Anal. Chem. 2005, 77, 1398-1406
Incorporation of Single-Wall Carbon Nanotubes into an Organic Polymer Monolithic Stationary Phase for µ-HPLC and Capillary Electrochromatography Yan Li, Yuan Chen, Rong Xiang, Dragos Ciuparu, Lisa D. Pfefferle, Csaba Horva´th,† and James A. Wilkins*
Department of Chemical Engineering, Yale University, New Haven, Connecticut 06520-8286
Single-wall carbon nanotubes (SWNT) were incorporated into an organic polymer monolith containing vinylbenzyl chloride (VBC) and ethylene dimethacrylate (EDMA) to form a novel monolithic stationary phase for highperformance liquid chromatography (HPLC) and capillary electrochromatography (CEC). The retention behavior of neutral compounds on this poly(VBC-EDMA-SWNT) monolith was examined by separating a mixture of small organic molecules using micro-HPLC. The result indicated that incorporation of SWNT enhanced chromatographic retention of small neutral molecules in reversed-phase HPLC presumably because of their strongly hydrophobic characteristics. The stationary phase was formed inside a fused-silica capillary whose lumen was coated with covalently bound polyethyleneimine (PEI). The annular electroosmotic flow (EOF) generated by the PEI coating allowed peptide separation by CEC in the counterdirectional mode. Comparison of peptide separations on poly(VBC-EDMA-SWNT) and on poly(VBC-EDMA) with annular EOF generation revealed that the incorporation of SWNT into the monolithic stationary phase improved peak efficiency and influenced chromatographic retention. The structures of pretreated SWNT and poly(VBC-EDMASWNT) monolith were examined by high-resolution transmission electron microscopy, Raman spectroscopy, scanning electron microscopy, and multipoint BET nitrogen adsorption/desorption. Carbon nanotubes (CNT) have been widely recognized as quintessential nanomaterials with high strength and unique topologically controlled electronic properties.1-5 Since their discovery in 1991, they have stimulated intensive research into potential high-impact applications such as nanoelectronic devices, catalyst supports, biosensors, and hydrogen storage. * Corresponding author. Tel: (203) 432-4373. Fax: (203) 432-4372. E-mail:
[email protected]. † Author deceased. (1) Iijima, S. Nature 1991, 354, 56-58. (2) Haddon, R. C. Acc. Chem. Res. 2002, 35, 997. (3) Dai, H. Surf. Sci. 2002, 500, 218-241. (4) Yu, M.-F.; Files, B. S.; Arepalli, S.; Ruoff, R. S. Phys. Rev. Lett. 2000, 84, 5552-5555. (5) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435-445.
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CNT have also shown potential in chromatographic applications. Several studies focused on size separation and purification of CNT themselves using chromatography.6-14 Different types of chromatography were used to remove nanoparticles from CNT samples and to separate functionalized CNT by length. Recently, electrochemical detectors with CNT modified electrodes were used in liquid chromatography15-17 and capillary electrophoresis18,19 achieving significantly lower operating potentials and yielding substantially enhanced signal-to-noise characteristics. Because carbon materials have long been used as adsorbents for trapping or separation of volatile organic compounds, we decided that, because of their unique physicochemical properties, CNT might offer interesting opportunities for the development of new stationary-phase materials for chromatography. Kartsova20 first discussed the sorption properties of CNT and therefore the possibility of using CNT as a stationary-phase component in chromatography. However, it was noted that most CNT were (6) Duesberg, G. S.; Muster, J.; Krstic, V.; Burghard, M.; Roth, S. Appl. Phys. A: Mater. Sci. Process. 1998, A67, 117-119. (7) Duesberg, G. S.; Burghard, M.; Muster, J.; Philipp, G.; Roth, S. Chem. Commun. (Cambridge) 1998, 435-436. (8) Duesberg, G. S.; Blau, W.; Byrne, H. J.; Muster, J.; Burghard, M.; Roth, S. Synth. Met. 1999, 103, 2484-2485. (9) Holzinger, M.; Hirsch, A.; Bernier, P.; Duesberg, G. S.; Burghard, M. Appl. Phys. A: Mater. Sci. Process. 2000, 70, 599-602. (10) Chen, J.; Rao, A. M.; Lyuksyutov, S.; Itkis, M. E.; Hamon, M. A.; Hu, H.; Cohn, R. W.; Eklund, P. C.; Colbert, D. T.; Smalley, R. E.; Haddon, R. C. J. Phys. Chem. B 2001, 105, 2525-2528. (11) Niyogi, S.; Hu, H.; Hamon, M. A.; Bhowmik, P.; Zhao, B.; Rozenzhak, S. M.; Chen, J.; Itkis, M. E.; Meier, M. S.; Haddon, R. C. J. Am. Chem. Soc. 2001, 123, 733-734. (12) Zhao, B.; Hu, H.; Niyogi, S.; Itkis, M. E.; Hamon, M. A.; Bhowmik, P.; Meier, M. S.; Haddon, R. C. J. Am. Chem. Soc. 2001, 123, 11673-11677. (13) Chattopadhyay, D.; Lastella, S.; Kim, S.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2002, 124, 728-729. (14) Farkas, E.; Anderson, E. M.; Chen, Z.; Rinzler, A. G. Chem. Phys. Lett. 2002, 363, 111-116. (15) Xu, J.; Wang, Y.; Xian, Y.; Jin, L.; Tanaka, K. Talanta 2003, 60, 11231130. (16) Zhang, W.; Xie, Y.; Ai, S.; Wan, F.; Wang, J.; Jin, L.; Jin, J. J. Chromatogr., B 2003, 791, 217-225. (17) Zhang, W.; Wan, F.; Xie, Y.; Gu, J.; Wang, J.; Yamamoto, K.; Jin, L. Anal. Chim. Acta 2004, 512, 207-214. (18) Wang, J.; Chen, G.; Wang, M.; Chatrathi, M. P. Analyst (Cambridge, U. K.) 2004, 129, 512-515. (19) Wang, J.; Chen, G.; Chatrathi, M. P.; Musameh, M. Anal. Chem. 2004, 76, 298-302. (20) Kartsova, L. A.; Makarov, A. A. Russ. J. Appl. Chem. (Translation of Zhurnal Prikladnoi Khimii) 2002, 75, 1725-1731. 10.1021/ac048299h CCC: $30.25
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insoluble in the known solvents, which restricted their application in chromatography. In 2003, Li21 studied CNT as a gas chromatographic column packing and concluded that CNT showed stronger retention of compounds but lower efficiency. The structure of CNT resembles one or more graphite sheets rolled up into a cylinder that consists of hexagon-rich sp2 carbon at the wall and a few pentagons at the caps, getting more sp3-like as tube diameters become smaller (
