Elsevier

Thin Solid Films

Volume 519, Issue 2, 1 November 2010, Pages 784-789
Thin Solid Films

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

https://doi.org/10.1016/j.tsf.2010.07.125Get rights and content

Abstract

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.

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 [3]. 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], [10], [11].

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], [13], [14]. Biegunski et al. immobilized tyrosinase onto poly(indole-5-carboxylic acid) modified Pt disc electrode to catalyse the oxidation of catechol [15]. Li et al. developed an electrochemical method to detect DNA hybridization on poly(indole-5-carboxylic acid) conducting polymer film [16]. Tüken et al. synthesized polyindole on nickel-coated mild steel and investigated its corrosion performance [17]. Yabuki et al. prepared amperometric glucose sensor by electrochemical polymerization of indole 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 by electrochemical and various spectroscopic methods [11].

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 [9]. According to some authors, C2 and C3 positions are the 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 [11]. 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.

Section snippets

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  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

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

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

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

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