Thin Solid Films 519 (2010) 784–789
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Preparation and characterization of poly(indole-3-carboxaldehyde) ﬁlm at the glassy carbon 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, Turkey Gazi University, Faculty of Arts and Science, Department of Chemistry, Ankara, Turkey Ankara University, Faculty of Science, Department of Chemistry, Ankara, Turkey d Dumlupınar University, Faculty of Arts and Science, Department of Chemistry, Kütahya, Turkey e Dicle University, Faculty of Arts and Sciences, Department of Chemistry, Diyarbakır, Turkey b c
a r t i c l e
i n f o
Article history: Received 5 December 2009 Received in revised form 30 July 2010 Accepted 30 July 2010 Available online 27 August 2010 Keywords: Glassy carbon Poly(indole-3-carboxaldehyde) Electrochemical impedance spectroscopy X-ray photoelectron spectroscopy Reﬂection-absorption infrared spectroscopy Contact angle Ellipsometry
a b s t r a c t 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 a modiﬁed electrode. Modiﬁcation was performed using cyclic voltammetry (CV) scanning from 0.0 V to 2.0 V at 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 the electrochemical impedance spectroscopy. The modiﬁed surface was analyzed by reﬂection-absorption infrared spectroscopy and compared with the spectrum of the monomeric In3C. Elemental composition of the surface was determined by X-ray photoelectron spectroscopy. Contact angle measurements was also performed to check the changes in hydrophobic character of the bare GC and compared to that of In3C-GC surface. Thickness of the oligomeric/polymeric ﬁlm was investigated by ellipsometric measurements and a surface conﬁned polymerization mechanism was proposed. © 2010 Published by Elsevier B.V.
1. Introduction In recent years, several heteroatom containing organic molecules such as pyrrole, carbazole, indole have received increasing attention as material coatings, because they are easily electrografted to the surfaces forming conductive ﬁlms [1,2]. As a modiﬁer compound, indole family has received more interest because of its advantages of having fairly good thermal stability and high redox activity [3,4]. When indole and its derivatives are exposed to electrochemical oxidation in various electrolytes, conductive ﬁlms are produced on the electrode surfaces [5,6]. Kelaidopoulou et al. investigated anodic polymerization and redox properties of N-methyl-N′-(3-indol-1-yl-propyl)-4,4′-bipyridinium . Mezlova et al. prepared some conducting materials with electropolymerization of thieno[3,2-b]indole, 6-methoxythieno[3,2-b]indole and N-methylthieno[3,2-b]indole on Pt disc electrode. Talbi et al. investigated redox behavior of poly(indole-5-carboxylic acid) modiﬁed ITO and Pt electrodes [7,8]. The polymerization mechanism and the different possibilities of monomer linkages in polymeric indole derivatives were reported in the literature [4,9–11].
⁎ Corresponding author. Tel.: + 90 312 2021114; fax: + 90 312 2122279. E-mail address: [email protected]
(E. Hasdemir). 0040-6090/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.tsf.2010.07.125
Indole coated surfaces have some more properties such as selectivity, sensitivity and stability. So, they have widespread applications compared to the bare surfaces and can be used as selective electrodes which are sensitive to the various cationic and anionic inorganic species, as a biosensor for biological molecules or as a protection of metallic surfaces against corrosion [12–14]. Biegunski et al. immobilized tyrosinase onto poly(indole-5-carboxylic acid) modiﬁed Pt disc electrode to catalyse the oxidation of catechol . Li et al. developed an electrochemical method to detect DNA hybridization on poly(indole-5-carboxylic acid) conducting polymer ﬁlm . Tüken et al. synthesized polyindole on nickel-coated mild steel and investigated its corrosion performance . Yabuki et al. prepared amperometric glucose sensor by electrochemical polymerization of indole derivatives at the glassy carbon (GC) electrode . Poly(2methylindole) was synthesized by direct anodic oxidation of 2methylindole in LiClO4/acetonitrile solution and characterized by electrochemical and various spectroscopic methods . Many controversial results have been reported concerning the site of linkages of polyindoles with different substituents in C1, C2, C3 and C5 positions of the indole monomer. A survey of the literature reveals that the polymerization mechanism of indole derivatives is not fully understood. Saraji and A. Bagheri report that the possibilities of monomer linkages are involved through the pyrrole ring in C2 and C3 positions . According to some authors, C2 and C3 positions are the
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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 Pt electrode . Others propose the C1 and C3 sites as the most probable coupling 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 oxidative electro polymerization of indole-3-carboxaldehyde (In3C) monomer. The modiﬁed surface was analyzed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), reﬂection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), contact angle measurements and ellipsometry. An electro polymerization mechanism and a surface layer structure were also proposed 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 was performed. All solutions, which were used in electrochemical and modiﬁcation experiments, were prepared at the concentration of 1 mM. Ultra pure quality of water with a resistance of 18.3 MΩ cm (Human Power I+ puriﬁcation system) was used in preparations of aqueous solutions, cleaning of the glassware and polishing the electrodes. All the test solutions were deaerated by passing high purity argon (99.999%) before the electrochemical experiment. During the experiments, argon gas was passed over the surface of the test solutions in order to avoid the reentrance of oxygen into the solution. All experiments were carried out at room temperature (25 ± 1 °C).
