Electrochemical Modification of Glassy Carbon Electrode Using

Dec 10, 1997 - Blocking Effect of 4-Nitrophenyl and 4-Carboxyphenyl Groups. Coralie Saby ...... (a) Downard, A. J.; Roddick, A. D. Electroanalysis 199...
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Langmuir 1997, 13, 6805-6813

6805

Electrochemical Modification of Glassy Carbon Electrode Using Aromatic Diazonium Salts. 1. Blocking Effect of 4-Nitrophenyl and 4-Carboxyphenyl Groups Coralie Saby,† Bertha Ortiz,† Gilles Y. Champagne,‡,§ and Daniel Be´langer*,† De´ partement de Chimie, Universite´ du Que´ bec a` Montre´ al, Case Postale 8888, succursale Centre-Ville, Montre´ al, Que´ bec, Canada H3C 3P8, and Institut de Recherche d’Hydro-Que´ bec, IREQ, 1800 Monte´ e Ste-Julie, Varennes, Que´ bec, Canada J3X 1S1 Received October 23, 1996. In Final Form: August 11, 1997X The effect of a 4-carboxyphenyl or a 4-nitrophenyl thin film at the surface of a glassy carbon electrode on their electrochemical responses in the presence of various electroactive probes has been investigated. The grafting of a substituted phenyl group to a glassy carbon electrode was achieved by electrochemical reduction of the corresponding substituted phenyldiazonium derivative in acetonitrile. The blocking properties of the film depend primarily on electrostatic and electrolyte/solvent effects. Permselectivity for the 4-carboxyphenyl film can be achieved by controlling the dissociation of the carboxy group. The substituted phenyl layer is much more compact and less permeable in contact with a nonaqueous solvent than with an aqueous solvent presumably because the layer is poorly solvated. Electrochemical impedance measurements indicate that the kinetics of electron transfer are slowed down when the time used to modify the glassy carbon electrode is increased. Cyclic voltammetry and X-ray photoelectron spectroscopy measurements for 4-nitrophenyl- or 4-carboxyphenyl-modified glassy carbon electrode have indicated close to monolayer coverage for the substituted phenyl groups. The presence of a peak at 400 eV on the nitrogen 1s core level spectra was tentatively attributed to the reaction of phenol groups present at the glassy carbon electrode surface with the diazonium salt since this peak is not observed for the unmodified glassy carbon electrode.

Introduction Chemically modified electrodes have been the subject of numerous studies in the last 2 decades since they may find applications in electrocatalysis, corrosion protection, thin film optical devices, integrated circuits, information storage, and sensing.1-6 A recent approach for electrode modification involves the spontaneous adsorption of thiols on gold electrode surfaces to form the so-called selfassembled monolayers, SAMs.7,8 The adsorption of thiol results in the formation of a gold-sulfur bond that is characterized by a partial covalent character.7,9 However, recent studies have shown that these monolayers are not totally stable and that they can be oxidatively or reductively desorbed.10 SAMs are interesting on their own because of their ease of preparation and also because this * To whom all correspondence should be addressed. E-mail: [email protected]. † Universite ´ du Que´bec a` Montre´al. ‡ Institut de Recherche d’Hydro-Que ´ bec, IREQ. § Current address: Argo-Tech Productions Inc., 1560 rue de Coulomb, Boucherville, Que´bec, Canada J4B 7Z7. X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Murray, R. W. Electroanal. Chem. 1984, 13, 191 and references therein. (2) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Hu, S. Langmuir 1987, 3, 932. (3) Barendrecht, E. J. Appl. Electrochem. 1990, 20, 175. (4) Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; Techniques of Chemistry Series; Wiley-Interscience: New York, 1992; Vol. 22. (5) Lane, R. F.; Hubbard, A. T. J. Phys. Chem. 1973, 77, 1401. (6) Lane, R. F.; Hubbard, A. T. J. Phys. Chem. 1973, 77, 1411. (7) (a) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (b) Goldenberg, L. M. J. Electroanal. Chem. 1994, 379, 3. (c) Zhong, C. J.; Porter, M. D. Anal. Chem. 1995, 67, 709A. (8) Finklea, H. O. Electroanal. Chem. 1996, 19, 109. (9) Lewis, M.; Tarlov, M.; Carron, K. J. Am. Chem. Soc. 1995, 117, 9574. (10) Everett, W. R.; Fritsch-Faules, I. Anal. Chim. Acta 1995, 307, 253 and references therein.

S0743-7463(96)01033-5 CCC: $14.00

first layer can be further modified to build up more complex chemical structure. On the other hand, carbon electrodes are usually modified by an oxidative procedure that generates oxygen functionalities which can be used to do further chemistries.1,11 Since electrochemical or chemical oxidation may damage the carbon surface, a new method involving the electrochemical reduction of a phenyldiazonium derivative was developed.12 This surface modification procedure involves the formation of a diazonium radical, followed by the formation of a covalent bond to the glassy carbon, GC, electrode according to

R-C6H4N2+ + e- f N2 + R-C6H4•

(1)

GC + R-C6H4• f GC-C6H4-R

(2)

where R is a para substituent such as nitro, carboxy, etc. Close to a monolayer coverage can be achieved by this procedure.12-14 These modified electrodes have been subsequently used to immobilize glucose oxidase12b and to control protein adsorption by appropriate selection of the R group.13 More recently, chemical modification of polyaniline films has been performed by nucleophilic substitution reactions with substituted phenyldiazonium ions.15 The blocking behavior of 4-carboxyphenyl and 4-nitrophenyl groups was also recently communicated.16 In addition, a large number of substituted phenyldiazo(11) McCreery, R. L. Electroanal. Chem. 1991, 17, 221. (12) (a) Delamar, M.; Hitmi, R.; Pinson, J.; Save´ant, J. M. J. Am. Chem. Soc. 1992, 114, 5883. (b) Bourdillon, C.; Delamar, M.; Demaille, C.; Hitmi, R.; Moiroux, J.; Pinson, J. J. Electroanal. Chem. 1992, 336, 113. (c) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1997, 119, 201. (13) (a) Downard, A. J.; Roddick, A. D. Electroanalysis 1995, 7, 376. (b) Downard, A. J.; Roddick, A. D.; Bond, A. M. Anal. Chim. Acta 1995, 317, 303. (14) (a) Liu, Y.-C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254. (b) Chen, P.; McCreery, R. L. Anal. Chem. 1996, 68, 3958. (c) Liu, Y.-C.; McCreery, R. L. Anal. Chem. 1997, 69, 2091. (15) Liu, G.; Freund, M. S. Chem. Mater. 1996, 8, 1164.

