Peptide-Mediated Formation ofSingle-Wall Carbon NanotubeCompositesMark J. Pender, † Laura A. Sowards, † Jeffrey D. Hartgerink, ‡

Morley O. Stone, † and Rajesh R. Naik* ,†

Air Force Research Laboratory, Materials and Manufacturing Directorate,3005 Hobson Way, Wright-Patterson Air Force Base, Ohio 45433-7702, andDepartment of Chemistry, Rice UniVersity, 6100 Main Street, Houston, Texas 77005

Received September 23, 2005; Revised Manuscript Received November 9, 2005

ABSTRACT

The formation of silica- and titania-coated single-wall carbon nanotubes (SWNTs) using a mutlifunctional peptide to both suspend SWNTs anddirect the precipitation of silica and titania at room temperature is demonstrated.

Carbon nanotubes (CNTs), both multiwall and single-wall,are proving to be critical components for emerging electronic,sensor, and structural systems.1 In many instances, the keyto success is the development of methods for the manipula-tion and functionalization of CNTs that do not compromisetheir structural or electronic properties. Coating single-wallcarbon nanotubes with oxide-based ceramics to form eitherhetero-nanomaterials with targeted properties or nanocom-posite materials has been demonstrated.2 Titanium dioxideis of particular interest for photovoltaics3 where single-wallcarbon nanotubes could aid electron transport through dye-sensitized solar cells or as supports for photocatalysts4 wherehigh surface areas and chemical inertness are importantfactors. Although there are several examples of titaniumdioxide on the surface of multiwall carbon nanotubes(MWNTs),5 there are few examples for single-wall carbonnanotubes (SWNTs).6 This is due, in large part, to the poorsolubility of individual SWNTs in solutions required forprocessing and a lack of specificity for titania formationexclusively at the surface of the SWNTs. Here, we demon-strate a multifunctional peptide that can suspend SWNTs asindividuals and precipitate silica or titania from water-solubleprecursors at the surface of the nanotubes without covalentfunctionalization of the carbon nanotubes under mild condi-tions.

The P1 dodecapeptide identified from a combinatorialphage peptide display library has a strong affinity forSWNTs. The P1 peptide is able to suspend SWNTs producedby the HiPco process in aqueous solution to form a

homogeneous dark-colored solution. We exploited the abilityof P1 peptide to coat and suspend SWNTs in the design ofa multifunctional peptide (P1R5) that not only binds toSWNTs but also contains a peptide domain that is capableof precipitating inorganics from precursor solutions specif-ically on carbon nanotube surfaces.7,8 Generally, the use ofpeptides to precipitate inorganics avoids severe reactionconditions such as high temperature or the use of harshchemicals. This biomimetic approach is of particular interestfor aqueous suspensions of SWNTs, which can precipitateeasily with modest changes in environmental parameters. TheR5 peptide domain, the repeat peptide unit of the silaffinpolypeptide involved in diatom biosilicification, has beenused as a template for precipitating silica from an alkoxideprecursor solution.7,8

There are several examples in the literature of biomol-ecules binding to the surface of single-wall carbon nano-tubes.9 In some cases these have been through covalentmodification, but others have taken advantage of the am-phiphilic nature of designed peptides to coat, separate, andsolubilize carbon nanotubes. More recent efforts havedemonstrated that cyclic peptides with a reversible disulfidelinker via thiol-containing groups at the termini provide stablesuspensions of SWNTs. Raman spectroscopy and AFM datasuggested that cyclic peptides of varying lengths provideddiameter-selective enrichment from the original nanotubesample.9i An advantage of using peptides to coat or func-tionalize carbon nanotubes is that additional functionalitiescan be engineered into the peptide by inserting other peptidedomains or epitopes, which can then be used in synthesis ofcomposites and other heterostructures for a variety ofapplications.

* Corresponding author. E-mail: rajesh.naik@wpafb.af.mil.† Wright-Patterson Air Force Base.‡ Rice University.

