# Nanomaterials-Based Ion-Imprinted Electrochemical Sensors for Heavy Metal Ions Detection: A Review
## 1 Introduction
As a branch of MIT, IIT has been regarded as a promising method for synthesizing materials owing to its capacity for realizing ion recognition with high specificity and selectivity [20]. IIPs differ from MIPs by altering the organic template molecules to ions [21]. The imprinting process is performed by copolymerization of functional monomers and a cross linker in the presence of templates [22]. There are differences in the choice of functional monomers between MIPs and IIPs. For MIPs, they depend on the molecular structure of the template, and the common functional monomers include methacrylic acid (MAA), methyl methacrylate, vinylphenol, pyrrole(Py), toluene, resorcinol, o-phenylenediamine (OPD), phenol, etc. [23]. However, for IIPs, the functional monomers including MAA, acrylic acid (AA), acrylamide (AM), 4-vinylpyridine (4-VP), etc., are divided into two categories that one with double bonds, and the other with functional groups such as amino groups and hydroxyl groups, which can chelate with metal ions [24]. IIPs have some benefits similar to those of MIPs, including high chemical stability, ease of preparation, low cost, predetermined recognition ability, repeatability, and adaptability to harsh environment, so as to improve the selectivity and sensitivity of electrochemical sensors (ECSs) [25]. IIPs have special coordination or electrostatic interaction functions compared to most MIPs and are usually compatible with aqueous media [26]. Moreover, the memory effect generated during the preparation of IIPs has ensured the recognition of the target metal ions with high selectivity [21; 27]. In a word, combining the advantages of electrochemical sensors such as high sensitivity, low cost and easy miniaturization, IIECSs constructed with IIPs as the sensitive material of electrochemical sensor not only has the advantages of simple preparation process and easy operation, but also has better selectivity, sensitivity and stability than the conventional ECSs [19]. Today, the unparalleled achievements of nanotechnology have given researchers a great opportunity to apply nanomaterials to the preparation of electrodes to obtain ultra sensitive surfaces with higher selectivity [28]. Nanomaterials are classified as zero-, one-, two-, and three-dimensional nanostructures that exist as single, fused, aggregated, or spherical, tubular, and irregularly shapes [29]. They show characteristics of unique and remarkable electrical conductivity, strong electrocatalytic activity, favorable mechanical properties, good stability, high surface energy and easy functionalization [15]. The charge transfer of imprinted polymer on the electrode surface can be effectively improved by using nanomaterials functionalized imprinted electrode and the synthesized novel nanomaterials can fine-tune the selectivity and sensitivity of the electrode system [18; 30; 31]. The nanomaterial-improved sensors also exhibit size dependence and high functionalization and the sensitivity and selectivity can be significantly improved by anchoring of organic ligands to the surfaces, formation of nanocomposites, and covalent functionalization of bare electrodes [6]. Although IIPs have many advantages, they also have some disadvantages such as poor electrical conductivity and electrocatalytic activity, so it is very necessary to improve the low sensitivity of the corresponding sensor by using various nanomaterials. Heavy metal ions imprinted electrochemical sensors (HMIIECSs) prepared from nanomaterial-modified electrodes have been widely utilized in the field of HMIs detection. Hence, the present review aimed to briefly narrate the fundamentals and current preparation methods of nanomaterials-based HMIIECSs for HMIs detection, especially those prepared by carbon-based nanomaterials (such as carbon nanotubes, graphene and graphitic carbonitride nanomaterials), noble-metal nanomaterials (such as gold and silver nanomaterials), and metal oxide nanomaterials (such as zinc oxide), magnetic iron oxide nanomaterials, and silica nanomaterials.