Ferrocene Peapod Modified Electrodes - American Chemical Society

Aug 8, 2006 - Preparation, Characterization, and Mediation of. H2O2. Nijuan Sun, Lunhui Guan, Zhujin Shi, Nanqiang Li, Zhennan Gu, Zhiwei Zhu, Meixian...
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Ferrocene Peapod Modified Electrodes: Preparation, Characterization, and Mediation of H2O2 Nijuan Sun, Lunhui Guan, Zhujin Shi, Nanqiang Li, Zhennan Gu, Zhiwei Zhu, Meixian Li,* and Yuanhua Shao

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

Electrochemical properties of a new nanomaterial ferrocene (Fc) peapod, Fc-filled single-walled carbon nanotubes (Fc@SWNTs), have been investigated in an aqueous solution in detail by preparing different kinds of Fc@SWNTsmodified glassy carbon electrodes (Fc@SWNTs/GCE and Fc@SWNTs-gel/GCE). One pair of surface-confined redox waves corresponding to the couple of Fc/Fc+ is obtained, which indicates that Fc encapsulated inside SWNTs retains electrochemical activity. The Fc@SWNTs-gel/GCE shows better electrochemical reversibility due to the existence of room temperature ionic liquid (RTIL). Furthermore, it shows excellent mediation of H2O2 based on Fc/Fc+ used as electron-transfer mediators for oxidation of H2O2 to O2 and reduction to H2O, suggesting specific properties of Fc@SWNTs due to a combination of Fc and SWNTs. The interaction between Fc and SWNTs is also characterized by UV-vis-NIR spectrometry and Raman spectrometry. A Fc@SWNTs-based sensor for H2O2 with a determination limit of 5 µM is fabricated, and it shows good stability and reproducibility. This work not only demonstrates that the Fc peapod is a new kind of functional nanomaterial but also appears promising in constructing novel chemical and biosensors and fuel cells. Carbon nanotubes (CNTs) are a novel form of metallic or semiconducting nanowires. Their impressive mechanical and electronic properties have opened the way for the development of new nanotechnologies ranging from high-strength composite materials to field emission devices.1-5 Meanwhile, they are also suitable candidates for fabrication of electrochemical sensing devices and biosensors because they can promote electron transfer (ET) of some electroactive substances.6-11 Our previous study has * To whom correspondence should be addressed. Tel: +86-10-62757953. Fax: +86-10-62751708. E-mail: [email protected]. (1) Ajayan, P. M. Chem. Rev. 1999, 99, 1787-1799. (2) Ajayan, P. M.; Charlier, J. C.; Rinzler, A. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 14199-14200. (3) Rao, C. N. R.; Satishkumar, B. C.; Govindaraj, A.; Nath, M. ChemPhysChem 2001, 2, 78-105. (4) Sun, Y. P.; Fu, K. F.; Lin, Y.; Huang, W. J. Acc. Chem. Res. 2002, 35, 10961104. (5) Zhao, J. J.; Chen, Z. F.; Zhou, Z.; Park, H.; Schleyer, P. V.; Lu, J. P. ChemPhysChem 2005, 6, 598-601. (6) Gooding, J. J.; Wibowo, R.; Liu, J. Q.; Yang, W. R.; Losic, D.; Orbons, S.; Mearns, F. J.; Shapter, J. G.; Hibbert, D. B. J. Am. Chem. Soc. 2003, 125, 9006-9007.

