CROSS REFERENCE TO RELATED APPLICATION

[001] This application claims the benefit of priority under 35 USC § 1 19(e) to United States Provisional Patent Application No. 61/499,697 titled "HIGH SELECTIVITY CUM YIELD GEL ELECTROPHORESIS SEPARATION OF SINGLE-WALLED CARBON NANOTUBES USING A CHEMICALLY SELECTIVE POLYMER DISPERSANT," filed June 22, 201 1, the disclosure of which is hereby incorporated by reference in its entirety for all purposes.

FIELD OF THE INVENTION

[002] The present invention is in the field of carbon nanotubes and more particularly, to methods for separating metallic from semiconducting single-walled carbon nanotubes (SWNTs).

BACKGROUND OF THE DISCLOSURE

[003] The unique electronic, mechanical and thermal properties of single-walled carbon nanotubes make them potentially suitable for many applications in electronics, optics and other areas of materials science. All current state-of-art single-walled carbon nanotube synthesis methods, however, produce mixtures containing both metallic and semiconducting nanotube species, which are unsuitable for many electronic applications. In addition, the future industrial use of nanotubes in electronics is likely to involve self- assembly processes based on their electronic properties for the integration of millions of nanotubes into functional circuits, which require single-walled carbon nanotubes with homogeneous electronic properties. In the light of such needs, the separation of metallic from semiconducting single-walled carbon nanotubes after synthesis has gained particular attention.

[004] Many metallicity-based single-walled carbon nanotubes separation techniques have been reported including chromatography, density-gradient ultracentrifugation and selective chemistry using biomolecules, amines or aromatic molecules. These methods, however, have a number of drawbacks including low process yields, low purity and/or degradation of the original properties of the single-walled carbon nanotubes in the separated samples.

[005] Alternatively, because single-walled carbon nanotube dimensions resemble those of some biomacromolecules, there have been a number of efforts to use techniques from the life sciences to sort single-walled carbon nanotubes by their physical and electronic properties. Electrophoretic separation is one technique that has recently been applied for the separation of metallic from semiconducting single-walled carbon nanotubes. These electrophoretic separation methods apply an electric field to mixtures of single-walled carbon nanotubes suspended by a suitable surfactant or dispersant, which separate different species that carry differential charges by their relative motion with respect to each other.

[006] A recent study has shown that metallic and semiconducting single-walled carbon nanotubes may be effectively separated using agarose gel electrophoresis (AGE) when applied in conjunction with the use of a suitable surfactant. Among the various gels and surfactants tested, detectible separation took place only when the combination of sodium dodecyl sulfate and agarose gel was used. The reasons for the effectiveness of sodium dodecyl sulfate in agarose gel electrophoresis of single-walled carbon nanotubes are not clearly understood. Field effect transistors (FETs) made with semiconducting single- walled carbon nanotubes purified with sodium dodecyl sulfate-assisted agarose gel electrophoresis, however, indicate that better separation efficiency is needed to achieve field effect transistors performance comparable to those reported for field effect transistors made using semiconductor-single-walled carbon nanotubes purified using other highly selective separation techniques. Thus, what is needed in the art are new methods for improving the efficiency of agarose gel electrophoresis in separating metallic from semiconducting single-walled carbon nanotubes.

SUMMARY OF THE INVENTION

[007] The present invention addresses these needs by providing new methods for improving the efficiency of agarose gel electrophoresis in separating single-walled carbon nanotubes using a chemically pre-selective dispersant in place of sodium dodecyl sulfate.

[008] Thus, in one embodiment the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, by a) subjecting a mixture of metallic and semiconducting single walled nanotubes and chondroitin sulfate (CS) in an electrophorese gel (matrix) to an electric uniform electric field (electrophoresis); and b) isolating the separated metallic and semiconducting single-walled carbon nanotubes.

[009] In another embodiment, the disclosure provides the use of chondroitin sulfate for the separation of metallic from semiconducting single-walled carbon nanotubes by gel electrophoresis.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 shows the following: Fig. 1(a) The bottom fraction of the gel (greenish) is enriched in metallic single- walled carbon nanotubes while the top fraction of the gel (pinkish) contains predominantly semiconducting nanotubes. Fig. 1(b) UV-vis-NIR spectra of P2/Pristine, P2/CS-A and of P2/SDS gel fractions after gel electrophoresis.

[0011] Figure 2 shows the Raman spectra of pristine and separated P2 single-walled carbon nanotubes with 633 nm source laser: Fig. 2(a) RBM, Fig. 2(b) G band.

