- Email: [email protected]
Covalent modification of glassy carbon electrode during electrochemical oxidation process of 4-aminobenzylphosphonic acid in aqueous solution
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 585 (2005) 301–305 www.elsevier.com/locate/jelechem
Short communication
Covalent modification of glassy carbon electrode during electrochemical oxidation process of 4-aminobenzylphosphonic acid in aqueous solution Guocheng Yang a
a,b
, Baifeng Liu a, Shaojun Dong
a,b,*
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China Received 20 April 2005; received in revised form 8 July 2005; accepted 20 September 2005 Available online 25 October 2005
Abstract 4-Aminobenzylphosphonic acid (4-ABPA) was covalently grafted on a glassy carbon electrode (GCE) through two types of reaction mechanisms viz. amine oxidation and the Kolbe-like reaction during the electrooxidation process in 0.1 M KCl aqueous solution. Two irreversible oxidation peaks were observed at about 0.75 and 0.90 V in cyclic voltammograms between 0.5 and 1.1 V. X-ray photoelectron spectroscopy (XPS) shows the presence of two types of N 1s environment after oxidation of 4-ABPA on the GCE surface. The quantum chemistry calculation proves that both of the NH2 and CH2 groups in 4-ABPA molecule can perform the oxidation reaction in turn. From this, we found and substantiated that the Kolbe-like reaction of benzylphosphonic acid group in 4-ABPA molecule can take place in aqueous solution. Ó 2005 Elsevier B.V. All rights reserved. Keywords: 4-Aminobenzylphosphonic acid; Covalent modification of carbon electrode; Electrochemical oxidation of 4-aminobenzylphosphonic acid in aqueous solution; Two types of reaction mechanisms; The quantum chemistry calculation
1. Introduction The modification of highly ordered mono- or multi-layers on carbon materials surface has been paid great attention because it plays an important role in catalytic, analytical and biotechnological applications [1]. In the recent ten years, free radical grafting method has often been adopted to achieve the formation of covalent bonds between carbon surface and the modifier. The modification process is carried out by electrochemical oxidation of amine-containing compounds, arylacetates or aliphatic alcohols or by electrochemical reduction of diazonium salts in nonaqueous solution [2] or in aqueous solution [3–6]. One irreversible oxidation or reduction peak appears under *
Corresponding author. Tel.: +86 431 5262101; fax: +86 431 5689711. E-mail address: [email protected] (S. Dong).
0022-0728/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.09.017
electrochemical process, the covalent grafting of modifier on glassy carbon (GC) surface takes place only once during one potential cycle [7–9]. In this paper, we achieved the covalent modification of GC surface using 4-aminobenzylphosphonic acid (4ABPA) as a modifier in 0.1 M KCl aqueous solution through two types of reaction mechanisms viz. amine oxidation and the Kolbe-like reaction during electrochemical oxidation process.
2. Experimental section 2.1. Reagents 4-Aminobenzylphosphonic acid (4-ABPA) and Ru(NH3)6Cl3 were purchased from Sigma. The solution of 4-ABPA
302
G. Yang et al. / Journal of Electroanalytical Chemistry 585 (2005) 301–305
was freshly prepared for each modification. Other reagents were of analytical reagent grade and used as received. Water was purified using Millipore Mili-Q purification system in all the experiments.
3. Results and discussion
2.2. Electrochemical measurements
Fig. 1(a) shows cyclic voltammograms on a freshly polished GCE in 0.1 M KCl aqueous solution with 0.75 mM 4-ABPA at 10 mV s1. Two irreversible oxidation peaks are observed at about 0.75 and 0.90 V, respectively, from 1st to 4th cycles, the peak currents diminish gradually and the peak potentials shift to more positive with increasing the potential scanning cycles. Due to the continual potential scanning to 20th cycles, the two peaks almost disappear. This indicates the formation of a coating on the electrode surface. Fig. 1(b) shows cyclic voltammograms on GCE at 10 mV s1 in a 0.1 M KCl aqueous solution with different concentrations of 4-ABPA. The peak currents at about 0.75 and 0.90 V increase gradually and the peak potentials shift to more positive with increasing 4-ABPA concentration in the solution. Under the pretreatment of GCE with polishing and rinsing carefully, the modification process was repeated many times; however, the results always present two irreversible oxidation peaks. This indicates that the covalent grafting of 4-ABPA on GC surface appears to be twice during one potential scanning cycle.
