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Electrochemical and in Situ FTIR Spectroscopic Investigation on the Electrochemical Transformation of 4-Aminothiophenol on a Gold Electrode in Neutral Solution C. Retna Raj, Fusao Kitamura, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Received May 21, 2001. In Final Form: August 15, 2001 The electrochemical transformation of the self-assembled monolayer of 4-aminothiophenol (4-ATP) has been studied in neutral solution by means of electrochemical and in situ FTIR measurements, and the results are compared with those in acidic solution. The electrochemical transformation in neutral solution results in the formation of a surface-confined aniline dimer, 4′-mercapto-N-phenylquinone diimine as in the case of acidic solution. The redox peak corresponding to the aniline dimer shifts by -56 mV while increasing the solution pH by 1 unit, suggesting that protons and electrons take part in the redox reaction in the ratio of 1 to 1. The surface coverage of the aniline dimer obtained in the neutral solution is relatively low ((1.3 ( 0.3) × 10-10 mol cm-2) when compared to that in 0.1 M HClO4 solution ((2.4 ( 0.3) × 10-10 mol cm-2). Very similar in situ FTIR spectral features were obtained in both acidic and neutral solutions, indicating that the reaction mechanism follows the same head-to-tail coupling in neutral solution as in the case of acidic solution. The electrochemical and FTIR spectroscopic studies of the model compound, N-phenyl-1,4-phenylene diamine, support the formation of aniline dimer on the electrode surface during the electrochemical transformation of 4-ATP in neutral and acidic solutions. The electroproduced diimine undergoes hydrolysis to yield the monoimine while holding the electrode at high positive potential.
Introduction The chemisorption of thiols and disulfides on gold (Au) surfaces has attracted considerable interest, as it results in the formation of well-organized monolayers.1-4 The selfassembling technique is a convenient method to prepare a stable and well-organized monolayer with controllable thickness and desired function. The self-assembled monolayers (SAMs) of long-chain alkanethiols form densely packed ordered films and can be used to engineer the surface energy of the metal surface.1-4 SAMs have been used as the basis for pH-dependent electrostatic binding of ions, molecular recognition, as platforms for further surface chemical modification, and for fundamental studies of electron transfer.1-6 SAMs containing more than one chemical species hold great promise as a scheme for * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-045-924-5489. Tel: +81045-924-5404. (1) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (c) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409. (2) (a) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (b) Hubbard, A. T. Chem. Rev. 1988, 88, 633. (c) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753. (d) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (3) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (b) Bain, C. D.; Whitesides, G. M. Adv. Mater. 1989, 1, 506. (c) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (d) Ulman, A. Chem. Rev. 1996, 96, 1533. (e) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109. (4) (a) Bain, C. D.; Whitesides, G. M. Science 1988, 240, 62. (b) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (c) Diem, T.; Czajka, B.; Weber, B.; Regen, S. L. J. Am. Chem. Soc. 1986, 108, 6094. (d) Sabatani, E.; Rubinstein, I.; Moaz, R.; Sagiv, J. J. Electroanal. Chem. 1987, 219, 365. (e) Sabatini, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663.
preparing patterns of surface active groups.7 These highly organized and structured interfaces can, in principle, provide the means to control the chemical and physical properties of the interfaces for a variety of heterogeneous processes.8 The SAMs of aromatic thiols have also been used in the recent years to modify the coinage metal surfaces for a variety of applications.2 Aromatic thiols are interesting for several reasons; the molecules are highly anisotropic, and the intermolecular interactions are expected to be stronger than those between the alkanethiols, which may lead to different molecular packing structure. Higher electrical conductance would be expected in the aromatic thiol, as the electrons are delocalized in the benzene ring.9 The adsorption of thiophenol, benzyl mercaptan, mercaptopyridine, and p-biphenyl mercaptan on platinum, silver, and gold was investigated, and a layered structure consistent with the formation of the (x3 × x3)R30° adlayer on Au(111) has been observed.2 (5) (a) Nahir, T. M.; Clark, R. A.; Bowden, E. F. Anal. Chem. 1994, 66, 2595. (b) Rebek, J. J. Acc. Chem. Res. 1990, 23, 399. (c) Rubenstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (d) Bent, S. F.; Schilling, M. L.; Wilson, H. E.; Katz, H. E.; Harris, A. L. Chem. Mater. 1994, 6, 122. (e) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (6) (a) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (7) (a) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (b) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (c) Stranick, S.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (8) (a) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13. (b) Notoya, T.; Poling, G. W. Corrosion 1979, 35, 193. (c) Zisman, W. A. In Friction and Wear; Davis, R., Ed.; Elsevier: New York, 1959. (d) Kaelble, D. H. Physical Chemistry of Adhesion; Wiley-Interscience: New York, 1971. (9) Jin, Q.; Rodriguez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J. Surf. Sci. 1999, 425, 101.
