J. Phys. Chem. C 2009, 113, 6033–6036
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Probing Interaction between ssDNA and Carbon Nanotubes by Raman Scattering and Electron Microscopy Masayuki Shoda,† Shunji Bandow,*,† Yusei Maruyama,‡ and Sumio Iijima† Department of Materials Science and Engineering, Nanofactory, Meijo UniVersity, 1-501 Shiogamaguchi, Tenpaku, Nagoya 468-8502, Japan, and Research Center for Micro/Nano Technology, Hosei UniVersity, Midorichou, Koganei, Tokyo 184-0003, Japan ReceiVed: December 12, 2008; ReVised Manuscript ReceiVed: January 27, 2009
We report experimental results of Raman scattering showing that single-stranded DNA (ssDNA) effectively modifies the electronic structure of metallic carbon nanotubes with the diameter of 1.5-1.6 nm but not for the same diameter class of semiconducting tubes. Experimental fact of drastic decrease on the G-band Raman intensity associated with only the metallic tubes, which give asymmetric BWF line shape, can be seen when the nanotubes were soaked in aqueous solution of ssDNA. The G-band signal from semiconducting tubes indicates neither change of intensity nor of peak position. To explain these results, we introduce possible energy scheme for metallic tubes that the doping band of ssDNA locates near below the Fermi level, but it is located just in between the first van Hove energy gap of semiconducting tubes. Transmission electron microscopy shows flappinglike motion of ssDNA in the nanotubes. Such motion of ssDNA is probably triggered by a Coulomb interaction between the electron beam and ssDNA, and this motion can be suppressed when ssDNA@nanotube is wrapped in cationic surfactant. 1. Introduction Deoxyribonucleic acid (DNA) acts as the surfactant for dispersing the carbon nanotubes in aqueous solvent.1 The mechanism of solubilization is an important subject to clarify, and the theoretical studies based on both the molecular dynamic simulation and tight binding calculation demonstrate how to interact DNA with nanotube2,3 and whether the electron charge transfer between DNA and nanotube occurs.4 Experimentally, the time dependence on the wrapping of single wall carbon nanotubes (SWNTs) in DNA over three months was carefully studied by using photoluminescence (PL) and transmission electron microscopy (TEM), which indicate, respectively, PL switch-on phenomenon after about 1 month of dispersion and helically wrapped DNA images around SWNTs.5 Atomic force microscopy study for DNA molecules around multiwall carbon nanotubes also revealed wrapping phenomenon and indicated that a close wrapping to the tube wall was able to see for the tubes with the diameter smaller than ca. 4.5 nm.6 Furthermore, the electrochemical insertion of single-stranded DNA (ssDNA) molecule into interior of nanotube was confirmed by TEM observation.7 According to the electrochemical results, simultaneous irradiation of radio frequency and static electric fields is important for inserting ssDNA into nanotubes, and the insertion of ssDNA is mainly taken place on the anode electrode where the open-ended nanotubes are coated. This may be caused by a Coulomb interaction because of negatively charged phosphoric acid group which constitutes a backbone of DNA. As noted above, nanotubes interactwell with DNA molecules, and the hybridization on the electronic feature is worth using in the nanoscale electronics application. In the present study, we show a modification of the electronic states of metallic * To whom correspondence should be addressed. E-mail: bandow@ ccmfs.meijo-u.ac.jp. † Meijo University. ‡ Hosei University.
nanotubes due to interaction with ssDNA via the Raman scattering experiments, and high-resolution TEM imaging clears up that the motion of ssDNA in the nanotube is controllable only by using surfactants. 2. Experimental Section Purified SWNTs and ssDNA from calf thymus were purchased, respectively, from a Meijo-NanoCarbon and a SigmaAldrich (product no. D8899, length of ∼50 kb, and purity of ∼65% and up). Prior to use, SWNTs, mat-like formed aspurchased purified SWNTs, were heat treated in dry air at 460 °C for 1 h (HT460NT) in order to open the windows8 for inserting ssDNA in the interior of nanotube. Next such heattreated SWNT-mats were directly put into aqueous solution of ssDNA, and the solution was well sonicated by using conventional ultrasonic bath for 30 min. Then the solution was left at room temperature for 1 day, and the supernatant was just sucked upwithoutcentrifugation.ThissampleisnotedasssDNA@HT460NT. Here we did not control the pH of ssDNA solution, so that the pH is a neutral. In addition, the SWNTs do not dissolve individually in the solution but they form mostly the bundles. The sample for the Raman scattering was prepared by drying the colloidal suspension on the quartz plate and surplus ssDNA was removed by soaking the sample in distilled water. For a control experiment, HT460NT sample was soaked in distilled water and dried up (HT460NT-WT) in order to see the effect of adsorbed water. Similar method was used to prepare the samples for TEM. Briefly the colloidal suspension was dropped onto the electron microgrid and surplus ssDNA adhering outside the SWNTs was rinsed out several times by soaking the electron microgrid in distilled water. In addition, other two samples for TEM observation were prepared by soaking ssDNA@HT460NT on the electron microgrid in 0.01 wt % aqueous solutions of anionic (sodium dodecyl benzene sulfonate, SDBS) and cationic (benzalkonium chloride, BKC) surfactants for only a few seconds. Then the electron microgrid was taken out and dried.