2.3.3. Characterization of poly(indole-3-carboxaldehyde) modiﬁed GC electrode RAIRS spectra of indole-3-carboxaldehyde monomer and poly (indole-3-carboxaldehyde) ﬁlm were acquired by a Bruker Tensor 27 spectrometer equipped with a grazing angle and ATR accessories. The spectra of the solids were obtained in KBr pellets with DTGS detector. RAIRS spectra of poly(indole-3-carboxaldehyde) ﬁlm obtained using a grazing angle accessory (VeeMAX™ II, Pike Technologies) with an incident angle of 37° and a liquid N2-cooled MCT detector. Bare GC surface spectra were obtained similarly and subtracted from the modiﬁed surface spectra to obtain the ﬁlm spectra of the grafted poly (indole-3-carboxaldehyde). A Kratos ES300 electron spectrometer with MgKα X-rays (nonmonochromatic) is used for XPS measurements, which are carried out at 90° electron take-off angle. Electrochemical impedance spectroscopic experiments were carried out with a Gamry Reference 600 workstation equipped with a PCI4/300 potentiostat in conjunction with EIS 300 software. GC electrode before and after modiﬁcation with In3C were characterized in 1 mM ferrocyanide/1 mM ferricyanide redox couple via EIS methods. EIS data were measured at 100 kHz to 0.1 Hz at 10 mV wave amplitude and at an electrode potential of 0.215 V, the formal potential of ferrocyanide/ferricyanide redox couple. Ellipsometric measurements of the ﬁlm thickness were performed with an ELX-02C/01R model high precision discrete wavelength ellipsometer. The wavelength was 532 nm for all experiments. The thickness values of poly(indole-3-carboxaldehyde) ﬁlms at the GC-20 (Tokai, Japan) surfaces were determined from the average of the measurements using incidence angle of 70°. The contact angle of water drops on the poly(indole-3-carboxaldehyde) ﬁlms was measured with a model G-III contact-angle meter (Kernco Instrument Co., Inc., El Paso, TX). The one-liquid method (air– liquid drop–organic layer system) was used in the measurements.
2.2. Apparatus A conventional three-electrode cell system was used in electrochemical experiments. BAS Model GC disc electrodes with a geometric area 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 tetraﬂuoroborate (TBATFB) in acetonitrile) (BAS) reference electrodes were used in aqueous and nonaqueous solutions, respectively. Auxiliary electrode was a Pt wire. Electrochemical experiments were performed using a PCI4/300 potentiostat/galvanostat (Gamry Instruments, PA, USA) equipped with a BAS C3 cell stand. 2.3. Procedure 2.3.1. Cleaning of the working electrode surfaces Before modiﬁcation, the GC electrodes were cleaned with ﬁne wet emery papers (Buehler with grain size of 4000), and then were polished with alumina paste (particle size 0.1 μm and 0.05 μm in water) on Buehler polishing microcloth pads. Polished GC electrodes were ﬁrst sonicated in ultra pure water and then in a mixture of 1:1 (v/v) isopropyl alcohol/acetonitrile (IPA + MeCN) (Riedel) between each polishing step for 5 min. 2.3.2. Preparation of poly(indole-3-carboxaldehyde) modiﬁed GC electrode In3C monomer (supplied from Aldrich) was dissolved in 0.05 M TBATFB in acetonitrile. GC electrodes were anodically modiﬁed by CV in 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, modiﬁed electrode was rinsed with a stream of acetonitrile to remove any physisorbed and unreacted materials from the electrode surface. Indole-3-carboxaldehyde modiﬁed electrode was stored in acetonitrile until use.