10.1021/la010746q CCC: $20.00 © 2001 American Chemical Society Published on Web 10/20/2001
Electrochemical Transformation of 4-ATP
It has been shown that the 4-aminothiophenol (4-ATP) undergoes electrochemical oxidation in acidic solution at the electrode surface.10-12 Hayes and Shannon have shown that the surface-bound 4-ATP undergoes electrochemical transformation to produce a new surface species, 2-(4′mercaptophenylamino)benzoquinone, which exhibits reversible redox properties.10 Later, Lukkari and co-workers investigated the electrochemistry of 4-ATP and suggested a different mechanism for the electrochemical oxidation.11 Furthermore, they have shown that the electrochemical oxidation of 4-ATP at pH g 6 does not yield the redox species giving a reversible redox response, which is distinct from the case of an acidic solution, probably due to the formation of electroinactive products.11 During the course of the present investigation, two reports appeared discussing the electrochemical oxidation of aniline-terminated alkanethiol monolayers in aqueous acidic and nonaqueous media.13 According to these reports, the electrochemical oxidation of an aniline-terminated alkanethiol monolayer results in the formation of polymer, dimer, and hydrolyzed dimer.13 Our group is interested in the development of electrochemical sensors based on SAMs of functionalized thiols and disulfides, and we are currently studying the SAMs of different redox active and redox inactive thiol/disulfides for the construction of efficient electrochemical sensors for the detection of biomolecules such as dopamine, NADH, and so forth.14 The electrocatalytic oxidation of NADH has received considerable interest, as it has been required for more than 400 oxidoreductases.15 It has been shown that the diimines are excellent candidates for the electrocatalytic oxidation of NADH.16 During the course of our investigations, we observed that the electrochemical transformation of 4-ATP in neutral solution yields surfaceconfined redox active diimine and it efficiently catalyzes the oxidation of NADH.17 As the electrochemical transformation of 4-ATP in neutral solution yields a redox active surface-confined diimine, we are interested to characterize the monolayer by electrochemical and in situ FTIR measurements and compare the results with those in acidic solution. Although various methods such as cyclic voltammetry, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and electroreflectance spectroscopy have been used for the characterization of the structure, the composition, and the packing of SAMs,1-13,18-20 the use of in situ FTIR spectroscopy for the characterization of monolayers received considerable (10) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688. (11) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705. (12) (a) Batz, V.; Schneeweiss, M. A.; Kramer, D.; Hagenstrom, H.; Kolb, D. M.; Mandler, D. J. Electroanal. Chem. 2000, 491, 55. (b) Kuwabata, S.; Fukuzaki, R.; Nishizawa, M.; Martin, C. R.; Yoneyama, H. Langmuir 1999, 15, 6807. (13) (a) Schomburg, K, C.; McCarley, R. L. Langmuir 2001, 17, 1983. (b) Schomburg, K, C.; McCarley, R. L. Langmuir 2001, 17, 1993. (14) (a) Gobi, K. V.; Tokuda, K.; Ohsaka, T. J. Electroanal. Chem. 1998, 444, 145. (b) Raj, C. R.; Gobi, K. V.; Ohsaka, T. Bioelectrochem. Bioenerg. 2000, 51, 181. (c) Raj, C. R.; Ohsaka, T. J. Electroanal. Chem. 2001, 496, 44. (d) Raj, C. R.; Tokuda, K.; Ohsaka, T. Bioelectrochem. Bioenerg. 2001, 53, 183. (e) Raj, C. R.; Ohsaka, T. Bioelectrochem. Bioenerg. 2001, 53, 251. (15) White, H. B., III Evolution of coenzymes and the origin of pyridine nucleotides; Academic Press: New York, 1982. (16) Kitani, A.; So, Y.-H.; Miller, L. L. J. Am. Chem. Soc. 1981, 103, 7636. (17) Raj, C. R.; Ohsaka, T. Electrochem. Commun., in press. (18) (a) Fenter, P.; Eisenberger, P.; Li, J.; Camillone, N., III; Bernasek, S.; Scoles, G.; Ramanarayanan, T. A.; Liang, K. S. Langmuir 1991, 7, 2013. (b) Bandyopadhyay, K.; Vjayamohanan, K.; Venkatramanan, M.; Pradeep, T. Langmuir 1999, 15, 5314. (c) Hu, K.; Chai, Z.; Whitesell, J. K.; Bard, A. J. Langmuir 1999, 15, 3343. (b) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500.
