Covalent Modification of a Glassy Carbon Surface by 4-Aminobenzoic

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Covalent Modification of a Glassy Carbon Surface by 4-Aminobenzoic Acid and Its Application in Fabrication of a Polyoxometalates-Consisting Monolayer and Multilayer Films Jianyun Liu, Long Cheng, Baifeng Liu, and Shaojun Dong* Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, People’s Republic of China Received October 13, 1999. In Final Form: May 23, 2000 4-Aminobenzoic acid (4-ABA) was covalently grafted on a glassy carbon electrode (GCE) by amine cation radical formation in the electrooxidation process of the amino-containing compound. X-ray photoelectron spectroscopy measurement proves the presence of 4-carboxylphenylamine monolayer on the GCE. The redox responses of various electroactive probes were investigated on the 4-ABA-modified GCE. Electron transfer to Fe(CN)63- in solutions of various pHs was studied by both cyclic voltammetry and electrochemical impedance analysis on the modified electrode. Changes in the solution pH value result in the variation of the terminal group charge state, based on which surface pKa values are estimated. The 4-ABA-modified GCE was used as a suitable charged substrate to fabricate polyoxometalates-consisting (POM-consisting) monolayer and multilayer films through layer-by-layer assembly based on electrostatic attraction. Cyclic voltammetry shows the uniform growth of these three-dimensional multilayer films. Taking K10H3[Pr(SiMo7W4O39)2]‚xH2O (abbreviated as Pr(SiMo7W4)2), for example, the preparation and electrochemical behavior of its monolayer and multilayer film had been investigated in detail. This modification strategy is proven to be a general one suitable for anchoring many kinds of POMs on the 4-ABA-modified GCE.

Introduction Modification of the carbon surface is an important objective in electrochemistry and material science. Much attention has been paid to covalently modified electrodes on which some functional groups such as carboxyl and amino groups are grafted for catalytic, analytical, and biotechnological applications.1-4 Recently, free radical grafting methods for modifying carbon surfaces are of great interest because compact and stable monolayers can be obtained. Up to now, there are mainly two methods for modifying the carbon surface through free radical grafting: one is the electrochemical reduction of diazonium salts to result in covalent attachment of aryl radicals onto the carbon surface;5-7 the other is the electrochemical oxidation of amine-containing compounds to lead to amine cation radical and subsequently form a carbon-nitrogen linkage on the carbon surface.8 The amine cation radicals are grafted onto carbon surface in a monolayer level.8,9 This surface modification method has been recently used to achieve electrocatalysis9 and to create a protective coating on the electrode from being fouled by protein adsorption.10 In addition, Aramata et al. immobilized * To whom correspondence should be addressed: Fax: +86-4315689711. E-mail: [email protected]. (1) Oyama, N.; Yap, K. B.; Anson, F. C. J. Electroanal. Chem. 1979, 100, 233. (2) Fujihira, M.; Tamana, A.; Osa, T. Chem. Lett. 1977, 64, 361. (3) Evans, J. F.; Kuwana, T. Anal. Chem. 1979, 51, 358. (4) Elliott, C. M.; Marrese, C. A. J. Electroanal. Chem. 1981, 119, 395. (5) Allongue, P.; Delamar, M.; Desbat, B.; Fagebarme, O.; Hitmi, R.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1997, 119, 201. (6) Delamar, M.; Hitmi, R.; Pinson, J.; Save´ant, J.-M.; J. Am. Chem. Soc. 1992, 114, 5883. (7) Moiroux, C.; Pinson, J. J. Electroanal. Chem. 1992, 336, 113. (8) Barbier, B.; Pinson, J.; Desarmot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137, 1757. (9) Deinhammer, R. S.; Ho, M.; Anderegg, J. W.; Porter, M. D. Langmuir 1994, 10, 1306. (10) Downard, A. J.; Mohamed, A. Electroanalysis 1999, 11, 418.

