Modified Glassy Carbon Electrode

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Spectroscopy Letters

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Spectroscopic and Electrochemical Characterization of BenzoylglycineModified Glassy Carbon Electrode: Electrocatalytic Effect Towards Dioxygen Reduction in Aqueous Media

Aybüke A. İsbir-Turana; Zafer Üstündağb; Haslet Ekşia; Remziye Güzelc; Ali Osman Solakad a Ankara University, Faculty of Science, Department of Chemistry, Tandogan, Ankara, Turkey b Dumlupınar University, Faculty of Art and Sciences, Department of Chemistry, Kütahya, Turkey c Dicle University, Faculty of Arts and Sciences, Department of Chemistry, Diyarbakır, Turkey d Faculty of Engineering, Department of Chemistry Engineering, Kyrgyz-Turk Manas University, Bishkek, Kyrgystan Online publication date: 18 March 2011 To cite this Article İsbir-Turan, Aybüke A. , Üstündağ, Zafer , Ekşi, Haslet , Güzel, Remziye and Solak, Ali Osman(2011)

'Spectroscopic and Electrochemical Characterization of Benzoylglycine-Modified Glassy Carbon Electrode: Electrocatalytic Effect Towards Dioxygen Reduction in Aqueous Media', Spectroscopy Letters, 44: 3, 158 — 167 To link to this Article: DOI: 10.1080/00387010.2010.487508 URL:

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Spectroscopy Letters, 44:158–167, 2011 Copyright # Taylor & Francis Group, LLC ISSN: 0038-7010 print=1532-2289 online DOI: 10.1080/00387010.2010.487508

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Spectroscopic and Electrochemical Characterization of BenzoylglycineModified Glassy Carbon Electrode: Electrocatalytic Effect Towards Dioxygen Reduction in Aqueous Media Aybu¨ke A. I˙sbir-Turan1, ¨ stu¨ndag˘2, Zafer U Haslet Eks¸ i1, Remziye Gu¨zel3, and Ali Osman Solak1,4 1

Ankara University, Faculty of Science, Department of Chemistry, Tandogan, Ankara, Turkey 2 Dumlupınar University, Faculty of Art and Sciences, Department of Chemistry, Ku¨tahya, Turkey 3 Dicle University, Faculty of Arts and Sciences, Department of Chemistry, Diyarbakır, Turkey 4 Kyrgyz–Turk Manas University, Faculty of Engineering, Department of Chemistry Engineering, Bishkek, Kyrgystan

ABSTRACT Present work aims to create a benzoylglycine (BG)-modified glassy carbon (GC) substrate exploiting the electroreduction of diazonium salts. Dopamine was used to confirm the attachment of benzoylglycine molecules onto the glassy carbon surface by observing the blockage of the electron transfer using cyclic voltammetry (CV). BG-modified GC surface (BG-GC) was also characterized by Raman spectroscopy and electrochemical impedance spectroscopy (EIS) techniques. The ellipsometric thickness of the BG film was measured as approximately 14 nm for seven CV cycles. The electrocatalytic effect of BG-GC electrode surface against dioxygen reduction was investigated. The catalytic effect for dioxygen reduction of the BG-GC surface was compared with that of 2-benzo[c]cinnoline, 2-benzo[c]cinnoline 6-oxide- and diethylene glycol bis(2-aminophenyl)ether-modified GC surfaces to clarify the mechanism of catalysis of the surfaces in terms of molecular structure. KEYWORDS benzoylglycine modification, dioxygen reduction, electrocatalytic effect, electrochemical impedance spectroscopy, Raman spectroscopy


Received 4 March 2010; accepted 18 April 2010. Address correspondence to Ali Osman Solak, Faculty of Science, Department of Chemistry, Ankara University, 06100, Tandogan, Ankara, Turkey. E-mail: [email protected]

