pubs.acs.org/Langmuir © 2009 American Chemical Society
Aromatic Electron Acceptors Change the Chirality Dependence of Single-Walled Carbon Nanotube Oxidation Fritz J. Knorr, Wei-Chun Hung, and Chien M. Wai* Department of Chemistry, University of Idaho, Moscow, Idaho 83844 Received October 24, 2008. Revised Manuscript Received July 29, 2009 Single-walled carbon nanotubes (SWNTs) dispersed in sodium dodecyl sulfate (SDS) suspensions exhibit diameterdependent protonation and oxidative quenching of their E11 fluorescence. This nanotube-diameter-based difference in solution chemistry is substantially changed when complexed with aromatic electron-accepting compounds such as nitrobenzene, o-nitrotoluene, 2,4-dinitrotoluene, and 9-nitroanthracene. SWNTs were suspended in aqueous SDS, and their emission spectra were measured as a function of pH and concentration of oxidizing agent (hypochlorite or hydrogen peroxide) to observe their protonation and oxidation behavior. The chirality dependence of the protonation and oxidation behavior became substantially reduced upon the addition of nitroaromatic compounds to the aqueous suspension. This suggests the possibility of forming an electron donor-acceptor (EDA) complex, where the SWNT is the electron donor and nitroaromatic compounds are the acceptor, and the resulting supramolecular complex exhibits different redox behavior than the uncomplexed SWNT.
Introduction Since their discovery in 1991, single-walled carbon nanotubes have demonstrated many remarkable electronic and mechanical properties.1 In addition, SWNTs also display interesting chemical and spectroscopic behavior. O’Connell et al. demonstrated that SWNTs can be unbundled and individual nanotubes can be dispersed and stabilized by aggressive sonication in aqueous solutions of common laboratory detergents.2 This discovery has allowed the observation of the near-infrared (NIR) bandgap fluorescence of semiconducting SWNTs because their fluorescence is not observed when the nanotubes aggregate into bundles. Since that discovery, SWNTs have been demonstrated to be unbundled and suspended in a variety of media, including aqueous ionic and nonionic surfactants, polymers, DNA, proteins, and carbohydrates,3-8 as well as simply dissolved into some organic solvents.9 In aqueous media, the detergents are thought to form micelles around the nanotubes, preventing the reaggregation of the unbundled nanotubes. Although the detergent suspension isolates individual carbon nanotubes and prevents reaggregation, it also leaves them available for reaction with the surrounding aqueous phase. When the semiconducting SWNTs are unbundled and fluorescent, their *Corresponding author. E-mail:
[email protected]. (1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603–605. (2) 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, 593–596. (3) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269–273. (4) Li, L.-J.; Nicholas, R. J.; Chen, C.-Y.; Darton, R. C.; Baker, S. C. Nanotechnology 2005, 16, S202–S205. (5) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; McLean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Matter. 2003, 2, 338–342. (6) Zheng, M.; Jogota, A.; Strano, M. S.; Santos, A. P.; Barone, P.; Chou, S.; Samsonidze, G. G.; Semke, E. D.; Usrey, M.; Walls, D. J. Science 2003, 302, 1545– 1548. (7) Witus, L. S.; Rocha, J. R.; Yuwono, V. M.; Paramonov, S. E.; Weisman, R. B.; Hartgerink, J. D. J. Mater. Chem. 2007, 17, 1909–1915. (8) Yan, L. Y.; Poon, Y. F.; Chan-Park, M. B.; Chen, Y.; Zhang, Q J. Phys. Chem. C 2008, 112, 7579–7587. (9) Hasan, T.; Scardaci, V.; Tan, P.; Rozhin, A. G.; Milne, W. I.; Ferrari, A. C. J. Phys Chem C 2007, 111, 12594–12602.
