Single-stranded DNA (ssDNA) shows remarkable selectivity when coated onto single-walled carbon nanotubes. Although many previous reports have focused on the applications of these structures, very few reports have analyzed their structure and composition.
Study: Compositional Analysis of ssDNA-Coated Single-Wall Carbon Nanotubes through UV Absorption Spectroscopy. Image Credit Oselote/Shutterstock.com
An article published in Nano Letters analyzed ssDNA-coated single-walled carbon nanotubes via ultraviolet (UV) absorption and measured the total carbon content.
The results revealed average concentrations of ssDNA and single-walled carbon nanotubes in the samples that helped deduce the average mid-UV absorptivity of single-wall carbon nanotubes for three different samples of small-diameter carbon nanotubes produced via the CoMoCAT and HiPco growth processes.
The measured absorptivity values quantified the ssDNA-to-single-walled carbon nanotube mass ratio, suggesting the average number of carbon atoms (of carbon nanotubes) suspended on single ssDNA strands of T30G or T15GT15 oligos.
Furthermore, spectrophotometry was employed to measure the absolute concentrations of similar single-walled carbon nanotube samples in aqueous sodium dodecyl sulfate (SDS), a typical surfactant that is transparent in the mid-UV.
Replica exchange molecular dynamics (REMD) simulations of one ssDNA strand were compared with the experimental parameters. Results revealed a concurrence between the computed and experimentally measured single-walled carbon nanotube carbon atoms per strand of ssDNA.
What Are Single-Walled Carbon Nanotubes?
Single-walled carbon nanotubes are one-dimensional (1D) carbon materials where graphene sheets roll up to form hollow tubes with atomic-thickness walls. Owing to its chemical structure and dimensional constraints, this material exhibits exceptional mechanical, electrical, thermal, and optical properties.
The electrical properties of carbon nanotubes depend on their lattice orientation which is given by two parameters (n,m). The disaggregation of single-walled carbon nanotubes is a vital step in engineering and scientific applications. Hence, raw single-walled carbon nanotubes are coated with polymers (for example, ssDNA) and surfactants and dispersed into liquid suspensions for their facile disaggregation.
Solute concentration plays a critical role when working with a solution or suspension. However, determining this concentration is difficult for most samples of single-walled carbon nanotubes owing to their inhomogeneous compositions. Although optical absorption measurements are inexpensive, quick, and non-destructive techniques for determining the solute concentration, knowledge of the absolute absorptivity values is required.
The (n,m)-specific values for near-infrared transitions are available for a wide range of semiconducting species. These values cannot be used to determine the concentration of single-walled carbon nanotubes because of the difficulty in estimating the metallic single-walled carbon nanotubes, the incomplete collection of known semiconducting absorptivity, and crowded near-infrared absorption spectra.
As all (n,m) species exhibit robust absorption in the UV range, UV spectroscopy may provide a more suitable spectral area for calculating the overall concentration of single-walled carbon nanotubes.
Analysis of ssDNA-Coated Single-Wall Carbon Nanotubes
The present study demonstrated a convenient method to measure UV extinction coefficients for single-walled carbon nanotube samples with small diameters grown via CoMoCAT and HiPco growth processes. The overall carbon concentration of single-walled carbon nanotubes and ssDNA in the dialyzed sample was measured by employing a total carbon analyzer.
Moreover, adding SDS to the sample enabled the removal of ssDNA from the single-walled carbon nanotube and facilitated the measurement of pure ssDNA concentration via UV absorption spectroscopy, which resulted in an unperturbed ssDNA spectrum.
Furthermore, the difference in total carbon concentration and ssDNA helped compute the absolute concentration of single-walled carbon nanotubes, which could be applied to determine the UV extinction coefficients of single-walled carbon nanotubes.
A spectrophotometric assay of single-walled carbon nanotubes was conducted based on the obtained UV extinction coefficients, which could be useful in various applications. Additionally, the mass ratio quantification of surface-adsorbed ssDNA to single-walled carbon nanotubes indicated the number of carbon atoms covered by one ssDNA strand.
The experimental validation of the REMD simulations of ssDNA structures on single-walled carbon nanotubes depended on the total carbon content. Thus, the experimental results supported the simulations and provided a deeper understanding of the structures of ssDNA oligos covering single-walled carbon nanotubes.
Conclusion
In conclusion, the composition of single-walled carbon nanotubes distributed in ssDNA oligos was quantified by combining measurements of total carbon content and UV absorption spectroscopy. The analysis provided the absolute UV extinction coefficient values for three separate single-walled carbon nanotube sample batches.
The resulting coefficient values can be used to perform UV absorption experiments to determine the absolute concentrations of single-walled carbon nanotubes in aqueous SDS solutions. The present study also provided the relative masses of single-walled carbon nanotubes and ssDNA in samples that had free ssDNA removed.
Comparing the observed ratios with REMD predictions revealed the average number of carbon atoms per ssDNA strand, which helped determine the typical number of nanotube carbon atoms suspended per strand.
Reference
Alizadehmojarad, A. A., Bachilo, S. M., Weisman, R. B. (2022). Compositional Analysis of ssDNA-Coated Single-Wall Carbon Nanotubes through UV Absorption Spectroscopy. Nano Letters. Available at: https://pubs.acs.org/doi/10.1021/acs.nanolett.2c02850
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