aminopyridyl compounds on glassy carbon to bridge cobalt(II) tetraphenylporphyrin.11 The surface charge states of modified electrodes are crucial to their behavior and potential applications. Takehara et al.12 systematically studied the effect of terminally substituted alkanethiol self-assembled monolayers (SAMs) on gold electrodes on the redox responses of Fe(CN)63-, Ru(NH3)63+, and uncharged ferrocenedimethanol (FcDM). Crooks et al.13 reported the pHdependent electrostatic interaction of the terminal NH3+ and COO- groups of SAMs with oppositely charged probe molecules. Previously, there has not been a detailed study on the electrostatic effect of the charged carbon surface, which is important in practical application. In the present work, 4-aminobenzoic acid (4-ABA) was immobilized onto a carbon electrode surface by amine cation radical formation in anhydrous ethanol solution. The resulting chemically modified GCE is negatively charged under appropriate pH conditions, which was characterized by cyclic voltammetry and electrochemical impedance analysis. Taking Fe(CN)63- as a probe, the effects of solution pH on the 4-ABA/GCE were studied in detail and the surface pKa values were determined. The 4-ABA-modified GCE is stable and can be used as a charge-rich precursor to assembly oppositely charged species by layer-by-layer electrostatic interaction. Polyoxometalates (POMs) are inorganic metal-oxygen cluster anionic compounds with potential applications in the areas of molecular electronics, catalysis, and medicine.14-16 Practical applications of POMs in these areas depend on the suitable immobilization of the POMs. (11) Tanaka, H.; Aramata, A. J. Electroanal. Chem. 1997, 437, 29. (12) Takehara, K.; Takemura, H.; Ide, Y. Electrochim. Acta 1994, 39, 817. (13) Jones, T. A.; Perez, G. P.; Johnson, B. J.; Crooks, R. M. Langmuir 1995, 11, 1318. (14) Pope, M. T. Heteropoly and Isopoly Oxometalates; SpringerVerlag: Berlin, 1983.

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Recently, POM-composed three-dimensional layered films are attractive for sensing and electronic device because of the thickness controllability and composite adjustability.17-20 Generally, adsorption of the first POMs layers on substrates is responsible for the stability of the multilayer films. Sun et al. fabricated multilayer films consisting of phosphomolybdic anion on a gold electrode coated with a precursor aminoethanethiol SAM.21 However, the precursor SAMs were unstable and easily oxidized in air,22 which were disadvantageous to further modification of POMs. Moreover, the potential window of a gold electrode is limited to a relatively positive range due to its low overpotential for hydrogen evolution. Therefore, a gold electrode is not very suitable for studying the electrochemical behavior of the compounds whose redox reactions occur at negative potentials, like most of the tungsten-based POMs. However, carbon electrode is free of this problem because of its wide potential window. Therefore, the 4-ABA-modified GCE is a suitable precursor for the fabrication of POM monolayer and multilayer films. Herein, a quaternized poly(4-vinylpyridine complexed with [Os(bpy)2Cl]2+/+) (abbreviated as QPVP-Os) was alternately deposited with Pr(SiMo7W4)2 (as a representative of various POMs) to build up multilayer structures on the 4-ABA/GCE. Pr(SiMo7W4)2 is one of the series of heteropoly complexes containing a lanthanide, which has a special bis-Keggin structure with large molecular mass and good catalytic properties.14 This paper describes the first study on the preparation and electrochemical property of a Pr(SiMo7W4)2-containing multilayer film. It is worth noting that a wide variety of POMs have been successfully immobilized on GCEs to form multilayer films, including the transition metal substituted polyoxometalates, which are difficult to immobilize on electrodes.23 Experimental Section Reagents. 4-Aminobenzoic acid (4-ABA) and ruthenium hexaamine chloride (Ru(NH3)63+) were purchased from Aldrich. The absolute ethanol was dried over 3A molecular sieve before use. The solution of 4-ABA was freshly prepared for each modification. Lithium perchlorate was dried at about 90 °C before use. Pr(SiMo7W4)2 was synthesized and purified as ref 24; Dawson-type R-K6P2W18O62‚14H2O (P2W18) as ref 25; Keggintype H5SiMo11VO40 (SiMo11V) as ref 26; and KnH[ZnW11Co(H2O)O39]‚xH2O (ZnW11Co) as ref 27. Polycation, QPVP-Os, was provided by professor Xi Zhang (Jilin University, China). All of the other chemicals were of reagent grade and used as received. Water used for preparation of aqueous solution was purified using Millipore Milli-Q water purification system. The following buffers of various pHs used in impedance experiments were kept a constant ionic strength of 0.1 M KCl: 0.1 M phosphate buffer (PBS) of pH 7 was prepared from 0.1 M (15) Pope, M. T.; Mu¨ler, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. (16) Sadakane, M.; Steckhan, E. Chem. Rev. 1998, 98, 219. (17) Kuhn, A.; Mano, N.; Vidal, C. J. Electroanal. Chem. 1999, 462, 187. (18) Caruso, F.; Kurth, D. G.; Volkmer, D.; Koop, M. J.; Mu¨ler, A. Langmuir 1998, 14, 3462. (19) Cheng, L.; Niu, L.; Gong, J.; Dong, S.Chem. Mater. 1999, 11, 1465. (20) Ichinose, I.; Tagawa, H.; Mizuki, S.; Lvov, Y.; Kunitake, T. Langmuir 1998, 14, 187. (21) Sun, C.; Zhang, X. J. Electrochim. Acta 1998, 43, 943. (22) Schoenfisch, M. H.; Penberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502. (23) Shiu, K. K.; Anson, F. C. J. Electroanal. Chem. 1991, 309, 115. (24) Zhou, B.; Shan, Y.; Liu, Z.; Zhang, X. J. Inorg. Chem. (Chinese) 1992, 3, 315. (25) Wu, H. J. Biol. Chem. 1920, 43, 189. (26) Altenan, J. J.; Pope, M. T.; Prados, R. A.; So, H. Inorg. Chem. 1975, 14, 417. (27) Liu, J. F.; Wang, Y.; Yang, Q. H.; Chen, Y. G.; Li, M. X.; Liu, C. G.; Yang, S. T. Polyhedron 1996, 15, 717.

