Different modified electrodes were prepared using multi-walled carbon nanotubes (MWCNTs) prepared and purified according to different procedures. Since MWCNTs purification and functionalization is a key point for the electroanalytical performance of the final devices, MWCNTs were previously characterized from the morphological, surface and electrochemical point of view. Different parameters, such as surface area, pore volume, mesoporosity (responsible of thin layer diffusion mechanism), covalent acidity, charge transfer resistance, amorphous impurities contribute to the final performances in terms of higher currents and more reversible behaviour. The best purification method consists in a treatment in sulphonitric mixture for 24 h with the removal of amorphous carbon. The new modified electrodes were successfully employed in the detection of o-toluidine, a hazardous and toxic water pollutant, reaching a sensitivity of 9.2 mu A cm (2) ppm (1), a limit of detection of 0.16 ppm and apparent recovery factors very close to 100%. Their application in a complex matrix to monitor o-toluidine, during its photocatalytic removal by ZnO nanopowders, was also investigated. (C) 2014 Elsevier Ltd. All rights reserved.