Thin Solid Films 519 (2010) 784–789

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Preparation and characterization of poly(indole-3-carboxaldehyde) film at the glassycarbon surface

Didem Deletioğlu a, Erdoğan Hasdemir b,⁎, Ali Osman Solak c, Zafer Üstündağ d, Remziye Güzel e

a Mustafa Kemal University, Faculty of Arts and Science, Department of Chemistry, Antakya, Turkeyb Gazi University, Faculty of Arts and Science, Department of Chemistry, Ankara, Turkeyc Ankara University, Faculty of Science, Department of Chemistry, Ankara, Turkeyd Dumlupınar University, Faculty of Arts and Science, Department of Chemistry, Kütahya, Turkeye Dicle University, Faculty of Arts and Sciences, Department of Chemistry, Diyarbakır, Turkey

⁎ Corresponding author. Tel.: +90 312 2021114; fax:E-mail address: hasdemir@gazi.edu.tr (E. Hasdemir)

0040-6090/$ – see front matter © 2010 Published by Edoi:10.1016/j.tsf.2010.07.125

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 December 2009Received in revised form 30 July 2010Accepted 30 July 2010Available online 27 August 2010

Keywords:Glassy carbonPoly(indole-3-carboxaldehyde)Electrochemical impedance spectroscopyX-ray photoelectron spectroscopyReflection-absorption infrared spectroscopyContact angleEllipsometry

Indole-3-carboxaldehyde (In3C) monomer was oxidized by electrochemical methods at the glassy carbon(GC) electrode in 0.05 M tetrabutylammonium tetrafluoroborate in acetonitrile, with the aim to prepare amodified electrode. Modification was performed using cyclic voltammetry (CV) scanning from 0.0 V to 2.0 Vat a scan rate of 50 mV s−1 for 10 cycles in 1 mM In3C monomer solution. The modified GC surface (In3C-GC)was characterized by CV response of potassium ferricyanide and ferrocene redox probes as well as by theelectrochemical impedance spectroscopy. The modified surface was analyzed by reflection-absorptioninfrared spectroscopy and compared with the spectrum of the monomeric In3C. Elemental composition ofthe surface was determined by X-ray photoelectron spectroscopy. Contact angle measurements was alsoperformed to check the changes in hydrophobic character of the bare GC and compared to that of In3C-GCsurface. Thickness of the oligomeric/polymeric film was investigated by ellipsometric measurements and asurface confined polymerization mechanism was proposed.

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1. Introduction

In recent years, several heteroatom containing organic moleculessuch as pyrrole, carbazole, indole have received increasing attention asmaterial coatings, because they are easily electrografted to the surfacesforming conductive films [1,2]. As a modifier compound, indole familyhas received more interest because of its advantages of having fairlygood thermal stability and high redox activity [3,4].When indole and itsderivatives are exposed to electrochemical oxidation in variouselectrolytes, conductive films are produced on the electrode surfaces[5,6]. Kelaidopoulou et al. investigated anodic polymerization and redoxproperties of N-methyl-N′-(3-indol-1-yl-propyl)-4,4′-bipyridinium [3].Mezlova et al. prepared some conducting materials with electropoly-merization of thieno[3,2-b]indole, 6-methoxythieno[3,2-b]indole andN-methylthieno[3,2-b]indole on Pt disc electrode. Talbi et al. investi-gated redox behavior of poly(indole-5-carboxylic acid) modified ITOandPt electrodes [7,8]. Thepolymerizationmechanismand thedifferentpossibilities of monomer linkages in polymeric indole derivatives werereported in the literature [4,9–11].

