## 3 Results and Discussion
### Preparation of IIP and NIP-Based Electrodes  
Mercury oxidation on gold electrodes yields two peaks: the first peak is attributed to the oxidation reaction of Hg(0) to Hg(I) (in the form Hg\({}^{2+}\)) while the second peak is due to the transformation of Hg(I) into Hg\({}^{2+}\)[68; 7; 76]:  
\[1/2\text{Hg}_{2}{}^{2+}\rightarrow\text{Hg}^{2+}+\text{e}^{-}\text{ (or Hg(I)}\rightarrow\text{Hg(II)}+\text{e}^{-})\]  
Before preparing the films on gold electrodes, we first characterized the signature of mercury with a gold disk working electrode in a solution containing mercury and KCl 0.1 M. The response obtained by SWV shows that the oxidation of mercury on gold gives two peaks at 0.061 and 0.71 V (vs. Ag/AgCl) (Figure 2). The position of the two peaks depends on the pH of the solution, the nature of the working electrode, the electrolyte and the layer or film deposited on gold. In the following, we will consider the most intense peak located at 0.71 V (vs. Ag/AgCl), which corresponds to Hg\({}^{2+}\). This dominant peak means that most of the mercury is converted to Hg\({}^{2+}\).  
A thin PPy barrier layer was deposited by chronoamperometry (CA) for 2 s before Au-ZnO-IIP electro-formation to prevent the complexation of mercury by Au-ZnO electrodes (Figure 3A, black curve). Both ZnO-IIP and ZnO-NIP were prepared by the same technique (Figure 3A, red curve) and then compared using SWV (Figure 3B). On the one hand, one can notice the obvious difference between the responses of ZnO-IIP and ZnO-NIP, i.e., mainly for the peak current corresponding to a potential of around 0.62 V (vs. Ag/AgCl) in the case of ZnO-IIP will be considered for quantitative monitoring of mercury. On the other hand, the removal of Hg\({}^{2+}\) from ZnO-IIP by EDTA leads to a flat SWV response, close to that of ZnO-NIP (Figure 3B). This means that all receptor sites have been released and thus can be reused for re  
Figure 2: Characteristic peaks of mercury oxidation on gold electrode in H\({}_{2}\)O/KCl 0.1 M solution. Conditions: E-pulse: 0.01 V, E-step: 0.01 V, frequency: 25 Hz, equilibration time: 2 s.  
In order to showcase the central role of ZnO nanorods, we prepared two different IIPs: the first was prepared directly on a bare gold electrode and the second IIP was prepared on Au-ZnO. After extraction of the metal ions from these two IIPs in EDTA solution, they were incubated in \(10^{-3}\) M mercury solution for 20 min for the rebinding test.  
From the results displayed in Figure 4, one can notice the important role of ZnO NRs thanks to the spaces between the NRs. ZnO NRs will be considered as a substrate for depositing the polymer matrix instead of depositing it directly on the gold electrode.  
Square wave voltammograms presented in Figure 4 show that the current intensity of the IIP prepared on ZnO nanorods is twice as high as that prepared on bare gold electrodes. Given that the surface area used on the bare gold electrode to form Au/Hg-IIP is identical to that used to form Au-ZnO/Hg-IIP, this can be related to the increase in the surface area thanks to ZnO nanorods, which makes it possible to deposit more IIP and thus a large number of artificial sites to capture more Hg\({}^{2+}\) ions.