## 3 Results and Discussion
### Metal Ion Detection  
To check the chemical reversibility and intermediate species formation at the Ti/TiO\({}_{2}\)-NT and Ti/TiO\({}_{2}\)-SR electrodes, cyclic voltammetry curves were recorded in the presence of Zn(II), Cd(II), Pb(II), Cu(II), and Hg(II) mixed ionic solution (Figure 2). At the electrode's surface, only four reduction peaks could be observed, which correspond to the reduction of Cd(II), Pb(II), Cu(II), and Hg(II) ions, respectively, to metal zero valent. In the reverse scan, the corresponding oxidation peaks are observed. The peaks are well separated in terms of potential without evidence of intermetallic species formation, probably due to the large surface of the electrode, which allows for multiple reduction sites for the ions. The absence of Zn (II) ions could be due to the start of an HER reaction at the electrode surface at lower potential. The potential of metal ions can be tailored to the energy levels of a semiconductor's conduction band to improve the selectivity and sensitivity of electrode materials. As shown in Figure 3A, the potentials of Zn(II), Cd(II), Pb(II), Cu(II), and Hg(II) ions exceed that of the conduction band, facilitating spontaneous photoreduction under light irradiation. Both the electrode material (Ti/TiO\({}_{2}\)-NT and Ti/TiO\({}_{2}\)-SR) and the wavelength of light irradiation play a crucial role in the adsorption and reduction of metal ions. In the absence of UV light, due to limited adsorption as a result of oxygen's interaction with the metal ions, both electrodes showed only two minor stripping peaks at \(-0.76\) V and \(-0.23\) V, corresponding to Pb and Cu, respectively (Figure 3B). Conversely, exposing the electrodes to different wavelengths of light led to an increased response of the electrodes to metal ions. Furthermore, it was observed that the wavelength of light irradiation has an important influence on electrode selectivity and sensitivity. For the Ti/TiO\({}_{2}\)-NT electrode, the stripping peak currents for Pb and Cu were found to increase upon irradiation and reach a maximum at 426 nm. The Ti/TiO\({}_{2}\)-SR electrode showed different behavior when irradiated with different wavelengths. At 372 nm and 389 nm, the electrode responds only to Pb and Cu ions, whereas the use of irradiation at higher wavelengths (426 nm and 450 nm) led to the appearance of a new oxidation peak at around 0.2 V, which corresponds to the re-dissolution of reduced mercury (Figure 3C). This finding underlines the beneficial effect of light irradiation on the electrode surface. However, no stripping peaks were observed for Zn(II) and Cd(II) ions. Despite its theoretical feasibility, the thermodynamic driving force for the photoreduction of these ions is low. Competitive adsorption at the binding sites is probably the cause of the absence of Zn and Cd ions. This suggests that the presence of Pb(II), Cu(II), or Hg(II) ions has a significant influence on the sorption of Zn and Cd ions. With the aim of improving the detection of metal ions on Ti/TiO\({}_{2}\)-NT and Ti/TiO\({}_{2}\)-SR electrodes, the influence of pH and accumulation was investigated. Figure 4A,B show the response of the Ti/TiO\({}_{2}\)-NT and Ti/TiO\({}_{2}\)-SR electrodes at different pH values of the metal ion photo-accumulation solution. A strong influence of pH on the electrodes' response to metal ions is observed, with the optimal response being at pH = 6. This value is in the range of the zero-charge point (PZC) of TiO\({}_{2}\)[26]. According to [PERSON] [26], a surface below this value is positively charged due to the presence of Ti-OH\({}^{2+}\) and negatively charged (TiO\({}^{-}\)) at pH values above 6. Therefore, below a PZC of 6, a lower response of the electrodes is observed, probably due to the electrostatic repulsions between the metal ions and the electrode surface.