Cyclic voltammetry was performed with a CHI 660 electrochemical workstation (USA) in a conventional three-electrode electrochemical cell. GC electrode (3 mm diameter) was obtained from Tokai Corp. and used as the working electrode. A twisted platinum wire and Ag/AgCl electrode were used as the auxiliary and the reference electrode, respectively. GC electrode was polished with 1.0-, 0.3- and 0.05-lm a-Al2O3 powders successively and sonicated in water for about 2 min after each polishing step. 2.3. Electrode modification The electrochemical modification of a GCE was performed in 0.1 M KCl solution containing 0.75 mM 4ABPA by potential scanning between 0.5 and 1.1 V (vs. Ag/AgCl). After modification, the electrode was successively rinsed with Milli-Q water and sonicated for 2 min in water to remove the physically adsorbed species. The 4-ABPA-modified GC plates were ready for characterization [10–13].
3.1. Covalent modification of GCE with 4-ABPA in aqueous solution
3.2. X-ray photoelectron spectroscopy of the 4-ABPAmodified GCE
2.4. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) measurement was carried out with an ESCALAB-MKII spectrometer (VG Co., UK) with an Al Ka X-ray radiation as the X-ray source for excitation. The data were obtained at room temperature, and typically the operating pressure in the analysis chamber was below 109 Torr with analyzer pass energy of 50 eV. The resolution of the spectrometer was 0.02 eV.
X-ray photoelectron spectroscopy (XPS) analysis results show that two types of N 1s environment exist at the GC plate grafted with 4-ABPA through 20 potential cycles after the sonication for 2 min. As shown in Fig. 2, the characteristic peak of the NH2 group for pure 4-ABPA powder appears at 400.8 eV. But after the oxidation of 4-ABPA on GC electrode, the peak of N 1s at 399.6 eV corresponds to the NH group [14], and the peak of N 1s at 402.4 eV to the NHþ 3 group [15]. The characteristic peaks of C 1s, O 1s and
10
a
40
8
6
2
4
3 4
2
20
I / µA
I / µA
b
1 30
4
20
3
10
2 1
0
0 0.5
0.6
0.7
0.8
0.9
E / V vs. Ag/AgCl
1.0
1.1
1.2
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
E / V vs. Ag/AgCl
Fig. 1. Cyclic voltammograms on a freshly polished GCE in 0.1 M KCl aqueous solution: (a) with 0.75 mM 4-ABPA from top to bottom: 1st, 2nd, 3rd, 4th and 20th cycles, (b) with different concentrations of 4-ABPA: (1) 0.25, (2) 0.50, (3) 0.75 and (4) 1.0 mM. Scan rate: 10 mV s1.
G. Yang et al. / Journal of Electroanalytical Chemistry 585 (2005) 301–305
303
paring the charge distributions of each group in the 4ABPA molecule, respectively, 0.103911 for PO3H2, 0.178941 for CH2, 0.2736 for C6H4 and 0.198603 for NH2 as shown below [17],
H2N
CH2
PO3H2
-0.178941 0.103911 0.273633 The charge distribution of 4-ABPA molecule
-0.198603
Fig. 2. (a) XPS of N 1s for pure 4-ABPA powder, (b) XPS of N 1s for a GC plate modified through 20 potential cycles with 0.75 mM 4-ABPA in 0.1 M KCl aqueous solution after the sonication for 2 min, and (c) lines fitted from b.
