# 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
Noble-metal NPs modified GR has been applied for the construction of HMIIECSs because metal NPs exhibit high catalytic activity through the size effect and GR can transfer the electrons gained during the catalysis of metal NPs to the electrodes, thus speeding up the catalytic process [101]. Among the noble-metal NPs, Au NPs are the most widely used in the detection of metal ions because of their advantages of high chemical stability and easy preparation [101]. [PERSON] et al. developed an ultrasensitive electrochemical platform based on imprinted CS/AuNPs/GR nanocomposites by electropolymerization (electrodeposition for 180 s at \(-\)1.2 V) for trace monitoring Cd(II) ions in drinking water and milk samples [25]. The CS/AuNPs/GR/GCE platform showed high sensitivity, excellent selectivity, good stability, and repeatability. Under the optimized conditions, its linear range varied from 0.1 to 0.9 \(\upmu\)M and the LOD was 1.62 \(\times\) 10\({}^{-4}\)\(\upmu\)M. The method was applied for sensing Cd(II) ions in real samples, and the results demonstrated that the developed CS/AuNPs/GR/GCE was a promising electrochemical platform for the quantitative analysis of Cd(II) ions in food samples. [PERSON] et al. introduced a novel Zn\({}^{2+}\)-ion selective electrode based on a nano-sensing layer composed of silver NPs decorated GR nanosheets (GNS@Ag NPs), Zn\({}^{2+}\)-IIP NPs, 1-butyl-1-methylpyrrolidine di (trifluoromethylsulfonyl) imide IL, and graphite powder (GP) [103]. Zn\({}^{2+}\)-IIP NPs as the efficient sensing agent was synthesized by using Zn\({}^{2+}\) as the template ion, methanol/ethanol (3/1; \(v/v\)) as the porogen solvent, MAA as the monomer, EGDMA as the crosslinker and AIBN as the initiator. Under the optimized experimental conditions, the lower LOD of the proposed potentiometric sensor was 1.93 \(\times\) 10\({}^{-1}\)\(\upmu\)g/L, and the linear analysis range was from 2.62 \(\times\) 10\({}^{-1}\) to 6.54 \(\times\) 10\({}^{-5}\)\(\upmu\)g/L. The zinc ion-selective sensor proposed in this paper has been successfully applied to the high-sensitivity determination of trace Zn\({}^{2+}\) in environmental and biological samples. In the electrochemical analysis of HMIs, GO prepared by the GR precursor system is more common because many oxygen functional groups (such as carboxyl and hydroxyl groups) are introduced into its GR monolayer, which can improve the dispersibility in solutions and provide enough binding sites for adsorbing HMIs [29]. Therefore, many ECSs for HMIs based on GO and its composites have been developed continuously [104]. In 2018, [PERSON] et al. fabricated Hg(II)-imprinted polymeric- and composite based sensors and investigated their efficiency base on four polymeric systems such as (1) polystyrene, (2) poly(vinylpyrrolidone), and (3) Styrene-co-vinylpyrrolidone polymer, and IIP composite of Styrene-co-vinylpyrrolidone polymer and GR sheets [105]. The ion-imprinted polystyrene system was an ideal receptor for the detection of mercury ions in solution, and the LOD was 2 ppm. The sensitivity of the IIP copolymer system was further improved by recombination with GO, and the LOD of the recombination system was estimated to be about 1 ppm. To sum up, the sensitivity and selectivity of IIPs and composite were in the following order: composite > polystyrene > copolymers > poly(vinyl- pyrrolone). The results indicated that by combining the IIT with the interdigital transducers, a very sensitive, selective, low-cost and user-friendly sensor system can be made for any HMI. [PERSON] et al. designed a CS/GO-IIP/GCE by the dip coating method for highly sensitive and selective detection of Cu\({}^{2+}\), and the preparation process was shown in Figure 7[65]. The Cu\({}^{2+}\) IIPs were synthesized by dropping CS/GO/Cu\({}^{2+}\) composite on the GCE surface and chemically crosslinked with epichlorohydrin. The electrical conductivity of the sensor was improved and the electrochemical signal was amplified attributed to the introduction of GO.