[0012] Figure 3 shows the following: Fig. 3(a) Transfer characteristics (V _dS = 0.3V) of I _ds vs. V _gs for bottom-gated field effect transistors fabricated using the P2/CS-A/Top. Fig. 3 (b) Output characteristics, I _d s vs. Vd _S measured at gate voltages of 40V (top) to -40V (bottom) with 10V intervals. Fig. 3(c) Histogram of on/off ratios of 20 devices made with P2/CS-A/Top single-walled carbon nanotubes (black) and 20 devices made with pristine single- walled carbon nanotubes (white). Fig. 3(d) The trend of on/off ratio with device mobility for the devices fabricated from P2/CS-A/Top.

[0013] Figure 4 shows the following: (a) FESEM (Fig. 4(a)I) and AFM (Fig. 4(a)II) images of single-walled carbon nanotubes separated by gel electrophoresis. Fig. 4(b) AFM image of pristine single-walled carbon nanotubes. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0014] To improve the efficiency and process yield of agarose gel electrophoresis in separating single-walled carbon nanotubes by metallicity, it has been found that a chemically pre-selective dispersant may be used in place of sodium dodecyl sulfate. It has been shown both experimentally and theoretically that amine containing compounds preferentially adsorb onto metallic nanotubes. The use of a chemically selective dispersant that preferentially disperses and suspends metallic nanotubes may significantly improve the purity of the separated nanotubes achieved using the inherently high yield agarose gel electrophoresis process.

[0015] Thus, it has been found that considerably better separation to obtain 95% semiconducting single-walled carbon nanotubes may be achieved using chondroitin sulfate such as chondroitin sulfate A (CS-A) or any of the other member of the chondroitin sulfate family such as chondroitin sulfate B, chondroitin sulfate C, chondroitin sulfate D, chondroitin sulfate E or mixtures thereof as the dispersant in agarose gel electrophoresis, rather than the previously reported sodium dodecyl sulfate. Earlier studies demonstrated nanotube dispersability using chondroitin sulfate A, a charged biomacromolecule. The present work extends the use of chondroitin sulfate such as chondroitin sulfate A to metallicity-based separation of single-walled carbon nanotubes, specifically using arc discharge single-walled carbon nanotubes. The effectiveness of a dispersant such as chondroitin sulfate A for agarose gel electrophoresis separation to achieve higher separation efficiency than sodium dodecyl sulfate can be demonstrated by ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy, Raman spectroscopy and field effect transistor results. The separation yield achieved with chondroitin sulfate A assisted agarose gel electrophoresis is also rather high (25%). The effectiveness of chondroitin sulfate A for agarose gel electrophoresis separation is found to be correlated to the zeta-potential of the single-walled carbon nanotube/dispersant hybrid.

[0016] In one embodiment the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, by a) subjecting a mixture of metallic and semiconducting single walled nanotubes and chondroitin sulfate (CS) in an electrophorese gel (gel matrix) to uniform electric field (electrophoresis); and b) isolating the separated metallic and semiconducting single-walled carbon nanotubes. The electrophorese gel (matrix) can be any suitable gel matrix, for example, an agarose gel, a starch gel or a polyacrylamide gel. The mixture of metallic and semiconducting single walled nanotubes can be any mixture of carbon nanotubes, for example, an as- synthesized mixture, meaning a mixture that has been obtained from a preparation reaction of carbon nanotubes. The as-synthesized mixture may, for example, be obtained from an electrical arc discharge method/reaction. The chondroitin sulfate might be any suitable chondroitin sulfate, for example, chondroitin sulfate A, chondroitin sulfate B, chondroitin sulfate C, chondroitin sulfate D, chondroitin sulfate E or any mixture thereof.

[0017] In one aspect, the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, wherein the metallic and semiconducting single-walled carbon nanotubes are purified arc discharge P2 or P3 single-walled carbon nanotubes.

[0018] In another aspect, the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, wherein the metallic and semiconducting single walled nanotubes are dispersed using sonication into an aqueous solution containing about 2% wt chondroitin sulfate A (CS-A), centrifuged to remove single walled carbon nanotube bundles, and the supernatant is collected to provide a solution of P2/CS-A single walled nanotubes.

[0019] In another aspect, the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, wherein the metallic and semiconducting single walled nanotubes are dispersed using sonication into an aqueous solution containing about 1% wt to about wt 5% , for example, 2 % wt or wt 3 % chondroitin sulfate A (CS-A), centrifuged to remove single walled carbon nanotube bundles, and the supernatant is collected to provide a solution of P2/CS-A single walled nanotubes.

[0020] In another aspect, the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, wherein the solution of P2/CS-A single walled nanotubes is dispersed into a similar volume of about 2 wt % agarose solution, melted through microwave heating and cooled to provide a P2/CS-A agarose gel. [0021] In another aspect, the disclosure provides methods for separating metallic from semiconducting single -walled carbon nanotubes, wherein the solution of P2/CS-A single walled nanotubes is dispersed into a similar volume of about 1 wt % to about 5 wt% agarose solution, melted through microwave heating and cooled to provide a P2/CS-A agarose gel.