P 2p for a GC plate grafted with 0.75 mM 4-ABPA in 0.1 M KCl aqueous solution after 20 potential cycles occur at 284.6, 532.1 and 132.6 eV, respectively, and they are similar to those occurred at 284.5, 532.2 and 132.8 eV for pure 4-ABPA powder. Based on what we see in Fig. 2(c), which shows the fitted XPS of N 1s for a GC plate grafted with 4ABPA, the XPS integral peak areas of the NH group and the NHþ 3 group are 3421.84 and 2948.84, respectively. Because certain amounts of 4-ABPA molecule adsorbed physically on GC plate are cleaned out after the sonication for 2 min, the ratio of the NH group to the NHþ 3 group grafted finally on GC surface is 3421.84:2948.84 (1.16:1). This indicates that two close active sites are present at the 4ABPA molecule. 3.3. The confirmation of the active site by quantum chemistry calculation We carried out quantum chemistry calculation of the 4-ABPA molecule with the GAUSSIAN 98 program [16] in order to confirm the active sites in the molecule. By com-
we can see that the charge distribution of the NH2 and CH2 groups is close, so they both have the sufficient activity to actuate the oxidation reaction one after the other. The charge distribution of the NH2 group is more than that of the CH2 group, so the oxidation reaction of the NH2 group has precedence over that of the CH2 group; in other words, the oxidation potential of the NH2 group to its radical is lower than that of CH2 to its radical. 3.4. The charge states of the terminal phosphonic acid and amine groups on GCE The charge states of the terminal phosphonic acid and amine groups on GCE can be judged by electrochemical behaviors of the redox probe. As shown in Fig. 3, for the GCE modified with 4-ABPA after four potential cycles, 3 the electron transfer of the FeðCNÞ6 is partly blocked 3þ and that of the RuðNH3 Þ6 is almost not changed in comparison with the bare GCE. This indicates that on the one hand 4-ABPA/GCE surface charges negatively in PBS buffer solution of pH 7.0, on the other hand, GC surface possesses unsheltered region. In the case of GCE modified with 4-ABPA after 20 potential cycles, GC surface is covered with 4-ABPA more closely, the electron transfer of the 3 FeðCNÞ6 is completely blocked and that of the Ru3þ ðNH3 Þ6 is not changed. It is known that the amine group displays the XPS results in the NHþ 3 form. The terminal
Fig. 3. Cyclic voltammograms on bare (solid line), 4-ABPA/GCE (dashed line, after four potential cycles) and (dotted line, after 20 potential cycles) in: (a) 3þ 1 5 mM FeðCNÞ3 6 and (b) 2 mM RuðNH3 Þ6 solutions buffered by 0.1 M PBS (pH 7.0). Scan rate: 100 mV s .
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G. Yang et al. / Journal of Electroanalytical Chemistry 585 (2005) 301–305
Scheme 1. Grafting of 4-ABPA on a GCE surface.
phosphonic acid group exhibits PO2 3 in PBS buffer solution of pH 7.0 because not only do the negative charges of the PO2 3 group need to counteract the positive charges of the NHþ 3 group but also 4-ABPA/GCE completely 3 blocks the electron transfer of the FeðCNÞ6 . 3.5. The description of electrode modification process In view of the above-mentioned facts, the reaction mechanism of covalent grafting of 4-ABPA on GCE is described as: when the potential scans to about 0.75 V, 4ABPA is stimulated to produce amine radical leading to the formation of carbon–nitrogen bonds on GC surfaces as shown in Scheme 1. This peak potential is ascribed to oxidize the NH2 group by one-electron to its radical [18– 21]. When the potential scans to about 0.90 V, the Kolbelike reaction [22–25] of benzylphosphonic acid group in 4-ABPA molecule takes place and transfers one electron to GCE, giving rise to HPO3 and the 4-aminobenzyl radical, and the radical produced is immobilized on GC surface immediately through carbon–carbon bonds. HPO3is very unstable, it rapidly translates H3PO4 [26]. Via repetitive potential scans, 4-ABPA and 4-aminobenzyl can be adequately grafted on GC surface. 4. Conclusions In conclusion, we have successfully performed the covalent modification of 4-ABPA on GCE surface in aqueous solution. 4-ABPA is another new compound except for 4aminobenzoic acid, 4-aminobenzo-15-crown-5 ether and 4-aminobenzene sulfonic acid, which could be covalently grafted on GCE surface in aqueous solution. The synthesis method in aqueous solution is quite simple and avoids the complicated treatments of the organic solvent and the saturated solution in reference electrode. The modification
process of 4-ABPA through two types of reaction mechanisms, viz. amine oxidation and the Kolbe-like reaction, is proposed for the first time. Moreover, the theoretical calculation proves that both of the NH2 and benzyl groups in 4-ABPA molecule can perform the oxidation one after the other; this agrees well with the experimental results. From this, we find and substantiate that the Kolbe-like reaction of benzylphosphonic acid group in 4-ABPA molecule can take place in aqueous solution. Acknowledgments This work was supported by the National Science Foundation of China (Nos. 20275036 and 20210506). We are grateful to Dr. Yongqing Qiu at Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University for the quantum chemistry calculation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
J.-M. Zen, A.S. Kumar, D.-M. Tsai, Electroanalysis 15 (2003) 1073. A.J. Downard, Electroanalysis 12 (2000) 1085, and references therein. F. Geneste, C. Loinet, G. Jezequel, New J. Chem. 11 (2002) 1539. F. Geneste, M. Cadoret, C. Moinet, G. Jezequel, New J. Chem. 26 (2002) 1261. X. Li, Y. Wan, C. Sun, J. Electroanal. Chem. 569 (2004) 79. K. Morita, A. Yamaguchi, N. Teramae, J. Electroanal. Chem. 563 (2004) 249. B. Barbier, J. Pinson, G. Desarmot, M. Sanchez, J. Electrochem. Soc. 137 (1990) 1757. M. Delamar, R. Hitmi, J. Pinson, J.-M. Save´ant, J. Am. Chem. Soc. 114 (1992) 5883. M.A. Hayes, W.G. Kuhr, Anal. Chem. 71 (1999) 1720. O. Kahn, Nature 378 (1995) 667. O. Sato, T. Iyoda, A. Fujishima, K. Hashimoto, Science 272 (1996) 704. C. Lin, A.B. Bocarsly, J. Electroanal. Chem. 300 (1991) 325. D.M. Zhou, H.-X. Ju, H.-Y. Chen, J. Electroanal. Chem. 408 (1996) 219.