© 1997 American Chemical Society

6806 Langmuir, Vol. 13, No. 25, 1997

nium salts have been prepared from the corresponding aniline and characterized electrochemically in the last decades.17,18 Since we are interested in applications of such modified carbon electrodes in electrocatalysis and electroanalysis, it is relevant to investigate in detail their blocking behavior.12c,14b,16 On the other hand, the blocking or barrier properties of long-alkyl-chain thiols on gold has been studied extensively.8 Their barrier properties depend on several parameters such as the length of the alkyl chain, chemical nature of the terminal group, and order of the monolayer and electrolyte. In this paper, we report cyclic voltammetry and electrochemical impedance results on the blocking behavior of glassy carbon electrodes modified by either 4-nitrophenyl or 4-carboxyphenyl in both aqueous and nonaqueous media. We also report a study of the electrochemical preparation of 4-nitrophenyl-modified glassy carbon electrode from a (4-nitrophenyl)diazonium fluoroborate solution and the characterization of the modified surface by X-ray photoelectron spectroscopy, XPS. Experimental Section Reagents. Acetonitrile was distilled over calcium hydride and stored over activated molecular sieves. Tetrabutylammonium tetrafluoroborate (NBu4BF4) and tetraethylammonium tetrafluoroborate (NEt4BF4) (Aldrich) were heated at 50 °C under vacuum for 8 h. Potassium ferricyanide, potassium ferrocyanide, hexaamineruthenium(III) chloride, hydroquinone, and ferrocene (Aldrich) were used as received. Tetraethylammonium ferricyanide was synthesized according to a published procedure.19 Tetraethylammonium ferrocyanide was synthesized by adaptation of a published procedure.19 Ten mmol (4.42 g) of K4Fe(CN)6 and 40 mmol (9.18 g) of Et4NClO4 were placed in methanol (200 mL). The mixture was stirred for 24 h under a nitrogen atmosphere. Following that, the mixture was filtered through a pad of Celite and the clear pale yellow filtrate was concentrated to ca. 10 mL under reduced pressure (rotary evaporator, 40 °C). The resulting yellow oil was shaken with 100 mL of diethyl ether, and the formation of a yellow precipitate occurs readily. The precipitate was filtered and dried under vaccuum (yield 10%). All other chemicals were analytical reagent grade (Anachemia or Aldrich). Synthesis of the Diazonium Tetrafluoroborate Derivative.20 The corresponding aniline (0.01 mol) was dissolved by warming into 3 mL of concentrated hydrochloric acid (12 M) and 14 mL of water. A precipitate was obtained by cooling to 0 °C. This precipitate disappeared after slow addition of a solution containing 0.752 g of sodium nitrite (0.011 mol) in 4 mL of water. The solution was filtered, and 1.48 g (0.013 mol) of sodium tetrafluoroborate was added. The slurry was cooled below 0 °C, filtered by suction, and washed with ice water and cold ether. The powder was dried. Recrystallization was carried out with a mixture of acetonitrile and ether. The diazonium salt was kept in a desiccator, at 3 °C, over phosphorus pentaoxide. NMR (CD3CN): two doublets ((4-nitrophenyl)diazonium tetrafluoroborate, 8.60-8.63 and 8.73-8.76 ppm; (4-carboxyphenyl)diazonium tetrafluoroborate, 8.39-8.42 and 8.55-8.59 ppm). In agreement with the literature data,21 the diazonium function was detected by IR spectroscopy at about 2290 cm-1. Electrode Preparation and Procedure. Glassy carbon electrodes (AIMCOR, Pittsburgh, Grade GC-10) were prepared from 3-mm-diameter rods embodied into epoxy resin (Hysol, 56C) and were used as working electrodes. Platinum gauze and a (16) Saby, C.; Champagne, G. Y.; Be´langer, D. Meeting Abstracts, Electrochemical Society, San Antonio, TX, Oct 6-11, 1996; Electrochemical Society: Pennington, NJ, 1996; Vol. 96-2, p 1120. (17) Elofson, R. M.; Gadallah, F. F. J. Org. Chem. 1969, 34, 854. (18) (a) Elofson, R. M.; Gadallah, F. F.; Cantu, A. A.; Schulz, K. F. Can. J. Chem. 1974, 52, 2430. (b) Elofson, R. M.; Gadallah, F. F.; Schulz, K.; Laidler, J. K. Can. J. Chem. 1984, 62, 1772. (19) Mascharak, P. K. Inorg. Chem. 1986, 25, 247. (20) Ruddy, A. W.; Starkey, E. B. Org. Synth. 1955, 3, 665. (21) Bruno, T. J.; Svoronos, P. D. N. CRC Handbook of basic tables for chemical analysis; CRC Press: Boca Raton, FL, 1989.