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demonstrated that single-walled carbon nanotube (SWNT)-modified electrodes prepared by casting method have a good preference toward oxidation or reduction of some biomolecules, such as dopamine, cytochrome c, and DNA12-15 because the high-aspect ratios of the tubes may present a steric effect for more efficient redox reactions of biomolecules. Hence, CNTs have been used widely in the construction of electrochemical sensors to improve responses of some electroactive substances. So far, there have been several methods in addition to casting,12-17 which were developed to prepare CNT-modified electrodes; for example, selfassembled;18-21 sol-gel;22-26 and employing polymers,27-31 bio(7) Wang, J.; Deo, R. P.; Poulin, P.; Mangey, M. J. Am. Chem. Soc. 2003, 125, 14706-14707. (8) Katz, E.; Willner, I. ChemPhysChem 2004, 5, 1085-1104. (9) Guldi, D. M.; Rahman, G. M. A.; Jux, N.; Tagmatarchis, N.; Prato, M. Angew. Chem., Int. Ed. 2004, 43, 5526-5530. (10) Guldi, D. M.; Rahman, G. M. A.; Zerbetto, F.; Prato, M. Acc. Chem. Res. 2005, 38, 871-878. (11) Wang, J. Electroanalysis 2005, 17, 7-14. (12) Luo, H. X.; Shi, Z. J.; Li, N. Q.; Gu, Z. N.; Zhuang, Q. K. Anal. Chem. 2001, 73, 915-920. (13) Wang, Q.; Jiang, N.; Li, N. Q. Microchem. J. 2001, 68, 77-85. (14) Wang, J. X.; Li, M. X.; Shi, Z. J.; Li, N. Q.; Gu, Z. N. Anal. Chem. 2002, 74, 1993-1997. (15) Wang, J. X.; Li, M. X.; Shi, Z. J.; Li, N. Q.; Gu, Z. N. Electroanalysis 2004, 16, 140-144. (16) Gong, K. P.; Zhu, X. Z.; Zhao, R.; Xiong, S. X.; Mao, L. Q.; Chen, C. F. Anal. Chem. 2005, 77, 8158-8165. (17) Chen, J.; Du, D.; Yan, F.; Ju, H. M.; Lian, H. Z. Chem.sEur. J. 2005, 11, 1467-1472. (18) Burghard, M.; Duesberg, G.; Philipp, G.; Muster, J.; Roth, S. Adv. Mater.1998, 10, 584-588. (19) Patolsky, F.; Weizmann, Y.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 2113-2117. (20) Diao, P.; Liu, Z. F. J. Phys. Chem. B 2005, 109, 20906-20913. (21) Song, C.; Xia, Y. Y.; Zhao, M. W.; Liu, X. D.; Huang, B. D.; Li, F.; Ji, Y. J. Phys. Rev. B 2005, 72, 165430. (22) Gong, K. P.; Zhang, M. N.; Yan, Y. M.; Su, L.; Mao, L. Q.; Xiong, Z. X.; Chen, Y. Anal. Chem. 2004, 76, 6500-6505. (23) Gavalas, V. G.; Law, S. A.; Ball, J. C.; Andrews, R.; Bachas, L. G. Anal. Biochem. 2004, 329, 247-252. (24) Shi, Q. C.; Peng, T. Z.; Zhu, Y. N.; Yang, C. F. Electroanalysis 2005, 17, 857-861. (25) Tan, X. C.; Ll, M. J.; Cai, P. X.; Luo, L. J.; Zou, X. Y. Anal. Biochem. 2005, 337, 111-120. (26) Yang, M. H.; Yang, Y. H.; Liu, Y. L.; Shen, G. L.; Yu, R. Q. Biosens. Bioelectron. 2006, 21, 1125-1131. (27) Zhang, X. F.; Liu, T.; Sreekumar, T. V.; Kumar, S.; Moore, V. C.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2003, 3, 1285-1288. (28) Wang, J.; Musameh, M. Anal. Chem. 2003, 75, 2075-2079. (29) Wang, J.; Dai, J. H.; Yarlagadda, T. Langmuir 2005, 21, 9-12. (30) Qin, S. H.; Qin, D. Q.; Ford, W. T.; Zhang, Y. J.; Kotov, N. A. Chem. Mater. 2005, 17, 2131-2135. 10.1021/ac060396i CCC: $33.50

© 2006 American Chemical Society Published on Web 08/08/2006

macromolecules,32-35 or nanoparticles36-39 to form nanocomposites with CNTs. Recently, a novel preparation method for chemically modified electrodes was proposed; namely, rubbing a kind of gel40-43 onto the surface of an electrode. This type of gel can be formed by grinding CNTs and room temperature ionic liquid (RTIL) together. Clearly, quite a number of applications of CNTs in electrochemistry have been reported in the past few years. In addition, as a consequence of the unique hollow structure of CNTs, their endo functionalization has attracted much more attention lately.44 It is fascinating to insert small molecules into the cavities of CNTs to modulate their properties and to develop new types of nanomaterials. In 1998, Luzzi et al. first reported that fullerenes (C60) could be trapped inside CNTs.45 Since then, various species (including atoms, molecules, and chemical compounds) have been successfully encapsulated into SWNTs.46-50 Recently, we have succeeded in introducing ferrocene (Fc) into the inner hollow spaces of SWNTs to obtain a new type of hostguest supramolecular system, ferrocene peapods (Fc@SWNTs).51 That is, Fc is the pea and SWNT is the pod. As is well-known, Fc can act as a good ET mediator in certain electrocatalytic reactions, and SWNTs can serve as an excellent transducer in promoting ET of some electroactive substances. Thus, the Fc@SWNTs, resulting from combination of Fc and SWNTs, are expected to have possible applications in the fabrication of nanoelectronic devices, such as chemical and biosensors. In this work, the Fc@SWNT-based modified electrodes are prepared by using two methods, casting (Fc@SWNTs/GCE) and rubbing (Fc@SWNTs-gel/GCE), and their electrochemical prop(31) Joshi, P. P.; Merchant, S. A.; Wang, Y. D.; Schmidtke, D. W. Anal. Chem. 2005, 77, 3183-3188. (32) Ortiz-Acevedo, A.; Xie, H.; Zorbas, V.; Sampson, W. M.; Dalton, A. B.; Baughman, R. H.; Draper, R. K.; Musselman, I. H.; Dieckmann, G. R. J. Am. Chem. Soc. 2005, 127, 9512-9517. (33) Hobbie, E. K.; Bauer, B. J.; Stephens, J.; Becker, M. L.; McGuiggan, P.; Hudson, S. D.; Wang, H. Langmuir 2005, 21, 10284-10287. (34) Xie, H.; Ortiz-Acevedo, A.; Zorbas, V.; Baughman, R. H.; Draper, R. K.; Musselman, I. H.; Dalton, A. B.; Dieckmann, G. R. J. Mater. Chem. 2005, 15, 1734-1741. (35) Chen, C. H.; Li, H.; Zhu, W.; Zhang, Q. X. Acta Phys.-Chim. Sin. 2005, 21, 1067-1072. (36) Chen, W. X.; Lee, J. Y.; Liu, Z. L. Electrochem. Commun. 2002, 4, 260265. (37) He, Z. B.; Chen, J. H.; Liu, D. Y.; Tang, H.; Deng, W.; Kuang, W. F. Mater. Chem. Phys. 2004, 85, 396-401. (38) Hrapovic, S.; Liu, Y. L.; Male, K. B.; Luong, J. H. T. Anal. Chem. 2004, 76, 1083-1088. (39) Wiyaratn, W.; Hrapovic, S.; Liu, Y. L.; Surareungchai, W.; Luong, J. H. T. Anal. Chem. 2005, 77, 5742-5749. (40) Zhao, F.; Wu, X.; Wang, M. K.; Liu, Y.; Gao, L. X.; Dong, S. J. Anal. Chem. 2004, 76, 4960-4967. (41) Zhao, Q.; Zhan, D. P.; Ma, H. Y.; Zhang, M. Q.; Zhao, Y. F.; Jing, P.; Zhu, Z. W.; Wan, X. H.; Shao, Y. H.; Zhuang, Q. K. Front. Biosci. 2005, 10, 326334. (42) Zhang, Y. J.; Shen, Y. F.; Li, J. H.; Niu, L.; Dong, S. J.; Ivaska, A. Langmuir 2005, 21, 4797-4800. (43) Zhao, Y. F.; Gao, Y. Q.; Zhan, D. P.; Liu, H.; Zhao, Q.; Kou, Y.; Shao, Y. H.; Li, M. X.; Zhuang, Q. K.; Zhu, Z. W. Talanta 2005, 66, 51-57. (44) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853-1859. (45) Smith, B. W.; Monthioux, M.; Luzzi, D. E. Nature 1998, 396, 323-324. (46) Jeong, G. H.; Hirata, T.; Hatakeyama, R.; Tohji, K.; Motomiya, K. Carbon 2002, 40, 2247-2253. (47) Monthioux, M. Carbon 2002, 40, 1809-1823. (48) Satishkumar, B. C.; Taubert, A.; Luzzi, D. E. J. Nanosci. Nanotechnol. 2003, 3, 159-163. (49) Vostrowsky, O.; Hirsch, A. Angew. Chem., Int. Ed. 2004, 43, 2326-2329. (50) Chikkannanavar, S. B.; Luzzi, D. E.; Paulson, S.; Johnson, A. T. Nano Lett. 2005, 5, 151-155. (51) Guan, L. H.; Shi, Z. J.; Li, M. X.; Gu, Z. N. Carbon 2005, 43, 2780-2785.

