CC BY-NC-ND
0Available online at www.sciencedirect.com
ScienceDirect
Physics Procedia 2 (2005») 1241-120408
www.elsevier.com/locate/procedia
Proceedings of the JMSM 2888 Conference
Complexations of polyoligothiophenes films with transition metals, and their use for electrocatalysis of ascorbic acid
N. Maouchea*, S. Chellib, B. Nessarka, S. Aeiyachb
a Laboratoire d'Electrochimie et Matériaux (LEM), Université de Sétif 19000, Algérie b Laboratoire Interfaces, Traitements, Organisation et Dynamique des Systèmes, CNRS - Université Paris 7, Denis Diderot, Bâtiment Lavoisier,
15 rue Jean de Baïf, 75205 Paris Cedex 13, France.
Received 1 January 2009; received in revised form 31 July 2009; apcepted 31 August 2009
Abstract
Chemically modified electrodes prepared by electropolymerization of oligothiophenes such as: 2,2'-bithiophene (BT), 2,2'-bithiophene-5-carboxylic acid (BTCA) and terthiophene aldehyde (TTCHO) on platinum (Pt) electrodes, in acetonitrile solution containing 0.1 M tetrabutylammonium perchlorate (TBAP) and 0.01 M of monomer, are characterized by cyclic volammetry (CV), and X-ray photoelectron spectroscopy (XPS) measurements. By immersing the prepared modified electrodes in transition metals (Cu2+, Co2+ and Ag+) solutions, the metal ions were complexed with films. The electrochemical response shows clearly, the presence of oxidation and reduction peaks corresponding to metallic couple redox. XPS technique reveal that the films complexed with metal ions and determine the mode of the connection with film's atoms. The obtained polyoligothiophenes-metal modified electrodes exhibited good electrocatalytic properties towards ascorbic acid (AA) oxidation after their complexation with metallic ions. The electrocatalytic response was evaluated by cyclic voltammetry with regard to the film nature, the metallic ion nature, immersion time, ascorbic acid concentration, and other variables. The results reveal that the catalytic activity of Ag+ complexed with BTCA thin-film is the best toward AA oxidation and it can be detected a very low concentration (~1 pM), of AA in a solution which can be utilized as an efficient electrochemical sensor. © 2009 Elsevier B.V. All rights reserved
PACS: Type pacs here, separated by semicolons ;
Keywords: Ascorbic acid; Cyclic voltammetry; Modified electrodes; Oligothiophenes; Electrocatalysis; XPS
1. Introduction
Redox polymer-modified electrodes have been widely investigated because of their potential application in electrocatalysis, sensors, energy conversion and storage, electronic displays and devices [1,2]. Electroactive polymeric films have acquired wide interest, since there simple generation on the electrode Surface [3]. Moreover,
* Corresponding author. Tel: +213 7 7125 4374; fax: +213 3692 5133. E-mail address: m_naima2001 @yahoo.fr
doi:10.1016/j.phpro.2009.11.087
the increased number of active sites in the polymer film rendered an enhanced electrochemical process at its surface than at the monolayer-modified electrode.
The ascorbic acid is one of the most important compounds which has received much attention in recent years [47], it was studied in electrocatalysis using different modified electrodes. It has been shown that the content of AA in biological fluids can be used to access the amount of oxidation stress in human metabolism and excessive oxidative stress has been linked to cancer, diabetes and hepatic disease [8]. But, it is known that accurate determination of AA using conventional electrodes is very difficult because of its high overpotential, poor reproducibility due to fouling effect caused by the oxidized products of AA, low selectivity and sensitivity [9]. Morever, conducting polymers (CPs) systems do not have any intrinsic electrocatalytic activity, but they have been used as host matrices for different chemical species such as metal particles [10], complex ions [11,12], and enzymes [13], through which these systems acquire catalytic activity. Therefore, several approaches have been used to modify the electrode surfaces which include ion exchange polymers [14]. But there are few reports of the metal ion complex with CPs, due to the weak interaction between a legating atom or CPs with metal ions. However, the functional groups, such as amine, imine, and carboxylic acid can be used as ligands for the metal ion complexation [15,16]. CPs having a carboxylic acid functional group can coordinate with a metal ion to form the coordination complex. In case of thiophenes films it exist a strong interaction between the sulfur atom of thiophene ring with metallic ions as silver (Ag) [17,18] and copper (Cu) [19,20].
