Purification of Single-Wall Carbon Nanotubes by Controlling the

Apr 13, 2012 - adsorbability was exerted using 0.05−1% sodium deoxycholate (DOC). The results show that the adsorbability depends on the concentrati...
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Purification of Single-Wall Carbon Nanotubes by Controlling the Adsorbability onto Agarose Gels Using Deoxycholate Atsushi Hirano,†,‡ Takeshi Tanaka,† Yasuko Urabe,†,‡ and Hiromichi Kataura*,†,‡ †

Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8562, Japan JST, CREST, Kawaguchi, Saitama 332-0012, Japan

‡

S Supporting Information *

ABSTRACT: One of the key challenges to the industrialization of single-wall carbon nanotubes (SWCNTs) is the commercial-scale production of highly purified SWCNTs separated into metallic and semiconducting species. In the present study, the purification of SWCNTs, i.e., the removal of amorphous carbon or bundled SWCNTs, was performed by quantifying and controlling their adsorbability onto agarose gel. The quantification of the adsorbability was achieved by assuming the Langmuir isotherm, and control over the adsorbability was exerted using 0.05−1% sodium deoxycholate (DOC). The results show that the adsorbability depends on the concentration of DOC. At a low DOC concentration (approximately 0.05%), impurities such as amorphous carbon or bundled SWCNTs were preferentially adsorbed onto the gels, whereas, at an intermediate DOC concentration (ca. 0.25%), high-purity SWCNTs were mainly adsorbed onto the gels. Thus, the impurities, which are difficult to remove by conventional methods, could be separated from unpurified SWCNTs by controlling the adsorbability, leading to the extraction of high-purity SWCNTs. In the purification, diameter-selective separation of SWCNTs was also observed. The purification method using a gel column can be conducted simply and continuously, so that it can be applied for the high-throughput production of high-purity SWCNTs.

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INTRODUCTION Purification of single-wall carbon nanotubes (SWCNTs), including separation, is necessary to fully exploit the mechanical, thermal, and electrical properties1−5 of these materials because as-prepared SWCNTs are obtained as mixtures of not only SWCNTs but impurities such as amorphous carbon or bundled SWCNTs, which lead to the deterioration of the material’s properties. Although approaches for the purification or separation of SWCNTs have been already reported, such as ultracentrifugation,6−10 dielectrophoresis,11−13 gel electrophoresis,14,15 selective oxidation,16,17 amine extraction,18,19 aromatics extraction,20,21 polymer wrapping,22,23 DNA wrapping,24−28 and protein wrapping,29 simpler and more high-throughput methods are needed for commercial-scale production of high-purity SWCNTs. One approach is to use methods featuring gel columns because of their simplicity and adaptable architecture. Previously, our group demonstrated the separation of SWCNTs into metallic species and semiconducting species (metal/ semiconductor separation) using gel columns. Agarose gel columns were useful for metal/semiconductor separation due to the simple and cost-effective separation systems.30 In addition, columns containing allyl dextran-based gel have also been used for separation, which promoted the large-scale single-chirality separation of semiconducting SWCNTs.31 Currently, the mechanism of separation using both gels has not fully been clarified, although there have been several studies in this direction.31−33 © 2012 American Chemical Society

Although such separation methods are useful to obtain separated SWCNTs, i.e., metallic and semiconducting species, the purity of SWCNTs is limited because of the presence of coexisting impurities. In particular, the purity of metallic SWCNTs tends to be lower than that of semiconducting species because metallic SWCNTs are commonly collected as the flow-through.30,31 The flow-through contains amorphous carbon or bundled SWCNTs besides debundled metallic SWCNTs, leading to lower purity. In this context, the present study aimed to improve the purity of SWCNTs with a gel column. The gels used were agarose gel and allyl dextran-based gel, which are used for the separation of SWCNTs, as mentioned above. Because the adsorbability of the impurity onto the gels is expected to be different from that of SWCNTs onto the gels, control over adsorbability may be a key to achieving high-quality purification. Thus, the adsorbability was controlled using DOC, which alters the interaction between SWCNTs and the gels,34 where the adsorbability was experimentally quantified for the first time as a dimensionless parameter using the Langmuir isotherm.32 The results show that the adsorbability of SWCNTs onto the agarose gel actually depended on the concentration of DOC, whereas adsorbability onto the allyl dextran-based gel hardly depended on the DOC concentration. At approximately 0.25% DOC, high-purity SWCNTs were adsorbed onto the Received: February 10, 2012 Revised: April 11, 2012 Published: April 13, 2012 9816