erties are characterized in aqueous solutions. The experimental results show that the reversibility of the Fc@SWNTs-gel/GCE is better than that of the Fc@SWNTs/GCE due to the existence of RTIL. Due to the interaction between Fc and SWNTs in Fc@SWNTs, Fc@SWNT modified on the GCE is more stable than Fc adsorbed onto the outside of SWNTs modified on the GCE. The interaction has also been characterized by UV-vis-NIR spectrometry and Raman spectrometry. Furthermore, mediation of some small molecules by Fc@SWNTs has also been investigated, especially for H2O2. The Fc@SWNTs-gel modified electrode shows excellent mediation for the oxidation and reduction of H2O2, and the possible mechanism is proposed. We have further constructed a H2O2 sensor based on the Fc@SWNTs, and the performance of the sensor with respect to sensitivity, linear range, and response time is presented and discussed. EXPERIMENTAL SECTION Chemicals. The ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6) and ferrocene (Fc) were from Acros. Tetrabutylammonium perchlorate (Bu4NClO4) was obtained from Fluka. Lithium perchlorate (LiClO4), potassium dihydrogenphosphate (KH2PO4), disodium hydrogenphospate (Na2HPO4), tetrabutylammonium bromide (Bu4NBr), and N,N-dimethylformamide (DMF), as well as other reagents that were purchased from Beijing Chemical Co., China, were all analytical grade or better. All reagents were used without further purification. Doubly distilled water was further purified with a quartz apparatus. Aqueous solutions were prepared with triply distilled water. Preparation of Fc@SWNTs. Fc@SWNTs were synthesized and purified as described previously.51 SWNTs were synthesized first by the dc arc discharge method with YNi2 as catalyst and then purified.52 Because the caps of SWNTs were burned out during the process of purification and defects were increased on the wall, therefore, the purified SWNTs already had a sufficient number of entrance holes for encapsulating. Fc was inserted into the SWNTs by the vapor diffusion method. To remove Fc coated on the SWNT surface, the SWNT was washed with diethyl ether by sonication several times until the solvent was colorless. Preparation of the Modified Electrodes. Two methods have been employed to prepare the Fc@SWNT-modified electrodes. The first one is the casting method in which 3 mg of Fc@SWNTs was dispersed in 3 mL of DMF to obtain a uniform suspension of ∼1 mg mL-1 by sonication for ∼30 min. Glassy carbon (GC) electrodes were first polished with 0.05-µm alumina slurry and then washed ultrasonically in triply distilled water and ethanol for a few minutes. The GC electrodes were coated by casting 10 µL of the above Fc@SWNTs suspension and dried under an infrared lamp. This modified electrode is denoted as Fc@SWNTs/ GCE. Similarly, a suspension of SWNTs in DMF (1 mg mL-1) and SWNT-modified GC electrodes (SWNTs/GCE) was also prepared by this method. To compare the differences between Fc@SWNTs and ferrocene adsorbed onto the outside of SWNTs, Fc/SWNTs modified onto a GC electrode were also prepared (denoted as Fc/SWNTs/ GCE). The procedure is that the pretreated GC electrode was modified by casting 20 µL of Fc (1 × 10-3 mol L-1) in acetonitrile (52) Li, H. J.; Feng, L.; Guan, L. H.; Shi, Z. J.; Gu, Z. N. Solid State Commun. 2004, 132, 219-224.