In the same meaning, we are interested in this work to prepare modified electrodes using a conducting oligothiophenes (poly-oligothiophenes) and their transition metal complexes (polyoligothiophenes-M). Hence, the films were electrosynthesized from the corresponding monomers and complexed with metal ions (Cu2+, Co2+, Ag+) to get polyoligothiophenes-M. The resultant polyoligothiophene-M modified electrode was characterized using cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS), and finally, they were utilized for electrocatalysis of AA. Various experimental parameters, which affect the AA detection, were studied
2. Experimental
Materials
All chemical products and solvents (2,2'-bithiophene (BT), 2,2'-bithiophene-5-carboxylic acid (BTCA) and terthiophene aldehyde (TTCHO), (Acros organics); CuCl2.2H2O, CoCl2.2H2O, AgNO3 (Aldrich) and acetonitrile (Prolabo) were analytical grade and were used without further purification.
Measurements
Electrochemical experiments were carried out in a three-electrode cell employing a platinum (Pt) disc 0.03 cm2 in area, as working electrode, a conventional saturated aqueous potassium chloride-calomel electrode (SCE) as reference electrode and a steel grid as an auxiliary electrode. The working electrode was polished before each experiment by cloth polishing with alumina slurry (0.1 pm), and rinsed with acetone and ethanol. The supported electrolyte was 0.1 M tetrabutylammonium perchlorate (TBAP) dissolved in argon-purged acetonitrile. Measurements were carried out on an EG&G PAR 273 A potentiostat-galvanostat.
XPS spectra were recorded using a Thermo VG ESCALAB 250 system equipped with a micro-focused, monochromatic Al Ka X-ray source (1486.6 eV) and a magnetic lens which increases the sensitivity of the apparatus. The specimens were pressed against double-sided adhesive tapes mounted on sample holders, and then pumped overnight in the fast-entry lock at 5x10-8 mbar before introduction into the analysis chamber. A 650 |im X-ray beam was used at a power of 10 mAx15 kV. The spectra were acquired in the constant analyser energy mode, with pass energies of 150 and 40 eV for the survey and the narrow regions, respectively. All samples (powders and plates) were subjected to charge compensation achieved with an electron flood gun combined with an argon ion gun. The argon partial pressure was 2x10 mbar in the analysis chamber. Advantage software, version 3.51, was used for digital data acquisition and processing. The peak binding energy positions were calibrated by setting the C1s peak maxima to 285.0 eV. The apparent surface compositions were determined by considering the integrated areas of the core-level peaks and their respective sensitivity factors.
3. Results and discussion
3.1. Preparation of modified electrodes by polyoligothiophenes
At first, the Polyoligothiophene film was grown on platinum electrodes through electropolymerization, and then obtained coated electrode was immersed in a solution containing transition metal. Prior to electropolymerization, platinum electrodes were polished with special cloth with 0.1 pm alumina slurry, and rinsed with acetone and ethanol. The Polyoligothiophene film at the cleaned electrodes was grown through electropolymerization of corresponding monomer (10-2 M) in a 0.1 M TBAP/CH3CN solution by cycling the potential from 0 V to 1.6 V for several cycles (example formation of BTCA dimer at Pt electrode fig.la). The resultant electrode was washed with acetonitrile for the removal of excess adhered monomer. The Polyoligothiophene-M complex was made by immersing the electrode in a solution containing 10-2 M of CuCl2, CoCl2 or AgNO3 during 10 min. After that the Polyoligothiophene-M complex-modified electrode was washed carefully with distilled water. We note that the film formed from BTCA monomer is very adherent, thin, stable with coppery color, compared with PBT and PTTCHO films which are very smooth and bulky.
The schematic representation of the preparation of the dimer BTCA-Cu complex on the electrode is shown in Figure. 1b.
