| Single-walled carbon nanotubes (CNTs) are a one-dimensional nanomaterial with advantageous mechanical, optical, and electronic properties. An effective method of dispersing individual nanotubes in aqueous solution is to wrap CNTs with single-stranded DNA. To study the structure of this CNT--DNA, we employed atomic force microscopy. Results suggest DNA strands are helically wrapped with &sim14-nm pitch and arranged end-to-end in a single layer along the CNT with a structure independent of the wrapping DNA length and sequence. Labeling the wrapping DNA with quantum dots demonstrated the useful functionalization of CNTs in a nondestructive manner and suggests nearly complete CNT surface coverage with DNA. To study CNT solution electrochemistry, we employed metal-mediated cyclic voltammetry. Oxidation of CNT--DNA by tris(2,2'-bipyridine)ruthenium(III) and similar electrogenerated oxidants was found to complete a catalytic cycle, enhancing metal oxidative peak current compared to a voltammogram of the metal alone. We observed an increase in this current enhancement at higher nanotube concentration, slower experimental scan rate, and higher metal redox potential. These observations were shown via digital simulation to be consistent with electron transfer involving different rate constants (on the order of 10 3--105 M-1s-1) reflecting the varying redox potentials of valence band electrons within one CNT chiral type and within the distribution of CNTs present in our sample. The lowest observed CNT oxidation potential was between 460 and 645 mV versus Ag/AgCl and, above this potential, the number of transferred electrons increased exponentially with the redox potential of the metal mediator, suggesting that more electrons are accessible to stronger metal oxidants. For example, an oxidant with 645-mV redox potential oxidized CNTs by &sim200 electrons per nanotube, while a stronger oxidant (with 1080-mV redox potential) oxidized CNTs by &sim2000 electrons per nanotube. This result is attributed to the electronic band structure of CNTs the stronger oxidant shifts the Fermi level deeper into the CNT valence band. These electron transfer and structural findings are anticipated to benefit the purification of heterogeneous CNT samples, controlled tuning of CNT optical--electronic properties, and development of CNT transistors, charge storage devices, and chemical sensors. |
