SpringerOpen Newsletter

Receive periodic news and updates relating to SpringerOpen.

Open Access Nano Express

Dispersion of single-walled carbon nanotubes modified with poly-l-tyrosine in water

Mio Kojima1, Tomoka Chiba2, Junichiro Niishima3, Toshiaki Higashi4, Takahiro Fukuda1, Yoshikata Nakajima1, Shunji Kurosu4, Tatsuro Hanajiri14, Koji Ishii5, Toru Maekawa14* and Akira Inoue14*

Author Affiliations

1 Bio-Nano Electronics Research Centre, Toyo University 2100, Kujirai, Kawagoe, Saitama 350-8585, Japan

2 Faculty of Life Sciences, Toyo University 1-1-1 Izumino, Itakura-machi, Oura-gun, Gunma 374-0113, Japan

3 Graduate School of Life Sciences, Toyo University, 1-1-1 Izumino, Itakura-machi, Oura-gun, Gunma 374-0113, Japan

4 Graduate School of Interdisciplinary New Science, Toyo University, 2100, Kujirai, Kawagoe, Saitama 350-8585, Japan

5 Asylum Technology Co. Ltd., 3-20-12 Yushima, Bunkyo-ku, Tokyo 113-0034, Japan

For all author emails, please log on.

Nanoscale Research Letters 2011, 6:128  doi:10.1186/1556-276X-6-128

The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/6/1/128


Received:23 July 2010
Accepted:10 February 2011
Published:10 February 2011

© 2011 Kojima et al; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In this study, complexes composed of poly-l-tyrosine (pLT) and single-walled carbon nanotubes (SWCNTs) were produced and the dispersibility of the pLT/SWCNT complexes in water by measuring the ζ potential of the complexes and the turbidity of the solution were investigated. It is found that the absolute value of the ζ potential of the pLT/SWCNT complexes is as high as that of SWCNTs modified with double-stranded DNA (dsDNA) and that the complexes remain stably dispersed in the water at least for two weeks. Thermogravimetry analysis (TGA) and visualization of the surface structures of pLT/SWCNT complexes using an atomic force microscope (AFM) were also carried out.

Nano Express

Carbon nanotubes (CNTs) are promising functional nanomaterials, which may initiate new industries in the twenty-first century, thanks to their unique properties, such as extremely high thermal conductivity and mechanical strength, conducting or semiconducting characteristics depending on their chirality, and so on and, therefore, there is a variety of possible applications of CNTs to a wide range of areas including bio-related ones: e.g., the development of biosensing, bioelectrochemical and biomedical devices, and drug delivery systems [1]. However, one serious problem arises when CNTs are used in bio-related fields, that is, their poor dispersibility in water. There have been quite a few studies aiming at improving the dispersibility of CNTs in water by attaching foreign molecules, such as DNA [2-6], proteins [7-10], polymers [11-18], surfactants [19-22], and other compounds [23-26] to CNTs. Wang et al. used poly-l-lysine for the improvement of the dispersibility of CNTs and investigated the effects of the pH and temperature on the dispersion of poly-l-lysine/single-walled carbon nanotube (pLL/SWCNT) complexes in water [27]. The pLL/SWCNT complexes showed reversible changes in dispersibility against the pH of the water, that is, the complexes dispersed stably in the case of pH < 9, whereas they coagulated to form clusters or bundles in the case of pH > 9. Li et al. investigated the effect of the surface modification of oxidized multi-walled carbon nanotubes (MWCNTs) with pLL on the dispersibility of the complexes and showed that the pLL/MWCNT complexes remained stably dispersed in water for at least 10 days [28]. Cousins et al. showed that N-(fluorenyl-9-methoxycarbonyl)/tyrosine/CNT (Fmoc/Tyr/CNT) complexes dispersed stably in water [29]. In this article, focus is laid on the creation of poly-l-tyrosine (pLT)/SWCNT complexes, supposing that pLT can be adsorbed onto SWCNTs via the interactions among six-membered rings [29], and the dispersibility of the complexes in water is investigated, which has not so far been carried out. The surfaces of SWCNTs with pLT have been modified and the dispersibility of the complexes by measuring the ζ potential of pLT/SWCNT complexes and the turbidity of the solution were evaluated. Thermogravimetry analysis (TGA) and visualization of the surface structures of pLT/SWCNT complexes using an atomic force microscope (AFM) were then carried out.

