Evaluation of Critical Parameters in the Separation of Single-Wall Carbon Nanotubes through Selective Adsorption onto Hydrogels

The electronic-type separation of single-wall carbon nanotubes (SWCNTs) by selective adsorption onto hydrogel surfaces has become a highly researched method. However, large variations in both SWCNT dispersion and separation protocols have been reported, making it difficult to elucidate whether changes to dispersion state or separation parameters are responsible for the observations reported in the literature. This study systematically evaluates the role that hydrogel type (dextran- or agarose-based) and SWCNT loading concentration have on separation quality, throughput, and reproducibility. For dextran-based gels, increased dextran concentration rather than cross-linker concentration improves retention and selectivity, suggesting that hydroxyl groups rather than linker molecules are the active adsorption sites. SWCNTs have much stronger interactions with agarose- than dextran-based gels. This stronger adsorption capacity combined with the improved retention of SWCNTs at higher agarose concentration suggests that the double helices formed by agarose are important to the adsorption of nanotubes. The quality and selectivity of the separations are heavily influenced by the initial concentration of SWCNTs; the purity of the metallic fraction falls rapidly as SWCNT concentration increases from 20 to 70 mg/L in Sephacryl 200 HR columns. The reproducibility of the separation is greatly affected by the amount of cross-linking in the gel, but restabilization of the hydrogel remains a critical concern for continued use. In general, the highest-throughput separations are obtained with dextran-based gels, while agarose-based gels produce higher purity fractions of metallic SWCNTs (>98%) and better reproducibility in consecutive separations.