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Figure 1. Absorption spectra (A) and concentrations (B) of the SWCNTs dispersed in 0.025, 0.05, 0.1, 0.25, 0.5, and 1% DOC. (C) Fluorescence spectra of the SWCNTs dispersed in 0.25% DOC. (D) Fluorescence intensity of representative chiral SWCNTs contained in the solution dispersed in 0.05, 0.1, 0.25, 0.5, and 1% DOC solutions.

path length of 10 mm. In the present study, the absorption peaks at approximately 940−1350 nm and 620−940 nm were assigned to the first and second optical transitions of the semiconducting species, designated as the S11 and S22 bands, respectively, whereas the absorption peak at approximately 400−620 nm was assigned to the first optical transition of metallic SWCNTs, designated as the M11 band.32,35 Measurement of Fluorescence Spectra. Fluorescence spectra of the SWCNTs were measured with a spectrofluorometer (Nanolog, Horiba) equipped with a liquid-nitrogencooled InGaAs near-IR alloy detector. The spectra were used for the assignment of the SWCNTs to the corresponding chiralities based on the data of the single-chiral semiconducting SWCNTs.27 The dispersion samples were diluted 250-fold into each DOC solution before the measurement. Batch Adsorption of SWCNTs onto Gels. To assess the adsorbability of SWCNTs onto the gels, a batch adsorption of the materials was conducted as follows: the gel beads (125 μL bed volume) were mixed with the dispersed SWCNT solutions (1375 μL) in the presence of various concentrations of DOC at room temperature by gentle rotation for different periods of time, and subsequently the mixtures were briefly centrifuged (500g for 1 min). Finally, the resulting supernatants were collected as the unadsorbed fractions. The absorbance of the fractions was used to assess the adsorbability of the SWCNTs onto the gels. The gels used here were agarose gel (Sepharose 2B, GE Healthcare) and allyl dextran-based gel (Sephacryl S200 HR, GE Healthcare) Adsorption and Elution of SWCNTs Using Agarose Gel Column. Adsorption and subsequent elution of the SWCNTs dispersed at each concentration of DOC were

agarose gel and were collected by subsequent elution. The purification system was, furthermore, applied to the metallic SWCNTs, which are difficult to purify by conventional methods. The present method can be conducted simply and continuously; therefore, it will be useful for the highthroughput production of high-purity SWCNTs.

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MATERIALS AND METHODS Dispersion of SWCNTs. Raw SWCNTs produced by highpressure catalytic CO (HiPco) decomposition were purchased from NanoIntegris and were used as the starting material for the following methods. Aliquots of 30 mg of HiPco SWCNTs were predispersed in 30 mL of purified water with DOC (Sigma-Aldrich) using an ultrasonic processor (Nanoruptor NR-350, Cosmo Bio) for 1 min at a power of 350 W. Subsequently, the solutions were dispersed using an ultrasonic homogenizer (Sonifire 250D, Branson) equipped with a 0.5 inch flat tip for 1 h at a power density of 20 W cm−2. To prevent heating during sonication, the bottle containing the sample solution was immersed in a bath of water at 18 °C. The dispersed sample solution was centrifuged at 210 000g for 1 h using an ultracentrifuge (S80AT3 rotor, Hitachi Koki) to remove the residue of the catalytic metal particles, the nanotube bundles and other impurities. However, the impurities such as amorphous carbon or bundled SWCNTs were not completely removed. The upper 70% of the supernatant was collected and used for the following measurements. Measurement of Absorption Spectra. Absorption spectra of the SWCNTs were recorded over a wavelength range of 300−1350 nm using an UV−vis−NIR spectrophotometer (UV-3600, Shimadzu) with a quartz cell that had a 9817

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Figure 2. Absorption spectral changes of SWCNTs that did not adsorb onto the agarose gel (A) and Sephacryl (B) over time in the presence of 0.25% DOC. (C) Absorbance changes for the samples at 620 nm over time.

