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2-BENZO[C]CINNOLINE AND 2-BENZO[C]CINNOLINE 6-OXIDE MODIFIEDGLASSY CARBON ELECTRODES:ELECTROCATALYTIC REDUCTION OFDIOXYGEN IN AQUEOUS MEDIAAybüke A. İsbir-Turan a , Zafer Üstündağ b , Emine Kılıç a , Remziye

Güzel c , Öznur Uçkan a & Ali Osman Solak a da Faculty of Science, Department of Chemistry , Ankara University ,Ankara, Turkeyb Faculty of Art and Sciences, Department of Chemistry ,Dumlupınar University , Kütahya, Turkeyc Faculty of Arts and Sciences, Department of Chemistry , DicleUniversity , Diyarbakır, Turkeyd Faculty of Engineering, Department of Chemical Engineering ,Kyrgyz-Turk Manas University , Bishkek, KyrgyzstanPublished online: 16 Mar 2011.

To cite this article: Aybüke A. İsbir-Turan , Zafer Üstündağ , Emine Kılıç , Remziye Güzel , ÖznurUçkan & Ali Osman Solak (2011) 2-BENZO[C]CINNOLINE AND 2-BENZO[C]CINNOLINE 6-OXIDE MODIFIEDGLASSY CARBON ELECTRODES: ELECTROCATALYTIC REDUCTION OF DIOXYGEN IN AQUEOUS MEDIA,Instrumentation Science & Technology, 39:2, 149-160, DOI: 10.1080/10739149.2010.545854

To link to this article: http://dx.doi.org/10.1080/10739149.2010.545854

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2-BENZO[C]CINNOLINE AND 2-BENZO[C]CINNOLINE 6-OXIDEMODIFIED GLASSY CARBON ELECTRODES: ELECTROCATALYTICREDUCTION OF DIOXYGEN IN AQUEOUS MEDIA

Aybuke A. Isbir-Turan,1 Zafer Ustundag,2 Emine Kılıc,1 Remziye Guzel,3

Oznur Uckan,1 and Ali Osman Solak1,4

1Faculty of Science, Department of Chemistry, Ankara University, Ankara, Turkey2Faculty of Art and Sciences, Department of Chemistry, Dumlupınar University,Kutahya, Turkey3Faculty of Arts and Sciences, Department of Chemistry, Dicle University,Diyarbakır, Turkey4Faculty of Engineering, Department of Chemical Engineering, Kyrgyz-Turk ManasUniversity, Bishkek, Kyrgyzstan

& Benzo[c]cinnoline (BCC) molecules were electrochemically grafted onto a glassy carbon (GC)surface in nonaqueous media, and the modified surface was characterized using cyclic voltammetry(CV) with redox probes. Blockage of the electron transfer on the modified surface was observed usingredox probes. Electrocatalytic effect of 2BCC modified GC (2BCC-GC) electrode surface towards tothe electrochemical reduction of dioxygen was also investigated. A mechanistic scheme for the elec-trochemical catalysis was proposed. To clarify the mechanism of the dioxygen reduction, a less basicfilm of 2-benzo[c]cinnoline 6-oxide (2BCCNO) molecules was also prepared at the glassy carbonsurface. The effect of electrochemical catalysis of dioxygen reduction at the 2BCC-GC surfacewas compared to that at the 2BCCNO-GC surface.

Keywords 2-benzo[c]cinnoline modified electrode, benzo[c]cinnoline, cyclic voltam-metry electrocatalytic dioxygen reduction, electrochemical modification

INTRODUCTION

Electrochemical surface modification of conductive materials such ascarbon and metal surface with organic and inorganic molecules has receivedextensive interest because of the multitude of applications they can beapplied to, including as model systems for understanding the electron trans-fer mechanism, nanobiosensor, molecular electronics, bioelectronics, and

Address correspondence to Ali Osman Solak, Faculty of Science, Department of Chemistry, AnkaraUniversity, 06100, Tandogan, Ankara, Turkey. E-mail: aliosman.solak@gmail.com

Instrumentation Science and Technology, 39:149–160, 2011Copyright # Taylor & Francis Group, LLCISSN: 1073-9149 print/1525-6030 onlineDOI: 10.1080/10739149.2010.545854

Instrumentation Science and Technology, 39:149–160, 2011Copyright # Taylor & Francis Group, LLCISSN: 1073-9149 print/1525-6030 onlineDOI: 10.1080/10739149.2010.545854