Liu et al. KH2PO4 and 0.1 M K2HPO4; buffers of pH 2.5-5.5 from 0.1 M C6H4COOKCOOH (adjusted with 0.1 M HCl or NaOH) and pH 1-2.5 solutions from 0.1 M HCl (adjusted with 0.1 M NaOH). Electrochemical Measurements. Cyclic voltammetry was performed with a CHI 600 electrochemical workstation (USA) in a conventional three-electrode electrochemical cell using glassy carbon (GC 2000, 3-mm diameter, Tokai Corp., Japan) as the working electrode, twisted platinum wire as the auxiliary electrode, and the Ag/AgCl reference electrode in aqueous media or Ag/Ag+ (0.01 M AgNO3) reference electrode in anhydrous ethanol solutions. The GCEs were polished with 1.0- and 0.3-µm R-Al2O3 powders, successively and sonicated in water for about 3 min after each polishing step. Finally, the electrodes were sonicated in ethanol, washed with ethanol, and dried with highpurity nitrogen stream immediately before use. Modification Procedure. The electrochemical modification of a glassy carbon electrode was performed in anhydrous ethanol solutions containing 3 mM 4-ABA and 0.1 M LiClO4 by scanning between 0 and +0.9 V (vs Ag/Ag+). After the modification, the electrode was successively rinsed with ethanol and Milli-Q water and sonicated for 10 min in water to remove the physically adsorbed materials. The 4-ABA-modified GCEs were then ready for characterization or for further modification according to the published precedures.19,28 Briefly, the 4-ABA/GCE was first placed in QPVP-Os +0.1 M acetate buffer (pH 3.8) and then scanned between 0.6 and -0.1 V at a scan rate of 100 mV s-1 for 25 cycles. The modified electrode with QPVP-Os layer was then placed in 0.1 M Pr(SiMo7W4)2 + 0.1 M acetate buffer (pH 3.8), resulting in one layer of Pr(SiMo7W4)2 by scanning similar to the conditions used above. When the resulting electrode was placed alternately in QPVP-Os and Pr(SiMo7W4)2 solutions, the Pr(SiMo7W4)2 multilayers were formed. The cyclic potential sweeps for 25 cycles proved to be sufficient for loading Pr(SiMo7W4)2) and QPVP-Os since the cyclic potential sweeps for more cycles did not increase the peak currents of Pr(SiMo7W4)2) at all. The multilayer films of other POMs were assembled with the same steps as those used above. X-ray Photoelectron Spectroscopy (XPS). XPS measurement was performed on an ESCALAB-MKII spectrometer (VG Co., U.K.) with a Al KR X-rays 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 10-9 Torr with an analyzer pass energy of 50 eV. The resolution of the spectrometer was 0.2 eV. Electrochemical Impedance Analysis. Electrochemical impedance measurements were performed with a PARC M398 electrochemical impedance system (EG & Princeton Applied Research, Princeton, NJ) consisting of a potentiostat Model 273 and a computer-controlled PAR Model 5210 lock-in amplifier with EG&G AC impedance software. An ac voltage of 10 mV in amplitude with a frequency range from 0.1 Hz to 60 kHz was superimposed on the dc potential and applied to the studied electrodes. The dc potential was always set up at the formal potential of Fe(CN)63-/4-. Experimental data of the electrochemical impedance plot were analyzed by applying the nonlinear leastsquares fitting to the theoretical model represented by a Randles equivalent electrical circuit.

