Noncovalent Functionalization of Graphite and Carbon Nanotubes

Stroock et al.36 have shown that polymer multilayers are not formed on surfaces presenting oligo (ethylene glycol) groups. Finally, we demonstrated th...
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NANO LETTERS

Noncovalent Functionalization of Graphite and Carbon Nanotubes with Polymer Multilayers and Gold Nanoparticles

2003 Vol. 3, No. 10 1437-1440

Alvaro Carrillo, Jeffrey A. Swartz, Jason M. Gamba, and Ravi S. Kane* The Howard P. Isermann Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180

Nirupama Chakrapani, Bingqing Wei, and Pulickel M. Ajayan* Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 Received June 6, 2003; Revised Manuscript Received August 5, 2003

ABSTRACT This paper describes a strategy for functionalizing graphite and carbon nanotube surfaces with multilayered polymeric films. Poly(amphiphiles) adsorb noncovalently onto these surfaces from aqueous solutions, due to hydrophobic interactions. The covalent attachment of a second polymer layer to this initial adsorbed layer results in the formation of a cross-linked polymer bilayer; additional layers can be deposited by the covalent or electrostatic attachment of polyelectrolytes. We used these multilayered polymer films to mediate the attachment of gold nanoparticles to graphite, single-walled nanotube (SWNT), and multiwalled nanotube (MWNT) surfaces. This approach provides a convenient method for attaching other nanostructures, biological molecules, or ligands to carbon nanotubes.

Carbon nanotubes have numerous potential applications as a result of their outstanding structural, mechanical, and electrical properties.1-5 Strategies for functionalizing carbon nanotubes are critical for the pursuit of these applications. One of the most well developed approaches for functionalizing nanotubes involves the introduction of carboxylic acid groups onto their surfaces via an acid treatment.6,7 These carboxylic acid groups may be used to covalently attach proteins,8 oligonucleotides,9,10 polymers,11 and nanocrystals12 to carbon nanotubes. Jiang et al.13 have also adsorbed polymers onto acid-treated nitrogen-doped carbon nanotubes. It is particularly desirable to develop methods for functionalizing nanotubes noncovalently, to retain their attractive electronic and mechanical properties. Fu et al. have reported a method of coating SWNTs with a thin layer of SiO2.14 Chen et al.15 demonstrated that the commercially available molecule 1-pyrenebutanoic acid, succinimidyl ester could be used to immobilize proteins on the surfaces of single-walled carbon nanotubes. An adsorbed pyrene-linked initiator has been used to form a noncovalent polymer coating on carbon nanotubes.16 Nanotubes have also been functionalized by the adsorption of surfactants and polymer monolayers.17-23 * Corresponding authors. Kane: telephone (518) 276-2536; fax (518) 276-4030; e-mail [email protected]. Ajayan: telephone (518) 276-2322; fax (518) 276-8554; e-mail: [email protected]. 10.1021/nl034376x CCC: $25.00 Published on Web 09/19/2003

© 2003 American Chemical Society

We describe an approach for functionalizing nanotubes noncovalently with polymer multilayers. The formation of polyelectrolyte multilayers represents a strategy for functionalizing surfaces with stable and uniform ultrathin films.24-33 The thickness of each individual layer depends on the choice of the polymer and may be further tuned by varying the pH and ionic strength of the solution used for the deposition. We used polymer multilayers to mediate the attachment of gold nanoparticles to SWNTs and MWNTs, creating hybrid nanostructures. These polymer films are also compatible with established techniques for immobilizing ligands and for generating protein-resistant surfaces.34,35 Our strategy for functionalizing hydrophobic graphite and carbon nanotube surfaces (Scheme 1) was similar to the one used by Stroock et al.36 to form polymer multilayers on hydrophobic (methyl-terminated) self-assembled monolayers of hexadecanethiolate on gold. We reasoned that polymers such as hydrolyzed-poly(styrene-alt-maleic anhydride) (hPSMA)36 would adsorb noncovalently onto graphite or carbon nanotube surfaces from aqueous solutions via hydrophobic interactions. The noncovalently attached layer of h-PSMA contains carboxylic acid groups that can be used to attach a second polymer (e.g., polyethyleneimine) covalently, forming a cross-linked polymer bilayer. The covalent cross-linking increases the stability of the polymer

Scheme 1.

