Adsorption of single-ringed N- and S-heterocyclic aromatics on carbon nanotubes

Adsorption of single-ringed N- and S-heterocyclic aromatics on single-walled carbon nanotubes (SWCNTs) was examined to explore the potential of using carbon nanotubes (CNTs) as drug carriers and environmental adsorbents. Adsorbates included pyrimidine, 2-aminopyrimidine, 4,6-diaminopyrimidine, thiophene, benzene and aniline. Adsorbents included pristine SWCNTs, oxidized SWCNTs, and nonporous graphite. Adsorption of N- and S-heterocyclic aromatics was significantly enhanced by non-hydrophobic interactions. Particularly, the –NH2-substituted compounds exhibited much stronger (up to 2 orders of magnitude) adsorption affinities to oxidized SWCNTs than benzene, even though they are much less hydrophobic. The π–π coupling or electron donor–acceptor (EDA) interactions are likely adsorption-enhancement mechanisms for all six compounds. The lone-pair electrons of the N heteroatoms and the –NH2 group can enable n–π EDA interactions with SWCNT surfaces. Lewis acid–base interactions are another significant adsorption-enhancement mechanism for the –NH2-substituted compounds (and possibly for pyrimidine) on SWCNTs. For the N-heterocyclic aromatics, adsorption affinity is highly dependent on the O-functionality of the SWCNT surface and on solution pH, due to the speciation reactions of both adsorbates and SWCNT surface O-functional groups, indicating that selective adsorption of N-heterocyclic aromatics is possible by combining the surface functionality of CNTs and solution chemistry.