3. Results and discussion 3.1. Electrochemical modiﬁcation and characterization of In3C-GC surface Electrochemical deposition of In3C ﬁlms on a GC electrode was carried out in acetonitrile containing 0.05 M TBATFB as a supporting electrolyte. Fig. 1 shows the multi sweep cyclic voltammograms for 1 × 10− 3 M In3C monomer (inset of Fig. 1) solution on GC. In the ﬁrst cycle, an irreversible oxidation peak of In3C appears at about +1.4 V vs. Ag/Ag+ reference electrode. Peak current gradually decreases in the subsequent potential cycles and reaches a steady state value after 10 cycles. This gradual decrease is ascribed to progressive accumulation
Fig. 1. Multi sweep cyclic voltammograms of 1 × 10− 3 M In3C at the GC surface at the GC electrode (vs. Ag/Ag+, sweep rate: 50 mV/s).
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which takes place as a consequence of electrode derivatization with In3C and implies that the deposition of In3C layer at the GC surface proceeds as the number of potential cycles increases . 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 peaks from 0 to +1.8 V potential range. This implies that the ﬁlm is electro inactive against positive scans up to + 1.8 V. Electro inactivity of the ﬁlm in this range is an advantage for the availability of using this modiﬁed surface as an electrode in the positive potential region for oxidation processes. Therefore, this modiﬁed electrode is very useful for the electrochemical oxidation of some biologically important molecules such as dopamine, uric acid and ascorbic acid. Preliminary experiments have shown the usefulness of this surface for analytical application and this work is under investigation. Electrochemical characterization of In3C-GC surface was carried out using ferricyanide and ferrocene redox probes. The electrochemistry of the In3C-GC and bare GC with redox active molecules in solution are shown in Fig. 2. It is well known that the electrochemical reaction of electroactive probes is efﬁciently blocked, suppressed or electron transfer rate is altered at some modiﬁed electrodes [21–24]. In the voltammograms of indole-3-carboxaldehyde modiﬁed electrode, the ferrocene and ferricyanide peaks are disappeared, because the heterogeneous electron transfer rate of these redox probes is completely blocked at the In3C modiﬁed surface. 3.2. EIS studies on the In3C-GC surface It is well known that electrochemical alternating current impedance technique is a useful tool for studying the interface properties of surface modiﬁed electrodes [25–27]. So, EIS was used to characterize the In3C-GC surface and investigate the pH effects of the terminal pyrrolic 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 of 1 mM ferrocene in acetonitrile with a background of 0.05 M TBATFB.