400 600 800 1 000 1 200 1 400 1600
E (mV/SCE) (b)
Fig. 1. (a) Consecutive Cyclic voltammograms of BTCA 5x10-3 M in 0.1 M TBAP/CH3CN at Pt. Scan rate = 50 mV s-1,(b) Schematic drawing of the preparation of dimmer BTCA-Cu complex at the platinum electrode.
4. Characterization of the obtained films
4.1. Electrochemical characterization
The electrochemical analysis of the obtained films was carried out in CH3CN/TBAP (0.1 M) and H2O/LiClO4 (0.1 M) solutions. For BTCA film in CH3CN/TBAP, the curves do not show any oxidation and, or reduction peaks in the potential range from -300 to 1600 mV, in two analysis mediums, indicating that there is no electroactivity corresponding to the doping-dedoping of these films (Fig.2Aa, 2Ba). However, after their immersion in a solution containing Cu2+ during 10 min, the analysis in CH3CN/TBAP, displayed oxidation and reduction peaks at 590 and 340 mV (Fig.2Ab), respectively. In addition, the difference between the oxidation and reduction potentials, AEp= 255 mV, showed that the process is quasi-reversible and the ratio of anodic to cathodic peak currents (Ipc/Ipa = 1) corresponds to a one-electron process. These results can be attributed to the Cu2+/Cu+ redox couple incorporated (trapped or complexed) into the BTCA dimer films [21]. In H2O/LiClO4, similar observations to those of the first case were observed, but the two redox peaks shifted to more negative potentials, at 100 and -100 mV, respectively (Fig.2Bb). The related CV shows behaviour similar to that observed for CuCl2 in aqueous solution, consisting of electrochemically irreversible redox peaks Cu2+/Cu+ at potential +100 and -100 mV(Fig. 2Bb). It is interesting to note that these peaks shift to more negative potentials compared to that obtained in CH3CN and have good electrochemical stability under repetitive cyclic voltammetry. These results, in aqueous solution, imply that the BTCA film-Cu2+ has rather good electrochemical stability, is more electroactive and has high porosity, which is necessary to allow the oxidation-reduction of Cu2+ complexing or trapping with BTCA dimer films.
0 200 400 600 800 1000 1200 1400 1600 1800
E (mV/SCE)
E (mV/SCE)
Fig.2. (a) Cyclic voltammogram of BTCA film and (b) BTCA film-Cu+2in: (A) 0.1 M TBAP/CH3CN, (B) 0.1 M LiClO4/H2O, at 100 mV s-1
A similar study was carried out by immersing BTCA thin-films in an aqueous solution of CoCl2 or AgNO3. The redox properties of BTCA thin-film-Co2+ or BTCA thin-film-Ag+ in aqueous solution exhibit features similar to those observed for BTCA thin-film-Cu2+, which confirms the presence of metallic ions in BTCA thin-films [22-24].
The same results are obtained when the films are PBT and P3TCHO and after their immersion in metallic solution, the corresponding voltammograms show the presence of the oxidation and reduction peak attributed to the corresponding metal. (Data not shown).
4.2. XPS spectra analysis
To verify and confirm the presence of metal ions in the films, XPS measurements were carried out on the free BTCA thin-film, BTCA thin-film-Cu2+ and other films, in order to determine the form of copper present in the film close to the surface.
The survey spectra of BTCA thin-film and after its immersion in a 0.1 M CuCl2 solution for 10 min are shown in Fig.3. All the expected elements such as C1s, O1s, S2p in the dimer film can be seen (Fig. 3a). Additional peaks can be detected in BTCA thin-film-Cu2+ at 935.2 and 955.0 eV corresponding to Cu 2p3/2 and 2p1/2 (Fig.3b), respectively, which indicates that Cu2+ species present in the BTCA-Cu2+ complex-modified surface.
when we compare the binding energy (BE) values of the O1s in BTCA and BTCA film-Cu2+, we found out that the oxygen BE is shifted about 0.8 eV (from 532.5 to 531.7 eV) due to O-Cu bonding [25], resulting from the complexation of a -COO- functional group of BTCA with Cu2+.
