# Nanomaterials-Based Ion-Imprinted Electrochemical Sensors for Heavy Metal Ions Detection: A Review
## 3 Application of Nanomaterials in HMIIECSs
### Carbon-Based Nanomaterials
#### 3.1.2 Graphene Materials  
Graphene (GR) is made up of a single sheet of carbon atoms arranged in sp\({}^{2}\), which are covalently bonded to each other and arranged in an ordered hexagonal lattice [96]. GR has the characteristics of large specific surface area, high porosity, many active centers, high charge transport rate, good electrical conductivity, high mechanical stability, and good electrocatalytic and photocatalytic activity [97]. GR is prepared by a low-cost, simple, green and environmentally friendly method, which is easy to functionalize compared to metals [98]. Various structures of GR, including graphene oxide (GO), reduced graphene oxide (rGO), GR quantum dots, GR nanoplates, GR nanoribbons, and three-dimensional GRs exhibit various properties and extend the application of GR from energy equipment and water treatment technology to sensors, as shown in Figure 6[99]. [PERSON] et al. combined the advantages of graphene, IIPs and electrochemical techniques to develop a highly sensitive and selective IIECS (Pd(II)-IIM/GR/GCE) for amperometric determination of Pd\({}^{2+}\) in complex substrates [100]. In this study, GR was directly modified on the electrode surface by GO electrodeposition, which can enhance the electron transfer and sensitivity of the sensor. Subsequently, IIP was synthesized on this modified surface by in situ polymerization with allylurea (NAU) as a functional monomer, EGDMA as a crosslinking agent, and AIBN as an initiator. By measuring the i-t curve of Pd\({}^{2+}\), it was found that the linear response range of the designed electrochemical platform to Pd\({}^{2+}\) was 2.0 \(\times\) 10\({}^{-8}\)-2.0 \(\times\) 10\({}^{-4}\) M, and the LOD was 6.4 \(\times\) 10\({}^{-9}\) M. Interference studies have shown that other metal ions did not interfere with the determination of Pd\({}^{2+}\) even if their concentration was 15 times higher than that of Pd\({}^{2+}\). The sensor has been successfully used for the determination of Pd\({}^{2+}\) in the catalysts and factory samples with RSD of less than 3.3% (n = 5) and recoveries in the range of 99.2-106.5%. In general, the direct use of pristine GR for electrochemical sensing has many problems such as low sensitivity, interference from other substrates, and easy agglomeration. Therefore, GR-based composite nanomaterial sensors have been extensively studied [101]. The sensitivity of HMIs sensing will be increased by using sensor based on polymer modified GR with wealthy reactive sites because in this case the analyte can be preconcentrated on the electrode surface [101]. In 2018, [PERSON] et al. developed an ion-imprinted electrochemical platform (CS-GR-IIP) based on CS-GR nanocomposites for the determination of hexavalent chromium [62]. CS possesses non-toxicity, good biodegradability, superior biocompatibility, favorable flexibility, excellent film forming capacity, and abundant metal chelating sites, so it is very suitable for preparing stable IIM on the electrode surface with various modification materials [102]. The IIPs were prepared by one-step electrodeposition. The electrochemical behavior of CS-GR-IIP was studied by using CV, electrochemical impedance spectroscopy (EIS) and DPV. The linear range of CS-GR-IIP/electrode was 1.0 \(\times\) 10\({}^{-9}\)-1.0 \(\times\) 10\({}^{-5}\) M, and the LOD was 6.4 \(\times\) 10\({}^{-10}\) M (S/N = 3). In the presence of Zn\({}^{2+}\), Co\({}^{2+}\), Cu\({}^{2+}\), Ni\({}^{2+}\), Mn\({}^{2+}\), MnO\({}_{4}\)\({}^{-}\), C\({}_{2}\)O\({}_{4}\)\({}^{2-}\), S\({}_{2}\)O\({}_{6}\)\({}^{2-}\) and MoO\({}_{4}\)\({}^{2-}\), the sensor had high  
Figure 6: Structure of (**a**) GR, (**b**) GO, (**c**) rGO and (**d**) GQDs. Reprinted with permission from Ref. [99]. Copyright 2021, the Springer Nature. selectivity, excellent stability and good repeatability for the detection of Cr\({}^{6+}\). It has been successfully applied to the detection of Cr\({}^{6+}\) in tap water and river water.