NANOLETTERS

2006Vol. 6, No. 1

40-44

10.1021/nl051899r CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 12/21/2005

Peptides with a strong affinity for SWNTs produced bythe HiPco (high pressure carbon monoxide) method wereisolated from a phage peptide library. After four rounds ofpanning with increased stringency, we were able to identifypeptides that exhibited strong binding to SWNTs (Table 1).Similar to work published previously,10 we identified peptidesthat were enriched in aromatic, charged, and hydroxyl-containing amino acids. Recently, Zorbas et al. have shownthat aromatic residues play an important role in dispersingSWNTs in aqueous solutions possibly byπ-stacking interac-tions.11 Of all of the peptides identified, the P1 peptide wasthe dominant sequence to emerge from the panning experi-ments. The P1 peptide is able to disperse SWNTs in waterby sonicating the HiPco SWNTs in the presence of the P1peptide to form a homogeneous stable suspension whencompared to a control peptide. Such suspensions are stableto 50 °C as determined by variable-temperature circulardichroism experiments and are also stable over a pH rangeof 4-10. The P3 peptide, also isolated from the phage displayexperiments, was not able to provide suitable suspensionsof SWNTs because it contained only two aromatic residues,one of which is located outside of the central hydrophobicdomain.

To introduce an additional functionalitity to the P1 peptide,we designed a fusion peptide that contained the SWNT-binding P1 domain and the alkoxide precipitating R5 domain.The R5 peptide repeat unit of the silica precipitating protein,silaffin, is capable of precipitating silica from a hydrolyzedalkoxide precursor (silicic acid).7,8 The resulting silicaprecipitate is composed of a network of amorphous silicananoparticles. The multifunctional P1R5 peptide should beable to coat and suspend SWNTs and also be able toprecipitate silica on the surface of the carbon nanotubes.Similar to the P1 peptide, a homogeneous water-stablesuspension of SWNTs can also be obtained using the P1R5peptide. Suspensions of individual SWNTs and small bundlesin aqueous solutions at concentrations comparable to SDS(10 mg/L and higher as determined via absorption at 763nm) were obtained repeatedly using the P1R5 peptide.Typically, a 1:1 (w/w) of peptide to cleaned SWNTs in wateris bath sonicated for 25 min followed by centrifugation at14 000 rpm for 30 min. The centrifugation step allows forthe precipitation of large bundles of SWNTs from thesolution. The supernatant solution containing the P1R5-coated SWNTs was transferred carefully to a fresh tube(Figure 1A). The disparity in the intensity of the colored

solution between the P1R5 peptide and the control peptidedispersed in the tube is indicative of the amount of dispersedSWNTs. An aliquot of the P1R5-dispersed SWNTs was spincoated on freshly cleaved mica and analyzed by AFM. TheAFM micrograph in Figure 1B shows the presence of amixture of individual and small bundles of carbon nanotubes.Height analysis of 95 tubes and bundles revealed that 43%of the structures analyzed were less than 2 nm in height,whereas 96% of all structures were below 10 nm in height.This indicates that the P1R5-coated suspension is a mixtureof individual and small bundles of SWNTs. In Figure 1C,individual nanotubes coated with the P1R5 peptide can beobserved; however, the peptide does not appear to be dis-tributed uniformly along the nanotube surface. Such ag-gregates on the surface could arise during the drying process.

The circular dichroism (CD) spectrum of P1 in water byitself exhibited a random coil structure as evidenced by the

Table 1. SWNT binding peptides

a The aromatic residues are highlighted in yellow. The R5 domain isitalicized. The tetraglycine linker is underlined.

Figure 1. (A) Optical micrograph of supernatant solutions of P1R5peptide (left) and a nonspecific control peptide (right) dispersedSWNTs. The darkness of the solution is qualitatively indicative ofthe amount of SWNTs in solution. (B) AFM micrograph of SWNTssuspended by a P1R5 peptide deposited on a mica substrate. (C)P1R5 peptide coating the nanotubes (arrows); the asterisk indicatesa peptide unit.