200000 150000 100000 50000 0
Energie de liaison (eV)
Energie de liaison (eV)
Fig.3. XPS survey spectra of: (a) dimer BTCA thin-film and (b) dimer BTCA thin-film after immersion in 0.1 M CuCl2 solution for 5 min
The BE of S2p was observed at 163.9 eV for the free BTCA. After complexation, this peak is slightly shifted to a higher energy, indicating that sulfur S atoms may involve the complexation. However, the degree of binding energy shifted was larger in the case of O1s peak than that observed in the S2p peak indicated that the complex formation between Cu(II) and BTCA dimer occurred through the direct formation of Cu-O bonds. However, for the PBT-Cu, and PTTCHO-Cu, the most important shift is observed for the S atom and no modification is observed for the O atom after complexation. This indicates that the complexation is occurred between the S-Cu. The complexation between sulphur of thiophene and copper ions is discussed in earlier studies, where a series of copper thiophene polymer complexes were characterized by the electrochemical method. They suggested complexation between the copper species and sulphur and the oxidation state of copper as Cu(II) [19,26].
The corresponding Binding Energy of the oligothiophenes films and their copper complexes are grouped in the table.
Table: Data of XPS spectra of oligothiophenes films and their copper complexes (binding energy/BE (eV).
Element C O S Cu
System C1s O1s S2p Cu2p3/2 Cu2p1/2
BTCA 285 531.3 163.9
BTCA-Cu 285 530.4 164.2 935.4 955.4
PBT 285 532.8 163.9
PBT-Cu 285 532.2 162.5 935.3 955.1
PTTCHO 285 232.3 164.3
PTTCHO-Cu 285 533.2 161.5 935.5 955.2
The same results are obtained with the Cobalt and silver, which are detected electrochemically by cyclic voltammetry and XPS measurements. The presence of metallic ions confirms the complexation of this later with polyoligothiophenes films. The advantage of polythiophenes is that sulfur atom of polyoligothiophene chain or the carboxylate group strongly acts by the interaction with metallic ion and form the composite materials which may be use in several applications
4.3. Electrocatalytic oxidation of ascorbic acid at modified electrodes
Many compounds have been found to be active electron transfer mediators of the electocatalytic oxidation of AA [4,27,28]. In our case also, the electrocatalytic activity of poly-oligothiophenes films modified by ion metals has been studied toward ascorbic acid oxidation in a 0.1 M phosphate buffer solution at pH = 7 and thus in the absence and presence of AA.
Poly-oligothiophenes films were immersed for 10 min in various solutions containing CuCl2, CoCl2 and AgNO3; then the electrode was removed, rinsed with distilled water and transferred to a freshly prepared ascorbic acid solution. Cyclic voltammetry was used to examine the electroactvity of film-metal ion complexes deposited on Pt electrodes.
The cyclic voltammograms of 0.2 M phosphate buffer containing 5 mM of AA at different modified electrodes show clearly that there is no observed response when the films are not modified by metallic ions (Fig.4A). This justifies that the polythiophenes films are not active in aqueous medium and do not present any catalytic effect and effectively blocked the electronic transfer between AA in solution and the underlying platinum electrode surface. Except a shoulder was shown by the voltammogramm at about 138 mV, which may be due to oxidation of AA, on the BTCA thin-film. However, when these polymer films are modified by the incorporation of copper for example (Fig.4B), the irreversible oxidation peak of AA appeared and occurred at 343 mV at PTTCHO-Cu modified Pt surface (curve Bc), and at 125 mV at BTCA-Cu film (curve Bb), whereas no response was observed at PBT-Cu electrode (curve Ba).
E (mV/ECS) E (mV/SCE)
Fig.4. Cyclic Voltammograms of 5 mM of AA in 0.2 M phosphate buffer at different films: (A) unmodified: (a) PBT , (b) BTCA, (c) PTTCHO, (B) modified by copper (a) PBT-Cu , (b) BTCA-Cu, (c) PTTCHO-Cu, at pH 7, v = 50 mV.s-1
In order to have an idea on the effect of immersing time, the electrode modified by PTTCHO-Cu was taken as an example, to test its electrocatalytic evolution. The latest electrode was immersed in copper solution for 2, 5, 20 min; the corresponding voltammograms are presented in figure.5.