First of all, the authors produced pLT/SWCNT complexes. The SWCNTs were brayed, the average diameter and length of which were, respectively, 2 nm and 10 μm (Shenzhen Nanotech Port Co., Ltd., Baoan, Shenzhen, China), in an agate mortar and then dispersed in distilled water (DW). The mass concentration of SWCNTs was 1.0 mg ml-1. The SWCNTs solution was sonicated using an ultrasonic cleaner (W-113 Ultrasonic multi cleaner, Honda Electronics Co., Ltd., Aichi, Japan) at 4°C for 8 h. The pLT (MP Biomedicals, LLC, Solon, OH, USA), the molecular weight of which varied from 12,000 to 35,000, was dissolved in 0.02 M KOH aqueous solution. The mass concentration of pLT was 1.0 mg ml-1. The SWCNTs and pLT solutions were mixed together at 30°C by shaking them at a frequency of 20 Hz by a shaker (Bio-Shaker BR-40LF, Taitec Co., Ltd., Aichi, Japan) for 1 h. After incubation, the pLT and SWCNT solution mixture was centrifuged at 1.5 × 104 rpm at 4°C for 30 min using a micro-centrifuge (MX-301, Tomy Seiko Co., Ltd., Tokyo, Japan), and then the supernatant was removed. The same volume of distilled water as that of the removed supernatant was added to the pLT/SWCNT solution, and this procedure was repeated three times so that any pLT, which had not been adsorbed earlier onto SWCNTs, was removed. The ζ potential of pLT/SWCNT complexes was measured using the dynamic scattering method (Zetasizer nano-zs, Malvern Instruments Ltd., Worcestershire, UK). The turbidity of the pLT/SWCNT complex solution was also measured using a spectral photometer (U-2001 Spectrophotometer, Hitachi High-Technologies Corp., Tokyo, Japan). Note that the normalized turbidity is defined as follows: log10(Iin/Iout)t/log10(Iin/Iout)t=0, where Iin and Iout are, respectively, the powers of the incident and transmitted laser beams of 600-nm wavelength, and the turbidity at time t is normalized by that at the initial time t = 0. To compare the ζ potential and turbidity of the present pLT/SWCNT complexes with those of other materials, the following solutions were also prepared: (1) SWCNTs dispersed in DW (0.1 mg ml-1), (2) SWCNTs dispersed in 0.1% Polyoxyethylene(10)Octylphenyl Ether (TritonX-100) (Wako Pure Chemical Industries, Ltd., Osaka, Japan) solution, and (3) double-stranded DNA (dsDNA)/SWCNT complexes dispersed in DW. See Appendix for the preparation of SWCNTs dispersed in TritonX-100 solution and the production of the dsDNA/SWCNT complexes. The thermogravimetric analysis (TGA) (DTG-60H, Shimadzu Corp., Tokyo, Japan) at a heating rate of 5 K min-1 from 20°C up to 800°C in nitrogen was carried out. The surface structures of pLT and pLT/SWCNT complexes using an atomic force microscope (AFM) were also visualized (MFP-3D, Asylum Research Co., Santa Barbara, CA, USA) through an ac non-contact mode by putting a drop each of pLT and pLT/SWCNT solutions onto a silicon substrate and then evaporating them naturally.