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fluorescence can be used to monitor chemical reactions between the nanotubes and the surrounding solution. Individual SWNTs in a detergent suspension have been shown to undergo reversible acid-base reactions with the surrounding aqueous phase, reminiscent of molecular acid-base chemistry.2,10-15 It was found that upon protonation the E11 NIR fluorescence is reversibly quenched. The mechanism of that quenching is thought to be through the oxidative removal of electrons occupying the highestenergy van Hove singularity of the valence band.15 The resulting localized holes disrupt the radiative exciton, quenching the fluorescence. It has been established that the reaction of SWNT with Hþ is not simply protonation and that adsorbed O2 on the surface of the nanotube is necessary for the oxidative quenching of the E11 fluorescence of SWNTs with acid.14 The samples used in this study were handled in contact with air, so the carbon nanotubes are assumed to have molecular O2 adsorbed on them. SWNT fluorescence also can be reversibly quenched through oxidation with common oxidizers in the aqueous phase of the detergent suspension.16-21 In addition to reversible oxidation, fluorescence quenching has also been used as a monitor to observe other reactions with aqueous-detergent-suspended SWNTs, (10) Strano, M. S.; Usrey, M. L.; Barone, P. W.; Heller, D. A. In Applied Physics of Carbon Nanotubes; Rotkin, , S., Subramoney, S., Eds.; Springer: New York, 2005. (11) Strano, M. S.; Huffman, C. B.; Moore, V. C.; O’Connell, M. J.; Haroz, E. H.; Hubbard, J.; Miller, M.; Rialon, K.; Kittrell, C.; Ramesh, S.; Hauge, R. H.; Smalley, R. E J. Phys Chem. B 2003, 107, 6979–6985. (12) Cognet, L.; Tsyboulski, D. A.; Rocha, J. R.; Doyle, C. D.; Tour, J. M.; Weisman, R. B. Science 2007, 316, 1465–1468. (13) Weisman, R. B.; Bachilo, S. M.; Tsyboulske, D. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 1111–1116. (14) 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, 15269–15276. (15) Nish, A.; Nicholas, R. J. Phys. Chem. Chem. Phys. 2006, 8, 3547–3551. (16) Miyata, Y.; Mainwa, Y.; Kataura, H. J. Phys. Chem. B 2006, 110, 25–29. (17) Zhang, M.; Yudasaka, M.; Miyauchi, Y.; Maruyama, S.; Iijima, S. J. Phys. Chem. B 2006, 110, 8935–8940. (18) Song, C.; Pehrsson, P. E.; Zhao, W. J. Phys. Chem. B 2005, 109, 21634– 21639. (19) Tu, X.; Pehrsson, P. E.; Zhao, W. J. Phys. Chem. C 2007, 111, 17227–17231. (20) O’Connell, M. J.; Eibergen, E. E.; Doorn, S. K. Nat. Mater. 2005, 4, 412– 417. (21) Zheng, M.; Diner, B. A. J. Am. Chem. Soc. 2004, 126, 15490.
Published on Web 08/12/2009
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including interactions with transition-metal cations22 and covalent reaction with diazonium salts.23 The bandgap fluorescence quenching due to the protonation or oxidation of detergent-suspended SWNTs has been shown to depend upon the chirality of the nanotube.10 In an SDS suspension (SWNT/SDS), the larger-diameter, smaller-bandgap semiconducting SWNTs have been shown to have a lower oxidation potential, and their fluorescence is quenched first upon lowering the pH of the aqueous suspension. This leads to the observation that upon lowering the pH of a detergent suspension of SWNTs the longer-wavelength fluorescence from these larger-diameter nanotubes is quenched first.11 This pattern is repeated for oxidation reactions that directly remove electrons without the involvement of protons.21 As the potential of the solution is made more oxidizing, the valence bands of the smaller-bandgap nanotubes are depleted first, followed by the smaller-diameter, largerbandgap nanotubes. The composition of the encapsulating micelle and other compounds adsorbed to the SWNT can alter the oxidation-induced fluorescence quenching behavior. For instance, SWNTs suspended in sodium dodecylbenzene sulfonate shows much less acid-induced quenching of the NIR fluorescence than do SDS suspensions.12 Indeed, some surfactants render the SWNTs effectively inert toward protonation quenching of E11 fluorescence whereas other surfactant preparations can actually reverse the pattern whereby the fluorescence is quenched at higher pH.24 The present work shows that intermolecular interactions between an SWNT and an aromatic electron acceptor, such as o-nitrotoluene (ONT), 2,4-dinitrotoluene (DNT), 9-nitroanthracene (9-NA), and nitrobenzene (NB), could change the redox behavior of the nanotubes. In this case, the oxidation potentials of the valence bands appear to shift so that the fluorescence quenching due to protonation or oxidation for all chiralities of semiconducting SWNTs happens simultaneously and the chirality dependence of the oxidation of the valence band is removed. In the presence of benzene or toluene, the pattern of oxidative fluorescence quenching is similar to that observed for the SWNTs alone. We postulate that the SWNT and the aromatic electron acceptor form an electron donor-acceptor complex, and this resulting complex does not have a chiralityor diameter-dependent difference in the redox potential of the valence band.