G. Yang et al. / Journal of Electroanalytical Chemistry 585 (2005) 301–305 [14] A.J. Wagner, G.M. Wolfe, D.H. Fairbrother, Appl. Surf. Sci. 219 (2003) 317. [15] J.-B. Dorota, N.K. Bozena, T. Wxodzimierz, Polymer 44 (2003) 5797. [16] M.J. Frisch et al., GAUSSIAN 98 (Revision A.9), Gaussian Inc., Pittsburgh, PA, 1998. [17] The charge distribution of each group in 4-ABPA molecule was obtained through the optimization by means of the ab initio Hartreee–Fock (HF) method on 6-31G(d) basis set level. [18] R. Nordberg, R.G. Albridge, T. Bergmark, U. Ericson, J. Hedman, C. Nordling, B.J. Siegbahn, Ark. Kemi. 28 (1968) 257. [19] R.S. Deinhammer, M. Ho, J.M. Anderegg, M.D. Porter, Langmuir 10 (1994) 1306.
305
[20] A.J. Downard, A. Mohamed, Electroanalysis 11 (1999) 418. [21] M. Masui, H. Sayno, Y. Tsuda, J. Chem. Soc. B (1968) 973. [22] C.P. Andrieux, F. Gonzalez, J.-M. Save´ant, J. Am. Chem. Soc. 119 (1997) 4292. [23] S.M. MacDonald, S.G. Roscoe, Electrochim. Acta 42 (1997) 1189. [24] C.P. Andrieux, F. Gonzalez, J.-M. Save´ant, J. Electroanal. Chem. 498 (2001) 171. [25] H. Johann, B. Michael, G. Andras, K. Hermann, L. Kornelia, M. Gtinter, R. Franz, W. Karin, Tetrahedron 54 (1998) 2059. [26] S.V. Kononova, M.A. Nesmeyanova, Biochemistry (Moscow) 67 (2002) 184.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 585 (2005) 301–305 www.elsevier.com/locate/jelechem
Short communication
Covalent modification of glassy carbon electrode during electrochemical oxidation process of 4-aminobenzylphosphonic acid in aqueous solution Guocheng Yang a
a,b
, Baifeng Liu a, Shaojun Dong
a,b,*
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China Received 20 April 2005; received in revised form 8 July 2005; accepted 20 September 2005 Available online 25 October 2005
Abstract 4-Aminobenzylphosphonic acid (4-ABPA) was covalently grafted on a glassy carbon electrode (GCE) through two types of reaction mechanisms viz. amine oxidation and the Kolbe-like reaction during the electrooxidation process in 0.1 M KCl aqueous solution. Two irreversible oxidation peaks were observed at about 0.75 and 0.90 V in cyclic voltammograms between 0.5 and 1.1 V. X-ray photoelectron spectroscopy (XPS) shows the presence of two types of N 1s environment after oxidation of 4-ABPA on the GCE surface. The quantum chemistry calculation proves that both of the NH2 and CH2 groups in 4-ABPA molecule can perform the oxidation reaction in turn. From this, we found and substantiated that the Kolbe-like reaction of benzylphosphonic acid group in 4-ABPA molecule can take place in aqueous solution. Ó 2005 Elsevier B.V. All rights reserved. Keywords: 4-Aminobenzylphosphonic acid; Covalent modification of carbon electrode; Electrochemical oxidation of 4-aminobenzylphosphonic acid in aqueous solution; Two types of reaction mechanisms; The quantum chemistry calculation
1. Introduction The modification of highly ordered mono- or multi-layers on carbon materials surface has been paid great attention because it plays an important role in catalytic, analytical and biotechnological applications [1]. In the recent ten years, free radical grafting method has often been adopted to achieve the formation of covalent bonds between carbon surface and the modifier. The modification process is carried out by electrochemical oxidation of amine-containing compounds, arylacetates or aliphatic alcohols or by electrochemical reduction of diazonium salts in nonaqueous solution [2] or in aqueous solution [3–6]. One irreversible oxidation or reduction peak appears under *
Corresponding author. Tel.: +86 431 5262101; fax: +86 431 5689711. E-mail address: [email protected] (S. Dong).