Nano Lett., Vol. 6, No. 1, 2006 41

negative ellipitcity at 200 nm. However, the presence ofSWNTs had a significant impact on the secondary structureof the P1 peptide (Supporting Information Figure S2A).Notably, the signal in the region that could be attributed toaromatic residues between 225 and 245 nm disappears inthe presence of SWNTs. In contrast, the SWNTs had verylittle impact on the secondary structure of the P1R5 peptideas determined by CD (Supporting Information Figure S2B).This may be explained by the fact that the structure of theP1R5 peptide is dominated by the random coil structure ofthe longer R5 domain. The aromatic residues of the P1domain, responsible for binding to the surface of thenanotubes, account for only 14% of the total amino acidresidues of P1R5 (vs 42% for P1) and it is therefore of littlesurprise that the overall structure of P1R5 varies only slightlyin the presence of SWNTs. One would expect that thehydrophobic pocket, which is commensurate with the nano-tube diameter, would experience little or no disruption ofthe secondary structure in the presence of SWNTs.

Surfactants and pluronics have allowed for the suspensionof low concentrations (mg/L) of individual and small bundlesof SWNTs in aqueous solutions,12 whereas functionalizedSWNTs have allowed for maximal concentrations of 0.8 mg/mL in organic solvents. However, extensive functionaliztionof SWNTs compromises the electronic property because ofan increase in the sp3 character of the nanotubes carbons.13

Unlike common surfactants or organic functionalities usedto suspend carbon nanotubes, the use of bifunctional peptidessuch as the P1R5 peptide represents a new route by whichone can selectively functionalize the surface of SWNTs and,ultimately, other nanoparticles. The flexibility of usingpeptides allows the inclusion of a tremendous amount offunctionality using peptide design. There is a growing wealthof literature illustrating highly substrate-specific peptidetemplates that bind to a wide variety of inorganics and, morerecently, a conducting polymer from which to construct suchmultifunctional peptides.14

The P1R5-coated SWNTs when exposed to the silicic acidprecursor solution resulted in the formation of a precipitate.In control experiments, using SDS-suspended SWNTs or acontrol peptide, little or no precipitate formation wasobserved. The precipitate was collected and analyzed bySEM and TEM. As seen in Figure 2A, the SEM micrographof the precipitate indicates the presence of SWNTs embeddedwithin the silica matrix. One can observe the carbonnanotubes sticking out of the silica matrix. The product wassonicated for 15 s with a tip sonicator in water to preparematerial for study by transmission electron microscopy. Fromthe TEM micrograph in Figure 2B, there is evidence of bothindividual and small bundles of SWNTs coated with silicaproduct; however, it does appear that the precipitationreaction leads to agglomeration of SWNTs. The elementalcomposition of the product using energy-dispersive spec-troscopy (EDS) confirmed the presence of a silica coatingon the SWNTs (Figure 2C).

Recently, a silica precipitating polypeptide was shown tobe able to template the synthesis of titanium dioxide usinga water-stable alkoxide-like conjugate of titanium, titanium

(IV) bis-(ammonium lactate)-dihydroxide (Ti[BALDH]).15

Exposure of the P1R5-dispersed SWNTs to Ti[BALDH] alsoresulted in the formation of a precipitate. In contrast, Ti-[BALDH] mixed with SDS-dispersed SWNTs yielded noprecipitate. This indicated that the P1R5 peptide was requiredfor the precipitation activity. SEM analysis of the productshowed the presence of a mat of SWNTs that was embeddedin the matrix (Figure 3A). The sonicated product whenanalyzed by TEM showed that individual and small bundlesof SWNTs were embedded within the titania matrix (Figure3B). Closer examination of certain regions within the productindicated the presence of small nanoparticles (2-4 nm) thatpartially coat the SWNT (Figure 3B inset). Examination ofthe sample using EDS confirmed the presence of titaniumand oxygen (Figure 3C). In Figure 4, a typical Ramanspectrum (514 nm, the intensity is plotted log-scale in orderto show the highly intense SWNT G peak at∼1600 cm-1

as well as the less intense peaks) peaks indicative of anataseTiO2 (peaks around 640, 515, and 397 cm-1) along withtypical SWNT peaks are present. Raman spectroscopy shows

Figure 2. SWNTs coated with silica. (A) SEM micrograph SWNTscoated with silica. (B) TEM micrograph of silica-coated SWNTs.(C) Energy-dispersive spectrum of the precipitate shown in A.

