Carbon nanotubes (CNTs) have unique physical and chemical properties that drive their use in a variety of commercial and industrial applications. CNTs are commonly oxidized prior to their use to enhance dispersion in polar solvents by deliberately grafting oxygen-containing functional groups onto CNT surfaces. In addition, CNT surface oxides can be unintentionally formed or modified after CNTs are released into the environment through exposure to reactive oxygen species and/or ultraviolet irradiation. Consequently, it is important to understand the impact of CNT surface oxidation on the environmental fate, transport, and toxicity of CNTs. In this review, we describe the specific role of oxygen-containing functional groups on the important environmental behaviors of CNTs in aqueous media (e.g., colloidal stability, adsorption, and photochemistry) as well as their biological impact. We place special emphasis on the value of systematically varying and quantifying surface oxides as a route to identifying quantitative structure−property relationships. The role of oxygen-containing functional groups in regulating the efficacy of CNT-enabled water treatment technologies and the influence of surface oxides on other carbon-based nanomaterials are also evaluated and discussed.

The unique physicochemical and luminescent properties of carbon dots (CDs) have motivated research efforts toward their incorporation into commercial products. Increased use of CDs will inevitably lead to their release into the environment where their fate and persistence will be influenced by photochemical transformations, the nature of which is poorly understood. This knowledge gap motivated the present investigation of the effects of direct and indirect photolysis on citric and malic acid−based CDs. Our results indicate that natural sunlight will rapidly and non-destructively photobleach CDs into optically inactive carbon nanoparticles. We demonstrate that after photobleaching, • OH exposure degrades CDs in a two-step process that will span several decades in natural waters. The first step, occurring over several years of • OH exposure, involves depolymerization of the CD structure, characterized by volatilization of over 60% of nascent carbon atoms and the oxidation of nitrogen atoms into nitro groups. This is followed by a slower oxidation of residual carbon atoms first into carboxylic acids and then volatile carbon species, while nitrogen atoms are oxidized into nitrate ions. Considered alongside related CD studies, our findings suggest that the environmental behavior of CDs will be strongly influenced by the molecular precursors used in their synthesis.

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The analysis of the environmental behavior and toxicity of metal oxide nanoparticles (MONPs) is complicated by high metal concentrations in natural matrices.

Cationic amphiphilic polymers are often used to coat nanoparticles as they increase chemical stability in solution and exhibit membrane disruption activities. Among these, poly­(oxonorbornenes) (PONs) are tunable membrane disruptors. They can be constructed with either one amine-terminated side chain and one hydrophobic alkyl side chain (PON-50) or two amine-terminated side chains (PON-100) on each repeat unit and can then be conjugated to gold nanoparticles using O-(2-carboxyethyl)-O′-(2-mercaptoethyl) heptaethylene glycol (HEG) spacers. While the amine content and membrane disruption activity of PONs can be controlled, the detailed structural properties of PONs conjugated to gold nanoparticles remain less understood. To address this, we performed molecular dynamics simulations of PON-50 and PON-100 to determine the nonbonded energies of PON structures as a function of amine composition. We found increasing energetic stabilization with decreasing amine composition. These results were consistent with experimental observations obtained with X-ray photoelectron spectroscopy (XPS) in which PON-100 was found to have the lowest conjugation efficiency to gold surfaces out of a range of PON amination ratios. Computationally obtained energetics suggest that replacing the aliphatic amine groups with aromatic amine groups can reverse this behavior and lead to more stable PON structures with increasing amine content. We also found that the curvature of the gold nanoparticle surface affects interactions between the surface and the amine groups of PON-50. Increasing curvature decreased these interactions, resulting in a smaller effective footprint of the HEG-PON-50 structure.