Figure 1a shows the sedimentation of SWCNTs in DW and the dispersion of pLT/SWCNT complexes in DW. Note that the mass concentrations of SWCNTs were 0.1 mg ml-1 in both cases. The pLT/SWCNT complexes remained stably dispersed in DW for at least 14 days at 25°C, whereas the SWCNTs without any surface modification gradually coagulated to each other, and finally, were completely sedimented in DW (see also Figure 1c). Figure 1b shows the ζ potential of each material: i.e. (a) SWCNTs in DW, (b) SWCNTs in TritonX-100 solution, (c) dsDNA/SWCNT complexes in DW, and (d) pLT/SWCNT complexes in DW. Note that the data shown in Figure 1b were taken 14 days after each material had been dispersed in DW. The absolute value of the ζ potential of pLT/SWCNT complexes in DW was higher than that of SWCNTs in TritonX-100 solution and was slightly lower than that of dsDNA/SWCNT complexes in DW. It is known that the particles disperse stably in water when the absolute value of the ζ potential of each particle is greater than 20 mV [30,31], with which the present results coincide: that is, for the pLT/SWCNT and dsDNA/SWCNT complexes, the absolute values of the ζ potentials were, respectively, 42.3 and 46.2 mV, and the complexes remained stably dispersed in DW for al least 14 days: however, SWCNTs in DW, the absolute value of the ζ potentials of which was 13.0 mV, finally got sedimented (see also Figure 1a). SWCNTs in TritonX-100 solution, the absolute value of the ζ potential of which was 30.0 mV, also got dispersed stably. The time variation of the turbidity of each solution is shown in Figure 1c. The turbidity of the pLT/SWCNT in DW, which was slightly lower than that of the dsDNA/SWCNT in DW, but higher than that of the SWCNTs in TritonX-100 solution, was almost constant for 14 days, whereas the turbidity of the SWCNTs in DW immediately decreased due to quick coagulations and sedimentations of SWCNTs in DW. There is a clear correlation between the turbidity of each solution and the ζ potential of each material in the solution: that is, the higher the absolute value of the ζ potential of a material is, the higher the turbidity of the solution becomes (see Figure 1b, c). The TGA data obtained for pLT, SWCNTs, and pLT/SWCNTs complexes are shown in Figure 2. Both pLT and pLT/SWCNT complexes started decomposing at 300°C, whereas SWCNTs did not decompose at least up to 800°C. Note that the decomposition temperature of pLT obtained by the present TGA analysis, that is, 300°C, coincides with that measured previously [32]. pLT was definitely adsorbed onto SWCNTs judging by the TGA data of pLT/SWCNT complexes. The quantum calculations of the interactions between a single tyrosine molecule and a [6,6] SWCNT were carried out by the PM3 method (Gaussian03, Gaussian Co., Pittsburgh, PA, USA). As shown in Figure 3, a tyrosine molecule was adsorbed onto a SWCNT via the interactions between the six-membered rings as in the case of Fmoc/Tyr/SWCNT complexes [29]. The gap between the six-membered rings was approximately 0.45 nm, which is quite similar to that between graphitic layers [33]. It is supposed that the decomposition temperature of the pLT adsorbed onto SWCNTs was almost the same as that of pLT, that is, 300°C (see Figure 2), since the interactions between the rings are not very strong [29]. Judging by the weight loss obtained by the TGA analysis (Figure 2), 0.2 μg of pLT was adsorbed onto 1 μg of SWCNTs on average. AFM images of pLT and pLT/SWCNT complexes are shown in Figure 4. pLT without any immobilizations onto SWCNTs folded by itself to form sphere-like structures (see Figure 4a). The whole surfaces of the SWCNTs were covered with pLT, and the thickness of the pLT layers adsorbed onto SWCNTs varied from 1 to 4 nm (Figure 4b, c). It is supposed that pLT was adsorbed onto SWCNTs via the interactions between six-membered rings as mentioned above, and pLT folded to form sphere-like structures on the surfaces of SWCNTs, the thickness of which varied cyclically in the axial direction of the SWCNTs (Figure 4c). A TEM image of a pLT/SWCNT complex is also shown in the Additional file 1, where the mass concentration of pLT in DW was set at 0.2 mg ml-1 to obtain a clearer image. The pLT/SWCNT complexes dispersed stably in water thanks to the polar -OH group in tyrosine. In the case of tryptophan/SWCNT complexes, on the other hand, they did not disperse in water due to the hydrophobic group in tryptophan although it was adsorbed onto SWCNTs via the interactions among six-membered rings. Biomolecules such as enzymes can be attached to pLT, and therefore enzyme/pLT/SWCNT complexes can be produced so that new biosensors and devices may be developed in combination with SWCNT electronics [34]. The authors will be carrying out spectroscopic analyses such as Raman and Infrared spectroscopies of pLT/SWCNT complexes so that the structures of and conformational changes in pLT immobilised on SWCNTs may be clearly understood. The authors will also be investigating the adsorption of various biomolecules, viruses, and bacteria onto pLT/SWCNT complexes so that the complexes may be used as adsorbers, filters, or screening devices for organic molecules, viruses, and bacteria. The authors will also be measuring the electric and electronic properties of the complexes so that the above mentioned biosensors may be developed.