Experimental Section HiPco SWNTs were purchased from CNI (batch no. R0524, Houston, TX) and used as received without further purification. No attempt was made to exclude oxygen from the SWNTs or any of the solutions. Sodium dodecyl sulfate (Fisher Scientific), o-nitrotoluene (Aldrich), 2,4-dinitrotoluene (Aldrich), 9-nitroanthracene (Aldrich), benzene (EMD), toluene (EMD), and nitrobenzene (Aldrich) were used as received without further purification. The 1% SDS solution was adjusted to pH 7.00 with 1 M NaOH and 1 M HCl. The various concentrations of aromatic chemicals were prepared in a 1% SDS solution. To prepare the SWNT/SDS dispersions, 1 mg of SWNTs was placed into 30 mL of a 1% SDS solution in water and sonicated for 20 min with a tip sonicator (Cole-Parmer CPX 130) at 100 W acoustic power. The suspension was centrifuged (Beckman Coulter Microfuge 16) for 30 min at 16 000g. One milliliter aliquots of (22) Brege, J. J.; Gallaway, C.; Barron, A. R. J. Phys. Chem. C 2007, 111, 17812– 17820. (23) Doyle, C. D.; Rocha, J-D. R.; Weisman, R. B.; Tour, J. M J. Am. Chem Soc. 2008, 130, 6795–6800. (24) Duque, J. G.; Cognet, L.; Parra-Vasquez, N. G.; Nicholas, N.; Schmidt, H. K.; Pasquali, M. J. Am. Chem. Soc. 2008, 130, 2626–2633.
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the SWNT supernatant were taken and diluted with the 1% SDS solution by a factor of 2. One milliliter of the above supernatant was then mixed with 100 μL of an aromatic compound in the 1% SDS aqueous solution in a vial, and the solution was stored in a box overnight for fluorescence measurements. The pH was adjusted using a 0.1 M solution of HCl added to the unbuffered sample. The pH was measured with a pH meter (IQ Scientific Instruments IQ 240). Stock solutions of hydrogen peroxide (30% from EMD) and 0.001 M NaOCl (Fisher Scientific) were prepared daily for use as oxidizers. Emission spectra were measured in 1 cm optical path length cuvettes at room temperature with a Jobin Yvon Nanolog 916B fluorescence spectrometer equipped with an IGA 512 InGaAs near-IR detector. All of the emission spectra shown in this study were treated with baseline correlation. Absorption spectra were obtained using a Guided Wave (Rancho Cordova, CA) model LS-E-VIS-NIR spectrometer and a Shimadzu UV-3101PC UV-vis-NIR scanning spectrophotometer.