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attention only in the recent years.21-24 In this paper, we report the electrochemical transformation of 4-ATP on a Au electrode in neutral and higher pH solutions and the characterization of the electrochemically derived monolayer by electrochemical and in situ FTIR techniques. Experimental Section Materials. 4-Aminothiophenol (4-ATP), thiophenol (TP), and N-phenyl-1,4-phenylenediamine (NPPD) were purchased from Aldrich and were used as received. All other chemicals used in this investigation were of Analar grade and used without further purification. Deionized water (Milli-Q system, Millipore, Japan) was used to prepare the electrolyte solution. All the electrochemical measurements were carried out under a nitrogen atmosphere. NaH2PO4, Na2HPO4, and HClO4 were used to prepare the supporting electrolyte. Voltammetric Measurements. Electrochemical studies were performed using a two-compartment three-electrode cell with a polycrystalline Au working electrode (diameter 1.6 mm), a Pt wire auxiliary electrode, and a NaCl saturated Ag/AgCl reference electrode. Cyclic voltammograms were recorded using a computercontrolled electrochemical analyzer (BAS 100B/W). The Au working electrodes were polished with alumina powder (1.0 and 0.06 µm) and sonicated in water for 5-10 min. The polished electrodes were then electrochemically cleaned by cycling the potential scan between -0.2 and 1.5 V in 0.05 M H2SO4 at the scan rate of 10 V/s for 10 min or until the cyclic voltammogram characteristic of a clean Au electrode was obtained. The SAMs of 4-ATP and TP were fabricated by immersing the cleaned Au electrode into an ethanol solution of 10 mM of the respective thiols for 1 h. In Situ FT-IR Measurements. The in situ FTIR spectroscopic measurements were carried out in a so-called SEIRAS (surface enhanced infrared absorption spectroscopy)25 mode with an FTS6000 FT-IR spectrometer (Bio Rad Laboratories) equipped with a liquid N2 cooled MCT detector. A 20 nm thick Au film vacuumevaporated on the base plane of a Si hemicylindrical prism was used as the working electrode. The evaporation of Au on the Si prism was performed in a vacuum of 4 × 10-3 Pa from a tungsten boat by thermal heating with a deposition rate of 0.05 nm/s. A series of SEIRA spectra synchronized with the potential sweep at the rate of 50 mV/s was obtained in the kinetics mode, in which each interferogram was acquired in every 0.27 s at the spectral resolution of 8 cm-1. Potential-difference SEIRA spectra were obtained with 64 scans and were coadded and averaged at the sample and the reference potentials, respectively. An Ag|AgCl and a Pt wire were used as the reference and the auxiliary electrodes, respectively. All the FTIR measurements were performed at room temperature under an Ar atmosphere. The gold electrode was modified with 4-ATP by exposing the electrode surface to an ethanol solution of 4-ATP (10 mM) for 30 min. The theoretical calculations were performed using the PC GAMESS version26 of the GAMESS (US) QC package.27 The (19) (a) Barner, B. J.; Corn, R. M. Langmuir 1990, 6, 1023. (b) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (20) (a) Creager, S. E.; Steiger, C. M. Langmuir 1995, 11, 1852. (b) Laibinis, P. E.; Bain, C. D.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 7663. (21) (a) Stole, S. M.; Porter, M. D. Langmuir 1990, 6, 1199. (b) Popenone, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (c) Bae, I. T.; Sandifer, M.; Lee, Y. W.; Tryk, D. A.; Sukenik, C. N.; Scherson, D. A. Anal. Chem. 1995, 67, 4508. (d) Yang, D. F.; AlMazani, H.; Morin, M. J. Phys. Chem. B 1997, 101, 1158. (22) (a) Anderson, M. R.; Gatin, M. Langmuir 1994, 10, 1638. (b) Mielczarski, J. A.; Mielzarski, E.; Zachwieja, J.; Gases, J. M. Langmuir 1995, 11, 2787. (23) (a) Sato, Y.; Ye, S.; Haba, T.; Uosaki, K. Langmuir 1996, 12, 2726. (b) Ye, S.; Yashiro, A.; Sato, Y.; Uosaki, K. J. Chem. Soc., Faraday Trans. 1996, 92, 3813. (c) H.-Z, Yu.; Zhang, H.-L.; Liu, Z.-F.; Ye, S.; Uosaki, K. Langmuir 1998, 14, 619. (24) (a) John, S. A.; Kitamura, F.; Tokuda, K.; Ohsaka, T. Langmuir 2000, 16, 876. (b) Arihara, K.; Kitamura, F.; Ohsaka, T.; Tokuda, K. J. Electroanal. Chem. 2000, 488, 117. (c) Gobi, K. V.; Kitamura, F.; Tokuda, K.; Ohsaka, T. J. Phys. Chem. B 1999, 103, 83. (d) Nanbu, N.; Kitamura, F.; Ohsaka, T.; Tokuda, K. J. Electroanal. Chem. 2000, 485, 128. (25) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861. (26) Granovsky, A. A. http://classic.chem.msu.su/gran/gamess/ index.html.