Schematic Showing the Procedure Used to Prepare Multilayered Polymer Filmsa

Figure 1. PIERS spectrum of a polymer multilayer ((hPSMA-cPEI)2-PAA)41 deposited on an HOPG surface.

a (i) Deposit a layer of h-PSMA and rinse with deionized water; (ii) covalently attach a layer of PEI and rinse with deionized water; (iii) repeat steps (i) and (ii) to deposit a second bilayer, and (iv) deposit a layer of PAA.

layer.36 These steps may be repeated to build a multilayered polymeric film, consisting of alternating layers of polyanions and polycations. We first demonstrated and optimized the proposed strategy using graphite surfaces. A graphite surface is a reasonable model for the surface of a nanotube, which can be considered to be a rolled graphene sheet. The deposition of layers was carried out using the flow cell reactor setup described by Stroock et al.36 A chip of highly ordered pyrolytic graphite (HOPG) was introduced into the reactor. Polymer multilayers were formed by using the following steps: (1) a 0.15 wt % solution of h-PSMA (Mw 350,000, Aldrich) was introduced into the reactor, followed by a 30 min pause. This step results in the adsorption of the polymer via hydrophobic interactions; (2) 300 mL of deionized water were flushed through the cell; (3) 40 mL of a solution containing 0.15 wt % poly(ethyleneimine) (Mw 750,000, Aldrich) (PEI), 30 mg/mL 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride (EDC), and 5 mg/mL N-hydroxysuccinimide (NHS) were introduced, followed by a 30 min pause. This step results in the formation of a cross-linked polymer bilayer; (4) 300 mL of deionized water were flushed through the cell; (5) 40 mL of the h-PSMA solution were introduced, followed by a 30 min pause; (6) steps 2, 3, and 4 were repeated, thereby covalently attaching a second layer of PEI; (7) 40 mL of a 0.15 wt % solution of poly(acrylic acid) (Mw 90,000, Acros) (PAA) were introduced, followed by a 30 min pause; (8) the sample was removed from the flow cell reactor and rinsed with deionized water. The functionalization of HOPG with polymers was confirmed by using polarized infrared internal reflectance 1438

spectroscopy (PIERS).36,37 Figure 1 shows the IR spectrum for a polymer multilayer deposited on HOPG as described above. We assign the strong peak at 1736 cm-1 to the carbonyl group of carboxylic acids. We assign the peaks at 1660 cm-1 and 1566 cm-1 to the CdO stretch and the N-H bending modes of amide bonds respectively;37,38 these peaks confirm the presence of amide cross-links in the multilayered polymeric film. The bare and polymer-coated HOPG surfaces were further characterized by measuring sessile contact angles of water.39 The measured sessile contact angle of water on an untreated HOPG surface, 81 ( 4°, is comparable to the value reported previously.40 The value of the sessile contact angle was found to be 54 ( 3° for an HOPG surface functionalized with two bilayers ((hPSMA-c-PEI)2),41 and 42 ( 2°, for a PAAterminated surface ((hPSMA-c-PEI)2-PAA). The decrease in the value of the contact angle reflects the increase in the hydrophilicity of the surface following the formation of the polymer multilayer. To test the uniformity of coverage, we functionalized polymer-coated HOPG surfaces with fluorescein. Fluorescein cadaverine (Molecular Probes) was coupled to a PAA-terminated surface ((hPSMA-c-PEI)2-PAA) by using a procedure described previously by Metallo et al.37 The fluorescence micrograph (Figure 2a) illustrates that the polymer multilayer was deposited uniformly on the HOPG surface. Having demonstrated the ability to functionalize HOPG surfaces with polymer multilayers, we next used multilayers to mediate the attachment of gold nanoparticles to HOPG and nanotube surfaces. Hybrid nanostructures, composed of nanotubes and nanoparticles may be used in electronic, magnetic, or catalytic applications. For instance, the functionalization of carbon nanotubes with metal or semiconductor nanoparticles might be used to influence their electronic properties. Functionalization of HOPG, SWNT, and MWNT surfaces with nanoparticles also serves to confirm the presence of functional groups on the modified surfaces. For these experiments, we used a multilayered film having the structure (h-PSMA-c-PEI)2-PAA-PEI;41 this film was synthesized by depositing an additional PEI layer onto the multilayer film ((hPSMA-c-PEI)2-PAA) described above. Nano Lett., Vol. 3, No. 10, 2003