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and then 10 µL of SWNTs suspension in DMF (1 mg mL-1) and allowed to dry under an infrared lamp. Another way to fabricate a modified electrode is the rubbing method. On the basis of the reported procedure,53 1 mg of Fc@SWNTs was mixed with 10 µL BMIPF6 by grinding in an agate mortar for about 20 min to form a black gel. The Fc@SWNTs gel was placed on a smooth glass slide. A pretreated GC electrode was rubbed over the gel, and the gel was mechanically attached to the surface. Then the gel on the electrode surface was smoothed with a spatula to leave a thin gel film on the GC electrode surfaces; thus, the gel modified GC electrode (denoted as Fc@SWNTs-gel/GCE) was fabricated. Similarly, the SWNTsgel-modified electrodes (SWNTs-gel/GCE) were also prepared. Electrochemical and Other Measurements. A CHI 660A electrochemical workstation (CH Instruments) with a conventional three-electrode cell was used to perform electrochemical measurements. The working electrode was a glassy carbon electrode with a diameter of 4 mm. A KCl saturated calomel electrode was used as the reference electrode and a platinum electrode as the auxiliary electrode. All electrochemical experiments were conducted at the ambient temperature (20 ( 2 °C). Inductively coupled plasma atomic emission spectrometry (ICP-AES) measurement was performed on a Profile spectrometer (Leeman). High-resolution transmission electron microscope (HRTEM) images were obtained using a Hitachi-9000 NAR at an acceleration voltage of 100 kV. For HRTEM observation, Fc@SWNTs-gel was dispersed into triply distilled water by sonication for 1 h. Meanwhile, Fc@SWNTs were dispersed into DMF by sonication for 2 h and subsequently centrifugated, and then the suspension was preserved. A drop of the solution was then dropped on a holeycarbon film of 300-mesh copper grid and allowed to dry in air. Ultraviolet visible and near-infrared (UVvis-NIR) absorption spectra were recorded by a Shimadzu model UV-3100 spectrophotometer at 200-1300 nm after Fc@SWNTsgel and SWNTs-gel were spread on quartz sheets to form thin films. RTILs do not influence the properties of SWNTs, on the basis of reported literature.53 Raman spectra for Fc@SWNTs and SWNTs, which were planished on glass slides separately after being ground into powders, were recorded on a Renishaw 1000 micro-Raman system with an excitation wavelength of 632.8 nm.

Figure 1. Cyclic voltammograms of different modified electrodes in the aqueous solution containing 0.1 M LiClO4: SWNTs/GCE (a), Fc/SWNTs/GCE (b), and Fc@SWNTs/GCE (c). Scan rate is 0.05 mV/s.

RESULTS AND DISCUSSION Electrochemical Characterization of the Modified Electrodes. (1) Fc@SWNTs/GCE. The electrochemical behaviors of the Fc@SWNTs/GCE were investigated by cyclic voltammetry in the aqueous solution. LiClO4, LiCl, KCl, KH2PO4-Na2HPO4, and Bu4NBr were chosen as the respective supporting electrolyte. It was found that the modified electrode in the solution of LiClO4 has the largest current responses and the best stability. One possible reason is that the n-doping of SWNTs with Li+ as counterions may happen. The reversible insertion of Li+ into SWNTs proceeds up to a stoichiometry of Li1.23C6, resulting in great enhancement of the electrical conductivity of the SWNTs.54 In addition, anions of the supporting electrolytes may have an influence on the stability of the modified electrode. The modified

electrodes are more stable when the electrolyte is LiClO4, rather than LiCl, because the ferrocinium ion is unstable in chloride salts.55,56 The Fc@SWNTs/GCE shows one pair of redox peaks with E1/2 ) 283 mV vs SCE (Figure 1, curve c) in the aqueous solution containing 0.1 M LiClO4, indicating that SWNTs can act as molecular wires to allow electrical communication between the electrode and redox-active guest Fc encapsulated inside the SWNTs. Both oxidation and reduction peaks shift ∼60 mV in the positive direction in comparison with those of the Fc/SWNTs/ GCE (Figure 1, curve b). The separation between the anodic and cathodic peak potentials (∆Ep) for the Fc@SWNTs/GCE at a scan rate of 0.05 V s-1 is over 100 mV. Oxidation and reduction peak currents increase linearly with an increase in the scan rate in the rage of ∼0.01 to 1 V s-1, which shows the characteristic of surfaceconfined waves. It can be seen from Figure 1 that the total width at half-height of either the anodic or cathodic peak obtained for the Fc@SWNTs/GCE is ∼300 mV, much larger than the theoretical value of a reversible process (90.6 mV at 25 °C). This is probably due to the interactions of π-π stacking and van der Waals forces between the Fc themselves and between Fc and SWNTs. Similar results were obtained for fullerene peapodmodified electrodes in organic solvent.57 Unlike the Fc/SWNTs/ GCE electrode, whose peak currents decrease rapidly, the peak currents of the Fc@SWNTs/GCE decrease slowly during electrochemical scanning. This means that the electrochemical responses of the Fc@SWNTs/GCE are more stable. In the case of Fc/SWNTs/GCE, the redox peaks disappeared after continuous 50 cyclic scans, since the adsorbed Fc leached from the film into the solution, whereas the peak currents for the Fc@SWNTs/GCE decreased less than 15% under the same conditions. To investigate the reason for the slow decrease of the peak currents of the Fc@SWNTs/GCE, the Fc@SWNTs/GCE was scanned continuously for 4000 cycles at a scan rate of 100 mV s-1 in the range of -0.2-0.8 V, which resulted in the peak currents’ decreasing nearly 80%, as compared to those of the initial scanning.