equal concentrations of both redox partners on the surface of the electrodes, these measurements were carried out at their formal 4− redox potentials of 0.215 V for Fe(CN)3− solution (1 mM 6 /Fe(CN)6 each) as redox probe in Britton–Robinson (BR) buffer containing 0.1 M KCl as supporting electrolyte and buffering the ionic strength . Fig. 3 shows the Nyquist and complex capacitance plots of the redox couple on In3C modiﬁed GC surface and the inset shows the equivalent electrical circuit model of the electrochemical process at the In3C modiﬁed surface, obtained using Gamry EIS300™ software. Nyquist plot for the probe on bare GC surface is a straight line at the entire range of frequency region and at all pHs (not shown in the ﬁgure) with a very small semicircle at the high-frequency region indicating a small charge transfer resistance (23 Ω) and almost fully diffusion-controlled process. The impedance plot for the probe system 4− of Fe(CN)3− 6 /Fe(CN)6 on In3C-GC surface shows a larger semicircle at the high-frequency region with a straight line at the low frequency region and at all lower pHs. The experimental data of the electrochemical impedance plot were analyzed by applying the nonlinear least squares ﬁtting to the equivalent electrical circuit including a solution resistance (Rs), a ﬁlm capacitance (Cf), a coating resistance (Rf), a charge transfer resistance (Rct), a double layer capacitance (Cdl) and a Warburg resistance (W), as shown in the inset of Fig. 3 . The values of Rct and Cf are the best for the measurement of the changes in the surface structure with the effect of pH. These values have been found from the ﬁtting process of the experimental data points with the theoretical electrical circuit. It is evident that pH has a signiﬁcant inﬂuence on the properties of the modiﬁed surface, appearing especially on the charge transfer resistance and ﬁlm capacitance 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)3−/4− of BR buffer, a) pH: 2, b) pH: 4, c) pH: 6, d) pH: 8, e) pH: 10. Inset is the 6 equivalent electrical circuit for the electrochemical system.
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constant between pH 2–6 (approximately 1.60 × 10− 5 μF/cm2) and increases gradually with the increase in pH of the solution, from 1.60 × 10− 5 μF/cm2 at pH 6 to 3.77 × 10− 5 μF/cm2 at pH 10. On the other hand, Rct value decreases with the increase in pH, from pH 2 to pH 6, and then stays almost constant. The charge transfer resistance for 1 mM Fe(CN)3−/4− solution on In3C-GC electrode was found to be 6 782 Ω and 3609 Ω at pH 10 and pH 2, respectively. We interpret this increase in Rct upon addition of H+ moieties, as being due to the interaction of H+ with the aldehydic carbonyl oxygen atoms of the polymeric In3C ﬁlm (Fig. 7), disrupting the N⋯H⋯O bond and putting the surface ﬁlm into a protonated form through O atoms in lower pH values. The higher values of Rct at lower pHs are probably due to the electrostatic repulsions between the negatively charged redox probes and nonbonding electrons of oxygen atoms, in that state they are more effective compared to the H-bonded state. It is important to emphasize that the dramatic changes in Rct, Cf and contact angles occur around at pH 6, which is the pKa value of the indole-3carboxaldehyde in solution. When In3C layer is in the neutral state (at pH 10), the doping Fe(CN)3−/4− ion transfer is negligible and polymer 6 chain has no charges. The EIS results are related to the regulated surfaces of In3C ﬁlms produced by the proton concentration of solution, which is conformity with our contact angle studies. The electrode coverage (θ) is a key factor that can be used to estimate the surface state of the electrode, and the charge transfer resistance is also related to it . The electrode coverage can be calculated by (1 − θ) = R0ct/Rct, where R0ct is the charge transfer resistance at the bare GC electrode and Rct is the charge transfer resistance 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 4− (Rct = 20 Ω) in Fe(CN)3− solution at pH of 2, from ﬁtting 6 /Fe(CN)6 the spectra to the equivalent circuit. The corresponding electrode coverage value was estimated as 99.5% using the above equation. This suggests that the permeability of the modiﬁed surface is low due to the compact and multilayer nature of the In3C nanoﬁlm at pH 2.