even at 0.05% DOC, indicating the high efficiency of dissolving isolated SWCNTs.38,39 Here, it should be noted that the intensity of the spectra was substantially reduced at 0.025% DOC, indicating a decrease in dispersibility. To illustrate the detailed dependence of dispersibility on the DOC concentration, the concentration of the dispersed solution was plotted in Figure 1B, which was calculated from the standard curves determined for raw SWCNTs. The concentration at 0.025% DOC was lower than that at higher concentrations of DOC. It is important to note that the concentration at approximately 0.05% or 0.1% DOC was the highest among all concentrations of DOC used in the present study. The decrease in dispersibility at higher concentration of DOC may be associated with the attractive depletion interaction between SWCNTs and surfactants reported in previous studies.37,40−42 As expected, in the absorption spectra in Figure 1A, the chirality of the SWCNTs in the solutions seemed to be independent of the DOC concentration between 0.05 and 1% DOC. Thus, the assignment of the chirality was performed for the SWCNTs dispersed at each concentration of DOC, except at 0.025% DOC, by measuring their fluorescence spectra. Figure 1C exhibits the fluorescence spectra at 0.25% DOC, which indicates the presence of (7,6), (9,4), and (7,5) SWCNTs as the main components. The profiles of the fluorescence spectra were essentially identical to those of the 0.05−1% DOC solutions (see Supporting Information Figure S1); therefore, the chirality of the SWCNTs contained in the solution was independent of the DOC concentration. Figure 1D shows the relative fluorescence intensity of the representative chiralities divided by the absorbance of the corresponding SWCNTs at 620 nm in each DOC solution. Interestingly, the intensities at 0.05% DOC were significantly reduced, although the absorbance values were retained, as shown in Figure 1A. The fluorescence intensities of the SWCNTs dispersed in 0.05% DOC solution were not recovered by transferring the samples from the 0.05% to the 1% DOC solution (data not shown). These results suggest that the SWCNTs dispersed at 0.05% DOC were irreversibly bundled, despite retaining their dispersibility. In addition, the slight decrease in the fluorescence intensity at the higher concentration of DOC is attributable to the association of SWCNTs by the depletion effect, as mentioned above. As shown above, the dispersibility of the SWCNTs was retained in 0.05−1% DOC solutions, so the adsorbability of the SWCNTs onto the gels was investigated in the DOC solutions. Figure 2A,B shows changes in the absorption spectra obtained from the supernatants after batch adsorption onto agarose gel

performed using an agarose gel column. The column consisted of a 6 mL syringe containing the agarose gel beads (600 μL bed volume). After the gel column was equilibrated with each concentration of DOC solution, 6 mL of the dispersed SWCNT solutions was applied to the top of the column. Subsequently, 6 mL of each DOC solution was added to the top of the column. The unadsorbed SWCNTs flowed through the column and were collected from the bottom of the column as a flow-through fraction. The SWCNTs that were adsorbed onto the gels were desorbed and eluted from the column by the addition of more concentrated DOC. Measurements of Raman Spectra. Raman spectra for 488 nm excitation were measured using a triple monochromator (PDPT3-640S, Photon Design) equipped with a chargecoupled device detector. The spectra of samples were taken at excitation wavelengths of 488 nm using a power of 20 mW. Raman spectra for 532 nm excitation were measured using a confocal Raman microscope (XploRA, Horiba Jobin−Yvon). The spectra of the samples were taken at excitation wavelengths of 532 nm using a power of 0.2 mW (0.1%).

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RESULTS AND DISCUSSION

Gel columns are useful for the separation of SWCNTs because of their simplicity and adaptable architecture. Previously, our group showed that SWCNTs dispersed in sodium dodecyl sulfate solutions can be separated into metallic and semiconducting species using agarose gels and allyl dextran-based gels.30,31,34 In these studies, DOC was used only for the elution of SWCNTs adsorbed onto the gels. Nevertheless, DOC can be successfully used for the separation of SWCNTs using densitygradient ultracentrifugation,6,8,36 suggesting the preferential interaction of DOC with SWCNTs. In the present study, the purification of SWCNTs using gel columns was performed in a binary system featuring DOC and water. To understand the adsorbability of SWCNTs onto the gels in DOC solutions, the dispersibility of SWCNTs in the solution should first be investigated because it is one of the most important parameters reflecting the interaction between SWCNTs and DOC.37 In the present study, the dispersibility was defined as the absorbance of the SWCNT solutions obtained from supernatants after sonication and subsequent ultracentrifugation at each concentration of DOC. Figure 1A shows the absorption spectra of the SWCNTs diluted 10-fold into each DOC solution. The spectra show characteristic peaks assigned to the M11 band at approximately 400−620 nm and to the S11 and S22 bands at approximately 940−1350 nm and 620−940 nm, respectively.32 The dispersibility was retained 9818

