Anal. Chem. 2006, 78, 3820-3826
High-Performance Carbon Composite Electrode Based on an Ionic Liquid as a Binder Norouz Maleki,* Afsaneh Safavi,* and Fariba Tajabadi
Department of Chemistry, College of Sciences, Shiraz University, Shiraz, 71454, Iran
Ionic liquid, n-octylpyridinum hexafluorophosphate (OPFP) has been used to fabricate a new carbon composite electrode with very attractive electrochemical behavior. This type of carbon electrode has been constructed using graphite mixed with OPFP as the binder. The electrode has combined advantages of edge plane characteristics of carbon nanotubes and edge plane pyrolytic graphite electrodes together with the low cost of carbon paste electrodes and robustness of metallic electrodes. It provides a remarkable increase in the rate of electron transfer of different organic and inorganic electroactive compounds and offers a marked decrease in the overvoltage for biomolecules such as NADH, dopamine, and ascorbic acid. It also circumvents NADH surface fouling effects as well as furnishing higher current density for a wide range of compounds tested. Depending on the choice of the electrolyte, the electrode can have the ion-exchange property and adsorptive characteristics of clay-modified electrodes. The proposed electrode thus allows sensitive, low-potential, simple, low-cost, and stable electrochemical sensing of biomolecules and other electroactive compounds. Scanning electron microscopy images indicate significant improvement in the microstructure of the proposed electrode compared to carbon paste electrodes. Such abilities promote new opportunities for a wide range of electrochemical and biosensing applications. Carbon electrodes are widely used in electroanalytical investigations because of their chemical inertness, relatively wide potential window, low background current, and suitability for different types of analysis.1-3 Carbon materials that have been widely used in the preparation of solid electrodes include glassy carbon, carbon fibers, carbon black, several forms of graphite from graphite powder to the highly oriented pyrolytic graphite, and carbon nanotubes.4-12 Based on these varieties of carbon, several * Corresponding authors. E-mail:
[email protected]; safavi@ chem.susc.ac.ir. Tel: +098-711-6305881. Fax: +098-711-2286008. (1) McCreery, R. L. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1991; Vol. 17, pp 221-374. (2) McCreery, R. L. In Laboratory Techniques in Electroanalytical Chemistry, 2nd ed.; Kissinger, P. T., Heineman, W. R., Eds.; Dekker: New York, 1996; Chapter 10. (3) Wang, J. Electroanalytical Chemistry, 2nd ed.; Wiley: New York, 2000. (4) Merkoci, A.; Pumera, M.; Llopis.: Perez, B.; Valle, M. D.; Alegret. S. Trends Anal. Chem. 2005, 24, 826-838. (5) Gooding, J. J. Electrochim. Acta 2005. 50, 3049-3060. (6) Gong, K.; Yan, Y.; Zanhg, M.; Su, l.; Xiong, S.; Mao, L. Anal. Sci. 2005, 21, 1383-1393.
3820 Analytical Chemistry, Vol. 78, No. 11, June 1, 2006
types of carbon electrodes that are suitable for electrochemical applications are available. Among these, glassy carbon (GC) and carbon paste (CP) are the most popular electrodes.1-3 The common carbon paste electrodes (CPE) usually employ paraffin oils as nonelectrolytic binders. These nonpolar pasting liquids fulfill some of the important criteria for a suitable electrode, such as chemical inertness, insulating property, nonvolatility, and water immiscibility.13-15 Another successful class of binders comprising liquid organophosphates has the attractive property of high ion-pairing ability.16-19 However, they suffer from less stability and rather atypical signal-to-noise characteristics requiring special pretreatment. A robust (solid phase) carbon paste electrode has been introduced by preparing