(53) Fukushima, T.; Kosaka, A.; Ishimura, Y.; Yamamoto, T.; Takigawa, T.; Ishii, N.; Aida, T. Science 2003, 300, 2072-2074. (54) Claye, A. S.; Fischer, J. E.; Huffman, C. B.; Rinzler, A. G.; Smalley, R. E. J. Electrochem. Soc. 2000, 147, 2845-2852.

(55) Ju, H. X.; Leech, D. Phys. Chem. Chem. Phys. 1999, 1, 1549-1554. (56) Abbott, N. L.; Whitesides, G. M. Langmuir 1994, 10, 1493-1497. (57) Sun, N. J.; Guan, L. H.; Shi, Z. J.; Zhu, Z. W.; Li, N. Q.; Li, M. X.; Gu, Z. N. Electrochem. Commun. 2005, 7, 1148-1152.

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Figure 2. Cyclic voltammograms of (A) the Fc@SWNTs-gel/GCE and (B) the SWNTs-gel/GCE in the aqueous solution containing 0.1M LiClO4. Scan rate is 0.1 V s-1.

The concentration of Fe in the solution was determined to be 66.3 µg L-1 by ICP-AES, whereas the concentration of Fe in the solution which the modified electrode was just dipped in but without electrochemical scanning could not be detected. This result gave an explanation for the slow decrease of the peak currents; that is, a portion of Fc inside SWNTs can exit into the solution during the electrochemical scanning. What should be emphasized is that the peak currents for the Fc@SWNTs/GCE dropped relatively fast during the first 50 cycles. Another possible reason for this is attributed to desorption of the trace amount of Fc strongly adsorbed outside of SWNTs from the film into the solution, which is similar to that for the Fc/SWNTs/GCE. The decrease was slower as the scanning went on. It implied that an equilibrium of the interactions between Fc themselves and between Fc and SWNTs was established during the process of the electrochemical scanning. (2) Fc@SWNTs-gel/GCE. From the above experiments, we can see clearly that the reversibility of the voltammetric responses for the Fc@SWNTs/GCE in the aqueous solution is not as good as that in acetonitrile solution containing 0.1 M Bu4NClO4 reported previously.51 The separation between the anodic and cathodic peak potentials (∆Ep) at a scan rate of 0.05 V s-1 is ∼55 mV in acetonitrile solution, whereas it is over 100 mV in aqueous solution (Figure 1, curve c). To improve the reversibility in the aqueous solution, the Fc@SWNTs-gel/GCE was prepared. Figure 2 shows a pair of well-defined redox peaks at E1/2 ) 234 mV for the Fc@SWNTs-gel/GCE in the aqueous solution containing 0.1 M LiClO4, and ∆Ep is ∼50 mV. Compared to Fc@SWNTs/GCE, the reversibility of the redox peaks was improved greatly. One of the reasons for this is the addition of BMIPF6. According to the reported literature,53 when grinding BMIPF6 and Fc@SWNTs together, a uniform gel is formed due to the uniform dispersion of the Fc@SWNTs in the medium of ionic liquid. The HRTEM images shown in Figure 3 further verify this. Figure 3A shows the image of Fc@SWNTs clearly; however, in Figure 3B, Fc@SWNTs could not be seen when mixed with BMIPF6, which indicates that the gel is formed. The round spots in Figure 3B might be congeries of the ionic liquid BMIPF6. This needs to be further investigated. The formation of the gel suggests a moderate interaction between BMIPF6 and SWNTs. As Wallace et al. stated,

Figure 3. HRTEM images of Fc@SWNTs (A) and Fc@SWNTsgel (B).

large amounts of RTILs might weaken the intertube junctions by coalescence of liquid bridges,58 which are junctions between nanotubes themselves formed by capillary forces, leading to an untangling of the heavily entangled SWNT bundles. On the other hand, RTILs that consist of organic cations and various anions have ionic conductivity and good thermal and chemical stability. Therefore, great enhancement of conductivity from a combination of ionic conductivity of the BMIPF6 and electrical conductivity of the SWNTs also leads to an improvement in the reversibility. As a consequence, these gels from mixtures of RTILs and SWNTS are suitable and can be used to improve voltammetric responses. The stability of the Fc@SWNTs-gel/GCE was also investigated in a 0.1 M LiClO4 solution by cyclic voltammetry in the potential (58) Whitten, P. G.; Spinks, G. M.; Wallace, G. G. Carbon 2005, 43, 18911896.