[0022] In another aspect, the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, wherein the P2/CS-A agarose gel is placed into an electrophoresis apparatus and subjected to an electric field at about 200 V for about 1 to about 4 hours.

[0023] In another aspect, the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, wherein after electrophoresis, the gel is cut into three (top, middle and bottom) fractions, wherein the top fraction containing predominantly the semiconducting nanotubes with agarose gel is dried, stirred in a solution of chlorosulfonic acid, and the solution is added to cold deionized water to provide solids.

[0024] In another aspect, the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, wherein the solids are collected, filtered, washed with water until neutral, washed with ethanol, dried at about 100 °C to provide a residue.

[0025] In another aspect, the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, wherein the residue is added a solution of about 1 M sodium hydroxide, stirred for about 30 minutes, filtered, washed with water until neutral, washed with ethanol, and dried at about 100°C to provide the isolated purified single-walled carbon nanotubes.

[0026] In another aspect, the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, wherein the semiconducting single- walled carbon nanotubes are isolated in about 95% purity.

[0027] In another aspect, the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, wherein the semiconducting single- walled carbon nanotubes are isolated in about 95% to about 98% purity. [0028] In another aspect, the disclosure provides methods for separating metallic from semiconducting single-walled carbon nanotubes, wherein the semiconducting single- walled carbon nanotubes are isolated in about 95% to 100% purity.

[0029] In another embodiment, the disclosure provides purified semiconducting single- walled carbon nanotubes prepared by the method of a) subjecting a mixture of metallic and semiconducting single walled nanotubes and chondroitin sulfate in an electrophorese gel to an uniform electric field (electrophoresis); and b) isolating the separated metallic and semiconducting single-walled carbon nanotubes.

[0030] Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material. These cylindrical carbon molecules have unusual properties, which are valuable for applications in nanotechnology, electronics, optics, and other fields of material science and technology.

[0031] Nanotubes are members of the fullerene structural family, which also includes the spherical buckyballs. Their name is derived from their long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called graphene. These sheets are rolled at specific and discrete ("chiral") angles, and the combination of the rolling angle and radius, i.e., their geometries, decides the nanotube properties; for example, whether the individual nanotube shell is a metal or semiconductor. Nanotubes may also be categorized as single walled nanotubes (SWNTs) and multi walled nanotubes (MWNTs).

[0032] Single-walled carbon nanotubes may be prepared using the electric arc discharge method. The ratio of semiconducting to metallic single-walled carbon nanotubes produced through this method is usually about 2 to 1. The nanotubes may be produced using a catalyst such as a nickel/yttrium catalyst. P2 and P3 single-walled carbon nanotubes are materials of the same quality, but with different characteristics: P2- processing produces high purity material with almost no functionalities, whereas P3- material has the same carbon and metal content purity, but it is more compatible with further chemical processing. P2 single-walled carbon nanotubes are obtained through a purification route involving air oxidation and subsequent treatment in acid to remove the catalyst. As a result, the P2-processing produces high purity materials with the most intrinsic behavior (the treatment does not dope the SWNTs). It does not have attached acidic functionalities and it is less dispersable in water and other solvents. By contrast, a specific feature of P3-SWNT is the presence of carboxylic acid groups (about 3-6%), which makes this material dispersible in water and other solvents and more compatible with further chemical processing including composite applications.

[0033] Chondroitin sulfate is a class of sulfated glycosaminoglycan (GAG) compounds composed of chain of alternating sugars (N-acetylgalactosamine (GalNAc) and D- glucuronic acid (GlcA)). It is usually found attached to proteins as part of a proteoglycan. A chondroitin chain can have over 100 individual sugars, each of which can be sulfated in variable positions and quantities. Examples of various chondroitin sulfates are shown below.

Chondroitin Sulfate

[0034] The chondroitin sulfate family was originally isolated well before the structure was characterized, leading to changes in terminology with time. Early researchers identified different fractions of the substance with letters A, B, C, D and E. For example, chondroitin sulfate A is also known as chondroitin-4-sulfate, wherein R^H; R ₂ =S0 ₃ H and R ₃ =H. Chondroitin sulfate B is an old name for dermatan suflate, which is no longer classified as a form of chondroitin sulfate. Although the name "chondroitin sulfate" suggests a salt with a sulfate counter anion, this is not the case, as sulfate is covalently bonded to the sugar. Commercial preparations of chondroitin sulfate typically are the sodium salt. A summary of the various chondroitin sulfate compounds that can be used in the present invention is provided in Table 1. Table 1 : Chondroitin Sulfate Compounds