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chemical sensors.[1–6] Delamar et al., in their pioneering paper, showed thataryl diazonium salts could be reductively grafted onto glassy carbon (GC)surfaces to yield covalently attached layers.[7] Following the appearance ofthis article, modification with diazonium chemistry has progressively beenattracting researchers to construct different kinds of organic films on thecarbonaceous and metal substrates. The application of the method is prim-arily due to the ease with which diazonium salts bearing a wide range of func-tional groups can be synthesized, combined with the high stability of theresulting layers with regard to both long terms storage and a broad potentialwindow. As a result, the electrografting with aryl diazonium salt reductionmethod is unique due to the characteristic electrochemical behavior ofthe molecules used as modifiers that generate the films on the sub-strate.[7–11] This technique is based on the reduction of the aryldiazonium salts (Ar --N þ

2 ) to produce free radicals, which react immediatelywith the carbon or metal surfaces.[3] Thus, C–C or C–metal bonds on thecarbon or metal surfaces are formed between modifiers and the substrate,respectively.

Modified electrodes can be characterized by a variety of techniquessuch as X-ray photoelectron spectroscopy (XPS),[12] electron spin reson-ance (ESR),[13] and Raman spectroscopy,[12–15] in addition to the electro-chemical techniques[2,3] used to elucidate the chemical composition ofthe resulting surface films and the dependence of film properties on theelectrochemical grafting conditions. In addition to these techniques,atomic force microscopy (AFM),[16] electrochemical impedance spec-troscopy (EIS),[17,18] and scanning electrochemical microscopy (SECM)[19]

are other useful techniques used in characterization of surface films.Dioxygen reduction finds wide applications in electrochemical technol-

ogies such as fuel cell and biosensors.[20] Therefore, dioxygen reductionreaction (ORR) has been studied extensively on various types of electrodesdue to the importance of this reaction in many industrial and energy conver-sion applications.[21,22] Investigation of the kinetics of dioxygen reductionon carbon electrodes containing surface-bound molecules, such as phenan-threnequinone on GC,[23] 1,4-naphthoquinone on highly ordered pyroliticgraphite (HOPG),[24] and 1,4-dihydroxy-9,10-anthraquinone derivatives onGC,[25] has received increasing attention, since the attachment of thesemolecules greatly increases the rate of ORR. Most of the articles in the litera-ture dealing with the dioxygen reduction are about the surface-confinedanthraquinone (AQ) derivatives.

The investigation of dioxygen reduction at carbon electrodes, whichcontains surface-bound quinones, has gained attention after the observationof the catalytic reduction at the quinone modified surface. Vaik et al.reported that a larger value of the rate constant of O2 reduction resultedat the AQ modified electrode, producing H2O2 with a 100% yield.[23]

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Sarapuu et al. studied O2 reduction at the AQ-modified GC electrodesurface in aqueous media and reported that the two-electron reduction ofoxygen to hydrogen peroxide was observed, and the catalytic activity ofthe electrodes for O2 reduction was dependent on the AQ surface concen-tration.[26] Yang and McCreery reported that when the surface is blocked byan organic film, the electrode serves only to reduce dioxygen to the super-oxide remaining in aqueous solution.[21] A survey of the literature revealsthat the electroreduction of O2 at 2-benzo[c]cinnoline-modified GC surfacehas not yet been investigated.[27]

In our earlier studies, we showed that benzo[c]cinnoline derivativesshowed reversible electrochemical behavior and could act as redox media-tors.[28–31] Considering the high kinetics of the electron transfer of benzo[c]-cinnoline derivatives, we figured out that the benzo[c]cinnoline-modifiedsurfaces would catalyze the dioxygen reduction in solution. Based on thisobservation, in the present work, we studied the electrocatalytic reductionof dioxygen by cyclic voltammetry (CV) at glassy carbon electrodes modi-fied with 2-benzo[c]cinnoline using the reduction of its diazonium salt.The preparation and characterization of this surface have been describedin our previous articles.[2,32] We describe in this article the electrocatalyticeffects of 2BCC-GC surface toward dioxygen reduction and propose amechanism of catalysis of ORR at this surface.

