pH-controlled carbon nanotube aggregation/dispersion based on intermolecular i-motif DNA formation.

In this work, we report a new strategy to manipulate the aggregation and dispersion of carbon nanotube in solution via formation of intermolecular i-motif (four-stranded C-quadruplex) structures in a pH dependent manner. Firstly, single-stranded (ss) DNAs containing two stretches of cytosine (C)-rich domains are covalently linked to carbon nanotubes. At pH 8.0, DNAs are at random coil state, which enhance the dispersion of multi-wall carbon nanotubes (MWNTs) in water; after changing pH to 5.0, the intermolecular i-motif structures formed by the C-rich ssDNAs on neighboring carbon nanotube could drive the MWNTs aggregate. This process is reversible and the transition process has been verified by circular dichroism (CD) spectroscopy, gel electrophoresis and transmission electron microscopy (TEM). Considering the mechanical properties of carbon nanotube, this finding will benefit many application research fields, such as artificial muscle, functional nano-devices and so on.