Enriched surface acidity for surfactant-free suspensions of carboxylated carbon nanotubes purified by centrifugation

• Boehm titrimetric method for oxidized CNT suspensions purified by centrifugation.
• Surface acidity affects CNT biodegradation rates and CNT toxicity profiles.
• Centrifugation of surfactant-free samples of oxidized CNTs enriches surface acidity.

It is well known that surfactant-suspended carbon nanotube (CNT) samples can be purified by centrifugation to decrease agglomerates and increase individually-dispersed CNTs. However, centrifugation is not always part of protocols to prepare CNT samples used in biomedical applications. Herein, using carboxylated multi-walled CNTs (cMWCNTs) suspended in water without a surfactant, we developed a Boehm titrimetric method for the analysis of centrifuged cMWCNT suspensions and used it to show that the surface acidity of oxidized carbon materials in aqueous cMWCNT suspensions was enriched by ∼40% by a single low-speed centrifugation step. This significant difference in surface acidity between un-centrifuged and centrifuged cMWCNT suspensions has not been previously appreciated and is important because the degree of surface acidity is known to affect the interactions of cMWCNTs with biological systems.

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