In contrast to DA, riboflavin quenches SWCNT fluorescence . Fluorescence of pristine SWCNTs decreased by 76 % in response to riboflavin (100 µM) and G d high-SWCNTs fluorescence decreased by 93 %. Hence, the riboflavin response was stronger for G d -SWCNTs than for pristine SWCNTs, which was opposite as for dopamine. To describe the fluorescence modulation in DNA-SWCNTs, many models have been discussed . One prominent model assumes a change in the DNA conformation, induced by the formation of bonds between the hydroxy groups of dopamine and the phosphate backbone of the DNA . A side effect of the covalently bound DNA in G d -SWCNTs could be a smaller degree of conformational freedom of the corona phase. Flexibility of the corona phase is expected to correlate inversely with defect density. The limited ability of the corona phase to change its conformation could therefore decrease the sensor response. The fact that the sensor response of G d -SWCNTs depends on defect density, with the lowest response occurring at the greatest defect density (Figure ) supports this hypothesis for dopamine. Riboflavin, on the other hand, quenched G d -SWCNTs more efficiently, but the difference in sensor responses of pristine and G d -SWCNTs was smaller. These different patterns suggest different sensing mechanisms. One explanation could be that for riboflavin the redox properties and the related optical properties play a role .