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Figure 3. Relationship between initial concentration and final concentration of the SWCNTs that did not adsorb onto the agarose gel (A) and Sephacryl (B) at each concentration of DOC.

and allyl dextran-base gel (Sephacryl) in the presence of 0.25% DOC for different periods of time. It has been reported that the gels provide effective metal/semiconductor separation of SWCNTs in the presence of sodium dodecyl sulfate.9,30−32,34 The absorbance of agarose gel was significantly reduced over the incubation time, unlike that of Sephacryl, indicating that the SWCNTs were more strongly adsorbed onto the agarose gel than onto the Sephacryl. Figure 2C shows changes in the absorbance at 620 nm with incubation time. The absorbance values for both gels almost reached a minimum after 6 h, indicating that adsorption equilibration was reached. In the same way, the concentration of SWCNTs obtained from the supernatants was determined at different concentrations of DOC. The relationship between the initial concentration of SWCNTs before mixing with the gels and the final concentration of SWCNTs after mixing with the gels for 6 h with each DOC solution is plotted in Figure 3. For the agarose gel, the final concentrations at 0.05%, 0.1%, and 0.25% DOC appeared to increase nonlinearly with the initial concentration (i.e., slightly concave up), whereas those at 0.5% and 1% increased linearly with the initial concentration. This result indicates that SWCNTs were adsorbed onto the agarose gel substantially at 0.05%, 0.1%, and 0.25%, but they did so only marginally at 0.5% and 1%. On the contrary, for Sephacryl, the final concentrations at each concentration of DOC increased linearly with the initial concentration, and those at 0.5% and 1% increased almost in proportion to the initial concentration, indicating that the SWCNTs were insignificantly adsorbed onto Sephacryl. By assuming that adsorption follows the Langmuir isotherm, the relationship between the initial concentration and the final concentration was utilized to quantitatively assess the adsorbability of SWCNTs, as demonstrated in our previous study;32 specifically, the initial concentration and the final concentration are related by the following equation: c = c0 − θα

θ=

(2)

where K is the equilibrium constant between the unadsorbed and adsorbed forms. By combining eqs 1 and 2, the relationship between the initial concentration of the SWCNTs and the final concentration of the SWCNTs can be described as follows:

c=

⎧ Vgel 1⎪ 1 ⎨c0 − −α 2⎪ K Vsol ⎩ +

⎫ 2 ⎛ Vgel ⎞ 4c ⎪ 1 −α ⎜c 0 − ⎟ + 0⎬ K Vsol ⎠ K ⎪ ⎝ ⎭

(3)

In the present study, the relationships between the initial and final concentrations of the SWCNTs were experimentally obtained as shown in Figure 3, so that regression coefficients such as α and K should have been obtained by fitting eq 3 to the experimental data. However, it was difficult to determine the parameters accurately by the fitting because of errors in the experiment. Accordingly, in place of the parameters, an adsorption parameter was used in the present study, which was defined using the initial slopes, i.e., derivatives, of the curves as follows. The derivative of the final concentration with respect to the initial concentration is obtained using the following equation: ⎧ ⎪ dc 1⎪ = ⎨1 + dc 0 2⎪ ⎪ ⎩

Vgel Vsol

Kc 1 + Kc

c0 −

(c

0

−

Vgel

1 K

− αV +

1 K

− αV

sol

Vgel 2 sol

)

2 K

+

4c 0 K

⎫ ⎪ ⎪ ⎬ ⎪ ⎪ ⎭

(4)

When c0 is around zero, eq 4 is described as follows: (1)