Identification Ri, R ₂ and R ₃

Chondroitin sulfate A (CS-A)

Ri = H; R ₂ = S0 ₃ H; and R ₃ = H (Chondroitin-4-Sulfate)

r

Chondroitin sulfate C (CS-C)

R, = S0 ₃ H; R ₂ = H; and R ₃ = H (Chondroitin-6-Sulfate)

Chondroitin sulfate D (CS-D)

Ri = S0 ₃ H; R ₂ = H; and R ₃ = SO3H

(Chondroitin-2,6-Sulfate)

Chondroitin sulfate E (CS-E)

Ri = SO3H; R ₂ = SO3H; and R ₃ = H (Chondroitin-4,6-Sulfate)

[0035] During the gel electrophoresis of the chondroitin sulfate A dispersed P2 nanotubes, some single-walled carbon nanotubes migrated down the agarose gel while other single-walled carbon nanotubes remained immobile and stuck in the upper part of the agarose gel. The gel could be visibly seen to have three fractions after the gel electrophoresis. The nanotubes solution extracted from the bottom gel fraction was greenish in color while the nanotubes solution extracted from the top or "stuck" portion of the gel was pinkish (Figure la). The two nanotube fractions ("top" and "bottom") were analyzed with UV-vis-NIR and Raman spectroscopy.

[0036] For UV-vis-NIR spectroscopy, pristine arc discharge P2 single-walled carbon nanotubes were used as the reference sample (#1 , Figure lb) and absorbance peaks in the 600-800 nm and 920-1050 nm ranges due to the first van Hove electronic transitions of metallic nanotubes (Mn) and the second van Hove electronic transition of semiconducting nanotubes (S ₂₂ ), respectively, were observed. For the chondroitin sulfate A dispersed P2 nanotubes extracted from the top fraction after agarose gel electrophoresis (hereafter denoted as P2/CS-A/Top, #2, Figure lb), the Mn peak was selectively suppressed. Measurement of the relative areas of Mn and S ₂₂ bands above the baseline showed that the metallic content of nanotubes in P2/CS-A/Top fraction was drastically decreased from 22% (in the pristine sample) to 5% (compare #1 with #2, Figure lb). The UV-vis-NIR spectrum of chondroitin sulfate A dispersed nanotubes from the bottom fraction (hereafter denoted as P2/CS-A/Bottom, #3, Figure lb) showed increased content of 40% metallic nanotubes. Hence, after agarose gel electrophoresis, the top agarose fraction was composed predominantly of semiconducting nanotubes with ~ 95% purity as most of the metallic nanotubes migrated down under the electric field. The yield of the semiconducting P2/CS-A/Top was measured to be about 25% (based on the initial nanotube feed).

[0037] For comparison, the gel electrophoresis separation of the sodium dodecyl sulfate dispersed P2 solutions was investigated. Sodium dodecyl sulfate has been suggested by a number of previous studies as the only surfactant effective for agarose gel electrophoresis. The metallic content in the nanotubes extracted from the top gel fraction, as calculated from its UV-VIS-NIR spectrum (hereafter denoted as P2 NDS/Top, #4, Figure lb), decreased from 22% (in pristine P2) to 15% after agarose gel electrophoresis. The decrease is significantly lower than the corresponding decrease achieved using chondroitin sulfate A, indicating the more efficient separation achieved using chondroitin sulfate A.

[0038] Raman spectroscopy with 633nm (1.96 eV) wavelength laser was also used to characterize the separation. The 1.96 eV line has been reported to resonate with both metallic (Mn) and semiconducting (S33) species of P2 nanotubes, which have a reported average diameter of 1.4 nm. P2 single-walled carbon nanotubes exhibit Raman bands due to the radial breathing mode (RBM) in the 100-200 cm ^"1 region and G mode in the 1500- 1600 cm ^{" 1} region (Figure 2). The pristine sample (Figure 2a) showed two major peaks at 155 cm ^{" 1} and 170 cm ^"1 due to semiconducting species with diameters of 1.58 and 1.45 nm and one peak at 200 cm ^"1 due to metallic species with 1.22 nm diameter. The

227 I

diameters of species were calculated using the equation: co _RSM jl +C _e df . The d,

two dominant semiconducting species and the dominant metallic species are postulated to be (12, 1 1 ), (13, 8) and ( 13, 4), respectively.