Indole coated surfaces have some more properties such asselectivity, sensitivity and stability. So, they have widespreadapplications compared to the bare surfaces and can be used asselective electrodes which are sensitive to the various cationic andanionic inorganic species, as a biosensor for biological molecules or asa protection of metallic surfaces against corrosion [12–14]. Biegunskiet al. immobilized tyrosinase onto poly(indole-5-carboxylic acid)modified Pt disc electrode to catalyse the oxidation of catechol [15]. Liet al. developed an electrochemical method to detect DNA hybridiza-tion on poly(indole-5-carboxylic acid) conducting polymer film [16].Tüken et al. synthesized polyindole on nickel-coated mild steel andinvestigated its corrosion performance [17]. Yabuki et al. preparedamperometric glucose sensor by electrochemical polymerization ofindole derivatives at the glassy carbon (GC) electrode [18]. Poly(2-methylindole) was synthesized by direct anodic oxidation of 2-methylindole in LiClO4/acetonitrile solution and characterized byelectrochemical and various spectroscopic methods [11].

Many controversial results have been reported concerning the siteof linkages of polyindoles with different substituents in C1, C2, C3 andC5 positions of the indole monomer. A survey of the literature revealsthat the polymerization mechanism of indole derivatives is not fullyunderstood. Saraji and A. Bagheri report that the possibilities ofmonomer linkages are involved through the pyrrole ring in C2 and C3positions [9]. According to some authors, C2 and C3 positions are the

Fig. 1. Multi sweep cyclic voltammograms of 1×10−3 M In3C at the GC surface at theGC electrode (vs. Ag/Ag+, sweep rate: 50 mV/s).

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most probable sites of polymerization [4,10,18]. Udum et al.reports that although 2-substituted indole shows polymer coating,3-substituted indole does not give any polymeric deposit at the Ptelectrode [11]. Others propose the C1 and C3 sites as themost probablecoupling sites in the indole polymerization mechanism [19,20].

This paper presents a study of the electro deposition of thin poly(indole-3-carboxaldehyde) films on GC electrode by the oxidativeelectro polymerization of indole-3-carboxaldehyde (In3C) monomer.The modified surface was analyzed by cyclic voltammetry (CV),electrochemical impedance spectroscopy (EIS), reflection-absorptioninfrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy(XPS), contact angle measurements and ellipsometry. An electropolymerization mechanism and a surface layer structure were alsoproposed and compared with the literature.

2. Experimental

2.1. Reagents and chemicals

All chemicals were of the highest purity available from Merck,Fluka or Riedel chemical companies and no further purification wasperformed. All solutions, which were used in electrochemical andmodification experiments, were prepared at the concentration of1 mM. Ultra pure quality of water with a resistance of 18.3 MΩ cm(Human Power I+ purification system) was used in preparations ofaqueous solutions, cleaning of the glassware and polishing theelectrodes. All the test solutions were deaerated by passing highpurity argon (99.999%) before the electrochemical experiment.During the experiments, argon gas was passed over the surface ofthe test solutions in order to avoid the reentrance of oxygen into thesolution. All experiments were carried out at room temperature (25±1 °C).

2.2. Apparatus

A conventional three-electrode cell system was used in electro-chemical experiments. BASModel GC disc electrodes with a geometricarea of 0.071 cm2 were used as working electrodes. Ag/AgCl/KCl(sat)(BAS) and Ag/Ag+ (0.01 M in 0.05 M tetrabutylammonium tetra-fluoroborate (TBATFB) in acetonitrile) (BAS) reference electrodeswere used in aqueous and nonaqueous solutions, respectively.Auxiliary electrode was a Pt wire. Electrochemical experimentswere performed using a PCI4/300 potentiostat/galvanostat (GamryInstruments, PA, USA) equipped with a BAS C3 cell stand.

2.3. Procedure

2.3.1. Cleaning of the working electrode surfacesBefore modification, the GC electrodes were cleaned with fine wet

emery papers (Buehler with grain size of 4000), and then werepolished with alumina paste (particle size 0.1 μm and 0.05 μm inwater) on Buehler polishing microcloth pads. Polished GC electrodeswere first sonicated in ultra pure water and then in a mixture of 1:1(v/v) isopropyl alcohol/acetonitrile (IPA+MeCN) (Riedel) betweeneach polishing step for 5 min.