⎛ dc ⎞ 1 = ⎜ ⎟ Vgel ⎝ dc 0 ⎠ c = 0 1 + αK V 0

where c and c0 are the final concentration and the initial concentration of SWCNTs, respectively; Vgel and Vsol are the volumes of the gel fraction and the aqueous solution fraction, respectively; and α is the saturation weight of the adsorbed SWCNTs per unit gel volume. θ is the fractional coverage of the gels, which is given by the following equation based on the Langmuir isotherm:

sol

(5)

which corresponds to the initial slope of the final concentration (c) versus the initial concentration (c0). Thus, the dimensionless adsorption parameter (αK) is given by the following equation: 9819

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1

( ) dc dc 0

c0 = 0

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⎞ ⎟ − 1⎟ ⎟ ⎟ ⎠

To examine the purification of SWCNTs, adsorption of SWCNTs onto, and subsequent elution from, agarose gel were conducted at each concentration of DOC using an agarose gel column. Figure 4A shows the absorption spectra of the SWCNTs eluted by 2% DOC after adsorption onto the agarose gel at each DOC solution. As expected from the values of the adsorption parameter exhibited in Table 1, the absorbance values decreased with increasing DOC concentrations. Every spectrum depicted explicit peaks of the M11, S11, and S22 bands, so that both metallic and semiconducting SWCNTs were determined to be present in the eluates. In Figure 4B, the absorbance values at 620 nm (left axis) were plotted against the DOC concentration. The absorbance decreased nonlinearly with the DOC concentration, indicating that the adsorbability onto the agarose gel in the column was promptly reduced with increasing DOC concentrations. Here, it is also noted that spectral shapes depended on the DOC concentration (Figure 4A), so that the SWCNTs contained in the solutions should vary in chirality. To identify the chirality of the SWCNTs in the solution, fluorescence spectra were measured as shown in Figure 4C. Consistent with the absorption spectra, the fluorescence spectra exhibited different profiles; namely, the spectra of the SWCNTs indicated the presence of (7,6), (10,2), (8,7), and (9,5) SWCNTs at 0.05% DOC, the presence of (8,6) SWCNTs at 0.1% and 0.25% DOC, the presence of (7,6) SWCNTs at 0.5% DOC, and the presence of (7,6), (9,4), and (7,5) SWCNTs at 1% DOC as the majority of the SWCNTs. Although the detailed mechanism underlying these results is not presently clear, the concentration dependence may be ascribed to the diameter-selective interaction between SWCNTs and DOC, as observed in previous studies,34,43,44 leading to the adsorbability onto the agarose gel. In any case, the most notable result described here

(6)

Using values of the initial slopes for the experimental data in Figure 3, the adsorption parameters (αK) were calculated from eq 6 (Table 1), where Vgel and Vsol are 125 μL and 1,375 μL, Table 1. Dimensionless Adsorption Parameter αK Describing the Adsorption of SWCNTs onto Agarose Gel and Sephacryl αK DOC (%) 0.05 0.10 0.25 0.50 1.00

agarose gel 6.7 7.5 6.0 2.0 2.1

± ± ± ± ±

0.4 0.6 0.0 0.8 1.2

Sephacryl 2.0 1.7 2.3 0.5 0.5

± ± ± ± ±

0.2 0.2 0.7 0.0 0.1

respectively (see Materials and Methods). It is important to note that, in the present study, the adsorption parameter, i.e., the quantitative adsorbability, of the SWCNTs adsorbed onto the gels has been reported for the first time using experimental data. As expected, for the agarose gel, the values of the parameters at 0.05%, 0.1%, and 0.25% DOC were significantly higher than those at 0.5 and 1%, whereas the values of the parameters for Sephacryl were low overall. The results indicate that, physicochemically, the SWCNTs are meaningfully adsorbed onto the agarose gel compared to Sephacryl. Thus, the significant adsorbability of SWCNTs onto agarose gel can be applied to the purification of SWCNTs in DOC solutions.

