Single stranded DNA (ssDNA) and single walled carbon nanotubes (SWNT) interact in solution, under sonication, to form a charged hybrid structure, DNA/SWNT[1
]. The aromatic nucleobases are believed to π-stack with the nanotube’s graphene side walls[1
]. By acting as a scaffold, single walled nanotubes confine and orient DNA molecules, thus opening the door to many applications in nano- and biotechnology[6
]. We have reported that each of the four nucleobases (Guanine, Cytosine, Adenine, and Thymine) orient in distinct ways with respect to the nanotube’s long axis[3
]. Both AFM images and spectroscopic studies of DNA/SWNTs have suggested that the DNA spontaneously wraps itself around nanotubes[2
]. But absent evidence that all of the bases in a DNA molecule are associated with the nanotube’s surface, one must consider the possibility that not all of the DNA bases are π-stacked with the nanotube’s graphene side walls or that DNA may not always assume a simple helical conformation around the nanotube[14
Synthetic single stranded oligonucleotides (homopolymers or simple sequences typically of lengths <100 bases) have been widely used in forming DNA/SWNTs. It has been reported that various ssDNA polymers of alternating sequences facilitate the separation of nanotubes by electronic property[1
]. This suggests that the nanotube’s electronic state and the base composition of the DNA determine the properties of the resulting DNA-nanotube hybrid[4
]. To date, the influence of individual nucleobases over the molecular interaction between DNA and a SWNT remains to be quantified.
Many applications involving DNA/SWNT will require controlling both the assembly and disassembly of the hybrid. For example, if DNA is used to sort nanotubes by diameter, chirality, or electronic behavior, the DNA must ultimately be removed to recover clean, sorted nanotubes[11
]. This is especially critical if the nanotubes are to be assembled into electronic devices. Furthermore, the effectiveness of DNA/SWNT hybrids as vehicles for gene or drug delivery[7
] hinges on the nanotubes’ ability to release their cargo within the cell. Thus, understanding the binding and unbinding of the DNA bases with SWNTs, along with the factors that contribute to the stability of the ensemble, will be important in choosing the correct base composition, length, and solution conditions for the many potential applications of these hybrids.
A few experimental measurements have been made to characterize the factors that determine the association and dissociation of DNA to SWNTs. Some half-life times for flocculation of DNA/SWNT held at 90°C were reported[26
], as were the base dependent efficiencies of different DNAs to disperse nanotubes during ultrasonication[1
]. Not only are these results qualitative because the environment that the DNA and nanotubes experience during sonication cannot be precisely duplicated, but we suspected that dispersability trends may not necessarily correlate with quantitative measures of binding strength. Complementary base-pairing between ssDNA in solution and ssDNA bound to a nanotube[28
] has been examined, but a simple direct assay of DNA-SWNT hybrid stability and its dependence on the specific nucleobases of the DNA is still lacking.
We present a rapid analytical method to quantify the association and dissociation of ssDNA to single walled nanotubes. By probing the specific base dissociation temperatures of homo-oligonucleotide/SWNT hybrids, the thermodynamics of processes that govern the stability of DNA/SWNT in solution is elucidated. Furthermore, we demonstrate control over the hybrid assembly and disassembly by tuning specific solution conditions such as ionic strength and free DNA concentration.