# 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
Table 2 lists the composition and main results of this kind of HMIIECSs. In the indirect methods, IIPs are synthesized first by bulk, precipitation, or suspension polymerization and so on and eluted the template ions, and then immobilized on the electrode surface by means of a medium, which can be divided into a coating method (modifying IIPs onto the electrode surface by various coating methods) and combination method (mixing IIPs with electrode materials in a certain proportion and then modifying the electrode) [19]. In the coating method, a uniform IIM will be formed on the electrode surface after the solvent volatilizes. [PERSON] et al. fabricated an Eu(III) ion-imprinted sensor on a screen printed electrode (SPE) (Eu(III)-IIM/PC/SPE) by using a poly catechol (PC) film as a signal amplifying element and an Eu(III) IIM as the recognition material [72]. In this work, the Eu(III) IIM was prepared via dripping the imprinted sol on the surface of PC/SPE, drying to be solidified under the ambient temperature, and then eluting Eu(III) in HCl. The prepared HMIIECS based on the coating method has good recognition, but the thickness of IIM could not be controlled, and long response time was required for \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Target** & **Electrode** & **Nanomaterial** & **Monomer** & **Electrochemical Method** & **Linear Range** & **LOD (μM)** & **Ref.** \\ \hline Co\({}^{2+}\) & GCE & Fe\({}_{3}\)O\({}_{4}\) & AAM & DPCSV & 5.0 \(\times\) 10\({}^{-4}\)-0.02 and & 1.0 \(\times\) 10\({}^{-4}\) & [61] \\ Cr\({}^{\text{\tiny{ph}}}\) & GE & GR & CS & DPV & 1.0 \(\times\) 10\({}^{-3}\)-10 & 6.4 \(\times\) 10\({}^{-4}\) & [62] \\ Ce\({}^{\text{\tiny{ph}}}\) & SPCE \({}^{\text{\tiny{a}}}\) & Fe\({}_{3}\)O\({}_{4}\) & But-2-enodic acid bis-(2-amino-ethyl)-amide & DPASV & 1.78 \(\times\) 10\({}^{-3}\)-4.45 \(\times\) 10\({}^{-2}\) & 5.0 \(\times\) 10\({}^{-4}\) & [63] \\ Gd\({}^{3+}\) & SPCE & Fe\({}_{3}\)O\({}_{4}\) & But-2-enodic acid bis-(2-amino-ethyl)-amide & DPASV & 4.71 \(\times\) 10\({}^{-3}\)-6.02 \(\times\) 10\({}^{-2}\) & 1.2 \(\times\) 10\({}^{-4}\) & [63] \\ Zn\({}^{2+}\) & PGE \({}^{\text{\tiny{b}}}\) & MWCNTs & BAAP & DPASV & 4.21 \(\times\) 10\({}^{-4}\) & 1.5 \(\times\) 10\({}^{-3}\)-0.364 & [64] \\ Cu\({}^{2+}\) & PGE & MWCNTs & BAAP & DPASV & 2.5 \(\times\) 10\({}^{-4}\) & 1.54 \(\times\) 10\({}^{-3}\)-0.375 & [64] \\  & GCE & GO & CS & DPASV & 0.5–100 & 0.15 & [65] \\ \hline \hline \end{tabular}  
* SICE \({}^{\text{\tiny{a}}}\): Screen-printed carbon electrode; PGE \({}^{\text{\tiny{b}}}\): Pencl graphite electrode; BAAP \({}^{\text{\tiny{c}}}\): Bis-(2-acrylolamino-ethyl)-phosphinic acid.  
\end{table} 
Table 1: Summary of the HMIIECSs prepared by in-situ polymerization. detection. By contrast, the combination method is widely used in the field of analysis and detection, among which the most classical way is the modified CPE. This kind of HMIIECSs has the advantages of cheap materials, low residual current and easy surface renewal, but also has some shortcomings such as poor accuracy and poor reproducibility. [PERSON] et al. prepared IIP-CPE for voltammetry detection of Hg\({}^{2+}\) by the combination method [73]. In detail, the Hg\({}^{2+}\) IIP was synthesized by thermal bulk polymerization with Hg(NO\({}_{3}\))\({}_{2}\) as the template, 1,5-diphenylcarbazone (DPC) as the ligand, and EGDMA as the monomer. Then, the imprinted graphite powder with the oil wax was mixed at a ratio of 7:3 (weight ratio), and the formed carbon paste was inserted into a cylindrical electrode holder glass (d = 3 mm) and contacted with a copper rod to obtain IIP-CPE. [PERSON] et al. synthesized the Hg-IIP by using Py as the functional monomer, methanol as the porogen, EGDMA and sodium persulfate as the crosslinker and the initiator, respectively and constructed the modified electrode (CPE-IIP) by packing a composite mixture of 0.02 g of IIP, 0.18 g of graphite powder, and 0.08 g of paraffin oil into a plastic tube (0.50 mm in diameter and 25 mm in depth) situated at one end of the tube [32].