Results and Discussion Covalent Modification of GCE with 4-ABA. Figure 1 shows cyclic voltammograms of 3 mM 4-ABA in 0.1 M LiClO4 ethanol solution on a GCE at 10 mV s-1. The 4-ABA was found to oxidize at about +0.73 V. This oxidation wave is attributed to one-electron oxidation of the amino group to its cation radical.8-10,29 When the potential is repeatedly scanned, the peak gradually diminishes. This indicates the formation of a coating on the electrode surface. Scheme 1 shows the oxidation of 4-ABA and grafting on GCE. X-ray Photoelectron Spectroscopy of the 4-ABAModified GCE. The attachment of amine cation radicals (28) Cheng, L.; Dong, S. Electrochem. Commun. 1999, 1, 159. (29) Masui, M.; Sayno, H.; Tsuda, Y. J. Chem. Soc. B 1968, 973.

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Figure 1. Cyclic voltammograms on a freshly polished GCE in LiClO4 ethanol solution with 3 mM 4-ABA for the (1) first, (2) second, and (4) fourth cycles. Scan rate: 10 mV s-1.

Figure 3. Cyclic voltammograms on bare (;) and 4-ABAmodified GCE (- - -) in (A) 5 mM Fe(CN)63- and (B) 2 mM Ru(NH3)63+ solutions buffered by 0.1 M PBS (pH 7.0) and for the 4-ABA-modified GCE in 0.1 M PBS without the redox couples (...). Scan rate: 100 mV s-1. Figure 2. X-ray photoelectron spectra of N1s energy levels for (a) a glassy carbon plate modified with 4-ABA and (b) the (a) electrode after being sonicated for 10 min, respectively. Scheme 1. Grafting of 4-ABA on a Glassy Carbon Electrode

to a carbon electrode has been proven by XPS analysis.8,9 Curve a of Figure 2 shows the XPS spectrum for 4-ABAmodified GCE. A characteristic N1s peak appears at about 398.6 eV, indicating that 4-ABA has been immobilized on the GCE surface. The binding energy of the peak maximum (398.6 eV) is consistent with the formation of a carbonnitrogen bond between the amine cation radical and an aromatic moiety of the glassy carbon surface.8,9,30 The thus-grafted 4-ABA thin film is stable and very difficult to remove from the electrode surface. After ultrasonic rinsing for 10 min in ethanol, CH3CN, H2O, or PBS, the XPS (N1s) signal is almost unchanged (curve b of Figure 2) as compared with that of curve a. The 4-ABA film also remained stable after a 2-month exposure to air (this stability is verified by the fact that the modified electrode can still block the electrochemical reaction of (30) Nordberg, R.; Albridge, R. G.; Bergmark, T.; Ericson, U.; Hedman, J.; Nordling, C.; Siegbahn, K.; Lindberg, B. J. Ark. Kemi. 1968, 28, 257.