42 Nano Lett., Vol. 6, No. 1, 2006

little, if any, change in the G:D ratio, often indicative of thedegree of covalent functionalization, between the startingnanotube material and the final product. The remainingresidue and staggered weight losses during the thermogra-vametric analysis to 900°C in air of the composite suggeststhat the material is 29% SWNTs, 21% peptide, and 44%titania. The remainder of the weight is attributed to loss offree water and water from condensation of the titania gel.

Preliminary photoconductivity experiments were per-formed on the titania/SWNT composite material, SWNTscoated with P1R5, and P1R5-derived titania. Wet pastes ofeach were applied to a prefabricated four-electrode surfboard. All samples were dried in a vacuum oven for at least3 h at 90°C. Of note was the lack of conductivity of thecomposite material. The titania/SWNT material had aresistance on the order of megaohms, whereas the resistanceof the SWNTs alone was∼300 ohms. This suggests thatthe SWNTs are indeed well isolated in the titania matrix.The titania control sample was not conductive in the realm

probed. A very small white light response was obtained forthe composite. Although the signal was far too weak to obtaina spectrum, the response did decrease as expected when thebeam was blocked. The photocurrent was estimated to be atleast 5 orders of magnitude below the dark current. Furtherstudies are underway to improve the photovoltaic response.

The results presented here represent the first example ofa peptide-mediated route for SWNT modification with silicaor titania and represents a broader route to selectivelyfunctionalized SWNTs with a variety nanomaterials such asAg, Au, and semiconductor quantum dots. Future efforts willexplore the use of other bifunctional peptides to make SWNTcomposite materials, further probing the structure andinteraction of the peptide at the surface of the nanotube, anda focus on morphology control of the products formed inthese reactions.

Acknowledgment. Funding for this work was providedby the Air Force Office of Scientific Research (BIC program)and the Wright Brothers Institute. We thank Pamela Lloyd,Barney Taylor and Marlene Houtz for technical assistance.We dedicate this manuscript to Professor Rick Smalley.

Supporting Information Available: Experimental meth-ods, circular dichroism data, AFM micrograph of P1R5peptide, SEM micrograph of titania and titania-coatedSWNTs. This material is available free of charge via theInternet at http://pubs.acs.org.

References

(1) (a) Harris, P.Carbon Nanotubes and Related Structures: NewMaterials for the Twenty-First Century; Cambridge UniversityPress: Cambridge, U.K., 2001. (b)Carbon Nanotubes: Synthesis,Structure, Properties and Applications; Dresselhaus, M. S., Dressel-haus, G., Avouris, P., Eds.; Springer: Berlin, 2001. (c) Special issueon Carbon Nanotubes.Acc. Chem. Res.2002, 35, 997-1113.

(2) (a) Fu, Q.; Lu, C.; Liu, J.Nano Lett.2002, 2, 329-32. (b) Seeger,T.; Kohler, T.; Frauenheim, T.; Grobert, N.; Ru¨hle, M.; Terrones,M.; Seifert, G.Chem Commun. 2002, 1, 34-35. (c) Han, W.; Zettl,A. Nano Lett.2003, 3, 681-683. (d) Hernadi, K.; Ljubovic´, E.; Seo,J. W.; Forro, L. Acta Mater. 2003, 51, 1447-1452. (e) Sun, J.; Gao,L.; Li, W. Chem Mater.2002, 14, 5169-5172. (f) Sun, J.; Gao, L.