Results and Discussion Figure 1 shows the fluorescence spectra and the oxidative quenching behavior of the SWNT/SDS alone and in the presence of ONT. Figure 1a shows the previously reported diameter dependence of the protonation-induced quenching of the bandgap fluorescence of semiconducting SWNTs.13 In general, the larger-diameter, the smaller-bandgap nanotubes are quenched at lower proton concentration and higher pH. This leads to the observed pattern of the longer-wavelength emission quenching first as acid is added to the SWNT/SDS suspension. This chirality-dependent quenching pattern is substantially altered upon the addition of a low concentration of aromatic electron acceptor, ONT, to the aqueous suspension. In Figure 1b, it shows that in this case the diameter dependence of acid quenching of the fluorescence of semiconducting SWNT is substantially reduced, to the point of being nearly eliminated. In the presence of the aromatic electron acceptor, all chiralities of SWNTs are quenched roughly the same amount, indicating a relative leveling of the redox potentials of all chiralities of SWNTs. Figure 2 shows the oxidative fluorescence quenching behavior of the SWNT/SDS E11 emission upon titration with hypochlorite or hydrogen peroxide. In Figure 2a,c, the chirality-dependent behavior is very similar to oxidation under acidic conditions, and it is similar to that reported by Zheng et al. for the oxidation of SWNT/SDS by Ir4þ.21 Again, the pattern is that the fluorescence from the larger-diameter nanotubes is quenched first. Figure 2b,d shows the pattern of oxidative fluorescence quenching of SWNT/ SDS due to oxidation in the presence of 1 mM ONT. In the presence of the aromatic electron acceptor, the fluorescence is quenched by the oxidizer, but there is a substantially reduced chirality-dependent difference in the behavior of the different nanotubes. The electrochemical potential of all of the chiralites of the semiconducting SWNTs is nearly leveled in the presence of an aromatic electron acceptor. Figure 2b,d also shows that the degree of quenching is higher for hypochlorite than for hydrogen peroxide even though the molar concentration of the latter is higher than that of the former (about 9 10-6 M NaOCl vs 7.9 10-5 M H2O2). This can be attributed to the fact that NaOCl is a stronger oxidizing agent than H2O2. In the NIR absorption spectra, the chirality-dependent bleaching pattern of the SWNT/SDS suspension is also observed upon addition of H2O2 in the absence of ONT. In contrast to the oxidative bleaching pattern of the control sample, the oxidative bleaching of the SWNT/SDS suspension in the presence of ONT likewise shows the substantially reduced chirality dependence (Supporting Information). Langmuir 2009, 25(18), 10417–10421
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Figure 1. SWNT/SDS fluorescence oxidative quenching pattern upon addition of acid. Excitation wavelength at 650 nm. (a) SWNT/SDS suspension. (b) SWNT/SDS suspension in the presence of 1 mM ONT.
Figure 2. SWNT/SDS fluorescence oxidative quenching pattern upon addition of NaOCl and H2O2. Excitation wavelength at 650 nm. (a) Oxidation of the SWNT/SDS suspension with NaOCl. (b) Oxidation of the SWNT/SDS suspension with NaOCl in the presence of 1 mM ONT. (c) Oxidation of the SWNT/SDS suspension with H2O2. (d) Oxidation of the SWNT/SDS suspension with H2O2 in the presence of 1 mM ONT.
The concentration of SWNTs in the SDS suspension determined by NIR absorption spectroscopy was about 9.5 mg L-1 (Supporting Information). The diameter-independent oxidation phenomenon depends on the amount of ONT relative to that of Langmuir 2009, 25(18), 10417–10421
SWNT in the SDS suspension system. When the concentration of ONT is low, only a fraction of the SWNTs could interact with ONT. The net result is a mixture of the two oxidation patterns (i.e., partially diameter-dependent behavior is observed). When DOI: 10.1021/la901813t
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Figure 3. Relative intensities of all chiralities in the SWNT/SDS suspension upon addition of H2O2. Excitation wavelength at 650 nm. (a) Oxidation of the SWNT/SDS suspension with H2O2. (b) Oxidation of the SWNT/SDS suspension with H2O2 in the presence of 0.30 mM ONT. (c) Oxidation of the SWNT/SDS suspension with H2O2 in the presence of 0.93 mM DNT. (d) Oxidation of the SWNT/SDS suspension with H2O2 in the presence of 0.12 mM 9-NA. (e) Oxidation of the SWNT/SDS suspension with H2O2 in the presence of 16.5 mM NB.
