Comparison of the Quality of Aqueous Dispersions of Single Wall

Herein we report a quantitative comparison of the relative ability of “wrapping ...... Gilman, J. W.; Maupin, P. H.; DeLong, H. C.; Truloved, P. C. ...
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Langmuir 2008, 24, 5070-5078

Comparison of the Quality of Aqueous Dispersions of Single Wall Carbon Nanotubes Using Surfactants and Biomolecules Reto Haggenmueller,† Sameer S. Rahatekar,† Jeffrey A. Fagan,O Jaehun Chun,O Matthew L. Becker,O Rajesh R. Naik,‡ Todd Krauss,§ Lisa Carlson,§ John F. Kadla,£ Paul C. Trulove,| Douglas F. Fox,|,∇ Hugh C. DeLong,⊥ Zhichao Fang,# Shana O. Kelley,# and Jeffrey W. Gilman*,† National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627, UniVersity of British Columbia, Biomaterials Chemistry, VancouVer, British Columbia V6T 1Z4, Canada, Department of Chemistry, U.S. NaVal Academy, Annapolis, MD21402, Air Force Research Laboratory/Air Force Office of Scientific Research, Arlington, Virginia 22203, and Faculty of Medicine, Biochemistry, Faculty of Pharmacy, UniVersity of Toronto, Toronto, Ontario M5S 1A8, Canada ReceiVed September 28, 2007. In Final Form: December 12, 2007 The use of single wall carbon nanotubes (SWCNTs) in current and future applications depends on the ability to process SWCNTs in a solvent to yield high-quality dispersions characterized by individual SWCNTs and possessing a minimum of SWCNT bundles. Many approaches for the dispersion of SWCNTs have been reported. However, there is no general assessment which compares the relative quality and dispersion efficiency of the respective methods. Herein we report a quantitative comparison of the relative ability of “wrapping polymers” including oligonucleotides, peptides, lignin, chitosan, and cellulose and surfactants such as cholates, ionic liquids, and organosulfates to disperse SWCNTs in water. Optical absorption and fluorescence spectroscopy provide quantitative characterization (amount of SWCNTs that can be suspended by a given surfactant and its ability to debundle SWCNTs) of these suspensions. Sodium deoxy cholate (SDOCO), oligonucleotides (GT)15, (GT)10, (AC)15, (AC)10, C10-30, and carboxymethylcellulose (CBMC-250K) exhibited the highest quality suspensions of the various systems studied in this work. The information presented here provides a good framework for further study of SWCNT purification and applications.

Introduction Single wall carbon nanotubes (SWCNTs) have attracted a remarkable amount of attention recently due to the significant potential they offer to provide quantum improvements in applications such as catalysis, solar energy, medical diagnostics, drug delivery, optical sensing, nanoelectronics, coatings, and polymer nanocomposites.1 Advances in purification,2 functionalization,3 and characterization4 are critical to developing a fundamental understanding of the structure-property relationships that govern the performance of these novel materials in the above applications. Specifically, enhancing the dispersion of individual SWCNTs in solutions enables many other SWCNT investigations such as (1) separation of SWCNTs by length and chirality, (2) preparation of SWCNT devices, (3) preparation of SWCNT polymer composites, (4) SWCNT environmental and health effects studies, and (5) SWCNT functionalization. For * To whom correspondence should be addressed. E-mail: [email protected]. † National Institute of Standards and Technology, Fire Research Division. O National Institute of Standards and Technology, Polymers Division. ‡ Air Force Research Laboratory. § University of Rochester. £ University of British Columbia. | U.S. Naval Academy. ⊥ Air Force Research Laboratory/Air Force Office of Scientific Research. # University of Toronto. ∇ Present address: American University, Department of Chemistry, Washington, DC 20016. (1) Baughman, R. H.; Zakhidov, A. A.; deHeer, W. A. Science 2002, 297, 787-792. (2) Haddon, R. C.; Sippel, J.; Rinzler, A. G.; Papadimitrakopoulos, F. Mater. Res. Soc. Bull. 2004, 29, 252-259. (3) Banerjee, S.; Hemraj-Benny, T.; Wong, S. S. AdV. Mater. 2005, 17, 1729. (4) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P. Carbon Nanotubes: Sythesis, Structure Properties and Applications; Springer: Berlin, 2001.

these reasons a number of groups have reported solubilization of SWCNT using different noncovalent surface complexation chemistries that do not alter the fundamental properties of the SWCNT. The types of molecules used for this purpose include surfactants, small molecule complexing agents, and organic biopolymers and oligomers. Various anionic, cationic, and nonionic surfactants have been used and were proposed to solubilize SWCNTs5-9 through interaction of the organic (alkyl or hydrophobic) groups on the surfactant with the SWCNT surface and interaction of the ionic portion of the surfactant with the aqueous phase. Wang et al.9 suggested that there exists an optimal surfactant concentration for dispersion, which results from competition between maximization of surfactant adsorption onto SWCNT surfaces and a micelle-mediated depletion interaction between adjacent SWCNT bundles. Debundling of SWCNTs is also reported in N-methyl-2pyrrolidone.10 Organic aromatic polymers have also been used to solubilize SWCNTs;11 here an interaction between the aromatic groups on the polymer backbone or side chain and the surface of the SWCNT affects solubilization. Notably, a variety of biopolymers have also been used to (5) Wenseleers, W.; Vlasov, I. I.; Goovaerts, E.; Obraztsova, E. D.; Lobach, A. S.; Bouwen, A. AdV. Funct. Mater. 2004, 14 (11), 1105-1112. (6) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3 (2), 269-273. (7) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2003, 3 (10), 1379-1382. (8) Kocharova, N.; a¨ritalo, T. A.; Leiro, J.; Kankare, J.; Lukkari, J. Langmuir 2007, 23 (6) 3363-3371. (9) Wang, H.; Zhou, W.; Ho, D. L.; Winey, K. I.; Fischer, J. E.; Glinka, C. J.; Hobbie, E. K. Nano Lett. 2004, 4, 1789-1793. (10) Giordani, S.; Bergin, S. D.; Nicolosi V.; Lebedkin, S.; Kappes M. M.; Blau W. J.; Coleman, J. N. J. Phys. Chem. B 2006, 110, 15708-15718. (11) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265-271.

