Anal. Chem. 2005, 77, 8158-8165
Rational Attachment of Synthetic Triptycene Orthoquinone onto Carbon Nanotubes for Electrocatalysis and Sensitive Detection of Thiols Kuanping Gong,† Xiaozhang Zhu,† Rui Zhao, Shaoxiang Xiong, Lanqun Mao,* and Chuanfeng Chen*
Center for Molecular Science, Institute of Chemistry, the Chinese Academy of Sciences, Beijing 100080, China
This study demonstrates a novel electrochemical method for sensitive determination of biological thiols including homocysteine, cysteine, and glutathione based on rational functionalization of single-walled carbon nanotubes (SWNTs) with synthetic triptycene orthoquinone (TOQ). Unlike previous strategies used for the functionalization of the carbon nanotubes to fabricate new kind of electrochemically functional nanostructures, the method demonstrated here is essentially based on understanding of the redox properties inherent in the SWNTs themselves. It is demonstrated that the electrochemical oxidation of the thiols at the SWNT-modified electrode is redoxmediated by the quinone-like functional groups at the tube ends and that the low density of such functional groups leads to a follow-up oxidation of the thiols at a more positive potential at the electrode. To mimic the redox properties of the SWNTs and thus to increase the catalytic sites onto the SWNTs, we rationally choose the synthetic TOQ and attach such a compound onto the SWNTs. As a result, it is found that the rational attachment of TOQ onto the SWNTs substantially results in a sufficient electrocatalysis toward the thiols at a low potential of 0.0 V with enhanced sensitivities (i.e., almost by a factor of 10-fold) for the determination of such kind of species in relative to those at the SWNT-modified electrode. The high sensitivity and the good stability as well as the high reproducibility of the TOQ/SWNT-modified electrodes substantially make them very useful for reliable and durable determination of the biological thiols. Considerable attention has been drawn to the determination of biological thiols including homocysteine, cysteine, and glutathione because of their great physiological and pathological importance.1 Electrochemical methods have been proved useful for such a purpose because of their good analytical properties, such as ease in automation, high sensitivity, and capability to be readily integrated with other techniques, for example, HPLC and * Corresponding author. Phone: +86-10-62646525. Fax: +86-10-62559373. E-mail:
[email protected]. † Also in Graduate School of the CAS. (1) (a) Schafer, F. Q.; Buettner, G. R. Free Radicals Biol. Med. 2001, 30, 11911212. (b) Brattstro ¨m, L.; EL Wilcken, D. Am. J. Clin. Nutr. 2000, 72, 315323. (c) Diaz-Arrastia, R. Arch. Neurol. 2000, 57, 1422-1427. (d) D’Angelo, A.; Selhub, J. Blood 1997, 90, 1-11. (e) Still, R. A.; McDowell, I. F. W. Clin. Pathol. 1998, 51, 183-188.
8158 Analytical Chemistry, Vol. 77, No. 24, December 15, 2005
capillary electrophoresis, for multianalysis.2 However, such methods have been limited mainly by the sluggish electrochemical process of the thiols at common electrodes, e.g., glassy carbon and gold electrodes.3 Although improvements in the electrochemical responses of these compounds have been recently sought by the use of mercury and diamond electrodes, enzyme-based biosensors, and chemically modified electrodes,4 an electrochemical method for sensitive determination of the thiols still remains very challenging. Other groups and we have reported that the uses of carbon nanotubes (CNTs) could facilitate sensitive electrochemical determinations of the thiols at a low potential.5 More remarkably, as demonstrated in our earlier work,5a at the multiwalled carbon nanotubes (MWNTs) properly purified in an acidic solution, the oxidation of homocysteine, a typical biological thiol, undergoes two steps; the first step occurs at a very low potential of 0.0 V, while the second is at +0.35 V (vs Ag/AgCl). Such a low-potential oxidation of biological thiols is particularly attractive for their determination, and this potentiality greatly activates our current interest in investigation on the electrochemical process of the thiols at the CNTs, aiming at getting a guide principle for designing a new kind of electrochemically functional nanostructure that can be used for sensitive determination of such kind of species. This study demonstrates a novel electrochemical method for sensitive determination of the thiols including homocysteine, cysteine, and glutathione. The method is essentially based on the (2) (a) Inoue, T.; Kirchhoff, J. R. Anal. Chem. 2002, 74, 1349-1354. (b) Lakritz, J.; Plopper, C. G.; Buckpitt, A. R. Anal. Biochem. 1997, 247, 63-68. (3) (a) Rabenstein, D. L.; Yamashita, G. T.; Anal. Biochem. 