Simultaneous Chirality and Enantiomer Separation of Metallic Single

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Simultaneous Chirality and Enantiomer Separation of Metallic SingleWall Carbon Nanotubes by Gel Column Chromatography Takeshi Tanaka,* Yasuko Urabe, Takuya Hirakawa, and Hiromichi Kataura Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562, Japan

Downloaded by UNIV OF MANITOBA on September 4, 2015 | http://pubs.acs.org Publication Date (Web): September 1, 2015 | doi: 10.1021/acs.analchem.5b02563

S Supporting Information *

ABSTRACT: We report the chirality and enantiomer separation of metallic single-wall carbon nanotubes (SWCNTs) using gel chromatography, which has been the last remaining issue in SWCNT separation that has yet to be achieved. The key to the separation is summarized as the following three points: (i) the use of a preseparated metallic SWCNT mixture to eliminate the semiconducting SWCNTs that are more interactive with the gel; (ii) the reduction of the concentration of dispersant to increase the interaction between the metallic SWCNTs and the gel; and (iii) the use of a long column to increase the number of interaction sites that enhance the slight differences between metallic SWCNT species. Using these three separation conditions, we obtained chirality-sorted metallic SWCNTs, especially (10,4) metallic SWCNTs were highly enriched. Circular dichroism spectra demonstrated the enantiomer separation of metallic SWCNTs. The discrimination of the enantiomers is derived from the dextran in the gel, which is the only enantiomeric moiety in this system. This is the first report on the enantiomer separation of metallic SWCNTs and will contribute to progress in the fundamental physics and applications of SWCNTs.

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study, we report the chirality separation of metallic SWCNTs by gel chromatography using a long column and the first enantiomer separation of metallic SWCNTs.

ingle-wall carbon nanotubes (SWCNTs) have attracted great interest for various applications due to their superior mechanical, electrical, optical, and chemical properties.1,2 The structure of SWCNTs, defined by chiral vectors (n,m), determines their electrical and optical properties.3,4 Over the past decade, various separation methods for metallic/semiconducting and single-chiral SWCNTs have been developed.5−15 In particular, for electronic and optical applications, homogeneous semiconducting SWCNTs are required, and many reports on the separation of single-chirality semiconducting SWCNTs have been reported.5,7,11−15 The singlechirality enantiomer (handedness) separation of semiconducting SWCNTs has also been reported by selective extraction,16 density gradient ultracentrifugation (DGU),12 and gel column chromatography.17 The single-chirality enantiomer-separated SWCNTs are truly single-structure, making them very important for understanding the basic physics of SWCNTs, such as through studies of a single crystal of SWCNTs. Although there have been a limited number of reports on chirality separation for metallic SWCNTs, the DGU method selectively extracts a mixture of armchair metallic SWCNTs,18 and single-chirality armchair metallic SWCNTs (6,6) and (7,7) have been isolated by SWCNTs/DNA chromatography19 and an aqueous two-phase separation (ATP).11 Interestingly, armchair-type (n,n) metallic SWCNTs were selectively separated from other metallic SWCNTs in these reports. Very recently, Zheng et al. reported the separation of nonarmchair single-chiral (7,4) metallic SWCNTs by ATP using DNA as a dispersant.20 As far as we know, however, there have been no reports on the enantiomer separation of metallic SWCNTs. True single-chiral metallic SWCNTs should also be important for basic science and applications. In the present © XXXX American Chemical Society

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EXPERIMENTAL SECTION Preparation of Metallic SWCNT Mixture. HiPcoSWCNTs (raw soot, RO513, NanoIntegris) were dispersed in 0.5% sodium dodecyl sulfate (SDS, 99%, Sigma-Aldrich) aqueous solution (30 mg SWCNT/30 mL of SDS solution) using the tip-type homogenizer for 30 min and ultracentrifuged (415000 × g for 30 min at 25 °C) to remove bundles and impurities. The upper 80% supernatant was recovered as a SWCNT dispersion. A column (50 mL medical plastic syringe, Terumo) was plugged with cotton and filled with 30 mL of gel beads (Sephacryl S-200 HR, GE Healthcare). After the column was equilibrated with 0.5% SDS solution, 11.5 mL of the SWCNT dispersion was applied to the column. Then, successively 10, 5, and 5 mL of 0.5% SDS solutions were poured to the column, and the solutions from the outlet were recovered as early-, middle-, and late-eluted fractions, respectively. This separation was repeated eight times, and obtained early- and late-eluted fractions were combined separately and designated as Metal 1 and Metal 2 fractions, respectively. The Metal 1 fraction (∼80 mL) was filtered through a membrane with 0.2 μm pores and concentrated by ultracentrifugation (after 3 h at 415000 × g, the supernatant was removed as much as possible). The concentrated metallic Received: July 8, 2015 Accepted: August 26, 2015