[0039] After agarose gel electrophoresis, the RBM region of P2/CS-A/Top showed a dramatically reduced metallic peak near 200 cm ^"1 but strong semiconducting peaks at 155 and 170 cm ^"1 , corroborating the enrichment of the top gel fraction with semiconducting nanotubes. The single-walled carbon nanotubes obtained from the bottom gel (P2/CS- A/Bottom) showed the persistence of metallic single-walled carbon nanotubes at 200 cm ^{" 1} and two RBM peaks due to semiconducting nanotubes at roughly 170 cm ^"1 and -157 cm ^"1 (Figure 2a) with the latter being dramatically reduced. [0040] All samples exhibited two Raman G bands (G ⁺ and G ^~ ) at around 1590 cm ^"1 and 1 550-1580 cm ^{" 1} corresponding to atomic displacement along the tube axis (longitudinal) and radial directions, respectively (Figure 2b). For the pristine and bottom fraction samples, the G ^" bands at approximately 1550-1580 cm ^'1 , which are generally used as signatures of metallic single-walled carbon nanotubes, were broad due to strong coupling in the density of states and each fitted by a Breit-Wigner-Fano line shape. For the semiconducting P2/CS-A/Top, the decreased width of the G ^" band corroborates the decreased metallic nanotubes content in this fraction.

[0041] To characterize the electronic properties of P2/CS-A/Top nanotubes after agarose gel electrophoresis, the separated semiconducting P2 single-walled carbon nanotubes were applied in network field effect transistors. In addition, for comparison purposes, a similar set of devices was also fabricated using un-separated P2 (pristine) single-walled carbon nanotubes. A total of 20 field effect transistors were made and tested using each of the above solutions. Figure 3a shows the transfer curve of a representative field effect transistor device made using semiconducting P2/CS-A/Top single-walled carbon nanotubes obtained by gel electrophoresis. The output characteristic drain current (Ij) vs. drain voltage { V _d ) measured at gate voltages from -40 V to 40 V with 10 V intervals for the representative device is shown in Figure 3b. The field effect transistors made using the separated semiconducting P2/CS-A/Top nanotubes exhibited p-type behavior with on/off ratios ranging from 10 of 10 (Figure 3c) and mobilities ranging from ~2 to 8 cm ² /Vs (Figure 3d). This is significantly better than the properties (e.g. the maximum mobility of 0.7 cm ² Vs with corresponding on/off ratio of 10 ⁴ ) reported by others for devices made using semiconducting single-walled carbon nanotubes obtained through gel electrophoresis with sodium dodecyl sulfate surfactant and the properties of the reference field effect transistors made using the un-separated field effect transistors (Figure 3c), confirming the efficacy of the chondroitin sulfate A assisted agarose gel electrophoresis technique.

[0042] Figure 4a shows the Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FESEM) images of the P2/CS-A/Top single-walled carbon nanotubes deposited between the source and drain electrodes in a representative field effect transistor device. Individual nanotubes were readily distinguishable. Moreover, the AFM image of pristine P2 single-walled carbon nanotubes deposited in a representative field effect transistor is shown in Figure 4b. The density of pristine and P2/CS-A/Top single- walled carbon nanotubes deposited was kept constant at around 10 tubes/μιη ² .

[0043] Available literature has reported a wide range of mobilities (1 -164 cm ² /Vs) and on/off ratios (10° to 10 ⁶ ) for the field effect transistors fabricated with semiconducting single-walled carbon nanotubes purified by various separation techniques. There is usually a tradeoff between mobility and on/off ratio and a good transistor needs to have both high mobility and high on/off ratio. It has been recently reported that considerably higher mobilities may be achieved when relatively longer semiconducting nanotubes are used in field effect transistors. Using relatively longer pure semiconducting nanotubes produced through selective gel-based filtration, mobilities as high as 100 cm ² /Vs with on- off ratio of 10 ⁵ can be achieved. A further filtration step with the purified semiconducting nanotubes to remove the shorter nanotubes, however, can considerably reduce the overall yield of the purification process. Due to percolation effects and the fringing contributions to capacitance, the apparent mobility of single-walled carbon nanotube network-based field effect transistors can vary widely because of differences in several factors such as surface coverage and average length and not just because of purity of semiconducting versus metallic nanotubes.

[0044] Agarose gel electrophoresis separation relies basically on the differential movement between semiconducting and metallic single- walled carbon nanotubes; the latter moving faster towards the anode electrode. Therefore, evaluating the electrophoretic mobility of various single-walled carbon nanotube/surfactant hybrids can be used to assess the suitability of various surfactants for agarose gel electrophoresis. The Smoluchowski equation (3) can be used to correlate the measured free solution mobility of a particle (μο) to the zeta-potential (ζ) and the dielectric constant of the medium (ε _ηη ): o = (3) where η is the viscosity of the solution. When the medium is gel, however, the actual traveling distance for single-walled carbon nanotubes is much longer than that of the free solution because the single-walled carbon nanotubes are to pass through a randomly distributed network of pores. By taking this into account, the following relationship (4) has been developed to estimate the actual electrophoretic mobility of single-walled carbon nanotubes in the long fiber gels ( _g ) based on the free solution mobility (μο) of single-walled carbon nanotubes:

where / is the length of the gel fibers, c is the gel concentration and LN and are respectively the length and diameter of the nanotubes. The electrophoretic mobility of single-walled carbon nanotubes in gel may be calculated by substituting Equation 3 in Equation 4.