Saby et al. Ag/AgCl (3 M NaCl) electrode were used as counter and reference electrodes, respectively. All potentials were reported versus the Ag/AgCl reference electrode. The glassy carbon electrode surface was cleaned by polishing with Buehler 1 µm diamond paste and Buehler 0.05 µm alumina slurry (Tech-Met Canada). After polishing, the electrode was washed with water and ultrasonicated for 5 min in acetonitrile. Electrochemical modification of the glassy carbon electrode was carried out in acetonitrile containing the diazonium salt and either 0.1 M NBu4BF4 or NEt4BF4 at -0.7 V vs Ag/AgCl. After the modification, the electrode was washed by rinsing in acetonitrile and ultrasonicating into acetonitrile. The solutions containing the various electroactive probes were hydroquinone (1 mM; KCl, 1 M; pH 0), ferricyanide (1 mM; KCl, 0.1 M; phosphate buffer; pH 7), ruthenium hexaamine (1 mM; KCl, 0.1 M; phosphate buffer; pH 7; deaerated), ferrocene (1 mM; NBu4BF4, 0.1 M; acetonitrile), and ferricyanide (5 mM; NBu4BF4, 0.1 M; acetonitrile). The following buffers were used in the experiments performed at various pHs: (0.1 M KH2PO4 + 0.1 M KCl, adjusted with 0.1 M NaOH) pH 7; (0.1 M C6H4COOKCOOH + 0.1 M KCl, adjusted with 0.1 M NaOH) pH 4.5 and 5; (0.1 M C6H4COOKCOOH + 0.1 M KCl, adjusted with 0.1 M HCl) pH 2.5-4; and (0.1 M KCl, adjusted with 0.1 M HCl) pH 1-2. Measurements in acetonitrile media were carried out with either 0.1 M NBu4BF4 or NEt4BF4 as the electrolyte, and the solutions were purged with extra dry nitrogen gas for 30 min. Electrochemical impedance measurements were performed in a 5 mM Fe(CN)63-/5 mM Fe(CN)64-/0.1 M KCl, phosphate buffer, pH 7, at 0.25 V or open circuit potential and a 5 mM Fe(CN)63-/5 mM Fe(CN)64-/0.1 M Bu4NBF4/acetonitrile solution at -0.55 V in the frequency range 65 kHz to 0.05 Hz using a 10 mV sine-wave amplitude. The surface area of the glassy carbon electrode was determined by methylene blue adsorption.22 An average value of 0.0975 cm2 was found and indicates that the surface area is about 1.4 times its geometric area. The interfacial capacitance of the bare and modified electrodes was evaluated by electrochemical impedance spectroscopy. Instrumentation and Procedure. Electrochemical measurements were performed in a one-compartment cell using the three-electrode configuration. Cyclic voltammetry was performed using either a potentiostat/galvanostat Model M273 (EG&G Princeton Applied Research) or an electrochemical interface SI1287 (Solartron Instruments) interfaced with a PC, and the electrochemical setups were controlled with the Model M270 (EG&G) and DC Corrware (Scribner Associates, version 1.2) softwares, respectively. Electrochemical impedance experiments were made with an Electrochemical Interface SI1287 and a Frequency Response Analyser SI1255 (Solartron Instruments) controlled by the Zplot software (Scribner Associates, version 1.2). The Zview software (Scribner Associates, version 1.2) was used to analyze the impedance data. X-ray photoelectron spectra (XPS) were obtained on glassy carbon plates (GLCP-10; The Electrosynthesis Co.) with an Escalab 220i XL from VG equipped with a hemispherical analyzer and an Al anode (KR X-rays at 1483.6 eV) used at 12-14 kV and 10-20 mA. The data were obtained at room temperature and typically the operating pressure in the analysis chamber was below 1 × 10-9 Torr. FTIR measurements were performed with a Michelson Series FTIR spectrometer (Bomem Hartmann & Braun) MB series and NMR measurements with a Gemini 300MHz spectrometer (Varian Instruments).

Results and Discussion Electrochemistry of (4-Nitrophenyl)diazonium Tetrafluoroborate in Acetonitrile at a Glassy Carbon Electrode. Cyclic voltammograms in a 1 mM (4nitrophenyl)diazonium tetrafluoroborate, 0.1 M TEABF4/ acetonitrile solution at a glassy carbon electrode are shown in Figure 1. The first sweep (- -) gave an irreversible reduction wave at -0.05 V, which was attributed to the formation of the 4-nitrophenyl radical from the diazonium derivative.12a,c The electrochemical behavior of aryldiazonium cations has been studied in great detail by Elofson and co-workers, who have observed aryl radical formation (22) Laviron, E. Bull. Soc. Chim. Fr. 1967, 3717.

Modification of Glassy Carbon Electrode

Langmuir, Vol. 13, No. 25, 1997 6807

Figure 1. Cyclic voltammograms of a glassy carbon electrode in a 1 mM (4-nitrophenyl)diazonium tetrafluoroborate, 0.1 M TEABF4/acetonitrile solution at a scan rate of 100 mV/s: first scan (- -); sixth scan (s).

Figure 2. Cyclic voltammetry of bare (s), 4-carboxyphenyl (- -), and 4-nitrophenyl (- O -) modified glassy carbon electrode in 1 mM Fe(CN)63-, 0.1 M KCl, pH 7, phosphate buffer at 10 mV/s. The electrodes were modified at -0.7 V for 240 s in 5 mM of the corresponding diazonium salt solution in 0.1 M NBu4BF4/acetonitrile.

and release of nitrogen upon reduction:18 +

N N •

+ e–

(3)

+ N2

NO2

NO2

Elofson et al.23 have also shown that aromatic compounds are arylated by the electrochemically generated radical according to X •

+

(4)

O2N X

NO2

Since carbon electrodes also have an aromatic character,11 a similar reaction can consequently occur and lead to the formation of a covalent bond between the carbon electrode and the 4-nitrophenyl moiety. The first wave at -0.05 V is greatly attenuated on the second scan and disappeared completely on the sixth scan (Figure 1, s). This observation indicates an inhibition of the electron transfer by the nitrophenyl group grafted at the carbon surface. A similar behavior was observed for the (4-carboxyphenyl)diazonium tetrafluoroborate except that the first reduction wave was detected at a slightly more negative potential of -0.16 V, as expected for a less electron-withdrawing group (COOH vs NO2). In the case of the (4-nitrophenyl)diazonium species, the first wave was followed by the reversible cathodic wave at -1.2 V corresponding to the reduction of the nitro group to the radical anion.12a The oxidation wave observed on the return sweep is attributed to the oxidation of the radical anion. As expected, this wave is not detected with the (4-carboxyphenyl)diazonium cation. The surface coverage of an attached nitrophenyl group was evaluated by integration of the cyclic voltammogram in 0.1 M Bu4NBF4/acetonitrile, and a value of 18 × 10-10 mol/cm2 was found (see Table 2). This value is in good agreement with previously reported values and can be considered to be in the monolayer region.12,14 Cyclic Voltammetry of 4-Nitrophenyl- and 4-Carboxyphenyl-Modified Glassy Carbon Electrode in the Presence of Electroactive Redox Species. The inhibition of the current during the second sweep of the cyclic voltammetry experiment of Figure 1 prompted us (23) Gadallah, F. F.; Elofson, R. M. J. Org. Chem. 1969, 34, 3335.