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Figure 4. UV-vis-NIR spectra of SWNTs and Fc@SWNTs. Curve a is the spectrum of pristine SWNTs-gel film, and curve b is the spectrum of Fc@SWNTs-gel film. Both spectra are shown after an identical background was subtracted from the raw data.

range of about +0.8 to -0.2 V at 0.1 V s-1. A loss of less than 5% of the original activity was observed after continuous 50 scans, which suggests that the stability of the Fc@SWNTs-gel/GCE is better than that of the Fc@SWNTs/GCE. Furthermore, its stability is dependent on the thickness of the gel on the electrode surface. The thinner the gel is, the more stable the Fc@SWNTs-gel/GCE will be. The stable Fc@SWNTs-gel/GCE should have better performance in its practical application. Cyclic voltammograms of the Fc@SWNTs-gel/GCE in 0.1 M LiClO4 solution at different scan rates were recorded. Linear variations in the oxidation and reduction peak currents with the scan rate in the range of 5 to 1000 mV s-1 were obtained and indicate the redox processes of Fc encapsulated into the SWNTs are surface-confined waves and show thin-layer electrochemical characteristics. Similarly, both oxidation and reduction peaks are very broad, and the total width at half-height of either the anodic or cathodic peak is ∼320 to 350 mV at 0.1 V s-1. This is probably due to the presence of the intermolecular interaction between Fc and SWNTs, which is in good agreement with the other hostguest supramolecular system.51 Optical Characterization. To further characterize the interaction between Fc and SWNTs in Fc peapods, UV-vis-NIR spectrometry and Raman spectrometry were performed. Figure 4 displays UV-vis-NIR spectra of SWNTs and Fc@SWNTs. Here, two spectra are shown after an identical background was subtracted from the raw data. The absorption spectrum of SWNTs is essentially identical to that already reported;59,60 that is, two prominent absorption peaks of SWNTs at ∼1.2 and 1.7 eV are observed. The peak at 1.2 eV is attributed to the electronic transition between the pair of Van Hove singularity from the valence band to conduction band vs2 f cs2 in semiconducting SWNTs. The feature at 1.7 eV is assigned to the transition between the first pair of singularity vm1 f cm1 in metallic SWNTs.61 In the absorption spectrum of Fc@SWNTs, there is no new peak (59) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Metals 1999, 103, 2555-2558. (60) Minami, N.; Kazaoui, S.; Jacquemin, R.; Yamawaki, H.; Aoki, K.; Kataura, H.; Achiba, Y. Synth. Metals 2001, 116, 405-409. (61) Kazaoui, S.; Minami, N.; Jacquemin, R.; Kataura, H.; Achiba, Y. Phys. Rev. B 1999, 60, 13339-13342.

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observed, including the characteristic peaks of Fc at 2.8 eV, only absorption decreased intensities at 1.2 and 1.7 eV. These spectral changes indicate that Fc has been encapsulated inside the SWNTs, resembling the other substances such as K, Cs, I2, TCNQ doped SWNTs.60, 62 Curve a in Figure 5A displays the Raman spectrum for a SWNT sample with tube diameter distributed from 1.3 to 1.4 nm, observed by TEM, which has three major characteristic regions similar to that described elsewhere:63 the radial breathing modes (RBM), the tangential modes (the so-called G bands), and the disorder-induced D mode. Among them, the RBM observation in the Raman spectrum provides direct evidence that a sample contains SWNTs. The RBM involves a radial displacement of the carbon atom of the SWNT. Using an excitation wavelength of 632.8 nm, two peaks of the RBM observed at 147.1 and 162.6 cm-1 in pristine SWNTs appear at 153.9 and 164.6 cm-1, respectively, in Fc@SWNTs (Figure 5B). The blue shift is estimated to be a consequence of the filling of the conduction band, following the charge-induced C-C bond contraction and breaking of van der Waals forces. Furthermore, an intensity reduction in the RBM of the SWNTs indicates molecular encapsulation, according to the literature.64 The tangential G band features for SWNTs consist of two main regions: one peaked at 1583 cm-1 (G+) and the other peaked at ∼1550 cm-1 (G-), as shown in Figure 5C, curve a. The G+ feature is associated with carbon atom vibrations along the nanotube axis, and its frequency G+ is sensitive to charge transfer from dopant additions to the SWNTs. When compared to the G band of SWNTs, an obvious upshift in G+ for Fc@SWNTs was observed in Figure 5C, curve b. This is in agreement with the feature of alkali-doped SWNT described previously,62 whose G-band upshifts from 1588 to 1600 cm-1. The Raman shift might be attributed to the following two processes. The first involves changes in the intra- and intertube force constants induced by structural reorganizations of the SWNT-Fc system, including expansion or deformation of the hexagonal lattice, hardening of the lattice via tube-Fc interactions, structural disorder, etc. The second process relates to charge transfer, which directly modifies the intratube force constants and leads systematically in all-carbon Fc-doped systems to a softening of the modes involving C-C stretching.65 Therefore, we assign the upshift observed for this spectrum to a dominant contribution of structural effects between Fc and SWNTs, mainly not to the transfer of electrons from Fc molecules to the carbon π (π*) states in the tubules, suggesting the interaction between Fc and the side walls of SWNTs. The above results indicate that Raman shifts of the RBM and the G band are sensitive to intercalation of doping species inside SWNTs, and could be used for characterization of peapods. Mediation of H2O2 by Fc@SWNTs. As is well-known, Fc is a good ET mediator in various artificial electron-transfer systems. (62) Takenobu, T.; Takano, T.; Shiraishi, M.; Murakami, Y.; Ata, M.; Kataura, H.; Achiba, Y.; Iwasa, Y. Nat. Mater. 2003, 2, 683-688. (63) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Phys. Rep.-Rev. Sec. Phys. Lett. 2005, 409, 47-99. (64) Kataura, H.; Maniwa, Y.; Abe, M.; Fujiwara, A.; Kodama, T.; Kikuchi, K.; Imahori, H.; Misaki, Y.; Suzuki, S.; Achiba, Y. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 349-354. (65) Bendiab, N.; Spina, L.; Zahab, A.; Poncharal, F.; Marliere, C.; Bantignies, J. L.; Anglaret, E.; Sauvajol, J. L. Phys. Rev. B 2001, 63, 153407.

