94 CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 2: NANOSTRUCTURED MATERIALS as sodium cholate to disperse the SWNTs in solution, and a linear density gradient was formed from varying concentrations of water and iodixanol (C 35 H 44 I 6 N 6 O 5 ). After 12 h of ultracentrifugation at 288,000g, the SWNTs formed distinctive colored bands, which could be isolated. The observed differ- ences in color represent SWNTs sorted by physical and electronic structure. Additionally, Yanagi et al. (2008) utilized sucrose as a gradient medium to separate metallic and semiconducting SWNTs by DGU (sucrose-DGU). Employing a low temperature and carefully controlling the concentration of surfactant, metallic and semiconducting SWNTs were isolated with purities of 69% and 95%, respectively (as de- termined by optical absorption spectra). 3. PROPERTIES OF CARBON NANOTUBES 3.1. PHYSICAL/MECHANICAL PROPERTIES The structure of carbon nanotubes (CNTs), which are composed completely of sp 2 -hybridized carbon atoms, endows these materials with unique properties. For example, CNTs have a tensile strength that is 100 times stronger than steel and 10 times stronger than Kevlar; at the same time, the elastic Young's modulus is 7 times that of steel (Sgobba and Guldi 2009). Moreover, CNTs are lighter than aluminum while having a surface area up to 1500 m 2 g -1 . Finally, CNTs remain thermally stable even at tempera- tures above 1000°C, and with a value of 6000 W m -1 K -1 , the thermal conductivity of a CNT is double