Figure 3. Cyclic voltammetry of bare (s), 4-carboxyphenyl (- -), and 4-nitrophenyl (- O -) modified glassy carbon electrode in 1 mM Ru(NH3)63+, 0.1 M KCl, pH 7, phosphate buffer at 10 mV/s. The electrodes were modified at -0.7 V for 240 s in 5 mM of the corresponding diazonium salt solution in 0.1 M NBu4BF4/acetonitrile.

to evaluate the cyclic voltammetry behavior of an electroactive, soluble redox couple at 4-nitrophenyl- and 4-carboxyphenyl-modified glassy carbon electrode. Cyclic voltammetry experiments were carried out in aqueous and nonaqueous media with four electroactive probes: two outer-sphere inorganic [Fe(CN)63- and Ru(NH3)63+], one organic (hydroquinone), and one organometallic (ferrocene) redox systems. Figures 2-4 show the cyclic voltammograms in aqueous solutions for Fe(CN)63-, Ru(NH3)63+, and hydroquinone, respectively, before and after subsequent modification with either 4-nitrophenyl or 4-carboxyphenyl. The data of Figure 2 clearly show that the electrochemical reaction of Fe(CN)63- is completely blocked when a glassy carbon electrode is modified with either a 4-nitrophenyl or a 4-carboxyphenyl group. In this case, the characteristic oxidation/reduction waves of the redox couple are not detected. On the other hand, the reversibility of the Ru(NH3)63+/2+ redox couple is not affected by the 4-carboxyphenyl groups (Figure 3, - -), but the redox reaction of Ru(NH3)63+/2+ is completely suppressed at the GC-4-nitrophenyl electrode (Figure 3, - O -). The electrochemical response of hydroquinone/ benzoquinone is little affected by the 4-carboxyphenyl group but is significantly attenuated by the 4-nitrophenyl group (see Figure 4). The blocking properties of the substituted phenyl groups were also evaluated in nonaqueous electrolyte. Figure 5 shows that the electrochemical response of ferrocene and ferricyanide is completely blocked at the modified electrode. The cyclic

6808 Langmuir, Vol. 13, No. 25, 1997

Figure 4. Cyclic voltammetry of bare (s), 4-carboxyphenyl (- -), and 4-nitrophenyl (- O -) modified glassy carbon electrode in 1 mM hydroquinone, 1 M KCl, and 1 M HCl at 10 mV/s. The electrodes were modified at -0.7 V for 240 s in 5 mM of the corresponding diazonium salt solution in 0.1 M NBu4BF4/ acetonitrile.

Figure 5. Cyclic voltammetry of bare (s), 4-carboxyphenyl (- -), and 4-nitrophenyl (- O -) modified glassy carbon electrode in (A) 5 mM ferrocene, 0.1 M NBu4BF4/acetonitrile at 25 mV/s and (B) 5 mM ferricyanide, 0.1 M NBu4BF4/acetonitrile at 50 mV/s. The electrodes were modified at -0.7 V for 240 s in 5 mM of the corresponding diazonium salt solution in 0.1 M NBu4BF4/acetonitrile.

voltammograms for the redox probe at the bare glassy carbon electrode are also included in Figure 5. It should be noticed that the electrochemical response of ferricyanide in nonaqueous media appears at more negative potentials (by almost 1 V) than that in aqueous solution.19 The difference in the blocking behavior of the substituted phenyl groups reported above can be explained by considering electrostatic interactions between the modified surface and the electroactive probe8,24-29 and electrolyte/ (24) (a) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1995, 67, 2767. (b) Cheng, Q; Brajter-Toth, A. Anal. Chem. 1996, 68, 4180. (25) (a) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (b) Miller, C.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 5225.

Saby et al.

Figure 6. Plot of the anodic peak current of the cyclic voltammogram as a function of solution pH. The cyclic voltammogram was recorded at a 4-carboxyphenyl-modified glassy carbon electrode in a solution containing 1 mM Fe(CN)63- and the appropriate buffer solution at a scan rate of 10 mV/s. The anodic peak currents were corrected for the background current.

solvent effects.24,30 In a pH 7 solution, the 4-carboxyphenyl group is expected to dissociate to some extent if we assume that its pKa is about the same as that of benzoic acid (pKa 4.2). Thus, for a negatively charged 4-carboxyphenyl film (at pH 7), the Ru(NH3)63+ probe should not be prevented from reaching the underlying glassy carbon electrode surface. In this case the response of Ru(NH3)63+ is almost undistinguishable from that at the bare electrode (Figure 3). On the other hand, the absence of response of Fe(CN)63is attributed to the negative Donnan potential which is established at the film surface as a result of the high negative charge density of the COO- groups.13b,24,26 In order to confirm this hypothesis, cyclic voltammograms for a glassy carbon/4-carboxyphenyl electrode were recorded in 1 mM Fe(CN)63- solutions of various pHs. Figure 6 shows that the anodic peak current (background corrected) of the cyclic voltammogram increases when the solution pH is lowered. This increase is related to the protonation of the carboxylate groups present at the electrode surface which are then uncharged and can no longer prevent the Fe(CN)63- redox species from reaching the underlying glassy carbon electrode surface. Similar pH-modulated electrochemical responses for Fe(CN)63have been reported in the literature for alkanethiols having COOH and NH2 terminal groups.8,24,31,32 In addition, sigmoidal curves such as the one shown in Figure 6 have been obtained for other measurable parameters (e.g., contact angle, mass variation) of self-assembled monolayers at the electrode surface.33,34 The pKa of the 4-carboxyphenyl films estimated from the inflection point of the Ipa-pH curve of Figure 6 is 2.8. This is slightly smaller than the value expected for benzoic acid in solution (26) Redepenning, J.; Tunison, H. M.; Finklea, H. O. Langmuir 1993, 9, 1404. (27) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409. (28) Godinez, L. A.; Castro, R.; Kaifer, A. E. Langmuir 1996, 12, 5087. (29) Madoz, J.; Kuznetzov, B. A.; Medrano, F. J.; Garcia, J. L.; Fernandez, V. M. J. Am. Chem. Soc. 1997, 119, 1043. (30) Anderson, M. R.; Evaniak, M. N.; Zhang, M. Langmuir 1996, 12, 2327. (31) Sun, L.; Johnson, B.; Wade, T.; Crooks, R. M. J. Phys. Chem. 1990, 94, 8869. (32) Takehara, K.; Takemura, H.; Ide, Y. Electrochim. Acta 1994, 39, 817. (33) Wang, J.; Frostman, L. M.; Ward, M. D. J. Phys. Chem. 1992, 96, 5224. (34) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675.

