## 3 Results and Discussion
### Topographical Characterization  
The atomic force microscopy (AFM) measurements have been carried out in order to monitor the topographic changes after each surface functionalization step. The evidence of nanorod growth is not the purpose herein, that being clearly shown in a previous work [47]. Therefore, we can use standard tapping mode AFM probes instead of high aspect ratio ones. The robustness of the former allows comparative investigation of several samples' surfaces without changing the probe. Two-dimensional AFM images of (\(5\times 5\)) \(\upmu\)m\({}^{2}\) have thus been achieved on the same sample, before (bare gold substrate) and after each functionalization step (dazonium salt, ZnO nanoparticles, ZnO nanorods, then IIP) and finally after extraction of the Hg\({}^{2+}\) template (Figure 6).  
Figure 6: The generated \(5\times 5\)\(\upmu\)m\({}^{2}\) AFM images of (**A**) bare gold electrode, (**B**) gold electrode-dazonium salt, (**C**) Au/Diazo/ZnO nanoparticles, (**D**) Au/Diazo/ZnO nanorods, (**E**) Au/Diazo/ZnO NRs-IIP, (**F**) extracted Au/Diazo/ZnO NRs-IIP.  
The AFM images (Figure 6A-F) exhibit a granular-like aspect and seem different enough in both shape and size. The grafting of diazonium salt (Figure 6B) leads to the formation of a layer veiling the granular appearance. The latter is again visible (Figure 6C) after grafting the ZnO nanoparticles, with higher contrast than for the bare gold surface (Figure 6A). Figure 6D, which corresponds to ZnO nanorod growth, exhibits the largest size of surface singularities, which seems reduced by the electropolymerization of IIP (Figure 6E). The extraction of template from the IIP film (Figure 6F) leads to a different aspect that is probably due to the local rearrangement of the IIP.  
Despite the usefulness of AFM images, qualitative description remains limited due to the high contrast that may exist in the presence of multiple defects with high amplitudes. Measurements should therefore be considered from a quantitative point of view by considering relevant roughness statistical parameters, such as rms roughness (_Sq_) and peak-valley height (_Sp-Sv_). Values of these two parameters are presented in Figure 7.  
As expected, each surface modification leads to a noticeable change in roughness parameters. The most important one occurs after diazonium salt grafting. This first step of functionalization led to an increase in (_Sq_) and (_Sp-Sv_) parameter values by a factor of 5.7 and 7.8, respectively. The increase in roughness continued with the grafting of ZnO NPs, but to a lesser extent, by a factor of 1.7 for (_Sq_), while the (_Sp-Sv_) value remains relatively stable. The growth of the nanorods generated a variation in the two considered parameters by a factor of 2.9 and 2.4, respectively. The electro-polymerization of IIP produced a coating effect resulting in a significant decrease in the values of the two roughness parameters by a factor of 4.7 for (_Sq_) and 3.3 for (_Sp-Sv_). These two parameters decreased by a factor of around 1.5 only, after the analyte extraction step. This slight decrease is probably due to the local rearrangements which occur once the Hg\({}^{2+}\) ions are extracted from the polymer matrix.  
The corresponding 2D images of ZnO NPs (Figure 8a) and ZnO NRs (Figure 8b), show the dimensions of the ZnO NPs and NRs. The diameter of the NPs is between 70 and 80 nm, while that of the obtained NRs is in the range of 300-350 nm (the same diameter was obtained in our previous work [47]). Figure 8c,d represents 2D and 3D images for the same analysis area on a surface of (80 \(\times\) 80 \(\upmu\)m\({}^{2}\)). The growth of NRs is generally homogeneous, and the height after growth of NRs reaches 325 nm (Figure 8d).  
Figure 7: Root-mean-square height (_Sq_) and peak-valley height (_Sp-Sv_) values after the different functionalization steps, from bare gold substrate to electrosynthesis of IIP and the further extraction of the imprinted Hg\({}^{2+}\) template ions.