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Figure 1. Cyclic voltammograms obtained during the electrochemical transformation of 4-ATP in 0.1 M phosphate buffer solution (pH 7.2). The potential was cycled between -0.2 and 0.7 V at the scan rate of 100 mV/s. The inset shows the cyclic voltammograms obtained after electrochemical transformation in 0.1 M phosphate buffer (pH 7.2). Scan rate: 100, 300, 500, 700, 900, and 1200 mV/s.
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Figure 2. Cyclic voltammograms of the electrochemically derived monolayer of 4′-mercapto-N-phenylquinone diimine at different pHs. Scan rate: 100 mV/s. The inset shows the plot of formal potential against the solution pH: (a) 9.4, (b) 8.3, (c) 7.2, (d) 6.2, (e) 4.9, and (f) 2.7.
geometry optimization was performed by the HF or the GVB method using the DZV basis set. The vibrational frequencies and IR intensities were calculated upon the optimized geometries. All the theoretical spectra were depicted after a scale factor of 0.89 was multiplied to the calculated frequencies.
Results and Discussion Electrochemical Transformation of 4-ATP. Figure 1 shows the cyclic voltammograms obtained for the 4-ATP-Au electrode while cycling the electrode potential between -0.2 and 0.7 V in 0.1 M phosphate buffer (pH 7.2). In the first sweep, a large irreversible anodic wave (X) at ca. 0.5 V was observed and the intensity of this wave gradually decreased while sweeping the potential continuously. At the same time, a symmetrical reversible redox peak (Y) centered at ca. 0.23 V was observed in the subsequent sweeps and it attained the steady state within 5-7 cycles. In addition, an irreversible wave (Z) at ca. -0.17 V was observed and it disappeared gradually upon repeated cycling. The electrode reaction associated with wave Z is not necessary for the formation of the redox wave Y. The electrode modified with TP did not show any of the above such electrochemical responses confirming that the results observed at the electrode modified with the 4-ATP SAM originate from the oxidation of the surfaceconfined amine. The redox peak Y shows the cyclic voltammogram characteristic of a surface-confined redox species, and it negatively shifts by ca. 56 mV as the solution pH is increased by 1 unit (Figure 2). The peak current linearly increases with the sweep rate below 1000 mV/s, and the peak-to-peak separation is typically small although not zero. The surface coverage of the redox peak Y in neutral pH (pH 7.2) was calculated by integrating the area of the anodic/cathodic peak, and it was found to be (1.3 ( 0.2) × 10-10 mol cm-2. As can be seen from Figure 2, an observable decrease in the surface coverage of the redox peak Y was found in acidic pH. The most probable reason for the observed decrease could be the hydrolysis of the surface-bound diimine to monoimine, as the hydrolysis is more favorable in acidic solution. (27) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gorden, M. S.; Jensen, J. J.; Koseki, S.; Matsunaga, N.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347.
Figure 3. Cyclic voltammograms obtained during the electrochemical transformation of 4-ATP in 0.1 M HClO4. The potential was cycled between -0.1 and 0.9 V at the scan rate of 100 mV/s. The inset shows the cyclic voltammograms obtained after the electrochemical transformation in 0.1 M HClO4. Scan rate: 100, 300, 500, 700, 900, and 1100 mV/s.
The electrochemical oxidation of 4-ATP in acidic solution (0.5 M HClO4) has been observed, and it involves the headto-tail coupling of a radical cation, which results in the formation of a redox species, 4′-mercapto-4-aminodiphenylamine.11 The surface-confined 4′-mercapto-4-aminodiphenylamine undergoes further transformation, hydrolysis, while standing in acidic solution or applying high positive potential to give the corresponding iminoquinone which is further hydrolyzed eventually to 1,4-benzoquinone and the surface-bound 4-ATP.11 Moreover, it has been shown that the whole oxidation process is strongly affected by the solution pH and the electrochemical oxidation of 4-ATP at pH g 6 does not yield a redox species giving a reversible redox response, probably due to the formation of electroinactive products.11 However, as shown in Figure 1, we observed that the 4-ATP SAM undergoes electrochemical transformation in neutral pH (0.1 M phosphate buffer, pH 7.2) and produces a stable surfaceconfined redox species as in the case of acidic solution. The electrochemical transformation of 4-ATP was also
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Figure 4. Cyclic voltammograms of the electrochemically transformed monolayer of the 4-ATP electrode in 0.1 M HClO4. Cyclic voltammograms were recorded after holding the electrode potential at 0.75 V for different time intervals: (a) 0, (b) 15, (c) 30, (d) 45, (e) 90, and (f) 135 min. The inset shows the plot of peak current at 0.5 and 0.2 V against time. Electrochemical transformation of 4-ATP was carried out in 0.1 M HClO4.