Figure 2. Functionalization of HOPG, SWNT, and MWNT surfaces: (a) fluorescence micrograph of polymer-coated HOPG surfaces functionalized with fluorescein; (b) SEM image of gold nanoparticles immobilized on polymer-coated HOPG; (c) SEM image of substrate-grown SWNTs (the arrow points to a SWNT bundle); (d)-(f) SEM images of gold nanoparticles immobilized on polymer-coated SWNTs; and (g) TEM image of gold nanoparticles immobilized on polymer-coated MWNTs.

A polymer-coated HOPG surface and an untreated HOPG sample (control) were immersed in aqueous suspensions of gold nanoparticles (40 nm in diameter, Ted Pella), and the samples were then thoroughly rinsed with water. The negatively charged gold nanoparticles were expected to bind to the PEI-terminated surfaces via electrostatic interactions.42-44 SEM studies (Figure 2b) confirmed the attachment of gold nanoparticles to polymer-coated HOPG. In contrast, an insignificant number of nanoparticles were attached to the control sample. Nano Lett., Vol. 3, No. 10, 2003

We also used the procedure described above to functionalize substrate-grown SWNTs (Figure 2c). SWNTs were grown between prepatterned, metal deposited locations on silicon substrates by using a chemical vapor deposition (CVD) process.45 Briefly, Fe or Co thin films (5-10 Å thickness), which serve as catalysts for nanotube growth, were deposited on submicron Si and SiO2 pillars patterned using synchrotron-radiation lithography. The CVD process was carried out at temperatures between 800 °C and 950 °C for 1-2 min using methane (CH4) as a carbon source.45 The suspended nanotube networks were identified to be SWNT bundles based on characterization by transmission electron microscopy (TEM) and Raman spectroscopy. These substrategrown SWNTs may have applications as sensors.46-48 The change in the electrical conductance of SWNTs following the adsorption of small molecules or biomolecules could be exploited to design molecular sensors based on SWNT arrays. Applications in biosensing will require methods for functionalizing the SWNTs with biospecific ligands, while preventing the nonspecific adsorption of biomolecules. The sample chip was subjected to the film deposition procedure described above to obtain a (h-PSMA-c-PEI)2PAA-PEI41 coating, and was then immersed in an aqueous suspension of gold nanoparticles (40 nm in diameter). The sample was then thoroughly rinsed with water. A noncoated SWNT sample was used as a control. The abundant presence of immobilized gold nanoparticles on the polymer-coated SWNTs (Figure 2d-f), compared to the insignificant presence of nanoparticles in a noncoated sample demonstrates that the polymer multilayer mediates the immobilization of the nanoparticles onto the SWNTs. We note that gold nanoparticles are also immobilized on the polymer-coated substrate. This deposition may be prevented by functionalizing the substrate with oligo(ethylene glycol)-terminated silanes49 prior to the deposition of the polymer multilayer. Stroock et al.36 have shown that polymer multilayers are not formed on surfaces presenting oligo (ethylene glycol) groups. Finally, we demonstrated the ability to immobilize gold nanoparticles onto MWNTs (Figure 2g). MWNTs, grown using a chemical vapor deposition process, were sonicated in acetone, and a drop of the resulting suspension was allowed to dry on a transmission electron microscopy (TEM) grid. After forming a (h-PSMA-c-PEI)2-PAA-PEI41 coating by the procedure described above, the sample was immersed in an aqueous suspension of gold nanoparticles (5 nm in diameter) and then thoroughly rinsed with water. The TEM micrograph (Figure 2g) illustrates the deposition of gold nanoparticles on MWNTs. In summary, we have demonstrated a strategy for functionalizing graphite, SWNT, and MWNT surfaces noncovalently with polymer multilayers and gold nanoparticles. The noncovalent method of functionalization is important for preserving the mechanical and electrical properties of carbon nanotubes. Functionalization with polymer multilayers allows the introduction of reactive groups (e.g., carboxylic acid or amine groups) onto the nanotube surface. These reactive groups may be used to attach biospecific ligands,34 as well as groups that resist the nonspecific adsorption of 1439

