Enhancement of Water Solubility of Single-Walled Carbon Nanotubes

Jul 31, 2008 - ... Thorbjørn Terndrup Nielsen , Luis Echegoyen , and Alex Fragoso ... Marco Dionisio , Jan M. Schnorr , Vladimir K. Michaelis , Rober...
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J. Phys. Chem. C 2008, 112, 13079–13083

13079

Enhancement of Water Solubility of Single-Walled Carbon Nanotubes by Formation of Host-Guest Complexes of Cyclodextrins with Various Guest Molecules Tomoki Ogoshi,*,† Mitsuhiro Ikeya,† Tada-aki Yamagishi,† Yoshiaki Nakamoto,† and Akira Harada*,‡ Graduate School of Natural Science and Technology, Kanazawa UniVersity, Kakuma-machi, Kanazawa, 920-1192, Japan, and Graduate School of Science, Osaka UniVersity, Toyonaka, 560-0043, Japan ReceiVed: February 18, 2008; ReVised Manuscript ReceiVed: June 16, 2008

Solubilization of single-walled carbon nanotubes (SWNTs) by physical adsorption of cyclodextrins (CDs), guest molecules, and host-guest complexes was investigated. As guest compounds, sodium adamantanecarboxylate (AdCNa), sodium dodecylbenzenesulfonate (SDBS), sodium ursodeoxycholate (UdCNa), sodium ferrocenecarboxylate (FeCNa), and TritonX-405 were employed. As host compounds, R-, β-, and γ-cyclodextrins (R-, β-, and γ-CDs) were used. By using CDs as a solubilizing agent, SWNTs were insoluble in aqueous media. In the presence of AdCNa and FeCNa, solubility of SWNTs was dramatically increased by formation of host-guest complex with β- or γ-CDs. By using SDBS-CD complexes, solubility of SWNTs was also enhanced compared to only guest molecules. On the other hand, solubility of SWNTs was decreased by formation of host-guest complexes of UdCNa and TritonX-405 with CDs. Formation of host-guest complex mostly resulted in enhancement of water solubility of SWNTs. Introduction Since the discovery of single-walled carbon nanotubes (SWNTs), tremendous interest in SWNTs remains strong because of their unique structural, electrical, and mechanical properties.1 However, their applications have been extremely limited due to their low solubility in solvents. Therefore, solubilization of SWNTs has been one of hot topics for the past few years. For solubilization of SWNTs in solvents, chemical modification of SWNTs, and physical adsorption of organic molecules on SWNT surfaces are useful strategies. Chemical modifications of SWNTs such as sidewall halogenations,2 cycloadditions,3 radical additions,4 “click” coupling,5 and defectsite amidation/esterification reactions6 were reported. In terms of physical adsorption of organic compounds on SWNT surface, soluble SWNTs in aqueous media were obtained by using π-π interaction between SWNTs and polynuclear aromatic compounds, such as pyrene and naphthalene derivatives.7 Successful simple solubilization of SWNTs with several types of surfactants such as sodium dodecylsulfate (SDS), TritonX-405, and sodium dodecylbenzenesulfonate (SDBS) was reported by Smalley and co-workers.8 Recently, we first reported on solubilization of SWNTs by physical adsorption of inclusion complex between β-cyclodextrin (β-CD) and adamantane derivative (Scheme 1).9 By sonication of SWNTs with the inclusion complex between adamantane derivative and β-CD, SWNTs were dispersed and soluble in aqueous media. In contrast, SWNTs were insoluble with only adamantane derivative or β-CD. Formation of host-guest complex was achieved by adding CD enhanced water solubility of SWNTs. The interaction between AdCNaβ-CD complex and SWNT surface should be mainly a hydrophobic interaction because host-guest complexes have both hydrophobic and hydrophilic segments and SWNTs were soluble * To whom correspondence should be addressed. E-mail: ogoshi@ t.kanazawa-u.ac.jp. Tel: +81-76-234-4775. Fax:+81-76-234-4800. † Kanazawa University. ‡ Osaka University.