Figure 6. (A) 3-D perspective plot showing the SEIRA spectra of the interface as a function of applied potential. Spectra were obtained during the first two potential cycles between 0 and 0.9 V at 50 mV/s in 0.1 M HClO4. (B) SEIRA spectra obtained during the first cycle at different potentials. The spectrum obtained at 0 V was used as the reference. Figure 5. Cyclic voltammograms of the electrochemically transformed monolayer of the 4-ATP electrode in 0.1 M phosphate buffer (pH 7.2) before (a) and after (b) holding the electrode potential at 0.4 V for 95 min.
carried out at different pHs, and we found that the electrochemical oxidation of 4-ATP results in the formation of surface-confined redox species giving a reversible redox response at all the pHs studied in this investigation (up to pH 9.4). The electrochemical oxidation of 4-ATP in 0.1 M HClO4 was also carried out, and it results in the formation of a redox species, which exhibits two distinct redox peaks at around 0.25 and 0.5 V (Figure 3). These results are in complete agreement with those reported by Lukkari and co-workers.11 The peak observed at 0.5 V corresponds to the aniline dimer, whereas the peak at 0.25 V corresponds to the hydrolyzed dimer, iminoquinone.11 The surface coverage of the redox peak Y was determined to be (2.4 ( 0.2) × 10-10 mol cm-2. To assess the reaction mechanism for the electrochemical transformation of 4-ATP in neutral solution, it is worth considering the different reaction pathways for the electrochemical oxidation of aniline. Generally, the elec-
trooxidation mechanisms are envisaged to occur via the initial formation of an aniline radical cation, which can then couple to form ring dimers, by three parallel pathways. The head-to-tail coupling of the radical cation results in the formation of N-phenyl-1,4-phenylenediamine, and the tail-to-tail coupling of the radical cations yields benzidine. These two coupling reactions are favorable only in acidic solution.28 The head-to-head coupling of the radical cations would result in the formation of hydrazobenzene, which is more favorable in basic solutions.29 In the present investigation, the tail-to-tail coupling of the radical cations could not be expected as the 4-ATP molecules are chemisorbed on the Au electrode through sulfur. If the electrooxidation of 4-ATP in neutral solution involves head-to-head coupling, the product should be azobenzene and the azobenzene is expected to give a redox response at more negative potentials with a large peak-to-peak separation.23c Such an electrochemical (28) (a) Bacon, J.; Adams, R. N. J. Am. Chem. Soc. 1968, 90, 6596. (b) Hand, R. L.; Nelson, R. F. J. Am. Chem. Soc. 1974, 96, 850. (29) (a) Matsuda, Y.; Shono, A.; Iwakawa, C.; Ohshiro, Y.; Agawa, T.; Tamura, H. Bull. Chem. Soc. Jpn. 1971, 44, 2960. (b) Desideri, P. G.; Lepri, L.; Heimler, D. J. Electroanal. Chem. 1971, 32, 225.
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Figure 8. Theoretical IR spectra of 4-ATP, protonated 4-ATP, and the cation radical obtained by the one-electron oxidation based on ab initio MO calculations.
Figure 7. (A) 3-D perspective plot showing the SEIRA spectra of the interface as a function of applied potential. Spectra were obtained during the first two potential cycles between -0.2 and 0.7 V at 50 mV/s in 0.1 M phosphate buffer (pH 7.2). (B) SEIRA spectra obtained during the first cycle at different potentials. The spectrum obtained at -0.2 V was used as the reference.
response was not observed for the electrochemically transformed monolayer of 4-ATP, and therefore the headto-head coupling reaction pathway can be excluded. Therefore, the reaction mechanism for the electrochemical transformation of 4-ATP in neutral solution could involve the head-to-tail coupling of the radical cations and the reaction product would be very similar to that in acidic solution. That is, the redox peak Y observed at 0.23 V in neutral solution can be ascribed to the redox reaction of the 4′-mercapto-N-phenylquinone diimine/4′-mercapto4-aminodiphenylamine couple, which involves two electrons and two protons. To support our above conclusion on the electrochemical transformation of 4-ATP in neutral solution, the cyclic voltammograms of a model compound, N-phenyl-1,4phenylenediamine (NPPD) have been recorded in acidic and neutral solutions at a Au electrode. The cyclic voltammogram of NPPD is well-defined (E° ) 0.47 V) in acidic solution (0.1 M HClO4) whereas it is quasi-reversible in neutral solution (E° ) 0.09 V), in complete agreement with the previous report.30 The cyclic voltammogram obtained for the electrochemically transformed 4-ATPAu electrode in acidic solution is comparable (E° ) 0.52 V) with that of NPPD in acidic solution, whereas it is slightly different in neutral solution (E° ) 0.23 V). (30) Male, R.; Allendoerfer, R. D. J. Phys. Chem. 1988, 92, 6237.
Figure 9. SEIRA spectra obtained for the cation radical in (a) 0.1 M HClO4 at 0.735 V and (b) 0.1 M phosphate buffer (pH 7.2) at 0.44 V.