Figure 4. (A) Absorption spectra of the SWCNTs eluted by 2% DOC after adsorption onto the agarose gel in the column at 0.05, 0.1, 0.25, 0.5, and 1% DOC. (B) Absorbance at 620 nm of the eluates (dotted line, left axis) and fluorescence intensity of representative chiral SWCNTs contained in the eluates (solid lines, right axis). (C) Fluorescence spectra of each eluate. 9820

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Figure 5. Absorbance spectra of the SWCNTs eluted stepwise by each concentration of DOC after adsorption onto the agarose gel in the column at 0.05% (A), 0.1% (B), 0.25% (C), 0.5% (D), and 1% (E) DOC.

Figure 6. Absorbance spectra (A) and Raman spectra (B,C) of the unpurified (dotted lines) and purified (solid lines) metallic SWCNTs. Absorbance spectra were normalized at 300 nm. The purified metallic SWCNTs were obtained by the addition of 0.5% DOC solution after the adsorption at 0.25% DOC. Inset, photographs of each SWCNT solution in vials after concentration by the centrifugation. Raman spectra were taken at excitation wavelength of 488 nm (B) and 532 nm (C).

corresponding to the M11, S11, and S22 bands, suggesting that high-purity SWCNTs were eluted. The SWCNTs eluted with the 1% and, subsequently, 2% DOC solution showed insignificant spectra. Thus, the majority of the debundled SWCNTs were eluted with the 0.5% DOC solution. Similar absorption spectra for the SWCNTs eluted by each concentration of DOC after adsorption at 0.1% DOC were obtained as shown in Figure 5B. Briefly, the eluate obtained by the addition of 0.25% DOC exhibited featureless spectra and that obtained by the subsequent addition of 0.5% DOC solution showed the characteristic spectrum with sharp peaks. Here, it should be noted that the spectrum of the solution eluted with the 0.5% DOC solution after adsorption in 0.25% DOC showed characteristic peaks, such as the M11, S11, and S22 bands, with significant intensities (Figure 5C), whereas the spectra of the solution obtained by the addition of 1% and, subsequently, 2% DOC solution showed marginal intensities. These results lead to an important conclusion; specifically, high-purity SWCNTs can be most efficiently obtained through adsorption at 0.25% DOC and successive elution at 0.5% DOC under the present conditions. Incidentally, the spectra of the SWCNTs eluted with each DOC solution after adsorption at 0.5 and 1% DOC exhibited marginal intensities (Figure 5D,E), as expected from the results shown in Figure 4B. Fluorescence spectra corresponding to the respective absorption spectra should also be useful in understanding the results described above (see Supporting Information, Figure S2). Here, an important perspective on the purification of SWCNTs arises; namely, the purification of SWCNTs via adsorption at 0.25% DOC and subsequent elution described above can be applied for the removal of impurities from metallic SWCNTs. Currently, although SWCNTs are easy to

is the dependence of the spectral intensity on the DOC concentration (see color scale). The intensity of the representative chiralities is shown in Figure 4B (right axis). Despite the continuous decrease in the absorbance (left axis) with the DOC concentration, the fluorescence intensity of each chiral SWCNT showed a peak at approximately 0.25% DOC. The results suggest that the apparent adsorbability at approximately 0.05% DOC observed from the absorbance in Figure 4A,B is due to the adsorption of impurities such as amorphous carbon or bundled SWCNTs. Thus, the purity of SWCNTs adsorbed onto the agarose gels with approximately 0.25% DOC appeared to be highest in this concentration region. Incidentally, the fluorescence spectrum at 1% DOC was similar to the control spectrum, as shown in Figure 1C, implying that bundled species remaining in the dispersed SWCNTs may also be adsorbed onto the agarose gel at 1% DOC. As mentioned above, it was suggested that the presence of ca. 0.05% DOC resulted in the adsorption of amorphous carbon or bundled SWCNTs, and the presence of ca. 0.25% DOC resulted in the adsorption of rich SWCNTs. Accordingly, stepwise elution seems to be useful for obtaining high-purity SWCNTs. Figure 5 shows the absorbance spectra of the SWCNTs eluted stepwise at each concentration of DOC after adsorption onto agarose gel in the column at 0.05%, 0.1%, 0.25%, 0.5%, and 1% DOC. As shown in Figure 5A, the absorption spectra of the eluates obtained by the addition of 0.1% and subsequent 0.25% DOC solution after the adsorption at 0.05% DOC were featureless, suggesting that amorphous carbon or bundled SWCNTs were mainly eluted under these conditions.45,46 The spectra of the SWCNTs obtained by subsequent elution at 0.5% DOC exhibited sharp peaks 9821