the electrode using melted phenanthrene mixed with graphite. The mixture was then left to harden, and after packing in a holder, the paste was modified in situ.20 In recent years, clay-modified electrodes have been introduced. In these electrodes, clays are mixed with carbon paste to enhance the adsorptive and ion-exchange properties of the electrode.21-25 To improve the ion-pairing property of the CP electrodes, they have also been modified by the addition of lipophilic chains of surfactants. However, these electrodes could only be used by (7) Rubianes, M. D.; Rivas, G. A. Electrochem. Commun. 2003, 5, 689-694. (8) Valentini, F.; Amine, A.; Orlanducci, S.; Terranova, M. L.; Palleschi. G. Anal. Chem. 2003, 75, 5413-5421. (9) Antiochia, R.; Lavagnini, I.; Magno, F.; Valentini, F.; Palleschib. G. Electroanalysis 2004, 16, 1451-1458. (10) Wang, J.; Musameh, M. Anal. Chem. 2003, 75, 2075-2079. (11) Chicharro, M.; Sanchez, A.; Bermejo, E.; Zapardiel, A.; Rubianes, M. D.; Rivas. G. A. Anal. Chim. Acta 2005, 543, 84-91. (12) Arai, S.; Endo, M. Electrochem. Commun. 2003, 5, 797-799. (13) Adams, R. N. Anal. Chem. 1958, 30, 1576-1576. (14) Rice, M. E.; Galus, Z.; Adams, R. N. J. Electroanal. Chem. 1983, 143, 89102. (15) Svancara, I.; Vytras, K.; Barek, J.; Zima. J. Crit. Rev. Anal. Chem. 2001, 31, 311-345. (16) Kalcher, K. Electroanalysis 1990, 2, 419-433. (17) Kalcher, K.; Kauffmann, J.-M.; Wang, J.; Svancara, I.; Vytras, K.; Neuhold, C.; Yang, Z. Electroanalysis 1995, 7, 5-22. (18) Svancara, I.; Kalcher, K.; Diewald, W.; Vytras, K. Electroanalysis 1996, 8, 336-342.. (19) Svancara, I.; Konvalina, J.; Schachl, K.; Kalcher, K.; Vytras, K. Electroanalysis 1998, 10, 435. (20) Diewald, W.; Kalcher, K.; Neuhold, C.; Svancara, I.; Cai, X. Analyst 1994, 119, 299-304. (21) Kalcher, K.; Grabec, I.; Raber, G.; Cai, X.-H.; Tavcar, G.; Ogorevc, B. J. Electroanal. Chem. 1995, 386, 149-156. (22) Kula, P.; Navratilova, Z. Fresenius J. Anal. Chem. 1996, 354, 692. (23) Walcarius, A. Anal. Chim. Acta 1999, 388, 79-91. (24) Zen, J. M.; Senthil Kumar, A. Anal. Chem. 2004, 205A-211A.. (25) Kilinc Alpat, S.; Yuksel, U.; Akcay, H. Electrochem. Comm. 2005, 7, 130134. 10.1021/ac060070+ CCC: $33.50
© 2006 American Chemical Society Published on Web 04/26/2006
applying in situ modification, since the surfactants are readily soluble in aqueous media.26-29 The ability of carbon nanotube-modified electrodes to promote electron-transfer reactions and to offer resistance to surface fouling has been documented in connection with important biomolecules.30-33 Recently, definitive evidence has been provided for the reactive sites of multiwalled carbon nanotubes as residing in electron transfer from edge planelike sites, which occurs at defects in the carbon nanotubes and at the end of nanotubes.34-36 In a recent report, Compton et al. explained the observed improvement in electrochemistry and low susceptibility to electrode fouling at carbon nanotubes to be the result of the presence of high edge plane density on carbon nanotubes.36 This led Compton et al. to propose the use of edge plane pyrolytic graphite as an electrode substrate for electroanalysis.36 The use of ionic liquids (IL) for electrode preparation is rather limited. They are mainly concerned with either in situ deposition of ionic liquid during the oxidation of neutral organic liquid microdroplets 37,38 or modification of the pyrolytic graphite electrode surface with deposition of a microdroplet or thin layer of 1-methyl-3-(2,6-(S)-dimethylocten-2-yl)imidazolium tetrafuoroborate ionic liquid.39 A recent report also explained the electrochemistry of dopamine on a glassy carbon electrode modified by a gel containing multiwalled carbon nanotubes and ionic liquid.40 While this paper was under preparation, a