Figure 5. Raman spectra for pristine SWNTs (a) and Fc@SWNTs (b), radial breathing mode (B) and tangential G mode (C) of the Fc@SWNTs and pristine SWNTs obtained by an excitation at 632.8 nm (1.96 eV) with baseline correction.

However, one of the problems is the stability of ferrocene immobilized on the matrix because it tends to leak out gradually. From the experimental results mentioned above, the Fc@SWNTsgel/GCE not only retains the redox activity of Fc but also shows good stability and conductivity. Here, the electrochemistry of some small molecules on the Fc@SWNTs-gel/GCE has been explored. It has been found that the Fc@SWNTs-gel/GCE shows mediation of some small molecules, such as hydrogen peroxide, dopamine, adenine, and ascorbic acid. Among them, excellent mediation of H2O2 oxidation and reduction by Fc@SWNTs has been observed and investigated in detail. The cyclic voltammagrams of the Fc@SWNTs-gel/GCE in the absence and presence of H2O2 are shown in Figure 6A. Both anodic and cathodic peak currents increase with an increase in the concentration of H2O2, and the oxidation and reduction peak potentials have no obvious change. These results suggest mediation of the reduction and oxidation of H2O2 by Fc@SWNTs because Fc can act as a good ET mediator.66,67 This is different from previous reports, in which some modified electrodes dis(66) Mao, L. Q.; Arihara, K.; Sotomura, T.; Ohsaka, T. Chem. Commun. 2003, 2818-2819. (67) Tian, Y.; Mao, L. Q.; Okajima, T.; Ohsaka, T. Anal. Chem. 2004, 76, 41624168.

played mediation of H2O2 oxidation,68,69 and some showed mediation of H2O2 reduction.70,71 By far, very few modified electrodes have shown the bifunctional mediation of H2O2 redox.72,73 Such a bifunctional mediation of the Fc@SWNTs-gel/GCE shows specificity of Fc inside SWNTs for the dismutation of H2O2; namely, both the oxidation of H2O2 to O2 and the reduction to H2O are mediated by the coupling of Fc/Fc+. The possible mechanism is shown in Figure 7. The causes of the results are based on the following two considerations: one side is specific for Fc@SWNTs, resulting from a combination of Fc and SWNTs. SWNTs seem to act as an ET promoter. It is known that the oxidation of H2O2 does not show a clearly defined wave at a bare GC electrode,72,74 and our results supported this. Only a small shoulder was observed at potentials where large background anodic currents (68) Limoges, B.; Degrand, C. J. Electrochem. Chem. 1997, 422, 7-12. (69) Wang, B. Q.; Li, B.; Wang, Z. X.; Xu, G. B.; Wang, Q.; Dong, S. J. Anal. Chem. 1999, 71, 1935-1939. (70) Lei, C. H.; Deng, J. Q. Anal. Chem. 1996, 68, 3344-3349. (71) Xu, J. Z.; Zhu, J. J.; Wu, Q.; Hu, Z.; Chen, H. Y. Electroanalysis 2003, 15, 219-224. (72) Itaya, K.; Shoji, N.; Uchida, I. J. Am. Chem. Soc. 1984, 106, 3423-3429. (73) Tsiafoulis, C. G.; Trikalitis, P. N.; Prodromidis, M. I. Electrochem. Commun. 2005, 7, 1398-1404. (74) Taha, Z.; Wang, J. Electroanalysis 1991, 3, 215-219.

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Figure 6. A: Cyclic voltammograms of the Fc@SWNTs-gel/GCE in the absence of H2O2 (a) and in the presence of 5 mM (b) and 10 mM (c) H2O2 at a scan rate of 50 mV s-1 in 0.1 M LiClO4. B: The bare GC electrode (a, b) and the SWNT-gel/GCE (c, d) in the absence of H2O2 (a, c) and in the presence of 10 mM H2O2 (b, d) at a scan rate of 50 mV s-1 in 0.1 M LiClO4. These experiments were conducted under a nitrogen atmosphere.

Figure 7. Reaction schemes of the oxidation of H2O2 to O2 (A) and the reduction of H2O2 to H2O (B) mediated by the Fc@SWNTs are on the left. Typical current-time response of the Fc@SWNTs-gel/ GCE for 100 µM H2O2 in a mixed solution containing 0.1 M LiClO4 and 25 mM PBS (pH 7.2) at +500 (a) and 0 mV (b) vs SCE are on the right. The detection was carried out in a stirring electrolytic cell. These experiments were conducted under nitrogen atmosphere.

commenced (Figure 6B, curve b), whereas the oxidation of H2O2 at the SWNT-gel/GCE (Figure 6B, curve d) is shifted to somewhat more negative potentials than that for the bare GC electrode in a manner similar to the oxidation of H2O2, and the reduction of H2O2 at the SWNT-gel/GCE occurs at much more positive potentials than that for the bare GC electrode. Having a catalytically active surface and very high aspect ratio (length over diameter), SWNTs can increase the surface area of the electrode, so the background voltammetric response for the SWNT-coated surface is stronger than that for the bare surface (Figure 6B, curves a and c). These positively suggest that the oxidation and reduction of H2O2 are promoted by SWNTs, which also provides a base for mediation of H2O2 at the Fc@SWNTs-gel/GCE with Fc as a mediator. On the other side, SWNTs in this system function as nanoreactors. The purified SWNTs had sufficient entrance holes so that H2O2 could enter into the tubes and react with ferrocene. This 6056 Analytical Chemistry, Vol. 78, No. 17, September 1, 2006