Modification of Glassy Carbon Electrode

(pKa 4.2).35 We have no clear explanation for the small shift of the pKa to a lower value upon grafting the 4-carboxyphenyl group at the surface of a glassy carbon electrode. However, it is possible that some specific interfacial effect between the carbon surface and the carboxylate functionalities or the phenyl ring of the layer might play an important role. On the other hand, the cyclic voltammogram for a glassy carbon/4-carboxyphenyl electrode was also recorded in 1 mM Ru(NH3)63+ at pH 1 (see the Supporting Information). In contrast to the cyclic voltammogram in pH 7 (Figure 3, - -), the 4-carboxyphenyl layer becomes blocking for the pH 1 Ru(NH3)63+ solution. A similar behavior was recently reported for thioctic acid monolayer and was related to the electrolyte effect.24 The suppression of the electrochemical response for hydrophilic species such as Ru(NH3)63+ and Fe(CN)63- at the 4-nitrophenyl-modified electrode may be due to the hydrophobicity of the film. The electrochemistry of nitroaromatic compounds has been widely studied in the past, and it is well-known that their electrochemical reduction leads to the formation of phenylhydroxylamine and aniline depending on the experimental conditions.36 The reduction of nitro to amine can also occur for the electrochemically generated nitrophenyl layer.12c,14a Thus, although the 4-nitrophenyl group is expected to undergo some chemical modification upon cycling in aqueous media, our results suggest that the resulting film still retains a significant hydrophobic character. In the presence of a hydrophobic electroactive probe such as hydroquinone at pH 0, the blocking effect is less pronounced (see Figure 4). At such a pH, the 4-carboxyphenyl is almost fully protonated and hydroquinone can reach the glassy carbon electrode and gives a response. On the other hand, at the 4-nitrophenyl-modified electrode the response of hydroquinone is significantly suppressed. The suppression cannot be related to hydrophobic effects since the 4-nitrophenyl group is more hydrophobic than the 4-carboxyphenyl group. In addition, the electrochemistry of hydroquinone is complex as it comprises consecutive electrochemical and chemical steps and yields cationic intermediates.37 This suppression is not fully understood yet, but it might be due to the fact that the 4-nitrophenyl film is thicker or more compact than the 4-carboxyphenyl film and seems to rule out any significant hydrophobic effects. Cyclic voltammetry recorded in acetonitrile containing hydrophobic (ferrocene) and hydrophilic (Fe(CN)63-) electroactive probes38 for modified carbon electrodes shows complete inhibition of their electrochemical responses in organic media (Figure 5). This agrees with the presence of a compact and dense layer. This is in contrast with long-alkyl-chain thiols which spontaneously form a monolayer on gold and are permeable to ferrocene due to the solvation of the monolayer coating.27 The cyclic voltammetry data for the 4-carboxyphenyl-modified electrode when it acts as a barrier suggest that the layer is more compact in organic media than in an aqueous solution. The interfacial capacitance can be used to evaluate the compactness or the extent of solvent and electrolyte permeation in a layer.30 Interfacial capacitance was evaluated by electrochemical impedance spectroscopy (35) Petrov, J. G.; Mo¨bius, D. Langmuir 1996, 12, 3650. (36) Cyr, A.; Huot, P.; Belot, G.; Lessard, J. Electrochim. Acta 1990, 35, 147. (37) Vetter, K. J. In Electrochemical Kinetics; Academic Press: New York, 1967. (38) The electron transfer rate, k°, for 5 mM Fe(CN)63-/Fe(CN)64- in 0.1 M Bu4NBF4/acetonitrile as evaluated by electrochemical impedance spectroscopy is 0.000 13 cm/s. The experimental RCT value of 4315 Ω was used for the calculation.

Langmuir, Vol. 13, No. 25, 1997 6809 Table 1. Interfacial Capacitancea for Bare and 4-Carboxyphenyl-Modified Glassy Carbon Electrode in the Presence of Electroactive Species Evaluated from the Electrochemical Impedance Data capacitancea (µF/cm2) ferricyanide/ferrocyanide electrode

aqueousb

nonaqueousc

bare modifiedd

28.4 68.5

13.0 9.9

a The capacitance, C, was calculated from43 Cφ ) T/{(R -1 + S RCT-1)1-φ}. b 5 mM K3Fe(CN)6/5 mM K4Fe(CN)6/0.1 M KCl; 0.1 M phosphate; pH 7. c 5 mM (Et4N)3Fe(CN)6/5 mM (Et4N)4Fe(CN)6/ 0.1 M Bu4NBF4/acetonitrile. d The glassy carbon electrode was modified at -0.7 V for 240 s in 5 mM of the diazonium salt solution, 0.1 M NBu4BF4/acetonitrile.

(vide infra) for the 4-carboxyphenyl and the bare electrodes in the presence of Fe(CN)63- in both aqueous and nonaqueous media. Table 1 shows that the capacitance increases for aqueous media and decreases for organic media following grafting of the substituted phenyl group. A detailed investigation of the capacitance of a substituted phenyl group modified glassy carbon electrode is currently underway in our laboratory, and the results will be reported elsewhere. Nonetheless, the smaller capacitance of the modified electrode in organic media (9.9 µF/cm2) than that in an aqueous solution (68.5 µF/cm2) is in agreement with a less permeable and less solvated film in a nonaqueous medium.30 Nonetheless, the blocking effect of either 4-nitrophenylor 4-carboxyphenyl-modified electrode is quite surprising in light of previous results reported in the literature for SAMs8,25,39 and the proposed monolayer structure of these substituted phenyl modified electrodes.12-14 Nevertheless, in addition to the results presented above, the barrier properties of the 4-carboxyphenyl film have been recently demonstrated for negatively charged biological molecules such as ascorbate.13b On the other hand, the barrier effects of several electrode modifiers such as pyridine40 and thiol derivatives25,41,42 have been investigated in detail. It was found that a significant blocking effect is only observed when the alkyl chain of the modifier consists of at least six carbon atoms.25 The van der Waals interactions between the long hydrophobic alkyl chains permit the “selfassembling” of close-packed monolayers with good barrier properties. The packing of the film is very important as was shown when the blocking properties of thioctic acid (1,2-dithiolane-3-pentanoic acid) and hexanethiol were compared.24 Thioctic acid, a disulfide which forms a less closely packed film, is less blocking than hexanethiol because of the absence of a second alkyl chain in the asymmetric thioctic acid. In contrast to long-alkyl-chain thiols, thioaromatic monolayers on gold, which bear some resemblance to the system investigated in our work in the sense that a phenyl group is present at the electrode surface, do not show good barrier properties in the presence of ferricyanide.41 Thus, the data of Figure 2 differ significantly from that reported for the Au/thiophenol electrode. Even if this is unexpected, if only a monolayer of 4-nitrophenyl or 4-carboxyphenyl is present at the surface of the glassy carbon electrode, these results (39) (a) Taira, H.; Nakano, K.; Maeda, M.; Takagi, M. Anal. Sci. 1993, 9, 199. (b) Uchida, I.; Ishiho, A.; Matsue, T.; Itaya, K. J. Electroanal. Chem. 1989, 266, 455. (40) (a) Lipkowski, J. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O’M., White, R. E., Eds.; Plenum Press: New York, 1992; Vol. 23, p 1. (b) Bizotto, D.; McAlees, A.; Lipkowski, J.; McCrindle, R. Langmuir 1995, 11, 3243. (41) Sabatini, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (42) Mahir, T. M.; Bowden, E. F. Electrochim. Acta 1994, 39, 2347.