preliminary report on construction of a carbon paste electrode with an immidazoliumbased ionic liquid appeared in the literature,41 in which it was shown that the presence of ionic liquid causes an increase in sensitivity of the response toward potassium ferricyanide used as a probe. However, the very high background of this electrode greatly limits its application. To improve the electrochemical performance, the electrode was bulk-modified via dispersion of Keggin-type phosphopolyoxomolybdate, which acts as an electrocatalyst, but this modification did not improve the high background of the electrode. (26) Digua, K.; Kauffmann, J.-M.; Delplancke, J.-L. Electroanalysis 1994, 6, 451458. (27) Digua, K.; Kauffmann, J.-M.; Delplancke, J.-L. Electroanalysis 1994, 6, 459462. (28) Svancara, I.; Foret, P.; Vytras K. Talanta 2004, 64, 844-852. (29) Hattori, T.; Nakayama, M. Electroanalysis 2005, 17, 613-618. (30) Musameh, M.; Wang, J.; Merkoci, A.; Lin, Y. Electrochem. Commun. 2002, 4, 743-746. (31) Wang, J.; Li, M.; Shi, Z.; Li, N.; Gu, Z. Anal. Chem. 2002, 74, 1993-1997. (32) Yu, X.; Chattapadhyay, D.; Galeska, I.; Papdimitrakopoulos, F.; Rusling, J. F. Electrochem. Commun. 2003, 5, 408-411. (33) Gooding, J. J.; Wibowo, R.; Liu, J.; Yang, W.; Losic, D.; Orbons, S.; Mearns, F.; Shapter, J.; Hibbert, B. D. J. Am. Chem. Soc. 2003, 125, 9006-9007. (34) Nugent, J. M.; Santhanam, K. S. V.; Rubio, A.; Ajayan, P. M. Nano Lett. 2001, 1, 87-91. (35) Banks, C. E.; Moore, R. R.; Davies, T. J.; Compton, R. G. Chem. Commun. 2004, 16, 1804-1805. (36) Banks, C. E.; Compton, R. G. Analyst 2005. 130, 1232-1239. (37) Marken, F.; Webster, R. D.; Bull, S. D.; Davies, S. G. J. Electroanal. Chem. 1997, 437, 209-216 (38) Marken, F.; Compton, R. G.; Goeting, C. H.; Foord, G. S.; Bull, S. D.; Davies, S. G. Electroanalysis 1998, 10, 821-826. (39) Wadhawan, J. D.; Schro ¨der, U.; Neudeck, A.; Wilkins, S. J.; Compton, R. G.; Marken, F.; Consorti, C. S.; de Souza, R. F.; Dupont, J. J. Electroanal. Chem. 2000, 493, 75-83. (40) Zhao, Z.; Gao, Y.; Zhan, D.; Liu, H.; Zhao, O.; Kou, Y.; Shao, Y.; Li, M.; Zhuang, Q.; Zhu, Z. Talanta 2005, 66, 51-57 (41) Liu, H.; He, P.; Li, H.; Sun. C.; Shi, L.; Liu, Y.; Zhu, G.; Li, J. Electrochem. Commun. 2005, 7, 1357-1363.
In the present paper, a new strategy for the fabrication of a novel carbon ionic liquid electrode (CILE) based on the use of the pyridinium-based ionic liquid, n-octylpyridinum hexafluorophosphate, as binder is explained, which not only provides very low background comparable to common carbon paste electrodes but also shows surprisingly high electrochemical performance. The electrode has a combination of many good features reported for previous variations of CPE, carbon nanotubes, and edge plane pyrolytic graphite electrodes all in one electrode. One of the very important and potent characteristics of the proposed electrode is its ability to lower the overpotential of electroactive compounds, particularly biomolecules, and to increase the rate of electrontransfer processes, which is believed to be due to the modification of the microstructure of the electrode surface by using ionic liquid as the binder. EXPERIMENTAL SECTION Reagents. 1-Methylimidazole, 1-idooctane, pyridine, diethyl ether, and paraffin oil were obtained from Merck. Ammonium hexafluorophosphate and graphite powder (particle size 50%OPFP CILE > GCE > CPE. Wettability of several electrodes was measured as approximate wetting angle and is shown in Table 1. The wetting angle of CILE (45) Fukushima, T.; Kosaka, A.; Ishimura, Y.; Yamamoto, T.; Takigawa, T.; Ishii, N.; Aida, T. Science 2003, 300, 2072-2074. (46) Zhao, Q.; Zhan, D.; Ma, H.; Zhang, M.; Zhao, Y.; Jing, P.; Zhu, Z.; Wan, X.; Shao, Y.; Zhuang, Q. Front. Biosci. 2005, 10, 326.