10.1021/la703008r CCC: $40.75 © 2008 American Chemical Society Published on Web 04/29/2008

Comparison of the Quality of Aqueous Dispersions

effectively disperse SWCNT in aqueous solutions. Zheng et al. demonstrated the effectiveness of single-strand oligonucleotides to suspend individual SWCNT, enabling successful ion exchange chromatography.12 It was further shown by Heller et al.13 that the oligonucleotides exhibit conformational polymorphism on SWCNT surfaces due to the presence of divalent cations. Ikeda et al.14 showed that aqueous SWCNT solubility significantly depends on the number of phosphate groups and the type of base in the ologonucleotide. Longer DNA (number of base pairs .100) with random base pair sequences have also been shown to achieve good dispersion of SWCNTs.15 Phage display was used to identify peptides with selective affinity for SWCNTs.16 A number of other biopolymers have been shown to be effective complexation agents for SWCNTs, including chitosan17,18 and cellulose derivatives.19 Ishibashi and Nakashima20 evaluated a range of sugar and steroid compounds for their ability to disperse and exfoliate SWCNTs. Nepal and Gerckerer21 reported similar work for dispersing SWCNTs using different types of proteins to disperse SWCNTs. It is critical that SWCNT suspensions be carefully characterized. Important issues include the relative amount of individual SWCNT versus SWCNT ropes or bundles in the suspensions and also the net concentration. The maximum relative solubility of these SWCNT complexes varies by as much as 2 orders of magnitude over the range of methods developed to date and strongly depends on the processing conditions used to obtain the SWCNT suspensions. For example, Islam et al. reported SWCNT-sodiumdodecyl benzene sulfonate suspensions with up to 20 mg/mL SWCNT concentration5 whereas Moore et al. obtained ∼0.01 mg/mL.7 These methods have been developed in different laboratories under different processing conditions often using different SWCNT sources; therefore, it is very difficult to make a quantitative comparison of the various methods of solubilizing SWCNTs. In order to make such a comparison, it is necessary to compare the systems using the same processing and characterization conditions from the same source of SWCNT. Previously, Tan et al.22 used a resonance ratio method, obtained from UV-vis-NIR spectra, to compare the quality of dispersions of SWCNTs using cationic, anionic, and nonionic surfactants. Wenseleers5 et al. also used the absorption peak at ∼1710 nm as a measurement of the relative concentration of SWCNTs using different types of bile salt surfactants. In this study, we report a quantitative comparison of the relative ability of “wrapping polymers” including oligonucleotides, peptides, lignin, chitosan, and cellulose and surfactants such as cholates, ionic liquids, and organosulfates to solubilize SWCNTs in water. In an effort to fully characterize the relative solubilization effectiveness of these various complexation approaches, we (12) Zheng, M.; Jagota, A.; Strano, M. S.; Santos, A. P.; Barone, P.; Chou, S. G.; Diner, B. A.; Dresselhaus, M. S.; Mclean, R. S.; Onoa, G. B.; Samsonidze, C. C.; Semke, E. D.; Usrey, M.; Walls, D. J. Science 2003, 302, 1545-1548. (13) Heller, D. A.; Jeng, E. S.; Yeung, T. K.; Martinez, B. M.; Moll, A. E.; Gastala, J. B.; Strano, M. S. Science 2006, 311, 508-511. (14) Ikeda, A.; Hamano, T.; Hayashi, K.; Kikuchi, J. Org. Lett. 2006, 8 (6), 1153-1156. (15) Gigliotti, B.; Sakizzie, B.; Bethune, D. S.; Shelby, R. M.; Cha, J. N. Nano Lett. 2006, 6, 159-164. (16) Wang, S.; Humphreys, E. S.; Chung, S. Y.; Delduco, D. F.; Lustig, S. R.; Wang, H.; Parker, K. N.; Rizzo, N. W.; Subramoney, S.; Chiang, Y. M.; Jagota, A. Nat. Mater. 2003, 2, 196-200. (17) Yang, H.; Wang, S. C.; Mercier, P.; Akins, D. L. Chem. Commun. 2006, 14, 1425-1427. (18) Takahashi, T.; Luculescu, C. R.; Uchida, K.; Ishii, T.; Yajima, H. Chem. Lett. 2005, 34 (11), 1516-1517. (19) Minami, N.; Kim, Y.; Miyashita, K.; Kazaoui, S.; Nalini, B. Appl. Phys. Lett. 2006, 88, 093123-1-093123-3. (20) Ishibashi, A. and Nakashima, N. Chem.-Eur. J. 2004, 12 (29), 75957602. (21) Nepal, D. and Geckeler K. E. Small 2006, 3 (7), 1259-1265. (22) Tan, Y. Q.; Resasco, D. E. J. Phys. Chem. B 2005, 109, 14454-14460.