[0045] As can be seen in equations 3 and 4, for a given gel concentration, dispersion medium properties and nanotube length and diameter, the electrophoretic mobility of single-walled carbon nanotubes is basically controlled by the particle charge (zeta- potential) of the nanotubes. Pristine single-walled carbon nanotubes are naturally nonionic and possess very low (close to zero) zeta-potential in neutral solutions (pH ~7). Hence, the majority of the charge content of the single- walled carbon nanotube/surfactant dispersions originates from the anionic surfactants coating the nanotubes. Therefore, the use of highly charged surfactants with higher selectivity for metallic than semiconducting nanotubes (such as chondroitin sulfate A) can result in the higher charge and thus higher mobility of metallic than semiconducting single-walled carbon nanotubes, leading to their separation.

[0046] Based on the above discussion, the separation efficiency in gel electrophoresis may be directly proportional to the zeta-potential (particle charge) of the single-walled carbon nanotube/surfactant hybrid and the selectivity of surfactants for the metallic nanotubes. The zeta-potentials of the various single-walled carbon nano tube/surfactant dispersions considered in the present study were measured and are shown in Table 2.

Table 2: Zeta-potentials of single- walled carbon nanotubes dispersed with different dispersing agents.

* sodium dodecyl sulfate (SDS), sodium cholate hydrate (SC), sodium dodecyl benzene sulfonate (SDBS), chondroitin sulfate A (CS-A), chondroitin sulfate B (CS-B), and chondroitin sulfate C (CS-C).

[0047] Table 2 shows the P2/CS-A and P2/SDS hybrids possess considerably higher zeta-potentials than do other P2/dispersant combinations. The performance of a number of other dispersants including the other isomers of chondroitin sulfate (i.e. CS-B and CS- C) and heparin sodium salt was also investigated. Heparin sodium salt was not an effective dispersion agent and therefore, proved to be unsuitable for agarose gel electrophoresis separation. In addition, although CS-B and CS-C are good dispersants for single-walled carbon nanotubes, the use of these surfactants in agarose gel electrophoresis resulted in a significantly less efficient separation compared to chondroitin sulfate assisted agarose gel electrophoresis; CS-B and CS-C possess relatively lower charge densities compared to chondroitin sulfate and this is confirmed by zeta-potential measurements. Moreover, these results confirmed that pristine has a very low (near zero) zeta-potential and therefore, the zeta-potentials measured for single- walled carbon nanotubes/surfactant dispersions originate mainly from the surfactants charge. The higher charge of sodium dodecyl sulfate and chondroitin sulfate A leads to significantly higher electrostatic forces exerted (by the electric field) on metallic single- walled carbon nanotubes dispersed using these two dispersants and therefore, leads to a more efficient gel electrophoresis separation compared to those dispersed using other dispersing molecules that result in single-walled carbon nanotubes/surfactant hybrids with less charge densities. [0048] Without wishing to be bound by theory, it is believed that the better separation achieved using a dispersant from the chondroitin family such as chondroitin sulfate A compared to sodium dodecyl sulfate may originate from the groups present in the chondroitin sulfate A molecular structure. It has been established both experimentally and theoretically that amine containing compounds are able to select neutral metallic single-walled carbon nanotubes. A simulation based on the different adsorption modes of methylamine on neutral single-walled carbon nanotube via local density functional theory (DFT) and ultrasoft pseudopotential plane-wave methods has shown that regardless of the adsorption mode, the magnitude of the adsorption energy on metallic tubes is always larger than on semiconducting tubes. This larger adsorption energy in turn has been proved experimentally to mean a stronger interaction of amine containing compounds with metallic tubes than with semiconducting tubes. In the chondroitin sulfate A structure, the selectivity for metallic nanotubes may be augmented by the hydrophobic backbone of chitosan that interacts favorably with the hydrophobic nanotube surface.