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present study could not be achieved without using this parameter. In other words, such a physicochemical approach was found to be useful for the purification of SWCNTs. A goal for future studies is to determine the thermodynamic parameters of the interaction between SWCNTs and gels, which could improve the understanding of the mechanism of the purification of SWCNTs using gels and result in applying these methods to the high-throughput purification of SWCNTs.

separate into metallic and semiconducting species using gel columns,30,31 the purity has been significantly limited because the metallic species are eluted together with amorphous carbon or bundled SWCNTs in the conventional methods. Thus, the simple purification procedure developed in the present study was applied to metallic SWCNTs. Metallic SWCNTs were prepared using a Sephacryl column based on the method previously reported by our group (see Supporting Information for details of the procedure).31 The prepared metallic SWCNTs (hereafter called unpurified metallic SWCNTs) in 0.25% DOC were applied to the agarose column, and the adsorbed metallic SWCNTs were subsequently eluted with 0.5% DOC. The respective spectra normalized at 300 nm are shown in Figure 6A. The unpurified metallic SWCNTs exhibited peaks corresponding to the M11 band and the broad absorption decreasing with increasing wavelength, indicating the coexistence of metallic SWCNTs and impurities such as typical amorphous carbon or bundled species.45,46 On the contrary, the SWCNTs purified using the agarose gels showed sharp peaks corresponding to the M11 band and the reduced absorbance derived from the impurities. In particular, the absorbance at approximately 1300 nm was significantly reduced. Thus, it was found that the impurities mixed with the metallic SWCNTs were readily removed in the agarose column using DOC. Photographs of the unpurified and purified metallic SWCNTs concentrated by centrifugation provide an intuitive understanding of the quality of the purification (Figure 6A inset). The dark color of the impurities seen in the unpurified sample was apparently reduced in the purified sample. Here, it would be interesting to compare the purity of the metallic SWCNTs with that reported in the literature. The index of the purity attained was defined here as the ratio of the absorbance at 500 nm to the absorbance at 300 nm (A500 nm/A300 nm). The ratio of the purified metallic sample obtained in the present study was approximately 0.9. The ratios of the unpurified metallic sample prepared here and two other metallic samples of HiPco obtained by agarose gel electrophoresis14 and density-gradient ultracentrifugation8 were ca. 0.6, 0.4, and 0.6, respectively, explicitly indicating the usefulness of the present method for purification. Fits to the optical absorption spectra of the samples were shown in Figure S4 (see Supporting Information). Figure 6B,C shows Raman spectra for the unpurified and purified metallic SWCNTs. The assignment of the SWCNTs to the corresponding chiralities was based on the data from the literature.47,48 The spectra of unpurified metallic SWCNTs taken at excitation wavelengths of 488 nm exhibited three sharp peaks corresponding to (7,7), (8,5), and (8,2) (Figure 6B). It was found that the relative intensity of (7,7) SWCNTs was increased by the purification. The spectra of unpurified metallic SWCNTs taken at an excitation wavelength of 532 nm exhibited three sharp peaks corresponding to (9,6), (10,4), and (9,3) (Figure 6C). The relative intensity of (10,4) SWCNTs was increased by the purification. Thus, larger diameter metallic SWCNTs seem to be adsorbed onto the gels at 0.25% DOC, similar to semiconducting SWCNTs as mentioned above. Taken together, SWCNTs that have a larger diameter than (7,7) SWCNT with a diameter of ca. 0.96 nm tend to be adsorbed onto the gel at 0.25% DOC, whether it is metallic or semiconducting. In conclusion, the adsorbability of SWCNTs onto agarose gel was quantified by a dimensionless adsorption parameter under the assumption of the Langmuir isotherm. The simple and effective purification of metallic SWCNTs demonstrated in the

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ASSOCIATED CONTENT

* Supporting Information S

Details of fluorescence spectra of the SWCNTs dispersed and eluted in DOC solutions, a procedure for the preparation of unpurified metallic SWCNTs, and fits to the absorption spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-29-861-2551. Fax: +81-29-861-2786. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by KAKENHI (23651122) of MEXT of Japan to T.T.

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