Dear Sasya,

I guess titration of an aqueous dispersion with NaOH or KOH should work.

for biosensor technology?

Titration is the easiest way, however, its accuracy may not be sufficient if the functionalization is low. Also, simple titration with indicator may be impossible due to deep color of the nanotubes dispersion, and using pH-sensitive electrode may be complicated by the nanotubes sedimentation onto the electrode surface.

I would suggest to try titration first, for sure, and if it does not work (result is close to zero or not realistic), the alternative might be elemental analysis (oxygen), or IR/ONMR, but in case of spectroscopy methods some external standard will be necessary for calibration (some aromatic carboxylic acid, I guess).

I thought especially about measuring conductivity. Long ago, during my PhD we determined content of 1-2 µMol COOH/g cellulose, assisted by a colleague.

Dear Sasya, have a look to the papers of Boehm for the overview of available methods (e.g. http://dx.doi.org/10.1016/S0008-6223(01)00165-8) and read standardised descritpion of the Boehm titration in more late papers. Its combination with IR, XPS and temperature-programmed desorption could help.

You can do the FTIR and XPS measurements of the tubes to determine the functional groups....

Also, the intensity of D and G band indicates the success of functionalization method.