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prepared SWCNT suspensions processed under the same conditions. In all cases the SWCNT complex solutions were mixed using long sonication times. Removal of poorly solubilized SWCNT aggregates was accomplished using centrifugation. The quantity of SWCNT in solution in each case was characterized using absorption spectroscopy. Optical absorption and fluorescence methods were evaluated to characterize the quality of these solutions. The resolutions of the peaks for the different chiralities in the absorption spectra were compared, as were the fluorescence intensities. A composite of these measures of solubilization effectiveness is presented. Experimental Section Materials. The imidazolium salts were prepared in our laboratory and were of the general form 1,2-dimethyl-3-X-imidzolium-Br, or -Cl with X ) C4H9, C8H17, C10H21, C12H25, C14H29, and C16H33. The syntheses were published previously23 (except for DMHdImCl obtained from Aldrich). The nomenclature DMBI-Br is used for the imidazolium salts, where B is substituted with the corresponding short form of the alkyl chain (Dd, dodecyl; Td, tetradecyl, and Hd, hexadecyl). Sodiumdodecyl sulfonate (SDS), sodiumdodecyl benzene sulfonate (SDBS), sodium cholate (SCO), sodium deoxycholate (SDOCO), chitosan (20 and 200k Brookfield viscosity, deacetylation ≈ 75-85%), carboxymethyl cellulose (CBMC, Mw ) 90 and 250k) were obtained from Sigma-Aldrich. Lignosulfonate sodium salts (#907: Mw 13k, SO3Na/1000 unit wt of lignin ≈ 1.2; #825: Mw ) 3.5k, SO3Na/1000 monomer units of lignin ≈ 2.0) were obtained from MeadWestvaco. The ssDNA with 30 and 20 nucleotides (sequences: (GT)15, (GT)10, (AC)15, (AC)10, T30, T20, C30, C20, A30, A20) were purchased from Integrated DNA Technologies (G, guanine; C, cytosine; T, thymine; A, adenine). ssDNA with 10 nucleotides (G10, T10, C10, A10) was synthesized using an ABI 394 DNA/RNA synthesizer according to standard automated solid-phase techniques.24 The peptide P1R5 has the amino acid sequence “SSKKSGSYSGSKGSKRRILGGGGHSSYWYAFNNKT”.25 SWCNTs (CoMoCAT, SouthWest Nanotechnologies Inc., Norman, OK, Batch NI-5-A001, S-P95-02-Dry) were purchased from SouthWest Nanotechnologies and used as received.26 Typical purification by SouthWest Nanotechnologies, Inc. includes oxidation of the Co and Mo catalysts and removal of the catalysts and the silica support by dissolution. Methods. All SWCNT suspensions were made at 0.5 mg of SWCNT/mL of water. SWCNT-DNA complexes were suspended in a buffer solution (3 mM NaN3, 200 mM NaCl, 40 mM Tris (2amino-2-hydroxymethyl-1,3-propanediol) in ultrapurified water, buffered to pH 7 with HCl), at a SWCNT-DNA ratio of 1:1 by mass. SWCNT-Chitosan suspensions were prepared in a 2% acidic acid solution (pH 2.85), at a SWCNT-Chitosan ratio of 1:10 by weight as described in the literature.18 All other surfactant-SWCNT systems were made at 0.5 mg/mL SWCNT in a 5 mg/mL surfactant solution (ratio 1:10 SWCNT-surfactant by mass.) A typical suspension volume of 6 mL in a 15-mL tube was used for all experiments. A tip sonicator was used for 2 h at a power output of 1.5 W/mL (Sonics VibraCell, 3 mm tip), with the tube immersed in an ice bath. Suspensions that appeared stable after sonication, i.e., no macroscopic phase separation observed, were centrifuged at 21 000g for 2 h. The resulting supernatant was used for further measurements. In some cases the solutions were filtered through DVPF membrane filters (0.45 µm) prior to analysis. UV-vis-NIR spectra of stable suspensions were recorded in transmission mode on a Perkin-Elmer Lambda 950 spectrometer using a 1 mm quartz cuvette. Data were recorded at 1 nm increments, (23) Fox, D. M.; Awad, W. H.; Gilman, J. W.; Maupin, P. H.; DeLong, H. C.; Truloved, P. C. Green Chem. 2003, 5, 724-727. (24) Lapierre, M. A.; O’Keefe, M.; Taft, B. J.; Kelley, S. O. Anal. Chem. 2003, 75, 6327-6333. (25) Pender, M. J.; Sowards, L. A.; Hartgerink, J. D.; Stone, M. O.; Naik, R. R. Nano Lett. 2006, 6 (1), 40-44. (26) Kitiyanan, B.; Alvarez, W. E.; Harwell, J. H.; Resasco, D. E. Chem. Phys. Lett. 2000, 317 (3-5), 497-503.

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with an instrument integration time of 0.2 s per increment. The incident light was circularly polarized prior to the sample compartment, and the instrument was corrected for both the dark current and background. The SWCNT concentration in suspensions after sonication, centrifugation, and filtering was determined from optical absorption spectroscopy27 using the Beer-Lambert law: A ) Rlc