thumbnailFigure 1. Dispersion of SWCNTs in distilled water. (a) Dispersion of SWCNTs in distilled water 14 days after the preparation. Left: SWCNTs without any surface modification. SWCNTs coagulated to each other and finally sedimented. Right: SWCNTs modified with pLT. pLT/SWCNT complexes remained stably dispersed for at least 14 days. (b) ζ potentials of SWCNTs in DW, SWCNTs in TritonX-100 solution, dsDNA/SWCNT complexes in DW, and pLT/SWCNT complexes in DW. The ζ potentials were measured 14 days after the preparation. The ζ potential of pLT/SWCNT complexes in DW is slightly lower than that of dsDNA/SWCNT complexes in DW, but higher than that of SWCNTs in TritonX-100 solution. (c) Time variations of the turbidity of SWCNTs in DW, SWCNTs in TritonX-100 solution, dsDNA/SWCNT complexes in DW, and pLT/SWCNT complexes in DW. The turbidity of SWCNTs in TritonX-100 solution, dsDNA/SWCNT complexes in DW, and pLT/SWCNT complexes in DW hardly changed for 14 days, whereas SWCNTs without any surface modification sedimented quickly. The turbidity of pLT/SWCNT complexes in DW was slightly lower than that of dsDNA/SWCNT complexes in DW, but higher than that of SWCNTs in TritonX-100 solution.

thumbnailFigure 2. TGA curves of SWCNTs, pLT, and pLT/SWCNT complexes. pLT by itself and pLT adsorbed onto SWCNTs decomposed at 300°C, which suggests that pLT is adsorbed rather weakly onto the surfaces of the SWCNTs.

thumbnailFigure 3. Interaction between a single tyrosine molecule and a [6,6]SWCNT calculated by the PM3 method. Tyrosine can be adsorbed onto the surface of SWCNT via the interactions among six-membered rings.

thumbnailFigure 4. AFM images of pLT and pLT/SWCNT complexes. (a) AFM image of pLT and the height distribution along line A-B. PLT folded to form sphere-like structures on the surface of an Si substrate. (b) AFM image of pLT/SWCNT complexes and the height distribution along line C-D. (c) AFM image of a pLT/SWCNT complex and the height distribution along line E-F. The thicknesses of pLT adsorbed onto the SWCNT varied cyclically in the axial direction of the SWCNT.

Additional file 1. TEM image of a pLT/SWCNT complex. The mass concentration of pLT was set at 0.2 mg ml-1 in DW to obtain a clearer image.

