98 CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 2: NANOSTRUCTURED MATERIALS 3.3.3. Raman Spectroscopy The Raman spectra for SWNTs consist of two types of peaks (Amer 2007). The first group occurs at <300 cm -1 ; this area represents the radial breathing mode of the SWNT. The second class corresponds to the G band in the tangential surface modes, of which there exist two orders. The first order lies between 1350 and 1580 cm -1 , and the second order is observable around 2700 cm -1 . Based on the position of the Raman bands, one can glean information regarding both the chemical structure of the nanomaterial and information regarding the interactions between the SWNTs and their local environ- ments. The latter aspect can be exploited to create a new generation of Raman-based nanosensors that employ SWNTs. 4. CHEMICAL MODIFICATION AND FUNCTIONALIZATION OF CARBON NANOTUBES 4.1. INTRODUCTION In general, the doping of SWNTs, like that of other graphene-based species, is amphoteric (Sgobba and Guldi 2009). Therefore, based on the chemical nature of the dopant or the electrochemical potential