Fe(CN)63-). The only way to remove the film is to mechanically polish the electrode. Blocking Effect of the 4-ABA-Modified GCE on Electrochemical Behavior of Redox Probes. Two oppositely charged redox probes, Fe(CN)63- and Ru(NH3)63+, were chosen to investigate the effects of the 4-ABA film on their electrochemical behavior. Figures 3A and 3B show the cyclic voltammograms of Fe(CN)63- and Ru(NH3)63+ in neutral aqueous solution, respectively, on a bare GCE (solid line) and the 4-ABA/GCE (dashed line). Figure 3A clearly shows that the electron transfer of Fe(CN)63- is completely blocked on the 4-ABA/GCE. On the other hand, Ru(NH3)63+ displays a well-defined redox response, and the peak current Ip and peak-to-peak separation ∆Ep ()Epa - Epc) on the 4-ABA/GCE are comparable to those observed on a bare GCE, indicating no blocking effects on the redox reaction of Ru(NH3)63+ on the modified electrode surface. The different effects of the 4-ABA/GCE on the electrochemical behavior of the two oppositely charged redox probes can be explained by the electrostatic interactions between the modified surface and the electroactive probes.31-35 In pH 7.0 solution, the carboxyl group of the modified GCE surface is expected to fully dissociate if we assume that its pKa is near to that of 4-aminobenzoic acid (pKa 4.6).36 Thus, on the negatively charged 4-ABA film, (31) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1995, 67, 2767. (32) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1996, 68, 4180. (33) Saby, C.; Ortiz, B.; Champagne, G. Y.; Belanger, D. Langmuir 1997, 13, 6805. (34) Molinero, V.; Calvo E. J. J. Electroanal. Chem. 1998, 445, 17. (35) Madoz, J.; Kuznetzov, B. A.; Medrano, F. J.; Garcia, J. L.; Fernandez, V. M. J. Am. Chem. Soc. 1997, 119, 1043. (36) Brill, H. C. J. Am. Chem. Soc. 1921, 43, 1320.

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Figure 4. Plot of (a) Ip and (b) ∆Ep of the cyclic voltammograms as a function of the solution pH. The cyclic voltammograms were obtained on a 4-ABA/GCE in 1 mM Fe(CN)63- solutions buffered at different pHs.

Ru(NH3)63+ should be able to get access to the underlying GCE surface. It is consistent with the fact that trace Ru(NH3)63+ can be accumulated on the COO--terminated electrode surface.37 However, the response of Fe(CN)63was not observed. This suggests that a negative Donnan potential is established at the film surface as a result of high negative charge density of the COO- group.31,38,39 Thus, the electrostatic repulsion resists access of Fe(CN)63to the electrode surface and blocks its electron transfer on the 4-ABA/GCE. Effects of Solution pH on the Blocking Property of the 4-ABA-Modified Electrode. To confirm the electrostatic interaction, Fe(CN)63- was used as a probe to test the effect of solution pH on its electrochemical response. At pH ) 1.0-2.0, Fe(CN)63- has a fast response on 4-ABA-modified GCE (∆Ep < 80 mV), which is close to an ideal electrochemical behavior of Fe(CN)63- on bare GCE. In this case, the carboxyl groups on the 4-ABAmodified electrode surface is fully protonated; thus, the electrode surface is uncharged and no longer prevents Fe(CN)63- from reaching the underlying GCE surface to proceed electron transfer. In contrast, with increasing pH, the Ip and ∆Ep of Fe(CN)63- in the corresponding cyclic voltammograms change markedly, due to the increasingly negative charge on the 4-ABA/GCE surface. The Ip gradually diminishes, and the ∆Ep substantially increases, based on the fact that the surface pKa of the 4-ABA film grafted on the GCE surface can be conveniently estimated from the relationship between Ip or ∆Ep and the corresponding pH. The surface pKa of the 4-ABA film is estimated to be about 3.1 from Ip-pH curve (a) in Figure 4. A similar pKa (3.0) is also obtained from the ∆Ep-pH curve (b) in Figure 4. These results are smaller than those for 4-aminobenzoic acid in solution (pKa ) 4.6).36 The pH-dependent electrochemical responses of Fe(CN)63were reported for alkanethiols SAMs bearing dissociable terminal groups.31,40,41 The surface pKa values of the SAM terminal groups are larger than those of the corresponding molecules in solution. However, our results are in line with that of Saby33 who determined the surface pKa values of aryl-carboxylate-modified GCE (pKa ) 2.8). Electrochemical Impedance Measurement of Fe(CN)63-/4- on 4-ABA-Modified GCE under Different (37) Downard, A. J.; Roddick, A. D. Electroanalysis 1995, 7, 376. (38) Redepenning, J.; Tunison, H. M.; Finklea, H. O. Langmuir 1993, 9, 1404. (39) Downard, A. J.; Roddick, A. D.; Bond, A. M. Anal. Chim. Acta 1995, 317, 303. (40) Petrov, J. G.; Mo¨bius, D. Langmuir 1996, 12, 3650. (41) Doblhofer, K.; Figura, J.; Fuhrhop, J. Langmuir 1992, 8, 1811.