Format: DOC Size: 1.3MB Download file

This file can be viewed with: Microsoft Word ViewerOpen Data

In summary, pLT/SWCNT complexes were produced and it was found that the complexes dispersed stably in water, which coincided with the results of the measurement of the ζ potential of the complexes in DW and the turbidity of the pLT/SWCNT aqueous solution. The dispersibility of pLT/SWCNT complexes was as high as that of dsDNA/SWCNT complexes. pLT was adsorbed onto the SWCNTs via rather weak interactions among six-membered rings according to the TGA data, quantum calculations, and AFM images. AFM images showed that the surfaces of the SWCNTs were completely covered with pLT, and the thickness of the pLT on the SWCNTs varied cyclically in the axial direction. The result of this study suggests that any polypeptide, in which some aromatic amino acids are included, can be adsorbed onto SWCNTs via the interactions among six-membered rings and that the dispersibility and other physical and chemical properties of polypeptide/SWCNT complexes can be altered by choosing some appropriate amino acid sequence depending on the users' purposes.

Abbreviations

AFM: atomic force microscope; dsDNA: double-stranded DNA; DW: distilled water; pLT: poly-l-tyrosine; SWCNTs: single-walled carbon nanotubes; TGA: thermogravimetry analysis.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

MK designed the study and carried out the experiment. TC participated in the dispersion experiment. JN participated in the dispersion experiment. TH performed AFM observation. TF carried out the TGA experiment and TEM observation. YN participated in the AFM observation. SK carried out the quantum calculation. TH participated in the design of the study. KI participated in the design of the study and AFM observation. TM participated in the design of the study, coordinated the study and wrote the manuscript. AI participated in the design of the study and coordinated the study. All authors read and approved the final manuscript.

Appendix

Preparation of SWCNTs dispersed in TritonX-100 solution

0.1% TritonX-100 and 0.02 M KOH were mixed with 0.15 mg ml-1 SWCNTs by a mixer (VORTEX-GENIE2, model G-560, Scientific Industries Inc., Bohemia, New York, USA) at 30°C for 1 h.

Production of dsDNA/SWCNT complexes

0.15 mg ml-1 of SWCNTs and 0.5 mg ml-1 of dsDNA were both dispersed in water. The above two solutions were mixed together by shaking them at 30°C at a frequency of 20 Hz for 1 h. After incubation, the dsDNA and SWCNT mixtures were centrifuged at 1.5 × 104 rpm at 4°C for 30 min, the supernatant was taken away, and DW was added to remove unadsorbed dsDNA molecules. The above washing procedure was repeated five times.

Acknowledgements

The authors would like to thank the Ministry of Education, Culture, Sports, Science and Technology, Japan, for supporting this study with a Grant for the High-Tech Research Centres since 2003.

References

  1. Yang W, Thordarson P, Gooding JJ, Ringer SP, Braet F: Carbon nanotubes for biological and biomedical applications.

    Nanotechnology 2007, 18:412001. Publisher Full Text OpenURL

  2. Zheng M, Jagota A, Semke ED, Diner BA, Mclean RS, Lustig SR, Richardson RE, Tassi NG: DNA-assisted dispersion and separation of carbon nanotubes.

    Nat Mater 2003, 2:338. PubMed Abstract | Publisher Full Text OpenURL

  3. Nakashima N, Okuzono S, Murakami H, Nakai T, Yoshikawa K: DNA dissolves single-walled carbon nanotubes in water.

    Chem Lett 2003, 32:456. Publisher Full Text OpenURL

  4. Chen Y, Liu H, Ye T, Kim J, Mao C: DNA-directed assembly of single-wall carbon nanotubes.

    J Am Chem Soc 2007, 129:8696. PubMed Abstract | Publisher Full Text OpenURL

  5. Noguchi Y, Fujigaya T, Niidome Y, Nakashima N: Single-walled carbon nanotubes/DNA hybrids in water are highly stable.

    Chem Phys Lett 2008, 455:249. Publisher Full Text OpenURL

  6. Jeng ES, Barone PW, Nelson JD, Strano MS: Hybridization kinetics and thermodynamics of DNA adsorbed to individually dispersed single-walled carbon nanotubes.

    Small 2007, 3:1602. PubMed Abstract | Publisher Full Text OpenURL

  7. Asuri P, Karajanagi SS, Sellitto E, Kim SY, Kane RS, Dordick JS: Water-soluble carbon nanotube-enzyme conjugates as functional biocatalytic formulations.

