A highlight of the PTC project is our success in purification of well-defined SWCNT populations using liquid phase separation methods. Although we use multiple techniques and methods, ion exchange chromatography, rate-zonal and isopycnic ultracentrifugation, size exclusion chromatography, our primary current method is aqueous two-phase extraction (ATPE). This technique was invented within the PTC project for the separation of SWCNTs, and with it we can rapidly sort many SWCNT populations by metallicity, diameter, species (i.e. (n,m) index), and even enantiomeric handedness (i.e. right-handed or left handed twist). Licensing opportunities for this technology can be discussed through the NIST Technology Transfer Office.
ATPE utilizes the phenomena of spontaneous phase separation above a critical concentration line for pairs of water soluble polymers (or polymer-salt mixtures) to generate spatial separation of dispersed solutes with differing chemical affinities for the two split phases. Advantages of ATPE are that it is rapid, scalable, utilizes dispersants that enable high concentration processing, and is exquisitely tunable. There are two primary versions of the ATPE method. In one version multiple types of surfactants are competed against each other to control the partitioning of the SWCNTs within the two-phase system. In the second version, short oligomers of specified sequence DNA are used to disperse the SWCNTs, with certain combinations of DNA and SWCNT (n,m) structure forming “recognition” hybrids with significantly distinct solvation energies compared to all other hybrids.
Separation of SWCNTs by controlling the nature and amounts of competing surfactants allows for the isolation of metallic from semiconducting nanotubes (including double-wall nanotubes) as well as fractionation by nanotube diameter. Results of both types of separations are shown in Figure 2. Fractionation by these vectors are often critical for technology development. For instance, CNT digital logic applications require high purity populations of solely semiconducting SWCNTs, and sensing applications benefit from spectral response dominated by a single component of known response.
Separation of the SWCNT structures via DNA-ATPE enables direct isolation of specific (n,m) structures, often in a single step, specified even to which enantiomer of the structure (left or right-handed) is purified. This method typically results in extremely pure materials in which contaminant structures are absent, providing high quality samples for intrinsic characterization. The interface of DNA with the SWCNT structure also provides benefits for various technical applications, and is interesting for future systematic control of the nanotube-solution interface.
The SWCNT populations produced in this project have and are being utilized in multiple developmental technologies and for characterization of intrinsic material properties beyond the capacity to list in this space. See the publications list below for details. Efforts in these areas and involving other separation and characterization methods are ongoing.