biomolecules,35 for applications in biosensors. The strategy may also be used to attach other nanostructures such as semiconductor nanocrystals or magnetic nanoparticles to carbon nanotubes. Acknowledgment. R.S.K. acknowledges the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research via grant PRF # 37972-G5. P.M.A. and B.W. acknowledge funding from the NSF-NSEC at RPI and Philip Morris USA. We thank Prof. Paul Laibinis for the use of the IR instrument in his laboratory, and Jiehyun Seong for assistance with the measurements. We thank Ananthakrishnan Sethuraman, in Prof. Georges Belfort’s group, for assistance with contact angle measurements. We thank Yung Joon Jung for providing substrate-grown SWNT samples. We thank Dr. Richard Cole at the Wadsworth Center for assistance with the fluorescence microscopy. References (1) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, Ph. Carbon nanotubes: synthesis, structure, properties, and applications; Springer: Berlin, New York, 2001. (2) Ajayan, P. M. Chem. ReV. 1999, 99, 1787-1799. (3) Dekker, C. Phys. Today 1999, 52, 22-28. (4) Yakobson, B. I.; Smalley, R. E. Am. Sci. 1997, 85, 324-337. (5) Baughman, R. H.; Zakhidov, A. A.; Heer, W. A. d. Science 2002, 297, 787-792. (6) Liu, J.; Rinzler, A. G.; Dai, H. J.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253-1256. (7) Zhao, W.; Song, C.; Pehrsson, P. E. J. Am. Chem. Soc. 2002, 124, 12418-12419. (8) Huang, W.; Taylor, S.; Fu, K.; Lin, Y.; Zhang, D.; Hanks, T. W.; Rao, A. M.; Sun, Y.-P. Nano Lett. 2002, 2, 311-314. (9) Baker, S. E.; Cai, W.; Lasseter, T. L.; Weidkamp, K. P.; Hamers, R. J. Nano Lett. 2002, 2, 1413-1417. (10) Nguyen, C. V.; Delzeit, L.; Cassell, A. M.; Li, J.; Han, J.; Meyyappan, M. Nano Lett. 2002, 2, 1079-1081. (11) Riggs, J. E.; Guo, Z.; Carroll, D. L.; Sun, Y.-P. J. Am. Chem. Soc. 2000, 122, 5879-5880. (12) Banerjee, S.; Wong, S. S. Nano Lett. 2002, 2, 195-200. (13) Jiang, K.; Eitan, A.; Schadler, L. S.; Ajayan, P. M.; Siegel, R. W.; Grobert, N.; Mayne, M.; Reyes-Reyes, M.; Terrones, H.; Terrones, M. Nano Lett. 2003, 3, 275-277. (14) Fu, Q.; Lu, C.; Liu, J. Nano Lett. 2002, 2, 329-332. (15) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838-3839. (16) Go´mez, F. J.; Chen, R. J.; Wang, D.; Waymouth, R. M.; Dai, H. Chem. Commun. 2003, 190-191. (17) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265-271. (18) Shim, M.; Kam, N. W. S.; Chen, R. J.; Li, Y.; Dai, H. Nano Lett. 2002, 2, 285-288. (19) Star, A.; Steuerman, D. W.; Heath, J. R.; Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 2508. (20) Bandyopadhyaya, R.; Nativ-Roth, E.; Regev, O.; Yerushalmi-Rozen, R. Nano Lett. 2002, 2, 25-28.

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NL034376X

Nano Lett., Vol. 3, No. 10, 2003