During recent years, chemically modified electrodes, especially glassy carbon (GC), have received increased attention due to their potential use in analytical and technical applications. Unique features such as compactness, chemical resistance to corrosion, mechanical stability, and its impermeability to fluids and gases made GC the choice of electrode material.[1,2] Furthermore, its electrochemical activity, easy modification, low cost, high applicability, low background currents over a wide potential range have increased the importance of carbon electrodes especially in electroanalysis and electrosynthesis.[3,4] The covalent modification of GC electrodes using 158

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aryl diazonium salts was first developed by Pinson and his group.[5] It is a well-known fact that aryl diazonium salts are reduced with a one-electron mechanism on the carbon surfaces. This reduction causes the formation of the covalent bonds of aryl groups with carbon atoms in the electrode of graphitic structure. Downward has reviewed this modification method for carbon electrodes like GC, carbon fiber, highly oriented pyrolytic graphite (HOPG), and porous graphite carbon disk electrode.[6] The procedure of this modification has been applied to various carbon electrodes including GC with small revisions.[7–10] Carbon electrode surfaces can also be derivatized by two kinds of wellestablished procedures. First, strong oxidation methods are employed at various media like HNO3 and KMnO4.[11] However, strong oxidation causes oxygenated functions to be formed on the surface, producing a nonspecific derivatization. Second method is the electrochemical oxidation of primary and secondary amines, alcohols, carboxylates, and hydrazides.[12–15] The method with aryl diazonium salt reduction is the simplest one of the aforementioned modification procedures. The grafting mechanism and bonding between the substrate and organic molecules with aryl groups was discussed by Pinson and Podvorica in a review paper.[16] All of these methods lead to the strong covalent attachment of –NHR, -OR, -CH2R, –NHR, and –aryl groups on the surface of carbon electrodes. Modified electrodes have in some cases electrocatalytic effects and in other cases blocking effects on the electron transfer reactions. It is believed that these electrodes enhance the electron transfer rate by the reaction that reduces the overpotential.[17] There are many studies on the electrocatalytic properties of various modified electrodes in the literature.[18–21] The major advantage of the electrochemical modification by cyclic voltammetry (CV) is its simplicity and that the modified electrodes have long time stability.[16,22–26] These electrodes can be used as sensors for the voltammetric differentiation of dopamine (DA) and ascorbic acid, for the alkali metal determination, dopamine, polysulfosalicylic acid, etc.[27–30] In a recent paper, I˙sbir et al. investigated that benzo[c]cinnolinemodified surface shows an electrocatalytic effect towards some podand-type molecules.[31] The most important application of the modified electrodes, probably, is the dioxygen reduction. 159

The oxygen reduction reaction plays significant role in various application area such as fuel cells, gas sensors, and the electrosynthesis of hydrogen peroxide.[32] Vaik et al. reported that a larger value of the rate constant of O2 reduction resulted in the antraquinonemodified electrode, producing H2O2 with a 100% yield.[33] Sarapuu et al. studied O2 reduction at antraquinone-modified GC electrode surface in aqueous media and reported that the two-electron reduction of oxygen to hydrogen peroxide was observed and the catalytic activity of the electrodes for O2 reduction was dependent on the antraquinone surface concentration.[34] A very interesting behavior of the modified electrodes is the current-voltage responses of the monolayer surfaces, such as rectification, negative differential resistance, conductance switching, and various electron transfer mechanisms.[35–37] In some cases, the modified electrodes might have exhibited much slower electron transfer than the unmodified GC electrodes due to several factors such as structure of the modifier, compactness, and film thickness. However, when a negative potential excursion is applied to relatively high values, the modified electrodes exhibit much faster electron transfer kinetics than before. Solak et al. reported the conductance-switching behavior of several organic monolayers on carbon surfaces. In another study, they concluded that in the presence of a negative voltage applied between a graphitic conductor and a metallic top contact, a monolayer of nitroazobenzene, nitrobiphenyl, or biphenyl switched from a high-resistance state to one with resistance at a factor of 10 or lower.[36,37] Characterization of modified GC surfaces can be performed by various spectroscopic methods such as X-ray photoelectron spectroscopy (XPS),[31,38] electron spin resonance (ESR),[39] Raman spectroscopy,[35–38] and RAIRS.[40] Raman spectroscopy has the advantage of providing structural information about the monolayer at the GC electrode surface.[41] Therefore, this very powerful technique is generally used to acquire the surface spectra of the modified GC electrode for characterization. Atomic force microscopy (AFM),[38] scanning tunneling microscopy (STM),[33] electrochemical impedance spectroscopy (EIS),[42,28] and scanning electrochemical microscopy (SECM)[14] are other useful techniques used for the characterization of the modified surfaces. Electrochemical methods have also been used to characterize the organic molecules bonded to the surface in two different Spectroscopic and Electrochemical Characterization