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DOI: 10.1021/acs.analchem.5b02563 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry SWCNT dispersion (∼7 mL) was diluted with the same amount of water and was sonicated using an ultrasonic bath briefly. Separation of Metallic SWCNTs. Metallic SWCNT separation was conducted by using a column chromatography system (AKTA explorer 10S, GE Healthcare) equipped with Hiprep 16/60 Sephacryl S-200 HR prepacked column (60 cm in length, 120.5 mL in bed volume, GE Healthcare). Before the column, the Tricorn 10/50 column filled with ∼4.3 mL of Sephacryl S-200 HR gel was connected as a guard column. After the equilibration of the column with 0.3% SDS solution, 5 mL of concentrated Metal 1 fraction was applied to the column at a flow rate of 0.5 mL/min. The applied SWCNTs were successively eluted by 0.3% SDS, 0.4% SDS, 0.5% SDS, and 2% sodium cholate solutions (250 mL each). Eluted samples were collected at 2, 5, and 10 mL of fraction volumes, as indicated in Figure 2a. The elution of the SWCNTs were traced by the optical absorption at 280 and 550 nm. For Metal 2 separation, three open columns filled with 2 mL of the gel aligned in series were used, because the amount of the concentrated Metal 2 fraction was not enough for the separation using chromatography system. Optical Measurements. Optical absorbance spectra were measured using an ultraviolet−near-infrared spectrophotometer (UV-3600, Shimadzu). Photoluminescence excitation spectra were measured using a spectro-fluorometer (Nanolog, Horiba) equipped with a liquid-nitrogen-cooled InGaAs near-IR alloy detector. Raman spectra for 488 nm excitation were measured using a triple monochromator (PDPT3−640S, Photon Design) equipped with a charge-coupled device detector. Raman spectra for 532 and 633 nm excitations were measured using a confocal Raman microscope (XploRA, Horiba). Circular dichroism (CD) spectra were recorded using a Jasco model 820 CD spectrometer equipped with a detector covering the wavelength range of 200−900 nm.

Figure 1. (a) Absorption spectra of metallic SWCNTs separated from semiconducting SWCNTs. The spectra were normalized at 280 nm. (Inset) Photographs of each separated metallic SWCNT solution. (b, c) Photoluminescence excitation spectra of HiPco-SWCNTs/SDS used for metal/semiconductor separation (b) and of separated metallic SWCNTs (Metal 1; c).

relatively larger diameter metallic SWCNTs than the latter. Photoluminescence measurement is highly sensitive to semiconducting SWCNTs and detects only semiconducting SWCNTs. The result of the photoluminescence measurement showed no semiconducting SWCNTs in the Metal 1 fraction, confirming again high-purity metallic SWCNTs (Figure 1b,c, see also Supporting Information, Figure S1, for the different height scale spectrum of Figure 1c). Separation of Metallic SWCNTs. As described above, metallic SWCNTs have a low affinity to the gel compared with semiconducting SWCNTs. The concentration of SDS was then lowered to 0.3% (close to the critical micelle concentration [CMC], 8.2 mM [0.24%] at 25 °C)22 to increase the interaction between the metallic SWCNTs and the gel in the second separation for metallic SWCNTs. The SDS layer on the SWCNT surface is thought to be thin at the low SDS concentration, resulting in the increase of the interaction; however, the SWCNTs likely form irreversible aggregation at the concentration lower than the CMC. We used chromatography equipment with a 60 cm long column in this separation. Very small difference in the interaction was discriminated during the migration of the metallic SWCNTs through the long column. The Metal 1 fraction containing metallic SWCNTs of various chiralities was concentrated by ultracentrifugation and diluted with the same volume of water to 0.25% SDS. The resulting concentrated SWCNT dispersion was injected into the long column equilibrated with 0.3% SDS aqueous solution, and aqueous solutions containing 0.3%, 0.4%, and 0.5% SDS and 2% sodium cholate (SC) were successively applied to the column (Figure 2a). The elution of the metallic SWCNTs was traced by the optical absorption at 280 and 550 nm. A sharp strong peak at ∼40 mL of retention volume appeared after the injection, and then broad and small peaks (60−250 mL)