1989, 180, 259263. (b) D’Eramo, J. L.; Finkelstein, A. E.; Boccazzi, F. O.; Fridman, O. J. Chromatogr., B 1998, 720, 205-210. (c) Vandeberg, P. J.; Johnson, D. C. Anal. Chem. 1993, 65, 2713-2718. (d) White, P. C.; Lawrence, N. S.; Davis, J.; Compton, R. G. Electroanalysis 2002, 14, 89-98. (e) Nekrassova, O.; Lawrence, N, S.; Compton, R. G. Talanta, 2003, 60, 1085-1095. (f) White, P. C.; Lawrence, N. S.; Tsai, Y. C.; Davis, J.; Compton, R. G. Mikrochim. Acta 2001, 137, 87-91. (4) (a) Inoue, T.; Kirchhoff, J. R. Anal. Chem. 2002, 72, 5755-5760. (b) Terashima, C. Rao, T. N.; Sarada, B. V.; Kubota, Y.; Fujishima, A. Anal. Chem. 2003, 75, 1564-1572. (c) Mao, L.; Yamamoto, K. Electroanalysis 2000, 12, 577-582. (d) Spa˜taru, N.; Sarada, B. V.; Popa, E.; Tryk, D. A.; Fujishima, A. Anal. Chem. 2001, 73, 514-519. (e) Nekrassova, O.; Allen, G. D.; Lawrence, N, S.; Jiang, L.; Jones, T. G.; Compton, R. G. Electroanalysis 2002, 14, 1464-1469. (f) Zen, J.; Kumar, A. S.; Chen, J. Anal. Chem. 2001, 73, 1169-1175. (5) (a) Gong, K.; Dong, Y.; Xiong, S.; Chen, Y.; Mao, L. Biosens. Bioelectron. 2004, 20, 253-259. (b) Moore, R. R.; Banks, C. E.; Compton, R. G. Analyst 2004, 129, 755-758. (c) Lawrence, N, S.; Deo, R. P.; Wang, J. Talanta 2004, 63, 443-449. 10.1021/ac0512397 CCC: $30.25
© 2005 American Chemical Society Published on Web 10/29/2005
Scheme 1. Structure of Triptycene Orthoquinone (TOQ, A), Schematic Illustration of Attachment of TOQ onto Carbon Nanotubes (B), and Enlarged Tube Ends with Oxygen-Containing Moieties (C)
electrocatalytic activity of the adsorptive adduct prepared by rational attachment of synthetic triptycene orthoquinone (TOQ, structure shown in Scheme 1) onto single-walled carbon nanotubes (SWNTs) toward the thiols. Effort thus far on electrochemistry and electroanalytical chemistry of the CNTs has revealed that the CNTs represent a new kind of carbon-based materials and are useful for electroanalytical applications, such as electrocatalysis, electrochemical sensors, and biosensors.6 However, the low density of electrochemically functional groups on the tube surface greatly hampers their potential electrochemical applications. It is known that the CNTs consist of seamlessly rolled-up graphene sheets of carbon, exhibiting a special sidewall curvature and possessing a π-conjugative structure with a highly hydrophobic surface.7 These properties essentially allow them to interact with some organic compounds through π-π electronic or hydrophobic interaction(s) to form new nanostructures with novel properties.8 Such a strategy could also be used to attach some kinds of electrochemical redox species onto the CNTs to endow the CNTs with novel electrochemical properties and functions, offering a new route to electrochemical nanodevices.9 For example, we have prepared an electrochemically functional CNT-based nanostructure by attaching electroactive methylene blue onto the CNTs and found that the prepared nanostructure possesses good electrochemical properties.9a However, a close survey of the literature indicates that the choices of the target compounds for (6) (a) Gong, K.; Yan, Y.; Zhang, M.; Xiong, S.; Mao, L. Anal. Sci. In press. (b) Gooding, J. J. Electrochim. Acta 2005, 50, 3049-3060. (c) Zhao, Q.; Gan, Z.; Zhuang, Q. Electroanalysis 2002, 14, 1609-1613. (d) Wang, J. Electroanalysis 2005, 17, 7-14. (e) Dai, L.; Soundarrajan, P.; Kim, T. Pure Appl. Chem. 2002, 74, 1753-1772. (7) (a) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483-487. (b) Journet, C.; Maser, W. K.; Bernier, P.; Loiseau, A.; delaChapelle, M. L.; Lefrant, S.; Deniard, P.; Lee, R.; Fischer, J. E. Nature 1997, 388, 756-758. (c) Poh, W. C.; Loh, K. P.; Zhang, W.; Triparthy, S.; Ye, J.; Sheu, F. Langmuir 2004, 20, 5484-5492. (8) (a) Guldi, D. M.; Rahman, G. M. A.; Jux, N.; Tagmatarchis, N.; Prato, M. Angew. Chem., Int. Ed. 2004, 43, 5526-5530. (b) Satake, A.; Miyajima, Y.; Kobuke, Y. Chem. Mater. 2005, 17, 716-724. (c) Guldi, D. M.; Rahman, G. M. A.; Jux, N.; Balbinot, D.; Hartnagel, U.; Tagmatarchis, N.; Prato, M. J. Am. Chem. Soc. 2005, 127, 9830-9838. (d) Zhang, J.; Lee, J. K.; Wu, Y.; Murray, R. W. Nano Lett. 2003, 3, 403-407. (e) Basiuk, E. V.; RybakAkimova, E. V.; Basiuk, V. A.; Acosta-Najarro, D.; Saniger, J. M. Nano Lett. 2002, 2, 1249-1252.
the functionalization of the CNTs have been primarily based on either the structural property of the compounds, to ensure their strong interaction with the CNTs and thereby stable adsorption onto the CNTs, or their excellent properties, or both.10 Very different from those strategies reported so far, the method demonstrated in this study for the preparation of an electrochemically functional nanostructure (Scheme 1) is essentially based on the understanding of redox properties inherent in the CNTs themselves. To the best of our knowledge, this is the first study on the rational design of the CNTs to produce an electrochemically functional nanostructure that is useful for electroanalytical applications. These demonstrations are envisaged to provide a new electrochemical method for sensitive determination of the biological thiols and offer a new concept for rational functionalization of the CNTs to prepare electrochemically functional nanostructures for electrochemical applications. EXPERIMENTAL SECTION Chemicals and Materials. Homocysteine (Hcys), cysteine, and glutathione (reduced form, GSH) were all purchased from Sigma. Other chemicals were of at least analytical grade and used without further purification. Aqueous solutions were prepared with doubly distilled water. SWNTs (purity, 95%; diameter,
