# Nanomaterials-Based Ion-Imprinted Electrochemical Sensors for Heavy Metal Ions Detection: A Review
## 3 Application of Nanomaterials in HMIIECSs
### Silica Nanomaterials  
Silica is the most commonly used core material for the preparation of MIPs because of its advantages such as thermal stability, biocompatibility, high permeability, and stability under acidic conditions, and its surface modification can be achieved by several silane coupling agents such as 3-aminopropylriethoxysilane (APTES), methacryloxyp-ltrimethoxysilane (MPS), and vinyltriethoxysilane (VTES) via conjugation the active hydroxyl groups on the silica particle surface [136]. These modification groups can improve the reactivity of silica surface, which is beneficial to receive IIP shell. [PERSON] et al. prepared Zn(II) IIP by radical polymerization on the surface of vinyl silica particles and applied them to fabricate CPEs for monitoring Zn\({}^{2+}\) by DPV [137]. Vinyl functionalized silica particles were synthesized using the sol-gel method and the IIP particles were prepared by radical polymerization. Different morphological and elemental techniques were employed for their characterization. The ion-imprinted particles were used to fabricate CPEs as Zn\({}^{2+}\) sensors. The improved Zn\({}^{2+}\) sensor showed a linear response in the concentration range of 6.12 \(\times\) 10\({}^{-9}\)-4.59 \(\times\) 10\({}^{-8}\) M, and the LOD and LOQ of the electrode were 1.351 \(\times\) 10\({}^{-8}\) M and 4.094 \(\times\) 10\({}^{-8}\) M, respectively. The proposed sensor had high selectivity and sensitivity to trace Zn\({}^{2+}\), and can be used for the detection of Zn\({}^{2+}\) in aqueous solution. [PERSON] et al. proposed an electrochemical sensor based on Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)@IIP modified GCE for the determination of Cd\({}^{2+}\) by DPV [138]. Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)@IIP was prepared by using the copolymerization approach including cadmium ions, EGDMA, AIBN, 4-VP, and 2-aminobenzimidazole. Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)@IIP modified GCE as shown in Figure 14. By grafting IIP onto the functional MNPs, the advantages of combining IIP with core-shell Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) NPs as well as their relative surface area and adsorption capacity were improved. The linear range of response to Cd\({}^{2+}\) was 0.008-0.05 \(\upmu\)M, and the LOD was 1 \(\times\) 10\({}^{-4}\)\(\upmu\)M. The platform has been successfully applied to the determination of trace Cd\({}^{2+}\) in environmental water samples. [PERSON] et al. homogeneously coated SiO\({}_{2}\) on the Fe\({}_{3}\)O\({}_{4}\) core to form the Fe\({}_{3}\)O\({}_{4}\)/SiO\({}_{2}\) NPs through a revised Stober method, then crosslinked the CS film incorporating Cu\({}^{2+}\) to Fe\({}_{3}\)O\({}_{4}\)/SiO\({}_{2}\) with glutaraldehyde to obtain Fe\({}_{3}\)O\({}_{4}\)/SiO\({}_{2}\)/CS/IIP, and finally dispersed the above Cu\({}^{2+}\)-IIP nanocomposites in Nafion solution and modified GCE by dip-coating to fabricate the electrochemical sensor denoted as Fe\({}_{3}\)O\({}_{4}\)/SiO\({}_{2}\)/CS/Nafion/GCE IIP [47]. During the electrochemical measurement, different experimental parameters were optimized. The sensor showed a wide linear detection range of 0.01-20 uM, a low LOD of 5 nM, and a significant reproducibility with RSD of 3.3%. The research showed that the sensor based on Fe\({}_{3}\)O\({}_{4}\)/SiO\({}_{2}\)/CS imprinted polymer had the potential to monitor the pollution of other toxic metal ions in a harsh environment. [PERSON] et al. synthesized a new nanometer IIP with ion recognition ability using SiO\({}_{2}\)-coated magnetite nanomaterials (SCMNPs) as the core and supporting material of the nanostructure and it was applied to modify CPE for Hg\({}^{2+}\) detection [139]. Highly selective and sensitive interactions between nano-IIP and Hg\({}^{2+}\) ions enhanced the deposition of target ions on the electrode surface, thereby enhancing the electrochemical signal. Square wave stripping voltammetry (SWV) was used for the rapid, simple, accurate, sensitive and highly selective determination of Hg\({}^{2+}\). The linear range and LOD were 0.20-1600.0 ng/mL, 0.04 ng/mL, respectively.