Although the formal potential of the model compound does not match exactly with the formal potential of the electrochemically transformed monolayer, the observed results support the fact that the electrochemical transformation of 4-ATP in both neutral and acidic solutions involves the head-to-tail coupling. The charge associated with the anodic wave for the oxidation of the surface-confined amine relates with the surface coverage of the 4-ATP monolayer on a Au electrode. The surface coverage was calculated to be (2.0 ( 0.3) × 10-9 mol cm2 at all the pHs studied here, and this is in close agreement with the recent report by Mandler and co-workers.12a It has been suggested that the large surface coverage obtained for 4-ATP could be due to the oxidation of more than one monolayer.12a As the dimerization reaction requires two 4-ATP molecules, the surface coverage of the dimer should be less than the original surface concentration of 4-ATP, even if 100% conversion
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Figure 10. SEIRA spectra obtained for the 4-ATP electrode after several potential cycles in (A) 0.1 M HClO4 and (B) 0.1 M phosphate buffer (pH 7.2) at different potentials.
Figure 11. (A) SEIRA spectra obtained for the 4-ATP electrode before and after cycling the potential between 0 and 0.9 V in 0.1 M HClO4. The spectrum obtained for the electrode at 0 V after oxidative desorption of 4-ATP at 1.3 V for 30 s was used as the reference: (a) before cycling the potential, (b) after several cycles, (c) after holding the electrode potential at 0.75 V for 100 min, and (d) spectrum obtained for NPPD (KBr disk). (B) SEIRA spectra obtained for the 4-ATP electrode before and after cycling the electrode potential between -0.2 and 0.7 V in 0.1 M phosphate buffer (pH 7.2). The spectrum obtained for the electrode at -0.2 V after oxidative desorption of 4-ATP at 1.3 V for 30 s was used as the reference: before potential cycling (a) and after several potential cycles (b).
has been considered. Although the surface coverage of 4-ATP obtained from the oxidation wave of 4-ATP remained almost the same at all the pHs, the surface coverage associated with the reversible redox wave Y was found to be different. For instance, the electrode which was electrochemically transformed in 0.1 M HClO4 gives the surface coverage of (2.4 ( 0.3) × 10-10 mol cm-2 at pH 1, whereas the electrode which was electrochemically transformed in neutral pH (0.1 M phosphate buffer, pH 7.2) gives the surface coverage of (1.3 ( 0.3) × 10-10 mol cm-2 at the neutral pH. This could be due to the difference in the efficiency of the head-to-tail coupling reaction between the radical cations. The head-to-tail coupling reaction of aniline radical cations is more favorable in acidic solution. Although the surface coverage is low at neutral and higher pHs, the head-to-tail coupling reaction actually occurs on the electrode surface even at neutral and higher pHs, which is unlikely to occur in solution during the electropolymerization of aniline. The origin of cathodic wave Z observed during the electrochemical oxidation of 4-ATP is not clearly known, and it may be due to the formation of some side reaction products. In
acidic solution, the wave Z disappeared within 5-7 cycles, whereas at neutral and higher pHs it disappeared after 10-12 cycles. Figure 4 shows the effect of applied potential on the cyclic voltammogram of an electrochemically transformed monolayer of 4-ATP in 0.1 M HClO4. The electrode potential was held at 0.75 V for different time intervals, and then the cyclic voltammograms were recorded. As can be seen from Figure 4, the peak current at 0.53 V decreased and the peak current at 0.25 V increased while holding the potential at 0.75 V. These voltammetric changes are associated with hydrolysis of the surfacebound diimine to iminoquinone.11 The imines are highly susceptible to hydrolysis in aqueous acidic media and at high positive potentials.11,12a,31 The electroproduced quinonediimine undergoes hydrolysis to the corresponding monoimine, which can be further hydrolyzed to benzoquinone and surface-bound 4-ATP while holding the potential at 0.75 V. The hydrolyzed diimine (iminoquinone) exhibits reversible (2e-, 2H+) redox behavior and gives (31) March, J. Advanced Organic Chemistry; Wiley: New York, 1985.
