The distribution of nanoparticles in different aqueous environments is a fundamental problem underlying a number of processes, ranging from biomedical applications of nanoparticles to their effects on the environment, health, and safety. Here, we study distribution of carbon nanotubes (CNTs) in two immiscible aqueous phases formed by the addition of polyethylene glycol (PEG) and dextran. This well-defined model system exhibits a strikingly robust phenomenon: CNTs spontaneously partition between the PEG- and the dextran-rich phases according to nanotube's diameter and metallicity. Thermodynamic analysis suggests that this chirality-dependent partition is determined by nanotube's intrinsic hydrophobicity and reveals two distinct regimes in hydrophobicity-chirality relation: a small diameter (<1 nm) regime, where curvature effect makes larger diameter tubes more hydrophobic than small diameter ones, and a large diameter (>1.2 nm) regime, where nanotube's polarizability renders semiconducting tubes more hydrophobic than metallic ones. These findings reveal a general rule governing CNT behaviors in aqueous phase and provide an extremely simple way to achieve spatial separation of CNTs by their electronic structures.

Aqueous two-phase extraction is demonstrated to enable isolation of single semiconducting and metallic single-wall carbon nanotube species from a synthetic mixture. The separation is rapid and robust, with remarkable tunability via modification of the surfactant environment set for the separation.

Sorting single-wall carbon nanotubes (SWCNTs) of different chiralities is both scientifically interesting and technologically important. Recent studies have shown that polymer aqueous two-phase extraction is a very effective way to achieve nanotube sorting. However, works published to date have demonstrated only separation of surfactant-dispersed SWCNTs, and the mechanism of chirality-dependent SWCNT partition is not well understood. Here we report a systematic study of spontaneous partition of DNA-wrapped SWCNTs in several polymer aqueous two-phase systems. We show that partition of DNA-SWCNT hybrids in a given polymer two-phase system is strongly sequence-dependent and can be further modulated by salt and polymer additives. With the proper combination of DNA sequence, polymer two-phase system, and partition modulators, as many as 15 single-chirality nanotube species have been effectively purified from a synthetic mixture. As an attempt to provide a unified partition mechanism of SWCNTs dispersed by surfactants and by DNA, we present a qualitative analysis of solvation energy for SWCNT colloids in a polymer-modified aqueous phase. Our observation and analysis highlight the sensitive dependence of the hydration energy on the spatial distribution of hydrophilic functionalities.

The armchair carbon nanotube is an ideal system to study fundamental physics in one-dimensional metals and potentially a superb material for applications such as electrical power transmission. Synthesis and purification efforts to date have failed to produce a homogeneous population of such a material. Here we report evolutionary strategies to find DNA sequences for the recognition and subsequent purification of (6,6) and (7,7) armchair species from synthetic mixtures. The new sequences were derived by single-point scanning mutation and sequence motif variation of previously identified ones for semiconducting tubes. Optical absorption spectroscopy of the purified armchair tubes revealed well-resolved first- and second-order electronic transitions accompanied by prominent sideband features that have neither been predicted nor observed previously. Resonance Raman spectroscopy showed a single Lorentzian peak for the in-plane carbon-carbon stretching mode (G band) of the armchair tubes, repudiating the common practice of using such a line shape to infer the absence of metallic species. Our work demonstrates the exquisite sensitivity of DNA to nanotube metallicity and makes the long-anticipated pure armchair tubes available as seeds for their mass amplification.

Single-stranded DNA wrap helically around individual single-walled carbon nanotubes to form DNA/CNT hybrids, which are both stable and dispersible in aqueous solution. Subjected to ion-exchange chromatography, a hybrid elutes at an ionic strength that depends on the electronic character and diameter of the core nanotube, thus providing a mechanism for separating nanotubes by chirality. We present a theoretical model for this separation process that explains all the salient features observed experimentally to date, and provides accurate predictions for critical elution salt concentration. The competition between adsorption on the stationary phase and counterion condensation in the mobile phase is characterized by estimating the difference in free energy between the two states of the hybrid. Parametric study of the DNA wrapping geometry, SWNT dielectric properties, hybrid length, and diameter indicate that the elution is most sensitive to the hybrid's effective charge density, primarily governed by the DNA helical pitch. The model correctly predicts hybrids with metallic nanotubes are weaker binding than hybrids with semiconducting nanotubes and larger diameter nanotubes are eluted at later times.