(1)

where A is the absorbance, R the extinction coefficient, l the path length, and c the concentration. To determine R, absorbance spectra were recorded of SDBS-SWCNT suspensions immediately after sonication, before centrifugation, to prevent SWCNT precipitation. The absorbance of these suspensions reflect 100% of the initial known SWCNT amount (0.5, 0.2, 0.1, 0.05, 0.01 mg/mL SWCNT), with a negligible amount of SWCNT precipitated. Absorbance values at 891 nm, where CoMoCAT SWCNTs do not have peaks of SWCNT interband transitions, are plotted as a function of concentration. The result is a straight line, and using the Beer-Lambert law, R was obtained from the slope of the linear least-square fit to A, resulting in R ) 2.064 ( 0.05 mL/mg‚mm. The same procedure was applied to SDBS-, CBMC-250K-, DMHdIm-Cl-, and (GT)15-SWCNT suspensions with various concentrations (0.05-0.001 mg/mL, determined with above R) after sonication and centrifugation to establish that this method is also valid for suspensions after centrifugation at low concentrations. The least-square fits to absorbencies in the vicinity of λ ) 891 nm result in comparable slopes, more specifically, 2.204 ( 0.02, 2.044 ( 0.017, 2.219 ( 0.013, and 2.167 ( 0.015 mL/mg‚mm for SDBS-, CBMC-, DMHdIm-Br-, and (GT)15-SWCNT, respectively. This demonstrates the validity of this method for determining the SWCNT concentration in these suspensions. The average of the five measurements, 2.14 ( 0.1 mL/mg‚mm, is used to determine the concentration of the various SWCNT suspensions after centrifugation. Photoluminescence excitation (PLE) spectroscopy maps and line scans were obtained using a JY-Horiba nanolog-3 spectrofluorometer and were corrected for the instrument’s source spectral distribution, detector spectral response, and the absorbance of the filter that is used to restrict the scattered excitation light from the NIR monochromators and detector. Excitation wavelength was scanned in 10 nm increments using a 450 W xenon lamp through a 10 nm slit. The emission was collected at 90° and measured using a liquid N2-cooled InGaAs detector over 4 nm increments through a 10 nm slit.

Results Visual inspection of SWCNT suspensions can provide an initial qualitative evaluation of the effectiveness of a suspension. In general if sonication followed by centrifugation results in a homogeneous, black suspension, which is stable after several hours, then the suspension is considered by many as a ‘good suspension’. However, more detailed characterization of the quality of the suspension is necessary. Figure 1 shows SWCNT suspensions after sonication for the imidazolium salt series with varying alkyl-chain length. Dodecyl appears to be the minimum alkyl chain length necessary to suspend SWCNT; shorter molecules do not yield suspended SWCNT. Only solutions with visibly suspended SWCNT without agglomerates were further processed and characterized. Absorption. The UV-vis-NIR absorption behavior of the SWCNT solutions was used first to compare the ability of the surfactants to suspend SWCNT. Specifically, the efficiency was compared for each system. The efficiency is the original SWCNT concentration used for the suspensions (0.5 mg/mL) divided by (27) Attal, S.; Thiruvengadathan, R.; Regev, O. Anal. Chem. 2006, 78, 80988104.

Figure 1. Imidazolium-Br-SWCNT suspensions in water, ∼10 min after tip sonication. The numbers refer to the length of the alkyl chain of the imidazolium salt.

Figure 2. Efficiency, expressing amount of SWCNT in suspension after sonication and centrifugation. The typical error in the efficiency is (2%, based on uncertainties of extinction coefficient; error bars in the figure are one standard deviation of several suspensions, prepared under comparable conditions (note that the olignucleotides are used in a 1:1 mass ratio with SWCNTs whereas for the rest of the surfactants, a 10:1 mass ratio is used to achieve concentration above critical micelle concentration).

the concentration after sonication and centrifugation (see Figure 2). The measure of efficiency indicates clear trends for various surfactants, including differences based on chemical composition and molecular mass. The ssDNA samples with bases GT, AC, and C have comparable efficiencies of ∼30%, independent of the chain length. The suspensions with the base T show a marked decrease in efficiency with decreasing length of the ssDNA strand. SWCNT-DNA complexes with bases A and G show ≈5% efficiency for all tested ssDNA lengths. The solublization efficiencies of the imidazolium salts surpass the ssDNA for alkyl chain length longer than dodecyl. As illustrated in Figure 1, DMDdIm-Br has the critical alkyl chain (C12) length to suspend SWCNT. Shorter imidazolium salts are not able to suspend SWCNT, whereas longer ones demonstrate a much higher efficiency. SDBS and SDS are comparable to the imidazolium salts with long alkyl chains. SDOCO and the CBMC reach the highest solublization efficiencies of the studied surfactant systems. The small difference of a hyroxy group between SDOCO and SCO results in an efficiency change of 17%. Chitosan and lignosulfonate show reduced efficiencies as compared to SDOCO and CBMC. Higher molecular weight increases the efficiency for both CBMC and chitosan. The polypeptide P1R5 yields ≈7% suspended SWCNT. We also tested peptides that consist of

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Figure 4. Ratio of absorbance intensity of (6,5) peak versus absorbance intensity of the baseline at ∼ 905 nm for the various surfactant systems.

Figure 3. Optical absorption spectra of SWCNT-surfactant systems (a) ssDNA and (b) others, normalized at 1685 nm and shifted for clarity.

hydrophobic phenylalanine (F) and hydrophilic lysine (K) amino acids with the sequences: FKFKFKFKFK, FFFFFKKKKK, and KKKFFFFKKK. The SWCNT-peptide mixtures separated immediately after sonication, indicating that more complex peptides like P1R5 are necessary to suspend SWCNT. In addition to the determination of the efficiency, the resolution of the optical absorption spectra provide an estimate of the quality of the SWCNT suspensions. Here the resolution of the absorption peaks (excitonic interband transitions for SWCNT chiralities) of SWCNTs were compared. It has been reported that clear absorption peaks can only be observed when SWCNTs are exfoliated from the bundle; this reduces electronic intertube coupling, which obscures fine structure in the spectra.28 Here, the primary band gap absorptions E11 are discussed for the stronger peaks. Clearly resolved peaks are present for all ssDNA (Figure 3a), with GT appearing most sharp and separated. Decreasing chain length slightly reduces the peak quality, i.e., compare C30 vs C10 and T30 vs T10. Base type and their combination are important; the peaks in A30 and G10 spectra are distinctively weaker and more broad than GT or AC. SDOCO shows peaks better resolved than for (GT)15 (Figure 3b), indicating better (28) Reich, S.; Thomsen, C.; Ordejon, P. Phys. ReV. B 2002, 65, 1554111-11.

