# Nanomaterials-Based Ion-Imprinted Electrochemical Sensors for Heavy Metal Ions Detection: A Review
## 3 Application of Nanomaterials in HMIIECSs
### Carbon-Based Nanomaterials  
#### 3.1.1 Carbon Nanotube Materials  
Carbon nanotubes (CNTs) are composed of two defined structural groups, single-walled carbon nanotubes (SWCNTs) and MWCNTs as shown in Figure 4[75; 76]. CNTs are sp\({}^{2}\) hybridization based on carbon atoms, which not only have unique electrical properties and low resistance, but also have significant stiffness and large axial strength, high active center specific surface area, controllable pore size distribution and excellent adsorption performance[77]. They can form complexes with HMIs, making it suitable for the development of electroanalytical systems specially used for the detection of HMIs[78]. Moreover, CNTs have higher electronic conductivity in electron transfer reactions and better electrochemical and chemical stability in aqueous and non-aqueous solutions, so they are suitable for the modification of various electrodes[79]. [PERSON] et al. reported a voltammetric sensor with a novel IIP NPs and MWCNTs-modified glassy carbon electrode (GCE-IIP-MWCNTs) for selective recognition and sensitive determination of mercury ions by DPV[81]. The Hg\({}^{2+}\) IIP nanobeads were prepared by precipitation polymerization, in which 5,10,15,20-tetrakis(3-hydroxyphenyl)-porphyrin (THPP) was added into the mixture of acetonitrile/dimethylsulfoxide as the porogen solvent, MAA as the functional monomer, EGDMA as the crosslinker monomer, and Hg\({}^{2+}\) as the template. The IIP NPs in MWCNT/GCE acted as both selective inducer and pre-concentrator, making the sensor have higher selectivity and lower LOD. The designed electrochemical sensor had a wide linear response range of \(1\times 10^{-8}\)-\(7.0\times 10^{-4}\) M and a LOD of 5.0 nM. [PERSON] et al. synthesized a simple and selective thallium-imprinted polymer (TI-IP) by bulk polymerization with EGDMA as the crosslinking monomer, MAA as the functional monomer and AIBN as the initiator, and utilized the CPE modified with nano-sized TI-IP and CNTs to fabricate the electrochemical sensor for the selective preconcentration and determination of TI(I) by differential pulse anodic stripping voltammetry (DPASV)[82]. Under the optimized conditions, the suggested TI-IP-MWCNT-CPE exhibited a wide concentration range of 3.0-240 ng/mL and low LOD of 0.76 ng/mL, respectively. In addition, it had good characteristics such as the simple preparation, reproducible surface renewal by simple polishing, excellent selectivity, high sensitivity and suitable stability and was successfully applied for the electrochemical determination of trace amounts of TI(I) in water, hair and synthetic samples. [PERSON] et al. prepared a voltammetric sensor for the determination of Cr\({}^{3+}\) ion by modifying a CPE with Ce-IP, which was synthesized via precipitation polymerization using VP as the complexing ligands, MAA as the functional monomers, divinylbenzene (DVB) as  
Figure 4: The structures of (**a**) MWCNTs and (**b**) SWCNTs. Reprinted with permission from Ref. [80]. Copyright 2009, the American Chemical Society. the crosslinker and AIBN as the radical starter [83]. Then, the CPE was impregnated with Ce-IP and used for extraction and subsequent determination of Ce\({}^{3+}\). The square wave voltammetry (SWV) showed that the response of this electrode was more significant than that of electrodes modified with non-IFs. The addition of MWCNTs to the Ce-IP modified electrode further improved the signal and increased the sensitivity of the method. Ce-IP-MWCNTs-CPE showed a linear response to Ce\({}^{3+}\) in the concentration range of 1.0 to 25 pM with a LOD of 10 pM, and this method has been successfully applied to the determination of trace amounts of Ce\({}^{3+}\) in drinking water and seawater. Again, [PERSON] et al. developed an inexpensive and simple potentiometric sensor based on CPE modified with IIP and MWCNTs to selectively recognize Cr\({}^{3+}\)[84]. The all-solid-state Cr\({}^{3+}\)-selective sensor was obtained by using IIP as an ionophore.