Optical absorption spectra of 2 μg/mL SWCNT dispersions in NMP (a) after 20 minutes sonication only and (b) after 20 minutes sonication followed by centrifugation. The as-prepared spectra (solid curves) were measured immediately after the dispersions were prepared and the after 35 days spectra (dashed-dotted curves) were measured for the same dispersion 35 days after it was prepared with (a) sonication only or (b) sonication and centrifugation. The absorbance of the dispersions was measured with respect to the blank solvent in absorbance units (a.u.).