2.3.2. Preparation of poly(indole-3-carboxaldehyde) modified GCelectrode

In3C monomer (supplied from Aldrich) was dissolved in 0.05 MTBATFB in acetonitrile. GC electrodes were anodically modified by CVin the potential range from 0.0 V to 2.0 V at a scan rate of 50 mV s−1

for 10 scans in 1 mM In3C solution. After the preparation, modifiedelectrode was rinsed with a stream of acetonitrile to remove anyphysisorbed and unreacted materials from the electrode surface.Indole-3-carboxaldehyde modified electrode was stored in acetoni-trile until use.

2.3.3. Characterization of poly(indole-3-carboxaldehyde) modified GCelectrode

RAIRS spectra of indole-3-carboxaldehyde monomer and poly(indole-3-carboxaldehyde) film were acquired by a Bruker Tensor 27spectrometer equipped with a grazing angle and ATR accessories. Thespectra of the solids were obtained in KBr pellets with DTGS detector.RAIRS spectra of poly(indole-3-carboxaldehyde) film obtained using agrazing angle accessory (VeeMAX™ II, Pike Technologies) with anincident angle of 37° and a liquid N2-cooled MCT detector. Bare GCsurface spectra were obtained similarly and subtracted from themodified surface spectra to obtain the film spectra of the grafted poly(indole-3-carboxaldehyde).

A Kratos ES300 electron spectrometer with MgKα X-rays (non-monochromatic) is used for XPS measurements, which are carried outat 90° electron take-off angle.

Electrochemical impedance spectroscopic experiments were car-ried out with a Gamry Reference 600 workstation equipped with aPCI4/300 potentiostat in conjunction with EIS 300 software. GCelectrode before and after modification with In3C were characterizedin 1 mM ferrocyanide/1 mM ferricyanide redox couple via EISmethods. EIS data were measured at 100 kHz to 0.1 Hz at 10 mVwave amplitude and at an electrode potential of 0.215 V, the formalpotential of ferrocyanide/ferricyanide redox couple.

Ellipsometric measurements of the film thickness were performedwith an ELX-02C/01R model high precision discrete wavelengthellipsometer. The wavelength was 532 nm for all experiments. Thethickness values of poly(indole-3-carboxaldehyde) films at the GC-20(Tokai, Japan) surfaces were determined from the average of themeasurements using incidence angle of 70°.

The contact angle of water drops on the poly(indole-3-carbox-aldehyde) films wasmeasuredwith a model G-III contact-angle meter(Kernco Instrument Co., Inc., El Paso, TX). The one-liquidmethod (air–liquid drop–organic layer system) was used in the measurements.

3. Results and discussion

3.1. Electrochemical modification and characterization of In3C-GCsurface

Electrochemical deposition of In3C films on a GC electrode wascarried out in acetonitrile containing 0.05 M TBATFB as a supportingelectrolyte. Fig. 1 shows the multi sweep cyclic voltammograms for1×10−3 M In3C monomer (inset of Fig. 1) solution on GC. In the firstcycle, an irreversible oxidation peak of In3C appears at about+1.4 V vs.Ag/Ag+ reference electrode. Peak current gradually decreases in thesubsequent potential cycles and reaches a steady state value after 10cycles. This gradual decrease is ascribed to progressive accumulation

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which takes place as a consequence of electrode derivatization withIn3C and implies that the deposition of In3C layer at the GC surfaceproceeds as the number of potential cycles increases [21].

The surface CVs of the poly(indole-3-carboxaldehyde) films,carried out in a monomer-free acetonitrile+0.05 M TBATFB solution,demonstrate that the deposited film at the surface showed no peaksfrom 0 to +1.8 V potential range. This implies that the film is electroinactive against positive scans up to +1.8 V. Electro inactivity of thefilm in this range is an advantage for the availability of using thismodified surface as an electrode in the positive potential region foroxidation processes. Therefore, this modified electrode is very usefulfor the electrochemical oxidation of some biologically importantmolecules such as dopamine, uric acid and ascorbic acid. Preliminaryexperiments have shown the usefulness of this surface for analyticalapplication and this work is under investigation.

