Solid stated 13C NMR can also be used to analyze functionalized nanotubes. As a result of functionalizing SWNTs with groups containing a carbonyl group, a slight shift toward higher fields (lower ppm) for the sp2 carbons is observed. This shift is explained by the perturbation applied to the electronic structure of the whole nanotube as a result of the modifications on only a fraction of the nanotube. At the same time, a new peak emerges at around 172 ppm, which is assigned to the carboxyl group of the substituent. The peak intensities could also be used to quantify the level of functionalization. Figure 3 shows these changes, in which the substituents are –(CH2)3COOH, –(CH2)2COOH, and –(CH2)2CONH(CH2)2NH2 for the spectra Figure 3b,Figure 3c, and Figure 3d, respectively. Note that the bond between the nanotube and the substituent is a C-C bond. Due to low sensitivity, the peak for the sp3 carbons of the nanotube, which does not have a high quantity, is not detected. There is a small peak around 35 ppm in Figure 3, can be assigned to the aliphatic carbons of the substituent.
For substituents containing aliphatic carbons, a new peak around 35 ppm emerges, as was shown in Figure 3, which is due to the aliphatic carbons. Since the quantity for the substituent carbons is low, the peak cannot be detected. Small substituents on the sidewall of SWNTs can be chemically modified to contain more carbons, so the signal due to those carbons could be detected. This idea, as a strategy for enhancing the signal from the substituents, can be used to analyze certain types of sidewall modifications. For example, when Gly (–NH2CH2CO2H) was added to F-SWNTs (fluorinated SWNTs) to substitute the fluorine atoms, the 13C NMR spectrum for the Gly-SWNTs was showing one peak for the sp2 carbons. When the aliphatic substituent was changed to 6-aminohexanoic acid with five aliphatic carbons, the peak was detectable, and using 11-aminoundecanoic acid (ten aliphatic carbons) the peak intensity was in the order of the size of the peak for sp2 carbons. In order to use 13C NMR to enhance the substituent peak (for modification quantification purposes as an example), Gly-SWNTs was treated with 1-dodecanol to modify Gly to an amino ester. This modification resulted in enhancing the aliphatic carbon peak at around 30 ppm. Similar to the results in Figure 3, a peak at around 170 emerged which was assigned to the carbonyl carbon. The sp3 carbon of the SWNTs, which was attached to nitrogen, produced a small peak at around 80 ppm, which is detected in a cross-polarization magic angle spinning (CP-MAS) experiment.
F-SWNTs (fluorinated SWNTs) are reported to have a peak at around 90 ppm for the sp
3 carbon of nanotube that is attached to the fluorine. The results of this part are summarized in
Table 2 (approximate values).
TABLE 2: Chemical shift for different types of carbons in modified SWNTs. Note that the peak for the aliphatic carbons gets stronger if the amino acid is esterified. Data are obtained from: H. Peng, L. B. Alemany, J. L. Margrave, and V. N. Khabashesku,J. Am. Chem. Soc., 2003, 125, 15174; L. Zeng, L. Alemany, C. Edwards, and A. Barron, Nano. Res., 2008, 1, 72; L. B. Alemany, L. Zhang, L. Zeng, C. L. Edwards, and A. R. Barron, Chem. Mater., 2007, 19, 735.
Group | δ (ppm) | Intensity |
sp2 carbons of SWNTs | 120 | Strong |
–NH2(CH2)nCO2H (aliphatic carbon, n=1,5, 10) | 20-40 | Depends on ‘n’ |
–NH2(CH2)nCO2H (carboxyl carbon, n=1,5, 10) | 170 | Weak |
sp3 carbon attached to nitrogen | 80 | Weak |
sp3 carbon attached to fluorine | 90 | Weak |
The peak intensities that are weak in
Table 2 depend on the level of functionalization and for highly functionalized SWNTs, those peaks are not weak. The peak intensity for aliphatic carbons can be enhanced as the substituents get modified by attaching to other molecules with aliphatic carbons. Thus, the peak intensities can be used to quantify the level of functionalization.