SCHEME 1: Enhancement of Water Solubility of SWNTs by Formation of Host-Guest Complex

in aqueous media. However, reasons and mechanisms of enhancement of SWNTs water solubility by formation of host-guest complex were unclear when we previously reported the findings as the communication.9 Therefore, in this study, we investigated water solubility of SWNTs by using various host-guest complexes between guests and CD derivatives. In any case, formation of a host-guest complex upon addition of CD effected a change in water solubility of SWNTs. The effects on solubilization ability of SWNTs using host-guest complexes were able to be divided into three classes. (1) Soluble SWNTs were obtained with host-guest complexes, while SWNTs were insoluble with only guest molecules (Class I). (2) Guest compounds enabled solubilization of SWNTs in aqueous solutions. Moreover, solubility of SWNTs was largely increased by forming host-guest complex (Class II). (3) SWNTs became insoluble in aqueous media by formation of host-guest complex, while SWNTs were soluble with only guest molecules (Class III). We discuss effect of the structure of host-guest complexes on water solubility of SWNTs. Experimental Section Materials. We purchased SWNTs produced by the method of high-pressure decomposition of carbon monoxide (HiPco Process, the length and diameter of pristine SWNTs are about 20-2000 nm and 0.8-1.2 nm, respectively) from Carbon

10.1021/jp801455e CCC: $40.75  2008 American Chemical Society Published on Web 07/31/2008

13080 J. Phys. Chem. C, Vol. 112, No. 34, 2008 SCHEME 2: Chemical Structures of (A) Cyclodextrins (CDs) and (B) Guest Compounds

nanotechnologies, Inc., Texas. The HiPco SWNTs were purified according to a previous paper.10 Measurements. The 1H NMR spectra were recorded at 270 MHz with a JEOL-JNM EX270 spectrometers. Two-dimensional ROESY NMR spectra were recorded with a Varian Unity Inova Plus 600 NMR. UV-vis absorption spectra were recorded with a JASCO V-630 spectrophotometer in quartz cuvette (1 cm path length). Deionized distilled water was used as blank. Tapping mode atomic force microscopy (TM-AFM) was taken on multimode SPA 400 (SEIKO Instruments). Nanoprobe cantilevers (SI-DF20, SEIKO Instruments) were utilized. The radius of the tip is under 15 nm. TM-AFM images were recorded at a scan rate of 2 Hz. The sample was prepared by slow evaporation on substrate overnight at room temperature. Mica was used as a substrate for TM-AFM measurements. Solubilization of SWNTs with Host-guest Complexes. Typical experimental procedure for solubilization of SWNTs is as follows. To a suspension of SWNTs (1.0 mg) in an aqueous solution (5.0 mL), guest compounds and cyclodextrin derivatives were added, and then the resulting solution was sonicated in low-energy ultrasonic bath (Bransonic 2510) for 2 h at room temperature. After the sonication, insoluble SWNTs were removed by centrifugation at 15 000 g for 1 h. In cases of sodium adamantanecarboxylate (AdCNa) and sodium ferrocenecarboxylate (FeCNa), 20 mg of the guest and CDs (1 eq to the guest) was used. The condition was as that in our previous report.9 In sodium ursodeoxycholate (UdCNa), sodium dodecylbenzenesulfonate (SDBS) and Triton X-405, 2 wt % of the guest with respect to the amount of SWNTs and CDs (1 equivalent to guest) was employed. The experimental condition was as that in the procedure reported by Smalley and co-workers.8 Result and Discussion Solubilization of SWNTs by Using Cyclodextrins (CDs). The solubilization ability of SWNTs in aqueous solution with only CDs or guests was investigated. As host compounds, R-, β-, and γ-cyclodextrins (R-, β-, and γ-CDs) were used (Scheme 2A). As guest molecules, sodium adamantanecarboxylate (Ad-