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ways. One is using surface sensitive molecules such as dopamine,[37] and the second is using molecules bearing electroactive groups.[7] In the present paper, we demonstrate the possibility of in situ electrografting the benzoylglycine (BG) on GC electrode using its diazonium salt and characterize the surface by CV, EIS, RAIRS, and Raman spectroscopy. Ellipsometric BG film thicknesses vs. number of CV cycles and the possibility of the catalytic effects of the film towards dioxygen reduction were also studied.


Electrochemical impedance spectroscopy (EIS) measurements were performed using a Gamry 300 electrochemical interface (Gamry 300, USA) equipped with a PCI4=300 potentiostat in conjuction with EIS 300 software. AC voltage of 10 mV in amplitude with a frequency range from 0.1 Hz to 100 kHz was superimposed on the DC potential and applied to the electrodes. The DC potential of 0.100 V, which is the formal potential of ferricyanide=ferrocyanide in 0.1-M KCl, was set up. Experimental data of the electrochemical impedance plot were analyzed by applying the nonlinear least squares fitting to the theoretical model represented by an equivalent electrical circuit.

Chemicals and Reagents

Electrode Cleaning and Polishing

Ultra pure quality of water with a resistance of 18.3 MX cm (Human Power 1þ purification system, S. Korea) was used in preparation of solutions and electrode cleaning. Benzoylglycine diazonium salt (BG DAS) was synthesized from [N(p-aminobenzoyl) glycine] (ABG) (Fluka). All chemicals were of the highest purity available from Merck, Fluka, or Riedel chemical companies. In all experiments, all the solutions were thoroughly deoxygenated by bubbling with purified Argon gas (99.999%) for 10 min prior to the voltammetric measurements and an Argon gas blanket was maintained over the solutions during the electrochemical experiment. All the experiments were performed at room temperature (25  1 C). Britton-Robinson (BR) buffer was used when the dioxygen reduction reaction was studied in different acidity levels.

The GC electrodes were prepared by polishing to reach a smooth and mirror-like appearance with a minimum roughness, first with fine wet emery paper (grain size 4000) for 2 min, then with 1.0-mm and 0.3-mm alumina slurry (Baikowski Int. Corp., USA) on micro cloth pads (Buehler, Lake Bluff, IL, USA) for approximately 5 min and then washed with ultra pure water. After the initial polishing, the GC electrodes were resurfaced using 0.05-mm alumina slurry to prepare a smoother surface. Polished GC electrodes were sonicated in ultra pure water at least twice and then once in a mixture of 1:1 (v=v) isopropyl alcohol=acetonitrile (IPA þ MeCN) (Riedel) purified with an equal volume of Norit A–activated carbon that filtered thorough Whatman filter paper No. 1 (Maidstone, England) for 10 min each.