EXPERIMENTAL

Reagents and Chemicals

Diazonium salts of 2-benzo[c]cinnoline and 2-benzo[c]cinnoline 6-oxidetetrafluoroborate were synthesized from the precursors 2-aminobenzo[c]cinnoline and 2-aminobenzo[c]cinnoline 6-oxide, respectively, in fluorobo-ric acid by the reaction with NaNO2.[2,32] 2-Aminobenzo[c]cinnoline 6-oxidewas synthesized with the procedure that was reported by Kılıc and Aktan.[33]

All solutions of the diazonium salts were prepared as 1 mM in aqueous andnonaqueous media. Britton–Robinson (BR) buffer solution was used inexperiments carried out in aqueous media. Purified argon gas (99.999%)was thoroughly purged into the solutions to deoxygenate the media for10 min just before the electrochemical measurements. During the electro-chemical experiments, an argon blanket was maintained over the solutionto supply an inert atmosphere. Room temperature of 25�C� 1�C was main-tained in recording all voltammograms. Ultrapure water with a resistance of18.3 MX � cm (Human Power 1þ purification system) was used for the prep-aration of aqueous solutions. The highest purity chemicals were suppliedfrom Merck, Fluka, or Riedel chemical companies and were used withoutany purification.

Electrocatalytic Reduction of Dioxygen 151

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Instrumentation

GC electrodes of 3-mm-diameter disks (BAS Model MF-2012) with ageometric area of 0.071 cm2 were purchased from Bioanalytical Systems,West Lafayette, IN, USA, and used as working electrodes in modificationand electrochemical characterization. In spectroscopic characterization,Tokai GC-20 glassy carbon electrodes with an O-ring delimited area of0.30 cm2 were used. Either an Ag=Agþ (0.01 M in 0.1 M TBATFB in acetoni-trile) or an Ag=AgCl=KCl(sat) reference electrode (Bioanalytical Systems,West Lafayette, IN, USA) was used in aqueous and nonaqueous media,respectively. Platinum auxiliary electrode was used in all electrochemicalmeasurements. Calibration of Ag=Agþ nonaqueous reference electrodewas checked with ferrocene standard solution once a month, regularly.All electrochemical measurements were carried out with a BAS CV-50 Welectrochemical analyzer (Bioanalytical System Inc., Lafayette, IL, USA).Spectra-Physics’ Raman spectrometer was used to record Raman data.

Preparation and Modification of GC Electrode Surfaces

GC electrodes were polished successively with 0.3 and 0.05 mm aluminaslurries prepared from dry alumina powder (Baikowski Int. Corp., Char-lotte, NC, USA) and Human Power 1þ ultrapure water (18.3 MX � cm) onmicrocloth pads. The electrodes were thoroughly rinsed and sonicated inHuman Power 1þ ultrapure water for 5 min at least twice and then oncewith a mixture of 1:1 (v=v) isopropyl alcohol:acetonitrile (IPAþMeCN)(Riedel). The IPAþMeCN mixture was purified in an equal volume ofNorit A activated carbon. Before the modification step, cleaned andpolished electrodes were dried under an argon gas stream.

Electrochemical modification of the GC electrodes was conducted indeaerated acetonitrile containing 1 mM of the aryl diazonium salts in0.1 M TBATFB by cycling between þ0.4 V and �0.8 V four times at a scanrate of 200 mV s�1. To remove any physisorbed and unreacted materialsfrom the electrode surface, modified electrodes were first rinsed and thensonicated with acetonitrile following the preparation. Modified electrodeswere stored in acetonitrile until use.

RESULTS AND DISCUSSION

2-Benzo[c]cinnoline-Modified GC Electrodes

GC electrode surfaces were modified with 1 mM 2BCC DAS solution byCV in acetonitrile containing 0.1 M TBATFB as supporting electrolyte. BCCmolecules were attached to the carbon surface by covalent bonds through

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the 2-position of the ring.[2] Scheme 1 shows the reaction path of themodification of the GC surface with 2BCC DAS.