Electrochemical characterization of In3C-GC surface was carriedout using ferricyanide and ferrocene redox probes. The electrochem-istry of the In3C-GC and bare GC with redox active molecules insolution are shown in Fig. 2. It is well known that the electrochemicalreaction of electroactive probes is efficiently blocked, suppressed orelectron transfer rate is altered at some modified electrodes [21–24].In the voltammograms of indole-3-carboxaldehyde modified elec-trode, the ferrocene and ferricyanide peaks are disappeared, becausethe heterogeneous electron transfer rate of these redox probes iscompletely blocked at the In3C modified surface.

3.2. EIS studies on the In3C-GC surface

It is well known that electrochemical alternating current imped-ance technique is a useful tool for studying the interface properties ofsurface modified electrodes [25–27]. So, EIS was used to characterizethe In3C-GC surface and investigate the pH effects of the terminalpyrrolic groups of In3C film on the redox probe behavior. To ensure

Fig. 2. Cyclic voltammograms of bare and modified electrodes in redox probe solutions.The electrodes are (a) in a solution of 1 mM ferricyanide in 0.1 M KCl, (b) in a solution of1 mM ferrocene in acetonitrile with a background of 0.05 M TBATFB.

equal concentrations of both redox partners on the surface of theelectrodes, these measurements were carried out at their formalredox potentials of 0.215 V for Fe(CN)63−/Fe(CN)64− solution (1 mMeach) as redox probe in Britton–Robinson (BR) buffer containing0.1 M KCl as supporting electrolyte and buffering the ionic strength[28]. Fig. 3 shows the Nyquist and complex capacitance plots of theredox couple on In3C modified GC surface and the inset shows theequivalent electrical circuit model of the electrochemical process atthe In3C modified surface, obtained using Gamry EIS300™ software.Nyquist plot for the probe on bare GC surface is a straight line at theentire range of frequency region and at all pHs (not shown in thefigure) with a very small semicircle at the high-frequency regionindicating a small charge transfer resistance (23 Ω) and almost fullydiffusion-controlled process. The impedance plot for the probe systemof Fe(CN)63−/Fe(CN)64− on In3C-GC surface shows a larger semicircle atthe high-frequency region with a straight line at the low frequencyregion and at all lower pHs.

The experimental data of the electrochemical impedance plot wereanalyzed by applying the nonlinear least squares fitting to theequivalent electrical circuit including a solution resistance (Rs), afilm capacitance (Cf), a coating resistance (Rf), a charge transferresistance (Rct), a double layer capacitance (Cdl) and a Warburgresistance (W), as shown in the inset of Fig. 3 [25].

The values of Rct and Cf are the best for the measurement of thechanges in the surface structure with the effect of pH. These valueshave been found from the fitting process of the experimental datapoints with the theoretical electrical circuit. It is evident that pH has asignificant influence on the properties of the modified surface,appearing especially on the charge transfer resistance and filmcapacitance values. The film capacitance value of the surface is almost

Fig. 3. Nyquist (a) and complex capacitance (b) plots on In3C-GC surface in 1 mM Fe(CN)63−/4−of BR buffer, a) pH: 2, b) pH: 4, c) pH: 6, d) pH: 8, e) pH: 10. Inset is theequivalent electrical circuit for the electrochemical system.

Table 1Some characteristic IR bands and their assignments for solid In3C in KBr pellets andIn3C film deposited at the GC surface.

