Mixed properties and strong intermolecular π-π interactions are unique problems of SWNTs.41 Also unstable immobilization of SWNTs on the substrate and low reproducibility of device are major obstacles in development of SWNTs based biosensors.3537
First, only sc-SWNTs were selectively dispersed by wrapping polymer to solve the mixed properties and bundling caused by π-π interaction of SWNTs. Click reaction was used to solve the remaining problems of low stability and reproducibility of SWNTs film.
For this, polyfluorene polymers known to have high selectivity of semiconducting tubes with large diameters were synthesized.39 This polymer was designed to have long alkyl chain lengths of C12 and azide groups at the end of side chains, which enables to separate large diameter SWNTs with a narrow band gap and form chemical bonds with alkyne groups by Click reaction.
To prepare this polyfluorene derivatives, a monomer called FD-N3 having C12 chain length and azide groups at the end of chains was synthesized from 2,7-dibromofluorene.From 2,7-dibromofluorene, FD- Br was synthesized by alkylation of 1,12-dibromododecane under the reaction TBAB-NaOH catalytic system.42 43 The bromine of FD-Br was substituted with azide group using sodium azide in DMF solution to obtain FD-N3. From 2,7-dibromo-9,9-didodecyl-9H-fluorenes, FD-pinacolborane was synthesized by condensation reaction with bis(pinacolato)diboron.44 The alternating polymer P(FD-N3) was synthesized using FD-N3 and FD-pinacolborane monomers through Suzuki coupling. This synthesized fluorene polymer with azide group was used to purify sc-SWNTs of the plasma torch SWNTs (Nanointegris, RN-220).
Rn220 having large diameters was used because it is known to have high performances than small diameter SWNTs. Polyfluorene, the wrapping polymers, are known to effectively separate sc-SWNTs.
However, if the wrapping polymer contains azide groups of its side chain, it is difficult to separate high- purity sc-SWNTs because metallic SWNTs are also have interactions with polymers containing high concentrations of nitrogen atoms, due to the rich electrons condition of the nitrogen.5 Among the results of the selective dispersions of sc-SWNTs using the conjugated polymer wrapping method, there are some results that the purity of sc-SWNTs was increased as metallic SWNTs are selectively removed by silica gels.45Using the affinity of silica gel and m-SWNTs, high-purity of sc-SWNTs were selectively separated.
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For selective sorting of sc-SWNTs, plasma torch SWNTs were dispersed in toluene solution of P(FD- N3) polymer with silica gel by horn sonication. After that, supernatant solution of sc-SWNTs wrapped with polyfluorene was obtained by ultracentrifuge. Remaining unwrapped polymer in the extracted suspension was removed by filtration, and enriched SWNTs on filter paper were re-dispersed in toluene.
For unsorted SWNTs, as a reference sample, raw SWNTs bundle was dispersed in sodium dodecylsulfate (SDS) solution dissolved in deionized (DI) water.
The results of selective sc-SWNTs dispersion by conjugated polymers were confirmed by UV-vis and Raman spectrum. The absorption spectrum of the enriched SWNTs was measured to identify the selectively separated species of sc-SWNTs from the solution by UV-vis spectroscopy. As shown in Figure 3.1, peaks of enriched SWNTs showed sharpen semiconducting transition peaks in 700-1100 nm, 450-550 nm bands for S11, S22 respectively after sorting processes. In sample separated by only toluene, slight peak is observed in the M11 (600-750 nm) region, but no peak is observed in the sample separated by silica and toluene.
Figure 3.1. UV-vis-NIR absorption spectra of raw SWNTs and sorted sc-SWNTs
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For a more detailed analysis of the selective sc-SWNTs dispersion, Raman spectrum was obtained as shown in Figure 3.2. In radial breathing mode (RBM), lattice vibrations of SWNTs is an inherent properties among specific SWNTs. RBM relative intensity can be used to describe the electrical properties of carbon nanotubes.46In the RBM region, sorted sc-SWNTs with 0.9–1.5 nm diameters were investigated with Raman shifts on 100-300 cm–1. With an excitation energy of 1.59 eV (780 nm), the resonant Raman scattering was measured. Since metallic SWNTs with a diameter of 1.3-1.5 nm resonated with an excitation energy of 1.59 eV in this RBM region, the resonance with metallic SWNTs appears near 160 cm-1, as shown in Figure 2.16.47 After the enrichment process using toluene with silica, these metallic peaks wasn’t observed from the Raman spectrum in the same range.
Figure 3.2. Raman scattering spectras of raw SWNTs and sorted sc-SWNTs (laser source 780nm)
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The damage of SWNTs structure also can be confirmed in RBM mode as shown in Figure 3.3. The Raman scattering spectrum of SWNTs is shown by two strong peaks at 1580 and 2700 cm−1, named G band and G’ band which is represented to intrinsic electrical properties of SWNTs.48 In presence of defects in SWNTs, D band (D means defect or disorder) is observed at about 1350 cm-1. In separated sc-SWNTs sample, the intrinsic peaks of G and G' bands were observed, but the D bands indicating defects were not observed. This result means that m-SWNTs were removed and only undamaged sc- SWNTs were selectively enriched by conjugated polymer wrapping method.
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Raman shift
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Figure 3.3. Raman scattering spectra of sorted sc-SWNTs (laser source 532nm)
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