## 3 Results and Discussion
### Electrochemical Analysis  
The modification of PGE with HAP and copper oxide was examined by the cyclic voltammetry (CV) method. CuSO\({}_{4}\) precursor precipitates as a single phase of polycrystalline CuO in alkaline medium. Pure copper nanostructured particles can be prepared by this coprecipitation method [44]. Figure 3 show voltammetric responses of the unmodified and modified electrode with HAP and copper oxide at different concentrations and ratios. It is then known that HAP retains copper ions when modified with copper [34]. Figure 2: SEM images of (**a**) unmodified PGE at 300\(\times\) and (**b**) 80,000\(\times\), EDX image of (**c**) PGE at 1300\(\times\). SEM images of (**d**) HAP-modified PGE at 300\(\times\) and (**e**) at 20,000\(\times\). EDX image of (**f**) HAP-modified PGE at 12,000\(\times\). SEM images of (**g**) HAP- and copper-oxide-modified PGE at 300\(\times\) and (**h**) at 400\(\times\). EDX image of (**i**) HAP- and copper-oxide-modified- PGE at 3000\(\times\). Figure 3: CV analysis of unmodified PGE and **(a)** HAP-PGE modified in 0.05 M CuSO\({}_{4}\) and different concentrations of NaOH; **(b)** HAP-PGE modified in 0.1 M CuSO\({}_{4}\) and different concentrations of NaOH; **(c)** HAP-PGE modified in 0.2 M CuSO\({}_{4}\) and different concentrations of NaOH; **(d)** HAP-PGE modified in 0.3 M CuSO\({}_{4}\) and different concentrations of NaOH; **(e)** HAP-PGE modified in 0.5 M CuSO\({}_{4}\) and different concentrations of NaOH; **(f)** HAP-PGE modified in 0.7 M CuSO\({}_{4}\) and different concentrations of NaOH; and **(g)** HAP-PGE modified in 1 M CuSO\({}_{4}\) and different concentrations of NaOH. The electrochemical response of unmodified and modified electrodes was characterized using the electron transfer redox probe [Fe(CN)\({}_{6}\)]\({}^{3-/4-}\) with cyclic voltammetry. The important parameters in CV graphs to evaluate the efficiency of the modified electrode are the separation of the anodic and cathodic peak potentials (peak-to-peak separation, \(\Delta\)E\({}_{\text{p}}\) = E\({}_{\text{pa}}-\) E\({}_{\text{pc}}\)), and peak currents. Electrolytic solutions have an intrinsic resistance in the electrochemical cell. While potentiostats can compensate for most of this solution resistance (R\({}_{\text{c}}\)), some of the uncompensated resistance (R\({}_{\text{u}}\)) remains. During electrochemical measurement, potential recorded by the potentiostat does not belong to the analyte alone. This uncompensated potential is also recorded. This phenomenon is called ohmic drop. A sign of ohmic drop in cyclic voltammogram is increased peak-to-peak separation. In reversible redox couple [Fe(CN)\({}_{6}\)]\({}^{3-/4-}\), \(\Delta\)E\({}_{\text{p}}\) was calculated to be 58 mV, theoretically. Therefore, the \(\Delta\)E\({}_{\text{p}}\) of the modified electrode is expected to be close to this theoretical value [45]. The final aim of electrode modification is the enhancement of the peak current in order to improve analytical sensitivity. These increased redox peak current and decreased peak-to-peak separation indicate the facilitated electron transfer process and larger electrode surface area [27]. Upon modification of PGE surfaces with CuO in different conditions, the voltammetric profiles are considerably altered. In Figure 3, the effect of concentration of alkali on the coprecipitation of CuO was evaluated electrochemically. The peak current of the unmodified PGE was 1.903 \(\pm\) 0.11 mA. When 0.05 M CuSO\({}_{4}\) was precipitated in NaOH at different concentrations, the best electrode response was obtained at 1.817 \(\pm\) 0.22 mA when it was precipitated in 1.0 M NaOH (Figure 3a). For the different concentrations of CuSO\({}_{4}\) at 0.1, 0.2, 0.3, 0.5, 0.7, and 1.0 M, the best electrode responses were obtained in 1.0 M (Figure 3b, 1.502 \(\pm\) 0.36 mA), 1.0 M (Figure 3c, 2.119 \(\pm\) 0.18 mA), 1.0 M (Figure 3d, 2.397 \(\pm\) 0.44 mA), 0.5 M (Figure 3e, 2.465 \(\pm\) 0.39 mA), 0.5 M (Figure 3f, 2.11 \(\pm\) 0.16 mA), and 0.5 M NaOH (Figure 3g, 2.18 \(\pm\) 0.22 mA), respectively.