A puzzle that remains to be solved is the origin of the sequence dependence observed in a number of experiments. To answer this question, one first needs to understand how individual nucleobases interact with CNTs.

In a systematic theoretical study that was recently published in Nanotechnology it was found that the interaction strength between DNA/RNA nucleobases and CNTs is strongly dependent on two factors: the polarizability of the nucleobase molecule and the curvature of the CNT surface.

The dependence of the binding energy on the polarizability of the nucleobase is a strong indicator that the binding appears to be exclusively based on van der Waals interactions. Several other results of the study, such as the comparatively large equilibrium distance of 0.32 nm, the negligible amount of charge transfer between the binding partner and the lack of hybridization, as apparent from the electronic density of states, all point towards a van-der-Waals bound system.

The role played by the curvature of the CNT was made clear by comparing the results between the authors' latest study for a small-diameter CNT and the results obtained in an earlier investigation by the same authors for a flat graphene sheet, effectively standing in as a model for large-diameter CNTs. The relative distance between atoms on the carbon nanostructure surfaces and the biological molecule is the key here: the higher the curvature, the further the separation and hence the lower the nano-bio interaction. This explains why the binding energy of the nucleobase molecules was found to be significantly reduced for the high-curvature CNT as compared to the flat graphene sheet.