[0049] Gel electrophoresis has several advantages over other reported separation methods such as density-gradient ultracentrifugation (DGU), and chemical approaches. Unlike density-gradient ultracentrifugation and chemical techniques, gel electrophoresis using for example, chondroitin sulfate A, as shown here, offers both high single-walled carbon nanotube purity and high separation yield. While density-gradient ultracentrifugation may result in highly pure semiconducting single-walled carbon nanotubes, the separation yield is significantly lower (usually 1 -2%) than that of the chondroitin sulfate A assisted agarose gel electrophoresis (-25%). Moreover, chemical approaches are only applicable to specific tubes with smaller diameters such as CoMoCAT and the purity achieved with larger diameter nanotubes, such as arc discharge single-walled carbon nanotubes investigated here, needed for high performance transistors and related electronic devices, is usually not high enough. Gel electrophoresis can be applied to a very broad range of nanotubes with various diameters and lengths to achieve high yield and purity.

[0050] In summary, a novel separation technique based on chondroitin sulfate assisted agarose gel electrophoresis that can achieve high purity (95%) separation of 5ew-single- walled carbon nanotubes from met-single-walled carbon nanotubes has been developed in the present invention. Due to the better separation efficiency, field effect transistors made, for example, from chondroitin sulfate A assisted agarose gel electrophoresis showed considerably better performance (mobilities of ~2 to 8 cm ² /Vs and on/off ratios from 10 to 10 ) as compared with those of the previously proposed sodium dodecyl sulfate assisted agarose gel electrophoresis process (mobility of 0.7 cm ² /Vs and 10 ⁴ on/off ratio). Further, the chondroitin sulfate A assisted agarose gel electrophoresis technique has considerably higher yield of about 25% (in the order of 5-1 Ox) than other highly selective separation methods which hitherto have low yields. The proposed chondroitin sulfate dispersant possess amine containing groups and a high degree of sulfation, which pre-select metallic nanotubes and cause good mobility and dispersion of individual nanotubes, respectively. The zeta-potential measurements showed that P2/CS- A hybrid has a considerably high zeta-potential which, together with the known chemical selectivity of chondroitin sulfate A for metallic nanotubes, results in high yield and high purity separation.

EXAMPLES

[0051] Experimental Methods

[0052] Materials: Chondroitin sulfate A (CS-A), sodium dodecyl sulfate (SDS), chondroitin sulfate B (CS-B), chondroitin sulfate C (CS-C), sodium cholate hydrate (SC), sodium dodecyl benzene sulfonate (SDBS) and heparin sodium salt were purchased from Sigma-Aldrich (Singapore) at the highest commercially available purity and used without any further purification. Purified arc discharge single-walled carbon nanotubes (P2) were purchased from Carbon Solutions, Inc. (Riverside, CA, USA) and used without any further purification. Agarose fine powder purchased from Nacalai Tesque (Japan) was used to fabricate the agarose gel. Buffer QG was purchased from Qiagen (Singapore).

[0053] Preparation of P2/CS-A single-walled carbon nanotubes solution: Purified arc discharge (P2) nanotubes were dispersed in an aqueous solution containing 2 wt % of chondroitin sulfate (CS-A) as dispersant using probe sonication (SONICS, VCX-130) at 100 W in a water-ice bath for 3 h. After sonication, the mixture was centrifuged (Sartorius, SIGMA@3K30) at 50,000 g for 1 h to remove single-walled carbon nanotube bundles. The supernatant was collected for use in gel electrophoresis. [0054] Gel Electrophoresis: The agarose gel was made by adding 1 wt % agarose to 0.2 wt % surfactant in > 2 Transport Buffer (TB) buffer solution. TB buffer solution was prepared using 48.5mM boric acid and 50mM tris hydroxymethyl aminomethane. The pH of the TB buffer was adjusted to 8.5. The agarose gel solution was melted by microwave heating and fed into a space (0.3 cm wide) sandwiched by 2 glass plates (20 cm by 20 cm), filling about 2/3 of its height. The sample was then cooled at room temperature to form the gel. The remaining 1/3 of the plate height was filled with a mixture of single-walled carbon nanotube solution and agarose solution. The mixture was prepared by dispersing about 20 ml of the previously prepared single-walled carbon nanotube solution (0.3 mg/ml) into a similar volume (20 ml) of 2 wt % agarose solution and then melting the resultant mixture through microwave heating. The plate was next placed in the gel electrophoresis apparatus (PROTEAN II XI, Bio Rad Laboratories, Singapore) filled with the ^χ 2 TB buffer solution. The sample was subjected to electric field at 200 V for about 4 hours.

[0055] Acid Treatment: chlorosulfonic acid was used for removal of agarose gel from the semiconducting carbon nanotubes after electrophoresis. After electrophoresis, the gel was cut into three (top, middle and bottom) fractions. The top fraction (containing predominantly semiconducting nanotubes) with agarose gel was dried in vacuum at 100 °C overnight. The dried agarose/single-walled carbon nanotubes were added to chlorosulfonic acid and stirred for 3 days at room temperature. The solution was drop- wise added to -400 ml of cold deionized water. The solids were collected by vacuum filtration and washed with water until neutrality was achieved. The solids were next washed with ethanol and dried at 100°C. 1M NaOH was then added to the residue filtered in a beaker. The nanotube residue was stirred for 30 min and filtered. The residue was washed with water again until neutrality. The solids were then washed with ethanol followed by drying at 100 °C.