10.1021/jp8109572 CCC: $40.75 2009 American Chemical Society Published on Web 03/25/2009
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TABLE 1: Detected Signal Strengths of Resonant Raman Scattering from the Nanotubes with Different Diameters S2 gap (semiconducting tubes)
M1 gap (metallic tubes)
laser wavelength (energy)
1.3-1.4 nm in dia. 1.2-1.4 eV
1.5-1.6 nm in dia. 1.0-1.2 eV
1.5-1.6 nm in dia. 1.5-1.8 eV
785 nm(1.58eV) 1064nm(1.17eV)
weak weak
faint intense
intense faint
Raman spectra were recorded by using a Bruker RFS100 Fourier transform spectrometer equipped with 785- and 1064nm lasers. TEM images were recorded by a high-speed CCD camera (a Soft Imaging System VELETA-S) mounted on a JEOL 2010F transmission electron microscope operated at 120 kV. 3. Results and Discussion 3.1. Raman Scattering. As shown in parts a and b of Figure 1, two large peaks associated with radial breathing mode (RBM) Raman scattering were detected for both laser excitations at 785 and 1064 nm with corresponding tube diameters of ca. 1.5-1.6 and 1.3-1.4 nm. For a 785-nm excitation, a low wavenumber peak is assignable to originate in metallic tubes, and a high wavenumber one is semiconducting tubes, while for a 1064 nm excitation both two RBM peaks are associated with semiconducting tubes from the following reason based on the resonant conditions. Energy gaps9 between first van Hove singularities of metallic tubes (M1) with diameters of 1.5-1.6 nm are about 1.5-1.8 eV, and those between the second ones of semiconducting tubes (S2) are about 1.0-1.2 eV. Moreover S2 gaps of 1.3-1.4 nm diameter tubes lie about 1.2-1.4 eV. Therefore, the laser excitation at 785 nm (1.58 eV) effectively picks up the metallic tubes of 1.5-1.6 nm diameters and may detect the Raman signal associated with the semiconducting tubes of 1.3-1.4 nm diameters. On the other hand, the metallic tubes with 1.5-1.6 nm diameters cannot be picked up by a laser excitation at 1064 nm (1.17 eV), which intensely picks up the semiconducting tubes of 1.5-1.6 nm diameters and weakly the semiconducting tubes of 1.3-1.4 nm diameters. We summarize above considerations in Table 1. Tangential mode Raman spectra (G band) are in parts a and b of Figure 2, which were recorded by the laser excitations at 785 and 1064 nm, respectively. One can easily find broadened spectral features in Figure 2a as compared with those in Figure 2b. As noted above, M1 gaps of 1.5-1.6 nm metallic tubes just match up with the laser excitation at 785 nm but not with the excitation at 1064 nm. With regard to the G band Raman signal from the metallic tubes, the line broadening occurs due to phonon scattering associated with the conduction electrons.10 This is known as the Breit-Wigner-Fano (BWF) resonance, and the BWF line shape is to be asymmetric. Since it is not necessary to consider the phonon scattering associated with the conduction electrons for the semiconducting tubes, symmetric line shape can be adopted to the spectral feature. Therefore, spectral curve fittings of Figure 2a were carried out by using three Lorentzian components (thin dotted lines) and a BWF component (thick dotted line). The former components are associated with the semiconducting tubes of 1.3-1.4 nm diameters and the latter asymmetric component is from the metallic tubes of 1.5-1.6 nm diameters. As seen in Figure 2a, noticeable spectral change cannot be found for the samples of HT460NT and HT460NT-WT. This means that the soaking of nanotubes in water does not give significant effect on the G-band Raman feature. Beside HT460NT-WT, the ssDNA@HT460NT sample gives crucial change in the G-band feature, which is