For modification with the diazonium salts of 2-benzo[c]cinnoline and2-benzo[c]cinnoline 6-oxide tetrafluoroborate, 200 mV s�1 of a scan rateand potential range from þ0.4 V to �0.8 V were chosen. We show that peakcurrent of the irreversible wave decreases as the number of potential cyclesincreases, just as in our previous work in our articles. In the beginning ofthe reduction step, the peak current decreases in each potential cycle andthen almost reaches a steady state value after four cycles.[2,32] It was knownthat these kind of current-potential curves belong to the electrochemicalreduction of diazonium salts onto carbonaceous and metal electrodes.[3,10,32]

The modified surface was characterized electrochemically usingdopamine (DA), ascorbic acid (AA), RuðNH3Þ2þ=3þ

6 , and FeðCNÞ3�=4�6

redox probes.[3,4,16,17,34–36] Further evidence of the successful derivatiza-tion of BCC is the catalytic reduction of dioxygen observed at �0.5 V onthe modified surface while it is absent on the bare GC surface. Detaileddiscussion of this catalytic surface reaction will be given below.

To confirm the attachment of BCC molecules to the GC surface, Ramanspectroscopy was used, and assignment of the peaks and evaluation ofthe Raman spectra was given in our previous articles and literature citedtherein.[2,32]

Electrocatalytic Reduction of Dioxygen at a 2BCC-GCElectrode Surface

Covalently bound benzo[c]cinnoline on the GC surface was observed tobe very active for ORR. Figure 1 shows CV waves for 0.1 M H2SO4 solution atthe bare GC and BCC-modified GC electrode, and also emphasizes that thebare GC electrode does not give a distinct peak as compared to the modi-fied surface. CV of dioxygen reduction at the modified electrode shows theORR wave is centered at �0.4 V in sulfuric acid solution. It has been pro-posed that oxygen is reduced at the unmodified GC at �0.5 V (vs. SCE)

SCHEME 1 The reaction path of derivatization between glassy carbon electrode surface and 2-benzo[c]-cinnoline diazonium salt in acetonitrile.

Electrocatalytic Reduction of Dioxygen 153

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due to the catalytic effects of the quinone surface groups present on thenative GC surface due to atmospheric oxidation.[26,37] In our experiments,no remarkable peak was observed at the above-mentioned potential rangeon carefully polished and cleaned GC surface, as can be seen in Figure 1.Therefore, underivatized glassy carbon is not a very effective dioxygenreduction electrocatalyst, as was observed by Tse et al.[38]

CV waves depend on the acidity level of the medium and become lessreversible as the pH of the solution of BR buffer is increased (Figure 2a).Cyclic voltammograms of a saturated O2 in 0.1 M H2SO4 solution at a 2-benzo[c]cinnoline modified glassy carbon electrode at different scan rateswere recorded between 0.01 V s�1 and 50 V s�1. The linearity of the peakcurrent vs. the square root of the scan rate indicates that the reductionof dioxygen is diffusion controlled (Figure 2b).

A very interesting observation of ORR at the modified electrode is thedependence of peak current to the surface coverage of BCC at GC. In orderto achieve smaller surface coverage of BCC at the GC electrode, the switch-ing potentials were made less negative, beginning from �0.065 V, duringthe surface derivatization scans. The switching potentials were �0.065 V,�0.250 V, �0.350 V, and �0.500 V, which were selected as more positiveand more negative potentials relative to the peak potential value of deriva-tization wave. Figure 3 shows the cyclic voltammetric responses of ORR atdifferent BCC surface coverages. It is clearly seen in Figure 3 that thereduction rate of oxygen is directly related to the surface coverage ofBCC molecules.

When the GC electrode surface is modified with BCC film, it acts as acatalyst to reduce dioxygen to H2O2 in acidic media. As Scheme 2 shows,

FIGURE 1 CV voltammogram of dioxygen reduction at the (a) 2-benzo[c]cinnoline modified glassycarbon electrode surface and (b) bare glassy carbon electrode surface in 0.1 M H2SO4 (air saturated).Scan rate is 200 mV s�1, reference electrode is Ag=AgCl=KCl(sat).

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as a result of a one-electron reduction of protonated BCC molecule on thesurface, a cation radical is formed. This intermediate form of BCC cationradical is responsible for the electrocatalytic activity observed for dioxygenreduction.

FIGURE 2 (a) Dependence of dioxygen reduction wave on pH at a scan rate of 200 mV s�1. (b) Changeof dioxygen peak current with the square root of scan rate (v1=2) in 0.1 M H2SO4 (air saturated) at the2BCC modified GC electrode.

