The marked differences in the sensor performances could be related to the electrocatalyst composition and its structure and stability, which would greatly affect the electrochemical oxidation of organic compounds of different nature in the
wastewater samples (Table S1 in the Supporting Information). Previous works that describe the use of Ni catalysts show their lack of working stability and so the developed sensors were applied as single-use devices. Regarding
other reported approaches using NiCu and CoO and CuO/AgO, these appear to be more stable and could be used for extended periods from 1 week (NiCu when the sensor was stored in DI water at RT) up to several months (CuO/AgO when the sensor was stored in air at RT). Therefore, taking into account that a single PS-MWCNT composite electrode for each of the inorganic catalysts was used to carry out the analysis of the whole set of real water samples, the COD measurements presented in Table 2 may have been seriously affected by such limited stability. From these results, the CuO/AgO-based sensor was selected for the subsequent experiments directed to a more in-depth analytical assessment and validation as an electrochemical sensor device for the analysis of COD in wastewater samples.