DNA Sequence Mediates Apparent Length Distribution in Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) functionalized with short single-stranded DNA have been extensively studied within the last decade for biomedical applications due to the high dispersion efficiency and intrinsic biocompatibility of DNA as well as the photostable and tunable fluorescence of SWCNTs. Characterization of their physical properties, particularly their length distribution, is of great importance regarding their application as a bioengineered research tool and clinical diagnostic agent. Conventionally, atomic force microscopy (AFM) has been used to quantify the length of DNA–SWCNTs by depositing the hybrids onto an electrostatically charged flat surface. Here, we demonstrate that hybrids of DNA–SWCNTs with different oligomeric DNA sequences ((GT)6 and (GT)30) differentially deposit on the AFM substrate, resulting in significant inaccuracies in the reported length distributions of the parent solutions. Using a solution-based surfactant exchange technique, we placed both samples into a common surfactant wrapping and found identical SWCNT length distributions upon surface deposition. Additionally, by spin-coating the surfactant-wrapped SWCNTs onto a substrate, thus mitigating effects of electrostatic interactions, we found length distributions that did not depend on DNA sequence but were significantly longer than electrostatic deposition methods, illuminating the inherent bias of the surface deposition method. Quantifying the coverage of DNA molecules on each SWCNT through both absorbance spectroscopy and direct observation, we found that the density of DNA per SWCNT was significantly higher in short (GT)6–SWCNTs (length < 100 nm) compared to long (GT)6–SWCNTs (length > 100 nm). In contrast, we found no dependence of the DNA density on SWCNT length in (GT)30–SWCNT hybrids. Thus, we attribute differences in the observed length distributions of DNA–SWCNTs to variations in electrostatic repulsion induced by sequence-dependent DNA density.