Ogoshi et al. CNa), sodium ferrocenecarboxylate (FeCNa), sodium dodecylbenzenesulfonate (SDBS), sodium ursodeoxycholate (UdCNa), and Triton X-405 were employed (Scheme 2B). Experimental condition was as that in the procedure reported by Smalley and co-workers.8 (Same solubilization procedure of SWNTs was carried out except for addition of CDs.) Typical experimental procedure for solubilization of SWNTs is as follows. To suspension of SWNTs (1.0 mg) in aqueous solution (5.0 mL), guest compounds or CDs (1 eq to the guest) were added, and then the resulting solution was sonicated for 2 h at room temperature. After the sonication, insoluble SWNTs were removed by centrifugation. UV-vis absorption spectra for the supernatants prepared are shown in Figure 1. Absorbance intensities from SWNTs at 500 nm are summarized in Table 1. With host compounds such as R-, β-, and γ-CDs, supernatants after sonication with SWNTs were colorless. Characteristic SWNT van Hove singularities, which were found in homogeneous dispersion of SWNTs, were not observed (Supporting Information). Thus, with only CD derivatives, SWNTs were slightly soluble in aqueous solution. These observations were already reported.11,12 Considering that the diameter of CDs is about 0.45-0.85 nm (R-CD, 0.45 nm; β-CD, 0.7 nm; γ-CD, 0.85 nm) and the outer diameter of Hipco SWCNT is about 1.0 nm,13 the cavity of CDs is too small to form pseudo-rotaxane structure between SWNT as the axle and CDs as the ring. Moreover, with large-ring CD such as η-CD composed of 12 glucopyranose units (diameter of η-CD is about 1.8 nm) SWNTs were soluble in aqueous media by forming an inclusion complex with SWNTs like a pseudo-rotaxane complex.12 Solubilization of SWNTs by Using Host-guest Complexes (Class I). We investigated changes in solubilization ability of SWNTs by formation of host-guest complex with CDs. By using only guest such as AdCNa (Figure 1A(a)) as a solubilizing agent, SWNTs were slightly soluble. Because of the bulkiness of the adamantyl groups, AdCNa might be hardly adsorbed to SWNT surface. Because AdCNa forms a stable complex with CDs, solubilization of SWNTs with the AdCNa-CDs complex was examined. With AdCNa-β-CD (Figure 1A(c)) or AdCNaγ-CD (Figure 1A(d)) complexes, the supernatant was homogeneous black solution and typical SWNT van Hove singularities were observed, while SWNTs were insoluble with AdCNa-R-CD (Figure 1A(b)). Considering that SWNTs are insoluble with only AdCNa or CDs, formation of the host-guest complexes with β-CD or γ-CD enhances solubility of SWNTs. From 2D ROESY NMR of AdCNa-β-CD complex in D2O, prospective structure of AdCNa-β-CD complex is shown in Scheme 3a (2D ROESY NMR spectrum is shown in Supporting Information). Hydrophobic adamantyl moiety was included into the cavity alongside. Therefore, stacking between SWNT and AdCNa included into β-CD cavity might be possible. The same trends were also observed in FeCNa (Figure 1B). SWNTs were insoluble with only FeCNa (Figure 1B(a)) due to bulkiness of ferrocenyl moiety. By using FeCNa-R-CD complex as a solubilizing agent, SWNTs were hardly soluble (Figure 1B(b)). In contrast, by formation of host-guest complex of FeCNa with β-CD (Figure 1B(c)) or γ-CD (Figure 1B(d)), SWNTs were soluble. Solubility of SWNTs with FeCNa-γ-CD complex was better than that with FeCNa-β-CD complex. These observations should result from structure of the CD-ferrocene complexes.14 Since ferrocene forms 2:1 host-guest complex with R-CD (Scheme 3b), hydrophobic ferrocenyl part was almost covered by R-CDs. Therefore, binding between hydrophobic ferrocenyl moiety of the complex and SWNT might be hard. With β-CD

Solubilization of SWNTs with CD-Guest Complexes

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Figure 1. UV-vis spectra of the supernatants after sonication with SWNTs in aqueous media by using guest or host-guest complexes. (A) AdCNa (a) without host, (b) with R-CD, (c) with β-CD, and (d) with γ-CD; (B) FeCNa (a) without host, (b) with R-CD, (c) with β-CD, and (d) with γ-CD; (C) SDBS (a) without host, (b) with R-CD, (c) with β-CD, and (d) with γ-CD; (D) UdCNa (a) without host, (b) with R-CD, (c) with β-CD, and (d) with γ-CD; (E) TritonX-405 (a) without host, (b) with R-CD, (c) with β-CD, and (d) with γ-CD.

