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

Single-walled carbon nanotubes (SWCNTs) have unique photophysical properties and promise many novel applications. Their functionalization is crucial, but the organic phase around SWCNTs is poorly understood. Noncovalent functionalization with single-stranded DNA (ssDNA) is one of the most used approaches to solubilize SWCNTs in water, and variation of ssDNA sequences leads to major advances in separation of SWCNT chiralities and SWCNT-based sensors. However, the exact number of adsorbed ssDNA molecules on ssDNA/SWCNT complexes and consequently the surface coverage are not precisely known. Here, we determine the number of adsorbed/bound ssDNA molecules per SWCNT for different ssDNA sequences. For this purpose, we directly quantify free and bound/adsorbed ssDNA and the concentration of SWCNTs using an approach based on filtration, absorption spectroscopy, and atomic force microscopy. We found that the number of adsorbed ssDNA molecules on 600 nm long (6,5)-SWCNTs varies between similar to 860 for (GT)(5) and similar to 130 for (A)(30). Interestingly, there are large differences in the average SWCNT segment lengths one ssDNA molecule occupies. It varies between sequences of the same length from similar to 2.1 nm (T)(30) and similar to 3.2 nm (C)(30) to similar to 4.6 nm (A)(30). The sequence (GT)(15) occupied an average SWCNT segment of similar to 2.3 nm. In contrast, for (GT)(x) repeats, we found a linear decrease of the number of adsorbed ssDNA molecules with sequence length x. We also correlated surface occupation with the ssDNA/SWCNT near-infrared (nIR) fluorescence responses to analytes such as dopamine, H2O2, riboflavin and pH changes. In most cases, the nIR fluorescence responses did not correlate with the number of adsorbed ssDNA molecules, indicating that the exact structure of the crowded corona around the SWCNT is crucial for photophysical changes. The occupied SWCNT segment length per ssDNA molecule is also an important parameter for molecular dynamics (MD) simulations. (GT)(15) ssDNA adsorbed and stacked on top of each, when using the determined parameters, in contrast to simulations with an excess SWCNT surface. This result highlights the importance of knowing the number of adsorbed ssDNA molecules per SWCNT. In summary, we present a versatile and direct assay to determine the amount of ssDNA molecules adsorbed on SWCNTs, report those numbers, and evaluate how they are related to photophysical properties.