0022-0728/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.09.017
electrochemical process, the covalent grafting of modifier on glassy carbon (GC) surface takes place only once during one potential cycle [7–9]. In this paper, we achieved the covalent modification of GC surface using 4-aminobenzylphosphonic acid (4ABPA) as a modifier in 0.1 M KCl aqueous solution through two types of reaction mechanisms viz. amine oxidation and the Kolbe-like reaction during electrochemical oxidation process.
2. Experimental section 2.1. Reagents 4-Aminobenzylphosphonic acid (4-ABPA) and Ru(NH3)6Cl3 were purchased from Sigma. The solution of 4-ABPA
302
G. Yang et al. / Journal of Electroanalytical Chemistry 585 (2005) 301–305
was freshly prepared for each modification. Other reagents were of analytical reagent grade and used as received. Water was purified using Millipore Mili-Q purification system in all the experiments.
3. Results and discussion
2.2. Electrochemical measurements
Fig. 1(a) shows cyclic voltammograms on a freshly polished GCE in 0.1 M KCl aqueous solution with 0.75 mM 4-ABPA at 10 mV s1. Two irreversible oxidation peaks are observed at about 0.75 and 0.90 V, respectively, from 1st to 4th cycles, the peak currents diminish gradually and the peak potentials shift to more positive with increasing the potential scanning cycles. Due to the continual potential scanning to 20th cycles, the two peaks almost disappear. This indicates the formation of a coating on the electrode surface. Fig. 1(b) shows cyclic voltammograms on GCE at 10 mV s1 in a 0.1 M KCl aqueous solution with different concentrations of 4-ABPA. The peak currents at about 0.75 and 0.90 V increase gradually and the peak potentials shift to more positive with increasing 4-ABPA concentration in the solution. Under the pretreatment of GCE with polishing and rinsing carefully, the modification process was repeated many times; however, the results always present two irreversible oxidation peaks. This indicates that the covalent grafting of 4-ABPA on GC surface appears to be twice during one potential scanning cycle.
Cyclic voltammetry was performed with a CHI 660 electrochemical workstation (USA) in a conventional three-electrode electrochemical cell. GC electrode (3 mm diameter) was obtained from Tokai Corp. and used as the working electrode. A twisted platinum wire and Ag/AgCl electrode were used as the auxiliary and the reference electrode, respectively. GC electrode was polished with 1.0-, 0.3- and 0.05-lm a-Al2O3 powders successively and sonicated in water for about 2 min after each polishing step. 2.3. Electrode modification The electrochemical modification of a GCE was performed in 0.1 M KCl solution containing 0.75 mM 4ABPA by potential scanning between 0.5 and 1.1 V (vs. Ag/AgCl). After modification, the electrode was successively rinsed with Milli-Q water and sonicated for 2 min in water to remove the physically adsorbed species. The 4-ABPA-modified GC plates were ready for characterization [10–13].
3.1. Covalent modification of GCE with 4-ABPA in aqueous solution
3.2. X-ray photoelectron spectroscopy of the 4-ABPAmodified GCE
2.4. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) measurement was carried out with an ESCALAB-MKII spectrometer (VG Co., UK) with an Al Ka X-ray radiation as the X-ray source for excitation. The data were obtained at room temperature, and typically the operating pressure in the analysis chamber was below 109 Torr with analyzer pass energy of 50 eV. The resolution of the spectrometer was 0.02 eV.