Electrocatalytic Reduction of Dioxygen 155

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The dioxygen reduction reaction follows the reaction pathways given here:

HBCCþðsurfaceÞ þ e� þ Hþ ! ðH2BCCÞþ�ðsurfaceÞ ðIÞ

ðH2BCCÞþ�ðsurfaceÞ þ O2 ! HO�

2ðsolnÞ þ HBCCþðsurfaceÞ ðIIÞ

SCHEME 2 Schematic representation of electrocatalytic activity of 2-benzo[c]cinnoline modified glassycarbon surface towards dioxygen reduction in acidic media.

FIGURE 3 Cyclic voltammograms of dioxygen reduction reaction in 0.1 M H2SO4 (air saturated) at dif-ferent surface coverages of BCC obtained in different switching potentials during derivatization scans.The numbers indicate switching potentials. Scan rate is 200 mV s�1 vs. Ag=AgCl=KCl(sat).

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HO�2ðsolnÞ þ Hþ þ e� ! H2O2 ðIIIÞ

Net reaction: O2 þ 2Hþ þ 2e� ! H2O2; ðIVÞ

where BCCHþðsurfaceÞ is the protonated surface benzo[c]cinnoline molecule in

acidic solution. Since the pKa value of BCC is 2.20[39] in acidic media, thesurface-bound BCC molecules are in their protonated form as shown in reac-tion (I). As the pH is increased to about 5, the 2BCC-GC surface becomesalmost unprotonated and ORR is almost blocked (Figure 2a). The formationreaction of HO2 (II) involves proton transfer, and the formation is more favor-able in acidic medium due to the positively charged surface. As shown inFigure 2a, as the pH of the solution of BR buffer increases to 10, the ORR waveis completely blocked. In acidic solution, thereversible character of the ORRreaction indicates that HO2 � formed in reaction (II). This intermediate pro-duct is easily reduced to the H2O2 in acidic media according to reaction(III). As mentioned above the, net reaction is the catalytic reduction of dioxy-gen to H2O2 as shown in reaction (IV).

To be certain about the effect of the charged surface on the reactionpath, a less basic moiety, namely 2-benzo[c]cinnoline 6-oxide, was electro-grafted onto the GC surface just to clarify the mechanism proposed above.Figure 4 demonstrates a profound change in the electrocatalysis of dioxygenreduction on 2-benzo[c]cinnoline– and 2-benzo[c]cinnoline 6-oxide–modified GC (2BCCNO-GC) electrodes compared to the bare GC electrode.Since the basicity of BCCNO is less than that of BCC, the 2BCCNO-GCelectrode surface is less protonated compared to the 2BCC-GC surface.

FIGURE 4 Voltammograms of dioxygen reduction reaction at 2-benzo[c]cinnoline (2BCC-GC) and2-benzo[c]cinnoline 6-oxide (2BCCNO-GC) modified glassy carbon electrodes in O2 saturated 0.1 MH2SO4 solution. Scan rate is 200 mV s�1, vs. Ag=AgCl=KCl(sat).

Electrocatalytic Reduction of Dioxygen 157

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Therefore, dioxygen reduction reaction at the 2BCC-GC surface is catalyzedmore than that at the 2BCCNO-GC surface. The catalytic activities of the2BCC-GC and 2BCCNO-GC surfaces toward the dioxygen reduction reac-tion are not expected to be identical when the stabilities of the surfacecation radicals of these two films are taken into consideration.

CONCLUSION

A GC electrode was modified with a 2-benzo[c]cinnoline molecule andcharacterized by CV using redox probes. Various redox probes were used tosupport the presence of the films on the surface. The reduction of O2 toH2O2 in aqueous media was investigated with 2-benzo[c]cinnoline-modified GC electrode using CV technique. It was investigated that2BCC-GC electrode electrocatalyzes the dissolved dioxygen reduction,and it behaves as a good voltammetric sensor in dioxygen reduction inacidic media. The electrochemical reaction took place by a two-electronprocess, forming H2O2. It was also observed that the kinetics of ORR weredepended on pH. The linearity of the peak potential vs. scan rate indicatesthat reduction of dioxygen is diffusion controlled. To clarify the mech-anism, the dioxygen reduction peak on the 2BCC-GC surface was com-pared with one on the 2BCCNO-GC surface.

ACKNOWLEDGMENTS

This work was supported by the Ankara University Scientific ResearchFund with Project Grant numbers of 2000-07-05-019 and 2003-07-05-084;DPT Project Grant number DPT 2003K12019011-5; and TUBITAK(Scientific and Technological Research Council of Turkey) project number106T622.

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