Figure 8. Steady-state current-time responses of the Fc@SWNTsgel/GCE (a) and SWNT-gel/GCE (b) at 0 mV (A) and +500 mV (B) vs SCE in a mixed solution containing 0.1 M LiClO4 and 25 mM PBS (pH 7.2) upon successive addition of 100 or 200 µM H2O2. The experiments were carried out in a stirring electrolytic cell under nitrogen atmosphere.

mechanism can explain pretty well the observation of bifunctional mediation of H2O2 at the Fc@SWNTs-gel/GCE. As shown in Figure 7, the Fc@SWNTs-gel-modified electrode displays a typical amperometric response for H2O2. An obvious anodic current was recorded at the Fc@SWNTs-gel-modified

electrode at +500 mV when 100 µM H2O2 was added into the solution containing 0.1 M LiClO4 and 25 mM phosphate buffer solution to generate O2 (Figure 7a). Meanwhile, a cathodic response of the generation of H2O at 0 V (Figure 7b)was also recorded. In addition, the electrocatalysis process referred to O2 and H+, and the effects of O2 and pH on amperometric response were investigated. No obvious change in the current response was observed (Figures not shown), indicating O2 and pH do not interfere with the determination of H2O2. Figure 8 displays the amperometric responses of successive addition of H2O2 in solution at the (a) Fc@SWNTs-gel and (b) SWNT-gel modified electrodes. Well-defined steady-state current responses with an increase in the H2O2 concentration at the Fc@SWNTs-gel/GCE are recorded at the applied potentials of 0 (A) and +0.5 (B) V versus SCE, and the steady-state current responses are obtained within 2 s. From this figure, we can see that higher sensitivity is observed at +500 mV (note the different scale); hence, it has been used for the determination of H2O2. The amperometric response to H2O2 concentration over the range from 20 µM to 33.8 mM was investigated. The determination limit is down to 5 µM. The linear ranges from 5 µM to 0.5 mM (R2 ) 0.997) and also from 0.5 to 4.8 mM (R2 ) 0.995) were obtained. In addition, the relative standard deviation is 6.2% for eight replicates of 1 mM H2O2 solution. These indicate the Fc@SWNTsgel/GCE has good stability and repeatability, which shows possible applications in fabrication of an H2O2 detector or chemical or biosensor, even a fuel cell. CONCLUSION Electrochemical properties of Fc@SWNTs have been explored by preparing Fc@SWNTs-based modified electrodes (Fc@SWNTs/ GCE and Fc@SWNTs-gel/GCE) and employing electrochemical methods. A pair of surface-confined redox peaks of Fc inside SWNTs in the aqueous solution has been obtained, and the total width at half-height of either the anodic or cathodic peak is very (75) Zhang, S. X.; Fu, Y. Q.; Sun, C. Q. Electroanalysis 2003, 15, 739-746. (76) Yamamoto, K.; Zeng, H. S.; Shen, Y.; Ahmed, M. M.; Kato, T. Talanta 2005, 66, 1175-1180. (77) Fernandez, L.; Carrero, H. Electrochim. Acta 2005, 50, 1233-1240.

broad. These indicate that SWNTs can act as molecular wires to allow electrical communication between the electrode and redoxactive guest Fc encapsulated inside SWNTs, and the interaction exists between Fc and SWNTs. The optical characterization of Fc@SWNTs by UV-vis-NIR spectrometry and Raman spectrometry also verifies the interaction. When compared with the Fc@SWNTs/GCE, the better response was obtained for the Fc@SWNTs-gel/GCE. It suggests the existence of RITLs can improve electrochemical reversibility of the modified electrode due to ionic conductivity of RITLs and formation of the gel leading to an untangling of the heavily entangled SWNT bundles. When compared to the Fc-modified electrodes reported previously,75-77 the Fc@SWNTs-gel/GCE is not only more stable and convenient, but also shows bifunctional mediation of H2O2 due to Fc inside the SWNTs acting as an ET mediator. Both the oxidation of H2O2 to O2 and the reduction to H2O is mediated by the couple of Fc/Fc+; namely, dismutation of H2O2 is observed at the modified electrode, implying that the Fc@SWNTs-gel/GCE has specific properties due to a combination of SWNTS and Fc. Hence, a Fc@SWNTs-based sensor for H2O2 with a determination limit of 5 µM was fabricated, and it showed good stability and reproducibility. The results not only appear promising in constructing new biosensors and fuel cells, but also found a base in developing novel small molecules filling carbon nanotube materials with specific properties. In this work, only mediation of some small molecules at the Fc@SWNTs-gel/GCE has been performed, and further investigation on mediation of biomacromolecules is under way. ACKNOWLEDGMENT This work has been supported by the National Natural Science Foundation of China (20575004, 20371004 and 20235010) and the State Key Laboratory of Electroanalytical Chemistry of Changchun Institute of Applied Chemistry, Chinese Academy of Sciences.

Received for review March 2, 2006. Accepted July 17, 2006. AC060396I

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