Noncovalent Modification of Carbon Nanotubes with Ferrocene-Amino Acid Conjugates for Electrochemical Sensing of Chemical Warfare Agent Mimics Mohammad A. K. Khan, † Kagan Kerman, ‡ Michael Petryk, § and Heinz-Bernhard Kraatz* ,†,‡ Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, SK, S7N 5C9 Canada, Department of Chemistry, The University of Western Ontario, 1151 Richmond Street, London, ON, N6A 5B7 Canada, and DRDC Suffield, P.O. Box 4000, Station Main, Medicine Hat, AB, T1A 8K6 Canada The electrochemical detection of chemical warfare agent (CWA) mimics was explored using multiwalled carbon nanotubes (MWCNTs) on indium tin oxide (ITO) surfaces in connection with ferrocene-amino acid conjugates. Various ferrocene-amino acid conjugates were synthe- sized and utilized as the recognition layer for the detection of CWA mimics. The ferrocene-amino acid conjugates were noncovalently attached to the pretreated MWCNTs on the ITO surface and reacted with CWA mimics, upon which the electrical properties of the MWCNTs and the Fc group were affected significantly. Alternating current voltammetry and capacitance-based detection offered large dynamic ranges for the detection of methylphospho- nic acid, diethyl cyanophosphonate, ethylmethylphospho- nate, and pinacolyl methylphosphonate in water. Electro- chemical measurements showed dramatic changes upon the electrostatic interaction between the CWA mimics and the ferrocene-amino acid conjugates immobilized on MWCNTs on ITO surfaces. Electrochemical sensing in connection with MWCNTs is shown to be a promising analytical tool for the trace-level detection of CWA mimics in aqueous solutions. In the past decade, electrochemical sensors have become very popular due to their suitability to miniaturization, low-power consumption, and high sensitivity. Sensors generally use a transducer modified with a recognition layer that is sensitive to the analytes of interest. Carbon nanotubes (CNTs) have become extremely useful for this purpose due to their nanoscale diameter with promising electrical and electromechanical properties. 1-5 Due to their high aspect ratio, simple adsorption of molecules causes significant changes in the electrical properties of CNTs. The modification of CNTs has been carried out using covalent and noncovalent bonds. For example, sodium dodecyl sulfate can be adsorbed with noncovalent forces on the multiwalled CNT (MWCNT) surface and form rolled-up half-cylinders with the alkyl chains pointing toward the MWCNT. 6 As a result, the surfactant molecules are loosely packed around the MWCNTs. Nonspecific hydrophobic interactions are believed to be involved in this phenomenon. Molecules containing aromatic groups or electron- rich environments have been reported to modify nanotubes via π-π stacking interactions with the graphite surface. For example, sodium dodecylbenzenesulfonate was reported to interact with single-walled carbon nanotubes (SWCNTs) via π-π stacking interactions. 7 Biological molecules have also been reported to interact noncovalently with the surface and interior of SWCNTs without changing their native reactivity. 8-14 The electronic proper- ties of CNTs along with the specific recognition properties of the immobilized biosystems can form a robust sensor platform for different analytes. Oligonucleotides, 9,10 small proteins such as streptavidin, 11 metallothionein, 12-14 and DNA 9-11 have been re- ported to adsorb to the surface and interior of the MWCNTs due to nonspecific interactions (e.g., van der Waals interaction). CNTs have been utilized for the detection of organic vapor as chemiresistors, where resistance changes in the CNTs were recorded in real time upon exposure to vapors. 15,16 These studies * To whom correspondence should be addressed. E-mail: hkraatz@uwo.ca. † University of Saskatchewan. ‡ The University of Western Ontario. § DRDC Suffield. (1) Ijima, S.; Ichihashi, T. Nature 1993, 363, 603. (2) Ajayan, P. M. Chem. Rev. 1999, 99, 1787. (3) Wong, S. S.; Harper, J. D.; Lansbury, J., P. T.; Lieber, C. M. J. Am. Chem. Soc. 1998, 120, 603. (4) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (5) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678. (6) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593. (7) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269. (8) Davis, J. J.; Green, M. L. H.; Hill, O. A. H.; Leung, Y. C.; Sadler, P. J.; Sloan, J.; Xavier, A. V.; Tsang, S. C. Inorg. Chim. Acta 1998, 272, 261. (9) Tsang, S. C.; Guo, Z.; Chen, Y. K.; Green, M. L. H.; Hill, O. A. H.; Hambley, T. W.; Sadler, P. J. Angew. Chem. Int. Ed. Engl. 1997, 36, 2198. (10) Guo, Z.; Sadler, P. J.; Tsang, S. C. Adv. Mater. 1998, 10, 701. (11) Balavoine, F.; Schultz, P.; Richard, C.; Mallouh, V.; Ebbesen, T. W.; Mioskowski, C. Angew. Chem., Int. Ed. Engl. 1999, 38, 1912. (12) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838. (13) Tsang, S. C.; Davis, J. J.; Green, M. L. H.; Hill, H. A. O.; Leung, Y. C.; Sadler, P. J. J. Chem. Soc., Chem. Commun. 1995, 2579. (14) Lago, R. M.; Tsang, S. C.; Lu, K. L.; Chen, Y. K.; Green, M. L. H. J. Chem. Soc., Chem. Commun. 1995, 1355. (15) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801. (16) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. Anal. Chem. 2008, 80, 2574-2582 2574 Analytical Chemistry, Vol. 80, No. 7, April 1, 2008 10.1021/ac7022876 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/26/2008