Important differences in the sensor performance are clearly seen. The CoO-based sensor did not produce analytical signals above its LOD (36 mg L −1 a). Although a good calibration curve was obtained by using glucose as standard, the CoO nanoparticles appeared not to catalyze the oxidation of the organic compounds present in these real samples. In the case of the Ni-based sensor, a lack of stability could be the cause of a progressive loss of response and in turn of sensitivity with the analysis of the samples being carried out in the order shown in Table 2. Thus, a COD value of 278 mg L −1 was estimated for the first sample whereas COD values below the sensor LOD were given for the last two ones. In spite of the NiCu-based sensor showing the best analytical performance in glucose standard solutions, the COD values measured in the real samples were between 30% and 80% lower than those provided by the accredited laboratory, which indicated a nonquantitative oxidation of the organic matter presented in this kind of sample by this catalyst. Just the CuO/AgO-based sensor showed the best analytical correlation, with three out of the five COD values provided by the composite-derived sensor being within the calculated confidence interval (95%) and the other two close to these interval limits.