## 3 Results and Discussion
### Electrochemical Sensing of Mercury
Figure 11: Peak current variation versus Hg\({}^{2+}\) concentration; electrochemical experimental data were fitted by a combined model (one binding site + Hill model). The inset is added to highlight the goodness of the fit according to the proposed model. The incubation of the extracted film each time in 10\({}^{-4}\) M solution of a single chosen ion (Figure 12A) shows that ZnO-IIP is specific for Hg\({}^{2+}\). The competition tests (Figure 12B) were carried out by incubating the extracted film in a solution containing equal concentrations (10\({}^{-4}\) M) of Cu\({}^{2+}\), Hg\({}^{2+}\) and Pb\({}^{2+}\) ions. The tests have shown that in the presence of Cu\({}^{2+}\) and/or Pb\({}^{2+}\), the extracted film detects Hg\({}^{2+}\) highly selectively. One can note the existence of some competition, since in Figure 12A, the height of the Hg\({}^{2+}\) peak is 106 uA but slightly decreases to 94 uA in the presence of both Cu\({}^{2+}\) and Pb\({}^{2+}\) at the same concentration of 10\({}^{-4}\) M (Figure 12B). If the Hg\({}^{2+}\) concentration is decreased to 10\({}^{-6}\) M, one can note a decrease in the height of the peak to 77 uA in the presence of 10\({}^{-4}\) M Pb\({}^{2+}\) (Figure 12B). Quantitatively speaking, there is a clear selectivity of the sensor to Hg\({}^{2+}\); indeed, competition of other ions to the artificial receptors occurs but remains marginal. This is very important to prove, because in most fundamental studies, selectivity tests are usually conducted in single metal ion solutions and not in mixtures [87]. In a mixture containing 10\({}^{-4}\) M of Pb\({}^{2+}\) and 10\({}^{-6}\) M of Hg\({}^{2+}\), we note that despite the low concentration of mercury with the presence of other ions, the sensor remains specific to Hg\({}^{2+}\), hence the importance of shaping the receptor sites with this template. This also shows that there is no competitive chemisorption of Pb\({}^{2+}\) which hampers the response of the sensor to the Hg\({}^{2+}\) ions. The performance of the IIP sensors designed so far is compared to that of a number of other sensors described in the literature (Table 2). Embedding ZnO nanorods in the IIP film provided a unique LOD, as low as 1 pM. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Electrode** & **Analytical Method** & **Electrolyte** & **LOD** & **Ref** \\ \hline Glassy carbon electrode & Square wave anodic & & & \\ GC/SH/AuNPs & stripping voltammetry (SWASV) & HCl solution & 10 nM & [13] \\ \hline Carbon-paste electrode & Differential pulse & & & \\ (CPE) & voltammetry (DPV) & HCl solution & 0.52 nM & [31] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of the analytical performance of Au-ZnO/Hg-IIP to that of other handpicked relevant sensors reported in the literature. Figure 12: Evaluation of sensor’s selectivity. (**A**): SWV curves in H\({}_{2}\)O/KCl 0.1 M solution of ZnO/Hg(II)-IIP electrodes incubated in 10\({}^{-4}\) M solutions of either mercury, cadmium, lead or copper ions for 20 min. (**B**): Competition tests. Conditions: E-pulse: 0.01 V, E-step: 0.01 V, frequency: 25 Hz, equilibration time: 2 s.