[0056] Zeta-Potential Measurement: Zeta-potential measurements were performed with a BROOKHAVEN-Zeta PALS system with irradiation from a solid state laser (660nm wavelength). All the P2/dispersant suspensions were prepared as described in Section 2.2. The sample cell was filled with the P2/dispersant suspension and the zeta-potential measurement was performed without additional electrolyte at 25 °C. [0057] Characterization: Raman spectroscopy was performed with a Renishaw inVia Raman microscope equipped with 633-nm (1.96 eV) laser wavelength. Optical absorption measurements were performed with a Varian Cary 5000 UV-vis-NIR spectrophotometer. All single-walled carbon nanotube samples, except pristine, were treated with Buffer QG before the UV-vis-NIR scans. A quantitative measure of the semiconducting purity of single-walled carbon nanotubes was obtained by considering the AA(S)/[AA(S) +AA(M)] ratio, where AA(S) and AA(M) are, respectively, the areas of the S ₂₂ (E ^S 22) an Mu (E ^M n) absorption spectral bands after subtraction of the baseline. Moreover, the Throughput Yield (TPY) was calculated as the weight of the semiconducting single-walled carbon nanotubes gained after gel electrophoresis divided by the weight of the starting original single-walled carbon nanotubes sample.

[0058] The effectiveness of agarose gel electrophoresis separation was also evaluated by characterization of the bottom-gated single-walled carbon nanotube Field Effect Transistors (FET) fabricated using the acid treated semiconducting single-walled carbon nanotubes. The acid treated semiconducting single- walled carbon nanotubes were re- dispersed in a 1 % solution of a (1 :4) mixture of SDS and SC using a cup-horn sonicator (SONICS, VCX-130) at 100 W in a water-ice bath for 1 h. The mixture was then centrifuged (SIGMA @ 3 30) at 30,000 g for 1 h. The supernatant single- walled carbon nanotube suspension was collected and used for fabricating field effect transistors by drop casting. The FET devices were bottom contact with channel length and width of 50 μπι and a 300 nm S1O2 layer as the gate dielectric. The source and drain electrodes were made of 60-nm Au coated on top of 10-nm Ti. In the drop-cast procedure, the single- walled carbon nanotube suspension was dropped onto the devices, followed by drying and rinsing with de-ionized water. Besides the field effect transistors made with the semiconducting single-walled carbon nanotubes separated by agarose gel electrophoresis, a similar set of devices (similar electrode dimensions and nanotubes density (~10 tubes/μη ² )) were also fabricated using the un-separated (pristine) single- walled carbon nanotubes as the reference field effect transistors.

[0059] Atomic Force Microscopy of single-walled carbon nanotubes on bottom-gated field effect transistors was performed with a MFP 3D microscope (Asylum Research) in AC mode. Field Emission Scanning Electron Microscopy of the field effect transistors was performed with a JEOL JSM-6700F microscope. The field effect transistors were coated with platinum before FESEM.

[0060J The on/off ratio and mobility of the field effect transistors were also measured to characterize their performance. The device property measurements were carried out under ambient conditions using a Keithley semiconductor parameter analyzer Model 4200-SCS. Mobility was estimated using the coupling model which has been proved to provide a more analytical and rigorous measure for capacitance than the commonly used parallel plate model, especially when the average spacing between single-walled carbon nanotubes is large compared to the thickness of the dielectric. In this model, the capacitance per unit area can be calculated using the following equation:

C ^~ ' + In

^{' 2πε} ο ^ε ο, R nD where CQ (4X 10 ^{" i0} [F/M]) stands for quantum capacitance of nanotubes, D is the areal density of nanotubes and is measured to be about 10 tubes/μηι, L _ox is the Si0 ₂ thickness, ε _οχ is the dielectric constant of Si0 ₂ , £¾ is the permittivity of free space (so ⁼ 8.85 ^χ 10 ² F/m) and R is nanotubes radius. Once the capacitance is calculated, mobility may be calculated using the following equation: μ = x— ^d - (2)

C xW xV _d dV _g where is the drain current, V _g is the gate voltage, V _d is the drain voltage , L is the channel length and W is the channel width.

[0061] Although the disclosure has been described with reference to the above examples, it will be understood that modifications and variations are encompassed within the spirit and scope of the disclosure. Accordingly, the disclosure is limited only by the following claims.