Table 1 Some characteristic IR bands and their assignments for solid In3C in KBr pellets and In3C ﬁlm deposited at the GC surface. Solid In3C bands
In3C-GC surface bands
3172 cm− 1 3049 cm− 1 2824 cm− 1, 2934 cm− 1 1579 cm− 1 1446 cm− 1 1130 cm− 1 1230 cm− 1
3220 cm− 1 3094 cm− 1 2865 cm− 1, 2938 cm− 1 1590 cm− 1 Vanished 1122 cm− 1 1260 cm− 1
N–H stretch Aromatic C–H stretch Aldehydic C–H stretch C N stretch C C aromatic stretch C–H bending N–H bending
Comparison of both spectra given in Fig. 4 shows that the bands corresponding to pyrrolic N–H stretching exist in both spectra of solid monomer (3172 cm− 1) and ﬁlm (3220 cm− 1) at the GC surface with a higher frequency shift at the surface. This is an important IR evidence of the polymerization coupling sites, suggesting that pyrrolic N does not play a role neither in the attachment to the GC surface nor between the polymerized monomers . Aromatic C–H stretching bands were exhibited in monomer (3049 cm− 1) and ﬁlm (3094 cm− 1) spectra with about 50 cm− 1 higher frequency shift for the later, indicating that the attachment to the GC surface may be through the phenyl ring . IR data demonstrate that In3C is deposited at the GC surface as a multilayer ﬁlm in which the pyrrolic N–H functionality persists. Moreover, the band at 1579 cm− 1 for the monomer which is assigned to the C N stretch is persisted 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 is vanished in the surface spectrum suggesting that the aromatic ring is involved in bonding to the surface, presumably as a result of π–π interactions between the graphitic rings systems of the GC . These observations suggest that the pyrrolic ring in indole-3-carboxaldehyde monomer was not involved either in the electropolymerization and the bonding to the GC surface. 3.4. X-ray photoelectron spectroscopic data for the indole-3-carboxaldehyde ﬁlm at the GC surface
3.3. RAIRS spectra of the In3C-GC surface To characterize the In3C-GC surface and to investigate the polymerization coupling sites, IR spectra of monomers in KBr pellets and In3C ﬁlms on the GC surface were recorded and compared to each other. The resulting RAIRS spectra of poly(indole-3-carboxaldehyde) ﬁlm and IR of solid indole-3-carboxaldehyde as KBr pellets are partly shown in Fig. 4 and the assignments of the main IR bands are given in Table 1. The assignments of the IR bands were performed using Bruker Tensor 27 spectrometer library and various IR tables. For RAIRS measurements the In3C modiﬁed surface was prepared on a Tokai GC 20 plates.
Fig. 4. High-frequency region IR spectra of (A) solid In3C in KBr pellets, (B) In3C ﬁlm deposited at the GC surface.
To characterize the poly(indole-3-carboxaldehyde) modiﬁed GC surface, XPS experiments were performed for the elemental distribution, 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 and O1s peaks in the survey spectrum of the modiﬁed surface conﬁrmed that the indole-3-carboxaldehyde moieties were attached on the GC surface. N1s peak in the core level spectrum for In3C-GC surface was observed 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 the percentages 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 N and O elements arising from the pyrrolic and aldehydic groups are signiﬁcantly higher on the modiﬁed surface than that of bare GC surface. Comparison of the percentages of C on the modiﬁed and bare surfaces also follows the same trend and drops signiﬁcantly as the In3C ﬁlm covers the bare GC surface. Increasing of the percentage of N1s peak and decreasing of that of C1s peak is expected phenomena due to the accumulation of In3C molecules at the GC surface. Appearance of the O1s and N1s peaks at the unmodiﬁed GC surface are likely due to surface oxidation during modiﬁcation process  and structural contamination during industrial fabrication of GC , respectively.
3.5. Contact angle measurements at In3C-GC surface Fig. 6. Change in the contact angle of water on GC and In3C-GC as a function of the pH.
The contact angles of the ﬁlm were affected by the hydrophobic and hydrophilic properties, heterogeneity and the roughness of the surface. Hence contact angles are important constants of liquid–solid systems providing valuable information on the change of the properties of surfaces . Therefore, the surface characterization of the In3C-GC electrode was investigated by contact angle measurements. Table 2 summarizes data from measurements of contact angles on both surfaces in buffered solutions with different pHs. The water-drop contact angles for the bare GC surface was measured between 76° and 83° in different pHs ranging from 2 to 10, indicating the hydrophobic character of the bare GC surface (Table 2). Indole-3-carboxaldehyde ﬁlms are more hydrophilic with contact angles between 58° and 61° than bare GC surface, 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 more hydrophilic than bare GC surface at every pH values as seen in Table 2. The most interesting feature of contact angle measurements is its pH dependency, as shown in Fig. 6. This behavior suggests the presence of the surface conﬁned functional groups which are acidic or basic character. Pyrrolic groups are probably responsible for this behavior. As Fig. 6 shows, water contact angle pass through a maximum against pH which is probably the pKa value of the surface. The pKa value of the indole-3-carboxaldehyde has been reported as 6.04 .