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rise to a redox peak at 0.25 V. A close examination of Figure 4 shows that the extent of decrease in the peak current at 0.53 V is not consistent with the increase in the peak current at 0.25 V, which suggests the further hydrolysis of monoimine while holding the potential at 0.75 V. Furthermore, a significant increase in the charging current was observed after holding the potential at 0.75 V for 135 min. This can be either due to the further hydrolysis of monoimine or due to the desorption of some surface-confined molecules from the electrode surface. The effect of applied potential on the electrochemical behavior of the electrode prepared in neutral solution has also been examined. As shown in Figure 5, the peak potential shifted to a less positive potential when the electrode potential was held at 0.4 V, suggesting that the redox species on the electrode surface undergoes further transformation. Unlike the electrochemical behavior of the electrode in acidic solution, only one peak has been observed in neutral solution while holding the electrode potential. These voltammetric changes can be ascribed to the hydrolysis of surface-bound diimine to the monoimine (vide infra). The surface coverage of the redox peak Y before and after holding the electrode potential at 0.4 V for 95 min was determined by integrating the area under the corresponding anodic peaks. An obvious decrease in the surface coverage was observed after holding the electrode potential, which implies the fact that holding the electrode potential at 0.4 V leads to the loss of electroactive species. The loss could be either due to hydrolysis of iminoquinone to the 1,4-benzoquinone and 4-ATP or due to the desorption of monolayer from the electrode surface. As the charging current remained the same before and after holding the electrode potential, the latter may not be the reason for the observed decrease in the surface coverage. The small peak observed at around -0.12 V at the neutral pH remained unaltered while holding the electrode potential. However, this peak disappeared while cycling the potential between -0.2 and 0.7 V (within 10-15 cycles). In Situ FTIR Studies. To understand the electrochemical transformation of 4-ATP, the in situ FTIR measurements were carried out in both neutral and acidic solutions. Figures 6 and 7 show a series of SEIRA spectra of the 4-ATP-Au electrode obtained during the first and second potential cycles in 0.1 M HClO4 and 0.1 M phosphate buffer (pH 7.2) solutions. During a sweep of the potential to the positive direction, prominent absorptions at 1587 and 1636 cm-1 were observed, which did not appear in the second potential cycle. Since these bands appear in the potential region where the large anodic current flows on its cyclic voltammogram (Figures 1 and 3), they can be ascribed to either the oxidation product or some quasi-stable intermediate state that further undergoes the electrochemical reaction to give the final product. We ascribe these bands to the intermediate, as they disappeared in the second cycle and during the potential excursion to the positive potential limit where the product is thought to be stably present. The vibrational analysis based on ab initio MO calculations has been carried out to support the observed results. Figure 8 shows the vibrational analysis data obtained for the neutral and protonated forms of 4-ATP and for the one-electron oxidized product of 4-ATP (cation radical). Comparison of these results with the experimental results confirms the fact that the bands observed at 1587 and 1636 cm-1 correspond to the cation radical. The calculated spectra of the cation radical (Figure 8) closely match with the experimentally observed spectra of the cation radical in both acidic and neutral solutions (Figure 9). The disap-
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Figure 12. Theoretical IR spectra obtained for the different isomers (A and B) of the mercaptoaniline dimer by the ab initio MO calculation.
pearance of these bands in the second sweep indicates that the cation radical undergoes dimerization at the electrode surface to give the dimeric product. The negativegoing bands at 1490 and 1600 cm-1 attributable to 4-ATP that is originally present on the electrode surface decreased after the oxidation reaction, confirming that the surfacebound 4-ATP underwent electrochemical reaction at the electrode surface. Figure 9 compares the spectra obtained for the intermediate cation radical in acidic and neutral solutions. The spectral features are very similar in both acidic and neutral solutions. Figure 10 compares the spectra of the 4-ATP-Au electrode obtained after several potential cycles in acidic and neutral solutions. A drastic spectral change was observed at potentials where the oxidation of surface-confined species takes place; the negative-going bands at 1589, 1516, and 1488 cm-1 start to appear and further develop at 0.6 and 0.4 V in acidic and neutral solutions, respectively. Since the reference spectrum was obtained at potentials well negative of the redox peak, these bands can be ascribed to the reduced state of the couple. The negative-going bands at 1516 and 1488 cm-1 correspond to the C-H and N-H bending modes, and the band observed at 1589 cm-1 corresponds to the C-C stretching mode of the benzene ring. Although there is some difference in relative band intensities between acidic and neutral solutions, similar spectral features indicate that the electrochemical transformation of 4-ATP in neutral and acidic solutions follows the same reaction pathway and they can be related to the redox reaction of the surface-confined 4′-mercapto-4-aminodiphenylamine/4′-mercapto-N-phenylquinone diimine couple. Figure 11 compares the potential-difference SEIRA spectra of the 4-ATP-Au electrode before and after potential cycling in acidic and neutral solutions. All spectra were measured at 0 V, referring to the bare surface at 0 V, which was established by oxidative desorption of adsorbates at 1.35 V for 30 s. On the basis of our cyclic voltammetric results presented above and those reported
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Figure 13. SEIRA spectra obtained for the hydrolyzed aniline dimer in (A) acidic and (B) neutral solutions. Scheme 1. Schematic Representation for the Electrochemical Transformation of the 4-ATP SAM on a Au Electrode and the Follow-Up Chemical Reactiona
a (a) Ethanol solution 10 mM 4-ATP for 1 h. (b) Cycling the electrode potential between -0.2 and 0.7 V (5-7 cycles) in 0.1 M phosphate buffer (pH 7.2) at the scan rate of 100 mV/s. The oxidation of surface-confined aniline and the subsequent head-to-tail coupling of the oxidized aniline (cation radical) occur during the potential cycling. (c) Holding the electrode potential at 0.4 V for 90 min. NPQD: 4′-mercapto-N-phenylquinone diimine; NPQM: 4′-mercapto-N-phenylquinone monoimine.