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RESULTS AND DISCUSSION Preparation of Metallic SWCNT Mixture. In the existing metallic/semiconducting SWCNT separation and singlechirality separation of semiconducting SWCNTs using polysaccharide gel, the gel selectively adsorbs semiconducting but not metallic SWCNTs.8,10,13,14,21 In the present study, metallic SWCNTs were first separated from semiconducting SWCNTs (see details in Supporting Information). A HiPcoSWCNT/0.5% sodium dodecyl sulfate (SDS) aqueous dispersion was prepared by ultrasonication and ultracentrifugation. The SWCNTs in the dispersion are covered with SDS molecules and stably exist as an isolated state in the solution. The dispersion was applied to an open column containing Sephacryl S-200 HR gel beads (GE Healthcare) equilibrated with 0.5% SDS solution, and metallic SWCNTs were collected as unbound fractions. The early and late eluted fractions designated as Metal 1 and Metal 2 showed red wine and orange colors, respectively (Figure 1a, insets). The optical absorption spectra demonstrated M11 peaks derived from the first optical transition of metallic SWCNTs (approximately 420−650 nm) but not S11 and S22 peaks derived from the first and second optical transitions of semiconducting SWCNTs (approximately 900−1350 and 600−900 nm, respectively),4 indicating the high-purity separation of metallic SWCNTs (Figure 1a). The peaks of the Metal 1 fraction (470−620 nm) were at longer wavelengths compared with those of the Metal 2 fraction (420−570 nm), namely, the former fraction contained B


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Analytical Chemistry

Figure 2. (a) Chromatogram of metallic SWCNT separation using a 60 cm column. Colored arrowheads indicate the fractions used for the subsequent optical measurements (Figures 3 and 4). Fractions are numbered following the order A1−A12, B1−B12, and, finally, J1−J12. (b) Picture of the separated metallic SWCNT solutions. The solutions of separately separated fractions from the Metal 2 fraction are also shown, Nos. 8, 10, and 12.

Figure 3. Absorption spectra (a) and Raman spectra (b−d) of separated metallic SWCNTs. Vertical broken lines in (a) correspond to laser wavelengths (488, 532, 633 nm) used for Raman measurement in (b)−(d).

SDS was then increased to 0.4% to elute the metallic SWCNTs adsorbed onto the gel at 0.3% SDS. The concentration change of the eluent could be detected as a change of conductivity in the chromatogram at approximately 330 mL of retention volume. We again detected a sharp peak and subsequent small peaks after elution by 0.4% SDS. The elution solution was changed from 0.4% SDS to 0.5% SDS, and the last SWCNTs that remained adsorbed to the gel were ultimately eluted by 2% SC. The solutions obtained in the different fractions showed

followed during the 0.3% SDS elution. It was thought that the SWCNTs in the sharp peak barely interacted with the gel and proceeded quickly through the column, whereas the SWCNTs in the subsequent broad peaks interacted with the gel and slowly migrated through the column. Although SWCNTs are insoluble and cannot exist in the absence of surfactants in water, the present separation can be classified in a kind of micellar chromatography.23 The 0.3% SDS solution was continuously flowed until no absorbance was detected. The concentration of C


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Figure 4. (a) Normalized CD spectra of separated metallic SWCNTs. Each CD spectrum is divided by the value of the optical absorption at 280 nm. Corresponding optical absorption spectra are superimposed as thin gray curves. (b, c) Optical absorption spectra (b) and CD spectra (c) for the fractions of F8 and H5. (d) Structural models of enantiomers of (9,3) metallic SWCNTs. To clarify the handedness, corresponding C−C bonds are highlighted by red lines.