3.6. Ellipsometric thickness measurements of In3C ﬁlms Ellipsometry is a valuable tool in the characterization of organic and inorganic ﬁlms and in the determination of polymeric or molecular ﬁlm thicknesses at the nanoscales . Ellipsometric measurements of In3C ﬁlm thickness on GC was performed by ﬁtting for a four-phase model consisting of graphite/GC substrate/In3C ﬁlm/air. Refractive indices were taken as 3.0841 for graphite, 1.9000 for GC substrate, 1.4600 for In3C ﬁlm and 1.0000 for air. In the calculation of the ﬁlm thickness based on the four-phase model, the main assumption is the presence of reasonable correlation between the thickness and its refractive index . Extinction coefﬁcients are −1.7820 for graphite, −0.8100 for GC substrate, 0.0000 for organic layer and 0.0000 for air. Ellipsometric thickness of In3C ﬁlm was measured as 79.14 ± 0.40 nm, indicating that a multilayer was formed at the GC electrode surface. To check the homogeneity of the modiﬁed surface, measurements were performed at four different spots. Standard deviation of the thickness shows that a
Table 2 The results of contact angle measurements for bare GC and In3C-GC surfaces in different pHs. pH
83.30 ± 0.17 61.99 ± 0.54
75.63 ± 0.39 57.77 ± 2.25
83.37 ± 0.48 69.18 ± 0.65
82.33 ± 1.22 67.11 ± 0.51
80.30 ± 0.50 58.91 ± 0.85
reasonably well homogeneity is obtained for the In3C ﬁlms at the GC surface. 3.7. Grafting mechanism and the structure of the modiﬁed surface From electrochemical data, it is clear that a ﬁlm is formed at the GC electrode which passivated the surface against monomer oxidation (Fig. 1). This surface ﬁlm did not show electroactivity when the electrode was taken out of the monomer solution after modiﬁcation and placed into a solution of background electrolyte without monomer. The presence of N–H stretching band in the spectra of In3C and its deposited polymer at the GC surface deﬁnitely excludes any possible role of the N–H group as polymerization sites at the GC surface. Dependence of the contact angles on pH suggest the presence of the surface conﬁned acidic or basic groups which can only be the N–H functionality in pyrrole ring. With regard to electrochemical, IR and contact angle measurement data, the most probable coupling sites were proposed to be the pyrrolic C2 and phenyl C5 or C6 positions. Involvement of the C2 carbon in the polymerization of indole derivatives has been reported in the literature [4,9,10,18]. Fig. 7 shows the structure of the In3C ﬁlm on GC surface proposed in the light of our experimental data and the reports of the literature. The proposed structure is one which has been grafted as oligomeric/polymeric In3C of various monomer repeat units with H-bond interactions of CHO and NH functionalities of pyrrole rings. 4. Conclusions The data acquired from RAIRS, XPS, ellipsometry, electrochemical experiments and contact angle measurements support the view that the indole-3-carboxaldehyde monomer was grafted to the GC surface during electro-oxidation of monomer solution as a short chain oligomers rather than long chain polymers. The most probable coupling sites were proposed to be the pyrrolic C2 and phenyl C5 or C6 positions. In the ﬁrst anodic scan of the monomer, In3C is oxidized to give radicals which attack the GC surface from the C5 or C6 carbon to be grafted as a monolayer at the surface. The radicals present in the acetonitrile solution attacks the In3C monolayer through the C2 position forming an oligomeric multilayer with an ellipsometric thickness of about 80 nm. The In3C ﬁlm is electro-inactive against positive scans up to + 1.8 V and useful as an electrode in the oxidation region. Acknowledgments This work was supported by the TUBITAK (Scientiﬁc and Technological Research Council of Turkey) project number 106T622. The authors thank Dr. Selda Keskin, from Middle East Technical
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Fig. 7. The proposed structure of the In3C oligomer/polymer ﬁlm on GC surface.
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