6810 Langmuir, Vol. 13, No. 25, 1997

Saby et al. Table 2. Charge-Transfer Resistance, RCT, Values Extracted from the Analysis of the Electrochemical Impedance Data A. Effect of Growth Conditions modification conditions time [diazonium] (s) E (V) (mM)

Figure 7. Complex impedance plots for a bare and 4-carboxyphenyl-modified glassy carbon electrode in 5 mM Fe(CN)63-/4at a potential of 0.25 V. The glassy carbon electrodes were modified at -0.7 V during 30 (s), 60 (- -), and 240 (- O -) s from a 5 mM (4-carboxyphenyl)diazonium tetrafluoroborate, 0.1 M NBu4BF4/acetonitrile.

demonstrate clearly the role of electrostatic repulsion in the 4-carboxyphenyl/ferricyanide case and the role of solvation (electrolyte/solvent effects) to explain the barrier properties of the grafted layer. Electrochemical Impedance Spectroscopy of 4-Nitrophenyl- and 4-Carboxyphenyl-Modified Glassy Carbon Electrode in the Presence of Ferri-Ferrocyanide. Electrochemical impedance spectroscopy measurements can be used to evaluate the effect of 4-nitrophenyl and 4-carboxyphenyl groups on the kinetics of a redox reaction at a glassy carbon electrode. Figure 7 presents the complex impedance plots for a bare electrode and various (4-carboxyphenyl)-modified glassy carbon electrodes at a potential of 0.25 V in the presence of 5 mM Fe(CN)63-/4-. The impedance plot for the bare electrode is characterized by a semicircle at high frequency and a low-frequency Warburg line at an angle of 45°. The semicircle corresponds to a parallel combination of the charge-transfer resistance, RCT, with the double layer capacitance, CDL, while the linear response is related to mass transport effects. The impedance plots for glassy carbon electrodes modified by electrochemical reduction of a (4-carboxyphenyl)diazonium salt at -0.7 V for various times differ significantly from that of the bare electrode. The diameter of the semicircle increases with an increase of the electrolysis time, and only about half of a semicircle is obtained for the longer electrolysis time t ) 240 s used in our work. The Warburg line is not observed for the modified electrodes in the frequency range 60 kHz to 0.05 Hz used here. Qualitatively, the increase of the semicircle indicates that the electrode kinetics become slower as the glassy carbon electrode is modified with a substituted phenyl group. The impedance data will be dealt with quantitatively below. A similar modification of the complex impedance plot has been reported previously for Au electrodes following modification with long-alkyl-chain thiols.38,41,42 The impedance data are in good agreement with the cyclic voltammetry results of Figure 2. An electrical equivalent circuit for both bare and modified electrodes is shown in the inset in Figure 8 and comprises the solution resistance, RS, the charge-transfer resistance, and a constant phase element (CPE) which is characterized by the pseudocapacitance T and the exponent φ.43 The CPE was used instead of a pure capacitor (43) Brug, G. J.; van den Eeden, A. L. G.; Sluyters-Rehbach, M.; Sluyters, J. H. J. Electroanal. Chem. 1984, 176, 275.

bare electrode 4-nitrophenyl- 240 modified 240 electrode 240 60 30 10 4-carboxy240 phenyl240 modified 240 electrode 60 30 10

o.c. -0.7 -0.7 -0.7 -0.7 -0.7 o.c. -0.7 -0.7 -0.7 -0.7 -0.7

5 5 1 1 1 1 5 5 1 1 1 1

k° Γb (cm/s) (mol cm-2)

RCTa (Ω)

422 0.0013 3 × 10-10 1.5 × 103 1.3 × 106 18 x10-10 9.1 × 104 3.8 × 104 2.8 × 104 2.3 × 104 3.5 × 103 4.0 × 105 5.4 × 104 4.7 × 104 4.2 × 104 4.1 × 104

B. Effect of the Solution pH

4-carboxyphenyl modified

electrodec

pH

RCTd (Ω)

7 5 4 3 2 1

4.0 × 105 1.5 × 105 4.7 × 104 1.0 × 104 7.7 × 103 6.8 × 103

a From the electrochemical impedance data obtained in the presence of a 5 mM Fe(CN)63-/5 mM Fe(CN)64-/0.1 M KCl, phosphate buffer, pH 7 solution at 0.25 V. b The surface coverage was obtained by integration of the cathodic wave of the cyclic voltammogram recorded in 0.1 M Bu4NBF4/acetonitrile at a scan rate of 100 mV/s. c The glassy carbon electrode was modified at -0.7 V for 240 s in 5 mM of the diazonium salt solution in 0.1 M NBu4BF4/acetonitrile. d From the electrochemical impedance data obtained in the presence of a 5 mM Fe(CN)63-/5 mM Fe(CN)64-/0.1 M KCl, phosphate buffer, pH 7 solution at open circuit (o.c.).