X-ray photoelectron spectroscopy (XPS) analysis results show that two types of N 1s environment exist at the GC plate grafted with 4-ABPA through 20 potential cycles after the sonication for 2 min. As shown in Fig. 2, the characteristic peak of the NH2 group for pure 4-ABPA powder appears at 400.8 eV. But after the oxidation of 4-ABPA on GC electrode, the peak of N 1s at 399.6 eV corresponds to the NH group [14], and the peak of N 1s at 402.4 eV to the NHþ 3 group [15]. The characteristic peaks of C 1s, O 1s and
10
a
40
8
6
2
4
3 4
2
20
I / µA
I / µA
b
1 30
4
20
3
10
2 1
0
0 0.5
0.6
0.7
0.8
0.9
E / V vs. Ag/AgCl
1.0
1.1
1.2
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
E / V vs. Ag/AgCl
Fig. 1. Cyclic voltammograms on a freshly polished GCE in 0.1 M KCl aqueous solution: (a) with 0.75 mM 4-ABPA from top to bottom: 1st, 2nd, 3rd, 4th and 20th cycles, (b) with different concentrations of 4-ABPA: (1) 0.25, (2) 0.50, (3) 0.75 and (4) 1.0 mM. Scan rate: 10 mV s1.
G. Yang et al. / Journal of Electroanalytical Chemistry 585 (2005) 301–305
303
paring the charge distributions of each group in the 4ABPA molecule, respectively, 0.103911 for PO3H2, 0.178941 for CH2, 0.2736 for C6H4 and 0.198603 for NH2 as shown below [17],
H2N
CH2
PO3H2
-0.178941 0.103911 0.273633 The charge distribution of 4-ABPA molecule
-0.198603
Fig. 2. (a) XPS of N 1s for pure 4-ABPA powder, (b) XPS of N 1s for a GC plate modified through 20 potential cycles with 0.75 mM 4-ABPA in 0.1 M KCl aqueous solution after the sonication for 2 min, and (c) lines fitted from b.
P 2p for a GC plate grafted with 0.75 mM 4-ABPA in 0.1 M KCl aqueous solution after 20 potential cycles occur at 284.6, 532.1 and 132.6 eV, respectively, and they are similar to those occurred at 284.5, 532.2 and 132.8 eV for pure 4-ABPA powder. Based on what we see in Fig. 2(c), which shows the fitted XPS of N 1s for a GC plate grafted with 4ABPA, the XPS integral peak areas of the NH group and the NHþ 3 group are 3421.84 and 2948.84, respectively. Because certain amounts of 4-ABPA molecule adsorbed physically on GC plate are cleaned out after the sonication for 2 min, the ratio of the NH group to the NHþ 3 group grafted finally on GC surface is 3421.84:2948.84 (1.16:1). This indicates that two close active sites are present at the 4ABPA molecule. 3.3. The confirmation of the active site by quantum chemistry calculation We carried out quantum chemistry calculation of the 4-ABPA molecule with the GAUSSIAN 98 program [16] in order to confirm the active sites in the molecule. By com-
we can see that the charge distribution of the NH2 and CH2 groups is close, so they both have the sufficient activity to actuate the oxidation reaction one after the other. The charge distribution of the NH2 group is more than that of the CH2 group, so the oxidation reaction of the NH2 group has precedence over that of the CH2 group; in other words, the oxidation potential of the NH2 group to its radical is lower than that of CH2 to its radical. 3.4. The charge states of the terminal phosphonic acid and amine groups on GCE The charge states of the terminal phosphonic acid and amine groups on GCE can be judged by electrochemical behaviors of the redox probe. As shown in Fig. 3, for the GCE modified with 4-ABPA after four potential cycles, 3 the electron transfer of the FeðCNÞ6 is partly blocked 3þ and that of the RuðNH3 Þ6 is almost not changed in comparison with the bare GCE. This indicates that on the one hand 4-ABPA/GCE surface charges negatively in PBS buffer solution of pH 7.0, on the other hand, GC surface possesses unsheltered region. In the case of GCE modified with 4-ABPA after 20 potential cycles, GC surface is covered with 4-ABPA more closely, the electron transfer of the 3 FeðCNÞ6 is completely blocked and that of the Ru3þ ðNH3 Þ6 is not changed. It is known that the amine group displays the XPS results in the NHþ 3 form. The terminal
Fig. 3. Cyclic voltammograms on bare (solid line), 4-ABPA/GCE (dashed line, after four potential cycles) and (dotted line, after 20 potential cycles) in: (a) 3þ 1 5 mM FeðCNÞ3 6 and (b) 2 mM RuðNH3 Þ6 solutions buffered by 0.1 M PBS (pH 7.0). Scan rate: 100 mV s .