exfoliated SWCNTs for the SDOCO suspensions. Peaks for SCO suspensions are weaker and more broad than that for SDOCO and (GT)15. The lignosulfonates and CBMC show peaks comparable to (GT)15. Chitosan, SDBS appear somewhat broader but still show peak separation. Well-resolved peak features in these spectra suggest good dispersion. SDS and imidazolium salts show poor peak resolution, indicating SWCNT bundling. The peptide P1R5-SWCNT spectra show an intermediate peak definition. The peak shifts are similar to previous reports with SDBS most blue-shifted and lignosulfonate and P1R5 most redshifted (refer to Table 1). Filtration of SWCNT solutions through 0.45 µm filters presumably removes larger objects such as long SWCNT ropes and agglomerates that are still in solution after centrifugation. However, the absorption spectra of centrifuged and filtered samples are roughly comparable with no marked changes in general line shape. Furthermore, we found no systematic dependence between filtration yield and surfactant structure; all filtration yields fell between 75 and 100 mass fraction %. Thus, filtration gives no improvement in solution quality. Peak Ratios. The ratio between the absorbance for the (6,5) SWCNT and the baseline absorbance at ∼905 nm (or at the corresponding wave length, considering the spectra shift for the various surfactants), reflects the absorption peak resolution and may be a quantitative indicator of suspension quality. We propose that a higher peak ratio indicates better SWCNT separation in suspensions. A similar approach was used by Tan et al.22 to study the effectiveness of different types of anionic and cationic surfactants to debundle SWCNTs. Figure 4 shows the peak ratio data which reveal significantly different rankings of the various surfactants as compared to that derived from the efficiencies (Figure 2). SDOCO, (GT)15, and (GT)10 have the highest ratio of ≈2.45, indicating the best separation of SWCNT in these suspensions. ssDNA bases AC and C are similar with a ratio of ≈2.2; T shows a decreasing ratio with decreasing ssDNA length, similar to the observed decrease in efficiency. A30 is much better than A20 and A10 (near zero, not shown) consistent with the efficiency. The imidazolium salts and SDS have among the lowest ratios; this is indicative of agglomerated SWCNT. SDBS, SCO, CBMC, and chitosan have comparable ratios of ≈2.0. The peak ratio of lignosulfonate almost reaches that of (GT)15 and (GT)10,

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absorption (6.5), nm (GT)15 (AC)15 C30 T30 A30 G10 DMHdImCl DMHdImBr SDS SDBS SDOCO SCO CBMC250K CBMC90K Chitosan 200K Chitosan 20K Lignosulfonate 825 Lignosulfonate 907 Peptide P1R5

991 991 994 992 993 995 1003 1003 989 978 983 982 989 989 993 995 998 999 1003

shift from (GT)15 nm meV 0 0 2 0 2 3 11 11 -2 -13 -8 -9 -2 -2 2 3 7 8 12

0 0 -3 -1 -2 -4 -14 -14 3 17 10 12 3 3 -2 -4 -9 -10 -15

indicating a comparable SWCNT dispersion (based on this measurement). The peptide P1R5 has a peak ratio somewhat below most of the other biomolecules and SDBS. The peak ratio of suspensions after filtering through 0.45 µm filters are the same or worse than before filtering, with only Lignosulfonate 825 slightly improved after filtering. Fluorescence. The characteristic van Hove singularities in the band structure, which depend on the SWCNT chirality, result in the sensitive dependence of fluorescence on the excitation wavelength, and enables the precise identification of the SWCNT chiralities.29 Furthermore, bundling suppresses fluorescence by quenching within a bundle, where metallic tubes present provide an efficient nonradiative pathway for photoexcited electrons.30,31 This makes fluorescence spectroscopy an important tool for the characterization of SWCNT suspensions. In order to attempt to use the fluorescence intensities of various SWCNT surfactant systems as a measure of suspension quality the measurement conditions have to be adequately adjusted, since the intensity of the SWCNT fluorescence is dependent on SWCNT concentration in suspension. Figure 5A demonstrates for a SWCNT-SDBS suspension that the fluorescence intensity markedly varies with concentration. Figure 5B illustrates this dependence for SWCNT-SDBS, SWCNT-(GT)15, SWCNTCBMC, and SWCNT-DMHdImCl suspension at the fluorescence intensity of (6,5) SWCNTs. The fluorescence intensity increases from low concentrations, to an intensity maximum at 0.01 mg/mL SWCNT, followed by a decrease toward higher concentrations. This behavior seems to be independent of the surfactant system. The decrease in the fluorescence intensity at higher concentration of SWCNTs is most likely due to the reabsorption effect reported by Rickard et al.32 This is in strong contrast to the linearly increasing absorption with concentration that follows the Lambert-Beer law. The nonlinearity of the (29) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley, R. E.; Weisman, R. B. Science 2002, 298, 2361-2366. (30) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297 (5581), 593-596. (31) Maeda, Y.; Kimura, S.-i.; Hirashima, Y.; Kanda, M.; Lian, Y.; Wakahara, T.; Akasaka, T.; Hasegawa, T.; Tokumoto, H.; Shimizu, T.; Kataura, H.; Miyauchi, Y.; Maruyama, S.; Kobayashi, K.; Nagase, S. J. Phys. Chem. B 2004, 108 (48), 18395-18397. (32) Rickard, D.; Giordani, S.; Blau, W. J. and Coleman J. N. J. Luminesc. 2007, 128 (1), 31-40.