represented by a disappearance of BWF component as shown in the bottom spectrum of Figure 2a, while the Lorentzian components do not indicate distinct change. These experimental facts suggest that the ssDNA aggressively interacts with the metallic tubes but not so much with semiconducting tubes. Similar phenomena were observed in the Raman spectra recorded at 1064 nm laser excitation. That is, no recognizable spectral change can be seen in Figure 2b, whose spectra are all associated with the semiconducting tubes. From these results, it can be concluded that the ssDNA selectively interacts with the metallic tubes and modifies the electronic structure. Possible energy scheme is that the doping level of ssDNA locates near the Fermi level of metallic tube and acts as either electron acceptor or electron donor. It can be expected that the electron charge transfer from the π-band of metallic tube to the doping level of ssDNA (electron acceptor) would occur and downshifts the Fermi level, or the electron charge transfer from the doped ssDNA (electron donor) to π*band of metallic tube would be taken place that upshifts the Fermi level. On the other hand, for the semiconducting tubes, the doping level would locate just in between the S1 gap (0.5-0.7 eV for 1.6-1.3 nm diameter tubes). In such energy scheme, the doping band of ssDNA does not overlap with the π or π* bands of semiconducting nanotubes, and hence the electron charge transfer may not be taken place. Schematics of the electronic density of states (DOS) for the nanotubes with a doping level of ssDNA as an electron acceptor are in parts a and b of Figure 3, which are, respectively, drawn for metallic and semiconducting tubes. Theoretical argument for the DOS modification of nanotubes wrapped in DNA also predicts that the doping band of DNA would locate near below the Fermi level,4 and this work indicates that, in some specific cases, such as for (8,2) and (7,4) metallic tubes, electron charge transfer as large as 0.2-0.4e (e is the electron charge) between DNA and nanotube is taken place, but in some other cases it is no more than 0.05e. These theoretical predictions indeed match up with the present results from the Raman scattering. As described above, laser excitation at 785 nm selectively picked up the change of electronic structure of metallic tubes and at 1064 nm cleared up that the semiconducting tubes did not interact so much with ssDNA. Since the double-resonant 2D-band (or D′-band) Raman scattering is also very susceptible to the modification of electronic structure of graphene,11-13 we indicate here the 2D-band Raman spectra recorded at 785 and 1064 nm excitations in parts a and b of Figure 4, respectively. In Figure 4a, one can easily find drastic decrease of 2D-band intensity when exposing the ssDNA to the nanotube, whose decrease should originate in the modification of the band structure of metallic tubes. On the other hand, almost no change in 2D-band intensity can be seen in Figure 4b, whose Raman spectra are all associated with the semiconducting tubes. Hence, we can conclude from these Raman analyses that the metallic tubes well interact with ssDNA. 3.2. TEM. A stringlike object was clearly caught by TEM observation as shown in Figure 5a which was recorded for ssDNA@HT460NT and the length of this object is estimated
Interaction between ssDNA and Nanotubes
Figure 1. RBM Raman spectra. Raman signals were recorded by the laser excitations at 785 nm (a) and 1064 nm (b). Fitting component with thick dotted lines in part a is from metallic tubes and with thin dotted lines from semiconducting tubes. Assignment details are described in text (see also Table 1).