Solid In3C bands In3C-GC surface bands Assignments

3172 cm−1 3220 cm−1 N–H stretch3049 cm−1 3094 cm−1 Aromatic C–H stretch2824 cm−1, 2934 cm−1 2865 cm−1, 2938 cm−1 Aldehydic C–H stretch1579 cm−1 1590 cm−1 C N stretch1446 cm−1 Vanished C C aromatic stretch1130 cm−1 1122 cm−1 C–H bending1230 cm−1 1260 cm−1 N–H bending

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constant between pH 2–6 (approximately 1.60×10−5 μF/cm2) andincreases gradually with the increase in pH of the solution, from1.60×10−5 μF/cm2 at pH 6 to 3.77×10−5 μF/cm2 at pH 10. On theother hand, Rct value decreases with the increase in pH, from pH 2 topH 6, and then stays almost constant. The charge transfer resistancefor 1 mM Fe(CN)63−/4− solution on In3C-GC electrode was found to be782 Ω and 3609 Ω at pH 10 and pH 2, respectively. We interpret thisincrease in Rct upon addition of H+ moieties, as being due to theinteraction of H+ with the aldehydic carbonyl oxygen atoms of thepolymeric In3C film (Fig. 7), disrupting the N⋯H⋯O bond and puttingthe surface film into a protonated form through O atoms in lower pHvalues. The higher values of Rct at lower pHs are probably due to theelectrostatic repulsions between the negatively charged redox probesand nonbonding electrons of oxygen atoms, in that state they aremore effective compared to the H-bonded state. It is important toemphasize that the dramatic changes in Rct, Cf and contact anglesoccur around at pH 6, which is the pKa value of the indole-3-carboxaldehyde in solution. When In3C layer is in the neutral state (atpH 10), the doping Fe(CN)63−/4− ion transfer is negligible and polymerchain has no charges. The EIS results are related to the regulatedsurfaces of In3C films produced by the proton concentration ofsolution, which is conformity with our contact angle studies.

The electrode coverage (θ) is a key factor that can be used toestimate the surface state of the electrode, and the charge transferresistance is also related to it [27]. The electrode coverage can becalculated by (1−θ)=Rct

0 /Rct, where Rct0 is the charge transfer

resistance at the bare GC electrode and Rct is the charge transferresistance at the In3C modified electrode under the same conditions.We have calculated a higher charge transfer resistance (Rct=3609 Ω)value for the In3C-GC surface than that for the bare GC surface(Rct=20Ω) in Fe(CN)63−/Fe(CN)64− solution at pH of 2, from fittingthe spectra to the equivalent circuit. The corresponding electrodecoverage value was estimated as 99.5% using the above equation. Thissuggests that the permeability of the modified surface is low due tothe compact and multilayer nature of the In3C nanofilm at pH 2.

3.3. RAIRS spectra of the In3C-GC surface

To characterize the In3C-GC surface and to investigate thepolymerization coupling sites, IR spectra of monomers in KBr pelletsand In3C films on the GC surface were recorded and compared to eachother. The resulting RAIRS spectra of poly(indole-3-carboxaldehyde)film and IR of solid indole-3-carboxaldehyde as KBr pellets are partlyshown in Fig. 4 and the assignments of the main IR bands are given inTable 1. The assignments of the IR bands were performed using BrukerTensor 27 spectrometer library and various IR tables. For RAIRSmeasurements the In3C modified surface was prepared on a Tokai GC20 plates.

Fig. 4. High-frequency region IR spectra of (A) solid In3C in KBr pellets, (B) In3C filmdeposited at the GC surface.