PL excitation (6,5), nm

PL emission (6.5), nm

569 574 570 570 570 570 569 567 559 559 570 570 570 570 576 577 -

995 995 998 995 1000 1001 987 980 985 985 998 999 999 998 1007 1003 -

Stokes shift nm meV 4 4 5 4 -3 -2 -2 2 2 3 9 10 6 4 9 4 -

-5 -5 -6 -4 3 2 3 -3 -3 -4 -11 -13 -8 -4 -11 -5 -

fluorescence with varying concentration also means that the linear absorption should not be used to normalize fluorescence data taken at different SWCNT concentrations. For this reason fluorescence measurements were performed at 0.01 mg/mL SWCNT concentration to eliminate variations of fluorescence intensity as a function of concentration. The fraction of debundled SWCNTs is known to vary as the concentration of SWCNTs in the suspension varies.33-34 To determine if the suspension quality of the concentration series used for Figure 5 remains constant, absorption spectra were recorded. Upon normalizing at ∼891 nm, the absorption spectra show similar line shapes (Figure 5C), indicating similar SWCNT dispersion. To compare the fluorescence of various systems, excitationfluorescence maps and line scans at the main excitation wavelength were measured. Fluorescence was measured for the best ssDNA samples and all the other surfactants and biomolecules that remained suspended sufficiently long to allow fluorescence measurements (>3 days). The short-chain ssDNA-SWCNT samples show agglomeration ∼3 weeks after centrifugation, while longer chain ssDNA-SWCNTs were stable over a longer period. The (GT)15 shows the best stability of the ssDNA. The stability of the SWCNT suspensions is best (several month) for biomolecules of high molecular weight and for short surfactants (SDS, SDBS, SDOCO, DMHdIm-Cl). The peptide agglomerated soon after centrifugation (∼3 days). Since PL spectral acquisition takes >4 h, steady-state measurements were not possible for this system. The fluorescence maps show the emission for each tube chirality in the sample and their relative intensity. The data are plotted on a linear color scale. The excitation wavelength is a measure of the energy at the edge of the second lowest sub-band (E22); the emission energy corresponds to the energy of electronhole pairs at the edge of the lowest sub-band (E22).35 Several peaks are clearly visible, each corresponding to specific SWCNT chiralities. The fluorescence map of the SDOCO-SWCNT suspension (Figure 6 A) shows the chiralities present in the sample: (6,5), (7,5), (8,3), (8,4), and (7,6). (33) Giordani S.; Bergin S. D.; Nicolosi V.; Lebedkin S.; Kappes M. M.; Blau W. J.; Coleman J. N. J. Phys. Chem. B 2006, 110 (32), 15708-15718. (34) Cathcart H.; Quinn S.; Nicolosi V.; Kelly J. M.; Blau W. J; and Coleman J. N. J. Phys. Chem. C 2007, 111 (1), 66 -74. (35) Lefebvre, J.; Fraser, J. M.; Homma, Y.; Finnie, P. Appl. Phys A: Mater. 2004, 78, 1107-1110.

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Figure 5. (A) Fluorescence of SDBS-SWCNT suspensions in water as a function of SWCNT concentration. (B) Fluorescence intensity at ∼995 nm, (6,5) peak, for the surfactants (GT)15 (b), SDBS (2), CBMC (f), and DMHdIm-Cl (O). (C) Absorbance of SDBS-SWCNT suspensions as a function of concentration, normalized at ∼891 nm. The lowest concentrations are at the detection limit, causing increased noise.

The 3-D maps clearly show the relative fluorescence intensity, with SDOCO being the strongest. (GT)15 and SDBS show an overall reduced fluorescence. SDBS shows a peak intensity distribution similar to (GT)15 for the (6,5), (7,5), and (8,3), but at lower intensities. Additionally, the spectrum of SDBS shows the irregular “bundle” peak at 1150 nm, first described by Torrens et al.36 This special fluorescence peak has not been observed in any other system tested here. Table 1 lists the positions of the E11 absorption, E22 excitation, and E11 fluorescence wavelength. The comparison of E11 of the absorption and fluorescence yields the Stokes shift, which ranges from (-13 to +3) meV. The Stokes shifts are in the range of previous reports,30 and the relatively wide distribution demonstrates the important influence of the environment on SWCNT fluorescence. As already indicated in the fluorescence concentration plot and the 3-D maps, the different surfactant systems show different levels of maximum intensity. Fluorescence intensity measurements extracted from line-scans and maps across the (6,5) and (7,5) peaks of all systems are summarized in Figure 7. SDOCO has by far the highest intensity for the (6,5) (0.098 arbitrary unit) and (7,5) (0.076 arbitrary unit) SWCNT. The fluorescence of the (6,5) SWCNT in (GT)15 and SCO are comparable, with intensities of ∼0.045 arbitrary units. The (7,5) peaks of (GT)15 and SCO and the other ssDNA are in the same range, with intensities of ∼0.035 arbitrary units. This is well above the intensities of the systems comprising short surfactants (other than SCO and SDOCO) and the other macro-biomolecules. The SWCNTs suspended using imidazolium salts show poor fluorescence intensity. SDBS, SDS, CBMC, and chitosan have overall similar (36) Torrens, O. N.; Milkie, D. E.; Zheng, M.; Kikkawa, J. M. Nano Lett. 2006, 6 (12), 2864-2867.