Electrodes and Equipment

Synthesis of BG Diazonium Salt

A traditional three-electrode cell system was used in all electrochemical experiments. Ag=AgCl=KCl(sat) and Ag=Agþ (0.01 M in 0.1-M TBATFB in acetonitrile) reference electrodes were used in aqueous and nonaqueous solutions, respectively. Ag=Agþ reference electrode was calibrated to ferrocene standard solution, and a Pt wire counter electrode was employed. BAS Model MF-2012 (Bioanalytical Systems, USA) and Tokai GC-20 (Tokai, Japan) glassy carbon electrodes were used BAS as working electrodes. The BG-modified GC electrode is designated as BG-GC electrode with a geometric area of 0.071 cm2. CV technique was performed with a BAS CV-50 W electrochemical analyzer (Bioanalytical Systems, West Lafayette, IN, USA) equipped with C3 Cell Stand.

BG diazonium salt (BG DAS) was synthesized from [N(p-aminobenzoyl)glycine] (ABG) (Fluka) in voltammetric cell, and 0.5-M HCl (Carlo Erba) was used during the synthesis for in situ derivatization.[29] Temperature of the experiment was maintained at þ4 C. NaNO2 (Merck) was dissolved in water at 0 C and then added into voltammetric cell content drop by drop, stirred for 1 hr keeping temperature below þ4 C. Initial concentrations of ABG and NaNO2 in voltammetric cell were adjusted to 1:4 molar ratio, respectively.

A. A. I˙sbir-Turan et al.

Electrode Modification Electrode surfaces were modified in the same media by using CV in the potential range from 160

þ0.4 V to 0.8 V with a scan rate of 200 mV s1 for 3 cycles vs. Ag=AgCl=KCl(sat). The electrode was then taken out and immediately rinsed with acetonitrile to remove any physisorbed and unreacted precursors from the electrode surface. Benzoylglycine-modified electrode (BG GC) was then ready for use and was stored in acetonitrile. Derivatized electrodes were not sonicated due to the risk of stripping or damaging the film at the surface.

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Raman Spectroscopic Studies Raman spectra were collected with a Raman spectrometer (Spectra-Physics, USA) with 15-cm focal-length cylindrical lens. A 514.5-nm argon laser (Arþ) with a power of 45 mW was used. For the detection, cooled GaAs photomultiplier tube equipped with the necessary photon-counting electronics and a U-1000 Jobin Yvon 1-m double grating spectrometer were used as detector and spectrometer, respectively. All the Raman data were evaluated by Labview software. Origin program was used for processing of Raman spectra.

Ellipsometric Measurements Ellipsometric measurements of the film thickness were performed with an ELX-02C=01 R Model highprecision discrete wavelength ellipsometer (Dr. Riss Ellipsometerbau GmbH, Ratzeburg, Germany). The wavelength was 532 nm for all experiments. The thickness values of BG-GC films at the GC surface were determined from the average of the measurements using incidence angle of 70 degrees.

SCHEME 1 The reaction path of electrochemical derivatization of glassy carbon surface with benzoylglycine diazonium salt in aqueous medium.

Figure 1 shows the CV derivatization curves obtained in a reaction mixture of ABG and NaNO2 in a 0.5-M-HCl solution. The voltammograms were taken at a scan rate of 200 mV s1 in the potential range from þ0.4 V to 0.8 V vs. Ag=AgCl=KCl(sat). As can be seen from Fig. 1, the irreversible peak current decreases with the number of potential cycles. This voltammetric behavior shows the characteristic features of electrochemical reduction of diazonium salts at carbon or metal electrodes in aqueous and nonaqueous media.[29,44] The peak current decreases with the potential cycles and reaches the steady state value after almost 3 cycles. Gradual decrease in peak current implies that the formation of the BG layer at the GC surface proceeds as the number of potential cycles increases. The number of scans for a complete formation of a monolayer depends on the structural differences of the molecules

RESULTS AND DISCUSSION Modification of Glassy Carbon Electrode The surface of the GC electrode was modified with BG using CV. The BG molecules were attached to the carbon surface by covalent bonds forming a multilayer film as ellipsometric measurements showed. Since it was impossible to isolate the diazonium salt of N(p-aminobenzoyl)glycine, the derivatization of the GC electrode was carried out while diazotization reaction was occurring in aqueous media according to the method described by Morita et al.[29] The reaction path of the electrochemical derivatization of GC surface with the BG DAS is given in Scheme 1. 161

