Single-walled carbon nanotubes (SWNTs) have been attracting extensive interest because of their novel and unique chemical and physical properties. However, the present technologies for SWNTs synthesis always produce samples with both enantiomers in equal amounts. Since the optically active properties of SWNTs are closely correlated to their helical structures, structural control of SWNTs is very important for their potential applications in photonics and photoelectronics. A wide variety of methods have been devised so far for the non-covalent separation of SWNTs according to their left-and right-handed structure. In this paper, they are classified into the following five sections according to the separation methods: ion exchange chromatography, nanotweezers selective extraction, density-gradient ultracentrifugation (DGU), conjugated polymers wrapping and small molecules adsorption. We reviewed the recent advances in this research field, including the separation mechanisms. The relationship between these methods and the results of the optical and (n,m) enhancements of the extracted SWNTs is analyzed, and it may be useful for designing the novel host molecules to obtain single structure SWNTs in view of practical applications. At last, the existing problems and the direction of development of the regarding studies are pointed out.

Contents
1 Introduction
2 Helical structures of SWNTs
3 Non-covalent methods to separate chiral SWNTs
3.1 Ion exchange chromatography
3.2 Nanotweezers selective extraction
3.3 Density-gradient ultracentrifugation
3.4 Conjugated polymers wrapping
3.5 Small molecules adsorption
4 Conclusion and outlook