    Biotechnol Bioeng 2006, 95:804. PubMed Abstract | Publisher Full Text OpenURL

  8. Matsuura K, Saito T, Okazaki T, Ohshima S, Yumura M, Iijima S: Selectivity of water-soluble proteins in single-walled carbon nanotube dispersions.

    Chem Phys Lett 2006, 429:497. Publisher Full Text OpenURL

  9. Nepal D, Geckeler KE: pH-sensitive dispersion and debundling of single-walled carbon nanotubes.

    Small 2006, 2:406. PubMed Abstract | Publisher Full Text OpenURL

  10. Tsai TW, Heckert G, Neves LF, Tan Y, Kao DY, Harrison RG, Resasco DE, Schmidtke DW: Adsorption of glucose oxidase onto single-walled carbon nanotubes and its application in layer-by-layer biosensors.

    Anal Chem 2009, 81:7917. PubMed Abstract | Publisher Full Text OpenURL

  11. Kitano H, Tachimoto K, Anraku Y: Functionalization of single-walled carbon nanotube by the covalent modification with polymer chains.

    J Colloid Interface Sci 2007, 306:28. PubMed Abstract | Publisher Full Text OpenURL

  12. Rice NA, Soper K, Zhou N, Merschrod E, Zhao Y: Dispersing as-prepared single-walled carbon nanotube powders with linear conjugated polymers.

    Chem Commun (Camb) 2006, 21:4937. Publisher Full Text OpenURL

  13. Sinani VA, Gheith MK, Yaroslavov AA, Rakhnyanskaya AA, Sun K, Mamedov AA, Wicksted JP, Kotov NA: Aqueous dispersions of single-wall and multiwall carbon nanotubes with designed amphiphilic polycations.

    J Am Chem Soc 2006, 127:3463. Publisher Full Text OpenURL

  14. Lillehei PT, Kim JW, Gibbons LJ, Park C: A quantitative assessment of carbon nanotube dispersion in polymer matrices.

    Nanotechnology 2009, 20:325708. PubMed Abstract | Publisher Full Text OpenURL

  15. Kang MS, Shin MK, Ismail YA, Shin SR, Kim SI, Kim H, Lee H, Kim SJ: The fabrication of polyaniline/single-walled carbon nanotube fibers containing a highly-oriented filler.

    Nanotechnology 2009, 20:85701. Publisher Full Text OpenURL

  16. Rouse JH: Polymer-assisted dispersion of single-walled carbon nanotubes in alcohols and applicability toward carbon nanotubes/sol-gel composite formation.

    Langmuir 2005, 21:1055. PubMed Abstract | Publisher Full Text OpenURL

  17. Alpatova AL, Shan W, Babica P, Upham BL, Rogensues AR, Masten SJ, Drown E, Mohanty AK, Alocilja EC, Tarabara VV: Single-walled carbon nanotubes dispersed in aqueous media via non-covalent functionalization: effect of dispersant on the stability, cytotoxicity, and epigenetic toxicity of nanotube suspensions.

    Water Res 2010, 44:505. PubMed Abstract | Publisher Full Text OpenURL

  18. Singh RK, Kumar J, Kumar A, Kumar V, Kant R, Singh R: Poly(3-hexylthiophene): Functionalized single-walled carbon nanotubes:(6,6)-phenyl-C61-butyric acid methyl ester composites for photovoltaic cell at ambient condition.

    Solar Energy Mater Solar Cells 2010, 94:2386. Publisher Full Text OpenURL

  19. Wallace EJ, Sansom MSP: Carbon nanotube self-assembly with lipids and detergent: a molecular dynamics study.

    Nanotechnology 2009, 20:45101. Publisher Full Text OpenURL

  20. O'Connell MJ, Bachilo SM, Huffman CB, Moore VC, Strano MS, Haroz EH, Rialon KL, Boul PJ, Noon WH, Kittrell C, Ma J, Hauge RH, Weisman RB, Smalley RE: Band gap fluorescence from individual single-walled carbon nanotubes.