to be about 8-9 nm or more if it straightens. We consider this stringlike object to be an image of ssDNA molecule, and similar stringlike images have been also recorded by Okada et al.7 using a sample of electrochemically doped DNA into the nanotube. However, the length of our stringlike object corresponds to ∼30 b, which is considerably shorter than the mean length of ssDNA used (∼50 kb). We consider that the end of a piece of ssDNA just gets into the nanotube. Frames in the columns of Figure 5 were extracted from recorded movies at intervals of 2 s. From a series of these snap shots, we can find that ssDNA molecule is flapping inside the nanotube, which is easily recognized when we look at the positions marked by the arrowheads in Figure 5a. Such motion of ssDNA can be also seen for the sample of ssDNA@HT460NT soaked in SDBS (ssDNA@HT460NTSDBS) as shown in Figure 5b (see also at the arrowhead positions). On the other hand, as shown at the arrowhead positions in Figure 5c, ssDNA is rather stable in the nanotube when the sample was soaked in BKC (ssDNA@HT460NTBKC). We consider that this suppression of motion for ssDNA is most probably due to Coulomb attraction between negatively charged backbone of ssDNA and positively charged adsorbed BKC, which will stabilize ssDNA on the inside wall of nanotube. Wrapping the nanotube in anionic surfactant of SDBS will give repulsive force to ssDNA, and the irradiation of the electron beam would destabilize ssDNA in the nanotube. This will be a reason for observing the frequent flapping motions of ssDNA for the samples of ssDNA@HT460NT and ssDNA@
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Figure 2. G band Raman spectra. Raman signals were recorded by the laser excitations at 785 nm (a) and 1064 nm (b). Deconvolutions of spectra were carried out by using three Lorentzian components drawn by thin dotted lines and an asymmetric BWF component drawn by thick dotted lines. Decrease of BWF component associated with metallic tubes can be recognized for ssDNA@HT460NT in part a.
Figure 3. Schematic of the doping level of ssDNA as an electron acceptor. Doping of ssDNA to metallic tube (a) and to semiconducting tube (b).
HT460NT-SDBS. Present observation of suppressing motion is to be convincing evidence suggesting that the moving object is ssDNA. As concluded in the Raman scattering, ssDNA molecules interact well with metallic tubes and would modify the electronic structure of metallic tubes but not for semiconducting ones. Direct imaging of ssDNA using TEM certainly demonstrates the insertion of ssDNA in the nanotube. However, type
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Shoda et al. was branched off from the nanotube bundle and rare to see clear ssDNA images in such nanotube. Most of ssDNA molecules were attached on the outer surface of nanotube bundle, and therefore it was hard to recognize ssDNA in the tubes even if numerous ssDNA would certainly be inserted. Taking into account such situation by TEM, modification on the Raman spectra observed for metallic tubes does not originate only in the inserted ssDNA but mostly in the ssDNA molecules wrapping around the nanotube bundles. Summary
Figure 4. 2D-band Raman spectra. Raman signals were recorded by the laser excitations at 785 nm (a) and 1064 nm (b). Drastic decrease of 2D-band intensity can be seen only for ssDNA@HT460NT in part a.
Interaction between SWNTs and ssDNA molecules can be picked up by resonant Raman scattering using a 785-nm laser excitation that selectively probes the metallic tubes with the diameters of 1.5-1.6 nm. Intensity on the BWF component in the G band associated with metallic tubes was weakened after soaking the sample in aqueous solution of ssDNA for 1 day, while the Lorentzian component in the G band associated with semiconducting tubes did not indicate any change in intensity and peak position. We concluded from these facts that the doping band of ssDNA would locate near below the Fermi level of metallic tube and noticeable electron charge transfer was taken place between metallic tubes and ssDNA. On the other hand for the semiconducting tubes, the doping band of ssDNA would locate in between S1 gap, which causes negligible small electron charge transfer. TEM images of individual ssDNA molecules clearly showed that the flapping motion of ssDNA under the electron beam irradiation was suppressed when the sample was soaked in cationic surfactant of BKC. On the other hand, anionic surfactant of SDBS did not give much effect on the flapping motion of ssDNA. To explain these facts, we introduced that the Coulomb interaction between negatively charged backbone of ssDNA and surfactant was playing an important role. Supporting Information Available: Videos of ssDNA@ HT460NT, ssDNA@HT460NT-SDBS, and ssDNA@HT460NTBKC. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 5. Snap shots of ssDNA in nanotube. (a) ssDNA@HT460NT, (b) ssDNA@HT460NT soaked in aqueous solution of anionic surfactant of SDBS (ssDNA@HT460NT-SDBS), and (c) that soaked in cationic surfactant of BKC (ssDNA@HT460NT-BKC). From top to bottom in the column, each frame was recorded at intervals of 2 s. For parts a and b, one can find that string-like ssDNA molecule is flapping inside the nanotube (see the positions marked by arrowheads), but ssDNA stays (c). The scale bar in part c represents 2 nm.
identification of the nanotube that tells whether metallic or semiconducting is not so easy like Raman scattering. In addition, it was rather difficult to find an isolated single nanotube that
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