Photoreactivity of Carboxylated Single-Walled Carbon Nanotubes in Sunlight: Reactive Oxygen Species Production in Water

Abstract

Very limited information exists on transformation processes of carbon nanotubes in the natural aquatic environment. Because the conjugated pi-bond structure of these materials is efficient in absorbing sunlight, photochemical transformations are a potential fate process with reactivity predicted to vary with their diameter, chirality, number and type of defects, functionalization, residual metal catalyst and amorphous carbon content, and with the composition of the water, including the type and composition of materials that act to disperse them into the aqueous environment. In this study, the photochemical reactions involving colloidal dispersions of carboxylated single-walled carbon nanotubes (SWNT-COOH) in sunlight were examined. Production of reactive oxygen species (ROS) during irradiation occurs and is evidence for potential further phototransformation and may be significant in assessing their overall environmental impacts. In aerated samples exposed to sunlight or to lamps that emit light only within the solar spectrum, the probe compounds, furfuryl alcohol (FFA), tetrazolium salts (NBT2+ and XTT), and p-chlorobenzoic acid (pCBA), were used to indicate production of 1O2, O2.-, and .OH, respectively. All three ROS were produced in the presence of SWNT-COOH and molecular oxygen (3O2). 1O2 production was confirmed by observing enhanced FFA decay in deuterium oxide, attenuated decay of FFA in the presence of azide ion, and the lack of decay of FFA in deoxygenated solutions. Photogeneration of O2.- and .OH was confirmed by applying superoxide dismutase (SOD) and tert-butanol assays, respectively. In air-equilibrated suspensions, the loss of 0.2 mM FFA in 10 mg/L SWNT-COOH was approximately 85% after 74 h. Production of 1O2 was not dependent on pH from 7 to 11; however photoinduced aggregation was observed at pH 3.

Origin and Perspectives of the Photochemical Activity of Nanoporous Carbons

“…Aqueous colloidal dispersions of bare and functionalized CNTs have been reported to generate ROS including singlet oxygen, superoxide anion, and hydroxyl radicals in light within the solar spectrum under oxic conditions. Purified CNTs were found to be generally more photoactive, producing singlet oxygen and superoxide, while little or no photoactivity was observed for raw CNTs 22, 23, 24. CNTs can also act as scavengers of hydroxyl or superoxide radicals,74, 75, 76, 77 or undergo photochemical transformations (e.g., decarbonilation) as in the case of oxidized multiwalled CNTs 24…”

“…In 2009, Luo et al reported the self‐photoactivity of carbon nanotubes under visible light, attributing the behavior to the presence of structural defects and vacancies 21. Later, other authors reported the ability of aqueous suspensions of carbon nanotubes to generate reactive oxygen species (ROS) including singlet oxygen, superoxide anion, and hydroxyl radicals upon illumination with light within the solar spectrum 22, 23, 24…”

Potential scenarios for nanomaterial release and subsequent alteration in the environment

“…However, degradation processes of these materials have not been fully investigated, so altering the composite material and releasing CNT are difficult to evaluate [53]. First results indicate that CNT can be released on photochemical degradation (process 1) of CNT-containing composites [80,81]. The PM-ENM composite material will be subjected to UVA/B radiation and abrasive forces while in use that will weaken molecular bonds and release CNT over an extended period.…”

“…The PM-ENM composite material will be subjected to UVA/B radiation and abrasive forces while in use that will weaken molecular bonds and release CNT over an extended period. These released CNT (PW-ENM) would make their way into waste water treatment plants or be directly deposited into environmental compartments where they would undergo transformation by photochemistry (process 1) [81], oxidation (process 2), adsorption of NOM [33] and other organic colloids [34], biotransformation (process 7), and continued abrasive forces (process 8). These transformative processes would change CNT aggregation, dispersability, and interaction with biota in the environmental compartment.…”

Synthesis and Characterization of Novel Plasmonic Ag/AgX-CNTs (X = Cl, Br, I) Nanocomposite Photocatalysts and Synergetic Degradation of Organic Pollutant under Visible Light

“…The modest photocatalytic activity of CNTs could be due to the ability of CNTs to produce the reactive oxygen species (ROSs) under irradiation. 41 Apparently, the photocatalytic activity of Ag/AgBr-CNTs is markedly improved by the loading of AgBr onto the CNTs surface. The photocatalytic removal of TBP was also performed using Ag/AgCl-CNTs and Ag/AgI-CNTs as photocatalysts ( Figure 7b).…”