## 3 Results and Discussions
### Electrochemical Impedance Spectroscopy of ITO/CuO Electrodes  
Electrochemical impedance spectroscopy (EIS) was employed at the open-circuit potential to obtain information on electron transfer between the electrolyte and the electrode surface. The Nyquist plots of the three synthesized \(CuO\) electrode types (NPs, NRs, and NSs) tested with the PBS solution without glucose (see Figure 7) were measured in the frequency range of 0.1 Hz to 100 kHz, revealing distinct features. The low-frequency region (Figure 7a) displays a linear region with an inset showing the equivalent circuit (R\({}_{\mathrm{s}}\): solution resistance, R\({}_{\mathrm{ct}}\): charge-transfer resistance, C\({}_{d}\): double-layer capacitance; Z\({}_{\varpi}\): Warburg impedance) [49; 50]. The straight lines in the low-frequency region correspond to Z\({}_{\varpi}\), indicating electrolyte diffusion within the \(CuO\) structures. In particular, the \(CuO\) NPs exhibit the lowest slope, which implies the fastest diffusion rate in three morphologies. The high-frequency regions of the Nyquist plots for \(CuO\) NPs, NRs, and NSs (Figure 7b-d) exhibit approximate semicircles, with corresponding equivalent series resistance (ESR) values of 60 \(\Omega\), 56 \(\Omega\), and 60 \(\Omega\), respectively. These ESR values are reflected by the the intercept on the real axis at high frequency, encompassing the electrolyte resistance, intrinsic electrical resistance of the spray-coated \(CuO\), and the contact resistances between spray-coated \(CuO\) and \(ITO\) electrodes [49]. The analysis of R\({}_{\mathrm{ct}}\) estimated from the diameters of the semicircles on the real axis reveals the highest R\({}_{\mathrm{ct}}\) value for \(CuO\) NPs. This indicates  
Figure 6: XRD pattern of the uncompleted samples after 15 h of hydrothermal process.  
that \(ITO/CuO\) NRs and \(ITO/CuO\) NSs exhibit significantly higher electrochemical activity compared to \(ITO/CuO\) NPs. Although the surface-area-to-volume ratio of \(CuO\) NPs is the largest, their particle-like structure leads to a diminished ability to transport charge from the \(CuO\) nanomaterials' interface with the electrolyte to the \(ITO\) electrode. In contrast, this transport capability is much more efficient in the \(CuO\) NRs and \(CuO\) NR forms.  
For the photoelectrochemical response, the EIS characterizes three morphological types of \(ITO/CuO\) electrodes with a 150 nM glucose in PBS solution under the illuminated and non-illuminated conditions of the 395 nm light source (Figure 8). The results reveal significant changes in the slope line at low frequencies in the response of \(CuO\) NPs and \(CuO\) NRs materials (Figure 8a,b) to light stimulation, while \(CuO\) NSs demonstrated relatively minor variations (Figure 8c). This could be attributed to the relatively large size of the \(CuO\) NSs, which limited the possibility of generating electron-hole pairs. Consequently, the recombination of these electron-hole pairs is substantial, resulting in the reduced production of electrons that participate in the reaction with glucose. All of the above impedance results explain the best photoelectrochemical performance of the \(CuO\) NRs electrode for glucose sensing.  
Figure 7: (**a**) Electrochemical impedance spectroscopy of bare \(ITO\) and three morphologies \(ITO/CuO\) nanomaterials without glucose with the inserted equivalent circuit in the low-frequency region. Semicircular shapes in high-frequency Nyquist plots of (**b**) \(ITO/CuO\) NPs, (**c**) \(ITO/CuO\) NRs, and (**d**) \(ITO/CuO\) NSs electrodes.