Influence of Surface Oxides on the Colloidal Stability of Multi-Walled Carbon Nanotubes: A Structure−Property Relationship

Abstract

As with all nanomaterials, a large fraction of the atoms in carbon nanotubes (CNTs) reside at or near the surface. Consequently, surface chemistry will play a crucial role in determining the fate and transport of CNTs in aquatic environments. Frequently, oxygen-containing functional groups (surface oxides) are deliberately grafted into the CNT surface to promote colloidal stability. To study the influence that both the oxygen concentration and the oxygen functional-group distribution have on the colloidal stability of multiwalled carbon nanotubes (MWCNTs), a suite of oxidized MWCNTs (O-MWCNTs) were created using different oxidizing agents and reaction conditions. Stable colloidal suspensions were prepared by low-power sonication of O-MWCNT powders in Milli-Q water. Results from TEM, AFM, DLS, and XPS measurements revealed that, irrespective of the surface chemistry, the colloidal suspensions were composed of individual nanotubes with comparable length distributions. The critical coagulation concentrations (CCC) of O-MWCNTs that exhibited different surface chemistries were measured with time-resolved dynamic light scattering (TR-DLS) using NaCl as the electrolyte. Over a range of environmentally relevant pH values, linear correlations were found to exist between the CCC, total oxygen concentration, and surface charge of O-MWCNTs. In contrast to surface charge, electrophoretic mobility did not prove to be a useful metric of colloidal stability. Information obtained from chemical derivatization studies, carried out in conjunction with XPS, revealed that the distribution of oxygen-containing functional groups also influences the colloidal stability of O-MWCNTs, with carboxylic acid groups playing the most important role. This study highlights the fact that quantitative relationships can be developed to rationalize the influence of surface chemistry on the behavior of nanomaterials in aquatic environments.

Methodological considerations for testing the ecotoxicity of carbon nanotubes and fullerenes: Review

“…Toxicity attributed to C 60 in those studies is instead more likely linked to THF decomposition products, as demonstrated in a study with zebrafish [71] and further confirmed in subsequent research [72]. Results showing that (THF-nC 60 ) does not generate oxidative injury (or any other toxic effects) when THF and THF decomposition products are removed [72] resulted in a convincing rejection of the hypothesis that C 60 was responsible for the toxicity reported in studies that have used THF-nC 60 . Despite this evidence, numerous articles continue to cite studies that have used THF-nC 60 to indicate toxicity of C 60 (see, for example, Kahru and Dubourguier [73]).…”

“…Perhaps the principal example of an artifact that impacted the interpretation of nanoecotoxicology results was with tetrahydrofuran-dispersed fullerenes (THF-nC 60 ). The hypothesis that THF-nC 60 can induce oxidative injury in aquatic organisms was supported in early studies [22,70] but has subsequently been refuted as techniques for investigating toxicity of C 60 have been refined.…”

“…Perhaps the principal example of an artifact that impacted the interpretation of nanoecotoxicology results was with tetrahydrofuran-dispersed fullerenes (THF-nC 60 ). The hypothesis that THF-nC 60 can induce oxidative injury in aquatic organisms was supported in early studies [22,70] but has subsequently been refuted as techniques for investigating toxicity of C 60 have been refined. Toxicity attributed to C 60 in those studies is instead more likely linked to THF decomposition products, as demonstrated in a study with zebrafish [71] and further confirmed in subsequent research [72].…”

“…Many other detailed reviews of NP characterization have discussed this topic in depth, including Hassellov et al [56] and Tiede et al [57]. Surface chemistry, particularly the presence of functional groups on CNPs, can alter the physicochemical properties of CNPs completely and influence environmental fate [58][59][60] and toxicity of these materials. Functionalized CNPs may be the starting material for ecotoxicity tests, and these characteristics should be verified prior to testing; alternatively, changes in surface functionalization may occur during testing and should be evaluated when possible.…”

“…Changes at the cellular level as a result of different surface modifications do not, however, necessarily translate to similar effects in whole organisms. For example, the cytotoxicities of nC 60 fullerene aggregates (fullerene aggregates produced by stirring in water) and hydroxylated fullerenes varied by several orders of magnitude, but neither was found to have toxic effects when instilled in rat lungs [66].…”