# Nanomaterials-Based Ion-Imprinted Electrochemical Sensors for Heavy Metal Ions Detection: A Review
## 1 Introduction  
Rapid industrial development and urbanization have brought about worldwide environmental pollution, including the almost ubiquitous contamination of heavy metal ions (HMIs), which are non-biodegradable and carry the risk of entering the human body through the food chain and poses a significant threat to human health when released into open areas [1, 2]. The hazard extent of HMIs is closely related to their type, form, exposure time and dose, so their maximum allowable concentration in the environment is also different. Drinking water quality standards recommended by the World Health Organization (WHO), the European Union or the Environmental Protection Agency usually set maximum pollutant limits between the pm and ppb regimes [3]. Common toxic HMIs include Hg\({}^{2+}\), Cd\({}^{2+}\), Pb\({}^{2+}\), Cu\({}^{2+}\), Co\({}^{2+}\), Cr\({}^{3+}\), Mn\({}^{2+}\), and so on. For example, Hg\({}^{2+}\) can cause damage to kidneys, nervous system and heart system [4]. Pb\({}^{2+}\) ingestion in the body can cause serious harm to the kidney, learning disabilities, decreased intelligence quotient (IQ), growth retardation and anemia [5]. Therefore, we need to develop inexpensive, simple, and sensitive methods for HMIs detection. Currently, there are many kinds of conventional methods for detecting HMIs including surface-enhanced Raman spectroscopy (SERS), atomic absorption spectrometry (AAS),capillary electrophoresis (CE), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), ion chromatography-ultraviolet vs spectrometry (IC-UV-vis), inductively coupled plasma mass spectrometry (ICP-MS), microprobe (MP), and X-ray fluorescence spectroscopy (XPS) [6; 7]. However, these detection techniques have some drawbacks, such as high cost, time-consuming and complex instrument operation [8]. Due to the portability, low cost, reusability, good sensitivity, and high selectivity of ion-imprinted electrochemical sensors (IIECSs), which have overcome the limitations of the above methods, they have been developed for the HMIs detection in recent years [9]. It is the current research focus to further improve the selectivity of IIECSs and reduce the detection limit. Molecular imprinting is an advanced technology that enables the preparation of molecularly imprinted polymers (MIPs) from tailored binding sites that complement the shape, size, and functional groups of template molecules [10]. The concept of \"molecular imprinting\" can be traced back to [PERSON]'s research on imprinted silica gel for targeting a selective sorption of organic molecules in 1937 [11]. Then, being inspired by [PERSON]'s theory of antibody formation, [PERSON] proposed to use a very similar method to prepare adsorbents with specific recognition sites for dye molecules in silica gel in 1949 [12]. In 1972, [PERSON] and [PERSON] prepared the first MIP based on covalent interaction for theophylline molecular separation [13]. MIPs have the merits including high temperature and pressure resistance, acid-base resistance property, good stability in organic solvents, strong reproducibility and regenerative ability and controllable film thickness, which makes them have great potential as sensor sensing elements [14; 15]. Hence, MIPs have played a prominent role as an analytical tool in the development of custom sensors. Molecularly imprinted electrochemical sensors (MIECSs) combining the advantages of molecular imprinting technique (MIT) and electrochemical analysis were born [16]. MIECSs have attracted extensive attention owing to their specific recognition, rapidity, simplicity, and low cost, and have been successfully used to detect various molecules ranging from dangerous small molecules to biological macromolecules [17; 18]. Ion imprinting technique (IIT) is a new technology emerging on the basis of MIT, and the materials produced using this technique are called ion-imprinted polymers (IIPs) [19].