Figure 3. SWNTs coated with titania. (A) SEM micrographshowing a mat of carbon nanotubes embedded in the titania matrix.(B) TEM micrographs of image of SWNTs coated with titaniafollowing sonication in water revealing the coating on the tubesand the connectivity between titania features. The energy dispersivespectrum of the mat shown in A. The Si peak is due to the TEMgrid.

Figure 4. Typical Raman spectrum at 514 nm of the titania-coatedSWNT product. They axis is plotted log scale to incorporate boththe intense and smaller features. Arrows highlight peaks attributedto the presence of titania (anatase). The asterisk represents thesubstrate (silicon) peak.

Nano Lett., Vol. 6, No. 1, 2006 43

Carbon 2003, 41, 1063-1068. (g) Huang, Q.; Gao, L.J. Mater.Chem. 2004, 14, 2536-2541. (h) Colorado, R., Jr.; Barron, A. R.Chem. Mater.2004, 16, 2691-2693. (i) Zhao, B.; Hu, H.; Mandal,S. K.; Haddon, R. C.Chem. Mater.2005, 17, 3235-3241.

(3) Jang, S.-R.; Vittal, R.; Kim, K.-J.Langmuir2004, 20, 9807-9810.(4) (a) Wang, W.; Serp, P.; Kalck, P.; Faria, L.J. Appl. Catal. A2005,

56, 305-312.(5) (a) Vincent, P.; Brioude, A.; Journet, S.; Rabaste, S.; Purcell, S. T.;

Le Brusq, J.; Plenet, J. C.J. Non-Cryst. Solids2002, 311, 130-137.(b) Lee, S.; Sigmund, W. M.Chem. Commun.2003, 6, 780-781.(c) Huang, Q.; Gao, L.J. Mater. Chem.2003, 13, 1517-1519. (d)Jitianu, A.; Cacciaguerra, T.; Benoit, R.; Delpeux, S.; Be´guin, F.;Bonnamy, S.Carbon2004, 42, 1147-1151. (e) Jitianu, A.; Caccia-guerra, T.; Berger, M.-H.; Benoit, R.; Be´guin, F.; Bonnamy, S.J.Non-Cryst. Solids2004, 345, 596-600. (f) Brioude, A.; Vincent,P.; Journet, C.; Plenet, J. C.; Purcell, S. T.Appl. Surf. Sci. 2004,221, 4-9. (g) Kedem, S.; Schmidt, J.; Paz, Y.; Cohen, Y.Langmuir2005, 21, 5600-5604.

(6) (a) Banerjee, S.; Wong, S. S.Nano Lett.2002, 2, 195-200. (b) Li,X.; Niu, J.; Zhang, J.; Li, H.; Liu, Z.J. Phys. Chem. B2003, 107,2453-2458. (c) Sun, J.; Iwasa, M.; Gao, L.; Zhang, Q.Carbon2004,42, 885-901.

(7) Brott, L. L.; Naik, R. R.; Pikas, D. J.; Kirkpatrick, S. M.; Tomlin,D. W.; Whitlock, P. W.; Clarson, S. J.; Stone, M. O.Nature2001,413, 291.

(8) Kroeger, N.; Deutzmann, R.; Sumper, M.Science1999, 286, 1129-1132.

(9) (a) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H.J. Am. Chem. Soc.2001, 123, 3838. (b) Shim, M.; Kim, N. W. S.; Chen, R. J.; Li, Y.;Dai, H. Nano Lett.2002, 2, 285. (c) Pantarotto, D.; Partidos, C. D.;Graff, R.; Hoebeke, J.; Briand, J.-P.; Prato, M.; Bianco, A.J. Am.Chem. Soc.2003, 125, 6160. (d) Dieckmann, G. R.; Dalton, A. B.;Johnson, P. A.; Razal, J.; Chen, J.; Giordano, G. M.; Munoz, E.;Musselman, I. H.; Baughman, R. H.; Draper, R. K.J. Am. Chem.Soc. 2003, 125, 1770-1777. (e) Chou, S. G.; Ribeiro, H. B.; Barros,E. B.; Santos, A. P.; Nezich, D.; Samsonidze, G. G.; Fantini, C.;Pimenta, M. A.; Jorio, A.; Plentz, F.; Dresselhaus, M. S.; Dresselhaus,G.; Saito, R.; Zheng, M.; Onoa, G. B.; Semke, E. D.; Swan, A. K.;Unlu, M. S.; Goldberg, B. B.Chem. Phys. Lett. 2004, 397, 296. (f)Lin, Y.; Taylor, S.; Li, H. P.; Fernando, K. A. S.; Qu, L. W.; Wang,W.; Gu, L. R.; Zhou, B.; Sun, Y. P.J. Mater. Chem.2004, 14, 527.(g) Zheng, M.; Jagota, A.; Strano, M. S.; Santos, A. P.; Barone, P.;