TABLE 1: Absorbance Intensities from SWNTs at 500 nm in an Aqueous Solution with Host-Guest Complexes CDs/guests none AdCNa FeCNa SDBS UdCNa TritonX-405 none R-CD β-CD γ-CD

0.89 1.87

0.23 0.77

0.27 0.43 0.57 0.49

0.95 0.49 0.47 0.32

0.58 0.42 0.08 0.04

SCHEME 3: (a) Structure of AdCNa-β-CD Complex Estimated from 2D ROESY Measurement; structures of inclusion complexes of ferrocene with CDs: the complexes with r-CD (b), β-CD (c), and γ-CD (d)

or γ-CD, ferrocene forms 1:1 inclusion complex. In the case of FeCNa-β-CD complex, ferrocene locates into the cavity alongside (Scheme 3c). Direction of ferrocene formed the complex with γ-CD is perpendicular to γ-CD cavity (Scheme 3d). Among these complexes, the perpendicular direction of ferrocene along

γ-CD cavity might be most easily adsorbed to SWNT surface. In both AdCNa and FeCNa used as guests, soluble SWNTs were obtained with host-guest complexes, while SWNTs were insoluble with only guest molecules (Class I). Enhancement of Solubility of SWNTs by Formation of Host-guest Complexes (Class II). We also examined solubility of SWNTs by formation of host-guest complexes of SDBS with CDs. With SDBS (Figure 1C(a)), SWNTs were soluble in aqueous media as previous reported by Smalley and co-workers.8 SDBS is typical and commonly used solubilizer of SWNTs in aqueous solution. Since dodecyl group of SDBS forms host-guest complex with CDs, we used SDBS-CD complexes for solubilization of SWNTs. By formation of host-guest complexes between SDBS and CDs (Figure 1C(b-d)), solubility of SWNTs was increased (Class II), which should be in relation with the structure of the host-guest complex. From 2D ROESY measurements (Supporting Information), the prospective structure of the SDBS-β-CD complex is shown in Scheme 4. Dodecyl chain of SDBS was partially included into β-CD cavity. Smalley and co-workers reported mechanism of solubilization of SWNTs with surfactants.8 Surfactant coverage around SWNT inhibited aggregation of SWNTs. By formation of host-guest complexes

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SCHEME 4: Structure of SDBS-β-CD Complex Estimated by 2D ROESY Measurement

Figure 2. (a) TM-AFM image of SWNTs with FeCNa-γ-CD complex. (b) Height profile along dash line in panel a.

SCHEME 5: (a) Prospective Structure of UdCNa-β-CD Complex Estimated from 2D ROESY Measurement; Structures of Inclusion Complexes of (b) TritonX-405-r-CD Complex and (c) TritonX-405-β-CD Complex; the Direction of CD Formed Complex Is Random in Case of TritonX-405 As Guest

SWNTs was lower than using R-CD-TritonX-405 complex and typical SWNT van Hove singularities were hardly observed. Site-selective complexation of TritonX-405 with R-CD or β-CD results in these observations. The hydrophobic end group of TritonX-405 should be adsorbed to the SWNT surface for solubilization of SWNTs. Therefore, formation of inclusion complexes of hydrophobic part of TritonX-405 with β- and γ-CDs inhibits adsorption of the hydrophobic part onto SWNT surface. In contrast, complexation of hydrophilic PEO chain with R-CD should hardly affect the bind of hydrophobic part onto SWNT surface, thus solubilization ability of SWNTs was not changed even with R-CD. TM-AFM study. From TM-AFM measurement, nanotubes solubilized by FeCNa-γ-CD complex were observed and average size of the tubes was about 1.0-1.1 nm (Figure 2). Considering that the average diameter of the HiPco SWNT is 0.8-1.2 nm,13 the height measurement suggests that the nanotubes observed are individual SWNTs. Dispersion of SWNTs in aqueous media by using host-guest complexes as solubilizer was confirmed. Conclusions

