# 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
Graphite is oxidized to graphite oxide, subsequently stripped to GO and eventually reduced to rGO. Although both GO and rGO contain oxygen functional groups, such as epoxy, hydroxyl and carboxyl groups, rGO contains less oxygen functional groups after reduction of GO. These oxygen-containing groups influence their electrochemical properties in terms of electron transfer rates or molecular adsorption/desorption, and provide anchor sites for enzymes or other species-specific sensing applications [107]. Although GO and rGO are widely used to detect different target molecules in electrochemistry, rGO shows more advantages in the electrochemical applications. The main characteristic of rGO is that its sp\({}^{2}\) carbon content is higher than that of GO. The natural result is the presence of a large \(\pi\) orbital, which allows for higher heteroelectron transfer rates than do GO [108]. [PERSON] et al. prepared IIP on the surface of rGO by surface imprinting technology, using MAA as the functional monomer, EGDMA as the crosslinker, 2,2\({}^{\prime}\)-(9E,10E)-1,4-dihydroxy-anthrace-9,10-dimethylene)bis(hydrazi-1-carbontionamide) (DDBRT) as the chelating agent and ammonium persulfate (APS) as the initiator [109]. The FE-SEM and transmission electron microscope (TEM) images of rGO and rGO-IIP nanocomposites were shown in Figure 9. rGO-IIP/GCE was used for the detection of Hg\({}^{2+}\) in a linear range of 0.07-80 \(\mu\)g/L with a LOD of 0.02 \(\mu\)g/L (S/N = 3), lower than the standard of the WHO. The electrochemical sensor had the advantages of high precision, selectivity, precision and reproducibility, and was applicable for the determination of mercury (II) ions in different water samples. Figure 8: Cyclic voltammograms of GCE, GO@GCE, NIP/GO@GCE, unleached IIP/GO@GCE and leached IIP/GO@GCE in Cd(II) solution buffered with acetate. Reprinted with permission from Ref. [106]. Copyright 2021, the Royal Society of Chemistry. [PERSON] et al. designed two IIECS for both detection of Cd\({}^{2+}\). One was based on ion-imprinted polypyrrole and reduced graphene oxide (PPV/rGO) composites [67]. The IIP was prepared by a two-step method and used to modify the electrode (IIP/rGO/GCE). Specially, rGO was first electrodeposited on the surface of GCE by CV, and then IIP/rGO/GCE was obtained by electropolymerization of Py and template Cd\({}^{2+}\) on the surface of rGO, and imprinted template Cd\({}^{2+}\) were removed by the electrochemical method. In this study, experimental conditions such as deposition time, pH, supporting electrolyte, accumulation time, and potential were optimized. Cd\({}^{2+}\) was detected by SWASV. The experimental results showed that the sensor had good linearity in the range of 1-100 ug/L, and the LOD was 0.26 ug/L. The sensor exhibited good stability, reproducibility and selectivity, and was triumphantly applied to the determination of trace Cd\({}^{2+}\) in the practical spiked water samples. Another sensor (PoPD/ERGO/IIP/GCE) was based on Cd\({}^{2+}\) imprinting, which was prepared by electropolymerization of poly (o-phenylenediamine) (PoPD) and electrochemical reduction of graphene oxide (ERGO) composites, for selective and sensitive determination of trace Cd\({}^{2+}\) in water [68]. In the range of 1-50 ng/mL, the SWASV dissolution peak showed a good linear relationship with the Cd\({}^{2+}\) concentration, and the LOD was 0.13 ng/mL (S/N = 3). The PoPD/ERGO/GCE has been successfully applied to the determination of trace Cd\({}^{2+}\) in real water samples. In contrast, the PPy/rGO/IIP/GCE had a wider linear range, while the PoPD/ERGO/IIP/GCE had a lower LOD. In addition, [PERSON] et al. prepared a novel, simple and very sensitive carbon paste sensor by using rGO nanosheets, alumina nanomaterials and Hg\({}^{2+}\) imprinted polymers as electrode modifiers, and ionic liquids as adhesives [110]. The developed sensor was used for potential determination of Hg\({}^{2+}\) in a variety of real samples.