Dispersion of single-walled carbon nanotubes modified with poly-l-tyrosine in water
© Kojima et al; licensee Springer. 2011
Received: 23 July 2010
Accepted: 10 February 2011
Published: 10 February 2011
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.
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 . 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 . 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 . Cousins et al. showed that N-(fluorenyl-9-methoxycarbonyl)/tyrosine/CNT (Fmoc/Tyr/CNT) complexes dispersed stably in water . 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 , 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(I in/I out)t/log10(I in/I out) t=0, where I in and I out 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.
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.
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.
atomic force microscope
single-walled carbon nanotubes
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.
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