Comparison of both spectra given in Fig. 4 shows that the bandscorresponding to pyrrolic N–H stretching exist in both spectra of solidmonomer (3172 cm−1) and film (3220 cm−1) at the GC surface with ahigher frequency shift at the surface. This is an important IR evidence ofthepolymerization coupling sites, suggesting that pyrrolic Ndoes not playa role neither in the attachment to the GC surface nor between thepolymerized monomers [7]. Aromatic C–H stretching bands wereexhibited in monomer (3049 cm−1) and film (3094 cm−1) spectra withabout 50 cm−1 higher frequency shift for the later, indicating that theattachment to theGC surfacemay be through the phenyl ring [29]. IR datademonstrate that In3C is deposited at theGCsurface as amultilayerfilm inwhich the pyrrolic N–H functionality persists. Moreover, the band at1579 cm−1 for the monomer which is assigned to the C N stretch ispersisted in the surface spectra at 1590 cm−1(not shown in Fig. 4).Comparison of both spectra also shows that the bands at 1446 cm−1

corresponding to aromatic C C stretch in the monomer spectrum isvanished in the surface spectrum suggesting that the aromatic ring isinvolved in bonding to the surface, presumably as a result of π–πinteractions between the graphitic rings systems of the GC [30]. Theseobservations suggest that the pyrrolic ring in indole-3-carboxaldehydemonomer was not involved either in the electropolymerization and thebonding to the GC surface.

3.4. X-ray photoelectron spectroscopic data for the indole-3-carboxaldehydefilm at the GC surface

To characterize the poly(indole-3-carboxaldehyde) modified GCsurface, XPS experiments were performed for the elemental distribu-tion, and the results were shown in Fig. 5. As can be seen from Fig. 5,three main XPS bands are appeared at 285 eV (C1s), 400.2 eV (N1s),and 530 eV (O1s) in the survey spectrum. The presence of C1s, N1s andO1s peaks in the survey spectrum of the modified surface confirmedthat the indole-3-carboxaldehyde moieties were attached on the GCsurface. N1s peak in the core level spectrum for In3C-GC surface wasobserved at 400.2 eV and attributed to the pyrrole nitrogen binding

Fig. 5. XPS spectra of a) bare GC, b) the indole-3-carboxaldehyde modified GC and thepercentages of C, N, O elements at the both surfaces.

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energy [31,32]. As the inset table in Fig. 5 shows, the percentages of Nand O elements arising from the pyrrolic and aldehydic groups aresignificantly higher on the modified surface than that of bare GCsurface. Comparison of the percentages of C on the modified and baresurfaces also follows the same trend and drops significantly as theIn3C film covers the bare GC surface. Increasing of the percentage ofN1s peak and decreasing of that of C1s peak is expected phenomenadue to the accumulation of In3C molecules at the GC surface.Appearance of the O1s and N1s peaks at the unmodified GC surfaceare likely due to surface oxidation during modification process [33]and structural contamination during industrial fabrication of GC [34],respectively.

Fig. 6. Change in the contact angle of water on GC and In3C-GC as a function of the pH.

3.5. Contact angle measurements at In3C-GC surface

The contact angles of the filmwere affected by the hydrophobic andhydrophilic properties, heterogeneity and the roughness of the surface.Hence contact angles are important constants of liquid–solid systemsproviding valuable information on the change of the properties ofsurfaces [35]. Therefore, the surface characterization of the In3C-GCelectrode was investigated by contact angle measurements. Table 2summarizes data frommeasurements of contact angles onboth surfacesin buffered solutions with different pHs. The water-drop contact anglesfor the bare GC surface was measured between 76° and 83° in differentpHs ranging from 2 to 10, indicating the hydrophobic character of thebare GC surface (Table 2). Indole-3-carboxaldehyde films are morehydrophilic with contact angles between 58° and 61° than bare GCsurface, as expected due to the pyrrole and aldehyde functionalities.

For In3C-GC contact angle values are lower than that for bare GC,thus contact angle measurements showed that In3C-GC was morehydrophilic than bare GC surface at every pH values as seen in Table 2.The most interesting feature of contact angle measurements is its pHdependency, as shown in Fig. 6. This behavior suggests the presence ofthe surface confined functional groups which are acidic or basiccharacter. Pyrrolic groups are probably responsible for this behavior.As Fig. 6 shows, water contact angle pass through a maximum againstpHwhich is probably the pKa value of the surface. The pKa value of theindole-3-carboxaldehyde has been reported as 6.04 [36].