An increase in the intensity of the AA oxidation peaks with immersion time of the electrode is observed. The behavior is explained by the quantity of copper complexed or incorporated which, increases also with immersion time. A catalytic effect is thus well observed, showing that modification of certain polyoligothiophenes by incorporation of metallic ion can play a catalyst role due to the electroactive surface which becomes more important.
E (mV/SCE)
Fig.5. Cyclic voltammograms of 5 mM of AA in 0.2 M phosphate buffer at PTTCHO-Cu modified Pt electrode for different immersion time;
(a) 0, (b) 2, (c) 5, (d) 20 min, at v = 50 mV s-1
The effect of the metallic cation nature is studied by using the Pt/BTCA as the electrocatalytic support of AA. As shown in figure 6, there is no peak corresponding to the oxidation of AA on a free BTCA film (fig. 6a). However, irreversible oxidation peaks at ca. 125, 285 and 305 mV were obtained on BTCA thin-film-Cu2+, BTCA thin-film-Ag+ and BTCA thin-film-Co2+, respectively, corresponding to AA oxidation. These results suggest that the metal ion plays an intermediate to transport the electron transfer between AA and electrode and indicating that a good electronic communication was achieved between metallic ion and the underlying Pt electrode through BTA thin-film.
It should be noted that the oxidation peak current on BTCA thin-film-Ag+ is approximately twice that on BTCA thin-film-Cu2+and BTCA thin-film-Co2+. This finding reveals that the catalytic activity of Ag+ complexed with BTA thin-film is the best toward AA oxidation and BTCA thin-film-Ag+ can be detected a very low concentration of AA in a solution (~1 ||M). In contrast, the oxidation of AA on BTCA thin-film-Cu2+ was shifted to negative potential
about 200 mV compared to that at BTCA thin-film-Co2+. This result suggests that electron transfer between AA and BTCA thin-film by the intermediary of Cu2+ is easier than that by Co2+.
120100-
SO. 60. 40. 20.
Fig.6. Cyclic voltammograms of 5 mM AA in phosphate buffer solution (0.2M PBS, pH = 7) at various modified electrodes surfaces: (a) BTCA thin-film; (b) BTCA thin-film-Cu2+, (c) BTCA thin-film-Ag+ and (d) BTCA thin-film-Co2+; v = 50 mV s-1
Figure.7 shows the cyclic voltammograms of Pt/BTCA-Ag electrode at various AA concentrations dissolved in 0.2 M phosphate buffer. From the cyclic voltammograms, we can see that electrocatalytic currents increase with increasing AA concentration and accompanied by a displacement of potential towards more positive values. The positive shift of oxidation peak potential indicates a kinetic limitation existing in the reaction between Pt/ BTCA-Ag and AA [29]. The experimental results indicate that electrocatalytic currents increase linearly with AA concentration in the range from (10-6-10-2 M).
We can say that the catalytic effect is caused by transition metals incorporated or complexed in polythiophenes films which play an important role in enhancing the catalytic activity of the films.
-5-100 100 200 300 400 500 600 700
E (mV/SCE)
Fig.7. Cyclic voltammograms corresponding to various concentration of AA, in phosphate buffer solution (0.2 M PBS, pH = 7) at Pt/BTCA-
Ag modified electrode, at v = 50 mV s-1
5. Conclusion
The preparation and characterization of transition metal-oligothiophenes thin films on platinum electrodes are demonstrated in this work. Electrochemistry can provide the evidence of the presence of surface-bound electroactive species but cannot provide the exact type of chemical bonding. The XPS measurement is very useful to check the presence of any chemical bonding at the electrode surface. In this study, XPS spectra clearly showed that the dimer BTCA-Cu(II) complex was formed through the Cu-O bond formation. Thus, Cu(II) was chemically but not physically adsorbed on electrode modified surface. However, for the PBT and PTTCHO the complexation occurred between S atom and the metal.