by Lukkari and co-workers, the product of the electrochemical transformation would be 4′-mercapto-4-aminodiphenylamine. To confirm the reaction product in acidic and neutral solutions, we performed the vibrational analysis taking into account the possible molecular conformations, and the results are shown in Figure 12. Unfortunately, both results shown in Figure 12 did not perfectly match with the experimental results. However they, especially those for conformation A, roughly represent the spectral feature for bands at 1488 (calcd 1503), 1516 (calcd 1543), and 1589 (calcd 1585) cm-1 suggesting that the product of the reaction would be 4′-mercapto-4aminodiphenylamine. As the spectral features are very similar in both acidic and neutral solutions, we conclude that the electrochemical transformation of 4-ATP in neutral solution follows the same reaction pathway as in acidic solution. The weak absorption bands in neutral solution can be due to the low reaction yield of the dimerization reaction, which is supported by low surface coverage in neutral solution compared to that in acid
solution. To further confirm these results, the spectral features are compared with the FTIR spectrum of the model compound, NPPD. As shown in Figure 11, the spectrum of NPPD is very similar to that obtained for the electrochemically transformed 4-ATP electrode in both acidic and neutral solutions, which confirms the fact that electrochemical transformation leads to the formation of a head-to-tail aniline dimer and the product is very similar in acidic and neutral solutions. The band at 1600 cm-1 for NPPD and 1587 cm-1 for the surface-confined amine can be assigned to the NH2 deformation vibration of a primary amine.32 The band at around 1517 cm-1 has, probably, two contributions: aromatic CdC stretching and NH deformation of the secondary amine.32 As mentioned earlier, the electroproduced diimines are easily hydrolyzed to form quinonemonimine, and this is (32) (a) Socrates, C. Infrared Characteristic Group Frequencies; Wiley: New York, 1994. (b) Cases, F.; Huerta, F.; Garces, P.; Morallon, E.; Vazquez, J. L. J. Electroanal. Chem. 2001, 501, 186.
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thought to proceed more easily in acidic solution and at high positive potential. Figure 13A shows the SEIRA spectrum obtained for the electrochemically transformed 4-ATP-Au electrode after several potential cycles between 0 and 0.9 V in 0.1 M HClO4 (note that the 4-ATP-Au electrode was considered as the reference and the reference potential was 0 V). At 0.3 V where the smaller oxidation wave was observed on the cyclic voltammogram, a positivegoing band appears at 1677 cm-1. Since the band falls well in that of the quinone group, we ascribe this band for the quinone group, which is oxidized before the main oxidation reaction takes place. Figure 13B shows the SEIRA spectrum obtained for the 4-ATP electrode after several cycles in 0.1 M phosphate buffer. We could not observe any characteristic band for the hydrolyzed dimer when the spectra were measured at 0 V. However, at 0.2 V we noticed a weak band at 1651 cm-1, which can be due to the hydrolyzed dimer. On the basis of the cyclic voltammetric and in situ FTIR spectral results, it is concluded that as shown in Scheme 1 the electrochemical transformation of 4-ATP in neutral solution (0.1 M phosphate buffer, pH 7.2) involves the head-to-tail coupling of surface-confined radical cations as in the case of acidic solution. The electrochemical oxidation of 4-ATP in neutral solution leads to the formation of a surface-confined diimine, 4′-mercapto-Nphenylquinone diimine, which undergoes hydrolysis to yield the monoimine while holding the electrode potential at 0.4 V. As shown in the scheme, both diimine and monoimine undergo reversible redox reaction in neutral pH.
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Conclusions The electrochemical transformation of 4-ATP on a Au electrode in neutral pH results in the formation of a surface-confined redox active SAM of 4′-mercapto-Nphenylquinone diimine, which exhibits a pH dependent redox response. The redox peak shifts by -56 mV while increasing the solution pH by 1 unit, suggesting that the redox reaction involves two protons and two electrons. In situ FTIR spectroscopy is used to characterize the electrochemically transformed monolayer of 4-ATP for the first time, and the results support the formation of an aniline dimer by head-to-tail coupling of the radical cations. Very similar spectral features are observed in acidic and neutral solutions, which demonstrates the fact that the electrochemical transformation follows the same pathway in both acidic and neutral solutions. The headto-tail coupling of the radical cations occurs at the electrode surface in neutral and higher pH solutions, which is unlikely to occur during the electropolymerization of aniline. Acknowledgment. The present work was financially supported by Grants-in-Aid for Scientific Research (No. 12875164), “Scientific Research (A)” (No. 10305064), and “Scientific Research (B)” (No. 12608) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. C.R.R. thanks the VBL of TIT for the fellowship. LA010746Q
