Oxygen-containing functionalities on the surface of multi-walled carbon nanotubes quantitatively determined by fluorescent labeling

Control over the type and concentration of functional groups on carbon nanotubes (CNTs) require the use of reliable and sensitive analytical methods to detect, identify and quantify the functionalities on the material. Here we report the results of the selective quantification of aldehyde (together with ketone), carboxylic, and alcohol groups on arc-produced multiwalled carbon nanotubes (MWCNTs) using fluorescent labeling of surface species (FLOSS), combined with surface area and thermogravimetric analysis. The high sensitivity of the fluorescence spectroscopy combined with the selectivity of the chemistry of covalent attachment, allowed us to determine that as-produced MWCNTs contain ∼1.1 at.% carboxylic groups, ∼2.0 at.% aldehydes (and ketones) and <2.0 at.% hydroxyls. Surprisingly, and contrary to the behavior of single walled carbon nanotubes, these concentrations do not appear to increase for acid purified MWCNTs but rather decrease to 0.4 at.% for carboxylic groups; 1.6 at.% for aldehydes (and ketones) and <3.0 at.%, for hydroxyls. Possible explanations for the observation that the acid purified MWCNTs have a lower level of the functionalities compared to the as-produced material are discussed.