FIGURE 1 Cyclic voltammetric curves for the derivatization of glassy carbon surface with BG DAS in 0.5-M HCl vs. Ag=AgCl= KCl(sat). Scan rate is 200 mV s1. Spectroscopic and Electrochemical Characterization

that were electrografted to the substrate. For example, one cycle derivatization was found to be enough for biphenyl and nitrobiphenyl[36] and 4-nitrophenyl.[15] Derivatization of GC with BG was almost completed at the end of the first cycle. But to obtain a complete and compact BG film at the GC surface free from pinholes, three cycles CV derivatization were performed. A reversible wave observed at about 0.4 V during the 2nd and 3rd derivatization scans was attributed to the catalytic dioxygen reduction in the de-oxygenated solution, which stands for the oxygen free control experiment. It is clear that dissolved oxygen was not fully removed from the solution during 10-min purging with argon gas.

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Electrochemical Characterization of BG-GC-modified Electrode Dopamine (DA) is a unique molecule used in the modified surface characterization.[36,44,45] It exhibits fast electron transfer kinetics at the bare GC surface, but the electron transfer is completely blocked at the most modified surfaces.[44] It needs to be adsorbed on the surface for oxidation. It has been reported that for rapid oxidation, DA requires an adsorption site and electroinactive at GC surfaces completely covered by nitrophenyl or trifluoromethyl monolayers.[46,47] It is also well known that the electrochemical reactions 3=4 2þ=3þ of FeðCNÞ6 and RuðNH3 Þ6 redox couples were blocked almost completely or suppressed at 4-nitrophenyl-modified GC electrode.[29,48,49] Figure 2 shows the voltammograms of DA in 0.1-M H2SO4 on bare GC and BG-modified GC surface. The reduction

is almost completely dead at the BG-GC, while it shows an almost reversible oxidation wave at bare GC surface. The oxidation and reduction blockage of DA is a very reliable proof of the presence of BG molecules at GC surface. As is seen from Fig. 2, no significantly observable DA response is present at the BG-GC surface.

Characterization of BG-GC-modified Electrode by EIS Electrochemical impedance measurement is another effective electrochemical technique for the characterization of the modified electrodes.[48] The EIS measurements were performed in the range of frequencies from 0.1 Hz to 100 kHz. The response 3=4 of 1-mM FeðCNÞ6 solution as a redox probe was shown in the complex-impedance presentation, and the results were interpreted in terms of an equivalent electrical circuit. Figure 3 shows the 3=4 Nyquist diagram of 1-mM FeðCNÞ6 solution in 0.1-M KCl at the BG-GC and bare GC electrodes. Compared with the bare GC electrode, BG-modified GC electrode showed a very different impedance diagram with a higher charge transfer resistance, indicating the formation of BG-blocking layer at the GC surface. The equivalent circuit compatible with the results is shown in Fig. 3 (inset), comprising the solution resistance (Ru), charge-transfer resistance (Rct), Warburg impedance (Wd), and a constant potential element (CPE) associated with the 3=4 as a redox probe. The equivalent circuit FeðCNÞ6

FIGURE 3 Electrochemical impedance spectra at bare GC and FIGURE 2 Cyclic voltammograms of 1 mM dopamine (DA) vs. Ag=AgCl=KCl(sat) reference electrode at a) bare GC, b) BG-GC electrodes in 0.1-M H2SO4. Scan rate is 200 mV s1.

A. A. I˙sbir-Turan et al.


BG-GC electrodes in 1:0mM FeðCNÞ6 in 0.1-M KCl at 0.100 V vs. Ag=AgCl=KCl(sat). Inset is the equivalent circuit. Solid line is the simulation of the equivalent circuit.