molecular sieve.10,13,21 The fractionation range of Sephacryl S200 HR gel (up to ∼2 × 104 Da for linear double-stranded DNA and ∼8 × 104 Da for dextran polymer) is too small for SWCNTs (for example, for 100 nm to over 1 μm SWCNTs [diameter, ∼1 nm], to the values are ∼1.3 × 105 Da to over ∼1.3 × 106 Da) to separate based on the difference in size. In this chirality separation of metallic SWCNTs, the SWCNTs were adsorbed to the gel and eluted by different concentrations of the surfactant. As a different experiment, batch separation was also conducted. In this separation, a metallic SWCNT mixture solution containing a much larger amount of SWCNTs than the amount that can be adsorbed by the gel was mixed (overloading condition), and the adsorbed SWCNTs were eluted and analyzed by optical absorption (Supporting Information, Figure S3). The adsorbed fraction contained metallic SWCNTs having absorption peaks at 455 and 504 nm, indicating that the chirality separation of metallic SWCNTs occurred. This result also clearly shows that the metallic SWCNT chirality separation is caused by the selective adsorption of the SWCNTs by the gel. To obtain further information on the chirality in each separated fraction, the Raman spectra in radial breathing mode region were measured using 488, 532, and 633 nm lasers (Figure 3b, c, and d, respectively). Although not all types of metallic SWCNTs were detected at high sensitivity because of the limitation of the excitation laser wavelengths, the Raman spectra showed that each fraction contained different chiralities of metallic SWCNTs. Combined with the results of the absorption spectra and Raman spectra, it was found that (10,4) metallic SWCNTs were highly enriched in fraction D3.25

different colors ranging from purple to red to orange (Figure 2b). Optical Absorption and Raman Spectra of Separated Metallic SWCNTs. This color difference suggests the chirality sorting of metallic SWCNTs. Figure 3a shows the absorption spectra of the selected fractions indicated by the colored arrowheads in Figure 2a. In each fraction, there is no clear absorption peak of semiconducting SWCNTs in the S11 and S22 regions (Supporting Information, Figure S2). The enrichment of the metallic SWCNTs with 630 nm optical absorption could be observed in the early eluted fraction A5. Through the elution of 0.3−0.5% SDS and to 2% SC, the earlier-eluted SWCNTs showed optical absorption peaks of longer wavelengths, whereas the later-eluted SWCNTs showed peaks of a shorter wavelength in the M11 region, indicating the chirality sorting of metallic SWCNTs. The wavelengths roughly correspond to the diameters of the SWCNTs; that is, larger diameter SWCNTs show longer wavelengths in each optical transition.4 A difference in the spectra from 300 to 380 nm, which is probably derived from the second optical transition of the metallic SWCNTs (M22), could also be observed. In fraction D3, the peak at 552 nm in M11 that was derived from family 24 metallic SWCNTs (2n + m = 24) was highly enriched, whereas fraction F6 mainly contained metallic SWCNTs with optical absorption at 506 nm derived from family 21 (2n + m = 21).24 Although the Sephacryl S-200 HR gel was originally developed for size exclusion column chromatography to separate biomolecules, in the metal/semiconductor separation of SWCNTs and the chirality separation of semiconducting SWCNTs, the gel acts as a selective adsorbent, not as a D


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highly (10,4)-enriched fraction D3. Interestingly, the absorption spectra (Figure 4b) and Raman spectra (Figure 3b−d) of fractions F8 and H5 were almost identical, indicating that the chirality breakdown (n,m) of the SWCNTs in each fraction was the same; however, corresponding CD spectra showed an opposite pattern (Figure 4c). These results suggest that fractions F8 and H5 contained different isomers of metallic SWCNTs from each other, as shown in Figure 4d. The Sephacryl gel is composed of allyl dextran cross-linked by N,N′-methylene bis(acrylamide), with dextran being a biopolymer and chiral compound consisting of α-D-glucan. Importantly, only an achiral surfactant, SDS, was used before using 2% SC as an eluent in this enantiomer separation. These facts suggest that the enantiomer separation of SWCNTs originates from the ability to discriminate based on handedness of the dextran moiety of the gel. This enantiomer separation of metallic SWCNTs again supports the supposition that the mechanism of this separation is the selective interaction between SWCNTs and the gel (especially dextran moiety), not size exclusion. In this study, we obtained (10,4)-enriched metallic SWCNTs in fraction D3; however, we could not obtain single-structure metallic SWCNTs, namely, chirality (n,m) and enantiomerseparated metallic SWCNTs. The purity of metallic (n,m) SWCNTs should be improved by repeated separations or utilizing a longer column (or columns connected in series). We also demonstrated that the overloading method was effective to achieve chirality separation for even metallic SWCNTs (Supporting Information, Figure S3). By combining these methods, high-purity true single-structure metallic SWCNTs will be obtained in the near future.

