# 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
In order to investigate the ion-electron conversion capability of graphene nanosheets and ionic liquids on the electrode surface, EIS measurements were conducted. After optimization, the LOD of the nanocomposite sensor was 1.95 \(\times\) 10\({}^{-9}\) M, and the linear analysis range was 4.00 \(\times\) 10\({}^{-9}\)-1.30 \(\times\) 10\({}^{-3}\) M. Therefore, it was suggested to use the sensor for the determination of Hg\({}^{2+}\) content in actual samples. [PERSON] et al. prepared a new IIECS for detecting Cd(II) based on nitrogen-doped rGO (N-rGO)-CS modified GCE via the one step deposition method [33]. The sensor was constructed by co-depositing N-rGO-CS-Cd(II) on the surface of the GCE with Cd(II) as the template ion, N-rGO as the conductivity enhancer, CS as the functional monomer, sodium  
Figure 9: (**a**) FE-SEM image of rGO, (**b**) FE-SEM image of rGO-IIP, (**c**) TEM image of rGO and (**d**) TEM image of rGO-IIP. Reprinted with permission from Ref. [109]. Copyright 2016, Elsevier. tripolphosphate as the crosslinker and ethylene diamine tetraacetic acid (EDTA) as the eluent to remove template ions. Owing to the synergistic effect of N-RGO, the conductivity of the electrode was enhanced, and the detection ability and sensitivity of Cd(II) was also improved ascribed to the target ion holes. The electrochemical properties of Cd(II)-IIECS were evaluated by CV, EIS, and DPV, and the linear range was obtained in 0.01-0.1 \(\upmu\)M with the LOD of 3.51 nM (S/N = 3). It also showed that the developed Cd(II)-IIECS had high sensitivity, good reproducibility, well stability and was suitable for practical application. Graphene quantum dots (GQDs) consisting of a monolayer or a few layers of graphene are one of the latest frontiers of carbon based nanomaterials due to their excellent and unique properties including low toxicity, high solubility in many solvents, excellent electronic properties, strong chemical inertness, large specific surface area, rich functionalized edge location, good biocompatibility, and low cost and versatility [111; 112]. They offer interesting properties for current electrochemical biosensing, as well as their ability to be modified by attractive surface chemistries and other modifiers/nanomaterials [113]. GQDs are highly electroactive and can reduce the overpotential of the analyte; therefore, they can be doped into MIP ECSs to enhance the current response [15]. In 2021, [PERSON] et al. used thiourea derivative-functionalized GQDs (GQDTU) as functional monomers, and prepared mercury ion-imprinted polymer materials (GQDTU-IIP) by using suspension polymerization technology, and with which being used to modify GCE, a novel nanosensor was synthesized for the electrochemical sensing of Hg(II) ions [114]. The preparation process is shown in Figure 10. The introduction of IIPs improved the selectivity of the sensor to Hg(II) ions, and the interference of other HMIS is negligible. CV and DPV were used to detect Hg\({}^{2+}\). By DPV, the linear response range of Hg\({}^{2+}\) concentration was 5.0 \(\times\) 10\({}^{-8}\)-2.3 \(\times\) 10\({}^{-5}\) M, and the LOD was 23.5 nM. However, by CV, the linear response ranges of 6 \(\times\) 10\({}^{-8}\)-8.5 \(\times\) 10\({}^{-7}\) M and 1.4 \(\times\) 10\({}^{-6}\)-7 \(\times\) 10\({}^{-6}\) M were obtained, and the LOD was 30.2 nM.