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model is a modified Randles circuit in which CPE is substituted with the ideal capacitor. The presence of the CPE was discerned from the shape of the semicircle with its center below the real axis and from the low-frequency end deviation in the Nyquist plot.[50] Appearance CPE instead of a pure capacitor in the equivalent circuit is probably due to the surface roughness and inhomogeneity of the monolayer.[50,51] The fitted curve in Fig. 3 is shown with the solid line in accordance with the equivalent circuit. The value 3=4 of Rct is 19.94 kohm for FeðCNÞ6 at the BG-GC surface, which is the resistance to the electron transfer through the BG film. The value of Rct is around 20 3=4 at the bare GC, indicating ohms for the FeðCNÞ6 the occurrence of the modification of the surface and blocking the electron transfer through the BG film.

Characterization of BG-GC-modified Electrode by Raman Spectroscopy Raman spectroscopy has the advantage of providing structural information about the film at the GC surface. Therefore, this very powerful technique was used to acquire spectra at the surface of the modified GC electrode. Raman spectra of bare GC, BG-GC, and solid ABG were shown in Fig. 4. First the spectrum of the polished GC was acquired (Fig. 4a), and then BG-modified GC surface spectrum was

obtained (Fig. 4b) under the same conditions. In the range of 600–1800 cm1, bare GC has two characteristic intense Raman bands at 1360 cm1 and 1600 cm1,[41] as shown in the Fig. 4a. The spectrum that belongs only to the surface species was obtained by subtracting the spectrum of bare GC from the spectrum the modified GC. The resultant spectrum is shown in Fig. 4c. A spectrum of the model compound, N(p-aminobenzoyl)glycine (ABG), which is the precursor of the diazonium salt, is also shown in Fig. 4d. Inspection of Fig. 4 reveals the close similarities between the model compound and its bound form on the GC surface. BG-GC surface exhibits most of the characteristic Raman bands observed for ABG, including 1255 cm1 (C-H deformation);[52] 1300 cm1 (C-N stretch);[52] 1397 cm1 (C-C stretch);[53] 1561 cm1 and 1621 cm1 (benzene ring stretch);[52] and 1727 cm1 (C=O stretch).[52] In most cases, the bound BG bands observed on the surface are downshifted by 1–10 cm1 relative to ABG. The relatively large up-shift for the C=O stretch (from 1700 cm1 to 1727 cm1) is the expected result, because it is well known that the C=O stretch depends significantly to the molecular structure.[53–55] For the derivatization of the GC surface, the Raman spectra results in Fig. 4 support the conclusion that the BG molecules form a film at the surface.

Film Thickness Measurements by Ellipsometry The thickness of the BG film was conveniently measured using an ellipsometer ELX-02C=01R type, a laser wavelength of 532 nm, and an incidence angle of 70 degrees. Figure 5 shows the ellipsometric film

FIGURE 4 Raman spectra of a) bare GC, b) BG modified GC, c)

FIGURE 5 Ellipsometric film thickness (using a refractive

subtraction of a from b, d) solid ABG (a precursor for the diazonium salt). A 514.5-nm argon laser was used.

index of n = 1.4600) for the BG films vs. the number of CV cycles. Solid lines represent linear regression of the experimental points.


Spectroscopic and Electrochemical Characterization

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thicknesses as a function of the number of CV cycles employed during modification. An excellent linear correlation was obtained for the film thickness vs. number of cycles. This figure suggests that the same amount of material was deposited on each CV cycle until seven cycles had occurred. Beyond the seventh cycle, irregular thicknesses and significant deviations from linearity were obtained. It stayed generally constant, but desorption of the film was also observed in some occasions. Ellipsometric measurements show that the BG film grafted to the GC surface is obviously a multilayer of 14 nm rather than a monolayer. In the calculation of film thickness, fitting was performed for a four-phase model consisting of graphite substrate=glassy carbon (70 nm)=BG film=air. Refractive indices are 3.0841 for graphite, 1.9000 for GC, 1.4600 for BG organic layer, and 1.0000 for air, assuming thickness and refractive indices are reasonably correlated for all films. Extinction coefficients are 1.7820 for graphite, 0.8100 for GC, 0.0000 for BG organic layer, and 0.0000 for air.

