# 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
Adding an appropriate amount of MWCNTs to the electrode composition was found to be necessary for observing the Nernstian response. The optimized electrode composition was 76.7% graphite, 14.3% binder, 5% IIP, and 4% MWCNTs. The electrode response was significantly improved owing to the combination of MWCNTs with IIP. The proposed sensor displayed a Nernstian slope of 20.2 \(\pm\) 0.2 mV decade\({}^{-1}\) in the working concentration range of 1.0 \(\times\) 10\({}^{-6}\)-1.0 \(\times\) 10\({}^{-1}\) M with a LOD of 5.9 \(\times\) 10\({}^{-7}\) M and a fast response time of less than 40 s. It showed a stable potential response in the pH range of 2-5 with high selectivity to some interfering ions, and has been successfully applied to the Cr\({}^{3+}\) in real samples (seawater, river water and soil) measure. [PERSON] et al. synthesized a new nano-silver-IP, namely Ag-IIP-MWCNTs, for stripping voltammetric detection of Ag\({}^{+}\)[85]. The electrochemical process was based on the accumulation of Ag\({}^{+}\) ions on the surface of a nanosized silver IIP and MWCNTs modified CPE. After optimization, the Ag\({}^{+}\) concentration showed a linear relationship between 0.5 \(\times\) 10\({}^{-9}\)-2.8 \(\times\) 10\({}^{-7}\) M, and the LOD was 1.2 \(\times\) 10\({}^{-10}\) M (serial number = 3). The Relative standard deviation (RSD) of 9 consecutive measurements on different electrodes was 2.8%, and the interference study showed that several common metal ions did not interfere with the quantitative determination of Ag\({}^{+}\). Due to the high selectivity and sensitivity of this system, it was applied to the determination of Ag\({}^{+}\) in environmental water samples and standard materials. [PERSON] et al. introduced the development of a highly selective Pb\({}^{2+}\) ion electrochemical sensor based on a drop-coated GCE and an IIP suspension loaded with 1-(2-pyridazo)-2-naphthol (PAN) and MWCNTs [86]. Competitive adsorption studies on binary Pb\({}^{2+}\)/Cu\({}^{2+}\), Pb\({}^{2+}\)/Cd\({}^{2+}\), Pb\({}^{2+}\)/Ni\({}^{2+}\), Pb\({}^{2+}\)/Ni\({}^{2+}\), Pb\({}^{2+}\)/Zn\({}^{2+}\) mixtures revealed their respective relative selectivity coefficients (\(k^{\prime}\)) 301, 13.3, 9.5, 63.0 and 133.3, thus indicating that IIP-PAN had higher selectivity than NIP. The presence of potentially interfering metal ions was also investigated and demonstrated that the proposed electrode was very tolerant. In the range of 5.0-10.0 \(\upmu\)g/L, the LOD and limit of quantification (LOQ) were 0.16 \(\upmu\)g/L and 0.50 \(\upmu\)g/L, respectively. The developed sensor was effectively applied in water samples and synthetic urine with recoveries ranging from 95% to 103%. [PERSON] et al. prepared Pb\({}^{2+}\)-IIP NPs with the precipitation polymerization technique with EGDMA (as the crosslinking agent) and AIBN (as the radical initiator), and then constructed the sensor (IIP/MWCNT-CPE) by thoroughly mixing IIP (7% \(w/w\)), MWCNT (6% \(w/w\)), graphite powder (74.8% \(w/w\)) and paraffin oil (12.2% \(w/w\)) and being transferred to a hole (2.00 mm in diameter, 3 mm in depth) which was located at the end of the electrode body [87]. The synthesis process was simple. Pb\({}^{2+}\) was adsorbed and reduced to a metal form by square wave anodic stripping voltammetry (SWASV). The results showed that the presence of MWCNTs and IIP nanomaterials in the CPE enhanced the analytical signal of the electrode. The obtained IIP/MWCNT-CPE exhibited a linear range of 1.0 \(\times\) 10\({}^{-11}\)-8.0 \(\times\) 10\({}^{-8}\) M, the detection limit of 3.8 pM (S/N = 3), excellent sensitivity of 20683 A M and high selectivity toward Pb\({}^{2+}\) over common potential interfering HMIs. Since the strong van der Waals hinders the dispersion in solution, raw CNTs are rarely used directly to absorb metal ions, and proper surface treatment of CNTs can improve their dispersibility, such as vinyl functionalization CNTs [29]. [PERSON] et al. designed seven HMIIECS based on vinyl-functionalized MWCNTs.