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). 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.