is the least showing good adherence of water to the electrode. This is expected as water adheres much better to the surface of ionic OPFP than nonionic organic materials such as paraffin oil in CPE. The CILE displays a broad potential window and a low background current. The anodic limit is extended to ∼1.3 V (vs Ag/AgCl) in phosphate buffer, HCl, KCl, and PF6- electrolytes, while the cathodic limit is at -1.0 V (vs Ag/AgCl). Moreover, the current density at CILE is higher than on CPE and even GCE. The high wettability of CILE could be responsible for obtaining higher sensitivity on CILE. It is generally believed that the addition of any pasting liquid decreases the electron-transfer rates as compared to the dry carbon limit.14 However, as will be described later, the presence of OPFP as the pasting liquid in CILE causes a great improvement in the rate of heterogeneous electron-transfer processes. To test the applicability of the electrode, different categories of compounds such as Fe(CN)63-/4-, Feaq3+/2+, dopamine, catechol, ascorbic acid, and NADH were tested. These redox systems were selected according to their known sensitivity or nonsensitivity toward the electronic properties, electrode surface microstructure, and surface chemistry. 47-52 The voltammetric data have been summarized in Table 2. Figures S3-S8 (Supporting Information) also show cyclic voltammograms for Fe(CN)63-/4-, ascorbic acid, Fe 3+/2+, dopamine, catechol, and NADH, respectively, at a CILE with 50% graphite/ OFPF ratio, CPE, GCE, and Pt electrodes. The results in Table 2 show the superiority of CILE to CPE, GCE, and Pt electrodes in terms of both provision of better reversibility and higher sensitivity. The oxidation peak current for each analyte on CILE (50% graphite/OFPF ratio) varied linearly with (scan rate)1/2 from 50 to 500 mV/s (r > 0.998) indicating semi-infinite linear diffusion of the reactant to the interface. To avoid ion-pairing and ion-exchange property in CILE, the studies were performed in the presence of the common ion PF6-. However, if it is desired to have an ion-exchange or ion-pairing property, the experiments can be performed in an electrolyte free from PF6-(cf. Table 2 for the results). To show the wide applicability and high electrochemical performance of CILE, the electrochemical behavior of different systems is described in detail. (47) Kneten, K. R.; McCreery, R. L. Anal. Chem. 1992, 64, 2518-2524. (48) Chen, P.; Fryling, M. A.; McCreery, R. L. Anal. Chem. 1995, 67, 31153122 (49) Chen, P.; McCreery, R. L. Anal. Chem. 1996, 68, 3958-3965. (50) Ranganathan, S.; Kuo, T.-C.; McCreery, R. L. Anal. Chem. 1999, 71, 35743580. (51) Yang, H.-H.; McCreery, R. L. Anal. Chem. 1999, 71, 4081-4087. (52) Hunt-DuVall, S.; McCreery, R. L. Anal. Chem. 1999, 71, 4594-4602.
Analytical Chemistry, Vol. 78, No. 11, June 1, 2006
3823
Table 2. Comparison of CILE Response with Pt, CP, and GCa ∆Ep (mV) (RSD) Ip (µA/cm2) (RSD)
CPE GCE Pt CILE (PF6- 0.05 M) CILE (no PF6- ) a
Ep (mV) (RSD) Ip (µA/cm2) (RSD)
Fe(CN)6 3-/4-
dopamine
catechol
ascorbic acid
NADH
235(2.65) 92.5(2.87) 68(3.15) 159.4(3.46) 64(2.83) 171.6 (3.13) 66(2.92) 163.5(3.63) 61(2.85) 169.3(3.24)
140(3.02) 117.6(3.45)
295(2.35) 223.2(2.73)
150(3.47) 114.4(3.62) 90(3.47) 124.3(3.58) 76(2.47) 148.8(3.17)
308(3.52) 169.6 (3.72) 135(3.17) 373.2(3.42) 74(2.94) 425.2(3.21)
430(2.84) 112.6(3.27) 380(3.66) 142.5(3.84) 410(3.38) 38.2 (3.65) 210 (3.73) 148.4(3.88) 90 (3.16) 185.4(3.32)
495(1.83) 115.3(2.17) 574(2.25) 101.4(2.74) 720(2.57) 88.2(2.71) 410(2.46) 118.0(2.67) 387(1.98) 124.0(2.24)
Cyclic voltammograms were recorded at 100 mV s-1 except for NADH, which was at 50 mV s-1.
