Structure-Based Carbon Nanotube Separations by Ion-Surface Interactions
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0923-V05-06
Structure-Based Carbon Nanotube Separations by Ion-Surface Interactions Steven R Lustig1, Anand Jagota2, Constantine Khripin2, and Ming Zheng1 1 DuPont, Wilmington, DE, 19880 2 Lehigh University, Bethlehem, PA, 18015
ABSTRACT Single-stranded DNA wraps helically around individual single walled carbon nanotubes to form a DNA/CNT hybrid, which is both stable and dispersible in aqueous solution. Subjected to ion-exchange chromatography, a hybrid elutes at an ionic strength that depends on the electronic band structure of the core nanotube, thus providing a mechanism for separating nanotubes by chirality. We present experimental data and a theoretical model for this separation process on dielectric substrates 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 indicates 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. INTRODUCTION Considerable progress has been made separating metallic from semiconducting SWNTs as well as making separations between semiconducting SWNTs of differing bandgap. Zheng and co-workers [1,2] report DNA-assisted dispersions and separations of SWNTs. Bundled SWNT ropes can be dispersed in water by their sonication in the presence of single-stranded deoxyribonucleic acid (ssDNA) oligomers. Spectroscopy and atomic force microscopy provide strong evidence that single SWNT are individually dispersed in water. Molecular modeling suggests that aqueous ssDNA can bind to CNTs through π -stacking and hydrophobic forces, resulting in helical wrapping around the CNT surface. We refer to the composition of ssDNA wrapped around a single SWNT as a DNA/CNT hybrid. Ion exchange chromatography (IEC) of these hybrids separates fractions according to the electronic structure of the interior SWNT. Most recently it was discovered that certain ssDNA sequences markedly improve IEC separations both between metallic and semiconducting CNTs as well as between semiconducting CNTs of different diameter and electronic bandgap [2]. THEORY The charge density on the DNA/CNT hybrid is primarily controlled by the geometry of wrapping the ssDNA molecule around the SWNT core. As we have noted previously [1], molecular simulations suggest that ssDNA can adopt a number of helical conformations around
the SWNT. Variability in wrapping geometry, if uncorrelated with the SWNT chirality, will introduce variability on the charge density along the hybrid that will be likely greater
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