Figure 5. (A) Impedance plots on a 4-ABA-modified GCE in 5 mM Fe(CN)63-/4- solutions of different pHs: (a) pH 7.05, (b) pH 5.08, (c) pH 3.36, (d) pH 2.67, and (e) pH 2.1. The top inset shows the equivalent circuit model for the analysis of the impedance data. The bottom inset shows the impedance plot of a bare GCE in 5 mM Fe(CN)63-/4- PBS (pH 7.0). (B) Plot of charge-transfer resistance Rct as a function of solution pH.

pH Conditions. To further probe the effect of pH on the modified surface and to obtain more kinetic information, the electrochemical impedance experiment was performed in equimolar Fe(CN)63-/4- solutions (5 mM) of various pHs with constant ionic strength (0.1 M). In the case of a reversible one-electron electrode reaction of Fe(CN)63-/4solution, the electrode system can be described by a simple equivalent circuit,42 which is shown in the top inset of Figure 5A. CPE represents a constant phase element, which is used instead of a pure capacitor in the equivalent circuit due to microscopic surface roughness and inhomogeneity, Rct, the charge-transfer resistance, ZW, Warburg impedance, and Rs, the solution resistance. The impedance data were analyzed using this electrical equivalent circuit. At a bare GCE, a typical complex-impedance plot (bottom inset of Figure 5A) is shown as a high-frequency semicircle and a low-frequency Warburg line at an angle of 45°. Figure 5A shows the impedance plots on the 4-ABA/ GCE in solutions with various pHs, which differ significantly from that on a bare electrode. Rct, the diameter of the semicircle, increases dramatically with the increase of pH, due to inhibition of the electron transfer. When the pH value increases to 6.0, the Warburg line is not observed (42) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980; p 316.

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for the 4-ABA-modified electrode. The impedance results are in good agreement with the increasing irreversibility of cyclic voltammograms (not shown) in the same conditions. The Rct values calculated from impedance analysis data in solution with various pHs are plotted against pH in the range between 1.1 and 4.2, as shown in Figure 5B. It is interesting that the pKa value (pKa ) 2.9) obtained from the Rct-pH plot is very close to those obtained from the Ip-pH and ∆Ep-pH plots (vide supra). But when pH is larger than 4.2, Rct increases dramatically, which cannot be applied to analyze the pKa (Rct ) 9.0 × 104 Ω in pH 7.05 solution). It is possible that the full deprotonation of carboxyl groups causes some surface structure changes on the 4-ABA film, which completely blocks the electrode surface, although the reason for this blocking is not very clear. As described above, Rct at high frequencies is expected to increase due to inhibition of the electron transfer.43 The increase in the charge-transfer resistance is related to the electrode coverage and is given by44

(1 - θ) ) R0ct/Rct

Figure 6. Cyclic voltammograms on a POM monolayer film Pr(SiMo7W4)2/QPVP-Os/4-ABA/GCE in pH 3.8 acetate buffer at different scan rates: 20, 50, 100, 200, 300, 400, 600, and 800 mV s-1 (from inside to outside), respectively. The inset shows a relationship of the first reduction peak currents vs the scan rates.

