Interactions of Secondary DNA and Initial DNA on Single-Walled Carbon Nanotube Surfaces Studied by Photoluminescence, Atomic Force Microscopy, and Electrophoresis

We examined the interactions of initial single-stranded DNA (ssDNA) and secondary ssDNA molecules on single-walled carbon nanotubes (SWNTs). Thymine 30-mers (T30) and 30-mers from a partial sequence of φx174 DNA (φ30) were used to prepare the DNA-SWNT hybrids. First, the hybrids were annealed at various temperatures without secondary DNA to evaluate the stability of the hybrids. As a result, aggregates of SWNTs were formed in the T30-SWNT hybrids, even at 54°C, although the φ30-SWNT hybrids were stable up to 84°C. Second, we added secondary DNA molecules during the annealing procedure. We reacted adenine 30-mers (A30) with the T30-SWNT hybrids and characterized the samples by combining agarose gel electrophoresis with/without ethidium bromide and atomic force microscopy (AFM) as well as near-infrared photoluminescence (PL) spectroscopy. Cross-links appeared to form among the SWNTs because of nonspecific hybridization of T30 and A30. PL measurements revealed clear shifts in the PL emission wavelength of SWNTs. However, when complementary φ30 DNA (cφ30) was reacted with φ30-SWNT hybrids, there was no significant difference in the PL spectra after the reaction, although electrophoresis suggested the hybridization of the cφ30 and φ30 DNA molecules. Our results suggest that the hybridization manner of DNA molecules with unnatural sequences greatly differs from that of natural DNA molecules.