No abstract

Single-walled carbon-nanotubes (SWNTs) exhibit unique electronic, optical, and mechanical properties; however, a narrow availability of chirally pure (single electronic structure) material can limit effective integration within novel devices and schemes. This work focuses on understanding, modeling, and advancing methodology used to generate preparative quantities of single-chirality SWNT: the iterative adsorption/desorption of SWNT from aqueous surfactant suspensions to/from hydrogel microspheres. Commercially available hydrogel microspheres (Sephacryl S200) were sorted by radius and exposed to SWNT, affording a direct correlation between microsphere surface area and quantity of SWNT adsorbed using differential absorbance spectroscopy. This relationship elucidates a SWNT/gel purification scheme interaction mechanism exclusively involving the gel surface. High-concentration surfactant was used to elute SWNT from the gel with desorption efficiencies dependent on both SWNT chirality and hydrogel microsphere radius, ranging from 25–45%. A thermodynamic model for SWNT desorption that accounts for hydrogel microsphere curvature effects is presented and suggests that (when compared with experimental data) SWNT with greater than ∼41% of their length adsorbed to a hydrogel surface bind irreversibly, while others are desorbed in the presence of high-concentration surfactant. These findings inspired the generation and application of mechanically fractured hydrogel microspheres (exhibiting greater surface area) for use as SWNT purification media in per-iteration quantities far less than traditional gels. A 10-iteration SWNT purification procedure demonstrated a marked improvement in process efficiency, as mechanically fractured gels afford a 10-fold reduction in gel-media use (the major expense of the process) while yielding equivalent SWNT purification.

Despite the commercial availability of many different hydrogel formulations, the effective gel-based purification of single-walled carbon nanotubes (SWNT) remains exclusive to the gel Sephacryl S-200. In this study, 12 commercially available gels and two custom-synthesized gels were investigated for their ability to effectively purify SWNT, as determined through quantification of SWNT adsorption, elution, chiral selectivity, and overall process efficiency. The ability of each gel to separate SWNT was found to correlate with physiochemical properties, such as hydrogel pore size, the presence of ionic ligands, and both polysaccharide backbone and cross-linker compositions. While Sephacryl S-200 demonstrated superior separation efficiency and chiral selectivity among the gels studied, Superose 6 was found to adsorb more SWNT than Sephacryl S-200 per cm2 of the gel surface area and exhibited a unique preference for the (7,3) and (7,5) SWNT chiralities, in contrast to the established selectivity of Sephacryl S-200 for the (6,5) chirality. Collectively, this work both identifies gels that exhibit unique SWNT chiral selectivity and provides insights into the rational design of gels tailored for SWNT purification.

Sephacryl-based separation of single-walled carbon nanotubes (SWCNTs) has afforded the enrichment of samples according to both type (metal/semiconducting) and band structure (chirality). However, Sephacryl-based procedures suffer from low process efficiency, in that some of the purified SWCNTs remain irreversibly bound on the gel. This work focuses on the elucidation of mechanistic aspects relating to the irreversible retention of SWCNT by Sephacryl S200 and identification of strategies to reduce material loss. By probing system parameters such as SWCNT or sodium dodecyl sulfate (SDS) concentration during adsorption, loading ratio (number of SWCNTs vs. number of gel binding sites), and the effect of kosmotropic additives on process elution efficiency, this work identifies inhomogeneous features belonging to SWCNTs as the primary factor which promotes their irreversible retention. Process elution efficiency is found to be negatively correlated with increased SDS surfactant concentration and increased SWNT loading, and methods are identified to mitigate material losses in Sephacryl-based SWCNT separation schemes. Application of NaF, a strong kosmotropic agent, to elute SWCNTs from Sephacryl is found to increase process elution efficiency from 54% to 88% compared to SDS surfactant alone. Collectively, this work both improves upon and provides valuable insight into the purification of SWNT with Sephacryl.