1, 2014. Minor text revisions were made in the last paragraph of the Experimental Section and in the Acknowledgment. The corrected version was reposted on May 2, 2014.

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Characterization by Electrochemical and X-ray Photoelectron

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