in the equivalent circuit that represents the electrochemical system due to microscopic surface roughness and inhomogeneity in surface and kinetics. Experimentally, the CPE is expressed by the clockwise rotation of the complex plane plot for a smooth electrode through an angle R ) 90°(1 - φ). Impedance data were analyzed44,45 using this electrical equivalent circuit, and Table 2 shows that the charge-transfer resistance, RCT, increases dramatically following modification of the glassy carbon electrode from a value of 422 Ω for the bare electrode to 1.3 × 106 Ω after electrochemical modification for 240 s from a 5 mM 4-nitrophenyldiazonium/acetonitrile solution at a potential of -0.7 V. A decrease of RCT is observed when a lower concentration of the diazonium salt (1 mM) is used while keeping the same electrolysis time. For the same 1 mM diazonium concentration, an increase of RCT is noticed for the 4-carboxyphenyl-modified electrode when the electrolysis time is increased from 10 to 240 s. A similar trend is observed in the case of the 4-nitrophenyl group although the amplitude of the difference in RCT values for short (10 s) and long (240 s) electrolysis time is smaller than for that the 4-carboxyphenyl group. The increase of RCT with electrolysis time demonstrates that the film becomes more compact upon an increase of the electrolysis time. The higher RCT for the 4-nitrophenyl group in comparison to that for the 4-carboxyphenyl group (see Table 1 for 240 s, -0.7 V, 5 mM) is in agreement with a more compact and less permeable layer. Table 2A also shows that the RCT value for the electrode left in the diazonium solution at (44) Groult, H.; Devilliers, D.; Vogler, M.; Chemla, M. J. Appl. Electrochem. 1994, 24, 870. (45) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods; John Wiley & Sons: New York, 1980; p 316.

Modification of Glassy Carbon Electrode

Langmuir, Vol. 13, No. 25, 1997 6811

Figure 8. Equivalent circuit used for the analysis of the electrochemical impedance data where RΩ is the solution resistance, RCT is the charge-transfer resistance, T a pseudocapacitance, and φ is the phase angle of the constant-phase element.

open circuit increases in comparison to the bare electrode, suggesting that the glassy carbon electrode is coated with an organic layer in these experimental conditions. The effect of solution pH on the RCT values was also evaluated in the pH 1-7 range. In agreement with the cyclic voltammetry data of Figure 6, Table 2B shows a decrease of RCT as the pH is lowered, indicating a more facile redox reaction for ferricyanide. The electrochemical response of thin films can be controlled by either electron tunneling through the thin film25 or diffusion of electroactive probe within defects or pinholes of the film8,40,41 or through film discontinuities.46 In principle, the pinhole model can be used to analyze the RCT data and estimate the coverage of the grafted film.41,47 However, a rigorous analysis does not allow the use of this model because the rate at which basal and edge planes are modified by the diazonium reaction differs significantly.12c,14a As expected, the rate is smaller for the less reactive basal planes. Thus, the coverage that can be evaluated from the RCT data would represent the coverage of the more reactive basal planes since the electron-transfer kinetics of Fe(CN)63- are sluggish at the basal planes.11 In a recent study, the electron-transfer kinetics of Fe(CN)63- were shown to be slowed down significantly at the 4-nitrophenyl-modified electrode,14b although the extent of the suppression was smaller than that reported in the present study. The difference might be due to the different experimental conditions (electrolysis potential and diazonium concentration) used for the electrochemical modification. Another factor to consider is the “reactivity” of the bare glassy carbon which is evidenced by the heterogeneous rate constant, k°, for the Fe(CN)63- redox reaction. From the RCT value (422 Ω) of Table 2 for the bare glassy carbon electrode, a k° of 0.0013 cm/s can be computed. This value is much lower than that reported (k° ) 0.08 cm/s) earlier by Chen and McCreery.14b On the other hand, we found a higher k° value (0.0073 cm/s) when the same electrolyte (1 M KCl/ H2O) was used instead of the phosphate buffer. It is clear that the surface properties of our glassy carbon are different because the rate constant is slightly smaller. This indicates that the surface is not as “clean” as that used in ref 14b. These observations are interesting and (46) Badia, A.; Back, R.; Lennox, R. B. Angew. Chem., Int. Ed. Engl. 1994, 33, 2332. (47) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatini, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660.

Figure 9. X-ray photoelectron survey spectrum for a 4-nitrophenyl-modified glassy carbon electrode modified from a 5 mM (4-nitrophenyl)diazonium tetrafluoroborate, 0.1 M NBu4BF4/acetonitrile.

suggest that the barrier properties of these layers might be related to the k° of the bare electrode, with the film being more permeable when the k° of the bare electrode is higher. These are surprising results, and experiments designed to understand these phenomena are currently underway in our laboratory. X-ray Photoelectron Spectroscopy of 4-Nitrophenyl- and 4-Carboxyphenyl-Modified Glassy Carbon Electrode. XPS was used to characterize the surface of the glassy carbon electrode following the electrochemical modification with the phenyldiazonium derivatives. The XPS survey spectrum recorded for a glassy carbon electrode modified with a 5 mM 4-nitrophenyldiazonium tetrafluoroborate/acetonitrile solution at a potential of -0.7 V for 240 s and shown in Figure 9 exhibits the characteristic C 1s, O 1s, and N 1s peaks at about 285, 530, and 400 eV, indicating that the 4-nitrophenyl moieties have been immobilized at the surface of the glassy carbon electrode. Figure 10 shows the N 1s core level spectra from the electrodeposited film (curve c) and from a glassy carbon electrode that was only soaked in the 4-nitrophenyldiazonium solution (curve b). The N 1s signal shows two peaks at 400 and 406 eV for both electrodes. On the one hand, the peak at 406 eV can be attributed to the nitrogen of a nitro group.14,48 On the other hand, the peak (48) Elliott, C. M.; Murray, R. W. Anal. Chem. 1976, 48, 1247.

6812 Langmuir, Vol. 13, No. 25, 1997

Saby et al.

presence of the peak at 400 eV,48 but this explanation can be ruled out since the XPS peak at 400 eV is also present in the case of a surface immersed in the (4-carboxyphenyl)diazonium salt which does not bear a nitro function (Table 3). Thus, the origin of the peak at 400 eV is unclear. However, a tentative explanation can be given when considering the reaction of diazonium with phenol or naphthol51 according to52 N N Ar O–

OH

+ ArN+2

Figure 10. Nitrogen 1s core level spectra for (a) a bare glassy carbon electrode, (b) a glassy carbon electrode soaked in the 5 mM (4-nitrophenyl)diazonium tetrafluoroborate, 0.1 M NBu4BF4/acetonitrile solution, and (c) a glassy carbon electrode modified in the same solution at a potential of -0.7 V for 240 s. Table 3. XPS Results of Glassy Carbon Electrodes Modified with 4-Substituted Phenyl Groups potential/time/ [diazonium] -0.7 V/30 s/5 mM -0.7 V/240 s/5 mM -0.7 V/240 s/1 mM immersed