Fe(CN)63-/4- behaves anomalously on carbon and metal electrodes and does not involve simple electron transfer in which the electrode acts as a source or sink for electrons.53 It has been documented that, at carbon-based electrodes, the redox process of the Fe(CN)63-/Fe(CN)64- redox couple is a typical inner-sphere reaction in which the electron-transfer kinetics is generally determined by several factors including tunneling and electrostatic and electrocatalytic effects.1,49 The observed improved electrontransfer kinetics at the CILE electrode, which is even much better than on some reported carbon nanotubes,8,9 is likely associated with the interaction (e.g., electrostatic interaction) between the Fe(CN)63-/Fe(CN)64- inner-sphere redox couple and the pyridinium cation at the CILE electrode. To the best of our knowledge, this is the first composite carbon electrode with a response similar to that of platinum electrode toward the Fe(CN)64-/Fe(CN)63system. AA oxidation is chemically irreversible so Ep,ox rather than ∆Ep is reflective of the electrode kinetics. As can be seen from the comparison between CPE and CILE (in the presence of PF6-), a decrease in the oxidation peak potential of ∼200 mV was obtained for AA at CILE. This decrease was even much more pronounced (∼340 mV) in the absence of PF6- in the electrolyte. McCreery et al.1,47 have demonstrated that AA oxidation is an inner-sphere reaction with electron-transfer kinetics sensitive to the electrode surface. In neutral media, AA is negatively charged. Thus, the enhanced electron-transfer kinetics at the CILE electrodes could be considered to be a consequence of the electrostatic interaction of the cationic OPFP films present on the electrode. As has already been demonstrated for other carbon electrodes, the state of the surface of the electrode plays an important role in the electrode kinetics. In another study, Feaq3+/2+ was selected as a system sensitive to surface oxides. The untreated CILE already shows a higher degree of reversibility toward Feaq3+/2+ compared to CPE. For treatment of CILE, an activation procedure was performed by holding the electrode fresh surface at an anodizing potential of 1.2 V for 10 min and then the cyclic voltammogram was initiated in the oxidation direction. If electron transfer for a particular redox system is dependent on an oxygencontaining surface functional group, its kinetics should be faster if the surface oxides/graphite ratio is increased. Previously it had (53) Rao, T. N.; Yagi, I.; Miwa, T.; Tryk, D. A.; Fujishima, A. Anal. Chem. 1999, 71, 2506-2511
3824 Analytical Chemistry, Vol. 78, No. 11, June 1, 2006
Figure 4. Cyclic voltammograms of 5 mM Fe3+/2+ in 0.1 M H2SO4 at (a) CPE after 10-min anodizing at 1.2 V, (b) Pt, and (c) CILE after 10-min anodizing at 1.2 V. Scan rate 100 mV s-1.
been indicated that Feaq3+/2+ exhibits these characteristics.49 A significant difference between cyclic voltammograms of Feaq3+/2+ with CILE and CPE was observed upon anodizing the electrodes (Figure 4). In fact, with anodizing CILE at a constant potential of 1.3 V, the response to Feaq3+/2+ is highly improved. This result shows that the CILE surface is much more susceptible to oxidation upon anodization than the CPE surface. In the case of dopamine and catechol compounds that display a quasi-reversible behavior at a CPE and Pt electrode, a dramatic improvement in the reversibility was found at the CILE. The ∆Ep for dopamine decreased from 140 mV on CPE and 150 mV on platinum electrodes to 90 mV and that of catechol decreased from 295 mV (on CPE) to 135 mV. The electrochemical reversibility even improved in the absence of PF6- in the electrolyte. These findings demonstrate that the presence of OPFP provides an excellent electrochemical reactivity. Studies on NADH oxidation with CILE show a decrease in overvoltage of NADH oxidation to ∼0.4 V versus Ag/AgCl reference electrode (Figure S8, Supporting Information). The shape and position of this peak is in very good agreement with the responses obtained by other workers on edge plane pyrolytic graphite 36 and film modified multiwalled carbon nanotubes GCE.30 This could be a good evidence for the beneficial role of OPFP as a suitable charge-transfer bridge, residing on graphite particles.
Table 3. Comparison of the Electrochemical Reactivity of CILE with Previously Reported Electrodes ∆Ep (mV)
Ep (mV)
electrode
Fe(CN)63-/4-
dopamine
catechol
ascorbic acid
CPE GCPE a,55 MWCNTPEb,7 SWCNTPEc,8 BBDd,56 GCE Pt EPPGe,36 CILE (PF6- 0.05 M) CILE (no PF6-)
235 95
140
295 287
430
90 71 68 64 66 61
100 164 480 150
95 308
210 404 736 380 410
90 76
135 74
210 90
239
NADH 495 546 574 720 ∼360 410 387
a Glassy carbon paste. b Multiwall carbon nanotube paste electrode. c Single-wall carbon nanotube electrode. d Boron-doped polycrystalline diamond thin-film electrode. e Edge plane pyrolytic graphite.
