Electrochemical Characterization. Cyclic voltammetric experiments were carried out to evaluate the electrochemical
performance of the fabricated composite-derived sensors. These were compared with the response of the bare Pt
electrode and the sensor based on a PS-MWCNTs composite without electrocatalysts. All the voltammetric signals recorded in a 0.1 M KNO solution containing 1.0 mM ferricyanide showed the reversible faradaic signal of the ferrocyanide/ ferricyanide redox pair, with the peak potential difference being between 88 and 112 mV and the anodic to cathodic peak current ratio being around 1 in all the cases. Such values indicate good and fast electron transfer processes between the composite-based sensors and the solution (Figure S2 in the Supporting Information). In addition, the sensors based on NiCu alloy, CoO, and CuO/AgO gave peak currents of the ferricyanide reduction process of −5.16, −5.37, and −5.32 μA, which correspond with an active area of 3.24, 3.38, and 3.34 mm 2 , respectively (estimated by applying the Randles-Sevcik equation and using the diffusion coefficient of 7.01 × 10−1 for potassium ferricyanide in 0.1 M KNO). These values were similar to those obtained with the bare Pt electrode (3.603−6 cm2mm2) and the sensor without metallic catalysts (3.42 mm).These results demonstrate the benefits of the MWCNTs aconducting material for the COD sensors. Likewise, it shows that the incorporation of the catalytic inorganic nanoparticles did not affect the electronic transfer of the resulting sensor devices. An exception to this behavior was observed with the Ni-based sensor, whose peak current value was −3.75 μA. This corresponds with an electrochemical active area of 2.36 mm, this being 34% smaller than that of the bare Pt electrode. This decrease could be caused by the heterogeneous distribution of the different components of this composite material. Nevertheless, such kind of differences should not have an effect on the analytical performance of this sensor device.