Although it is difficult to compare the Raman spectra obtained from the different excitation wavelengths, we can compare the ratios of the peak intensities of the spectra obtained from the same excitation wavelength. From these spectra, we could roughly estimate the separation order, which is related to the interaction strength between the SWCNT and the gel. The order was (14,2), (11,8), (13,1), (13,4), (11,5), (11,2), (10,4), (9,3), (9,6), (7,7), (8,5), and (8,2). The earlier eluted SWCNTs have weaker interactions with the gel, while later eluted SWCNTs have stronger interactions. We then analyzed the relationships between the interaction strengths of (n,m) nanotubes and their chiral angle (θ), diameter (Dt), and smallest bond curvature radius (Rc, Rc = Dt/(2 cos2) (30 − θ); Supporting Information, Figure S4),26 and found good correlation between the interaction and the smallest bond curvature, similar to the case of the chirality separation of semiconducting SWCNTs using the gel.13,14 These results suggest that the difference in the π electron state on the SWCNTs’ surface, which is derived from the bond curvature, affects the interaction between the SWCNTs and SDS molecules and consequently the interaction between the SWCNTs and the gel in both the metallic and semiconducting chirality separations of SWCNTs. Raman spectra of the high wavenumber region in each fraction showed the characteristic shape of metallic SWCNTs, namely, a broad asymmetric (BreitWigner-Fano) G-peak at approximately 1550−1580 cm−1, in addition to a sharp G+ peak at approximately 1590 cm−1 (Supporting Information, Figure S5).27 Separation for the larger diameter SWCNTs, for example, arc-discharge SWCNTs with ∼1.5 nm in diameter, must be difficult because the larger diameter SWCNTs contain a larger number of chiral species, and the difference in the bond curvature becomes small. In order to improve the resolution of the separation, a longer column or columns connected in series should be used. Circular Dichroism Analysis of Separated Metallic SWCNTs. So far, reports on the enantiomer separation of SWCNTs have been limited t o semiconducting SWCNTs.12,16,17 To determine whether the enantiomer separation of metallic SWCNTs occurred in this separation, circular dichroism (CD) spectra of the separated samples were measured. In the case of general molecules, the value of the molar CD is used for a comparison of the enantiomer purity; however, in the case of single-chirality semiconducting SWCNTs, CD normalized by the absorbance of S22 is used28 because of the difficulty in determining the molar concentration of SWCNTs due to their heterogeneity of length, that is, molecular weight. Each fraction separated here contained a few types of metallic SWCNTs. The CD spectra were normalized with the optical absorbance at 280 nm, where all types of SWCNT show absorption and can be used for the quantification of SWCNTs.29 The normalized CD spectra are shown in Figure 4a. The sample before separation (Metal 1) showed no clear CD peaks. Although fractions A5 and A8, which showed no significant enrichment of specific metallic SWCNT species in the optical absorbance, showed no clear peaks of CD, some fractions (e.g., from C5 to D3) showed clear positive and negative peaks in the M11 and M22 regions. The absolute values of M22 in the CD spectra tend to show higher values than M11. This relationship is similar to that of the enantiomer separation of semiconducting (6,5) SWCNTs, where the value of S33 is higher than that of S22.12,17 The highest intensity in the M22 region of the CD spectra was more than 10 mdeg, obtained in

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CONCLUSION The simultaneous separation based on chirality and enantiomers of metallic SWCNTs was achieved by dextran-based gel chromatography using a long column. A metallic SWCNT mixture was used as a starting material for the separation, and the separation was conducted at a low SDS concentration close to its CMC to increase the interaction between the gel and the metallic SWCNTs. We obtained highly (10,4)-enriched metallic SWCNTs, which showed the highest CD intensity, indicating enantiomer separation. The discrimination between the enantiomers should be derived from the optically active dextran. This is the first report of enantiomer separation of metallic SWCNTs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b02563. Supplementary Figures S1−S5 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. E


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(29) Hirano, A.; Tanaka, T.; Urabe, Y.; Kataura, H. ACS Nano 2013, 7, 10285−10295.

ACKNOWLEDGMENTS This work was supported in part by KAKENHI (23651122, 25220602, and 26286009) of MEXT of Japan and by CREST of JST of Japan.


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Simultaneous Chirality and Enantiomer Separation of Metallic Single

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