A sensitive and practical fluorimetric test for CNTs acidic sites determination

A sensitive and practical colorimetric test for oxidized carbon nanotubes (CNTs) is described. The assay is based on direct labeling with the commercially available fluorescent dye thionin acetate (THA). This strategy offers the possibility for quantitation of acidic sites in a very fast and easy way. Since the abundance of carboxylic groups, which increases with treatment time, temperature and the degree of lattice defectiveness, is critical for the final application, it is important to determine the prevalence of surface acidic groups. However, there are some difficulties associated with monitoring the oxidative reaction. Although a number of analytical techniques such as IR, Raman and XPS have been applied to address these shortcomings, a routine monitoring tool without the need for specialized hardware has been difficult to find. Some groups have reported acid–base titration of acidic sites of different CNTs; however, these techniques suffer from several shortcomings: (i) CNTs must be properly dispersed in the aqueous solution; (ii) they do not meet the requirements of a fast routine test, because they are multistep sequences. In the present communication we report experiments on the use of fluorescence labelling reactions for acidic group determination on CNTs. It is a very sensitive, practical, fast and accurate method to titrate carboxylic groups on carbon nanotube samples based on their specific interaction with a cationic dye such as thionin. The specificity of the interaction is supported by Raman spectroscopy and thermogravimetric analysis. The method is particularly useful as a routine test to monitor the oxidation processes of both single and multiwalled CNTs.