(1)

where θ is the apparent electrode coverage, assuming that all the currents are passed via bare spots on the electrode44 and R0ct and Rct represent the charge-transfer resistance measured on a bare and a film-covered electrode, respectively. In 0.1 M KCl solution with 5 mM Fe(CN)63-/4- (pH 7.05), R0ct is 316 Ω based on the fitting results with the electrical equivalent circuit. Under the same conditions, Rct on 4-ABA/GCE is about 9.0 × 104 Ω. Using eq 1, the coverage was calculated to be 99.7%. Fabrication of Monolayer and Multilayer Assemblies Composed of QPVP-Os and POMs on the 4-ABA/GCE. Through the attachment of 4-ABA to GCE, a stable and negatively charged surface can be achieved at pH > pKa of the 4-ABA film. An interesting application of the 4-ABA/GCE is that it can serve as a charge-rich precursor for electrostatic adsorption of oppositely charged compounds. In this paper, we used cationic polymer QPVP-Os and anionic compounds Pr(SiMo7W4)2 as modifiers. Since the surface pKa of the 4-ABA-modified GCE is about 3.1 as mentioned above, pH 3.8 solution is enough for the carboxyl groups on the 4-ABA film to dissociate and adsorb QPVP-Os. QPVP-Os in pH 3.8 solution was first adsorbed on the 4-ABA/GCE by electrostatic attraction. Similarly, Pr(SiMo7W4)2 monolayer was then formed on the QPVP-Os/4-ABA/GCE. Figure 6 shows cyclic voltammograms of the Pr(SiMo7W4)2 monolayer in pH 3.8 acetate buffer at different scan rates. It exhibits three couples of symmetrical redox waves with formal potentials ((Epa + Epc)/2) being 0.249, 0.074, and -0.154 V, which are similar to those of Nd(SiMo7W4)2 and should be assigned to three two-electron molybdate-centered redox reactions (MoVI/V).45,46 The peak currents are proportional to scan rates up to 800 mV s-1, as shown in the inset of Figure 6, taking the first reduction peak as a representative. Moreover, the formal potentials are independent of scan rates and the currents for the cathodic waves are almost equal to those for corresponding anodic waves. It suggests that the redox reactions exhibit reversible surface pro(43) Amatore, C.; Save´ant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39. (44) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663. (45) Cheng, L.; Zhang, X.; Xi, X.; Liu, B.; Dong, S. J. Electroanal. Chem. 1996, 407, 97. (46) Dong, S.; Cheng, L.; Zhang, X. Electrochim. Acta 1998, 43, 563.

Figure 7. Cyclic voltammograms on POM multilayer films n Pr(SiMo7W4)2/n QPVP-Os/4-ABA/GCE in pH 3.8 acetate buffer with increasing of layer number: n ) 1, 3, 5, 7, 9, and 11 (curves from inside to outside), respectively. Scan rate: 100 mV s-1. The inset shows the relationship of layer number vs the first reduction peak current.

cesses on the Pr(SiMo7W4)2/QPVP-Os/4-ABA/GCE.47 The peak potential full-width at half-maximum of peak current, Efwhm, is 58 mV for the first reduction wave at 50 mV s-1, which is larger than the theoretically predicted value (90/n) of 45 mV for an ideal two-electron surface wave.47 This may result from an interaction between Pr(SiMo7W4)2 anions in the film, which is consistent with the result of the surface-confined POMs studied previously.19,48,49 Calculated from the charge consumption of MoVI/V in the film during the electrolysis, the apparent surface coverage of Pr(SiMo7W4)2 immobilized on QPVP-Os/4-ABA/GCE is 1.44 × 10-10 mol cm-2 (the geometry areas of the GCE 0.07 cm2), which is at the level of monolayer coverage. Figure 7 shows cyclic voltammograms of n Pr(SiMo7W4)2/ n QPVP-Os/4-ABA/GCE multilayers (curves from inside to outside: n ) 1, 3, 5, 7, 9, and 11, respectively) in pH 3.8 buffer solution. With the number of Pr(SiMo7W4)2 layers deposited increasing, the peak currents increase markedly. Taking the first reduction peak as a representative, the redox peak current has a good linear relationship with the number of layers, as shown in the (47) Murray, R. W. Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, pp 191-368. (48) Kuhu, A.; Anson, F. C. Langmuir 1996, 12, 5481. (49) Dong, S.; Cheng, L.; Zhang, X. Electrochim. Acta 1998, 43, 563.

