Carbon Nanotube Mediated Redox Reactions in Water

Hsin-Se Hsieh, Purdue University

Abstract

The continuing advances made in both application and production of carbon nanotubes (CNTs) will inevitably result in their release into the natural environment. Therefore, evaluating potentially adverse environmental effects of CNTs is necessary in order to develop better risk assessments for these emerging nanomaterials. Although some formations have been reported to cause cytotoxicity, such as induction of oxidative stress, there is no solid understanding of how CNTs may induce DNA damage. Considering that they are composed of large arrays of conjugated π bonds, it was hypothesized that CNTs suspended in water can shuttle electrons from dissolved electron donors (i.e., reducing agents) to electron acceptors (i.e., oxidizing agents), effectively catalyzing redox reactions in aquatic environments. Based on the hypothesis, this work investigated whether single-walled carbon nanotubes (SWCNTs) can participate in this type of light-independent redox reaction in water. Based on a detailed examination of electron transfer from two biological electron donors to either carboxylated SWCNTs or nonfunctionalized SWCNTs with further transfer to molecular oxygen forming reactive oxygen species (ROS), a detailed mechanism of ROS production by SWCNTs was proposed. The production of ROS also contributed to the cleavage of supercoiled DNA plasmids in the suspension of carboxylated SWCNTs, presumably caused by a site-specific generation of hydroxyl radical. The catalytic role of nonfunctionalized SWCNTs to shuttle electrons was demonstrated to be dependent on the type of dispersant used to suspend the SWCNTs in water, with cetyltrimethylammonium bromide and Suwannee River natural organic matter both effectively enhancing electron transfer. In addition to the catalytic reduction of molecular oxygen, forming reactive oxygen species (i.e., superoxide anion and hydrogen peroxide), SWCNTs were demonstrated to catalyze the reduction of nitrobenzene to aniline and N-phenylhydroxylamine in the presence of sodium sulfide, a ubiquitous reducing agent produced mainly from biological anaerobic reactions. The reduction of nitrobenzene occurred through an apparent pseudo-first-order decay process. The presence of molecular oxygen was shown to partially inhibit the reduction of nitrobenzene, providing evidence that molecular oxygen is a more facile electron acceptor to receive electrons from sulfide and/or negatively charged SWCNTs. Because the products of SWCNT-catalyzed redox reactions might have different stabilities and/or mobilities compared to the parent compounds, the reactivity of SWCNTs with organic chemicals could significantly alter the fate and transport of the parent/product pollutant mixture, complicating the overall risks associated with SWCNTs. Overall, the results presented in this study provide evidence that SWCNTs are not inert materials in aquatic environments even in the absence of light, and suggest the need for additional comprehensive studies to investigate the potential roles that CNTs and other carbon-based nanomaterials might play in facilitating dark redox reaction.

Degree

Ph.D.

Advisors

Jafvert, Purdue University.

Subject Area

Environmental Health|Environmental science|Environmental engineering

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