intensities; the lignosulfonates’ intensities are somewhat reduced (which could be caused by absorption and quenching of lignosulfonate below 800 nm, even though the SWCNT were diluted in water for the florescence map). Figure 7 also indicates differences in preferred debundling between (6,5) and (7,5) SWCNT. SDOCO, SCO, (GT)15, and SDS strongly prefer (6,5) over (7,5). CBMC shows a preference for (7,5). Overall, SDOCO has intensities of ≈100% (≈0.040 au) higher than the other systems, SWCNT in suspension. The filtered samples show lower fluorescence than the centrifuged counter parts ((GT)15 and CBMC-250K), which correlates with the reduced peak ratio of the absorption spectra. Both the imidazolium salts, SDS, SDBS, SCO, and SDOCO are surfactant molecules that may enclose the SWCNT in micelles or semi-micelles.6,30,37 ssDNA wraps around SWCNT,11,37 which is also suggested to be the case for chitosan,18 CBMC,19 and lignosulfonate.39 Wrapping molecules also build more stable complexes with SWCNT than micelle forming surfactants. To probe the stability, and indirectly the complexing mechanism, SWCNT suspensions with CBMC-250K, lignosulfonate-825, and SDBS were diluted in water (diluted to 0.02 mg/mL surfactant molecule, or 0.06 mM SDBS, 0.0001 mM CBMC, 0.057 mM lignosulfonate-825) or corresponding surfactant or biomolecule solution (5 mg/mL or 14 mM SDBS) and the absorbance and fluorescence was measured. The critical micelle concentration of SDBS in water at room temperature is ∼2 mM.40 Dilution (37) Matarredona, O.; Rhoads, H.; Li, Z.; Harwell, J. H.; Balzano, L.; Resasco, D. E. J. Phys. Chem. B 2003, 107, 13357-13367. (38) Zheng, M.; Jagota, A.; Semke, E. D.; McLean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338-342. (39) Liu, Y.; Gao, L.; Sun, J. J Phys. Chem. C 2007, 111, 1223-1229. (40) Saiyad, A. H.; Bhat, S. G. T.; Rakshit, A. K. Colloid Polym. Sci. 1998, 276, 913-919.

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Figure 6. (A) 2-D fluorescence of SDOCO-SWCNT (B) combined 2-D/3-D map of SDOCO-SWCNT, (C) combined 2-D/3-D map of (GT)15-SWCNT, and (D) combined 2-D/3-D map of sodium dodecyl benzene sulfonate-SWCNT suspensions at 0.01 mg/mL concentration. All intensity scales are -0.003-0.1 arbitrary units.

fluorescence for dilutions in water or biomolecule solution (Figure 8A and B). This high stability indicates wrapping of the biomolecules around the SWCNT.

Discussion

Figure 7. Fluorescence intensity by chirality, left bar (6,5) and right bar (7,5). Obtained from fluorescence maps and line scans. The typical error in the efficiency is (7%, based on varying sensitivity of detectors.

below the critical value results in bundling of SWCNTs (Figure 8c) as SDBS is no longer able to support the micelles around SWCNT, resulting in shifted, wider absorption spectra and quenched fluorescence (Figure 8C). Such aggregation of SWCNTs well below the critical micellar concentration of surfactants has also been reported by McDonald et al.41 On the other hand, CBMC and lignosulfonate remain stable suspensions; this is supported by the similar line shape of absorption and

In this work a broad range of surfactants and biomolecules are used to prepare dispersions of SWCNTs. Since we propose to use absorption and fluorescence data to evaluate the effectiveness of these dispersion agents in producing concentrated suspensions of single nanotubes in water, an important question is, what is the effect of the change in the environment around SWCNTs (due to change in the type of surfactants) on the absolute intensity of the absorption peaks and the photoluminescence peaks? Unfortunately, due to the nature of this study, we cannot decouple the effect of the change in environment around SWCNTs, due to changing the dispersing agent, from the effect of debundling on the absorption and fluorescence spectra. Absorption may be a good tool to distinguish between suspensions with marked differences in roping and agglomeration, as several groups have asserted that well-resolved absorption spectra from SWCNTs can only be obtained if the SWCNT suspension contains a major fraction of debundled SWCNTs.28,29 It has been reported that the absorption peak intensity is less sensitive to changes in environment around SWCNTs (due to changes in the type of surfactant) as compared to the fluorescence peak intensity.42 Furthermore, the changes in absorption peak intensity due to (41) McDonald T. J.; Engtrakul C.; Jones M.; Rumbles G.; and Heben M. J. J. Phys. Chem. B 2006, 110, 25339-25346. (42) Gordana Dukovic G.; White B. E.; Zhou Z.; Wang F.; Jockusch S.; Steigerwald M. L.; Heinz T. F.; Friesner R. A.; Turro N. J.; Brus L. E. J. Am. Chem. Soc. 2004, 126 (46), 15269-15276.

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Figure 9. UV-vis NIR absorption peak ratios and fluorescence intensity of the various surfactant-SWCNT systems (lines are a visual aid only and do not imply a linear relationship).