The Electrocatalytic Effect of BG-GC Electrode Towards Dioxygen Reduction Oxygen reduction reaction (ORR) has been extensively studied on various types of modified electrodes for the purpose of energy conversion applications.[56] The reduction of dioxygen plays an important role in electrochemical technology, such as zinc=air batteries and the air cathode.[57] Dioxygen reduction was studied by Salimi et al. in aqueous media, and they reported that when adsorbed on the surface of preanodized GC electrodes, various antraquinone derivatives show enhanced catalytic activity for the electroreduction of dioxygen.[58] Catalytic activity of the methylphenyl-modified GC surfaces for the dioxygen reduction was also studied by Yang and McCreery.[57] Since the attachment of quinone type molecules on carbon electrodes forming a film increases the rate of ORR, the investigation of the kinetics of ORR on GC[33,58] and HOPG[60] has received increasing attention. Covalently bound benzoylglycine film on the GC surface was observed to be active towards ORR. Figure 6 shows the CV waves for the dioxygen reduction on the GC and BG-modified GC electrode A. A. I˙sbir-Turan et al.

FIGURE 6 Cyclic voltammetric waves for the dioxygen reduction on (a) bare GC and (b) BG-GC electrode in air saturated 0.1-M H2SO4 vs. Ag=AgCl=KCl(sat). Scan rate is 200 mV s1.

in acidic media. The CV wave of oxygen reduction at the modified electrode is centered at 0.4 V in airsaturated sulfuric acid solution as is seen in Fig. 6. There are discrepancies in the literature about the presence of dioxygen reduction wave on the native GC electrode in aqueous media. These discrepancies might be the result of air oxidation of GC surface forming quinoid and other carbonyl groups. Therefore, it is a matter of careful polishing and cleaning of the GC surfaces. For example, it has been proposed by Sarapuu et al.[34] that oxygen is reduced at the unmodified GC at 0.5 V (vs. SCE) due to the catalytic effects of the quinone surface groups present on the native GC surface. On the other hand, Dias et al. claimed the absence of oxygen wave at the GC surface in Britton-Robinson buffer in the voltage range of þ0.5 V to 1.2 V vs. SCE.[60] In our experiments, no observable peak was recorded at the aforementioned potential range on carefully polished and cleaned GC surface, as can be seen in Fig. 6. Therefore underivatized GC is not a very good dioxygen reduction electrocatalyst, as was also observed by Tse et al.[61] We studied the electrochemical behavior of dissolved dioxygen reduction in aqueous media to investigate the ability of the BG-GC-modified electrode as a potential electrocatalyst for the reduction of dioxygen. The dioxygen reduction reaction follows a mechanism at the BG-modified-GC surface according to the similar reaction mechanism that was proposed for the 2-benzo[c]cinnoline-modified GC surface that is under research. The ORR was observed to be dependent on the acidity level of the medium, which indicates that the reduction was catalyzed by the protonated surface–confined BG 164

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molecules, BGHþ ðsurfaceÞ , in acidic solution. As the pH was increased to the value of 5, which is greater than pKa of GC, the BG-GC surface becomes almost unprotonated, and ORR is completely blocked (Fig. 7). This observation is in accordance with the pKa value of BG, which is 3.62.[62] In the light of literature and our experimental observations, the mechanism of the ORR at the BG-GC surface can be summarized as follows. A cation radical of the surface bound molecule is formed by accepting an electron from the electrode. This moiety interacts with the dioxygen to form hydrogen superoxide ðHO2 Þ. The mechanism of ORR is depicted in Scheme 2, and the reactions are given as follows. When GC electrode surface is modified with BG film, it acts as a catalyst to reduce dioxygen to H2O2 in acidic media. This intermediate form of BG cation radical is responsible for the electrocatalytic activity observed for dioxygen reduction.  þ þ BGHþ ðsurfaceÞ þ e þ H !BGH2ðsurfaceÞ þ BGHþ 2ðsurfaceÞ þ O2 !BGHðsurfaceÞ þ HO2 HO2 þ e þ Hþ !H2 O2 Net reaction: O2 þ 2e þ 2Hþ !H2 O2


where BGHð surfaceÞþ is the protonated surface benzoylglycine molecule in acidic solution. Since the pKa value of BG is 3.62,[62] in acidic media the surface-bound BG molecules are in their protonated form as shown in reaction (I). As the pH is increased to about 5, the BG-GC surface becomes almost