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at about 0.3 V in Figure 8 A, B, and D, which derives from OsIII-centered redox reaction in QPVP-Os.19 As to SiMo11V-composed multilayers, the OsIII-centered redox wave is overlapped with the redox waves of SiMo11V and not seen separately, just like that of Pr(SiMo7W4)2. From Figure 8, one can also clearly see the uniform growth of these POM multilayer films. Therefore, the 4-ABA film grafted on GCE can be used as a precursor film on which nearly all kinds of POMs have been successfully immobilized in multilayer structures including transition metal substituted POMs ZnW11M (M ) Co, Mn, Cu). The detailed characterization and application of POM multilayer films will be reported in another place. Conclusion

Figure 8. Cyclic voltammograms on multilayer films n POMs/n QPVP-Os/4-ABA/GCE with POMs: (A) ZnW11Co in pH 3.8 buffer (n ) 1, 2, 3, and 4), (B) P2W18 in pH 2.4 buffer (n ) 2, 4, 6, and 8), (C) SiMo11V in 0.5 M H2SO4 (n ) 1, 3, 5, and 7), and (D) SiW12 in 0.1 M H2SO4 (n ) 2, 4, 6, and 8), respectively. Scan rate: 100 mV s-1.

inset of Figure 7. It indicates that uniform and homogeneous multilayer films have been fabricated on the 4-ABA-modified carbon substrate. Moreover, when the number of Pr(SiMo7W4)2 layers is larger than 11, the corresponding current slightly deviates from linearity. It is possible that Pr(SiMo7W4)2 deposited in the outmost layer is so far from the electrode surface that electron transport through the film is difficult. In addition, it is worth noting that the first pair of Mo-centered redox waves is larger than that of the other two, which should result from the overlapping with the OsIII-centered redox reaction of QPVP-Os occurring at the same potential. Other POMs-containing monolayer and multilayer films have also been successfully fabricated on 4-ABA/GCE using the same polycation as counterion. Figure 8 shows cyclic voltammograms of multilayer films containing different POMs: (A) ZnW11Co in pH 3.8 buffer solution, (B) P2W18 in pH 2.4 buffer, (C) SiMo11V in 0.5 M H2SO4, and (D) SiW12 in 0.1 M H2SO4. They all exhibit well-defined reversible electrochemical behavior similar to the corresponding POMs in solution. There are similar redox waves

We have demonstrated a novel method to chemically modify GCEs via the electrooxidation of 4-ABA. XPS, cyclic voltammetry, and electrochemical impedance analysis have been used to characterize the 4-ABA-modified electrodes. The dissociable terminal carboxyl groups on the 4-ABA/GCE play a major role in blocking the redox response of the anionic probe Fe(CN)63- while favoring the response of the cationic probe Ru(NH3)63+. By changing the solution pH, this blocking effect can be controlled. Cyclic voltammetry and electrochemical impedance experiments have been reliably used as convenient and precise methods to estimate the surface pKa of the 4-ABA film grafted on GCE. The 4-ABA-modified GCE, which is stable and negatively charged at high pH (pH > pKa), has been used as a charged precursor substrate to assemble POMscontaining multilayer films by layer-by-layer assembling technique. These multilayer films have good uniformity and stability, and the POMs in the multilayers retain their electrochemical behavior. This modification strategy has been proven to be a general one suitable for anchoring many kinds of POMs on 4-ABA-modified GCE. Moreover, POMs with particular structure, Ln(SiMo7W4)2 and transition metal substituted POMs that are usually difficult to immobilize on electrodes23 can also be anchored by this technique. Work in our lab is in progress on the preparation, characterization, and applications of some other kinds of multilayer assemblies on 4-ABA-modified carbon electrodes according to the above-discussed concepts. Acknowledgment. This work is supported by the National Science Foundation of China. We are grateful to Dr. Z. Cheng for his help with the electrochemical impedance analysis. LA9913506