Figure 8. Absorbance and fluorescence (573 nm excitation) spectra of (A) CBMC-250K-SWCNT, (B) lignosulfonate825-SWCNT, and (C) SDBS-SWCNT. Solid lines: dilution in 5 mg/mL biomolecule or surfactant solution; dotted lines: dilution in water to 0.02 mg/mL.

change in the environment around SWCNTs were found to be significantly less for small diameter SWCNTs, i.e. (6,5) and (7,5). The CoMoCat SWCNTs used here are indeed rich in (6,5) and (7,5) species. Hence, we believe that the change in the intensity of the absorption peak and the resolution of the peak is mainly because of the change in the fraction of debundled SWCNTs present in the suspension. As seen from Figures 3 and 4, (GT)15, (GT)10, and SDOCO show very well resolved absorption peaks and high absorption intensity peak ratios, indicating the presence of largely debundled SWCNTs. SDS- and imidazolium-based surfactants show broader absorption peaks and low absorption peak ratios, indicating the presence of a large fraction of bundled SWCNTs. The rest of the surfactants show intermediate absoption peak resolution and peak ratios. Photoluminescence intensity from a SWCNT dispersion is strongly reduced when the SWCNTs are bundled.29 However, PL intensity is more sensitive to changes in the environment

Figure 10. (A) Plot of Absorption peak ratio and efficiency, (B) plot of fluorescence intensity and efficiency. (3) SDOCO, (O) (GT)15 ([) DMHdImCl, (9) SDS, (*) SDBS, (g) CBMC-250K, (rightfacing triangle) Chitosan-200K, (2) Lignosulfonate 825 (lines are a visual aid only and do not imply a linear relationship).

around the SWCNTs as compared to absorption intensity. Hence, it cannot be completely ruled out that the change in the PLE intensity is due to the change in environment. We may not be able to use absolute intensity of PLE as the measure of debundling

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of SWCNTs using different surfactants, but the PLE peak intensity in general shows a similar trend (Figure 7) to that of the peaks ratios from UV-vis-NIR absorption spectra. Figure 9 compares the UV-vis-NIR absorption peak ratios with the fluorescence intensity showing that the two methods overall show a direct relationship for CBMCs, cholates, and lignosulfonates. This seems to indicate that the effect of debundling of SWCNTs may have a more significant effect on the PL intensity than the effect of change in environment due different types of surfactants. Furthermore, the lack of the bundle peak at 1150 nm in the (GT)15 and SDOCO PL maps (Figure 6) supports the assertion that these suspensions contain fewer bundles than the SDBS suspension. While it should be recognized that comparison between surfactant systems may be difficult for the reasons discussed above, both absorbance and PL characterization appear to be useful in evaluating the relative effectiveness of a series of samples within a given surfactant system. The plots for absorption peak ratio versus efficiency and PL intensity versus efficiency are shown separately in panels A and B of Figure 10, respectively. These data reveal that in some systems efficiency is directly related to debuldling, but in others it is not. For example, as shown by Figure 10A, the DNA, lignosulphonates, cholates, chitosans, and imidazoliums show an increase in the absorption intensity ratio with increasing efficiency, but the sulfonates and CBMC surfactants do not. Hence, there may not be a direct relationship between efficiency and debundling. Some surfactants such as SDS and imidazolium salts are effective in suspending bundles of SWCNTs in aqueous medium, which explains their high efficiency, but they are not effective in debundling the SWCNTs after suspending them. This is apparent from the poor resolution of the absorption peaks for SWCNTs dispersed with SDS and imidazolium salts. This means that the surfactants that can suspend agglomerates may, or may not, also be capable of debundling. As presented above, there are significant differences in efficiency and suspension quality for the various systems studied here. The complexing mechanism, micellar or wrapping, does not result in markedly better efficiency or quality. However, the chemical composition and molecule size of the surface molecule does affect the SWCNT suspension properties. SDOCO demonstrates an overall better suspension quality than the other surfactants evaluated. ssDNA show an overall good suspension quality, if the ssDNA is long enough (g20-mer) and consists of the specific bases (GT, AC, C, or T). Length is also important

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for the other biomolecules, as CBMC and chitosan are better with higher molecular mass. Alkyl chain length is important for small surfactant molecules as seen in the imidazolium salts. The lignosulfonates show that chemical composition can be more important than length, as the lignosulfonate with the higher sulfonation level (825) yield better efficiency and quality.

Conclusions In evaluating SWCNT suspensions one has to consider efficiency and quality. Characterization of dispersion efficiency is important in applications requiring SWCNT quantities larger than the analytical level. Quality, individual SWCNT versus bundles, is crucial to enable (1) separation of SWCNT by length43 and chirality, (2) preparation of SWCNT devices, (3) preparation of SWCNT polymer composites, (4) SWCNT environmental and health effects studies, and (5) SWCNT functionalization. Optical absorption spectroscopy is a quick and easy way to evaluate the surfactant efficiency and also gives an indication on the suspension quality. Fluorescence spectroscopy is also a sensitive method for assessing SWCNT bundling; however, effects such as local environment around SWCNT, reabsorption at high SWCNTs concentration may affect the overall PL intensity. When directly comparing the various complexation agents studied in this work, SDOCO, (GT)15, and CBMC-250K results produced good overall suspension quality. The information presented here provides a good base for further study of SWCNTs. Acknowledgment. We thank Barry Bauer (NIST) and Erik Hobbie (NIST) for useful discussions. J.W.G. thanks the Air ForceOfficeofScientificResearchforfunding(F1ATA06300J0001). This work was carried out by the National Institute of Standards and Technology (NIST), an agency of the US government and by statute is not subject to copyright in USA. The identification of any commercial product or trade name does not imply endorsement or recommendation by NIST. The policy of NIST is to use metric units of measurement in all its publications, and to provide statements of uncertainty for all original measurements. In this document, however, data from organizations outside NIST are shown, which may include measurements in non-metric units or measurements without uncertainty statements. LA703008R (43) Fagan, J. A.; Simpson, J. R.; Bauer, B. J.; De Paoli Lacerda, S. H.; Becker, M. L.; Chun, J.; Migler, K. B.; Hight Walker, A. R.; Hobbie, E. K. J. Am. Chem. Soc. 2007; 129 (34), 10607-10612.