Therefore, it can be concluded that the main reason for the dramatic improved electrochemical properties of CILE compared to CPE is the result of the replacement of a nonconducting paraffin oil layer on graphite that is blocking access to the graphite electrocatalytic sites, with the OPFP layer, which not only acts favorably toward some electrochemical systems (such as Fe(CN)64-/Fe(CN)63-, ascorbic acid, dopamine, and catechol) but shows very good electrochemical reactivity toward surfacesensitive systems such as NADH and Feaq3+/2+. Further studies are desired for elucidating the structure-reactivity relation of the new CILE material. We are currently performing more studies in order to address the electron-transfer mechanism in CILE.
Figure 5. Cyclic voltammogram of NADH (a) before and (b) after leaving CILE for 30 min in phosphate buffer (pH 7.4) containing NADH (1.2 mM) and PF6- (0.05 M).
The CILE displays good resistance to surface fouling common to NADH oxidation, thus imparting high stability onto NADH measurements. The voltammetric response of the CILE electrode before and after being left in a 1.2 mM solution of NADH for 30 min was investigated. The results are depicted in Figure 5. As is obvious, CILE offers a highly stable signal over the entire operation (RSD ) 5%). In contrast, the CPE and GCEs display a rapid loss of their activity upon being left in NADH solution (not shown). The decrease in peak height and potential shift to more positive values for CPE and GCEs is indicative of adsorption phenomena, which has been widely reported to happen at glassy carbon substrates.30 Such minimization of NADH passivation effects is in agreement with early observation at carbon nanotube electrodes.30,54 The response at the CILE was very similar to the edge plane pyrolytic graphite reported by Compton et al.36 showing similar Ep values with only a relatively negligible decrease in peak height. This could be due to the presence of a layer of OPFP on graphite, which acts as a protective layer promoting the resistance to passivation. (54) Wang, J.; Deo, R. P.; Poulin, P.; Mangey, M. J. Am. Chem. Soc. 2003, 125, 14706-14707. (55) Wang, J.; Kirgoz, U. A.; Mo, J. W.; Lu, J.; Kawad, A. N.; Muck. A. Electrochem. Commun. 2001, 3, 203-208. (56) Granger, M. G.; Witek, M.; Xu, J.; Wang, J.; Hupert, M.; Hanks, A.; Koppang, M. D.; Butler, J. E.; Lucazeau, G.; Mermoux, M.; Strojek, J. W.; Swain, G. M. Anal. Chem. 2000, 72, 3793-3804.
CONCLUSION The very interesting feature of CILE is that it provides very promising signals for different categories of electroactive compounds and biomolecules. This behavior is very important because common electrode modifications usually result in improvement of reactivity of the electrode toward a certain type or group of compounds. Table 3 shows a comparison between CILE performance and some important previously reported electrodes. As could be inferred from this table, CILE displays very promising electrochemical reactivity toward different compounds. It combines many good features of the electrochemical properties of different types of carbon electrodes such as GCE, carbon nanotubes, and edge plane pyrolytic graphite with the various advantages of carbon paste electrodes such as low cost, low background, simplicity of preparation, surface modification, and renewal. The very favorable electrochemical response, higher reversibility, sensitivity, and selectivity observed for CILE toward biomolecules together with its resistance to electrode fouling make it a good candidate for construction of biosensors. Large potential window, low resistance, high wettability, high sensitivity, low background, controllable ion-pairing and extraction properties, rigidity, stability, resistance to fouling, favorable electrode surface morphology, ease of preparation, renewal, miniaturization, and low cost are among other advantages of this electrode. Due to its mechanical strength, CILE can be used as an effective flow-through detector for flowing streams. Since the mixture of melted OPFP with graphite could be easily shaped in a mold, fabrication of different shapes and sizes of electrodes is Analytical Chemistry, Vol. 78, No. 11, June 1, 2006
3825
quite feasible. Current efforts in our research group are also aimed at miniaturizing this electrode. ACKNOWLEDGMENT The authors acknowledge the support of this work by the Iranian Ministry of Science, Research and Technology. SUPPORTING INFORMATION AVAILABLE Effect of heating CILE on the response of Fe(CN)63-/4- and NADH, cyclic voltammograms for Fe(CN)63-/4-, ascorbic acid, Fe
3826
Analytical Chemistry, Vol. 78, No. 11, June 1, 2006
3+/2+,
dopamine, catechol, and NADH, respectively, at a CILE with 50% graphite/OFPF ratio (in the absence and presence of PF6-), CPE, GCE. and Pt electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.
Received for review January 10, 2006. Accepted March 23, 2006. AC060070+