of SDBS with CDs, coverage around SWNTs might become stable due to hydrogen bonding between CDs formed host-guest complexes. Reducing Solubility of SWNTs by Formation of Host-guest Complexes (Class III). In the presence of biosurfactant of UdCNa (Figure 1D(a)), SWNTs were also soluble as studied by Nakashima and co-workers.15 With the host-guest complex between UdCNa and CDs, SWNTs were slightly soluble and typical SWNT van Hove singularities were not clearly observed (Figure 1D(b-d)). Judging from the high solubility of SWNTs with UdCNa via hydrophobic binding of the steroid group onto SWNT surface, the formation of host-guest complex between UdCNa and CDs decreased solubilization ability of SWNTs. Scheme 5a shows the prospective structure of the UdCNa-βCD complex estimated from 2D ROESY NMR measurement (Supporting Information). The steroid part of UdCNa is mostly included into β-CD cavity. Thus, adsorption of the steroid part of UdCNa onto SWNT surface might be inhibited by the formation of the complex with β-CD. Complexation of TritonX-405 with R-CD or β-CD was previous studied.16 TritonX-405 composes of PEO chain and iso-octyl group through a benzene ring. Therefore, R-CD binds a PEO chain and β-CD binds a hydrophobic end group (an isooctyl and a phenyl group) specifically (Scheme 5). In this work, solubilization of SWNTs using host-guest complexes of TritonX-405 with CDs was examined (Figure 1E). In TritonX405 (Figure 1E(a)), SWNTs were soluble in aqueous media as previous reported by Smalley and co-workers.8 With host-guest complex between TritonX-405 and R-CD (Figure 1E(b)), SWNT van Hove singularities were observed and the solubility of SWNTs slightly decreased compared to that with TritonX-405. By using the host-guest complexes of TritonX-405 with β-CD (Figure 1E(c)) and γ-CD (Figure 1E(d)), the solubility of

Solubilization ability of SWNTs by using host-guest complexes was investigated. The trends of solubilization ability of SWNTs with host-guest complexes were divided into three classes. In case of AdCNa and FeCNa used as a guest compound, solubility of SWNTs was dramatically increased by forming host-guest complex with β- or γ-CDs (Class I). By using SDBS-CD complexes, solubility of SWNTs was also enhanced compared to only SDBS (Class II). On the other hand, solubility of SWNTs was decreased by formation of host-guest complexes such as UdCNa-CDs or TritonX-405-CDs complexes (Class III). In case of Class I and Class II, hydrophobic part was not spatially covered by CD cavity, therefore hydrophobic part was able to bind to SWNT surface. CD should enhance hydrophilicity for dispersion in aqueous media. On the other hand, in UdCNa and TritonX-405 (Class III), binding site of hydrophobic part was fully included into CD cavity. Thus, solubilization ability of SWNTs was decreased by formation of host-guest complexes between these guests and CDs. Since trends of Class I and Class II were majority compared to that of Class III, formation of host-guest complex upon addition of CDs mostly enhanced solubility of SWNTs. The solubilization procedure using CDs as an additive is so simple, effective and a very promising strategy for increasing water solubility of SWNTs that the procedure will enlarge solubilization techniques of SWNTs and be industrially applicable. Acknowledgment. We thank Professor Kohshin Takahashi (Kanazawa University) for TM-AFM measurements, and Dr. Yoshinori Takashima (Osaka University) and Dr. Tomokazu Umeyama (Kyoto University) for 2D ROESY NMR measurements and helpful comments. This work was supported by the Kinki Invention Center and a Grant-in-Aid for Young Scientist (WAKATE-B-1975011) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

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