![]() ![]() As controls, we prepared both empty (end-capped) SWCNTs and water-filled end-opened nanotubes. After exposure to either n-hexane or cyclohexane, the opened nanotubes were then stabilized as individual particles in water by the surfactant sodium deoxycholate for ensemble measurements or deposited on a substrate for single nanotube hyperspectral imaging. 1a) and bright PL from the end-opened nanotubes (vide infra). The amount of oxidative defects introduced, if any, is negligible, as evidenced by the nearly unchanged Raman D/G peak ratio (Supplementary Fig. ![]() We note that the nanotubes retain their structural integrity during this oxidative opening process. This opening step is required as the ends of raw SWCNTs are typically capped or blocked, which would prevent the molecules from entering the pore 18, 21. To prepare samples of molecule-filled SWCNTs, we first thermally oxidized the nanotubes to open their ends, and then incubated these end-opened SWCNTs in cyclohexane or n-hexane (see “Methods” for details). 1c, and combining ab initio molecular dynamics simulations, we uncover a molecular level of insights for the nanopore selectivity. By capturing the optical response of the nanotube, as schematically illustrated in Fig. In contrast, for SWCNT pores only 0.025 nm larger, both molecules are able to enter and the selectivity is lost. Our series of experiments further confirm that even a trace amount of 0.1% n-hexane in 99.9% (by volume) cyclohexane can be selectively captured and removed from the mixture. This is despite the fact that plausible filling configurations of both molecules suggest that neither molecule should be able to enter the rigid pore when observed from the view of KD (Fig. Here, we show that n-hexane is able to enter (6,5)-SWCNT, while cyclohexane is excluded. Additionally, the excitonic photoluminescence (PL) of semiconducting SWCNTs is sensitive to both the exterior and interior environments of the hollow nanotube 17, 18 due to changes in the dielectric microenvironment 19, as well as from molecule-induced strain 20. For example, (6,5)-SWCNT has a van der Waals pore size of just 0.422 nm, which is even smaller than n-hexane (KD ≈ 0.43 nm) 14, 15 and cyclohexane (KD ≈ 0.60 nm) 15, 16. Many of these SWCNT nanopores have a pore size comparable to small molecules. These nanopores are tunable in size within the sub-nm range based on their individual atomic structure (i.e., nanotube chirality, as defined by a pair of integers ( n, m)) 11, and they exhibit intriguing molecular transport properties 12, 13. The SWCNT nanopores are chemically inert, structurally rigid (Young’s modulus >1 TPa) 10, and atomically smooth cylinders, featuring inflexible pores that are well-defined by the nanotube cylinder that is constructed from a conjugated sp 2 carbon lattice. However, in examining nanopores defined by single-wall carbon nanotubes (SWCNTs), we find unambiguous evidence that molecules can adapt their conformation to enter smaller pores. It is generally believed that the selectivity enabling such separations arises from the ability of molecules with smaller kinetic diameter (KD) to enter the pore while larger ones are excluded 6, 7, 8, 9. Synthetic porous materials, such as zeolites, have also demonstrated an impressive level of molecular sieving capabilities for chemicals, allowing the separation of minor components from heterogeneous mixtures 5. Biological systems, for example, have evolved a diverse array of specialized nanoscale protein channels that can allow only selected ions and molecules to cross the cell membrane 4. Nanopores play an important role in chemical separations and selective mass transport that underlie many basic biological functions and industrial processes 1, 2, 3. ![]()
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