## 3 Results and Discussion
### Metal Ion Detection
\begin{table}
\begin{tabular}{l l l l l} \hline \hline Electrode & \multicolumn{3}{c}{Ti/TiO\({}_{2}\)-NT} \\ \hline Wavelength (nm) & \multicolumn{3}{c}{389} \\ \hline Metal Ion & Linear Dependence Equation & Linear Domain (M) & Limit of Detection (M) \\ Pb & i(\(\mu\)A) = 1.82 + 1.1 \(\times\) 10\({}^{8}\) C(M); R\({}^{2}\) = 0.994 & \(10^{-8}\div 10^{-6}\) & \(8\times 10^{-9}\) \\ Cu & i(\(\mu\)A) = 4.2 + 1.89 \(\times\) 10\({}^{7}\) C(M); R\({}^{2}\) = 0.990 & \(5\times 10^{-8}\div 10^{-6}\) & \(3\times 10^{-8}\) \\ \hline Wavelength (nm) & \multicolumn{3}{c}{426} \\ \hline Metal ion & Linear dependence equation & Linear domain (M) & Limit of detection (M) \\ Pb & i(\(\mu\)A) = 15.9 + 1.42 \(\times\) 10\({}^{8}\) C(M); R\({}^{2}\) = 0.992 & \(10^{-8}\div 10^{-6}\) & \(5\times 10^{-9}\) \\ Cu & i(\(\mu\)A) = 2.40 + 3.49 \(\times\) 10\({}^{7}\) C(M); R\({}^{2}\) = 0.990 & \(10^{-8}\div 10^{-6}\) & \(7\times 10^{-9}\) \\ \hline Electrode & \multicolumn{3}{c}{Ti/TiO\({}_{2}\)-SR} \\ \hline Wavelength (nm) & \multicolumn{3}{c}{389} \\ \hline Metal ion & Linear dependence equation & Linear domain (M) & Limit of detection (M) \\ Pb & i(\(\mu\)A) = 1.06 + 2.1 \(\times\) 10\({}^{7}\) C(M); R\({}^{2}\) = 0.990 & \(3\times 10^{-7}\div 2\times 10^{-6}\) & \(2\times 10^{-7}\) \\ Cu & i(\(\mu\)A) = 2.4 + 5.13 \(\times\) 10\({}^{7}\) C(M); R\({}^{2}\) = 0.994 & \(5\times 10^{-8}\div 10^{-6}\) & \(8\times 10^{-9}\) \\ \hline Wavelength (nm) & \multicolumn{3}{c}{426} \\ \hline Metal ion & Linear dependence equation & Linear domain (M) & Limit of detection (M) \\ \hline Pb & i(\(\mu\)A) = 0.87 + 10\({}^{7}\) C(M); R\({}^{2}\) = 0.989 & \(2\times 10^{-7}\div 4\times 10^{-6}\) & \(1\times 10^{-7}\) \\ Cu & i(\(\mu\)A) = 6.5 + 5 \(\times\) 10\({}^{7}\) C(M); R\({}^{2}\) = 0.990 & \(5\times 10^{-9}\div 5\times 10^{-7}\) & \(3\times 10^{-9}\) \\ Hg & i(\(\mu\)A) = 1.5 + 5.6 \(\times\) 10\({}^{7}\) C(M); R\({}^{2}\) = 0.992 & \(5\times 10^{-9}\div 2\times 10^{-6}\) & \(4\times 10^{-9}\) \\ \hline \end{tabular}
\end{table} 
Table 1: The linear dependence equation of the calibration curve, linear range, and detection limit obtained for the Ti/TiO\({}_{2}\)-NT and Ti/TiO\({}_{2}\)-SR electrodes at light irradiation wavelengths of 389 nm and 426 nm. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Electrode** & \multicolumn{1}{l}{**Target Analyte**} & \multicolumn{1}{l}{**Linear Range**} & \multicolumn{1}{l}{**LOD**} & \multicolumn{1}{l}{**Ref.**} \\ \hline TiON/TiO\({}_{2}\) & Pb\({}^{2+}\) & \(10^{-5}\) to \(10^{-1}\) M & \(10^{-5}\) M & [27] \\ \hline CoFe\({}_{2}\)O\({}_{4}\)/CNS heterojunction-modified & Pb\({}^{2+}\) & 1–10 \(\mu\)M & 0.213 \(\mu\)M & [28] \\ nickel foam & Cd\({}^{2+}\) & 1–10 \(\mu\)M & 0.316 \(\mu\)M & [18] \\ \hline Glassy carbon/poly(2,2’-(ethan-1,2-diylbis(2-(azulen-2-ylamino)-2-oxoethyl))azadiy)diacetic acid & Cu\({}^{2+}\) & 0.003–2\({}_{\mu}\)M & 0.4 nM & [18] \\ \hline Metal-free g-C3N4/carbon & Cd\({}^{2+}\) & 0–0.7 \(\mu\)M & 2.1 nM & \\ black composite & Pb\({}^{2+}\) & 0–0.3 \(\mu\)M & 0.26 nM & [29] \\ \hline Zr/ZrO\({}_{2}\) nanotube & Cu\({}^{2+}\) & 0.05–2 \(\mu\)M & 40 nM & [17] \\ Hg\({}^{2+}\) & 0.4–10 \(\mu\)M & 100 nM & \\ \hline Ti/TiO\({}_{2}\) & Cd\({}^{2+}\) & 0.6–13.2 \(\mu\)M & 160 nM & \\ Pb\({}^{2+}\) & 0.6–13.2 \(\mu\)M & 140 nM & [13] \\ Cu\({}^{2+}\) & 0.6–13.2 \(\mu\)M & 100 nM & \\ \hline Ti/TiO\({}_{2}\) nanotubes & Pb\({}^{2+}\) & 0.01–1 \(\mu\)M & 5 nM & \\ Cu\({}^{2+}\) & 0.01–1 \(\mu\)M & 7 nM & This work \\ \hline Ti/TiO\({}_{2}\) self-reduced nanotubes & Pb\({}^{2+}\) & 0.2–4 \(\mu\)M & 100 nM & \\ Cu\({}^{2+}\) & 0.005–0.5 \(\mu\)M & 3 nM & This work \\ Hg\({}^{2+}\) & 0.005–2 \(\mu\)M & 4 nM & \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison of photo-assisted heavy metal detection between the developed Ti/TiO\({}_{2}\)-NT/TiO\({}_{2}\)-SR electrodes and other previously reported electrodes.