# Nanomaterials-Based Ion-Imprinted Electrochemical Sensors for Heavy Metal Ions Detection: A Review
## 2 Preparation of IIPs
### Preparation Principle of HMIIECSs
#### 2.3.2 Preparation Methods of HMIIECS  
According to the different electrode modification methods, HMIIECS can be prepared by the direct and indirect methods [19]. The direct method refers to that of forming IIPs directly on the electrode surface, which can be divided into in situ polymerization and electropolymerization [19; 58]. In situ polymerization is a method of mixing functional monomers, template ions and initiators under certain conditions such as light or heat, and then applying them to the electrode surface for polymerization to form ion-imprinted membranes (IIMs). The thickness, hydrophobicity and permeability of IIPs prepared by this method can be artificially controlled and these IIPs are usually non-conductive, so conductive additives such as graphene or GO etc. are added to the pre-polymerized mixture [24]. The HMIIECSs prepared by this method presents simple and good recognition performance, but their IIMs are easy to fall off and their reproducibility is poor. Table 1 shows the comparison of this class of HMIIECSs. Electropolymerization means that the electrode is placed in a solution mixed with the functional monomers and the template ions in a certain proportion. Under the condition of applying electric potential or current, the functional monomers will polymerize on the electrode, and the template ions will be embedded in the polymeric film to form an IIM on the electrode surface. It is a good alternative approach, which can directly produce a uniform and selective film on the transducer surface and can overcome the problems  
Figure 2: Construction and detection principle of HMIIECSs. Adapted with permission from Ref. [57]. Copyright 2021, Elsevier. associated with the fixation procedure [59]. Additionally, the synthesis of IIMs can be easily controlled by electrochemical methods to obtain the controllable amount of imprinted cavities. The mechanism of electropolymerization is still incompletely understood, nevertheless, the simplified one of the electroactive monomer, such as thiophene (Th) or Py involving alternate chemical (C) and electrode (E) reaction steps can be discussed [60]. As reported in this reference, taking Th electropolential polymerization as an example (shown in Figure 3), Th is firstly electrooxidized to form a free radical cationic II in step (E), which appears as an anode peak of high positive potential (Ep,a = 1.6 V vs. the saturated calomel electrode, SCE). In the second step (C) reaction, II reacts with the monomer I to form a protonic dimer of the free radical cation III. In the third step (E\({}^{\prime}\)) reaction, III is electrooxidized to the so-called double-charged \(\sigma\)-dimer IV, which cancels out the two protons to form a neutral dimer V at the following (C\({}^{\prime}\)) step. This dimer of V is then oxidized to form a free radical cation VI in the (E\({}^{\prime\prime}\)) step. In turn, VI reacts with I in the (C\({}^{\prime\prime}\)) step. The trimer IX is formed by these steps following the ECE\({}^{\prime}\)C\({}^{\prime}\)E\({}^{\prime\prime}\)C\({}^{\prime\prime}\) mechanism in sequence. It is advantageous that the electrooxidation potential of the dimer V (Ep,a = 1.1 V vs. SCE) is lower than that of the monomer I. Therefore, many researchers are interested in this technique and have achieved good results in the water quality of HMIs detection. [PERSON] et al. reported a sensitive and highly selective voltammetric sensor for the determination of copper(II) in water samples by electropolymerization of p-phenylenediamine (PPD) with Cu\({}^{2+}\) as the template ions on the surface of screen-printed electrodes [59]. Cyclic voltammetry (CV), differential pulse voltammetry (DPV), scanning electron microscopy (SEM) analysis, and FTIR-ATR measurements were employed to characterize the proposed sensor. Good detection performance was obtained in acetate buffer (50 mM, pH = 5) that the linear range of Cu\({}^{2+}\) was 0.95-244 nM with a LOD of 2.7 nM, and the analytical application in a spiked water sample revealed a good recovery of 95-105%.