Sonication-mediated association of DNA with single-walled carbon nanotubes: Characterization and potential biological applications

The conjugation of DNA to single-walled carbon nanotubes (SWCNTs) is representative of an important link between biology and nanotechnology. Sonication-mediated conjugation has evolved as an effective method for water-soluble dispersion of SWCNTs by single-stranded DNA. Here characterization of the DNA:SWCNT conjugates will be discussed using results obtained from various spectroscopic and microscopy methods. A systematic study was carried out by altering various parameters in the sonication-mediated preparation allowing importance of SWCNT preparation methods, sonication time, and chemical treatment to the SWCNTs to be determined. There have been several reports of a helical wrapping of the DNA about the SWCNTs; however there is little experimental evidence that elucidated the chemical nature of the interaction between DNA and the SWCNTs. The key factor in this method of dispersion is sonication, which is known to create radicals in aqueous solutions. Sonication also damages the sidewalls of SWCNTs as shown by transmission electron microscopy. In order to study the importance of radical generation by sonication, the free radical inhibitors Trolox (a vitamin E derivative) and ascorbic acid (vitamin C) were added to the reaction mixture. In the presence of these free radical inhibitors dispersion of SWCNTs by DNA was prevented. These results suggest that generation of radical intermediates plays an important role in sonication-mediated dispersion of DNA:SWCNTs. It is hypothesized the DNA becomes covalently anchored to the SWCNT through a radical mechanism at one or few point along the DNA. This covalent anchor allows the DNA to unwrap around the SWCNT, yet remain attached. Once unwrapped, the DNA is available for hybridization to a complementary DNA. This attachment can be exploited to build upon DNA:SWCNTs creating a platform for the attachment of multiple moieties creating a multivalent carrier. A DNA:SWCNT was hybridized to two DNA strands of different sequences each complementary to the attached DNA. Each hybridized DNA contained a different fluorescent tag. To study the ability of DNA:SWCNTs to act as multivalent cellular carriers, MCF-7 cells were treated with the dual-labeled hybrids. Using fluorescence microscopy, both fluorophores appear to be co-localized within the cells.