Liquid-phase processing of single-wall carbon nanotubes (SWCNTs) generally results in the exposure of their core to the environment (opening) due to the energy input necessary for purification and solubilization, and this often affects the SWCNT properties [1-3]. For instance, for aqueous processing, SWCNTs are routinely getting filled with water, which leads to significant redshifts to, and inhomogeneous broadening of, the electronic transitions of the SWCNTs, as well as a substantial decrease of their fluorescence quantum efficiency [1]. Selection of (remaining) empty (end-capped) SWCNTs to avoid these detrimental effects is possible by means of ultracentrifugation, but is a natively low yield process [4]. In this work, adverse filling is prevented by intentionally prefilling the SWCNTs with alkanes, serving as a passive and highly homogeneous spacer. In contrast to water and other encountered fillants, the alkane core is of low dielectric nature and therefore only weakly affects the electronic structure of the SWCNTs, and hence effectively simulates empty core conditions. This yields highly resolved optical spectra with strongly shifted peak positions compared to water filled SWCNTs (see Figure 1), and fluorescence efficiencies approaching those of empty SWCNTs. It is demonstrated that this can be realized using a wide variety of linear as well as cyclic alkanes, in combination with various SWCNT materials having very different diameter distributions.

References
[1] Cambré, S.; Wenseleers, W. Separation and Diameter-Sorting of Empty (End-Capped) and Water-Filled (Open) Carbon Nanotubes by Density Gradient Ultracentrifugation.  Angew. Chem. Int. Ed. 2011, 50, 2764–2768.
[2] Cambré, S.; Santos, S. M.; Wenseleers, W.; Nugraha, A. R. T.; Saito, R.; Cognet, L.; Lounis, B. Luminescence Properties of Individual Empty and Water-Filled Single-Walled Carbon Nanotubes. ACS Nano 2012, 6, 2649.
[3] Duque, J. G.; Oudjedi, L.; Crochet, J. J.; Tretiak, S.; Lounis, B.; Doorn, S. K.; Cognet, L. Mechanism of Electrolyte-Induced Brightening in Single-Wall Carbon Nanotubes. J. Am. Chem. Soc. 2013, 135 (9), 3379–3382.
[4] Fagan, J.A; Huh, J.Y.; Simpson, J.R.; Blackburn, J.L.; Holt, J.M.; Larsen, B.A.; Walker, A.R.H. Separation of Empty and Water-Filled Single-Wall Carbon Nanotubes.  ACS Nano 2011, 5, 3943–3953.

Figure 1. Optical absorption spectra of electric Arc SWCNTs filled with linear alkanes (C_nH_2n+2) of different lengths, compared to the spectra of water-filled, ethyl acetate-filled (rinsing solvent) and empty Arc SWCNTs. The improved spectral resolution upon alkane encapsulation compared to water filling is clearly visible.