###### Abstract  
A biomimetic, ion-imprinted polymer (IIP) was prepared by electropolymerization of pyrrole at the surface of gold electrodes decorated with vertically grown ZnO nanorods. The vertical growth of the nanorods was achieved via an ultrathin aryl monolayer grafted by reduction of diazonium salt precursor. Pyrrole was polymerized in the presence of L-cysteine as chelating agent and Hg\({}^{2+}\) (template). Hg\({}^{2+}\)-imprinted polypyrrole (PPy) was also prepared on a bare gold electrode in order to compare the two methods of sensor design (Au-ZnO-IIP vs. Au-IIP). Non-imprinted PPy was prepared in the same conditions but in the absence of any Hg\({}^{2+}\) template. The strategy combining diazonium salt modification and ZnO nanorod decoration of gold electrodes permitted us to increase considerably the specific surface area and thus improve the sensor performance. The limit of detection (LOD) of the designed sensor was \(\sim\)1 pM, the lowest value ever reported in the literature for gold electrode sensors. The dissociation constants between PPy and Hg\({}^{2+}\) were estimated at [K\({}_{\text{d1}}\) = (7.89 \(\pm\) 3.63) mM and K\({}_{\text{d2}}\) = (38.10 \(\pm\) 9.22) pM]. The sensitivity of the designed sensor was found to be 0.692 \(\pm\) 0.034 uA.pH\({}^{-1}\). The Au-ZnO-IIP was found to be highly selective towards Hg\({}^{2+}\) compared to cadmium, lead and copper ions. This sensor design strategy could open up new horizons in monitoring toxic heavy metal ions in water and therefore contribute to enhancing environmental quality. Keywords:ionic imprinted polypyrrole; zinc oxide nanorods; mercury ions; diazonium salts; electrochemical sensor +
Footnote †: journal: Applied  
by the United States Environmental Protection Agency (USEPA), European Union (EU) and World Health Organization (WHO). The environmental monitoring of mercury is very much important to avoid any further damage to the ecosystem [12]. The WHO has set a maximum permissible limit of (1 ug-L-1/5 nM) for mercury [13]. Developing reliable, sensitive and selective methods for the determination of mercury concentrations in food and drinking water is thus of particular significance. Electrochemical sensors [4; 14; 15; 16; 17] are an excellent alternative combining miniaturization, speed and low cost. Field analysis of metal pollutants (even within a processing plant, for example) will be possible thanks to portable, wireless and ultra-sensitive potentiostats [18]. Ion imprinted polymers (IIPs) constitute a new class of sorbents possessing selectivity and affinity for the separation, pre-concentration or removal of target ions [14; 16; 19; 20; 21]. IIPs are prepared by the formation of specific recognition sites in the framework of organic polymers and usually prepared by the bulk polymerization method [16; 22]. Template ions are then removed, leaving cavities or \"artificial receptor sites\" in the polymer layer that have appropriate shape, size and orientation of the functional groups for ion recognition at the rebinding stage [23; 24; 25]. For this reason, imprinted polymers are coined \"plastic antibodies\" for their ability to mimic antibodies. One of the most common applications of IIPs is solid phase extraction (SPE), which permits the pre-concentration of metal ions traces in environmental samples like natural water or seawater [26; 27; 28]. IIPs as electrochemical sensors have shown remarkable selectivity, good thermal stability, low cost and excellent sensitivity [29; 30; 31; 32; 33; 34]. The current challenge for most scientific researchers is to improve the performance of electrochemical sensors, including the limit of detection, in order to detect heavy metal ions at trace levels and by using very simple and inexpensive methods. To address this issue, we reasoned to increase the electroactive surface using nanoparticles to form a nanostructured sensing layer [35; 36].