Quantification of the Number of Adsorbed DNA Molecules on Single-Walled Carbon Nanotubes

Abstract

Single-walled carbon nanotubes (SWCNTs) have unique photophysical properties and promise many novel applications. Noncovalent functionalization with single-stranded DNA (ssDNA) is one of the most used approaches and variation of ssDNA sequences lead to major advances in separation of SWCNT chiralities and SWCNT-based sensors. However, the exact number of adsorbed ssDNA molecules on ssDNA/SWCNT complexes vary in literature and are overall not precisely known. Here, we determine the number of adsorbed ssDNA molecules per SWCNT for different ssDNA sequences. For this purpose, we directly quantified free and adsorbed ssDNA and the concentration of SWCNTs using an approach based on filtration, absorption spectroscopy, and atomic force microscopy. The number of adsorbed ssDNA molecules on 600 nm long (6,5)-SWCNTs varied between ∼860 for (GT)5 and ∼130 for (A)30. The sequence (GT)15 occupied an average SWCNT segment of ∼2.3 nm, while the SWCNT segment length of (GT)x repeats increased linear with sequence length x. Molecular dynamics simulations with experimentally determined values as parameters showed that (GT)15 ssDNA stacked on top of each other on the SWCNTs in contrast to simulations with an excess SWCNT surface. By knowing the number of adsorbed ssDNA molecules per SWCNT it is further more possible to address ratio-specific DNA conjugation approaches. In summary, these results provide novel quantitative insights into the complex structure of DNA/SWCNT hybrids.