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 −NH2, −OH, −SO3H/–SO3
–Na+, −NO2, 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, SO3
–Na+- and NO2-functionalized naphthalene show significantly
higher interaction forces with M SWNTs compared with S SWNTs; the
−SO3H 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.