However, some analytes with positively charged amine groups could exhibit relatively stable binding through interactions with ssDNA phosphate groups, but elicited small (e.g., 5+) or no (e.g., YY+) optical response. Conversely, some analytes without charged amine groups (e.g., C) were able to generate a strong optical response. This indicated that the perturbation cross section of an analyte is not a simple function of its ability to bind to ssDNA-SWCNT conjugates through electrostatic interactions. Instead, the characteristics of substituents on the aryl group plays an important role, consistent with experimental observation that substituents significantly influence optical response. Further analysis revealed that proximity of polar substituents to nanotube surfaces correlated positively with optical response. This indicates that molecules that have a high perturbation cross section (e.g., DA + , F + ) not only stably bind ssDNA-SWCNT conjugates but exhibit higher density of polar substituents close to the nanotube surface (Fig. ).