/ \ (c)
(b) r\l
E (mV/SCE)
The obtained modified electrodes are stable and show electrocatalytic properties for the oxidation of AA, which may find its promising application as an electrochemical sensor. Furthermore, the method for the fabrication of the metal oligothiophenes thin-films is simple and can be used for the preparation of other metal and others polymers thin-films.
References
[1] C. M. Casado, I. Cuadrado, M. Moran, B. Alonso, B. Garcia, B. Gonzalez, J. Losada, Coord. Chem. Rev. 53 (1999) 185.
[2] L. M. Goldenberg, M.R. Bryce, M.C. Petty, J. Mater. Chem. 9 (1999) 1957.
[3] R. W. Murray, Annu. Rev. Mater. Sci. 14 (1984) 145.
[4] A. Sivanesan, P. Kannan, S. Abraham John, Electrochim. Acta 52 (2007) 8118.
[5] Y. Sha, L. Qian , Y. Ma, H. Bai, X. Yang, Talanta 70 (2006) 556.
[6] P. R. Roy, M. S. Saha, T. Okajima, T. Ohsaka, Electrochim. Acta 51 (2006) 4447.
[7] D.W. Martin Jr., in: D.W. Martin Jr, P.A. Mayes, V.W. Rodwell (Eds.), Harper's Review of Biochemistry, 19th ed., Lange, Los Altos, CA,
[8] J. Koshiishi, T. Imanari, Anal. Chem. 69 (1997) 216.
[9] R.N. Adams, Anal. Chem. 48 (1976) 1126A.
[10] M. Gholamian, A. Q. Contractor, J. Electroanal. Chem. 289 (1990) 69.
[11] H. Laborde, J.-M. Léger, C. Lamy, J. Appl. Electrochem. 24 (1994) 219.
[12] H. D. Abruna, P. Denisevich, M. Umana, T. J. Meyer, R. W. Murray, J. Am. Chem. Soc. 103 (1981) 1.
[13] M. Umana, J. Waller, Anal. Chem. 58 (1986) 2979.
[14] J. M. Zen, Y.J. Chen, C.T. Hsu, Y.S. Ting, Electroanalysis 9 (1997) 1009.
[15] F. A. Cotton, G. Wilkinson, 1988. Advanced .Inorg Chem. Wiley, NewYork.
[16] R. C. Mehrotra, R. Bohra, 1983. Metal Carboxylates. Academic Press, London.
[17] C. J. Lee, M. R. Karim, M.S. Lee, Mater. Lett. 61 (2007) 2675.
[18] Z. Mousavi, J. Bobacka, A. Lewenstam, A. Ivaska, J. Electroanal. Chem. 593 (2006) 219.
[19] A. Czerwinski, D.D. Cunningham, A. Amer, J.R. Schrader, C.V. Pham, H. Zimmer, H.B. Mark Jr., S. Pons, J. Electrochem. Soc. 134 (1987) 1158.
[20] D. Guay, G. Tourillon, E. Dartyge, A. Fontaine, H. Tolentino, J. Electrochem. Soc. 138 (1991) 339.
[21] P.V.Bernhardt, N. L. Kilah, Polyhedron 26 (2007) 392.
[22] G. G. Wallace, H. Zhao, C. O. Too, C. J. Small, Synth. Met. 84 (1997) 323.
[23] K. Takeshita, W. Wernet, N. Oyama, J. Electrochem. Soc. 141 (1994) 2004.
[24] B. Ballarin, R. Seeber, L. Tassi, D. Tonelli, Synth. Met. 114 (2000) 279.
[25] B. Wang, F. Gao, H. Ma, Journal of Hazardous Materials 144 (2007) 363.
[26] Z. Mekhalif, J. Delhalle, P. Lang, F. Garnier, R. Caudano, J. Electrochem. Soc. 146 (1999) 2913.
[27] D. Ravi Shankaran, K. Iimura, T. Kato, Sensors and Actuators B 94 (2003) 73.
[28] H. Wang, Y. Huang, Z. Tan, X. Hu, Analyt. Chim. Acta 526 (2004) 13.
[29] C-X. Cai1, K-H. Xue. Microchemical, Journal 61 (1999) 183.