    Science 2002, 297:593. PubMed Abstract | Publisher Full Text OpenURL

  21. Chen RJ, Bangsaruntip S, Dr1uvalakis KA, Kam NWS, Shim M, Li Y, Kim W, Utz PJ, Dai H: Noncovalent functionalization of carbon nanotubes for highly specific electronic biosensors.

    Proc Natl Acad Sci USA 2003, 100:4984. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  22. Shen K, Curran S, Xu H, Rogelj S, Jiang Y, Dewald J, Pietrass T: Single-walled carbon nanotube purification, pelletization, and surfactant-assisted dispersion: a combined TEM and resonant micro-raman spectroscopy study.

    J Phys Chem B 2005, 109:4455. PubMed Abstract | Publisher Full Text OpenURL

  23. Maeda Y, Kanda M, Hashimoto M, Hasegawa T, Kimura S, Lian Y, Wakahara T, Akasaka T, Kazaoui S, Minami N, Okazaki T, Hayamizu Y, Hata K, Lu J, Nagase S: Dispersion and separation of small-diameter single-walled carbon nanotubes.

    J Am Chem Soc 2006, 128:12239. PubMed Abstract | Publisher Full Text OpenURL

  24. Dumonteil S, Demortier A, Detriche S, Raes C, Fonseca A, Rühle M, Nagy JB: Dispersion of carbon nanotubes using organic solvents.

    J Nanosci Nanotechnol 2006, 6:1315. PubMed Abstract | Publisher Full Text OpenURL

  25. Yang H, Wang SC, Mercier P, Akins SL: Diameter-selective dispersion of single-walled carbon nanotubes using a water-soluble, biocompatible polymer.

    Chem Commun (Camb) 2006, 7:1425. Publisher Full Text OpenURL

  26. Gao C, He H, Zhou L, Zheng X, Zhang Y: Scalable Functional Group Engineering of Carbon Nanotubes by Improved One-Step Nitrene Chemistry.

    Chem Mater 2009, 21:360. Publisher Full Text OpenURL

  27. Wang D, Chen L: Temperature and pH-responsive single-walled carbon nanotube dispersions.

    Nano Lett 2007, 7:1480. PubMed Abstract | Publisher Full Text OpenURL

  28. Li J, He WD, Yang LP, Sun XL, Hua Q: Preparation of multi-walled carbon nanotubes grafted with synthetic poly(L-lysine) through surface-initiated ring-opening polymerization.

    Polymer 2007, 48:4352. Publisher Full Text OpenURL

  29. Cousins BG, Das AK, Sharma R, Li Y, McNamara JP, Hillier IH, Kinloch IA, Ulijn RV: Enzyme-activated surfactants for dispersion of carbon nanotubes.

    Small 2009, 5:587. PubMed Abstract | Publisher Full Text OpenURL

  30. Riddick TM: Zeta-Meter Manual. New York: Zeta-Meter Inc; 1968.

  31. Freitas C, Müller RH: Effect of light and temperature on zeta potential and physical stability in solid lipid nanoparticle (SLN™) dispersions.

    Int J Pharm 1998, 168:221. Publisher Full Text OpenURL

  32. Port WS, Burrell BW: Preparation and Study of Systematic Protein-Like Materials for High Performance Adhesive Systems. AVSD-0288-70-RR. Huntsville: NASA; 1970.

  33. Ruoff RS, Lorents DC, Chan B, Malhotra R, Subramoney S: Single crystal metals encapsulated in carbon nanoparticles.

    Science 1993, 259:346. PubMed Abstract | Publisher Full Text OpenURL

  34. Hwang JS, Kim HT, Son MH, Oh JH, Hwang SW, Ahn D: Electronic transport properties of a single-wall carbon nanotube field effect transistor with deoxyribonucleic acid conjugation.

    Physica E 2008, 40:1115. Publisher Full Text OpenURL