the concentration of ONT reaches a certain value, the reduced chirality-dependent oxidation behavior becomes predominant as shown in the Supporting Information. Therefore, we are able to determine a critical concentration for ONT that is about 3.2 10-6 moles/mg of SWNT corresponding to approximately 1 ONT molecule per 27 carbon atoms. However, this is obviously an estimate because the SDS suspension contains both bundled and individual SWNTs and ONT may attach to them differently. The preparation of the suspension by sonication in detergent, followed by centrifugation, likely leaves small bundles of SWNTs in the sample. To test whether the observed changes in electrochemical behavior were due to an effect involving small bundles, the oxidative quenching of the SWNT/SDS suspension was studied using three different centrifugation times (30, 60, and 90 min) at 16 000g. The results showed that the oxidative quenching behavior of the SWNT/SDS-ONT system did not change with different centrifugation times as shown in the Supporting Information. In addition, we also studied the oxidative quenching of the SWNT/SDS suspension followed by ultracentrifugation at about 100 000g for 18 h. The reduction in chirality-dependent oxidation behavior of the SWNT/SDS-DNT system remains unchanged (Supporting Information). For these reasons, we believe that under our experimental conditions with 30 min of centrifugation the bundled SWNTs did not contribute to the observed fluorescence of our SWNT/SDS system. In addition to ONT, the diameter-independent oxidation behavior was also studied for other nitroaromatic electron acceptors including DNT, 9-NA, and NB with varying concentrations. Figure 3 shows the relative intensities, with the (7,5) chirality as the reference, for all chiralities of the SWNTs upon addition of H2O2 in the presence of each of the nitroaromatic compounds. In all four cases, the chirality dependence of the oxidative quenching behavior is substantially reduced. It should be noted that the concentration of each nitroaromatic compound used for the oxidative quenching experiments was chosen from 10420 DOI: 10.1021/la901813t
our concentration-dependent studies shown in the Supporting Information. In summary, we observe that the SWNT/SDS suspension in the presence of nitroaromatic electron acceptors exhibits substantially changed chirality-dependent fluorescence quenching behavior, where the difference in oxidation potential between the different diameters of SWNTs is dramatically reduced. One hypothesis for this change is that the relative oxidation potentials of the valence bands of the various chiralities are shifted to become more or less equal upon addition of nitroaromatics. We believe that this is an electrochemical effect of the formation of a complex between the SWNT and the aromatic electron acceptor because the effect is seen when the oxidation is done by acid and adsorbed oxygen or direct oxidation by hypochlorite or hydrogen peroxide. This situation, where the relative redox potentials of the valence and conduction bands of the various chiralities of SWNT/ SDS shift upon complexation with an electron acceptor, is illustrated in cartoon form in Figure 4. In the SDS suspension alone, the redox potentials of the different chiralities of semiconducting SWNTs are such that the midpoints between their respective valence and conduction bands are roughly equal.15 This pattern is also repeated for SWNTs in solution in DMSO with no surfactant.25 In our model, SWNTs would be acting as the electron donor and nitroaromatics would be acting as the acceptor. The resulting complex has an emission spectrum that is very similar to the emission spectrum of the uncomplexed SWNT/ SDS. However, the oxidation behavior of the complex is different from that of the uncomplexed SWNT in that the oxidation of the valence band is much less sensitive to chirality. The oxidation potentials of all chiralities are relatively leveled to a potential closer to that of the smallest-diameter nanotube in the sample, the (8,3) chirality. The oxidation potential of the smallest-diameter (25) Paolucci, D.; Franco, M. M.; Iurlo, M.; Marcaccio, M.; Prato, M.; Zerbetto, F.; Penicaud, A.; Paolucci, F. J. Am. Chem. Soc. 2008, 130, 7393–7399.
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sence of benzene or toluene is similar to that in the control sample as shown in the Supporting Information. Benzene and toluene do not contain electron-withdrawing groups and are not considered to be electron acceptors in the conventional sense. The SWNT fluorescence peaks in this case also do not show measurable shifts. However, because of the π-π interactions, benzene and toluene could be attached to the SWNT surfaces and might have some minor effects on SWNT oxidative quenching behavior that are not clear at the present time.
