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Carbon nanotubes (CNTs) are a unique and versatile class of nanomaterials with enormous commercial potential. This has been responsible for a rapid increase in their production rates and, as a result, larger quantities of CNTs will inevitably find their way into the environment. The ecological and toxicological impacts of these nanomaterials are, however, still poorly understood.
Many purification and functionalization strategies incorporate oxygen-containing functional groups into the surface of CNTs, and similar modifications can occur after CNTs are released into the environment through exposure to oxidizing agents. Surface analytical techniques have been developed to quantify both the concentration of oxygen and distribution of surface oxides produced on CNTs by different oxidative treatments. The influence that surface oxides exert on the colloidal stability and sorption properties of CNTs in aquatic environments will also be discussed.
A suite of CNTs with different levels of oxygen content were prepared by refluxing pristine CNTs in HNO3 solutions of various concentrations; X-ray photoelectron spectroscopy (XPS) showed that the surface oxide concentration increased from 3% for the pristine nanomaterials to 12.5% for CNTs treated in ~16M HNO3. The corresponding variation in oxide distribution has been probed using chemical derivatization in conjunction with XPS. UV-vis spectroscopy shows that well-defined relationships exist between the level of CNT surface oxidation and their colloidal stability; specifically, more highly oxidized CNTs remain stable over a wider range of aquatic conditions. The sorption properties of CNTs also display systematic variations as the level of surface oxidation increases: adsorption of 14C labeled-naphthalene, a hydrophobic organic chemical, decreased linearly while divalent heavy metal contaminants like Zn2+ showed an increasing affinity towards more highly oxidized CNTs. Relationships that exist between specific surface oxides (e.g. carboxylic acid groups), and CNT behavior will also be discussed. Our studies highlight the fact that even comparatively small changes in surface chemistry are responsible for pronounced changes in CNT properties.
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