FIGURE 7 Dependence of dioxygen reduction wave at the BG-GC surface on pH, scan rate is 200 mV s1 vs. Ag=AgCl=KCl(sat). pH values were adjusted in air saturated BR buffer solution. 165

SCHEME 2 Schematic representation of electrocatalytic activity of benzoylglycine modified glassy carbon surface towards dioxygen reduction in acidic media.

unprotonated, and ORR is completely blocked as it shown in Fig. 7. The formation reaction of HO2 involves proton transfer, and the formation is more favorable in acidic medium due to the positively charged surface (II). As shown in Fig. 7, as pH of the solution of BR buffer increases to 7, ORR wave is almost blocked. Hydrogen superoxide is easily reduced to the H2O2 in acidic media according to reaction (III). The net reaction is the surfacecatalyzed reduction of dioxygen to H2O2 (IV). To investigate the ORR catalysis at the derivatized surface in terms of the molecular structure of the film, four different types of films were prepared and compared for the dioxygen reduction in 0.1-M H2SO4. CV voltammograms in air-saturated 0.1-M H2SO4 at a scan rate of 200 mV s1 after the immobilizations of 2-benzo[c]cinnoline (2BCC),[31] 2-benzo[c]cinnoline 6-oxide (2BCCNO), benzoylglycine (BG), and diethylene glycol bis(2-aminophenyl)ether (DGAE) are shown in Fig. 8.[22] DGAE-GC, 2BCCNOGC, and 2BCC-GC surfaces were prepared using the solid diazonium salts of 2BCC-DAS, 2BCCNO-DAS, and DGAE-DAS as described in our previous papers.[22,31] DGAE film, which has no nitrogen atom, does not catalyze the dioxygen reduction.[22] Figure 8 clearly shows that the electron transfer for dioxygen reduction is more rapid for 2BCC-GC surface than for 2BCCNO-GC and BG-GC surfaces. Different electrode surfaces could, of course, influence the rate of electron transfer due to the different electronic properties or due to the different thicknesses of the films. Catalytic effects of the 2BCCNO-GC and 2BCC-GC surfaces for the dioxygen reduction were fully investigated and submitted for publishing. Spectroscopic and Electrochemical Characterization

FIGURE 8 CV voltammograms in air saturated 0.1-M H2SO4 at a scan rate of 200 mV s1 after the immobilization of benzoylglycine (BG), 2-benzo[c]cinnoline (2BCC), 2-benzo[c]cinnoline 6-oxide (2BCCNO) and diethylene glycol bis(2-aminophenyl)ether (DGAE).

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CONCLUSIONS A sensor for the determination of dioxygen was developed based on the BC-GC electrode. BG was immobilized on the surface of GC by electrochemical reduction of its diazonium salt using in situ modification in aqueous media during the synthesis. Raman spectroscopy, ellipsometry, CV, and EIS were utilized for the characterization of the BG-GC electrode surface. The effects of the pH on the dioxygen reduction reaction were discussed. To investigate the ORR catalysis at the BG-GC surface in terms of the molecular structure of the film, four different types of surface films at GC were prepared and compared for the catalytic effect of the dioxygen reduction in acidic media.

ACKNOWLEDGMENTS This work was supported by Ankara University Scientific Research Fund with Project Grant number 2003-07-05-084 and TUBITAK (Scientific and Technological Research Council of Turkey) project with a number of 106T622.

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