Conclusions Figure 4. Redox potential of the conduction and valence bands of SWNT/SDS as prepared (-) and as complexed with nitrotoluene (---). The oxidation potentials of the valence bands of all chiralities are equalized upon complexation with nitrotoluene.
nanotube observed in this sample, the (8,3) chirality, probably does not seem to change within the accuracy of this study, although we were not able to access oxidation potentials that could completely quench (or bleach) this chirality. The formation of electron donor-acceptor (EDA) complexes26 between SWNTs and electron donors and acceptors has been reported previously.27 Raman spectroscopy has been used to show that solvents with electron-donating and electronwithdrawing solvents can donate and withdraw electrons from SWNTs.28 The isotherms of nitroaromatics show a much stronger adsorption to SWNTs than to other aromatic compounds,29 indicating a strong intermolecular interaction between SWNTs and nitroaromatic electron acceptors. The stability of the EDA complex, in addition to more commonly reported π-π stacking interactions with carbon nanotubes, may explain the stability of supramolecular assemblies of SWNTs and moieties with electronaccepting capacity30 and the ability of small molecules9 and chromophores31 to unbundle and solublize SWNTs. This might also be related to the report that nitrobenzene was able to change the gate voltage of field-effect transistor devices with semiconducting SWNTs as the conducting channels.32 The effects of benzene and toluene on the oxidative quenching of SWNTs were also studied in the SDS system. The oxidative quenching behavior of the SWNT/SDS suspension in the pre(26) Mulliken, R. S.; Person, W. B. Molecular Complexes; A Lecture and Reprint Volume; Wiley-Interscience: New York, 1969. (27) Guldi, D. K.; Rahman, G. M. A.; Zerbetto, F.; Prato, M. Acc. Chem. Res. 2005, 38, 871–878. (28) Shin, H.-J.; Kim, S. M.; Yoon, S.-M.; Benayad, A.; Kim, K. K.; Kim, S. J.; Park, H. K.; Choi, J.-Y.; Lee, Y. H. J. Am. Chem. Soc. 2008, 130, 2062–2066. (29) Chen, W.; Duan, L; Zhu, D Environ. Sci. Technol. 2007, 41, 8295–8300. (30) Liu, Z.; Sun, X.; Nakayama-Ratchford, N.; Dai, H. ACS Nano 2007, 1, 50– 56. (31) Chitta, R.; Sandanayaka, A. S. D.; Schumacher, A. L.; D’Souza, L.; Araki, Y.; Ito, O.; D’Souza, F. J. Phys. Chem. C 2007, 111, 6947–6955. (32) Star, A.; Han, T. R.; Gabriel, J. C. P.; Bradley, K.; Gruner, G. Nano Lett. 2003, 3, 1421–1423.
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We have demonstrated that the addition of a small amount of an aromatic electron acceptor such as o-nitrotoluene, 2,4-dinitrotoluene, 9-nitroanthracene, and nitrobenzene could drastically change the relative oxidation potentials of the valence bands of the different chiralities of SWNTs. We attribute this to the formation of a donor-acceptor complex, where SWNTs serve as the electron donor and the nitroaromatic compounds serve as the acceptor. This is the first report of the adsorption of a small molecule changing the redox behavior of SWNTs in suspension. EDA complex formation by SWNTs may help to explain other observations, such as solublization. Because the energy of interaction of EDA complexes can be considerably larger than the dispersion forces of π-π stacking, it may be that electronacceptor moieties will be the most favorable approach for the noncovalent functionalization of SWNTs for supramolecular assemblies. The observed shifting of the valence band potentials must necessarily also shift the conduction band potentials, as shown in Figure 4. Therefore, the formation of an EDA complex may provide a mechanism by which to adjust the redox potentials of both the valence and conduction bands and affect photoinduced charge-transfer reactions involving SWNTs.33,34 Acknowledgment. This work was supported by AFOSR (FA9550-06-1-0526) and by DURI-AFRL (FA8650-06-D5401). We thank Dr. Paszczynski and Dr. Lee (University of Idaho) for their assistance with the ultracentrifugation instrument and Gracy Elias (Idaho National Laboratory) for the NIR absorption spectra measurements. Supporting Information Available: Normalized NIR absorption and emission spectra of SWNT/SDS electronacceptor complexes and additional spectroscopic data. Analysis of the quenching behavior using the Stern-Volmer equation. This material is available free of charge via the Internet at http://pubs.acs.org. (33) Kongkanand, A.; Kamat, P. V. J. Phys. Chem. C 2007, 111, 9012–9015. (34) D’Souza, F.; Chitta, R.; Sandanayaka, A. S. D.; Subbaiyan, N. K.; D’Souza, L.; Araki, Y.; Ito, O. J. Am. Chem. Soc. 2007, 129, 15865–15871.
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