  Even though ultrasonication is considered to be an effective method to
disperse carbon nanotubes (CNTs), its devastating effects on the nanotubes are
often neglected. Here, even mild ultrasonication is found to rapidly extract
the inner single-wall CNTs (SWCNTs) from the outer shells of the double-wall
CNTs (DWCNTs). As-synthesized DWCNTs are gently solubilized in a surfactant
solution, strictly avoiding any ultrasonication, followed by two consecutive
density gradient ultracentrifugation (DGU) steps to obtain a purified colloidal
solution of isolated DWCNTs. The latter is carefully selected based on in situ
resonant Raman (RRS) and fluorescence (PL) spectroscopy, measured as a function
of depth directly in the ultracentrifuge tube after DGU. These purified DWCNTs
are ultrasonicated in successive time steps while intermittently probing the
sample via RRS and PL spectroscopy. These results unravel the very fast
increasing yet saturating extraction mechanism that leads to the formation of
fluorescing SWCNTs. A statistical high-resolution transmission electron
microscopy study confirms the drastic increase in SWCNTs after ultrasonication,
and evidences that ultrasonication forms SWCNTs from both the inner and outer
shells of the DWCNTs. This study demonstrates how easily ultrasonication
extracts SWCNTs from individually solubilized DWCNTs, unavoidably complicating
any further spectroscopic studies on DWCNTs severely.