Chou, S. G.; Diner, B. A.; Dresselhaus, M. S.; McLean, R. S.; Onoa,G. B.; Samsonidze, G. G.; Semke, E. D.; Usrey, M.; Walls, D. J.Science2004, 302, 1545. (h) Zorbas, V.; Ortiz-Acevedo, A.; Dalton,A. B.; Yoshida, M. M.; Dieckmann, G. R.; Draper, R. K.; Baughman,R. H.; Jose-Yacaman, M.; Musselman, I. H.J. Am. Chem. Soc. 2004,126, 7222-7227. (i) 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. (j) 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.

(10) (a) Wang, S.; Humphreys, E. S.; Chung, S.-Y.; Delduco, D. F.; Lustig,S. R.; Wang, H.; Parker, K. N.; Rizzo, N. W.; Subramoney, S.;Chiang, Y.-M.; Jagota, A.Nat. Mater. 2003, 2, 196-200. (b) Kase,D.; Kulp, J. L., III; Yudasaka, M.; Evans, J. S.; Iijima, S.; Shiba, K.Langmuir2004, 20, 8939-8941.

(11) Zorbas, V.; Smith, A. L.; Xie, H.; Ortiz-Acevedo, A.; Dalton, A. B.;Dieckmann, G. R.; Draper, R. K.; Baughman, R. H.; Musselman, I.H. J. Am. Chem. Soc.2005, 127, 12323-12328.

(12) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley,R. E. Nano Lett.2003, 3, 1379-1382.

(13) (a) Chiang, W.; Brinson, B. E.; Smalley, R. E.; Margrave, J. L.;Hauge, R. H.J. Phys. Chem. B2001, 105, 1157-1161. (b) Hirsch,A. Angew. Chem., Int. Ed.2002, 41, 1853-1859. (c) Bahr, J. L.;Tour, J. M.J. Mater. Chem.2002, 12, 1952-1958. (d) Strano, M.S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan,H.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E.Science2003, 301, 1519. (e) Dyke, C. A.; Tour, J. M.J. Phys. Chem. A2004, 108, 1151-1159. (f) Hudson, J. L.; Casavant, M. J.; Tour, J.M. J. Am. Chem. Soc.2004, 126, 11158-11159.

(14) (a) Dickerson, M. B.; Naik, R. R.; Stone, M. O.; Cai, Y.; Sandhage,K. H. Chem. Commun.2004, 1776-1777. (b) Naik, R. R.; Jones, S.E.; Murray, C. J.; McAuliffe, J. C.; Vaia, R. A.; Stone, M. O.AdV.Func. Mater. 2004, 14, 25-30. (c) Sarikaya, M.; Tamerler, C.;Schwartz, D. T.; Baneyx, F.Annu. ReV. Mater. Res.2004, 34, 373-408. (d) Sanghvi, A. B.; Miller, K. P-H; Belcher, A. M.; Schmidt,C. E. Nat. Mater.2005, 4, 496-502.

(15) Sumerel, J. L.; Yang, W.; Kisailus, D.; Weaver, J. C.; Choi, J. H.;Morse, D. E.Chem. Mater.2003, 15, 4808-4809.

NL051899R

44 Nano Lett., Vol. 6, No. 1, 2006