Application of Chemical Force Microscopy for Finding Selective Functional Groups for Discriminating Different Electronic Type Single-Walled Carbon Nanotubes

We recently reported a noncovalent chemistry bead-based method that can sort single-walled carbon nanotubes (SWNTs) to ultrahigh purity. The method is based on the higher affinity of functionalized agarose beads for M(etallic) carbon nanotubes in acidic pH versus S(emiconducting) SWNTs based on the selective charge reversal of metallic carbon nanotubes. However, fundamental understanding of the relative selective affinity of various functional groups for certain electronic type nanotubes remains somewhat lacking. We show that the chemical force microscopy (CFM) technique can measure the subtle differences between various common functional groups (such as −NH 2, −OH, −SO 3 H/–SO 3 – Na +, −NO 2, etc .) and the different electronic types of SWNTs. We show that the amine-functionalized alkane has significantly higher interaction forces with S SWNTs. On the other hand, SO 3 – Na + - and NO 2 -functionalized naphthalene show significantly higher interaction forces with M SWNTs compared with S SWNTs; the −SO 3 H substitution on an alkane, however, shows no significant selectivity for any single electronic type of SWNTs. We discovered two novel molecules (sodium 4-amino-1-naphthalenesulfonate and 1-amino-4-nitronaphthalene) that are able to have significantly higher interaction force with M SWNTs and provide complete electronic type discrimination over the entire nanotube diameter range. We also show that the CFM platform can be applied to distinguish between M or S tubes from an as-grown SWNTs mixture in air. The platform can also be applied for studying the effect of solvent (water) on the selectivity. It is anticipated that our new CFM method using functionalized tips will be able to accelerate the development of noncovalent separation strategies for improved nanotube electronic type separation.