## 3 Results and Discussion
### Electrochemical Sensing of Mercury  
SWV was used to investigate the electrochemical response of the ZnO/Hg\({}^{2+}\)-IIP sensor towards mercury in solution at different concentrations. Sensor response was recorded, via the characteristic peak current, for concentrations between 10\({}^{-12}\) and 10\({}^{-3}\) M (Figure 10). The sensor limit of detection (LOD) was determined from the lowest concentration that gave a visible signal [81; 82], instead of the method based on the signal to noise ratio of 3, as usually done. Here, the obtained LOD of 1 pM is much lower than the WHO maximum permissible limit in drinking water of (1 \(\mu\)g.L\({}^{-1}\)). To confirm the performance of the sensor, this Hg\({}^{2+}\) detection step was conducted on three different ZnO/Hg\({}^{2+}\)-IIP, and the average calibration curve is shown in Figure 11 (variations of output signal of electrochemical IIP-based sensor versus Hg\({}^{2+}\) concentrations). Figure 10: Follow-up of peak current variations (determined from SWV measurements in H\({}_{2}\)O/KCI 0.1 M solution) according to the potential. Conditions: E-pulse: 0.01 V, E-step: 0.01 V, frequency: 25 Hz, equilibration time: 2 s.  
Figure 9: Hg-IIP response after different incubation times in 10\({}^{-4}\) M mercury solution/H\({}_{2}\)O/KCI 0.1 M.  
The dissociation constant \(K_{d}\) is an important parameter that determines the degree of affinity between the polymer and the template. Different analytical models were tested to fit the experimental data of the electrochemical measurements. The most appropriate one was a combination of the one site model and Hill model (Figure 11). The choice of this model makes it possible to take into consideration the mechanical restructuring of polymers at the nanometric scale [83]. \[Y(C)=\frac{A_{1}\times C}{K_{d1}+C}+\frac{A_{2}\times C^{\alpha}}{K_{d2}^{ \alpha}+C^{\alpha}}\]  
where \(y\) (\(C\)) is the output sensor's signal response (current) for a given concentration (\(C\)) of Hg\({}^{2+}\). \(K_{d1}\), and \(K_{d2}\) are the first and second dissociation constants, \(A_{1}\) and \(A_{2}\) are empiric constants, and \(\alpha\) is an empiric ponderation exponent. The values obtained for \(K_{d1}\) and \(K_{d2}\) were in the order of (\(7.9\pm 3.6\)) mM and (\(38.1\pm 9.2\)) pM, respectively. The value of \(K_{d2}\) is consistent with that linked to the binding of Hg\({}^{2+}\) to the Cys4 site in CP-CCCC peptide, which shows that the IIP can imitate biological entities in terms of affinity for heavy ions [71; 83]. The \(\alpha\) value of \(0.06\pm 0.002\), indicates positive cooperative interactions between Hg\({}^{2+}\) ions and other binding sites [71]. The sensor sensitivity, computed from the slope at the origin of current/concentration curves (i.e., at lower concentrations), was in the order of \(0.692\pm 0.034\) uA.pM\({}^{-1}\). This value shows that this LCys-PPy sensor prepared on Au-ZnO to capture mercury is very sensitive compared to several sensors intended to detect heavy metals [84; 85; 86]. In addition to these interesting metrological features, we also tested the selectivity of the sensor by comparing the recognition of the targeted Hg\({}^{2+}\) to lead, cadmium and copper ions. The rationale for this set of ions is the following: Cd\({}^{2+}\) has an ionic radius close to that of Hg\({}^{2+}\); Cu\({}^{2+}\) has a lower ionic radius than Hg\({}^{2+}\), whereas Pb\({}^{2+}\) is a larger ion than the targeted Hg\({}^{2+}\). We first incubated the extracted film in a solution containing an ion of the chosen ions with a concentration of \(10^{-4}\) M (Figure 12A). The second step was to mix several ions with Hg\({}^{2+}\) with the same concentration of each ion (Figure 12B, black curve) and conduct another test in a solution containing Pb\({}^{2+}\) and Hg\({}^{2+}\) with a concentration of Pb\({}^{2+}\) greater than 100 times that of Hg\({}^{2+}\) (Figure 12B, red curve).