Highly-Selective Harvesting of (6,4) SWCNTs Using the Aqueous Two-Phase Extraction Method and Nonionic Surfactants.

Monochiral single-walled carbon nanotubes (SWCNTs) are indispensable for advancing the technology readiness level of nanocarbon-based concepts. In recent times, many separation techniques have been developed to obtain specific SWCNTs from raw unsorted materials to catalyze the development in this area. This work presents how the aqueous two-phase extraction (ATPE) method can be enhanced for the straightforward isolation of (6,4) SWCNTs in one step. Introducing nonionic surfactant into the typically employed mixture of anionic surfactants, which drive the partitioning, is essential to increasing the ATPE system's resolution. A thorough analysis of the parameter space by experiments and modeling reveals the underlying interactions between SWCNTs, surfactants, and phase-forming agents, which drive the partitioning. Based on new insight gained on this front, a separation mechanism is proposed. Notably, the developed method is highly robust, which is proven by isolating (6,4) SWCNTs from several raw SWCNT materials, including SWCNT waste generated over the years in the laboratory.

Molecular dynamics simulations reveal single-stranded DNA (ssDNA) forms ordered structures upon adsorbing onto single-walled carbon nanotubes (SWCNT).

Replica exchange molecular dynamics were used to observe the adsorption of single-stranded DNA (ssDNA) onto the surface of single-walled carbon nanotubes (SWCNTs). The assembly of these systems has garnered interest as a method by which SWCNTs can be separated based on chirality. While the exact mechanism of separation is yet unknown, it is hypothesized that the structure of the ssDNA layer pays an important role. Characterization of such an adsorbed layer has been a matter of recent work with the focus being on atomic level detail or such as base-stacking and hydrogen bonding. In this manuscript, we detail a new observation of ssDNA organization and demonstrate how it can be used to infer additional information about the way in which such biopolymers wrap around the cylindrical SWCNT.

Interaction of DNA-Complexed Boron Nitride Nanotubes and Cosolvents Impacts Dispersion and Length Characteristics.

Processing boron nitride nanotubes (BNNTs) for applications ranging from nanomedicine to electronics generally requires dispersions of nanotubes that are stable in various compounds and solvents. We show that alcohol/water cosolvents, particularly isopropyl alcohol (IPA), are essential for the complexation of BNNTs with DNA under mild bath sonication. The resulting DNA-wrapped BNNT complexes are highly stable during purification and solvent exchange from cosolvents to water, providing potential for the versatile liquid-phase processing of BNNTs. Via molecular dynamics simulations, we demonstrate that IPA assists in the solvation of BNNTs due to its pseudosurfactant nature by verifying that water is replaced in the solvation layer as IPA is added. We quantify the solvation free energy of BNNTs in various IPA/water mixtures and observe a nonmonotonic trend, highlighting the importance of utilizing solvent-nanotube interactions in nanomaterial dispersions. Additionally, we show that nanotube lengths can be characterized by rheology measurements via determining the viscosity of dilute dispersions of DNA-BNNTs. This represents the bulk sample property in the liquid state, as compared to conventional imaging techniques that require surface deposition and drying. Our results also demonstrate that BNNT dispersions exhibit the rheological behavior of dilute Brownian rigid rods, which can be further exploited for the controlled processing and property enhancement of BNNT-enabled assemblies such as films and fibers.

Solvation Free Energy of Self-Assembled Complexes: Using Molecular Dynamics to Understand the Separation of ssDNA-Wrapped Single-Walled Carbon Nanotubes.

Molecular dynamics simulations were used to characterize the self-assembly of single-stranded DNA (ssDNA) on a (6,5) single-walled carbon nanotube (SWCNT) in aqueous solution for the purpose of gaining an improved theoretical understanding of separation strategies for SWCNTs using ssDNA as a dispersant. Four separate ssDNA sequences, ((TAT)4, TTA(TAT)2ATT, C12, (GTC)2GT), at various levels of loading, were chosen for study based on published experimental work showing selective extraction of particular SWCNT species based on the ssDNA dispersant sequence. We develop a unique workflow based on free energy perturbation (FEP) and use this to determine the relative solubility of these complexes due to the adsorption of the ssDNA on the SWCNT surface, and hence, rank the favorability of separations observed during experiments. Results qualitatively agree with experiments and indicate that the nucleobase sequence of the adsorbed ssDNA greatly affects the free energy of complex solvation which ultimately drives SWCNT separation. Further, to elucidate the underlying physics governing the ssDNA-SWCNT solubility rankings, we also present calculations for four structural characteristics of ssDNA adsorption. We demonstrate that a unique type of intra-strand hydrogen bonding is the most important factor contributing to the stability of the ssDNA-SWCNT complexes and show how these adsorption characteristics are coupled with the FEP results.

Solvation of Carbon Nanoparticles in Water/Alcohol Mixtures: Using Molecular Simulation To Probe Energetics, Structure, and Dynamics.

Molecular dynamics simulations were used to examine the solvation behavior of buckminsterfullerene and single-walled carbon nanotubes (SWCNT) in a range of water/alcohol solvent compositions at 1 atm and 300 K. Results indicate that the alcohols assume the role of pseudosurfactants by shielding the nanotube from the more unfavorable interactions with polar water molecules. This is evident in both the free energies of transfer (ΔΔGwater→xOH = -68.1 kJ/mol and -86.5 kJ/mol for C60 in methanol and ethanol; ΔΔGwater→xOH = -345.6 kJ/mol and -421.2 kJ/mol for the (6,5)-SWCNT in methanol and ethanol) and the composition of the solvation shell at intermediate alcohol concentrations. Additionally, we have observed the retardation of both the translational and rotational dynamics of molecules near the nanoparticle surface through use of time correlation functions. A 3-fold increase in the residence times of the alcohol molecules within the solvation shells at low concentrations further reveals their surfactant-like behavior. Such interactions are important when considering the complex molecular environment present in many schemes used for nanoparticle purification techniques.