3.6. Ellipsometric thickness measurements of In3C films

Ellipsometry is a valuable tool in the characterization of organic andinorganic films and in the determination of polymeric ormolecular filmthicknesses at the nanoscales [37]. Ellipsometric measurements of In3Cfilm thickness on GC was performed by fitting for a four-phase modelconsisting of graphite/GC substrate/In3C film/air. Refractive indiceswere taken as 3.0841 for graphite, 1.9000 for GC substrate, 1.4600 forIn3Cfilmand1.0000 for air. In the calculation of thefilm thickness basedon the four-phase model, the main assumption is the presence ofreasonable correlation between the thickness and its refractive index[38]. Extinction coefficients are −1.7820 for graphite, −0.8100 for GCsubstrate, 0.0000 for organic layer and 0.0000 for air. Ellipsometricthickness of In3C filmwasmeasured as 79.14±0.40 nm, indicating thata multilayer was formed at the GC electrode surface. To check thehomogeneity of themodified surface,measurementswere performed atfour different spots. Standard deviation of the thickness shows that a

Table 2The results of contact angle measurements for bare GC and In3C-GC surfaces in differentpHs.

pH 2 4 6 8 10

GC 83.30±0.17 75.63±0.39 83.37±0.48 82.33±1.22 80.30±0.50In3C-GC 61.99±0.54 57.77±2.25 69.18±0.65 67.11±0.51 58.91±0.85

reasonably well homogeneity is obtained for the In3C films at the GCsurface.

3.7. Grafting mechanism and the structure of the modified surface

From electrochemical data, it is clear that a film is formed at the GCelectrode which passivated the surface against monomer oxidation(Fig. 1). This surface film did not show electroactivity when theelectrode was taken out of the monomer solution after modificationand placed into a solution of background electrolyte withoutmonomer.

The presence of N–H stretching band in the spectra of In3C and itsdeposited polymer at the GC surface definitely excludes any possiblerole of the N–H group as polymerization sites at the GC surface.Dependence of the contact angles on pH suggest the presence of thesurface confined acidic or basic groups which can only be the N–Hfunctionality in pyrrole ring.

With regard to electrochemical, IR and contact angle measurementdata, the most probable coupling sites were proposed to be thepyrrolic C2 and phenyl C5 or C6 positions. Involvement of the C2 carbonin the polymerization of indole derivatives has been reported in theliterature [4,9,10,18]. Fig. 7 shows the structure of the In3C film on GCsurface proposed in the light of our experimental data and the reportsof the literature. The proposed structure is onewhich has been graftedas oligomeric/polymeric In3C of various monomer repeat units withH-bond interactions of CHO and NH functionalities of pyrrole rings.

4. Conclusions

The data acquired from RAIRS, XPS, ellipsometry, electrochemicalexperiments and contact angle measurements support the view thatthe indole-3-carboxaldehyde monomer was grafted to the GC surfaceduring electro-oxidation of monomer solution as a short chainoligomers rather than long chain polymers. The most probablecoupling sites were proposed to be the pyrrolic C2 and phenyl C5 orC6 positions. In the first anodic scan of the monomer, In3C is oxidizedto give radicals which attack the GC surface from the C5 or C6 carbon tobe grafted as a monolayer at the surface. The radicals present in theacetonitrile solution attacks the In3C monolayer through the C2

position forming an oligomeric multilayer with an ellipsometricthickness of about 80 nm. The In3C film is electro-inactive againstpositive scans up to+1.8 V and useful as an electrode in the oxidationregion.

Acknowledgments

This work was supported by the TUBITAK (Scientific andTechnological Research Council of Turkey) project number 106T622.The authors thank Dr. Selda Keskin, from Middle East Technical

Fig. 7. The proposed structure of the In3C oligomer/polymer film on GC surface.

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University (Ankara), for acquiring the XPS spectra and Serkan Demircifrom Gazi University, for contact angle measurements.

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