N 1s(406 eV)/ N 1s(406 eV)/ N 1s/ O 1s/ N 1s(400 eV) C 1s C 1s C 1s 4-Nitrophenyl 2.13 2.16 1.71 1.41

0.089 0.093 0.071 0.008

0.131 0.136 0.113 0.013

4-Carboxyphenyl -0.7 V/240 s/5 mM immersed

0.022 0.006

0.29 0.21

at 400 eV seems to be related to the modification procedure because such a peak is not observed for a bare glassy carbon electrode. This result is in contrast with those of Liu and McCreery14a who have observed a peak at 400 eV for their glassy carbon electrodes. This difference might be explained by the different sources of glassy carbon used although Porter and co-workers have not observed a peak at 400 eV for their bare glassy carbon electrodes.49 In addition, a recent study has shown that a peak at 400 eV was also observed in the case of silicon modified with either 4-nitrophenyl or bromophenyl groups.50 Table 3 gives relevant atomic ratios extracted from XPS core level spectra measured for glassy carbon electrodes modified in various experimental conditions (electrolysis time, 4-substituted diazonium concentration, and nature of substituent). Table 3 shows that the ratio of the N 1s(400 eV) and N 1s(406 eV) peaks for the electrode modified with the 4-nitrophenyl group is not constant. This suggests that the electrochemical reduction of the diazonium salt induces a modification of the molecule such as the loss of nitrogen (see eq 1). Moreover, Table 3 indicates that the N 1s(406 eV)/N 1s(400 eV) ratio is not equal to 0.5, the value expected if the substituted phenyldiazonium would only be adsorbed at the electrode surface without further chemical transformation. The peak at 400 eV might be related to the presence of tetrabutylammonium groups of the tetraalkylammonium cations from the diazonium salt. However, the peak position is not in agreement with the value expected (401.5 eV) for such cations.48 Reduction of nitroaromatics into the XPS spectrometer chamber can account for the (49) Deinhammer, R. S.; Ho, M.; Anderegg, J. W.; Porter, M. D. Langmuir 1994, 10, 1306. (50) Henry de Villeneuve, C.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415.

(5)

The phenol groups present at the surface of the glassy carbon electrode can undergo a coupling reaction with the diazonium and yield a hydrazine. The N 1s core level XPS spectrum of a hydrazine derivative displays one peak at 400 eV.48 This is in agreement with our experimental data of Figure 9 and the coupling reaction described above. The fraction of the diazonium that couples with glassy carbon according to eq 2 can be estimated from the N 1s(406 eV)/N 1s(400 eV) ratio. A value of about 75% can be computed from the data of Table 3 for the electrode modified at -0.7 V for 240 s in the 5 mM (4-nitrophenyl)diazonium solution. The area ratio of N 1s(406 eV)/C 1s for the modified electrode is 0.089 and is close but slightly higher than the value (0.055) reported by Liu and McCreery.14a This result indicates that close to a monolayer of substituted phenyl groups is present at the surface of the glassy carbon electrode.14a Even if the sum of the peaks at 400 and 406 eV is considered, the amount of diazonium derived material will be close to a monolayer. This is in agreement with other recent investigations with substituted phenyl modified glassy carbon electrodes.12-14 On the other hand, a N 1s(406 eV)/C 1s ratio of 0.167 is expected for a 4-nitrophenyl group. However, this ratio is larger than the values reported in Table 3 because the underlying glassy carbon electrode can contribute to the C 1s signal as well. Since the mean free path for C 1s photoelectrons is 32 Å, the sampling depth (3 times the mean free path) should be 96 Å.53 Thus, it is clear that the N 1s(406 eV)/C 1s ratio should be much lower than 0.167 if only a monolayer of 4-nitrophenyl is present at the surface of a glassy carbon electrode. Moreover, the sampling depth of 96 Å and the N 1s(406 eV)/C 1s ratio lower than 0.167 indicate that the substituted phenyl film thickness is not larger than 96 Å. This can be taken as an indirect evidence that close to monolayer coverage is achieved, in agreement with previous studies.12,14 It is interesting to note that monolayer coverage is also achieved at silicon (111) electrode.50 Conclusion Glassy carbon electrodes modified by electrochemical reduction of (4-nitrophenyl)diazonium fluoroborate and (4-carboxyphenyl)diazonium fluoroborate have been characterized by cyclic voltammetry, electrochemical impedance spectroscopy, and XPS. Such modified electrodes show a blocking behavior in the presence of a negatively charged redox couple such as Fe(CN)63-, and this blocking effect can be controlled by an appropriate choice of the conditions used for the modification of the electrode and the pH of the solution. Electrolyte/solvent effects have (51) Elofson, R. M.; Edsberg, R. L.; Mecherly, P. A. J. Electrochem. Soc. 1950, 97, 167. (52) Zollinger, H. Acc. Chem. Res. 1973, 6, 335. (53) Sun, F.; Castner, D. G.; Mao, G.; Wang, W.; McKeon, P.; Grainger, D. W. J. Am. Chem. Soc. 1996, 118, 1856.

Modification of Glassy Carbon Electrode

also been put forward to explain the behavior of Ru(NH3)63+ in aqueous solution and ferricyanide and ferrocene in nonaqueous media. The substituted phenyl layer is much more compact and less permeable in contact with a nonaqueous solvent presumably because the layer is poorly solvated and acts as a barrier for both ionic (Fe(CN)63-) and hydrophobic (ferrocene) redox species. Cyclic voltammetry and XPS measurements have confirmed that close to monolayer coverage can be achieved with this procedure. Acknowledgment. This work was supported by the Natural Sciences and Engineering Research Council (NSERC) and Hydro-Que´bec (Research Partnerships Program-Collaborative Research Development Grant)

Langmuir, Vol. 13, No. 25, 1997 6813

and through an equipment grant by NSERC for the impedance setup. C.S. acknowledges partial financial support from UQAM for a PAFACC postdoctoral fellowship. We also acknowledge Drs. Save´ant and Pinson for providing reprints of their work prior to publication. Supporting Information Available: Figure of cyclic voltammetry of bare (s) and 4-carboxyphenyl (- -) modified glassy carbon electrodes in 1 mM Ru(NH3)63+, 0.1 M KCl, 0.1 M HCl, pH 1, at 10 mV/s. The electrodes were modified at -0.7 V for 240 s in 5 mM of the corresponding diazonium salt solution in 0.1 M NBu4BF4/acetonitrile (1 page). Ordering information is given on any current masthead page. LA961033O