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G. Yang et al. / Journal of Electroanalytical Chemistry 585 (2005) 301–305
Scheme 1. Grafting of 4-ABPA on a GCE surface.
phosphonic acid group exhibits PO2 3 in PBS buffer solution of pH 7.0 because not only do the negative charges of the PO2 3 group need to counteract the positive charges of the NHþ 3 group but also 4-ABPA/GCE completely 3 blocks the electron transfer of the FeðCNÞ6 . 3.5. The description of electrode modification process In view of the above-mentioned facts, the reaction mechanism of covalent grafting of 4-ABPA on GCE is described as: when the potential scans to about 0.75 V, 4ABPA is stimulated to produce amine radical leading to the formation of carbon–nitrogen bonds on GC surfaces as shown in Scheme 1. This peak potential is ascribed to oxidize the NH2 group by one-electron to its radical [18– 21]. When the potential scans to about 0.90 V, the Kolbelike reaction [22–25] of benzylphosphonic acid group in 4-ABPA molecule takes place and transfers one electron to GCE, giving rise to HPO3 and the 4-aminobenzyl radical, and the radical produced is immobilized on GC surface immediately through carbon–carbon bonds. HPO3is very unstable, it rapidly translates H3PO4 [26]. Via repetitive potential scans, 4-ABPA and 4-aminobenzyl can be adequately grafted on GC surface. 4. Conclusions In conclusion, we have successfully performed the covalent modification of 4-ABPA on GCE surface in aqueous solution. 4-ABPA is another new compound except for 4aminobenzoic acid, 4-aminobenzo-15-crown-5 ether and 4-aminobenzene sulfonic acid, which could be covalently grafted on GCE surface in aqueous solution. The synthesis method in aqueous solution is quite simple and avoids the complicated treatments of the organic solvent and the saturated solution in reference electrode. The modification
process of 4-ABPA through two types of reaction mechanisms, viz. amine oxidation and the Kolbe-like reaction, is proposed for the first time. Moreover, the theoretical calculation proves that both of the NH2 and benzyl groups in 4-ABPA molecule can perform the oxidation one after the other; this agrees well with the experimental results. From this, we find and substantiate that the Kolbe-like reaction of benzylphosphonic acid group in 4-ABPA molecule can take place in aqueous solution. Acknowledgments This work was supported by the National Science Foundation of China (Nos. 20275036 and 20210506). We are grateful to Dr. Yongqing Qiu at Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University for the quantum chemistry calculation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
J.-M. Zen, A.S. Kumar, D.-M. Tsai, Electroanalysis 15 (2003) 1073. A.J. Downard, Electroanalysis 12 (2000) 1085, and references therein. F. Geneste, C. Loinet, G. Jezequel, New J. Chem. 11 (2002) 1539. F. Geneste, M. Cadoret, C. Moinet, G. Jezequel, New J. Chem. 26 (2002) 1261. X. Li, Y. Wan, C. Sun, J. Electroanal. Chem. 569 (2004) 79. K. Morita, A. Yamaguchi, N. Teramae, J. Electroanal. Chem. 563 (2004) 249. B. Barbier, J. Pinson, G. Desarmot, M. Sanchez, J. Electrochem. Soc. 137 (1990) 1757. M. Delamar, R. Hitmi, J. Pinson, J.-M. Save´ant, J. Am. Chem. Soc. 114 (1992) 5883. M.A. Hayes, W.G. Kuhr, Anal. Chem. 71 (1999) 1720. O. Kahn, Nature 378 (1995) 667. O. Sato, T. Iyoda, A. Fujishima, K. Hashimoto, Science 272 (1996) 704. C. Lin, A.B. Bocarsly, J. Electroanal. Chem. 300 (1991) 325. D.M. Zhou, H.-X. Ju, H.-Y. Chen, J. Electroanal. Chem. 408 (1996) 219.
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